Ewi_sokromo_2008.

Transcript

Dynamic and Transient study in Mid Voltage networks in relation to Distributed Generations MSc Graduation thesis of Ivo Sokromo Delft, October 2008 University of Technology Delft Electrical Power Engineering, Electrical Power Systems Supervisors: Dr. Ir. G.C. Paap Ir. F. Wiercx Acknowledgement I have done this work with very good support and encouragement of my thesis guides, Dr. Ir. G. C. Paap and Ir. F. Wiercx. Both of my thesis guides have been excellent guides in their own specific ways and I am very grateful I have done this work under their supervision. I would also like to thank R. Kielstra of Agriport A7 for his corporation and support in this project. I also thank my parents, Cherel, my uncle and aunt and my cousins for their continuous love and encouragement which carried me throughout my time in Delft. And I would like to end with a big Thank You to all my friends who made my student life in Delft a pleasant time. Table of contents Abstract Chapter 1 Introduction…………………………………………………………. - 1 §1.1 Background………………………………………………………………. - 1 §1.2 The Agriport A7 network………………………………………………… - 2 §1.3 Matrix of events to study…………………………………………………. - 3 §1.3.1 Short circuits………………………………………………………. - 3 §1.3.2 Voltage dips……………………………………………………….. - 4 §1.3.3 Load variations…………………………………………………….. - 5 §1.3.4 Synchronization fault……………………………………………… - 5 Chapter 2 The Network Model…………………………………………………. - 7 Chapter 3 Results of the Simulations…………………………………………… - 11 §3.1 Case study 1: A three phase short circuit on the 150kV side of the Agriport sub station……………………………………………….………. - 11 §3.2 Case study 2: A three phase short circuit in the 20kV rail of the Agriport sub station……………………………………….………………. - 14 §3.3 Case study 3: A three phase short circuit in the 20 kV installation at a farmer ……….………………………………………………….……. - 18 §3.4 Case study 4: A voltage dip of 0.2pu occurs for 0.3 seconds in the 150kV grid ……….………………………………………………………. - 23 §3.5 Case study 5: A voltage dip of 0.4pu occurs for 0.3 seconds in the 150kV grid ……….………………………………………………………. - 23 §3.6 Case study 6: A load change of 2MW occurs at Farmer1…………..……. - 26 §3.7 Case study 7: A sudden load loss of 18MW occurs at Farmer1…….……. - 30 §3.8 Case study 8: A loss of 3.3MW generation power occurs at Farmer2…… - 32 §3.9 Case study 9: Loss of all generation power at Farmer2 occurs ……..…… - 34 §3.10 Case study 10: Connecting an unloaded generator with an angle difference ……….…………………………………………….…………. - 36 §3.11 Case study 11: Opening and re-closing of a generator circuit breaker of a loaded generator.……………………………………………………. - 39 Chapter 4 Adjustments in the Network ……………………………….…….… .- 43 §4.1 Adjusting the generator power controller ………………………………... - 43 §4.1.1 Case study 1 with K = 0.3…………………………………………. - 44 §4.1.2 Case study 1 with K = 3………………………………………….... - 46 §4.2 Adjusting the excitation voltage (Efd)………………………………......... - 48 §4.2.1 Case study 1 with limited Efd and K = 0.3……..…………………. - 49 §4.2.2 Case study 2 with limited Efd………………………………...….... - 50 §4.2.3 Case study 3 with limited Efd………………………………...….... - 52 §4.3 Increasing the series coils .……………………………………………….. - 53 §4.4 Adjusting the protection time settings……………………………………. ..- 61 §4.4.1 Short circuit at Farmer1 with adjusted protection time settings…… - 62 §4.4.2 Case study 2 with adjusted protection time settings………………. - 64 Chapter 5 Conclusions and Recommendation……………………….………… - 67 References………………………………………………………………………… - 71 Appendix..………………………………………………………………………… - 73 Abstract By increasing numbers of distributed generation, that moreover also become larger individually, the need arises for a better picture of the behavior of such generators during transients like short circuits, load changes, synchronizing faults, island operation, phase opposition, etc. Delft University of Technology (DUT) has a network simulator installed, called the Real-Time Digital Simulator (RTDS), on which real-time calculations on networks and components can be carried out. This makes it better possible to simulate both dynamic and transient phenomena in networks. It is for example possible to simulate grids with a considerable amount of small synchronous generators and study the response and behavior due to disturbances in the system. In this thesis simulations of short circuits, load changes and synchronizing faults will be carried out in a network with distributed generation. The purpose of this thesis is to find out if a network remains a stable system after a disturbance and what happens with the generators if a disturbance occurs. For an existing power system the interest went out to the private network of Agriport A7 which is owned by the local farmers. Agriport A7 is a modern project for large-scale greenhouse activities and a company for agribusiness. Agriport A7 lies in the north of the Netherlands. The generator units in this network are CHP units (Combined Heat and Power) of approximately 3 to 8 MW. The heat and CO2 produced is used for the plants in the greenhouses. The electricity generated is used for the lighting of the greenhouses. When there is excess of electrical energy or when there is no lighting needed, the electrical energy is delivered to the Dutch grid. By doing so, selling electrical energy is also an important income for the farmers. For normal operation generators of the farmers in Agriport A7 are connected to the system. Disturbances can take place in the 150 kV distribution grid, far away from the Agriport network or in the network itself. During disturbances it is likely that high currents flow through the system, resulting in protection operating, which can lead to disconnection of one or more generators. This is very unpleasant for the farmers. However, protection is implemented in each power system, because high currents through a power system component during disturbances -for example a short circuit- put high stress on the construction of power system component. The component will be damaged when the stress is larger than the strength of the component. This is also not what the farmers want. Disturbances that will be simulated in this research are three phase short circuits in the 150 kV grid and on different locations in the 20 kV network of Agriport A7. Further more voltage dips in the power system will be simulated. Voltage dips can be caused by short circuits (far) away from the Agriport network. Also disturbances like generator outage, for example when there is an error in the fuel supply for the prime mover, will be simulated and a situation where the load (lighting) at the farmers changes drastically. Also fault synchronizing of a generator will be observed. For each disturbance the behavior of the generators within one farmer and between neighbors farmers are studied. The conclusions in the end will indicate if the system remains stable during disturbances and if generators and how many generators will be disconnected because of the protection devices. For this study, not the whole Agriport network is modeled in the RTDS, but only a part of the network. Based on findings of this network part, conclusions are drawn for the larger network. Chapter 1 Introduction §1.1 Background In recent years, distributed generation has evolved to a significant part of the overall electric power generation capacity, especially by wind energy converters and cogeneration plants. The increasing costs of energy and the societal emphasis on environmental issues have also stimulated the growth of small combined heat and power (CHP) generation for residential, commercial and industrial applications as well as for greenhouses. For greenhouses the concept of total energy installations (including a further optimization of co-generation through the exploitation of CO2 from the exhaust-gasses, the trading of electricity and the storage of heat) is gaining more and more interest. The growth of small combined heat and power (CHP) generation is seen as being on the threshold of a new revolution in the large scale electric infrastructures. A further penetration of dispersed generation into distribution-grids as well as into (sub) transmission-grids is therefore a reality. In the past, until today, it was preferred that during each disturbance in the network, the distributed generators were immediately isolated from the grid, to keep the operational condition of the network simple and clear, safe and suitable for autoreclosing. The risk that small generators would feed the short circuit was eliminated and the fault isolation procedure remained straight forward. However, the onset of dispersed and renewable energy sources are forcing system operators to re-consider the contribution of the distributed power plants to system services as voltage control, reactive power support, frequency support, frequency control, etc. In this thesis the focus is only set on the behavior of a small network where a number of CHP’s is installed. -1- Chapter 1 Introduction §1.2 The Agriport A7 network For this thesis the network of Agriport A7 is chosen. The network of Agriport A7 is coupled to the 150kV grid in a sub station. In the sub station the voltage is transformed from 150kV to 20kV with three transformers of 80MVA each. The 20kV grid in Agriport A7 has a radial structure. 20kV cables connect the sub station with the 20 kV rail at each farmer. In series with each 20 kV cable a coil is placed to reduce short circuit currents. The CHP’s at each farmer deliver electrical energy at a voltage of 10 kV and with a power angle of 0.99 (almost 1). Each CHP is connected to the 20 kV rail through a 10/20 kV step-up transformer. At each farmer also a radial network of 20 kV cables is installed. These cables feed the 20kV/440V transformers which energize the lighting in the greenhouses and other equipment. The lighting operates with a power factor of 0.98. The 150 kV circuit breakers in the transformer station are SF6 circuit breakers. All breakers in the 20 kV network are vacuum breakers. The following figure depicts the Agriport A7 network with one farmer in more detail. Figure 1.1 The Agriport A7 network -2- Chapter 1 Introduction §1.3 Matrix of events to study A disturbance in a power system results in current and voltage changes in the system. The severity of the effect depends on the type of disturbance in the power system. The types of disturbances are arranged in a way that the worst effects are observed first followed by the disturbances with less worse effects. First the effects of short circuits are observed. Only the worst case is taken into account, thus only three phase short circuits are studied. Then the effects of voltage dips are observed, also the effects of load changes are observed and to conclude the effects of fault synchronizing of a generator. The observations are carried out in the following case studies. In each case the behavior of the generators is studied. Will the generators stay connected to the system or will they be disconnected because of the protection system? And what effect does a disturbance at one farmer have on a neighbor farmer? §1.3.1 Short circuits
Case study 1: A three phase short circuit on the 150kV side of the Agriport sub station. A three phase short circuit is simulated in the 150kV rail in the sub station of Agriport. This will result in the voltage becoming 0 volts. An example when this can occur is when digging activities cause damage to the 150kV cables feeding the Agriport sub station.
Case study 2: A three phase short circuit in the 20kV rail of the Agriport sub station. This can occur for example when an excavator damages a 20 kV cable very close to the 20 kV bus of the sub station in Agriport.
Case study 3: A three phase short circuit in the 20 kV installation at a farmer. This can occur for example when an excavator damages a 20 kV cable very close to the 20 kV bus at a farmer. Also here the effects on the generators of a neighbor farmer will be observed. -3- Chapter 1 Introduction §1.3.2 Voltage dips A voltage dip is a short and sudden decrease of the voltage. Voltage dips occur when for example there is a short circuit in the electrical power system. A voltage dip causes, for example, the lighting to dim for a very short period, which can be noticed by the naked eye. For this study the voltage dips are caused by short circuits in the 150kV grid away from the Agriport network. The closer the short circuit to the Agriport grid, the bigger the voltage dip; when there is a voltage dip of 0.4pu the short circuit location is closer than when the voltage dip is 0.2pu. In this thesis, when mentioning a voltage dip of 0.4pu, the voltage drops to 0.6pu. This means that when a voltage dip of 0.4pu occurs in the 150kV system the voltage will become 90kV for a short period. Analogue, when a voltage dip of 0.2pu occurs in the 150kV system the voltage will become 120kV for a short period. The following figure gives an example of voltage dip areas in the Netherlands caused by a short circuit in Diemen (South of Amsterdam). Figure 1.2 Extension of the voltage dip caused by a three phase short circuit in Diemen Voltage dips rarely occur. The following table gives an indication how often voltage dips occur. It is a dip-duration table for the year 2007 in the Dutch High Voltage grid with the average number of dips over all measuring locations. -4- Chapter 1 Introduction Table 1.1 Dip-duration Voltage dips, average number of dips in 2007 dip [pu] duration [seconds] 0.02 – 0.1 0.1 – 0.5 0.1 – 0.3 4 1.7 0.3 – 0.6 0.4 0.5 0.6 – 0.99 0.9 0.5 For clarity of the table; a voltage dip between 0.1pu and 0.3pu with a duration of 0.02 to 0.1 seconds occurred on an average of 4 times per year. A voltage dip usually occurs for just a very short time because in normal operation the protection systems in the 150kV grid will isolate the fault very quickly (shorter than 0.3 seconds). To analyze the effects of voltage dips in the 150kV grid, the following study cases are carried out:
Case study 4: A voltage dip of 0.2pu occurs for 0.3 seconds in the 150kV grid.
Case study 5: A voltage dip of 0.4pu occurs for 0.3 seconds in the 150kV grid. §1.3.3 Load variations To observe load variations in the network of the farmers, two types of load variations are considered. The first one is the switching on and off of the lighting in the greenhouses. The lighting in the greenhouses is controlled by a ‘climate computer’. Switching on and off is done in steps of 2 MW. Also a case when all lighting (18MW) switches off because of a disturbance in the lighting control is observed. The other type of load variations is the outage of one or more loaded generators. Outage of a loaded generator can occur for example when the prime mover shuts down when there is an error in the fuel system or when the prime mover or the generator gets overheated. For the study of load changes the following case studies are defined:
Case study 6: A load change of 2MW occurs at Farmer1.
Case study 7: A sudden load loss of 18MW occurs at Farmer1.
Case study 8: A loss of 3.3MW generation power (one generator) occurs at Farmer2.
Case study 9: Loss of all generation power (9.9MW) occurs at Farmer2. -5- Chapter 1 Introduction §1.3.4 Synchronization fault The chance of synchronization faults occurring in practice is very small because of generator protection. But to complete the study and to observe the responses of the generators they are also simulated. These responses also show that synchronization is very important. In this thesis the following synchronization faults are simulated: An unloaded generator which is not connected to the grid has the same voltage amplitude and sequence as the grid and with the same frequency. The generator will be coupled to the grid with an angle difference between the voltage of the generator and the voltage of the grid. Another synchronization fault that will be observed is opening and directly closing the circuit breaker of a loaded generator. By doing so a fault switching action of the operator is simulated. This brings us to the following case studies:
Case study 10: Connecting an unloaded generator with an angle difference.
Case study 11: Opening and closing of a generator circuit breaker of a loaded generator. -6- Chapter 2 The Network Model The electrical power system of two farmers is studied. The case studies mentioned in chapter 1 are carried out and the responses of the generators and the protection schemes are observed. The results are presented in chapter 3. In this chapter the network model that is used is described in more detail. Because of limitations of the RTDS only a part of the network is modeled. The network is a cable network. The network contains the 150kV slack node (representing the 150kV distribution grid), one 80MVA, 150/20kV transformer and two 20kV cables, each feeding a farmer. In each cable a coil is installed to reduce the short circuit currents. The generators generate electrical power at 10.5kV and each generator is connected through a 3.5MVA, 20.5/10.75kV step-up transformer. One farmer has lighting for his greenhouses, which are 18MW with a power factor of 0.98, and also has load for own auxiliaries and all the load is modeled as one bulk load of 19.14MW, 3.96MVar, which is connected to the 20kV rail at the farmer. The other farmer2 has no lighting, but does have a load of 0.8MW, 0.27MVar. This network to be analyzed is depicted in figure 2.1. Figure 2.1 Network to analyze -7- Chapter 2 The Network Model The first farmer (for convenience called Farmer1) has 3 generators, each generating 3MW at a power factor of 0.99. Farmer2 has 3 generators, each generating 3.3MW, also at a power factor of 0.99. All generators have a nominal current of 207A at a power factor of 0.8. The generators have a nominal speed of 1500rpm. The cable (cable1) that connects Farmer1 to the Agriport sub station is 960 meters long. The cable (cable2) that connects Farmer2 to the sub station has a length of 2210 meters. It can already be noted that cable2 is longer than cable1, thus the impedance of cable2 is larger than the impedance of cable1. The coils in series with the cables have reactance X = 0.3ohm and resistance R = 0.0015ohm The impedances of the cables in the network model are small and it was therefore necessary to model the network on one single rack of the RTDS. Because of limited numbers of processors on one rack the network model needed to be simplified. It is done by modeling the three small generators at each farmer as one larger generator which is three times larger than a smaller generator. Simulations in RTDS confirmed that the responses of three small generators are identical with the responses of the simplified large generator. The controls of the generators are identical and kept simple; an excitation system to control the voltage and a governor system to control the power. The generator excitation and governor systems are taken from examples of the Power System Dynamics course. The excitation system of a generator provides the energy for the magnetic field that keeps the generator in synchronism with the power system. In addition to maintaining the synchronism of the generator, the excitation system also affects the amount of reactive power that the generator may absorb or produce. The scheme of the excitation system is shown in figure 2.2. The excitation field voltage (Efd) is limited between 0 and 7. The parameters have the following values: Kf = 0.03, Tf = 1, KA = 300, KE = 1, TE = 1.25. The parameters are chosen arbitrarily. -8- Chapter 2 The Network Model Figure 2.2 Exciter and AVR The governor continuously monitors the generator speed and controls the throttle of valves that adjust the fuel flow into the prime mover in response to changes in system speed or frequency. The scheme of the governor system is shown in figure 2.3. The parameters have the following values: K = 1, T1 = 0.5. Also these parameters are chosen arbitrarily. Figure 2.3 Speed control and valve Protection schemes are important in each power system, thus they are also modeled. However, based on the scope of this thesis (transients) the focus is set on the protection with fast tripping times only. These are the over current protections tripping within 1 second. Other protections like over voltage protection, which take approximately 4 seconds to operate are not modeled. The protection of the load is also much slower than the protection of the cables, or the generators or the 80MVA transformers. Therefore they are not modeled. In the protection scheme the following is set: Circuit Breaker Transformer1 (20kV side): I >> 5kA, 0.0 sec Circuit Breaker Cable1 and Cable 2: I >> 2kA, 0.9 sec Circuit Breaker Generator (plus step-up transformer): IGen > 217A, 1 sec IGen >> 518A, 0.3 sec -9- Chapter 2 The Network Model - 10 - Chapter 3 Results of the Simulations The network as described in chapter 2 is analyzed and in this chapter the results of the simulations are presented. This is done for each of the case studies mentioned in chapter 1. The results are presented with plots. The scale of the graphs axes is chosen in such a way that when possible a comparison can be made between related cases or between the responses of the generators of Farmer1 and Farmer2. Based on the scope of this thesis the time of observation is 1.5 seconds. As mentioned earlier only protections with a trip time within 1 second are modeled. The responses of the 3MW generators are identical as well as the responses of the 3.3MW generators. Because of this, for each farmer the responses of only one generator are shown. For Farmer1 we chose the responses of Generator3 and for Farmer2 we chose for the responses of Generator5. The currents of the generators are plotted in separate graphs. All currents shown are the RMS phase currents. The active power P output of a 3MW and a 3.3MW generator are plotted in one graph, as is done for the generators reactive power Q output. Also the voltage profiles of both the 20kV busses at each farmer and the voltages at the generator terminals are plotted in one figure. In the figures presenting the voltages, 20Bus1 and 20Bus2 are the voltage profiles of the 20kV busses of Farmer1 and Farmer2 respectively. G3Bus and G5Bus are the RMS voltages at the terminals of Generator3 (Farmer1) and Generator5 (Farmer2) respectively. The voltage profile of the 150kV side is not studied. §3.1 Case study 1: A three phase short circuit on the 150kV side of the Agriport sub station In this case study the effects of a short circuit that occurs on the 150kV side of the sub station are studied. Figure 3.1.1 shows where in the network the short circuit occurs by means of a red arrow. The protection is modeled as such that when a short circuit occurs on the 150kV side, the 150kV circuit breaker of the 80MVA transformer opens instantly. This results in island operation of the two farmers. The load of both farmers - 11 - Chapter 3 Results of the Simulations Figure 3.1.1 Short circuit on the 150kV rail is 19.94MW. The total generation of both farmers is 18.9MW. That means that the generators together must increase 1.04MW to maintain active power balance between load and generation. The disruption brings the generators out of their balanced operation point and the generator speed start to oscillate (see figure 3.1.2a). It appears that the oscillation does not damp out, but the amplitude stays constant (see figure 3.1.2b). Also the other generator output quantities oscillate in island operation. The oscillation in island mode is not a desired situation. One reason for the oscillations is the setting of the governor controls, which add to the effect of the oscillations. a) after switch to island operation b) island operation Figure 3.1.2 Machine speed Figure 3.1.3 shows the active power P and reactive power Q output of the generators. The fluctuation in the speed can be seen in the machine active power P output and also in the machine currents (figure 3.1.4). - 12 - Chapter 3 Results of the Simulations Figure 3.1.3 Generators P and Q output The peak in the active power P output (the sudden decrease) is the result of the short circuit in the 150kV side of the sub station which is switched off immediately. Also the peak in the reactive power Q output is the result of the short circuit, because at the moment that the short circuit occurs, the system voltage decreases rapidly for a short time (see figure 3.1.5a). Figure 3.1.4 shows the generator currents and figure 3.1.5 shows the voltage profiles when the short circuit occurs. The peak downwards that is first seen in figure 3.1.5a is the moment the short circuit occurs. The peak has very short duration because the 150kV transformer circuit breaker opens shortly after the short circuit is detected. After opening of the transformer circuit breaker the voltages at the farmers also show oscillations. The oscillations in the voltages do not damp out when in island operation, like mentioned before (see figure 3.1.5b). Figure 3.1.4 Generator currents - 13 - Chapter 3 Results of the Simulations a) after switch to island operation b) island operation Figure 3.1.5 Voltage profiles In island mode the generator voltages of Farmer1 oscillate between 8.8kV and 11.6kV, and the generator voltages of Farmer2 oscillate between 9.1kV and 11.9kV. The 20kV bus voltages of Farmer1 oscillate between 16.8kV and 22.8kV, and the 20kV bus voltages of Farmer2 oscillate between 17.4kV and 23.2kV. The voltage at the 20kV busses should be 20kV±10% and in island operation the voltage peaks are outside the voltage interval. As mentioned earlier, the oscillations during island operations can be a result of improper settings of the generator’s governor controls. In chapter 4 we will see that a different setting of the governor controls result in better results. §3.2 Case study 2: A three phase short circuit in the 20kV rail of the Agriport sub station A short circuit occurs in the 20kV bus in the Agriport sub station. The short circuit is depicted in figure 3.2.1. Figure 3.2.1 Short circuit in the 20kV bus at Agriport - 14 - Chapter 3 Results of the Simulations First the 20kV circuit breaker of the 80MVA transformer opens. This is because the short circuit causes a very high current flow through the 80MVA transformer, resulting in opening the transformer breaker (see figure 3.2.2). Figure 3.2.2 shows the currents through the 20kV windings of the transformer. Figure 3.2.2 Currents through the 80MVA transformer; Short circuit 20kV Agriport sub station The short circuit is first fed by the grid and the generators. This is shown by the steep peak in the 80MVA transformer currents and the generator currents. After switching off the 80MVA transformer the short circuit is still fed by the generators, resulting in high generator currents right after the peak (see figure 3.2.3). The generators are disconnected because of over current relay tripping. Figure 3.2.3 Generator currents; Short circuit 20kV Agriport sub station - 15 - Chapter 3 Results of the Simulations The generators active power P and reactive power Q output are shown in figure 3.2.4. The active power P output reduces to almost zero, because of the short circuit. The short circuit causes the 20kV bus voltages to become almost zero volts (see figure 3.2.5). The generator voltage control detects this voltage drop and increases the generator excitation power to improve the generator voltage. This results in the increase of reactive power Q output, hence the increase of the generator currents. Figure 3.2.4 Generators P and Q output; Short circuit 20kV Agriport sub station Figure 3.2.5 show spikes in the generator’s terminal voltages. These spikes are the transient recovery voltages (TRVs), because the generator circuit breakers open. After the generator circuit breakers open the terminal voltages increase. Figure 3.2.5 Voltage profiles; Short circuit 20kV Agriport sub station - 16 - Chapter 3 Results of the Simulations The increase of the terminal voltages is a result of the excitation system. During the period that the generators feed the short circuit, the terminal voltage is very low and the excitation system increases the excitation field voltage (Efd) in order to maintain the nominal terminal voltages. When the generator circuit breakers open, the fault is cleared and the terminal voltage suddenly increase to nominal voltage, but the Efd is still too high, thus the terminal voltages keep increasing. The excitation system again lowers the Efd and the terminal voltages then decrease to nominal value. It appears that the generator terminal voltages reach a maximum of a little more than 1.5 times the nominal value (16.4kV). These are the results with the settings of the exciter as mentioned in chapter 2. In chapter 4 we will see that the generator terminal voltages do not reach high values with different setting of the excitation system. Figure 3.2.6 shows the cable currents. The cable currents during the short circuit do not exceed 2kA, thus the cable breakers do not trip. Figure 3.2.6 Cable currents; Short circuit 20kV Agriport sub station When the short circuit occurs, the generators active power output reduces to almost zero, because they cannot deliver the active power to the grid anymore. The mechanical power input of the prime mover is not in balance anymore with the generator power output. The difference in power results in the increase of the machine speed. The generator speed does not increase to infinity because in this model the governor adjusts the mechanical torque as a response to changes in the machine speed. - 17 - Chapter 3 Results of the Simulations Figure 3.2.7 Generator speed; Short circuit 20kV Agriport sub station It must be noted that the over speed protection of the prime movers is not implemented. In practice the prime movers are protected against sudden large speed changes, mostly by reducing or shutting down the fuel input. The generator circuit breaker might also trip in a shorter time than these simulations show. §3.3. Case study 3: A three phase short circuit in the 20 kV installation at a farmer This case study is divided into two parts because there are two farmers with a different cable length connecting them to the sub station. The cable to Farmer1 is shorter than the cable to Farmer2. In the first part a short circuit is simulated in the 20kV rail at Farmer1 and in the second part a short circuit is simulated at Farmer2. Part A A short circuit occurs in the 20kV bus at Farmer1. This is depicted in figure 3.3.1. Figure 3.3.1 Short circuit at Farmer2 - 18 - Chapter 3 Results of the Simulations First the 20kV circuit breaker of the 80MVA transformer opens, because of high short circuit currents flowing through the 80MVA transformer (see figure 3.3.2) and also because the trip time is much faster than the over current relay of the cables and the generators. Figure 3.3.2. Current through the 80MVA transformer; Short circuit 20kV Farmer1 After switching off the 80MVA transformer the short circuit is still fed by the generators. The generator currents increase as shown in figure 3.3.3. The generators of Farmer1 are disconnected due to the fast tripping (I >> 518A, 0.3 sec) of the over current relay and the generators of Farmer2 due to tripping of the over current relay (I > 217A, 1 sec). Figure 3.3.3 Generator currents; Short circuit 20kV Farmer1 - 19 - Chapter 3 Results of the Simulations The generators of Farmer1 are disconnected from the grid earlier than the generators of Farmer2. This makes sense, because the short circuit is at Farmer1 and hence the magnitude of the generator currents here is higher than that at Farmer2. The generator currents increase because the generators reactive power Q output increases as a result of the generators exciters. This is shown in figure 3.3.4. Figure 3.3.4 Generators P and Q output; Short circuit 20kV Farmer1 The currents in the two cables feeding the farmers are depicted in figure 3.3.5. Cable1 first carries the short circuit current from the grid and the generators of Farmer2. The contribution of the grid to the short circuit current is switched off when the 80MVA transformer circuit breaker opens. After that only the short circuit currents from the generators of Farmer2 flow through cable1 until those generators are isolated from the network. Figure 3.3.5 Cable currents; Short circuit 20kV Farmer1 - 20 - Chapter 3 Results of the Simulations At the first moment of the short circuit the currents in cable1 exceed 2kA, but the protection of cable1 is not activated because its trip time is slower than the generators protection and the 80MVA transformer protection. After switching off the 80MVA transformer the currents in both cables do not exceed the value of 2kA, thus the over current relay of the cables do not trip. The short circuit causes the voltages of the 20kV busses to become almost zero. The short circuit is at Farmer1 and his 20kV bus reaches very close to zero volts. In figure 3.3.6 it can be seen that the voltage of the 20kV bus at Farmer2 is a little higher than the voltage at Farmer1 during the disturbance. This is because of the cable impedance between the two farmers. The voltages at the 20kV busses are zero only when all the generators at both farmers are isolated from the grid. Also in this figure is seen that the generator terminal voltages increase after clearing of the fault. Again this is the result of the generator’s excitation system. The effect of a short circuit at Farmer1 is a blackout at both farmers. In the following part the effects of a short circuit in the 20kV bus at Farmer2 is observed. Figure 3.3.6 Voltage profiles; Short circuit 20kV Farmer1 - 21 - Chapter 3 Results of the Simulations Part B In the second part of this case study a short circuit occurs in the 20kV bus at Farmer2. (see figure 3.3.7) Also in this case the current through the 80MVA transformer exceeds 5kA and the 20kV circuit breaker opens (see figure 3.3.8). The transformer currents are lower compared to the transformer currents in part A of this case study because cable2 is longer than cable1. Figure 3.3.7. Short circuit at Farmer2 The larger impedance of cable2 reduces the short circuit currents. In principle the responses of the generators are similar to part A of this case study, with the difference that the generators of Farmer2 are switched off earlier than those of Farmer1. The effect of a short circuit at Farmer2 is also a blackout at both farmers. The responses are presented in the following figures. Figure 3.3.8 Current through the 80MVA transformer; Short circuit 20kV Farmer2 - 22 - Chapter 3 Results of the Simulations Figure 3.3.9 Generator currents; Short circuit 20kV Farmer2 Figure 3.3.10 Generators P and Q output; Short circuit 20kV Farmer2 Figure 3.3.11 Cable currents; Short circuit 20kV Farmer2. - 23 - Chapter 3 Results of the Simulations Figure 3.3.12 Voltage profiles; Short circuit 20kV Farmer2 A short circuit at the 20kV bus of either Farmer1 or Farmer2 results in a blackout at both farmers. However, this is not desirable for the farmer where the short circuit did not occur, because it means that his production process will be disturbed. In chapter 4 measures are discussed to prevent a blackout at the farmer where the short circuit did not occur. §3.4 Case study 4: A voltage dip of 0.2pu occurs for 0.3 seconds in the 150kV grid When a voltage dip of 0.2pu occurs in the 150kV grid the voltage becomes 0.8pu (which is equal to 120kV in the 150kV system). This dip has duration of 0.3 seconds as mentioned earlier in chapter 1. The voltage dip in the 150kV grid results also in a voltage dip in the network of the farmers. This is seen in figure 3.4.1. The minimum voltage of the 20kV bus is 16.7kV at Farmer1 and 17kV at Farmer2. The voltage dip in the 20kV bus is 0.17pu at Farmer1 and 0.15pu at Farmer2. The voltage of the generators at Farmer1 reaches a minimum value of 8.7kV and at Farmer2 the minimum is 8.9kV. The dip in the generator voltage at Farmer1 is 0.13pu and 0.11pu at Farmer2. These numbers indicate that Farmer1 has a larger voltage dip compared to Farmer2, and comparing the per unit values, the voltage dips in the Agriport network are smaller than the 0.2pu dip in the 150kV grid. - 24 - Chapter 3 Results of the Simulations Figure 3.4.1 Voltage profiles; 0.2pu voltage dip The generators of both farmers stay connected to the grid during and after the voltage dip. During the voltage dip the generators exciters increase the Efd, resulting in the generators delivering extra reactive power Q to increase the voltage. After 0.3 seconds the dip ends. The generators show an instant decrease in reactive power Q, but because the Efd is still high the generator reactive power Q output increases again and then decreases as the exciter controls the Efd to get the generator terminal voltage to nominal level. The response of generator active power P and reactive power Q output is shown in figure 3.4.2. The up and down response of the voltage profiles after the voltage dip is related to the up and down response of the generator Q output. During the disturbance the generator active power P output fluctuates around its operating point. The reactive power Q output however shows large increase, which is also the reason why the generator currents increase. The response of the generator currents is shown in figure 3.4.3. Figure 3.4.2 Generators P and Q output; 0.2pu voltage dip - 25 - Chapter 3 Results of the Simulations Figure 3.4.3 Generators currents; 0.2pu voltage dip When a voltage dip occurs, the generators experience a “shock”, a disruption. The disruption brings the generators out of their pre-fault operating. This can be compared to the mass-spring system. When a balanced mass-spring system is disturbed, the mass-spring system starts oscillating until it reaches its new balance point. Here the generators experience two disruptions. The first one is when the voltage dip starts and the second one is when the voltage dip ends. In both cases the generator active power P outputs starts oscillating. The oscillation is also seen in the generators speed. Figure 3.4.4 Generators speed; 0.2pu voltage dip - 26 - Chapter 3 Results of the Simulations §3.5 Case study 5: A voltage dip of 0.4pu occurs for 0.3 seconds in the 150kV grid Just like the voltage dip in case study 4, this larger voltage dip also results in a voltage dip in the Agriport network. This is shown in figure 3.5.1. The voltage dip of 0.4pu results in the same responses like the 0.2pu dip, but with larger amplitudes. The voltage dip in the 20kV bus at Farmer1 is 13kV (0.35pu) and 13.3kV (0.34pu) at Farmer2. The voltage dip in the generator voltage at Farmer1 is 7.3kV (0.27pu) and 7.16kV (0.28pu) at Farmer2. Just like in case study 4 the per unit voltage dips in the Agriport network are smaller than the voltage dip in the 150kV grid. Figure 3.5.1 Voltage profiles; 0.4pu voltage dip The responses of this case study show the same responses as that in case study 4, but with considerably larger amplitudes. The plots are presented below. The same explanations of case study 4 apply. Also in this case all the generators stay connected to the grid during and after the voltage dip. Figure 3.5.2 Generators P and Q output; 0.4pu voltage dip - 27 - Chapter 3 Results of the Simulations Figure 3.5.3 Generators currents; 0.4pu voltage dip Figure 3.5.4 Generators speed; 0.4pu voltage dip §3.6 Case study 6: A load change of 2MW occurs at Farmer1 The load (lighting) at Farmer1 is switched on and off in steps of 2MW. In this case study both the increase and decrease of 2MW in load at Farmer1 is observed. The responses of the generator active power P outputs are shown in figure 3.6.1. In this plot the response of the grid active power (PGRID) is added. In figure 3.6.1 it can be seen that the generators active power P outputs do not show changes because the difference of 2MW power is provided by the grid. - 28 - Chapter 3 Results of the Simulations a) 2MW load decrease b) 2MW load increase Figure 3.6.1 Generator and Grid Power Also the generator currents, the voltages at the 20kV busses and the generator terminals as well as the generator speed do not show (significant) changes. This is demonstrated in the following plots. Figure 3.6.2a Generator currents; 2MW load decrease Figure 3.6.2b Generator currents; 2MW load increase - 29 - Chapter 3 Results of the Simulations a) 2MW load decrease b) 2MW load increase Figure 3.6.3 Voltage profiles a) 2MW load decrease b) 2MW load increase Figure 3.6.4 Generator speed §3.7 Case study 7: A sudden load loss of 18MW occurs at Farmer1 In this case study the total load (lighting) at Farmer1 switches off abruptly, for example due to an error. The sudden loss of 18MW load results in small fluctuation in the generators active power P output. The sudden load change is balanced by the grid. Figure 3.7.1 displays this. - 30 - Chapter 3 Results of the Simulations Figure 3.7.1 Generator and Grid Power; 18MW load loss The following plots show the responses of the generator currents, the voltages at the 20kV busses and the generator terminals and the generator speed. Figure 3.7.2 Generator currents; 18MW load loss Figure 3.7.3 Voltage profile; 18MW load loss - 31 - Chapter 3 Results of the Simulations Figure 3.7.4 Generator speed; 18MW load loss All generators remain connected to the network and the system remains stable after a loss of 18MW load. There is only fluctuation noticeable in the generator active power P output, the generator currents and the generator speed. The fluctuation damps out fast. §3.8 Case study 8: A loss of 3.3MW generation power occurs at Farmer2 A generator of Farmer2 is switched off at full load (for example due to an error of the machine fuel system). Seen from the grid this has the same effect as a load increase of 3.3MW. Figure 3.8.1 shows that the sudden change in power is balanced by the grid and that the generators show small fluctuation. The amplitudes of the fluctuation are larger at Farmer2, compared to Farmer1. The generators remain connected to the grid and the system is stable. Figure 3.8.1 Generator and Grid Power; 3.3MW generation loss - 32 - Chapter 3 Results of the Simulations The responses of the generator currents, the network voltages and the generator speed are presented in the following plots. Figure 3.8.2 Generator currents; 3.3MW generation loss Figure 3.8.3 Voltage profile; 3.3MW generator loss Figure 3.8.4 Generator speed; 3.3MW generator loss - 33 - Chapter 3 Results of the Simulations The generator active power P output and the generator currents show small fluctuation, which damp out. The voltages in the network and the generator speed do not show (significant) change. §3.9 Case study 9: Loss of all generation power at Farmer2 occurs The effects of the loss of all generators of Farmer2 at full load are now observed. Seen from the grid this event is equal to a load increase of 9.9MW (3 times 3.3MW). Figure 3.9.1 shows again that the change in power is balanced by the grid. The active power P output of the generators of Farmer1 show small fluctuation which damp out. This fluctuation is also seen in the generator currents (figure 3.9.2). Figure 3.9.1 Generator and Grid Power; 9.9MW generation loss Figure 3.9.2 Generator currents Farmer1; 9.9MW generation loss - 34 - Chapter 3 Results of the Simulations The voltages in the 20kV network do not show changes. Only a very small peak is noticeable in the voltage of the generators of Farmer2 at the moment when the 9.9MW generation is switched off (figure 3.9.3). Figure 3.9.3 Voltage profile; 9.9MW generator loss The generators of Farmer1 show only small fluctuation in speed (figure 3.9.4). The speed of the generators of Farmer2 increase from the moment they are switched off but the speed does not increase to infinity, because the power controller is regulating the generator speed. Figure 3.9.4 Generator speed; 9.9MW generator loss - 35 - Chapter 3 Results of the Simulations §3.10 Case study 10: Connecting an unloaded generator with an angle difference The following synchronization fault is simulated and studied: An unloaded generator which is not connected to the grid has the same voltage amplitude and sequence as the grid voltage and also the generated voltage has the same frequency as the voltage of the grid. The generator will be coupled to the grid with an angle difference between the voltage of the generator and the voltage of the grid. The following angle differences are observed: -180°, -120°, -60°, -20°, +20°, +60°, +120°, +180°. The angle difference is the angle of the voltage at the grid side minus the angle of the voltage at the generator side of the generator circuit breaker. When a generator is connected to the grid with an angle difference, high currents will flow through the generator in order to get the voltages of the grid and the generator synchronized. The magnitude of the generator currents at the moment of closing the generator circuit breaker depends on the angle difference. For the fault synchronization a generator of 3.3MW, for convenience called Generator6, is selected. All other generators are connected to the grid and operate at full load. Table 3.1 shows with which of the above mentioned angle differences result in the opening of the generator circuit breaker of the ‘faulted generator’. Table 3.1 Voltage difference and response of generator protection -180° “Generator6” switched off due to protection yes Other generators switched off due to protection no -120° yes no -60° yes no -20° yes no +20° no no +60° Yes no +120° Yes no +180° Yes no Voltage angle difference It appears that in all cases the other generators stay connected to the grid. Figure 3.10.1 demonstrates what the responses of Generator6-currents are when fault synchronizing. The responses in figure 3.10.1 are of fault synchronizing with a voltage angle difference of -120° degrees. It can be seen that the RMS-currents have - 36 - Chapter 3 Results of the Simulations rectangular shapes (figure 3.10.1a). This is because the generator currents are not sinusoidal, but they contain a DC-component, which is shown in figure 3.10.1b. a) RMS currents b) instantaneous currents Figure 3.10.1 Currents of Generator6 at -120° angle difference The worst case scenario is when closing the generator breaker if the voltage angle difference is 180°. 180° is also the maximum angle difference which can appear when closing the generator breaker. Now attention will be paid to the other generators in the worst case scenario. First the currents of the other generators will be observed. The fault synchronizing is at a 3.3MW generator of Farmer2. The effect of the fault synchronizing is therefore bigger on the generators of Farmer2. The currents of the generators of Farmer2 show fluctuations with larger amplitudes compared to the generators of Farmer1. The currents of the generators of Farmer1 do not exceed the tripping value of the generator protection, so they stay connected. And the currents of the generators of Farmer2 exceed the tripping value of 217A, but not long enough to trip the generator breaker. Figure 3.10.2 Generator currents; Fault synchronizing, -180° angle difference - 37 - Chapter 3 Results of the Simulations In the worst case scenario, fault synchronizing of Generator6 does not result in opening the generator circuit breakers of the other generators. The responses of other quantities are now presented in the following figures. Figure 3.10.3 Generators P and Q output; Fault synchronizing, -180° angle difference Figure 3.10.4 Voltage profiles; Fault synchronizing, -180° angle difference Figure 3.10.5 Generator speed; Fault synchronizing, -180° angle difference - 38 - Chapter 3 Results of the Simulations Connecting a generator with an angle difference of 180° does not result in opening the generator circuit breakers of the other generators. When Generator6 is connected to the grid with an angle difference smaller than 180°, the responses of the other generator currents have the same shape, but with smaller amplitudes. It can therefore be concluded that in case of fault synchronizing with angle differences smaller than 180° the other generators also stay connected to the grid. In case of fault synchronizing with angle differences smaller than 180°, also the responses of the generator active power P and reactive power Q outputs, the voltage profiles and the generator speed have the same shape, but with smaller amplitudes. The low frequency in the responses is due to the disruption that the generators experience. The high frequency in figure 3.10.3 and figure 3.10.5 is 50Hz and is because of the DC component in the generator currents of the generators of Farmer2. §3.11 Case study 11: Opening and re-closing of a generator circuit breaker of a loaded generator In this case study opening and directly re-closing the circuit breaker of a loaded generator will be simulated. The generator circuit breaker is open for just 0.5 seconds. By doing so a fault switching action of the operator is simulated. The responses will be observed. This opening and re-closing of the generator circuit breaker is done for a 3.3MW generator of Farmer2, for convenience called Generator6. First attention is paid to the currents of Generator6 (see figure 3.11.1). Before opening the generator circuit breaker the generator currents have a constant value. When the generator circuit breaker is opened, the generator currents drop to zero kA. When reclosing the generator circuit breaker it again shows that the generators RMS-currents have rectangular shapes. This is because the generator currents are not sinusoidal, but they contain a DC-component (figure 3.11.1b), similar like the generator currents in the previous case study when the generator circuit breaker is closed with an angle difference between the generator voltage and the grid voltage. - 39 - Chapter 3 Results of the Simulations a) RMS currents b) instantaneous currents Figure 3.11.1 Currents of Generator6 The effect of opening and re-closing of the generator circuit breaker is that Generator6 is disconnected because of the over current protection of the generator. Now the effects on the other generators will be observed. First the generator currents of the other generators are presented in figure 3.11.2. It can be seen that the currents of the generators show oscillations after opening and after closing of the Generator6 circuit breaker. The currents of the generators of Farmer2 show oscillations with higher amplitudes compared to the generator currents of Farmer1. In Farmer2 the generator currents exceed the trip value of 217A, but for a period shorter than the tripping time. So the generators of Farmer2 stay connected to the grid. Also the generators of Farmer1 stay connected to the grid, because their currents do not even exceed the trip value of 217A. Figure 3.11.2 Generator currents; Fault synchronizing, opening and re-closing circuit breaker - 40 - Chapter 3 Results of the Simulations It can be noted that the responses of the generator currents are a combination of case study 8 (loss of 3.3MW generation) and case study 10 (connecting a generator with an angle difference). After opening of the generator circuit breaker of Generator6, the responses of the generator currents look like that in case study 8. After closing the Generator6 circuit breaker the responses of the generator currents look like that in case study 10. This behavior can also be seen in the generator active power P and reactive power Q output, the generator speed and the voltage profiles. Figure 3.11.3 Generators P and Q output; Fault synchronizing, opening and re-closing circuit breaker Figure 3.11.4 Voltage profiles; Fault synchronizing, opening and re-closing circuit breaker - 41 - Chapter 3 Results of the Simulations Figure 3.11.5 Generator speed; Fault synchronizing, opening and re-closing circuit breaker From case studies 10 and 11 it can be seen that these types of fault synchronizing do not result in disconnecting the ‘not faulted’ generators from the grid. However it is still important to synchronize because without synchronizing very high currents flow through the ‘faulted generator’ and other generators experience disturbances as the simulations above demonstrated. - 42 - Chapter 4 Adjustments in the Network Chapter 3 presented the responses of the generators and it can be seen that a voltage dip of 0.2pu and 0.4pu, load changes, outages of generators and fault synchronizing of a generator do not result in opening the generator circuit breakers. When a short circuit occurs on the 150kV side of the Agriport sub station the generators circuit breakers do not open and the farmers will operate in island mode. Then the generators of both farmers supply power to the load at both farmers. But the oscillations in island operation are not what we want. In this chapter we will make adjustments to prevent the oscillations in island operation. Also, when a short circuit occurs on the 20kV side of the Agriport sub station or in the 20kV busses at either one of the farmers, the effect is a black out at both farmers. This is not desirable. Black out at both farmers occurred in case study 2 and case study 3 and in this chapter some adjustments will be made to the existing network to prevent a black out at both farmers. §4.1 Adjusting the generator power controller In chapter 3 we saw oscillations in the generator outputs when the farmers operate in island mode. This is not a desirable situation for the farmers, because the generators experience a swinging output and the voltages have swinging magnitudes. This can be because of the governor control settings. In the governor scheme shown in figure 2.3 in chapter 2, K is the inverse of the droop R. The droop is the magnitude of the slope of the speed-versus-power output characteristic of a generating unit (see figure 4.1.1). Figure 4.1.1 Speed-governing characteristic of a generating unit - 43 - Chapter 4 Adjustments in the Network In practice the per unit droop Ru is mostly used. The formula for Ru is: Ru = ∆f fR ∆P PR ∆f / f R ∆P / PR : the change in frequency : the rated frequency (50Hz) : the change in active power : the rated active power Ru is the magnitude of the slope of the speed-versus-power output characteristic when the frequency axis and the power-output axis are each scaled in per unit of their respective rated values. The unit of R is Hz/MW and the unit of K is thus MW/Hz. In chapter 2 is arbitrarily chosen for K is 1. That means that when the frequency changes with 1Hz, the generator output changes with 1MW. The droop R is thus 1Hz/MW. For a generator with 3MW rated output and 50Hz system frequency, the Ru is 6%. When the generators of the farmers are connected to the grid it is desired that the generators have a larger Ru because we do not want the generators to react on load changes and therefore frequency changes in the grid. If Ru is large, for example Ru = 20%, the droop R = 3.33Hz/MW and K = 0.3MW/Hz. That means that when the frequency changes with 1Hz, the generator output changes with only 0.3MW. But when operating in island mode, we would like the frequency to be constant and a small Ru is desired. If Ru is small, for example Ru = 2%, the droop R = 0.33Hz/MW and K = 3MW/Hz. That means that when the generator output changes with 1MW, the frequency changes with only 0.33Hz. In the following, the results of the simulations for case study 1 are done with K = 0.3 (large Ru) and K = 3 (small Ru), and the effects in island operation are observed. §4.1.1 Case study 1 with K = 0.3 When the short circuit occurs in the 150kV grid, the 150kV circuit breaker of the 80kV transformer opens and the generators operate in island mode. In figure 4.1.2 we observe the machine speed. In the time of observation it shows that the machine speed oscillates after the 150kV transformer circuit breaker opens (figure 4.1.2a). The oscillation damps out and in island operation the machine speed becomes 1478rpm (figure 4.1.2b). That means that the frequency in the system of the farmers is 49.27Hz in island operation. - 44 - Chapter 4 Adjustments in the Network a) after switch to island operation b) island operation Figure 4.1.2 Machine speed, K=0.3 Figure 4.1.3 shows the generators active power P output and it can be seen that after opening of the 80MVA transformer circuit breaker the P-outputs also show oscillations which damp out. In steady state we see that each generator has approximately 0.2MW increase of active power P. With the same Ru for all generators, the load difference will be divided equally over the generators. The difference between load and generated active power P is 1.04MW. Each generator will thus experience a P-output change of 1.04/6 = 0.17MW. This is close to the measured value of 0.2MW. a) after switch to island operation b) island operation Figure 4.1.3 Generators P output, K=0.3 The voltages at the farmers also show oscillations because of the response of the swinging speed of the generator (figure 4.1.4). The oscillations in the voltages damp out and in steady state the voltages reach the values as before the disturbance - 45 - Chapter 4 Adjustments in the Network occurred. The highest value of the generator voltages during the oscillations is 11.5kV (Farmer1) and 11.8kV (Farmer2). And the highest value of the 20kV busses at the farmers is 22.6kV (Farmer1) and 23.2kV (Farmer2). These high values only appear for a short time and therefore acceptable. Figure 4.1.4 Voltage profiles, K=0.3; Switch to island operation Plots of the other quantities are not shown. The responses of the other quantities are similar as shown in § 3.1, with the difference that the oscillations damp out. §4.1.2 Case study 1 with K = 3 We have seen the responses for operation in island mode with K = 0.3 as governor setting. In this paragraph a large K is chosen (K = 3) and again we look what happens when the generators operate in island mode. The results are presented below. In figure 4.1.5 we observe the machine speed. Also here it shows that the machine speed oscillates after the 150kV transformer circuit breaker opens (figure 4.1.5a). The oscillation damps out and in island operation the machine speed becomes 1497.8rpm (figure 4.1.5b). This means that the frequency in the system of the farmers is 49.93Hz in island operation. - 46 - Chapter 4 Adjustments in the Network a) after switch to island operation b) island operation Figure 4.1.5 Machine speed, K=3 Figure 4.1.6 shows the generators active power P output and it can be seen that after opening of the 80MVA transformer circuit breaker the P-outputs also show oscillations which damp out. In steady state we again see that the difference in load and generated active power is divided equally over the six generators, as was seen in §4.1.1. a) after switch to island operation b) island operation Figure 4.1.6 Generators P output, K=3 Also with a large K the voltages show oscillations because of the response of the excitation system (figure 4.1.7). The oscillations in the voltages damp out and in steady state the voltages reach the values as before the disturbance occurred. The highest value of the generator voltages during the oscillations is 11.9kV (Farmer1) and 12.1kV (Farmer2). And the highest value of the 20kV busses at the farmers is 23.4kV (Farmer1) and 23.9kV (Farmer2). The high voltage values occur for a very - 47 - Chapter 4 Adjustments in the Network short time and are therefore acceptable. The next paragraph will show how to avoid the high voltages after switching to island operation. Figure 4.1.7 Voltage profiles, K=3; Switch to island operation Analogue to §4.1.1, the responses of the other quantities are similar as shown in §3.1, with the difference that the oscillations damp out. Adjusting K in the governor results in preventing oscillations which were seen in §3.1. If we look at the frequency in island operation, we see that the frequency is very close to 50Hz for either K = 0.3 (Ru = 20%) or K = 3 (Ru = 2%), so for this case choosing K = 0.3 would be proper. The generators then do not experience big influences on frequency changes in the grid and when switching to island operation the frequency is still close to 50Hz. In §4.1.1 and §4.1.2 we see that the generator voltages and the 20kV buses reach high values after switching to island operation. The maximum values are close to the values observed in case study 1 in §3.1. In the following we will adjust the excitation voltage (Efd) for a better result in the voltage profiles. §4.2 Adjusting the excitation voltage (Efd) Chapter 3 taught us that when a short circuit occurs, the system voltage will show a sudden decrease and during this period the generator reactive power Q output increases, which cause the increase of the generator currents. The generators are - 48 - Chapter 4 Adjustments in the Network disconnected because of the high currents that flow through the generator during short circuit. We will now reduce the generator currents during short circuits. This means that reactive power Q output should be smaller when a short circuit occurs. This is done by lowering the limits of the Efd of the generator exciter. When the generators outputs are 3MW and 3.3MW with a power factor of 0.98, the currents are 170A and 183A respectively and the Efd is 2.1pu. In chapter 3 the limits of the Efd are: 0pu < Efd < 7pu. In this paragraph 0pu < Efd < 3pu will be used and case studies 2 and 3 are carried out. To see what effect this has in island operation, also case study 1 will be carried out. We start with case study 1. §4.2.1 Case study 1 with limited Efd and K = 0.3 In §4.1 we saw that the voltages at the farmers reach high values after switching to island operation. This is because of the increase of reactive power, due to the response of the excitation system. By lowering the limits of the Efd, the generator reactive power Q output does not increase so fast compared to higher limit setting of the Efd. We start with observing the generators active power P and reactive power Q output (figure 4.2.1). We pay closer attention to the reactive power Q output and if we compare figure 4.2.1 with figure 3.1.2 we see that in figure 4.2.1 after opening of the 80MVA transformer circuit breaker the reactive power Q output slowly increases. Figure 4.2.1 Generators P and Q output; Case study 1, adjusted limits Efd - 49 - Chapter 4 Adjustments in the Network Therefore the generator voltages also increase slowly as well as the 20kV busses (see figure 4.2.2). Thus by lowering the Efd limits the generator voltage do not reach high voltages after switching to island operation, as was seen before. Figure 4.2.2 Voltage profiles; Case study 1, adjusted limits Efd §4.2.2 Case study 2 with limited Efd At first the short circuit is fed by the grid and the generators. The 20kV circuit breaker of the 80MVA transformer opens almost immediately and after that the short circuit is still fed by the generators. The Efd limits of the generators are lowered so that the generator reactive power Q output does not keep increasing like was shown in figure 3.2.3. The active power P and reactive power Q outputs are presented in figure 4.2.3. When the short circuit occurs, the reactive power Q output has a steep increase and after opening of the 20kV circuit breaker of the 80MVA transformer the generators reactive power Q decreases. Figure 4.2.3 Generators P and Q output; Case study 2, Adjusted limits Efd - 50 - Chapter 4 Adjustments in the Network The decrease in the generator reactive power Q output determines the profile of the generator currents. The generator currents are shown in figure 4.2.4. Figure 4.2.4 Generators currents; Case study 2, adjusted limits Efd Figure 3.2.4 in chapter 3 showed us that the generator terminal voltages reach high values after the generators are disconnected from the grid. This is due to the response of the voltage control as was explained in §3.2. Figure 4.2.5 now shows the effect of lower limits of Efd on the voltages at the farmers. It appears that in the time of observation the generator terminal voltages do not reach high values as was seen in figure 3.2.4. Figure 4.2.5 Generators currents; Case study 2, adjusted limits Efd - 51 - Chapter 4 Adjustments in the Network This paragraph learns us that the generators are still disconnected because the generator currents are higher than 217A for longer than 1 second. After adjusting the limits of the Efd, the effect of a short circuit in the 20kV bus at Agriport is still a black out at both farmers. §4.2.3 Case study 3 with limited Efd The effect of a short circuit in the 20kV bus at either one of the farmers resulted in a total black out, as was seen in case study 2 in the previous chapter. Case study 3 is again carried out, but now with lowered Efd limits. In chapter 3 case study 3 taught us that the effects of a short circuit at Farmer1 are similar to the effects of a short circuit at Farmer2. Therefore in this paragraph only the responses of a short circuit at Farmer1 are shown. The responses of a short circuit at Farmer2 are similar, analogue to case study 3 in chapter 3. Also now the 20kV circuit breaker of the 80MVA transformer opens quickly because of the high short circuit current through the transformer. The responses of the active power P and reactive power Q outputs (figure 4.2.6) look almost the same as in figure 4.2.3 in the previous paragraph. The active power P becomes almost zero and the reactive power Q does not increases because of the lower limits of the generator Efd. Figure 4.2.6 Generators P and Q output; Case study 3, adjusted limits Efd Eventually both the active power P and reactive power Q become zero when the generators are disconnected. The generator circuit breakers open due to the high generator currents. The generator currents are shown in figure 4.2.7. - 52 - Chapter 4 Adjustments in the Network Figure 4.2.7 Generators P and Q output; Adjusted limits Efd The voltage profile for this case is the same as shown in figure 4.2.5. The voltage profiles are the same, because the 20kV circuit breaker of the 80MVA transformer opens instantly, whether the short circuit is in the 20kV bus in the sub station or in the 20kV bus at one of the farmers. After adjusting the limits of the Efd, the effect of a short circuit in the 20kV bus at either farmer is still a black out at both farmers. Lowering the limits of the Efd did not prevent a total black out if a short circuit occurs in the 20kV bus at the Agriport sub station or at a farmer. This is because the circuit breaker of the 80MVA transformer opens first. In the next paragraph we take measures to reduce the short circuit current through the 80MVA transformer and observe the effects in the system. §4.3 Increasing the series coils In the previous parts it showed that when a short circuit occurs in the 20kV network of the farmers, the 20kV circuit breaker of the 80MVA transformer opens because the short circuit currents exceed the tripping value of 5kA. If we now reduce the short circuit currents through the 80MVA transformer such that the 20kV circuit breaker does not open, we can look what the response are of the generators and the protection of the cables. The short circuit currents through the 80MVA transformer are reduced by increasing the coils in series with the cables feeding the farmers. By increasing the coils the impedance is made larger and larger impedance will reduce the short circuit currents. - 53 - Chapter 4 Adjustments in the Network This however, will only work for short circuits at the farmers in our network model. If a short circuit occurs in the 20kV bus of the Agriport sub station, increasing the coils will have no effect on the short circuit current through the 80MVA transformer. Hence the responses with increased series coils are only observed for case study 3. Case study 3 is done for several sizes of the coils. Each time the coils at both cables are adjusted. First the coils are made two times the original size, then three times, then four, five and six times the original size. The limitations for the generators Efd as mentioned in §4.2 are applied. Table 4.1 demonstrates the effects on the peak of the short circuit current through the 80MVA transformer (20kV side) when the coils are increased and also if the protection opens the 20kV circuit breaker of the transformer or not. Table 4.1 Short circuit currents with increasing coils and response of transformer protection Coil size [times original size] 1 Short circuit at Farmer1 Isc,peak 80MVA effect transformer transformer [kA] circuit breaker 8.94 opens Short circuit at Farmer2 Isc,peak 80MVA effect transformer transformer [kA] circuit breaker 8.04 opens 2 7.62 opens 6.97 opens 3 6.66 opens 6.17 opens 4 5.92 opens 5.55 stays closed 5 5.33 stays closed 5.04 stays closed 6 4.86 stays closed 4.62 stays closed When the coil is made twice and three times its original size and a short circuit occurs at either one of the farmers, the short circuit current through the 80MVA transformer is still high enough to trip the transformer protection. And the responses of the generators are according to case study 3 in chapter 3 and the result is a black out at both farmers. If the coil size is four times larger, a short circuit at Farmer1 results in instant opening of the 20kV circuit breaker of the 80MVA transformer and the end result is a black out at both farmers. But a short circuit at Farmer2 does not trip the transformer protection and the 20kV circuit breaker stays closed. - 54 - Chapter 4 Adjustments in the Network Next a closer look is taken at the responses of the generators when the coils are four times larger and the short circuit occurs at Farmer2. The short circuit is in the 20kV bus at Farmer2 and the currents of all the generators increase instantly. The generator currents at Farmer2 have higher amplitude. This is shown in figure 4.3.1. The circuit breakers of the generators of Farmer2 open because of fast tripping of the generator because vv Figure 4.3.1 Generator currents; Short circuit 20kV Farmer2, coils 4 times increased protection (IGen >> 518A, 0.3 sec). The currents of the generators of Farmer1 also increase and the generator protections of these generators trip because of fast tripping (IGen >> 217A, 1 sec). The generator active power P and reactive power Q are presented in figure 4.3.2 and we see that the reactive power Q have a negative slope after the sudden increase. Figure 4.3.2 Generators P and Q output; Short circuit 20kV Farmer2, coils 4 times increased - 55 - Chapter 4 Adjustments in the Network Now attention is paid to the currents through cable1 and cable2 (see figure 4.3.3). First the short circuit is fed by the grid and the generators. The generators of Farmer2 switch off first and the generators of Farmer1 follow shortly after that. When the generators of Farmer2 are isolated, the short circuit is still fed by the grid and the short circuit current flows through cable2. The protection of cable2 trips and the circuit breaker of cable2 opens. The short circuit is then isolated and there is no current through cable2. After isolation of the fault, there is still current flowing through cable1 because of the load at Farmer1. Figure 4.3.3 Cable currents; Short circuit 20kV Farmer2, coils 4 times increased If we look at the current through the 80MVA transformer in figure 4.3.1, we see that the transformer circuit breaker does not open. The short circuit current flows through the 80MVA transformer until the circuit breaker of cable2 opens. After the fault is isolated the transformer current has a steep decrease and what remains is the load current for the load of Farmer1. Figure 4.3.4 Current through the 80MVA transformer; Short circuit 20kV Farmer2, coils 4 times increased - 56 - Chapter 4 Adjustments in the Network When increasing the coils to four times the original size and simulating a short circuit at the 20kV bus of Farmer2, the effect is a black out at Farmer2 and at Farmer1 the generators get disconnected, but the load (lighting) is still in operation. This can also be seen in the voltage profiles at both farmers, shown in figure 4.3.5. After the disturbance the 20kV bus voltage of Farmer2 becomes zero and the 20kV bus of Farmer1 stays energized. Figure 4.3.5 Voltage profiles; Short circuit 20kV Farmer2, coils 4 times increased Simulations are also done with coils five and six times the original coil size. The effects on the protection are summarized in table 4.2a and table 4.2b for all mentioned coil sizes. The table’s show which circuit breakers open and which ones stay closed and what effect the short circuit has on the generators. Table 4.2a gives a summary for a short circuit occurring at Farmer1 and table 4.2b for a short circuit occurring at Farmer2. Table 4.2a Effects when a short circuit occurs at Farmer1 Coil size CB T80MVA CB Cable1 CB Cable2 1 opens stays closed stays closed 2 opens stays closed stays closed 3 opens stays closed stays closed 4 opens stays closed stays closed 5 stays closed opens stays closed 6 stays closed opens stays closed [times original size] - 57 - effect on generators all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers overall effect black out at both farmers black out at both farmers black out at both farmers black out at both farmers black out at Farmer1 only black out at Farmer1 only Chapter 4 Adjustments in the Network Table 4.2b Effects when a short circuit occurs at Farmer2 Coil size CB T80MVA CB Cable1 CB Cable2 1 opens stays closed stays closed 2 opens stays closed stays closed 3 opens stays closed stays closed 4 stays closed stays closed opens 5 stays closed stays closed opens 6 stays closed stays closed opens [times original size] effect on generators overall effect all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers all disconnect at both farmers black out at both farmers black out at both farmers black out at both farmers black out at Farmer2 only black out at Farmer2 only black out at Farmer2 only The results of the simulations show that if the coils are five and six times the original size, and when a short circuit occurs at a farmer, only the circuit breaker of the cable feeding that farmer opens. This is an improvement because the faulted bus gets isolated from the grid and the 20kV bus of the farmer without the fault stays energized. However we also see that for all mentioned sizes of the coils, all the generators of both farmers are disconnected from the grid and it does not matter at which farmer the short circuit occurs. This is because of the high currents flowing through the generators. The high currents are because of the increase of the generators reactive power Q outputs. By increasing the coils, only the short circuit currents through the 80MVA transformers are reduced. When there is a short circuit at Farmer1 the responses of the generators look the same like in figure 4.3.1 and 4.3.2 with the difference that responses of the generators of Farmer1 and Farmer2 are switched with each other. This means that for example the currents of the generators of Farmer1 show the same response like the currents of the generators of Framer2 when the short circuit occurs at Farmer2. This is demonstrated in figure 4.3.6 and figure 4.3.7, where we will look at responses of a short circuit at Farmer1 and with the series coils five times the initial size. - 58 - Chapter 4 Adjustments in the Network Figure 4.3.6 Generator currents; Short circuit 20kV Farmer1, coils 5 times increased Figure 4.3.7 Generators P and Q output; Short circuit 20kV Farmer1, coils 5 times increased The responses of the cable currents and the 80MVA transformer currents and the voltage profiles are presented in figure 4.3.8, 4.3.9 and 4.3.10 respectively. Figure 4.3.8 Cable currents; Short circuit 20kV Farmer1, coils 5 times increased - 59 - Chapter 4 Adjustments in the Network Figure 4.3.9 Current through the 80MVA transformer; Short circuit 20kV Farmer1, coils 5 times increased Figure 4.3.10 Voltage profiles; Short circuit 20kV Farmer1, coils 5 times increased The responses of a short circuit at Farmer2 with coils five times larger look similar to the responses of a short circuit at Farmer2 with coils four times larger and are therefore not presented. Plots of the responses for simulations with six times larger coils are also not presented. The responses are the same like when the coils are five times larger. The essential difference is that the short circuit current through the 80MVA transformer is reduced and the 20kV transformer circuit breaker does not open. Making the coils larger will only reduce the short circuit currents through the 80MVA transformer, and the generators at both farmers will still be disconnected. In the next paragraph we will adjust the protection time settings to prevent a black out at both farmers. - 60 - Chapter 4 Adjustments in the Network §4.4 Adjusting the protection time settings Until this point the system analysis is done with the protection settings presented in chapter 2. In this paragraph we will adjust the protection time settings for a more favorable system operation. This means that when a short circuit occurs at Farmer1, the circuit breakers of the generators of Farmer2 do not open, and the other way around. The original time settings of the generators protection are kept. Only the time settings of the 80MVA transformer protection and of the cables protection are adjusted. When a short circuit occurs at one farmer, we do not want the 80MVA transformer circuit breaker to be the first to open, because this will result in a black out at both farmers, as was seen in case study 2 in chapter 3. We also want the circuit breaker of the cable feeding the farmer with the fault to open before the generators of the other farmer get disconnected. This means that the trip time of the cable protection should be the shortest. The tripping time of the 80MVA transformer protection is then chosen to be later than the tripping time of the generator protection to make sure that even when the generators of the non-faulted farmer switch off, the 20kV bus stays energized. The following protection settings are thus modeled: - Circuit Breaker Transformer1 (20kV side): I >> 5kA, 0.4 sec - Circuit Breaker Cable1 and Cable 2: I >> 2kA, 0.25 sec - Circuit Breaker Generator (plus step-up transformer): IGen > 217A, 1 sec IGen >> 518A, 0.3 sec It can be noted that only the very fast tripping times of the transformer and cable protection are adjusted. We have seen that by lowering the limits of the generators excitation voltage, the generators reactive power Q outputs are lower and hence the generator currents are lower than with higher limits for the excitation voltage. The following simulations are done with adjusted time settings of the protections and with lowered limits of the generators excitation voltage. The coils in series with the cables have their initial value. - 61 - Chapter 4 Adjustments in the Network §4.4.1 Short circuit at Farmer1 with adjusted protection time settings We first start with a short circuit at the 20kV bus of Farmer1. For convenience the currents through the 80MVA transformer and the cables are presented in one figure (figure 4.4.1). It shows that 0.25 seconds after the short circuit occurs the currents through cable1 have a steep fall to zero amperes and stay zero amperes. The circuit breaker of cable1 thus opens after 0.25 seconds. Before the cable1 circuit breaker opens, the short circuit current from the grid flows through the 80MVA transformer and through cable1. The short circuit current through cable1 is the sum of the short circuit current from the grid and from the generators of Farmer2. It can be seen that the contribution of the generators to the short circuit current is significantly small compared to the contribution of the grid. Figure 4.4.1 80MVA transformer currents and cable currents; Short circuit 20kV Farmer1, adjusted protection time settings When the cable1 circuit breaker opens, the short circuit is isolated from the grid, but the generators of Farmer1 still feed the short circuit. Because of the lower limits of the excitation voltage the currents of the generators of Farmer1 have a negative slope. The currents of the generators are presented in figure 4.4.2 and we see that the generators of Farmer1 get switched off. After opening of cable1 circuit breaker the currents of the generators of Farmer2 show a peak. After the peak, the currents oscillate around the initial current value before the short circuit occurred at Farmer1. The generators of Farmer2 stay connected to the grid. - 62 - Chapter 4 Adjustments in the Network Figure 4.4.2 Generator currents; Short circuit 20kV Farmer1, adjusted protection time settings Figure 4.4.3 shows the responses of the generator’s output. During the short circuit the active power P output of the generators of Farmer1 is almost zero. The generator currents are thus mainly because of the reactive power Q output, hence the response of the reactive power Q seem similar to the currents of the generators of Farmer1. We now look at the responses of the generators of Farmer2. The current peak that occurs in the generators after fault clearing is a result of the response of the generators reactive power Q outputs, which shows a sudden steep fall after fault clearing. The reactive power Q has a sudden steep fall, because after fault clearing the voltage jumps back to nominal value and the generator suddenly have to decrease the Q output. It is also seen that during the short circuit the generators still deliver active power P to the grid. After the fault is cleared, the active power P shows oscillations. These oscillations are also seen in the generator currents. Figure 4.4.3 Generator P and Q output; Short circuit 20kV Farmer1, adjusted protection time settings - 63 - Chapter 4 Adjustments in the Network The voltage profiles are shown in figure 4.4.4. It shows that the 20kV bus of Farmer2 stays energized. And the voltages at the terminals of the generators of Farmer2 again return to the nominal value. Figure 4.4.4 Voltage profiles; Short circuit 20kV Farmer1, adjusted protection time settings With the adjusted protection time settings Farmer2 stays on line and Farmer1 has a black out. Simulations show that when a short circuit occurs at Farmer2, the result is that Farmer1 stays on line and Farmer2 has a black out. §4.4.2 Case study 2 with adjusted protection time settings We will now look what happens when a short circuit occurs at the 20kV bus in the sub station with the adjusted protection time settings. Again the currents through the 80MVA transformer and the cables are presented in one figure (figure 4.4.5). The 20kV circuit breaker of the 80MVA transformer opens first. The short circuit is still on the 20kV bus station and is fed by the generators. The currents through the cables are too small to trip the cable circuit breakers. Figure 4.4.5 80MVA transformer currents and cable currents; Short circuit 20kV sub station, adjusted protection time settings - 64 - Chapter 4 Adjustments in the Network The currents of the generators are presented in figure 4.4.6 and we see that the generators circuit breaker open because of the fast tripping time of the over current protection. Figure 4.4.6 Generator currents; Short circuit 20kV sub station, adjusted protection time settings Figure 4.4.7 shows the responses of the generators output. The active power P reduces to almost zero because of the short circuit. During the short circuit the voltages also reduce to almost zero (see figure 4.4.8) and the voltage controller increases the excitation voltage which results in increase of reactive power Q, as was seen in previous cases. Figure 4.4.7 Generator P and Q output; Short circuit 20kV sub station, adjusted protection time settings - 65 - Chapter 4 Adjustments in the Network Figure 4.4.8 Voltage profiles; Short circuit 20kV sub station, adjusted protection time settings When a short circuit occurs at the 20kV bus of the sub station the cable circuit breakers will not open, because the currents do not exceed the trip value. But the 20kV circuit breaker of the 80MVA transformer opens and circuit breakers of all the generators open. The result is thus a black out at both farmers. With the new settings for the trip time of the transformer and the cable protection, we see good results when a short circuit occurs at one of the farmers; only the farmer with the short circuit gets isolated from the grid. When we look at all the voltage profiles we see in some cases a very short peak. The peaks seen in the voltage profiles are the transient recovery voltages (TRV’s) at the moments of switching. In this thesis no detailed attention is paid to the transient recovery voltages. In appendix A a few plots of the TRV’s are depicted. - 66 - Chapter 5 Conclusions and Recommendations The case studies mentioned in chapter 1 are carried out in the network model which is presented in chapter 2. This network model is a small portion of the complete network of the farmers in Agriport A7. The network is simplified because of the necessity to model the network on one rack of the RTDS. The results of the case studies are presented in chapter 3 and are based on the data that was available and based on the network model in chapter 2. The goal of this study is to observe the responses of the generators when a transient disturbance occurs. Attention is paid to the generator active power P and reactive power Q outputs, the generator currents, the generator mechanical speed and the generator terminal voltages. Also the voltages of the 20kV and 10kV busses at each farmer are observed. Further more is observed whether the generators stay connected to the grid or whether the generator protection will open the generator circuit breaker. And to complete the analysis, also attention is paid to the protection of the 80MVA transformer and the cables feeding the farmers. We have seen that all the generators in the network model stay connected when a voltage dip of 0.2pu and 0.4pu occurs in the 150kV grid. The effect of a 0.4pu voltage dip is more severe than a voltage dip of 0.2pu. It can be concluded that all generators stay on line for voltage dips smaller than 0.4pu. The generators also stay connected to the grid when there is load change in the Agriport network. The largest load change observed is a load loss of 18MW. The study showed that the generators experience a disruption out of their balanced operation point, but they stay connected to the grid. The load change is quickly balanced by the grid. It can be concluded that for load losses smaller than 18MW the generators also stay connected to the grid. Generation loss can also be seen as load change. Seen from the grid a generation loss is similar to increase of load. The largest generation loss observed is a generation loss of 9.9MW. The results showed that the other generators in the network also experience a disruption out of their balanced operation point, but they stay connected - 67 - Chapter 5 Conclusions and Recommendations to the grid. Just like a load loss, the generation loss is quickly balanced by the grid. It can be concluded that for generator losses smaller than 9.9MW the generators also stay connected to the grid. Fault synchronizing of one generator does not result in disconnecting of the other generators in the Agriport network. However we have seen that the other generators experience a disruption out of their balanced operation point. And the disruption is more severe for the generators at the farmer with the ‘faulted generator’. In this study we have seen that the effects on the ‘faulted generator’ and the other generators in the network can not be neglected and it proves that synchronizing is very important. The most severe effects are observed when a short circuit is simulated. A short circuit at either one of the farmers results in a black out at both farmers. In chapter 4 several measures are taken to prevent this. One measure was to increase the coils in series with the cables. The effect is that the short circuit current through the 80MVA transformer decreases if a short circuit occurs at one of the farmers. To prevent a black out at the farmer without the short circuit, the coils should be at least 5 times the initial size in the network model, and this is very large. The best solution to prevent a black out at both farmers is to arrange the protection time settings. The problem in the original model is that the protection of the 80MVA transformer is much faster then the cable protections and the generator protections. It is important to prevent the generators from damage so the protection settings of the generators don’t change. By adjusting the time settings and making the very fast tripping time of the cable protection shorter than the 80MVA transformer protection, a favorable situation is reached. The result is that only the farmer with the short circuit gets isolated from the grid and the other farmer stays on line. But for a short circuit in the 20kV bus of the sub station the result is a still a black out at both farmers. The network is modeled such that when a short circuit occurs in the 150kV grid, the Agriport network will operate in island mode. With proper setting of the generator’s governor, it is possible to operate in island mode without oscillation, as was seen in chapter 4. In chapter 4 we observed the responses of the generator with different settings of the power controls and it can be recommended that the generator’s - 68 - Chapter 5 Conclusions and Recommendations governor can be set with K = 0.3 (large droop). The generators will then not experience big influences in frequency changes in the grid when connected and in island operation there would be very small deviation in the system frequency. It can also be recommended to adjust the voltage controller. The limits of the excitation voltage Efd can be set to a lower value and this will result in slower increase in generator reactive power Q output and hence slower increase in generator currents. This also has positive effects on the voltages at the generator terminals and the 20kV bus voltages at the farmers. Compared to a higher voltage limit of the excitation voltage, the voltages will not reach high values during disturbances and when the generator is disconnected. The TRV’s (transient recovery voltages) are not observed in this thesis, but they are present when switching actions occur. In the appendix a very brief demonstration of the TRV’s is given. Further study on the TRV’s is recommended. In this thesis attention is paid to the behavior of small synchronous generators. For future study, a dynamic and transient study on a network with windmills or a combination of windmills and synchronous generators can be carried out. - 69 - Chapter 5 Conclusions and Recommendations - 70 - REFERENCES [1] A. Janssen, Q. Bui-Van, F. Iliceto, M. Waldron, S. de Azevedo Morais, B.Middleton, J. Jäger, F. Gallon, M. Glinkowski, “Changing Network Conditions and System Requirements, Part 1 The impact of distributed generation on equipment rated above 1 kV”, Cigre WG A3.13, 2008 [2] P. Kundur, “Power System Stability and Control”, McGraw-Hill, Inc., 1994 [3] J.J. Grainger, W.D. Stevenson, “Power System Analysis”, McGraw Hill Inc., 1994 [4] P.C. Sen, “Principles of Electric Machines and Power Electronics”. (2nd ed.), Wiley, 1997. [5] P.L.J. Hesen, R. Oto, “Spanningskwaliteit in Nederland 2007”, Kema Nederland BV, 2008 [6] W. Kling, P. Bresesti, I. Valadè, D. Canever, R. Hendriks, “Transmission systems for offshore wind farms in the Netherlands” - 71 - - 72 - Appendix Transient Recovery Voltage The time of observation in this thesis is chosen to be 1.5 seconds. To present the currents and voltages we chose for RMS values, because if we would present the instantaneous currents and the voltages it would be inconvenient to see the response during transient disturbances due to the 50Hz. When observing the voltage profiles of the different case studies, we saw that there are spikes when there were switching actions of the circuit breakers. These spikes are the transient recovery voltages (TRV’s). In this appendix we will briefly discuss the voltages during the switching actions of the generator circuit breakers. The instantaneous voltages are presented. We will take a look at the voltages at both farmers. First the time of observation is 1.5 seconds and we will zoom in at the moment of switching. The voltages on both sides of the generator circuit breaker are presented. Also the voltages at the terminals of the generator and the voltages across the generator circuit breaker are presented. We only zoom into the voltage across the generator circuit breaker. In the following figure is shown at which points in the system we observe the voltages. Figure A1. Points of voltage observation in the system - 73 - Appendix A short circuit is simulated at the 20kV bus of Farmer1 (case 3a in chapter 3) and observations of the transient recovery voltage are done. We first look at the voltages at Farmer 1. Figure A2 and A3 show respectively the voltages at the grid side and the generator side of the generator circuit breaker. Figure A2. Voltage Generator CB, 20kV bus Grid side, Farmer1 Figure A3. Voltage Generator CB, 20kV bus Generator side, Farmer1. The voltage across the generator circuit breaker is the difference between the voltage at the grid side of the circuit breaker and the voltage at the generator side and is presented in figure A4. The voltage of phase A shows the highest peak just after the generator circuit breaker opens. The peak values of transient voltages of phase A, B and C are respectively 4.1, 2.5 and 1.9 times the nominal amplitude of the instantaneous voltage. - 74 - Appendix Figure A4. Voltage across the Generator CB, Farmer1. Figure A5 shows the voltages at the generator terminals. After the generator circuit breaker opens, the peaks of the transient voltages of phase A, B and C are respectively 1.6, 1.6 and 1.7 times the nominal amplitude of the instantaneous voltage. Figure A5. Voltage Generator terminals, Farmer1. In figure A4 the voltage over the generator circuit breaker is presented. The time of observation is 1.5 seconds. In figure A6 we zoom in some cycles after opening of the generator circuit breaker. A high frequency component caused by the step-up transformer at one side of the generator circuit-breaker and a low frequency component caused by the 150/20 kV transformer and the cables at the other side of the circuit-breaker are visible. The amplitude of the high frequency component reduces and becomes zero after more than 8 cycles. - 75 - Appendix Figure A6. Zoom into the voltage across the Generator CB, Farmer1. In figure A7 the transient recovery voltage is shown in more detail. It must be noted that the scale of the U-axes in figure A7 is not the same. The frequency of the high frequency component is approximately 9kHz. Figure A7. Voltage across the Generator CB, Farmer1; zoom in transient. We will now observe the voltages at Farmer2 with the short circuit occurring at Farmer1. Figure A8 and A9 show respectively the voltages at the grid side and the generator side of the generator circuit breaker. And figure A10 shows the voltage across the generator circuit breaker. In Figure A10 we see that the voltage across the circuit breaker of phase C shows the highest peak just after the generator circuit breaker opens. The peak values of transient voltages of phase A, B and C are respectively 2.2, 1.6 and 4.2 times the nominal amplitude of the instantaneous voltage. - 76 - Appendix Figure A8. Voltage Generator CB, 20kV bus Grid side, Farmer2 Figure A9. Voltage Generator CB, 20kV bus Generator side, Farmer2. Figure A10. Voltage across the Generator CB, Farmer2. Figure A11 shows the voltages at the generator terminals. After the generator circuit breaker opens, the peaks of the transient voltages of phase A, B and C are respectively 1.3, 2.0 and 1.3 times the nominal amplitude of the instantaneous voltage. - 77 - Appendix Figure A11. Voltage Generator terminals, Farmer2. In figure A12 we zoom into the voltage across the circuit breaker, some cycles after opening of the generator circuit breaker. Also at Farmer2 a high frequency component is visible. The amplitude of the high frequency component reduces and becomes zero after more than 8 cycles. Figure A12. Zoom into the voltage across the Generator CB, Farmer2. In figure A13 the transient recovery voltage is shown in more detail. It must be noted that the scale of the U-axes in figure A13 is not the same. Also at Farmer2 the frequency of the high frequency component is approximately 9kHz. - 78 - Appendix Figure A13. Voltage across the Generator CB, Farmer2; zoom in transient. In other cases where the generator circuit breaker opens due to a disturbance, the same voltage profiles will be seen. - 79 -