Transcript
Subject, Course & Code: Power Systems 3: ENEL4PS S2
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UNIVERSITY OF KWAZULU-NATAL Electrical, Electronic and Computer Engineering Discipline FINAL EXAMINATIONS: SEPTEMBER 2014 ENEL4PS3 – POWER SYSTEMS 3 DURATION: 3 HOURS
TOTAL MARKS: 100
Examiners: Dr I E Davidson Prof Komla Folly
(Internal) (External)
Instructions: This examination paper consists of 4 pages (inclusive of this front page) Answer FOUR out of FIVE questions ONLY. All questions carry equal marks Please take note of the formulae provided below, some of which you may find useful in answering the questions in this examination. Any formulae that do not appear below are considered required knowledge for this course. No other formula sheets will be allowed into the examination. Gauss Seidel Iterative Algorithm: For j I, where yij Yij , yij Yii
x1 x0
Newton Raphson equation:
f x0
df where f ' x0 f x0 dx 0 '
d 2 E 'V Machine Swing Equation, M 2 Pm sin X dt Angular momentum, M
2GH
0
2 H c 0 f 0 Pm
Critical clearing time, t c
Accelerating torque (power acting) Pacc Ps Pe and Pe
Power delivered (1-Φ), Pe P12
V1V2 sin 12 X 12
3-phase synchronous machine, P3
3Vt E f Xs
Developed torque, T
sin
Q
3Vt E f Xs
P3
syn
cos
E 'V sin X
3 Vt E f sin syn X s
3Vt
2
Xs
1 3 1 3 Operator, a j a2 j a 3 1.0 j 0 2 2 2 2 Va 1 1 1 Va 0 Va 0 1 1 1 Va 1 2 a Va1 Symmetrical components, Vb 0 1 a a 2 Va Phase Voltages, Vb 1 a 3 Vc 1 a a 2 Va 2 Vc 0 1 a 2 a Va Z Z Zb Zc Zc Za Z Z Z Zb Zc Zc Za Z ZB a b – Y transformation: Z A a b Za Za Zb Zb Z Z Zb Zc Zc Za Z ZC a b Zc Zc Z B ZC Z AZC Z AZ B Reverse transformation Y – : Z a Zb Zc Z A Z B ZC Z A Z B ZC Z A Z B ZC All symbols have their usual meaning.
Subject, Course & Code: Power Systems 3: ENEL4PS S2
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Question 1
[25 Marks]
(a.)
Fault studies are extremely important in power systems. What information do fault studies provide and how is this used practically in power systems design and operation? Mention the assumptions used in fault studies. [4]
(b.)
A 20MVA 11kV 3-Φ generator has X '' j 20% . It is connected through a Δ-Υ transformer to a high voltage transmission line having a total series reactance of 100Ω. At the load end of the line is a Υ-Υ step-down transformer. Both 3-Φ transformers are composed of single-phase (1-Φ) transformers connected for three-phase operation. Each 1-Φ transformer is rated 8333kVA 11/132kV and combines to have a reactance of 10%. The load represented as an impedance drawing 15000kVA at 12.5kV and 86.6% power factor lagging. (i.) Draw the one-line diagram and mark the kVbase in the various parts of the power system. [1] (ii.) Draw the positive sequence impedance diagram showing all the impedances in per unit. [2] (iii.) Using per unit system, and choosing a base of 15MVA 12.5kV in the load circuit, determine the voltage (in per unit, and kV) at the terminals of the generator. [8]
(c.)
List and briefly discuss any two practical methods for controlling the load bus voltage?
Question 2
[4]
[25 Marks]
(a.)
What is a SMART Grid? What are the driving force(s) behind the development of SMART Grids? Clearly identify the energy resources that can be harnessed in SMART Grids? [8]
(b.)
List any six major technologies in use for enabling SMART Grids? Discuss the advantages and disadvantages of SMART Grid technology. [6]
(c.)
A factory takes 600 kVA at a lagging power factor of 0.6. Discuss specific methods you will adopt to improve power factor (power quality) at this factory. [3] An engineer proposes to install a synchronous motor and use it to raise the power factor to 0.9 lagging. If this new motor is taking 200 kW. Calculate:
(d.)
(i.)
The corresponding apparent power (in KVA) taken by the motor.
[2]
(ii.)
The power factor at which it operates?
[2]
Why is power quality such an important issue? What are the sources of quality problems?
[4]
Subject, Course & Code: Power Systems 3: ENEL4PS S2
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Question 3
[25 Marks]
(a.)
What technical factors constrain the amount of electric power that can be delivered through a specific high voltage transmission route? [4]
(b.)
How might the transmission capacity of a HVAC or HVDC line be enhanced using FACTS device(s)? [Be specific and use detailed illustrations] [8]
(c.)
In a Micro-Grid network, two loads LD1 and LD 2 connected in parallel are supplied from a generator at 415V, 50 Hz. The power requirement for LD1 is 12 kW at 0.8-power factor lagging, while LD 2 requires 15 kVA at 0.6 power factor lagging. A capacitor is then added to the circuit and connected in parallel to the load to provide power factor correction. (i.)
Sketch a simple circuit of the network before and after the installation of the capacitor. [1]
(ii.)
Draw and label a power triangle (or phasor diagram) showing real (P), reactive (Q) and complex power (S) before and after the installation. [1]
Calculate the following: (iii.) The value of reactive power kVAr and the capacitance required for the generator to operate at 0.95 power factor lagging. [5] (iv.) The generator current I g without the capacitor in the circuit.
[3]
(v.)
[3]
The total load current I L with the capacitor in the circuit.
Question 4 (a.)
What is Artificial Intelligence? Briefly discuss basic principles with applications of any TWO of the following artificial intelligence techniques in power and energy systems: [6]
(b.)
[25 Marks]
Knowledge-based systems Fuzzy systems ANNs Evolutionary Computing
Using suitable illustrations, clearly explain any TWO of these applications of AI techniques:
Intelligent Systems for Demand Forecasting Artificial Intelligence Techniques for Voltage Control Adaptive techniques using Neural Networks Protection and System Control Neuro-expert System Applications in Power Systems
[6]
Subject, Course & Code: Power Systems 3: ENEL4PS S2
(c.)
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A Wye-connected synchronous generator has the following per unit sequence reactance: X 0 0.09 X1 0.22 and X 2 0.36 . The neutral point of the machine is grounded through a reactance of 0.09 per unit. The machine is running at no-load with rated terminal voltage when it suffers an unbalanced fault. The fault currents out of the machine are I a 0 , Ib 3.75150 and I c 3.7530 all in per unit with respect to phase a line-to-neutral voltage. Determine:
(i.)
The terminal voltages in each phase of the machine with respect to ground.
[8]
(ii.)
The voltage of the neutral point of the machine with respect to ground.
[3]
(iii.) What type of fault occurred in part (i.) above?
Question 5
[2]
[25 Marks]
(a.)
Discuss the basic operation of a turbo-alternator in a typical coal-fired power plant. Using suitable block diagrams, sketch and label energy-conversion sequence from coal to the output electrical energy. Discuss the function and operation of each element in this bulk electricity generation scheme. [9]
(b.)
3-phase synchronous generators are widely used as the preferred choice as bulk electric power generation plants. List and briefly describe any six inherent advantages synchronous generators have in bulk power generation? [6]
(c.)
A 3-, 5-kVA, 208 V, 4-pole, 60Hz, Y-connected synchronous machine has negligible stator winding resistance, and a synchronous reactance of 8 per phase at a rated terminal voltage. The machine is first operated as a generator in parallel with a 3-, 208 V, 60Hz power supply. (i.)
Determine the excitation voltage per phase E and the power angle when the machine is delivering rated kVA at 0.8 power factor lagging. [2]
(ii.)
If the field excitation current is now increased by 20% (without changing the prime mover power), find the stator current, power factor, reactive kVA supplied by the machine. [2]
With the field current as in (i), the prime mover power is slowly increased.
(d.)
(iii.)
What is the steady-state (or static) stability limit?
[2]
(iv.)
What are the corresponding values of the stator (or armature) current, power factor, and reactive power at this maximum power transfer condition? [2]
Carry out a critical comparison and identify the key advantages and disadvantages of standalone power networks and interconnection power networks? [2] END OF EXAM
