Transcript
Temperature dependency of wave propagation velocity in MV power cable Li, Y.; Wouters, P.A.A.F.; Wagenaars, P.; van der Wielen, P.C.J.M.; Steennis, E.F. Published in: 18th International Symposium on High Voltage Engineering (ISH 2013), August 25-30 2013, Seoul
Published: 01/01/2013
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Citation for published version (APA): Li, Y., Wouters, P. A. A. F., Wagenaars, P., Wielen, van der, P. C. J. M., & Steennis, E. F. (2013). Temperature dependency of wave propagation velocity in MV power cable. In 18th International Symposium on High Voltage Engineering (ISH 2013), August 25-30 2013, Seoul (pp. 1861-1866). Seoul.
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Download date: 23. Oct. 2017
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TEMPERATURE DEPENDENCY OF WAVE PROPAGATION VELOCITY IN MV POWER CABLE 1*
1
2
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Yan LI , Peter A. A. F. Wouters , Paul Wagenaars , Peter C. J. M. van der Wielen and 1,2 E. Fred Steennis 1 Eindhoven University of Technology, P.O. Box 513, 5600 MB Eindhoven, Netherlands 2 DNV KEMA Energy & Sustainability, P.O. Box 9035, Arnhem 6800 ET, Netherlands *Email: Abstract: Propagation velocity of high frequency signals, e.g. from partial discharge, is a vital parameter for time domain power cable diagnostic techniques. The propagation velocity is mainly dependent on the permittivity of the insulation material, which can be affected by external parameters like temperature or water ingress. This paper focuses on the influence of temperature on the propagation velocity in medium voltage (MV) cables. Laboratory scale tests are performed for both PILC and XLPE cable. Test results show that the high frequency signal propagation velocity for XLPE will increase with the temperature rise while PILC has opposite behaviour. The variation of propagation velocity of XLPE is confirmed by data of a power cable subjected to strong load cycling monitored over eight months. 1
power cable via 50 Ω coaxial cable. The injected pulse and its reflections are recorded by an oscilloscope. Heating is accomplished by a 10 kW DC current source, capable of generating 600 A at a maximum voltage of 16 V. It is connected to the conductor from the second segment to the last cable segment.
INTRODUCTION
In time domain reflectometry measurements, e.g. in partial discharge diagnostic techniques for power cable [1-3], propagation velocity should be accurately known for precise defect location. On one hand, the propagation velocity can be affected by temperature. On the other hand, information on variability in propagation velocity informs on changing conditions as temperature variation by e.g. cable loading or insulation ageing. Two sets of laboratory scale test are presented to evaluate the temperature effect on wave propagation velocity in both XLPE (Section 2) and PILC cables (Section 3). The effect is compared after translating measured temperature to temperature inside the insulation (Section 4). In addition, propagation time data are extracted from Smart Cable Guard systems [4] installed on live cable connections with XLPE insulation (Section 5). The field data, with variation recorded over a year is then compared with the load profile. The observed temperature effect on propagation velocity will be briefly further discussed in Section 6. 2
Figure 1: Test circuit for TDR on heated XLPE cable; the connector numbers correspond to the connector types illustrated in Figure 2.
LABORATORY TEST ON XLPE CABLE
2.1 Test circuit Six segments of 12/20kV XLPE single core (aluminium) cable are connected to form the test circuit. Each piece is about 12 m resulting in a cable circuit of about 72 m. The test circuit is open at both sides. Pulse signals are injected into the cable via one open end and reflections are recorded at the same end (time domain reflectometry, TDR). Due to space limitation, the test circuit is half inside the test room and half outside in open air. Figure 1 illustrates the test circuit. An 8 ns wide pulse is injected into the
(1)
(2)
(3)
Figure 2: Connector types applied for combining XLPE cable segments; numbers correspond with the connectors indicated in Figure 1
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Temperature sensors are attached on the surface of the cable outer jacket to measure the outer sheath temperatures of indoor and outdoor cables. These sensors are shown in Figure 1 as dots and labeled as T1 and T2. Letters A-G indicate the reflection points of TDR measurements shown later in Figure 3. Figure 2 shows the connector types. Connector type 3 was improvised for the present measurements. The cable earth screen connections were realized with short wires parallel to the connectors for all types.
14.4C,12.0C 20.4C,17.0C 27.6C,21.9C 37.3C,27.0C 48.7C,31.8C
Amplitude [V]
0.1 0.05 0 -0.05 -0.1 -0.15 -0.2 0.95
1
2.2 Test results
Amplitude [V]
After heating for about 3 hours, the cable outer jacket temperature has increased from 9 °C to 48 °C (indoor) and from 8 °C to 30 °C (outdoor). TDR signals are recorded continuously. Results at different temperatures are shown in Figure 3. All reflected signals are labelled with symbols to identify the reflections coming from the transition between coaxial cable and power cable (A), the 5 cable connectors (B-F) and the open far end (G). Reflection C (from connector type 1), E (from connector type 2) and G (from open end) are zoomed in and shown in Figure 4. It is qualitatively observed that with the increase of temperature, the propagation time decreases, indicating that the propagation velocity has increased.
10
10.8C,9.0C
8
20.8C,17.4C
Amplitude [V]
0.6
B
2
DE
A
1 Time [s]
1.5
2
14.4C,12.0C 20.4C,17.0C 27.6C,21.9C 37.3C,27.0C 48.7C,31.8C
Amplitude [V]
0.6 0.4 0.2
-0.2 0.76
0.77 0.78 Time [s]
0.79
1.5
Insulation temperature conversion: A thermal ladder network can be used to model the temperature distribution in radial direction of the cable [6]. Each layer of cable can be modelled by a thermal resistor and a thermal capacitor. The heat source can be represented as a current source. According to the thermal ladder network, logarithmic temperature distribution across the dielectric material is assumed. Based on this model, the time response of the temperature in the middle of dielectric material is shown in Figure 5. The measured temperature of the outdoor cable jacket is then converted to outdoor temperature at
0
0.75
1.4 1.45 Time [s]
In order to analyse the measured result, two issues have to be addressed. The first issue is the temperature correction, since the temperature sensors are attached on the outer jacket of the XLPE cable. The propagation velocity is mainly determined by permittivity of the insulation. Therefore, the measured jacket temperature will be converted to the temperature at the midpoint of the insulation. The second issue concerns the quantitative calculation of the pattern shift at different temperatures.
Figure 3: Reflection patterns at different temperatures; the legend indicates first the indoor temperature, next the outdoor temperature, letters A-G correspond to the ones in Figure 1
0.8
1.35
2.3 Analysis
F G
0.5
0
Figure 4: (a) depicts reflection C from connector 1; (b) shows reflection E from connector 2 and (c) illustrates reflection G from open end
0 -2 0
14.4C,12.0C 20.4C,17.0C 27.6C,21.9C 37.3C,27.0C 48.7C,31.8C
0.2
(c)
48.8C,32.1C
C
0.4
-0.2 1.3
40.8C,28.8C
4
1.1
(b)
30.9 C,23.4 C
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1.05 Time [s]
0.8
(a)
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midpoint of the insulation, Table 1. The outdoor temperature is used since the propagation time between the connector 1 and open far end will be used for analysis in order to exclude the indoor and outdoor temperature difference. Tjx is the jacket temperature of the XLPE cable, and Tdx is the temperature at the middle of insulation of the XLPE cable; t is the heating time according to Figure 5.
0
-6
12 1 2 10 3 8 4 6
-8
4
-10
2
-12
0
-14
-2
-2
15 time delay [ns]
-4
T [C]
10
5
0 0
-16 0
0.5
1
1.5 t [h]
2
2.5
0.5
3
3
Table 1: Insulation temperature for XLPE; the outer jacket temperature Tjx is converted to the midpoint value Tdx of the insulation reached at time t (see Figure 5) 1
2
3
4
Tjx (°C)
12.0
17.0
21.9
27.0
31.8
t (h)
0.27
0.60
1.15
2.24
5.47
Tdx (°C)
23.6
30.3
35.3
40.4
45.2
1.5 2 time [s]
2.5
3
-4 3.5
Figure 6: Time shift of reflection patterns of XLPE cable at different temperatures
Figure 5: Derived temperature difference between the midpoint of XLPE insulation and the outer jacket surface for single core XLPE cable
Ref.
1
amplitude [V]
reflection from the far end. This might be caused by the indoor and outdoor temperature difference and lower signal to noise ratio. Directly past this peak, structures arise which travelled additional length along a not heated part. Later also the heated part is included resulting in a steeper time shift again.
LABORATORY TEST ON PILC CABLE
3.1 Test circuit The laboratory scale cable setup at DNV KEMA is utilized for PILC cable test. The setup consists of 50 m three core XLPE cable and 70 m three core PILC cable (outdoor). 100 ns wide pulse is injected into the cable. Due to the setup limitation, the current source for heating (same as for XLPE test) is connected to the earth screen of cables, which is lead for PILC and copper for XLPE. Figure 7 shows the test setup. Since the resistivity of lead is much higher than copper, heating power distributes mainly along the PILC cable. Due to power source limitation, about 130 A current could be loaded to the cable. Two temperature sensors are attached on the surface of the PILC and the XLPE cable separately.
Quantitative time shift: The minimum square error (MSE) [5] detection with time window is applied to get the time shift between two patterns. When time shift between pattern P (reference) and Q (at elevated temperature) is calculated, a time window (length t) taken from the complete record will be shifted along pattern Q back and forth to find the best match in pattern P, i.e. where the minimum error occurs. The time window length is chosen to be 1500 ns to get a reasonable flat result excluding error from noise. Figure 6 shows the calculated time shift along the pattern between heated cable and reference cable. The numbers in the legend correspond to the patterns in Table 1. The pattern shifts more as the temperature difference increases. The shift becomes most nd rd apparent as from the 2 – 3 reflection (at 700 ns, B-C in Figure 3). This is where the heated part of the cable begins. The time delay increases after each clear reflection from joint due to longer propagation distance. After the steep part the time delay slowly increases around and just after the
Figure 7: Test setup for TDR on heated PILC cable
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3.2 Test results
1
Approximately 3 hours heating increased the temperature of the PILC cable with about 25 °C and the XLPE with about 5 °C. The measured result is shown in Figure 8. In the pattern, the injected pulse is located around 2.1 μs. The first reflection near 2.6 μs is from the coaxial cable connection to the power cable; the reflection around 4.1 μs arises from the transition joint, and the reflection around 5 μs from the open end. The later reflections are secondary or higher order reflections. The transition joint reflection and the far end reflection are depicted enlarged in Figure 9. It is observed that with the increase of temperature, the propagation time increases, indicating a lower propagation velocity, which is opposite to the observations for XLPE insulation.
Amplitude [V]
0 -2
4
Time [s]
6
8
T [C]
1.5 1
Amplitude [V]
0.5
PILC:33.1C XLPE:25.8C PILC:38.2C XLPE:26.1C
0 0
PILC:43.0C XLPE:27.3C
-0.2
PILC:48.1C XLPE:28.8C 5 5.05 5.1 5.15 Time [s]
PILC:28.1 C XLPE:24.9 C
-0.1
4.95
2
PILC:23.5C XLPE:24.0C 0
4.9
2.5
10
Figure 8: Reflection patterns at different temperatures; the legend indicates first the indoor temperature, next the outdoor temperature
PILC:43.0C XLPE:27.3C
Insulation temperature conversion: Applying a similar approach as for the XLPE cable heating test, the temperature difference between the insulation midpoint and jacket of PILC cable is shown in Figure 10. The measured outer jacket temperature for PILC cable is converted to temperature at the midpoint of the paper insulation as shown in Table 2.
PILC:48.1C XLPE:28.8C
2
PILC:38.2C XLPE:26.1C
0.2
3.3 Analysis
PILC:43.0C XLPE:27.3C
-4 0
PILC:33.1C XLPE:25.8C
Figure 9: (a) enlargement of reflection from PILC to XLPE transition joint; (b) enlargement of reflection from open end
PILC:38.2C XLPE:26.1C
2
PILC:28.1C XLPE:24.9C
0.4
(b)
PILC:33.1 C XLPE:25.8 C
4
PILC:23.5C XLPE:24.0C
4.85
PILC:28.1C XLPE:24.9C
0.6
0
PILC:23.5C XLPE:24.0C
6
Amplitude [V]
0.8
PILC:48.1C XLPE:28.8C
-0.3
0.5
1
1.5 t [h]
2
2.5
3
Figure 10: Derived temperature difference between the midpoint of paper insulation and the outer jacket surface for the three-core PILC cable
-0.4 -0.5 4.05
4.1
4.15 4.2 Time [s]
4.25
Table 2: Middle of insulation temperature for PILC
(a)
Ref.
1
2
3
4
5
Tjp (°C)
23.5
28.1
33.1
38.2
43.0
48.1
t(h)
0.08
0.35
0.60
1.05
1.83
3.13
Tdp (°C)
25.0
30.1
35.3
40.4
45.2
50.3
Quantitative time shift: The previously described MSE method is applied to the PILC cable patterns to find the time shift. The results are shown in
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of the PILC cable is affected more by temperature. Also this figure suggests that the variation is not linear and becomes more pronounced at higher temperatures.
Figure 11 for five measurements. The time shift increases with propagation time and temperature difference. The steep rise observed directly after the reflection from the injection cable is caused by the time window beginning to cover the reflection from the PILC to XLPE transition point, which gives a clear shift. The increase in time delay directly after the first reflection is where the heated cable starts.
50
2
time shift [%]
1 2 3 4 5
-XLPE PILC
10
0
1.5
1
0.5
amplitude [V]
time delay [ns]
100
2.5
0 0
5
10
15
20
25
temperature difference [ C]
Figure 12: Propagation time variation comparison; the absolute values are indicated -10 10
5
Propagation time data from installed Smart Cable Guard system recorded over a year is compared with load profile data over the same time period. The propagation time and load of a XLPE cable st section (4.8 km long with 6 RMUs) from May 1 , th 2010 to January 14 2011 is shown in Figure 13. It shows that though the load does not have a clear summer winter cycle, the propagation time is lower at summer time and higher at winter time. This indicates a clear ambient temperature variation. On top of the ambient temperature effect, the load also influences the propagation time. The peak load th around July 9 , 2010 corresponds to a lower propagation time at that period. A weekly trend is observed. The propagation time variation is opposite compared to the load current variation. Besides, the variation within each day is observed. The relatively high load on Thursday and Monday correlates with the relatively large drop in propagation time. Further, there is a delay between the increase in current and the thermal response.
2010
31.8
31.7 31.6
705 -0 10
703 -0
31.4
10
16: 35 10: 40 2010 28-11-
701
2010
-0
01-10-
10
05:44
629
-2010
01-08
-0
02: 43
-0
2010
10
31.4
628
31.6
31.5
30-05-
0
705
30-12-
-0
1
10
2010
703
21-10-
-0
0
00:40
0 10
2010
0 5: 35:0
628
13-08-
0 6: 10:0
-0
1
0 5: 05:0
Saturday
10
2010
0 8: 25:0
Propagation time [s] Load [A]
Load [A] Propagation time [
s]
0 04-06-
12:40
500
500
701
Test results show that the XLPE and PILC cable have an opposite temperature effect on propagation velocity. With the conversion from measured jacket temperature to conductor temperature, it is possible to compare both cables. For XLPE, the time shift is calculated from reflection C (from connector 1 in Figure 1) to G (from open end) utilizing the outdoor part (consistent with the temperature data in Table 1). For PILC, the time shift is derived from the reflections at both ends of the PILC cable. The comparison between the XLPE and PILC cable’s propagation time variation is shown in Figure 12. The deviations caused by different time window lengths are indicated in the error bar. The velocity
-0
COMPARISON OF THE TEMPERATURE EFFECT OF PILC AND XLPE CABLE
10
Figure 11: Time shift of reflection patterns of PILC cable at different temperatures 4
FIELD DATA
629
8
-0
4 6 time [s]
10
2
10
0 0
Figure 13: Propagation time and load of XLPE cable for year and week cycle
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This thermal response time of typically 3 hours is more apparent when the cable load starts compared to when the load is switched off probably because the temperature rise due to higher load is faster than the temperature decrease caused by lighter load. It should be noted that the load profile is not the same over the complete cable section. The measured current represents the highest loaded part of the section.
9
[1] G. M. Hashmi, R. Papazyan, M. Lehtonen: “Comparing Wave Propagation Characteristics of MV XLPE Cable and Covered-Conductor Overhead Line using Time Domain Reflectometry Technique,” 2007 International Conference on Electrical Engineering (ICEE '07), pp.1-6, 11-12 April 2007 [2] G. M. Hashmi, R. Papazyan, M. Lehtonen: “Determining wave propagation characteristics of MV XLPE power cable using time domain reflectometry technique,” 2009 International Conference on Electrical and Electronics Engineering (ELECO 2009), pp.I-159-I-163, 58 Nov. 2009 [3] S. Markalous, T. Strehl, C. Herold, T. Leibfried: “Enhanced signal processing for conventional and unconventional PD measuring methods: Wavelet de-noising, automatic detection algorithms and averaging for arrival time-based PD location in transformers and power cables,” 2008 International Conference on Condition Monitoring and Diagnosis (CMD 2008), pp.1115-1118, 21-24 April 2008 [4] Peter C.J.M. van der Wielen and E. Fred Steennis: “Risk-controlled application of current MV cable feeders in the future by intelligent continuous diagnostics,” in 2011 IEEE Power and Energy Society General Meeting, pp.1-6, 24-29 July 2011 [5] F. Censi, G.Calcagnini, M. D’ Alessandro, M. Triventi, P. Bartolini: “Comparison of alignment algorithms for P-Wave coherent averaging,” Computers in Cardiology, 2006 , pp.921-924, 17-20 Sept. 2006 [6] George J. Anders: “Rating of Electric Power Cables in Unfavorable Thermal Environment,” pp.1-75, May 2005, Wiley-IEEE Press [7] V. Dubickas, H. Edin: “On-line time domain reflectometry measurements of temperature variations of an XLPE power cable,” 2006 IEEE Conference on Electrical Insulation and Dielectric Phenomena, pp.47-50, 15-18 Oct. 2006 [8] C. Fanggao, G. A. Saunders, R. N. Hampton, S. M. Moody and A. M. Clark: “The effect of hydrostatic pressure and temperature on the permittivity of crosslinked polyethylene”, Seventh International Conference on Dielectric Materials, Measurements and Applications, pp.267-270, 1996 [9] I. Mladenovic, C. Weindl, C. Freitag: “Comparison of parametric partial discharge and dissipation factor characteristics of MV PILC cables,” Conference Record of the 2012 IEEE International Symposium on Electrical Insulation (ISEI), pp.319-322, 10-13 June 2012
Unfortunately, at present there is no correlated PILC data available for propagation time and load current. From the laboratory tests a clearer effect is expected here. 6
DISCUSSION
The velocity change of the XLPE cable with temperature can be attributed to the real part of the permittivity ε of the XLPE. The decrease of velocity in percentage is in agreement with the ε decrease in the measured temperature range [7,8], which is about 0.7-3% in the temperature range of 20 °C to 60 °C. For PILC, ε increases with temperature [9]; however quantitative analyses on the temperature dependence of ε in high frequency range is scarce in literature. Furthermore, it is acknowledged that the insulation parameters of paper insulation can vary a lot depending on manufacturer, production year, country and even from cable to cable. Further study is needed to demonstrate the velocity dependence of ε for PILC cable in live circuits. Another factor that may affect the velocity is the change in dimension of the cable. This may have an effect on the pressure on the dielectrics. 7
CONCLUSION
Experiments show that high frequency signals propagates faster with higher temperatures for XLPE cable and slower for PILC. It is observed that the temperature effect on propagation velocity is more significant for PILC than for XLPE. The temperature dependence of the real part of permittivity (ε) dominates the velocity change for XLPE cables. For PILC, further work is needed to explain the origin of the velocity dependency with temperature. 8
REFERENCES
ACKNOWLEDGMENTS
The authors would like to thank DNV KEMA Energy and Sustainability, Enexis, Alliander and Locamation for their financial support.
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