Advanced
Insulation Design and Reliability Evaluation of ±80kV HVDC XLPE Cables
Insulation Design and Reliability Evaluation of ±80kV HVDC XLPE Cables
Journal of Electrical Engineering and Technology. 2014. May, 9(3): 1002-1008
Copyright © 2014, The Korean Institute of Electrical Engineers
  • Received : May 25, 2013
  • Accepted : February 23, 2014
  • Published : May 01, 2014
Download
PDF
e-PUB
PubReader
PPT
Export by style
Article
Author
Metrics
Cited by
TagCloud
About the Authors
Chae-Kyun Jung
Corresponding Author: Power Transmission Laboratory, KEPCO Research Institute, Korea. (chekyun@kepri.re.kr)
Hung-Sok Park
Power Transmission Laboratory, KEPCO Research Institute, Korea. (parkhs@kepco.co.kr,jwkang@kepri.re.kr)
Ji-Won Kang
Power Transmission Laboratory, KEPCO Research Institute, Korea. (parkhs@kepco.co.kr,jwkang@kepri.re.kr)

Abstract
This paper describes insulation design and its reliability evaluation of ±80kV HVDC XLPE cable. Recently, the construction of HVDC transmission system, which is combined overhead line with underground cable, has been completed. This system is installed with existing 154 kV AC transmission line on the same tower. In this paper, the lightning transient analysis is firstly reviewed for selection of basic impulse insulation level and nominal insulation thickness. Then the electrical performance tests including load cycle test and superimposed impulse test based on CIGRE TB 496 are performed to evaluate the reliability of newly designed HVDC cable. There is no breakdown for ±80kV HVDC XLPE cable during electrical performance test. Finally, this system is installed in Jeju island based on successful electrical performance test (Type test). After installation tests are also successfully completed.
Keywords
1. Introduction
HVDC is a preferred technology for transmitting large amount of power across long distance. HVDC results in overall higher efficiency and reliability than AC system transmitting the same amount of power. One of the biggest advantages of HVDC transmission is efficiency. HVDC transmission can supply more electricity with less power losses than AC system.
In Korea, the construction of hybrid transmission system has been completed. This system consists of existing 154 kV AC transmission line and newly installed ±80kV HVDC transmission line on the same tower. HVDC transmission line is combined overhead line and underground cable.
In previous papers [1 - 3] , the lightning analysis and basic impulse insulation level were investigated including surge arrester type and capacity for ±80kV HVDC transmission line. Specially, the new DC cable with nano-composite XLPE insulation was developed to improve improve the insulation efficiency as well as to decrease the impact on the space charge.
In this paper, firstly, the basic impulse level and insulation thickness are reviewed in detail based on the transient analysis results for ±80kV HVDC cables. Then, the performance tests including load cycle test and superimposed impulse test were performed based on CIGRE TB 496 [4] for evaluating the reliability of newly designed HVDC cable. Finally, the qualified cable was installed in HVDC test area in Jeju. The test after installation was also successfully completed in 2012. The final commissioning test of converter station is now in progress.
2. ±80kV HVDC Transmission Line
± 80 kV HVDC transmission line in the hybrid transmission system is connected from Hanlim converter station to Geumak converter station in Juju island. The length of overhead DC line section with ACCC 160 mm 2 is 4 km, underground cable section with nano-composite XLPE insulation is 0.5 km. Fig. 1 shows the diagram of power system in Jeju island. In this figure, the red line means ±80kV HVDC transmission line.
PPT Slide
Lager Image
Power system diagram of Jeju island
Fig. 2 shows the picture and diagram of ±80kV HVDC XLPE cable. This cable consists of conductor (①), inner semiconducting layer (②), nano-composite XLPE insulation (③), outer semiconducting layer (④), metal sheath (⑤) and oversheath (⑥)
PPT Slide
Lager Image
±80kV HVDC XLPE cable
Fig. 3 shows the detailed configuration of the system. As shown in this figure, the system consists of a 154 kV AC transmission line and a ±80kV HVDC transmission line on the same tower. In addition, the HVDC transmission line is connected to underground cables.
PPT Slide
Lager Image
Detailed system configuration
3. Basic Impulse Insulation Level
In previous papers [2 - 3] , the surge arrester for ±80kV HVDC transmission line has been already selected by various lightning overvoltage analysis. The required maximum continuous operation voltage (MCOV) is 59.4 kV rms . Therefore, the surge arrester with the rated voltage of 78 kVrms and MCOV of 63.1 kV rms is selected for ±80kV HVDC transmission line. The nominal discharge current is also selected by10 kA based on IEC 60099-4 [5] . These kinds of surge arrester have been installed at cable head (joint between overhead lines and underground cables) and converter station inlet.
In this paper, EMTP simulation is used for lightning analysis and insulation design of ±80kV HVDC cables. The EMTP model is illustrated in Fig. 4 . Four story tower model and back flashover model are used for lightning analysis. The arching horns modeled by TACS and MODELS are also applied in this simulation model. The applied lightning current and waveform are 80 kA and 2/70 ㎲. This lightning surge strikes on grounding wire of HVDC overhead transmission line section at 1 km from cable head.
PPT Slide
Lager Image
Captured EMTP Simulation
The lightning overvoltages according to surge arrester installation are compared in Table 1 . The lightning overvoltage with surge arrester is significantly decreased from 647 kV to 182 kV. At this moment, the surge arrester does not exceed the nominal discharge current of 10 kA. The discharge current measured at the positive pole of cable head is 8.99 kA. Fig. 5 shows the lightning overvoltage with surge arrester in cable head as well as converter station inlet.
Lightning overvoltage according to arrester installation
PPT Slide
Lager Image
( ) : discharge current of surge arrester
PPT Slide
Lager Image
Lightning overvoltage with surge arrester
The basic impulse insulation level is calculated based on EMTP simulation result and several standards of IEC 60071-1 [6] , IEC 60071-5 [7] and CIGRE TB 496 [4] .
In this paper, DC voltage source is not considered for EMTP simulation because the calculation error is occurred by complicated impedance matrix. Therefore, for superimposed DC source, the Bahder’s coefficient is applied as expressed in Eq. (1) [6 - 8] .
PPT Slide
Lager Image
where, K is Bahder’s coefficient. It generally has a coefficient 0< K <1. K =0.5 is applied based on Electra 86 recommendation in this paper [8] . Uo is the rated DC voltage, Ui means the maximum lightning impulse voltage to grounding acting alone and Up is the maximum transient voltage to ground with lightning superimposed on U0 . Table 2 shows the lightning overvoltage considering DC source superimposition. The overvoltage superimposed DC rated voltage can be calculated by Eq. (2). It is critical when the polarity is opposite to Uo .
PPT Slide
Lager Image
where, Uc is calculated lightning overvoltage when the DC rated voltage is not considered for simulation. Uk is the overvoltage considering DC source superimposition. As shown in Table 2 , the overvoltages considering superimposition of opposite DC rated voltage are 222 kV and 219 kV, respectively. They are 40 kV higher than the overvoltage without DC superimposition. Therefore, the maximum overvoltage by DC source superimposition of 222 kV can be selected to coordinatedly withstand overvoltage for the basic impulse insulation level of ± 80 kV nano-composite HVDC XLPE cable.
Lightning overvoltage considering DC superimposition
PPT Slide
Lager Image
Lightning overvoltage considering DC superimposition
Then, the safety margin of 20 % considering accuracy of EMTP model and other uncertain factors based on IEC 60071-5 [7] is applied for the calculation of the basic impulse insulation level. Finally, the standard basic impulse insulation level for ± 80 kV nano-composite HVDC XLPE cable can be selected by IEC 60071-1 [6] .
The maximum lightning overvoltage considering safety margin of 20% is 266.4 kV. From this result, the standard basic impulse insulation level of 325 kV for ± 80 kV nanocomposite HVDC XLPE cable can be finally selected.
4. Insulation thickness
The insulation thickness for ± 80 kV nano-composite HVDC XLPE cable is calculated considering impulse withstand voltage ( Uimp ) based on the standard basic impulse protection level ( Ubil ) of 325 kV and design stress for impulse voltage ( ELimp ). Eq. (3) shows the calculation for impulse withstand voltage and the minimum insulation thickness can be calculated by Eq. (4).
PPT Slide
Lager Image
where Uimp is calculated impulse withstand voltage (kV), Ubil is standard basic impulse protection level, A is the tolerance of arrester protection level (recommended by CIGRE TB 189), K is Bahder’s coefficient, U0 is the DC rated voltage, K1 is the repetition deterioration coefficient, K2 is the temperature coefficient, and K3 is the overall safety factor for uncertainties.
PPT Slide
Lager Image
where t is minimum insulation thickness (mm) and ELimp is the design stress for impulse voltage.
The calculated impulse withstand voltage ( Uimp ) of 411.13 kV can be calculated by Eq. (3). Finally, the minimum insulation thickness of 10.8 mm is also calculated by Eq. (4). Therefore, the nominal insulation thickness of 12 mm is selected in this paper for ± 80 kV nano-composite HVDC XLPE cable.
5. Electrical Performance Test
The electrical performance test (Type test) should be performed on test cable based on CIGRE TB 496 for evaluating the reliability of newly designed ±80kV HVDC XLPE cable [4] . Electrical performance tests are made before supplying on a general commercial basis of cable system in order to demonstrate satisfactory performance to meet the intended application. Once accessory with respect to materials, manufacturing process, design electrical stress levels, which might adversely change the performance characteristics [9] .
Fig. 6 shows the flow chart for electrical performance test of ±80kV HVDC XLPE cable. The order of electrical performance test is 1 st load cycle test, polarity reversal test, 2 nd load cycle test, superimposed impulse voltage test including switching and lightning impulse, and DC test. Fig. 7 shows the system diagram for electrical performance test. The test is performed in Underground Test Lab in KEPCO Power Testing Center. Fig. 8 shows the real picture of installed test cable for performance test.
PPT Slide
Lager Image
Flow chart for electrical type test
PPT Slide
Lager Image
System diagram for electrical type test
PPT Slide
Lager Image
Real picture of installed test cable for type test
The conditions for load cycle tests including 1 st 24-hour load cycles, polarity reversal cycles and 2 nd 48-hour load cycles are as follows:
  • - Eight 24-hour load cycles at negative polarity at 1.85·U0(148 kV)
  • - Eight 24-hour load cycles at positive polarity at 1.85·U0(148 kV)
  • - Eight 24-hour load cycles with polarity reversal cycles at 1.45·U0(116 kV)
  • - Three 48-hour load cycles at positive polarity at 1.85·U0(148 kV)
24-hour load cycles consist of an 8 hours heating period and a 16 hours cooling period. During the last 2 hours of heating, the conductor temperature shall be at 70℃ which is suggested by manufacturer. 48-hour load cycles consist of a 24 hours heating period and a 24 hours cooling period. During the last 18 hours of heating period the conductor temperature should be more than 70℃. 48-hour load cycles are only required as part of the test procedure to ensure that electrical stress inversion is well advanced within the cycle. For a polarity reversal, starting with positive voltage, the voltage polarity shall be reversed three times every “24 hours” load cycle (evenly distributed) and one reversal shall coincide with the cessation of loading current in every “24 hours” load cycle. The recommended time duration for a polarity reversal is within 2 minutes.
During all load cycle tests, there are no breakdown of ± 80 kV HVDC XLPE cable and interruption of testing. The load cycle test is successfully completed. Figs. 9 and Fig. 10 show the results of polarity reversal cycle test and 2 nd 48 hour heat cycle test.
PPT Slide
Lager Image
Polarity reversal cycle test results
PPT Slide
Lager Image
2nd 48 hour heat cycle test results
Then, the superimposed impulse voltage tests are performed. The switching surge withstand test and lightning impulse withstand test are performed on ± 80 kV HVDC XLPE cable. During the impulse tests, conductor temperature of more than 70℃ shall be reached for a minimum of 10 hours before the voltage impulses are applied and shall be maintained throughout the duration of the test. Fig. 11 shows the test circuit diagram for superimposed impulse test. The voltage and temperature maintained for superimposed impulse test are shown in Fig. 12 . Also, the conditions for switching and lightning impulse test are as follows:
PPT Slide
Lager Image
Circuit diagram for superimposed impulse test
PPT Slide
Lager Image
Maintaining voltage and temperature for superimposed impulse test
- Switching impulse test (10 times each)
  • ·Test objective at Uo, 10 consecutive impulses to -144 kV±3%
  • ·Test objective at -Uo, 10 consecutive impulses to 144 kV±3%
  • ·Waveform (front/tail) : 200-300㎲ / 1000-4000㎲
- Lightning impulse test (10 times each)
  • ·Test objective atUo,10 consecutive impulses to -325 kV±3%
  • ·Test objective at -Uo, 10 consecutive impulses to 325 kV±3%
  • ·Waveform (front/tail) : 1-5 ㎲ / 40-60㎲
During superimposed impulse tests, there are no breakdown of ±80kV HVDC XLPE cable and interruption of testing. The superimposed impulse test is successfully completed. Figs. 13 and Fig. 14 show the waveforms of the first and last negative switching impulse test and positive lightning impulse, respectively.
PPT Slide
Lager Image
The first and last negative superimposed switching impulse waveform
PPT Slide
Lager Image
The first and last positive superimposed lightning impulse waveform
± 80 kV HVDC XLPE cable is installed in Jeju island based on successful electrical performance test including load cycle tests and superimposed impulse voltage test. The construction has been already completed. After installation tests including DC withstand voltage test, nondestructive test and TDR (Time Domain Reflectrometer) test are also successfully completed. Fig. 15 shows a DC withstand voltage test of real HVDC cables in Jeju. Fig. 16 shows the TDR test results during after installation test. As shown in this figure, the inserted signal from cable head is reflected back from termination of converter station inlet. There is no abnormal signal between the first and the second reflections. The time difference between these two reflections is 5.37㎲. Therefore, the propagation velocity of real HVDC cables can be calculated as 186.2 m/㎲. This velocity can be applied for fault location when the fault occurs on this HVDC cable.
PPT Slide
Lager Image
DC withstand voltage test in real HVDC cable system
PPT Slide
Lager Image
TDR test result during after installation test
6. Conclusions
In this paper, the insulation design and its reliability evaluation of newly developed nano-composite ± 80kV HVDC XLPE cable is reviewed. The results are summarized as follows;
  • 1) The maximum overvoltage including safety margin and DC superimposition is 266.4kV. The basic impulse insulation level of 325 kV can be finally selected based on IEC 60071-1.
  • 2) The calculated minimum insulation thickness is 10.8 mm. The nominal insulation thickness of 12 mm is selected for ± 80 kV nano-composite HVDC XLPE cable.
  • 3) The electrical performance test is performed based on CIGRE TB 496 for evaluating the reliability of newly designed ±80kV HVDC XLPE cable.
  • 4) During all load cycle tests including 1stand 2ndload cycle and polarity reversal, there are no breakdown of ± 80 kV HVDC XLPE cable and interruption of testing.
  • 5) During all superimposed impulse tests including switching and lightning impulse tests, there are no breakdown of ±80kV HVDC XLPE cable and interruption of testing.
  • 6) ±80kV HVDC XLPE cable is installed in Jeju island based on successful electrical performance test. The construction and after installation tests including DC withstand voltage test, non-destructive test and TDR are also successfully completed.
BIO
Chae-Kyun Jung worked at University of Siegen as a post-doctor researcher from 2006 to 2007. He has been working at the KEPCO Research Institute from 2007 where he is currently a senior researcher in the Power Transmission Laborotory.
Hung-Sok Park has been working at the KEPCO Research Institute from 2008 where he is currently a researcher at Power Transmission Laborotory. He is mainly interested in researching testing and opeartion of power cables.
Ji-Won Kang has been working at the KEPCO Research Institute from 1993 where he is currently a Principal researcher in the Power Transmission Laborotory. His current research interests are power systems operation, analysis and diagnosis of power cable systems.
References
2009 EPRI Technical Report, “AC to DC Power Testing Line Conversion”
Jung C. K. , Kang J. W. , Shin K. Y. , Kim D. H. , Lee D. I. 2011 “Lightning Analysis on HVDC/HVAC Hybrid Transmission System in Korea” 2011 ISH Conference proceedings
Jung C. K. , Park H. S. , Kang J. W. , Kim D. H. , Shim J. W. , Moon B. S. , Lee D. I. 2012 “Basic Impulse Insulation Level Review of ±80k Nano-composite DC XLPE Cables” CIGRE Colloquium on HVDC and Power Electronic Systems
2012 Electra 496, “Recommendation for testing DC extruded cable systems for power transmission at a rated voltage up to 500 kV”, CIGRE WG B1.32
2009 IEC 60099-4, “Metal-Oxide Surge Arresters without Gaps for AC Systems”
2006 IEC 60071-1, “Insulation coordination-Part 1: Definition, Principles and rules”
2002 IEC 60071-5, “Insulation coordination-Part 5: Procedures for high-voltage direct current(HVDC)”
1994 Electra 86, Overvoltage on HVDC Cables, CIGRE J/W Group 33/21/14.16
SANDEN B. 2012 “Recommendations for testing HVDC extruded cable systems for power transmission at a rated voltage up to 500 kV” CIGRE Colloquium on HVDC and Power Electronic Systems