PARTIAL DISCHARGE MONITORING SYSTEM FOR CLP POWER STRATEGIC 400kV GIS EXTENSION AT BLACK POINT POWER STATION

T. Ito, F. Taniguchi, M. Kamei S.Chu, K.L. Lee, K. Wakabayashi

S.L. Ip, D.L.S. Yeung

Mitsubishi Electric Corporation Japan

Email: Ito.Takashi@cb.MitsubishiElectric.co.jp

Mitsubishi Electric (H.K.) Ltd. Hong Kong

Email: wakabayashi@mehk.meap.com

CLP Power Hong Kong Limited Hong Kong

Email: slip@clp.com.hk

ABSTRACT

CLP Power Hong Kong Limited (CLP Power) has reinforced its 400kV power grid by introducing new extension switchgear to the Black Point Power Station (BKP). Since the circuits are of strategic importance to provide more generation infeed capacity and any fault must be avoided by all means, if possible, to ensure high supply reliability, CLP Power has created a cooperative arrangement with Mitsubishi Electric, the original equipment manufacturer of the 400kV gas insulated switchgear (GIS) for BKP to establish an effective solution using the condition-based maintenance (CBM) approach. So an On-line partial discharge (PD) monitoring system has been installed with operation and maintenance support from the manufacturer. The design of the PD coupler has been integrated with the GIS design, with high sensitivity and reliability, while meeting recognized standards. The system can also cope with future expansion. Establishing a methodology on how to diagnose the insulation healthiness of GIS using the PD monitoring system is of high importance to the CBM practice. The PD monitoring system has been working satisfactorily since its full commissioning in June 2008. There is also a good collaboration between the teams in building up the future CBM requirements through the analysis of PD monitoring data and providing expertise support.

The paper is intended to share (a) the information on key technical requirements, system design, architecture, layout arrangement and provisions on the GIS of the PD monitoring and, (b) the experience in factory acceptance test, site acceptance test and subsequent operation of the system.

Keywords: Condition Monitoring, Partial Discharge Monitoring, PD Coupler, CBM

1. INTRODUCTION

CLP Power is a power utility in Hong Kong SAR. Operating a vertically integrated electricity generation, transmission, distribution and retail business, it supplies electricity to about 80% of local

population. Transmission network of CLP Power consists of 554km of 400kV transmission circuits by end 2008. GIS with one-and-a-half breaker design is adopted in CLP Power’s 400kV substations.

In order to achieve a high level of supply reliability, CLP Power’s 400kV grid network is connected as ring configuration which is also interconnected with the South China Grid. To meet the growing generation in the interconnected system in mid 2008 and beyond, a strategic project was initiated in 2005 to reinforce the 400kV power grid by introducing new GIS extension to the Black Point Power Station (GIS bay Nos. 8 & 9 extensions), thus enabling the installation of 400kV series reactors for fault level containment.

To ensure high level of availability of the strategic 400kV GIS extension, condition monitoring through an on-line PD monitoring system for the GIS is considered an essential tool to achieve the following purposes:-

• To act as an early warning system to show evolving problems on the GIS;

• To optimise the requirement of inspections and maintenance on GIS based on its prevailing condition; and

• To prevent costly GIS failures through planned follow-up action.

The condition monitoring data of GIS could facilitate proactive and condition-based approach for plant equipment maintenance. The available data can be used as a feedback to plant maintenance optimization.

2. KEY TECHNICAL REQUIREMENT

CLP Power considered the following are the key technical requirements for the PD monitoring system to align with its condition monitoring and future development strategies:-

• PD monitoring system with internal PD couplers of high sensitivity and reliability for on-line condition monitoring, but be able to effectively distinguish genuine PD signals from spurious

background noise and other interference sources; • Addition of internal PD couplers not affecting the

insulation integrity of the GIS; • Design and calibration of PD couplers complying

with recognized international/ national standards, e.g. IEC, CIGRE recommendations, National Grid Transco internal specification;

• PD monitoring system with expansion capability to cope with future needs in the same substation;

• Advanced PD monitoring system with user-friendly application software and expert system for detection and location of PD sources inside the GIS, identification of their nature and event logging; and

• Life expectancy of PD couplers compatible with that of the GIS.

3. PRINCIPLES OF PD DETECTION AND INTERPRETATION

PD takes place in GIS locally at non-uniform electric field due to the presence of defect when the field strength concentration is strong enough to cause ionization. The PD current in SF6 gas has rise times that can be less than one hundred picoseconds. The first pulse creates electromagnetic wave propagation in GIS which frequency components are above 2 GHz. Permissible PD in GIS is 5 pico-Coulomb (pC) according to the recent IEC2271-203 standard.

The most sensitive PD detection method is a technique utilizing electromagnetic wave coupling device, PD coupler, installed outside or inside GIS. The method is called “Ultra High Frequency (UHF) method” and on-line PD monitoring system taking advantage of the UHF method is available with 5 pC detection sensitivity. The UHF method also provides advantage that the type of defect can be interpreted by the signal fingerprint on applied AC voltage so called “Point-On Wave (POW)”. [1]

PD is an indication of insulation degradation and must be located and removed before it leads to breakdown. The on-line PD monitoring system is useful not only to ensure the perfect installation of GIS during High Voltage AC (HVAC) test but also to avoid breakdown when GIS is in service operation. Moreover the online PD monitoring is regarded as a potential solution for insulation life estimation of GIS by CBM practice.

There will be occasional switching operations in a substation and the high-level switching transient signal will swamp comparatively weak PD signal. The PD monitoring system is required not to give false alarm by switching operation but must initiate PD alarm when actual PD is found.

4. SYSTEM DESIGN AND ARCHITECTURE

The on-line PD monitoring system of Mitsubishi Electric consists of three main functional components, PD coupler, PD signal processing panel (PDP) and human-machine interface (HMI).

PD coupler picks up UHF PD electromagnetic signals and provides PD electric signals to PDP through coaxial cable. There are internal and external type couplers. The internal type coupler is shown in Figure 1. Because of the fact that the coupler is installed in GIS shield chamber, the signal to noise (S/N) ratio can be better than the external type. The design life of the PD coupler is expected to be same as that of the GIS so that the coupler can be used to meet the CBM practice without the need for replacement. The location of PD coupler considers sufficient coverage with 5 pC detection sensitivity for GIS by taking account of the UHF signal attenuation, which should be less than 30 dB. To meet this criterion, the couplers are usually placed within 30 meter interval in straight busbar or with a shorter spacing if a higher level attenuation is anticipated in situation where components, breaker chamber, T-shaped branch and elbow busbar are located in between.

Figure 1 Internal Type PD Coupler

PDP accommodates electronic devices such as amplifier, signal processing board and network communication device. PDP is located in the proximity of the relevant PD coupler in order to avoid UHF signal attenuation in coaxial cable connecting from the coupler. The length of the coaxial cable must be less than 30 m. One PDP unit can connect up to a maximum of 18 PD couplers. Therefore, more PDP units are arranged in case there are a lot of PD couplers in the PD monitoring system.

The HMI is continuously collecting the information from PDP to provide real-time presentation of PD monitoring for operators. The data display updating rate at the HMI is 1 second while the data archiving interval is 5 seconds in consideration of limited storage capacity. The integrated information is referred during HVAC test as well as CBM. The data network transmission uses optical fiber to avoid damage to the PD monitoring electronics from surge over high voltage in substation.

The PD monitoring system initiates PD alarm when the PD coupler(s) has detected continuous PD activity for a certain period of time. The alarm criterion, which default set time is 30 minutes, works as a software filter to eliminate false PD alarm by unexpected intermittent background noise interference. Occasional switching operations can be filtered out by the facility.

As the project was relating to the GIS extension, due consideration has also been taken into account in the overall design of the PD monitoring system for the existing ‘aged’ GIS. In aged GIS, potential insulation deterioration that can cause evolving PD by long-term operation stress is regarded as insulator-concerned defects such as crack, sphere and peel-off void. Mitsubishi Electric’s research and experience have revealed that such defects will generate signal mostly in very high frequency (VHF) band. Having taken into account of this fact, the PD monitoring system installed at Black Point Power Station comes with a wide UHF detection bandwidth ranging from 200MHz to 1300MHz so as to enable PD for GIS of all ages can be detectable.

5. LAYOUT ARRANGEMENT AND PROVISIONS ON GIS

CLP Power decided to integrate the internal type PD coupler into the extension portion in Black Point 400kV GIS. The location of the PD coupler which provides sufficient coverage with 5 pC sensitivity for entire extension circuit was designed and proposed by Mitsubishi Electric.

Figure 2 shows the location of the PD couplers in Bay No. 9 extension portion of GIS. The PD couplers were arranged basically at the intervals of few tens of a meter in consideration of UHF signal attenuation. For load breaker switch (LBS), the couplers were located on both ends of the LBS chamber because of high attenuation in LBS chamber which is more than 10dB. At the interface point with the 400kV series reactor, which was supplied by other manufacturer, the PD couplers were also provided so as to enable operators conduct quick trouble shooting in case of PD alarm by determining which high voltage equipment PD is taking place.

Figure 2 PD Coupler Arrangement for Bay No.9 Extension Portion of Black Point 400kV GIS

Plan View(see Extract A below for PD Coupler Arrangement at LBS)

Extract A(PD Coupler Arrangement at LBS)

Side View(see Extract B below for PD Coupler Arrangement with Series Reactor)

Extract B(PD Coupler Arrangement with Series Reactor)

Installation of PD Couplers at Elbow Busbar End Covers

SeriesReactor

SeriesReactor

LBS

LBS

LBS LBS

Existing Overhead Line

PD Coupler 9-1

PD Coupler 9-2Lower Busbar

PD Coupler 9-10Upper Busbar

PD Coupler 9-11

PD Coupler 9-3(Lower Busbar)

PD Coupler 9-9(Upper Busbar)

PD Coupler 9-1

PD Coupler 9-11

PD Coupler 9-6 PD Coupler 9-8

PD Coupler 9-7

PD Coupler 9-5 PD Coupler 9-4

Figure 3 Scheme of On-line PD Monitoring System for Black Point 400kV GIS Extension

During the commissioning stage of the PD monitoring system, we had occasionally experienced noise disturbance in some PD couplers in Bay Nos. 8 and 9 where the GIS is of outdoor installation. The fixed noise frequency band was specified as in 400MHz and 800MHz bands. A special band-stop noise filter was designed and installed. The disturbance was perfectly rejected while the sensitivity of 5 pC was kept guaranteed.

The overall PD monitoring system for Black Point extension GIS is shown in Figure 3. Totally 69 PD couplers and 5 PDPs are arranged. The HMI is located in the Protection Room. The system is ready for future expansion capable for up to 216 PD couplers.

6. FACTORY ACCEPTANCE TEST AND SITE ACCEPTANCE TEST

At factory, Mitsubishi Electric conducted to determine the test pulse voltage level for CIGRE 5 pC sensitivity verification test. The voltage level was 60V for the 400kV GIS and the PD monitoring system with 5 pC detection sensitivity was fully verified with the test voltage at site.

The PD monitoring system was used for the HVAC test of the GIS on site to ensure that the GIS is free from PD after installation. There was a noteworthy experience to share with:

The experience was a flashover occurred during HVAC test on Bay No. 9 GIS extension portion of GIS. The flashover occurred 20 seconds after the application of a maximum test voltage of 520 kV. Taking into account of the fact that the PD monitoring system did not detect any signal in advance of the flashover because of such a short duration, we concluded that the defect should have been caused by a metallic particle left inside a GIS chamber during installation. The decision was made by referring to the paper on the voltage-time (V-t) characteristics of metallic particles in GIS which have random chance failure pattern, where flashover can happen without PD in advance. [2]

The experience gives noteworthy suggestion that the current on-line PD monitoring technology did not work perfectly enough. The future PD monitoring development will probably demand a faster data acquisition and interpretation.

Figure 4 False PD Alarm Event during Operation

7. OPERATION EXPERIENCE

In order to establish a methodology on how to diagnose insulation performance of the GIS using on-line PD monitoring system, CLP Power and Mitsubishi Electric have created cooperative arrangement in the CBM approach. Mitsubishi Electric is entitled to access the PD monitoring system at Black Point Power Station via the Internet for remote monitoring.

The cooperative scheme aims to train CLP Power’s operators to have high-level expertise on diagnosis on insulation healthiness by taking full advantage of the information from the PD monitoring system. The new approach of exchanging periodical report based on actual case study stimulates the utility to learn knowledge on PD monitoring effectively and efficiently. Since full commissioning in June 2008,

many switching operations have been made and these did not initiate any false PD alarm as intended.

There was one time the PD monitoring system gave out a false PD alarm. The false PD alarm was triggered by the PD coupler located at the extreme end of the busbar connecting to the series reactor. As shown in Figure 4, high repetition rate of over 5,000 pulses per second run over the PD alarm set time. The POW patterns of the signals are completely random and the repetition rates are over 5,000 pulses per second which is not possible for metallic particle PD of which POW pattern is also random. Verification of data recorded in the PD monitoring system by Mitsubishi Electric revealed the cause should have been due to external disturbance, most probably by mobile phone used in the proximity of the PD coupler.

22 Dec 2008 13:00~14:00

PD Alarm initiated at 14:19:34

PD Alarm stopped at 14:20:50

Repetitive Signal (Noise) detected by the PD coupler.

22 Dec 2008 14:00~15:00

22 Dec 2008 15:00~16:00

No PD pattern in the recorded signals.

The PD coupler, PDC9-8B, detected continuous repetitive signal.

Trend Plot Display

Trend Data Display

8. WAY FORWARD

CLP Power is looking for better asset management with longer life expectancy of the plant equipment. PD monitoring system can provide an effective solution in CBM. At a relatively small cost in the overall price of the main equipment, it would be beneficial to incorporate PD monitoring system as a packaged solution with plant equipment procurement and installation. First of all, the PD couplers of high sensitivity and reliability can be optimally integrated into the plant equipment initially. This can, however, hardly be achieved in subsequent retrofitting work. Besides, the CM data obtained since the commissioning of the plant equipment can facilitate proactive and condition-based approach for plant equipment maintenance, thus rendering the extensionof plant life possible.

As an enhancement to the current on-line PD monitoring technology, the future PD monitoring development will probably be equipped with a faster data acquisition and interpretation so as to enable any PD with short duration during the HVAC test of the GIS on site can be detected.

In the long term, it is opted to have an integrated equipment performance management IT platform for recording and evaluating (through an expert system), the CM results and equipment performance parameters to provide predictive advice for asset management decisions, e.g. maintenance, replacement, etc.

9. CONCLUSION

The on-line PD monitoring system installed for the 400kV GIS extension at Black Point Power Station has been working stably and reliably since its commissioning. CLP Power and Mitsubishi Electric have been collaborating well in establishing an effective CBM practice with continuous support from the supplier in the operation phase.

10. ACKNOWLEDGEMENTS

The authors would like to thank the Management of Mitsubishi Electric and CLP Power for the encouragement and endorsement for the publication of this paper.

11. REFERENCES

[1] J.S. PEARSON, B.F. HAMPTON and A.G.SELLARS: ‘A CONTINUOUS UHF MONITOR FOR GAS-INSULATED SUBSTATIONS’, IEEE Trans on Electrical Insulation, Vol. 26, No. 3, June 1991

[2] T. HATTORI : ‘A STUDY ON EFFECTS OF CONDUCTING PARTICLES IN SF6 GAS AND TEST METHODS FOR GIS’, IEEE Trans. On Power Delivery, Vol. 3, No. 1, January 1988