Application of Combined PD Sensor for GIS PD Detection and Condition Monitoring

Chen Min1*, Koji Urano1, Li You-Cheng 2 and Atsuhide Jinno3 1 Affiliation: SE Technology Limited, Hong Kong, China

2 Affiliation: Zhuhai Electric Power Corp., China 3 Affiliation: J-Power Systems Corp., Japan

*E-mail: chen@se-tek.com

Abstract--It was reported previously that partial discharge

(PD) existing in GIS can be localized with a PD localization method based on the time difference of PD detection, in which more than one pulse current sensors are installed at the GIS sealing end or cable termination. However, the localization accuracy is not satisfactory since the PD propagation speed is too high and not average inside GIS. To increase the accuracy combined PD sensors to detect pulse current and to catch the acoustic signal were applied for the on-site PD localization. The result shows that higher location accuracy can be obtained.

Index Terms--acoustic emission (AE), AE sensor (AES), capacitive coupling sensor (CCS), GIS, high frequency current transformer (HFCT), spacer, partial discharge (PD), combined PD sensor, PD localization (PDL), PD measurement (PDM)

I. INTRODUCTION HIS paper introduces a newly developed partial discharge (PD) localization method for locating the PD existing in

GIS on-site, in which capacitive coupling sensors (CCS) and high frequency current transformer (HFCT) installed at the GIS sealing end and GIS spacer to detect the PD pulse current, and acoustic emission sensor (AES) installed on the surface of the GIS enclosure to catch the acoustic emission signal from PD, are combined for application. It is successfully applied the method on-site to detect and localize some real PDs existing in the GIS under the condition of commercial service operation.

With the improvement of the on-site PD Measurement (PDM) system and PD sensor in recent years, it has become more convenient and easier to carry out PDM and PD localization (PDL) without outage of the power system and any changes in connection of the operating apparatus, also so-called the online PDM. This paper reports a practical PDM conducted for a GIS in which combined PD sensors were installed on the cable sealing end of GIS [1] and the GIS connection flange with an insulation spacer. As an example, the on-site PDM shows the composition of whole measurement system, including a set of high function, portable PD signal processing unit, wide-band optical-electrical transmitters and the PD sensing circuits, and the new combined PD sensors.

When a detected PD is considered as not immediate threat leading to a destructive insulation problem based on its magnitude, trailing this kind of PD signal is very important because the trend of the PD will affect the protection measures being taken, for which a newly developed long-term on-line PD monitoring system was introduced and recommended. The

paper introduces not only the configuration and specification of the newly developed PD sensor, but also the long-term on-line PD monitoring system, automatic data save, automatic transmitting function and the remote monitoring performance. Based on the long-term, real-time monitoring results, the judgment of that how long the GIS can continue its service operation or it should be stopped soon for maintenance or replacement, can be obtained more correctly and confidently.

II. PD SENSOR It is reported previously that pulse current detection method

can be used for the PD measurement for GIS in which there are cable sealing end and the PD sensor like CCS and HFCT can be installed at the sealing end to detect the PD signal. Furthermore, when a PD is found, with multiple PD sensors installed at the same phase where the PD is, the PD site can be localized based on the detection time difference [1]. However, the localization accuracy of ±2m by only using the sensor at sealing end is not satisfactory since the PD propagation speed inside GIS is too high and not average. In order to increase the localization accuracy the new application of combined PD sensors was proposed. The combined PD sensor consists of three kinds such as a) the well-known CCS and HFCT to detect pulse current from the GIS sealing end; 2) a newly developed PD sensor combined CCS with HFCT installed on the GIS spacer also to detect the pulse current; 3) the also well-known AES installed on the surface of the GIS enclosure to catch the acoustic emission signal of PD.

As shown in Fig. 1, the new combined PD sensor consists of a metal tape as capacitive electrode that is put on the surface around the GIS spacer and a wire attached with a HFCT is connected with the tape and the enclosure or the grounding. As a CCS for PD pulse before, but here the tape and GIS core conductor form an individual capacitor in the spacer between the GIS enclosures of its both sides. Since

T

AECC

HFCT

Fig. 1 PDM and PDL with new combined PD sensors

2008 International Conference on Condition Monitoring and Diagnosis, Beijing, China, April 21-24, 2008

978-1-4244-1622-6/08/$25.00 ©2008 IEEE

there is a given quantity of insulation spacers in GIS, this kind of combined PD sensor can be used on many positions on GIS and it is very helpful to search or approach PD site when it is found.

III. EXPERIMENT WITH A SCALE MODEL As feasibility study, an experimental system was set up in

lab before going to the field. It is shown in Fig. 2 and Fig. 3, a scale model, a dummy GIS unit was made for the experiment. The purpose is to confirm the characteristics of the sensing circuit. With this model system tow or three PD sensors can be used in a test at same time for comparison, so that the factors such as the covered area of the coupling tape on the GIS spacer (the coupling capacitance), the length of the each connection wire, the grounding positions and their formation affecting the PDM sensitivity can be observed and studied. The experiment started with that the connection flange of model GIS insulated by the insulation spacer. it means the both sides of enclosure at spacer is fully isolated liks a cable insulation joint. Here the well-known foil electrodes as CCS is installed at the flange and the PDM sensitivity can be evaluated by injecting test PD pulse into GIS model from the flange or the model endings with a PD pulse generator (PG). When the signal detection magnitude of the injection of 10pC, for example, from the flange (indirect injection) is two times of the signal detection magnitude of the same injection level but from the ending (direct injection), that means the GIS model can be for normal experiment. Then, to simulate the real GIS connection, some metallic bolts and nuts are installed on the flange and both sides of the enclosure are shorted electrically. From here, the comparison experiment of CCS and the new kind of PD sensor combined CCS with HFCT was started. The simulated PD signal can be only injected into core conductor and the enclosure from the model ending. The pulse waveform and its frequency distribution of the simulated PD detected are shown in Fig. 4. The PDM detection sensitivity of 50-200pC with the new kind of combined PD sensor installed at the spacer was confirmed successfully.

IV. VERIFICATION ON-SITE In order to confirm if the application level of the combined

PD sensor meets the practical utility on-site a verification PD test based on the substantial 230kV GIS was conducted on-site. As shown in Fig. 5, a portable PDM system was used. Its multiple detection channels were connected to two combined PD sensors installed at the GIS connection flange with insulation spacer, of that one HFCT was combined to detect the pulse current flowing between the spacer and the enclosure and another HFCT was combined to detect the pulse current flowing between the spacer and the grounding. Since both sides of the enclosure at the spacer is fully shorted by those bolts and nuts on the flange and two connection bars, simulated PD pulse could not injected into the GIS from the flange, either direct injection like that for the scale model in lab. Therefore, for this test on the GIS on-site, the evaluation

method is different. For the scale model GIS since the PD pulse can be injected into the core conductor directly, the focus of the detection is to see how strong the detected signal and how wide the detection frequency band can be obtained; but on-site, the focus is on how many MHz of the detected

GIS Scale Model (7m)

Fig. 2 Schematic of experiment with GIS scale model

Foil electrodes

Amp

PG

PC

PG

OSC

SPA

HFCT

Grounding metal sheet

1m HFCT

PC PDM

Fig. 3 Experiment with a scale GIS model

Fig. 4 Detected frequencies and waveform with new PD sensor

Fig. 5 Verification PDM based on substantial GIS on-site

Fig. 6 Frequency distribution of injected PD signal

c b a

a: PD signal injected to the HFCT not connected to flange b: PD signal injected to the HFCT connected to flange c: PD signal injected to GIS flange

Fig. 8 PDM and PDL on GIS in commercial operation

signal will be lost by the shortage on the flange. So the pulse injection is with same way that the PG is also connected to the flange as the indirect injection used for foil electrodes for cable joint. When the PD pulse was injected to the flange, most of the component frequency of the signal was bypassed, compared with that if the PD pulse is injected only to the HFCT not install on the flange. As in Fig. 6 the green curve in the photo of the spectrum analyzer, frequency distribution detected with the combined PD sensor on the substantial GIS was very similar to that of the scale model test in lab. As the evaluation result, when PD pulse of 100pC is injected to the

GIS flange, the injected signal was detected at 0.25V from the PD sensor combined with HFCT connected with CCS on the spacer. It is considered that, although the detection sensitivity of using combined PD sensor for the shorted joint of GIS is only around 1/10 of that obtained from the insulation joint, it is still helpful when a definite PD inspection or PD localization is required for GIS. As shown as in Fig. 7, some typical corona signals with “120o-phase-difference pattern” detected with the new PD sensor installed on the PT flange.

V. PDM AND PDL FOR GIS IN OPERATION Three cases of practical PDM and PDL conducted in recent

months are introduced briefly here. The first PDM and PDL

were conducted in 110kV substation with main transformers and GIS in it. It is shown in Fig. 8, that the well-known PD sensors, C-arms were used for PDM firstly. Two or three PD signals at different phase were found. Then PD localization was done based on the time difference of the detection time of at the two ending bays as the upper in the Fig. however the area where the PD located was not so small for the focus of

examination, the PDL with higher accuracy was required. At the step the new PD sensors described above were applied as the lower in the Fig., and the injection of simulated PD pulse was also tried at the spacer where the PD sensor is for confirming PD propagation following route inside the GIS. It was determined that two or three PDs were sited in the GIS section between the outgoing bushing and GCB unit as shown as the upper in Fig. 9. Based on the later dismantling and examination, it was confirmed that some discharge traces were found on the surface of the insulation spacer as shown as the lower in Fig. 9.

The second case describes a PDL on 220kV GIS substation was conducted with multiple PD sensors combined with pulse

Corona detected

Fig. 7 Corona detected with combined PD sensor

Fig. 9 Successful PDM/PDL verified by practical dismantling

Some discharge traces can be seen clearly on the surface of the insulation spacer after dismantling. It may be from PD, considered.

PD site

current sensor and acoustic emission sensor (AES). Similarly, a PD signal was clearly detected with CCS and HFCT installed at the cable sealing end and it was localized as in the ending case side but not in the cable side. To further locate the PD site precisely, four AE sensors were applied at the same time because the propagation speed of AE signal is much slower than that of pulse current in GIS. The application principle of AES is simple that for which detection channels the higher and the earlier the signal is detected the AES of the channel is the nearest to the PD site. As Fig. 10, a four-channel high speed digital oscilloscope was used to measure the 4-channel AE signals for the PD localization. Here the trigger signal is the key point. The portable PDM main unit developed and made by SET can provides a slower pulse signal at lower frequency more than that of AE signals. It is converted from and synchronized with the detected PD pulse by CCS or HFCT usually, with which the oscilloscope can be easily trigged by the electrical signal of real PD, catch the AE signals, and the timing of each channel can be observed simply and precisely based on the trigger time base. Otherwise, it may firstly require the signal identification as PD judgment if using the AE signal to trig the oscilloscope. This is a successful example that applied combined PD sensors to localize a real PD on-site, in which, based on the trigger signal detected by CCS, and the detection order of the four AE signals, the positions of four AESs were changed and moved step by step, approaching to the PD site until reaching the goal.

The last case in this paper shows an online PD monitoring system applied for a 400kV cable sealing end of GIS. Some suspicious PD signals were detected with CCS at the sealing end about one year ago, but it was not easy to determine the best way of dealing with the case because the doubtful PD signals were unstable and intermittent. For this case, a kind of newly developed long-term online PD monitoring system [1] [2] [3] was used to chase the doubtable and abnormal signals as in Fig. 11, in which the suspicious PD signals, corona and the other abnormal signals were observed and analyzed down to the minutest details. With the newest IT technology the online PDM system can auto-save all the data and send out variable reports to the operators. It is successful to help the power company to keep safely the power equipment and their accessories operating on with more confidence.

VI. CONCLUSION 1) A new combined PD sensor method for the PDM and PDL for GIS is introduced in which the well-known CCS and HFCT, the well-known AES and a new kind of combined PD sensor developed are applied. All the sensors can be installed on GIS during its commercial operation, without any outage. 2) Experiments with a scale GIS model was conducted in laboratory for feasibility study. It shows that, the injected PD pulse can be detected clearly with the developed PD sensor. The apparent PDM sensitivity in lab is 25-200pC.

3) A verification test based on a substantial 220kV GIS without outage was conducted, in which the combined PDM sensors were installed on the GIS flange with epoxy spacer, and the simulated PD pulse was injected from the shorted flange. It shows the injected PD pulse can be detected clearly. The apparent PDM sensitivity in lab is around 50-200pC. 4) Serial practical PDM and PDL for several GIS of 110kV, 220kV and 400kV in substations were conducted during service operation. Some PD signals and PD-similar signals were detected. It is successful to localize the PD with the combined PD sensors for times. The judgments were verified by the GIS examination after the PD measurement. Finally, a

CCSPD trigger signal

AES

AES

Cable sealing end

Fig. 10 Successful PDL using AES and CCS on sealing end

Daily report, weekly report and monthly report by online PDM

Combined PD sensors used for GIS sealing end

Local station of online PDMS

Fig. 11 Online PD monitoring system in service operation

successful online PDM system was applied practically.

VII. REFERENCE [1] M. Chen, K. Urnao, K. Choy: “Application of Long-term Online

Partial Discharge Monitoring System for GIS” in Proceeding, CMD2006 Changwon Korea.

[2] M. Chen, K. Urnao, X. Liao, L. Shi: “Application of On-Line Partial Discharge Measurement at Main Transformer in P/S”, International conf. ISH2005 Beijing, Proceeding p. 479.

[3] M. Chen, K. Urano, A. Jinno: “Basic Technique of On-line Partial Discharge Measurement for Power Cables with a Portable PDM System “International Conf. on Electrical Engineering 2003.

[4] M. Chen, K. Hirotsu, H.Nishima, A. Miyazaki, M. Yagi, S. Kobayashi: “Development of Partial Discharge Automated Locating System for Power Cable”, 1998 IEEE Conf. on Electrical Insulation and Dielectric Phenomena, Vol.2, 424-427.