Proceedings of the 7th International Conference on Properties and Applications of Dielectric Materials June 1-5 2003 Nagoya
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On-line Partial Discharge Monitor System of XLPE Power Cable Based on Virtual Instrument Cheng Yonghong’, Chen Xiaolin, Zhang Shaofeng, and Lv Yihang State Key Laboratory of Electrical Insulation for Power Equipment Xi’an Jiaotong University, Xi’an, China 710049 ‘E-mail
[email protected],edu.cn Abstract: High voltage XLPE power cable is a very important part of power system. The degree of electric-tree aging in the cable can be estimated effectually by measuring partial discharge. An on-line partial discharge monitor system of XLPE power cable was developed based on the virtual instrument designing conception. This system has the advantage of wide analog bandwidth, excellent anti-interference ability, good stability, powerful analysis, friendly interface, convenient operation, etc, it can detect the quantity of partial discharge (PD) in XLPE power cable. It holds utility value in power system.
anti-interference ability, good stability, powerful analysis, kiendly interface, convenient operation, etc, it can detect the quantity of partial discharge (PD) in XLPE power cable. It holds very vital value in power system. HARDWARE The hardware consists of high fiequency current sensors, preamplifier, signal cable, signal processing board, DAQ board, industrial computer, database sewer and network server etc. The eamework of hardware was showed in Fig. 1.
Key words: sensor, high-rate DAQ board, virtual insmunent
INTRODUCTION High voltage power cable is a very important part of power system. With the development of high and ultra-high voltage transmission, the cross-linked polyethylene (XLPE) cables are applied more and more widely for their especial advantage in insulation. When the cables are in service, partial discharge ( PD) often occurs in the insulation part of cables, due to partial centralize of electric field stress caused by their inner impurity, the convexity of semiconductor layer and the collection of space charge, etc “I. Therefore, the degree of electric-tree ageing in the cable can be estimated effectually by measuring partial discharge. Based on the virtual instrument designing conception, using the PCI-5112 digitizing acquisition board produced by National Instrument in USA, we have developed a new on-line PD detection and diagnosis system. The hardware of the system consists of sensors, fore-end signal processing boards, rear-end signal processing board, DAQ board and industrial computer, etc. The part of sofhvare employs LabWindows/CVI program language, which realizes the functions of digital processes (including FIR filter and LMS self-adapting filter) to acquitted signal, statistic analysis to discharge signal, auto-store of data and inquiry o f historical data, monitoring through network, fault diagnosis, etc. And multiple user models were designed according to actual need of using field, and different user model has different administration authority. This system has been launched into practical application in Xi’an electric power company, and takes on good effect. This system has the advantage of wide analog bandwidth, excellent
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Fig1 The block diagram ofthe hardware components
PD signal couples in high frequency current sensors and is transferred into voltage pulses. The signal output by sensors is very weak, so to long distance transmission it is amplified by preamplifier, then is conducted by high frequency and low loss coaxial cable into signal processing board, which is in the main-control room. The signal is send into industrial computer after processing in signal processing board. The digital controls card in industrial computer control the signal
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processing board and DAQ board.
Communication Cables
Sensor
There is a long distance between the XLPE cable to main-control room, so to effective transmission and anti-interference, 50Q coaxial cable was used which present very low attenuation at IOMHz as we had experimented.
The sensor developed, showed in Fig 2, was based on the theory of Rogowski coil. It was set on the earth wire of cables, so was isolated to high voltage side and didn't change the earth model ofthe cable. It is a well-accepted way of signal coupling method'*'.
Signal preprocessing unit The system can monitor the PD signals occurring in three phase cables. When the three-way signals are transmitted to the signal-conditioning unit, only one-way signals can be selected by the operator or automatically. Then, they are filtered by a filter circuit to remove the interfering signals. And then, according to the amplitude of input signals of the digitizing acquisition board, the gain amplifier is selected automatically. At last, the signals are transmitted to the acquisition board. Fig.4 shows the block diagram of signal processing unit.
Fig.2 Schematic diagram ofsensor
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Fig.3 Amplitude-frequencyresponse characteristics curves of sensor
The sensors employed high permeability and low loss material. There lies a best match of resistance R and numbers of turns of coils N make the frequency bandwidth of sensors reaching to suitable width and keeping well responsive sensitivity 13'. So, altering the match of resistance R and numbers of t u n s of coils N could design sensors with best amplitude frequency response characteristics. As the test analysis shows, the sensors we developed hold a better amplitude frequency response characteristic and wide frequency bandwidth, and with a 3db bandwidth of 100kHz-15MHz. So they were effective for extracting the PD signal of cable insulation barrier. Preamplifiers Fore-end amplifier amplifies the signal yielded by sensors, and makes the signal to suitable amplitude when it gets signal-processing board after long distance transmission, so that the quantization distortion of N D conversion can be reduced. The amplifier used in the system enlarge the signal to IO times, and has a frequency bandwidth of 100!dfz-15MHz, which is anastomotic to sensor.
1 Industrial Computer Fig.4 The block diagram of signal processing unit
Digitizing Acquisition Board A digitizing acquisition board called DAQ Scope 5112 is used, which is produced by National Instrument Company in USA. Its sample rate is 100MS/s ADC with the analog bandwidth of 20MHz. Its ADC resolution is 8-bit and the memory on board is 16Mbyte. The sample rate is about 7 times of the highest frequency of PD signals, so the PD signals can be sampled without distortion according to the Nyquist Theorem. Digital Control Board A digital control board called digital U0 6503 is used, which is also produced by National Instrument Company in USA. There are three ports on the board and each port includes an 8-bit control word. The three
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control words can control the multiple switches, the gain adjustment and the frequency range of filters. The control functions can be realized easily by program.
reserve the PD si nals The PD signals can be detected correctly with it I4f, '
There are not only basic PD parameters, such as max discharges, mean discharges and discharge numbers, but also statistical parameters such as skewness and kurtosis in parameter calculating part. Skewness describes degree of symmetry of random variable distributing: and it is a statistical variable. Kurtosis is described the degree of distributing of a random variable concentrating on the average, or the rising speed of random variable i.e. varying gradient of distributing function. The experimental results show that the skewness and kurtosis are one of characteristics of XLPE cable PD, they correlate with the PD status of insulation close, and can indicate the insulation status of XLPE cables.
Sewer There are a database server and a web server in my PD detector. The database server is connected to the industrial computer and the web server. The PD data are transmitted to it from the computer and they are managed in order by it. Thus, the local computer and the remote computers can query history PD data easily. The web server is coMected to Internet and it services for communication by network.
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SYSTEM SOFTWARE
Software components
The fault diagnosis part can give reference advice by the calculated PD parameters, linking with the data of operation history of XLPE cables.
Using a virtual instrument programming language named LabWindows/CVI, the measuring and controlling software of detecting system is developed. The software components include initialization, data acquisition, digital signals processing, parameter calculating, data storage, data query, printing, fault diagnosis and communication by network. Fig3 shows the block diagram of the software components.
The communicate part includes two functions. One is transmitting PD data in real-time method by the data sock technique. Another is transmitting PD data if a remote computer demand by Internet based on the web technique. Appearance of the software platform Fig. 6 shows the appearance of the software platform of the detecting system. It is a typical interactive interface of a virtual instrument. It is composed of a menu bar, a tool bar, an IP address bar, a waveform area, a parameter area and an alarm area for failure.
Digital Singal Processing Calculating Parameters
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Fig.6 The interactive appearance ofthe software platform Fig. 5 The block diagram of the software componenU
The style of the appearance looks like the explorer of Windows operating system. The menu bar and the tool bar can fulfill the setting of the parameters, which include the bandwidth of the filter, the detecting time, the number of the phase-windows, and so on. On the right of the interface, there are 12 statistical parameters, which indicate the phase distribution of discharge signals. There are three green round indicating lamps, they can give an alarm for failure of the XLPE power cable according to the number of the number of the
A FIR digital filter and a LMS adaptive filter are used in digital signals processing part. The FIR digital filter can remove the interfering signals in 6equency domain and the PD signals can be reserved. The bandwidth of FIR digital filter can be changed in software panel easily. The LMS adaptive filter can process data in optimization method and it can separate the PD signals 6om the interfering signals. Thus, the digital signals processing part can remove effectively interfering signals and
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discharge, the maximum and mean amplitude of the discharge. We can see that the interface of the software platform is interactive and easy to use. PRACTICAL RESULTS This system has been launched into practical application in Xi'an electric power company. We determined that disturbing signal frequency was under 40OkHz after measuring it in the sense. So the cut-off frequency of FIR filter was set to 0400kHz.LMS self-adapting filter was set to IO orders, and with a self-adapting factor of 0.0001, so that periodical disturbing signal can be efficiently eliminated. Signal processing board has a bandwidth of 100kHz-IOMHz, that was the widest frequency bandwidth. Fig.7 shows a three-dimensional distribution (N-@-Q)of a typical discharge. The number of discharge, the phase of discharge and the amplitude of discharge can be observed by this distribution. Fig.8 shows the signal of PD detected without filter. Fig.9 shows the signal of PD detected after FIR filter and LMS self-adapting filter. The mode of operation should be set to auto-measurement, and the time of measurement at a time is set to a power current cycle. The system measured once in a minute, in which PD parameters are recorded automatically. So operation crews can look over and print the PD factor and history of record.
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Fig.9 Signal of PD detected aRer filter
CONCLUSIONS An on-line partial discharge system of XLPE, with the upper cut-off frequency of ISMHz, is detailed in paper. This system based on virtual instrument has the advantage of wide analog bandwidth, excellent anti-interference ability, good stability, high intelligence, etc, it satisfies the on-line monitor for partial discharge (PD) in XLPE power cable. The results of examination in laboratory and field showed that the system could pick-up PD signals effectively, displayed PD parameters directly. It indicates the detecting system can realize the intelligence monitor and diagnosis of PD of XLPE cable. ACKNOWLEDGEMENT We would like to thank State Power Company of China for financial Support. REFERENCES [I] Liu Yaonan, Qiu Changrong; "Technique of Insulation test", Mechanism Publishing Company in China, 1994
[Z] F. H. Kreuger, et al, "Partial discharge detection in highvoltage equipment", London Buttenvorths, 1989
Fig.7 Three-dimensional distribution of discharge
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[3] K. Fukunaga, M. Tan, et al, "New partial discharge detection method for live UHVl EHV cable joints", IEEE Trans. on Electrical Insulation, Vol. 27, No.3, 1992, P.669-674
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[4] Zheng Nanning, "Technology of digital signal processing '0Xi'an Jiaotong University Press, 1995
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Fig 8 Signal of PI)detected without filter
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