Ultraviolet Corona Discharge Detection Based on ...

Extended Summary 本文は pp.357-366

－5－

Ultraviolet Corona Discharge Detection Based on Photomultiplier

Liao Yongli Non-member (Tsinghua University, [email protected])

Wang Liming Non-member (Tsinghua University, [email protected])

Wang Ke Non-member (Tsinghua University, [email protected])

Wang Canlin Non-member (Tsinghua University, [email protected])

Guan Zhicheng Non-member (Tsinghua University, [email protected])

Keywords : ultraviolet, photomultiplier, detection, corona, water droplets, polymer insulators

Polymer insulators have come into service widely over the past several decades in the worldwide. Housings and sheds of polymer insulators are always under comprehensive effect of surface discharge. Corona discharge is a significant threat to the integrity of polymer insulators because of the organic nature of the housing material. So it is significant to detect corona discharge in order to ensure the insulators’ working safety.

This paper presents the development of a novel corona discharge detection system based on photomultiplier tube, which has good functions of distance detection for corona discharge in determined region from surface of high voltage equipment and corona characteristics data analysis. The structure of the detection system is shown in Fig.1, which is made up of optical receiving and locating part, photomultiplier, data regulation and data processing.

The discharge position of polymer insulators is located by the PMT accompanied with optical receiving and locating system, in order to acquire feeble UV light signal from the surface of polymer insulators. A proper electric signal is obtained by the way of amplifying and filtering the output signal from PMT, and then the electric pulse signal can be observed in the terminal, computer with data acquisition card or oscilloscope. The signals can be used to judge the degree of corona discharge.

National Instrument’s LabView® software is used to develop the analysis program. The program consists of four modules: (1) Initialization of the DAQ card; (2) Collection of the discharge pulse data; (3) Separation of the discharge pulse data from noises; (4) Analysis and display of the discharge pulse data. Characteristics of UV corona light are taken from the pulse data in the program: (1) peak value in one plus; (2) number of pulses whose peak current is above corresponding value; (3) interval distribution between adjacent pulses. Pulses group interval

distribution can be used to identify polarity of AC corona when positive and negative corona are both generated.

Verification experiments were carried out for the efficiency of corona detection using PMT. There is good correlation between the UV corona light and corona current magnitude as shown in Fig.2. The corona discharge magnitude can be seen to be proportional to the magnitude of UV corona light picked up by PMT. Other than corona leakage current, UV corona light can be also taken as characteristic for detecting corona discharge. Furthermore, mean peak value and number of pulses whose peak value is above threshold are extracted from the basic data. Both of the two characteristics can be used to quantify the development of corona discharge.

The detection experiments of corona discharge on polymer insulators have been carried out. The results of investigation on polymer insulators suggested that detection for the region between metal end fitting and first shed should be emphasized and the measurements of corona discharge distribution along insulators can be used to learn about the degradation condition. Additionally, water droplets generate negative corona easily and trend to make negative corona intensity increase faster.

The effect of distance and humidity on detection of corona discharge using PMT has been investigated. It is shown that the number and magnitude of UV corona light pulses are reduced with increasing relative humidity (fog density) or distance from the point of interest and thus there is a threshold discharge magnitude below which corona can not be observed. The relation between magnitude of pulses from UV light source and distance showed a 1/d2 dependence, which is expected from the dependence of the solid angle subtended by the receiver with respect to distance.

Finally, useful results have been obtained from field inspection. The usefulness of the corona detection system is confirmed. However, more research on accepted standards to be used as guide lines to decide what action to take after inspection is still required.

Fig. 1. Schematic diagram of UV corona discharge detection system: 1-polymer insulator; 2-optical part; 3-photomultiplier; 4-signal regulation; 5-data processing

(a) (b)

Fig. 2. Correlation between current pulses and light pulses: (a): waveform, (b) magnitude

© 2008 The Institute of Electrical Engineers of Japan. 357


Liao Yongli＊,＊＊ Non-member

Wang Liming＊ Non-member Wang Ke＊,＊＊ Non-member

Wang Canlin＊,＊＊ Non-member

Guan Zhicheng＊ Non-member

High voltage equipments, especially polymer insulators may be getting into aging conditions due to the existence of corona discharge on the surface after a long term of running, which would accelerate the deterioration of the surface insulation performance, and even make equipments step into calamity ultimately. So it is significant to detect corona discharge on surface to ensure insulators’ stable running. This paper presents the development of a novel corona discharge detection system based on photomultiplier tube (PMT), which has good functions of distance detection for corona discharge in determined region from surface of high voltage equipment and corona characteristics data analysis. In the verification experiments, it was shown that UV corona light can be also taken as characteristic for detecting corona discharge, other than corona leakage current detection, and a linear relationship was shown between the light magnitude and the current magnitude. Furthermore, mean peak value and number of pulses whose peak value is above threshold are extracted from the basic data, which can be used to quantify the development of corona discharge. The results of investigation on polymer insulators suggested that detection of the region between metal end fitting and first shed should be emphasized. The measurements of corona discharge distribution along insulators can be used to learn about the degradation conditions. Detection of polymer insulators in lab and field inspection experience are both soundly verifying the usefulness of the corona detection system.

Keywords : ultraviolet, photomultiplier, detection, corona, water droplets, polymer insulators

1. Introduction

Polymer insulators have come into service widely in the electric utilities industry over the past several decades. However the aging resistibility of polymer materials is not ideal enough, in comparison with nonpolymer materials. Housings and sheds of polymer insulators are always under comprehensive effect of surface discharge, such as corona discharge, ultraviolet and acid rain. All of this will make polymer materials come into a serious aging conditions, and even result in serious problem in future. Thus it is very important to make the assurance of insulators’ stable running.

Corona discharge is a significant threat to the integrity of polymer insulators because of the organic nature of the housing material(1). For transmission polymer insulators, corona can be existed for a long time due to an inadequate hardware design, damaged hardware, or deficient interfaces which are caused by improper design or manufacturing(2). Corona would accelerate the deterioration of insulation. So it is significant to detect corona discharge in order to ensure the insulators working safety.

Currently, there are many methods for detecting insulators, such as leakage current measurement, acoustic emission detection, Infrared (IR) thermography, electric-field measurement, ultraviolet (UV) imaging technology, visual observation, etc. Leakage current measurements are often carried out to evaluate the

performance and degree of aging polymer insulators in laboratory and field studies(3)(4). It is a contact detection method and can be disturbed by the ground current. Acoustic emission detection is disturbed easily by background noise, so it is hard to be used to determine the precise location of the discharge on insulators. IR thermography cannot be used to detect defects in early stage. The reliability of IR thermography is reduced when the insulators is performed under hot and sunny conditions. Electric field measurement along the insulator has been more successfully applied for ceramic than non-ceramic insulators. UV imaging inspection technology can allow users to observe corona discharge in full daytime without interference from solar radiation. The intensity of corona can not be quantified accurately(5)(6). Visual observation is subjectively identified by users.

This paper is concerned with the development of a novel corona discharge detection system, which has good functions of distance detection for corona discharge in determined region and corona characteristics data analysis. Investigations were made on the measurement of the ultraviolet corona discharge performance along the surface of polymer insulators in laboratory for finding method to learn about the degradation conditions. Moreover, field tests were performed with the system, which soundly verified the validity of the system for detecting corona.

2. Corona Discharge Detection System

The photomultiplier tubes (PMTs) are extremely sensitive light detectors providing a current output proportional to light intensity. Photomultipliers are used to measure any process which directly or indirectly emits light. With advantages of large area light

* Graduate School at Shenzhen, Tsinghua University Xili Town, Nanshan District, Shenzhen, Guangdong province, 518055, P.R. China

** Department of Electrical Engineering, Tsinghua University Beijing, 100084, P.R. China

Paper

358 IEEJ Trans. FM, Vol.128, No.5, 2008

detection, high gain and the ability to detect single photons, PMTs were applied to observe the streamer propagation in air and surface discharge in laboratory(7)(8).

Corona is a luminous partial discharge due to ionization of air that occurs where electric field strength exceeds a critical extent. Multiple effects, such as ultraviolet (UV) radiation, impingement of charged particles, and liberation of ozone, are taken for responsibility by corona discharge. Wavelength of corona light emission ranges from 240 to 400 nm. Most of the emission is in the UV, with a small part in the visible spectrum region. So some corona can be seen at nighttime or in the laboratory under total darkness. UV radiation from the sun in the 240-300 nm spectral range is denoted as “solar blind” range, which is totally absorbed by the ozone in the atmosphere. Thus wavelength of sunlight ultraviolet reaching the earth is mostly above 300nm. In solar blind region, the UV light emitted from corona is very weak. However, detection of this spectral range is not affected by solar light and thus a PMT sensitive in this range is not affected by sun radiation. It is these UV signals generated by corona in the blind range (240nm-300nm) that are used by the photomultiplier (PMT) to detect corona discharge.

The discharge position of polymer insulators can be located by PMT accompanied with proper optical part, in order to acquire ultraviolet light signal polymer insulators. A proper electric signal can be obtained by the way of amplifying and filtering the output signal from PMT, and then the electric signal can be observed in the terminal (computer with data acquisition card or oscilloscope), in order to judge the degree of corona discharge.

The structure of detection system is shown in Fig.1, which is made up of optical receiving and locating part, photomultiplier, data regulation and data processing. 2.1 Hardware Design The hardware includes four

units: optical part, photomultiplier, signal regulation and data processing. 2.1.1 Optical part The design of the optical part is shown

in Fig.2. The design consists of two light channels: visible channel and UV channel. The visible channel is used to locate corona emission. The UV channel is used to transmit and focus UV light. As it is shown in Fig.2(a), the light including UV light generated by corona and sun is divided into two beams. One beam is reflected by folding mirror and then received by visible CCD; the other beam transmits through UV channel. It is focused by a set of concave mirror and convex mirror, and finally collected by photomultiplier. The appearance of the optical part is displayed in

Fig.2(b). The diameter of concave mirror and convex mirror were chosen as 85mm, 34.4mm. The curvature radius of concave mirror and convex mirror were chosen as 200.4mm, 144.2mm. The distance between concave and convex mirrors was selected to be 60mm. The focal point is 90mm apart from the centre of convex mirror. According to these parameters, the light-path diagram of these two mirrors is shown in Fig.2(c).

The optical part can realize remote measurement for confirmed detection region and the detection region is related with the distance from the detection system, which is expressed as: D=d/100, where D (m) is the diameter of the region which can be distinguished by the optical part, and d (m) is the distance from the detection system. For instance, the region with diameter 30cm is designed to be distinguished from the distance 30m by the optical part. 2.1.2 Photomultiplier The selected PMT is sensitive to

feeble UV light and transform the UV light signals into current signals. The typical spectral response curve of the photomultiplier used in the corona detection system is 110~310nm according with “solar blind” range so as to detect corona discharge in the daylight, and the max quantum efficiency is up to 10%. The max gain of the photomultiplier can reach 107A/lm. The magnitude of anode current from photomultiplier is about 100µA and dark current is only 0.3nA when gain is set 107A/lm, which is indicated that low background noise is generated when measuring. Single photon response time is 20ns. In short, the selected photomultiplier has anti-interference ability for detecting corona and high response speed will be obtained, attempting to realize detection in daylight. 2.1.3 Signal regulation The output current signal with

max amplitude 100µA is a feeble and fast narrow pulse (typical pulse width is only 100ns), and it is not easy to collect it directly, hence signal regulation part, as shown in Fig.3, is applied to make it fit for data acquisition. Signal regulating unit is used to make current signals output from photomultiplier into suitable voltage signals, with suitable magnitude and pulse width, for acquired easily by DAQ card.

Figure 3 shows the schematic diagram of signal regulation, and the regulation circuit is processing of 500kHz upper cut-off

(a) (b)

Fig. 1. Photograph and schematic diagram of UV corona discharge detection system : (a) photograph, (b) schematic; 1-polymer insulator; 2-optical part; 3-photomultiplier; 4-signal regulation; 5-data processing

(a) (b)

(c)

Fig. 2. Design and photograph of the optical part : (a) schematic, (b) photograph, (c) light-path diagram


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frequency. Experiment is presented that the pulse width is about 1µs after regulation. Considering the interval between adjacent corona light pulses is at least 5µs, low filtering of regulation part will not produce bad effect on corona detection. 2.1.4 Data processing unit A typical waveform of single

pulse signal after regulation is shown as Fig.4, when the gain of the photomultiplier is set 104A/lm. It is a negative voltage pulse, with rise time about 700ns and pulse width about 1.2µs.

NI DAQ card (PCI-5112) is used to acquire pulse signals after regulation. The card is a high performance digitizer with parameters of 8 bits (resolution), 100MHz (bandwidth), 100MS/s (max real-time sample rate). For a single pulse signal, DAQ card can collect at least 10 data sampling points in the interval of pulse width, which is enough for the requirements of data analysis. With the application of the selected DAQ card, waveform of corona pulse signals can be real-time sampled and some important information such as magnitude and number of corona pulse signals can be analyzed and stored. 2.2 Program Design Virtual instrument (VI) is applied

to analyze corona signals in the design and National Instrument’s LabView® software is used to develop the analysis program. The program consists of four modules: (1) Initialization of the DAQ card; (2) Collection of the discharge pulse data; (3) Separation of the discharge pulse data from noises; (4) Analysis and display of the discharge pulse data. Software flow chart is presented in Fig.5.

In the process of corona detection, noise is generated from dark current of photomultiplier and solar light, which may disturb corona UV signals. Setting threshold value is used to reject the noise in the program. Before detection users can observe waveform of noise in the data collection terminal and choose a suitable threshold value based on observed noise level to reject the noise. Normally the level is below 100mV. Hence, the pulse with peak value above noise level can be definite as corona pulse.

Cubic spline function is used for interpolation to obtain peak value of pulses. In the program, some analysis is carried out and some characteristics are taken from these corona pulse data.

With this definition, some characteristics are taken from the pulse data in the program: (1) peak value in one plus; (2) number of pulses whose peak value is above corresponding values; (3) interval distribution between adjacent pulses.

More information can be obtained from these characteristics. Waveform of the pulse current and its characteristics are saved in separated files in computer and can be recalled by the software. 2.3 Experimental Verifications Verification experi-

ments are carried out for the efficiency of ultraviolet corona light using photomultiplier. The results showed that there was good numerical correlation between the ultraviolet corona light and corona leakage current. Ultraviolet corona light can be taken as characteristic for detecting corona discharge, other than corona leakage current. Thus the corona detection system can realize quantitative detection of corona discharge by measuring ultraviolet corona light.

Needle-plate discharge experiments under DC voltage are taken to observe the relationship between ultraviolet corona light pulse and corona leakage current pulse. The electrode arrangement consists of a needle of radius 1mm and a plane of diameter 25cm. The gap clearance of needle and plate is 30mm. UV corona light pulse is picked up by corona detection system, and current pulses are collected by a sensor resistance. Figure 6 shows contrast waveform between ultraviolet light pulse and leakage current pulse in a single DC corona discharge. In comparison with current pulse, single ultraviolet light pulse has a longer duration (greater

Fig. 3. Schematic diagram of signal regulation

Fig. 5. Flow chart of data processing program

Fig. 4. Typical waveform after regulation

(a)

(b)

Fig. 6. Contrast waveform between one leakage current pulse and UV light pulse: (a) DC positive corona discharge, (b) DC negative corona discharge


pulse width, caused mainly by signal regulation circuit) and lags a little in time domain (caused mainly by inherent response delay in PMT). In the observation, one light pulse strictly corresponds to one current pulse. When number of current pulses increases or decreases, the number of UV light pulses increases or decreases accordingly.

By changing the DC voltage applied on needle-plate electrode, the corona leakage current pulse magnitude is varied and its corresponding UV light pulse magnitude is also varied. As shown in Fig.7, the magnitude of single current pulse can be seen to be proportional to that of UV light pulse. Figure 8 represents the distribution statistic of DC negative corona pulses. Figure 8 (a) shows the histogram of the number of corona current pulses between － 50mV and － 190mV, and Fig.8 (b) shows the histogram of the number of the UV light pulses between －87mV and －420mV. It is seen that the histogram of corona current pulses and UV light pulses is similar. To sum up, there is strong relation between UV light pulses and corona leakage current pulses in quantity, magnitude and number distribution. The characteristics (quantity, magnitude and number distribution) taken from UV corona light pulses can be also used to quantify the development of corona discharge, other than the characteristics of corona leakage current.

In addition, the gain of the photomultiplier for measuring UV negative corona light is much bigger than that for UV positive corona light in Fig.7. It is found that under the same condition UV light pulse magnitude of positive corona discharge is much bigger than that of negative corona discharge. When the corona discharge appears on the surface of polymer insulators, intensity of negative

corona light is always weaker than that of positive corona, which is considered as the polarity effect. Therefore, it is usually difficult to detect both positive and negative corona discharge accurately in a single acquisition sequence. Detection of positive corona discharge should be concerned more. 2.4 Statistics on Interval between Adjacent Pulses The detection system can not distinguish the polarity of the

corona discharge only from the light pulse. The waveform of UV light pulse colleted by detection system is always negative pulse, whether positive corona or negative corona. But the polarity of

(a)

(b)

Fig. 7. Corresponding relationship of the magnitude between current pulses and UV light pulses: (a) the relationship for the positive corona (gain of 104), (b) the relationship for the negative corona (gain of 105)

(a)

(b)

Fig. 8. Distribution statistic of current pulses and light pulses: (a) current pulses, (b) UV light pulses

(a)

(b)

Fig. 9. Corona discharge pulses interval distribution in AC power: (a) unipolar pulses, (b) bipolar pulses


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leakage current pulses can be identified by waveform, when AC voltage is applied to electrodes. Polarity of the corona discharge can be identified through the characters of interval distribution between adjacent pulses, which is taken from pulse data.

Statistics on interval between adjacent pulses are displayed as Fig.9. X axes is followed with interval between adjacent pulse (s) and Y axes is followed with pulses number on specific interval. Corona light pulses are always emitted in group, so the interval between adjacent pulse is around 0ms, but when pulse groups develop, one pulse is transferring next one and the interval distribution will be presented in two cases: (1) The polarity of following pulse group is the same as the previous one, that is only unipolar pulses emitted, as Fig.9 (a) displayed, light pulses will be distributed around with power period interval 20ms or multiple of 20ms. (2) The polarity of following pulse group is different from the previous one; that is bipolar pulses emitted, as shown in Fig.9 (b), the average pulses interval distribution will be about half power period 10ms.

Consequently, pulses group interval distribution can be used to identify polarity of AC corona when positive and negative corona are both generated.

3. Experimental Setup

The experimental test setup used in the present study is illustrated in Fig.10. It is made up of two parts, test platform and

measure system. High frequency corona current pulses are picked up by Rm (noninductive resistance) and colleted by digital oscilloscope. UV corona light on insulator is detected by PMT, and pulses data is analyzed on computer.

A 110kV/100kN rated silicone rubber insulators are chosen for these experiments. The silicone rubber insulator is shown in Fig.11 and the silicone rubber insulator with water droplets on is presented in Fig.12. The structural height of the insulators is 1200mm. The insulator has 12 big sheds and 12 small sheds. The AC voltage applied on insulators was increased gradually in steps of 5 or 10 kV. The UV corona light pulses and their corresponding corona leakage current pulses were recorded simultaneously. The distance between the point of interest and the PMT was selected to be 3m for convenience.

ANSYS® calculation result of electric potential distribution along a dry and clean 110kV rated polymer insulator is shown in Fig.13. The calculating polymer insulator is in the absence of a grading ring. It is shown that 40～50% electric potential is centralizing in the portion between metal end fitting and first shed(9). The region of the triple junction of high voltage end fitting, sheath and air has the highest electric field as shown in Fig.14 and it is most likely to cause corona, especially in the presence of water droplets(10). Hence detecting the region between metal end

Fig. 13. Electric potential 2-D distribution along dry and clean polymer insulator

Fig. 14. Electric field strength along the shank portion between high voltage end fitting and first shed

(a) (b) (c)

Fig. 12. Experimental polymer insulators: (a) good performance insulators with water droplets, (b) natural degradation insulators, (c) natural degradation insulators with water droplets

Fig. 11. FXBW 110 kV/100 kN polymer insulator

Fig. 10. Experimental test setup; T1 - insulating transformer; CB - control board; T2 - self-coupling voltage divider; NF - power filter; T3 - non-PD HV testing transformer; Z - low-pass resistance; EVM - electrostatic volt meter; C1, C2 - capacitance for voltage divider; CX - polymer insulator; Rm - 50 Ω noninductive resistance；C - 50 Ω coaxial cable; PMT - corona detecting system; OSC - digital oscilloscope; PC - desktop computer


fitting and first shed was emphasized and water drop corona on polymer insulator was also investigated by PMT in these experiments.

4. Results and Discussions

4.1 Correlation between Corona Light and Current The corona discharge magnitude is varied when the applied

voltage is changing. The correlation between UV corona light and corona leakage current, when AC voltage is applied on dry and clean polymer insulator, is illustrated in Fig.15. The results of Fig.15 (b) are manifested that the corona discharge magnitude can be seen to be proportional to the magnitude of UV corona light picked up by PMT. 4.2 Characteristic for Quantifying Corona Discharge UV corona light pulses are always presented in group under AC

voltage. From the statistics on interval between adjacent Pulses, most of the interval is around 0ms, and pulses develop from one group to another in time domain, as shown in CH2 of Fig.16. CH1 and CH2 represent corona leakage current pulses and UV corona light pulses respectively.

In order to quantify corona discharge, specific features should be extracted from the basic pulse information. When detecting corona discharge, light pulses are acquired and stored in PC after signal regulation. Some basic pulse information, such as peak value, pulse number, and pulse appearing time, are stored in PC. After additional processing of the pulses data, mean peak value of pulses and number of pulses whose peak value is above threshold are extracted from the basic data. Both of the two characteristics can be used to quantify the development of corona discharge. The time interval for collection of pulses data is chosen as 30s in

Fig.17 and Fig.18. Figure 17 shows the number of UV corona light pulses colleted

by the detection system, as a function of voltage applied on dry polymer insulator. When applied voltage is increased to 65kV, the region around the first shed is first detected. The region is in the portion between metal end fitting and first shed, as ANSYS® calculation results indicate that the region is strongest. The number of pulses from the region around the first shed is increased when voltage is applied more. Furthermore, as seen, corona discharge is detected in the region around the second shed, third shed, fourth shed at 75kV, 95kV and 105kV respectively. The number of pulses is increased after discharge takes place with applied more voltage. Mean peak value of pulses is also increased as number of pulses when applied voltage is increased, as illustrated in Fig.18. Corona discharge is developed from shed located in high voltage end to sheds in lower voltage regions. The discharge development showed that number and mean peak value of pulses were both used to be specific feature for quantifying corona discharge. Hence these two characteristics are used to detect the intensity of corona discharge by system.

Increasing voltage applied on polymer insulators represent an enhancement of electric field along insulators. The enhancement of electric field is essentially the same as that as the result of degradation of polymer insulators. From observation of Fig.17 and Fig.18, corona discharge is developing along insulators from high voltage end to ground with enhancement of electric field. The results of an observation on degradation polymer insulators under dry conditions (Fig.12 (b)), which have been served for at least 10 years, showed that corona discharge was detected in the region around the second shed and the third shed at 60kV and 85kV

(a)

(b)

Fig. 15. Correlation between current pulses and light pulses: (a) waveform, (b) peak value

Fig. 17. Number of UV corona light pulses as a function of voltage applied on dry polymer insulator

Fig. 16. UV corona light pulses in AC voltage


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respectively. Corona discharge is developing faster along natural degradation insulators than that along good performance insulators. Therefore, it is concluded that the measurements of corona discharge distribution along polymer insulators can be used to learn about the degradation condition of polymer insulators. Comparing with the measurements of corona development along good performance insulators, the characteristics from corona discharge along insulators in service can be used for evaluation of aging degree. Further investigation is still required for aiming a more extensive analysis of the corona light characteristics for degradation degree recognition purpose. 4.3 Water Droplets Corona Corona discharge on

polymer insulators in the presence of water droplets is different from that under dry condition. Results of observation on polymer insulators with water droplets (Fig.12(a)) shows that positive and negative corona discharge were both generated on insulator. Compared to dry conditions, negative corona discharge appears on the surface and is detected in evidence, as shown in Fig.19. When insulators is under wet conditions, interval between pulse groups is concentrated around 0.01s or multiple of 0.01s (Fig.19(b)), manifesting the appearance of negative corona discharge.

When water droplets are on polymer insulators, they may enhance the electric field nearby (11). In the same time, they will be also deformed, and always elongated along the direction of electric field under applied voltage. These distortions shorten the insulating distance resulting in the generation of corona discharge easily. Hence corona discharge was detected in low applied voltage. Furthermore, water droplets on the surface trend to make

negative corona discharge intensity increase faster. In Fig.20, mean peak value of negative corona pulses is close to 1/3 of that of positive corona pulses. By contrast with it, in Fig.21, mean peak value of negative corona pulses are almost coming up to that of positive corona pulses. Hence, corona status of polymer insulators under wet conditions is greatly different from that under dry conditions. Water droplets make effect on the electric field nearby and make the discharge complex. In addition, corona discharge may be generated by water droplets in some other region besides the region between high voltage end fittings and first shed. For example, discharge near ground end can be detected in evidence. The measurements of corona discharge distribution along polymer insulators used to learn about the insulation condition of polymer insulators should be reconsidered when polymer insulators are under wet conditions. 4.4 Effect of Distance Magnitude of UV corona light

pulses decreases when increasing the distance between the PMT and insulators. A constant UV light source, which can keep the same intensity, was used to account for the distance factor. Due to the size of the laboratory, the testing distance was limited to 10m. Meanwhile, the gain of PMT and the amplifier are kept in constant during the test. The result is shown in Fig.22. As the distance increases from 2m to 10m, the UV amplitude decreases from 700mV to 70mV. The peak value of pulses is proportional to 1/d2, where d stands for the distance. 4.5 Effect of Humidity Observation can be hampered

under foggy conditions. The transmitted UV light can be scattered and absorbed by water vapor in the air before reaching receiver, which decrease the intensity of UV light received. It is getting worse when small droplets exist under foggy conditions. It can be concluded that for the same discharge magnitude, the number and magnitude of UV corona light pulses are reduced with increasing

(a)

(b)

Fig. 18. Mean peak value of UV corona light pulses as a function of voltage applied on dry polymer insulator: (a) first shed, (b) other sheds

(a)

(b)

Fig. 19. Number statistics as a function of interval between adjacent pulses at 85 kV (1.34 p.u.): (a) dry conditions, (b) wet conditions


relative humidity (fog density). Actually, field inspection of corona discharge can be effected by humidity. Data reported in this paper were observed in laboratory with 70~75% relative humidity.

5. Field Inspection

In practice, strong solar light has obvious disturbance on the detection of feeble corona discharge when the corona detection system is applied for field inspection. Thus it is suggested that field inspection is carried out at evening or very early morning. A utility experienced in Quanzhou power substation in China at evening, almost under dark condition, where the relative humidity was about 70% and the temperature was about 25. As shown in Fig.23, corona discharge on the region between metal end fitting and the first shed of 220kV PT insulators was detected and no

significant sound was heard. The distance away from the region of interest is about 15m. Figure 24 shows the waveform of corona discharge obtained from the region. Then it was found that the region of interest was wet and polluted. The feeble invisible corona discharge was detected by the system. It is indicated that such work can give warning information of the surface insulation conditions, where may be expected serious failure in future.

In Fig.25, inspection for bundled conductors with strain insulator strings was carried out from distance at early evening. The distance between the region of interest and the detection system was close to 15m. DC voltage was applied on the conductors and increased gradually in steps of 5 or 10kV, as illustrated Fig.26. The maximum voltage value was increased to 1000kV. In the whole process, corona on four regions was detected by the system, as marked in Fig.27. Before detection, background noise was measured and the result is showed in Fig.28. It is seen that there are seldom pulses with peak value above 180mV. Thus threshold

Fig. 23. Field inspection for 220 kV PT insulators

Fig. 24. Corona discharge waveform of the region between metal end fitting and the first shed from 220 kV PT insulator

Fig. 25. Field inspection for bundled conductors with strain insulator strings

Fig. 22. Correlation between UV light magnitude and distances

Fig. 20. Water droplets corona by PMT at 63.5 kV

Fig. 21. Water droplets corona by PMT at 95 kV


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value is set to 180mV for analyzing. In Table 1, onset corona voltage in four regions is recorded as 600kV, 950kV, 900kV and 580kV respectively. When corona discharge appears on the conductor surface, big number and mean peak value of pulses will be recorded in the system, compared with the background noise. It is analyzed that such corona discharge firstly appears due to some rough surface or damaged surface. Abnormal surface and structure make onset corona presented. In short, such experience soundly verified the usefulness of the corona detection system.

6. Conclusion

The development of a novel corona detection system is

presented. The system has good functions of distance detection for corona discharge in determined region and corona characteristics data analysis, which had been confirmed in the field test. The application of the system helps to lean about the insulation surface conditions of high voltage equipments.

Verification experiments were carried out for the efficiency of corona detection using photomultiplier. The results showed that there was good correlation between ultraviolet corona light and corona leakage current. Hence, other than corona leakage current detection, UV corona light can be also taken as characteristic for detecting corona discharge. Thus the corona detection system can realize quantitative detection for corona discharge by measuring ultraviolet corona light. Mean peak value and number of pulses whose peak value is above threshold are extracted from the basic data, which can be used to quantify the development of corona discharge.

For polymer insulators under dry conditions, it is suggested that detection for the region between metal end fitting and first shed should be emphasized and the measurements of corona discharge distribution along insulators can be used to learn about the degradation condition of polymer insulators under dry conditions. Further investigation is on the way aiming a more extensive analysis of the corona light characteristics for degradation degree recognition purpose.

Program of calculation for statistics on interval between adjacent pulses is developed. The statistics can be used to identify polarity of AC corona. The results of water droplet corona show that corona status of polymer insulators under wet conditions is greatly different from that under dry conditions. Water droplets generate negative corona easily and trend to make negative corona intensity increase faster. Water droplets make effect on the electric field nearby and make the discharge complex.

The effect of distance and humidity on detection of corona discharge using PMT has been investigated. The relation between magnitude of pulses from UV light source and distance showed a 1/d2 dependence, which is expected from the dependence of the solid angle subtended by the receiver with respect to distance.

Useful results have been obtained from field inspection. The usefulness of the corona detection system is confirmed. However, more research on accepted standards to be used as guide lines to decide what action to take after inspection is still required.

Acknowledgement The authors are grateful to National Natural Science Foundation

of China for supporting the research presented in the paper. (No. 90210030). FJEPTRI (Fujian Electric Power Test & Research Insitute) and WHVRI (Wuhan High Voltage Research Institute) are also greatly appreciated.

(Manuscript received Oct. 30, 2007, revised Jan. 4, 2008)

References

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Fig. 28. Measurement for background noise; Number distribution with peak value of pulses

Table 1. Detection Results for bundled conductors with strain insulator strings

Region 1 2 3 4

Voltage (kV) 600 950 900 580

Number 165 183 546 303

Mean Peak Value (mV) 218 302 283 216

(a) (b)

Fig. 27. Corona discharge on bundled conductors: (a) three detected regions on conductors, (b) detected regions on the conductor end

Fig. 26. HVDC power equipment


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Liao Yongli (Non-member) was born in Chongqing , China, in 1984. He received his B.S. degree in Electrical Engineering from Tsinghua University, China in 2006. He is currently pursing his M.S. degree in department of Electrical Engineering of Tsinghua University continuously. His research covers high voltage insulation and electrical discharge.

Wang Liming (Non-member) was born in Zhejiang province, China, in 1963, and received the B.S., M.S., and Ph.D. degrees in high voltage engineering from the Department of Electrical Engineering, Tsinghua University, Beijing, P.R. China, in 1987, 1990, and 1993, respectively. He has worked at Tsinghua University since 1993. His major research fields are high voltage insulation and electrical discharge, flashover mechanisms of

contaminated insulators, and application of pulsed electric fields.

Wang Ke (Non-member) was born in Sichuan province, China, in 1983. He received his B.S. degree in Electrical Engineering from Tsinghua University, China, in 2005 and he is currently working toward his Ph.D in department of Electrical Engineering of Tsinghua University. His research is focused on the surface micro-discharge of outdoor insulation and the application of PMT in corona detecting.

Wang Canlin (Non-member) was born in Hunan province, China, in 1983. He received his B.S. degree in Electrical Engineering from Huazhong University of Science and Technology, Wuhan, China, in 2004, and he graduated with the M.S. degree in Electrical Engineering from Tsinghua University, Beijing, China, in 2007. Now he is working in Hunan Electric Power Dispatch & Communication Center, Changsha, China. His research

interests include electric power system technology and surface micro-discharge of outdoor insulation.

Guan Zhicheng (Non-member) was born in Jilin province, China in 1944, and received the B.S., M.S., and Ph.D. degrees in high voltage engineering from the Department of Electrical Engineering, Tsinghua University, Beijing, P.R. China, in 1970, 1981, and 1984, respectively. Now he is Vice President of Tsinghua University Council and Dean of Graduate School at Shenzhen, Tsinghua University. His major research fields are high voltage

insulation and electrical discharge, composite insulators and flashover of contaminated insulators, high voltage measurement and application of plasma and high voltage technology in biological and environment engineering.

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