IEEE Transactions on Electrical Insulation Vol. 25 No. 2, April 1990 405

Optical Discharge Detection in SFG-insulated Systems

D. F. Binns Univ. of Salford, Salford, U.K.

A. H. Mufti King Abdulaeie Univ., Jeddah, Saudi Arabia

and N. H. Malik King Saud Univ., Riyadh, Saudi Arabia

ABSTRACT It is well known that corona discharges can occur in SF6-filled compressed gas insulated systems (GIS) due to the electrode and insulator surface imperfections and the presence of free conducting particles. If such discharges are allowed to continue, these can lead to ultimate failure of the insulation. There- fore an early detection and consequent prevention of such dis- charges is highly desirable. This paper describes an opto- electronic system for detecting and locating corona discharges in SF6 filled GIS. The system consists of optical fiber links, photomultiplier light detectors, operational amplifier and a computer-based data collection system. Various factors which can influence the optical discharge detection sensitivity are in- vestigated. It is shown that an optical system can detect dis- charges which are located even in the hidden areas in GIS. The optically measured discharge onset voltages are verified by us- ing the conventional electrical discharge detect ion techniques.

1. INTRODUCTION

SF6 gas has excellent electrical, chemical and thermal properties and is extensively being used as an insulat- ing and arc-quenching medium in compressed gas insu- lated systems (GIS). The major drawback of this gas is its high sensitivity to the presence of microscopic regions of high field. Such regions can occur due to the presence of foreign free and fixed conducting particles as well as electrode and insulator surface imperfections. Depending upon several factors such as the nature, the location, and the extent of the field non-uniformity, the gas pressure and the type of applied electrical stress, the discharge in- ception in SFs-filled GIs is usually in the form of corona. If such corona discharges go undetected, these can lead to ultimate failure of the insulation. Therefore, in order

to improve the life and system reliability, there is con- siderable interest in the early detection and consequent prevention of partial discharges in GIS.

Several methods for discharge detection in GIS have been discussed in the literature. These methods can be broadly classified as chemical [l-41, acoustic [5-91, elec- tromagnetic [l, 10,111, electrical [5,12,13] and optical [l, 14,151. The chemical methods are based on the identifica- tion of SF6 decomposition byproducts. Their main draw- back is that, depending upon the discharge level, these methods can take anywhere from seconds to hours after the fault initiation to respond. Moreover, these meth- ods are not suitable for discharge location. The acoustic methods are suitable for discharge location with reason- able accuracy for discharge levels of x 30 pC. Oscillat- ing particles can also be identified using such techniques.

0018-0367/00/0400-405$1.00 @ 1900 IEEE

406 Binns et al.: Optical Discharge Detection in SFs-insulated Systems

However, such methods do not provide any direct indica- tion of the discharge magnitude in pC, have low sensitiv- ity, and are difficult to apply in the presence of external noise and vibrations.

The electromagnetic methods are based on the prin- ciple that any discharge (partial or complete) will cause an electromagnetic transient in the GIs. The wave shape of the transient voltage or the current will depend upon the location of the fault, the system configuration, the type and location of the sensor, as well as the nature of the fault. Electromagnetic methods are also affected by external electromagnetic interference, have modest detec- tion sensitivity, and are somewhat complicated to apply for practical systems. The conventional electrical par- tial discharge detection method is well established and, using this technique, very accurate discharge level mea- surements are possible with a high degree of sensitivity. However, this method is also prone to external electrical noise, cannot be used easily for the discharge location and is not convenient for field measurements in large systems.

The optical method is based on the detection of emit- ted radiation during the discharge and is potentially very attractive for GIS because it is not affected by any exter- nal electrical and mechanical noise. This paper describes a prototype optical system and its performance in the de- tection of partial discharges in SF6 filled systems at high gas pressures. Using such a system, different types of discharges have been investigated. The paper reports on the results of such investigations. It is shown that such an optical system has excellent sensitivity and can be used for detection of several types of low level discharges (x 5 pC) in SFe insulated systems.

2. BASIC PRINCIPLE OF OPTICAL DETECTION

HE optical discharge detection method is based upon T the detection of light produced as a result of var- ious ionization, excitation and recombination processes in a gaseous discharge. The amount of emitted light as well as its wavelength depend, among other factors, on the gas, its pressure, and the electric field strength. The number of photons produced are generally proportional to the number of excited atoms or molecules present in a discharge at a given instant. These photons spread in all directions in the gas and are partially absorbed by the gas. Furthermore, in enclosed systems such as GIS, there is also the possibility of reflections from internal surfaces which can affect the resulting light intensity. Hence, the photon's beam intensity Ip in the gas is given by

1 - Durhlng 2 Adjurtablo

3 -Window 4 ,Cyllndricoi

Durbar 15.4mm

5 -Tub. 76.2mm i/D

Sharp point

Figure 1. A sectional view of the pressure vessel.

n

I~ = 10 exp[-kzfl n R, exp[-kz;] (1) ;=I

where Io is the beam intensity at X = 0, K is the pho- toabsorption coefficient of the gas, R; < 1 is the coeffi- cient of reflection from a surface reached after traveling a distance X; in the gas, Xf is the distance that the light travels after the last reflection, and n is the total num- ber of reflections suffered by the light beam. K depends upon the light wavelength, gas type, and on the gas pres- sure P. Generally, K increases with P and a t gas pres- sures of practical interest, Ip reduces appreciably with z. Such low levels of light can usually be detected over a certain range of z values by using a suitable photomul- tiplier (PM) detector. The detection of photon number d J ( t ) is very sensitive to the geometrical arrangement of the photomultiplier G, the transmission coefficient D of the medium, and the electron efficiency of the multiplier cathode A . For a photon flux of J p = d J ( t ) / d ( t ) the detector current J d is given as [16]

IEEE Transactions on Electrical Insulation Vol. 25 No. 2, April 1000 407

Figure 2. Some of the sharp points used in the tests. Mag- nification 164x. (a) and (b) show fresh sharp points whereas (c) shows a sharp point after be- ing extensively used.

Thus, a low-noise optical discharge detection system should have suitable values of the parameters G , A, and D in order to have the required detection sensitivity. Such a detection method requires a penetration in the GIS system to provide viewing access to the inside of the equipment. The number of penetrations will depend upon the equipment size and the application strategy. Due to the enclosed nature of the GIS systems, the light reflected from the enclosure and other components could be used for detecting discharges which are located behind the viewing windows. For this reason, internal surfaces with high values of the reflection coefficient R, are de- sirable. Thus, an optical system for discharge detection

in GIS may involve optical windows, light guides, pho- tomultiplier tubes, signal amplifiers, and radiometers. A good match should exist between the frequency response of each component and the characteristics of light emis- sion from the discharge. In SFs-filled systems, the dis- charge light can have a wavelength over the range of 200 to 800 nm. Therefore, the viewing ports, optical guides, windows and the cathodes of the PM tubes should have proper responses over this range of wavelength.

1000 t--

- t ' 2 x ' 1 - l o r

1 , 1 , 1 , 1 1 1 t 1 ~ 1 1 1 1

applied voltago ( k V r m r )

'i!j 2 0 94 28 32 36 4 0 4 4 48

Figure 3. Variation of light intensity with applied voltage and the angle of light guide for the bottom sharp point and the end window.

3. EXPERIMENTAL SYSTEM AND PROCEDURES

3.1 ELECTRODE ARRANGEMENT

N order to investigate typical discharges in GIS system, I a 1 m long model busbar was used. It consisted of a 25/75 mm diameter coaxial system which is about 0 . 1 ~ the size of typical 500 kV equipment. It was filled with pure SFs a t P = 0.3 MPa and could be tested with 50 Hz voltages of < 50 kV,,,. The outer cylinder was made of steel whereas the inner busbar consisted of interchange- able aluminum or brass rods of different surface finishes. The outer cylinder had three flanges welded to it to which quartz windows were bolted. Each window was 50 mm in diameter and 6 mm thick. These quartz ports were

408 Binns et al.: Optical Discharge Detection in SFG-insulated Systems

claimed to transmit light between 120 and 4500 nm (10% cut off points) and were blanked off and light tight when not in use. When used, they had an assembly bolted to them containing an optical guide mounted in a fabric- lined swivel bearing. The bearing allowed the axis of the optical guide to rotate over a conical volume of 30' half angle.

0 L L

/ over f ivs days .-

appliod voltago (kV rrns)

Figure 4. Corona light emission from different sharp point tips.

Three corona point locations were used as shown in Figure 1. The top corona point was located opposite to the top window, behind the busbar, representing a dis- charge source in a hidden area. The middle corona point was directed towards the busbar a t right angle to the middle window simulating a discharge source at an an- gle to the viewing port. The bottom corona point was directed towards the busbar with the end-window facing the tube. All three corona points could pass light to- wards the end-window from different distances and with the central busbar having a shadowing effect in each case. Thus many relative positions of discharge points and light guide were available varying from direct axial line of sight a t a separation of 0.12 m to the reflected light only, at 1 m distance. The aim was to determine whether a corona source could be detected by using the bearing to which the optical guide was connected and able to be directed anywhere within a region.

Adjustable sharp points were used to simulate the fixed particles on both electrodes. These sharp points could be controlled to vary the protrusion height. However, for all

-,3- w i t h fluorescent w i thout fluorescent

I , I , I , I I I I I I I I i d 20 24 28 32 36 4 0 44 48

applied voltage (kV rrns)

Figure 5. The effect of fluorescent paint (cream white) on the light characteristics for a fresh sharp point.

the tests a standard protrusion height of 3 mm was used. An alternate arrangement was used to simulate an iso- lated free conducting particle facing the HV busbar with an adjustable gap between the particle and the busbar. All the sharp points were made from aluminum and brass using an electrolytic process. For a fresh point, the tip diameter was about 1 pm. However, when subjected to discharges, the point eroded, increasing the radius to 10 pm before it was remade. Figure 2 shows photographs of two fresh points as well as an extensively used protrusion.

3.2 OPTICAL DETECTION SYSTEM.

Flexible light guides of two materials, each 2 m long, were used in this work. One was a liquid light guide 4.5 mm in diameter and capable of transmitting wavelengths between 230 and 770 nm (10% cut off points). The other was a glass fiber guide of 3.5 mm diameter with a spectral range of 400 to 2400 nm. The light guides were connected to the photomultiplier (PM) tubes.

The P M tubes used consisted of two side-on types (Ha- mamatsu R446 and R955) with multialkali cathode ma- terial, having a photocathode area of 24x8 mm2 and a head-on type (R376) with a photocathode of 25 mm di- ameter. The spectral response of the photocathodes of

IEEE Transactions on Electrical Insulation

1000

- loo-- E :

3 -

N

Y \ -

0 'E - : 10- e : 0 -

0

.-

s i

sharp point f i n e d on the enclosure

1 " ' 1 1 1 ' ~ ' 1 ' " I ' ' , I 4 8 12 16 20 24 2 8 32 3 6 4'2

applied v o l t a g e ( k V r m r )

Figure 6. Detection of corona light produced from (a): Sharp point fixed at the inner surface of the enclosure. (b): Sharp point fixed on the HV busbar. (c): Floating sharp point near the HV busbar.

-

-

all three tubes was approximately from 200 to 800 nm and the quartz windows of these tubes were claimed to transmit down to 120 nm (10% cut off point). The cur- rent amplifications of the PM tubes were in the range of 5x10' to lx107. The PM tubes were operated at ambient temperature. The system was calibrated a t 300 nm.

1 1

The PM tubes were used with a control unit contain- ing an amplifier and BCD output giving a light quantity averaged over 0.45 s. A series of outputs was fed to a BBC (Acorn) computer programmed to analyze 40 read- ings taken over a 16 s period for a constant applied volt- age. The light output was measured as irradiance in five ranges ( 2 ~ l O - l ~ to 2x10-" W/cm2). The highest sensi- tivity range was used to detect the discharge inception.

I I I I I I , I I I I I I l ? , ' - l ,

Vol. 25 No. 2, April 1990

3.3 MEASUREMENT PROCEDURES

The pressure vessel was evacuated to about 0.1 Pa be- fore SF6 was admitted to a total pressure of 0.3 MPa (3 bar). For mixtures of SF6 with Nz, Nz was admitted first to produce a defined pressure which was topped up with SF6 to the usual total pressure of 0.3 MPa. The optical measurements were made by increasing the voltages in

X I 0

I X ,o' c

1 , to I I

x o I * ' 0'

e -e'B

409

1 kV steps. For each applied voltage a set of 40 irradi- ance readings were taken and each set was repeated three times. The results given here are the average of all 120 measurements for any given experimental condition. To assess the detection sensitivity, the optical inception volt- ages were verified, using an electrical discharge detection method employing an E. R. A. Model 3 partial discharge detect or.

4. RESULTS AND DISCUSSION

4.1 DISCHARGE DETECTION FROM B O T T O M WINDOW

s mentioned earlier, the bottom sharp point could be A directly viewed from the bottom window without any physical obstruction. In this case the distance between the sharp point and the light guide inlet was about 13 cm. This arrangement was used to investigate the general discharge characteristics of various types of sources as well as methods which may be used to increase the detection sensitivity.

41 0 Binns et al.: Optical Discharge Detection in SFG-insulated Systems

1

loo0l

/' r/bottom " " " " " " ' f " '

s top sharp point liquid light guide

,4- j ' t x ,

4.1.1 DISCHARGE FROM FIXED POINT

When the sharp point was fixed to the inside surface of the enclosure or to the HV bus, it was possible to detect discharges from their very inception. It was fur- ther observed that the measured irradiance level varied with the applied voltage, the gas pressure, the radius, height and material of the sharp point, the angle and dis- tance between the light guide and the discharge source as well as the characteristics of the photomultiplier tube and the light guide. Generally the light irradiance increased with the applied voltage above the discharge onset level as shown in Figure 3. This Figure clearly shows that the maximum irradiance is measured when the light guide is directed towards the sharp point (curve A). As the light guide is rotated away from the discharge source, the irra- diance level decreases as shown in curves B and C. It is important to note that in case of curve B the measured irradiance is due to reflected light only. Consequently, the discharge is detected at a threshold voltage of 40 kV,,, for curve B as compared to 38 kV,,, for curve A. This feature can be helpful for discharge detection when the discharge source is not in front of the window, as will be discussed later.

Based upon numerous measurements it was observed that generally the variation of the light intensity with

l o o o r

. I

/ /

1 /' a ) positive half cycles

b) negat ive half cycles

1 10 100

discharge l e v e l (PC 1

Figure 9. Variation of discharge level with the irradiance for different sharp point locations. Curves 1,2 and 3 are for sharp points at the grounded enclosure, at the HV bus and adjacent to the HV bus respec- tively. (a) and (b) refer to the positive and the negative half cycles of the supply voltage respec- tively.

applied voltage can be expressed according to a mathe- matical relation. Taking the intensity IO a t voltage Vo as a threshold, the intensity I at a voltage V can be approx- imated as

(3) I = Io 1o(v-v")/vl'"

In these tests Io was generally in the range of 1 to ~ O X ~ O - ' ~ W/ cm2, and I ranged from 1 to ~ O O O X ~ O - ' ~ W/cm2. This equation implies a tenfold increase in the light in- tensity when the voltage is doubled.

The gas pressure has significant influence on both the nature of the corona phenomenon as well as the level of the light emission associated with such a discharge. It is well known that as a result of increasing gas pressure, light absorption in the gas medium is increased. Further- more, instead of continuous corona, infrequent discharges occur. It was observed during the present investigations that the intensity of the corona light increases as the pres- sure is decreased in a nonlinear fashion as expected from Equation 1.

The sharp point's tip radius has a significant influ- ence upon the light emission. A fresh sharp point caused lower discharge inception voltage. Moreover, it produced

IEEE Transactions on Electrical Insulation Vol. 25 No. 2, April 1000 411

loo0p point bottom point sharp -

10

middle Y E

U .-

middlo sharp ,’ 0-- middle point - // .? sharp point

.- top sharp point 1, U \

/- top sharp point

/ *

top sharp

16 20 24 2 8 32 36 40 44 48 0.1

applied v o l t a g e ( k V rms)

Figure 10. Corona light produced in SFs/Nz mixtures from three sharp points used one at a time. In each case the light was detected from the bottom window.

a higher light intensity for a given value of the applied voltage. Figure 4 shows the irradiance for a fresh sharp point (shown in Figure 2 (a)), for the same point but af- ter being used frequently over a period of five days as well as for a highly used point which had a much larger ra- dius, of - 5 pm. Hence, the corona light emission greatly depends upon the dimensions of the discharge source. It was further observed that the light irradiance was also af- fected by the material of the sharp point. Generally sharp points made from aluminum had higher irradiance levels as compared to similar sharp points made from brass. Since GIS are usually constructed from aluminum, it is anticipated that the active discharge point will also be of the same material. This could result in an increased detection sensitivity for practical GIs.

SFG emits a considerable amount of radiation in the UV and near UV range. It appears that the relative frac- tion of the emission in the visible range to that in the UV range increased with the applied voltage. Therefore, to detect the discharge near its inception, it is preferable to have a system which can detect UV light with higher sen- sitivity. This may be achieved by using suitable waveg- uides as well as PM detectors and fluorescent paint on the GIS enclosure. These factors were investigated and it was noted that fluorescent paint (cream/white) on the chamber walls can be used to absorb the UV light from

the discharge and re-emit it as longer wavelength radia- tion which could be detected with better sensitivity. Such a paint can also increase the light reflection from the en- closure surfaces, thereby increasing the overall irradiance level. Figure 5 shows that use of such paint can help con- siderably to detect discharge at an early stage. The use of such paint seems to increase the light intensity 5 x . It is obvious that the use of such a paint does imply some modifications to the conventional GIs.

The characteristics of the light guide as well as the PM also influence the sensitivity of the system. When a liquid light guide (calcium chloride and water) was used instead of glass fibers, the detection sensitivity increased, and the onset was recorded at lower voltages. It should be pointed out that whereas the glass fibers transmit down to 360 nm, the liquid light guide can transmit down to wave- lengths of 230 nm. Moreover, in the lower range of wave- lengths, the transmission losses associated with the liquid light guide are lower than those corresponding to the glass fibers. Consequently, the light intensity recorded with the liquid guide was almost twice the value for the glass guide when directed at the same corona source. Similarly, the PM having the better spectral response in the UV and near UV range was found to be more suitable for detec- tion purposes. It was further noted that PM type 955, which used a quartz window, has a higher current ampli- fication factor, a wider bandwidth, and a better radiant sensitivity a t lower wavelengths, and so proved to be a more sensitive optical detector. Thus, the most impor- tant factors in the choice of photomultiplier tube for dis- charge detection in SFc filled GIS are the photocathode material, the window material and the current amplifica- tion factor.

It should be pointed out that the corona point used for most of these measurements was roughly 1 pm in diam- eter and 3 mm in height. For such a small sharp point, the field decreases very rapidly with distance away from the tip. Consequently, the active discharge volume where the ionization coefficient a is greater than the attach- ment coefficient r] is very small and the discharge magni- tude in terms of charge as well as light emission is very small. Keeping these factors in mind, it is clear that the detection system has good sensitivity and is capable of detecting such low level discharges a t an early stage.

4.1.2 DISCHARGE FROM FLOATING POINTS

In SFs-filled systems, typical discharge sources are rough electrodes and fixed as well as free particles. With a fixed sharp point on the enclosure as the discharge source, it was always possible to detect the discharge at an early

412 Binns et al. : Optical Discharge Detection in SF6-insulated Systems

stage as discussed earlier. This was also true for a fixed point on the HV conductor as well as for a floating sharp point near the HV bus.

Figure 6 shows the irradiance for a fixed point located on (a) the HV conductor, (b) the grounded enclosure and (c) for a point floating in potential near the HV conduc- tor. In case (c), the distance from the particle tip to the HV conductor was 4 mm. This Figure clearly shows that a sharp point fixed to the HV conductor starts to dis- charge a t one-third the voltage for a similar sharp point on the enclosure wall. This is consistent with the gra- dient at the busbar being three times as large as a t the enclosure surface. When the particle is near the HV con- ductor, its inception voltage is slightly higher than that of case (b).

However, in this case the onset voltage varied from one test run to the next one. Starting with a new test point, the onset voltages for three successive runs were 9, 11.5 and 14 kV,,,, respectively. The most probable reason for this behavior is the alteration in the point tip radius which can occur as a result of microdischarges between the particle and the HV bus. It is clear from Figure 6 that once the discharge starts and the voltage is some- what higher than the inception level, the irradiance level corresponding to different test runs is similar. This Fig- ure also shows that the irradiance magnitude for the free sharp point is lower than that for the fixed one on the HV busbar.

4.1.3 DETECTION RANGE

Sharp points were alternatively introduced at the top and middle locations and the light guide was connected to the bottom window in order to determine the detection range. A very weak light emission signal was observed from a corona point located in SFG gas a t 0.3 MPa at a distance of 50 cm (middle location). However, it was not possible to detect a discharge which was located more than 50 cm away from the window in SF6 gas a t P = 0.3 MPa. However, in an SF6/Nz mixture containing 2.5% nitrogen it was possible to detect the discharges as far away as 1 m from the window, as will be discussed later. Thus, in order to increase the optical detection range, it may be necessary to increase the light emission from corona discharges in SFG by using a small amount of suitable gas additives. Such gas mixtures should have electrical, chemical and thermal characteristics similar to SF6, but should improve the light emission.

4.2 DETECTION FROM MIDDLE WINDOW

The middle window is located at a right angle to the middle discharge point and is 5 cm away from it. Figure 7 shows the irradiance measurements for different orienta- tions of the light guide. If the light guide is directed towards the sharp point, the threshold for light emission is recorded a t 33 kV,,,. However, as shown in curve C, when the light guide is swiveled to the opposite side, the light intensity is reduced and the threshold voltage becomes 38 kV,,,. When the light guide is moved to measure the reflected light on the other side of the tube, curve B is obtained which has much lower intensity as well as higher threshold voltage level. Thus it is possible to detect discharges which are close to the window from the reflected light only. However this will have somewhat lower sensitivity. The difference between curves A and C can also be used to determine the discharge location in a practical system.

From the middle window it was possible to detect weak light signals when the discharge sources were located at the top or bottom of the bus. Moreover, similar to the case of bottom window and bottom discharge source, the corona light intensity measured from this window was affected by all factors discussed earlier. Similarly it was also possible to detect discharges from fixed points on the HV bus as well as floating points near the HV bus.

4.3 DETECTION FROM THE TOP WINDOW

The top sharp point is located behind the HV bus. Figure 8 shows three curves for different light guide po- sitions. Curve C is for the case when the light guide is fixed vertically in the top window facing the HV bus bar. In order to receive more light, the light guide is rotated in the bearing towards the side of the tube and the results of curve B are obtained. When the light guide is directed towards the bottom of the tube to receive the scattered light, the third curve is obtained. It is clear from this Fig- ure that the differences between the three curves as well as the three threshold voltages are not very significant. This is due to the fact that direct light is not received for any of the three positions since the discharge source is hidden from the window.

4.4 ELECTRICAL DISCHARGE DETECTION

The results so far indicate that, when artificial defects are present in the system, it leads to discharges which

IEEE Transactions on Electrical Insulation Vol. 25 No. 2, April 1990 413

can be detected using the optical method. To determine if the discharges are being detected near their inception voltage or not, some of the optical measurements were verified using a conventional electrical partial discharge detection method. In these measurements an E. R. A. partial discharge detector was used and the discharge on- set voltages as well as discharge levels were measured for several types of corona discharges.

It was observed that when the discharge point was lc+ cated in front of the window (as in the case of the bottom window in Figure 1) the two sets of onset voltages agreed well. This was true for all locations of the sharp points i.e. for discharge points located a t the ground enclosure, a t the HV conductor or near the HV bus (i.e. with a gap between the sharp point and the HV conductor). Fur- thermore, for very sharp points which were used in this study, the discharge levels a t the inception voltages were generally < 5 p c for SF6 a t a gas pressure of 0.3 MPa. This indicates an excellent detection sensitivity for the optical discharge detection system for discharges which are located in front of the windows.

For discharges which were located behind the optical windows (such as the top sharp point in Figure 1) it was found that the light is detected somewhat after the dis- charge onset is observed by the electrical detector. For example, for a sharp point located in the ground enclosure behind the top window, the discharge onset at P = 0.1 MPa was at 12.5 kV,,, whereas the received light was detected at 18 kV,,, . At this voltage the discharge mag- nitude was 2.5 pC on the positive half cycle. Similarly a t 0.3 MPa the corona pulse from electrical detector is ob- served a t 31 kV,,, whereas the received light is detected a t 39 kV,,, . At 39 kV,,, the discharge magnitudes were 3.5 and 4.9 pC on both the positive and negative half cy- cles. Thus it is clear that although a discharge in the hidden area is not detected at its inception it is possible to detect such discharges by the optical method well be- fore breakdown even when these have magnitudes of as low as x 2 pC.

For a given sharp point and its location, the discharge magnitude in pC depends upon the applied voltage level as well as the polarity of the applied voltage. The dis- charge level during the two half cycles is different and varies with the applied voltage in different fashions. For SF6 a t 0.3 MPa, the maximum discharge level varied lit- tle with the applied voltage when the sharp point was at a negative potential. However, when the sharp point was a t a positive potential, the maximum discharge level in- creased appreciably with the applied voltage level. This was true for all sharp point locations investigated i.e. at the ground enclosure, a t the HV bus or near the HV bus.

For both polarities, the repetition rate of pulses increased with the applied voltage. Since the measured irradiance level was averaged over a period of 0.45 s, it exhibited the resultant light emission from many discharge pulses of different magnitudes and both polarities. Figure 9 shows the variation of the irradiance level with the maximum discharge level measured during the positive as well as the negative half cycle of the applied voltage for sharp point located at

1. the inside surface of the grounded enclosure, 2. the outside surface of HV bus, and 3. adjacent to the HV bus with a 4 mm gap.

The results of Figure 9 were obtained using the end window and the bottom sharp point location. For other sharp point locations, the results were similar but with lower values of the irradiance level.

4.5 DISCHARGES IN SFs/N2 M IXTU RES

In order to enhance the light emission in SF6 insulated equipment a small amount of certain additive can be used to give enhanced visible light and increase the light emis- sion from reflection. Nitrogen appears to be one such ad- ditive since SF6/Nz mixture with high SFG content have dielectric strength which is approximately similar to that of SFG. However, such mixtures exhibit higher light emis- sions [14,17]. Consequently the detection range can be increased considerably. As mentioned earlier, with SF6 alone it was not possible to detect any discharge which was located more than 50 cm away from the window. However, with an S F ~ / N Z mixture containing as low as 2.5% Nz, it was possible to detect a discharge as far as 1 m away from the window. For mixtures with higher Nz content the light irradiance increased even more. Fig- ure 10 shows that the detection sensitivity increased ap- preciably for a mixture containing 5% of Nz. Thus, SFG/ Nz mixtures increase appreciably the light produced dur- ing a discharge. This property could be exploited for increasing the sensitivity as well as distances over which discharges could be detected, using the optical methods. However, since such an increase in light irradiance means an increase in the number of photons released, there may also be a possibility of an increase in the number of free electrons produced by the photon-related processes in the gas volume and at the electrode surfaces, resulting in a greater probability of initiating partial discharges in the GIs. Furthermore, when Nz is added to SFG, the corona inception voltages may be reduced depending upon the Nz percentage. Since GIS equipments are usually de- signed to operate under partial discharge free conditions,

414 Binns et al.: Optical Discharge Detection in SF6- insuh ted Systems

the N2 content must be limited to < 3% since such mix- tures exhibit corona inception voltages which are almost similar to those for pure SFG.

5. CONCLUSIONS

every sensitive discharge detection system based upon A optoelectronics is presented for detecting low-level corona discharges from fixed as well as floating parti- cles in compressed SF6 GIs equipment at high gas pres- sures. Such a detector can sense corona discharges of < 5 pC located a t distances of 6 50 cm. For application of this technique to large practical systems, there will be a need to use either multiple access windows and/or to use suitable additives which could enhance the light emis- sion without reducing the insulation performance of the system.

The light emission in an SF6 discharge depends upon the applied voltage level, the radius of curvature of the active electrode, the material of the electrode as well as the gas pressure. The sensitivity of the optical detection system is further influenced by the characteristics of the fiber optic waveguide, the photomultiplier tube, viewing windows and the angle at which the light enters the wave- guide.

REFERENCES

[l] G. Carlson, J . Houston, W. Davis, M. Perry and T . Rautenberg, “Fault sensors for SF6 equipment”, Proc. 42nd American Power Conf., Vol. 42, No. 111, Chicago, April, pp. 615-619, 1980.

[2] S. Kusumoto, S. Itoh, Y. Tsuchiya, H. Mukae, S. Matsuda and K. Takahashi, “Diagnostic technique of gas insulated substations by partial discharge de- tection”, IEEE Trans. PAS, Vol. 99, pp. 1456-1463, 1980.

[3] S. Tominaga, H. Kuwahara, K. Hirooka and T. Yosh- ioka, “SF6 gas analysis technique and its application for evaluation of internal conditions in SF6 gas equip- ment”, IEEE Trans. PAS, Vol. 100, pp. 4196-4206, 1981.

[4] J . Braun and F. Chu, “Novel low cost SF6 arcing by-product detectors for field use in gas-insulated switchgear”, IEEE Trans. PWRD, Vol. 1, pp. 81-85, 1986.

151 R. Bartnikas and J. McMahon (Eds.), Engineering Dielectrics Vol. 1, ASTM Press, USA, pp. 327-408, 1981.

[6] H. Graybill, J . Cronin and E. Field, “Testing of gas insulated substations and transmission lines”, IEEE Trans. PAS, Vol. 93, pp. 404-413, 1974.

[7] L. Lundgaard, L. Hegerberg, J . Kulsetas and A. Rein, “Ultrasonic detection of particle movement and par- tial discharges in gas insulated apparatus”, Proc. 3rd Int. Symp. on Gaseous Dielectrics, Knoxville, Ten- nessee, USA, March, pp. 300-306, 1982.

[8] R. Vanhaeren, G. Stone, J. Meehan and M. Kurtz, “Preventing failure in outdoor distribution class metal- clad switchgear”, IEEE Trans. PAS, Vol. 104, pp.

[9] S. Yanabu, H. Okubo, S. Matsumoto, H. Aoyagi and N. Kobayashi, “Metallic particle motion in 3-phase SF6 gas insulated bus”, IEEE Trans. PWRD, Vol. 2,

101 S. Boggs, G. Ford and F. Chu, “Coupling device for the detection of partial discharge in a gas insulated switchgear”, IEEE Trans. PAS, Vol. 100, pp. 3969- 3973, 1981.

111 S. Boggs, F. Chu, G. Ford and C. Law, “Improved diagnostic test and fault location techniques for gas insulated switchgear”, 42nd American Power Conf., Vol. 42, No. 110, Chicago, April, pp. 609-614, 1980.

[12] D. Konig, C. Neumann, H. Suiter, H. Lipkin and P. Schmidhuber, “Partial discharge measurements of SF6 insulated high voltage metal enclosed switchgear on site: A study based on fundamentals and expe- riences available up to now”, CIGRE, Vol. l , No.

131 J. Hill and R. Cresswell, “Partial discharge testing of HV metal-clad compound filled switchgear in service in London”, IEE Conf. Publ. No. 94, pp. 163-170, 1973.

141 T. Teich and D. Bronston, “Light emission from elec- tron avalanches in electronegative gases and Nitro- gen”, IEE Conf. Publ. No. 90, pp. 335-337, 1972.

[15] B. Cox, “Partial discharge detection in GIS by an op- tical technique”, Proc. Int. Symp. on GIs, Toronto, Canada, pp. 341-349, 1985.

[16] E. D. Olser, Modern optical methods ofanalysis, Mc- Graw Hill, 1975.

[17] T. H. Teich and R. Braunlich, UV radiation from electron avalanches in sF6 with small admixtures of nitrogen, Gaseous Dielectrics IV, Pergamon Press,

2706-2712, 1985.

pp. 101-106, 1987.

23-09, pp. 1-17, 1980.

pp. 71-81, 1984.

Manuscript was received on 7 Sep 1988, in final form 14 Oct 1989.