An investigation has been made in order to develop the location technique of corona discharge in insulating oil, on behalf of the development of the UHV transformer. This paper describes the principle of analysing the corona location by making use of electrostatic probes. The corona produces a space charge in the oil, and induces charges on the probes electrostatically. Thus, the location as well as the space charge quantity of the corona can be analyzed, based on the induced charges of the probes. For this investigation, a needle-plane electrode and four probes were arranged in the oil, and a lightning impulse voltage was applied to the gap. Then, the corona produced in the gap was investigated by measuring the corona current and the induced charges on the probes, and then taking photographs of the corona. From these experiments, it was proved that this technique was quite useful for analyzing the location of the corona

HAYASHI, Muneaki

ISA, Hiromu

MINO, Yoshiaki

UENOSONO, Chikasa

YAMAMOTO, Osamu

English

Kyoto University Research Information Repository

RIGHT:URL:CITATION:AUTHOR(S):ISSUE DATE:TITLE:Studies on Location Technique ofCorona and Measurement of SpaceCharge in Transformer OilHAYASHI, Muneaki; YAMAMOTO, Osamu; ISA,Hiromu; MINO, Yoshiaki; UENOSONO, ChikasaHAYASHI, Muneaki ...[et al]. Studies on Location Technique of Corona and Measurementof Space Charge in Transformer Oil. Memoirs of the Faculty of Engineering, KyotoUniversity 1984, 46(1): 14-241984-03-31http://hdl.handle.net/2433/28126014 Mem. Fae. Engg., Kyoto Univ. Vol. 46 No. 1 (1984) Studies on Location Technique of Corona and Measurement of Space Charge in Transformer Oil By Muneaki HAYASHI*, Osamu YAMAMOTO*, Hiromu IsA**, Yoshiaki Mrno* and Chikasa UENOsoNo* (Received June 28, 1983) Abstract An investigation has been made in order to develop the location technique of corona discharge in insulating oil, on behalf of the development of the UHV transformer. This paper describes the principle of analysing the corona location by making use of electrostatic probes. The corona produces a space charge in the oil, and induces charges on the probes electrostatically. Thus, the location as well as the space charge quantity of the corona can be analyzed, based on the induced charges of the probes. For this investigation, a needle-plane electrode and four probes were arranged in the oil, and a lightning impulse voltage was applied to the gap. Then, the corona produced in the gap was investigated by measuring the corona current and the induced charges on the probes, and then taking photographs of the corona. From these experiments, it was proved that this technique was quite useful for analyzing the location of the corona. I. Introduction In a large capacity power transformer, if the corona discharge occurs in the oil, the pressboard and the insulating oil which construct the transformer insulating system, are reduced in their insulation performance by the corona. So far many researchers have investigated the corona discharge phenomena in insulating oil. Recently, associated with an increase of the transmission line voltage, the insulation distance increases on the inside of the transformer. However, the transformer size is restricted severely, because of its transportation and installation space prob-lems. On the occasion of a design and test for a compact transformer, one of the most important problem is a corona discharge developed from the weak points, eg., edges of windings, so that the identification of the corona location has become necessary. Hitherto, the detection of the corona has been tried by means of acoustic emission etc., but it can not be performed practically. * Kyoto University ** Osaka Technical colledge Studies on Location Techinique of Corona and Measurement of Space Charge in Traniformer Oil 15 In this paper, the principle to identify the location of the space charge produced in the oil by a corona discharge is described, using the induced charge on the electro-static probes arranged in the oil-tank. Based on the induced charges on the probes, the corona location as well as its space charge quantity can be analyzed. Further-more, through the application of this method, the detection of the corona generated in a need-leplane gap in an oil-tank made of acryl, is performed. The effectiveness of this method is established in the results. 2. Principle of Corona Location and Measurement of its Charge Q.uantity The space charge, generated by the corona in the oil-tank, is considered to be distributed along the path of the corona. However, if the volume of the corona space is small and is measured from a sufficient distance, the space charge can be regarded as a point charge. As shown in Fig. 1, a metal sphere with a radius a is arranged above the plane as a probe, and grounded to the earth. Let the charge of the point charge be Q,, the charge induced electrostatically on the probe P be q', and the induced charge by -Q. (image charge of Q, due to the plane electrode) be q", as shown in the figure. r' -05 (X, Y,-Z) Fig. l. Relation of the corona charge Q, and the induced charge q on the probe P. Then, the relationships between these charges are given as follows: q=-(a/r')Q,, q=(a/r")Q, (1) q=q' +q" (2) Hence, q= (-a/r' +a/r") Q, (3) where, r', r" represent the distance between P and Q,, P and -Q. respectively. 16 Muneaki HAYASHI et al. Provided that Q, is known, its location being (X, Y, Z) and that 3 probes (P1, i = 1, 2, 3) are arranged around the charge, the following relations are held: { I aQ, -.1=============-v (X-x1) 2 +(Y-y1) 2 +(Z-z1) 2 - v(X-x1) 2 +(Y~y1) 2 +(Z+z1) 2 }-qi=O, (i=l, 2' 3) (4) where (x1, y;, Zi) denote the coordinates of the center of the probe P,. When q;'s are measured and substituted into Eq. (4), these equations can be solved and the coordinate of Q. can be found. In this paper, Brent's method has been utilized for numerical calculation. Sometimes, Q. in the transformer may not be measurable, and in such cases, there exist 4 unknown quantities. Hence, in order to find them, a fourth probe must be arranged, and the fourth equation with the same form as Eq. (4) is established as follows. Thus, the 4 unknown quantities can be solved numerically. { I aQ, ---,,============ v (X-x1) 2 + (Y-y,) 2 + (Z-z1) 2 _ _ 1 I }-q1 =0. (i=l,2,3,4) (5) v (X-x1) 2 +(Y-y1) 2 +(Z+z,) 2 Here, since the probes have been arranged together, the electric charges are induced mutually between them. In the experiment, however, the probes were separated far from each other, so that the mutual effects are ignored in this paper for simplicity. 3. Experimental Apparatus and Method 3-1. Experimental method The electrode arrangement which constructs the needle-plane gap is shown in Fig. 2. This needle-plane gap (gap length: 71 mm) is set in the oil-tank. The (3) osc 0.022µF J_ ,-----13) ::.---tt---+-<2) -~-1) (a) Side view (b) Top view Fig. 2. Experimental apparatus. Studies on Location Techinique of Corona and Measurement of Space Charge in Tran.iformer Oil 17 shape of the tank is cubic, made of acryl with a 20 cm side length. The oil used here is the JIS No. 2 in which 1:=2.2, Pv=l013 0,cm, and is of ca. 8 l.. A needle (No. 2 for cotton cloth on the market) was used as the point electrode, and was sharpened making the point at an angle 30°, and the curvature at 0.3 mm. The plane electrode has a rectangular form with a 20 cm side length, and was set in the inside bottom of the tank. Because the plane electrode is too small to be re-garded as an infinitely large plane, an auxiliary electrode is arranged outside the tank, as shown in Fig. 2. Each of the four electrostatic probes (P1 -P4) for measuring space charge quantity and its location had a 5 mm radius, and was arranged at a different height above the plane and at a different distance from the electrode axis respectively. They all were grounded through capacitor C with 20000 pF. Provided that the center of the plane electrode is the origin, the coordinate of each probe (center) is shown in Table I. The applied impulse voltages to the gap have the form of ± 1.4/74 (µs). When the voltage was of a 50 kV peak, the corona stretched itself ca. 10 mm. Table 1. Location of the probes x-axis (mm) y-axis (mm) z-axis (mm) P1 40 -30 21 P2 0 40 21 P3 -26. 5 -30 21 P4 0 -61 103 Note, x-,_y-axis; horizontal plane, z-axis; vertical (gap)-axis. The charge quantities q1 -q4 induced on the probes due to the corona were ob-tained through the terminal voltage of the capacitor, where two 2-beam oscilloscopes (TEK 7844, 556) were utilized for the voltage measurement. The induced charge on the probe depends not only on the corona charge Qs, but also on the applied voltage to the needle electrode. In this experiment, however, the component due to the applied voltage was canceled by the use of a differential amplifier set in the oscilloscopes, and only the component due to the corona charge could be meas-ured. The space charge quantity produced by the corona can also be obtained by the time integration of the current flowing from the plane to the earth through the resistance R (10 D), where R is used to measure the current of the plane i, as shown in Fig. 2. Here, the time integrated value (Ji dt) is denoted by Q. However in general, Q is an apparent value, and is a little different from the true corona charge Qs- Under the condition that the gap distance is much larger than the 18 Muneaki HAYASHI et al. plane-electrode diameter, the relation between Q., and Q. 1s held generally as follows: (6) In this experiment, however, the gap length is relatively small compared with the size of the plane electrode, and the needle electrode is sufficiently fine, so that Q. can be regarded as equal to Q.,. Then; (a) Photograph of the corona (v; Average corona region) (b) q -(0.43 nC/div.), i (2 mA/div.) (2 µs/div.) Fig. 3. A example of measurement on the current of the plane i, the induced charge on the probe q and the photograph associated with the corona in the insulation oil (Applied voltage; +50 kV). (nC) 20 5 0-0 5 0 100 q=2.9xlO-zQ5 0 0 q=2.5x!Ci2 Q5 0 0 200 300 400 500 600 Q 5 (nC) Fig. 4. Relations between amount of space charge Q, and the induced charge on the probe ( r; = I 00 mm, e; positive corona, Q; negative corona). Studies on Location Techinique of Corona and Measurement of Space Charge in Transformer Oil 19 For the observation of the form of the corona with a high sensitivity, an image-intensifier was utilized along with a still camera. The photograph and the oscillograms of q and i associated with the corona, which were obtained by the experiment as described above, are shown in Fig. 3. The relation between Q, and q for the positive and the negative corona respectively is shown in Fig. 4, where the positive and the negative voltage was applied to the gap respectively, changing its peak value. 3-2. Experimental results For the needle electrode described above, a positive impulse voltage (50 kV) was applied to produce the corona in oil. The induced charge (q1) on each probe (P1) was measured, and at the same time the corona form was observed. From the obtained data, the location and the charge quantity of the corona were calculated by Eqs. (4), (5). They were compared with the form taken by the still camera and the Q, obtained by the current method (Eq. (7)). The experiment was also executed under the condition that a craft paper with a 0.65 mm or 4.0 mm thickness was inserted at 30 mm level under the needle tip, where the craft papers were stuck together by synthetic-rubber glue in order to get the necessary thickness. Case a. Q, known: By measuring the induced charges (q1 -q3) and Q, (due to Eq. (7)) and substituting these values into Eq. (4), the equation has been solved simultaneously in order to obtain the corona location as shown in Fig. 5. The corona region is depicted in this figure, based on the picture taken by the still camera. The coronal location obtained by the calculation described above is shown by the marks (0), which have been obtained by applying impulse voltage five times to the oil gap. These five marks are contained in the corona region of this figure, and it is deduced that the locating method executed here is effective. The corona locating method described above was applicable also where craft paper was inserted between the electrodes, and the experimental results are shown in Fig. 5(b), (c). These two graphs correspond to 0.65 mm and 4.0 mm in thickness of paper respectively. From Fig. 5(c), it can be seen that the corona region taken by the still camera does not include the charge location. Therefore, the locating method is not applicable in a case where there is thick paper between the electrodes. It may come from the reason that the difference between the dielectric constants of the oil and the paper is ignored in Eq. (4). Case b. Q, unknown: By measuring the induced charge (q1 -q4), and sub-stituting these values into Eq. (5), the location (X, Y, Z) and the space charge quantity (Q,) can be obtained by solving simultaneous equations. The result 20 z !! II " " " " (mm)!! ~ 20 Muneaki HAYASHI et al. 50 40 z (mm) Y 40 20 -40 -20 corona region 20 40(mml -+---+---0--+---+---4-'- X -20t -40f (a) Without paper z II ii " ,, ,,(mm) i: corona region 7 corona region 40 (mm) y 40 20 paper -40 -20 20 40 (mm) -+---=+----+----+----+- X -40 -20 40(mm) -+---+---o,-1!---+----+-x 20 ::1 -20 --40 (b) With paper of 0.6 mm thickness (c) With paper of 4 mm thickness Fig. 5. Position of the corona charge evaluated by using Eq. (4) (Applied voltage; +50 kV). Studies on Location Techinique of Corona and Mearurernsnt of Space Charge in Transformer Oil 21 50 40 z !! :: " ,, ,,y (mm) 40 20 -20 -40 corona region (a) Without paper z corona region ----~- _____ e_a_per (mm) y 40 20 -40 -20 20 40 (mm) -+---+---0-+----+'----+-x -20 -40 (b) With paper of 4 mm thickness Fig. 6. Position of the corona charge evaluated by using Eq. (5) (Applied voltage; +so kV). is shown with good accuracy, as shown in Fig. 6(a), where only the probe P4 was set at a higher level than the needle tip. If all probes are set in level lower than the needle tip, the calculated location of the corona will be obtained outside the corona region, as mentioned later. The space charge quantity produced by the corona was obtained as 4 7. 7 ~50.3 nC by the probe method, and as 49.0 ~51.2 nC by the current method (Eq. (7)). Therefore, both values obtained by the two methods approximately coincide with each other, and this probe method seems effective for charge measurement. In the case where the papers are inserted between the electrodes, the measure-ment results are shown in Fig. 6(b), where the paper is 4 mm in thickness, and the corona location has been obtained in close vicinity to the needle tip i.e. the corona region. Then, this locating method seems to be effective also where the paper exists, and the paper is thick. From these results, it is deduced that the method utilizing the electrostatic probes is effective for locating the corona discharge and for measuring the space charge quantity. 4. Consideration on Probe Arrangement and Measuring Sensitivity of Corona Location The variation of the induced electric charge q1 on the probes were calculated 22 Muneaki HAYASHI et al. for a point charge Q, ( =1 C) using Eq. (3). The results are shown in Fig. 7(a), (b), (c), where the location of the point charge is moved along the x-,y-, z-axis respec-tively. For example, when Q, is moved along the x-axis as shown in Fig. 7(a), the curves of q1 and q3 intersect each other, since the distance between P 1 and Q.,, or between P 3 and Q.,, increases or decreases oppositely to each other, where P 1 and P3 are arranged facing each other along the x-axis. On the contrary, since the distances between P2 and Q.1 , and between P4 and Q.. change in same manner according to the change in x, the curves of q2 and q4 run parallel to each other. u N 'o )( '7 Let the angle made by curves q1 and q3 be O :r:• Then, the sensitivity of the probe, -4.0 -4.0 qz Q4 u N 'Q-3_ q2 ~ q, ·-.,. q4 qll q, -2.0 q, -1b -5 0 5 110 -10 -5 0 5 10 x (mm) y (mm) (a) Position of the charge (x, 0, 68 mm) (b) Position of the charge (0, y, 68 mm) u 'l'o -4. 3 -3.0 .,. -2.0 65 70 z (mm) (c) Position of the charge (0, 0, z mm) Fig. 7. Variation of q1 with the position of a point charge Q, (=I C). Studies on Location Technique of Corona and Measurement of Space Charg, in Transformer Oil 23 against the change of the Q, location along the x-axis, increases with an increase in 0,,. From the curves in Fig. 7, it is obtained that O,,=a45°, 011 •• 75° and o.· .67°. It is deduced from the experimental results mentioned earlier, that these angles are sufficient for analyzing the location of Q,. When z4 (coordinate of probe P4) is changed from 103 mm to 35 mm ( cf. Table. I), the curve q4 is shown by the broken line in Fig. 7(c), then O, is obtained as 15°. These experimental results of the corona location are shown in Fig. 8, where the corona location on the x- and y- axis is exactly performed, but not on the z-ax1s. 50 40 z f (mm) Y 40 20 region -40 -20 20 40 (mm) --t----<>--o-+-.. ---+---+-x -20 -40 Fig. 8. Evaluated position of the corona charge with the probe P,. located at (0, -61, 35 mm). From these results, it is deduced that the angles made by the 4 curves of (qi, q3), (q2, q4) and (q3, q4) are preferably more than 45°, refering to 0,, in Fig. 7(a). 5. Conclusion In this paper, the measuring method of space charge quantities and its location in the oil-tank has been described, based on the induced charge quantities on the probes arranged around the electrodes. The results have been obtained as follows; I) In case the space charge quantity due to the corona is known, the corona location and its quantity can be measured by using induced charge quantities on the 3 probes arranged in the tank. 2) In case the space charge quantity is unknown, they, also, can be measured by 4 24 Muneaki HAYASHI et al. probes in the same manner as described above. 3) Consideration of the sensitivity of the probes due to their arrangement was carried out, based on the calculating results regarding the induced charge on each probe, and an effective arrangement has been determined. Acknowledgment The authors are thankful for the contributions to this experiment given by Mr. Shigeyoshi Ishitoku and Mr. Yukihiko Shimono, students of Kyoto University, and also for useful discussions and support given by Mr. Shoji Hirabayashi and Mr. Tsutomu Tani, Mitsubishi Electric Co .. Reference 1) O, Yamamoto, C. Uenosono and Muneaki Hayashi: "Space Charge Measurement Using a Small Sphere as a Probe", Memoirs of the Faculty of Engineering, Kyoto University, Vol. XLIV, Part 4, 1982. 

Studies on Location Technique of Corona and Measurement of Space Charge in Transformer Oil

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Studies on Location Technique of Corona and Measurement of Space Charge in Transformer Oil

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