Effect of Humidity on Partial Discharge Characteristics Zainuddin Nawawi* Yuji Muramoto Naohiro Hozumi and Masayuki Nagao Proceedings of the 7 th International Conference on Properties and Aplications of Dielectric Materials 1-5 June 2003 장 성 수

Abstract  The effect of humidity on Partial Discharge (PD) characteristics in a void with insulation-insulation configuration  Test Configuration  CIGRE Method-II specimen with various levels of relative humidity  The statistical time lag under periodical square HV pulses  With related to the change of relative humidity  The relation among surface resistance of samples  PD characteristics and the discharge time lags

Introduction  Partial Discharge (PD) characteristics  One factor for the improvement and the diagnosis of insulating materials  Affected by contamination, particularly absorbed moisture, nature of the ambient medium, temperature and humidity  The purpose of this paper  To clarify the effect of relative humidity on the PD characteristic in a air gap with insulation-insulation configuration  Investigation of the PD behaviors and the effect of humidity through measurements of PD Vi and Ve, PD patterns and the discharge time lags

Experimental Methods  Test Specimen and Humidity Control (Fig. 1) 1. Specimen : CIGRE method II type 2. Insulation material : Polymethyl methacrylate (PMMA) sheet of 1mm thick 3. Polyimide spacer of 125 μm thick (to form a gap where discharge takes place) 4. Two slits with 10 mm wide to control relative humidity 5. Saturated salt solution for humidity conditioning of the air surrounding specimens Fig. 1 Schematic diagram of the specimen

Experimental Methods  Measurement of PD patterns (Fig. 2) 1. Function Generator 2. High Voltage Amplifier -Amplified the output of the function generator -Generated the sinusoidal high voltage waveform -Applied to the spherical electrode of the specimen 3. PD inception voltage (Vi) - The applied ac voltage was raised at a rate of 70 V/sec until the discharge occurred 4. PD extinction voltage (Ve) - The applied voltage was decreased at the same rate until the discharge disappear

Experimental Methods Fig. 2 Schematic diagram of the measurement system

Experimental Methods  Measurement of Discharge Time Lag 1. Test configuration for discharge time lag - Same setup as shown in Fig. 2 - Applied square HV pulses with 10 ms in pulse width, 20 ms interval of pulses 2. Discharge time lag - The time between the leading edge of square pulse and the discharge pulse - The measurements through 64 pulses were performed - The test was performed under 33, 75 and 98% of relative humidity

Experimental Results and Discussion  Surface Resistance of Sample 1. To evaluate the effect of humidity on surface resistance of void 2. The surface current measurement - At 10 minutes after 10KV dc voltage was applied to the main electrode using electronic pico ammeter 3. Results : Fig. 3 - The surface resistance decreases with increasing relative humidity Fig. 3 Surface resistance as a function of RH

Experimental Results and Discussion  The PD Inception and PD Extinction Voltages 1. The effect of relative humidity on the Vi and Ve 2. With increasing of relative humidity - Vi decreased, and Ve increased 3. Major factors to determine the Vi and Ve - Geometry, spacing between electrode - The type and homogeneity of the insulation - The type of the contamination on the surface of the insulation and the ambient condition 4. Ve at 33, 75 and 98% of RH are about 0.7Vi, 0.8Vi and 0.9Vi, respectively 5. At humid condition, Vi occurred at lower voltage Fig. 4 PD inception voltage and PD extinction voltage as a function of relative humidity

Experimental Results and Discussion  The effect of humidity on PD magnitude 1. The average of PD magnitude Qave decreases with increase of relative humidity 2. In positive cycle - 33% RH is about 530pC, at 75 and 98% RH are 240pC and 120pC 3. In negative cycle - At 33% RH is about 640pC, at 75 and 98% RH are similar to PD magnitude in positive cycle Fig. 5 The average of PD magnitude as a function of relative humidity

Experimental Results and Discussion  The plots (Fig. 6) of charge intensity in positive and negative half cycles, to clarify the effect humidity on PD magnitude; 1. Fig. 6 shows the plots of charge intensity and cumulative probability 2. The charge intensity shift to smaller value with increase of RH 3. A slight difference between positive discharges and negative ones; - it seems that the effect of humidity on the PD magnitude was significantly Fig. 6 Plots of charge intensity in positive and negative half cycles

Experimental Results and Discussion  The plots (Fig. 7) of number of discharge pulses as a function of charge intensity, to clarify the effect humidity on PD magnitude; 1. Fig. 7 shows the plots of the number of PD pulses as a function of charge intensity 2. The number of PD pulses; - Increased with increase of RH - For positive PD, charge intensity becomes too small to be detected 3. In the experiment, the PD magnitude was measured 1 min after 10KVrms was applied to avoid degradation effect of sample material Fig. 7 Charge intensity of negative and positive cycle as a function of discharge pulses

Experimental Results and Discussion  The Effect of Humidity on Discharge Time Lag 1. Fig. 8 shows the Laue plots of the discharge time lags with various levels of relative humidity 2. The discharge time lag becomes shorter with increase of RH 3. The statistical discharge time lag at 37% of residual probability - For positive PD at 33, 75 and 98% RH are about 2.8, 1.6 and 0.41ms - For negative PD at 33, 75 and 98% RH are about 2.9, 1.6 and 0.42ms, respectively Fig. 8 Laue plots of the discharge time lag for PD in negative and positive cycles at 20KVp-p

Experimental Results and Discussion  The Effect of Humidity on Discharge Time Lag 1. Fig. 9 shows the discharge time lag under the same applied voltage the different levels of RH 2. The reduction on the discharge time lags; - The RH plays in determination of the PD magnitude 3. Based on experimental results; 1)Increase of the RH 2)Lower of the surface resistance of the void 3)Easier release of initial electron to decrease discharge inception voltage and leading to a large number of small discharge Fig. 9 The discharge time lag as a function of relative humidity under step voltage 20KVp-p

Conclusions  Measurement of the PD patterns in a void of insulation- insulation configuration with various level of RH  The PD characteristics in positive and negative cycles  Increase of RH in the void  Significant change of the PD characteristics  The decrease of the discharge time lags  Based on experimental results 1. Increase of the RH 2. Decrease of the surface resistance of the void 3. Easier release of initial electron from surface of void 4. At humid condition, - PD inception voltages decrease - PD extinction voltages increase - Leading to decrease in PD magnitude