Measurement xxx (xxxx) xxx Contents lists available at ScienceDirect Measurement journal homepage: www.elsevier.com/locate/measurement Resistive component extraction of leakage current in metal oxide surge arrester: A hybrid method Abdullah Munir a, b, Zulkurnain Abdul-Malek a, *, Rai Naveed Arshad a a Institute of High Voltage and High Current, School of Electrical Engineering, Faculty of Engineering, Universiti Teknologi Malaysia (UTM), 81310 Johor Bahru, Malaysia b Department of Electrical Engineering, NED University of Engineering and Technology, Karachi, Pakistan A R T I C L E I N F O A B S T R A C T Keywords: Leakage current Resistive component of leakage current Condition monitoring of surge arresters Leakage current based condition monitoring is the most widely used method to ascertain the ageing of on-site Metal Oxide Surge Arrester (MOSA). The extraction of resistive component of leakage current is necessary for the exact health diagnostic of arrester. In particular, the third harmonic component of resistive current is known to be directly correlated with the ageing of arrester. Although, some existing methods are still used to extract the resistive leakage current but their computational procedures and accuracies have some limitations. This paper is aimed to propose a newly developed technique, referred as hybrid method for the extraction of resistive leakage current without acquiring the grid’s voltage. This method was developed on SIMULINK software and then validated experimentally on 120 kV rated MOSA manufactured by ABB. Results have shown that the proposed method is 3.2% more accurate than the modified shifted current method. It can be implemented to develop a highly precise condition monitoring module for online monitoring of MOSAs. 1. Introduction Surge Arresters act as overvoltage limiting devices to protect the transmission and distribution system from high voltage surges caused by severe lightning and switching operations [1,2]. Therefore, their con­ dition monitoring is vital to ensure the reliability of the power system [3,4]. The most popular type of gapless MOSA contains zinc oxide element due to its highly nonlinear voltage-current characteristics and faster conduction response for high voltage surges [5,6]. Fig. 1 shows a typical voltage-current ‘V-I’ characteristic curve of zinc oxide surge arrester. This V-I curve is divided into three regions based on the applied voltage, as shown in Fig. 1. The region I is referred as low current or steady state region due to the steady state operation of MOSA. Four different voltage levels are defined in the low current re­ gion. The first point ‘Vc’ is the initial voltage, at which the total leakage current of MOSA is mainly capacitive. while other points are the phase to earth voltage ‘VP-N’, continuous operating voltage ‘VCOV’ and rated voltage ‘Vrated’. The region II is known as the flat region, because a small increase in the voltage increases the leakage current significantly. The temporary over voltage ‘VTOV’ occurs in this region. On the other hand, region III is called the high current region due to the discharging of lightning current by MOSA. In this region, the residual or discharge voltage is represented as ‘V10′ . As the zinc oxide impedance cannot be designed infinity; therefore, it draws leakage current at the standard operating voltage [7,8]. The leakage current is acquired in two ways; offline and online. The offline method requires the disconnection of arrester from the grid, while the online technique monitors the leakage current of on-site arrester [9,10]. In most of the grid stations, it is challenging to acquire the high voltage signal; therefore, it is preferable to measure online arrester’s leakage current [9,11,12]. The total leakage current is decomposed into its resistive and capacitive components [13]. The capacitive component depends on the applied voltage. At the same time, the resistive current also depends upon zinc oxide characteristics, ambient temperature, and other envi­ ronmental factors [14–16]. Any deterioration of the insulating proper­ ties of MOSA causes an increase in the resistive leakage current [17,18]. Therefore, it is used as the most reliable indicator of MOSA deterioration [10,15,19,20]. Several techniques have been proposed for the extraction of resistive component of leakage current such as conventional method [21], capacitive current compensation method [23], modified shifted current method (MSCM) [24,27], least square method [25], linear regression method [26], and circuit-based method [7]. A comparative analysis of * Corresponding author. E-mail addresses: abdullah.munir@neduet.edu.pk (A. Munir), zulkurnain@utm.my (Z. Abdul-Malek). https://doi.org/10.1016/j.measurement.2020.108588 Received 11 August 2020; Received in revised form 29 September 2020; Accepted 5 October 2020 Available online 12 October 2020 0263-2241/© 2020 Elsevier Ltd. All rights reserved. Please cite this article as: Abdullah Munir, Measurement, https://doi.org/10.1016/j.measurement.2020.108588 A. Munir et al. Measurement xxx (xxxx) xxx these techniques is presented in table 1. In the conventional method, the resistive current is measured at the instant when the voltage signal reaches its maximum value. Although, this method is considered as the benchmark procedure to acquire the resistive current with maximum accuracy but it is not feasible for on-site measurement [10]. In the capacitive current compensation method, the capacitive current is compensated by adjusting the amplitude and phase angle of the refer­ ence signal. However, the presence of harmonics in the capacitive cur­ rent may interfere with the measurement of the nonlinear resistive current [28]. MSCM is based on the waveform analysis of total leakage current using the orthogonal relation between its resistive and capacitive com­ ponents [27,29]. Only the fundamental component of capacitive current gets compensated in this procedure [30]. The algorithm is initiated by reproducing a leakage current waveform with a phase shift of π/2 rad. Actual leakage current is added with the reproduced leakage current to produce a total current waveform. The time at which the peak value of total current occurs is noted. The magnitude of capacitive current is approximated at a time delay of π/2 rad to the peak time of total current waveform. The generated capacitive current waveform is subtracted from the actual leakage current to acquire its resistive component. In this method it is assumed that no phase shift exists between the total leakage current and its capacitive part. In actual the leakage current is lagged by its capacitive component by some angle at rated voltage due to the harmonics. The results of this technique are more accurate as compared to the compensation technique. However the accuracy of re­ sults is affected by the approximation of capacitive current magnitude and ignorance of phase angle between the total and capacitive leakage currents at the rated voltage In least square method, time-domain equations of the resistive and capacitive currents are derived, followed by the least squares applica­ tion to estimate the capacitance and resistance of MOSA [25]. The ac­ curacy of the proposed method is decreased with the rise of resistive leakage current at the rated voltage condition. To reduce the error of least square method, the authors in [26] have proposed linear regression method. Initially, linear and differential mathematical equations of the resistive and capacitive leakage currents are derived in time domain, which are then converted into linear regression model to determine the resistance and capacitance of MOSA. Although, the results of this method are better than the capacitive current compensation method but, the accuracy is limited to noiseless voltage only. Similarly, the circuitbased analytical method estimates the resistive and capacitive currents based on the dimensions of surge arrester [7]. However, the algorithms of least square, linear regression, and circuit-based methods are not validated for online measurement of resistive leakage current. Based on the above discussion, it is concluded that reference signal and complex implementation steps are required to extract the resistive current in these methods and the results are not accurate enough [31]. Therefore, it Table 1 Comparative analysis of the resistive current extraction techniques. Conventional Method Capacitive Current Compensation Method Modified Shifted Current Method Least Square Method Linear Regression Method Circuit-based Method Principle of Measurement Limitation Reference IR is measured at the peak instant of the voltage IR is determined by adjusting the magnitude and phase of a reference signal to compensate IC The theoretical basis of this technique resides on 180 degrees phase-shifted capacitive current injection to the measured leakage current in order to extract IR Time domain equations are formulated to determine IR and IC linear and differential mathematical equations of IR and IC are derived in time domain IR and IC are determined by using the circuit element values of MOSA It is not applicable for online measurement [21] The presence of harmonics in voltage disrupts the accuracy [22,23] Consideration of a constant phase between IC and IT at all voltage levels decreases the accuracy of measurement [5,14,24] Complicated steps are required to implement this technique for online monitoring Accuracy is affected by neglecting the distortion from the applied voltage [25] The performance of method is not tested for online measurement [26] [7] is highly desirable to propose a new method with simple steps and better accuracy. This paper intends to introduce a new ‘Hybrid Method’ to extract the resistive leakage current without measuring the arrester’s voltage. It is simulated in SIMULINK software and then tested experimentally on 120 kV rated MOSA. The obtained results are compared with the conven­ tional and modified shifted current methods. This comparison has revealed that the proposed method is more accurate than the existing method. More straightforward computational steps and high accuracy of this technique can lead to design an effective online condition moni­ toring module of MOSA. 2. Hybrid method to extract the resistive current The simplified equivalent circuit of MOSA consists of the parallel combination of capacitance ‘C’ and nonlinear resistance ‘R’ as shown in Fig. 2 [32,33]. The total leakage current ‘IT’ can be represented as the Fig. 1. V-I characteristic curve of a typical MOSA. 2 A. Munir et al. Measurement xxx (xxxx) xxx IR = IT − IC The algorithm to extract the resistive component from the total leakage current is illustrated as; (a) Filter the total leakage current signal to suppress the high frequency noise and distortion above 1 kHz. (b) Determine the peak value of the total leakage current. (c) Get the zero crossing time reference of the filtered signal. (d) Determine the funda­ mental frequency ‘f’ of the leakage current. (e) Determine the magnitude of IC using Eq. (4) and the phase angle between IT and IC using Eq. (3). (f) Generate the capacitive leakage current waveform having the magni­ tude and phase angle obtained in (e). (g) Subtract the capacitive current waveform from the total leakage current to obtain the resistive current waveform. Fig. 4 illustrates the block diagram of the proposed method to extract the resistive leakage current of on-site MOSA. Initially the leakage current is measured using a highly precise current sensor ALCL-40D. Low pass filter is used to eliminate the high frequency noise and distortion from the acquired current signal above 1 kHz. A simple 1st order passive low pass filter is designed in Simulink with a resistor (160 kΩ) and capacitor (1nF). After eliminating the noise, peak value detector is applied to detect the peak value of the filtered leakage current. Zero crossing detector is used to determine the zero crossing time reference of the filtered leakage current signal. Frequency detector is aimed to determine the fundamental frequency of the leakage current signal. Fig. 2. Simplified equivalent circuit of MOSA. phasor sum of the nonlinear resistive ‘IR’ and capacitive components ‘IC’ as shown in Eq. (1). The capacitive component is subtracted from the total leakage current to determine its resistive component. (1) IT = I R + I C θ = sin− 1 IC 3. Methodology (2) IT The algorithm of proposed method is developed in Simulink software to determine the resistive leakage current, as shown in Fig. 5. This technique is proposed for monitoring the online surge arresters. However, experiments are carried out offline in high voltage laboratory. To investigate the performance of proposed technique at different voltage levels, the leakage current of a 120 kV rated MOSA manufac­ tured by ABB is measured at voltages from 10 kV to 120 kV. Table 2 shows the technical characteristics of the studied surge arrester including different voltage levels of the low current region. The block diagram of the experimental set up is shown in Fig. 6. A step-up trans­ former (240 V/250 kV) is used to apply the voltage to the tested MOSA. The applied voltage is measured by a capacitive voltage divider with a ratio of 686:1. A digital Picoscope kit is used to display and export the measured signals to MATLAB. Both leakage current and voltage across the arrester are measured in this experiment. Voltage measurement is required to execute the conventional technique to extract the resistive leakage current. A highly sensitive current sensor ALCL-40D is used to measure the total leakage current of MOSA. Its technical specifications are illustrated in table 3. Current sensor is specifically designed for the measurement of arrester’s leakage current at power frequency of 45–65 Hz only with least effects of electromagnetic interference. Eq. (6) is used to determine the leakage current ‘IT’ from the output voltage ‘VS’ of sensor. The output characteristic curve of the sensor is shown in Fig. 7. (3) φ = 90 − θ (5) Fig. 3 shows the phasor diagram of IT with phase angles ‘θ’ and ‘φ’ between the peak values of ‘IT’ and ‘IR’ and ‘IT’ and ‘IC’ respectively. IT leads IR by angle ‘θ’ and lags IC by angle φ. The resistive component of leakage current exceeds from its capacitive component with the rise in applied voltage. Therefore, the decrease in capacitive leakage current decreases the angle ‘θ’ and consequently increases the angle ‘φ’ at the rated voltage, as seen in Eqs. (2) and (3). As the approximation of the capacitive component of leakage current produces error in the extraction of resistive current from the modified shifted current method. Therefore, in hybrid method, the capacitive current magnitude is determined by calculating its equivalent capaci­ tance, which is directly dependent on the dimensions such as area ‘A’, height ’d’ and permittivity ’∊’ of the metal oxide element. After finding the magnitude, phase angle φ is calculated to lead the capacitive current by total leakage current. This method is known as hybrid, because it combines the circuit based method with the waveform analysis of shifted current method. The peak values of capacitive and nonlinear resistive currents are determined using Eqs. (4) and (5) respectively. √̅̅̅ √̅̅̅ ( 2)(2πf )(∊A)Vrms Ic = 2ωCVrms = (4) d IT = (2.6)(Vs ) (6) 4. Results and discussion 4.1. Simulation results The proposed method is initially tested on the simulated leakage current waveforms to extract its resistive component. Leakage current waveforms of 120 kV rated MOSA are simulated in MATLAB at voltage levels 10 kV, 93 kV, 98 kV, and 120 kV respectively. The simulated leakage current waveform along with its extracted resistive component are shown in Fig. 8(i)–(iv). It is observed that resistive leakage current is increased with the rise in voltage from 12.36 µA at 10 kV to 2150.42µA at 120 kV as shown in 8(i) and 8(iv) respectively. The simulation results have revealed that the proposed method works well for the extraction of Fig. 3. Phasor diagram of total leakage current. 3 A. Munir et al. Measurement xxx (xxxx) xxx Fig. 4. Block diagram of the proposed method. Fig. 5. Simulink model of the proposed method. Table 2 Technical characteristics of the tested 120 kV rated MOSA. Initial voltage (rms) VC 10 kV Phase-to-Earth Voltage (rms) Continuous Operating Voltage (rms) Rated Voltage (rms) Temporary Over Voltage (rms) Radius of MO disc Height of MO column VP-N VCOV Vrated VTOV r d 93 kV 98 kV 120 kV 130 kV 0.03 m 1m resistive leakage current at all voltage levels. 4.2. Experimental results The proposed method is then applied to extract the resistive component from the total leakage current of 120 kV rated MOSA experimentally. Fig. 9(i)–(iv) show the experimental results of total leakage, capacitive and resistive currents waveforms at 10 kV, 93 kV,98 kV and 120 kV respectively. At the initial voltage ‘VC’, the total leakage current is mainly capacitive in nature having minimum contribution of Fig. 6. Experimental set up for the measurement of leakage current. 4 A. Munir et al. Measurement xxx (xxxx) xxx The variation of the calculated capacitive component of leakage current with the applied voltage is shown in Fig. 10. It is observed that the capacitive current changes linearly with the applied voltage unlike the resistive component, which changes in non-linear manner. A linear increase in the capacitive leakage current from 108.90 µA at 10 kV to 1296 µA at 120 kV is plotted in Fig. 10. It is due to the fact that capacitance of MOSA is independent of the applied voltage, as depicted in Eq. (4). Using the technical specifications of MOSA as mentioned in Table 2, its capacitance is found to be 0.0245nF. Although, IC is observed to increase linearly with the rise in applied voltage. But, to validate the dominance level of IR at the rated voltage, ration of IC/IT and IR/IT are plotted in Figs. 11 and 12 respectively. Fig. 11 shows the decreasing trend of the ratio (IC/IT) and theta ‘θ’ with the applied voltage. It is evident that the contribution of capacitive component in total leakage current is high at the initial voltage. But this contribution is decreased with the increase of nonlinear resistive current at higher voltage levels. It is noticed that the ratio IC/IT is decreased from 99.36% at 10 kV to 51.6% at the rated voltage condition. Due to decrease in the capacitive current, the phase angle ‘θ’ is also decreased from 83.52 degree at 10 kV to 31.06 degree at the rated voltage. Likewise, the variation of the ratio (IR/IT) and phi ‘φ’ with the applied voltage is shown in Fig. 12. It is noticed that the ratio IR/IT is increased from 11.27% at 10 kV to 85.65% at 120 kV. It is due to the fact that resistive leakage current exhibits a nonlinear increase with the rise in voltage level. Consequently, the angle ‘φ’ is also increased from 6.48 degree at 10 kV to 58.94 degree at the rated voltage. Due to the capacitive nature of total leakage current, the lagging angle φ between IT and IC is small at the initial voltage. However, the increase of IR also increases the angle φ considerably at the rated voltage as shown in Fig. 12. Table 3 Technical specifications of current sensor. Measuring Range AC 1µA–50A Inside Diameter Turns Frequency Dimensions 37 mm 2400 45–65 Hz 134.5(W) × 165.5(H) × 61(D) mm Leakage Current (μA) 3000 2500 2000 1500 1000 500 0 0 100 200 300 400 500 600 700 800 900 1000 Output Voltage (μV) Fig. 7. Output characteristic curve of current sensor. the resistive current as shown in 9(i). With the increase in voltage, the dominance of capacitive component gradually decreases and total leakage current becomes highly resistive at the rated voltage as shown in 9(iv). It is due to the fact that the resistive component of leakage current increases considerably with the rise in applied voltage. The variation in resistive current is noted from 12.36µA at 10 kV to 2151µA at 120 kV in 9(i) and 9(iv) respectively. It is also observed that the distortion in leakage current signal is more pronounced at the higher voltage level due to the rise in nonlinear current. On the basis of findings, it is evident that the simulation and experimental results are closely related with each other. 4.3. Comparison of experimental results Table 4 shows the acquired values of the capacitive and resistive components of total leakage current using the hybrid, conventional and modified shifted current methods at voltages from 10 kV to 120 kV. The Fig. 8. Simulation results of the proposed method (i) Leakage, capacitive, and resistive currents at 10 kV rms; (ii) Leakage, capacitive, and resistive currents at 93 kV rms; (iii) Leakage, capacitive, and resistive currents at 98 kV rms; (iv) Leakage, capacitive, and resistive currents at 120 kV rms. 5 A. Munir et al. Measurement xxx (xxxx) xxx Fig. 9. Experimental results of the proposed method (i) Leakage, capacitive, and resistive currents at 10 kV rms; (ii) Leakage, capacitive, and resistive currents at 93 kV rms; (iii) Leakage, capacitive, and resistive currents at 98 kV rms; (iv) Leakage, capacitive, and resistive currents at 120 kV rms. Fig. 12. Increasing trend of the ratio IR/IT and φ with the applied voltage. Fig. 10. Variation of capacitive leakage current with the applied voltage. (IC/IT) %,θ(degrees) 120 methods is observed. At VC, the resistive currents determined from the proposed and modified shifted current methods are 0.8% and 1.2% higher than the conventional method’s value. Similarly, at Vrated, the difference in the parameter IR acquired from the proposed and shifted current method is 5% and 8.2% with the conventional method. The obtained values of the resistive current in Table 4 are used to plot V-IR characteristic curves of MOSA as shown in Fig. 13. A significant rise in resistive leakage current is observed beyond VCOV. It is noted from Fig. 13 that the V-IR curves produced by the proposed and conventional methods are closely related with each other. It is due to the fact that almost similar results are obtained from both methods. On the other hand, characteristic curve by the modified shifted current method is deviated from the conventional curve as shown in Fig. 13. To determine the actual difference in the results of modified current method w.r.t the conventional technique, percentage errors are evalu­ ated, which are plotted in Fig. 14. The curves as shown in Fig. 14 can be divided into three regions. Region I ranging from 10 kV to 30 kV shows linear increase in the % error for both methods. Region II starting from 30 kV to 90 kV displays (Ic/IT)*100 Theta 100 80 60 40 20 0 10 20 30 40 50 60 70 80 90 93 Applied Voltage (kV) rms 98 100 110 120 Fig. 11. Decreasing trend of the ratio IC/IT and θ with the applied voltage. calculated values of the phase angles θ and φ at voltage level from 10 kV to 120 kV are also provided in the Table 4. A close coordination between the resistive current obtained from the proposed and conventional 6 A. Munir et al. Measurement xxx (xxxx) xxx Table 4 Computed values of the capacitive and resistive leakage currents using proposed, conventional and modified shifted current methods. Vrms (kV) IT (µA) Proposed Method IC (µA) IR (µA) θ (Degree) 10 20 30 40 50 60 70 80 90 93 98 100 110 120 109.60 219.30 336.20 458.00 579.10 704.20 828.10 955.10 1126.30 1185.60 1253.00 1298.10 1510.00 2511.25 108.90 217.80 326.70 435.60 544.50 653.40 762.30 864.01 972.00 1012.70 1067.20 1089.20 1188.00 1296.00 12.36 25.60 79.35 141.48 197.17 262.61 323.50 406.80 569.00 616.50 656.57 706.31 932.07 2151.00 83.52 83.30 76.35 72.00 70.09 68.10 67.00 64.80 59.65 58.67 58.40 57.03 51.90 31.06 Conventional Method Modified Shifted Current Method φ (Degree) IC (µA) IR (µA) IC (µA) IR (µA) 6.48 6.70 13.65 18.00 19.91 21.90 23.00 25.20 30.35 31.33 31.60 32.97 38.10 58.94 108.91 218.12 327.10 436.55 542.80 656.17 765.98 869.91 984.16 1027.56 1086.10 1110.20 1220.35 1455.58 12.25 22.65 77.65 138.50 201.80 255.60 314.70 394.30 547.70 591.40 624.80 672.70 889.30 2047.30 108.89 218.06 326.42 434.93 545.53 652.31 770.75 876.85 996.24 1009.77 1060.34 1029.10 1171.11 1185.00 12.40 23.20 80.50 143.50 194.30 265.30 302.80 378.60 525.40 621.30 667.60 719.20 953.20 2215.00 proposed method is 3.2% more accurate than the modified current method in extracting the resistive leakage current. Moreover, the proposed method is also applied to determine the most sensitive indicator of arrester ageing, i.e., the third harmonic component of resistive current. The variation of third harmonic current with the applied voltage from 70 kV to 120 kV based on proposed and modified current method is shown in Fig. 15. It is found that the proposed method can extract the resistive third harmonic current more efficiently as compared to modified shifted current method. At the rated voltage, the third harmonic component extracted from the proposed and modified current method are found to be 861.5µA and 827.9µA respectively. Therefore, a difference of 33.60 µA is computed in the value of third harmonic current extracted from both methods. Based on the difference, it is established that the proposed method is 4% more accurate than the modified current method to determine the third harmonic resistive current at the rated voltage. 5. Conclusion Fig. 13. V-IR Characteristic Curve of 120 kV Rated MOSA. Third Harmonic Current (μA) Existing methods to determine the resistive leakage current are less accurate and require complex computational procedures. In this paper, a hybrid method for the extraction of the resistive leakage current is proposed. The algorithm of proposed method is simulated on Simulink and its accuracy is validated experimentally on 120 kV rated MOSA. Experimental results have established that the new method works well for the extraction of resistive component from the metal oxide surge arrester’s total leakage current without measuring the voltage signal. It minimizes the errors in generating the capacitive current waveform by calculating the actual capacitance of metal oxide element. The accuracy of extracted resistance by proposed method is improved by introducing the exact angle between the total leakage current and its capacitive component, which is not considered in the previous technique. At the Fig. 14. Relative error of the resistive leakage current using the proposed and modified shifted current methods. approximately a constant error in the values of IR at all voltage levels obtained from both methods. Region III ranging from 93 kV to 120 kV shows a significant rise in the error for the modified current method. On the other hand, the proposed method exhibits a constant error in the region III after VCOV. It is observed that the % error produced by the proposed method at high voltage region is significantly less than the modified current method. Moreover, the relative errors produced by the modified shifted current and proposed methods at the rated voltage are 8.2% and 5% respectively. Based on the findings, it is inferred that the Proposed Method 1000 900 Conventional Method 800 Modified Shifted Current Method 700 600 500 400 300 200 100 0 70 80 90 93 98 100 Applied Voltage (kV) rms 110 120 Fig. 15. Variation of third harmonic resistive current with the applied voltage. 7 A. 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