Iranian Journal of Electrical & Electronic Engineering, Vol. 11, No. 1, March 2015 79 Leakage Current Analysis of Polymer and Porcelain Housed Metal Oxide Surge Arresters in Humid Ambient Conditions K. Mokhtari*, M. Mirzaie* (C.A.) and M. Shahabi* Abstract: This paper aims to measure and analyze of the leakage current of 20 kV polymer and porcelain metal oxide surge arresters under humid ambient conditions by applying different voltages to the arresters terminal. The characteristics of the leakage currents at that stage have been investigated when changes in the ambient humidity were introduced in an artificial fog chamber. It is assumed that magnitude of the noise level during the tests is constant. The frequency and resistive component peak efficient analysis can then be done on the leakage current signal. The idea behind this is to get indicators for investigating of surge arrester behavior in humid conditions. Two important indicators were obtained to evaluate the behavior of the surge arrester in humid conditions. Keywords: FFT Amplitude, Humidity, Leakage Current, Metal Oxide Surge Arrester. 1 Introduction1 Lightning and switching surges and also temporary overvoltages are the main reasons for outages in overhead transmission lines, distribution lines and in substations such as UHV GIS [1]. Metal-Oxide (MO) surge arresters is an important overvoltage protection device in power systems [2], that frequently being used in power transmission and distribution facilities for surge protection of equipment. Today, there are several different types of surge arresters and all perform in a similar manner. Most distribution surge arrester types have polymeric or porcelain housing and a mechanical structure without spark gaps. Surge arresters are known that nonlinear voltage-current characteristics of metal oxide varistors become degraded due to the continuous application of AC or due to transients with currents larger than the varistors ratings. In addition MO arresters are inherently faster-acting than the gapped type, since there is no time delay due to series air gaps extinguishing the current [3]. The MO surge arresters can degrade during its service due to passage of surge currents, moisture ingress, pollution on the external surface, and overvoltages. Thus, is obvious the importance of the arresters’ condition monitoring, since this process is capable to guarantee the reliability of the system. Nowadays, the determination of the condition of Iranian Journal of Electrical & Electronic Engineering, 2015. Paper first received 7 Nov. 2014 and in revised form 18 Dec. 2014. * The Authors are with the Department of Electrical and Computer Engineering, Babol University of Technology, Babol, Iran. E-mails: k.mokhtari@stu.nit.ac.ir, mirzaie@nit.ac.ir and shahabi.m@nit.ac.ir. gapless MO arresters is achieved using many alternative methods, such as ultrasonic and radio interference detection [4], partial discharge and electromagnetic radiation measurement [5, 6], thermovision methods [7] and certainly the leakage current measurement. As it has already been mentioned earlier, the majority of diagnostic methods are based on the measurements of the leakage current. The total leakage current of a MO surge arrester consist of resistive and capacitive current components, where the capacitive component is much bigger than the resistive. Increase of the applied voltage and the increase of ambient temperature, degradation or aging in the arrester results in an increase to the resistive component of the leakage current, while the capacitive part has little change [8-10]. An increase of the resistive current can be considered as an indicator of the arresters condition, and with the continued operation time it can cause failures or permanent degradation [11]. The leakage current measuring procedures can be divided into two different groups [12], Online monitoring and offline monitoring. Offline measurements can be performed with voltage sources that are specially suited for this purpose, for example, mobile AC or DC test generators. Good accuracy may be obtained by using the offline methods, provided that a sufficiently high test voltage is used. The direct measurement can be performed using a reference voltage signal (a procedure that demands simultaneous measurement of the voltage) or by an appropriate compensation method. The third harmonic analysis [12], which does not need reference voltage signal, is based on the fact that the leakage current contains harmonics, due to the nonlinearity of the