18-04-2012, 01:08 PM
ZnO gapless surge arresters
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INTRODUCTION:
ZnO gapless surge arresters are widely used in electrical substations for power equipment protection. During a surge, the arrester provides a safe path for the surge to flow to the grounding system but at the same time the surge causes the non linear varistor to suffer degradation or ageing. The corresponding leakage current that flows due to its gapless configuration can be used for the purpose of monitoring the condition of the surge arrester. A high leakage current magnitude, in particular, the third harmonic resistive current component, indicates a severely degraded arrester which then has to be immediately replaced.
Degradation of metal oxide surge arresters in service can be grouped into three basic types:
i) degradation under the influence
of the operating voltage,
ii) degradation caused by impulse
currents and
iii) degradation due to chemical reactions with the
surrounding atmosphere
DIAGNOSTIC METHODS FOR MO ARRESTERS
Measurement of the Reference Voltage
Measuring of the v-I Characteristics
Power Loss Measurement
Determining the Third Order Harmonic Content of the Leakage Current
Measurement of the Temperature of the MO Insert of an Arrester
Various methods have been proposed and applied in order to measure the arrester leakage current when the arrester is on-line (on-site) or off-line (off-site) [1-12]. The on-line measurement method can give a fast assessment on the condition of the arrester and therefore can be used for the predictive maintenance. A suitable leakage current measurement device should not only able to measure the total leakage current but also able to accurately determine or extract the resistive current component including its higher harmonics. A technique to extract the resistive component from the total leakage current without the need of any other measured signals is known as the Modified Shifted Current Method (MSCM) [12]. Basically, using waveform manipulations, the MSCM is able to accurately determine the resistive component of a given total leakage current. Hence, from the resistive leakage current component, the third harmonic of the resistive current can be obtained and thereafter the arrester degradation can be determined.
This paper describes the usage of the MSCM in a portable device for on-line or in-field surge arrester degradation study.
MODIFIED SHIFTED CURRENT METHOD:
The MO surge arrester could be represented as a simplified equivalent circuit model comprises of a capacitance branch in parallel with a non-linear resistive branch as shown in Fig. 1. The total leakage current (IX) of the arrester is given by a vector sum of a capacitive component (IC)which does not vary with the degradation of the arrester and the resistive component (IR) which varies with the degradation of the MO surge arrester