ISA Scar10
Application
SCAR 10 is used to check regularly the efficiency of the metal oxide surge arrester in service that is installed on the high voltage transmission and distribution networks.
SCAR 10 performs the test according to the IEC standard 60099-5A1 ED. 1.0 Section 6: Diagnostic indicators of metal-oxide surge arresters in service – Method B1 and B2.
On-line leakage current measurement methods
SCAR10 allows to carry out the control while the surge arrester is in service, analyzing by means of a special current clip-on transformer the leakage current in the surge arrester ground connection. The values of this current normally range from fractions of milliampere to a few milliamperes and are characterized by a third harmonic distortion whose value is an indicator of the deterioration of the surge arrester.
The resistive component of this leakage current may increase due to different stresses causing aging and finally causing arrester failures.
Third harmonic with compensation
In case there is a doubt that the third harmonic comes from a heavy distortion of the voltage rather than the surge arrester itself, the auxiliary measuring probe provided with the SCAR 10allows to measure this content quite easily in order to perform compensation. This problem does not apply to HV arresters, as on HV lines the voltage distortion is almost negligible (typically0.2%), while the limit for the arrester is 2% up. For MV arresters, the measurement of the third harmonic content in the voltage is easily performed connecting the instrument to the secondary ofa P.T. of the line under test. The environment during the measure was characterized by the presence of high electric and magnetic fields. In order to minimize their effects on the measurement, the clip-on transformer is completely screened; also, the signal amplifier is located on the transformer’s handle. Thanks to this arrangement, the cable connecting the clip-on transformer to the measuring instrument is less sensitive to the electric and magnetic fields.
