Omicron CP TD1, Reference Manual

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Capacitance and DF Measurement of Overvoltage Arresters
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10 Capacitance and DF Measurement of
Overvoltage Arresters
The purpose of a surge (lighting) arrester is to limit the over voltages that may
occur across transformers and other electrical apparatus due either to lightning
or switching surges. The upper end of the arrester is connected to the line or
terminal that has to be protected, while the lower end is solidly connected to
ground. The arrester is composed of an external tube out of porcelain or
compound tubing containing an arrangement of stacked discs that are
composed of a silicon carbide or metal oxide material. This material has a
resistance that decreases dramatically with increasing voltage.
Arresters are effectively switching devices that serve as an insulator under
normal conditions and as a conductor under over voltage conditions. After an
over voltage condition is cleared, the arrester must return to its normal insulating
condition. The measurement of capacitance and resistance is an effective
method of evaluating the integrity of an arrester and isolating potential failure
hazards. This test reveals conditions that could affect the protective functions of
the arrester, such as the presence of moisture or salt deposits.
Temperature
It is recommended to measure the capacitance and the resistance of the
arrester at the same temperature as the measurement that was made as a
benchmark.
Voltage
The measurement should be made exactly at the same voltage level that the
fingerprint measurement was made. Normally the test voltage is 10 kV.
Figures 10-1, 10-2, and 10-3 show the dissipation factor, the capacitance and
the parallel resistance of a lightning arrester, dependent on the frequency.