HIPOT, CORONA, AND BIL BASICS AND TUTORIALS

Any insulation system must be able to tolerate a continually applied voltage, a transient overvoltage, and a surge voltage. Furthermore, it must be free of partial discharge (corona) under the worst-case operating conditions.

 The hipot test is typically a 1-min application of a 50- or 60-Hz voltage between all conductors and ground, during which the system must not fail shorted or show a fl uctuating leakage current. There may, of course, be displacement currents from the capacitance to ground.

Absent a specific c high-potential test specification, a rule of thumb is a 1-min, 60-Hz applied sinusoidal voltage of twice rated rms voltage plus 1000 V for equipment rated 600 V or less and 2.25 times rated voltage plus 2000 V for ratings of 601 V and above.

The ability to withstand surge voltages is defined by a test wave with a 1.2 μ s rise time to peak and a 50 μ s fall to half voltage. This test approximately defInes a basic insulation level (BIL) for the system. The test is a single application of this wave, and the requirement to pass is simply freedom from breakdown.

Yet another test is the voltage at which a certain level of corona begins. This is detected by the appearance of impulse discharge currents on an oscilloscope as the applied voltage is slowly raised.

The voltage at which these currents appear is the onset or inception level, and the cessation of the impulses as the voltage is reduced is the offset or extinction voltage.  Standardized metering circuits in commercial corona testers allow these impulse currents to be quantified in micro-coulombs of current-time integral.

A simple corona tester can be made that is sufficient for most purposes with only a hipot tester, a filter, and a coupling circuit. The noise filter can be made with a high-voltage resistor and capacitor, and the current demand should be kept below the maximum rating of the hipot tester.

The RF choke (RFC) can be any small inductor of from 1 to 100 mH inductance, and the high-pass R/C filter can be used to eliminate the fundamental current from the oscilloscope.

Some tinkering of these components can be expected. In operation, corona will be indicated by the appearance of noise spikes as the voltage is raised. The unit can be tested with some twisted hookup wire.

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