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.
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