the instrument by interference current, then the anti-interference capacity of
instrument cannot be measured.
3) It is not advised to apply interference with an almost HV conductor as it
may result in close range point discharge. This kind of discharge resistance is
nonlinear resistance, which is likely to produce co-channel interference.
XII. Discussion on variable frequency measurement
1. Variable frequency measurement
An accurate and reliable result may be reached by a variable frequency
even if the interference is extremely serious. For example, if 55Hz
measurement is adopted, only 55Hz signal is allowed by measurement system
and 50Hz interference signal is restrained effectively. The reason lies in that
the measurement system is likely to distinguish different frequencies and the
effect of selective frequency measurement can be explained by the following
simple calculation:
When a sine wave superimposes on another one, whose frequency is
twice as that of the former, the high frequency is interference and its amplitude
is ten times as much as that of low frequency.
Y=1.234sin(x+5.678°)+12.34sin(2x+87.65°)
Supposing that x=0/90/180/270°,
4 measurement valued will be
available.
Y0=12.4517，Y1= -11.1017，Y2=12.2075，Y3= -13.5576,
Calculating: A=Y1 - Y3=2.4559，B=Y0 - Y2=0.2442, then:
φ=tg-1(B/A)=5.678°
V=
A2+B2 / 2=1.234
This is just the phase and amplitude of low frequency and the interference
is restrained. There are as much as tens of thousands of measurement points
on actual wave forms. It is a very demanding calculation and the result reflects
the overall features of waveform.
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