2–14
M-0420 Instruction Book
59I
Peak Overvoltage
Unless otherwise noted, an overvoltage relay is
presumed to operate based on the rms value of
voltage. There is, however, a system phenomenon
known as ferroresonance that can occur on a
lightly loaded islanded system. As the name
implies, a system experiencing ferroresonance is
in resonance, but the inductance is highly
nonlinear, being variable as the transformer core
cycles in and out of magnetic saturation. At this
time, the voltage waveform will be expected to
be very rich in harmonics, to the extent that it is
possible that the peak voltage of the non-sinusoidal
wave will be dangerously high, even though the
rms value of the same voltage remains in an
acceptable range.
Because it is necessary to describe voltage for
this purpose in terms of the peak value of volt-
age (not rms), it is convenient to define the pa-
rameter setpoints in per unit of the peak of the
nominal sinusoidal waveform. Then, with the peak
value of a nominal 120 V sine wave being 120 x
S
2 = 170 V, a 1.2 per unit setting would initiate
tripping action at 1.2 x 170 = 204 V, as instanta-
neously detected. Note that, because of the non-
sinusoidal voltage waveform expected, the rms
value of this voltage cannot be so simply calcu-
lated as 204 V/
S
2 = 144 Volts, but may be very
much less than that value. One suggestion is that
this function be set at 1.3 to 1.5 per unit with the
time delay set to operate very quickly, i.e., at
less than 10 cycles of delay.
■
NOTE: When 69.3 V is chosen for the VT sec-
ondary voltage, 1 pu peak voltage is equivalent
to 69.3 x
S
2 = 98 V.
N
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h
s
Table 2-4
Peak Overvoltage (59I) Setpoint Ranges
Summary of Contents for M-0420
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