4.35
Date Code 20150130 Instruction Manual SEL-787 Relay
Protection and Logic Functions
Basic Protection
NOTE: The cosine filter provides
excellent performance in removing dc
offset and harmonics. However, the
bipolar peak detector has the best
performance in situations of severe
CT saturation when the cosine filter
magnitude estimation is significantly
degraded. Combining the two
methods provides an elegant solution
for ensuring dependable short circuit
overcurrent element operation.
The phase instantaneous overcurrent elements (50Pn1 through 50Pn4; see
Figure 4.19) normally operate using the output of the one cycle cosine filtered
phase current. During severe CT saturation, the cosine filtered phase current
magnitude may be substantially reduced because of the distorted secondary
waveform. If the overcurrent element relied only on the output of the cosine
filtered secondary current, this may severely delay and may even jeopardize
the operation of any high-set instantaneous overcurrent element. For any
phase instantaneous overcurrent element in the SEL-787 relay set above eight
times the relay current input rating (40 A in a 5 A relay), the overcurrent
element also operates on the output of a bipolar peak detector if the current
waveform is highly distorted, as is the case with severe CT saturation. This
ensures fast operation of the 50P phase overcurrent elements even with severe
CT saturation.
PHASE IOC DELAY 0.00–5.00 sec 50Pn3D := 0.00
PH IOC TRQCTRL SEL
OGIC 50Pn3TC := 1
PHASE IOC LEVEL OFF, 0.50–96.00 A
a
,
0.10–19.20 A
b
50Pn4P := OFF
50Pn4P := OFF
PHASE IOC DELAY 0.00–5.00 sec 50Pn4D := 0.00
PH IOC TRQCTRL SEL
OGIC 50Pn4TC := 1
a
For I
NOM
=5 A.
b
For I
NOM
=1 A.
Table 4.7 Winding n Maximum Phase Overcurrent Settings (n = 1 or 2)
(Sheet 2 of 2)
Setting Prompt Setting Range
Setting Name :=
Factory Defa ul t
To Next Page
