Method Of Operation; Current And Voltage Input (See Figure 2.1) - Siemens 7SN71 Instruction Manual

Transient earth-fault relay 7SN71
2

Method of operation

2.1

Current and voltage input (see figure 2.1)

The summation current of the cable-type CT or of the
three phase-current CTs in Holgreen circuit is trans
formed into a proportional voltage in the input circuit
and is supplied to positive and negative comparators.
As soon as the positive or negative trigger threshold
is exceeded, the memory for the corresponding half-
cycle is set while that of the other is blocked
The three phase voltages form the virtual starpoint of
the voltage transformer delta via the capacitors in
start-delta connection. Hence, the displacement vol
tage of the system is located between this point and
the supplied earth potential of the transformers. This
voltage is supplied to the positive and negative com
parators via an input transformer. As soon as the
positive or negative trigger threshold is exceeded and
upon simultaneous pickup of a current trigger the
corresponding memory is set while the other is
blocked .
2
Siemens AG March 1997
Method of operation
Instruction Manual Order No.: E50410-A0007-U501-A1-7691
In order to prevent incorrect signalling during swit
ching operations in the system, it is verified whether
the displacement voltage still exceeds approx. 25 V
after 70 ms. Only after this verification is the evalua
tion of the set memories released. If this is not the
case, the result are discarded and the memories are
. reset .
1 3
This function can be disabled by withdrawing the
front plug t ". Since in isolated networks the displa
v
cement voltage is present for only a short period of
time after a transient earth-fault, the front plug t
should in this case also be withdrawn.
If after approx. three seconds the displacement vol
tage persists above 25 V
tinuous earth fault" is released
2
v
, the annunciation Con
eff
.
4
2 - 1
eff
"