ABB RELION RET670, Applications Manual

Page: 181 / 1016

To calculate the sensitivity at operating voltage, refer to equation 28 which is
acceptable as it gives around 10% minimum operating current, ignoring the current
drawn by the non-linear resistor.
( )
600
200 0 4 20 60 .5.4
5
IP approx A
= × ° + × - ° £
EQUATION1885-ANSI V1 EN-US
(Equation 28)
Where 200mA is the current drawn by the IED circuit and 50mA is the current drawn
by each CT just at pickup. The magnetizing current is taken from the magnetizing
curve for the current transformer cores which should be available. The current value at
TripPickup is taken.
7.2.4.6 Alarm level operation
M16850-196 v6
The 1Ph High impedance differential protection HZPDIF (87) function has a separate
alarm level, which can be used to give alarm for problems with an involved current
transformer circuit. The setting level is normally selected to be around 10% of the
operating voltage
TripPickup .
As seen in the setting examples above the sensitivity of HZPDIF (87) function is
normally high, which means that the function will in many cases operate also for short
circuits or open current transformer secondary circuits. However the stabilizing resistor
can be selected to achieve sensitivity higher than normal load current and/or separate
criteria can be added to the operation, like a check zone. This can be either another
IED, with the same HZPDIF (87) function, or be a check about the fault condition,
which is performed by a ground overcurrent function or neutral point voltage function.
For such cases where operation is not expected during normal service the alarm output
should be used to activate an external shorting of the differential circuit avoiding
continuous high voltage in the circuit. A time delay of a few seconds is used before the
shorting and alarm are activated. Auxiliary relays with contacts that can withstand high
voltage shall be used, like RXMVB types.
The metrosil operating characteristic is given in the following figure.
1MRK 504 163-UUS A
Section 7
Differential protection
Transformer protection RET670 2.2 ANSI 175
Application manual