The calculations are made with the worst situations in mind and a minimum operating
voltage V
R
is calculated according to equation 18
EQUATION1531-ANSI V1 EN-US (Equation 18)
where:
IF
max is the maximum through fault current at the secondary side of the CT
Rct
is the current transformer secondary winding resistance and
Rl
is the maximum loop resistance of the circuit at any CT.
The minimum operating voltage has to be calculated (all loops) and the IED function is
set higher than the highest achieved value (setting TripPickup
). As the loop resistance
is the value to the connection point from each CT, it is advisable to do all the CT core
summations in the switchgear to have shortest possible loops. This will give lower
setting values and also a better balanced scheme. The connection in to the control room
can then be from the most central bay.
For an internal fault, all involved CTs will try to feed current through the measuring
branch. Depending on the size of current transformer, relatively high voltages will be
developed across the series resistor. Note that very high peak voltages can appear. To
prevent the risk of flashover in the circuit, a voltage limiter must be included. The
voltage limiter is a voltage dependent resistor (Metrosil).
The external unit with stabilizing resistor has a value of either 6800 ohms or 1800
ohms (depending on ordered alternative) with a sliding link to allow adjustment to the
required value. Select a suitable value of the resistor based on the VR voltage
calculated. A higher resistance value will give a higher sensitivity and a lower value a
lower sensitivity of the relay.
The function has a recommended operating current range 40 mA to 1.0A for 1 A inputs
and 200 mA to 5A for 5A inputs. This, together with the selected and set value, is used
to calculate the required value of current at the set TripPickup and R series values.
The CT inputs used for 1Ph High impedance differential protection
HZPDIF (87) function, shall be set to have ratio 1:1. So the parameters
CT
secx
and CT
primx
of the relevant channel x of TRM and/or AIM shall
be set equal to 1 A by PST in PCM600; The parameter CTStarPointx
may be set to ToObject.
Section 7 1MRK 504 163-UUS A
Differential protection
160 Transformer protection RET670 2.2 ANSI
Application manual
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