ABB RELION 670 SERIES Applications Manual page 144

Section 7
Differential protection
138
(
> × +
VR IF max Rct
EQUATION1531-ANSI V1 EN
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 and CT
secx
set equal to 1 A by PST in PCM600; The parameter CTStarPointx may be
set to ToObject.
The tables 16, 17 below show, the operating currents for different settings of operating
voltages and selected resistances. Adjust as required based on tables 16,
between as required for the application.
)
Rl
of the relevant channel x of TRM and/or AIM shall be
primx
1MRK 506 369-UUS -
(Equation 16)
17 or to values in
Line distance protection REL670 2.2 ANSI
Application manual