ABB RET670 Applications Manual page 143

1MRK504116-UUS C
Application manual
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, circulation is not possible, due to the high impedance. 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 shorting 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.
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
be set equal to 1 A by PST in PCM600; The parameter CTStarPointx
may be set to ToObject.
The tables 44, 45 below show, the operating currents for different settings of operating
voltages and selected resistances. Adjust as required based on tables 44,
in between as required for the application.
Minimum ohms can be difficult to adjust due to the small value
compared to the total value.
Normally the voltage can be increased to higher values than the calculated minimum
TripPickup with a minor change of total operating values as long as this is done by
adjusting the resistor to a higher value. Check the sensitivity calculation below for
reference.
of the relevant channel x of TRM and/or AIM shall
primx
Section 3
IED application
45 or to values
137