ABB REG670 Applications Manual page 158

Section 7
Differential protection
152
The minimum operating voltage has to be calculated (all loops) and the IED function
is set higher than the highest achieved value (setting U>Trip). 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 UR 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 U>Trip and SeriesResitor values.
The CT inputs used for 1Ph High impedance differential protection
HZPDIF 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 23, 24 below show, the operating currents for different settings of operating
voltages and selected resistances. Adjust as required based on tables 23,
values 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
U>Trip 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.
Table 23: 1 A channels: input with minimum operating down to 40 mA
Operating Stabilizing
voltage resistor R
U>Trip
ohms
20 V 500
40 V 1000
60 V 1500
Table continues on next page
Generator protection REG670 2.2 IEC and Injection equipment REX060, REX061, REX062
of the relevant channel x of TRM and/or AIM shall
primx
Operating Stabilizing
current level 1 resistor R
A ohms
0.040 A --
0.040 A --
0.040 A 600
1MRK 502 071-UEN -
24
Operating
current level 1
A
--
--
0.100 A
Application manual
or to