ABB Relion 670 series, Applications Manual

Page: 102 / 760

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
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.
The tables 12 , 13 below show, the operating currents for different settings of operating
voltages and selected resistances. Adjust as required based on tables 12 , 13 or to 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 12: 1 A channels: input with minimum operating down to 40 mA
Operating
voltage
U>Trip
Stabilizing
resistor R
ohms
Operating
current level
1 A
Stabilizing
resistor R
ohms
Operating
current level
1 A
20 V 500 0.040 A -- --
40 V 1000 0.040 A -- --
60 V 1500 0.040 A 600 0.100 A
80 V 2000 0.040 A 800 0.100 A
100 V 2500 0.040 A 1000 0.100 A
150 V 3750 0.040 A 1500 0.100 A
200 V 5000 0.040 A 2000 0.100 A
Section 6 1MRK 505 343-UEN B
Differential protection
96
Application manual