ABB Relion 670 Series Applications Manual page 112

Section 6
Differential protection
are made with the worst situations in mind and a minimum operating voltage V
according to equation
>
VR IF
EQUATION1531-ANSI V1 EN-US
where:
IF max
Rct
Rl
The minimum operating voltage has to be calculated (all loops) and the IED function is set higher
than the highest achieved value (setting
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
The tables 12,
and selected resistances. Adjust as required based on tables 12,
required for the application.
Normally the voltage can be increased to higher values than the calculated minimum
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.
106
14
( )
× +
max Rct Rl
is the maximum through fault current at the secondary side of the CT
is the current transformer secondary winding resistance and
is the maximum loop resistance of the circuit at any CT.
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
relevant channel x of TRM and/or AIM shall be set equal to 1 A by PST in PCM600;
CTStarPointx may be set to ToObject .
The parameter
13 below show, the operating currents for different settings of operating voltages
Minimum ohms can be difficult to adjust due to the small value compared to the
total value.
TripPickup ). As the loop resistance is the value to the
13
1MRK 511 358-UUS A
is calculated
R
(Equation 14)
CT CT
and of the
secx primx
or to values in between as
TripPickup
Application manual