ABB Relion REC670 Applications Manual page 94

Section 6
Differential protection
calculations are made with the worst situations in mind and a minimum operating voltage U
is calculated according to equation
>
UR IF
EQUATION1531 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
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
The tables 12,
voltages and selected resistances. Adjust as required based on tables 12,
between as 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.
88
( )
× +
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.
The CT inputs used for 1Ph High impedance differential protection HZPDIF
function, shall be set to have ratio 1:1. So the parameters
the relevant channel x of TRM and/or AIM shall be set equal to 1 A by PST in
PCM600; The parameter
13 below show, the operating currents for different settings of operating
Minimum ohms can be difficult to adjust due to the small value compared to
the total value.
14
U>Trip ). As the loop resistance is the value to
U>Trip and SeriesResitor values.
CTStarPointx may be set to ToObject .
1MRK 511 358-UEN A
R
(Equation 14)
CT CT
and of
secx primx
13 or to values in
U>Trip
Application manual