For a through fault one current transformer might saturate when the other CTs still
will feed current. For such a case a voltage will be developed across the measuring
branch. The calculations are made with the worst situations in mind and a
minimum operating voltage U
R
is calculated according to equation
11
EQUATION1531 V1 EN-US (Equation 11)
where:
IF
max is the maximum through fault current at the secondary side of the CT
Rct
is the current transformer secondary winding resistance and
Rl
is the maximum loop resistance of the circuit at any CT.
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
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 10, 11 below show, the operating currents for different settings of
operating voltages and selected resistances. Adjust as required based on tables 10,
11 or to values in between as required for the application.
Section 6 1MRK 505 388-UEN B
Differential protection
76 Busbar protection REB650 2.2 IEC
Application manual
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