When the R value has been selected and the TripPickup
value has been set, the
sensitivity of the scheme IP can be calculated. The IED sensitivity is decided by the
total current in the circuit according to equation
19.
( )IP n IR Ires lmag= × + +
å
EQUATION1747 V1 EN-US (Equation 19)
where:
n is the CT ratio
IP primary current at IED pickup,
IR IED pickup current (U>Trip/SeriesResistor)
Ires is the current through the voltage limiter and
ΣImag is the sum of the magnetizing currents from all CTs in the circuit (for example, 4 for
restricted earth fault protection, 2 for reactor differential protection, 3-5 for autotransformer
differential protection).
It should be remembered that the vectorial sum of the currents must be used (IEDs,
Metrosil and resistor currents are resistive). The current measurement is insensitive to
DC component in fault current to allow the use of only the AC components of the fault
current in the above calculations.
The voltage dependent resistor (Metrosil) characteristic is shown in Figure
61.
Series resistor thermal capacity
SEMOD54734-336 v6
The series resistor is dimensioned for 200 W
.
Care shall be exercised while testing to
ensure that if current needs to be injected continuously or for a significant duration of
time, check that the heat dissipation Vxxx Series Resistance value does not exceed 200
W. Otherwise injection time shall be reduced to the minimum.
Section 7 1MRK 504 163-UUS A
Differential protection
162 Transformer protection RET670 2.2 ANSI
Application manual
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