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
37.
Series resistor thermal capacity
SEMOD54734-336 v6
The series resistor is dimensioned for 200 W. Preferable the U>Trip
2
/
SeriesResistor should always be lower than 200 W to allow continuous activation
during testing. If this value is exceeded, testing should be done with a transient
faults.
Section 6 1MRK 505 388-UEN B
Differential protection
78 Busbar protection REB650 2.2 IEC
Application manual
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