ABB RET670 Applications Manual page 145

1MRK504116-UUS C
Application manual
IR IED pickup current
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 45.
Series resistor thermal capacity
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 3
IED application
139