( )IP n IR Ires lmag= × + +
å
EQUATION1747 V1 EN (Equation 17)
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
55.
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 7 1MRK 506 369-UUS -
Differential protection
140 Line distance protection REL670 2.2 ANSI
Application manual
To Next Page
Loading...
