MV design guide - Schneider Electric

Switchgear
definition
Current transformer
Differential protection
Many manufacturers of differential protection relays recommend
class X CT's.
c Class X is often requested in the form of:
Vk ≤ a . If (Rct + Rb + Rr)
The exact equation is given by the relay manufacturer.
Values characterising the CT
Vk : Knee-point voltage in volts
a : asymmetry coefficient
Rct : max. resistance in the secondary winding in Ohms
Rb : loop resistance (feed/return line) in Ohms
Rr : resistance of relays not located in the differential part
of the circuit in Ohms
If : maximum fault current seen by the CT
in the secondary circuit for a fault outside of the zone
to be protected
If = Isc
Kn
Isc : primary short-circuit current
Kn : CT transformation ratio
What values should If be given to determine Vk?
c The short-circuit current is chosen as a function of the application:
v generator set differential
v motor differential
v transformer differential
v busbar differential.
c For a generator set differential:
v if Isc is known: Isc short-circuit current for the generator set on its own
If = Isc
Kn
relay
CT G CT
v if the Ir gen is known: we will take
If =
7 • Ir gen
Kn
v if the Ir gen is unknown: we will take
If = 7 • Isr (CT)
Isr(CT) = 1 or 5 A
c For motor differential:
v if the start-up current is known: we will take
Isc = I start-up
relay
CT M CT
If = Isc
Kn
v if the Ir motor is known: we will take
If = 7 • Ir
Kn
v if the Ir motor is not known: we will take
If = 7 • Isr (CT)
Isr(TC) = 1 or 5 A
Reminder
Ir : rated current
Schneider Electric
Merlin Gerin MV design guide
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