MV design guide - Schneider Electric

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definition
Current transformer
Primary operating current (Ips)
An installation's primary operating current I (kA) (for a transformer
feeder for example) is equal to the CT primary operating current (Ips)
taking account of any possible derating.
c If:
S : apparent power in kVA
U : primary operating voltage in kV
P : active power of the motor in kW
Q : reactive power of capacitors in kvars
Ips : primary operating current in A
c We will have:
v incomer cubicle
v generator set incomer
v transformer feeder
v motor feeder
S
Ips =
e• U
S
Ips =
e• U
Ips =
S
e• U
Ips =
P
e• U • cosϕ • η
η : motor efficiency
Example:
A thermal protection device for a motor
has a setting range of between 0.6 and
1.2 • IrTC. In order to protect this motor,
the required setting must correspond to
the motor's rated current.
c If we suppose that Ir
for the motor = 45 A,
the required setting is therefore 45 A;
v if we use a 100/5 CT, the relay will
never see 45 A because:
100 • 0.6 = 60 > 45 A.
v if on the other hand, we choose a
CT 75/5, we will have:
0.6 <
45
< 1.2
75
and therefore we will be able to set our
relay. This CT is therefore suitable.
If you do not know the exact values of ϕ and η,
you can take as an initial approximation: cos ϕ = 0.8 ; η = 0.8.
v capacitor feeder
1.3 is a derating coefficient of 30% to take account of temperature rise
due to capacitor harmonics.
Ips =
1.3 • Q
e•U
v bus sectioning
The current Ips of the CT is the greatest value of current that can flow in
the bus sectioning on a permanent basis.
Rated primary current (Ipr)
The rated current (Ipr) will always be greater than or equal to the
operating current (I) for the installation.
c Standardised values:
10 -12.5 - 15 - 20 - 25 - 30 - 40 - 50 - 60 - 75 and their multiples
and factors.
c For metering and usual current-based protection devices, the rated
primary current must not exceed 1.5 times the operating current.
In the case of protection, we have to check that the chosen rated current
enables the relay setting threshold to be reached in the case of a fault.
N.B.: current transformers must be able to withstand 1.2 times the rated current on a constant
basis and this as well must be in conformity with the standards.
Schneider Electric
Merlin Gerin MV design guide
55