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CT requirement
The GRT100 does not require the use of dedicated CTs nor the use of CTs with an identical ratio.
The GRT100 can share the CTs with other protections and the different ratios are adjusted by
setting.
The general CT requirements are set for the through-fault stability which comes up when any CTs
saturate under very large through-fault currents. To ensure correct operation of the GRT100 for
such through-fault currents, the factor Ks of each CT is required to satisfy the following
conditions:
Ks ≧ 1 when Tc ≦ 150ms
or
Ks ≧ 5 when Tc ≦ 200ms
where,
Ks = ratio of CT knee point voltage to CT secondary probable voltage under the maximum
through-fault current
= Vk / {(R
CT
+ R
L
+ R
B
+ R
O
)(I
F
max / CT ratio)}
Tc = d.c. time constant of primary circuit
Vk = knee point voltage of CT
R
CT
= resistance of CT
secondary
winding
R
L
= loop resistance of cable between CT and relay
R
B
= ohmic load of GRT100 (i.e. 0.1 ohm for 1A rating and 0.012 ohm for 5A rating)
R
O
= ohmic load of other series-connected relays (if any)
I
F
max = maximum through-fault current
For example, if the following parameters are given:
Vk = 800 V, CT ratio = 1,200/1, R
CT
= 5.0 ohm, R
L
= 3.0 ohm, R
B
= 0.1 ohm,
R
O
= 0 ohm (i.e. no series-connected relays) and I
F
max = 40kA
then the factor Ks is calculated as:
Ks = 800/{(5.0 + 3.0 + 0.1)×(40,000/1,200) }
= 800/270
= 3.0
This shows that the GRT100 operates correctly for all the faults under the condition that the d.c.
time constant of the primary circuit is less than 200ms.