Toshiba GR200 Series Instruction Manual page 41

Current transformer requirements
2.1.4
GRB200 does not have the request to use dedicated CTs or CTs with an identical ratio. The
busbar protection IED can share CTs with other protections and different CT ratios can be
accommodated by setting.
The general CT requirements are dictated by through-fault stability, which is the
parameter of interest when CTs saturate under very large through-fault currents. To ensure
correct operation of the IED for such through-fault currents, the factor Ks for each CT is
required to satisfy the following conditions:
Ks ≥ 1 when Tc ≤ 200ms
Or
Ks ≥ 2 when Tc ≤ 250ms
where,
Tc = d.c. time constant of primary circuit
Ks = Ratio of CT knee point voltage to CT secondary probable voltage under the
maximum through-fault current
=
( ��
+ ��
����
Vk = CT knee point voltage
R = CT secondary winding resistance
CT
R : loop resistance of cable between the CT and relay
L
R : ohmic load of CH (i.e. 0.1 ohm for 1A rating and 0.012 ohm for 5A rating)
B
R : ohmic load of other series-connected relays (if any)
O
I Fmax : maximum through-fault current
Example: If the following parameters are given, the value of Ks is obtained by
equation (2.1-5). That is,
Vk = 800 V, CT ratio = 1200/1, R
R = 0 ohm (i.e. no series-connected IEDs) and I Fmax = 40kA
O
Then the factor Ks is calculated as follows:
Ks = 800 / {(5.0 + 3.0 + 0.1)×(40,000/1,200)}
= 800/270
= 3.0
This shows that the DIF element will operate correctly for all faults under the condition
that the d.c. time constant of the primary circuit is less than 250ms.
��
��
����������
) ×
+ �� + ��
�� �� ��
���� ����������
- 27 -
= 5.0 ohm, R = 3.0 ohm, R
CT L
Centralized GRB200
(Soft: 030, 031, 032, 033, 034)
6F2S1931 (0.20)
(2.1-5)
= 0.1 ohm,
B