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FAQ01558 of Measuring / Motor Protective Relays FAQ

FAQ No. FAQ01558

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Question

Please describe the current setting for the timing element in the K2CA Over-Current Relay(OCR).

Answer



The operating current for the timing element is usually set to approximately 150% of the current value for the contract power. The tap value is obtained from the following formula:

ITAP: The setting tap value (A) for the OCR timing element
I1: The current value for the contract power (equals contract power (kW)/√3 x 6.6 (kV))
ICT: Primary rated current of the current transformer (secondary current is 5 A)

Because the tap value in the K2CA Series can be set in relatively fine increments of 3, 3.5, 4, 5.4, 5, or 6 A, it is quite easy to make the 150% setting to provide optimal overload protection.

The following table shows the relationship between the contract power and the setting tap. The α value in this table is the proportion of the current for the contract power and the OCR operating current, indicating the tap values near the 150% point. The tap value can be easily changed by turning a rotary switch. There is no need to insert a spare tap to prevent an open circuit on the CT2 side as in previous systems.

Contract Power vs. K2CA Timing Element Current Setting

Contract
power
(kW)
CT1 primary current (A)
51015203040
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
5061363136
3.5159
10061383.51213138
4138
4.51553.5161
5172
20061373.51203137
4137
4.51543.5160
5171
30061374122
4.5137
5153
500
750
1,000
1,500
Contract
power
(kW)
CT1 primary current (A)
5075100150200300
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
Tap
(A)
α
(%)
50
100
2003171
3003.51343172
4153
4.5172
50061373.51203137
4137
4.51543.5160
5172
750613741223137
4.5137
3.5160
5152
1,00061373.51203137
4137
4.51543.5160
5171
1,500613741223137
4.5137
3.5160
5153

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