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) | |||||||||||
| 5 | 10 | 15 | 20 | 30 | 40 | |||||||
| Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
|
| 50 | 6 | 136 | 3 | 136 | ||||||||
| 3.5 | 159 | |||||||||||
| 100 | 6 | 138 | 3.5 | 121 | 3 | 138 | ||||||
| 4 | 138 | |||||||||||
| 4.5 | 155 | 3.5 | 161 | |||||||||
| 5 | 172 | |||||||||||
| 200 | 6 | 137 | 3.5 | 120 | 3 | 137 | ||||||
| 4 | 137 | |||||||||||
| 4.5 | 154 | 3.5 | 160 | |||||||||
| 5 | 171 | |||||||||||
| 300 | 6 | 137 | 4 | 122 | ||||||||
| 4.5 | 137 | |||||||||||
| 5 | 153 | |||||||||||
| 500 | ||||||||||||
| 750 | ||||||||||||
| 1,000 | ||||||||||||
| 1,500 | ||||||||||||
| Contract power (kW) |
CT1 primary current (A) | |||||||||||
| 50 | 75 | 100 | 150 | 200 | 300 | |||||||
| Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
Tap (A) |
α (%) |
|
| 50 | ||||||||||||
| 100 | ||||||||||||
| 200 | 3 | 171 | ||||||||||
| 300 | 3.5 | 134 | 3 | 172 | ||||||||
| 4 | 153 | |||||||||||
| 4.5 | 172 | |||||||||||
| 500 | 6 | 137 | 3.5 | 120 | 3 | 137 | ||||||
| 4 | 137 | |||||||||||
| 4.5 | 154 | 3.5 | 160 | |||||||||
| 5 | 172 | |||||||||||
| 750 | 6 | 137 | 4 | 122 | 3 | 137 | ||||||
| 4.5 | 137 | |||||||||||
| 3.5 | 160 | |||||||||||
| 5 | 152 | |||||||||||
| 1,000 | 6 | 137 | 3.5 | 120 | 3 | 137 | ||||||
| 4 | 137 | |||||||||||
| 4.5 | 154 | 3.5 | 160 | |||||||||
| 5 | 171 | |||||||||||
| 1,500 | 6 | 137 | 4 | 122 | 3 | 137 | ||||||
| 4.5 | 137 | |||||||||||
| 3.5 | 160 | |||||||||||
| 5 | 153 | |||||||||||