How to Select the Current Transformer Ratio in High-Voltage Switchgear

The ratio selection of the high-voltage side current transformer is closely related to the transformer capacity. Based on the rated current of the transformer, consider the overload margin and select the standard ratio upward.

Calculation Process (Taking a 1600kVA, 10kV Transformer as an Example)

Three-phase transformer rated current formula:
Ie = S / (√3 × U)
Where S = 1600 kVA, U = 10 kV

Ie = 1600 / (√3 × 10) ≈ 92 A

The conventional overload margin for transformers is 1.3 to 1.5 times. 1.5 times is suitable for occasions with impact loads and frequent full loads; 1.3 times can be used for stable loads. Here, calculated at 1.5 times:
92 A × 1.5 = 138 A

The primary side of the current transformer uses standard specifications. There is no 138A standard. Select the nearest standard primary current upward: 150A. Therefore, the ratio of the current transformer in this case is 150/5A.