1. Selection of Current Transformers
1.1 Current transformer rating
When the current transformers used for transformer differential protection are composed of transformers with different voltage levels, different ratios, and different models on each side, the inconsistent transient characteristics of each transformer can lead to maloperation or failure to operate.
Typically, current transformers on the 500kV side are selected with protection considering transient characteristics, i.e., TP class. Current transformers on the 220kV and below sides are generally selected with protection class, i.e., P class (5P or 10P, representing composite errors of 5% or 10%, respectively). TPY and TPZ type transformers have air gaps in their cores, resulting in large residual magnetism and easy saturation. Transformer differential protection is composed of TP and P class transformers of different voltage levels. When there is a fault outside the high-voltage side and the short-circuit current is relatively large, the protection may malfunction due to the different currents transmitted by the transformers on each side.
Therefore, transformers of the same class should be selected on all sides of the transformer. Ideally, TP-class transformers should be used on all sides, which is technically best. However, the rated current on the low-voltage side is large, and TP-class transformers are expensive, making it economically impractical. Furthermore, the large size of TP-class transformers on the low-voltage side significantly restricts installation, especially during retrofitting. Since 5P-class transformers have higher accuracy than 10P-class transformers but are similarly priced, 5P-class transformers are generally chosen.
The issue of current transformer rating should also be extended to other differential protection systems, such as differential protection for high-voltage lines and differential protection for busbars. When selecting the rating of current transformers, it's not sufficient to focus on a single line or side; all current transformers in all differential circuits must be considered comprehensively. Particular attention should be paid to the current transformers of bypass switches, taking into account the current transformers of all bypass switches, especially when bypassing the circuit breaker of the main transformer.
1.2 Current transformer turns ratio
For the selection of steady-state parameters of Class P current transformers, a redundancy of 1.5 to 2.0 times should be considered. For example, if the rated current is 6000A to 8000A, a transformation ratio of 12000 to 15000.5 (or 1)A should be selected; if the maximum short-circuit current multiple is 8 to 10, a rated limit factor of 20 should be selected; if the secondary load is 15VA, a rated capacity of 30VA should be selected. Thus, the current transformer selection would be 5P20230VA, 15000.5A, which is beneficial for improving the transient characteristics of Class P current transformers.
With the advancement of social and economic development, electricity consumption is constantly increasing. When selecting the current transformer ratio, the situation after the load increases should also be considered, so the redundancy should be appropriately increased.
