Do you know how to choose the current transformer ratio?

Selection of current transformer ratio needs to consider the system requirements, technical parameters and application scenarios, the specific principles are as follows:

Rated voltage matching

Primary voltage needs to be the same as the nominal voltage of the system, allowing high voltage transformers to be used in low voltage systems (e.g., 10kV transformer in 6kV system), but need to ensure that the secondary load resistance and insulation level meets the requirements.

Selection of rated current

Priority standard series:

5,10,15,20,30,40,50,75,100,150,200,300,400,500,600,800,1000,1500,2000,3000,4000,5000,7500,10000A and other standard ratings to choose.

Measurement and protection:

Measurement circuits are commonly used in class S (10~110% of rated current), while protection circuits need to meet the requirements of short-circuit currents and may need to increase ratios (e.g., 750/5A for measurement, 1000/5A for protection).

Secondary current system

1A: Suitable for new systems, reduces line losses (60% reduction in line losses) and improves transmission accuracy over long lines. - 5A system: Suitable for old system renovation, reducing the risk of secondary open circuit voltage.

Ratio Adjustment Mode: Ratio change is achieved by series/parallel connection of secondary winding taps or primary taps (e.g., doubling the ratio of the series connection and halving the ratio of the secondary winding taps). - Multi-purpose transformer: The same plant can be configured with 1A and 5A secondary current to meet different needs.

Influence of short-circuit current: The maximum short-circuit current should be considered for the protection circuit, which may require the selection of higher ratios or special types of transformers. - Environment and economy: Combine with the installation environment (temperature, humidity, etc.) and maintenance cost, choose the product with high cost performance.

Conclusion:

The choice of ratio should balance accuracy, safety, economy and flexibility. Priority should be given to following the national standard sequence and choosing the right type according to the measurement/protection requirements.