Selection Essentials: Six Key Factors
There are six key factors when selecting a current transformer (CT): transformation ratio, accuracy class, rated burden (capacity), voltage class, secondary wiring configuration, and verification certificate.
Industry experts advise checking against these criteria: transformation ratio (primary current vs. secondary 5A or 1A; primary current should be 1.2–1.5 times the load current), accuracy class (0.2S/0.5S for metering; 5P/10P for protection), rated burden (matching secondary load), voltage class (insulation matching), secondary wiring (star/delta), and statutory verification certificate (for metering applications).
Accuracy and transformation ratio are the most critical aspects of CT selection.
Six Key Points
- Transformation Ratio: Primary current = 1.2–1.5 times the load current; secondary current = 5A (standard) or 1A (for long-distance transmission);
- Accuracy Class: 0.2S/0.5S for metering (high precision) and 5P/10P for protection; select based on application;
- Rated Burden: Secondary burden (VA) must match the instrument or protection relay load to prevent overloading;
- Voltage Class: Insulation level (e.g., 0.66kV, 10kV, 35kV) must match the system voltage;
- Secondary Wiring: Star (three-phase four-wire) or Delta (protection);
determine based on wiring design; - Verification Certificate: Statutory verification certificate required for metering applications (mandatory verification) to ensure regulatory compliance.
Selection Highlights
First, ensure a reasonable transformation ratio.
Transformation ratio: Set to 1.2–1.5 times the load current (to ensure metering accuracy and prevent saturation caused by under-sizing).
Accuracy matching: Select based on application—metering (0.2S/0.5S) or protection (5P/10P).
Capacity matching: Match secondary capacity (VA) to the load (to prevent errors) and verify.
Certificate compliance: Metering units require statutory verification certificates to ensure traceability.
Pitfalls to Avoid
Excessive transformation ratio: Ratios that are too high lead to inaccurate metering;
stick to the 1.2–1.5x range.
Insufficient accuracy: Using protection-class units for metering results in high errors;
ensure accuracy matches the application.
Missing verification certificates: Metering units without certificates are non-compliant;
statutory verification is mandatory.
Trends and Reminders
Instrument transformers are moving toward “intelligence,” driven by trends such as electronic transformers (digital sampling) and online metering (IoT).
For those selecting equipment, the recommendation is to “check six key points, ensure a reasonable transformation ratio, and match accuracy.”
Accurate CT selection ensures reliable metering and protection.
