Selection Parameters: Seven Key Checks
When selecting a voltage transformer (PT), there are seven parameters to verify: primary voltage, secondary voltage, accuracy class, rated capacity, wiring configuration, insulation type, and fuse protection.
Industry experts advise checking against this list: primary voltage (system voltage such as 10kV or 35kV), secondary voltage (standard 100V or 100/√3V), accuracy (0.2/0.5 for metering; 3P/6P for protection), capacity (VA matched to the load), wiring (V-V or star connection), insulation (dry-type or oil-immersed), and fuses (primary protection).
Voltage and accuracy are the most critical factors in PT selection.
The Seven Parameters
- Primary voltage: System voltage (e.g., 10kV, 35kV, 110kV) matched to the power grid;
- Secondary voltage: Standard 100V (line-to-line) or 100/√3V (phase-to-neutral) matched to instruments;
- Accuracy class: 0.2/0.5 for metering, 3P/6P for protection; selected based on application;
- Rated capacity: Secondary capacity (VA) matched to instrument or protection loads to prevent overloading;
- Wiring configuration: V-V (two-phase) or star (three-phase) connection design;
- Insulation type: Dry-type (indoor) or oil-immersed (outdoor);
selected based on the environment; - Fuse protection: Primary fuse (short-circuit protection) configuration.
Key Selection Points
The first point is voltage matching.
Primary/secondary voltage matching (between system and instruments) to prevent mismatch.
Second, accuracy matching: select based on application—metering (0.2/0.5) or protection (3P/6P).
Third, capacity matching: match secondary capacity (VA) to the load to prevent errors and ensure proper verification.
Fourth, fuse protection: configure primary fuses (to prevent short circuits) for reliable protection.
Pitfalls to Avoid
First, “voltage mismatch”: secondary voltage mismatch leads to instrument errors;
ensure standard voltage alignment.
Second, “insufficient accuracy”: using protection-class units for metering results in high errors;
ensure accuracy matches the application.
Third, “lack of fuse protection”: absence of primary fuses poses short-circuit risks;
ensure protection is configured.
Trends and Reminders
Instrument transformers are moving toward digitalization, driven by electronic voltage transformers (digital sampling) and smart substations (merging units).
For those selecting equipment, the recommendation is to verify seven key parameters, specifically focusing on voltage and accuracy matching.
Proper PT selection ensures reliable metering and protection.
