Current transformer (CT) plays a critical role in protection and metering circuits - ensuring their health is essential for safe and reliable operation of high-voltage systems. It is essential to test and diagnose once a year on CTs. Here are important test items we should do during testing CTs.
Insulation Resistance (IR) Test
Purpose: Assess insulation health between primary, secondary, and core.
Method: High-voltage DC applied using a Megger Insulation Tester.
Interpretation:
High IR → Healthy insulation
Low IR → Possible moisture or insulation deterioration
CT Analyzer Testing( CT Parameters analyzer)
Comprehensive testing performed using Megger CT Analyzer, which automatically measures and analyzes all electrical characteristics of the CT, including:
1. Winding Resistance (WR):
Evaluates the resistance of secondary windings to detect loose connections or shorted turns.
(Measured automatically by CT Analyzer with temperature correction applied.)
2. Ratio Test:
Confirms the actual turns ratio matches the nameplate ratio.
3. Phase Error / Phase Displacement:
Measures angular deviation between primary and secondary currents - essential for accurate metering and protection.
4. Excitation (Magnetization / Saturation) Curve:
Determines the knee-point voltage and CT core behavior under fault conditions.
5. Burden & Accuracy Class Verification:
Confirms the CT maintains accuracy under rated burden as per IEC / IEEE standards.
6. Polarity Test:
Verifies the correct orientation between primary and secondary terminals.
7. Demagnetization Function:
Automatically demagnetizes the CT core after testing to restore accurate characteristics.
Capacitance & Dissipation Factor (C&DF / Tan Delta) Test
Purpose: Evaluate insulation dielectric condition and detect early aging.
Method: High-voltage AC applied; Capacitance and Tan Delta (Dissipation Factor) measured.
Interpretation:
Stable capacitance → Healthy insulation
Increased Tan Delta → Possible moisture, heat, or contamination