How to ensure correct wiring of three-phase current transformers?
Dec 17, 2025| 1. Reducing Polarity Principle
Current transformers use a reducing polarity marking method. The primary winding L1 and secondary windings K1 and K2 are of the same polarity. If the terminal markings are unclear, they can be determined by voltage measurement: connect the primary winding to a low-voltage DC power supply, and the secondary winding to a voltmeter. When the switch is closed and the voltmeter deflects forward, the primary terminal connected to the positive terminal of the power supply and the secondary terminal connected to the positive terminal of the voltmeter are of the same polarity.
2. Star/Delta Connection Methods
Star Connection: The secondary sides of the three transformers are connected in a Y-shape. This allows measurement of the current in each phase and is suitable for systems above 10kV. Neutral grounding can be used for relay protection.
Delta Connection: Used for differential protection, offering high sensitivity, but requires paired matching.
3. Polarity Determination Method
External Markings: L1 and K1, L2 and K2 are of the same polarity.
Measurement Method: Using a multimeter in milliampere mode, connect the red probe to the assumed K1 and the black probe to K2. Touch the positive and negative terminals of the dry cell battery to L1 and L2 respectively. If the multimeter deflects positively, then the assumed K1 and L1 are the same terminals.
4. Key Points to Ensure Correct Wiring:
For systems above 10kV, the non-polar terminals of the current transformer must be grounded, with each phase grounded separately.
Current and voltage circuits must be connected separately to avoid mixing.
All ammeters must be grounded; this is a safety requirement.
