What are the different wiring methods for current transformers?

1. Single-phase connection
Features: Uses a single current transformer, directly reflecting the current of one phase.
Applicable scenarios: Suitable for systems with balanced three-phase loads, such as measuring current or connecting load protection devices in low-voltage power lines.
Limitations: If the system is unbalanced, the measurement results may be inaccurate.
2. Two-phase V-connection (Two-phase incomplete star connection)
Features: Uses two current transformers (usually connected to phases A and C), calculating the current of the unconnected phase (e.g., phase B current is -Ib) through the common line current.
Advantages: Saves one transformer, resulting in higher economic efficiency.
Applicable scenarios: Three-phase three-wire systems with ungrounded neutral points (such as 10kV high-voltage circuits), widely used for measuring three-phase current and overcurrent protection.
Limitations: Cannot detect single-phase ground faults.
3. Two-phase current difference connection
Features: The common line current on the secondary side of the two transformers (phases A and C) is the difference between the two phase currents (Ia-Ic), which is √3 times the phase current.
Advantages: High sensitivity, suitable for overcurrent protection.
Applicable scenarios: Three-phase three-wire systems with ungrounded neutral points, referred to as "two-phase one-relay connection" in relay protection.
4. Three-phase star connection (Three-phase complete star connection)
Features: Uses three transformers, with the secondary sides connected in a star configuration, directly reflecting the current of each phase.
Advantages: Can measure symmetrical/asymmetrical currents, detect all types of short-circuit faults, and provide comprehensive protection functions.
Applicable scenarios: Three-phase three-wire or three-phase four-wire systems where the load may be unbalanced (such as TN systems), used for energy measurement and overcurrent protection.
Note: In high-voltage systems, the non-polar end must be grounded.