Wye vs Delta Connection in Current Transformer (CT) Secondaries: Pros and Cons?

The discussion addresses the differences between wye (Y) and delta (Δ) connections in current transformer (CT) secondaries, focusing on their applications, advantages, and disadvantages. Delta-connected CT secondaries are uncommon and primarily used in specific protection schemes for power transformers with delta/wye or wye/delta winding configurations. This connection compensates for phase displacement between high voltage (HV) and low voltage (LV) windings, ensuring correct current phase angles for differential protection relays (ANSI 87). Delta connections act as zero-sequence current traps, filtering out zero-sequence currents by circulating them within the delta loop, which prevents these currents from reaching protection relays. This characteristic is beneficial for protecting zig-zag transformers from internal faults using overcurrent relays. However, delta connections can alter current characteristics by diverting some current within the delta, potentially affecting measurement accuracy. Modern intelligent electronic devices (IEDs) in protection relays can perform phase correction digitally, reducing the need for delta-connected CT secondaries. Phase accuracy and fast relay response are critical for system stability; incorrect phase correction can cause false differential trips and prevent transformer energization. Overall, wye connections are more common in CT secondaries due to their straightforward current measurement, while delta connections are specialized solutions for phase displacement correction and zero-sequence current filtering in transformer protection schemes.
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