8.4 : Y-Y回路

Balanced four-wire Y-to-Y systems consist of three balanced Y-connected sinusoidal voltage sources and loads with the neutral wire connecting both the neutral nodes of the source with the load.

The load impedance for each phase equals the sum of the source, line, and load impedances. The source and line impedances are neglected to simplify the system.

Considering the positive phase sequence, phase voltages determine the three-line voltages

Kirchhoff's Voltage Law is applied to each phase, and the line currents are determined to have equal magnitudes and 120-degree phase differences.

Their sum equals zero, implying zero current in the neutral wire.

In a balanced Y system, the total power delivered to the three-phase load is three times the power delivered in each phase.

For an unbalanced Y-to-Y system with an unbalanced load, the line currents are unbalanced, resulting in a non-zero neutral current.

Another method of analyzing a balanced Y-to-Y system is considering one phase and analyzing its single-phase equivalent circuit to determine its line current. The phase sequence then yields the other two line currents.