In parallel operation, if the phase angle is not matched, what issue can occur?

• A. Excessive circulating currents between generators.
• B. Voltage instability across the bus.
• C. Increased rotor speed without control.
• D. Reduced reactive power support.

Answer: (A)

When generators are connected in parallel, they share the same bus voltage and must be synchronized in frequency and phase. If the phase angle between the machines’ internal voltages isn’t matched, there’s a time-varying difference in voltage around the parallel path. This drives circulating currents between the machines as each unit pushes current to correct the phase disparity. Those circulating currents flow even under balanced load and can cause unnecessary heating, increased losses, and mechanical/stability stresses on the governors and turbines as the machines fight to align their outputs. This is the primary issue that arises from phase-angle mismatch.

While phase-angle differences can lead to fluctuations in power transfer and potential voltage interactions, they do not inherently cause rotor speed to rise without limit or directly reduce reactive power support as the main effect. The most immediate and distinguishing consequence of an unmatched phase angle is the excessive circulating currents between generators.