Before paralleling a generator to a bus, what three conditions must be matched?

• A. Voltage, frequency, and power factor.
• B. Voltage, frequency, and phase angle (or phase rotation).
• C. Phase sequence, voltage, and current.
• D. Voltage, current, and phase rotation.

Answer: (B)

Synchronizing a generator to a bus means making its output match the bus exactly in three aspects: the voltage magnitude, the system frequency, and the phase relationship (phase angle or phase rotation) between the two sources. Matching voltage ensures there’s no large current surge when connected because the two voltages sit at the same potential. Matching frequency keeps the same number of cycles per second so power flow remains steady and the systems stay in step. Matching the phase angle (and confirming the same phase rotation) ensures the waveforms align in time so the instantaneous voltages cross zero together; if the angle is off or the phase sequence is wrong, large circulating currents can flow, causing stress or damage and potentially tripping breakers. That’s why the correct requirements are voltage, frequency, and phase angle (or phase rotation).