Checks the three quantities a synchroscope or sync-check relay (ANSI device 25) monitors before permitting a generator paralleling breaker to close: voltage difference, frequency difference (slip), and phase-angle difference between the incoming generator and the bus/system it's joining.
Standards followed
General synchronizing practice (the quantities monitored by ANSI device 25 sync-check relays) — acceptance windows are application/relay-setting specific, not one fixed standard, so all three thresholds here are user-adjustable
What it can do
- Voltage difference (%) between generator and bus, checked against an adjustable threshold.
- Frequency difference (slip, Hz) between generator and bus, checked against an adjustable threshold, plus the resulting 'beat period' — the time between successive in-phase instants.
- Phase-angle difference, checked against an adjustable threshold.
- Overall go/no-go verdict across all three checks.
Example problem
Generator at 415V/50.05Hz, bus at 412V/50.00Hz, 3° phase angle difference, with 5% voltage, 0.2Hz frequency and 10° phase thresholds.
- Voltage difference = |415−412|/412 × 100 = 0.73% — within the 5% threshold.
- Frequency difference = |50.05−50.00| = 0.05Hz — within the 0.2Hz threshold; beat period = 1/0.05 = 20s.
- Phase angle difference = 3° — within the 10° threshold.
All three checks pass — safe to close, with the phase angle drifting through zero roughly every 20 seconds.
Want the full step-by-step version?
The default thresholds shown are commonly-cited illustrative starting points, not a fixed standard — actual acceptance windows depend on generator size, prime mover type, and your protection engineer's or utility interconnection agreement's specific requirements. Always use your sync-check relay's actual configured settings, or your utility's interconnection requirements, for a real paralleling decision — this tool doesn't replace the relay.