Grid Resynchronization Check

The RMS voltage on the microgrid side must closely match the main grid side. A significant difference causes a reactive power surge upon breaker closure, potentially damaging transformer windings and sensitive power electronics.

• Typical tolerance: ±5% of nominal voltage
• Risk of mismatch: High inrush currents and voltage collapse
• Control action: Adjust smart inverter reactive power output or tap changer settings

Example: A 480V microgrid must be within 456V–504V of the utility side before the sync check relay allows closure.

The microgrid frequency must be nearly identical to the main grid's nominal frequency (50 Hz or 60 Hz). Even a small slip frequency creates a phase angle drift that can cause severe power oscillations and generator shaft stress.

• Typical tolerance: ±0.05 Hz to ±0.1 Hz
• Risk of mismatch: Sub-synchronous resonance and protective relay tripping
• Control action: Adjust governor setpoints on dispatchable generators or grid-forming inverters

Example: A 60 Hz system requires the microgrid to be between 59.95 Hz and 60.05 Hz before synchronization is permitted.

The voltage sine waves on both sides of the point of common coupling (PCC) breaker must be in phase. Closing with a phase angle difference forces the grids to instantaneously align, creating a synchronizing torque that can snap generator shafts or trigger protection.

• Typical tolerance: ±5° to ±20° depending on system stiffness
• Risk of mismatch: Catastrophic mechanical failure of rotating machinery
• Control action: Fine-tune frequency slightly to let the phase angle drift into alignment

Example: A sync check relay set to 10° will block breaker closure until the voltage zero-crossings on both sides are within 463 microseconds of each other.

The rotational order of the three phases (A-B-C vs A-C-B) must be identical on both sides of the breaker. A phase sequence mismatch causes motors to spin in reverse and creates a bolted fault condition upon closure.

• Verification method: Phase rotation meter or synchroscope
• Risk of mismatch: Instantaneous overcurrent trip and motor damage
• When checked: During initial commissioning and after any wiring changes

Example: A microgrid rewired after maintenance must pass a phase sequence test before the sync check relay is armed, as this is a hard-wired safety interlock, not a dynamic parameter.

The ANSI 25 relay is the protective device that continuously monitors voltage, frequency, and phase angle on both sides of the PCC breaker. It issues a permissive close signal only when all parameters fall within programmable deadbands for a set dwell time.

• Dwell time: Ensures stable alignment, not a transient coincidence (typically 100 ms to 1 sec)
• Live bus / Dead bus logic: Handles scenarios where one side is energized and the other is not
• Integration: Communicates with the microgrid controller via IEC 61850 GOOSE messaging

Example: A SEL-700G relay configured with a 5% voltage window, 0.1 Hz slip limit, and 15° phase angle maximum will supervise all automatic reconnection attempts.

The slip frequency is the small intentional difference between microgrid and main grid frequency used to rotate the phase angle into alignment. A positive slip means the microgrid is slightly faster, causing its voltage phasor to advance relative to the grid.

• Optimal slip: 0.05 Hz to 0.1 Hz for controlled synchronization
• Zero slip risk: Phase angle may never align if perfectly matched
• Control strategy: The microgrid controller adjusts a grid-forming inverter or generator governor to maintain a target slip until the phase angle enters the deadband

Example: At 0.1 Hz slip, the phase angle sweeps 36° per second, meaning a 10° window will be hit every 278 milliseconds, allowing fast and reliable synchronization.