Synchrocheck

The absolute difference in voltage amplitude between the two sides of the breaker. Closing with a large ΔV causes a sudden inrush of reactive power, leading to high transient currents that can mechanically stress transformer windings and generator shafts.

• Typical Setting: 0.5% to 10% of nominal voltage.
• Risk: Excessive magnetizing inrush current in transformers.
• Measurement: Calculated as |V_bus - V_line|.

The angular displacement between the voltage sine waves on either side of the breaker. This is the most critical parameter; closing at a large angle creates an instantaneous vector difference, resulting in severe mechanical torque and winding stress.

• Typical Setting: 5° to 20° for general synchronizing; < 5° for large turbine generators.
• Risk: Catastrophic shaft shear and winding displacement.
• Measurement: Continuously compared using zero-crossing detection or phasor calculation.

The difference in system frequency (Hz) between the incoming source and the running bus. A non-zero Δf causes the phase angle to constantly drift. The relay must predict the zero-phase crossing and issue the close command in advance to compensate for the breaker's mechanical operating time.

• Typical Setting: 0.05 Hz to 0.5 Hz.
• Risk: Out-of-phase closure if slip accelerates beyond the breaker's closing window.
• Advanced Logic: Monitors slip stability (dF/dt) to ensure the frequency difference is not diverging.

A permissive mode that allows closure when one or both sides of the breaker are de-energized. This logic bypasses the standard synchrocheck parameters to enable black start restoration or energizing a dead section of the network.

• Live-Dead (L-DB): Energizing a dead bus from a live line.
• Dead-Live (D-LB): Energizing a dead line from a live bus.
• Dead-Dead (D-DB): Closing on two dead circuits, typically blocked unless explicitly enabled for restoration.

A critical setting that inputs the mechanical operating time of the circuit breaker (from close coil energization to main contact touch). The relay uses this value to calculate the advance angle, issuing the close command slightly before the phase angle reaches zero so that electrical connection occurs precisely at the point of synchronism.

• Typical Value: 40 ms to 100 ms for medium-voltage breakers.
• Calculation: Advance Angle = 360° × Δf × T_close.

A configurable timeout that defines how long the relay will wait for the three parameters to align within the set limits. If synchronization is not achieved within this window, the close command is aborted to prevent a stale or unsafe closure attempt.

• Purpose: Prevents indefinite hunting during unstable grid conditions.
• Typical Setting: 1 to 10 seconds.
• Integration: Often linked to the auto-recloser sequence logic.

An automation function that automatically closes a tripped circuit breaker after a preset dead time. Synchrocheck supervision is critical for auto-reclose schemes on lines connecting two active sources to prevent out-of-phase re-energization. The recloser must receive a permissive 'close enable' signal from the synchrocheck relay before issuing the close command. For transient faults on radial lines, a simple dead-line check may suffice, but for interconnected grids, strict voltage magnitude, phase angle, and frequency difference limits must be satisfied.

A two-step control security mechanism that prevents unintended switching operations. The operator first selects a specific switchgear object, and the system returns a positive confirmation before the operate command is accepted. For circuit breaker closing sequences, SBO integrates with synchrocheck by validating that the selected breaker's associated synchronism-check function is healthy and enabled before allowing the operate step. This prevents manual closing attempts when the synchrocheck relay is out of service or bypassed.

A safety logic function that prevents dangerous switching operations by evaluating the real-time status of connected equipment. For a circuit breaker closing operation, interlocking rules verify that associated earthing switches are open and disconnectors are in the correct position before a close command is permitted. Synchrocheck acts as an additional permissive condition within the interlocking logic chain. If voltage conditions across the breaker are unsafe, the interlocking scheme blocks the close command regardless of other switchgear positions.

A device that interfaces with instrument transformers to digitize analog current and voltage signals, synchronizes them with a common time source, and publishes them as Sampled Values (SV) on the process bus. In a digital substation, the synchrocheck function resides in a bay-level IED that subscribes to SV streams from the bus-side and line-side merging units. The accuracy of the synchrocheck decision depends directly on the time synchronization accuracy and sampling fidelity of these merging units.

A network protocol defined by IEEE 1588 used to synchronize clocks throughout a substation network with sub-microsecond accuracy. For synchrocheck functions operating on digitized Sampled Values, PTP ensures that voltage measurements from the bus and line sides are aligned to a common time reference. A 1 µs timing error translates to a 0.018° phase angle error at 50 Hz, which is critical when permissible phase angle differences for breaker closure are often set to 10-20°.

A network redundancy protocol that provides seamless failover by duplicating frames over two independent, parallel Ethernet networks. For synchrocheck functions relying on GOOSE messages for interlocking or Sampled Values for voltage measurement, PRP ensures zero recovery time if one network path fails. The receiving IED discards the duplicate frame, maintaining uninterrupted synchronism-check operation even during a single network fault. This is essential for maintaining breaker closing availability in critical interconnection points.