Synchronism-check permits closing only when the electrical conditions on both sides of an open breaker are acceptable. It protects equipment from severe current and torque caused by out-of-phase closing.
Learning objectives
Set voltage, angle and slip limits, define dead/live bus logic and coordinate command delay with predicted angle movement.
Core engineering principles
Both sides require valid voltage measurement
Separate VT sources identify source and bus conditions. Fuse failure, wrong VT selection or phase rotation can create a false permissive.
Angle limit depends on system strength
A strong interconnected system can produce very high closing current from a modest angle difference. Generator shafts and transformers may impose tighter limits.
Frequency difference predicts angle movement
Slip frequency causes the phase angle to change during relay, communication and breaker closing time. Compensation or predictive closing may be required.
Dead-bus conditions are separate logic
Energising a dead bus from an approved live source may bypass angle comparison, but the scheme must reliably distinguish dead bus from failed VT.
Close time is part of the calculation
The relay decision occurs before main-contact touch. Breaker closing time and variation determine the actual angle at energisation.
Engineering application method
- Step 1: Identify voltage sources on both breaker sides.
- Step 2: Define live, dead and VT-healthy thresholds.
- Step 3: Set voltage difference, angle and slip limits.
- Step 4: Include command path and breaker closing time.
- Step 5: Test live-live, dead-live, wrong phase and VT-failure cases.
Practical example
A 10° angle at relay output can become 25° by contact touch if slip is high and the breaker is slow. Predictive compensation or a smaller permissive window is needed.
Common mistakes
- Checking angle without slip.
- Using one VT for both sides.
- Treating a failed fuse as dead bus.
- Ignoring breaker close-time scatter.
- Enabling dead-bus close from either side without source rules.
Design and review checklist
- Are both VT circuits independent and healthy?
- What angle and slip are permitted?
- Is closing time included?
- Who may energise a dead bus?
- Are wrong-phase tests performed?
Standards basis and official sources
- IEC 60255-1:2022 — Common requirements for measuring relays and protection equipment.
- IEC 61869-1:2023 — General requirements for instrument transformers and low-power instrument transformers.
- IEEE C37.2-2022 — Device function numbers, acronyms and contact designations.
Engineering note: Verify the contracted standard edition, amendments, manufacturer evidence and project-specific studies before applying these principles to a supplied assembly.