Synchronism Check Function vs. Synchronizing Function: Roles, Differences, and Applications

​1. Introduction to Synchronism Check Function in Line Protection relays​

          The ​synchronism check function​ (coded SYN or 25) is a safety interlock that prevents circuit breaker closing during unsynchronized conditions. It blocks closing commands when voltage difference (ΔU), frequency difference (Δf), or phase-angle difference (Δθ) exceed thresholds, thereby avoiding high inrush currents and power oscillations. Its ​core objective​ is to validate electrical synchronization between two sources (e.g., busbar and line/generator sides) before permitting manual/automatic closing.

​2. Introduction to Synchronizing Function in Synchronizing relays​
          The ​synchronizing function​ in auto-synchronizing relays enables smooth interconnection of independent power sources. As a closed-loop control system, it achieves near-zero deviations (ΔU ≈ 0%, Δf ≈ 0 Hz, Δθ ≈ 0°) through real-time monitoring, dynamic parameter adjustments, and predictive closing. Its ​core value​ lies in active regulation and sub-second timing control.

​3. Application Scenarios of Line Protection Synchronism Check​

• ​Manual Closing/Interconnection: Validates synchronization before human-initiated breaker closure.
• ​Auto-Reclosing: Enables reclosing only if sources synchronize after a fault trip.
• ​Bus Transfer Schemes: Checks synchronicity for automatic transfer to standby sources.
• ​Generator Synchronization (Backup)​: Secondary verification in simple systems.

​4. Application Scenarios of Synchronizing relays​
The ​synchronizing function​ specializes in precision-critical tasks:

• ​Generator Synchronization: Essential for fossil/nuclear/hydro units.
• ​Black Start Restoration: Rebuilds de-energized networks.
• ​Tie-Line Interconnection: Enables stable regional grid interlinks.

​5. Working Principles: Line Protection Synchronism Check​

• ​Measurement: Monitors voltages (Ubus, Uline) via PTs.
• ​Calculation: Computes real-time ΔU, Δf, and Δθ.
• ​Prediction: Estimates Δθclose at future contact closure:
  Δθclose = Δθnow + ωs × tclose
• ​Interlock Logic:
  Enables closing if Δθclose, ΔU, Δf within limits.
  Blocks closing + alarms on deviation.

​6. Working Principles: Synchronizing relays​

• ​Startup: Verifies voltage validity & receives sync command.
• ​Parameter Regulation​ (key differentiator):
  ​Coarse Adjustment: Reduces Δf < 0.3 Hz, ΔU < 5%.
  ​Fine Tuning: Optimizes to Δf < 0.1 Hz, ΔU < 1%.
• ​Closing Timing:
  Calculates slip angular velocity (ωs) and Δθclose.
  Issues closing pulse when Δθclose enters window (e.g., |±2°|).
• ​Post-Closing Check: Confirms successful interconnection.

​7. Key Differences Between the Two Functions​
​Line Protection relay Synchronism Check:

• Role: Safety gatekeeper (“Can we close?”)
• Action: Passive verification
• Precision: Tolerates Δθ < 20°
• Actuation: Enables external closing circuits
• Primary Use: Transmission auto-reclosing & manual switching

​Synchronizing Device Function:

• Role: Active interconnector (“Achieve perfection & close”)
• Action: Closed-loop voltage/frequency/phase regulation
• Precision: Targets Δθ < 2°
• Actuation: Directly issues closing commands
• Primary Use: Generator synchronization & tie-line paralleling

​Conclusion​
          The ​synchronism check​ in line protection relay provides vital safety interlocks during routine grid operations. Conversely, the ​synchronizing function​ in dedicated equipment enables mission-critical generator/tie-line interconnections through closed-loop control. While synchronism checks prevent catastrophic errors, synchronizing relay achieve near-perfect synchronization via active parameter regulation and predictive algorithms.