Synchronism Check 25 (RSYN): What It Is and How It Works

What function 25 (synchronism check) is

Closing a breaker that ties two energized parts of the system is safe only if the two voltages are similar in magnitude, frequency and angle. If they are not, at the instant of closing a current flows that depends on the vector difference between the two voltages. That current can damage transformers, cause torque shocks on generator shafts and operate protections that have nothing to do with the switching.

Function 25 measures the voltage on each side of the breaker and grants a close permission only when all three differences are within limits. It does not close the breaker by itself: it supervises manual, remote and automatic reclosing commands.

• ANSI/IEEE C37.2: function 25, synchronizing or synchronism-check device.
• IEC 61850: logical node RSYN (synchronism check or synchronizing).
• IEC 60617: it has no symbol as widely used as other functions; on drawings it usually appears with the manufacturer’s acronym or as “SYNC”.

Where it is applied

• Transmission lines with automatic reclosing, so the second terminal closes only if the two parts of the system are still in synchronism.
• Bus couplers and breaker-and-a-half or ring bus arrangements, where a close can tie two parts of the system together.
• Generators, as supervision of the automatic synchronizer and of manual closing.
• Solar, wind and storage plants at their point of interconnection, especially when they can operate as an island.
• Industry with cogeneration or with standby plants that are paralleled with the grid.

How it works

The relay receives a voltage from each side of the breaker (bus and line, or bus and generator) and calculates three differences:

• ΔV: magnitude difference.
• Δf: frequency difference, which also defines the slip rate.
• Δφ: angle difference.

If all three stay inside their windows for a minimum time, permission is granted. When the two systems are separate and have different frequencies, the angle rotates continuously. Some relays calculate when the angle will pass through zero and advance the close command by the breaker operating time. This is called synchronized closing, and it is done by a synchronizer, not by a simple check.

Dead-voltage modes. To energize a line or a bus that is dead, the function permits closing if the enabled combination is met:

• Live bus and dead line.
• Dead bus and live line.
• Both dead.

Each mode has live-voltage and dead-voltage thresholds and is enabled according to the operating philosophy. On lines with reclosing, it is common for one terminal to energize the line in dead-line mode and the other to close with synchronism check. More detail in automatic reclosing 79.

Compensation. If the voltages come from different sides of a transformer or from different phases (for example, a phase-to-neutral voltage on one side and a phase-to-phase voltage on the other), the relay needs angle and magnitude compensation. Without it, the measured angle will never match.

Setting criteria

• ΔV, Δf and Δφ windows: they depend on the location in the network, the system strength and the equipment. For distributed generation, standards such as IEEE 1547 set synchronization limits according to the size of the resource; in transmission they are set by the utility or the study.
• Maximum angle on lines: in a meshed system there can be a steady angle between terminals due to load flow. The window must allow it without accepting dangerous closures; this is confirmed with load flow studies.
• Supervision time and, if there is synchronized closing, the actual breaker closing time measured in tests.
• Live and dead voltage thresholds, taking into account voltages induced by parallel lines.
• Phase selection and compensation of the voltage transformer ratio and angle.

The final values come from the coordination study and the utility’s requirements.

How each manufacturer implements it

• Siemens SIPROTEC with DIGSI: synchronism check can work as an independent function or tied to breaker control, with its dead-voltage modes.
• SEL with AcSELerator QuickSet: synchronism elements with angle and slip windows, whose permissives are used in the programmable close logic.
• Relion with PCM600: in the Hitachi Energy 670 and 650 series, a synchronism block with separate functions for synchronism check, energizing check and synchronizing; in the ABB 615 and 620 series, synchronism and energizing check.
• GE Vernova (Multilin) with EnerVista: synchrocheck elements with dead-voltage modes, combinable with FlexLogic.
• Schneider MiCOM or Easergy with MiCOM S1 or Easergy Studio: synchronism check tied to reclosing and control.

In practice the differences are how the reference voltage is selected, how the angle is compensated and whether the permission acts on every close or only on automatic ones.

Why it trips or blocks

The 25 does not trip: what clients report is that it will not let the breaker close. Typical causes:

• Wrong phase selection or missing angle compensation.
• Reversed polarity of a voltage transformer, which adds 180° to the measured angle. The voltage transformer polarity check rules it out.
• VT fuse or miniature circuit breaker open, which the relay reads as dead voltage, or which blocks the function.
• Actual load angle larger than the set window.
• Dead-voltage mode not enabled for the actual condition.
• Induced voltage on a de-energized line, which prevents it from being recognized as dead.

And the opposite, more serious case: an out-of-phase close because the function was disabled or its windows were too wide. If that happens, besides the relay, the breaker should be checked with circuit breaker testing.

How it is tested

With secondary injection and a test set such as the Omicron CMC 356, two independent voltage systems are injected. The usual checks are:

• The ΔV, Δf and Δφ limits, moving one variable at a time.
• Each dead-voltage mode and its thresholds.
• Angle and magnitude compensation.
• If there is synchronized closing, the advance time with the actual breaker.
• That the permission supervises the right close commands: manual, remote and reclosing.

Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.

References

• IEEE C37.2, Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations.
• IEEE C37.104-2022, Guide for Automatic Reclosing on AC Distribution and Transmission Lines.
• IEEE C37.102, Guide for AC Generator Protection.
• IEEE 1547, Standard for Interconnection and Interoperability of Distributed Energy Resources with Associated Electric Power Systems Interfaces.
• IEC 61850-7-4, Compatible logical node classes and data object classes.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions