Synchro check or sync check relays are used to verify that voltage, phase angle, frequency and phase rotation across two sides of a breaker are the same prior to closing the breaker. Synchrocheck relays ensures that bus and line side voltages are within programmed differentials of voltage magnitude, phase angle, frequency and phase rotation is the same. The permissive from this relay can then be used for either manual or automatic source paralleling. Out of ‘sync’ closing can create significant impact on the power system and in worst case can damage the equipment due to high short circuit currents. The sync check relay is ANSI element ‘25’.

The basic method of synchronism check in a modern digital relay is shown in the figure below. Three phase input (running source) from one side of breaker is fed to the three-phase voltage input of the relay. A separate single-phase input from the other side of the breaker is used to provide the ‘sync’ voltage input (incoming source) to the relay.

Synchrocheck Relay Settings

For successful synchronization, voltage, frequency, phase rotation and phase angle of the two sources must meet the thresholds set per the user programmable settings in the relay.

Voltage: Voltage settings for sync check relays have a voltage HI and voltage LO settings. If measured voltages across the breaker fall within this band then the corresponding relay word bits will be asserted (TRUE) indicating healthy voltage.

Frequency: If the voltages are found to be good in the step above, relay then calculates ‘slip frequency’ which is the difference of frequency between the running phase voltage (fp) and incoming sync voltage (fs) input.

For example, if the running frequency is 60Hz and the incoming frequency is 60.1Hz the slip frequency is -0.1Hz and the slip angle is 360. This means that in a time period of one second, incoming source (fs) will zoom past running source (fp) by 36 degree. Or in other words the angular distance between running source and incoming source changes by 36 degrees in one second.

Frequency parameter to set in the relay will be ‘Max Slip Frequency’. If the measured slip is greater than the set value, the synchrocheck relay will not give permissive for breaker to close. While two utility sources should have very low slip (<0.005 Hz), a utility-generator or a generator-generator combination can have higher slip frequency threshold.

• Phase Rotation: The phase rotation can be ABC or ACB. One way to program phase rotation check is to use ANSI 59Q (negative sequence overvoltage) element in the breaker close logic. If negative sequence voltage is detected (Opposite phase rotation will result in high negative sequence overvoltage) the breaker close logic will prevent breaker from closing.

• Phase Angle: Phase angle settings for sync check relay is Max Angle. If the angle between the two sources is greater than this value, close permissive is not given. Within this there could be two options depending on the relay.

Modes of Operation for Sync Check Relays

Synchronism check relays can include various operating modes beyond the standard sync check function, allowing the output contacts to close when either the line or bus is de-energized.

The primary options available are:

1. Normal Synchronism Check: Ensures both the line and bus are live and within acceptable voltage, frequency, and phase angle limits before closing.

2. Dead Line, Live Bus: Allows closing when the bus is live, but the line is de-energized.

3. Dead Bus, Live Line: Permits closing when the line is live, but the bus is de-energized.

4. Dead Line, Dead Bus: Allows closing when both the line and bus are de-energized, depending on system requirements.

    

The pickup indicator lights up when the voltage difference between the Line and Bus meets the preset criteria, or under dead-bus, dead-line conditions. The voltage thresholds at which the line or bus are considered “live” can be adjusted independently within the relay settings.

A common setting is 30 volts, with an adjustable range from 0 to 120 volts. In each mode of operation, if both sources are live, a normal synchronism check is performed. Sync relays with multiple operating modes can also be configured to only monitor for the standard sync check function.

Testing and Maintenance of Synchro Check Relay

Same as with any different IEEE class of relay, testing of Synchrocheck protective relays should always start with a detailed visual and mechanical inspection. The specific items to check and inspect will differ based on the relay type—whether it is electromechanical, solid-state, or microprocessor-based. Each type has unique components and failure points that need to be assessed accordingly during inspection.

Check the functional operation of each element in the protection scheme by performing the following steps:

1. Verify the closing zone at rated voltage.

2. Measure the maximum voltage differential that allows closing at zero degrees.

3. Confirm the set points for live line, live bus, dead line, and dead bus conditions.

4. Determine the time delay settings.

5. Check the advanced closing angle.

6. Test and verify the control functions for dead bus/live line, dead line/live bus, and dead bus/dead line conditions.

These checks ensure that each protection element operates correctly within its designed parameters.

Various test units can follow this procedure, you need to be sure that your kit will be able to provide a selectable output frequency and a standalone voltage source to compare to.

If you want to add functional testing, or check full operation of sensing elements and intercommunication of relays on a larger scale – IEC 61850 protocol could help. Advanced secondary injection test sets like EuroSMC Quasar or Omicron CMC 356 are great tools to cover single-phase and 3-phase sync check routine.