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What is the difference between a Phasor Measurement Unit (PMU) and a Synchro-check relay?

I need to know if Synchro-check relay can be used as replacement for PMU in a Synchrophasor data system.

We will be incorporating this synchrophasor data system with Automatic Generation Control (AGC) of a Diesel Generation during grid synchronization.

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Disclaimer

It sounds like you are working with utility-scale power generation. You can cause expensive equipment damage if you get this wrong.

Therefore, if in doubt, consult a power electrical engineer who is qualified to practice in this field, in your country.


A phasor measurement unit is a control device. It outputs numerical measurements. Its job is to provide numerical inputs for a control system.

A synchronism check relay is a protection device. It compares voltages on each side of a switch. Its job is to ensure the switch can only be closed if the voltage magnitude and phase are equal on each side of the switch.

The output of a sync check relay is binary:

  • YES, the voltages are matched - switch can be closed.
  • NO, the voltages are not matched. - the switch must not be closed.

Synchronism check is an important protection function, so it is kept as simple as possible.


A sync check relay outputs a binary yes-no signal.

Your automatic generator controller would require numerical voltage inputs - or at least four binary signals, "VOLTS RAISE", "VOLTS LOWER", "FREQUENCY RAISE", "FREQUENCY LOWER".

The sync check relay can't provide these signals.

A sync check relay cannot, and should not, be used to replace a phasor measurement unit.


In general, it is poor practice to mix control functions with protection functions - it is best to have the protection functions separate and as simple as possible. This reduces the chance that a fault on the control circuits will disable the important protection functions.

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  • \$\begingroup\$ Sir Li-aung Yip, is my understanding correct? PMU controls Synch check relay. Synch Check relay controls breaker. Thank you! \$\endgroup\$ – ADP Oct 24 '15 at 1:25
  • \$\begingroup\$ Your understanding is not correct. The PMU does not control sync check. Recall my remark above, that the sync check relay should be as simple as possible, to minimize exposure to faults. I do not want my sync check relay to fail because of PMU failure. Therefore, there should be a hard-wired VT connection to the synch check relay with no other devices in-between. \$\endgroup\$ – Li-aung Yip Oct 24 '15 at 13:52

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