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.