When to use a synchroniser with a synch check relay

When doing sync check across a 22kV CB (30MW generator on one side, utility on the other) is it only sufficient to use a synchroniser relay or should a synchroniser be used in conjunction with a synch check relay?

I have always followed the path of both. Their outputs are effectively in series so both relays must issue a close release before closing can occur. The sync check relay’s settings have a slightly bigger margin than the synchroniser relay. A mate of mine reckons he’s always found the synchroniser only to be satisfactory.


Your mate may be right, but where will he be when it all goes wrong?

A check synchroniser is peanuts compared to the risk of destroying your 30MVA Generator.

Why not always use both devices? and why?

Well it is a question of cost, practicality and risk.

Start at the bottom; consider a 500kVA 400V distributed generator. Say on a landfill site. Manufactured as a factory packaged set, with the generator (synchronising) breaker located in the control panel. Auto synchroniser is part of the generator controller.

This is a relatively low cost set, little chance of any on site wiring changes causing a faulty synchronisation, and if it did, it is (usually) an unmanned set, so low chance of damage to personnel.

Assuming that the set is small compared to the rest of the power system, then a faulty synchronisation will result in damage to the 500kVA set, not to the 100MVA set 50 miles away.

Now consider your 30MVA 22kV set.

Big investment. Long lead time. Difficult discussion with boss when it all goes wrong……

High voltage breaker in a high voltage room, remote from the set. Control panel, remote from the set, in a different room.

Much higher chance of wiring errors during installation not being discovered during commissioning……

Much higher chance of modifications to site wiring closing that generator breaker when it should not close……. (“Why do we need to re commission the set when we just fitted an XYZ in the control panel”)

So: Auto Synchroniser in control panel in control room
Check Synchroniser in HV breaker panel in HV switchgear room. As you say, The sync check relay’s settings have a slightly bigger margin than the synchroniser relay. This should allow the auto sych to operate without the check synch opening.

Also remember the philosophy, Auto Synchroniser is a control device, so it is in the control panel. Check Synchroniser is a protective device, so it is in the protection panel.

Above is a snippet.

@Hoxton, Thank you for this incredibly thorough and logical reply.