When to use a synchroniser with a synch check relay 2

[veritas]

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.

Any comments?

Thanks in advance.

 

[Hoxton]

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.

 

[ScottyUK]

Very true Hoxton. On a reasonably large machine the cost of a check-sync relay is trivial. It provides valuable backup to the auto synchroniser, items which are definitely not infallible, and it prevents someone doing something silly and forcing an out-of-step closure of the breaker - probably while they're trying to figure out what is wrong with the auto synchroniser. It may be difficult to explain to the authorities why bits of the LP turbine are in a field half a mile away, let alone explain it to the insurers. ;-)

 

[veritas]

Hoxton and Scotty - many thanks. Appreciated.

 

[Marmite]

Agree with what the others have said, but also these days the check synch function doesn't need to be a discrete relay, it can simply be a function within an existing IED, which makes the cost very minor.
Regards
Marmite

 

[davidbeach]

Marmite is right on, the cost of that synch check function should be just the cost of two wires to an output of an existing relay.Those two wires are just about the cheapest insurance you can buy. Hoxton covered the issue well.

 

[slavag]

Synchrocheck function is part of most newer generator protective terminals or GCB bay control units.
From my point of view synchrochek function is not function of size of generator or cost, it is must.
and from my experience, it will be connected to other winding or phases of VT ( PT ) than autosynchronaisez .

 

[cranky108]

Do you trust having both functions in the same box? I would not, which is the point of having both.

 

[Marmite]

None of us are advocating having the auto synchroniser and check synch in one box, Cranky.

Regards
Marmite

 

[veritas]

I typically have them in separate systems. If part of feeder schemes, one goes with the Main 1 scheme and the other with the Main 2 scheme. Also separate DC supplies.

 

[waross]

I think that this is what cranky was refering to by "both functions in the same box"

but also these days the check synch function doesn't need to be a discrete relay, it can simply be a function within an existing IED

the cost of that synch check function should be just the cost of two wires to an output of an existing relay

Synchrocheck function is part of most newer generator protective terminals or GCB bay control units.

These are valid comments but may introduce a common point of failure.
I share cranky's concern, and when the budget permits or management allows I prefer the added protection of a seperate sync-check function.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Are synchronising relays getting integrated into bay computers these days? I haven't seen that before. Sync-check relays plenty times but not a synchroniser. Not sure how likely a 'one box' solution actually is.

 

[davidbeach]

The SEL-700G can do both auto synch and synch check in the same box. I'd use two relays, only one of which can be use to do the auto synch, so the other one can do the synch check. So, yes, all in one box is possible, but letting the fox guard the hen house is always a bad idea.

 

[ScottyUK]

Thanks David - my little bit of learning for today.