Relay vs PLC contorl for M-T-M transfer scheme

[rockman7892]

In MV & LV Swgr is there a preferred method or better industry practice for implementing a M-T-M breaker transfer scheme when considering relay based control vs a dedicated PLC? Are there pros and cons to each or is it typically a customer preference?

 

[davidbeach]

What do you want it to do? There’s myriad ways an M-T-M scheme can work; some functions better suited to the protection system and others that are fine for a control system to handle. The choice of relays will also enter the equation, a top end relay can do all sorts of control without issue while a lower end relay might do just fine with the protection but not have any control possibility.

 

[thermionic1]

I suppose if your relay is OOS for whatever reason and you need a transfer...

On the other hand, a M-T-M transfer scheme can be implemented with SEL relays and mirrored bits with a few serial cables.

The PLC route would need some loss of voltage sensing relays, Auto/Manual switches, associated wiring, etc. If the protection relays have enough available programmable push buttons & display points it is less wiring.

If this is a new installation / design either is probably just as easy. If you are trying to install into an existing location, consider that the protection may be disabled during installation and testing. One way or the other, the transfer scheme has to be wiring into the MTM trip / close scheme.

I would also consider the end user preference and comfort with pressing buttons on relays. Some non utility maintenance staff may be wary of touching a relay.

 

[DM61850]

If you used relays for your MTM scheme, you will have all the functions you care about built into it and you can include protective functions into those relays as well. I am coming from a protecton background and not a controls background but I would use a relay for any function that involved any relaying. The only PLCs that I have been used in protective schemes were interacting with mechanical relays and it was just to avoid needing to include hardwired logic. You can put all that logic into a microprocessor relay so I fail to see why anyone would use a PLC unless it was already there for some other function. You can get 3 SEL-751 relays for under 3k and that will be enough for your scheme and you'll be able to use them for breaker failure and other relaying.

I don't know what kind of facility you work at but if it falls under NERC jurisdiction there may be reasons that you wouldn't want protective function put into a PLC. At one company that I worked at, they had a generating plant that had protective functions built into the DCS to protect the transformer from overloading. The unit tripped on these functions. The problem that you run into when you do stuff like this is that there are NERC standards that dictating how protective relaying needs to be set and tested. If you start putting protective relaying functions into your PLC or DCS, you are not creating a segregation between your plant controls and plant relaying. Without segregation, you potentionally could drag your plant controls system under the NERC umbrella. I know you said that it was a dedicated PLC but if you use one that is undedicated, you potentially could get involved in stuff you never intended.