Transformer Primary Protection - Relay vs Fuse

[rockman7892]

When considering transformer primary protection on a 13.8kV or 4.16kV transformer can better protection be achieved with a fuse or a relay?

From what I've heard and read the fuse provides better short circuit protection but does not provide good overload protection while the relay provides better overload protection but does not provide good short circuit protection.

Is there a better or preferred method for how transformer is protected on primary or does it come down to cost and system topology? A lot of times I see them protected on the primary with both fuse and relay with the relay located at the switchgear feeder and the fuse located at a MV switch at the transformer location. I'm assuming using this fuse and relay combination protection provides the best protection but the fuse may cause coordination issues with upstream relays?

From an overload perspective I have always viewed this as being provided by the secondary protection, so any overload protection on the primary is just backup protection for the secondary protection.

 

[davidbeach]

You can have a current limiting fuse, which means it fully interrupts in less than half a cycle, while the fastest you'll get with a relay and breaker is about 3-4 cycles. There are no other advantages of using a fuse. The right relay, particularly a differential relay, will allow tripping for certain faults that the fuse would never respond to; with those faults and fuses you simply have to wait for the fault to become something else that the fuses will blow for.

 

[wbd]

The other items to consider is cost and the real estate needed. For a fuse it is rather simple to install and maintain as they usually on the structure where the primary comes in. For a relay you would need a breaker or circuit switcher and that comes with a lot of additional cost and maintenance requirements. Unless it is some very critical application, I usually see fuses on transformers up to 25MVA.

 

[rockman7892]

In terms of primary protection providing overload protection I typically see relay pickups and fuses being sized at about 1.5x-2x transformer primary full load current. In the US the NEC allows for up to 300% primary overload protection.

Should a lot of emphasis be put on the primary protective device in terms of overload protection or is this primarily the role of the secondary protection?

If the relay or curve or fuse curve sits under the transformer through-fault damage curve then it appears that they would both provide the same level of protection for through faults. I suppose the only difference would be a currently limiting fuse may be able to react quicker for internal transformer faults and perhaps minimize the damage that may otherwise occur with relay 50/51 protection?

 

[jghrist]

If this is for an industrial setting where NFPA 70E must be followed for arc hazard analysis, a breaker on the primary side may considerably lower arc incident energy. Where high side fuses are the fault clearing devices for low side faults, very high incident energy can result. With a high side breaker, CTs on the secondary can operate a relay for low side faults and provide much faster clearing times.