Generator Inter-turn protection

[RRaghunath]

Generator inter-turn protection is not widely employed these days. What could be the reasons!
Is it because of the change in the design of present day design of generators, meaning no two turns are being laid in the same stator conductor slot etc.?
Appreciate your help!

 

[RRaghunath]

Looking forward to have opinion of Mr. Muthu (Eng-tips handle 'Edison123') the rotating machinery expert as well as others on the forum.

 

[waross]

Cost.
Back in the days before T frame motors were available, I was working in a lumber mill.
In the early days of mill electrification, motors were treated brutally.
We had a lot of motor burn-outs.
Mostly in the 5 HP to 75 HP range.
Original motors did not have extra inter-phase insulation. Re-winds did have inter phase insulation.
It made a difference.
We tried to use rewound motors for more demanding applications.

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

 

[edison123]

Hi Mr Raghunath

Unlike earth fault, phase to phase fault, etc., inter turn faults are difficult to detect due to their localized nature. Normally, these turn faults eventually turn into earth fault puncturing ground wall insulation or phase to phase to fault in end winding and EF/differential will pick it up.

In large 2 pole generators, the stator winding is mostly of single turn Roebel bars and hence the concept of inter turn insulation failure does not apply here. Multi turn diamond coils are more in common in multipolar hydro generators. In those cases, I have seen what is called cross differential protection being used to detect turn faults, which of course adds to the cost of the protection. Here, the stator winding is usually split into two halves with each half having its own identical CT and their secondaries are cross connected to detect tun faults. (Of course, for still better detection & protection, you can split the winding into more parallel paths with each section with its own set of CT's but that would be still additional cost driver.)

The schematic of cross differential for interturn protection.

Hope I have answered your question (of why it is not being included now a days), which, thanks to bean counters, boils down to additional cost justification for the interturn protection but I would consider it a critical protection to be included in the project design stage.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Thanks Muthu Sir, for the insight.
You mentioned "but I would consider it a critical protection to be included..." - I suppose you mean only for machines with multiple turn diamond coils.
Appreciate if you could advice which other machines (apart from Hydro-generators) are manufactured with multi-turn diamond coils.
Do Synchronous motors in 10-30MW range qualify??
Thanks once again.

 

[davidbeach]

The winding configuration that Muthu shows doesn't need a separate 87 as shown; it can be protected using a conventional generator differential. On the single connection at the terminal end of the machine use a full sized CT and at the neutral end, on one of the splits, use a CT with half the rating. The most recent I did it was 6000:5 on the terminal and 3000:5 on the neutral end of one split and tell the relay that it has a 6000:5 on both ends. Works well.

 

[edison123]

Yes, this protection is applicable for multi-turn diamond coils, which are used in induction motors, induction generators, synchronous motors, synchronous generators, which are basically the same stators.

david - I doubt conventional differential will pick up localized turn faults until they turn into earth fault or phase to phase fault.

Muthu

http://www.edison.co.in

 

[RRaghunath]

Thanks Sir.
Coming to protection scheme, the connection shown is for High impedance circulating current differential protection and can be implemented in any numerical relay with three phase overcurrent elements. In olden days, MCAG34 (Alstom make electro-mechanical relay) used to be popular for this application.
The numerical generator protection relays such as 7UM62 of Siemens are offering the inter-turn protection based on voltage differential protection principle that can be applied the split winding terminals are not accessible for mounting CTs in each of the windings.

 

[edison123]

Hi Raghunath. I am aware of basic concept of protection theory than the nuts & bolts of protection relays. Modern numeric protection could do all sorts of things, I guess.

Muthu

http://www.edison.co.in