Zone 2 time delays 1

[Mbrooke]

For an obsolete sub-transmission system chunk without active breaker failure schemes, would anyone see an issue reducing Zone 2 delays down to 8-10 cycles instead of the typical 20-25 cycle delay? In so far the reduced delay appears to be advantageous but I am sure I am missing something.

 

[krisys]

You might loose some selectivity with neighboring breakers. If you feel safety has precedence in your case, you may sacrifice a small selectivity.

 

[Mbrooke]

I would normally agree, but if all the other breakers follow the same time delay and are of the same type, in theory they should co-ordinate with on another?

 

[bacon4life]

Seems kind of strange to reduce the coordination margin for obsolete system. This does raise the risk of a larger outage for a slow to operate breaker or relay.

 

[Mbrooke]

I agree lol, but new generation is being added to the 69 and 115kv system. Simulated testing predicts that faults persisting longer that 15 cycles results in loss of synchronism and poll slipping for slipping for generation units near the fault.

 

[marks1080]

Wouldn't the generator have some kind of protection on it to take itself out?

 

[stevenal]

I wasn't aware that 20 to 25 was typical. What is your source? I assume you are coordinating with non-delayed zone 1 or other instantaneous protection downstream. You need to allow time for the downstream relay and breaker to operate plus a bit of margin. The Alstom/GE NPAG has good information on this. Link

 

[Mbrooke]

The generation does have protection, hence why it shuts down. However, when a fault occurs on the system there is a limited amount of time the generation can handle it before loosing syncronism as determined by fault studies, hence why newer systems are being built with differential or communication assisted tripping to speed up clearing time.


Zone 2 always coordinates with a non delayed Zone 1 which is set to about 85% for any given line being protected. 20-25 Cycles is typical for zone 2 as its usually coordinating with breaker failure. BF is set to 10 cycles allowing any given breaker to clear in 3 cycle plus a safety margin which then derives the zone 2 timing. I say usually- because in this case due to the age of the 69kv system and parts of the 115kv system there is currently no active BF protection, however over the years the bulk oil breakers have been replaced with 3 cycle SF6 breakers speeding up the overall clearing time.

The 69kv system is fed from the 115kv system, and the 115 from 345kv which contains the bulk generation. However, for various reasons generation is being added to the lower levels necessitating faster clearing times.

 

[davidbeach]

It seems like a communications assisted tripping scheme would be a better approach than simply speeding up the zone 2 time. I'd also work on getting in that missing BF protection.

Do you know how long all these breakers actually take to clear faults, particularly the first fault in many months? Not every breaker trips in the advertised time on the first try. Do you now have a system with your 20-25 that works well but will be rife with over tripping at 8 or 9 cycles? All those spec values look nice on paper, but sometimes reality likes to throw a curve ball. Pushing Murphy causes him to have tendency to push back.

 

[Mbrooke]

Communication would be ideal, but adding PLC, pilot or OPGW to an existing system takes cost. In so far the only practical option (for the budget) to my knowledge would be a leased private phone line which I am considering.


I'm glad you brought up tripping speed. New SPS2 SF6 breakers are advertised to trip in 3 cycles, but the fault studies are simulated with a 5 cycle breaker tripping time (6 total). 3.5 cycles on 2 cycle 362kv breakers.

The idea is that since BF is rare, and already covered by zone 2/3/4, and a 10 cycle BF timer has proven well on the 345kv system (breakers that work are opening before that) reducing the zone 2 time just above the maximum worst case zone 1 clearing (8-10 cycles) would provide fast enough tripping speeds to prevent generation instability. Of course the other option is keeping a 25 cycle zone 2 at the risk of tripping local generation.