LA DWP Substation Transformer Impedances

[kartracer087]

Hello,

Just was curious if anyone knows approximately what LADWP's RS substation transformer impedances are. I know their breakers at their substations were rated 25,000A for many years and the IEEE standard rates those at a test X/R of 17. Based on that the minimum impedance considering infinite primary would be :

25,000A interrupting at X/R = 17 considering a system X/R ration of 30 ---> maximum allowable current that the breaker could safely interrupt would be somewhere in the range of 21,000A symmetrical given IEEE test X/R for breakers is 17 which is less than estimated system X/R of 30, so my de-rate to 21,000A is suitable. X/R for transformers with 100MVA base is about 30-35, roughly. So it would need to interrupt roughly 21,000A given the X/R of 30 to not exceed the tested interrupting capability of the breaker.

My figuring on 100MVA base (100/130/160MVA transformer): 230kV-34.5GY/19.92

21,000A = 1,255MVA -----> 100MVA/1255MVA = 7.97% minimum impedance.

Of course, if anyone here knows roughly what their bus current fault levels are you could back-calculate in the same way. I do know that they use S&C Vista switches on their distribution system so I'd imagine fault currents would be limited to approximately 25,000A in a non-bus tied situation (off of one transformer).

LADWP's stations are usually double bus-double breaker or breaker and half and their new station breakers are rated 40kA symmetrical. So in a tied situation you would still have half the current interrupted by one breaker and the other half by the other. The only exception to this would be if you had a situation per my attached sketch. In this case the full 40kA would be required to be interrupted by the breaker in question, so in this case if the interrupting ratings of the breakers are 40kA symmetrical, you would realistically need to reduce fault currents down more so that when two transformers are tied and one breaker would be required to interrupt the fault on a line bay, you would be no greater than say 37kA. In that situation, maximum fault current from each transformer would be limited to about 18,500A. Doing the same analysis as above, on 100MVA base, impedance would be 1105MVA short circuit or 9.05% minimum impedance.

Anyone have any ideas on what their impedance values actually are per spec on 100MVA base?

 

[davidbeach]

I don't know what LADWP does, but you'd have a horrible time trying to back into transformer sizes and impedances on our system based on what we tell people to use for equipment ratings. One goal, probably common to essentially all utilities, is that we never want to have to go back and tell a customer that their equipment has become underdutied because of a change we've made.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[kartracer087]

True, I also believe there are some trade-offs for the utility practice, I know some utilities have equipment that would be overdutied by calculation. An example of that would be fuse cutouts, its often that the available short circuit current near a substation exceeds the asymmetrical duty of the cutout, for example. It is a far less common practice, though, to not take into consideration interrupting duties of station breaker with respect to system design practices because they are the most important device on the circuit in terms of being able to clear a fault. A fuse cutout may fail from an excessive short circuit, for example, but ultimately it will still extinguish the fault and worst case scenario then the breaker will trip on a high fault if for some strange reason the fuse can't interrupt the fault. The worst case is the fuseholder would rupture but still extinguish the fault.

The other thing is sometimes utilties give the users their "closed tie" fault currents which are much greater than usual operating conditions where circuits are not tied. Hypothetically this is the worst case scenario, but one that is incredibly rare and you are operating under controlled conditions many times when circuits are tied in a situation where they normally are not.

 

[davidbeach]

I would certainly think/hope that equipment sizing duties for primary service customers would be for that rare tie closed case. It exists. Faults happen. Murphy loves corner cases.

I’ll see your silver lining and raise you two black clouds. - Protection Operations

 

[dpc]

There are ANSI standard impedances for large transformers based on the primary voltage. Probably a reasonable starting point. Or you could ask them.

 

[cranky108]

Besides it is typical to purchase breakers with higher ratings, because it reduces the number of maintenance cycles needed in respect to replacing breaker contacts.
Then again, maybe that breaker was the lowest bid at the time they were purchased. Or they may purchase breakers for the worst in there system application.