Supplying network load 2

[Mbrooke]

Is it always absolutely necessary to parallel MV buses when supplying spot and secondary network feeders? No phase angle differences on the 138kv bus.


Certain scenarios will require a bus section to be taken out of service (network stays on through N-2) breaking the normally closed topology. I'd like to eliminate running bus or cable to tie the two ends together forming a ring.

 

[davidbeach]

There’s at least one network system where four source buses are only paralleled at the 115kV level. The transformer LTCs all step together but otherwise the 13kV buses are independent.

 

[Mbrooke]

Good to hear its been put to the test out in the real world. Otherwise its normally closed operation with everything in service.

 

[wroggent]

It will depend a lot on how the network transformers are distributed across the substation transformers and their relative sizes and impedances. How well the load on the buses and feeders are balanced will make a difference too. It doesn't take much voltage magnitude or angle difference to make things go awry. At the last place I worked none of the buses were connected on the 12kV side - a decision made decades ago to limit available short circuit current. Conditions that could actually occur varied based on the substation in question but we had only one set of standards for new construction to keep things simple. The possibility of circulating current on 3+ network transformer sites severely limited the capacity of our installations: 2x 2MVA for a 3000A service, 3x 2MVA for a 5000A service, 4x 2MVA for a 7000A service and 5x 2MVA for a 8000A service (all 480V). Doesn't look to bad but in reality the planners that put that standard together assumed (incorrectly) the NEC would prohibit our customers from loading those services to more than 80% of their rating. Also, our 2MVA transformer ratings were based on a 55C rise, for n-1 conditions they utilized the 65C rating. If your network protectors have not been set to operate in this type of environment, you will likely end up with some of them being stuck open indefinitely, which could also cause overloading problems. You could also experience pumping in the network protectors.

 

[bacon4life]

I don't know if my experience is at all applicable to your system...

At least one Grid network is fed from two different stations. This configuration makes regulating voltage more difficult as well as increases the likelihood of pumping network protectors. Each LTC step difference between the buses will drive a large amount of reactive flow. Also, be sure the equipment has capacity to handle the expected change in both real and reactive power flows.

Anecdotally, the angle difference on the 15 kV bus is much more impacted by the number of transformers in parallel at each substation and by the loading on those transformers than by the angle difference in a few miles of 115 kV transmission. When taking a transformer offline at one of our station that feeds a mix of loads, some radial load is shifted to a different substation so that the loading of the still in service transformer stays constant, which keeps the network flows constant.

 

[Mbrooke]

@Wroggent: Its 5 65MVA transformers, all of equal impedance and tap in sync. Each transformer feeds its own bus section separated by circuit breakers with two feeds coming from each bus section for a meshed secondary network and one feed from each section for spot networks consisting 5 transformers. In the least I do not need to worry about the spots despite contingencies pushing them to 100% loading?


@bacon4life: While applicable here, I can grantee you with certainty this scenario will come up within 24-72 hours lol. Noted and bookmarked- thank you!