T&D Upgrading Plan Philosophy

[CKent]

I am a T&D planning engineer for an electric utility.
I am doing bench marking/ cross referencing involving different utility practice for planning distribution facilities.

From my previous experience, our subtransmission system (115 kV) is looped while our distribution system (34.5 kV or 13.8 kV) is radial with N.O. connection from other source for load shifting in case of troubles. Back there, one of our basic criteria is to plan for expansion when the distribution line reaches approx. 70% loading. Expansion could either be by upgrading the involve feeder or expansion from another source which can unload the affected feeder through load shifting. We evaluate base on actual peak loading and not based on applied connected loads being serve by the feeders.

Currently, I am working for an electric utility away from my country but which is having different system and somehow different philosophy. Here, we have a system which is independent having our own generation and T&D system. We have inter-tie with a larger network/system @ 380kV. However, from 115 kV system dowstream, all are radial but with redundancy. Community areas are serve by 13.8 kV loop (2-feeders with N.O. connection in between for redundancy). For connecting new or additional loads, I would evaluate the feeder based on actual historical peak load of the loop to check if capacity is still available for the new connection. Of course with due consideration of the possible future load increase based on the connected load. Ideally, I would recommend favorably if peak load has not reached 80%-85% and the potential for increase is still there due to the connected transformer capacities.

However, operations people will always argue and sometimes reject recommendation saying that the same feeder is over committed considering the connected load (despite peak loading way below the capacity of the single feeder). I know that their consideration is mostly on technical side and somehow not considering the economic side of this philosophy.

We are planning to propose to adapt an official philosophy which considers the peak load as a means to evaluate utilization of facilities (not only feeders but also dist. XF).

In this regard, I am bench marking to gather prevailing philosophy on this issue. We would appreciate if somebody can inform of their followed philosophy in their utility planning or if there are other utility company following the committed load/connected load instead of the peak load in their planning criteria?

Thanks!
CK

 

[jghrist]

By connected load, do you mean the total kVA of connected distribution transformers or the rating of loads connected to the transformers?

If the former, then the actual circuit loading would depend on the sizing practices for the transformers. If they are sized conservatively, then using connected load will require unnecessarily large distribution facilities.

If by connected, you mean the total rated loads connected to the transformers, then you have a very difficult problem determining this for a distribution system of any size.

If you have good records of actual feeder loads, these are the easiest and best sources for sizing the facilities. You might use connected transformer capacity to consider changes, but you should relate the connected capacity to actual feeder peak loads. Most distribution analysis programs will have the capability to allocate feeder peak loads according to connected transformer capacity.

In any case, you should consider the ability to serve the loads under contingency switching and the cost of losses. Loading circuits to their thermal capacity is seldom the most economical practice.

 

[CKent]

Thanks jghrist...

original design of the system (when the area was developed some 30+ yrs ago) was that transformers & feeders are sized according to the calculated expected load with consideration of the lot areas to be used..

however, as users materialized, they are applying more than what was assumed in the design...so theoretically, we can say that right now xfs will have applied connected load of approx. 150% - 200% of its design capacity...same goes with the feeder...this is not always the case but it is prevalent...

however, my concern is that why should I plan for upgrading the existing facilities, say feeder or dist XF, when the actual loading is very much less than its capacity. should I consider upgrading because of applied connected load...it is like reserving the capacities of the facilities so that anytime this applied load requires it they can get it...but my point is that historical figures will show good utilization pattern..it is unlikely that in a month or even in a year time, suddenly load will increase significantly without any new loads being connected (applied) to the system...

switching is considered as I mentioned in the original post that the system is designed for redundancy so that if one source experience problem, the other source can still carry the entire load...this is not the case however, for dist XF since each XF will serve distinct/specific load...

does utilities plan for expansion or addition of facilities based on design/applied/connected (which may be unutilized) load or the actual peak load?

 

[jghrist]

original design of the system (when the area was developed some 30+ yrs ago) was that transformers & feeders are sized according to the calculated expected load with consideration of the lot areas to be used..
however, as users materialized, they are applying more than what was assumed in the design...so theoretically, we can say that right now xfs will have applied connected load of approx. 150% - 200% of its design capacity...same goes with the feeder...this is not always the case but it is prevalent...

I'm confused. If the transformers and feeders were sized according to the lot area, then they weren't sized according to connected load. I assume that there was some historical load data based on lot size.

What then do you mean by the connected load being 150% - 200% of design capacity? This implies that the transformers and feeders are overloaded, but then you later say that the loading is much less than the capacity.