138/69kV Substation Design 4

[tefaber]

We are in the beginning stages of designing a 138 to 69kV substation on a greenfield site and would appreciate any input as to the design/layout. The substation is to be designed for an ultimate build-out of three 28MVA transformers, but will probably only see two of them ever installed. The substation will only have one source initially, but will be designed for a second source to be installed at a later date.

I am considering two basic designs. The first is a single bus with breakers on the incoming lines and a breaker for each of the transformers. I don't like this design since a bus fault couldn't be easily isolated and repaired. The second design is a ring bus. This would give me the bus flexibility I am looking for at a comparable cost (at least in my estimation). Any thoughts? Is it atypical to see this type of design at the 69kV level. Is there anything I am overlooking?

Thanks for the help,

 

[dpc]

I think the two obvious choices at these voltages are a ring bus or a breaker-and-a-half. The decision probably rests on the number of feeders planned. For only a few feeders, the ring bus will be less expensive and provide good operating flexibility. As the number of feeders increases, the ring bus can start to look less attractive because of the need to open up the ring to take a feeder out of service. The breaker-and-a-half scheme doesn't have that weakness, but does require more breakers.

Other factors would include the nature of the loads being served; ability to feed these loads from other sources, finances, etc.

 

[tefaber]

Thanks,

I like that answer. Would your answer change if the incoming lines were 69kV instead of 138?

We will be taking six feeders out of this substation. The client only has one other substation to date, covering about 40MVA of load, so this project is far overdue.

One more question if you don't mind: I will run this by our PE, but if you could comment....

For the ring bus, this was the layout I was considering:

| | |
XF3 XF2 XF1
|--X1--|--X2--|--X3--|--X4--|
L1-| |-L2
|-----------XTIE---------------|

where X1, etc...= Breaker
L1, etc = 69kV line

My question is this, would I need an additional breakers on each of the incoming lines or would isolation on the lines, in case of a fault, be handled by a switch alone?

 

[davidbeach]

In a ring bus configuration, you should alternate loads and sources to the greatest extent possible. In your 3 load, 2 source, case, you should have one load between the sources one direction and two loads between the sources the other direction. This way the ring has more operational flexibility than the configuration you show.

 

[tefaber]

Thanks David, that's a great point...I appreciate the input.

 

[RalphChristie]

For a possible 3 trsf, 2 source substation, I would go for multiple bus sections with bus tie arrangement. (or what you call a ring bus arrangement) If you are going to incorporate bus-protection, then your incomers and bus-tie must have breakers. Also if you want to parallel your two sources. If you are just going to feed from one source at a time, and are not incorporating bus-protection, you can have switches at your incomers.
(Just keep in mind I am also just familiar up to 66kV... )

 

[cuky2000]

A ring bus arrangement provides better reliability and more flexibility in the substation operation than the multiple buses with tie breaker arrangement.

Future breaker(s) may be considering in the layout. In addition, the station could be designed for future upgrade to breaker 1 1/2 configuration with minimum investment.

I hope this could help your final decision.

 

[dpc]

I agree with David Beach's comment on the configuration. For the number of positions you have at this sub, I would lean toward the ring bus, based on what little I know about it.

You would just have a disconnect on the incoming and outgoing lines. For a fault down the line, you have to trip both breakers on the ring bus. Then you can open the disconnect and close the ring back in if desired.

Also, if you lay out the substation to anticipate it, it is not too difficult to convert the ring bus to a breaker-and-a-half scheme later on if the number of positions increased beyond what is practical for a ring bus.

If you have access to J.L. Blackburn's Protective Relaying book, he has a good discussion on the relative merits of each type of bus configuration.

 

[tefaber]

Thanks all-this is all very good information.

I did want to clarify one point, however...whether a breaker or switch will be needed on each incoming line.

So with the ring configuration, a basic layout may look like this: (Sorry for the bad dwg...not exactly Autocad quality)

XF1 XF2
|--X2-- |
| |
X1 X3
| |
L1-X7-| |-X4-L2
| |
X6 X5
|----------|
|
XF3
The breakers located on either side of the transformers are no-brainers...but the breakers X7 and X4 are the ones I'm not sure I need. The normal operation (at the onset) will be from one source only. In the future, we will setup a ring system of the substations (Since there is only one right now, that doesn't make sense yet, of course). So the normal mode of operation, I believe, will see both lines energized. If there is a fault on either Line 1 (L1) or Line 2 (L2), Either Breakers X1 and X6 or X3 and X5 would trip to clear it...having breakers X7 and X4 seem redundant. Thoughts?

 

[rcw retired EE]

tefaber - You're correct. The line breakers are redundant in a ring bus.

If the initial installation does not require all of the feeders or lines in the ring bus, just bus across the future breaker space. You may want to install disconnects for the future breakers to minimize shutdowns.

As others have suggested, try to lay out the ring so it can convert to breaker-and-half in the future.

In my experience, ring bus substations are very popular in the US but are not common elsewhere. Single-breaker, double-bus, or double-breaker, double-bus systems were the prevalent designs in the European and Asian countries where I worked.

Ring bus installations get expensive if you use live tank breakers and still want CT's on each side of each breaker since you need 6 free standing CT's for each breaker. Dead tank breakers have all of the CT's in the bushings.

Bus protection is not used on a ring bus. Each section of bus between the breakers is considered as a part of the associated line, feeder or transformer and is protected by the transformer differential relays or the line relays.

 

[RalphChristie]

Download RUS Bulletin 1724E-300, Design Guide for Rural Substations:

http://www.usda.gov/rus/electric/pubs/1724e300/1724e300.pdf

(It is a rather big file - 10MB)

Chapter 4.6, from pg. 129
Advantages and disadvantages of different bus-configurations.

I see in Blackburn's book they use ring-busses more on higher-voltage substations, while multiple bus sections with bus tie arrangement more in use is on lower voltage substations.

 

[tefaber]

Thanks for all the input, it has been very helpful...