Ring Bus

[peetey]

I am a journeyman meterman with a utility. I want to learn about ring bus substation configuration and how to meter it with solid state meters. We currently use 2 types of mechanical meters. One is a 6 potential 6 current mechanical meter and the other is a 3 potential 6 current mechanical meter. No one currently makes a 6 current meter so to replace these requires a 2 meter 4 quadrant solution. This is where I need help. I need to understand the CT placement and how the power is totalized to yield a "NET". Is there a website or a book or something to learn about ringbus? Peetey

 

[Rodmcm]

Can you please explain or draw out a 'ring bus 'substation, the terminology is not known to me

 

[jmmee97]

Rodmcm,
The Rural Utilities Service has a good explanation of different bus configurations; ring bus being one. Look at their "Design Guide for Rural Substations", RUS Bulletin 1724E-300 under the Electric Program at usda.gov/rus/. The guide can be downloaded, but is a little large; 764 pages. If you think of a square with circuit breakers at the middle of each side and lines or transformers feeding in/out at each corner you can visualize a four-position ring bus.

peetey,
We take CTs from the two breakers protecting the outgoing line and combine their outputs. We then go to the meter with the resulting currents. This is a pretty simple explanation, because you may have 30 amps through breaker "A" and twenty amps through breaker "B", but only have 10 amps going out the line. Just remember that the sum of currents coming in and out at a node must equal zero. Hope this helps.

 

[scottf]

Jmmee97-

Do you use the configuration for revenue metering or just for indication metering??

IMO, paralleling CTs is a highly questionable practice for revenue metering.

 

[jmmee97]

Scottf,
Indication metering. We are metered on the high side of our transformers feeding into the ring.

 

[ScottyUK]

Hi scottf,

I'm not questioning your comment re. paralleling of CTs for metering purposes, but I wonder if you could expand on the reasons why. Paralleling CTs for protection purposes is routine in any form of differential scheme, so the principle is well-tried. What causes the accuracy to degrade when two or more presumably matched revenue class CTs are paralleled?



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If we learn from our mistakes,
I'm getting a great education!

 

[scottf]

ScottyUK-

The problem with paralleling CTs for revenue metering is that it's darn near impossible to predict/measure the burden that one CT places on the other, as well as the magnetizing effect that one CT has on the other. You simply can't add the errors of each CT for a worst-case error, but rather the errors are exponential functions of one another. Throw in that each CTs effect on the other changes as a function of the current through each CT and you've got quite a mess.

Clearly for protection applications, this is not a significant concern, but when accuracy classes such as 0.3 or 0.15 (IEEE) or 0.2/0.5 (IEC) are to be maintained, it's a big problem.

Hopefully, that's enough detail to help explain, as I'm not sure I could offer too much more in such a forum...or rather, I'm not sure I understand the math involved enough to offer too much more

 

[ScottyUK]

Hi scottf,

Makes sense now. I started trying to sketch out the problem and develop some equations on paper, but quickly turned hideously complex with a lot of unknowns. I shall accept your explanation!



----------------------------------

If we learn from our mistakes,
I'm getting a great education!

 

[peetey]

jmmee97, Thank you for the info on Ring Bus. I guess what I need is a diagram of a typical setup with the placement of the Ct's and how to meter with a 2 meter setup. I'm hoping to find someone with this knowledge who is willing to share their experience. Peetey

 

[mc5w]

You could contact Russ Grenier russ.grenier@ge.com about General Electric's solutions to this problem. You could also try contacting ABB's metering people and a few others.