FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
(OP)
Hello,
I am in the process of reviewing specification for a GSU transformer for a hydropower plant. As calculated, the primary current on the low voltage side(13.8 kv)is equal to 6521 amp and the secondary current on the high voltage side (241.5 kv) is equal to 373 amp. What ratio bushing CTs are required on the primary as well as the secondary size? How the CT ratios are determined based on GSU transformer primary and secondary current? What accuracy class CT should be used and how it is determined for this particular application?
Below is the information about the GSU transformer.
1 phase transformer
90 kVA
13.8 kV/241.5 kV
I am in the process of reviewing specification for a GSU transformer for a hydropower plant. As calculated, the primary current on the low voltage side(13.8 kv)is equal to 6521 amp and the secondary current on the high voltage side (241.5 kv) is equal to 373 amp. What ratio bushing CTs are required on the primary as well as the secondary size? How the CT ratios are determined based on GSU transformer primary and secondary current? What accuracy class CT should be used and how it is determined for this particular application?
Below is the information about the GSU transformer.
1 phase transformer
90 kVA
13.8 kV/241.5 kV






RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
Specifying the BCT for the application is better than over-specifying it and making the CT difficult to produce. This is especially true in higher current CTs and this is coming from a CT designer. But please, whatever you do, make the continuous current and bushing ratings part of your specification.
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
If they're to be used for metering (specifically revenue metering) then the sizing should be done differently.
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
For the HV side with a max current of 373A, it gets a little more tricky. As you know, tough to get good metering performance when you get down to that current level on a bushing CT, depending on how much physical room is available. To do it correctly for revenue metering, would need to get the burden rating as low as possible and possibly use some kind of extended-range or 0.15 class rating with a slightly higher ratio, but that is able to cover down to the lower currents. As we all know, the current may not be at or near the max most of the time.
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
To the OP, with regards to accuracy that depends on the protection scheme and what's up and down stream of it. The higher ratio will have no trouble delivering 0.3B1.8/C800 class. And if you have IPB protection a much higher ratio CT may also be needed to match upstream generator CTs. As for the lower ratio CT, well depending on what you are needing its size can get out of hand. If for metering as I mentioned earlier it shouldn't be a problem obtaining 0.15 class with a low or medium burden, and on the protection side stay as low as you can possibly get away with. Requesting C800 for this ratio will make it quite large in size. If that becomes a problem in size then perhaps increase the ratio to 600:5. Again this is all generally speaking, you still have to coordinate with other devices. These are best answered by a systems guy.
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
They're frequently mounted on the oil side of the bushing in a GSU transformer, at least over here in IEC-land. There are exceptions, but not many.
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
RE: FINDING A CORRECT BUSHING CT RATIO FOR A GSU TRANSFORMER
The issue of temp rise is a valid concern, but normally there is enough physical space in a power transformer install to use a secondary wire of sufficient size to keep the Cu losses low enough at RF4 or RF3. Given how transformers are so often over-sized and the load on the transformer can vary so much, if you really want to have good metering performance across the entire operating range, going with a lower ratio and higher RF is definitely preferred from a metering perspective when possible.