For protective relaying, unless you really have a need to have a CT ratio that closes matches the expected maximum load and may change over time, you might consider single-ratio CTs.
A lot of the reasons for using multi-ratio CTs are no longer as important as they used to be, with the increased use of digital relaying and metering.
One of the reasons to set a MRCT below the maximum is the limitation for low pickup values. Such conditions come into play on long sub-transmission, or distribution lines, where a ground setting needs to be low enough to protect the end of the line.
Another case is when someone orders a "standard" ct ratio for all applications, without consitering how it applies to low fault current applications.
As some relays have a bottom limit for pickup of about 0.25 to 0.5 amps, it is still wise to order MRCT's if the situation allows.
David,
I really hadn't thought of that. Specifying MR CTs in transformers and breaker bushings is just a matter of course. Do you have any idea how much money can be saved if single ratio CTs are used? Are single ratio relay class bushing CTs readily available in the USA?
We do it fairly routinely and have not had any major problems. Sometimes we specify single ratio and get MR anyway. As long as it has the specified accuracy at the specified ratio, we accept it. I've never really analyzed the cost savings aspect. We like single ratio because it prevents someone from changing to a much smaller ratio in the future and giving up most of the relaying accuracy they assume they have.
I agree that you have to use a little common sense to make sure the minimum relay settings will work.
We see so many substation with a 1200/5 CT connected at 400/5 because they think it will be more accurate that way. In the old days when we have directly driven ammeters, and limited tap range in the relays it was more important to match the CT ratio to the expected max current.
Also had not considered the concept. I always am worried about transformer diff ratios, but I also know what I expect to do going into the project. But like you say that you sometimes still get the MRBCT's, it is the standard, and I guess will be until we set about changing specs.
Seems better design up front might save a few bucks at some point.
In some cases there are contradiction between requirements of relay protection settings and tariff metering. I had such case last year:
Short 110 kV cable line with 10 MVA power transformer on the end, protected by line differential protection. Short circuit level on 110 kV is over 25 kA.
In the same time expected load for first year of operation will be about 2-3 MVA, so load current is about 10 A primary.
Solution was to use CT's with 800/1 A class 5P30 on protective cores and 200/1 A, class 0.2S on metering cores. Guys from metering division would be more happy if we use 100/1 A, but according to the manufacturer it cannot be produced in reasonable size and cost.
Actually these CT's were very special type:
primary switching only on cores 3 and 4 and primary and secondary switching on cores 1,2 and 5:
core 1 and 2: 100-200/1, 200-400/1 A
core 3 and 4: 400-800/1 A
core 5: 800-1600/1 A (needed for existing electromechanical busbar differential protection)
We used higher primary tap, so used ratios are 200/1 for cores 1 and 2, 800/1 for cores 3 and 4 and 1600/1 for core 5
According to me this case is good example that even when using last generation of numerical technology careful choice of CT's and secondary circuits is still necessary.
------------------------ It may be like this in theory and practice, but in real life it is completely different.
The favourite sentence of my army sergeant
Do you put metering and relaying on the same CT?
We do some for non-revenue metering, in which the MRCT comes in. But for revenue we use deadicated CT's.
Another idea, which most people won't like, is you can use aux. CT's to gain ratio changes.
But an idea is to purchase single ratio CT's, and use aux. CT's to tap the ratio down if needed on the relay panels.
There is no appreciable price difference between MR and SR bushing CTs, i.e. 2000:5A MR vs simply 2000:5.
I don't any reason from an engineering perspective one wouldn't use MR bushing CTs, due to the dramatically increased flexibility.
Dual ratio CTs (100/200:5A for example) are typically used for metering and may also have a protection rating. Don't see any reason why a dual-ratio CT would be used in a bushing CT application.
