Breaker/CT/Relay Mismatch

[Sunny_92]

Can anyone see a reason why an engineer would spec 1200:5 multi-ratio CTs and an OCR with 0.3-2.4A (1A nom.) tap range for a 600A 15kV breaker?

Seems like you would want the primary ratings to match as well as the secondary ratings. Was this sloppy engineering or do you think it was intended?

 

[SG-1]

An ANSI 1200:5 CT can be connected for;
1200:5
1000:5
900:5
800:5
600:5
500:5
400:5
300:5
200:5
100:5

Engineers can make mistakes. The CT is sized according to the expected load & so that it will not saturate during a fault. The engineer may not have had complete information about the load when he had to pick the parts.

 

[Tony S]

What is the loading on this breaker?

The range of the OLR would give a span of 72→576A with not a lot of scope for fine tuning.

 

[Sunny_92]

The circuit feeds a number of transformers totaling 26.15 MVA, but I don't know the actual load. The CTR is set to 1200:5, the 51P pickup is set at 2 (480A pri), and the 50P pickup is set at 12 (5,760A pri) with a 1.0 sec delay.

This circuit resides in a substation that contains 4 other similar circuits. All were previously OCBs and they are now slowly replacing them with VCBs. The first one that was replaced was done by a different company. When it came time to replace 2 more OCBs, we got the job, and the customer told the project manager to install the same equipment that was put in for the first one. Because of time constraints, the PM ordered the exact same equipment without consulting engineering, and we got what was described in the OP.

Seems like poor engineering, or poor QC the first time around. I was under the impression that the company that spec'd the original equipment did good work (though I'm not saying they don't), so I'm a little surprised.

I'm a fairly new engineer, so I'm constantly trying to learn from other people's work. It can just be tough sometimes discerning between good and bad work, so any insight is appreciated!

 

[Sunny_92]

I should also mention that they use these relays for sub-metering their tenants. When their load is low (say 50A pri, 0.21A sec), they're outside the current range of the relay (0.3-2.4 A) so the accuracy is likely poor.

 

[Bugman1400]

If the CT is to be used in a bus diff circuit, it typically has a ratio that is the same as the other breaker CTs. The other breakers may be 1200A or the mains may be 1200A.

 

[Sunny_92]

If the CT is to be used in a bus diff circuit, it typically has a ratio that is the same as the other breaker CTs. The other breakers may be 1200A or the mains may be 1200A.

Good point. There's no bus differential protection here though, not even a main breaker.

 

[Bugman1400]

Good point. There's no bus differential protection here though, not even a main breaker.

If the CT is to be used for revenue metering, I would expect the CT ratio to be close to expected load and have revenue class. If the CTs are to be used for check metering then, I would expect the CTs to be close to the load but, without the revenue class.

 

[topgone]

Unless you can find a schematic diagram of that system, you can't arrive at any conclusion/analyses, just plain guesswork.