Transformer Protection
Transformer Protection
(OP)
Dear All,
In transformer protection with fuse and LV power circuit breakers on the secondary, I understand that the maximum fuse rating on the primary has to be maximum 250% of the full load and the maximum LTP is 125% of the secondary full load. The instantaneous set up is dictated by short circuit rating at that location.
I did not find any information about any rules to how to set up (based on what criteria) the short circuit pick up and delay for the breaker. I appreciate any feedback on this.
I also noticed that IEEE uses ANSI point as another point that needs to be shown on the graph. There are multipliers which applies to this as well( 16.6 for 3.75MVA and 17.6 for 750KVA XFR). What are these numbers and what they represent? Thank you very much for any answers on the above questions.
Regards,
In transformer protection with fuse and LV power circuit breakers on the secondary, I understand that the maximum fuse rating on the primary has to be maximum 250% of the full load and the maximum LTP is 125% of the secondary full load. The instantaneous set up is dictated by short circuit rating at that location.
I did not find any information about any rules to how to set up (based on what criteria) the short circuit pick up and delay for the breaker. I appreciate any feedback on this.
I also noticed that IEEE uses ANSI point as another point that needs to be shown on the graph. There are multipliers which applies to this as well( 16.6 for 3.75MVA and 17.6 for 750KVA XFR). What are these numbers and what they represent? Thank you very much for any answers on the above questions.
Regards,






RE: Transformer Protection
RE: Transformer Protection
RE: Transformer Protection
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RE: Transformer Protection
I came across some hand calculation for breaker settings which was done 10-15 years ago and I don't think that they had used any software for coordination purpose at that time.
The "CT" turn ratio was also considered in breaker settings.
What I don't understand is that all softwares now a days can change the graph to get the best coordination and they don't ask for CT turn ratio.
How this factor comes to play in coordination of relays?
RE: Transformer Protection
If you have relay connected to a CT, the CT ratio must be taken into account in order to even plot the curve. If you have the wrong CT ratios, your results are meaningless.
RE: Transformer Protection
RE: Transformer Protection
Software has a lot of data that is just built in and the mfg can just e-mail a curve files that will work with your program. Proper training for understanding relay curve methods is alway required as well as training using the software. Maybe two or three weeks of training for each and a electrical engineering degree in power.