Transformer Protection

[Humble2000]

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,

 

[Borti]

I suggest reading IEEE Std. 242 (Buff Book). There is a good chapter on transformer protection.

 

[davidbeach]

When you get done with the Buff Book, you should follow up with the Red Book (IEEE Std. 141).

 

[SilverArc]

There are different rules when it comes to below and above 600 V transformer protection.

http://img221.imageshack.us/my.php?image=fuseau2.jpg

 

[Humble2000]

I have started reading about protection and coordination .Thanks for the references.
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?

 

[dpc]

You don't need any software to do relay coordination. The software will give you prettier pictures and make it easier to check coordination options, but it's certainly not necessary and probably better to learn to do it by hand first.

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.

 

[Borti]

The software packages I have worked with always asked for the ct ratio. The only time the ct ratio was not required was when you use LV breakers. Since you mentioned LTP and 750 kV transformer, it appears that you are dealing with small distribution transformers. You will have to enter breaker, sensor or plug rating or all of them. The long time pickup setting is actually a multiplier of the sensor, plug or breaker rating.

 

[advidana]

Software is nice. I did a lot of hand calcs for years and years. It is very time consuming. You need the graph paper which you cannot find when you need it. An idea of where to find the breaker curve data needed from the manufactures, and on and on.

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.