Modeling 2 pot open delta

Have the 12kv, 3 phase fault currents for a service to a pump station that is 240v, 3ph, 4wire. There are 2 25kva transformers, 12kv to 240/120 in an open delta configuration. I'm not really sure how to approach modelling this. Turns out this can't be modelled in Etap so I have to do the calcs by hand. It's getting through 2 single phase transformers with 3 phase fault currents and back to 3 phase that has me scratching my head.

Thanks for any help.

After a lot of research I have modeled the two pot open delta as a 3 phase system. There were (2) single phase 25kva transformers connected as 3 phase. I modelled it as a three phase 75kva transformer with the same impedance. It provides fault currents that are higher than using a 50kva or the single phase manual calcuations. However, the actual 6T fuse was used in the 75kva model and the sc study shows the fuse as operating beyond its rating. My concern is how does this translate to the validity of the arc flash hazard category determined for the first customer circuit breaker?

Any thoughts?

OTI has provided a work around to model the open delta and will be adding the ability to model this in the next release. The interesting thing is that the work around provides lower short circuit currents than modelling the system as a three phase transformer. The down side is the work around doesn't mimic the bus layout of the system to allow an AF study. If I use a smaller three phase transformer that provides the same short circuit current as the work around, there is a big difference in AF results. The larger transformer provides higher sc currents with a hazard category of 2, the smaller transformer provides lower sc currents with a hazard category of 3.

Any thoughts?

The full load rating of an open delta is only 57.7% of an equivalent closed delta rating, which is equal to 86.6% of the sum of the indivdual transformers.

Have you tried using a 43.3kVA 3-phase transformer with your impedances?

Yes - The three phase transformer I used was 75kva, the 43.3kva 3phase xf provided the same short circuit currents as the work around. The 75kva gives a higher short circuit current and the hazard category of 2. The 43.3 gives lower short circuit current and a hazard category of 3 and this is a 240 volt system. The clearing times are over 1 second which is the problem. The 70E table notes show max fault currents but clearing times of about 2 or 3 cycles. Using the 75kva gives conservative results for specifying short circuit ratings of devices such as panelsboards, fuses, etc. My concern is using the AF results of the 75 kva.

As NFPA70E reaching smaller industries, customers are requesting studies for smaller systems as far as kva, 208 and 240 volt and also many single phase systems. Not all software manufacturers have single phase utility sources/generators or can model the open deltas. It's making it tough to meet the customers needs.

Appreciate your thoughts, suggestions and opinions.

We model single phase networks as three phase networks since IEEE - 1584 has no equations for single phase networks. I think this approach is the best and acceptable solution until IEEE-1584 is revised.

How do you handle the protective devices such as CB's? just use a 3 pole instead of a 2 pole? OTI can model a single phase system and calc single phase sc currents but the AF calcs are 3 phase. When you speak of modelling a single phase as 3 phase are you referring to the actual model itself?

Yes, I would model the single phase as a three phase system. Then use a three phase breaker instead of a single phase. For most protective devices, the device time current characteristic is exactly the same for three and single pole devices.

You have to realize that IEEE-1584 will give you a value for the AF energy, but even then it is an approximation subject to various constants, test setups, and factors. The equations are curve fitting equations from test results.

There have been many discussions about how accurate and realistic they are. In short, they are the best tool we have. Since there is no IEEE recongnized tool (equations) for single phase systems, I feel that it is acceptable to model a three phase system as a single phase one.

Any thoughts on the difference in hazard categories between the 75kva model and the 43.3kva? Does Category 3 on a 240 volt system seem logical to those who have done this for years?

Similar to Moutaineer, I have a 2 pot transformer bank with open delta secondary, 240V. The two transformers are 50kVA. If I model it three phase 150kVA, the NFPA 70E exception no longer applies. Should I use the exception or not?

Zach

I would say no, because I would think the simple interpretation of the exception was intended for an isolation transformer. Your arrangement is as an autotransformer. The center leg would have a different impedance than the outer legs. The answer might be different if you had three transformers in a Wye configuration.

The upcoming ETAP 11 has many new capabilities including the open, delta transformer modeling.