Calculating Secondary Inrush Current for Isolation Transformer

[bellington]

Good morning to my electrical math wizards,

I have a 3-phase 500 kVA isolation transformer 480 delta fed by a EPC 500 kW inverter powered by a 1107 kWH battery, to 480 wye fed to switch gear. There is a 450 kW generator to charge the battery when solar is inadequate. In black start conditions, the generator breaker typically trips, unless I open the battery inverter breaker on the primary side of the transformer, then close the generator switch gear breaker, then close the transformer breaker.

The company that installed the equipment keeps discussing the need for a coordination study. But, it is my thoughts that the secondary, wye, side of the transformer may actually have a higher inrush current than the delta primary. I also think that while setting a longer delay on the generator may prevent the generator breaker from tripping, I don't want the generator being exposed to that current.

1. Are there reliable calculations that will provide the inrush current and the length of time the generator would be exposed to that current?

2. What is the least costly and most effective way of limiting the inrush current on the output side of the generator?

 

[synchro]

Is it feasible to start the engine of the generator with the generator output already connected to the wye side of the transformer? This would ramp up the generator output, which could then reduce the peak inrush current it experiences.

Bye the way, what configuration is the generator output (e.g. , delta, grounded wye, ungrounded wye)?

 

[jim dungar]

Yes the 'secondary side' of a reverse fed transformer may experience higher inrush currents than the normal primary. The manufacturer might be able to help estimate this value, as it depends on the transformer construction. Load currents are almost never factored into inrush current calculations.

However you may also experience hysteristics. This is a issue of the core remaining magnetized when the transformer is deenergized. This magnetism can be out of sync with inrush current when it is energized causing the OCPD to operate. This is a random occurence, even though it may happen regularly, it cannot be predicted.

 

[winnie]

1) My non-expert understanding is that to calculate inrush you would need a good equivalent circuit model of the transformer. You would probably need information from the transformer manufacturer.

2) I know that in theory you can eliminate transformer inrush current with carefully timed switching to connect the transformer to the supply; there might even be packaged systems that implement this.

3) If you have an unloaded transformer, then you can do a resistive pre-charge; first you connect the supply to the transformer through a set of resistors, then you bypass the resistor to fully power the transformer, and finally you connect the load to the transformer. I don't know if there are packaged approaches to this or if you'd need to build a system.

4) Are you sure that you are dealing with a transformer inrush issue, and not the load (the battery charger) causing significant current flow when it is first energized? Your symptom is that the breaker trips when the load is connected to the transformer, but doesn't trip when it isn't connected. So it could be that the initial current to the battery charger is the problem, or the combination of transformer inrush and battery charger energizing.

 

[bellington]

Is it feasible to start the engine of the generator with the generator output already connected to the wye side of the transformer? This would ramp up the generator output, which could then reduce the peak inrush current it experiences.

Bye the way, what configuration is the generator output (e.g. , delta, grounded wye, ungrounded wye)?

I'm not sure. The last time I had to do a black start, the programming would not allow me to manually close the switch gear breaker with no voltage on the bus, either from the battery, or from a running generator. If we can program that breaker to close and remain closed during generator startup, that may be a viable solution.

However, the issue will become more significant as we must migrate from (1) 450 kW generator, to multiple smaller ones. The generator was originally merely a backup to solar and grid power from a neighboring island. That submarine cable has been damaged and may never become a functional reality. So, we need generators that can charge the battery in low solar production, AND be capable of running the island without the battery, AND still have backup. Obviously, I do not want to attempt to set breakers on 150 kW generators to attempt to carry that load. Again, starting one generator in black start with breaker closed may solve that.

Thanks for the suggestion.

If I remember correctly, the generator is grounded wye.

 

[bellington]

1) My non-expert understanding is that to calculate inrush you would need a good equivalent circuit model of the transformer. You would probably need information from the transformer manufacturer.

2) I know that in theory you can eliminate transformer inrush current with carefully timed switching to connect the transformer to the supply; there might even be packaged systems that implement this.

3) If you have an unloaded transformer, then you can do a resistive pre-charge; first you connect the supply to the transformer through a set of resistors, then you bypass the resistor to fully power the transformer, and finally you connect the load to the transformer. I don't know if there are packaged approaches to this or if you'd need to build a system.

4) Are you sure that you are dealing with a transformer inrush issue, and not the load (the battery charger) causing significant current flow when it is first energized? Your symptom is that the breaker trips when the load is connected to the transformer, but doesn't trip when it isn't connected. So it could be that the initial current to the battery charger is the problem, or the combination of transformer inrush and battery charger energizing.

Thanks for the input.

I will send an email to the manufacturer for calculations and possible solutions.

As for item 4., I am fairly certain that the 500 kW inverter contacts remain open until the battery is brought on line. There is however, another 480 to 240/120 50 kW transformer connected to the battery side of the isolation transformer. That transformer has several power supplies connected to it. It seems as though opening the breaker between the battery and the isolation transformer is the only means available to prevent the generator breaker from tripping. Regardless, I would like to do something so that black start could be simplified.

Do you agree that I do not want to push the generator breaker to a higher delay as a means for compensating for this current?

Thanks,

Byron

 

[winnie]

Do you agree that I do not want to push the generator breaker to a higher delay as a means for compensating for this current?

Thanks,

Byron

I don't know enough to comment one way or the other. I don't know how much harm the transformer inrush will cause to the generator, if any. If the 'excessive' inrush doesn't harm the generator (say due to torque transients), then I don't see it harming anything else.

-Jonathan