Deadbus Closing Standby Generator to Large Traffo Load

[GerH]

I am looking into how to estimate if I can energise a large unloaded transformer load, about 50MVA made up of individual 1 to 4MVA units. When the mains fails all the LV loads will be shed throughout the factory.

So far I have been able to test with 16MVA with no problem. The remaining load is no available for test. The running load once energised was about 90kW, 140kVAr. The standby plant is made up of 2.5MVA units and I planned to use the first up.

I also tried enrgising one additiona 4MVA unit and it seemed to create a karger transient volatge dip. Why ?

Any ideas please.

 

[dpc]

One thing to keep in mind is that the transformer inrush is not consistent - it depends on the phase angle of the voltage at the instant the transformer is energized. So, for any one transformer, the inrush will usually be much higher in one phase than the other two.

Beyond that, I'm not really clear on what your system looks like and what size generator you have.

 

[busbar]

A rule of thumb for coordinating upstream overcurrent devices with transformer inrush is allow 12xFLC for 0.1 second and 25xFLC for 0.01 second. Would that be applicable in your case? You are probably well aware that transfomer inrush is of low PF and singificant second-harmonic current/DC offset...however short-lived, not the tidiest standby-genset load.

 

[alehman]

Assuming all transformers get de-energized at the exact same time, they will all have the same relative residual flux, so
I would think you can treat it as one large 50MVA transformer for a rough analysis. If there is significant impedance in the feeders to the transformers, this will be less accurate.

 

[GerH]

Inrush seems noto to be a significant issue. As the genset is only 2.5MVA it's reactance is relatively large and so the in the dominant influence i.e. I cannot see the inrush currents tripping any over-current devices. The genset has a long duration short circuit capability > 5 secs. So I have tended to inhore that aspect.

The genset manufacturer said I should check the offload reactive lagging kvars is within the rating of the genset. It seems we have very good transformers with low losses and low running reactive loads i.e. only about 1% of the traffo ratings. I can generatye up to something like 1250 kVAR so this should not be a limitation.

What I am concerned is if I am missing something else that e.g. would cause an instability. I laso feel there should be some way to calculate this.

 

[jbartos]

Comment on the previous posting. Any larger capacitors might cause instability as well as loads with periodic torques/loads.

 

[alehman]

I don't think there will be instability problems unless your AVR has problems with low power factor. You might check with the mfg. if you think that may be the case. If you have significant other loads, power factor may not be a concern at all. The inrush would be my main concern. This can sometimes cause problems with the generator protection, particularly differential overcurrent.

 

[GerH]

Thanks to all.

 

[jbartos]

Suggestion: The 4MVA unit needs to have checked the AVR function if it is properly working, designed or fine tuned.