Generators with step up/down transformers.

[hockeyoligist2]

We have a lot of generators, some portable, some permanent. Several have transformers to step up or down the voltage. I got to thinking, what kind of damage may result from starting them with the load of the transformers? I'm talking about just the transformer with no load on the secondary side.

The permanent ones have auto switch gear after the transformer. The portables plug in to manual switch gear and sometimes someone may forget to turn off the breaker before starting.

Could/would this damage or shorten the life of the generator?

 

[Jraef]

Not really. No load on the transformer means just the magnetizing inrush, maybe 2 cycles of high current but at very low power factor. Shouldn't mean a thing to the generator.

 

[kingpb]

Not really. No load on the transformer means just the magnetizing inrush, maybe 2 cycles of high current but at very low power factor. Shouldn't mean a thing to the generator.

The transformer is always going to have some residual flux density and when the transformer is re-energized the incoming flux will add to the already existing flux. If the voltage is applied at the zero crossing, the inrush current could be very high and last for several seconds. The result would be that the generator cannot respond to the demand and trip. I agree, there would not be any damage to the generator, but for generator sizing purposes this effect must be taken into consideration.

 

[rcwilson]

If the breaker to the transformer is closed before the generator runs, the inrush current will be negligble. As the generator spins up and builds voltage, the transformer magnetizing current will come up smoothly also. Voltage is not applied suddenly as happens when closing a breaker.

 

[kingpb]

If the breaker to the transformer is closed before the generator runs, the inrush current will be negligble. As the generator spins up and builds voltage, the transformer magnetizing current will come up smoothly also. Voltage is not applied suddenly as happens when closing a breaker.

I can't say I've seen an arrangement where the transformer is energized as the generator is started, or without some type of generator breaker or switch. Could you provide some examples where that would be used.

 

[steve66]

If the breaker to the transformer is closed before the generator runs, the inrush current will be negligble. As the generator spins up and builds voltage, the transformer magnetizing current will come up smoothly also. Voltage is not applied suddenly as happens when closing a breaker.

That's not the case I've ever seen. The generator gets up to full speed and voltage, then the transfer switch switches, and bang.

It can cause the breaker feeding the transformer to trip. Especially if the magnetic setting on the breaker is set to a fairly low value.

Of course, its not really any worse than just suddenly applying power to a transformer by turning on the main breaker. But with a generator and a ATS, there is usually monthly testing that gives the breaker lots of chances to trip.

And there is that transfer back to normal power, which can happen very fast, and the normal power may not be at the same phase angle. I think this is even more likely to trip a breaker.

 

[rcwilson]

I can't say I've seen an arrangement where the transformer is energized as the generator is started, or without some type of generator breaker or switch. Could you provide some examples where that would be used.

On large generators (>100 MVA) when the client wants to save the $1M+ cost of a generator breaker, the Generator Step Up (GSU) Transformer is direct connected to the generator at 13.8 kV or 18 kV and the synchronizing is done by the high voltage breaker at 115 kV up to 500 kV. On starting, the generator spins up and a small voltage of about 3% appears due to residual magnetism in the rotor, but there is hardly any current flow. When the excitation turns on at 95-100% speed, the voltage ramps up to 100%, and a small, almost immeasurable, current flows to the transformer. There is hardly any inrush current.

Some generators also have an excitation transformer (2.5 MVA ? 7.5 MVA) direct connected to the generator terminals. It powers the AC-DC Exciter power supply that feeds the DC field current to the rotor through large brushes. During commissioning tests, we disconnect the generator and energize the GSU and excitation transformer by backfeeding from the 230kV system. I have to set the instantaneous overcurrent trips at 12x to get past the transformer inrush. But when the generator energizes the same transformers, a 0.05 setting is fine. The inrush current is negligible because the voltage builds slowly, relatively speaking.

The OP was wondering if a transformer could get damaged by being connected directly to a generator - no transfer switch or breaker closing. I say it is no problem.

But it would be very unusual at low voltage to have a direct generator to transformer connection like this.

 

[templdl]

From my point of view this should be a non issue. Having worked with MCCBs, ATS, transformers with over 20 years of experience the only issue that I picked up in the previous posts was a breaker that tripped instantaneously (magnetically) when a transformer was energized. The poster pinted out that it was most likely the result of the mag setting set too low. Most TM breakers are shipped with the mag trip setting set in the lowest setting and the installer never changes it. Quite often this does not pose to be an issue but may result in nuisance tripping. As far as I can tell the only place in the NEC that refers to a mag setting is with MCPs in section 430 otherwise one would set the either at the maximum setting or just high enough not to nuisance trip. Some attempt to coodinate devices that are in series with limited success.
Bottom line is that there should be no issues when using a step up transformer with a generator as it's done all of the time.

 

[kingpb]

On large generators (>100 MVA) when the client wants to save the $1M+ cost of a generator breaker, the Generator Step Up (GSU) Transformer is direct connected to the generator at 13.8 kV or 18 kV and the synchronizing is done by the high voltage breaker at 115 kV up to 500 kV. On starting, the generator spins up and a small voltage of about 3% appears due to residual magnetism in the rotor, but there is hardly any current flow. When the excitation turns on at 95-100% speed, the voltage ramps up to 100%, and a small, almost immeasurable, current flows to the transformer. There is hardly any inrush current.

Some generators also have an excitation transformer (2.5 MVA ? 7.5 MVA) direct connected to the generator terminals. It powers the AC-DC Exciter power supply that feeds the DC field current to the rotor through large brushes. During commissioning tests, we disconnect the generator and energize the GSU and excitation transformer by backfeeding from the 230kV system. I have to set the instantaneous overcurrent trips at 12x to get past the transformer inrush. But when the generator energizes the same transformers, a 0.05 setting is fine. The inrush current is negligible because the voltage builds slowly, relatively speaking.

The OP was wondering if a transformer could get damaged by being connected directly to a generator - no transfer switch or breaker closing. I say it is no problem.

But it would be very unusual at low voltage to have a direct generator to transformer connection like this.

I thought the OP related to DEG sets, not power generation facilities.:?

 

[rcwilson]

I thought the OP related to DEG sets, not power generation facilities.:?

Correct. I was responding to your request for examples and picked one I am most familiar with. But it was probably not totally applicable even though the physics are the same.

Maybe a better example is a 480V generator feeding a 208/120V transfer switch through a transformer. The generator output breaker provides overcurrent protection for the cables and transformer and another breaker at the transformer protects the 208V cables to the ATS. During normal operation both breakers are closed and the generator off. When the ATS signals an outage, the generator will start, ramp up voltage to the transformer and the ATS will transfer load after it senses correct voltage. Due to the ramped voltage, neither of the two breakers will ever trip on transformer inrush and nothing detrimental should happen to the transformers.

The above is essentially what the OP had: generators directly feeding transformers through closed circuit breakers.

 

[kingpb]

Thanks, good info. Something to take into consideration.:thumbsup:

 

[hockeyoligist2]

Thanks for all the info Y'all! If I'm reading them right, then I shouldn't be concerned. The one that was worrying me the most is an old 480v gen stepping up to 2400v.