slight mismatch in ratings between generator and transformer

[sigshane]

Hi all. a few years ago, I spent many hours trying to find the answer to this question. I seem to remember finding one, but now I can't remember the answer, nor the source for the answer.

We have 8 hydroelectric generators, rated at 13.8kV output, that are tied in two's to four oil-filled transformers, with low-side voltage rating of 13.2kV.

I am trying to find a good explanation why, when this plant was designed, the generators and transformers were not matched in terms of voltage...was this simply a logistics/availability issue, or is there some engineering explanation?

Thanks,

Shane

 

[templdl]

Hi all. a few years ago, I spent many hours trying to find the answer to this question. I seem to remember finding one, but now I can't remember the answer, nor the source for the answer.

We have 8 hydroelectric generators, rated at 13.8kV output, that are tied in two's to four oil-filled transformers, with low-side voltage rating of 13.2kV.

I am trying to find a good explanation why, when this plant was designed, the generators and transformers were not matched in terms of voltage...was this simply a logistics/availability issue, or is there some engineering explanation?

Thanks,

Shane

It is interesting that you have not provided more information about the transformer. Is this a step up or step down transformer? What is the output voltage? Is the output voltage correct or not?
Since the input as nd output voltages of a transformer are proportional I wonder why the output voltage hasn't been the issue here.
With the lack of information provided one would have to assume that the output voltage is incorrect unless thhere are sufficient transformer taps to compensate for the input voltage.

 

[charlie b]

It is interesting that you have not provided more information about the transformer. Is this a step up or step down transformer? What is the output voltage?

The answers are in the OP's description. The transformers are step-down. The primary is 13.8 kv. The secondary is 13.2 kv. I, for one, cannot imagine why they would have been designed this way. Perhaps the generator's output voltage is simply what the generator manufacturers could provide, and the transmission system is designed for 13.2.

 

[sigshane]

Sorry for that mess I posted before.

Four 75MW, 13.8kV generators, connected in pairs to two 189MVA transformers, 13.2kV:230kV, 3-phase.

Four 86MW, 13.8kV generators, connected in pairs to two 230MVA transformers, 13.2kV:230kV, 3-phase.

The transformers step up generator output for transmission to the grid when gens are running, and step down 230kV from the grid to feed our station service (plant power) when the gens are not running. Also, both types of transformers have high-side tap changers which allow for up to 5% variance up or down from the nameplate rating, which is where we have them set - 230kV.

the question was why are 13.8kV generators directly connected to 13.2kV transformer primaries. That's a 600 volt difference between generator and transformer - why?

I have attached a partial print of our plant to show the relationship between one of the transformers, and the two generators it is connected to.

Shane

 

[GoldDigger]

It is a 600V difference, but also less than 5% so it is OK.
Why there are two different nominal voltages is a better question than whether they can be interconnected (they can).

 

[Bugman1400]

What is the typical operating generator bus voltage? Is it 13.8kV or 13.2kV? If a 230/13.2kV xfmr operates at a 13.8kV gen bus voltage, it allows the primary voltage to run higher at no load (240kV). As you reach the full output of the generators, the higher primary voltage will be reduced by the xfmr impedance down towards the 230kV nominal...assuming the 230kV system operates at 230kV.

Let me know when you guys are ready for a relay upgrade.

 

[electrofelon]

I dont know what type of generators or turbines these are, and this is a bit out of my league anyway, but it could be a matter of the "mismatch" just shifting your theoretical efficency a bit. For example if this is an induction type generator, you have some slip in the system so you may just end up operating the turbine at a slightly different RPM due to different loading - different from its theoretical design.

 

[templdl]

Sorry for that mess I posted before.

Four 75MW, 13.8kV generators, connected in pairs to two 189MVA transformers, 13.2kV:230kV, 3-phase.

Four 86MW, 13.8kV generators, connected in pairs to two 230MVA transformers, 13.2kV:230kV, 3-phase.

The transformers step up generator output for transmission to the grid when gens are running, and step down 230kV from the grid to feed our station service (plant power) when the gens are not running. Also, both types of transformers have high-side tap changers which allow for up to 5% variance up or down from the nameplate rating, which is where we have them set - 230kV.

the question was why are 13.8kV generators directly connected to 13.2kV transformer primaries. That's a 600 volt difference between generator and transformer - why?

I have attached a partial print of our plant to show the relationship between one of the transformers, and the two generators it is connected to.

Shane

I figured as much per my previous reply.
Can you post a picture on the name plates for the transformer?
If the transformer has taps you can correct for the 13,200 under voltage with a 5% below normal tap.
Is you set up for load shaving to prevent being gigged by the POCO? I assume by your description you are not cogenerating with the POCO.

 

[topgone]

@sigshane,
There's no problem with your unit ratings. You can set your generator voltage at 13.2 kV and you don't even have to change the taps on your transformers,imo.

 

[GoldDigger]

@sigshane,
There's no problem with your unit ratings. You can set your generator voltage at 13.2 kV and you don't even have to change the taps on your transformers,imo.

I did not even think about the possibility that the generator voltage lists a default or maximum value and is subject to adjustment of 5% or more.

 

[Bugman1400]

I dont know what type of generators or turbines these are, and this is a bit out of my league anyway, but it could be a matter of the "mismatch" just shifting your theoretical efficency a bit. For example if this is an induction type generator, you have some slip in the system so you may just end up operating the turbine at a slightly different RPM due to different loading - different from its theoretical design.

These size units are typically synchronous.

 

[Bugman1400]

@sigshane,
There's no problem with your unit ratings. You can set your generator voltage at 13.2 kV and you don't even have to change the taps on your transformers,imo.

I think operating at 13.2kV would require at highside tap change at least 2 taps (probably the highest tap of 241500).

 

[GoldDigger]

If the transformer is, as it should be, specified as a step up transformer the taps will be on the low voltage side, yes?

 

[Bugman1400]

If the transformer is, as it should be, specified as a step up transformer the taps will be on the low voltage side, yes?

I think the NLTC taps are on the highside and are typically -5%,-2.5%,0,+2.5%,+5%. Look at the attached nameplate of GSU at the last hydroplant I did work for.

 

[Bugman1400]

I think the NLTC taps are on the highside and are typically -5%,-2.5%,0,+2.5%,+5%. Look at the attached nameplate of GSU at the last hydroplant I did work for.

see attached.

 

[GoldDigger]

Text is not readable.

 

[Bugman1400]

Looks good to me.....let me check from another computer.

 

[topgone]

I think operating at 13.2kV would require at highside tap change at least 2 taps (probably the highest tap of 241500).

The OP said his generators are rated 13.8 kV while his transformers are rated 13.2 kV. Setting the terminal voltage of the generators to 13.2 kV fits the transformer low-side voltage.

 

[Besoeker]

Hi all. a few years ago, I spent many hours trying to find the answer to this question. I seem to remember finding one, but now I can't remember the answer, nor the source for the answer.

We have 8 hydroelectric generators, rated at 13.8kV output, that are tied in two's to four oil-filled transformers, with low-side voltage rating of 13.2kV.

I am trying to find a good explanation why, when this plant was designed, the generators and transformers were not matched in terms of voltage...was this simply a logistics/availability issue, or is there some engineering explanation?

Thanks,

Shane

Do the generators have an AVR? Most I've dealt with do and can be adjusted to alter the open circuit voltage. Would that solve your problem - assuming that there is one.

 

[Bugman1400]

The OP said his generators are rated 13.8 kV while his transformers are rated 13.2 kV. Setting the terminal voltage of the generators to 13.2 kV fits the transformer low-side voltage.

Do you think the generator bus is operated at the generator rating or the xfmr low-side terminal rating? Think about the high-side to low-side ratio of the xfmr. Then, think about what the high-side voltage would be when you are generating at the full MW capability.