Autotransformer Application - Tertiary Needed? - LONG
Autotransformer Application - Tertiary Needed? - LONG
(OP)
We have a client with a cogen system that currently operates independently from the plant power system. The generator runs at 13.8 kV, with the output running directly to a 13.8 kV to 115 kV step-up transformer (delta on generator side, grounded wye on high side) and out onto the grid. The plant power system takes service from utility at 12.47 kV. It appears that all plant loads run from three-phase step down transformers, a mixture of wye-wye (both windings grounded) and delta-wye.
Due to low price current being paid for the generated power, owner wishes to connect the 13.8 kV system to the 12.47 kV so that the plant loads can be at least partially provided by their generator.
Existing utility 12.47 kV service is solidly grounded, but there should be very little neutral current, since all transformers are three-phase.
Generator is presently UNGROUNDED. A grounding resistor is existing that would allow conversion to low-resistance grounding (150 A max or so).
Options for an autotransformer to convert 13.8 to 12 kV:
Wye-connected three-legged core
Wye-connected five-legged core
Wye-connected with buried tertiary (For cost adder of about 90%)
Concern is what happens if there is any imbalance in the plant loading (on the wye-wye transformers) that would normally result in neutral current on the primary (12.47 kV) side.
The tertiary winding would address this, but the cost increase is significant - this nearly doubles the transformer cost.
Transformer supplier is suggesting a five-legged core, but I'm not sure how this helps since there is still no place for the zero sequence current to come from.
Three-legged core has a "phantom tertiary" effect that I think could handle a small level of neutral current. We would have to provide ground fault detection to trip on a 12.47 kV ground fault regardless, but still trying to determine if we really need the delta tertiary.
Any thoughts would be appreciated.
dpc
Due to low price current being paid for the generated power, owner wishes to connect the 13.8 kV system to the 12.47 kV so that the plant loads can be at least partially provided by their generator.
Existing utility 12.47 kV service is solidly grounded, but there should be very little neutral current, since all transformers are three-phase.
Generator is presently UNGROUNDED. A grounding resistor is existing that would allow conversion to low-resistance grounding (150 A max or so).
Options for an autotransformer to convert 13.8 to 12 kV:
Wye-connected three-legged core
Wye-connected five-legged core
Wye-connected with buried tertiary (For cost adder of about 90%)
Concern is what happens if there is any imbalance in the plant loading (on the wye-wye transformers) that would normally result in neutral current on the primary (12.47 kV) side.
The tertiary winding would address this, but the cost increase is significant - this nearly doubles the transformer cost.
Transformer supplier is suggesting a five-legged core, but I'm not sure how this helps since there is still no place for the zero sequence current to come from.
Three-legged core has a "phantom tertiary" effect that I think could handle a small level of neutral current. We would have to provide ground fault detection to trip on a 12.47 kV ground fault regardless, but still trying to determine if we really need the delta tertiary.
Any thoughts would be appreciated.
dpc






RE: Autotransformer Application - Tertiary Needed? - LONG
I would be looking at the monetary effect of 10% less output versus the cost of transformers and installation. Don't forget that you will be avoiding some transformer losses also. Any time that the fuel supply is less than 90% there may not be any loss of kW output at 12.47 kV versus 13.8 kV.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer Application - Tertiary Needed? - LONG
RE: Autotransformer Application - Tertiary Needed? - LONG
But in your application,I believe station start up will be done from utility supply and then switch over to generator load.So to match the vectors you have to go for a 2 winding delta /star transformer of 13.8/12.47 kV. In case Generator transformer is YNd1(if in US, other places YNd11)then this auxiliary supply transformer connection shall be Dyn1( or Dyn11)
RE: Autotransformer Application - Tertiary Needed? - LONG
That would work if they planned to operate isolated from the grid, but that is not the case. They will still have excess power they want to sell. If the generator voltage is reduced, their existing GSU will not be able reach the necessary transmission system voltage. As it is now, the transmission voltage runs extremely high, the generator voltage runs high and the GSU is on maximum tap.
prc,
Well, I have the quotes with that pricing. We are talking about an autotransformer, so I assume the tertiary would add significant core size? The plan for operation is to isolate from the 12.47 kV utility feeder (outage), switch the plant system over to the new autotransformer source, then bring up the generator. The cogeneration system depends on the plan power system (12.47 kV) for station service power, so it cannot start up without the plant power system being energized. Once running, it cannot switch back to the 12.47 kV source without an outage (not without adding a lot of additional complexity and utility issues. Perhaps I need to pursue additional quotes on the transformer, but the number of companies willing to build a transformer like this in the US is very small.
Thanks,
dpc
RE: Autotransformer Application - Tertiary Needed? - LONG
Have you considered using a three phase 13.8 kV voltage regulator? 12.47 is within the standard +- 10% reach of an off the shelf regulator, just lock it off at buck 8 or so. Might be a cheaper solution than a custom built transformer if you can find a suitable capacity. Don't forget the effective capacity of +- 10% regulator will be 10X the nameplate.
RE: Autotransformer Application - Tertiary Needed? - LONG
Thanks - We did consider the voltage regulator option. The actual generator voltage when running is significantly higher than nominal due to the 115 kV line running extremely high. So I'm concerned about running out of head room. I'll think about that option a little more. But I still would have a concern regarding neutral current, I think.
The tie to the 115 kV utility system is through a delta-wye transformer, so there is no good source of zero sequence current on the plant system when the 12.47 kV tie to the utility is open - at least that I can see.
Regards,
dpc
RE: Autotransformer Application - Tertiary Needed? - LONG
With a wye autotransformer dropping 13.8 kV to 12.47 kV you may have an issue of a neutral shift if the phase loading is not equal. The voltage on the less loaded phase will tend to rise until the saturation limits further voltage rise. A delta winding will lessen the voltage swing. The phantom delta will help but it is more difficult to determine the amount, just better than nothing.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer Application - Tertiary Needed? - LONG
The plant is fed from a separate 12.47 kV distribution feeder. I'm thinking of using a small wye-delta transformer bank to stabilize the neutral instead of the tertiary winding in the autotransformer. I think this will be less expensive and accomplish the same thing.
dpc
RE: Autotransformer Application - Tertiary Needed? - LONG
I am concerned with power sharing issues and reactive power sharing issues.
The loading on a generator depends on the angular advance of the rotor slightly ahead of the grid wave form and on the impedances of the system and the generator.
If you are tied to the grid at the 115 kV level and at the 12,47 kV level, you will be able to load your generator but you will have no control over the power division between the two systems.
Likewise, with a fixed voltage ratio you will not be able to follow voltage variations on the 12.47 kV system and the reactive power and the power factor will vary with variations in the 12.47 kV grid voltage.
Are you able to drop the 12.47 kV service and feed the plant via the 115/13.8 kV system?
I'll keep thinking about this.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer Application - Tertiary Needed? - LONG
I'm probably not explaining the situation well. The utility 12.47 kV feeder will never be tied to the 115 kV transmission (at this site). We will shut down, open a disconnect to the 12.47 kV feeder, then switch over to the 115 kV source. Never in parallel.
dpc
RE: Autotransformer Application - Tertiary Needed? - LONG
I'm good with the wye delta bank.
Another thought. Can you buy a little more head room by going 12.47,13.8 and 14.4 kV on the autotransformer? Just a thought.
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer Application - Tertiary Needed? - LONG
Of course the design of such a non standard unit can be tricky,if in not in experienced hands.Why you are not considering the connection that I suggested? That is the how station start up and station auxiliary transformers are usually connected in all thermal stations.
RE: Autotransformer Application - Tertiary Needed? - LONG
The system will not be able to switch after startup. It will start up on the 115 kV source and stay on that connection. We did consider a two-winding transformer. This will certainly work, but the cost is about 3 times the autotransformer.
Also, I did consider the three-leg core and this is what the manufacturer originally proposed, but then they got nervous about it. They are pushing either the tertiary winding or a 4-leg or 5-leg core. Do you have any quantitative numbers on what the zero sequence impedance might be for the three-leg core autotransformer and its "phantom" tertiary? I have not been able to find much real data on this.
thanks,
dpc
RE: Autotransformer Application - Tertiary Needed? - LONG
We had a pretty bad unbalance (20% to 30% and at times more) on a 1.2 MW genset that lasted for over five years until routine testing may have shown rotor deterioration (I was not involved with the test and was somewhat suspicious of the results)
Our loads were mainly single phase and we had problems balancing the phases.
I would not expect a three phase plant load to present that much unbalance.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer Application - Tertiary Needed? - LONG
RE: Autotransformer Application - Tertiary Needed? - LONG
Yes, we have discussed grounding the generator through the 200 A resistor. But we really don't know the magnitude of imbalance in the plant load. I expect it to be small, but no one is quite sure and no data is available. The other issue is that the turbine-generator was purchased as a package from Germany with all controls, relaying, etc. Apparently at the time of installation, the German field engineer was emphatic about leaving the generator ungrounded - I have no idea why or if he even knew what he was talking about, but their drawings show it ungrounded. There are some advantages at this point in not re-opening that particular can of worms.
Thanks,
dpc
RE: Autotransformer Application - Tertiary Needed? - LONG
I believe the 85% figure would apply to a delta-wye transformer, not a wye-wye. At least that has always been my understanding.
RE: Autotransformer Application - Tertiary Needed? - LONG
Although this is a totally different voltage class and is just a single data point from 1975, I have test report for a McGraw-Edison 108 kV/120 kV 3 leg core autotransformer with Z0=0.64% almost identical to Z1=.639% for the nominal tap. Since Bill mentioned taps, I'll also mention the test report gives the positive sequence impedance for the 111kV and 114kV taps as 0.358 and 0.051 respectively, but the test report does not list the zero sequence impedance for these taps.
RE: Autotransformer Application - Tertiary Needed? - LONG
Regards
Marmite
RE: Autotransformer Application - Tertiary Needed? - LONG
Have you looked at the utility meter? The newer electronic meters may yield a wealth of information. I was having power issues at a department store. I paid a visit to one of the engineers in the distribution department. He sent a crew down to dump the information from the meter and then gave me a disc containing the three phase voltages, the three phase currents, the power and the reactive power, at 15 minute intervals for the preceding three months.
An inquiry to the local utility may bear fruit.
Yours
Bill
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Autotransformer Application - Tertiary Needed? - LONG
RE: Autotransformer Application - Tertiary Needed? - LONG
RE: Autotransformer Application - Tertiary Needed? - LONG
Bill
--------------------
"Why not the best?"
Jimmy Carter