Any Problems Associated with the Tap Changer Control?

[pwrtran]

Hi All

Does anybody see any problems on this temporary operation:
We are replacing 2 transformers on a single DESN station. Two existing 110kV/14.2kV 20MVA transformers will be replaced by two 30MVA transformers. First one is going to happen in spring and the second one is to happen in fall, which means in summer we will have two dissimilar transformers working in parallell for about 4 months. The existing transformer has a LTC on secondary with +/- 10% in 16 steps and the new transformer also has a LTC on secondary with +/- 20% in 32 steps. The per step voltage difference is 1.25% for both old and new transformers.
The existing LTC control scheme is very simple, neither Master/Flowwer nor circulating current or var control but a single AVR having multiple Raise/Lower outputs that command to change both transformers' tap simultaneously.
We plan to keep using the existing LTC control, however, during summer there will be two different transformers.

So my question is - except for the load sharing issues between two transformers, do you see any problems with regard to using the existing the LTC control during summer time?

Thanks in advance!

 

[davidbeach]

DESN=??

How do the transformer impedances match up?

 

[pwrtran]

davidbeach
DESN=dual element spot network, simply two transformers is parallel with bus tie closed.
Impedance:
20MVA one - 8.6%
30MVA one - 13%

 

[slavag]

Hi.
DESN, learn something new .

Im not like this solution, impedance difirence more than 10%.

That menans, load sharing will be:
x%=(20+30MVA)x( 20/8.6+30/13)=10.79%
TR1--> 20x(10.79/8.6)=25.09MVA
TR2--> 30x(10.79/13)=24.9MVA
sum= 25.09+24.9=50MVA

You can see, one Xfr will be overloaded, second will be underloaded.

Best Regards.
Slava

 

[waross]

Slava, at present, there are two 20 MVAs in service. The 30 MVA at 13% will be 8.67% That should be a very good match for the 8.6% transformer. The new 30 should share the load much as the outgoing 20 MVA transformer did. Performance while waiting for the second 30 MVA transformer should be much as if the 20 MVA was still in service.
This is assuming that the X/R ratios are close. If there is a significant difference between the X/R ratios the sum of the currents through the transformers may be greater than the load current resulting in more transformer heating than if the X/R ratios were matched.
This may be something to check if you are near or past 100% loading.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pwrtran]

Hi
We expect that the current splitting between two dissimilar xmfrs should be 50-50 roughly. Should the station loading exceed the summer LTR as Slava mentioned, we would not let it happen by means of transferring some load to other stations at distribution level.
What I really like to get advise from your expertise is - how do you foresee the performance/problems in terms of using the existing tap changer control during the change over?
Thanks!

 

[waross]

The per step voltage difference is 1.25% for both old and new transformers.

You have a good match on transformer impedances on a 20 MVA base. You have equal voltage steps per tap. You should be good to go.
You may consider inhibiting the new transformer from setting taps outside the range of the existing transformer. Keep it inside the 10% limits of the existing 20 MVA Xfrmr. As alluded to, if the X/R ratios are not equal the phase angle of the transformer voltage drops may differ. An X/R ratio difference may cause some circulating current at low load levels. Higher load levels will swamp out the circulating currents but may result in the sum of the transformer currents being slightly greater than the load current. I don't think that small differences in X/R ratios will be a problem. This may explain minor apparent errors that you may find in current sums.
I would expect any differences to be of a magnitude similar to small metering errors and may be mistake for metering error.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[pwrtran]

Thanks Bill !

 

[prc]

pwrtan,
one more point to check is impedance variation at extreme tappings.This can be different for the two transformers.This cab be checked from respective test reports