Electrical 1

[spdave2004]

IN OLTC we observed "OUT OF STEP" Alarm, we have two different transformer, when both are in parallel this type of alarm observed. what's the meaning of this alarm and why it occurs???

 

[marks1080]

If you are only gettin the alarm when the transformers are in parallel it is a good indication that the tap changers aren't syncing properly. It is important to keep the taps equal on transformers connected in parallel to avoid a circulating current that would be caused by the impedance difference of taps being unequal. What i just said assumes the two transformers are identicle. In some cases, where you have transformers from two different manufacturers, it might not be necessary to keep taps equal, as long as effective impedance from the winding is equal.

You may have a problem with the transducer on one of the transformers which tells the control circuit what tap it is on. It could also be a problem with the tap changer itself not making a solid connection to the tap.

You said the alarm doesn't come in when only one transformer is in service. Have you tried single service to your station with both transformers? If you haven't and you put the unit that hasn't been tested into service as a single supply and you get the same out of step alarm it is likely to be a problem with the tapchanger.

 

[Marmite]

If the tapchange scheme is master/follower then you would expect them to be in step, and if they go out of step you get an out of step alarm. However, there are other voltage control schemes such as circulating current, or negative reactance circulating current, where it is quite normal for the taps to be one or two steps apart even on identical transformers. It can be beneficial in some cases as it essentially gives you a half a tap voltage step which can reduce the number of tapping operations and therefore tapchanger maintenance. You need to give more details of the voltage control scheme.
Regards
Marmite

 

[ScottyUK]

It means that the two tapchangers, which should be tapping simultaneously, are not doing so and are on different taps. This usually results in large circulating currents.


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If we learn from our mistakes I'm getting a great education!

 

[spdave2004]

Hi Marks1080

Thanks for you comment.
what I observed, Primary voltage of both the Transformers are not exactly same, around 2000-3000V difference observed.
secondly AVR operate Tap changer to compensate secondary voltage. and beacuse of tap positions are different it gives Out of step alarm, is it right????? or actually problem in Tap changer???
Thanks once again.

 

[magoo2]

Whether the primary voltages match or not, that shouldn't affect what you're working with. With them paralleled, they are both connected to the same primary voltage, correct? Or are you just paralleling them on the secondary?

What Scotty told you is correct. With 2 paralleled transformers (primary and secondary), if one of the tap changers is out of step with the other, you develop large circulating currents.

 

[spdave2004]

Both the transformers has different source on primary side and we are paralleling secondary side only.

 

[marks1080]

What i mentioned were just possible (but likely) causes of your alarm. You need to get field staff to investigate.

 

[veritas]

Hi spdave2004

The master/follower or master slave schemes are intended to be applied where trfrs primaries are connected to the same busbar. If this is not the case then it would be unwise to use these OLTC philosophies as circulating currents are definitely possible even with the trfrs on the same tap.

Note also, that impedance does not play such an important role as turns ratio! Trfrs differing in impedance by up to 50% can still be paralleled, the only issue is that they will not be sharing the load current in proportion to their ratings. If the units are not heavily loaded this should not be a problem.

If the source voltages differ then I recommend you consider a tapchanging algorithm such as those found in the REG-D relay (from Eberle) which seeks to minimuse the circultaing current based on various principles. In your case the algorithm that regulates when the source voltages are different is based on the power factor of the system.

Here's their website:

http://www.a-eberle.de/index.php?lang=english

Hope this helps

 

[waross]

If the primary voltages are not the same the tap changers should be set to develop the same voltage on the secondaries. If the secondaries are connected in parallel with different voltages you can expect circulating currents. If the tap changers are matching different voltages from different primary sources the tap changers will not be on the same step.
Your "OUT OF STEP" alarm may possibly need to be turned off in this application.

Bill
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"Why not the best?"
Jimmy Carter

 

[veritas]

I agree with waross. This thus rules out master follower or master slave. As the trfrs cannot be set to independant whilst operating in parallel (tapchange runaway could result)a different voltage regulating algorithm is required.

Thus my recommendation that an algorithm such as those in the REG-D or similar is required.

Regards.