I am going to parallel 3 T/Fs( two 31.5MVA and one 30 MVA) which have different step voltages. Two identical TFs have 1.25% (21taps)step voltages and other has 1.5%(16 taps). i am going to use TAPCON 230 pro AVR model.
i am going to implement "master-follower' method. i set one of 30MVA TF) as the "master". what will happen if it trip off. Will there be "parallel failure' come ?.
How can i see the circulating reactive current?
I appriciate ur help.
janaka from sri lanka

From my limited knowledge with unmatched transformers, I expect that you'll have mis-matched impedances, as well as tap steps. Both of these will provoke circulating currents using a Master-Follower regime, which could be excessive . If these get too high, there's a risk of tripping directional overcurrent protection, as well as being very inefficient.

I'd recommend you have a look at alternative AVC relays such as "SuperTAPP", "MicroTAPP" and "SuperTAPP N+". This range was specifically developed by Fundamentals Ltd to manage exactly the situation you describe. Most are available through Siemens-Reyrolle. The operating algoritm is heavily biased towards reducing circulating currents. (Please note, I don't have any personal interest in these companies, but have worked alongside Fundamentals on a couple of projects).

Good luck

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Jonno

Attempting to parallel transformers which do not have the same impedance & tappings will be troublesome.

Different impedances will prevent the units sharing the load equally, resulting in one becoming overloaded before the other reaches full load.

The different tappings will result in circulating currents driving reactive power around the transformer loop as the transfomer with the higher terminal voltage attempts to 'drive' the other transformer in reverse. These currents could be significant and may approach an appreciable proportion of the max permitted load current, leading to a much lower combined capacity than you are expecting.

These currents will also result in elevated copper losses for the two transformers which will be present even with no load applied.

To see the circulating currents you need to be able to monitor the currents in the lines which link the units, pref. both in the primary & secondary. You can only really do this with no load applied since there is no way to distinguish the reactive current from the load current otherwise.

My advice, in short, is dont do it! As Jonathan says, if these transformers are protected using a directional over current scheme to detect faults & isolate only a faulty transformer then you risk maloperation of this scheme.

Thanks , anyway can I implement "Minimum Circulating method " for this case