To operate them in parallel on a continuous basis, they must have the same voltage ratio and a relatively close impedance; if they are polyphase they must have the same configuration (wye/wye, delta/wye, etc.) If they are not the same KVA rating, you will be limited by the smaller unit.
we have three power transformers of ratings 4MVA having impedance percentage of 4.97%, 2MVA of impedance 5.4%,another 2MVA of impedance 5.3%.the input voltage is 66KV and the output voltage is 11.5KV .The 4MVA and 2MVA of 5.3% impedance each have been connected to VCB's but the 2MVA of 5.4% impedance is connected to an MOCB. Can these above specified transformers be paralled?. if no, please tell us why.
thank you
The following conditions must exist for transformers to operate
satisfactorily in parallel:
1. Connection diagrams must be identical. Paralleling transformers
with different connection diagrams is similar to short circuiting
their secondary windings.
2. Voltage ratios must be the same. If voltage ratios are not the
same, circulating currents will flow in the secondaries with no or
little load, and the division of load will be improper.
3. Percent impedance, including primary and secondary leads to each
transformer, should be nearly equal. If the impedances are equal and
the turns ratios are identical, the paralleled transformers will
divide the load currents (properly) in proportion to their kVA
ratings. If the percent impedances are different, the transformer
with the lower percent impedance will take more than its proper share
of the load.
If the impedances are equal and
the turns ratios are identical, the paralleled transformers will
divide the load currents (properly) in proportion to their kVA
ratings. .
basically, to parallel up two system, some element should be looked 1) voltage level 2) phase sequence 3) phase angle 4) frequency. however in this case, consider that the voltage and frequency are the same. The concern is the load support by that transformer. There where the different rating come from. therefore, in order to parallel it, we need to control its load current because when these transformer are paralleled, they may share the same bus (bustie close). So they will share the same fault current and everything. So, what may we can do is we install a current limiter to control the current go through the less rating transformer. By this, we are possible to paralleled the two different rating of trasformer. Or other member has another idea?
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