What is the need of parallel operation? What are the conditions to be satisfied for parallel operation of transformers?
Answers
Parallel Operation of transformers
Two transformers are connected in parallel means that the two primary windings are connected to supply bus and the two secondary windings are connected to load bus-bars as shown in the figure.
Parallel Operation of Transformer
Parallel Operation of Transformer
Need of parallel Operation of Transformers
To supply a load in excess of the ratings of an existing transformer, Two or more transformers may be connected in parallel with the existing transformer. This is more economical connecting an extra small transformer in parallel instead of keeping an another large capacity transformer. The cost is also less for purchasing extra small rating transformer.
Parallel operation of transformers provides more reliability i.e. even in the failure or out off service of one transformer half of the bus load can be driven using signal transformer in emergency cases.
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Conditions for parallel operation of Transformers
There are various conditions that must fulfilled for the successful operation of transformers as follows.
The line voltage ratio of two transformers must be equal.
The per unit impedance of each transformer should be equal and they should have same ratio of equivalent leakage reactance to the equal resistance(X/R).
The transformers should have same secondary winding polarity.
The Transformers should have same phase sequence (Three phase transformer)
The transformers should have the zero relative phase replacement between the secondary line voltages.(Three phase transformers)
1. The line voltage ratios of the two transformers must be equal
This condition is used to avoid the inequality EMF induction at the two secondary windings. If the two transformers connected in parallel have slightly different voltage ratios, then due the inequality of induced emfs in the secondary voltages, a circulating current will flow in a loop format in the secondary windings. This current is greater than the no load current and will be quite high due to less leakage impedance during load. When the secondary windings are loaded, this circulating current will tend to unequal loading on two transformers and one transformer may be over loaded and another may be less loaded.
2) Equal per unit leakage impedance
If the ratings or line voltages are equal their per unit leakage impedance’s should be equal in order to have equal load sharing of the both transformers. If the ratings are unequal then the transformer which has less rating will draw more current and it leads to unequal load sharing. It may also lead to mismatch in line voltages due to voltage drops. In other words, for unequal ratings, the numerical values of their impedance’s should be in inverse proportional to their ratings to have current in them inline with their ratings.
A difference in the ratio of the reactance value to the resistance value of the impedance results in different phase angles of the currents carried by the two parallel transformers. Due to this phase angle difference between voltage and current, one transformer may be working on high power factor and another transformer may be working on lower factor. Hence real power sharing is not proportional between the two transformers.
3) The transformers should have same secondary winding polarity
The transformers should be properly connected with regards to their polarity. If they are connected with in correct polarities then the two emf’s induced in the secondary winding which are in parallel, will act together and produce a short circuit between the two of them. Total loss of power supply and high damage to the transformers.
4) The Transformers should have same phase sequence
In case of three winding transformers in-addition to the above conditions the phase sequence of line voltages of the both transformers must be identical for parallel operation. If the phase sequence is not correct in every voltage cycle each pair of phases will get shorted
.