Parallel Operation of Transformer

PARALLEL OPERATION OF TRANSFORMERS

INTRODUCTION:

For supplying a load in excess of the rating of an existing transformer, two or more transformers may be connected in parallel with the existing transformer. The transformers are connected in parallel when load on one of the transformers is more than its capacity. The reliability is increased with parallel operation than to have single larger unit. The cost associated with maintaining the spares is less when two transformers are connected in parallel. * It is usually economical to install another transformer in parallel instead of replacing the existing transformer by a single larger unit. The cost of a spare unit in the case of two parallel transformers (of equal rating) is also lower than that of a single large transformer. In addition, it is preferable to have a parallel transformer for the reason of reliability. With this at least half the load can be supplied with one transformer out of service.

CONDITION FOR PARALLEL OPERATION OF TRANSFORMER:

For parallel connection of transformers, primary windings of the Transformers are connected to source bus-bars and secondary windings are connected to the load bus-bars. * Various conditions that must be fulfilled for the successful parallel operation of transformers:

Same voltage Ratio & Turns Ratio (both primary and secondary Voltage Rating is same). 2. Same Percentage Impedance and X/R ratio. 3. Identical Position of Tap changer. 4. Same KVA ratings. 5. Same Phase angle shift (vector group are same). 6. Same Frequency rating. 7. Same Polarity. 8. Same Phase sequence.

Some of these conditions are convenient and some are mandatory. * The convenient are: Same voltage Ratio & Turns Ratio, Same Percentage Impedance, Same KVA Rating, Same Position of Tap changer. * The mandatory conditions are: Same Phase Angle Shift, Same Polarity, Same Phase Sequence and Same Frequency. * When the convenient conditions are not met paralleled operation is possible but not optimal.

1.SAME VOLTAGE RATIO & TURNS RATIO (ON EACH TAP):

If the transformers connected in parallel have slightly different voltage ratios, then due to the inequality of induced emfs in the secondary windings, a circulating current will flow in the loop formed by the secondary windings under the no-load condition, which may be much greater than the normal no-load current. * The current will be quite high as the leakage impedance is low. When the secondary windings are loaded, this circulating current will tend to produce unequal loading on the two transformers, and it may not be possible to take the full load from this group of two parallel transformers (one of the transformers may get overloaded). * If two transformers of different voltage ratio are connected in parallel with same primary supply voltage, there will be a difference in secondary voltages. * Now when the secondary of these transformers are connected to same bus, there will be a circulating current between secondary’s and therefore between primaries also. As the internal impedance of transformer is small, a small voltage difference may cause sufficiently high circulating current causing unnecessary extra I2R loss. * The ratings of both primaries and secondary’s should be identical. In other words, the transformers should have the same turn ratio i.e. transformation ratio.

If two transformers connected in parallel with similar per-unit impedances they will mostly share the load in the ration of their KVA ratings. Here Load is mostly equal because it is possible to have two transformers with equal per-unit impedances but different X/R ratios. In this case the line current will be less than the sum of the transformer currents and the combined capacity will be reduced accordingly. * A difference in the ratio of the reactance value to resistance value of the per unit impedance results in a different phase angle of the currents carried by the two paralleled transformers; one transformer will be working with a higher power factor and the other with a lower power factor than that of the combined output. Hence, the real power will not be proportionally shared by the transformers. * The current shared by two transformers running in parallel should be proportional to their MVA ratings. * The current carried by these transformers are inversely proportional to their internal impedance. * From the above two statements it can be said that impedance of transformers running in parallel are inversely proportional to their MVA ratings. In other words percentage impedance or per unit values of impedance should be identical for all the transformers run in parallel. * When connecting single-phase transformers in three-phase banks, proper impedance matching becomes even more critical. In addition to following the three rules for parallel operation, it …