Parallel operation of transformers is effected when both the HV and LV windings of two (or more) transformers are connected to the same set of HV and LV busbars respectively. Since connecting two impedances in parallel will result in a combined impedance which is very much less than either of the components (paralleling three identical transformers results in a combination which has an impedance of third that of each, individually) the primary result of this is to increase the fault level of the LV busbar.
If Z1=Z2=Z3=Z we will have Zt=Z/3 and Isct=3Isc where Isc is individual fault current and Isct is total fault current after paralleling. Care must therefore be taken to ensure that the fault capability of the LV switchgear is not exceeded.
In the study of the parallel operation of transformers, polarity and phase sequence play important parts, and so it is essential to consider these characteristics in some detail before passing on to the more general treatment of Parallel operation. The points to consider are the relative directions of the windings, the voltages in the windings and the relative positions of leads from coils to terminals. To understand how each of these factors interact it is best to consider transformer operation in instantaneous voltage terms relating directly to a phasor diagram, that is, by studying transformer polarity diagrams basing an explanation upon the instantaneous voltages induced in both windings, as this procedure avoids any reference to primary and secondary windings. This can be seen to be logical as transformer polarity and phase sequence are independent of such a distinction.
Unless fuse protection is provided each of the outgoing circuits would also need to be designed and cabled to withstand the full fault level of the paralleled transformers.
The satisfactory parallel operation of transformers is dependent upon five principal characteristics; that is, any two or more transformers which it is desired to operate in parallel should possess:
1. The same inherent phase angle difference between primary and secondary terminals.
2. The same voltage ratio.
3. The same percentage impedance.
4. The same polarity.
5. The same phase sequence.
To a much smaller extent parallel operation is affected by the relative outputs of the transformers, but actually this aspect is reflected into the third characteristic since, if the disparity in outputs of any two transformers exceeds three to one it may be difficult to incorporate sufficient impedance in the smaller transformer to produce the correct loading conditions for each unit.
Characteristics 1 and 5 only apply to polyphase transformers. A very small degree of latitude may be allowed with regard to the second characteristic mentioned above, while a somewhat greater tolerance may be allowed with the third, but the polarity and phase sequence, where applicable, of all transformers operating in parallel must be the same.
It is very desirable that the voltage ratios of any two or more transformers operating in parallel should be the same, for if there is any difference whatever a circulating current will flow in the secondary windings of the transformers when they are connected in parallel, and even before they are connected to any external load. Such a circulating current may or may not be permissible.
This is dependent firstly on its actual magnitude and, secondly, on whether the load to be supplied is less than or equal to the sum of the rated outputs of the transformers operating in parallel. As a rule, however, every effort should be made to obtain identical ratios, and particular attention should be given to obtaining these at all ratios when transformers are fitted with tappings.
In passing, it may be well to point out that when a manufacturer is asked to design a transformer to operate in parallel with existing transformers, the actual ratio of primary and secondary turns should be given, as this ratio can easily be obtained exactly. Such figures would, of course, be obtained from the works test certificate for the existing transformers.
It is to be noted that this flow of circulating current takes place before the transformers are connected up to any external load. A circulating current in the transformer windings of the order of, say, 5% of the full-load current may generally be allowed in the case of modern transformers without any fear of serious overheating occurring. It is sometimes very difficult to design new transformers to give a turns ratio on, say, four tappings identical to what an existing one may possess, and while it is desirable that the ratios should be the same, it is not necessary to insist on their being identical.
Circulating current control
Where two or more transformers of similar impedance are operated in parallel they will each provide an equal share of the load current. In the event of one of these transformers changing to a higher tapping position, a circulating current will flow between this transformer and the remaining units. This circulating current will appear as a lagging current from the unit which has changed taps.
It will be equally divided between the other transformers which are in parallel and will appear to these transformers as a leading current.
It is possible by judicious connection of current transformers to separate this circulating current from the load current and introduce it into components in the automatic voltage regulating (AVR) circuit. These are so connected into the AVR circuit such as to provide an additional voltage to the AVR which has tapped up and a subtractive voltage to the remaining AVRs controlling the parallel-connected transformers. Using this method and carefully adjusted components, transformers can be kept within close tapping positions of each other. There has been much development in the supervisory control of system voltages, and on some systems centralised control has been achieved by the operations of tapchangers by remote supervisory methods. This is usually confined to supervisory remote pushbutton control, with an indication of the
tapchanger position, but more complicated schemes have been installed and are being satisfactorily operated where tapchangers are controlled from automatic relays on their respective control panels, with supervisory adjustmentof their preset voltage and selection of groups operating in parallel, and with all necessary indications reported back by supervisory means to the central control room.