In the case of certain system transformers operating in parallel it is relatively common practice (particularly in transmission and distribution substations) to set the on-load tap changers with AVR system to give a ‘tap stagger’ so that the system voltage profile at the point where the transformers are located can be varied by adjusting the reactive load flows at that point. Such practice results in local circulating currents between the transformers irrespective of their load throughput.
Where two or more transformers with automatically controlled on-load tap changers are operating in parallel, it is normally necessary to keep them either on the same tapping position or a maximum of one tap step apart. If transformers are operated in parallel on different tappings circulating currents will be set up and in general one step is the most that can be tolerated.
Many different schemes of parallel control have been devised, several of which are in regular use. If it is considered necessary that all transformers must operate on the same tapping this can be achieved by a master–follower system or by a simultaneous operation method.
2-Parallel transformers bank protection
The most famous protection of parallel transformers bank is differential protection. From the standpoint of protective relaying, the operation of two transformer banks in parallel without individual breakers is to be avoided. In order to obtain protection equivalent to that when individual breakers are used. To protect two equally rated banks as a unit, using only CT's on the source sides of the common breakers and a single relay is only half as sensitive as protecting each bank from its own CT's; this is because the CT ratios must be twice as high as if individual CT's were used for each bank, both banks being assumed to have the same rating, and as a result the secondary current for a given fault will be only half as high. If one bank is smaller than the other, its protection will be less than half as sensitive. With more than two banks, the protection is still poorer.
When parallel transformer banks having individual breakers are located some distance away from any generating station, a possibly troublesome magnetizing-current-inrush problem may arise. If one bank is already energized and a second bank is then energized, magnetizing-current inrush will occur-not only to the bank being energized but also to the bank that is already energized. Moreover, the inrush current to both banks will decay at a much slower rate than when a single bank is energized with no other banks in parallel.