GENERATOR NEUTRAL SWITCHING AND THE STORY OF THE BLACK CAT

Once upon a time, there was this guruji, religiously conducting classes for his disciples in his gurukul. One day, at around noon, when the classes were in full swing, a black cat was moving here and there across the classroom, making the customary ‘meow’ noise. It was disturbing the concentration of the students as well as that of the teacher. On the command of the guruji, one of the students, chased away the black cat out of the classroom. But, next day, the black cat came again and disturbed the class. Again, someone chased the black cat away. This was repeating for about a week or so. The guruji got fed up with the black cat on the 8th day and asked the students to tie up the black cat to a pole outside the classroom. The students did so. After the classes were completed, the black cat was released. This was repeated every day.

One day, the guru died. A new guru from a nearby gurukul was transferred to this school and took charge. On the first day, the new teacher was about to conduct the class, the troubling black cat was not around. But the students did not allow the new teacher to start the class. They went around, found some street black cat, brought it to the school, tied the new black cat to the same old pole outside the classroom exactly at mid-noon and now asked the new teacher to start his classes. The new teacher was baffled at this strange practice and questioned the students the logic behind the strange practice. The students replied: “We don’t know sir, the old teacher used to ask us to tie a? black cat to that pole, everyday at mid-noon. We had been doing it for years. We can’t and should not overrule the wise guruji’s command. There must be some logic in guruji’s command”.

The ongoing discussions in some professional forums about neutral switching when more than one AC generators are operating in parallel is akin to the above story of the black cat.

In earlier days, when generators of various makes & designs were synchronised, with their neutral also interconnected, there was this problem of circulating neutral currents between the synchronised generators, which was causing unnecessary overheating of the generator windings and was causing pr-mature insulation failure of the generators. To avoid the neutral circulating currents, it was practiced to connect only one generator’s neutral (normally the neutral of the largest capacity generator) to earth/ground and all the other generators’ neutrals were kept isolated, when in synchronisation.

But, over a period of time, the generator winding technology evolved and almost all AC generators were wound with two-third pole pitch. With two-third pole pitched winding, the zero sequence harmonic voltages or the triplen harmonic voltages are suppressed and thus neutral circulating currents became negligible when such generators were operating in parallel, with their neutrals interconnected.

But even today, a lot many of the generator assemblers/contractors/consultants/Engineers, still insist on neutral isolation, when more than one generator is operating in parallel, without understanding the logic, like the black cat tying students.

The IEEE Green Book well acknowledges this fact where it says that “Generators that are designed to suppress zero-sequence harmonics, usually by the use of a two-thirds pitch winding, will have negligible circulating currents when operated in parallel; therefore, it is often found practical to operate these types of generators with the neutral disconnect device closed. This simplifies the operating procedure and increases assurance that the system will be grounded at all times”.

Reference is also drawn to “Application Guidance Notes: Technical Information from Cummins Generator Technologies - AGN 022 – Conditions for Parallel Operation”, wherein it says that “Parallel operation of alternators with dissimilar winding pitches can result in circulating currents. All STAMFORD alternators and some AVK alternators are wound with a 2/3rd winding pitch with minimal triplen harmonics. There is no basic reason why any system consisting of these types of alternators cannot have their star points interconnected and in turn, the Neutral link continued to the distribution network’s Neutral (star point) connection at the Point of Common Coupling (PCC)”.

Let us not be the foolish students to tie a black cat to the pole every day exactly at mid-noon, without any logic. Let us be wise and understand the logic behind the practice and improve, as we evolve.

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K. SIVAKUMAR

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