Continuing in the previous post, on every occasion generators are in parallel, their positive and negative terminals are severally connected to the positive and negative sides of the bus-bars. These bus-bars are dense thick copper bars and that they act as positive and negative terminals for the entire power plant. If polarity of the inward generator isn't identical because the line polarity, as serious short-circuit can occur once S1 is closed.
Furthermore, paralleling a generator with opposite polarity meritoriously short-circuits it and results in damage brushes, a damaged commutator and a blacked-out plant. Generators that are trip up off the bus-because of an important fault current should be tested for reversed polarity before paralleling.
In the above fig is shown a shunt generator No. one connected through the bus-bars and supplying some of the load. For connecting generator No. two in parallel with it the subsequent procedure is adopted.
The armature coil of generator No. two is speeded by the prime-mover up to its rated value and so switch S2 is closed and circuit is completed by connecting a voltmeter V across the open switch S1. The excitation of the incoming generator No. two is modified until the voltmeter V reads zero. Then it implies that its terminal voltage is that the same as that of generator No. one or bus-bar voltage. Later this, switch S1 is closed and then the incoming machine is paralleled to the system. under these conditions, however, generator No. two isn't taking any load, as a result of its induced e.m.f. is that the same as bus-bar voltage and there is no flow of current between 2 points at identical potential. The generator is claimed to be ‘floating’ on the bus- bar. If generator No. two is to deliver any current, then its induced e.m.f. (E) ought to be bigger than the bus-bar voltage V. therein case, current equipped by it's I = (E − V)/Ra where Ra is the resistance of the armature circuit. The induced e.m.f. of the incoming generator is redoubled by strengthening its field until it takes its correct share of load. At identical time, it should be found necessary to weaken the sector of generator No. one to take care of the bus-bar voltage V constant.