Parallel Operation of DC Generators

In modern power system, power is usually supplied by many synchronous generators connected in parallel which helps to maintain continuity of plant operations. Now a day's, isolated one large generator is outdated. When we connect two generators in parallel, they have a tendency to remain in step. By doing some changes in their armature current and by connecting generators properly to the existing bus bar, this problem could be solved.

Connection of Parallel DC Generators

The generators in a power plant, connected by heavy thick copper bars, called bus-bars which act as positive and negative terminals. To connect the generators in parallel, Positive terminal of the generators are connected to the positive terminal of the bus-bars and negative terminals of generators are connected to negative terminal of the bus-bars, as shown in the figure.

To connect the 2 generators with the 1 existing working generators, first we have to bring the speed of the prime mover of the 2nd generator to the rated speed. At this point switch S4 is closed.

1. The circuit breaker V₂ (voltmeter) connected across the open switch S₂ is closed to complete the circuit. The excitation of the generator 2 is increased with the help of field rheostat till it generates voltage equal to the voltage of bus-bars.
2. The main switch S₂ is then closed and the generator 2 is ready to be paralleled with existing generator. But at this point of time generator 2 is not taking any load as its induced e.m.f. is equal to bus-bar voltage. The present condition is called floating, that means ready for supply but not supplying current to the load.
3. In order to deliver current from generator 2, it is necessary that its induced e.m.f. E should be greater than the bus-bars voltage V. By strengthening the field current, the induced e.m.f. of generator 2 could be improved and the current supply will get started. To maintain bus-bar voltage, the field of generator 1 is weakened so that value remains constant.

Field current I given by where, R_a is resistance of armature winding.

Load Sharing of Parallel Connected DC Generators

The load gets shifted to another generator by adjusting induced e.m.f., but in modern power plant, everything has been done by “sychroscope” which gives instruction to governor of the prime mover. Let us suppose that two generator having different load voltage. Then the load sharing between these generators will beThe value of current output depends upon the values of E₁ and E₃ which could be managed by field rheostats to keep the bus-bars voltage constant.

Advantages of Parallel Operation of DC Generators

Cost Effectiveness:- Electric power cost reduces when generator produce electricity according to their capacity and easily manage demand and supply requirement. If there is less requirement of power, one or more units can be taken on hold or startup visa versa. Smooth Supply of Power:- In case of breakdown of generator, the supply of power will not get disrupted. If something goes wrong in one generator, the continuity of power can be continued by other healthy units. Easy to Maintain:- The routine maintenance of generator is required time to time. But for that supply of power cannot be obstructed. In parallel generators the routine checkup could be done one by one. Easy to Increase Plant Capacity:- The demand of electricity is increasing day by day. To meet the requirement of power generation, additional new unit can run paralleled with the running units.

Precautions During Parallel Connection

1. The specification of each generator is different from one another. When they are synchronized together, their speed are locked into the overall speed of the system.
2. The entire load of the system should be distributed in all the generators.
3. There should be a controller for keeping check on parameters of the engine. This can be done with modern digital controllers which are available in market.
4. Voltage regulation in the whole system plays an important role. In case of voltage drop in one unit compare with other units, end up bearing the whole voltage load of the system of parallel generators.
5. While connecting terminals to the bus-bars, extra precaution should be made. If generator is connected with wrong polarity of the bar, it may result to a short circuit