Parallel Operation of DC Generators

Parallel Operation of DC Generators.

In most power stations it is necessary to supply power from several small units (generators) rather than from a single large unit, so that the number of working generators at any time can be varied according to the magnitude of the load on the station. This keeps the machines loaded up to their rated capacity and increases efficiency of operation.

The other advantages of several small units over single large unit are

Increased reliability
more convenient and economical repairing
small cost of stand by unit and
possibility of installation of an additional unit when required with the growth of the load.

There are two ways in which the de generators can be connected to the load, namely in series or in parallel.

When several generators are connected in series all the units deliver the same current and load on any unit is proportional to its terminal voltage.

The latter arrangement i.e., parallel arrangement is almost universally adopted, in which all the generators work at the same voltage and load on any unit is proportional to the current delivered by it.

1. Series Wound Generators in Parallel.

In running the series wound generators in parallel it is necessary to connect the armature of the two machines through a copper bar as shown in Fig. (a) This arrangement is used in braking position of a locomotive controllers.

The two motors are disconnected from the supply mains andare connected in par allel to the resistance.

The motors continue to run due to kinetic energy of train and act as generators.

The ki netic energy is con verted into electrical energy which is dissi pated as heat in the external resistance.

(a) series generators in parallel with equalising bar connection

2. Shunt Wound Generators in Parallel.

Shunt wound generators due to their drooping characteristics are best suited for parallel operation. The generator with more drooping characteristic carries small load.

While connecting two de shunt generators in parallel certain precautions are necessary to be taken.

The polarity of both the machines must be the same i.e., the positive terminal of one machine must be connected to the positive terminal of the other and similarly negative terminals must be connected together. Terminal voltage of both generators must be approximately equal.

The polarity and terminal voltage can be determined by a moving coil-parallel voltmeter.

When a number of de generators operate in parallel, +ve and -ve terminals of all the machines are connected to two separate heavy copper bars placed behind the switch panel. These copper bars known as bus-bars also may be regarded as +ve and -ve terminals for the whole power station.

The generators are connected to busbars through switches, and S, as shown in fig (b)

(b) Shunt Generators in Parallel Operation

3. Compound Wound Generators in Parallel.

The compound wound de generators are run in the same way as the shunt wound dc generators.

In this case, similar to the case of running of series generators in parallel, it is necessary to have equalising bars to prevent the series field from reversing.