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PARALLEL OPERATION OF INTERCONNECTED SYNCHRONOUS

GENERATORS

In order to assure continuity of the power supply within prescribed limits of frequency and voltage at all the load points scattered over the surface area, it becomes necessary in any modern power system to operate several alternators in parallel, interconnected by various transmission lines, in a well-coordinated and optimized manner such that the operation is most economical.

Requirements for parallel operation. In order to operate properly, ac polyphase generators must first meet several requirements before they may be connected in parallel.

1. They must have the same voltage rating.

2. They must have the same frequency rating. The speeds need not necessarily be the same.

3. The waveform of the voltages should be the same, and therefore the generators should be of the same type, though their kva ratings may differ.

4. In order to divide load in proportion to their ratings, the alternators should have prime movers whose speed-load characteristics are the same.

The above requirements are really those necessary for the selection of the alternators in the first place. It is only after those conditions are satisfied that we can begin to place the alternators in parallel to furnish current to a load. The actual procedure of paralleling alternators is known as synchronizing, and in performing this operation, certain other requirements must be met.

Synchronizing procedure. It may logically be assumed that one alternator is placed in parallel with one or more other alternators only when additional load requires it. Those alternators already carrying load are known as the running machines, while that which is to be placed in the system is known as the incoming machine. At the time of synchronizing, several conditions must be met.

1. The effective voltage of the incoming generator must be exactly equal to that of the others, or of the bus bars connecting them.

2. The phase rotation, or sequence of the running and incoming generators, must be the same.

3. The individual phase voltages which are to be connected to each other must be in exact phase opposition. This is the same as saying that dc generators must be connected + to + and - to -.

4. The frequencies should be the same, although it is more desirable that the frequencies at the instant of paralleling be almost, but not quite, identical.

In essence, the above conditions are all necessary in order to have no circulating currents between the generators at the time when their terminals are connected together.

A synchroscope or a bright-lamp synchronizing method or a dark-lamp synchronizing method is used for indicating the appropriate moment for synchronization. After the machine has been synchronized and is now a part of the system, it can be made to take its share of the active and reactive power by appropriate adjustments of its prime-mover throttle and field rheostat. The system frequency and the division of active power among the generators are controlled by means of prime-mover throttles regulated by governors and automatic frequency regulators, whereas the terminal voltage and reactive volt-ampere division among the generators are controlled by voltage regulators acting on the generator-field circuits and by transformers with automatic tap-changing devices.

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