In this topic, you study Parallel Operation of Alternator.
Electrical power requirements of industry, commercial establishments and individual consumers are almost exclusively met by the alternators. In nearly all the power stations, it is now an established practice to supply the electrical power with the help of a number of small alternators running in parallel instead of installing one large capacity alternator for this purpose.
Need for Parallel Operation
Following are the advantages of this practice of installing a large number of small alternators and running them in parallel to supply a common load :
Continuity of Service: Using several small alternators is more reliable than using a large single alternator. This is because if one alternator fails, the continuity of supply can be maintained by operating other alternators.
Efficient Operation: Efficiency of an electrical machine is maximum when it is operated at or near its full load. If several small alternators are used they can be added or put off depending upon the load requirement and thus they can be operated at or near their rated capacity.
Ease of Maintenance: If there are a large number of small alternators, repair and maintenance of individual alternators is more easy and convenient keeping the continuity of supply. This will not be possible if only one large alternator is installed.
Facility of Extension: Load growth can be easily handled by installing additional alternators as and when required without disturbing the original arrangement.
Small Cost of Standby Unit: Since the capacity of a standby unit needed is small, its cost is also less.
Conditions for Parallel Operation
Before connecting an alternator in parallel with others already in operation, following conditions must be fulfilled.
- Terminal voltage of the incoming alternator (i.e. an alternator which is to be connected in parallel) must be numerically equal to that of the system bus-bars to which it is to be connected.
- Frequency of the incoming alternator must be equal to the frequency of the system to which it is to be connected.
- Phase sequence of the incoming alternator must be the same as that of the system.
- Phase of the incoming alternator voltage must be identical with the phase Of the bus-bar voltage at the instant of connecting the alternator to the bus-bars. That is, the two voltages must be in phase with respect to the load circuit and in phase opposition relative to the local circuit. This is essential to avoid any circulating current between the incoming alternator and the bus-bars.