Automatic alternator synchronisation.doc
(Size: 189 KB / Downloads: 153)
The manual method of synchronization demands a skilled operator and the method is suitable for no load operation or normal frequency condition. Under emergency condition such as lowering of frequency or synchronizing of large machines a very fast action is needed, which may not be possible for a human operator. Thus there is a need of autosynchroniser in a power station or in an industrial establishment where generators are employed. This paper describes a microprocessor based set up for synchronizing a three phase alternator to a busbar. Also existing methods of synchronization are mentioned.
It is well known that electrical load on a power system or an industrial establishment, is never constant but it varies. To meet the requirement of variable load , economically and also for assuring continuity of supply the number of generating units connected to a system busbar are varied suitably . The connection of an incoming alternator to system bus, ie; synchronization requires fulfillment of the condition like the same phase sequence equality of voltages and frequency between the incoming machine and frequency between the in coming machine and busbar. In order to order to overcome the 9 technical drawbacks of the conventional synchronization methods we can introduce a microprocessor based system.
EXISTING METHODS OF SYNCHRONIZATION AND PRINCIPLE
a. Synchronizing Lamp
The operation of connecting an alternator parallel with another alternator or with a common busbar is known as synchronizing for proper synchronization of alternators the following three conditions must be satisfied
1. The terminal voltage of incoming machine must be the same as the busbar voltage.
2. The speed of the incoming machine must be same such that the frequency is equal to the busbar frequency.
3. The phase of the alternator voltage must be identical to the busbar voltage.
It means that the switch must be closed at the instant the two voltages are in correct phase.
Condition 1 can be checked with the help of voltmeter, frequency is adjusted by varying the prime mover speed. In the dark lamp method the lamps are connected across the alternator and busbar terminal. If the phase sequence is different, the lamps will brighten in a cyclic manner correct phase sequence is indicated by simultaneous darkening brightening of lamps. The switch is closed in the middle of the dark period. Once synchronized properly, the two alternators continues to run in synchronism.
The armature of the sychroscope will align itself so that the axis of windings are R and F are inclined at an angle equal to phase displacement between V and V’. If there any difference between the frequencies of V and V’ a pointer attached to the armature shaft will rotate at slip speed, and the direction of its rotation will indicate whether the incoming machine is running above or below synchronism. At synchronism, the pointer will remain stationary, but it must be brought to the particular position which indicates zero phase displacement between V and V’ before the main switch of the incoming generator is closed.
Synchronisation by means of manually operated switching served well enough when the individual generators were relatively small, but with the growth of system capacity, it becomes necessary to use automatic devices to ensure the closing of the main switch of the incoming machine at the proper instant.
The scheme introduced here is for the complete automation of synchronization i.e.; the adjustment of magnitude of voltage and frequency of incoming alternator is done automatically. When all the requirements of synchronisation are satisfied, closing of the main switch of the incoming machine is done by the automatic synchronizer
CRITERIA OF DESIGN
The auto synchronizer has been developed to carry out the following tasks related to the synchronization such as
I To check if the phase sequence of incoming machine is correct or otherwise, in case of wrong phase sequence, to terminate the further steps in the process and also to indicate corrective action.
II To check if frequency of incoming machine is equal to that of busbar and to adjust it to a value nearly equal to the busbar frequency.
III to check machine voltage is equal to that of busbar and to adjust it to a value nearly equal to the busbar voltage and
IV After ascertaining the fulfillment of the above condition, to give closing signal to the circuit breaker so that the breaker will close the exact inphase instant.
In addition, the auto synchronizer has been designed so that the alternator is started with in minimum voltage and minimum frequency conditions