Parallel operation of alternator is a method of adding an alternator in the existing live system with another alternator. When there is more demand for power to supply the load, there comes the need for an additional generator. So, additional synchronous generators are connected to the live system to deliver the required power.
If you are not aware of the construction of the alternator. Read here.
Generally, in a power system, alternators are connected in parallel with many other alternators. The operation of connecting two or more such alternators in parallel with one another needs proper synchronism. There are certain advantages of operating two or more alternators in parallel.
Before going into the parallel operation of alternator, we need to know, why the parallel operation is needed in power systems?
What is the Need for Parallel Operation?
Parallel operation of alternator is needed to satisfy one or many of the following.
- To supply big loads, parallel operation of alternators is essential, which cannot be fulfilled by a single alternator.
- During light load conditions, instead of operating all the alternators, it is better to operate one alternator and shut down the other alternators.
- If any one of the alternators is shut down because of any problem or maintenance, the other alternator in parallel is operated for continued supply.
- If there is an increase in the demand for electrical energy, then it is possible to add another alternator to meet the demand.
- Operating two or more alternators in parallel will reduce the operating cost and the cost of energy generation.
Since AC power systems are interconnected, there comes a need for the parallel operation of alternators. Whenever you visit generating stations like a thermal power plant, nuclear power station, etc, you can observe the alternators connected in parallel.
Conditions for Parallel Operation
We cannot simply connect two alternators in parallel with one another because many parameters like voltage, frequency and phase sequence of the voltage will come into play while paralleling. So, we need to consider those parameters while doing the parallel operations.
An improper synchronization can affect and collapse the entire power system and result in damage to generators, transformers and other power system components. For proper synchronization of alternators, the following three conditions must be satisfied.
- The terminal voltage of the incoming alternator must be the same as the bus-bar voltage.
- The speed of the incoming machine must have the same frequency as that of the bus-bar frequency.
- The phase sequence of the incoming alternator voltage must be identical to that of the bus-bar voltage. The switch must be closed at the instant the two voltages have correct phase relationships.
Synchronizing of Single Phase Alternators
The process of connecting an alternator in parallel with another alternator or with an infinite bus bar system is called synchronizing. The different ways of synchronizing the alternators are discussed below.
Dark Lamp Method
Let us consider, alternator-2 is to be synchronized with the alternator-1, which is already connected. This is done with the help of two lamps, Lamp1 and Lamp2, which are connected as shown in the below figure.
If the speed of the new alternator-2 is not brought up to that of the alternator-1, then its frequency will also be different, hence there will be a phase difference between the two alternator voltages.
This phase difference will be continuously changing with respect to the change in their frequencies. Due to this, the resultant terminal voltage will undergo changes.
The resultant voltage will be maximum for some time and it will be minimum for some time. Accordingly, the current will also be maximum and minimum.
Due to the changing current through the lamps, a flicker will be produced in the lamp, the frequency of the flicker being (f2-f1). The lamp will dark out and glow up alternately.
Darkness indicates that the two voltages E1 and E2 are in exact phase opposition and hence there is no resultant current through the lamps. Synchronizing is done in the middle of the dark period. Hence this method of synchronizing is called the Dark Lamp Method of synchronization.
Bright Lamp Method
In the above method of synchronization, it is sometimes very difficult to judge the correct instant of zero voltage. Hence, we have to move on to another efficient method of synchronization.
The bright lamp method of synchronization is a sharper and more accurate method of synchronization. In this method, the lamps, Lamp1 and Lamp2 are connected across the phase as shown in the figure below.
The same process as in the dark lamp method is repeated for the synchronization of two alternators in the bright lamp method. In this case, the lamp will glow brightest when the two voltages are in phase with the bus bar voltage because the resultant voltage across them will be twice the voltage of each alternator. Synchronizing is done in the middle of the bright period.
Synchronizing of Three Phase Alternators
Two bright and One dark method of synchronization
In this method, paralleling of alternators is done with three lamps(L1, L2, L3) and three switches(S1, S2, S3), each for three phases. Among these, one lamp(L2) is connected between the corresponding phase(Y) and the other two lamps(L1 and L3) are cross-connected between the other two phases(R and B), as shown in the figure below.
The prime mover of the incoming alternator-2 is started and brought up to its rated speed. The excitation of the incoming alternator-2 is adjusted in such a way that the incoming machine induces the rated voltage, which is equal to the Bus bar voltages.
The perfect moment to close the switch for synchronization of an alternator is obtained at the instant when the straight connected lamp(L2) becomes dark, and the cross-connected lamps(L1 and L3) equally glow brightly.
If the phase sequence is incorrect, no such instant will take place, and so all the lamps will be dark simultaneously.
Synchronization of alternators by means of a synchroscope provides a more accurate indication of synchronism than the lamps do. If the phase sequence is known to be correct, then a synchroscope is the best device to use.
The Synchronizing panel shown below contains two voltmeters, incandescent lamps, frequency indicating meter and synchroscope. Voltmeters are used to measure the voltages of incoming and running alternators, whereas the incandescent lamps check the phase difference. It has terminals to be connected to the two alternator terminals.
Synchroscope is single phase instrument used for indicating the difference of phase and frequency between two voltages. It is simply a split phase motor in which torque is developed if the frequencies of the two voltages differ.
Voltages from corresponding phases of the alternator-1 and alternator-2 are applied to the synchroscope. A pointer is attached to the rotating part of the instrument. It moves over the dial face on either a clockwise or anti-clockwise direction. Depending on which, you can say whether the new incoming alternator is fast or slow.
When the frequencies of the two alternators are equal, no torque is exerted on the rotating part of the instrument and so the pointer stops.
When the pointer stops in the vertical position, the frequencies are equal, the voltage will be in phase and so the paralleling switch may be closed.
But in practice, it is not possible to bring the pointer in the vertical position to make the two alternators to attain synchronism.
So, in order to synchronize, the speed of the incoming alternator should be adjusted as closely as possible. The paralleling switch is closed just before the pointer reaches the vertical position when travelling in the fast direction.
However, it is very important that parallel operation of alternator should be done as accurately as possible. If not, serious damage will happen to the alternators, resulting in damage to the coil.