Wednesday, January 11, 2017
Synchronization of alternator and methods of synchronization of alternator
What is meant by synchronization of alternator?
Connecting a group of alternators parallel to a bus bar and the alternators should have same voltage and frequency as that of bus-bar. This is called synchronization of alternator. There are some conditions to be satisfied by the alternators which are to be connected in parallel to bus-bar to be in synchronization.
Conditions for synchronization of alternators:
1. The terminal voltage of incoming alternator must be equal to the bus bar voltage.
2. The frequency of voltage generated by incoming alternator must be equal to busbar frequency.
3.The phase sequence of the three phases of the incoming alternator must be same as phase sequence of bus-bars.
4. The phase angle between the voltage generated by incoming alternator and voltage of bus-bar must be zero.
5. Always connect running alternator to bus-bar. If a stationary alternator is connected to bus-bar it will result in short circuit of stator winding.
The above conditions are to be satisfied by alternators to satisfy synchronization.
Why synchronization of alternators is necessary?
1.An alternator cannot deliver power to electric power system until its voltage,frequency,phase sequence and other parameters matches with the network to which the the alternator is connected.
2. The case of synchronization arises because we are connecting many alternators in parallel to supply the demanded load. So we need to match all the parameters of connected alternators with bus-bar to deliver power to load.
3. By synchronization we can match all the parameters of one alternator with the other alternator and also with the bus-bar and deliver the required power to load.
4. Synchronization of alternator is also called as paralleling of alternators.
Advantages of paralleling of alternators:
We get a common doubt why we need to supply the load by paralleling small units of alternator rather than using a single larger unit? This is because we have many advantages by doing so. They are:
Continuity of service:
In case of any damage to one of the alternators it can be removed.Supply to load is not interrupted because other alternators can supply the required load. But if u use a larger single unit even a small damage causes the interruption of supply.
Requirement of load:
As the load demanded is not same all the time, during light load periods we can run two or three alternators in parallel. When the demand is high we can add the required amount of alternators in parallel to meet the load demanded.
Reliability:
Several single units connected in parallel is more reliable than single larger unit because if a single unit gets damaged it can be removed and its work is compensated by other units which are running.
High efficiency:
An alternator runs efficiently when it is loaded at their rated value. By using required number of alternators for required demand i.e, light load or peak load we can load an alternator efficiently.
So because of above advantages we use paralleling of alternators.
Steps to connect alternators in parallel or synchronization of alternators:
1.Consider an alternator-1. It is supplying power to bus bar at rated voltage and frequency.
2. Now we need to connect another alternator let it be alternator-2 in parallel with the alternator-1. In order to match the frequency of alternator-2 with the frequency of bus-bar or alternator-1 (since alternator-1 and bus-bar are already in synchronism) we need to adjust the speed of alternator-2. Now the voltage of alternator-2 is to be matched with the voltage of bus-bar or voltage of alternator-1 (since alternator-1 and bus-bar are already in synchronism). For this purpose we need to vary the field rheostat until the voltage matches.
3. The three phase voltages generated by alternator must be same as the three phase voltages of bus-bar or alternator-1(since alternator-1 and bus-bar are already in synchronism).This can be achieved by matching the phase sequence and frequency of alternator-2 with bus bar or alternator-1(since alternator-1 and bus-bar are already in synchronism) phase sequence and frequency.
By following these steps synchronization of alternators is possible.
Methods for synchronization of alternators:
There are three methods for synchronization of alternators. These methods check whether the above mentioned conditions for synchronization of alternators is satisfied or not. The three methods are.
1. Three dark lamps method.
2. Two bright, One dark method.
3. Synchroscope method.
Three dark lamps method for synchronization of alternators:
Let us study synchronization of alternators using three dark lamps method in detail.
Circuit diagram for synchronization of alternators using three lamp method:
Procedure:
1. Consider alternator-1 is supplying power to load at rated voltage and rated frequency which means alternator-1 is already in synchronism with bus-bar.
2. Now we need to connect alternator-2 in parallel with alternator-1.
3. Across the 3 switches of alternator-2 three lamps are connected as shown in the figure.
4. To match the frequency of alternator-2 with the bus-bar frequency we need to run the prime mover of alternator-2 at nearly synchronous speed which is decided by the frequency of bus-bar and number poles present in alternator-2.
5. To match the terminal voltage of alternator-2 with bus-bar voltage we need to adjust the field current of alternator-2 until terminal voltage of alternator-2 matches with the bus-bar voltage. The required value of voltage can be seen in the voltmeter connected to bus-bar.
6.To know whether the phase sequence of alternator -2 matches with the bus-bar phase sequence we have a condition. If all the three bulbs ON and OFF concurrently then we say the phase sequence of alternator-2 matches with the phase sequence of bus-bar. If the bulbs ON and OFF one after the other then the phase sequence is mismatching.
7. To change the connections of any two leads during the mismatch of phase sequence first off the alternator and change the connections.
8. ON and OFF rate of bulbs depends upon frequency difference of alternator-2 voltage and bus-bar voltage. Rate of flickering of bulbs is reduced when we match the frequency of alternator-2 with bus-bar voltage by adjusting the speed of prime mover of alternator-2
9. If all the conditions required for synchronization are satisfied then the lamps will become dark.
10. Now close the switches of alternator -2 to synchronize with alternator-1.
11. Now the alternators are in synchronism.
Disadvantage of three dark lamps method for synchronization of alternators:
Flickering only says difference between frequency of voltages of alternator and bus bar but correct value of frequency of voltage of alternator cannot be found.
For example, if the bus bar frequency of voltage is 50 HZ and difference in frequency of voltage of bus-bar and alternator is 1 HZ the alternator frequency of voltage can be either 49 HZ or 51 HZ.
Two bright and one dark lamp method for synchronization of alternators:
Let us discuss synchronization of alternator using two bright and one dark lamp method.
Circuit diagram for synchronization of alternators using two bright and one lamp method:
Procedure:
1. Consider alternator-1 is supplying power to load at rated voltage and rated frequency which means alternator-1 is already in synchronism with bus-bar.
2. Now we need to connect alternator-2 in parallel with alternator-1.
3. Here lamp L-2 is connected similar to the three dark lamp method.
4. Lamps L-1 and and L-3 are connected in different manner. One end of lamp L-1 is connected to one of the phases other that the phase to which lamp L-2 is connected and the other end of lamp L-1 is connected to the phase to which lamp L-3 is connected.
5.Similarly one end of lamp L-3 is connected to a phase other than the phase to which lamp L-2 is connected and other end of lamp L-3 is connected to the phase to which lamp L-1 is connected as shown in the following circuit.
6. To match the terminal voltage of alternator-2 with bus-bar voltage we need to adjust the field current of alternator-2 until terminal voltage of alternator-2 matches with the bus-bar voltage. The required value of voltage can be seen in the voltmeter connected to bus-bar.
7. Depending upon the sequence of lamps L1,L2, L3 becoming dark and bright we can decide whether the alternator-2 frequency of voltage is higher or lower than bus-bar frequency.
8. If the sequence of bright and dark of lamps is L1-L2-L3 then the frequency of voltage of alternator-2 is higher than the bus-bar voltage. Now until the flickering reduces to a low value decreases the speed of prime mover of alternator-2.
9. If the sequence of bright and dark of lamps is L1-L3-L2 then the frequency of voltage of alternator-2 is less than the bus-bar voltage. Now until the flickering reduces to a low value increase the speed of prime mover of alternator-2.
10. When the L1 and L3 are equally bright and lamp L2 is dark then close the switches.
11. Now the alternators are in synchronism.
Disadvantage of two bright and one dark lamp method for synchronization of alternators:
Phase sequence of the alternator cannot be checked by this method.
Synchroscope method for synchronization of alternators:
Let us discuss synchronization of alternator using synchroscope method.
Circuit diagram for synchronization of alternators using synchroscope method:
Procedure:
1. A synchroscope is used to achieve synchronization accurately.
2. It is similar to two bright and one dark lamp method and tells whether the frequency of incoming alternator is whether higher or lower than bus bar frequency.
3. This contains two terminals they are a) existing terminal b) incoming terminal.
4. Existing terminals are to be connected to bus-bar or existing alternator here in the diagram it is alternator-1 and incoming terminals are connected to incoming alternator which is alternator-2 according to the diagram which we have considered.
5. Synchroscope has a circular dial inside which a pointer is present and it can move both in clockwise and anti clockwise direction.
6. To match the terminal voltage of alternator-2 with bus-bar voltage we need to adjust the field current of alternator-2 until terminal voltage of alternator-2 matches with the bus-bar voltage. The required value of voltage can be seen in the voltmeter connected to bus-bar.
7. Depending upon the rate at which the pointer is rotating the difference of frequency of voltage between incoming alternator and bus-bar can be known.
8. And also if the pointer moves anti clockwise then the incoming alternator is running slower and has frequency less than the bus bar or existing alternator frequency and if the pointer moves clock-wise then the incoming alternator is running faster and has frequency greater than bus-bar or existing alternator frequency. So by adjusting the speed of prime mover of incoming alternator we can match the frequency with bus bar or existing alternator frequency. Frequency matches when the pointer is straight up-wards. At this point close the switch.
9. Now both the alternators are in synchronism.
So by these three methods synchronization of alternators is checked.
Today in this post we have learnt what is meant by synchronization of alternator and methods of synchronization of alternator.
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