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Thursday, July 28, 2022

We have already discuss about  parallel operation of alternator/generator and its condition. Here we are going to describe about Synchronizing lamps || bright lamp method || dark lamp method where there are 3 lamp maybe bright or dark according to condition. For a incoming machine to be connected in infinite busbars having a number of running machine, both need to be same in voltage and frequency.  A standstill alternator shouldn't be connected to live busbar. If do so, short circuit will occur due to induced emf is zero in incoming machine. Generally, there are 2 methods for synchronization. They are as follows:-

Synchronizing Method

Synchronizing lamps

A cheap method for synchronization by using 3 lamp is called synchronizing lamps method. By using these three lamps, it is easy to check the condition for synchronization. This method is generally used for low power machine. When a generator is rotated at its rated speed by using prime mover, then it is ready to connect in the infinite busbar by adjusting the field current of infinite busbar to get its terminal voltage equal to busbar voltage.  Voltage maybe varies from 0 V to twice the phase voltage, so it is required to use suitable lamp.

There are three methods for parallel connection/synchronization. They are:-

Three dark lamp method

For this method, all three lamps are connected in parallel to the switches as A1 to A2, B1 to B2 and C1 to C2. If all the phase are connected properly, all lamps will bright and dark at same time. If not flicker at same time, the phase sequence is not corrected so need to change a phase and observe until all lamp flicker at same time. These 3 lamps are flicker at a rate which is equal to the difference in frequency of incoming machine and busbars. Now, the frequency of incoming machine need to be adjusted so that it will equal to busbar's frequency which is known by observing these 3 lamps. Adjusting the frequency until the lamps flicker at slow rate (i.e. less than one dark period per second). Finally, all switches are closed at the middle of dark period.

Three bright lamp method

For this method, all three lamps are connected in parallel to the switches as A1 to B2, B1 to C2 and C1 to A2. If all the phase are connected properly, all lamps will bright and dark at same time. If not flicker at same time, the phase sequence is not corrected so need to change a phase and observe until all lamp flicker at same time. These 3 lamps are flicker at a rate which is equal to the difference in frequency of incoming machine and busbars. Now, the frequency of incoming machine need to be adjusted so that it will equal to busbar's frequency which is known by observing these 3 lamps. Adjusting the frequency until the lamps flicker at slow rate (i.e. less than one bright period per second). Finally, all switches are closed at the middle of bright period.

Two-bright one dark lamp method

For this method, all three lamps are connected in parallel to the switches as A1 to A2, B1 to C2 and C1 to B2. Adjusting the frequency until the lamps flicker at slow rate. All switches are closed only when the straight connection lamp is dark and cross connection lamps are bright at the same time.

1. This method is cheap.

2. The phase sequence is easily determined.

1. The lamp filaments may burn out.

2. This method doesn't give information about which machine has high frequency.

3. It is difficult to operate or connect the incoming machine to infinite busbar due to requirement of proper connection at proper time.