Parallel operation of synchronous generator (Three dark lamp method)

When generating rating is less than the load requirement So the alternator is not capable of delivering energy to the load Now what to do? We need to add another alternator, But we cannot add directly before synchronizing.

Basic load

Parallel Running

Before that, we have to satisfy some condition. There are three conditions:

Conditions for Parallel operation:

 Condition -1: The terminal voltage of incoming alternator should be same as existing alternator or Bus-bar.

Condition-2: The phase sequence of the incoming alternator must be equal to the existing alternator or Bus-bar.

Condition-3: The frequency of the incoming alternator must be equal to the existing alternator or Bus-bar. 

We have to satisfy these three conditions before synchronizing.

So, Synchronizing there are three methods.

1. Three Dark lamp method.
2. Two Bright and one Dark lamp method.
3. Synchroscope.

Three Dark lamp Method:

Three Dark lamp method

So, Our first condition is, The terminal voltage of incoming alternator should be same as existing alternator or Bus-bar.

How to measure the voltage? We used to two voltmeters, One voltmeter is used on bus-bar and another used to the incoming alternator terminal. If two voltmeters reading is the same so, the condition one is fulfill. But the incoming alternator terminal voltage is not the same as bus-bar voltage, So we need to adjust the exciting field current in this case.

Our second condition is, The phase sequence of the incoming alternator must be equal to the existing alternator or Bus-bar.

we don't know the Phase sequence of bus-bar, Which is the R Y B. 

So, We used three lamps each across the switch to know the phase sequence. If the three light at same time On and off, On and off like flickering -phase sequence is correct. If the phase sequence is not correct, Interchange any two terminal.

Our third condition is,  The frequency of the incoming alternator must be equal to the existing alternator or Bus-bar. 

If the three lamps at the same time On and off, On and off slowly so, the frequency are the same. But the lamps On and off, On and off firstly so, frequency are not the same. In this case, controlling the prime mover speeds and field current can be fixed the problem.

When We satisfied all three conditions close the switch.