Question

Draw a power system with multiple sources and show how synchronization will be completed including the protection

Answer 1

Synchronization is a process involved in connecting two AC power supplies. AC power sources

such as transformers and generators need to be operated in parallel. This may be required
for greater reliability and capacity sharing.

In the picture, the incoming power source, a generator(#1) is being synchronized with a busbar which is already to a generator(#2) which is running.

For synchronizing three conditions need to be satisfied
1. Equal Voltages
By Equal voltages, we mean that the rms value of the voltages should be equal.

2. Equal frequencies
Equal frequencies implies that the two sources should have equal frequencies. A 50Hz source
can only be synchronized with another source whose frequency is also 50 Hz.

3. Same Phase Sequence
The sources should have similar phase sequence i.e. RYB and RYB or RBY and RBY.

4. Same Phase angle
The vectors of the two sources should have the same phase angle.

It is possible to bring the frequency, phase angle and voltage within synchronizing limits by adjusting the speed and voltage of the generator.

The circuit breaker connecting the two power sources can be closed when all these conditions are satisfied. The drawing shows vector diagrams of two three phase voltages whose phase sequences, phase angles, voltages and frequencies are equal

Special equipment and checks are used to ensure that the above conditions are satisfied

HOW TO SYNCHRONIZE THE POWER SOURCE;

Synchronizing of an incoming generator or alternator is very important before paralleling it with another generator. The synchronizing of the generator is done with the help of synchroscope or with three bulb method in case of emergency. It is of utmost importance that before paralleling the generators the frequency and voltage of the generators need to be matched. In this article we will describe the method for synchronizing generators on a ship.

There are two methods to synchronize generators on a ship – one is the normal and other is the emergency method

1. Synchroscope method

1. The synchroscope consists of a small motor with coils on the two poles connected across two phases. Let’s say it is connected in red and yellow phases of the incoming machine and armature windings supplied from red and yellow phases from the switchboard bus bars.
2. The bus bar circuit consists of an inductance and resistance connected in parallel.
3. The inductor circuit has the delaying current effect by 90 degrees relative to current in resistance.
4. These dual currents are fed into the synchroscope with the help of slip rings to the armature windings which produces a rotating magnetic field.
5. The polarity of the poles will change alternatively in north/south direction with changes in red and yellow phases of the incoming machine.
6. The rotating field will react with the poles by turning the rotor either in clockwise or anticlockwise direction.
7. If the rotor is moving in clockwise direction this means that the incoming machine is running faster than the bus bar and slower when running in anticlockwise direction.
8. Generally, it is preferred to adjust the alternator speed slightly higher, which will move the pointer on synchroscope is in clockwise direction.
9. The breaker is closed just before the pointer reaches 12 o clock position, at which the incoming machine is in phase with the bus bar

2. Emergency synchronizing lamps or three bulb method

This method is generally used when there is a failure of synchroscope. In case of failure a standby method should be available to synchronize the alternator, and thus the emergency lamp method is used.

Three lamps should be connected between three phases of the bus bar and the incoming generator should be connected as shown in the diagram:-

1. The lamps are connected only in this manner because if they are connected across, the same phase lamps will go on and off together when the incoming machine is out of phase with the switchboard .
2. In this method as per the diagram the two lamps will be bright and one lamp will be dark when incoming machine is coming in phase with the bus bar.
3. The movement of these bright and dark lamps indicates whether the incoming machine is running faster or slower.
4. For e.g. there is a moment when lamp A will be dark and lamp B & C will be bright, similarly there will be instance when B is dark and others are bright and C is dark and other two are bright. This example indicates that machine is running fast and the movement of the lamps from dark and bright gives an clockwise movement
5. Clockwise movement indicates fast and anti clockwise direction indicates slow running of incoming generator