Hello friends, I hope all of you are fine. In today’s tutorial, we are gonna have a look at the Parallel Operation of Synchronous Generator. Nowadays it is rare in an electrical system that any synchronous generator providing power to its loads lonely. In certain conditions, like if somewhere supply is disconnected from the grid, then there is a single generator can be used to provide power to the load, but its normal generator does not work alone. In normal working condition always more than one generators are working together in parallel. An example of this system is the united states of America grid station, where almost a thousand plus generators providing power to the load in system.

In today’s post, we will have a look at how a generator works in parallel, its features, connections, and some other conditions. So, let’s get started with a Parallel Operation of Synchronous Generator.

Parallel Operation of Synchronous Generator

• Before study the parallel operation of the generators, first we discuss the advantage of the parallel operation. That are described here.

Advantage of Parallel Operation of Synchronous Generator 

• More than one generator can supply power to the larger load.
• Many generators in a system enhance consistency of the system, in certain cases, if one generator is not working then there will be no effect on the total load, that can occur in case of the single generator.
• If more than one generators are working in the system and we have to repair someone from the system, we can easily remove it and can repair. Other will continue to deliver power to the load.
• If we have one generator of higher power rating in the system and it not providing power to the load at rated condition, that’s not good for system. Instead, if we use many generators of less power ratings, and few of them are working and providing power to the load at rated conditions. So, it will be good for the system to use more than one generators of less rating than one generator of a higher rating.

Conditions for Parallel Operation of Synchronous Generator

• In above-given diagram you can see that there are two generators are connected in parallel. First generator is G₁ provides electrical power to output with the second generator G₂, that is connected with the first by a switch S
• If we closed switch randomly at any time and connect these generators it can cause of serious disaster for both generator and load.
• And the voltage at the terminals of both generators are not the same, it will produce such high current that will damage generators.
• To eliminate this situation there should be all three phases of both generators should be at same voltage level and have same phase angles.
• It means phase a has an equal voltage level to the phase a’, and phase b is equal to phase b’ and c is equal to c’.

• To get this same matching of the phases we must follow some conditions that are described here.

1. 1. The root mean square (rms) voltage of these 2 generators should be same.
   2. The phase sequence of both of the generators should be same, which means a is connected with a’ of other generator, b with b’ and c with c’.
   3. The phase angle of two similar phases should be equal. Means (a) phase of both the generators have same angle, and so on.
   4. The frequency of the generator that is going to connect also called the ongoing generator should be a little bit higher than the generator that is already in working condition.

• Now we explain these 4 conditions with details.

Condition 1 and 3:

• To have similar voltage at the terminals of both generators there should be same value of root mean square voltage and their phase angle that are described in condition 3. The voltage at phases a and a’ will remains same during working if these to follow condition one and three.

Condition 2:

The phases that are to be connected have a similar phase sequence.

• If the phase sequence is not the same, (as shown in given diagram) or one phase like (a) phase is connected with the same phase but other 2 phases are not connected properly with the similar phases.
• If these 2 generators have this type of sequence then there will be nothing happen at phase a, as it is connected correctly but at other 2 phases b, c such short circuit will occur that will damage both the generators.
• To solve this issue only change the position of the 2 phases at any one generator out of the three phases.

Condition 4:

• If the frequency of the ongoing generator is not slightly higher than running the generator, there will huge amount of transient’s current will produce until the frequencies of both generators become equal.
• Frequencies of these 2 generators should be almost equal, but not exactly the have the same value, which means ongoing generator frequency should be a little bit higher than another generator.
• There should be some difference among the frequencies of 2 generators so that incoming generator phase angle will change according to the phase angle of the running generator.
• By this method, the angles among the voltage can be observed and the S₁ can be closed, when both generators have same value of the voltages.

Procedure for Paralleling Generators

• In given diagram there are generators G1 and G2, you can see that the G1 is already connected with the load and G2 has to connect with the system.
• To connect the generator G2 we must apply these steps.
• First, we have to adjust the voltage of the generator G2 equals to the system. For this monitor, the voltage of this generator by the volt-meter and set equal to the system.
• 2^nd is to check the phase sequence of the G2 and match it with the system frequency there are many methods to check the phase sequence some are described here.
  • First method is to connect small rating induction motors with both generator one by one and observe their rotation direction.
  • If the direction of the rotation is same for both generator then the sequence is same.
  • If the rotation direction of the motors is not same there is no same phase sequence, for same sequence just change the two phases’ connection of the incoming generator.

Three-Light-Bulb Method for Checking Phase Sequence 

• There is another method to check the phase sequence of the generator.
• In this method, there are 3 bulbs connected with the open terminals of the incoming generator with the switches that connect it with the system. it is shown in a given diagram.

• If there is the difference among the sequence of the phases, then bulb glows (it means high phases difference) then become darker (less phase difference).
• If these 3 bulbs glow and dim at a same time, then both of generator is connected with the same phases.
• If the bulbs do not glow at a time or glow one by one then there will be different phases are connected, to solve this problem one of the sequences should be changed.
• The other point for parallel operation is that the frequency of the incoming generator should a little bit higher than the working generator.
• The value of the frequency is set by the frequency meter, then measure from the phase changing (difference).

• The frequency of the coming generator should be slightly higher than the system so that when it connected with the system it works as a generator, not as a motor.
• When the frequencies of both the working and incoming generator are almost same, the 2 generators will vary phase according to each other gradually,
• The variations in the phase detect, and when the angles of the phases are the same, the switch that joins both the generator will close.

How can we tell when the 2 generators are in phase? 

• The simplest method is to watch the three bulbs that are connected with the circuitry as shown in above figure.
• When all three bulbs glow at a time there is no phase difference and phases are connected in same sequence.
• This method works but has some limitations, sometimes not work accurately.
• There is another technique can be used for checking the phase sequence, is the use of
• It is an instrument that calculates the phase angle difference among (a) phases of the generators.
• In given diagram, you can see the synchroscope.
• You can see the needle on the meter that will indicate the phases if it is at zero and a one-eighty degree than the voltages are in phases.
• As we studied earlier that the frequency of both the generators are different, so the angle of the phases will vary on the meter gradually.
• If the frequency of the incoming generator is higher than the working generator (that is in our case), so the angle of the phase will advance and the pointer (needle) on the meter will move in the clockwise direction.
• If the frequency of the incoming generator is less than the running generator, the needle of the meter will move in the anticlockwise direction.
• When the needle of the meter is either 0 degree or 180⁰ it means that the phase sequence is same, so we can close the switch to make contact among the generators.
• You can note that this meter tells about the only single phase, that is we discussed. It does not tell about the sequences of the phases.
• For high rating generators that are connected in power system, parallel operation of the incoming generator is done by the computer, synchroscope is not used for such systems.

You can also read some related topics to synchronous generator that are listed here.

Introduction to Synchronous Generator

Synchronous Generator Equivalent Circuit  

Synchronous Generator Phasor Diagram

Synchronous Generator Power and Torque

Synchronous Generator Parameters

Synchronous Generator Operating Alone

Synchronous Generator parallel with Large Power system

Synchronous Generator Parallel with same Size Generator

Synchronous Generator Ratings

Synchronous Generator Capability Curves

Synchronous Generator Transients

That is a complete article on the Parallel Operation of Synchronous Generator if you want to know something more can ask in comments. See you in the next tutorial Synchronous Generators in Parallel with Large Power Systems.