Parallel operation of generators means two or more generator of same or different size are running with their output cable connected onto a single electrical bus bar.

Paralleling is the process of synchronizing multiple power sources, usually two or more generators, and connecting them to a common bus.

Parallel operations of generator

The process of getting generators to run in parallel is known as synchronising

Synchronising of incoming generator or alternator with running current is very important before doing paralleling it with running generator.

Conditions required before synchronising or paralleling of generators

To synchronize a generator to the grid or with other generators, the following conditions must be met.

Conditions required for synchronising or parallel operation of generators

1.Frequency of incoming generator should be same as running generator
2.Voltage of incoming generator should be same as running generator
3.Phase angle should be same

1.Phase sequence:- The phase sequence of incoming generator must be same as that of running generator.

2.Voltage :-The root mean square (RMS ) voltage of incoming generator should be same as that RMS voltage of running generator or bus bar or electric grid.

What will happen if voltage of incoming generator more than voltage of  running generator or bus bar.There will be high reactive power that flows from running generator into the grid.

What will happen voltage of incoming generator less than voltage of running generator or bus bar.It cause incoming generator started absorbing reactive power from running generator or grid.

3.Frequency :- The frequency of incoming generator should be same as that of running generator or grid or bus bar.Improper frequency matching causes the prime mover to accelerate and decelerate rapidly, increasing the transient torque.

4.Phase angle :- The pahale angle between the voltage of incoming generator and voltage of running generator or bus bar  is zero.
This can be observed by comparing the occurrence of zero crossings or peaks in voltage waveforms.

Need of paralleling of generators

To increase total system size and redundancy, generators are used in parallel.Rather than a single large unit, most commercial power plants use several small units to generate electricity is more cost effective. This is known as Parallel Generator Operation. The following are the reasons why this practice is preferred.

Reason for use of generator in parallel

1.A group of alternators can supply a greater load than a single alternator.

2.When one machine is taken out of operation for routine maintenance and inspection, the remaining machines ensure that the supply remains continuously.

3.There is no interruption in power supply if the generator fails.

4.The parallel operation of the alternator lowers the operating and energy generation costs.

5.A number of machines can be added without disrupting the initial installation in order to meet the increasing future demand of the load.

6.During periods of low load, one or more alternators may shut down. As a result, the remaining alternator operates more efficiently at near or full load.

7.Thus,It ensures greater supply security and allows for overall economic generation.

Advantages of paralleling of generators

1.Increase in redundancy by parallel operation of generator

For critical loads, the redundancy inherent in the parallel operation of many generators is more reliable than a single generator unit. If a unit fails, backup loads are prioritized and redistributed among the system’s other generators.

Paralleling of generator increase the redundancy and make the system more reliable.

For example, suppose you have a total power requirement of 2000kVA, but you need to purchase a larger generator, say 3000kVA, to account for load acceptance concerns. Instead, you may buy 6 x 500kVA units and connect them in parallel. In the first solution, in the event of a single failure, the 3000kVA unit would not be able to run, and you would be without power.In the second scenario, if one of the six 500kVA units fails, you still have 2500kVA available, giving some redundancy.

2.Increase power; As the demand of electricity increasing day by day continuously so,we can increase the power of any plant by putting one more generator  in parallel with existing generator equipment.

Let us understand by a example :-Imagine you own a factory. You you have a single generator,  500kVA which you have had for 2 years. Business is really good and you want to expand, but you don’t have enough power. In this instance you might consider adding a second generator to your system, lets say another 500kVA. This will give you 1000kVA of power.

3.Increase efficiency :-When generators are loaded at their rated values, they run the most efficiently. Because of the operation of a few generators at light loads and more generators at high peak loads, the generators are efficiently loaded.

4. Reliability :- A group of small units is more reliable than a single large unit. This is due to the fact that if one alternator fails, the other alternators remain active, and thus the entire system is not shut down.

5.Ease of maintenance and service / Continuity service :-If one alternator requires periodic maintenance, breakdown, or repair, it must be shut down and removed from service. Because the other machines are running in parallel, there is no interruption in the supply of the load.

6.Load requirements :-The central station’s load requirements change on a regular basis. During low-load periods, only one or two generators are used to meet load demands. During peak-load demands, additional alternators are connected in parallel to meet the demand.

7.Reduce cost

What are the steps in paralleling of generators

To be synchronized, two generators must have an equal number of phases, the same phase angle, the same voltage, frequency, and the same voltage sine curve ( waveform ). This means that the RYB connection point of the bus bar should be connected to the RYB terminals of the incoming generator rather than the RBY terminals.

Paralleling of generator done by 3 method
2.Dark lamp method and 3.Two dark and one bright

1.Synchroscope method

Allmodern control panels, whether on a ship or in a large industry, have the capability of paralleling generators automatically, it is necessary to understand and prepare to do so manually in the event that this is not the case. A synchroscope is a device that measures the degree of synchronization, allowing us to close the breaker at right time.

Synchroscope method of paralleling of generator

It uses a pointer to represent the relative speed of the incoming machinery. If it moves anticlockwise, the speed of the incoming generator is less; if it moves clockwise, the speed is greater.

1.Start the incoming generator and wait till it’s speed  gradually increase until it reaches the required operational frequency.

2.Monitor the control panel to ensure that the incoming generator has the same voltage and frequency as the bus-bar.

3.Make sure that the phase sequences of your generators are the same.

4.Make any necessary adjustments to achieve identical voltage, frequency, and phase sequence.

5.Now, turn on the synchroscope and watch the pointer move.

6.If the pointer moves anticlockwise, the alternator speed increases; if it moves clockwise, the alternator speed decreases.

7.Maintain the incoming generator’s speed slightly higher than that of the load, so that the pointer moves very slowly clockwise.

8.Engage the breaker just before the pointer reaches midnight (12 o’clock); this is the point at which the incoming generator is in phase with the bus bar.

9.Once get parallel then start sharing loading on both generator.

10.When parallel operation done ,then decide ,you need to run both generator load or need to turned off the running generator.

11.If you have to turn off running generator,then lower the governing switch off running generator and simultaneously increasing for incoming generator.

12.Turn off or trip the breaker for the running generator; the generator is now in standby mode.

13.Turn off the synchroscope

2.Dark Lamp method

In the dark lamp method, all three bulbs are connected across the same phase, i.e. RR’, YY’, BB’, and a voltmeter is connected across two phases of the bus bar and terminals of the incoming generator.

Basic principle :-The basic idea behind how the dark lamp method works is that when the two generators are in sync, their net resultant frequency is zero.

Step :-

1.Start the incoming generator and wait for it coming to its rated speed.

2.Connect the three bulbs with the open terminal of incoming generator with switches which connect it to the system.(all three bulbs are connected across the same phase, i.e. RR’, YY’, BB’,)
It us clearly shown in figure below:-

3.Monitor the voltage if incoming generator us same as that of bus bar or running generator.

4.If not same,then incoming generator field current is now increased until the voltage across the machine terminals equals the bus bar voltage (by observing the readings on voltmeters).

5.Now ,we check phase sequence

A)If there is a difference in the phase sequence, the bulb glows (indicating a high phase difference) and then becomes darker (less phase difference).

B)If these three bulbs glow and dim at the same time, it means that both generators are connected to the same phases.

C)If the bulbs do not light up at the same time or light up one by one, this indicates that different phases are connected; to solve this problem, one of the sequences should be changed.

6.By changing the connection of any two lead of incoming generator after shuting the machine,the phase sequence can be changed.

7.Now,we check frequency

The incoming machine’s frequency is adjusted until the lamp flickers slowly. The rate of flickering should be less than one dark period per second. In the middle of their dark period, the synchronizing switch is closed after finally adjusting the incoming voltage.

3.Two Bright one dark lamp method

Steps To parallel Generators By Two Bright One dark Lamp Method

1.Start the incoming generator and wait it to come at its rated speed.

2.Connect the three lamps and a voltmeter to its terminals as shown in the figure above, using the two bright and one dark lamp method.

one dark and two bright lamp method have; one lamp connected in phase while others out of phase to the bus-bar i.e RB’, YY’, BR’.

3.Check the voltage of incoming generator via a voltmeter is same as that of bus bar or running generator.

4.If voltage is not same then Increases the voltage by changing the field current in the alternator until it reaches the voltage of the bus bar.

5.Monitor for the bulb flicker and decide  whether it follows an anticlockwise or clockwise pattern.

6.An anticlockwise rotation indicates slow speed, whereas a clockwise rotation indicates fast speed of the incoming generator in relation to the bus bar or running generator.

7.Change the alternator speed to reduce the flickering of the bulb to near zero or as low as possible.

8.Now, close the breaker when the bulb connected in phase is dark and the other two are equally bright.

9.After that, decide whether you need both generators running or if the running generator should be turned off.

10.This can be accomplished by lowering the governor switch on the running generator while increasing the governor switch on the incoming generator.

11.Disconnect the power to the lamps and, if applicable, turn off the step down transformer.

12.It is common practice for high voltage generators to connect these lamps through a step down transformer.

13.Turn off or trip the breaker for the running generator; the generator is now in standby mode.


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