What things should be considered when paralleling 2 or 3 generators ?

What things should be considered when paralleling 2 or 3 generators ?

How to do that (generators of different fuels i.e. of Natural Gas & Diesel & of different output powers) ?

thanks

The answers to your questions could be expanded into a long lecture. To put them as briefly as possible:

Before connecting their outputs together -
1. Frequencies and voltages have to be matched accurately
2. Phases have to be synchronised. This is at least as important as No.1
3. Phase directions have to be the same
4. They should have similar droop characteristics
5. There has to be some form of load distributing facility. This is particularly important if the generators are of different power ratings.

1) How to match the frequencies & voltages of two generators accurately ?

Because there is some variation or voltage drops in the output voltages of the generators , for e.g : 396 V to 400 V ...... if one's voltage is 398 V at a particular instant & other's 404 V then, what might happen ? Also, same with the frequencies, then, what might happen ?

thanks a lot

---------- Post added at 20:11 ---------- Previous post was at 20:08 ----------

1) How to match the frequencies & voltages of two generators accurately ?

Because there is some variation or voltage drops in the output voltages of the generators , for e.g : 396 V to 400 V ...... if one's voltage is 398 V at a particular instant & other's 404 V then, what might happen ? Also, same with the frequencies, then, what might happen ?

thanks a lot

danishdeshmuk said:

1) How to match the frequencies & voltages of two generators accurately ?

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These have to be monitored with meters. Frequency is controlled by engine rpm. Voltage is controlled by an AVR which will have some provision for adjusting the voltage. The adjustment may be a potentiometer or pushbutton or keypad.

Because there is some variation or voltage drops in the output voltages of the generators , for e.g : 396 V to 400 V ...... if one's voltage is 398 V at a particular instant & other's 404 V then, what might happen ?

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There will be circulating currents. This means that current will flow from the higher voltage to the lower voltage, and this results in wasted power. Since a generator is essentially a power source with a very low output impedance, the circulating currents can be very large even for small differences in voltage.

Also, same with the frequencies, then, what might happen ?

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Again wasted energy.

You did not mention phase synchronization. Even if frequency and voltage are perfectly matched, if the phases are not synchronized, it means that the instantaneous voltages are different from moment to moment. This results in high circulating currents when the outputs are paralleled. Once the outputs are tied together, the generators will tend to synchronize by themselves, but the initial shock can be severe and puts a lot of stress on the machines.

How to synchronize the phases ? what is meant by synchronization of phases ?

thanks

This means that the sine wave each generator is producing is identically timed to the others. i.e. they are all going through zero at the same time and all have their positive peak at the same time. . . Traditionally the display was like a clock with lamps in place of the clock digits. When the local generator was running at a slightly different frequency to the network, the lights would come on sequentially and appear to revolve around the dial. As the local frequency was brought in, the pattern would would slow down and ideally come to rest with the lamp on at the 12 O'clock position (position of phase co-incidence), then the local generator could be put on line.
Frank

Should we check this out with the help of oscilloscope ? if all the outputs of all the generators going to zero & going to max. 1 value at the same time , then, it means the generators phases are synchronizes , Am i correct ? And, then check to all of the three phases one by one ......... ?

thanks

Look at the diagram below. We have two AC voltages, both of 50Hz and 230V RMS. They have exactly the same voltage and frequency, but the voltages do not change at the same time. In other words, they are out of phase with each other. This causes a difference in their instantaneous voltages most of the time. If you simply connect the two together, it will cause huge circulating currents between them. They have to be brought in phase first. This can be done by slightly increasing and decreasing the engine rpm of one generator until they are in phase, that is, they are synchronized. The in-phase condition is met when there's no voltage difference between the two.



The synchronizing process may be done manually or by automated synchronizing aids. In the old days, operators would monitor the phase condition by observing a voltmeter or a light bulb connected between the two outputs while they fiddled with the rpm control of one generator (not both at the same time). When the voltage difference falls to almost zero, they will quickly switch the two outputs to a common bus.

---------- Post added at 01:51 ---------- Previous post was at 01:23 ----------

I started composing the above reply after post #5. It took some time, so I didn't see the others until I posted it. Time for quick answers to the questions quoted below:

phatcreators said:

Should we check this out with the help of oscilloscope ?

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An oscilloscope could be used but other methods are often more practicable. See my reply and chuckey's above for one method.

if all the outputs of all the generators going to zero & going to max. 1 value at the same time , then, it means the generators phases are synchronizes , Am i correct ?

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Yes.

And, then check to all of the three phases one by one ......... ?

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The three phases of the same generator have a fixed relationship so that if one phase is in sync with another generator, the other two will also be synchronized. However, the phases have to run in the same order for the two generators. The three phases are often called R, B and Y for Red, Blue and Yellow. If the phases of one generator are in the order R-B-Y, then the other generator also has to run in the same order. All three phases are usually monitored while synchronizing to make sure they are in the same order.

If one is in R-B-Y configuration then the other & also the third one should had the same configuration ? am i correct ?

thanks

why there's always some variation in the per phase amps of a 3 phase system ?

why we don't use the two phase system ?

thanks

What should do when rpms are not getting matched of the two generators ?

if one's rpm is 1000 rev/m & others 1500 rev/m ...... what else we can do to synchronize their rpms ?

danishdeshmuk said:

If one is in R-B-Y configuration then the other & also the third one should had the same configuration ? am i correct ?

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Yes. But remember that it's not the names of the phases that are important, but the order in which their voltages change.

dineshdeshmuk said:

why there's always some variation in the per phase amps of a 3 phase system ?

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If I understand your question correctly, it's because they don't always have the same load. A 3-phase motor will always draw the same current from each phase, but each of the phases may be given separate single-phase loads such as lighting, household appliances like TVs, computers, etc. An important (and often difficult) job of power distribution technicians is to try to balance the loads so that each phase has an approximately equal load.

why we don't use the two phase system ?

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It;s because a 3-phase system is more efficient and more convenient for many applications. For example, a 3-phase motor can be simpler than a single-phase or 2-phase motor. Rectified 3-phase voltage has less ripple and is more easily filtered. Some applications may not require any filtering. Sending 3-phase power over a distance requires smaller and lighter wires because the load currents are distributed over 3 lines.

munzir said:

What should do when rpms are not getting matched of the two generators ?

if one's rpm is 1000 rev/m & others 1500 rev/m ...... what else we can do to synchronize their rpms ?

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It's the AC frequency, not the actual rpm, that's important. The frequency is determined by the rpm and the number of poles.

if no. of poles are different then ??

If for the same design (means speed) the number of poles are different then the frequency will differ. You can not give parallel operation to generators with different frequency. They are rather to be synchronised for frequency to be same.
Number of poles and speed both relate to frequency.

most important frequency

amplitude

all should be in same phase...


if any one thing is different there will be current flow from one source to another....

Synchronization of generators is a standard experiment in electrical power labs, as shown in this experiment description:


The typical synchronizing instrumentation is also shown in this Wikipedia article
Alternator synchronization - Wikipedia, the free encyclopedia

To decide about the practical parallelization capability of generators, some additional information is needed, I think
- type of generator (synchronous, with electrical or permanent magnet excitation, or asynchronous)
- does it have a voltage control option independent of speed regulation?
- kind of speed controller

danish,

What things should be considered when paralleling 2 or 3 generators ?

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Can I ask if this is related to a particular project or is a general question ?. As if related to a particular project, please, provide the all specifications of the machine to be supplied from, as it may lead some other better guidance. And if a general question then it is ok.

should run any generator to its 80% output power ? & not exceed that limit .......... is it a right thumb rule

danishdeshmuk said:

should run any generator to its 80% output power ? & not exceed that limit .......... Is it a right thumb rule

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is it 50% of the rated value or 80% ... I been thinking half of the rated load....

Do you have any valid reason for running the generators to 50% or 80%, if it is not recommended by the manufacturer ?. The manufacturer knows the rated operation and always tell in the Operation's manual. Some times you will likely see high oil consumption, bore glazing and possibly high combustion blowby with continued use at low loads. Performance won't be an issue. The recommended ratings should always be followed by the manufacturer/supplier. And the accompanied operations manual must be consulted for.