generator droop settings

Hi

You should always start at about 4% droop (that is 4% of full load current) ie. When the machine is running at full load, the speed must reduce by 4% of normal running rpm, and if unstable, increase bit by bit until load sharing stabilises.

To explain droop, I'll begin with speed (governor). Kw is a function of speed. If you have 2 parallel machines, and increase the speed on one, it takes more load. Setting droop, will cause it to slow down slightly as it reaches a higher load, causing it to give that load to the other machine - whose droop setting will cause the same action - thereby giving the impression on load sharing. In fact they are continually handing off load to each other. Without speed droop, you will get one machine stealing all of the load from the other. This will cause one machine to possible overload, and the other to go into reverse power - both situations should be avoided.

The same is true for Voltage droop which you set on the AVR. This does reactive load sharing, and ensures that the voltages remain similar. If the voltages go out of whack, it causes cross currents - which will detract from the capacity of the sets. The importance of reactive load sharing becomes very evident when parallel with the utility. If your set voltage is lower than the mains voltage, it causes VAR flow into your machine/s. You will end up paying for electricity that you cannot use.

Voltage droop in standalone parallel sets (not connected to the grid) will keep the voltages stable. Without it, you end up will rapidly swinging voltages - and tripping as a result.

If you want proper load sharing - have a look at the Inteligen controller - http://www.comap.cz

I have installed many of these - and they are remarkable.

I hope this answered your question.

Regards

John

Thanks John.

I understand that droop of 4 % is recommended for DGs using mechanical governors. I have a electronic governor on the machine and the droop is 1 %.

What I want to know is that this droop is OK for parallel running or should I increase the droop? If yes , to what value, without compromising the advantages gained by electronic governor on quality of power supply.

CEngWannabe has covered a lot of the important points, but often without an explanation. Assuming that each generator can be set to automatic or manual mode, but not coupled in anyway....

When running two generators in parallel, one should be in manual control, taking the major part of the load, the other should be in automatic mode to handle possible changes in the whole load.

The manual machine should for example be set up for taking 75% of the load. If load changes are expected at some point in time that could reduce the load to being near to the present 75% or increase beyond say 150%, then someone must be around to change the load on the manual machine to suit!! Otherwise the automatic machine could either drop off on reverse current or become overloaded....

If a completely automatic load sharing system is in place for 2 or more generators, then the above can be forgotten....but read the manual well beforehand!!!

Hi Guys

Thanks for your comments. In answer to the response from Andy_Germany

"When running two generators in parallel, one should be in manual control, taking the major part of the load, the other should be in automatic mode to handle possible changes in the whole load."

- This can only work if you are guaranteed a base load of at least 75% of one machines full load capacity all of the time.

I have often run two machines with electronic governors on pure droop load sharing - and it works well. Running in base load is a recipe for disaster if the load goes below the base load setting. The reason for this - is that the base load machine will try to keep its output at 75% - causing the second machine to go into reverse power and tripping. There are ways around this - but unless you are 100% sure of the load conditions over a long period - its advisable to avoid this configuration.

What you need to look for when commissioning:

1. Droop of 1% may not be enough. Even though you have electronic governors, you still have a time constant in the machine. This is the amount of time the machine takes to respond to a load change - not the amount of time the governor takes. A droop of 4% is still recommended for electronic governors.

2. When commissioning - start at 4%. Apply a step load change and see what happens. You can then start to tweak the settings. Remember that no two machines are identical - so one is bound to be more responsive than the other. This should be adjusted in the PID for the speed control loop - but will never be perfect - and will always differ slightly from the other machine. Your droop adjustments then give you another adjustment to even things out under normal running conditions.

3. Never have too little droop. More is always better than less. You will not lose much from the machine - but you will always reduce your headaches. Too little droop makes the system a lot more susceptible to instability. The last think you need is when a large step load comes on, the machines start swinging violently - this will almost certainly cause a trip. The major advantage of an electronic governor is not to allow less droop - its the ability to finely tune the control PID to get the best (quickest and most stable) response to load change. The other advantage of electronic governors is that the ones fitted with load sharing lines or control inputs for load sharing will allow very precise load sharing - whereas a mechanical governor will have had an electric motor fitted to it - and the speed change takes a lot longer because of this. Electronic governors also allow you to do fancy control scenario's with multiple machines - which is usually for very specialised applications.

The way to ensure that your droop settings will not affect your intentional operation of the plant - is to calculate the full load of the machine - less 4%. Multiply by 2 for 2 machines. Make sure that the value you get is not less than the full load of the plant.

Hope this helps

John

Hi John,

I mentioned just that problem when load sharing!! but thanks anyway for re-enforcing that though as it was forgotten in your(?) previous entry. It is of utmost importance & significance of course, regards, Andy

Hi Andy

No insult/disrespect to you intended. I saw your comment about it - and decided to reinforce it. I have seen what it can do - and its quite scary. :)

Regards

John

I agree, but then should we not tell them what happens when two (big) generators are linked OUT OF PHASE?

That gets even scarier.......

Regards,

Andy

I once saw the result of out-of-phase sync between two 500kw machines. The newer of the two ended up with a shattered coupling, twisted alternator shaft and twisted crankshaft - and the engine block split in two.

I have heard though that machines have completely left their mountings before when it happened.

Wouldn't want to be standing next to it when that happens :)

I blew up a boiler (accidentally) and that was scary enough. Fortunately it was only the fire box. If it was the pressure side - I wouldn't be here typing this.

I was once told that the Royal Navy had such an accident, must have been in the late 50s or early 60s at a guess, one generator left its bed and tried to go through the ships side.....the phase synchronizing lamps had been wrongly connected to the selector switches...

Maybe someone on CR4 can tell us whether or not the facts are true......I have not done a web search...yet!

Yes, it would be interesting to know if its true. The one I saw was caused when there was an engine room fire. Two generators and the main switchboard were replaced. The new syncroscope was different to the old one, and the chief engineer thought he was doing fine - until the lights went out.

I did a quick search on Google and found nothing, but I doubt if the Navy would have published it anyway......! I heard it only by word (untrustworthy?) of mouth when I was still serving.......

It could be true, certainly the magnetic/electrical forces concerned are horrifically strong.......