Generator Sets

You should replace all three power plants with a single unit that can supply all of the kw you need. The three units you are using have a slight frequency variation and are not in phase since you are using three voltage entities in parallel. One lone unit instead of individual units in parallel will solve the problem you have encountered.

Hello Mansour,

More details required,..

a) What is the type of alternator winding connections star or delta?

b) What is the size (kva) all three generators,

c) Is no load voltage of all generators same?

d) Is droop setting of Caterpillar AVr same as that of Waukesha generator AVR?

e) What is the pf ( power factor) displayed on caterpillar generator? is it same as that of waukesha generator?

f) If the alternator is Star winding then are neutrals bonded to ground ?

If you give this data, problem can be analysed.

Cheers.

a- the winding are star connected

b- waukesha are 1200KVa,cat. is 720

c- no load voltage is 480 for all

d- droop is the same.

e- pf is the same for all.

f- the problem comes to the surface when the three nutrals are bonded togethe and then to the ground

g-

Ahmed,

When you say that the power factor is the same for all how did you determinate it ? Did you have an (inductive) load ? Usually the power factor of a power net connected generator can be varied (by varying the excitation voltage/current).

If you have neutral currents then pf cannnot be same....check on the display of caterpilar engine ( EMCP) .

Also please check the AVR droop settings and possibly the CAT generator is having zero AVR droop.

cheers.

dear Sir,

Generally Only one Neutral - of the MAster DG shall be in circuit ( i.e. Earthed solidly). Other 2 DG set's ( Slave )Neutral shall be floating, when run in parallel. Then, problem of circulating current shall be eliminated.

The size of the resistor will depend on the voltage. But this is just masking the problem. If the Waukesha and Caterpillar have different pitch, that may be your problem. (I assume the high neutral current exists with no load.) At any rate fixing the problem instead of covering it up will save you from future grief.

The caterpiller seem to have un-balanced phases due to this the neutral of this when synchronised with others have the floating neutral at a voltage- defined by its emfs

This un-balances - pl check - may even create negative sequences

a) Leave the neutral of the caterpiller floating - short term solution

b) repair the caterpiller generator.

Putting and inductor or resistor is only to generate the neutral voltahes through IR (and the losses) or wLI (again losses)

This is a common issue when the pitch wind doesn't match each generator (alternator). First, to be sure the installation is sound, with all paralleled adjust the voltage on each machine slowly until the minimum amps are being consumed. This means no circulating currents between the units; all current is going out to the load. Then see if the two Waukeshas paralleled to each other have high neutral current. then Cat to W # 1; Then Cat to W # 2, and trend to see if it is only when the Cat is put on the bus.

If you determine it is only when the Cat is on the bus, or the 2 Ws are greatly different in age and also had neutral current between them (possibly different alternator manufacturer) then pitch wind is VERY likely the cause. Two typical pitches are 2/3 and 5/6. Kato was famous for 5/6 pitch winds. Marathon, Lima, and others were 2/3. Cat has switched generator suppliers 3 times in the past 20 years so you may have anything there. Ask each alternator manufacturer to tell you the pitch wind they use by serial number.

Then tell us the results here.

the problem appears only when putting the cat on the bus

From what I've read, I think PetroPower is correct in his assessment.

Does your amp-probe have the capability to measure frequency? If not, perhaps it can be done with an DMM by measuring the frequency of the slight voltage imposed at a connection point. If you measure the frequency and find it is around 180 Hz (or 150 Hz for 50 Hz systems), then this would verify PetroPower's assessment.

I think what PetroPower is talking about is winding pitch as compared to pitch wind. The document at http://www.bmcoi.com/CatLit/Power/TECHNICAL%20PAPERS/GEN.%20WINDING%20PITCH%20-%20LEKX3115.PDF has a good explanation of this, and it does sound like you are experiencing a 3rd harmonic circulating current caused by different pitch windings between the different generators just as PetroPower described.

It sound like a reactor in the neutral would help alleviate this problem, but I could not tell you the size.

Since you measured a current (probably the rms value) in the wire connecting the 3 neutrals, to insert a reactor (inductance) or resistance between one of the neutrals and ground will probably not make a difference in my oppinion. To connect together the 3 neutral points is basically to make them equipotentially. To ground them is to bring all points connected together to the potential of the ground (earth) (assumed to be 0).

As shown in one of the responses before, one reason for having a current in the connecting wire between the neutrals is the pitch. Different pitches between generators lead basically to different wave shapes (different componence of harmonics) in the case of the involved generators, so there would be a current between the 2 points corresponding to the difference in amplitudes for a particular harmonic.

To reduce that current, a better method would be to insert a reactance on the connector between the neutral points of the generators, that should act like a filter for the harmonics in cause.

One way to determinate the harmonic component in case of each involved generator whould be to plot the wave in the stand-alone situation as well as connected and to compare them. Also to get the harmonics componence of the measured current between the neutral points, to see which harmonics are present in it and what the contribution of each is.

I tend agree with Petro Power.Its the winding pitch which can make the difference.Most alternator manufacturers use 2/3 now adays to eliminate 3rd harmonics.I also thank the other respondent for the article ref which is excellent.Good discussion going on.

You need to confirm the pitch of each generator winding is the same. What kind of load are you feeding? Not sure why you are connecting the neutrals, are you really doing this? Neutrals should each be grounded at the generator. I assume you are feeding all 3 wire loads (480 v, three phase, three wire). Do you have 277 v loads off the parallelled bus?, If not why would you bring out the neutral?