100KVA Generator Proper Grounding

Dear jeabellera,

First you must confirm in your generator single line diagram, Does it Gounding with resistance or without resistance?

Hi Febriansasi, As I checked the Neutral to Grounding Point (Tapping Point) it seem that the Neutral is completely isolated. when I measure it don't give any reading at all, and as I inspect it, the neutral plate is separated by a porcelain insulator. A I don't see any other wire or winding connected to it.

What have the generator manufacturers said about this?

Have you bonded the neutral and ground together?

Hi wareagle, I have not bond it yet. And Im not sure if it is safe? Also only 2 tapping point of the Ground on the entire Gen. 1) at Frame (leg) and 2) at the housing of the alternator. Also, when I try to measure the resistance between the Neutral Plate and the grounding tapping point is like it not connected by any means. Does it mean I need to ground it also separately?

There is possibility of 3rd harmonics or its multiple current in your load. These current when flow through neutral are producing voltage drop.

Please use a True RMS digital multimeter and measure voltage between neutral and system earth and phase to earth. You may find measurements similar to noticed with Mains supply.

If above is the case, nothing to worry as these 3rd harmonic voltages are not going to harm your machine - only you may find some humming in intercom (telephones) if telephone lines are running parallel to earth conductor or Bus Bar or neutral cables.

Your next question can be source of third harmonics. Most probably, the stator winding of alternator is not 2/3 pitch. But nothing to worry as far as you are not synchronising and going to run this DG Set in parallel with other DG Sets.

Best regards

Ramesh Kapur

even though you are using the same earth but all you need to check is the resistance between your transformer and your earth. that may be high.

It is always the duty of the machine supplier to provide installation guidelines to customer and it is the duty of the service engineer commissioning the machine to thoroughly check the installations before commissioning the machine. Here, in your case, neither of this happened. It is always recommended to install the genset with 4 nos of earthings. 1) Alternator neutral 2) Panel neutral 3) Generator body ground 4) Panel body ground. Note: All these earthings should be separate and for particular use only. Also, you need to balance your connected load on each phase. So, that equal load is connected on each phase. In your case, due imbalance load and ungrounded neutral, you are facing neutral shift problem on your genset. Hope, this article would be useful to you.

Which code says you will have to keep all earthing separate? The OP has measured the voltages in both load and as well as idle running condition. I am sorry, in my opinion, your reply does not measure up.

You sound like, you were the one measuring the voltages on load as well as in idle condition. Try connecting the earthing as said and you will create your own code.

My advise - please go back to the basics. I do not want to write my own earthing codes. If we are talking of TN-S system here (as is evident), there are enough documentation as it is and various interpretation thereof. No need for me to muddy the water further.

Hi Anonymous. So point the lad in the right direction then. One day you may require an sensible answer and you might not appreciate getting this type of answer. Point us all in the right direction. Help costs very little but time and effort. You took effort and time to post this, why not post the actual road we ALL should be on. I too might enjoy learning from another. So muddy the water!!!

This topic has been discussed ad nauseum in this forum. Search engine in CR4 forum will deliver volumes of discussion that will keep us all busy for nights together. Maybe we can have a discussion once #DS reviews the information. Thanks.

Dear Anonymous; That's what I wish to convey. Go to basics. OP has clearly quoted that, he has not connected neutral (ie star point of the generator) to earth. In a TN earthing system, one of the points in the generator or transformer is connected with earth, usually the star point in a three-phase system (Refer this link : http://en.m.wikipedia.org/wiki/Earthing_system). As far as seperate earthing to be used is concerned, that is not mandatory. But, doing so is the safest and complication free method. But, earthing the neutral is mandatory. Hope the water is crystal clear now.

Dear #DS,

It was & is not my intention to get into a slanging match and this is going to be the last post from me in this matter. Your original recommendation was strong and absolute (It is always recommended to install the genset with 4 nos of earthings. 1) Alternator neutral 2) Panel neutral 3) Generator body ground 4) Panel body ground. Note: All these earthings should be separate and for particular use only) and in post #15 (You mean to say that you have common earthing for neutrals of grid and generator…….. It is advisable to use separate earthing for neutrals of each power sources. ).

Then to compound the matter further, you tried to defend the indefensible (Try connecting the earthing as said and you will create your own code).

However in your last post, you have slightly watered down your stand (As far as separate earthing to be used is concerned, that is not mandatory). Hopefully, once you fully read the code and assimilate, your understanding and interpretation may change. I know for sure that each and every time I read the codes, my interpretation changes.

Dear IQ,

It was & is not my intention to be rude and curt. If at all my reply had come out like that, I apologise. The subject matter is vast and cannot be compressed into replies in forum format. Best is to allow the forum followers to read, digest and interpret the codes and discussion threads. Education is a continuing exercise & we learn something each day.

Dear Anonymous, No need to apologise at all. We all get frustrated. We cannot all know everything, so depend on others more knowledgeable for help. I also wanted to know the correct answer as all pointed to earthing for myself. Enjoy your day and certainly, No hard feelings at all. Have a good day.

Dear Develop Service, As I explain on my other reply, I can only see two (2) tapping point of the Ground on the entire generator system. 1.) Base-Leg of the Alternator 2.) On top of the stator or housing of rotor. (which is the same metal structure) But, we also ground the chassis, which make it a 3-ground point. But I did not ground the neutral. As for the neutral, How do I ground it? Can I ground it directly but separately? meaning I will put a separate ground cable and earth it directly with its own rod?

Ofcourse, ground it directly. Your generator is a LT (415V) so you do not require NGR as in case of HT. And you do not require 3 body grounds for generator body. Disconnect one from body and connect it to neutral with the help of flexible cable. This will surely solve you problem.

Clarify that this new installed DG is running in synchronizing with main commercial supply or DG is running independently?

If Two no of DG are running in parallel then only one DG Neutral is to be earthed. Second DG netural to be kept isolated.

Pl do not confuse the person who had asked guidance on reason and solution for factitious voltage appearing at Neutral of DG set.

1. This has nothing to do with whether it is in parallel with grid or not.

2. For 3 phase 4 wire system neutrals of all source of power shall be grounded so that single phase loads are shared equally by all sources in parallel.

Dear powersolutionFBD, I dont want to create confusion but want to discuss the matter in detail. Kindly find the comments of PWSlack which is more clear about two source paralling

"If the genset is operating in parallel with some other source of power then only one neutral should be connected. If the other supply's neutral is connected and this genset is operating in assist mode, then one must expect strange voltages on its own neutral point, it not being connected to anything. Don't worry about them."

Also I had faced the fluctuation in Neutral voltage when I tried to synchronize two Gen sets which is due to its AVR is not responding.

I think PWSlack answer is more useful for us.

It is true for high voltage machines, wherein we have impedance earthing to limit fault currents.

For LT machines opening neutral is last resort when there is no other solution. Both for safety of equipment and personnel.

And it is just a 100kVA DG Set (very small DG for parallel operation that too with Grid, which requires a very special speed governor and special AVR).

Also please note if his DG set had been running in parallel with Mains, he would not have noticed any problem of neutral shift and interference in communication system. for your information reasons being that Grid has a infinite source of Power when compared with 100kVA DG Set. Thus it will decide the status of neutral as well as absorb all harmonics because of almost near Zero short circuit impedance.

Hope you understood the request to please not to create any confusion.

Best regards,

Ramesh

...which would account for the 40V.

Hi Ritz, There is only one GEN, the 100KVA, the other supply is the commercial source which was tapped on their distribution transformer nearby. The GEN will NOT be running simultaneously and also NOT in parallel. This is just a standby supply. And all the lines (L1,L2,L3,N) of GEN and Commercial are completely isolated from each other by the ATS Contactor. Only the ground are shared.

Hi, fistly, start at a common point and a beginning before jumping around, it is advisable to check your earthing and knowing West Africa you have no good earthing connection. Normally for W Africa 1 ohm and less is good, if not 10 ohm is acceptable. So do an earth resistance test before doing anything else.

Secondly, you may be picking up stray currents from other equipment or cables, so you cannot determine much with simply checking voltages.

Thirdly, what other steel structures are in the ground? This will all help to reduce the earth voltages but if the ground (soil) is poorly conducting, (which I have no doubt it is), you need to install a longer earth rods or more of them, or a earth mat of copper conductor and discover the source of the earth leakage. Check all the earthing, check your cable earthing, check your Tfx and gen set earthing before looking to harmonics, etc. Experience of Africa and power systems around Africa. And new gen set installed, all points to earthing connections.

Hi IQ, Yes I agree that somehow Africa Area poor grounding soil. But it our location, I'm near the coastline and rain is frequent which makes the soil a little bit wet and possibly a good conductor at all. And for the Grounding Resistance the Value is lower that 5 ohm in our existing grounding for the other supply, which we are currently using with our commercial supply. This is the same grounding I used to test the generator. As for stray voltage, as I mentioned, it is only happening on the Generator. If the stray is there, why was it not coming when Normal supply is being used or tested. At 100KVA Gen - With or without the load, the voltage is the same 35-50 volts. using <5ohm Grounding BUT at Commercial, with/without the load, it is 1-2 volts only. By the way, I used 3 set of 2-meter pure copper grounding rod, which is 1 meter below the ground, making the entire Grounding-Rod buried under completely for better grounding.

Dear all,

I am in UAE, the same problem I am also facing with 800KVA generator. But in my case it is just reverse.

In case of generator the Neutral to ground voltage is 0.8V and in case of mains from transformer the neutral to earth voltage is 15V. If the connected load is not balanced for each phases the the same voltage between Neutral-to-Earth should appear with generator mode too.

Please be informed that the neutral of generator and transformer is not shared. Both are connected through an ATS panel.

The generator set has 4 earths. 1) Alternator neutral 2)panel neutral 3) & 4) body earth

Er, what problem?

The problem is with Neutral to Earth voltage is 15V. Due to this N-E voltage many display devices are malfunctioning and EMI is introduced in unwanted locations like communication modules.

You mean to say that you have common earthing for neutrals of grid and generator. And you connect either of the neutrals (ie of grid or of genset) to earth through auto transfer switch (ATS). Doesn't that sound complicated? It is advisable to use separate earthing for neutrals of each power sources. That too near to source itself.

The <...Earth...> conductor is there only to take, safely, fault currents until the moment the circuit protective device(s) operate(s). That is its job.

Display devices should be powered from line and neutral, in the case of mains-powered ones, and from low voltage supplies such as 24VDC for some others; it is common to bond the negative downstream of the 24VDC power supply to local earth within the panel.

The problem <...many display devices are malfunctioning and EMI is introduced in unwanted locations...> seems to indicate something is wrong with the supply arrangements to the <...many display devices...>.

Get the installation checked over by a qualified local Electrician without any delay - as in, today ends at midnight sort of speed - and ideally before anyone gets hurt/killed.

As I expressed earlier, EMI in communication system points to 3rd harmonics in supply when on DG Set. To be sure:

1. Ask supplier of DG Set about information on Pitch of Stator Coils. This information is also available in Data Sheet of 3 phase Synchronous Alternator. Earlier practice was to manufacture LV alternators with 2/3 pitch, this makes the alternator expansive (whole pole pitch is fully utilized) and to reduce the cost and be competitive, practice is to use 7/8 pitch. Machine with 2/3 pitch will result in Zero Third harmonics (Pitch factor is proportional to Sin of angle. Full pitch = 180 Deg. 2/3 pitch = 120 Deg. for fundamental (50Hz) and 120 x 3 = 360 Deg for 3rd and its multiple. Sin of 360 = 0, Hence zero 3rd and its multiple induced in stator winding.

For any other frequency the sum of three phase currents = Zero (whether balance or unbalance) and hence no current flows in Neutral.

But in case if 3rd harmonics are generated in 3 phase system, the current becomes zero degree in all three phases Phase angle = 120 deg for three phase fundamental, but for 3rd and multiples = 3 x 120 = 360 deg. That means the vector sum of 3rd harmonics will be 3 times phase harmonics current. Combined effect of 3 times line inductance, increase in AC resistance of earth path due to skin effect etc. is voltage appearing in between Neutral and Earth.

2. You can check this either by True RMS meter or by getting Harmonic analysis carried out of DG Output wave form.

In above case to avoid hum in telephone circuits or communication circuits - two solutions I can think of:

1. To avoid interference from Conduction of harmonics - install a Low Pass filter in Power Supply of Communication circuit (not very expansive - you can fabricate of your own - inductor in series and capacitor in parallel).

2. To protect from interference from Induced Harmonics, any communication wire in laid in parallel will be power supply shall be encased in MS or GI conduit. (While typing this I remember, I had once similar problem sometime in 2002 from 11kV, MaK(Caterpillar) + AVK alternator DG set for Videocon in Surat. The Power Cables were single core and not laid in Trifoil (not possible at termination point). These were causing induced voltages in control cables running in close vicinity and problem in PLC. I took a 3" GI conduit, Split it, opened and placed around control cables where these were close to power cables (only 2 to 3 feet length) and problem was solved).

I hope I could help you better. Trying to give explanation and solutions from symptoms.

Best regards,

Ramesh

What reading do you get between lines and neutral and also between lines? Are they consistent? If yes, check whether the point you call ground is solidly bonded to the earth mass/grid. I suspect a case of poor contact of the common ground as your line to ground voltage is varying. Please check with extended leads with common lead connected to a good earth electrode. I think you will get consistent reading. This is of course subject to your D.G. Neutral also being solidly earthed. Why are you saying your DG neutral is not earthed like commercial power?

If it weren't earthed, for instance when the genset is in assist mode, then 40V neutral-to-earth would not be a surprise.

Given the replies so far the absence of any additional supporting information from the original poster is, however.

You are absolutely right. However it is long shot that a 100 KVA d.g set will be operated in synchronous mode with other d.g's. Cost of providing synchronising facility may even be more than the cost of d.g itself. Seen in the light of this assumption, the neutral to earth voltage is because of either poorly tied neutral earth or body earth. The ball is in the OP's court.

...who has gone quiet. It is because of a fatality, a loss of care, or is it embarrassment, one might wonder.

I am sorry that I did not noticed that you have not bonded your neutral to earth. Pl do that first. As you have asked, Yes it is required to be earthed the way we do on neutral of commercial supply.

Reasons for connecting neutral to earth are:

1. Fixed reference for complete insulation system with respect to earth (termed as insulation co-ordination).

2. Return path for all leakage currents to earth of insulations of generator stator coils, cables and equipment.

3. Dump any static build up in the distribution system. When any conductor is insulated from earth it can pick up stray statics which will keep on adding and thus raising static voltage of that conductor (in your case your generator winding and whole LV distribution system). When we impose voltage generated by the DG on top of Static charge - it can reach breakdown value of insulation.

4. Safety of personnel. If neutral is not earthed and any phase (say R phase) develops an earth fault - the voltage of metal parts, like base of DG set, body of thee generator the casing of switchgear etc, will be at voltage off phase R. In this case if any technician touches any other phase by accident, he is subjected to phase to phase voltage which is fettle.

5. For proper functioning of protections - if we do not bond neutral to earth and any phase in distribution goes to ground, there will not be any return path for fault current, thus fault current will be practically zero and fuse will not blow out or MCCB will not trip. Moreover this will result in unsafe position like 4 above.

Hence please bond neutral to earth immediately. Your problem of Neutral to Earth voltage will be solved.

The phase to neutral voltage can be different when

1. Loads are unbalanced: This is due to voltage drop in each phase is proportional to current in that phase.

2. some time above is increased if sensing of AVR is single phase and not on average of three phase voltages.

I have been discussing with you 3rd harmonics, cause of their generation and remedies without realizing that Neutral of 100kVA DG Set was so far floating and not grounded - sorry about that

Best regards,

Ramesh

That is what I am stressing on from my first post itself.

floating neutral. ground it per NEC or BS7671. problem solved guaranteed

http://cr4.globalspec.com/thread/85330