grounding generator netural 4

[farang1]

Hi Everyone,
Now thet we have rewired a resort on one of our neighboring islands and added grounding all over(frames, panels and equipment) I now need to ground the Netural. How do I determine which conductor on the generator is the netural? We have driven 3 ea. 2 meter long ground rods near the generators and have run a common ground to all circuit pannels and have driven additional ground rods at pannel locations.

Any help or suggestions apperciated.

Thanks
Farang

 

[alehman]

It is not possible to answer this question without seeing the generator nameplate. It should be clearly identified though.

Seriously - hire an engineer. Arbitrarily running ground conductors to everything is not a good idea. You need an engineer to look at how your distribution system is grounded in order to determine how the generator should be grounded.

 

[Tristram01]

Suggestion: consult the vendor, and read the generator manual.

 

[Elecme]

You are mixing two grounding systems in one, and you now have two separate grounding paths. Even this may affect the operation of the circuit breakers. You really need to get an engineer (Specialist in grounding) to see and decide after

Elecme

 

[buzzp]

I agree, driving multiple grounds arbitrarily can and will cause problems with all sorts of equipment.

 

[farang1]

The Generator is a Chineese 1 phase 20 KW 220 vac Generator with no other info.
The grounds were not placed arbitrarily but placed where we nad no path to ground due to rock and soil/sand conditions and results of grounding tests. The intergrety of the ground may vary with the tides and weather. Grounds are driven at all Panel locations and used to bond equipment and the panel. All ground rods are tied together with a common ground wire. These are only for safety and a referene for our ELCB,s. The grounds also are connected to the generator frames.

When we rewired we chose one conductor marked it and called it netural and used it throughout the site. They
were hit by lightning 2 weeks ago and the circuit breakers operated and shutdown the system and there was no dammage. There was 2 years ago when equipment and wiring was severly dammaged the generator field regulators were blown and several fires resulted by a ligntning strike.

Agian how do I determin which conductor is the netural???

The system is as safe as we can make it under the circumstances and the conditions we have. Almost all locations are right on the beach or with in 30 meters, all get salt spray, we use silicone and heat shrink on all connections as well as main ELCB's on each panel.

If more can be done we will gladly propose it to the owners. Right now we war concerned with perminent power coming in 2 yeras and we want to be ready to install it with out rewiring.

Any advice will be apperciated, I am an Electrical Engineer but have been in backwater places for to long and need help on occation, I am all they heve in most cases. That is why I need help, especially on grounding and bonding issues.

Thanks
Farang

 

[alehman]

If the generator has 3-wire output, measure the voltages. Line-to-neutral would be 110V, line-to-line would be 220. You can determine by elimination which is neutral. Then check if it is bonded to the frame.

If the output is 2 wire, see if you can determine with resistance test or voltage measurement if one wire or the other is connected to the generator frame. If so, that is the neutral. If not, it doesn't matter which you use as neutral, unless there is an output circuit breaker.

If one generator is the sole source of supply for a circuit, you should connect the neutral to ground at the generator.

You said "generators" - is there more than one? Are they interconnected in any way? Where are the ELCB's located in the system? Does each generator feed multiple buildings?

If there is a circuit breaker (service disconnect) at each building, normally you should also provide a neutral-to-ground bond at the disconnects. However, if your system has ELCB's upstream, neutral to ground bonding will reduce or eliminate effectiveness of the earth leakage sensing upstream of the bond. It is in violation of the NEC to have neutral-ground bonds downstream of ELCB devices. In this case you must run a grounding conductor from the source of supply to the ground bus at the service disconnect at each building. Per the NEC that "equipment ground" must be routed with the supply conductors.

I don't know if this helps. You may also want to obtain a copy of the 2002 NEC and read up on sections 230 (services) and 250 (grounding). This covers details of grounding for various configurations.

 

[skiier]

I agree with everyone on the gnding.Multiple earth gnding points are not good. too many multiple paths to gnd can cause problems at fault time. Copper conductors used to bond distribution panels, generators, t/f's, and other equipment to one "central" earth point is a good idea though. Choose the best gnding point you believe you have with any of your gnd rods. Tie all your gnd bonds to this point and disconnect all those other gnd rods. Ensure that you have a solid gnd bond from any equipment, etc, to this common earth point.At ditribution panels disconnect any gnd bonds from neutral bars.The nuetral conductor is that conductor being common to your generator supply that is gnded. for example, a single phase 120/240v generator will have 2 windings with 4 leads. If we wire these windings "additive" polarity or in series we get a 3 wire system producing 120/240v. The center lead is gnded at the point of generation (or as closely as possible)to the central earth gnd point.This center lead only carries the unballanced current of the load(s) and is often referred as the "neutral" conductor. If your generator is strictly 220v then you will only have 2 leads out of your generator and all loads are strictly 2 wire. If this is the case then you have no "nuetral" and you must pick a conductor to gnd and makes this the gnded conductor (usually a white wire).In these 2 wires systems the gnded conductor is often mistakenly called the "neutral". So which is it? Do you have a "neutral" or a "Gnded conductor"? Oh yes it is also common to "float" the generator above gnd with no earth connection but this is not recommended for safety reasons and the added expense gnd fault arrangements.

Multiple gnd points create multiple gnd paths for faults. Copper conductors have a billion times less resistance than the earth so never substitute a possible gnd rod fault path for a good copper bond path. I mean the fault will find its way to the best earth point so why not give it a low resistance path. By bonding at multiple points to earth you offer the current a chance to split and travel thru earth or building structures or plumbing or electrical equipment or whatever. Use one central earth point only.

 

[farang1]

Thanks for the most helpfull information. I will read the NEC refrenced, make sure the grounds are common and to one point at the generators.

There are two generators being driven by one prime mover, I have seperated each one so it supplies seperate loads and buildings. Our elcb's measure fault current line to netural and are adjustable up to 30ma.

I would love to be able to parallel the 2 generators and have a combined capacity of 40 kw, but have been reluctant because of the netural and the grounded netural problem. If I can I would like to supplt all loads from the generators running in parallel (connections done at the MDB,
I would still be able to connect one generator when the load is low, (day time)and energise the others breaker when the load increased. Comments or suggestions apperciated. Not much access to refrence materials or other qualified people here.

Thanks Agian and Merry Christmas

Farang

Thanks

 

[jbartos]

Suggestion: Please, clarify the power supply system, namely:
1. Is it single phase, one hot (phase) one neutral, neutral grounded system, two wire system; or
2. Single phase, one hot (phase) one neutral (ungrounded), ground, three wire system.

For safety purposes, the equipment ground should be implemented. It means to ground all conductive metallic parts and bond them together.

The system ground. If the system is grounded, the neutral should be grounded to the equipment ground near the generator at one point only.

If generators supposed to be paralleled to obtain relatively more power, then they have to be synchronized first. The grounding system should be common and tied. The short circuit current rating of the equipment should be verified whether or not it will withstand the short circuit current without any damage.

The power distribution system should also have arrestors to be protected against lightning. the lightning arrestors are normally located on the incoming side of the power distribution system, downstream of the first circuit breaker protecting the transmission line.

The multiple neutral grounds should be avoided. The neutral conductor should be of sufficiently large size to guarantee low resistance return path.

It would be good to follow: NFPA 70-2002 National Electrical Code (NEC) or similar electrical safety code.
Visit

http://www.nfpa.org

for NEC.

 

[alehman]

Comments to skiier and jbartos,
You both mention it is best to avoid multiple neutral grounds. This is certainly true in some situations. Under the NEC, if there is no ground fault protection (ELCB) and no common grounding conductors between building, the neutral must be grounded at the service disconnect of each building (if one exists). Otherwise, you must bond at the source and run equipment grounding conductors from the source to each building.

In either case the frame of the building and water piping (if applicable) must be connected to the service ground.

 

[jbartos]

Comment on the previous posting: It depends on the power distribution system voltage system. I agree with NEC that covers one type of power distribution systems, namely, hot wire, neutral wire and equipment ground wire. There are other systems being used in various Countries. E.g. there is a two wire power distribution system where the neutral is also ground conductor. This power distribution system permits multiple neutral grounds since the protective device will sense the ground or neutral and ground fault current. The multiple neutral grounds are improving the return path to the source thus allowing the protective device to sense higher current and trip. If the neutral happened to be long and the return path had the high resistance, the protective device might or might not be able to trip.

 

[buzzp]

In a previous post I mentioned the practice of using multiple ground rods is not common and should be avoided. This is the case in most installations, but in this one (based on the little info I have and assuming I understand things as installed) is very acceptable due to this being a sort of generation facility. He has made a ground mat. As far as finding the neutral conductor, your best method might be to measure the current in the neutral. If you have current flow in the conductor then it is likely the neutral since there will be some current returned (presumably there will be some loads other than pure resistive). The ground lead should have next to none. This is a very basic statement without knowing how the grounding/bonding is handled exactly. Other than that, I can not think of an easy method to find the neutral vs. ground without tracing cables.

 

[vmac100]

i will have to say that a 2m earth-rod is inadequate.how deep did sink the earth-electrode?and did you measure the earth resistance?here in my company,the maximum acceptable value for earth resistance is 4 ohms.as for finding the neutral conductor,why not call on the manufacturer's representative?it's no good toying with these things.remember that electricity is a 'monster'

 

[skiier]

General question.

It's been a while since I studied parallel generator theory so correct me if I am wrong please.
The load shared by each generator will be in direct proportion to it's KVA rating - assuming that each generator has the same proportional internal characteristics such as impedance and voltage levels. I don't know the details about your installation - the distance between generators and the load on each - but would I be correct in stating that it would be wasteful and less efficient for current from one generator to go to a load that it did not supply before you connected your generators in parallel?
What I mean is you will be getting a current between generators as the load varies that was not present (or even needed for that matter) before you parallelled those generators.
For example. Gen1 has a capacity of 20KVA and has a static load of 15KVA. Gen2 has a capacity of 30KVA and has a static load of 5KVA.
When these 2 generators are paralleled the load on Gen1 will become 20/50*20=8KVA and the load on Gen2 will become
30/50*20=12KVA. What happens is Gen2 takes some load away from Gen1 and you produce a current in the conductors between the generators. This cuurent flows from one generator to another and then out to the load from each "tie point". Note that this current didn't exist before and wasn't even neccasary.
The only time that there would be no current between generators is when the loads connected to Gen1 and Gen2 are in equal proportion to their KVA ratings. For example, Gen1 now has a static load of 10KVA. Gen2 now has a static load of 15KVA. When these 2 generators are paralleled the load on Gen1 will become 20/50*25=10KVA and the load on Gen2 will become 30/50*25=15KVA. The load sharing is unchanged and no current flow exists between generators.I am assuming the loads connected to Gen1 and Gen2 are right near each generator and in the same building.
Think about it. If you had a load right next to Gen1 it might be receiving a portion of it's power from Gen2 somewhere a long ways away. Is this efficient?

Also, would it be common sense that the "tie conductors" between generators would have to rated for the capacity of the entire system. Instead of rating conductors at 20 or 30KVA do we not need to rate them at 50KVA?
However, if you think that you are approaching full load on either generator than you might consider paralleling for capacity concerns only.
Just wanted to show another angle on this. Any comments or replys would be appreciated?

 

[skiier]

Oh yes. When you parallel generators if you lose one generator does the other generator not shutdown to protect itself?
Lose one, lose them all.
I live in ontario. I saw our grid go down last year for just that very reason. Wouldn't it be wiser, cheaper, and safer not to parallel your generators?
Just another thought.

 

[skiier]

Comment to Jbartos.
"E.g. there is a two wire power distribution system where the neutral is also ground conductor."
In North America the neutral conductor is that conductor of a multi phase system that carries the unbalanced load. I understand about neutrals as grounds but I am trying to make a different point here.
The man is asking which conductor is the "nuetral". I see people in the industry commonly referring to the "grounded conductor" or "white wire" in a 2 wire as the "nuetral". So maybe in this case he is looking for something he doesn't have in his system - a nuetral! If he has a 2 wire system connected as you say then he has a "combination nuetral/ground" conductor. If he grounds a generator conductor at the supply and no where else then in a 2 wire system he has no "nuetral" but he instead has "grounded conductor".
I just want to help this guy to determine what he has.
If you have a 2 wire generator with one lead grounded at the source stopping looking for your nuetral- you don't have one!

 

[farang1]

The system is operating as normaly or as much as I can expect.
ELCB's work and when there is a short the breakers cut. It was necessary to install multiple ground rods due to the rocks and sanddy soil, the ground resistance depends on the weather and the tide level, I get no less than 20 onms at the generator. I desided not to pararel the generators for the reason you suggested, if I have a problem in one generator I still have one good one.

I still have not grounded the netural, there is no generator electrical connection to the generator frame, although I have grounded the frames of the generators.

I have wired the loads, buildings, ect. following strict color coding, with the hot and netural marked and hooked up as if the netural was grounded.

When/if main power does come the systejm as wired should be able to be connected with little problem, at lease I hope so. If the local "technicians" have been warned to follow the color code we gave them when making repairs or additions to the system, they must also keep a log of all repairs or alterations to the system which we check when we make the journey to the island every month.

I would like to thank all the members for the help and advice, I beleve the system is as good as it can be under the circumstances. Any additional advice or help is always apperciated.

Farang
Thailand

 

[davidbeach]

skiier,

Whether the loss of one of a pair of paralleled generators means the loss of both depends on many factors. If the control system is designed to deal with that situation, you should be able to have some load remain following the loss of one generator. Best case is you are running N+1 generators and the loss on one means a brief voltage sag while the remaining generator(s) respond to the increased load. With two generators supplying a combined load of more that the rating of the smaller generator, you should have a load management system designed to deal with the loss of a generator. It may mean that all load has to be shed instantaneously, and the the highest priority loads picked back up. But this is still better that dividing the load between two unparalleled generators as the highest priority loads will have a higher reliability.

 

[buzzp]

For synchronous machines this is why they make governors and excitation systems.
If one generator goes down it is NOT desirable to have the other go done because the problem gets bigger as more generators are taken off line (assuming a problem with the load side of the generator). Both need seperate breakers so one can be isolated from the other if need be. Paralleling generators is not a problem, assuming you have control over both machines and associated breakers/disconnects.