Earthing of parallel generators 1

[AusLee]

Hello,

I have 2 generators entering one syncho panel supplying two ATS. All breakers are 3 poles, throughout.

I will hard wire the neutral of every generator to the generator's easrthing pole and connect this link to the earthing bar of the synchro panel.

I will connect the earthing bar of the synchro panel to the earthing bar of the generator room, which is connected on both sides to separate earth pits.

Finally, i will connect the eathing bars of the 2 ATS to the earthing bar of the synchro panel.

Questions: is the above correct and if not what is missing or what is wrong? Another thing, shall i connect the neutral to earth in every generator panel, and then do it again in the synhcronizing panel?
help.
Thank you for your

 

[davidbeach]

Neutral should be connected to ground (earth) EXACTLY ONCE; never more, never less. That connected only once applies regardless of position of the transfer switches.

 

[waross]

What davidbeach said.
If you are adding generators to an existing installation there is probably already in place a connection from neutral to ground. This will be sufficient as long as your generators do not exceed the rating of the existing service. You may want to recheck the existing ground-neutral connection to verify that it has sufficient ampacity to satisfy current codes. If you are in North America check NEC. If you ground the neutral at one point you need only three pole transfer switches. If you ground at more than one point you may only have one ground at a time in service. Then you need to switch the neutral in the transfer switches so that only one connection is in use at any one time.
Some switch manufacturers will only supply late break early make switching for the neutral. This adds a lot of extra cost and with a switch there is always the posibility of a switch failure. I've seen a few open neutrals. An open neutral is usually expensive in the way of ruined equipment.
Run appropriately sized grounding cables to the generator frames for operator safety. Do Not Connect This Safety Ground to the Neutral at the Generator.
Re-read davidbeach's post. It is concise and accurate.
As far as your other ground connections, without knowing what codes you are working under and how the equipment is placed in relation to other equipment, it's hard to say, but Just Ground The Neutral Once in the Whole System. Some codes will not allow you to "Daisy Chain" the ground conductor from one piece of major equipment to another and if it is allowed it is still a bad idea. In the event that a piece of equipment is removed or suffers a meltdown, the next piece of equipment may be left ungrounded. Multiple neutral grounding creates both code issues and protection issues. Tell us what codes you work under and someone will probably offer advice on your other grounding connections.
yours

 

[AusLee]

Thank you for your answers.

I work under IEC, the system is TN-S.

The generators are a new addition. I have connected each generator frame to the earthing bar, and have connected the neutrals at the synchronizing panel to the earthing bar at the synchro. This way it the two neutrals will be connected only once to earth. The switches are 3 poles and i would appreciate if you point me to asupplier of late break early make breaker.

 

[AusLee]

Hello again, i have uploaded a schematich of the intended installation on the following address:

http://www.geocities.com/eaccari/index.html

The ATS is a two way 4 pole switch: either its Emergency or Normal side is transfered on the load side. It has no "intermidiate OFF position".

The circuit breakers in the synchro panel are 3 pole.

As per your comments, the link between the generator neutral and the earthing bars in the enerator room is better removed. "They" argue that since the generator has no circuit breaker, in case of an internal short-circuit to earth current will not leave the generator on to the synchro panel and trip the breaker, rather current is like do be drawn from the synchro panel, towards the short circuit.

So removing the connection between the neutral of the generator and the generaotr's earthing bar will not "reduce the fault current, which the more it is the faster the trip and the less the damage", again because if the fault is before the synchro panel, there is no breaker in this "fault loop" and the generator will function under short-circuit condition until something melts inside or some security function in its panel board will trip at least the excitation.

"They" go further and say, that since it's useless to have it or not to have it, better have it in order to "force" the zero voltage of the generator. Otherwise, given a small difference in the length of cables from G1 and G2 to the synchro panel, the latter will have a hard time regulating the two sets of voltage, having both ends of each floating at the source.

Thus given, is the proposed installation fangerous in any form and does it breach any article in NEC - if so can you please point it out?

 

[waross]

Hello AusLee
You're getting closer. Some one local may contradict me, but in North America the following will apply. If the system already has a connection from neutral to ground, this is sufficient. Remove the grounds at the synchronizing panel.
Re the late break-early make. Some manufacturers have 4 pole switches or breakers that are approved for simultaneous breaking of the neutral. Some manufacturers go with the late break-early make solution. See your ATS supplier for a four-pole transfer switch approved for neutral switching. I don't recommend this solution. If your transfer switches have been purchased, I would be most surprised if you can find a fourth pole that is field installable.
If your codes allow, just connect all the neutrals together. That is, the existing system neutrals and the generator neutrals. A separate ground conductor is recommended from the main ground bus to each piece of equipment. If the location of the equipment makes this very expensive, you may be able to run a common grounding conductor and tap off of it for each piece of equipment. The important point is that one piece of equipment may be removed or disconnected without affecting the grounding of any other equipment.
yours

 

[alehman]

As stated previously, there should be exactly one neutral-ground bond.

Multiple 3-pole ATS's with neutrals fed from common sources, create a situation with multiple neutral-ground bonds. If you must take neutrals to the loads, the only proper solution is 4-pole ATS's.

 

[AusLee]

ok - i understand you but i would really appreciate it if you correct me based ont the schematic i put on the above address, this way i can understand better.

Thank you in advance.

 

[waross]

Hello alehman

Multiple 3-pole ATS's with neutrals fed from common sources, create a situation with multiple neutral-ground bonds. If you must take neutrals to the loads, the only proper solution is 4-pole ATS's.

Considering the number of standby generator systems that I have installed, serviced and viewed with interest, that have had the neutrals taken to the loads and were able to satisfy codes without the confusion of multiple grounds and switched neutrals, and considering also the advice from the other responders to this question I must disagree with your statment.
respectfully

 

[ScottyUK]

Auslee, waross,

Draw out the system as a mesh of impedances, including both those of the copper conductors and those of the paths between earthing bars through bulk of earth. You will see that there are a number of parallel paths, some through copper neutral conductors, and some through a mix of copper earthing conductors and the bulk of earth.

Any neutral current flowing will divide between the neutral conductors and the parallel paths through the earthing system in inverse proportion to the impedances of the paths. So if you have any neutral current, your earthing system is also a current-carrying conductor, a function which it is expressly excluded from being except under fault conditions.

If any of your breakers use a differential technique to detect an earth fault, the flow of current through the parallel path can lead to protection misoperation. It is also a code violation to provide multiple earthing of the neutral conductor on a TNS system, at least over here in the UK.

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[AusLee]

Thank you scotty, i agree with you and i think there will be some nuisance tripping. Therefore can you please correct me on the schematic i put on the website, tell me which connections to kepp, make or remove?

 

[ScottyUK]

AusLee,

My opinion will vary a little depending on where your breakers and protection CTs are located. I think I understand that your generator has no local breaker, and the first breaker is at the sync panel? Does the generator have any form of zone protection - either REF or stator differential? If it does, extend the protected zone out to the first breaker by relocating the CTs from the generator. Check your CT VA rating will cope with the extra burden from the interconnect cables. You then protect the winding and cables from the majority of faults without going to the expense of a full generator protection relay. If you don't have some form of fast-acting zone protection, I would seriously consider adding it if this is a large generator.

Without any knowledge whatsoever of Australian code requirements, I am inclined to say bond the neutral to ground at the sync panel and to lose the bonds at the generators. Furthermore I would run a heavy earth bond cable between the earth bars at the generator site and the sync panel, unless you have sufficient CPC equivalent in the cable armour and can guarantee its integrity.

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[AusLee]

ScottyUK - Australian standards are now IEC clones, they even show the original IEC text barred and the local replacement, mostly relaxing the contraints.

The CTs are like this: there is one on the neutral bar at the back of the synchro panel, and the CTs of the phases are in the circuit breakers.

The generators are large. I do not know if they have an internal protection relay and will ask for this as soon as i can, this relay will be the only protection against a fault in the rotor/stator; this is the one that will cut out the excitation, right? in this case i have this relay protecting the generator and the circuit breaker in the SP protecting the load side.

The bond between the earth bar of G1 and that of G2 and the earth bar in the synchro is many times more the code requirements.

I understand what you say about taking out the link between the neutral point of the generators and their corressponding bars, and for this i have 4 questions:

1. once taken out, the function of these bars is only the equipotential bond of the chassis to earth, thus protection a person from entereing in contact with an extraneous conductor?

2. In case thre is no protection relay in the gen, and in case of a short circuit in the stator to earth, then whether the link between the generator neutral and the generator bar is present or not is irrelevant because in such case no current will be leaving the generator anyway since the short circuit is inside, and this is why this realy is not only needed but required?

3. In case i have this relay, then will having the link between the generator's neutral and earth bar play an active role in "enforcing the zero reference" or not?

4. And finally, we still uphold BS 7671 and if the installation is in breach of that or the IEE Wiring Regulation, 16th edition (the Guide) then i will duely report it if you can please point it out?

Thank you.

 

[waross]

Hello AusLee, ScottyUK,
Scotty, re the bond at the sync panel, I understand that this is an existing installation that may already have the required connection from neutral to ground. Other than that, 100%
A good rule of thumb that covers Canadian installations is
1> Design the ground system completely.
2> Design the neutral system completely and separately from the ground system.
3> Make one connection from neutral to ground at the service entrance panel.
No parallel paths, no need for 4 pole transfer switches, no worry about possible switch failure or contact welding resulting in an open neutral.
Other than that, AusLee, I'll leave you in the very capable hands of ScottyUK. He has the applicable codes and understands your situation. Thanks ScottyUK.
respectfully

 

[ScottyUK]

Hi AusLee,

I'm at home at present, so I don't have my references to hand - I'll have a look tomorrow if I have time. I'll try to answer your questions:

1) The generator frame earth connection could, in the event of a major stator earth fault right at the top of the winding, see virtually the full prospective fault current from both generators. The frame earth is a vital part of the protection for the machine, in addition to providing personnel protection.

2) In the event of a stator earth fault, current flow will depend on other available sources - you may see current infeed to the fault from the other genset through the phase connection of the faulted machine, so you can't categorically say that "no current will be leaving the generator". You will definitely see neutral current returning to the machine through the neutral-earth bond. This sort of fault is best detected using a high speed differential scheme using CTs located at the extremities of the protected zone which will detect the mismatch between current entering and leaving the windings, and extend that protection out to cover the cables.

For your application I suggest the extremities of the protected zone would be on the neutral conductor immediately prior to the connection to the main earth terminal, and on the phase conductors immediately before the generator breaker. You may need a dedicated group of CTs for this function if the breaker CTs are associated with an internal tripping unit within the breaker.

3) Not quite sure what you mean here: are you saying that "they" (yeah, I work with "them" too...) think that they are improving the ability to clear a fault by forcing the neutral voltage at the generator to be at earth potential? If that is the case, I can't see any benefit, only problems: if there is a stator earth fault you need a means to detect it quickly, initiating an OPEN command signal to the generator breaker AND tripping the genset. "Their" proposal doen't do anything to assist that requirement. Depending on your level of required protection, you want might to consider breaker fail protection to clear the board in the event of the faulted generator's breaker failing to open within a certain time, rather than motor the engine. Either way the generator should not trip unless the breaker has confirmed it has opened, otherwise you will need engine repairs in additon to stator repairs.

Note: Even a dedicated gen. diff relay will not detect stator faults right down to the first few turns of the winding, so there is some small risk of an undetected stator earth fault occuring. There are 100% stator diff. relays out there, but I have never seen one applied to an LV machine, nor most MV machines. They only become commonplace on multi-MW HV generators where the capital risk is much higher.

4) Can't remember offhand - will have a look.

----------------------------------

I don't suffer from insanity. I enjoy it...

 

[Aquarius]

Unless you corrected the schematic you have posted on the web quoted above , it is wrong.

You have excellent advice here from people :

1 One neutral only
2 Design Earthing/Ground system separately
3 Design neutral system
4 Position of CT and type of protection is important if you do not want to have unnecessary trips

Re. 4 pole CBs - there are cases that if you do not have them you run into a protection problem , this is a bit difficult to explain without a schematic.