480 3ph 400kVA generator 4

[itsmoked]

A generator had been setup to run a facility and had run it many times.

The facility toasted as I mentioned elsewhere. Now they are reconnecting the generator to temporarily run some of the functional remains.

The "industrial" electricians are seeking permission to "ground the generator's neutral" at the generator. The generator has only ever had it's ground lug grounded and the three phases hooked to the facility, never a neutral.

It's a Volvo Penta.

I've never looked in the connection compartment. Would there normally be a neutral? I was assuming the generator was built delta.

If there is no actual neutral star point in the windings is there a problem grounding the 'neutral' lug?
If there actually is a star point in the windings that is connected to the neutral lug is there any reason why it shouldn't be bonded as the sole source for the building?

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

I think every 3-phase generator I've seen has been star connected, or some variant of star such as series or parallel windings for multi-voltage machines.

If it is a star-connected machine then the neutral should be bonded to the main earth terminal. This is important because without a path back to the generator for fault current the protection may not operate for an earth fault.

 

[dpc]

It's most likely a wye-connected machine. Delta would be uncommon. It's possible the neutral is grounded internally. If not, there should be neutral lead brought out. If the facility had been operating as an ungrounded 480 V system (this used to be common), then grounding the neutral could introduce some unintended consequences. In theory it should be OK, but there may not be adequate equipment grounding conductors to handle this new available fault current. If they have 277 V loads, you have no choice - neutral must be grounded.

Curious as to why to the reasons the electricians have for wanting to ground the neutral. Sounds like fun.

Dave

 

[waross]

It will be a twelve lead set. (Some very old TelCo standby sets were 10 lead but there are not many left.)
A normal and preferred standby setup is:
Utility supply to the revenue metering.
Reveue metering to a three pole ATS.
ATS to the main service.
Solid neutral, grounded at the main service.
Generator to the three pole ATS.
Solid neutral.
This complies with the rule that the neutral must be grounded in one place only.
Alternate connection:
Utility supply to and through the revenue metering to the main service.
Neutral grounded at the main service.
Main service to a four pole ATS.
Neutral switched at the four pole ATS.
Generator neutral grounded.
Generator to the four pole ATS.
Generator neutral switched at the four pole ATS.

Quick answer:
The plant neutral may be grounded at the main service as per code.
If a solid (unswitched neutral) is used the neutral is not grounded at the generator.
If the generator neutral is grounded at the gen-set, then the neutral must be switched to comply with the rule that the neutral must be grounded at one point only.
My preference is a solid neutral and no grounded neutral at the generator.
Not knowing how the set was originally connected and not knowing if the transfer switch will be in use, I have to say;
"It depends."
AND.
If possible consult your local AHJ.
There may be regional amendments or interpretations to the code.
The electricians may be confused or they may know more than both of us as to local practice.
Hope that this helps.

Notes;
The generator will have six 138.5 Volt windings that will be rated for 120 Volts.
At 277/480 Volts there will be two 138.5 Volt windings in series for each phase.
For 129/208 Volts the AVR will drop the winding voltage to 120 Volts.
Your 400 KVA gen-set will be capable of 347 KVA at 120/208 Volts. (400 KVA x 120/138.5 Volts = 347 KVA).
Hi Scotty.
You see the big generators and I see the small ones.
I appreciate and enjoy sharing information with you concerning the differences between big and small.
There are two common delta connections with the smaller sets.
A delta connection is used to supply a four wire delta system. 120/240 Volts delta.
A double delta connection is often used for a single phase conversion.
The double delta connection is often shown on the nameplate of small machines. (Single phase conversion is not something that you are likely to see on your large machines unless you get into electric railways.) grin



Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

Ah, thanks guys.

Unfortunately the power center where the service drop split out to 5 buildings is now a twisted heap of slagged metal.

Hence this generator is going to be used prime for the next 8 months running one of the production facilities. Meanwhile there are two more 400kVA generators each running different other buildings rented for $4.9k/mo.

Curious as to why to the reasons the electricians have for wanting to ground the neutral. Sounds like fun.

They've stated they want to do it "for code compliance". So I believe it's going to be THE bonding location for the next few months.

I just find it interesting that this generator has run the entire facility in the past for weeks at a time yet never had any neutral run to the building. They just put in a large chiller about 2 months ago and I looked at the generator noting two large cables per phase and ground only coming out of the generator. Yet now the neutral is being focused on by the industrial electricians company.

Makes total sense that it's a wye wound generator as you guys are describing. This place was built in 1920 and grew to 300kVA over many decades.

At any rate I'll not apply any brakes to them getting the neutral involved or bonded to ground. Hopefully dpc's grounding conductor concerns don't get realized.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Hi Bill,

I've been in the oil & gas game for a while since my old station closed, me having left prior to closure when I saw the writing on the wall. I do get to play with the little machines now and then. In fact everything I work with is a little machine, relatively speaking.

I was deliberately excluding the single-phase variants from my comments - that is the only place I've seen delta-connected generators in the UK, and even then the zig-zag connection is more commonly used. We don't use the corner-grounded delta that exists in North America - the first time I came across it I did a double-take thinking "What the heck...?" and I still think it's weird.

 

[waross]

Hi Scotty.
Actually the more common four wire delta is 120/240 Volt delta, not corner grounded.
One transformer of the 240 Volt delta supplies 120:240 Volt single phase loads with the center point being grounded.
Our famous "Wild Leg" is high in respect of both the neutral and the ground.
Keith.
Originally the system ground would have been at the main service.
When the original gen-set was installed there should have been a neutral cable run to provide a system ground when the plant was running on the generator.
The genset installation may have been non-compliant and the deficiency either not noticed or overlooked when the chiller was installed.
Or, the system may have been an ungrounded 480 Volt system.
That is not a good idea.
If there are no 277 Volt legs and this will be the prime power, then grounding at the genset saves the cost of running a neutral conductor to the main panel.
It sounds as if the electricians have it right.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

The genset installation may have been non-compliant and the deficiency either not noticed or overlooked when the chiller was installed.

I think you're absolutely correct. An oversight originally and these electricians have discovered it. I'm pretty sure the service drop off that 300kVA transformer has only three wires and maybe a ground. They must make 240 with a now missing-in-action transformer generating a neutral with that for all the non-480V stuff.

Thanks!

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

Caveat:
There may be differences between the installation standards for an equipment grounding conductor and the system grounding conductor from the wye point to the ground grid.
The conductor grounding/connecting the neutral to the ground grid should be installed to comply with system grounding standards.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

This is all temporary and the replacement building will have all this, no doubt, engineered.

We'll see what happens.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[ScottyUK]

Bill - thanks. Equally weird!

Keith - sorry for dragging your thread into the weeds!

 

[waross]

Deeper into the weeds.
Scotty, the background may make it a little less weird.
As I understand it, your single phase circuits are derived from one phase of a three phase supply.
As such, the circuits are grounded at one end.
While there is no neutral in a two wire single phase circuit, one wire is the grounded neutral of the three phase supply and may still be called the neutral.
Our normal single phase supply is a center tapped transformer with the center tap being the neutral which is grounded.
To this may be added a second transformer in open delta to supply a relatively small three phase load.
Note; regional differences, open delta = two transformer delta, not the three transformer delta with an open corner which is called a broken delta on this side of the pond.
At one time in the distant past, this was a common supply for large upscale homes.
The reason; In the early days of air conditioning, single phase A/C units were not available in the larger sizes that are now available.
The open delta was used to supply three phase for residential AC units.
Single phase AC units are available in much larger sizes now and residential three phase is a thing of the past.
As for corner grounded delta;
I have never seen a grounded delta system in the wild.
I have seen lots of four wire delta systems.
I have encountered a number of older homes that were once supplied by three phase four wire delta sources.
The old three phase AC units have long ago failed and have been replaced with the now available large single phase AC units.
The service has been reverted to a single phase service, but in some homes the third phase wire is still in place and abandoned at the weatherhead.
Every third breaker position in the panel is unused.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[itsmoked]

No worries Scotty! I got the fine answers I needed.

Keith Cress
kcress -

http://www.flaminsystems.com