Generator with Voltage at Neutral?

[bozkirkurdu]

I have some difficult times at a construction site. One of the generators which generates power for some part of the site has voltage at its neutral and its grounding connection (while the ground cable was disconnected).

We found out the problem yesterday by chance when an electrician got an electric shock from neutral. I measured voltages between the following points and ended up with the figures below:

1) Voltage between any 2 phases: 380V
2) Voltage between any phase and neutral: 220V
3) Voltage between ground connection of the generator and phase 1: 380V (while the ground cable was disconnected)
4) Voltage between ground connection of the generator and phase 2: 380V (while the ground cable was disconnected)
5) Voltage between ground connection of the generator and phase 3: 20V (while the ground cable was disconnected)
6) Voltage between ground connection of the generator and neutral: 200V (while the ground cable was disconnected)

When the grounding connection of the generator is connected to the ground (which is less than 2 Ohms) nothing changes.
The voltage is real since I can light a bulb between ground connection and neutral.

Can anybody tell me what the problem could be? And what can I do to fix it?

I appreciate any help regarding this.

Aziz

 

[itsmoked]

Well, well, a nicely worded question will all the info. Very nice. Thanks.

Looks like your phase 3 is shorted to the housing/frame.
Look for a frayed wire or a bad three phase cable plugged in. Or a flying wire.

Have you unplugged everything from the generator to isolate it and see if the problem is the generator or a connected load?

 

[bozkirkurdu]

Well, I disconnected the load to see that the failure was not coming from the load side. It is not. But I have not disconnected the short wiring between the alternator and the generator panel. I can not see this wiring completely. A small possibility but maybe, the insulation of phase3 cable has damaged due to vibration of the engine.

I wonder if anything can happen inside the alternator which can cause this problem.

 

[waross]

Hello bozkirkurdu
Are your new heaters on this generator? Are the control transformers connected phase to ground instead of phase to neutral?
In any event, I would look for a single phase load that is connected phase to neutral instead of phase to ground.
Yours

 

[bozkirkurdu]

Waross,
I have the problem there even when the load is disconnected. I did all the measurement while the load was isolated from the generator.

 

[waross]

bozkirkurdu
What current flows in the ground connection? If this is internal to the generator it may be serious.
My thoughts are;
It is possible, but unusual for a fault to pass enough current to light a light bulb, but not do progressive damage.
It is quite possible a device connected to neutral may cause this type of issue, without doing any damage. A volt meter, or some control device.
You didn't mention the size of the generator. Does it have any auxilliary devices such as heaters or a battery charger fed from another source?
From your description I wonder if the neutral is normally grounded.
You may have an internal ground at the 200 volt point of a winding, but even if it was high resistance, I would expect it to lead to failure quite rapidly.
Can you isolate the generator leads from the control leads and megger the generator windings. If you have a winding failing, it's best to not wait for a complete failure. A burn-out may do arcing damage to the stator.
yours

 

[itsmoked]

Waross;

[highlight]In any event, I would look for a single phase load that is connected phase to neutral instead of phase to ground.[/highlight]

Why would you want to hook any load to ground?

I agree it's pull the leads and meggar time. Or even just pull the leads and ohm it.

 

[waross]

Hi itsmoked
I wouldn't want to hook any load to ground. However in the third world, many technicians and or electricians cannot understand the difference.
Duhh it wurks don't it??
Stuff happens.
Is there a forum for stories you won't believe. I cleaned up after a local expert connected a residential standby generator. The owner asked him, "Do you know how to connect a generator."
"Sure. It's just like a motor." He didn't bother with a ground or a neutral. Good-by TVs, Stereos, and a bunch of other expensive stuff.
yours

 

[bozkirkurdu]

One moment, one moment. No load was actually connected to the ground. A ground is a ground, right? It is our PE (Protective Earth). In our case, the system was PE+N; i.e. PE and N was not connected together. That is OK.

The generator is a 150kVA, 376A, 3-phase generator. While making the voltage measurements, the auxiliary power to battery charger and the engine coolant heater was disconnected completely. The voltage measurements I gave in my first mail are all done when the generator has no connection with any load or any auxiliary power source.

I get the feeling from the discussions here that there is something wrong with the winding of phase3 and the chasis of the generator. I think the best way to find out this to make a Meger test.

 

[waross]

Yes. Now is the time to megger.
For the first test, megger everything to ground. If the reading is not acceptable, then open the common neutral connection and megger each winding seperately. Don't forget any voltmeters in the generator panel and the AVR.

I am used to seeing system neutrals connected to ground. On a stand-alone set, the connection is in the generator connection box or at the generator circuit breaker. With a transfer switch the connection between neutral and ground is often made at the transfer switch. Sometimes electricians take a shortcut and use the ground for the neutral instead of running another wire.

 

[bozkirkurdu]

For me, a ground is a ground and a neutral is a neutral. Even if I come accross with a system with grounding and system neutral connected together, a seperate grounding cable should be connected to the ground connection of the unit to be energized.

Anyway, after meger measurement (waiting for weather conditions to be better to stand outside) I will let you know the result.

Thank you for all the comments.

 

[waross]

I agree with you completely. My point is/was that in some parts of the world, there are no codes and no standards.
The most worrying reading is the 20 volts from phase three to neutral. A reading of zero volts would be more encouraging.
It is sounding more and more as if you have a winding going to ground internally. I want to be sure that the problem is not external to the generator. External problems are often relatively easy and cheap to fix. Internal generator winding problems are usually more expensive and time consuming to fix.

In this part of the world there aren't many ground wires and the ground wires that are installed are used as neutrals. I know the difference between a ground and a neutral but how do you explain the difference to a local expert who has just proved that there is no difference. He used the ground for a neutral and it worked, ergo there is no difference.
A couple of years ago, I came across a 1500 KW generator that had the ground cable connected to the radiator frame. The radiator frame is mounted on vibration insulators which are also electrical insulators. This was a new installation supervised by a local professional engineer.
I have no idea what local standards apply in your area and no idea what connections may have been made with out your knowledge. That's the reason for the silly questions. Sorry.

 

[bozkirkurdu]

Waross, I did not mean to discourage you. Your questions were not silly, if I wrote something that made you feel like this, I am sorry. Maybe this is because english isn't my first language.

I know about this grounding and neutral problems and I sometimes have difficulties to explain the difference and the importance of both to my own technicians. They sometimes have tendency to choose shortcuts. I just didn't want the discussion to go into a wrong direction. Everything written here is always helping. I appreciate that.

By the way, I still could not make the meger measurements. Hopefully, today.

 

[waross]

bozkirkurdu
No problem. Just getting to know you and finding that your first replies are quite accurate. There are many things that would be obvious if I was on site, and there are things that I may assume that are in error. That part of the reason for the questions. I didn't realise that english was not your first language.
Post in when the weather gets better.
yours