Floating Neutral

[bdenner]

Scenario:

A reasonably large farm is located at the end of an Isaan village, it's 3 phase power meter is at the end of the distribution line (220v Ph to N and 380v Ph to Ph "on a good day") about 1.2 km from the step down transformer and it's star point (neutral).

 

Feed from the meter has been installed using 4 x 28 sq mm aluminum aerial cables, the first user in the main house about 300 meters down:-

4 x ACs

Very large swimming pool and lawn reticulation whose pumps are timed to only come on late night.

And everything else a well appointed home and workshop comes with.

The whole system is reasonably well balanced.

 

Now the problems begin.

 

The aerial cables continue for about 1.1 km with various users down the line, bore pumps, lighted sheds, and at the end is a large cattle yard with various buildings including employee living accommodation, trying to balance this lot is not going so well. At times a diesel generator has to be used in places.

 

Question: Will earthing the neutral at the cattle yard, to stop it floating all over the place, help smooth things out?

 

BTW, the monthly power bill for this lot is in excess of 30K Baht. The electrical authority have been reasonably helpful and done as much as they can within budgetary constraints.

[Crossy]

No, grounding the neutral won't improve the voltage changes due to imbalance and volt drop. It will help keep the neutral nearer ground potential but this is really a minor safety improvement.

 

Can you draw out the arrangement with cable sizes and lengths, I'm a bit confused about where the feed for the cattle yard is coming from.

 

I suspect that a 3-phase AVR is your likely solution. IMHO is the ThaiVisa AVR king.

[bdenner]

Thank you.

 

The 28 mm feeders going all the way to the cattle yards are direct from the meter box with just a 3 phase isolator on them at the house distribution point. Most all sub feeders to various "safety cut" distribution panels, down the line, use 16 mm sq cable.

 

 

                Home          Users

                   l          

Meter--------l--//-----l----l-----l------------l Yards 28mm sq all the way

 

          300m  Isolator         1.1km

 

Edit, they have tries several single phase voltage conditioners without much success.

Edited August 10, 2014 by bdenner

[IMHO]

What size is the meter, and how much are you paying per unit of electricity including ft and VAT?

 

Doing some basic math on your bill based on 5 baht/unit, 30,000 Baht/mth = 6,000 units = 200 units per day.

Even if averaging out this load over 20 hours, it still means an average load of around 10,000watts - or 20,000watts if only averaging out over 10 hours/day.

 

That's a massive load.

 

I suspect the 28mm2 alu cable is well undersized for the task (which would also explain why an AVR can't fix it) - Crossy?

Edited August 10, 2014 by IMHO

[Crossy]

Hmmm, I think IMHO is right that the cable is well undersized, a 10kW 3-phase load, assuming it's reasonably well balanced (about 18A per phase) is going to need 70mm² Al cable to keep volt drop manageable for a 1.1km run.

 

But before running out and buying cable a few checks would be in order.

 

During a period of maximum load measure the Phase-Neutral voltages at the meter, or as close to it as you can get (whilst there check the meter rating 5-15, 15-45 or 30-100).

 

Also go and buy a clamp-on ammeter, they are not expensive, you need to know the current in each phase and the neutral at the same time as you make the voltage checks. Clamp each cable in turn.

 

Record these readings.

 

Make the same measurements at the cattle yard and the main house.

 

Come back here with some numbers.

 

A couple of other questions:-

 

What are the symptoms of the problem?

Where are these symptoms observed?

When are the symptoms observed?

You say some voltage conditioners were tried, what model and rating, where were they installed?

 

One last point, 28mm² is a non-standard cable size, please check again, could it be 25mm² Al?

 

 

[bdenner]

The electrical authority installed the 3 phase conductors upstream of the meter, 28 mm, to the village transformer (was initially just a typical Isaan single phase reticulation) and sized the meter to the user requirements. The user installed their feeders at the same size. I have not discussed the makeup of their power bill with them.

 

As the cable sizing matches the authorities can't do much about that other than paying them to run the 2.2 kv (guess) feeders down and installing their own stepdown transformer.

 

I carried out some reticulation changes yesterday basically giving the yard accommodation a dedicated phase to take the stress of the 2 freezers and 2 refrigerators down there.

[Crossy]

Have you made any measurements of voltage and current use, without these we are playing a guessing game.

 

What are the actual loads in use?

 

Please verify the cable size and manufacturer.

 

EDIT How far from the transformer is the meter?

 

[bdenner]

Thank you See notes:

Hmmm, I think IMHO is right that the cable is well undersized, a 10kW 3-phase load, assuming it's reasonably well balanced (about 18A per phase) is going to need 70mm² Al cable to keep volt drop manageable for a 1.1km run. I'm thinking that is out of the question, a new authority HT feed and their own transformer would be more practical.

 

But before running out and buying cable a few checks would be in order. On going now for some 6 months

 

During a period of maximum load measure the Phase-Neutral voltages at the meter, or as close to it as you can get (whilst there check the meter rating 5-15, 15-45 or 30-100). I have a 15-45 at my home and this one is at least 2 times larger 30-100? And yes between 11:00 to 14:00 the voltage at the meter drops as low as 195 VAC P to N on one of the phases (Imbalance up stream).

 

Also go and buy a clamp-on ammeter, they are not expensive, you need to know the current in each phase and the neutral at the same time as you make the voltage checks. Clamp each cable in turn. Been ongoing, using a clamp on to help with balancing and check loads at different locations. Biggest problem is the surge when the bore pumps kick in. 3 phase bore pumps are on order, hopefully that will help the situation.

 

Record these readings. On going

 

Make the same measurements at the cattle yard and the main house. On going, have so many numbers my head is spinning.

 

Come back here with some numbers. No point - they are not much good, around midday yesterday after we made some changes the accommodation on it's dedicated phase was at 198 - 203 VAC at the time of measurement the load was only 6 odd amps.

 

A couple of other questions:-

 

What are the symptoms of the problem? Was fridges cutting out and lights flickering

Where are these symptoms observed? The yard

When are the symptoms observed? Welding and pumps as I said the accommodation has been removed from these phases.

You say some voltage conditioners were tried, what model and rating, where were they installed? Chinese junk rated at about 60 amps, used on the bore pumps.

 

One last point, 28mm² is a non-standard cable size, please check again, could it be 25mm² Al? Will have another look using my glasses this time!

 

 

There is a very good Earth installation at the yard (2 x 2 meter rods connected to the distribution panel with 16 mm cable) all GPO's are earthed

 

The neutral was floating up to 8 VAC there so I bonded it to the Earth.

 

[bdenner]

Have you made any measurements of voltage and current use, without these we are playing a guessing game. See previous post

 

What are the actual loads in use? Most everything a well equipped cattle, poultry (including incubators) etc. property and large home has.

 

Please verify the cable size and manufacturer. Will get that on my next visit it is at least 25mm.

 

EDIT How far from the transformer is the meter? As stated in the OP about 1.2 km. Do not know how many additional users are connected to the supply but the farm is at the end.

 

 

 

[Crossy]

Not being a farmer I have no idea what loads are associated with "a well equipped cattle, poultry (including incubators) etc. property" the large home I can handle
 
Running 25kV 3-phase to the yard and giving it its own Tx would certainly fix the issue, but could well blow the budget into next century

 

Welding and large single-phase pumps are nasty loads, we have 3 HP single-phase pump for the garden irrigation, even on our normally good and stable supply the lights flicker and the PC UPS beeps when the pump starts up (SWMBO is watering the lawn right now, incoming supply is 33A).

 

Moving the accommodation to its own phase sounds like a smart move, the 3-phase pumps will also help. Adding an AVR to the accommodation supply will also help.

 

If the voltages at the meter aren't too far out of kilter you could score a significant improvement by doubling (or more) up the cables running from the meter to the yard, just pull 4 new wires along the same poles fixed to the existing insulators, ignore where the branches go to other properties (leave them connected to the existing cable). Al cable is not hideously expensive and it may provide relief at a fraction of the cost of running the 25kV and a Tx.

 

 

 

 

[IMHO]

I think first off, we need a verification of the cable size both before and after the meter. Along with Crossy, I've also never heard of 28mm2 cable - standard sizes are 25mm2, 35mm2, 50mm2 etc.

 

Even for a short 80M run I had to pay for, the PEA still used 50mm2, even though the only meter it services is my 30/100. It's hard to imagine a PEA/MEA using such small cable for a 1.2KM run.

Edited August 11, 2014 by IMHO