Practical Engineering

Neutral of a three phase supply..... that's a new one for me. Three phase power contains 3 hots which also serve the purpose of being a neutralesque wire for the other wires.

Thanx brother. I don't know what kind of systems you have but here we have three phase, four wire systems. Due to unbalanced load conditions, we have a neutral return so that any unbalance current would flow back through this neutral and believe here we have a huge neutral currents.

The obvious way to prevent the problem is to have a perfectly balanced load. Assuming you can not achieve that perfection, you must maintain the neutral.

If the three phase supply and loads are wired as a "Wye - 4 wire" system and you open the neutral then the load is seen as a "Delta - 3 wire" system with unequal load in the three legs. The secondaries that had been more heavily loaded now have some vector current from the other two phases (because your virtual neutral still exists). You can see this if you do some vector analysis with effective load conversions of unbalanced legs of a delta-wye system.

To say it another way, opening the neutral feeder does not remove the connections that the various loads share. You won't get phase to phase voltage because of the shared "virtual neutral" line connection. But the phase to phase voltage will attempt send currents proportional to the load it sees in the effective combination. Draw it out before and after the broken neutral using unbalanced legs in a "Wye" and then a load converted to "Delta". Does this make sense to you?

I use 240 volt heaters connected in a wye connection and if you loose one heater the heat output (amperage) goes down a third on the other two heaters. If I use a wye connection on the heaters with the neutral connected to the wye and loose a heater, the other two continue to draw full amperage (I know the heaters will now see 277 volts).

Thanx brother Joe. It would surely take me sometime to understand your point. But my point here is, as I expound it more clearly that we have a 3 phase, 4 wire system. Everything is connected. No changes can be brought about in the system. Load is absolutely unbalanced. This is the most common system here in India. Now the neutral breaks at the source end i.e. at the distribution-transformer end. Now all of a sudden, all the loads experience a very high voltage. If it was 230 V earlier, now it is 315 V. Why does this happen? What are the various protection schemes that can be applied considering the fact that the system cannot be altered.

What are the various protection schemes that can be applied considering the fact that the system cannot be altered.

You can fuse each leg with the maximum amperage allowed.

Now the neutral breaks at the source end i.e. at the distribution-transformer end. Now all of a sudden, all the loads experience a very high voltage. If it was 230 V earlier, now it is 315 V.

When you lose the neutral, you read phase to phase instead of phase to neutral.

I see now what you have. You could possibly put an overvoltage trip on the circuit. If you want to keep your power running you would probably have to get some fairly expensive equipment to drop the voltage back down, and you still probably wouldn't get the current flow you need if the system is unbalanced.

Dear Emran,

Hope you understand a delta / star (Δ / γ ) transformer where the primary is delta wound and secondary star wound with a neutral point. In India HT consumers normally get 3 phase 3 wire power at High Voltage .( 11/33/ 66/110 kV), supplied from delta / delta (Δ / Δ )transformers with out neutral. Normal LT distribution by Electrical distribution company (in most of the cases state electricity boards) supply power from 11/ 0.44 kV Delta /Star ( Δ / γ )transformer for the Low tension consumers. This is called a 440 volts 3 phase 4 wire system where the voltage between the phase is 440 volts and phase to neutral is 230 volts. Since there are large number of 230 volts single phase connection provided, it is not practicable to balance the loading on all three phases equally.Even though they try to provide balance loading it cannot remain balanced as all the load will not remain on, hence this results in neutral voltage and current. In a proper distribution the neutral is earthed at the transformer and the neutral potential remains at earth potential so long as neutral wire and cable are perfect. When the line neutral get dis connected from transformer neutral, the line neutral remain floating.The floating neutral will try to have a new null point causing neutral shifting. This will cause to vary the voltages ( increase or decrease)in different phases varying with the loads on each phase. The load will fluctuate and voltage also will fluctuate accordingly and will results in damage to equipments.

Thanx everybody for your good comments. Your comments really helped me to analyse what actually happens. What your comments did, it activated my mind to probe into the matter. I did simple analysis by constructing a 3 phase 4 wire system with simple resistive loads connected between the three load lines. Then I broke the neutral by marking a "X" at the source end. Then I looked for the current path for each supply line through the loads. It helped me a lot in giving me a rough idea about what actually happens. O.K What about the products in the Market that can be employed like the Overvoltage protection device? Can a Servo Voltage stabilizer help?

Thanx Abraham, your comment really made sense to me. Since I experienced a voltage rise, so there can be voltage drop as well. It will take a bit of more time for me to analyse your point better. Fine. You are an Indian, and you are the most qualified person to give me the protection scheme for such a problem. And I don't mean that you say we need an overvoltage relay. But what is available in the market.

Hi Emran,

I shall be glad to recommend protection system available in the Indian market. But you have to specify your exact load / equipment that you want to protect. The protection equipment has to be cost effective depending on the cost of equipment to be protected.If it is only simple domestic system an ELCB with MCB protection is sufficient. For a computer or costly electronic items CVT can protect. So give details.

Thanx for reply Sir,

Actually it is a three phase, 4 wire, supply going to a domestic load of not more than 4 KVA to the house of one bureaucrat. Then the load is distributed on all the three phases with neutral as common.

Emran,

You might try a network search using "AC voltage regulation" in your search. The type of equipment you need is very common. I did this and the first site I looked at http://www.elect-spec.com/klr_y.htm has such equipment. Very reasonable prices. Good luck with your quest.

Thanx for your comments. It really helped me a lot to understand the point although not 100%. I need to do a bit of more homework.

Now I want to ask rather a bit embarrassing question. It is embarrassing because I feel that I ought to know about these things.

What is the difference between MCB, MCCB, and RCCB. What are the operating principles of the three?