Reduction in Neutral Current Due to Unbalancing of Loads

Neutral current is simply a function of differing single phase loads attached to a 3-phase supply. To eliminate the neutral current, arrange for the phase currents to be equal.

<...destroying earthing path...> This phrase is meaningless.

Quote" single phase transformers are installed on various 11 kv feeders emanating from 33/11 KV sub-station. the s/s is having transformer as delta-star"

Do not follow your posting. Are the transformers single phase or 3 phase delta?

the 33/11 KV transformer is three ph. (delta-star) and the 11 KV/LT transformers are single ph.( for domestic 1-ph supply) and three ph.(delta-star)( for 3-ph supply to pumps/industrial motors etc.), the problem of unbalancing is faced during single phase supply hours. for a limited hours 3-ph supply available to the feeders/lines and for rest period only one phase made on (rest two phases cut from 33 KV s/s end(sending end)).

You cannot eliminate neutral current when operating on single phase. In single phase operation the neutral current will be equal to the phase current.

This sounds like a rough and ready set up. Deviation from normal practice rarely results in satisfactory performance.

What is being rested? What is the expected advantage of this arrangement?

For 11kV/LT single phase transformer is the primary connected between two phases,in which case if utility cut 2phases there will be no power or between phase/earth or phase/neutral and is the neutral properly earthed at the pole?.

The unbalancing is reduced by balancing. You may never get it fully balanced but improvement should be possible.

I don't follow what you mean about earth path destruction.

The neutral of 33/11 KV transformer (delta-star) is grounded to earth.contineous flow of unbalance/neutral current (which is substantial) to earth ultimately destroys the earthing.

It would appear that BIHARILALGUPTA is talking out is is a single wire earth return system which in my part of the world (Western Australia) would be called a Rural Supply. It is commonly found in broardacre farming areas due to minimal capital structure required for large areas of coverage.

As others have said, if you are only outputting one phase it all has to come back through the earth/ neutral at the one transformer. To reduce this the two additional phases need to be used in a balanced fashion. The earth return neutral is then cancelled, to a balance in the mass of earth, away from the 11kV transformer.

If OP is running a SWER system then he hasn't said so.

It's all guess work until the OP gets it straight and can communicate it. Whatever "it" may be....

BIHARILALGUPTA, can you post a sketch of your arrangement please?

You are correct it hasn't been clarified, but it is either SWER or MEN (Multiple earth neutral) with extremely bad neutrals. Either way it still won't balance without the other phases.

With this kind of rural distribution - 11kV 3 phase long two wire feeds can string off from the core 3 phase feeder. In these cases the local retriculation transfromer looks like it has single phase primary and secondary windings. Apart from having only one coil (each side) there is not much difference to those with three phases. the primary is connected across two phases and secondary supplies a standard sine wave as one connection is the primary and the other is connected to the neutral and earth. It is not possible for a transfromer with a single primary winding to produce a three phase output as the flux within the iron core is all in the one phase.

All output current flows alone active and neutral (two wire retriculation) or active and earth (in a one wire -SWER system, no nuetral). This current is reflected into the primary circuit which looks like a single phase but really a segment of a 3 phase network. The sum of all these segments (phase currents) should balance themselves out at the 3 phase source. these 3 phase feeds are normally delta - without a neutral - therefore imbalances in the load appear as differences in the phase currents, and perhaps voltage drop across phase to phase when extreame.

??????

<...destroys the earthing...>

This phrase is meaningless too.

With such a dispensation you will never be able to control the unbalance. This is rather extremely rudimentary, inefficient and unsafe system of load distribution and should be avoided. It would be better to install small 3-phase distribution transformers for each village and connect loads to it as per standard practices followed for such load distribution systems.

B B Raina

You are right when you say less efficient but it's economical and safe particularly if you operate MEN. Insulation levels are the same, less conductors to string out less insulators and thinner poles (less physical loads). All this depends on the maximun demand for the load and should onluy be used for local retriculation for a few dwellings not so much a village. distribution for village size loads should be 3 phase especially if they are Kilometers apart. but it will come down to cost v's need.

Unbalance loads due to random use of the system are to be expected. But surely the system should be able to cope. The worst unbalance load must occur when there is no load on two phases. Perhaps due to fault conditions causing single phase breakers to trip. Either way leaving the neutral/earth to carry the full load of the remaining single phase.

Destroying the earth path is one thing, but not the transformer as well. To some extent the loss of the earth path will protect the transformer because it reduces the neutral current. Something must be wrong with the over-current protection system to let a power transformer fail.

An unbalance load is a consequence of your system design and there is not much you can do about it, other than to devise a means to protect the system when the neutral current gets too high.

Biharilalgupta,

As stated in other posts, in any system where you don't have control over how balanced the load is the neutral should be the same size as the phase conductors. A good size ground rod or grid doesn't hurt either.

Is there lightning protection on your system? Could the damage be caused by lightning strikes? Lightning doesn't always come in huge destructive flashes.

This is not a new system but all over the countries except North Americas; where domestic distribution in 9KV delta to 220Vdelta and each phase is Center-tapped to give 2x 110V from center-tap and 220V phase-to-phase;

While the system you are talking is 11KV Delta / 220V Star [Y] 3+N +E

To every house 1Ph*+N +E 220V

It is the PowerSupply company's responsibillity to distribute evenly not to overload any one phase in a street.

And mind that ideal balancing cannot be thought as domestic requirements cannot be compared to 3Ph balanced loads like big motors or industrial loads.