Load Deference In 400KVA Distribution Transformer

Do you mean "Difference"?? Are we talking two transformers in parallel and load sharing?? Or does "load" mean phase current?? What are Ir, Is and It?? You have not given all essential information here. If currents given are for phases R, S and T - and they are all less than rated current - there is no problem (except you have not given a rated current!).

Hi 67model.

1-I am sorry,U r right Imean difference in current for phases R,S,Tand thy are all less than rated current.

2-This case for individual transformer.

thanks.

Hi,

you mean 21,..% is normal/not ? for long term above 20% is gonna reduce work rated of transformer, you can distribute the load in order to get the balance

Hi

Do u mean by (get the balance) that there are no diference in current between phases,if so how I get it for the distribution transformer.

Thanks

The primary current of a transformer is the vector sum of no-load current and reflected secondary current.

Please take the trouble to develop equivalent circuit diagram of your transformer, the secondary quantities can be referred to primary side.

So long as the rated current limit of the primary of the transformer is not exceeded the loading although unbalanced is tolerable.

In a glance the unbalance figures provided by you are rather frequent in a distribution transformer.

If your transformer has 415/240V secondary (often used for 50 Hz system), the rated secondary current is 400000/(415*1,732) = 556 amps/phase (greater than any of the currents you list). But if it were a 460/266 V secondary (often used for 60 Hz), the R phase would be overloaded. Unless an individual phase is overloaded, the transformer will not be affected by 100% rated current difference between phases. It is possible that a very big current difference could contribute to an unsatisfactory load voltage on one phase, but this is unlikely with currents you listed. Therefore, the numbers you give seem allowable. It is possible, if this is a public supply, that there are rules for the voltage delivered to the consumer. Also, the supply company may have some load balance objectives (in its planning) to avoid undesirable losses or voltage variation, but these cannot be known without contact with them - they will not be rules which cannot be violated anywhere.

Hi.

This is for 11KV/0.4Kv ( 50 HZ ) and this is for public supply so please tell me if this diference is allowabel.I can re distribute the load to reduce the percent to 9%.

Thanks.

I would be surprised if there is any rule which makes it "not allowable". In the normal way of the world, an electricity supplier has no control over the consumer's loads. These vary with time of day, season of year, factory orders, holidays etc.. But consider that each phase of your on-site supply feed has resistance r in each phase of its supply cables and transformer. For your given loads 550, 400, 350 amps the losses (I squared R) will be 550x 550 x r + 400x 400 x r + 350x 350 x r = 585000 x r. If you change the loads to 500, 400, 400 amps, the losses will fall to 570000 x r, that is 15000/585000 = 0.025 in 1 reduction. If your losses are 1 % of the load of 287 kW at the existing load balance i.e. 287 x 0. 01 = 2.9 kW, then the reduction will be 2.9 x 0.025 = 0.072 kW. Over a year (8760 hours), that is 0.072 x 8760 = 635 kW-hours. At home, this 635 kW-h would cost me about £60 per year. I have left out the consideration that the cables in the most loaded phase will be more than proportionally increased in resistance: also that with a completely balanced load there is no current in neutral to add its cable losses. Particularly if your are metered at the high voltage input of the 400 kVA transfo, you have an cost interest in reducing losses. This is just an illustration of a general truth that balanced loads are more economic (all through a power system) than badly balanced ones.

Dear model.

Thanks for this discussion , actually I have another equation about this operating condition that is the effect of the Neutral current on the transformer winding when its work with this unbalaced load .

best regard.