Unbalanced Loads

Unbalanced loads affect the power factor. Power factor is a system measurement which has effects on the service back to the supplier. There are a number of things to consider.

I recommend the IEEE 3000 Standards Collection and also the IEEE Color Book series. The 3000's are replacing the color books slowly but surely. You can read about it at the links I provided.

If the books are too difficult to understand, I would recommend a solid education in the electrical field.

There is definitely math involved.

This is very clear from question that connections are in Delta-Star.

So Secondary is in Star and Primary is in Delta . So even if there is unbalanced load in secondary i.e. star even then there will be balanced load in Delta as in Delta there is no neutral so unbalanced load will keep floating inside delta which will ultimately get balanced.

But in Star there is neutral so unbalance current get floating in Neutral.

An unbalanced secondary will result in a reduced power factor on the supply side, i.e., unbalance.

Active, reactive, and apparent power calculations show this.

The currents in the primary winding are proportional with the currents in secondary winding. If the sum of secondary currents is not zero then also sum of the three primary winding will be different [not zero].A zero sequence current will circulate in close circuit through primary winding. However, since the line current is the difference-vectorial-between adjacent windings the sum of line currents will be zero.

The negative sequence will influence the primary line currents, I guess.

..at last a voice of reason.

Yes, sorry, I got sucked into that misconception again.

I overstated the relationship, considering the effects on both sides the transformer.

Power factor is not related to efficiency nearly as much as a balanced load.

You are right RAMconsultant, it is a very simple issue. However, there are some others which complicate the subject. See [for instance]:

"Propagation of Unbalance in Electric Power Systems"

http://ieeexplore.ieee.org/xpl/login.jsp?tp=&arnumber=4424221&url=http%3A%2F%2Fieeexplore.ieee.org%2Fxpls%2Fabs_all.jsp%3Farnumber%3D4424221

This, it seems to me, is their opinion about unbalance propagation:

"A. General aspects

In the issue of unbalanced voltage and currents propagation

through the distribution system, the main element is the

transformer. Transformer connection influences in different

ways this phenomenon: there are transformers through which

unbalance propagate unchanged and contrary, transformers that

reduce the unbalance factors.

The solving of the unbalanced operating state of a threephase

transformer means the finding of the following

quantities:

- primary and secondary phase-to-phase voltages;

- primary and secondary line currents;

- current through the neutral conductor (if it is any).

To establish the solution of this problem, the following data

are required:

- transformers rated parameters;

- primary phase-to-phase voltages (considered symmetric);

- consumers parameters that produce the unbalanced

loading.

Because finding the solution of this issue in the general case

is complicated, the analysis will be made considering that the

supply voltage system is perfectly symmetric; the transformer's

magnetization current is neglected and the transformer is a

linear load, in consequence the superposition of effects is

admitted."

...and your point is???

I think it depends on how accurate result it is required. Neglecting voltage unbalance it could be a considerable error-in my opinion. However the calculation presented-even simplified as it was said-it is sophisticated enough involving matrix and so on.

HI believe his point was this: "...Transformer connection influences in different ways this phenomenon: there are transformers through which unbalance propagate unchanged and contrary, transformers that reduce the unbalance factors...", note the word "reduce" not "eliminate".

Unfortunately at the end of this section the authors destroy his argument by saying "...Because finding the solution of this issue in the general case is complicated, the analysis will be made considering that the supply voltage system is perfectly symmetric..." Well, duh, how do you expect to see any influence the load side might have if you don't allow the supply side to change?

Here's the original paper, it contains a nice introduction to symmetrical components, and provides a mathematical "proof" of something that is well known, how a zig-zag secondary winding "traps" zero sequence currents, but does not eliminate them. What's missing from the paper is any correlation with actual data, an experiment that can be easily performed in any college AC Machines lab.

Unfortunately OP's question was about delta-wye (and made no mention of a ground), but as an educational exercise you can use the formulas to calculate the following two cases: phase A open on the high side and then phase A open on the low side, and present your results, it is unlikely that you will find that an unbalance on one side has no unbalance on the other. The authors use the term "unbalance factor" for a reason.

I don't bother answering anonomous posts. Oh darn, I just did!

If your fundamental still not clear whether secondary unbalance load will results into primary or not then visit link below and see connections and will clear your doubts:-

http://electricalsystembasics.com/transformer-connection-types-applications/