Presence of Circulating Currents in Delta Winding

[nvchinai]

I have read some where that whenever there is a mixture of 3 phase and 1 phase loads on the secondary of a power transformer, a DYn transformer is preferred, as it allows for the flow of circulating current within the Delta winding.

Though I am not sure, but a logical outcome of this is that no matter what the degree of unbalance on the Secondary grounded Star side, the primary Delta side line currents will always be balanced, because the circulating current adjusts the degree of unbalance by flowing within the Delta winding and thus keeps the balance of the primary line currents unaffected.


Is what I am saying above correct ?

 

[Phil Corso]

nvchinai:

Consider a single phase-to-neutral secondary load of 1.0 per-unit magnitude. Then, two of the primary delta-windings will each carry 0.577 per-unit, but the third will carry zero.

For the worst-case scenario, consider a phase-phase secondary load of 1.0 per-unit magnitude, then as above, two of the primary delta-windings will carry equal current, and the third will be zero. Only this time, the primary current magnitudes will be 1.0 per-unit!

Regards, Phil Corso

 

[mike_kilroy]

there is a lot of info out there to google and read about circulating currents, their causes and effects. example:

http://books.google.com/books?id=_r2O7D-rzBwC&pg=SA9-PA157&lpg=SA9-PA157&dq=what+causes+transformer+circulating+current&source=bl&ots=0EwudGh9Wy&sig=19F1XR7YxlN8rZK6PUSQOr3NjWM&hl=en&sa=X&ei=MXnLT578Jsae2gWG9dzZCw&sqi=2&ved=0CFgQ6AEwBg#v=onepage&q=what%20causes%20transformer%20circulating%20current&f=false

section 15.4.1.3 shows that yes, uneven loads on secondary Y will cause primary circulating currents since all 3 windings are connected, but it will not cause all 3 input currents to be equal since the total Z of the other 'parallel' windings will be higher than the leg with the higher sec load; this causes circuilatin currents but not even distribution of input current.

For example, when we design a transformer with multiple secondaries, including 1 phase ones, we still try to make each phase as equal as possible. we would not want to build a xfmr with 2kva 1ph, 2kva 1ph, and 20 kva 1ph loads on each output leg. now this xfmr also had a 200kva 3ph sec, this mismatch would not be too big to live with.

 

[nvchinai]

Thanks Mike. The link you have sent is not showing me the relevant section to which you are referring me.I am not even sure which book this section refers to. Is there some way you can end this info as an attachment ?

 

[mike_kilroy]

dont think I can copy due to copyright laws but pix of webpage prob ok.

 

[nvchinai]

Thanks Mike. I need a more detailed analysis to either reject my hypothesis or have it accepted by others.Unfortunately I can't get any standard book that has done this. A book titled ELECTRICAL MACHINES (2nd Edn.) by I J Nagrath and D P Kothari, TATA Mc GrawHill, New Delhi, 1985 discusses the condition under which a Zero Sequence Circulating Current can exist in a Delta winding.i.e when there is a mixture of both 3 phase and 1 phase loads simultaneously on the grounded star side of DYn transformer.

 

[steve066]

As post #2 pointed out, an unbalance on the secondary will result in an unbalance in the primary.

The advantage of a delta -wye is that harmonics cancel in the primary, but the fundamental frequency does not cancel in the primary.

 

[Phil Corso]

Mike_Kilroy
The referenced text illustrates that slight construction differences will produce a small circulating-current. In fact zero-sequence current-flow, is always present, but its magnitude is of course dependent on the capacitance-to-ground of the secondary system, especially if low voltage! Neither is of major concern.

If you subscribe to the theory that a transformer can be modeled with an induced EMF in each winding, and they are equal in magnitude, and displacement, then their total vectoral sum is zero!
In conclusion, unbalanced secondary-loading will not produce a significant circulating-current in the delta-windings. It will, however, affect the line voltage-drops depending on their parameters!
Regards, Phil Corso

 

[nvchinai]

My hypothesis is based on the following logic:

Since the secondary load is unbalanced (mixture of 3 phase balanced load + 1-phase load), the system is unbalanced and may be resolved into positive , Negative and Zero Sequence networks, suitably interconnected.
Now, the Positive and Negative sequence currents are a balanced set of currents and when reflected on the primary Delta side, result in balanced line currents.However the Zero sequence currents when reflected on the primary Delta side cannot flow in the lines and is confined as circulating current within Delta.
Therefore the line currents on the Delta now being only the being sum of Positive and Negative sequence currents are balanced !! It was on the basis of this logic that I stated the following:

"no matter what the degree of unbalance on the Secondary grounded Star side, the primary Delta side line currents will always be balanced, because the circulating current adjusts the degree of unbalance by flowing within the Delta winding thereby ensuring that the balance of the primary line currents is unaffected "

 

[steve66]

My hypothesis is based on the following logic:

Since the secondary load is unbalanced (mixture of 3 phase balanced load + 1-phase load), the system is unbalanced and may be resolved into positive , Negative and Zero Sequence networks, suitably interconnected.
Now, the Positive and Negative sequence currents are a balanced set of currents and when reflected on the primary Delta side, result in balanced line currents.However the Zero sequence currents when reflected on the primary Delta side cannot flow in the lines and is confined as circulating current within Delta.
Therefore the line currents on the Delta now being only the being sum of Positive and Negative sequence currents are balanced !! It was on the basis of this logic that I stated the following:

"no matter what the degree of unbalance on the Secondary grounded Star side, the primary Delta side line currents will always be balanced, because the circulating current adjusts the degree of unbalance by flowing within the Delta winding thereby ensuring that the balance of the primary line currents is unaffected "

Are you saying that the currents into the 3 lines supplying the delta will always be equal? That's not correct. It is easy to find an example where that isn't true. For example, a load between two phases on the output, and no connection to the other phase.

 

[nvchinai]

Steve, what I am saying is that when there is a mixture of 3 phase loads and 1 phase loads simultaneously on the secondary STAR side which is grounded, then the primary side line currents of the Delta are balanced.

 

[mike_kilroy]

I believe a simple test would disprove this quickly. circulating currents will try to even current loads out but no way can a Z of 2 coils in series cause equal current to current in 1 coil alone. Sure it will try, and it will help, but it cant make the current equal in all 3 inputs.

 

[Smart $]

I believe a simple test would disprove this quickly. circulating currents will try to even current loads out but no way can a Z of 2 coils in series cause equal current to current in 1 coil alone. Sure it will try, and it will help, but it cant make the current equal in all 3 inputs.

I've not studied this issue and have some doubts in principles here...

I feel you are comparing the delta primary windings' currents as if they were secondary windings. On delta secondaries, a single phase, line-to-line load current is shared among the three secondary windings at a ratio of 1z for the direct winding and 2z for the two indirect windings. Correct?

Now, when the delta is the primary and the secondary wye-configured, I feel the preceeding is not true of the delta primary. Consider a single phase, line-to-neutral load current. It is only across one secondary winding, say A to N. The correlating primary winding is connected A-C. If the primary current was shared with windings connected B-A and C-B, would that not also induce a current on secondary windings B-N and C-N? ...and just where is that secondary current going to go if there is no connected load? ...or does current flow on all three primary windings regardless of secondary current? Also is there a difference in this regard between a bank of three single-phase transformers and one three-phase transformer.

I see circulating currents in delta windings as follows. Winding currents are out-of-phase. At each winding connection (KCL node) you have the line conductor and the ends of two windings. There will be instances where the current of one winding is incoming to the node (say positive) while the current of the other winding is outgoing (say negative). We also note that the line current at those instances is equal to the sum of the windings' currents. But since at those instances one winding current is positive and the other negative with respect to the node, the sum (line current) will be less than sum of their absolute values. What I'm saying is, for example, at one instance you have positive current of 10A from one winding and a negative current of 5A "from" the other (i.e. -5A with respect to the node). The line current would be 5A. So it's pretty obvious that 5A of 10A incoming current is the same current as the outgoing 5A. Now if by chance at that same instance there is 5A (or more) across the third winding in the same direction, you'd have 5A of circulating current (at that instance).

I know for a fact that delta primary line currents are not always equal.