Circulating Current
Circulating Current
(OP)
Hello,
I'm having a problem understanding 'circulating current' in power systems. When two substations are conected through a switch, there may be a voltage difference between the sites. When the switch is closed, creating a parallel, it is said that current flows between the sub stations, known as 'circulating current'.
Surley, in an AC system, the current flows no where as it is reversing direction 50 times a second (in the UK!). So what excatly is 'circulating current'?
Thanks in advance.
I'm having a problem understanding 'circulating current' in power systems. When two substations are conected through a switch, there may be a voltage difference between the sites. When the switch is closed, creating a parallel, it is said that current flows between the sub stations, known as 'circulating current'.
Surley, in an AC system, the current flows no where as it is reversing direction 50 times a second (in the UK!). So what excatly is 'circulating current'?
Thanks in advance.






RE: Circulating Current
From dc to microwaves, current flows due to difference in potential. If you parallel two sources that are operating at different voltages, a current will flow between the two such that the voltage drop in the impedance between the two sources equals the voltage difference. Just draw out simple circuit diagram and see for yourself.
David Castor
www.cvoes.com
RE: Circulating Current
I should of said 50 cycles a second, which of course means 100 changes of direction a second...
I understand why a current would flow, and understand that in a DC system, but in AC system, if the electrons are oscillating back and forward past a fixed pint in the cable, then how can current be said to flow?
RE: Circulating Current
Think of it another way:- If you have a voltage difference between two points in a closed circuit with an AC supply, then if there was no current flow no AC circuit would ever work, so all your home appliances would fall over to start with.
desertfox
RE: Circulating Current
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Circulating Current
As far as the electrons, they move in one direction for half a cycle and the movement produces heat or do other work. That work is done and may not be recalled. Then the electrons move in the other direction and do work and/or produce heat.
The electrons just have to move. They don't have to continue moving and they don't have to always move in the same direction.
But, are you really an electrical professional? This really sounds like the kind of question that teachers, instructors and professors often have to cope with.
Bill
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"Why not the best?"
Jimmy Carter
RE: Circulating Current
See this related thread:thread238-125257: Electricity basics
Rafiq Bulsara
http://www.srengineersct.com
RE: Circulating Current
RE: Circulating Current
RE: Circulating Current
I am a graduate Electrical Engineer working for a large UK Distribution network, which is why I'm trying to connect what I learned at university to what I'm told at work.
Below is what I think, please correct me if I'm wrong!
From what is taught in universty, and also indicated in jghrist's post, we treat AC curret as if it is DC and has a direction of flow, but in reality it does not because it is constantly reversing back and forward. I completly understand that if there is a voltage difference, and a path, current will flow. The difference being in AC that this voltage is constantly changin polarity, therefore the current changes too.
So when we talking about circulating current, what is happening here? The current cannot be 'flowing' from one sit to another, as it's AC. So does the voltage difference cause the amplitude of the load current to increase? Or am I on the wrong path?
The terms may have been accepted many years ago, but I'm only starting out and like to ask questions!
Thanks
RE: Circulating Current
http://w
Another, physical circulating current can flow in the delta winding of a transformer, as discussed above.
RE: Circulating Current
If a light bulb is connected to an ac source, something is obviously happening, because the filament heats up and emits light. AC current flows in the circuit due to the voltage of the source.
In ac circuits, the phase angle of the current in relation to the voltage at each end must also be considered. The phase angle is related to real power versus reactive power.
My suggestion is take out your textbooks and review your ac circuit theory.
RE: Circulating Current
What does not agree? You are the only one here having difficulty grasping the concept. The current does flow in AC and is measurable as it would indicated by an ammeter in circuit or a clamp on meter around a conductor.
You need bone up on the understanding of the "current" and "waves", both electric and physical.
Rafiq Bulsara
http://www.srengineersct.com
RE: Circulating Current
Thanks to the posters who posted with constructive posts which were very helpful (ijl,jghrist, marks1080).
What was the point of the comments asking if I'm a power professional and I should 'bone up on my theory'?
If I'm the only person having problems understanding the basics of AC current, then why are some posters saying that there is a direction to the overall flow of current (marks1080)?
My questions is about why people in the industry talk about current 'circulating' around a loop when AC current flows back and forward 100 times a second, it never actually flows constantly in one direction like DC.
I did 'bone' up on my theory, which says that AC flows back and forward with no overall direction, yet people talking about it having direction. Was that really such a stupid question?
I'm not asking how a light bulb works or how AC current provides power to an appliance, the current flowing back and forward does this, with the energy provided by the generation plant.
I've come the conclusion that 'circulating current' is actually reactive power flowing in the system, and not current.
RE: Circulating Current
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Circulating Current
In my opinion :)
The current doesn't flow back and forward, the current flow from a source to a demand or a fault. How do you think directional relays/protections know where the fault is if the current is going random in each direction?
You're talking about circulating current, that's an reactive current that appears when you have for instance 2 transformers feeding on the same busbar. If the transformers have the same characteristics but are on 'very' different taps, you'll get a circulating current flowing from the transformer with higher voltage output to the other, that's because with very few exceptions the reactive power will flow from an end with the higher voltage level to the end with lower voltage level.
That's called tap staggering and sometimes is used to control the voltage by circulating a current between transformers. You could get the same situation if you have 2 transformers with different impedances feeding on the same busbar.
With the reactive power and reactive current it is different, reactive power actually oscillates back and forth between the generators and loads, also there's a permanent exchange between capacitors electric field and inductors magnetic field.
However, it is common practice to think of reactive power as flowing in a certain direction. By convention, when a generator is exchanging reactive power with an inductive load we say that positive reactive power is flowing from the generator to the load.
If these conventions for the flow of reactive power are used we can further say that reactive power will normally flow from the high voltage to the low voltage point.
Hope this helps!
May you grow up to be righteous, may you grow up to be true...
RE: Circulating Current
May you grow up to be righteous, may you grow up to be true...
RE: Circulating Current
There's nothing to it, really. We do not need to complicate things and a good way to avoid complication is to use standard terminology.
To question that, more than hundred years after its introduction is just moot. (And this is not an "Einstein" situation. He was on to something real).
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Circulating Current
Skogsgurra, are you really saying that as an engineer, if I do not understand something, I should not try and understand it? That is ridiculous, I'm not questioning AC theory, I'm trying to understand it! The fact that a lot of the posts on this thread differ in their opinion makes me want to understand this all the more.
RE: Circulating Current
Germany - Uruguay :)
Skogsgurra,
It is NOT reactive power?:)
The resulting circulating current from tap staggering it's not reactive power? Could you please expand on this?
May you grow up to be righteous, may you grow up to be true...
RE: Circulating Current
In what way does reactive power oscillate back and forth between a generator and a reactive load, like a shunt reactor? Reactive power constantly goes from the generator to the load. Reactive power never flows from the shunt reactor to the generator.
The original post mentioned "circulating" current flowing between station when a switch is closed between them. This could well be a flow of real power, not just reactive power. The power flows from one substation to the other, or "circulates" from one to the other.
RE: Circulating Current
David Castor
www.cvoes.com
RE: Circulating Current
I think it would be great if you tried to understand this. But, the way you are trying is, shall we say... trying.
If you read the posting rules, you will find that this site is for practicing engineers and work-related problems. Not for hobbyists and their personal problems.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Circulating Current
It may be wiser to just say;
"Hi Gunnar, can I buy you a cool one in the pub? It's a much better place to waste our time."
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: Circulating Current
Make it large - and cool
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: Circulating Current
The electric current is directed movement of electric charges, not random, so current will always flow from a source to a load or to a fault.
Reactive power is constantly being exchanged between those devices that produce it and those devices that store it in their electric and magnetic fields. Half the time the power is stored in electric or magnetic fields, the other half the power returns to the source. Over time the average reactive power flow is zero. Because of this and X being 10 time higher than R in transmission systems, you could say reactive power doesn't travel well, and try to place devices that produce near devices that store it.
May you grow up to be righteous, may you grow up to be true...
RE: Circulating Current
RE: Circulating Current
My post with the link vanished so here it is again with an additional link which might be useful.:-
http://www.play-hookey.com/ac_theory/
http://www.school-for-champions.com/science/ac.htm
desertfox
RE: Circulating Current
RE: Circulating Current
Forget about the word "Circulating" first. just imagine that you hold one end of a long rope and then move your hand fast up and down, you see a wave going away from you. If the reference line is the center of the wave, you see each single point on the rope is up and down, however, the wave or the energy is moving forward. That is what happened in the AC current.
RE: Circulating Current
BTW, slimer3k, I remember the late George S. Patton once said, and I quote: "If everyone is thinking alike, then somebody isn't thinking.". Learn to think out-of-your-box, these people are thinking people, hence they all speak differently. My two cents.
RE: Circulating Current
Now let get back to the word "circulating" - it is trapped between two transformers, or you can say it is looped inside two transformer windings. It is not directly measurable current, it is separated by math.
Whether or not it is real power or reactive power is determined by the load. Circulating current between paralleled transformers is not pure reactive, because of the very small R value compared to X value of the transformer windings, we can say it is "reactive" or Var flow. However, when the circulating current passing through the winding R, it creates heat, thus, the losses.
Real or reactive power is just a matter of whether or not the current phase angle going ahead of or behind of the voltage that's it, do not try to complicate it with the direction.
Hope it would help you slimer3k!
RE: Circulating Current