Circulating Current 1

[slimer3k]

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.

 

[dpc]

The current may be reversing 50 times a second (100 actually) but it is still flowing.

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

http://www.cvoes.com

 

[slimer3k]

Thanks for the reply,

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?

 

[desertfox]

Hi slimer3k

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

 

[Skogsgurra]

We have accepted the terminology of the trade many years ago. And never had any problems with that. Many of the words used are carried over from DC systems to AC systems. Just accept it.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[waross]

There are transformer connections that may result in a current circulating in a delta winding. In the simplest form the circulating current may add to the load current on one winding and subtract from the load current in the other windings. Easy to say, but the phase angle of the circulating current must be compared to the the phase angle of the load currents and the impedance of the transformer winding must be considered to determine the actual current in each phase winding. It gets more complicated. But on some transformer banks a primary voltage imbalance of about the same percentage as the transformer impedance voltage may result in a circulating current of about full load current with no load applied. Much more primary voltage imbalance and the transformer bank may fail from excess current and heat.
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
--------------------
"Why not the best?"
Jimmy Carter

 

[rbulsara]

Do not confuse change in polarity of current or voltage wave with change in direction of flow of energy. Also actual electrons do not "flow" just gets disturbed. AC power (electromagnetic wave) flow is very much like a wave in a body of water, in part at least.

See this related thread:thread238-125257



Rafiq Bulsara

http://www.srengineersct.com

 

[marks1080]

slimer3K: I understand your confusion, but if you think about it, if what you're saying was true how would current ever 'flow' in an AC system? The AC signal may oscillate, but there is a still a direction to the overall flow.

 

[jghrist]

We assign a direction to ac current, but it is really sort of a mathematical construct instead of physical when you discuss current by itself. As has been indicated, the electrons move one direction and then another, regardless of the ac current "direction". Power flow does have a direction, and we generally assign the direction of real power flow to the current.

 

[slimer3k]

Thanks for all the replies, they are all useful, even if they do not agree!

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

 

[ijl]

The circulating current in question may be a mathematical construction used to calculate the current in two parallel, similar (but not identical) paths, see for example

http://www.cdnova.com/Portals/0/Link 1 - App11- Introduction to Parallel Control.pdf

Another, physical circulating current can flow in the delta winding of a transformer, as discussed above.

 

[dpc]

"Circulating" current in an ac system is no different than any other type of ac 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.

 

[rbulsara]

Slimer3K:

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

 

[slimer3k]

What is going on here? I've asked a question, and most of the poeple posting seem to be getting all jumped up and annoyed that I asked this question!

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.

 

[Skogsgurra]

Well, if that is what you think. Then do not expect to have many followers.

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[m3ntosan]

slimer3k,

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...

 

[m3ntosan]

oops! Failed to see you already concluded it’s a reactive power thing started the post and then went to see the world cup final ...

May you grow up to be righteous, may you grow up to be true...

 

[Skogsgurra]

Sorry. It is NOT reactive power. It is just a current superimposed on other currents and used as a mathematical tool to describe the unbalance between circuits.

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

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[slimer3k]

Thanks m3ntosan, I'll be having a think about your post! By the way, if you were off to watch the world cup final you will have a long wait, it's tomorrow....

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.

 

[m3ntosan]

slimer3k,

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...