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.

 

[jghrist]

The current doesn't flow back and forward, the current flow from a source to a demand or a fault.

What is your definition of current? Not the movement or flow of electrical charge? In ac, doesn't the charge move back and forth?

reactive power actually oscillates back and forth between the generators and loads

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.

 

[dpc]

Best of luck to you in your new career. If you plan to be a practicing power engineer and have a philosophical issue with the notion of ac current flow and conventional ac circuit theory, you may find your work frustrating .





David Castor

http://www.cvoes.com

 

[Skogsgurra]

Slimer (and now also m3)

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

http://www.gke.org

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

 

[waross]

I was going to suggest that looking at the instantaneous polarities of both the current and voltage may be helpful.
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

 

[Skogsgurra]

Thanks Bill! They will learn...

Make it large - and cool

Gunnar Englund

http://www.gke.org

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

 

[m3ntosan]

jghrist,

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...

 

[slimer3k]

You've all been very helpful. Thank you.

 

[desertfox]

Hi slimer3k

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

 

[trchambe]

I believe the best way to envision this problem is to break everything down into discrete components. The earth, busbar, etc through which the circulating current flows must be thought of as a discrete component of some impedance in parallel with the intended path. If you'll recall your studies of mesh analysis, there aren't really two types or values of current 'flowing' through a given component. However, this system can be analyzed or thought of as though there are two sources or currents. In reality however, this method of evaluation and way of speaking is simply to mathematically account for the voltage drop in such a scenario. Multiple currents through the same component do not exist outide of a mathematical model.

 

[pwrtran]

Hi slimer3k

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.

 

[burnt2x]

Good analogy, pwrtran!
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.

 

[pwrtran]

slimer3k

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!

 

[stevenal]

AC current is sinusoidal, and increasing the magnitude of the current increases the height of the sine wave. To determine the direction, the wave must be compared against another sine wave, such as voltage. By convention, current direction is the same as power, flowing from higher voltage to lower. Circulating current between substations is primarily reactive due to the reactance of the wires connecting the two substations. The average of any sinusoid is zero, and no unit of charge gets very far away from its origin. This does not mean, however, that current does not flow in AC systems; it does.