AC Current Polarity

[big john]

So in the past whenever I used to think about AC current flow, I always envisioned it like the electrons were ping-pong-balls hanging from a string: The would only move back and forth as the voltage on the conductor went from positive to negative and back again.

But then I got to thinking about CTs: If you don't pay attention to polarity and install a CT backwards, the AC current flows through it in the wrong direction and will mess up metering. If the current only flowed back and forth, it would be impossible for it to move in the "wrong" direction, because it flows in both directions.

So, now I have to change my mental image: Is the current moving back and forth as the voltage changes, but it's actually moving in one direction more than the other? Think about a ping-pong-ball on a string mounted inside in a moving car.

Is that an accurate analogy? If it is accurate, why would that be? And if it's not, how is it possible for an AC current transformer to tell which direction current is flowing?

-John

 

[kwired]

I would say an individual CT does not care which way you pass the conductor through it. When trying to monitor usage with CT watt hour meter I can imagine that proper polarity may be important although I don't know much about how the meter actually works. May depend on the meter itself. If it needs to know both voltage and current and they are not in phase with each other it may not give proper output.

 

[SG-1]

Think about the polarity with time stopped. The polarity is instanteous. When H1 is positive then X1 is positive. When H2 is positive then X2 is positive.
Freeze Time. ( Stop the ping pong ball in time )

If the current ping pong ball is at the top and the voltage ping pong ball is at the top the power is flowing in one direction.

If the current ping pong ball is at the bottom & the voltage ping pong ball is at the top then the power is flowing in the other direction.

There are two ping pong balls to watch.

 

[winnie]

I think that kwired nails it. The electrons are moving back and forth in a balanced fashion; there is no over-all direction to the current flow. The only reason that we care about the orientation of the CT is the _relative_ polarity of the current measurement with respect to the relevant voltage measurement or the other current measurements in the system.

Think about DC power: simply voltage times current. This is the same as the instantaneous power in an AC system. The relative polarity of voltage and current tells you the direction of power flow...if you flip the CT then the meter will run backward.

-Jon

 

[K2500]

The polarity will also come into play if you're using secondary turn ratio modification.

 

[SG-1]

Big John, in order to know which direction the current is flowing the meter or relay must compare the current waveforms with voltage waveforms. Polarity is very important if you are using the CT for metering, protection, or control.

 

[PetrosA]

This is one I have trouble visualizing too. Is it sort of like what happens to the bass signal when you reverse the polarity of one of the speakers? (bass cancels out when one is reverse wired).

 

[richxtlc]

This is one I have trouble visualizing too. Is it sort of like what happens to the bass signal when you reverse the polarity of one of the speakers? (bass cancels out when one is reverse wired).

Sound is the vibration of molocules on air, and the loss of the bass when you reverse the wire is the basic principle of active noise cancellation. By reversing the sound it cancels the sounds that produced it.
With CTs the polarity it marked so, as stated previously, when H1 is positive, X1 is positive, to insure that differential, directional, and power relays function for faults inside their zone of protection.
With respect to metering, real power is always positive, no matter whether the sine wave is positive or negative, (+ x + = +, and - x - = +). The only time power has a negative number is for reactive power when current and voltage are out of phase and therefore you can have + x - = - or - x + = -.

 

[big john]

Okay... So let me see if I understand this:

Say you had an ammeter hooked to a CT. You could reverse the polarity on that CT and the ammeter would always read the same alternating current. CT polarity never applies to current measurement, because there is not such thing as alternating current polarity.

However! Connect that same CT to a watt meter that's also hooked to some PTs, and then the wattmeter is comparing the relative measurements of voltage and current to determine the direction of power flow (not current flow). This is where CT polarity is important, because if the CT is backwards, then the relative measurements will show power flowing in the wrong direction.

Am I close? I was originally just thinking of a CT connected to an ammeter, and couldn't understand why polarity would ever matter to an ammeter.

-John

 

[K8MHZ]

Okay... So let me see if I understand this:

Say you had an ammeter hooked to a CT. You could reverse the polarity on that CT and the ammeter would always read the same alternating current. CT polarity never applies to current measurement, because there is not such thing as alternating current polarity.

However! Connect that same CT to a watt meter that's also hooked to some PTs, and then the wattmeter is comparing the relative measurements of voltage and current to determine the direction of power flow (not current flow). This is where CT polarity is important, because if the CT is backwards, then the relative measurements will show power flowing in the wrong direction.

Am I close? I was originally just thinking of a CT connected to an ammeter, and couldn't understand why polarity would ever matter to an ammeter.

-John

There are directional watt meters. We use the all the time in radio.

 

[Cold Fusion]

Okay... Say you had an ammeter hooked to a CT. You could reverse the polarity on that CT and the ammeter would always read the same alternating current. CT polarity never applies to current measurement, because there is not such thing as alternating current polarity. ...

The ammeter reads the same either polarity, because ammeters are designed to read rms values - which is just the magnitude of the current.

An ammeter can not see any phase angle relationships. It would have to have a connection to the voltage source to do that. Then the ammeter could see the magnitude and the phase angle. This is as you discussed in the third paragraph. Those are pretty expensive ammeters (usually called analyzers)

cf

 

[SG-1]

Okay... So let me see if I understand this:

Say you had an ammeter hooked to a CT. You could reverse the polarity on that CT and the ammeter would always read the same alternating current. CT polarity never applies to current measurement, because there is not such thing as alternating current polarity.

However! Connect that same CT to a watt meter that's also hooked to some PTs, and then the wattmeter is comparing the relative measurements of voltage and current to determine the direction of power flow (not current flow). This is where CT polarity is important, because if the CT is backwards, then the relative measurements will show power flowing in the wrong direction.

Am I close? I was originally just thinking of a CT connected to an ammeter, and couldn't understand why polarity would ever matter to an ammeter.

-John

You have nailed it, John.

 

[rcwilson]

Okay... So let me see if I understand this:

Say you had an ammeter hooked to a CT. You could reverse the polarity on that CT and the ammeter would always read the same alternating current. CT polarity never applies to current measurement, because there is not such thing as alternating current polarity.

.......

Am I close? I was originally just thinking of a CT connected to an ammeter, and couldn't understand why polarity would ever matter to an ammeter.

-John

Close, but not quite. Think of a ground fault relay that works by adding the three phase current signals together on a 1000A load.

Each one is signalling 1000 amps and the ammeter in the circuit shows 1000 Amps for each phase. The relay (or the hard wired connection) adds the three currents together. As we all know, with three balanced phases A + B + C = 0, so the relay doesn't trip.

But turn the B phase CT around so the polarity is reversed. The ammeter still reads 1,000, 1000, 1,000. but the relay is now unknowingly adding A + (-B) + C = 2B = 2000 Amps and keeps tripping on ground fault. Note that if it was a fancy ammeter that calculated neutral current, it would read 2000 A.

Back when I was testing 480V ground fault systems for a living, about 20% of the installations had CT polarity problems that would have caused phony trips.

Reversed current leads can also cause false trips of some current imbalance relays used to protect large motors from overheating.

 

[big john]

...But turn the B phase CT around so the polarity is reversed. The ammeter still reads 1,000, 1000, 1,000. but the relay is now unknowingly adding A + (-B) + C = 2B = 2000 Amps and keeps tripping on ground fault. Note that if it was a fancy ammeter that calculated neutral current, it would read 2000 A....

Hold on, though, because you're on the verge of confusing me again. The ground-fault relays you're talking about, did they have voltage brought to them as well? Was there a reference for the relay to use to determine the direction of power flow on each phase?

If not, then if the current on each phases is alternating in both directions (as AC has a tendency to do), how on earth does the GF relay know which phase is out of "polarity?"

-John

 

[SG-1]

I believe the type of ground fault relay Mr. Wilson is refering to is using 3 CTs. The relay is comparing the waveforms from each CT to the other CTs as reference. The current from each is added to together. If one CT is backwards then that current is subtracted and the relay will not perform as expected.

If more than one CT is used polarity must be observed or the equipment will malfunction.

 

[nicholaaaas]

I would say an individual CT does not care which way you pass the conductor through it. When trying to monitor usage with CT watt hour meter I can imagine that proper polarity may be important although I don't know much about how the meter actually works. May depend on the meter itself. If it needs to know both voltage and current and they are not in phase with each other it may not give proper output.

Depending on the CT is does care. Emon Demon CT's only work if you install them in one direction

 

[K8MHZ]

Depending on the CT is does care. Emon Demon CT's only work if you install them in one direction

Could this have something to do with Lenz's Law and the direction of the windings in the CT?

 

[K8MHZ]

Depending on the CT is does care. Emon Demon CT's only work if you install them in one direction

Here is some good info about polarity of CT's.

http://www.kilowattclassroom.com/Archive/CurrentTransf.pdf

Lenz's Law was a shot in the dark, it seems that CT's have more than just a coil in them.

 

[zog]

OK, hope these work. #1 is a visualization of DC electron flow, #2 represents AC.

While the classic definition of current flow is the flow of electrons the electrons do not really "flow", what really "flows" is the energy transfered, think of a line of billiards balls touching each other, when the first one is hit by the cue ball there is little movement (Besides the one on the end) but the energy is transfered from one to the next.

OK, edit here, my GIF's did not animate so hard to show my point.

 

[Chamuit]

How about the wave at a stadium. The people (electrons) just stand up and down, but the motion (current) of the wave "moves" around the stadium. Too simple?