# How does a current transformer system know the direction of the power?

I know that it is important to connect a current transformer the correct way so it does not appear that the power goes from source to load. But how does this work exactly? Since it is AC system it measures the current is going back and forth (alternating current) so how can it be any different if the CT is connected one way or another?

If it is connected the other way around (connected wrong) the secondary side of the CT would depict the sinusodial wave as negative when the measured sinus, in reality, is positive and depict the sinus wave as positive when the measured sinus, in reality, is negative. But since an AC signal toggles from negative to positive all the time. Then how does this give any information about the power direction in the wires?

Does it use the CT measurment together with some voltage measurement or something? but since you can have different kinds of loads like resistive, capacitive or inductive then it does not make sense since the current can lead or drag etc.

Can anyone bright help me understand?

P.S. By "connected wrong" I mean the two leads on the primary side being opposite to each other (pin 1 and 2 on primary switched) and the two leads on the secondary side being opposite to each other (pin 1 and 2 on secondary switched).

• Power flow direction is dependent on both current and voltage. Current isn't the only thing alternating. You can connect a current transformer backwards but this would be pointless since you want the side with only one turn to be where the current being measured is. This is so the impedance of the transformer interferes with the current being measured as little as possible. Sep 25, 2022 at 22:04
• I added info about how i mean by connected wrong in question. I think I was unclear there.
– aaq
Sep 25, 2022 at 22:29
• Those edits just have more to do with the flux produced by the current in both coils cancelling each other out. It's kind of like Newton's equal and opposite reaction. One coil wound in a particular direction, with current in a particular direction will produce a magnetic flux in a particular direction. This flux travels through the core through the center of a second coil. This locks in the flux direction through the second coil, and the second coil can only be in one wound in one direction or the other. Sep 25, 2022 at 22:48
• That leaves only the direction of current in the second coil to play with and it will be such that the flux generated cancel out the flux of the first coil. So if you want to know how the transformer "knows", then it doesn't any more than the box you're pushing knows it is being pushed by your hand rather than the other way around. Sep 25, 2022 at 22:48
• It's the product of voltage and current that gives you power and its polarity. However, there are so many binary decisions (CT primary sense, secondary sense, voltage polarity, current polarity, meaning of positive or negative flow) standing between connecting it up and getting it right first time, that it's important to check that your meter reads import or export power correctly before agreeing to pay any bills for power based on it. Sep 26, 2022 at 10:15

For current measurement the polarity of the connection doesn't matter. The CT will drive an AC current meter and it won't care.

For power measurement the polarity does matter and must be set up to match the voltage measurement. The power is calculated from $$P = \frac 1 T \int_0^T VI dt$$ and that number can have a positive, negative or zero result for non-zero values of V and I depending on their relative phases.

By "connected wrong" I mean the two leads on the primary side being opposite to each other (pin 1 and 2 on primary switched) and the two leads on the secondary side being opposite to each other (pin 1 and 2 on secondary.

Simultaneously reversing the polarity on both sides of a transformer will have no effect as the relative phase remains unchanged.

• Sorry, not to great with math.. but then I understand it as the problem has to do with voltage being a positive number and current (because of incorrect installed CT) has a neagative number (correct?). But with capacitive or inductive it is not aligned anyway. Does it have to do with it being more than 90 degrees off? (I think this is max capacitive or inductive can be "off" sync with voltage)
– aaq
Sep 25, 2022 at 22:28
• "Simultaneously reversing the polarity on both sides of a transformer will have no effect as the relative phase remains unchanged." are you sure? if the voltage measurement is correct and the CT is reversed the power should be incorrect..or?
– aaq
Sep 25, 2022 at 22:35
• I'm sure. -1 x -1 = 1. See if electronics.stackexchange.com/a/126402/73158 is of any help. Sep 25, 2022 at 22:38
• @aaq If the current is going into the system when the voltage is high, and out when the voltage is low, power is going in. If it's vice versa, power is going out. If you swap both the current and voltage you'll get the same direction of power. Sep 26, 2022 at 2:22

The output from a current transformer contains no information of the direction of the power flow. The direction of the power flow can be decided if one has at the same time available the polarity of the momentary voltage between the wires ( 2 wires assumed) and the directions of the currents in the wires at the same moment of the time. The currents in the 2 wires are assumed to be as many amperes, but to have opposite directions as Kirchoff's current law says

The rule The power flows at a certain moment just to the direction of the current in that wire which has at the same moment of the time the plus pole of the voltage between the wires:

If the load is resistive the momentary AC current direction changes just at the same moment as the voltage direction, so the power flow direction stays the same.

Reactive loads take current so that there's a phase shift between the current and voltage, so the power flows a part of the AC cycle towards the load and a part back to the voltage source.

A practical wattmeter measures at the same time the voltage between the wires and the current. It averages the momentary power, but it can show negative value if one swaps either the voltage measurement wires or current measurement wires. That's because the momentary power is the momentary voltage and current multiplied. Change one's polarity, the polarity of the power changes.

Current transformers are used if the current is too high to be measured directly or if one doesn't want to break one of the wires to insert the current measurement device. AC current clamp is actually an easy to insert current transformer plus a meter.

Here's a drawing how the momentary primary and secondary current directions are related (It's based on the Lentz Law of induction):

It's drawn as very loose to show the winding direction. A practical current transformer should have markings for the directions because the winding direction is not visible. Unfortunately the markings vary and they can be unclear, so one must measure the direction if he wants the polarity be surely valid for power meters. It can be done for ex. with a battery and voltmeter. See it here: https://www.quora.com/What-is-the-checking-of-CT-polarity