# Voltage and Current relationship in generation

Please could someone explain why the current is 180 Deg out of phase with the voltage when measured at the generation point and then in phase with the voltage when measured at the load point. I am purely talking about a resistive load. We can talk about VAr (Reactive Power, leading, Lagging, inductive, capacitive) later. I would really appreciate if someone who studied this could share a bit of their knowledge. I can provide examples of sine waves recorded that verify my above statement if required. I have the data, just having a hard time trying to get my head around it. thanks in advance - Mark

Thanks for the response. Brian and The Photon are relating to what im saying. By convention current entering a load is positve and leaving a power source is negative thus 180 out of phase. To answer some other questions about my post. The measurements are recorded on a Transient Fault Recorder that has micro second accuracy. The system is a HVDC Interconnector and it has Inverter and Rectifier Modes. In Rectifier mode (drawing power from the grid) the Current is in phase with the Voltage but in Inverter mode (delivering power to the grid) the Current is 180 deg out of Phase with the voltage. I understand that it all has to do with the direction of the current and how its represented but i bet if i was a few hundered yards up the road at the first load point and i was able to look at the wave forms on an oscilloscope the Voltage and Current would be in phase again. Maybe it is just the way the CT's are orientated. I might have just answered my own question

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For a resistive load, current and voltage are in-phase. It's impossible to explain why they might be 180º out of phase other than by suggesting a measurement error. – Andy aka Mar 29 '13 at 11:58
How are you measuring the current and voltage? – Leon Heller Mar 29 '13 at 12:14
I think it's a simple convention error : if current is flowing INTO the load, it is necessarily flowing OUT of the generator... – Brian Drummond Mar 29 '13 at 12:15
@BrianDrummond, I think you have the answer. – The Photon Mar 29 '13 at 16:44

As Brian says in comments, it's a matter of convention.

In network analysis, it's a common convention that the current associated with a device (generator or load in this case) has a positive sign when it flows in to the device. This appears to be the convention your text is using.

Notice that this convention means that when a device is providing power to the circuit (like a voltage source or generator does), it will have a negative current. This seems anti-intuitive, but it's just a consequence of choosing a common sign convention for device currents regardless of whether they are sources or loads.

And, as Brian says, if current is flowing into the load, it is necessarily flowing out of the generator. So with the network theory convention, the signs of the two currents must be opposite, which is the same as a 180 degree phase change.

i bet if i was a few hundered yards up the road at the first load point and i was able to look at the wave forms on an oscilloscope the Voltage and Current would be in phase again.

Because you would be measuring the current in the direction away from the generator and towards the load. If you turned your current probe around and measured the current going towards the generator, it would be 180 degrees different.

In your original measurement, when you measured at the generator, you had the current probe/sensor pointed toward the generator; when you measured at the load, you had the probe/sensor pointed toward the load. So you got results that differed by 180 degrees.

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Thanks, I updated my post slightly – user22555 Mar 30 '13 at 16:16

Your question can't be answered because it is based on incorrect assumptions and therefore makes no sense.

For a resistive load, the voltage and current are always in phase, whether at the generator or at the load. If what you have between the generator and the resistive load has significant inductance, then there can be a phase shift at the generator, but then the load as seen by the generator isn't resistive anymore. However, this can only at most push the phase shift up to but never at 90°. At exactly 90°, no net power is actually being delivered by the generator. Having a resistor somewhere dissipating power, whether at the end of a long inductive transmission line or not, would be violation of conservation of energy, thermodynamics, and a few other felonies of physics.

If you have data that supposedly shows a 180° difference between the generator and at a resistive load, then you got a polarity flipped somewhere. Note that 180° is a sign flip, or negation. Let's say you have two wires coming out of the generator labeled A and B. The voltage of A with respect to B and B with respect to A will be exactly the negative of each other. With a repeating AC voltage, this can be expressed as a phase shift of 180°. Somewhere along the line, you lost track of which wire is A and which is B and flipped a measurement.

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Voltage is a potential difference which has to be measured between two nodes in a circuit.

The the two nodes span across a device or network which is reactive, then there will be a phase shift between the current flow across the device and the voltage. If the device is purely reactive (inductor or capacitor with no series resistance) then the phase shift is 90 degrees.

If the two nodes span a device or network which is purely resistive, then that voltage is in phase with the current flowing across that network.

It is not possible to make voltage and current out of phase, and then send them that way through a wire. The phasing behavior is exhibited by the reactive device.

For instance if we have a two-terminal device which is made by connecting a capacitor in series with a resistor, then that will have a phase shift depending on the R and the C value (somewhere less than 90 degrees). But that resistor taken individually has no phase shift across it, and the capacitor has 90 degrees.

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