AC Theory and Direction of Current Flow - Page 1
Forum section: electronics / projects

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: April 02, 2025, 07:59:36 am
content: I have some questions that I would value answering by someone truly qualified in AC theory.Suppose I have a PV system on my property, and I am connected to the grid.When my PV production exceeds my own usage I have export, which is allowed.Q1: Given that AC current 'flows' back and forth, am I correct that the DIRECTION of current flow in the line from my property to the grid will be distinguished by its PHASE?Q2: Am I correct that(a) The current direction is determined by the instantaneous voltage difference (∆V) between the two sources?(b) A ∆V is essential to drive direction?(c) And that this voltage difference is determined by the (∆V) between instantaneous voltage of PV and grid, where V for each = V.sin(w.t + ø) with w being angular frequency and ø being phase angle?(d) The difference (voltage differential driving the electron movement) thus being one minus the other?(e) The current direction is defined by whether that difference is +ve or -ve?(f) If the phase angle and frequency were the same no current flows as ∆V would be zero.(g) So, another way of putting it is that a small phase angle delta is what drives the voltage differential?Thank you.

author: Someone
author_rank: Super Contributor
timestamp: April 02, 2025, 10:12:49 am
content: Go back to an infinite vacuum with spherical cows, all waveforms as frequency locked sine waves, and superconducting wires:A sink has the current flowing in phase with the voltage + when +, - when -, positive powerA source has the current flowing out of phase with the voltage - when +. + when -, negative powerSomething in-between where there is phase shift isimaginary

author: 6SN7WGTB
author_rank: Regular Contributor
timestamp: April 02, 2025, 11:39:17 am
content: OK, so to answer my own question, there are only two possible phase angles in THIS situation: 0˚ at SINK and 180˚ at SOURCE.Correct?But, to your final point, does the phase angle along the conductor between my PV export at 180˚ and grid at 0˚ shift from 180 to 0? (On the basis of that at my end it MUST be 180˚ and at the infinite grid it MUST be 0˚).

author: jwet
author_rank: Frequent Contributor
timestamp: April 02, 2025, 05:39:06 pm
content: This is a pretty good question really. Its probably more fruitful to think about AC power in terms of moving energy or power that sine wave voltage waveforms. If you put a resistor across an AC source, it will dissipate RMS power just as if the source was DC. It doesn't sink for a half cycle and then source for half cycle. Its all sink.I think the adjustment to have power flow out of your grid tied PV house and onto the grid is for you to be completely in phase with the grid but at a slightly higher voltage. The will make you a relative source and energy will flow out. You maintain your "delta V" to limit your power output in a closed loop way. Fooling with phase would create losses I believe.I thought I was an AC theory expert but these kind of common sense questions can trip you up.Post what you learn.

author: ajb
author_rank: Super Contributor
timestamp: April 02, 2025, 06:32:15 pm
content: Thinking about this in terms of ∆V between the inverter and the grid is difficult, because the impedance across which that voltage appears is complex (in the common sense and in the mathematical one), and therefore it's difficult to relate to power. You have a bunch of wiring inside and outside of the building, with some transformers in the mix, plus all of your household loads that are in parallel with the PV inverter, plus loads in other adjacent buildings all interacting over that impedance network, and the impedances are constantly changing as those loads fluctuate.It makes more sense to simplify the grid to a voltage source with zero impedance, meaning it can sink or source any amount of current with no change in voltage, and then think about the PV<->Grid interaction in terms of current flow. The inverter's goal is to apply as much power to the grid as the PV panels can generate, so once it determines what the available input power is, it can measure the grid voltage and compute an output current setpoint that will produce the desired output power. Making current the controlled value at the power stage comes with other benefits anyway, like built-in current limiting/overload protection. In reality, the grid and other wiring/equipment between it and the PV system do have impedance, so the voltage measured at the inverter will change in response to the inverter's power output, but the inverter is already measuring that and can compensate accordingly to maintain the power setpoint.In reality, even without a PV installation, power flow between grid and building loads isn't necessarily one-way. Reactive loads (particularly motors) have a current waveform that is out of phase with voltage. In the case of an inductive load the voltage crosses zero before the current: between those two zero crossings, power flows back into the grid from the load. The ratio of forward to reverse power flow depends on the phase shift, and is related to power factor.

author: Picuino
author_rank: Super Contributor
timestamp: April 02, 2025, 06:35:59 pm
content: The sign criterion is used for direct current and voltage and also for instantaneous alternating current and voltage.Assuming: a DC current flows from the positive of a battery to the negative of the battery when a resistor is connected (positive current convention).Whenever a current flows in the direction of entering through the positive of an element and leaving through the negative pole, we are delivering power to the element.Whenever a current flows in the direction of entering through the negative pole of an element and exiting through the positive pole, the element is delivering power to the rest of the circuit.In alternating current, when the angle is 0º or 180º, power is always being absorbed or delivered.With +-90º between voltage and current there is only pure reactive power. During half of the cycle power is absorbed and during the other half of the cycle it is returned (the same power that has been absorbed).

author: bdunham7
author_rank: Super Contributor
timestamp: April 02, 2025, 07:01:13 pm
content: Quote from: 6SN7WGTB on April 02, 2025, 07:59:36 amThe term "phase" can mean between two voltages of the same frequency or between the voltage and the current. The sign designations are a bit arbitrary but there are traditions. For the grid connected to your house (solar or not) let's consider the current supplied to around zero phase when you are consuming power and around 180 in phase when you are exporting it. This would be for a resistive load or to be more precise, a power factor near 1.0. If your load is inductive, the current phase may be a bit negative, if it is capacitive then it will be positive, if it reaches + or - 90 degrees you have no net power consumption (discounting parasitic losses) and at any magnitude greater than 90 degrees you are exporting.Quotea) yesb) yes, but these voltage differential may be very small as the grid is a very, very low impedance load...c) yes, but I think you're going astray hered) yes...e) yes, the instantaneous voltagef) no, this is where you're going wrongg) no, the answer here is complicated, but the simplistic, not-quite-correct answer is that what drives it is a voltage differential.On a nice bright sunny day with everybody on the grid only using PFC-corrected appliances so that there are no power factor issues, your solar system should be producing voltage that is nearly perfectly in phase with the grid and just a small bit higher than the grid so as to drive current into it. The voltages are both in phase, the current is at 180 degrees.Now reality isn't so pretty and it depends on your solar system and what levels of control it has--some of them may actually produce current up to 15 degrees out of phase on demand and then the voltages won't be in phase anymore, etc.

author: TimFox
author_rank: Super Contributor
timestamp: April 02, 2025, 07:24:42 pm
content: Looking at the problem of AC through a pair of wires between source (generator) and load (sink) as a measurement, I use the following model:Cut the cable (wire pair or coax) with an imaginary plane perpendicular to the cable.The two observable quantities (as a function of time) visible on an appropriate oscilloscope are the voltage between the wires (one defined as “low” or the reference for voltages, such as the coax shield or the “neutral” wire) and the current through the other (“high”) wire through that plane.There are two physical directions when crossing the plane: choose one (e.g. left-to-right) as the positive direction. Use a current probe around that wire, oriented so that a positive current in that positive direction gives a positive voltage to drive the second input channel of the oscilloscope.Connect the voltage across the cable (high with respect to low) to the first input channel.In general (due to partially reactive loads, etc.) the phase angle between the two sinusoids will differ from 0 or 180 degrees, and the direction of power flow can be visualized from the orientation of the current probe with respect to the wires.This should work for DC and frequencies low enough that the spacing between the two wires can be ignored.

author: IanB
author_rank: Super Contributor
timestamp: April 02, 2025, 08:06:00 pm
content: Quote from: 6SN7WGTB on April 02, 2025, 07:59:36 amEssentially, yes.If we picture a traditional rotating generator that wants to feed power into the grid, we open up the governor to feed more power into the turbine, which makes the generator try to go faster and "lead" the grid. The attempt to lead by the generator opens up a small phase angle, which creates a voltage differential, which causes power to flow from the generator to the grid.There are additional complexities regarding voltage regulation, but the basic principle of power transfer has phase angle at its core.The corollary of this is that when grid demand exceeds supply, the grid frequency will slow down, and when supply exceeds demand the grid frequency will speed up.

author: bdunham7
author_rank: Super Contributor
timestamp: April 02, 2025, 08:51:30 pm
content: Quote from: IanB on April 02, 2025, 08:06:00 pmIt is true that generators on the grid may behave something like what you're suggesting, but consider the following hypothetical:You have an isolated system (microgrid consisting entirely of incandescent lamps and space heaters) with a large engine driving a large generator via a very rigid shaft, to which there is also affixed a smaller generator. Both of the generators are three phase AC with a DC-excited rotor and are physically exactly in phase with each other. Each generator can be connected or disconnected from the microgrid and the DC rotor current can be varied from zero to the maximum all via a control panel. So you start the engine and get the microgrid going with only the larger generator. Later on, you want to switch the smaller generator in and have it produce some power while equally reducing the power of the large generator. Can you do it?