# What determines the phase difference between two voltages at an AC bus?

At an AC bus, the power flow is given by the following equation :

In normal operation, the voltages (V1, V2), as well as the inductive resistance (x), are constants. So the parameter δ defines the power flow.

I would like to ask if the value of δ is constant. If not, what are the parameters that determine its value?

• I think you should be a bit more precise and descriptive in your question. I believe you are asking for power flowing at some other element (probably a transmission line?), as you have two different voltages (so two different buses? do you know what a bus is?). X is denoted as reactance, not inductive resistance. The formula you gave only holds if the line has no resistance, it's an approximation. Also, delta is not a parameter, but a state of your system, just like V1 and V2. Even though you can assume V1 and V2 to be constants, this may not hold in "normal operation" – xuva Mar 20 '19 at 14:48

$$\\delta\$$ is called the power angle. It is the difference in phases between $$\ V_1 \$$ and $$\ V_2 \$$. You are correct that depending on its value, positive or negative, it determines the direction of power flow.
$$\delta = \phi_2 - \phi_1$$
Since $$\ \phi_2 \$$ and $$\ \phi_1 \$$ can vary, $$\ \delta \$$ is therefore variable.