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In electrical engineering, particularly power engineering, voltage regulation is a measure of change in the voltage magnitude between the sending and receiving end of a component, such as a transmission or distribution line. Voltage regulation describes the ability of a system to provide near constant voltage over a wide range of load conditions. The term may refer to a passive property that results in more or less voltage drop under various load conditions, or to the active intervention with devices for the specific purpose of adjusting voltage. Vnl is voltage at no load and Vfl is voltage at full load.

The percent voltage regulation of an ideal transmission line, as defined by a transmission line with zero resistance and reactance, would equal zero due to Vnl equaling Vfl as a result of there being no voltage drop along the line. This is why a smaller value of Voltage Regulation is usually beneficial, indicating that the line is closer to ideal. The Voltage Regulation formula could be visualized with the following: “Consider power being delivered to a load such that the voltage at the load is the load’s rated voltage VRated, if then the load disappears, the voltage at the point of the load will rise to Vnl. Voltage regulation in transmission lines occurs due to the impedance of the line between its sending and receiving ends.

Transmission lines intrinsically have some amount of resistance, inductance, and capacitance that all change the voltage continuously along the line. Both the magnitude and phase angle of voltage change along a real transmission line. The short line approximation ignores capacitance of the transmission line and models the resistance and reactance of the transmission line as a simple series resistor and inductor. IR in the short line approximation, different from the medium and long line.

The medium length line approximation takes into account the shunt admittance, usually pure capacitance, by distributing half the admittance at the sending and receiving end of the line. This configuration is often referred to as a nominal – π. The long line approximation takes these lumped impedance and admittance values and distributes them uniformly along the length of the line. The long line approximation therefore requires the solving of differential equations and results in the highest degree of accuracy.

In the voltage regulation formula, Vno load is the voltage measured at the receiving end terminals when the receiving end is an open circuit. The sending and receiving end voltages are thus the same.

This value is what the voltage at the receiving end would be if the transmission line had no impedance. The voltage would not be changed at all by the line, which is an ideal scenario in power transmission. Vfull load is the voltage across the load at the receiving end when the load is connected and current flows in the transmission line. IZline is nonzero, so the voltages and the sending and receiving ends of the transmission line are not equal.

The effects of this modulation on voltage magnitude and phase angle is illustrated using phasor diagrams that map VR, VS, and the resistive and inductive components of Vline drop. In all cases the line resistance R causes a voltage drop that is in phase with current, and the reactance of the line X causes a voltage drop that leads current by 90 degrees. These successive voltage drops are summed to the receiving end voltage, tracing backward from VR to VS in the short line approximation circuit. The vector sum of VR and the voltage drops equals VS, and it is apparent in the diagrams that VS does not equal VR in magnitude or phase angle.

Voltage phasor diagrams for a short transmission line serving lagging, in-phase, and leading loads. The diagrams show that the phase angle of current in the line affects voltage regulation significantly. The phase angle difference between sending and receiving end is minimized, however.