Voltage regulation

This is why a smaller value of Voltage Regulation is usually beneficial, indicating that the line is closer to ideal.

The long line approximation therefore requires the solving of differential equations and results in the highest degree of accuracy.

The entire short line model is an open circuit in this condition, and no current flows in an open circuit, so I = 0 A and the voltage drop across the line given by Ohm’s law Vline drop = IZline is 0 V. The sending and receiving end voltages are thus the same.

Now Vline drop = IZline is nonzero, so the voltages and the sending and receiving ends of the transmission line are not equal.

The current I can be found by solving Ohm’s law using a combined line and load impedance:

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.

Three power factor scenarios are shown, where (a) the line serves an inductive load so the current lags receiving end voltage, (b) the line serves a completely real load so the current and receiving end voltage are in phase, and (c) the line serves a capacitive load so the current leads receiving end voltage.

The diagrams show that the phase angle of current in the line affects voltage regulation significantly.

In-phase current in (b) does little to affect the magnitude of voltage between sending and receiving ends, but the phase angle shifts considerably.

Real transmission lines typically serve inductive loads, which are the motors that exist everywhere in modern electronics and machines.

Sometimes, the term voltage regulation is used to describe processes by which the quantity VR is reduced, especially concerning special circuits and devices for this purpose (see below).

Depending on law and local practice, actual service voltage within a tolerance band such as ±5% or ±10% may be considered acceptable.

For example, a long feeder with a high concentration of DG at the end will experience significant current injection at points where the voltage is normally lowest.

In a study conducted jointly by the National Renewable Energy Laboratory (NREL) and Electric Power Research Institute (EPRI), when volt-VAR control was added to a distribution feeder with 20% PV penetration, the diurnal voltage swings on the feeder were significantly reduced.

Under no load, when no current flows through the secondary coils, Vnl is given by the ideal model, where VS = VP*NS/NP.