Voltage regulation

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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.

Electrical power systems[edit]

In electrical power systems it is a dimensionless quantity defined at the receiving end of a transmission line as:

\text{Percent } VR = \frac{|V_{nl}| - |V_{fl}|}{|V_{fl}|} \times 100[1]

where Vnl is voltage at no load and Vfl is voltage at full load. A smaller value of Voltage Regulation is usually beneficial.

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."

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).

Electronic power supply parameters[edit]

The quality of a system's voltage regulation is described by three main parameters:

Parameter Symbol Description
Line regulation Sv Measure of the ability to maintain a constant output voltage, regardless of changes to the input voltage
Load regulation Ro Measure of the ability to maintain a constant output voltage, regardless of the size of the system's load
Temperature dependence ST Measure of the ability to maintain a constant output voltage, regardless of variations in temperature of electrical components within the system, especially semiconductor based devices.

Distribution feeder regulation[edit]

Electric utilities aim to provide service to customers at a specific voltage level, for example, 220V or 240V. However, due to Kirchhoff's Laws, the voltage magnitude and thus the service voltage to customers will in fact vary along the length of a conductor such as a distribution feeder (see Electric power distribution). Depending on law and local practice, actual service voltage within a tolerance band such as ±5% or ±10% may be considered acceptable. In order to maintain voltage within tolerance under changing load conditions, various types of devices are traditionally employed:[2]

• a load tap changer (LTC) at the substation transformer, which changes the turns ratio in response to load current and thereby adjusts the voltage supplied at the sending end of the feeder;

voltage regulators, which are essentially transformers with tap changers to adjust the voltage along the feeder, so as to compensate for the voltage drop over distance; and

capacitors, which reduce the voltage drop along the feeder by reducing current flow to loads consuming reactive power.

A new generation of devices for voltage regulation based on solid-state technology are in the early commercialization stages.[3]

Distribution regulation involves a "regulation point": the point at which the equipment tries to maintain constant voltage. Customers further than this point observe an expected effect: higher voltage at light load, and lower voltage at high load. Customers closer than this point experience the opposite effect: higher voltage at high load, and lower voltage at light load.

Complications Due to Distributed Generation[edit]

Distributed generation, in particular photovoltaics connected at the distribution level, presents a number of significant challenges for voltage regulation.

Typical voltage profile expected on a distribution feeder with no DG. This voltage profile results from the fact that current through feeders with no DG decreases with distance from the substation.

Conventional voltage regulation equipment works under the assumption that line voltage changes predictably with distance along the feeder. Specifically, feeder voltage drops with increasing distance from the substation due to line impedance and the rate of voltage drop decreases farther away from the substation.[4] However, this assumption may not hold when DG is present. 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. If the load is sufficiently low, current will flow in the reverse direction (i.e. towards the substation), resulting in a voltage profile that increases with distance from the substation. This inverted voltage profile may confuse conventional controls. In one such scenario, load tap changers expecting voltage to decrease with distance from the substation may choose an operating point that in fact causes voltage down the line to exceed operating limits.[5]

Comparison of 24 hour voltage swings on a feeder with no PV, 20% PV and 20% PV with volt-VAR control.

The voltage regulation issues caused by DG at the distribution level are complicated by the fact that utilities generally have very little monitoring equipment along distribution feeders. The relative scarcity of information on distribution voltages and loads makes it difficult for utilities to make adjustments necessary to keep voltage levels within operating limits.[6]

Although DG poses a number of significant challenges for distribution level voltage regulation, if combined with intelligent power electronics DG can actually serve to enhance voltage regulation efforts.[7] One such example is PV connected to the grid through inverters with volt-VAR control. 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.[8]

See also[edit]


  1. ^ Gönen, Turan (2012). Electrical machines with MATLAB(R). CRC Press. p. 337. ISBN 978-1-43-987799-9. 
  2. ^ von Meier, Alexandra (2006). Electric Power Systems: A Conceptual Introduction. Wiley-IEEE. pp. 184–188. ISBN 0471178594. 
  3. ^ "Greentechmedia article on voltage-correcting grid sensor". Retrieved May 4, 2013. 
  4. ^ von Meier, Alexandra (2006). Electric Power Systems: A Conceptual Introduction. Wiley-IEEE Press. p. 186. ISBN 0471178594. 
  5. ^ "Power Quality Impact of Distributed Generation: Effect on Steady State Voltage Regulation". p. 7. CiteSeerX: 
  6. ^ "Statistics of voltage drop in radial distribution circuits: a dynamic programming approach" (PDF). p. 1. Retrieved May 5, 2015. 
  7. ^ "Impact of Distributed Generation on Voltage Profile in Deregulated Distribution System" (PDF). p. 6. Retrieved May 5, 2015. 
  8. ^ "Updating Interconnection Screens for PV System Integration" (PDF). p. 20. Retrieved May 5, 2015.