At its most basic level, voltage is a simple concept. Voltage, also referred to as electric potential difference or electromotive force (emf), is the difference in electric potential between two points. In the International System of Units (SI), one Volt equals one Joule of work per Coulomb of charge. In practical applications, voltage is simple, but not that simple, especially when alternating currents or dynamic operating conditions are involved.

There are numerous sources of voltage, including static charge build-ups, movement of a magnetic field, electrochemical processes including fuel cells and batteries, piezoelectric devices, heat-induced emf across metal junctions, the photovoltaic effect, and so on.

Kirchhoff’s voltage law states that the directed sum of the potential differences (voltages) around any closed loop is zero. Kirchhoff’s voltage law results from the lumped-element model and depends on the model being applicable to the specific situation. When the model is not applicable, the law does not apply. And the voltage law relies on the fact that the action of time-varying magnetic fields is confined to individual components, such as inductors. In reality, the induced electric field produced by an inductor is not confined, but the leaked fields are often negligible. Finally, it is most applicable to DC circuits and becomes more problematic for AC circuits, especially as the frequency increases.

Voltage and alternating currents

Several voltage measurements are commonly used with AC systems, including peak voltage, peak-to-peak voltage, and root means square (RMS) voltage. For alternating electric current (regardless of the waveform), RMS equals the value of the constant direct current that would produce the same power dissipation in a resistive load.

Two- and three-phase AC voltages

Single-phase AC voltages can deliver a relatively limited amount of power since all the current has to be carried using the line and neutral conductors. This power limitation is not a concern for most residential uses, but industrial systems, and some residential appliances, often require more power than single-phase AC can be delivered. For those uses, multi-phase power is often needed.

The available power can be doubled by using two-phase power found in residential settings in the U.S. Called a split-phase connection where two 120 Vac phases are used 180° apart to provide 240 Vac line-to-line. Split phase connections are only common in the U.S. and are rarely found in Asia or Europe, where the normal single-phase AC voltage is already 220 to 240 Vac.

In industrial settings where even higher power levels are required, AC power is delivered using three phases. This distributes the current over three instead of one set of wires, allowing for smaller and less expensive wiring. The three voltage phases are shifted 120° with respect to each other to balance the load currents.

Out of phase voltage and current

Reactive power is not consumed by the device and goes back and forth between the power supply and the load. Sometimes called wattless power, reactive power takes power away from the current due to the phase shift created by capacitive and/or inductive components. This phase shift reduces the amount of active power available to perform the work. Reactive power has the units of volt-amperes reactive (VAr). There is no reactive power in a DC circuit.

Current and voltage are “in phase” in polarity and time when current and voltage waveforms pass zero simultaneously. Current and voltage are always in phase when the load is purely resistive. When a load has one or more reactive components such as an inductor, motor, or capacitor and resistance, a phase shift between the voltage and current signal occurs. This lag is the phase difference; it can be positive or negative.

Reactive power is one source of power quality concerns. The power factor of an ac circuit is defined as the ratio of real power to apparent power. As power is transferred along a transmission line, it does not consist purely of real power that can do work once transferred to the load but rather consists of a combination of real and reactive power, called apparent power. The power factor describes the amount of real power transmitted along a transmission line relative to the total apparent power flowing in the line. In an electric power system, a load with a low power factor draws more current than a load with a high power factor for the same amount of useful power transferred. The higher currents increase the energy lost in the distribution system and require larger wires and other distribution equipment.

More power quality concerns

Power factor is only one of the power quality concerns with ac power distribution. Other power quality concerns include voltage sags, brownouts, harmonics, spikes, and surges. Spikes are short-duration rapid positive or negative voltage transitions superimposed on the AC waveform. Spikes can damage any sensitive electronics they reach. Electrical noise can appear as either common-mode noise arising from supply line and ground disturbances or normal-mode noise created by line-line or line-neutral disturbances. Harmonic distortion is important in power systems, where a low harmonic distortion means higher power factor, lower peak currents, and higher efficiency.

A surge occurs when the AC voltage significantly exceeds its normal level for at least one cycle. Conversely, sags are drops in the mains supply that can last for several cycles. A sag that occurs over an extended period of time is called a brownout.  Surges and sags can be caused by insufficient stiffness in the voltage source.

Voltage stiffness

Initially a power quality concept from the AC power distribution world, stiffness reflects the electrical supply’s source impedance. The stiffer a voltage is, the less impact changing loads will have on the voltage. This is true both at the primary frequency (60Hz) and at the harmonics. With the increased use of distributed energy resources such as photovoltaics, the stiffness of power converters is becoming increasingly important. An electronic power converter or power supply with a very-low internal resistance is sometimes called a “stiff” power supply.

In many instances, different types of loads in the same system benefit from varying levels of voltage stiffness. Electronic equipment that is dynamic in its operation often benefits from a stiff supply, with enough reserve capacity and a fast closed-loop response that prevents the output drop induced by a load transient from becoming so large that the supply output drops out of regulation, but that also recovers quickly and smoothly from sudden load decreases. There’s no single number that identifies a “good” or “bad” level of output voltage stiffness in power supplies; it’s application-dependent.

The same is true in AC power distribution, where voltage stiffness is important but can’t be absolutely quantified as “good” or “bad.” The optimal situation is where the stiffness of the distribution voltage enables the various loads to use all of the power delivered. While that’s not possible in the real world, it is possible to ensure about 99% power transfer and consumption with an economically viable system. To reach that goal, the total source impedance (all of the impedances summed together when looking back from the point of common coupling toward the generator) need to be 1 percent of the value of the load impedances (all the impedances of the load’s combined looking toward them from the point of common coupling). With electric utility source impedances in the range of 0.1 to 0.5 Ohm, 99% power transfer is often achievable.

Unfortunately, distributed energy resources (DERs) such as photovoltaic systems, wind turbines, and local motor-generators (M-G sets) used for backup power are not nearly as stiff as the overall electric utility distribution voltage. DERs and M-G sets lack the “electrical inertia” that large hydro plants or nuclear power plants have. When a large load turns on while running off an M-G set, the voltage often drops, as does the fundamental power frequency for a period.

Research is still underway to understand the impact that DERs have on voltage stiffness. It is a complex issue with many questions, such as what happens with solar photovoltaics and wind turbines when they are islanded? And what happens when the DER is feeding the grid, and the power flow changes direction back and forth as loads change or clouds and wind change? Power quality-related issues such as voltage stiffness could have important implications for future DER installations.

Summary

The concept and definition of “voltage” start off relatively straightforward with the SI definition and Kirchhoff’s voltage law. But once practical systems and applications are considered, it quickly becomes complex. AC systems can be single-phase, two-phase, or three-phase, and varying power quality challenges arise, complicating the efficient use of electricity. In addition, the concept of voltage stiffness is important to understand relative to the efficiency of power delivery via electricity distribution networks and the impact that DERs have on those networks.

References

Fundamentals of Electric Power Measurements, Yokogawa
Kirchhoff’s Voltage and Current Laws, Analog Devices
Root mean square, Wikipedia
Understanding Three Phase Voltage, Pacific Power
Voltage, Wikipedia
Why Do I Need a UPS?, Kohler Power