Voltage sags and swells are common power quality problems that affect electrical networks. They can cause equipment malfunction, data loss, and energy waste. In this article, you will learn what causes voltage sags and swells, how they affect different types of loads, and what solutions are available to mitigate them.
Voltage sags and swells are deviations from the nominal voltage level in an electrical network. A voltage sag is a decrease in the rms voltage below 90% of the nominal value for a short duration, usually less than a second. A voltage swell is an increase in the rms voltage above 110% of the nominal value for a short duration, usually less than a second. Voltage sags and swells can be symmetrical, affecting all phases equally, or asymmetrical, affecting one or more phases differently.
Sag is a decrease in rms voltage or current at the power frequency, typically 0.1 to 0.9 per unit, for duration of 0.5 cycle to 1 minute. Swell is an increase in rms voltage or current at power frequency, typically 1.1 to 1.8 per unit, for duration of 0.5 cycle to 1 minute.
Voltage sags and swells may have various causes, both internal and external to the network. For example, short circuits, ground faults, and arc faults can cause voltage sags or swells in the faulted or adjacent sections of the network. Additionally, switching operations, such as opening or closing circuit breakers, disconnectors, or contactors, can cause transient voltage changes. Furthermore, changes in load demand or characteristics can result in voltage sags or swells due to the variation of network impedance or power factor. Finally, intermittent generation from renewable sources like wind or solar can lead to voltage sags or swells due to fluctuation of output power or connection/disconnection of sources.
An example of voltage sags and current fluctuations in residential areas too close to industrial or commercial building usage : When there is single phasing of motor loads and other fixed unbalanced loads on ungrounded three phase systems, heat tracing of pipelines using single phase and auto transformer supplies and harmonic distortions generated by transformers , variable speed drive motors (pump stations) and the list goes on. Also I have found that capacitor banks and surge protection including ups power supplies to protect electronic deices such as computer loads and life safety/security systems to be helpfull.
Voltage sags and swells can affect different types of loads in various ways, depending on their sensitivity, functionality, and protection. For instance, motors may experience torque loss or mechanical stress due to voltage sags, while overvoltage or overcurrent can be caused by voltage swells. Computers may reset, freeze, or shut down due to voltage sags, while overheat, malfunction, or damage may be caused by voltage swells. Lighting can dim, flicker, or extinguish due to voltage sags, while brighten, glare, or burn out may be caused by voltage swells. All of these issues can result in data loss, reduced visibility, energy waste, eye strain, discomfort, safety hazards and more.
Most prevalent problem associated with sags and swells is equipment shutdown. In many industries with critical process loads, even instantaneous short duration sag can cause process shutdowns requiring hours to restart.
Various solutions exist for voltage sags and swells, which depend on the source, severity, and frequency of the problem. Fault clearing devices, such as fuses, circuit breakers, or relays, can quickly isolate the faulted section of the network and restore normal voltage. Switching control devices, such as timers, sensors, or logic controllers, coordinate switching operations to minimize voltage disturbances and balance network conditions. Load management devices like capacitors, reactors, or filters adjust load impedance or power factor to reduce voltage variations and improve network efficiency. Finally, voltage regulation devices like transformers, regulators, or compensators keep voltage level within a desired range by adjusting tap position, output voltage, or injected voltage.
Sag is often sensed by electronic process controllers equipped with fault detection circuitry, which initiates shutdown of other, less sensitive loads. The application challenge is maintaining controller during sags. For this purpose, usually, the electronic controller will be served with a constant-voltage transformer, or other mitigating device, to provide adequate voltage to the controller during a sag. Voltage liming or current limiting devices will be used to mitigate swelling.
Improving power quality by reducing voltage sags and swells can have several benefits for electrical networks and their users, such as equipment protection, data security, and energy conservation. Equipment protection devices, such as surge arresters or limiters, can prevent or reduce damage to equipment components. Data security devices, like uninterruptible power supplies or backup generators, can prevent data loss or corruption. Additionally, energy conservation devices like meters or controllers can measure and analyze energy consumption to reduce energy waste and lower operating costs.
Harmonic peaks from motor loads and the grid can damage electronic equipment and if the computer loads are separated from other power loads as well as other redundant measures it can simplify the troubleshooting maintenance process as well as protect the different types of loads, power , resistive (heating) and computers.
In summary, voltage sags & swells are disturbances in electrical networks that are best controlled using electronic power correction devices such as UPS. This defines the reason UPS power is described by some designers as clean power (i.e., raw power with disturbances filtered off). It's very important to consider this when designing electrical power systems for office workstations and most importantly data centres.