Power Quality Explained
Power Quality Defined
What is meant by power quality is really voltage quality as measured at the customer's meter. High quality AC voltage is of consistent amplitude, frequency, and waveform within specified standards and, if three-phase, phase-balanced. Such voltage is well-matched to the loads receiving it. Amplitude in the US needs to be 114-126V, frequency in the US must be very close to 60 cycles/second, and waveform must maintain a smooth sinusoidal shape. Similar standards apply in areas with 50-cycle, 240V AC. Within this broad framework are a host of variables that may be detected, measured, and corrected to maintain power quality.
Why is Power Quality Important?
Degradations in power quality can waste energy, damage equipment, shorten equipment lifetimes, interrupt sensitive processes, and cause faulty equipment performance. The development of electronic equipment has raised the bar for power quality, and the continued rapid rate of technology innovation increases the importance of high power quality for demanding, sensitive applications. Power quality failures can disrupt data integrity in server farms, induce manufacturing robots to commit physical errors, damage microchip fabrication plants production runs, cause motors to run too hot (and increase cooling costs), cause annoying flicker in lights, charge customers for power that doesn't actually benefit them, and waste energy that is expensive to supply at peak times
What Degrades Power Quality?
Traditionally, lightning strikes and other weather events, as well as utility equipment malfunctions (eg, tap changer failure, capacitor malfunction, lines down) were a significant cause of power quality failures, as were impacts of some user behaviors--starting up large induction motors, for example, or ramping up loads that were not synchronous with the frequency on the grid. Increasingly, challenges to power quality also come from distributed generation across the grid. Variable additions of voltage from residential rooftop photovoltaic, industrial cogeneration, and utility-owned large-scale renewables put pressure on the utilities to cope with unpredictable increases and decreases in total voltage available. As the grid becomes smarter, and therefore more dependent on sophisticated electronic technology, it too becomes more susceptible to impaired power quality, and can amplify or extend power quality issues as well as repair them. Here's a quick overview of the major ways in which power quality can be diminished:
OVERVOLTAGE - Overvoltage is voltage amplitude greater than 110% of nominal voltage for at least one minute. Traditionally it has been caused by system errors such as tap changer settings or sudden cessation of heavy loads. Now, it is also a consequence of distributed generation. It can damage electronic equipment or trigger automatic shutdowns of equipment.
UNDERVOLTAGE - Undervoltage is voltage amplitude less than 90% of nominal voltage for at least one minute. It is most frequently caused by overloading in periods of high demand. It can be damaging to electronic equipment and well as motors.
VOLTAGE SAGS & INTERRUPTIONS - A sag is a drop greater than 10% in voltage amplitude for a period between half-a-cycle and one minute. A drop of greater than 90% for a similar time period is called an interruption. These are the single leading cause of power quality disturbances, and are most often caused by line faults. The standard is expressed as occurrences per customer per year, and customers are told what to expect. Typical predicted rate of occurrence would be between 0.5 and 5 times/year. Many types of equipment can malfunction or be damaged, either by a single episode, or the accumulated consequences of sags and interruptions.
VOLTAGE SWELLS - A swell is the mirror image of a sag: an increase in line voltage to between 110% and 180% of the nominal voltage for a period between 0.5 cycle and 1 minute. The most common cause is sudden loss of large load. Swells are much less common than sags, but potentially more damaging to equipment.
VOLTAGE UNBALANCE - In three-phase systems, the difference in voltage between the three phases should be less than 3%. Unbalance has many possible causes, and results in three-phase motors running too hot, or failing, or having shortened life.
HARMONIC DISTORTION - The fundamental frequency is the one that does useful work, but both current and voltage can be provoked into increasing amplitudes within the waveform at frequencies that are integer multiples of the fundamental frequency--these are harmonics. The third and fifth harmonics are the most significant sources of trouble, and can increase current on the neutral wire, overheat motors, or slightly affect motor speed. The sum of the amplitudes of all harmonics is called Total Harmonic Distortion, and the higher it is the poorer power quality becomes. THD should be less than 5% of the fundamental, with no more than 3% for any single harmonic.
RAPID VOLTAGE FLUCTUATION - When the power system experiences ongoing rapid changes in load, it can generate corresponding rapid changes in voltage, called voltage fluctuation. The primary negative consequence is visible flicker in lighting systems, which can give people headaches, be tiring, and reduce concentration and productivity. The relevant standards deal with human perception of visible flicker. Controlling rapid voltage fluctuation can diminish flicker.
VOLTAGE FREQUENCY - Alternating current changes direction at a frequency determined fundamentally by the rotation of generators, and closely controlled across entire grids. Certain loads, however, can affect frequency, and many loads depend on precisely stable frequency. Typically, a motor will continue to run with frequency 5-10% high or low, but its performance will change, which can have consequences for whatever work the motor is doing. Distributed generation and energy storage are new tools to address frequency stability, and therefore power quality.
TRANSIENTS - Transients can be impulsive--a brief increase in voltage in one direction--or oscillatory--a brief increase in both directions. Common causes are lightning strikes, substantial switching on the system, or even the interaction between power factor correction capacitors and inductive loads. Transients can damage equipment, and can inadvertently turn on backup power systems.
How Can Power Quality Be Defended and Restored?
Different threats require different responses, because no one technology solves every problem. Transients, for example, are partially blocked by surge suppressors, though large transients can overwhelm them. Harmonic distortion can be countered via an array of active and passive methods, and typically requires a tuned response to specific conditions. An uninterruptible power supply can correct for interruptions and deep sags, at the expense of efficiency. The best single response to the most significant threats to power quality is to use high performance AC voltage regulators on the secondary, which is the Pacific Volt approach. Our regulators respond with one millisecond speed to overvoltage, undervoltage, voltage sags and swells, rapid voltage fluctuation. Our large transient suppressors control transients. Adding our regulators on secondary lines calms all of these power quality issues, knocking down the lion's share of the problem, simply and reliably.