Power Quality

Your guide to better power quality

Without proper power, an electrical device may malfunction, fail prematurely, or not operate at all.

By Amin Almasi

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Power quality determines how fit electrical power is to run consumer devices. Synchronizing the voltage frequency and phase allows electrical systems to function as they're intended to without significant loss of performance or life. The term is used to describe electric power that drives an electrical load and the load's ability to function properly. Without the proper power, an electrical device (or load) may malfunction, fail prematurely, or not operate at all. There are many ways in which electric power can be of poor quality and many more causes of such poor quality power.

The electric power system, in general, comprises electricity generation (AC power), electric power transmission, and ultimately electricity distribution to electric users (such as VSD electric motors) that are the electric power's end users. The electricity then moves through the end user's hardware and wiring system until it reaches the load. The complexity of moving electric energy from the point of production to the point of consumption – combined with variations in related factors such as generation and demand – provides many opportunities for the quality of supply to be compromised.

And although "power quality" may be a convenient term, it is the quality of the voltage – rather than power or electric current – that is actually described by the term. Power is simply the flow of energy, and the current demanded by a load is largely uncontrollable.

The quality of electrical power may be described as a set of parameter values, such as:

  • Continuity of service
  • Variation in voltage magnitude (see below)
  • Transient voltages and currents
  • Harmonic content in the waveforms for AC power

It can be useful to think of power-quality issues as compatibility problems. To address these, consider: “Is the equipment that is connected to the grid compatible with the events on the grid?” Alternatively, ask: “Is the power delivered by the grid, including the events, compatible with the equipment that is connected?” Compatibility problems always have at least two solutions; here, the options are either clean up the power or make the equipment tougher.

Power quality

Ideally, AC voltage is supplied by the plant network (whether electric power is imported to a plant from a national or local network or generated in a power generation unit) as sinusoidal, having an amplitude and frequency given by standards or system specifications. Further, AC voltage ideally is supplied with an impedance of near zero ohms at all frequencies. No real-life power source is ideal, however, and power sources generally can deviate in the following ways:
Variations in the peak or RMS (root mean square) voltage are important to different types of equipment as well as to electrical power consumers. When the RMS voltage exceeds the nominal voltage by even 10% to 20% (or more) for 0.5 cycles to 30 seconds (or even more), the event is usually called a "swell." A "sag" describes the opposite situation: The RMS voltage is below the nominal voltage 10% to 20% or more for 0.5 cycles to 30 seconds. Random or repetitive variations in the RMS voltage by a relatively large value – between 90% and 100% of the nominal value, for example – can produce a phenomenon known as "flicker" in lighting equipment. Flicker is rapid visible changes of light level.

Abrupt, very brief increases in voltage, called "spikes," "impulses," or "surges," are generally caused by large inductive loads being turned off or by lightning.

Undervoltage occurs when the nominal voltage drops below 90% for more than 60 seconds. The term "brownout" describes voltage drops that land somewhere between full power (say for instance, full load or bright lights) and a blackout (no power). It comes from the noticeable to significant dimming of regular incandescent lights during system faults or overloading etc., when insufficient power is available to achieve full brightness. Brownouts typically reflect a reduction in system voltage taken by the power generation unit, the utility or the system operator in an effort to decrease demand or to increase system operating margins.

"Overvoltage" occurs when the nominal voltage rises above 110% for more than 60 seconds.

Variations in frequency and in the wave shape (the latter usually described as harmonics) also are relevant to power quality discussions. The two following impedances are important in power quality of a plant:

  • Nonzero low-frequency impedance – when a load draws more power, the voltage drops.
  • Nonzero high-frequency impedance – when a load demands a large amount of current and then stops demanding it suddenly, there will be a dip or spike in the voltage because of the inductances in the power supply line.

Each of the above-mentioned power quality problems has a different cause. Some problems result from the shared infrastructure or network. For example, a fault on the network may cause a dip that will affect some customers – the higher the level of the fault, the greater the number affected. A problem on one customer’s unit or facility may cause a transient that affects all other customers on the same subsystem. Problems such as harmonics may¬¬ arise within the customer’s own installation and propagate onto the network and affect other customers. In most general forms, harmonic problems can be dealt with by a combination of good design practice and proven reduction equipment.

Consequences of poor power quality

We can identify the following main contributors to low-voltage poor power quality:

  1. Reactive power, as it loads up the supply system unnecessarily
  2. Harmonic pollution, as it causes extra stress on the networks and makes installations run less efficiently
  3. Load imbalance, as the unbalanced loads may result in excessive voltage imbalance, causing stress on other loads connected to the same network and leading to an increase of neutral current and neutral-to-earth voltage buildup
  4. Fast voltage variations, which lead to flicker.

Poor power quality can be described as any event related to the electrical network that ultimately results in financial losses and operational problems. Possible consequences of poor power quality include:

  • Unexpected power supply failures (breakers tripping, fuses blowing).
  • Equipment failure or malfunctioning
  • Overheating of equipment such as transformers and electric motors, leading to a shortening of their lifespan.
  • Damage to sensitive equipment (such as control systems or data processing units), which may result in electronic communication problems and interferences.
  • Health issues and reduced efficiency among personnel because of the visual disruption caused by flicker.

Solutions for better power quality

Power conditioning, or modifying power, is an valuable way to obtain better power quality. For example, an uninterruptible power supply (UPS) can be used if there is a transient (temporary) condition. However, cheaper UPS units create poor-quality power themselves. High-quality UPS units utilize a double conversion topology that breaks down incoming AC power into DC, charges the batteries, and then remanufactures an AC sine wave. This remanufactured sine wave could be of higher quality than the original AC power feed. A surge protector or simple capacitor or varistor can protect against most overvoltage conditions, while a lightning arrestor protects against severe spikes. Harmonic filters, used to remove harmonics, are an additional powerful power conditioning tool.

Another important concept is the smart grid. Modern systems use sensors called “phasor measurement units” (PMU) distributed throughout their network to monitor power quality and in some cases respond automatically to them. Using such smart grid features as rapid sensing and automated self-healing of anomalies in the network can result in higher-quality power and less downtime. This use simultaneously supports power from intermittent power sources and distributed generation, which, if unchecked, would degrade power quality.

Control systems and data processing

Plants also should pay attention to control systems and data processing units that consume electric power and could be affected by power quality. The tolerance of data-processing equipment to voltage variations is often characterized by a curve known as CBEMA (Computer and Business Equipment Manufacturers Association) curve, which gives the duration and magnitude of voltage variations that can be tolerated.

Around 40 years ago, CBEMA developed one of the most frequently employed power acceptability curves as a guideline for members' design of their power supplies. Basically, the CBEMA curve was derived to describe the tolerance of mainframe computer business equipment to the magnitude and duration of voltage variations on the power system; it eventually became a standard design target for sensitive equipment to be applied on the power system and a common format for reporting power quality variation data.

The CBEMA curve stipulates that the voltage needs to remain within the upper and lower curves. These curves create what is known as the tolerance envelope. The amount of voltage allowed is dependent on its duration and the curves approach steady-state values of 87% and 106%. The CBEMA curve was derived from experimental and historical data taken from mainframe computers. The best scientific interpretation of the curve can be given in terms of a voltage standard applied to the DC bus voltage of a rectifier load.

The envelope that is plotted on the curve serves as a guide for allowable and unacceptable voltage excursions. That reference is important because power engineers and operators know what is necessary before problems occur with customer loads. It is also vital to designers and manufacturers of machines to ensure that the machine would not crash or malfunction within the envelope. Within the envelope, the device should be sturdy enough to tolerate acceptable voltage excursions.

If there is a voltage spike and it is quick enough to cause an issue, there would not be any physical damage since the large current associated with momentary overvoltage will not have enough time to overheat components and damage them. If the voltage is significantly reduced but it occurs relatively fast, the device will practically continue to function; basically, too much voltage for too long will cause components to overheat and damage the machine. Voltage values above the envelope are supposed to cause malfunctions such as insulation failure, over-excitation, and overvoltage trip. In addition, too little voltage for too long of a period of time will lead to the device shutting off. A solution to prepare for low voltage would be for valuable equipment to have a backup battery. The problem with this is that although this is reasonable for light electronics, it gets to be expensive for large machines.

Harmonics in electrical systems

Harmonics voltages and currents in an electric power system are a result of nonlinear electric loads mainly VFD (variable frequency drives), switching mode power devices or similar systems. Another source of harmonics is the power generation unit.

A pure sinusoidal voltage is a conceptual quantity produced by an ideal AC generator built with finely distributed stator and field windings that operate in a uniform magnetic field. Because neither the winding distribution nor the magnetic field is uniform in a working AC machine, voltage waveform distortions are created, and the voltage-time relationship deviates from the pure sine function.

The distortion at the point of generation is usually very small (about 1% to 2%), but nonetheless it exists. Because this is a deviation from a pure sine wave, the deviation is in the form of a periodic function, and by definition, the voltage distortion contains harmonics. When a sinusoidal voltage is applied to a certain type of load, the current that the load draws is determined by the voltage and impedance, and it follows the voltage waveform. These loads are referred to as linear loads; examples of linear loads are resistive heaters, incandescent lamps, and constant speed induction and synchronous motors. In contrast, some loads cause the current to vary disproportionately with the voltage during each cyclic period. These are classified as nonlinear loads, and the current that they take has a nonsinusoidal waveform. When there is significant impedance in the path from the power source to a nonlinear load, these current distortions will also produce distortions in the voltage waveform at the load. However, in most cases where the power delivery system is functioning correctly under normal conditions, the voltage distortions will be quite small and can usually be ignored.

Harmonic frequencies in the power grid are a frequent cause of power quality problems. Harmonics in power systems result in increased heating of equipment and conductors, misfiring in variable-speed drives, and torque pulsations in motors. Reduction of harmonics is always desirable. Current harmonics are caused by nonlinear loads. When a nonlinear load such as a rectifier is connected to the system, it draws a current that is not necessarily sinusoidal. The current waveform can become quite complex, depending on the type of load and its interaction with other components of the system. Regardless of how complex the current waveform becomes, as described through Fourier series analysis, it is possible to decompose it into a series of simple sinusoids, which start at the power system fundamental frequency and occur at integer multiples of the fundamental frequency. Further examples of nonlinear loads include variable-speed drives (VSDs), common control and data processing equipments, fluorescent lighting, battery chargers and similar.

Voltage harmonics are caused mostly by current harmonics. A nonlinear load will not directly cause voltage harmonics unless it is injecting power. However, the voltage provided by the voltage source will be distorted by current harmonics due to source impedance. If the source impedance of the voltage source is small, current harmonics will cause only a small voltage harmonics. One of the major effects of power system harmonics is to increase the current in the system. In addition to the increased line current, different pieces of electrical equipment can suffer effects from harmonics on the power system.

Electric motors experience losses because of hysteresis and losses due to eddy currents set up in the iron core of the motor. These are proportional to the frequency of the current. Because the harmonics are at higher frequencies, they produce higher core losses in a motor than the power frequency would. This results in increased heating of the motor core, which (if excessive) can shorten the life of the motor. The fifth harmonic usually causes a CEMF (counter electromotive force) in large motors that acts in the opposite direction of rotation. The CEMF is not large enough to counteract the rotation; however, it does play a small role in determining the resulting rotating speed of the motor.

Waveform distortion can be mathematically analyzed to show that it is equivalent to superimposing additional frequency components onto a pure sine wave. These frequencies are harmonics (integer multiples) of the fundamental frequency and can sometimes propagate outward from nonlinear loads, causing problems elsewhere on the power system.'

Harmonic filters are necessary for the elimination of the harmonic waves and for the production of the reactive power at line-commutated converter stations.

At plants with six pulse line-commutated converters, complex harmonic filters are usually necessary because there are odd-numbered harmonics of the orders “6n+1”and “6n-1” produced on the AC side and even harmonics of order “6n” on the DC side. At 12 pulse converter stations, only harmonic voltages or currents of the order “12n+1” and “12n-1” (on the AC side) or “12n”(on the DC side) result. Harmonic filters are tuned to the expected harmonic frequencies and consist of series combinations of capacitors and inductors. Voltage sourced converters generally produce lower intensity harmonics than line commutated converters. As a result, harmonic filters are generally smaller or may be omitted altogether.