Power quality can basically be understood as the ability of electrical energy to properly power equipment or any devices that consume electricity—particularly in terms of frequency synchronization and voltage levels that allow electrical systems to operate as intended, without significant loss of performance or lifespan. The term “power quality” is commonly used to describe both the quality of the electrical energy delivered to a load and the ability of that load to function properly. Without proper power supply, an electrical device (or load) may malfunction, fail prematurely or simply not work. There are many aspects in which electrical energy can be of poor quality and many causes for such poor quality.
Power quality has become a widely used term today, although there is still no consensus on its exact definition. Its meaning can vary depending on the context and point of view. Electric utility companies currently define power quality based on statistical indicators and voltage variation limits established by ANEEL (Brazil’s National Electric Energy Agency).
For the end consumer, power quality means having a continuous and efficient supply of energy. In general, the term is used to encompass a wide variety of disturbances in electrical systems, which have long been a concern for both utilities and industries.
Equipment manufacturers may define power quality as the characteristics of the energy supply that meet the needs of their equipment—which can vary from one manufacturer to another.
With regard to compliance with steady-state voltage levels, this measurement uses Resolution No. 505 of November 26, 2001, which defines limits for the effective voltage value at the delivery or connection point. These values are classified as adequate, precarious, and critical. For a preliminary analysis and in accordance with Article 5 of this resolution, the measured voltage (reading) must be between 95% and 105% of the nominal operating voltage at the delivery or connection point.
Nowadays, the measurement of electrical power quality is determined by the sensitivity and performance of consumer equipment. From this perspective, satisfactory power quality is defined as that which ensures the continuous, safe, and appropriate operation of electrical equipment and associated processes. For a more accurate evaluation of equipment, it is always necessary to verify the operating limits of the equipment and compare them with the values obtained through energy monitoring.
Main Disturbances Associated with Power Quality:
At any point in an ideal three-phase electrical system, voltages should be permanently perfectly sinusoidal, balanced, with constant amplitude and frequency. Any deviation in these parameters is considered a power quality issue (disturbance).
Therefore, when there are problems, what actually occurs is a deviation in the waveform of the supply voltage—meaning power quality is equivalent to voltage quality. With this understanding, and given that there are standards for voltage, measuring power quality becomes a more straightforward task.
The limit for total harmonic distortion (THD) of voltage is based on IEEE Std 519/1992 – Harmonic Control in Electric Power Systems, where the general limit is: 5% for voltages up to and including 69 kV, 2.5% for voltages between 69 kV and 161 kV and 1.5% for voltages at or above 161 kV.
As for total harmonic distortion of current, a reference value of 15% is used. This is an empirical threshold where transformer lifespan begins to degrade and it may indicate the potential for problems in the electrical installation.
When the measured value approaches this reference and the consumer has capacitors installed, it is recommended to measure directly at the capacitors to check whether the harmonic distortion is affecting their lifespan as well.
• Total Harmonic Distortion of Voltage: 5% (Recommended to be below 5% according to IEEE Std 519-1992)
• Total Harmonic Distortion of Current: 15% (Recommended to be below 15% according to transformer manufacturers)
• Voltage Unbalance: 2% (Recommendation by ONS – National System Operator, Submodule 2.2, with a K-Factor less than or equal to 2%, www.ons.org.br)
• Current Unbalance: 10% (Recommended to be below 10% according to transformer manufacturers)
It should be noted that the above limits may vary depending on the needs and characteristics of each installation.
Other Disturbances That Should Also Be Assessed in a Power Quality Measurement:
• Transients, either impulsive or oscillatory types.
• Short-duration voltage variations, which may be instantaneous, momentary, or temporary.
• Long-duration voltage variations, which may fall into three categories: interruptions, sustained undervoltages (sags), or sustained overvoltages (swells).
• Voltage unbalance, typically caused by poor distribution of single-phase loads, which results in negative sequence voltages in the circuit.
• Waveform distortions, which can be classified into five types: DC offset, harmonics, interharmonics, notching, and noise.
• Voltage fluctuations, which are systematic variations in the RMS voltage supply values (within the range of 0.95 to 1.05 pu) and may be random, repetitive, or sporadic.
• System frequency variations, defined as deviations from the system’s fundamental frequency (either 50 Hz or 60 Hz).
Electromagnetic phenomena resulting from sudden changes in the operating conditions of an electrical power system. Typically, the duration of a transient is very short, but its significance is high, as it subjects equipment to severe voltage and/or current stress. There are two types of transients: impulsive, caused by lightning strikes, and oscillatory, caused by switching operations.
An impulsive transient (typically caused by lightning strikes) can be defined as a sudden change in the steady-state conditions of voltage, current, or both, characterized by unidirectional impulses in polarity (either positive or negative) and a frequency very different from that of the power grid.
In distribution systems, the most likely path for lightning strikes is through a phase conductor, either on the primary or secondary side, causing high overvoltages in the system. A direct strike to a phase conductor can also generate short-duration undervoltages (“sags”) and interruptions.
High transient overvoltages can also be generated by lightning currents flowing along the ground conductor, causing the following problems:
• Elevation of local ground potential relative to other grounds, reaching several kilovolts. Electronic equipment connected between two ground references—such as computers connected to modems—can be damaged when subjected to high voltage levels.
• Induction of high voltages in phase conductors when lightning currents travel through cables en route to ground.
An oscillatory transient is characterized by a sudden change in the steady-state conditions of voltage and/or current, exhibiting both positive and negative polarity values. These transients usually result from line energization, switching of inductive currents, fault clearing, switching of capacitor banks and transformers, etc.
Medium-frequency oscillatory transients can be caused by: “Back-to-back” capacitor energization (resulting in transient currents in the tens of kHz), circuit breaker operations during fault clearing and the system’s response to an impulsive transient.
Short-duration voltage variations can be categorized as instantaneous, momentary, or temporary changes. These voltage variations are generally caused by the energization of large loads that require high inrush currents or by intermittent faults in system cable connections. Depending on the location of the fault and system conditions, the result may be a temporary voltage drop (“sag”), a voltage rise (“swell”), or even a complete interruption of the electrical system.
A short-duration interruption occurs when the supply voltage drops to a value below 0.1 pu (per unit) for a period not exceeding 1 minute. These are typically caused by faults in the power system, equipment failures, or malfunctioning control systems. Some interruptions may be preceded by a sag if they are due to faults in the supplying system. The sag occurs in the time window between the onset of the fault and the operation of the system’s protective device.
For example, consider a short-circuit in the utility’s supply system. As soon as the protective device detects the short-circuit current, it commands the line to be de-energized in order to eliminate the fault current. Only after a short delay does the automatic reclosing of the circuit breaker or recloser take place. However, if the fault persists after reclosing, a sequence of reclosing attempts may be performed to clear the fault. The illustration below shows a typical reclosing sequence with standard delay settings. If the fault is temporary, the protection equipment may not complete all programmed reclosing operations, and energy supply is not fully interrupted.
As a result, most consumers—especially residential—will not notice the interruption. However, sensitive loads (e.g., computers and other electronic devices) are more vulnerable, unless the facility is equipped with UPS units (uninterruptible power supplies), which can prevent serious consequences for the operation of such equipment.
Statistical data show that about 75% of faults in overhead lines are temporary. In the past, this percentage was not a major concern. However, with the growing use of electronic loads such as inverters and computers, this figure has become significant in power system optimization studies. These temporary faults are now considered a leading cause of equipment outages, disrupting production processes and causing considerable financial losses in industrial environments.
A short-duration voltage sag is characterized by a reduction in the RMS voltage, between 0.1 and 0.9 pu at the fundamental frequency, with a duration from 0.5 cycle to 1 minute. The figure next illustrates a typical short-duration undervoltage caused by a phase-to-ground fault. A voltage drop of 80% is observed over approximately 3 cycles, until the substation protection equipment operates to eliminate the fault current.
A short-duration overvoltage, or “swell,” is defined as an increase in the RMS voltage to between 1.1 and 1.8 pu, at the system frequency, lasting from 0.5 cycrle to 1 minute. Swells are generally associated with fault conditions in the power system. The adjacent figure (not shown here) illustrates a swell caused by a phase-to-ground fault. This phenomenon can also occur due to the disconnection of large load blocks or the energization of large capacitor banks, although such causes are less frequent compared to phase-to-ground faults in transmission and distribution networks.
Short-duration overvoltages are characterized by their magnitude (RMS values) and duration. The severity of a swell during a fault condition depends on the location of the fault, the system impedance, and the grounding scheme. Its duration is closely linked to the settings of protection devices, the nature of the fault (permanent or temporary), and its location within the power grid.
Consequences of short-duration overvoltages on equipment can include component failures, depending on how frequently the disturbance occurs. Electronic devices, including ASDs (Adjustable Speed Drives), computers, and electronic controllers, may fail immediately under such conditions. Transformers, cables, busbars, switching devices, potential transformers (PTs), current transformers (CTs), and rotating machines may experience reduced lifespan. A short voltage rise may cause misoperation in certain relays, while others may remain unaffected. A swell affecting a capacitor bank can often result in equipment damage.
Given this, the primary concern lies with electronic equipment, as these overvoltages can damage internal components, leading to malfunction or, in extreme cases, complete failure. It’s important to emphasize once again that a device’s tolerance is not only determined by the magnitude of the overvoltage but also by its duration, as illustrated by the CBEMA curve, which shows typical microcomputer tolerance to voltage variations.
In light of the problems caused by short-duration overvoltages, this aspect of power quality requires close attention from consumers, manufacturers, and utility companies, in order to eliminate or minimize the consequences of this phenomenon.
Imbalances can be defined as the maximum deviation from the average of three-phase currents or voltages, divided by the average of the three-phase currents or voltages, expressed as a percentage. These imbalances usually originate in distribution systems, which often have improperly distributed single-phase loads, resulting in negative sequence voltages in the circuit. This issue worsens when three-phase-fed consumers have poor internal load distribution, imposing unbalanced currents on the utility’s circuit.
These factors negatively impact the quality of the power supply, and some consumers may experience voltage imbalance in their electrical systems. Such voltage imbalances can lead to undesirable issues in the operation of equipment, notably:
• Induction Motors: In analyzing the effects of unbalanced voltages on an induction motor, only the effects produced by negative sequence voltages are considered. When combined with the effects of positive sequence voltage, they result in a pulsating torque on the machine’s shaft (see figure), and in the overheating of the machine. A direct consequence of this temperature rise is the reduction in the motor’s expected service life, as the insulating material deteriorates more rapidly under high temperatures in the windings.
• Synchronous Machines: As in the previous case, the negative sequence current flowing through the stator of a synchronous machine creates a rotating magnetic field at the same speed as the rotor, but in the opposite direction to that defined by the positive sequence. Consequently, voltages and currents induced in the field windings, damper windings, and on the rotor iron surface will have a frequency twice that of the power grid, significantly increasing losses in the rotor.
• Rectifiers: An AC/DC rectifier bridge, whether controlled or not, injects characteristic harmonic currents into the AC grid (of orders 5, 7, 11, 13, etc.) under nominal operating conditions. However, when the supply system is unbalanced, the rectifiers begin to generate, in addition to the characteristic harmonics, the third harmonic and its multiples. The presence of the third harmonic and its multiples in the electrical system is highly undesirable, as it can lead to unexpected resonances, causing damage to various types of equipment.
Waveform distortion is defined as a steady-state deviation from a purely sinusoidal waveform at the fundamental frequency, and it is mainly characterized by its spectral content. There are five main types of waveform distortions:
• Harmonics: Sinusoidal voltages or currents at frequencies that are integer multiples of the fundamental frequency (50 or 60 Hz) at which the power system operates. These harmonics distort the voltage and current waveforms and originate from non-linear devices and loads connected to the power system.
• Interharmonics: Voltage or current frequency components that are not integer multiples of the system’s fundamental frequency (50 or 60 Hz). These can appear as discrete frequencies or as a broad spectral band. Interharmonics can be found in networks of various voltage levels. Their main sources include static power converters, cycloconverters, induction motors, and arc-based equipment. Carrier signals on power lines can also be considered interharmonics. The effects of this phenomenon are not well understood, but it is believed they can interfere with carrier signal transmission and cause visual flicker in displays such as cathode ray tubes.
• DC Offset: The presence of direct current (DC) voltage or current in an alternating current (AC) system is called “DC offset.” This phenomenon can result from the normal operation of half-wave rectifiers. DC levels in AC networks can cause transformer core saturation, leading to additional losses and reduced equipment lifespan.
• Notching: A voltage disturbance caused by the normal operation of power electronic equipment when current is commutated from one phase to another. This phenomenon can be identified by analyzing the harmonic content of the affected voltage. The frequency components associated with notching are very high and, therefore, cannot be measured by standard harmonic analysis equipment.
• Noise: Defined as an unwanted electrical signal containing a wide spectral range with frequencies below 200 kHz, superimposed on phase voltages or currents, or present in neutral conductors. Noise in power systems can be caused by power electronic devices, control circuits, arc equipment, solid-state rectifiers, and switched-mode power supplies. It is often associated with improper grounding.
Technically, a harmonic is a component of a periodic wave whose frequency is an integer multiple of the fundamental frequency (in the case of electrical energy, 60 Hz). The best way to explain this is through the adjacent illustration. In this figure, we see two curves: a normal sine wave representing a “clean” current waveform, and a smaller wave representing a harmonic. This smaller wave represents the fifth-order harmonic, meaning its frequency is 5 x 60 Hz, or 300 Hz.
In the second illustration, we see the sum of the two curves. This resulting curve clearly shows the harmonic distortion of the voltage waveform due to the presence of harmonics.
Harmonic distortions run counter to the goals of power quality promoted by an electric utility, which must supply its consumers with a purely sinusoidal voltage, with constant amplitude and frequency. However, when power is supplied to certain consumers who introduce distortions into the power system, it affects not only the responsible consumer but also others connected to the same electrical network.
In the past, harmonics were not a major concern. Non-linear loads were rarely used, and equipment was more resistant to the effects of harmonics. However, in recent years, with the rapid development of power electronics and the use of more efficient energy-saving methods, the harmonic content in systems has increased, causing various undesirable effects on equipment, compromising both power quality and energy efficiency.
Thus, it is important to mention the various types of non-linear electrical loads that have been increasingly implemented in the Brazilian power system:
• discharge lamp lighting circuits;
• arc furnaces;
• static compensators using saturated reactors;
• DC motors controlled by rectifiers;
• induction motors controlled by forced-commutation inverters;
• electrolysis processes using uncontrolled rectifiers;
• synchronous motors controlled by cycloconverters;
• high-frequency induction furnaces;
• induction furnaces controlled by saturated reactors;
• heating loads controlled by thyristors;
• AC motor speed controlled by stator voltage;
• saturated-core voltage regulators;
• computers;
• household appliances with switched-mode power supplies, etc.
As mentioned, harmonic distortions cause significant losses in industrial facilities. The most critical are productivity and sales losses due to production stoppages caused by unexpected failures in motors, drives, power supplies, or simply circuit breaker tripping.
Below is a more detailed list of such issues:
• Capacitors: fuse blowing, reduced lifespan.
• Motors: shortened service life, inability to reach full power.
• Fuses/Breakers: false or incorrect tripping, damaged components.
• Transformers: increased losses, leading to reduced capacity and shorter lifespan.
• Meters: potential for incorrect readings and higher bills.
• Telephones: interference.
• Synchronous Machines: Overheating of the pole shoes, caused by the circulation of harmonic currents in the damper windings.
• Drives/Power Sources: Malfunctions due to multiple zero crossings and failure in circuit commutation.
• Excessive loading of the neutral conductor, especially in installations with many electronic devices and poorly designed grounding systems.
The main problems caused by harmonics, however, occur in capacitor banks, which can lead to resonance conditions, resulting in overvoltage at the capacitor terminals.
As a result of this overvoltage, the insulation of the capacitor units degrades, and in extreme cases, the capacitors may be completely damaged. Additionally, consumers connected to the same point are exposed to dangerous voltages, even if they do not have harmonic-polluting loads in their installation. Even in the absence of a resonance condition, a capacitor always represents a low-impedance path for harmonic currents, and is therefore constantly subjected to overload and excessive overheating.
Voltage fluctuations correspond to systematic variations in the effective values of supply voltage within the range of 0.95 to 1.05 per unit (pu). These fluctuations are generally caused by industrial loads and manifest in different forms, such as:
• Random Fluctuations: Caused by arc furnaces, where the amplitude of the oscillations depends on the melting state of the material and the short-circuit level of the installation.
• Repetitive Fluctuations: Caused by welding machines, rolling mills, mine elevators, and railways.
• Sporadic Fluctuations: Caused by the direct startup of large motors.
The main effects on electrical systems resulting from these fluctuations include power and torque oscillations in electric machines, decreased equipment efficiency, interference with protection systems, and “flicker” or light flickering.
Frequency variations in an electrical system are defined as deviations from the fundamental system frequency (50 or 60 Hz). The system frequency is directly associated with the rotational speed of the generators supplying the system. Small frequency variations may be observed as a result of the dynamic balance between load and generation in the event of a disturbance (typically within the range of 60 ± 0.5 Hz). Frequency deviations that exceed the normal operating range may be caused by faults in transmission systems, the loss of a large load block, or the shutdown of a major generation source.
In isolated systems, however—such as on-site industrial generation—in the event of a disturbance, the magnitude and duration of machines operating outside their rated speed can lead to more significant frequency deviations.