Basics of how to minimize harmonics
The August 14, 2003 blackout emphasized that power quality should not be taken lightly. The blackout was caused by, and affected the power grid — our nation’s electrical infrastructure. The issues raised by this event should and will come under scrutiny. Just as the nation looks at its power quality issues, plant engineers must look at power quality issues inside the plant.
Harmonics is a buzzword thrown about freely these days. But what are harmonics, and why do we hear so much about them? What is the big deal? Why are they so important?
What are harmonics?
Harmonics are multiples of a fundamental frequency. In music, they are called octaves, and are usually desirable. But in a plant’s electrical power distribution system, they are unwanted.
Harmonics cause trouble when combined with the fundamental electrical waveform. Since these harmonics are multiples of the 60-Hz fundamental power frequency, harmonic frequencies can be 2-times at 120-Hz, 3-times at 180-Hz, and so on. When harmonics mix with the fundamental, they distort the sine wave (Fig. 1).
IEEE defines harmonic content as “a measure of the presence of harmonics in a voltage or current waveform expressed as a percentage of the amplitude of the fundamental frequency at each harmonic frequency. The total harmonic content is expressed as the square root of the sum of the squares of each of the harmonic amplitudes.”
What causes harmonics?
Nonlinear loads are the primary causes of harmonics. These nonlinear loads include, but are not limited to, variable speed drives, solid-state controls for heating and other applications, switched-mode power supplies like those found in virtually every computerized piece of equipment, static uninterruptible power supply (UPS) systems, electronic ballasts, electronic test equipment, and electronic office machines.
Nonlinear loads draw short bursts of current each waveform cycle, thereby distorting the sinusoidal waveform. Harmonic voltages are the result of harmonic currents interacting with power system impedance.
Why are they harmful to equipment?
The detrimental effects of harmonics include overheating of transformers, power cables, motors, and drives. They cause inadvertent thermal tripping of relays and protective devices. Harmonics can even cause logic faults in digital devices and incorrect voltage and current meter measurements. Any of these damaging results can cause downtime in your plant.
Section 6 of IEEE Standard 519-1992 describes how harmonic currents increase heating in motors, transformers, and power cables. According to the specification, harmonics can cause electrical losses in transformer cores and motor rotors resulting from hysteresis and eddy currents, making them overheat. Motors experience torque reduction. High harmonics cause electronic equipment to operate erratically.
Harmonics affect different equipment differently. Some of the detrimental effects caused by harmonics are:
Capacitors — Capacitors operate as sinks to increased harmonics and harmonic frequencies. Supply system inductance can resonate with capacitors at some harmonic frequencies, causing large currents and voltages to develop at these frequencies. Increased currents and voltages cause breakdown of dielectric material within capacitors, which, in turn, causes the capacitors to heat. As capacitor dielectrics dry out, they are less capable of dissipating heat, and become even more susceptible to damage from harmonics. As this deterioration continues, short circuits or capacitor explosions can occur.
Transformers — Harmonic voltages cause higher transformer voltage and insulation stress, resulting in transformer heating, reduced life, increased copper and iron losses through hysteresis and eddy currents, and insulation stress.
Motors — Harmonic voltages produce magnetic fields that rotate at speeds corresponding to the harmonic frequencies, resulting in increased losses, motor heating, mechanical vibrations and noise, pulsating torques, increased eddy current and hysteresis losses in stator and rotor windings, reduced efficiency, reduced life, and voltage stress on motor winding insulation.
Circuit breakers — Harmonics may prevent blowout coils from operating properly; circuit breakers could fail to interrupt current properly; or breakers could fail completely.
Watt-hour meters — Induction disks are calibrated for accurate operation on the fundamental frequency only. Harmonics generate additional torque on these disks, causing improper operation and incorrect readings.
Electronic and computer-controlled equipment — Some electronic equipment depends on zero-crossing or voltage peaks for proper operation. Harmonics can alter these parameters, causing erratic operation and premature equipment failure.
Besides equipment damage, you could be smacked with stiff penalties from your power company for noncompliance with IEEE Standard 519. The IEEE specification requires that harmonic distortion of the current waveform be limited to 5%. However, some engineers feel that operating a plant with harmonic distortion this high can cause significant energy losses and shortened equipment life; and recommend that total harmonic distortion should not exceed 1.5% under normal conditions.
How can you minimize harmonics?
A power quality site survey can help you determine what, if any, power quality problems your plant has on both sides of the power meter. Most surveys require the installation of power quality monitoring equipment or software. Not only does the survey help determine the presence and the extent of harmonics, but it also reveals other power quality problems such as voltage sags, power interruption, flicker, voltage unbalance, transients, poor wiring, and poor or inadequate grounding.
Harmonics can be minimized — and to some extent prevented — by:
Designing electrical equipment and systems to prevent harmonics from causing equipment or system damage
Analyzing harmonic symptoms to determine their causes and devise solutions
Identifying and reducing or eliminating the medium that is transmitting harmonics
Using power conditioning equipment to mitigate harmonics and other power quality problems when they occur.
When the electrical transmission and distribution system acts as a conduit for harmonics, any user connected to the grid could be responsible for generating them. In this case, work with your utility to identify sources of harmonics and minimize their influence on your plant’s electrical system.
However, if harmonics are generated within your plant, it’s up to you to mitigate them effectively. Attacking the harmonics problem at the source is always the best way to go. At your plant, minimizing harmonics is better for your equipment and the price you pay for electricity. Beyond that, it is your responsibility to keep your harmonics from feeding back into the electrical distribution medium, thereby affecting power quality of others connected to the grid.
One way to minimize harmonics that are generated inside your plant is to reduce or eliminate them before they occur. Variable-speed drives traditionally have been harmonic-generating culprits. However, companies are designing drives that operate at reduced harmonic levels (Fig. 2).
A delta-wye transformer can be installed in parallel with a delta-delta transformer to effectively convert two synchronized 6-pulse VFDs to a 12-pulse application. Some drive applications are specified with line reactors and isolation transformers to provide additional inductive reactance that helps minimize harmonics.
Utilities use harmonic filters to minimize harmonics on their distribution systems. Filters can be used inside the plant as well. Typically, harmonic filters are either passive or active. Passive harmonic filters use inductors and capacitors to block harmonics or shunt them to ground, depending on the configuration and application. As frequency increases, the impedance of an inductor also increases, whereas the impedance of a capacitor decreases. Passive filters may become ineffective if harmonics change due to varying loads.
Line reactors and transformers are used for limited harmonic control with ac drives. However, most of them are installed to protect the drive from transients. Significant harmonic control can only be achieved when the inductor has been sized correctly, when the source impedance is low, or when the drive does not contain an integrated dc bus choke.
A passive harmonic filter may contain a series/shunt capacitor/inductor network and a series inductor or transformer. This type of filter often is added to an electrical system as a peripheral to a drive system. It must be tuned to the individual drive. Multiple drives require multiple filters.
Active harmonic filters are sometimes called active power line conditioners. Rather than block or shunt harmonic currents, active filters attempt to condition them. Active harmonic filters monitor and sense harmonic currents electronically, and generate corresponding waveforms to counter the original harmonic currents (See sidebar titled “How active harmonic filters work”). The generated waveform is injected back into the electrical supply to cancel the harmonic current generated by the load.
Ideally, electrical systems would be designed so that harmonics are not produced. Some equipment available today features circuitry that can reduce the generation of harmonics. Active and passive filters can help minimize harmonics.
Acknowledgements PLANT ENGINEERING magazine extends its appreciation to ABB, Inc.; EPRI; Rockwell Automation; Siemens Energy & Automation; and Square D/Schneider Electric for the use of their materials in the preparation of this article.
How active harmonic filters work
Typical active harmonic filters use analog power electronics and digital logic to sense and inject current, cancel harmonics, and provide reactive power. If sized properly, active harmonic filters can reduce harmonics below the limits specified in IEEE 519-1992 and improve power factor within your plant. Active harmonic filters cancel harmonics by dynamically injecting inverted (180-deg out of phase) current into the ac line, improving electrical system stability (Fig. A).
Generally, an active harmonic filter is installed on the ac lines in parallel to the loads that produce the offending harmonics (Fig. B).
For 3-phase, 3-wire power systems, current transducers (CTs) on two of the three phases provide the control logic with the shape of the current waveform just upstream of the load (Fig. C).
The active harmonic filter logic removes the 60-Hz fundamental frequency from this waveform. The resulting waveform is inverted and used to direct the firing of the insulated gate bipolar transistors (IGBTs). This inverted waveform is injected back into the ac line (Fig. D).
The result is a cancellation of the current harmonics in the upstream electrical system (Fig. E). Since voltage harmonics are the result of current harmonics flowing through source impedances, they are also dramatically reduced.
Modern active harmonic filters are designed using components similar to those found in VFDs, including power semiconductors, dc link capacitors, buses, and fuses. The IGBTs use pulse width modulation (PWM) at an appropriate switching speed. An internal filter blocks this frequency from entering the ac lines and decouples the active harmonic filter from the rest of the system so no harmful interaction occurs.
Most active harmonic filters are scalable and can be sized to control existing or anticipated harmonic current in a system for one or many loads. The rated output current of an active harmonic filter is equal to the square root of the sum of the squares of the harmonic and reactive currents at the bus. When the total harmonic current exceeds the rating of a single active harmonic filter, additional units can be installed in parallel. Active harmonic filtration is just one of the benefits of a power correction system.