- By John Cherney
- Mar 01, 2004
Water treatment plants and pumping stations supported by variable-frequency drives (VFDs) may be unwittingly threatened with power system problems including interruptions, interference, downtime and even disruption of instrumentation and other sensitive equipment due to an elusive culprit called harmonic distortion, an undetected condition that has been said to cost between $16 to $40 billion in annual losses due to downtime, equipment failures and malfunctioning systems.
Although harmonic distortion is hard to see and measure, it is no mere gremlin. It is a very real problem that often accompanies the installation of equipment with motors controlled by VFDs, which are an integral part of today's water and wastewater power systems because they save energy, provide added control and reduce mechanical and electrical stress during starting and stopping of loads. However, unless certain precautions are taken, VFDs also can induce harmful harmonic distortion.
Harmonic distortion can cause incorrect meter readings, nuisance tripping of zero sensing circuits, motor bearing failure in aeration blowers and pumps, blown fuses on power factor-corrected systems and interference with telephones and other communications systems. The potential for most such problems is undetected -- until the equipment fails.
Potential for Major Trouble at Water Treatment Plants
Equipment failures and disturbances are particularly perilous for water treatment plants. These plants typically treat millions of gallons daily using high horsepower, high harmonics-producing VFD loads intermixed with precision controls. The components that make up the wastewater flow that is treated by municipal wastewater treatment plants typically include the following: domestic wastewater (sewage), industrial wastewater discharged after preliminary treatment by industrial facilities in the community, stormwater runoff and infiltration/inflow (which is water that enters the sewer system through leaking joints, cracks, breaks or porous walls). Since these environments are highly automated and operated with small maintenance staffs, operational problems caused by harmonic distortion, such as the disabling of an aeration blower or faulty measurement of solid waste, can spell major trouble.
Pumping stations also potentially face problem amounts of total harmonics because virtually their entire electric power load comes from VFDs. Since pumping stations often are located in commercial or industrial areas, the harmonic distortion they generate often can induce power system disturbances that affect nearby businesses and homes. This, of course, produces a stream of irate complaints from the community -- and, quite possibly, subsequent interruption of pumping station operation.
What is Harmonic Distortion?
Harmonic distortion is a complex condition, and an elusive one. Power system technicians can't see harmonic distortion with a standard meter, but they can readily bear witness to its effects, which include increased overheating of conductors, motors, transformers and capacitor banks; sensitive instrumentation disruptions; equipment damage; and production interruptions.
Essentially, harmonic distortion is electrical noise that occurs in power distribution networks. This noise is created by alternating current (AC) VFDs with non-linear power loads, loads that draw current with a waveform that does not conform with the shape of the supply voltage waveform.
Understanding the meaning of the terms "linear loads" and "nonlinear loads" is important to understanding harmonics distortion. Linear loads are loads that draw current with a waveform that is the same shape as the supply voltage waveform. A relevant example of this is a three-phase induction motor where the shape of the current wave and the voltage wave are the same. Three-phase motors are linear loads and, as a result, the current and voltage waveforms take the same shape. Nonlinear loads, however, are loads that draw current with a waveform that is a different shape than that of its supply voltage. The most relevant example of this is the VFD.
With VFDs, the shape of the current wave is different from the voltage wave. Thus, it is said that VFDs cause distortion of the AC line simply because they are nonlinear loads -- that is, they don't draw sinusoidal current from the line. For a standard six-pulse diode bridge rectifier, as typical for most pulse wave modulation (PWM) inverters, the waveform takes a slightly different form.
How Do VFDs Distort Waveforms?
VFDs, as well as direct current (DC) adjustable speed drives, lighting ballasts, uninterruptable power supplies (UPS) and other nonlinear loads, have dissimilar, nonlinear voltage and current waveforms. VFDs draw their current from only the peaks of the AC line.
This is explained by understanding a basic PWM VFD circuit. The incoming AC supply voltage is rectified (converted to DC voltage) by a diode bridge consisting of six (or more) diodes, each responsible for a positive or negative half cycle of one of the three incoming phases (six total half cycles). This energy is stored in a capacitor called a DC bus or DC link. This capacitor, in turn, is discharged by current flowing to power the motor.
The three-phase input feeds a six-diode (rectifier) bridge that charges the DC bus. Six "pulses" of current flow during each cycle of the three-phase AC sine wave, hence the name six-pulse VFD.
A pulse of current flows into the VFD whenever the input voltage sine wave exceeds the DC bus voltage. When additional diodes are installed and fed by a phase-shifting voltage source 12-, 18- and 24- pulse systems are the result. In these systems, the number of pulses is increased while the magnitude of each is reduced and the total harmonic current is reduced.
What do Harmonics Look Like?
The term line harmonics is defined as sinusoidal frequency components contained on the line voltage or current waveform. Each component is an integer (2,3,5,7...11) multiple of the line input frequency. A pure sine wave has no harmonics. When a wave becomes distorted, we say that harmonics are present in this distorted waveform. The harmonics of primary concern from a PWM VFD are the 5th, 7th, 11th, 13th and so on. The frequencies of these harmonics are 300 hertz (Hz), 420 Hz, 660 Hz and 780 Hz. This set of harmonics can be classified as low-order harmonics with frequencies below 1.0 kilohertz (kHz).
These waves of different frequencies are simply added to each other to form a resultant waveform. The actual (resultant) waveform appears on an oscilloscope as a single, distorted wave. (See Figure 1 below
Current and Voltage Harmonics
Now that we understand where harmonics come from and what they look like, it is essential to distinguish between two very different types of harmonics: current harmonics and voltage harmonics.
The Institute of Electrical and Electronics Engineers (IEEE) has attempted to define acceptable levels of current harmonic distortion and voltage harmonic distortion. These limits are now the basis by which many systems are defined. The intent of IEEE Standard 519 is to make users responsible for limiting current harmonics injected into the power system, hence limiting voltage distortions.
- Current Harmonics. We saw above that current harmonics are actually generated by the pulsing current flow into the VFD. It can be said that the VFD injects current harmonics back onto the AC line.
- Voltage Harmonics. These are not created directly by the VFD but by the flow of the harmonic current through the source impedance of the distribution system, thus causing voltage distortion. As a result, the line side of a VFD becomes a combination of both current and voltage distortion with very dynamic interrelating effects. An exception to this is the VFD with an active front end. In this case voltage harmonics are directly added to the AC line.
Harmonic Frequency Categories
Further subdividing current and voltage harmonics are the frequency at which they exist. Three general categories are defined below.
- Low order - 300 Hz to 1000 Hz. The highest magnitude of harmonics in at the VFD input is considered low order. The harmonics of primary concern from a PWM VFD are the 5th, 7th, 11th, 13th and so on. The frequencies of these harmonics are 300 Hz, 420 Hz, 660 Hz and 780 Hz. These low-order harmonics generally are the largest magnitudes present on the line.
- High Order - 1000 Hz to 150 kHz. In most systems, higher-order harmonics, such as the 17th (1020Hz), 19th (1140 Hz), 23rd (1380 Hz) and so on, will be of lower magnitude than those mentioned above.
- Radio frequency - 150 kHz to 30 MHz. Clearly the lowest magnitude harmonics in any VFD system. Harmonics in the radio-frequency range do not contribute to the overall harmonic levels as defined by IEEE-519. Though these components should not be overlooked and some systems are very sensitive to these small amplitude harmonic distortions, radio frequency should be dealt with separately.
Although there are many different effects of harmonic distortion that may result in potential problems, the single most damaging effect of the current harmonics caused by many VFDs is the excitation of system resonance, which causes instability. IEEE Standard 519 states: "System resonant conditions are the most important factors affecting harmonic levels."
Most power-system environments are continually in a state of flux, where new technologies are being introduced by adding new production equipment. In many cases this equipment is highly sophisticated, employing computer-controlled operations and microprocessor-controlled monitor systems that are highly dependent on clean power. Yet, the companies installing the equipment may not have the experience or personnel to understand how it affects their power system.
Too often, this results in harmonic current distortion that destabilizes the power system and equipment attached to it. Because the users have no way of metering total harmonic distortion (THD) within their system, they figure the problem must belong to the power company.
Efforts to manage the rampant harmonic distortion that came with the influx of VFDs are what lead to the writing of standard IEEE 519. Because users' power distribution systems that have severe harmonic distortion were causing problems for electric power suppliers, the IEEE decided to pressure those users to clean up their systems through power conditioning or the use of VFDs that create lower amounts of harmonic distortion. Otherwise, they will continue to experience the consequences of power system problems and, in the future, possibly pay increasingly stiff penalties for non-compliance.
This economic need for clean use power distribution quality is amplified by the potential for power factor penalties. Another incentive for incorporating VFD technology into the distribution environment is to lower power factor. Yet, companies with erratically performing equipment due to harmonic distortion problems are subject to higher power factor charges.
Measuring and Reducing Distortion
Harmonics can be measured with handheld meters designed exclusively for this characteristic. Most facilities own more popular standard meters that only measure the fundamental frequency component that is the useable part of the total harmonic spectrum. Harmonic meters are considerably more expensive but are quite simple to use.
Many standard harmonic reduction solutions are available, including reactors, isolation transformers, filters, active devices and multi-pulse VFD packages. All have their strengths and weaknesses, and should be carefully considered by qualified engineers after an analysis of power system characteristics -- often a complicated and expensive affair.
Although applying component-level solutions provides some level of harmonic reduction, they do not necessarily meet the strict guidelines of IEEE 519, not to mention adding to overall equipment costs, further complicating the system -- even causing harmful leading power factor and system resonant problems.
Another method of keeping harmonic distortion within safe and IEEE 519-compliant levels is to use VFDs with a sufficiently high "pulse count." Most general-purpose VFDs employ a six-diode input power section, and are hence called six-pulse VFDs. Pulse "multiplication" is achieved in six-pulse increments, such as 12, 18, 24, etc. The six-pulse and 12-pulse systems will not meet harmonic levels outlined in IEEE-519; and, if either is installed, additional harmonic-reduction devices are required. The 18-pulse VFD design is a self-contained solution that typically reduces THD to less than 5 percent -- well within IEEE 519 compliance.
Eighteen-pulse VFDs are available from several suppliers and the technology is well known throughout the industry. The differentiation between these various VFDs comes down to each manufacturer's unique design. Saftronics, a Fort Meyers, Fla.-based company, manufactures a package that is designed to reduce harmonic distortion to meet IEEE 519 requirements. The package minimizes interference that can impair sensitive electronic equipment such as computers, sensors and communications devices. At the same time, it stabilizes power factor to help users avoid leading power factor complications and utility penalties based on power factor and peak demand charges. But whether you choose an 18-pulse VFD or another option, ridding your plant of the nuisance of harmonic distortion can be a harmonious action at your water treatment plant.
- Electrical Power Research Institute's Power Electronics Applications Center (EPRI
PEAC) -- www.epri-peac.com
This article originally appeared in the 03/01/2004 issue of Environmental Protection.