Manufacturers must produce an exploding array of products faster than ever while reducing costs and increasing operational efficiencies to previously unreachable levels. To meet these demands, companies are increasing the amount of automation within their plants. While this additional electronic equipment is helping boost production, it is also contributing to an increase in harmonic distortion problems. Variable frequency drives often comprise a large portion of this new equipment, so drive users are taking a close look at harmonics issues and evaluating potential mitigation strategies.
Harmonics are deviations from the base AC line voltage and current waveforms. Most electrical circuits in North America operate at 60-Hz frequency. A harmonic frequency is a multiple of this base frequency. In a 60-Hz system, for example, the second harmonic would be 120 Hz, the third would be 180 Hz, and so on. Any frequency other than the base creates distortion. The more harmonics present, the more distortion in the electrical waveform.
That's the technical definition. In its most basic description, however, harmonic distortion is simply electrical noise. Non-linear loads, which are drawing power throughout a facility, create this noise. If the plant’s non-linear load is significantly larger than the linear load, there is a potential for harmonic distortion problems.
While drives certainly contribute to harmonic problems, they aren't the only causes. In fact, any equipment that converts alternating current (AC) to direct current (DC) can create harmonic distortion. This includes many of the systems used on the plant floor, such as welders, arc furnaces, process controllers, lighting systems, DC power supplies and uninterruptible power supplies.
Many industrial operations, such as electric welding, can generate harmonics.
Harmonics problems usually aren't catastrophic, but they can cause equipment overheating, erratic system operation, nuisance tripping and noise transfer to other loads. This can lead to equipment damage and production downtime. In extreme cases, utilities can impose penalties on power users or even disconnect a facility from the power grid until harmonic problems have been resolved.
Clearing the confusion
Although awareness and concern about harmonics has grown considerably in recent years, it’s often misunderstood, inaccurately diagnosed and ineffectively mitigated. Therefore, before rushing out and purchasing additional hardware to solve a perceived harmonic problem, it's important to first address a few basic questions.
- Is there actually a harmonic problem within the system?
- If so, exactly what is causing the problem?
- Based on the application and the identified cause, which of the many solutions is the best one?
As the number of drives used in automation systems grows, so too does the misconception that they are always the single largest cause of harmonic problems. While drives comprise a sizable portion of non-linear loads, they are only one element in the harmonic equation. That's why, before making any final conclusions, it's important to analyze every electrical load that has the potential to cause problems. The bottom line is that not all installations have a problem, and not all problems are drive-related. In fact, there are hundreds of thousands of drives operating in the field without necessitating harmonic mitigation.
Because utility power feeds supply multiple customers, a harmonic problem in one location can severely distort the power supply and cause problems for others. IEEE519-1992 is the North American harmonics standard for power utilities, and is often imposed on large consumers and medium voltage systems. Total harmonic distortion limits are recommended based upon the type of installation. This specification also has become common in low-voltage systems.
The ability to measure harmonics is important for isolating the cause and recommending potential remedies. In many cases, utilities use special meters that are connected to the power lines feeding a plant to measure the total harmonic distortion. Other instruments, such as high-speed spectrum analyzers, determine the waveforms and help isolate the root of a harmonic problem. Once users have identified the problem, they can implement a number of approaches to reduce or eliminate the effects of these unwanted frequencies (harmonics).
Eliminating bad vibes
As the occurrence of harmonics becomes more prevalent, drive manufacturers are offering more advanced solutions to address the problem. Some are small and inexpensive, while others require additional hardware and are more technically complex. While many of these solutions exceed IEEE 519-1992 requirements and may provide added benefits, such as improved power factor, it's important to evaluate the requirements of the application and select the solution that offers the best fit.
A typical plant installation may include drives alongside other electrical elements
Drives using pulse width modulation technology with the least distortion today are 12- and 18-pulse AC variable frequency drives. They produce little transient noise and can offer a total harmonic distortion of less than three percent. And with a 0.95 power factor, or better, these drives eliminate the need for power factor correction capacitors and tuned inductors.