How can you minimize harmonic distortion? Is the concern financial, or is your equipment at risk? Here are some basics you need to know.
With so much equipment and knowledge available in this industry, specialization is sometimes necessary. But even with specialized training, many new technicians still lack the hands-on experience to troubleshoot power quality problems. Added to these concerns, we're seeing more standards, guidelines, and operating instructions for power quality than most of us have time to study.
While utility deregulation and the popularity of AC/DC drives are thrusting power quality into the limelight, competition among utilities has heightened the concern of penalties from high distortion levels. How can you minimize this distortion?
First, let's understand what we're really talking about when we speak of power quality problems. Though we have many culprits for poor power quality (harmonics, voltage spikes, voltage unbalance, and power factor), we can usually blame these problems on voltage and current harmonic distortion.
You can understand harmonic distortion if you break it down into fundamentals. The most common reference is total harmonic distortion (THD). THD is the ratio of the root-mean-square of the harmonic content to the root-mean-square value of the fundamental quantity, expressed as a percent of the fundamental. Simply put, it's the rms value of the signal with the line frequency (fundamental) removed. A perfect 60 Hz sine wave would have 0% THD; so anything other than the fundamental line frequency is a harmonic distortion.
Harmonic distortion is not difficult to understand, if you remember distortion is comprised of signals other than the 60 Hz fundamental, as well as harmonic multiples of that same 60 Hz signal.
Positive, negative, and zero sequence harmonics exist in all 3-phase systems. Positive sequence harmonics creates a magnetic field in the direction of rotation. The magnetic field the fundamental harmonic develops must be in the direction of rotation. Otherwise, the motor would run backward. So, the fundamental is a positive sequence harmonic.
Negative sequence harmonics develop magnetic fields in the opposite direction of rotation. This reduces torque and increases the current demand required for a given load. The rotation of the magnetic field the second harmonic develops is in reverse order. Rather than advancing in the order of 1-2-3, its sequence is 3-2-1.
Zero sequence harmonics creates a single-phase signal that does not produce a rotating magnetic field of any kind. Though this signal performs no real work, it can still increase overall current demand and generate heat.
You can more easily understand sequence harmonics by studying the basic relationship between these sequences. The positive, negative, and zero sequences simply repeat.
Harmonic frequencies are primarily the result of nonlinear (switching) loads, such as computers, fluorescent lighting, and variable frequency drives (VFDs). The presence of harmonics in a distribution system results in excessive heat from the increased current demands. A load designed to pull 100A at full capacity may now draw 120A if the harmonic distortion is high.
Excessive zero sequence harmonics collects back at the transformer, leading to overload and possible failure. It returns to the source through the neutral bus, and if excessive, can generate substantial heat and even fires.
In an effort to avoid such catastrophic events, many companies modify their distribution systems. Two popular options are to install k-transformers, which handle the larger loads generated by harmonics, or increase the mil size of their neutral to accommodate larger current levels. Though these efforts do nothing to diminish harmonics, they do reduce the failure risk. Removing harmonics requires the installation of filtering mechanisms.
Some of the newer VFDs that use IGBTs can exceed line voltage by a tremendous amount in less than a microsecond. Older, Class B insulation systems have low tolerance for this rapid rise-time and can fail quickly.
IEEE 519 power quality standards provide in-depth references to the legal levels of harmonic distortion. The IEEE 519-1992 standard for power quality states the current distortion at the point of common coupling (PCC) should not exceed a predetermined value. Though this sounds straightforward, do you know where the point of common coupling is? And when was the last time you identified where the current transformer (CT) circuit is, not to mention, the CT ratio? Unfortunately, as in-depth as this is, it's still difficult to determine how to apply this information to individual situations. Simply put, voltage distortion is the primary concern.
Current distortion is given little attention in the standards. Current distortion, however, is limited by specific standards at the point of common coupling (PCC). The PCC is the location in the utility distribution system that splits to provide power to separate customers. The standards should prevent the poor quality of one customer's power influencing another's through the PCC.
General guidelines in Table 3.3.1 of IEEE 519-1992 recommend less than 5% voltage THD for systems operating at less than 69kV. They further recommend the individual harmonic voltage distortion to be less than 3%.
Experience is the best teacher when it comes to power quality. Passing on this expertise to new employees in the power quality field is the best way to keep it from disappearing with retirement.
Bethel is a Product Development Manager with PdMA Corp., Tampa, Fla.