Variable frequency drives (VFDs) are uniformly present in modern industrial plants. Their presence is the result of increased awareness of the environmental effect of high power consumption and the need to reduce the cost of energy.
From the viewpoint of the utility-supplied power system, the typical 6-pulse VFD, which is a nonlinear load, draws a distorted current like the one represented by the waveform in Fig. 1 at right. This waveform is very rich in 5th and 7th harmonics and contains lesser amounts of the 11th and 13th harmonics. Higher order harmonics are present in smaller quantities and only cause problems in rare cases where high order resonant conditions exist in the system.
Various techniques have been developed for reducing harmonic currents because they don't produce useful work and generate losses in the power system. Consulting and specifying engineers are tasked with balancing the capital costs in harmonic mitigation equipment against the effectiveness of the proposed mitigation method. There's a clear need for solutions that are flexible, scalable, and easily designed and applied.
IEEE 519 and harmonic limits. Technical literature extensively discusses the multitude of problems caused by harmonic currents. Specifically, it's common knowledge that harmonic currents that flow toward the 60 Hz power supply encounter the impedance inherent in the system and produce distorted voltages. IEEE Standard 519 — 1992, “Recommended Practice and Requirements for Harmonic Control in Electrical Power Systems,” provides specific limits for current and voltage distortion at the point of common coupling (PCC), which is the point of interconnection between two customers of a utility company (Fig. 2).
However, no industry standard exists for current and voltage distortion limits of the internal electrical busses of something like a water treatment plant. The consulting and specifying engineer may specify the limits for internal busses, such as motor control centers that supply drive loads, based on first-hand experience. But some choose to specify the same limits as noted in IEEE Standard 519, which is a conservative specification for an internal bus.
Applying the IEEE Standard 519 limits to the individual drives is an even more conservative approach, forcing drive manufacturers to supply 18-pulse drives or incorporate broadband filtering on individual drives. Proponents of this method may argue that when individual drives meet the harmonic limits, the total system is guaranteed to meet the limits. But this guarantee adds a significant capital cost to the plant.
Harmonic mitigation strategies. The challenge of mitigating harmonics is designing the most cost-effective solution that will help the system meet the imposed limits. Let's look at some strategies that may or may not meet this objective.
Add inductance. The oldest as well as most effective and economical harmonic mitigation strategy is the addition of inductance (typically 3% to 5% on the drive base) in the individual drive circuits. This added inductance (in the form of a reactor or transformer) directly reduces the amount of harmonic currents produced by the drive (Fig. 3). Reactors are hardy, durable, and economical, and their application may be sufficient to reduce the harmonic distortion at the motor control center to acceptable levels.
Use of phase-shifting transformers. Delta-wye connected transformers are the second best way to address harmonic mitigation if the system's design is such that multiple 6-pulse AC drives operate at the same time and at similar load levels. However, this strategy isn't effective where different drives cycle on and off individually. The delta-wye transformer produces phase displacement in the current such that the 5th and 7th harmonics are shifted by 180°. With a judicious combination of delta-wye transformers and reactors, it's possible to cancel the 5th and 7th harmonic currents at the motor control center (Fig. 4).
Use of passive (tuned) filters. Individually tuned filters draw harmonics at their tuned frequency but inject reactive volt-amperes (VAR) into the system at a frequency level of 60 Hz. However, modern voltage source drives run at close to 100% displacement power factor and the motor control center bus may not need power factor correction. The net effect is that the filter system may never come on line because the need for reactive VARs may not rise high enough to engage the filter control system. Furthermore, loss of capacitance due to blown fuses or damaged capacitor cells may shift the tuned frequency of the individually tuned filter, putting the filter in a resonant condition with the power system.
Use of active filters. If the inductor and inductor-transformer strategies don't reduce the harmonic levels below the specified values, you can add active filters to control the current distortion on the motor control center bus. The active filter is a shunt-connected device composed of insulated gate bipolar transistor (IGBT) switching devices. It determines the exact current that would cancel the harmonics present in the circuit and synthesizes and injects that current into the system so the source doesn't have to supply distorted current (Fig. 5).
By comparison, individually tuned passive filters are primarily effective at their tuned frequency. Their effectiveness diminishes at other frequencies. It's possible to install multiple passive filters to remove the 5th, 7th and 11th harmonics, but such a strategy isn't always workable or effective economically.
Active filters, on the other hand, can be set to cancel all harmonics, to correct power factor, or both, giving you full control of the filter. These filters will never exacerbate or produce resonant conditions in the power system.
Which system will you specify? Because no uniform harmonic mitigation standard exists for the busses inside a plant, the specification of limits and the choice of technology are your prerogative, with system design and component selection that best meets the needs of your client.
A cost-benefit analysis will most likely show that inductors are the best choice as the first line of defense. You may be able to use phase-shifting transformers in conjunction with inductors to cancel the 5th and 7th harmonics where the drive units operate simultaneously. If inductors and inductor-transformer combinations aren't sufficient to reduce harmonic levels to acceptable values, you can strategically deploy active filters on the motor control center busses. You can effectively size these filters to remove the remaining harmonics on the bus.
This methodology simplifies the design of harmonic mitigation and produces a cost-effective system.
Tajali, P.E., is senior staff engineer of power systems engineering at Square D/Schneider Electric in Nashville, Tenn.
