Taming The Ubiquitous Nonlinear Load (August 2000)
As electronic hardware proliferates within the workplace, this equipment is more susceptible to, and sometimes the cause of, a building’s power-quality problems. Excessive distortion is manifested as voltage fluctuations and non-sinusoidal supply voltages within the building’s power distribution grid. While power-quality issues have been given little attention in the past, today it is widely recognized that current distortion must be controlled to protect the building’s power system and electronic office equipment. A building’s electrical system can tolerate only a limited amount of current distortion, and vsd’s can contribute to the problem.
As more and more variable-speed motor control devices are specified for chillers, hvac equipment motors, and auxiliary hvac pumping systems, an increasing amount of attention is being paid to ensure that vsd’s don’t have a negative impact on power quality.
This article discusses the basics of harmonic current distortion, explains how the design and construction of vsd’s impact the amount of harmonics generated, and describes the technologies currently available to mitigate these electrical anomalies.
Electronic Pollution: The Causes and Effects of Harmonic CurrentA nonlinear load uses high-speed electronic switches, such as diodes, thyristors, or transistors, to convert the alternating current (ac) supply voltage to a constant direct current (dc) level. Loads that utilize these electronic switches to convert ac to dc have the unwanted side effect of generating harmonic currents onto the building’s power grid. These harmonic currents in turn have the capability to cause harmonic voltages to be generated, and the net result is distortion – current distortion and its resultant voltage distortion. In extreme situations, distortion-derived problems caused by nonlinear loads such as vsd’s include overheated power cabling, burned-out neutral conductors, and the premature failure of a building’s power transformer.
Less obvious occurrences, but much more common, are nuisance problems. A typical example is electronic equipment that mysteriously shuts off or fails to operate reliably. Nuisance problems are often a warning that the building’s electrical system is being burdened by excessive current distortion. As more and more electronic equipment is added to the building, the problem will grow until the nuisance becomes intolerable.
The amount of current distortion that a building can sustain is a complex function of numerous variables, including the following:
- The number and types of loads (both linear and nonlinear);
- Capacitors placed on the electrical system for power-factor correction;
- Length and size of the cabling that spans between the building’s power source and the various building loads; and
- The building’s short-circuit capacity.
The percentage of total harmonic current distortion is a function of the following variables: the magnitude of the 50 Hz or 60 Hz (fundamental) current required by the load and the measure of the magnitude of current at frequencies other than the fundamental. The load’s harmonic current, expressed as a percentage of the load’s fundamental current, is the percent of total harmonic current distortion (% current THD). For example, suppose the current drawn by a load contained 10 amperes at a frequency of 180 Hz (this is the third harmonic of the 60 Hz supply — 3 times 60 Hz is 180 Hz), and 40 amperes of current at the power main’s frequency of 60 Hz. Then the % current THD of the current drawn by the load would be:
(10 amps ÷ 40 amps) x 100 = 25% current THD
The ratio of linear loads to nonlinear loads within the building will directly influence the % current THD seen at the utility interface. As the size of the vsd increases, the generated current distortion will have a more significant impact on the building’s power system. For instance, a 20-hp drive for a fan will not be as critical as a 500-hp drive for a centrifugal chiller. Additionally, the length of the power cabling, the existence and location of power factor correction capacitors, and the short-circuit capacity of the building all have the capability to influence system resonances within the building which can amplify specific harmonic currents and generate significant and damaging harmonic voltages.
Table 1 reveals the typical % current THD magnitudes and the actual input current waveshapes associated with various types of common electronic vsd’s. The baseline used for comparison is a typical induction motor operated directly from the building’s power supply. All comparisons are done at full load and full speed.
Cleaning Up Electrical Loads with Filter TechnologyTo avoid harmonic problems associated with the power source, the vsd should exhibit a % current THD and input current waveform similar to the standard induction motor. But can equipment manufacturers, building owners, and contractors reduce or eliminate sources of current distortion? The answer to both of these questions is undeniably, “Yes: Simply use ‘clean’ electrical loads.”
A low % current THD number implies a low level of current harmonics — and a clean load. A clean electrical load minimizes current harmonics. By limiting the amount of current harmonics each load is permitted to generate, the worries associated with voltage harmonics are reduced. How can this be accomplished?
Some electronic vsd manufacturers provide computer programs that attempt to analyze the building’s power system to determine if adding their product’s nonlinear load will push the building “over the edge,” rendering it unreliable. Often, these analytical programs indicate that an inductor or isolation transformer is needed to isolate the nonlinear load from the rest of the power system.
But beware: These programs use a very simplified approach for the analysis of a very complex problem. Manufacturers are aware of this and usually attach a disclaimer, which limits or totally eliminates their liability. That’s because using inductors and transformers merely applies a bandage to the wound. They do little to reduce the harmonic current distortion to a level that brings true peace of mind. Inductors and transformers deal with the existing problem; they do not eliminate the root cause.
As Table 1 indicates, the standard induction motor is the cleanest load; it has been this way for the past 100 years. The magnitude of the inductor (or reactor) used within the drive may alter the level of % current THD of the load by a small degree. But inductors and transformers alone, no matter whether 6- or 12-pulse technology is used, cannot match the extremely low level of % current THD of the induction motor.
Active harmonic filters are electronic filters that eliminate the root cause of harmonic distortion problems: current distortion. Active filters are much smaller than passive filters, and by combining state-of-the-art power semi-conductors with signal processing technology, the performance of an active filter can far surpass the capability offered by any passive technology. Active harmonic filters are designed to supply the harmonic currents required by nonlinear loads. By including this technology within the enclosure of the offending nonlinear load, the usual harmonic currents associated with that load are effectively blocked from flowing outside the boundary of the offending equipment. This eliminates the root cause of harmonic problems at the source, and eliminates any problems associated with distortion on the building power grid.
Active Filter Technology Surpasses Passive TechniquesActive Filter Technology Surpasses Passive Techniques Active filter technology (AFT), which is optional on some vsd-equipped chillers, offers an alternative to passive, external harmonic filtration. By using state-of-the-art electronics, AFT surpasses such passive techniques as transformers, inductors, and capacitors to maintain clean, low-distortion power lines.
As a result, AFT provides an extremely low level of % current THD. AFT even rivals the % current THD benchmark exhibited by the standard induction motor. Table 2 illustrates the results of this technology breakthrough.
In many cases, current THD levels of 30% or lower are acceptable. As Table 2 shows, even without AFT, the base vsd employs a six-pulse converter with a large inductive and capacitive dc filter to provide a nominal, “middle of the road” level of % current THD. With the typical vsd, no optional inductors or transformers are needed to achieve this basic level of % current THD protection.
AFT provides an unprecedented level of security against power line harmonic troubles. A vsd with optional AFT can help maintain your building’s power system integrity and help your building comply with the IEEE 519-1992 Standard (see sidebar).
ConclusionUtilization of the % current THD as a harmonic distortion benchmark is the simplest way to avoid problems with harmonics on the power grid. As explained above, control of the current THD will ensure that voltage harmonics are kept in line. Keeping the % current THD of all nonlinear electronic loads to a minimum is the best method for avoiding troubling harmonic problems.
It is not sufficient to simply specify that line reactors or some other type of harmonic reduction should be applied. As shown in Table 1, the reference point can be dramatically different depending on the drive technology used. A specification should be based on % current THD, with a 30% maximum being sufficient for many applications. If the highest level of protection is required, the drive should be specified to meet a % current THD of 5% maximum at full load. ES
The Standard (519) IssueThe Institute of Electrical and Electronic Engineers (IEEE) has studied the problem of voltage and current distortion for over 18 years. To assist building owners and contractors who are concerned about the integrity of the power system, the IEEE issued a “Guide for Applying Harmonic Limits on Power Systems,” which was first published in 1981 as IEEE 519–1981. This standard focused on the issue of the system voltage, placing limits on parameters such as notch depth and the percent total harmonic distortion of the voltage waveform seen by other utility customers (the point of common coupling).
A few years after its first publication, the standards committee changed its focus from voltage distortion to current distortion. The reason for this change was simple: current distortion is the root cause of voltage distortion. Voltage distortion is the result of supplying harmonic load currents from a power source containing a finite impedance.
Since all power distribution systems have a finite impedance (they cannot source infinite current), the harmonic voltage dropped across this impedance is distributed throughout the building. In many cases, this distortion is also carried by the common incoming power feed beyond the offending building to neighboring buildings. If current distortion can be limited (or better yet eliminated), the system’s voltage distortion will also be reduced (or eliminated), and the reliability and integrity of the power source can be maintained.
The updated version of this standard, published in 1992 (IEEE 519–1992), addresses the source of the problem: current distortion. The standard places strict limits on the level of current distortion permitted at the utility interface. The only variables that remain in this latest standard are knowledge of the short-circuit capacity of the building and knowledge of the building’s peak-demand requirement.