Harmonic Distortion in Electrical Systems
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providing insights for today’s hvac system designer Engineers Newsletter volume 47–1 A primer for non-electrical engineers Harmonic Distortion in Electrical Systems The term harmonics is used to describe a Voltage is determined at the The quest to lower electrical energy distortion in the fundamental voltage and/ transformer serving the building. Many consumption of HVAC and other or current waveform supplied from a utility voltage choices are possible but once electrically-driven equipment has led or generator. In technical terms it's a fixed by the transformer the voltage to the introduction of 'non-linear' electrical loads to the electrical grid. mathematical way to describe the downstream of that transformer remains Harmonic distortion caused by distortion. In a practical sense it gives us relatively constant. There are factors increasing non-linear loads can result terminology to talk about the problems, which will alter the average voltage but in issues in a building's electrical both potential and real, due to the these tend to be short term. system. proliferation of energy saving devices. This newsletter provides a simplified Current, or amperage, depends on the explanation of the causes of harmonic supplied voltage and the electrical loads distortion by taking the reader through in the building. For a given building, as some electrical system basics and Start with the basics the electrical load increases so does the moving on to what harmonic distortion current flow. A combination of current, means and why it matters. It's intended for those with little or no Before we talk about the distortion let's voltage, and power factor are used to experience with electrical systems. back up and look at what is being determine the power used by the distorted. Distortion can happen in any building. electrical system regardless of how the Frequency is determined on a country- power is supplied to the system. For this by-country basis. The United States, for discussion we assume electrical power is example, uses 60 Hz, other countries being supplied to the building from the may use 50 Hz, but within a distribution common electrical grid. Harmonics on grid the utility supplying the power will systems supplied by onsite generators stay with one frequency. This frequency have some unique problems as discussed is called the fundamental frequency. It in an earlier Engineers Newsletter, "How is stable and consistent even when VFDs Affect Genset Sizing", volume 35-1. voltage or current change. Power is supplied to most buildings from an electrical utility. The utility provides power via an electrical distribution grid with wires going to each building. The key components of the supplied power are the voltage, current, and frequency. ©2018 Trane. All rights reserved. 1 Figure 1 shows one cycle of a Figure 1. One cycle of a 60 Hz periodic waveform fundamental 60 Hz waveform. It's called a periodic waveform because of the repeating nature. The horizontal axis is time. As time passes the wave repeats over and over in the same shape. The shape can be mathematically described as a sine wave. As shown in Figure 2 each complete cycle of the wave represents 360 time degrees of rotation. Counting the number of complete wave cycles per Figure 2. The number of complete wave cycles per second yields the frequency of the wave second yields the frequency of the wave. The Y axis is used to define one wave cycle magnitude. Alternating power, or AC power, means that the voltage supplied varies 0 90 180 270 360 90 180 270 360 between positive and negative values as (0) (0) shown in Figure 3. This defines the fundamental voltage waveform supplied to the building. one wave cycle The final piece for a basic understanding of power supply is the current signal. Figure 3. Positive and negative voltage variation in alternating power The utility defines the fundamental peak value frequency and voltage but the current + signal is dependent on the load. The relationship between the voltage waveform and the current waveform is 0 dependent on the type of electrical load. time This relationship is key to understanding how harmonic currents are created. - peak value Figure 4. Waveform difference between current and voltage magnitude (resistive load). Types of electrical loads v Linear loads draw current evenly and in i proportion to voltage throughout the duty cycle; the sinusoidal waveform of v the incoming power remains intact. i There are three types of linear loads. We'll start with resistive loads. Electric resistance heaters are a common example of a resistive load. For these time loads the waveforms for voltage and Figure 5. Current signal that is shifted slightly from the voltage signal (inductive load). current are different only in magnitude as shown in Figure 4. v Inductive loads, e.g., common i electrical motors, result in a current signal that is shifted slightly (Figure 5) v from the voltage signal. This shift is called lagging because for a given point i on the time scale, the current waveform passes through that point after the voltage waveform passes the same point. time 2 Trane Engineers Newsletter volume 47-1 providing insights for today’s HVAC system designer A third type of linear load is capacitive Figure 6. Resistive loads always consume positive power loads. A capacitive load shifts the current signal to lead the voltage signal. There aren't many work-producing loads that have a capacitive character but capacitors are sometimes added to power electrical systems to balance the inductive loads. When the voltage and current 90º180º 270º 360º waveforms line up, as they do with resistive loads, the voltage multiplied by current current is always positive (Figure 6). However when the voltage and current waveforms are shifted, as with voltage inductive loads, there are occasions when the product of voltage times current is negative (Figure 7). The negative portion (caused as stored Figure 7. Current and voltage waveforms shifted (inductive) consume positive and negative power. energy is released) doesn't contribute to the positive work done by the load. current lagging the voltage by 30º The non-productive power is indicated by the displacement power factor. Adding capacitors to systems with power inductive loads improves the average displacement power factor of the power system by shifting the combined waveform toward unity. 90º180º 270º 360º Displacement power factor is defined as the ratio of positive work actually done current (true power) to the positive work that would have been done if the waveforms aligned. voltage When voltage and current waveforms To a "non-electrical" engineer this work, this current still needs to be are not aligned some fraction of the concept may not make sense. To better generated and transmitted by the utility current isn't doing positive work. The understand, it's helpful to think of company. extra current must be generated by the inductors and capacitors as energy utility and transmitted through the storage devices. They affect the current Non-linear loads distort the original electrical distribution system even by temporarily storing some of the current and voltage waveforms by though the current isn't doing positive energy internally. An inductive load, drawing current in instantaneous pulses work. Anytime current travels though such as a motor, inherently stores that are disproportionate to voltage. the electrical grid there are losses energy as the voltage approaches the associated with the resistance of the positive or negative maximum. As the Switch-mode power supplies (SMPS), system. voltage drops back toward zero, the found in computers, servers, monitors, stored energy is released back onto the printers, photocopiers, telecom Although we’re discussing linear grid delayed in time. systems, broadcasting equipment, and electrical loads, the concept of current variable-speed motors and drives, are flow that doesn't do positive work is A capacitor works just the opposite. By examples of non-linear loads. Single- important to understand. shifting the current value in time relative phase, non-linear loads are prevalent in to the voltage, these devices affect the office equipment, while three-phase, current flow without doing any actual non-linear loads are widespread in larger work. As stated earlier, even though the electrical systems. shifted current isn't doing any positive providing insights for today’s HVAC system designer Trane Engineers Newsletter volume 47–1 3 Non-linear electric loads are Figure 9. Resultant waveform for the combination of fundamental and 3rd harmonic characterized by a non-constant resistance during the applied voltage waveform. Because the resistance is not constant the resulting current waveform fundamental 60 hz waveform does not match the applied voltage resultant waveform waveform. Each of the various non-linear 180 hz waveform (3rd harmonic) loads have a unique resistance characteristic, and thus, a unique current waveform shape. The common SMPS load consists of a 2-pulse (full wave) rectifier bridge (to convert AC to DC) and a large filter capacitor on its DC bus. This load draws current in short, high-amplitude pulses Harmonics. As mentioned earlier, the same sign, e.g., both are positive, the that occur around the positive and presence of harmonics in electrical magnitudes add. When the waves have negative peaks of voltage. The resulting systems means that current and voltage opposite signs the values subtract. The current waveform is shown in Figure 8. are distorted and deviate from sinusoidal result is the dotted blue wave. waveforms. Figure 8. Common SMPS current waveform The harmonic frequencies are always To demonstrate we’ll start with a integer multiples of the fundamental. fundamental 60 Hz sine wave, similar to Figure 10 shows the resulting waveform the one shown in Figure 1, and add a when second, third and fourth harmonics second sine wave with a frequency of 180 are added to the fundamental waveform.