Answers .Power . .Quality . Volume 2 Number 2 February 1995 Introduction flow towards the power source for one half of the cycle and away from the power source for Concerns about the effects of lighting prod- the other half. At 60 Hz, the voltage wave ucts on power distribution systems have completes a cycle every 1/60th of a second, focused attention on power quality. Poor or approximately every 17 milliseconds (ms). power quality can waste energy and the Problems with a utility’s generators or capacity of an electrical system; it can harm distribution system can cause serious power both the electrical distribution system and quality problems such as voltage drops and devices operating on the system. transients, both of which can reduce the life The National Lighting Product Information of lighting systems and other electrical Program (NLPIP) prepared this issue of equipment. High levels of distortion (devia- Lighting Answers to help lighting specifiers tion from a sine wave) in the distribution and consumers better understand power system can also harm electrical equipment. quality, so that they can more confidently Unlike voltage drops and transients, howev- select energy-efficient lighting products. er, distortion often is caused by electric devices operating on the system. For a specific electric device, the term What is power quality? power quality describes the extent to which the device both distorts the voltage waveform For an electrical distribution system, power and changes the phase relationship between quality is the extent to which line voltage is a * voltage and current. A device with ideal sine wave of constant amplitude . Figure 1 power quality characteristics neither distorts * Terms in italics are shows the waveform of a 120-volt (V), 60- the supply voltage nor affects the voltage- defined in the hertz (Hz) line voltage of ideal power quality. current phase relationship. glossary on p. 7. In an alternating current circuit, electrons . Figure 1 Voltage waveform for a 120-V, 60-Hz power supply with ideal power quality A smooth sine wave is 170 characteristic of undistorted voltage. At the 120 frequency of 60 Hz, the Amplitude wave repeats every 16.7 ms. The amplitude is 170 V; the root-mean-square (rms) value of the wave is 0 120 V. 4.2 8.3 12.5 16.7 20.8 25 Voltage (V) Voltage Time (ms) 120 170 1 How do lighting systems affect power Figure 2 quality? A highly distorted current waveform Most incandescent lighting systems do not 465 reduce the power quality of a distribution system because they have sinusoidal current waveforms that are in phase with the voltage es) waveform (the current and voltage both increase and decrease at the same time). Fluorescent, high-intensity discharge 4.2 8.3 12.5 16.7 20.8 (HID), and low-voltage incandescent lighting Time (ms) systems, which use ballasts or transformers, ent (milliamper may have distorted current waveforms. Fig- Curr ure 2 shows an example of a highly distorted –465 current waveform typical of some electronic ballasts for compact fluorescent lamps. De- . vices with such distorted current waveforms draw current in short bursts (instead of draw- Both lighting manufacturers and building ing it smoothly), which creates distortion in owners can take steps to improve power the voltage. These devices’ current waveforms quality. Most electronic ballasts for full-size also may be out of phase with the voltage fluorescent lamps have filters to reduce waveform. Such a phase displacement can current distortion. Some electronic ballasts reduce the efficiency of the alternating cur- for compact fluorescent lamps have high rent circuit. In Figure 3, the current wave current distortion, but contribute very little to lags behind the voltage wave. During part of voltage distortion because of their low power. the cycle the current is positive while the Magnetic ballasts for fluorescent and HID voltage is negative (or vice versa), as shown lamps typically have lagging current. Some in the shaded areas; the current and voltage magnetic ballasts contain capacitors that work against each other, creating reactive resynchronize the current and voltage, which power. The device produces work only during eliminates reactive power. Building owners the time represented by the non-shaded parts also can install capacitors in their building of the cycle, which represent the circuit’s distribution systems to compensate for large active power. loads with lagging current. Reactive power does not distort the volt- age. However, it is an important power quali- ty concern because utilities’ distribution systems must have the capacity to carry reactive power even though it accomplishes no useful work. Figure 3 Phase displacement and reactive power Phase displacement = 2.1 ms or 45˚ 170 When a device’s current waveform is out of phase with the voltage wave- = voltage form, the difference = current between the two is the 4.2 8.3 12.5 16.7 20.8 25 = reactive power phase displacement. oltage (V) generated in V Time (ms) this area The shaded areas represent the reactive power that results. -170 2 Figure 4 Illustrating harmonics a. 3 ft 2 ft b. 1 ft The distorted waveform in Figure 4a can be de- portions of the distorted wave result when the funda- scribed by the sum of one sine wave with frequency mental and harmonic cancel each other out. 1 Hz and amplitude 2 feet (ft), which is the funda- mental, and a second sine wave with frequency 3 Hz The distorted wave in Figure 4a is similar to what a and amplitude 1 ft, which is the third-order harmonic. rope would look like if one person shook one end at The two component waves are shown in Figure 4b. the frequency and amplitude of the fundamental wave shown in Figure 4b, while another person shook the The “peaks” and “troughs” of the distorted wave re- other end at the frequency and amplitude of the third- sult when a peak or trough of the fundamental coin- order harmonic shown in Figure 4b. cides with a peak or trough of the harmonic. The flat . What are harmonics? Utilities typically supply voltage with less than 2% THD. However, current THD for A harmonic is a wave with a frequency that is electronic devices may be very high, often an integer multiple of the fundamental, or over 100%. Table 1 on p. 5 lists current THD main wave. Any distorted waveform can be from several types of lighting loads, as well described by the fundamental wave plus one as from common office equipment, as mea- or more harmonics, as shown in Figure 4. A sured by NLPIP. Devices with high current distorted 60-Hz current wave, for example, THD contribute to voltage THD in propor- may contain harmonics at 120 Hz, 180 Hz, tion to their percentage of a building’s total and other multiples of 60 Hz. The harmonic load. Thus, higher-wattage devices can whose frequency is twice that of the funda- increase voltage THD more than lower- mental is called the second-order harmonic; wattage devices. If harmonic distortion is a the third-order harmonic has a frequency concern for a lighting system, NLPIP recom- three times that of the fundamental, and so mends that specifiers use electronic ballasts forth. with filters to minimize THD. Highly distorted current waveforms con- The recommended maximum allowable tain numerous harmonics. The even harmon- voltage THD at the point where a building ic components (second-order, fourth-order, connects to the utility distribution system is etc.) tend to cancel out each other’s effects, 5% (IEEE 1992). Figure 5 on p. 4 shows that but the odd harmonics tend to add in a way voltage THD reaches this limit when approx- that rapidly increases distortion because the imately half the building’s load has current peaks and troughs of their waveforms often THD of 55%, or when approximately one- coincide. The lighting industry calls its most quarter of the building’s load has current common measure of distortion total harmon- THD of 115%. ic distortion (THD). The sidebar “Defining total harmonic distortion and harmonic fac- tor” on p. 4 gives formulas for calculating THD. 3 Figure 5 Voltage THD resulting from 55% and 115% current THD (Adapted from Verderber et al. 1993, © 1993 IEEE) 10 115% current THD IEEE 519 limit 5 55% current THD Voltage distortion at Voltage service entrance (%) 10 20 30 40 50 Percent of total load The IEEE voltage THD limit is theoretically exceeded when approximately 47% of the total load in a building has 55% current THD or when approximately 26% of the load has 115% current THD. What is power factor? Power factor is a measure of how effectively a is called normal power factor (NPF). Mag- device converts input current and voltage netic and electronic ballasts for fluorescent into useful electric power. It describes the lamps may be either HPF or NPF. HPF bal- combined effects of current THD and reac- lasts usually have filters to reduce harmonics tive power from phase displacement. A de- and capacitors to reduce phase displacement. vice with a power factor of unity (1.0) has 0% On average these additional components add current THD and a current draw that is syn- about 16% to the retail costs of ballasts (Dorr chronized with the voltage. Resistive loads et al. 1994). such as incandescent lamps have power NLPIP measured power factor for several factors of unity. A device is said to have high types of lighting loads, and for common office power factor (HPF) if the power factor is 0.9 equipment; these data are shown in Table 1. or greater. Power factor between 0.5 and 0.9 Defining total harmonic distortion (THD) and harmonic factor Ballast manufacturers, electric utilities, and standards organiza- where I1 is the rms of the fundamental current waveform, tions define THD differently, which has caused some confusion in I2 is the rms of the second-order harmonic current waveform the lighting industry.