Electric Power Quality and Lighting

Electric Power Quality and Lighting

Introduction

Concerns about the effects of lighting products on power distribution systems have focused attention on power quality. Poor power quality can waste energy and the capacity of an electrical system; it can harm both the electrical distribution system and devices operating on the system.

There are many elements in a power system that affects two major parameters; power factor and harmonics. Electric motors, some lighting fixtures, transformers and other inductive and capacitive appliances introduce reactive power to the system, and thus involved in damaging the power factor. These components need reactive power to work.

Nonlinear loads like UPS, computer systems, fluorescent fixtures, CFLs, digital electronics, etc. are distorting current waveforms and introducing harmonics to the power system.

This technical article will help lighting specifiers and consumers better understand power quality, so that they can more confidently select energy-efficient lighting products.

What is power quality?

For an electrical distribution system, power quality is the extent to which line voltage is a sine wave of constant amplitude. Figure 1 shows waveform of a 120-volt (V), 60-hertz (Hz) line voltage of ideal power quality. In an alternating current circuit, electrons flow towards the power source for one half of the cycle and away from the power source for the other half.

At 60 Hz, the voltage wave completes a cycle every 1/60th of a second, or approximately every 17 milliseconds (1/50th of a second, or 20 milliseconds in 50 Hz systems). Problems with a utility’s generators or distribution system can cause serious power quality problems such as voltage drops and transients, both of which can reduce the life of lighting systems and other electrical equipment. High levels of distortion (deviation from sine wave) in the distribution system can also harm electrical equipment.

Unlike voltage drops and transients, however, distortion often is caused by electric devices operating on the system.

For a specific electric device, the term power quality describes the extent to which the device both distorts the voltage waveform and changes the phase relationship between voltage and current. A device with ideal power quality characteristics neither distorts the supply voltage nor affects the voltage-current phase relationship.

Voltage waveform for a 120V, 60Hz power supply with ideal power quality

Figure 1 - Voltage waveform for a 120V, 60Hz power supply with ideal power quality


A smooth sine wave is characteristic of undistorted voltage. At the frequency of 60 Hz, the wave repeats every 16.7 ms. The amplitude is 170V; The root-mean-square (rms) value of the wave is 120V.


How do lighting systems affect power quality?

Highly distorted current waveform

Figure 2 - Highly distorted current waveform

Most incandescent lighting systems do not reduce the power quality of a distribution system because they have sinusoidal current wave forms that are in phase with the voltage waveform (the current and voltage both increase and decrease at the same time).

Florescent, high intensity discharge (HID), and low-voltage  incandescent lighting systems, which use ballasts or transformers, may have distorted current waveforms. Figure 2 shows an example of a highly distorted current waveform typical of some electronic ballast for compact fluorescent lamps. Devices with such distorted current waveforms draw current in short bursts (instead of drawing it smoothly), which creates distortion in the voltage. These devices current waveforms also may be out of phase with the voltage waveform.

Such a phase displacement can reduce the efficiency of the alternating current circuit. In Figure 3, the current wave lags behind the voltage wave.

During part of the cycle the current is positive while the voltage is negative (or vice versa), as shown in the shaded areas; the current and voltage work against each other, creating reactive power. The device produces work only during the time represented by the non-shaded parts of the cycle, which represent the circuit’s active power.

Reactive power does not distort the voltage. However, it is an important power quality concern because utilities’ distribution systems must have the capacity to carry reactive power even though it accomplishes no useful work.

Both lighting manufacturers and building owners can take steps to improve power quality. Most electronic ballasts for full-size fluorescent lamps have filters to reduce current distortion. Some electronic ballasts for compact fluorescent lamps have high current distortion, but contribute very little to voltage distortion because of their low power.

Phase displacement reactive power

Figure 3 - Phase displacement reactive power


Magnetic ballasts for fluorescent and HID lamps typically have lagging current. Some magnetic ballasts contain capacitors that resynchronize the current and voltage, which eliminates reactive power. Building owners also can install capacitors in their building distribution systems to compensate for large loads with lagging current.


What are harmonics?

A harmonic is a wave with a frequency that is an integer multiple of the fundamental, or main wave. Any distorted waveform can be described by the fundamental wave plus one or more harmonics, as shown in Figure 4. A distorted 60 Hz current wave, for example, may contain harmonics at 120 Hz, 180 Hz, and other multiples of 60 Hz (in 50 Hz systems these can 100 Hz, 150 Hz, other multiples of 50 Hz).

The harmonic whose frequency is twice that of the fundamental is called the second-order harmonic; the third-order harmonic has a frequency three times that of the fundamental, and so forth.

Illustrating harmonics

Figure 4 - Illustrating harmonics


Note - A distorted waveform in Figure 4a can be described by the sum of one sine wave with frequency 1 Hz and amplitude 2 ft, which is the fundamental, and a second sine wave with frequency 3 Hz and amplitude 1 ft, which is the third order harmonic. The two component waves are shown in Figure 4b.

Highly distorted current waveforms contain numerous harmonics. The even harmonic components (second-order, fourth-order, etc.) tend to cancel out each other’s effects, but the odd harmonics tend to add in a way that rapidly increases distortion because the peaks and troughs of their waveforms often coincide. The lighting industry calls its most common measure of distortion total harmonic distortion (THD).


Total harmonic distortion (THD) and harmonic factor

Ballast manufacturers, electric utilities, and standard organizations define THD differently, which has caused some confusion in the lighting industry. For example IEEE defines THD given in IEEE 1035-1989 as follows:

THD formula-1

Where:

  • I1 is the root-mean-square (rms) of the fundamental current waveform
  • I2 is the rms of the second-order harmonic current waveform
  • I3 is the rms of the third-order harmonic current waveform, etc.

Or as defined in IEC 61000-2:

THD formula 2

Where I1 is the fundamental current waveform.

On the other hand, ANSI and CSA use the below formula to calculate THD:

THD formula 3

Where:

  • I1 is the rms of the fundamental current waveform,
  • I2 is the rms of the second-order harmonic current waveform
  • I3 is the rms of the third-order harmonic current waveform, etc.

As we can see, according to the second definition, THD is always less than 100%. The table below gives some conversions between the two definitions.

THD (%) as commonly reported by manufacturers (IEEE 1035-1989)THD (%) as defined by CSA and IEC
55
2019.6
3230.5
5044.7
10070.7
15083.2

Utilities typically supply voltage with less than 2% THD. However, current THD for electronic devices may be very high, often over 100%. Table 1 lists current THD from some lighting loads, as measured by NLPIP. Devices with high current THD contribute to voltage THD in proportion to their percentage of a building’s total load. Thus, higher-wattage devices can increase voltage THD more than lower wattage devices. If harmonic distortion is a concern for a lighting system, NLPIP recommends that specifiers use electronic ballasts with filters to minimize THD.

The recommended maximum allowable voltage THD at the point where a building connects to the utility distribution system is 5% (IEEE 1992).

Figure 5 shows that voltage THD reaches this limit when approximately half the building’s load has current THD of 55%, or when approximately one quarter of the building’s load has current THD of 115%.

Voltage THD resulting from 55% and 115% current THD

Voltage THD resulting from 55% and 115% current THD


To be continued in technical article part 2…

References:

• National Lighting Product Information Program;
• American National Standards Institute;
• Schneider Electric – Electrical Installations Guide


author-pic

Sufi Shah Hamid Jalali - B.Sc. in Electrical Engineering. Designer of websites. Currently working as Electrical Engineer at Aga Khan Foundation, Afghanistan. Likes to write and share knowledge of Electrical Engineering. Knows Primavera P6, Graphics, Professional AutoCAD and Project Management.



2 Comments

  1. [...] Electric Power Quality and Lighting Continued from first part: Electric Power Quality and Lighting (part 1)What is power factor?Power factor is a measure of how effectively a device converts input current [...]


  2. Edvard
    May 26, 2012

    Nice article, thanks for sharing!

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