A case study survey of harmonic currents generated from a computer centre in an office building

Architectural Science Review, Sept, 2007 by Ming-Yin Chan, Ken K.F. Lee, Michael W.K. Fung

Abstract: Due to the growing use of non-linear load equipment and new technologies in buildings, harmonic currents generated in distribution systems pose a new problem for electrical engineers. This is a serious problem when power quality is a prime concern. The problem is due to some non-linear loads showing different current waveforms when supplied by a distorted voltage. This paper summarises the results of a case study survey in an office building with a large number of connected computers, a major source of harmonics. The scope of work included site measurement and analyses. The characteristics and effects of harmonic distortion of load current and voltages on distribution systems are discussed. It was found that on most occasions, careful planning and design can minimise the risk of harmonic-related losses in electrical systems. However, this does not always guarantee satisfaction. A bank of capacitors may be used to improve power factors in electrical systems, though in some cases such a bank may make the situation worse. An alternative is filters, but the position of filters is also crucial. Based on the case study, the paper discusses alternatives and provides some practical solutions to the problem of harmonics in office buildings.

Keywords: Computer centres, Harmonic distortion, Office buildings, Power quality

Introduction

The subject of power quality has been given increased attention over the past decade. Broadly defined, power quality refers to the degree to which voltages and currents in a system represent sinusoidal waveforms. Harmonics have become a serious concern for electrical engineers following the wide use of electronic appliances. The quality of electrical power in commercial and industrial installation is undeniably decreasing. In addition to external disturbances, such as outages, sags and spikes due to switching and atmospheric phenomena, there are inherent, internal causes specific to buildings that result from the combined use of linear and non-linear loads. Solid examples of degradation are:

*** Untimely tripping of protection devices

*** Harmonic overloads

*** Voltage and current distortion

*** Temperature rise in conductors and generators

*** Reliability of low-voltage AC systems

The above disturbances are well documented and are directly related to the proliferation of loads consuming non-sinusoidal current, referred to as "non-linear loads" (Singh & Verma, 2007). The harmonic currents generated by non-linear loads cause voltage distortion as they interact with the impedance of electrical distribution systems.

With the increasing use of solid-state circuit equipment, harmonic distortion in supply systems becomes more frequent and severe due to non-linear characteristics of such circuits (Singh & Verma, 2007). Well known non-linear devices include converters, inverters, electronic-ballast, and lifts and especially computer equipment. These voltage or current distortions may cause unsafe and unreliable electrical power supplies, malfunction of equipment, overheating of conductors and can reduce the efficiency, and life of most connected loads (Frewin, 1991; Maza-Ortega, Gomez-Exposito, Trigo-Garcia, & Burgos-Payan, 2005). Therefore, harmonic distortion is an undesirable effect for electrical systems.

"Clean" power refers to voltage and current waveforms that represent pure sine waves and are free of any distortion. "Dirty" power refers to voltage and current waveforms that are distorted and do not represent pure sine waves. Alternating current power supply has always suffered from the effects of harmonics. The harmful effects at the tee-off point, lighting and socket outlet circuits of electrical distribution systems are documented in many publications (Elmoudi, 2006; Newcombe, 1994a).

How Harmonic Distortions are Formed

A harmonic is defined as "a sinusoidal component of a periodic wave or quantity having a frequency that is integral multiples of the fundamental frequency" (IEEE, 1995). Harmonics can be voltage and/or current related and present in an electrical system in multiples of the fundamental frequency. If the fundamental frequency is 50 Hz, the second harmonic is 100 Hz, the third harmonic is 150 Hz, and so on. The second harmonic is negative-sequence, the third is zero-sequence and the fourth is positive-sequence (Newcombe, 1994b). Table 1 shows the order and sign of harmonics. It is important to note that each type of harmonic has different effects on power distribution systems. The most common effects on power systems are shown in Table 2 (Newcombe, 1994a).

To investigate harmonic distortions, total harmonic distortions (THD) and total demand distortions (TDD) are used for evaluation. IEEE Standard 519-1992 (IEEE, 1992) recommends limits on the level of harmonics at the consumer or "point of common coupling" (details are shown below in Table 3). The approach is to limit consumer's current distortions based on relative size of loads. The terminal power supplier's voltage distortions based on the voltage level are also considered. The voltage distortion is the second limitation for the quality of voltage that a utility company must furnish the user. Five percent of voltage distortion is the general guideline (IEEE, 2002).