a case study survey of harmonic currents generated from a

© 2007 University of Sydney. All rights reserved. www.arch.usyd.edu.au/asr

Architectural Science Review Volume 50.3, pp 274-280

A Case Study Survey of Harmonic Currents Generated from a Computer Centre

in an Office Building

Ming-Yin Chan*†, Ken KF Lee** and Michael WK Fung*

* Department of Building Services Engineering, Hong Kong Polytechnic University, Hunghom, Hong Kong, China** Maxim’s Caterer Limited, Hong Kong, China

† Corresponding author: Tel: (852) 2766 5836; Fax: (852) 2765 7198; Email: [email protected]

Submitted 20 November 2006; accepted 22 May 2007

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

IntroductionThe 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).

275Harmonic Currents in Office BuildingsMing-Yin Chan, Ken K.F., Lee and Michael W.K., Fung

Table 1: Classification of Harmonics (IEEE, 1995).

14


2nd

Harmoni

c

3rd

Harmoni

c

4th

Harmoni

c

5th

Harmoni

c

6th

Harmoni

c

7th

Harmoni

c

8th

Harmoni

c

50 Hz 100 150 200 250 300 350 400

Sequenc

e

− 0 + − 0 + −

14


2nd

Harmoni

c

3rd

Harmoni

c

4th

Harmoni

c

5th

Harmoni

c

6th

Harmoni

c

7th

Harmoni

c

8th

Harmoni

c

50 Hz 100 150 200 250 300 350 400

Sequenc

e

− 0 + − 0 + −

How Harmonic Distortions are FormedA 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).

Total harmonic distortion (voltage or current) is expressed as a ratio of fundamental (IEC, 1982). It is represented by equation (1)

Total Demand Distortion (TDD) is expressed as a percentage of the customer’s average maximum demand current level, rather than as a percentage of the fundamental, in order to provide a common

basis for evaluation over a period of time (IEEE, 1992).

(1)

Where Ih = Magnitude of individual harmonic components (root mean squared amperes)

(2)

h = Harmonic order IL = Maximum demand load current (root mean squared

amperes)According to IEC 61000-3-2 and IEC 555-2, current harmonic

limits for lighting are reproduced in Table 4. For an overall (lamp plus ballast) power factor of 0.9, the third harmonic can be 25%

15

Table 2: Effects of Harmonics (Wagner, 1992).

Sequence Effect on a Motor Effects on the Power Distribution System

Positive Creates forward-rotating magnetic

field

Heating

NegativeCreates a reverse-rotating magnetic

field

Heating

Motor problems

Zero None

Heating

Creates current in the neutral of a 3-

phase, 4-wire system

16

Table 3: Current Distortion Limits (IEEE, 1992).

Load ratio

Isc/IL

Harmonic order (odd harmonics)

h<11 11<h<17 17<h<23 23<h<35 h<11 THD

<20 4 2 1.5 0.6 0.3 5

20<50 7 3.5 2.5 1 0.5 8

50<100 10 4.5 4 1.5 0.7 12

100<1000 12 5.5 5 2 1 15

>1000 15 7 6 2.5 1.4 20

Even harmonics are limited to 25% of the odd harmonic limits above

Isc = maximum short-circuit current at PCC

IL = maximum demand load current at PCC

Table 2: Effects of Harmonics (Wagner, 1992).

Table 3: Current Distortion Limits (IEEE, 1992).

4

(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).

Total harmonic distortion (voltage or current) is expressed as a ratio or

fundamental (IEC, 1982). It is represented by equation (1)

VTHD =h

h

h

V

V!"

=2

2

× 100% (1)

Total Demand Distortion (TDD) is expressed as a percentage of the customer’s

average maximum demand current level, rather than as a percentage of the fundamental, in

order to provide a common basis for evaluation over a period of time (IEEE, 1992).

TDD =

I

I

h

h

L

2

2=

!

"× 100% (2)

Where Ih = Magnitude of individual harmonic components (root mean squared

amperes)

h = Harmonic order

IL = Maximum demand load current (root mean squared amperes)

According to IEC 61000-3-2 and IEC 555-2, current harmonic limits for lighting

are reproduced in Table 4. For an overall (lamp plus ballast) power factor of 0.9, the third

harmonic can be 25% or more, assuming total voltage harmonic distortion is around 1%

(IEC, 1982).

4

(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).

Total harmonic distortion (voltage or current) is expressed as a ratio or

fundamental (IEC, 1982). It is represented by equation (1)

VTHD =h

h

h

V

V!"

=2

2

× 100% (1)

Total Demand Distortion (TDD) is expressed as a percentage of the customer’s

average maximum demand current level, rather than as a percentage of the fundamental, in

order to provide a common basis for evaluation over a period of time (IEEE, 1992).

TDD =

I

I

h

h

L

2

2=

!

"× 100% (2)

Where Ih = Magnitude of individual harmonic components (root mean squared

amperes)

h = Harmonic order

IL = Maximum demand load current (root mean squared amperes)

According to IEC 61000-3-2 and IEC 555-2, current harmonic limits for lighting

are reproduced in Table 4. For an overall (lamp plus ballast) power factor of 0.9, the third

harmonic can be 25% or more, assuming total voltage harmonic distortion is around 1%

(IEC, 1982).

Architectural Science Review Volume 50, Number 3, September 2007276

or more, assuming total voltage harmonic distortion is around 1% (IEC, 1982).

For neutral current in systems close to balanced loading this definition leads to THD of several hundred or even thousand percent, because the fundamental component is small. Expressing THD in relation to total root mean squared currents would be preferable. In this way, the percentage would not exceed 100% (Burnett, 1994; Ortiz Rivera, 2004).

Research Methodology

Most of the work on the harmonics survey assumes a single load, linear or non-linear fed from a power system. They can be categorised into time domain or frequency domain techniques. They also proposed the measurements of the voltage and load current where the load is connected. However, there are buses that individually feed many loads. These loads may or may not be the source of harmonics and may interfere with each other (Alammari, Soliman, & El-Hawary, 2004). The site measurement in this paper followed the guidelines of IEC 6100-4-7 for the measurement of instantaneous value of harmonic content, power and power factor (IEC, 2002). It is the standard procedure for measurement of harmonics with multiple sources. The recommended measurement interval is three seconds (very short interval) and ten minutes (short interval). Since the main loads are quasi-stationary (slowly varying), the time interval for the measurements was taken as one minute. Due to the variability of harmonics on weekdays and weekends, the instrument analysed data and records at different sampling periods. To measure the load characterization under different supply locations, an instrument “Energytest 2020E” was used. The instrument enabled arbitrary waveforms to be injected in single-phase, two-phase and three-phase loads in cases of three-phase or single-phase tests. The instrument also had a harmonics measurement module that was used to obtain the consumed current spectrum for the load under different test conditions. In order to carry out the test, it was necessary to establish a wide range of harmonic voltage measurements. The testing procedures were set as follows:

1. Monitor existing values of harmonics and check against recommended levels

2. Identify equipment that generates harmonics3. Observe existing background levels and track the trends of

voltage and current harmonics (daily)4. Make measurements with and without nonlinear loads

connected, and determine the harmonic driving point impedance at given locations

The site used for this case study was a major office building in Hong Kong, where several levels serve as a computer centre. The non-linear loads under investigation included fluorescent lighting, small power equipment, computers and printers. They were fed from the 250A TPN plug-in unit (schematic line diagram - detail ‘A’ of Figure 1) and were distributed from MCCB to individual MCB board. The site measurements of this building were mainly taken at the tee-off, lighting and socket outlet points. In order to have full load and part load conditions, the measurements of harmonic were under both working and non-working hours (night time). The studied parameters included power factor, voltage and current harmonic.

Three periods of measurement were from 28-31 October 2005, 4-7 November 2005 and 6 March 2006. A multiple clamp was used to take the measurements. The harmonic data (Accuracy: ±5.0% + 2 digit) was recorded with time intervals of one minute. Since the background harmonics changed from time to time, the parameters were analyzed with regard to the full scale of the current. Three different points were taken at the same time. The instrument was able to measure voltages, currents, active powers, inductive and capacitive reactive powers, total harmonic distortion, inductive and capacitive power factors, and analogue or impulse parameters.

Results and DiscussionLighting Load

The lighting equipment in the building consisted of fluorescent lamps and electronic ballast. Harmonic current generated in electronic ballasts was due to the operation of a single-phase diode-bridge rectifier, which is commonly found in most electronic power supplies. The measured values in normal working hours are shown in Table 6. The THDI was around 13%. The THDv and third harmonic ranged from 3.7% to 4.2% and 8.2V to 9.4V respectively. Compared with the recommended standard, the harmonic voltage distortion was acceptable for the 5% value (IEEE, 1992) and the third harmonic was also within the limit of 30λ% (where λ is the power factor of the lamp circuit) in IEC 555-2 (IEC, 1982). In non-working hours, the THDv and third harmonic were reduced to zero when the non-linear loads were switched off.

The corresponding neutral current was also examined. Results showed that it was composed of the third harmonic. Taking the

17

Table 4: Harmonic limits for Lighting (IEC Standard 555-2) (IEC, 1982).

Harmonic

order (n)

Maximum permissible harmonic current expressed as a

percentage of the input current at the fundamental

frequency (%)

2 2

3 30λ*

5 10

7 7

9 5

11 < n < 39 (odd

harmonics only)3

* λ is the circuit power factor

Table 4: Harmonic limits for Lighting (IEC Standard 555-2) (IEC, 1982).


18

Table 5: Harmonic limits of COP for Energy Efficiency of Electrical Installation

(EMSD, 2000).

19

Table 6: Compatibility Levels for Individual Harmonic Voltages in the Low-Voltage

Public Network According to IEC 61000-2-2*.

Table 5: Harmonic limits of COP for Energy Efficiency of Electrical Installation (EMSD, 2000).

Table 6: Compatibility Levels for Individual Harmonic Voltages in the Low-Voltage Public Network According to IEC 61000-2-2*.

Figure 1: Electrical installation the office building.


measurement data in the computer centre the third harmonic current (12.2A) in neutral was approximately 1.9 times the fundamental current (6.54A). The predominance of third harmonic current was due to the in-phase summing of phase values (IEC, 1982). This dynamic situation enhanced the production of third harmonic current caused by the presence of third harmonic voltage distortion in phase voltage waveform (Liew, 1989).

Socket Outlets (Computer Loads)The small power supplies were mainly socket outlet computers

and printers. The results of measurement showed that the dominant harmonics were third, fifth and seventh harmonics. The highest harmonic current distortion for R–phase, Y-phase and B-phase was in the region of 48%, 38% and 71%. It was excessively high when it was compared with the recommended value of 15% (IEEE, 1992). This was mainly due to the switched mode power supplies of computers, which had a capacitor and diode bridge as fundamental components in the input stage. The wide range of phase THDI from 38% − 71% was caused by the effect of attenuation and the diversity of current harmonics was caused by variation in power levels (Mansoor, 1995). The neutral current (35.97A) with an unbalanced current (12.53A) was lower than the design values (100A) in the distribution system. If additional computer appliances were installed, the risk of excessive neutral current would occur due to an increase

in harmonic current distortion.

Tee-off Points of MCCB The overall effects due to lighting, socket outlets and other

loads were observed at the main tee-off point. Similar to the small power loads, the dominant harmonics were third, fifth and seventh. The measured results of the THDI (51% to 54% − Phase R, 53% to 55% - Phase Y & 47% to 49% - Phase B) were still greater than the recommended value of 15% in IEEE 519, but they were lower than the small power load of the socket outlets. The lighting and computers were connected at the same supply source and this supply source was known as the “Point of Common Coupling”. A cancellation of fifth and seventh harmonics current occurred between these two types of non-linear loads. Obviously, the harmonic distortion was due to the cancellation and absorption effect of different relative phase angles of harmonic content. The neutral wire (62.96A) is dominated by triple harmonics (54.8A), rather than unbalanced current (20.75A). The cumulative effect of non-linear loads on the floor contributed to large distortion. It caused the temperature to rise and polluted the AC waveform. As a whole, the overall THDs in different equipment are said to be satisfactory, but it will cause heating of neutral conductors. The final outcome is a waste of energy.

20

Table 7: Results of site measurement (January to February 2006).

Date Current P.F. II

(A) rms (A) h1 h3 h5 h7 h9 h11 THD

28/10 - 31/10 L1 51.96 0.85 45.52 100 50.3 15.9 5.05 5.78 0.198 53.52

L2 6.25 0.95 6.17 100 13.13 0 0 0 0 13.2

L3 17.51 0.78 14.26 100 53.9 17 5.5 6.5 0 70.11

4/11 - 7/11 L1 55.72 0.85 49.6 100 52 14.94 5.12 5.38 0 48.6

L2 8.89 0.92 8.7 100 13.8 0 0 0 0 13.8

L3 19.23 0.78 16.94 100 52 15.05 6.73 6.73 0 48.47

6/3 (Test 1) L1 62.32 0.88 58.77 100 43.9 27.65 16.2 7.1 0 54.83

L2 9.92 0.99 9.9 100 7.12 0 0 0 0 7.12

L3 31.16 0.92 28.27 100 30.03 18.25 11.2 6.3 0 37.4

6/3 (Test 2) L1(N) 62.96 _ 20.75 100 264.14 80.82 44.77 27.04 11.1 100

L2(N) 20.4 _ 6.54 100 183.48 1.3 0.5 0 0 183.5

L3(N) 35.97 _ 12.53 100 180.9 37.2 24 11.5 3.1 196.2

Voltage P.F. VI

(V) rms (V) h1 h3 h5 h7 h9 h11 THD

28/10 - 31/10 L1 225.1 _ 224.9 100 4.18 0 0 0 0 4.18

L2 225.2 _ 224.9 100 4.19 0 0 0 0 4.19

L3 225.2 _ 224.8 100 4.18 0 0 0 0 4.19

4/11 - 7/11 L1 224.8 _ 224.6 100 4.08 0 0 0 0 4.08

L2 224.8 _ 224.6 100 4.07 0 0 0 0 4.07

L3 224.7 _ 224.5 100 4.07 0 0 0 0 4.07

6/3 L1 225.2 _ 224.9 100 3.82 0 0 0 0 3.82

L2 225.4 _ 225.5 100 3.64 0 0 0 0 3.64

L3 225.4 _ 224.8 100 3.71 0 0 0 0 3.71

% of harmonic distortion (In/II)

% of harmonic distortion (Vn/VI)

Table 7: Results of site measurement (January to February 2006).


ConclusionsProblems of harmonics from non-linear loads continue to grow

with modern office buildings. It becomes a new challenge for building design engineers because there is a drastic change of electrical equipment installed inside modern buildings. The increasing use of electronic equipment may cause the distortion to reach unacceptable levels in future. The most common problem is the harmonic distortion caused by non-linear loads such as electric household appliances, lighting, personal computers or speed control units for motors. In a well balanced system, the vector sum of the currents in the neutral was zero or close to zero. In general, even harmonics, (2nd and 4th) do not cause problems. The odd multiples of the third harmonic are added together in the neutral and can cause overheating even with balanced loads, which is extremely undesirable.

Non-linear loads draw power at a low power factor with large harmonic currents. Reduction of continuous disturbances due to harmonics often requires the use of harmonic filters. In most instances, power factor correction capacitors can be installed in the form of a harmonic filter bank to provide both power factor correction and harmonic filtering capabilities. However, the application of capacitors in the presence of harmonic generating

equipment may produce undesirable effects. Capacitors can amplify certain harmonics if there is parallel resonance between the capacitor bank and source of harmonic equipment. The result would be excessive capacitor currents, capacitor fuse blowing and excessive voltage distortion in the system. Special consideration must be given to the application of capacitors to a power system that contains harmonic generating equipment.

Three site measurements were taken at an office building with a large number of computers connected. The phenomenon of harmonics generated by non-linear loads of lighting, socket outlets and tee-off point were also investigated. Harmonic distortion is currents at frequencies of odd multiples of the fundamental frequency. It is usually expressed in terms of percentage of the fundamental. The AC input currents drawn by non-linear loads were made of a fundamental sine wave plus a number of harmonic sine wave (multiples of the fundamental frequency). Frequency inverter of air conditioning systems and fluorescent lighting are well-known sources of harmonics; others are computers and equipment fitted with switch mode power supplies (Francisco, 2006). Higher distortion values may result in the malfunction of control equipment and power supply which in turn can lead to production and process interruption that can have high economical

22

Current / flexible clamps connected at three different points

Current / flexible clamps connected at three different points

Figure 2: Site measurement.

L1 – Tee-off point

(Phase Red)

L3 – S/O point (Phase

Red)

L2 – Lighting

point (Phase

Red)

L1 – Tee-off

point (Phase

Blue)

L2 – Lighting

point (Phase

Blue)

L3 – S/O point

(Phase Blue)

Figure 2: Site measurement.


impacts. Harmonic currents considerably affect the neutral wire of electric installations and pollute the AC waveform. Therefore, remedial actions taken in isolation can greatly improve the overall power quality.

A current controlled power converter can be used in the mitigation or active filtering of harmonic currents and voltages, provided that it is at an appropriate point. Otherwise, the effect is not sound and it does not relieve the problem efficiently. This was not stressed in the past. A power electronic converter intended for load balancing, source balancing and harmonic compensation can be connected in parallel with the voltage source close to the source of unwanted reactive and harmonic currents. This may lead to a higher cost due to using a number of filters or converters. In return, it could provide a better elimination effect.

The survey was conducted in a typical office building with many computers connected. The data obtained from the survey indicates that it is a problem commonly found in many similar office buildings in Hong Kong. The office building was built in the 1990’s and was well planned to eliminate harmonics at the design stage, but it did not work satisfactorily. The main reason is the improper position of converters and filters. Although there are many variations of power quality problems, the most significant power quality issues are voltage unbalance and harmonic current. Balanced three-phase currents do not generate neutral currents and provide a reliable and safe operation. All neutral components, including neutral terminal and neutral bus-bars, should be sized for additional harmonic current (HKEMSD, 2000). This will lead to oversize of conductors, therefore, it is imperative to identify the various harmonic generating sources and to understand their load characteristics. Whatever the source of disturbances, the effects must be minimized or eliminated. Failure to do so will result in an increase in downtime, lost production, increased cost of equipment maintenance, or frequent replacement or failed equipment. Therefore, designers should take into consideration these issues when designing a power distribution system.

References Alammari, R.A., Soliman, S.A., & El-Hawary, M.E. (2004). Identification

of individual types of harmonic loads in an electric power system bus. Electrical Power and Energy Systems, 26(7), 545-548.

Burnett, J. (1994). Survey of power quality in high-rise air-conditioned buildings. IEE Conference Publication No. 399, October, 163-168.

Electrical and Mechanical Services Department, Hong Kong (HKEMSD) (2000). COP for Energy Efficiency of Electrical Installation. Hong Kong: Hong Kong Special Autonomous Region Government.

Elmoudi, A.A. (2006). Evaluation of power system harmonic effects on transformers: Hot spot calculation and loss of life estimation. Unpublished PhD Thesis, Helsinki University of Technology.

Francisco, C.D.L.R. (2006). Harmonics and Power Systems. London: CRC Taylor & Francis.

IEC std. 555-2 (1982). Disturbances in supply systems caused by household appliances and similar electrical equipment. Part 2: Harmonics. Geneva: International Electrotechnical Commission.

IEC std. 61000-4-7 (2002). General guide on harmonics and inter-harmonics measurements and instrumentation, for power supply systems and equipment connected thereto. Geneva: International Electrotechnical Commission

Liew, A.C. (1989). Excessive neutral current in three-phase fluorescent lighting circuit. IEEE Transaction Industrial Applications, 25(4), 776-778.

Institution of Electrical and Electronic Engineers (1995). Standard 1159: Recommended Practices for Monitoring Electric Power Quality. London: IEEE.

Institution of Electrical and Electronic Engineers (1992). Standard 519: Recommended practices and requirements for harmonic control in electrical power systems. Washington, DC: IEEE.

Mansoor, A. (1995). An investigation of harmonics attenuation and diversity among disturbed single-phase power electronic loads. IEEE Transaction Industrial Applications, 10(1), 467-473.

Maza-Ortega, J.M., Gomez-Exposito, A., Trigo-Garcia, J.L., & Burgos-Payan, M. (2005). Parameter estimation of harmonic polluting industrial loads. International Journal of Electrical Power & Energy Systems, 27(9-10), 635-640.

Newcombe, C. (1994a). Evaluating harmonics problems in commercial and industrial facilities. Asia Engineer, 22(3), 20-23.

Newcombe, C. (1994b). Take control of harmonics. Asia Engineer, 22(6), 41-42.

Ortiz Rivera, E.I. (2004). Study of power quality: Problems in commercial buildings in Puerto Rico. 2004 IEEE International Conference on Electric Utility Deregulation, Restructuring and Power Technologies (DRPT). Hong Kong, April, 301-305.

Singh, B., & Verma, V. (2007). An improved hybrid filter for compensation of voltage and current harmonics for varying rectifier loads. International Journal of Electrical Power & Energy Systems, 29(4), 312-321.

a case study survey of harmonic currents generated from a

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