A Laboratory for Power Quality Analysis

A laboratory for power quality analysisJulio Barros, Daniel Cando and Iker DuranaDepartamento de Electronica y Computadores, Universidad de Cantabria, Santander, SpainE-mail: [email protected]

Abstract This paper describes a laboratory designed for electrical power quality analysis. Among the differenttypes of disturbances in voltage supply that the laboratory allows us to generate are harmonics, voltage dips andshort interruptions in voltage supply, voltage imbalance and frequency deviations. Using this laboratory we can testsoftware for analysis, detection and classification of power quality disturbances and also study their effectson equipment.

Keywords frequency deviations; harmonics; power quality; voltage dips and short interruptions; voltage imbalance

The term power quality refers to a wide variety of electromagnetic phenomenathat characterize the voltage and current at a given time and at a given locationon the power system. The International Electrotechnical Commission classifiesthe different types of low frequency electromagnetic disturbances that affectthe electrical power quality in the following groups:1 harmonics and inter-harmonics, voltage fluctuations, voltage dips and short interruptions in voltagesupply, voltage imbalance, frequency deviations, low frequency inducedvoltage and d.c. in a.c. networks.

These disturbances affect in different ways the voltage characteristics ofelectricity supplied and their relevance is growing steadily due to the increasinguse of non-linear electrical/electronic loads. The effects of power quality disturb-ances on equipment and on the power system itself and their economic impor-tance in industrial processes have focused growing attention to this issue. TheEuropean Community has developed different standards to define the charac-teristics of voltage at the user supply terminals and the compatibility levels forlow-frequency conducted disturbances in power supply systems.2–7

This paper describes a laboratory designed for electrical power quality analy-sis. The laboratory is able to generate different types of disturbances in voltagewaveform and thus test different software for analysis, detection and classifi-cation of disturbances. It can also be used to study their effects on electricalloads, protection systems or measurement systems.

Structure of electrical power quality laboratory

The structure of the laboratory designed for electrical power quality analysisis shown in Fig. 1. It is made up basically of an a.c. programmable powersource used to feed any type of electrical/electronic, single phase/three phaseload and a data acquisition system used for data handling and analysis.

A personal computer controls the programmable a.c. power source/analyzerthrough an HP-IB interface. The HP-6834B power source produces singlephase/three phase arbitrary waveforms with programmable amplitude, fre-

International Journal of Electrical Engineering Education 38/3

211A laboratory for power quality analysis

Fig. 1 Structure of power quality analysis laboratory.

quency and waveshape and is used to generate a voltage supply with differenttypes of disturbances. The maximum ratings of the power source are300 V r.m.s., 4500 VA and 0–5 kHz output frequency range.8

The data acquisition system designed uses a DT-01EZ data acquisition boardand voltage and current transducers to measure voltage and currents in theelectrical load supplied by the a.c. power source.

The data acquisition board is installed in a second personal computer. Thisis a general-purpose board with 16 single-end/8 differential input channels with12-bit resolution and 27.5 kHz throughput, two analog output channels witha resolution of 12 bits and 29.7 kHz throughput and 16 digital input/outputlines.9

A LEM LV 25-P voltage transducer based on the Hall effect is used tomeasure a.c. and d.c. voltages. An external resistor R1 selected by the user andinstalled in series with the primary circuit is used to produce a current pro-portional to the measured voltage (Fig. 2(a)). The external resistor should becalculated to produce a primary current of 10 mA at the nominal voltage tobe measured and thus obtain the transducer optimun accuracy. The turns ratiois 2500 : 1000, producing an analog output current IM of 25 mA.

A measuring resistor RM in series with the power supply zero is used toderive a voltage output (Fig. 2(a)). This voltage output is connected to one of


212 J. Barros, D. Cando and I. Durana

Fig. 2 (a) LV 25-P voltage transducer, (b) L A 25-NP current transducer.

the input channels of the DT-01EZ board. The LV 25-P voltage transducer issuitable for measuring nominal voltages from 10 to 500 V.

The current transducer used in the laboratory is the LEM LA 25-NP. Thisis a multirange current transducer based on Hall effect technology that allowselectronic measurement of d.c. and a.c. current with galvanic isolation. Usinga set of primary connection pins the range of nominal currents capable of beingmeasured is 25/12/8/6/5 A with an output current Is of 25 mA. The outputcurrent is fed through a measuring resistor RM in series with the power supplyzero to derive a voltage output (Fig. 2(b)). This voltage output is connected toone of the input channels of the data acquisition board.

Both types of transducers, voltage and current, have linear frequency responsein the frequency range in which they are used. Any type of linear/non-linearsingle phase/three phase electrical/electronic load can be used in the laboratorywith a maximun power of 4500 VA.

Software for generating and analyzing electrical power quality disturbances

A software application in Microsoft Visual Basic has been developed to controlthe types of power quality disturbances generated by the laboratory.



Harmonics, voltage dips and short interruptions in voltage supply, voltageimbalance and frequency deviation of different magnitudes can be applied toany electrical load connected to the a.c. programmable power source.

The a.c. power source is controlled through a built-in HP-IB interface andthe functions of this interface are implemented through SCPI commands.10The HP-6840 board is used for HP-IB interface. When the software applicationis started the a.c. power source is set to its single phase or three-phase modeand nominal voltage and frequency are defined.

Figure 3 shows the window for voltage supply disturbances generation. Theuser is requested to choose one type of disturbance in the voltage waveformto be generated by the laboratory.

To control the data acquisition system used for data handling and analysisthe DTx-EZ software package is used.11 DTx-EZ allows us to access thecapabilities of the DT-01 EZ board using Microsoft Visual Basic or MicrosoftVisual C++. It provides two controls: data acquisition custom control andplotting custom control. The first control facilitates data acquisition functionsand the second one provides a high-speed plotting control useful for plottingfixed or floating-point data in a Visual Basic application.

Harmonics in voltage supplyHarmonic voltages are sinusoidal voltages with a frequency equal to an integermultiple of the fundamental frequency of the supply voltage. Harmonics areproduced by the connection of non-linear loads to the distribution systems.These loads are drawn on non-linear currents which in combination with theline impedances cause a harmonic voltage drop and disturb the sinusoidalgenerated supply voltage.

The harmonics generated in this way spread through the distribution systemand greatly affect protection systems, electrical machines and transformers,cables, measurement equipment, etc., also leading to interference in communi-

Fig. 3 W indow for voltage supply disturbances generation.



cation systems. The lifetime of devices might be reduced, the power factordecreased and sensitive loads could malfunction or even be damaged.12

According to European Standard EN50160 (Ref. [2]) harmonic voltages canbe evaluated in two different ways:

1 individually by their relative amplitude (Uh) related to the fundamental

voltage U1 , where h is the harmonic order

uh=

Uh

U12 globally, for example by the total harmonic distortion factor THD, calcu-

lated using the following expression:

THD=S ∑40

h=2(uh)2

Standard EN50160 and IEEE-519 (Ref. [13]) define the limits for individualvoltage distortion and for total voltage distortion in percentages under normaloperating conditions.

If the user of the laboratory chooses the option for harmonic generation inthe voltage supply, the application first requests the higher harmonic order tobe generated and then the amplitude and phase of the different harmonics, ascan be seen in Fig. 4. The voltage supply waveform generated by the powersource is also shown. There is no restriction on the harmonic order nor on thetotal harmonic distortion in the voltage supply generated using the laboratory.

Figure 5 shows the voltage waveform generated by the laboratory when a0.92% third harmonic, 3.06% fifth harmonic, 1.21% seventh harmonic, 0.39%ninth harmonic and 0.11% eleventh harmonic are selected by the user.

Fig. 4 W indow for harmonic generation in voltage supply.



Fig. 5 Example of voltage waveform generated using the laboratory.

Voltage dips and short interruptions in voltage supplyA voltage dip is a sudden decrease in the voltage amplitude followed by areturn to the initial level after a short time, while a short interruption in voltagesupply is a complete voltage cut off during a time interval necessary forautomatic restoration of supply. Voltage dips and short interruptions may beproduced by faults in the supply network, both permanent and transitory,rapidly varying loads or equipment taking high current at starting.

A voltage dip is defined by a pair of data: duration and magnitude. Theduration corresponds to the period during which the r.m.s. values measuredremain less than 90% of the declared value (U

N). The depth of a voltage dip

is defined as the difference, expressed as a percentage of the declared voltage,between the minimum r.m.s. value during the voltage dip and the declaredvalue. If the supply voltage value drops below 1% of the declared voltage theevent is considered an interruption, otherwise it is classified as a voltage dip.

According to Ref. [3], the lower limit for dip duration is set to 10 ms (halfa 50 Hz cycle), because this is the minimim time period over which an r.m.s.value can be calculated. The upper limit of duration is set to 60 s.

The window for voltage dips and short interruptions generation in voltagesupply is shown in Fig. 6. First of all the user is requested to choose among asingle phase, two-phase or three-phase voltage dip. Then the elapsed time, stepduration, voltage magnitude and voltage slew must be defined for each of thesteps of the voltage dip or for a short interruption in voltage supply. There areno restrictions on the voltage dips that can be generated. Complex voltagedips with many steps and different durations can be generated using thelaboratory.

Figure 7 shows the schematic representation of the voltage dip generatedusing the values established in Fig. 6. There are five steps involved in thegeneration of this voltage dip. In step 1 the output voltage is set to 230 V for



Fig. 6 W indow for voltage dips and short interruptions generation in voltage supply.

Fig. 7 Schematic representation of a voltage dip.

1 s. In step 2 the output voltage is set to 110 V for 50 ms. In step 3 the outputvoltage is set to 85 V for 150 ms. In step 4 the output is set to 170 V for 20 msand, finally, in step 5 the output voltage is set again to 230 V. The voltage slewin each transition of voltage dip is set to its maximum value, 9.9e+37, toproduce sudden transitions between the different steps. After the voltage dip isfinished, the output voltage returns to the settings in effect before the voltagedip was executed.



Voltage imbalanceThe imbalance of a three-phase supply voltage consists of a loss of symmetryof the phase voltage vectors (magnitude and/or phase) created mainly by anunbalance of the load. The primary source of voltage imbalance less than 2%is imbalanced single phase loads on a three-phase circuit. Voltage imbalancecan also be the result of capacitor bank anomalies, such as a blown fuse onone phase of a three-phase bank. Several voltage imbalances, greater than 5%,can result from single-phasing conditions.

The imbalance of the supply voltage Uuis defined by the negative sequence

component Uiexpressed in p.u. or percentage of the positive sequence compo-

nent Ud(U

u=U

i/U

d).

Voltage imbalance can be generated without any restriction using the labora-tory. The user is requested to introduce the desired voltage magnitude andphase angle for each phase. Voltage imbalance is calculated and is also shownto the user.

Frequency deviationsThe power system frequency is directly related to the rotational speed of thegenerators of the system. At any instant, the frequency depends on the balancebetween the load and the capacity of the available generation. When thisdynamics changes, small changes in frequency occur. The size of the frequencyshift and its duration depends on the load characteristics and the response ofthe generation system to load changes. In general, utilities maintain very closecontrol of the power system frequency.

Frequency variations that go beyond accepted limits for normal steady-stateoperation of the power system are normally caused by faults on the bulk powertransmission system, a large block of load being disconnected, or a large sourceof generation going off-line.

Off-nominal power system frequencies can be generated using the laboratory.The user is requested to introduce the new voltage supply frequency in Hz andthe frequency slew in Hz/s. If the user wants a sudden transition to the newsupply frequency, frequency slew must be set to its maximum value (9.9e+37).Lower values of frequency slew are used to produce slower transitions to thenew values.

Using the power quality analysis laboratory

Different types of experiments can be carried out using the laboratory. Userscan generate power quality disturbances for testing software for harmonicanalysis, harmonic tracking, voltage dips analysis, frequency measurement,frequency tracking, voltage imbalance, disturbance detection and classification,effects of power quality disturbances on equipment, etc.

The following experiments provide examples of using the power analysislaboratory:



1 Effects of harmonics in voltage supply on the performance of inductionmotors.

2 Measurement of voltage dips and short interruptions in voltage supply.

Effects of harmonics in voltage supply on the performance of induction motorsThe aim of this experiment is to study how the operation performance of ainduction motor is affected by harmonic distortion in voltage supply. To carryout the test a three-phase, two-pole induction motor is used. The motor isloaded with a prony break and a motor speed sensor and torque sensor arealso used.

Tests have been made when the motor is fed using the programmable a.c.power source, with a pure 50 Hz sinusoidal voltage, and when it is fed with apure 50 Hz sinusoidal voltage and 5%, 10% and 15% voltage distortion factorof harmonics from order 3 to 13. FFT analysis is performed on the motorvoltage and current. In each test, input current, power factor, real power,reactive power, apparent power and efficiency measurements are made toinvestigate the effect of non-sinusoidal voltages in the electrical and mechanicalperformance of the motor.

Table 1 and Fig. 8 show the motor efficiency obtained when the motor is fedwith different harmonics and several voltage distortion factors. In all eventsthe harmonic voltage is in phase with the fundamental component of thesupply voltage.

As can be seen from the results, harmonic order and voltage distortion factoraffect motor performance in a different way. Lower motor efficiency resultsfrom lower order harmonics, particularly 3rd and 5th harmonics, and alsofrom higher voltage distortion factors. Motor efficiency increases for higherharmonic order.

A new aspect taken into account in this experiment is the influence of therelative phase difference between harmonics in the voltage supply. For a givenvoltage distortion factor, the phase difference between harmonics modify thewaveform of the voltage supply, being more flattened than a perfect sinusoidalwave or with higher peak value than the pure sinusoidal. In this way, currents

TABLE 1 Motor eYciency under diVerent voltage distortion factors

Voltage distortion factor (%)

Harmonic

order 5 10 15

1° 80.77 80.77 80.77

1°+3° 70.95 70.48 69.87

1°+5° 75.37 74.39 73.37

1°+7° 77.46 76.60 74.44

1°+9° 78.21 77.34 75.25

1°+11° 79.72 79.44 78.67

1°+13° 79.84 79.65 78.92



Fig. 8 Motor eYciency under diVerent voltage distortion factors.

drawn by loads connected to the mains are also modified. To take into accountthis effect, the voltage crest factor (VCF) is defined as:

VCF=Peak voltage

r.m.s. voltage

A pure sinusoidal waveform has a VCF equal to 1.41.To study the effect of voltage crest factor in the motor performance different

tests have been made, specially for low order harmonics (3rd and 5th), forconstant voltage distortion factors (10%) and different crest factors. Inputcurrent, input current harmonics, current harmonic distortion factor and cur-rent crest factor depend on the crest factor of the voltage supply, and also themotor efficiency as can be seen from Ref. [15]. There is no linear relationbetween voltage crest factor in voltage supply and total harmonic distortioncurrent of the motor. From the tests, in general higher voltage crest factorresults in higher total harmonic distortion current and in lower motor efficiency.

Measurement of voltage dips and short interruptions in voltage supplyThe purpose of this experiment is to develop a software application to measurevoltage dips and short interruption in voltage supply following the specifica-tions given in Ref. [3].

To carry out the experiment the a.c. power source of the laboratory isprogrammed to generate voltage dips of different depths and durations. Usingthe data acquisition system, voltage magnitude is continuously monitoredthrough LV 25-P voltage transducers. The selected sampling rate of voltagesupply is 3.2 kHz, with a 10 ms sampling window.16

Every half-cycle of voltage supply the r.m.s. voltage is computed using theexpresion:

Vrms=SWNi=1 v2(n)

N



where v(n) are the voltage samples and N is the number of samples taken inthe sampling window (32 samples in our case).

The r.m.s. voltage computed in this way is compared with the threshold forvoltage dip detection (this threshold is set at 10% below the nominal voltage).If no voltage dip in voltage supply is detected the software application doesnot take any action. Otherwise, in case of a voltage dip, r.m.s. voltage, durationand depth of voltage dip are computed and stored in memory. The date andtime of occurrence of a voltage dip is also stored in memory and a statisticalanalysis is also carried out, classifying the voltage dip as shown in Table 2,where Nij represents the number of voltage dips belonging to the correspondingclasses of depth and duration.

To test the software application developed, the a.c. power source of thelaboratory is programmed to generate the voltage dip of Fig. 5. As can be seenin this figure, the duration and depth of this voltage dip are 220 ms and 61.36%(difference expressed in % of the declared voltage between the minimum r.m.s.value during the voltage dip, 85 V, and the declared value, 220 V) respectively.The voltage dip is correctly detected and classified by the software applicationdeveloped as shown in the window for statistical classification of the voltagedip (Fig. 9) and in the window for voltage dip representation of Fig. 10.

TABLE 2 Statistical classification of voltage dips

Dips/ 10<100 100<500 0.5<1 1<3 3<20 20<60

Duration (ms) (ms) (s) (s) (s) (s)

10%<depth<15% N11 N21 N31 N41 N51 N61

15%<depth<30% N12 N22 N32 N42 N52 N62

30%<depth<60% N13 N23 N33 N43 N53 N63

60%<depth<99% N14 N24 N34 N44 N54 N64

Fig. 9 W indow for statistical classification of voltage dips.



Fig. 10 W indow for voltage dips representation.

Conclusions

An electrical power quality analysis laboratory has been described. Harmonicsin voltage supply, voltage dips, short interruptions in voltage supply, voltageimbalance and frequency deviations can be generated in a programmable wayby the user of the laboratory. This laboratory has proved to be a powerfultool for testing software for analysis, detection and classification of powerquality disturbances and also for studying their effects on electrical loads,protection systems or measurement systems.

Acknowledgements

The authors wish to thank the Spanish Commission of Science and Technology(CICYT) for the support of the research project ‘Cancelacion activa de armon-icos de tension y corriente en redes de distribucion de energıa electrica’(TIC96-0728) of which the present paper is a part.

References

1 IEC Technical Committee 77, Working Group 6, Classification of Electromagnetic

Environments.

2 European Standard EN 50160, Voltage Characteristics of Electricity Supplied by Public

Distribution Systems.

3 UNIPEDE, Application Guide to the European Standard EN 50160 on Voltage Characteristics

of Electricity Supplied by Public Distribution Systems, September 1994.

4 European Standard EN 61000-2-1: Entorno electromagnetico para las perturbaciones conduc-

idas de baja frecuencia y la transmision de senales a traves de las redes de suministro publico.

5 European Standard EN 61000-2-2: Niveles de compatibilidad para las perturbaciones

conducidas de baja frecuencia y la transmision de senales a traves de las redes generales de

alimentacion de baja tension.

6 European Standard EN 61000-3-2. Electromagnetic Compatibility (EMC) — Part 3: Limits —

Section 2: Limits for harmonic current emissions.



7 European Standard EN 61000-4-7. Electromagnetic Compatibility (EMC). Part 4: Testing

and measurement techniques — Section 7: General guide on harmonics and interharmonics

meassurements and instrumentation for power supply systems and equipment connected

thereto.

8 User’s Guide for HP-6834B, AC Power Source/Analyzers, Hewlett-Packard, 1996.

9 DT-01EZ User’s Manual, Data Translation, Inc., 1997.

10 Programming Guide for HP-6834B, AC Power Source/Analyzers, Hewlett-Packard, 1996.

11 DTx-EZ User’s Manual, Data Translation, Inc., 1997.

12 IEEE Task Force on Effects of Harmonics on Equipment, ‘Effects of harmonics on equipment’,

IEEE T rans. Power Delivery, 8 (2) (1993), 672–680.

13 IEEE Recommended practices and requirements for harmonic control in electric power

systems. Standard 519.

14 IEEE Recommended practice for monitoring electric power quality. Standard 1159.

15 J. Barros, V. M. Moreno and D. Cando, ‘Effects of harmonics in voltage supply on the

performance of induction motors’, in Proc. 1999 IEEE Int. Symp. on Diagnostics for Electrical

Machines, Power Electronics and Drives (SDEMPED’99). 1–3 September 1999, Gijon, Spain,

pp. 573–576.

16 I. Durana, Sistema de medida, analisis y registro en tiempo real de huecos de tension en redes

electricas, Proyecto fin de carrera, Escuela Tecnica Superior de Ingenieros Industriales y de

Telecomunicacion, Universidad de Cantabria, March 1999.


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A Laboratory for Power Quality Analysis