Ward Jewell

Power QualityLaboratory Testing

Power quality laboratory testing usually servesone of three purposes: to test electrical devicesfor their performance in the presence of power qual-ity disturbances, to test power quality mitigation de-vices for their ability to mitigate disturbances, or todetermine the magnitude and types of power qualitydisturbances produced by a device connected to thepower system. The power quality lab must, therefore,be able to recreate operating conditions, both dis-turbed and undisturbed, that a device will experiencein the field.

Several types of equipment are needed to repro-duce the various disturbances. Measuring instru-ments must then capture disturbances and monitorthe performance of the device under test. To testlarge devices, it is sometimes more practical to takethe test and monitoring equipment to the device tobe tested rather than moving and reconnecting thedevice in the lab.

The IEEE Emerald Book categorizes power qual-ity disturbances. The power quality lab should beable to reproduce and measure these disturbances.For lab testing purposes, it is useful to categorize thedisturbances as voltage or current disturbances and ashigh- or low-frequency disturbances. The categorydetermines what equipment will generate and mea-sure the disturbance in the lab.

Low-Frequency TestingA typical system used to generate low frequency dis-turbances is shown in Figure 1. The disturbance iscreated at low voltage by an arbitrary waveform gen-erator. The generator may be a commercial device orit may be a PC with waveform generation softwaredriving digital-to-analog converters. Typical outputsare 10 V.

Disturbance characteristics are fed to the wave-form generator. The characteristics may be of a stan-dard waveform, or they may be taken from disturbance datacollected in the field. The second option is particularly usefulwhen a device is malfunctioning only in one location. Distur-bances measured at that location are recreated in the lab, allow-ing the device to be analyzed and mitigation options to be testedfor those specific disturbances.

The disturbance is then converted to appropriate levels by anamplifier. The frequency limits of the test system are set by thebandwidth of the amplifier. The limits on the Wichita State Uni-versity (WSU) low-frequency system, for example, are around22 kHz. This limit allows harmonic testing to over the 300th har-monic of 60 Hz and momentary outages or sags of down toaround 50 s or less than 1/300th of a 60 Hz cycle.

The amplifier used depends on the type of disturbance to becreated. Some disturbances require a voltage source amplifier,while others require a current source. The voltage amplifier pro-

IEEE Power Engineering Review, February 2002 0272-1724/02/$17.002002 IEEE 13

This article is part of a series of articles on power quality appearing in this issueand the the August, September, and November 2001 issues of IEEE Power Engi-neering Review. W. Jewell is with Wichita State University, Wichita, Kansas, USA.

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duces a voltage that has the same shape as the output of thewaveform generator and a magnitude appropriate for the deviceunder test, e.g., 120 V rms for a 120 V device. The current am-plifier similarly produces a current with the shape of the wave-form generator output. Table 1 lists various disturbances and thetype of amplifier used to create them. A three-phase device willrequire three amplifiers, one per phase.

Voltage Source TestingOutages, sags, and swells are all voltage disturbances and, thus,require a voltage source amplifier. The output of the amplifier isconnected directly to the device under test, as shown in Figure 1.The disturbances are applied to the device and its performance ismeasured.

Harmonic distortion, except for devices used to detect ormeasure current, is also a voltage disturbance, and the voltagesource amplifier is again used. The distorted voltage waveformfrom the amplifier is applied directly to the device under test.While harmonic current production of a device under test maybe an issue, devices designed to be connected to the electricpower grid are voltage source devices and, thus, see harmonicvoltages. A pure sinusoidal voltage is usually used to determinethe harmonic current production of a device.

Current Source TestingA current source amplifier is needed in two power quality test-ing situations. The first of these is to test devices used to sense ormeasure current. Current relays, watt-hour meters, and currenttransformers are some examples. These all present a very lowimpedance to current and require a current amplifier that gener-ates a current waveform into a low impedance. A common ap-plication is determining the response of current transformers toharmonic current distortion.

The second application for a current source amplifier is toproduce radiated electromagnetic interference (EMI). The mag-netic field that creates EMI is produced by current, so a specificcurrent waveform is needed to generate the magnetic field. Thecurrent may be passed through a Helmholtz coil to create a uni-form field or through one or more conductors in a bundle to cre-ate interference in nearby conductors.

Some devices will require both voltage source and currentsource amplifiers. A watt-hour meter is one example, with bothvoltage and current sensing coils. A three-phase distance relayrequires three voltage and three current sources. The arbitrarywaveform generator, in this case, must generate six waveforms.

High-Frequency TestingBecause of bandwidth limits on the arbitrary waveform genera-tor and amplifiers, higher frequency tests require specializedequipment. High-frequency voltage disturbances include light-

ning and switching transients, capacitor switching, arcing, andelectrostatic discharge.

Standard tests for lightning and switching transients and ca-pacitor switching are described in IEEE Standard C62.45. Thetest waveforms used in these tests have frequencies in the MHzrange. Specialized surge generators are used to generate thesedisturbances. The generators allow the equipment to be poweredduring the tests while preventing the applied surge from beingcoupled to the external power system. Surge magnitude, shape,and phase angle are all controllable.

Arcing and other high-frequency noise are generated by anelectrical fast transients generator. This device also allows theequipment under test to be powered, and the fast transients areapplied to the voltage waveform. Magnitude, frequency, phaseangle, and duration of the transient pulses are all controllable.

Electrostatic discharge, such as that produced by walking on acarpet and then touching a grounded device, producing a smallspark, can cause failures in chip-level electronic devices. To testdevices for this, a controlled spark is produced by a device thatgenerates a high static voltage and then discharges through theequipment under test. The static voltage produced is controllable.

Many devices are in use today that emit high-frequency radi-ation, and such devices may cause problems for some types ofequipment. In particular, low-voltage control and communica-tion lines may be affected by radiated EMI. High-frequency ra-diated EMI testing requires specialized field generators,measuring equipment, and shielded rooms for reliable results.Such testing is usually done by a commercial lab that specializesin high-frequency EMI testing.

Field TestingBecause of size, weight, and power limitations, it can be diffi-cult to bring large equipment into a laboratory. Voltage sags areoften the most important power quality issue with this type ofequipment, and a voltage sag generator can be taken to theequipment under test.

14 IEEE Power Engineering Review, February 2002

Table 1. Amplifiers for low-frequency disturbances

Voltage Source Current Source

Outage Harmonic current distortionCurrent probesCurrent transformersCurrent-sensing relaysCurrent-measuring instruments

Sag

Swell

Harmonicdistortion Radiated electromagnetic interference (EMI)

DisturbanceCharacteristics

ArbitraryWaveformGenerator

Amplifier(Voltage or

Current)

DeviceUnderTest

Monitors

Figure 1. Low-frequency disturbance generator

Computer-Controlled Switch

VariableAutotransformer

DeviceUnder Test

NormalSource

Figure 2. Voltage sag generator

A typical sag generator is shown in Figure 2. The sag genera-tor consists of variable autotransformers and computer-controlledswitches that switch the transformers in and out of the circuit.

The equipment under test is powered from its normal linesource. The autotransformers are set at the desired sag voltage,and the device is switched to the autotransformers for the de-sired duration of the sag. Operation of the equipment is moni-tored during the sag, and mitigation options may be appliedduring the tests.

Laboratory LoadsControllable mechanical and electrical loads are needed in thelab for power quality testing. When testing motors or motordrives, for example, the motor must be loaded to produce realis-tic results. The mechanical load is usually a dynamometer,which provides variable speed and torque loading for the motor.

Power supplies and rectifiers need to drive electrical loadswhen tested. Linear electrical loads include variable resistors,inductors, and capacitors, that can be combined in series or par-allel to produce a desired impedance. Nonlinear loads can oftenbe simulated by a rectifier bridge with variable linear loads con-nected to its dc side. Sometimes the actual load the device wasdesigned to drive, such as fluorescent lights, is used to load thedevice.

Measurement and Monitoring EquipmentA variety of instruments are used to measure the disturbancescreated in the lab by test equipment and by disturbing loads inthe lab. Other instruments measure the performance of a devicebeing tested. Many tests require measurement of numerous pa-rameters during a test.

Multichannel data loggers with appropriate probes and trans-ducers are very convenient for monitoring both power qualitydisturbances in the lab and the performance of the equipmentunder test. Commercial data loggers are available, or a computerwith data acquisition hardware and software may be used. In theWSU lab, for example, the same digital-to-analog converterhardware that generates waveforms for low-frequency testinghas 16 analog data acquisition channels for data logging. Thecomputer and data acquisition hardware and software speedmust be high enough to sample the disturbances generated.

For high-frequency transient tests, a digital storage oscillo-scope capable of triggering on transients is used. The scopebandwidth should be 250 MHz or higher to accurately catch thehigh-frequency transients. Lower bandwidth scopes may missthe peaks and ringing generated by transient voltages. At leasttwo differential channels are needed, one to monitor voltage andone for current.

Power and energy meters are used for some power qualitytests. Meters must be able to withstand and properly measure thepower quality disturbances applied in the lab. Because of thedistorted waveforms used in the power quality tests, all metersmust read true rms values. Digital multimeters are used to set upthe tests and provide visual indications of parameters during thetests.

Test ProtocolsPower quality testing may be done to determine how a device re-sponds to a particular waveform or disturbance. This is usually

done to troubleshoot a device that is not working properly. Inthis case, a power quality monitor is used at the field site to de-termine what disturbances are present. These disturbance char-acteristics are then reproduced in the lab and applied to thedevice under test.

Power quality tests are often used to characterize the re-sponse of a device to a specific set of power quality distur-bances. This allows manufacturers or users to verify that thedevice will operate through most common disturbances once in-stalled. A number of test protocols have been developed forthese tests.

Examples include test protocols developed for a large num-ber of devices by the Electric Power Research Institute (EPRI)Power Electronics Applications Center in Knoxville, Tennes-see. ANSI Standard C62.45 and Underwriters LaboratoriesStandard UL 1449 specify tests for low-voltage surgesuppressors. The semiconductor industry, through Semiconduc-tor Equipment and Materials International (SEMI), has devel-oped standards for sag testing equipment used in semiconductormanufacturing.

Future DevelopmentsWhile this article describes the most common power qualitytests, it is by no means an exhaustive discussion. Many distur-bances and types of equipment require creative application ofexisting test equipment, and others require new types of equip-ment to be developed. Such development is ongoing at WSUand other power quality labs.

References

WSU Power Quality Lab. Available: http://www.engr.twsu.edu/pqlab.

Power Quality Disturbances

IEEE Recommended Practice for Powering and Grounding SensitiveElectronic Equipment (IEEE Emerald Book), IEEE Standard 1100,1999.

Field Voltage Sag Testing

A. McEachern, Voltage sag generators: not as simple as they seem,Power Quality Assurance Magazine, p. 22, Jul./Aug. 2001. Avail-able: http://industryclick.com/magazine.asp?magazineid=286&SiteID=13.

D. Dorr, D. Nastasi, How a sag generator helped solve an elevatorproblem, Power Quality Assurance Magazine, May 2001. Avail-able: http://industryclick.com/magazine.asp?magazineid=286&SiteID=13.

Test Protocols

EPRI PEAC Corp. Available: http://www.epri-peac.com.

IEEE Guide on Surge Testing for Equipment Connected to Low-VoltageAC Power Circuits, IEEE Standard C62.45, 1992.

Specification for Semiconductor Processing Equipment Voltage SagImmunity, SEMI Standard F47-0200. Available: http://www.semi.org.

Transient Voltage Surge Suppressors, UL Standard 1449, 1996. Avail-able: http://www.ul.com.

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