New Equipment Provides Detailed Data on AC Power Quality

The era of the jeans-clad power-company lineman clipping his trusty Simpson 260 to an entry box and declaring the AC power in good health is rapidly fading. The ever-expanding array of appliances attached to the AC lines has brought an end to the blithe assumption that a clean, steady sinusoid of constant amplitude is available for every device we add to the power load. When appliances do not function reliably, the quality of the input power often is suspect.

Power problems that must be addressed include interference imposed on power lines by appliances as well as the related characterizations of harmonic content and flicker. The IEC1000-3 standard spells out the allowable levels of interference caused by an electrical appliance that presents up to a 16-A load. One component of this standard, IEC1000-3-2, deals with harmonic currents caused by appliances. Another, IEC1000-3-3, addresses flicker and became effective on June 1.

These standards provide an indication of what levels of power problems an approved appliance is likely to cause. But there is a fly in the ointment. The levels will not be exceeded if the power-line impedance is equal to or less than the standard test situation. This means resolution of power-line problems must include examination of the appliance and the power distribution.

If either the power line or the appliance were perfect, there would be no problem. In practice, both power distribution and load contribute to the problem, requiring the technician to evaluate both potential causes. A modern power analyzer makes this evaluation straightforward and repeatable and provides documentation of the tests performed.

Troubleshooting Aids

Power analyzers are useful tools in both product development and power- distribution troubleshooting. As a developmental test instrument, the power analyzer is used to verify that an appliance does not generate power-line problems beyond allowable limits. As a troubleshooting instrument, the power analyzer isolates the cause of observed power problems.

Corrective action or exoneration of the AC power as the problem requires that a cause-and-effect relationship be established between events on the AC line and the problem occurrence. Once such a relationship is established, characterization, location, and resolution of the responsible source of the troublesome power-line event are the next logical steps.

Power analyzers are the tools of choice to identify and characterize AC line problems. As a first step, establishing the cause-and-effect relationship requires monitoring both the power and the operation of the device that is malfunctioning.

Once the malfunction occurs in time coincidence with a power-line anomaly, a similar technique can be used to associate the observed anomaly with its source. The nature of the anomaly helps identify the type of source suspected.

Next, the time that the anomaly occurs is compared with the time of operation of the suspected source. A good power analyzer notes and records the time and nature of an event on the power line. This allows the technician to direct his attention solely to documenting the appliance operation.

Then, the time of the observed malfunction can be compared to the recorded time of the power anomaly to establish the correlation of the malfunction and the power-line event. The nature of the recorded event will indicate the probable cause. The characteristics of the event also will help to establish whether the problem is source or load-related.

Source vs Load Problems

Source problems are defined as anything on the AC power side of the analyzer connection. This is an important definition because distortions caused by other electrical distribution paths in the building can become source problems.

On the other hand, the power company will classify such a situation as a load problem. For example, if an analyzer is connected at the outlet where a cranky computer is connected while an air conditioner connected to another branch circuit causes a voltage dip, the analyzer perceives the problem as a source problem. If the power analyzer is connected at the main power distribution connection, it will identify the problem as a load problem.

For this reason, several independent power-analyzer measurements will assist in narrowing the location of an internal problem. Deployment of the analyzer as close to the power source for the building will better document an external problem source.

Sources of Problems

There are many common internal sources of AC line problems, and more are added daily. Older structures have smaller gauge wires and, quite possibly, heavier corrosion loads at points of connections that will make transient load problems worse by presenting greater source impedance.

Lamp dimmers, solid-state ballasts, and switching power supplies impose harmonic distortion. Laser printers and air conditioners cause brief overloads that result in abrupt voltage sags. High current, mechanical contacts on appliances such as water heaters generate transient spikes. Large reactive motors cause power factor variations. All these loads distort the available power input to other devices on the AC line and, in some cases, radiate to interfere with RF devices or even corrupt nearby wire data transmission.

There also are many external distortion sources. Power-line leakage and corona cause harmonic distortion. Insufficient wire size or poor connections contribute to voltage sags during load transients. This is a prevalent problem in areas where aluminum conductors have been in place for extended periods of time. Lightning protection devices can cause multiple brief power interruptions. Switching of power-factor correction capacitors by the utility causes discontinuities in harmonic content and voltage.

Flicker

Flicker is a relatively new class of AC power distortion. Flicker differs from other power distortions because it deals with the effect of the power quality on humans rather than on equipment.

Since most buildings use electrical lighting, power fluctuations cause lighting fluctuations. The result of this can be simply annoying, producing headaches and eye fatigue. The discovery that cyclical light variations with a frequency close to 9 Hz can trigger epileptic seizures in susceptible individuals precipitated an attempt to quantify and specify allowable flicker limits.

Unfortunately, there is no such thing as a standard person. Each individual’s response to flicker differs. There also is very little available information about how the parameters of amplitude duration and frequency of flicker interact to affect the human nervous system.

Added to this is the varying physical response of different types of lighting to voltage fluctuations. For example, fluorescent lights react quicker to voltage change than do incandescent lamps, but the magnitude of the reaction is less.

IEC1000-3 is the EC document that addresses flicker. It includes a block diagram of a flicker meter and shows how the AC power parameters must be measured to produce the specified numbers as opposed to a device construction guide. Since the intent of this standard is to put a limit on the effect of an appliance on the observed flicker, the standard assumes a nominal line impedance, lighting response, and human response.

A device that passes this test is in no way guaranteed not to cause any problem under any circumstance. But it does provide a standard test and benchmark by which the characteristics of one appliance may be numerically compared to another.

The test performed by the flicker meter is simple in theory, complex in execution. The complexity comes from trying to reduce a large number of variables: frequency, amplitude, duration, and human response to a single- number designated flicker. By definition, flicker values are scaled. A flicker value of 1 means half of the human population considers the variation to be objectionable and half does not.

Technically, the test as presented in IEC1000-3-3 is defined only for voltages in the range of 220 to 250 VAC, 50 Hz. The source impedance for the flicker test is defined as 0.24 W resistive in series with 0.5 W reactive on hot lines and 0.16 W resistive in series with 0.1 W reactive on neutral lines. The load appliance is limited to devices drawing less than 16 A.

Changes induced by manual switching, such as turning on the device, are not considered. A change of the relative steady-state voltage exceeding 3%, the maximum relative voltage exceeding 4%, or a duration of change greater than 200 ms is, by itself, a failure of the test. Beyond these limits, a value of the 10-minute flicker value of 1 or more or a 2-hour flicker value of 0.65 or greater constitutes a failure of the test.

The rather complex calculation of the flicker value is defined by the block diagram of a flicker meter in the IEC868 document rather than by a formula. Using this block diagram, several manufacturers of power analyzers have incorporated the flicker meter into their instruments. When such an instrument is used, the necessary calculation is performed automatically, yielding a single- number output—the flicker rating of the device under test.

Selecting a Power Analyzer

A power analyzer also can quickly characterize the harmonic current load, power factor, and flicker characteristics of any appliance suspected of causing a problem. Power analyzers are available in a broad range of capabilities and cost. The first and most obvious differentiation among analyzers is the choice between a stand-alone and a PC-based unit. The stand-alone unit is easier to set up and move around when used to isolate a problem, but also is generally more costly. The PC-based unit is less expensive since it uses the resources of the host computer, but it is better suited to bench-type testing rather than troubleshooting existing AC power problems.

The selection of a power analyzer is determined by its intended application. For appliance testing, several manufacturers outlined some minimum specifications.

Herman vanEijkelenburg, product marketing manager at California Instruments, had two suggestions: Seek sample rates of about 100 kS/s to capture fast transients. Also useful is the capability to acquire continuous data for 16-cycle intervals without permitting any gap between successive intervals for periods of 2.5 minutes or more.

“All inputs must be fully isolated from each other and earth,” said Georg Zimmer of Zes Zimmer. He also advocated a minimum bandwidth of 6 kHz and an accuracy of 0.05% of the measurement range if IEC approval measurements will be made with the instrument. He added a recommendation for traceable calibration certificates.

Addressing the measurement of power factor was Dick Troberg, senior product specialist at Fluke. “The power-factor function must display the traditional power factor now known as displacement power factor or cosine as well as the true power factor which takes into account the kVA generated by harmonic content.”

Report generation during appliance testing is essential. When testing for approval under IEC100-3-2 for harmonic content limits, a minimum test data set includes a pass or fail determination, voltage and frequency of the power source, and a list of observed harmonics.

Desirable additions to this include graphics depicting the observed data with any out-of-tolerance measurements highlighted. These reports can be produced on a built-in printer or, in the case of PC-hosted machines, output to the regular printer used with the PC.

Another alternative is to output the test results to disk, where any data of interest can be called up for examination later. A visual display is a convenient addition, especially for troubleshooting, but a permanent record is necessary for any approval-type testing application.

Summary

When the intended application is for troubleshooting, size, cost, and rapid data display become paramount. Data storage and report generation are still required, but the emphasis shifts to concise, rapid display.

According to Rich Bingham, manager of products and technology at Dranetz-BMI, “Local data storage on power quality monitors is used for troubleshooting in the field as well as temporary storage until it can be transferred to a PC for further analysis and report generation. The use of the PC allows more detailed and extensive analysis as well as incorporation of the data into standard and custom reports.

“The type of report and detail needed depend on the application,” he continued. “Often times, the problem is solved right in the field by looking at a time plot and corresponding harmonic analysis. Other times, it requires a cycle- by-cycle examination of the magnitude, phase, and harmonic characteristics of the voltage and current on all phases during the event.”

Power Analyzers

Power Analyzer Provides

IEC 1000 Tests and Reports

The WT2000 Series features a 500-kHz bandwidth, an accuracy of 0.04%, and an RS-232-C or GPIB interface. The instrument is designed to accomplish IEC1000-3-2 power and harmonic measurements as well as IEC1000-3-3 flicker measurements. Three 32-bit DSPs implement 8,192-point FFT analysis to provide true simultaneous power measurement on three-phase power. An optional built-in printer to document test results is available. Starts at $12,925. Yokogawa Corp. of America, (770) 253-7000.

PC-Based Test System

Simplifies Compliance Testing

The Compliance Test System includes a high-performance AC power source used in conjunction with a PC-based measurement system to provide a cost-effective test solution for CE Marking test requirements in the design and production phases. Flicker measurement in accordance with IEC868 is included in the programming. A Windows-based program runs the test, logs the test data to disk, and prints a pass or fail test report. Starts at $9,975. California Instruments, (800) 422-7693.

Analyzer Provides Log

Of Field Measurements

The PQR D-50 is a troubleshooting instrument to document power anomalies. It automatically monitors a single-phase power line and records a log of power disturbances in a battery-backed memory with time and date stamp. Software accompanying the unit allows retrieval, storage, and analysis of recorded data on a standard PC operating under Windows 95 or NT. Additional recorder channels for humidity and temperature monitoring are provided. $995. PowerTronics, (603) 483-5876.

Field Power Harmonic Analyzer

Optimized for Troubleshooting

The Model 41B is a rugged, battery-powered, hand-held unit designed for quick and accurate field measurements for troubleshooting and problem resolution on single- or three-phase power systems. It features an alpha numeric and graphical LCD readout. Direct readings of harmonic content are provided in addition to voltage, amperage, frequency, and power. It can store eight waveforms for later uploading to a PC. A direct interface to Epson or HP printers also is provided. Call company for price. Fluke, (800) 443-5853.

New Software Automates

IEC Compliance Testing

The EN61000 Windows Software runs on a standard PC under Windows and controls the company’s PM3000A Power Analyzer and an AC test source via an RS-232-C or IEEE 488 bus. It performs all required tests and calculations for IEC EN61000-3 compliance including voltage fluctuation and steady-state and fluctuating harmonics. Recording and displaying test results are accomplished without operator intervention. Printed reports can be produced in several formats including comparison with approval limits. $395. Voltech Instruments, (919) 461-1701.

Copyright 1998 Nelson Publishing Inc.

June 1998

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