As the world population grows and energy consumption increases, nations have become increasingly concerned with the future availability of electrical power. One way to minimize the effects of the threatened shortfall is to reduce consumption by using more energy-efficient lighting and motor drive systems.
However, the more efficient electrical systems often use sophisticated semiconductor-based electronic circuits that produce current harmonics. This affects power quality and creates a problem for others on the public utility networks. Computers, TVs, fax machines, and printers are among the major offenders.
Government Standards for Harmonics in Europe
Several years ago, the International Electrical Committee (IEC) issued Standard 555.2 dealing with the effect of harmonics on the electric distribution system. Later, that standard was refined and reissued as IEC 61000-3-2. Since Jan. 1, 1996, most electrical devices sold in the member countries of the European Union (EU) have met this standard.
In general, an IEC directive does not have the legal force of law. However, the EU issues Euro Norms (EN), and these are legally binding. The relevant enforceable standard for harmonics is EN 61000-3-2. Individual member countries have issued identical national norms that carry the same legal enforceability.
Penalties for violating an EN standard range from hefty fines to jail time. In cases where the manufacturer is not located in the EU, the distributor or authorized agent is liable. To further the penalty process, local customs agencies can prevent equipment that does not have proof of compliance from entering the country. For example, conformance to the EMC low-voltage directive for harmonics is one of the several factors that must be met before a product is acceptable.
A confusing issue is the nature of the products that fall within the IEC standards. While the original specifications focused primarily on consumer products and excluded most professional and industrial equipment, IEC 61000-3-2 includes almost all electrical products with a rated input current up to 16 A rms per phase and an input voltage of 220 V or higher.
Many manufacturers—even those making the worst harmonic current generators—believe that their products are exempt from EN requirements if they are subassemblies and not deliverable consumer products. This is a mistake, because the ruling applies to power supplies as well as finished products.
There are no government regulations controlling harmonic generation for products sold in the United States and Canada. The per capita consumption of electricity is much higher in these two countries than in Europe; a big part of the difference is the greater use of air conditioning and electrical heating, both relatively kind to the power line. The committee responsible for IEEE 519-2000, IEEE Recommended Practices and Requirements for Harmonic Control in Electrical Power Systems, constantly evaluates the situation, but there is general agreement that mandates are not required.
EN 61000-3-2 Amendment 14
The 1995 standard defined four product classes, each with its own performance criteria. Class D pertained to TVs, PCs and their monitors, and a host of other product types. Class C comprised all lighting products, including dimmers, with an active input power >25 W. All portable electric tools were Class B. Class A had all motor-driven equipment, most domestic appliances, and virtually all three-phase equipment that draws no more than 16 A rms per phase.
Several key questions came from designers, testers, and users of the 1995 standard. For example, should the voltage source be checked for compliance continuously during a test or only prior to the test? Since Class D limits were specified in terms of milliamps per watt (mA/W), what value of power (W) should be used to calculate those limits? Can the tester use the power rating specified on the rating plate or the level measured during a pre-test rather than monitor the wattage throughout the test sequence?
In response to these difficulties with the standard, Amendment 14, the so-called Millennium Amendment to EN 61000-3-2, was adopted in late 1999 and went into effect Jan. 1, 2001. The new document changed the classification of many types of products from Class D to the less strict Class A, leaving only TVs, PCs, and PC monitors in Class D. Also, it changed the way that limits are computed for Classes C and D.
The amendment references the measurement method instructions of IEC 61000-4-7 Test and Measurement Techniques—General Guide on Harmonics and Inter-Harmonics Measurements and Instrumentation for Power Supply Systems and Equipment Connected Thereto. This document is likely to be harmonized in 2002.
There is a three-year grace period for adoption of Amendment 14. “For most manufacturers, however, postponement is not an attractive option. The relaxation of some requirements has encouraged many of them to start using the amended standard immediately,” according to Herman van Eijkelenburg, vice president of product development at California Instruments.
Harmonic Testing
The amended IEC 61000-3-2 standard specifies the test setup of Figure 1 (see the November 2001 issue of Evaluation Engineering) for measurement of harmonics. Power is applied to the DUT from a sine-wave source using a resistive component and a reactive component in each line. The degree of conformity to the standard is determined by the level of harmonics generated by the product and measured across points A and B. Test limits for a given product depend on its class.
The current harmonic limits for each class, based on the European 230-V, 50-Hz power distribution norm, are shown in Table 1 (see below). Limits prove to be most stringent for Class D equipment. Since many of these products use low-cost switching power supplies with rectifier capacitor inputs, each unit exhibits relatively high odd harmonics. Thousands of them in use simultaneously can cause significant power-line problems.
Table 1. Harmonic Limits for Each Equipment Class
Harmonic (n) of 50-Hz Fundamental
(A)
(A)
(% of Fundamental)
(mA/W)
Class C and Class D limits are not specified in absolute values but as a percentage of the current at the fundamental frequency. The third harmonic limit also is a function of the power factor (PF), so it is harder to meet the limits as the power factor deteriorates. Neither Class C nor Class D products must be checked for even-harmonic current levels.
Since Class C and D limits are load dependent, the power level and PF must be determined for each test. In case of fluctuating power levels, it may be necessary to measure the power level during the test and adjust the pass-fail limits dynamically.
Class A and Class B limits are not related to the power level of the product being tested but instead expressed as the maximum acceptable currents at various harmonic frequencies. Class B levels are 1.5× those of Class A.
For the 21st and higher order odd harmonics, the average value for each may exceed the limits by up to 50% under specified conditions. The harmonic must be measured over the full observation period and smoothed for 1.5 s. The partial odd-harmonic current (POHC) cannot be greater than the calculated limits, and none of the 1.5-s smoothed harmonics can exceed the limits by more than 50%. Currents are disregarded if less than 0.6% of the input current or below 5 mA.
The quantification of the POHC is based on this equation:
Steady-State vs. Transitory Harmonics
Quasisteady-state and transitory harmonic tests are specified in EN 61000-3-2. The transitory tests recognize equipment in which power demands vary and allow temporary extension of current limits by as much as 50% if such elevated levels do not occur more than 10% of the time. This requirement imposes more demands on the power analyzer than on the AC power source.
Steady-state harmonics are generated by equipment with a constant current draw, such as fluorescent lighting fixtures. Some other products, like a laser printer whose heating element kicks in whenever a page prints, have fluctuating power demands and require transitory harmonics testing.
Measurement of the total harmonic current (THC) is not specifically required by the standard. However, harmonic emissions tests must be conducted with the controls or programs on the product set to the mode that is expected to produce the maximum THC, and the tester will find this quantity helpful in defining that mode.
THC, the summation of rms values of the 2nd through the 40th harmonics, is calculated as follows:
Must You Redesign to Reduce Harmonics?
Harmonic reduction methods of a few months ago may prove unnecessary now. “Designers should test Class D equipment to the amended standard before introducing power factor correction or other harmonic reduction techniques,” said Jonathan Francis, marketing manager at Voltech Instruments. “Such redesigns may no longer be needed. Also, although the harmonic limits have not changed, products are more likely to comply with the new standard because some of the limit checking is performed against the average reading rather than the instantaneous levels.
“When a product does not comply the first time it is tested,” he continued, “the design engineers need to know the nature of the failure before embarking on redesign. Software that provides clear and transportable diagnostic information is extremely valuable. For example, a transitory harmonic failure might be corrected by using a simple inrush-limiting resistor on the equipment that generated the transient. Where the averaged harmonics exceed the limit, power-factor correction should be considered. For a marginal failure, passive power-factor correction using an inductor may be sufficient rather than going to a complex circuit.”
Test-System Architecture
Fortunately for engineers who develop products for the European market, test systems with versatile software packages are available to evaluate harmonics in the development laboratory. This helps avoid surprises when the product is taken to the EN-approved laboratory. Suppliers have developed software that addresses the basic standard of Amendment 14. In most cases, the test equipment includes additional versatility such as the capability to conduct flicker tests per EN 61000-3-3.
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November 2001