The newest industrial, commercial, and consumer electronic equipment requires AC power that is closer to an ideal source than ever before. While avoiding unexpected operation from power variations, products must not be the cause of power disturbances.
Unfortunately, power quality has deteriorated over recent decades rather than improve. This is a global problem with governments in every region of the world adapting and enforcing EMC standards. The International Electrotechnical Commission (IEC) standards define the requirements used in Europe as well as the United States, Japan, and other regions. At the heart of the issue are problems that involve harmonics, flicker, and immunity to various supply fluctuations and anomalies with the associated testing to verify compliance. And, the standards are in a constant state of flux.
EN/IEC Standards
Whenever standards change or have additions approved, interest increases in verifying the compliance of existing products to the new standard. Of course, if a new product is developed, it must be tested to ensure its initial conformance with the existing standards. Among the hot areas for new products are medical instrumentation and cloud computing storage, but any commercial, industrial, or consumer product may require testing to meet EMC standards.
IEC 61000 establishes the requirements and procedures for harmonics, flicker, and other supply variations. CENELEC, the European Committee for Electrotechnical Standardization, provides the enforcement arm for IEC-developed standards in Europe. Approved standards have the same number as the IEC standards with a EuroNorm EN prefix instead of IEC. Table 1 presents a summary of some of the key sections in EN 61000.
Table 1. EN Specifications Addressing Several EMC Issues Including Harmonics and Flicker
If not properly designed or rated, electrical equipment often will malfunction when harmonics occur in an electrical system. Power systems designed to function at 60 Hz in the United States and 50 Hz in many other countries may experience unsatisfactory operation and, in some instances, failure when subjected to voltages and currents that contain substantial harmonic frequency elements. Quite often, the operation of electrical equipment may seem normal, but under a certain combination of conditions, the impact of harmonics is enhanced with damaging results. As a result, EMC requirements address product immunity and emissions.
Products must be designed for the normal and periodically abnormal variations in grid power. Predictable operation, such as survival, auto-resume, and manual restart under worst-case variances provide acceptable design criteria. EN 61000-4-11 and EN 61000-4-13 are the most relevant immunity test standards.
When a piece of equipment produces excessive harmonics or voltage variations, it can affect other equipment. As a result, there are harmonics and flicker requirements per EN 61000-3-2 and EN 61000-3-3, respectively, to limit their harmful effect on other products and distortion on the power network.
IEC 61000 defines four test classes—A, B, C, and D—that have distinct harmonic current limits. Classes-B, -C, and -D describe specific products and product families. All other products and motor-driven equipment are automatically categorized as Class-A.
While the limits for Class-A and Class-B equipment are given in Arms, Class-C and Class-D limits vary with the power level of the tested product. There are no limits for even harmonics in either Class-C or -D except for the 2nd harmonic in Class-C. The 3rd harmonic limit of Class-C depends on the product’s power factor. Table 2 shows the types of products that fall into each class.
Table 2. The IEC Class Designations
In contrast to harmonics, voltage variations, called flicker, can cause problems to lighting and other products. Personal computers, for example, can withstand voltage drop out for 40 to 50 ms and a dip of 40% for about 80 ms. EN 61000-3-3 provides the specific requirements for products.
IEC specifications are in a constant state of renewal and upgrading with frequent new editions. The flicker measurement standard 61000-4-15 Edition 2.0 was published in late 2010. According to Mathieu van den Bergh, secretary of IEC SC 77A/WG1 involved in several task forces for measurement equipment within that group as well as IEC SC 77A/WG2, many changes in IEC specifications are expected in 2011. These include IEC/TR 60725, 61000-3-2, 61000-3-3, and 61000-3-12 and IEC 61000-3-15.
A new edition of IEC/TR 60725 for reference impedances and public supply network impedances will be completed in 2011. Several amendments to 61000-3-2 will be approved in 2011. One of the key amendments will move variable-speed drive-based refrigerators into Class-D.
New work has started on 61000-3-3 to make provisions for low-power, high-efficiency lighting because of the transition to compact fluorescent (CFL) and light-emitting diode (LED)-based lighting. CFLs and LEDs have a response to voltage fluctuation that differs from the standard incandescent bulbs, which provided the basis for the flicker standard. The different thermal responses of CFLs and LEDs require changes in the flicker standard to accommodate these differences. It probably will take a couple of years to complete this work.
For 61000-3-12, the standard for >16-A harmonics, a new edition will be approved within the next six months. It will have a new test table for adjustable-speed motor drives. There are several types of motor drives, and one of them is a low-capacitance drive that has a square wave current. The new table addresses the potential issues associated with the square wave.
In addition to changes due to more efficient lighting and motor controls, other well-known activities that could instigate standards changes include electric vehicle (EV) and plug-in hybrid vehicle charging; dispersed generation addressed in IEC 61000-3-15 for wind, solar, and EV-generated power; and cloud computing’s server banks and cooling.
A User Example
Whether the standards have changed or a new product has been developed, test engineers must determine how they will test the product to ensure conformation to the latest version of the standard. Companies that want to obtain the CE mark must pass these tests. A user example shows the types of testing that must be performed and how the testing was accomplished.
A customer that makes information infrastructure equipment including high-end data storage products had to consider the full range of EMC tests. The company designs and manufactures a range of small to very large storage systems used in data centers. A compliance test system (CTS) provided the required testing for the company’s newest network storage system.
For its products, the company must perform voltage dips and interrupts as well as various immunity and emissions tests. With the critical role that the data storage systems provide, the manufacturer has to ensure uptime in excess of 99.999%.
While very concerned with immunity, the company’s products also involve emissions because excessive harmonics from multiple storage platforms could affect the quality of the power network of the whole data center. Having harmonics and other power quality aspects under control also adds to the efficient operation of computer storage products.
This is critically important to any facility with a larger number of data storage systems since it minimizes the facility’s power consumption for the data center and avoids unnecessary cooling costs. While complying with the standards is essential, in many EMC tests, the company’s internal requirements go beyond the standard requirements to ensure a high level of performance and reliability.
Figure 1. A CTS Consisting of an AC Power Source, a PACS, and a PC-Based Data Acquisition System
As shown in Figure 1, AMETEK Programmable Power’s CTS 3.2 Series is a turnkey compliance test system for EN/IEC 61000-3-2 including A14, EN/IEC 61000-3-3, and various EN/IEC 61000-4 AC immunity tests. The system consists of a programmable AC power source, a power analysis conditioning system (PACS), and a PC-based data acquisition system. CTS software based on Windows technology performs the IEC tests and generates detailed test reports that are stored on disk to allow post-test analysis.
The data acquisition system used to implement the required IEC compliance measurements has a high-speed digital signal processor. A special signal conditioning and isolation unit provides easy connection between the AC source output and the EUT. Figure 2 shows the actual test configuration.
Figure 2. Typical Functional Test Setup for Performing EMC Tests
This approach to EMC testing achieved two of the company’s goals:
•?The capability to support future versions of test standards by installing new PC software to reduce the risk of product obsolescence as the test standards evolve.
•?The capability to provide a complete data record for each test since the data is streamed to a hard disk in real time.
The IEC-compliant power analyzer provided detailed information on voltage and current. Measurements of both harmonics and interharmonics were made in real time with no measurement gaps to fully conform to the latest revision of the IEC 61000-4-7 test standard.
AC source voltage and EUT power were monitored continuously during the entire test. The voltage distortion and current harmonic data were checked against IEC class limits for pass or fail detection. With the CTS, full compliance testing for current harmonics emissions of the Class-D product was conducted that included conformance with Amendment 14. In addition to the waveforms and harmonic levels on the PC screen, the system displays relevant power parameters and test conditions (Figure 3).
Figure 3. Example Screen Capture of the Voltage and Current Waveform of an EUT Operating at 230 VAC, 50 Hz
The company will retain the test limits in a password-protected database that can be updated in the future without the need to change software. In addition, test engineers can make other software modifications to accommodate revisions in the IEC harmonics by simply installing new PC software. No harmonics testing software resides in system firmware which would require more costly field upgrades.
The IEC 61000-4-15-compliant flicker meter was accomplished with the CTS software. An IEC 60725-compliant reference impedance was implemented in the iX Series AC Source using a programmable output impedance.
In contrast to a lumped reference impedance, the programmable impedance provided improved accuracy and the capability to support different national standards the company wanted to address without switching lumped reference impedance hardware. EN 61000-3-3 flicker results were measured and displayed in real time during the entire test. As a result, there was no need to wait until the end of the test to know if the EUT passed or failed.
Precompliance voltage dips and interruptions per EN 61000-4-11 were determined. For full compliance, the electronic output switch option EOS1 was added to the system. Testing to the EN 61000-4-13 interharmonics standard also was performed on the CTS system by adding the -413 option.
These are just a few examples of the tests that were performed. However, additional immunity test standards are supported on all iX-based CTS products.
In-Progress EMC Standards
Generally, EMC test standards address products that consume power rather than produce it. However that changes with distributed renewable energy sources such as solar and wind power. IEC 61000-3-15 Limits—Assessment of low-frequency electromagnetic immunity and emission requirements for dispersed generation systems in LV network targets these power sources. This document is in the development phase. Anything that is distributed and puts power back on the grid will have to conform to this new power quality standard.
To make matters worse, power quality problems will be amplified when equipment that puts power back onto the grid comes online. Since the inverters can easily output 5,000 W or more, they have a much greater impact on EMC than a TV, refrigerator, HVAC unit, or any product that draws 1,000 W or less. As a result, the level of noise, harmonics, and flicker from the inverters sending power back onto the utility is extremely important. A new aspect for these types of power sources is the anti-islanding requirement.
Islanding occurs when a portion of an electric power system is energized solely by a local power system through the common point while it is electrically separated from the rest of the power grid. Since unintentional islanding of a distributed power source may cause power quality issues, requirements for anti-islanding have been developed.
Standards generally take a long time from initial identification until final approval and release with many industry experts involved in their development. This certainly is the case with IEC 61000-3-15. When it is released sometime in 2011, the industry’s leading test companies are prepared to offer test equipment to qualify products to the latest revision. For the software-based CTS solution, revised testing involves a software upgrade and not the replacement of a dedicated hardware-based analyzer.
Conclusions
Manufacturers should test any grid-connected equipment for harmonic susceptibility. To obtain the CE mark, the testing is required. While EMC testing has focused almost exclusively on the impact of grid power sources on products and the products’ impact on the grid, future requirements also will add qualifying equipment that puts energy onto the grid. In general, the interest in EMC testing increases whenever the standards change. Unfortunately, changes in standards and data requirements to conform to those standards can make hardware-based test equipment obsolete.
To avoid increased costs, companies investing in EMC test equipment should consider software-based testers that can be upgraded rather than replaced. Similar to other products, when software can be easily installed on an EMC tester, the disruptive impact of a standard change is minimized. For those selecting EMC equipment to qualify a new product, the software selection criteria can assist in reducing the qualification efforts.
About the Author
Eric Turner is the product marketing manager and head of Renewable Energy Initiatives at AMETEK Programmable Power. His career includes 10 years of international sales in more than 45 countries and extensive experience with design and test of sophisticated missile guidance systems. e-mail: [email protected]