Low Harmonic Technology Reducing harmonic currents for SED2 Building Technologies s 1
Harmonic distortion damages equipment and creates a host of other problems The use of variable speed drives (VSD) in HVAC systems has proven both efficient and cost-effective. However, the generation of harmonic currents on the AC line can lead to problems and increased installation cost. Building design specifications often require compliance with local installation guidelines and standards such as IEEE519.2 (USA), EA G5/4 (UK and Hong Kong), EN 61000-2-2 (Europe) and IEC (AS/NZ) 61000-3-6 (used globally).
■ How conventional VSDs work To understand harmonics, one must first understand how a conventional VSD works and how its power flows. Conventional VSDs consist of a 6-pulse diode rectifier, DC link capacitor, IGBT inverter and a processor-based controller. The diode rectifier is used to convert AC line voltage to a constant fixed level DC voltage. The DC link capacitor acts as a filter to smooth the DC link voltage and help keep it constant.
Harmonic considerations for HVAC applications ■ Common problems that arise from harmonics Harmonics can create many problems in a facility. They can cause additional motor heating as well as higher RMS currents through connected transformers and feeder equipment. Sensitive equipment such as instrumentation, computers and communications systems may fail to function correctly or get damaged.
■ Harmonic current spectrum: The fundamental problem All periodic waveforms can be represented by a set of sinusoidal waveforms consisting of the fundamental frequency plus various other harmonic frequencies. The AC line harmonic currents, with a 6-pulse bridge, have characteristic frequencies at 6n±1 times the fundamental frequency where n is an integer. A 6-pulse VSD on a balanced supply will only generate odd order harmonics from the 5th order upwards (5th, 7th, 11th, 13th, 17th, etc).
The SED2 reduces input line harmonic currents without using line reactors or DC link chokes Siemens has introduced the SED2, a family of VSDs designed specifically for the HVAC market. Typical applications for this drive are fan and pumps with variable torque load characteristics. The fact that these applications are low dynamic by nature means the overall topology of the SED2 now provides a new method of reducing the input line harmonic currents without the addition of extra components.
15O 10O Current (%) 5O O -5O -10O -15O Time Current waveform of a typical 6-pulse VSD vs. the SED2 At the input terminal of a 6-pulse VSD, THD can be in excess of 120 % without any filtering (i.e. AC line reactors or DC link chokes). The SED2 produces only 29 % THD. Total harmonic distortion reduction comparison In tests between a typical 6-pulse VSD with and without AC line reactors and DC link chokes versus the SED2, the Siemens Easy Drive demonstrates superior harmonic performance.
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