Mitigate damaging harmonics with effective UPS design
It’s important to understand what creates these harmonics and how the uninterruptible power supply design mitigates them in the power system.
When looking at power quality in a facility, harmonics must be considered. Harmonics are created by nonlinear equipment in the electrical system such as uninterruptible power supply (UPS) units, which create current harmonics on the input. It’s important to understand what creates these harmonics and how the transformerless UPS design mitigates these harmful harmonics in the power system.
The main purpose of the converter section of the UPS is to convert the ac utility power to dc power. The amount of harmonics can vary greatly, depending on the type of converter. The most popular power electronics used for this process are the diode (6-pulse), the thyristor (SCR 6-pulse and SCR 12-pulse) and the insulated-gate bipolar transistor (IGBT). The IGBT converter has proven to reduce harmonics without the use of a transformer.
The 12-pulse SCR rectifier uses an isolation transformer in combination with two 6-pulse SCR rectifiers. The 30-deg phase shift provided by the transformer is the main purpose of the isolation transformer. The two 6-pulse SCR rectifiers will alternate to create twice as many pulses. The 12-pulse SCR rectifier will produce fewer harmonics than the single 6-pulse SCR rectifier, allowing for a smaller harmonic filter.
The diode bridge converter is similar to the 6-pulse SCR rectifier, except the diodes are natural commutation (natural on and natural off). The benefits of the diode bridge are better efficiencies and lower harmonics as compared to a traditional SCR rectifier.
Every rectifier, regardless of the technology or power electronics used, will produce harmonics. For all technologies but the IGBT, these harmonics are greater than desired for most electrical systems including the backup generator. Therefore, an input filter is required to reduce the harmonics to less than 10% of the total harmonic distortion of the input current. The input filter is comprised of an inductor with a parallel capacitor. If there is a transformer, it helps add inductance to the line to further reduce the harmonics.
Figure 1 above shows reflected harmonic distortion on input current waveforms from their respective rectifier or converter technology.
Although some manufacturers have started using IGBTs in the converter section, most are still switching the IGBTs based on the traditional SCR logic. The benefit of using a slower switching speed for the IGBT converter is a higher efficiency (less switching means less switching losses). In this case, the IGBT converter will still produce larger amounts of harmonics, which will require the input harmonic filter (similar to the harmonic filter for the SCR rectifiers).
The problem with traditional harmonic filters on the UPS converter is the leading power factor at small loads. When the UPS system is operating at a reduced load, the ratio of capacitance in the input filter to the load becomes very large and will produce a leading power factor from the UPS. This leading power factor can result in generator compatibility issues. To eliminate these issues, an active input filter (switching harmonic filter capacitors in and out of the circuit depending on the load) will need to be used, or the input filter would need to be disconnected. Both result in increased harmonic content.
Line-to-line noise produced by the high-frequency switching inside the UPS system is easily filtered using the small input and output filter of the UPS. However, high-frequency line-to-neutral components are not suppressed due to an absence of a common connection between the three phases (neutral connection).
To filter these components, a virtual neutral is createdby connecting the common point of each of the filter capacitors to a common point. By virtue of this connection, the common-mode harmonics are passed through the virtual neutral of the UPS system.
In addition to the input connection, the virtual neutral is also tied to the common point of the output filter, where the common-mode harmonics are canceled by the output of the inverter. During battery operation, the input contactor for the UPS system is opened, and the common-mode harmonics are eliminated from the equation.
The potential of the virtual neutral is derived from the three phases on the input. A capacitor is added between the system ground and the virtual neutral. Under normal conditions, this capacitor will have minimal potential across the terminals and minimal current, as the potential of the virtual neutral and the system ground is the same.
The output-phase voltage referenced to ground will be the same as the output-phase voltage referenced to the virtual neutral. The input common-mode harmonics are introduced through the virtual neutral, but they are canceled by the output common-mode harmonics.
Traditional diode and thyristor rectifiers produce greater than desired harmonic content for most electrical systems including the backup generator, thus requiring an input filter to mitigate harmonics. An even more effective way to reduce harmonics is with an IGBT converter, which can keep harmonics to less than 10% iTHD without adding the extra inductance of a transformer. The small filters on the input and output of the UPS can also be used to create a virtual neutral allowing common-mode harmonics to pass through to the inverter. The harmonics are then canceled by the output of the inverter.
The Bottom Line:
It’s important to understand what creates harmonics and how the transformerless UPS design mitigates these harmful harmonics in the power system.
Every rectifier, regardless of the technology or power electronics used, will produce harmonics. Therefore, an input filter is required to reduce the harmonics to less than 10% of the total harmonic distortion of the input current.
Small filters on the input and output of the UPS can also be used to create a virtual neutral allowing common-mode harmonics to pass through to the inverter. The harmonics are then canceled by the output of the inverter.
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John Steele serves an engineering manager for Mitsubishi Electric Power Products Inc. in Warrendale, Pa. Steele has been affiliated with Mitsubishi since December 2007. He holds a BS in electrical engineering from West Virginia University. Jenya DeBenedetti is a project engineer at Mitsubishi Electric Power Products, Inc. in Warrendale, Pa. since 2013. She earned a bachelor’s degree in electrical engineering from University of Pittsburgh. Karla E. Bert, P.E., is a project engineer at Mitsubishi Electric Power Products, Inc. in Warrendale, Pa. since 2011. She earned a bachelor’s degree in electrical engineering from Messiah College in Pennsylvania.