The steady increase in non-linear loads on the power supply network such as, for instance, variable speed motor drives (VSDs), raises questions about power quality and reliability. In this respect, a great deal of attention has been focused on harmonics as they overload the network infrastructure, cause reliability problems on equipment and system level, and waste energy. Passive and active harmonic filters are used to keep these harmonic problems in check.
The use of both active and passive technology is justified. The difficulty is in how to select and deploy harmonic filters correctly, which is key to achieving a satisfactory performance. This article explains which specifications are suitable when it comes to choosing active and passive harmonic filters and which mistakes need to be avoided.
Power quality and harmonics
The load exerted by harmonics on the power network infrastructure has increased dramatically over the past few years. Harmonic currents are caused by non-linear loads. A non-linear load is a consumer of electricity that draws a non-sinusoidal current from the supply network when supplied with a sinusoidal voltage. These harmonic currents flow in addition to the 'active' sinewave, generate additional losses in electrical installations, and can result in thermal overload.
Harmonic currents flow through the system impedance, resulting in non-sinusoidal voltage drops that can compromise network voltage quality. Sensitive loads, such as medical devices or IT infrastructure, can have their operation affected if the voltage supply is distorted.
Measures for reducing harmonics are implemented nowadays in order to resolve this issue and comply with national and international standards at every level of the network infrastructure. In this article, we will focus solely on the use of passive and active harmonic filters in low-voltage installations.
Typical topologies of non-linear loads
In our further observations we will be focusing on the six pulse rectifier bridge. The three-phase rectifier has a key role to play because a significant portion of the electrical energy is drawn from loads with this type of front-end circuit. One typical application that can be mentioned is the variable speed motor drive, which has been used for years with high growth rates in almost every industrial sector.
The most common six-pulse rectifier topologies are shown in Figure 1. Topology A does not include any magnetic components for smoothing the current. Topology B is operated using an upstream AC inductor Lac, usually in the form of a laminated line-reactor. Topology C has a built-in DC choke Ldc, which is often integrated in higher power motor drives. In all three topologies the grid, including the line impedance, is depicted on the left-hand side. On the right the constant power sink 'P=const' represents the active power drawn from the DC/AC inverter and motor that is taken, for example, to be 20 kW.
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Figure 1: Common non-linear load topologies (six-pulse bridge rectifiers): A - without chokes; B - with AC reactor Lac; C - with DC-link choke Ldc.
Figure 2 shows the input current i for topologies A, B and C from a balanced three-phase power line. Zline represents the equivalent impedance of a distribution transformer, distribution line, fuses, etc., which are assumed to be half inductance and half resistive in our example (L=18 uH; R=6 mOhm). The following values are used:
DC-link capacitor Cdc: 2000 uF
AC-reactor (topology B) Lac: 500 uH (2 percent)
DC-link choke (topology C) Ldc: 1 mH
(Click on Image to Enlarge)
Figure 2: Input current i (white) and its active ia (green) and reactive ib (red) components for topologies A, B and C from Figure 1. All specified values are in ARMS.
Note: LabVIEW-based software specially developed by Schaffner was used to decompose the waveforms in Figures 2 and 6, which was presented for the first time at the IEEE International SPEEDAM 2008 (Virtual Laboratory for Harmonics Filtering Visualization).