Typical non-linear loads

We will be focusing on the six pulse rectifier bridge. The 3 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 20kW.

Common non-linear load topologies (six-pulsebridge rectifiers): A – without chokes; B– withAC reactor Lac; C– withDC-link chokeLdc.

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 grid when supplied with a sinusoidal voltage. These harmonic currents flow in addition to the “active” sine-wave, generate additional losses in electrical installations, and can result in thermal over-load.

Harmonic currents flow through the system impedance, resulting in non-sinusoidal voltage drops that can compromise grid 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.

ZDDQ Related Power Quality Solutions

◆Lv Active Harmonic Filter

◆Lv Static Var Generator

◆Hv STATCOM

◆Lv Automatic Power Factor Correction

◆Hv Automatic Power Factor Correction