Power electronics is a key technology in terms of energy conversion in industrial motor drive systems. Electrical motor drives consume considerable amount of global electrical energy and majority of them are equipped with power electronics technology. Typically a three-phase diode rectifier is employed as the first conversion stage following with a voltage source inverter (Fig. 1). Simplicity, reliability, robustness, and being cost-effective are the main reasons that the majority of the systems apply a diode rectifier at the front-end. However, non-linear characteristics of the diode rectifier cause harmonic input current, which has undesirable consequences. This problem becomes significant when a large number of industrial converters and variable speed drives are connected to a Point of Common Coupling (PCC), where other users are also connected to the same network. The other problem associated with the harmonic distortion is unnecessary losses and heat in power system transformers and nuisance tripping of circuit breakers and over-stressing of power factor correction capacitors. Therefore, maintaining the level of the generated current harmonics within the acceptable limits and recommended by standards, such as IEC61000, has been always a challenge. This becomes more evident with applications such as adjustable-speed drives where majority of them still using conventional ac-dc conversion stage.
The solution is suitable filtering which can remedy the harmonic issues. This has been started by the application of passive filtering which traditionally started with employing ac-side or dc-side inductance (see Figure 1). Passive filters are a cost effective solution at low power (below 10 kW), but they are bulky, worsen the system dynamic and behavior, introduce resonance in a power system and their performance depends on load profile and the power system configuration.
To increase the system performance and nullify the aforementioned drawbacks combining active and passive methods (hybrid) or pure active methods are the feasible solutions. Varied harmonic mitigation solutions have been introduced and developed over the years. These solutions have emerged from case to case, from the simplest solutions such as passive filters to fully active methods. The common feature that these entire solutions share is that harmonic mitigation is not possible without additional costs and engineering.
So far not many active methods have been employed in industry as the simplicity and cost are one of the main desirable features in product development. However, recent technological progresses in semiconductor and passive components have changed the perception of active methods. As a result, the cost of these devices has been fallen due to increase in demand and presence of many competitors in the market. Moreover, complying with the new and future standard regulations and increasing the system power density are pushing the industry towards employing active filtering. Therefore, developing suitable harmonic mitigation solutions become quite vital.
