Although the preponderance of electronic equipment in our professional environment makes work more convenient, these devices complicate demands on facility wiring and power utilities. Most facilities employ a variety of devices such as multiple switch mode power supplies, motors, fans, and other nonlinear loads. Among the adverse effects of multiple nonlinear loads are voltage distortion, excessive neutral return currents, reduced utilization of available power, and power factor penalties
Harmonic currents in particular are receiving more attention as a critical power quality concern, with an estimated 60% of electricity now passing through nonlinear loads. Ironically, the equipment used to boost productivity and efficiency is also generating increases in non-productive power consumption, power pollution, and low power factor. Additionally, the same equipment producing the harmonic distortion is also highly susceptible to its damaging effects.
Power factor correction (PFC) techniques include both passive and active solutions for eliminating harmonic distortion and improving power factor. The passive approach uses inductors, transformers, capacitors, and other passive components to reduce harmonics and phase shift. The passive approach is heavier and less compact than the active approach, which is finding greater favor due to new technical developments in circuitry, superior performance, and reduced component costs.
Traditional passive PFC solutions used at the system level — where multiple subsystems or nonlinear loads are involved — have proven themselves unfeasible economically as well as architecturally. Specially corrected transformers are effective only for certain harmonic frequencies and most passive filters, once installed and tuned, are difficult to upgrade and may generate harmful system resonance. As for active PFC techniques, they must be applied to each individual power supply or load in the system, which complicates architecture and results in high system cost.
Unlike traditional PFC techniques, active power filtering (APF) supplies only the harmonic and reactive power re- quired to cancel the reactive currents generated by nonlinear loads. In this case, only a small portion of the energy is processed, resulting in greater overall energy efficiency and increased power processing capability.
APF utilizes harmonic or current injection to achieve PFC. Unlike designs that process all the power presented to the converter — due to the fact that they are in series or cascade with the AC line — APF can be accomplished parallel to the line. The APF device determines the harmonic distortion on the line, and injects specific currents to cancel the reactive loads.
This technique has been used for years in high-power, three-phase systems, but high costs and complicated highspeed circuitry made it impractical for low-level power systems. However, new techniques that utilize simpler circuitry are making active power filtering more attractive and advantageous for low power, single-phase systems.
The APF is connected in parallel to the front end or AC input of the system, and corrects all loads directly from the AC line. Experimental results using a variety of nonlinear loads show that this type of APF provides excellent harmonic filtering that complies with international harmonic regulations (see Figure 1).
In the application of multiple power supplies and reactive loads, the efficiency and cost-effectiveness of the parallel APF becomes very attractive (see Figure 2). In a typical rack system containing multiple and varied loads, the different currents combine and may cancel harmful harmonics automatically. This reduces the percentage of current that must be injected by the APF to cancel reactive currents, and results in efficiencies that may exceed 95%.
Using active PFC techniques, each load in the system would require its own PFC converter in series, which would need to be capable of processing all the power to each load. The reduction in quantity and power handling capability of the components, as well as the greater inherent reliability of the APF parallel technique, becomes obvious in the example of a 1,000 W system. In this case, the in-series PFC devices would need to process approximately 1,250 watts of power.
Because APF correction is accomplished parallel to the AC line, it only needs to correct for the combined reactive currents, which may be as little as 15 to 20% of the actual load in the same 1,000 W system. This means that a 200 W corrector would be sufficient for the job. Clearly, when very small converters are able to correct very large loads, the economics of APF power factor correction become extremely attractive. An added benefit is that, unlike a series correction device, if the APF fails, an interruption device placed in series with the current injector will open, leaving power available to the system.
Active power filters provide a cost-effective, reliable, and flexible solution for power quality control. Since the APF only processes the reactive and harmonic current, power loss and component rating are typically lower when compared to other power factor correction methods.