The modern world is full of goods and electric energy is just one of them. Producers want to sell goods and clients want to buy, but, of course, energy should be of good quality. Why is quality so important? Products of advanced technology need a stable and clean supply. Disturbances can cause failures, such as hangs in telecommunication devices, additional losses in power lines, increased current in neutral wires, resonance phenomena, and even production shutdowns brought on by improper operation of protection systems. All these cases cost us billions of dollars each year. What is behind power disturbances? They are mainly caused by nonlinear loads. Their presence in the network is the reason for the deformations of voltage and currents waveforms. A few decades ago, there were mainly stable large power industrial loads. Deformations caused by such types of loads are relatively easy to fix by using passive filters. At present, a very large number of small nonlinear loads (like phone and computer power supplies, led lights, etc.) cause more stochastic disturbances. The best way to reduce this type of disturbances is the application of active power filters. APFs connected to power systems, depending on control strategy and configuration, can realize higher harmonic reduction, and reactive power compensation or symmetrization in power supply systems.
Keywords: Active power filters; Hybrid active power filters; Unified power flow controllers; Harmonics reduction; Reactive power compensation; Power quality.