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There has been a growing demand for using active power filters (APFs) for grid-connected converter systems based on nonconventional energy sources such as solar, wind, and fuel cells. In addition to power quality conditioning, the APF can also be used for bidirectional active power exchange with the three-phase four-wire grid and, therefore, acts as a multifunctional compensator. This paper presents a four-leg shunt multifunctional grid-connected power quality compensator. The compensator reference currents are generated using the instantaneous p-q-r theory. This paper proposes a simple modification to the conventional theory to eliminate the source neutral current from the three-phase four-wire power system. A digital controller is used to implement dead-beat current control. The outputs of the digital current controller are used as reference voltages for a 3-D space vector modulator (3D-SVM). A new approach for pulse width modulation waveform generation in 3D-SVM is proposed. All the intermediate steps are discussed from the viewpoint of implementation on a digital signal processor field-programmable gate array platform. The multifunctional compensator is analyzed, simulated, and tested experimentally for the most practical conditions in the power system, and the results are presented. Index Terms--Active power filter (APF), active rectifier, deadbeat current control, multifunctional compensator, p-q-r theory, 3-D space vector modulation.

A scheme for controlling parallel-connected inverters in standalone ac supply system is presented in this paper. This scheme is suitable for control of inverters in distributed source environments such as in isolated ac systems, large and distributed uninterruptible power supply (UPS) systems,photo-voltaic systems connected to ac grids, and low-voltage dc power transmission meshes. A key feature of the control scheme is that it uses feedback of only those variables that can be measured locally at the inverter and does not need communication of control signals between the inverters. This is essential for the operation of large ac systems, where distances between inverters make communication impractical. It is also important in high-reliability UPS systems where system operation can be maintained in the face of a communication breakdown. Real and reactive power sharing between inverter scan be achieved by controlling two independent quantities - the power angle, and the fundamental inverter voltage magnitude.Simulation results obtained with the control scheme are also presented.