There are many types of distributed-generation (DG) sources that cannot be connected to the network directly and use an inverter-based interface. In harmonic compensators [like active power filters (APFs)], the coupling transformer disrupts the compensation capability, therefore the transformerless inverter schemes have grown up in recent years. A new cascaded multilevel transformerless inverter topology is introduced in this study to connect the DG sources to power utility. The proposed circuit of this study can compensate the current harmonic of the non-linear load (as in APF) and inject maximum possible active power as a DG source interface, simultaneously. A fixed gain called K C is obtained based on the load nature, and is used to adjust the set point of the inverter between two operating modes. Proposed scheme has been simulated in Matlab/Simulink to evaluate the circuit performance both in the maximum active power injection mode and the load harmonic compensation mode. Then a 2.2 kW single-phase prototype of the circuit is used for experimental evaluation of the study. Both simulative and experimental results prove that such a circuit minimises the total harmonic distortion of the source side current to an acceptable margin, while injecting the maximum possible active power.

This study presents a new voltage control algorithm for multilevel inverters which are used to connect the distributed generation (DG) sources (like photovoltaic cells) to the network, which is called the voltage look-up table method (VLUTM). This method can build up an almost sinusoidal voltage at the inverter output with the least number of switching in the power circuit, and the best possible harmonic spectrum without the need to any big filtering. On the other hand, in some specific applications where the DG has been set up beside a non-linear load, VLUTM can make a reference voltage based on the reference current to take a desired current form the DG. This way the DG can compensate the major low-order load current harmonics and improve the source side current THD greatly. Decreasing the switching frequency as much as possible, which leads to better efficiency and less EMI problems, the capability of implementation both in single phase and three phase circuitry, and the very simple and desired current control, would be the major benefits of this method. The simulation results taken from Matlab/Simulink and the experimental results on a 1 kVA experimental prototype are presented to evaluate the performance of this method.

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In this paper a new lead-lag supplementary damping controller is introduced for damping power system oscillations using SVC, which it's gain is controlled with a mamdani type fuzzy logic controller. Here the global signals are used as the input of damping controller, because some of the oscillating modes aren't seen in the local signals. The residue index function method is used to select this input signal. The phase compensation method is used to design the lead-lag controller parameters. To validate this controller, some computer simulations are used on a 4-machine 2-area test system. The results show the effectiveness of applying the fuzzy gain in damping controller.

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