This paper deals with the control of distribution energy resource (DER) connected to a grid connected system feeding nonlinear loads. The DER is controlled to provide power quality improvement capabilities, namely, power factor correction, harmonic reduction, and load balancing. The developed controller is based on modification of synchronous reference theory. It possesses unique features such as filtration and fundamental voltage extraction from the polluted grid voltages to eliminate harmonics and unbalance. Additionally, the control is implemented over the square of DC link voltage. The distributed generation source is realized as a voltage source converter. Both simulation and experimental results prove the effectiveness of control algorithm with nonlinear loads. The control algorithm works well for power factor correction, harmonic reduction, and operation under unbalanced load condition. The dynamics of the system under load change and load unbalancing are well depicted. 1. Introduction A number of benefits of renewable energy resources exist, namely, high reliability, modular in nature, and environment friendly operation. These advantages have ensured increased acceptance of distributed resources in the world [1–12]. The renewable energy sources (wind, solar, etc.) are currently being interfaced to the conventional systems using voltage source inverters (VSCs) [2–4]. There are several advantages of distributed resources over conventional sources so a high emphasis is nowadays paid to renewable energy sources (RES) and their interconnection to grid. These sources are connected at the low or medium voltage levels. A primary energy source generally in the form of wind energy, photovoltaic cell, fuel cell, battery, and so forth is connected at the primary of the converter which converts this power and supplies it to the load in islanded mode. A number of such voltage source converters may be connected in parallel and to the utility grid also. Control of such distributed energy sources [5–13] is an important control aspect. Very recently the concept of multifunctional distributed generators has been introduced which is controlled to provide two equally important tasks of providing power delivery as well as power quality improvement. Active power filters [1, 12, 14] provide reactive power compensation and power quality improvement only. The concept is to flexibly control the distributed generator resource as a source and extract maximum power from it and also provide compensation current from it. Lately, power converters in both voltage and current
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