Applications of 4-leg inverter are found not only in active and unbalanced load compensators but also as an interface for transferring power generated by renewable energy sources in distributed generation system and microgrids. However existing distributed generation systems commonly use 3-leg inverters. In this paper, the possibility of using a 4-leg inverter interface along with the existing units with 3-leg inverter is explored. This forms the distributed generation system with hybrid inverter interfaces and is studied in detail in presence of unbalanced loads. Analysis of a hybrid system is presented in respect of sequence component analysis of terminal voltages. It is shown that if the expected unbalanced loads are fed from the 4-leg inverter side, the performance of the system is improved in respect of negative and zero sequence components in terminal voltages.
This paper is concerned with the distributed line interactive operation of 4-leg inverters. Generally, a 3-leg inverter is used as an inverter interface in distributed systems or microgrids. However 3-leg inverters are incapable in effectively handling the unbalance and nonlinearity in the loads. The 4-leg inverters, on the other hand, can control zero sequence currents and give effective DC link utilization. This feature is utilized here to improve the emergency operation of line interactive system. In case of failure of main power supply, that is in emergency operation, 4-leg inverters can maintain balanced and sinusoidal voltages even if the load is unbalanced and nonlinear. Instead of a dedicated compensator designed for handling load unbalance or nonlinearity, the system is inherently made capable of handling such loads. For line interactive operation the transitions between normal and emergency mode are taken care of by frequency restoration scheme. This allows seamless transitions without any change in control algorithm.
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