Capacity enhancement and operational flexibility are two of the important limitations of the centralized shunt APF ( ) unit. These limitations can be conquered by the operation of multiple APF units in parallel and connected back to back by a common DC link capacitor. In that case, a circulating current (CC) flows within the units. This CC flow becomes out of control when the units operate in hysteresis based current controlled mode. One of the difficulties of this CC flow control or reduction is the variable switching frequency of the units. In this paper, the model for CC flow is derived by the switching dynamics study of the units. It is found that the selection of design parameters plays an important role in the amount of CC flow. Detailed simulation, analysis, and real-time performance show how the selection of design parameters affects the CC flow and the reduction of CC flow can also be achieved at an acceptable level by the proper selection of design parameters. 1. Introduction The power quality, at all time, is a matter of concern where a number of nonlinear, harmonics producing, and sophisticated loads are connected in an electrical distribution network. In that case, for power quality (PQ) improvement, shunt active power filters ( ) are finding greater applications as interfacing and compensating devices in the distributed network. The power rating and switching frequency of the converters are determined by the magnitude of harmonic currents and required filter bandwidth. In high power applications, the filtering task cannot be performed for the whole spectrum of harmonics by using a single converter due to the limitations on switching frequency and power rating of the semiconductor devices. Therefore, compensating the reactive harmonic components to improve the power quality of the distributed network as well as to avoid the large capacity centralised , parallel operation of multiple low power units is increasing. Different controlling mechanisms and topologies are available in handling the difficulties of parallel operation of either in active load sharing or distributed mode. Although the harmonic current compensation is the primary function of parallel APF, it can also be used as a compensator for voltage harmonics. A detailed technical review on parallel operation of for current and voltage harmonic compensation in a distributed generation (DG) network has been done in [1, 2] where the pros and cons of the different control methods have been discussed. For these cases, there is no physical/electrical link between the units. Another kind
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