Low harmonics, decoupled histeresis type current control of a multiconverter consisting of a parallel transformerless connection of VSC converters

The increasing number of applications of high power converters to power systems, energy systems and motor drives can be efficiently implemented by the use of multiconverters consisting of the connection of smaller size units. This paper studies the transformerless parallel connection of VSC units (PCTL). It is a simple topology, requiring no specially designed transformer. It can also be easily upgraded and operate with faulted units, increasing the system availability. However, its high individual current ripple requires an improved current controller. The intervals of high ripple caused by fixed frequency PWM methods can be avoided by imposing a constant amplitude ripple by the use of an hysteresis controller. However, its operation is disturbed by the coupling between power converter phases. After deriving the necessary conditions for inputs decoupling, this paper implements a decoupling controller by the use of PLL (phase locked loop) synchronization of power converters associated to the reduction of the “fictitious zero sequence current”. Simulation results confirm the good performance of this controller even for transient conditions