Parallel multicell converters for high current: Design of intercell transformers

Abstract

Duality rules show that the multilevel output voltage of series multicell converters transposes in the form of a multilevel input current in parallel converters. However the duality of these families of converter is only partial and parallel converters offer an extra advantage which is multilevel output behavior. When the inductors are coupled together, it is even possible to reduce the ripple of internal quantities, i.e. individual cell currents. The magnetic components at the heart of these converters obey very particular rules and need a specific design procedure. The leakage inductance of this component plays an important part in this design and because it involves flux trajectories in the air it is difficult to predict its value and optimize the design. It is also required to evaluate the core losses, the copper losses and the heat exchange surface to determine which number of cells should be used to minimize total weight. Such a procedure is described and used to compare designs with coupled and uncoupled inductors.

Impact of PWM methods and load configuration in the design of intercell transformers used in parallel three-phase inverters

“…However, when considering a double load (circuit of Fig 1b), the ICT flux ( d ) will also depend on the neutral point voltages (V N1 and V N2 ) and on the load parameters, as shown in (3). If the load is a motor, the related parameters are the EMF (V E1 and V E2 ) and the mutual and self inductances of the motor (M and L).…”

“…However, if massive parallelization is necessary, the use of regular inductors is not recommended due to the increase of the current ripple in each cell [3]. A better solution is the use of InterCell Transformers (ICT), which magnetically couple the currents of all cells, which results in the reduction of its ripple amplitude and the increase of its frequency [3].…”

“…Paralleling inverters and interleaving the switching signals result in a parallel multilevel inverter since the output current has reduced ripple, increased apparent frequency and the output voltage present different levels when the load has inductive characteristics [3].…”

“…However, usually the most important value related to the ICT design is the magnetic flux in the ICT magnetic core since it is one of the most important values related the final ICT size and weight [3]. Moreover, low and high frequency currents flowing through the ICT have an important role in its design.…”

“…Most of the concepts and techniques developed for series multilevel converters can be applied to its parallel counterpart [3]. For three-phase applications, PWM techniques such as space vector, carrier disposition and injection of zero sequence signals can be applied to parallel multilevel converters [4] [5].…”