PROJECTS Simulation of MV VFD Based on CHB Topology
One of the common topologies used as a voltage source converter is CHB (Cascaded H-Bridge) topology (called also “Multi-Level H-Bridge”). This topology is based on connecting multiple cells in series; each one consists of a basic H-Bridge circuit. The amount of cells is chosen according to the desired number of voltage levels that are required at the inverter output, and according to the devices’ capability to deliver power. Each cell has its own DC link supply and actually can operate independently from the other cells. This is a major advantage because it allows the system to overcome on faults by disabling only the faulty cells and continue working with the rest cells (with some adjustments).
In addition, the topology gives the ability to use low cost low voltage semiconductors rather than the more expensive medium voltage components. Its main drawbacks are the high number of required devices, and the complexity of the transformer design which has to feed separately every cell in the system.
A2Z Consulting ltd. team developed an advanced model and simulated a three phase inverter based on the CHB topology, connected to an induction motor and a load, while each phase is composed of series connection of H-bridge cells. The PWM control unit behavior was modeled and simulated as well. This model allowed us to investigate different aspects of the topology, including the behavior under fault conditions. The model is used today in the development of new Medium Voltage VFD.