Patent ID: 8564994

Claim:
A method of controlling a NPC (Neutral Point Clamped) multilevel inverter, including: three parallel switching arms connected between a positive line and a negative line of a bus, each switching arm including four power switches linked in series, the power switches being controlled by a control module for variable speed drive based on a control voltage vector with aim of controlling an electrical load, the control module using a hexagonal vector space defining voltage vectors achievable by control combinations for the switching arms, the control combinations being defined in six identical triangles covering the vector space, each triangle of the vector space including two vertices defining a first side of the triangle and an intermediate point situated on the first side, the two vertices and an intermediate point each corresponding to a single control combination for the switching arms, two capacitors connected in series between the positive line and the negative line of the bus and a mid-point defined between the two capacitors, a current passing through the mid-point, the method comprising: determining a position of the control voltage vector in one of the six identical triangles covering the hexagonal vector space; regulating the electrical potential of the mid-point; wherein when the inverter operates in overmodulation, the regulating the electrical potential of the mid-point comprises: determining the positive or negative sign of the product of current passing through the mid-point times its electrical potential; decomposing the control voltage vector (1) by using the voltage vector of the vector space joining the intermediate point of the triangle when the sign of the product is negative or (2) by using the two voltage vectors joining the two vertices of the triangle when the sign of the product is positive.