Control of a Hybrid Asymmetric Multilevel Inverter for Competitive Medium-Voltage Industrial Drives

In symmetric multilevel inverters, there is a tradeoff between the output quality and the reliability and efficiency of the converter. New asymmetric and hybrid solutions, using different voltages and devices in various parts of the inverter, promise significant improvements for medium-voltage industrial drives. This paper investigates such a hybrid asymmetric nine-level inverter. It consists of a three-phase three-level integrated gate-commutated thyristor inverter (main inverter), with a two-level insulated-gate bipolar transistor H-bridge (subinverter) in series with each phase. To keep the power part simple and the efficiency high, the subinverters have no feeding from the net and can only supply reactive power. This is a very interesting solution in terms of power quality, efficiency, reliability, and cost. But the nonsupplied intermediate-circuit capacitors form an unstable system. This paper proposes a control method to stabilize their voltages. Power balancing is guaranteed by varying the common-mode voltage, using an online nonlinear model-predictive controller. The controller predicts the system evolution as a function of the control inputs. A cost function of system and control quantities is iteratively minimized in real time, to find the optimal control to apply to the system. Simulations and measurements demonstrate stable behavior in steady state and during transients. Precharging of the nonsupplied capacitors is also an issue to consider. This paper proposes a startup method that charges them in parallel with the supplied ones, without any additional equipment. Measurements show its successful application in the proposed drive system.

Investigation and control of a hybrid asymmetric multi-level inverter for medium-voltage applications

Martin Veenstra

Power-electronic inverters are becoming popular for various industrial drives applications. In recent years also high-power and medium-voltage drive applications have been installed. However, the existing solutions suffer from some important drawbacks. Hybrid asymmetric multi-level inverters promise significant improvements for medium-voltage applications. This dissertation investigates such a hybrid inverter. To simplify the topology, some inverter parts are deprived of their feeding from the net and can only supply reactive power. The non-supplied intermediate-circuit capacitor voltages are inherently unstable and require a suitable control method for converter operation, preferably without influence on the load. Apart from normal operation, also converter start-up is an issue to consider, for which it is desirable to limit additional equipment. In this dissertation, we investigate the behaviour of this new inverter, and develop methods to obtain its reliable operation for the considered applications. These methods include modulation, voltage stabilization and start-up. We establish suitable models for their foundation. The principle achievement of this work is the development of a control method to stabilize a multitude of capacitor voltages which have no equilibrium state. Power balancing is performed by varying the common-mode output voltage, using a non-linear model-predictive controller. This method, which is new to power electronics, is applied to our hybrid asymmetric nine-level inverter driving an asynchronous motor. Computer simulations and measurements on an experimental drive system demonstrate stable behaviour in steady-state and during transients over the whole operating range. The obtained results prove the possible implementation of such a complex control algorithm for fast real-time operation. As second important accomplishment, this thesis proposes a start-up method that charges the non-supplied intermediate-circuit capacitors in parallel with the supplied ones, without additional equipment. Measurements show its successful application in the investigated drive system.

EPFL2003

Control of a Hybrid Asymmetric Multi-Level Inverter for Competitive Medium-Voltage Industrial Drives

Alfred Rufer, Martin Veenstra

Hybrid asymmetric multi-level inverters promise significant improvements for medium-voltage industrial drives. Floating sub inverters without feeding can only supply reactive power. An on-line non-linear model-predictive controller stabilizes their intermediate-circuit voltages in steady-state and during transients, as demonstrated by simulations and measurements. A pre-charge method assures converter start-up without additional equipment.

2003

Non-Equilibrium State Capacitor-Voltage Stabilization in a Hybrid Asymmetric Nine-Level Inverter: Non-Linear Model-Predictive Control

Alfred Rufer, Martin Veenstra

In symmetric multilevel inverters, there is a tradeoff between the output quality and the reliability and efficiency of the converter. New asymmetric and hybrid solutions, using different voltages and devices in various parts of the inverter, promise significant improvements for medium-voltage applications. This paper investigates such a hybrid asymmetric nine-level inverter. It consists of a three-phase three-level main inverter, with a two-leg two-level sub inverter in series with each phase (Fig. 1). To keep the power part simple and the efficiency high, the sub inverters have no feeding from the net and can only supply reactive power. But the nonsupplied intermediate-circuit capacitors form an unstable system. This paper proposes a control method to stabilize their voltages. Power balancing is guaranteed by varying the common-mode voltage, using an on-line nonlinear model-predictive controller. The controller predicts the system evolution as a function of the control inputs. A cost function of system and control quantities is iteratively minimized in real time, to find the optimal control to apply to the system. Simulations and measurements demonstrate stable behaviour in steady state and during transients. The originality of this paper is the application of nonlinear model-predictive control in power electronics.

2005

Investigation and control of a hybrid asymmetric multi-level inverter for medium-voltage applications

Martin Veenstra

EPFL2003