Predictive Control of Power Converters and Electrical Drives

The electronics group has been pioneering this research line since 2004 in the power electronics community. It has been recognized as a promising and powerful tool to control power converters and drives. The reason for this lies in the nonlinear and discrete nature of power converters, which have limited control actions (switching states). This enables to evaluate predictive models of the system over a one or more future time intervals to select the most appropriate control action according to a predefined cost function. The cost function can hold any mathematical term that can model variables, constraints and non-linarites of the system, even in the frequency domain. This flexibility brings many powerful control tools to power electronics, which were before limited to a few degrees of freedom provided by classic linear controllers with modulation stage. Our group has embraced this potential and applied predictive control to a wide variety of power converters, motor drives and applications. A major breakthrough has been the inclusion of different dynamics (electrical and mechanical) into a single predictive control algorithm, eliminating the use of any linear controller. The research carried out by our group in the field of MPC of power converters and drives covers the following topics: current control of three-phase, multi-phase and multi-level inverters; predictive power control, predictive torque and speed control of AC machines, and control of active filters, among others.


Matrix Converters

Matrix converters are AC-AC power converters, also known as direct converters. Their main characteristic is the ability to directly connect the input phases of the converter (usually a three-phase supply) to the output phases (usually the phases of a motor) through bidirectional power semiconductor switches. The direct connection enables the elimination of the DC link stage; in other words, there is no need of energy storage components such as capacitors or inductors. This allows an important volume reduction, and consequently an increase in power density. It s for this reason these converters have found acceptance in aerospace applications. At Powerlab, research in this area has been pursued since its beginnings; several contributions have been made since then, mainly related to: advanced control and modulation methods, sensorless control of matrix-converter-fed AC-machine drives, current control with non-linear and unbalanced loads, and active filters, among others.


Multilevel Converters

Multilevel converters were conceived to address the need to increase the operating voltage of power converters without exceeding the limits of existing power semiconductors. This is achieved by arranging the semiconductors and capacitors in special configurations, which can distribute the stress more appropriately and generate different output voltage levels. This also improves significantly the quality of the generated voltage waveforms and enables the reduction of the switching frequency of the power semiconductors. Improvements in power quality and efficiency, together with higher voltage to operation have made these converters very popular in high-power medium-voltage drives, such as pumps, fans, conveyors, train traction and wind energy conversion. At Powerlab much work on design, modeling, topology development, modulation and control of these converters has been carried out since 2002. Powerlab is recognized as one of the most active groups in this area, with dozens of contributions, which include: new modulation methods, fault-tolerant operation, and their use in high performance applications such as regenerative conveyor belts, photovoltaic energy conversion systems and HVDC transmission. Powerlab has organized several special sections in journals, tutorial courses at conferences and highly cited review articles related to this type of converters.


Renewable energy conversion systems (wind & photovoltaic)

Wind and photovoltaic grid-connected energy systems are the two fastest growing renewable energy sources of the last decade. This is due to cost reduction and the development of key technologies used in wind and photovoltaic energy systems, together with subsidies and environmental awareness. In addition, the cost increase of fossil fuels, their limited reserves and adverse impact on the environment, have contributed to the competitiveness and penetration of renewables to the grid. The interest in renewable energy systems and the search for more cost effective, efficient and reliable solutions has driven the industry towards larger wind turbines in the multimegawatt range, and utility-scale PV plants. This has been achieved mainly through economy of scale and higher energy conversion efficiency by research and development of new technology. In this respect, power converters and their control are key enabling technologies for renewable energy conversion systems. The inherent variable nature of wind and solar irradiation, the efficient operation of the conversion system following the maximum power point, and grid connection requirements (synchronization, power control and low voltage ride through), are some of the challenges addressed by the power converter and control system. At Powerlab research on both Wind and Photovoltaic energy conversion systems has been performed actively over the last years. The main focus has been the development of new multilevel converter interface systems to the grid, particularly oriented to large-scale high power systems. Several contributions have been made over the last years, including a boon on wind energy conversion systems.


Motor Drive Control

Most of the generated electrical energy in the world, is used to produce motion in industrial processes, most of which do not require control of the dynamic behavior, and are known as standard motor applications. However, there is a steadily growing share of adjustable speed drives fed and controlled by power converters. Among the reasons are the cost reduction of power electronics, the increase of its realiability and most of all the energy efficiency that variable speed operation provides. The development of control strategies for high-performance AC drives reached maturity over the past three decades. Currently, two technologies largely dominate the industrial market: Field Oriented Control (FOC) and Direct Torque Control (DTC). These strategies were developed in the 70’s and 80’s respectively. Although both strategies have differences in their operating principles an operation, both have the same control objectives: to achieve decoupled control of torque and flux to achieve high dynamic performance speed control. High performance drives have found wide spread acceptance in demanding applictions such as train traction, wind turbines, electric and hybrid electric vehicles, steel rolling mills and flywheel energy storage, to name a few. At powerlab several research lines have led to improvements to motor drive control, particularly in relation to sensorless speed control, PWM based DTC, multilevel-converter-fed drives, and more recently predictive torque and flux control.


Smart Grids

Smart grids are a new concept wich involves  the gathering and  distribution of information about the behavior of all participants (suppliers and consumers) in a electrical grid in order to improve the efficiency, reliability and sustainability of electricity services.

 

 

At Powerlab research on Smart Grids is recent. The main focus are the converter related topics, particularly HVDC and FACTS.