Model predictive control for advanced multilevel power converters in smart-grid applications

In the coming decades, electrical energy networks will gradually change from a traditional passive network into an active bidirectional one using concepts such as these associated with the smart grid. Power electronics will play an important role in these changes. The inherent ability to control power flow and respond to highly dynamic network will be vital. Modular power electronics structures which can be reconfigured for a variety of applications promote economies of scale and technical advantages such as redundancy. The control of the energy flow through these converters has been much researched over the last 20 years. This thesis presents novel control concepts for such a structure, focusing mainly on the control of a Cascaded H-Bridge converter, configured to function as a solid state substation. The work considers the derivation and application of Dead Beat and Model Predictive controllers for this application and scrutinises the technical advantages and potential application issues of these methodologies. Moreover an improvement to the standard Model Predictive Control algorithm that include an intrinsic modulation scheme inside the controller and named Modulated Model Predictive Control is introduced. Detailed technical work is supported by Matlab/Simulink model based simulations and validated by experimental work on two converter platforms, considering both ideal and non-ideal electrical network conditions

Model predictive control for advanced multilevel power converters in smart-grid applications

In the coming decades, electrical energy networks will gradually change from a traditional passive network into an active bidirectional one using concepts such as these associated with the smart grid. Power electronics will play an important role in these changes. The inherent ability to control power flow and respond to highly dynamic network will be vital. Modular power electronics structures which can be reconfigured for a variety of applications promote economies of scale and technical advantages such as redundancy. The control of the energy flow through these converters has been much researched over the last 20 years. This thesis presents novel control concepts for such a structure, focusing mainly on the control of a Cascaded H-Bridge converter, configured to function as a solid state substation. The work considers the derivation and application of Dead Beat and Model Predictive controllers for this application and scrutinises the technical advantages and potential application issues of these methodologies. Moreover an improvement to the standard Model Predictive Control algorithm that include an intrinsic modulation scheme inside the controller and named Modulated Model Predictive Control is introduced. Detailed technical work is supported by Matlab/Simulink model based simulations and validated by experimental work on two converter platforms, considering both ideal and non-ideal electrical network conditions

Indirect model predictive current control techniques for a direct matrix converter

The direct matrix converter has twenty-seven available switching states which implies that the implementation of predictive control techniques in this converter requires high computational cost while an adequate selection of weighting factors in order to control both input and output sides of the converter. In this paper, two indirect model predictive current control strategies are proposed in order to simplify the computational cost while avoiding the use of weighting factors. Both methods are based on the fictitious dc-link concept, which has been used in the past for the classical modulation and control techniques of the direct matrix converter. Simulated results confirm the feasibility of the proposed techniques demonstrating that they are an alternative to classical predictive control strategies for the direct matrix converter

Modulated model predictive control for active split DC-bus 4-leg power supply

This paper proposes a constant switching frequency Finite Control Set Model Predictive Control (FCS-MPC), formally Modulated FCS-MPC or M2PC, for a 4-leg inverter having an Active Split DC-bus on the fourth leg. The great advantage of MPC over linear control schemes is the very fast transient response it is capable to produce; it also can handle general constrained nonlinear systems with multiple inputs and outputs in a unified and clear manner. These features are highly valuable in power electronic converters used to supply the electrical utility loads in micro-grids. However, one of the main drawback of the MPC is its variable switching frequency, above all in system with accurately tuned output power filters (i.e. switching traps), which is the case when stable voltage waveforms with very low harmonic content are required. The proposed investigation relates with the application of a constant switching frequency variant of the MPC to a 4-leg inverter with a specifically tuned filter to assure high quality voltage supply even in case of non-linear and unbalanced loads

Active Magnetic Bearing system design featuring a Predictive current control

Active Magnetic Bearing (AMB) technology is becoming attractive for several reasons such as high speed operations, high reliability and vibrations exemption. Moreover, AMB can behave as active vibration dampers and provide a real-time control of the shaft. For all these advantages, AMBs are particularly attractive for high power - high speed applications. These desirable features come at the cost of an increased complexity of the system, which now includes a power electronic converter and a control system dedicated to the AMBs. This paper focus on the overall system design, from the AMB design, to the power electronic converter design and control, for an AMB featuring Wheatstone bridge winding configuration. The magnetic design has been developed analytically and validated by means of Finite Elements simulation, to generate up to 2kN of axial forces. The power conversion system is based on three full bridges, one to magnetize the bearing and two to control the axial forces independently on the x and y axes. In order to achieve high bandwidth current control able to generate the desired orthogonal forces, a predictive control strategy has been proposed, for the several advantages it can provides such as fast dynamic response, no need of modulation, easy inclusion of nonlinearities and constraints of the system, possibility of incorporating nested control loops in only one loop and the flexibility to include other system requirements in the controller. The control system has been validated in Matlab/PLECS simulation, including the effect of parameters mismatches in the coils

Evaluation of isolated DCDC converter topologies for future HVDC aerospace microgrids

High performance power conversion equipment is currently gaining an increasing interest for aircraft applications. In particular, isolated bidirectional DC/DC converters are often proposed for modern HVDC aircraft distribution systems. For such reason an evaluation of several isolated DC/DC converter topology is carried out considering the proposed application, interfacing a 270V DC network with a 28V DC network. A trade off evaluation has been carried out for three different topologies and an experimental prototype has been manufactured for the selected conversion architecture. Simulation and experimental results are provided in order to validate the trade off and the design of the proposed converter

Evaluation of isolated DC/DC converter topologies for future HVDC aerospace microgrids

High performance power conversion equipment is currently gaining an increasing interest for aircraft applications. In particular, isolated bidirectional DC/DC converters are often proposed for modern HVDC aircraft distribution systems. For such reason an evaluation of several isolated DC/DC converter topology is carried out considering the proposed application, interfacing a 270V DC network with a 28V DC network. A trade off evaluation has been carried out for three different topologies and an experimental prototype has been manufactured for the selected conversion architecture. Simulation and experimental results are provided in order to validate the trade off and the design of the proposed converter

Grid Parameter estimation using Model Predictive Direct Power Control

This paper presents a novel Finite Control Set Model Predictive Control (FS-MPC) approach for grid-connected converters. The control performance of such converters may get largely affected by variations in the supply impedance, especially for systems with low Short Circuit Ratio (SCR) values. A novel idea for estimating the supply impedance variation, and hence the grid voltage, using an algorithm embedded in the MPC is presented in this paper. The estimation approach is based on the difference in grid voltage magnitudes at two consecutive sampling instants, calculated on the basis of supply currents and converter voltages directly within the MPC algorithm, achieving a fast estimation and integration between the controller and the impedance estimator. The proposed method has been verified, using simulation and experiments, on a 3-phase 2-level converter

Modulated model predictive control for a 7-level cascaded h-bridge back-to-back converter

Multilevel Converters are known to have many advantages for electricity network applications. In particular Cascaded H-Bridge Converters are attractive because of their inherent modularity and scalability. Predictive control for power converters is advantageous as a result of its applicability to discrete system and fast response. In this paper a novel control technique, named Modulated Model Predictive Control, is introduced with the aim to increase the performance of Model Predictive Control. The proposed controller address a modulation scheme as part of the minimization process. The proposed control technique is described in detail, validated through simulation and experimental testing and compared with Dead-Beat and traditional Model Predictive Control. The results show the increased performance of the Modulated Model Predictive Control with respect to the classic Finite Control Set Model Predictive Control, in terms ofcurrent waveform THD. Moreover the proposed controller allows a multi-objective control, with respect to Dead-Beat Control that does not present this capability

Model predictive control for Active-Bridge-Active-Clamp (ABAC) converter

The Dual-Active-Bridge is a well-established isolated, bidirectional DC/DC topology suitable for applications where high efficiency, galvanic isolation and large voltage conversion ratios are required. However, in low voltage high power cases, the output current ripple is significant and large filtering capacitance is needed. As an alternative to the standard Dual-Active-Bridge, the Active-Bridge-Active- Clamp (ABAC) converter is presented in this paper. The ABAC converter overcomes the current ripple limitation of Dual Active Bridge by presenting a current interleaved structure. An average switching model is developed for the ABAC converter by neglecting the dynamic on the high frequency link, and a Model Predictive Control (MPC) is proposed. The control features a reduced prediction horizon and a fixed switching frequency. Finally, simulation results for a 10kW ABAC converter are provided to validate the theoretical claims