Stefan Almer

In this article, a novel point of view on grid-forming control with current controlled power electronic converters is presented. A control concept based on the emulation of the combined effect of a virtual synchronous condenser and a parallel-connected virtual current source is proposed. It combines the positive effects of virtual synchronous generator control, such as the provision of inertia, voltage support, the possibility of islanded operation, and good control performance in weak grids, with the benefits of vector current control, such as fast current limitation, improved synchronization properties, and fast setpoint tracking. The proposed control concept is simply a modification of standard vector current control. Therefore, it can be implemented easily on all control platforms with minor software changes. The control method is verified with analytical models, time-domain simulations, and experimental results.

This brief considers model predictive control of voltage source converters with LC filter. The converter model describes the nonlinear effect of the switching on the converter state, and the model predictive control (MPC) problem is thus nonlinear and nonconvex. An in-depth analysis reveals a convex structure of the converter model, and a nonlinear variable transformation is introduced, which allows to equivalently reformulate the MPC problem as a convex, quadratically constrained quadratic problem (QCQP). Thus, a problem, which has previously been formulated as a nonconvex problem and solved approximately, can be solved exactly without approximation error. This brief also contains experimental results showing the practical applicability of the approach: The QCQP is solved in real time at a kilohertz sampling rate using the projected gradient and constraint linearization method FalcOpt.

Summary form only given. The past two decades have witnessed enormous advances in solving model predictive control (MPC) problems on embedded controller hardware. Besides featuring limited computational resources, the main challenge in embedded MPC applications is to make the underlying numerical optimization run highly reliably without any user interaction. Today, theory, algorithms and implementations have reached a level of maturity that enables the use of embedded MPC technology in commercial products. This survey provides an overview of successful industrial applications of embedded MPC from various application domains. A couple of examples are discussed in detail in order to illustrate the use of different problem formulations and the associated solution approaches. The description of industrial applications is complemented by a concise overview of engineering tools for embedded MPC. Finally, we discuss challenges in embedded MPC that currently limit a wider spread of the technology in industry.

This paper considers torque regulation of a variable-speed synchronous machine fed by a line-commutated rectifier and a load-commutated inverter. The proposed control approach is model-predictive control where both the rectifier and inverter firing angles are considered as control inputs. Conventional controllers assign different tasks to the rectifier and inverter firing angle. In contrast, the model-predictive controller coordinates the firing angles and this improves the dynamic performance and disturbance rejection. In particular, the proposed controller handles line side under voltage conditions better than a conventional PI controller. The nonlinear model-predictive torque controller has been implemented on an embedded system and applied in an experimental test bed. The experiments confirm that the controller is able to successfully ride through line side under voltage conditions.

This paper presents an application of model predictive control to a variable-speed drive system operating in the multi-megawatt range. The variable-speed drive system comprises a synchronous machine fed by a line-commutated rectifier and a load-commutated inverter. The control task is to regulate the dc-link current, and hence the machine torque, to ensure the machine speed follows a given reference. The proposed control approach is model predictive control, where both the rectifier- and inverter-firing angles are considered as control inputs. The nonlinear model predictive torque controller has been implemented on an embedded system and applied in an industrial-scale pilot plant installation. The experiments show the successful operation of model predictive control on a plant with more than 48 MW power.

The paper considers optimal pulse-width modulation of a single phase voltage source inverter with LC filter. The approach is based on model predictive control which requires a non-convex optimization problem to be solved in real-time. A new optimization algorithm for solving the problem is presented. The proposed algorithm utilizes an invariance property of the LC filter to recast the non-convex optimization problem as a convex quadratic problem. The novel algorithm has significant advantages compared to previously applied methods since it has lower complexity and is, under certain assumptions, guaranteed to find the global optimum.

This paper considers model predictive control (MPC) of switched mode power converters and uses a hybrid prediction model that represents the switching ripple of the converter state. The goal of the approach is to achieve both high dynamic performance and low harmonic distortion of the output at steady state. It is argued that the previously published approaches to hybrid MPC of power converters are sensitive to plant/model mismatch, and an alternative problem formulation is suggested. The new formulation describes the converter state in terms of dynamic Fourier coefficients. This allows to quantify and feed back more information about the switched converter waveform, and can thus reduce the dependence on the system model. The suggested approach is evaluated in both simulation and experiment, and it is shown to be less sensitive to plant/model mismatch than approaches from the literature.

The paper at hand considers the design of a controller for torque regulation of a variable speed synchronous machine fed by a line commutated rectifier and a load commutated inverter. The control approach is model predictive control where a constrained optimal control problem is solved to minimize the deviation of the torque from its reference. The dynamic model of the converters and drive is nonlinear and considers both the rectifier and inverter firing angles as control input. Controlling both firing angles simultaneously, as opposed to in a cascaded manner, implies improved potential for dynamic performance and disturbance rejection. In particular, the controller handles voltage dips on the line better than a conventional PI controller. The nonlinear MPC solution is implemented through on-line optimization. The optimization algorithm has been implemented on an embedded system and has been shown to execute sufficiently fast for the targeted control frequency.

This paper considers model predictive control of a class of piecewise affine systems and introduces a method to reduce the complexity of the optimisation problem which is solved online. The optimisation problem is a mixed-integer quadratic program which is parameterised by the initial state of the system dynamics. The complexity of the problem solved online is reduced by moving part of the computational burden offline. In the offline computations, the structure of the mixed-integer problem is explored to determine a polytopic partition of the set of initial states. Each polytope of the partition is then considered and it is determined which integer combinations can/cannot be optimal, given that the initial state is contained in the current polytope. The result of the offline computations is that each polytope is associated with the set of potentially optimal integer combinations. The online solution of the mixed-integer problem consists of a two-step procedure: first, it is determined which polytope contains the initial state. Then, the mixed-integer problem is solved, taking into account only those integer combinations associated with the current polytope. The solution approach is shown to give a significant complexity reduction when applied to hybrid control of switched-mode power converters.