Feed-Forward based Control in a DC-DC Converter of Asymmetric Multistage Stacked Boost Architecture

A new feed-forward control technique for a DC-DC step-up converter based on the asymmetric Multistage Stacked Boost Architecture (MSBA) is proposed. The devel-oped feed-forward controller directly handles the load current and the perturbations that may be caused by the dynamic operation of the converter. Thus, there is no time delay for estimating the error in a control process and therefore, both robust steady-state operation and high dynamic performance can be achieved. The asymmetric MSBA DC-DC converter is a single output high-voltage topology that embodies several in-series active voltage balancing circuits. Thus, there is a need for an improved balancing control method. This can be ac-complished by replacing the conventional Proportional-Integral (PI) controllers with feed-forward type and therefore improved balancing between the stages can be attained, be-cause the steady state errors are suppressed and the dynamic response can be enhanced. A thorough theoretical analysis is conducted for verifying the stability of the suggested feed-forward control technique in an asymmetric MSBA converter and selective simulation and experimental results are demon-strated in order to validate the effectiveness and feasibility of the proposed control scheme.

DC-DC Converter based on the Asymmetric Multistage Stacked Boost Architecture with Feed-Forward Control for Photovoltaic Plants

Georgios Mademlis, Alfred Rufer, Gina Kristin Steinke

A new feed-forward control technique for a DC-DC step-up converter based on the asymmet-ric Multistage Stacked Boost Architecture (MSBA) is presented in this paper. The proposed closed loop control scheme can provide improved accuracy in the steady-state operation and high dynamic performance. The asymmetric MSBA converter is a single output high-voltage DC-DC step-up converter that comprises several in-series connected capacitors with active voltage balancing circuits. The converter can attain high voltage ratios and thus, it is a suita-ble solution for the high voltage transmission requirements of the large photovoltaic (PV) plants. Selective simulation and experimental results are presented in order to validate the effectiveness and the operational improvements of the proposed control system.

2016

Modélisation et réglage d'un entraînement à haute performance par un moteur réluctant

Michel Girardin

Usually, each phase of the direct reluctance motor is fed separately by its own inverter producing a squarewave phase current depending on the rotor position. In the present work, a star-connection of the three stator phases is used and the direct reluctance motor is fed by a three-phase switched inverter. The space phasor theory shows that three-phase sinewave currents can produce a constant electromagnetic torque. In this case, the rotating reference frame rotates at half of the rotor speed and in the opposite direction. This command strategy allows to reduce the stator deformation due to the electromagnetic radial force which is very high in this type of motor and which is mainly responsible for the acoustic noise. The electromagnetic torque control is realised by a sliding mode control of the three phase currents. In the rotating reference frame, it is possible to show that the eight commutation states of the switched inverter are placed around an ellipse in the complex plane of the derivatives of the space phasor of the phase current. A study of this ellipse has allowed to determine the maximal torque as a function of the speed for a given continuous voltage. It is important to insure that the torque reference will never exceed this limitation in order to avoid a pullout phenomenon, highly unwanted in the speed control. Despite the fact that the resolver inside the casing of the direct reluctance motor produces a small periodic error, it is accurate enough for the position measurement and to produce the three current references. However, the derivative of the position measurement provides a speed evaluation with such a ripple that the speed control cannot be outstanding. So, for the speed measurement, an additional laser rotary encoder is used. The experimental study in the steady-state mode of the speed control shows the presence of an electromagnetic torque ripple. This is a residual ripple caused by the dissimilarity between the stator poles teeth-shapes and the magnetic saturation effect. Although this torque ripple is rather low compared with the usual command strategy, two methods are proposed in order to compensate it. The first method is based on the use of a correction factor depending on the tooth angle. First, in steady-state conditions, the correction factor is stored off-line in a table. Then it is used in-line as a feedforward disturbance compensation. The second method of reducing the residual torque ripple consists in implementing an observer of variable disturbance. This is a disturbance observer able to determine the frequency content of the torque ripple. For instance, in order to observe the continuous component, the fundamental wave as well as the harmonics 2, 3, and 6, the observer must have ten poles. The main difficulty to deal with such an observer is the fact that each pole moves quickly as a function of the motor speed. So, in order to guarantee the observer stability over a large range of speed, the poles are located dynamically regarding to the motor speed by the way of a discontinuous adaptation of the feedforward coefficients. The theoretical study and the numerical simulation have allowed to formulate some criteria about the way of locating the poles. Experimental results confirm the efficiency of the observer of variable disturbance in order to reduce the electromagnetic torque ripple.

EPFL2000

Medium Voltage DC-DC Converter for Power Collection Links

Gina Kristin Steinke

Nowadays electrical energy generation is an important topic discussed in the society. Since technologies like wind or solar energy generation are most of the time placed far away from the city centres one point to focus on is the transmission of the energy opening opportunities for DC-distribution within the energy distribution. Focussing on solar plants as application it became obvious that to achieve high power levels a large surface area is used. Covering such a large area, where the energy is generated, means that the power must be collected over long distances, which leads to a low efficiency if the transmission is done under low voltage conditions. A better efficiency could be expected by increasing the voltage to a medium voltage level using a DC-DC converter. The introduced converter is based on the current diverter topology, which is used to equalize the voltage between different cells. To use this topology as a step-up converter a boost stage was added as input stage forming the Multistage Stacked Boost Architecture. To ensure that this topology would be a possible solution for the before mentioned challenges calculations were done to verify a satisfying efficiency of the converter. In addition an Energetic Macroscopic Representation was done and a control was designed. This control uses two cascaded controllers in each stage with feedback of the load current to generate the PWM. Since that control relies on the measurement of nine state variables, it was considered important to introduce a second control. The first step was to introduce a State-Space control, which is based on the description of system with n first-order differential equations. Then the research continued into a control strategy using an observer. The observer is basically a copy of the state space system; it has the same input and almost the same differential equations. An extra term compares the actual measured output to the estimated output; this will cause the estimated states to approach the values of the actual states. To verify simulation results and the working of the proposed controls a low voltage prototype was built. The measurements that were conducted all included normal steady-state operation as well as test to verify the correct functioning during perturbations such as variations of the load. All the measurement showed that the controls feasible. In addition, there were only very small resonant phenomena that are of no consequence for the correct functioning of the converter. To conclude, the work that has been done for this thesis included considering applications for medium voltage DC-DC converters. In addition, the problems of the approach of having huge solar plants use a low voltage distribution inside were shown. A converter that could be used to elevate said voltage to a medium voltage level was introduced - The Multistage Stacked Boost Architecture. This topology was then investigated in detail and calculations for the efficiency of the topology were conducted. Three different dedicated controls were investigated. A control based on the feed forwarding of the output current, a state-space control and an observer control. All control schemes were first tested with simulations and then verified on a low voltage prototype. The measurement results obtained with the different controls proved satisfactory, showing that the Multistage Stacked Boost Architecture is a reliable and interesting topology.