Alfred Rufer

A supercapacitor-based energy-storage system for elevators with soft commutated interface

Power variations and energy criteria have been the main motivations for developing regenerative drive converters for elevators. A more performant solution for power-smoothing can be easily found by using a supercapacitor based storage device, connected to the intermediary circuit of a variable speed drive system. In this paper, power and energy considerations are made for the design of the storage tank and regarding the maximum power demand from the feeding network. For the power-conversion circuit, which is necessary to compensate the voltage variations of the supercapacitors by discharging and charging, a high efficiency converter topology is proposed which allows the bidirectional energy flow under soft-commutation conditions, and offers also a good flexibility for the optimal sizing of the supercapacitor voltage level. The typical behaviour of the special converter is given, together with an analyze of the advantages related to the specific application.

Multivariable-PI-Based

This paper presents a linear direct-quadrature current control strategy for voltage source converters (VSCs) in a rotating reference frame (RRF). The described method is based on multivariable-proportional-integral (PI) regulators and provides fast dynamics and a zero steady-state error. Contrary to the well-known conventional PI-based control strategies in RRFs, the presented method provides practically decoupled axes with a superior disturbance rejection capability. Moreover, its implementation is relatively simple and does not impose excessive structural complexity compared to its conventional PI-based competitors. The method is applicable to both single- and three-phase systems and also to anisotropic three-phase systems, e.g., synchronous motors with different direct and quadrature impedances driven by VSCs. Implementing a three-phase test system, the performance of the presented method is experimentally evaluated.

dq

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 nonlinear 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.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.

Current Control of Voltage Source Converters With Superior Axis Decoupling Capability

This paper deals with asymmetrical multi-level inverters. The investigated topologies consist of series connected cells with different input voltages. Some combinations of different input voltages have been proposed to improve the converter resolution. This paper has a general approach. It describes the rules to obtain a uniform step with these topologies. It points out the switching problem of some of these asymmetrical solutions, describes the rules to avoid this problem, resulting in a solution where the number of commutations and hence the switching losses may be reduced. For this purpose, it also proposes an adapted optimal control strategy based on a geometrical approach, and a scheme for the control of multi-level inverters.

Control of a hybrid asymmetric multi-level inverter for competitive medium-voltage industrial drives

This paper compares different cascaded and multilevel topologies to interface supercapacitors to a dc bus in regenerative braking applications. It is shown that the modular multilevel dc/dc converter (MMC) can benefit from both reduced voltage and increased frequency across the inductor to reduce its weight and volume when using phase shifting modulation. The proposed control method is able to balance supercapacitor voltage while providing precise output current control. The converter topology and control method are validated with simulations and experimental results.

Design and control of asymmetrical multi-level inverters

In this paper, the analytical solution for the submodule voltage ripple equations of a modular multilevel converter (MMC) is derived, based on the knowledge of the external voltage/current magnitudes, and enhancing a concept previously presented in the literature. In order to achieve high accuracy, all passive elements of the converter, common-mode voltage injection as well as intentionally imposed circulating current harmonics are taken into consideration. The natural charge level mechanism of the capacitor voltages is also explained. As application examples, the three- as well as the two-phase grid-connected MMC cases are chosen. The control of line and circulating currents is also discussed and two respective independent feedback loops are formed. The concept of fictive-axis emulation is tailored for the two-phase MMC case, in order to achieve vector control of the line current and therefore straightforward desired injection of active and reactive power. Finally, the development of a reduced-scale laboratory prototype is presented and a full set of experimental results are provided, verifying the aforementioned concepts.

Design and Control of a Modular Multilevel DC/DC Converter for Regenerative Applications

This paper proposes and studies new strategies for an increase of the energetic efficiency of an urban transportation network fed with catenaries. A simulation tool has been developed to determine the energy efficiency on the global transportation system (vehicles, feeding line and substations). The system energy consumption is identified by calculating the energy injected by the substations to the feeding line. It takes into account all the line losses and the interactions between the different vehicles on the line. The development of a new tool was necessary as other simulation tools calculate the energy consumption of the tram without taking in account the line properties and the interactions between the vehicles. Using this simulation tool, the energetic efficiency of a tram is studied while on board of the vehicle a supercapacitive energy storage has been added. The energy storage allows local energy recovery and injection of energy to the catenaries during braking phases. The results of simulations with or without on-board energy storage and with overhead lines with high resistive or standard behavior are presented. The results show that the global energy efficiency of the system (trams, feeding line and substations) is increased by a factor between 19.4 and 25.6% (depending on the vehicle auxiliaries power) by adding a supercapacitive energy storage for both cases presented in this paper: high and standard line resistance. The use of a high line resistance instead of a standard line resistance increases the efficiency of the total system by an additional 5%.

Accurate Capacitor Voltage Ripple Estimation and Current Control Considerations for Grid-Connected Modular Multilevel Converters

This paper presents a vector control strategy for regulating the current of grid-tied voltage source converters (VSCs) in a rotating reference frame. The proposed approach is based on shaping the open-loop and closed-loop transfer matrices of the system. Solving a constrained convex optimization problem, the shaping is achieved, which guarantees the stability of the closed-loop system. The designed controller results in the desired dynamic performance and decouples the direct and quadrature (dq) current axes. The structure of the proposed controller is similar to that of its predecessors and consists of four proportional-integral controllers. The performance of the method is evaluated based on simulation and experimental results. It is confirmed that its dynamic performance is better than that of the previously proposed approaches, and it results in the decoupled current axes.

Study and simulation of the energy balance of an urban transportation network

A configurable converter topology is presented as front-end converter for a locomotive that can be operated as well on a high voltage AC-system as on a medium voltage DC-system. The configurable converter is realized on the base of a multilevel chain, where all modules are fed through reversible and isolated DC-DC stages. The medium frequency transformers used in the isolation stages of the DC-DC converters allow a significant weight reduction of the locomotive, in comparison with the classical low-frequency voltage-transformers. A first solution using hard switching techniques is presented, using a three-times silicon-conversion. A second solution based on ZVT/ZCS techniques is then presented, with the advantage of having ideally only a two times silicon conduction. Prototype modules have been realized with association of diodes and IGBTs in order to provide reverse-blocking devices. First results of the two-stage conversion are also presented.

Decoupled dq-Current Control of Grid-Tied Voltage Source Converters Using Nonparametric Models

This paper proposes a power converter architecture for the implementation of an ultrafast charging station for electric vehicles (EVs). The versatile converter topology is based on the concept of the power electronic transformer. For the direct transformerless coupling to the medium-voltage grid, a cascaded H-bridge (CHB) converter is utilized. On the level of each submodule, integrated split battery energy storage elements play the role of power buffers, reducing thus the influence of the charging station on the distribution grid. The power interface between the stationary split storage stage and the EV batteries is performed through the use of parallel-connected dual-half-bridge dc/dc converters, shifting the isolation requirements to the medium-frequency range. By choosing several different submodule configurations for the parallel connection, a multiport output concept is achieved, implying the ability to charge several EVs simultaneously without the use of additional high-power chargers. All possible charging station operating modes among with the designed necessary control functions are analyzed. The state-of-charge self-balancing mode of the delta-connected CHB converter is also introduced. Finally, the development of a downscaled laboratory prototype is described, and preliminary experimental results are provided.