Philippe Ladoux

Phase-controlled multilevel converters based on dual structure associations

Multilevel converters, like neutral-point-clamped inverters or multilevel choppers, are particularly attractive in high-power applications. Nevertheless, in these structures, all switches are confronted to commutation stresses caused by their turn-on and turn-off control. Furthermore, the methods to balance the capacitor voltages or to control the neutral point voltage are complex enough. In this paper, the authors propose new multilevel converters based on series connection of zero-current-source (ZCS) inverter cells and parallel connection of zero-voltage-source (ZVS) inverters. These dual structure associations give soft-switching operation for all switches and allow the use of semiconductors, normally destined for medium-power applications, in high-power converters (up to 1 MW). The authors consider the structure design for several topologies to achieve DC-DC or DC-AC converters. The simulation results validate the simplicity of phase control techniques and give out the principal features of different topologies.

Railway traction power supply from the state of the art to future trends

In this paper, the authors show how high power static converters contribute to the voltage quality in railways. The first part of the paper gives an overview of power electronics solutions currently used by French National Railways and Swiss Federal Railways such as frequency converter for 15 kV/16.7 Hz system and line voltage booster or unbalance compensator for 25 kV/50 Hz system. Operating results for such devices are presented. The second part of the paper is dedicated to future trends in railway power supply. Innovative solutions concerning DC or AC systems are proposed. In each case, prototypes and experimental results are presented.

Medium frequency transformer for railway traction: Soft switching converter with high voltage semi-conductors

Nowadays, in railway traction, in order to reduce the weight and the size of the input transformer, several structures using intermediate medium frequency stages are under consideration. The major problem is to reach a high switching frequency with high voltage devices. In this paper, the authors propose a soft commutated topology based on dual structure association. The switching losses are calculated considering 6.5 kV IGBTs in soft commutation mode.

Comparative study of variant topologies for MMC

On the base of the MMC principle, this work considers different topologies of sub module (or elementary converter) which make the High Voltage AC/DC converter more flexible and easy suitable respect to different voltage and current levels. A topological study is consolidated and extended on three different topologies proposed as elementary converter and their employment for the MMC structure is treated. For each cell, topology advantages and limits are discussed in terms of current and voltage. Under SPWM modulation conditions, an analytical approach for the power losses evaluation is proposed for the three topologies used in the MMC structure. A comparison on the efficiency is proposed by fixing the DC voltage and the operating power. By considering the basic topology of the MMC (composed by elementary cells) also an evaluation of the power losses is performed by considering the staircase modulation which is becoming more and more diffused especially for a high number of levels.

Chopper-Controlled Steinmetz Circuit for Voltage Balancing in Railway Substations

This paper presents a new technique in voltage balancing for single-phase railway substations. Using the concept of chopper-controlled impedance, the authors study the feasibility of an active Steinmetz circuit based on ac choppers. The low power losses in semiconductor devices make the proposed topology an interesting alternative to classical solutions such as those based on voltage source inverters. Design criteria are presented using information obtained from measurements carried out on a substation of the French National Railways. Finally, the study is validated by means of simulations and experimental results carried out on a 1.5 kVA prototype.

Outlook for SiC devices in traction converters

Silicon (Si) IGBTs are widely used in railway traction converters. In the near future, Silicon Carbide (SiC) technology will push the limits of switching devices in the three directions: higher blocking voltage, higher operating temperature and higher switching speed. All in all, these large-gap components should improve the traction chain efficiency and the power-weight ratio. Thus, the topology of the traction converter should be reconsidered from the input stage to the inverter including the DC bus. In this paper, the authors show the prospects for developments in traction voltage source inverters and active front end converters.

Considerations on boost inductor design in back-to-back converters for renewable energy

In this paper, considerations on converter inductor design in PWM back-to-back converter based on Synchronous Active Front End (SAFE) are discussed. This is considered one of the most efficient converter topology that successfully manages the hard trade-off between high power size and power quality requirements at the Point of Common Coupling (PCC). In renewable energy, in wind farm, this trade-off is getting harder due to the increasing size of generators. Despite all converter design parameters influence the performances in terms of harmonic generation, especially the converter inductor assumes a key role. The design of the inductance is carried out evaluating an admissible range according to imposed power quality requirements and power rate desired.

Reactive power compensation in railways using active impedance concept

In this paper a new active impedance topology, employing AC-Choppers is proposed for reactive power compensation in 25 KV/50Hz railways. The good behaviour of AC-AC converters in terms of losses respect to other solutions like VSI based STATCOM, make this solution really interesting in high power single phase systems like railway networks. The concept is applied to a substation in the French Railways. A design method is proposed and simulation results confirm the good working of this low cost and low losses solution. Finally, experimental results, achieved on a 1.2MVAR prototype are presented.

Power Quality Improvement in ac Railway Substations: The concept of chopper-controlled impedance.

The 25-kV/50-Hz ac single-phase supply is a widely used railway system in France with a length of 9,698 km. Overhead lines are supplied by substations drawing power from two phases of a three-phase utility. They behave as nonlinear and time-varying loads and represent one of the most important sources of voltage unbalance for the electricity-transmission network. In the case of weak networks, railway operators are required to install compensation systems in substations to satisfy utility regulations and to avoid penalties regarding voltage unbalance and reactive power consumption. The limits are established by the energy provider with a view to guaranteeing an acceptable power quality to other customers.

Evaluation of high voltage SiC diodes in a medium frequency AC/DC converter for railway traction

Nowadays, the typical railway traction chain, which operates under alternative power supplies, includes a heavy and bulky transformer. In order to reduce the size and the weight of the input transformer, a medium frequency topology, which involves switches in soft commutation mode, is proposed. A test bench, based on one elementary stage of the complete structure, was built. It allows characterizing new high voltage Silicon Carbide devices in soft switching conditions. Switching losses measurements are performed to determine the optimal switching frequency and the operating limits of the converter.