Javier Pereda

High-Frequency Link: A Solution for Using Only One DC Source in Asymmetric Cascaded Multilevel Inverters

Multilevel inverters are in state-of-the-art power conversion systems due to their improved voltage and current waveforms. Cascaded H-bridge (CHB) multilevel inverters have been considered as an alternative in the medium-voltage converter market and experimental electric vehicles. Their variant, the asymmetrical CHB (ACHB) inverter, optimizes the number of voltage levels by using dc supplies with different voltages. However, the CHB and ACHB inverters require a large number of bidirectional and isolated dc supplies that must be balanced, and as any multilevel inverter, they reduce the power quality with the voltage amplitude. This paper presents a solution to improve the already mentioned drawbacks of ACHB inverters by using a high-frequency link using only one dc power source. This single power source can be selected according to the application (regenerative, nonregenerative, and with variable or permanent voltage amplitude). This paper shows the experimental results of a 27-level ACHB inverter with a variable and single dc source, but the strategy can be applied to any ACHB inverter with any single dc source. As a result, the reduction of active semiconductors, transformers, and total harmonic distortion was achieved using only one dc power source.

Asymmetrical Multilevel Inverter for Traction Drives Using Only One DC Supply

The main advantage of asymmetrical multilevel inverters is the optimization of levels with a minimum number of power supplies. However, this optimized multilevel system still needs a large number of isolated and floating dc supplies, which makes these converters complicated to implement in electric vehicles (EVs), because the system will require many independent battery packs. In this paper, a very simple scheme, based on a small and cheap high-frequency link (HFL), allows the utilization of only one power supply for the complete multilevel inverter drive, with an inherent regulation of the voltages supplied among the H-bridges. It also allows voltage control with the full number of levels if the dc source is of a variable voltage characteristic. This paper is focused on a 27-level asymmetric inverter, but the strategy, using only one power supply, can be applied to converters with any number of levels. In particular, an asymmetrical 27-level converter needs nine isolated power supplies, and the proposed system reduces these nine sources to only one: the battery car. The topology also permits full regenerative braking working as a three-level converter. The proposed system is intended for application in EVs from power ratings up to 150 kW. Simulations and experimental results show the feasibility to implement this “one-source” multilevel system.

Cascaded Multilevel Converters: Optimal Asymmetries and Floating Capacitor Control

Cascaded multilevel (CM) converter is a series connection of several inverters that together generate multiple voltage levels with controllable frequency, phase, and amplitude. Its main advantages are high power, reliability, and power quality. However, it has considerable drawbacks such as high number of components, many isolated power sources, decreasing voltage quality with the modulation index, and regeneration in some series inverters at specific modulation indexes, even when the machine is motoring. The authors propose to improve any CM topology through two solutions: use optimal voltage asymmetries (ratios), higher than conventional ones; replace the voltage sources by floating capacitors balanced with a new control (PI controller) and/or a high-frequency link. This paper presents theoretical analysis and experimental results of CM converters with increased voltage-quality (levels), some of them keeping this high quality and avoiding regeneration in motor mode at any motor operation point, using the proposed voltage asymmetries and simplifying or eliminating some voltage sources. Experimental results show a reduction of components, an improved voltage quality, and a satisfactory behavior in stationary and dynamic operation.

PWM Method to Eliminate Power Sources in a Nonredundant 27-Level Inverter for Machine Drive Applications

A nonredundant three-stage 27-level inverter using ldquoHrdquo converters is analyzed for medium- and high-power machine drive applications. The main advantage of this converter is the optimization of levels with a minimum number of semiconductors. However, the system needs six bidirectional and isolated power supplies and three more unidirectional if the machine is not using regenerative braking. In this paper, these nine power supplies are reduced to only four, all of them unidirectional, using three strategies: 1) the utilization of independent and isolated windings for each phase of the motor; 2) the utilization of independent input transformers; and 3) the most important of them, the application of special pulsewidth modulation (PWM) strategies on the 27-level converter, to keep positive average power at the medium power bridges and zero average power at the low-power bridges. The generation of this PWM and control of this multiconverter was implemented using DSP controllers, which give flexibility to the system.

23-Level Inverter for Electric Vehicles Using a Single Battery Pack and Series Active Filters

Cascaded H-bridge (CHB) multilevel inverters have been conceived as an alternative to reduce total harmonic distortion (THD) in medium-voltage drives. The reduced THD makes them useful for electric vehicle (EV) applications, but the main problem with the CHB is the large amount of isolated power sources required to feed each of the H-bridges. An improved variant known as the asymmetrical CHB (ACHB) inverter uses H-bridges of different sizes and then needs fewer isolated power sources than the CHB. However, in battery-powered EVs, only one power supply (fuel cell or battery pack) is desirable. This work presents a solution to solve the problem, operating some of the small H-bridges (Aux-bridges) as series active filters and using a small high-frequency link (HFL). With this solution, only one dc source is required to feed the inverter, and if the control is adjusted to work at particular switching points, more than 98% of power is transferred through the larger H-bridges (MAIN bridges). The proposed ACHB topology can produce any number of levels, and the M AIN bridges always commutate at fundamental frequency. As the number of levels must remain constant for all output voltages, a variable dc source is required to control the amplitude of the motor voltage. This work shows some simulations and experiments on a 2-kW 27-level ACHB working with only 23 levels. The concept is being implemented in a small EV with an ACHB drive of 18 kW.

Direct Torque Control for sensorless induction motor drives using an improved H-Bridge multilevel inverter

This paper presents the application of a Sensorless Direct Torque Control (DTC) for an induction motor, using an improved H-Bridge multilevel inverter with 27-Levels. The inverter topology reduces the power sources from nine to only four active sources and three ultracapacitors. The power sources are unidirectional and non-redundant; scaled in power of three to optimize the number of voltage levels with a minimum of semiconductors and power sources. For the improved inverter topology, two additional control strategies are introduced; 1) an Inhibit Negative Currents (INC) controller, which solves the regeneration problem when unidirectional sources are used; and a 2) Proportional-Integral (PI) controller, which keeps the ultracapacitor voltages at the reference value. Both controls work on a Pulse Width Modulation (PWM) signal, where the INC control decides the levels among the PWM operates, and the PI controller changes the duty-cycle. A closed-loop estimator called Model Reference Adaptive System (MRAS) was used for the speed estimation, with the advantage of using the stator voltages and currents already obtained for the DTC. The application of the system was simulated and implemented in Matlab®/Simulink® software using the industrial controller AC800PEC from ABB, obtaining satisfactory results. The multilevel inverter was specially designed and built for this application.

27-level converter for electric vehicles using only one power supply

The main advantage of asymmetrical multilevel inverters is the optimization of levels with a minimum number of power supplies. However, this optimized multilevel system still needs a large number of isolated and floating DC supplies, which makes these converters complicated to implement in electric vehicles (EVs), because the system will require many independent battery packs. In this paper, a very simple scheme, based on a small and cheap high frequency link (HFL), allows the utilization of only one power supply for the complete multilevel inverter drive, with an inherent regulation of the voltages supplied among the H-bridges. It also allows voltage control with full number of levels if the DC power supply is of variable voltage characteristic. This work is focused on a 27-level asymmetric inverter but the strategy, using only one power supply, can be applied to converters with any number of levels. In particular, an asymmetrical 27-level converter needs nine isolated power supplies and the proposed system reduces these nine sources to only one: the battery car. The topology also permits full regenerative braking working as a three-level converter. The proposed system is intended for application in electric vehicles from power ratings up to 150 kW. Simulations and experimental results show the feasibility to implement this “one-source” multilevel system.

A Methodology to Obtain a Synthetic Driving Cycle through GPS Data for Energy Analysis

This paper proposes a methodology to obtain a synthetic driving cycle by using GPS data as input. The aim is to obtain a driving cycle that is representative of a given condition when used for energy consumption studies. The methodology begins with a data acquisition phase, where an adequate sampling frequency is proposed. Velocity raw data are processed and clustered to capture a mobility global pattern. A synthetic driving cycle is generated from the clustered information using a two level energy model optimization. For a case study, a representative synthetic driving cycle was determined for a Bus Rapid Transit (BRT) route.

Energy evaluation of different inverter topologies and modulations used on electrical vehicles

This paper evaluates different inverter topologies and modulations, which use hard switching techniques applied to an electric powertrain to reduce inverter losses. Three inverter topologies and three modulation techniques are evaluated through a simulation. Some inverter configurations are in research level and not used commercially on powertrains yet, but are studied to contrast its energy-efficiency advantages. The methodology proposed simulates and evaluates the powertrain behavior that uses one permanent magnet synchronous motor with a field oriented controller, with an ECE-15 driving cycle. This methodology uses a multi-disciplinary analysis scheme that combines the vehicle dynamics with the electric powertrain. Then, the total harmonic distortion and the power losses are compared. As a result, different indicators allow suggesting an inverter configuration that extends driving autonomy.

Optimal asymmetry for cascaded multilevel converter with cross-connected half-bridges

DC-AC conversion has been revolutionized by multi-level converters due to their high power quality, high modularity and high voltage operation with standard semiconductors. New multilevel topologies, voltage asymmetries and control techniques have been developed in the last decade to improve the performance of these power converters. This paper proposes a novel voltage asymmetry for a three-phase cascaded multilevel converter based on cross-connected half-bridge cells. The proposed asymmetry is compared with the state-of-the-art asymmetry in the same topology and with several asymmetries and topologies as the conventional Cascaded H-Bridge (CHB), the Asymmetric Cascaded H-Bridge (ACHB) and the Neutral Point Clamped (NPC). This paper presents a comparison and analysis of voltage levels generated, the number of IGBTs and isolated voltage supplies required. Simulation results are presented for a converter of 11 kV and 4 MVA. The proposed configuration increase the number of voltage levels in three-phase loads, reducing the Total Harmonic Distortion (THD) under 1%. On the other hand, the total blocking voltage was not reduced, but the number of isolated power sources required for the proposed topology is very low in comparison with other topologies.