Modulation Strategy Assessment for 3-Level Unidirectional Rectifiers in Electric Vehicle Ultra-Fast Charging Applications

This paper proposes a complete analysis and comparison of the most significant pulse-width modulation (PWM) strategies for unidirectional 3-level rectifiers. The basic operation of the converter is described and the stresses on the major passive components (i.e. DC-link capacitors, differential-mode inductors, common-mode chokes) are calculated, highlighting the general performance trade-off of each modulation strategy. This analysis is applied to a rectifier for electric vehicle (EV) ultra-fast charging connected to the European low-voltage grid (i.e. 50 Hz, 400 V line-to-line), adopting a 650 V DC-link. The best candidates concerning different performance metrics are identified and the most suitable strategy for EV battery charging is selected

Digital Current-Control Schemes

The paper is about comparing the performance of digital signal processor-based current controllers for three-phase active power filters. The wide use of nonlinear loads, such as front-end rectifiers connected to the power distribution systems for dc supply or inverter-based applications, causes significant power quality degradation in power distribution networks in terms of current/voltage harmonics, power factor, and resonance problems. Passive LC filters (together with capacitor banks for reactive power compensation) are simple, low-cost, and high-efficiency solution

Optimal Design of Grid-Side LCL Filters for Electric Vehicle Ultra-Fast Battery Chargers

This paper proposes a complete design procedure for LCL filters intended for electric vehicle (EV) ultra-fast battery chargers. The basic modeling of LCL filters is reported and the optimal ratio between grid-side and converter-side inductance is discussed. The design methodology is based on the identification of all parameter constraints, which allow to graphically determine the filter design space. Once the available space is identified, the feasible design which minimizes the total required inductance is selected, since inductors dominate the overall LCL filter volume, loss and cost. The proposed design procedure is directly applied to a 50 kW, 20 kHz 3-level unidirectional rectifier for a modular EV ultra-fast charger. The performances of the selected design, in terms of harmonic filtering and current control dynamics, are verified by means of simulation in PLECS environment, proving the validity of the proposed design methodology

Optimal Design of Grid-Side LCL Filters for Electric Vehicle Ultra-Fast Battery Chargers

This paper proposes a complete design procedure for LCL filters intended for electric vehicle (EV) ultra-fast battery chargers. The basic modeling of LCL filters is reported and the optimal ratio between grid-side and converter-side inductance is discussed. The design methodology is based on the identification of all parameter constraints, which allow to graphically determine the filter design space. Once the available space is identified, the feasible design which minimizes the total required inductance is selected, since inductors dominate the overall LCL filter volume, loss and cost. The proposed design procedure is directly applied to a 50 kW, 20 kHz 3-level unidirectional rectifier for a modular EV ultra-fast charger. The performances of the selected design, in terms of harmonic filtering and current control dynamics, are verified by means of simulation in PLECS environment, proving the validity of the proposed design methodology

Self-Commissioning Algorithm for Inverter Non-Linearity Compensation in Sensorless Induction Motor Drives

In many sensorless field-oriented control schemes for induction motor (IM) drives, flux is estimated by means of measured motor currents and control reference voltages. In most cases, flux estimation is based on the integral of back-electromotive-force (EMF) voltages. Inverter nonlinear errors (dead-time and on-state voltage drops) introduce a distortion in the estimated voltage that reduces the accuracy of the flux estimation, particularly at low speed. In the literature, most of the compensation techniques of such errors require the offline identification of the inverter model and offline postprocessing. This paper presents a simple and accurate method for the identification of inverter parameters at the drive startup. The method is integrated into the control code of the IM drive, and it is based on the information contained in the feedback signal of the flux observer. The procedure applies, more in general, to all those sensorless ac drives where the flux is estimated using the back-EMF integration, not only for IM drives but also for permanent-magnet synchronous motor drives (surface-mounted permanent magnet and interior permanent magnet). A self-commissioning algorithm is presented and tested for the sensorless control of an IM drive, implemented on a fixed-point DSP. The feasibility and effectiveness of the method are demonstrated by experimental result

Grid-Forming Inverter with Simplified Virtual Synchronous Compensator Providing Grid Services and Grid Support

This paper proposes a Simplified Virtual Synchronous Compensator (S-VSC) model with grid-forming capabilities for microgrid applications. Previous works have shown how the S-VSC can provide grid services (i.e., virtual inertia, current harmonic compensation and reactive support during faults) in grid-feeding configuration. In this paper, the S-VSC model is extended to a grid-forming converter to demonstrate its capability to work in island as well, thus representing a promising solution for the control of a microgrid. The control algorithm is validated on a 15 kVA inverter connected to a scaled microgrid

An eDrive-Based Estimation Method of the Laundry Unbalance and Laundry Inertia for Washing Machine Applications

The estimation of the laundry unbalance and laundry inertia is fundamental in washing machine applications. On the one hand, the estimation and management of the laundry unbalance play a pivotal role in reducing mechanical stress and noise during the spinning phase. On the other hand, the laundry inertia’s estimation, performed at the beginning of the washing cycle, allows for the determination of the proper amounts of water and detergent, the water temperature, and the tumbling time. In this way, good washing performance is obtained, avoiding the waste of energy and resources. Moreover, at the end of the washing cycle, the laundry inertia’s accurate estimation is needed to properly manage the spinning phase. With the aim of optimizing the washing performance, this paper proposes a novel method to estimate the laundry unbalance and laundry inertia. The proposed approach does not require additional sensors, since it uses the already implemented motor control scheme, enhanced by a dedicated position-tracking observer. Experimental results have been carried out on a commercial horizontal-axis direct-drive washer, demonstrating the validity of the proposed solution

Power Decoupling Method for Grid Inertial Support Provided by Ultra-Fast Bidirectional Chargers

The Active Front-End (AFE) converter unit of ultra-fast battery chargers can contribute to the inertial frequency response by embedding the Virtual Synchronous Machine (VSM) control algorithm. However, the injection of inertial active power involves a non-negligible reactive power contribution due to the active-reactive power coupling, thus increasing the current output of the converter. Therefore, this paper proposes an active-reactive power decoupling solution to minimize the AFE current rating for frequency support

Design Space Optimization of a Three-Phase LCL Filter for Electric Vehicle Ultra-Fast Battery Charging

State-of-the-art ultra-fast battery chargers for electric vehicles simultaneously require high efficiency and high power density, leading to a challenging power converter design. In particular, the grid-side filter, which ensures sinusoidal current absorption with low pulse-width modulation (PWM) harmonic content, can be a major contributor to the overall converter size and losses. Therefore, this paper proposes a complete analysis, design and optimization procedure of a three-phase LCL filter for a modular DC fast charger. First, an overview of the basic LCL filter modeling is provided and the most significant system transfer functions are identified. Then, the optimal ratio between grid-side and converter-side inductance is discussed, aiming for the maximum filtering performance. A novel design methodology, based on a graphical representation of the filter design space, is thus proposed. Specifically, several constraints on the LCL filtering elements are enforced, such that all feasible design parameter combinations are identified. Therefore, since in low-voltage high-power applications the inductive components typically dominate the overall filter volume, loss and cost, the viable LCL filter design that minimizes the total required inductance is selected. The proposed design procedure is applied to a 30 kW, 20 kHz 3-level unidirectional rectifier, employed in a modular DC fast charger. The performance of the selected optimal design, featuring equal grid-side and converter-side 175 µH inductors and 15 µF capacitors, is verified experimentally on an active front-end prototype, both in terms of harmonic attenuation capability and current control dynamics. A current total harmonic distortion (THD) of 1.2% is achieved at full load and all generated current harmonics comply with the applicable harmonic standard. Moreover, separate tests are performed with different values of grid inner impedance, verifying the converter control stability in various operating conditions and supporting the general validity of the proposed design methodology

Dead-Time Effect on Two-Level Voltage Source Virtual Synchronous Machines

The Virtual Synchronous Machine (VSM) concept represents a valid solution to integrate renewable energy sources into the grid to provide straightforwardly grid services (e.g., inertial behavior, harmonic sink), grid support during faults and island operation. Under non–ideal (symmetric and sinusoidal) operating conditions, VSMs can behave as harmonic and un- balance sinks, improving the voltage quality at the point of connection to the grid. However, the inverter dead–time alters the harmonic and unbalance sink capability of voltage source VSMs. To demonstrate the negative influence of the dead– time effect, this paper uses a simplified method to predict the ideal behavior of voltage source VSMs under non–ideal grid voltage conditions. The paper demonstrates through experiments that: (1) the inverter dead–time effect limits the harmonic and unbalance sink capability of voltage source VSMs under non– ideal grid voltage conditions and (2) a dead–time compensation is needed to make the voltage source VSMs behave according to the theoretical analysis. Two experimental tests under a 5% grid voltage unbalance and a 10% grid voltage fifth harmonic distortion validate the negative influence of the dead–time and the beneficial effect of its compensation