A communication-free decentralized control for grid-connected cascaded pv inverters

This paper proposes a communication-free decentralized control for grid-connected cascaded PV inverter systems. The cascaded PV inverter system is an AC-stacked architecture, which promotes the integration of low voltage (LV) distributed photovoltaic (PV) generators into the medium/high voltage (MV/HV) power grid. The proposed decentralized control is fully free of communication links and phase-locked loop (PLL). All cascaded inverters are controlled as current controlled voltage sources locally and independently to achieve maximum power point tracking (MPPT) and frequency self-synchronization with the power grid. As a result, control complexity as well as communication costs are reduced, and the system’s reliability is greatly enhanced compared with existing communication-based methods. System stability and dynamic performance are evaluated by small-signal analysis to guide the design of system parameters. The feasibility and effectiveness of the proposed solution are verified by simulation tests

Model predictive control of a microgrid with energy-stored quasi-Z-source cascaded H-bridge multilevel inverter and PV systems

This paper presents a new energy management system (EMS) based on model predictive control (MPC) for a microgrid with solar photovoltaic (PV) power plants and a quasi-Z-source cascaded H-bridge multilevel inverter that integrates an energy storage system (ES-qZS-CHBMLI). The system comprises three modules, each with a PV power plant, quasi-impedance network, battery energy storage system (BESS), and voltage source inverter (VSI). Traditional EMS methods focus on distributing the power among the BESSs to balance their state of charge (SOC), operating in charging or discharging mode. The proposed MPC-EMS carries out a multi-objective control for an ES-qZS-CHBMLI topology, which allows an optimized BESS power distribution while meeting the system operator requirements. It prioritizes the charge of the BESS with the lowest SOC and the discharge of the BESS with the highest SOC. Thus, both modes can coexist simultaneously, while ensuring decoupled power control. The MPC-EMS proposed herein is compared with a proportional sharing algorithm based on SOC (SOC-EMS) that pursues the same objectives. The simulation results show an improvement in the control of the power delivered to the grid. The Integral Time Absolute Error, ITAE, achieved with the MPC-EMS for the active and reactive power is 20 % and 4 %, respectively, lower than that obtained with the SOC-EMS. A 1,3 % higher charge for the BESS with the lowest SOC is also registered. Furthermore, an experimental setup based on an OPAL RT-4510 unit and a dSPACE MicroLabBox prototyping unit is implemented to validate the simulation result

Analysis, Modeling and Control of Stacked DC-AC Converters

Stacked DC-AC converters offer several unique advantages including i) the ability to create power systems with voltage level specifications many orders above the device voltage ratings, ii) flexibility in system design, and iii) modularity for system expansion. The benefits of stacked DC-AC system can be further extended by making the converters bi-directional using circuit design and control techniques. This thesis identifies and analyses the challenges that come along with the advantages of stacked DC-AC systems. These challenges are then addressed by introducing circuit architectures and control techniques. Simple, yet powerful models are presented to understand the overall system operation. Unlike DC-DC stacking, DC-AC stacking requires line frequency information to synchronize the AC ports and build up the voltage across the stack. Rather than using phase-locked loop (PLL) which requires additional wiring or using power-line communication (PLC) which requires additional hardware, this thesis describes the design and implementation of a self-synchronizing stacked DC-AC converter system that achieves synchronization without additional wiring or hardware or any means of communication. The research presented in this thesis can be applied to off-grid systems for supporting standalone loads using either PV or batteries at the DC port. A virtual droop control technique that enables the stacked system to process power flow in both the directions is also presented. Such a control scheme when implemented, makes the DC-AC converter operate as an AC-battery. One straightforward application of this technique is energy storage integration with the grid. There are significant challenges in extending the stacked architecture to the case of DC to three-phase AC systems. Unlike single-phase stacking, having an isolated DC source such as a battery or a PV panel at the DC port is not a sufficient condition for three-phase AC stacking. A new multilevel DC to three-phase AC architecture is presented, which includes phase-to-phase isolation using a multi-port active-bridge DC-DC converter. This approach achieves required isolation using compact high-frequency transformers and eliminates the need for bulk energy storage or bulky line-frequency transformers. Furthermore, distributed control techniques are developed to achieve system level goals from local measurements

Modular Medium-Voltage Grid-Connected Converter with Improved Switching Techniques for Solar Photovoltaic Systems

© 1982-2012 IEEE. The high-frequency common magnetic-link made of amorphous material, as a replacement for common dc-link, has been gaining considerable interest for the development of solar photovoltaic medium-voltage converters. Even though the common magnetic-link can almost maintain identical voltages at the secondary terminals, the power conversion system loses its modularity. Moreover, the development of high-capacity high-frequency inverter and power limit of the common magnetic-link due to leakage inductance are the main challenging issues. In this regard, a new concept of identical modular magnetic-links is proposed for high-power transmission and isolation between the low and the high voltage sides. Third harmonic injected sixty degree bus clamping pulse width modulation and third harmonic injected thirty degree bus clamping pulse width modulation techniques are proposed which show better frequency spectra as well as reduced switching loss. In this paper, precise loss estimation method is used to calculate switching and conduction losses of a modular multilevel cascaded converter. To ensure the feasibility of the new concepts, a reduced size of 5 kVA rating, three-phase, five-level, 1.2 kV converter is designed with two 2.5 kVA identical high-frequency magnetic-links using Metglas magnetic alloy-based cores

Power Electronic Architecture for Multi-Vehicle Extreme Fast Charging Stations

Electric vehicles (EV) are quickly gaining popularity but limited driving range and a lack of fast charging infrastructure are preventing widespread use when compared with gas powered vehicles. This gave rise to the concept of multi-vehicle extreme fast charging (XFC) stations. Extreme fast charging imposes challenges in the forms of power delivery, battery management, and energy dispatch. The extreme load demand must be handled in such a way that users may receive a timely charge with minimal impacts on the electric grid. Power electronics are implemented to address these challenges with highly power dense and efficient solutions. This work explores a power electronic architecture as one such solution. The system consists of three parts: a cascaded H-bridge (CHB) active rectifier that interfaces to a medium voltage (MV) grid, a dual active bridge (DAB) based solid state transformer (SST) that provides isolation and forms a low voltage DC (LVDC) bus, and full bridge DC-DC converters configured as partial power converters (PPC) that interface with the vehicle battery