Hybrid Microgrid Based on Solid State Transformer

One promising topology for solid state transformer (SST) is a modular multilevel cascaded converter, in which submodule is composed of dual active bridge (DAB) and H-bridge. For SST application in PV system, the efficiency could be severely affected especially for DAB due to the wide voltage and power range of PV panels. Thus, the motivation of this paper is to deal with the control strategy to improve DAB efficiency inside SST for PV application. Instead of utilizing time-domain based analysis method, which requires complex modeling process, this paper models DAB under frequency domain by fully considering the effect of both fundamental and harmonic frequency component. To achieve high efficiency operation, a multiorder reactive-current suppression (MRS) strategy is proposed by controlling phase shift angle as well as the duty cycles of primary and secondary side H-bridges. In terms of DAB controller design inside SST, the small signal model under MRS is established and a notch filter is implemented to suppress the second order line frequency fluctuation in the control loop. Finally, a 10 kV/1 MW SST prototype is introduced along with the system control structure and implementation method. The experiment of the submodule and SST confirms the effectiveness of proposed method.

A Solid-State Transformer as Enabling Technology for Microgrids for Integration into the Power Distribution System: An Overview

This paper presents the design of new highfrequency transformer isolated bidirectional dc-dc converter modules connected in module-cascaded solid state transformer (SST). A phase-shift dual active bridge (DAB) converter is employed to achieve high-frequency galvanic isolation, bidirectional power flow, and zero voltage switching (ZVS) of all switching devices, which leads to low switching losses even with high-frequency operation. Bidirectional DC-DC converter is crucial to the power transmission in SST. The proposed DAB converter consists of two three-leg bridges and a high-frequency transformer with winding shunting taps. Furthermore, the commutation inductance connected in series with the transformer in the conventional DAB converter is integrated into the transformer windings, thus, enjoying smaller volume and higher power density. By changing the number of primary winding turns, leakage inductance, which is the key parameter in the energy transfer process of DAB converter, can be adjusted on a large scale, enabling the possibility of reducing loss over wide load range. Additionally, time-sharing circuit topology and operation mode are adopted to adapt to the output power by detecting the output current as the feedback. As a result, the efficiency at both light and heavy load can be significantly improved compared with the conventional DAB converter, and, therefore, high efficiency over wide load range and high power density can be achieved. Besides, the additional bridge arm structure along with a flexible control scheme provides possible high-level fault tolerance. Finally, the simulation results on a 2-kW DAB converter module switching at 10kHz are presented to validate the theoretical analysis.

In this paper, agent-aggregators cooperation has been introduced in Smart Microgrid (SMG) consisting of household users, Photo Voltaic array (PV), and Local Energy Storage System (LESS). The SMG has Energy Management System called agent deal with two aggregators to purchase the energy for the consumers. Each aggregator supplying a limited level of energy from different energy resource, where one aggregator depend on Community Energy storage System to provide power in one direction to the users and the other aggregator supplying power from the utility grid. The job of the agent is to decide the optimum amount of purchasing power from the aggregators at the lowest cost to supply the users and to charge the LESS taking into account the constraints of supplying limitations for both CESS and utility grid at Point of Common Coupling. Each aggregator supplies a limited level of energy from different energy resource, where one aggregator depend on the Community Energy storage System (CESS) to provide power in one direction to the users and the other aggregator supplying power from the utility grid. The job of the agent is to decide the optimum amount of purchasing power from the aggregators at the lowest cost to provide the users and to charge the LESS taking into account the constraints of providing limitations for both CESS and utility grid at Point of Common Coupling (PCC).A model-free Reinforcement Learning (RL) method is used as an essential strategy to achieve the proposed task using Deep Q-Network (DQN). The simulation result is done and the results are verified using Improved Particular Swarm (IPSO) as a benchmark method. The obtained results show the efficiency and effectiveness of the method used

The three-phase Solid State Transformer (SST) is one of the key elements in the Future Renewable Electric Energy Delivery and Management (FREEDM) System. The SST consists of an input rectifier, Dual Active Bridge (DAB) bidirectional dc-dc converter and followed by an inverter for ac voltage output. The DAB converter is a bidirectional dc-dc converter using high frequency transformers to step up or down the voltages at high frequency for a reduction in size while maintaining high efficiency and reliability. The single phase high-frequency high-voltage transformer for two-level DAB has been reported by the authors [2]. In this paper, high frequency three-phase transformer for multi-level (3-level) and multi-phase (three-phase) DAB is designed for three-phase multilevel SST. There are two proposed topologies and for each topology the high voltage and high frequency transformer is designed at 3 kHz and 20 kHz. The proposed transformer designs for electromagnetic, thermal and mechanical analysis are validated with the simulation of Finite Element Analysis software.

Hybrid microgrids (HMG) combine AC and DC microgrids using an interlinking converter (IC) that controls the power flow between the networks and the utility grid. These HMGs allow the interconnection of variable AC and DC sources and loads. Modular Multilevel Converter (MMC) based IC enables to integrate energy storage (ES) elements into their structure, instead of the connection to the AC and DC microgrids directly. In this paper, the features of the MMC topology with respect to its two degrees of freedom control (grid and circulating current control) are explored to regulate both AC and DC microgrids in a decoupled manner. The possible power management strategies to operate under both dispatched and undispatched utility grid-connected modes, as well as islanded mode are discussed. The control strategies for the IC to achieve resilient operation under utility grid-connected low voltage ride-through (LVRT) scenarios and grid forming mode under unbalanced loads are also provided. The proposed control schemes for HMG with MMC based IC are validated through controller hardware-in-the-loop (C-HIL) simulation, and the results are explained.

The voltage of point of common coupling (PCC) connects industrial distribution networks to the utility grid. Asymmetric sags often occur at PCC due to large load changes. To tackle this challenge, the energy storage system (ESS) is employed to improve the voltage stability of industrial distribution networks. However, there is lack of effective control methods to improve the stability of grid since the unstable energy control rate of ESS. Passivity-based control (PBC), which makes the unstable objects and energy storage resources into a passive system, is applied to improve the voltage stability. PBC is an energy-based approach, which aims to render the closed-loop system with a given storage function. In this paper, a PBC strategy based on euler-lagrange (EL) model is proposed for industrial distribution networks under asymmetrical utility grid voltage sag conditions. According to the passive control theory, the passivity based controller is designed by a new positive/negative sequence damping injection method. The power distribution of parallel-connected multiple inverters has been investigated to achieve coordinated operation. Simulation results verify the effectiveness of the proposed control strategy, such as faster response, good stability, and better voltage support performance.

Nowadays, AC/DC hybrid microgrids are introduced as a possibilistic approach to aggregate the distributed generations, energy storage systems, and AC/DC load. Controlling of the hybrid microgrids is quite challenging and the main challenge is to ensure a precise power division between the two AC and DC sides. In hybrid microgrids, interlinking converters can also provide the compensability of the harmonics of the AC side and utility grid. To lessen the initial investments and volume, reduce maintenance costs, and improve the converter efficiency in the microgrid, this letter uses the multifunctionality concept to design a simpler control system for interlinking converter of the grid connected hybrid microgrid. The multifunctionality control system can balance the produced power, compensate the current harmonics caused by the grid-side nonlinear load, and fix the DC bus voltage. The multifunctional control implements in the synchronous reference frame by cascade control loops and the PI and hysteresis controller are used to control the voltage and current loops respectively. Simulation results verify the acceptable performance of the proposed control system.

This paper presents an experimental study with a pilot hybrid microgrid system that is proposed and implemented in Tagajo campus of Tohoku Gakuin University, Japan. The developed microgrid system is designed to be able to supply power to both DC and AC loads simultaneously by renewable power sources such as PV and wind power generations, etc. Meanwhile, in case that the renewable power is insufficient to supply both these loads, a diesel generator is automatically actuated as backup power source. For the purpose of power balance and voltage stabilization, two types of energy storage devices with different response characteristics and cost, which are secondary battery and EDLC (Electrical Double-layer Capacitor) respectively, are introduced into this system. In this work, a coordinated control strategy for these energy storages were proposed and verified by a series of experimental studies with the developed hybrid microgrid system. The purpose of the experimental study addressed in this paper is to confirm the islanded operation properties of this system for stable power supply to both AC and DC loads, even in case with considerable power fluctuation of renewable sources and significant power change in loads. As a result of this study, it is illustrated that the developed hybrid microgrid and its control systems can work properly and resiliently as being designed.

To guarantee an uninterruptable power supply, a microgrid must be able to operate in both islanded and grid‐connected modes. Hence, it is required to synchronize the voltage phasor of the point of common coupling of the microgrid with the utility grid. Therefore, a distributed cooperative hierarchal control structure is proposed that seamlessly synchronized an islanded microgrid with the utility grid. A proposed method uses a leader‐follower based consensus to regulate the voltage phasor while ensuring proportionate load sharing. A synchronization controller is designed that uses an adjustable parameter to eliminate the voltage magnitude and phase mismatch, and transmit the regulated compensation signal to the leader DGs. The leader DGs transmits these signals to the follower DGs through a cyber‐channels. Moreover, a Massachusetts Institute of Technology rule and Lyapunov function based adaptive controllers are designed to update the secondary control law parameters in case of any disturbance or uncertainty. These adaptive techniques do not require prior knowledge about the microgrid topology, loads, and impedances thus enhancing the system performance. Furthermore, a small signal stability analysis is presented to facilitate the controller parameter design criteria. Finally, the robustness and effectiveness of proposed structure is verified through simulation results in different scenarios.

Concerns about climate changes and environmental air pollution are leading to the adoption of new technologies for transportation, mainly based on vehicle electrification and the interaction with smart grids, and also with the introduction of renewable energy sources (RES) accompanied by energy storage systems (ESS). For these three fundamental pillars, new power electronics technologies are emerging to transform the electrical power grid, targeting a flexible and collaborative operation. As a distinctive factor, the vehicle electrification has stimulated the presence of new technologies in terms of power management, both for smart homes and smart grids. As the title indicates, this book chapter focuses on the role of off-board EV battery chargers in terms of operation modes and contextualization for smart homes and smart grids in terms of opportunities. Based on a review of on-board and off-board EV battery charging systems (EV-BCS), this chapter focus on the off-board EV-BCS framed with RES and ESS as a dominant system in future smart homes. Contextualizing these aspects, three distinct cases are considered: (1) An ac smart home using separate power converters, according to the considered technologies; (2) A hybrid ac and dc smart home with an off-board EV-BCS interfacing RES and ESS, and with the electrical appliances plugged-in to the ac power grid; (3) A dc smart home using a unified off-board EV-BCS with a single interface for the electrical power grid, and with multiple dc interfaces (RES, ESS, and electrical appliances). The results for each case are obtained in terms of efficiency and power quality, demonstrating that the off-board EV-BCS, as a unified structure for smart homes, presents better results. Besides, the off-board EV-BCS can also be used as an important asset for the smart grid, even when the EV is not plugged-in at the smart home.

This article proposes a Distributed-control Architecture (D-CA) and an operation sequence with start-up strategies for a Digital Signal Processor (DSP)-based Solid-State Transformer (SST). Although various control techniques for SSTs have been reported in earlier studies, there is still a lack of research covering comprehensive content, including hierarchical control architectures and operation sequences with start-ups considering the implementation of DSPs. Therefore, this article addresses the following factors of SST. First, the D-CA is described for the design of the hierarchy between control boards. With the D-CA, because sub-boards are in charge of their corresponding DC-link voltage balancing control individually, the computational burden on the master board can be reduced. Second, the operation sequence of the SST system is explained based on the SST with D-CA. The step of DC-link voltage balance is considered throughout the entire operation sequence for safe driving. Furthermore, the PWM start-up strategies for a Cascade H-bridge Multilevel (CHM) converter and Dual Active Bridge (DAB) converter are proposed to prevent switching pulse errors caused by DSP operating characteristics. These start-up strategies reduce the current surges. The validity of the proposed D-CA and operation sequence with start-up strategies are verified by experimental results.

In this paper, the mathematical modelling and control of isolated bi-directional multi-stage, on-board charger for the electric vehicle (EV) with unity power factor operation is presented. The presented charger topology is proficient to work in grid to vehicle (G2V) mode and vehicle to grid (V2G) mode as well. The EV charger interacts with the utility grid to exchange the adequate amount of active and reactive power required in the both modes of operation. The mathematical modelling of converter is obtained in both G2V and V2G modes to obtain the state to control and output to control transfer functions through state space averaging method. These transfer functions are further used to design the controller parameters and examine the overall dynamics of the system. In this topology an active front end (AFE) rectifier is employed to maintain the adequate constant DC link voltage, which is controlled by current mode control technique. An isolated dual active bridge (DAB) converter is used to exchange the bi-directional power flow between DC link and EV battery, which is controlled by modified single phase shift modulation technique. The overall control is implemented in Simulink model to validate bi-directional power exchange between the utility grid and EV battery for a 230 V, 50 Hz, 5 kW EV charger.

Power quality and stability assessment of hybrid microgrid and electric vehicle through a novel transformation technique

The electrical distribution network is continuously changing due the fact that in recent years the distributed generation sources interacting with it have increased, this impacts direcly the energy consumption dynamics because the network does not just deliver energy but can also receive it. These new operating conditions requires new and modern conditioning stages and power flow control rules to ensure optimal system performance. One of the alternatives currently being studied and developed to face this challenge are Solid State Transformers (SST). These devices are composed by Power Electronic Converters and have diverse attractive features which make them a vital, world-class study case; one of his main requirements its to ensure a proper operation on the SST power flux direction. This paper describes a control proposal applied to a Dual Active Bridge (DAB) converter, which is part of an SST; the proposal serves as the starting point to establish new control rules that satisfy the basics criteria that a bidirectional converter needs to meet.