The coexistence of ac and dc subgrids in a hybrid microgrid is likely given that modern distributed sources can either be ac or dc. Linking these subgrids is a power converter, whose topology should preferably be not too unconventional. This is to avoid unnecessary compromises to reliability, simplicity, and industry relevance of the converter. The desired operating features of the hybrid microgrid can then be added through this interlinking converter. To demonstrate, an appropriate control scheme is now developed for controlling the interlinking converter. The objective is to keep the hybrid microgrid in autonomous operation with active power proportionally shared among its distributed sources. Power sharing here should depend only on the source ratings and not their placements within the hybrid microgrid. The proposed scheme can also be extended to include energy storage within the interlinking converter, as already proven in simulation and experiment. These findings have not been previously discussed in the literature, where existing schemes are mostly for an ac or a dc microgrid, but not both in coexistence.

Autonomous Control of Interlinking Converters with Energy Storages in Hybrid AC-DC Microgrids

Amalgamation of distributed energy resources (DER) with power system is developing as an emerging power framework. It is ecumenically based on the paramount issues such as extensive use of technologies based on distributed energy resources, shortage of fossil fuel in future, liberalism of electrical service industries and custom vigilance on environmental impact of the conventional method of generating electrical power. These issues are transmuting the concept of conventional power generation globally and introduce incipient challenges in the generation and distribution system. The interconnection of distributed generators has initiated the concept of microgrid which is the assemblage of interconnected distributed generators, intermediate storage units and loads. The pragmatic application of microgrid faces numerous technical challenges. Scheming with a congruous protection scheme is one of the solemn challenges in a microgrid framework. The level of fault current in both the modes of operation, active distributed generation, two-way flow of power, increased value of impedance and quality of communication network are the profuse challenges in scheming an efficacious protection technique. The protection scheme must be reliable, selective, fast and susceptible in both the working modes. This paper presents the meticulous study of the architecture of AC microgrid, DC microgrid and hybrid microgrid along with the associated protection issues and solutions. It also provides the censorious assessment of available challenges in the protection of microgrid in both grid-tied & islanded mode and available protection strategies for both AC microgrid and DC microgrid. An all-inclusive review has been done in each section considering the latest developments along with the future progression in the field of microgrid protection.

Microgrid protection: A comprehensive review

Design of integral terminal sliding mode controller for the hybrid AC/DC microgrids involving renewables and energy storage systems

Owing to the smart lifestyle, environmental consciousness, and dwindling fossil fuel supplies, there is a huge demand for clean and green energy. Microgrid (MG) is a crucial approach to renewable and clean energy. Because of the success of the AC utility grid and the growing demand for critical loads, it is very convenient to provide AC/DC MG that can easily satisfy both the alternating current (AC) MG and the direct current (DC) MG requirements. Due to uncertainty in load variations, main grid failures, intermittent power generations from renewable energy sources (RESs), the synchronization and interconnection of different power converters are the paramount issues in the control of AC/DC MG. This article presents an overview-oriented state of the art in the recent advancement in control strategies of AC/DC MG and its associated power converters control. Based on recent research, this article summarizes unobstructed views on different topologies, types of power converters, power converter controls, and control strategies of AC/DC MG. Finally, it identified some future challenges that need to be addressed in order to develop a sustainable and reliable control strategy for AC/DC MG. This article will serve as a guide for researchers.

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A Comprehensive Review on Power Converters Control and Control Strategies of AC/DC Microgrid

The hybrid AC/DC microgrid is considered to be the more and more popular in power systems as increasing DC loads. In this study, it is presented that a hybrid AC/DC microgrid is modelled with some renewable energy sources (e.g. solar energy, wind energy), typical storage facilities (e.g. batteries), and AC, DC load, and also the power could be transformed smoothly between the AC and DC sub-grids by the bidirectional AC/DC converter. Meanwhile, coordination control strategies are proposed for power balance under various operations. In grid-connected mode, the U–Q (DC bus voltage and reactive) or PQ method is adopted for the bidirectional AC/DC converter according to the amount of exchange power between AC and DC system in order to improve the DG utilisation efficiency, protecting the converter and maintain the stable operation of the system. In islanded mode, V/F control is applied to stabilising the entire system voltage and frequency, achieving the power balance between the AC and DC systems. Finally, these control strategies are verified by simulation with the results showing that the control scheme would maintain stable operation of the hybrid AC/DC microgrid.

https://ietresearch.onlinelibrary.wiley.com/doi/pdf/10.1049/joe.2018.8505

Coordination control of hybrid AC/DC microgrid

In this paper, a grid-connected AC/DC hybrid microgrid with some renewable energy sources (PV, fuel cell), energy storages and loads is proposed. The hybrid microgrid consists of both ac microgrid and dc microgrid. A bi-directional AC/DC converter is used to link the ac microgrid and dc microgrid by regulating the power between them. The dc side of the PV array and fuel cell through the DC/DC converter connected to the dc bus. A battery act as energy storage device is connected to the dc bus through the bi-directional DC/DC converter. The PV array of ac side is connected to the ac bus through the DC/DC/AC converter. The proposed hybrid microgrid operates in grid-connected mode by the ac microgrid. Then, the model and control strategy of each module are analyzed and the simulation test-bed is realized using MATLAB/Simulink. The influence of the system operation is simulated from the steady state of the system, the transient process of the switching of the fuel cell, the change of the solar irradiance in the both ac and dc sides, and the change of the AC and DC loads. Finally, the results for simulation analysis of operation characteristics and transient process of grid-connected operation under different conditions are presented with the final conclusions.

Simulation analysis of grid-connected AC/DC hybrid microgrid

The present invention relates to the technical field of distribution network terminal testing and more specifically relates to an automatic test method and test device for a distribution network terminal. The method comprises a step of preparing an analog quantity, a switch quantity and a communication signal for testing, a step of reading test sequence files and outputting the analog quantity and switch quantity according to a time sequence according to the requirements of the test sequence files, a step of allowing the test device to monitor the switch quantity outputted by the distribution network terminal and related information uploaded by communication in real time, a step of determining the test of the distribution network terminal is qualified if all test files are correctly executed after the test of the distribution network terminal is completed, and a step of confirming the test of the distribution network terminal is failed if at least one test sequence file is failed to execute after the test of the distribution network terminal is completed. The characteristics and test items of the distribution network terminal and the computer technology are utilized, analog quantity input, switch quantity input, switch quantity output and a communication test are integrated in one test device, and thus the closed-loop and automatic test of the distribution network terminal are realized.

Automatic test method and test device for distribution network terminal

This system has made several improvements on the basis of the traditional substation monitoring system, providing a new solution for the construction of the new ubiquitous power Internet of Things. This system focuses on the monitoring of illegal intrusion by external personnel, which is ignored by the traditional substation monitoring system. The entire system is designed as a three-tier structure of edge node-regional master node-server, and the computing pressure is distributed to edge nodes through edge computing. And realize the monitoring of outsiders through target detection and face recognition technology, and finally use the raft algorithm to ensure the data reliability of the entire system. This article mainly discusses the hypotheses proposed for the realization of the above functions, describes the structure and interaction design of the system, and briefly describes its hardware and software interface design ideas.

A new type of substation real-time detection system based on edge computing and RAFT consensus algorithm

The invention discloses a control method and system for an alternating current-direct current hybrid distributed renewable energy system. The method comprises the following steps: acquiring data information stored in a microgrid management system connected with a microgrid, wherein the data information comprises microgrid load information, distributed energy generating capacity information and electric vehicle historical data information; and extracting electric vehicle characteristic value data contained in the electric vehicle historical data information, determining a user response intention according to the electric vehicle characteristic value data, and performing intention level division on the user response intension from high to low. According to the method provided by the invention, the response degree of electric vehicle users is divided, historical data and real-time data are collected to predict the response degree of a regional user within a period of time, and the response degree is fed back to a power grid dispatching system. The electric vehicle users are notified in real time to respond to grid adjustment. The randomness and uncertainty of charging behaviors of theelectric vehicle users in the prior art are avoided, and exchange between electric vehicles and the power grid can be carried out purposefully and accurately.

Zhao Yang

Papers

Coordination control of hybrid AC/DC microgrid

Simulation analysis of grid-connected AC/DC hybrid microgrid

Automatic test method and test device for distribution network terminal

A new type of substation real-time detection system based on edge computing and RAFT consensus algorithm

Control method and system for alternating current-direct current hybrid distributed renewable energy system