System deployment and decentralized control of islanded AC microgrids without communication facility

J. Mod. Power Syst. Clean Energy (2019) 7(4):913–922 https://doi.org/10.1007/s40565-018-0475-4 System deployment and decentralized control of islanded AC microgrids without communication facility Baoquan LIU1 Abstract This paper proposes a novel system deployment principle for master/slave type islanded alternating current (AC) microgrids, with which decentralized control can be achieved without communications. The net power of a microgrid, including active and reactive power, is metered and compensated locally and independently by its units. This can benefit a microgrid regarding system expandability, flexibility, and plug-and-play. The proposed strategy is demonstrated in a typical islanded AC microgrid with diesel generators, renewable generation, and hybrid storage. A diesel generator set with constant speed governor and static exciter runs to build up and dominate the main AC bus. An ultra-capacitor unit suppresses fastvarying power fluctuations, and the battery shares part of the slow-varying power component. The diesel generator set only provides slow-varying power within a lower limit, which can avoid dramatic accelerations and decelerations and low load-rate operation. Finally, simulations on MATLAB/Simulink are carried out to verify the proposed strategy in typical scenarios. Keywords System deployment, Decentralized control, AC microgrid, Diesel, Hybrid storage CrossCheck date: 20 September 2018 Received: 2 May 2018 / Accepted: 20 September 2018 / Published online: 20 November 2018  The Author(s) 2018 & Baoquan LIU 1 School of Electrical and Information Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China 1 Introduction Along with the accelerated development of distributed generation (DG), such as photovoltaic solar arrays, wind turbines, etc., the microgrid has gradually become a promising framework for distributed energy harvesting and utilization, especially in rural and remote areas that a utility grid cannot reach [1–3]. A microgrid is typically a low voltage network consisting of DGs, local loads, energy storage, and other auxiliary infrastructure, aiming to power a certain area (e.g., an island, an industrial park, or a residential quarter) [4–6]. To achieve power balance and further economical operational objectives, centralized to distributed control frameworks can be applied [7, 8]. Centralized control requires an advanced central controller, which is responsible for achieving common goal(s) of the system. Through communication facilities, the central controller manages all units by collecting information, making decisions, and sending instructions [9, 10]. Control is always implemented hierarchically with two or three layers. The microgrid in [11] applies a master/slave architecture, where a master converter is assigned to dominate the AC bus and also performs as the central controller to drive other slave converters. Droop-based control is another system for organizing microgrid, which enables active and reactive power sharing without communications. However, the system frequency and voltage in steady state deviate from their nominal values with load changes, which must be restored using a communication-based secondary control loop [12–14]. In [15], two secondary control schemes, a model predictive controller and a Smith predictor-based controller, are investigated. In [16], two control techniques, based on H? and l-synthesis theory, are developed as the secondary correction loop. 123 914 Distributed control has no central controller, and the information-collecting and decision-making authority is delegated to DGs and flexible loads, forming a multi-agent control architecture. In this case, DGs can operate with more autonomy, but communications, especially between adjacent units, are still necessary for accurate power sharing, frequency restoration, or to achieve economic targets [17, 18]. Reference [19] proposes a supervisory control scheme based on neighboring message exchange, which can achieve either precise power sharing or economic dispatch of a microgrid. In [20], construction rules for communication networks and their agents in a microgrid are proposed together with a systematic method to design control laws, which is verified in the study. Generally, energy management of a microgrid with centralized or distributed control always relies on wired or wireless communications to achieve power balance, frequency restoration, or to achieve profitable targets, which will be costly and prone disturbances when it attempts to contact every unit in a complex, scattered, and unorganized system. If faults occur in the communication channels (for example, total net power information errors), the microgrid will have difficulties balancing the power within this system, likely leading to system collapse. Furthermore, the communication network, which has a particular structure and specific protocols, is relatively exclusive and not flexible for plug-and-play. Reducing the communication dependency in microgrid operations is always a challenging problem. This work proposes a novel system deployment principle for master/slave type islanded alternating current (AC) microgrids, with which fully decentralized control without communications can be achieved. The total net power, including active and reactive power, can be metered and compensated for locally and independently by the microgrid units. This design can benefit a microgrid regarding system expandability, flexibility, and plug-and-play. A typical small-scale AC microgrid is considered for demonstration. The assumed units include a diesel generator set, renewable generation, critical loads, hybrid energy storage (battery and ultra-capacitor), and flexible loads. The diesel generator set runs to build up and dominate the AC bus, assuring constant frequency and voltage. With the deployment principle and the designed power flow control scheme, net power of the microgrid is separated into fastvarying and slow-varying components, which are then shared and compensated independently and locally. The rest of this work is organized as follows. Section 2 presents the proposed system deployment principle in a typical master/slave type islanded AC microgrid. Section 3 illustrates the decentralized power flow control scheme without communications, and Section 4 provides the simulation results in typical scenarios using the 123 Baoquan LIU MATLAB/Simulink platform. Finally, Section 5 presents the conclusions. 2 System deployment principle of islanded AC microgrids 2.1 Units of microgrid Up to now, diesel generators remain the de-facto power supplies for emergency and remote area power utilization with proven reliability [21, 22]. A typical islanded AC microgrid is constructed in this work using a diesel generator set with constant speed governor and static exciter to act as the grid-forming unit (master unit). Hybrid energy storage and renewable generation (wind turbines in thi (...truncated)