Active Stabilization Control of Multi-Terminal AC/DC Hybrid System Based on Flexible Low-Voltage DC Power Distribution

Wei Deng, Wei Pei, Luyang Li
2018 Energies  
Multi-terminal AC/DC interconnection will be an important form of future distribution networks. In a multi-terminal AC/DC system, if scheduled power for the AC/DC converter exceeds limits this may result in instability of the DC network. In order to overcome these limitations and avoid an unstable situation during coordinated control, this paper proposes a general active stabilization method for a low-voltage multi-terminal AC/DC hybrid system. First, the typical coordinated control modes for a
more » ... control modes for a hybrid system are analyzed. Second, a multi-level active stabilization controller, using the Lyapunov method, is introduced, and a feedback law allowing large signal stability is proposed. Finally, a system simulation model is further established, and the proposed active stabilization method is tested and verified. Study results show that only low stabilizing power with a slight influence on the DC network dynamic can improve the system's stability and ensure stable system voltage. Energies 2018, 11, 502 2 of 20 network, which connects to the external power grid through an intelligent energy management (IEM) interface [11] . Moreover, the micro-grid of Osaka University in Japan [12] and the European universal and flexible power-management system (UNIFLEX) [13] have also put forward AC/DC power-distribution systems. Coordinated control is an important foundation for the stable operation of a multi-terminal AC/DC hybrid system [14] . How to achieve the coordinated operation of various types of distributed generation, loads, energy storage, and AC/DC converters has become one of the technological challenges of AC/DC hybrid system development. To date, there have been some achievements in the field of coordinated control in a multi-terminal AC/DC hybrid system. In [15] , coordination-control algorithms in a hybrid AC/DC micro-grid for converters, such as a PV panel, wind-turbine generator (WTG) with a double-fed induction generator (DFIG), and battery, are studied, and have been modeled and verified using Simulink in MATLAB. In [16], a hybrid structure for an AC grid-connected micro-grid, with a DC connection based on back-to-back (B2B) converters, is studied, a control scheme for the utility-interfacing voltage-source converter (VSC) and DC micro sources is proposed, and different operating scenarios, even faults inside or outside the micro-grid, are also investigated. In [17] , typical operation modes for low-voltage (LV) AC/DC micro-grids are proposed, and a coordination-control method of utility-interfacing VSC, storage energy, PV, and direct-driven WTG with a permanent magnet synchronous generator (PMSG) under each operation mode, are put forward. In [18] , an improved virtual-impedance control method is proposed for bi-directional power converters in hybrid AC/DC microgrids operated in island mode, in order to reduce the circulating current and for accurate power-sharing. In [19], a new droop control scheme is investigated for a hybrid microgrid formed by multiple AC and DC sub-grids, in order to ensure active power-sharing and the autonomous operation of the hybrid microgrid. In [20], the power-sharing control issues in hybrid AC/DC microgrids are discussed, the drawbacks of conventional voltage droop methods are described, and a new frequency droop-based strategy is proposed to share power in hybrid microgrids. This research mainly takes the DC bus voltage signal (DBS) as the judgment criterion in order to propose a hierarchical or coordinated control strategy based on different operating system modes, and adjusts each converter to ensure power balance under various conditions. However, this method can only make DC voltage maintain the ideal reference value when the utility grid is normal; in other cases, DC voltage will deviate from the ideal reference point, which obeys a differential regulation. Nevertheless, a DC network contains a lot of constant power load (CPL) in practical applications, and CPL has negative impedance characteristics. Therefore, DC voltage deviating from the ideal reference point may aggravate DC-voltage fluctuations, and even lead to the collapse of the whole system [21, 22] . Tools allowing large signal-stability analysis of a DC-power system with CPLs, such as the Takagi-Sugeno (TS) multi-modeling, block-diagonalized quadratic Lyapunov function, Brayton-Moser's mixed potential function, and reverse-trajectory tracking, have been introduced [22] , and a general active control method is proposed for multi-CPL DC power networks in order to ensure the system is stabilized at an operating point, which would otherwise be unstable [23] . However, the above studies mainly focus on AC/DC systems with low power and multi-loads. There is still a lack of effective coordinated control methods for a multi-terminal AC/DC hybrid system with multi-sources, high power and higher voltage. Using a multi-terminal AC/DC hybrid system based on flexible LV DC power distribution, this paper establishes an electrical equivalent model and the state-space model required for coordinated control. An active stabilization-control method using the Lyapunov theory is then proposed, and feedback laws are designed to ensure system global stability, a wide operational boundary with lower control cost, stable voltage of the DC network, and the normal operation of each piece of equipment in some operating conditions that would otherwise be unstable. This paper is structured as follows: Section 2 outlines the typical structure and coordinated control mode of the LV multi-terminal AC/DC hybrid system. Section 3 carries out system-stability analysis in order to study the stable boundary. Section 4 proposes the multi-level active stabilization control method. Section 5 shows the corresponding simulation results and analysis. Conclusions are drawn in Section 6. Energies 2018, 11, 502 3 of 20 Coordinated Control Mode of Low-Voltage (LV) Multi-Terminal AC/DC Hybrid System System Structure The typical structure of a LV multi-terminal AC/DC hybrid system is shown in Figure 1 . The utility-interfacing VSC is the interconnection interface between the AC and DC systems. The AC side of each utility-interfacing VSC is connected to some AC line or AC node located in the corresponding AC system, and the DC side of each utility-interfacing VSC is connected to the DC system. It is noteworthy that AC systems do not exist with interconnections directly between each other, and each AC system has the independent voltage and frequency support provided by its internal utility grid, respectively. The AC system can absorb power from the DC system or inject power into the DC system through the utility-interfacing VSC, according to the power instructions from the dispatch agency. Based on the power exchange between each AC and DC system, load-balancing and power-flow optimization among multiple AC systems can be achieved. The DC sides of the VSC1, VSC2, and VSC3 are connected to each other through the DC system, which can build the multi-terminal interconnection structure. A DC device, such as a PV, WTG, EV, or battery energy-storage system (BESS) is usually integrated into the DC system through the DC/DC converter. The typical structure of a LV multi-terminal AC/DC hybrid system is shown in Figure 1 . The utility-interfacing VSC is the interconnection interface between the AC and DC systems. The AC side of each utility-interfacing VSC is connected to some AC line or AC node located in the corresponding AC system, and the DC side of each utility-interfacing VSC is connected to the DC system. It is noteworthy that AC systems do not exist with interconnections directly between each other, and each AC system has the independent voltage and frequency support provided by its internal utility grid, respectively. The AC system can absorb power from the DC system or inject power into the DC system through the utility-interfacing VSC, according to the power instructions from the dispatch agency. Based on the power exchange between each AC and DC system, load-balancing and powerflow optimization among multiple AC systems can be achieved. The DC sides of the VSC1, VSC2, and VSC3 are connected to each other through the DC system, which can build the multi-terminal interconnection structure. A DC device, such as a PV, WTG, EV, or battery energy-storage system (BESS) is usually integrated into the DC system through the DC/DC converter.
doi:10.3390/en11030502 fatcat:oeqrzwpxkvgr5i2as7nwcqrzbm