Abstract
The multi-terminal AC/DC system will become one of the important forms of the future power grid. The negative impedance characteristic caused by the constant power load in the DC network will reduce the power transfer capacity between the terminals, especially when a grid fault occurs in AC system at any terminal. Energy storage has played an important role in improving the stability of AC and DC systems. This paper proposes an additional control method based on an energy storage system to improve system power transfer capacity with low cost. The state space model of two-terminal AC/DC system is established, and the feedback laws for additional control are further designed by Lyapunov theory. Furthermore, the additional control strategies based on the energy storage system is built, without changing the existing control system of each control object. Finally, the corresponding system simulation model is established by Matlab/Simulink for analysis and verification. The research results show that the proposed additional control method is effective. The power transfer limitation can be overcome by only adding small damping energy with the stable DC voltages under large disturbances, and the power transfer capacity between the terminals can be significantly improved with low control cost.
Highlights
Due to the rapid development and wide application of the renewable energy, new material and, power electronic technology, and the growing requirement for high power supply quality, reliability, and efficiency, the current AC power distribution system is facing great challenges like diversified requests for power customers, modularized connection of the distributed renewable energy, and complex control of power flow, etc
On the purpose of solving the above problem, this paper proposes an additional control technology of the energy storage system (ESS) to overcome the negative impedance feature and the limit of VSC power transfer under DC network resonance, and improve the stability of the overall multi-terminal AC/DC system by generating only a small amount of additional power for ESS
Based on the above variables, the additional current instruction value iadd formed by Equation (8) is sent to the ESS to improve the power transfer capacity of the multi-terminal AC/DC system through the additional control, without changing the existing structure of the ESS controller and its parameters
Summary
Due to the rapid development and wide application of the renewable energy, new material and, power electronic technology, and the growing requirement for high power supply quality, reliability, and efficiency, the current AC power distribution system is facing great challenges like diversified requests for power customers, modularized connection of the distributed renewable energy, and complex control of power flow, etc. On one hand, in a multi-terminal AC/DC hybrid system, when the DC/DC converter uses closed-loop control to regulate the load voltage, it presents the dynamic characteristic of constant power load (CPL) [12] Both the DC/DC converter and the DC/AC VSC under the P&Q control mode appear the negative impedance feature, which will further aggravate voltage changes under fluctuation of the DC voltage, and even destabilize the system in some serious cases [13,14]. Aiming at the DC voltage oscillation caused by poor interaction between the power electronic device and the DC network in the multi-terminal DC power distribution system, an active damping control strategy based on the inner current loop is proposed which can improve the ability of mutual power compensation among several power electronic equipment and significantly expand the stability domain [19].
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