Adaptive droop control based power flow regulation and optimization in multi-terminal high voltage DC system

Minimization of the total transmission loss of an interconnected AC-DC grid plays an important role in the economic operation of the AC-DC grid. Different from the conventional AC grid where the transmission loss is usually minimized by reactive power regulation, the transmission loss of a meshed AC-DC grid can be optimized by adjusting the active power exchange between the AC and DC grids. Additionally, smaller DC voltage deviation after grid disturbances is very desirable since it can bring less impact to the operations of AC-DC grid. This thesis firstly presents two improved sequential power flow algorithms for modular multilevel converters (MMCs) based AC-DC grid under DC power-voltage droop control. An optimization algorithm is then proposed to minimize the total loss of the AC-DC grid and the overall DC voltage deviation after the change of operating conditions. Adaptive droop control is used in the proposed optimization algorithm in which the power references are control variables solved from the optimal AC-DC power flow. Active power sharing and voltage regulation are two of the major control challenges in the operation of the voltage source converter based multi-terminal high-voltage DC (VSC-MTDC) system. Conventional droop control methods for power-sharing in an MTDC grid lead to voltage deviation from the nominal value. Moreover, the power-sharing is inaccurate in the droop-controlled MTDC system. This thesis proposes two novel autonomous control methods to regulate average DC voltage and share the power burden proportionally, using the adaptive droop control strategy. The proposed Method I utilizes DC grid lossy model with the local voltage droop control (LVDC) strategy, while the proposed Method II adopts a modified common voltage droop control (MCVDC) based on DC grid lossless model. The regulation of active power flowing through one or multiple DC lines plays an important role to guarantee secure and economic operations of MTDC grids. This thesis proposes a new method to regulate DC line power flow based on the adaptive DC voltage droop control strategy in which the voltage references of the voltage droop controllers vary autonomously at post-contingencies. The main advantage of the proposed method is that it can avoid installation of extra equipment and thus the associated losses and costs in the power-converter-based power flow control methods. The proposed control approach does not require solving online global AC-DC power flow equations, leading to autonomous control.Applied Science, Faculty ofEngineering, School of (Okanagan)Graduat