Optimal Coordinated Voltage Control Of AC/DC Connected Wind Farms

Nowadays,energy is an essential ingredient of the socio-economic development.Due to the depletion of fossil fuels and growing concerns on environmental issues,the utilization of clean and non-pollution sustainable energy becomes a significant task and inevitable trend for the social development.Wind power is currently considered as one of the most technically mature and commercially promising renewable power generation.In the past two decades,with the growing sophistication of wind energy technology and cost reduction,wind power has gained favor with many national gov?ernments and is thus widely developed around the world.In general,wind power is exploited in a centralized manner and integrated into power systems in the form of wind farms.The inherent stochastic and intermittent nature of wind power might bring a number of technical and economic challenges toward system operation while it provides the clean energy.Among them,voltage control and management is the important foundation of the secure and stable operation of wind farms.With the increasing penetration of wind power,its fluctuation could not only cause severe fluctuation of the point of common coupling(PCC)but also make the terminal voltage of wind turbines violate the feasible range,which might further lead to trip-off events.Wind farm voltage control has become a crucial part that affects the development of wind power.Especially for large-scale wind farms,the optimal voltage control problems should be further investigated.In this dissertation,the research on coordinated control of power and voltage control for wind farms is conducted,with the aim of regulating the wind farm network voltages within the feasible range while improving the economic benefits of system operation.Particularly,in addition to the traditional centralized voltage control schemes,the dis-tributed optimization-based distributed and decentralized optimal control schemes are proposed which are more flexible and robust.The main contents and contributions of this dissertation are summarized as follows:(1)The centralized coordinated voltage control scheme is proposed for voltage-source converter-based high-voltage direct current(VSC-HVDC)connected wind farms based on the model predictive control(MPC).The proposed control method optimally coordinates wind turbines(WTs)and wind farm side VSC to regulate the voltage pro-file of the offshore wind farm networks within the feasible range while reducing the power loss of the system by using the reactive power and voltage regulation capability of VSC and WTs.Moreover,due to the high R/X ratio,the impact of active power on voltage is considered to achieve better control performance.The simulation results show validate the effectiveness of the proposed method under both normal and start-up operation.(2)The hierarchical distributed MPC scheme for VSC-HVDC connected wind farm voltage control is proposed based on the alternating direction method of multipliers(ADMM)algorithm.In this scheme,the conventional centralized optimal control prob-lem is decomposed into a unconstrained optimization problem and multiple parallel small-scale constrained optimization problems and is solved using a hierarchical dis-tributed iterative method via the ADMM framework.This method can alleviate the computational burdens of the central controller and improve the information security.The simulation results validate the optimality and effectiveness of the proposed con-trol scheme.Besides,the good convergence performance of the ADMM algorithm can ensure the feasibility of the proposed method for real-time control.(3)The fully distributed control algorithm for optimally coordinated voltage con-trol of alternating current(AC)-connected wind farms is proposed based on the dual decomposition and generalized fast gradient algorithm to regulate the voltage profile of wind farm network by optimally coordinating WTs and reactive power compen-sators inside the wind farm.Firstly,the original centralized reactive power/voltage optimization problem is transformed into an equivalent problem with the separable structure.Secondly,the optimization problem can be decomposed into multiple paral-lel subproblems based on the dual decomposition theory,corresponding to each con-trolled unit,i.e.,WTs and reactive power compensators.Finally,the generalized fast gradient method is used to efficiently solve the dual problem in a distributed manner.The proposed method can eliminate the requirement of a central controller and distant communications while guaranteeing the optimality.The simulation results validate the effectiveness of the proposed control scheme.Moreover,the fast convergence of the distributed algorithm ensures its feasibility for real-time control.(4)A decentralized(local)optimization framework for voltage control of wind farms is proposed based on the gradient projection(GP)algorithm.Firstly,the op-timal power flow(OPF)-based optimal voltage control problem is built.Then,the im-proved gradient projection method—scaled gradient projection(SGP)method is used to iteratively solve the optimal coordinated voltage control problem.By specifically designing the weighting matrixes in the optimal control problem,the global coupling gradient information can be locally obtained.Moreover,the optimality of the itera-tive update under fixed point is proven and the sufficient condition for the closed-loop stability of the control algorithm(i.e.,convergence of the iterative update)is given.Compared with the centralized and distributed control schemes,the proposed scheme eliminates the requirement and reliance on communications,reducing the construc-tion and operation cost and improving the robustness.The simulation results validate the convergence and good dynamic voltage control performance of the proposed algo-rithm.