Hierarchical Coordinated Voltage Control In Distribution Networks With Multi-Microgrids

In response to the global energy crisis and increasingly serious environmental pollution problems,a large number of Renewable Energy Sources(RES)are connected to the grid using Distributed Generation(DG)technology,making the traditional distribution network transform from passive to active.The massive penetration of DGs brings more flexibility and controllability to the distribution network,effectively improving the economics and reliability.At the same time,large-scale and different kinds of DGs connect to distribution networks,which will make the operation and control more complicated.The fluctuation and uncertainty of the RES itself will also cause a series of safety problems to the system,such as over/under-voltage problems.The operation and control of distribution network are facing great challenges.Microgrid(MG),as an independent autonomous system containing DG,energy storage system and loads,can improve the acceptance of RES in distribution network through flexible control technology.MG technology has been widely used in distribution network operation.Considering distribution networks and multi-microgrids,both of them are independent utilities with their own operators and operational control strategies.However,due to the physical structure of the distribution network and MG,the control strategy of distribution network will influence the optimal operation of MG,and vice versa.In some cases,contradiction may exist due to the lack of consistency,i.e.dispatching orders of distribution networks couldn't guarantee the maximum profit of MG.Therefore,it is necessary to coordinate the voltage control strategy to realize optimal operation of the whole network.To this end,the paper proposes a hierarchical and coordinated voltage optimization control strategy for active distribution networks with multi-microgrids.The main work and results are as follows:(1)Considering the voltage optimization control in distribution networks with multi-microgrids,a bi-level optimization model is formulated,in which the objective of the upper level is to minimize the network loss in distribution networks,and the objective of the lower level is to maximize profit in MG.The optimization problem is solved based on the game theory method,and the convergence is achieved through interaction of power exchange at the Point of Common Coupling(PCC),which ensures the autonomy and commercial confidentiality of the MG.Besides.the coordination among transformers,capacitor banks,photovoltaic units and microgrids are realized.The optimization problems in distribution network and MG are solved by Dicopt solver in General Algebraic Modeling System(GAMS)software.The results show that the bi-level optimization model in voltage control successfully mitigate the voltage rise problems while maximizing the economic benefits of the MGs.(2)In order to realize coordinated optimal voltage control between distribution networks and multi-microgrids,a bi-level optimization model is formulated considering electricity price incentive mechanism.The objective of the upper level is to minimize the voltage regulation cost in distribution networks,which includes the cost of the power losses and the cost of purchasing the ancillary service;the objective of the lower level is to maximize the economic benefits in MG,including the profit from the sale of electricity and the ancillary services provision.The distribution network has established a reward mechanism to encourage the MG to provide voltage support.The MG weighs its own economic benefits and the additional gains from voltage regulation,and make a decision.Considering the problem that the distribution network can't adaptively adjust the reward price and will easily fall into the local optimum,an improved coordinated voltage control strategy based on the price-reactive power curve is proposed.The results show that the improved coordinated voltage control strategy can realize the coordinated optimal operation in distribution networks with multi-microgrids.Under the premise of ensuring voltage safety,the voltage regulation cost of the distribution network is further reduced and the economic benefits of the MG are increased slightly to achieve a win-win situation.(3)In order to achieve the coordination between voltage control effect and grid communication requirements,a combined central and local real-time voltage control strategy is proposed,which is the modification and improvement of the local voltage control curve.The two-stage voltage control framework in MG is constructed.The voltage reference at PCC is obtained using previous reactive power plan,and the local voltage control curve is corrected based on the optimal power flow algorithm and the curve fitting method.A local voltage control strategy combining P(?)and Q(?)curves is proposed to realize coordinated control of active and reactive power of controllable loads.The voltage control curves under different fitting methods are compared and results show that the voltage control curve obtained by piecewise linear curve fitting method has a higher fitting precise and performs better in mitigating voltage rise and reducing power losses than others.