Multi-level Coordinated Voltage Control Considering Reactive Power Support Capability Of Active Distribution Networks

In order to cope with global environmental pollution and energy shortage,a large number of distributed renewable energy sources are connected to the distribution network,which makes the traditional passive distribution network into a multi-source and highly controllable active distribution network.Active distribution network is a distribution network with active control and management of its internal distributed power,which can solve problems such as fault crossing,voltage control and power quality in active distribution network.At the same time,the operation of the Transmission and distribution network is no longer independent,and the Transmission System Operator(TSO)will not be able to regard the distribution network as a fixed load.The operation and control of the active distribution network will have a great impact on the voltage stability of the transmission network.In order to ensure the safe and stable operation of the system,it is of great theoretical value and practical significance to study the cooperative voltage control strategy of transmission and distribution including active distribution network.Due to the increasing penetration rate of new energy,the connection between active distribution network and transmission network is increasingly close.Firstly,the necessity of transmission and distribution coordination is reflected in the fact that traditional thermal power units are replaced by distributed power sources,and the controllable resources of transmission network are reduced.In order to improve the voltage stability of the system,new control means should be sought.As the advanced stage of the development of intelligent distribution network technology,active distribution network can control the internal distributed power supply autonomously and coordinate,and actively participate in voltage control to ensure the safe operation of network.Secondly,due to the physical structure connection between active distribution network and transmission network,their respective control strategies will influence each other.The transmission network reduced by thermal power units will not be able to guarantee the stability of the bus voltage at the public connection point of transmission and distribution under emergency condition,and the power recovery problem caused by active distribution network restoring its own voltage will also reduce the voltage of transmission network.The voltage of the transmission network exceeds the limit and even crashes.Therefore,the active distribution network should be included in the collaborative control of transmission and distribution,and control resources in the transmission network and active distribution network should be reasonably coordinated to improve the voltage stability of the system.Therefore,aiming at the transmission and distribution system with active distribution network,this paper proposes the multi-level voltage coordination control strategy of the system.The main research work is as follows:(1)Based on the maximum transferable power theory,the influence of active distribution network access on the static voltage stability margin of the transmission network is considered,and the power margin is calculated for the system with active distribution network combined with PV curve.Among them,the distributed PV in the active distribution network can adjust its reactive power output in the calculation of continuation power flow according to the voltage change of the common connection point,so as to give a certain support to transmission network.At the same time,considering the power feasible domain of distributed photovoltaic inverter,its own power decoupling can be controlled independently,the photovoltaic inverter can actively reduce its active power to obtain a larger reactive power output range.Calculating the influence of different photovoltaic active power output on the static voltage stability margin of the system.(2)In order to cope with the reduction of transmission network regulation measures and make full use to the supporting capacity of active distribution network,this paper proposes a three-level voltage coordination control strategy for transmission and distribution including distributed power supply access.Distributed PV in active distribution network can provide voltage support during system failure.In this paper,distributed photovoltaic control in active distribution network,generator automatic voltage control in transmission network and on-load regulator transformer tap control are modeled.Secondly,the three-level voltage coordination control model is established.The transmission grid level takes the minimum voltage deviation and the minimum control cost as the target.At the same time,it dispatches reactive power resources in the transmission grid and sends reactive power demands to the active distribution network.At the distribution network level,the goal is to minimize the deviation between the reactive power support provided and the demand received,and to minimize the reactive power output and active power reduction in photovoltaic.At the same time,the weight of photovoltaic is determined by using the normalization method according to the characteristics of different photovoltaic junction points.The photovoltaic level adjusts its own operation mode according to the received reactive power support instruction of the distribution network,and carries out reactive power output support on the premise of ensuring its normal operation.(3)After voltage control,a large amount of fast reactive power is occupied,making the dynamic reactive power reserve of the system decrease and unable to respond to emergencies quickly and effectively.In order to remove the fast reactive power occupation in the system and improve the dynamic reactive power reserve,a reactive power reserve optimization scheme based on multi-reactive power source coordination is proposed.Considering the weak area of the system,load partition is carried out.Based on the partition,dynamic reactive power reserve calculation based on V-Q curve is established to determine the minimum area of dynamic reactive power reserve after reactive voltage control.After the minimum dynamic reactive power reserve of the system is determined,the slow reactive power that is not put into use in the emergency control is used to replace the reactive power of the fast response equipment,and the effective reactive power reserve of the fast response equipment is increased,so as to improve the dynamic reactive power reserve of the system.At the same time,the dynamic reactive power reserve of the system before and after displacement is compared with the V-Q curve,which proves that the displacement method can improve the individual effective reactive power reserve and the system dynamic reactive power reserve under the premise of guaranteeing the voltage level,and improve the ability of the power grid to deal with disturbance.