Research On Multi-levelVoltage Control And Islanding Frequency Stability Of Distribution Network With Distributed Photovoltaics

With the in-depth promotion of the "carbon peak,carbon neutral" strategy,the installed capacity of distributed photovoltaic(PV)continues to increase,and the penetration rate of PV in the distribution network is increasing year by year.When the distribution network is in different operating states,the active participation of photovoltaic grid-connected inverters in voltage and frequency stability control not only helps to improve power quality and enhance the friendly support ability of subsequent access to new energy for the grid,but also has important significance in building a new type of power system and ensuring the reliable and high-quality development of energy in the new era.After a high proportion of distributed photovoltaic power is connected to the distribution network,current distribution networks mainly improve power quality by adjusting inverter control strategies to address voltage and frequency stability issues.Decentralized control often has poor results.The stability and support ability of voltage and frequency in the distribution network at this time needs to be further improved.However,the need to process a large amount of data during centralized and unified control leads to low efficiency,frequent inverter actions,and poor pertinence in voltage regulation during centralized control,and poor power quality at individual nodes.In response to the above issues,combined with the results of distribution network cluster division,this paper conducts research using distribution network clusters containing distributed photovoltaic as an intermediate unit,focusing on various control strategies for distributed photovoltaic grid-connected inverters,in order to improve the voltage stability of the distribution network and ensure the voltage and frequency stability of the cluster during parallel and offline operation.The main research content and results of this article are as follows:First of all,considering the lack of controllable resources in the distribution network,this article determines the research object as the node clusters in the distribution network,and takes photovoltaic grid-connected inverters as the main control object for research.It establishes multiple control strategy models for inverters,laying a foundation for the later research on voltage adjustment in the distribution network and improving the stability of voltage and frequency during cluster islanding and off-grid operation.Subsequently,this paper proposes a cluster voltage deviation index based on sensitivity analysis method for voltage fluctuations in distribution networks,which is used for cluster operation status evaluation and cluster selection in the process of voltage coordinated adjustment.The operation status of clusters is clarified,and a multi-level voltage control method based on multi cluster coordination is proposed.Through the control mode switching control of distributed photovoltaic grid connected inverters within each cluster and coordinated voltage regulation between clusters,the goal of improving the voltage stability of each cluster in the distribution network is achieved.Time domain simulations were conducted in a classic IEEE 33 node model and a 41 node model of an actual distribution network.The effectiveness of the multilevel voltage control strategy proposed in this paper was verified by comparing the unified power optimization control strategy with the decentralized Droop control strategy.Finally,considering that a cluster with a high level of active power balance can operate in an isolated island mode,this paper proposes a seamless switching technology and an active support method for voltage and frequency to achieve long-term stable operation of the cluster in response to the impact of cluster parallel and off grid switching voltage and frequency and the stability of voltage and frequency during isolated island operation.Simulations have verified the stability of voltage and frequency during parallel and off grid switching,and the effectiveness of the proposed support method for improving voltage and frequency stability when the cluster is operating in an isolated island.