Research On Control Strategies And Operation Modes Of Soft Open Point

Energy structure upgrading prompts a continuous growth of renewable energy in power production and ever-growing electricity replacement in power consumption.Under this trend,a high proportion of distributed renewable power generation and active loads such as electric vehicles are introduced to distribution network,along with their inherent randomness,inter-mittency and time-space shifting characteristics,which pose great challenge on the safe and economic operation of the system.Traditional distribution systems enhance their initiative by means of network reconfiguration and closed-loop operation but cannot avoid the subsequent shortcomings of prolonged outage and sharper short-circuit current.As a primary side device based on fully controlled power electronics,soft open point(SOP)replaces the tie switch be-tween different feeders or different distribution networks of various voltage level and enables optimal system operation and service restoration by applying appropriate control strategies.In terms of both device development and system scheduling,this paper studies control strategies and operation modes of soft open point,and main contents are as follows.1.In terms of SOP’s modeling and control strategies: with a double-end SOP being an example,mathematical model of the voltage source converter(VSC)is derived in three different coordinates and two steady-state vector relations are derived to describe SOP’s grid-connected operation.Power and DC-link voltage control of VSC are studied in response to grid-connected operation.A second-order high pass filter is designed and passive inverter control is studied in response to one end of SOP’s isolated operation.In request of adaptation to variable operating scenarios,an isolation detection method based on voltage amplitude and frequency is proposed and a synchronization control without PPL is devised.Device-level simulation is formulated and afterwards testifies the effectiveness of overall control logic and smooth switching among different control strategies under combined scenarios,and demonstrates the SOP’s success in tracking references under whether single or complex operating scenarios.2.In terms of SOP’s contributions to distribution system’s optimal operation: typical distri-bution power flow models are analyzed and compared and lead to a conclusion that approximate power flow models have general conservativeness and computational errors that is in positive correlation of load level.Therefore,precise power flow models and the loss-involved SOP model are selected in which quadratic constraints are relaxed into second-order conic forms.Static cases prove SOP’s significant improvement of system voltage profile and reduction of power loss.Dynamic cases prove SOP’s ability to accommodate and balance DGs’ fluctuating power output,and furthermore,indicates a multi-end SOP maintains its performance at a lower capacity in comparison with a set of double-end SOPs with the same total end amount.In light of tie switches’ long existence,network reconfiguration models are studied in formulations of both improved simulated annealing and mixed-integer second-order cone programming,with the ev-idence showing the latter formulation’s superiority.A dynamic hybrid optimization of SOP and tie switches is then proposed and case studies confirm that SOP dominates the contribution to optimal objective’s realization without the necessity of tie switches’ frequent operation,indi-cating SOP’s possibility to enhance system stability and extend mechanical longevity.3.In terms of SOP-equipped distribution system’s service restoration: firstly it is proposed that the effectiveness of service restoration is largely dependent on fault location in flexible distribution systems.Several restoration scenarios are discussed,among which the restorable scenarios are further studied.As studies present an optimal solution space problem when for-mulating the restoration objective in the traditional way,a system loss-accounted restoration model of flexible distribution network is proposed.Cases demonstrate a SOP of larger capac-ity or a SOP closer to outage center is more advantageous to restoration process.In deciding two detailed schemes when both SOP and network reconfiguration are functioning,case results indicate that SOP performs better on a radially configured network.Based on the aforemen-tioned model,an adjustable robust service restoration model accounting for uncertainties of DG output and load demands is proposed,with the column and constraint generation algorithm being employed to iteratively solve the master and sub-problem both in forms of mixed-integer second-order cone programming.Worst-case test and Monte Carlo simulation are undertaken,demonstrating merits of SOP-based robust restoration strategy in defending system uncertainty and disturbances,stabilizing system performance and improving voltage profile.