Research On Operation Optimization Of Power Electronic Transformer Embedded Ac/dc Distribution Network

Nowadays,a new round of energy revolution characterized by the utilization of renewable energy is developing,and the penetration of renewable energy in the distribution network is increasing.In the future,the distributed generators,energy storage systems and micro-grids will be ubiquitous in the distribution network.Compared with the traditional AC distribution network,the AC/DC hybrid distribution network has the advantages of offering a plug-in interface for the distributed generators.Furthermore,it can improve the system’s power flow controllability,power quality and power transmission capacity.The AC/DC hybrid distribution network has been proven to be an effective technical means to cope with system uncertainties.Power Electronic Transformer(PET)is expected to be a critical hub equipment of AC/DC hybrid distribution network because of its flexible power flow controllability.Targeted at the steady-state operation of multi-port PET-based AC/DC hybrid distribution network,this thesis carries out the research from the following three aspects: the power flow calculation,flexible operation optimization,and the load transfer assessment.Firstly,the power flow calculation algorithm of the PET-based AC/DC distribution network is studied.The steady-state equivalent model of multi-port PET is established based on its topological structure at first,and then the control modes and corresponding equivalent method of PET in power flow calculation are given according to the characteristic of the network connected to the PET port.Based on the above studies,the steady-state power flow model of PET-based AC/DC hybrid distribution network is proposed.An alternating iterative power flow algorithm is then designed for this model.The case study is carried out in an AC/DC hybrid distribution network which is transformed from the IEEE 33 case.The simulation results analyze the proposed power flow algorithm and the effect of PET in power flow calculation.Secondly,a day-ahead and intra-day multi-time scale scheduling model for AC/DC distribution networks is proposed from the perspective of flexibility.The effect of four types of flexibility resources on flexibility balance is analyzed.In the day-ahead scheduling layer,the multiple scenarios technique is used to simulate the uncertainties of renewable energy and load.With full consideration of the system’s flexibility and operational cost,the day-ahead scheduling layer makes plan of reserved flexibility resources.In the intra-day scheduling layer,the model predictive control method is introduced to ensure the rolling scheduling strategy fits the operating status of the system while the cost of adjusting the day-ahead scheduling strategy is minimized.Based on the convex relaxation technique,the proposed non-convex nonlinear optimization scheduling problem is transformed into a second-order cone programming problem.The results of the case study verify the effectiveness of the proposed optimal scheduling model.Finally,a load transfer assessment method for the AC/DC hybrid distribution network is proposed,in order to quantitatively evaluate the system load transfer situation reasonably and intuitively after distribution network failures.A PET-based load transfer strategy for AC/DC distribution network under N-1 predicted failures is proposed.According to the characteristics of the AC/DC hybrid distribution network,a proper system load transfer assessment index based on risk theory is developed.Meanwhile,an evaluation framework for AC/DC hybrid distribution network is established.The system load transfer assessment index that can avoid voltage limit violations and line congestions can then be calculated by optimal power flow.The case study shows that the static ecurity assessment method and the proposed index is feasible.