Research On Resonance Analysis And Suppression Methods Of Hybrid AC/DC Distribution Network

With the establishment of the "double carbon" target and the rapid development of power electronic technologies,the hybrid AC/DC distribution network has become an important part of the future power system with its advantages of high energy efficiency,better power quality,and easier distributed power access.As the main object for multi types of loads and renewable energy access,the hybrid AC/DC distribution network presents the characteristics of multiple nodes,complex topologies,and diverse operation scenarios.The complex interaction between equipment,between equipment and grid,and between the AC and DC sub-network introduce challenges to resonance analysis and suppression.This paper conducts in-depth research on the resonance interaction analysis and suppression method of the hybrid AC/DC distribution network,including impedance modeling and resonance interaction analysis,influencing factors analysis,resonance suppression safety zone analysis,and research on resonance suppression methods basedon the active damper.The specific contents are as follows:1)Modeling and resonance interaction analysis of hybrid AC-DC distribution networkTo accurately reflect the interaction between nodes of AC and DC subnets,a mathematical modeling method for hybrid AC/DC distribution networks is proposedbased on the node voltage equation.The corresponding relationship between the voltage of each node of the AC and DC subnets and the injected current of the nodes is reflected through a block matrix,avoiding deviations from the subnet’s equivalence.Aiming at observing the source side converter resonance characteristics to input side disturbance,a disturbance source transfer function is defined to present the corresponding relationship between the system equivalent disturbance source and the converter input side disturbance source,then the traditional node voltage equation is modified.Based on the proposed model,resonance interaction between AC and DC subnets is analyzed,and the quantitative index of interaction intensity between and within subnets is defined.According to the research,both the AC and DC subnets resonance peaks exist in the corresponding frequency of the model interactive part in addition to the local subnets.Due to the frequency coupling between the AC and DC subnets,the resonance peak of the interaction part will correspond to the new response frequency or the new disturbance frequency triggering resonance.Besides,resonance amplification might happened during the disturbances transmission to another subnet,and the interconnecting converter(ILC)accessing node is not always the largest responsive node in that subnet.Finally,a simulation model is built to verify the effectiveness of the proposed model and analysis conclusions.2)Resonance influencing factors analysis methodModal sensitivity analysis(MSA)is an important part of the influencing factor analysis,which can reflect the complex interaction of the hybrid AC/DC distribution network on the component scale.Since existing MSA is merely applicable to the traditional RLC circuit,based on the mathematical model in Chapter 2,the modal impedance sensitivity(MIS)calculation method for the disturbance source transfer function,the converter equivalent admittance,and its internal parameters are proposed.The proposed method extends the MIS application scenario and realizes the global MIS calculation of the hybrid AC/DC distribution network parameters.Besides,to avoid errors generated by differential approximation to the MFS calculation process,an MFS calculation method based on the solution of non-homogeneous equations is proposed.The proposed method realizes the solution of critical variables of MFS through the general solution expression of non-homogeneous equations,avoids the impact of frequency resolution on MFS,and owns higher accuracy and calculation efficiency.To strengthen the comprehensiveness of MSA,the sensitivity of resonance peak impedance to parameters is defined as modal resonance sensitivity(MRS),which has a good insight into resonance amplification under resonance frequency transfer.Finally,the effectiveness of the proposed method is verified by the simulation model in Chapter 2.3)Safety zone and effectiveness analysis of resonance suppression strategiesInteraction usually involves participation of multiple devices or links,and the responses are also closely related to scenarios.Compared with traditional resonance,the resonance interaction has a complex genesis and diverse characteristics,and the difficulty of resonance suppression is significantly increased.Existing resonance suppression strategies pay less attention to their effectiveness under different application scenarios,and it is difficult to unify suppression methods for interactive resonance with different features.In order to analyze the effectiveness of the resonance suppression method,the resonance suppression safety zone for the admittance changes of AC and DC subnets is defined with the admittance changes at different frequencies as the independent variable.The safety zone boundary for the admittance changes is derived,and a method for analyzing the effectiveness of the resonance suppression strategy is proposed.The proposed method can directly determine the rationality of the resonance suppression strategy through the distribution of admittance changes in the security domain.and provide optimization suggestions,which is universal for resonance suppression methods that can be represented by impedance models.Furthermore,considering that the obtained d-axis and q-axis safety zone of the AC subnet require iteration when used in the design of resonance suppression strategies,a conservative safety region of the AC subnet considering the consistency of the d-axis and q-axis parameters is defined and derived to simplify the analysis process.The obtained criterion avoids the iterative process and can accurately lock the regions with higher resonance suppression efficiency around the origin of the admittance change,which is valuable for guiding the resonance suppression strategies design.Finally,taking typical resonance suppression strategies as examples,the effectiveness of the resonance suppression safety zone is demonstrated.4)Research on resonance suppression method based on the active damperIn order to solve the problem that resonance interaction suppression methods under different scenarios are difficult to unify,based on the characteristic of the impedance distribution of each node in the impedance model,the resonance suppression is carried out by means of connecting an active damper at the main resonance nodes,a control strategy design method for active dampers with strong versatility for different characteristic mutual resonances is proposed.The proposed control strategy adds an admittance remodeling part on the basis of the traditional double closed control loop,so the obtained mathematical model of the active damper can achieve decoupling of fixed and remodeled admittance,avoiding the interference of the fixed and remodeled admittance parameters coupling since the former owns limited flexibility on impedance remodeling design,which achieves flexible adjustment of its amplitude and phase distribution through a second-order and a lead-lag transfer function.By designing the parameters in the remodeled admittance transfer function according to the resonance suppression safety zone distribution,the proposed method achieve targeted resonance suppression for different scenarios.Finally,simulation verifies the effectiveness of the proposed method.