| With the rapid increase of renewable power penetration and electric vehicles,the dynamic behavior of distribution networks is becoming more complex.To ensure the safe and stable operation of the power system,it is significant to monitor the operating status of the distribution networks in real time.However,the dynamic behaviors detected by measurements at the distribution networks is the coupling between the dynamic behaviors of major grid and local grid.It is able to reflect the dynamics of high-voltage(HV)grids while containing the dynamics of the local low-voltage(LV)grids.This phenomenon has been verified by the recorded data of the WAMS Light(Wide-area Measurement System Light),which is developed by Shandong University and deployed at the demand side.In order to improve the availability of the measured data at distribution networks and use the data reflecting different dynamics for different scenarios,it is necessary to study the coupling characteristics and decoupling approaches of the dynamic behaviors of HV and LV grids.The main contents is listed below:(1)To reveal the ability of the measured data at LV grids to reflect the dynamics of the HV grids,the correlation of frequency,voltage and phase angle responses of HV and LV grids after large disturbances is analyzed from the perspectives of theoretical level and mathematical statistics.Based on the definition of dynamic frequency definition and transmission matrix theory,the functional relationship between frequency,voltage and phase angle between HV and LV grids after large disturbances is proposed.The validation of the theoretical analysis is proved through simulation.Using the simulation data,the correlation is quantified by correlation coefficients and linear regression.(2)The coupling mechanism of the dynamic behavior of high and low voltage grids is proposed based on superposition theorem and compensation current method.On the basis of the mechanism and transmission matrixes,the response magnitude of HV and LV grids to large disturbances and local disturbances is compared.Through the global sensitivity analysis,the main factors affecting the magnitude of the response of the HV grid to local disturbances are discovered,which are the local disturbances,short-circuit capacity of HV bus and the load capacity of the fault LV bus.The mathematical model of dynamic responses of high and low voltage grids is established.The physical meaning of various parts of the model is explained and the features of the parts in different disturbances or under different grid parameters is discussed.The validity of the coupling mechanism and response magnitude analysis is verified by examples.(3)The response characteristics of LV grids to large and local disturbances is analyzed.The response trajectory to various typical disturbances is described in detail.A parameter that accurately reflect the propagation range of the disturbances in the low-voltage side is proposed,with which the propagation range of the two types of disturbances is quantified.The validity of the analysis in the response characteristics is verified by numerical simulations.(4)Considering the occurrence of multiple local disturbances,an approach to decouple the dynamics of high and low voltage grids in voltage and phase angle is proposed.The mathematical nature of the decoupling problem of voltage and phase angle is analyzed based on its characteristics.According to the significant differences in the propagation range between large and local disturbances,this decoupling problem is transformed into a problem of estimating the solution of contradictory equations.Then use complex least squares is used to estimate the solution and get the phasor,voltage and phase angle response of each local disturbance.The validity of the decoupling method is testified through an example. |
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