Analysis And Suppression Of Microgrid Oscillation

The intermittency of new energy,voltage sag of power grid and the broadband coupling of multi-power electronic converters can easily cause oscillations to the microgrid.Oscillations at least will lead to the decline of power quality in the power grid,even causing a chain reaction,resulting in the system off-line protection and loss of new energy generation power.At the same time,it will become a source of interference,which will lead to the instability of the whole system.Those complicated control processes and parameter difference,such as the transiting of multiple control modes for distributed power generation system including the process of grid-connection,off-grid,black-start,or low-voltage ride-through,the different filter structures and parameters for different structure microgrid,the different startup transient characteristics of diesel or inverter generation systems with different energy sources,would cause the mechanism of oscillations inside the multi-inverter microgird system to become more complicated.In this paper,the occurrence scenarios of oscillation include grid-connection mode,isolated island mode,voltage sag,low voltage ride through and reclosing process.In grid-connected operation state,aiming at the oscillation problem caused by the operation transition of power electronic converter interface from constant power control mode to current limiting protection mode when voltage sag and low voltage process appears,the judging method of oscillation,the prediction method of resonance frequency and resonance mechanism are researched based on the impedance model,get the mechanism is the grid harmonic suppression constraint conditions changed during voltage sag condition.Oscillation suppression measures are discussed and an oscillation suppression strategy is proposed that is adjusting the output power of grid-connected inverter.The strategy can eliminate oscillation simply and quickly.According to the oscillation information of voltage sag or short circuit fault,the method of real-time online monitoring and identification by extracting the characteristics of harmonic footprint is researched;two kinds of harmonic footprint characteristic identification strategy including voltage sag and voltage not dropping type are put forward,the restricted voltage harmonic criterion and the scalar differential criterion of current transient harmonic footprint characteristic are proposed.Simulation and hardware in loop experiments verify the effectiveness and rapidness of unified harmonic oscillation monitoring and identification suppression strategy.Aiming at the problem of oscillation between inverters,inverters and load when inverters in droop control mode in off-grid state,the mechanism of oscillation and its suppression strategy are researched.A complete impedance model of the droop control inverter is proposed,including power loop,voltage loop and current loop,and the influence range of control parameters and droop coefficients on oscillation is given.The method of oscillation judging method is analyzed through order reduction model in high-frequency,and finding the key line connection node as the whole oscillation priority judging condition strategy is proposed,which can simplify the judging process when analyzing the influence of the same line and different line length.A low-frequency order reduction model is proposed to analyze the influence of different load types on inverter droop coefficients.The order is significantly reduced relative to the complete model in the reference under the condition of constant impedance load,it has the advantage of simplified operation in large-scale micro-grid applications.On this basis,the parameter boundary problem of droop coefficient is discussed based on the proposed model.The characteristics of different black start sources,the selection of black start sources and the start process of the system are researched.The equivalent impedance model of inverter for diesel generator set is researched.Based on this model,how to avoid oscillation during the process of the current source control inverter and load loading is analyzed.Aiming at taking energy storage inverter as the black start source,the influence factors of oscillation during the black start process of the inverter is researched,the risk of oscillation introduced by phase locked loop after the energy storage system is added and the countermeasures are discussed.For the oscillation risk after black start process in reclosing stage,voltage and frequency preset adjustment scheme is researched in order to reduce the reclosing instantaneous oscillation.The dual voltage frequency deviation adjustment scheme with the boundary conditions for oscillation suppression is proposed which using analytic hierarchy process(ahp)to find Pareto optimal solutions,and the particle swarm algorithm is used to optimize the results.Hardware in the loop together with the experiment platform are used to verify the effectiveness of the equivalent inverter impedance model for the synchronous in analyzing oscillation and the dynamic process of parallel operation of the energy storage inverter.Through the double optimization,the voltage deviation within 5V and frequency deviation close to 0 can be realized to reduce oscillation in reclosing process.