Research On Stabilization Control Of Grid-connected Converter Under Weak Grid

Grid-connected voltage source converters have been widely accepted as the interface between recent distributed power generation and traditional power grid,taking an more and more important role.However,unexpected problems are also brought.The grid-connected power electronics converters utilize high order power filters,having faster control dynamics,which may cause stability problems in higher frequency.What's worse,the converters are increasingly connected to the weak grid.The weak grid usually has higher impedance and grid impedance wide variation may also happen.The grid impedance may interact with converter,causing harmonic instability.There are still several challenges for system stabilization.The converter impedance model needs to be built and the potential unstable frequency range should be predicted.The potential unstable region should be eliminated in a high efficiency way.The system stability should also be kept under grid impedance wide variation.Fewer sensors should be used to reduce system cost and the robustness under comverter parameter variation should also be kept.Currently,the existing stability criterion could not predict the potential unstable region.In order to solve this problem,this paper first build the impedance model of grid-connected converter with converter side current control and grid side current control.By using frequency domain passivity theory,the non-passive region is also derived.The main infulence factor of non-passive region is also predicted.By using the impedance based stability analysis method,the potential harmonic instability caused by the non-passive region is also analyzed.Different stabilization methods are also proposed based on changing the system control delay and reducing the non-passive region.In oder to solve the stability problem caused by the non-passive region,this paper also studies the non-passive region elimination method based on active damping.The converter impedance models including passive and active damping are derived.The impacts of passive damping and active damping on converter non-passive region are also analyzed.By using capacitor current feedback active damping,the non-passive region is reduced,which can achieve a similar effect in a high effieiency way.This paper also analyzes the effect of control delay on the active damping and converter output impedance.It can be found that the digital control delay can degrade the effect of active damping and enlarge the non-passive region.So this paper propose an improved active damping method.By adding delay compensation component,system stability can be further improved.In order to further improve the system stability under grid impedance wide variation,this paper proposed enhanced passivity based damping methods,which can ensure system stability margin.An optimal active damping design method is first studied according to the traditional passivity theory.Then the effect of grid impedance wide variation is also analyzed.The limitation of traditional passivity frequency is also pointed.On this basis,this paper proposes to use enhanced passivity to further limit the phase of converter output admittance.The impedance shaping method based on hybrid damping and combined active damping is also proposed.System stability margin can be ensured while system efficiency is also considered.This paper proposes a robust sensorless damping method to reduce the system cost.The system robustness under converter parameter variation is also considered.A sensorless active damping method based on grid current feedback filter is firs t proposed.Then this paper analyzes the effect of converter parameter variation on the sensorless active damping.It can be found that the inductance variation can bring additional stability problem.Then this paper uses low-pass flitered grid voltage feedforward and series damping resistor to achieve full frequency passivity.The robustness under inductance variation and stability margin in the high frequency are also improved.The robustness under capacitance variation is also verified.This paper systematically studies the modeling and stability problems of grid-connected converter system under weak grid.Stabilization control methods are also proposed.The simulation and experiment results verify the proposed method.