Research On Stability Of Key Equipment In AC/DC Distribution Network Based On Impedance Analysis Method

Nowadays,driven by the targets of‘carbon peak’and‘carbon neutral’,more and more power sources,loads and storage devices are equipped with DC characteristics while the traditional power distribution system is gradually developing into AC/DC distribution network which is mainly formed by more converters.Caused by the interaction between the converters and the power grid,the internal mechanism of system stability has changed,presenting a more complex oscillation ranged from wide frequency.Since the existing power frequency phasor method is no more suitable,it is necessary to make research on the stability of the key power electronic equipment in the AC/DC distribution network.Therefore,the typical voltage source converter with virtual synchronous control method is chosen.Firstly,the modeling of control system in the virtual synchronous converter is studied.The primary frequency regulation and voltage regulation characteristics of traditional synchronous generators/motors,as well as the mechanism of inertia support and damping regulation are analyzed to build the uniform power control loops including VSG inverter and VSM rectifier mode.Meanwhile,to reflect the rapidity and accuracy of power electronic control,the output voltage and current dual control loop is established.Secondly,in view of the power coupling problem caused by the changing characteristics of line impedance,the mechanism of transmitting power is analyzed which points out that the power coupling system is a steady-state operating point offset problem.Considering the idea of second-order voltage response compensation,a power decoupling control strategy based on adaptive virtual impedance is proposed.By estimating the steady-state operating point,the values are updated in real time.The simulation further verifies the necessity of introducing the adaptability.Furthermore,based on the proposed improved virtual synchronous control system,the sequence impedance models in both VSG inverter and VSM rectifier mode are established,considering the double mirror coupling frequency and the effect of the phase angle disturbance during the coordinate transformation.The frequency sweep simulation model verifies the correctness of the proposed sequence impedance.It points out that the positive sequence impedance of the converter has a resonance peak response and large phase fluctuations close to50Hz which is easy to cause the oscillation under the weak grid.Decreasing J and D_Qappropriately is meaningful to enhance the system stability.Finally,based on the classification of system stability types,an impedance-based analysis method is applied to study the static stability and small signal stability of the virtual synchronous converter system.Under the 50Hz frequency,the maximum transmission power and the minimum SCR is calculated based on the line impedance characteristics,pointing out that the converter system static stable area will be decreased under the weak grid,which may cause the static instability.Furthermore,under the static stability condition,the positive and negative sequence impedance model is used to analyze the small signal stability near the rated steady-state operating point.It shows that the converter system is easy to occur harmonic oscillation close to 50Hz frequency under the weak grid.At last,the impedance reshaping method is introduced in current loop and its effectiveness is verified by the simulation results.