Research On Transient Stability Analysis And Optimization Methods Of HVDC Transmission System For New Energy

The connection of new energy sources to HVDC transmission system makes the system structure as well as the operation mode more variable,and the dynamic characteristics more complex.Especially when the large disturbance happened,the new energy sources will show different behavior from the conventional power generator,which causes the essential change of the fault transient characteristics of the system,leading to a serious threat to the safety and stability of the system.On the one hand,the fault of the inverter side system may cause commutation failure(CF),resulting in large areas of off-line of the new energy sources,.On the other hand,the DC side topology of the system is complex and the fault transition process is rapid.Therefore,once the control strategy is not properly chosen,it will lead to multiple reclosing-on-faults,which will cause the system to swing with large power angle and suffer from high fault current impact,seriously threatening the safety and stability of the system.At present,in view of the traditional high voltage DC transmission system,there are numerous CF detection methods and stability analysis methods after commutation failure,but research on commutation failure prediction method as well as the stability analysis method of DC transmission system for new energy under the condition of commutation failure or multiple reclosing-on-fault of DC system is less.Most of the existing methods can only reveal the external characteristics of the fault transient,and the interaction analysis of the equipment and its internal control units in the transient process is still insufficient.To sum up,the extraction of fault transient characteristics of DC transmission system for new energy and its instability mechanism explanation needs to be set off from the coupling effect between control units and equipment,through the energy storage and interaction in each independent unit,reveals the essential characteristics and physical meaning,and put forward the transient characteristic extraction and optimization method and the stability analysis method.The main contents and achievements are as follows.1)Concerning the difficulty in predicting commutation failure in HVDC transmission system,firstly the operation characteristic of valve-side current in the commutation process is analyzed.Then,according to the characteristic of cross commutation between Y bridge and D bridge of 12-pulse inverter,the model of accumulated energy difference between commutation inductances is built.On this basis,considering the instantaneous values of real-time firing angle and valve-side current in the DC system,the commutation failure prediction structure is constructed,which combined with the variation rate of valve-side current can predict commutation failure.The proposed method can effectively predict commutation failure in LCC-HVDC system,and lays a foundation for stability analysis and optimization of the system under large disturbance.2)Concerning that when fault occurs on the receiving-end AC line of the system,it is difficult to analyze the influence of commutation failure and key parameters of the control links on the system stability quantitatively,firstly considering the influence of the conducting state of converter valves on the system’s transient operation characteristic,the energy function of the system is established,which takes into account the effects of DFIG phase-locked loop(PLL),current inner loop,power outer loop and DC-side constant current control link.Then,the dissipation energy component in the energy function which characterizes the system stability is extracted,and the quantitatively how the variation of the key control parameters affects the system stability is analyzed.3)Concerning the problem that HVDC restart failure can result in DC power interruption,which will further cause the transient power angle of rectifier-side AC system to go unstable,firstly the dynamic energy equations of DC terminal and AC filter in the whole process of DC line restart failure are built,including the ’restart at fault’stage,phase-shifted triggering stage and deionization stage.On this basis,according to Lyapunov’s second theorem,the variation rate of the aperiodic component of generator dynamic energy is extracted,which characterizes the accumulation or consumption trend of total system energy.Then,the effects of system components on transient stability are analyzed quantitatively.4)When the LCC-HVDC transmission system restarts at fault,the high restart DC pulsed current will deliver a second impact on the system,causing the transient power angles of rectifier-side generators to swing.In order to solve this problem,by considering different stages in the whole fault recovery process,the detailed expressions of the dissipation energy injected from the generator to the system in the stages of’phase-shifted trigger’,’deionization’ and ’restart’ are derived.On this basis,the system energy optimization model considering the control unit of DC rectifier is established.With the accumulated dissipation energy of the system reaching the minimum value as the object,the optimal rectifier control parameters are calculated,which realized the goal that the amplitude of the DC pulsed current and the peak-valley difference of the power angle swing curve during the ’restart at fault’ stage been greatly reduced,which effectively improve the fault transient characteristics of the system.