Research On Control Strategy And Engineering Application Of Subsynchronous Oscillation Dynamic Suppressor

With the increasing penetration of renewable energy sources,especially the wind power generation,and broad application of the high-power power electronic technology,electric power system with renewable energy sources(EPSRE)is evolved,which is constituted by complex AC/DC system.In EPSRE,the characteristic of multi energy resources and multi conversions makes new subsynchronous oscillation(SSO)problems prominent constantly.In order to solve these new SSO problems,a new mitigation method of subsynchronous oscillation suppressor(SSO-DS)based on voltage source converter(VSC)technology is proposed in this paper.Focusing on this method,the following researches are carried out:The mechanism of new SSO problems which occur in Chinese power system recently is analyzed at first.Aiming at the frequent subsynchronous oscillation(FSSO)for large-scale thermal power plant cluster transmitted by high voltage direct current(HVDC)and AC hybrid series compensated line,a comprehensive analysis method combined system damping characteristic with shaft torsional response characteristic based on forced oscillation principle is proposed.Analysis results indicate that the combined interaction of forced response and free response under low damping is the root cause of FSSO.Aiming at subsynchronous control interaction(SSCI)in the doubly fed induction generator(DFIG)-based wind farm with fixed series compensation(FSC),based on single-machine infinite bus system,the SSCI mechanism is analyzed by eigenvalue analysis.Its root cause includes negative resistors effect related to the rotor resistor and current inner loop of rotor side converter(RSC)in subsynchronous frequency range,and subsynchronous modulation effect of the converters.The SSCI risk between DFIG and static synchronous compensators(STATCOM)in case of DFIG-based wind farm interfaced with weak AC system is analyzed.If the STATCOM controller induces a subsynchronous voltage component from the weak AC system distance,STATCOM will generate a corresponding subsynchronous component,which forms a closed-loop.If the closed-loop is unstable,the SSCI occur.The analysis above not only reveal the mechanism of the new SSO problem,but also lays the basis for SSO-DS damping controller design.The fundamental principle and operation characteristic is analyzed.Aiming at the subsynchronous shaft torsional interation of large scale thermal po wer generator,damping current modulation stragtegy is studied.Based on the analysis of existed double frequency modulation method,a single frequency modulation method injecting subsynchronous current or supersynchronous current is proposed,and the key parameters design is analyzed.For different shunt ratio of different system,the design method of optimal damping control strategy is proposed.Based on the time domain simulation platform of PSCAD/EMTDC,comparison results of different damping modulation strategy are carried out,which demonstrate the effectiveness of the design method of optimal damping control strategy.Aiming at the wide frequency range characteristic of SSCI in DFIG-based wind farm with FSC,the difficulty in extracting control input signal,and the drawback of the poor robust characteristic,corresponding damping control strategy of SSO-DS is studied.Considering different compensation degree and wind speed,residue-based analysis is used to identify the most effective input signals to the subsynchronous damping controller(SDC)and VSC input(d or q current order)to which the SDC must be connected.Root locus analysis is used to obtain the SDC parameters so that the subsynchronous mode damping is positive,however,there is a possibility that damping this mode can exacerbate the supersynchronous mode.In order to overcome this drawback,a modified design scheme is proposed.It is achieved by blocking the supersynchrnous component from the input to the SDC using a band-stop filter.The effectiveness of the SDC is verified by time domain simulation based on PSCAD/EMTDC.The results demonstrate the proposed SSO-DS can mitigate the SSCI significantly in DFIG-Based wind farms.A 10 k V/20 MVA SSO-DS device developed is applied in Hulunbuir power plant.Based on the proposed damping current modulation stragtegy,the control strategy of SSO-DS is designed.Damping characteristic and optimal damping control parameters are analyzed by real time digital and physical closed-loop simulation platform.Field test and the engineering application result indicate that SSO-DS successfully solves the FSSO problem nagging Hulunbuir power plant for many years.Besides,it can enhance the modal convergence speed in large disturbance.The results validate the effectiveness of SSO-DS.