Research On Transient Voltage Active Support Technology Of Hybrid Multi-converter Consisting Of Controlled Voltage/Current Source Converter

In power system with high penetration renewables,transient voltage is the key factor affecting the stable operation of converter and DC transmission system.With the reduction of synchronous machines,it is an inevitable trend to achieve transient voltage support through the control of converters.Controlled current source converter which is mainly used in renewable energy field at present lacks transient voltage support capability.The filed can have voltage source characteristics by using controlled voltage source converter.Reasonable combination of controlled voltage source converter and controlled current source converter can realize active transient voltage support.Therefore,transient voltage active support technology of hybrid multi-converter consisting of controlled voltage/current source converter is researched in this paper.The transient voltage active support of renewable energy stations is realized through the research for the key technology problems which include the transient voltage support characteristics of hybrid multi-converter,the transient voltage support capability improvement of hybrid multi-converter,the transient stability of of hybrid multi-converter,capacity configuration of of hybrid multi-converter and reactive power compensation,and decentralized cooperative control of hybrid multi-converter and reactive power compensation.The main research contents mainly include the following parts :1)Transient voltage support characteristics of hybrid multi-converter are analyzed.In order to rationally and fully utilize controlled voltage/current converter to realize transient voltage support,the control characteristics of controlled voltage/current converter are analyzed and compared.The hybrid multiconverter system composed of controlled voltage/current source converter is proposed.In the hybrid multiconverter system,the two controlled source characteristics are complemented.The influence of voltage source on transient voltage and the transient voltage support characteristics of hybrid multi-converter are further analyzed.2)Transient voltage support strategy for hybrid multi-converter consisting of controlled voltage/ current source converter is studied.The transient control of controlled voltage/current source converter is improved based on the requirement of transient voltage support to reduce the transient fluctuation and improve its transient reactive power output capability.Aiming at the problem that the controlled voltage source converter loses the voltage source characteristics due to current limitation under transient conditions,the flexible limiting control is proposed to ensure the voltage source characteristics at the initial stage of transient and limit the overshoot of the converter current.Aiming at the problem that the active power fluctuation of the controlled voltage source converter under transient conditions,the transient active damping control is proposed to reduce the power fluctuation of the controlled voltage/current source converter under transient conditions.The flexible limiting combined with the transient active damping control improve the voltage source stability degree of the hybrid multi-converter under transient conditions.In order to improve the transient support capacity and accuracy of the transient station,the adaptive compensation control for controlled current source converter is proposed to balance the reactive power output demand of the converter and the converter capacity limit.In order to further improve the phase tracking capability of the converter under transient conditions,the improved strategy of phase-locked loop dynamic characteristics based on phase difference interval is proposed.Finally,the transient control performance of the hybrid multi-machine converter system and the proposed transient voltage support strategy is analyzed and verified by simulation and experiment.3)The stability of hybrid multi-machine converter system is studied.In order to guarantee the reliability of active support of transient voltage,the stability of hybrid multi-machine converter system under transient voltage conditions is studied from two aspects of resonance stability and transient power angle stability.Aiming at the resonance stability problem caused by impedance changing under transient conditions,based on the impedance model of the controlled voltage /current source converter,the impedance analysis model of the hybrid multi-converter system is constructed.Based on the global admittance,the resonance stability analysis and determination method of the hybrid multi-converter system are studied.The influence of system impedance,converter number and control bandwidth on resonance instability is analyzed.The harmonic suppression strategies based on virtual filter capacitor current feedback and global damping are analyzed and compared.Aiming at the problem of power angle stability caused by the introduction of controlled voltage source converter,the influence of transient voltage on the power limit of hybrid multi-converter system is analyzed.The power model of hybrid multi-converter system is builded.Based on the phase diagram analysis method,the relationship between power limit and transient power angle stability of hybrid multi-converter system is studied.The converter is easy to enter the limiting state due to the large transient voltage change.The influence of power oscillation superimposed on the limiting characteristics of the controlled voltage source converter on the transient voltage support is further studied.In order to ensure the transient power angle stability of the hybrid multi-converter system,the power limit is taken as the core.The active power control target control of the controlled voltage/current source converter based on the transient voltage change is proposed based on the aggregation model of the multi-converter.Finally,the resonance stability,transient power angle stability and its suppression strategy are analyzed and verified by simulation.4)Capacity configuration and distributed collaborative control strategy for hybrid multi-converter system and reactive power compensation are studied.On the basis of guaranteeing the transient voltage support capability and reliability of hybrid multi-converter system,the capacity allocation of hybrid multiconverter and reactive power compensation equipment and the distributed cooperative control strategy are studied to improve the support capacity of each renewable station which is the voltage source of power grid.Controlled voltage source converter is used as voltage source.Controlled current source converter and static var generator(SVG)compensate internal voltage source quickly at the beginning of transient condition.Static var compensator(SVC)satisfies transient reactive compensation capacity at the later stage of transient condition.To ensure sufficient reactive power capacity,the capacity configuration method of hybrid multiconverter system and reactive power compensation is studied.The capacity configuration of controlled voltage/current converter is studied based on the demand of phase tracking.The steady-state regulation and transient reactive power compensation requirement are used as the capacity configuration basis of SVG and SVC respectively.In order to fully utilize the active transient voltage support capability of hybrid multiconverter system and reactive power compensation,based on the existing transient voltage control and stability analysis,the transient improved voltage control strategy for hybrid multi-converter system,SVG and SVC are proposed to realize the decentralized cooperative control of hybrid multi-converter system and reactive power compensation.Through typical transient voltage scenario simulation,the proposed capacity configuration and distributed collaborative control strategy are verified and analyzed.