Research And Implementation Of Multi-HVDC Coordinated Transient Stability Control For AC/DC Hybrid Power System

High Voltage Direct Current(HVDC)transmission has become the main channel for power exchange among regions and grid interconnection in China.By October 2016,there have been 26 HVDC transmission lines in operation in the State Grid(SG)and the China Southern Power Grid(CSPG).In AC/DC hybrid power systems,the transient power angle instability,caused by severe HVAC or HVDC faults,is still the main security and stability risk and the major factor to limit the power transmission capacity.HVDC system can endure short-term overload and has the ability to adjust large amount of transmission power within a very short period.These characteristics make HVDC become a promising measure to improve the transient stability in the the AC/DC hybrid power system.This paper focuses on the theory and application of transient stability control strategies implemented on multi-HVDC.Systematic studies,including the stability control mechanism,the designing of HVDC transient stability control schemes,the control adaptability analysis to different oscillation modes,the coordination of multi-HVDC controls as well as the engineering application of the proposed control strategy in the CSPG,are all introduced in detail in the thesis.The main contents are as follows.1)The multi-machine AC/DC hybrid system under a certain instability mode is equivlent to a one-machine-infinite-bus(OMIB)system based on the extended equal area criterion(EEAC),and on this basis working mechanism of the HVDC transient power control on the power system transient stability is analyzed.Accordingly,a Bang-Bang-type transient stability control(denoted as BTSC)is put forward.Key links in the BTSC design,including the low voltage blocking loop,the boundary of the power modulation amplitude,the power modulation rate,the dead-band settings etc.,are discussed in detail.The effect of BTSC on transient stability enhancement is fully verified in a two-region single-HVDC test system.2)Towards the application in complicated multi-machine AC/DC hybrid power system,the influence of HVDC power modulation on the dynamic power balance of any synchronous generators is analyzed.A new index,named as the HVDC power influence coefficient,is proposed to select and integrate the wide-area control input signal.The analytic expression to calculate the HVDC power influence coefficient,as well as the features of the coefficient,is elaborated.An enhanced transient stability control scheme,the energy function based transient stability control(denoted as,ETSC)is proposed based on the idea of accelerating the transient energy dissipation.Tests on the feasibility of the ETSC are carried out in a three-area single-HVDC test system.Key issues,including variations of the proposed HVDC power influence coefficient,integration of the input signals,control adaptability to the fault location changes,are discussed.3)To facilitate the coordination design of multi-HVDC transient stability controls,a normative description on the structure of multi-area multi-HVDC interconnected power grid is proposed firstly.Then the interactions of ETSCs on different HVDCs at different oscillation modes and in different network topologies are analyzed.Accordingly,a coordinated ETSC strategy for multi-HVDC links(denoted as CETSC)is put forward.Towards different network topologies and HVDC control ranges,CETSC coordinates the transient stability control behavior of different HVDC links from the aspects of the input siginal integration and the maximum power control magnitude.Two test systems,i.e.a three-area two-HVDC test system and a four-area three-HVDC test system,are designed to formulate different network topologies and to testify the performances of the proposed CETSC scheme.4)Application of the CETSC strategy in the CSPG is explored.The control schemes on four HVDC links in the power grid,i.e.the Chusui HVDC,the Guiguang I HVDC,the Guiguang II HVDC and the Tianguang HVDC,are designed respectively.The HVDC power influence coefficients are calculated and the main control parameters are set.Performance and adaptability of the CETSC are firstly testified on the PSCAD/EMTDC platform with well-constructed CSPG simulation network and detailed HVDC control and protection models.Then a prototype control device based on the CETSC strategy is developed with the collaboration with the Southern Power Grid Electric Power Research Institute and the Beijing Sifang Automation Company.Finally dynamic performance of the prototype control device is tested on the real time digital system(RTDS)simulation platform in the key laboratory of CSPG.The test results show that the CETSC device can effectively improve the transient stability of CSPG and increase the maximum transmission capability of the power grid.The research works and development achieved in this thesis can help promoting the researches and the practical application of the HVDC transient stability control,which will be one of the most powerful means to improve the transient stability of the AC/DC hybrid power system and to enhance the power transmission capacity across regions.