Research On Frenquency Control Strategy Of Thermal Power-HVDC Islanding System

HVDC(High Voltage Direct Current)transmission has the significant advantages of economical efficiency,rapid response,and high controllability,therefore,it is widely applied for long-distance power transmission from the large-scale energy bases to the load centers.If the sending system is composed primarily of several thermal power generating units,and there is no electrical connection with main AC network,it is called the Thermal Power under Islanding Operation,and the system is called the DC islanding system.Its lower effective short circuit ratio and little rotating inertia means the poor anti-disturbance ability,and further causes a series of technical issues,such as steady control,frequency stability,over-voltage,and sub-synchronous resonance.Furthermore,it would greatly increase the possibility of the outage of the generation units and the HVDC system,which would bring enormous difficulties to the stability control,device security and engineering decisions.With the concentrated development of large-scale thermal power bases in China for the long-distance transmission only,and the introduction of international electric energy,the significance and importance of the research and experiment on the Thermal Power under Islanding Operation are becoming more and more prominent.Moreover,there is no experience about the thermal power HVDC islanding system at home and abroad.In the past,the research on the frequency stability of HVDC islanding system mainly relied on the real-time simulation of the actual HVDC control protection system or the off-line simulation of the electromagnetic transient model.Therefore,the actual result on control and protection system is obtained by using the exhaustive method,and the parameters of the control system are optimized by using the heuristics method.Significantly,this mthod is long time-consuming and high cost,and even has a blind area of the conclusions and understanding,which would be failed to be the real guidance for the engineering practice,neither in the qualitative aspect nor in the quantitative aspect.Based on the physical concept,the physical nature of the frequency characteristics of the thermal power HVDC system under islanding operation is revealed.The simplified model,such as d.c transmission system,steam turbine sets,and servomotor system,would decrease the order of the close-loop system of the frequency control system,therefore,the equivalent transfer function model with the consistent characteristics has been established.According to the different control strategies and parameters of the primary frequency modulation and HVDC additional control,the research on the frequency stability and the optimized control parameters of the thermal power dc islanding system is expounded in this paper based on the root trajectory analysis,which reveals the mechanism of frequency stability of the islanding system.Taking into account the safety of the power system and equipments,and the control complexity of the interaction between the thermal power unit and the HVDC system,a detailed and accurate electromagnetic transient with the actual characteristics of generating units,AC lines,and DC lines is proposed in this paper by taking the ±500kV Huliao HVDC transmission project as an example.According to the analysis of the AC/DC fault,the key problems of the islanding operation could be revealed from three aspects.First,it should meet the requirement of the control index related to the steady-state frequency under the stable or normal operation.Then,it should meet the requirement of the control index related to the system recovery ability and the frequency fluctuation range if N-1 fault occurs.Thirdly,the coordination with safety measures in serious disturbance,the operating conditions and the operation control strategy of the islanding system should be given with the different control parameters and cooperations,respectively.Aiming at the requirement of the coordinated starting of single generating unit with the HVDC system,and considering the capacity and safety of equipments,this paper studies the frequency stability problem caused by the different response characteristics at the starting process.Because of the effect of boiler and turbine,it is difficult to raise the starting load to the required value when synchronising.Therefore,two aspects mentioned could be combined to solve this issue comprehensively.One is to further decrease HVDC system starting power,another is to increase the initial capacity of the generating unit.Moreover,simulation verification has been done,such as the activated result of the controller in the generating units and the coordination time for the generating unit and HVDC system.The research lays a theoretical foundation for the selection of the islanding operation,which provides the necessary technical basis for centralizing the development of the large-scale thermal power basis and the long-distance transmission.