Research On The Mechanism Of Subsynchronous Oscillations Caused By HVDC Transmission System In Power System

This paper mainly investigates the mechanism of subsynchronous oscillationsinpower system caused by LCC-HVDC transmission system.In multi-machine AC/DC hybrid system,LCC-HVDC transmission system may interact with synchronous machine,wind turbine generators and other DC systems.The strength of the interaction directly affects the stability of the subsynchronous oscillation of the system.In this paper,according to the interaction object with LCC-HVDC transmission,the subsynchronous oscillation problemsin power system caused by LCC-HVDC transmission system are classfied,and the interaction objects are divided into "object subsystem" and "feedback subsystem",based on this a closed-loop interconnected system model is established.Based on the closed-loop interconnect model,the open-loop mode coupling theory and the near strong mode coupling theory are used to explain the stability mechanism.It is pointed out that when the open-loop mode of the LCC-HVDC system is close to the open-loop mode of other parts of the system,the open-loop mode coupling will occur,at this time the interaction between the subsystems is very strong,which leads to the subsynchronous oscillationsin the power system.At the same time,the existing open-loop mode stabilization method is improved,and the universality research idea is proposed.The main contributions of this paper are as follows:(1)Aiming at the problem of shafte torsional caused by the interaction between LCC-HVDC transmission system and shaft system ofgenerator,a "single input and single output" closed-loop interconnection model is established,and the open-loop mode stabilization judgement method is used for mechanism analysis and stability judgement.At present,the complex torque coefficient method is used to analyze the shafte torsional problem.The corresponding "negative damping" mechanism also formed a consensus.In this paper,it is proved that the open-loop mode stabilization judgement method and the complex torque coefficient method based on the open-loop mode coupling theory are equivalent.The analysis results obtained by the two methods can be mutually verified,thus providing the open-loop mode stabilization judgementmethod in theoretical support.(2)The existing open-loop mode stabilization method is derived when the relative distance of the open-loop modes is negligible,that is,the open-loop modes are approximately coincident.However,the open-loop mode relative distance has a decisive influence on the prediction accuracy of the method.In the actual system,the open-loop modes aren't coincident in most cases.In this paper,based on the existing open-loop mode deterministic method,the improved method of considering the relative distance of the open-loop mode is derived,and the prediction accuracy of the improved method is clarified by calculation.And the scope of application is far superior to existing methods.(3)The interaction between LCC-HVDC transmission systems and wind generators,and other DC systems belongs to the interaction problems between electrical subsystems.At present,only the mode analysis method can be used to determine the stability of such problems,and the corresponding research ideas and methods are not effective to givein-depth exploration of the mechanism.For the first time,the HVDC transmission system is proposed as the "object subsystem",and other parts of the system are proposed as the "feedback subsystem".Following this research idea,a "four-input and four-output" closed-loop interconnection model is established.Based on the model,the approximate strong-mode coupling theory can be used to explain the dynamic interaction between the HVDC transmission system and any parts of the system,and the formula is given for predicting the position of the closed-loop mode based on the open-loop mode information.At the same time,a general analysis step isgiven.The advantages of the above research ideas are verified by practical examples.