Study Of Sub-synchronous Torsional Interaction With Parallel Hybrid HVDC Transmission System

With the continuous development and construction of HVDC transmission projects in China,The Line-Commuted Converter HVDC(LCC-HVDC)and the Voltage Source Converter based HVDC(VSC-HVDC)coexist in many engineering scenarios due to their respective advantages and characteristics.When converter stations of LCC-HVDC and VSC-HVDC are connected to the same or near-electrical distance AC bus,a parallel hybrid HVDC transmission system is formed.However,when there is a thermal generation unit near the parallel hybrid HVDC sending system,sub-synchronous oscillation(SSO)will cause in some certain conditions.And that will bring new challenges to the safe and stable operation of the power system,which should be more concerned.Firstly,this paper introduces the structure of parallel hybrid HVDC transmission sending system and the working principle of each unit in the system,mainly including the rotor shaft model of the thermal generation unit,and the working principle and control strategies of the LCC-HVDC unit and VSC-HVDC unit.Secondly,this paper builds a small signal mathematical model of the synchronous generator including external network information,therefore the transmission relationship between the generator rotor speed and the electromagnetic torque is obtained.At the same time,the equivalent output impedance models of the power network and the two HVDC units are also established.By using complex torque coefficient method and time-domain simulation method,this paper analysis the effects of the controller bandwidth,when the two HVDC units are running alone,and the AC grid strength and the HVDC operation mode,when the two HVDC units are operating cooperatively,on the electrical damping characteristics of the synchronous machine.Finally,the influence of the feed-forward compensations in the AC current controller of the VSC-HVDC unit on the electrical damping characteristics of the synchronous machine is analyzed.It shows that the q-axis voltage feed-forward plays a decisive role on the change of the system electrical damping.A subsynchronous oscillation suppression scheme,which using the q-axis grid voltage processed by a bandpass filter is proposed.A model of back-to-back parallel hybrid HVDC system in a province is established in PSCAD/EMTDC.The time-domain simulation method is used to screen the subsynchronous oscillation risk of different faults in the power grid circuit.The suppression scheme proposed in this paper is used and the results are verified.