Studying Of Shaft Torsion Of Three Gorges Hydro-generator Unit Caused By A.c.&d.c. Hybrid Transmission System

Three Gorges power system has the features of great unit, a.c. & d.c. hybrid transmission and flexible operation modes. Therefore, there are great interests in its possible problems, especially the operation characteristics, power harmonics and system stability etc. which have been widely studied. In 1977, when the HVDC transmission system in parallel with the a.c. transmission system at Square Butte was commissioned, the subsynchronous torsion oscillation of the turbine-generator unit shaft caused by HVDC system was found firstly. Through studying, the mechanism of the subsynchronous torsion oscillation caused by the controller of HVDC system has been understood. At present, more and more attentions are paid to the influence of Three Gorges a.c. & d.c. hybrid transmission system on the hydro-generator shaft torsion by engineers and researchers. In this paper, the situations of the hydro-generator shaft torsion oscillation are analyzed under different operation conditions. The research results are valuable for the safe and stable operation of Three Gorges power system. Based on the model of mass-spring, the mathematic model of Three Gorges hydro-generator with the shaft is established. Using Matlab simulation software, according to the mathematic models of step-up transformer, HVDC system and a.c. transmission system, the simulation model of Three Gorges hydro-generator unit and a.c. & d.c hybrid transmission system is built. Using the simulation model, the shaft torsion oscillations are simulated, during some typical disturbances in a.c. & d.c. hybrid transmission system (e.g. the three-phase to ground fault at a.c. transmission system, the fault to ground at HVDC and the start of HVDC system etc.). As the results, these disturbances may not damage the shaft of generator unit severely, because of the big inertia of hydro-generator unit shaft segments. However if the shaft segments of hydro-generator units are subjected to the transient torsional stresses time after time, the life-time of the shaft will be influenced. To the exclusion of studying the effect of the system disturbance on the shaft torsion oscillation, the effects of non-characteristic harmonics caused by HVDC system on the shaft torsion oscillation of hydro-generator unit are investigated in this paper especially. When sending end and receiving end of HVDC system operate at different frequencies, the harmonics within the range of subsynchronous frequency will appear at a.c. transimission line, which are called as interharmonics as well. Entering the stator of the generator and acting with the flux of rotor, such harmonics will induce the subsynchronous ripple torque which may cause subsynchronous torsion oscillation of hydro-generator unit. At the same time, the complex torque coefficient method realized by time domain simulation, i.e. the test signal method, is adopted to study the influence of the interharmonics on the electrical damping of the hydro-generator unit within the range of subsynchronous frequency. The results show that the shaft will be subjected to a continuous subsynchronous torsional oscillation when the frequency of the subsynchronous ripple torque caused by the interharmonics is close to the nature torsional frequencies of the shaft segments, which threaten the safe and stable operation of Three Gorges power system. In the end, the influences of some operation and control parameters of a.c& d.c. hybrid transmission system on the electrical damping of the generator unit are analyzed by the test signal method.