Research On Sub-synchronous Resonance (SSR) Problems In Flexible Series Compensated Lines

Series capacitor compensation of flexible ac transmission lines has been recognized asuseful and economical means to increase long distance power transfer capability and toimprove system stability. However, this method may lead to sub-synchronous resonance(SSR) problems and cause a great damage to the turbine-generator system. During theNational12th Five-year Plan period, with the rapid growth of large energy bases in WesternChina, more series compensated transmission lines are needed in order to sent large amountof long distance and heavy load electric power from West to East. The problems of SSRwill be more sever than ever. In such circumstances, this dissertation studies the powersystem monitoring and suppression of sub-synchronous resonance. The main works areorganized as follows.1. Studies the mechanism of subsynchronous resonance in flexible transmissionsystem with series compensation device, analyzes the electrical parameters characteristicsunder the mechanism, identify the main factors affecting the oscillation characteristics.Effect of induction generator and the interaction of torsional vibration and transient torqueamplification effect are analyzed in detail and explain their hazards respectively.2. Sub-synchronous resonance monitoring methods are study analyzed, includingthose gear point sensor monitoring methods and based on Prony identification and testsignal method of sub-synchronous resonance monitoring method. Mechanical measurementmethod directly measures the rotor mechanical parameters, so the accuracy of measurementis high, its main drawback is that monitoring devices is complex; need lots of measuringpoint, so excellent speed measuring devices are needed. And as to sub-synchronousresonance monitoring method base on the test signal and Prony identification method, themeasurement signal can obtain each variable signal curve changes over time intuitively,while Prony identification method can obtain all kinds of useful information of fault signal.The combination of both methods and mutual comparison, confirmed the correctness ofeach, can make time domain information of SSR more complete.3. Sub-synchronous resonance major suppression measures are study analyzed. One ofthose is the natural suppression method of TCSC, which do not require the installation ofadditional equipment, only need pay attention to control the TCSC trigger delay angle and control parameters can achieve better suppression effect. The second scheme is jointlycontrolled SEDC and IMDU, which is a relatively new sub-synchronous resonancesuppression scheme. It is added to the excitation system after SEDC operating suitable forgenerator speed signal phase shift; zoom in, so that it can provide a positive damping in thesmall damping. And IMDU through a special induction motor to balances of mechanicaland electrical power after large disturbances, thereby suppressing SSR. Based onPSCAD/EMTDC time-domain simulation results show that SEDC has been unable toprevent the occurrence of SSR when the system is subjected to large disturbances. However,at which point IMDU-SEDC joint suppression scheme can guarantee system remainedstable in the steady state and transient fault occurs.