Study On Protection Of Large Variable Speed Pumped Storage Unit

The pumped storage is the most mature large-capacity energy storage technology at present.With the development of new-generation power system construction,the demand for peak shaving of energy storage is increasing rapidly,and the role of pumped storage will become increasingly prominent.Variable-speed pumped storage unit can control the magnitude,relative position and motor speed of excitation magnetic field by adjusting the amplitude,phase and frequency of AC excitation current,which can obtain better regulation performance than the normally constant speed pumped storage unit,and is more conducive to the rapid regulation of active power in new generation power system.Ensuring the safety and reliability of large variable speed pumped storage units is the basis for realizing their contribution to frequency modulation and stability of power system.The research on relay protection technology for variable speed pumped storage units in the domestic is still in its initial stage,and there is a lack of literature available for reference.In this thesis,the key technology of relay protection for large variable-speed pumped storage units is studied,and the main work is as follows.(1)This thesis proposes an internal short-circuit fault protection method for rotor windings of variable speed pumped storage unit based on the harmonic characteristics of branch circulating currents on the stator side.Aiming at the interphase and interturn short-circuit faults of rotor windings of variable speed pumped storage unit and based on the multi-circuit theory,this thesis analyzes the fault mechanism.The internal faults of rotor windings are detected by specific fractional harmonics in different branch circulating currents on the stator side.The design method and protection criterion of digital filter are proposed.The dynamic simulation test results show that this method can reliably respond to various interphase and interturn short-circuit faults of rotor windings.(2)This thesis proposes the main protection scheme of variable speed pumped storage unit based on fiber optic current transducer(FOCT).Aiming at the problems of insufficient sensitivity of short-circuit fault protection of stator winding and maloperation of protection caused by easy saturation of electromagnetic current transformer during pumping start-up,the main protection scheme of generator-motor unit is constructed by using the flexible installation and excellent low-frequency current transmission characteristics of FOCT.The FOCT can realize accurate measurement of partial branch or full branch current of stator winding.On the basis of the conventional main protection scheme,a certain number of functions such as complete/incomplete phase-segregated transverse differential protection and incomplete longitudinal differential protection are added to completely eliminate the dead zone of internal fault protection for stator winding.Compared with the protection scheme based on conventional electromagnetic current transformer,this new protection scheme can significantly improve the operation speed and sensitivity of differential protection.At the same time,the FOCT can accurately measure the zero-sequence current of the generator-motor terminal.The internal and external stator grounding faults can be quickly identified by the zero-sequence power direction criterion,which can greatly improve the speed of fault detection on site.(3)This thesis proposes a principle of pole disagreement protection of terminal circuit breaker based on voltage phasor difference.It establishes a single-machine infinity system model,and analyzes the electrical characteristics of single-phase and two-phase broken conductor faults of generator terminal circuit breaker(GCB)of variable speed pumped storage unit by means of symmetrical component method.Then,it adopts the voltage phasor difference on both sides of the broken conductor to detect the pole disagreement fault of GCB.The results of dynamic simulation test show that this method can quickly and sensitively identify the pole disagreement faults of GCB at the initial stage of grid connection and during disconnection operation.This principle does not require the three-phase pole disagreement contact of the circuit breaker,and is not affected by the disconnection of the voltage transformer.(4)This thesis proposes a stator grounding location method based on the distribution characteristics of stator fundamental potential.It analyzes the distribution characteristics of the internal fundamental potential when the stator winding is grounded,studies the relationship among the fundamental potential,the fundamental zero-sequence voltage,the grounding transition resistance and the zero-sequence circuit impedance characteristic parameters of the stator winding,and proposes a fault location method based on the distribution characteristics of the fundamental potential of the stator,with the location error not exceeding 5%.The feasibility and accuracy of this location method are verified by dynamic simulation tests and field cases.(5)This thesis proposes a protection function optimization scheme of variable speed pumped storage unit.It comprehensively analyzes the adaptability of the protection principle of constant-speed pumped storage units in the application of variable-speed pumped storage units,gives the discrimination and verification methods of the operating conditions of variable-speed pumped storage units,logically optimizes some protection functions in protection criteria,operating condition locking design and other aspects,and establishes a typical protection configuration scheme for large-scale variable-speed pumped storage units.(6)This thesis designs the first domestic protection dynamic simulation system of variable speed pumped storage unit and engineering prototype of protection device.The dynamic simulation system can comprehensively simulate various faults such as phase-to-phase fault,turn-to-turn fault and grounding fault on the stator side and rotor side,as well as various abnormal operation modes,which creates conditions for fault mechanism,fault feature analysis,research and verification of protection criteria.The engineering prototype of protection device has passed the verification of the third-party type test and dynamic simulation test.The prototype can correctly and sensitively respond to all kinds of faults and abnormalities,and has the engineering demonstration application conditions.