Analysis On Loss-of-excitation Process And Research On Protetion Method Of Synchronous Generator

With the expansion of the unit capacity of generator and the scale of power grid,the excitation system fault is more and more harmful to the safety and stable operationof generator and power grid. Currently, there are three excitation protection criteria oflarge-capacity synchronous generators, namely static stability boundary impedancecircle criterion and static stability boundary minimum excitation voltage criterion andasynchronous boundary impedance circle criterion，which all have shortages in theprotection principle. The first two criteria are based on small-disturbance static stability.The third criterion only proves measuring impedance can enter in the impedance circlein the situation of complete loss-of-excitation, but does not exclude the possibility ofentering in the impedance circle in other situations. Despite various improvements, theexcitation protection criteria above still exist the possibility of maloperation ormisoperation. So, according to the principle of large-disturbance stability, it is verysignificant in science and engineering application to study deeply the behavior ofgenerator and power grid after synchronous generator loss-of-excitation, and putforward the excitation protection principle of generator.By using numerical simulation and dynamic simulation experiment two methods,the dynamic process of synchronous generator after loss-of-excitation is analyzed, andthe principle、design principles、configuration scheme of loss-of-excitation protectionbased on the power angle and slip are studied. Based on the right action of protection,the resynchronization control of synchronous generator is introduced to expand theprotection functions.Firstly, on the basis of the reasearch on the mathematical models of the equivalentsystem、 synchronous generator and excitation system, the models of numericalsimulation and dynamic simulation experiment are built to study the excitation faults ofsynchronous generator.Secondly, by changing the input power of the motor、the equivalent contactreactance and the excitation decay time constant，using the numerical simulationmethod, different degrees of partial loss-of-excitation and complete loss-of-excitationdynamic process of synchronous generator are analyzed. The results show: for thedifferent degrees partial loss-of-excitation of synchronous generator, the input power ofthe motor is smaller, the equivalent contact reactance is smaller, which is more helpful to keep synchronization of synchronous generator; while the change of the excitationdecay time constant does not influence the final operation of loss-of-excitationsynchronous generator, but it could influence the loss-of-excitation dynamic process ofsynchronous generator. At the same time, the results of numerical simulation areverified by the dynamic simulation experiment method, and when the completeloss-of-excitation fault of generator is caused by rotor winding short circuit and opencircuit, the recorded diagram prove effectively: the larger asynchronous power will appearat the primary stage of the synchronous generator loss-of-excitation. So, when theproblems related to the synchronous generator loss-of-excitation are analyzed andstudied, the asynchronous power and the effects on dynamic power-angle characteristicfrom the asynchronous power should be considered at the primary stage of thesynchronous generator loss-of-excitation.Thirdly, by the numerical simulation analysis of generator three-phase short circuit,the low voltage lockout condition is introduced, which is established on the directlymeasuring power-angle excitation protection method. It is effective to avoidmisoperation of the directly measuring power-angle excitation protection causing bythree-phase short circuit nearby, and make the directly measuring power-angleexcitation protection more perfect.Lastly, based on the normal monitoring、control and adjustment after fault on thesynchronous generator by the power angle and slip, using numerical simulation anddynamic simulation experiment two methods, the resynchronization control ofsynchronous generator after losing step is studied. the results of two methods all prove:the way of active power-reducing control is helpful for resynchronization ofloss-of-excitation synchronous generator, which creates the condition to expand thefunctions of the directly measuring power-angle excitation protection.