Research On The Load Control Strategy For Large Thermal Power Unit In Accident Frequency

The electric power system structure of china has formed the pattern of “power exchange between South China and North China, nationwide interconnection of power grids in China”, power system stability has been a major strategic problem, which concerned the social safety and development, maintaining frequency stability is one of the most important tasks for generators. For example, when faults occur in shanghai power grid’s long-distance transmission, the local units need to respond the load variation, reduce the frequency fluctuations, and maintain the stable operation of the power grid. So mining the unit’s heat storage capacity and analyzing the dynamic characteristics of the ultra supercritical unit under different conditions to improve the control logic design of unit operation is a practical research. The following works have been carried out in this thesis.The intermediate point superheat degree dynamic model of ultra supercritical units caused by primary frequency regulation is given. The water wall is divided by the splitting criterions of the IAPWS-IF97, and establish the dynamic model by using the equations of mass and energy conservation, which can affectively avoid the issue of model switching between the subcritical state and the supercritical state. According to the operation data and structure parameters of a 1000 MW ultra supercritical once-through boiler, the model parameters are calculated and simulated, which can provide theoretical basis for the improvement scheme of control system of the primary frequency regulation.The load dynamic model of ultra supercritical units caused by condensate throttling is given. Based on the mass conservation and energy conservation of low-pressure heating system, the dynamic relationship between the condensate flow and load of the ultra supercritical units was studied, and subsequently a mathematical model was established between above two variable. According to a 1000 MW ultra supercritical unit’s thermal balance and structure parameters, through simulation experiments and comparative analysis of plant running data to verify the accuracy of the model, with which the goal can be achieved to provide theoretical basis for the design of condensate throttling’s module and returned logic.The control system of ultra supercritical units responding the load variation fast in accident frequency is designed and improved. Based on the control system of primary frequency regulation, the monitoring module of intermediate point’s superheat degree is added for increase the amplitude and duration of primary frequency regulation. For the power unit which is at high load operation or the type of the full-opening governing valve, on the basic of the traditional coordinated control system, the condensate throttling module is designed for the effective utilization of the turbine’s heat storage and making up for the deficiency of the power unit’s primary frequency regulation in accident frequency.