Study On Primary Frequency Modulation Margin And Improving Methods Of Supercritical Thermal Power Unit

At present,in China's power structure,the proportion of wind power and photovoltaic power generation continues to rise,and the rapid development of long-distance UHV transmission have put forward higher requirements for the safe operation of the power grid.Under the guidance of the"two rules",the regional grid security operations and ancillary service policies are becoming stricter in order to cope with the network source coordination problem under the new situation.Among them,the traditional thermal power units play an important role in peak regulation and frequency modulation.The development of the smart grid also poses a greater challenge for the primary frequency modulation of thermal power units.It's the basic requirement for the development of smart grids to have more flexible adjustment space and stable efficiency in the future.Therefore,it is significant to improve the primary frequency modulation capability of thermal power units.In the current power structure,the units participating in the primary frequency modulation function of the power grid are mainly thermal power units.The primary frequency modulation function of the thermal power unit is mainly realized by the heat storage capacity of the unit.Although the supercritical unit has the advantages of high operating parameters and superior performance,compared with the steam drum furnace,the heat storage capacity of the direct current furnace is limited.At present,the supercritical units in China's existing thermal power units account for a large proportion,and the total installed capacity of only 660,000 to 1 million kilowatts of supercritical generating units has exceeded 100 million kilowatts.Therefore,how to mine the primary frequency modulation capability of a supercritical unit is of great significance.In this paper,a frequency modulation margin is used as a new primary frequency performance evaluation index,that is,the maximum instantaneous output that the unit can provide under various working conditions.Based on the structural data,design data and historical operation data of boilers and steam turbines of a supercritical unit in a power plant,the corresponding mathematical model is constructed.The model is simulated by Simulink software to calculate and mine the primary frequency modulation margin.Under the relevant design conditions,making the high pressure regulating valve opening degree step test.The higher the unit load,the greater the influence of the valve step on the steam pressure and the smaller the influence on the steam temperature.The step range of the valve under three operating conditions was determined by constraining the main steam temperature and pressure and the separator outlet temperature and pressure.The corresponding primary frequency modulation margin is obtained by simulation,and the three-dimensional calculation surface map and numerical calculation expression of the unit's primary frequency modulation margin are constructed,which provides a quantitative reference for the unit's primary frequency modulation control strategy ad ustment.A related exploration was carried out for the improvement of the unit's primary frequency modulation margin.The first is to study the effect of the unit's regenerative extraction on the output of the unit and the methods of improving the unit's primary frequency modulation margin.Study on the increase of primary frequency modulation margin by the regenerative steam extraction of the thermal power unit.Based on the steam-water cycle of the unit,the theory of the entransy dissipation resistance is introduced,and the heat transfer process of the high-pressure heater and the low-pressure heater is Expressed by the entransy dissipation resistance.Assuming that the condenser condensate parameters of the condenser and the shaft seal heater are constant,the constraint equation containing the entransy dissipation resistance of each heater is derived.Taking the steam extraction amount of all levels as the independent variables,according to the turbine and heater variable working condition model,the calculation expression of the heat rate of the unit is derived,and as the optimization target,the Lagrangian operator is introduced to optimize the calculation.Finally,the optimal distribution value of the steam extraction amount of all levels under the four working conditions are obtained,the heat consumption rate of the unit is reduced,the upper limit of the frequency modulation margin is increased,and the primary frequency modulation margin correction coefficient calculation expression ? after the optimization ofthe heat recovery system is fitted out.For the heating season of the unit,the influence of heating extraction and throttling on the output of the unit is studied,and the method of increasing primary frequency modulation margin of the unit is analyzed.Five kinds of equivalent circuit models of the thermal system fluid network under the heating condition are constructed,and the flow resistance of the heating extraction pipe is changed to realize the influence of the throttling of the heating extraction on the pressure and flow of the entire circuit.Assuming that the temperature of each steam extraction point of the steam turbine is constant,the enthalpy of each node after throttling is obtained,and the output of the steam turbine is obtained.Changing of the throttling flow,and the change value of the turbine output is recorded.The results show that under the five kinds of heating and steaming conditions,the variation of the turbine output is basically linear with the heating and pumping throttling percentage,and as the flow of heating extraction decreases,the effect of reducing the same heat extraction on the turbine output is greater.Finally,through the fitting of the calculated data,the influence of the heating extraction throttling on the turbine output and the thermal conversion efficiency of the unit,that is,the calculation expressions of AP,and ?,are obtained,and then the complete calculation expression of the primary frequency modulation margin is obtained.These study results lay the foundation for the practical engineering application of a frequency modulation margin.