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Key factors affecting low frequency oscillation of power system in steam turbine speed governing system

News by ninechip | On 2018-10-22 17:44
 Steam turbine speed governing system is composed of many links. The function and parameters of each link have different effects on low-frequency oscillation of power system. The key factors affecting low-frequency oscillation of power grid can be summarized as follows.
   The following is the steam distribution function of Steam Turbines. Steam turbine inlet control is accomplished by a number of control valves. The relationship between the lift of the control valve and the flow through the turbine is regarded as the flow characteristics of the turbine. The flow characteristics of modern large-scale steam turbine are nonlinear. In order to achieve accurate control, the steam distribution function is used to modify the flow characteristics of the steam turbine in DEH to linearize the control commands and power output of the steam turbine, so as to improve the linearity of the regulating action of the steam turbine. When the steam distribution function is correct, the turbine opportunity shows good control performance. Otherwise, it will appear such as the swing of control valve, large load fluctuation when the steam distribution mode is switched. Most steam turbines participate in low-frequency oscillation accidents of power system, which is the root of the problem.
   The mismatch between steam distribution function and flow characteristics of steam turbine often occurs, especially in newly built units, retrofitted units or aging units which have been running for many years. This problem is more common. The most direct impact is that the local speed inequality of steam turbine is too small or too large. The local speed inequality reflects the proportional relationship between the change of steam intake and the change of turbine speed caused by the change of valve stroke within a certain opening range of the turbine governing valve; the low frequency oscillation of the system will be caused by the small speed inequality delta of the turbine. The simulation results show that the oscillation is basically unavoidable when the local speed inequality reaches 1%. It is free.
   The unequal local speed of steam turbine will have a great influence on the control performance of steam turbine. The problems of low frequency oscillation caused by the overlap of turbine governing valves, the switching of steam distribution modes, the valve activity test, the primary frequency modulation function and the load adjustment of steam turbine are mostly caused by the unequal local speed of steam turbine deviating from the normal value. That is. The overlap degree of control valve in steam distribution function of steam turbine is unreasonable, and the change of actual steam flow caused by the change of the same flow command easily exceeds the design value, which makes the local speed unequal sudden decrease, and causes the control valve to shake under the DEH regulation. Similar questions often occur in the primary frequency regulation function. Generally speaking, the unequal rate of primary frequency modulation speed of large steam turbines is required to be 4-5%. If the steam distribution function does not match the flow characteristics of the steam turbine, the unequal rate of local speed of the steam turbine may be small, which may easily induce low-frequency oscillation of the speed governing system.
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