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Key words: steam turbine; single valve control system; sequence valve control system; low frequency oscillation
With the continuous development of economy and society, the power system is also undergoing continuous development. It has made important contributions to the development of human society. The fundamental problem of the power system is the stability issue. The constant introduction of the rapid excitation system and the continuous expansion of the power grid scale In the power system, unstable oscillations continue to occur. This unstable oscillation has become a major hidden danger in the safe operation of the power grid, which may cause major power outages and huge economic losses. There are many problems in the development of the power system, such as low-frequency oscillation caused by the switching of the control mode of the steam turbine valve. The low-frequency oscillation caused by the switching of the control valve of the steam turbine is caused by the rapid excitation system. In order to solve this low-frequency oscillation problem, it is necessary to study the relevant examples of low-frequency oscillation, to grasp the relevant mechanism of the low-frequency oscillation caused by the switching of the control mode of the turbine valve, and to prevent the influence of the low-frequency oscillation on the safety of the power grid.
1 Establishing turbine model for valve flow characteristics In general, large-scale thermal power plants adopt two control systems: single-valve control and sequential valve control. A single-valve control system means that all valves maintain the same degree of opening. In this case, Turbine can evenly heat, but it will consume a lot of energy, and the cost is higher. The sequence valve control mode refers to the high pressure control valve of the unit being turned on in sequence during the operation of the steam turbine. This will cause the unit to be heated unevenly, but the unit loss will be small. Therefore, the two valve control methods should be combined. A single-valve control system should be used during the turbine load-up phase, and the sequence valve control method should be used after the unit temperature is increased.
In the process of steam turbine operation, there is a positive correlation between the valve opening and the steam flow through the valve. The higher the steam flow, the greater the opening of the valve. When the steam flow is close to one hundred, the valve opening varies with steam flow. The rate of change has accelerated . However, the valve opening degree and the steam flow rate are non-linearly related. Therefore, it is necessary to introduce the valve flow rate correction network function to calculate the numerical relationship between the two.
2 Examples of Low-Frequency Oscillations Caused by the Switching of Turbine Valve Control Modes 2.1 Analysis of the Causes of Low-Frequency Oscillations Caused by the Switching of Turbine Valve Control Modes Two turbogenerators in a power plant in southern China are taken as examples. Both generators have a rated power of 330MW. The two steam turbines were put into service after being inspected without errors. One of the turbines was operated with a load of 220 MW, and the other was operated with a load of 230 MW. When the two steam turbines work for a period of time, one of the valve control modes is switched back and forth between the single valve control mode and the sequence valve control mode. At this time, the active power of the turbine in the valve switching state fluctuates in the range of 186-279MW, and low-frequency oscillation occurs.
2.2 Example analysis of the low-frequency oscillation caused by the switching of the control mode of the steam turbine valve In order to analyze the low-frequency oscillation caused by the switching of the control mode of the steam turbine valve, it is necessary to use the tool of time-domain simulation. First of all, an effective power system model must be established to match the self-excitation excitation method so that the frequency change of the steam turbine can be expressed in the form of simulation. In the process of simulation, the initial active power of the turbine generator is 220MW, and the single-valve control method is used initially. After a certain period of time, after the sequential valve control method is used, the frequency vibration of the turbine generator produces a back-and-forth swing of equal amplitude. The oscillation disappeared after the power frequency control system was exited, indicating the influence of the switching of the control mode on the low-frequency oscillation of the turbine.
3 Analysis of Low Frequency Oscillation Mechanism Caused by Valve Control Mode Switching of Steam Turbine 3.1 Analysis of Negative Damping Mechanism The negative damping mechanism refers to the fact that after the valve control mode changes to generate a disturbance, the stability of the steam turbine autonomous system will change accordingly, and the damping is positive. In this case, the amplitude of oscillation will be attenuated, and in the case of negative damping, the amplitude of oscillation will gradually increase. In this process, the frequency of the low frequency oscillation is very close to the frequency vibration of the system in the natural state.
There is also a special case of underdamping, that is, when the frequency of the disturbance is the same as the natural frequency of the system, the system may generate resonance-type under-damped low-frequency oscillations. If the damping of the system is zero or smaller, the unbalanced torque occurs due to the influence of the disturbance, so that the solution of the system is in the form of an equal amplitude oscillation. When the frequency of the disturbance is equal to or close to the natural frequency of the system, the response will be It is amplified by resonance and causes resonant low-frequency oscillation . Resonance-type low-frequency oscillations are ultimately due to inadequate system damping, and are therefore a special case of underdamped low frequency oscillations.
3.2 Forced Oscillation Theoretical Analysis The negative damping mechanism is based on the premise of a steam turbine autonomous system. The forced oscillation theory uses external forces to apply periodic disturbances to generate oscillations in a non-autonomous system of a steam turbine. In this case, the positive and negative damping have no influence on whether the turbine oscillates or not. If the damping is negative, the turbine will still oscillate. However, the magnitude of the damping still affects the amplitude of the vibration. The premise of the forced oscillation theory must have a premise that there is a vibration source, and only the vibration source exists, and the steam turbine will oscillate, and the frequency of the forced power oscillation of the power system is the same as the vibration frequency of the disturbance source.
3.3 Mechanism analysis of low-frequency oscillation of steam turbine The reason for low-frequency oscillation of steam turbine is that the valve control mode is changed from single-valve control mode to sequential valve control mode. Therefore, the sequential valve control mode has a crucial influence on the oscillation of the steam turbine, and the valve flow characteristics under the sequential valve control mode need to be studied in-depth . Through analysis we know that the valve flow characteristic curve of the turbine under the sequential valve control mode is different from the valve flow characteristic curve provided by the steam turbine manufacturer, and the gap between the CV and TCV curves is large. Since the valve flow correction function of the original steam turbine control system is based on the valve flow characteristic curve provided by the manufacturer, when the single valve control mode of the steam turbine is converted to the sequential valve control mode, the adjustment valve and the original flow correction Deviations between functions will cause the regulator valve to fluctuate more, resulting in low-frequency oscillations of the turbine.
After the steam turbine has a single-valve control mode converted to a sequential valve control mode, the data shows that the control valve opening fluctuates within a large range of 25% - 100%, and the large fluctuation of the valve leads to the fluctuation of the steam turbine flow, the steam turbine The fluctuation of the flow, in turn, causes periodic fluctuations in the mechanical power of the turbine.
Returning to the case mentioned in China above, the low-frequency oscillations of a unit with 220MW load operation are oscillations of a single machine relative to the power grid, and its low-frequency oscillation frequency is 0.171Hz. At this time, the local mode low-frequency oscillation frequency is about 1Hz. The unit's low frequency oscillation frequency is different from the unit's local mode low frequency oscillation frequency. Analyzing this data, we know that the low-frequency oscillations generated by this unit are not due to local damping caused by negative damping, but rather to the mechanical power generation of the turbine caused by the change of valve control from single-valve control to sequential valve control. The periodic changes, which are far from the characteristics of the valve flow curve provided by the manufacturer, the deviation between the two eventually leads to low frequency vibration of the turbine and the forced power oscillation of the power system.
4 Conclusion With the extensive use of power systems in social life, there are many problems in the power system that need to be solved, among which the low-frequency oscillation caused by the switching of the control mode of the steam turbine valve is a question worth analyzing. The switching of the control mode of the steam turbine valve will cause the low-frequency oscillation of the power system, which will adversely affect the normal operation of the power system. The low-frequency oscillation theory of steam turbine is divided into negative damping mechanism and forced oscillation theory. The general large-scale thermal power plant adopts single valve control mode and sequence valve control mode. When the valve control mode is changed from a single-valve control mode to a sequence valve control mode, the turbine will generate low-frequency oscillation. This article uses a steam turbine of a certain power plant in the south. The low-frequency vibration generated is taken as an example for research. Research shows that there is a large deviation between the valve flow characteristics generated by the turbine under the control of the sequential valve control method and the flow characteristics of the valve provided by the manufacturer. As a result, under the theory of forced oscillation, the steam turbine's regulating valve generates large oscillations, which in turn leads to continuous oscillation of the turbine's mechanical power, resulting in the phenomenon of low-frequency oscillation of the steam turbine and adversely affecting the normal operation of the power system. In order to prevent this from happening, it is necessary to strengthen the maintenance and maintenance of the steam turbine and periodically correct and update the valve flow characteristics of the valve control system of the steam turbine to ensure the normal operation of the power system.
ReferencesXu Yanhui, Ma Wei, Deng Xiaowen, Cai Sun. Example and mechanism analysis of low frequency oscillation caused by switching of control mode of steam turbine valve[J]. Electric Power Automation Equipment, 2015, 35(03): 170-174.
Xu Yanhui, Wang Zhenzhen, Weng Hongjie. An example and mechanism analysis of low frequency oscillation induced by a frequency modulation experiment[J]. Automation of Electric Power Systems, 2013, 37(23): 119-124.
DONG Qing, ZHANG Ling, YAN Xiangwu, LIU Xue. Automatic determination method of forced resonance low frequency oscillation source in power grid[J]. Proceedings of the CSEE, 2012, 32(28): 68-75+17.
"Feed Yeast","Yeast powder"
Feed Yeast,Brewers yeast feed grade,yeast powder feed,Dry Yeast powder
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