Due to the advantages of the DEH system with its ability to manage and control a wide variety of steam turbine valves, most power plants control turbines through the DEH system, converting the flow to Turbine valve opening, so the valve flow and opening there is a certain relationship between the flow characteristics of the valve. If the actual flow of the valve during operation of the steam turbine deviates from the original flow characteristic curve, the control of the steam turbine may deviate, thus affecting the safety and load capacity of the entire turbine unit, which may eventually cause severe tremors in the generating unit system and severely damage the ongoing High-speed high-load operation of the unit. However, in practice, the steam turbine units in various power plants are manufactured and installed in different processes, with different degrees of wear of the valves. There is a difference between the theoretical design stroke and the actual stroke of the valve, which causes the deviation of the valve flow rate from the original flow rate characteristic curve . Therefore, it is necessary to study the steam turbine valve and adjust its flow characteristic curve so as to maintain the stability of the operation of the steam turbine unit and avoid the potential hazards so as to promote the development and expansion of power plants with the development of the times.
1, the turbine valve flow characteristics analysis Turbine through the flow part of the design is based on economic power, from the turbine power formula:
It can be seen that adjusting the amount of steam entering the steam turbine D0 can adjust the output, and at the same time, the work done by steam Δhtmac can be adjusted. Two different steam distribution modes can change D0 and Δhtmac. In the unit to run the valve, the nozzle is used with the way the steam, the first turbine for the regulation level, the regulation level can be divided into several nozzle groups, when the steam through the main valve after the full open, you can Turn on the regulating valve and turn on the regulating stage. Under normal circumstances, a regulating valve to control a nozzle group, and a nozzle group of 3 to 6 groups. When the load is very small, only open a regulating valve, that is, only the first nozzle group into the steam, part of the intake is very small; when the load increases so that the first regulating valve close to fully open, the second regulating valve open, the first Two nozzle group can enter the steam and some into the steam increase, and so on. Therefore, only part of the opening of the steam valve to adjust the larger throttle, and fully open the steam flow in the valve has been minimized, so only part of the load, the unit's economy is better, which is the nozzle with the main characteristics of steam . Due to the partition between the nozzle groups, even if all the regulating valve is fully open, the regulating stage still only partially enters the steam, that is, there is some intake loss at the regulating stage at maximum power [1].
Second, the valve flow characteristics of the optimization method Obviously, the steam turbine valve itself is a flow characteristics, which is an objective fact that can not be changed at random. Therefore, the most direct way to optimize the valve flow is to optimize the flow characteristics curve of the actual work, but there are differences among different steam turbine units and the optimization methods are not the same. Therefore, there are A certain degree of complexity and maneuverability. In this regard, the power plant staff can only be based on the actual flow curve of the valve to determine and identify the actual flow characteristics of the scientific analysis of the curve to optimize valve flow, to maintain the stability of the power system. The specific operation is as follows: before optimizing the flow curve of the valve, summarize the work of the predecessors, and measure the actual flow rate of different valves of each power system during operation and draw the flow characteristic curve thereof And optimize the actual flow curve of the valve, and then through the DEH system on the turbine valve issued a professional valve flow instructions and valve opening instructions. Under this operation, we can manage and optimize the actual flow curve according to the steam turbine valves of different power plants so as to enhance the steam flow control of the steam turbine, maintain the stability of the power system and promote the development of all power plants. In addition, the optimization of the flow curve of the turbine valve also needs to be based on the relationship of each valve curve, such as single and forward valve. At present, the curve of each valve of steam turbine unit mainly exists single and the valve is set proportionally. In the known theoretical research, there are mainly two ways to manage the valve curve. The first one is single and the valve adopts a reasonable proportion and Offset correction mode; The second is a single, along the different valves with different management curves.
In fact, the curves of the valve opening and the steam flow obtained from the tests done by the existing research show that there is a non-linear relationship between the two. An important method to adjust the turbine regulating system is to adjust the flow curve of the steam turbine valve. According to the adjustment of the valve curve, the flow rate command is changed to the corresponding valve position command. Therefore, in the established steam turbine model, the flow characteristics of the valve are essentially corresponding to the relationship between the equivalent valve position and the steam flow rate, and in this case, the valve flow characteristic suffered by the system is usually ignored, Therefore, the establishment of turbine and turbine speed control system to establish mathematical model must consider whether the adjustment of turbine valve characteristics will have an impact [2].
Third, the steam turbine to improve the control strategy Steam turbine control on the existing problems to be improved, the need to be based on a large number of facts, so the unit for many simulation experiments and practical experiments, draw the following conclusions: When the unit power occurs In the case of persistent fluctuations, in order to calm the continuous fluctuations quickly, the automatic control of the load of the steam turbine unit needs to be switched to the manual control. However, due to the manual switching of the operation mode by the control mode, the uncertainty is increased to some extent Of people as subjective factors. In 2008, there was an accident caused by the low-frequency oscillation of the unit in China Southern Power Grid. The accident happened at Honghe Power Plant's Unit 2 at that time, and its power had been continuously fluctuating for more than 6 minutes. However, during these six minutes , The crew of Unit 2 did not find and did not adopt the planned interventions, which eventually caused irreparable damage to the plant. From this accident, we can see that while improving the control strategy of steam turbine unit, we must strengthen the awareness of staff and try to avoid the influence of subjective factors on the control strategy of steam turbine.
From the point of view of the control strategy of the steam turbine, we can consider that the fluctuation of steam turbine power is caused by the over-regulation of the controller. Therefore, after the proportional-integral-derivative control of the control system equipped with steam turbine, Turbine rate limiting, by limiting the speed of the method to avoid due to excessive adjustment caused by vibration fluctuations. After this improvement, it is possible to achieve a turbine control strategy that reduces over-regulation of the controller in the event of poor valve flow characteristics, thereby further suppressing the power fluctuations of the turbine unit. Only when both the improved unit control strategy and the original unit control strategy are in normal operation, can the improved unit control strategy not affect the adjustment quality of the control system; only when the turbine valve flow and the actual flow characteristic curve do not match each other, Security and variable load capacity will be affected. After the improvement of the control strategy, the continuity and linearity of the turbine load and steam flow curves have been improved, which further improves the automation level and production efficiency of steam turbine units [3].
V. CONCLUSIONS Through the study on the influence of steam turbine valve's own characteristics on the power system, it is found that when the flow characteristics of the steam turbine valve play an unfavorable role, the turbine unit will fluctuate within a certain range. When the fluctuating frequency and power system When the frequency of vibration occurs, it may cause the low frequency and oscillation of the power grid. By improving the control strategy of steam turbine unit system, the unit power fluctuation can be effectively suppressed.
references:
[1] Jiao Jingdong. Steam turbine valve flow characteristics of the power system and its control analysis [J] Technology Innovation Herald, 2012 (27): 76.
2.Sheng Sheng, Liu Fu-ping, Liu Wulin, Xun Xin, Wang Bo-chun, Li Jin-bai.Effects of steam turbine valve flow characteristics on power system and control strategy [J] .Automation of Electric Power Systems, 2012,36 (7): 104-109 .
[3] Xu Houda. Steam turbine valve flow characteristics of the power system and its control strategy [J]. Information Construction, 2015 (12): 273.
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