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Keywords: Flow characteristics; Single valve; Multiple valves; Degree of overlap 1 Foreword Datang Changchun Thermal Power Development Co., Ltd. No. 6 turbine is a subcritical, single-shaft, intermediate reheat, condensing steam turbine produced by Harbin Turbine Co., Ltd. , equipped with four high-pressure governor (GV). Both the DCS system and the DEH control system adopt the XDPS-400 control system of Shanghai Xinhua Control Engineering Co., Ltd.
2 DEH load control principle According to the unit load requirements, the DEH control system calculates the flow value corresponding to the main steam parameters at that time. After the high and low load limits, it is output to the valve management program and converted into the corresponding valve opening through the valve management program. . In single-valve operation, the total flow signal of the turbine is added to each high-pressure regulation valve on average; when the sequence valve is controlled, the steam flow to the turbine is the sum of the flow of each valve. It will be added to GV1-GV4 in sequence, each valve. In order to open and close, the adjacent two valves have a certain degree of overlap when they are opened. It is generally considered that when the valve pressure ratio P2/P1 = 0.95 to 0.98, the valve is fully open. The selection of the overlap degree is subject to comparison of the schemes. Generally, when the pressure ratio P2/P1=0.85 to 0.90 before a valve is opened to the front and rear of the valve, the latter valve starts to open properly, and the valve flow characteristic curve is the flow and the valve opens. Degree conversion function.
3 Valve Flow Characteristic Curve Necessity Necessity Since Unit 6 was put into operation in 2010, due to the continuous operation of the unit for a long time, the high-temperature and high-pressure steam will continuously scour the flow passage of each high-pressure speed control valve, resulting in the failure of the valve function curve of the steam turbine. Pre-factory and valve characteristics completely match, in addition to the shutdown process or when the high-speed speed control valve is closed, the rapid closing of the valve will cause a certain degree of impact on the throttling parts of the valve core, which will affect the flow characteristics of the valve. It is necessary to analyze, correct, and optimize valve functions through tests, and to meet the requirements of safe and stable operation of steam turbines, and to improve the operational economic efficiency of the units.
On September 9, 2016, State Grid Jilin Electric Power Co., Ltd. Power Dispatching Control Center and Jilin Electric Power Research Institute Co., Ltd. jointly organized technical personnel to perform a frequency tuning performance test on Unit 6. On October 15, 2016, the Jilin Electric Power Research Institute transmitted the electronic version of the test report of Unit 6 frequency modulation to the company. According to the conclusion of the test report, Unit 6 operated in valve control mode, power control mode, and coordinated control. Under the circumstance, the requirements of the "Frequency Tuning Test Guidelines for Thermal Power Generating Units" (Q/GDW669-2011) can not be guaranteed once. It is necessary to optimize the valve flow characteristics of the DEH system of unit 6.
4 Setting of Flow Characteristic Curve In June 2014, during the overhaul of Unit 6, the valve flow characteristic test was conducted, and the original valve flow characteristic curve was modified through the test data.
4.1 Test procedure (1) The AGC and CCS remote control methods are removed during the test.
(2) DEH in the valve position control mode (MW, IMP circuit removal), by the operator to change the valve position command target value to achieve changes in the test conditions.
(3) The boiler must maintain a constant main steam pressure throughout the test. This pressure is the main steam pressure at which the valve is fully opened and the load does not exceed the pressure at the beginning of the test.
(4) DEH specifies the valve position point by point. After the furnace control adjusts the steam pressure, DEH collects data.
(6) Test sequence: valve full open → single valve drop test → (valve full open) valve switching → sequence valve drop test → valve full open → DEH recovery. From the start of the valve to the lowest point of the fully open condition, and then fully open after the valve switching, from the fully open condition to the lowest point, once again fully open and cut back to the single valve.
(7) Restore the DEH configuration before the test.
5 Test data analysis Changchun Erfeng 6 unit adopts XDPS-400 control system of Shanghai Xinhua Control Engineering Co., Ltd. 5.1 Valve flow characteristic test data analysis under single valve control mode Valve flow characteristic test data under single valve control mode 1 can be seen from Table 1, the calculated valve actual flow and valve flow instructions there is a deviation, the difference varies between -10.89% to -0.18%, affecting the stability of the unit control. This shows that the valve flow characteristic curve under the current single-valve control method is not consistent with the actual situation and needs to be optimized.
Since the unit has reached the minimum steady combustion load when the GV1~GV4 command drops to 31.505% in the test, only the valve flow characteristics of 31.505% and above are optimized for GV1~GV4 commands. According to the test data, after reasonably simplifying calculations, the original pre-opening degree is retained, and the new valve flow characteristic function is fitted under the single valve control mode that is more consistent with the actual situation. Table 2 shows the new valve characteristic curve diagram and the original The comparison of the valve characteristic diagram is shown in Fig. 1.
5.2 Analysis of valve flow characteristics under sequential valve control mode Analysis of valve flow characteristics under sequential valve control mode After the data is compiled, as shown in Table 3, it can be seen from Table 3 that the actual flow rate of the valve and the valve flow rate command are calculated. There are deviations that affect the stability of the unit control. This shows that the valve flow characteristic curve in the current class-sequence valve control mode does not match the actual situation and needs to be optimized.
As the GV1~GV4 command drops to 41.5% during the test, the unit has reached the minimum stable combustion load. In the sequential valve control mode, when the valve flow command is 87%, the GV4 valve command is 0%, and when the valve flow command is 67%. The GV3 valve command is 0%. Therefore, the valve flow rate of 35.5% and above are optimized. According to the test data, after reasonably simplifying the calculation, the original pre-opening degree is retained, and the new valve flow characteristic function under the sequential valve control mode that matches the actual situation is shown in Table 4.
6 Conclusions The flow characteristic curves before and after correction are drawn through Table 1 and Table 2, and the unit is subjected to a frequency test test. A certain change has taken place through the single valve curve and the forward valve curve. Under the same load command, the single valve When the control method, the valve opening is 3% to 4% smaller than before the setting. In the case of the forward valve control mode, the opening degree of GV1 and GV2 increases relatively and the curve becomes gentler. The opening degree of GV3 and GV4 changes greatly, and the overlapping degree changes. Small. In terms of coordination and control, after the automatic commissioning of the unit, the actual load response of the DEH is significantly higher than before, meeting the requirements for coordinated control.
References [1] Steam Turbine Digital Electro-hydraulic Control System [M] Xiao Zenghong, Xu Feng China Electric Power Press, 2003
[2] Wang Wenkuan Research on Valve Overlap Degree in Turbine Control System [J] Turbine Technology, 2008, 55(2): 156~158
Abstract: Turbine valve flow characteristics are inconsistent with the actual flow characteristics, affecting the unit load control accuracy and primary frequency modulation capability. Taking the 200MW unit of Datang Changchun Second Thermal Power Co., Ltd. as an example, the valve flow rate was tested under the single valve control mode and the sequential valve control mode, and the test data was analyzed and optimized. The optimized valve flow characteristics curve obtained better continuity and linearity, and improved the unit load control accuracy and primary frequency pass rate.