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Shenzhen Yueliangwan Gas Turbine Power Plant VEGA206 gas-steam combined cycle unit was introduced by French ALSTOM company. The turbine is manufactured by the French company RATAUTEVA-BLISSEMENT, model TC363 FV140, single cylinder, impulsive, pure condensing. It was put into operation in September 1992.
In February 2002, due to the aging of the control system, it was reformed, the hydraulic system and the implementing agencies, and the measuring device kept the original equipment. The digital electrohydraulic control system (DEH) was changed to the ABB Bailey Symphony system. The turbine was verified by several start-up tests that the starting speed was normally controlled and good results were achieved.
First, the speed of shaking
The unit is a daily peaking unit. After the transformation of the control system is completed, when the steam turbine is stopped (1 to 12) h and when it is restarted for more than 48 hours, the rotation speed is controlled well, and the initial rotation speed is within 200 r/min; when it reaches 300 r/min, the rotation speed value is within ± Within 5r/min, the rotational speed after stabilization is within ±1r/min. This operating condition is called normal operation. When the steam turbine is stopped (24~36)h restarts, when the speed of revolution is increased to (400~500)r/min and the speed is increased to 3000r/min, the speed control quality is poor, and the speed swing value reaches ±20r/ Min, this operating condition is called a problem condition.
Second, there is a problem with the speed adjustment
When the unit starts, it adopts two synchronized main steam regulating valves (adjusting the door) to adjust and control. Its speed regulation loop is similar to that of other typical steam turbine DEH systems. The rotational speed regulation logic frame is shown in Figure 1. 
When the steam turbine begins to run, the DEH control system is switched on and the speed setpoint starts to increase by a given slope. However, if the adjustment has not yet been started, a deviation between the speed setpoint and the measured value will occur. The speed proportional integral differential adjustment algorithm (speed PID) output gradually increases according to the calculation result. The output of the regulating valve position command is also gradually increased through calculation of the flow-lift curve, and then the servo valve proportional integral differential adjustment algorithm (servo valve PID) is used to calculate the command. Output, making the door gradually open, the turbine starts to turn, through the PID adjustment, and gradually meet the speed set value.
The servo valve command output is the offset of the electro-hydraulic servo valve coil, which is the opening and closing speed of the door. In this system, the FC55 function block is configured using ABB Bailey's HSS03 electro-hydraulic drive module. The output range of the FC55 function block is -112 to 112. The negative value indicates that the door is closed and the positive value indicates that the door is open. The greater the absolute value, the faster the door is opened or closed.
In order to ensure that the door is closed tightly, the crew usually uses a servo-command valve output of 112 before slewing. When the valve position command output increases from 0, the servo valve signal command output decreases from 112 until it is reduced to less than 0, but the actual valve position will not change. During this period, since the speed setting value is increased by a certain slope, the actual adjustment of the door is not open, and the rotation speed does not increase. As a result, the difference between the rotation speed setting value and the measured value continuously increases, and the speed PID is added. The integral effect, the output of the throttle valve position command of the speed PID loop will increase rapidly. In this way, when the steam turbine begins to rush, it will inevitably produce a speed disturbance, a large instantaneous speed, and then gradually consistent with the set value.
The rotation speed setting value changes from a certain slope rising speed to a constant speed. The steam amount required for the steam turbine speeding up process is much larger than the steam amount required when the rotation speed is kept constant. Therefore, the steam turbine speed is increased to 3000 r/min. When the speed is fixed, the door needs to be turned off, and there is a certain delay in the door-moving operation, which causes a certain disturbance to the rotation speed.
In general, within the allowable range of the regulation system, the proportional constant of the PID of the servo valve is set to a larger value to improve the flexibility of the response of the regulating gate; and when the speed is close to 3000 r/min, the slope of the speed setting value is increased. For a relatively small value, adopting these two measures can improve the rotational speed control level when the unit is turned.
Third, the cause analysis
After analysis, it was found that when the combined cycle turbine is started under the problem conditions, the instability of the speed control is related to the characteristics of the flow-lift curve and the arrangement position of the door of the turbine unit.
(1) Based on the analysis of operational data, especially during the initial ramp-up phase and the ramp-up to constant speed, the commands issued by the DEH system to the electro-hydraulic servo valve are somewhat different under normal operating conditions and problematic operating conditions. Under the normal working conditions, the initial speed increase phase is switched over, and the door opening degree is basically 0.5%, and in the problematic condition, it is 1.5% or more. The command output of the electro-hydraulic servo valve is approximately -20 under normal conditions; and under the problematic conditions, the output reaches -45. At this time, the turbine has been driven, even when the door is closed and it cannot be closed again. Next, the FC55's order also requires the door to be closed.
(2) Before starting up, the valve position of the initial state of adjusting the door by the linear differential transmitter (LVDT) feedback under normal and problem conditions is different. Under normal operating conditions, the feedback signal of the valve position adjustment valve is basically between -0.4% and 0%, while it is between 0.4% and 0.5% under the problem conditions.
(3) Compare the flow rate-lift curve of this unit with other units. It is found that when the steam turbine unit is at a small opening, the larger flow rate can be used and the flow rate is larger than that of other units. The adjustment is not like the other units. When the opening is less than 1%, there is a flow dead zone with a flow rate of zero. As long as the actual valve position has a certain degree of opening, there will be a certain amount of steam passing through the door. When the unit is accelerating at a rate of 400 r/min, the door opening is only required to be 1% to 1.5%, and the opening at 3000 r/min is less than 1%. The flow-lift curve of the turbine governor gate is compared with the flow-lift curve of a certain steam turbine governor gate as shown in Fig. 2. 
Under the problem conditions, since the feedback value of the initial valve position of the door-adjustment is already 0.4%, the command output of the servo valve will decrease from 112 after the unit is turned and the command output of the door-control valve must reach 0.4%, thus making the hysteresis. As the time goes by, the overshoot amount during the overturning increases, and the perturbation during the initial stage of overturning also increases. What's more serious is that once the actual speed exceeds the set speed, the command output of the speed PID will quickly drop. In the case of large disturbances, the output of the valve position command reaches the lower limit value 0 of the flow-lift curve. At this time, the increase in door opening at 0.4% is actually tight and cannot be closed. Therefore, the PID command output of the servo valve will increase instead of staying close to zero. Once the actual speed is low, although the command output of the speed PID starts to rise, and the servo valve PID command output cannot rise quickly with it, it will produce a certain hysteresis, causing the command output of the servo valve to start to rise, and causing the door opening to rise too quickly. is too big. Therefore, in the case of rushing, there is a phenomenon in which the regulating door is over-opened and sometimes completely closed so that the rotational speed fluctuates greatly. In this phenomenon, when the rotational speed rises to 3000 r/min, the perturbation due to the change in lift rate also exists.
If the door opening is less than 1%, there will be no large fluctuations in speed due to the presence of a dead zone in the flow. Because the output minimum value after the calculation of the flow-lift curve is near 1%, the phenomenon of regulation delay will not occur due to the deviation of the feedback position of the LVDT during the initial acceleration.
In the analysis of reasons why the deviation of the feedback value of LVDT reached about 1% before the rotation under different working conditions, other factors such as unstable performance of the LVDT and poor installation and fixing were excluded. Basically, the deviation of the feedback value of the LVDT is considered to be caused by the inconsistency of the expansion and contraction of the valve body and the valve stem. The arrangement of the main steam control valve of this unit is shown in Fig. 3. 
One end of the LVDT of the unit is fixed on the casing of the oil motive, and the other end is fixed in the middle of the valve stem. The regulating door is arranged on the side of the turbine cylinder body, and the valve body and the cylinder block are directly connected. After the unit is shut down, the valve body of the door is adjusted to dissipate heat slowly, and the oil engine heats up quickly. When the engine is started within the shutdown (0 to 12) h, the temperature difference between the valve body and the oil drive is not large, and the LVDT indication does not show a positive deviation; When the engine is restarted within 24 to 36 hours of shutdown, the temperature of the oil motor has dropped more. When the door is still closed tightly, the distance between the two ends of the LVDT is reduced. The above-mentioned positive deviation appears; the door is adjusted more than 48 hours after shutdown. The body of the valve is sufficiently heat-dissipated, and the lower part of the valve stem wrapped by it is also cooled and shortened, so that the distance between the two ends of the LVDT is elongated, so that the positive deviation indicated by the LVDT is reduced, and the rotation speed is controlled more than normal.
Fourth, treatment measures
Under the problematic conditions, the root cause of rotational speed fluctuations during the start-up of this unit is due to the inconsistence of expansion and contraction due to the influence of temperature on the throttle body and stem.
In order to make the improvement measures simple and easy, an improvement is made from the aspect of control logic, that is, to adjust the opening degree of the LVDT feedback before the turbine steering system memory is remembered, and to make a corresponding translational correction on the flow-lift curve, so that each time The flow-lift curves at start are in accordance with the current actual opening of the door.
After the control logic system adopts the above-mentioned improvement measures, the verification of the starting conditions after a number of shutdowns (12 to 24) hours has a good effect.
Abstract : This paper analyzes the phenomenon of poor quality of the speed control during the start-up of Unit 5 of Yueliangwan Gas Turbine Power Plant under different working conditions, finds out the reasons, and improves the control system. The improved starting speed is normally controlled.