What are the different types of load control for power plant units, and what are their respective characteristics?

The last edit to this post was made by zgj2405 on 2011-7-3 at 13:32. Load control method: The main function of the unit’s main controller is to select an appropriate load control method based on the operating conditions and requirements of the unit. It receives the actual load command N0 sent by the load command processing unit, as well as signals related to the unit’s actual power generation capacity NE, the main steam pressure PT, and its set value P0. Through certain calculation circuits, it determines the main control commands MB and MT for the boiler and turbine, thereby implementing the corresponding load control method and fulfilling the task of load control. Please refer to Figure 10-1. The main boiler controller consists of two parts. (1) Boiler main controller: The arithmetic circuit that calculates the boiler main control command MB. (2) Turbine main controller: The arithmetic circuit that calculates the turbine main control command MT. I. Load control methods Load control methods can be divided into two categories: separate control of the turbine and boiler, and coordinated control of the turbine and boiler. 1. Separate control of the turbine and boiler: By separate control, it is meant that there is only one control parameter for a given variable. There are two types of separate control for the turbine and boiler: load control with the boiler following (referred to as boiler-following control or furnace-following control), and load control with the turbine following (referred to as turbine-following control or machine-following boiler control). These are the two basic control methods. The working principle and main features are introduced below respectively: (1) BOILER FOLLOW method. The basic working principle of the boiler follow method is that the turbine regulates the output electrical power of the unit, while the boiler regulates the steam pressure. Figure 10-6 shows a schematic diagram of the boiler follow mode; based on Figure 10-6, its block diagram 10-7 can be drawn. Figure 10-6: Schematic diagram of the boiler following mode. Figure 10-7: Block diagram of the boiler following mode. N0 – The unit load requirement command, generated by the load command processing unit. The control object refers to the broader concept of control objects, including machine and furnace control systems. When the load command (power setpoint) N0 changes, the main controller of the turbine first issues a command MT to adjust the valve opening, thereby changing the amount of steam supplied to the turbine and ensuring that the electrical power output NE of the unit quickly approaches N0. When the valve opening changes, the steam pressure PT changes immediately. At this point, the boiler’s main controller issues control commands MB based on the deviation in steam pressure, thereby adjusting the boiler’s combustion rate (and accordingly, the feedwater flow rate) so that PT returns to the set value P0. In the steady state, NE = N0 and PT = P0. When the combustion rate is perturbed (internal disturbance), the steam pressure changes, resulting in a deviation, and the steam flow rate also changes. On the turbine side, the control valves must operate in order to maintain the output electrical power; as a result, the variations in steam pressure are further intensified, leading to an increased deviation and greater fluctuations in steam pressure. The characteristic of the boiler follow-up mode is that, when N0 changes, its good load adaptability is achieved due to the use of the boiler’s heat storage capacity, which is beneficial for regulating the power output of the unit; however, steam pressure fluctuations are relatively large ； When there is an internal disturbance (flame rate perturbation), the steam pressure fluctuates significantly. For large unit systems, the heat storage capacity of the boiler is relatively reduced. When the load requirement command N0 changes by a small amount, it is possible to make full use of the boiler’s heat storage within the allowable range of steam pressure in order to quickly adapt to changes in load, which is also beneficial for regulating the grid frequency. When the load demand command N0 changes significantly, the steam pressure fluctuates greatly, which can affect the normal operation of the boiler. Especially for DC boilers, their heat storage capacity is much smaller than that of drum boilers; it is practically impossible to adapt to large load variations by using a boiler-following approach. When the boiler equipment in the unit is operating normally but the unit’s output electrical power is limited due to abnormal operation of the turbine components, the boiler-following method can be employed. (2) Turbine follow (TURBINE FOLLOW, abbreviated as TF) mode: The basic working principle of the turbine follow mode is that the boiler adjusts the output electrical power of the unit, while the turbine adjusts the steam pressure. Figure 10-8 shows a schematic diagram of the turbine follow-up mode, and based on Figure 10-8, its block diagram 10-9 can be drawn. Figure 10-8: Schematic diagram of the turbine following mode. Figure 10-9: Block diagram of the turbine following mode. When the load command N0 changes, the boiler main controller first issues a command MB to change the boiler’s combustion rate (and accordingly, the feedwater flow rate). After the pre-operational pressure PT is changed, the main controller of the turbine issues a command MT to adjust the valve opening, thereby changing the amount of steam flowing into the turbine and altering the electrical power output NE of the unit, so that it aligns with the load command N0. In the final steady state, NE = N0 and PT = P0. When the combustion rate is perturbed, the steam pressure changes, resulting in deviations; the steam flow also changes, and accordingly the electrical power output of the unit changes as well. The main controller of the turbine acts on the valves in order to maintain the steam pressure; as a result, the variations in steam flow are further intensified, leading to greater fluctuations in the electrical power output of the unit and an increased deviation. This causes significant fluctuations in the output power. The characteristic of the turbine follow-up mode is low steam pressure fluctuations. However, since the heat storage capacity of the boiler is not utilized, and the output power of the unit only changes when the combustion rate of the boiler changes, resulting in significant delays, its ability to adapt to load variations is poor, which is not conducive to handling variable loads or participating in grid frequency regulation. This control method is suitable for unit plants with a base load, or when the units have just been put into operation; it is used to maintain a relatively stable steam pressure in the units, thereby creating conditions for their stable operation. When the turbine equipment in the unit is operating normally but the unit’s output electrical power is limited due to abnormal operation of the boiler equipment, the turbine follow mode can be employed. 2. The boiler-following and turbine-following load control methods, which are forms of coordinated control between the machine and the boiler, represent two extreme control approaches. In operating under variable load conditions, each of these methods has shortcomings in terms of controlling electrical power output and steam pressure. Therefore, when adopting control strategies, the inherent interconnections of the system components as well as the differences in the dynamic characteristics of the machine and the boiler should be taken into account, and a coordinated control approach for the machine and boiler should be employed. The control strategy of the coordinated control approach is to allow certain fluctuations in steam pressure, so as to make full use of the boiler’s heat storage capacity and enable the unit to adapt more quickly to the load requirements of the power grid. However, there are limits to the use of the boiler’s heat storage capacity here; it is necessary to ensure that the deviation of the pressure before the machine from the set value does not exceed the allowable limit. Therefore, the coordinated control method enables the unit to adapt more quickly to the load requirements of the power grid, while also ensuring that steam pressure fluctuations remain within acceptable limits. There are three common approaches for boiler-turbine coordinated control: a coordinated control approach based on boiler following, a coordinated control approach based on turbine following, and a comprehensive coordinated control approach. (1) The coordination control method based on boiler following has been discussed earlier. The characteristic of the load control method using the boiler following approach is that it provides a fast load response when there are changes in load requirements, but it causes large fluctuations in the pressure ahead of the machine. Therefore, a limiting element is provided for the signal that regulates the opening degree of the turbine valves (the output of the turbine main regulator PIT), as shown in Figure 10-10. http://rgtjjpkc.njit.edu.cn/admins/attachment/article/attachment/Mon_1003/581595_7776c189ed6.jpg (a) (b) Figure 10-10: (a) Coordination control method based on boiler following ； (b) The block diagram 10-11 of the dead-zone nonlinear element can be drawn from Figure 10-10. Figures 10-11: Block diagram of the coordination control method based on boiler follow-up. During dynamic operation, when the steam pressure deviation falls within the insensitive region of the nonlinear element, it has no effect on MT; when the deviation exceeds this range, the nonlinear element restricts MT, thereby limiting further changes in the turbine valve opening and thus achieving the goal of limiting the steam pressure deviation. The size of the insensitive zone roughly reflects the allowable range of variation in the main steam pressure during unit operation. This control method is an improvement based on the boiler follow-up mode. Since the turbine and the boiler take coordinated control actions to maintain the steam pressure together, this type of control is referred to as a coordinated control method based on boiler follow-up. It can be seen from the restrictive effect of steam pressure deviations on the turbine control valves that, although this helps to keep steam pressure fluctuations within acceptable limits, it also slows down the response speed of the output electrical power. In essence, the improvement in steam pressure control quality is achieved at the cost of reduced response performance of the output electrical power. In this sense, the result of coordinated control is to achieve control quality in both electrical power and steam pressure. (2) Coordination control method based on turbine following. As mentioned earlier, the characteristics of the load control method using turbine following are as follows: when the load requirement changes, the response speed is slow, but the main steam pressure fluctuates little. To speed up the load response time of such a control system, it is necessary to make use of the heat storage capacity of the boiler. To this end, based on the turbine-following approach, the main steam pressure is allowed to vary within a certain range, thereby forming a coordinated control method based on turbine following, as shown in Figure 10-12. Figures 10-12: (a) Coordinated control method based on turbine following; (b) Characteristics of the limiting nonlinear element. The block diagram 10-13 can be drawn from Figures 10-12. Figures 10-13 Block diagram of the coordinated control scheme based on turbine follow-up. While the power deviation signal is fed to the boiler regulator PIB, it is also sent to the turbine regulator PIT through a nonlinear element. In the dynamic process, the signal can be regarded as part of the set value for main steam pressure. When >0 (an increase in the unit’s output power is required), the main steam pressure setpoint decreases. The main turbine regulator PIT issues a command to open the control valves, thereby increasing the unit’s output power ； When