A coal-fired unit RB trip control method adaptive to full-mill operation

Abstract

A method for controlling RB mill tripping in coal-fired power units, adaptable to full mill operation, belongs to the field of power plant unit control technology. This method, based on the original RB control strategy, adds an RB control strategy under full mill operation conditions. Using the RB target load as a benchmark, it distinguishes between the maximum and minimum load-carrying capacities of the mill units. By comparing the maximum and minimum load-carrying capacities of the pre-reserved mill units, it accurately calculates whether the last mill needs to be tripped. If tripping the last mill is still necessary, a delay time is designed to better adapt to changes in RB operating conditions to further reduce disturbances in furnace combustion, thus reducing disturbances during the RB process and further improving the adaptability of the RB control strategy.

Description

Technical Field

[0001] This invention belongs to the field of power plant unit control technology, specifically relating to a method for controlling RB mill skipping in coal-fired units that is suitable for full mill operation. Background Technology

[0002] RUNBACK is an existing auxiliary equipment failure load reduction function for thermal power units, abbreviated as RB. Its function is to quickly reduce the unit load to the maximum output that all operating auxiliary equipment can achieve when a major auxiliary equipment trips due to a failure, and the unit's maximum load capacity is significantly lower than the current actual load. It also controls operating parameters within permissible ranges to maintain unit operation. The main equipment in the pulverizing system consists of a coal feeder and a coal mill. The coal feeder transports coal to the coal mill via a hopper, where it is ground and then blown into the furnace for combustion. Depending on the unit's auxiliary equipment, for example, a pulverizing system might have 4-6 sets of auxiliary equipment, two blowers, two steam-driven feedwater pumps, and one electric feedwater pump. Generally, the following RUNBACK functions are set based on unexpected trips of operating auxiliary equipment: fuel RB (coal mill trip), forced draft fan RB, induced draft fan RB, primary air fan RB, and feedwater pump RB. Units with only a single auxiliary equipment do not require RB control. After RB is triggered, the unit control mode switches to turbine follow mode (TF). The boiler command automatically and rapidly reduces to the RB target value at a pre-designed rate. The turbine controls the main steam pressure, and the load command tracks the actual active power. Generally, the target value of fuel RB is the load capacity of the still-operating pulverizing system. The RB of forced draft fan, induced draft fan, and primary air fan is 50% of the rated load (Pe). The feedwater pump RB is divided into two cases: successful electric pump start-up and electric pump not started. The target load for successful electric pump start-up is the feedwater load capacity of one steam pump and one electric pump. The target load for electric pump not started is 50% of the rated load. There are also different RB load reduction rates corresponding to different RB types. Fuel RB is designed to be 50%Pe / min, forced draft fan RB and induced draft fan RB are designed to be 100%Pe / min, primary air fan RB is designed to be 200%Pe / min, feedwater pump RB (electric pump start-up) is designed to be 100%Pe / min, and feedwater pump RB (electric pump not started) is designed to be 200%Pe / min.

[0003] In order to quickly reduce the load, the non-fuel RB is designed with a mill skipping logic in addition to rapidly reducing the boiler command to the RB target value. The pulverizing system mainly includes a coal feeder and a coal mill. Non-fuel RBs need to trip the coal mills synchronously. Depending on the location of the pulverizing system, there are tripped mills and reserved mills. The tripped mills trip one by one in a pre-designed order. The first coal mill trips directly. To reduce the disturbance to the furnace combustion, the second coal mill will trip after a certain interval. The interval is set according to the operating conditions of each unit and is generally a fixed value corresponding to the RB type. According to the design, one pulverizing system is configured as a backup. For example, if six pulverizing systems are configured, only five pulverizing systems are needed to operate at full rated load. The forced draft / induced draft fan RB, primary air fan RB, and feedwater pump RB (electric pump not started) will ultimately retain three coal mills. The feedwater pump RB (electric pump started together) will retain four coal mills or not trip depending on the pump capacity. The minimum number of coal mills retained by the RB is three. Three tripped mills are set, and the corresponding coal mills are tripped according to the RB type and the operating status of the coal mill. Similarly, for units with five pulverizing systems, the forced draft / induced draft fan RB and feedwater pump RB (electric pump not started) will ultimately retain three coal mills. The primary air fan RB, due to its significant impact on primary air pressure, will ultimately retain only two coal mills to prevent excessive mill blockage. The feedwater pump RB (electric pumps running in tandem) will retain four coal mills or not trip at all, depending on the pump capacity. Since a maximum of two mills can trip consecutively and the standby mill is variable, the first mill tripped by RB will trip directly, while the tripping intervals for the other mills will be consistent.

[0004] In recent years, due to the volatility of coal prices, most power plants have opted for lower-calorific-value coal to save on fuel costs. Since the calorific value is significantly lower than that of the designed coal, standby mills need to be turned on to operate at full load, and even with all mills running, they cannot operate at full rated load. Currently, full mill operation is the norm for thermal power units.

[0005] The existing RB control strategy is still designed based on the operation mode with backup mills. The load-carrying capacity of the pulverizing system is matched with that of auxiliary equipment such as fans and feedwater pumps. The number of mills to be retained and the mill tripping interval are designed as fixed values. However, with all mills now operating, it has become the norm for thermal power units. The RB control strategy lacks flexibility. Burning low-calorific-value coal may cause a mismatch between the load-carrying capacity of the pulverizing system and auxiliary equipment such as fans and feedwater pumps in the current RB control strategy. In addition, under the condition of all mill operation, non-fuel RBs need to trip one more mill, especially for units with six pulverizing systems. Currently, most thermal power units are equipped with six pulverizing systems, which may require a maximum of three consecutive trips of coal mills. Especially when burning low-calorific-value coal, the load-carrying capacity of the pulverizing system decreases. Under operating condition B, the load-carrying capacity of the remaining operating mills may be lower than the target load for RB. Furthermore, the original tripping interval only considered the interval between the second and first mills. According to the current RB control strategy, the tripping intervals for the second and third mills are the same. The most dangerous phase in the actual RB process is the moment the first two mills trip, which may cause the unit to trip because the auxiliary machine's main control parameters exceed protection limits. The consecutive tripping of three mills in a short period will significantly disrupt combustion in the furnace, potentially causing other important parameters to exceed safe limits. Based on the accumulated experience from six mill RB tests, the first two mills should trip as quickly as possible, while the tripping of the third mill can be appropriately delayed to reduce disturbance to furnace combustion. The current RB control strategy does not consider the tripping interval of the third mill, or whether it is necessary to trip the third mill. Summary of the Invention

[0006] In view of the above-mentioned problems in the prior art, the purpose of this invention is to provide a method for controlling RB mill skipping in coal-fired power units that is suitable for the operation of the entire mill group.

[0007] The present invention provides the following technical solution: a method for controlling RB mill tripping in a coal-fired unit that is adapted to full mill operation. The method is based on the existing RB control strategy and adds a matching calculation between the mill's load capacity and the RB target load. When the mill's load capacity is less than the RB target load, the RB mill tripping is interrupted to prevent the situation where the load capacity of the mill that is kept running is less than the RB target load. The interval time of RB mill tripping under full mill operation mode is set accordingly.

[0008] Furthermore, the method includes the following steps:

[0009] S1, the load capacity of the computer group pulverizing system, including the number of operating units, fuel load capacity, minimum load capacity of the mill group, and RB reserved load capacity of the mill group;

[0010] S2, RB Mill Trip Sequence (Second Mill): When one mill in the first pulverizing system trips, after receiving a trip signal from the first pulverizing system and a delay interval of N seconds, and if the number of operating mills in the mill group is still greater than the reserved number, the mill tripping process continues based on the set second mill tripping strategy; otherwise, the mill tripping process is terminated.

[0011] S3, RB Mill Trip Sequence (Third Mill): When the mill tripping process is scheduled to proceed to the second mill tripping stage according to the strategy, the RB signal is triggered and the number of operating mills is greater than the reserved number, the RB mill tripping process continues. If the first and second pulverizing systems have tripped and after a delay interval of M seconds, and the number of operating mills is still greater than the reserved number, the mill tripping process continues based on the third mill tripping strategy; otherwise, the mill tripping process is terminated.

[0012] S4. When the above mill tripping process is executed according to the strategy until the third mill trips, the RB mill tripping interval time of the third mill is designed differently to delay the tripping of the third mill and reduce the disturbance to combustion.

[0013] Furthermore, the specific process of S1 is as follows:

[0014] S101. Calculate the number of running mills and compare it with the number of mills reserved by RB mill tripping. If the number of running mills is greater than the number of mills reserved by RB mill tripping, it is considered that the number of running mills is greater than the number of reserved mills, and RB mill tripping needs to be triggered. If the number of running mills is equal to the total number of mills, it is determined to be the full mill operation mode. Otherwise, it is the non-full mill operation mode. The falling edge of the switch signal is delayed and held for t1 time.

[0015] S102. Based on the number of pulverizing systems still in operation, the rated load capacity of each mill, and the fuel load capacity of the BTU correction coefficient computer group, the minimum load capacity is calculated by multiplying the fuel load capacity by the minimum load ratio coefficient A of the coal mill; the load capacity of the pre-reserved mill group is calculated by multiplying the number of RB skipped mills retained by the rated load capacity of each mill.

[0016] S103. When the RB target load value is less than the reserved mill group's load capacity, it is considered that the reserved mill group's load capacity matches the RB target load, triggering the reserved mill group's load capacity being higher than the RB target. If the RB target load is greater than the reserved mill group's load capacity, and at the same time the RB target load is less than the current fuel load capacity and greater than the current operating mill group's minimum load capacity, it is considered that the reserved mill group's load capacity is too low and does not match the set RB target load, stopping the trip of the third coal mill and triggering the third mill trip interruption signal.

[0017] Furthermore, in S2, the second mill skipping strategy includes the following two cases:

[0018] S201. In non-full-mill operation mode, if the load-carrying capacity of the reserved mill group is higher than the target load of RB, the second mill will continue to trip; otherwise, the operation will be interrupted.

[0019] S202. In full mill operation mode or when the primary air blower RB is activated, the second mill trip will continue.

[0020] Furthermore, in S3, the third mill skipping strategy includes the following two cases:

[0021] S301. In non-full mill operation mode, when the third mill in the trip sequence is in operation, the number of operating coal mills is greater than the number of reserved mills, and the load capacity of the reserved mill group is higher than the RB target condition, the pulse signal is triggered to trip the third mill.

[0022] S302. In full mill operation mode, when the RB target value is within the load capacity range of the reserved mill group or when the primary fan RB is activated, a pulse signal is triggered to trip the third mill.

[0023] Furthermore, the specific process of S4 is as follows:

[0024] When the tripping process is executed according to the strategy until the third coal mill trips, the RB tripping interval time of the third mill is designed differently. The tripping interval time T2 of the third mill is added on the basis of the original tripping interval time T1. The tripping interval time in the non-full mill operation mode is still T1. In the full mill operation mode, the tripping interval time of the second mill is T1 and the tripping interval time of the third mill is T2. The T2 time is set according to the operation of each unit and each system. The T2 time is longer than the T1 time to delay the tripping of the third mill to reduce the disturbance to combustion.

[0025] Furthermore, in S4, the mill skipping intervals T1 and T2 are set according to the RB type. The forced draft / induced draft fan RB, primary air fan RB, and feedwater pump RB are all set with different values ​​according to the unit parameter operating characteristics. The main control parameter of the forced draft / induced draft fan RB is the furnace negative pressure, the main control parameter of the primary air fan RB is the primary air pressure, and the main control parameter of the feedwater pump RB for supercritical units is the superheat, while the main control parameter for subcritical units is changed to the steam drum water level.

[0026] Furthermore, in S4, a direct trip protection without delay is set for the third mill. Before the first and second mills have tripped and the third mill has not tripped, if the main control parameters of the tripping auxiliary machine have exceeded the danger adjustment limit, or if the tripping signal of the first mill has tripped and the tripping signal of the second mill has been triggered but the coal mill equipment has not tripped correctly, the tripping interval time is switched to 0, and the tripping signal of the third mill is immediately triggered. The judgment that the main control parameters of the tripping auxiliary machine exceed the danger adjustment limit adds a t2s rising edge delay to prevent signal jump.

[0027] By employing the above-described technology, the beneficial effects of the present invention compared to the prior art are as follows:

[0028] This invention adds an RB control strategy under full mill operation conditions to the original RB control strategy. Based on the RB target load, it distinguishes between the maximum and minimum load capacity of the mill group. By comparing the maximum and minimum load capacity of the pre-reserved mill group, it accurately calculates whether the last mill needs to be tripped. If the last mill still needs to be tripped, in order to further reduce the disturbance of furnace combustion, a delay time that is more adaptable to changes in RB operating conditions is designed to reduce the disturbance in the RB process; thus further improving the adaptability of the RB control strategy. Attached Figure Description

[0029] Figure 1 This is a logical schematic diagram illustrating the calculation of the number of operations in the powder-making system of the present invention;

[0030] Figure 2 A schematic diagram illustrating the logic for setting the number of RB skip mills to retain in this invention;

[0031] Figure 3 This is a logical schematic diagram of the RB target load calculation of the present invention;

[0032] Figure 4 This is a logical diagram illustrating the calculation of fuel carrying capacity and minimum carrying capacity of the grinding unit according to the present invention.

[0033] Figure 5 This is a schematic diagram of the tripping interruption logic of the third mill in this invention;

[0034] Figure 6 This is a schematic diagram of the RB skip mill control logic of the present invention;

[0035] Figure 7 This is a schematic diagram showing the setting of the primary blower RB skipping interval time in this invention;

[0036] Figure 8 This is a schematic diagram showing the setting of the RB milling interval time for the blower / induced draft fan of the present invention;

[0037] Figure 9 This is a schematic diagram showing the setting of the RB skipping interval time for the water pump of the present invention.

[0038] Explanation of symbols in the attached diagram:

[0039] The values ​​are represented as analog signals, and the dashed lines represent digital signals (0 and 1).

[0040] OR: OR;

[0041] AND: phase and;

[0042] NO: negate;

[0043] A: Constant;

[0044] T-ON: Rising edge delay, meaning that after the uplink input changes from 0 to 1, the output will be 1 after a set delay time. If the uplink input changes from 1 to 0, no delay is needed and the output will be 0.

[0045] T-OFF: Falling edge delay. After the uplink input changes from 1 to 0, the output is 1 within the set delay time. After the delay time is exceeded, the output switches to 0. If the uplink input is 0, no delay is needed to output 0 directly.

[0046] T: Switching function. When the input switch signal is 1, the YES terminal is selected for output; when the input switch signal is 0, the NO terminal is selected for output.

[0047] H: When the input analog quantity is greater than a set value, the output switch quantity is 1;

[0048] L: When the input analog quantity is less than a set value, the output switch quantity is 1;

[0049] H / L: Input analog quantity is greater than a certain value 1 and less than a certain value 2, output switch quantity 1;

[0050] DIGCOUNT: Counter, counts the total number of 1 input switches;

[0051] >: Select the largest value from the input for output;

[0052] <: Select the smaller value from the input for output;

[0053] ×: Multiplication;

[0054] SUB: Output the result of subtracting two analog inputs;

[0055] SUM: Sum of the input analog quantities;

[0056] f(x): A linear function in one variable;

[0057] PULSE: Pulse. Detailed Implementation

[0058] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0059] Conversely, this invention encompasses any substitutions, modifications, equivalent methods, and solutions made within the spirit and scope of the invention as defined in the claims. Furthermore, to provide a better understanding of the invention, certain specific details are described in detail below. However, those skilled in the art will fully understand the invention even without these detailed descriptions.

[0060] Example:

[0061] Please see Figure 1-9 This embodiment provides a method for controlling RB mill tripping in coal-fired power units that is suitable for full-grid operation. This method can be applied to coal-fired power generating units with six pulverizing systems. In this embodiment, the RB mill tripping sequence and the final number of mills retained remain unchanged. The main difference is the addition of a matching calculation between the mill's load capacity and the RB target load during the RB process. When the mill's load capacity is significantly less than the RB target load, the RB mill tripping is interrupted to prevent situations where the load capacity of the retained operating mill is less than the RB target load. The interval between the tripping of the last mill in the RB process under full-grid operation is also reasonably set. Fuel RB refers to unexpected mill tripping and is not considered in this embodiment. After the RB signal (non-fuel) is triggered, the first mill in sequence still trips directly. This method also considers RB control under non-full-grid operation conditions. The RB mill tripping control solution suitable for a wide range of operating conditions is as follows:

[0062] S1. The load capacity of the precision computer-controlled milling system, including the number of operating mills, fuel load capacity, minimum load capacity of the mill group, and RB reserved load capacity of the mill group. Details are as follows:

[0063] S101. Calculate the number of operating mills and compare it with the number of mills reserved by RB mill tripping. If the number of operating mills is greater than the number of mills reserved by RB mill tripping, it is considered that the number of operating mills is greater than the number of reserved mills, and RB mill tripping needs to be triggered. If the number of operating mills is equal to six, it is determined to be the full mill operation mode. Otherwise, it is the non-full mill operation mode. The falling edge of the switch signal is delayed for 120s.

[0064] S102. Based on the number of pulverizing systems still in operation, the rated load capacity of each mill, and the fuel load capacity of the BTU correction coefficient computer group, the minimum load capacity is calculated by multiplying the fuel load capacity by the minimum load ratio coefficient A of the coal mill; the load capacity of the pre-reserved mill group is calculated by multiplying the number of RB skipped mills by the rated load capacity of each mill.

[0065] S103. When the RB target load value is less than the reserved mill group's load capacity, it is considered that the reserved mill group's load capacity matches the RB target load, triggering the reserved mill group's load capacity higher than the RB target signal. If the RB target load is greater than the reserved mill group's load capacity, and at the same time satisfies that the RB target load is less than the current fuel load capacity and greater than the current operating mill group's minimum load capacity, it is considered that the reserved mill group's load capacity is too low and does not match the set RB target load, stopping the trip of the third coal mill and triggering the third mill trip interruption signal.

[0066] S2, RB (non-fuel RB) trip sequence: Second mill. If the RB (non-fuel) signal is triggered and the number of operating mills is greater than the reserved number, a pulse signal is immediately sent to trip one mill. After receiving the trip signal from the first pulverizing system and a delay interval of N seconds, if the number of operating mills is still greater than the reserved number, the tripping process continues based on the set second mill tripping strategy; otherwise, the tripping process is aborted. The second mill tripping strategy is as follows:

[0067] S201. In non-full-mill operation mode, if the load-carrying capacity of the reserved mill group is higher than the target load of RB, the second mill will continue to trip; otherwise, the operation will be interrupted.

[0068] S202. In full mill operation mode or when the primary air blower RB is activated, the second mill trip will continue.

[0069] S3, RB (non-fuel RB) trip sequence: third mill. If the RB (non-fuel) signal is triggered and the number of operating mills is greater than the reserved number, the RB process tripping procedure continues. If the first and second pulverizing systems have already tripped, and after a delay interval of M seconds, the number of operating mills is still greater than the reserved number, the tripping procedure continues based on the third mill tripping strategy; otherwise, the tripping procedure is terminated. The third mill tripping strategy is as follows:

[0070] S301. In non-full mill operation mode, when the third mill in the trip sequence is in operation, the number of operating coal mills is greater than the number of reserved mills, and the load-carrying capacity of the reserved mill group is higher than the RB target condition, the pulse signal is triggered to trip the third mill.

[0071] S302. In full mill operation mode, when the RB target value is within the load capacity range of the reserved mill group or when the primary fan RB is activated, a pulse signal is triggered to trip the third mill.

[0072] S4. When the above mill tripping process is executed according to the strategy until the third coal mill trips, the RB mill tripping interval time for the third mill is designed differently. An additional tripping interval time T2 is added to the original mill tripping interval time T1. In non-full-mill operation mode, the mill tripping interval time remains T1. In full-mill operation mode, the mill tripping interval time for the second mill is T1, and the mill tripping interval time for the third mill is T2. The T2 time is set according to the operating conditions of each unit and system. The T2 time is longer than the T1 time to delay the tripping of the third mill and reduce disturbance to combustion.

[0073] S401. As a supplement to S4, the mill skipping interval time T1 and T2 can be set separately according to the RB type. The forced / induced draft fan RB, primary air fan RB and feedwater pump RB can all be set with different values ​​according to the unit parameter operating characteristics. The main control parameter of the forced / induced draft fan RB is the furnace negative pressure, the main control parameter of the primary air fan RB is the primary air pressure, and the main control parameter of the feedwater pump RB for supercritical units is the superheat, while the main control parameter for subcritical units is replaced by the steam drum water level.

[0074] S402. As a supplement to S4, a direct trip protection without delay is set for the third mill. If the main control parameters of the tripping auxiliary machine have exceeded the danger adjustment limit before the first and second mills have tripped and the third mill has not tripped, or if the tripping signal of the first mill has been triggered but the coal mill equipment has not tripped correctly, the tripping interval time is switched to 0, and the tripping signal of the third mill is immediately triggered. The judgment that the main control parameters of the tripping auxiliary machine exceed the danger adjustment limit is supplemented with a 1-second rising edge delay to prevent signal jump.

[0075] Based on the original RB control strategy, an additional RB control strategy under full mill operation is added. Using the RB target load as a benchmark, the maximum and minimum fuel load capacities for each mill are calculated. These are compared with the fuel load capacities of the mills pre-reserved by the RB to accurately determine whether the fuel load capacities of the pre-reserved mills are lower than the RB target load. If the fuel load capacity is lower than the RB target load, and simultaneously the fuel load capacity before the last mill trips is greater than the RB target value and the minimum fuel load capacity is less than the RB target value, then the tripping of the third mill is interrupted, and the fuel master control reduces the coal quantity to the value corresponding to the RB target load. Otherwise, the tripping procedure for the third mill continues. If the tripping of the third mill continues, and both the first and second mills have tripped and the main control parameters of the tripping auxiliary equipment are within controllable ranges, the tripping interval is appropriately extended. If the second mill fails to trip correctly or the main control parameters of the tripping auxiliary equipment exceed the safe range within the interval, then the third mill is immediately tripped. Under the above control strategy, when the calorific value of the coal used by the unit is relatively poor, such as when the forced draft / induced draft fan RB or the feedwater pump RB is running, one less mill can be tripped, and the tripping of the third coal mill can be delayed after the first two mills have tripped, thereby reducing combustion disturbances and avoiding situations where the fuel carrying capacity is lower than the target load of RB.

[0076] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for controlling RB mill skipping in a coal-fired power unit adaptable to full mill operation, characterized in that, The method is based on the existing RB control strategy and adds a matching calculation between the mill set load capacity and the RB target load. When the mill set load capacity is less than the RB target load, the RB mill tripping is interrupted to prevent the situation where the load capacity of the mill set in operation is less than the RB target load. The method also sets the interval time for RB mill tripping under full mill operation mode. The method includes the following steps: S1, the load capacity of the computer group pulverizing system, including the number of operating units, fuel load capacity, minimum load capacity of the mill group, and RB reserved load capacity of the mill group; S2, RB Mill Trip Sequence (Second Mill): When one mill in the first pulverizing system trips, after receiving a trip signal from the first pulverizing system and a delay interval of N seconds, and if the number of operating mills in the mill group is still greater than the reserved number, the mill tripping process continues based on the set second mill tripping strategy; otherwise, the mill tripping process is terminated. S3, RB Mill Trip Sequence (Third Mill): When the mill tripping process is scheduled to proceed to the second mill tripping stage according to the strategy, the RB signal is triggered and the number of operating mills is greater than the reserved number, the RB mill tripping process continues. If the first and second pulverizing systems have tripped and after a delay interval of M seconds, and the number of operating mills is still greater than the reserved number, the mill tripping process continues based on the third mill tripping strategy; otherwise, the mill tripping process is terminated. S4. When the above mill tripping process is executed according to the strategy until the third mill trips, the RB mill tripping interval time of the third mill is designed differently to delay the tripping of the third mill and reduce the disturbance to combustion.

2. The method for controlling RB mill skipping in a coal-fired power unit adapted to full mill operation as described in claim 1, characterized in that, The specific process of S1 is as follows: S101. Calculate the number of operating mills and compare it with the number of mills reserved by RB mill tripping. If the number of operating mills is greater than the number of mills reserved by RB mill tripping, it is considered that the number of operating mills is greater than the number of reserved mills, and RB mill tripping needs to be triggered. If the number of operating mills is equal to the total number of mills, it is determined to be the full mill operation mode. Otherwise, it is the non-full mill operation mode. The falling edge of the switch signal is delayed and held for t1 time. S102. Based on the number of pulverizing systems still in operation, the rated load capacity of each mill, and the fuel load capacity of the BTU correction coefficient computer group, the minimum load capacity is calculated by multiplying the fuel load capacity by the minimum load ratio coefficient A of the coal mill; the load capacity of the pre-reserved mill group is calculated by multiplying the number of RB skipped mills retained by the rated load capacity of each mill. S103. When the target load value of RB is less than the pre-reserved mill set load capacity, it is considered that the load capacity of the pre-reserved mill set matches the target load of RB, triggering the signal that the load capacity of the pre-reserved mill set is higher than the target load of RB. If the RB target load is greater than the reserved mill's load capacity, and at the same time the RB target load is less than the current fuel load capacity and greater than the minimum load capacity of the currently operating mill, it is considered that the reserved mill's load capacity is too low and does not match the set RB target load. The third coal mill will then be tripped, triggering the third mill trip interruption signal.

3. The method for controlling RB mill skipping in a coal-fired power unit adapted to full mill operation as described in claim 1, characterized in that, In S2, the second mill skipping strategy includes the following two cases: S201. In non-full-mill operation mode, if the load-carrying capacity of the reserved mill group is higher than the target load of RB, the second mill will continue to trip; otherwise, the operation will be interrupted. S202. In full mill operation mode or when the primary air blower RB is activated, the second mill trip will continue.

4. The method for controlling RB mill skipping in a coal-fired power unit adapted to full mill operation as described in claim 3, characterized in that, In S3, the third mill skipping strategy includes the following two cases: S301. In non-full mill operation mode, when the third mill in the trip sequence is in operation, the number of operating coal mills is greater than the number of reserved mills, and the load capacity of the reserved mill group is higher than the RB target condition, the pulse signal is triggered to trip the third mill. S302. In full mill operation mode, when the RB target value is within the load capacity range of the reserved mill group or when the primary fan RB is activated, a pulse signal is triggered to trip the third mill.

5. The method for controlling RB mill skipping in a coal-fired unit adapted to full mill operation as described in claim 4, characterized in that, The specific process of S4 is as follows: When the tripping process is executed according to the strategy until the third coal mill trips, the RB tripping interval time of the third mill is designed differently. The tripping interval time T2 of the third mill is added on the basis of the original tripping interval time T1. The tripping interval time in the non-full mill operation mode is still T1. In the full mill operation mode, the tripping interval time of the second mill is T1 and the tripping interval time of the third mill is T2. The T2 time is set according to the operation of each unit and each system. The T2 time is longer than the T1 time to delay the tripping of the third mill to reduce the disturbance to combustion.

6. The method for controlling RB mill skipping in a coal-fired power unit adapted to full mill operation as described in claim 5, characterized in that, In S4, the mill skipping intervals T1 and T2 are set according to the RB type. The forced draft / induced draft fan RB, primary air fan RB, and feedwater pump RB are all set with different values ​​according to the unit parameter operating characteristics. The main control parameter of the forced draft / induced draft fan RB is the furnace negative pressure, the main control parameter of the primary air fan RB is the primary air pressure, and the main control parameter of the feedwater pump RB for supercritical units is the superheat, while the main control parameter for subcritical units is changed to the steam drum water level.

7. The method for controlling RB mill skipping in a coal-fired power unit adapted to full mill operation as described in claim 6, characterized in that, In S4, a direct trip protection without delay is set for the third mill. Before the first and second mills have tripped and the third mill has not tripped, if the main control parameters of the tripping auxiliary machine have exceeded the dangerous adjustment limit, or if the tripping signal of the first mill has tripped and the tripping signal of the second mill has been triggered but the coal mill equipment has not tripped correctly, the tripping interval time is switched to 0, and the tripping signal of the third mill is immediately triggered. The judgment that the main control parameters of the tripping auxiliary machine exceed the dangerous adjustment limit adds a t2s rising edge delay to prevent signal jump.

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