Coal-fired unit RB jump mill control method suitable for full mill unit operation

Abstract

The invention discloses a coal-fired unit RB jump mill control method suitable for full mill unit operation, and belongs to the technical field of power plant unit control. According to the method, on the basis of an original RB control strategy, an RB control strategy under a full-mill operation condition is added, RB target loads serve as the standard, the highest loading capacity and the lowest loading capacity are distinguished for the loading capacity of a mill set, the highest loading capacity and the lowest loading capacity of the pre-reserved mill set are compared, whether the last coal mill needs to be tripped or not is accurately calculated, and the final coal mill needs to be tripped or not is judged. If the last coal mill still needs to be tripped, in order to further reduce disturbance of hearth combustion, delay time more suitable for RB working condition changes is designed, and disturbance in the RB process is reduced; and the adaptability of the RB control strategy is further improved.

Description

Technical Field

[0001] The invention belongs to the technical field of power plant unit control, and in particular relates to a RB mill skipping control method for a coal-fired unit suitable for full mill group operation. Background Art

[0002] RUNBACK is the existing auxiliary machine fault load reduction function of thermal power units, referred to as RB. The function of this function is that when some of the main auxiliary machines of the unit trip due to a fault, and the maximum load capacity of the unit is significantly lower than the current actual load, the unit coordinated control system will quickly reduce the unit load to the maximum output that all the operating auxiliary machines can actually achieve, and control the operating parameters within the allowable range to keep the unit running. The main equipment of the pulverizing system is the coal feeder and the coal mill. The coal feeder conveys coal to the coal mill through the hopper, and after being ground by the coal mill, it is blown into the furnace for combustion. According to the situation of the unit's auxiliary machines, for example, the pulverizing system will be configured with 4-6 sets, two fans, two steam-driven feed water pumps and one electric feed water pump. Generally, the following RUNBACK functions are set according to the accidental tripping of the operating auxiliary machines: fuel RB (pulverizer tripping), forced draft fan RB, induced draft fan RB, primary fan RB and feed water pump RB. In addition, units with only a single auxiliary machine do not need to be configured with RB control. After RB is triggered, the unit control mode switches to the turbine follower mode (TF), the boiler command automatically drops to the RB target value at a designed rate, the turbine controls the main steam pressure, and the load command tracks the actual active power. Generally speaking, the target value of fuel RB is the load capacity of the pulverizing system that is still in operation, the RB of the blower, the induced draft fan and the primary fan are 50% of the rated load (Pe for short), and the feedwater pump RB is divided into two cases: the electric pump is successfully started and the electric pump is not started. The target load of the successful electric pump is the feedwater load capacity of a steam pump and an electric pump, and the target load of the electric pump is 50% of the rated load. There are different RB load reduction rates corresponding to different RB types. The fuel RB is designed to be 50% Pe / min, the RB of the blower and the induced draft fan are designed to be 100% Pe / min, the RB of the primary fan is designed to be 200% Pe / min, the RB of the feedwater pump (electric pump start-up) is designed to be 100% Pe / min, and the RB of the feedwater pump (electric pump not started) is designed to be 200% Pe / min.

[0003] In order to reduce the load quickly, the non-fuel RB is designed with a skip-mill logic in addition to quickly reducing the boiler command to the RB target value. The pulverizing system mainly includes coal feeder and coal mill. Non-fuel RB needs to trip the coal mill synchronously. According to the location of the pulverizing system, tripping mills and reserved mills are set. The tripping mills trip one by one in the designed order. The first coal mill trips directly. In order to reduce the disturbance of furnace combustion, the second coal mill will trip after a certain interval. The interval time is set according to the operation status of each unit, which is generally a fixed value corresponding to the RB type. According to the design, one set of pulverizing systems configured by the unit is used as a backup. For example, six sets of pulverizing systems are configured, and only five sets of pulverizing systems are needed with full rated load. The forced / induced draft fan RB, primary fan RB and feed water pump RB (electric pump not started) finally retain three coal mills. The feed water pump RB (electric pump started together) retains four coal mills or does not trip according to the capacity of the electric pump. The minimum number of coal mills retained by RB is three, and three tripping mills are set. The corresponding coal mills are tripped according to the RB type and the operation status of the coal mill. Similarly, the unit with five sets of pulverizing systems, including the induced draft fan RB and the water pump RB (electric pump not started), will eventually retain three coal mills. The primary fan RB has a greater impact on the primary air pressure. In order to prevent the running coal mills from blocking too many mills, only two coal mills will be retained. The water pump RB (electric pump joint start) will retain four coal mills or not trip the mill according to the capacity of the electric pump. Since at most two mills can trip continuously and the standby mill is variable, the first mill will trip directly when RB trips, and the trip intervals set for other mills are the same.

[0004] Due to the volatility of coal prices in recent years, most power plants have chosen more economical low-calorific value coal in order to save fuel costs. Since the calorific value is significantly lower than the designed coal type, the standby mill needs to be turned on to carry out full load, and even full mill operation cannot carry out full rated load. Currently, full mill operation has become the norm for thermal power units.

[0005] The existing RB control strategy is still designed based on the operation mode with spare mills. The pulverizing system matches the load capacity of auxiliary machines such as fans and water pumps. The number of retained mills and the time interval between mill trips are designed to be fixed values. However, full-mill operation has become the norm for thermal power units. The RB control strategy is not flexible enough. The use of coal with lower calorific value may cause the load capacity of the pulverizing system in the current RB control strategy to not match that of auxiliary machines such as fans and water pumps. In addition, non-fuel RB needs to trip one more mill under full-mill operation, especially for units equipped with six pulverizing systems. At present, most thermal power units are equipped with six pulverizing systems, and at most three pulverizers need to be tripped continuously. Especially when using coal with lower calorific value, the load capacity of the pulverizing system decreases, and R The load carrying capacity of the operating mill group retained under condition B may be lower than the RB target load. In addition, the interval time originally set for tripping the mill only considers the interval time between the second mill and the first mill. According to the current RB control strategy, the tripping interval time of the second and third mills is the same. The most dangerous stage in the actual RB process is the tripping moment of the first two mills. The main control parameters of the tripping auxiliary machine may exceed the protection limit, causing the unit to trip. The continuous tripping of three mills in a short period of time will cause great interference to the combustion in the furnace, and may cause other important parameters to exceed the safety range. According to the current accumulated experience of the six-mill RB test, the first two mills should be tripped as soon as possible, and the third mill can be appropriately delayed to reduce the disturbance to the combustion in the furnace. The current RB control strategy does not consider the tripping interval time of the third mill, and whether it is necessary to trip the third mill. Summary of the invention

[0006] In view of the above problems existing in the prior art, the object of the present invention is to provide a RB mill skipping control method for a coal-fired unit that is suitable for full mill group operation.

[0007] The present invention provides the following technical solution: a RB mill skipping control method for a coal-fired unit adapted to full-mill group operation, wherein the method adds a matching calculation between the mill group load capacity and the RB target load on the basis of the existing RB control strategy, and interrupts the RB mill skipping when the mill group load capacity is less than the RB target load to prevent the situation where the load capacity of the remaining running mill group is less than the RB target load, and correspondingly sets the interval time of the RB mill skipping under the full-mill operation mode.

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

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

[0010] S2, RB mill skipping sequence second mill: When a coal mill in the first pulverizing system trips, after receiving the tripping signal of the first pulverizing system and the delay interval N seconds, and the number of running mills in the mill group is still greater than the number of reserved mills, the mill skipping process will continue to be executed based on the set second mill skipping strategy, otherwise the mill skipping process will be terminated:

[0011] S3, RB mill skipping sequence for the third mill: When the mill skipping process is executed according to the strategy until the second coal mill trips, the RB signal is triggered and the number of coal mills in operation is greater than the number of reserved units, the RB mill skipping process continues to be executed; after receiving the tripping signal of the first and second pulverizing systems and the delay interval is M seconds, and the number of running mills is still greater than the number of reserved units, the mill skipping process continues to be executed based on the third mill skipping strategy, otherwise the mill skipping process is terminated:

[0012] 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 of the third mill is designed differently to delay the tripping of the third mill to reduce the disturbance to combustion.

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

[0014] S101, calculate the number of running units, and compare it with the number of units reserved for RB mill skipping. If the number of running units is greater than the number of units reserved for RB mill skipping, it is considered that the number of running units of the coal mill is greater than the number of units reserved, and RB mill skipping needs to be triggered. If the number of running units is equal to the total number of units, it is determined to be a full mill operation mode, otherwise it is a non-full mill operation mode, and the falling edge of the switch signal is delayed for t1 time;

[0015] S102, the fuel load capacity of the group operation is calculated according to the number of pulverizing systems still in operation, the rated load capacity of each mill and the BTU correction coefficient; the minimum load capacity is calculated by multiplying the fuel load capacity by the minimum load ratio coefficient A of the coal mill; the reserved mill group load capacity is calculated by multiplying the number of RB mills reserved by the rated load capacity of each mill;

[0016] S103, when the RB target load value is less than the pre-reserved grinding group load capacity, it is considered that the pre-reserved grinding group load capacity matches the RB target load, triggering the pre-reserved grinding group load capacity higher than the RB target signal; if the RB target load is greater than the pre-reserved grinding group load capacity, and at the same time meets the RB target load is less than the current fuel load capacity and greater than the current operating grinding group minimum load capacity, it is considered that the pre-reserved grinding group load capacity is too low and does not match the set RB target load, and the third coal mill is stopped and tripped, triggering the third mill tripping interruption signal.

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

[0018] S201, in the non-full mill operation mode, when the reserved mill group load capacity is higher than the RB target load, the second mill tripping is continued, otherwise it is interrupted;

[0019] S202: In full-mill operation mode or when primary fan RB is actuated, the second mill tripping operation continues.

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

[0021] S301, in the non-full mill operation mode, when the third mill in the tripping sequence is in operation, the number of running 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, all the conditions are met, a pulse signal is triggered to trip the third mill;

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

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

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

[0025] Furthermore, in S4, the mill jumping interval times T1 and T2 are set respectively according to the RB type, and the forced / induced draft fan RB, primary fan RB and feed water pump RB are set with different values ​​according to the operating characteristics of the unit parameters. The main control parameter of the forced / induced draft fan RB is the furnace negative pressure, the main control parameter of the primary fan RB is the primary air pressure, the main control parameter of the feed water pump RB supercritical unit is the superheat, and the main control parameter of the subcritical unit is changed to the drum water level.

[0026] Furthermore, in S4, the third mill is provided with direct tripping protection without delay. When the first and second mills have tripped but before the third mill has tripped, the main control parameters of the tripping auxiliary machine have exceeded the dangerous adjustment limit, or the first mill has tripped, 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 triggered immediately. The judgment that the main control parameters of the tripping auxiliary machine exceed the dangerous adjustment limit increases the rising edge delay of t2s to prevent signal jumping.

[0027] By adopting the above technology, compared with the prior art, the beneficial effects of the present invention are as follows:

[0028] The method of the present invention adds an RB control strategy under the full-mill operation condition on the basis of the original RB control strategy, distinguishes the maximum load capacity and the minimum load capacity for the mill group based on the RB target load, compares the maximum and minimum load capacities of the reserved mill group, and accurately calculates whether it is necessary to trip the last coal mill. If it is still necessary to trip the last coal mill, in order to further reduce the disturbance of furnace combustion, a delay time that is more suitable for changes in RB conditions is designed to reduce disturbances in the RB process, thereby further improving the adaptability of the RB control strategy. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 A logical diagram of the operation quantity calculation of the milling system of the present invention;

[0030] Figure 2 A logical schematic diagram of setting the number of reserved RB mills according to the present invention;

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

[0032] Figure 4 It is a logical schematic diagram of the calculation of the fuel carrying capacity and the minimum carrying capacity of the grinding group of the present invention;

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

[0034] Figure 6 It is a schematic diagram of the RB jump grinding control logic of the present invention;

[0035] Figure 7 It is a schematic diagram of setting the interval time of the primary fan RB grinding of the present invention;

[0036] Figure 8 It is a schematic diagram of setting the RB grinding interval time of the supply / induced draft fan of the present invention;

[0037] Fig. 9 It is a schematic diagram of setting the interval time of the water supply pump RB grinding in the present invention.

[0038] Description of the symbols in the accompanying drawings:

[0039] It is implemented as an analog quantity, and the dotted line is the switch quantity (0 and 1);

[0040] OR: phase or;

[0041] AND: phase and;

[0042] NO: negate;

[0043] A: constant;

[0044] T-ON: rising edge delay, that is, after the uplink input changes from 0 to 1, the output is 1 for the time set by the delay. If the uplink input changes from 1 to 0, the output is 0 without delay.

[0045] T-OFF: Falling edge delay. After the uplink input changes from 1 to 0, the output is 1 within the delay time. After the delay time, the output switches to 0. If the uplink input is 0, the output is 0 directly without delay.

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

[0047] H: The input analog value is greater than a certain set value, and the output switch value is 1;

[0048] L: The input analog value is less than a certain set value, and the output switch value is 1;

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

[0050] DIGCOUNT: counter, counts the total number of input switch quantity 1;

[0051] >: select the largest value from the input and output it;

[0052] <: select the smaller value from the input and output it;

[0053] ×: multiplication;

[0054] SUB: subtract the two analog inputs and output;

[0055] SUM: sum of the input analog quantity;

[0056] f(x): a linear function of one variable;

[0057] PULSE: Pulse. DETAILED DESCRIPTION

[0058] In order to make the purpose, technical solution and advantages of the present invention more clearly understood, the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments of the specification. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0059] On the contrary, the present invention covers any substitution, modification, equivalent method and scheme made on the essence and scope of the present invention as defined by the claims. Further, in order to make the public have a better understanding of the present invention, some specific details are described in detail in the detailed description of the present invention below. Those skilled in the art can fully understand the present invention without the description of these details.

[0060] Example:

[0061] See also Figure 1-9 , this embodiment provides a RB mill skipping control method for coal-fired units that are adapted to full-mill operation. The method can be applied to coal-fired thermal power generating units equipped with six sets of pulverizing systems. In the method of this embodiment, the order of RB mill skipping and the final number of retained units still maintain the control strategy without change. The main difference is that the matching calculation of the mill group load capacity and the RB target load is added during the RB process. When the mill group load capacity is obviously less than the RB target load, the RB mill skipping is interrupted to prevent the situation where the load capacity of the retained mill group is less than the RB target load, and the interval time for the tripping of the last coal mill of RB under the full-mill operation mode is reasonably set. The fuel RB is an accidental tripping of the coal mill, which is not considered in the scope of this embodiment. After the RB signal (non-fuel) is triggered, the first mill in the sequence still trips directly. This method takes into account the RB control under non-full-mill operation conditions. The RB mill skipping control solution that adapts to a wide range of conditions is as follows:

[0062] S1. Accurately calculate the load capacity of the milling system, including the number of units in operation, fuel load capacity, minimum load capacity of the mill group, and RB reserved mill group load capacity. Details are as follows:

[0063] S101, calculate the number of running units, and compare it with the number of units reserved for RB mill skipping. If the number of running units is greater than the number of units reserved for RB mill skipping, it is considered that the number of running units of the coal mill is greater than the number of units reserved, and RB mill skipping needs to be triggered. If the number of running units is equal to six, it is determined to be a full mill operation mode, otherwise it is a non-full mill operation mode, and the falling edge delay of the switch signal is maintained for 120 seconds;

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

[0065] S103, when the RB target load value is less than the pre-reserved grinding group load capacity, it is considered that the pre-reserved grinding group load capacity matches the RB target load, triggering the pre-reserved grinding group load capacity higher than the RB target signal; if the RB target load is greater than the pre-reserved grinding group 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 grinding group minimum load capacity, it is considered that the pre-reserved grinding group load capacity is too low and does not match the set RB target load, and the third coal mill is stopped and tripped, triggering the third mill tripping interruption signal.

[0066] S2, RB (non-fuel RB) tripping sequence for the second mill. When the RB (non-fuel) signal is triggered and the number of coal mills in operation is greater than the number of reserved units, a pulse signal is immediately sent to trip one coal mill. After receiving the tripping signal of the first pulverizing system and the delay interval of N seconds, and the number of running mills is still greater than the number of reserved units, the mill skipping process is continued based on the set second mill skipping strategy, otherwise the mill skipping process is terminated; the second mill skipping strategy is as follows:

[0067] S201, in the non-full mill operation mode, when the reserved mill group load capacity is higher than the RB target load, the second mill tripping is continued, otherwise it is interrupted;

[0068] S202: In full-mill operation mode or when primary fan RB is actuated, the second mill tripping operation continues.

[0069] S3, RB (non-fuel RB) tripping sequence of the third mill. RB (non-fuel) signal is triggered and the number of coal mills in operation is greater than the number of reserved units, and the RB process mill skipping process continues to execute. After receiving the tripping signal of the first and second pulverizing systems and the delay interval is M seconds, and the number of mills in operation is still greater than the number of reserved units, the mill skipping process continues to execute based on the third mill skipping strategy, otherwise the mill skipping process is terminated; the third mill skipping strategy is as follows:

[0070] S301, in the non-full-mill operation mode, when the third mill in the tripping 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 conditions are all met, a pulse signal is triggered to trip the third mill.

[0071] S302: In the full-mill operation mode, when the RB target value is within the load capacity of the reserved mill group or the primary fan RB is actuated, 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 of the third mill is designed differently. On the basis of the original mill tripping interval T1, the third mill tripping interval T2 is added. The mill tripping interval in the non-full mill operation mode is still T1. The mill tripping interval of the second mill in the full mill operation mode is T1, and the mill tripping interval of the third mill is T2. T2 is set according to the operation status of each unit and system. T2 is longer than T1, delaying the tripping of the third mill to reduce the disturbance to combustion.

[0073] S401. As a supplement to S4, the grinding intervals T1 and T2 can be set separately according to the RB type. The forced / induced draft fan RB, primary fan RB and feed water pump RB can all be set to different values ​​according to the unit parameter operation characteristics. The main control parameter of the forced / induced draft fan RB is the furnace negative pressure, the main control parameter of the primary fan RB is the primary air pressure, the main control parameter of the feed water pump RB supercritical unit is the superheat, and the main control parameter of the subcritical unit is changed to the drum water level.

[0074] S402. As a supplement to S4, set the third mill to have direct tripping protection without delay. If the main control parameters of the tripping auxiliary machine have exceeded the dangerous adjustment limit before the first and second mills have tripped and the third mill has not tripped, or if 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 will be switched to 0 and the tripping signal of the third mill will be triggered immediately. The judgment that the main control parameters of the tripping auxiliary machine have exceeded the dangerous adjustment limit will increase the rising edge delay of 1s to prevent signal jumps.

[0075] On the basis of the original RB control strategy, the RB control strategy under the full mill operation condition is added. Taking the RB target load as the benchmark, the maximum fuel load capacity and the minimum fuel load capacity are calculated for the mill group load capacity, and compared with the fuel load capacity of the RB pre-reserved mill group, it is accurately determined whether the fuel load capacity of the RB pre-reserved mill group is lower than the RB target load. If the fuel load capacity is lower than the RB target load, and at the same time, 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, the tripping of the third mill is interrupted, and the fuel master control reduces the coal volume to the corresponding value of the RB target load, otherwise the tripping process of the third mill is continued. If the tripping of the third mill is continued, the first and second mills have tripped and the main control parameters of the trip auxiliary machine are within the controllable range, and the tripping interval time is appropriately extended, if the second mill does not trip correctly or the main control parameters of the trip auxiliary machine exceed the safe range during the interval time, the third mill will be tripped immediately. Under the above control strategy, when the calorific value of the coal burned by the unit is relatively poor, for example, when the forced / induced draft fan RB and the feed water pump RB, one less mill can be tripped, and the tripping of the third mill can be delayed after the first two mills have tripped, so as to reduce the disturbance of combustion and avoid the situation where the fuel load capacity is lower than the RB target load.

[0076] The above description is only 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 in the protection scope of the present invention.

Claims

1. A coal-fired unit RB mill skipping control method adapted to full mill group operation, characterized in that: The method adds the matching calculation of the grinding group load capacity and the RB target load on the basis of the existing RB control strategy, interrupts the RB mill jump when the grinding group load capacity is less than the RB target load, prevents the situation that the load capacity of the grinding group remaining in operation is less than the RB target load, and sets the interval time of the RB mill jump in the full mill operation mode.

2. The RB mill skipping control method for a coal-fired unit adapted to full mill group operation according to claim 1 is characterized in that: The method comprises the following steps: S1. Computer group milling system load capacity, including the number of operating units, fuel load capacity, minimum mill load capacity, and RB reserved mill load capacity; S2, RB mill skipping sequence second mill: When a coal mill in the first pulverizing system trips, after receiving the tripping signal of the first pulverizing system and the delay interval N seconds, and the number of running mills in the mill group is still greater than the number of reserved mills, the mill skipping process will continue to be executed based on the set second mill skipping strategy, otherwise the mill skipping process will be terminated: S3, RB mill skipping sequence for the third mill: When the mill skipping process is executed according to the strategy until the second coal mill trips, the RB signal is triggered and the number of coal mills in operation is greater than the number of reserved units, the RB mill skipping process continues to be executed; after receiving the tripping signal of the first and second pulverizing systems and the delay interval is M seconds, and the number of running mills is still greater than the number of reserved units, the mill skipping process continues to be executed based on the third mill skipping strategy, otherwise the mill skipping process is terminated: 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 of the third mill is designed differently to delay the tripping of the third mill to reduce the disturbance to combustion.

3. The RB mill skipping control method for a coal-fired unit adapted to full mill group operation according to claim 2 is characterized in that: The specific process of S1 is as follows: S101, calculate the number of running units, and compare it with the number of units reserved for RB mill skipping. If the number of running units is greater than the number of units reserved for RB mill skipping, it is considered that the number of running units of the coal mill is greater than the number of units reserved, and RB mill skipping needs to be triggered. If the number of running units is equal to the total number of units, it is determined to be a full mill operation mode, otherwise it is a non-full mill operation mode, and the falling edge of the switch signal is delayed for t1 time; S102, the fuel load capacity of the group operation is calculated according to the number of pulverizing systems still in operation, the rated load capacity of each mill and the BTU correction coefficient; the minimum load capacity is calculated by multiplying the fuel load capacity by the minimum load ratio coefficient A of the coal mill; the reserved mill group load capacity is calculated by multiplying the number of RB mills reserved by the rated load capacity of each mill; S103, when the RB target load value is less than the pre-reserved grinding group load capacity, it is considered that the pre-reserved grinding group load capacity matches the RB target load, triggering a signal that the pre-reserved grinding group load capacity is higher than the RB target; If the RB target load is greater than the pre-reserved grinding group load capacity, and at the same time the RB target load is less than the current fuel load capacity and greater than the current minimum load capacity of the operating grinding group, it is considered that the pre-reserved grinding group load capacity is too low and does not match the set RB target load, and the third coal mill is stopped and tripped, triggering the third mill tripping interruption signal.

4. The RB mill skipping control method for a coal-fired unit adapted to full mill group operation according to claim 2 is characterized in that: In S2, the second mill skipping mill strategy includes the following two situations: S201, in the non-full mill operation mode, when the reserved mill group load capacity is higher than the RB target load, the second mill tripping is continued, otherwise it is interrupted; S202: In full-mill operation mode or when primary fan RB is actuated, the second mill tripping operation continues.

5. The RB mill skipping control method for a coal-fired unit adapted to full mill group operation according to claim 4 is characterized in that: In S3, the third mill skipping strategy includes the following two situations: S301, in the non-full mill operation mode, when the third mill in the tripping sequence is in operation, the number of running 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, all the conditions are met, a pulse signal is triggered to trip the third mill; S302: In the full-mill operation mode, when the RB target value is within the load capacity of the reserved mill group or the primary fan RB is actuated, a pulse signal is triggered to trip the third mill.

6. A coal-fired unit RB mill skipping control method adapted to full mill group operation according to claim 5, characterized in that: The specific process of S4 is as follows: When the mill skipping process will be executed according to the strategy until the third coal mill trips, the RB mill skipping interval of the third mill is designed differently; the third mill tripping interval T2 is added on the basis of the original mill skipping interval T1, the mill skipping interval in non-full mill operation mode is still T1, the mill skipping interval of the second mill in full mill operation mode is T1, and the mill skipping interval of the third mill is T2. T2 is set according to the operating conditions of each unit and system. T2 is longer than T1, delaying the tripping of the third mill to reduce disturbance to combustion.

7. A coal-fired unit RB mill skipping control method adapted to full mill group operation according to claim 6, characterized in that: In S4, the grinding intervals T1 and T2 are set respectively according to the RB type, and the forced draft fan RB, the primary fan RB and the feed water pump RB are all set with different values ​​according to the parameter operation characteristics of the unit. The main control parameter of the forced draft fan RB is the furnace negative pressure, the main control parameter of the primary fan RB is the primary air pressure, the main control parameter of the feed water pump RB supercritical unit is the superheat, and the main control parameter of the subcritical unit is changed to the drum water level.

8. The RB mill skipping control method for a coal-fired unit adapted to full mill group operation according to claim 7, characterized in that: In the S4, the third mill is set to have direct tripping protection without delay. When the first and second mills have tripped but before the third mill has tripped, the main control parameters of the tripping auxiliary machine have exceeded the dangerous adjustment limit, or the first mill has tripped, 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 triggered immediately. The judgment that the main control parameters of the tripping auxiliary machine exceed the dangerous adjustment limit increases the rising edge delay of t2s to prevent signal jumps.

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