High-cycle generator tripping optimization method for transient frequency evaluation index F

Abstract

The invention discloses a high-cycle generator tripping optimization method for a transient frequency evaluation index F, and relates to the technical field of electrical engineering. According to the high-cycle generator tripping optimization method for the transient frequency evaluation index F, a provided piecewise linearization single-machine equivalent model based on parameter aggregation and model identification is adopted, a high-cycle generator tripping basic scheme is obtained through setting, scheme optimization can be conveniently achieved by adopting a provided transient stability evaluation index, and therefore an optimization scheme is obtained, and on the basis, the high-cycle generator tripping optimization method for the transient frequency evaluation index F is achieved. The characteristic that a power plant side has abundant measurement information is considered, a high-cycle generator tripping acceleration action strategy based on a df / dt criterion is provided, the theoretical basis of df / dt acceleration is analyzed, the characteristics of active unbalance reflected by the df / dt criterion of a sending-end power grid are calculated, a setting value of an acceleration action is determined, and an acceleration action high-cycle scheme is obtained; false operation caused by frequency abrupt change due to machine end short circuit, load feedback and abnormal measurement links is considered, and frequency abrupt change locking is introduced, so that an acceleration action strategy is more perfect.

Description

Technical Field

[0001] The present invention relates to the field of electrical engineering technology, and in particular to a high frequency shear machine optimization method for a transient frequency evaluation index F. Background Art

[0002] In a grid structure where a large amount of electricity is transmitted from one grid to another grid, the concepts of a sending grid and a receiving grid are formed. In the early stages of grid development, the sending grid was mostly a structure where several units of a single power plant transmitted electricity through a step-up transformer and transmission lines. However, with the development of the grid, the sending grid has gradually evolved from a single power plant to a complex grid consisting of multiple power plants, multiple generators of different types, and a certain amount of local load.

[0003] my country's energy distribution is relatively concentrated and inversely distributed with the load. Large-scale thermal power, hydropower, and wind power energy bases far away from the load center will emerge more and more, which determines that the characteristics of large installed capacity, small load, and high output of the sending-end power grid are becoming more and more prominent. For example, the total installed capacity of the sending-end power grid in northern Shanxi reached 10,139.75MW in 2013, while the peak load was only 1,560MW, and the transmission ratio under normal operation reached 72.98%.

[0004] The Aba, Ganzi, and Ertan areas in western Sichuan are typical hydropower-based power grids, with their external transmission ratio exceeding 50% [11. A larger range of high-powered power grids include provincial power grids such as the Yunnan and Guizhou power grids, where the external transmission ratio can reach 50% of the total power generation under different operating modes.

[0005] The construction of power transmission channels at power bases often lags behind the construction of power sources. In order to reduce power congestion and make full use of existing grid facilities to improve economic benefits, the power grid operation and management departments need to close the power transmission channels to the maximum transmission capacity.

[0006] In this case, the relatively weak regional sending-end power grid is easily disconnected from the receiving-end power grid under serious faults, forming an isolated grid. The sending-end isolated grid is often accompanied by excess power, which causes the frequency to rise. In the early stage, the sending-end power grid can quickly close the regulating valve or trip the generator to ensure the safety of the power grid through the over-speed protection (OPC) of the generator when the grid is isolated [4 South]. However, for the regional high-output sending-end power grid, simply relying on the unit protection of each power plant to trip the sending-end unit can no longer meet the requirements of frequency emergency control [-91. On July 7, 2006, the southern Guiyang power grid was isolated after a fault. Due to the inappropriate setting of the delay point and exit point of the OPC action in the system, coupled with the unreasonable speed limit setting of the primary frequency regulation, the high-frequency instability of the sending-end power grid was caused, and the system continued to oscillate.

[0007] Simulation analysis of the frequency stability of the isolated grid of Yunnan Power Grid also shows that the setting value of the existing OPC is likely to cause the protection of multiple large units to operate simultaneously, resulting in a sudden drop in frequency, which will trigger low-frequency load reduction and expand the scope of the accident.

[0008] In recent years, the use of high-frequency cutting as a measure for emergency frequency control in the sending-end area has gradually been applied to engineering practice. However, these high-frequency cutting control strategies lack systematic configuration principles, and the scheme setting mostly relies on a large number of offline simulation analyses. The method of trial matching before verification is adopted, which is heavily dependent on operation and scheduling experience. The optimization of the high-frequency cutting scheme cannot be guaranteed, and the verification workload is large, the calculation speed is slow, and the configuration method is not systematic and generally applicable. Summary of the invention

[0009] The purpose of the present invention is to solve at least one of the technical problems existing in the prior art and to provide a high frequency shear machine optimization method of a transient frequency evaluation index F, which can solve the above-mentioned problem.

[0010] To achieve the above object, the present invention provides the following technical solutions: A method for optimizing high-frequency shearing machine with transient frequency evaluation index F, comprising the following steps: S1 Coordination configuration principle of emergency control measures for frequency stability at the sending end

[0011] Configuration principles of three-level safety and stability control measures

[0012] The safety and stability of the power grid operation is calculated and analyzed in accordance with the third-level stability standard of the "Guidelines for Safety and Stability of Power Systems". When the sending-end power grid is disconnected from the main grid and operates in an isolated grid, as a safety and stability control measure to solve the problem of isolated grid operation, the first consideration is to take interlocking or remote cutting measures to quickly cut off the unit and restore the system to stability;

[0013] In the case that the normal action of the interlocking cutting machine cannot meet the active power balance or the interlocking cutting machine partially or completely refuses to operate, consider taking high-frequency cutting measures. When the frequency reaches the fixed value, the generator is cut off after a certain delay, and finally the active power balance is achieved, and the system frequency is restored to an acceptable range. In addition, the coordination of the system remote interlocking cutting machine, high-frequency cutting machine and 103% overspeed limit protection functions of the thermal power unit must also be considered to avoid accidents that cause the 103% overspeed limit protection to cause a sudden drop in the power grid frequency;

[0014] S2 high frequency cutting machine configuration principle

[0015] Frequency emergency control measures mainly include interlocking shedding, high-frequency shedding, low-frequency load shedding, generator set overspeed protection and low-frequency protection. Under severe power imbalance, the coordination of these measures is particularly important. The arrangement of high-frequency shedding and the setting of delay are limited by the maximum deviation of system frequency, steady-state deviation, and protection setting value. At the same time, the selection of shedding often determines the performance of the system after the accident.

[0016] S3 high frequency cutting machine solution coordination configuration and optimization, high frequency cutting machine solution setting

[0017] Steady-state frequency calculation: The action of the high-frequency shear machine should restore the system steady-state frequency to between 50Hz and 50.5Hz. Taking a certain regional power grid as an example, considering the extreme extreme situation, after the regional power grid is isolated, if the security control fails to work correctly, the total power balance is 6686.5MW, and the remaining power accounts for 72.99% of the total power generation output. The single-machine equivalent model is used to calculate the system frequency steady-state response. According to the final value theorem, it can be obtained Figure 1 ;

[0018] Therefore, according to the equivalent parameters of a single machine, the steady-state frequency corresponding to different machine cutting amounts is obtained as follows: Figure 2 As shown in the table, it can be seen from the calculation results that the total amount of cutting in all rounds of weekly cutting should be close to 70% of the total power generation. Considering the situation of insufficient safety control cutting and the refusal of a certain round of high weekly cutting, a certain amount of cutting should be appropriately increased as a backup;

[0019] When the safety control function is correct and the machine is cut off by 3600MW, the calculation result of the machine cut off when the system frequency is restored to the allowable operating range is as follows: Figure 3 As shown, under the condition that the safety control operates correctly, the total amount of high-frequency cutting should be close to 30% of the total power generation. Considering different operating modes and the situation of cutting refusal, a certain amount of cutting should be appropriately increased as a backup;

[0020] S4S4 high frequency cutting machine solution optimization

[0021] After the basic scheme is determined, the level difference, cutting quantity and cutting selection are optimized according to the frequency stability index F;

[0022] Differential Optimization

[0023] Keep the first and last round setting values, each round delay and the amount of machine cutting unchanged, and compare the influence of equal step difference, small step difference first and then large step difference and large step difference first and then small step difference on the value. Scheme 1: adopt equal step difference, the step difference is not 0.2Hz, scheme 2: the step difference of the first three rounds is 0.15Hz, and the step difference of the last two rounds is 0.25Hz, scheme three: the first three rounds are 0.25Hz, and the last two rounds are 0.15Hz. Calculate the index F in three cases, the results are as follows Figure 7As shown in the figure, it can be concluded from the results that the three differential schemes can meet the requirements, and the differential setting of scheme 2 is more conducive to frequency stability;

[0024] Optimization of the cutting sequence of S5 unit;

[0025] S6 acceleration strategy based on df / dt criteria

[0026] For areas with large installed capacity, small load and high transmission, once an isolated grid occurs, the active power imbalance is very serious and the frequency rises very quickly. In order to ensure the adaptability of the high-frequency shedding scheme, a certain level difference and delay are usually reserved between each round, which makes the high-frequency shedding control untimely in some extreme cases. On the other hand, considering that the high-frequency shedding device is installed in the power plant, compared with the low-frequency load shedding where the actuator is installed in the 10kV or 35kV substation, the high-frequency shedding action device has rich electrical quantities (especially frequency information) for analysis and judgment. Therefore, a high-frequency shedding acceleration action scheme based on the dffdt criterion is proposed to improve the frequency dynamic response in the sending end area.

[0027] S7 frequency mutation lockout

[0028] Abnormal conditions such as short-circuit faults, load feedback, and measurement link abnormalities may cause frequency mutations and cause df / dt criterion malfunctions, so device locking should also be considered. Taking the three-phase metallic short-circuit grounding at the generator end as an example, at the moment of the fault, the generator rotor accelerates, causing the frequency to rise too fast in a short time. The simulation results are as follows: Fig.17 As shown:

[0029] from Fig.17 It can be seen that the short-term frequency change rate of the system is Fig.18 ;

[0030] If this happens during the high frequency period of the isolated grid, it may cause the acceleration action strategy based on the df / dt criterion to malfunction. Based on the above analysis and the maximum frequency change rate that the system may produce, the threshold value of the df / dt lockout is set to 5Hz / s, thus the high frequency shear acceleration action strategy considering the lockout can be obtained. Fig.19 ;

[0031] This acceleration action strategy is used to adaptively configure the high-frequency cutting machine optimization, and the high-frequency cutting machine optimization scheme using df / dt acceleration is obtained. Its control strategy table is the same as the optimization scheme.

[0032] Under the control of the calculation acceleration action strategy, the system frequency stability index can be obtained as F=0.0007, which is much smaller than the indicators of the basic solution and the optimization solution.

[0033] Preferably, S1 level 3 safety includes formulating three-level safety and stability control measures for high-frequency problems caused by isolated grid operation:

[0034] First-level safety and stability control measures: emergency interlocking cutting measures in case of serious failure

[0035] Hydropower units are given priority. When the capacity of hydropower units is insufficient, some thermal power units are cut off. In principle, each power plant retains one unit in operation. The interlocking cutting requires a faster action speed. When the monitoring line fault trips, the cutting action is immediately executed according to the local strategy table. The interlocking cutting strategy ensures that over-cutting will not occur and will not cause low-frequency problems in the regional power grid.

[0036] Second level safety and stability control measures: High frequency cutting of generator after disconnection

[0037] The main generating units are equipped with high-frequency shedding measures; after the high-frequency shedding of the generating units can be correctly operated according to the rotation and delay, the power grid will not have frequency stability problems and will not be over-cut, resulting in low-frequency load shedding of the regional power grid;

[0038] Level 3 safety and stability control measures: Coordination of thermal power unit steam turbine control and safe operation of power grid. In the grid-connected operation mode, the 103% overspeed function of the OPC of the thermal power unit is locked or the action delay is increased, and the 110% overspeed protection is retained. The first and second level measures are used to ensure that the grid frequency is stable and the OPC does not act.

[0039] The first-level control measures and the second-level control measures should be independent of each other and effective independently. The second-level control measures can serve as a supplement or backup for the first-level control measures.

[0040] Preferably, the S2 configuration principle includes the coordination principle of frequency emergency control measures

[0041] High-frequency generator cutting measures should be set up to focus on the high-frequency situations that may occur after the provincial or regional power grid is disconnected from the main grid;

[0042] The system frequency emergency control first considers taking interlocking or remote cutting measures to quickly cut off the unit to restore the system stability; when the normal action of interlocking cutting cannot meet the active power balance or the interlocking cutting fails to work partially or completely, consider taking high-frequency cutting measures. When the frequency reaches a fixed value, the generator is cut off after a certain delay, and finally the active power balance is achieved, and the system frequency is restored to an acceptable range;

[0043] The high frequency cutter can be used as a backup for the chain cutter to maintain the system frequency stability. The high frequency cutter and the chain cutter or remote cutter can be configured in a repeated manner.

[0044] The coordination between the high-frequency shear and the turbine overspeed protection should be considered. The configuration of the high-frequency shear should effectively prevent the system frequency from rising to the action value of the OPC protection; avoid the OPC preemptive action causing the generator to have repeated "oscillations" and affect the safety of the unit and the stability of the system; in order to avoid short-term frequency increases, the OPC should also be set with an appropriate delay to prevent frequent protection actions;

[0045] High-frequency cutting machine needs to be coordinated with low-frequency load reduction measures to avoid the high-frequency cutting machine causing the low-frequency load reduction device to operate, thus reducing the control cost;

[0046] High-frequency shedding measures need to be coordinated with oscillation disconnection measures; high-frequency shedding action has a great impact on the action time of the oscillation disconnection device, and in some cases may even affect the action behavior of the disconnection device, delaying the timing of the disconnection device action, especially for power plants that are close to the oscillation center, whose shedding action has a great impact on the action of the tie-line oscillation disconnection device; in order to ensure the correct action of the disconnection device, the high-frequency shedding action frequency of power plants near the oscillation center should be appropriately increased or the action time should be extended on the premise of ensuring the safety of the unit;

[0047] High frequency cutting measures should be regularly checked and adjusted according to the grid structure and maximum exchange power changes;

[0048] Since the system load level, external transmission level and unit startup mode all have an impact on the high-frequency shedding effect, the high-frequency shedding plan should try to adapt to most external transmission scales and operating modes: it should also try to minimize the large changes in the power flow in the entire network caused by high-frequency shedding measures;

[0049] Principles of round configuration, frequency setting and delay setting

[0050] When configuring a high-frequency cutting machine solution, the rounds should be arranged reasonably. Too many rounds will affect the control effect due to action delay, and too few rounds will easily cause overcutting. Generally, 3 to 5 rounds are appropriate;

[0051] Since the normal operating frequency of the unit is between 49.5 and 50.5 Hz, the threshold value of the first round of high frequency cutting should be higher than 50.5 Hz;

[0052] High frequency cutting machine measures should be able to effectively prevent the frequency from rising to the OPC action value (generally set to 3090r / m, i.e. 51.5Hz). Considering the sampling accuracy and calculation error of the device, it is generally calculated as not exceeding 51.3Hz;

[0053] The high-frequency shedding measures should not cause the low-frequency load shedding device to operate. According to the current low-frequency first round configuration of the power grid, the system frequency is not less than 49Hz. Considering the sampling accuracy and calculation error of the device, it is generally calculated as not less than 49.25Hz;

[0054] Generally, the abnormal operation time allowed for large units with a frequency higher than 51.5Hz shall not exceed 30 minutes in total, and each time shall not exceed 30 seconds. Considering the frequency rise and fall time and a certain slope, the total delay of the units cut in the frequency band of 50.5-51.5Hz shall generally not exceed 10 seconds;

[0055] In order to avoid malfunction of the device caused by sudden frequency changes during the measurement process, the delay of each round of the high frequency cutting machine should generally be greater than 200ms;

[0056] Principles of cutting machine selection

[0057] Priority should be given to cutting off hydropower units, including pumped storage units in power generation status;

[0058] For power grids with wind turbines, the output of wind turbines is volatile due to the intermittent nature of wind energy. At the same time, the presence of double-fed asynchronous wind turbines and permanent magnet synchronous wind turbines reduces the inertia of the system and does not participate in the primary and secondary frequency regulation of the system. When the system is isolated, the proportion of wind turbines is greater, which is not conducive to the stability of the system frequency. Therefore, wind turbines are given priority in high-frequency shedding rounds.

[0059] Large-capacity units are removed first;

[0060] When the high-frequency machine cutting plan for the entire network is formulated, one unit of the power plant is retained to avoid power outages in the entire power plant during high-frequency operation.

[0061] Preferably, the S3 optimization scheme specifically includes the first and last rounds of adjustment

[0062] Assuming that the safety control operates correctly and that all the machine cut-off quantities are first-round operations, the single-machine equivalent model is used to calculate the system frequency response under different first-round starting frequencies. The results are as follows: Figure 4 As shown;

[0063] It can be concluded that setting the start value to 50.6 Hz is beneficial to reducing the maximum system frequency and also helps to reduce the fluctuation of system frequency during dynamic process;

[0064] The setting of the last wheel setting value of the high-frequency shear machine should be based on the maximum frequency allowed by the system as the upper limit. According to the configuration principle, the high-frequency shear machine should effectively avoid OPC action. The OPC setting value of the steam turbine of the sending-end power grid is 51.5Hz, with a delay of 3s. Therefore, the final wheel setting value is initially set to 51.4Hz.

[0065] Set the rounds, level differences and cutting amount for each round

[0066] The number of high-frequency cutting rounds cannot be too many or too few. Too many rounds will result in too few cuttings in each round and it will be impossible to configure suitable units. At the same time, due to errors in frequency measurement, too small frequency intervals are likely to cause over-level actions. Therefore, according to the configuration principle, considering selectivity and reliability, a total of 5 rounds are set. Since the cutting capacity of the sending-end power grid should be configured to be 3000MW when the security control is correctly operated, a certain capacity is considered as a backup, and a total of 3900MW of cutting capacity is configured. The first three rounds set a cutting capacity of 3000MW, which is evenly distributed in each round, corresponding to the cutting capacity that should be configured when the security control is correctly operated. The cutting capacity of each round is 1000MW, and the last two rounds are configured with a total of 900MW, the fourth round is 500MW, and the fifth round is 400MW. Assuming that the frequency intervals between each round are equal, it can be obtained from the setting value of 50.6Hz in the first round and 51.4Hz in the last round that each round The step difference is 0.2Hz. On this basis, considering the different cutting amounts of security control chain cutting with different interconnection line currents, according to the configuration principle, the security control cutting amount is configured in the high-frequency cutting amount. Through repeated configuration, the high-frequency cutting is used as a backup for security control, which ensures the frequency stability of the sending-end power grid when the security control refuses to operate; considering that the security control cutting needs to be as fast as possible, the security control cutting amount of 3600MW is evenly configured in the first three rounds of high-frequency cutting; according to the configuration principle, the delay of each round is not less than 0.2s. For the situation of serious imbalance of active power at the sending end, controlling the active power imbalance as soon as possible is conducive to frequency stability. Therefore, the delay of the first three rounds of cutting takes the minimum value of 0.2s. The frequency will not drop rapidly after the cutting action. Due to the inertia of the unit, the frequency response takes a period of time. Therefore, the delay is appropriately increased in the last two rounds, 1.6s for the fourth round and 1s for the fifth round; thus, the basic scheme of high-frequency cutting is obtained, such as Figure 5 ;

[0067] The transient frequency stability evaluation index F of the high-frequency cutting basic scheme is calculated by using the equivalent model. It is obtained that: the high-frequency cutting three-round action, F = 0.0478 meets the frequency response requirements. The actual high-frequency cutting scheme should also determine the units to be cut in each round. According to the cutting amount of each round, considering the unit operation management requirements, the feasibility of high-frequency cutting and avoiding the situation of full shutdown of the whole plant, the units to be cut in each round are configured, and the high-frequency cutting basic scheme is obtained as follows Figure 6 shown.

[0068] Preferably, S5 includes that the actual sending-end power grid has 1099.75MW of wind power installed capacity, and the installed capacity has grown rapidly in recent years. According to the principle of selecting high-frequency cutting units, in the case of isolated grid at the sending end, the wind turbines are kept running, which is not conducive to the stability of the frequency of the sending-end power grid. Therefore, optimization is performed in the cutting sequence, and wind turbines are preferentially configured in the first round of high-frequency cutting. At present, the actual generating capacity of the wind turbines in the sending-end power grid is only 556.5MW, not more than 1000MW. Therefore, the running wind turbines are concentrated in the first round of the high-frequency strategy. When the frequency and cutting amount of each round are approximately the same, the values ​​of index F and index E of the basic scheme (prioritized cutting of thermal power units and retaining wind turbines) and the optimized scheme (prioritized cutting of wind power) can be compared to obtain the following: Figure 8 Results shown;

[0069] From the comparison of the results in the figure, it can be seen that the priority removal of wind turbines not only has a better frequency dynamic response, but also can obtain better economy; from a theoretical analysis, since the inertia of wind turbines does not act on the system frequency response, it is an "implicit" inertia. Therefore, compared with the priority removal of thermal power, the priority removal of wind power makes the inertia of the isolated grid system larger, which is conducive to frequency emergency control and can obtain better frequency response; the indicators are consistent with the theoretical analysis, and the high frequency cutting optimization scheme is obtained by combining S1, S2, and S3. Fig. 9 shown.

[0070] Preferably, S6 includes an initial frequency response characteristic analysis

[0071] The single-machine equivalent model is used to analyze the frequency response characteristics at the initial moment. The rotor motion equation of the normalized generator is linearized at the operating point to obtain Fig.10 ;

[0072] Among them, △Pm(t) is the equivalent machine mechanical power increment, △Pe(t) is the equivalent machine electromagnetic power increment, △Pe(d) is the unbalanced power increment acting on the generator rotor, H is the equivalent machine inertia time constant, D is the equivalent load damping factor representing the load frequency regulation effect of the equivalent system, f* is the frequency per unit value of the equivalent system. At t=0+, when the disturbance occurs, the frequency of the system has not changed yet, and the load frequency regulation effect has not yet taken effect. The unbalanced power can be expressed as Fig.11 ;

[0073] The equation shows that at the initial moment of disturbance, the rate of change of the power system frequency is proportional to the active unbalance, and the proportional factor is the system inertia time constant; by setting the appropriate df / dt threshold, the program can determine the size of the active unbalance by itself, so as to implement the corresponding acceleration strategy for the high-frequency shedding scheme; from the equivalent calculation results, it can be seen that the total inertia time constant of the system based on the product is = 2H = 63.982s; therefore, in the case of isolated grid, the maximum value of the frequency change rate generated by the acceleration power is: Fig.12

[0074] The threshold value of the f / dt acceleration strategy is set. From the power flow data of the sending-end grid, it can be seen that if the safety control fails to operate correctly after the grid is isolated, the maximum active unbalance that may occur in the system is △PD=6686MW, which is converted to the per-unit value △PD*=6.686 based on the system benchmark capacity Sn=1000MVA;

[0075] Converting the per-unit value of the frequency change rate to a nominal value yields: Fig.13 ;

[0076] When the safety control is correctly operated and the machine is cut to 3600MW, assuming that the inertia of the sending end system is reduced proportionally, it is: Fig.14 ;

[0077] The same calculation can be obtained. In this case, Fig.15 ;

[0078] Since the large external transmission mode is selected for calculation, when the power grid operates in other modes, the frequency change rate should be less than 3.862Hz / s when the same fault occurs; therefore, 3Hz / s can be selected as the threshold. When the frequency change rate is greater than the threshold, the high-frequency shedding rounds are accelerated. As can be seen from Section 4.2.2, the correct safety control action corresponds to the first three rounds of high-frequency shedding. Therefore, based on the local quantity measurement of the power plant, the frequency change rate is judged on the premise of satisfying the first round of high-frequency shedding. If the frequency change rate is higher than 3Hz / s, the second and third rounds are accelerated. If the frequency is higher than 2Hz / s and lower than 3Hz / s, the second round is accelerated. Thus, the high-frequency shedding acceleration action strategy based on the df / dt criterion is obtained as follows Fig.16 .

[0079] Compared with the prior art, the present invention has the following beneficial effects:

[0080] (1) The high-frequency shedding optimization method of the transient frequency assessment index F. This scheme introduces the configuration principles of the three lines of defense for frequency stability at the sending end and the configuration and optimization methods of the high-frequency shedding as the main control measure. Guided by the high-frequency shedding configuration principle, the proposed piecewise linearized single-machine equivalent model based on parameter aggregation and model identification is adopted to adjust the basic high-frequency shedding scheme. The proposed transient stability assessment index can be used to easily optimize the scheme, thereby obtaining the optimized scheme. On this basis, considering the characteristics of rich measurement information on the power plant side, a high-frequency shedding acceleration action strategy based on the df / dt criterion is proposed. The theoretical basis of df / dt acceleration is analyzed, and the characteristics of the active unbalance reflected by the df / dt criterion of the sending-end power grid are calculated. The setting value of the acceleration action is determined, and the accelerated action high-frequency scheme is obtained. At the same time, the false operation caused by the frequency mutation caused by the short circuit at the machine end, load feedback, and abnormal measurement link is considered, and the frequency mutation lockout is introduced to make the acceleration action strategy more perfect. BRIEF DESCRIPTION OF THE DRAWINGS

[0081] The present invention is further described below in conjunction with the accompanying drawings and embodiments:

[0082] Figure 1 It is a schematic diagram of the public structure of the present invention;

[0083] Figure 2 It is a schematic diagram of the present invention;

[0084] Figure 3 It is a schematic diagram of the present invention;

[0085] Figure 4 It is a schematic diagram of the present invention;

[0086] Figure 5 It is a schematic diagram of the present invention;

[0087] Figure 6 It is a schematic diagram of the present invention;

[0088] Figure 7 It is a schematic diagram of the present invention;

[0089] Figure 8 It is a schematic diagram of the present invention;

[0090] Fig. 9 It is a schematic diagram of the present invention;

[0091] Fig.10 It is a schematic diagram of the formula of the present invention;

[0092] Fig.11 It is a schematic diagram of the formula of the present invention;

[0093] Fig.12 It is a schematic diagram of the formula of the present invention;

[0094] Fig.13 It is a schematic diagram of the formula of the present invention;

[0095] Fig.14 It is a schematic diagram of the formula of the present invention;

[0096] Fig.15 It is a schematic diagram of the formula of the present invention;

[0097] Fig.16 It is a schematic diagram of the process of the present invention;

[0098] Fig.17 It is a schematic diagram showing a curve of the present invention;

[0099] Fig.18 It is a schematic diagram of the formula of the present invention;

[0100] Fig.19 Schematic diagram of the algorithm flow of the present invention. DETAILED DESCRIPTION

[0101] This section will describe in detail the specific embodiments of the present invention. The preferred embodiments of the present invention are shown in the accompanying drawings. The purpose of the accompanying drawings is to supplement the description of the text part of the specification with graphics, so that people can intuitively and vividly understand each technical feature and the overall technical solution of the present invention, but it cannot be understood as a limitation on the scope of protection of the present invention.

[0102] In the description of the present invention, it should be understood that descriptions involving orientations, such as up, down, front, back, left, right, etc., and orientations or positional relationships indicated are based on the orientations or positional relationships shown in the accompanying drawings, and are only for the convenience of describing the present invention and simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, be constructed and operated in a specific orientation, and therefore should not be understood as a limitation on the present invention.

[0103] In the description of the present invention, "greater than", "less than", "exceed" etc. are understood as not including the number itself, and "above", "below", "within" etc. are understood as including the number itself. If there is a description of "first" or "second", it is only used for the purpose of distinguishing technical features, and cannot be understood as indicating or implying relative importance or implicitly indicating the number of the indicated technical features or implicitly indicating the order of the indicated technical features.

[0104] In the description of the present invention, unless otherwise clearly defined, terms such as setting, installing, connecting, etc. should be understood in a broad sense, and technicians in the relevant technical field can reasonably determine the specific meanings of the above terms in the present invention based on the specific content of the technical solution.

[0105] The present invention provides a technical solution: a high frequency shear machine optimization method for transient frequency evaluation index F, comprising the following steps: S1 sending end frequency stability emergency control measure coordination configuration principle

[0106] Configuration principles of three-level safety and stability control measures

[0107] The safety and stability of the power grid operation is calculated and analyzed in accordance with the third-level stability standard of the "Guidelines for Safety and Stability of Power Systems". When the sending-end power grid is disconnected from the main grid and operates in an isolated grid, as a safety and stability control measure to solve the problem of isolated grid operation, the first consideration is to take interlocking or remote cutting measures to quickly cut off the unit and restore the system to stability;

[0108] In the case that the normal action of the interlocking cutting machine cannot meet the active power balance or the interlocking cutting machine partially or completely refuses to operate, consider taking high-frequency cutting measures. When the frequency reaches the fixed value, the generator is cut off after a certain delay, and finally the active power balance is achieved, and the system frequency is restored to an acceptable range. In addition, the coordination of the system remote interlocking cutting machine, high-frequency cutting machine and 103% overspeed limit protection functions of the thermal power unit must also be considered to avoid accidents that cause the 103% overspeed limit protection to cause a sudden drop in the power grid frequency.

[0109] In response to the high-frequency problems caused by isolated grid operation, three-level safety and stability control measures are formulated:

[0110] First-level safety and stability control measures: emergency interlocking cutting measures in case of serious failure

[0111] Hydropower units are preferentially cut off. When the capacity of hydropower units is insufficient, some thermal power units are cut off. In principle, each power plant keeps one unit in operation. Chain cutting requires a faster action speed. When the monitoring line fault trips, the cutting action is immediately executed according to the local strategy table. The chain cutting strategy ensures that over-cutting will not occur and will not cause low-frequency problems in the regional power grid.

[0112] Second level safety and stability control measures: High frequency cutting of generator after disconnection

[0113] The main generating sets are equipped with high-cycle shedding measures; after the high-cycle shedding of the generating sets can operate correctly according to the turns and delays, the power grid will not have frequency stability problems and will not over-cut, causing low-frequency load shedding action in the regional power grid.

[0114] The third level of safety and stability control measures: Coordination of steam turbine control of thermal power units with safe operation of power grid. In the grid-connected operation mode, the 103% overspeed function of the OPC of the thermal power unit is locked or the action delay is increased, and the 110% overspeed protection is retained. The first and second level measures are used as much as possible to ensure that the grid frequency is stable and the OPC does not operate.

[0115] The first-level control measures and the second-level control measures should be independent of each other and effective independently. The second-level control measures can serve as a supplement or backup for the first-level control measures.

[0116] S2 high frequency cutting machine configuration principle

[0117] The measures for frequency emergency control mainly include interlocking machine cutting, high-frequency machine cutting, low-frequency load reduction, generator set overspeed protection and low-frequency protection. Under severe power imbalance, the coordination of these measures is particularly important. The arrangement of high-frequency machine cutting rounds and the setting of delays are subject to the maximum deviation of system frequency, steady-state deviation, and protection setting value. At the same time, the choice of machine cutting often determines the performance of the system after the accident.

[0118] Principles of coordination and cooperation for frequency emergency control measures

[0119] High-frequency generator cutting measures should be set up to focus on the high-frequency situations that may occur after the provincial or regional power grid is disconnected from the main grid;

[0120] System frequency emergency control first considers taking interlocking or remote cutting measures to quickly cut off the unit and restore the system to stability. In the case that the normal action of interlocking cutting cannot meet the active power balance or the interlocking cutting partially or completely refuses to operate, consider taking high-frequency cutting measures. When the frequency reaches a fixed value, the generator is cut off after a certain delay, and finally the active power balance is achieved, and the system frequency is restored to an acceptable range;

[0121] The high frequency cutter can be used as a backup for the chain cutter to maintain the system frequency stability. The high frequency cutter and the chain cutter or remote cutter can be configured in a repeated manner.

[0122] The coordination between the high-frequency shear and the turbine overspeed protection should be considered. The configuration of the high-frequency shear should effectively prevent the system frequency from rising to the action value of the OPC protection. Avoid the OPC preemptive action causing the generator to repeatedly "oscillate", affecting the safety of the unit and the stability of the system. In order to avoid short-term frequency increases, the OPC should also set an appropriate delay to prevent frequent protection actions;

[0123] High-frequency cutting machine needs to be coordinated with low-frequency load reduction measures to avoid the high-frequency cutting machine causing the low-frequency load reduction device to operate, thus reducing the control cost;

[0124] High-frequency cutting measures need to be coordinated with oscillation disconnection measures. High-frequency cutting action has a great impact on the action time of the oscillation disconnection device. In some cases, it may even affect the action behavior of the disconnection device and delay the action timing of the disconnection device. In particular, the cutting action of the power plant that is close to the oscillation center has a great impact on the action of the interconnection line oscillation disconnection device. In order to ensure the correct action of the disconnection device, the high-frequency cutting action frequency of the power plant near the oscillation center should be appropriately increased or the action time should be extended under the premise of ensuring the safety of the unit;

[0125] High-frequency cutting measures should be regularly checked and adjusted as necessary according to changes in the grid structure and maximum exchange power.

[0126] Since the system load level, transmission level and unit startup mode all have an impact on the high-frequency shedding effect, the high-frequency shedding plan should try to adapt to most transmission scales and operating modes: it should also try to minimize the large changes in the power flow in the entire network caused by high-frequency shedding measures.

[0127] Principles of round configuration, frequency setting and delay setting

[0128] When configuring a high-frequency cutting machine solution, the rounds should be arranged reasonably. Too many rounds will affect the control effect due to action delay, and too few rounds will easily cause overcutting. Generally, 3 to 5 rounds are appropriate;

[0129] Since the normal operating frequency of the unit is between 49.5 and 50.5 Hz, the threshold value of the first round of high frequency cutting should be higher than 50.5 Hz;

[0130] High frequency cutting machine measures should be able to effectively prevent the frequency from rising to the OPC action value (generally set to 3090r / m, i.e. 51.5Hz). Considering the sampling accuracy and calculation error of the device, it is generally calculated as not exceeding 51.3Hz;

[0131] The high-frequency shedding measures should not cause the low-frequency load shedding device to operate. According to the current low-frequency first round configuration of the power grid, the system frequency is not less than 49Hz. Considering the sampling accuracy and calculation error of the device, it is generally calculated as not less than 49.25Hz;

[0132] Generally, the abnormal operation time allowed for large units with a frequency higher than 51.5Hz shall not exceed 30 minutes in total, and each time shall not exceed 30 seconds. Considering the frequency rise and fall time and a certain slope, the total delay of the units cut in the frequency band of 50.5-51.5Hz shall generally not exceed 10 seconds;

[0133] In order to avoid malfunction of the device caused by sudden frequency changes during the measurement process, the delay of each round of the high-frequency cutting machine should generally be greater than 200ms.

[0134] Principles of cutting machine selection

[0135] Priority should be given to cutting off hydropower units, including pumped storage units in power generation status;

[0136] For power grids with wind turbines, the output of wind turbines is volatile due to the intermittent nature of wind energy. At the same time, the presence of double-fed asynchronous wind turbines and permanent magnet synchronous wind turbines reduces the inertia of the system and does not participate in the primary and secondary frequency regulation of the system. When the system is isolated, the proportion of wind turbines is greater, which is not conducive to the stability of the system frequency. Therefore, wind turbines are given priority in high-frequency shedding rounds.

[0137] Large-capacity units are removed first;

[0138] When the high-frequency machine cutting plan for the entire network is formulated, one unit of the power plant is retained to avoid power outages in the entire power plant during high-frequency operation.

[0139] S3 high frequency cutting machine solution coordination configuration and optimization

[0140] High frequency cutting machine program setting

[0141] Steady-state frequency calculation: The action of the high-frequency shear machine should restore the system steady-state frequency to between 50Hz and 50.5Hz. Taking a certain regional power grid as an example, considering the extreme extreme situation, after the regional power grid is isolated, if the security control fails to work correctly, the total power balance is 6686.5MW, and the remaining power accounts for 72.99% of the total power generation output. The single-machine equivalent model is used to calculate the system frequency steady-state response. According to the final value theorem, it can be obtained Figure 1 ;

[0142] Therefore, according to the equivalent parameters of a single machine, the steady-state frequency corresponding to different machine cutting amounts is obtained as follows: Figure 2 As shown in the table, it can be seen from the calculation results that the total amount of cutting in all rounds of weekly cutting should be close to 70% of the total power generation. Considering the situation of insufficient safety control cutting and the refusal of a certain round of high weekly cutting, a certain amount of cutting should be appropriately increased as a backup;

[0143] When the safety control function is correct and the machine is cut off by 3600MW, the calculation result of the machine cut off when the system frequency is restored to the allowable operating range is as follows: Figure 3 As shown, when the safety control is in correct action, the total amount of high-frequency cutting should be close to 30% of the total power generation. Considering different operating modes and the situation of cutting failure, a certain amount of cutting should be appropriately increased as a backup.

[0144] First and last round adjustment

[0145] Assuming that the safety control operates correctly and that all the machine cut-off quantities are first-round operations, the single-machine equivalent model is used to calculate the system frequency response under different first-round starting frequencies. The results are as follows: Figure 4 As shown;

[0146] It can be concluded that setting the start value to 50.6 Hz is beneficial to reducing the maximum system frequency and also helps to reduce the fluctuation of system frequency during dynamic process;

[0147] The setting of the last wheel setting value of the high-frequency shear machine should be based on the upper limit of the maximum frequency allowed by the system. According to the configuration principle, the high-frequency shear machine should effectively avoid OPC action. The OPC setting value of the steam turbine of the sending-end power grid is 51.5Hz, with a delay of 3s. Therefore, the initial setting value of the last wheel is 51.4Hz.

[0148] Set the rounds, level differences and cutting amount for each round

[0149] The number of high-frequency shedding rounds cannot be too many or too few. Too many rounds will result in too few sheddings in each round and inability to configure appropriate units. At the same time, due to errors in frequency measurement, a small frequency interval is prone to overstepping. Therefore, according to the configuration principle, considering selectivity and reliability, a total of 5 rounds are set. Since the shedding amount that the sending-end power grid should configure when the security control is correctly operated is 3000MW, considering a certain capacity as a backup, a total of 3900WM of shedding is configured. The first three rounds are set with a shedding amount of 3000MW, which is evenly distributed in each round, corresponding to the shedding amount that should be configured when the security control is correctly operated. The amount of cutting in each round is 1000MW, 900MW in the last two rounds, 500MW in the fourth round, and 400MW in the fifth round. Assuming that the frequency intervals between each round are equal, it can be obtained from the setting value of 50.6Hz in the first round and 51.4Hz in the last round that the step difference of each round is 0.2Hz. On this basis, considering that the amount of cutting varies with the flow of the tie line due to the different safety control chain cutting, according to the configuration principle, the safety control cutting amount is configured in the high-frequency cutting amount. Through repeated configuration, the high-frequency cutting is used as a backup for safety control, which ensures the frequency stability of the power grid at the sending end when the safety control refuses to operate. Considering that the safety control cutting needs to be as fast as possible, the safety control cutting amount of 3600MW is evenly configured in the first three rounds of high-frequency cutting. According to the configuration principle, the delay of each round is not less than 0.2s. For the case of serious imbalance of active power at the sending end, controlling the active power imbalance as early as possible is conducive to frequency stability. Therefore, the delay of the first three rounds of shedding takes the minimum value of 0.2s. The frequency will not drop rapidly after the shedding action. Due to the inertia of the unit, the frequency response takes a period of time. Therefore, the delay of the last two rounds is appropriately increased, 1.6s for the fourth round and 1s for the fifth round. The basic scheme of high-frequency shedding is obtained, such as Figure 5 ;

[0150] The transient frequency stability evaluation index F of the high-frequency cutting basic scheme is calculated by using the equivalent model. It is obtained that: the high-frequency cutting three-round action, F = 0.0478 meets the frequency response requirements. The actual high-frequency cutting scheme should also determine the units to be cut in each round. According to the cutting amount of each round, considering the unit operation management requirements, the feasibility of high-frequency cutting and avoiding the situation of full shutdown of the whole plant, the units to be cut in each round are configured, and the high-frequency cutting basic scheme is obtained as follows Figure 6 shown.

[0151] S4 high frequency cutting machine solution optimization

[0152] After the basic plan is determined, the level difference, cutting quantity and cutting selection are optimized according to the frequency stability index F.

[0153] Differential Optimization

[0154] Keep the first and last round setting values, each round delay and the amount of machine cutting unchanged, and compare the influence of equal step difference, small step difference first and then large step difference and large step difference first and then small step difference on the value. Scheme 1: adopt equal step difference, the step difference is not 0.2Hz, scheme 2: the step difference of the first three rounds is 0.15Hz, and the step difference of the last two rounds is 0.25Hz, scheme three: the first three rounds are 0.25Hz, and the last two rounds are 0.15Hz. Calculate the index F in three cases, the results are as follows Figure 7 As shown, it can be concluded from the results that the three step difference schemes can meet the requirements, and the step difference setting of scheme 2 is more conducive to frequency stability.

[0155] Optimization of the cutting sequence of S5 unit

[0156] The actual sending-end power grid has 1099.75MW of wind power installed, and the installed capacity has grown rapidly in recent years. According to the principle of selecting high-frequency cutting units, in the case of isolated grid at the sending end, the wind turbines remain in operation, which is not conducive to the stability of the sending-end power grid frequency. Therefore, optimization is performed in the cutting sequence, and wind turbines are given priority in the first round of high-frequency cutting. At present, the actual generating capacity of wind turbines in the sending-end power grid is only 556.5MW, not more than 1000MW. Therefore, the operating wind turbines are concentrated in the first round of the high-frequency strategy. When the frequency and cutting amount of each round are approximately the same, the values ​​of index F and index E of the basic scheme (prioritized cutting of thermal power units and retaining wind turbines) and the optimized scheme (prioritized cutting of wind power) can be compared to obtain the following: Figure 8 Results shown.

[0157] From the comparison of the results in the figure, it can be seen that the priority removal of wind turbines not only has a better frequency dynamic response, but also can obtain better economy. From a theoretical analysis, since the inertia of wind turbines does not act on the system frequency response, it is an "implicit" inertia. Therefore, compared with the priority removal of thermal power, the priority removal of wind power makes the inertia of the isolated grid system larger, which is conducive to frequency emergency control and can obtain a better frequency response. The indicators are consistent with the theoretical analysis. Combining S1, S2, and S3, the high-frequency machine removal optimization scheme is obtained as follows Fig. 9 shown.

[0158] S6 acceleration strategy based on df / dt criteria

[0159] For areas with large installed capacity, small load and high transmission, once an isolated grid occurs, the active power imbalance is very serious and the frequency rises very rapidly. In order to ensure the adaptability of the high-frequency shedding scheme, a certain step difference and delay are usually required between each round, which makes the high-frequency shedding control untimely in some extreme cases. On the other hand, considering that the high-frequency shedding device is installed in the power plant, compared with the low-frequency load reduction in which the actuator is installed in the 10kV or 35kV substation, the high-frequency shedding action device has rich electrical quantities (especially frequency information) for analysis and judgment. Therefore, a high-frequency shedding acceleration action scheme based on the dffdt criterion is proposed to improve the dynamic response of the frequency in the sending end area.

[0160] Initial frequency response characteristics analysis

[0161] The single-machine equivalent model is used to analyze the frequency response characteristics at the initial moment. The rotor motion equation of the normalized generator is linearized at the operating point to obtain Fig.10 ;

[0162] Among them, △Pm(t) is the equivalent machine mechanical power increment, △Pe(t) is the equivalent machine electromagnetic power increment, △Pe(d) is the unbalanced power increment acting on the generator rotor, H is the equivalent machine inertia time constant, D is the equivalent load damping factor representing the load frequency regulation effect of the equivalent system, f* is the frequency per unit value of the equivalent system. At t=0+, when the disturbance occurs, the frequency of the system has not changed yet, and the load frequency regulation effect has not yet taken effect. The unbalanced power can be expressed as Fig.11 ;

[0163] The equation shows that at the initial moment of the disturbance, the rate of change of the power system frequency is proportional to the active unbalance, and the proportional factor is the system inertia time constant. By setting the appropriate df / dt threshold, the program can determine the size of the active unbalance by itself, so as to implement the corresponding acceleration strategy for the high-frequency shedding scheme; from the equivalent calculation results, it can be seen that the total inertia time constant of the system based on the product is = 2H = 63.982s. Therefore, in the case of an isolated grid, the maximum frequency change rate of the system due to the acceleration power is: Fig.12

[0164] The threshold value of the f / dt acceleration strategy is set. From the power flow data of the sending-end power grid, it can be seen that if the safety control fails to operate correctly after the grid is isolated, the maximum active unbalance that may occur in the system is △PD=6686MW, which is converted into a per-unit value △PD*=6.686 based on the system benchmark capacity Sn=1000MVA.

[0165] Converting the per-unit value of the frequency change rate to a nominal value yields: Fig.13 ;

[0166] When the safety control is correctly operated and the machine is cut to 3600MW, assuming that the inertia of the sending end system is reduced proportionally, it is: Fig.14 ;

[0167] The same calculation can be obtained. In this case, Fig.15 .

[0168] Since the large external transmission mode is selected for calculation, when the power grid operates in other modes, the frequency change rate should be less than 3.862Hz / s when the same fault occurs. Therefore, 3Hz / s can be selected as the threshold. When the frequency change rate is greater than the threshold, the high-frequency machine cutting rounds are accelerated. As can be seen from Section 4.2.2, the correct action of the safety control corresponds to the first three rounds of high-frequency machine cutting. Therefore, based on the local measurement of the power plant, the frequency change rate is judged on the premise that the first round of high-frequency machine cutting is satisfied. If the frequency change rate is higher than 3Hz / s, the second and third rounds are accelerated. If the frequency is higher than 2Hz / s and lower than 3Hz / s, the second round is accelerated. The high-frequency machine cutting acceleration action strategy based on the df / dt criterion is obtained as follows Fig.16 .

[0169] S7 frequency mutation lockout

[0170] Abnormal conditions such as short-circuit faults, load feedback, and measurement link abnormalities may cause frequency mutations and cause df / dt criterion malfunctions, so device locking should also be considered. Taking the three-phase metallic short-circuit grounding at the generator end as an example, at the moment of the fault, the generator rotor accelerates, causing the frequency to rise too fast in a short time. The simulation results are as follows: Fig.17 As shown:

[0171] from Fig.17 It can be seen that the short-term frequency change rate of the system is Fig.18 ;

[0172] If this happens during the high frequency period of the isolated grid, it may cause the acceleration action strategy based on the df / dt criterion to malfunction. Based on the above analysis and the maximum frequency change rate that the system may produce, the threshold value of the df / dt lockout is set to 5Hz / s, thus the high frequency shear acceleration action strategy considering the lockout can be obtained. Fig.19 ;

[0173] This acceleration action strategy is used to adaptively configure the high-frequency cutting machine optimization, and the high-frequency cutting machine optimization scheme using df / dt acceleration is obtained. Its control strategy table is the same as the optimization scheme.

[0174] Under the control of the calculation acceleration action strategy, the system frequency stability index can be obtained as F=0.0007, which is much smaller than the indicators of the basic solution and the optimization solution.

[0175] Furthermore, this scheme introduces the configuration principles of the three lines of defense for frequency stability at the sending end, as well as the configuration and optimization methods of high-frequency shedding as the main control measure. Guided by the high-frequency shedding configuration principles, the proposed piecewise linearized single-machine equivalent model based on parameter aggregation and model identification is adopted to adjust the basic high-frequency shedding scheme. The proposed transient stability evaluation index can be used to conveniently optimize the scheme, thereby obtaining the optimized scheme. On this basis, considering the characteristics of rich measurement information on the power plant side, a high-frequency shedding acceleration action strategy based on the df / dt criterion is proposed, the theoretical basis of df / dt acceleration is analyzed, the characteristics of the active unbalance reflected by the df / dt criterion of the sending-end power grid are calculated, the setting value of the acceleration action is determined, and the accelerated action high-frequency scheme is obtained. At the same time, the false operation caused by the frequency mutation caused by the short circuit at the machine end, load feedback, and abnormal measurement link is considered, and the frequency mutation lockout is introduced to make the acceleration action strategy more perfect.

[0176] The embodiments of the present invention are described in detail above with reference to the accompanying drawings. However, the present invention is not limited to the above embodiments. Various changes can be made within the knowledge scope of ordinary technicians in the technical field without departing from the purpose of the present invention.

Claims

1. A high frequency shear machine optimization method for transient frequency evaluation index F, characterized by: The following steps are included: S1 sending-end frequency stability emergency control measures coordination configuration principle Configuration principles of three-level safety and stability control measures The safety and stability of the power grid operation is calculated and analyzed in accordance with the third-level stability standard of the "Guidelines for Safety and Stability of Power Systems". When the sending-end power grid is disconnected from the main grid and operates in an isolated grid, as a safety and stability control measure to solve the problem of isolated grid operation, the first consideration is to take interlocking or remote cutting measures to quickly cut off the unit and restore the system to stability; In the case that the normal action of the interlocking cutting machine cannot meet the active power balance or the interlocking cutting machine partially or completely refuses to operate, consider taking high-frequency cutting measures. When the frequency reaches the fixed value, the generator is cut off after a certain delay, and finally the active power balance is achieved, and the system frequency is restored to an acceptable range. In addition, the coordination of the system remote interlocking cutting machine, high-frequency cutting machine and 103% overspeed limit protection functions of the thermal power unit must also be considered to avoid accidents that cause the 103% overspeed limit protection to cause a sudden drop in the power grid frequency; S2 high frequency cutting machine configuration principle Frequency emergency control measures mainly include interlocking shedding, high-frequency shedding, low-frequency load shedding, generator set overspeed protection and low-frequency protection. Under severe power imbalance, the coordination of these measures is particularly important. The arrangement of high-frequency shedding and the setting of delay are limited by the maximum deviation of system frequency, steady-state deviation, and protection setting value. At the same time, the selection of shedding often determines the performance of the system after the accident. S3 high frequency cutting machine solution coordination configuration and optimization, high frequency cutting machine solution setting Steady-state frequency calculation: The action of the high-frequency shear machine should restore the system steady-state frequency to between 50Hz and 50.5Hz. Taking a certain regional power grid as an example, considering the extreme extreme situation, after the regional power grid is isolated, if the security control fails to work correctly, the total power balance is 6686.5MW, and the remaining power accounts for 72.99% of the total power generation output. The single-machine equivalent model is used to calculate the system frequency steady-state response. According to the final value theorem, Figure 1 can be obtained; Therefore, the steady-state frequencies corresponding to different machine cutting amounts are obtained according to the equivalent parameters of a single machine, as shown in Figure 2. From the calculation results in the table, it can be seen that the total machine cutting amount of all rounds of weekly machine cutting should be close to 70% of the total power generation. Considering the insufficient machine cutting of safety control and the refusal of a certain round of high weekly machine cutting, a certain amount of machine cutting should be appropriately increased as a backup; When the safety control is correctly operated and the machine is cut off at 3600MW, the calculation result of the machine cut amount when the system frequency is restored to the allowable operating range is shown in Figure 3. Therefore, when the safety control is correctly operated, the total high-frequency machine cut amount should be close to 30% of the total power generation. Considering different operating modes and the situation of machine cut refusal, a certain amount of machine cut should be appropriately increased as a backup; S4 high frequency cutting machine solution optimization After the basic scheme is determined, the level difference, cutting quantity and cutting selection are optimized according to the frequency stability index F; Differential Optimization Keep the setting value of the first and last rounds, the delay of each round and the amount of machine cutting unchanged, and compare the influence of equal step difference, small step difference first and then large step difference and large step difference first and then small step difference on the value. Scheme 1: adopt equal step difference, the step difference is not 0.2Hz, scheme 2: the step difference of the first three rounds is 0.15Hz, and the step difference of the last two rounds is 0.25Hz, scheme three: the first three rounds are 0.25Hz, and the last two rounds are 0.15Hz. Indicator F is calculated in three cases, and the results are shown in Figure 7. From the results, it can be concluded that the three step difference schemes can meet the requirements, and the step difference setting of scheme 2 is more conducive to frequency stability; Optimization of the cutting sequence of S5 unit; S6 acceleration strategy based on df / dt criteria For areas with large installed capacity, small load and high transmission, once an isolated grid occurs, the active power imbalance is very serious and the frequency rises very quickly. In order to ensure the adaptability of the high-frequency shedding scheme, a certain level difference and delay are usually reserved between each round, which makes the high-frequency shedding control untimely in some extreme cases. On the other hand, considering that the high-frequency shedding device is installed in the power plant, compared with the low-frequency load shedding where the actuator is installed in the 10kV or 35kV substation, the high-frequency shedding action device has rich electrical quantities (especially frequency information) for analysis and judgment. Therefore, a high-frequency shedding acceleration action scheme based on the dffdt criterion is proposed to improve the frequency dynamic response in the sending end area. S7 frequency mutation lockout Abnormal conditions such as short-circuit faults, load feedback, and abnormal measurement links may cause frequency mutations and cause df / dt criterion malfunctions, so device locking should also be considered. Taking the three-phase metallic short-circuit grounding at the generator end as an example, at the moment of the fault, the generator rotor accelerates, causing the frequency to rise too fast in a short time. The simulation results are shown in Figure 17: From Figure 17, we can see that the short-term frequency change rate of the system is shown in Figure 18; If this happens during the high frequency period of the isolated grid, it may cause the acceleration action strategy based on the df / dt criterion to malfunction. According to the above analysis and the maximum frequency change rate that the system may produce, the threshold value of df / dt lockout is set to 5Hz / s, thus we can get the high frequency cutting machine acceleration action strategy considering lockout Figure 19; This acceleration action strategy is used to adaptively configure the high-frequency cutting machine optimization, and the high-frequency cutting machine optimization scheme using df / dt acceleration is obtained. Its control strategy table is the same as the optimization scheme. Under the control of the calculation acceleration action strategy, the system frequency stability index can be obtained as F=0.0007, which is much smaller than the indicators of the basic solution and the optimization solution.

2. The high frequency shear machine optimization method of the transient frequency evaluation index F according to claim 1 is characterized in that: S1 Level 3 safety includes formulating three levels of safety and stability control measures for high-frequency problems caused by isolated grid operation: First-level safety and stability control measures: emergency interlocking cutting measures in case of serious failure Hydropower units are given priority. When the capacity of hydropower units is insufficient, some thermal power units are cut off. In principle, each power plant retains one unit in operation. The interlocking cutting requires a faster action speed. When the monitoring line fault trips, the cutting action is immediately executed according to the local strategy table. The interlocking cutting strategy ensures that over-cutting will not occur and will not cause low-frequency problems in the regional power grid. Second level safety and stability control measures: High frequency cutting of generator after disconnection The main generating units are equipped with high-frequency shedding measures; after the high-frequency shedding of the generating units can be correctly operated according to the rotation and delay, the power grid will not have frequency stability problems and will not be over-cut, resulting in low-frequency load shedding of the regional power grid; Level 3 safety and stability control measures: Coordination of thermal power unit steam turbine control and safe operation of power grid. In the grid-connected operation mode, the 103% overspeed function of the OPC of the thermal power unit is locked or the action delay is increased, and the 110% overspeed protection is retained. The first and second level measures are used to ensure that the grid frequency is stable and the OPC does not act. The first-level control measures and the second-level control measures should be independent of each other and effective independently. The second-level control measures can serve as a supplement or backup for the first-level control measures.

3. The high frequency shear machine optimization method of the transient frequency evaluation index F according to claim 1 is characterized in that: S2 configuration principles include: frequency emergency control measures coordination principle High-frequency generator cutting measures should be set up to focus on the high-frequency situations that may occur after the provincial or regional power grid is disconnected from the main grid; The system frequency emergency control first considers taking interlocking or remote cutting measures to quickly cut off the unit to restore the system stability; when the normal action of interlocking cutting cannot meet the active power balance or the interlocking cutting fails to work partially or completely, consider taking high-frequency cutting measures. When the frequency reaches a fixed value, the generator is cut off after a certain delay, and finally the active power balance is achieved, and the system frequency is restored to an acceptable range; The high frequency cutter can be used as a backup for the chain cutter to maintain the system frequency stability. The high frequency cutter and the chain cutter or remote cutter can be configured in a repeated manner. The coordination between the high-frequency cut-off and the turbine overspeed protection should be considered. The configuration of the high-frequency cut-off should effectively prevent the system frequency from rising to the action value of the OPC protection; avoid the OPC preemptive action causing the generator to repeatedly "oscillate" and affect the safety of the unit and the stability of the system; in order to avoid short-term frequency increases, the OPC should also be set with an appropriate delay to prevent frequent protection actions; High-frequency cutting machine needs to be coordinated with low-frequency load reduction measures to avoid the high-frequency cutting machine causing the low-frequency load reduction device to operate, thus reducing the control cost; High-frequency shedding measures need to be coordinated with oscillation disconnection measures; high-frequency shedding action has a great impact on the action time of the oscillation disconnection device, and in some cases may even affect the action behavior of the disconnection device, delaying the timing of the disconnection device action, especially for power plants that are close to the oscillation center, whose shedding action has a great impact on the action of the tie-line oscillation disconnection device; in order to ensure the correct action of the disconnection device, the high-frequency shedding action frequency of power plants near the oscillation center should be appropriately increased or the action time should be extended on the premise of ensuring the safety of the unit; High frequency cutting measures should be regularly checked and adjusted according to the grid structure and maximum exchange power changes; Since the system load level, external transmission level and unit startup mode all have an impact on the high-frequency shedding effect, the high-frequency shedding plan should try to adapt to most external transmission scales and operating modes: it should also try to minimize the large changes in the power flow in the entire network caused by high-frequency shedding measures; Principles of round configuration, frequency setting and delay setting When configuring a high-frequency cutting machine solution, the rounds should be arranged reasonably. Too many rounds will affect the control effect due to action delay, and too few rounds will easily cause overcutting. Generally, 3 to 5 rounds are appropriate; Since the normal operating frequency of the unit is between 49.5 and 50.5 Hz, the threshold value of the first round of high frequency cutting should be higher than 50.5 Hz; High frequency cutting machine measures should be able to effectively prevent the frequency from rising to the OPC action value (generally set to 3090r / m, i.e. 51.5Hz). Considering the sampling accuracy and calculation error of the device, it is generally calculated as not exceeding 51.3Hz; The high-frequency shedding measures should not cause the low-frequency load shedding device to operate. According to the current low-frequency first round configuration of the power grid, the system frequency is not less than 49Hz. Considering the sampling accuracy and calculation error of the device, it is generally calculated as not less than 49.25Hz; Generally, the abnormal operation time allowed for large units with a frequency higher than 51.5Hz shall not exceed 30 minutes in total, and each time shall not exceed 30 seconds. Considering the frequency rise and fall time and a certain slope, the total delay of the units cut in the frequency band of 50.5-51.5Hz shall generally not exceed 10 seconds; In order to avoid malfunction of the device caused by sudden frequency changes during the measurement process, the delay of each round of the high frequency cutting machine should generally be greater than 200ms; Principles of cutting machine selection Priority should be given to cutting off hydropower units, including pumped storage units in power generation status; For power grids with wind turbines, the output of wind turbines is volatile due to the intermittent nature of wind energy. At the same time, the presence of double-fed asynchronous wind turbines and permanent magnet synchronous wind turbines reduces the inertia of the system and does not participate in the primary and secondary frequency regulation of the system. When the system is isolated, the proportion of wind turbines is greater, which is not conducive to the stability of the system frequency. Therefore, wind turbines are given priority in high-frequency shedding rounds. Large-capacity units are removed first; When the high-frequency machine cutting plan for the entire network is formulated, one unit of the power plant is retained to avoid power outages in the entire power plant during high-frequency operation.

4. The high frequency shear machine optimization method of the transient frequency evaluation index F according to claim 1 is characterized in that: The S3 optimization scheme specifically includes the first and last rounds of adjustment Assuming that the safety control operates correctly and that all the machine cut-off quantities are first-round operations, the single-machine equivalent model is used to calculate the system frequency response under different first-round starting frequencies. The results are shown in Figure 4. It can be concluded that setting the start value to 50.6 Hz is beneficial to reducing the maximum system frequency and also helps to reduce the fluctuation of system frequency during dynamic process; The setting of the last wheel setting value of the high-frequency shear machine should be based on the maximum frequency allowed by the system as the upper limit. According to the configuration principle, the high-frequency shear machine should effectively avoid OPC action. The OPC setting value of the steam turbine of the sending-end power grid is 51.5Hz, with a delay of 3s. Therefore, the final wheel setting value is initially set to 51.4Hz. Set the rounds, level differences and cutting amount for each round The number of high-frequency cutting rounds cannot be too many or too few. Too many rounds will result in too little cutting in each round and inability to configure suitable units. At the same time, due to errors in frequency measurement, a small frequency interval is prone to overstepping. Therefore, according to the configuration principle, considering selectivity and reliability, a total of 5 rounds are set. Since the cutting capacity of the sending-end power grid should be configured to be 3000MW when the security control is correctly operated, a certain capacity is considered as a backup, and a total of 3900MW of cutting capacity is configured. The first three rounds set a cutting capacity of 3000MW, which is evenly distributed in each round. Corresponding to the cutting capacity that should be configured when the security control is correctly operated, the cutting capacity of each round is 1000MW, and the last two rounds are configured with a total of 900MW, the fourth round is 500MW, and the fifth round is 400MW. Assuming that the frequency interval between each round is equal, it can be obtained from the setting value of 50.6Hz in the first round and 51.4Hz in the last round that the level difference of each round is The frequency of the power grid is 0.2Hz. On this basis, the situation that the amount of power cut by the safety control chain cutting varies with the different interconnection line currents is considered. According to the configuration principle, the amount of power cut by the safety control is configured in the high-frequency power cut. By repeated configuration, the high-frequency power cut is used as a backup for the safety control, which ensures the frequency stability of the power grid at the sending end when the safety control refuses to operate. Considering that the safety control cutting needs to be as fast as possible, the 3600MW of safety control cutting is evenly configured in the first three rounds of high-frequency power cut. According to the configuration principle, the delay of each round is not less than 0.2s. For the situation of serious imbalance of active power at the sending end, controlling the active power imbalance as soon as possible is conducive to frequency stability. Therefore, the delay of the first three rounds of cutting takes the minimum value of 0.2s. The frequency will not drop rapidly after the cutting action. Due to the inertia of the unit, the frequency response takes a period of time. Therefore, the delay is appropriately increased in the last two rounds, 1.6s for the fourth round and 1s for the fifth round. The basic scheme of high-frequency power cut is obtained, as shown in Figure 5. The transient frequency stability evaluation index F of the high-cycle cutting basic scheme is calculated by using the equivalent model, and it is obtained that: the high-cycle cutting three-round action, F = 0.0478 meets the frequency response requirements. The actual high-cycle cutting scheme should also determine the units to be cut in each round. According to the cutting amount of each round, the unit operation management requirements, the feasibility of high-cycle cutting and the avoidance of the whole plant shutdown are considered, and the units to be cut in each round are configured. The high-cycle cutting basic scheme is shown in Figure 6.

5. The high frequency shear machine optimization method of the transient frequency evaluation index F according to claim 1 is characterized in that: S5 includes the actual sending-end power grid with 1099.75MW of wind power installed capacity, and the installed capacity has grown rapidly in recent years. According to the principle of selecting high-frequency cutting units, in the case of isolated grid at the sending end, the wind turbines are kept running, which is not conducive to the stability of the sending-end power grid frequency. Therefore, optimization is performed in the cutting sequence, and wind turbines are given priority in the first round of high-frequency cutting. At present, the actual generating capacity of wind turbines in the sending-end power grid is only 556.5MW, not more than 1000MW. Therefore, the running wind turbines are concentrated in the first round of high-frequency strategy. When the frequency and cutting amount of each round are approximately the same, the values ​​of index F and index E of the basic scheme (prioritized cutting of thermal power units and retained wind turbines) and the optimized scheme (prioritized cutting of wind power) can be compared to obtain the results shown in Figure 8. From the comparison of the results in the figure, it can be seen that the priority removal of wind turbines not only has a better frequency dynamic response, but also can obtain better economy; from a theoretical analysis, since the inertia of wind turbines does not act on the system frequency response, it is an "implicit" inertia. Therefore, compared with the priority removal of thermal power, the priority removal of wind power makes the inertia of the isolated grid system larger, which is conducive to frequency emergency control and can obtain a better frequency response; the indicators are consistent with the theoretical analysis, and the high-frequency machine removal optimization scheme is obtained by combining S1, S2, and S3 as shown in Figure 9.

6. The high frequency shear machine optimization method of the transient frequency evaluation index F according to claim 1 is characterized in that: S6 includes frequency response characteristics analysis The single-machine equivalent model is used to analyze the frequency response characteristics at the initial moment. The rotor motion equation of the normalized generator is linearized at the operating point to obtain Figure 10; Among them, △Pm(t) is the equivalent machine mechanical power increment, △Pe(t) is the equivalent machine electromagnetic power increment, △Pe(d) is the unbalanced power increment acting on the generator rotor, H is the equivalent machine inertia time constant, D is the equivalent load damping factor representing the load frequency regulation effect of the equivalent system, f* is the frequency per unit value of the equivalent system, at t=0+, when the disturbance occurs, the frequency of the system has not changed in time, and the load frequency regulation effect has not yet taken effect, and the unbalanced power can be expressed as Figure 11; The equation shows that at the initial moment of disturbance, the rate of change of the power system frequency is proportional to the active unbalance, and the proportional factor is the system inertia time constant; by setting the appropriate df / dt threshold, the program can determine the size of the active unbalance by itself, so as to implement the corresponding acceleration strategy for the high-frequency shedding scheme; from the equivalent calculation results, it can be seen that the total inertia time constant of the system based on the product is = 2H = 63.982s; therefore, in the case of isolated grid, the maximum frequency change rate of the system due to the acceleration power is: Figure 12 The threshold value of the f / dt acceleration strategy is set. From the power flow data of the sending-end grid, it can be seen that if the safety control fails to operate correctly after the grid is isolated, the maximum active unbalance that may occur in the system is △PD=6686MW, which is converted to the per-unit value △PD*=6.686 based on the system benchmark capacity Sn=1000MVA; Converting the per-unit value of the frequency change rate to a nominal value yields: Figure 13; When the safety control is correctly operated to cut off the machine to 3600MW, assuming that the inertia of the sending end system is reduced proportionally, it is: Figure 14; The same calculation can be performed, and in this case, Figure 15 can be obtained; Since the large external transmission mode is selected for calculation, when the power grid operates in other modes, the frequency change rate should be less than 3.862Hz / s when the same fault occurs; therefore, 3Hz / s can be selected as the threshold. When the frequency change rate is greater than the threshold, the high-frequency machine shedding rounds are accelerated; from Section 4.2.2, it can be seen that the correct safety and control action corresponds to the first three rounds of high-frequency machine shedding. Therefore, based on the local quantity measurement of the power plant, the frequency change rate is judged on the premise of satisfying the first round of high-frequency machine shedding. If the frequency change rate is higher than 3Hz / s, the second and third rounds are accelerated. If the frequency is higher than 2Hz / s and lower than 3Hz / s, the second round is accelerated. The high-frequency machine shedding acceleration action strategy based on the df / dt criterion is obtained as shown in Figure 16.

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