New energy leading system safety cost quantification method

Abstract

The invention relates to the technical field of power markets, in particular to a new energy induction system safety cost quantification method, which comprises the following steps: S1, quantifying peak regulation cost required by a system under new energy access: quantifying peak regulation auxiliary service cost; s2, quantifying the frequency modulation cost required by the system under new energy access: accounting the frequency modulation auxiliary service cost; s3, quantifying the standby cost required by the system under new energy access: determining the standby auxiliary service capacity, and calculating the total cost of the standby auxiliary service; s4, new energy induced system safety cost quantification: summarizing peak regulation cost, frequency modulation cost and standby cost, calculating the total amount of system safety external cost under new energy access, and providing a basis for new energy auxiliary service cost allocation; according to the method, the new energy consumption capability is improved, the wind and light abandoning and load loss risks are reduced, stable operation of a power system is guaranteed, and important support is provided for power market reform and new energy high-proportion access.

Description

Technical Field

[0001] The present invention relates to the technical field of electric power market, and in particular to a method for quantifying system safety costs caused by new energy. Background Art

[0002] At present, various provinces have not yet formed a clear method to quantify the safety costs caused by renewable energy, which has led to unreasonable cost allocation. In some areas, this unreasonable sharing method has dampened the enthusiasm of market players and led to a decrease in the number of power generators willing to participate in the ancillary service market. The lack of power generators to provide ancillary services will lead to safety issues such as unstable grid currents and large-scale power outages, which will threaten the normal operation of the power system in serious cases. Therefore, we should accelerate the research on reasonable methods to quantify the safety costs caused by renewable energy to promote the stable and healthy operation of the power market.

[0003] Existing studies have established a series of capacity calculation models for peak-shaving, frequency regulation, and standby ancillary services, such as constructing an optimal credible interval estimation model for renewable energy grid-connected output to calculate the capacity of ancillary services caused by renewable energy grid-connected output. For peak-shaving, frequency regulation, and standby capacity calculations, on this basis, only the capacity of each ancillary service under the influence of volatility and uncertainty is clarified. From the existing research, current studies mostly start from the calculation of various types of ancillary service capacity and cost calculation, and lack quantification of various types of system safety costs under high proportion of renewable energy access. In view of this, this paper establishes a method to quantify the system safety cost caused by renewable energy, quantifies the impact of renewable energy access on power grid safety, and on this basis, clarifies the incremental safety cost required for the system under renewable energy access. Summary of the invention

[0004] The present invention provides a method for quantifying the safety cost of a new energy-induced system.

[0005] A method for quantifying system safety costs caused by new energy sources comprises the following steps:

[0006] S1, quantify the peak-shaving cost required by the system under the access of new energy: calculate the peak-shaving capacity demand caused by the grid connection of new energy based on the optimization scheduling model, and quantify the peak-shaving auxiliary service cost through the segmented pricing mechanism;

[0007] S2, quantify the frequency regulation cost required by the system under the access of new energy: calculate the impact of new energy output fluctuations and forecast deviations on frequency regulation capacity, and use the capacity compensation + mileage compensation method to calculate the frequency regulation auxiliary service cost;

[0008] S3, quantify the backup cost required by the system under the access of new energy: calculate the spinning reserve capacity caused by new energy based on the grid load forecast deviation, determine the backup ancillary service capacity based on the spinning reserve capacity and price, and calculate the total cost of backup ancillary services;

[0009] S4. Quantification of System Security Costs Caused by New Energy: Aggregate peak shaving costs, frequency regulation costs, and reserve costs, calculate the total external system security costs under the access of new energy, and provide a basis for the allocation of new energy ancillary service fees.

[0010] Optionally, the quantification of peak shaving costs required by the system under the access of new energy in S1 includes:

[0011] S11. Calculation of Peak Shaving Capacity of the System Caused by New Energy: Calculate the peak shaving capacity demand caused by the grid connection of new energy based on the optimal dispatch model, consider the impact of load changes on peak shaving capacity, and optimize the allocation of peak shaving resources by flexibly adjusting units;

[0012] S12. Quantification of Peak Shaving Costs of the System Caused by New Energy: Adopt a stepped segmented pricing mechanism to calculate the peak shaving ancillary service costs based on the peak shaving capacity demand.

[0013] Optionally, the target condition of the system peak shaving capacity is the minimization of the total system cost, expressed as:

[0014]

[0015] where c is the total system cost, G c 、G Pu 、Ω D are the sets of thermal power units, paid peak shaving units, and paid peak shaving gears respectively, are the peak shaving volume and quotation of unit i in the o-th segment of paid peak shaving respectively, y i,t 、z i,t are 0-1 state variables indicating whether unit i is started / stopped at time t respectively, are the start-up / stop costs of unit i at time t respectively, and are the load shedding cost and curtailment cost of wind and solar power of the unit respectively.

[0016] Optionally, the peak shaving ancillary service cost is expressed as:

[0017] C TF =P re ·P N ;

[0018] where C TF is the total peak shaving service cost, P re is the peak shaving capacity caused by the grid connection of new energy, and P N is the gear price corresponding to the N capacity segment.

[0019] Optionally, the quantification of frequency regulation costs required by the system under the access of new energy in S2 includes:

[0020] S21, Calculation of the frequency regulation capacity of the new energy-induced system: Calculate the impact of the new energy output fluctuation and prediction deviation on the frequency regulation capacity, and determine the total frequency regulation demand caused by the new energy connection;

[0021] S22, Quantification of the frequency regulation cost of the new energy-induced system: Adopt the capacity compensation + mileage compensation method, and combine the frequency regulation demand and price to calculate the frequency regulation ancillary service price caused by the new energy grid connection.

[0022] Optionally, the calculation of the frequency regulation capacity of the new energy-induced system in S21 includes:

[0023] S211, Calculate the AGC frequency regulation capacity demand: Based on the power generation and consumption balance of the whole network, calculate the frequency regulation capacity demand P load , the new energy output component P energy , the tie line planned regulation component P line , the unit power generation plan regulation component P G-plan , and calculate the AGC frequency regulation capacity demand, expressed as:

[0024] P af =P load -P energy -P line -P G-plan ;

[0025] S212, Calculate the total frequency regulation capacity caused by the new energy connection: The frequency regulation demand caused by the new energy output fluctuation includes the predicted output change and the prediction deviation caused by the high volatility. The predicted output change is determined by calculating the difference between the initial and end outputs of each time period, and the prediction deviation is calculated based on the cumulative distribution curves and proportion coefficients of wind power and photovoltaic power. By comprehensively considering the new energy output change and prediction deviation, determine the total frequency regulation capacity demand caused by the new energy connection, expressed as:

[0026] P1=L N,t+Δt -L N,t

[0027] P2=α fd ·Φ fd +α gd ·Φ gd

[0028] P energy =P1+P2;

[0029] Among them, P1 is the frequency regulation capacity caused by the predicted output change of the new energy in each time period, P2 is the frequency regulation capacity caused by the high volatility and high uncertainty of the new energy power generation, P energry is the total frequency regulation capacity caused by the new energy connection, L N,t is the predicted output of the new energy at time t, L N,t+Δt is the predicted output at time t+Δt, Φfd and Φ gd are the cumulative distribution curves of the prediction deviations of wind power and photovoltaic power respectively, and α fd and α gd are the proportion coefficients of the prediction deviations of wind power and photovoltaic power.

[0030] Optionally, the frequency regulation ancillary service price is expressed as:

[0031] C TP = P af ·P AGC ·L AGC + P mil ·p mil ·K;

[0032] Among them, C TP is the total cost of frequency regulation services, P af is the frequency regulation capacity induced by the grid connection of new energy, P AGC is the capacity compensation price, L AGC is the unit operation rate, P mil is the frequency regulation mileage, p mil is the frequency regulation mileage price, and K is the frequency regulation performance index.

[0033] Optionally, the spinning reserve capacity is expressed as:

[0034]

[0035] Among them, Φ eql is the probability distribution function of the prediction deviation of the equivalent grid load, and σ eql is the standard deviation of the prediction deviation of the equivalent grid load;

[0036] The reserve ancillary service capacity is expressed as:

[0037]

[0038] Among them, is the spinning reserve ancillary service induced solely by the original grid load without the access of new energy power generation. σ1 is the original grid load, and Φ1 is the probability distribution of the prediction deviation of the original grid load;

[0039] The total cost of the reserve ancillary service is expressed as:

[0040] C BY = P sr ·P BY ;

[0041] Among them, C BY is the total cost of the reserve ancillary service, P sr is the reserve capacity induced by the grid connection of new energy, and P BY is the reserve service price.

[0042] Optionally, the total external cost of system security is expressed as:

[0043] C safe = C TF + C TP + C BY ;

[0044] where C safe is the total external cost of system security under the access of new energy.

[0045] Advantages of the present invention:

[0046] In the present invention, by constructing a cost calculation model for peak shaving, frequency modulation, and reserve auxiliary services, the impact of new energy access on system security is accurately quantified, making up for the deficiency of the existing research in the lack of systematic quantification of the security costs caused by new energy. Based on the optimal scheduling model, combined with the output fluctuation characteristics of new energy and the prediction deviation of grid load, the demand capacity of auxiliary services caused by new energy is accurately calculated, and a reasonable pricing mechanism is adopted to quantify the relevant costs, providing a scientific basis for the reasonable sharing of new energy auxiliary service costs, and helping to enhance the economy and security of power grid operation.

[0047] In the present invention, by clarifying the incremental security cost required by the system under the access of new energy, the scheduling of peak shaving, frequency modulation, and reserve resources can be optimized, the new energy consumption capacity can be improved, the risks of wind and light abandonment and load loss can be reduced, the stable operation of the power system can be guaranteed, and important support can be provided for the power market reform and the high-proportion access of new energy. Description of the Drawings

[0048] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for the description of the embodiments or the prior art. Obviously, the drawings in the following description are only those of the present invention. For those of ordinary skill in the art, other drawings can be obtained based on these drawings without creative efforts.

[0049] Figure 1 It is a schematic flow chart of the quantification method according to the embodiment of the present invention. Detailed Embodiments

[0050] The present invention will be described in detail below in conjunction with the drawings and specific embodiments. At the same time, it should be noted here that in order to make the embodiments more detailed, the following embodiments are the best and preferred embodiments. For some well-known technologies, those skilled in the art can also adopt other alternative methods for implementation; moreover, the drawings are only for more specifically describing the embodiments and are not intended to specifically limit the present invention.

[0051] It should be noted that in the specification, the mention of "an embodiment", "embodiment", "exemplary embodiment", "some embodiments", etc. indicates that the described embodiment may include specific features, structures or characteristics, but not necessarily every embodiment includes such specific features, structures or characteristics. Additionally, when combining an embodiment to describe a specific feature, structure or characteristic, the realization of such feature, structure or characteristic in combination with other embodiments (whether explicitly described or not) should be within the knowledge of those skilled in the relevant art.

[0052] Generally, terms can be understood at least in part from their use in the context. For example, at least in part depending on the context, the term "one or more" used herein can be used to describe any feature, structure or characteristic in a singular sense, or can be used to describe a combination of features, structures or characteristics in a plural sense. Additionally, the term "based on" can be understood as not necessarily intended to convey a set of exclusive factors, but instead, at least in part depending on the context, allows for the existence of other factors that may not be explicitly described.

[0053] As Figure 1 shown, a method for quantifying the safety cost of a new energy induced system includes the following steps:

[0054] I. Quantify the peak shaving cost required by the system under new energy access:

[0055] 1.1 Calculate the peak shaving capacity of the new energy induced system:

[0056] The quantification of the system peak shaving capacity is solved through an optimization scheduling model. If the system lacks peak shaving capacity, corresponding curtailment costs of wind and light will be generated during low load periods, and corresponding load shedding costs will be generated during high load periods. Therefore, corresponding flexible regulating units are introduced to carry out paid peak shaving to minimize the peak shaving purchase cost, load shedding cost, and curtailment costs of wind and light.

[0057] Therefore, the objective condition of the system peak shaving capacity quantification model is the minimization of the total system cost, as shown in the following formula:

[0058]

[0059] In the formula: c is the total system cost; G c , G Pu , Ω D are respectively the sets of thermal power units, paid peak shaving units, and paid peak shaving gears; are respectively the peak shaving volume and quotation of unit i during the o - th stage of paid peak shaving; y i,t , z i,t are respectively 0 - 1 state variables indicating whether unit i is started / stopped at time t; are respectively the start - up / stop costs of unit i at time t; and They are the load shedding cost and the curtailment cost of wind and solar power of the unit respectively.

[0060] 1.2 Quantification of the peaking cost induced by new energy in the system:

[0061] The peaking price clearing mechanism takes 50% of the declared capacity as the node. The prices below 50% output are declared in gradually increasing brackets, with each 10% as a bracket, forming a total of 5 brackets of prices, and the price clearing curve is monotonically decreasing; the prices above 50% output are divided into 5 - 10 segments, and the price clearing curve is monotonically increasing. Therefore, the peaking ancillary service cost induced by the grid connection of new energy is: C TF = P re ·P N ;

[0062] In the formula: C TF is the total peaking service cost, P re is the peaking capacity induced by the grid connection of new energy, and P N is the bracket price corresponding to the N - capacity segment.

[0063] II. Quantification of the frequency modulation cost required by the system under the access of new energy:

[0064] 2.1 Calculation of the frequency modulation capacity induced by new energy in the system:

[0065] In the load - generation balance, the components of the power generation side participating in frequency regulation can be summarized into 4 types: the tie - line planned component, the component of the unit following the day - ahead plan, the component of the unit following the intraday rolling plan, and the AGC regulation component. Therefore, from the perspective of the overall network power generation and consumption balance, the AGC frequency modulation capacity demand can be based on the overall network load. By calculating the frequency modulation capacity demand P load caused by load changes, the new energy output component P energy , the tie - line planned regulation component P line , and the unit power generation plan regulation component P G-plan , the AGC frequency modulation capacity demand can be obtained. The frequency modulation capacity is:

[0066] P af = P load - P energy - P line - P G-plan (2)

[0067] Among them, the frequency modulation demand P energyIt can be divided into two parts: on the one hand, it is the predicted output change of new energy power generation. The frequency regulation demand caused by the output change of new energy power generation in each period can be reflected by calculating the difference in the output magnitudes at the initial and end moments of each period; on the other hand, the high volatility and high uncertainty of new energy power generation will lead to deviations in the prediction of new energy output. For this part of the deviation, wind power and photovoltaic power are calculated separately, and the distribution requirements covering a certain probability are set through the proportion coefficient method, so as to obtain the predicted deviation values of wind power and photovoltaic power output in this period, which are used as the frequency regulation demand generated by the prediction deviation in this period:

[0068]

[0069] Among them, P1 is the frequency regulation capacity caused by the predicted output change of new energy in each period, P2 is the frequency regulation capacity caused by the high volatility and high uncertainty of new energy power generation, and P energry is the total frequency regulation capacity caused by the access of new energy; L N,t is the predicted output of new energy at time t, and L N,t+Δt is the predicted output at time t+Δt; Φ fd and Φ gd are the cumulative distribution curves of the prediction deviations of wind power and photovoltaic power respectively; α fd and α gd are the proportion coefficients of the prediction deviations of wind power and photovoltaic power. During the day, α fd takes 0.4, and α gd takes 0.6; at night, α fd takes 1, and α gd takes 0.

[0070] 2.2 Quantification of the system frequency regulation cost caused by new energy:

[0071] The clearing of the frequency regulation ancillary service price mostly adopts the two-part method of "capacity compensation + mileage compensation". Among them, the mileage compensation is mainly calculated based on the frequency regulation mileage, introducing the frequency regulation performance index, and calculating according to the frequency regulation mileage, frequency regulation performance and mileage unit price; the capacity compensation is mainly calculated based on the invoked capacity, and the two parts of the income are added up to obtain the final frequency regulation ancillary service price: C TP =P af ·P AGC ·L AGC +P mil ·p mil ·K;

[0072] In the formula: C TP is the total cost of the frequency regulation service, P af is the frequency regulation capacity caused by the grid connection of new energy, P AGC is the capacity compensation price, L AGC is the unit operation rate of the unit; P mil is the frequency regulation mileage, and p milIt is the frequency modulation mileage price, and K is the frequency modulation performance index.

[0073] III. Quantifying the required reserve cost of the system under new energy access:

[0074] 3.1 Calculation of the system reserve capacity induced by new energy:

[0075] When the positive spinning reserve of the system is insufficient, load shedding accidents will occur. When the negative spinning reserve configuration of the system is insufficient, the phenomenon of abandoning new energy power generation will be triggered. Under the specified maximum allowable new energy power generation abandonment rate P lore and load shedding rate P lolp conditions, the spinning reserve capacity caused by the predicted deviation of the equivalent grid load is:

[0076]

[0077] Φ eql is the probability distribution function of the predicted deviation of the equivalent grid load, and σ eql is the standard deviation of the predicted deviation of the equivalent grid load.

[0078]

[0079] is the spinning reserve auxiliary service solely induced by the original grid load without new energy power generation access. σ1 is the original grid load, and Φ1 is the probability distribution of the predicted deviation of the original grid load.

[0080] Therefore, the spinning reserve auxiliary service capacity of the new energy power generation access system is:

[0081]

[0082] 3.2 Quantifying the reserve cost induced by new energy:

[0083] The total cost of the reserve auxiliary service is obtained by multiplying the required reserve capacity by the reserve price: C BY = P sr ·P BY ;

[0084] In the formula: C BY is the total cost of the reserve auxiliary service, P sr is the reserve capacity induced by the new energy grid connection, and P BY is the reserve service price.

[0085] IV. Quantifying the system security cost induced by new energy:

[0086] Based on the total cost of the peak shaving service, the total cost of the frequency modulation service, and the total cost of the reserve auxiliary service, the total amount of the system security external cost under new energy access can be obtained: C safe = C TF + CTP +C BY ;

[0087] wherein, C safe is the total amount of the external costs of system security under the access of new energy.

[0088] The present invention covers any alternatives, modifications, equivalent methods and solutions made on the essence and scope of the present invention. For the public to have a thorough understanding of the present invention, specific details are described in detail in the following preferred embodiments of the present invention, and those skilled in the art can fully understand the present invention without these detailed descriptions. In addition, well-known methods, processes, procedures, components and circuits, etc. are not described in detail to avoid unnecessary confusion to the essence of the present invention.

[0089] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A method for quantifying the safety cost of a new energy induced system, characterized in that, It includes the following steps: S1. Quantify the peak regulation cost required by the system under new energy access: Calculate the peak regulation capacity demand caused by new energy grid connection based on the optimal scheduling model, and quantify the peak regulation ancillary service cost through the stepped pricing mechanism; S2. Quantify the frequency modulation cost required by the system under new energy access: Calculate the impact of new energy output fluctuations and prediction deviations on the frequency modulation capacity, and adopt the capacity compensation + mileage compensation method to account for the frequency modulation ancillary service cost; S3. Quantify the reserve cost required by the system under new energy access: Calculate the spinning reserve capacity caused by new energy based on the grid load prediction deviation, determine the reserve ancillary service capacity according to the spinning reserve capacity and price, and calculate the total reserve ancillary service cost; S4. Quantify the system security cost caused by new energy: Summarize the peak regulation cost, frequency modulation cost and reserve cost, calculate the total system security external cost under new energy access, and provide a basis for the allocation of new energy ancillary service fees.

2. A method for quantifying the safety cost of a new energy-induced system according to claim 1, characterized in that, The quantification of the peak regulation cost required by the system in S1 includes: S11. Calculate the peak regulation capacity of the system caused by new energy: Calculate the peak regulation capacity demand caused by new energy grid connection based on the optimal scheduling model, consider the impact of load changes on the peak regulation ability, and optimize the peak regulation resource allocation by flexibly adjusting the units; S12. Quantify the peak regulation cost of the system caused by new energy: Adopt the stepped pricing mechanism and calculate the peak regulation ancillary service cost based on the peak regulation capacity demand.

3. A method for quantifying the safety cost of a new energy-induced system according to claim 2, characterized in that, The target condition of the system peak regulation capacity is the minimization of the total system cost, expressed as: Among them, c is the total system cost, G c , G Pu , Ω D are the sets of thermal power units, paid peaking units, and paid peaking levels respectively, are the peaking volume and quotation of unit i for paid peaking in the o-th section, y i,t , z i,t are the 0-1 state variables indicating whether unit i is started / stopped at time t, are the start-up / stop costs of unit i at time t, and are the load shedding cost and the cost of abandoning wind and light of the unit respectively.

4. A method for quantifying the safety cost of a new energy-induced system according to claim 3, characterized in that, The peak regulation ancillary service cost is expressed as: C TF = P re ·P N ; Among them, C TF is the total cost of peak shaving service, P re is the peak shaving capacity caused by the grid connection of new energy, P N is the gear price corresponding to the N capacity segment.

5. A method for quantifying the safety cost of a new energy-induced system according to claim 4, characterized in that, The quantification of the frequency modulation cost required by the system in S2 includes: S21. Calculate the frequency modulation capacity of the system caused by new energy: Calculate the impact of new energy output fluctuations and prediction deviations on the frequency modulation capacity, and determine the total frequency modulation demand caused by new energy access; S22. Quantify the frequency modulation cost of the system caused by new energy: Adopt the capacity compensation + mileage compensation method, combine the frequency modulation demand and price, and calculate the frequency modulation ancillary service price caused by new energy grid connection.

6. A method for quantifying the safety cost of a new energy induced system according to claim 5, characterized in that, The calculation of the frequency modulation capacity of the system caused by new energy in S21 includes: S211. Calculate the AGC frequency regulation capacity requirement: Based on the power generation and consumption balance of the whole network, calculate the frequency regulation capacity requirement P caused by the load change load , the new energy output component P energy , the planned regulation component P of the tie line line , the planned regulation component P of the unit power generation plan G-plan , and calculate the AGC frequency regulation capacity requirement, expressed as: P af = P load -P energy -P line -P G-plan ; S212. Calculate the total frequency modulation capacity caused by new energy access: The frequency modulation demand caused by new energy output fluctuations includes the predicted output change and the prediction deviation caused by high volatility. The predicted output change is determined by calculating the difference between the initial and end outputs of each period, and the prediction deviation is calculated based on the cumulative distribution curves and proportion coefficients of wind power and photovoltaic power. By comprehensively considering the new energy output change and prediction deviation, determine the total frequency modulation capacity demand caused by new energy access, expressed as: P1 = L N,t+Δt -L N,t P2 = α fd ·Φ fd +α gd ·Φ gd P energy = P1 + P2; Among them, P1 is the frequency regulation capacity caused by the predicted output changes of new energy at each time period, P2 is the frequency regulation capacity caused by the high volatility and high uncertainty of new energy power generation, and P energry is the total frequency regulation capacity caused by the access of new energy, L N,t is the predicted output of new energy at time t, L N,t+Δt is the predicted output at time t+Δt, Φ fd and Φ gd are the cumulative distribution curves of the prediction deviations of wind power and photovoltaic power respectively, α fd and α gd are the proportion coefficients of the prediction deviation of wind power and the prediction deviation of photovoltaic power.

7. A method for quantifying the safety cost of a new energy-induced system according to claim 6, characterized in that, The frequency modulation ancillary service price is expressed as: C TP = P af ·P AGC ·L AGC + P mil ·p mil ·K; Among them, C TP is the total cost of frequency regulation service, P af is the frequency regulation capacity induced by the grid connection of new energy, P AGC is the capacity compensation price, L AGC is the unit operation rate, P mil is the frequency regulation mileage, p mil is the frequency regulation mileage price, and K is the frequency regulation performance index.

8. A method for quantifying the safety cost of a new energy induced system according to claim 7, characterized in that, The spinning reserve capacity is expressed as: Among them, Φ eql is the probability distribution function of the power grid equivalent load prediction deviation, and σ eql is the standard deviation of the power grid equivalent load prediction deviation; The reserve ancillary service capacity is expressed as: Among them, is the spinning reserve ancillary service solely caused by the original load of the power grid without the access of new energy power generation. σ1 is the original load of the power grid, and Φ1 is the probability distribution of the prediction deviation of the original load of the power grid; The total reserve ancillary service cost is expressed as: C BY = P sr · P BY ; Among them, C BY is the total cost of backup auxiliary services, P sr is the backup capacity caused by the grid connection of new energy, P BY is the backup service price.

9. A method for quantifying the safety cost of a new energy induced system according to claim 8, characterized in that, The total system security external cost is expressed as: C safe = C TF + C TP + C BY ; Among them, C safe is the total external cost of system security under the access of new energy.

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