A matching method for grid-connected transformers in photovoltaic power plants

Abstract

This invention relates to the field of grid-connected transformer matching, and specifically to a matching method for grid-connected transformers in photovoltaic power plants. The invention determines a transformer library based on the grid connection point voltage and inverter output voltage; obtains the rated breaking current of the circuit breaker and the system short-circuit impedance; and solves for the first short-circuit impedance interval by combining the grid connection point voltage and fluctuation range. It then obtains the harmonic spectrum of the inverter under different loads, analyzes the dangerous short-circuit impedance corresponding to each harmonic, determines the circuit's safe impedance interval, and determines the short-circuit impedance sequence by combining the first short-circuit impedance interval. Finally, it selects candidate transformers from the transformer library, analyzes the annual insulation aging economic loss based on the inverter output power within a preset period, calculates the total discounted cost based on the annual loss electricity cost analysis based on the no-load and load loss power of each candidate transformer, and calculates the levelized energy expenditure based on the total power generation on the high-voltage side, thus determining the matched transformer and improving the reliability of the selection results.

Description

Technical Field

[0001] This invention relates to the field of grid-connected transformer matching, and specifically to a matching method for grid-connected transformers in photovoltaic power plants. Background Technology

[0002] With the large-scale and centralized development of the photovoltaic power generation industry, the grid-connected installed capacity of photovoltaic power stations continues to increase. As the core power equipment connecting photovoltaic power stations to the public power grid, the selection and matching effect of grid-connected transformers directly affects the safe and stable operation of power stations, power quality, and economic benefits throughout their entire life cycle.

[0003] Existing technologies, such as Chinese Patent Publication No. CN110277784B, disclose a matching method and device for photovoltaic grid-connected transformers. This method calculates the cost per kilowatt-hour of the photovoltaic power generation system corresponding to each photovoltaic grid-connected transformer based on the price of each transformer and the power generation on the high-voltage side within a preset period. The photovoltaic grid-connected transformer with the lowest cost per kilowatt-hour is then identified as the matching transformer for the photovoltaic power generation system, thereby maximizing the benefits of the photovoltaic power generation system.

[0004] However, the existing technology has the following problems: 1. The existing technology only considers the cost per kilowatt-hour and ignores the risk of harmonic resonance. At the same time, it does not take into account the joint constraints of the circuit breaker's breaking capacity, voltage fluctuation range and short-circuit impedance, which may lead to the selection not meeting the requirements of power grid safety and power quality, and may cause harmonic amplification, excessive short-circuit current or voltage exceeding the limit.

[0005] 2. Existing technologies mostly rely on fixed lifespan and static loss methods when calculating the cost per kilowatt-hour, without considering the transformer thermal life loss caused by photovoltaic power output fluctuations. They also fail to discount costs incurred at different points in time, making it difficult to truly reflect the economics of the entire life cycle, which may result in suboptimal matching results. Summary of the Invention

[0006] This invention aims to address the shortcomings of existing technologies by providing a matching method for grid-connected transformers in photovoltaic power plants. The invention determines a first short-circuit impedance range based on circuit breaker breaking capacity and voltage fluctuations, obtains a safe impedance range by combining harmonic hazardous short-circuit impedances to screen transformers, calculates the economic losses from insulation aging and total discounted costs based on thermal life loss rates, and finally determines the matching transformer based on minimizing levelized energy expenditure.

[0007] To achieve the above objectives, the present invention adopts the following technical solution: the transformer library is determined based on the grid connection point voltage and the inverter AC side output voltage, the rated circuit breaking current of the grid connection point circuit breaker and the system short-circuit impedance are obtained, and the first short-circuit impedance interval is solved by combining the grid connection point voltage and the fluctuation range.

[0008] Obtain the harmonic spectrum of the inverter under different loads, analyze the dangerous short-circuit impedance corresponding to each harmonic, determine the safe impedance range of the circuit based on the dangerous short-circuit impedance corresponding to each harmonic, and determine the short-circuit impedance sequence in combination with the first short-circuit impedance range.

[0009] Based on the short-circuit impedance sequence, candidate transformers are selected from the transformer library. The annual thermal life loss rate of each candidate transformer is analyzed according to the AC side output power of the inverter within a preset period. The annual insulation aging economic loss is calculated in combination with the initial purchase cost.

[0010] The no-load loss power and load loss power of each candidate transformer are obtained to analyze the annual loss electricity cost. The total discounted cost is calculated by combining the annual insulation aging economic loss. The levelized energy expenditure is calculated by combining the total power generation on the high-voltage side. The matching transformer is determined based on the levelized energy expenditure.

[0011] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention determines the transformer library based on the grid connection point voltage and the inverter AC side output voltage, obtains the rated circuit breaking current of the grid connection point circuit breaker and the system short-circuit impedance, and solves the first short-circuit impedance range by combining the grid connection point voltage and fluctuation range, so as to avoid grid safety accidents such as equipment damage and line tripping caused by excessive short-circuit current.

[0012] (2) This invention obtains the harmonic spectrum of the inverter under different loads, analyzes the dangerous short-circuit impedance corresponding to each harmonic, determines the safe impedance range of the circuit based on the dangerous short-circuit impedance corresponding to each harmonic, and determines the short-circuit impedance sequence in combination with the first short-circuit impedance range, thereby avoiding the risks of harmonic resonance and harmonic amplification, ensuring the power quality at the grid connection point, and improving the reliability of the selection results.

[0013] (3) The present invention selects candidate transformers from the transformer library based on the short-circuit impedance sequence, analyzes the annual thermal life loss rate of each candidate transformer according to the AC side output power of the inverter within a preset period, and calculates the annual insulation aging economic loss in combination with the initial purchase cost, which makes up for the defect of the traditional scheme that ignores the thermal aging cost, and makes the economic analysis fit the actual operating conditions of the transformer.

[0014] (4) This invention analyzes the annual power loss cost by obtaining the no-load loss power and load loss power of each candidate transformer, calculates the total discounted cost by combining the annual insulation aging economic loss, calculates the levelized energy expenditure by combining the total power generation on the high-voltage side, and determines the matching transformer based on the levelized energy expenditure. This solves the problem of distortion in traditional static cost calculation and takes into account both the safety and economy of the selection result. Attached Figure Description

[0015] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the description of the embodiments will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0016] Figure 1 This is a schematic diagram of the method steps of the present invention;

[0017] Figure 2 This is a schematic diagram of the steps involved in obtaining the safe impedance range in this invention.

[0018] Figure 3 This is a schematic diagram of the steps in the method for obtaining the annual thermal life loss rate in this invention. Detailed Implementation

[0019] Various exemplary embodiments of the present invention will now be described in detail with reference to the accompanying drawings. It should be noted that, unless otherwise specifically stated, the relative arrangement, numerical expressions, and values ​​of the components and steps set forth in these embodiments do not limit the scope of the invention. Furthermore, it should be understood that, for ease of description, the dimensions of the various parts shown in the drawings are not drawn to actual scale.

[0020] The following description of at least one exemplary embodiment is merely illustrative and is in no way intended to limit the invention or its application or use. Techniques, methods, and apparatus known to those skilled in the art may not be discussed in detail, but where appropriate, such techniques, methods, and apparatus should be considered part of the specification.

[0021] In all examples shown and discussed herein, any specific values ​​should be interpreted as merely exemplary and not as limitations. Therefore, other examples of exemplary embodiments may have different values.

[0022] Please see Figure 1 As shown, the present invention provides a matching method for grid-connected transformers in photovoltaic power plants, including: S1, determining the transformer library and solving the first short-circuit impedance range.

[0023] The grid connection voltage level of a photovoltaic power station and the AC output voltage of the inverter are the core constraints on the rated voltage ratio of the grid-connected transformer. Mismatch in voltage parameters will directly lead to the transformer being unable to connect to the grid and inverter circuit normally. At the same time, the grid connection circuit breaker has a fixed rated breaking current limit. The inherent short-circuit impedance of the system, the allowable fluctuation range of the grid voltage, and the dynamic change characteristics of photovoltaic output will jointly restrict the value of the transformer's short-circuit impedance. If the transformer's short-circuit impedance is not properly selected, it is easy to cause grid safety faults such as the system short-circuit current exceeding the breaking capacity of the circuit breaker and the grid connection voltage exceeding the limit.

[0024] Based on this, the specific implementation steps of S1 include: S11, determining the transformer library based on the grid connection point voltage and the AC side output voltage of the inverter.

[0025] Obtain the rated voltage of the photovoltaic power station's grid connection point from the power grid dispatch system or grid connection agreement, and obtain the rated output voltage of the inverter's AC side from the inverter's technical parameters. Select transformers with the corresponding rated voltages on both sides based on the rated voltage of the grid connection point and the rated output voltage of the inverter's AC side to form a transformer library.

[0026] S12. Obtain the rated breaking current of the circuit breaker at the grid connection point and the system short-circuit impedance, and solve for the first short-circuit impedance range by combining the grid connection point voltage and fluctuation range. The specific implementation steps are as follows: S121. The product of the rated breaking current of the circuit breaker at the grid connection point and the preset safety margin coefficient is recorded as the maximum allowable short-circuit current of the circuit. Based on the ratio of the transformer's rated capacity to the system short-circuit capacity, the per-unit value of the system short-circuit impedance is obtained.

[0027] In this embodiment, the preset safety margin coefficient is set to 0.9. The implementer may also set other specific values, but they must be less than 1 to ensure that the circuit breaker does not operate in the extreme state, to leave a safety margin for the circuit breaker, and to avoid the actual short-circuit current exceeding the rated breaking capacity of the circuit breaker due to short-circuit current calculation errors or changes in system operating conditions.

[0028] S122. Calculate the minimum total short-circuit impedance per unit value based on the maximum allowable short-circuit current, grid connection point voltage, and transformer rated capacity. Calculate the difference between this value and the system short-circuit impedance per unit value to obtain the lower limit of the short-circuit impedance range.

[0029] The formula for calculating the per-unit value of the minimum total short-circuit impedance is as follows: .

[0030] in This is the maximum allowable short-circuit current of the circuit. Represents the voltage at the grid connection point. Representing the transformer's rated capacity, this formula is based on the premise that the per-unit value of the three-phase short-circuit current equals the per-unit value of the equivalent voltage at the short-circuit point divided by the per-unit value of the total impedance at the short-circuit point. , This is the per-unit voltage value at the fault point before the short circuit, usually taken as the rated voltage, hence the per-unit value is 1. This is the per-unit value of the short-circuit current. And because... , Therefore ;make Substituting these values ​​into the expression for the minimum total short-circuit impedance per unit, we obtain the expression for this minimum total short-circuit impedance. This is the effective value of the rated breaking current of the circuit breaker. The reference current is used for per-unit value calculation, and its expression is based on existing technology.

[0031] S123. Obtain the per-unit value of the voltage fluctuation range at the grid connection point from the power grid system's backend database, and record the ratio of the maximum power change of the system's photovoltaic output to the rated capacity of the transformer as the per-unit value of the power change. In this embodiment, for example, the per-unit value of the voltage fluctuation range is 5%, which represents the maximum permissible unidirectional deviation of the voltage relative to the rated voltage.

[0032] S124. Calculate the ratio of the per-unit value of voltage fluctuation range to the per-unit value of power change, and record the difference between this and the per-unit value of system short-circuit impedance as the upper limit of the short-circuit impedance range.

[0033] S125. The upper limit and lower limit of the interval based on the short-circuit impedance constitute the first short-circuit impedance interval.

[0034] This invention determines the transformer library based on the grid connection point voltage and the inverter AC side output voltage, obtains the rated breaking current of the grid connection point circuit breaker and the system short-circuit impedance, and solves the first short-circuit impedance range by combining the grid connection point voltage and fluctuation range, so as to avoid grid safety accidents such as equipment damage and line tripping caused by excessive short-circuit current.

[0035] S2. Analyze the dangerous short-circuit impedances corresponding to each harmonic to determine the short-circuit impedance sequence.

[0036] Considering that photovoltaic inverters generate multi-frequency harmonic currents during operation with load changes, when the circuit impedance formed by the system, transformer, and grid-connected capacitor bank is matched to a specific value, it is very easy to cause harmonic resonance and harmonic amplification problems, which degrade power quality and burn out power equipment. Different frequency harmonics correspond to specific dangerous impedance ranges, which are prohibited from operation. Therefore, it is necessary to first identify the main harmful harmonics, delineate the prohibited short-circuit impedance range, and then combine it with the first short-circuit impedance range mentioned above to screen out a qualified short-circuit impedance sequence that simultaneously meets the requirements of grid safety and harmonic suppression.

[0037] Based on this, the specific implementation steps of S2 include: S21, obtaining the harmonic spectrum of the inverter under different loads and analyzing the dangerous short-circuit impedance corresponding to each harmonic. The specific implementation steps are as follows: S211, measuring the harmonic spectrum output by the AC side of the photovoltaic inverter under various set loads, and extracting the current content of each harmonic. This is prior art and will not be described in detail here.

[0038] S212. Obtain all harmonics whose harmonic content rate is greater than a set content rate threshold under each load, and record them as harmonics to be prevented. Record the union of the harmonics to be prevented under each load as the set of harmonic orders to be prevented. In this embodiment, the content rate threshold is set to 5%, meaning that when the content rate of a certain harmonic is greater than 5%, it is determined to be a harmonic to be prevented. The implementer can also set other specific values ​​according to the actual situation.

[0039] S213. Obtain the capacity of the parallel capacitor bank at the grid connection point and the voltage at the grid connection point to calculate the fundamental capacitive reactance. Based on the number of each harmonic in the set of harmonics to be prevented, infer the dangerous short-circuit reactance.

[0040] It should be noted that the formula for calculating the fundamental capacitance is as follows: .

[0041] in Represents fundamental frequency capacitance. This represents the capacity of the parallel capacitor bank, which is existing technology and will not be described in detail here.

[0042] Furthermore, the calculation formula for the dangerous short-circuit reactance is as follows: .

[0043] in Represents a dangerous short-circuit reactor. The representative system reactance can be obtained from the power grid's backend database. This represents the order of the i-th harmonic.

[0044] S214. Based on the capacity of each transformer in the transformer library, obtain each reference impedance. Based on each reference impedance, convert the dangerous short-circuit reactance into a dangerous short-circuit impedance in percentage form corresponding to each transformer capacity. Specifically, calculate the ratio of the dangerous short-circuit reactance to the reference impedance of each transformer, convert it into a percentage form, and record it as the dangerous short-circuit impedance.

[0045] S22. Determine the safe impedance range of the circuit based on the dangerous short-circuit impedance corresponding to each harmonic. For example... Figure 2 As shown, the specific implementation steps are as follows: S221, obtain the dangerous short-circuit impedance corresponding to each harmonic at each transformer capacity, record the difference between it and the preset half-width as the minimum value of the extended interval, record the sum of the dangerous short-circuit impedance and the preset half-width as the maximum value of the extended interval, and combine the minimum value of the extended interval to obtain the extended interval corresponding to the dangerous short-circuit impedance. In this embodiment, the preset half-width is set to 0.5% to avoid false safety gaps caused by numerical calculation errors or measurement noise, and to merge adjacent dangerous intervals; the implementer can also set the half-width to other specific values, such as adjusting it within the range of 0.2% to 1%.

[0046] S222. Calculate the extended range of dangerous short-circuit impedance corresponding to all harmonics under each transformer capacity, and combine them to form the short-circuit impedance prohibition range.

[0047] S223. Obtain the remaining range after removing the short-circuit impedance prohibited range, and record it as the circuit safe impedance range under each transformer capacity.

[0048] S23. Determine the short-circuit impedance sequence based on the first short-circuit impedance interval. The specific implementation steps include: S231. Extracting the standard short-circuit impedance sequence of the transformer from the power grid system's backend database, and obtaining the intersection of the circuit safety impedance interval and the first short-circuit impedance interval for each transformer capacity. The standard short-circuit impedance sequence refers to the standard short-circuit impedance percentage sequence specified in the transformer industry standard, such as 4%, 6%, 8%, 10%, etc.

[0049] S232. If a standard short-circuit impedance sequence exists in the intersection, then the standard short-circuit impedance sequence shall be taken as the qualified short-circuit impedance, and all qualified short-circuit impedances shall be combined to form the short-circuit impedance sequence under the corresponding transformer capacity.

[0050] This invention obtains the harmonic spectrum of the inverter under different loads, analyzes the dangerous short-circuit impedance corresponding to each harmonic, determines the safe impedance range of the circuit based on the dangerous short-circuit impedance corresponding to each harmonic, and determines the short-circuit impedance sequence in combination with the first short-circuit impedance range, thereby avoiding the risks of harmonic resonance and harmonic amplification, ensuring the power quality at the grid connection point, and improving the reliability of the selection results.

[0051] S3. Screen candidate transformers, analyze the annual thermal life loss rate of each candidate transformer, and calculate the annual economic loss due to insulation aging.

[0052] Considering the randomness and fluctuation of photovoltaic power plant output, the transformer load rate will change in real time with the photovoltaic output. The load fluctuation, coupled with the change of ambient temperature, will cause the transformer winding hot spot temperature to change continuously. High winding temperature will accelerate the aging of insulation materials and shorten the actual service life of equipment. Traditional selection methods ignore the hidden economic losses caused by thermal aging and cannot truly reflect the full life cycle cost of transformer. Therefore, it is necessary to calculate the thermal life loss rate based on actual operating load and temperature data to quantify the economic losses corresponding to insulation aging.

[0053] Based on this, the specific implementation steps of S3 include: S31, screening candidate transformers from the transformer library based on the short-circuit impedance sequence, and analyzing the annual thermal life loss rate of each candidate transformer according to the inverter AC side output power within a preset period. For example... Figure 3 As shown, the specific implementation steps include: S311, for each candidate transformer, if there is a transformer whose short-circuit impedance is equal to a certain short-circuit impedance in the short-circuit impedance sequence under the transformer capacity, then the transformer is recorded as a candidate transformer.

[0054] S312. Obtain the power factor of each candidate transformer from the transformer library, and calculate the load factor of the transformer based on the inverter AC side output power at each time point within the preset period, the rated capacity of each candidate transformer, and the power factor.

[0055] It should be noted that the formula for calculating the load rate is as follows: .

[0056] in This refers to the AC output power of the inverter. The power factor at the grid connection point of the photovoltaic power station can be obtained from the backend database of the photovoltaic power station monitoring system. This formula is an existing formula for calculating the load rate, and will not be described in detail in this invention.

[0057] Furthermore, in this embodiment, the preset period is one year, and each time point is one hour. The implementer can also set other specific values.

[0058] S313. Obtain the load rate change sequence and ambient temperature change sequence of the transformer within a preset period. Obtain the hot spot temperature rise and rated hot spot temperature of the candidate transformer under rated load from the power grid system background database, and calculate the winding hot spot temperature at each time point. Specifically, obtain the square of the load rate change at a certain time point, multiply it by the hot spot temperature rise under rated load, and record the sum of the product and the temperature at that time point as the winding hot spot temperature.

[0059] S314. Calculate the relative aging rate of each candidate transformer based on the winding hot spot temperature and rated hot spot temperature at each time point. Integrate the relative aging rate at all time points within the preset period to obtain the total aging life loss. Record the ratio of this to the number of years in the preset period as the annual thermal life loss rate.

[0060] It should be explained that the formula for calculating the relative aging rate is as follows: .

[0061] in (t) represents the relative aging rate at time point t. This represents the winding hotspot temperature at time point t. Represents the rated hot spot temperature. The temperature constant is typically taken as 6~10℃, and in this embodiment, it is taken as 8℃. Furthermore, in this embodiment, the preset period is 1 year, therefore the preset period is 1 year.

[0062] S32. Calculate the annual economic loss due to insulation aging based on the initial purchase cost. The specific calculation method is as follows: the product of the initial purchase cost of each candidate transformer and the annual thermal life loss rate is taken as the annual economic loss due to insulation aging of the candidate transformer within the preset period.

[0063] This invention selects candidate transformers from a transformer library based on short-circuit impedance sequences, analyzes the annual thermal life loss rate of each candidate transformer based on the inverter's AC side output power within a preset period, and calculates the annual insulation aging economic loss in conjunction with the initial purchase cost. This invention overcomes the shortcomings of traditional solutions that ignore thermal aging costs, and makes the economic analysis more in line with the actual operating conditions of the transformer.

[0064] S4. Analyze annual loss electricity costs, calculate levelized energy expenditure, and determine the matching transformer.

[0065] Considering that there are fixed no-load losses and load losses that vary with the load throughout the operation of a transformer, the lost electricity will generate continuous electricity expenses based on the time-of-use electricity price in the region; at the same time, money has time value, and the annual expenses under different service years need to be discounted; and the transformer purchase cost, aging loss, and operating loss need to be evaluated in conjunction with the power generation throughout the entire life cycle. A single static cost cannot objectively evaluate the comprehensive benefits of the equipment.

[0066] Based on this, the specific implementation steps of S4 include: S41, obtaining the no-load loss power and load loss power of each candidate transformer to analyze the annual electricity loss cost. The specific implementation steps are as follows: S411, calculating the electricity loss at each time point based on the no-load loss power and load loss power of each candidate transformer combined with the load rate change sequence, and obtaining the time-of-use electricity price table for the region where the photovoltaic power station is located.

[0067] S412. Based on the time-of-use electricity price table, obtain the electricity price at each time point, and multiply it with the electricity loss at the corresponding time point to calculate the electricity loss charge at each time point.

[0068] S413. Integrate the electricity loss cost at each time point within the preset period to obtain the total electricity loss cost, and calculate the annual electricity loss cost by comparing it with the number of years in the preset period.

[0069] S42. Calculate the total discounted cost based on the annual economic loss due to insulation aging. The specific implementation steps include: S421. Obtain the design service life of each candidate transformer from the transformer database. If the total aging life loss within the preset period is less than 1, calculate the product of the total aging life loss and the design service life to obtain the service life depreciation amount. Record the difference between the design service life and the service life depreciation amount as the estimated service life.

[0070] S422. If the total aging life loss is greater than or equal to 1, the transformer loss is determined to be abnormal and does not meet the requirements, and it is removed from the candidate transformers. It should be noted that if all candidate transformers are removed due to the total aging life loss being greater than or equal to 1, it is determined that there is currently no transformer that meets the requirements, a matching failure warning message is output, and the matching process is terminated.

[0071] S423. Calculate the sum of annual power loss and annual insulation aging economic loss, and record it as the total annual cost. Calculate the discounted cost for each service life based on the current annual interest rate and the expected service life of each candidate transformer.

[0072] In a preferred embodiment of the present invention, the formula for calculating the discounting cost is as follows: .

[0073] in This represents the discount expense in year a. This represents the total annual cost for year a. This represents the number of years from the current year to the year a, where a = 1, 2, 3, ..., W, and W is the expected lifespan.

[0074] in Representative process The current discounted cost can be obtained by taking the ratio of the total annual cost in year a to the total multiple of the years.

[0075] S424. Calculate the sum of the discount costs for each useful life and sum it with the initial purchase cost to record the total discount cost.

[0076] S43. Calculate the levelized energy expenditure (LEE) based on the total power generation on the high-voltage side, and determine the matching transformer based on the LEE. The specific implementation steps are as follows: S431. Calculate the total power generation on the high-voltage side of each candidate transformer during its expected service life, based on the power generation on the high-voltage side within a preset period and the expected service life of each candidate transformer.

[0077] S432. The ratio of the total discounted cost of each candidate transformer to the total power generation on the high-voltage side is denoted as the levelized energy expenditure, and the transformer with the minimum levelized energy expenditure among the candidate transformers is denoted as the matched transformer.

[0078] This invention analyzes the annual power loss cost by acquiring the no-load loss power and load loss power of each candidate transformer, calculates the total discounted cost by combining the annual insulation aging economic loss, calculates the levelized energy expenditure by combining the total power generation on the high-voltage side, and determines the matching transformer based on the levelized energy expenditure. This solves the problem of distortion in traditional static cost calculation and takes into account both the safety and economy of the selection results.

[0079] The above embodiments can be implemented, in whole or in part, by software, hardware, firmware, or any other combination thereof. When implemented using software, the above embodiments can be implemented, in whole or in part, in the form of a computer program product.

[0080] Those skilled in the art will recognize that the modules and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0081] In addition, the functional modules in the various embodiments of this application can be integrated into one processing module, or each module can exist physically separately, or two or more modules can be integrated into one module.

[0082] The above description is merely a specific embodiment of this application, but the scope of protection of this application is not limited thereto. Any variations or substitutions that can be easily conceived by those skilled in the art within the scope of the technology disclosed in this application should be included within the scope of protection of this application. Therefore, the scope of protection of this application should be determined by the scope of the claims.

[0083] Finally, the above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A matching method for a grid-connected transformer in a photovoltaic power station, characterized in that, include: The transformer library is determined based on the grid connection point voltage and the inverter AC side output voltage. The rated circuit breaking current of the grid connection point circuit breaker and the system short-circuit impedance are obtained. The first short-circuit impedance interval is solved by combining the grid connection point voltage and the fluctuation range. Obtain the harmonic spectrum of the inverter under different loads, analyze the dangerous short-circuit impedance corresponding to each harmonic, determine the safe impedance range of the circuit based on the dangerous short-circuit impedance corresponding to each harmonic, and determine the short-circuit impedance sequence in combination with the first short-circuit impedance range. Based on the short-circuit impedance sequence, candidate transformers are selected from the transformer library. The annual thermal life loss rate of each candidate transformer is analyzed according to the AC side output power of the inverter within a preset period. The annual insulation aging economic loss is calculated in combination with the initial purchase cost. The no-load loss power and load loss power of each candidate transformer are obtained to analyze the annual loss electricity cost. The total discounted cost is calculated by combining the annual insulation aging economic loss. The levelized energy expenditure is calculated by combining the total power generation on the high-voltage side. The matching transformer is determined based on the levelized energy expenditure.

2. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 1, characterized in that, The solution method for the first short-circuit impedance interval includes: The product of the rated breaking current of the circuit breaker at the grid connection point and the preset safety margin factor is recorded as the maximum allowable short-circuit current of the circuit. The per-unit value of the system short-circuit impedance is obtained based on the ratio of the rated capacity of the transformer to the system short-circuit capacity. The minimum total short-circuit impedance per unit value is calculated based on the maximum allowable short-circuit current, grid connection point voltage, and transformer rated capacity. The difference between this value and the system short-circuit impedance per unit value is then used to calculate the lower limit of the short-circuit impedance range. The per-unit value of the voltage fluctuation range at the grid connection point is obtained from the background database of the power grid system. The ratio of the maximum power change of the system's photovoltaic output to the rated capacity of the transformer is recorded as the per-unit value of the power change. Calculate the ratio of the per-unit value of the voltage fluctuation range to the per-unit value of the power change, and record the difference between this and the per-unit value of the system short-circuit impedance as the upper limit of the short-circuit impedance range; The first short-circuit impedance interval is formed by the upper and lower limits of the short-circuit impedance interval.

3. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 1, characterized in that, The analysis methods for the dangerous short-circuit impedance corresponding to each harmonic include: Measure the harmonic spectrum of the AC output of the photovoltaic inverter under various load settings, and extract the current content of each harmonic from it; Obtain each harmonic with a harmonic content rate greater than a set content rate threshold under each load, and record them as harmonics to be prevented. Record the union of the harmonics to be prevented under each load as the set of harmonic orders to be prevented. Obtain the capacity of the parallel capacitor bank at the grid connection point and the voltage at the grid connection point to calculate the fundamental capacitive reactance. Based on the number of each harmonic in the set of harmonics to be prevented, infer the dangerous short-circuit reactance. Based on the capacity of each transformer in the transformer library, each reference impedance is obtained. Based on each reference impedance, the dangerous short-circuit reactance is converted into dangerous short-circuit impedance in the form of a percentage corresponding to each transformer capacity.

4. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 3, characterized in that, The method for obtaining the safe impedance range of the circuit includes: Obtain the critical short-circuit impedance corresponding to each harmonic at each transformer capacity, record the difference between it and the preset half-width as the minimum value of the extended range, record the sum of the critical short-circuit impedance and the preset half-width as the maximum value of the extended range, and combine the minimum value of the extended range to obtain the extended range corresponding to the critical short-circuit impedance. The extended range of dangerous short-circuit impedance corresponding to all harmonics under each transformer capacity is statistically analyzed, and these ranges are collectively formed into the short-circuit impedance prohibition range. Obtain the remaining range after removing the short-circuit impedance forbidden range, and record it as the circuit safe impedance range for each transformer capacity.

5. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 1, characterized in that, The method for obtaining the short-circuit impedance sequence includes: Extract the standard short-circuit impedance sequence of transformers from the power grid system's backend database, and obtain the intersection of the circuit safety impedance range and the first short-circuit impedance range for each transformer capacity; If a standard short-circuit impedance sequence exists in the intersection, then that standard short-circuit impedance sequence is taken as the qualified short-circuit impedance, and all qualified short-circuit impedances are combined to form the short-circuit impedance sequence under the corresponding transformer capacity.

6. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 5, characterized in that, The methods for obtaining the annual thermal life loss rate of each candidate transformer include: For each candidate transformer, if there exists a transformer whose short-circuit impedance is equal to a certain short-circuit impedance in the short-circuit impedance sequence under that transformer capacity, then that transformer is recorded as a candidate transformer. The power factor of each candidate transformer is obtained from the transformer library. The load factor of the transformer is calculated based on the AC output power of the inverter at each time point within the preset period, the rated capacity of each candidate transformer, and the power factor. Obtain the load rate change sequence and ambient temperature change sequence of the transformer within a preset period, obtain the hot spot temperature rise and rated hot spot temperature of the candidate transformer under rated load from the power grid system background database, and calculate the winding hot spot temperature at each time point. The relative aging rate of each candidate transformer is calculated based on the winding hot spot temperature and the rated hot spot temperature at each time point. The relative aging rate at all time points within the preset period is integrated over time to obtain the total aging life loss. The ratio of this to the number of years in the preset period is recorded as the annual thermal life loss rate.

7. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 6, characterized in that, The calculation method for the annual economic loss due to insulation aging includes: The product of the initial purchase cost of each candidate transformer and the annual thermal life loss rate is taken as the annual insulation aging economic loss of the candidate transformer within the preset period.

8. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 1, characterized in that, The analysis method for the annual electricity loss cost includes: Based on the no-load loss power and load loss power of each candidate transformer combined with the load rate change sequence, the power loss at each time point is calculated to obtain the time-of-use electricity price table for the region where the photovoltaic power station is located. The electricity price at each time point is obtained based on the time-of-use electricity price table. The electricity price at each time point is then multiplied by the electricity loss at the corresponding time point to calculate the electricity loss charge at each time point. The total electricity loss cost is obtained by integrating the electricity loss cost at each time point within the preset period. The annual electricity loss cost is then calculated by comparing this total cost with the number of years in the preset period.

9. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 6, characterized in that, The method for obtaining the total discount cost includes: The design service life of each candidate transformer is obtained from the transformer library. If the total aging life loss within the preset period is less than 1, the product of the total aging life loss and the design service life is calculated to obtain the service life depreciation amount. The difference between the design service life and the service life depreciation amount is recorded as the expected service life. If the total aging life loss is greater than or equal to 1, the transformer loss is deemed abnormal and does not meet the requirements, and it is removed from the list of candidate transformers. Calculate the sum of annual power loss and annual insulation aging economic loss, and record it as the total annual cost. Calculate the discounted cost for each service life based on the current annual interest rate and the expected service life of each candidate transformer. Calculate the sum of the discount costs for each useful life, and sum this with the initial purchase cost to record the total discount cost.

10. The matching method for a grid-connected transformer in a photovoltaic power station according to claim 9, characterized in that, The method for determining the matching transformer includes: Based on the high-voltage side power generation within a preset period and the expected service life of each candidate transformer, calculate the total high-voltage side power generation of each candidate transformer within its expected service life. The ratio of the total discounted cost of each candidate transformer to the total power generation on the high-voltage side is denoted as the levelized energy expenditure, and the transformer with the minimum levelized energy expenditure among the candidate transformers is denoted as the matched transformer.

Images