Method for calculating equivalent conductor temperature rise and correcting loss rate of underground cable of photovoltaic power station

Abstract

The application relates to the technical field of photovoltaic cable loss calculation, and discloses a photovoltaic power station underground cable equivalent conductor temperature rise calculation and loss rate correction method, which comprises the following steps: obtaining daily periodic load current data of underground cables of a photovoltaic power station; performing transient thermal simulation on the underground cables under a target laying mode to obtain daily temperature change data after the conductor temperature reaches periodic stability; based on the same loss equivalent temperature principle, the equivalent conductor temperature of the cable is calculated by using the daily periodic load current data and the stable daily temperature change data; the resistance value of the cable conductor is corrected according to the equivalent conductor temperature, and then the corrected cable loss rate is calculated. The actual loss of the conductor under the periodic current is equivalent to the loss under a certain constant temperature, so that the equivalent conductor temperature rise is determined, and the accuracy of the underground cable loss calculation of the photovoltaic power station can be effectively improved.

Description

Technical Field

[0001] This invention relates to the field of photovoltaic cable loss calculation technology, and in particular to a method for calculating the temperature rise of the equivalent conductor and correcting the loss rate of underground cables in photovoltaic power plants. Background Technology

[0002] With the rapid development of the global clean energy industry, photovoltaic (PV) power plants, as the core carrier for the large-scale development and utilization of solar energy, have seen continuous expansion in installed capacity. System power generation efficiency and life-cycle economic benefits have become core considerations in the design, construction, and operation phases of PV power plants. Underground power cables, as a crucial transmission link connecting inverters and prefabricated substations in PV power plants, bear the core function of power transmission. The power loss generated by their conductor resistance is a major component of the line loss in PV systems, directly affecting the overall system performance ratio, power output, and ultimate return on investment.

[0003] Currently, mainstream professional design software is widely used in photovoltaic power plant design to calculate cable losses. This software typically derives the cable loss rate based on conductor resistance values ​​under standard test conditions and the actual load rate of the cable. This calculation method has a significant inherent technical flaw: it fails to consider the temperature rise effect caused by the Joule heating of the cable conductor during current flow, and the further increase in conductor resistance due to this temperature rise. While conductor resistance increases linearly with temperature, the output power of a photovoltaic power plant exhibits a clear diurnal periodicity due to factors such as solar radiation intensity and weather conditions. Cables operate under intermittent, periodic loads for extended periods. Furthermore, the added effect of underground ambient temperature means that the actual conductor temperature during operation is far higher than under standard test conditions. Consequently, the cable loss value calculated using traditional methods is significantly lower than the actual operating loss.

[0004] The aforementioned technical deficiencies can trigger a series of chain reactions in engineering practice, particularly in international photovoltaic projects with stringent line loss assessment requirements. These projects typically have clear and stringent individual assessment standards for both system performance ratio and AC / DC cable line loss. If traditional methods are used to underestimate cable loss, it can lead to overestimation of system efficiency in the initial design, resulting in substandard line loss during actual operation and consequently, substantial penalties. Furthermore, if the design is passively adjusted later in the project to meet line loss requirements, it often necessitates increasing the cable cross-section, significantly increasing costs for cable material procurement, laying, and earthwork, severely impacting the project's economic benefits. In addition, many existing temperature rise calculation methods rely on steady-state thermal simulation models, which cannot accurately simulate the dynamic temperature rise process of cable conductors under periodic loads in photovoltaic power plants. They also fail to adequately consider the mutual heating effects when multiple cables are laid in parallel, resulting in significant deviations between calculation results and actual operating conditions, making it difficult to meet the demands of high-precision engineering design.

[0005] Therefore, developing a method that can accurately calculate the temperature rise of the equivalent conductor of underground cables in photovoltaic power plants and scientifically correct the loss rate has become a technical problem that urgently needs to be solved in this field. Summary of the Invention

[0006] To address the aforementioned issues, this invention proposes a method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants. This method equates the actual loss of the conductor under periodic current to the loss at a constant temperature, thereby determining the equivalent conductor temperature rise. This invention can effectively improve the accuracy of loss calculation for underground cables in photovoltaic power plants.

[0007] The technical solution adopted in this invention is as follows: A method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants includes: Obtain daily periodic load current data of underground cables in photovoltaic power plants; Transient thermal simulation was performed on underground cables under the target laying method to obtain daily temperature variation data after the conductor temperature reached periodic stability. Based on the principle of equivalent temperature with the same loss, the equivalent conductor temperature of the cable is calculated using daily periodic load current data and stable daily temperature change data. The resistance value of the cable conductor is corrected based on the equivalent conductor temperature, and then the corrected cable loss rate is calculated.

[0008] Furthermore, the acquisition of daily periodic load current data of underground cables of photovoltaic power stations includes: collecting current data for the entire day according to the stepping method of duration and corresponding current; distinguishing the current characteristics of power generation periods and non-power generation periods, so that the collected current data presents a daily periodic variation pattern, with the current peak corresponding to the period with the highest solar radiation intensity, and the current being zero during non-power generation periods.

[0009] Furthermore, the transient thermal simulation of the underground cable under the target laying method to obtain daily temperature change data after the conductor temperature reaches periodic stability includes: building a cable laying simulation model using transient thermal simulation software, setting the cable laying method, ambient ground temperature, and rated current carrying conditions; continuously running the cable laying simulation model until the cable conductor temperature shows a stable periodic fluctuation state, and extracting conductor temperature change data for a complete natural day after the temperature stabilizes.

[0010] Furthermore, the transient thermal simulation of underground cables under the target laying method also includes: for the scenario of multiple cables being laid in parallel, simulating and obtaining the daily temperature change data of the conductor of each cable; and selecting the daily temperature change data corresponding to the cable with the highest temperature among all cables as the basis data for subsequent calculations.

[0011] Furthermore, the calculation of the equivalent conductor temperature of the cable based on the principle of equivalent temperature with the same loss, using daily periodic load current data and stable daily temperature change data, includes: calculating the instantaneous conductor loss corresponding to the current at each moment within a cycle based on the linear relationship between conductor resistance and temperature; summing the instantaneous conductor losses at all moments within a cycle to obtain the total loss within the cycle; calculating the root mean square value of the load current within a cycle; deriving the equivalent conductor resistance based on the total loss within the cycle and the root mean square value of the current; and calculating the equivalent conductor temperature by combining the equivalent conductor resistance with the linear relationship between conductor resistance and temperature.

[0012] Furthermore, the calculation of the equivalent conductor temperature of the cable based on the principle of equivalent temperature with the same loss, using daily periodic load current data and stable daily temperature change data, includes: determining whether the daily temperature fluctuation range within one cycle after the conductor temperature stabilizes is within a preset threshold range; if the fluctuation range is within the preset threshold range, directly taking the average temperature of one natural day after the conductor temperature stabilizes as the equivalent conductor temperature.

[0013] Furthermore, the step of correcting the resistance value of the cable conductor based on the equivalent conductor temperature and then calculating the corrected cable loss rate includes: calculating the corrected conductor resistance per unit length by substituting the equivalent conductor temperature into the linear relationship between conductor resistance and temperature; replacing the conductor resistance value under standard test conditions with the corrected conductor resistance per unit length; and calculating the corrected cable loss rate by combining the actual length of the cable and the operating current parameters.

[0014] Furthermore, after calculating the corrected cable loss rate, the method further includes: calculating the theoretical cable loss rate under standard test conditions, comparing the corrected cable loss rate with the theoretical loss rate under standard test conditions, and calculating the loss difference rate between the two; and assessing the degree of loss underestimation when the conductor temperature rise effect is not considered based on the loss difference rate.

[0015] Furthermore, after calculating the corrected cable loss rate, the method further includes: applying the corrected cable loss rate to the system efficiency assessment of the photovoltaic power station, selecting and designing the cable cross-section based on the corrected loss rate, and adjusting the cable design parameters.

[0016] Furthermore, when performing transient thermal simulation on underground cables under the target laying method, the annual average measured ground temperature at the cable burial depth is preferentially used as the environmental ground temperature parameter for transient thermal simulation; when there is no measured ground temperature data, the ground temperature parameter at the cable burial depth is calculated based on the local air temperature data.

[0017] The beneficial effects of this invention are as follows: 1. This invention, through a technical solution combining transient thermal simulation and the equivalent temperature principle of the same loss, effectively solves the problem of insufficient accuracy in calculating the equivalent temperature rise of cable conductors under periodic loads in existing technologies, thus significantly improving the accuracy of loss calculations for underground cables in photovoltaic power plants. This invention uses transient thermal simulation to model the dynamic temperature rise process of cables under periodic loads, accurately capturing the daily periodic fluctuations in conductor temperature with current changes. Compared to traditional steady-state thermal simulation methods, it better reflects the actual intermittent operation of cables in photovoltaic power plants. Simultaneously, based on the equivalent temperature principle of the same loss, it equates the complex, time-varying conductor loss process to a loss process at a constant temperature, achieving quantification and simplification of dynamic losses. This, to a certain extent, compensates for the shortcomings of traditional design methods that neglect conductor temperature rise effects, making the loss calculation results closer to engineering reality.

[0018] 2. This invention balances computational accuracy with engineering practicality, adapting to application scenarios with varying design stages and accuracy requirements. It provides two methods for calculating equivalent conductor temperature. The accurate calculation method based on instantaneous loss accumulation calculates conductor loss time-by-time and derives equivalent resistance and temperature, yielding highly accurate equivalent temperature results, suitable for photovoltaic power station projects with high accuracy requirements for line loss calculation. The simplified calculation method based on daily average temperature utilizes the characteristic of small daily temperature fluctuations after photovoltaic cable temperature stabilizes, directly using the daily average temperature as the equivalent temperature. This effectively simplifies the calculation process, improves engineering calculation efficiency, and is suitable for rapid estimation in the preliminary design stage of a project. Furthermore, for common engineering scenarios involving parallel laying of multiple cables, this invention fully considers the mutual heating effect between adjacent cables, selecting the cable with the highest temperature for subsequent calculations. This ensures the conservatism of temperature rise and loss assessment, helping to reduce risks such as cable overheating and shortened lifespan caused by overly risky designs, thus improving the safety of engineering design.

[0019] 3. This invention provides a more scientific basis for the engineering design of photovoltaic power plants, helping to achieve a balance between engineering risk control and cost optimization. The cable loss rate, corrected for equivalent conductor temperature, more accurately reflects the actual operating loss level of the cable. Based on this result, system efficiency assessment and power generation prediction can yield more realistic and reliable design indicators, helping to reduce the risks of substandard system efficiency and line loss penalties caused by underestimating losses using traditional methods. Simultaneously, in the cable cross-section selection stage, using the corrected loss rate to calculate voltage drop and power loss allows for the selection of more reasonable cable cross-section specifications while meeting line loss and voltage drop design requirements. This reduces material waste and cost increases caused by blindly increasing cable cross-sections, better balancing the safety and economy of the project.

[0020] 4. This invention has a wide range of applications and is easy to promote and apply in engineering practice. The technical solution of this invention is applicable to the design of underground cables for photovoltaic power plants of different laying methods, voltage levels, and scales. It has good applicability to foreign photovoltaic projects with strict line loss assessment requirements. At the same time, the method of this invention can be implemented using existing mature cable thermal simulation software without the need for additional complex hardware equipment. It has low requirements for professional operation and can be well integrated into the existing photovoltaic power plant design process. It is easy for engineering technicians to master and apply, and it has a certain positive effect on improving the accuracy of cable design and the quality of engineering construction in the photovoltaic industry. Attached Figure Description

[0021] Figure 1 This is a flowchart of a method for calculating the temperature rise of the equivalent conductor and correcting the loss rate of an underground cable in a photovoltaic power station, according to Embodiment 1 of the present invention.

[0022] Figure 2 This is the temperature rise curve (2000 hours) of the three cables in Embodiment 2 of the present invention.

[0023] Figure 3 This is the temperature rise curve of the three cables in Embodiment 2 of the present invention (500 hours after reaching 55°C).

[0024] Figure 4 This is the temperature rise curve of the three cables in Embodiment 2 of the present invention (ground temperature 25°C, 2000 hours). Detailed Implementation

[0025] To provide a clearer understanding of the technical features, objectives, and effects of the present invention, specific embodiments are now described. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the invention, not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0026] Example 1 like Figure 1 As shown, this embodiment provides a method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants, including: Obtain daily periodic load current data of underground cables in photovoltaic power plants; Transient thermal simulation was performed on underground cables under the target laying method to obtain daily temperature variation data after the conductor temperature reached periodic stability. Based on the principle of equivalent temperature with the same loss, the equivalent conductor temperature of the cable is calculated using daily periodic load current data and stable daily temperature change data. The resistance value of the cable conductor is corrected based on the equivalent conductor temperature, and then the corrected cable loss rate is calculated.

[0027] Preferably, the acquisition of daily periodic load current data of underground cables of photovoltaic power stations includes: collecting current data for the entire day according to a step format of duration and corresponding current; distinguishing the current characteristics of power generation periods and non-power generation periods, so that the collected current data presents a daily periodic variation pattern, with the current peak corresponding to the period with the highest solar radiation intensity, and the current being zero during non-power generation periods.

[0028] Specifically, based on the solar radiation patterns of the area where the photovoltaic power station is located, the 24 hours of the day are divided into multiple continuous time intervals, each time interval corresponding to a constant current value, forming a step-current curve of duration-current. Among them, the current value is set to zero during the non-power generation periods from dawn to sunrise and from sunset to night. After sunrise, the current gradually increases with the solar radiation intensity, reaching its peak at noon when the solar radiation intensity is the highest. In the afternoon, the current gradually decreases with the decrease in solar radiation intensity, until it drops to zero at sunset.

[0029] It should be noted that the daily periodic load current data collected using the step format can accurately reproduce the actual characteristics of the photovoltaic power plant's output power changing with solar radiation, providing input conditions that conform to engineering practice for subsequent transient thermal simulation and equivalent temperature calculation, and avoiding calculation errors caused by distortion of the current model.

[0030] Preferably, the step of performing transient thermal simulation on the underground cable under the target laying method to obtain daily temperature change data after the conductor temperature reaches periodic stability includes: building a cable laying simulation model using transient thermal simulation software; setting the cable laying method, ambient ground temperature, and rated current carrying conditions; continuously running the simulation model until the cable conductor temperature exhibits a stable periodic fluctuation state; and extracting conductor temperature change data for a complete natural day after the temperature stabilizes.

[0031] Specifically, a cable transient thermal simulation software was selected to build a basic cable model based on the structural parameters, insulation type, and conductor material of the target cable. The cable laying method, laying spacing, and burial depth were set according to the actual engineering laying scenario. The rated current carrying conditions of the cable and the previously obtained daily periodic load current curves were input. The simulation was started and continuously run, monitoring the temperature change process of the cable conductor. When the conductor temperature change curves of several consecutive natural days completely overlapped, it was determined that the conductor temperature had reached a periodic stable state. At this point, the hourly change data of the conductor temperature for the last complete natural day was extracted as the basis for subsequent calculations.

[0032] It should be noted that transient thermal simulation can simulate the dynamic temperature rise process of cables under periodic loads, accurately capture the fluctuation law of conductor temperature with current change, and is more consistent with the actual operating conditions of photovoltaic power station cables than steady-state thermal simulation, which can significantly improve the accuracy of temperature data.

[0033] More preferably, the transient thermal simulation of the underground cable under the target laying method further includes: for the scenario of multiple cables being laid in parallel, simulating and obtaining the daily temperature change data of the conductor of each cable; and selecting the daily temperature change data corresponding to the cable with the highest temperature among all cables as the basis data for subsequent calculations.

[0034] Specifically, when building a simulation model of multiple cables laid in parallel, the relative position and laying spacing of each cable are accurately set, and the mutual heating effect between adjacent cables is fully considered. After the simulation is completed, the daily temperature change data of each cable after the temperature stabilizes are extracted. The daily maximum temperature, daily average temperature and other parameters of all cables are compared to determine the cable that is most affected by the mutual heating effect and has the highest temperature. The temperature data of this cable is then used for subsequent equivalent temperature and loss calculations.

[0035] It should be noted that when multiple cables are laid in parallel, the cable in the middle position is most significantly affected by the mutual heating effect of the cables on both sides, and its temperature is significantly higher than that of the edge cables. Using the temperature data of the hottest cable for calculation can ensure the conservatism of cable loss and temperature rise assessment, avoid the design bias caused by selecting the data of the cable with the lower temperature, and improve the safety of the project.

[0036] More preferably, when setting the ambient ground temperature of the cable, the annual average measured ground temperature at the cable burial depth is used as the simulated ambient ground temperature parameter; when there is no measured ground temperature data, the ground temperature parameter at the cable burial depth is calculated based on the local air temperature data.

[0037] Specifically, for projects with suitable conditions, ground temperature monitoring points are set up at the cable burial depth to continuously monitor ground temperature data for at least one calendar year, calculate the annual average ground temperature, and use it as the simulation input; for projects without measured ground temperature data, the environmental ground temperature at the cable burial location is estimated by referring to the multi-year average air temperature data released by the local meteorological department and according to the conversion relationship between air temperature and ground temperature at the underground burial depth.

[0038] It should be noted that ambient ground temperature is one of the key factors affecting the temperature rise of cable conductors. Using ground temperature parameters that conform to the actual engineering situation for simulation can effectively reduce calculation errors caused by deviations in environmental parameters. At the same time, using the annual average ground temperature for calculation can take into account temperature changes in different seasons, making the calculation results more representative.

[0039] Preferably, the calculation of the equivalent conductor temperature of the cable based on the principle of equivalent temperature with the same loss, using daily periodic load current data and stable daily temperature change data, includes: calculating the instantaneous conductor loss corresponding to the current at each moment within a cycle based on the linear relationship between conductor resistance and temperature; summing the instantaneous conductor losses at all moments within a cycle to obtain the total loss within the cycle; calculating the root mean square value of the load current within a cycle; deriving the equivalent conductor resistance based on the total loss within the cycle and the root mean square value of the current; and calculating the equivalent conductor temperature by combining the equivalent conductor resistance with the linear relationship between conductor resistance and temperature.

[0040] Specifically, firstly, the temperature coefficient of resistance and the resistance per unit length at 20℃ (reference value) are determined based on the conductor material; then, the time intervals of the daily periodic load current are divided, and the instantaneous conductor loss within each time interval is calculated based on the current value and conductor temperature value; the instantaneous losses of all time intervals are summed to obtain the total conductor loss within a complete daily cycle; simultaneously, the root mean square value of the load current within that daily cycle is calculated; based on the principle of equivalent loss, the total loss is equal to the square of the root mean square value of the current multiplied by the equivalent conductor resistance, thus deriving the equivalent conductor resistance; finally, the equivalent conductor resistance is substituted into the linear relationship between conductor resistance and temperature to calculate the equivalent conductor temperature.

[0041] It should be noted that this method strictly follows the core principle of equivalent loss. By calculating and accumulating instantaneous losses over time periods, it can accurately obtain the equivalent conductor temperature under periodic loads, achieving the highest calculation accuracy. It is suitable for photovoltaic power station projects with high requirements for line loss calculation accuracy.

[0042] Preferably, the method of calculating the equivalent conductor temperature of the cable based on the principle of equivalent temperature with the same loss using daily periodic load current data and daily temperature change data after stabilization further includes: determining whether the daily temperature fluctuation range within one cycle after the conductor temperature stabilizes is within a preset threshold range; if the fluctuation range is within the preset threshold range, the average temperature of one natural day after the conductor temperature stabilizes is directly taken as the equivalent conductor temperature.

[0043] Specifically, hourly temperature data of the conductor are extracted for one natural day after the temperature stabilizes, and the difference between the maximum and minimum temperature values ​​of that day is calculated to obtain the daily temperature fluctuation range. This fluctuation range is compared with a preset threshold. When the fluctuation range is less than or equal to the preset threshold, the arithmetic mean of the temperature at all times of that day is directly calculated and used as the equivalent conductor temperature. When the fluctuation range is greater than the preset threshold, the equivalent conductor temperature is obtained using the aforementioned precise calculation method.

[0044] It should be noted that after the temperature of the photovoltaic power station cable reaches stability under cyclic load, the daily fluctuation range of the conductor temperature is usually small. At this time, using the daily average temperature as the equivalent conductor temperature greatly simplifies the calculation process, and the calculation error is within the allowable range of the project. This can significantly improve the efficiency of engineering calculations and is suitable for rapid estimation in the preliminary design stage.

[0045] Preferably, the step of correcting the resistance value of the cable conductor based on the equivalent conductor temperature and then calculating the corrected cable loss rate includes: calculating the corrected conductor resistance per unit length by substituting the equivalent conductor temperature into the linear relationship between conductor resistance and temperature; replacing the conductor resistance value under standard test conditions with the corrected conductor resistance per unit length; and calculating the corrected cable loss rate by combining the actual length of the cable and the operating current parameters.

[0046] Specifically, the equivalent conductor temperature obtained from the aforementioned calculation is substituted into the linear formula for conductor resistance as a function of temperature to calculate the conductor resistance per unit length at the equivalent temperature; the conductor resistance value under the standard test conditions used in the traditional design method is replaced with this corrected resistance value; based on the actual cable laying length, system operating voltage, and load current parameters, the corrected cable loss rate considering the conductor temperature rise effect is calculated according to the cable loss calculation formula.

[0047] It should be noted that traditional design methods calculate losses using conductor resistance under standard test conditions, without considering the resistance increase effect caused by conductor temperature rise, which may underestimate the actual loss. After correcting the resistance value by equivalent conductor temperature, the calculated loss rate is closer to the actual operating loss of the cable, which can effectively improve the accuracy of cable loss calculation.

[0048] Preferably, after calculating the corrected cable loss rate, the method further includes: calculating the theoretical cable loss rate under standard test conditions; comparing the corrected cable loss rate with the theoretical loss rate under standard test conditions to calculate the loss difference rate between the two; and assessing the degree of loss underestimation when the conductor temperature rise effect is not considered based on the loss difference rate.

[0049] Specifically, using the conductor resistance value under standard test conditions, combined with the same cable length and operating current parameters, the theoretical cable loss rate under the traditional method is calculated; the loss difference rate is obtained by subtracting the theoretical loss rate from the corrected loss rate and then dividing by the theoretical loss rate; based on the magnitude of the loss difference rate, the degree of loss underestimation caused by neglecting the conductor temperature rise effect in the traditional design method is quantitatively evaluated.

[0050] It should be noted that by calculating the loss difference rate, the impact of conductor temperature rise on cable loss can be intuitively reflected, helping designers to clearly understand the limitations of traditional design methods, providing a quantitative basis for adopting the modified calculation method, and also providing a reference for the economic analysis and risk assessment of the project.

[0051] Preferably, after calculating the corrected cable loss rate, the method further includes: applying the corrected cable loss rate to the system efficiency assessment of the photovoltaic power station; selecting and designing the cable cross-section based on the corrected loss rate; and adjusting the cable design parameters according to the corrected loss rate for projects with strict line loss assessment requirements to reduce the risk of line loss assessment.

[0052] Specifically, the corrected cable loss rate is incorporated into the calculation system of photovoltaic power station system efficiency to obtain more accurate prediction results of total system efficiency and power generation. When selecting cable cross-sections, the voltage drop and power loss of the cable are calculated based on the corrected loss rate, and the minimum cable cross-section that meets the requirements of line loss and voltage drop is selected. For scenarios with strict line loss assessment requirements, such as international projects, design margins are reserved in advance based on the corrected loss rate, and the cable cross-section is appropriately increased when necessary to ensure that the actual operating line loss meets the assessment requirements.

[0053] It should be noted that applying the corrected loss rate to engineering design can avoid problems such as substandard system efficiency and line loss assessment penalties caused by underestimating losses. At the same time, it can optimize cable cross-section selection while meeting design requirements, control engineering construction costs, and balance the safety and economy of the project.

[0054] Example 2 Currently, mainstream photovoltaic design software calculates AC / DC cable losses using conductor resistance values ​​under STC (Standard Test Conditions, 25°C) and then calculates the cable loss rate based on the load factor. This method has a significant drawback: it fails to consider the effect of increased resistance in the cable conductor due to temperature rise after current flow. In reality, for every 1°C increase in conductor temperature, the resistance of copper conductors increases by approximately 0.393%. In high-temperature environments during summer, the conductor temperature of underground cables may far exceed 25°C, resulting in actual losses significantly higher than the software-calculated values.

[0055] To address this issue, this embodiment provides a method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants. Based on the cable conductor temperature rise data obtained from transient thermal simulation software, a method for calculating the equivalent temperature with the same loss is proposed, and a 3×240mm... 2 The examples of three and seven copper core cables laid directly in parallel quantify the impact of conductor temperature rise on loss calculation, as explained in detail below.

[0056] I. Periodic Load Model The output power of a photovoltaic power station varies with solar irradiance, exhibiting a clear diurnal periodicity. The typical diurnal periodic current curve used in this embodiment is shown in Table 1. This current curve uses a duration-current step definition format; the first column represents the duration of the current, and the second column represents the corresponding per-unit current. The current curve exhibits a triangular wave shape, with zero current in the early morning and at night. The peak current of 1.0 pu occurs around noon, and a relatively high current level is maintained during the day. The effective power generation period is approximately 6.25 hours out of a 24-hour period, with zero current for the remaining 17.75 hours.

[0057] Table 1 - Daily Periodic Current Curve

[0058] II. Cable Transient Temperature Rise Simulation 2.1 Simulation Conditions Transient thermal simulation is performed using transient thermal simulation software, under the following conditions: (1) Cable type: 33kV 3×240mm 2 Copper core cross-linked polyethylene insulated power cable (3C33CU240). (2) Laying method: Direct burial; (3) Ground temperature: 40°C; (4) Current and waveform: The rated current is continuously applied when the conductor temperature is 90 degrees Celsius, and the waveform is the periodic current described above. (5) Laying method: Three or seven wires are laid in parallel with a spacing of 200mm.

[0059] 2.2 Equivalent Temperature Calculation Method and Definition The linear relationship between the resistance of a copper conductor and temperature is as follows: R(θ) = R20 × [1 + α × (θ- 20)] (1) Where: R20 = ρ / A = 0.0175 / 240 = 7.292×10 -5 Ω / m (resistance per unit length of conductor at 20°C), α = 0.00393 / °C (temperature coefficient of copper conductor), θ is the conductor temperature.

[0060] The equivalent temperature θeq refers to the loss generated by a conductor with a root mean square current Irms passing through it at a constant temperature, which is equal to the average loss generated by an actual periodic current at varying temperatures.

[0061] Mathematical expression: Σ[I²(t)]× R(θeq) = Σ[I²(t) × R(θ(t))](2) Therefore, the equivalent resistance can be obtained: R(θeq) = Σ[I²(t) × R(θ(t))] / Σ[I²(t)](3) The equivalent temperature can then be obtained: θeq = 20 + (R(θeq) / R20 - 1) / α(4) Simulation results show that once the cable conductor temperature stabilizes, the temperature fluctuation range is small, generally within 1°C. For specific projects, to simplify calculations, directly using the daily average temperature after conductor temperature stabilization minimizes error and significantly simplifies the calculation process.

[0062] 2.3 Three-Cable Scheme The simulation parameters for the three-cable parallel laying scheme are as follows: the rated current corresponding to a conductor temperature of 90°C is 241.84A.

[0063] The transient thermal simulation software simulated 2000 hours of operation. After 700 hours, the conductor temperature tended to stabilize periodically. Due to the daily load rate not being 1, the highest conductor temperature was much lower than 90 degrees Celsius. The graph shows that the cable continuously heated up within the first 700 hours of loading, but this represents a relatively small percentage of the 8760 hours of operation throughout the year. Therefore, the data from the last 24 hours after the conductor temperature stabilized can be used for analysis. Using the conductor temperature of this segment to calculate the annual conductor loss rate would be more conservative. The conductor temperature rise of the cable is as follows... Figure 2 , Figure 3 As shown in Table 2.

[0064] Table 2 - Equivalent Temperature Calculation Results for the Three-Cable Scheme

[0065] The middle cable (Cable 2) has the highest temperature due to the mutual heating effect between the two cables, with an equivalent temperature of 54.81°C.

[0066] For comparison, at a ground temperature of 25°C, under the periodic current shown in Table 1, when a continuous rated current of 270A corresponding to a conductor temperature of 90°C is applied, the temperature rise of the conductor over 2000 hours is shown in the figure below. It can be seen that the lower the ground temperature, the lower the steady-state cable conductor temperature.

[0067] 2.4 Seven-Cable Scheme The simulation parameters for the seven-cable parallel laying scheme are as follows: the rated current corresponding to a ground temperature of 40°C and a conductor temperature of 90°C is 136.32A.

[0068] Data from the last 24 hours after the temperature stabilized were analyzed, and the results are shown in Table 3.

[0069] Table 3 - Equivalent Temperature Calculation Results for the Seven-Cable Scheme

[0070] The central cable (Cable 4) is most affected by the mutual heating effect of the six cables on both sides, with an equivalent temperature of 59.69°C.

[0071] III. Loss Analysis Cable loss was calculated using photovoltaic design software under STC conditions (25°C). The hottest cable among multiple cables was used as the benchmark, and a comparative analysis is shown in Table 4.

[0072] Table 4 - Comparison of Loss at Equivalent Temperature and Loss under STC Condition

[0073] The formula for calculating the loss difference rate is: ΔP% = (Peq - PSTC) / PSTC × 100% (5) IV. Conclusion (1) Under periodic load, the conductor temperature of the underground cable of the photovoltaic power station exhibits obvious daily periodic fluctuations with the change of current. The transient thermal simulation software can accurately obtain the conductor temperature change process.

[0074] (2) The equivalent temperature method with the same loss proposed in this embodiment equates the periodically changing conductor loss to the loss at a constant temperature, providing a simple and accurate correction method for engineering calculations. Using the daily average temperature under steady-state conditions simplifies the calculation.

[0075] (3) Using three 3×240mm 2 Taking parallel direct burial of copper core cables as an example, the equivalent temperature of the hottest cable is 54.81°C, which is 29.81°C higher than the STC condition (25°C). When seven cables are laid in parallel, the equivalent temperature of the hottest cable is 59.69°C, which is 34.69°C higher than the STC condition. If the conductor temperature rise effect is not considered, the loss of the three-cable scheme will be underestimated by about 11.49%, and the loss of the seven-cable scheme will be underestimated by about 13.37%. This difference cannot be ignored in the line loss assessment of large-scale photovoltaic power plants.

[0076] (4) It should be noted that the ground temperature changes throughout the year. The simulation calculation can be based on the average ground temperature throughout the year. Generally, it is necessary to measure the ground temperature at the cable burial depth. If this data is not available, the ground temperature can be considered to be about 15°C lower than the air temperature.

[0077] (5) The lower the ground temperature, the lower the temperature at which the conductor reaches a steady state under periodic current.

[0078] Example 3 This embodiment is based on embodiment 1: This embodiment provides a computer device, including a memory and a processor. The memory stores a computer program, and the processor executes the computer program to implement the method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants as described in Embodiment 1. The computer program can be in the form of source code, object code, executable file, or some intermediate form.

[0079] Example 4 This embodiment is based on embodiment 1: This embodiment provides a computer-readable storage medium storing a computer program. When executed by a processor, this computer program implements the method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants as described in Embodiment 1. The computer program can be in the form of source code, object code, executable file, or some intermediate form. The storage medium includes any entity or device capable of carrying computer program code, a recording medium, a computer memory, a read-only memory (ROM), a random access memory (RAM), an electrical carrier signal, a telecommunication signal, and a software distribution medium, etc.

[0080] The above description is merely a preferred embodiment of the present invention. It should be understood that the present invention is not limited to the forms disclosed herein and should not be construed as excluding other embodiments. It can be used in various other combinations, modifications, and environments, and can be altered within the scope of the concept described herein through the above teachings or related technologies or knowledge. Modifications and variations made by those skilled in the art that do not depart from the spirit and scope of the present invention should be within the protection scope of the appended claims.

[0081] It should be noted that, for the sake of simplicity, the foregoing method embodiments are described as a series of actions. However, those skilled in the art should understand that this application is not limited to the described order of actions, as some steps may be performed in other orders or simultaneously according to this application. Furthermore, those skilled in the art should also understand that the embodiments described in the specification are preferred embodiments, and the actions and modules involved are not necessarily essential to this application.

Claims

1. A method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants, characterized in that... include: Obtain daily periodic load current data of underground cables in photovoltaic power plants; Transient thermal simulation was performed on underground cables under the target laying method to obtain daily temperature variation data after the conductor temperature reached periodic stability. Based on the principle of equivalent temperature with the same loss, the equivalent conductor temperature of the cable is calculated using daily periodic load current data and stable daily temperature change data. The resistance value of the cable conductor is corrected based on the equivalent conductor temperature, and then the corrected cable loss rate is calculated.

2. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, The acquisition of daily periodic load current data of underground cables in photovoltaic power plants includes: Current data is collected throughout the day using a stepping method that combines duration and corresponding current. The current characteristics of power generation periods and non-power generation periods are distinguished to make the collected current data exhibit a daily periodic variation pattern, with the current peak corresponding to the period of highest solar radiation intensity and the current being zero during non-power generation periods.

3. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, The transient thermal simulation of the underground cable under the target laying method, to obtain daily temperature variation data after the conductor temperature reaches periodic stability, includes: A cable laying simulation model was built using transient thermal simulation software. The cable laying method, ambient ground temperature, and rated current conditions were set. The cable laying simulation model was run continuously until the cable conductor temperature showed a stable periodic fluctuation state. The conductor temperature change data for a complete natural day after the temperature stabilized were captured.

4. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 3, characterized in that, The transient thermal simulation of underground cables under the target laying method also includes: For scenarios where multiple cables are laid in parallel, the daily temperature variation data of the conductor of each cable is simulated and obtained separately; the daily temperature variation data of the cable with the highest temperature among all cables is selected as the basis for subsequent calculations.

5. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, The equivalent conductor temperature of the cable, calculated based on the principle of equivalent temperature with the same loss, using daily periodic load current data and stabilized daily temperature variation data, includes: Based on the linear relationship between conductor resistance and temperature, the instantaneous conductor loss corresponding to the current at each moment within a cycle is calculated. The instantaneous conductor loss at all moments within a cycle is summed to obtain the total loss within the cycle. The root mean square value of the load current within a cycle is calculated. Based on the total loss within the cycle and the root mean square value of the current, the equivalent conductor resistance is derived. The equivalent conductor temperature is calculated by combining the equivalent conductor resistance with the linear relationship between conductor resistance and temperature.

6. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, The equivalent conductor temperature of the cable, calculated based on the principle of equivalent temperature with the same loss, using daily periodic load current data and stabilized daily temperature variation data, includes: Determine whether the daily temperature fluctuation range within one cycle after the conductor temperature stabilizes is within the preset threshold range; if the fluctuation range is within the preset threshold range, directly take the average temperature of one natural day after the conductor temperature stabilizes as the equivalent conductor temperature.

7. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, The step of correcting the resistance value of the cable conductor based on the equivalent conductor temperature, and then calculating the corrected cable loss rate, includes: Based on the linear relationship between conductor resistance and temperature, the corrected conductor resistance per unit length is calculated by substituting the equivalent conductor temperature. The corrected conductor resistance per unit length is then used to replace the conductor resistance under standard test conditions. Combined with the actual cable length and operating current parameters, the corrected cable loss rate is calculated.

8. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 7, characterized in that, After obtaining the corrected cable loss rate through calculation, the process also includes: Calculate the theoretical loss rate of the cable under standard test conditions, compare the corrected cable loss rate with the theoretical loss rate under standard test conditions, and calculate the loss difference rate between the two; based on the loss difference rate, assess the degree of underestimation of loss when the conductor temperature rise effect is not considered.

9. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, After obtaining the corrected cable loss rate through calculation, the method further includes: applying the corrected cable loss rate to the system efficiency assessment of the photovoltaic power station, selecting and designing the cable cross-section based on the corrected loss rate, and adjusting the cable design parameters.

10. The method for calculating the equivalent conductor temperature rise and correcting the loss rate of underground cables in photovoltaic power plants according to claim 1, characterized in that, When performing transient thermal simulation on underground cables under the target laying method, the annual average measured ground temperature at the cable burial depth is preferentially used as the environmental ground temperature parameter for transient thermal simulation. When there is no actual measured ground temperature data, the ground temperature parameters at the cable burial depth are calculated based on local air temperature data.

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