Method for evaluating transmission distance of a collection line of a medium-voltage offshore wind farm

Abstract

The application provides an evaluation method for power transmission distance of a medium-frequency offshore wind farm power collection line. Firstly, based on the structure parameters of the medium-frequency submarine cable, the equivalent thermal resistance method is used to calculate the rated current of each cross-section submarine cable, and the cross-section of the medium-frequency power collection line is preliminarily selected according to the rated current and the loop current. Then, according to the cross-section of the preliminarily selected power collection submarine cable, the reactive and active values of the power collection loop are calculated. When P 损 ≤ 5% P 额定 , it is considered that the active output is basically not affected; finally, the preliminary economic evaluation of the submarine cable is carried out. If the total investment of the medium-frequency power collection line submarine cable is not more than 110% of the power frequency scheme, it is considered that the power transmission distance of the power collection line is reasonable, otherwise the power transmission range of the power collection submarine cable or the medium-frequency frequency needs to be recalculated. The application provides a more accurate calculation method for the power transmission range and the cross-section selection of the medium-frequency grid-connected type offshore wind power project, thereby providing a basis for the design of the medium-frequency type offshore wind power project.

Description

TECHNICAL FIELD

[0001] The present application belongs to the technical field of offshore wind power flexible DC transmission, in particular to a method for evaluating the transmission distance of a medium-frequency offshore wind farm power collection line, and is suitable for the field of offshore wind power generation. BACKGROUND

[0002] With the development of large-scale and deep-sea offshore wind power in China, offshore wind power transmission is becoming increasingly important, and its investment proportion is also increasing. Flexible DC transmission technology is a transmission technology developed in recent years, which has many advantages such as system stability, decoupling of AC and DC systems, independent control of active and reactive power, and flexible regulation mode, and is particularly suitable for grid-connected power transmission of long-distance and large-capacity deep-sea offshore wind farms. At present, several land flexible DC technology demonstration projects have been constructed and put into operation in China, but offshore wind farm flexible DC transmission is still in its infancy in China.

[0003] In order to further optimize the economic cost of DC transmission and reduce the volume and weight of transmission and transformation equipment, according to the principle of electromagnetic induction, increasing the operating frequency of transmission and transformation equipment is a feasible scheme. When offshore wind power adopts DC transmission to access the onshore power grid, the offshore wind farm and the onshore AC power grid are asynchronous, and their operating frequencies are completely independent. Therefore, the offshore wind farm can select a more economically beneficial frequency value and increase the operating frequency to the medium-frequency range (100-400 Hz) to form a medium-frequency grid-connected offshore wind DC transmission system.

[0004] However, for the medium-frequency grid-connected offshore wind DC transmission system, although the costs of offshore converter stations (including electrical equipment) and wind turbines are decreasing, due to the increase of the frequency of the offshore wind farm, the reactive power of the power collection line will increase proportionally with the frequency, and the active power transmission of the submarine cable of the power collection line will decrease. Therefore, the original submarine cable cross-section selection suitable for power frequency cannot be directly applied to the medium-frequency power collection line. If the power collection line distance is too long and the line capacity is too large, the reactive power of the single-circuit line will be too large, and even reactive power compensation devices may need to be configured in the offshore converter station for inductive compensation, which will affect the economy and rationality of the project.

[0005] Therefore, the selection of the cable cross-section of the medium-frequency power collection line and the determination of the transmission distance range of the power collection line are particularly important, especially the confirmation of the transmission distance of the power collection line, which is closely related to the selection of the wind farm range and the offshore converter station. It is urgent to develop this technology to guide the scheme development, economic calculation and subsequent engineering implementation of the medium-frequency grid-connected DC transmission project. SUMMARY

[0006] The purpose of the present application is to provide an evaluation method for the power transmission distance of the power collection line in the medium frequency tracking network type offshore wind power DC transmission system, and to provide evaluation basis for the rationality of the cross section selection of the medium frequency submarine cable and the economic rationality of the medium frequency power collection system, so as to provide basis for the design of the medium frequency type offshore wind power project.

[0007] To this end, the above-mentioned purpose of the present application is realized by the following technical scheme:

[0008] The evaluation method for the power transmission distance of the medium frequency offshore wind power field power collection line is characterized in that the method comprises the following steps:

[0009] S1, based on the structural parameters of the medium frequency submarine cable, the equivalent thermal resistance method is used to calculate the rated current of each cross section of the medium frequency submarine cable;

[0010] S2, the cross section of the medium frequency power collection line is preliminarily selected according to the rated current and the loop current;

[0011] S3, when the selection of the cross section of the medium frequency submarine cable is compared with the power frequency scheme: the cross section of the submarine cable is flat or only increases by one or two sections, it is preliminarily considered that the selection of the cross section of the submarine cable is reasonable; if it exceeds two sections, the economic benefit is relatively poor, and the medium frequency needs to be reselected.

[0012] S4, according to the selected parameters of the preliminarily selected power collection submarine cable, the reactive power and active power values of the power collection submarine cable are calculated:

[0013] S5, when P 损 ≤ the first threshold value, it is considered that the active output is basically not affected, the power transmission distance of the power collection line is reasonable or the frequency is reasonable, no reactive power compensation device needs to be configured, and the layout position of the wind power plant and the converter station is not affected; wherein P 损 is the active power loss of the wind farm side and the converter station interface side;

[0014] If the active power loss is too large, the cross section of the power collection line is appropriately adjusted to meet the demand of active power transmission;

[0015] S6, the economic evaluation of the submarine cable is carried out to evaluate the economic rationality of the submarine cable power collection line;

[0016] S7, if the total investment of the increased power collection line submarine cable exceeds the second threshold value of the power frequency scheme of the same scale power collection line, it is considered that the radius of the power collection line is unreasonable or the frequency is unreasonable, and the boundary of the power collection line needs to be redefined or the medium frequency is reevaluated.

[0017] While the above technical scheme is used, the present application can also use or combine the following technical scheme:

[0018] As a preferred technical solution of the present application: in step S5, the first threshold value is 5% P 额定 .

[0019] As a preferred technical solution of the present application: in step S7, the second threshold value is 10%.

[0020] The present application provides a method for evaluating the power transmission distance of a medium-frequency offshore wind farm power collection line, which first calculates the rated current of each cross-section of the medium-frequency cable based on the structural parameters of the medium-frequency cable, and preliminarily selects the cross-section of the medium-frequency power collection line according to the rated current and the loop current; then, according to the cross-section of the preliminarily selected power collection cable, the reactive and active values of the power collection loop are calculated, and when P 损 ≤5% P 额定 , it is considered that the active output is basically not affected; finally, the cable is preliminarily evaluated in terms of economy, and if the total investment of the medium-frequency power collection line cable does not exceed 110% of that of the power frequency scheme, it is considered that the power transmission distance of the power collection line is reasonable, otherwise the power transmission range of the power collection cable or the medium-frequency frequency needs to be recalculated.

[0021] The present application has the following beneficial effects:

[0022] (1) The present application provides a relatively accurate calculation method for the power transmission range and cross-section selection of the power collection line of a medium-frequency grid-connected offshore wind power project, thereby providing a basis for the design of the medium-frequency offshore wind power project.

[0023] (2) Based on the determination of the power transmission range of the power collection line, the present application can also be used to judge the rationality of the layout position of the offshore converter station and the rationality of the wind farm planning layout.

[0024] (3) The application scope of the present application can be extended to cables of other frequencies than power frequency, i.e. the present application can be applied to the evaluation of medium-frequency power collection cables, high-frequency power collection cables or low-frequency power collection cables. BRIEF DESCRIPTION OF DRAWINGS

[0025] Figure 1 The flowchart of the method for evaluating the power transmission distance of a medium-frequency offshore wind farm power collection line provided by the present application is shown.

[0026] Figure 2 The structure diagram of a typical three-phase alternating current cable is shown.

[0027] Figure 3 The schematic diagram of the relationship between the capacitive current in the cable and the load current flowing through the cable is shown.

[0028] Figure 4 The schematic diagram of the 66kV power collection line layout applied in the implementation example is shown.

[0029] Figure 5This is a diagram showing the economic evaluation results of submarine cables after assessment. Detailed Implementation

[0030] To describe the invention in more detail below, in conjunction with... Figures 1 to 5 The technical solution of the present invention will be described in detail through a typical case.

[0031] Summary of methods for evaluating the transmission distance of medium-frequency offshore wind power collection lines as follows: Figure 1 The process is shown below.

[0032] First, this invention simplifies the submarine cable structure, selects key structural layers, and uses the equivalent thermal resistance method to calculate the current carrying capacity of the submarine cable.

[0033] like Figure 2 The diagram shows a typical three-phase AC submarine cable structure. In the diagram, number 1 is the water-blocking copper conductor, number 2 is the conductor shield, number 3 is the XLPE insulation layer (cross-linked polyethylene insulation material), number 4 is the insulation shield layer, number 5 is the semi-conductive water-blocking tape layer, number 6 is the alloy lead sheath layer, number 7 is the PE sheath layer, number 9 is the cable inner filling material, number 10 is the bag layer, number 11 is the PP inner padding layer, number 12 is the armored steel wire and asphalt layer, and number 13 is the PP outer sheath layer.

[0034] The equivalent thermal resistance method calculates the cable current carrying capacity under various laying conditions by establishing a simplified cable thermal circuit model. Its calculation is mainly based on steady-state temperature field theory, with the following assumptions: (1) the earth surface is an isothermal surface; (2) the cable surface is an isothermal surface; (3) the superposition principle applies. The steady-state allowable current carrying capacity I of an AC submarine cable is... rat The calculation formula is as follows:

[0035]

[0036] in:

[0037] R is the AC resistance of the conductor at the highest operating temperature, in Ω / km;

[0038] W d This is due to dielectric loss, which is generally negligible.

[0039] λ1 represents the power loss in the metal sheath of the AC cable;

[0040] λ2 represents the loss of the steel wire armor.

[0041] T1 is the insulation thermal resistance;

[0042] T2 is the thermal resistance of the inner lining layer;

[0043] T3 represents the thermal resistance of the outer cladding layer;

[0044] T4 represents the external thermal resistance;

[0045] θc The highest operating temperature of the conductor is 90°C;

[0046] θ 01 The ambient temperature of the laying area refers to the temperature of the surrounding medium under normal conditions.

[0047] In the above formula (1), the alternating current resistance R is calculated as follows:

[0048] R = R dc (1+k s +k p ) (Formula 2)

[0049] Where:

[0050] R dc is the direct current resistance at a given calculation temperature, Ω / km;

[0051] k s is the skin effect coefficient;

[0052] k p is the proximity effect coefficient;

[0053] In the above formula (2), the direct current resistance R dc is calculated as follows:

[0054]

[0055] Where,

[0056] ρ is the resistivity of the conductor, determined by the material of the conductor, Ω.m;

[0057] A is the nominal cross-sectional area of the conductor, m 2 ;

[0058] T is the temperature of the conductor, ℃;

[0059] α is the temperature coefficient of resistance, determined by the material of the conductor, ℃ -1 ;

[0060] In the above formula (2), the skin effect coefficient k s is as follows:

[0061]

[0062] Where, z is calculated as follows:

[0063]

[0064] Where,

[0065] f is the frequency, Ω.m;

[0066] α z1 when the conductor is a general copper core stranded conductor or a copper core sector conductor; 0.43 when the conductor is a segment conductor or a segment conductor;

[0067] In the above formula (2), the proximity effect coefficient k p Generally, for single-core cable, take 1; for three-core cable, calculate as shown in the following formula:

[0068]

[0069]

[0070] In the above formula (6), (7):

[0071] d c is the diameter of the conductor layer, m;

[0072] S ij is the axial spacing of the conductor, m;

[0073] α p is 0.8 for stranded copper core and aluminum core, and 0.37 for segment conductor;

[0074] According to the above method, the boundary current of 66kV submarine cable at typical frequencies of 50Hz, 100Hz, 150Hz, 200Hz, etc. is calculated as shown in Tables 1-3. From the calculation results, it can be seen that in the frequency range studied, the unit length resistance of the submarine cable increases with the increase of frequency, the unit length inductance decreases with the increase of frequency, and the influence of frequency on the unit length resistance and inductance of the submarine cable becomes significant with the increase of the cross section of the conductor. At the same frequency, the larger the cross section of the cable conductor, the smaller the unit length resistance and the unit length inductance, and the larger the unit length capacitance.

[0075] As shown in Tables 1-3, the selection of the cross section of the submarine cable needs to be increased with the increase of frequency, but generally, the corresponding submarine cable cross section can be selected within the cross section range close to the power frequency or one to two steps larger than the power frequency, for example, from 3x70mm 2 to 3x95mm 2 or 3x120mm 2 However, in terms of economy, increasing one step is within the range that the economy can accept, and if it is increased by two steps or even higher, the economy will be obviously insufficient. Therefore, the selected cross section is bounded by not more than two steps of the power frequency cross section.

[0076] Table 1: Analysis table of data of 66kV submarine cable laid on seabed at frequency

[0077]

[0078] Table 2: Analysis table of data of 66kV submarine cable laid in J-shaped pipe at frequency

[0079]

[0080] Table 3 cable laying data analysis table

[0081]

[0082] Secondly, according to the calculation results of the sea cable boundary current-carrying, according to the layout of the wind farm, the preliminary selection of the power collection line is carried out, and the long-distance sea cable reactive power is checked to confirm the transmission range of the sea cable. The main two boundary conditions for checking the transmission distance are:

[0083] One is that the sea cable voltage drop ΔU≤5%U 额定 , this checking is a conventional checking scheme, which is not expanded in the present application.

[0084] The second is to evaluate the active loss of the sea cable transmission, mainly using the following method:

[0085] As shown in the equivalent circuit shown in Figure 3 , let the total capacitance of the cable to the ground be C cab , the system voltage on the line be U S , the voltages at both ends of the cable be U S1 and U S2 , and the active current component of the cable transmission be I Load . Then the following relationship is obtained:

[0086] The current flowing through the distributed capacitance on the cable is:

[0087]

[0088] In the above formula (8), since ω=2πf, ω changes with the change of frequency f, and ω increases with the increase of frequency.

[0089] Considering that the cable impedance is low, U S ≈U S1 ≈U S2 , formula (8) can be simplified as

[0090] I C =U S ωC Cab (formula 9)

[0091] If the power supply at both ends of the cable is not limited by the output power, it can be considered that the capacitance current of the cable is provided by the power supply at both ends, and the current at both ends of the cable is:

[0092]

[0093] Adding the load current, the total current at both ends of the cable is shown in the following formula, and Each shall not exceed I rat :

[0094]

[0095] With Figure 4 1000 MW wind farm as an example, the reactive and active output of the nearest section of sea cable from the collector line to the converter station is verified, and the calculation results are shown in the following table, which shows that the reactive power loss is basically within the acceptable range, and the collector line is less affected by frequency, and does not need to be equipped with reactive power compensation device. For larger capacity wind farms, when P 损 ≤5%P 额定 , it is considered that it basically does not affect the active output.

[0096] Among them,

[0097]

[0098] Table 4 Calculation results of active / reactive power difference at the beginning and end of a typical wind farm

[0099]

[0100] Finally, the investment of the collector line is calculated, and the 1000 MW wind farm shown in Figure 4 is taken as an example. Due to the high unit price of large cross-section sea cable, the cost of 3×800mm 2 and 3×1000mm 2 cross-section sea cable is doubled or even more than the unit price of small cross-section sea cable. Considering the maximum cross-section of the sea cable, it is selected to 3×630mm 2 , and the sea cable price level is basically the same. If larger cross-section sea cable is needed, the number of sea cable loops is increased for comparison. The investment of the collector sea cable changes with frequency as shown in the following table:

[0101] Table 5 Comparison of medium frequency collector sea cable cost with frequency increase

[0102]

[0103] As shown in Table 5 and Figure 5 , with the increase of frequency, the collector sea cable shows a slow upward trend, and there is a significant increase at the frequency of 130Hz, because at this frequency, the cross-section of part of the collector sea cable increases by one grade, but this increase does not change the overall slow upward trend. Therefore, for the 1000 MW wind farm, Figure 3The 1000MW wind farm shown, with the rising frequency of the power collection cable cost shows an upward trend, but the overall rising cost is limited, therefore, the existing power collection arrangement, the cable selection scheme is reasonable, the power collection radius is within a reasonable range. For larger scale wind farms, if the total investment of the medium frequency power collection line cable does not exceed 110% of the power frequency scheme, the power transmission distance of the power collection line is reasonable, otherwise the power transmission range of the power collection cable or the medium frequency is re-calculated.

[0104] The above examples are only a more optimal technical solution of the present application, and those skilled in the art should understand that the technical solutions or parameters in the examples can be modified or replaced without departing from the principles and essence of the present application, and all should be covered within the protection scope of the present application.

Claims

1. A method for evaluating the transmission distance of a power collection line of an intermediate frequency offshore wind farm, characterized in that: The method comprises the following steps: S1, based on the intermediate frequency submarine cable structure parameters, using the equivalent thermal resistance method to calculate the rated current of each cross-section of the intermediate frequency submarine cable; S2, according to the rated current and the loop current, preliminarily selecting the cross-section of the intermediate frequency current collection line; S3, when the selection of the intermediate frequency submarine cable cross-section is compared with the power frequency scheme: the submarine cable cross-section uses the same size or only increases by one or two cross-sections, it is preliminarily considered that the selection of the submarine cable cross-section is reasonable; if it exceeds two cross-sections, the economic benefit is relatively poor, and the intermediate frequency needs to be reselected; S4, according to the selected parameters of the selected submarine cable, the reactive power and active power of the submarine cable are calculated: S5、when P 损 ≤ the first threshold value, it is considered that the active output is not substantially affected, the power transmission distance of the collection line is reasonable or the frequency setting is reasonable, the reactive compensation device does not need to be configured, and the layout position of the wind farm and the converter station is not affected; wherein, P 损 is the active loss of the wind farm side and the converter station interface side. If the active power loss is too large, the cross-section of the current collection line is adjusted appropriately to meet the demand of active power transmission; S6, the economic evaluation of the submarine cable is carried out to evaluate the economic rationality of the submarine cable current collection line; S7, if the total investment of the increased submarine cable of the current collection line exceeds the second threshold value of the power frequency scheme of the same scale current collection line, it is considered that the radius of the current collection line is unreasonable or the frequency is unreasonable, and the boundary of the current collection line needs to be re-established or the intermediate frequency needs to be evaluated.

2. The method for assessing the power transmission distance of an intermediate frequency offshore wind farm power collection line according to claim 1, characterized in that: In step S5, the first threshold value is 5% P 额定 .

3. The method for assessing the power transmission distance of an intermediate frequency offshore wind farm power collection line according to claim 1, characterized in that: In step S7, the second threshold value is 10%.

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