Method for Selecting Voltage Sequence of Flexible DC Transmission for AC Network Connection of Wide-Area New Energy Base

Abstract

The present invention discloses a method for selecting voltage sequences for AC network connection and flexible DC transmission of a wide-area new energy base, which relates to the technical field of voltage level selection; the method includes the following steps: collecting data of the wide-area new energy base; judging the critical installed capacity of new energy: calculating the critical installed capacity S(k) of new energy at different voltage levels max , and controlling the installed capacity of new energy within S(k) max ; designing the line transmission capacity and the number of circuits: calculating the critical transmission distance of the line to obtain the relationship table between the critical transmission distance of the line and the line transmission capacity; according to the relationship table and the collected data of the wide-area new energy base, designing the line transmission capacity and the number of circuits; judging the system strength and economy for the distance between the k-th voltage level and the (k + 1)-th voltage level; selecting the voltage level with greater system strength and lower cost as the aggregation voltage level. The present invention is applicable to the AC network connection and DC transmission system without conventional power support in the wide-area new energy base of Shagehuang

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage level selection, and particularly relates to a method for selecting voltage sequences for flexible DC transmission of AC networking in a wide-area new energy base. Background Technique

[0002] The research on DC transmission systems for new energy export scenarios at home and abroad has become increasingly in-depth, and the sub-module hybrid MMC has gradually become a key technology for new energy DC export. Different from the new energy export scenarios in deserts, gobi, and wastelands supported by power grids and coal-fired power plants, the new energy export scenarios in deserts, gobi, and wastelands without the support of conventional power sources not only have the characteristics of larger scale and wider area, but also have problems such as difficult networking, insufficient economy, and reliability.

[0003] The collector voltage level sequence of renewable energy bases will largely affect the construction and operation costs of substations. Different collector voltage level sequences result in different grid structures, electrical equipment configurations, and losses in wind farms and photovoltaic power plants, and there are significant differences in costs. Therefore, for the large-scale new energy collection voltage levels in deserts, gobi, and wastelands that are not connected to the main grid, it is an important issue that urgently needs to be optimized and studied.

[0004] Currently, the voltage levels of the upcoming new energy AC networking are generally unified with those of the main grid. For the large-scale new energy export scenarios in deserts, gobi, and wastelands, different from the new energy export scenarios in deserts, gobi, and wastelands supported by power grids and coal-fired power plants, there are no reference cases for selecting the collector voltage levels of the sending-end power grid that can ensure both the stability of the power grid and good economy.

[0005] The present invention proposes a method for selecting voltage sequences for flexible DC transmission of AC networking in a wide-area new energy base. By analyzing the power limit transmission distance, considering the voltage deviation constraint, and taking into account the impact of electrical distance and economy on the system, the present invention proposes the voltage level formation rules and methods applicable to the sending-end power grid framework of renewable energy bases in desert, gobi, and wasteland areas, and optimizes the collector voltage level sequence of the new energy system in desert, gobi, and wasteland areas. Summary of the Invention

[0006] The purpose of the present invention is to provide a method for selecting voltage sequences for flexible DC transmission of AC networking in a wide-area new energy base to solve the problems in the prior art such as the lack of a method for selecting the collector voltage levels of the sending-end power grid that can ensure both the stability of the power grid and good economy for large-scale new energy export scenarios in deserts, gobi, and wastelands as mentioned in the above background technique.

[0007] To achieve the above object, the present invention is implemented by adopting the following technical solutions:

[0008] The present invention proposes a method for selecting voltage sequences for flexible DC transmission of AC networking in a wide-area new energy base, including the following steps:

[0009] S1. Collect data of wide - area new - energy bases: the regional area of new - energy power stations, the installed capacity ratio of wind power and photovoltaic power, the distance from the collection point to the converter station, the parameters of power transmission and transformation equipment, the installed capacity of new - energy and the rated transmission power;

[0010] S2. Judgment of the critical installed capacity of new - energy: According to the installed capacity ratio of wind power and photovoltaic power, calculate the short - circuit current at the k - th voltage level to obtain the critical installed capacity S(k) of new - energy at different voltage levels max ; If the installed capacity of new - energy is greater than S(k) max , then increase one voltage level and recalculate S(k) max , and control the installed capacity of new - energy within the range of S(k) max ;

[0011] S3. Design of line transmission capacity and number of circuits: Calculate the critical transmission distance of the line to obtain the relationship table between the critical transmission distance of the line and the line transmission capacity; According to the relationship table and the data of the wide - area new - energy base collected, design the line transmission capacity and the number of circuits;

[0012] S4. Judgment of the maximum voltage level: Judge whether k is less than the maximum voltage level k max ; If k is k max , directly select k max as the collection voltage level;

[0013] Judge whether the distance from the collection point to the converter station is less than the critical transmission distance of the line; If it is judged that the distance from the collection point to the converter station is greater than the critical transmission distance of the line, then increase one voltage level and go back to S2;

[0014] S5. Calculate the system strength and economy: When k is less than the maximum voltage level k max , and the distance from the collection point to the converter station is less than the critical transmission distance of the line, judge the system strength and economy for the distance between the k - th voltage level and the k + 1 - th voltage level;

[0015] S6. Determine the collection voltage level: Comprehensively compare the system strength and economy. If the system strength of the k - th voltage level is greater and the cost is lower than that of the k + 1 - th voltage level, then select the k - th voltage level as the collection voltage level, otherwise select the k + 1 - th voltage level as the collection voltage level.

[0016] Preferably, the critical installed capacity of new - energy in S2 is restricted by the equipment's bearing capacity, and the specific calculation is as follows:

[0017]

[0018] n total =n WT1 +n WT2 +n PV

[0019] Wherein, a is the per-unit value of the short-circuit current that the doubly-fed wind turbine can provide, b is the per-unit value of the short-circuit current that the direct-drive wind turbine and the photovoltaic can provide, n WT1 is the number of doubly-fed wind turbines, n WT2 is the number of direct-drive wind turbines, n PV is the number of photovoltaics, U is the AC collection voltage level, I k_max is the maximum short-circuit current that the equipment can withstand, I k_MMC1 is the short-circuit current provided by the network-forming flexible DC converter valve, I k_MMC1 is the short-circuit current provided by the grid-following flexible DC converter valve;

[0020] Preferably, in S2, the installed capacity of new energy is controlled within the range of S(k) max Specifically:

[0021] P 2 +Q 2 ≤S(k) max 2

[0022] Wherein, P is the active power output on the AC side of the new energy grid-following converter, and Q is the reactive power output on the AC side of the new energy grid-following converter.

[0023] Preferably, based on controlling the installed capacity of new energy within the range of S(k) max The calculation of the allowable power output range for the AC networking of the new energy base is carried out:

[0024] Combining the maximum and minimum active power outputs, the maximum and minimum reactive power outputs, the power factor limit, the static voltage stability constraint, the voltage deviation constraint, the equipment tolerance constraint, the maximum output capacity constraint of the converter and the grid connection requirements of the new energy station, the common area is determined in the P-Q coordinate system, and the maximum power output capacity of the converter within this range is expressed by the following formula:

[0025]

[0026] Wherein, are the maximum active / reactive power outputs of new energy respectively; are the minimum active / reactive power outputs of new energy respectively; is the minimum / maximum allowable power factor angle of the output, U i is the voltage at the grid connection point of the new energy unit; E is the grid voltage.

[0027] Preferably, in S3, the design of the line transmission capacity and the number of circuits is as follows:

[0028] The critical installed capacity of new energy is limited by the bearing capacity of equipment, and the maximum power output capacity is restricted by the power transmission limit constrained by voltage deviation; therefore, the critical transmission distance of the line is used as the judgment basis;

[0029] The calculation of the critical transmission distance of the line is shown in the following formula:

[0030]

[0031] In the formula, L cr is the critical transmission distance of the line; X T is the reactance of the transformer, and X u is the reactance of the AC transmission line per unit length, and n is the number of parallel lines.

[0032] Preferably, the calculation of the system strength in S5 is specifically as follows: The system strength is analyzed using the capacity short-circuit ratio index; the capacity short-circuit ratio is inversely proportional to the impedance, and the impedance value is used as the judgment criterion:

[0033]

[0034] In the formula: SCR-S i (k) represents the voltage support strength at the grid connection point i under the kth voltage level; S ac,i represents the short-circuit capacity provided by the AC system to the grid connection point i; S eq,i is the equivalent grid-connected capacity of new energy at the grid connection point i; represents the nominal voltage; represents the actual operating voltage of the ith node before new energy is grid-connected; respectively represent the apparent power of new energy injected into nodes i and j; respectively represent the self-impedance and mutual impedance of node i in the system equivalent impedance matrix, respectively represent the voltages at the grid connection points i and j.

[0035] Preferably, the calculation of the economy in S5 is specifically as follows:

[0036] When conducting an economic comparison, a life-cycle cost model is constructed, including the initial investment cost, operation cost, and scrap cost; the operation cost includes the operation loss cost C O and the maintenance cost C M ; the mathematical model of the total cost C total is described as:

[0037]

[0038] In the formula: C I is the initial investment cost; C O is the operation loss cost; C Mis the maintenance cost; C D is the scrapping cost; n c is the annual life of the power transmission and transformation equipment; r is the investment recovery rate of the power industry.

[0039] Preferably, the initial investment cost C I is divided into the construction costs C tr of the ordinary transformer and the converter transformer, and the line construction cost C line ; The calculation formula is:

[0040] C I (k) = ∑R tr S + R line l line ;

[0041] In the formula: R tr is the unit capacity cost of the converter transformer and the ordinary transformer; S is the transformer capacity; R line is the cost per unit length of the line; l line is the line length.

[0042] Preferably, the maintenance cost C M and the scrapping cost C D are based on the initial investment cost;

[0043] C M (k) = f m C I

[0044] In the formula: f m [[ID=5३]]is the project maintenance rate;

[0045] Estimated from engineering experience, the scrapping cost is 3% - 5% of the initial investment cost;

[0046] C D (k) = (3% - 5%)C I .

[0047] Preferably, the operation loss cost introduces the levelized cost per kilowatt-hour, and the calculation formula is:

[0048] C O (k) = P loss T time C price

[0049] P loss (k) = P loss_T +P loss_line

[0050] In the formula: P loss is the total operation loss, including line loss and transformer loss; T time is the reasonable utilization hours in the whole life cycle; Cprice is the electricity cost; P loss_T is the transformer loss; P loss_line is the line loss.

[0051] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0052] (1) The present invention provides a method for selecting the voltage level sequence of large-scale desert, gobi and waste island new energy collection not connected to the main grid. By exploring the relationship between the electrical distance, new energy capacity and voltage level, the critical transmission reactance and distance under different voltage levels are studied. Then, based on the short-circuit current calculation, the critical installed capacity of new energy under different voltage levels is obtained. On this basis, considering the influence of electrical distance and economy on the system, the selection rules and methods of the collection voltage level suitable for the grid framework of the sending-end power grid of the renewable energy base are proposed. The advantages of the technical solution provided by the present invention are that: to a great extent, the construction and operation costs of the station are reduced, the electrical equipment configuration and losses are reduced, the strength of the system is enhanced, which has great significance for the project. The present invention can be widely applied in the field of multi-voltage level AC networking and DC sending of large-scale onshore new energy bases in a wide area.

[0053] (2) In the present invention, by analyzing the power limit transmission distance, it is obtained that the line transmission distance is inversely proportional to the rated power of new energy, and the voltage deviation is the main factor restricting the power limit transmission distance. On this basis, the formation rules and methods of the voltage level suitable for the grid framework of the sending-end power grid of the renewable energy base in the desert, gobi and waste area are proposed, and the feasibility and advantages of the voltage level sequence optimization are discussed, which have great significance for reducing the construction and operation costs of the station, the electrical equipment configuration and losses.

[0054] (3) In the present invention, on the basis of fully considering the critical transmission reactance, distance and the critical installed capacity of new energy under different voltage levels, the influence of electrical distance and economy on the system is considered; when calculating the economy, the full life cycle cost is fully constructed, paying attention to both the initial investment and the operation cost and the scrap cost; the proposed voltage sequence selection method is applicable to the AC networking and DC sending system of the wide-area new energy base in the desert, gobi and waste without the support of conventional power sources.

[0055] (4) In the present invention, the maximum and minimum active power outputs, the maximum and minimum reactive power outputs of the new energy station, as well as the limitations of the power factor, the static voltage stability constraint, the voltage deviation constraint, the equipment bearing capacity constraint, the maximum output capacity constraint of the converter and the grid connection requirements of the new energy station are comprehensively considered. Description of the Drawings

[0056] Figure 1 is the topology diagram of the AC networking and flexible DC sending of the wide-area new energy base in the present invention;

[0057] Figure 2 It is a flowchart of the voltage sequence selection method for the AC networked flexible DC transmission of a wide - area new - energy base in the present invention;

[0058] Figure 3 It is a theoretical analysis diagram of the critical transmission distance in the present invention;

[0059] Figure 4 It is a diagram of the allowable power output range of the new - energy system in the present invention;

[0060] Figure 5 It is a simulation analysis diagram of the 500 - kV critical transmission distance in the present invention;

[0061] Figure 6 It is a simulation analysis diagram of the 220 - kV critical transmission distance in the present invention;

[0062] Figure 7 It is a comparison diagram of the electrical distances under the voltage - level optimization in the present invention;

[0063] Figure 8 It is an economic comparison diagram under the voltage - level optimization in the present invention. Specific embodiments

[0064] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all of the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without making creative efforts belong to the scope of protection of the present invention.

[0065] Embodiment 1:

[0066] Refer to Figure 1 , the new - type power - system structure diagram is as follows:

[0067] The sending end is a wind - solar integrated base, which is networked through multiple voltage levels in AC, gathered at the hybrid MMC converter station for DC transmission, and then inverted into the AC power grid through a long - distance overhead line to the receiving - end MMC converter station.

[0068] Refer to Figure 2 , the voltage - sequence selection method for the AC networked flexible DC transmission of a wide - area new - energy base is as follows:

[0069] 1) Input initial parameters: Input the known regional area of the new - energy power station, the installed capacity ratio of wind power and photovoltaic power, the addresses of the collection point and the converter station, and the parameters of the power - transmission and transformation equipment; the installed capacity of new energy and the rated transmission power.

[0070] 2) Judgment of the critical installed capacity of new energy, design of the line transmission capacity and the number of circuits:

[0071] Calculate the short-circuit current according to the installed capacity ratio of wind power and photovoltaic power, and obtain the critical installed capacity S(k) of new energy at different voltage levels max If the installed capacity of new energy is greater than S(k) max , then increase one voltage level (k = k + 1).

[0072]

[0073] In the formula, a is the per-unit value of the short-circuit current that a doubly-fed wind turbine can provide (times the rated current), b is the per-unit value of the short-circuit current that a direct-drive wind turbine and photovoltaic can provide (times the rated current), n WT1 is the number of doubly-fed wind turbines, n WT2 is the number of direct-drive wind turbines, n PV is the number of photovoltaics, n total = n WT1 + n WT2 + n PV , U is the AC collection voltage level, I k_max is the maximum short-circuit current that the equipment can withstand, I k_MMC1 is the short-circuit current provided by the network-forming flexible DC converter valve, I k_MMC1 is the short-circuit current provided by the grid-following flexible DC converter valve.

[0074] Calculate the power output capacity of the converter under various constraint conditions through formula (1). From the power operation range diagram ( Figure 4 ) drawn by this formula, it can be seen that the critical installed capacity of new energy is mainly restricted by the equipment's tolerance, and the maximum power output capacity is mainly restricted by the power transmission limit considering voltage offset constraints. Therefore, in the voltage level selection method, the critical transmission distance of the line is used as the judgment basis.

[0075] Comprehensively considering the maximum and minimum active power outputs, maximum and minimum reactive power outputs, power factor limits, static voltage stability constraints, voltage offset constraints, equipment tolerance constraints, maximum output capacity constraints of the converter, and grid connection requirements of the new energy power station, determine the common area in the P-Q coordinate system. The maximum power output capacity of the converter within this range can be expressed by formula (1):

[0076]

[0077] In the formula, are the minimum active / reactive power outputs of new energy respectively; are the maximum active / reactive power outputs of new energy respectively; is the minimum / maximum allowable power factor angle of the output.

[0078] Through the stability analysis considering voltage offset, obtain the critical transmission distance L of the line crTable of relationship with line transmission capacity. When the new energy installed capacity is within S(k) max range, according to the relationship table and the input initial parameters, design the line transmission capacity and the number of circuits. When k is less than the maximum voltage level k max , and the distance L from the collection point to the converter station is less than L cr , judge the electrical distance and economy for the distance between the k-th voltage level and the (k + 1)-th voltage level; if the distance L from the collection point to the converter station is greater than L cr , then increase one voltage level (k = k + 1); when k is k max , and the new energy critical installed capacity is satisfied, then select k max as the collection voltage level.

[0079] Among them, the critical transmission distance is shown in Equation (2):

[0080]

[0081] In the formula, L cr is the critical transmission distance of the line; X cr is the critical impedance of the line; U i is the voltage at the grid connection point of the new energy unit; E is the grid voltage; P is the active power output from the AC side of the new energy grid-connected converter; X T is the transformer reactance, X u is the reactance of the unit length of the AC transmission line, and n is the number of parallel circuits of the line.

[0082] 3) Determination of AC voltage level considering system strength and economy:

[0083] ① System strength:

[0084] The voltage at the grid connection point of the multi-infeed system uses the capacity short-circuit ratio index to analyze the voltage support strength of the grid connection point i of the new energy multi-infeed system. In a pure new energy system, the short-circuit ratio of the system is still inversely proportional to the impedance of the system. The smaller the impedance, the larger the new energy short-circuit ratio. Therefore, the impedance size can be used as a judgment criterion.

[0085]

[0086] In the formula: SCR-S i (k) represents the voltage support strength at the grid connection point i under the k-th voltage level; S ac,i represents the short-circuit capacity provided by the AC system to the grid connection point i; S eq,i equivalent grid-connected capacity of new energy at the grid connection point i; represents the nominal voltage; represents the actual operating voltage of the i-th node before new energy grid connection; respectively represent the apparent power of new energy injected into nodes i and j; respectively represent the self-impedance and mutual impedance of node i in the equivalent impedance matrix of the system, respectively represent the voltages at the grid connection points i and j.

[0087] ② Economy

[0088] When conducting an economic comparison of the new energy AC networking and DC power transmission plan, it is necessary to construct a life cycle cost model. The total cost C total The mathematical model of can be described as:

[0089] C total = C I + C O + C M + C D (4)

[0090] In the formula: C I is the initial investment cost; C O is the operation loss cost; C M is the maintenance cost; C D is the scrap cost.

[0091] A large amount of data research is required for the initial investment cost, operation cost, and scrap cost. Considering the time value of funds, the cash flow discount calculation formula is:

[0092]

[0093] In the formula: n c is the annual life of the power transmission and transformation equipment; r is the investment recovery rate of the power industry; C D is the scrap cost, which is taken as 4% here.

[0094] Among them, the operation loss cost introduces the levelized cost of electricity, which does not need to be converted. The cash flow discount calculation formula is:

[0095]

[0096] In the formula: P loss is the total operation loss, including line loss and transformer loss; T time is the reasonable utilization hours in the whole life cycle; C price is the electricity cost.

[0097] A. Initial investment cost C I

[0098] C I is divided into the construction cost C tr of ordinary transformers and converter transformers, line(Involving both AC and DC lines), the calculation formula is:

[0099] C tr = ∑R tr S (7)

[0100] C line = ∑R line l line (8)

[0101] In the formula: R tr is the cost per unit capacity of the converter transformer and the traditional transformer; S is the transformer capacity; R line is the cost per unit length of the line; l line is the line length.

[0102] B. Operating cost

[0103] The operating cost includes the operation loss cost C O and the maintenance cost C M in two parts. The calculation formula is:

[0104] C O = P loss T time C price (9)

[0105] C M = f m C I (10)

[0106] In the formula: f m is the engineering maintenance rate.

[0107] C. Scrap cost

[0108] Estimated from engineering experience, it is about 3% - 5% of C I .

[0109] 4) Comprehensive comparison

[0110] Comprehensively comparing the short - circuit ratio and the life - cycle cost, if the short - circuit ratio of the k - th voltage level is larger and the cost is lower than that of the (k + 1) - th voltage level, then select the k - th voltage level.

[0111] The gathering voltage level suitable for the sending - end power grid framework of the renewable energy base in the sandy, gobi and desert areas is 220 kV or 500 kV, and both the sending - end and receiving - end converters are modular multilevel converters.

[0112] Example 2:

[0113] Refer to Figure 2Select the voltage level according to the provided voltage sequence scheme selection process, and perform specific voltage selection at the 500 kV and 220 kV voltage levels.

[0114] Select the voltage level according to the project parameters in Table 1.

[0115] Table 1 System parameters

[0116]

[0117]

[0118] Substitute the system parameters into Equation (2) in Example 1 to obtain the critical transmission distance. The theoretical analysis diagram of the critical transmission distance is as Figure 3 shown.

[0119] From Figure 3 it can be seen that curve P S-500 and P S-220 are the static voltage stability power transmission limits at the 500 kV and 220 kV voltage levels, and P L-500 and P L-220 are the power transmission limits under voltage deviation constraints. Under normal operating conditions, the static voltage stability power transmission limit is greater than the power transmission limit under voltage deviation constraints, and the curves all show a downward trend as the transmission distance increases.

[0120] Substitute the system parameters into Equation (1) in Example 1 to obtain the allowable power output range of the new energy system. The diagram of the allowable power output range of the new energy system is as Figure 4 shown.

[0121] By comparing and analyzing Figure 4 the allowable power output range of new energy under different operating conditions in

[0122] it can be known that:

[0123] (1) Under different voltage levels, the critical installed capacity of new energy is different. When the doubly-fed wind turbine provides 3 times the rated current, and the direct-drive wind turbine and photovoltaic provide 1.5 times the rated current, taking the North China Power Grid with the Inner Mongolia Power Grid as an example, the critical installed capacity of new energy at the 220 kV voltage level is about 5.8 GW, and the critical installed capacity of new energy at the 500 kV voltage level is about 20 GW. This method is also applicable to the 330 kV and 750 kV voltage levels in the northwest region.

[0124] Experimental verification:

[0125] To verify the consistency between the critical transmission power obtained by quantitative calculation of this equivalent method and the simulation analysis considering the detailed power electronic model, a detailed power electronic model was established in PSCAD / EMTDC. The simulation results are as Figure 5 、 6 shown.

[0126] Under the 500 kV voltage level, when a single circuit transmits 1250 MW of power, three working conditions with transmission distances of 50 km, 70 km, and 75 km are selected respectively. When the transmission distance is 50 km, the AC voltage and frequency waveforms are as Figure 5 (a) and 5(b) shown, and both can remain stable; when the transmission distance is 70 km, the AC voltage and frequency waveforms are as Figure 5 (c) and 5(d) shown, the AC voltage has a small fluctuation, and the frequency fluctuates within the range of 50 ± 0.2 Hz; when the transmission distance is 75 km, the AC voltage and frequency waveforms are as Figure 5 (e) and 5(f) shown, the AC voltage and frequency collapse, and the system becomes unstable. The theoretically calculated critical transmission distance at this time is 70 km. When the transmission distance is less than 70 km, the system is stable; when the transmission distance is greater than 70 km, the system is unstable. In summary, under the 500 kV voltage level, the simulation analysis is consistent with the theoretical analysis results.

[0127] Under the 220 kV voltage level, when a single circuit transmits 500 MW of power, three working conditions with transmission distances of 40 km, 60 km, and 65 km are selected respectively. When the transmission distance is 40 km, the AC voltage and frequency waveforms are as Figure 6 (a) and 6(b) shown, and both can remain stable; when the transmission distance is 60 km, the AC voltage and frequency waveforms are as Figure 6 (c) and 6(d) shown, the AC voltage has a small fluctuation, and the frequency fluctuates within the range of 50 ± 0.2 Hz; when the transmission distance is 65 km, the AC voltage and frequency waveforms are as Figure 6 (e) and 6(f) shown, the AC voltage and frequency collapse, and the system becomes unstable. The theoretically calculated critical transmission distance at this time is 60 km. When the transmission distance is less than 60 km, the system is stable; when the transmission distance is greater than 60 km, the system is unstable. In summary, under the 220 kV voltage level, the simulation analysis is consistent with the theoretical analysis results.

[0128] The present invention further adopts case analysis:

[0129] Case I: A single group of 1000 MW is networked, divided into 5 groups in total, and each group has wind-solar complementarity (the wind-solar ratio is 3:2) to improve the channel utilization rate;

[0130] Case II: A single cluster of 1250 MW is networked, divided into 4 clusters in total, and each cluster has a balanced combination of wind and light (the ratio of wind to light is 3:2) to improve the channel utilization rate.

[0131] After the voltage sequence selection method proposed in the present invention, the comparison results of the electrical distance and economy of the following two cases at the 220 kV and 500 kV voltage levels are as Figure 7 、 8 shown.

[0132] As Figure 7 can be seen, in Case I: when networking a single cluster of 1000 MW, when the distance is less than 56.4 km, the electrical distance of 220 kV is closer and the system strength is better; in Case II: when networking a single cluster of 1250 MW, when the distance is less than 92 km, the electrical distance of 220 kV is closer and the system strength is better. At the same distance, the electrical distance in Case II is less than that in Case I, indicating that different cluster divisions have an impact on the system strength. In addition, using double-circuit lines at the 220 kV voltage level is more conducive to safety and stability.

[0133] As Figure 8 can be seen, in Case I: when networking a single cluster of 1000 MW, when the distance is less than 66 km, the economy is better at the 220 kV voltage level. In Case II: when networking a single cluster of 1250 MW, when the distance is less than 52 km, the economy is better at the 220 kV voltage level. For the 220 kV collection voltage level, Case II will increase the number of line circuits while improving the grid voltage strength, so the initial and maintenance costs will increase, and when the line is too long, it will cause the adverse effect of excessive line corridor occupation.

[0134] In summary, when determining the distance L between the collection point and the converter station, the cluster division needs to make full use of the critical transmission capacity of each line; for occasions where new energy resources are concentrated such as in deserts, gobi, and wastelands, when the distance L ≤ 52 km, it is recommended to reduce the collection voltage sequence to 220 kV; when the distance 52 ≤ L ≤ 60 km, it is necessary to combine different cluster division situations and equipment models, and according to different scenarios, the decision-maker determines the weight of technical economy, conducts system strength and economy judgments, and obtains a more optimal voltage sequence plan; when the distance L is far, it is recommended to consider the construction of the positive and negative poles at separate locations to reduce the AC distance and improve the system strength.

[0135] This analysis conclusion has a certain margin. After reducing the collection voltage level, the transformation ratio of the converter transformer decreases, and its cost will decrease to some extent. The critical distance L for the better full-life cycle cost of 220 kV will be greater than 52 km.

[0136] The above is only for helping to understand the method of the present invention and its core concept. However, the protection scope of the present invention is not limited thereto. For those of ordinary skill in the art in the technical scope disclosed by the present invention, any equivalent substitution or change made according to the technical solution of the present invention and its inventive concept should be covered within the protection scope of the present invention. In summary, the content of this specification should not be construed as a limitation on the present invention.

Claims

1. A method for selecting voltage sequences for flexible DC transmission in AC network connection of a wide-area new energy base, characterized in that It includes the following steps: S1. Collect data of the wide-area new energy base: the regional area of new energy power stations, the installed capacity ratio of wind power and photovoltaic power, the distance from the collection point to the converter station, the parameters of power transmission and transformation equipment, the installed capacity of new energy, and the rated transmission power; S2. Judgment of critical installed capacity of new energy: Based on the proportion of wind power and photovoltaic installed capacity, k Calculate the short-circuit current at each voltage level to obtain the critical installed capacity of new energy at different voltage levels S ( k ) max If the installed capacity of new energy is greater than S ( k ) max , then increase the voltage level and recalculate S ( k ) max , control the installed capacity of new energy S ( k ) max within the scope; The critical installed capacity of new energy is restricted by the equipment's bearing capacity, and the specific calculation is as follows: n total = n WT1 + n WT2 + n PV Wherein, a is the per-unit value of the short-circuit current that the doubly-fed wind turbine can provide, b is the per-unit value of the short-circuit current that the direct-drive wind turbine and the photovoltaic can provide, n WT1 is the number of doubly-fed wind turbines, n WT2 is the number of direct-drive wind turbines, n PV is the number of photovoltaics, U is the AC collection voltage level, I k_max is the maximum short-circuit current that the equipment can withstand, I k_MMC1 is the short-circuit current provided by the network-forming flexible DC converter valve, I k_MMC1 is the short-circuit current provided by the grid-following flexible DC converter valve; S3. Design the line transmission capacity and the number of circuits: Calculate the critical transmission distance of the line to obtain the relationship table between the critical transmission distance of the line and the line transmission capacity; Based on the relationship table and the data of the wide-area new energy base collected, design the line transmission capacity and the number of circuits; S4. Maximum voltage level judgment: Judge whether k is less than the maximum voltage level k max ; If k is k max , directly select k max as the aggregated voltage level; If k is less than the maximum voltage level k max , determine whether the distance from the collection point to the converter station is less than the critical transmission distance of the line; if it is determined that the distance from the collection point to the converter station is greater than the critical transmission distance of the line, increase the voltage level by one level and return to S2 again; S5. Calculate the system strength and economy: When k is less than the maximum voltage level k max , and the distance from the collection point to the converter station is less than the critical transmission distance of the line, judge the system strength and economy for the k th voltage level and the distance of the k +1th voltage level; S6. Determine the aggregated voltage level: By comprehensively comparing the system strength and economy, if the k -th voltage level has a greater system strength and lower cost than the k +1-th voltage level, then select the k -th voltage level as the aggregated voltage level; otherwise, select the k +1-th voltage level as the aggregated voltage level.

2. The method for selecting voltage sequence of flexible DC transmission for AC network connection in wide-area new energy bases according to claim 1, wherein In the above S2, the installed capacity of new energy is controlled within S ( k ) max The specific range is as follows: P 2 +Q 2 ≤ S ( k ) max 2 Wherein, P is the active power output from the AC side of the new energy grid-connected converter, and Q is the reactive power output from the AC side of the new energy grid-connected converter.

3. The method for selecting voltage sequences for flexible DC transmission in AC networking of a wide-area new energy base according to claim 2, wherein Based on controlling the new energy installed capacity within S ( k ) max range, calculate the allowable power output range for AC networking of the new energy base: Integrating the maximum and minimum active power outputs, maximum and minimum reactive power outputs, power factor limits, static voltage stability constraints, voltage deviation constraints, equipment capacity constraints, maximum output capacity constraints of converters, and grid connection requirements of new energy power stations, in the P - Q coordinate system, the common area is determined, and the maximum power output capacity of the converter within this range is expressed by the following formula: Wherein, and are the maximum active / reactive power outputs of the new energy respectively; and are the minimum active / reactive power outputs of the new energy respectively; and are the minimum / maximum allowable power factor angles of the output, U i is the grid connection point voltage of the new energy unit; E is the grid voltage.

4. The method for selecting voltage sequences of flexible DC transmission for AC network connection in a wide-area new energy base according to claim 1 or 3, characterized in that The design of the line transmission capacity and the number of circuits in S3 is specifically as follows: The critical installed capacity of new energy is restricted by the equipment's bearing capacity, and the maximum power output capacity is restricted by the power transmission limit constrained by voltage deviation; Therefore, the critical transmission distance of the line is used as the judgment basis; The calculation of the critical transmission distance of the line is shown in the following formula: In the formula, L cr is the critical transmission distance of the line; X cr is the critical impedance of the line; U i is the voltage at the grid connection point of the new energy unit; E is the grid voltage; P is the active power output from the AC side of the new energy grid-connected converter; X T is the transformer reactance, X u is the reactance of the AC transmission line per unit length, n is the number of parallel circuits of the line.

5. The method for selecting voltage sequences of AC network connection and flexible DC transmission in a wide-area new energy base according to claim 1, wherein The calculation of the system strength in S5 is specifically as follows: Analyze the system strength by using the capacity short-circuit ratio index.

6. The method for selecting voltage sequences for AC network connection and flexible DC transmission of a wide-area new energy base according to claim 1, wherein The calculation of the economy in S5 is specifically as follows: When conducting an economic comparison, a life-cycle cost model is constructed, including initial investment costs, operating costs, and scrapping costs; operating costs include running loss costs C O and maintenance costs C M The total cost C total is mathematically modeled as: Wherein: C I is the initial investment cost; C O is the operation loss cost; C M is the maintenance cost; C D is the scrapping cost; n c is the annual life of the power transmission and transformation equipment; r is the investment recovery rate of the power industry.

7. The method for selecting voltage sequences for flexible DC transmission in AC network connection of wide-area new energy bases according to claim 6, wherein Initial investment cost C I Construction costs are divided into those of ordinary transformers and converter transformers C tr , line construction costs C line ; The calculation formula is: Wherein: R tr are the unit capacity costs of the commutation transformer and the ordinary transformer; S is the transformer capacity; R line is the construction cost per unit length of the line; l line is the line length.

8. The method for selecting voltage sequences for AC networked flexible DC transmission from a wide-area new energy base according to claim 7, characterized in that Maintenance cost C M and scrap cost C D based on the initial investment cost; C M (k)=f m C I Wherein: f m is the engineering maintenance rate; Estimated from engineering experience, the scrapping cost is 3% - 5% of the initial investment cost; C D (k)=(3%~5%)C I 。 9. The method for selecting voltage sequence of flexible DC transmission for AC networking of wide-area new energy bases according to claim 6, characterized in that The operation loss cost introduces the levelized cost of electricity, and the calculation formula is: C O (k)=P loss T time C price P loss (k)=P loss_T +P loss_line Wherein: P loss is the total operating loss, including line loss and transformer loss; T time is the reasonable utilization hours in the whole life cycle; C price is the electricity cost; P loss_T is the transformer loss; P loss_line is the line loss.

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