Voltage sequence selection method for alternating-current networking flexible direct transmission of wide-area new energy base

Abstract

The invention discloses a voltage sequence selection method for alternating current networking flexible direct transmission of a wide-area new energy base, and relates to the technical field of voltage grade selection. The method comprises the following steps: collecting wide-area new energy base data; new energy critical installed capacity judgment: calculating new energy critical installed capacity S (k) max of different voltage grades, and controlling the new energy critical installed capacity to be within the range of S (k) max; line transmission capacity and loop number design: calculating a line critical transmission distance to obtain a relation table between the line critical transmission distance and the line transmission capacity; according to the relation table and the collected wide-area new energy base data, line transmission capacity and number of turns are designed; performing system strength and economy judgment on the distance between the kth voltage level and the (k + 1) th voltage level; and selecting a voltage level with high system strength and low cost as a collection voltage level. The method is suitable for an alternating current networking direct current sending-out system which is suitable for a Sagomean wide-area new energy base and is not supported by a conventional power supply.

Description

Technical Field

[0001] The present invention belongs to the technical field of voltage level selection, and in particular relates to a method for selecting a voltage sequence transmitted by a flexible direct current (DC) transmission system of an AC network of a wide-area new energy base. Background Art

[0002] The research on DC transmission system for renewable energy transmission scenarios is becoming more and more in-depth at home and abroad, and submodule hybrid MMC is gradually becoming a key technology for renewable energy DC transmission. Different from the Shagohuang renewable energy transmission scenario supported by power grids and coal-fired power plants, the Shagohuang renewable energy transmission scenario without conventional power supply support is not only larger in scale and wider in area, but also has the problems of difficulty in networking, lack of economy and reliability.

[0003] The collection voltage level sequence of renewable energy bases will greatly affect the construction and operation costs of the station. Different collection voltage level sequences lead to different grid structures, electrical equipment configurations and losses of wind farms and photovoltaic farms, and there are large differences in costs. Therefore, for large-scale Shago deserted islands that are not connected to the main grid, the collection voltage level of new energy is an important issue that needs to be optimized and studied.

[0004] The voltage level of the new energy AC network that is about to be built is generally unified with the voltage level of the main grid. For the large-scale Shagohuang isolated island new energy transmission scenario, which is different from the above-mentioned Shagohuang new energy transmission scenario supported by power grids and coal-fired power plants, there is no reference case for the selection of the sending-end power grid collection voltage level that ensures the stability of the power grid and has good economy.

[0005] The present invention proposes a method for selecting a voltage sequence for flexible direct current transmission of a wide-area new energy base AC network. 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, a voltage level formation law and method suitable for the sending-end power grid of a renewable energy base in the Shagohuang area are proposed, and the collection voltage level sequence of the new energy system in the Shagohuang area is optimized. Summary of the invention

[0006] The purpose of the present invention is to provide a method for selecting a voltage sequence for flexible direct current transmission of an AC network of a wide-area new energy base, so as to solve the problems raised in the above background technology that the prior art has not yet proposed a method for selecting the collection voltage level of the sending-end power grid that ensures the stability of the power grid and has good economy for large-scale new energy transmission scenarios on deserted islands.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] The present invention proposes a method for selecting a voltage sequence sent by a flexible direct current (DC) network of a wide-area new energy base, comprising the following steps:

[0009] S1. Collect data on wide-area new energy bases: the area of ​​new energy stations, the proportion of wind power and photovoltaic installed capacity, 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. Determination of critical installed capacity of new energy: According to the proportion of wind power and photovoltaic installed capacity, the short-circuit current is calculated 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 the voltage level by one level and recalculate S(k) max , control the installed capacity of new energy at S(k) max within the scope;

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

[0012] 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 To collect voltage levels;

[0013] 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;

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

[0015] S6. Determine the aggregate voltage level: comprehensively compare the system strength and economy. If the kth voltage level has greater system strength and lower cost than the k+1th voltage level, then select the kth voltage level as the aggregate voltage level; otherwise, select the k+1th voltage level as the aggregate voltage level.

[0016] Preferably, the critical installed capacity of new energy in S2 is limited by the bearing capacity of the equipment, and is specifically calculated as follows:

[0017]

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

[0019] Where a is the per-unit value of the short-circuit current that the double-fed wind turbine can provide, b is the per-unit value of the short-circuit current that the direct-drive wind turbine and photovoltaic can provide, and n WT1 is the number of doubly fed wind turbines, n WT2 is the number of direct-drive fans, n PV is the number of photovoltaic cells, U is the AC collection voltage level, I k_max is the maximum short-circuit current that the equipment can withstand, I k_MMC1 The short-circuit current provided to the grid-type flexible DC converter valve, I k_MMC1 Provide short-circuit current for grid-following flexible DC converter valves;

[0020] Preferably, in S2, the installed capacity of new energy is controlled at S(k) max The scope is as follows:

[0021] P 2 +Q 2 ≤S(k) max 2

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

[0023] Preferably, based on controlling the installed capacity of new energy in S(k) max Within the range, the allowable power output range of the new energy base AC network is calculated:

[0024] Taking into account the maximum and minimum active output, maximum and minimum reactive output, power factor limit, static voltage stability constraint, voltage offset constraint, equipment bearing capacity constraint, maximum output capacity constraint of converter and grid connection requirements of new energy stations, the common area is determined in the PQ coordinate system. The maximum power output capacity of the converter within this range is expressed as follows:

[0025]

[0026] In the formula, They are the maximum active / reactive output of new energy respectively; They are the minimum active / reactive output 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, the line transmission capacity and number of circuits in S3 are designed as follows:

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

[0029] The calculation of the critical transmission distance of the line is shown as follows:

[0030]

[0031] Where, L cr is the critical transmission distance of the line; X T is the transformer reactance, 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 size is used as a criterion for determination:

[0033]

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

[0035] Preferably, the economic efficiency is calculated in S5 as follows:

[0036] When making economic comparisons, a full life cycle cost model is constructed, including initial investment cost, operating cost and scrapping cost; operating cost includes operating loss cost C O and maintenance cost C M Two parts; total cost C total The mathematical model is described as:

[0037]

[0038] Where: C I is the initial investment cost; C O is the operating 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 The construction cost C is divided into ordinary transformer and converter transformer tr , Line construction cost C line ; The calculation formula is:

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

[0041] Where: R tr is the unit capacity cost of converter transformer and 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 scrap cost C D Based on initial investment cost;

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

[0044] Where: f m is the engineering maintenance rate;

[0045] According to engineering experience, the scrapping cost is estimated to be 3% to 5% of the initial investment cost;

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

[0047] Preferably, the operating loss cost is introduced into the levelized cost of electricity, 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] Where: P loss is the total operating loss, including line loss and transformer loss; T time Reasonable utilization hours for 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 present invention has the following beneficial effects:

[0052] (1) The present invention provides a method for selecting a voltage level sequence for a large-scale isolated island of Shago that is not connected to the main grid. By exploring the relationship between electrical distance, new energy capacity and voltage level, the critical transmission reactance and distance at different voltage levels are studied. Then, based on the short-circuit current calculation, the critical installed capacity of new energy at different voltage levels is obtained. On this basis, taking into account the impact of electrical distance and economy on the system, a law and method for selecting the voltage level of the collection that is suitable for the power grid at the sending end of a renewable energy base are proposed. The technical solution provided by the present invention has the advantages of: to a large extent, it reduces the construction and operation costs of the station, reduces the configuration and loss of electrical equipment, and enhances the strength of the system, which is of great significance to the project. The present invention can be widely used in the field of multi-voltage level AC networking and DC transmission of large-scale new energy bases on wide areas.

[0053] (2) In the present invention, by analyzing the power limit transmission distance, it is concluded that the line transmission distance is inversely proportional to the rated power of new energy, and that voltage deviation is the main factor constraining the power limit transmission distance. On the basis of the above, the voltage level formation law and method of the power grid at the sending end of the renewable energy base in the Shagohuang area are proposed, and the feasibility and advantages of voltage level sequence optimization are discussed, which is of great significance for the construction and operation costs of the station, the configuration of electrical equipment and the reduction of losses.

[0054] (3) The present invention takes into account the impact of electrical distance and economy on the system on the basis of fully considering the critical transmission reactance, distance and critical installed capacity of new energy under different voltage levels; when calculating the economy, the full life cycle cost is fully constructed, focusing on both the initial investment and the operating cost and scrapping cost; the proposed voltage sequence selection method is suitable for the Shagohuang wide-area new energy base, an AC network DC transmission system without conventional power support.

[0055] (4) The present invention integrates the maximum and minimum active output, maximum and minimum reactive output, power factor limitation, static voltage stability constraint, voltage offset constraint, equipment bearing capacity constraint, maximum output capacity constraint of the converter and grid connection requirements of the new energy station. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 It is the topological diagram of the flexible direct current transmission of the AC network of the wide-area new energy base in the present invention;

[0057] Figure 2 It is a flow chart of the method for selecting the voltage sequence sent by the flexible direct current of the wide-area new energy base AC network in the present invention;

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

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

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

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

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

[0063] Figure 8 This is a comparison chart of the economic performance under voltage level optimization in the present invention. DETAILED DESCRIPTION

[0064] The following will be combined with the drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without creative work are within the scope of protection of the present invention.

[0065] Embodiment 1:

[0066] See also Figure 1 , the structure diagram of the new power system is as follows:

[0067] The sending end is a wind and solar integrated base, which is connected through multi-voltage AC networking and collected to the hybrid MMC converter station for DC transmission, and then inverter-connected to the AC grid at the receiving-end MMC converter station via long-distance overhead lines.

[0068] See also Figure 2 The method for selecting the voltage sequence sent by the flexible direct current in the AC network of the wide-area new energy base is as follows:

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

[0070] 2) Determination of critical installed capacity of new energy, design of line transmission capacity and number of times:

[0071] According to the proportion of wind power and photovoltaic installed capacity, the short-circuit current is calculated 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 one voltage level is added (k=k+1).

[0072]

[0073] Where a is the per-unit value (multiple of rated current) of the short-circuit current that the doubly-fed wind turbine can provide, b is the per-unit value (multiple of rated current) of the short-circuit current that the direct-drive wind turbine and photovoltaic can provide, and n is WT1 is the number of doubly fed wind turbines, n WT2 is the number of direct-drive fans, n PV is the photovoltaic number, 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 The short-circuit current provided to the grid-type flexible DC converter valve, I k_MMC1 Provides short-circuit current for grid-following flexible DC converter valves.

[0074] The power output capacity of the converter under various constraints is calculated by formula (1). The power operating range diagram ( Figure 4 ) It can be seen that the critical installed capacity of new energy is mainly limited by the equipment's bearing capacity, and the maximum power output capacity is mainly constrained by the power transmission limit considering the voltage offset constraint. Therefore, in the voltage level selection method, the critical transmission distance of the line is used as the basis for judgment.

[0075] Taking into account the maximum and minimum active output, maximum and minimum reactive output, power factor limit, static voltage stability constraint, voltage offset constraint, equipment bearing capacity constraint, maximum output capacity constraint of converter and grid connection requirements of new energy stations, the common area is determined in the PQ coordinate system. The maximum power output capacity of the converter within this range can be expressed by formula (1):

[0076]

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

[0078] By considering the stability analysis of voltage deviation, the critical transmission distance L of the line is obtained. crRelationship table with line transmission capacity. When the installed capacity of new energy is S(k) max Within the range, the line transmission capacity and number of circuits are designed according to the relationship table and the input initial parameters. max , and the distance L from the collection point to the converter station is less than L cr When the distance between the kth voltage level and the (k+1)th voltage level is determined by electrical distance and economic efficiency; if the distance L from the collection point to the converter station is greater than L cr When k is k, the voltage level is increased by one level (k=k+1); when k is k max , and when the critical installed capacity of new energy is met, k is selected max For the collection voltage level.

[0079] The critical transmission distance is shown in formula (2):

[0080]

[0081] Where, L cr is the critical transmission distance of the line; X cr is the critical impedance of the line; U i is the grid connection point voltage of the new energy unit; E is the grid voltage; P is the active power output on 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, and n is the number of parallel lines.

[0082] 3) Determination of AC voltage level taking into account system strength and economy:

[0083] ① System strength:

[0084] The capacity short-circuit ratio indicator is used to analyze the voltage support strength of the grid connection point i of the multi-infeed system. In a pure renewable 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 short-circuit ratio of the renewable energy. Therefore, the impedance can be used as a criterion for judgment.

[0085]

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

[0087] ②Economical

[0088] When comparing the economic feasibility of new energy AC networking and DC transmission solutions, it is necessary to build a full life cycle cost model. total The mathematical model can be described as:

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

[0090] Where: C I is the initial investment cost; C O is the operating loss cost; C M is the maintenance cost; C D For scrapping costs.

[0091] The initial investment cost, operating cost, and scrapping cost all require a lot of data research. Considering the time value of money, the cash flow discount calculation formula is:

[0092]

[0093] Where: n c is the annual life of power transmission and transformation equipment; r is the investment recovery rate of power industry; C D For the scrapping cost, 4% is taken here.

[0094] The operating loss cost is introduced into the levelized cost of electricity, and no discount is required. The cash flow discount calculation formula is:

[0095]

[0096] Where: P loss is the total operating loss, including line loss and transformer loss; T time Reasonable utilization of hours for the entire life cycle; C price For electricity cost.

[0097] A. Initial investment cost C I

[0098] C I The construction cost C is divided into ordinary transformer and converter transformer tr , Line construction cost C 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] Where: R tr is the unit capacity cost of converter transformer and 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.Operational costs

[0103] Operating costs include operating loss costs C O and maintenance cost C M 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] Where: f m It is the project maintenance rate.

[0107] C. Scrap cost

[0108] According to engineering experience, it is about C I 3% to 5%.

[0109] 4) Comprehensive comparison

[0110] Comprehensively compare the short-circuit ratio and the full life cycle cost. If the kth voltage level has a larger short-circuit ratio and lower cost than the (k+1)th voltage level, then the kth voltage level is selected.

[0111] The collection voltage level of the power grid at the sending end of the renewable energy base in the Shagohuang area is 220kV or 500kV, and the converters at the sending and receiving ends are all modular multi-level converters.

[0112] Embodiment 2:

[0113] See also Figure 2The voltage sequence scheme selection process provided is used to select the voltage level, and specific voltage selection is performed at 500kV and 220kV voltage levels.

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

[0115] Table 1 System parameters

[0116]

[0117]

[0118] Substituting the system parameters into formula (2) in Example 1, the critical transmission distance can be obtained. The critical transmission distance theoretical analysis diagram is as follows: Figure 3 shown.

[0119] Depend on Figure 3 It can be seen that curve P S-500 and P S-220 is the static voltage stability power transfer limit at 500kV and 220kV voltage levels, P L-500 and P L-220 is the power transmission limit under the voltage offset constraint. Under normal operating conditions, the static voltage stability power transmission limit is greater than the power transmission limit under the voltage offset constraint, and the curves show a downward trend with the increase of transmission distance.

[0120] Substituting the system parameters into formula (1) in Example 1, the allowable power output range of the new energy system can be obtained. The allowable power output range of the new energy system is shown in FIG. Figure 4 shown.

[0121] Through comparative analysis Figure 4 The allowable power output range of new energy under different operating conditions can be known:

[0122] (1) The critical installed capacity of new energy is different at different voltage levels. When the double-fed wind turbine provides 3 times the rated current, the direct-drive wind turbine and photovoltaic provide 1.5 times the rated current, the critical installed capacity of new energy at the 220kV voltage level of the North China Power Grid, taking the Inner Mongolia Power Grid as an example, is about 5.8GW, and the critical installed capacity of new energy at the 500kV voltage level is about 20GW. This method is also applicable to the 330kV and 750kV voltage levels in the northwest region.

[0123] (2) Under normal operating conditions, the allowable power output range of the new energy system is mainly constrained by static voltage stability constraints and site grid connection requirements. If the power devices of the new energy unit are limited by electrical stress and can only provide a short-circuit current of 1.2 pu, the critical installed capacity of new energy will be further expanded, but it is not the main factor affecting the allowable power output range of the new energy system.

[0124] Experimental verification:

[0125] In order to verify the consistency between the critical transmission power quantitatively calculated by the equivalent method and the simulation analysis considering the detailed power electronic model, a detailed power electronic model was established in PSCAD / EMTDC. Figure 5 , 6 shown.

[0126] At 500kV voltage level, when transmitting 1250MW power in a single circuit, three transmission distances of 50km, 70km and 75km are selected. When the transmission distance is 50km, the AC voltage and frequency waveforms are as follows: Figure 5 (a) and 5(b), both can remain stable; when the transmission distance is 70km, the AC voltage and frequency waveforms are as follows Figure 5 As shown in (c) and 5(d), the AC voltage fluctuates slightly and the frequency fluctuates within the range of 50±0.2Hz. When the transmission distance is 75km, the AC voltage and frequency waveforms are as follows Figure 5 As shown in (e) and 5(f), the AC voltage and frequency collapse and the system becomes unstable. Theoretical calculations show that the critical transmission distance at this time is 70km. When the transmission distance is less than 70km, the system is stable; when the transmission distance is greater than 70km, the system becomes unstable. In summary, at the 500kV voltage level, the simulation analysis is consistent with the theoretical analysis results.

[0127] At 220kV voltage level, when transmitting 500MW power in a single circuit, three transmission distances of 40km, 60km and 65km are selected. When the transmission distance is 40km, the AC voltage and frequency waveforms are as follows: Figure 6 (a) and 6(b), both can remain stable; when the transmission distance is 60km, the AC voltage and frequency waveforms are as follows Figure 6 As shown in (c) and 6(d), the AC voltage fluctuates slightly and the frequency fluctuates within the range of 50±0.2Hz. When the transmission distance is 65km, the AC voltage and frequency waveforms are as follows Figure 6 As shown in (e) and 6(f), the AC voltage and frequency collapse and the system becomes unstable. Theoretical calculations show that the critical transmission distance at this time is 60km. When the transmission distance is less than 60km, the system is stable; when the transmission distance is greater than 60km, the system becomes unstable. In summary, at the 220kV voltage level, the simulation analysis is consistent with the theoretical analysis results.

[0128] The present invention further adopts example analysis:

[0129] Case I: A single cluster of 1000MW is networked and divided into 5 clusters, each of which has wind and solar power complementing each other (the ratio of wind and solar power is 3:2) to improve channel utilization;

[0130] Case II: A single cluster of 1250MW is networked and divided into four clusters, each with wind and solar power complementing each other (the ratio of wind and solar power is 3:2) to improve channel utilization.

[0131] The comparison results of electrical distance and economic efficiency of the two cases under 220kV and 500kV voltage levels after the voltage sequence selection method proposed by the present invention are as follows: Figure 7 , 8 shown.

[0132] Depend on Figure 7 It can be seen that in case Ⅰ: when a single group of 1000MW is networked, when the distance is less than 56.4km, the electrical distance of 220kV is closer and the system strength is better; case Ⅱ: when a single group of 1250MW is networked, when the distance is less than 92km, the electrical distance of 220kV is closer and the system strength is better. At the same distance, the electrical distance of case Ⅱ is smaller than that of case Ⅰ, indicating that different sub-groups have an impact on the system strength. In addition, at the 220kV voltage level, double-circuit lines can be used, which is more conducive to safety and stability.

[0133] Depend on Figure 8 It can be seen that, in case I: when a single group of 1000MW is networked, when the distance is less than 66km, the economic performance is better at the 220kV voltage level. Case II: when a single group of 1250MW is networked, when the distance is less than 52km, the economic performance is better at the 220kV voltage level. For the 220kV collection voltage level, case II will increase the number of lines while increasing the voltage strength of the power grid, thus increasing the initial and maintenance costs. When the line is too long, it will cause the adverse effect of the line corridor occupying too much space.

[0134] In summary, when the distance L between the collection point and the converter station is determined, the subgroup needs to make full use of the critical transmission capacity of each line; for places where new energy resources are concentrated, such as Shagohuang, when the distance L≤52km, it is recommended to reduce the collection voltage sequence to 220kV; when the distance is 52≤L≤60km, it is necessary to combine different subgroup situations and equipment models, and according to different scenarios, the decision makers should determine the technical and economic weights, make system strength and economic judgments, and obtain a better voltage sequence plan; when the distance L is far, it is recommended to consider the construction of positive and negative poles to reduce the AC distance and improve the system strength.

[0135] The conclusion of this analysis leaves a certain margin. After lowering the collection voltage level, the conversion ratio will decrease, and its cost will decrease. The critical distance L for a better life cycle cost of 220kV will be greater than 52km.

[0136] The above description is only used to help understand the method of the present invention and its core essence, but the protection scope of the present invention is not limited thereto. For those skilled in the art in the art, equivalent replacement or change according to the technical solution and inventive concept of the present invention within the technical scope disclosed by the present invention should be included in the protection scope of the present invention. In summary, the content of this specification should not be understood as limiting the present invention.

Claims

1. A method for selecting a voltage sequence sent by a flexible direct current (DC) network of a wide-area new energy base, characterized in that: The following steps are involved: S1. Collect data on wide-area new energy bases: the area of ​​new energy stations, the proportion of wind power and photovoltaic installed capacity, 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. Determination of critical installed capacity of new energy: According to the proportion of wind power and photovoltaic installed capacity, the short-circuit current is calculated 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 the voltage level by one level and recalculate S(k) max , control the installed capacity of new energy at S(k) max within the scope; S3. Line transmission capacity and number of loops design: Calculate the critical transmission distance of the line and obtain the relationship table between the critical transmission distance of the line and the line transmission capacity; Design the transmission capacity and number of lines based on the relationship table and the collected wide-area new energy base data; 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 To collect voltage levels; 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; S5. Calculate system strength and economy: When k is less than the maximum voltage level k max , and when the distance from the collection point to the converter station is less than the critical transmission distance of the line, the system strength and economy are determined for the distance between the kth voltage level and the k+1th voltage level; S6. Determine the aggregate voltage level: comprehensively compare the system strength and economy. If the kth voltage level has greater system strength and lower cost than the k+1th voltage level, then select the kth voltage level as the aggregate voltage level; otherwise, select the k+1th voltage level as the aggregate voltage level.

2. The method for selecting the voltage sequence sent by the flexible direct current of the wide-area new energy base AC network according to claim 1 is characterized in that: The critical installed capacity of new energy in S2 is limited by the equipment's bearing capacity, and the specific calculation is as follows: n total =n WT1 +n WT2 +n PV Where a is the per-unit value of the short-circuit current that the double-fed wind turbine can provide, b is the per-unit value of the short-circuit current that the direct-drive wind turbine and photovoltaic can provide, and n WT1 is the number of doubly fed wind turbines, n WT2 is the number of direct-drive fans, n PV is the number of photovoltaic cells, U is the AC collection voltage level, I k_max is the maximum short-circuit current that the equipment can withstand, I k_MMC1 The short-circuit current provided to the grid-type flexible DC converter valve, I k_MMC1 Provides short-circuit current for grid-following flexible DC converter valves.

3. The method for selecting the voltage sequence sent by the flexible direct current of the wide-area new energy base AC network according to claim 2 is characterized in that: The control of the installed capacity of new energy in S2 is S(k) max The scope is as follows: P 2 +Q 2 ≤S(k) max 2 Where P is the active power output on the AC side of the new energy grid-connected converter, and Q is the reactive power output on the AC side of the new energy grid-connected converter.

4. The method for selecting the voltage sequence sent by the flexible direct current in the wide-area new energy base AC network according to claim 3 is characterized in that: Based on controlling the installed capacity of new energy in S(k) max Within the range, the allowable power output range of the new energy base AC network is calculated: Taking into account the maximum and minimum active output, maximum and minimum reactive output, power factor limit, static voltage stability constraint, voltage offset constraint, equipment bearing capacity constraint, maximum output capacity constraint of converter and grid connection requirements of new energy stations, the common area is determined in the PQ coordinate system. The maximum power output capacity of the converter within this range is expressed as follows: obj.P max =maxP out In the formula, They are the maximum active / reactive output of new energy respectively; They are the minimum active / reactive output 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.

5. The method for selecting the voltage sequence sent by the flexible direct current in the wide-area new energy base AC network according to claim 1 or 4, characterized in that: The line transmission capacity and number of circuits in S3 are designed as follows: The critical installed capacity of new energy is limited by the equipment's bearing capacity, and the maximum power output capacity is constrained by the power transmission limit of the voltage offset constraint; therefore, the critical transmission distance of the line is used as the basis for judgment; The calculation of the critical transmission distance of the line is shown as follows: Where, L cr is the critical transmission distance of the line; X cr is the critical impedance of the line; U i is the grid connection point voltage of the new energy unit; E is the grid voltage; P is the active power output on 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, and n is the number of parallel lines.

6. The method for selecting the voltage sequence sent by the flexible direct current of the wide-area new energy base AC network according to claim 1 is characterized in that: The calculation of the system strength in S5 is specifically as follows: the system strength is analyzed using the capacity short-circuit ratio index.

7. The method for selecting the voltage sequence sent by the flexible direct current of the wide-area new energy base AC network according to claim 1 is characterized in that: The economic efficiency is calculated in S5 as follows: When making economic comparisons, a full life cycle cost model is constructed, including initial investment cost, operating cost and scrapping cost; operating cost includes operating loss cost C O and maintenance cost C M Two parts; total cost C total The mathematical model is described as: Where: C I is the initial investment cost; C O is the operating 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.

8. The method for selecting the voltage sequence sent by the flexible direct current of the wide-area new energy base AC network according to claim 7 is characterized in that: Initial investment cost C I The construction cost C is divided into ordinary transformer and converter transformer tr 、Line construction cost C line ; The calculation formula is: C I (k)=ΣR tr S+ΣR line l line Where: R tr is the unit capacity cost of converter transformer and ordinary transformer; S is the transformer capacity; R line is the cost per unit length of the line; l line is the line length.

9. The method for selecting the voltage sequence sent by the flexible direct current in the wide-area new energy base AC network according to claim 8 is characterized in that: Maintenance cost M and scrap cost C D Based on initial investment cost; C M (k)=f m C I Where: f m is the engineering maintenance rate; According to engineering experience, the scrapping cost is estimated to be 3% to 5% of the initial investment cost; C D (k)=(3%~5%)C I 。 10. The method for selecting the voltage sequence sent by the flexible direct current in the wide-area new energy base AC network according to claim 7, characterized in that: The operating loss cost is introduced into 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 Where: P loss is the total operating loss, including line loss and transformer loss; T time Reasonable utilization hours for 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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