Large-scale new energy AC networking power grid strength evaluation method

Abstract

The invention discloses a large-scale new energy AC networking power grid strength evaluation method, and relates to the technical field of power grid strength evaluation. The method comprises the following steps: enabling all nodes except grid-connected nodes and grid-connected equipment to be equivalent to a multi-infeed system by adopting a Thevenin theorem, and analyzing influence factors of voltage fluctuation caused by grid connection of the multi-infeed system; calculating a voltage intensity evaluation value of the new energy grid-connected node; calculating a critical stability standard value of the new energy grid-connected node; judging whether the grid-connected node is stable or not by judging whether the voltage intensity evaluation value of the new energy grid-connected node is greater than the critical stability standard value corresponding to the node of the new energy grid-connected node or not; and judging whether the large-scale new energy AC networking power grid is stable or not by judging whether all the nodes are stable or not. The method is suitable for evaluating the voltage strength of the sending-end multi-voltage-level alternating-current network frame which is sent out by a large-scale new energy through a network construction type direct-current converter station and is not supported by a conventional power supply and not connected by an alternating-current main network, the logic is simple, and the method has pertinence.

Description

Technical Field

[0001] The present invention belongs to the technical field of power grid strength assessment, and in particular relates to a method for assessing the strength of a large-scale new energy AC networking power grid. Background Art

[0002] The hinterland of the desert is usually richer in renewable energy, but unlike the edge of the desert, such areas face no conventional power supply support and no supporting coal-fired power plants, and the stable operation of their renewable energy bases faces great challenges. Grid-forming (GFM) control is widely considered to be an effective means to solve the problem of insufficient grid strength, but the strength analysis of the AC grid with flexible DC converters providing the main voltage support is an urgent problem to be solved.

[0003] Grid strength is often used to measure the voltage fluctuation caused by the connection of equipment to the grid, that is, the interaction between the grid and the connected equipment. The greater the grid strength, the smaller the impact of the connected equipment on the grid connection point voltage. As an important indicator to measure the strength of the grid, the short circuit ratio (SCR) can be used to evaluate the grid strength by comparing it with the critical short circuit ratio (CSCR) of the system. At present, the evaluation of the grid strength of the renewable energy multi-feed grid-connected system is still in the research stage. Usually, the stability of the voltage of the node of the AC grid supported by the power supply or the main grid is considered, and only the grid connection point of the renewable energy converter station is treated as a PQ node, which cannot reflect the impact of the converter station control strategy on the grid stability.

[0004] In traditional short-circuit ratio calculation, short-circuit capacity is regarded as the product of grid rated voltage and node short-circuit current, where short-circuit current is the quotient of grid-connected node no-load voltage and grid-connected equipment and system equivalent impedance. Due to the stress constraints of power electronic devices, general converter design requires 1.5 times overcurrent capacity. Therefore, for the voltage strength assessment of large-scale new energy multi-voltage level AC collection grids with a high proportion of power electronic equipment, based on the traditional short-circuit ratio calculation, the calculated value of grid-connected node short-circuit current may be higher than the short-circuit current that the system can provide, resulting in the problem of too large calculated value of node short-circuit capacity, and the reliability of voltage strength assessment of new energy base grid connection points is difficult to guarantee.

[0005] In order to overcome the problem of voltage strength assessment of multi-voltage level AC collection grids of new energy bases provided with main voltage support by the above-mentioned grid-type DC converters, the present invention provides a method for evaluating the strength of large-scale new energy AC networking grids. The applicable scenario is the DC transmission system after AC collection of multiple stations and multiple voltage levels in a large-scale new energy base through a grid-type converter station, wherein the AC collection grid does not contain conventional power support and does not contain AC main grid connection; the evaluation method is to take into account the remaining grid-connected equipment except the grid-connected node through the Thevenin equivalent, and use the voltage fluctuation caused by the grid-connected equipment and the equivalent impedance of the grid-connected equipment to evaluate the voltage strength of the grid-connected node. Summary of the invention

[0006] The purpose of the present invention is to provide a method for evaluating the grid strength of a large-scale new energy AC network, so as to solve the problem of voltage strength evaluation of a large-scale new energy multi-voltage level AC collection grid with a high proportion of power electronic equipment in the prior art proposed in the above background technology. When calculated based on the traditional short-circuit ratio, the calculated value of the short-circuit current of the grid-connected node may be higher than the short-circuit current that the system can provide, thereby causing the problem of the calculated value of the node short-circuit capacity being too large, resulting in the problem that the reliability of the voltage strength evaluation of the grid-connected point of the new energy base is difficult to ensure.

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

[0008] The present invention proposes a method for evaluating the strength of a large-scale new energy AC network, comprising the following steps:

[0009] S1. Using Thevenin's theorem, all nodes except the grid-connected nodes and grid-connected equipment are equivalent to a multi-feed system, and the influencing factors of voltage fluctuation caused by the grid connection of the multi-feed system are analyzed;

[0010] S2. Considering the influencing factors of voltage fluctuation, based on Thevenin equivalent and Ohm's law, calculate the voltage strength assessment value of the new energy grid-connected node;

[0011] S3. Calculate the critical stability standard value of the new energy grid-connected node based on the relationship between transmission power and static voltage stability;

[0012] S4. Determine whether the grid-connected node is stable by determining whether the voltage strength evaluation value of the new energy grid-connected node is greater than the critical stability standard value corresponding to the node itself, and evaluate the strength of the grid-connected node;

[0013] S5. According to S2-S4, determine whether all nodes are stable in turn, and determine whether the large-scale new energy AC networking power grid is stable according to whether all nodes are stable.

[0014] Preferably, the S1 is as follows:

[0015] The AC collection grid does not contain conventional power support, such as coal-fired power plants; and the AC collection grid does not contain AC main grid connection, that is, it is not connected to the existing AC grid through transmission lines; in the AC collection grid, only the grid-type flexible DC converter station is used to provide the main voltage support for the AC grid, and reactive compensation equipment and other equipment with a high proportion of power electronics and voltage support capabilities can be connected, which only needs to be equivalent to grid-connected equipment; all other grid-connected equipment except this grid-connected node, that is, other new energy stations and supporting equipment, are taken into account through the Thevenin equivalence.

[0016] The large-scale renewable energy AC network grid is regarded as a multi-feedback system, and it is assumed that there are no other grid-connected devices before the grid-connected devices are connected to the grid. That is, before the grid-connected devices are connected to the grid, the grid-connected nodes are in a no-load state. According to the Thevenin equivalence theorem, the relationship between the voltages before and after the grid-connected points of the renewable energy bases in the multi-feedback system satisfies:

[0017]

[0018] Among them: U i is the voltage amplitude of the grid-connected node after the new energy is connected to the grid, is the no-load voltage amplitude of the grid-connected node of the multi-feed system before the new energy is connected to the grid, They are the actual working voltage and no-load voltage of the grid-connected node before and after the new energy is connected to the grid. is the actual no-load voltage of node j, Z device，j is the equivalent grid-connected impedance of the grid-connected device at node j, Z jj is the network node self-impedance of grid-connected node j, Z ij is the network node mutual impedance between grid-connected node i and node j.

[0019] Furthermore, based on the above analysis, the influencing factors in S1 include the voltage fluctuation caused by the grid-connected device itself and the voltage fluctuation caused by the grid-connected devices at other nodes on the grid-connected node voltage. The degree of influence of the grid-connected devices at other nodes is related to the impedance between them and the grid-connected nodes.

[0020] Preferably, S2 is specifically as follows:

[0021] Taking into account the influence of other grid-connected equipment, the voltage strength evaluation value of the new energy grid-connected node i is calculated by comprehensive impedance and grid-connected node voltage fluctuation; based on the Thevenin equivalent and Ohm's law, the calculation method of the voltage strength evaluation value of the new energy grid-connected node i is obtained:

[0022]

[0023] Among them: The node voltage before and after the new energy grid-connected node is obtained through power flow calculation Z ij represents the mutual impedance between grid-connected nodes i and j, Zjj represents the self-impedance of the other grid-connected nodes j, Z device，j It represents the equivalent grid-connected impedance of the grid-connected equipment, and SCRZii represents the voltage strength assessment value of the new energy grid-connected node i.

[0024] Preferably, S3 is specifically as follows:

[0025] For the critical stability of the system, the voltage static stability limit expression corresponding to the power-voltage relationship considering reactive power compensation is obtained from the relationship between the transmission power and the static voltage stability:

[0026]

[0027] Among them: U R In order to meet the minimum value of the receiving terminal voltage when the power transmission of the power grid is maximum, U s is the sending end voltage, Qk is the reactive power generated by the reactive power compensation equipment, is the power factor angle, x is the power transfer impedance;

[0028] When the new energy station is connected to the grid, the sending end voltage is U s At this time, the equivalent grid-connected impedance of the new energy station is Z device Considered as power transfer impedance x, the maximum voltage drop is obtained according to the voltage static limit;

[0029] According to the voltage static stability limit, the critical stability standard value of the new energy grid-connected node is obtained:

[0030]

[0031] Among them, U i is the voltage amplitude of the grid-connected node after the new energy is connected to the grid, U R is the voltage static stability limit, CSCRZ i Represents the critical stability standard value of the new energy grid-connected node i.

[0032] Preferably, the S4 is specifically as follows:

[0033] If the voltage strength assessment value of the new energy grid-connected node is judged to be greater than the critical stability standard value corresponding to the node itself, then the grid-connected node is stable, and the greater the voltage strength assessment value exceeds the critical stability standard value, the more stable the corresponding grid-connected node is and the greater its strength is;

[0034] Preferably, the S5 is specifically as follows:

[0035] If the voltage strength assessment values ​​of all nodes of the large-scale new energy AC network power grid are greater than their corresponding critical stability standard values, the large-scale new energy AC network power grid is judged to be stable; if there is a new energy grid-connected node whose voltage strength assessment value is less than the critical stability standard value corresponding to its own node, the large-scale new energy AC network power grid is judged to be unstable.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] The present invention provides a method for evaluating the strength of a large-scale new energy AC network. The evaluation method uses the Thevenin equivalence theorem, starting from the perspective of equivalent impedance of grid-connected equipment, and based on the voltage fluctuation caused by the grid connection of new energy equipment, evaluates the voltage strength of the above-mentioned AC grid at the impedance level. The present invention can effectively avoid the problem of excessive short-circuit capacity of nodes in new energy AC grids with a high proportion of power electronic equipment, and avoid the impact of current limiting on the grid voltage strength evaluation caused by the grid-building type flexible DC converter station, making the voltage strength evaluation of large-scale new energy AC collection grids more reasonable and accurate. It is suitable for the voltage strength evaluation of the sending-end multi-voltage level AC grid sent by large-scale new energy in Shagohuang without conventional power support and AC main grid connection through a grid-building type DC converter station. The logic is simple and targeted. BRIEF DESCRIPTION OF THE DRAWINGS

[0038] Figure 1 It is a schematic diagram of the large-scale new energy base in the present invention, after the AC of multiple stations and multiple voltage levels are collected, the DC is sent through the grid-type converter station;

[0039] Figure 2 It is a schematic diagram of the system and the grid connection point port of the new energy base after being equivalently applied with the Thevenin equivalent theorem in the present invention;

[0040] Figure 3 It is a schematic diagram of a single-port and multi-port equivalent circuit diagram after equivalently using the Thevenin equivalent theorem in the present invention;

[0041] Figure 4 It is a schematic diagram of a power transmission model corresponding to the power-voltage relationship considering reactive power compensation in the present invention. DETAILED DESCRIPTION

[0042] 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.

[0043] Embodiment 1:

[0044] A method for evaluating the voltage strength of the AC grid at the sending end without conventional power supply support and AC main grid connection, which is sent by a large-scale renewable energy multi-voltage level convergence of power electronic equipment with a high proportion through a grid-type DC converter station, is as follows:

[0045] Step 1: Use the Thevenin theorem to treat all nodes and devices except the grid connection point as equivalent to a multi-feed system, and analyze the influencing factors of voltage fluctuation caused by the voltage before and after the grid connection point of the multi-feed system;

[0046] Figure 1 The present invention provides a method for evaluating the voltage strength of a large-scale new energy AC network. The method is aimed at a large-scale new energy base with multiple stations and multiple voltage levels. After the AC is collected, the DC is sent out through a grid-type converter station. The grid structure is radial, which is the main structure used in the current large-scale new energy base AC collection grid. Figure 1 As shown in the figure, the AC collection grid does not contain conventional power support and does not contain AC main grid connection, that is, it is not connected to the existing AC grid through transmission lines, and only relies on the grid-type flexible DC converter station to provide the main voltage support of the AC grid. It can be connected to reactive compensation equipment and other equipment with a high proportion of power electronics with voltage support capabilities, and it only needs to be equivalent to grid-connected equipment. Among them, the new energy base converter station is mainly a voltage source converter station (VSC) controlled by the grid-following (GFL) type, which does not have voltage support capabilities. The sending-end DC converter station of the grid-type control itself has a current limiting link, so the short-circuit current it provides is limited.

[0047] For a large-scale new energy base with multiple stations, the AC grid can be regarded as a multi-feed system. According to the Thevenin equivalence theorem, the relationship between the voltage before and after the grid connection point of the new energy base in the multi-feed system satisfies:

[0048]

[0049] Among them: U i is the voltage amplitude of the grid-connected node after the new energy is connected to the grid, is the no-load voltage amplitude of the grid-connected node of the multi-feed system before the new energy is connected to the grid, They are the actual working voltage and no-load voltage of the grid-connected node before and after the new energy is connected to the grid. is the actual no-load voltage of node j, Z device，j is the equivalent grid-connected impedance of the grid-connected device at node j, Z jj is the network node self-impedance of grid-connected node j, Z ij is the grid-connected node i and node, the network node mutual impedance.

[0050] It can be seen that the relationship between the voltage of the grid-connected node after the new energy is connected to the grid and the no-load voltage of the grid-connected node before the new energy is connected to the grid in the multi-feed system is not only related to the actual working voltage and no-load voltage of the grid-connected node, but also affected by the voltage of the other grid-connected nodes, and the degree of influence is related to the impedance between the grid-connected nodes.

[0051] Since electrical distance can characterize the degree of electrical connection between nodes, the greater the impedance between the two, the weaker the connection. For example, infinite resistance is equivalent to disconnection; therefore, impedance can also be represented by electrical distance.

[0052] Figure 2 The Thévenin theorem is used to equate all nodes and devices except the grid connection point to the system. The equipment with a high proportion of power electronics such as reactive compensation equipment with voltage support capability, and other new energy station equipment, etc., can be connected to the grid connection point port equivalent, combined with the newly added new energy bases that need to be incorporated. At this time, the newly added nodes for the grid connection of new energy stations are in a no-load state. Therefore, only the impact of the system itself needs to be considered to obtain the no-load voltage; and then the voltage fluctuation caused by the access of the grid connection point equipment can be analyzed through the mutual influence between the system itself and the newly added grid connection points.

[0053] Step 2: Calculate the evaluation value of the voltage intensity of the new energy grid-connected node i;

[0054] Figure 3 The single-port and two-port Thevenin equivalent circuits are used to illustrate the mutual influence between the system itself and the newly added grid-connected points when deriving the multi-port Thevenin equivalent circuit. According to Ohm's law, for a single-port Thevenin equivalent circuit, the voltage fluctuation caused by the addition of grid-connected equipment can be calculated by voltage division; for a two-port Thevenin equivalent circuit, the voltage fluctuation caused by the addition of grid-connected equipment needs to consider the influence of other grid-connected points; and so on, if all new energy ports are port-equivalent, the influence of other new energy base grid-connected nodes on the voltage fluctuation of this grid-connected point can be considered.

[0055] Based on the above-mentioned Thevenin equivalent and Ohm's law, a method for calculating the evaluation value of the voltage intensity of the new energy grid-connected node i can be obtained:

[0056]

[0057] Among them: The node voltage before and after the new energy grid-connected node is obtained through power flow calculation Combined with the mutual impedance Z between the grid-connected nodes ij , the self-impedance Z of other grid-connected nodes jj and the equivalent grid impedance Z of the grid-connected equipment device，j , taking into account the influence of other grid-connected equipment, the evaluation value of the voltage intensity of the new energy grid-connected node i can be calculated by comprehensive impedance and grid-connected node voltage fluctuation.

[0058] Step 3: Calculate the critical stability standard value of the new energy grid-connected node i;

[0059] Figure 4 This is the power transmission model corresponding to the power-voltage relationship of reactive power compensation when the present invention obtains the critical stability standard. For high voltage systems, the influence of resistance can be ignored; when considering resistance, the voltage static stability limit expression only has the product term of active power and resistance, which has little effect on the final calculation result, so it is ignored. The actual input reactive power at the sending end can be reduced by the compensation equipment, thereby reducing the reactance voltage drop, which is beneficial to reducing the voltage difference between the sending and receiving ends. When the same active power is transmitted, the voltage drop at the sending and receiving ends is less. Combining the proposed formula, the voltage static stability limit of the grid voltage can be obtained, which is used to obtain the critical stability standard.

[0060] The critical stability of the system, that is, the judgment standard of the grid voltage strength, can be derived from the relationship between the transmission power and the static voltage stability. For this purpose, the voltage static stability limit expression corresponding to the power-voltage relationship considering reactive power compensation is obtained:

[0061]

[0062] Among them: U R In order to meet the minimum value of the receiving terminal voltage drop when the grid transmission power is maximum, U s is the sending end voltage, Qk is the reactive power generated by the reactive power compensation equipment, is the power factor angle, and x is the power transfer impedance.

[0063] When the new energy station is connected to the grid, the sending end voltage is U s At this time, the equivalent grid-connected impedance of the new energy station is Z device It can be regarded as the impedance x in equation (3). Based on the voltage static limit, the maximum voltage drop can be obtained.

[0064] As the transmission power of new energy stations increases, the voltage drop of the grid-connected nodes increases, and the corresponding equation (2) The larger the voltage is, the smaller the voltage strength assessment value is, and the more unstable the system is. Therefore, the critical stability standard value of the new energy grid-connected node i can be obtained according to the voltage static stability limit obtained by (3):

[0065]

[0066] Step 4: Assessment of the strength of the power grid for large-scale new energy AC networking;

[0067] If the voltage strength assessment value of the new energy grid-connected node is judged to be greater than the critical stability standard value corresponding to the node itself, then the grid-connected node is stable, and the greater the voltage strength assessment value exceeds the critical stability standard value, the more stable the corresponding grid-connected node is and the greater its strength is;

[0068] If the voltage strength evaluation values ​​of all nodes of the large-scale renewable energy grid are greater than their corresponding critical standard stability values, the large-scale renewable energy AC grid is considered stable. If there is a renewable energy grid-connected node whose voltage strength evaluation value is less than the critical stability standard value corresponding to its own node, the system is unstable.

[0069] The method in the present invention does not start from the perspective of traditional short-circuit capacity, but from the perspective of impedance and voltage fluctuations caused by the connection of new energy to the grid, and provides a method for evaluating the strength of the power grid for large-scale new energy AC networking, which is simple in logic and targeted.

[0070] 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 evaluating the strength of a large-scale new energy AC network, characterized in that: The steps include: S1. Using Thevenin's theorem, all nodes except the grid-connected nodes and grid-connected equipment are equivalent to a multi-feed system, and the influencing factors of voltage fluctuation caused by the grid connection of the multi-feed system are analyzed; S2. Considering the influencing factors of voltage fluctuation, based on Thevenin equivalent and Ohm's law, calculate the voltage strength assessment value of the new energy grid-connected node; S3. Calculate the critical stability standard value of the new energy grid-connected node based on the relationship between transmission power and static voltage stability; S4. Determine whether the grid-connected node is stable by determining whether the voltage strength evaluation value of the new energy grid-connected node is greater than the critical stability standard value corresponding to the node itself, and evaluate the strength of the grid-connected node; S5. According to S2-S4, determine whether all nodes are stable in turn, and determine whether the large-scale new energy AC networking power grid is stable according to whether all nodes are stable.

2. A large-scale new energy AC network power grid strength assessment method according to claim 1, characterized in that: The S1 is specifically as follows: The large-scale renewable energy AC network grid is regarded as a multi-feedback system, and it is assumed that there are no other grid-connected devices before the grid-connected devices are connected to the grid. That is, before the grid-connected devices are connected to the grid, the grid-connected nodes are in a no-load state. According to the Thevenin equivalence theorem, the relationship between the voltages before and after the grid-connected points of the renewable energy bases in the multi-feedback system satisfies: Among them: U i is the voltage amplitude of the grid-connected node after the new energy is connected to the grid, is the no-load voltage amplitude of the grid-connected node of the multi-feed system before the new energy is connected to the grid, They are the actual working voltage and no-load voltage of the grid-connected node before and after the new energy is connected to the grid. is the actual no-load voltage of node j, Z device，j is the equivalent grid-connected impedance of the grid-connected device at node j, Z jj is the network node self-impedance of grid-connected node j, Z ij is the grid-connected node i and node, the mutual impedance of the network nodes.

3. A large-scale new energy AC network power grid strength assessment method according to claim 2, characterized in that: The influencing factors in S1 include the voltage fluctuation caused by the grid-connected device itself and the voltage fluctuation caused by the grid-connected devices at other nodes on the grid-connected node voltage. The influence degree of the grid-connected devices at other nodes is related to the impedance between them and the grid-connected node.

4. A large-scale new energy AC network power grid strength assessment method according to claim 3, characterized in that: The S2 is specifically as follows: Taking into account the influence of other grid-connected equipment, the voltage strength evaluation value of the new energy grid-connected node i is calculated by comprehensive impedance and grid-connected node voltage fluctuation; based on the Thevenin equivalent and Ohm's law, the calculation method of the voltage strength evaluation value of the new energy grid-connected node i is obtained: Among them: The node voltage before and after the new energy grid-connected node is obtained through power flow calculation Z ij represents the mutual impedance between grid-connected nodes i and j, Z jj represents the self-impedance of the other grid-connected nodes j, Z device，j Indicates the equivalent grid impedance of the grid-connected equipment, SCRZ i It represents the voltage strength evaluation value of the new energy grid-connected node i.

5. A method for evaluating the strength of a large-scale new energy AC network according to claim 4, characterized in that: The S3 is as follows: For the critical stability of the system, the voltage static stability limit expression corresponding to the power-voltage relationship considering reactive power compensation is obtained from the relationship between the transmission power and the static voltage stability: Among them: U R In order to meet the minimum value of the receiving terminal voltage when the power transmission of the power grid is maximum, U s is the sending end voltage, Q k Reactive power generated by reactive power compensation equipment, is the power factor angle, x is the power transfer impedance; When the new energy station is connected to the grid, the sending end voltage is U s At this time, the equivalent grid-connected impedance of the new energy station is Z device Considered as power transfer impedance x, the maximum voltage drop is obtained according to the voltage static limit; According to the voltage static stability limit, the critical stability standard value of the new energy grid-connected node is obtained: Among them, U i is the voltage amplitude of the grid-connected node after the new energy is connected to the grid, U R is the voltage static stability limit, CSCRZ i Represents the critical stability standard value of the new energy grid-connected node i.

6. A large-scale new energy AC network power grid strength assessment method according to claim 1, characterized in that: The S4 is specifically as follows: If the voltage strength assessment value of the new energy grid-connected node is judged to be greater than the critical stability standard value corresponding to the node itself, then the grid-connected node is stable, and the greater the voltage strength assessment value exceeds the critical stability standard value, the more stable the corresponding grid-connected node is and the greater its strength is.

7. A method for evaluating the strength of a large-scale new energy AC network according to claim 6, characterized in that: The S5 is specifically as follows: If the voltage strength assessment values ​​of all nodes of the large-scale new energy AC network power grid are greater than their corresponding critical stability standard values, the large-scale new energy AC network power grid is judged to be stable; if there is a new energy grid-connected node whose voltage strength assessment value is less than the critical stability standard value corresponding to its own node, the large-scale new energy AC network power grid is judged to be unstable.

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