A method for assessing the grid strength of large-scale renewable energy AC grids

Abstract

This invention discloses a method for assessing the grid strength of large-scale renewable energy AC grids, belonging to the field of grid strength assessment technology. The method includes: using Thevenin's theorem to treat all nodes except the grid-connected nodes and grid-connected equipment as equivalent to a multi-infeed system, analyzing the influencing factors of voltage fluctuations caused by the grid connection of the multi-infeed system; calculating the voltage strength assessment value of the renewable energy grid-connected nodes; calculating the critical stability standard value of the renewable energy grid-connected nodes; determining whether the grid-connected node is stable by judging whether its voltage strength assessment value is greater than its corresponding critical stability standard value; and determining the stability of the large-scale renewable energy AC grid based on the stability of all nodes. This invention is applicable to the voltage strength assessment of multi-voltage level AC grids at the sending end of large-scale renewable energy-based DC converter stations in desert areas without conventional power support or AC main grid connections. It is logically simple and highly targeted.

Description

Technical Field

[0001] This invention belongs to the field of power grid strength assessment technology, and in particular relates to a method for assessing the power grid strength of large-scale new energy AC grids. Background Technology

[0002] The heart of deserts is typically richer in renewable energy sources, but unlike the desert edges, these areas lack conventional power sources and supporting coal-fired power plants, posing a significant challenge to the stable operation of their renewable energy bases. Grid-forming (GFM) control is widely considered an effective means of addressing insufficient grid strength, but the strength analysis of AC grids primarily supported by flexible DC converters remains an urgent problem to be solved.

[0003] Grid strength is commonly used to measure voltage fluctuations caused by equipment connected to the grid, i.e., 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. The short-circuit ratio (SCR), as an important indicator of grid strength, can be evaluated by comparing it with the system's critical short-circuit ratio (CSCR). Currently, the assessment of grid strength for multi-infeed renewable energy systems is still in the research stage. Typically, it considers the voltage stability of AC grid nodes connected to the renewable energy multi-infeed system that are supported by power sources or the main grid structure, and only treats the renewable energy converter station connection point as a PQ node, failing to reflect the impact of converter station control strategies on grid stability.

[0004] In traditional short-circuit ratio calculations, short-circuit capacity is considered as the product of the grid's rated voltage and the node's short-circuit current, where the short-circuit current is the quotient of the grid-connected node's no-load voltage and the equivalent impedance of the grid-connected equipment and the system. Due to stress constraints on power electronic devices, converters are generally designed with a 1.5 times overcurrent capacity requirement. Therefore, for the voltage strength assessment of large-scale new energy multi-voltage level AC aggregation grids with a high proportion of power electronic equipment, the calculated value of the grid-connected node's short-circuit current based on traditional short-circuit ratio calculations may be higher than the short-circuit current that the system can provide, resulting in an overestimation of the node's short-circuit capacity. This makes it difficult to guarantee the reliability of the voltage strength assessment at the grid connection points of new energy bases.

[0005] To overcome the problem of assessing the voltage strength of multi-voltage-level AC aggregation grids in renewable energy bases where grid-type DC converters provide the main voltage support, this invention provides a method for assessing the grid strength of large-scale renewable energy AC grids. The applicable scenario is a large-scale renewable energy base with multiple stations and multiple voltage levels, where AC power is aggregated and then transmitted to a DC system via a grid-type converter station. This AC aggregation grid does not contain conventional power supply support or AC main grid connections. The assessment method considers all grid-connected equipment except for the grid-connected node using Thevenin equivalents, and assesses the voltage strength of the grid-connected node using voltage fluctuations caused by equipment grid connection and the equivalent impedance of the grid-connected equipment. Summary of the Invention

[0006] The purpose of this invention is to provide a method for assessing the grid strength of large-scale new energy AC grids, in order to solve the problem of voltage strength assessment of large-scale new energy multi-voltage level AC aggregation grids with high proportion of power electronic equipment in the prior art mentioned in the background. When using traditional short-circuit ratio calculation, the calculated value of the short-circuit current at the grid connection node may be higher than the short-circuit current that the system can provide, resulting in an overestimation of the short-circuit capacity at the node, which makes it difficult to guarantee the reliability of voltage strength assessment at the grid connection point of new energy bases.

[0007] To achieve the above objectives, the present invention employs the following technical solution:

[0008] This invention proposes a method for assessing the grid strength of large-scale renewable energy AC grid connections, comprising the following steps:

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

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

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

[0012] S4. Determine whether the grid-connected node is stable by judging whether the voltage strength assessment value of the new energy grid-connected node is greater than the critical stability standard value corresponding to its own node, and evaluate the strength of the grid-connected node.

[0013] S5. Based on S2-S4, determine whether all nodes are stable in sequence, and determine whether the large-scale new energy AC grid is stable based on whether all nodes are stable.

[0014] Preferably, S1 is specifically as follows:

[0015] The AC aggregation grid does not include conventional power source support, such as coal-fired power plants; and the AC aggregation grid does not have AC main grid connections, meaning it is not connected to the existing AC grid via transmission lines; the AC aggregation grid relies solely on the grid-type flexible DC converter station to provide the main voltage support for the AC grid, and can connect to equipment with high power electronics ratios and voltage support capabilities, such as reactive power compensation equipment, which can be equivalently treated as grid-connected equipment; all other grid-connected equipment, namely other new energy power plants and supporting equipment, except for this grid-connected node, are all considered using Thevenin equivalents.

[0016] The large-scale renewable energy AC grid is considered as a multi-infeed system, and it is assumed that there are no other grid-connected devices at the grid-connected node before the device is connected to the grid, that is, the grid-connected node is in an unloaded state before the device is connected to the grid; according to Thevenin's equivalent theorem, the voltage relationship before and after the grid connection point of the renewable energy base in the multi-infeed system satisfies:

[0017]

[0018] Among them: U i This refers to the voltage amplitude at the grid connection node after the new energy source is connected to the grid. For multi-infeed systems, the no-load voltage amplitude of the grid-connected node before the new energy source is connected to the grid is given. These are the actual operating voltage and no-load voltage of the grid-connected node before and after the new energy source is connected to the grid. Z represents the actual no-load voltage at node j. device，j Z is the equivalent grid-connected impedance of the grid-connected equipment at node j. jj Z is the network node self-impedance of grid-connected node j. ij Let i be the mutual impedance between the network nodes i and j.

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

[0020] Preferably, step S2 is as follows:

[0021] Considering the influence of other grid-connected equipment, the voltage intensity assessment value of the new energy grid-connected node i is calculated by combining impedance and grid-connected node voltage fluctuation; based on Thevenin equivalent and Ohm's law, the calculation method for the voltage intensity assessment value of the new energy grid-connected node i is obtained:

[0022]

[0023] Among them: the node voltage before and after grid connection of the new energy grid-connected node is obtained through power flow calculation. Z ij Z represents the mutual impedance between grid-connected nodes i and j.jj Z represents the self-impedance of the remaining grid-connected node j. device，j SCRZii 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, step S3 is 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 derived from the relationship between transmitted power and static voltage stability:

[0026]

[0027] Among them: U R To meet the minimum voltage drop at the receiving end when the grid transmission power is at its maximum, U s Where Q is the sending-end voltage, and Qk is the reactive power generated by the reactive power compensation equipment. θ is the power factor angle, and x is the power transmission impedance.

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

[0029] Based on 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 U represents the voltage amplitude at the grid-connected node after the new energy source is connected to the grid. R For the voltage static stability limit, CSCRZ i This represents the critical stability standard value of new energy grid-connected node i.

[0032] Preferably, step S4 is as follows:

[0033] If the voltage strength assessment value of a new energy grid-connected node is greater than the critical stability standard value corresponding to its own node, then the grid-connected node is stable. 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 the strength.

[0034] Preferably, step S5 is as follows:

[0035] If the voltage strength assessment value of all nodes in a large-scale renewable energy AC grid is greater than its corresponding critical stability standard value, the large-scale renewable energy AC grid is considered stable; if the voltage strength assessment value of a renewable energy grid node is less than its corresponding critical stability standard value, the large-scale renewable energy AC grid is considered unstable.

[0036] Compared with the prior art, the beneficial effects of the present invention are:

[0037] This invention provides a method for assessing the grid strength of large-scale renewable energy AC grids. This method utilizes Thevenin's equivalent theorem, starting from the equivalent impedance of grid-connected equipment, and assesses the voltage strength of the aforementioned AC grid at the impedance level based on voltage fluctuations caused by the grid connection of renewable energy equipment. This invention effectively avoids the problem of excessive short-circuit capacity at renewable energy AC grid nodes with a high proportion of power electronic equipment, and avoids the impact of current limiting on the grid voltage strength assessment caused by grid-type flexible DC converter stations, making the voltage strength assessment of large-scale renewable energy AC aggregation grids more reasonable and accurate. It is applicable to the voltage strength assessment of multi-voltage-level AC grids at the sending end of large-scale renewable energy grids in desert areas without conventional power supply support or AC main grid connection, transmitted via grid-type DC converter stations. The logic is simple and highly targeted. Attached Figure Description

[0038] Figure 1 This is a schematic diagram of the AC power collection of multiple stations and voltage levels in a large-scale new energy base in this invention, which is then transmitted to DC power through a grid-type converter station.

[0039] Figure 2 This is a schematic diagram of the system after equivalence using Thevenin's equivalence theorem and the grid connection point of the new energy base in this invention;

[0040] Figure 3 This is a schematic diagram of the single-port and multi-port equivalent circuit diagrams after applying Thevenin's equivalent theorem in this invention;

[0041] Figure 4 This is a schematic diagram of the power transmission model corresponding to the power-voltage relationship considering reactive power compensation in this invention. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] Example 1:

[0044] A method for assessing the voltage strength of the sending-end AC grid of a large-scale, multi-voltage-level, grid-connected DC converter station with a high proportion of power electronic equipment and no conventional power supply support or AC main grid connection is as follows:

[0045] Step 1: Using Thevenin's theorem, all nodes and equipment except the grid connection point are equivalent to a multi-feed system, and the influencing factors of voltage fluctuations caused by the voltage before and after the grid connection point of the multi-feed system are analyzed.

[0046] Figure 1 This invention provides a method for assessing the voltage intensity of a large-scale new energy AC grid, targeting a scenario where multiple stations and voltage levels in a large-scale new energy base aggregate AC power, and then transmit DC power through a grid-type converter station. The grid structure is radial, which is the main structure currently used in large-scale new energy base AC aggregation grids. For example... Figure 1 As shown, the AC collection grid does not include conventional power supply support and does not have AC main grid connections. That is, it is not connected to the existing AC grid via transmission lines, but relies solely on grid-type flexible DC converter stations to provide the main voltage support for the AC grid. It can connect to high-power-electronics equipment with voltage support capabilities, such as reactive power compensation equipment, and can be treated as equivalent to grid-connected equipment. The converter stations in the new energy base are mainly voltage source converters (VSCs) using grid-following (GFL) control, and do not have voltage support capabilities. The grid-type controlled sending-end DC converter station itself has a current limiting circuit, therefore its short-circuit current supply is limited.

[0047] For a large-scale renewable energy base with multiple substations, the AC grid can be considered as a multi-infeed system. According to Thevenin's equivalent theorem, the voltage relationship before and after the grid connection point of the renewable energy base in a multi-infeed system satisfies:

[0048]

[0049] Among them: U i This refers to the voltage amplitude at the grid connection node after the new energy source is connected to the grid. For multi-infeed systems, the no-load voltage amplitude of the grid-connected node before the new energy source is connected to the grid is given. These are the actual operating voltage and no-load voltage of the grid-connected node before and after the new energy source is connected to the grid. Z represents the actual no-load voltage at node j. device，j Z is the equivalent grid-connected impedance of the grid-connected equipment at node j. jj Z is the network node self-impedance of grid-connected node j. ij Let i be the grid-connected node and node i be 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 grid connection and the no-load voltage of the corresponding grid-connected node before the new energy grid connection in the multi-infeed 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 them, the weaker the connection. For example, an infinite resistance is equivalent to a break. Therefore, impedance can also be represented by electrical distance.

[0052] Figure 2 The Thevenin theorem is used to equate all nodes except the grid connection point to the system as equivalent equipment. High-power-electronics equipment with voltage support capabilities, such as reactive power compensation equipment, and other renewable energy plant equipment, can be equated to the grid connection point ports of the newly added renewable energy base. At this point, the nodes added for grid connection to the renewable energy plant are in an unloaded state. Therefore, only the influence of the system itself needs to be considered to obtain the voltage under no-load conditions; then, the voltage fluctuations caused by the connection of grid connection point equipment can be analyzed through the interaction 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 diagram uses single-port and two-port Thevenin equivalent circuits to illustrate the mutual influence between the system itself and 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 adding grid-connected equipment can be calculated using voltage divider; for a two-port Thevenin equivalent circuit, the voltage fluctuation caused by adding grid-connected equipment needs to consider the influence of other grid-connected points; and so on. If all new energy ports are equivalently represented, 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 Thevenin equivalent and Ohm's law mentioned above, a method for calculating the voltage intensity assessment value of new energy grid-connected node i can be obtained:

[0056]

[0057] Among them: the node voltage before and after grid connection of the new energy grid-connected node is obtained through power flow calculation. Combining the mutual impedance Z between grid-connected nodes ij The self-impedance Z of the remaining grid-connected nodes jj The equivalent grid-connected impedance Z of the grid-connected equipment device，j Taking into account the influence of other grid-connected equipment, the voltage strength assessment value of the new energy grid-connected node i can be calculated by combining the impedance and the voltage fluctuation of the grid-connected node.

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

[0059] Figure 4 This invention presents a power transmission model considering reactive power compensation in the power-voltage relationship when determining the critical stability criterion. For high-voltage systems, the influence of resistance can be ignored; when considering resistance, the voltage static stability limit expression only adds a product term of active power and resistance, which has little impact on the final calculation result and is therefore ignored. Compensation equipment can reduce the actual reactive power input at the sending end, thereby reducing the reactance voltage drop and thus reducing the voltage difference between the sending and receiving ends. When transmitting the same active power, the voltage drop between 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 determine the critical stability criterion.

[0060] The critical stability of the system, i.e., the criterion for judging the grid voltage intensity, can be derived from the relationship between transmitted power and static voltage stability. Therefore, the voltage static stability limit expression corresponding to the power-voltage relationship considering reactive power compensation is derived as follows:

[0061]

[0062] Among them: U R To meet the minimum voltage drop at the receiving end when the network transmission power is at its maximum, U s Where Q is the sending-end voltage, and Qk is the reactive power generated by the reactive power compensation equipment. θ is the power factor angle, and x is the power transmission impedance.

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

[0064] As the transmission power of new energy power plants increases, the voltage drop at the grid connection node also increases, corresponding to equation (2). The larger the voltage, the smaller the assessed voltage strength, and the more unstable the system. Therefore, the critical stability standard value of the new energy grid-connected node i can be obtained based on the voltage static stability limit obtained in (3):

[0065]

[0066] Step 4: Assessment of the grid strength of large-scale renewable energy AC grids;

[0067] If the voltage strength assessment value of a new energy grid-connected node is greater than the critical stability standard value corresponding to its own node, then the grid-connected node is stable. 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 the strength.

[0068] The large-scale renewable energy AC grid is considered stable if the voltage intensity assessment values ​​of all nodes are greater than their corresponding critical standard stability values. Conversely, if the voltage intensity assessment value of any renewable energy grid-connected node is less than its corresponding critical stability standard value, the system is considered unstable.

[0069] This invention does not start from the traditional short-circuit capacity perspective, but from the perspective of impedance and voltage fluctuations caused by the grid connection of new energy sources. It presents a grid strength assessment method for large-scale new energy AC grid connections, which is logically simple and targeted.

[0070] The above description is only for the purpose of helping to understand the method and core essence of the present invention, but the scope of protection of the present invention is not limited thereto. For those skilled in the art, any equivalent substitutions or modifications made within the technical scope disclosed in the present invention, based on the technical solution and inventive concept, should be covered within the scope of protection of the present invention. Therefore, the content of this specification should not be construed as a limitation of the present invention.

Claims

1. A method for assessing the grid strength of large-scale renewable energy AC grids, characterized in that, Includes the following steps: S1. Using Thevenin's theorem, all nodes except the grid-connected nodes are equivalent to the grid-connected equipment in a multi-feed system. The factors affecting voltage fluctuations caused by grid connection of the multi-feed system are analyzed. S2. Considering the influencing factors of voltage fluctuations, calculate the voltage intensity assessment value of the new energy grid-connected node based on Thevenin's equivalent and Ohm's law. Taking into account the influence of other grid-connected equipment, the calculation of the new energy grid-connected node is based on the comprehensive impedance and grid-connected node voltage fluctuation. i The voltage intensity assessment value; based on Thevenin equivalent and Ohm's law, the new energy grid connection node is obtained. i Calculation method for voltage strength assessment value: Among them: the node voltage before and after grid connection of the new energy grid-connected node is obtained through power flow calculation. , , Z ij Indicates grid connection node i ,node j mutual impedance between Z jj Represents the remaining nodes j Self-impedance, Z device, j express j The equivalent grid-connected impedance of the node grid-connected equipment, SCRZ i Indicates the grid connection node of new energy sources i Voltage strength assessment value; for j Node voltage before grid connection; S3. Based on the relationship between transmission power and static voltage stability, calculate the critical stability standard value of the new energy grid-connected node; For the critical stability of the system, the voltage static stability limit expression corresponding to the power-voltage relationship considering reactive power compensation is derived from the relationship between transmitted power and static voltage stability: in: U R To meet the minimum voltage drop at the receiving end when the power grid transmission power is at its maximum, U s This is the sending voltage. Q k The reactive power generated by the reactive power compensation equipment. The power factor angle, x Power transmission impedance; When a new energy power station is connected to the grid, the sending-end voltage is U s At this time, the equivalent grid connection impedance of the new energy power station Z device Considered as power transfer impedance x Based on the voltage static stability limit, the maximum voltage drop amplitude is obtained; Based on the voltage static stability limit, the critical stability standard value of the new energy grid-connected node is obtained: in, U i This refers to the voltage amplitude at the grid connection node after the new energy source is connected to the grid. U R For voltage static stability limit, CSCRZ i Indicates the grid connection node of new energy sources i The critical stability standard value; S4. Determine whether the grid-connected node is stable by judging whether the voltage strength assessment value of the new energy grid-connected node is greater than the critical stability standard value corresponding to its own node, and evaluate the strength of the grid-connected node. S5. Based on S2-S4, determine whether all nodes are stable in sequence, and determine whether the large-scale new energy AC grid is stable based on whether all nodes are stable.

2. The method for assessing the grid strength of large-scale new energy AC grids according to claim 1, characterized in that, S1 is specifically as follows: The large-scale renewable energy AC grid is considered as a multi-infeed system, and it is assumed that there are no other grid-connected devices at the grid-connected node before the device is connected to the grid, that is, the grid-connected node is in an unloaded state before the device is connected to the grid; according to Thevenin's equivalent theorem, the voltage relationship before and after the grid connection point of the renewable energy base in the multi-infeed system satisfies: in: U i This refers to the voltage amplitude at the grid connection node after the new energy source is connected to the grid. For multi-infeed systems, the no-load voltage amplitude of the grid-connected node before the new energy source is connected to the grid is given. , These represent the no-load voltage and actual operating voltage of the grid-connected node before and after the new energy source is connected to the grid. for j Actual no-load voltage at the node Z device, j for j The equivalent grid-connected impedance of the node grid-connected equipment, Z jj For nodes j Network node self-impedance, Z ij For grid connection nodes i and nodes j Network node mutual impedance.

3. The method for assessing the grid intensity of large-scale new energy AC grids according to claim 2, characterized in that, The influencing factors in S1 include voltage fluctuations caused by the grid-connected equipment itself and voltage fluctuations caused by the grid-connected equipment of other nodes to the voltage of the grid-connected node. The degree of influence of the grid-connected equipment of other nodes is related to the impedance between them and the grid-connected node.

4. The method for assessing the grid intensity of large-scale new energy AC grids according to claim 1, characterized in that, S4 is specifically as follows: If the voltage strength assessment value of a new energy grid-connected node is greater than the critical stability standard value corresponding to its own node, then the grid-connected node is stable. 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.

5. The method for assessing the grid strength of a large-scale new energy AC grid according to claim 4, characterized in that, S5 is specifically as follows: If the voltage strength assessment value of all nodes in a large-scale renewable energy AC grid is greater than its corresponding critical stability standard value, the large-scale renewable energy AC grid is considered stable; if the voltage strength assessment value of a renewable energy node is less than its corresponding critical stability standard value, the large-scale renewable energy AC grid is considered unstable.

Images