A Method and Equipment for Quantifying the Voltage Support Strength of a Flexible DC Grid Connection for Offshore Wind Power

Abstract

The present invention discloses a method and device for quantifying the voltage support strength of a flexible DC grid connection for offshore wind power, including: obtaining the parameters of the offshore collection system, the parameters of the onshore power grid, and the parameters of the flexible DC grid connection system of the offshore wind power project to be evaluated; calculating the onshore AC grid impact factor according to the onshore power grid parameters; calculating the minimum support power adjustment factor of the offshore wind power flexible DC transmission system according to the offshore collection system parameters and the flexible DC grid connection system parameters; and finally calculating the flexible DC grid connection short-circuit ratio index of the offshore wind power to be evaluated according to the onshore AC grid impact factor, the minimum support power adjustment factor, and the grid parameters. This method considers the influence relationship of the active and reactive power sources of the onshore power grid and the flexible DC transmission system on the voltage stability of the grid connection point, provides a quantitative analysis standard for the voltage support strength of offshore wind power connected to the grid through the flexible DC transmission system, and provides support for analyzing the voltage stability of the power grid in actual engineering.

Description

Technical Field

[0001] The present invention relates to the field of power grid planning and analysis, and particularly to a method and device for quantifying the voltage support strength of a flexible DC grid connection for offshore wind power. Background Art

[0002] Offshore wind power in China has entered a period of rapid development, and large-capacity long-distance offshore wind power will become the future development trend. Due to the flexible DC transmission (VSC-HVDC, hereinafter referred to as flexible DC) technology having the ability of long-distance power transmission and voltage source characteristics, it has become the main technical approach for long-distance offshore wind power grid connection. However, the voltage stability characteristics shown by new energy grid-connected through the flexible DC transmission system are different from those of traditional AC power grids or conventional DCs. Therefore, it is urgent to conduct a quantitative analysis of the voltage support strength for the offshore wind power grid-connected system through flexible DC.

[0003] Currently, for the voltage support strength index problem of the offshore wind power grid-connected system through flexible DC, there are mainly three types of indexes: local voltage stability index (L index), sensitivity index, and simulation index. However, there are problems such as cumbersome calculation, difficulty in applying to actual large power grids, and being affected by the operation mode. The short circuit ratio (SCR) refers to the system short-circuit capacity divided by the equipment capacity. So, a large short circuit ratio means the equipment is connected to a strong system, indicating that the switching of the equipment has little impact on the system. And the short-circuit capacity is numerically equal to the system admittance value under the unit voltage condition, which is the reciprocal of the Thevenin equivalent impedance of the system. The larger the short-circuit capacity, the smaller the Thevenin equivalent resistance of the system, and the switching of loads, shunt capacitors, or reactors will not cause large changes in the voltage amplitude. Therefore, the system is relatively strong. Therefore, using the short circuit ratio as one of the important indexes for quantifying the system voltage support ability can make a more accurate assessment of the system. However, in the current short circuit ratio calculation, the interactive influence between the flexible DC system and the onshore power grid is not considered comprehensively enough, and it fails to accurately reflect the support ability of the flexible DC system for the grid voltage. Summary of the Invention

[0004] The technical problem to be solved by the present invention is: to propose a method and device for quantifying the voltage support strength of an offshore wind power grid-connected through flexible DC considering the land-sea interactive influence, which can accurately reflect the voltage support strength of the offshore wind power grid-connected system through flexible DC and provide a reference for actual engineering.

[0005] To solve the above technical problem, the technical solution adopted by the present invention is:

[0006] A method for quantifying the voltage support strength of a flexible DC grid connection for offshore wind power, comprising:

[0007] Obtaining the parameters of the offshore wind farm collection system, the onshore power grid topology and parameters, and the parameters of the offshore wind power flexible DC transmission system;

[0008] Calculate the influencing factors of the onshore AC grid that affect the voltage support strength according to the onshore grid topology and parameters.

[0009] Calculate the minimum support power adjustment factor of the HVDC transmission system for offshore wind power according to the parameters of the offshore wind farm collection system and the HVDC transmission system for offshore wind power.

[0010] Calculate the short-circuit ratio index of the HVDC grid-connected offshore wind power according to the influencing factors of the onshore AC grid, the minimum support power adjustment factor, and the grid parameters, which is used to measure the voltage support strength of the HVDC transmission system for offshore wind power.

[0011] To solve the above technical problems, another technical solution adopted by the present invention is:

[0012] A voltage support strength quantification device for HVDC grid-connected offshore wind power includes a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, each step in the voltage support strength quantification method for HVDC grid-connected offshore wind power as described above is implemented.

[0013] The beneficial effects of the present invention are as follows: Based on the parameters of the offshore wind farm collection system, the onshore grid topology and parameters, and the parameters of the HVDC transmission system for offshore wind power, calculate the influencing factors of the onshore AC grid that affect the voltage support strength and the minimum support power adjustment factor of the HVDC transmission system for offshore wind power. Subsequently, construct a short-circuit ratio model for HVDC grid-connected offshore wind power, provide a quantitative analysis standard for the voltage support strength of the HVDC transmission system for offshore wind power, and provide support for analyzing the grid voltage stability in actual engineering. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] Figure 1 It is a voltage support strength quantification method for HVDC grid-connected offshore wind power in an embodiment of the present invention;

[0015] Figure 2 It is a schematic diagram showing the influence of the output of offshore wind power on the onshore grid when an AC fault occurs in the offshore power supply in an embodiment of the present invention;

[0016] Figure 3 It is a schematic diagram showing the influence of the onshore grid on the output of offshore wind power in the offshore power supply under an AC fault in an embodiment of the present invention;

[0017] Figure 4 It is a schematic diagram of the structure of a voltage support strength quantification device for HVDC grid-connected offshore wind power in an embodiment of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0018] To describe in detail the technical content, the achieved objectives, and the effects of the present invention, the following is described in conjunction with the embodiments and with reference to the accompanying drawings.

[0019] Please refer to Figure 1 , a method for quantifying the voltage support strength of a flexible DC grid connection for offshore wind power, including:

[0020] Obtain the parameters of the offshore wind farm collection system, the onshore power grid topology and parameters, and the parameters of the offshore wind power flexible DC transmission system;

[0021] According to the onshore power grid topology and parameters, calculate the onshore AC power grid influence factor affecting the voltage support strength;

[0022] According to the parameters of the offshore wind farm collection system and the parameters of the offshore wind power flexible DC transmission system, calculate the minimum support power adjustment factor of the offshore wind power flexible DC transmission system;

[0023] According to the onshore AC power grid influence factor, the minimum support power adjustment factor and the power grid parameters, calculate the short-circuit ratio index of the offshore wind power connected to the grid through the flexible DC, which is used to measure the voltage support strength of the offshore wind power flexible DC transmission system.

[0024] As can be seen from the above description, the beneficial effects of the present invention are as follows: Based on the parameters of the offshore wind farm collection system, the onshore power grid topology and parameters, and the parameters of the offshore wind power flexible DC transmission system, calculate the onshore AC power grid influence factor affecting the voltage support strength and the minimum support power adjustment factor of the offshore wind power flexible DC transmission system, and then construct a short-circuit ratio model for the offshore wind power flexible DC grid connection, providing a quantitative analysis standard for the voltage support strength of the offshore wind power connected to the grid through the flexible DC transmission system, and providing support for analyzing the voltage stability of the power grid in actual engineering.

[0025] Further, the onshore power grid topology and parameters include grid line parameters, node active power and reactive power; the parameters of the offshore wind power flexible DC transmission system include the offshore collection point of the flexible DC transmission system, the onshore connection point, and the rated capacity.

[0026] Further, the calculation method of the onshore AC power grid influence factor includes:

[0027] Calculate the system impedance matrix according to the onshore power grid topology and parameters, and calculate the equivalent power of the active and reactive power injected into each node in the system on the connection point i of the offshore wind power flexible DC transmission system according to the relationship between the system impedance matrix and the current and voltage:

[0028]

[0029] Among them, P eq,ij represents the equivalent active output power of the active source at node j to the grid connection node i; Q eq,ij represents the equivalent reactive output power of the reactive source at node j to the grid connection node i; U i is the voltage of the grid connection node; U jis the node voltage of any active or reactive power source node; Z ij is the equivalent impedance between the grid connection point and each active and reactive power source; Z ii is the self-impedance of the grid connection point; P j is the output power of the onshore active power source; Q j is the output power of the onshore reactive power source;

[0030] Furthermore, the onshore AC grid impact factor RAIF ij is expressed as:

[0031]

[0032] Furthermore, the calculation method of the minimum support power adjustment factor of the offshore wind power VSC-HVDC transmission system includes:

[0033] Considering the reactive power support of the offshore converter station in the VSC-HVDC transmission system, the single-fed new energy extended short-circuit ratio index is defined:

[0034]

[0035] In the formula, S ac,w is the short-circuit capacity of the offshore wind power offshore collection point, Q vsc,s is the reactive power support capacity of the offshore converter station, P w is the active power output of the offshore wind power; Calculate the specific value of S ac,w according to the parameters of the offshore wind farm collection system, and calculate the specific value of Q vsc,s according to the parameters of the offshore wind farm collection system; The minimum support power adjustment factor is defined as RSCR w Take the output power of the offshore wind power when its critical value CRSCR w is reached:

[0036]

[0037] Furthermore, the calculation method of the short-circuit ratio index of the offshore wind power connected to the grid through VSC includes:

[0038]

[0039] In the formula, OWVSCR i represents the short-circuit ratio of the offshore wind power connected to the grid through VSC, S ac,i is the short-circuit capacity of the offshore wind power grid connection point, which can be calculated from the grid parameters; Q eq,i is the equivalent reactive power at the grid connection point, P eq,i is the equivalent active power at the grid connection point;

[0040] Among them, Q eq,iIt is the sum of the reactive power support capacity of the onshore converter station in the HVDC flexible system and the equivalent reactive power of other reactive power sources in the onshore power grid:

[0041]

[0042] In the formula, Q vsc,r is the reactive power support capacity of the onshore converter station; Q j is the output power of onshore reactive power sources, and RAIF ij is the influence factor of the onshore AC power grid;

[0043] P eq,i includes the power transmitted by the HVDC flexible connection from the offshore wind farm, the equivalent active power of other active power sources in the onshore power grid, and the minimum support power adjustment factor of the HVDC flexible system:

[0044]

[0045] In the formula, P i represents the power transmitted by the HVDC flexible connection from the offshore wind farm; P j represents the output power of onshore active power sources; RAIF ij represents the influence factor of the onshore AC power grid; P m is the minimum support power adjustment factor;

[0046] According to the above formula, the short-circuit ratio of the HVDC flexible connection of the offshore wind farm to the grid can be specifically expressed as:

[0047]

[0048] Another embodiment of the present invention provides a voltage support strength quantification device for the HVDC flexible connection of the offshore wind farm, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, it realizes each step in the above-mentioned voltage support strength quantification method for the HVDC flexible connection of the offshore wind farm.

[0049] The voltage support strength quantification method and device for the HVDC flexible connection of the offshore wind farm provided by the present invention can be applied to the scenario of voltage support strength quantification analysis for the HVDC flexible connection system of the offshore wind farm, and can accurately reflect the support ability of the HVDC flexible system for the grid voltage. The following is illustrated through specific embodiments:

[0050] Embodiment 1

[0051] Please refer to Figure 1 , a voltage support strength quantification method for the HVDC flexible connection of the offshore wind farm, including:

[0052] S1. Obtain the parameters of the offshore wind farm collection system, the onshore power grid topology and parameters, and the parameters of the HVDC flexible transmission system of the offshore wind farm.

[0053] S2. Obtain the onshore AC grid interaction factor (OAIF) based on the onshore grid data; the onshore AC grid interaction factor is used to evaluate the impact of the active power output and reactive power output of the onshore AC grid on the grid connection point. Specifically:

[0054] According to the relationship between the impedance matrix and current and voltage in the power flow equation, the following formula can be obtained:

[0055] U n×1 =Z n×n ·I n×1 ;

[0056] In the formula, U n×1 is the system node voltage matrix, Z n×n is the system impedance matrix, and I n×1 is the system injection node current matrix; all nodes in the system can be divided into active power source nodes, reactive power source nodes, and other nodes; therefore, combining the above formula, we have:

[0057]

[0058] In the formula, U i is the grid connection node voltage; Z ij is the equivalent impedance between the grid connection point and each active power source and reactive power source, Z ii is the self-impedance of the grid connection point; Z ik is the equivalent impedance between the grid connection point and the set of other system nodes except the active power source nodes and reactive power source nodes; N is the total number of system nodes, J is the set of system active power sources and reactive power source nodes, K is the set of other system nodes except the active power sources and reactive power source nodes, I j is the current injected into node j, and I k is the current injected into node k; from the above formula, it can be seen that its equivalent model can be deduced as:

[0059]

[0060]

[0061] Furthermore, we get:

[0062]

[0063] In the formula, U j is the node voltage of any active power source or reactive power source node; I j is the current injected into node j; S j is the apparent power of the active and reactive power sources, S eq,ijis the equivalent output apparent power of the active power source and the reactive power source to the grid connection point i. The active power and reactive power of the equivalent output of the active power source and the reactive power source to the grid connection point i are as follows:

[0064]

[0065] In the formula, P eq,ij represents the equivalent output active power of the active power source to the grid connection node i; Q eq,ij represents the equivalent output reactive power of the reactive power source to the grid connection node i; P j is the output power of the onshore active power source; Q j is the output power of the onshore reactive power source; Δφ ij is the voltage phase angle difference between node i and node j; Since Δφ ij ≈0, the above formula can be further simplified to:

[0066]

[0067] That is, the influence factor of the onshore AC power grid is obtained as:

[0068]

[0069] Among them, RAIF ij is the influence factor of the onshore AC power grid.

[0070] S3. The flexible DC transmission system has the capabilities of independent active and reactive power control and reactive power voltage support. To reflect the above influences, the influence factor of the offshore wind power flexible DC transmission system on the onshore power grid voltage support is calculated. Define the single-infeed new energy extended short-circuit ratio index:

[0071]

[0072] In the formula, S ac,w is the short-circuit capacity of the offshore wind power offshore collection point, Q vsc,s is the reactive power support capacity of the offshore converter station, P w is the output active power of the offshore wind power. The minimum support power adjustment factor is defined as RSCR w Take the output power of the offshore wind power when its critical value CRSCR w is reached:

[0073]

[0074] P m reflects the minimum support degree of the active power output of the offshore power source to the onshore power grid.

[0075] S4. According to the onshore AC power grid influence factor and the minimum support power adjustment factor, the short-circuit ratio of the offshore wind power connected to the grid through the flexible DC is obtained. Specifically:

[0076] Considering the impact of offshore power sources on the voltage stability of onshore power grids in grid-connected systems, and linking the impact of active and reactive power sources in onshore AC power grids on the voltage support ability at the grid connection point based on the above-mentioned onshore AC grid impact factor, the offshore wind power and VSC-HVDC grid-connected system short circuit ratio (OWVSCR) is defined as:

[0077]

[0078] Among them, OWVSCR i represents the offshore wind power and VSC-HVDC grid-connected short circuit ratio; S ac,i is the short circuit capacity at the grid connection point of offshore wind power, which can be calculated from grid parameters; Q eq,i is the equivalent reactive power at the grid connection point, and P eq,i is the equivalent active power at the grid connection point.

[0079] Meanwhile, according to the above-mentioned onshore AC grid impact factor, the equivalent reactive power at the grid connection point is calculated, and we get:

[0080]

[0081] Among them, Q vsc,r is the reactive power support capacity of the onshore converter station; Q j is the output power of onshore reactive power sources, and RAIF ij is the onshore AC grid impact factor;

[0082] And according to the above-mentioned onshore AC grid impact factor and the minimum support power adjustment factor, the equivalent active power at the grid connection point is calculated, and we get:

[0083]

[0084] In the formula, P i represents the power output from offshore wind power through VSC-HVDC; P j represents the output power of onshore active power sources; RAIF ij represents the onshore AC grid impact factor; P m is the minimum support power adjustment factor;

[0085] Finally, the offshore wind power and VSC-HVDC grid-connected short circuit ratio can be obtained as:

[0086]

[0087] Substituting the relevant calculated parameter values into the above formula, the specific value of the offshore wind power and VSC-HVDC grid-connected short circuit ratio can be obtained.

[0088] When the short - circuit ratio of the HVDC - connected offshore wind power is 2.2, the rated operating point is on the right side of the maximum power. At this time, dP i / dI d <0, and the grid - connected system cannot maintain stability; when the short - circuit ratio of the HVDC - connected offshore wind power is 2.75, the rated operating point basically coincides with the maximum power. At this time, dP i / dI d =0, and the grid - connected system is in a critical stable state; when the short - circuit ratio of the HVDC - connected offshore wind power is 3.0, the rated operating point is on the left side of the maximum power. At this time, dP i / dI d >0, and the grid - connected system maintains stable operation; the larger the short - circuit ratio value of the HVDC - connected offshore wind power, the larger the stable range of the maximum power curve, and the stronger the voltage support ability of the grid - connected system.

[0089] Since the short - circuit ratio of the HVDC - connected offshore wind power reflects the commutation station control mode of the HVDC - connected offshore wind power system by quantifying the reactive power support capacity and cannot take into account the dynamic characteristics of the grid - connected system, when quantifying the voltage support strength of the grid - connected system based on the short - circuit ratio of the HVDC - connected offshore wind power in this embodiment, a certain margin is considered. That is: when the short - circuit ratio of the HVDC - connected offshore wind power < 2.5, the grid - connected system is a weak system; when 2.5 ≤ the short - circuit ratio of the HVDC - connected offshore wind power < 2.75, the grid - connected system is a weak system; when the short - circuit ratio of the HVDC - connected offshore wind power ≥ 2.75, the grid - connected system is a strong system.

[0090] Embodiment 2

[0091] Please refer to Figure 4 , a device for quantifying the voltage support strength of HVDC - connected offshore wind power, including a memory, a processor, and a computer program stored on the memory and executable on the processor. When the processor executes the computer program, it realizes each step in the method for quantifying the voltage support strength of HVDC - connected offshore wind power as described in Embodiment 1.

[0092] The above are only the embodiments of the present invention, and do not limit the patent scope of the present invention. Any equivalent transformation made by using the content of the specification and drawings of the present invention, or directly or indirectly applied in the related technical fields, shall be included in the patent protection scope of the present invention by the same token.

Claims

1. A method for quantifying the voltage support strength of a flexible DC grid connection for offshore wind power, characterized in that Including: Obtaining the parameters of the offshore wind farm collection system, the topology and parameters of the onshore power grid, and the parameters of the offshore HVDC transmission system for wind power; Calculating the onshore AC grid impact factor affecting the voltage support strength according to the onshore power grid topology and parameters; Calculating the minimum support power adjustment factor of the offshore HVDC transmission system for wind power according to the parameters of the offshore wind farm collection system and the parameters of the offshore HVDC transmission system for wind power; Calculating the short-circuit ratio index of offshore wind power integrated into the grid through HVDC according to the onshore AC grid impact factor, the minimum support power adjustment factor, and the grid parameters, which is used to measure the voltage support strength of the offshore HVDC transmission system for wind power; The calculation method of the short-circuit ratio index of offshore wind power integrated into the grid through HVDC includes: ； In the formula, represents the short-circuit ratio of the offshore wind power integrated into the grid through the flexible DC transmission; S ac,i is the short-circuit capacity at the connection point of the offshore wind power, which can be calculated from the grid parameters; Q eq,i is the equivalent reactive power at the connection point; P eq,i is the equivalent active power at the connection point; Among them, Q eq,i is the sum of the reactive power support capacity of the VSC-HVDC converter station on land and the equivalent reactive power of other reactive power sources in the onshore power grid: ； In the formula, is the reactive power support capacity of the onshore converter station; is the output power of the onshore reactive power source, is the onshore AC grid impact factor; P eq,i Including the equivalent active power of the offshore wind power transmitted through the flexible DC transmission, the other active power sources of the onshore power grid, and the adjustment factor of the minimum support power of the flexible DC system: ； In the formula, represents the output power of offshore wind power transmitted through flexible DC; represents the output power of onshore active power sources; represents the influencing factor of onshore AC power grids; is the minimum support power adjustment factor; According to the above formula, the short-circuit ratio of offshore wind power integrated into the grid through HVDC can be specifically expressed as: ； In the formula, S ac,w is the short-circuit capacity of the offshore wind power offshore collection point; S ac,pcc is the rated capacity of the offshore wind power; CRSCR w is the critical value of the single-infeed new energy extended short-circuit ratio index RSCR w ; Q vsc,s is the reactive power support capacity of the offshore converter station.

2. A method for quantifying the voltage support strength of a flexible DC grid connection for offshore wind power, characterized in that, The calculation method of the onshore AC grid impact factor includes: Calculate the system impedance matrix based on the onshore power grid topology and parameters, and calculate the equivalent power of the active and reactive power injected at each node in the system into the connection point of the offshore wind power VSC-HVDC system according to the relationship between the system impedance matrix, current, and voltage. i of: ； Among them, represents the node j The equivalent active output power of the active power source to the grid-connected node i ; represents the node j The equivalent reactive output power of the reactive power source to the grid-connected node i ; is the grid-connected node voltage; is the node voltage of any active or reactive power source node; is the equivalent impedance between the grid connection point and each active and reactive power source, is the self-impedance of the grid connection point; is the onshore active power source output power; is the onshore reactive power source output power; Furthermore, the onshore AC grid influence factor is expressed as: 。 3. A method for quantifying the voltage support strength of a flexible HVDC grid connection for offshore wind power, characterized in that, The calculation method of the minimum support power adjustment factor of the offshore HVDC transmission system for wind power includes: Considering the reactive power support of the offshore converter station of the HVDC transmission system, defining the extended short-circuit ratio index of new energy with single-feed-in: ； In the formula, S ac,w is the short-circuit capacity of the offshore wind power offshore collection point, Q vsc,s is the reactive power support capacity of the offshore converter station, P w is the active power output of the offshore wind power; calculate according to the parameters of the offshore wind farm collection system S ac,w The specific value of, calculate according to the parameters of the offshore wind farm collection system Q vsc,s The specific value of; the minimum support power adjustment factor is defined as RSCR w Take its critical value CRSCR w The active power output of the offshore wind power at this time: 。 4. A voltage support strength quantification device for flexible DC grid connection of offshore wind power, comprising a memory, a processor, and a computer program stored on the memory and executable on the processor, characterized in that, When the processor executes the computer program, it realizes each step in a method for quantifying the voltage support strength of offshore wind power integrated into the grid through HVDC as described in any one of claims 1-3.

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