Method for calculating active power boundary of virtual synchronous type network construction wind turbine generator

Abstract

The invention discloses a method for calculating the active power boundary of a virtual synchronous type network construction wind turbine generator. The method comprises the following steps: 1, constructing a closed-loop transfer function; 2, calculating a phase angle increment; 3, calculating reactive power increment caused by phase angle change; 4, q-axis current is obtained through calculation, and a current limit contour line composed of the maximum d-axis current and the maximum q-axis current on the grid side is formed; 5, converting the current limit contour line into a grid-side active-reactive power boundary contour line; 6, adding the reactive power increment and the reactive power of the current unit, substituting the result into the active-reactive power boundary contour line of the network side, and obtaining the maximum active power boundary value of the network construction type wind turbine generator under the corresponding maximum network side terminal voltage limitation; according to the method, the mutual coupling influence of the active power and the reactive power of the grid-forming type wind turbine generator is considered, the active power boundary value of the grid-forming type wind turbine generator is obtained, and the situation that the unit operation exceeds the power limit due to the power coupling effect in the active and reactive power changing process of the unit is avoided.

Description

Technical Field

[0001] This invention belongs to the field of wind power generation technology, specifically relating to a method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine. Background Technology

[0002] Grid-connected wind turbines exhibit relatively independent phase angle-voltage characteristics by implementing controls such as virtual synchronous generators, droop, and DC voltage self-synchronization in the grid-side converter, thereby achieving active external support. The external characteristics of grid-connected wind turbines are similar to those of a voltage source; their voltage is controlled by the reactive power control loop, and their phase angle is controlled by the active power control loop. Their real-time output power is related to both voltage and phase angle.

[0003] Grid-connected wind turbines not only provide active support to the external power grid but also need to respond to power commands from the wind farm's energy management system. Typically, when responding to external active power commands, the active power-phase angle loop of the turbine operates; when responding to external reactive power commands, its reactive power-voltage loop operates. However, the power of a wind turbine under voltage source characteristics is related to both voltage and phase angle, causing fluctuations in reactive power when the turbine responds only to active power commands.

[0004] Wind turbines have an operating power limit, which must be adhered to when adjusting their power. However, for grid-connected wind turbines, due to their unique voltage-phase angle characteristics, their reactive power values ​​fluctuate when responding to active power commands, causing fluctuations in their total capacity and increasing the risk of exceeding their power limit. If transient power fluctuations cause the turbine to operate beyond its power limit, it may lead to risks such as converter and winding burnout, converter modulation instability, and runaway.

[0005] In patent document CN120767897A, the power limit of the grid-connected power source is considered to be a certain rated value; in patent document CN116014819B, the dynamic active power limit of the wind turbine is considered, but only the power limit between the generator-side power and the generator speed is constructed, without considering the grid-side power situation; in patent document CN120934096A, the dynamic power limit of the power station's external transmission is constructed through the node admittance matrix, current vector, and voltage vector of the renewable energy power station, but the case of grid-connected units within the power station is not considered. It is evident that further quantification of the power boundary of grid-connected wind turbines is needed to provide a feasible range for high-performance power control of grid-connected wind turbines, avoid the coupling effect of active and reactive power causing them to exceed their power limits, and ensure the safety of their power regulation process. Summary of the Invention

[0006] The technical problem this invention aims to solve is: addressing the issues of active and reactive power coupling in grid-connected wind turbines, reduced accuracy of their steady-state power boundaries, and increased risk of exceeding power limits during operation. This invention provides a method for calculating the active power boundary of virtual synchronous grid-connected wind turbines, considering the impact of power fluctuation characteristics on their operating range during active and reactive power regulation, and calculating the dynamic limits of active power regulation in grid-connected wind turbines. This provides a feasible solution for high-performance power control of grid-connected wind turbines.

[0007] To solve the above-mentioned technical problems, the technical solution proposed by this invention is as follows: A method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine generator consists of the following steps: Step 1) Construct the closed-loop transfer function between active power and phase angle of the grid-connected wind turbine. : (1) In the formula, A is the coefficient of the first-order term in the denominator of the transfer function, B is the constant term in the denominator of the transfer function, s is a complex variable, J is the inertia coefficient of the virtual synchronous generator control, D is the damping coefficient, and K is the inertia coefficient of the virtual synchronous generator control. Pθ Let f be the partial derivative of the active power output of the wind turbine with respect to the phase angle, and f be the grid frequency. Step 2) Obtain the phase angle increment under the active power command change from equation (1) using the Laplace transform of the second-order system. : (2) In the formula, ΔP ref The change in active power command is represented by m, the equivalent attenuation coefficient is represented by n, the equivalent oscillation frequency is represented by t, and the time operator for calculating the increment is represented by t, which is usually 0.2. Step 3) Calculate the phase angle increment The resulting increase in reactive power : (3) In the formula, K Qθ It is the partial derivative of the reactive power output of the wind turbine with respect to the phase angle; Step 4) Express the maximum terminal voltage of the grid-side converter using the grid-side d-axis and q-axis currents as equation (4): (4) Among them, V l max R is the maximum terminal voltage of the grid-side converter. l It is the grid-side equivalent resistance, L l It is the grid-side equivalent inductance, i ld i lq These are the d-axis and q-axis currents on the grid side, u ld ulq These are the d-axis and q-axis voltages on the grid side, respectively. Within the rated current range of the grid-side converter, the d-axis current is sampled at equal intervals and substituted into equation (4) to calculate the q-axis current, forming a current limit contour line composed of the maximum d-axis and q-axis currents on the grid side; Step 5) Based on the power calculation formula of the grid-side d-axis and q-axis currents shown in equation (5), calculate the active power and reactive power corresponding to each point within the current limit contour obtained in step 4). The calculation results represent the maximum active power and reactive power of the grid side corresponding to each d-axis and q-axis current limit. The calculated maximum active and reactive power results form the grid-side active-reactive power boundary contour. This contour consists of the grid-side active-reactive power limit under the maximum grid-side terminal voltage limit. (5) In the formula, P and Q are the active power and reactive power output by the wind turbine, respectively. Step 6) Increment the reactive power in Step 3) Add the result to the measured reactive power output of the unit, and substitute the result as the reactive power into the grid-side active-reactive power boundary contour line in step 5.2). Take the active power value of the corresponding point in the contour line as the maximum active power boundary value of the grid-type wind turbine.

[0008] Furthermore, step 4 is detailed below: Step 4.1) Based on the relationship between the line impedance, current and voltage on the grid side, the maximum terminal voltage formed by the d-axis current and q-axis current of the grid-side converter is expressed as Equation (4). Step 4.2) Within the rated current range, take the d-axis current of the grid side at fixed intervals, substitute them into equation (4) to solve, obtain and record the corresponding q-axis current results, and draw the point set formed by the d-axis current of the grid side and the solved q-axis current into a two-dimensional coordinate system with the q-axis current of the grid side as the x-axis and the d-axis current as the y-axis. Usually, 1 / 1000 of the rated current range value is taken as the fixed interval for taking the equidistant values. Step 4.3) The curve formed by the point set obtained in Step 4.2 is regarded as the current limit contour line composed of the maximum d-axis and q-axis currents on the network side.

[0009] Furthermore, step 5 is detailed below: 5.1) Substitute each point within the current limit contour line in step 4 into equation (5) to obtain the active power and reactive power corresponding to each point. The reactive power with a positive value is the inductive reactive power, and the reactive power with a negative value is the capacitive reactive power. Plot the point formed by the active power and the inductive reactive power in the right plane of a two-dimensional coordinate system with reactive power as the x-axis and active power as the y-axis. Plot the point formed by the active power and the capacitive reactive power in the left half plane of the coordinate system. 5.2) The curve formed by the set of points obtained in step 5.1) is regarded as the contour line of the active-reactive power boundary on the grid side. This contour line is composed of the active-reactive power limit on the grid side under the maximum grid-side terminal voltage limit.

[0010] Compared with existing methods for defining steady-state power boundaries, the advantages of this invention are: This invention considers the coupling between the active and reactive power output of grid-connected wind turbines, obtains the dynamic power boundary during the power coupling period, and clarifies the adjustable power range of grid-connected wind turbines, thereby ensuring the safety performance during power regulation of grid-connected wind turbines. Attached Figure Description

[0011] Figure 1 This is a flowchart of a method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine generator according to the present invention. Figure 2 This is a schematic diagram of the active power boundary of a virtual synchronous grid-connected wind turbine considering the grid power coupling characteristics in this invention. Figure 3 This is a schematic diagram of the verification results of the power boundary of the grid-type wind turbine in this invention under inductive reactive power conditions; where (a) is a time-active power schematic diagram; (b) is a time-reactive power schematic diagram; and (c) is a time-grid-side converter modulation ratio schematic diagram. Figure 4 This is a schematic diagram of the verification results of the power boundary of the grid-type wind turbine in this invention under capacitive reactive power conditions; where (a) is a time-active power schematic diagram; (b) is a time-reactive power schematic diagram; and (c) is a time-grid-side converter modulation ratio schematic diagram. Detailed Implementation

[0012] To verify the effectiveness of this invention, a comparison was made between grid-connected and grid-following wind turbines near the power boundary range, and the results under inductive reactive power conditions are presented as follows. Figure 3 The results shown are illustrated below, demonstrating the operation under capacitive reactive power conditions. Figure 4 As shown in the example, the maximum terminal voltage of the grid-side converter of the grid-connected wind turbine is 2.35 pu, the grid-side equivalent resistance is 1 p.u., and the grid-side equivalent inductance is 7.05 × 10⁻⁶. 3 The grid-side d-axis and q-axis voltages are 1p.u. and 0, respectively. Substituting these parameters into the method proposed in this invention, the grid-side active-reactive power boundary contour lines of the unit can be obtained. Figure 2As shown in the black semi-circular curve. If the unit power exceeds this boundary, the grid-side voltage will increase and the modulation ratio of the grid-side converter will also increase. If it exceeds the limit range, the unit will experience power oscillation, and in severe cases, it may even become unstable, leading to unit failure and shutdown.

[0013] When grid-connected and grid-following wind turbines with the same parameters operate near the current active power boundary, their active and reactive power are equal, and the modulation ratio of the grid-side converter is also equal. (See...) Figure 3 and Figure 4 The portion 4 seconds before. At this point, the unit's terminal voltage has reached near its limit range. Continued increases in current and power will cause the grid-side terminal voltage and modulation ratio to exceed their limits, leading to oscillations in the unit's operating power.

[0014] exist Figure 3 In the diagram, the initial active power of the unit 4 seconds prior was 1 p.u., the initial inductive reactive power was 0.32 p.u., and the initial modulation ratio was 0.98 p.u. Calculations and analysis show that when a grid-connected wind turbine outputs inductive reactive power, if the received active power command increases, the unit's phase angle increases, and the inductive reactive power decreases. (See...) Figure 3 (b) shows the black curve, where the active power boundary is temporarily widened, allowing the unit to rise to 1.04 pu. Figure 3 The black curve in (a) shows that the grid-side converter modulation ratio is lower during the active power increase process of the grid-connected unit. Figure 3 The black curve in (c) indicates that the unit did not experience power oscillations caused by excessively high terminal voltage or modulation ratio exceeding the limit during the power increase process (4-4.5 seconds). Compared to grid-connected units, there is no coupling relationship between the active and reactive power of grid-connected units, therefore, inductive reactive power is not reduced during active power regulation. Figure 3 The gray dashed line in (b) indicates that its active power only increased to around 1.03 pu. Figure 3 (a) shows the gray dashed line, and during the adjustment process, the grid-side converter of the unit reaches its limit voltage, resulting in a higher modulation ratio. Figure 3 The gray dashed line in (c) indicates a more pronounced power oscillation in the unit. The verification results are consistent with the content of this invention.

[0015] With the increasing electronic nature of power grids, the frequency of high-voltage faults is constantly rising. In this situation, wind turbines should output capacitive reactive power to provide voltage support to the external system. Figure 4 In the diagram, the initial active power of the unit 4 seconds prior was 1 p.u., the initial capacitive reactive power was 1 p.u., and the initial modulation ratio was 0.98 p.u. When the grid-connected wind turbine outputs capacitive reactive power, if the received active power command increases, its capacitive reactive power increases, see [reference needed]. Figure 4 (b) The black curve represents a temporary narrowing of the active power boundary, resulting in a lower range of active power that the unit can increase. Figure 4 The black curve in (a) indicates that the converter modulation ratio is high when the active power of the grid-connected unit increases, limiting the normal increase in active power. Therefore, during periods of sudden increase in capacitive reactive power fluctuations, the unit can only increase the active power to 1.01 pu. Figure 4 (c) The black curve; compared to grid-type units, there is no coupling relationship between the active and reactive power of grid-type units, therefore, capacitive reactive power does not suddenly increase during active power regulation. Figure 4 (b) The gray dashed line indicates a higher active power boundary, where the unit can be directly boosted to 1.03 pu. See [reference needed]. Figure 4 The gray dashed line in (a) indicates that the modulation ratio of the grid-side converter of the unit is lower during power regulation. Figure 4 The gray dashed line in (c) indicates that the verification results are consistent with the content of this invention.

Claims

1. A method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine, characterized in that, This method consists of the following steps: Step 1) Construct the closed-loop transfer function between active power and phase angle of the grid-connected wind turbine. : （1）， In the formula, A is the coefficient of the first-order term in the denominator of the transfer function, B is the constant term in the denominator of the transfer function, s is a complex variable, J is the inertia coefficient of the virtual synchronous generator control, D is the damping coefficient, and K is the inertia coefficient of the virtual synchronous generator control. Pθ Let f be the partial derivative of the active power output of the wind turbine with respect to the phase angle, and f be the grid frequency. Step 2) Obtain the phase angle increment under the active power command change from equation (1) using the Laplace transform of the second-order system. : （2）， In the formula, ΔP ref The change in active power command is denoted by m, where m is the equivalent attenuation coefficient, n is the equivalent oscillation frequency, and t is the time operator for calculating the increment. Step 3) Calculate the phase angle increment The resulting increase in reactive power : （3）， In the formula, K Qθ It is the partial derivative of the reactive power output of the wind turbine with respect to the phase angle; Step 4) Express the maximum terminal voltage of the grid-side converter using the grid-side d-axis and q-axis currents as equation (4): （4）， Among them, V l max R is the maximum terminal voltage of the grid-side converter. l It is the grid-side equivalent resistance, L l It is the grid-side equivalent inductance, i ld i lq These are the d-axis and q-axis currents on the grid side, u ld u lq These are the d-axis and q-axis voltages on the grid side, respectively. Within the rated current range of the grid-side converter, the d-axis current is sampled at equal intervals and substituted into equation (4) to calculate the q-axis current, forming a current limit contour line composed of the maximum d-axis and q-axis currents on the grid side; Step 5) Based on the power calculation formula of the grid-side d-axis and q-axis currents shown in equation (5), calculate the active power and reactive power corresponding to each point within the current limit contour obtained in step 4). The calculation results represent the maximum active power and reactive power of the grid side corresponding to each d-axis and q-axis current limit. The calculated maximum active and reactive power results form the grid-side active-reactive power boundary contour. This contour consists of the grid-side active-reactive power limit under the maximum grid-side terminal voltage limit. （5） In the formula, P and Q are the active power and reactive power output by the wind turbine, respectively. Step 6) Increment the reactive power in Step 3) Add the result to the measured reactive power output of the unit, and substitute the result as the reactive power into the grid-side active-reactive power boundary contour line in step 5.2). Take the active power value of the corresponding point in the contour line as the maximum active power boundary value of the grid-type wind turbine.

2. The method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine generator according to claim 1, characterized in that: Step 4 is as follows: Step 4.1) Based on the relationship between the line impedance, current and voltage on the grid side, the maximum terminal voltage formed by the d-axis current and q-axis current of the grid-side converter is expressed as Equation (4). Step 4.2) Within the rated current range, take the d-axis current of the grid side at fixed intervals, substitute them into equation (4) in turn to solve the problem, obtain and record the corresponding q-axis current results, and draw the point set formed by the d-axis current of the grid side and the solved q-axis current into a two-dimensional coordinate system with the q-axis current of the grid side as the x-axis and the d-axis current as the y-axis. Step 4.3) The curve formed by the point set obtained in Step 4.2 is regarded as the current limit contour line composed of the maximum d-axis and q-axis currents on the network side.

3. The method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine generator according to claim 2, characterized in that: In step 4.2), 1 / 1000 of the rated current range value is taken as the fixed interval for equidistant values.

4. The method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine generator according to claim 1, characterized in that: Step 5 is as follows: 5.1) Substitute each point within the current limit contour line in step 4 into equation (5) to obtain the active power and reactive power corresponding to each point. The reactive power with a positive value is the inductive reactive power, and the reactive power with a negative value is the capacitive reactive power. Plot the point formed by the active power and the inductive reactive power in the right plane of a two-dimensional coordinate system with reactive power as the x-axis and active power as the y-axis. Plot the point formed by the active power and the capacitive reactive power in the left half plane of the coordinate system. 5.2) The curve formed by the set of points obtained in step 5.1) is regarded as the contour line of the active-reactive power boundary on the grid side. This contour line is composed of the active-reactive power limit on the grid side under the maximum grid-side terminal voltage limit.

5. The method for calculating the active power boundary of a virtual synchronous grid-connected wind turbine generator according to any one of claims 1 to 4, characterized in that: In step 2), the time operator t for calculating the increment is 0.2.

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