Reactive current safety constraint method of grid-forming wind turbine generator

Abstract

The invention discloses a reactive current safety constraint method for a grid-forming wind turbine generator, and the method comprises the steps: obtaining a feasible range of the reactive current of the grid-forming wind turbine generator through calculating the influence of the following fluctuation of the active current on the feasible range of the reactive current of the grid-forming wind turbine generator under the grid-forming control characteristics; and the current during the reactive power resource adjustment period of the grid-forming wind turbine generator can be constrained according to the range, so that the safety is improved, the problems of capacity margin evaluation misaccuracy and current over-limit during the reactive power current adjustment period of the grid-forming wind turbine generator due to active current fluctuation are avoided, and the service life of the grid-forming wind turbine generator is prolonged. And the external power grid voltage can be supported more safely through reactive power resource adjustment by the grid-forming wind turbine generator.

Description

Technical Field

[0001] This invention belongs to the field of wind power generation technology, specifically relating to a method for reactive current safety constraint of grid-connected wind turbine units. Background Technology

[0002] With the continuous integration of distributed power sources such as wind, solar, and energy storage into the power grid, the degree of power electronics in the grid is constantly deepening, and voltage fluctuation problems such as overvoltage and undervoltage are becoming more frequent. Grid-connected wind turbines, using grid-based control instead of conventional grid-connected inverter control based on phase-locked loops, can respond to external voltage fluctuations and support the external grid voltage through their own reactive power regulation. They are an important component of power grids with high distributed power penetration. Grid-connected wind turbines use the grid voltage vector generated by grid control to guide the operation of the current loop, ultimately achieving regulation of the unit's output power. During power regulation, changes in the grid voltage vector cause simultaneous changes in both the active and reactive currents of the wind turbine.

[0003] When wind turbines adjust their reactive power, they must adhere to their current capacity limits. However, the active and reactive currents of grid-connected wind turbines change simultaneously, affecting the accuracy of reactive current capacity assessment. During reactive current adjustments, fluctuations in active current may cause the total current to exceed the maximum current capacity, posing a safety risk to wind turbine operation.

[0004] Patent document CN121076802A uses the critical short-circuit ratio of the power system as a safety constraint, proposing regulations on the open capacity of various new energy units from a system perspective. Patent document CN115986810B specifies the maximum stator-side reactive current of doubly-fed induction generator (DFIG) wind turbines, but does not consider the impact of grid-connected control on the reactive current capacity of wind turbines. Considering the risk of current over-limit caused by the active current of grid-connected wind turbines following changes in reactive current, it is necessary to further constrain the reactive current of grid-connected wind turbines to ensure that they can more safely support the external grid voltage through reactive resource regulation. Summary of the Invention

[0005] The technical problem to be solved by this invention is: to address the risk of current exceeding limits caused by the active current following action during the reactive current regulation of grid-connected wind turbines, this invention provides a method for the safe constraint of reactive current of grid-connected wind turbines, which dynamically constrains the reactive current of grid-connected wind turbines under the active current following action, ensuring that grid-connected wind turbines can safely utilize their reactive resources to support the grid voltage.

[0006] To solve the above-mentioned technical problems, the technical solution proposed by this invention is: a method for reactive current safety constraint of grid-connected wind turbine generators, which includes the following steps: 1. Construct the first-order equivalent transfer function between the reactive power command of the grid-connected wind turbine and the grid voltage amplitude: , In the formula, G(s) is the first-order equivalent transfer function between reactive power command and grid voltage amplitude in the complex frequency domain, s is a complex variable, and K q G is the reactive power-voltage droop factor. QV This is the partial derivative of reactive power with respect to the change in voltage amplitude; 2. Based on the Laplace transform, the first-order equivalent transfer function is transformed from the complex frequency domain to the time domain, and the grid voltage increment ΔV(t) under the reactive power command change is calculated: , In the formula, ΔQ ref The change in reactive power reference value is denoted by t, which is the time interval for calculating voltage increments, typically taken as t=0.3. 3. Calculate the increase in active current Δi of the grid-connected wind turbine caused by the increase in grid voltage based on the proportional-integral relationship between grid voltage and current. ac (t): , In the formula, K P K is the proportional gain of the voltage loop. I T represents the integral coefficient of the voltage loop. cur The equivalent time constant of the inner current loop; 4. Measure the output active current of the grid-connected wind turbine. This is compared with the active current increment Δi caused by the grid voltage increment in step 3. ac Adding (t) together, we obtain the predicted active current i of the grid-connected wind turbine. acp : , 5. Based on the relationship between the output voltage and current of the grid-connected wind turbine, the output voltage equation of the grid-connected wind turbine is derived as follows: , In the formula, R is the resistance of the output line of the grid-connected wind turbine, L is the inductance of the output line, and ω grid Let i be the grid angular frequency. re To output reactive current for grid-connected wind turbine generators, u gd u gq These are the d-axis and q-axis voltages of the power grid, respectively, V t For the output voltage of the grid-connected wind turbine generator set; 6. DC bus voltage U based on grid-connected wind turbine units dc Calculate the maximum terminal voltage V of the grid-connected wind turbine. t max : , 7. The predicted active current i of the grid-connected wind turbine in step 4. acp The maximum terminal voltage V of the grid-connected wind turbine in step 6 t max Substitute the output voltage equation of the grid-connected wind turbine in step 5 to solve the dynamic limit of reactive current of the grid-connected wind turbine. When the current of the grid-connected wind turbine exceeds this limit, the current is limited.

[0007] Furthermore, in step 3, the increase in active current Δi of the grid-connected wind turbine output caused by the increase in grid voltage is calculated based on the proportional-integral relationship between grid voltage and current. ac The specific steps for (t) are as follows: 3.1 The open-loop transfer function of the voltage PI regulator is constructed as follows: , 3.2. The current loop can be equivalently represented as a first-order inertial element as follows: , 3.3. Calculate the step response for 3.1 and 3.2 respectively and sum them to obtain: , This represents the sum of the step responses; 3.4 Multiply the above equation by the grid voltage increment ΔV(t) obtained in step 2 to obtain the grid-connected wind turbine output active current increment Δi. ac (t); The present invention has the following technical effects: This invention considers the active current fluctuation phenomenon of grid-connected wind turbines during reactive power regulation. Compared with the conventional rated current specification method, the advantage of this invention is that it calculates the impact of active current fluctuation on the feasible range of reactive current of grid-connected wind turbines under grid control characteristics, further obtaining the feasible range of reactive current of grid-connected wind turbines. Based on this range, the current of grid-connected wind turbines during reactive power regulation can be constrained, thereby improving its safety. This avoids the problem of inaccurate capacity margin assessment and current over-limit issues caused by active current fluctuations during reactive current regulation of grid-connected wind turbines. It enables grid-connected wind turbines to more safely support the external grid voltage through reactive power regulation. Attached Figure Description

[0008] Figure 1 This is a flowchart of a reactive current safety constraint method for grid-connected wind turbine generators according to the present invention. Figure 2The figures are verification diagrams of the effectiveness of the reactive current safety constraint method for grid-connected wind turbines in this invention. (a) and (b) are comparison results of reactive current adjustment near the current limit between grid-connected wind turbines and grid-connected wind turbines without reactive current constraint, respectively; (c) and (d) are comparison results of reactive current adjustment near the current limit between grid-connected wind turbines with reactive current constraint of this invention and grid-connected wind turbines without reactive current constraint, respectively. Detailed Implementation

[0009] To verify the effectiveness of this invention, the reactive power regulation results near the current limit were compared between grid-connected wind turbines with the same parameters, grid-connected wind turbines without current constraints, and grid-connected wind turbines implementing the reactive current constraint method of this invention. In the example used, the maximum terminal voltage of the grid-connected wind turbine was 1.84 pu, the wind turbine-side filter inductance was 700 p.u., the grid-side filter inductance was 5 p.u., the filter capacitor was 10 p.u., and the line resistance was 0.1 pu. For both grid-connected and grid-connected wind turbines with these parameters, they were operating at 1 p.u. active current and 1 p.u. inductive reactive current 2 seconds prior. The reactive current value corresponding to the inductive reactive power was negative. At 2 seconds, both received additional reactive power commands. The active current of the grid-connected wind turbine did not change accordingly, see [link to relevant documentation]. Figure 2 (a) shows the gray curve, where the reactive current is increased to 1.035 pu. See [reference needed]. Figure 2 (b) shows the gray curve; after receiving additional reactive power commands, the active and reactive currents of grid-connected wind turbines without any reactive current constraints increase under the influence of the grid voltage vector, affecting the operational range of reactive current. This leads to the grid-connected wind turbine terminal voltage rising beyond the limit, and the grid-connected wind turbine current oscillating. Figure 2 The black curves in (a) and (b) represent the reactive current safety constraint method of this invention. When the reactive current of the grid-connected wind turbine is implemented in the grid-connected wind turbine, considering the effect of the active current increasing along with the active current, the reactive current of the grid-connected wind turbine is limited to 1.015 pu. At this point, neither the active nor reactive current of the grid-connected wind turbine oscillates. (See figure...) Figure 2 The gray curves in (c) and (d) show that, compared with grid-connected wind turbines that do not implement reactive current constraints, safer reactive current regulation is achieved.

Claims

1. A method of reactive current safety constraint for grid integrated wind turbine generators, characterized in that, The method comprises the following steps: Step 1, constructing a first-order equivalent transfer function between the reactive power instruction of the grid-connected wind turbine and the grid voltage amplitude: ， In the formula, G(s) is a first-order equivalent transfer function between the reactive power instruction and the network voltage amplitude in the complex frequency domain, s is a complex variable, K q is a reactive-voltage droop coefficient, G QV is the partial derivative of the reactive power with respect to the voltage amplitude; Step 2, converting the first-order equivalent transfer function from the complex frequency domain to the time domain based on pull transformation, and calculating the grid voltage increment AV(t) under the change of the reactive power instruction: ， In the formula, AQ ref is the change amount of the reactive power reference value, and t is the time interval for calculating the voltage increment. Step 3, calculating the active current increment Δi of the grid-connected wind turbine output caused by the grid voltage increment based on the proportional-integral relationship between the grid voltage and the current ac (t): ， where K P is the proportional coefficient of the voltage loop, K I is the integral coefficient of the voltage loop, T cur is the equivalent time constant of the current inner loop; Step 4, Measure the grid-connected wind turbine output active current , which is added to the active current increment Δi ac (t) caused by the grid voltage increment in Step 3, to obtain the grid-connected wind turbine active current prediction i acp : ； Step 5, based on the relationship between the output voltage and the current of the grid-connected wind turbine, the output voltage equation of the grid-connected wind turbine is obtained: ， In the formula, R is the resistance of the output line of the grid-connected wind turbine, L is the inductance of the output line, and ω grid Let i be the grid angular frequency. re To output reactive current for grid-connected wind turbines, u gd u gq These are the d-axis and q-axis voltages of the power grid, respectively, V t For the output voltage of the grid-connected wind turbine generator set; Step 6, DC bus voltage U of the grid-connected wind turbine based dc Calculating the maximum terminal voltage V of the grid-connected wind turbine t max : ； Step 7, the active current prediction of the grid-connected wind turbine in step 4 is i acp , the maximum terminal voltage of the grid-connected wind turbine in step 6 is V t max Substitute the grid-connected wind turbine output terminal voltage equation in step 5, solve the reactive current dynamic limit of the grid-connected wind turbine, and when the grid-connected wind turbine current will exceed the limit, the current is limited and constrained.

2. The method of claim 1, wherein the method is used for constructing a wind turbine generator system, and the method further comprises: determining a reactive current safety constraint of the wind turbine generator system based on the reactive current safety constraint of the wind turbine generator system. The output active current increment Δi of the grid-connected wind turbine caused by the grid voltage increment calculated in step 3 based on the proportional-integral relationship between the grid voltage and the current ac The specific steps of (t) are as follows: Step 3.1, the open-loop transfer function of the voltage PI regulator link is constructed as: ； Step 3.2, the current loop is equivalent to a first-order inertia link as: ； Step 3.3, the step responses of 3.1 and 3.2 are calculated respectively and summed to obtain: ； denotes the sum of the step responses; Step 3.4, multiplying the above equation with the grid-forming voltage increment ΔV(t) obtained in Step 2, the grid-forming wind turbine output active current increment Δi ac (t) is obtained.

3. The method for constructing the reactive current safety constraint of wind turbine generator according to claim 1 or 2, characterized in that: In step 2, the time interval for calculating the voltage increment is t=0.3.

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