A method for improving output power of a converter under a weak power grid

Abstract

The present application relates to a kind of weak power grid under converter output power promotion method, by collecting in different current instruction under grid point voltage and grid current, a set of arrays characterized by grid impedance is calculated, select middle several data to obtain average, obtain the variable of characteristic grid impedance, establish the active current model corresponding to maximum power, obtain the active current value after the first iteration, then active current value is brought into the reactive current model corresponding to maximum power, obtain the reactive current value after the first iteration;Again, reactive current value is brought into the active current model, and the second iteration is carried out, so on and so forth, until the error of two iterations is less than set value, then stop iteration, obtain the reactive instruction required by converter output rated power under current grid impedance.The present application does not need to increase additional reactive compensation device, utilizes the control advantage of converter itself to carry out reactive compensation, improves the power transmission capacity of device, method is simple, applicable to engineering application.

Description

Technical Field

[0001] This invention relates to a method for increasing the output power of a converter under weak power grid conditions, belonging to the field of power electronics technology. Background Technology

[0002] With the continuous promotion and development of distributed generation systems, considering that these systems often use long-distance transmission lines and multiple transformers to interconnect and connect to the public power grid, these high-penetration distributed generation systems exhibit weak grid characteristics with low short-circuit ratios (SCR), meaning they contain non-negligible grid impedance. According to IEEE Standard 1204-1997, a grid with an SCR < 3 is considered a weak grid, and a grid with an SCR < 2 is considered an extremely weak grid.

[0003] Under weak power grid conditions, the equivalent impedance on the grid side is large. After the output current passes through the equivalent impedance on the grid side, a voltage drop is generated. The magnitude of the voltage drop is the product of the impedance and the output current. The equivalent voltage on the output side of the converter is equal to the difference between the voltage of the equivalent impedance on the grid side and the grid voltage. The larger the equivalent impedance, the lower the equivalent voltage on the output side of the converter, which will greatly reduce the output capacity of the converter.

[0004] Therefore, researching a reactive power compensation strategy for weak grid conditions, providing reactive power compensation to improve the power output capacity of converters when the grid's equivalent impedance is too high and reduces the steady-state output capability, is a challenge. Current research on power transmission of grid-connected converters under weak grid conditions mostly focuses on using reactive power compensation devices to improve power transmission capacity, lacking solutions that consider reactive current compensation control based on the grid-connected converter itself. Summary of the Invention

[0005] The purpose of this invention is to provide a method for increasing the output power of a converter under weak power grid conditions, so as to solve the problem of reduced converter output capacity under weak power grid conditions.

[0006] To achieve the above objectives, the present invention includes:

[0007] The technical solution of the present invention for a method to improve the output power of a converter under weak power grid conditions includes the following steps:

[0008] 1) Calculate the variable Ks that characterizes the grid impedance based on the output current and output voltage values ​​of the converter under weak grid conditions;

[0009] 2) Input the variable Ks and the preset initial value of the reactive current into the active current model corresponding to the maximum power of the converter to calculate the active current value, and then input the active current value into the reactive current model corresponding to the maximum power of the converter to calculate the reactive current value, which is recorded as the reactive current command value.

[0010] 3) Substitute the reactive current value of the previous generation into the active current model and repeat step 2). The obtained reactive current value is recorded as the new generation reactive current command value.

[0011] 4) If the absolute value of the difference between the reactive current command values ​​of two adjacent generations is less than the set value, then the reactive current command value determined by the new generation shall be the reactive current command required for the converter to output rated power under a weak power grid.

[0012] If the absolute value of the difference between the reactive current command values ​​of two adjacent generations is not less than the set value, then repeat steps 3) and 4) until the reactive current command value required for the rated output power of the converter is obtained.

[0013] The method of this invention does not require the addition of an extra reactive power compensation device. It utilizes the control advantages of the converter itself and combines mathematical iterative optimization algorithms to perform reactive power compensation, thereby improving the power transmission capability of the device. The method is simple to implement and suitable for engineering applications.

[0014] Further, in step 1), the variable Ks is calculated by the following method: within the set current command range, several sets of output current and output voltage of the converter under weak grid conditions are obtained, several sets of values ​​characterizing grid impedance are calculated, and at least two values ​​are selected to calculate the average value to obtain the variable Ks.

[0015] Furthermore, the range of the set current command is 0.6-0.9 times the rated current.

[0016] Furthermore, several sets of output currents are uniformly obtained within the set current command range according to a set step size.

[0017] Furthermore, the values ​​representing the grid impedance are arranged in order of magnitude, and the average of at least two of the middle values ​​is calculated.

[0018] Further, in step 2), the active current model corresponding to the maximum power of the converter is:

[0019]

[0020] Where: i is the iteration number. This is the reactive current command value. This is the initial value of the reactive current. The active current value is obtained from the i-th calculation.

[0021] Furthermore, the initial value of the preset reactive current value is... It is 0.

[0022] Further, in step 2), the reactive current model corresponding to the maximum power of the converter is:

[0023]

[0024] in, This represents the reactive current command value obtained from the i-th calculation. Let P be the active current value obtained from the i-th calculation. max This represents the maximum power value of the converter.

[0025] To address the issue of reduced converter output power under weak power grid conditions, this invention collects grid-connected voltage and current under different current commands, calculates an array representing grid impedance, sorts these values ​​from largest to smallest, selects the middle three coefficients and calculates their average to obtain the variable representing grid impedance, establishes an active current model corresponding to maximum power, and obtains the active current value after the first iteration.

[0026] Then Substituting this into the reactive current model corresponding to maximum power, we obtain the reactive current value after the first iteration. Then, substitute it into the active current model and perform a second iteration until the error between the two iterations satisfies the condition. Then stop iterating and obtain the reactive power command required for the converter to output rated power under the current grid impedance. This invention eliminates the need for additional reactive power compensation devices, utilizing the converter's own control advantages to perform reactive power compensation, thereby enhancing the device's power transmission capability. The method is simple and suitable for engineering applications. Attached Figure Description

[0027] Figure 1 This is a schematic diagram of the equivalent circuit on the AC side of a single-unit infinite bus system of a new energy grid-connected inverter.

[0028] Figure 2 This is a weak grid model with a 200kW energy storage converter, showing waveform diagrams of active power, active current, and reactive current.

[0029] Figure 3 This is a flowchart of the method for increasing the output power of a converter under weak power grid conditions according to the present invention. Detailed Implementation

[0030] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0031] The present invention provides a method for increasing the output power of a converter under weak power grid conditions. This method does not require the addition of an extra reactive power compensation device. It utilizes the control advantages of the converter itself and combines mathematical iterative optimization algorithms to perform reactive power compensation, thereby improving the power transmission capacity of the device. The method is simple to implement and suitable for engineering applications.

[0032] The equivalent circuit of the AC side of a single-unit infinite bus system for a new energy grid-connected inverter is as follows: Figure 1 As shown.

[0033] The voltage vector at the grid-connected PCC point is U. t ∠0 (oriented on the d-axis); X g (X g =wL g w is the fundamental angular frequency of the power grid, L g (Equivalent inductance on the grid side) is the equivalent impedance on the grid side; the infinite grid voltage vector is U. g ∠δ.

[0034]

[0035] Among them: I d For active current, I q For reactive current, U td For the voltage vector U at PCC point t The component of ∠0 on the d-axis, U tq Voltage vector U t The component of ∠0 on the q-axis, P out For active power output, U g For grid voltage, X g It is the equivalent impedance on the grid side.

[0036] When I q When X = 0, g The larger the value, the smaller the output power, therefore reactive power compensation is required.

[0037] Differentiating equation (1) with respect to the active current, we can obtain:

[0038]

[0039] Let I ref =0.6I N 0.05I N 0.9I N (Values ​​range from 0.6 to 0.9 in increments of 0.05), I N The rated current value of the converter; measure the output current I of the converter. abc and output voltage V abc Calculate the active power P ac and active current I d Standardize equation (1) and set X to a different value. g Convert to numerical k s The representation is based on a set of numerical values ​​calculated using the following formula:

[0040]

[0041] Where Idpu =I d / I dN P pu =P ac / P N I dpu Active current I d per-unit value, I dN Equal to rated current I N peak value; P pu =P ac / P N P pu P is the per-unit value of active power. N This is the rated power value of the converter.

[0042] array k s (i) Sort:

[0043] k s =sort(k s (4)

[0044] The `sort()` function sorts the values, selecting the three middle values ​​and calculating their average. The resulting value is:

[0045]

[0046] Based on formula (2), when the active power is at its maximum, the following active current model is established:

[0047]

[0048] Where i is the iteration number, let I qref initial value Calculate the active power according to formula (6) Substituting this into the following formula, the maximum power P can be calculated. max Corresponding I qref value:

[0049]

[0050] Let P max =1.0pu (i.e., rated power), which can be used to obtain the reactive current command corresponding to the rated power output. The I obtained from formula (7) qref Substitute it back into formula (6) for iteration, when I qref When the difference between two iterations is less than 0.05pu, the iteration stops, and the value of the last iteration is used as the command value for reactive current.

[0051] Construct a weak grid model with a 200kW energy storage converter and a grid voltage of 690V. Taking a short-circuit ratio of 1 as an example, analyze the relationship between active power, active current, and reactive current. Figure 2 As shown.

[0052] from Figure 2 As can be seen, a 200kW rated power output (and at its maximum) corresponds to approximately I reactive power. q = -0.65pu, active current is approximately I d =196A / (167A*1.414)=0.83pu.

[0053] Based on reactive current I q The power curve with a ratio of 0, combined with formulas (3) to (7) and algorithm iteration, can be used to calculate the command value corresponding to the rated power output under the condition of a short-circuit ratio of 1: I dref =0.82617 pu, I qref = -0.64698 pu, and Figure 2 The results obtained were basically consistent, proving the effectiveness of the algorithm in the method for improving the output power of converters under weak power grids in this invention.

[0054] The implementation process of this invention is as follows: Figure 3 As shown, the specific steps are as follows:

[0055] Step 1: In constant current mode, the current command ranges from 0.6 times the rated current to 0.9 times the rated current, with a step size of 0.05 times the rated power, and the output current I of the converter is measured. abc and output voltage V abc Calculate the active power P ac and active current I d Convert to the corresponding per-unit value and calculate the value according to the following formula:

[0056]

[0057] Step 2, for array k s Sort by size from smallest to largest:

[0058] k s =sort(k s )

[0059] Where `sort` is the sorting function, which selects the middle three values ​​and calculates their average.

[0060]

[0061] Step 3: Establish the active current model corresponding to the maximum power, where i is the iteration number, let I qref initial value Calculate active power

[0062]

[0063] Step 4: Establish a mathematical model of the reactive current corresponding to the maximum power, and... Substitute into the following formula to calculate P max corresponding Value, P at rated power max Take 1.0 pu.

[0064]

[0065] Step 5, determine I qref Does the value meet the following conditions:

[0066]

[0067] If the condition is met, exit the iteration; otherwise, return to step 3 and continue iterating.

[0068] The value of the last iteration is used as the command value for reactive current.

Claims

1. A method for increasing the output power of a converter under weak power grid conditions, characterized in that, Includes the following steps: 1) Calculate the variable Ks that characterizes the grid impedance based on the output current and output voltage values ​​of the converter under weak grid conditions; 2) Input the variable Ks and the preset initial value of the reactive current into the active current model corresponding to the maximum power of the converter to calculate the active current value, and then input the active current value into the reactive current model corresponding to the maximum power of the converter to calculate the reactive current value, which is recorded as the reactive current command value. 3) Substitute the reactive current value of the previous generation into the active current model and repeat step 2). The obtained reactive current value is recorded as the new generation reactive current command value. 4) If the absolute value of the difference between the reactive current command values ​​of two adjacent generations is less than the set value, then the reactive current command value determined by the new generation shall be the reactive current command required for the converter to output rated power under a weak power grid. If the absolute value of the difference between the reactive current command values ​​of two adjacent generations is not less than the set value, then repeat steps 3) and 4) until the reactive current command value required for the rated output power of the converter is obtained.

2. The method for increasing the output power of a converter under weak power grid conditions according to claim 1, characterized in that, In step 1), the variable Ks is calculated as follows: within the set current command range, several sets of output current and output voltage of the converter under weak grid conditions are obtained, several sets of values ​​representing grid impedance are calculated, and at least two values ​​are selected to calculate the average value to obtain the variable Ks.

3. The method for increasing the output power of a converter under weak power grid conditions according to claim 2, characterized in that, The set current command range is 0.6-0.9 times the rated current.

4. The method for increasing the output power of a converter under weak power grid conditions according to claim 3, characterized in that, Several sets of output currents are obtained evenly within the set current command range according to the set step size.

5. The method for increasing the output power of a converter under weak power grid conditions according to claim 2, characterized in that, Arrange the values ​​representing the grid impedance in ascending order, and then take the average of at least two of the middle values.

6. The method for increasing the output power of a converter under weak power grid conditions according to claim 1, characterized in that, In step 2), the active current model corresponding to the maximum power of the converter is: Where: i is the iteration number. This is the reactive current command value. This is the initial value of the reactive current. The active current value is obtained from the i-th calculation.

7. The method for increasing the output power of a converter under weak power grid conditions according to claim 4, characterized in that, The initial value of the preset reactive current value It is 0.

8. The method for increasing the output power of a converter under weak power grid conditions according to claim 1, characterized in that, In step 2), the reactive current model corresponding to the maximum power of the converter is: in, This represents the reactive current command value obtained from the i-th calculation. Let P be the active current value obtained from the i-th calculation. max This represents the maximum power value of the converter.

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