Converter DC side impedance measurement method based on secondary side disturbance

Abstract

The invention provides a converter direct current side impedance measurement method based on secondary side disturbance. The method is applied to a new energy grid-connected system which at least comprises a direct current constant voltage source, a converter and an alternating current power grid. The direct-current side of the converter is connected with a direct-current constant-voltage source, and the alternating-current side is connected into an alternating-current power grid The method comprises the following steps: determining a to-be-measured frequency point; before and after the direct current side of the converter is connected with a capacitor in parallel, direct current voltage and current responded by the direct current side of the converter when a harmonic disturbance signal with the same frequency as a to-be-measured frequency point is injected into the direct current side of the converter based on a grid-connected point PCC are obtained; the first disturbance voltage current and the second disturbance voltage current are converted from the direct-current voltage current before and after parallel connection of the corresponding specified capacitor; and according to the first disturbance voltage current and the second disturbance voltage current, obtaining the direct current side impedance of the converter corresponding to the frequency point to be measured through vector calculation. According to the invention, the method can replace a conventional impedance measurement device for measurement, thereby achieving the purposes of reducing the cost and reducing the operation complexity.

Description

Technical Field

[0001] The present invention relates to the technical field of power systems, and in particular to a method for measuring impedance of a DC side of a converter based on secondary side disturbance. Background Art

[0002] In recent years, my country's new energy industries such as wind power and photovoltaics have developed rapidly. The power system with large-scale new energy access dominated by voltage source converter (VSC) has subverted the traditional synchronous machine system. Due to the negative damping characteristics of the control loop of power electronic equipment, it poses a major threat to the stability of the system.

[0003] At present, the impedance method based on frequency domain is often used to analyze the new energy grid-connected system. The key to this method is to obtain the output impedance of the converter. In actual engineering, impedance measurement equipment is often used to measure the external characteristics of the converter port. Since there is no need to obtain the specific internal structure of the equipment, it is completely regarded as a "black box". However, impedance measurement equipment often requires huge investment and complex operation.

[0004] Therefore, it is necessary to propose a new method for obtaining the output impedance of the converter to replace the traditional impedance measurement equipment to measure the method, so as to achieve the purpose of reducing costs and operating complexity. Summary of the invention

[0005] The technical problem to be solved by the embodiments of the present invention is to provide a method for measuring the DC side impedance of a converter based on secondary side disturbance, which can replace the measurement method using traditional impedance measurement equipment, thereby achieving the purpose of reducing costs and operation complexity.

[0006] In order to solve the above technical problems, an embodiment of the present invention provides a method for measuring the DC side impedance of a converter based on secondary side disturbance, which is used in a new energy grid-connected system comprising at least a DC constant voltage source, a converter and an AC power grid; wherein the DC side of the converter is connected to the DC constant voltage source, and the AC side is connected to the AC power grid via a designated grid connection point PCC; the method comprises the following steps:

[0007] Determine the frequency point to be tested;

[0008] Before and after a designated capacitor is connected in parallel between the DC side of the converter and the DC constant voltage source, a DC voltage and a DC current that the DC side of the converter responds to when a harmonic disturbance signal having a frequency equal to the frequency point to be measured is injected into the grid connection point PCC are obtained, and a discrete Fourier transform is used to obtain a first disturbance voltage and a first disturbance current that are respectively transformed from the DC voltage and the DC current obtained before the designated capacitor is connected in parallel, and a second disturbance voltage and a second disturbance current that are respectively transformed from the DC voltage and the DC current obtained after the designated capacitor is connected in parallel are obtained;

[0009] According to the obtained first disturbance voltage, first disturbance current, second disturbance voltage and second disturbance current, the DC side impedance of the converter corresponding to the frequency point to be measured is obtained through vector calculation.

[0010] Among them, through the formula Calculate the DC impedance of the converter at the frequency to be measured in,

[0011] is the vector value of the DC side impedance of the converter, including amplitude and phase angle; is the vector value of the first disturbance voltage, including amplitude and phase angle; is the vector value of the first disturbance current, including amplitude and phase angle; is the vector value of the second disturbance voltage, including amplitude and phase angle; is the vector value of the second disturbance current, including amplitude and phase angle.

[0012] Wherein, the DC constant voltage source is an energy storage capacitor of the new energy power grid.

[0013] Wherein, the new energy power grid is a wind power generation grid or a photovoltaic power generation grid.

[0014] Wherein, the harmonic disturbance signal is a current disturbance signal or a voltage disturbance signal.

[0015] Implementing the embodiments of the present invention has the following beneficial effects:

[0016] Compared with the measurement method performed by traditional impedance measurement equipment, the present invention aims to require only one designated capacitor (such as a small capacitor) in the primary main circuit, and before and after the designated capacitor is connected in parallel, respectively measure and record the DC voltage and DC current in response to a harmonic disturbance signal with the same frequency as the frequency point to be measured at the grid connection point PCC of the converter, so as to calculate the DC side impedance of the converter corresponding to the frequency point to be measured, thereby replacing the measurement method performed by traditional impedance measurement equipment and achieving the purpose of reducing costs and operating complexity. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the drawings required for use in the embodiments or the description of the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, without paying creative labor, other drawings obtained based on these drawings still belong to the scope of the present invention.

[0018] Figure 1 A flow chart of a method for measuring the DC side impedance of a converter based on secondary side disturbance provided by an embodiment of the present invention;

[0019] Figure 2 A simplified circuit diagram of a new energy grid-connected system in a method for measuring the DC side impedance of a converter based on secondary side disturbance provided in an embodiment of the present invention;

[0020] Figure 3 A diagram of an application scenario of a method for measuring the DC side impedance of a converter based on secondary side disturbance provided in an embodiment of the present invention. DETAILED DESCRIPTION

[0021] In order to make the objectives, technical solutions and advantages of the present invention more clear, the present invention will be further described in detail below with reference to the accompanying drawings.

[0022] like Figure 1 As shown in the figure, a method for measuring the DC side impedance of a converter based on secondary side disturbance is proposed in an embodiment of the present invention, which is used for at least including a DC constant voltage source V dc (such as energy storage capacitors of new energy grids), converters VSC and AC grid u g On the new energy grid-connected system, such as Figure 2 shown in Figure 2 In the process, the DC side of the converter VSC is directly connected to the DC constant voltage source V dc , the AC side is connected to the AC grid through the designated grid connection point PCC g The new energy grid is a wind power grid or a photovoltaic power grid; the method comprises the following steps:

[0023] Step S1, determining the frequency point to be measured;

[0024] Step S2, before and after a designated capacitor is connected in parallel between the DC side of the converter and the DC constant voltage source, a DC voltage and a DC current in response to a harmonic disturbance signal having the same frequency as the frequency point to be measured is obtained based on the grid connection point PCC, and a discrete Fourier transform is used to obtain a first disturbance voltage and a first disturbance current respectively transformed from the DC voltage and the DC current obtained before the designated capacitor is connected in parallel, and a second disturbance voltage and a second disturbance current respectively transformed from the DC voltage and the DC current obtained after the designated capacitor is connected in parallel;

[0025] Step S3, obtaining the DC side impedance of the converter corresponding to the frequency point to be measured through vector calculation according to the obtained first disturbance voltage, the first disturbance current, the second disturbance voltage and the second disturbance current.

[0026] The specific process is as follows: in step S1, a plurality of frequency points to be measured are defined to form a frequency set, and one of the frequency points to be measured is randomly selected from the frequency set to perform transformer impedance measurement.

[0027] In step S2, the first step is to first determine that the first working scenario is that there is no designated capacitor connected in parallel between the DC side of the converter VSC and the DC constant voltage source; secondly, in the first working scenario, after the harmonic disturbance signal (such as current disturbance signal or voltage disturbance signal) at the same frequency as the frequency point to be measured is superimposed at the current sampling point (i.e., the grid connection point PCC) of the converter VSC, the voltage and current sensors are used to measure, and the DC voltage and DC current that the DC side of the converter VSC responds to when the above harmonic disturbance signal is injected at the grid connection point PCC are recorded. Finally, the DC voltage and DC current measured and recorded above are both subjected to discrete Fourier transform to obtain the corresponding first disturbance voltage (including amplitude and phase angle) and first disturbance current (including amplitude and phase angle).

[0028] Step 2: First, determine that the second working scenario is that a specified capacitor is connected between the DC side of the converter VSC and the DC constant voltage source; secondly, in the second working scenario, after the same harmonic disturbance signal (such as current disturbance signal or voltage disturbance signal) is superimposed at the current sampling point (i.e., the grid connection point PCC) of the converter VSC, measure through the voltage and current sensors, and record the DC voltage and DC current that the DC side of the converter VSC responds to when the above harmonic disturbance signal is injected at the grid connection point PCC. Finally, the DC voltage and DC current measured and recorded above are both subjected to discrete Fourier transform to obtain the corresponding second disturbance voltage (including amplitude and phase angle) and second disturbance current (including amplitude and phase angle).

[0029] It should be noted that the size of the specified capacitor is designed according to the actual situation, and its capacitance value needs to be smaller than the DC constant voltage source Vdc The energy storage size is approximately the DC constant voltage source V dc The energy storage value is between 1 / 20 and 1 / 10.

[0030] It should be noted that the reference positive directions of the first and second disturbance voltages are both connected to the DC constant voltage source V dc The potential level of the first and second disturbance currents remains consistent, that is, the positive and negative poles correspond to each other; the reference positive directions of the first and second disturbance currents are consistent with the directions flowing into the DC side ports of the converter VSC, that is, the direction flowing to the DC side of the converter VSC is positive; conversely, the direction flowing to the AC side of the converter VSC is negative.

[0031] In step S3, the formula Calculate the DC impedance of the converter VSC corresponding to the frequency to be measured in, is the vector value of the DC side impedance of the converter VSC, including the magnitude and phase angle; is the vector value of the first disturbance voltage, including amplitude and phase angle; is the vector value of the first disturbance current, including amplitude and phase angle; is the vector value of the second disturbance voltage, including amplitude and phase angle; is the vector value of the second disturbance current, including amplitude and phase angle.

[0032] In summary, by analogy, step S1 to step S3 are repeated to complete the measurement of the DC side impedance of the converter VSC at all the frequency points to be measured.

[0033] like Figure 3 As shown, the application scenario of a method for measuring the DC side impedance of a converter based on secondary side disturbance proposed in an embodiment of the present invention is further described as follows:

[0034] Figure 3 In the equation, A2 is the amplitude of the A-phase disturbance injected at the PCC point, ω2 is the angular frequency of the A-phase disturbance injected at the PCC point, is the phase of the A phase disturbance injected at PCC point; the same is true for phases B and C, which will not be repeated here. At this time, the DC constant voltage source V dc It is powered by a new energy grid and connected to a voltage stabilizing capacitor (see Figure 2 The capacitor C on the far right is used to form the

[0035] First, select a specific disturbance measurement frequency point.

[0036] Secondly, the harmonic disturbance signal at a specific disturbance measurement frequency is superimposed at the current sampling point of the converter VSC (i.e., the grid connection point PCC), and then the DC side port of the converter VSC will respond to generate disturbance voltage and disturbance current, so that the DC voltage and DC current of the DC side port of the converter VSC are measured and recorded by using voltage and current sensors, and the first disturbance voltage v corresponding to the response at the specific disturbance measurement frequency is further obtained by Fourier transform. dc1 The amplitude and phase angle of the disturbance current, and the first disturbance current response i corresponding to the specific disturbance measurement frequency are obtained. dc1 The amplitude and phase angle of the first disturbance voltage v dc1 The reference positive direction is consistent with the potential of the voltage stabilizing capacitor C on the DC side of the converter VSC; the first disturbance current i dc1 The positive direction is taken as the reference direction of the DC side port of the converter VSC.

[0037] Next, a small capacitor (not shown, with a capacitance value of 1 / 10 of the voltage-stabilizing capacitor C) is connected in parallel to the voltage-stabilizing capacitor C on the DC side of the converter VSC, and then the harmonic disturbance signal superimposed at the current sampling point (i.e., the grid connection point PCC) of the converter VSC is kept unchanged, and then the DC side port of the converter VSC will also respond to generate disturbance voltage and disturbance current, so that the DC voltage and DC current of the DC side port of the converter VSC are measured and recorded by using voltage and current sensors, and the second disturbance voltage v corresponding to the response at the specific disturbance measurement frequency is further obtained by Fourier transform. dc2 The amplitude and phase angle of the disturbance current, and the corresponding response of the second disturbance current response i under the specific disturbance measurement frequency are obtained. dc2 Amplitude and phase angle.

[0038] Finally, the DC side impedance of the converter VSC corresponding to the specific disturbance measurement frequency is calculated by the above formula.

[0039] Implementing the embodiments of the present invention has the following beneficial effects:

[0040] Compared with the measurement method performed by traditional impedance measurement equipment, the present invention aims to require only one designated capacitor (such as a small capacitor) in the primary main circuit, and before and after the designated capacitor is connected in parallel, respectively measure and record the DC voltage and DC current in response to a harmonic disturbance signal with the same frequency as the frequency point to be measured at the grid connection point PCC of the converter, so as to calculate the DC side impedance of the converter corresponding to the frequency point to be measured, thereby replacing the measurement method performed by traditional impedance measurement equipment and achieving the purpose of reducing costs and operating complexity.

[0041] A person skilled in the art can understand that all or part of the steps in the above-mentioned embodiment method can be completed by instructing related hardware through a program, and the program can be stored in a computer-readable storage medium, such as ROM / RAM, disk, CD-ROM, etc.

[0042] The above disclosure is only the preferred embodiment of the present invention, which certainly cannot be used to limit the scope of the present invention. Therefore, equivalent changes made according to the claims of the present invention are still within the scope of the present invention.

Claims

1. A method for measuring the DC side impedance of a converter based on secondary side disturbance, characterized in that: Used in a new energy grid-connected system comprising at least a DC constant voltage source, a converter and an AC power grid; wherein the DC side of the converter is connected to the DC constant voltage source, and the AC side is connected to the AC power grid via a designated grid connection point PCC; the method comprises the following steps: Determine the frequency point to be tested; Before and after a designated capacitor is connected in parallel between the DC side of the converter and the DC constant voltage source, a DC voltage and a DC current that the DC side of the converter responds to when a harmonic disturbance signal having a frequency equal to the frequency point to be measured is injected into the grid connection point PCC are obtained, and a discrete Fourier transform is used to obtain a first disturbance voltage and a first disturbance current that are respectively transformed from the DC voltage and the DC current obtained before the designated capacitor is connected in parallel, and a second disturbance voltage and a second disturbance current that are respectively transformed from the DC voltage and the DC current obtained after the designated capacitor is connected in parallel are obtained; According to the obtained first disturbance voltage, first disturbance current, second disturbance voltage and second disturbance current, the DC side impedance of the converter corresponding to the frequency point to be measured is obtained through vector calculation.

2. The method for measuring the DC side impedance of a converter based on secondary side disturbance according to claim 1, characterized in that: By formula Calculate the DC impedance of the converter at the frequency to be measured in, is the vector value of the DC side impedance of the converter, including amplitude and phase angle; is the vector value of the first disturbance voltage, including amplitude and phase angle; is the vector value of the first disturbance current, including amplitude and phase angle; is the vector value of the second disturbance voltage, including amplitude and phase angle; is the vector value of the second disturbance current, including amplitude and phase angle.

3. The method for measuring the DC side impedance of a current transformer based on secondary side disturbance according to claim 2, characterized in that: The DC constant voltage source is an energy storage capacitor of the new energy power grid.

4. The method for measuring the DC side impedance of a current transformer based on secondary side disturbance according to claim 3, characterized in that: The new energy power grid is a wind power generation grid or a photovoltaic power generation grid.

5. The method for measuring the DC side impedance of a current transformer based on secondary side disturbance according to claim 4, characterized in that: The harmonic disturbance signal is a current disturbance signal or a voltage disturbance signal.

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