A flexible DC-DC converter system with grid-side filtering and a parameter setting method

Abstract

This invention belongs to the technical field of flexible DC-DC converters, and relates to a flexible DC-DC converter system with grid-side filtering and a parameter setting method. It includes a transformer, a filter, and a flexible DC-DC converter. The flexible DC-DC converter has an AC terminal and a DC terminal, with a transformer positioned between the AC and DC terminals. The filter is located between the AC terminal and the transformer and is connected in parallel with the flexible DC-DC converter. The flexible DC-DC converter system contains two resonant points: one is a series resonant point between the transformer and the filter, and the other is a parallel resonant point between the filter and the flexible DC-DC converter. The impedance phase-frequency characteristics between the two resonant points exhibit a capacitive negative and weakly damped state. This effectively improves the damping characteristics of the high-frequency flexible DC-DC converter and suppresses high-frequency oscillations.

Description

Technical Field

[0001] This invention relates to a flexible DC converter system with grid-side filtering and a parameter setting method, belonging to the field of DC converter technology. Background Technology

[0002] Compared with traditional DC transmission, flexible DC transmission has advantages such as no need for reactive power compensation, no risk of commutation failure, ability to supply power to passive systems, independent control of active and reactive power, low harmonic levels, ease of constructing multi-terminal DC systems, and small footprint, thus leading to its rapid development in recent years. However, in practical applications, due to the inherent time delay of the flexible DC converter and its rapid power control, the system damping weakens or even becomes negatively damped, exhibiting capacitive weak damping characteristics, which may cause high-frequency oscillations in the flexible DC converter. Summary of the Invention

[0003] To address the aforementioned problems, the present invention aims to provide a flexible DC-DC converter system with grid-side filtering and a parameter setting method, which can effectively improve the damping characteristics of the high-frequency flexible DC-DC converter and suppress high-frequency oscillations.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: a flexible DC-DC converter system with grid-side filtering, comprising: a transformer, a filter, and a flexible DC-DC converter. The flexible DC-DC converter includes an AC terminal and a DC terminal, with a transformer disposed between the AC terminal and the DC terminal. The filter is disposed between the AC terminal and the transformer and connected in parallel with the flexible DC-DC converter. The impedance Z of the flexible DC-DC converter system is calculated using the following formula:

[0005]

[0006] Where R1, L1, and C1 are the resistance, inductance, and capacitance values ​​of the filter, respectively; j is the unit of a complex number; ω is the angular frequency; and L... MMC R is the equivalent inductance value of the impedance of the flexible DC converter. MMC Re|Z is the equivalent resistance value of the impedance of the flexible DC converter. s |Is the impedance Z s The real part, Im|Z s |Is the impedance Z s The imaginary part, Pha|Z s |Is the impedance Z s The phase.

[0007] Furthermore, the filter includes a resistor, a capacitor, an inductor connected in series and two branches connected in parallel. One branch is used for current detection, and the other branch is used to install a surge arrester. The resistor, capacitor, and inductor are used to adjust the high-frequency impedance characteristics of the flexible DC converter.

[0008] Furthermore, the formula for calculating the filter impedance is:

[0009]

[0010] Where R1, L1, and C1 are the resistance, inductance, and capacitance values ​​of the passive damping device, respectively, j is the unit of a complex number, and ω is the angular frequency.

[0011] Furthermore, the flexible DC transmission converter includes a three-phase upper arm module and a three-phase lower arm module. The positive DC terminal of the three-phase upper arm module serves as the first DC terminal of the flexible DC transmission converter. The three-phase AC lines of the three-phase upper arm module are connected one-to-one with the AC lines of the three-phase lower arm module, and the negative DC terminal of the three-phase lower arm module serves as the second DC terminal of the flexible DC transmission converter.

[0012] Furthermore, each arm of the three-phase upper arm module and the three-phase lower arm module is equipped with an inductor, and the inductor, filter, and transformer constitute an LCL filter circuit.

[0013] Furthermore, the formula for calculating the impedance characteristics of a flexible DC converter is as follows:

[0014] Z MMC =jωL MMC +R MMC

[0015] Among them, L MMC R is the equivalent inductance value of the impedance of the flexible DC converter. MMC ω represents the equivalent resistance value of the flexible DC converter impedance, j is the unit of a complex number, and ω is the angular frequency.

[0016] Furthermore, the flexible DC converter system contains two resonant points: one is the series resonant point of the transformer and the filter, and the other is the parallel resonant point of the filter and the flexible DC converter. The impedance phase frequency characteristics between the two resonant points exhibit a capacitive negative damping state.

[0017] This invention also discloses a parameter setting method for a grid-side filter in a flexible DC-DC converter system, used in any of the above-mentioned flexible DC-DC converter systems with grid-side filtering, comprising the following steps:

[0018] S1 obtains the equivalent inductance and equivalent resistance values ​​of the impedance of the flexible DC converter.

[0019] S2 determines the two resonant points of the flexible DC converter system and calculates the frequencies f1 and f2 at the two resonant points respectively;

[0020] S3 calculates the relationship between the filter's inductance L1 and capacitance C1 at the two resonant points.

[0021] S4 solves the relationship between the two resonant points in step S3 by combining the equations, and substitutes the frequencies f1 and f2 at the two resonant points in step S2 into the relationship in step S3 to obtain the inductance L1 and capacitance C1 of the filter.

[0022] S5 obtains the range of values ​​for the filter's resistance R1, provided that the phase angle near the right side of the parallel resonant point is not less than the preset angle θ, and the impedance of the flexible DC system does not exhibit negative damping characteristics at the parallel resonant frequency point.

[0023] Furthermore, one of the two resonant points is the series resonant point of the transformer and the filter, and the other is the parallel resonant point of the filter and the flexible DC converter. The relationship between the inductance L1 and the capacitance C1 of the filter at the series resonant point is as follows:

[0024]

[0025] The relationship between the inductance L1 and capacitance C1 of the filter with parallel resonant points is as follows:

[0026]

[0027] Where f2 is the frequency value at the parallel resonant point, L MMC This represents the equivalent inductance value of the impedance of the flexible DC converter.

[0028] Furthermore, in step S5, to ensure that the phase angle near the right side of the parallel resonant point is not less than the preset angle θ, the filter resistance R1 should satisfy the following formula:

[0029]

[0030]

[0031]

[0032] To ensure that the impedance of the flexible DC system does not exhibit negative damping characteristics at the parallel resonant frequency, the filter resistance R1 should satisfy the following equation:

[0033]

[0034] Among them, L MMC R is the equivalent inductance value of the impedance of the flexible DC converter. MMC Let be the equivalent resistance value of the flexible DC converter impedance, j be the unit of a complex number, ω be the angular frequency, L1, C1, and R1 be the inductance, capacitance, and resistance of the filter, respectively, and Re|Z s |Is the impedance Z s The real part, Im|Z s |Is the impedance Z s The imaginary part, Pha|Z s|Is the impedance Z s The phase.

[0035] This invention, by adopting the above technical solutions, has the following advantages: 1. This invention adds a high-frequency filter consisting of a capacitor, inductor, and resistor connected in series on the AC grid side of the converter transformer in the flexible DC converter station, which can effectively enhance the damping characteristics of the flexible DC converter in the high-frequency band. 2. In this invention, the flexible DC converter does not need to change its control strategy; high-frequency oscillation suppression can be achieved by adjusting the capacitor, inductor, and resistor parameters. 3. This invention has a wide influence on the high-frequency impedance characteristics of the flexible DC converter over a broad frequency band, eliminating the need for additional hardware filters at other frequencies. 4. This invention can be widely applied to solve the high-frequency oscillation problem of flexible DC converter stations connected to the AC grid or large-scale new energy power plants. Attached Figure Description

[0036] Figure 1 This is a structural diagram of a flexible DC-DC converter system with grid-side filtering in one embodiment of the present invention;

[0037] Figure 2 This is an impedance characteristic diagram of a flexible DC converter system with grid-side filtering in one embodiment of the present invention;

[0038] Figure 3 This is a structural diagram of a flexible DC-DC converter system with grid-side filtering in another embodiment of the present invention;

[0039] Figure 4 This is an impedance characteristic diagram of a flexible DC converter system with grid-side filtering in another embodiment of the present invention. Detailed Implementation

[0040] To enable those skilled in the art to better understand the technical direction of the present invention, the present invention is described in detail through specific embodiments. However, it should be understood that the specific embodiments are provided only for a better understanding of the present invention and should not be construed as limiting the present invention. In the description of the present invention, it should be understood that the terminology used is for descriptive purposes only and should not be construed as indicating or implying relative importance.

[0041] This invention provides a flexible DC-DC converter system with filtering and a method for setting the filtering parameters. By adding a filter to the transformer grid side of the flexible DC-DC converter system, it effectively suppresses high-frequency oscillations, thereby improving the damping characteristics of the system in the high-frequency range. Furthermore, by adaptively improving the filter parameters, it can enhance the high-frequency damping characteristics of the flexible DC-DC converter in the negative damping frequency band. The technical solution of this invention will be described in detail below through several specific embodiments.

[0042] Example 1

[0043] This embodiment includes a flexible DC-DC converter system with grid-side filtering, such as... Figure 1 As shown, it includes: a transformer, a filter, and a flexible DC-DC converter. The flexible DC-DC converter includes an AC terminal and a DC terminal. U in the figure... dc The voltage at the DC end is the voltage at the AC end. A transformer is installed between the AC end and the DC end. A filter is installed between the AC end and the transformer and is connected in parallel with the flexible DC converter.

[0044] The flexible DC-DC converter includes a three-phase upper arm module and a three-phase lower arm module. The positive DC terminal of the three-phase upper arm module serves as the first DC terminal of the flexible DC-DC converter. The three-phase AC terminals of the three-phase upper arm module are connected one-to-one with the AC terminals of the three-phase lower arm module. The negative DC terminal of the three-phase lower arm module serves as the second DC terminal of the flexible DC-DC converter.

[0045] The three-phase upper bridge arm module includes three structurally identical sub-modules: Phase A, Phase B, and Phase C. Similarly, the three-phase lower bridge arm module includes three structurally identical sub-modules: Phase A, Phase B, and Phase C. The positive terminals of the Phase A, Phase B, and Phase C upper bridge arm sub-modules are all connected to the positive DC terminal. The negative terminal of the Phase A upper bridge arm sub-module is connected to the positive terminal of the Phase A lower bridge arm module. The negative terminal of the Phase B upper bridge arm module is connected to the positive terminal of the Phase B lower bridge arm module. The negative terminal of the Phase C upper bridge arm module is connected to the positive terminal of the Phase C lower bridge arm module. The negative terminals of the Phase A, Phase B, and Phase C lower bridge arm modules are all connected to the negative DC terminal. The three-phase output terminals of the transformer—Phase A, Phase B, and Phase C—are connected to the Phase A, Phase B, and Phase C bridge arm sub-modules, respectively. The bridge arm sub-module includes an upper bridge arm and a lower bridge arm sub-module. Both the upper and lower bridge arm modules have the same structure, including an inductor L. arm and the inductor L arm N power sub-units SM1, SM2...SMn are connected in series, and the output terminal of the power sub-unit is connected to the positive DC terminal (corresponding to the upper bridge arm sub-module) or the negative DC terminal (corresponding to the upper bridge arm sub-module).

[0046] The impedance characteristic diagram of a flexible DC converter (excluding transformer) is shown below. Figure 2 As shown, the formula for calculating its impedance characteristics is:

[0047] Z MMC =jωL MMC +R MMC

[0048] Among them, L MMC R is the equivalent inductance value of the impedance of the flexible DC converter. MMC ω represents the equivalent resistance value of the flexible DC converter impedance, j is the unit of a complex number, and ω is the angular frequency.

[0049] The filter comprises a resistor, capacitor, and inductor connected in series, and two branches connected in parallel. One branch is used for current sensing, and the other branch is used to install a surge arrester. The resistor, capacitor, and inductor are used to adjust the high-frequency impedance characteristics of the flexible DC-DC converter. Specifically, the filter's capacitor, inductor, and resistor play a role in adjusting the high-frequency impedance characteristics of the flexible DC-DC converter. The filter's current sensing branch monitors the current in real time, providing monitoring and protection for the filter. The filter's surge arrester branch is used for overvoltage protection.

[0050] The formula for calculating the filter is as follows:

[0051]

[0052] Where R1, L1, and C1 are the resistance, inductance, and capacitance values ​​of the passive damping device, respectively, j is the unit of a complex number, and ω is the angular frequency.

[0053] The impedance Z of the flexible DC-DC converter system formed by the parallel connection of the flexible DC-DC converter and the filter branch is calculated using the following formula:

[0054]

[0055] Where R1, L1, and C1 are the resistance, inductance, and capacitance values ​​of the filter, respectively; j is the unit of a complex number; ω is the angular frequency; and L... MMC R is the equivalent inductance value of the impedance of the flexible DC converter. MMC This represents the equivalent resistance value of the impedance of the flexible DC converter.

[0056] The flexible DC-DC converter system contains two resonant points: one is the series resonant point of the transformer and filter, and the other is the parallel resonant point of the filter and the flexible DC-DC converter. The phase-frequency impedance characteristics between the two resonant points exhibit a capacitive negative and weakly damped state. Above the series resonant point, it exhibits an inductive weakly damped state, and below the parallel resonant point, it exhibits an inductive strongly damped state. Therefore, by adding a high-frequency filter, the phase-frequency characteristics of the flexible DC-DC converter system can be adjusted to capacitive in the frequency band where the system is prone to high-frequency oscillations, thereby greatly reducing the risk of high-frequency oscillations in the system.

[0057] Example 2

[0058] By optimizing the filter parameters, the damping characteristics of the flexible DC converter within a certain frequency band can be adjusted, thereby greatly reducing the risk of high-frequency oscillations after the flexible DC converter is connected to the AC grid or a large-scale new energy power plant.

[0059] Based on the same inventive concept, this embodiment discloses a parameter setting method for a grid-side filter in a flexible DC-DC converter system, used in any of the grid-side filtering flexible DC-DC converter systems in Embodiment 1, comprising the following steps:

[0060] S1 obtains the equivalent inductance and equivalent resistance values ​​of the impedance of the flexible DC converter.

[0061] S2 determines the two resonant points of the flexible DC converter system and calculates the frequencies f1 and f2 at the two resonant points respectively;

[0062] S3 calculates the relationship between the filter's inductance L1 and capacitance C1 at the two resonant points.

[0063] One of the two resonant points is the series resonant point of the transformer and the filter, and the other is the parallel resonant point of the filter and the flexible DC converter. The relationship between the inductance L1 and the capacitance C1 of the filter at the series resonant point is as follows:

[0064]

[0065] The relationship between the inductance L1 and capacitance C1 of the filter with parallel resonant points is as follows:

[0066]

[0067] Where f2 is the frequency value at the parallel resonant point, L MMC This represents the equivalent inductance value of the impedance of the flexible DC converter.

[0068] S4 solves the relationship between the two resonant points in step S3 by combining the equations, and substitutes the frequencies f1 and f2 at the two resonant points in step S2 into the relationship in step S3 to obtain the inductance L1 and capacitance C1 of the filter.

[0069] S5 obtains the range of values ​​for the filter's resistance R1, provided that the phase angle near the right side of the parallel resonant point is not less than the preset angle θ, and the impedance of the flexible DC system does not exhibit negative damping characteristics at the parallel resonant frequency point.

[0070] To ensure that the phase angle near the right side of the parallel resonant point is not less than the preset angle θ, the filter resistor R1 should satisfy the following formula:

[0071]

[0072]

[0073]

[0074] To ensure that the impedance of the flexible DC system does not exhibit negative damping characteristics at the parallel resonant frequency, the filter resistance R1 should satisfy the following equation:

[0075]

[0076] Among them, L MMC R is the equivalent inductance value of the impedance of the flexible DC converter. MMC Let be the equivalent resistance value of the flexible DC converter impedance, j be the unit of a complex number, ω be the angular frequency, L1, C1, and R1 be the inductance, capacitance, and resistance of the filter, respectively, and Re|Z s |Is the impedance Z s The real part, Im|Z s |Is the impedance Z s The imaginary part, Pha|Z s |Is the impedance Z s The phase. In this embodiment, the preset angle θ is preferably 87°.

[0077] The final value of the filter resistor R1 must simultaneously meet two conditions: not less than the preset angle and not exhibiting negative damping characteristics.

[0078] Example 3

[0079] To further illustrate the technical solution and effects of the present invention, this embodiment takes a flexible DC converter station as an example to provide a detailed description of the design method for the valve-side filter device parameters of the present invention.

[0080] This embodiment discloses a flexible DC-DC converter system with grid-side filtering, such as Figure 3 As shown, it includes: a transformer, a filter, and a flexible DC converter. The flexible DC converter includes an AC terminal and a DC terminal. U in the figure... dc The voltage at the DC end is used as the reference voltage. A transformer is installed between the AC end and the DC end. The filter is placed between the AC end and the transformer and connected in parallel with the flexible DC converter. In this embodiment, the frequency operating range of the filter is 200Hz to 1000Hz. 200Hz is the parallel resonance point of the flexible DC converter impedance and the high-frequency filter impedance, and 1000Hz is the series resonance point of the high-frequency filter. The frequencies of the parallel and series resonance points are f1 = 200Hz and f2 = 1000Hz, respectively. Substituting these values ​​into the calculation formulas for the inductance L1 and capacitance C1 of the grid-side filter in Embodiment 2, the filter inductance is calculated to be L1 = 6mH and the filter capacitance to be C1 = 4.24uF. To further enhance the high-frequency damping characteristics of the flexible DC converter, a series filter resistor can be added to the filter. Based on the embodiment shown, the filter resistor parameters of the high-frequency filter are calculated; in this embodiment, R1 = 3.42Ω. Figure 4The impedance characteristic diagram of the filtered flexible DC converter system in this embodiment shows that its high-frequency impedance characteristics are more obvious compared to the unfiltered flexible DC converter system, and it effectively suppresses high-frequency oscillations in the flexible DC converter system.

[0081] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific embodiments of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the claims of the present invention. The above content is only a specific embodiment of this application, but the protection scope of this application is not limited thereto. Any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in this application should be covered within the protection scope of this application. Therefore, the protection scope of this application should be determined by the protection scope of the claims.

Claims

1. A flexible DC-DC converter system with grid-side filtering, characterized in that, include: The system comprises a transformer, a filter, and a flexible DC-DC converter. The flexible DC-DC converter includes an AC terminal and a DC terminal. The transformer is located between the AC terminal and the DC terminal. The filter is located between the AC terminal and the transformer and is connected in parallel with the flexible DC-DC converter. The impedance Z of the flexible DC-DC converter system is calculated using the following formula: in, R 1. L 1 and C 1 represents the resistance, inductance, and capacitance values ​​of the filter, respectively, and j is the unit of a complex number. It is angular frequency. L MMC This represents the equivalent inductance value of the impedance of the flexible DC converter. R MMC This represents the equivalent resistance value of the impedance of the flexible DC converter. To ensure that the phase angle near the right side of the parallel resonance point is not less than a preset angle. Furthermore, under the premise that the impedance of the flexible DC converter system does not exhibit negative damping characteristics at the parallel resonant frequency point, the range of values ​​for the filter resistance R1 is obtained. When obtaining the range of values ​​for the filter's resistor R1, to ensure that the phase angle near the right side of the parallel resonance point is not less than a preset angle... The filter resistor R1 should satisfy the following formula: To ensure that the impedance of the flexible DC-DC converter system does not exhibit negative damping characteristics at the parallel resonant frequency, the filter resistance R1 should satisfy the following equation: in, L MMC This represents the equivalent inductance value of the impedance of the flexible DC converter. R MMC Let j be the equivalent resistance value of the flexible DC converter impedance, where j is the unit of a complex number. It is angular frequency. L 1. C R1 and R2 are the inductances of the filter, respectively. 、 Capacitors and resistors It is impedance Z s The real part, It is impedance Z s The imaginary part, It is impedance Z s The phase.

2. The flexible DC-DC converter system with grid-side filtering as described in claim 1, characterized in that, The filter includes a resistor, a capacitor, an inductor connected in series and two branches connected in parallel. One branch is used for current detection, and the other branch is used to install a surge arrester. The resistor, capacitor, and inductor are used to adjust the high-frequency impedance characteristics of the flexible DC converter.

3. The flexible DC-DC converter system with grid-side filtering as described in claim 2, characterized in that, The formula for calculating the impedance of the filter is: in, R 1. L 1 and C 1 represents the resistance, inductance, and capacitance values ​​of the passive damping device, respectively, and j is the unit of a complex number. It is angular frequency.

4. The flexible DC-DC converter system with grid-side filtering as described in claim 1, characterized in that, The flexible DC converter includes a three-phase upper arm module and a three-phase lower arm module. The positive DC terminal of the three-phase upper arm module serves as the first DC terminal of the flexible DC converter. The three-phase AC terminals of the three-phase upper arm module are connected one-to-one with the AC terminals of the three-phase lower arm module. The negative DC terminal of the three-phase lower arm module serves as the second DC terminal of the flexible DC converter.

5. The flexible DC-DC converter system with grid-side filtering as described in claim 4, characterized in that, Each arm of the three-phase upper arm module and the three-phase lower arm module is provided with an inductor, and the inductor, the filter, and the transformer constitute an LCL filter circuit.

6. The flexible DC-DC converter system with grid-side filtering as described in claim 5, characterized in that, The formula for calculating the impedance characteristics of the flexible DC converter is as follows: in, L MMC This represents the equivalent inductance value of the impedance of the flexible DC converter. R MMC Let j be the equivalent resistance value of the flexible DC converter impedance, where j is the unit of a complex number. It is angular frequency.

7. The flexible DC-DC converter system with grid-side filtering as described in any one of claims 1-6, characterized in that, The flexible DC converter system includes two resonant points: one is the series resonant point of the transformer and the filter, and the other is the parallel resonant point of the filter and the flexible DC converter. The impedance phase frequency characteristics between the two resonant points exhibit a capacitive negative damping state.

8. A method for setting parameters of a grid-side filter in a flexible DC converter system, characterized in that, A flexible DC-DC converter system for grid-side filtering according to any one of claims 1-7 includes the following steps: Obtain the equivalent inductance and equivalent resistance values ​​of the flexible DC converter. Determine the two resonant points of the flexible DC converter system and calculate the frequencies at each resonant point. f 1 and f 2; Calculate the inductance of the filter at the two resonant points. L 1. Capacitor of the filter C The relationship between 1; Solve the relationship between the two resonant points using the concatenation method, and determine the frequencies at the two resonant points. f 1 and f 2. Substitute the given relationship into the formula to obtain the inductance of the filter. L 1. Capacitor of the filter C 1; To ensure that the phase angle near the right side of the parallel resonance point is not less than a preset angle. Furthermore, under the premise that the impedance of the flexible DC converter system does not exhibit negative damping characteristics at the parallel resonant frequency point, the range of values ​​for the filter resistance R1 is obtained.

9. The parameter setting method for the network-side filter as described in claim 8, characterized in that, The two resonant points are, respectively, a series resonant point between the transformer and the filter, and a parallel resonant point between the filter and the flexible DC converter. The inductance of the filter at the series resonant point... L 1 and capacitor C The relationship between 1 and 2 is: The inductance of the filter at the parallel resonant point L 1 and capacitor C The relationship between 1 and 2 is: in, f 2 is the frequency value at the parallel resonant point. L MMC This represents the equivalent inductance value of the impedance of the flexible DC converter.

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