Improved output filter circuit of flexible voltage regulation device and improvement method of improved output filter circuit

Abstract

The invention relates to an improved output filter circuit of a flexible voltage regulation device and an improvement method of the improved output filter circuit. The improvement method comprises the following steps of deducing an output voltage expression of a parallel side of the flexible voltage regulation device; establishing a single-phase equivalent circuit of the parallel side of the flexible voltage regulation device; deriving an output current expression of the parallel side of the flexible voltage regulation device; analyzing circuit characteristics of different Trap filters; an improved output filter circuit structure of the flexible voltage regulation device is provided; deducing the coupling inductance constraint of the parallel side of the flexible voltage regulation device; deducing a current tracking capability constraint condition of the parallel side of the flexible voltage regulation device; deducing a total capacitance constraint condition of the parallel side of the flexible voltage regulation device; and deducing a Trap branch constraint condition of the parallel side of the flexible voltage regulation device. According to the invention, the switching harmonic current generated by SPWM modulation can be effectively eliminated, and the harmonic compensation performance of the flexible voltage regulation device is ensured.

Description

Technical Field

[0001] The present application relates to the field of power electronics technology, and in particular to an improved output filter circuit of a flexible voltage regulation device and an improvement method thereof. Background Art

[0002] As more and more power electronic equipment is connected to the power system, problems such as harmonic pollution and imbalanced power characteristics are exacerbated. However, precision instruments and sensitive equipment, such as computers, medical devices, and high-precision production equipment, place extremely high demands on power quality. To ensure the stable operation of complex and sensitive loads on the user side, power quality management devices are being widely researched.

[0003] Static Var Generators (SVGs) compensate for voltage fluctuations by outputting reactive current, improving the unity power factor of the power grid. However, in practical applications of distribution network voltage regulation, they suffer from limited voltage regulation range, slow response speed, and low sensitivity. Dynamic Voltage Restorers (DVRs) suppress various grid-side voltage instabilities, such as voltage flicker and voltage sag. However, due to limitations in DC energy storage units, DVRs have a limited voltage regulation range and cannot provide continuous voltage regulation for extended periods. Active Power Filters (APFs) are categorized as series and shunt types. The series type is connected to the main circuit via a transformer to track and compensate for voltage harmonics, while the shunt type is used for current harmonics. Flexible voltage regulators can compensate for both voltage and current simultaneously. Their topology consists of a series-side converter connected to the grid via an LC filter and a series transformer, while a shunt-side converter is connected to the grid via an L-type filter and a shunt transformer. The series and shunt converters are connected via DC capacitors. However, the SPWM modulation of traditional flexible voltage regulators will produce high-frequency switching ripple, which will affect the quality of the output harmonic compensation current waveform, seriously restricting its application and promotion in distribution networks. Summary of the Invention

[0004] The purpose of the embodiments of the present application is to provide an improved output filter circuit of a flexible voltage regulator and an improvement method thereof, which can effectively eliminate the switching harmonic current generated by SPWM modulation and ensure the harmonic compensation performance of the flexible voltage regulator.

[0005] To achieve the above objectives, this application provides the following technical solutions:

[0006] The present invention provides a method for improving an output filter circuit of a flexible voltage regulator, which includes the following specific steps:

[0007] Step 1: Derive the output voltage expression of the parallel side of the flexible voltage regulator;

[0008] Step 2: Establish a single-phase equivalent circuit on the parallel side of the flexible voltage regulator;

[0009] Step 3: Derive the output current expression of the parallel side of the flexible voltage regulator;

[0010] Step 4: Analyze the circuit characteristics of different trap filters;

[0011] Step 5: Propose an improved output filter circuit structure of the flexible voltage regulator;

[0012] Step 5: Derive the coupled inductance constraint on the parallel side of the flexible voltage regulator;

[0013] Step 6: Derive the current tracking capability constraints of the parallel side of the flexible voltage regulator;

[0014] Step 7: Derive the total capacitance constraint of the parallel side of the flexible voltage regulator;

[0015] Step 8: Derive the Trap branch constraints on the parallel side of the flexible voltage regulator.

[0016] 2. According to the method for improving the output filter circuit of a flexible voltage regulator device according to claim 1, step 1.1: deriving the output voltage expression of the parallel-side converter 1 of the flexible voltage regulator device, namely:

[0017] According to the principle of bipolar SPWM control modulation technology, assuming that the sine wave u MA is the modulation wave of phase A, and u MA =Msinω t t, M is the modulation ratio, 0≤M≤1; triangle wave u t is the carrier signal with an amplitude of 1 and an angular frequency of ω t The voltage of the midpoint of the bridge arm of phase A relative to the midpoint of the DC side is expressed as u iao1 , combined with Fourier transform, u iao1 The Fourier series expression of is:

[0018]

[0019] Where M represents the voltage modulation ratio; ω0 represents the angular frequency of the modulation signal; ω t represents the angular frequency of the carrier, x m Indicates the harmonic order of the carrier, x n Indicates the harmonic order of the fundamental wave, J n (x) is the Bessel function of the first kind, and its expression is:

[0020]

[0021] Step 1.2: Derive the output voltage expression of the flexible voltage regulator parallel-side converter 2, namely:

[0022] For converter 2, its carrier phase is π degrees different from that of converter 1. The voltage u of the midpoint of the A-phase bridge arm relative to the DC side midpoint is derived. iao2 The expression is:

[0023]

[0024] The specific steps of step 2 are:

[0025] Taking phase A as an example, u iao1 is the A-phase PWM voltage output by converter 1, u iao2 The A-phase PWM voltage output by converter 2 is used. The coupling coefficients of the coupled inductors L1 and L2 are k1 and k2 respectively. k1 and k2 are close to 1. A single-phase equivalent circuit of the parallel side of the flexible voltage regulator is established.

[0026] The specific steps of step 3 are:

[0027] Step 3.1: Output current i ao The switching harmonic current in the ao1 with i ao2 The switching harmonic current is determined by the sum of the switching harmonic currents in the PWM harmonic voltage. Assuming that the two pairs of coupled inductors are balanced and equal, i ao The harmonic current in iao1 with u iao2 The average value u iao-avg Determine, from formula (1) and formula (3) we can get:

[0028]

[0029] It can be seen from formula (4) that the carrier phase shift of π angle can eliminate all odd harmonics of the switching frequency and their sideband harmonics, but will not affect the amplitude and phase of the even harmonics of the switching frequency and their sideband harmonics as well as the fundamental wave;

[0030] Step 3.2: Determine the current output by the parallel side of the flexible voltage regulator. There are three main types:

[0031] 1) Compensation current i that needs to be injected into the grid a0 : Unaffected by the carrier phase, the fundamental currents output by the two conversion bridges are equal in phase. Since the current frequency is much lower than the switching frequency, the coupling inductor and the high-order filter branch have little effect on the fundamental current.

[0032] 2) Common mode current harmonic component i acm:This current is the even harmonic current of the switching frequency and its sideband harmonic current. Since the coupling coefficient of the coupled inductor is close to 1, the common mode component is mainly suppressed by the coupled inductor L1, while the suppression effect of the coupled inductor L2 is close to 0. The common mode current harmonic component i acm The expression is:

[0033]

[0034] 3) Differential mode current harmonic component i adm :This current is the odd harmonic current of the switching frequency and its sideband harmonic current. Since the coupling coefficient of the coupled inductor is close to 1, the differential mode component is mainly suppressed by the coupled inductor L2, while the suppression effect of the coupled inductor L1 is close to 0. This current will form a loop current between the two converters. Differential mode current harmonic component i adm The expression is:

[0035]

[0036] The specific steps of step 4 are:

[0037] Step 4.1: Compare the filtering characteristics of the LCL and single trap branches. The trap branch has an additional stopband frequency compared to the LCL, making it more effective at attenuating specific high-frequency harmonics.

[0038] Step 4.2: Compare the filtering characteristics of the trap branch with series passive damping. The damping resistor does not affect the filtering characteristics at high and low frequencies, but weakens the two resonant peaks, resulting in a weaker trapping effect at specific frequencies.

[0039] Step 4.3: Compare the filtering characteristics of the trap filter with and without the parallel RC damping branch. The trap filter with the parallel RC damping branch not only maintains the attenuation of the specific high-frequency switching ripple, but also effectively prevents system oscillation.

[0040] Step 4.4: Propose an improved output filter circuit structure of the flexible voltage regulator.

[0041] The specific steps of step 5 are:

[0042] The coupled inductor constraints on the parallel side of the flexible voltage regulator are derived. L1 mainly limits the ripple of even-order switching harmonics, and L2 mainly limits the ripple of odd-order switching harmonics. According to the Fourier decomposition expressions (1)-(3) of the PWM output voltage and the common-mode and differential-mode current expressions (5)-(6), the ripple current is limited to 10% of the rated current, that is:

[0043]

[0044] The specific steps of step 6 are:

[0045] Step 6.1: Ignore the resistance of the output inductor and derive the single-phase mathematical expression of the parallel side of the flexible voltage regulator as follows:

[0046]

[0047] Where, L is the total output inductance; K is the switching coefficient; U dc is the DC side voltage; u s is the AC power supply voltage;

[0048] Step 6.2: Use the average voltage of 4U in long-term operation dc / 9, the constraints of the current tracking capability under the voltage average value model are obtained as follows:

[0049]

[0050] Among them, (di c / dt) max is the maximum rate of change of the compensation current;

[0051] Step 6.3: Derive the constraints for current tracking capability, namely:

[0052]

[0053] The specific steps of step 7 are:

[0054] In order to avoid the converter's power factor being too low, the fundamental reactive power absorbed by the filter capacitor cannot be greater than 5% of the system's rated active power. The total capacitance constraint on the parallel side of the flexible voltage regulator is:

[0055]

[0056] Among them, P0 is the rated active power of a single phase; E0 is the effective value of the grid phase voltage; ω0 is the grid fundamental angular frequency.

[0057] The Trap branch constraint conditions on the parallel side of the flexible voltage regulator in step 8 are:

[0058]

[0059] Among them, ω t is the switching harmonic angular frequency to be filtered out by each Trap branch, C t(x) is the capacitance value of the xth Trap branch, ω t(x) is the angular frequency of the switching harmonics to be filtered out by the xth Trap branch, t x is the splitting factor determined by the ratio of the frequencies to be filtered out.

[0060] An embodiment of the present application also provides an improved output filter circuit of a flexible voltage regulation device, including a flexible voltage regulation device, wherein the series-side converter of the main circuit of the flexible voltage regulation device is connected in series to the power grid via an LC filter and a series transformer, and the parallel-side converter is connected to the power grid via an improved filter and a parallel transformer. The series and parallel converters are connected via DC capacitors, and the output filter circuit also includes two pairs of coupled inductors and three notch branches with parallel RC damping branches.

[0061] Compared with the prior art, the beneficial effect of the present invention is that the present application can effectively eliminate the switching harmonic current generated by SPWM modulation and ensure the harmonic compensation performance of the flexible voltage regulation device. BRIEF DESCRIPTION OF THE DRAWINGS

[0062] In order to more clearly illustrate the technical solutions of the embodiments of the present application, the following is a brief introduction to the drawings required for use in the embodiments of the present application. It should be understood that the following drawings only show certain embodiments of the present application and therefore should not be regarded as limiting the scope. For ordinary technicians in this field, other relevant drawings can be obtained based on these drawings without creative work.

[0063] Figure 1 This is a topological diagram of the flexible voltage regulator;

[0064] Figure 2 This is the topology diagram of the converter on the parallel side of the flexible voltage regulator;

[0065] Figure 3 It is the single-phase equivalent circuit of the parallel side of the flexible voltage regulator;

[0066] Figure 4 Comparison of Bode diagrams of filters with different structures;

[0067] Figure 5 This is a structural diagram of the improved output filter circuit of the flexible voltage regulator;

[0068] Figure 6 is the Bode plot of the output filter;

[0069] Figure 7 It is the grid A-phase current before and after the flexible voltage regulator is put into operation and the A-phase current waveform output by the flexible voltage regulator. DETAILED DESCRIPTION

[0070] The technical solutions in the embodiments of the present application will be described below in conjunction with the accompanying drawings. It should be noted that similar reference numerals and letters represent similar items in the following drawings. Therefore, once an item is defined in one drawing, it does not need to be further defined or explained in subsequent drawings.

[0071] The terms "comprises," "comprising," or any other variations thereof are intended to encompass non-exclusive inclusion, such that a process, method, article, or apparatus that includes a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus. In the absence of further limitations, an element defined by the phrase "comprising a ..." does not preclude the presence of additional identical elements in the process, method, article, or apparatus that includes the element.

[0072] The terms "first," "second," etc. are only used to distinguish one entity or operation from another entity or operation, and are not to be understood as indicating or implying relative importance, nor are they to be understood as requiring or implying any actual relationship or order between these entities or operations.

[0073] The topology diagram of the flexible voltage regulator is as follows: Figure 1 As shown; the topology diagram of the flexible voltage regulator parallel side converter is as follows Figure 2 As shown; the single-phase equivalent circuit of the parallel side of the flexible voltage regulator is as follows Figure 3 As shown; the Bode diagrams of filters with different structures are compared. Figure 4 As shown; the improved output filter circuit structure diagram of the flexible voltage regulator is as follows Figure 5 The output filter Bode diagram is shown as Figure 6 As shown; the grid A phase current before and after the flexible voltage regulator is put into operation and the flexible voltage regulator output A phase current waveform are as follows Figure 7 shown.

[0074] Step 1 includes:

[0075] Step 1.1: Derive the output voltage expression of the converter 1 on the parallel side of the flexible voltage regulator, namely:

[0076] The topology diagram of the flexible voltage regulator is as follows: Figure 1 As shown in Figure 1, the series side converter is connected to the main circuit through a transformer, and the parallel side converter is directly connected to the main circuit. Figure 2 As shown in the figure, the interleaved parallel converter can eliminate the odd-order switching harmonic current through the carrier phase shift technology, so that the component harmonic current flows on the inverter side and does not enter the subsequent links, which to a certain extent reduces the design pressure of the subsequent filtering link.

[0077] According to the principle of bipolar SPWM control modulation technology (taking phase A as an example), assuming that the sine wave u MA is the modulation wave of phase A, and u MA =Msinω t t (M is the modulation ratio, 0≤M≤1); triangle wave u t is the carrier signal with an amplitude of 1 and an angular frequency of ωt The voltage of the midpoint of the bridge arm of phase A relative to the midpoint of the DC side is expressed as u iao1 Combined with Fourier transform, the output voltage u iao1 The Fourier series expression of is:

[0078]

[0079] Where M represents the voltage modulation ratio; ω0 represents the angular frequency of the modulation signal; ω t represents the angular frequency of the carrier, x m Indicates the harmonic order of the carrier, x n Indicates the harmonic order of the fundamental wave. J n (x) is the Bessel function of the first kind, and its expression is:

[0080]

[0081] Step 1.2: Derive the output voltage expression of the flexible voltage regulator parallel-side converter 2, namely:

[0082] For converter 2, its carrier phase is π degrees different from that of converter 1. Derive the voltage u of the midpoint of the A-phase bridge arm relative to the DC side midpoint. iao2 The expression is:

[0083]

[0084] Step 2 includes:

[0085] Step 2: Take phase A as an example, u iao1 is the A-phase PWM voltage output by converter 1, u iao2 The A-phase PWM voltage output by converter 2, the coupling coefficients of the coupling inductors L1 and L2 are k1 and k2 respectively, k1 and k2 are close to 1, and a single-phase equivalent circuit of the parallel side of the flexible voltage regulator is established, as shown in Figure 3 shown.

[0086] Step 3 includes:

[0087] Step 3.1: Output current i ao The switching harmonic current in the ao1 with i ao2 The switching harmonic current is determined by the sum of the switching harmonic currents in , which in turn is determined by the PWM harmonic voltage. Assuming that the two pairs of coupled inductors are balanced and equal, then i ao The harmonic current in iao1 with u iao2 The average value u iao-avg Determine, from formula (1) and formula (3) we can get:

[0088]

[0089] It can be seen from formula (4) that a carrier phase shift of π can eliminate all odd harmonics of the switching frequency and their sideband harmonics, but will not affect the amplitude and phase of the even harmonics of the switching frequency, their sideband harmonics, and the fundamental wave.

[0090] Step 3.2: Determine the current output by the parallel side of the flexible voltage regulator. There are three main types:

[0091] 1) Compensation current i that needs to be injected into the grid a0 : Unaffected by the carrier phase, the fundamental currents output by the two conversion bridges are equal in phase. Since the current frequency is much lower than the switching frequency, the coupling inductor and the high-order filter branch have little effect on the fundamental current.

[0092] 2) Common mode current harmonic component i acm :This current is the even harmonic current of the switching frequency and its sideband harmonic current. Since the coupling coefficient of the coupled inductor is close to 1, the common mode component is mainly suppressed by the coupled inductor L1, while the suppression effect of the coupled inductor L2 is close to 0. Common mode current harmonic component i acm The expression is:

[0093]

[0094] 3) Differential mode current harmonic component i adm :This current is the odd harmonic current of the switching frequency and its sideband harmonic current. Since the coupling coefficient of the coupled inductor is close to 1, the differential mode component is mainly suppressed by the coupled inductor L2, while the suppression effect of the coupled inductor L1 is close to 0. This current will form a loop current between the two converters. Differential mode current harmonic component i adm The expression is:

[0095]

[0096] Step 4 includes:

[0097] Step 4.1: Compare the filtering characteristics of LCL and single Trap branch. The Trap branch has an additional stopband frequency compared to LCL, which is more effective in attenuating specific high-frequency harmonics. Figure 4 As shown in (a).

[0098] Step 4.2: Compare the filtering characteristics of the Trap branch with series passive damping. That is, the damping resistor will not affect its filtering characteristics in the high-frequency band and the low-frequency band, but will weaken the two resonance peaks, resulting in a weaker trap effect at a specific frequency, such as Figure 4 (b) shown.

[0099] Step 4.3: Compare the filtering characteristics of the Trap branch with and without the parallel RC damping branch. That is, the Trap filter with the parallel RC damping branch not only does not lose the attenuation of the specific high-frequency switching ripple, but also effectively prevents system oscillation, such as Figure 4 (c) shown.

[0100] Step 4.4: Propose an improved output filter circuit structure of the flexible voltage regulator, such as Figure 5 shown.

[0101] Step 5 includes:

[0102] Step 5: Derive the coupled inductor constraints on the parallel side of the flexible voltage regulator. L1 mainly limits the ripple of even-order switching harmonics, and L2 mainly limits the ripple of odd-order switching harmonics. According to the Fourier decomposition expressions (1)-(3) of the PWM output voltage and the common-mode and differential-mode current expressions (5)-(6), the ripple current limit is specified to be 10% of the rated current, that is:

[0103]

[0104] Step 6 includes:

[0105] Step 6.1: Ignore the resistance of the output inductor and derive the single-phase mathematical expression of the parallel side of the flexible voltage regulator as follows:

[0106]

[0107] Where, L is the total output inductance; K is the switching coefficient; U dc is the DC side voltage; u s is the AC power supply voltage.

[0108] Step 6.2: Use the average voltage of 4U in long-term operation dc By replacing / 9, we can get the constraints of current tracking capability under the voltage average value model as follows:

[0109]

[0110] Among them, (di c / dt) max is the maximum rate of change of the compensation current.

[0111] Step 6.3: Derive the constraints for current tracking capability, namely:

[0112]

[0113] Step 7 includes:

[0114] Step 7: To prevent the converter's power factor from being too low, the fundamental reactive power absorbed by the filter capacitor must not exceed 5% of the system's rated active power. The total capacitance constraint on the parallel side of the flexible voltage regulator is:

[0115]

[0116] Among them, P0 is the rated active power of a single phase; E0 is the effective value of the grid phase voltage; ω0 is the grid fundamental angular frequency.

[0117] Step 8 includes:

[0118] Step 8: The Trap branch constraints on the parallel side of the flexible voltage regulator are:

[0119]

[0120] Among them, ω t is the switching harmonic angular frequency to be filtered out by each Trap branch, C t(x) is the capacitance value of the xth Trap branch, ω t(x) is the angular frequency of the switching harmonics to be filtered out by the xth Trap branch, t x is the splitting factor determined by the ratio of the frequencies to be filtered out.

[0121] Example:

[0122] To verify the effectiveness of the designed filter circuit, a prototype flexible voltage regulator with a coupled inductor and a trap branch filter was built. To simulate the grid current waveform under harmonic pollution, a nonlinear load, a three-phase bridge diode rectifier, was used on the load side. The system parameters are shown in Table 1.

[0123] Table 1 System parameters

[0124]

[0125] The improved filter circuit parameters of the flexible voltage regulator are as follows:

[0126]

[0127] The waveforms of the grid phase A current and the flexible voltage regulator output phase A current before and after the flexible voltage regulator is put into operation are as follows: Figure 7 As shown. a_grid is the grid phase A current, i a_outThe flexible voltage regulator outputs phase A current. Before the flexible voltage regulator is activated, the grid current THD content is 21.72%. After the flexible voltage regulator is activated, the grid current THD content drops to 1.77%. This indicates that the waveform quality is significantly improved before and after compensation. The flexible voltage regulator effectively compensates for the grid current, verifying the performance of the designed improved filter.

[0128] The above description is merely an embodiment of the present application and is not intended to limit the scope of protection of the present application. For those skilled in the art, various modifications and variations of the present application are possible. Any modifications, equivalent substitutions, improvements, etc. made within the spirit and principles of the present application shall be included in the scope of protection of the present application.

Claims

1. A method for improving an output filter circuit of a flexible voltage regulator, characterized in that: The specific steps include: Step 1: Derive the output voltage expression of the parallel side of the flexible voltage regulator; Step 2: Establish a single-phase equivalent circuit on the parallel side of the flexible voltage regulator; Step 3: Derive the output current expression of the parallel side of the flexible voltage regulator; Step 4: Analyze the circuit characteristics of different trap filters; Step 5: Propose an improved output filter circuit structure of the flexible voltage regulator; Step 5: Derive the coupled inductance constraint on the parallel side of the flexible voltage regulator; Step 6: Derive the current tracking capability constraints of the parallel side of the flexible voltage regulator; Step 7: Derive the total capacitance constraint of the parallel side of the flexible voltage regulator; Step 8: Derive the Trap branch constraints on the parallel side of the flexible voltage regulator.

2. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: Step 1.1: Derive the output voltage expression of the converter 1 on the parallel side of the flexible voltage regulator, namely: According to the principle of bipolar SPWM control modulation technology, assuming that the sine wave u MA is the modulation wave of phase A, and u MA =Msinω t t, M is the modulation ratio, 0≤M≤1; triangle wave u t is the carrier signal with an amplitude of 1 and an angular frequency of ω t The voltage of the midpoint of the bridge arm of phase A relative to the midpoint of the DC side is expressed as u iao1 , combined with Fourier transform, u iao1 The Fourier series expression of is: Where M represents the voltage modulation ratio; ω0 represents the angular frequency of the modulation signal; ω t represents the angular frequency of the carrier, x m Indicates the harmonic order of the carrier, x n Indicates the harmonic order of the fundamental wave, J n (x) is the Bessel function of the first kind, and its expression is: Step 1.2: Derive the output voltage expression of the flexible voltage regulator parallel-side converter 2, namely: For converter 2, its carrier phase is π degrees different from that of converter 1. The voltage u of the midpoint of the A-phase bridge arm relative to the DC side midpoint is derived. iao2 The expression is:

3. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The specific steps of step 2 are: Taking phase A as an example, u iao1 is the A-phase PWM voltage output by converter 1, u iao2 The A-phase PWM voltage output by converter 2 is used. The coupling coefficients of the coupled inductors L1 and L2 are k1 and k2 respectively. k1 and k2 are close to 1. A single-phase equivalent circuit of the parallel side of the flexible voltage regulator is established.

4. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The specific steps of step 3 are: Step 3.1: Output current i ao The switching harmonic current in the ao1 with i ao2 The switching harmonic current is determined by the sum of the switching harmonic currents in the PWM harmonic voltage. Assuming that the two pairs of coupled inductors are balanced and equal, i ao The harmonic current in iao1 with u iao2 The average value u iao-avg Determine, from formula (1) and formula (3) we can get: It can be seen from formula (4) that the carrier phase shift of π angle can eliminate all odd harmonics of the switching frequency and their sideband harmonics, but will not affect the amplitude and phase of the even harmonics of the switching frequency and their sideband harmonics as well as the fundamental wave; Step 3.2: Determine the current output by the parallel side of the flexible voltage regulator. There are three main types: 1) Compensation current i that needs to be injected into the grid a0 : Unaffected by the carrier phase, the fundamental currents output by the two conversion bridges are equal in phase. Since the current frequency is much lower than the switching frequency, the coupling inductor and the high-order filter branch have little effect on the fundamental current. 2) Common mode current harmonic component i acm :This current is the even harmonic current of the switching frequency and its sideband harmonic current. Since the coupling coefficient of the coupled inductor is close to 1, the common mode component is mainly suppressed by the coupled inductor L1, while the suppression effect of the coupled inductor L2 is close to 0. The common mode current harmonic component i acm The expression is: 3) Differential mode current harmonic component i adm :This current is the odd harmonic current of the switching frequency and its sideband harmonic current. Since the coupling coefficient of the coupled inductor is close to 1, the differential mode component is mainly suppressed by the coupled inductor L2, while the suppression effect of the coupled inductor L1 is close to 0. This current will form a loop current between the two converters. Differential mode current harmonic component i adm The expression is:

5. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The specific steps of step 4 are: Step 4.1: Compare the filtering characteristics of the LCL and single trap branches. The trap branch has an additional stopband frequency compared to the LCL, making it more effective at attenuating specific high-frequency harmonics. Step 4.2: Compare the filtering characteristics of the trap branch with series passive damping. The damping resistor does not affect the filtering characteristics at high and low frequencies, but weakens the two resonant peaks, resulting in a weaker trapping effect at specific frequencies. Step 4.3: Compare the filtering characteristics of the trap filter with and without the parallel RC damping branch. The trap filter with the parallel RC damping branch not only maintains the attenuation of the specific high-frequency switching ripple, but also effectively prevents system oscillation. Step 4.4: Propose an improved output filter circuit structure of the flexible voltage regulator.

6. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The specific steps of step 5 are: The coupled inductor constraints on the parallel side of the flexible voltage regulator are derived. L1 mainly limits the ripple of even-order switching harmonics, and L2 mainly limits the ripple of odd-order switching harmonics. According to the Fourier decomposition expressions (1)-(3) of the PWM output voltage and the common-mode and differential-mode current expressions (5)-(6), the ripple current is limited to 10% of the rated current, that is:

7. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The specific steps of step 6 are: Step 6.1: Ignore the resistance of the output inductor and derive the single-phase mathematical expression of the parallel side of the flexible voltage regulator as follows: Where, L is the total output inductance; K is the switching coefficient; U dc is the DC side voltage; u s is the AC power supply voltage; Step 6.2: Use the average voltage of 4U in long-term operation dc / 9, the constraints of the current tracking capability under the voltage average value model are obtained as follows: Among them, (di c / dt) max is the maximum rate of change of the compensation current; Step 6.3: Derive the constraints for current tracking capability, namely:

8. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The specific steps of step 7 are: In order to avoid the converter's power factor being too low, the fundamental reactive power absorbed by the filter capacitor cannot be greater than 5% of the system's rated active power. The total capacitance constraint on the parallel side of the flexible voltage regulator is: Among them, P0 is the rated active power of a single phase; E0 is the effective value of the grid phase voltage; ω0 is the grid fundamental angular frequency.

9. The method for improving the output filter circuit of a flexible voltage regulator according to claim 1, characterized in that: The Trap branch constraint conditions on the parallel side of the flexible voltage regulator in step 8 are: Among them, ω t is the switching harmonic angular frequency to be filtered out by each Trap branch, C t(x) is the capacitance value of the xth Trap branch, ω t(x) is the angular frequency of the switching harmonics to be filtered out by the xth Trap branch, t x is the splitting factor determined by the ratio of the frequencies to be filtered out.

10. An improved output filter circuit of a flexible voltage regulator, characterized in that: It includes a flexible voltage regulator. The series-side converter of the main circuit of the flexible voltage regulator is connected in series to the power grid through an LC filter and a series transformer. The parallel-side converter is connected to the power grid through an improved filter and a parallel transformer. The series and parallel converters are connected through DC capacitors. The output filter circuit also includes two pairs of coupled inductors and three notch branches with parallel RC damping branches.

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