A method and system for common-mode interference escape suppression of a grid-connected converter

Abstract

The application discloses a grid-connected converter common-mode interference escape inhibition method and system, and belongs to the technical field of electromagnetic interference inhibition, which comprises the following steps: increasing the impedance of the strong-weak electric coupling path, or / and reducing the impedance of the common-mode interference backflow path. The application takes into account that the strong-weak electric coupling path is generated between the strong electric power side and the control system due to the existence of the primary and secondary side parasitic capacitances of the current transformer and the auxiliary power supply, so that the common-mode interference bypasses the common-mode inductance of the EMI filter to form escape, thereby seriously reducing the inhibition effect of the EMI filter; the impedance of the strong-weak electric coupling path is increased to directly reduce the escaped common-mode interference current, or / and the impedance of the common-mode interference backflow path is reduced to increase the backflow common-mode interference current and indirectly reduce the escaped common-mode interference current, so as to inhibit the common-mode interference escape. The application can achieve a good inhibition effect on the interference escape, thereby restoring the inhibition performance of the EMI filter without the need of additionally increasing the size of the filter inductance and capacitance.

Description

Technical Field

[0001] This invention belongs to the field of electromagnetic interference suppression technology, and more specifically, relates to a method and system for suppressing common-mode interference escape in grid-connected converters. Background Technology

[0002] With the increasing proportion of distributed renewable energy sources such as photovoltaics and wind power, the application of grid-connected converters, which are based on power semiconductor devices, in the power grid is gradually increasing. Although the application of grid-connected converters enables renewable energy power generation systems to be stably and reliably connected to the grid, the voltage and current changes generated during the high-speed switching of their power semiconductor devices also introduce a large amount of electromagnetic interference (EMI) into the power grid. EMI not only affects the reliability of the equipment itself, but also injects into the power grid through the equipment's grid connection, thereby affecting the normal operation of the entire power grid and its connected sensitive equipment. Therefore, in order to avoid the impact of EMI on the stable operation of equipment and the power grid, it is necessary to suppress the interference generated by the grid-connected converter during operation. EMI filters are commonly used to suppress common-mode interference between power equipment. In existing technologies, EMI filters are generally placed on the converter side of the grid-connected converter to suppress the impact of common-mode interference on the power grid. However, due to the special structure of the grid-connected converter, it has been found that the suppression effect of adding EMI filters on common-mode interference is not significant, and in some cases, the EMI filters are completely ineffective in suppressing common-mode interference. Summary of the Invention

[0003] In view of the shortcomings of the existing technology and the need for improvement, the present invention provides a method and system for suppressing common-mode interference escape in grid-connected converters, the purpose of which is to suppress common-mode interference escape and improve the common-mode interference suppression effect.

[0004] To achieve the above objectives, according to a first aspect of the present invention, a method for suppressing common-mode interference escape in a grid-connected converter is provided, comprising:

[0005] A common-mode interference model for a grid-connected converter is constructed. At the grid connection point, the model includes: the converter side, the grid side, and the load side. The control system of the grid-connected converter draws power from the high-voltage DC bus through an auxiliary power supply and collects load current information through a current transformer. The primary side of the current transformer is the load side. An EMI filter is installed on the converter side.

[0006] Increase the impedance of the strong and weak electrical coupling path, and / or reduce the impedance of the common-mode interference return path;

[0007] The strong-weak coupling path refers to the common-mode interference coupling path between the high-power side and the control system caused by the parasitic capacitances of the primary and secondary sides of the current transformer and the auxiliary power supply. The common-mode interference return path is the common-mode interference current generated by the common-mode interference source flowing through the ground capacitor in the converter side or the ground Y capacitor of the EMI filter, and then flowing through the ground capacitor C of the high-voltage DC bus. N The path then flows back to the common-mode interference source; the high-power side includes the load side and the high-voltage DC bus.

[0008] Furthermore, the increase in the impedance of the strong and weak electrical coupling path includes any one or more of the following methods:

[0009] Method 1: Connect a first common-mode inductor in series on the high-voltage input side or the low-voltage output side of the auxiliary power supply;

[0010] Method 2: Connect a second common-mode inductor in series with the secondary cable of the current transformer;

[0011] Method 3: Remove the Y capacitor from the auxiliary power supply.

[0012] Furthermore, it also includes:

[0013] Increase the value of the first common-mode inductance;

[0014] Or / and, increase the value of the second common-mode inductance.

[0015] Furthermore, in the first method, the first common-mode inductor is connected in series on the high-voltage input side of the auxiliary power supply.

[0016] Furthermore, the reduction of the impedance of the common-mode interference return path includes: connecting a Y-capacitor C to ground in parallel on the high-voltage DC bus. Y .

[0017] Furthermore, the ground-connected Y capacitor C Y The range of values ​​for is:

[0018] C N <C Y <3C N

[0019] Among them, C N This refers to the capacitance to ground of the high-voltage DC bus.

[0020] Furthermore, it also includes:

[0021] Reduce the Y capacitance to ground in the EMI filter y The EMI filter is a CL-type EMI filter;

[0022] Alternatively, increase the common-mode inductance of the EMI filter, wherein the EMI filter is an L-type EMI filter.

[0023] According to a second aspect of the present invention, a common-mode interference escape suppression system for a grid-connected converter is provided, for performing the method according to any one of the first aspects, the system comprising:

[0024] The model building module is used to build a common-mode interference model for a grid-connected converter. At the grid connection point, the model includes: the converter side, the grid side, and the load side. The control system of the grid-connected converter draws power from the high-voltage DC bus through an auxiliary power supply and collects load current information through a current transformer. The primary side of the current transformer is the load side. An EMI filter is installed on the converter side.

[0025] The strong-weak coupling path refers to the common-mode interference coupling path between the high-power side and the control system caused by the parasitic capacitances of the primary and secondary sides of the current transformer and the auxiliary power supply. The common-mode interference return path is the common-mode interference current generated by the common-mode interference source flowing through the ground capacitor in the converter side or the ground Y capacitor of the EMI filter, and then flowing through the ground capacitor C of the high-voltage DC bus. N The path then flows back to the common-mode interference source; the high-power side includes the load side and the high-voltage DC bus.

[0026] Furthermore, in the common-mode interference escape suppression module, increasing the impedance of strong and weak electrical coupling paths includes one or more of the following methods:

[0027] Method 1: Connect a first common-mode inductor in series on the high-voltage input side or the low-voltage output side of the auxiliary power supply;

[0028] Method 2: Connect a second common-mode inductor in series with the secondary cable of the current transformer;

[0029] Method 3: Remove the Y capacitor from the auxiliary power supply;

[0030] The impedance reduction of the common-mode interference return path includes: connecting a Y-capacitor C to ground in parallel on the high-voltage DC bus. Y .

[0031] According to a third aspect of the invention, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the method as described in any of the first aspects.

[0032] In summary, the above-described technical solutions conceived in this invention can achieve the following beneficial effects:

[0033] (1) The common-mode interference escape suppression method of the grid-connected converter of the present invention takes into account the strong and weak coupling paths generated between the high power side and the control system due to the presence of parasitic capacitances on the primary and secondary sides of the current transformer and the auxiliary power supply. The common-mode interference will return to the interference source through the strong and weak coupling paths, causing the common-mode interference to bypass the common-mode inductance of the EMI filter and escape. This leads to the problem of interference escape of the EMI filter installed on the converter side, which seriously reduces the suppression effect of the EMI filter. Since the common-mode interference current flows into the common-mode interference return path and into the strong and weak coupling path through the power grid after passing through the Y capacitor of the EMI filter or the ground capacitor on the converter side, the escape common-mode interference current is directly reduced by increasing the impedance of the strong and weak coupling path, or / and the return common-mode interference current is increased by decreasing the impedance of the common-mode interference return path, thereby indirectly reducing the escape common-mode interference current and suppressing the escape of common-mode interference. This achieves the suppression of the overall common-mode interference. Experiments have also shown that this method can improve the suppression effect of the overall common-mode interference.

[0034] When a comprehensive common-mode interference escape suppression scheme is formed by simultaneously increasing the impedance of the strong and weak electrical coupling paths and decreasing the impedance of the common-mode interference return path, the overall common-mode interference suppression effect can be further improved, allowing the EMI filter to restore its suppression performance. At the same time, it is not necessary to increase the size of the filter inductor and capacitor.

[0035] (2) Furthermore, adding a common-mode inductor to the high-voltage input side of the auxiliary power supply, adding a common-mode inductor to the secondary cable of the current transformer, and removing the Y capacitor of the auxiliary power supply itself can increase the impedance of the strong and weak electrical coupling path, which can directly suppress interference escape.

[0036] Adding a Y capacitor to ground on the high-voltage DC bus of the converter can reduce the impedance of the common-mode interference return path, increase the common-mode interference return current, and indirectly suppress interference escape.

[0037] The above method is simple to operate, and the added common-mode inductor and Y capacitor occupy little space, while effectively suppressing interference escape.

[0038] (3) Preferably, since the current on the input side of the auxiliary power supply is smaller than that on the output side, the volume and cost of the first common mode inductor can be reduced by connecting the first common mode inductor in series on the high voltage input side of the auxiliary power supply.

[0039] (4) Preferably, the larger the value of the first common-mode inductance, or / the larger the value of the second common-mode inductance, the better the common-mode interference suppression effect.

[0040] (5) Preferably, the parallel-connected Y capacitor C to ground Y Within the set range, it can achieve a good effect in suppressing interference escape.

[0041] (6) Furthermore, without considering cost, reduce the Y-capacitance to ground in the CL-type EMI filter. y Alternatively, increasing the common-mode inductance in the L-type EMI filter can improve the interference suppression effect to some extent.

[0042] In summary, the method of this invention simultaneously considers the electromagnetic interference path on the high-power side of the converter and the influence of strong and weak coupling paths on common-mode interference conduction. It also verifies the correctness of the strong and weak coupling paths discovered in this invention through experiments, and provides ways to increase the impedance of the strong and weak coupling paths and / or decrease the impedance of the common-mode interference return path, thus solving the problem of common-mode interference escape caused by the Y capacitor of the EMI filter, other capacitors to ground, and the strong and weak coupling paths. Attached Figure Description

[0043] Figure 1 This is a structural diagram of the grid-connected converter in an embodiment of the present invention.

[0044] Figure 2 The common-mode interference model for grid-connected converters, which includes strong and weak electrical coupling paths, is constructed for this invention.

[0045] Figure 3 This is a simplified model of the common-mode interference model of the grid-connected converter that includes strong and weak electrical coupling paths, constructed in this invention.

[0046] Figure 4 This is a comparison chart of measured and predicted interference on the weak current side of the grid-connected converter in an embodiment of the present invention.

[0047] Figure 5 This is a further simplified model of the common-mode interference model of the grid-connected converter used for interference escape analysis in the embodiments of the present invention.

[0048] Figure 6 This is a comparison diagram of interference escape phenomena on the converter side, grid side, and low-voltage side in an embodiment of the present invention.

[0049] Figure 7 This is a simplified common-mode interference model for grid-connected converters used for interference escape calculation in this embodiment of the invention.

[0050] Figure 8 This is a diagram showing the variation of the common-mode interference current on the power grid side with the Y capacitance of the CL filter in an embodiment of the present invention.

[0051] Figure 9 This diagram illustrates the interference escape suppression when a Y capacitor is added to the high-voltage DC bus in an embodiment of the present invention.

[0052] Figure 10 This is the common-mode EMI conduction path of the flyback power supply that operates independently after adding a Y capacitor in this embodiment of the invention.

[0053] Figure 11 This diagram illustrates the interference escape suppression when strong / weak electrical coupling or weak electrical path impedance is added in an embodiment of the present invention.

[0054] Figure 12 This is a diagram showing the common-mode interference suppression effect after using the common-mode interference escape comprehensive suppression scheme in an embodiment of the present invention.

[0055] Figure 13 This is a schematic diagram of the common-mode interference escape suppression method for grid-connected converters according to the present invention. Detailed Implementation

[0056] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention. Furthermore, the technical features involved in the various embodiments of this invention described below can be combined with each other as long as they do not conflict with each other.

[0057] In this invention, the terms "first," "second," etc., used in the invention and accompanying drawings are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence.

[0058] Figure 1 The grid-connected converter structure shown in this embodiment mainly includes: a converter side, a high-voltage DC side, and a low-voltage side. The high-voltage DC side includes a high-voltage DC bus, and the low-voltage side includes the grid-connected converter's control system. The control system draws power from the high-voltage DC bus of the power converter via an auxiliary power supply, which employs a flyback power supply structure. Simultaneously, the control system collects load current information through a current transformer, collects the current output of the power converter through a current Hall effect sensor, collects the high-voltage DC bus voltage information through a DC voltage Hall effect sensor, and collects the grid voltage magnitude and phase information through an AC voltage Hall effect sensor. Ultimately, it controls the switching action of the power converter to control energy flow. The common-mode interference escape suppression method for the grid-connected converter of this invention mainly includes:

[0059] A common-mode interference model for a grid-connected converter, including strong and weak current coupling paths, is constructed. At the grid connection point, the model is divided into three sides: the converter side, the grid side, and the load side. The primary side of the current transformer represents the load side, and the secondary side represents the weak current side. The primary side of the auxiliary power supply represents the high-voltage DC side (high-voltage input side), and the secondary side represents the weak current side (low-voltage output side). Figure 2 As shown;

[0060] Increase the impedance of the strong and weak electrical coupling path, and / or reduce the impedance of the common-mode interference return path;

[0061] Among them, the strong-weak coupling path is the common-mode interference coupling path between the high-power side and the control system caused by the parasitic capacitances of the primary and secondary sides of the current transformer and auxiliary power supply; the common-mode interference return path is that the common-mode interference current generated by the common-mode interference source flows through the ground capacitor in the converter side or the ground Y capacitor of the EMI filter, and then flows through the ground capacitor C of the high-voltage DC bus. N The path then flows back to the common-mode interference source; where the high-power side includes the load side and the high-voltage DC side.

[0062] Specifically, the three-phase lower bridge arm voltage in the grid-connected converter serves as a common-mode interference source. Figure 2 In the middle, common-mode interference originates from common-mode interference source U A U B U C Starting from the grid connection point, the current is split. One part flows through the grid side into the PE (system ground), and then returns to the common-mode interference source through the ground capacitor in the converter side, forming high-voltage side common-mode interference. The other part enters the load side, flows into the control system through the parasitic capacitance of the primary and secondary sides of the current transformer, and then flows into the high-voltage DC side through the parasitic capacitance of the auxiliary power transformer and its Y capacitor in the converter side, and finally returns to the common-mode interference source, forming low-voltage side common-mode interference.

[0063] Among them, the main conduction path of common-mode interference on the weak current side is the strong-weak coupling path generated by the parasitic capacitances of the primary and secondary sides of the current transformer and the auxiliary power supply. Specifically, the strong-weak coupling path includes the common-mode interference coupling path provided by the parasitic capacitances on the primary and secondary sides of the current transformer to the load side and the weak current side of the control system, and the common-mode interference coupling path provided by the parasitic capacitances on the primary and secondary sides of the auxiliary power supply transformer to the high-voltage DC side of the grid-connected converter and the weak current side of the control system. These two paths are the main causes of strong-weak coupling of common-mode interference in the grid-connected converter.

[0064] Figure 2 In the middle, C C C O C E C1 and C2 represent the capacitances to ground of each power terminal of the IGBT in the grid-connected converter; C1 and C2 represent the capacitances to ground of the filter inductors L1 and L2 in the grid-connected converter, respectively. t C represents the parasitic capacitance between the primary and secondary sides of a current transformer. f C represents the parasitic capacitance between the primary and secondary sides of the auxiliary power transformer. fy The Y capacitor, L, represents the connection between the primary side PGND and the secondary side SGND of the auxiliary power supply. ac L represents the parasitic inductance of the AC side cable. dc L represents the parasitic inductance of the DC-side cable. s L represents the parasitic inductance of the signal cable from the current transformer to the control system. p This indicates the parasitic inductance of the power supply cable from the auxiliary power output to the control system.

[0065] Specifically, to further illustrate the correctness of the strong-weak electrical coupling path discovered in this invention, the following will be... Figure 2 The common-mode interference model of the grid-connected converter, which includes strong and weak electrical coupling paths, is simplified as follows: Figure 3 As shown, for common-mode interference source U A U B U C Since the voltages of the upper and lower IGBTs in the grid-connected converter are complementary, only the three lower IGBT voltages are used as common-mode interference sources to reflect all voltage fluctuations in the system. The upper IGBT is effectively open-circuited at this time, allowing the three common-mode interference sources to be combined into an equivalent common-mode interference source. The voltage U of this equivalent common-mode interference source is... cm for:

[0066]

[0067] Among them, U A U B U C These represent the voltages of the midpoints of the bridge arms of phases A, B, and C relative to the negative busbar, respectively. Figure 3 In the middle, C N This indicates the original ground capacitance of the high-voltage DC bus.

[0068] In this embodiment of the invention, an impedance analyzer is used to measure the impedance amplitude-frequency characteristics of each component on the converter side. Since the supporting capacitor value of the high voltage DC bus of the grid-connected converter is large, it can be regarded as a short circuit in the conducted interference frequency band. Furthermore, the three-phase impedance on the AC side is symmetrical. Therefore, the positive and negative buses on the high voltage DC side and the three-phase output cables on the AC side can be combined. At this time, the impedance of each component on the DC side of the converter becomes 1 / 2 of the original impedance, and the impedance on the AC side of the converter becomes 1 / 3 of the original impedance.

[0069] Measuring the three-phase lower bridge arm voltage U in the actual device A U B U C The equivalent common-mode interference source U can be obtained through formula (1). cm In CST, create such Figure 3 The simplified model shown imports an equivalent common-mode interference source U. cm It can simulate common-mode interference on the weak current side. The measured and simulated waveforms of the common-mode interference current on the weak current side in the frequency domain are as follows: Figure 4 As shown in the figure, the blue line represents the measured waveform, and the red line represents the simulated waveform. By comparison, it can be found that within the 10kHz-10MHz range, the simulated waveform of the common-mode interference on the weak current side is basically consistent with the measured waveform. This indicates that the equivalent model can accurately simulate the common-mode interference of grid-connected converters containing strong-weak coupling paths within the 10kHz-10MHz range. That is, the strong-weak coupling path discovered in this invention is correct.

[0070] Based on the strong and weak electrical coupling paths obtained from the above analysis, an EMI filter is installed on the converter side of the grid-connected converter to analyze the interference escape path of the EMI filter. In this embodiment of the invention, the EMI filter includes a common-mode inductor L. m The CL-type EMI filter is formed by the Y capacitor to ground. In other embodiments, the EMI filter can also be of various types such as L-type and CLC-type.

[0071] right Figure 3 The simplified module shown is further simplified to obtain the following: Figure 5 The simplified circuit shown is used to analyze common-mode interference escape:

[0072] The specific simplification process includes:

[0073] 1) Connect the three capacitors C to ground on the converter side. O C1 and C2 are considered part of the Y-capacitance to ground of the EMI filter; in other embodiments, if an L-type EMI filter is used, although the EMI filter does not have a Y-capacitance to ground, the ground capacitance C on the converter side is still present. O C1 and C2 will cause common-mode interference, but the capacitance to ground C can still be reduced. O C1 and C2 are equivalent to "Y capacitors to ground", and the circuit diagram remains essentially unchanged after the equivalent circuit.

[0074] 2) Parasitic inductance L of the cables on the converter side and the low-voltage side dc L ac L s and L p It only affects the resonant point at high frequencies and does not affect the overall trend, so it can be ignored;

[0075] 3) The impedance of the combined grid-connected converter filter inductor L1 and filter inductor L2 is Z. L The parasitic capacitance C of the primary and secondary sides of the combined current transformer t Parasitic capacitance C of the primary and secondary sides of the auxiliary power transformer f For the weak current side, the strong-weak coupling capacitor C w .

[0076] Based on the above simplification, in this embodiment of the invention, a CL-type EMI filter is provided on the converter side of the grid-connected converter, such as... Figure 5 As shown, after adding a CL-type EMI filter, the common-mode interference current I generated by the common-mode interference source... y It will pass through the EMI filter to the Y capacitor C to ground y (When the EMI filter does not have a Y capacitor to ground, the common-mode interference current I) y Through the converter side, the capacitor to ground C OC1 and C2) flow into PE, at this time, the common-mode interference current I y It is divided into two parts: the return common-mode interference current I1 and the escape common-mode interference current I2; the return common-mode interference current I1 is transmitted through the high-voltage DC bus to ground capacitance C. N The current flows back to the interference source, forming a circulating current within the system without affecting the power grid; this path is denoted as the common-mode interference return path. The escaped common-mode interference current I2 flows to the power grid side and then through the strong-weak coupling capacitor C. w The current flows back to the interference source; obviously, the escaped common-mode interference current I2 will affect the power grid. Strong and weak current coupling capacitor C w And the Y-capacitor C to ground of the EMI filter y (Or the equivalent capacitance to ground Y of the EMI filter) creates a path for common-mode interference through the power grid, severely weakening the common-mode inductance L of the EMI filter. m The suppression effect causes interference escape problems. Even when using only an L-type EMI filter, the internal parasitic capacitance to ground in the circuit, such as C... O C1 and C2 can also form a path for interference escape. That is, when the EMI filter does not have a Y capacitor to ground, the common-mode interference current I... y Through the converter side, the capacitor to ground C O C1 and C2 flow into PE, which also constitutes a path for interference to escape, thereby reducing the performance of the EMI filter.

[0077] In this embodiment of the invention, the common-mode interference currents on the converter side, grid side, and load side are measured respectively, and the waveforms of each interference current are obtained as follows: Figure 6 As shown, it can be observed that in the 100kHz-7MHz frequency band, the common-mode interference current on the converter side is suppressed to a low level, but the common-mode interference current on the grid side and the load side remains relatively large and is basically the same in magnitude. This indicates that while the interference on the converter side is effectively suppressed due to the EMI filter being installed there, the interference can escape through the filter's Y-capacitor, the grid side, and strong / weak coupling paths, resulting in ineffective suppression of interference on the grid side and the load side. Therefore, directly adding an EMI filter on the converter side has little effect on suppressing common-mode interference. Even increasing the size of the filter inductor or capacitor does not improve the filter's suppression effect, and in some cases, the EMI filter may even be completely ineffective in suppressing common-mode interference.

[0078] based on Figure 5 The simplified model shown has a common-mode inductance L m Since the impedance is relatively large, for ease of analysis, it is treated as an open circuit, and the following is obtained: Figure 7 The simplified circuit shown is for ease of calculation. According to Kirchhoff's law (KCL), we can obtain:

[0079]

[0080] Among them, V lisn Z represents the common-mode interference voltage on the grid side. w C represents the strong and weak coupling capacitor. w The impedance, Z N C represents the capacitance to ground of the high-voltage DC bus. N The impedance, where, C N C represents the capacitance to ground of the high-voltage DC bus. w I1 represents the return common-mode interference current, and I2 represents the escape common-mode interference current.

[0081] From formula (2), we can obtain:

[0082]

[0083]

[0084] From formula (4), it can be seen that the Y capacitance C of the CL type EMI filter to ground is... y The larger the voltage, the greater the common-mode interference voltage V on the grid side. lisn The larger the capacitance to ground (Y), the more obvious the interference escape phenomenon becomes, and the weaker the interference suppression capability of the filter becomes.

[0085] In this embodiment of the invention, a CL-type EMI filter is installed in the actual device, maintaining the common-mode inductance L. m =5mH, respectively set Y capacitor C to ground y The parameters are 0nF, 0.1nF, 1nF, and 10nF. An EMI receiver is used to measure the common-mode interference current on the power grid side under different conditions. The measurement results are as follows: Figure 8 As shown, it can be seen that only the common-mode inductor, i.e., C... y When the capacitance is 0nF, the suppression effect on interference is relatively the best. However, when the Y-capacitor to ground is increased, the common-mode interference on the grid side increases, and the suppression effect of the EMI filter decreases. Furthermore, as C increases... y As the voltage increases, grid-side interference continues to rise, and the EMI filter's suppression effect weakens even more significantly, especially when C... y When the capacitance is 10nF, the grid-side interference is almost identical to that without a CL filter, meaning the filter essentially loses its suppression effect. This also proves that the Y capacitance to ground of the EMI filter causes common-mode interference escape, reducing or completely eliminating the EMI filter's suppression effect. Furthermore, the larger the Y capacitance, the more pronounced the escape phenomenon.

[0086] The above analysis also shows that reducing the Y capacitance to ground C in a CL-type EMI filter yIt can improve the interference suppression effect to a certain extent; when the Y capacitor C to ground y When the common-mode inductance is 0, the CL-type EMI filter becomes an L-type first-order EMI filter. Increasing the common-mode inductance can improve the common-mode interference suppression effect to some extent, but increasing the common-mode inductance will correspondingly increase the size and cost of the filter. Meanwhile, regardless of whether the Y-capacitor to ground is reduced or eliminated, the ground capacitance (such as C) existing inside the grid-connected converter will remain. O C1 and C2) can also become paths for interference escape, that is, reducing the Y capacitance to ground in the CL-type EMI filter. y Alternatively, increasing the common-mode inductance of the L-type EMI filter can suppress common-mode interference to some extent, but it is not the optimal solution.

[0087] This invention also attempts to install the EMI filter on the grid side, which can isolate the interference escape path between the grid side and the weak current side. However, the grid side is located on the main line of the grid, and its current is much larger than that of the converter side. The power requirements for the EMI filter are very stringent, and the common mode inductor will be very large. At the same time, the small Y capacitor to ground will also cause a large leakage current. Therefore, it is impractical to install the EMI filter on the grid side.

[0088] Therefore, a preferred approach adopted in this invention is to install an EMI filter on the converter side, such as... Figure 13 As shown, auxiliary measures (increasing the impedance of strong and weak electrical coupling paths, or / and reducing the impedance of common-mode interference return paths) are then used to suppress interference escape as much as possible, thereby achieving the best suppression effect.

[0089] Suppressing interference escape essentially involves reducing the escape common-mode interference current I2, and / or increasing the return common-mode interference current I1. Methods for reducing the escape common-mode interference current I2 include:

[0090] Method 1: Connect a first common-mode inductor in series on the high-voltage input side or low-voltage output side of the auxiliary power supply on the converter side; preferably, the larger the value of the first common-mode inductor, the better the common-mode interference suppression effect.

[0091] Method 2: Connect a second common-mode inductor in series with the secondary cable of the current transformer; preferably, the larger the value of the second common-mode inductor, the better the common-mode interference suppression effect.

[0092] Method 3: Remove the Y capacitor from the auxiliary power supply.

[0093] Any one or a combination of the above three methods can increase the impedance of the strong and weak electrical coupling path, thereby reducing the escape common-mode interference current I2.

[0094] Preferably, for method one, a first common-mode inductor is connected in series on the high-voltage input side of the auxiliary power supply, which can reduce the size and cost of the first common-mode inductor. Specifically, the current on the input side of the auxiliary power supply is smaller than that on the output side, so the winding of the first common-mode inductor added here can be thinner and the magnetic ring can be smaller, achieving a good suppression effect while saving cost and space. In contrast, adding a first common-mode inductor on the output side of the auxiliary power supply requires thicker winding and a larger magnetic ring to achieve the same suppression effect. In this embodiment of the invention, a 2.2mH first common-mode inductor is connected in series on the input side of the auxiliary power supply, and the magnetic ring material is manganese-zinc ferrite, with an interference escape suppression effect as shown in the figure. Figure 11 As shown by the red line, it can be seen that the system has a good suppression effect on common-mode interference in the 600kHz-8MHz frequency band.

[0095] Specifically, for method two, the secondary side of the transformer transmits a weak signal with a small current. Therefore, only a second common-mode inductor with a thin winding and a small magnetic ring needs to be connected in series to achieve a good suppression effect. In this embodiment of the invention, a 2.8mH second common-mode inductor is connected in series at this point, and the magnetic ring material is nanocrystalline. The interference escape suppression effect is as follows: Figure 11 As shown by the black line, it can be seen that the system has a good suppression effect on common-mode interference in the 100kHz-10MHz frequency band.

[0096] Specifically, regarding method three, such as Figure 10 The diagram shows a stand-alone flyback switching power supply. Its input side draws power from the AC grid via an uncontrolled rectifier. The high-speed switching action of its own switching devices is the source of common-mode interference. In most cases, adding a Y capacitor between PGND and SGND creates a low-impedance loop between the primary and secondary grounds, allowing the current flowing through C... ps Common-mode noise via C Y The current returns to the starting point of the displacement current and is converted into a manageable DM noise current. However, when the flyback power supply draws power from the DC bus of the grid-connected converter as an auxiliary power supply, the common-mode interference mainly comes from the three-phase converter, and its own common-mode interference is relatively small. Adding a Y capacitor at this time would reduce the isolation between the strong and weak current sides, increase the degree of strong-weak coupling, and thus increase the overall common-mode interference, while also worsening the interference escape problem. Therefore, removing the Y capacitor between the auxiliary power supply PGND and SGND can suppress common-mode interference escape. Removing the auxiliary power supply Y capacitor improves the interference escape suppression effect as follows: Figure 11 As shown by the brown line, the system exhibits good common-mode interference suppression within the 10kHz-10MHz frequency band. However, interference caused by the auxiliary power supply's own switching action can occur. Figure 11 The interference is marked with a red box, but this interference is negligible compared to the interference from the three-phase converter.

[0097] Ways to increase the return common-mode interference current I1 include:

[0098] A Y-capacitor C is connected in parallel to ground on the high-voltage DC bus of the converter. Y This reduces the impedance of the common-mode interference return path, thereby increasing the return common-mode interference current I1, which is equivalent to indirectly reducing the escape common-mode interference current I2.

[0099] As a preferred option, the parallel-connected Y capacitor C to ground Y The range of values ​​for is:

[0100] C N <C Y <3C N

[0101] Among them, C N This represents the original ground capacitance of the high-voltage DC bus. In this embodiment of the invention, C N =246.9pF, therefore, the parallel Y capacitor to ground C Y satisfy:

[0102] 246.9pF<C Y <740.7pF

[0103] In this embodiment of the invention, the Y capacitor C to ground... Y The suppression effect of interference escape at 470 pF, 1 nF, and 1.5 nF are as follows: Figure 9 As shown. It can be seen that when the parallel-connected Y capacitor C to ground... Y The interference escape suppression effect is best when the capacitance is 470pF. The parallel Y-capacitor C to ground... Y The larger the value, the smaller the increase in suppression effect. When the parallel-connected Y capacitor to ground C... Y If the value is too high, it may also generate excessive leakage current.

[0104] For the four preferred methods mentioned above, one or more can be used for interference escape suppression; the optimal method is to use all four methods simultaneously to form the optimal comprehensive common-mode interference escape suppression method. In this embodiment of the invention, the comprehensive common-mode interference escape suppression method used is as follows:

[0105] ① Connect a 470pF Y capacitor to ground in parallel on the high-voltage DC bus. Y ;

[0106] ② Connect a 2.2mH first common-mode inductor in series on the auxiliary power input side;

[0107] ③ Connect a 2.8mH second common-mode inductor in series with the secondary cable of the current transformer;

[0108] ④ Remove the auxiliary power supply Y capacitor.

[0109] Corresponding to Figure 13 ①-④ in the example Figure 12 As shown, the common-mode interference escape suppression effect is achieved after using this comprehensive suppression method. The blue line represents the original common-mode interference current on the power grid side, and the red line represents the common-mode interference current on the power grid side after installing the CL-type EMI filter. It can be observed that the EMI filter has almost no suppression effect. The black line represents the common-mode interference current on the power grid side after using the comprehensive common-mode interference escape suppression method in this embodiment of the invention. It can be observed that the common-mode interference current is well suppressed, with an average reduction of about 20dB. Therefore, the use of the comprehensive common-mode interference escape suppression method of this invention restores the original suppression performance of the EMI filter, achieving a good suppression effect without requiring additional parameters to the filter inductor and capacitor.

[0110] According to a second aspect of the present invention, a common-mode interference escape suppression system for a grid-connected converter is provided, for performing each step corresponding to the common-mode interference escape suppression method for a grid-connected converter in the above embodiments, the system comprising:

[0111] The model building module is used to build a common-mode interference model for the grid-connected converter. At the grid connection point, the model includes: the converter side, the grid side, and the load side. The control system of the grid-connected converter draws power from the high-voltage DC bus through an auxiliary power supply and collects load current information through a current transformer. The primary side of the current transformer is the load side. The EMI filter is located on the converter side.

[0112] The common-mode interference escape suppression module is used to increase the impedance of strong and weak electrical coupling paths, and / or reduce the impedance of the common-mode interference return path.

[0113] Among them, the strong-weak coupling path is the common-mode interference coupling path between the high-power side and the control system caused by the parasitic capacitances of the primary and secondary sides of the current transformer and auxiliary power supply; the common-mode interference return path is that the common-mode interference current generated by the common-mode interference source flows through the ground capacitor in the converter side or the ground Y capacitor of the EMI filter, and then flows through the ground capacitor C of the high-voltage DC bus. N The path then flows back to the common-mode interference source; the high-power side includes the load side and the high-voltage DC bus.

[0114] Furthermore, in the common-mode interference escape suppression module, the impedance of strong and weak electrical coupling paths is increased, including any one or more of the following methods:

[0115] Method 1: Connect the first common-mode inductor in series on the high-voltage input side or low-voltage output side of the auxiliary power supply;

[0116] Method 2: Connect a second common-mode inductor in series with the secondary cable of the current transformer;

[0117] Method 3: Remove the Y capacitor from the auxiliary power supply;

[0118] To reduce the impedance of the common-mode interference return path, the following measures are taken: A Y-capacitor C is connected in parallel to ground on the high-voltage DC bus. Y .

[0119] According to a third aspect of the present invention, a computer-readable storage medium is provided having a computer program stored thereon, which, when executed by a processor, implements the common-mode interference escape suppression method for grid-connected converters as described in the above embodiments.

[0120] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A method for suppressing common-mode interference escape in a grid-connected converter, characterized in that, include: A common-mode interference model for a grid-connected converter is constructed. At the grid connection point, the model includes: the converter side, the grid side, and the load side. The control system of the grid-connected converter draws power from the high-voltage DC bus through an auxiliary power supply and collects load current information through a current transformer. The primary side of the current transformer is the load side. An EMI filter is installed on the converter side. Increase the impedance of the strong and weak electrical coupling path, and reduce the impedance of the common-mode interference return path; The strong-weak coupling path refers to the common-mode interference coupling path between the high-power side and the control system caused by the parasitic capacitances on the primary and secondary sides of the current transformer and the auxiliary power supply. The common-mode interference return path is the common-mode interference current generated by the common-mode interference source flowing through the ground capacitor in the converter side or the ground Y capacitor of the EMI filter, and then flowing through the ground capacitor of the high-voltage DC bus. C N The path then flows back to the common-mode interference source; the high-power side includes the load side and the high-voltage DC bus; The increase in the impedance of the strong and weak electrical coupling path includes one or more of the following methods: Method 1: Connect a first common-mode inductor in series on the high-voltage input side or the low-voltage output side of the auxiliary power supply; Method 2: Connect a second common-mode inductor in series with the secondary cable of the current transformer; Method 3: Remove the Y capacitor from the auxiliary power supply; The impedance reduction of the common-mode interference return path includes: connecting a Y-capacitor to ground in parallel on the high-voltage DC bus. C Y ; The Y capacitor to ground C Y The range of values ​​for is: C N < C Y <3 C N ,in, C N This refers to the capacitance to ground of the high-voltage DC bus.

2. The method according to claim 1, characterized in that, Also includes: Increase the value of the first common-mode inductance; Or / and, increase the value of the second common-mode inductance.

3. The method according to claim 1, characterized in that, In the first method, the first common-mode inductor is connected in series on the high-voltage input side of the auxiliary power supply.

4. The method according to claim 1, characterized in that, Also includes: Reduce the Y capacitance to ground in the EMI filter C y The EMI filter is a CL-type EMI filter; Alternatively, increase the common-mode inductance of the EMI filter, wherein the EMI filter is an L-type EMI filter.

5. A common-mode interference escape suppression system for a grid-connected converter, characterized in that, The system for performing the method according to any one of claims 1-4, the system comprising: The model building module is used to build a common-mode interference model for a grid-connected converter. At the grid connection point, the model includes: the converter side, the grid side, and the load side. The control system of the grid-connected converter draws power from the high-voltage DC bus through an auxiliary power supply and collects load current information through a current transformer. The primary side of the current transformer is the load side. An EMI filter is installed on the converter side. The common-mode interference escape suppression module is used to increase the impedance of strong and weak electrical coupling paths and reduce the impedance of common-mode interference return paths. The strong-weak coupling path refers to the common-mode interference coupling path between the high-power side and the control system caused by the parasitic capacitances on the primary and secondary sides of the current transformer and the auxiliary power supply. The common-mode interference return path is the common-mode interference current generated by the common-mode interference source flowing through the ground capacitor in the converter side or the ground Y capacitor of the EMI filter, and then flowing through the ground capacitor of the high-voltage DC bus. C N The path then flows back to the common-mode interference source; the high-power side includes the load side and the high-voltage DC bus; In the common-mode interference escape suppression module, increasing the impedance of strong and weak electrical coupling paths includes any one or more of the following methods: Method 1: Connect a first common-mode inductor in series on the high-voltage input side or the low-voltage output side of the auxiliary power supply; Method 2: Connect a second common-mode inductor in series with the secondary cable of the current transformer; Method 3: Remove the Y capacitor from the auxiliary power supply; The impedance reduction of the common-mode interference return path includes: connecting a Y-capacitor to ground in parallel on the high-voltage DC bus. C Y ; The Y capacitor to ground C Y The range of values ​​for is: C N < C Y <3 C N ,in, C N This refers to the capacitance to ground of the high-voltage DC bus.

6. A computer-readable storage medium having a computer program stored thereon, characterized in that, When the program is executed by the processor, it implements the method as described in any one of claims 1-4.

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