Leakage current suppression method and system for non-isolated flexible interconnection device

Abstract

The invention discloses a leakage current suppression method and system for a non-isolated flexible interconnection device, and belongs to the technical field of leakage current processing, and the method comprises the steps: setting a common-mode loop in a topological structure of the non-isolated flexible interconnection device, separating the high-frequency leakage current and the low-frequency leakage current of the topological structure, and obtaining the high-frequency leakage current and the low-frequency leakage current of the topological structure; obtaining a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows; wherein the high-frequency leakage current is suppressed by adjusting the capacitance of the first loop; performing low-frequency leakage current closed-loop control on the second loop, and constructing a low-frequency leakage current impedance model; and based on the low-frequency leakage current impedance model, virtual impedance is introduced into the second loop, and the low-frequency leakage current is suppressed. Therefore, by implementing the application, the problems that high and low frequency leakage current is difficult to suppress at the same time and the leakage current suppression cost is relatively high in the prior art can be solved.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of leakage current processing, and particularly relates to a leakage current suppression method and system for a non-isolated flexible interconnection device. BACKGROUND

[0002] As a key equipment for realizing flexible interconnection between low-voltage transformer areas based on power electronic technology, the flexible interconnection device can guarantee power quality, improve power supply reliability, and realize flexible regulation of tidal flow in the case of large-scale access of photovoltaic and other distributed new energy to the power grid. In the low-voltage distribution network, due to the large size, low efficiency and high cost of the isolation transformer, the flexible interconnection device is seriously restricted in wide application in the low-voltage distribution network, so the isolation transformer is often omitted. However, the absence of the isolation transformer will cause the lack of electrical isolation between the converter and the distribution network, resulting in serious leakage current problems of the flexible interconnection device.

[0003] The flexible interconnection device without an isolation transformer is often referred to as a non-isolated flexible interconnection device, and its leakage current includes high-frequency leakage current and low-frequency leakage current. The high-frequency leakage current is generated by the photovoltaic transformer and is affected by the parasitic parameters of the converter (the parasitic parameters are additional capacitance, inductance and resistance parameters generated between electrical elements due to mutual connection); and the low-frequency leakage current is caused by the grounding of the two-side distribution network and is affected by the operation conditions of the two-side power grid, so different methods need to be used to suppress the above two kinds of leakage currents. However, the existing leakage current suppression method is difficult to suppress the high-frequency and low-frequency leakage currents at the same time, and needs to introduce additional hardware for assistance, resulting in high leakage current suppression cost. SUMMARY

[0004] The application provides a leakage current suppression method and system for a non-isolated flexible interconnection device, which can solve the problems of difficulty in simultaneously suppressing high-frequency and low-frequency leakage currents and high leakage current suppression cost in the prior art.

[0005] The first aspect of the application provides a leakage current suppression method for a non-isolated flexible interconnection device, which comprises:

[0006] A common-mode loop is arranged in the topology structure of the non-isolated flexible interconnection device to separate the high-frequency leakage current and the low-frequency leakage current of the topology structure, so as to obtain a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows; wherein the high-frequency leakage current is suppressed by adjusting the capacitance of the first loop;

[0007] The second loop is subjected to low-frequency leakage current closed-loop control, and a low-frequency leakage current impedance model is constructed;

[0008] Based on the low-frequency leakage current impedance model, a virtual impedance is introduced into the second loop to suppress the low-frequency leakage current.

[0009] The above scheme introduces a branch containing a capacitor on the circuit in the non-isolated flexible interconnection device, obtains a common-mode loop with lower impedance, because high-frequency leakage current tends to flow through the path with low impedance, and low-frequency leakage current does not flow through the common-mode loop with high low-frequency impedance, so the high-frequency leakage current and the low-frequency leakage current are successfully separated, providing basic conditions for subsequent separate management. For the separated high-frequency leakage current and low-frequency leakage, simultaneous management of the two currents can be achieved by simultaneously managing the first loop and the second loop. For the second loop, first construct the impedance model of the loop under the closed-loop control of the low-frequency leakage current, analyze the relationship between the equivalent output impedance and the low-frequency leakage current, and provide data support for suppressing the low-frequency leakage current by adjusting the equivalent output impedance. Then, through the low-frequency leakage current impedance model, find the virtual impedance that can suppress the low-frequency leakage current, realize the enhancement of the leakage current suppression capability of the non-isolated flexible interconnection device without additional hardware cost, and further reduce the leakage current suppression cost.

[0010] In a possible implementation method of the first aspect, a common-mode loop is set in the equivalent model to separate high-frequency leakage current and low-frequency leakage current of the equivalent model, to obtain a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows, specifically:

[0011] The common point of the filter capacitor of the converter on both sides of the topology structure and the midpoint of the DC side capacitor are directly connected to construct a common-mode loop;

[0012] The high-frequency leakage current and the low-frequency leakage current of the topology structure are separated through the common-mode loop;

[0013] Based on the topology structure after the leakage current is separated, the loop established by grounding the parasitic capacitor of the port converter is taken as the first loop, and the loop in communication with the grounding wires of the two terminal areas is taken as the second loop.

[0014] The above scheme connects the common point of the filter capacitor and the midpoint of the DC side capacitor to construct a high-frequency common-mode voltage cancellation path, providing a lower-impedance flow path for high-frequency leakage current. Because the branch constructed has high low-frequency impedance, low-frequency leakage current does not flow through the circuit, so the high-frequency leakage current and the low-frequency leakage current are successfully separated, and subsequent leakage current suppression can be performed without mutual influence.

[0015] In a possible implementation method of the first aspect, the common-mode loop is specifically:

[0016] The data model of the converter of the topology structure after the common-mode loop is added is:

[0017]

[0018] wherein u a1 , u b1 , u c1 , u n1 is the output voltage of the converter on the A, B, C, N four-phase side, L is the filter inductance on the converter side, i A1 , i B1 , i C1 , i N1 is the output current of the converter on the A, B, C, N four-phase side, i Ca1 , i Cb1 , i Cc1 , i Cn1 is the filter capacitor current of the converter on the A, B, C, N four-phase side, u dcm1 is the DC side midpoint voltage.

[0019] In a possible implementation method of the first aspect, the common-mode loop is used to separate the high-frequency leakage current and the low-frequency leakage current of the topology, and specifically:

[0020] The high-frequency leakage current flows through the capacitor branch in the common-mode loop to obtain the first loop, and the low-frequency leakage current does not flow through the capacitor branch, and the capacitor branch includes a DC excitation source, and the low-frequency resistance value of the capacitor branch is greater than the low-frequency resistance value in the topology.

[0021] In a possible implementation method of the first aspect, the second loop is subjected to low-frequency leakage current closed-loop control, and a low-frequency leakage current impedance model is constructed, and specifically:

[0022] The low-frequency leakage current closed-loop control is performed on the converter on one side of the second loop, and the equivalent output impedance of the second loop under the low-frequency leakage current closed-loop control is determined.

[0023] The relationship between the equivalent output impedance and the low-frequency leakage current is analyzed to construct the low-frequency leakage current impedance model.

[0024] The above scheme directly controls the low-frequency leakage current of the converter on one side of the second loop, and analyzes the change relationship between the equivalent output resistance and the low-frequency leakage current in the control process, so as to find the factors affecting the low-frequency leakage current and provide data support for subsequent low-frequency leakage current suppression.

[0025] In a possible implementation method of the first aspect, the relationship between the equivalent output impedance and the low-frequency leakage current is specifically:

[0026] The expression of the low-frequency leakage current is:

[0027] i cmg (s)=G cl(s)·I ref (s)-u cm_inv (s) / Z oi1 (s);

[0028] wherein i cmg is the low-frequency leakage current, G cl is a closed-loop transfer function of the low-frequency leakage current control, I ref is a given value of the low-frequency leakage current, u cm_inv is a common-mode equivalent in-out voltage of the inverter side, Z oil is the equivalent output impedance.

[0029] In a possible implementation method of the first aspect, based on the low-frequency leakage current impedance model, a virtual impedance is introduced in the second loop to suppress the low-frequency leakage current, specifically:

[0030] Based on the low-frequency leakage current impedance model, the virtual impedance is connected in series in the second loop to obtain an equivalent reshaped impedance of the second loop;

[0031] The low-frequency leakage current is suppressed by increasing the equivalent reshaped impedance to reduce the amplitude of the low-frequency leakage current.

[0032] The above scheme increases the equivalent output impedance in the loop by connecting the virtual impedance in series, so as to obtain a larger equivalent reshaped impedance. The increased equivalent reshaped impedance can increase the impedance amplitude at the low-frequency leakage current in the second loop, so as to reduce the amplitude of the low-frequency leakage current and achieve suppression of the low-frequency leakage current.

[0033] In a possible implementation method of the first aspect, the equivalent reshaped impedance is specifically:

[0034] Z oi (s)=Z oi1 (s)+Z oi2 (s)+Z s1 (s);

[0035] wherein Z oi is the equivalent reshaped impedance, Z oil is the equivalent output impedance of the second loop, Z oi2 is a common-mode equivalent impedance, and Z s1 is the virtual impedance.

[0036] The second aspect of the application provides a leakage current suppression system for a non-isolated flexible interconnection device, the system comprising: an equivalent model construction module, a leakage current separation module, an impedance model construction module, and a leakage current suppression module.

[0037] The equivalent model construction module is configured to model the topology of the non-isolated flexible interconnection device and construct a corresponding equivalent model.

[0038] The leakage current separation module is configured to separate the high-frequency leakage current and the low-frequency leakage current of the equivalent model by setting a common-mode loop in the equivalent model, to obtain a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows; and the high-frequency leakage current is suppressed by adjusting the capacitance of the first loop.

[0039] The impedance model construction module is configured to perform low-frequency leakage current closed-loop control on the second loop and construct a low-frequency leakage current impedance model.

[0040] The leakage current suppression module is configured to introduce a virtual impedance in the second loop based on the low-frequency leakage current impedance model, to suppress the low-frequency leakage current.

[0041] The third aspect of the present application provides a terminal device, which comprises a terminal device including a processor and a memory, the memory stores a computer program, and the processor implements the steps of the leakage current suppression method for a non-isolated flexible interconnection device according to any one of the embodiments of the present application when executing the computer program. BRIEF DESCRIPTION OF DRAWINGS

[0042] In order to more clearly illustrate the technical solutions of the present application, the drawings needed in the embodiments will be briefly introduced below. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative labor.

[0043] Figure 1 is a specific flowchart of a leakage current suppression method for a non-isolated flexible interconnection device according to an embodiment of the present application;

[0044] Figure 2 is a common-mode equivalent circuit of a non-isolated FID of a leakage current suppression method for a non-isolated flexible interconnection device according to an embodiment of the present application;

[0045] Figure 3 is a low-frequency leakage current closed-loop control block diagram of a leakage current suppression method for a non-isolated flexible interconnection device according to an embodiment of the present application;

[0046] Figure 4 is a low-frequency leakage current control impedance model of a leakage current suppression method for a non-isolated flexible interconnection device according to an embodiment of the present application;

[0047] Figure 5is a low-frequency leakage current control impedance model after series virtual impedance of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application;

[0048] Figure 6 is a low-frequency leakage current loop equivalent impedance amplitude-frequency response curve of different methods of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application;

[0049] Figure 7 is a low-frequency leakage current closed-loop control block diagram after impedance remodeling of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application;

[0050] Figure 8 is a FID system control block diagram of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application;

[0051] Figure 9 is a right side grid current and high-frequency leakage current experimental result of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application;

[0052] Figure 10 is a grid voltage with inconsistent amplitude of two areas of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application

[0053] Figure 11 is a right side grid current and low-frequency leakage current experimental result of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application, with inconsistent amplitude of two area grid voltages;

[0054] Figure 12 is a grid voltage with phase difference of two areas of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application

[0055] Figure 13 is a right side grid current and low-frequency leakage current experimental result of a leakage current suppression method for a non-isolated flexible interconnection device provided by an embodiment of the present application, with phase difference of two area grid voltages;

[0056] Figure 14 is a structure diagram of a leakage current suppression system for a non-isolated flexible interconnection device provided by an embodiment of the present application;

[0057] Figure 15 A structure diagram of a terminal device is provided by an embodiment of the present application. DETAILED DESCRIPTION

[0058] The technical solutions of the embodiments of this application will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of this application, and not all embodiments. Based on the embodiments of this application, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of this application.

[0059] It should be understood that the step numbers used in the text are for ease of description only and are not intended to limit the order in which the steps are performed.

[0060] First Embodiment

[0061] Flexible Interconnection Device (FID) is a key device based on power electronics technology that enables flexible interconnection between low-voltage distribution networks. In low-voltage distribution networks, FIDs typically use a three-phase four-wire system. Based on the presence or absence of an isolation transformer, low-voltage FID circuit topologies can be divided into back-to-back three-phase three-arm converters with isolation transformers and back-to-back three-phase four-arm converters without isolation transformers. However, the large size, low efficiency, and high cost of isolation transformers severely limit the widespread application of FIDs in low-voltage distribution networks. Therefore, non-isolated FIDs are now widely used. Although non-isolated FIDs effectively improve system compactness and efficiency, the lack of electrical isolation between the converter and the distribution network due to the absence of an isolation transformer leads to serious leakage current problems.

[0062] The purpose of this application is to suppress leakage current at a low cost based on non-isolated FID.

[0063] like Figure 1 As shown, to address the problems of difficulty in simultaneously suppressing high- and low-frequency leakage currents and high leakage current suppression costs in the prior art, the first embodiment of this application provides a detailed flowchart of a leakage current suppression method for non-isolated flexible interconnect devices. The leakage current suppression method for non-isolated flexible interconnect devices in this embodiment includes steps S1 to S3, detailed below:

[0064] Step S1: Set a common-mode loop in the topology of the non-isolated flexible interconnect device to separate the high-frequency leakage current and the low-frequency leakage current of the topology, and obtain the first loop through which the high-frequency leakage current flows and the second loop through which the low-frequency leakage current flows.

[0065] The leakage current of the non-isolated FID includes high-frequency leakage current and low-frequency leakage current. The high-frequency leakage current is formed by the parasitic capacitance to ground of the filter circuit of the two-sided converter, and is caused under the excitation of the common-mode voltage. The high-frequency leakage current is related to the modulation algorithm and switching frequency of the converter, and can be effectively suppressed by adding a common-mode inductor, but this will introduce additional power loss and system cost. The low-frequency leakage current is caused by the grounding of the two-sided power distribution network, and is affected by the operating conditions of the two-sided power grid, which is essentially different from the leakage current caused by the parasitic parameters in the photovoltaic converter. Since it is not affected by the parasitic parameters and has a low frequency, it is difficult to suppress by adding passive filter devices.

[0066] Therefore, the embodiment of the present application proposes a method of separating high and low frequency leakage currents and then suppressing them respectively, which can effectively suppress high and low frequency leakage currents without introducing additional hardware costs.

[0067] The topology structure of the non-isolated FID is formed by two three-level T-type three-phase four-bridge arm converters connected back-to-back, and the four-phase A, B, C and N of the double-sided converter is configured with a grid-side filter inductor L g and a filter capacitor C f , and the filter parameters of the two sides are the same. Taking the left rectifier in the topology structure as an example, the equivalent mathematical model of the converter when the traditional filter capacitor common point and the DC side capacitor midpoint are not connected is analyzed first. According to Kirchhoff's voltage law, the equivalent mathematical model of the converter can be expressed as:

[0068]

[0069] In the formula, u a1 , u b1 , u c1 , u n1 are the output voltages of the two-sided converter in the A, B, C and N four-phase, L g is the grid-side filter inductor, i A1 , i B1 , i C1 , i N1 are the output currents of the two-sided converter in the A, B, C and N four-phase, i Ca1 , i Cb1 , i Cc1 , i Cn1 are the grid-side currents of the two-sided converter in the A, B, C and N four-phase, and u gn1 is the grid voltage.

[0070] The equivalent mathematical model of the converter can also be expressed as:

[0071]

[0072] In the formula, C fis the filter capacitor current of the converter at phase A, u Ca1 is the filter capacitor current of the converter at phase A, u nN is the voltage across the parasitic capacitor.

[0073] The voltage across the parasitic capacitor is specifically expressed as:

[0074]

[0075] In the formula, C pv is the parasitic capacitor, i Ccm1 is the common-mode current of the filter capacitor, i.e., the high-frequency leakage current.

[0076] The high-frequency leakage current satisfies the following formula:

[0077] i Ccm1 = i Ca1 + i Cb1 + i Cc1 + i Cn1 ;

[0078] In the formula, i Ca1 , i Cb1 , i Cc1 , i Cn1 is the filter capacitor current of the converter at phase A, B, C, and N.

[0079] Adding the above two equivalent mathematical models of the converters can obtain an equivalent model of the common-mode loop:

[0080]

[0081] In the formula, u cm1 is the common-mode excitation source, i cm1 is the common-mode current of the converter side, i cmg is the low-frequency leakage current, which respectively satisfies:

[0082] i cm1 = i A1 + i B1 + i C1 + i N1 ;

[0083] i cmg = i a1 + i b1 + i c1 + i n1 ;

[0084] According to the above formula and considering the common-mode loop of the right inverter in the topology structure, the common-mode equivalent circuit of the conventional non-isolated FID system can be obtained as shown in Figure 2 (a). In Figure 2(a) The equivalent circuit shown contains two excitation sources: common-mode excitation source u cm1 and u cm2 It can be seen that there are both high-frequency leakage current loops and low-frequency leakage current loops in the equivalent circuit. The low-frequency leakage current loop in the red circle is the connection of the two-terminal area grounding line, and the high-frequency leakage current loop in the blue circle is the grounding of the parasitic capacitance of the port converter.

[0085] In Figure 2 (b) Based on the equivalent circuit shown in (a), the common-mode loop is set by connecting the common point of the filter capacitor of the converter on both sides and the midpoint of the DC side capacitor, and the data model of the converter of the topology structure after adding the common-mode loop is:

[0086]

[0087] In the formula, u a1 , u b1 , u c1 , u n1 are the output voltages of the converters on both sides in A, B, C, and N four phases, L is the filter inductance on the converter side, i A1 , i B1 , i C1 , i N1 are the output currents of the converters on both sides in A, B, C, and N four phases, i Ca1 , i Cb1 , i Cc1 , i Cn1 are the filter capacitor currents of the converters on both sides in A, B, C, and N four phases, and u dcm1 is the midpoint voltage on the DC side.

[0088] Adding each term in the above formula can obtain the common-mode excitation source expression after adding the common-mode loop:

[0089]

[0090] Combined with the topology structure of the traditional non-isolated FID provided in Figure 2 (a) and the common-mode excitation source expression after adding the common-mode loop, the common-mode excitation source expression after adding the common-mode loop can be obtained: Figure 2 (b) The topology structure diagram of the new non-isolated FID after adding the common-mode loop is shown in (a). In Figure 2 (b) The equivalent circuit shown contains four excitation sources: common-mode excitation source u cm1 and u cm2 , DC excitation source 4u dcm1 and 4u dcm2 . Because of the essential difference between the nature of the common-mode excitation source and the DC excitation source, for the capacitor, the DC excitation source 4u dcm1 and 4udcm2 For high frequency leakage current, the filter capacitor common point and the DC side capacitor midpoint are directly connected, two capacitor branches containing C f are added in the common mode equivalent circuit, which provides a lower impedance path for high frequency leakage current, separates high frequency leakage current from low frequency leakage current, and effectively suppresses high frequency leakage current.

[0091] In Figure 2 (b), the red loop is the second loop through which the low frequency leakage current flows, and the blue loop is the first loop through which the high frequency leakage current flows.

[0092] For low frequency leakage current, since the low frequency impedance of the two C f capacitor branches is high, the low frequency leakage current will not flow through this branch, but will form a common mode loop between the two-sided converters. Therefore, the generation mechanism of low frequency leakage current is more complex, and is jointly affected by the operating state of the left and right-sided converters and the power grid working condition. According to Figure 2 , the low frequency leakage current can be expressed as:

[0093]

[0094] In the formula, Z L is the impedance of the converter side inductance, and Z Lg is the impedance of the grid side inductance.

[0095] According to the above formula, as long as the common mode voltages u cm1 and u cm2 of the two-sided converters are not equal, the low frequency leakage current will not be 0. Since the low frequency impedance values of L and L g are usually small, i.e. the low frequency impedance on the circulating path is low, a small u cm1 and u cm2 difference will produce significant low frequency leakage current.

[0096] Therefore, by introducing a common mode loop in the topology of the non-isolated FID, the high frequency leakage current is added to the capacitor branch, and the separation of high and low frequency leakage currents is realized. The low frequency resistance value of the capacitor branch is smaller than the low frequency resistance in the topology, so the low frequency leakage current will not flow through this capacitor branch.

[0097] Step S2, low frequency leakage current closed loop control is performed on the second loop, and a low frequency leakage current impedance model is constructed.

[0098] To suppress low-frequency leakage current, closed-loop control is typically employed. The error between the reference and actual values ​​of the low-frequency leakage current is fed into the controller to generate a control variable, which is then superimposed on the modulation wave for carrier comparison. It is important to note that the low-frequency leakage current suppression strategy only needs to be applied to a single-sided converter; otherwise, the circulating current suppression strategy in both-sided converters may generate the same additional common-mode voltage component, thereby reducing the DC bus voltage utilization. Therefore, this embodiment selects the left-side converter of a non-isolated FID for low-frequency leakage current closed-loop control.

[0099] Figure 3 The closed-loop control mechanism for low-frequency leakage current is demonstrated. For example... Figure 3 As shown, G d (s) represents the digital control delay, including computational delay and PWM delay, G i (s) is the transfer function of the low-frequency leakage current controller, which is usually selected as a proportional-integral (PI) controller G. i_PI Or proportional resonant (PR) controller G i_PR , can be represented as:

[0100]

[0101] In the formula, K p K i and K r1 K r2 These represent the proportional-integral and resonant element gains, respectively, ω. c1 and ω c2 ω1 and ω2 are the resonant controller bandwidth coefficients and the resonant center frequencies.

[0102] Since low-frequency leakage current generally contains AC components with frequencies of 1 and 3 times the grid frequency, the center frequency of the resonant controller in this embodiment is selected as 50Hz and 150Hz.

[0103] exist Figure 3 In the middle, Z L (s) and Z Lg (s) represent the converter-side inductance L and the grid-side inductance L, respectively. g The impedance, Z C (s) represents the total impedance of the two capacitor branches of the left-side converter in a non-isolated FID, which can be expressed as:

[0104] Z C (s)=C f s / / (C f s+C pv / s);

[0105] Combined Figure 3 Under low-frequency leakage current closed-loop control, the low-frequency leakage current can be expressed as:

[0106] i cmg (s) = G cl (s) · I ref (s) - u cm_inv (s) / Z oi1 (s) ;

[0107] where i cmg is the low-frequency leakage current, G cl is the low-frequency leakage current control closed-loop transfer function, I ref is a given value of the low-frequency leakage current, and is selected as 0 to minimize the leakage current amplitude; u cm_inv is the common-mode equivalent input voltage on the inverter side, Z oil is the equivalent output impedance.

[0108] where the expressions of the closed-loop transfer function and the equivalent output impedance are:

[0109]

[0110] where T op (s) is the low-frequency leakage current control open-loop transfer function, which can be expressed as:

[0111]

[0112] According to the Norton equivalent model, the system impedance model based on low-frequency leakage current closed-loop control is shown in Figure 4 . In Figure 4 , in addition to the low-frequency leakage current control equivalent output impedance Z oi1 (s), the system also contains the common-mode equivalent impedance Z oi2 (s) of the right inverter. For low-frequency leakage current, the right filter capacitor C f branch can be ignored, so the common-mode equivalent impedance Zoi2(s) of the right inverter can be expressed as:

[0113] Z oi2 (s) = Z L (s) + Z Lg (s) ;

[0114] In the low-frequency band, the impedance value of Zoi2(s) is very small, and the damping effect on the low-frequency leakage current is limited, so the low-frequency leakage current is mainly affected by the low-frequency leakage current control equivalent output impedance Z oi1 (s). When G i (s) is a PI controller, Z oi1 (s) has no additional amplitude gain at 1 and 3 times the grid frequency, so the suppression effect on the low-frequency leakage current is limited. After using a PR controller, Z oi1(s) Increasing the impedance at 1 and 3 times the grid frequency is beneficial for suppressing low-frequency leakage current. However, the grid-side inductance Z Lg (s) will weaken the resonant gain of the PR controller, thereby reducing Z. oi1 (s) The amplitude at 1 and 3 times the grid frequency limits the suppression effect of low-frequency leakage current. Therefore, embodiments of this application achieve optimal suppression of low-frequency leakage current by introducing virtual impedance.

[0115] Based on the above expression for equivalent output impedance, the relationship between the equivalent output impedance and the low-frequency leakage current can be obtained, and thus the low-frequency leakage current impedance model can be derived.

[0116] Step S3: Based on the low-frequency leakage current impedance model, a virtual impedance is introduced into the second circuit to suppress the low-frequency leakage current.

[0117] To effectively suppress low-frequency leakage current, it is necessary to increase the equivalent impedance of the second loop through which the low-frequency leakage current flows. Therefore, this application employs a series-based common-mode virtual impedance reshaping method, which connects a virtual impedance in series in the low-frequency leakage current loop to increase the loop's equivalent impedance, thereby reducing the leakage current amplitude.

[0118] Figure 5 A low-frequency leakage current control impedance model is provided after series virtual impedance, where G i (s) The resonant environment in the PR controller used is the same, and the series virtual impedance Z is the same. s1 (s) can be expressed as:

[0119]

[0120] After introducing the series virtual impedance, the equivalent output impedance (i.e., the equivalent reshaping impedance) of the second circuit becomes:

[0121] Z oi (s)=Z oi1 (s)+Z oi2 (s)+Z s1 (s);

[0122] In the formula, Z oi Z is the equivalent reshaping impedance. oil Z is the equivalent output impedance of the second circuit. oi2 For the common-mode equivalent impedance, Z s1 The virtual impedance is referred to here.

[0123] Through the Figure 5 Amplitude-frequency response analysis of the low-frequency leakage current control impedance model with provided series virtual impedance shows that the amplitude of the equivalent impedance of the low-frequency leakage current loop increases significantly at 1 times and 3 times the grid frequency.

[0124] Figure 6 The amplitude-frequency response curves of the equivalent remodeling impedance of the second loop of the traditional closed-loop control method and the impedance remodeling method are given. The blue curve is the amplitude-frequency response curve of the equivalent remodeling impedance of the low-frequency leakage current controller G i (s) using a PI controller (Z io1_PI (s)) of the traditional closed-loop control method; the red curve is the amplitude-frequency response curve of the equivalent remodeling impedance of the low-frequency leakage current controller G i (s) using a PR controller (Z io1_PR (s)); and the yellow curve is the amplitude-frequency response curve of the low-frequency leakage current controller in the embodiment of the present application, in which the virtual impedance uses the parameters of the resonant controller, which are the same as the parameters of the resonant controller in the traditional low-frequency leakage current controller. It can be seen that, compared with the traditional closed-loop control method using the resonant controller, the impedance remodeling method proposed in the embodiment of the present application has a significantly improved impedance amplitude at the leakage current frequency under the same resonant gain, has a stronger damping effect on the low-frequency leakage current, and thus achieves a better leakage current suppression effect.

[0125] Further, after introducing the virtual impedance, the common-mode equivalent input voltage on the inverter side becomes:

[0126] u' cm_inv (s) = u cm_inv (s) + Z s1 (s) i cmg (s);

[0127] In the formula, the virtual impedance is fed back from the low-frequency leakage current to the common-mode equivalent input voltage u cm_inv on the inverter side, and is specifically shown in the control block diagram as shown in Figure 7 For convenience of implementation, the control block diagram shown in Figure 7 is equivalently transformed, the impedance remodeling loop is fed back to the output end of the controller, and the equivalent transformation compensation link G s (s) can be represented as:

[0128]

[0129] Because the above formula has a lead link, it is difficult to implement in actual situations, and thus is usually ignored. Therefore, the actual equivalent transformation compensation link can be represented as:

[0130] G s (s) = 1 + Z L1 (s) Z Cf (s);

[0131] The FID system control block diagram as shown in Figure 8 is also provided in the embodiment of the present application. In Figure 8In the specific implementation, the modulation strategy of the double-sided converter of the FID device adopts a sinusoidal pulse width modulation (SPWM) based on 3rd harmonic injection, which is used to improve the utilization rate of DC voltage and is equivalent to a three-dimensional space vector modulation (3D-SVM) algorithm. The left converter operates in a rectification mode, and an outer ring is used to maintain the stability of the DC bus voltage, and an inner ring is used to perform positive sequence, negative sequence and zero sequence component separation control on the grid current. The right converter operates in an inversion mode, and the grid-connected active and reactive power is the primary control target. A low-frequency leakage current suppression strategy is added to the left rectifier, the actual value of the low-frequency leakage current is sampled, and the difference between the actual value and the reference value is closed-loop controlled. On this basis, a virtual impedance is introduced to increase the impedance amplitude at the frequency of the low-frequency leakage current, thereby enhancing the leakage current suppression effect.

[0132] The implementation of the present application has the following beneficial effects:

[0133] The present application introduces a branch containing a capacitor in the circuit of the non-isolated flexible interconnection device, obtains a common-mode loop with lower impedance, because high-frequency leakage current tends to flow through the path with low impedance, low-frequency leakage current does not flow through the common-mode loop with high low-frequency impedance, so the high-frequency leakage current and the low-frequency leakage current are successfully separated, providing a basic condition for subsequent separate management. For the separated high-frequency leakage current and low-frequency leakage, simultaneous management of the first loop and the second loop can achieve simultaneous management of the two currents. For the second loop, first construct the impedance model of the loop under the closed-loop control of the low-frequency leakage current, analyze the relationship between the equivalent output impedance and the low-frequency leakage current, and provide data support for suppressing the low-frequency leakage current by adjusting the equivalent output impedance. Then, through the low-frequency leakage current impedance model, find the virtual impedance that can suppress the low-frequency leakage current, realize the enhancement of the leakage current suppression capability of the non-isolated flexible interconnection device without additional hardware cost, and further reduce the leakage current suppression cost.

[0134] Second embodiment

[0135] Further, in order to perform the leakage current suppression method for the non-isolated flexible interconnection device corresponding to the method embodiment to achieve the corresponding functions and technical effects, the present application provides an embodiment related to the experimental verification results of the leakage current suppression method for the non-isolated flexible interconnection device and the traditional leakage current suppression method. For the sake of convenience, only the part related to the present embodiment is shown.

[0136] In order to verify the effectiveness and correctness of the leakage current suppression method for the non-isolated flexible interconnection device proposed by the present application, the present application builds an experimental platform, and the specific implementation parameters are shown in Table 1:

[0137] Table 1 Experimental parameters

[0138]

[0139] In this experimental platform, the left converter of the non-isolated FID is in constant DC voltage mode, and the right converter is in constant power mode.

[0140] Firstly, the difference between the high-frequency leakage current suppression strategy and the traditional leakage current suppression strategy in the leakage current suppression method for the non-isolated flexible interconnection device is verified. Since the high-frequency leakage current is independent of the grid operating condition, the experimental verification can be carried out under the normal grid operating condition. Figure 9 (a) is the experimental result of the grid current and the high-frequency leakage current of the right converter using the conventional non-isolated FID topology. Due to the existence of the high-frequency common-mode loop, the high-frequency leakage current is generated under the excitation of the common-mode voltage, which causes the resonance of the grid current. After the high-frequency leakage current suppression strategy provided in the embodiment of the application is adopted, the common point of the filter capacitor is directly connected to the midpoint of the DC side capacitor, which provides a lower-impedance circulation path for the high-frequency leakage current. The amplitude of the high-frequency leakage current is greatly reduced, as shown in Figure 9 (b).

[0141] Figure 10 and Figure 11 The experimental results of two district grids with different amplitudes are given. As shown in Figure 10 , the left district grid operates in the rated voltage condition, and the voltage of the right district grid drops by 50%. If the conventional leakage current suppression method is used, the result is shown in Figure 11 (a), the three-phase grid current is severely distorted, and the amplitude of the low-frequency leakage current is about 50 A, which seriously threatens the safe and stable operation of the system; if the low-frequency leakage current suppression strategy with virtual impedance introduced in the embodiment of the application is used, the system operating state is obviously improved, and the result is shown in Figure 11 (b), the low-frequency leakage current is suppressed to below 1 A, and the three-phase grid current is sinusoidal and symmetrical, which indicates that the proposed control method can effectively adjust the impedance characteristics of the system under the unbalanced voltage condition of the two district grids, actively suppress the low-frequency leakage current, and maintain high power quality.

[0142] In order to further verify the adaptability of the low-frequency leakage current suppression strategy with virtual impedance introduced in the embodiment of the application under different types of grid operating conditions, Figure 12 and Figure 13 The experimental results of two district grids with phase difference are given. As shown in Figure 12 , the voltage phase of the left district grid is unchanged, and the voltage phase of the right district grid lags by 30°. Although the amplitudes of the two sides are symmetrical, the difficulty of synchronous control is increased due to the phase difference, which easily causes internal circulating current of the system. Under this condition, the conventional low-frequency leakage current suppression result is shown in Figure 13As shown in (a), the three-phase grid-connected current exhibits significant distortion, and the low-frequency leakage current amplitude remains large, approaching 50A, indicating that conventional control methods are insufficient to address the low-frequency circulating current problem caused by voltage phase difference. However, after adopting the low-frequency leakage current suppression strategy proposing the virtual impedance method in this application, the experimental results are as follows: Figure 13 As shown in (b), the three-phase current waveform is restored to a good sine waveform, the low-frequency leakage current is effectively suppressed, and the amplitude is controlled within 1A, which verifies the effectiveness of the proposed strategy under the condition of inconsistent voltage phase between the two transformer substations.

[0143] comprehensive Figures 10 to 13 The experimental results show that the low-frequency leakage current suppression strategy with virtual impedance proposed in this application can operate stably under different types of unbalanced grid conditions on both sides of the distribution network, and can significantly suppress low-frequency leakage current and maintain the power quality of the three-phase grid-connected current. This strategy has strong robustness and provides effective technical support for the safe and stable operation of flexible interconnection devices in complex distribution network environments.

[0144] Third Embodiment

[0145] Furthermore, in order to implement the leakage current suppression system for non-isolated flexible interconnect devices corresponding to the above method embodiments, and to achieve the corresponding functions and technical effects, Figure 14 A structural diagram of a leakage current suppression system for non-isolated flexible interconnect devices is provided. For ease of explanation, only the parts relevant to this embodiment are shown. The leakage current suppression system for non-isolated flexible interconnect devices provided in this application embodiment includes:

[0146] Leakage current separation module 201 is used to set a common mode loop in the topology of a non-isolated flexible interconnect device to separate the high-frequency leakage current and low-frequency leakage current of the topology, and obtain a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows; wherein, the high-frequency leakage current is suppressed by adjusting the capacitance of the first loop.

[0147] In this embodiment of the application, the common point of the filter capacitors on both sides of the topology and the midpoint of the DC-side capacitor are directly connected to construct a common-mode circuit.

[0148] The common-mode circuit separates the high-frequency leakage current and the low-frequency leakage current of the topology.

[0149] Based on the topology after leakage current separation, the loop established by grounding the parasitic capacitance of the port converter is taken as the first loop, and the loop connected to the grounding wires of the two substations is taken as the second loop.

[0150] The impedance model construction module 202 is configured to perform low-frequency leakage current closed-loop control on the second loop, and construct a low-frequency leakage current impedance model.

[0151] In the embodiments of the present application, the low-frequency leakage current closed-loop control is performed at the converter on one side of the second loop, and the equivalent output impedance of the second loop under the low-frequency leakage current closed-loop control is determined.

[0152] The low-frequency leakage current impedance model is constructed by analyzing the relationship between the equivalent output impedance and the low-frequency leakage current.

[0153] The leakage current suppression module 203 is configured to introduce a virtual impedance in the second loop based on the low-frequency leakage current impedance model, and suppress the low-frequency leakage current.

[0154] In the embodiments of the present application, the virtual impedance is connected in series in the second loop based on the low-frequency leakage current impedance model, to obtain an equivalent reshaped impedance of the second loop.

[0155] The low-frequency leakage current is suppressed by increasing the equivalent reshaped impedance to increase the amplitude of the low-frequency leakage current.

[0156] In some embodiments, the leakage current separation module 201 specifically includes:

[0157] The leakage current of the non-isolated FID includes high-frequency leakage current and low-frequency leakage current. The high-frequency leakage current is formed by the high-frequency common-mode loop of the parasitic capacitance of the filter circuit of the two-side converters, and is caused under the excitation of the common-mode voltage. The high-frequency leakage current is related to the modulation algorithm and switching frequency of the converter, and can be effectively suppressed by adding a common-mode inductor, but this will introduce additional power loss and system cost. The low-frequency leakage current is caused by the grounding of the two-side power distribution networks, and is affected by the operation conditions of the two-side power distribution networks, which is essentially different from the leakage current caused by the parasitic parameters in the photovoltaic converter. Since the low-frequency leakage current is not affected by the parasitic parameters and has a low frequency, it is difficult to suppress by adding passive filter devices.

[0158] Therefore, the embodiments of the present application propose a method of separating the high-frequency leakage current and the low-frequency leakage current, and then suppressing them respectively, to effectively suppress the high-frequency leakage current and the low-frequency leakage current without introducing additional hardware costs.

[0159] The topology of the non-isolated FID is formed by connecting two three-level T-type three-phase four-bridge arm converters back to back, and the four-phase A, B, C, and N of the double-side converters are configured with grid-side filter inductors L g and filter capacitors C f, and the filter parameters on both sides are the same. Taking the left rectifier in the topology as an example, first, the equivalent mathematical model of the converter when the public point of the filter capacitor and the midpoint of the DC side capacitor are not connected is analyzed, and according to the Kirchhoff voltage law, the equivalent mathematical model of the converter can be expressed as:

[0160]

[0161] wherein, u a1 , u b1 , u c1 , u n1 are the output voltages of the converters on both sides in A, B, C and N four phases, L g is the filter inductance on the grid side, i A1 , i B1 , i C1 , i N1 are the output currents of the converters on both sides in A, B, C and N four phases, i Ca1 , i Cb1 , i Cc1 , i Cn1 are the grid side currents of the converters on both sides in A, B, C and N four phases, and u gn1 is the grid voltage.

[0162] The equivalent mathematical model of the converter can also be expressed as:

[0163]

[0164] wherein, C f is the filter capacitor on the grid side of the converter, i Ca1 is the filter capacitor current of the converter in A phase, and u nN is the voltage across the parasitic capacitor.

[0165] The voltage across the parasitic capacitor is specifically expressed as:

[0166]

[0167] wherein, C pv is the parasitic capacitor, and i Ccm1 is the common-mode current of the filter capacitor, i.e. the high-frequency leakage current.

[0168] The high-frequency leakage current satisfies the following formula:

[0169] i Ccm1 =i Ca1 +i Cb1 +i Cc1 +i Cn1 ;

[0170] wherein, i Ca1 , i Cb1 , iCc1 Cn1 is the filter capacitor current of the converter in A, B, C, N phase.

[0171] The equivalent mathematical model of the two converters is added to obtain the common-mode loop equivalent model:

[0172]

[0173] In the formula, u cm1 is the common-mode excitation source, i cm1 is the common-mode current on the converter side, i cmg is the low-frequency leakage current, which satisfies:

[0174] i cm1 = i A1 + i B1 + i C1 + i N1 ;

[0175] i cmg = i a1 + i b1 + i c1 + i n1 ;

[0176] According to the above formula, and considering the common-mode loop of the right inverter in the topology, the common-mode equivalent circuit of the conventional non-isolated FID system can be obtained.

[0177] On the basis of the common-mode equivalent circuit, the common point of the filter capacitor of the converter on both sides of the circuit and the midpoint of the DC side capacitor are connected, and a common-mode loop is set in the system, so that the data model of the converter of the topology after adding the common-mode loop is obtained:

[0178]

[0179] In the formula, u a1 , u b1 , u c1 , u n1 are the output voltages of the converters on both sides in A, B, C, N four phases, L is the filter inductance on the converter side, i A1 , i B1 , i C1 , i N1 are the output currents of the converters on both sides in A, B, C, N four phases, i Ca1 , i Cb1 , i Cc1 , i Cn1 are the filter capacitor currents of the converters on both sides in A, B, C, N four phases, and u dcm1 is the midpoint voltage on the DC side.

[0180] ​Adding each term in the above formula can obtain the common-mode excitation source expression after adding the common-mode loop:

[0181]

[0182] The topology structure of the new non-isolated FID can be obtained by combining the topology structure of the traditional non-isolated FID and the common-mode excitation source expression after adding the common-mode loop. The equivalent circuit of the new non-isolated FID contains four excitation sources: the common-mode excitation sources u cm1 and u cm2 , the direct-current excitation sources 4u dcm1 and 4u dcm2 . Because the properties of the common-mode excitation sources and the direct-current excitation sources are essentially different, for the capacitor, the direct-current excitation sources 4u dcm1 and 4u dcm2 in series with the capacitor can be ignored. For the high-frequency leakage current, the common-mode equivalent circuit is increased by two capacitor branches containing C f , which provides a lower-impedance flow path for the high-frequency leakage current, and can separate the high-frequency leakage current from the low-frequency leakage current, thereby effectively suppressing the high-frequency leakage current.

[0183] For the low-frequency leakage current, because the low-frequency impedance of the two C f capacitor branches is high, the low-frequency leakage current does not flow through this branch, but forms a common-mode loop between the two-sided converters. Therefore, the generation mechanism of the low-frequency leakage current is more complex, and is jointly affected by the operating states of the left and right converters and the power grid conditions. The low-frequency leakage current can be represented as:

[0184]

[0185] In the formula, Z L is the impedance of the converter-side inductance, and Z Lg is the impedance of the grid-side inductance.

[0186] According to the above formula, as long as the common-mode voltages u cm1 and u cm2 of the two-sided converters are not equal, the low-frequency leakage current will not be 0. Because the low-frequency impedance values of L and L g are usually small, that is, the low-frequency impedance on the circulating path is low, a small difference between u cm1 and u cm2 will produce a significant low-frequency leakage current.

[0187] Therefore, by introducing a common-mode loop in the topology of the non-isolated FID, the high-frequency leakage current is made to flow to the added capacitor branch, realizing the separation of high-frequency and low-frequency leakage currents. Among them, the low-frequency resistance value of the capacitor branch is smaller than the low-frequency resistance in the topology, so the low-frequency leakage current will not flow through this capacitor branch.

[0188] In some embodiments, the impedance model construction module 202, in particular:

[0189] To realize low-frequency leakage current suppression, usually closed-loop control is performed on the low-frequency leakage current, and the error between the reference value and the actual value of the low-frequency leakage current is sent to the controller to generate a control variable, which is superimposed on the modulation wave for carrier comparison. It should be noted that only the low-frequency leakage current suppression strategy needs to be added in the single-sided converter, otherwise, the circulating current suppression strategy of the double-sided converter may generate the same additional common-mode voltage component, thereby reducing the utilization rate of the DC bus voltage. Therefore, the left converter of the non-isolated FID is selected for low-frequency leakage current closed-loop control in the embodiments of the present application.

[0190] According to the low-frequency leakage current closed-loop control mechanism, G d (s) is a digital control delay, including a calculation delay and a PWM delay, G i (s) is the transfer function of the low-frequency leakage current controller, which is usually selected as a proportional integral (PI) controller G i_PI or a proportional resonant (PR) controller G i_PR , which can be respectively expressed as:

[0191]

[0192] In the formula, K p , K i and K r1 , K r2 are the proportional integral and resonant link gains, ω c1 and ω c2 are the resonant controller bandwidth coefficients, and ω1 and ω2 are the resonant center frequencies.

[0193] Since the low-frequency leakage current generally mainly contains AC components with frequencies of 1 times and 3 times the grid frequency, the center frequencies of the resonant controller in the embodiments of the present application are selected as 50 Hz and 150 Hz.

[0194] Based on the low-frequency leakage current closed-loop control mechanism, Z L (s) and Z Lg (s) are the impedances of the converter-side inductance L and the grid-side inductance L g , respectively, Z C (s) is the total impedance of the two capacitor branches of the left converter of the non-isolated FID, which can be expressed as:

[0195] ZC (s) = C f s / (C f s + C pv s);

[0196] Recombine Figure 3 Under the low-frequency leakage current closed-loop control, the low-frequency leakage current can be expressed as:

[0197] i cmg (s) = G cl (s) · I ref (s) - u cm_inv (s) / Z oi1 (s);

[0198] In the formula, i cmg is the low-frequency leakage current, G cl is the closed-loop transfer function of low-frequency leakage current control, I ref is a given value of the low-frequency leakage current, which is usually selected as 0 to reduce the leakage current amplitude as much as possible; u cm_inv is the common-mode equivalent input voltage of the inverter side, Z oil is the equivalent output impedance.

[0199] The expressions of the closed-loop transfer function and the equivalent output impedance are:

[0200]

[0201] In the formula, T op (s) is the open-loop transfer function of low-frequency leakage current control, which can be expressed as:

[0202]

[0203] According to the Norton equivalent model, the system impedance model based on low-frequency leakage current closed-loop control is shown in Figure 4 In Figure 4 , in addition to the low-frequency leakage current control equivalent output impedance Z oi1 (s), the system also includes the common-mode equivalent impedance Z oi2 (s) of the right inverter. For low-frequency leakage current, the right filter capacitor C f branch can be ignored, so the common-mode equivalent impedance Zoi2(s) of the right inverter can be expressed as:

[0204] Z oi2 (s) = Z L (s) + Z Lg (s);

[0205] In the low-frequency band, the impedance value of Zoi2(s) is very small, and the damping effect on the low-frequency leakage current is limited, so the low-frequency leakage current is mainly affected by the low-frequency leakage current control equivalent output impedance Zoi1 (s) is a PI controller, Z i (s) is a PI controller, Z oi1 (s) has no additional magnitude gain at 1 times and 3 times the grid frequency, so the suppression effect on the low-frequency leakage current is limited. After adopting the PR controller, Z oi1 (s) increases at 1 times and 3 times the grid frequency, which is beneficial to the suppression of the low-frequency leakage current. However, Z Lg (s) will weaken the resonance gain of the PR controller, thereby reducing the magnitude of Z oi1 (s) at 1 times and 3 times the grid frequency, resulting in limited suppression effect on the low-frequency leakage current. Therefore, the embodiment of the present application realizes optimal suppression of the low-frequency leakage current by introducing a virtual impedance.

[0206] According to the expression of the equivalent output impedance described above, the change relationship between the equivalent output impedance and the low-frequency leakage current is known, and a low-frequency leakage current impedance model is obtained.

[0207] In some embodiments, the leakage current suppression module 203, in particular:

[0208] To realize effective suppression of the low-frequency leakage current, it is necessary to increase the equivalent impedance of the second loop through which the low-frequency leakage current flows. For this purpose, the embodiment of the present application adopts a series-based common-mode virtual impedance remodeling method, which serially connects a virtual impedance in the low-frequency leakage current loop to increase the equivalent impedance of the loop, and thereby reduce the leakage current magnitude.

[0209] According to the low-frequency leakage current control impedance model after the series virtual impedance, the same as the resonance environment in the PR controller used by Z i (s), the series virtual impedance Z s1 (s) can be expressed as:

[0210]

[0211] After introducing the series virtual impedance, the equivalent output impedance (i.e., the equivalent remodeling impedance) of the second loop becomes:

[0212] Z oi (s) = Z oi1 (s) + Z oi2 (s) + Z s1 (s) + Z

[0213] In the formula, Z oi is the equivalent remodeling impedance, Z oil is the equivalent output impedance of the second loop, Z oi2 is the common-mode equivalent impedance, and Z s1 is the virtual impedance.

[0214] The amplitude-frequency response analysis of the low-frequency leakage current control impedance model after the series virtual impedance shows that the amplitude of the low-frequency leakage current loop equivalent impedance is significantly increased at 1 times and 3 times the power grid frequency.

[0215] Further, after introducing the virtual impedance, the inverter side common-mode equivalent input voltage becomes:

[0216] u' cm_inv (s)=u cm_inv (s)+Z s1 (s)i cmg (s);

[0217] In the formula, the virtual impedance is fed back from the low-frequency leakage current to the inverter side common-mode equivalent input voltage u cm_inv To facilitate implementation, the impedance remodeling loop is fed back to the controller output, and the equivalent transformation compensation link G s (s) can be represented as:

[0218]

[0219] Because the above formula has a lead element, it is difficult to implement in actual situations, so it is usually ignored. Therefore, the actual equivalent transformation compensation link can be represented as:

[0220] G s (s)=1+Z L1 (s)Z Cf (s);

[0221] The embodiment of the application has the following beneficial effects:

[0222] The embodiment of the application introduces a branch containing a capacitor in the circuit in the non-isolated flexible interconnection device, obtains a common-mode loop with lower impedance, because the high-frequency leakage current tends to flow through the path with low impedance, the low-frequency leakage current does not flow through the common-mode loop with high low-frequency impedance, so the high-frequency leakage current and the low-frequency leakage current are successfully separated, providing a basic condition for subsequent separate management. For the separated high-frequency leakage current and low-frequency leakage, simultaneous management of the first loop and the second loop can achieve simultaneous management of the two currents. For the second loop, first, construct the impedance model of the loop under the low-frequency leakage current closed-loop control, analyze the relationship between the equivalent output impedance and the low-frequency leakage current, and provide data support for suppressing the low-frequency leakage current by adjusting the equivalent output impedance. Then, through the low-frequency leakage current impedance model, find the virtual impedance that can suppress the low-frequency leakage current, realize the enhancement of the leakage current suppression capability of the non-isolated flexible interconnection device without additional hardware cost, and further reduce the leakage current suppression cost.

[0223] Further, Figure 15 A structural diagram of a terminal device provided by an embodiment of the application is shown in FIG. 1. Figure 15As shown, the terminal device 3 of the embodiment comprises at least one processor 30 (only one is shown in the figure) and a memory 31, and a computer program 32 stored in the memory 31 and executable on the at least one processor, wherein the processor 30 implements the steps of the leakage current suppression method for a non-isolated flexible interconnection device according to any one of the embodiments of the present application when executing the computer program 32. Figure 15

[0224] The terminal device 3 can be a computing device such as a desktop computer, a cloud server, and a notebook computer, which can include but is not limited to the processor 30 and the memory 31. Figure 15 The terminal device 3 is only an example and does not constitute a limitation on the terminal device 3, and can include more or fewer components than shown.

[0225] The above-described specific embodiments further illustrate the purposes, technical solutions, and beneficial effects of the present application. It should be understood that the above-described embodiments are only specific embodiments of the present application and are not used to limit the protection scope of the present application. It is particularly pointed out that any modification, equivalent replacement, improvement, etc. made by those skilled in the art within the spirit and principles of the present application shall be included in the protection scope of the present application.​

Claims

1. A method for suppressing leakage current in non-isolated flexible interconnect devices, characterized in that, include: A common-mode loop is set within the topology of a non-isolated flexible interconnect device to separate the high-frequency leakage current and the low-frequency leakage current of the topology, resulting in a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows; wherein, the high-frequency leakage current is suppressed by adjusting the capacitance of the first loop. Low-frequency leakage current closed-loop control is performed on the second loop to construct a low-frequency leakage current impedance model; Based on the low-frequency leakage current impedance model, a virtual impedance is introduced in the second circuit to suppress the low-frequency leakage current.

2. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 1, characterized in that, The method involves setting a common-mode loop within the topology of the non-isolated flexible interconnect device to separate the high-frequency leakage current and the low-frequency leakage current of the topology, thereby obtaining a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows. Specifically: Connect the common point of the filter capacitors of the converters on both sides of the topology and the midpoint of the DC-side capacitor directly to construct a common-mode loop; The common-mode circuit separates the high-frequency leakage current and the low-frequency leakage current of the topology. Based on the topology after leakage current separation, the loop established by grounding the parasitic capacitance of the port converter is taken as the first loop, and the loop connected to the grounding wires of the two substations is taken as the second loop.

3. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 2, characterized in that, The common-mode circuit is specifically as follows: The converter data model of the topology after adding the common-mode loop is as follows: In the formula, u a1 u b1 u c1 u n1 Let L be the output voltage of the converter on both sides at phases A, B, C, and N, and let L be the filter inductance on the converter side. A1 i B1 i C1 i N1 i represents the output current of the converters on both sides in phases A, B, C, and N. Ca1 i Cb1 i Cc1 i Cn1 The current u represents the filter capacitor current in phases A, B, C, and N of the converters on both sides. dcm1 This is the DC side midpoint voltage.

4. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 2, characterized in that, The separation of high-frequency leakage current and low-frequency leakage current in the topology via the common-mode circuit specifically involves: The high-frequency leakage current flows through the capacitor branch in the common-mode circuit to form the first circuit, while the low-frequency leakage current does not flow through the capacitor branch; wherein, the capacitor branch includes a DC excitation source, and its low-frequency resistance value is greater than that of the low-frequency resistance in the topology.

5. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 1, characterized in that, The low-frequency leakage current closed-loop control of the second circuit, and the construction of a low-frequency leakage current impedance model, are specifically as follows: Low-frequency leakage current closed-loop control is performed at the converter on one side of the second circuit to determine the equivalent output impedance of the second circuit under low-frequency leakage current closed-loop control. By analyzing the relationship between the equivalent output impedance and the low-frequency leakage current, a low-frequency leakage current impedance model is constructed.

6. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 5, characterized in that, The relationship between the equivalent output impedance and the low-frequency leakage current is as follows: The expression for the low-frequency leakage current is: i cmg (s)=G cl (s)·I ref (s)-u cm_inv (s) / Z oi1 (s); In the formula, i cmg For the low-frequency leakage current, G cl For the closed-loop transfer function of low-frequency leakage current control, I ref Given the low-frequency leakage current, u cm_inv Z is the common-mode equivalent input / output voltage on the inverter side. oil The equivalent output impedance is given.

7. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 1, characterized in that, Based on the low-frequency leakage current impedance model, a virtual impedance is introduced into the second circuit to suppress the low-frequency leakage current, specifically as follows: Based on the low-frequency leakage current impedance model, the virtual impedance is connected in series in the second circuit to obtain the equivalent reshaping impedance of the second circuit. The low-frequency leakage current is suppressed by increasing the equivalent remodeling impedance to reduce the amplitude of the low-frequency leakage current.

8. The leakage current suppression method for non-isolated flexible interconnect devices according to claim 7, characterized in that, The equivalent remodeling impedance is specifically: Z oi (s)=Z oi1 (s)+Z oi2 (s)+Z s1 (s); In the formula, Z oi Z is the equivalent reshaping impedance. oil Z is the equivalent output impedance of the second circuit. oi2 For the common-mode equivalent impedance, Z s1 The virtual impedance is referred to here.

9. A leakage current suppression system for non-isolated flexible interconnect devices, characterized in that, include: Leakage current separation module, impedance model construction module, and leakage current suppression module; The leakage current separation module is used to set a common-mode loop in the topology of the non-isolated flexible interconnect device to separate the high-frequency leakage current and the low-frequency leakage current of the topology, thereby obtaining a first loop through which the high-frequency leakage current flows and a second loop through which the low-frequency leakage current flows; wherein, the high-frequency leakage current is suppressed by adjusting the capacitance of the first loop. The impedance model construction module is used to perform low-frequency leakage current closed-loop control on the second loop and construct a low-frequency leakage current impedance model. The leakage current suppression module is used to introduce a virtual impedance in the second circuit based on the low-frequency leakage current impedance model to suppress the low-frequency leakage current.

10. A terminal device, characterized in that, It includes a processor and a memory, the memory storing a computer program, and the processor executing the computer program to implement the steps of the leakage current suppression method for a non-isolated flexible interconnect device according to any one of claims 1 to 8.

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