Low-voltage current conversion system based on active zero sequence injection and leakage current control method

Abstract

The application discloses a low-voltage current conversion system based on active zero sequence injection, which comprises a direct current device, a direct current bus, a DC / AC converter, an LCL filter, an active zero sequence loop and an alternating current power grid. A leakage current control method of the low-voltage current conversion system based on active zero sequence injection comprises the following steps: obtaining a power grid phase, converting sample values of inverter side currents into dq0 axis components through Park transformation, controlling d-axis currents as active power, controlling q-axis currents as reactive power, controlling 0-axis as leakage current control based on active zero sequence current injection, obtaining a direct current component of an output voltage through active damping control, obtaining three-phase modulation waves through coordinate inverse transformation, and taking the three-phase modulation waves as input variables of a carrier modulation strategy. The application successfully suppresses leakage current, realizes LCL resonance suppression and system stability.

Description

Technical Field

[0001] This invention belongs to the technical field of non-isolated low-voltage converter systems, specifically relating to a low-voltage converter system based on active zero-sequence injection, and also to a leakage current control method for a low-voltage converter system based on active zero-sequence injection. Background Technology

[0002] With the continuous development and application of clean energy, low-voltage converter systems have been widely used in the field of distributed energy. These systems typically include components such as DC power generation equipment (e.g., solar photovoltaic panels, energy storage batteries), DC collection buses, DC / AC converters, LCL filters, and AC power grids, used to convert DC power into AC power and inject it into the power grid.

[0003] However, leakage current is a significant technical challenge in low-voltage converter systems. Leakage current can pose safety risks to the power grid and system equipment, thus requiring effective measures to suppress it and improve system safety and reliability.

[0004] Traditional leakage current suppression methods can generally be divided into two categories: modulation-based methods and topology-based methods. Improved modulation methods can stabilize common-mode voltage and reduce leakage current, but these methods share the drawback of high harmonic content in the output waveform, requiring large filters for filtering, resulting in large size and high cost. In topology-based methods, additional power switches and common-mode filters are usually required, which often leads to low efficiency and can easily introduce new system instability problems. Summary of the Invention

[0005] The purpose of this invention is to provide a low-voltage converter system based on active zero-sequence injection. Through the active zero-sequence loop, only one additional connection line is needed to form a zero-sequence current path, so that the leakage current forms a circulating current between the LCL filter and the DC / AC converter, thereby preventing the leakage current from flowing into the AC power grid. This achieves low-cost and high-efficiency suppression of leakage current.

[0006] Another objective of this invention is to provide a leakage current control method for low-voltage converter systems based on active zero-sequence injection. By actively injecting zero-sequence current in zero-sequence control, common-mode current is suppressed, leakage current generated by shunt is reduced, leakage current suppression effect is improved, the impact of leakage current on the system and equipment is reduced, and the stability and reliability of the system are improved.

[0007] The technical solution adopted in this invention is a low-voltage converter system based on active zero-sequence injection, including a DC power device. The output terminal of the DC power device is connected to the DC side of a DC / AC converter through a DC bus. Each phase of the AC side of the DC / AC converter is connected to the corresponding phase of the AC grid through an LCL filter. The filter capacitor of the LCL filter is connected to the negative terminal of the DC bus through an active zero-sequence loop. The DC power device generates DC power, producing a 750V DC voltage on the DC bus. The DC / AC converter converts the DC voltage into AC voltage. After being filtered by the LCL filter, the voltage enters the 380V AC grid. The leakage current of the filter capacitor of the LCL filter circulates through the active zero-sequence loop.

[0008] The invention is further characterized by:

[0009] Each LCL filter includes an inductor L T Inductor L T One end is connected to the AC side of the DC / AC converter, inductor L T The other end is connected to both a filter capacitor and an inductor L. g The filter capacitor is connected to one end of the active zero-sequence loop, and the inductor L... g Connect to the AC power grid.

[0010] A DC / AC converter is one of three types: a two-level converter, a three-level converter, or an NPC converter.

[0011] DC power equipment is either solar photovoltaic panels or energy storage batteries.

[0012] The leakage current control method for low-voltage converter systems based on active zero-sequence injection is implemented according to the following steps:

[0013] Step 1: Collect the data from the passing inductor L T Current and inductance L g The output voltage is calculated using a phase-locked loop (PLL) to determine the required rotation angle θ for control in the synchronous rotating coordinate system dq0. The inductor L... T The current is transformed by Park to obtain the DC component under the dq0 axis. The current and voltage are controlled to be in phase by rotating the angle θ.

[0014] Step 2: Obtain the DC component of the output voltage using a current PI controller and an active damping controller, and obtain the three-phase modulated wave through coordinate inverse transformation, which serves as the input variable for the carrier modulation strategy of the active zero-sequence loop.

[0015] Current PI controller transfer function G c (s) is represented as:

[0016]

[0017] Among them, Kp It is a proportional parameter, K i It is the integral parameter.

[0018] Active damped controller transfer function G LCL (s) is represented as:

[0019]

[0020] Among them, G iL2 It is the transfer function of the LCL filter, G d This results in a delay of 1.5 sampling periods during the calculation and pulse width modulation process, H i1 K is an active damping control parameter. pwm It is the DC / AC converter gain, G ic It is the transfer function from the converter output voltage to the capacitor current.

[0021] G ic G iL2 G d They are represented as follows:

[0022]

[0023] Where, ω r This is the resonant frequency of the system, expressed as:

[0024]

[0025] Among them, L T It is the inverter-side inductor, L g It is the grid-side inductance, C f It is a filter capacitor.

[0026] In the dq0 axis, the d-axis current is used for active power control, the q-axis current for reactive power control, and the 0-axis current for common-mode current control via active zero-sequence current injection. The reference value for the q-axis current is taken as 2πfC based on instantaneous reactive power theory. f U d .

[0027] Step 2 is as follows: The DC component under the dq0 axis obtained in Step 1 is obtained by active damping control to obtain the current output reference value. The difference between the actual current output value and the current output reference value is calculated to obtain the DC components of the output voltage on the d-axis, q-axis and 0-axis. Then, the three-phase modulation wave is obtained by inverse coordinate transformation. The DC component of the voltage on the 0-axis is injected into the active zero-sequence circuit for carrier modulation.

[0028] The beneficial effects of this invention are:

[0029] 1) In this invention, by using an active zero-sequence loop, only one additional connection line is needed to form a zero-sequence current path, so that the leakage current forms a circulating current between the LCL filter and the DC / AC converter, thereby preventing the leakage current from flowing into the AC power grid, and achieving low-cost and high-efficiency suppression of leakage current.

[0030] 2) The leakage current suppression technology based on active zero-sequence current injection used in this invention actively injects zero-sequence current in zero-sequence control to suppress common-mode current, reduce leakage current generated by shunt, improve leakage current suppression effect, reduce the impact of leakage current on system and equipment, and improve system stability and reliability.

[0031] 3) The active zero-sequence loop and leakage current suppression technology based on active zero-sequence current injection constructed in this invention are applicable to non-isolated low-voltage converter systems, including various DC devices such as solar photovoltaic panels and energy storage batteries, as well as various DC / AC converters such as two-level converters and three-level converters. They have broad application prospects, can effectively solve leakage current problems, improve the safety and stability of systems and equipment, and promote the development and application of related fields. Attached Figure Description

[0032] Figure 1 This is a schematic diagram of the low-voltage converter system structure based on active zero-sequence injection of the present invention;

[0033] Figure 2 This is the equivalent common-mode circuit diagram of the non-isolated grid-connected system based on the active zero-sequence loop of the present invention;

[0034] Figure 3 This is a schematic diagram of the low-voltage converter system and control structure based on active zero-sequence injection of the present invention;

[0035] Figure 4 This is a control block diagram of the leakage current suppression technology based on active zero-sequence current injection of the present invention;

[0036] Figure 5 This is the zero-axis equivalent differential circuit diagram of the present invention;

[0037] Figure 6 This is a waveform diagram of common-mode voltage and leakage current of a traditional low-voltage converter system under conventional current control.

[0038] Figure 7 This is a waveform diagram of the common-mode voltage and leakage current in a low-voltage converter system with an active zero-sequence loop added in this invention. Detailed Implementation

[0039] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments.

[0040] Example 1

[0041] This invention is based on a low-voltage converter system with active zero-sequence injection, such as... Figure 1 As shown, the equivalent common-mode circuit diagram is as follows: Figure 2 As shown, the system includes a DC power supply device. The output of the DC power supply device is connected to the DC side of a DC / AC converter via a DC bus. Each phase of the AC side of the DC / AC converter is connected to the corresponding phase of the AC grid via an LCL filter. The filter capacitor of the LCL filter is connected to the negative terminal of the DC bus via an active zero-sequence loop. The DC power supply device generates DC power, producing a 750V DC voltage on the DC bus. This DC voltage is converted to AC voltage by the DC / AC converter. After being filtered by the LCL filter, the AC voltage enters the 380V AC grid. The leakage current of the LCL filter capacitor circulates through the active zero-sequence loop. In this system, the neutral point of the LCL filter capacitor is short-circuited to the negative terminal of the DC bus, actively creating a zero-sequence current path so that the leakage current circulates between the LCL filter and the DC / AC converter, preventing leakage current from flowing into the AC grid.

[0042] The DC bus serves as an intermediate medium connecting DC equipment and the DC / AC converter, responsible for transmitting DC power to the DC / AC converter and transmitting the converted AC power to the improved LCL filter.

[0043] Each LCL filter includes an inductor L T Inductor L T One end is connected to the AC side of the DC / AC converter, inductor L T The other end is connected to both a filter capacitor and an inductor L. g The filter capacitor is connected to one end of the active zero-sequence loop, and the inductor L... g Connected to the AC power grid, the LCL filter serves as the system's filtering device.

[0044] Depending on the application scenario, DC / AC converters can be selected from various types such as two-level converters and three-level converters to convert DC power on the DC bus into AC power and output it to the improved LCL filter.

[0045] DC power equipment is either solar photovoltaic panels or energy storage batteries.

[0046] The inverter-side filter inductor, filter capacitor, grid-side filter inductor, and an improved neutral point connection method that short-circuits the neutral point of the filter capacitor and the neutral point of the DC bus split capacitor, together form a zero-sequence current loop, preventing leakage current from flowing into the AC grid. The filter inductor and filter capacitor constitute a filter network used to filter high-frequency noise flowing into the AC grid.

[0047] The AC power grid, serving as the system's power output terminal, is connected to the output of the improved LCL filter to receive filtered AC power. The AC power grid can be a city power grid, an industrial power grid, or other types of AC power source, used to supply power to external loads.

[0048] Example 2

[0049] DC / AC converter structure as follows Figure 1 As shown, it includes 6 switching transistors, which are arranged in pairs to form a bridge arm. The three bridge arms are arranged in parallel and connected to a DC capacitor. The midpoint of each of the three bridge arms is connected to an inductor L of an LCL filter. T The two ends of the DC capacitor are connected to the DC power equipment.

[0050] Example 3

[0051] like Figure 3 , Figure 4 As shown, a leakage current control method for a low-voltage converter system based on active zero-sequence injection is described. This suppression technique suppresses leakage current generation by actively injecting zero-sequence current on the inverter side. Specifically, it is implemented according to the following steps:

[0052] Step 1: Collect the data from the passing inductor L T Current and inductance L g The output voltage is calculated using a phase-locked loop (PLL) to determine the required rotation angle θ for control in the synchronous rotating coordinate system dq0. The inductor L... T The current is transformed by Park to obtain the DC component under the dq0 axis. The current and voltage are controlled to be in phase by rotating the angle θ.

[0053] Step 2: Obtain the DC component of the output voltage using a current PI controller and an active damping controller. Then, obtain a three-phase modulated wave through inverse coordinate transformation, which serves as the input variable for the carrier modulation strategy of the active zero-sequence loop. Specifically, the DC component along the dq0 axis obtained in Step 1 is processed by active damping control to obtain a current output reference value. The difference between the actual current output value and the current output reference value is calculated to obtain the DC components of the output voltage along the d, q, and 0 axes. Then, a three-phase modulated wave is obtained through inverse coordinate transformation. The DC component of the voltage along the 0 axis is injected into the active zero-sequence loop for carrier modulation. The equivalent differential circuit diagram along the zero axis is shown below. Figure 5 As shown, a zero-sequence current path is constructed by connecting the neutral point of the filter capacitor and the neutral point of the DC bus split capacitor. Through leakage current suppression control via active zero-sequence current injection on the 0 axis, the common-mode current is suppressed, reducing the leakage current generated by the shunt.

[0054] Current PI controller transfer function G c (s) is represented as:

[0055]

[0056] Among them, K p It is a proportional parameter, K i It is the integral parameter.

[0057] Active damped controller transfer function G LCL (s) is represented as:

[0058]

[0059] Among them, G iL2 It is the transfer function of the LCL filter, G d This results in a delay of 1.5 sampling periods during the calculation and pulse width modulation process, H i1 K is an active damping control parameter. pwm It is the DC / AC converter gain, G ic It is the transfer function from the converter output voltage to the capacitor current.

[0060] G ic G iL2 G d They are represented as follows:

[0061]

[0062] Where, ω r This is the resonant frequency of the system, expressed as:

[0063]

[0064] Among them, L T It is the inverter-side inductor, L g It is the grid-side inductance, C f It is a filter capacitor.

[0065] In the dq0 axis, the d-axis current is used for active power control, the q-axis current for reactive power control, and the 0-axis current for common-mode current control via active zero-sequence current injection. The reference value for the q-axis current is taken as 2πfC based on instantaneous reactive power theory. f U d .

[0066] Example 4

[0067] The common-mode voltage and leakage current waveforms of a traditional low-voltage converter system under conventional current control are shown below. Figure 6 As shown, when there is no zero-sequence loop and the active zero-sequence current control is cut off, the common-mode voltage ripple value is ±600V, and the effective value of its leakage current is 300mA, which far exceeds the 30mA required by VDE0126-1-1. At this time, a large amount of leakage current flows into the AC power grid, causing problems such as distortion of the grid current.

[0068] In a low-voltage converter system incorporating an active zero-sequence loop, a leakage current control method based on active zero-sequence current injection is implemented. The common-mode voltage and leakage current waveforms are shown below. Figure 7 As shown.

[0069] according to Figure 7 It can be seen that the grid-connected current remains stable, the power quality is good, the common-mode voltage fluctuation is within ±16V, and the effective value of the leakage current is about 0.62mA, which is far below the 30mA requirement of VDE0126-1-1.

[0070] The low-voltage converter system based on active zero-sequence injection, as described above, comprises: DC equipment, a DC bus, a DC / AC converter, an LCL filter, an active zero-sequence loop, and an AC power grid. The leakage current control method for this system includes: acquiring the grid phase; converting the sampled inverter-side current into dq0-axis components using Park transformation; using the d-axis current for active power control, the q-axis current for reactive power control, and the 0-axis for leakage current control based on active zero-sequence current injection; obtaining the DC component of the output voltage through active damping control; and obtaining the three-phase modulated wave through inverse coordinate transformation, which serves as the input variable for the carrier modulation strategy. This invention successfully suppresses leakage current, achieving LCL resonance suppression and system stability.

Claims

1. A leakage current control method for a low-voltage converter system based on active zero-sequence injection, characterized in that, The low-voltage converter system includes a DC power device. The output of the DC power device is connected to the DC side of a DC / AC converter via a DC bus. Each phase of the AC side of the DC / AC converter is connected to the corresponding phase of the AC grid via an LCL filter. The filter capacitor of the LCL filter is connected to the negative terminal of the DC bus via an active zero-sequence loop. The DC power device generates DC power, producing a 750V DC voltage on the DC bus. The DC / AC converter converts the DC voltage into AC voltage. After being filtered by the LCL filter, the AC voltage enters the 380V AC grid. The leakage current of the LCL filter capacitor circulates through the active zero-sequence loop. The LCL filter includes an inductor L T The inductor L T One end is connected to the AC side of the DC / AC converter, and the inductor L T The other end is connected to both a filter capacitor and an inductor L. g The filter capacitor is connected to one end of the active zero-sequence loop, and the inductor L g Connect to the AC power grid; The leakage current control method is implemented according to the following steps: Step 1: Collect the data from the passing inductor L T Current and inductance L g The output voltage is used to calculate the required coordinate rotation angle for control in the synchronous rotating coordinate system dq0 using a phase-locked loop. θ , inductor L T The current is transformed using the Park transform to obtain the DC component along the dq0 axis, and then rotated by an angle. θ Control the current and voltage to be in phase; Step 2: Obtain the DC component of the output voltage using a current PI controller and an active damping controller, and obtain the three-phase modulated wave through inverse coordinate transformation, which serves as the input variable for the carrier modulation strategy of the active zero-sequence loop. The active damping controller transfer function G LCL (s) is represented as: （2） in, G iL2 It is the transfer function of the LCL filter. G d This results in a delay of 1.5 sampling cycles during the calculation and pulse width modulation process. H i1 These are active damping control parameters. K pwm It is the DC / AC converter gain. G ic It is the transfer function from the converter output voltage to the capacitor current. K p It is a proportional parameter; The G ic , G iL2 , G d They are represented as follows: （3） in, ω r This is the resonant frequency of the system, expressed as: （4） in, L T It is the inverter-side inductor. L g It is the grid-side inductance. C f It is a filter capacitor.

2. The leakage current control method for a low-voltage converter system based on active zero-sequence injection according to claim 1, characterized in that, The DC / AC converter is one of a two-level converter, a three-level converter, or an NPC-type converter.

3. The leakage current control method for a low-voltage converter system based on active zero-sequence injection according to claim 1, characterized in that, The DC power equipment is a solar photovoltaic panel or an energy storage battery.

4. The leakage current control method for a low-voltage converter system based on active zero-sequence injection according to claim 1, characterized in that, The current PI controller transfer function G c (s) is represented as: （1） in, K p It is a proportional parameter. K i It is the integral parameter.

5. The leakage current control method for a low-voltage converter system based on active zero-sequence injection according to claim 1, characterized in that, In the dq0 axis, the d-axis current is used for active power control, the q-axis current for reactive power control, and the 0-axis current for common-mode current control via active zero-sequence current injection. The reference value for the q-axis current is taken as 2π based on instantaneous reactive power theory. f C f U d .

6. The leakage current control method for a low-voltage converter system based on active zero-sequence injection according to claim 1, characterized in that, Step 2 is as follows: The DC component under the dq0 axis obtained in Step 1 is obtained by active damping control to obtain the current output reference value. The difference between the actual current output value and the current output reference value is calculated to obtain the DC components of the output voltage on the d-axis, q-axis and 0-axis. Then, the three-phase modulation wave is obtained by inverse coordinate transformation. The DC component of the voltage on the 0-axis is injected into the active zero-sequence circuit for carrier modulation.

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