CN116960950A - Low-voltage converter system based on active zero sequence injection and leakage current control method - Google Patents

Low-voltage converter system based on active zero sequence injection and leakage current control method Download PDF

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Publication number
CN116960950A
CN116960950A CN202310840842.7A CN202310840842A CN116960950A CN 116960950 A CN116960950 A CN 116960950A CN 202310840842 A CN202310840842 A CN 202310840842A CN 116960950 A CN116960950 A CN 116960950A
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current
zero sequence
voltage
active
direct
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CN202310840842.7A
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Inventor
刘闯
毛廷瑞
蔡国伟
王汝田
王秀云
裴忠晨
于钦海
李瑞峰
王菁月
孔德昊
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Northeast Electric Power University
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Northeast Dianli University
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Publication of CN116960950A publication Critical patent/CN116960950A/en
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    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/001—Methods to deal with contingencies, e.g. abnormalities, faults or failures
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02H—EMERGENCY PROTECTIVE CIRCUIT ARRANGEMENTS
    • H02H9/00—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection
    • H02H9/02—Emergency protective circuit arrangements for limiting excess current or voltage without disconnection responsive to excess current
    • H—ELECTRICITY
    • H02—GENERATION; CONVERSION OR DISTRIBUTION OF ELECTRIC POWER
    • H02J—CIRCUIT ARRANGEMENTS OR SYSTEMS FOR SUPPLYING OR DISTRIBUTING ELECTRIC POWER; SYSTEMS FOR STORING ELECTRIC ENERGY
    • H02J3/00—Circuit arrangements for ac mains or ac distribution networks
    • H02J3/38—Arrangements for parallely feeding a single network by two or more generators, converters or transformers
    • H02J3/381—Dispersed generators

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  • Engineering & Computer Science (AREA)
  • Power Engineering (AREA)
  • Inverter Devices (AREA)

Abstract

The invention discloses a low-voltage converter system based on active zero sequence injection. The leakage current control method of the low-voltage converter system based on active zero sequence injection comprises the following steps: the method comprises the steps of obtaining a power grid phase, converting a sampling value of inverter side current into a dq0 axis component through Park conversion, controlling d-axis current to be active power, controlling q-axis current to be reactive power, controlling 0-axis current to be leakage current based on active zero sequence current injection, obtaining an output voltage direct current component through active damping control, obtaining a three-phase modulation wave through coordinate inverse conversion, and using the three-phase modulation wave as an input variable of a carrier modulation strategy. The invention successfully inhibits leakage current and realizes LCL resonance inhibition and system stability.

Description

Low-voltage converter system based on active zero sequence injection and leakage current control method
Technical Field
The invention belongs to the technical field of non-isolated low-voltage current transformation systems, and particularly relates to a low-voltage current transformation system based on active zero sequence injection and a leakage current control method of the low-voltage current transformation system based on active zero sequence injection.
Background
With the continuous development and application of clean energy, low-voltage variable flow systems are widely used in the field of distributed energy. These systems typically include components such as DC power generation equipment (e.g., solar panels, energy storage cells, etc.), DC collection buses, DC/AC converters, LCL filters, and an AC power grid for converting DC power to AC power and injecting into the power grid.
However, in low voltage variable current systems, leakage current problems are an important technical challenge. Leakage currents may cause safety risks for the grid and system equipment, so that effective measures need to be taken to suppress leakage currents, and safety and reliability of the system are improved.
Conventional leakage current suppression methods can be generally classified into two categories: modulation-based methods and topology-based methods. The improved modulation method can stabilize common-mode voltage and reduce leakage current, but the common disadvantages of the modulation methods are high harmonic content of output waveforms, large filter for filtering, large volume and high cost. In topology-based methods, additional power switching transistors and common mode filters are often required to be introduced, which often results in inefficiency and easy initiation of new system instability problems.
Disclosure of Invention
The invention aims to provide a low-voltage converter system based on active zero sequence injection, which can form a zero sequence current path through an active zero sequence loop by only needing an extra connecting wire, so that leakage current forms circulation between an LCL filter and a DC/AC converter, thereby avoiding leakage current from flowing into an alternating current power grid, and realizing the suppression of the leakage current with low cost and high efficiency.
The invention further aims to provide a leakage current control method of the low-voltage converter system based on active zero sequence injection, which is used for inhibiting common mode current, reducing leakage current generated by shunt, improving the leakage current inhibition effect, reducing the influence of leakage current on the system and equipment and improving the stability and reliability of the system by actively injecting zero sequence current in zero sequence control.
The invention adopts the technical scheme that the low-voltage converter system based on active zero sequence injection comprises direct-current energy equipment, wherein the output end of the direct-current energy equipment is connected with the direct-current side of a DC/AC converter through a direct-current collecting bus, each phase of the alternating-current side of the DC/AC converter is connected with a corresponding phase of an alternating-current power grid through an LCL filter, a filter capacitor of the LCL filter is connected with the negative pole of the direct-current collecting bus through an active zero sequence loop, the direct-current energy equipment sends out direct current, 750V direct-current voltage is generated on the direct-current collecting bus, the direct-current voltage is converted into alternating-current voltage through the DC/AC converter, the alternating-current voltage is filtered by the LCL filter and enters the 380V alternating-current power grid, and leakage current passing through the filter capacitor of the LCL filter circulates through the active zero sequence loop.
The invention is also characterized in that:
each LCL filter comprises an inductance L T Inductance L T One end is connected with the alternating current side of the DC/AC converter, and the inductor L T The other end is simultaneously connected with a filter capacitor and an inductor L g The filter capacitor is connected with one end of the active zero sequence loop, and the inductor L g And connecting an alternating current power grid.
The DC/AC converter is one of a two-level converter, a three-level converter and an NPC type converter.
The direct current energy source equipment is a solar photovoltaic panel or an energy storage battery.
The leakage current control method of the low-voltage converter system based on active zero sequence injection is implemented according to the following steps:
step 1, collecting the passed inductance L T Current and inductance L g The voltage of the output end calculates the coordinate rotation angle theta required by control under the dq0 synchronous rotation coordinate system through a phase-locked loop, and the inductance L is calculated T Performing Park conversion on the current to obtain a direct current component under the dq0 axis, and controlling the current and voltage to be in phase through the rotation angle theta;
and 2, obtaining an output voltage direct current component by using the current PI controller and the active damping controller, and obtaining a three-phase modulation wave by coordinate inverse transformation, wherein the three-phase modulation wave is used as an input variable of a carrier modulation strategy of the active zero sequence loop.
Transfer function G of current PI controller c (s) is expressed as:
wherein K is p Is a proportion parameter, K i Is an integral parameter.
Active damping controller transfer function G LCL (s) is expressed as:
wherein G is iL2 Is the transfer function of the LCL filter, G d Is a delay of 1.5 sample periods, H, which occurs during the computation and pulse width modulation process i1 Is an active damping control parameter, K pwm Is the DC/AC converter gain, G ic Is the transfer function of the converter output voltage to the capacitive current.
G ic ，G iL2 ，G d Expressed as:
wherein omega r Is of the systemThe resonant frequency, expressed as:
wherein L is T Is an inverter side inductance, L g Is the inductance of the power grid side, C f Is a filter capacitor.
d-axis current in dq0 axis is active power control, q-axis current is reactive power control, and 0 axis is common mode current control of active zero sequence current injection; the q-axis current reference value is 2 pi fC according to the instantaneous reactive power theory f U d 。
The specific process of the step 2 is as follows: and (3) obtaining a current output reference value by active damping control of the direct current component under the dq0 axis obtained in the step (1), carrying out difference between the current output actual value and the current output reference value to obtain voltage direct current components of the d axis, the q axis and the 0 axis, obtaining three-phase modulation waves by coordinate inverse transformation, and injecting the voltage direct current component of the 0 axis into an active zero sequence loop for carrier modulation.
The beneficial effects of the invention are as follows:
1) According to the invention, the zero sequence current path can be formed by the active zero sequence loop only by an extra connecting wire, so that the leakage current forms circulation between the LCL filter and the DC/AC converter, thereby avoiding the leakage current from flowing into an alternating current power grid, and realizing the suppression of the leakage current with low cost and high efficiency.
2) According to the leakage current suppression technology based on active zero sequence current injection, the zero sequence current is actively injected in zero sequence control, so that common mode current is suppressed, leakage current generated by shunt is reduced, the leakage current suppression effect is improved, the influence of the leakage current on a system and equipment is reduced, and the stability and reliability of the system are improved.
3) The active zero sequence loop and the leakage current suppression technology based on active zero sequence current injection constructed in the invention are suitable for non-isolated low-voltage converter systems, and comprise various direct-current devices such as solar photovoltaic panels, energy storage batteries and the like and various DC/AC converters such as two-level converters, three-level converters and the like, have wide application prospects, can effectively solve the leakage current problem, improve the safety and stability of the systems and the devices, and promote the development and application of related fields.
Drawings
Fig. 1 is a schematic diagram of a low-voltage converter system structure based on active zero sequence injection;
FIG. 2 is an equivalent common mode circuit diagram of the non-isolated grid-connected system based on an active zero sequence loop;
fig. 3 is a schematic diagram of a low-voltage converter system and a control structure based on active zero sequence injection in the invention;
FIG. 4 is a control block diagram of the leakage current suppression technique based on active zero sequence current injection in accordance with the present invention;
FIG. 5 is a zero-axis equivalent differential circuit diagram of the present invention;
FIG. 6 is a graph of common mode voltage and leakage current waveforms of a conventional low voltage converter system under conventional current control;
fig. 7 is a graph of common mode voltage and leakage current waveforms in a low voltage variable current system incorporating an active zero sequence loop in accordance with the present invention.
Detailed Description
The invention will be described in detail below with reference to the drawings and the detailed description.
Example 1
The invention discloses a low-voltage converter system based on active zero sequence injection, as shown in figure 1, an equivalent common mode circuit diagram is shown in figure 2, and the system comprises direct current energy equipment, wherein the output end of the direct current energy equipment is connected with the direct current side of a DC/AC converter through a direct current collecting bus, each phase of the alternating current side of the DC/AC converter is connected with a corresponding phase of an alternating current power grid through an LCL filter, a filter capacitor of the LCL filter is connected with the negative pole of the direct current collecting bus through an active zero sequence loop, the direct current energy equipment sends out direct current, a direct current voltage of 750V is generated on the direct current collecting bus, the direct current voltage is converted into an alternating current voltage through the DC/AC converter, the alternating current voltage is filtered by the LCL filter and enters the 380V alternating current power grid, and leakage current passing through the filter capacitor of the LCL filter circulates through the active zero sequence loop. In the system, the neutral point of the filter capacitor of the LCL filter is in short connection with the negative electrode of the direct current bus, and a zero sequence current path is actively formed, so that leakage current circulates between the LCL filter and the DC/AC converter, and the leakage current is prevented from flowing into an alternating current power grid.
The direct current bus is used as an intermediate medium for connecting direct current equipment and the DC/AC converter, and is responsible for transmitting direct current electric energy to the DC/AC converter and transmitting the converted alternating current electric energy to the improved LCL filter.
Each LCL filter comprises an inductance L T Inductance L T One end is connected with the alternating current side of the DC/AC converter, and the inductor L T The other end is simultaneously connected with a filter capacitor and an inductor L g The filter capacitor is connected with one end of the active zero sequence loop, and the inductor L g And the LCL filter is connected with an alternating current power grid and is used as a filtering device of the system.
According to different application scenes, the DC/AC converter can select a plurality of converters such as a two-level converter, a three-level converter and the like, converts the direct-current electric energy on the direct-current bus into alternating-current electric energy, and outputs the alternating-current electric energy to the improved LCL filter.
The direct current energy source equipment is a solar photovoltaic panel or an energy storage battery.
The inverter side filter inductor, the filter capacitor, the power grid side filter inductor and an improved neutral point connection mode of shorting the neutral point of the filter capacitor and the neutral point of the direct current bus split capacitor are used for preventing leakage current from flowing into an alternating current power grid by forming a zero sequence current loop. The filter inductance and the filter capacitance form a filter network for filtering high-frequency noise flowing into the alternating current power grid.
The alternating current power grid is used as a power output end of the system and is connected with an output end of the improved LCL filter to receive the alternating current power subjected to filtering treatment. The ac power grid may be a municipal power grid, an industrial power grid, or other type of ac power source for supplying power to external loads.
Example 2
The DC/AC converter structure is shown in figure 1, and comprises 6 switch tubes, wherein each switch tube comprises a bridge arm, three bridge arms are mutually parallel, a direct current capacitor is arranged in parallel, and the midpoints of the three bridge arms are respectively connected with an inductance L of an LCL filter T The two ends of the direct current capacitor are connected with direct current energy equipment.
Example 3
As shown in fig. 3 and fig. 4, the suppression technology can suppress the generation of leakage current by actively injecting zero-sequence current at the inverter side according to the leakage current control method of the low-voltage converter system based on active zero-sequence injection; the method is implemented according to the following steps:
step 1, collecting the passed inductance L T Current and inductance L g The voltage of the output end calculates the coordinate rotation angle theta required by control under the dq0 synchronous rotation coordinate system through a phase-locked loop, and the inductance L is calculated T Performing Park conversion on the current to obtain a direct current component under the dq0 axis, and controlling the current and voltage to be in phase through the rotation angle theta;
and 2, obtaining an output voltage direct current component by using the current PI controller and the active damping controller, and obtaining a three-phase modulation wave by coordinate inverse transformation, wherein the three-phase modulation wave is used as an input variable of a carrier modulation strategy of the active zero sequence loop. The specific process is as follows: and (3) obtaining a current output reference value by active damping control of the direct current component under the dq0 axis obtained in the step (1), carrying out difference between the current output actual value and the current output reference value to obtain voltage direct current components of an output d axis, a q axis and a 0 axis, obtaining three-phase modulation waves by coordinate inverse transformation, and injecting the voltage direct current component of the 0 axis into an active zero sequence loop for carrier modulation, wherein a zero axis equivalent differential circuit diagram is shown in figure 5. A zero sequence current path is constructed by connecting a filter capacitor neutral point and a direct current bus split capacitor neutral point, and leakage current suppression control of active zero sequence current injection of a 0-axis is adopted to suppress common mode current and reduce leakage current generated by shunt.
Transfer function G of current PI controller c (s) is expressed as:
wherein K is p Is a proportion parameter, K i Is an integral parameter.
Active damping controller transfer function G LCL (s) is expressed as:
wherein G is iL2 Is the transfer function of the LCL filter, G d Is a delay of 1.5 sample periods, H, which occurs during the computation and pulse width modulation process i1 Is an active damping control parameter, K pwm Is the DC/AC converter gain, G ic Is the transfer function of the converter output voltage to the capacitive current.
G ic ，G iL2 ，G d Expressed as:
wherein omega r Is the resonant frequency of the system, expressed as:
wherein L is T Is an inverter side inductance, L g Is the inductance of the power grid side, C f Is a filter capacitor.
d-axis current in dq0 axis is active power control, q-axis current is reactive power control, and 0 axis is common mode current control of active zero sequence current injection; the q-axis current reference value is 2 pi fC according to the instantaneous reactive power theory f U d 。
Example 4
The common-mode voltage and leakage current waveform diagram of the traditional low-voltage converter system under the traditional current control is shown in fig. 6, when the zero-sequence loop is not adopted and active zero-sequence current control is cut off, the common-mode voltage pulsation value is +/-600V, the effective value of the leakage current is 300mA, and the leakage current is far more than 30mA required by VDE0126-1-1, and at the moment, a large amount of leakage current flows into an alternating current power grid to cause the problems of network access current distortion and the like.
In the low-voltage converter system added with the active zero sequence loop, a leakage current control method based on active zero sequence current injection is put into, and the common-mode voltage and leakage current waveform diagram of the leakage current control method is shown in figure 7.
As can be seen from FIG. 7, the grid-connected current is kept stable, the power quality is good, the common-mode voltage fluctuation is within +/-16V, and meanwhile, the effective value of the leakage current is about 0.62mA, which is far lower than the 30mA requirement of VDE 0126-1-1.
By the mode, the low-voltage converter system based on active zero sequence injection comprises direct current equipment, a direct current bus, a DC/AC converter, an LCL filter, an active zero sequence loop and an alternating current power grid. The leakage current control method of the low-voltage converter system based on active zero sequence injection comprises the following steps: the method comprises the steps of obtaining a power grid phase, converting a sampling value of inverter side current into a dq0 axis component through Park conversion, controlling d-axis current to be active power, controlling q-axis current to be reactive power, controlling 0-axis current to be leakage current based on active zero sequence current injection, obtaining an output voltage direct current component through active damping control, obtaining a three-phase modulation wave through coordinate inverse conversion, and using the three-phase modulation wave as an input variable of a carrier modulation strategy. The invention successfully inhibits leakage current and realizes LCL resonance inhibition and system stability.

Claims (10)

1. The low-voltage conversion system based on active zero sequence injection is characterized by comprising direct-current energy equipment, wherein the output end of the direct-current energy equipment is connected with the direct-current side of a DC/AC converter through a direct-current collecting bus, each phase of the alternating-current side of the DC/AC converter is connected with a corresponding phase of an alternating-current power grid through an LCL filter, a filter capacitor of the LCL filter is connected with the negative electrode of the direct-current collecting bus through an active zero sequence loop, the direct-current energy equipment emits direct current, 750V direct-current voltage is generated on the direct-current collecting bus, the direct-current voltage is converted into alternating-current voltage through the DC/AC converter, the alternating-current voltage is filtered by the LCL filter and enters the 380V alternating-current power grid, and leakage current passing through the filter capacitor of the LCL filter is circulated through the active zero sequence loop.
2. The active zero sequence injection based low voltage conversion system according to claim 1, wherein each of the LCL filters comprises an inductance L T The inductance L T One end is connected with the alternating current side of the DC/AC converter, and the inductor L T The other end is connected with the filter at the same timeWave capacitance, inductance L g The filter capacitor is connected with one end of the active zero sequence loop, and the inductor L g And connecting an alternating current power grid.
3. The active zero sequence injection based low voltage conversion system according to claim 1, wherein the DC/AC converter is one of a two-level converter, a three-level converter, and an NPC type converter.
4. The active zero sequence injection-based low voltage conversion system according to claim 1, wherein the direct current energy source equipment is a solar photovoltaic panel or an energy storage battery.
5. The leakage current control method of the low-voltage converter system based on active zero sequence injection as claimed in claim 2, which is characterized by being implemented according to the following steps:
step 1, collecting the passed inductance L T Current and inductance L g The voltage of the output end calculates the coordinate rotation angle theta required by control under the dq0 synchronous rotation coordinate system through a phase-locked loop, and the inductance L is calculated T Performing Park conversion on the current to obtain a direct current component under the dq0 axis, and controlling the current and voltage to be in phase through the rotation angle theta;
and 2, obtaining an output voltage direct current component by using the current PI controller and the active damping controller, and obtaining a three-phase modulation wave by coordinate inverse transformation, wherein the three-phase modulation wave is used as an input variable of a carrier modulation strategy of the active zero sequence loop.
6. The method for controlling leakage current of a low-voltage converter system based on active zero sequence injection according to claim 5, wherein the current PI controller transfer function G c (s) is expressed as:
wherein K is p Is a proportion parameter, K i Is an integral parameter.
7. The leakage current control method of a low-voltage converter system based on active zero sequence injection according to claim 5, wherein the active damping controller transfer function G LCL (s) is expressed as:
wherein G is iL2 Is the transfer function of the LCL filter, G d Is a delay of 1.5 sample periods, H, which occurs during the computation and pulse width modulation process i1 Is an active damping control parameter, K pwm Is the DC/AC converter gain, G ic Is the transfer function of the converter output voltage to the capacitive current.
8. The leakage current control method of a low-voltage converter system based on active zero sequence injection according to claim 7, wherein the G ic ，G iL2 ，G d Expressed as:
wherein omega r Is the resonant frequency of the system, expressed as:
wherein L is T Is an inverter side inductance, L g Is the inductance of the power grid side, C f Is a filter capacitor.
9. The leakage current control method of the low-voltage converter system based on active zero sequence injection according to claim 7, wherein d-axis current in the dq0 axis is active power controlled, q-axis current is reactive power controlled, and 0-axis active zero sequence current is injectedCommon mode current control; the q-axis current reference value is 2 pi fC according to the instantaneous reactive power theory f U d 。
10. The leakage current control method of the low-voltage converter system based on active zero sequence injection according to claim 7, wherein the specific process of the step 2 is as follows: and (3) obtaining a current output reference value by active damping control of the direct current component under the dq0 axis obtained in the step (1), carrying out difference between the current output actual value and the current output reference value to obtain voltage direct current components of the d axis, the q axis and the 0 axis, obtaining three-phase modulation waves by coordinate inverse transformation, and injecting the voltage direct current component of the 0 axis into an active zero sequence loop for carrier modulation.
CN202310840842.7A 2023-07-10 2023-07-10 Low-voltage converter system based on active zero sequence injection and leakage current control method Pending CN116960950A (en)

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CN202310840842.7A CN116960950A (en) 2023-07-10 2023-07-10 Low-voltage converter system based on active zero sequence injection and leakage current control method

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Application Number Priority Date Filing Date Title
CN202310840842.7A CN116960950A (en) 2023-07-10 2023-07-10 Low-voltage converter system based on active zero sequence injection and leakage current control method

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