A three-phase four-bridge-arm converter non-sensing synchronous grid-connected system and method

Abstract

The present invention relates to a three-phase four-bridge-arm converter non-inductive synchronous grid-connected system and method, belonging to the technical field of converter grid-connected technology, including a sampling module, a coordinate transformation module, a positive and negative sequence separation module, an improved VSG control module, a dual-loop control module, a modulation module, an angular velocity integration module, a phase pre-synchronization module and a voltage pre-synchronization module; the present invention can realize non-inductive synchronous grid-connected according to the grid voltage and phase, the converter output voltage, through the invented VSG-based three-phase four-bridge-arm converter pre-synchronization method under asymmetric load, and reduce the fluctuation and instability of the system under asymmetric load conditions. In the implementation process, an attenuation function is introduced to avoid the sudden increase of active power in the early stage of pre-synchronization, effectively reduce the impact current during grid connection, and enhance the system stability and power quality.

Description

Technical Field

[0001] The invention relates to a three-phase four-bridge-arm converter non-inductive synchronous grid-connected system and method, belonging to the technical field of converter grid-connected technology. Background Art

[0002] In actual power systems, there are some single-phase loads and three-phase asymmetric loads. The four-leg converter is an alternative solution for powering and connecting asymmetric loads to the grid. With the continuous increase in electricity demand and the strengthening of environmental protection awareness, renewable energy represented by solar and wind energy has received more and more attention. The microgrid generated by the vigorous development of clean energy has the ability to operate stably in off-grid and grid-connected states, and can flexibly switch between independent operation and grid-connected modes.

[0003] However, when the grid-connected energy storage inverter switches between independent operation and grid-connected mode, it may cause current shock, which may endanger the safety and stability of the microgrid. Summary of the invention

[0004] In order to solve the above problems, the present invention provides a three-phase four-bridge-arm converter non-inductive synchronous grid-connected system and method, which can achieve the synchronization of the output voltage amplitude and phase of the three-phase four-bridge-arm converter with the power grid under asymmetric load at the time of grid connection, effectively reducing the impact current during grid connection and enhancing the system stability and power quality.

[0005] Terminology explanation:

[0006] 1. VSG: Virtual Synchronous Generator.

[0007] 2. PCC point: Point of Common Coupling, which means public connection point in Chinese, refers to the interface point between the power grid and the independent power source.

[0008] 3. PLL module: PLL stands for Phase-Locked Loop, which means phase-locked loop in Chinese.

[0009] 4. The abc coordinate system is a three-phase stationary coordinate system, the αβ coordinate system is a two-phase stationary coordinate system, and the dq coordinate system is a two-phase rotating coordinate system. The three can be transformed into each other. The transformation from the abc coordinate system to the αβ coordinate system is called Clarke transformation, and its transformation matrix is:

[0010] ;

[0011] The transformation from the abc coordinate system to the dq coordinate system is called Park transformation, and its transformation matrix is:

[0012] , wherei for d Axis and q The angle between the axes.

[0013] The technical solution of the present invention is as follows:

[0014] A three-phase four-bridge-arm converter non-sensing synchronous grid-connected system comprises a sampling module, a coordinate transformation module, a positive-negative sequence separation module, an improved VSG control module, a dual-loop control module, a modulation module, an angular velocity integration module, a phase pre-synchronization module and a voltage pre-synchronization module;

[0015] The sampling module collects the a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn , the three-phase AC current a, b, c output by the converter i a , i b , i c , the three-phase AC current a, b, c of the converter flow filter inductor i La , i Lb , i Lc And the a, b, c phase voltages of the grid output voltage u ga , u ga , u gc ;

[0016] The coordinate transformation module converts the input abc coordinate system into i La , i Lb , i Lc , u an , u bn , u cn , i a , i b and i c All are converted into values ​​of αβ coordinate system and input into the dual-loop control module;

[0017] The input terminal of the positive and negative sequence separation module inputs the a, b, and c phase voltages of the PCC point voltageu an , u bn , u cn , and the three-phase AC current a, b, c output by the converter i a , i b , i c The first output terminal outputs the positive sequence current of the three phases a, b, and c of the converter. i a + , i b + , i c + And input to the improved VSG control module, the second output terminal outputs the three-phase negative sequence current of converter a, b, c i a - , i b - , i c - The third output terminal outputs the three-phase positive sequence voltage of PCC points a, b, and c u an + , u bn + , u cn + The fourth output terminal outputs the three-phase negative sequence voltage of PCC points a, b, and c u an - , u bn - , u cn - ;

[0018] The phase pre-synchronization module includes a PLL module, and the a, b, and c phase voltages of the grid output voltage u ga , u ga , u gc The grid phase angle is obtained through the PLL module i g , the a, b, c phase voltages of the PCC point voltage u an , ubn , u cn and grid phase angle i g Input phase pre-synchronization module, the phase pre-synchronization module controls the a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn Grid phase angle i g Perform Park transformation on the reference angle to obtain the inverter voltage in the dq coordinate system u qn is zero; the angular frequency compensation value Δ output by the phase pre-synchronization module oh Input angular velocity integration module; the phase angle difference Δ output by the angular velocity integration module i The improved VSG control module is input to perform phase angle compensation on the VSG, and at the same time, an attenuation function is introduced to reduce the sudden increase in the active power output of the converter at the start of phase pre-synchronization.

[0019] The a, b, c phase voltages of the PCC point voltage u an , u bn , u cn And the a, b, c phase voltage of the grid output voltage u ga , u ga , u gc Input voltage pre-synchronization module, the voltage pre-synchronization module controls the RMS voltage of the converter U i and the RMS voltage of the grid U ga , U gb , U gc Keep consistent to achieve voltage pre-synchronization, the voltage difference Δ of the output of the voltage pre-synchronization module U Input the improved VSG control module to perform amplitude compensation on VSG;

[0020] The improved VSG control module is connected to the input end of the dual-loop control block, and the improved VSG control module adopts the improved VSG control strategy to control the complete voltage reference of the three-phase system of the converter. u refa , u refb , u refc, the improved VSG control module and dual-loop control module enter the modulation module to generate modulation waves to complete the pre-synchronization control of the VSG.

[0021] Preferably, the improved VSG control module comprises four input terminals and four output terminals, the first input terminal inputting the a, b, c phase voltages of the PCC point voltage u an , u bn , u cn The second input terminal inputs the positive sequence current of the three phases a, b, and c of the converter i a + , i b + , i c + , the third input terminal input phase difference Δ i , the fourth input terminal input voltage difference Δ U , the first output terminal outputs the complete voltage reference of phase a of the converter u refa The second output terminal outputs the complete voltage reference of the converter b phase u refb , the third output terminal outputs the complete voltage reference of the converter phase c u refc and connected to the input end of the dual-loop control block;

[0022] The expression of the improved VSG control strategy is as follows:

[0023] (1);

[0024] In the formula, P ref and P e are the active power reference value and active power calculated value at the PCC point respectively; similarly, Q ref and Q e are the reactive power reference value and reactive power calculation value at the PCC point; J and D 1 represent the virtual inertia constant and damping coefficient respectively; D 2 and K Represent the voltage regulation coefficient and the integral regulation coefficient respectively; s is the complex variable in the complex frequency domain signal obtained by Laplace transform of the time domain signal; oh n and ohis the reference angular frequency of the system and the angular frequency of the actual output; E m is the no-load electromotive force, i abc Indicates the three-phase AC current a, b, c output by the converter, including i a , i b and i c ; U ref and U m are the reference value and the measured value of the voltage RMS value respectively; and They are P e and Q e The DC components of active power and reactive power at the PCC point after removing the high-frequency components; Z v + is the positive sequence virtual impedance; t Indicates time; U refabc Represents the complete voltage amplitude reference of the three phases of the converter, including U refa , U refb , U refc ;

[0025] The purpose of the improved VSG control module is to use the DC components of active power and reactive power at the PCC point output by the converter. , and positive sequence virtual impedance Z v + Complete voltage reference for controlling the three phases of the converter u refa , u refb , u refc Specifically, the actual output angular frequency of the converter is calculated by formula (1): oh and the complete voltage amplitude reference of the three-phase system U refa , U refb , U refc , then the angular frequency oh The phase angle can be obtained by integrating I, Combined phase angle i and full voltage amplitude reference Urefa , U refb , U refc The complete voltage reference of the three-phase system can be obtained u refa , u refb , u refc .

[0026] Preferably, the phase pre-synchronization module includes two input terminals and one output terminal, and the first input terminal inputs the a, b, and c phase voltages of the grid output voltage. u ga , u ga , u gc , first of all u ga , u ga , u gc Performing Park transformation yields u gd , u gq , u g0 , among which u gq After passing through a PI controller, π Add, the sum after integration is 2 π The remainder is the grid phase angle. i g , the output angular frequency compensation value Δ oh ;

[0027] The inverter voltage is only when the phase of the grid voltage is equal to the phase of the converter output voltage. u qn is zero;

[0028] Angular frequency compensation Δ oh Input to the angular velocity integral module, the angular frequency compensation Δ oh Integrate over time to obtain and output the phase angle difference Δ i , input to the improved VSG control module, the phase angle difference Δ i The actual output angular frequency obtained by formula (1) oh The integral result phase angle i The addition can achieve phase angle compensation for the VSG.

[0029] Preferably, an attenuation function is introduced in the initial stage of pre-synchronization in the phase pre-synchronization module, and the attenuation function is:

[0030] (2);

[0031] In the formula, α Attenuation coefficient; t Indicates the time from the start of pre-synchronization ( t =0) elapsed time;

[0032] Assume the attenuation coefficient α The selected maximum value and the time Δ of power fluctuation at the beginning of pre-synchronization t , the difference between the maximum and minimum power fluctuations at the beginning of pre-synchronization Δ P , calculated active power value at PCC point P e There is a relationship between:

[0033] (3);

[0034] In the formula, k , β , c is a fixed constant;

[0035] Attenuation coefficient α In the calculation formula k , β , c The calculation method is:

[0036] (4);

[0037] In the formula, P e1 , P e2 , P e3 They are 10% rated power, 40% rated power, and 100% rated power respectively; α m1 , α m2 , α m3 They are the attenuation coefficient values ​​obtained when the output power is 10% of the rated power, 40% of the rated power, and 100% of the rated power; Δ t 1 , Δ t 2 , Δ t 3 They are the power fluctuation time corresponding to the output power of 10% rated power, 40% rated power and 100% rated power respectively; Δ P 1 , Δ P 2 , ΔP 3 They are the differences between the maximum and minimum values ​​of the power fluctuation corresponding to the output power of 10% rated power, 40% rated power, and 100% rated power respectively;

[0038] Taking the base 10 logarithm on both sides of the equal sign of the three sets of equations in the system, we get:

[0039] (5);

[0040] Solve formula (5) to get the attenuation coefficient α middle k , β , c .

[0041] Preferably, the voltage pre-synchronization module includes two input terminals and one output terminal, the first input terminal inputs the a, b, c phase voltages of the grid output voltage u ga , u ga , u gc The second input terminal inputs the a, b, c phase voltage of the PCC point voltage u an , u bn , u cn ,Will u ga , u ga , u gc The RMS voltage U ga , U gb , U gc and u an , u bn , u cn The RMS voltage U i Subtract the difference, and the difference is passed through the PI controller. The output of the PI controller is the voltage difference Δ U ;PI controller control U i and U ga , U gb , U gc Keep consistent, the output voltage difference Δ UInput into the improved SG control module and the no-load electromotive force obtained by equation (1) E m Add and change the no-load electromotive force of VSG E m , thereby changing the complete voltage amplitude reference of the three phases of the converter U refa , U refb、 U refc , so that the voltage output by the converter is consistent with the grid voltage.

[0042] Preferably, the coordinate transformation module includes nine input terminals and nine output terminals, and the first to third input terminals input the a, b, and c phase voltages of the PCC point voltage. u an , u bn , u cn , the fourth to sixth input terminals input three-phase alternating current a, b, c i a , i b , i c The seventh to ninth input terminals input the three-phase AC current of the filter inductor a, b, c i La , i Lb , i Lc ; The first to third output terminals output the PCC point voltage in the αβ coordinate system u α , u β , u 0 And input to the dual-loop control module, the fourth to sixth output terminals output the converter output current in the αβ coordinate system i α , i β , i 0 And input to the dual-loop control module, the seventh output terminal to the ninth output terminal output the converter current filter inductor current in the αβ coordinate system i Lα , i Lβ , i L0 And input to the dual-loop control module.

[0043] Preferably, the dual-loop control module includes four input terminals and three output terminals, and the first to third input terminals respectively input the complete voltage reference of the three-phase system of the converter u refa , u refb , u refc , the fourth input terminal inputs the nine output quantities output by the coordinate transformation module, and the first to third output terminals output the three-phase reference voltage of the converter without LC filtering and input it to the modulation module;

[0044] The dual-loop control module is equipped with a current control loop and a voltage control loop. The current control loop is a P controller, which uses the capacitor current i Cf As feedback signal; PR controller is used in voltage control loop, the resonant part is the core of PR controller, it can provide zero steady-state error for a specific frequency (such as grid frequency). The resonant controller has infinite gain at the resonant frequency (such as grid frequency), which can ensure that the voltage error at this frequency is fully compensated and achieve accurate voltage reference tracking.

[0045] Preferably, the modulation module includes six input terminals and eight output terminals, the first to third output terminals are the three-phase reference voltages of the converter when it is not LC filtered, the fourth input terminal is a constant, which means the carrier period, the fifth input terminal is a constant, which means the DC voltage amplitude, and the sixth input terminal is a constant, which means twice the sampling period; the first to eighth output terminals correspond to the switching signals of the upper and lower IGBTs of the first bridge arm to the upper and lower IGBTs of the fourth bridge arm respectively (the first bridge arm refers to a branch composed of T1 and T2 in the IGBT; the second bridge arm refers to a branch composed of T3 and T4 in the IGBT; the third bridge arm refers to a branch composed of T5 and T6 in the IGBT; the fourth bridge arm refers to a branch composed of T7 and T8 in the IGBT; IGBT refers to Insulated Gate Bipolar Transistor, which means insulated gate bipolar transistor in Chinese), using 3D-SVPWM modulation, the output signals of the modulation module are respectively input to the gate of the main circuit IGBT to drive the upper and lower IGBTs to be turned on.

[0046] A method for the three-phase four-bridge-arm converter non-sensing synchronous grid-connected system based on the above-mentioned method comprises the following steps:

[0047] (1) When pre-synchronization starts, the sampling module collects the a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn , the three-phase AC current a, b, c output by the converter ia , i b , i c , the three-phase AC current a, b, c of the converter flow filter inductor i La , i Lb , i Lc And the a, b, c phase voltages of the grid output voltage u ga , u ga , u gc ;

[0048] (2) The a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn , and the three-phase AC current a, b, c output by the converter i a , i b , i c Input to the positive and negative sequence separation module, the positive and negative sequence separation module outputs the three-phase positive sequence current of the converter a, b, c i a + , i b + , i c + and input to the improved VSG control module; i La , i Lb , i Lc , u an , u bn , u cn , i a , i b and i c The input coordinate transformation module outputs the value of each input quantity in the αβ coordinate system and inputs it into the dual-loop control module;

[0049] (3) Positive sequence current of three phases a, b, c i a+ , i b + , i c + and PCC point voltage u an , u bn , u cn Input to the improved VSG control module;

[0050] (4) Obtain the grid phase angle through the PLL module in the phase pre-synchronization module i g , the a, b, c phase voltages of the PCC point voltage u an , u bn , u c Grid phase angle i g The reference angle is converted into the inverter voltage in the dq reference frame through Park transformation u qn ; Construct a PI controller so that u qn = zero; PI controller outputs angular frequency compensation Δ oh , angular frequency compensation Δ oh Input to the angular velocity integral module, the angular frequency compensation Δ oh Integrate the time to get the phase angle difference Δ i , the phase angle difference Δ output by the phase pre-synchronization module i Enter the improved VSG control module;

[0051] (5) In the voltage pre-synchronization module, the PI controller controls the RMS voltage of the converter U i and the RMS voltage of the grid U ga , U gb , U gc To keep consistent, the voltage difference Δ output by the voltage pre-synchronization module U Input the improved VSG control module for amplitude compensation;

[0052] (6) In the improved VSG control module, the phase angle difference Δ i The actual output angular frequency of the converter obtained by formula (1) oh The integral result phase angle i Addition;

[0053] (7) In the improved VSG control module, the voltage difference Δ obtained by the voltage pre-synchronization module is U Input into the improved VSG control module and compared with the no-load electromotive force obtained by equation (1) E m Add and change the no-load electromotive force of VSG E m ; and introduce positive sequence virtual impedance, in the original no-load electromotive force E m Subtract the voltage drop of the current on the positive sequence virtual impedance from the reference voltage to obtain the complete voltage amplitude reference of the three-phase system. U refa , U refb , U refc ;

[0054] (8) The complete voltage amplitude of the three-phase system of the improved VSG control module is referenced U refa , U refb , U refc and phase angle i Input into the dual-loop control module and modulation module, and obtain the IGBT switching signal through 3D-SVPWM (three-dimensional space vector pulse width modulation);

[0055] (9) After the grid connection is completed, disconnect the phase pre-synchronization module and the voltage pre-synchronization module.

[0056] For any details not provided in the present invention, please refer to the prior art.

[0057] The beneficial effects of the present invention are:

[0058] The three-phase four-bridge-arm converter non-inductive synchronous grid-connected system and method of the present invention can realize non-inductive synchronous grid-connected according to the grid voltage and phase, converter output voltage, and the invented three-phase four-bridge-arm converter pre-synchronization method based on VSG under asymmetric load, and reduce the fluctuation and instability of the system under asymmetric load conditions. In the implementation process, the attenuation function is introduced to avoid the sudden increase of active power in the early stage of pre-synchronization, effectively reduce the impact current during grid connection, and enhance the system stability and power quality. BRIEF DESCRIPTION OF THE DRAWINGS

[0059] The drawings in the specification, which constitute a part of the present application, are used to provide further understanding of the present application. The illustrative embodiments of the present application and their descriptions are used to explain the present application and do not constitute improper limitations on the present application.

[0060] Figure 1 It is the control block diagram of the three-phase four-bridge-arm converter sensorless synchronous grid-connected system;

[0061] Figure 2 for P e Active output image without adding attenuation function when the power is 3000W;

[0062] Figure 3 for P e Active output image with attenuation function added when the power is 3000W;

[0063] Figure 4 for P e Active output image without adding attenuation function when the power is 5000W;

[0064] Figure 5 for P e Active output image with attenuation function added when the power is 5000W. DETAILED DESCRIPTION

[0065] In order to enable those skilled in the art to better understand the technical solutions in this specification, the technical solutions in the embodiments of the present invention are clearly and completely described below in conjunction with the drawings in the implementation of this specification, but are not limited to this. Anything not fully described in the present invention shall be based on the conventional technology in the art.

[0066] Example 1

[0067] A three-phase four-bridge-arm converter non-sensing synchronous grid-connected system, such as Figure 1 As shown, it includes a sampling module, a coordinate transformation module, a positive and negative sequence separation module, an improved VSG control module, a dual-loop control module, a modulation module, an angular velocity integration module, a phase pre-synchronization module and a voltage pre-synchronization module;

[0068] The sampling module collects the a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn , the three-phase AC current a, b, c output by the converter i a , i b , i c , the three-phase AC current a, b, c of the converter flow filter inductor i La , i Lb , i Lc And the a, b, c phase voltages of the grid output voltageu ga , u ga , u gc ;

[0069] The coordinate transformation module converts the input abc coordinate system into i La , i Lb , i Lc , u an , u bn , u cn , i a , i b and i c All are converted into values ​​of αβ coordinate system and input into the dual-loop control module;

[0070] The positive and negative sequence separation module includes two input terminals and four output terminals. The first input terminal inputs the a, b, and c phase voltages of the PCC point voltage. u an , u bn , u cn The second input terminal inputs the three-phase AC current a, b, c output by the converter i a , i b , i c , Clarke transform is performed on the two input ends to obtain i αβ0 and u αβ0 .by i αβ0 For example, i α and i β Through the second-order generalized integrator, we can get i α ‘ , qi α ‘ and i β ‘ , qi β ‘ , i α‘ and qi β ‘ Subtracting i α + , qi α ‘ and i β ‘ Subtracting i β + , i 0 + =0, i αβ0 + After reverse Clarke transformation, we can get i a + , i b + , i c + ; The first output terminal outputs the three-phase positive sequence current of the converter a, b, c i a + , i b + , i c + And input to the improved VSG control module, the second output terminal outputs the three-phase negative sequence current of converter a, b, c i a - , i b - , i c - The third output terminal outputs the three-phase positive sequence voltage of PCC points a, b, and c u an + , u bn + , u cn + The fourth output terminal outputs the three-phase negative sequence voltage of PCC points a, b, and c u an - , u bn - , u cn- ;

[0071] The phase pre-synchronization module includes a PLL module, and the a, b, and c phase voltages of the grid output voltage u ga , u ga , u gc The grid phase angle is obtained through the PLL module i g , the a, b, c phase voltages of the PCC point voltage u an , u bn , u cn and grid phase angle i g Input phase pre-synchronization module, the phase pre-synchronization module controls the a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn Grid phase angle i g Perform Park transformation on the reference angle to obtain the inverter voltage in the dq coordinate system u qn is zero; the angular frequency compensation value Δ output by the phase pre-synchronization module oh Input angular velocity integration module; the phase angle difference Δ output by the angular velocity integration module i The improved VSG control module is input to perform phase angle compensation on the VSG, and at the same time, an attenuation function is introduced to reduce the sudden increase in the active power output of the converter at the start of phase pre-synchronization.

[0072] Phase a, b, c voltages at PCC point u an , u bn , u cn And the a, b, c phase voltage of the grid output voltage u ga , u ga , u gc Input voltage pre-synchronization module, the voltage pre-synchronization module controls the RMS voltage of the converter U i and the RMS voltage of the grid U ga , U gb , U gcKeep consistent to achieve voltage pre-synchronization, the voltage difference Δ of the output of the voltage pre-synchronization module U Input the improved VSG control module to perform amplitude compensation on VSG;

[0073] The improved VSG control module is connected to the input end of the dual-loop control block. The improved VSG control module uses the improved VSG control strategy to control the complete voltage reference of the three-phase system of the converter. u refa , u refb , u refc , the improved VSG control module and dual-loop control module enter the modulation module to generate modulation waves to complete the pre-synchronization control of the VSG.

[0074] like Figure 1 In this case, a constant DC source is connected to a large capacitor in parallel C , used to stabilize the DC side voltage U dc . Eight IGBT switches T 1 ~ T 8 The upper and lower conduction output voltage is output to the common connection point (PCC) after LC filtering. j Phase voltage is recorded as u jn ( j =a,b,c), where the filter inductance values ​​connected to the output of the a, b, and c phase converters are L f , the fourth bridge arm to the neutral point N The inductance between L n . Current filter inductor j Phase current is i Lj . Each phase is connected to a filter capacitor C f They are all ideal capacitors and are star-connected. Then they are connected to a star-connected asymmetric load. Z load Each phase of the converter is connected to the AC grid. u ga , u ga , u gc The line impedance is Z line , after filtering, flows through the line impedance j Phase current is recorded as i j .

[0075] The present invention controls the a, b, and c phase voltages of the PCC point voltage. u an , u bn , u cn Grid phase angle i g The reference angle is obtained by Park transformation u qn Phase pre-synchronization is achieved by setting the RMS voltage of the converter to zero and voltage pre-synchronization is achieved by controlling the RMS voltage of the converter and the grid to be equal. An attenuation function is introduced to reduce the sudden increase in the active power output of the converter at the start of pre-synchronization, which effectively reduces the impact current during grid connection and enhances system stability and power quality.

[0076] The improved VSG control module includes four input terminals and four output terminals. The first input terminal inputs the a, b, and c phase voltages of the PCC point voltage. u an , u bn , u cn The second input terminal inputs the positive sequence current of the three phases a, b, and c of the converter i a + , i b + , i c + , the third input terminal input phase difference Δ i , the fourth input terminal input voltage difference Δ U , the first output terminal outputs the complete voltage reference of phase a of the converter u refa The second output terminal outputs the complete voltage reference of the converter b phase u refb , the third output terminal outputs the complete voltage reference of the converter phase c u refc and connected to the input end of the dual-loop control block;

[0077] In order to avoid voltage imbalance and frequency fluctuation, an improved VSG control strategy is adopted, including an improved virtual impedance method and a power calculation method. The expression of the improved VSG control strategy is as follows:

[0078] (1);

[0079] In the formula, P ref and P e are the active power reference value and active power calculated value at the PCC point respectively; similarly,Q ref and Q e are the reactive power reference value and reactive power calculation value at the PCC point; J and D 1 represent the virtual inertia constant and damping coefficient respectively; D 2 and K Represent the voltage regulation coefficient and the integral regulation coefficient respectively; s is the complex variable in the complex frequency domain signal obtained by Laplace transform of the time domain signal; oh n and oh is the reference angular frequency of the system and the angular frequency of the actual output; E m is the no-load electromotive force, i abc Indicates the three-phase AC current a, b, c output by the converter, including i a , i b and i c ; U ref and U m are the reference value and the measured value of the voltage RMS value respectively; and They are P e and Q e The DC components of active power and reactive power at the PCC point after removing the high-frequency components; Z v + is the positive sequence virtual impedance; t Indicates time; U refabc Represents the complete voltage amplitude reference of the three phases of the converter, including U refa , U refb , U refc ;

[0080] The purpose of the improved VSG control module is to use the DC components of active power and reactive power at the PCC point output by the converter. , and positive sequence virtual impedance Z v + Complete voltage reference for controlling the three phases of the converter u refa , urefb , u refc Specifically, the actual output angular frequency of the converter is calculated by formula (1): oh and the complete voltage amplitude reference of the three-phase system U refa , U refb , U refc , then the angular frequency oh The phase angle can be obtained by integrating I, Combined phase angle i and full voltage amplitude reference U refa , U refb , U refc The complete voltage reference of the three-phase system can be obtained u refa , u refb , u refc .

[0081] The phase presynchronization module includes two input terminals and one output terminal. The first input terminal inputs the a, b, and c phase voltages of the grid output voltage. u ga , u ga , u gc , first of all u ga , u ga , u gc Performing Park transformation yields u gd , u gq , u g0 , among which u gq After passing through a PI controller, π Add, the sum after integration is 2 π The remainder is the grid phase angle. i g , the voltage output by the four-leg inverter u an , u bn , u cn Grid phase angle i gThe reference angle is converted into the inverter voltage in the dq reference frame through Park transformation u qn , the inverter voltage u qn Subtract it from 0 and then construct a PI controller so that u qn is equal to zero. The output of the PI controller is the angular frequency compensation Δ oh , that is, the output angular frequency compensation value Δ of the output end of the phase pre-synchronization module oh ;

[0082] The inverter voltage is only when the phase of the grid voltage is equal to the phase of the converter output voltage. u qn The phase pre-synchronization module outputs the angular frequency compensation value Δ oh , and input to the angular velocity integration module, the angular frequency compensation Δ oh Integrate over time and output the phase angle difference Δ i , input to the improved VSG control module, the phase angle difference Δ i The actual output angular frequency obtained by formula (1) oh The integral result phase angle i The addition can achieve phase angle compensation for the VSG.

[0083] Preferably, an attenuation function is introduced in the initial stage of pre-synchronization in the phase pre-synchronization module to reduce the sudden increase in the active power output of the converter at the start of pre-synchronization, and the attenuation function is:

[0084] (2);

[0085] In the formula, α Attenuation coefficient; t Indicates the time from the start of pre-synchronization ( t =0) elapsed time;

[0086] Assume the attenuation coefficient α The selected maximum value and the time Δ of power fluctuation at the beginning of pre-synchronization t , the difference between the maximum and minimum power fluctuations at the beginning of pre-synchronization Δ P , calculated active power value at PCC point P e There is a relationship between:

[0087] (3);

[0088] In the formula, k , β , c is a fixed constant;

[0089] Attenuation coefficientα In the calculation formula k , β , c The calculation method is:

[0090] (4);

[0091] In the formula, P e1 , P e2 , P e3 They are 10% rated power, 40% rated power, and 100% rated power respectively; α m1 , α m2 , α m3 They are the attenuation coefficient values ​​obtained when the output power is 10% of the rated power, 40% of the rated power, and 100% of the rated power; Δ t 1 , Δ t 2 , Δ t 3 They are the power fluctuation time corresponding to the output power of 10% rated power, 40% rated power and 100% rated power respectively; Δ P 1 , Δ P 2 , Δ P 3 They are the differences between the maximum and minimum values ​​of the power fluctuation corresponding to the output power of 10% rated power, 40% rated power, and 100% rated power respectively;

[0092] Taking the base 10 logarithm on both sides of the equal sign of the three sets of equations in the system, we get:

[0093] (5);

[0094] Solve formula (5) to get the attenuation coefficient α middle k , β , c .

[0095] The y-axis value of the decay function changes with the horizontal axis time t The increase of Δ is from 0 to 1, and the speed of increase is from fast to slow. oh After the attenuation function, its value in the early stage of pre-synchronization is reduced, thereby reducing the phase angle difference Δ after integration i . Phase angle difference Δ iInput the improved VSG control module and the angular frequency of the converter obtained by formula (1) oh The integral result phase angle i Adding changes the phase angle of the converter i , VSG needs to adjust the active power it emits to change the angular frequency oh , thereby changing the phase angle i The addition of the attenuation function reduces the phase difference Δ at the start of pre-synchronization. i , thereby reducing the sudden increase in active power.

[0096] The voltage pre-synchronization module includes two input terminals and one output terminal. The first input terminal inputs the a, b, and c phase voltages of the grid output voltage. u ga , u ga , u gc The second input terminal inputs the a, b, c phase voltage of the PCC point voltage u an , u bn , u cn ,Will u ga , u ga , u gc The RMS voltage U ga , U gb , U gc and u an , u bn , u cn The RMS voltage U i Subtract the difference, and the difference is passed through the PI controller. The output of the PI controller is the voltage difference Δ U ;PI controller control U i and U ga , U gb , U gc Keep consistent, the output voltage difference Δ U Input into the improved SG control module and the no-load electromotive force obtained by equation (1) E m Add and change the no-load electromotive force of VSG E m, thereby changing the complete voltage amplitude reference of the three phases of the converter U refa , U refb、 U refc , so that the voltage output by the converter is consistent with the grid voltage.

[0097] The coordinate transformation module includes nine input terminals and nine output terminals. The first to third input terminals input the a, b, and c phase voltages of the PCC point voltage. u an , u bn , u cn , the fourth to sixth input terminals input three-phase alternating current a, b, c i a , i b , i c The seventh to ninth input terminals input the three-phase AC current of the filter inductor a, b, c i La , i Lb , i Lc ; The first to third output terminals output the PCC point voltage in the αβ coordinate system u α , u β , u 0 And input to the dual-loop control module, the fourth to sixth output terminals output the converter output current in the αβ coordinate system i α , i β , i 0 And input to the dual-loop control module, the seventh output terminal to the ninth output terminal output the converter current filter inductor current in the αβ coordinate system i Lα , i Lβ , i L0 And input to the dual-loop control module. L n for T 7 and T 8 The fourth bridge arm to the neutral point N In order to eliminate the inductance between L nThe three-phase coupling brought about by Clarke transformation is used to realize the conversion from the abc coordinate system to the αβγ coordinate system.

[0098] The dual-loop control module includes four input terminals and three output terminals. The first to third input terminals input the complete voltage reference of the three-phase system of the converter. u refa , u refb , u refc , the fourth input terminal inputs the nine output quantities output by the coordinate transformation module, and the first to third output terminals output the three-phase reference voltage of the converter without LC filtering and input it to the modulation module;

[0099] The dual-loop control module is equipped with a current control loop and a voltage control loop. The current control loop is a P controller, which uses the capacitor current i Cf As feedback signal; PR controller is used in voltage control loop, the resonant part is the core of PR controller, it can provide zero steady-state error for a specific frequency (such as grid frequency). The resonant controller has infinite gain at the resonant frequency (such as grid frequency), which can ensure that the voltage error at this frequency is fully compensated and achieve accurate voltage reference tracking.

[0100] The modulation module includes six input terminals and eight output terminals. The first to third output terminals are the three-phase reference voltages of the converter without LC filtering. The fourth input terminal is a constant, which means the carrier period. The fifth input terminal is a constant, which means the DC voltage amplitude. The sixth input terminal is a constant, which means twice the sampling period. The first to eighth output terminals correspond to the switching signals of the upper and lower IGBTs of the first bridge arm to the upper and lower IGBTs of the fourth bridge arm respectively (the first bridge arm refers to a branch composed of T1 and T2 in the IGBT; the second bridge arm refers to a branch composed of T3 and T4 in the IGBT; the third bridge arm refers to a branch composed of T5 and T6 in the IGBT; the fourth bridge arm refers to a branch composed of T7 and T8 in the IGBT; IGBT refers to Insulated Gate Bipolar Transistor, which means insulated gate bipolar transistor in Chinese). Using 3D-SVPWM modulation, the output signals of the modulation module are respectively input to the gate of the main circuit IGBT to drive the upper and lower IGBTs to be turned on.

[0101] Example 2

[0102] A method for a three-phase four-bridge-arm converter non-sensing synchronous grid-connected system based on Embodiment 1 comprises the following steps:

[0103] (1) When pre-synchronization starts, the sampling module collects the a, b, and c phase voltages of the PCC point voltage u an ,u bn , u cn , the three-phase AC current a, b, c output by the converter i a , i b , i c , the three-phase AC current a, b, c of the converter flow filter inductor i La , i Lb , i Lc And the a, b, c phase voltages of the grid output voltage u ga , u ga , u gc ;

[0104] (2) The a, b, and c phase voltages of the PCC point voltage u an , u bn , u cn , and the three-phase AC current a, b, c output by the converter i a , i b , i c Input to the positive and negative sequence separation module, the positive and negative sequence separation module outputs the three-phase positive sequence current of the converter a, b, c i a + , i b + , i c + and input to the improved VSG control module; i La , i Lb , i Lc , u an , u bn , u cn , i a , i b and i cThe input coordinate transformation module outputs the value of each input quantity in the αβ coordinate system and inputs it into the dual-loop control module;

[0105] (3) Positive sequence current of three phases a, b, c i a + , i b + , i c + and PCC point voltage u an , u bn , u cn Input to the improved VSG control module;

[0106] (4) Obtain the grid phase angle through the PLL module in the phase pre-synchronization module i g , the a, b, c phase voltages of the PCC point voltage u an , u bn , u c Grid phase angle i g The reference angle is converted into the inverter voltage in the dq reference frame through Park transformation u qn ; Construct a PI controller so that u qn = zero; PI controller outputs angular frequency compensation Δ oh , angular frequency compensation Δ oh Input to the angular velocity integral module, the angular frequency compensation Δ oh Integrate the time to get the phase angle difference Δ i , the phase angle difference Δ output by the phase pre-synchronization module i Enter the improved VSG control module;

[0107] (5) In the voltage pre-synchronization module, the PI controller controls the RMS voltage of the converter U i and the RMS voltage of the grid U ga , U gb , U gc To keep consistent, the voltage difference Δ output by the voltage pre-synchronization module U Input the improved VSG control module for amplitude compensation;

[0108] (6) In the improved VSG control module, the phase angle difference Δ i The actual output angular frequency of the converter obtained by formula (1) oh The integral result phase angle i Addition;

[0109] (7) In the improved VSG control module, the voltage difference Δ obtained by the voltage pre-synchronization module is U Input into the improved VSG control module and compared with the no-load electromotive force obtained by equation (1) E m Add and change the no-load electromotive force of VSG E m ; and introduce positive sequence virtual impedance, in the original no-load electromotive force E m Subtract the voltage drop of the current on the positive sequence virtual impedance from the reference voltage to obtain the complete voltage amplitude reference of the three-phase system. U refa , U refb , U refc ;

[0110] (8) The complete voltage amplitude of the three-phase system of the improved VSG control module is referenced U refa , U refb , U refc and phase angle i Input into the dual-loop control module and modulation module, and obtain the IGBT switching signal through 3D-SVPWM (three-dimensional space vector pulse width modulation);

[0111] (9) After the grid connection is completed, disconnect the phase pre-synchronization module and the voltage pre-synchronization module.

[0112] In order to verify the effectiveness of the sensorless synchronous grid-connected method of the three-phase four-bridge-arm converter based on the improved VSG proposed in this embodiment, a model is built in MATLAB / Simulink for simulation verification. The system parameters are shown in Table 1:

[0113] Table 1 Parameters of the sensorless synchronization system of the three-phase four-leg converter based on the improved VSG

[0114]

[0115] Tested α m1 , α m2 , α m3 The values ​​of are 1, 0.6, and 0.3. Based on formula (5), we can getk =0.01655924344, β =3.1164, c =0.0085128686, and then the attenuation coefficient at each output power can be obtained based on formula (3): α .

[0116] (6);

[0117] Simulation test of the phase angle difference between the converter and the grid (the grid phase angle difference refers to the grid phase angle i g The difference between the phase angle of the converter and the phase angle of the converter in this embodiment is 0°, and the power fluctuation time Δ at the beginning of the grid connection when the attenuation function is added and when the attenuation function is not added is 90° and 180°. t , the difference between the maximum and minimum power fluctuations Δ P The capability of reducing sudden increase in active power of this embodiment is compared and analyzed through two sets of simulation results.

[0118] The phase angle difference between the converter and the grid is 180° and P e When the power is 3000W, the comparison image of active output with and without adding attenuation function in the pre-synchronization stage is as follows: Figure 2 As shown in his 3, Figure 2 and Figure 3 The horizontal axis represents time. t , the unit is s ; The vertical axis is active power P , in W. In the figure, pre-synchronization occurs at 1s and grid connection is performed at 4s. Figure 2 The output active power image without adding the attenuation function, power fluctuation time Δ t =0.34s, the difference between the maximum and minimum power fluctuations Δ P =288W(Δ t , Δ P It can be read from the output active power image when the attenuation function is not added), and the power fluctuation is obvious. Figure 3 The figure in the figure shows the output active power image of the converter after adding the attenuation function. The attenuation coefficient is α =0.455, after adding the attenuation function, Δ t and Δ P It is basically zero, which effectively reduces the sudden increase in active power at the beginning of pre-synchronization.

[0119] The phase angle difference between the converter and the grid is 180° and P e When the active power is 5000W, the comparison image of active power output with and without adding attenuation function in the pre-synchronization stage is as follows: Figure 4 and Figure 5 As shown, Figure 4 and Figure 5 The horizontal axis represents time. t , the unit is s ; The vertical axis is active power P , in W. In the figure, pre-synchronization occurs at 1s and grid connection is performed at 4s. Figure 4 The output active power image without adding the attenuation function, power fluctuation time Δ t =0.4s, the difference between the maximum and minimum power fluctuations Δ P =413W, the power fluctuation is obvious. Figure 5 The figure in the figure shows the output active power image of the converter after adding the attenuation function. The attenuation coefficient is α =0.3, after adding the attenuation function, Δ t and Δ P It is basically zero, which effectively reduces the sudden increase in active power at the beginning of pre-synchronization.

[0120] The above is a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A three-phase four-bridge-arm converter non-sensing synchronous grid-connected system, characterized in that: It includes a sampling module, a coordinate transformation module, a positive and negative sequence separation module, an improved VSG control module, a dual-loop control module, a modulation module, an angular velocity integration module, a phase pre-synchronization module and a voltage pre-synchronization module; The sampling module collects the a, b, and c phase voltages u of the PCC point voltage an 、u bn 、u cn , the three-phase AC current a, b, c output by the converter is i a 、i b 、i c , the three-phase AC current i of a, b, c flowing through the filter inductor of the converter La 、i Lb 、i Lc And the a, b, c phase voltage u of the grid output voltage ga 、u ga 、u gc ; The coordinate transformation module converts the input i in the abc coordinate system La 、i Lb 、i Lc 、u an 、u bn 、u cn 、i a 、i b and i c All are converted into values ​​of αβ coordinate system and input into the dual-loop control module; The input terminal of the positive and negative sequence separation module inputs the a, b, c phase voltage u of the PCC point voltage an 、u bn 、u cn , and the three-phase AC current i a, b, c output by the converter a 、i b 、i c The first output terminal outputs the three-phase positive sequence current i of the converter a, b, c a + 、i b + 、i c + And input to the improved VSG control module, the second output terminal outputs the three-phase negative sequence current i of the converter a, b, c a - 、i b - 、i c - The third output terminal outputs the three-phase positive sequence voltage u of PCC points a, b, and c an + 、u bn + 、u cn + The fourth output terminal outputs the three-phase negative sequence voltage u of PCC points a, b, and c an - 、u bn - 、u cn - ; The phase pre-synchronization module includes a PLL module, and the a, b, and c phase voltages u ga 、u ga 、u gc The grid phase angle θ is obtained through the PLL module g , the a, b, c phase voltages u of the PCC point voltage an 、u bn 、u cn and grid phase angle θ g Input phase pre-synchronization module, the phase pre-synchronization module controls the a, b, c phase voltage u of the PCC point voltage an 、u bn 、u cn The grid phase angle θ g The inverter voltage u in the dq coordinate system is obtained by Park transformation for the reference angle qn is zero; the angular frequency compensation Δω output by the phase pre-synchronization module is input into the angular velocity integration module; the phase angle difference Δθ output by the angular velocity integration module is input into the improved VSG control module to perform phase angle compensation on the VSG, and at the same time, an attenuation function is introduced to reduce the sudden increase in the active power output of the converter at the start of the phase pre-synchronization; The a, b, c phase voltages u of the PCC point voltage an 、u bn 、u cn And the a, b, c phase voltage u of the grid output voltage ga 、u ga 、u gc Input voltage pre-synchronization module, the voltage pre-synchronization module controls the RMS voltage U of the converter i The voltage pre-synchronization is realized by keeping consistent with the root mean square voltage of the power grid. The voltage difference ΔU output by the voltage pre-synchronization module is input into the improved VSG control module to perform amplitude compensation on the VSG. The improved VSG control module is connected to the input end of the dual-loop control block. The improved VSG control module uses an improved VSG control strategy to control the complete voltage reference u of the three-phase system of the converter. refa 、u refb 、u refc , the improved VSG control module and dual-loop control module enter the modulation module to generate modulation waves to complete the pre-synchronization control of the VSG; The improved VSG control module includes four input terminals and four output terminals. The first input terminal inputs the a, b, and c phase voltages u of the PCC point voltage. an 、u bn 、u cn The second input terminal inputs the three-phase positive sequence current i of the converter a, b, c a + 、i b + 、i c + , the third input terminal inputs the phase angle difference Δθ, the fourth input terminal inputs the voltage difference ΔU, and the first output terminal outputs the complete voltage reference u of the a phase of the converter refa The second output terminal outputs the complete voltage reference u of phase b of the converter refb The third output terminal outputs the complete voltage reference u of the converter phase c refc and connected to the input end of the dual-loop control block; The expression of the improved VSG control strategy is as follows: Where P ref and P e are the active power reference value and active power calculation value at the PCC point respectively; Q ref and Q e are the reactive power reference value and reactive power calculation value at the PCC point; J and D1 represent the virtual inertia constant and damping coefficient respectively; D2 and K represent the voltage regulation coefficient and integral regulation coefficient respectively; s is the complex variable in the complex frequency domain signal obtained by Laplace transform of the time domain signal; ω n and ω are the reference angular frequency and actual output angular frequency of the system; E m is the no-load electromotive force, i abc Indicates the three-phase AC current a, b, c output by the converter, including i a 、i b and i c ; U ref and U m are the reference value and the measured value of the voltage RMS value respectively; and They are P e and Q e The DC components of active power and reactive power at the PCC point after removing the high-frequency components; Z v + is the positive sequence virtual impedance; t represents time; U refabc Represents the complete voltage amplitude reference of the three phases of the converter, including U refa , U refb , U refc ; The purpose of the improved VSG control module is to use the DC components of active power and reactive power at the PCC point output by the converter. and positive sequence virtual impedance Z v + The complete voltage reference u that controls the three phases of the converter refa 、u refb 、u refc ; The phase pre-synchronization module includes two input terminals and one output terminal. The first input terminal inputs the a, b, and c phase voltages u of the grid output voltage. ga 、u ga 、u gc , first of all, ga 、u ga 、u gc Perform Park transform to get u gd 、u gq 、u g0 , where u gq After passing through a PI controller, it is added to 100π, and the sum is integrated and then modulo 2π. The remainder is the grid phase angle θ g , the output angular frequency compensation Δω is output at the output end; Only when the phase of the grid voltage is equal to the phase of the converter output voltage, the inverter voltage u qn is zero; The angular frequency compensation Δω is input to the angular velocity integration module, and the angular frequency compensation Δω is integrated over time to obtain and output the phase angle difference Δθ, which is input to the improved VSG control module. The phase angle difference Δθ is added to the integral result phase angle θ of the actual output angular frequency ω obtained by formula (1) to achieve phase angle compensation for VSG. The attenuation function is introduced in the early stage of pre-synchronization in the phase pre-synchronization module, and the attenuation function is: and=1-e -αt (2) Where α is the attenuation coefficient; t represents the time elapsed from the start of pre-synchronization; Assume that the maximum value of the attenuation coefficient α and the time Δt of the power fluctuation at the beginning of the pre-synchronization stage, the difference ΔP between the maximum and minimum values ​​of the power fluctuation at the beginning of the pre-synchronization stage, and the active power calculation value P at the PCC point e There is a relationship between: In the formula, k, β, and γ are fixed constants; The calculation method of k, β, and γ in the attenuation coefficient α calculation formula is: Where P e1 , P e2 , P e3 They are 10% rated power, 40% rated power, and 100% rated power respectively; α m1 , α m2 , α m3 are the attenuation coefficient values ​​obtained when the output power is 10% of the rated power, 40% of the rated power, and 100% of the rated power, respectively; Δt1, Δt2, and Δt3 are the power fluctuation times corresponding to the output power of 10% of the rated power, 40% of the rated power, and 100% of the rated power, respectively; ΔP1, ΔP2, and ΔP3 are the differences between the maximum and minimum values ​​of the power fluctuations corresponding to the output power of 10% of the rated power, 40% of the rated power, and 100% of the rated power, respectively; Taking the base 10 logarithm on both sides of the equal sign of the three sets of equations in the system, we get: Solving formula (5) yields k, β, and γ in the attenuation coefficient α.

2. The three-phase four-bridge-arm converter non-sensing synchronous grid-connected system according to claim 1, characterized in that: The voltage pre-synchronization module includes two input terminals and one output terminal. The first input terminal inputs the a, b, and c phase voltages u of the grid output voltage. ga 、u ga 、u gc The second input terminal inputs the a, b, c phase voltage u of the PCC point voltage an 、u bn 、u cn , will u ga 、u ga 、u gc The root mean square voltage U ga , U gb , U gc with u an 、u bn 、u cn The root mean square voltage U i Subtract the difference, and the difference is passed through the PI controller. The output of the PI controller is the voltage difference ΔU. The PI controller controls U i and U ga , U gb , U gc Keeping the same, the output voltage difference ΔU is input into the improved SG control module and compared with the no-load electromotive force E obtained by formula (1) m Add and change the no-load electromotive force E of VSG m , thereby changing the complete voltage amplitude reference U of the three-phase converter refa , U refb、 U refc , so that the voltage output by the converter is consistent with the grid voltage.

3. The three-phase four-bridge-arm converter non-sensing synchronous grid-connected system according to claim 2, characterized in that: The coordinate transformation module includes nine input terminals and nine output terminals. The first to third input terminals input the a, b, and c phase voltages u of the PCC point voltage. an 、u bn 、u cn The fourth to sixth input terminals input three-phase alternating current a, b, c i a 、i b 、i c The a, b, c three-phase AC current i of the seventh to ninth input terminals is input into the filter inductor La 、i Lb 、i Lc ; The first to third output terminals output the voltage u of the PCC point in the αβ coordinate system α 、u β , u0 and input to the dual-loop control module, the fourth to sixth output terminals output the converter output current i in the αβ coordinate system α 、i β , i0 and input to the dual-loop control module, the seventh output terminal to the ninth output terminal output the converter current filter inductor current i in the αβ coordinate system Lα 、i Lβ 、i L0 And input to the dual-loop control module.

4. The three-phase four-bridge-arm converter non-sensing synchronous grid-connected system according to claim 3, characterized in that: The dual-loop control module includes four input terminals and three output terminals. The first to third input terminals respectively input the complete voltage reference u of the three-phase system of the converter. refa 、u refb 、u refc , the fourth input terminal inputs the nine output quantities output by the coordinate transformation module, and the first to third output terminals output the three-phase reference voltage of the converter without LC filtering and input it to the modulation module; The dual-loop control module is equipped with a current control loop and a voltage control loop. The current control loop is a P controller, which takes the capacitor current i Cf As feedback signal; PR controller is used in voltage control loop, which can provide zero steady-state error to grid frequency.

5. The three-phase four-bridge-arm converter non-sensing synchronous grid-connected system according to claim 4, characterized in that: The modulation module includes six input terminals and eight output terminals. The first to third output terminals are the three-phase reference voltages of the converter without LC filtering. The fourth input terminal is a constant, which means the carrier period. The fifth input terminal is a constant, which means the DC voltage amplitude. The sixth input terminal is a constant, which means twice the sampling period. The first to eighth output terminals correspond to the switching signals of the upper and lower IGBTs of the first bridge arm to the upper and lower IGBTs of the fourth bridge arm, respectively. Using 3D-SVPWM modulation, the output signals of the modulation module are respectively input to the gates of the main circuit IGBT to drive the IGBT to be turned on and off.

6. A method for the three-phase four-bridge-arm converter non-sensing synchronous grid-connected system according to claim 5, comprising the following steps: (1) When pre-synchronization starts, the sampling module collects the a, b, and c phase voltages u of the PCC point voltage. an 、u bn 、u cn , the three-phase AC current a, b, c output by the converter is i a 、i b 、i c , the three-phase AC current i of a, b, c flowing through the filter inductor of the converter La 、i Lb 、i Lc And the a, b, c phase voltages u of the grid output voltage ga 、u ga 、u gc ; (2) The a, b, and c phase voltages u of the PCC point voltage an 、u bn 、u cn , and the three-phase AC current i a, b, c output by the converter a 、i b 、i c Input to the positive and negative sequence separation module, the positive and negative sequence separation module outputs the three-phase positive sequence current i of the converter a, b, c a + 、i b + 、i c + And input to the improved VSG control module; i La 、i Lb 、i Lc 、u an 、u bn 、u cn 、i a 、i b and i c The input coordinate transformation module outputs the value of each input quantity in the αβ coordinate system and inputs it into the dual-loop control module; (3) a, b, c three-phase positive sequence current i a + 、i b + 、i c + and PCC point voltage u an 、u bn 、u cn Input to the improved VSG control module; (4) In the phase pre-synchronization module, the grid phase angle θ is obtained through the PLL module g , the a, b, c phase voltages of the PCC point voltage u an 、u bn 、u c The grid phase angle θ g The reference angle is converted into the inverter voltage u in the dq reference frame through Park transformation qn ; Construct a PI controller so that u qn = zero; the PI controller outputs an angular frequency compensation Δω, which is input into the angular velocity integration module. The angular frequency compensation Δω is integrated with time to obtain a phase angle difference Δθ, which is output by the phase pre-synchronization module and input into the improved VSG control module; (5) In the voltage pre-synchronization module, the PI controller controls the RMS voltage U of the converter. i and the RMS voltage of the power grid U ga , U gb , U gc To keep consistent, the voltage difference ΔU output by the voltage pre-synchronization module is input into the improved VSG control module for amplitude compensation; (6) In the improved VSG control module, the phase angle difference Δθ is added to the integral result phase angle θ of the actual output angular frequency ω obtained by the converter through formula (1); (7) In the improved VSG control module, the voltage difference ΔU obtained by the voltage pre-synchronization module is input into the improved VSG control module and compared with the no-load electromotive force E obtained by formula (1). m Add and change the no-load electromotive force E of VSG m ; and introduce positive sequence virtual impedance, in the original no-load electromotive force E m Subtract the voltage drop of the current on the positive sequence virtual impedance from the reference voltage U of the three-phase system. refa , U refb , U refc ; (8) The complete voltage amplitude of the three-phase system of the improved VSG control module is referenced to U refa , U refb , U refc The phase angle θ is input into the dual-loop control module and the modulation module, and the switching signal of the IGBT is obtained through 3D-SVPWM; (9) After the grid connection is completed, disconnect the phase pre-synchronization module and the voltage pre-synchronization module.

Images