Control method of cascaded photovoltaic inverter under grid inter-phase short circuit fault condition

Abstract

This invention discloses a control method for a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in the power grid, belonging to photovoltaic power generation technology in the field of electrical engineering. The steps are as follows: First, determine the fault type of the three-phase power grid and detect the ratio of the line voltage amplitude of the two faulty phases after the voltage drop to that of the two faulty phases before the voltage drop. Then, use a proportional resonant controller to control the grid-connected current. Second, adopt an adaptive zero-sequence voltage compensation control strategy based on active current injection to adaptively compensate the zero-sequence voltage according to the maximum positive-sequence active current actually output by the inverter. Third, control the average value of the DC bus capacitor voltage of the H-bridge converter by controlling a two-level full-bridge LLC converter with one input and m outputs. When a phase-to-phase short-circuit fault occurs in the power grid, this control method improves the adaptability of the cascaded photovoltaic grid-connected inverter to different output power and different grid voltage drop depths, and improves the system's low-voltage ride-through capability.

Description

Technical Field

[0001] This invention pertains to photovoltaic power generation technology in the field of electrical engineering, specifically relating to a control method for a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in the power grid. Background Technology

[0002] Data from the "2020 Global Renewable Energy Status Report" shows that in 2019, the newly installed capacity of photovoltaic (PV) power generation was approximately 115 GW, accounting for 57.79% of the total newly installed capacity of renewable energy. With a large number of PV power plants connected to the power system, both domestic and international standards require medium and large-scale PV grid-connected inverters to have low-voltage ride-through capability. This means that during periods of grid voltage dips, PV power plants need to inject reactive current in proportion to the voltage drop to support the grid voltage, while the remaining capacity is generated as active power to prevent large active power deficits in the grid that could affect the system's frequency stability.

[0003] Compared to traditional centralized inverters connected to the medium-voltage grid via power frequency transformers, grid-connected inverters based on a three-phase isolated cascaded H-bridge topology with a common DC bus offer significant advantages for large-scale photovoltaic power plants. These advantages include modular structure, low switching frequency, small filter size, high conversion efficiency, low output current harmonic content, and direct connection to the medium-voltage grid without a bulky power frequency transformer. Therefore, this modular cascaded photovoltaic inverter can achieve efficient, ultra-high power direct-connection to the medium-voltage grid, possessing broad development prospects and market potential. However, unlike traditional three-phase inverters, the topology of cascaded photovoltaic inverters is relatively unique, and the converter experiences active power backflow during low-voltage ride-through. Active power backflow is a unique problem of three-phase modular cascaded grid-connected inverters, and it is predicated on three-phase grid voltage asymmetry faults. Specifically, this manifests as one phase of the converter absorbing active power from the grid while the other two phases output active power back to the grid. This results in the system having no balanced operating point during a fault, causing the H-bridge DC bus voltage to continuously rise, and the inverter shutting down and disconnecting from the grid due to an overvoltage fault. Therefore, it is necessary to suppress the active power backflow of the three-phase cascaded photovoltaic inverter during grid dips, which is a necessary condition for achieving low voltage ride-through.

[0004] The paper "HD.Tafti, AI.Maswood, G. Konstantinou, CD.Townsend, P. Acuna, and J. Pou, 'Flexible control of photovoltaic grid-connected cascaded H-bridge converters during unbalanced voltage sags,' IEEE Trans.Ind.Electron., vol.65, no.8, pp.6229-6238, Aug.2018" proposes an unbalanced current injection control strategy aimed at suppressing active power fluctuations. This strategy achieves stable active power output by injecting an appropriate amount of negative sequence current. The paper "R. Sharma, and A. Das, Analysis of solar..." further explores this approach. "PV fed cascaded H-bridge converter during low voltage ride operation, 2020 IEEE International Conference on Power Electronics, Drives and Energy Systems (PEDES), Jaipur, India, Dec. 16-19, 2020. (R. Sharma, and A. Das, Analysis of low voltage ride operation of cascaded H-bridge converter powered by solar photovoltaic power, 2020 IEEE International Conference on Power Electronics, Drives and Energy Systems, Jaipur, India, December 16-19, 2020)" This paper derives a suitable expression for the zero-sequence voltage to ensure that the system can balance the DC bus voltage and interphase power of the three-phase star-connected cascaded H-bridge photovoltaic grid-connected inverter under asymmetrical grid sag conditions.The paper "H.Li, Z.Gao, S.Ji, Y.Ma, and F.Wang, An inrush current limit method for SiC-based multi-level grid-connected converter during Low voltage ride through, 2021 IEEE Applied Power Electronics Conference and Exposition (APEC), Phoenix, AZ, USA, June 14-17, 2021" proposes a PWM mask method to limit the inrush current of a SiC-based CHB multi-level grid-connected converter during the transient process of grid voltage change, and analyzes the threshold setting considering the delay of the control loop.

[0005] The literature “Tao Zhao, and Daolian Chen, Analysis and Suppression of Active Power Backflow of Three-Phase Common DC-Bus Cascaded H-Bridge PV Grid-Connected Inverter During LVRT,” IEEE Journal of Emerging and Selected Topics in Power Electronics, vol.10, no.1, pp.745-759, Feb.2022. (Tao Zhao and Daolian Chen, Analysis and Suppression of Active Power Backflow of Three-Phase Common DC-Bus Cascaded H-Bridge PV Grid-Connected Inverter During Low Voltage Ride-Up, IEEE Journal of Emerging and Selected Topics in Power Electronics, vol.10, no.1, pp.745-759) studies the mechanism of active power backflow in a three-phase isolated CHB PV grid-connected inverter during low voltage ride-up, derives the critical conditions for active power backflow under different grid types of faults and different voltage drop depths, and proposes an active current injection strategy to suppress active power backflow. However, under low light intensity conditions such as early morning, evening, and cloudy / rainy days, the actual output power of the photovoltaic array is far lower than the rated power, and the inverter cannot provide the positive-sequence active current necessary to suppress active power backflow. To address this, the literature “Tao Zhao, and Daolian Chen, Active Power Backflow Control Strategy for Cascaded Photovoltaic Solid-State Transformer During Low-Voltage Ride Through,” IEEE Trans. Ind. Electron., vol. 69, no. 1, pp. 440-451, Jan. 2022. (Tao Zhao and Daolian Chen, Active Power Backflow Control Strategy for Cascaded Photovoltaic Solid-State Transformer During Low-Voltage Ride Through, IEEE Industrial Electronics Journal, January 2022, Vol. 69, No. 1, pp. 440-451) proposes a low-voltage ride-through control strategy for cascaded photovoltaic solid-state transformers based on zero-sequence voltage compensation. This strategy compensates for a suitable zero-sequence voltage to offset the negative-sequence voltage's effect on the distribution of active power between phases in the three-phase converter, thereby suppressing active power backflow. This method does not impose specific requirements on the positive sequence active current, and theoretically it can still effectively suppress active power backflow even when the transmission power is low.However, when a phase-to-phase short-circuit fault occurs in the power grid, the amplitude of the modulated voltage after compensating for the zero-sequence voltage will be much greater than the rated amplitude of the grid voltage. This will cause the inverter to over-modulate within a large operating range, weakening the compensation effect of the zero-sequence voltage, leaving the system still at risk of active power backflow, and even causing the system to fail to operate normally and fail to achieve low voltage ride-through.

[0006] In summary, existing control methods for suppressing active power return current in grid-connected inverters with a three-phase isolated common DC bus cascaded H-bridge topology under phase-to-phase short-circuit fault conditions still have the following problems:

[0007] (1) Under low light intensity conditions such as early morning, evening, and rainy days, the actual output power of the photovoltaic array is far lower than the rated power, and the inverter cannot provide the positive sequence active current necessary to suppress active backflow.

[0008] (2) After adopting the existing zero-sequence voltage compensation strategy, the amplitude of the modulation voltage will be much greater than the rated amplitude of the grid voltage. This will cause the inverter to over-modulate within a large operating range, weakening the compensation effect of the zero-sequence voltage, making the system still at risk of active power backflow, and even causing the system to fail to operate normally and fail to achieve low voltage ride-through. Summary of the Invention

[0009] The technical problem this invention aims to solve is to overcome the limitations of the aforementioned solutions and propose a control method for cascaded photovoltaic inverters under grid phase-to-phase short-circuit fault conditions. The core of this method is an adaptive zero-sequence voltage compensation control strategy based on active current injection. This strategy adaptively compensates for the zero-sequence voltage according to the maximum positive-sequence active current actually output by the inverter; that is, the larger the positive-sequence active current, the smaller the compensated zero-sequence voltage amplitude. Compared to existing zero-sequence voltage compensation strategies, this method can reduce the peak value of the three-phase modulation voltage, decrease the risk of over-modulation, and thus expand the effective region for active power return current suppression.

[0010] To achieve the above objectives, the technical solution adopted by this invention is as follows:

[0011] A control method for a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in a power grid. The cascaded photovoltaic inverter is a grid-connected photovoltaic inverter based on a three-phase isolated common DC bus cascaded H-bridge topology, consisting of phases A, B, and C. Each of phases A, B, and C contains n modules, and all modules in phases A, B, and C have identical structures, where n is a positive integer greater than 1. Each module in phases A, B, and C consists of a one-input, m-output two-level full-bridge LLC converter and m H-bridge converters. The m output ports of the one-input, m-output two-level full-bridge LLC converter each... Each H-bridge converter is connected to a single H-bridge converter, resulting in a total of m H-bridge converters, where m is a positive integer greater than 1. Each H-bridge converter's input port is connected in parallel with a DC bus capacitor. The output ports of the m H-bridge converters are connected in series to form a single AC output port. The AC output ports of all modules in phases A, B, and C are connected in series, with one end connected together to form a common point. The other ends are each connected to a three-phase star-connected power grid via filter inductors. The input ports of all modules in phases A, B, and C are connected in parallel to form a common DC bus, and a photovoltaic array is connected in parallel to this common DC bus.

[0012] The control method includes fault detection and judgment of three-phase grid voltage, grid-connected current control, active current injection and adaptive zero-sequence voltage compensation control, and average value control of DC bus capacitor voltage of H-bridge converter. The specific steps are as follows:

[0013] Step 1: Fault detection and diagnosis of three-phase power grid voltage

[0014] Step 1.1: Sample the three-phase grid voltage to obtain the sampled value u of the three-phase grid voltage. gA u gB u gC ;

[0015] Step 1.2: Use a dual second-order generalized integrator frequency-locked loop to sample the three-phase grid voltage u obtained in Step 1.1. gA u gB u gC Phase-locked loop (PLL) is performed to obtain the phase angle ωt and angular frequency ω of the grid voltage. The sampled values ​​u of the three-phase grid voltage are then converted using synchronous rotating coordinate transformation. gA u gB u gC Converted to positive-sequence active power component u of grid voltage in synchronous rotating coordinate system dP Positive sequence reactive power component of grid voltage u qP Negative sequence active power component of grid voltage u dN and the negative sequence reactive power component of the grid voltage u qN The calculation formulas are as follows:

[0016]

[0017]

[0018] In addition, the amplitude U of the positive sequence phase voltage of the power grid P and the amplitude U of the negative sequence phase voltage of the power grid N The formula for calculation is:

[0019]

[0020] Step 1.3, based on the negative sequence active power component u of the grid voltage calculated in Step 1.2 dN Negative sequence reactive power component of grid voltage u qN The amplitude U of the positive sequence phase voltage of the power grid P and the amplitude U of the negative sequence phase voltage of the power grid N Determining the fault type in a three-phase power grid:

[0021] When U P +U N =U g andu dN When u > 0, if u qN =0, then a two-phase short circuit fault has occurred in the power grid; otherwise, other types of faults have occurred in the power grid.

[0022] When U P +U N =U g andu dN When u < 0, if u qN If U ≠ 0, then a two-phase short-circuit fault occurs in the power grid; otherwise, other types of faults occur. g This indicates the rated value of the phase voltage amplitude of the power grid;

[0023] Step 1.4, according to the power grid fault type detection method described in Step 1.3, when a two-phase short-circuit fault occurs in the power grid, the ratio D of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop can be calculated as follows:

[0024] D = 2u dP / U g -1

[0025] Step 2, Grid-connected current control

[0026] Step 2.1: Sample the three-phase grid current to obtain the sampled value i of the three-phase grid current. gA i gB i gc ;

[0027] Step 2.2, the sampled values ​​i of the three-phase grid current obtained in Step 2.1 are transformed by synchronous rotation coordinate transformation.gA i gB i gC Transformed into the α-axis component of the grid current in a two-phase stationary vertical coordinate system α and the β-axis component of the grid current i β The formula for its calculation is:

[0028]

[0029] Step 2.3, calculate the command value of the positive sequence reactive current. The formula is:

[0030]

[0031] In the formula, I N This indicates the rated value of the current amplitude in a three-phase power grid;

[0032] Step 2.4: Sample the common DC bus voltage and the photovoltaic array output current to obtain the sampled value U of the common DC bus voltage. PVT The sampled value I of the output current of the photovoltaic array PVT And calculate the actual output power P of the photovoltaic array. T The formula for its calculation is:

[0033] P T =U PVT I PVT

[0034] Step 2.5, calculate the command value of the positive sequence active current. The formula is:

[0035]

[0036] In the formula, express and The minimum value, express The square root of;

[0037] Step 2.6, calculate the command value of the α-axis component of the grid current. Command value of the β-axis component of the grid current The formula is:

[0038]

[0039] Step 2.7: The α-axis component i of the grid current is converted using proportional resonant controllers α and β, respectively. α The command value for the control is the α-axis component of the grid current. The β-axis component of the grid current i βThe command value for the control is the β-axis component of the grid current. The output value u of the proportional resonant controller α is obtained. α The output value u of the proportional resonant controller β β The formula for its calculation is:

[0040]

[0041] Among them, K IP K is the proportionality coefficient. II The resonant coefficient;

[0042] Step 2.8, take the sampled values ​​u of the three-phase grid voltage obtained in Step 1.1. gA u gB u gC Step 2.7 Calculate the output value u of the proportional resonant controller α a The output value u of the proportional resonant controller β β After coordinate transformation, the three voltages u in the three-phase coordinate system are obtained. cA u cB u cC The calculation formula is:

[0043]

[0044] Step 3: Active current injection and adaptive zero-sequence voltage compensation control

[0045] Step 3.1, based on the grid current α-axis component i obtained in step 2.2 α and the β-axis component of the grid current i β Calculate the positive-sequence active power component i of the grid current in the rotating coordinate system. dP and the positive-sequence reactive power component i of the grid current qp The formula for its calculation is:

[0046]

[0047] Step 3.2, based on the ratio D of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, calculated in Step 1.4, and the positive-sequence active component i of the grid current in the rotating coordinate system calculated in Step 3.1. dP and the positive-sequence reactive power component i of the grid current qP The adaptive compensation coefficient k is calculated using the following formula:

[0048]

[0049] Step 3.3, based on the ratio D of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, calculated in Step 1.4, and the three voltages u in the three-phase coordinate system calculated in Step 2.8. cA u cB u cC Step 3.1 calculates the positive-sequence active power component i of the grid current in the rotating coordinate system. dP and the positive-sequence reactive component of the grid current i qP The modulation voltage of the cascaded photovoltaic inverter is calculated using the adaptive compensation coefficient k obtained in step 3.2. The formula is:

[0050]

[0051] In the formula, when u dN <0 and u qN When u > 0, β = 2π / 3; when u > 0, β = 2π / 3; dN <0 and u qN When u < 0, β = -2π / 3; when u dN >0 and u qN When = 0, β = 0; arctan(i qP / i dP ) represents i qP / i dP The arctangent value;

[0052] Step 3.4: Calculate the modulation voltage of the cascaded photovoltaic inverter obtained in Step 3.3. Divide by the number of H-bridge converters in phases A, B, and C, respectively (m×n), to obtain the modulation voltage u of the phase A H-bridge converter. AH The modulation voltage u of the B-phase H-bridge converter BH The modulation voltage u of the C-phase H-bridge converter CH The calculation formulas are as follows:

[0053]

[0054] Step 3.5: Sample the DC bus capacitor voltage of all H-bridge converters in phases A, B, and C respectively, and obtain the following data: Sample value U of the DC bus capacitor voltage of the j-th H-bridge converter in the i-th module of phase A. HAij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase B. HBij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase C. HCij ,i=1,2,...,n,j=1,2,...,m;

[0055] Step 3.6: Calculate the modulation waves of all H-bridge converters in phases A, B, and C; let m be the modulation wave of the j-th H-bridge converter in the i-th module of phase A. Aij The modulation wave of the j-th H-bridge converter of the i-th module in phase B is m Bij The modulation wave of the j-th H-bridge converter of the i-th module in phase C is m. Cij If i = 1, 2, ..., n, j = 1, 2, ..., m, then m Aij m Bij m Cij The calculation formula is as follows:

[0056]

[0057] Step 4: Average value control of DC bus capacitor voltage of H-bridge converter

[0058] Step 4.1, based on the DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module of phase A obtained in step 3.5. HAij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase B. HBij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase C. HCij The average value U of the DC bus voltage of the m H-bridge converters in the i-th module of phase A is calculated. HAi The average value U of the DC bus voltage of the m H-bridge converters in the i-th module of phase B. HBi The average value U of the DC bus voltage of the m H-bridge converters in the i-th module of phase C. HCi The calculation formulas are as follows:

[0059]

[0060] Step 4.2: Using three identical two-level full-bridge LLC voltage controllers, respectively calculate the average value U of the DC bus voltage of the m H-bridge converters in the i-th module of phase A obtained in step 4.1. HAi The average value U of the DC bus voltage of the m H-bridge converters in the i-th module of phase B. HBi The average value U of the DC bus voltage of the m H-bridge converters in the i-th module of phase C. HCi Control is U PVT / N T The switching frequency f of the two-level full-bridge LLC converter with i-th input and m-th output of phase A is obtained. DAi The switching frequency f of the two-level full-bridge LLC converter with the i-th input and m-output of phase B is... DBiThe switching frequency f of the two-level full-bridge LLC converter with the i-th input and m-output of phase C is... DCi The calculation formulas are as follows:

[0061]

[0062] In the formula, N T K is the turns ratio of the primary and secondary windings of the high-frequency transformer in a two-level full-bridge LLC converter with one input and m outputs. DP K is the proportional gain of a two-level full-bridge LLC voltage controller. DI The integral coefficient of the two-level full-bridge LLC voltage controller.

[0063] The advantages of this invention over the prior art are: it proposes an adaptive zero-sequence voltage compensation based on active current injection, which can effectively reduce the overmodulation region of the inverter, and thus expand the effective region of active power return current suppression of the photovoltaic grid-connected inverter based on the three-phase isolated common DC bus cascaded H-bridge topology under the condition of grid phase-to-phase short-circuit fault. Attached Figure Description

[0064] Figure 1 This is a circuit diagram of a photovoltaic grid-connected inverter based on a three-phase isolated common DC bus cascaded H-bridge topology, implemented according to the present invention when n=2 and m=3.

[0065] Figure 2 This is the circuit diagram of the first module of phase A of the photovoltaic grid-connected inverter based on the three-phase isolated common DC bus cascaded H-bridge topology implemented in this invention when m=3.

[0066] Figure 3 This is a control block diagram of a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in the power grid, as implemented in this invention.

[0067] Figure 4 This is a flowchart of the control method for a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in the power grid, as implemented in this invention.

[0068] Figure 5 This is the rated value U of the grid phase voltage amplitude when phases B and C experience a phase-to-phase short-circuit fault in this invention. g =100V, when the ratio D = 0 of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, the three-phase power grid voltage sampling value u gA u gB u gC A waveform diagram.

[0069] Figure 6This is a waveform diagram of the normalized amplitude of the modulation voltage of phase A under the condition of phase-to-phase short circuit between phases B and C, using the existing zero-sequence voltage compensation strategy and the method proposed in this invention.

[0070] Figure 7 This is a schematic diagram of the overmodulation region of phase A when phases B and C experience a phase-to-phase short circuit, and the inverter modulation is 0.8696, using both the existing zero-sequence voltage compensation strategy and the method proposed in this invention.

[0071] Figure 8 This is a schematic diagram of the output waveform of phase A of the carrier phase-shifting sinusoidal pulse width modulation strategy implemented in this invention when n=1 and m=3. Detailed Implementation

[0072] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described clearly and completely below in conjunction with the accompanying drawings and embodiments.

[0073] Figure 1 This is a circuit diagram of a photovoltaic grid-connected inverter based on a three-phase isolated common DC bus cascaded H-bridge topology, implemented according to the present invention when n=2 and m=3. The cascaded photovoltaic inverter is a photovoltaic grid-connected inverter based on a three-phase isolated common DC bus cascaded H-bridge topology, consisting of phases A, B, and C. Each of phases A, B, and C contains n modules, and all modules in phases A, B, and C have identical structures, where n is a positive integer greater than 1. Each module in phases A, B, and C consists of a one-input, m-output two-level full-bridge LLC converter and m H-bridge converters. Each of the m output ports of the converter is connected to an H-bridge converter, for a total of m H-bridge converters, where m is a positive integer greater than 1. Each H-bridge converter's input port is connected in parallel with a DC bus capacitor. The output ports of the m H-bridge converters are connected in series to form a unit AC output port. The unit AC output ports of all modules in phases A, B, and C are connected in series, with one end connected together to form a common point, and the other end connected to the three-phase star-connected power grid through filter inductors. The input ports of all modules in phases A, B, and C are connected in parallel to form a common DC bus, and a photovoltaic array is connected in parallel on the common DC bus.

[0074] exist Figure 1 in, u gA u gB u gC i represents the sampled value of the three-phase power grid voltage. gA i gB i gC This represents the sampled value of the three-phase grid current, which is also the output current of the cascaded photovoltaic inverter, L. fIndicates the filter inductance; C xij U represents the DC bus capacitance of the j-th H-bridge converter in the i-th module of phase X. Hxij This represents the sampled value of the DC bus capacitor voltage of the j-th H-bridge converter in the i-th module of phase X, where X = A, B, C, i = 1, 2, j = 1, 2, 3; U PVT I represents the sampled value of the common DC bus voltage. PVT Sampled value of the output current of the photovoltaic array.

[0075] Figure 2 This is the circuit diagram of the first module of phase A of the photovoltaic grid-connected inverter based on a three-phase isolated common DC bus cascaded H-bridge topology implemented in this invention when m=3, with the main switch Q. A11 Q A12 Q A13 and Q A14 (Including anti-parallel diodes and parasitic capacitance) constitute the primary-side inverter bridge of a two-level full-bridge LLC converter with one input and three outputs. A11 L represents the input capacitor of a two-level full-bridge LLC converter with one input and three outputs. rAl C rA1 and L mA1 These represent the resonant inductance, resonant capacitance, and magnetizing inductance, respectively. T rA1 This represents a high-frequency isolation transformer where all three secondary windings have the same number of turns, and the number of turns in the primary winding is N times the number of turns in the secondary winding. T times, ; D RA1-11 ~D RA1-14 D RA1-21 ~D RA1-24 and D RA1-31 ~D RA1-34 Indicates the output rectifier diode; the three H converters are each controlled by a fully controlled switching device T. A1-11 ~T A1-14 T A1-21 ~T A1-24 and T A1-31 ~T A1-34 Composed of (including anti-parallel diodes), C A11 C A12 and C A13 These represent the DC bus capacitors of the three H-bridge converters.

[0076] Figure 3 This is a control block diagram of a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in the power grid, as implemented in this invention. It mainly includes: using a dual second-order generalized integrator frequency-locked loop to obtain the sampled values ​​u of the three-phase grid voltage. gA u gB u gC Calculate the angular frequency ω and phase angle ωt, and the positive sequence voltage amplitude U of the power grid.P and the magnitude of the negative sequence voltage of the power grid U N 1. Determine the type of power grid fault and calculate the ratio of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop. 2. Calculate the α-axis component i of the power grid current. α and the β-axis component of the grid current i β Command values ​​for calculating the α-axis component of the grid current. Command value of the β-axis component of the grid current The three voltages u in the three-phase coordinate system are obtained through grid-connected current control. cA u cB and u cC Calculate the adaptive zero-sequence voltage compensation coefficient k and the modulation voltage of the cascaded photovoltaic inverter. and To expand the effective region of power return current suppression, the modulation wave m of all three-phase H-bridge converters was calculated. Aij m Bij and m Cij (i = 1, 2, j = 1, 2, 3) and obtain the switching drive signals of all H-bridge converters and the average value control of the DC bus capacitor voltage of the H-bridge converter according to the carrier phase-shifted sinusoidal pulse width modulation strategy. The drive signals of all fully controlled switching devices in the two-level full-bridge LLC converter with one input and three outputs are obtained through frequency conversion modulation.

[0077] Figure 4 This is a flowchart of the control method for a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in the power grid, as implemented in this invention. It includes: fault detection and judgment of three-phase grid voltage, grid-connected current control, active current injection and adaptive zero-sequence voltage compensation control, and average value control of DC bus capacitor voltage of H-bridge converter.

[0078] See Figure 1 , Figure 2 , Figure 3 and Figure 4 The implementation process of this invention is as follows:

[0079] Step 1: Fault detection and diagnosis of three-phase power grid voltage

[0080] Step 1.1: Sample the three-phase grid voltage to obtain the sampled value u of the three-phase grid voltage. gA u gB u gC .

[0081] Step 1.2: Use a dual second-order generalized integrator frequency-locked loop to sample the three-phase grid voltage u obtained in Step 1.1. gA u gB u gCPhase-locked loop (PLL) is performed to obtain the phase angle ωt and angular frequency ω of the grid voltage. The sampled values ​​u of the three-phase grid voltage are then converted using synchronous rotating coordinate transformation. gA u gB u gC Converted to positive-sequence active power component u of grid voltage in synchronous rotating coordinate system dP Positive sequence reactive power component of grid voltage u qP Negative sequence active power component of grid voltage u dN and the negative sequence reactive power component of the grid voltage u qN The calculation formulas are as follows:

[0082]

[0083]

[0084] In addition, the amplitude U of the positive sequence phase voltage of the power grid P and the amplitude U of the negative sequence phase voltage of the power grid N The formula for calculation is:

[0085]

[0086] Step 1.3, based on the negative sequence active power component u of the grid voltage calculated in Step 1.2 dN Negative sequence reactive power component of grid voltage u qN The amplitude U of the positive sequence phase voltage of the power grid P and the amplitude U of the negative sequence phase voltage of the power grid N Determining the fault type in a three-phase power grid:

[0087] When U P +U N =U g andu dN When u > 0, if u qN =0, then a two-phase short circuit fault has occurred in the power grid; otherwise, other types of faults have occurred in the power grid.

[0088] When U P +U N =U g andu dN When u < 0, if u qN If U ≠ 0, then a two-phase short-circuit fault occurs in the power grid; otherwise, other types of faults occur. g This indicates the rated value of the phase voltage amplitude of the power grid.

[0089] Step 1.4, according to the power grid fault type detection method described in Step 1.3, when a two-phase short-circuit fault occurs in the power grid, the ratio D of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop can be calculated as follows:

[0090] D = 2u dP / U g -1

[0091] Figure 5 This is the rated value U of the grid phase voltage amplitude when phases B and C experience a phase-to-phase short-circuit fault in this invention. g =100V, when the ratio D = 0 of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, the three-phase power grid voltage sampling value u gA u gB u gC The waveform diagram shows that D=0 means the line voltage u of phases B and C is... BC =0, then the phase voltages of phases B and C should be equal, i.e., u gB =u gC Considering that the sum of the three-phase voltages is 0, i.e., u gA +u gB +u gC =0, then u gB =u gC =-0.5u gA .from Figure 5 It can be seen that the amplitude of phase A voltage is 100V, while the amplitudes of phase B and phase C voltages are 50V, and their phases are opposite to those of phase A voltage.

[0092] Step 2.1: Sample the three-phase grid current to obtain the sampled value i of the three-phase grid current. gA i gB i gC ;

[0093] Step 2.2, the sampled values ​​i of the three-phase grid current obtained in Step 2.1 are transformed by synchronous rotation coordinate transformation. gA i gB i gC Transformed into the α-axis component of the grid current in a two-phase stationary vertical coordinate system α and the β-axis component of the grid current i β The formula for its calculation is:

[0094]

[0095] Step 2.3, calculate the command value of the positive sequence reactive current. The formula is:

[0096]

[0097] In the formula, I N This indicates the rated value of the current amplitude in a three-phase power grid.

[0098] Step 2.4: Sample the common DC bus voltage and the photovoltaic array output current to obtain the sampled value U of the common DC bus voltage. PVT The sampled value I of the output current of the photovoltaic array PVT And calculate the actual output power P of the photovoltaic array. T The formula for its calculation is:

[0099] P T =U PVT I PVT

[0100] Step 2.5, calculate the command value of the positive sequence active current. The formula is:

[0101]

[0102] In the formula, express and The minimum value, express The square root of.

[0103] According to the power conservation principle, the active power transmitted to the grid by the three-phase cascaded photovoltaic solid-state transformer is:

[0104]

[0105] Therefore, the command value of positive sequence active current It can be calculated as follows:

[0106]

[0107] However, considering that photovoltaic grid-connected inverters generally require the maximum output current to not exceed 1.1 times the rated current, therefore, the maximum current is 1.1I. N The maximum positive-sequence active current command value that the inverter can output under phase-to-phase short-circuit fault conditions in the power grid can be calculated. In summary, the command value of the positive-sequence active current... Ultimately, it can be expressed as:

[0108]

[0109] In the formula, express and The minimum value, express The square root of.

[0110] Step 2.6, calculate the command value of the α-axis component of the grid current. Command value of the β-axis component of the grid current The formula is:

[0111]

[0112] Step 2.7: The α-axis component i of the grid current is converted using proportional resonant controllers α and β, respectively. α The command value for the control is the α-axis component of the grid current. The β-axis component of the grid current i β The command value for the control is the β-axis component of the grid current. The output value u of the proportional resonant controller α is obtained. α The output value u of the proportional resonant controller β β The formula for its calculation is:

[0113]

[0114] Among them, K IP K is the proportionality coefficient. II is the resonance coefficient.

[0115] In this embodiment, K IP =10, K II =300.

[0116] Step 2.8, take the sampled values ​​u of the three-phase grid voltage obtained in Step 1.1. gA u gB u gC Step 2.7 Calculate the output value u of the proportional resonant controller α α The output value u of the proportional resonant controller β β After coordinate transformation, the three voltages u in the three-phase coordinate system are obtained. cA u cB u cC The calculation formula is:

[0117]

[0118] Step 3: Active current injection and adaptive zero-sequence voltage compensation control

[0119] Step 3.1, based on the grid current α-axis component i obtained in step 2.2 α and the β-axis component of the grid current i β Calculate the positive-sequence active power component i of the grid current in the rotating coordinate system. dP and the positive-sequence reactive power component i of the grid current qP The formula for its calculation is:

[0120]

[0121] Step 3.2, based on the ratio D of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, calculated in Step 1.4, and the positive-sequence active component i of the grid current in the rotating coordinate system calculated in Step 3.1. dP and the positive-sequence reactive power component i of the grid current qP The adaptive compensation coefficient k is calculated using the following formula:

[0122]

[0123] Step 3.3, based on the ratio D of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, calculated in Step 1.4, and the three voltages u in the three-phase coordinate system calculated in Step 2.8. cA u cB u cC Step 3.1 calculates the positive-sequence active power component i of the grid current in the rotating coordinate system. dP and the positive-sequence reactive component of the grid current i qP The modulation voltage of the cascaded photovoltaic inverter is calculated using the adaptive compensation coefficient k obtained in step 3.2. The formula is:

[0124]

[0125] In the formula, when u dN <0 and u qN When u > 0, β = 2π / 3; when u > 0, β = 2π / 3; dN <0 and u qN When u < 0, β = -2π / 3; when u dN >0 and u qN When = 0, β = 0; arctan(i qP / i dP ) represents u qP / i d The arctangent value of P.

[0126] Figure 6 This is a waveform diagram of the normalized amplitude of the modulation voltage of phase A under the condition of phase-to-phase short circuit between phases B and C, using the existing zero-sequence voltage compensation strategy and the method proposed in this invention. Figure 7 This diagram illustrates the overmodulation region of phase A under the condition of a phase-to-phase short circuit between phases B and C, when the inverter modulation is 0.8696, using both existing zero-sequence voltage compensation strategies and the method proposed in this invention. In the diagram, R... P P represents the actual output power of the photovoltaic array. T With inverter rated power P N The ratio, i.e., R P =P T / PN Normalized amplitude representation of phase A modulation voltage amplitude The rated value U of the phase voltage amplitude of the power grid gN The ratio. From Figure 6 It can be seen that after adding the adaptive compensation coefficient k, the normalized amplitude of the A-phase modulation voltage decreases significantly within a certain range, which indicates that the A-phase modulation voltage... amplitude Reduce. In Figure 7 In the middle, curve C1 intersects the coordinate axis R. P The region enclosed by curve C2 and D represents the overmodulation region of the method proposed in this invention. The curve C2 intersects the coordinate axis R. P The region enclosed by D represents the overmodulation region using existing zero-sequence voltage compensation strategies. It is evident that the method proposed in this invention can effectively reduce the overmodulation region of the inverter compared to existing zero-sequence voltage compensation strategies, thereby expanding the effective region for active power return current suppression under grid phase-to-phase short-circuit fault conditions for photovoltaic grid-connected inverters based on a three-phase isolated common DC bus cascaded H-bridge topology.

[0127] Step 3.4: Calculate the modulation voltage of the cascaded photovoltaic inverter obtained in Step 3.3. Divide by the number of H-bridge converters in phases A, B, and C, respectively (m×n), to obtain the modulation voltage u of the phase A H-bridge converter. AH The modulation voltage u of the B-phase H-bridge converter BH The modulation voltage u of the C-phase H-bridge converter CH The calculation formulas are as follows:

[0128]

[0129] Step 3.5: Sample the DC bus capacitor voltage of all H-bridge converters in phases A, B, and C respectively, and obtain the following data: Sample value U of the DC bus capacitor voltage of the j-th H-bridge converter in the i-th module of phase A. HAij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase B. HBij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase C. HCij ,i=1,2,j=1,2,3.

[0130] Step 3.6: Calculate the modulation waves of all H-bridge converters in phases A, B, and C; let m be the modulation wave of the j-th H-bridge converter in the i-th module of phase A. Aij The modulation wave of the j-th H-bridge converter of the i-th module in phase B is m Bij The modulation wave of the j-th H-bridge converter of the i-th module in phase C is m. CijIf i = 1, 2, j = 1, 2, 3, then m Aij m Bij m Cij The calculation formula is as follows:

[0131]

[0132] After calculating the modulation waves of all H-bridge converters, the switching drive signals of all H-bridge converters can be obtained using a carrier phase-shifted sinusoidal pulse width modulation (CPWM) strategy. The aforementioned carrier phase-shifted sinusoidal pulse width modulation strategy refers to the commonly used and mature technology in cascaded H-bridge converters. Many documents describe carrier phase-shifted sinusoidal pulse width modulation in detail, such as the doctoral dissertation entitled "Research on Key Technologies of Photovoltaic Grid-Connected Inverters Based on Cascaded H-Bridge Topology and Their Applications" by Zhao Tao, a student at Hefei University of Technology, published in 2020. Figure 8 This is a schematic diagram of the output waveform of phase A of the carrier phase-shifting sinusoidal pulse width modulation strategy implemented in this invention when n=1 and m=3. In the diagram, m... A11 m A12 and m A13 These represent the modulation waves of the 1st, 2nd, and 3rd H-bridge converters in the first module of phase A, respectively. c1 u c2 and u c3 These represent the carriers of the 1st, 2nd, and 3rd H-bridge converters in phase A, respectively. HO1 u HO2 and u HO3 These represent the AC output voltages of the first, second, and third H-bridge converters in phase A, respectively. HT This represents the total output voltage on the AC side of phase A converter. As can be seen from the diagram, u... c2 The phase angle compared to u c1 Lag π / 3, u c3 The phase angle compared to u c2 A hysteresis of π / 3 means there is a phase shift between carrier waves. HO1 u HO2 and u HO3 Both are three-level waveforms, while u HT It is a 7-level stepped wave.

[0133] Step 4: Average value control of DC bus capacitor voltage of H-bridge converter

[0134] Step 4.1, based on the DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module of phase A obtained in step 3.5. HAij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase B.HBij The DC bus capacitor voltage sampling value U of the j-th H-bridge converter of the i-th module in phase C. HCij Given i = 1, 2, j = 1, 2, 3, calculate the average value U of the DC bus voltage of the three H-bridge converters in the i-th module of phase A. HAi The average value U of the DC bus voltage of the three H-bridge converters in the i-th module of phase B. HBi The average value U of the DC bus voltage of the three H-bridge converters in the i-th module of phase C. HCi The calculation formulas are as follows:

[0135]

[0136] Step 4.2: Using three identical two-level full-bridge LLC voltage controllers, respectively calculate the average value U of the DC bus voltage of the three H-bridge converters in the i-th module of phase A obtained in step 4.1. HAi The average value U of the DC bus voltage of the three H-bridge converters in the i-th module of phase B. HBi The average value U of the DC bus voltage of the three H-bridge converters in the i-th module of phase C. HCi Control is U PVT / N T The switching frequency f of the two-level full-bridge LLC converter with one input and three outputs in phase A is obtained. DAi The switching frequency f of the two-level full-bridge LLC converter with one input and three outputs in phase B is... DBi The switching frequency f of the two-level full-bridge LLC converter with one input and three outputs in phase C is... DCi The calculation formulas are as follows:

[0137]

[0138] In the formula, N T K is the turns ratio of the primary and secondary windings of the high-frequency transformer in a two-level full-bridge LLC converter with one input and three outputs. DP K is the proportional gain of a two-level full-bridge LLC voltage controller. DI K represents the integral coefficient of the two-level full-bridge LLC voltage controller. In this embodiment, K... DP =40, K DI =8000.

[0139] The switching frequency f of the two-level full-bridge LLC converter with one input and three outputs in phase A is calculated according to step 4.2. DAi The switching frequency f of the two-level full-bridge LLC converter with one input and three outputs in phase B is... DBiThe switching frequency f of the two-level full-bridge LLC converter with one input and three outputs in phase C is... DCi By employing a frequency conversion modulation strategy for a two-level full-bridge LLC converter, the switching drive signals for all one-input, three-output two-level full-bridge LLC converters can be obtained. The frequency conversion modulation strategy for the two-level full-bridge LLC converter refers to a commonly used strategy in two-level full-bridge LLC converters. Numerous studies have described the frequency conversion modulation strategy for two-level full-bridge LLC converters in detail, such as the master's thesis entitled "Research on Digitally Controlled Full-Bridge LLC Resonant Converters" by Qian Juan, a student at Nanjing University of Aeronautics and Astronautics, in 2013.

Claims

1. A control method for a cascaded photovoltaic inverter under phase-to-phase short-circuit fault conditions in a power grid, wherein the cascaded photovoltaic inverter is a grid-connected photovoltaic inverter based on a three-phase isolated common DC bus cascaded H-bridge topology, consisting of phase A, phase B, and phase C; phase A, phase B, and phase C each contain n There are three modules, and the structures of all modules in phases A, B, and C are completely identical. n The input is a positive integer greater than 1; each module of phases A, B, and C consists of one input channel. m Two-level full-bridge LLC converter with output and m It consists of an H-bridge converter, with one input. m Two-level full-bridge LLC converter with output of 1000 ohms m Each output port is connected to an H-bridge converter, totaling... m One H-bridge converter, m The integer is a positive integer greater than 1. Each input port of the H-bridge converter is connected in parallel with a DC bus capacitor of the H-bridge converter. m The output ports of the H-bridge converters are connected in series to form a unit AC output port; the unit AC output ports of all modules in phases A, B, and C are connected in series, with one end of each module connected together to form a common point, and the other end of each module connected to the three-phase star-connected power grid through a filter inductor; the input ports of all modules in phases A, B, and C are connected in parallel to form a common DC bus, and a photovoltaic array is connected in parallel on the common DC bus. Its features are, The control method includes fault detection and judgment of three-phase grid voltage, grid-connected current control, active current injection and adaptive zero-sequence voltage compensation control, and average value control of DC bus capacitor voltage of H-bridge converter. The specific steps are as follows: Step 1: Fault detection and diagnosis of three-phase power grid voltage Step 1.1: Sample the three-phase grid voltage to obtain the sampled values ​​of the three-phase grid voltage. , , ; Step 1.2: Use a dual second-order generalized integrator frequency-locked loop to sample the three-phase grid voltage values ​​obtained in Step 1.

1. , , Phase-locked loop (PLL) is performed to obtain the phase angle of the grid voltage. angular frequency The sampled values ​​of the three-phase grid voltage are then transformed using synchronous rotating coordinate transformation. , , Converted to positive-sequence active power components of grid voltage in a synchronous rotating coordinate system Positive sequence reactive power components of grid voltage Negative sequence active power component of grid voltage and the negative sequence reactive power component of the grid voltage The calculation formulas are as follows: In addition, the amplitude of the positive-sequence phase voltage of the power grid and the amplitude of the negative sequence phase voltage of the power grid The formula for calculation is: Step 1.3, based on the negative sequence active power component of the grid voltage calculated in Step 1.

2. Negative sequence reactive power components of grid voltage The amplitude of the positive sequence phase voltage of the power grid and the amplitude of the negative sequence phase voltage of the power grid Determining the fault type in a three-phase power grid: when and At that time, if If the fault occurs, a two-phase short-circuit fault will occur in the power grid; otherwise, other types of faults will occur in the power grid. when and At that time, if If the fault occurs, a two-phase short-circuit fault will occur in the power grid; otherwise, other types of faults will occur. This indicates the rated value of the phase voltage amplitude of the power grid; Step 1.4, based on the power grid fault type detection method in Step 1.3, when a two-phase short-circuit fault occurs in the power grid, the ratio of the line voltage amplitude of the two faulty phases after the voltage drop to the line voltage amplitude of the two faulty phases before the voltage drop is calculated. D It can be calculated as: Step 2, Grid-connected current control Step 2.1: Sample the three-phase grid current to obtain the sampled values ​​of the three-phase grid current. , , ; Step 2.2: The sampled values ​​of the three-phase grid current obtained in Step 2.1 are transformed by synchronous rotation coordinate transformation. , , Transformed into the α-axis component of the grid current in a two-phase stationary vertical coordinate system and grid current β Axial components The formula for its calculation is: Step 2.3, calculate the command value of the positive sequence reactive current. The formula for its calculation is: In the formula, This indicates the rated value of the current amplitude in a three-phase power grid; Step 2.4: Sample the common DC bus voltage and the photovoltaic array output current to obtain the sampled value of the common DC bus voltage. and the sampled value of the output current of the photovoltaic array And calculate the actual output power of the photovoltaic array. The formula for its calculation is: Step 2.5, calculate the command value of the positive sequence active current. The formula for its calculation is: In the formula, express and The minimum value, express The square root of; Step 2.6, calculate the command value of the α-axis component of the grid current. and grid current β Command values ​​of axis components The formula for its calculation is: Step 2.7, using the proportional resonant controller α and the proportional resonant controller respectively. β α-axis component of grid current The command value for the control is the α-axis component of the grid current. , to the grid current β Axial components Controlled to grid current β Command values ​​of axis components The output value of the proportional resonant controller α is obtained. Sum proportional resonant controller β Output value The formula for its calculation is: in, This is the proportionality coefficient. The resonant coefficient; Step 2.8: Sample the three-phase grid voltage values ​​obtained in Step 1.

1. , , Step 2.7 Calculate the output value of the proportional resonant controller α Sum proportional resonant controller β Output value After coordinate transformation, the three voltages in the three-phase coordinate system are obtained. , , The calculation formula is: Step 3: Active current injection and adaptive zero-sequence voltage compensation control Step 3.1, based on the α-axis component of the grid current obtained in Step 2.2 and grid current β Axial components Calculate the positive-sequence active component of the grid current in the rotating coordinate system. and the positive-sequence reactive component of the grid current The formula for its calculation is: Step 3.2: The ratio of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, calculated according to Step 1.

4. D Step 3.1 Calculation of the positive-sequence active power components of the grid current in the rotating coordinate system and the positive-sequence reactive component of the grid current Calculate the adaptive compensation coefficient The formula for its calculation is: Step 3.3: The ratio of the line voltage amplitude of the two fault phases after the voltage drop to the line voltage amplitude of the two fault phases before the voltage drop, calculated according to Step 1.

4. D Step 2.8 calculates the three voltages in the three-phase coordinate system. , , Step 3.1 calculates the positive-sequence active power components of the grid current in the rotating coordinate system. and the positive-sequence reactive component of the grid current and the adaptive compensation coefficients calculated in step 3.2 Calculate the modulation voltage of the cascaded photovoltaic inverter , , The formula for its calculation is: In the formula, when and hour, ;when and hour, ;when and hour, ; express The arctangent value; Step 3.4: Calculate the modulation voltage of the cascaded photovoltaic inverter obtained in Step 3.

3. , , Divide by the number of H-bridge converters in phases A, B, and C respectively. The modulation voltage of the A-phase H-bridge converter is obtained. The modulation voltage of the B-phase H-bridge converter Modulation voltage of C-phase H-bridge converter The calculation formulas are as follows: Step 3.5: Sample the DC bus capacitor voltage of all H-bridge converters in phases A, B, and C respectively, and obtain the following data: Phase A's... i The first module j DC bus capacitor voltage sampling value of each H-bridge converter Phase B i The first module j DC bus capacitor voltage sampling value of each H-bridge converter ; Phase C i The first module j DC bus capacitor voltage sampling value of each H-bridge converter , , ; Step 3.6: Calculate the modulation waves of all H-bridge converters in phases A, B, and C; denote the modulation wave of phase A. i The first module j The modulation wave of the H-bridge converter is Phase B i The first module j The modulation wave of the H-bridge converter is Phase C i The first module j The modulation wave of the H-bridge converter is , , ,but , , The calculation formula is as follows: Step 4: Average value control of DC bus capacitor voltage of H-bridge converter Step 4.1, based on the A phase obtained in step 3.5... i The first module j DC bus capacitor voltage sampling value of each H-bridge converter Phase B i The first module j DC bus capacitor voltage sampling value of each H-bridge converter Phase C i The first module j DC bus capacitor voltage sampling value of each H-bridge converter The calculation yields the first phase of phase A. i In each module m Average value of DC bus voltage of each H-bridge converter Phase B i In each module m Average value of DC bus voltage of each H-bridge converter Phase C i In each module m Average value of DC bus voltage of each H-bridge converter The calculation formulas are as follows: Step 4.2: Use three identical two-level full-bridge LLC voltage controllers to respectively convert the voltage of phase A obtained in step 4.1 into the voltage of phase A. i In each module m Average value of DC bus voltage of each H-bridge converter Phase B i In each module m Average value of DC bus voltage of each H-bridge converter Phase C i In each module m Average value of DC bus voltage of each H-bridge converter Control as The first phase of phase A was obtained. i One-way input m The switching frequency of the two-level full-bridge LLC converter with output voltage. Phase B i One-way input m The switching frequency of the two-level full-bridge LLC converter with output voltage. and the first phase of C i One-way input m The switching frequency of the two-level full-bridge LLC converter with output voltage. The calculation formulas are as follows: In the formula, It's all about inputting. m The turns ratio of the primary and secondary windings of the high-frequency transformer in a two-level full-bridge LLC converter with output voltage. The proportional gain of a two-level full-bridge LLC voltage controller. The integral coefficient of the two-level full-bridge LLC voltage controller.

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