Photovoltaic direct current system and power equalization method based on integrated three-port converter

Abstract

The application discloses a photovoltaic direct-current system based on an integrated three-port converter and a power balancing method, and aims at the problems of output voltage imbalance among modules caused by photovoltaic power mismatch, limited module regulation capacity, low efficiency, complex control and the like caused by traditional two-stage voltage regulation topologies or additional balancing units in a IIOS type photovoltaic direct-current system. The application provides a photovoltaic direct-current system based on an integrated three-port converter and a power balancing method. The system constructs a low-voltage direct-current bus by connecting the bidirectional port of the integrated three-port converter in parallel, so as to serve as a power exchange channel among modules. On this basis, the application provides a power balancing method based on output voltage droop control of the integrated three-port converter. The application solves the power balancing problem of the IIOS type system without an additional balancing unit through the cooperation of the photovoltaic direct-current system topology and the control method, and improves the operation efficiency and reliability of the photovoltaic direct-current system.

Description

Technical Field

[0001] This invention belongs to the field of new energy power generation technology, specifically relating to the power conversion topology and equalization control technology of photovoltaic medium-voltage DC power generation systems, and is particularly applicable to photovoltaic DC systems and power equalization methods based on integrated three-port converters in large-scale photovoltaic medium-voltage DC power generation scenarios. Background Technology

[0002] In large-scale photovoltaic medium-voltage DC power generation systems, modular design is key to achieving flexible system expansion and independent, reliable operation. In terms of topology selection, the IIOS-type photovoltaic medium-voltage DC system possesses significant technical advantages: its input allows each photovoltaic string to independently perform maximum power point tracking, and its unipolar structure enables high energy conversion efficiency; at the output, through multi-module cascading, it generates the high voltage gain required for large-scale photovoltaic DC systems without the need for extremely high transformer ratios.

[0003] In actual operation, the IIOS type photovoltaic medium-voltage DC system faces severe challenges in power mismatch and balance control:

[0004] First, the mismatch in photovoltaic input power can easily lead to system overvoltage risks. Because the photovoltaic arrays connected to each module are susceptible to localized shading, cloud cover, or varying degrees of aging, the input power of each module often differs significantly. In an IIOS structure, the output current of each module on the series side is forced to be identical. In this case, power mismatch will directly lead to uneven voltage distribution between modules, easily causing overvoltage damage to some components.

[0005] Secondly, existing equalization topologies and control schemes have significant limitations. To address power mismatch, traditional solutions typically employ a two-stage structure of "pre-stage boost + post-stage DC-DC" or add an independent equalization unit. This not only increases the number of energy conversion stages, leading to a significant decrease in overall system efficiency, but also significantly increases hardware costs and the complexity of the control system. Furthermore, the gain adjustment capability of traditional single-converter control is very limited, making it difficult to effectively balance independent MPPT tracking of each module and output voltage equalization across modules. Summary of the Invention

[0006] The purpose of this invention is to overcome the aforementioned shortcomings of existing technologies and provide a photovoltaic DC system and power balancing method based on an integrated three-port converter. This invention simplifies the hardware topology by sharing four switching transistors between a two-phase interleaved Boost converter and a full-bridge LLC resonant converter through topology integration innovation. A low-voltage DC bus is constructed in parallel with the bidirectional ports of each module to serve as a power exchange channel, enabling power redistribution between modules without additional balancing hardware. Simultaneously, an output voltage balancing method based on droop control is proposed, combining output voltage deviation with low-voltage bus reference value correction, and using frequency adjustment to achieve output voltage balancing between modules. This solves the problems of voltage imbalance, low system efficiency, and complex control caused by power mismatch in existing IIOS-type photovoltaic DC systems, significantly improving the operating efficiency, stability, and reliability of photovoltaic DC systems.

[0007] To solve the above-mentioned technical problems, the present invention provides the following technical solution:

[0008] A photovoltaic DC system based on an integrated three-port converter includes n independent photovoltaic arrays and modular three-port converter units connected one-to-one with each photovoltaic array; n is a positive integer greater than or equal to 2.

[0009] Each of the three-port converters adopts an integrated topology, which integrates a two-phase interleaved Boost converter and a full-bridge LLC resonant circuit. The two-phase interleaved Boost converter and the full-bridge LLC resonant circuit share a full-bridge switching unit composed of four switching transistors.

[0010] The input terminals of each three-port converter are connected to the output terminals of the corresponding photovoltaic array. The output terminals of each three-port converter are connected in series to form a medium-voltage DC bus. The bidirectional ports of each three-port converter are connected in parallel to form a common low-voltage DC bus. The low-voltage DC bus has no external load connected and serves only as a power exchange channel between the converter modules.

[0011] Furthermore, when there is a difference in the input power of each module, the module with excess power outputs power to the low-voltage DC bus through its bidirectional port, while the module with insufficient power absorbs power from the low-voltage DC bus through its bidirectional port, thereby realizing the redistribution of power among the modules through the low-voltage DC bus.

[0012] Furthermore, the output current of each converter module is consistent, and the output voltage is autonomously distributed according to its transmission power, satisfying the following relationship:

[0013] ;

[0014] In the formula, U ok U is the output voltage of the k-th converter module. MVDC P is the medium-voltage DC bus voltage.k The transmission power of the k-th module, P Σ Total system transmission power.

[0015] A power equalization control method applied to the above system includes the following steps:

[0016] S1. Each module independently achieves maximum power point tracking control: The controller of each module collects the output voltage and current of the corresponding photovoltaic array in real time, adjusts the duty cycle of the upper bridge arm switch in the full-bridge switching unit of the converter, and tracks the maximum power point of the corresponding photovoltaic array; at the same time, it controls the phase shift angle of the left and right bridge arms of the full-bridge switching unit to maintain at 180°.

[0017] S2. Output voltage equalization control based on droop control: The droop control method of the analog AC grid-connected inverter is used to correct the reference value of the low-voltage DC bus after droop control of the output voltage of each module converter. Each module compares the actual sampled value of the low-voltage DC bus voltage with the corrected reference value, and changes the voltage gain of the LLC resonant circuit of the corresponding module by frequency adjustment, so as to achieve the equalization of the output voltage of each module.

[0018] Furthermore, the two-phase interleaved Boost converter operates in continuous conduction mode, and the steady-state relationship between its input voltage and the low-voltage DC bus voltage is as follows:

[0019] ;

[0020] In the formula, U LV For low-voltage bus voltage, U pvk Let D be the output voltage of the k-th photovoltaic array. k Let be the duty cycle of the upper bridge arm switch in the k-th converter full-bridge switching unit.

[0021] Furthermore, in step S2, the reference value U of the low-voltage DC bus is corrected. LVref * The mathematical expression is:

[0022] ;

[0023] In the formula, U LVref U is the preset initial value of the low-voltage bus reference voltage. ok U is the output voltage of the k-th converter. oref K is the output voltage reference value, and K is the proportional gain coefficient.

[0024] Furthermore, each module monitors its own output voltage U in real time. ok and compare it with the rated reference voltage U oref The difference is used as a feedback quantity and is equivalently superimposed on the initial value U of the low-voltage bus reference voltage. LVrefAbove, generate the dynamic reference voltage command U corresponding to each module. LVref * The proportional-integral control U is adopted. LVn with U LVref * They are equal, as shown in the following expression:

[0025] ;

[0026] In the formula, U LV1 U LV2 ...U LVn This represents the actual low-voltage bus voltage value for each module.

[0027] Furthermore, in step S2, system power balance is achieved by constructing a collaborative mechanism for the deviation between the low-voltage bus voltage and the output voltage. The specific implementation logic is as follows:

[0028] (1) Low-voltage bus voltage reference value correction: The output voltage deviation is superimposed on the low-voltage bus reference value to generate differentiated low-voltage bus voltage reference values ​​for each module;

[0029] (2) Operating frequency adjustment: Each module compares the sampled value of the low-voltage bus voltage with the corrected reference value of each module to obtain the voltage difference of the low-voltage bus, and outputs the operating frequency f through the PI regulator. k ;

[0030] (3) Resonant converter gain adjustment: By adjusting the frequency to change the voltage gain of the resonant cavity, under the constraint of the low-voltage bus potential, the power flow direction is guided by adjusting the voltage gain of each module, and finally the system output voltage balance is achieved.

[0031] Furthermore, the expression for voltage gain is:

[0032] ;

[0033] In the formula, M k For a single module, N is the ratio of the primary to secondary windings of the isolation transformer, and f is the gain. n The normalized value of the operating frequency is the ratio of the operating frequency to the resonant frequency; Q is the quality factor; and k is the resonant frequency. m This is the ratio of the magnetizing inductance to the resonant inductance.

[0034] Furthermore, by adjusting the operating frequency, the LLC resonant cavity is always operated in the inductive impedance region, ensuring that the power switching transistors of the full-bridge switching unit achieve zero-voltage turn-on.

[0035] One or more technical solutions provided in the embodiments of this application have at least the following technical effects or advantages:

[0036] 1. Integrated topology design: The two-phase interleaved Boost converter and the full-bridge LLC resonant converter share four switching transistors, avoiding the device redundancy caused by the independent setting of the switching transistors in the front and rear stages in the traditional two-stage topology, effectively improving the power density and reliability of the system.

[0037] 2. No additional equalization hardware required: Utilizing the inherent low-voltage DC bus as the power exchange channel, power self-balancing is achieved through active adjustment of the bidirectional port, avoiding the problems of additional components, cumulative losses and current stress caused by traditional equalization circuits.

[0038] 3. Autonomous power allocation: By utilizing the consistent output current and stable total voltage of the series structure, the power of each module is automatically and rationally allocated.

[0039] 4. Enhanced system fault tolerance: When a single module fails, power can be redistributed or the faulty module can be bypassed through the low-voltage bus, while the remaining modules can still maintain continuous system operation. This is suitable for photovoltaic medium-voltage DC power generation scenarios with high reliability requirements. Attached Figure Description

[0040] Figure 1 This is a topology diagram of a photovoltaic DC system based on an integrated three-port converter.

[0041] Figure 2 The main waveforms of the integrated three-port converter are shown.

[0042] Figure 3 This is a diagram showing the main operating modes of an integrated three-port converter.

[0043] Figure 4 This is a schematic diagram of the low-voltage bus power interaction in a photovoltaic DC system based on an integrated three-port converter.

[0044] Figure 5 This diagram illustrates the power balancing control strategy for a photovoltaic DC system based on an integrated three-port converter.

[0045] Explanation of the symbols in the attached diagram: PV1—the first photovoltaic string; PV2—the second photovoltaic string; PV n —The nth photovoltaic string; U MVDC —Medium-voltage DC bus voltage; C pvk —The k-th photovoltaic output capacitor; C LVk —The kth low-voltage bus capacitor; S k1 —The first switch transistor of the kth module of the medium-voltage DC transformer, S k2 —Second switch transistor of the kth module of the medium-voltage DC transformer, S k3 —The third switch of the kth module of the medium-voltage DC transformer, S k4 —The fourth switch of the kth module of the medium-voltage DC transformer; L b1k—Inductors and L connecting the interleaved BOOST converter to the left arm of the full bridge b2k —The inductor connecting the interleaved BOOST converter to the right arm of the full-bridge; L rk —The resonant inductance of the kth module of the medium-voltage DC transformer; C rk —The resonant capacitance of the kth module of the medium-voltage DC transformer; T k —The kth module isolation transformer of the medium-voltage DC transformer; D k1 D k2 D k3 D k4 —Uncontrolled rectifier bridge diodes in the kth module of the medium-voltage DC transformer; C ok — Output capacitor of the kth module of the medium-voltage DC transformer; U LV —Low-voltage bus voltage; i LVk — Output current of the kth photovoltaic module of the medium-voltage DC transformer; U ok — Output voltage of the kth module of the medium-voltage DC transformer; i o —System output current; A—Connection point of first switch S1 and second switch S2; B—Connection point of third switch S3 and fourth switch S4; U AB —The voltage between connection point a and connection point b. Detailed Implementation

[0046] The technical solutions in the embodiments of the present invention will be clearly and completely described below. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0047] like Figure 1 As shown, this embodiment provides a photovoltaic-DC system based on an integrated three-port converter. The system consists of n isolated three-port converter unit modules. The input terminal is n photovoltaic arrays (PV1, PV2... PV...). n Each photovoltaic array is connected to a modular three-port converter. The bidirectional ports of each converter are connected in parallel to form a low-voltage DC bus. This bus has no external load and serves only as a power exchange channel between modules. The outputs of each converter are connected in series to form a medium-voltage bus. The main circuit topology of the three-port converter is an integrated structure of a two-phase interleaved Boost converter and a full-bridge LLC resonant converter, which share four switching transistors S. k1 ~S k4 A single converter module includes a photovoltaic unit input capacitor C. pvk Low-voltage bus capacitor C LVk BOOST converter interleaved inductor L b1k With L b2kFull-bridge inverter circuit S k1 ~S k4 LLC resonant cavity (composed of resonant inductor L) rk Resonant capacitor C rk With excitation inductance L mk Composition), isolation transformer T k Secondary uncontrolled rectifier bridge and output capacitor C ok The specific connection method is as follows: Low-voltage bus capacitor C LVk A parallel input capacitor C is connected across the bidirectional ports of the inverter full-bridge, and the output terminal of the photovoltaic unit is connected in parallel with the input capacitor C. pvk Series boost inductor L b1k L b2k Then, the first switching transistor S of the inverter full-bridge is connected to it respectively. k1 With the second switching transistor S k2 Connection point A and the third switch S k3 With the fourth switch S k4 Connection point B, connection point A, series resonant capacitor C rk With resonant inductor L rk Then connect the excitation inductor L mk Connection point B is connected to the magnetizing inductor L. mk Magnetizing inductance L mk Parallel connection to isolation transformer T k The ratio of the primary to secondary sides of the isolation transformer is N. The resonant cavity output is coupled to the secondary side via the isolation transformer. The uncontrolled rectifier full-bridge on the secondary side includes diode D. k1 D k2 D k3 D k4 Output capacitor C ok It is connected in parallel at both ends of the uncontrolled rectifier bridge on the secondary side.

[0048] First switching transistor S k1 With the second switching transistor S k2 Third switch S k3 With the fourth switch S k4 Complementary conduction, first switch S k1 The duty cycle of the drive signal is D k The phase shift angle between the first half-bridge unit and the second half-bridge unit is fixed at 180°; the first switching transistor S k1 Second switch S k2 Third switch S k3 Fourth switch S k4 Operating at the same operating frequency f s The schematic diagrams of the drive signals and related operating waveforms for the switching transistors S1, S2, S3, and S4 are shown below. Figure 2 As shown, in order to prevent the switching transistor from shooting through, Figure 2The drive signal diagram takes dead time into account. Under this control method, the operating mode diagram of the single-module three-port converter is as follows: Figure 3 As shown, the working principle of each mode of the three-port converter is as follows:

[0049] Mode I [t0-t1]: Before time t0, only S4 is on; at time t0, S1 is on; S2 and S3 remain off. The voltage at point A is +U. LV If the voltage at point B is 0V, then U AB +U LV .

[0050] L b1 Energy is released through S1, L b2 Storing energy. Establishing differential equations:

[0051] ;

[0052] Solving the equation yields the resonant current, which is a sine wave:

[0053] ;

[0054] Where the LC resonant angular frequency is The characteristic impedance is .

[0055] The excitation current increases linearly:

[0056] ;

[0057] Mode Ⅰ [t1-t2]: At time t1, S1 is off, while S2 is not yet on, entering the dead zone. The resonant current and the BOOST converter inductor current charge and discharge the junction capacitance of S1 and S2, causing the voltage across S2 to drop to zero. At the end of this stage, the anti-parallel diode of S2 turns on, preparing for zero-voltage turn-on.

[0058] Mode I / II [t2-t3]: At time t2, switch S2 is turned on, the voltage at point A is 0V, the voltage at point B is 0V, then U AB Reduce the voltage to 0V and establish the differential equation:

[0059] ;

[0060] Solving the equation yields the resonant current, i. r It no longer follows the original sinusoidal trajectory, but instead drops to the excitation current i at a faster slope. m .

[0061] ;

[0062] The excitation current still increases linearly:

[0063] ;

[0064] Modal IV [t3-t4]: At time t3, S2 and S4 remain on, U AB The voltage remains 0V, and the secondary rectifier diodes D1 and D4 are naturally turned off. The differential equation is as follows:

[0065] ;

[0066] At this time i r with i m They are equal. Solving the equation, we get:

[0067] ;

[0068] The resonant angular frequency at this time is:

[0069] ;

[0070] Mode V [t4-t5]: At time t4, S4 is off, while S3 is not yet on, only S2 remains on, entering the dead zone. The resonant current and the BOOST inductor current charge and discharge the junction capacitance of S3 and S4, causing the voltage across S3 to drop to zero. At the end of this stage, the anti-parallel diode of S3 turns on, preparing for zero-voltage turn-on.

[0071] Mode VI [t5-t6]: At time t5, S3 is turned on, while S2 and S3 remain on. The voltage at point A is 0V, and the voltage at point B is +U. LV , then U AB Reduced to -U LV Establish the differential equation:

[0072] ;

[0073] Solving the equation yields the resonant current, which is a sine wave:

[0074] ;

[0075] The second half of the mode is similar.

[0076] A schematic diagram of low-voltage bus power interaction is shown below. Figure 4 As shown, when there is a difference in the input power of each module, the module with excess power transmits power to the low-voltage DC bus through its bidirectional port, while the module with insufficient power absorbs power from the low-voltage DC bus through its bidirectional port, thereby automatically realizing the redistribution of power among the modules and maintaining the balance of the output voltage of each module. Figure 4 In this case, the input power of module 3 is lower than that of modules 1 and 2. At this time, modules 1 and 2 transmit power to the low-voltage DC bus through their bidirectional ports, while module 3 absorbs power from the low-voltage DC bus through its bidirectional ports.

[0077] The overall system control strategy is as follows: Figure 5 As shown, the system as a whole adopts a hybrid modulation control strategy of PWM and PFM, relying on the physical characteristics of the low-voltage bus to achieve large-scale automatic energy balance and adjust the duty cycle D. k Perform maximum power point tracking independently, using the operating frequency f s Balance voltage deviation.

[0078] Each three-port converter controller independently executes the MPPT algorithm, and each module controller acquires the output voltage U of the photovoltaic array in real time. pvk and output current i pvk The MPPT control module adjusts the real-time power output based on the low-voltage bus voltage U. LV Calculate and output the corresponding duty cycle instruction D k D k The duty cycle for the upper bridge arm is specified, and this duty cycle command is applied to the full-bridge switch S in the integrated topology. k1 ~S k4 By controlling the first and second boost inductors L b1k L b2k The energy storage and release ensure that the photovoltaic array always operates at its maximum power point. Meanwhile, the duty cycle D... k This determines the voltage transformation ratio between the input terminal and the low-voltage bus terminal.

[0079] Based on the steady-state relationship of the Boost converter in continuous conduction mode, the relationship between the input voltage and the low-voltage equalization bus voltage can be obtained as follows:

[0080] ;

[0081] The low-voltage DC bus voltage regulation and output voltage balancing are controlled collaboratively by various modules through frequency adjustment to maintain a stable low-voltage bus voltage. Each module monitors its own output voltage U in real time. ok Compare it with the system output rated reference value U oref Subtraction yields the deviation ΔU ok This deviation, after being processed by the proportional gain coefficient K, is then added to the initial reference value U of the low-voltage bus voltage. LVref The system generates correction commands for the low-voltage bus voltage reference in each module, converting the output imbalance into a dynamic disturbance to the low-voltage bus reference value. At this point, the corrected low-voltage bus reference voltage expression is:

[0082] ;

[0083] This scheme incorporates the output voltage of an integrated three-port converter into the overall regulation loop, using the potential changes at the output terminal to adjust the reference standard of the low-voltage DC bus in real time. The core purpose of introducing this mechanism is to establish flexible control tolerance for the low-voltage bus, absorbing sampling deviations between modules through deviation feedback at the output voltage terminal, thus ensuring the control stability of the parallel system.

[0084] Proportional-integral control U LVn with U LVref * They are equal, as shown in the following expression:

[0085] ;

[0086] The modules are physically connected in parallel via a low-voltage busbar.

[0087]

[0088] Thanks to the constraint that the low-voltage bus voltage of each module is equal, each module adjusts its operating frequency to change the voltage gain of the converter, thereby further achieving equal output voltage.

[0089] Under this control architecture, each module changes the operating frequency of the resonant cavity by comparing the consistent actual sampled potential of the bus with its own corrected dynamic reference value. Since the bidirectional ports of the transformers in each module are connected in parallel to achieve equal potential, the gain change caused by frequency adjustment affects the output voltage, altering the system voltage distribution. By adjusting the frequency to change the gain characteristics of the LLC resonant converter, consistent output voltage is achieved. The single-module voltage gain expression is:

[0090] ;

[0091] This solution ensures maximum input power for each module by independently performing maximum power point tracking (MPPT). It simulates the droop control method of an AC grid-connected inverter, adjusting the low-voltage DC bus reference value after controlling the droop of the integrated three-port converter's output voltage. In this way, the system establishes control tolerance in low-voltage bus voltage control, using output-side adjustment feedback to offset sampling deviations and external fluctuations between modules. At the physical level, the bidirectional ports of each module's converter are directly connected in parallel to construct the low-voltage DC bus. When each module detects that its output voltage deviates from its rated state, it adjusts the low-voltage bus reference to change the operating frequency of the resonant cavity. This frequency adjustment maps the converter's resonant gain to the output voltage of each module, achieving converter output voltage balance. Through the synergy of topology and control methods, the system achieves voltage balance through internal power self-balancing without requiring additional hardware units or inter-module information exchange, ensuring system operational stability and power balance.

[0092] It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the invention. Therefore, the embodiments should be considered in all respects as exemplary and non-limiting, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0093] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This way of describing the specification is only for clarity. Those skilled in the art should regard the specification as a whole, and the technical solutions in each embodiment have been appropriately combined to form other embodiments that are easy for those skilled in the art to understand.

Claims

1. A photovoltaic DC system based on an integrated three-port converter, characterized in that, It includes n independent photovoltaic arrays and modular three-port converter units connected to each photovoltaic array in a one-to-one correspondence; n is a positive integer greater than or equal to 2; Each of the three-port converters adopts an integrated topology, which integrates a two-phase interleaved Boost converter and a full-bridge LLC resonant circuit. The two-phase interleaved Boost converter and the full-bridge LLC resonant circuit share a full-bridge switching unit composed of four switching transistors. The input terminals of each three-port converter are connected to the output terminals of the corresponding photovoltaic array. The output terminals of each three-port converter are connected in series to form a medium-voltage DC bus. The bidirectional ports of each three-port converter are connected in parallel to form a common low-voltage DC bus. The low-voltage DC bus has no external load connected and serves only as a power exchange channel between the converter modules.

2. The photovoltaic DC system based on an integrated three-port converter according to claim 1, characterized in that, When there is a difference in the input power of each module, the module with excess power outputs power to the low-voltage DC bus through its bidirectional port, while the module with insufficient power absorbs power from the low-voltage DC bus through its bidirectional port, thereby realizing the redistribution of power among the modules through the low-voltage DC bus.

3. The photovoltaic DC system based on an integrated three-port converter according to claim 1, characterized in that, Each converter module outputs the same current, and the output voltage is autonomously distributed according to its transmission power, satisfying the following relationship: ； In the formula, U ok U is the output voltage of the k-th converter module. MVDC P is the medium-voltage DC bus voltage. k The transmission power of the k-th module, P Σ Total system transmission power.

4. A power equalization control method applied to a photovoltaic DC system based on an integrated three-port converter as described in claim 1, characterized in that, Includes the following steps: S1. Each module independently achieves maximum power point tracking control: The controller of each module collects the output voltage and current of the corresponding photovoltaic array in real time, adjusts the duty cycle of the upper bridge arm switch in the full-bridge switching unit of the converter, and tracks the maximum power point of the corresponding photovoltaic array; at the same time, it controls the phase shift angle of the left and right bridge arms of the full-bridge switching unit to maintain at 180°. S2. Output voltage equalization control based on droop control: The droop control method of the analog AC grid-connected inverter is used to correct the reference value of the low-voltage DC bus after droop control of the output voltage of each module converter. Each module compares the actual sampled value of the low-voltage DC bus voltage with the corrected reference value, and changes the voltage gain of the LLC resonant circuit of the corresponding module by frequency adjustment, so as to achieve the equalization of the output voltage of each module.

5. The power equalization control method according to claim 4, characterized in that, The two-phase interleaved Boost converter operates in continuous conduction mode, and the steady-state relationship between its input voltage and the low-voltage DC bus voltage is as follows: ； In the formula, U LV For low-voltage bus voltage, U pvk Let D be the output voltage of the k-th photovoltaic array. k Let be the duty cycle of the upper bridge arm switch in the k-th converter full-bridge switching unit.

6. The power equalization control method according to claim 4, characterized in that, In step S2, the reference value U of the low-voltage DC bus is corrected. LVref * The mathematical expression is: ； In the formula, U LVref U is the preset initial value of the low-voltage bus reference voltage. ok U is the output voltage of the k-th converter. oref K is the output voltage reference value, and K is the proportional gain coefficient.

7. The power equalization control method according to claim 6, characterized in that, Each module monitors its own output voltage U in real time. ok and compare it with the rated reference voltage U oref The difference is used as a feedback quantity and is equivalently superimposed on the initial value U of the low-voltage bus reference voltage. LVref Above, generate the dynamic reference voltage command U corresponding to each module. LVref * The proportional-integral control U is adopted. LVn with U LVref * They are equal, as shown in the following expression: ； In the formula, U LV1 U LV2 ...U LVn This represents the actual low-voltage bus voltage value for each module.

8. The power equalization control method according to claim 7, characterized in that, In step S2, system power balance is achieved by constructing a collaborative mechanism between the low-voltage bus voltage and the output voltage deviation. The specific implementation logic is as follows: (1) Low-voltage bus voltage reference value correction: The output voltage deviation is superimposed on the low-voltage bus reference value to generate differentiated low-voltage bus voltage reference values ​​for each module; (2) Operating frequency adjustment: Each module compares the sampled value of the low-voltage bus voltage with the corrected reference value of each module to obtain the voltage difference of the low-voltage bus, and outputs the operating frequency f through the PI regulator. k ; (3) Resonant converter gain adjustment: By adjusting the frequency to change the voltage gain of the resonant cavity, under the constraint of the low-voltage bus potential, the power flow direction is guided by adjusting the voltage gain of each module, and finally the system output voltage balance is achieved.

9. The power equalization control method according to claim 4, characterized in that, The expression for voltage gain is: ； In the formula, M k For a single module, N is the ratio of the primary to secondary windings of the isolation transformer, and f is the gain. n The normalized value of the operating frequency is the ratio of the operating frequency to the resonant frequency; Q is the quality factor; and k is the resonant frequency. m This is the ratio of the magnetizing inductance to the resonant inductance.

10. The power equalization control method according to claim 4, characterized in that, By adjusting the operating frequency, the LLC resonant cavity is always operated in the inductive impedance region, ensuring that the power switching transistors of the full-bridge switching unit achieve zero-voltage turn-on.

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