A single-stage self-voltage-equalizing IIOS converter for distributed photovoltaic MVDC collection and a control method thereof

Abstract

This invention discloses a single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation. The invention comprises N sub-modules, each a three-port structure integrating interleaved parallel Boost units and hybrid full-bridge LLC resonant units. The former implements MPPT (Multi-Level Testing) for the distributed photovoltaic array, while the latter provides isolated voltage boost. Each sub-module's first port connects to an independent photovoltaic array, its second port is connected in parallel to a low-voltage DC bus, and its third port is connected in series to a medium-voltage DC bus. A control method is also provided based on this converter. By employing sinusoidal modulation with consistent primary and secondary drive logic, the hybrid full-bridge LLC resonant unit operates in DC transformer mode, with a constant sinusoidal resonant current. Furthermore, the second ports of each sub-module are directly connected in parallel, thus enabling self-equalizing output voltage among the sub-modules. This invention only requires sampling the output voltage and current of each distributed photovoltaic array to achieve MPPT for each sub-module and self-equalizing output voltage among sub-modules, eliminating the need for sampling the voltage at the third port or additional voltage equalization circuitry.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and in particular to a single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation and its control method. Background Technology

[0002] With the advancement of the "dual-carbon" strategy, my country's photovoltaic (PV) installed capacity is expanding, making the efficient collection and transmission of PV power increasingly important. In distributed PV power collection systems, medium-voltage direct current (MVDC) collection is replacing traditional medium-voltage alternating current (MVAC) collection as a research hotspot due to its advantages in efficiency, stability, and power quality. In this system, because the output voltage of a single PV array is limited, multiple isolated DC-DC converter sub-modules are often cascaded to match the PV voltage with the MVDC bus. However, in actual operation, PV array panels may suffer from localized shading, aging, dust accumulation, or manufacturing defects, leading to PV array panel output power mismatch. This results in uneven output voltage across multiple series-connected sub-modules in the system, posing a risk of equipment damage due to overvoltage. To address the challenge of output voltage imbalance and achieve high voltage conversion ratios and high efficiency, there are currently two main structures for distributed medium-voltage direct current collection systems. The first structure is a modular configuration of input-independent and output-in-series (IIOS) power sharing units (PBUs) with isolated submodules (SMs). The IIOS combined converter input port is connected to the photovoltaic array to achieve independent maximum power point tracking (MPPT), and the PBUs achieve voltage equalization at the IIOS converter output. However, in this structure, the IIOS converter requires an additional PBU to achieve output voltage sharing, and the PBUs need to be synchronized or phase-shifted by a centralized controller, so it is not fully modular. The second structure uses a front-end DC-DC converter to achieve individual MPPT control and transfer photovoltaic energy to a shared DC bus. Then, a subsequent input-parallel-output-series (IPOS) converter connected to the shared DC bus adapts to the MVDC bus voltage. However, since all the power from the photovoltaic array undergoes two processing steps and there are many DC / DC converters, the efficiency is generally low and the cost is high.

[0003] The following prior art was obtained through searching and is presented as a reference document. The differences between this application and each of the prior art documents are listed below:

[0004] I. Technical comparison with patent CN112564080A "Novel IIOS Converter with Low-Loss LC-PBU":

[0005] 1. Patent CN112564080A uses a power balancing unit consisting of an inductor and a capacitor connected between every two adjacent photovoltaic sub-modules to balance the voltage on the output side of each isolated photovoltaic sub-module. This research, however, eliminates the need for a power balancing unit. Instead, it achieves self-equalization of output voltage between sub-modules by connecting the second ports of all sub-modules in parallel to a shared low-voltage DC bus and using sinusoidal modulation. This approach features a simple topology.

[0006] 2. Patent CN112564080A uses a main controller to sample the output current and voltage of the distributed photovoltaic array and determines the control phase shift angle of the input and output switches of each isolated DC / DC converter using the MPPT algorithm. Furthermore, it needs to collect the output voltage of the sub-modules and determine the phase shift of the output switches of adjacent sub-modules using the voltage difference and corresponding PI algorithms. In this study, each sub-module has an independent local controller instead of a centralized controller. The local controller only needs to sample the output current and voltage of the distributed photovoltaic array and determine the switch drive signal using the MPPT algorithm; the primary and secondary logics are identical. Compared to patent CN112564080A, this solution eliminates the need to sample the voltage at the sub-module output ports, resulting in simpler control logic. Moreover, the sub-modules achieve a fully modular design, offering better scalability and maintainability.

[0007] II. Technical Comparison with Patent CN113839564A "A Voltage Equalization Circuit Suitable for IIOS Type Photovoltaic DC Boost Collector System":

[0008] 1. Patent CN113839564A uses a DPBU (Power Distribution Unit) circuit, consisting of a half-bridge and two inductors, to connect two photovoltaic converter sub-modules. After each sub-module achieves independent MPPT (Multi-Level Testing) tracking, the output voltage of each sub-module needs to be sampled and compared with a reference voltage. A PI controller then generates the phase shift angle of the upper and lower bridge arm switches of the DPBU, thereby achieving output voltage balancing across the sub-modules. This research, however, only requires sampling the output voltage and current of each distributed photovoltaic array to achieve MPPT for each sub-module and self-equalizing output voltage between sub-modules, eliminating the need for sampling the voltage at a third port or additional voltage equalization circuitry. Compared to patent CN113839564A, this research significantly simplifies the topology and control logic, effectively reducing system cost and control complexity, and improving the reliability and dynamic response speed of sub-module voltage equalization.

[0009] 2. The isolated boost full-bridge converter topology used in patent CN113839564A achieves independent maximum power point tracking (MPPT). In contrast, this study achieves distributed MPPT through interleaved parallel boost units, followed by isolated boost through a hybrid full-bridge LLC resonant unit. Compared to the isolated boost full-bridge converter topology used in patent CN113839564A, the converter in this study has a higher boost ratio, lower submodule input current ripple, and combines the soft-switching characteristics and high-efficiency operation advantages of an LLC resonant converter, resulting in superior overall efficiency and dynamic performance.

[0010] 3. Comparison with the literature G. Ning et al., "Single-Stage IIOS Converter With Auto-Voltage-Sharing for Distributed Photovoltaic MVDC Collection System," in IEEETransactions on Power Electronics, vol. 39, no. 11, pp. 14172-14178, Nov.2024.

[0011] 1. The literature G. Ning et al., "Single-Stage IIOS Converter With Auto-Voltage-Sharing for Distributed Photovoltaic MVDC Collection System," in IEEE Transactions on Power Electronics, vol. 39, no. 11, pp. 14172-14178, Nov. 2024, employs a topology that integrates an interleaved parallel boost unit and a full-bridge LC resonant unit. Its resonant cavity is composed of a resonant inductor and a resonant capacitor connected in series and placed on the secondary side of the transformer, with a full-bridge uncontrolled rectifier circuit on the secondary side. This study, however, integrates an interleaved parallel boost unit and a hybrid full-bridge LLC resonant unit. The resonant cavity is located on the primary side of the transformer, and a hybrid full-bridge rectifier circuit is used on the secondary side. The two topologies differ significantly.

[0012] 2. In the literature, G. Ning et al., "Single-Stage IIOS Converter With Auto-Voltage-Sharing for Distributed Photovoltaic MVDC Collection System," in IEEE Transactions on Power Electronics, vol. 39, no. 11, pp. 14172-14178, Nov. 2024, the full-bridge LC resonant unit operates in DCX mode by ensuring the integrity of the forward and reverse resonant modes of the LC series resonant cavity, and ensuring that the resonant current is 0 in the zero-point current mode and the peak resonant voltage is less than the output voltage to reverse bias the rectifier diodes, thereby achieving self-equalization of the submodule output voltage. This study only requires ensuring that the two secondary-side switches have a simultaneous conduction period to enable the hybrid full-bridge LLC resonant unit to operate in DCX mode and achieve self-equalization of the submodule output voltage. Compared with this literature, the implementation conditions of DCX mode in this scheme are simpler, and the resonant current maintains a sinusoidal waveform under any operating condition within a wide input voltage range, effectively reducing current harmonics. Summary of the Invention

[0013] This invention provides a single-stage self-equalizing IIOS converter and its control method for distributed photovoltaic MVDC aggregation, aiming to solve the problems of complex topology, cumbersome control, and numerous power processing operations in existing distributed photovoltaic medium-voltage DC aggregation systems with power balancing units. This invention only requires sampling the output voltage and current of each distributed photovoltaic array. By employing sinusoidal modulation with consistent primary and secondary drive logic, the hybrid full-bridge LLC resonant unit operates in DC transformer mode, with a constant sinusoidal resonant current. Furthermore, the second ports of each submodule are directly connected in parallel, eliminating the need for sampling the voltage at the third port or additional voltage equalization circuits. This achieves self-equalizing output voltage between submodules, featuring a simple topology and easy control.

[0014] To achieve the above objectives, the technical solution adopted by the present invention is as follows:

[0015] A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation is disclosed. The single-stage self-equalizing IIOS converter comprises N sub-modules. Each sub-module is a three-port structure integrating interleaved parallel Boost units and hybrid full-bridge LLC resonant units. The former implements MPPT for distributed photovoltaics, while the latter implements isolated voltage boost. The first port of each sub-module is connected to an independent photovoltaic array. The second port of each sub-module is connected in parallel to a common low-voltage DC bus. The third port of each sub-module is connected in series to a medium-voltage DC bus. The voltage of the common low-voltage DC bus is V. L The medium-voltage DC bus voltage is Vo ;

[0016] The submodule includes an interleaved parallel Boost unit and a hybrid full-bridge LLC resonant unit. The first port is the input terminal of the interleaved parallel Boost unit, the second port is the output terminal of the interleaved parallel Boost unit and also serves as the input terminal of the hybrid full-bridge LLC resonant unit, and the third port is the output terminal of the hybrid full-bridge LLC resonant unit.

[0017] Furthermore, the interleaved parallel Boost unit includes an input capacitor C. i First boost inductor L b1 The second boost inductor L b2 Full-bridge inverter unit, intermediate capacitor C M The full-bridge inverter unit includes a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4; the hybrid full-bridge LLC resonant unit includes an intermediate capacitor C. M Full-bridge inverter unit, LLC resonant network, transformer T r Hybrid full-bridge rectifier unit and output capacitor C o The LLC resonant network includes a resonant inductor L. r Resonant capacitor C r Magnetizing inductance L m The hybrid full-bridge rectifier unit includes a fifth switch Q5 and a sixth switch Q6, and a first diode D. R1 Second diode D R2 Interleaved parallel Boost units and hybrid full-bridge LLC resonant units reuse the full-bridge inverter unit and intermediate capacitor C. M The second port, the submodule is a single-stage converter integrating interleaved parallel Boost units and hybrid full-bridge LLC resonant units.

[0018] Furthermore, the input capacitor C i The intermediate capacitor C is connected in parallel across the first port. M The full-bridge inverter unit, connected in parallel across the second port, consists of a first bridge arm and a second bridge arm. The first bridge arm is composed of a first switch Q1 and a second switch Q2 connected in forward series, and the second bridge arm is composed of a third switch Q3 and a fourth switch Q4 connected in forward series. The first and second bridge arms are connected in forward parallel to the middle port of the second port; the first boost inductor L... b1 The second boost inductor L is connected to the positive terminal of the first port and the midpoint A of the first bridge arm. b2 Connect to the positive terminal of the first port and the midpoint B of the second bridge arm.

[0019] Furthermore, the resonant inductor L r and resonant capacitor C rConnected in series at the midpoint A of the first bridge arm and the transformer T r On the primary winding terminal of transformer T, r The other end of the primary winding is connected to the midpoint B of the second bridge arm, and the transformer T r The primary and secondary windings are configured with the same terminals in the same direction, and the turns ratio of the primary and secondary windings is 1:n.

[0020] Furthermore, the hybrid full-bridge rectifier unit consists of a third bridge arm and a fourth bridge arm, the third bridge arm being composed of a fifth switch Q5 and a first diode D. R1 The bridge is composed of forward series connections, with the fourth arm consisting of the sixth switch Q6 and the second diode D. R1 Composed of forward series connections, transformer T r The secondary winding's corresponding terminal is connected to the midpoint C of the third bridge arm, and the other terminal is connected to the midpoint D of the fourth bridge arm. The output capacitor is C. o It is connected in parallel to the third port.

[0021] The present invention further provides a control method for a single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation, comprising the following steps:

[0022] The submodules in the converter adopt sinusoidal modulation with consistent primary and secondary driving logic. The duty cycle of the second switch Q2 and the fourth switch Q4 is D. The driving signals of the two switches on the first bridge arm, namely the first switch Q1 and the second switch Q2, are complementary. The driving signals of the two switches on the second bridge arm, namely the third switch Q3 and the fourth switch Q4, are complementary. At the same time, the phase of the driving signal of the second bridge arm lags behind the phase of the driving signal of the first bridge arm by half a switching cycle. The fifth switch Q5 on the third bridge arm of the secondary side has the same driving signal as the first switch Q1 on the first bridge arm of the primary side. The sixth switch Q6 on the fourth bridge arm of the secondary side has the same driving signal as the third switch Q3 on the second bridge arm of the primary side.

[0023] By controlling the conduction time of the drive signals of the fifth switch Q5 and the sixth switch Q6 to be greater than half a resonant cycle, i.e., 1-D>0.5, so that the fifth switch Q5 and the sixth switch Q6 can be simultaneously turned on, the voltage V between the midpoint of the third bridge arm and the midpoint of the fourth bridge arm can be further increased. CD The amplitude is V oN -V oN The three-level voltage of 0 and 0, since the primary and secondary driving logic of the submodule are the same, the voltage V between the midpoint of the first bridge arm and the midpoint of the second bridge arm. AB The amplitude is V LN -V LN A three-level voltage of 0 and 0, and voltage V AB and V CD Both are in phase;

[0024] The converter only needs to sample the output voltage and output current of each photovoltaic array, that is, the input voltage V at the first port of each submodule. inN and current I inN Then, through the independent controllers of each submodule, the duty cycles D of the second switch Q2 and the fourth switch Q4 are obtained using the maximum power point tracking algorithm. The gains of the first port and the second port are only related to the duty cycle D. The gain M1 of the interleaved parallel boost units in the converter is expressed as:

[0025] ;

[0026] By controlling the converter to operate under the resonant inductor L r and resonant capacitor C r A fixed constant series resonant frequency f r Below, the submodule hybrid full-bridge LLC resonant unit operates in DC transformer mode, and the excitation inductor L... m It does not participate in resonance, and the resonant current i Lr and resonant voltage V cr The resonant current i remains constant as a sine wave under all operating conditions. Lr Leading resonant current V cr Half a switching cycle, switching frequency f s Equal to the resonant frequency f r resonant frequency f r Represented as:

[0027] ;

[0028] In this mode, the voltage gain of the hybrid full-bridge LLC resonant unit is only related to the transformer T. r The voltage gain M2 of the hybrid full-bridge LLC resonant unit, which is related to the number of turns, is expressed as:

[0029] ;

[0030] Since the second ports of all submodules are connected in parallel to a common low-voltage DC bus, meaning the input voltage of the hybrid full-bridge LLC resonant unit in each submodule is equal, therefore V L1 =V L2 =...=V LN =V L Therefore, the proposed IIOS converter can achieve self-equalizing output voltage between sub-modules, i.e., V o1 = V o2 =...= V oN = V o / N, since the submodule is a single-stage converter integrated with interleaved parallel Boost units and hybrid full-bridge LLC resonant units, the voltage gain M of the submodule is expressed as:

[0031] .

[0032] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0033] 1. This invention only requires sampling the output voltage and current of each distributed photovoltaic array to achieve MPPT of each sub-module and self-equalizing output voltage between sub-modules, without the need to sample the voltage of the third port or additional voltage equalization circuit.

[0034] 2. This invention achieves complete modularity. Each submodule is identical and can operate in independent mode. Each submodule has its own local controller and there are no communication lines between them.

[0035] 3. The converter proposed in this invention has a fixed switching frequency, and the primary and secondary driving logics are simple and completely consistent.

[0036] 4. The converter proposed in this invention has a resonant current that is always sinusoidal over a wide voltage range, and the voltage gain is independent of the load. Therefore, the voltage gain and ZVS analysis are simplified. Attached Figure Description

[0037] Figure 1 This is a schematic diagram of the topology in an example of the present invention;

[0038] Figure 2 This is a typical working waveform diagram of a submodule in an example of the present invention;

[0039] Figure 3 This is the equivalent circuit diagram of the resonant cavity of the submodule in an example of the present invention;

[0040] Figure 4 The waveforms are typical of the resonant cavity of the submodule in this invention.

[0041] Figure 5 This is the equivalent circuit diagram of the converter submodule in mode 1 in an example of the present invention;

[0042] Figure 6 This is the equivalent circuit diagram of the converter submodule in mode 2 in an example of the present invention;

[0043] Figure 7 This is the equivalent circuit diagram of the converter submodule in mode 3 in an example of the present invention;

[0044] Figure 8 This is the equivalent circuit diagram of the converter submodule in mode 4 in an example of the present invention;

[0045] Figure 9 These are the waveform curves of each sub-module of the photovoltaic array in the system under different environments in the example of this invention. Detailed Implementation

[0046] To better illustrate the purpose and advantages of the present invention, the invention will be further described below with reference to the accompanying drawings and examples. The present invention will be described in further detail below with reference to the accompanying drawings and specific embodiments:

[0047] like Figure 1 As shown, this invention proposes a single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation, comprising N sub-modules. These N sub-modules are three-port structures integrating interleaved parallel boost units and hybrid full-bridge LLC resonant units. The former implements MPPT for distributed photovoltaics, while the latter implements isolated boost. Each sub-module's first port is connected to an independent photovoltaic array, its second port is connected in parallel to a common low-voltage DC bus, and its third port is connected in series to a medium-voltage DC bus. The voltage of the common low-voltage DC bus is V. L The medium-voltage DC bus voltage is V o .

[0048] Wherein, the first port is the input terminal of the interleaved parallel Boost unit, the second port is the output terminal of the interleaved parallel Boost unit and also serves as the input terminal of the hybrid full-bridge LLC resonant unit, and the third port is the output terminal of the hybrid full-bridge LLC resonant unit.

[0049] The interleaved parallel Boost unit includes an input capacitor C. i First boost inductor L b1 The second boost inductor L b2 Full-bridge inverter unit, intermediate capacitor C M The full-bridge inverter unit includes a first switch Q1, a second switch Q2, a third switch Q3, and a fourth switch Q4; the hybrid full-bridge LLC resonant unit includes an intermediate capacitor C. M Full-bridge inverter unit, resonant network, magnetizing inductor Lm, transformer T r Hybrid full-bridge rectifier unit and output capacitor C o The resonant network includes a resonant inductor L. r Resonant capacitor C r The hybrid full-bridge rectifier unit includes a fifth switch Q5 and a sixth switch Q6, and a first diode D. R1 Second diode D R2 The submodule is a single-stage converter that integrates interleaved parallel Boost units and hybrid full-bridge LLC resonant units, and multiplexes the full-bridge inverter unit with interleaved parallel Boost units and hybrid full-bridge LLC resonant units.

[0050] The input capacitor C i The intermediate capacitor C is connected in parallel across the first port. MThe full-bridge inverter unit, connected in parallel across the second port, consists of a first bridge arm and a second bridge arm. The first bridge arm is composed of a first switch Q1 and a second switch Q2 connected in forward series, and the second bridge arm is composed of a third switch Q3 and a fourth switch Q4 connected in forward series. The first and second bridge arms are connected in parallel in forward series across the second port. The first boost inductor L... b1 The second boost inductor L is connected to the positive terminal of the first port and the midpoint A of the first bridge arm. b2 Connect to the positive terminal of the first port and the midpoint B of the second bridge arm.

[0051] The resonant inductor L r and resonant capacitor C r Connected in series at the midpoint A of the first bridge arm and the transformer T r On the primary winding terminal of transformer T, r The other end of the primary winding is connected to the midpoint B of the second bridge arm, and the transformer T r The primary and secondary windings are configured with the same terminals in the same direction, and the turns ratio of the primary and secondary windings is 1:n.

[0052] The hybrid full-bridge rectifier unit consists of a third bridge arm and a fourth bridge arm. The third bridge arm consists of a fifth switch Q5 and a first diode D. R1 The bridge is composed of forward series connections, with the fourth arm consisting of the sixth switch Q6 and the second diode D. R1 Composed of forward series connections, transformer T r The secondary winding's corresponding terminal is connected to the midpoint C of the third bridge arm, and the other terminal is connected to the midpoint D of the fourth bridge arm. The output capacitor is C. o It is connected in parallel to the third port.

[0053] The present invention proposes a control method for a single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation, comprising the following steps:

[0054] like Figure 2 As shown, the converter submodule adopts sinusoidal modulation with consistent primary and secondary driving logic. The duty cycle of the second switch Q2 and the fourth switch Q4 is D. The driving signals of the two switches on the first bridge arm, namely the first switch Q1 and the second switch Q2, are complementary. The driving signals of the two switches on the second bridge arm, namely the third switch Q3 and the fourth switch Q4, are complementary. At the same time, the phase of the driving signal of the second bridge arm lags behind the phase of the driving signal of the first bridge arm by half a switching cycle. The fifth switch Q5 on the third bridge arm of the secondary side has the same driving signal as the first switch Q1 on the first bridge arm of the primary side. The sixth switch Q6 on the fourth bridge arm of the secondary side has the same driving signal as the third switch Q3 on the second bridge arm of the primary side.

[0055] like Figure 2 and Figure 3As shown, by controlling the conduction time of the drive signals of the fifth switch Q5 and the sixth switch Q6 to be greater than half a resonant cycle, i.e., 1-D>0.5, the fifth switch Q5 and the sixth switch Q6 can be turned on simultaneously, and the transformer T r The secondary voltage, i.e., the voltage V between the midpoint of the third bridge arm and the midpoint of the fourth bridge arm, is... CD The amplitude is V oN -V oN The three-level voltage of 0 and 0, since the primary and secondary driving logic of the submodule are the same, the voltage V between the midpoint of the first bridge arm and the midpoint of the second bridge arm. AB The amplitude is V LN -V LN The three-level voltage with and without 0, voltage V AB and V CD Both are in phase, due to the submodule transformer T r Since the primary and secondary switching transistors have the same drive signals and D < 0.5, the waveforms of the first half and second half of the switching cycle of the submodule are symmetrical and have similar working principles. Therefore, the working process within half a switching cycle can be divided into 4 modes, as follows:

[0056] Mode 1 (t0~t1): The current path of the mode is as follows Figure 4 As shown, at time t0, the second switch Q2 is turned off, and the resonant current i Lr Equal to the magnetizing inductor current i Lm Secondary current i s When the junction capacitance of the first switch Q1 and the fifth switch Q5 is equal to 0, the junction capacitance of the second switch Q2 is discharged, and the junction capacitance of the second switch Q2 is charged. In this mode, the voltage V between nodes A and B is... AB Voltage V between nodes C and D CD All are 0;

[0057] Mode 2 (t1~t2): The current path of the mode is as follows Figure 5 As shown, at time t1, the first switch Q1 and the fifth switch Q5 are turned on with zero voltage. In this mode, the voltage V between nodes A and B is... AB Voltage V between nodes C and D CD All are 0;

[0058] Mode 3 (t2~t3): The current path of the mode is as follows Figure 6 As shown, at time t2, the third switch Q3 and the sixth switch Q6 are turned off, and the resonant current i Lr Equal to the magnetizing inductor current i Lm Secondary current i s When the junction capacitance of the fourth switch Q4 is equal to 0, the junction capacitance of the third switch Q3 and the sixth switch Q6 is discharged, while the junction capacitances of the second and third switches Q3 and Q6 are charged. In this mode, the voltage V between nodes A and B is... AB Equal to the second port voltage, i.e., the low-voltage DC bus voltage VMN Voltage V between nodes C and D CD Equal to output voltage V ON ;

[0059] Mode 4 (t3~t4): The current path of the mode is as follows Figure 7 As shown, at time t3, the fourth switch Q4 is turned on with zero voltage. In this mode, the voltage V between nodes A and B is... AB Voltage V between nodes C and D CD All are 0.

[0060] By sampling the output voltage and current of the distributed photovoltaic array, i.e., the input voltage V at the first port of each submodule inN and current I inN Afterwards, each submodule's independent controller uses the maximum power point tracking algorithm to obtain the duty cycle D of the second switch Q2 and the fourth switch Q4. The gains of the first and second ports are only related to the duty cycle D. The gain M1 of the interleaved parallel boost units in the converter is expressed as:

[0061] .

[0062] like Figure 8 As shown, the magnetizing inductance L in the LLC resonant network m The converter does not participate in resonance throughout the entire process; it operates under the influence of the resonant inductor L. r and resonant capacitor C r A fixed constant series resonant frequency f r Below, the submodule hybrid full-bridge LLC resonant unit operates in DC transformer mode, and the resonant current i Lr The resonant current i remains constant as a sine wave under all operating conditions. Lr Leading resonant current V cr Half a switching cycle, switching frequency f s Equal to the resonant frequency f r resonant frequency f r Represented as:

[0063] .

[0064] Since the hybrid full-bridge LLC resonant unit of the submodule operates in DC transformer mode, the voltage gain of the hybrid full-bridge LLC resonant unit is only related to the transformer T. r The voltage gain M2 of the hybrid full-bridge LLC resonant unit, which is related to the number of turns, is expressed as:

[0065] .

[0066] Since the second ports of all submodules are connected in parallel to a common low-voltage DC bus, the input voltage of the hybrid full-bridge LLC resonant unit in each submodule is equal, i.e., VL1 =V L2 =...=V LN =V L Therefore, the proposed IIOS converter can achieve self-equalizing output voltage between sub-modules, i.e., V o1 =V o2 =...=V oN =V o / N.

[0067] Since the submodule is a single-stage converter integrating interleaved parallel Boost units and hybrid full-bridge LLC resonant units, the voltage gain M of the submodule is expressed as:

[0068] .

[0069] The low-voltage DC bus voltage V in the distributed medium-voltage DC photovoltaic collection system of this embodiment is... L Set to 250V, medium-voltage DC bus voltage V o The voltage was set to 750V. A simulation was performed using a IIOS single-stage converter built with three sub-modules as an example. The simulation parameters for a single sub-module are shown in Table 1.

[0070] Table 1 Submodule Parameters

[0071]

[0072] Figure 9 The simulation results show the input voltage, input power, output voltage at the third port, and duty cycle of the first submodule of the converter when the environment (light intensity, temperature) of the distributed photovoltaic array changes. It can be seen that the converter disclosed in this invention can automatically track the maximum power point when the environment of the distributed photovoltaic array changes, and can always achieve automatic voltage balancing among the submodules without sampling the voltage at the third port or additional voltage equalization circuit.

[0073] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation, characterized in that, The system comprises N sub-modules, each a three-port structure integrating interleaved parallel boost units and hybrid full-bridge LLC resonant units. The interleaved parallel boost units are used to implement MPPT (Multi-Level Photovoltaic Test) for distributed photovoltaic systems, while the hybrid full-bridge LLC resonant units are used for isolated boost. Each sub-module has its first port connected to an independent photovoltaic array, its second port connected in parallel to a common low-voltage DC bus, and its third port connected in series to a medium-voltage DC bus. The voltage of the common low-voltage DC bus is V. L The medium-voltage DC bus voltage is V o ; The first port is the input terminal of the interleaved parallel Boost unit, the second port is the output terminal of the interleaved parallel Boost unit and also serves as the input terminal of the hybrid full-bridge LLC resonant unit, and the third port is the output terminal of the hybrid full-bridge LLC resonant unit.

2. A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation according to claim 1, characterized in that: The submodule includes an interleaved parallel boost unit and a hybrid full-bridge LLC resonant unit; The interleaved parallel boost unit and the hybrid full-bridge LLC resonant unit reuse the full-bridge inverter unit and the intermediate capacitor C. M The second port, the full-bridge inverter unit, consists of the first switch Q1, the second switch Q2, the third switch Q3, and the fourth switch Q4. The interleaved parallel boost unit also includes the input capacitor C. i First boost inductor L b1 The second boost inductor L b2 The hybrid full-bridge LLC resonant unit also includes an LLC resonant network and a transformer T. r Hybrid full-bridge rectifier unit and output capacitor C o The LLC resonant network includes a resonant inductor L. r Resonant capacitor C r Magnetizing inductance L m The hybrid full-bridge rectifier unit includes a fifth switch Q5 and a sixth switch Q6, and a first diode D. R1 Second diode D R2 .

3. A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation according to claim 2, characterized in that: The input capacitor Ci is connected in parallel across the first port, and the intermediate capacitor C M The full-bridge inverter unit, connected in parallel across the second port, includes a first bridge arm and a second bridge arm. The first bridge arm is composed of a first switch Q1 and a second switch Q2 connected in forward series, and the second bridge arm is composed of a third switch Q3 and a fourth switch Q4 connected in forward series. The first and second bridge arms are connected in parallel across the second port. The first boost inductor L... b1 The second boost inductor L is connected to the positive terminal of the first port and the midpoint A of the first bridge arm. b2 Connect to the positive terminal of the first port and the midpoint B of the second bridge arm.

4. A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation according to claim 3, characterized in that: The resonant inductor L r and resonant capacitor C r Connected in series at the midpoint A of the first bridge arm and the transformer T r On the primary winding terminal of transformer T, r The other end of the primary winding is connected to the midpoint B of the second bridge arm, and the transformer T r The primary and secondary windings are configured with the same terminals in the same direction, and the turns ratio of the primary and secondary windings is 1:n.

5. A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation according to claim 4, characterized in that: The hybrid full-bridge rectifier unit includes a third bridge arm and a fourth bridge arm. The third bridge arm consists of a fifth switch Q5 and a first diode D. R1 The bridge is composed of forward series connections, with the fourth arm consisting of the sixth switch Q6 and the second diode D. R1 Composed of forward series connections, transformer T r The secondary winding's corresponding terminal is connected to the midpoint C of the third bridge arm, and the other terminal is connected to the midpoint D of the fourth bridge arm. The output capacitor is C. o It is connected in parallel to the third port.

6. A single-stage self-equalizing IIOS converter for distributed photovoltaic MVDC aggregation according to claim 5, characterized in that, Its control method includes the following steps: The submodules in the converter adopt sinusoidal modulation with consistent primary and secondary driving logic. The duty cycle of the second switch Q2 and the fourth switch Q4 is D. The driving signals of the two switches on the first bridge arm, namely the first switch Q1 and the second switch Q2, are complementary. The driving signals of the two switches on the second bridge arm, namely the third switch Q3 and the fourth switch Q4, are complementary. At the same time, the phase of the driving signal of the second bridge arm lags behind the phase of the driving signal of the first bridge arm by half a switching cycle. The fifth switch Q5 on the third bridge arm of the secondary side has the same driving signal as the first switch Q1 on the first bridge arm of the primary side. The sixth switch Q6 on the fourth bridge arm of the secondary side has the same driving signal as the third switch Q3 on the second bridge arm of the primary side. By controlling the conduction time of the drive signals of the fifth switch Q5 and the sixth switch Q6 to be greater than half a resonant cycle, i.e., 1-D>0.5, so that the fifth switch Q5 and the sixth switch Q6 can be simultaneously turned on, the voltage V between the midpoint of the third bridge arm and the midpoint of the fourth bridge arm can be further increased. CD The amplitude is V oN -V oN The three-level voltage of 0 and 0, since the primary and secondary driving logic of the submodule are the same, the voltage V between the midpoint of the first bridge arm and the midpoint of the second bridge arm. AB The amplitude is V LN -V LN A three-level voltage of 0 and 0, and voltage V AB and V CD Both are in phase; The converter only needs to sample the output voltage and output current of each photovoltaic array, that is, the input voltage V at the first port of each submodule. inN and current I inN Then, through the independent controllers of each submodule, the duty cycles D of the second switch Q2 and the fourth switch Q4 are obtained using the maximum power point tracking algorithm. The gains of the first port and the second port are only related to the duty cycle D. The gain M1 of the interleaved parallel boost units in the converter is expressed as: ； By controlling the converter to operate under the resonant inductor L r and resonant capacitor C r A fixed constant series resonant frequency f r Below, the submodule hybrid full-bridge LLC resonant unit operates in DC transformer mode, and the excitation inductor L... m It does not participate in resonance, and the resonant current i Lr and resonant voltage V cr The resonant current i remains constant as a sine wave under all operating conditions. Lr Leading resonant current V cr Half a switching cycle, switching frequency f s Equal to the resonant frequency f r resonant frequency f r Represented as: ； In this mode, the voltage gain of the hybrid full-bridge LLC resonant unit is only related to the transformer T. r The voltage gain M2 of the hybrid full-bridge LLC resonant unit, which is related to the number of turns, is expressed as: ； Since the second ports of all submodules are connected in parallel to a common low-voltage DC bus, meaning the input voltage of the hybrid full-bridge LLC resonant unit in each submodule is equal, therefore V L1 =V L2 =...=V LN =V L Therefore, the proposed IIOS converter can achieve self-equalizing output voltage between sub-modules, i.e., V o1 = V o2 =...= V oN = V o / N, since the submodule is a single-stage converter integrated with interleaved parallel Boost units and hybrid full-bridge LLC resonant units, the voltage gain M of the submodule is expressed as: 。

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