A Photovoltaic DC Boost Converter Based on a Dual-Bus Structure and Its Control Method

Abstract

The present invention relates to photovoltaic power generation and power electronic converter technologies, and particularly to a photovoltaic DC boost converter and a control method based on a dual-bus structure. The topology of the photovoltaic DC boost converter includes N photovoltaic arrays, N three-active-bridge converters, a low-voltage DC bus, and a medium-voltage DC bus. Among them, the first ports of the N three-active-bridge converters are connected to independent photovoltaic power supplies, the second ports are connected in parallel to the low-voltage DC bus, and the third ports are sequentially connected in series to the medium-voltage DC bus to achieve medium-voltage DC collection. When the photovoltaic input power is uneven, resulting in power mismatch of the photovoltaic DC boost converter, the control is used to transfer the mismatched power through the low-voltage DC bus, so as to achieve power balance and autonomous voltage equalization of the medium-voltage DC bus. Moreover, by adding a voltage offset in the control loop, the voltage equalization effect and the voltage stability of the low-voltage DC bus can be ensured simultaneously.

Description

Technical Field

[0001] The present invention belongs to the technical field of photovoltaic power generation and power electronic converters, and particularly relates to a photovoltaic DC boost converter and a control method based on a dual-bus structure. Background Art

[0002] As one of the main forms of current renewable resource development and utilization, with the continuous increase in the penetration rate of photovoltaic energy in the power grid and the increasingly urgent demand for large-scale aggregation and long-distance transmission, the efficiency and stability problems existing in photovoltaic AC boost aggregation technology can no longer well adapt to the development. Photovoltaic DC boost aggregation has the advantages of high efficiency and strong stability, and is more suitable for large-scale photovoltaic energy aggregation and grid connection and long-distance transmission.

[0003] At the present stage, in order to meet the requirements of high power rating and high voltage gain, the photovoltaic DC boost converters mostly adopt modular DC / DC converters connected in series on the output side of the sub-modules to meet the requirements. At the present stage, the multi-module cascaded photovoltaic DC boost converters are mainly divided into four types, namely, input-independent output-series (IIOS) type, input-parallel output-series (IPOS) two-stage conversion (TSC) type, and low-voltage bus based (LVBB) type. The IIOS type has a simple structure, few power conversion stages, and low losses. However, this converter structure has serious power mismatch problems, and the resulting uneven output voltage may lead to increased losses, power commutation and MPPT failure, and even damage to switching devices. The IPOS type solves the problem of uneven output voltage of the converter unit during power mismatch, but requires a large number of distributed MPPT devices, bringing problems such as difficult system operation and maintenance and increased costs. The TSC type adds a post-stage converter on the basis of the IIOS type for boosting and output voltage equalization. The LVBB type adds a pre-stage converter on the basis of the IPOS type for independent MPPT control. These two structures can solve the power mismatch problem to a certain extent, but require two-stage power conversion, which seriously affects the system efficiency.

[0004] The invention patent with the application publication number CN106787707A and the name "Embedded energy storage type multi-module series photovoltaic DC boost converter and application method" proposes a photovoltaic DC boost converter with a sub-module cascade structure. The conversion sub-module uses a two-port isolated full-bridge DC-DC converter, and a hybrid energy storage module and a corresponding bidirectional boost / buck converter are configured at the output end of each photovoltaic array to achieve power balance by controlling the output power of the hybrid energy storage module. However, the introduction of the hybrid energy storage module will greatly increase the cost, and the power balance ability is limited by the energy storage capacity.

[0005] The invention patent with the application publication number CN115051572A and the name "IIOS converter with series resonant type LC power self-balancing unit and method" and the invention patent with the application publication number CN114785145A and the name "Low input current ripple high gain low loss modular photovoltaic DC boost converter" are both applied to the scenario of DC collection and grid connection of photovoltaic energy. They both adopt the IIOS type structure with two-port converters as sub-modules, and both use a power balance unit to transmit photovoltaic mismatch power to achieve the output balance of the converter sub-modules. However, the switching tube of the power balance unit is integrated with the secondary side switching tube of the sub-module, which greatly increases the current stress borne by the secondary side switching tube when the power is uneven. Summary of the Invention

[0006] In view of the problems existing in the background technology, the present invention provides a photovoltaic boost converter based on a dual-bus structure and a power balance control method.

[0007] To solve the above technical problems, the present invention adopts the following technical solutions: A photovoltaic DC boost converter based on a dual-bus structure, the photovoltaic DC boost converter includes N photovoltaic arrays, N is a natural number greater than or equal to 2, and N three-active-bridge converters, a low-voltage DC bus, and a medium-voltage DC bus connected correspondingly.

[0008] In the above photovoltaic DC boost converter based on a dual-bus structure, the three-active-bridge converter includes a first switching tube S1, a second switching tube S2, a third switching tube S3, a fourth switching tube S4, a fifth switching tube S5, a sixth switching tube S6, a seventh switching tube S7, an eighth switching tube S8, a ninth switching tube S9, a tenth switching tube S 10 、an eleventh switching tube S 11 、a twelfth switching tube S 12 ; a first capacitor C1 on the primary side, a second capacitor C2 on the primary side, a first capacitor C3 on the secondary side; a first inductor L1 on the primary side, a second inductor L2 on the primary side, a first inductor L3 on the secondary side, a first coil n1 of the primary side of the high-frequency isolation transformer, a second winding n2 of the primary side of the high-frequency isolation transformer, a first winding n3 of the secondary side of the high-frequency isolation transformer;

[0009] The positive electrode of the primary first capacitor C1 is respectively connected to the drain of the first switching transistor S1 and the drain of the third switching transistor S3, and the negative electrode of the primary first capacitor C1 is respectively connected to the source of the second switching transistor S2 and the source of the fourth switching transistor S4;

[0010] The positive electrode of the primary second capacitor C2 is respectively connected to the drain of the fifth switching transistor S5 and the drain of the seventh switching transistor S7, and the negative electrode of the primary second capacitor C2 is respectively connected to the source of the sixth switching transistor S6 and the source of the eighth switching transistor S8;

[0011] The positive electrode of the secondary first capacitor C3 is respectively connected to the drain of the ninth switching transistor S9 and the drain of the eleventh switching transistor S 11 ; the negative electrode of the secondary first capacitor C3 is respectively connected to the source of the tenth switching transistor S 10 and the source of the twelfth switching transistor S 12 ;

[0012] The drain of the first switching transistor S1 is connected to the drain of the third switching transistor S3, the source of the first switching transistor S1 is connected to the drain of the second switching transistor S2, the source of the second switching transistor S2 is connected to the source of the fourth switching transistor S4, and the source of the third switching transistor S3 is connected to the drain of the fourth switching transistor S4; the drain of the fifth switching transistor S5 is connected to the drain of the seventh switching transistor S7, the source of the fifth switching transistor S5 is connected to the drain of the sixth switching transistor S6, the source of the sixth switching transistor S6 is connected to the source of the eighth switching transistor S8, and the source of the seventh switching transistor S7 is connected to the drain of the eighth switching transistor S8;

[0013] The drain of the ninth switching transistor S9 is connected to the drain of the eleventh switching transistor S 11 ; the source of the ninth switching transistor S9 is connected to the drain of the tenth switching transistor S 10 ; the source of the tenth switching transistor S 10 is connected to the source of the twelfth switching transistor S 12 ; the source of the eleventh switching transistor S 11 is connected to the drain of the twelfth switching transistor S 12 ;

[0014] One end of the primary first winding n1 of the high-frequency isolation transformer is connected to the source of the first switching transistor S1 and is connected to the primary first inductor L1, and the other end is connected to the drain of the fourth switching transistor S4; one end of the primary second winding n2 of the high-frequency isolation transformer is connected to the source of the fifth switching transistor S5 and is connected to the primary second inductor L2, and the other end is connected to the drain of the eighth switching transistor S8; one end of the secondary first winding n3 of the high-frequency isolation transformer is connected to the source of the ninth switching transistor S9 and is connected to the secondary first inductor L3, and the other end is connected to the drain of the twelfth switching transistor S 12 ;

[0015] In the above photovoltaic DC boost converter based on the dual-bus structure, both ends of the first capacitor C1 on the primary side form the first port of the three-active-bridge converter; both ends of the second capacitor C2 on the primary side form the second port of the three-active-bridge converter; both ends of the first capacitor C3 on the secondary side form the third port of the three-active-bridge converter.

[0016] In the above photovoltaic DC boost converter based on the dual-bus structure, the first port of the three-active-bridge converter serves as the energy input port, which is connected to the photovoltaic array to achieve MPPT control;

[0017] The second port of the three-active-bridge converter serves as the energy bidirectional flow port, which is connected in parallel to the low-voltage DC bus to absorb energy from the low-voltage DC bus or deliver energy to the low-voltage DC bus;

[0018] The third port of the three-active-bridge converter serves as the energy output port, which is connected in series to the medium-voltage DC bus in sequence.

[0019] In the above photovoltaic DC boost converter based on the dual-bus structure, the low-voltage DC bus is connected to the local load and transmits the mismatch power. The second ports of N three-active-bridge converters are connected to the low-voltage DC bus, and the mismatch power transfer between N three-active-bridge converters is achieved through power balance control, so that the output power at the third ports of N three-active-bridge converters is balanced, and then output voltage equalization is achieved.

[0020] In the above photovoltaic DC boost converter based on the dual-bus structure, the input powers of N photovoltaic arrays are the same, no mismatch power flows through the low-voltage DC bus, there is no mismatch power transfer between N three-active-bridge converters, and the input powers of all photovoltaic arrays are incorporated into the medium-voltage DC grid through one-stage power conversion;

[0021] If there are differences in the input powers of the photovoltaic arrays among N photovoltaic arrays, the mismatch power will be transferred from the three-active-bridge converter with a higher input power to the three-active-bridge converter with a lower input power through the low-voltage DC bus, and the output powers at the third ports of N three-active-bridge converters are equal;

[0022] Part of the mismatch power flowing through the low-voltage DC bus undergoes two-stage power conversion, and the remaining input power undergoes one-stage power conversion.

[0023] A power balance control method for a photovoltaic DC boost converter based on the dual-bus structure. When the power mismatch of the photovoltaic DC boost converter is caused by uneven photovoltaic input power, this control method enables the mismatch power to be transferred through the low-voltage DC bus to achieve power balance and autonomous voltage equalization on the medium-voltage side, and simultaneously achieves voltage equalization and voltage stability of the low-voltage DC bus by adding a voltage offset in the control loop.

[0024] In the above power balance control method of the photovoltaic DC boost converter based on the double-bus structure, the specific steps of the power balance control method of the photovoltaic DC boost converter are as follows:

[0025] Maximum power point tracking: Collect the output voltage and output current of N photovoltaic arrays to obtain the input voltage values V 1_i and input current values i 1_i at the first port of N three-active-bridge converters. Through the MPPT control module, obtain the input voltage reference value V 1ref_i of the first port; Utilize the difference between the input voltage value V 1_i at the first port of the three-active-bridge converter and the input voltage reference value V 1ref_i at the first port of the three-active-bridge converter. After passing through a PI controller and a limiting link, obtain the phase-shifted duty cycle D 13 , which is used as the duty cycle of the phase-shift angle between the drive signals of the first port and the third port of N three-active-bridge converters, so that N photovoltaic arrays operate at their maximum power points;

[0026] Output voltage equalization step: Collect the output voltage values V 2_i at the second port and the output voltage values V 3_i at the third port of N three-active-bridge converters. The output voltage reference values of the second ports of N three-active-bridge converters are V 2ref , and the output voltage reference value V 3ref at the third port of the three-active-bridge converter. Utilize the difference between the output voltage value V 3_i at the third port of the three-active-bridge converter and the output voltage reference value V 3ref at the third port of the three-active-bridge converter, and superimpose the output voltage reference value V 2ref at the second port of the three-active-bridge converter and the voltage offset Δv2 to obtain the instantaneous output voltage reference value V 2ref_i of the second ports of N three-active-bridge converters; Utilize the difference between the output voltage value V 2_i at the second port of the three-active-bridge converter and the instantaneous output voltage reference value V 2ref_i at the second port of the three-active-bridge converter. After passing through a PI controller and a limiting link, obtain the phase-shifted duty cycle D 12 , which is used as the duty cycle of the phase-shift angle between the drive signals of the first port and the second port of N three-active-bridge converters; where V 2ref_i = V 2ref + k vo (V 3_i - V 3ref_i ) - Δv2, and k vo is the amplification factor of the difference between V 3_i and V 3ref_i ;

[0027] The voltage offset is k j is the amplification factor of the difference between the actual average voltage and the reference value of the third port of the three-active-bridge converter.

[0028] In the above power balance control method of the photovoltaic DC boost converter based on the dual-bus structure, the control of the phase-shifted duty cycle D 12 includes:

[0029] When the output voltage value V of the third port of the three-active-bridge converter 3_i increases, the reference voltage V of the second port of the corresponding three-active-bridge converter 2ref_i increases accordingly, and the phase-shifted duty cycle D 12 increases, so that the power P transmitted from the first port of the three-active-bridge converter to the second port 12 increases, and the corresponding power P of the third port 13 decreases; vice versa.

[0030] In the above power balance control method of the photovoltaic DC boost converter based on the dual-bus structure, the control of the voltage offset Δv2 includes:

[0031] When the output voltage value V of the third port of the three-active-bridge converter 3_i increases, the corresponding voltage offset Δv2 of the second port of the three-active-bridge converter also increases, so that the instantaneous output voltage reference value V of the second ports of N three-active-bridge converters 2ref_i is approximately equal to the output voltage reference value V of the second port of the three-active-bridge converter 2ref .

[0032] Compared with the prior art, the beneficial effects of the present invention: A photovoltaic DC boost converter and a control method based on a dual-bus structure proposed by the present invention. Compared with the IIOS structure, through the transfer of the mismatched power by the low-voltage DC bus, the output powers of the converter units are made equal, thus fundamentally solving the power mismatch problem and improving the wide-range operation ability of the converter system; compared with the IPOS structure, the present invention does not require a distributed MPPT device and can achieve independent MPPT control. In addition, except for the mismatched power, the photovoltaic input power in the present invention only needs to go through one-stage power conversion, which is beneficial to the high-efficiency operation of the system.

[0033] The present invention uses a three-active-bridge converter with three ports as a sub-module, without an additional energy storage device, and transfers the mismatched power through a low-voltage DC bus, with a low system cost and strong power balancing ability.

[0034] The present invention uses a three-active-bridge converter with three ports as a sub-module, and the mismatched power is transferred through a low-voltage bus, without additionally increasing the current stress of the switching tubes. Description of the Drawings

[0035] Figure 1 Schematic diagram of the structure of the photovoltaic DC boost converter according to an embodiment of the present invention;

[0036] Fig. 2(a) is a schematic diagram of the power balance mechanism when the input powers of N independent photovoltaic arrays are the same according to an embodiment of the present invention;

[0037] Fig. 2(b) is a schematic diagram of the power balance mechanism when there are different input powers among N independent photovoltaic arrays according to an embodiment of the present invention;

[0038] Figure 3 Topological structure diagram of the three-active-bridge converter according to an embodiment of the present invention;

[0039] Figure 4 Power balance control block diagram according to an embodiment of the present invention;

[0040] Figure 5 Simulation waveform when power mismatch occurs in the system according to an embodiment of the present invention;

[0041] Figure 6 Simulation waveform when a fault occurs in the photovoltaic array according to an embodiment of the present invention;

[0042] Figure 7 Simulation waveform when a fault occurs in the three-active-bridge converter according to an embodiment of the present invention. Detailed implementation manners

[0043] The technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. All other embodiments obtained by those of ordinary skill in the art based on the embodiments of the present invention without creative efforts shall fall within the protection scope of the present invention.

[0044] It should be noted that, without conflict, the embodiments in the present invention and the features in the embodiments may be combined with each other.

[0045] The present invention will be further described below in conjunction with specific embodiments, but it is not a limitation of the present invention.

[0046] In order to enable the converter to achieve power balance and autonomous voltage equalization, and at the same time ensure the voltage stability of the low-voltage DC bus, this embodiment proposes a photovoltaic DC boost converter and control method based on a dual-bus structure. The topology of the photovoltaic DC boost converter includes N photovoltaic arrays, N three-active-bridge converters, a low-voltage DC bus, and a medium-voltage DC bus, where N is a natural number greater than or equal to 2. Among them, the first ports 1 of the N three-active-bridge converters are connected to the N photovoltaic arrays, the second ports 2 are connected in parallel to the low-voltage DC bus, and the third ports 3 are connected in series to the medium-voltage DC bus in sequence to achieve medium-voltage DC collection. When the photovoltaic input power is uneven, resulting in power mismatch of the photovoltaic DC boost converter, the control method of this embodiment enables the mismatched power to be transferred through the low-voltage DC bus to achieve power balance and autonomous voltage equalization on the medium-voltage side. And by adding a voltage offset in the control loop, the voltage equalization effect and the voltage stability of the low-voltage DC bus can be ensured at the same time.

[0047] This embodiment is implemented through the following technical solutions. A photovoltaic DC boost converter based on a dual-bus structure includes N independent photovoltaic arrays, N corresponding three-active-bridge converters, a low-voltage DC bus, and a medium-voltage DC bus. Among them, the first ports 1 of the N three-active-bridge converters are used as energy input ports and are independently connected to the N photovoltaic arrays. The second ports 2 are used as energy bidirectional flow ports and are connected in parallel to the low-voltage DC bus. The third ports 3 are used as energy output ports and are connected in series to the medium-voltage DC bus in sequence, where N is a natural number greater than or equal to 2.

[0048] The topology structure includes N three-active-bridge converters. The first ports 1 of the N three-active-bridge converters are connected to the photovoltaic arrays to achieve independent MPPT control, that is, maximum power point tracking control.

[0049] The second ports 2 of the N three-active-bridge converters are used as ports for energy bidirectional flow. The second ports 2 can absorb energy from the low-voltage DC bus or deliver energy to the low-voltage DC bus according to the photovoltaic power mismatch.

[0050] The third ports 3 of the N three-active-bridge converters are used as energy output ports. Under all operating conditions, the power obtained by the third ports 3 of the N three-active-bridge converters from the corresponding first ports 1 and second ports 2 are all equal to each other, and power balance and voltage equalization on the output side are achieved.

[0051] The three-active-bridge converter includes: the first switch tube S1, the second switch tube S2, the third switch tube S3, the fourth switch tube S4, the fifth switch tube S5, the sixth switch tube S6, the seventh switch tube S7, the eighth switch tube S8, the ninth switch tube S9, the tenth switch tube S 10 and the eleventh switch tube S 11 and the twelfth switch tube S 12; the first primary capacitor C1, the second primary capacitor C2, and the first secondary capacitor C3; the first primary inductor L1, the second primary inductor L2, and the first secondary inductor L3, the first primary coil n1 of the high-frequency isolation transformer, the second primary coil n2 of the high-frequency isolation transformer, and the first secondary coil n3 of the high-frequency isolation transformer.

[0052] A power balance control method for a photovoltaic DC boost converter based on a double-bus structure, comprising:

[0053] The maximum power point tracking step: sampling the output voltage and output current of N independent photovoltaic arrays to obtain the input voltage values V 1_i and input current values i 1_i at the first port 1 of the three-active-bridge converter, obtaining the input voltage reference value V 1ref_i of the first port 1 through the independent MPPT control module, and using the difference between the input voltage value V 1_i at the first port 1 and the input voltage reference value V 1ref_i of the first port 1 to obtain the phase-shifted duty ratio D 13 after passing through a PI controller and a limiting link, that is, the duty ratio of the phase-shift angle between the driving signals of the first port 1 and the third port 3 of N three-active-bridge converters, so that N photovoltaic arrays operate at their maximum power points.

[0054] The output voltage equalization step: sampling the output voltage values V 2_i , V 3_i at the second port 2 and the third port 3 of N three-active-bridge converters. The output voltage reference values of the second port 2 and the third port 3 of N three-active-bridge converters are V 2ref , V 3ref respectively. Using the difference between V 3_i and V 3ref , and superimposing V 2re and the voltage offset Δv2 to obtain the instantaneous output voltage reference value V 2ref_i of the second port 2 of N three-active-bridge converters. Then, using the difference between the output voltage value V 2_i at the second port 2 of the three-active-bridge converter and the instantaneous output voltage reference value V 2ref_i of the second port 2 of the three-active-bridge converter, obtaining the phase-shifted duty ratio D 12 after passing through a PI controller and a limiting link, that is, the duty ratio of the phase-shift angle between the driving signals of the first port 1 and the second port 2 of N three-active-bridge converters. Wherein, V 2ref_i =V 2ref +k vo (V 3_i -V 3ref_i )-Δv2, and k vo is the coefficient between V 3_i and V3ref_i The amplification factor of the difference. Among them, the voltage offset:

[0055]

[0056] k j is the amplification factor of the difference between the actual average voltage and the reference value of the third port 3 of the three-active-bridge converter.

[0057] The output voltage value V of the third port 3 of the three-active-bridge converter 3_i When it rises, the reference voltage V corresponding to the second port 2 of the three-active-bridge converter 2ref_i also rises accordingly, and the phase-shift duty cycle D 12 increases, making the power P i transmitted from the first port 1 to the second port 2 of the i-th three-active-bridge converter TAB 12 increase, and the power P of the corresponding third port 3 13 decrease, and vice versa. Therefore, when there is an internal power mismatch in the photovoltaic DC boost converter based on the dual-bus structure, for example, the photovoltaic input power of the i-th TAB i is greater than the photovoltaic input power of the j-th TAB j , V 2ref_i will rise, and V 2ref_j will decrease, making the output power of the final TAB i and TAB j at port 3 equal.

[0058] When the output voltage value V of the third port 3 of the three-active-bridge converter 3_i rises, the corresponding voltage offset Δv2 also rises accordingly, making the instantaneous output voltage reference value V of the second port 2 of the corresponding i-th three-active-bridge converter 2ref_i close to the output voltage reference value V of the second port 2 2ref . Since the second ports 2 of N three-active-bridge converters are connected in parallel to the low-voltage DC bus, the output voltage reference value V of the second port 2 of the three-active-bridge converter 2ref is also the rated voltage of the low-voltage DC bus at the same time. Therefore, the voltage offset Δv2 can keep the voltage of the low-voltage DC bus near the rated value, ensuring the voltage stability of the low-voltage bus when connecting the load. Among them, the low-bandwidth communication is used to transmit the output voltage value V 3_i of the third port 3 of N three-active-bridge converters, and the voltage offset Δv2 is calculated based on this.

[0059] In specific implementation, a topology of a photovoltaic DC boost converter based on a dual-bus structure Figure 1As shown in the figure, it includes N independent photovoltaic arrays, N three-active-bridge converters, a low-voltage DC bus, and a medium-voltage DC bus. Among them, the first port 1 of the N three-active-bridge converters serves as the energy input port and is independently connected to the N photovoltaic arrays to achieve independent MPPT control, that is, maximum power point tracking control; the second port 2 of the N three-active-bridge converters serves as the energy bi-directional flow port and is connected in parallel to the low-voltage DC bus to process the mismatched power, and can absorb energy from the low-voltage DC bus or deliver energy to the low-voltage DC bus according to the mismatch situation of the photovoltaic power. The third port 3 of the N three-active-bridge converters serves as the energy output port and is connected in series to the medium-voltage DC bus in sequence to achieve medium-voltage collection. Under all operating conditions, the power obtained by the third port 3 of the N three-active-bridge converters from the corresponding first port 1 and second port 2 are all equal to each other, so as to achieve power balance and voltage equalization on the output side. N is a natural number greater than or equal to 2.

[0060] The low-voltage DC bus can be used to connect local loads and transfer the mismatched power. The mismatched power is transferred from the second port 2 of the three-active-bridge converter with high input power to the second port 2 of the three-active-bridge converter with low input power through the low-voltage DC bus, so that the output power of the third port 3 of the N three-active-bridge converters is balanced, and then output voltage equalization is achieved.

[0061] When the input powers of the N independent photovoltaic arrays are the same, as shown in Fig. 2(a), no mismatched power flows through the low-voltage DC bus, and there is no energy exchange between the N three-active-bridge converters. The input power of all photovoltaic arrays is incorporated into the medium-voltage DC grid only after one-stage power conversion.

[0062] When there are unequal input powers among the N independent photovoltaic arrays, as shown in Fig. 2(b), the mismatched power will be transferred from the three-active-bridge converter with high input power to the second port 2 of the three-active-bridge converter with low input power through the low-voltage DC bus, and finally make the output powers of the third ports 3 of the N three-active-bridge converters equal. Only this part of the mismatched power flowing through the low-voltage DC bus undergoes two-stage power conversion, and the rest of the input power undergoes one-stage power conversion, which is beneficial to the high-efficiency operation of the photovoltaic DC boost converter system based on the dual-bus structure.

[0063] As Figure 3 shown, it is the topological structure diagram of the three-active-bridge converter, including: the first switch tube S1, the second switch tube S2, the third switch tube S3, the fourth switch tube S4, the fifth switch tube S5, the sixth switch tube S6, the seventh switch tube S7, the eighth switch tube S8, the ninth switch tube S9, the tenth switch tube S 10 、the eleventh switch tube S 11 、the twelfth switch tube S 12; the first primary capacitor C1, the second primary capacitor C2, and the first secondary capacitor C3; the first primary inductor L1, the second primary inductor L2, and the first secondary inductor L3, the first primary coil n1 of the high-frequency isolation transformer, the second primary coil n2 of the high-frequency isolation transformer, and the first secondary winding n3 of the high-frequency isolation transformer.

[0064] The positive electrode of the first primary capacitor C1 is respectively connected to the drain of the first switch tube S1 and the drain of the third switch tube S3, and the negative electrode of the first primary capacitor C1 is respectively connected to the source of the second switch tube S2 and the source of the fourth switch tube S4.

[0065] The positive electrode of the second primary capacitor C2 is respectively connected to the drain of the fifth switch tube S5 and the drain of the seventh switch tube S7, and the negative electrode of the second primary capacitor C2 is respectively connected to the source of the sixth switch tube S6 and the source of the eighth switch tube S8.

[0066] The positive electrode of the first secondary capacitor C3 is respectively connected to the drain of the ninth switch tube S9 and the drain of the eleventh switch tube S 11 and the negative electrode of the first secondary capacitor C3 is respectively connected to the source of the tenth switch tube S 10 and the source of the twelfth switch tube S 12 .

[0067] The drain of the first switch tube S1 is connected to the drain of the third switch tube S3, the source of the first switch tube S1 is connected to the drain of the second switch tube S2, the source of the second switch tube S2 is connected to the source of the fourth switch tube S4, and the source of the third switch tube S3 is connected to the drain of the fourth switch tube S4; the drain of the fifth switch tube S5 is connected to the drain of the seventh switch tube S7, the source of the fifth switch tube S5 is connected to the drain of the sixth switch tube S6, the source of the sixth switch tube S6 is connected to the source of the eighth switch tube S8, and the source of the seventh switch tube S7 is connected to the drain of the eighth switch tube S8.

[0068] The drain of the ninth switch tube S9 is connected to the drain of the eleventh switch tube S 11 , the source of the ninth switch tube S9 is connected to the drain of the tenth switch tube S 10 , the source of the tenth switch tube S 10 is connected to the source of the twelfth switch tube S 12 , and the source of the eleventh switch tube S 11 is connected to the drain of the twelfth switch tube S 12 .

[0069] One end of the first winding n1 on the primary side of the high-frequency isolation transformer is connected to the source of the first switch tube S1 and is connected to the first inductor L1 on the primary side, and the other end is connected to the drain of the fourth switch tube S4. One end of the second winding n2 on the primary side of the high-frequency isolation transformer is connected to the source of the fifth switch tube S5 and is connected to the second inductor L2 on the primary side, and the other end is connected to the drain of the eighth switch tube S8. One end of the first winding n3 on the secondary side of the high-frequency isolation transformer is connected to the source of the ninth switch tube S9 and is connected to the first inductor L3 on the secondary side, and the other end is connected to the drain of the twelfth switch tube S 12 The drain of. Both ends of the first capacitor C1 on the primary side form the first port 1 of the three-active-bridge converter; both ends of the second capacitor C2 on the primary side form the second port 2 of the three-active-bridge converter; both ends of the first capacitor C3 on the secondary side form the third port 3 of the three-active-bridge converter.

[0070] A power balance control method for a photovoltaic DC boost converter based on a dual-bus structure, as Figure 4 shown, includes the following steps:

[0071] Maximum power point tracking step: Sample the output voltage and output current of N independent photovoltaic arrays to obtain N input voltage values V 1_i and input current values i 1_i of the first port 1 of the three-active-bridge converter. Through the independent MPPT control module, obtain the input voltage reference value V 1ref_i of the first port 1. Use the difference between the input voltage value V 1_i of the first port 1 and the input voltage reference value V 1ref_i of the first port 1. After passing through the PI controller and the limiting link, obtain the phase-shifted duty cycle D 13 , that is, the duty cycle of the phase-shift angle between the drive signals of the first port 1 and the third port 3 of the N three-active-bridge converters, so that the N photovoltaic arrays operate at their maximum power points.

[0072] Output voltage equalization step: Sample the output voltage values V 2_i , V 3_i of the second port 2 and the third port 3 of the N three-active-bridge converters. The output voltage reference values of the second port 2 and the third port 3 of the N three-active-bridge converters are V 2ref , V 3ref respectively. Use the difference between the output voltage value V 3_i of the third port 3 and the output voltage reference value V 3ref of the third port 3, and superimpose the output voltage reference value V 2ref of the second port 2 and the voltage offset Δv2 to obtain the instantaneous output voltage reference value V 2ref_i of the second port 2 of the N three-active-bridge converters. Then use the output voltage value V 2_iThe output voltage reference value V 2ref_i The difference is obtained through the PI controller and the limit link to obtain the phase shift duty ratio D 12 , that is, the duty cycle of the phase shift angle between the driving signals of the first port 1 and the second port 2 of the N three-active bridge converters. Among them, V 2ref_i =V 2ref +k vo (V 3_i -V 3ref_i )-Δv2,k vo V 3_i With V 3ref_i The difference between the amplification factor. Among them, the voltage offset:

[0073]

[0074] k j is the amplification factor of the difference between the actual voltage average value of the third port 3 of the converter and the reference value.

[0075] The output voltage value V of the third port 3 of the three active bridge converters 3_i When it increases, the reference voltage value V corresponding to the second port 2 of the three active bridge converter 2ref_i Also increases, the phase shift duty ratio D 12 Increase, so that the i-th three-active bridge converter TAB i The power P transmitted from the first port 1 to the second port 2 12 Increase, the corresponding power P of the third port 3 13 Therefore, when the photovoltaic DC boost converter based on the double bus structure has an internal power mismatch, for example, the i-th TAB i The photovoltaic input power is greater than the jth TAB j The photovoltaic input power of the i-th TAB i The reference voltage value V of the second port 2 2ref_i Will rise, the jth TAB j The reference voltage value V of the second port 2 2ref_j will decrease so that the final i-th TAB i and the jth TAB j The output powers of the third ports 3 are equal.

[0076] The output voltage value V of the third port 3 of the three active bridge converters 3_i When the voltage increases, the corresponding voltage offset Δv2 also increases, so that the reference voltage value V corresponding to the second port 2 of the three-active bridge converter 2ref_i Close to the output voltage reference value V of the second port 2 2refSince the second ports 2 of N three-active-bridge converters are connected in parallel to the low-voltage DC bus, the reference output voltage V of the second port 2 2ref is also the rated voltage of the low-voltage DC bus. Therefore, the voltage offset Δv2 can keep the voltage of the low-voltage DC bus near the rated value, ensuring the voltage stability of the low-voltage bus when connecting the load. Among them, the low-bandwidth communication is used to transmit the output voltage value V of the third port 3 of N three-active-bridge converters 3_i , and the voltage offset Δv2 is calculated based on this.

[0077] Embodiment 1

[0078] As Figure 5 shown, the simulation waveforms when power mismatch occurs in the three-active-bridge converter system in Embodiment 1. A 10 kV system composed of 13 three-active-bridge converters was built in the MATLAB SIMULINK software. The rated voltage V of the medium-voltage bus of the system MVDC = 10.4 kV, the rated power P = 130 kW, the switching frequency f of the three-active-bridge converter s = 20 kHz, the rated voltage of the second port 2, that is, the rated voltage of the low-voltage DC bus V2 = V LVDC = 200 V, the rated voltage of the third port 3 V 3_i = 800 V, the transformer turns ratio 1:n2:n3 = 1:1:4, the leakage inductance of the transformer 1 winding L1 = 5 μH, the leakage inductance of the transformer 2 winding L2 = 5 μH, the leakage inductance of the transformer 3 winding L3 = 80 μH, the filter capacitors of the first, second, and third ports 1, 2, and 3 C1 = C2 = C3 = 1000 μF, and the rated power P of the TAB converter i = 10 kW. At t = 0.008 s, the photovoltaic input powers P1 and P2 suddenly decrease, and P3 and P4 suddenly increase. At this time, the output voltage values V of the corresponding three-active sub-module third ports 3 3_1 , V 3_2 rise, and V 3_3 , V 3_4 fall, but the instantaneous change values of V 3_1 , V 3_2 , V 3_1 , V 3_2 are not greater than 0.5 V, and return to the steady-state value within 0.04 s. Therefore, when there is power mismatch inside the converter, the system has a good output voltage equalization effect.

[0079] Embodiment 2

[0080] As Figure 6 shown, it is the simulation waveform when the photovoltaic array of the converter system fails in Embodiment 2. The situation of the photovoltaic array failure can still be attributed to the power mismatch problem. As Figure 6As shown in the figure, when the photovoltaic array 1 fails, the photovoltaic input power P1 corresponding to its TAB1 is 0, and the remaining non-failed photovoltaic arrays operate normally. At this time, the second port 2 of the TAB1 corresponding to the failed photovoltaic array 1 will absorb energy from the low-voltage DC bus, and the second port 2 of the three-active-bridge converter corresponding to the non-failed photovoltaic array will deliver energy to the low-voltage DC bus, ultimately making the output powers of the third ports 3 of the N three-active-bridge converters equal. Therefore, when the photovoltaic array fails, the output voltages of each three-active-bridge converter can still remain equal, and the converter operates stably.

[0081] Embodiment 3

[0082] As Figure 7 shown, it is the simulation waveform when the three-active-bridge converter in the converter system of Embodiment 3 fails. When TAB1 fails, it has no input power, so Figure 7 there is no input power P1 in it, and the voltage V 3_1 of the corresponding third port 3 is 0. The non-failed three-active-bridge converters still operate normally, but their voltages V 3_2 ~V 3_13 rise from 800V to 866.6V, and the voltage at the low-voltage bus terminal rises from 200V to 331V. Therefore, the converter system has a certain fault isolation ability and can still operate stably when the three-active-bridge converter fails.

[0083] The above are only the preferred embodiments of the present invention, and do not limit the implementation manners and protection scope of the present invention accordingly. For those skilled in the art, it should be able to realize that all the equivalent replacements and obvious changes made by using the content of the specification of the present invention should be included in the protection scope of the present invention.

Claims

1. A photovoltaic DC boost converter based on a dual-bus structure, characterized in that, The photovoltaic DC boost converter includes N photovoltaic arrays, where N is a natural number greater than or equal to 2, and N three-active-bridge converters, a low-voltage DC bus, and a medium-voltage DC bus connected correspondingly; The three-active-bridge converter includes a first switching transistor ( S 1), a second switching transistor ( S 2), a third switching transistor ( S 3), a fourth switching transistor ( S 4), a fifth switching transistor ( S 5), a sixth switching transistor ( S 6), a seventh switching transistor ( S 7), an eighth switching transistor ( S 8), a ninth switching transistor ( S 9), a tenth switching transistor ( S 10 ), an eleventh switching transistor ( S 11 ), a twelfth switching transistor ( S 12 ); a first primary capacitor ( C 1), a second primary capacitor ( C 2), a first secondary capacitor ( C 3); a first primary inductor ( L 1), a second primary inductor ( L 2), a first secondary inductor ( L 3), a first primary winding of the high-frequency isolation transformer ( n 1), a second primary winding of the high-frequency isolation transformer ( n 2), a first secondary winding of the high-frequency isolation transformer ( n 3); The positive electrode of the primary side first capacitor ( C 1) is respectively connected to the drain of the first switching transistor ( S 1), the drain of the third switching transistor ( S 3), and the negative electrode of the primary side first capacitor ( C 1) is respectively connected to the source of the second switching transistor ( S 2), the source of the fourth switching transistor ( S 4); The positive electrodes of the primary-side second capacitor ( C 2) are respectively connected to the drains of the fifth switching transistor ( S 5), the seventh switching transistor ( S 7), and the negative electrodes of the primary-side second capacitor ( C 2) are respectively connected to the sources of the sixth switching transistor ( S 6), the eighth switching transistor ( S 8); The positive electrodes of the first secondary capacitor ( C 3) are respectively connected to the drain of the ninth switching transistor ( S 9), the drain of the eleventh switching transistor ( S 11 ), and the negative electrodes of the first secondary capacitor ( C 3) are respectively connected to the source of the tenth switching transistor ( S 10 ), the source of the twelfth switching transistor ( S 12 ); The drain of the first switching transistor ( S 1) is connected to the drain of the third switching transistor ( S 3), the source of the first switching transistor ( S 1) is connected to the drain of the second switching transistor ( S 2), the source of the second switching transistor ( S 2) is connected to the source of the fourth switching transistor ( S 4), the source of the third switching transistor ( S 3) is connected to the drain of the fourth switching transistor ( S 4); the drain of the fifth switching transistor ( S 5) is connected to the drain of the seventh switching transistor ( S 7), the source of the fifth switching transistor ( S 5) is connected to the drain of the sixth switching transistor ( S 6), the source of the sixth switching transistor ( S 6) is connected to the source of the eighth switching transistor ( S 8), the source of the seventh switching transistor ( S 7) is connected to the drain of the eighth switching transistor ( S 8); The drain of the ninth switching transistor ( S 9) is connected to the drain of the eleventh switching transistor ( S 11 ), the source of the ninth switching transistor ( S 9) is connected to the drain of the tenth switching transistor ( S 10 ), the source of the tenth switching transistor ( S 10 ) is connected to the source of the twelfth switching transistor ( S 12 ), and the source of the eleventh switching transistor ( S 11 ) is connected to the drain of the twelfth switching transistor ( S 12 ).

2. The photovoltaic DC boost converter based on the double-bus structure according to claim 1, wherein The first winding of the primary side of the high-frequency isolation transformer ( n 1) has one end connected to the source electrode of the first switching transistor ( S 1) and is connected to the first inductor of the primary side ( L 1), and the other end is connected to the drain electrode of the fourth switching transistor ( S 4); one end of the second winding of the primary side of the high-frequency isolation transformer ( n 2) is connected to the source electrode of the fifth switching transistor ( S 5) and is connected to the second inductor of the primary side ( L 2), and the other end is connected to the drain electrode of the eighth switching transistor ( S 8); one end of the first winding of the secondary side of the high-frequency isolation transformer ( n 3) is connected to the source electrode of the ninth switching transistor ( S 9) and is connected to the first inductor of the secondary side ( L 3), and the other end is connected to the drain electrode of the twelfth switching transistor ( S 12 ).

3. The photovoltaic DC boost converter based on the double-bus structure according to claim 2, characterized in that The two ends of the primary side first capacitor ( C 1) form the first port of the three-active-bridge converter; the two ends of the primary side second capacitor ( C 2) form the second port of the three-active-bridge converter; the two ends of the secondary side first capacitor ( C 3) form the third port of the three-active-bridge converter.

4. The photovoltaic DC boost converter based on the double-bus structure according to claim 3, characterized in that, The first port of the three-active-bridge converter serves as the energy input port and is connected to the photovoltaic array to achieve MPPT control; The second port of the three-active-bridge converter serves as the energy bidirectional flow port and is connected in parallel to the low-voltage DC bus for absorbing energy from or delivering energy to the low-voltage DC bus; The third port of the three-active-bridge converter serves as the energy output port and is connected in series to the medium-voltage DC bus in sequence.

5. The photovoltaic DC boost converter based on the double-bus structure according to claim 4, characterized in that, The low-voltage DC bus is connected to the local load and transmits the mismatch power. The second ports of the N three-active-bridge converters are connected to the low-voltage DC bus. The mismatch power transfer between the N three-active-bridge converters is achieved through power balance control, so that the output powers of the third ports of the N three-active-bridge converters are balanced, and thus output voltage equalization is achieved.

6. The photovoltaic DC boost converter based on the double-bus structure according to claim 3, wherein The input powers of the N photovoltaic arrays are the same, no mismatch power flows through the low-voltage DC bus, there is no mismatch power transfer between the N three-active-bridge converters, and the input powers of all photovoltaic arrays are incorporated into the medium-voltage DC grid through one-stage power conversion; When there are differences in the input powers of the N photovoltaic arrays, the mismatch power will be transferred from the three-active-bridge converter with a higher input power to the three-active-bridge converter with a lower input power through the low-voltage DC bus, and the output powers of the third ports of the N three-active-bridge converters are equal; Part of the mismatch power flowing through the low-voltage DC bus undergoes two-stage power conversion, and the remaining input power undergoes one-stage power conversion.

7. The power balance control method of the photovoltaic DC boost converter based on the double-bus structure according to any one of claims 1-6, characterized in that, When the power mismatch of the photovoltaic DC boost converter is caused by uneven photovoltaic input powers, this control method enables the mismatch power to be transferred through the low-voltage DC bus, achieving power balance and autonomous voltage equalization on the medium-voltage side, and simultaneously achieving voltage equalization and voltage stability of the low-voltage DC bus by adding a voltage offset in the control loop.

8. The power balance control method of the photovoltaic DC boost converter based on the double-bus structure according to claim 7, characterized in that The specific steps of the power balance control method for the photovoltaic DC boost converter are as follows: Maximum power point tracking: Collect the output voltage and output current of N photovoltaic arrays to obtain the input voltage values at the first ports of N three-active-bridge converters. V 1_i and the input current values i 1_i , and obtain the input voltage reference value at the first port through the MPPT control module V 1ref_i ; Utilize the input voltage value at the first port of the three-active-bridge converter V 1_i and the difference between the input voltage reference value at the first port of the three-active-bridge converter V 1ref_i to obtain the phase-shifted duty cycle through the PI controller and the limiting link D 13 , which serves as the duty cycle of the phase-shift angle between the driving signals of the first ports of N three-active-bridge converters and the third ports of the three-active-bridge converters, enabling N photovoltaic arrays to operate at their maximum power points. Output voltage equalization step: Collect the output voltage values of the second ports of N three-active-bridge converters V 2_i and the output voltage values of the third ports V 3_i . The output voltage reference values of the second ports of N three-active-bridge converters are V 2ref and the output voltage reference values of the third ports of the three-active-bridge converters V 3ref . Use the output voltage value of the third port of the three-active-bridge converter V 3_i and the difference between the output voltage reference value of the third port of the three-active-bridge converter V 3ref , and superimpose the output voltage reference value of the second port of the three-active-bridge converter V 2ref and the voltage offset Δ v 2 to obtain the instantaneous output voltage reference values of the second ports of N three-active-bridge converters V 2ref_i . Use the difference between the output voltage value of the second port of the three-active-bridge converter V 2_i and the instantaneous output voltage reference value of the second port of the three-active-bridge converter V 2ref_i . After passing through a PI controller and a limiting link, obtain the phase-shifted duty cycle D 12 , which is used as the duty cycle of the phase-shift angle between the drive signals of the first and second ports of N three-active-bridge converters. Among them, V 2ref_i =V 2ref + k vo (V 3_i -V 3ref_i ) -Δ v 2, k vo is V 3_i the amplification factor of the difference between V 3ref_i . The voltage offset is ; k j is the amplification factor of the difference between the actual average voltage and the reference value of the third port of the three-active-bridge converter.

9. The power balance control method of the photovoltaic DC boost converter based on the double-bus structure according to claim 7, characterized in that, By phase-shifting duty cycle D 12 The control includes: Output voltage value of the third port of the three-active-bridge converter V 3_i When it rises, the reference voltage corresponding to the second port of the three-active-bridge converter V 2ref_i rises accordingly, and the phase-shift duty cycle D 12 increases, causing the power transmitted from the first port to the second port of the three-active-bridge converter P 12 to increase, and the corresponding power of the third port P 13 to decrease; Output voltage value of the third port of the three-active-bridge converter V 3_i When it decreases, the reference voltage of the second port of the three-active-bridge converter V 2ref_i Decreases accordingly, and the phase-shift duty cycle D 12 Decreases, making the power transmitted from the first port to the second port of the three-active-bridge converter P 12 Decrease, and the corresponding power of the third port P 13 Increases.

10. The power balance control method of the photovoltaic DC boost converter based on the double-bus structure according to claim 7, characterized in that, By means of a voltage offset Δ v The control of 2 includes: Output voltage value of the third port of the three-active-bridge converter V 3_i When it increases, the voltage offset Δ v 2 of the second port of the corresponding three-active-bridge converter also increases, making the instantaneous output voltage reference values of the second ports of N three-active-bridge converters V 2ref_i approximately equal to the output voltage reference value of the second port of the three-active-bridge converter V 2ref .

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