Medium-voltage direct-current power conversion system for photovoltaic grid connection and optimal trajectory control method

Abstract

The application discloses a medium-voltage direct-current power conversion system and an optimal trajectory control method for photovoltaic grid connection. The system adopts an input-independent output series structure, and a plurality of output ports of sub-topologies are connected in series to a medium-voltage direct-current bus, and the input ports are respectively connected to independent photovoltaic panels. The sub-topology comprises, in sequence, a full-bridge high-frequency inverter unit, a resonant network unit, a high-frequency transformer unit and a double full-bridge loop high-frequency rectifier unit. The double full-bridge loop high-frequency rectifier unit comprises a switch tube S 5 S 10 and a filter capacitor, the high-frequency transformer unit is composed of two transformers, and the two transformers are respectively connected with switch tubes S 5 S 6 in the bridge arm mid-point and the switch tube S 7 S 8 in the bridge arm mid-point and the switch tube S 9 S 10 The bridge arm mid-point forms a double full-bridge loop. The conversion range of soft switching of the converter is expanded, synchronous rectification is realized, the current stress is greatly reduced, and the backflow power is significantly inhibited.​​​​

Description

Technical Field

[0001] This invention belongs to the technical field of power conversion systems, specifically relating to a medium-voltage DC power conversion system for photovoltaic grid connection and an optimal trajectory control method. Background Technology

[0002] Compared with traditional AC power distribution systems, medium-voltage DC (MVDC) systems have the following outstanding advantages: First, they do not require consideration of issues such as synchronization, reactive power, and frequency stability, making them easier to use and coordinate with distributed energy sources; second, they can significantly reduce substation links, lower system losses, and improve overall energy utilization efficiency; third, they can provide higher power supply capacity and power quality, making them very suitable for application scenarios such as data centers, urban load centers, and large industrial parks.

[0003] Despite the significant advantages of medium-voltage DC systems, achieving efficient and reliable grid connection of photovoltaic power generation to medium-voltage DC grids still faces core technological bottlenecks. Among these, a high-efficiency, high-power-density power conversion system is the core equipment for achieving this goal. Existing multi-stage power conversion architectures suffer from efficiency bottlenecks, while traditional isolated DC-DC converters, when applied to medium-voltage, high-power scenarios, generally face the following problems: First, the voltage and current stress on switching devices is high, leading to reduced system reliability and high equipment costs; second, there is a serious return power problem, resulting in a significant decrease in converter operating efficiency; third, the soft-switching range is narrow, and switching losses are huge under high voltage and high power conditions, limiting the improvement of switching frequency and power density. Summary of the Invention

[0004] The purpose of this invention is to provide a medium-voltage DC power conversion system for photovoltaic grid connection and an optimal trajectory control method. It adopts a high-frequency transformer unit composed of two transformers. By introducing two transformers and constructing a double full-bridge circuit on the secondary side, a voltage level of twice the normal value can be obtained. Combined with the optimal trajectory control method, the current stress on the secondary side is greatly reduced, and the service life of active devices is extended.

[0005] The technical solution to achieve the purpose of this invention is as follows: A medium-voltage DC power conversion system for photovoltaic grid connection adopts an independent input and output series structure. Multiple sub-topologies have their output ports connected in series to a medium-voltage DC bus, and their input ports are connected to independent photovoltaic panels. Each sub-topology includes a full-bridge high-frequency inverter unit, a resonant network unit, a high-frequency transformer unit, and a dual full-bridge high-frequency rectifier unit connected in sequence. The full-bridge high-frequency inverter unit includes switching transistors. S 1 ~ S 4 The input terminal of the full-bridge circuit is connected to a filter capacitor. C H ; The dual full-bridge high-frequency rectifier unit includes switching transistors. S 5 ~ S 10 and filter capacitor C L Switching transistor S 5 ~ S 8 Constructing the first full-bridge circuit, the switching transistor S 5 , S 6 , S 9 , S 10 The second full-bridge circuit is formed by connecting the first and second full-bridge circuits in parallel; the high-frequency transformer unit consists of two transformers, each with a secondary side connected to a switching transistor. S 5 , S 6 Bridge arm midpoint and switching transistor S 7 , S 8 Full-bridge transformer at the midpoint of the bridge arm T 1 and connecting switch tube S 5 , S 6 Bridge arm midpoint, switch tube S 9 , S 10 Full-bridge transformer at the midpoint of the bridge arm T 2 .

[0006] In a preferred embodiment, the resonant network unit includes resonant capacitors connected in sequence. C r and resonant inductor L r resonant inductor L r The other end is connected to the switching transistor. S 1 , S 2 Midpoint of bridge arm, resonant capacitor C r The other end is connected to the primary side of the transformer.

[0007] This invention also discloses an optimal trajectory control method applied to the aforementioned medium-voltage DC power conversion system for photovoltaic grid connection, comprising the following steps: S01: Switching transistor controlling the full-bridge high-frequency inverter unitS 1 The pulse width is 0 to π, and the switching transistor... S 2 The pulse width is π to 2π, and the switching transistor... S 3 The pulse width is from α to π+α, and the switching transistor... S 4 The pulse width is from π+α to 2π+α, thereby generating an AC voltage. v ab It has three voltage levels: +V1, -V1, and 0, where V1 is the input voltage. v ab For switching transistors S 1 , S 2 Bridge arm midpoint and switching transistor S 3 , S 4 Voltage between midpoints of bridge arms; α is the voltage of the switching transistor. S 4 Lag switching transistor S 1 angle; S02: Control switch transistor S 5 The pulse width is from β to π+β, and the switching transistor... S 6 The pulse width is from π+β to 2π+β, and the switching transistor... S 7 The pulse width is from π+β+γ to 2π+β+γ, and the switching transistor... S 8 The pulse width is from β+γ to π+β+γ, and the switching transistor... S 9 The pulse width is from π+β+δ to 2π+β+δ, and the switching transistor... S 10 The pulse width is from β+δ to π+β+δ, and the switching transistor... S 5 Lag switching transistor S 1 Angle β, switching transistor S 8 Lag switching transistor S 5 Angle γ, switching transistor S 10 Lag switching transistor S 5 An angle of δ is generated, thus producing an alternating voltage. v cd It contains three voltage levels: +V2, -V2, and 0, and generates AC voltage. vce It has three voltage levels: +V2, -V2, and 0, where V2 is the output voltage. v cd For switching transistors S 5 , S 6 Bridge arm midpoint and switching transistor S 7 , S 8 Voltage between midpoints of bridge arms v ce For switching transistors S 5 , S 6 Bridge arm midpoint and switching transistor S 9 , S 10 Voltage between midpoints of bridge arms; S03: Based on the steady-state model of resonant current and output power, the optimal trajectory control relationship is obtained, and the optimal trajectory power of the converter is obtained.

[0008] In the preferred technical solution, the method for calculating the resonant current includes: Steady-state analysis was performed using the fundamental wave approximation method, and the following results were obtained. v ab , v cd and v ce The fundamental phasors of the three voltages, for v ab , v cd and v ce After normalization, the normalized phasor expression is as follows:

[0009] In the formula, yes v ab The normalized vector; yes v cd The normalized vector; yes v ce The normalized vector; M For voltage gain, k The turns ratio of the two transformers; Will and The two were merged into The simplified equivalent circuit diagram is as follows:

[0010] Wherein, phase angle for:

[0011] According to the equivalent circuit diagram, the normalized resonant current expression is:

[0012] in, ω s The switching angular frequency, t For time, I r,pu This is the peak value of the resonant current. This is the phase angle of the resonant current.

[0013] In the preferred technical solution, the peak value of the resonant current I r,pu and phase angle for:

[0014]

[0015] in, Q For quality factor, F This is the normalized switching frequency.

[0016] The preferred technical solution also includes: Normalized power was calculated P pu :

[0017] in, Q For quality factor, F This is the normalized switching frequency.

[0018] In the preferred technical solution, the calculation method for the optimal trajectory control relationship includes: Obtain the minimum resonant current I rms，pu ; Based on the obtained steady-state model of resonant current and output power, a model is established regarding... I rms，pu Lagrange's equation:

[0019] in, L For the Lagrange optimization function, λ It is a Lagrange multiplier. C It is a constant; To each Lmiddle , , and Partial derivative, we get:

[0020]

[0021]

[0022]

[0023] in,

[0024] Simplify and substitute P pu Obtain the optimal trajectory control formula and optimal trajectory power. P M :

[0025] By controlling v ab pulse width α And the transformer turns ratio k, for P M The size is controlled.

[0026] This invention further discloses a multi-mode non-return modulation system for a medium-voltage DC power conversion system for photovoltaic grid connection, used to realize the optimal trajectory control method for the aforementioned medium-voltage DC power conversion system for photovoltaic grid connection, comprising: The first control module controls the pulse width of the switching transistors of the full-bridge high-frequency inverter unit; The second control module controls the pulse width of the switching transistors in the dual full-bridge circuit high-frequency rectifier unit; The optimal trajectory control module obtains the optimal trajectory control relationship based on the steady-state model of resonant current and output power, and thus obtains the optimal trajectory power of the converter.

[0027] The present invention also discloses a computer storage medium storing a computer program, which, when executed, implements the wide-gain method described above for a medium-voltage DC power conversion system for photovoltaic grid connection.

[0028] Compared with the prior art, the significant advantages of this invention are: (1) The present invention proposes a medium-voltage DC power conversion system suitable for photovoltaic grid connection. The secondary side adopts a double full-bridge circuit topology, realizing five voltage levels, with the highest voltage level on the secondary side reaching 2 times. This effectively reduces the current stress on the secondary side and extends the service life of active devices.

[0029] (2) The present invention uses two transformers to form a high-frequency transformer unit. By controlling the turns ratio of the two transformers, an additional controllable degree of freedom k is added, which realizes a wide voltage input range and expands the soft switching conduction range of the converter.

[0030] (3) This invention utilizes the optimal trajectory control method to optimize the problems of excessive return power on the output side, excessive current stress on the secondary side of the converter, and low converter operating efficiency, achieving zero return power and low current stress, which greatly improves the operating efficiency and system safety of the system. It is suitable for photovoltaic grid connection, has a wide voltage adaptation range, and can achieve zero return power, low current stress, and wide-range soft switching in a high-efficiency power conversion system. Attached Figure Description

[0031] Figure 1 This is a schematic diagram of a medium-voltage DC power conversion system for photovoltaic grid connection in this embodiment; Figure 2 This is a circuit diagram of a medium-voltage DC power conversion system for photovoltaic grid connection according to this embodiment; Figure 3 This embodiment controls the switching transistor. S 1 ~ S 4 and switching transistor S 5 ~ S 10 The generated AC voltage waveform and resonant current waveform; Figure 4 This embodiment uses the equivalent circuit in the phasor domain established by FHA for the medium-voltage DC power conversion system used for photovoltaic grid connection. Figure 5 This is in the embodiment when M =0.4, k=1, V1=300V, V2=120V, P =200W, v ab , v cd 、v ce 、v mn , i r And the current waveforms of each switching transistor; Figure 6 This is in the embodiment when M =0.4, k=1, V1=300V, V2=120V, P =400W, v ab , vcd 、v ce 、v mn , i r And the current waveforms of each switching transistor. Detailed Implementation

[0032] The principle of this invention is as follows: This invention uses two transformers to form a high-frequency transformer unit. By introducing two transformers and constructing a double full-bridge circuit on the secondary side, a voltage level of twice the normal value is obtained. Combined with the optimal trajectory control method, the secondary side current stress is greatly reduced, significantly extending the service life of components. By controlling the turns ratio of the two transformers, an additional controllable degree of freedom k is added, realizing a wide voltage input range and expanding the soft-switching conduction range of the converter. The problems of excessive return current power on the output side, low converter operating efficiency, and high secondary side current stress of the converter are optimized, achieving zero return current power and low current stress, greatly improving the operating efficiency of the system.

[0033] Example 1: like Figure 1 As shown, a medium-voltage DC power conversion system for photovoltaic grid connection includes photovoltaic panels, a sub-topology, and a medium-voltage DC grid. The sub-topology comprises a full-bridge high-frequency inverter unit, a resonant network unit, a high-frequency transformer unit, and a dual full-bridge high-frequency rectifier unit connected in sequence. The optimal trajectory control method addresses the problems of excessive output-side return power, excessive secondary-side current stress of the converter, and low converter operating efficiency.

[0034] Photovoltaic panels convert light energy into electrical energy. The electrical energy is converted into high-frequency DC voltage through a low-frequency rectifier bridge. The high-frequency DC voltage is then converted into high-frequency AC voltage through a full-bridge high-frequency inverter unit. Finally, the high-frequency AC voltage is converted into high-frequency DC voltage through a resonant network unit, a high-frequency transformer unit, and a double full-bridge high-frequency rectifier unit, and then transmitted to the medium-voltage DC grid.

[0035] The high-frequency transformer unit consists of two high-frequency transformers connected together. By controlling the turns ratio of the two transformers, an additional controllable degree of freedom k is added, which realizes a wide voltage input range and expands the soft-switching conduction range of the converter.

[0036] The secondary side adopts a dual full-bridge circuit topology, enabling five voltage levels, with the highest secondary side voltage level reaching twice the rated voltage. Combined with optimal trajectory control methods, the secondary side current stress is effectively reduced.

[0037] Specifically, such as Figure 2 As shown, the core architecture of the system is characterized by an independent input and a series-connected output structure. Multiple sub-topologies have their output ports connected in series to a medium-voltage DC bus, while their input ports are connected via filter capacitors.C H Connecting independent photovoltaic panels, the sub-topology includes, in sequence, a full-bridge high-frequency inverter unit, a resonant network unit, a high-frequency transformer unit, and a dual full-bridge high-frequency rectifier unit. The full-bridge high-frequency inverter unit consists of switching transistors... S 1 ~ S 4 Body diode d S1 ~ d S4 and parasitic capacitance C S1 ~ C S4 Composition, switching transistor S 1 ~S 4 This forms a full-bridge circuit, with a filter capacitor connected to its input terminal. C H And the photovoltaic panel. The output voltage of the photovoltaic panel serves as the input voltage V1 of the full-bridge high-frequency inverter unit.

[0038] The resonant network unit consists of a resonant capacitor. C r Resonant inductor L r They are connected sequentially to form a whole.

[0039] The dual full-bridge high-frequency rectifier unit contains 6 switching transistors, which are... S 5 ~ S 10 Body diode d S5 ~ d S10 Parasitic capacitance C S5 ~ C S10 and filter capacitor C L Composition, can be regarded as a full-bridge circuit (switching transistor) S 5 ~ S 8 , T 1 ) and another full-bridge circuit (switching transistor) S 5 , S 6 , S 9 , S 10 and T 2 They are connected in parallel.

[0040] The high-frequency transformer unit consists of two transformers, one of which is connected to a switching transistor on its secondary side. S 5 , S 6 Bridge arm midpoint and switching transistor S 7 , S 8 Full-bridge transformer at the midpoint of the bridge arm T 1 Its variation ratio is n 1 :1 ; and connecting switching transistors S 5 , S 6 Bridge arm midpoint, switch tube S 9 , S 10 Full-bridge transformer at the midpoint of the bridge arm T 2 Its variation ratio is n 2 :1 .

[0041] like Figure 3 As shown, the switching transistors controlling the full-bridge high-frequency inverter unit S 1 The pulse width is 0 to π, and the switching transistor... S 2 The pulse width is π to 2π, and the switching transistor... S 3 The pulse width is from α to π+α, and the switching transistor... S 4 The pulse width is from π+α to 2π+α; this generates an AC voltage. v ab It contains three voltage levels: +V1, -V1, and 0. v ab For switching transistors S 1 , S 2 Bridge arm midpoint and switching transistor S 3 , S 4 The voltage between the midpoints of the bridge arms, α is the voltage of the switching transistor. S 4 Lag switching transistor S 1 angle.

[0042] Switching transistors controlling the high-frequency rectifier unit of the dual full-bridge circuit S 5The pulse width is from β to π+β, and the switching transistor... S 6 The pulse width is from π+β to 2π+β, and the switching transistor... S 7 The pulse width is from π+β+γ to 2π+β+γ, and the switching transistor... S 8 The pulse width is from β+γ to π+β+γ, and the switching transistor... S 9 The pulse width is from π+β+δ to 2π+β+δ, and the switching transistor... S 10 The pulse width is from β+δ to π+β+δ, and the switching transistor... S 5 Lag switching transistor S 1 Angle β, switching transistor S 8 Lag switching transistor S 5 Angle γ, switching transistor S 10 Lag switching transistor S 5 An angle of δ is generated, thus producing an alternating voltage. v cd It contains three voltage levels: +V2, -V2, and 0, and generates AC voltage. v ce It contains three voltage levels: +V2, -V2, and 0. v mn for v cd and v ce The superposition contains five voltage levels: +2V2, -2V2, 0, +V2, and -V2. v cd For switching transistors S 5 , S 6 Bridge arm midpoint and switching transistor S 7 , S 8 Voltage between midpoints of bridge arms v ce For switching transistors S 5 , S 6 Bridge arm midpoint and switching transistor S 9 , S 10 Voltage between midpoints of the bridge arms.

[0043] Specifically, the resonant currenti r Zero-crossing adjustment in the switching transistor S 5 The on / off point, thus causing the resonant current to interact with the switching transistor. S 5 In-phase operation eliminates the reactive power of the high-frequency rectifier full bridge.

[0044] In another embodiment, an optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection is applied to the aforementioned medium-voltage DC power conversion system for photovoltaic grid connection. The optimal trajectory control method includes the following steps: S01: Switching transistor controlling the full-bridge high-frequency inverter unit S 1 The pulse width is 0 to π, and the switching transistor... S 2 The pulse width is π to 2π, and the switching transistor... S 3 The pulse width is from α to π+α, and the switching transistor... S 4 The pulse width is from π+α to 2π+α, thereby generating an AC voltage. v ab It has three voltage levels: +V1, -V1, and 0, where V1 is the input voltage. S02: Switching transistor S 5 The pulse width is from β to π+β, and the switching transistor... S 6 The pulse width is from π+β to 2π+β, and the switching transistor... S 7 The pulse width is from π+β+γ to 2π+β+γ, and the switching transistor... S 8 The pulse width is from β+γ to π+β+γ, and the switching transistor... S 9 The pulse width is from π+β+δ to 2π+β+δ, and the switching transistor... S 10 The pulse width is from β+δ to π+β+δ, and the switching transistor... S 5 Lag switching transistor S 1 Angle β, switching transistor S 8 Lag switching transistor S 5 Angle γ, switching transistor S 10 Lag switching transistor S 5 An angle of δ is generated, thus producing an alternating voltage. v cdIt contains three voltage levels: +V2, -V2, and 0, and generates AC voltage. v ce It has three voltage levels: +V2, -V2, and 0, where V2 is the output voltage. S03: Based on the steady-state model of resonant current and output power, the optimal trajectory control relationship is obtained, and the optimal converter operating power trajectory is obtained.

[0045] Specifically, the methods for calculating the resonant current include: Steady-state analysis was performed using the fundamental wave approximation method, and the following results were obtained. v ab , v cd and v ce The fundamental phasors of the three voltages, for v ab , v cd and v ce After normalization, the normalized phasor expression is as follows:

[0046] In the formula, yes v ab The normalized vector; yes v cd The normalized vector; yes v ce The normalized vector; M For voltage gain, k The turns ratio of two transformers is defined as follows: ; Will and The two were merged into The simplified equivalent circuit diagram is obtained, such as Figure 4 As shown:

[0047] Wherein, phase angle for:

[0048] According to the equivalent circuit diagram, the normalized resonant current expression is:

[0049] Among them, the peak value of the resonant current I r,pu and phase angle for:

[0050]

[0051] in, Q For quality factor, F To normalize the switching frequency, ω s The switching angular frequency, t For time.

[0052] Specifically, the normalized power is calculated. P pu :

[0053] Specifically, the calculation methods for the optimal trajectory control relationship include: To obtain the minimum resonant current:

[0054] Based on the obtained steady-state model of resonant current and output power, a model is established regarding... I rms，pu Lagrange's equation:

[0055] in, λ It is a Lagrange multiplier. C It is a constant.

[0056] To each L middle , , and Partial derivatives yield:

[0057]

[0058] in,

[0059] Simplify the above four equations and substitute them into the equations. P pu The optimal trajectory control equation and optimal trajectory power can be obtained by simplification. P M :

[0060] control v abpulse width α And the transformer turns ratio k, for P M The size is controlled.

[0061] Another embodiment provides a multi-mode non-return modulation system for a medium-voltage DC power conversion system for photovoltaic grid connection, used to implement the optimal trajectory control method for the medium-voltage DC power conversion system for photovoltaic grid connection, comprising: The first control module controls the pulse width of the switching transistors of the full-bridge high-frequency inverter unit; The second control module controls the pulse width of the switching transistors in the dual full-bridge circuit high-frequency rectifier unit; The optimal trajectory control module obtains the optimal trajectory control relationship based on the steady-state model of resonant current and output power, and thus obtains the optimal trajectory power of the converter.

[0062] The specific optimal trajectory control method is the same as described above, and will not be repeated here.

[0063] In another embodiment, a computer storage medium stores a computer program that, when executed, implements the above-described optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection.

[0064] The specific implementation method can be the one described above, and will not be repeated here.

[0065] The following example illustrates how appropriate parameter design is needed for efficient converter operation. The parameter design is as follows: choose Q =1, F =1.6, ω S =200πkHz, L r =366.69μH, C r =17.68nF.

[0066] The design input voltage V1 is 300V, the output voltage V2 is 120V, and the rated power is 400W.

[0067] Simulations were performed based on the designed input and output voltages and power, and all switches were able to achieve soft switching.

[0068] To verify the correctness of the theory, simulation tests were conducted in PSIM.

[0069] when M =0.4, k=1, V1=300V, V2=120V, P =200W, v ab, v cd 、v ce 、v mn , i r and the current of each switch transistor i S1 - i S9 Waveform as Figure 5 As shown.

[0070] when M =0.4, k=1, V1=300V, V2=120V, P =400W, v ab , v cd 、v ce 、v mn , i r and the current of each switch transistor i S1 - i S9 Waveform as Figure 6 As shown.

[0071] After verification with simulation waveforms, it was found that the theory and the practice were consistent, proving that the invention is feasible.

[0072] The above embodiments are preferred embodiments of the present invention, but the embodiments of the present invention are not limited to the above embodiments. Any changes, modifications, substitutions, combinations, or simplifications made without departing from the spirit and principle of the present invention shall be considered equivalent substitutions and shall be included within the protection scope of the present invention.

Claims

1. A medium-voltage DC power conversion system for photovoltaic grid connection, characterized in that, The system employs an independent input and output series structure, with the output ports of multiple sub-topologies connected in series to a medium-voltage DC bus, and the input ports connected to independent photovoltaic panels. Each sub-topology includes, in sequence, a full-bridge high-frequency inverter unit, a resonant network unit, a high-frequency transformer unit, and a dual full-bridge high-frequency rectifier unit. The full-bridge high-frequency inverter unit includes a switching transistor. S 1 ~ S 4 Switching transistor S 1 ~S 4 A full-bridge circuit is formed, and a filter capacitor is connected to the input terminal of the full-bridge high-frequency inverter unit. C H Connect the photovoltaic panel later; The dual full-bridge high-frequency rectifier unit includes switching transistors. S 5 ~ S 10 and filter capacitor C L Switching transistor S 5 ~ S 8 Constructing the first full-bridge circuit, the switching transistor S 5 , S 6 , S 9 , S 10 The second full-bridge circuit is formed by connecting the first and second full-bridge circuits in parallel; the high-frequency transformer unit consists of two transformers, each with a secondary side connected to a switching transistor. S 5 , S 6 Bridge arm midpoint and switching transistor S 7 , S 8 Full-bridge transformer at the midpoint of the bridge arm T 1 and connecting switch tube S 5 , S 6 Bridge arm midpoint, switch tube S 9 , S 10 Full-bridge transformer at the midpoint of the bridge arm T 2 .

2. The medium-voltage DC power conversion system for photovoltaic grid connection according to claim 1, characterized in that, The resonant network unit includes resonant capacitors connected in sequence. C r and resonant inductor L r resonant inductor L r The other end is connected to the switching transistor. S 1 , S 2 Midpoint of bridge arm, resonant capacitor C r The other end is connected to the primary side of the transformer.

3. An optimal trajectory control method applied to the medium-voltage DC power conversion system for photovoltaic grid connection as described in claim 2, characterized in that, Includes the following steps: S01: Switching transistor controlling the full-bridge high-frequency inverter unit S 1 The pulse width is 0 to π, and the switching transistor... S 2 The pulse width is π to 2π, and the switching transistor... S 3 The pulse width is from α to π+α, and the switching transistor... S 4 The pulse width is from π+α to 2π+α, thereby generating an AC voltage. v ab It has three voltage levels: +V1, -V1, and 0, where V1 is the input voltage. v ab For switching transistors S 1 , S 2 Bridge arm midpoint and switching transistor S 3 , S 4 The voltage between the midpoints of the bridge arms, α is the voltage of the switching transistor. S 4 Lag switching transistor S 1 angle; S02: Control switch transistor S 5 The pulse width is from β to π+β, and the switching transistor... S 6 The pulse width is from π+β to 2π+β, and the switching transistor... S 7 The pulse width is from π+β+γ to 2π+β+γ, and the switching transistor... S 8 The pulse width is from β+γ to π+β+γ, and the switching transistor... S 9 The pulse width is from π+β+δ to 2π+β+δ, and the switching transistor... S 10 The pulse width is from β+δ to π+β+δ, and the switching transistor... S 5 Lag switching transistor S 1 Angle β, switching transistor S 8 Lag switching transistor S 5 Angle γ, switching transistor S 10 Lag switching transistor S 5 An angle of δ is generated, thus producing an alternating voltage. v cd It contains three voltage levels: +V2, -V2, and 0, and generates AC voltage. v ce It has three voltage levels: +V2, -V2, and 0, where V2 is the output voltage. v cd For switching transistors S 5 , S 6 Bridge arm midpoint and switching transistor S 7 , S 8 Voltage between midpoints of bridge arms v ce For switching transistors S 5 , S 6 Bridge arm midpoint and switching transistor S 9 , S 10 Voltage between midpoints of bridge arms; S03: Based on the steady-state model of resonant current and output power, the optimal trajectory control relationship is obtained, and the optimal trajectory power of the converter is obtained.

4. The optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection according to claim 3, characterized in that, Methods for calculating resonant current include: Steady-state analysis was performed using the fundamental wave approximation method, and the following results were obtained. v ab , v cd and v ce The fundamental phasors of the three voltages, for v ab , v cd and v ce After normalization, the normalized phasor expression is as follows: In the formula, yes v ab The normalized vector; yes v cd The normalized vector; yes v ce The normalized vector; M For voltage gain, k The turns ratio of the two transformers; Will and The two were merged into The simplified equivalent circuit diagram is as follows: Wherein, phase angle for: According to the equivalent circuit diagram, the normalized resonant current expression is: in, ω s The switching angular frequency, t For time, I r,pu This is the peak value of the resonant current. This is the phase angle of the resonant current.

5. The optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection according to claim 4, characterized in that, Peak resonant current I r,pu and phase angle for: in, Q For quality factor, F This is the normalized switching frequency.

6. The optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection according to claim 4, characterized in that, Also includes: Normalized power was calculated P pu : in, Q For quality factor, F This is the normalized switching frequency.

7. The optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection according to claim 6, characterized in that, The methods for calculating the optimal trajectory control relationship include: Obtain the minimum resonant current I rms，pu ; Based on the obtained steady-state model of resonant current and output power, a model is established regarding... I rms，pu Lagrange's equation: in, L For the Lagrange optimization function, λ It is a Lagrange multiplier. C It is a constant; To each L middle , , and Partial derivative, we get: in, Simplify and substitute P pu Obtain the optimal trajectory control formula and optimal trajectory power. P M : By controlling v ab pulse width α And the transformer turns ratio k, for P M The size is controlled.

8. A multi-mode non-return modulation system for a medium-voltage DC power conversion system for photovoltaic grid connection, characterized in that, The optimal trajectory control method for implementing the medium-voltage DC power conversion system for photovoltaic grid connection as described in any one of claims 3-7 includes: The first control module controls the pulse width of the switching transistors of the full-bridge high-frequency inverter unit; The second control module controls the pulse width of the switching transistors in the dual full-bridge circuit high-frequency rectifier unit; The optimal trajectory control module obtains the optimal trajectory control relationship based on the steady-state model of resonant current and output power, and thus obtains the optimal trajectory power of the converter.

9. A computer storage medium having a computer program stored thereon, characterized in that, When the computer program is executed, it implements the optimal trajectory control method for a medium-voltage DC power conversion system for photovoltaic grid connection as described in any one of claims 3-7.

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