A solid-state transformer based on flying capacitor and insulation optimization method

Abstract

This invention discloses a solid-state transformer based on a flying capacitor and an insulation optimization method, relating to the field of medium- and high-voltage DC power conversion technology. The proposed solid-state transformer FC-SST based on a flying capacitor is... N The system comprises several power conversion units connected in series. Each unit includes a medium-voltage side dual half-bridge, a common-mode flying capacitor, an internal flying capacitor, an intermediate frequency transformer, and a low-voltage side H-bridge. The midpoint of the common-mode flying capacitor in all units is uniformly connected to the medium-voltage system ground potential (GNDMV). The primary-side reference terminal of each MFT is connected to GNDMV, and the low-voltage side output terminals are connected in parallel. This invention uses the flying capacitor to block the DC voltage across the MFT, thereby reducing the steady-state operating voltage of the MFT to [value missing]. V dc / (4 N This invention significantly simplifies MFT insulation design, reduces manufacturing costs, and is highly modular, making it suitable for MVDC / LVDC applications such as data centers, fast charging for electric vehicles, and rail transportation.

Description

Technical Field

[0001] This application relates to the field of medium and high voltage DC power conversion technology, and in particular to a solid-state transformer based on a flying capacitor and an insulation optimization method. Background Technology

[0002] With the rapid proliferation of data centers, electric vehicle fast-charging stations, and high-speed electric traction systems, medium-voltage DC to low-voltage DC (MVDC / LVDC) power conversion has become a key enabling technology. Solid-state transformers (SSTs) offer significant advantages over traditional power frequency transformers, including smaller size, lighter weight, better control performance and functionality, higher power density, higher efficiency, and less reliance on copper and iron. Since these emerging applications essentially operate in DC mode, MVDC / LVDC solid-state transformers have become an urgent research priority.

[0003] However, in the engineering implementation of medium-voltage solid-state transformers, regarding insulation requirements, taking a 10 kV SST as an example, the intermediate frequency transformer (MFT) in the solid-state transformer (SST) must meet three types of electrical stresses: partial discharge requirements under normal operation (PD<10pC), 35 kV (2 U in +1 kV power frequency withstand voltage test (lasting several minutes) and 75 kV (6 U in Lightning impulse test (1.2 / 50μs pulse). Among the three types of electrical stress requirements, the partial discharge (PD) requirement of the MFT is the most stringent and is the core challenge of insulation design. The ISOP architecture, which is widely used in solid-state transformers, connects multiple converter units in series on the MV side, so that each unit only bears a portion of the bus voltage. However, this does not alleviate the insulation stress PD of the MFT. That is, each MFT must still withstand the insulation stress PD of the entire grid voltage level. MFT insulation design remains a major bottleneck.

[0004] Reducing the operating voltage of the partial discharge test (MFT) at the topology level to relax its partial discharge requirements is an existing technical approach to solving the MFT insulation problem. Capacitively coupled multi-cell stationary ... Summary of the Invention

[0005] Therefore, it is necessary to provide a solid-state transformer based on flying capacitor and an insulation optimization method to address the above-mentioned technical problems.

[0006] The following technical solution is adopted in this specification: This specification provides a solid-state transformer based on a flying capacitor, comprising multiple power conversion units connected in series; each power conversion unit includes: The common-mode flying capacitor branch consists of a first common-mode flying capacitor and a second common-mode flying capacitor connected in series, and the midpoint of the common-mode flying capacitor branch is connected to the medium-voltage system ground potential. The flying capacitor branch within a unit consists of the first flying capacitor within the unit and the second flying capacitor within the unit connected in series. The medium-voltage side dual half-bridge circuit includes a positive bridge arm and a negative bridge arm. The positive bridge arm is composed of a first switch and a second switch connected in series, and the negative bridge arm is composed of a third switch and a fourth switch connected in series. The midpoints of the positive and negative bridge arms are respectively connected to the two ends of the common-mode flying capacitor branch. One end of the positive and negative bridge arms is respectively connected to the two ends of the common-mode flying capacitor branch. The other ends of the positive and negative bridge arms are respectively connected to the two ends of the common-mode flying capacitor branch of the power conversion unit adjacent to the power conversion unit. The low-voltage side H-bridge circuit includes a first rectifier bridge arm and a second rectifier bridge arm. The first rectifier bridge arm is composed of a fifth switch and a sixth switch connected in series, and the second rectifier bridge arm is composed of a seventh switch and an eighth switch connected in series. The intermediate frequency transformer includes a primary winding and a secondary winding; the first end of the primary winding is connected to the midpoint of the flying capacitor branch in the unit, and the second end of the primary winding is connected to the ground potential of the medium voltage system; the two ends of the secondary winding are respectively connected to the midpoint of the first rectifier bridge arm and the midpoint of the second rectifier bridge arm. In this system, the medium-voltage system ground potential of all power conversion units is the same potential point, and the low-voltage side output terminals of all power conversion units are connected in parallel.

[0007] Furthermore, multiple power conversion units are connected in series and superimposed through the bias of the common-mode flying capacitor; wherein, the two ends of the outermost common-mode flying capacitor branch are connected to the positive and negative terminals of the medium-voltage DC bus, respectively, and the other ends of the positive and negative bridge arms of the innermost power conversion unit are connected to the medium-voltage system ground potential.

[0008] Furthermore, the steady-state operating voltage across the intermediate frequency transformer is 1 / (4) of the total input DC voltage of the system. N On the order of magnitude, of which, N This represents the number of power conversion units connected in series.

[0009] Furthermore, the first common-mode flying capacitor and the second common-mode flying capacitor achieve automatic voltage balancing through a built-in switched capacitor network; and through periodic charge transfer, the voltage of the first common-mode flying capacitor and the second common-mode flying capacitor are automatically kept equal.

[0010] Furthermore, the first to eighth switching transistors in each power conversion unit achieve full-bridge zero-voltage switching through a preset control strategy; The control strategy includes: The first, fourth, fifth, and eighth switching transistors constitute the first switching group, which conducts synchronously with a duty cycle of 50%. The second switch, the third switch, the sixth switch, and the seventh switch constitute a second switch group, which is complementary to the first switch group in conduction; A dead time is provided between the first switch group and the second switch group; and the switching frequency is set at the resonant frequency of the resonant network.

[0011] 0. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that each power conversion unit adopts an interleaved modulation scheme to control the gate drive signal, the interleaved modulation scheme comprising: No interleaving: The gate signals of all power conversion units are completely synchronized; Uniformly staggered: N The adjacent phase shift of each power conversion unit is 180°. N This is to reduce the size and capacitance requirements of the secondary output filter; 180° stagger: The gate signals of adjacent power conversion units are phase-shifted by 180° to reduce the size and capacitance requirements of the common-mode flying capacitor.

[0012] Furthermore, by performing pre-charge startup from the low-voltage side, the startup circuit includes: The first switch is connected between the medium-voltage DC bus and the medium-voltage input terminal of the solid-state transformer; The second switch is connected between the low-voltage auxiliary power supply and the low-voltage output terminal of the solid-state transformer; The third switch is connected between the low-voltage output terminal of the solid-state transformer and the load.

[0013] Furthermore, the pre-charge startup process includes: Close the second switch and disconnect the first switch and the third switch; A drive signal is applied to each power conversion unit on the low-voltage side, and the switching transistors of each power conversion unit on the medium-voltage side perform natural rectification charging of the flying capacitor on the medium-voltage side through the parasitic diodes of the switching transistors themselves. Control the output voltage of the low-voltage auxiliary power supply at a preset startup time. T start The internal linear ramp-up voltage from zero to the rated output voltage V o This ensures that the flying capacitors and main capacitors of all power conversion units are charged to their rated values. The total voltage of the capacitors on the medium-voltage side is detected. When the total voltage matches the actual voltage of the medium-voltage DC bus, the second switch is opened and the first switch is closed to connect to the medium-voltage DC bus in a non-inrush current manner. Close the third switch, connect the load, and enter the operating state.

[0014] Furthermore, a resonant network is connected between the medium-voltage side dual half-bridge circuit and the low-voltage side H-bridge circuit, and the resonant network is a CLLC resonant network.

[0015] This specification provides an insulation optimization method for solid-state transformers based on flying capacitors, including: Connect the first end of the primary winding of the intermediate frequency transformer to the midpoint of the series connection of the flying capacitor in the unit, and connect the second end of the primary winding of the intermediate frequency transformer to the ground potential of the medium voltage system. By having the flying capacitor and common-mode flying capacitor within the unit jointly bear the DC voltage stress, the intermediate frequency transformer is made to present only high-frequency AC voltage across its terminals. Since the ground potential output from the low-voltage side of each power conversion unit is close to the ground potential of the medium-voltage system, the actual potential difference between the primary and secondary sides of the intermediate frequency transformer is reduced, thereby reducing the isolation operating voltage that each intermediate frequency transformer needs to withstand, thus simplifying the partial discharge insulation design requirements.

[0016] The above-mentioned technical solutions adopted in this specification can achieve the following beneficial effects: This specification provides a solid-state transformer based on a flying capacitor, which consists of multiple power conversion units connected in series. Each unit includes a medium-voltage side dual half-bridge, a common-mode flying capacitor, an intra-unit flying capacitor, an intermediate frequency transformer, and a low-voltage side H-bridge. The midpoint of the common-mode flying capacitor in all units is uniformly connected to the medium-voltage system ground potential, and the primary-side reference terminal of each MFT is connected to the medium-voltage system ground potential. The low-voltage side output terminals are connected in parallel. This invention blocks the DC voltage across the MFT using the flying capacitor, reducing the steady-state operating voltage of the MFT to the order of Vdc / (4N), fundamentally eliminating the risk of partial discharge, significantly simplifying the MFT insulation design, reducing manufacturing costs, and exhibiting a high degree of modularity. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings:

[0018] Figure 1 A schematic diagram of a solid-state transformer based on a flying capacitor is provided for this specification; Figure 2 This is a schematic diagram of the circuit principle of the single-level FC-SST provided in this manual; Figure 3 This is a schematic diagram of the key steady-state operating waveform of a single-level FC-SST under full load conditions, provided in this specification. Figure 4 This specification provides a schematic diagram of a switched capacitor circuit in a single-level FC-SST; wherein, (a) is the equivalent circuit diagram when switching transistors Q1 and Q4 are turned on; and (b) is the equivalent circuit diagram when switching transistors Q2 and Q3 are turned on. Figure 5 The key simulation waveforms of the solid-state transformer provided in this manual under the same driving signal are shown. Figure 6 This document provides a comparative schematic diagram of different phase-shifting interleaving control strategies; wherein, (a) is a waveform diagram of the switching drive signal under different control strategies; and (b) is a comparison diagram of the output voltage ripple and the flying capacitor voltage ripple under different control strategies. Figure 7 A schematic diagram of a pre-charge startup scheme for a solid-state transformer provided in this specification; Figure 8 This manual provides a schematic diagram illustrating the effect of different transformer inductance ratios on voltage gain under full-load conditions. Figure 9 This is a schematic diagram of key simulation waveforms of a solid-state transformer during steady-state operation, as provided in this specification. Figure 10 This document presents a transient operating waveform diagram of a system under load switching and bidirectional power flow conditions.

[0019] Figure 11 This is a waveform diagram illustrating the startup process of a system employing a low-voltage side pre-charging strategy, as provided in this specification.

[0020] Figure 12 This is a physical schematic diagram of a hardware experimental prototype provided in this manual. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this specification clearer, the technical solutions of this application will be clearly and completely described below in conjunction with specific embodiments and corresponding drawings. Obviously, the described embodiments are only a part of the embodiments of this application, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments in this specification without creative effort are within the scope of protection of this application.

[0022] Improving the insulation level of the partial discharge transformer (MFT) based on the input series-output parallel (ISOP) architecture is another existing technical approach to solving the MFT insulation problem. Encapsulation materials such as epoxy resin, with a dielectric strength of approximately 20 kV / mm, far exceeding the 2 kV / mm of air, can meet the AC withstand voltage (WS) and lightning impulse (LI) requirements of the MFT with a relatively thin insulation layer. However, unavoidable voids during the encapsulation process can cause partial discharge in the MFT, making it extremely difficult to control the partial discharge below 10 pC. Two improvement schemes exist for addressing the partial discharge problem of the MFT: one is to improve the PD insulation level of the MFT by adding a complex shielding layer. While this scheme can improve the PD insulation level, it has high manufacturing complexity, high cost, and low yield. The second is to use an oil-immersed transformer, which can alleviate the PD problem of the MFT, but it carries the risk of oil leakage, is difficult to maintain, and is unsuitable for high power density applications.

[0023] In summary, among existing medium-voltage DC to low-voltage DC solid-state transformers, the intermediate frequency transformer faces the core technical challenge of stringent insulation requirements, specifically including: 0) Partial discharge problem of MFT: Under normal operating conditions, MFT must control the partial discharge level below 10 pC. Voids that are difficult to eliminate inside existing encapsulation materials (such as epoxy resin and silicone) can cause partial discharge of MFT under high electric fields, which seriously affects the long-term reliability and lifespan of MFT.

[0024] 0) AC withstand voltage and lightning impulse requirements for MFT: Taking 10 kV SST as an example, MFT must withstand 35 kV power frequency withstand voltage test and 75 kV lightning impulse test. The insulation design is difficult, the manufacturing process is complicated, and the cost is high.

[0025] 0) Inherent defects of traditional ISOP architecture: Although ISOP architecture can share the voltage stress of semiconductor devices, each MFT still needs to bear the insulation stress of the entire power grid voltage level, which cannot fundamentally alleviate the insulation problem of MFT.

[0026] To address the aforementioned technical problems, this invention proposes a novel topology of a flying capacitor solid-state transformer (FC-SST). By using a high-voltage flying capacitor to isolate the steady-state DC voltage across the MFT, the operating voltage and partial discharge requirement PD of the MFT in MVDC / LVDC SST are fundamentally reduced, while retaining the advantages of a modular and easily expandable system architecture.

[0027] The technical solutions provided by the various embodiments of this application are described in detail below with reference to the accompanying drawings.

[0028] Figure 1 This is a schematic diagram of a solid-state transformer circuit based on a flying capacitor, as described in this specification, specifically including: The FC-SST system operates in a multi-unit series mode to achieve power conversion from MVDC to LVDC. It consists of N FC-SST units connected in series, with the overall architecture as follows:

[0029] 1) Series expansion method: Taking unit 1 as the reference, unit 2 is expanded through a common-mode flying capacitor ( C m1 , C m2 The biases of the cells are sequentially stacked and connected in series from the left. More cells extend to the left in this manner, and each additional cell linearly increases the system's input voltage withstand capability.

[0030] 2) GNDMV Unified Connection: The midpoint of the common-mode flying capacitor branch of each unit is connected to the same medium-voltage system ground potential (GNDMV) in the circuit. This GNDMV node is also connected to the primary side reference terminal of each intermediate frequency transformer.

[0031] 3) Low-voltage side parallel output: The output terminals of all units on the low-voltage side are connected in parallel to supply power to the low-voltage bus; both the medium-voltage side and the low-voltage side can be used as power sources, and the system supports bidirectional power transmission.

[0032] 4) Independent operation of each unit: There is no high-frequency transformer coupling between units, and each unit can transmit power independently. FC-SST fundamentally eliminates the inter-unit circulating current problem caused by gate mismatch in CC-SST, does not require precise gate synchronization, has high system reliability, and is easy to modularly expand.

[0033] The independent operation of each unit also brings another important advantage: different phase-shift gate signals can be applied to different units to achieve interleaved modulation.

[0034] Because the LVDC output voltage and GNDMV have approximately the same ground potential, the isolation voltage across each intermediate frequency transformer is significantly reduced, fundamentally eliminating the PD stress of the intermediate frequency transformer. All PD stress is transferred to the high-voltage flying capacitor ( C m andC f The insulation manufacturing process is mature and simple, and there are a large number of commercially available products on the market. The intermediate frequency transformer only needs to meet the AC withstand voltage and lightning impulse requirements, which can be achieved through standard epoxy / silicone encapsulation process, without the need for complex shielding structures or oil immersion designs.

[0035] Quantitative analysis of system voltage distribution: Assume the system has N series-connected units, and the voltage gain of each unit is... M i The peak value of the input voltage of the i-th resonant slot is defined as V ti = M i V o .

[0036] Based on the single-unit working principle, the capacitor voltage within each unit can be derived step by step according to the following recursive relationship: The voltage across the flying capacitor within the i-th cell is: ; The common-mode flying capacitor voltage is: .

[0037] The boundary conditions are determined by the total DC link voltage borne by the outermost common-mode flying capacitor: V cm,2n-1 = V cm,2n = V dc / 2, Substituting into the recursive relation, we can obtain the relationship between the system output voltage and the input voltage: .

[0038] Specifically, when the turns ratio of each unit transformer n and voltage gain M i When all are set to 1, the input voltage of each unit resonant network is... That is, the steady-state operating voltage across the intermediate frequency transformer is only 1 / (4) of the total system input voltage. N ),like Figure 5 As shown.

[0039] To explain the system's working mechanism in detail, the circuit structure of a single FC-SST unit is described below. Figure 2 As shown.

[0040] Because the system operates in a multi-unit series mode to achieve MVDC to LVDC power conversion, the terms "MV side" and "LV side" are used to maintain consistency when describing a single unit, but in reality, each unit only handles a portion of the total MVDC voltage. The circuit structure of each unit is identical and consists of the following parts:

[0041] The common-mode flying capacitor branch consists of a first common-mode flying capacitor and a second common-mode flying capacitor connected in series. The midpoint of the common-mode flying capacitor branch is connected to the medium-voltage system ground potential (GND_MV). If it is the outermost power conversion unit, the common-mode flying capacitor branch is connected to both sides of the medium-voltage DC bus.

[0042] The medium-voltage side dual half-bridge circuit includes a positive bridge arm and a negative bridge arm; wherein, the positive bridge arm consists of a first switch transistor connected in series. Q 1 and second switching transistors Q The structure consists of two parts, with the negative bridge arm composed of a third switch transistor connected in series. Q 3 and the fourth switching transistor Q 4. Composition: The midpoint of the positive bridge arm is the midpoint between the first and second switching transistors, and the midpoint of the negative bridge arm is the midpoint between the third and fourth switching transistors. Among these, the common-mode flying capacitor... C m1 and C m2 As an input DC link, its midpoint forms the MV ground potential (GNDMV), connected to Q 2 and Q The node of the 4th node serves as the reference ground for the resonant network.

[0043] The flying capacitor branch within the unit is formed by the flying capacitor in the first unit connected in series. C f1 The flying capacitor in the second unit constitutes C f2 Since each unit actually only handles a portion of the total MVDC voltage, C f1 and C f2 The voltage at the input terminal of the resonant slot of this unit ( V t ) clamp located at ± V dc / 4 N (When all unit gains are the same), the DC component at both ends of the intermediate frequency transformer is blocked, so that the intermediate frequency transformer only withstands the amplitude of V dc / 4 N High-frequency square wave voltage.

[0044] Intermediate frequency transformer, including primary winding and secondary winding; V tThe resonant network is jointly applied by GNDMV and CLLC resonant network, which can achieve electrical isolation and efficient bidirectional power transmission. The resonant slot part can also be replaced by other topologies such as DAB. The core innovation lies in the flying capacitor structure and multi-unit connection method on the MV side, rather than the resonant network itself.

[0045] The low-voltage side H-bridge circuit consists of a first rectifier bridge arm and a second rectifier bridge arm connected in parallel; wherein, the first rectifier bridge arm is composed of a fifth switch transistor connected in series. Q 5 and the sixth switching transistor Q The second rectifier bridge arm consists of a seventh switch connected in series. Q 7 and 8 switch transistors Q It consists of 8 components; it connects to a CLLC network to achieve AC to DC conversion.

[0046] FC-SST utilizes only the zero-voltage state of the flying capacitor (AC terminal shorted to the midpoint of the MV DC link): 1) Q 1 、Q 4 、Q 5 、Q Eight switches form a switch group, which conduct synchronously with a duty cycle of 50%. 2) Q 2. Q 3. Q 6. Q 7 form a complementary switch group; 3) Insert dead time between the two groups t d The switching frequency is set near the resonant frequency.

[0047] Key steady-state operating waveforms are as follows Figure 3 As shown. In the above control mode, all 8 switching transistors on the MV side and LV side ( Q 1– Q 8) Both can achieve zero-voltage switching (ZVS): The resonant current charges and discharges the output capacitor (Coss) of the switching transistor during the dead time, causing the transistor to turn on only after the drain-source voltage drops to zero, thus eliminating switching losses. ZVS can be achieved under various load conditions, and the system has load-independent gain characteristics. Both bidirectional power flows support soft switching, significantly improving system efficiency. Furthermore, increasing Lm / Lr can effectively suppress gain shift caused by device parameter mismatch, such as... Figure 8 As shown.

[0048] Under ideal symmetry conditions, the average current at the midpoint of the common-mode flying capacitor is zero, thus maintaining its stability. V cm1 = V cm2In the event of actual parasitic parameter mismatch, FC-SST utilizes built-in mechanisms such as... Figure 4 The switched capacitor (SC) network mechanism shown in (a) and (b) achieves active voltage equalization:

[0049] In the two complementary switching states, the flying capacitor within the cell C f1 and C f2 (Series equivalent) Alternating parallel connection to C m1 and C m2 At both ends, charge is transferred from the capacitor with higher voltage to the capacitor with lower voltage through periodic charging and discharging, dynamically restoring equilibrium. Because... C m1 , C m2 The voltage directly determines the drain-source voltage stress of each switch on the MV side. This SC self-equalizing voltage mechanism ensures that even if there is parasitic mismatch, no switch will be subjected to overvoltage, and no additional active voltage equalization control circuit is required.

[0050] Because each unit operates independently, gate signals with different phase shifts can be applied to different units to achieve, for example... Figure 6 The three interleaved modulation schemes shown in (a) and (b) are as follows: 1) No interleaving: All unit gate signals are fully synchronized, the same as the CC-SST driving method. Theoretically, a single centralized high-frequency transformer can be shared, but as the number of units increases, precise synchronization becomes extremely difficult, and circulating currents are easily generated between units, requiring additional suppression.

[0051] 2) Uniform interleaving: The phase shift of adjacent N units is 180° / N (uniformly distributed). The LV-side output voltage ripple frequency is increased by N times, the peak-to-peak value is significantly reduced, and the output filter size and capacity requirements are reduced, making it suitable for applications with strict requirements on output voltage ripple.

[0052] 3) 180° interleaving: The gate signals of adjacent cells are phase-shifted by 180°. Under this symmetrical operation, the common-mode flying capacitor ( C m The high-frequency voltage ripple on the common-mode flying capacitor naturally cancels out, significantly reducing its size. Considering cost-effectiveness, this solution is the preferred choice because reducing the size of all common-mode flying capacitors has the most significant impact on overall power density improvement.

[0053] like Figure 7 As shown, FC-SST supports the following two pre-charging methods: MV side pre-charging: First, connect the pre-charge resistor branch and apply the rated gate drive signal to the system; All common capacitors (C m ) and flying capacitor ( C f ) will be charged by the pre-charge resistor ( R pre The charging rate is controlled by the RC time constant determined by the equivalent capacitance. Once the capacitor has charged to the rated MV voltage, disconnect the pre-charge resistor branch and close the main switch. K 1. Connect to the steady-state bus; then close the switch. K 3 is used to supply power to the load.

[0054] The main limitation of this method is that it requires the use of relatively expensive and bulky pre-charge resistors.

[0055] LV-side pre-charging (preferred option): Start-up can be achieved from the LV side using an auxiliary controllable power supply (APS). The specific steps are as follows: Close switch K 2. Connect the auxiliary controllable power supply, while keeping switches K1 and K3 in the off state; Apply gate drive signals to each cell on the LV side; at this time, the switching transistor on the MV side can receive the gate signal or remain inactive (off) – since its internal body diode will naturally rectify the high-frequency AC voltage, it can charge the primary side capacitor without affecting the pre-charging process. At the set time T start Internally, the voltage of the auxiliary controllable power supply is gradually ramped up from 0 to the rated output voltage. V o , making the capacitor C m and C f Slowly charge to its rated value; Detect the total voltage across the primary side capacitor; when this detected voltage matches the actual MV DC bus voltage, disconnect the switch. K 2. And close the main switch. K 1; Since the voltages on both sides are already matched before closing, no surge current will be generated; Finally, close the switch. K 3. Once the load is connected, the system enters normal operating mode.

[0056] Since high-voltage starting on the MV side is challenging and costly in practical engineering applications, the LV side pre-charging scheme is a more feasible and preferred starting method.

[0057] Based on the above technical solution, the beneficial effects brought about by the present invention through the following core technologies include: 1) Core topology: Dual half-bridge on the MV side combined with common-mode flying capacitor ( C m1 / C m2 ) and flying capacitors within the cell ( C f1 / C f2 The FC-SST unit topology of the MVDC / LVDC SST fundamentally reduces the operating voltage of the intermediate frequency transformer in the MVDC / LVDC SST by blocking the steady-state DC voltage across the intermediate frequency transformer through a flying capacitor, thereby eliminating the PD risk of the intermediate frequency transformer.

[0058] 2) Multi-unit series expansion connection method: N Each unit is connected in series and superimposed through common-mode flying capacitor bias; each unit C m The midpoint of each branch is connected to GNDMV, which is also connected to the primary reference terminal of each intermediate frequency transformer. The outputs of each unit on the LV side are connected in parallel. By using GNDLV ≈ GNDMV, the isolation voltage of the intermediate frequency transformer is significantly reduced, and the PD stress is transferred to the high-voltage flying capacitor.

[0059] 3) Common-mode flying capacitor (SC) automatic voltage equalization mechanism: based on the built-in switched capacitor (SC) network. C m1 / C m2 Automatic voltage balancing method – intra-cell flying capacitor C f1 / C f2 Alternating parallel connection in complementary switching state C m1 and C m2 Periodic charge transfer enables active voltage equalization, ensuring that the switching transistor is not over-voltaged and eliminating the need for an external voltage equalization control circuit.

[0060] 4) Full-bridge ZVS implementation method: Under a control strategy with a 50% duty cycle and a switching frequency close to the resonant frequency, all 8 switches on the MV and LV sides ( Q 1– Q 8) Methods to achieve ZVS: ZVS can be achieved in all load ranges, and the system has load-independent gain and bidirectional soft-switching characteristics.

[0061] 5) Three interlacing modulation schemes: no interlacing, uniform interlacing (180° / N Three gate phase shift schemes are proposed: 1), 2), and 3), with 180° staggered gate phase shift. In particular, the 180° staggered gate phase shift scheme is a method to naturally cancel the common-mode flying capacitor ripple and reduce the capacitor size.

[0062] 6) LV-side pre-charge startup strategy: Utilizing the inherent bidirectional power flow characteristics of FC-SST, an auxiliary low-voltage controllable power supply is used to charge all flying capacitors from the LV side ( C m and C f The pre-charging start-up method and the utilization method of the MV side body diode during the pre-charging process are described.

[0063] Beneficial effects: 1) Fundamentally eliminate the risk of partial discharge in intermediate frequency transformers: By blocking the steady-state DC voltage across the intermediate frequency transformer through a flying capacitor, the steady-state operating voltage of the intermediate frequency transformer is greatly reduced, the risk of partial discharge in the intermediate frequency transformer is eliminated, the encapsulation voids no longer pose a threat, and no special encapsulation process or complex shielding structure is required.

[0064] 2) Significantly simplified insulation system and reduced manufacturing costs for intermediate frequency transformers: Intermediate frequency transformers only need to meet WS and BIL requirements and can be packaged using standard commercial packaging (epoxy / silicone). This significantly reduces the difficulty of insulation design, greatly improves manufacturing yield, and reduces costs. PD stress is borne by commercially available high-voltage capacitors with mature technology.

[0065] 3) Modularity and scalability are superior to CC-SST: It adopts a distributed intermediate frequency transformer structure, each unit operates completely independently, there is no high frequency coupling, no inter-unit circulating current, which facilitates linear expansion of power level and modular thermal management.

[0066] 4) No need for precise gate synchronization, eliminating circulating current risks: There is no high-frequency transformer coupling between units, which completely eliminates the inter-unit circulating current problem caused by gate mismatch in CC-SST, making the control system simpler and more reliable.

[0067] 5) SC self-equalizing voltage mechanism, no need for active voltage equalization control: The common mode flying capacitor automatically equalizes voltage through the built-in SC network to ensure that the switching transistor is not over-voltaged, and the system operates safely and reliably in steady state.

[0068] 6) Full-bridge ZVS, high efficiency: All 8 switches on the MV and LV sides achieve ZVS, significantly reducing switching losses; ZVS can be achieved across the entire load range, and the system has load-independent gain and bidirectional soft-switching characteristics.

[0069] 7) Easy to start, economical and practical LV-side pre-charging: Supports two pre-charging schemes: MV-side and LV-side. The LV-side scheme only requires a small low-voltage controllable power supply and does not require a complex high-voltage starting device, making it suitable for rapid deployment in industrial sites.

[0070] 8) High-voltage capacitors use standard commercial components, have a stable supply chain, low manufacturing barriers, and are suitable for industrial mass production.

[0071] Example 1: Simulation Verification (5 units, 12 kV MVDC / 800 V LVDC, 500 kW) The simulation parameters are shown in the table below: Simulation verification content and results: 1) Steady-state operating waveform (180° interleaved modulation mode): such as Figure 9 The simulation results show that the FC-SST can successfully achieve ZVS for all switches in steady state; the steady-state isolation voltage across the intermediate frequency transformer ( V iso The voltage PD stress is limited to ±200 V, which is a significant reduction compared to the 12 kV input voltage; all capacitor voltages are automatically balanced with minimal ripple, verifying the effectiveness of the SC self-equalizing voltage mechanism.

[0072] 2) Load shedding transient response: such as Figure 10 As shown, the load is 500 kW from 0 to 50 ms; at 50 ms, the load suddenly drops to 250 kW, Vo fluctuates slightly but recovers quickly, all capacitor voltages remain stable during the transient process, and the current transitions smoothly. During 100–120 ms, the load smoothly switches from +250 kW to −200 kW (power flows in reverse, from the LV side to the MV side). V o The system maintained stable voltage across all capacitors, demonstrating its excellent dynamic performance and bidirectional power flow capability.

[0073] 3) LV-side pre-charge start-up waveform: such as Figure 11 As shown, the 0–80 ms period is the pre-charge phase, with full load applied at 80 ms. The system was safely started from the LV side using an 800 V auxiliary power supply (APS), and all capacitor voltages smoothly rose from zero to their rated values, verifying the feasibility and safety of the LV side pre-charge strategy.

[0074] Example 2: Physical Experiment Verification (2 units, 320 V MVDC / 40 V LVDC, 500 W prototype) Composition: Build a two-unit FC-SST step-down prototype, such as... Figure 12 As shown, the input is 320 V MVDC, the output is 40 VLVDC, and the rated power is 500 W, to verify the actual working performance of the topology.

[0075] Experimental verification content and results: 1) Steady-state operation experiment: Verify the realization of ZVS for all switching transistors, automatic balancing of capacitor voltages, and significant reduction of isolation voltage across the intermediate frequency transformer. The experimental results are consistent with the simulation.

[0076] 2) Load shedding transient test: Verify the dynamic response performance of the system under sudden load changes. V o The fluctuations quickly subsided, and the capacitor voltage remained stable.

[0077] 3) LV side pre-charge start-up experiment: Verify the start-up process of pre-charging the flying capacitor from the low voltage side through the auxiliary power supply. All capacitor voltages smoothly rise from zero to the rated value, and the start-up process is safe and reliable.

[0078] The experimental results agree well with the simulation results, verifying the correctness and practicality of the FC-SST topology.

[0079] The core differences between this solution and existing technologies include: 1) Difference from ISOP architecture: FC-SST blocks the DC voltage across the MFT via a flying capacitor, and the steady-state operating voltage of the MFT is only [missing information]. V dc / 4 N The magnitude, rather than the entire network voltage, fundamentally eliminates the PD stress of MFT, which is something the ISOP architecture cannot achieve.

[0080] 2) Differences from CC-SST: CC-SST is a centralized single transformer, and each unit is connected at high frequency through coupling capacitors, requiring precise gate synchronization; FC-SST is a distributed multi-MFT, and each unit operates independently, without high-frequency coupling, no synchronization required, no circulating current risk, and has a significantly higher degree of modularity.

[0081] 3) Difference from the hierarchical nested ISOP: The above scheme has power loops between each level, resulting in lower system efficiency; FC-SST transmits power independently in each unit, without power loops, resulting in high efficiency.

[0082] Based on the above solutions, the objectives of this invention include: 1) Fundamentally reduce the operating voltage and partial discharge requirements of MFT in MVDC / LVDC SST, completely eliminate the PD insulation risk of MFT, and simplify the design of MFT insulation system; 2) Automatic voltage balancing of each capacitor is achieved through the built-in switched capacitor (SC) mechanism of the flying capacitor; 3) Maintain the modularity and scalability of the ISOP architecture, with each unit operating completely independently, requiring no gate synchronization and eliminating inter-unit circulating current; 4) Using commercially available high-voltage capacitors reduces manufacturing difficulty and cost; 5) Supports two pre-charging schemes: MV side and LV side, with LV side pre-charging being more economical and practical.

[0083] The technical features of the above embodiments can be combined in any way. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

Claims

1. A solid state transformer based on flying capacitor, characterized in that, It includes multiple power conversion units connected in series; each power conversion unit includes: The common-mode flying capacitor branch consists of a first common-mode flying capacitor and a second common-mode flying capacitor connected in series, and the midpoint of the common-mode flying capacitor branch is connected to the medium-voltage system ground potential. The flying capacitor branch within a unit consists of the first flying capacitor within the unit and the second flying capacitor within the unit connected in series. The medium-voltage side dual half-bridge circuit includes a positive bridge arm and a negative bridge arm. The positive bridge arm is composed of a first switch and a second switch connected in series, and the negative bridge arm is composed of a third switch and a fourth switch connected in series. The midpoints of the positive and negative bridge arms are respectively connected to the two ends of the common-mode flying capacitor branch. One end of the positive and negative bridge arms is respectively connected to the two ends of the common-mode flying capacitor branch. The other ends of the positive and negative bridge arms are respectively connected to the two ends of the common-mode flying capacitor branch of the power conversion unit adjacent to the power conversion unit. The low-voltage side H-bridge circuit includes a first rectifier bridge arm and a second rectifier bridge arm. The first rectifier bridge arm is composed of a fifth switch and a sixth switch connected in series, and the second rectifier bridge arm is composed of a seventh switch and an eighth switch connected in series. The intermediate frequency transformer includes a primary winding and a secondary winding; the first end of the primary winding is connected to the midpoint of the flying capacitor branch in the unit, and the second end of the primary winding is connected to the ground potential of the medium voltage system; the two ends of the secondary winding are respectively connected to the midpoint of the first rectifier bridge arm and the midpoint of the second rectifier bridge arm. In this system, the medium-voltage system ground potential of all power conversion units is the same potential point, and the low-voltage side output terminals of all power conversion units are connected in parallel.

2. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, Multiple power conversion units are connected in series and superimposed through the bias of a common-mode flying capacitor; the two ends of the outermost common-mode flying capacitor branch are connected to the positive and negative terminals of the medium-voltage DC bus, respectively, and the other ends of the positive and negative bridge arms of the innermost power conversion unit are connected to the medium-voltage system ground potential.

3. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, The steady-state operating voltage across the intermediate frequency transformer is 1 / (4) of the total input DC voltage of the system. N On the order of magnitude, of which, N This represents the number of power conversion units connected in series.

4. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, The first common-mode flying capacitor and the second common-mode flying capacitor achieve automatic voltage balancing through a built-in switched capacitor network; and through periodic charge transfer, the voltage of the first common-mode flying capacitor and the second common-mode flying capacitor are automatically kept equal.

5. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, The first to eighth switching transistors in each power conversion unit achieve full-bridge zero-voltage switching through a preset control strategy; The control strategy includes: The first, fourth, fifth, and eighth switching transistors constitute the first switching group, which conducts synchronously with a duty cycle of 50%. The second switch, the third switch, the sixth switch, and the seventh switch constitute a second switch group, which is complementary to the first switch group in conduction; A dead time is provided between the first switch group and the second switch group; and the switching frequency is set at the resonant frequency of the resonant network.

6. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, Each power conversion unit uses an interleaved modulation scheme to control the gate drive signal, the interleaved modulation scheme including: No interleaving: The gate signals of all power conversion units are completely synchronized; Uniformly staggered: N The adjacent phase shift of each power conversion unit is 180°. N This is to reduce the size and capacitance requirements of the secondary output filter; 180° stagger: The gate signals of adjacent power conversion units are phase-shifted by 180° to reduce the size and capacitance requirements of the common-mode flying capacitor.

7. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, The startup circuit includes a pre-charge start-up mechanism from the low-voltage side, comprising: The first switch is connected between the medium-voltage DC bus and the medium-voltage input terminal of the solid-state transformer; The second switch is connected between the low-voltage auxiliary power supply and the low-voltage output terminal of the solid-state transformer; The third switch is connected between the low-voltage output terminal of the solid-state transformer and the load.

8. A solid-state transformer based on a flying capacitor as described in claim 7, characterized in that, The pre-charge start-up process includes: Close the second switch and disconnect the first switch and the third switch; A drive signal is applied to each power conversion unit on the low-voltage side, and the switching transistors of each power conversion unit on the medium-voltage side perform natural rectification charging of the flying capacitor on the medium-voltage side through the parasitic diodes of the switching transistors themselves. Control the output voltage of the low-voltage auxiliary power supply at a preset startup time. T start The internal linear ramp-up voltage from zero to the rated output voltage V o This ensures that the flying capacitors and main capacitors of all power conversion units are charged to their rated values. The total voltage of the capacitors on the medium-voltage side is detected. When the total voltage matches the actual voltage of the medium-voltage DC bus, the second switch is opened and the first switch is closed to connect to the medium-voltage DC bus in a non-inrush current manner. Close the third switch, connect the load, and enter the operating state.

9. A solid-state transformer based on a flying capacitor as described in claim 1, characterized in that, A resonant network, which is a CLLC resonant network, is also connected between the medium-voltage side dual half-bridge circuit and the low-voltage side H-bridge circuit.

10. An insulation optimization method for a solid-state transformer based on flying capacitors according to any one of claims 1-9, characterized in that, Includes the following steps: Connect the first end of the primary winding of the intermediate frequency transformer to the midpoint of the series connection of the flying capacitor in the unit, and connect the second end of the primary winding of the intermediate frequency transformer to the ground potential of the medium voltage system. By having the flying capacitor and common-mode flying capacitor within the unit jointly bear the DC voltage stress, the intermediate frequency transformer is made to present only high-frequency AC voltage across its terminals. Since the ground potential output from the low-voltage side of each power conversion unit is close to the ground potential of the medium-voltage system, the actual potential difference between the primary and secondary sides of the intermediate frequency transformer is reduced, thereby reducing the isolation operating voltage that each intermediate frequency transformer needs to withstand, thus simplifying the partial discharge insulation design requirements.

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