Energy storage type solid state transformer and control method thereof

Abstract

This application relates to the field of electrical automation equipment technology, and provides an energy storage solid-state transformer and its control method, comprising: three structurally identical units, each unit including a single-phase energy storage bridge arm composed of multiple energy storage sub-modules connected in series, a grid-side filter reactor, an AC soft-start circuit, a rectifier, and a bus capacitor; further comprising a resonant capacitor, a resonant inductor, and a matrix transformer, the primary branch of the resonant capacitor, resonant inductor, and matrix transformer forming a high-frequency circuit loop with the three single-phase energy storage bridge arms; the rectifier connecting the secondary side of the matrix transformer and the bus capacitor, all bus capacitors connected in parallel to form low-voltage DC positive and negative bus ports. The control method generates low-frequency modulation voltage and high-frequency modulation voltage through decoupling and superimposes them to form the final modulation voltage. This invention achieves deep integration of energy storage and solid-state transformers at the topology and control levels, reducing system costs and improving efficiency and reliability.

Description

Technical Field

[0001] This invention belongs to the field of electrical automation equipment technology, specifically relating to an energy storage solid-state transformer and its control method. Background Technology

[0002] With the explosive growth of AI computing power, the power of a single rack in a training AIDC has exceeded 100kW, placing unprecedented demands on the capacity, efficiency, and reliability of power supply systems. Solid-state transformers (SSTs), with their advantages of high frequency, multiple ports, and intelligent controllability, are considered core equipment for next-generation data center power supply architectures. Currently, deep integration of SSTs with energy storage systems has become a key path to solving core challenges such as high power density power supply and leaps in power supply reliability.

[0003] However, existing solutions for integrating solid-state transformers (SSTs) with energy storage systems all have significant limitations. For example, patent document CN121508105A discloses an energy regulation router for data centers, which directly connects the energy storage battery to the DC bus of the SST. While the integration is achieved in terms of control strategy, the topology remains separate. This results in the DC bus voltage being susceptible to wide-range fluctuations due to the charging and discharging of the energy storage, which is detrimental to the optimized design of subsequent converters. Similarly, patent documents CN119010600A and CN115632562A both disclose solid-state transformer topologies and mixing modulation methods based on modular multilevel converters (MMCs). These methods achieve decoupling of high and low frequency signals through common-mode and differential-mode decoupling or by setting high-frequency cancellation submodules. However, these solutions either rely on specific submodule topologies and complex paired control, or involve highly coupled submodule functions, resulting in complex design and equalization, and do not address the deep integration of energy storage units at the submodule level. Although patent document CN114070115B discloses a cascaded H-bridge energy storage system, the system does not provide electrical isolation and does not achieve the high-frequency isolation and voltage conversion functions required by solid-state transformers.

[0004] In summary, existing technologies have failed to simultaneously achieve distributed energy storage submodule cascading, centralized high-frequency isolation transformation, and complete decoupling of power frequency and high-frequency control in a single topology, making it difficult to balance system cost, efficiency, and reliability. Summary of the Invention

[0005] The purpose of this invention is to overcome the shortcomings of existing technologies, such as the separation of energy storage systems and solid-state transformers in topology, coupling in control strategies, lengthy energy paths, and high system costs. This invention provides an energy storage solid-state transformer and its control method, which integrates energy storage and power conversion at the topology and control strategy levels, reduces system costs, improves system efficiency, and supports direct access and efficient consumption of new energy sources.

[0006] To achieve the above objectives, the present invention provides the following technical solution:

[0007] It should be noted that the energy storage solid-state transformer provided by this invention can also be called an energy storage power electronic transformer; both refer to energy storage power electronic conversion devices that use high-frequency transformers to achieve electrical isolation and voltage transformation. For ease of description, this invention will uniformly use the term "energy storage solid-state transformer" in its description.

[0008] This invention proposes an energy storage type solid-state transformer, comprising:

[0009] Three identical units are connected to phases A, B, and C of a three-phase medium-high voltage AC power grid and the positive and negative buses of a low-voltage DC power grid, respectively.

[0010] Each of the units includes: a single-phase energy storage bridge arm, a grid-side filter reactor, an AC soft start circuit, a rectifier, and a bus capacitor; the single-phase energy storage bridge arm is composed of multiple energy storage sub-modules connected in series at their AC ports, one end of which is connected to the corresponding phase of the three-phase medium-high voltage AC grid through the grid-side filter reactor and the AC soft start circuit, and the other end is connected to the corresponding end of the single-phase energy storage bridge arm in the other two units to form a neutral point;

[0011] The energy storage solid-state transformer also includes: a resonant capacitor, a resonant inductor, and a matrix transformer. The primary side branches of the resonant capacitor, resonant inductor, and matrix transformer, together with the three one-phase energy storage bridge arms, form a high-frequency circuit loop.

[0012] The AC port of the rectifier is used to connect to the secondary AC port of the matrix transformer, and the DC port of the rectifier is connected to the bus capacitor.

[0013] The positive and negative terminals of the bus capacitors of the three units are connected in parallel to form the low-voltage DC positive and negative bus ports of the energy storage solid-state transformer.

[0014] Based on the above technical solution, the high-frequency circuit loop can be constructed in one of the following two ways:

[0015] Method 1: In each unit, the resonant capacitor, resonant inductor, primary branch of the matrix transformer, and AC port of one phase energy storage bridge arm are connected in series to form a high-frequency circuit loop. This method facilitates independent optimization of the resonant parameters of each phase.

[0016] Method 2: One end of each of the three resonant capacitors is connected to the other end, and the other end of each capacitor is connected to the connection point between the grid-side filter reactor and the energy storage bridge arm in each of the three units. The resonant inductor and the primary branch of the matrix transformer are connected in series, with one end connected to the interconnection point of the three resonant capacitors and the other end connected to the neutral point. This method can automatically balance the three-phase resonant current and simplify the power circuit layout.

[0017] Based on the above technical solution, the energy storage submodule includes an energy storage unit, a filter inductor, a filter capacitor, a DC soft-start circuit, power devices, and a bypass switch. The four power devices and the filter capacitor form an AC / DC power conversion circuit, with the bypass switch connected to its AC output port. The energy storage unit is connected in parallel with the filter inductor and the DC soft-start circuit, and then in parallel with the filter capacitor branch. The energy storage unit can be an energy storage carrier such as a battery cluster, supercapacitor, or lithium supercapacitor. The DC soft-start circuit consists of a bypass contactor and a pre-charge resistor connected in parallel, used to limit inrush current when the energy storage unit is put into operation. The bypass switch is used to bypass the energy storage submodule from the bridge arm in case of a fault, ensuring the system's redundant operation capability.

[0018] Furthermore, the matrix transformer comprises multiple discrete high-frequency transformers, with the primary windings of the multiple high-frequency transformers connected in series to form a high-voltage AC port, and the secondary windings of the multiple high-frequency transformers forming multiple secondary AC ports. Preferably, the matrix transformer includes 3 to 12 high-frequency transformers.

[0019] Furthermore, the rectifier is a diode uncontrolled rectifier bridge or a synchronous rectifier circuit. A synchronous rectifier circuit is preferred to improve rectification efficiency.

[0020] Secondly, the present invention also proposes a control method for the above-mentioned energy storage solid-state transformer, comprising the following steps:

[0021] Grid-side power control steps: control the active and reactive power injected into the three-phase medium-voltage AC grid, and generate the low-frequency modulation voltage of each of the three one-phase energy storage bridge arms by performing closed-loop control of the grid-connected current.

[0022] Low-voltage DC bus voltage control steps: Control the low-voltage DC bus voltage, and generate the high-frequency modulation voltage of each of the three one-phase energy storage bridge arms by performing closed-loop control of the low-voltage DC bus voltage.

[0023] Modulation voltage synthesis step: The low-frequency modulation voltage generated by the grid-side power control step and the high-frequency modulation voltage generated by the low-voltage DC bus voltage control step are added together to form the final modulation voltage of each of the three one-phase energy storage bridge arms.

[0024] Furthermore, the grid-side power control steps include:

[0025] Energy storage charging and discharging power control sub-step: The active current command for grid-connected current control is calculated by summing the energy storage charging and discharging power command and the low-voltage DC output power.

[0026] Energy storage phase-to-phase state of charge equalization sub-step: Calculate the amplitude and phase of the zero-sequence voltage based on the average state of charge of the energy storage sub-modules in the three units, and superimpose it onto the modulation voltage of each of the three one-phase energy storage bridge arms.

[0027] The state of charge balancing sub-step within the energy storage phase: Based on the difference between the state of charge of each energy storage sub-module within each phase energy storage bridge arm and the average state of charge of the energy storage sub-modules within that phase energy storage bridge arm, calculate the equalization voltage superimposed on the modulation wave of each energy storage sub-module.

[0028] Grid-connected current control sub-step: The low-frequency modulation voltage of each of the three one-phase energy storage bridge arms is generated by closed-loop control of the grid-connected current.

[0029] Furthermore, the low-voltage DC bus voltage control step includes:

[0030] Bus voltage closed-loop control sub-step: Perform closed-loop control on the low-voltage DC bus voltage to generate high-frequency modulated voltages for each of the three one-phase energy storage bridge arms;

[0031] Interphase current equalization sub-step: Calculate the compensation voltage of the high-frequency modulation voltage of each of the three units based on the difference between the DC-side output current of each of the three units and the average value of the DC-side output current of the three units.

[0032] The modulation voltages output from the bus voltage closed-loop control sub-step and the phase-to-phase current equalization sub-step are added together to obtain the final high-frequency modulation voltages of the three units.

[0033] Furthermore, the frequency of the low-frequency modulation voltage is 50Hz or 60Hz, and the frequency of the high-frequency modulation voltage is 5kHz to 50kHz.

[0034] Based on the above technical solution, the control method further includes a grid-connected / off-grid switching control step: when a grid fault is detected, the energy storage solid-state transformer is controlled to switch from current source mode to voltage source mode, and the energy storage submodule supplies power to the load through the one-phase energy storage bridge arm and the matrix transformer, with a switching time of no more than 1ms.

[0035] The beneficial effects of this invention are:

[0036] This invention deeply integrates the power units of the energy storage converter and the solid-state transformer at the topology level, sharing power devices, control platform and heat dissipation system, which can reduce the initial investment of the system by about 30%-40% and significantly reduce the footprint.

[0037] The converged architecture transforms the energy path from a multi-level, circuitous route to the shortest direct path, enabling energy exchange between the grid, energy storage, and loads to be achieved through the same power unit, significantly improving the overall system efficiency.

[0038] This invention decomposes the modulation voltage of each of the three one-phase energy storage bridge arms into low-frequency modulation voltage and high-frequency modulation voltage. The two are generated by independent control modules and then superimposed. The power frequency grid connection control and the high-frequency resonant transformation control are completely decoupled and can be optimized independently.

[0039] The system employs a multi-level equalization method that balances phase-to-phase state of charge through zero-sequence voltage equalization and phase-to-phase state of charge through modulation wave superposition equalization. This method achieves high equalization accuracy, does not affect grid-connected power quality, and significantly improves the available capacity and lifespan of the energy storage system.

[0040] After system integration, it is endowed with complete grid-type capabilities, which can actively provide inertia support and frequency response for the power grid, and complete seamless on-grid and off-grid switching within milliseconds, supporting direct access and efficient consumption of new energy sources. Attached Figure Description

[0041] Figure 1 This is a structural diagram of the energy storage solid-state transformer in Embodiment 1 of the present invention.

[0042] Figure 2 This is a structural diagram of the energy storage solid-state transformer in Embodiment 2 of the present invention. Detailed Implementation

[0043] The present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments.

[0044] Example 1:

[0045] Reference Figure 1 This embodiment provides an energy storage solid-state transformer, comprising three structurally identical units, which are respectively connected between phases A, B, and C of a three-phase high-voltage AC power grid and the low-voltage DC positive and negative busbars (Vdc+, Vdc-). For ease of description, the unit connecting phases A, B, and C can be referred to as the first unit, the second unit, and the third unit, respectively.

[0046] Reference Figure 1 The upper end of phase A bridge arm is sequentially connected to the first AC soft start circuit S. Sa and the first grid-side filter inductor L ga Phase A is connected to a three-phase medium-high voltage AC power grid; the upper end of phase B bridge arm is connected sequentially to the second AC soft starter circuit S. SbSecond grid side filter inductor L gb Connected to phase B of the power grid; the upper end of phase C bridge arm is connected in sequence to the third AC soft start circuit S. Sc and the third-side filter inductor L gc Connected to phase C of the power grid. Each AC soft starter circuit consists of a switch and a soft starter resistor connected in parallel, with the first AC soft starter circuit S... Sa For example, it includes a first switch Ka and a first soft-start resistor Ra, with Ka and Ra connected in parallel to achieve soft-start pre-charging during grid connection. The grid-side filter inductor L... ga L gb L gc Used to filter out high-frequency harmonics in grid-connected current.

[0047] Reference Figure 1 Each energy storage submodule has the same internal topology. Taking the first energy storage submodule SM of phase B as an example... b1 Let's take an example to illustrate. SM b1 Including power devices, DC bus capacitor C dc (or filter capacitor), energy storage unit, filter inductor L f And a DC soft-start circuit. The power devices form an H-bridge circuit, which consists of a first switch S1, a second switch S2, a third switch S3, and a fourth switch S4; wherein the upper ends of S1 and S3 are connected to the DC bus capacitor C. dc The positive terminal of S2 and the lower terminals of S4 are both connected to the DC bus capacitor C. dc The negative terminal. A bypass switch S is connected in parallel at the AC port of the H-bridge. bp This is used to bypass the submodule from the bridge arm in case of a submodule failure. The energy storage unit is connected in series with a filter inductor L. f After connecting to the DC soft start circuit, and the DC bus capacitor C dc Parallel connection. The energy storage unit can be an energy storage carrier such as a lithium battery, supercapacitor, or lithium supercapacitor. The DC soft start circuit consists of a DC switch K. dc and pre-charge resistor R dc The parallel configuration is used to power the DC bus capacitor C when the energy storage unit is put into operation. dc Soft charging is performed. The H-bridge circuit described above can achieve bidirectional power conversion between the energy storage unit and the AC port by controlling the on / off state of S1 to S4.

[0048] Reference Figure 1 To form a high-frequency power transmission path, this embodiment draws a high-frequency port from each phase arm and connects it to an external resonant network and matrix transformer. Specifically, taking phase A as an example, in its energy storage submodule SM... a1 The first AC terminal and the first AC soft start circuit S SaA connecting line is drawn from the neutral point N to form the first high-frequency lead; another connecting line is drawn from the neutral point N to form the second high-frequency lead. A first resonant inductor L is connected in series between the first and second high-frequency leads. ra First resonant capacitor C ra and the first matrix transformer T a The primary side of the series branch forms the high-frequency resonant circuit of phase A. ra Resonant capacitor C ra A resonant network is formed. The high-frequency power transmission path of phases BC is the same as that of phase A. Each of the three bridge arms is independently connected to the matrix transformer via a resonant circuit, and they form a common reference point with each other through the neutral point N.

[0049] Reference Figure 1 First matrix transformer T a Composed of n discrete high-frequency transformers T a1 T a2 ...T an The primary windings are connected in series, forming a complete primary series branch. One end of this primary series branch is connected to the first resonant capacitor C. ra One end is connected to the neutral point N. The secondary winding of each high-frequency transformer forms an independent secondary AC port. Each secondary AC port is connected to a rectifier, forming a multi-parallel structure. Taking the first matrix transformer T... a The first transformer T a1 For example, its secondary winding is connected to the first rectifier R. ec_a1 Rectifier R ec_a1 A full-bridge uncontrolled rectifier topology is adopted, with its AC input terminal connected to both ends of the secondary winding, and the DC output terminal connected in parallel with the first bus capacitor C. bus_a1 Similarly, transformer T an The secondary side is connected to the nth rectifier R. ec_an The nth bus capacitor C is connected in parallel at the DC output terminal. bus_an All n bus capacitors have their positive terminals connected to the positive terminal DC+ of the low-voltage DC bus, and their negative terminals connected to the negative terminal DC- of the low-voltage DC bus. The second matrix transformer T... b and the third matrix transformer T c Using the exact same structure, each circuit has n rectifiers and n bus capacitors, denoted as C. bus_b1 ~C bus_bn and C bus_c1 ~C bus_cn In this way, the positive terminals of the three-phase 3n bus capacitors are all connected to the same low-voltage DC positive bus DC+, and the negative terminals are all connected to the same low-voltage DC negative bus DC-, forming a unified low-voltage DC port.

[0050] The energy storage solid-state transformer proposed in this embodiment has a basic working principle including a power frequency grid-connected power conversion path and a high-frequency isolated power transmission path, which are decoupled and controlled by the low-frequency and high-frequency components of the bridge arm voltage, respectively.

[0051] From the power frequency grid connection side, each phase of the three-phase cascaded H-bridge consists of multiple energy storage sub-modules connected in series with their H-bridge AC outputs. Each sub-module can maintain a stable DC voltage on its DC bus capacitor. By injecting a power frequency sinusoidal signal into the modulation wave of each sub-module's H-bridge, each of the three-phase bridge arms can output its own power frequency AC voltage, which is then connected to the medium-high voltage power grid via an AC soft-start circuit and a grid-side filter inductor. By controlling the voltage of the three-phase bridge arms, the active and reactive power injected into the grid can be adjusted.

[0052] From the high-frequency isolation side, the upper and lower ends of each phase bridge arm are led out separately to form a high-frequency port, and the primary-side series branch of the resonant inductor, resonant capacitor, and matrix transformer is connected in series between these high-frequency ports. By superimposing high-frequency components into the modulation waves of each submodule H-bridge, a high-frequency AC voltage can be generated between the upper end of the three-phase bridge arm and the neutral point. Taking phase A as an example, when the bridge arm voltage contains a high-frequency component, this high-frequency voltage passes through the first resonant inductor L... ra and the first resonant capacitor C ra Applied to the first matrix transformer T a The primary windings of the matrix transformer generate a high-frequency resonant current in the primary circuit. This high-frequency current is induced in the secondary windings through the multiple primary windings of the matrix transformer. Each secondary winding is rectified by its own rectifier and then charges its corresponding bus capacitor. Since all the bus capacitors are connected in parallel to the same low-voltage DC bus, a low-voltage DC output is formed.

[0053] The control system of this energy storage solid-state transformer mainly includes two levels: the grid-side power control module, which is responsible for the grid-connected current waveform and the charging and discharging management of the energy storage unit; and the low-voltage DC bus voltage control module, which is responsible for maintaining the stability of the DC output voltage and realizing the power balance of the three-phase high-frequency transmission.

[0054] The goal of grid-side power control is to enable the device to inject specified active and reactive power into the grid according to instructions, while maintaining energy balance among the energy storage units. This part includes four stages: charging and discharging power calculation, grid-connected current regulation, inter-phase balancing, and intra-phase balancing.

[0055] The charging and discharging power calculation process is based on the total active power command P issued by the superior authority and the actual measured low-voltage DC side output power P. dc Calculate the total power reference that the device needs to absorb from or send to the grid. :

[0056]

[0057] Then, based on the grid voltage amplitude and reactive power command Q... ref The grid-connected active current reference value i was obtained through calculation. dref and reactive current reference value i qref The current closed-loop control outputs a control voltage, which generates the low-frequency fundamental voltage u required by the three-phase bridge arms. la u lb u lc .

[0058] The phase-to-phase balancing stage is used to address the issue of inconsistent state of charge (SOC) across the entire three-phase energy storage unit. Taking battery energy storage as an example, the average SOC of all submodules in each phase is calculated in real time and denoted as SOC. a SOC b SOC b Then calculate the overall average SOC. avg Based on the magnitude and sign of the three-phase state-of-charge deviation, a zero-sequence voltage component u0 is generated. The formula for calculating u0 is:

[0059]

[0060] In the formula: θ is the equilibrium coefficient. a Let A be the phase angle of the bridge arm current. The imbalance of three-phase SOC is defined as:

[0061]

[0062] Zero-sequence voltage u0 is simultaneously superimposed on u la u lb u lc The three-phase low-frequency modulation voltage is obtained above. , , .

[0063] The intra-phase balancing mechanism addresses the issue of inconsistent states of charge (SOC) among sub-modules within the same phase. Taking battery energy storage as an example, for the j-th sub-module, its SOC is denoted as SOC. aj SOC aj With SOC a The difference is multiplied by an equilibrium coefficient K a The voltage correction amount of this submodule is obtained. This correction is in phase or out of phase with the fundamental wave of the bridge arm current and is superimposed on the final modulation wave of the submodule. By adjusting the active power absorbed or emitted by the submodule, the state of charge (SOC) within the phase is gradually leveled out.

[0064] The low-voltage DC bus voltage control is responsible for stabilizing the low-voltage DC bus voltage at the set value and ensuring that the power transmitted in the three-phase high-frequency circuits is basically the same. It includes a bus voltage closed-loop control module and an inter-phase current balancing module.

[0065] Bus voltage closed-loop circuit measurement of low-voltage DC bus voltage U dc , with the given value U dcref The error is compared and fed into a proportional-integral regulator, which is then modulated in a closed loop and its output is fed into an H-bridge modulated high-frequency voltage amplitude U. high The bus voltage is adjusted by adjusting the amplitude of the high-frequency voltage.

[0066] The phase-to-phase current balancing circuit is used to prevent excessive or insufficient power transmission in a particular phase due to differences in three-phase parameters. For each phase, the bus capacitors of the rectifier bridge are connected in parallel, and the total branch current i of each phase after the parallel connection of the bus capacitors is measured. dca i dca i dca The average value of the DC output current of the three-phase main branch, i dc After subtracting from the 3-phase voltage, the compensation voltage amplitude for each phase of the high-frequency modulation voltage is calculated using a proportional controller. , , :

[0067]

[0068] In the formula, Kp is the proportional controller coefficient. The compensation voltage ( , , ) and high frequency voltage (U high The three-phase high-frequency modulated voltage is obtained by superposition. , , .

[0069] The low-frequency modulation voltage provided by the two control layers mentioned above ( , , ) and high-frequency modulated voltage ( , , They are directly added in the time domain to form the final reference voltage for each phase arm:

[0070]

[0071] Example 2:

[0072] The energy storage solid-state transformer proposed in this embodiment includes: an energy storage submodule, a grid-side filter reactor, an AC soft start circuit, a resonant capacitor, a resonant inductor, a matrix transformer, a rectifier, and a bus capacitor.

[0073] Figure 2 This is a system structure diagram of the energy storage solid-state transformer in this embodiment. Each phase arm is composed of multiple energy storage sub-modules SM connected in series at their AC ports. Taking phase A as an example, this phase arm consists of m energy storage sub-modules SM. a1 SM a2 SM am Composed in series. Each energy storage submodule has a first AC terminal and a second AC terminal, SM a1 The second AC terminal is connected to SM a2 The first AC terminal, and so on, up to SM am The second AC terminal is connected to the neutral point N of the three-phase star connection. a1 The first AC terminal serves as the upper end of phase A bridge arm, SM am The second AC terminal serves as the lower end of the A-phase bridge arm. The B-phase and C-phase bridge arms are respectively composed of the energy storage submodule SM. b1 ~SM bm and SM c1 ~SM cm The three phases are connected in series in the same way, and the lower ends of the three phase arms are connected to the neutral point N to form a star connection.

[0074] Reference Figure 2 The upper end of phase A bridge arm is sequentially connected to the first AC soft start circuit S. Sa and the first grid-side filter inductor L ga Phase A is connected to a three-phase medium-high voltage AC power grid; the upper end of phase B bridge arm is connected sequentially to the second AC soft starter circuit S. Sb Second grid side filter inductor L gb Connected to phase B of the power grid; the upper end of phase C bridge arm is connected in sequence to the third AC soft start circuit S. Sc and the third-side filter inductor L gc Connected to phase C of the power grid. Each AC soft starter circuit consists of a switch and a soft starter resistor connected in parallel, with the first AC soft starter circuit S... Sa For example, it includes a first switch Ka and a first soft-start resistor Ra, with Ka and Ra connected in parallel to achieve soft-start pre-charging during grid connection. The grid-side filter inductor L... ga L gb L gc Used to filter out high-frequency harmonics in grid-connected current.

[0075] Reference Figure 2 Each energy storage submodule has the same internal topology. Taking the first energy storage submodule SM of phase C as an example... c1 Let's take an example to illustrate. SM c1 Including power devices, DC bus capacitor C dc (or filter capacitor), energy storage unit, filter inductor Lf And a DC soft-start circuit. The power devices form an H-bridge circuit, which consists of a first switch S1, a second switch S2, a third switch S3, and a fourth switch S4; wherein the upper ends of S1 and S3 are connected to the DC bus capacitor C. dc The positive terminal of S2 and the lower terminals of S4 are both connected to the DC bus capacitor C. dc The negative terminal. A bypass switch S is connected in parallel at the AC port of the H-bridge. bp This is used to bypass the submodule from the bridge arm in case of a submodule failure. The energy storage unit is connected in series with a filter inductor L. f After connecting to the DC soft start circuit, and the DC bus capacitor C dc Parallel connection. Energy storage units can be lithium batteries, supercapacitors, lithium supercapacitors, or other energy storage carriers. The DC soft-start circuit consists of a DC switch K. dc and pre-charge resistor R dc The parallel configuration is used to power the DC bus capacitor C when the energy storage unit is put into operation. dc Soft charging is performed. The H-bridge circuit described above can achieve bidirectional power conversion between the energy storage unit and the AC port by controlling the on / off state of S1 to S4.

[0076] Reference Figure 2 To form a high-frequency power transmission path, this embodiment draws a high-frequency port from each phase arm and connects it to an external resonant network and matrix transformer. Specifically, in SM a1 The first AC terminal and the first AC soft start circuit S Sa A connecting wire is drawn from the first resonant capacitor C. ra In series, in SM b1 The first AC terminal and the first AC soft start circuit S Sb A connecting wire is drawn from the second resonant capacitor C. rb In series, in SM c1 The first AC terminal and the first AC soft start circuit S Sc A connecting line is drawn between them and the third resonant capacitor C. rc These three connecting wires are connected in series, and then connected in parallel to form the first high-frequency lead-in. A connecting wire is drawn from the neutral point N to form the second high-frequency lead-in. A first resonant inductor L is connected in series between the first and second high-frequency leads-in. r Together with the primary-side series branch of the first matrix transformer T, they form a high-frequency resonant circuit.

[0077] Reference Figure 2 The first matrix transformer T consists of n discrete high-frequency transformers T1, T2...T n The primary windings are connected in series, forming a complete primary series branch. One end of this primary series branch is connected to the first resonant inductor L. rOne end is connected to the neutral point N. The secondary winding of each high-frequency transformer forms an independent secondary AC port. Each secondary AC port is connected to a rectifier, forming a multi-parallel structure. Taking the first transformer T1 of the first matrix transformer T as an example, its secondary winding is connected to the first rectifier R. ec_1 Rectifier R ec_1 A full-bridge uncontrolled rectifier topology is adopted, with its AC input terminal connected to both ends of the secondary winding, and the DC output terminal connected in parallel with the first bus capacitor C. bus_1 Similarly, transformer T n The secondary side is connected to the nth rectifier R. ec_n The nth bus capacitor C is connected in parallel at the DC output terminal. bus_n The positive terminals of all n bus capacitors are connected to the positive terminal DC+ of the low-voltage DC bus, and the negative terminals are connected to the negative terminal DC- of the low-voltage DC bus. In this way, the positive terminals of all n bus capacitors are connected to the same low-voltage DC positive bus DC+, and the negative terminals are connected to the same low-voltage DC negative bus DC-, forming a unified low-voltage DC port.

[0078] The energy storage solid-state transformer in this embodiment has a basic working principle including a power frequency grid-connected power conversion path and a high-frequency isolated power transmission path, which are decoupled and controlled by the low-frequency and high-frequency components of the bridge arm voltage, respectively.

[0079] From the power frequency grid connection side, each phase of the three-phase cascaded H-bridge consists of multiple energy storage sub-modules connected in series with their H-bridge AC outputs. Each sub-module can maintain a stable DC voltage on its DC bus capacitor. By injecting a power frequency sinusoidal signal into the modulation wave of each sub-module's H-bridge, each of the three-phase bridge arms can output its own power frequency AC voltage, which is then connected to the medium-high voltage power grid via an AC soft-start circuit and a grid-side filter inductor. By controlling the voltage of the three-phase bridge arms, the active and reactive power injected into the grid can be adjusted.

[0080] From the high-frequency isolation side, the upper end of each phase bridge arm is individually led out and connected in parallel with a resonant capacitor to form a high-frequency port with the neutral point N. The resonant inductor and the primary-side series branch of the matrix transformer are then connected in series between this high-frequency port. By superimposing high-frequency components into the modulation waves of each submodule H-bridge, a high-frequency AC voltage can be generated between the upper end of the three-phase bridge arms and the neutral point. Taking phase A as an example, when the bridge arm voltage contains a high-frequency component, this high-frequency voltage passes through the first resonant capacitor C. ra and the first resonant inductor L r A high-frequency resonant current is generated in the primary circuit of the first matrix transformer T through its primary series windings. This high-frequency current is induced in the secondary windings through the multiple primary series windings of the matrix transformer. Each secondary winding is rectified by its own rectifier and then charges its corresponding bus capacitor. Since all bus capacitors are connected in parallel to the same low-voltage DC bus, a low-voltage DC output is formed.

[0081] The control system of this energy storage solid-state transformer mainly includes two levels: the grid-side power control module, which is responsible for the grid-connected current waveform and the charging and discharging management of the energy storage unit; and the low-voltage DC bus voltage control module, which is responsible for maintaining the stability of the DC output voltage and realizing the power balance of the three-phase high-frequency transmission.

[0082] The goal of grid-side power control is to enable the device to inject specified active and reactive power into the grid according to instructions, while maintaining energy balance among the energy storage units. This part includes four stages: charging and discharging power calculation, grid-connected current regulation, inter-phase balancing, and intra-phase balancing.

[0083] The charging and discharging power calculation process is based on the total active power command P issued by the superior authority and the actual measured low-voltage DC side output power P. dc Calculate the total power reference P that the device needs to absorb from or send to the grid. ref :

[0084]

[0085] Then, based on the grid voltage amplitude and reactive power command Q... ref The grid-connected active current reference value i was obtained through calculation. dref and reactive current reference value i qref The current closed-loop control outputs a control voltage, which generates the low-frequency fundamental voltage u required by the three-phase bridge arms. lowa u lowb u lowc .

[0086] The phase-to-phase balancing stage is used to address the issue of inconsistent state of charge (SOC) across the entire three-phase energy storage unit. Taking battery energy storage as an example, the average SOC of all submodules in each phase is calculated in real time and denoted as SOC. a SOC b SOC b Then calculate the overall average SOC. avg Based on the magnitude and sign of the three-phase state-of-charge deviation, a zero-sequence voltage component u0 is generated. The formula for calculating u0 is:

[0087]

[0088] In the formula: θ is the equilibrium coefficient. a Let A be the phase angle of the bridge arm current. The imbalance of three-phase SOC is defined as:

[0089]

[0090] Zero-sequence voltage u0 is simultaneously superimposed on ulowa u lowb u lowc The three-phase low-frequency modulation voltage is obtained above. , , .

[0091] The intra-phase balancing mechanism addresses the issue of inconsistent states of charge (SOC) among sub-modules within the same phase. Taking battery energy storage as an example, for the j-th sub-module, its SOC is denoted as SOC. aj SOC aj With SOC a The difference is multiplied by an equilibrium coefficient K a The voltage correction amount of this submodule is obtained. This correction is in phase or out of phase with the fundamental wave of the bridge arm current and is superimposed on the final modulation wave of the submodule. By adjusting the active power absorbed or emitted by the submodule, the state of charge (SOC) within the phase is gradually leveled out.

[0092] The low-voltage DC bus voltage control is responsible for stabilizing the low-voltage DC bus voltage at the set value and ensuring that the power transmitted in the three-phase high-frequency circuits is basically the same. It includes a bus voltage closed-loop control module and an inter-phase current balancing module.

[0093] Bus voltage closed-loop circuit measurement of low-voltage DC bus voltage U dc , with the given value U dcref The error is compared and fed into a proportional-integral regulator, which is then modulated in a closed loop and its output is fed into an H-bridge modulated high-frequency voltage amplitude U. high The bus voltage is adjusted by adjusting the amplitude of the high-frequency voltage.

[0094] The phase-to-phase current balancing circuit is used to prevent excessive or insufficient power transmission in a particular phase due to differences in three-phase parameters. For each phase, the bus capacitors of the rectifier bridge are connected in parallel, and the total branch current i of each phase after the parallel connection of the bus capacitors is measured. dca i dca i dca The average value of the DC output current of the three-phase main branch, i dc After subtracting from the 3-phase voltage, the compensation voltage amplitude for each phase of the high-frequency modulation voltage is calculated using a proportional controller. , , :

[0095]

[0096] In the formula, Kp is the proportional controller coefficient. The compensation voltage ( , , ) and high frequency voltage (U high The three-phase high-frequency modulated voltage is obtained by superposition. , , .

[0097] The low-frequency modulation voltage provided by the two control layers mentioned above ( , , ) and high-frequency modulated voltage ( , , They are directly added in the time domain to form the final reference voltage for each phase arm:

[0098]

[0099] The above are merely preferred embodiments of the present invention. The scope of protection of the present invention is not limited to the above embodiments. All technical solutions falling within the scope of the present invention's concept are within the scope of protection of the present invention. It should be noted that for those skilled in the art, any improvements and modifications made without departing from the principle of the present invention should also be considered within the scope of protection of the present invention.

Claims

1. An energy storage solid-state transformer, characterized in that, include: Three identical units are connected to phases A, B, and C of a three-phase medium-high voltage AC power grid and the positive and negative buses of a low-voltage DC power grid, respectively. Each of the units includes: a single-phase energy storage bridge arm, a grid-side filter reactor, an AC soft start circuit, a rectifier, and a bus capacitor; the single-phase energy storage bridge arm is composed of multiple energy storage sub-modules connected in series at their AC ports, one end of which is connected to the corresponding phase of the three-phase medium-high voltage AC grid through the grid-side filter reactor and the AC soft start circuit, and the other end is connected to the corresponding end of the single-phase energy storage bridge arm in the other two units to form a neutral point; The energy storage solid-state transformer also includes: a resonant capacitor, a resonant inductor, and a matrix transformer. The primary side branches of the resonant capacitor, resonant inductor, and matrix transformer, together with the three one-phase energy storage bridge arms, form a high-frequency circuit loop. The AC port of the rectifier is used to connect to the secondary AC port of the matrix transformer, and the DC port of the rectifier is connected to the bus capacitor. The positive and negative terminals of the bus capacitors of the three units are connected in parallel to form the low-voltage DC positive and negative bus ports of the energy storage solid-state transformer.

2. The energy storage solid-state transformer according to claim 1, characterized in that, In each of the units, the resonant capacitor, resonant inductor, primary branch of the matrix transformer, and AC port of the energy storage bridge arm are connected in series to form a high-frequency circuit loop.

3. The energy storage solid-state transformer according to claim 1, characterized in that, One end of each of the three resonant capacitors is connected to the other end, and the other end of each capacitor is connected to the connection point between the grid-side filter reactor and the energy storage bridge arm in the three units. The resonant inductor and the primary branch of the matrix transformer are connected in series, with one end connected to the connection point of the three resonant capacitors and the other end connected to the neutral point.

4. The energy storage solid-state transformer according to claim 1, characterized in that, The energy storage submodule includes an energy storage unit, a filter inductor, a filter capacitor, a DC soft start circuit, power devices, and a bypass switch. The four power devices and the filter capacitor form an AC-DC power conversion circuit, and the bypass switch is connected to its AC output port. The energy storage unit is connected in parallel with the filter inductor and the DC soft start circuit after being connected in series with the filter capacitor branch.

5. The energy storage solid-state transformer according to claim 1, characterized in that, The matrix transformer is composed of multiple discrete high-frequency transformers. The primary sides of the multiple high-frequency transformers are connected in series to form a high-voltage AC port, and the secondary sides of the multiple high-frequency transformers form multiple secondary AC ports.

6. The energy storage solid-state transformer according to claim 1, characterized in that, The rectifier is a diode uncontrolled rectifier bridge or a synchronous rectifier circuit.

7. A control method for an energy storage solid-state transformer according to any one of claims 1 to 6, characterized in that, Includes the following steps: Grid-side power control steps: control the active and reactive power injected into the three-phase medium-voltage AC grid, and generate the low-frequency modulation voltage of each of the three one-phase energy storage bridge arms by performing closed-loop control of the grid-connected current. Low-voltage DC bus voltage control steps: Control the low-voltage DC bus voltage, and generate the high-frequency modulation voltage of each of the three one-phase energy storage bridge arms by performing closed-loop control of the low-voltage DC bus voltage. Modulation voltage synthesis step: The low-frequency modulation voltage generated by the grid-side power control step and the high-frequency modulation voltage generated by the low-voltage DC bus voltage control step are added together to form the final modulation voltage of each of the three one-phase energy storage bridge arms.

8. The control method according to claim 7, characterized in that, The grid-side power control steps include: Energy storage charging and discharging power control sub-step: The active current command for grid-connected current control is calculated by summing the energy storage charging and discharging power command and the low-voltage DC output power. Energy storage phase-to-phase state of charge equalization sub-step: Calculate the amplitude and phase of the zero-sequence voltage based on the average state of charge of the energy storage sub-modules in the three units, and superimpose it onto the modulation voltage of each of the three one-phase energy storage bridge arms. The state of charge balancing sub-step within the energy storage phase: Based on the difference between the state of charge of each energy storage sub-module within each phase energy storage bridge arm and the average state of charge of the energy storage sub-modules within that phase energy storage bridge arm, calculate the equalization voltage superimposed on the modulation wave of each energy storage sub-module. Grid-connected current control sub-step: The low-frequency modulation voltage of each of the three one-phase energy storage bridge arms is generated by closed-loop control of the grid-connected current.

9. The control method according to claim 7, characterized in that, The low-voltage DC bus voltage control steps include: Bus voltage closed-loop control sub-step: Perform closed-loop control on the low-voltage DC bus voltage to generate high-frequency modulated voltages for each of the three one-phase energy storage bridge arms; Interphase current equalization sub-step: Calculate the compensation voltage of the high-frequency modulation voltage of each of the three units based on the difference between the DC-side output current of each of the three units and the average value of the DC-side output current of the three units. The modulation voltages output from the bus voltage closed-loop control sub-step and the phase-to-phase current equalization sub-step are added together to obtain the final high-frequency modulation voltages of the three units.

10. The control method according to claim 7, characterized in that, The low-frequency modulation voltage has a frequency of 50Hz or 60Hz, and the high-frequency modulation voltage has a frequency of 5kHz to 50kHz.

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