A solid state transformer and control signal generation method

Abstract

The application provides a solid-state transformer and a control signal generation method, relates to the technical field of solid-state transformers, and comprises a grid-side filter reactance, an AC / DC conversion submodule, an energy storage unit, a first DC conversion unit and a second DC conversion unit. The negative electrode port of the energy storage unit of the solid-state transformer is connected to a negative bus, the positive electrode port is connected to a primary positive bus and a secondary negative bus, and the secondary positive bus is connected to the positive bus of the AC / DC submodule, so that the secondary DC port of the second DC conversion unit only bears the difference between the intermediate DC bus voltage and the energy storage voltage, the second DC conversion unit no longer needs to process the full charging and discharging power of the energy storage unit, and the power conversion loss of the energy storage access DC bus link is effectively reduced.

Description

Technical Field

[0001] This invention relates to the field of solid-state transformer technology, and more particularly to a solid-state transformer and a method for generating control signals. Background Technology

[0002] With the increasing demand for computing power and the penetration of new energy sources, solid-state transformers are gradually becoming core devices connecting the power grid and computing loads. Integrating energy storage systems into solid-state transformers can provide bidirectional support for both the power grid and computing power. However, if the energy storage battery is directly connected in parallel to the intermediate DC bus, the wide voltage range variation during the charging and discharging process of the energy storage battery will cause significant fluctuations in the DC bus voltage, which is difficult to regulate, thus having a serious negative impact on the overall operating efficiency of the solid-state transformer.

[0003] To address the bus voltage fluctuation issue caused by direct parallel connection, existing solutions typically employ a bidirectional DC-DC converter to connect the energy storage unit to the intermediate DC bus of a solid-state transformer. This approach achieves matching between the energy storage voltage and the DC bus voltage by adding a full-power conversion stage, avoiding forced clamping of the intermediate DC bus voltage due to wide voltage variations at the energy storage end, thereby maintaining relatively stable operation of the DC bus voltage.

[0004] However, existing bidirectional DC-DC converters need to handle the entire charging and discharging power of the energy storage units during operation. This not only increases the hardware cost of the system but also leads to high conduction losses and multiple energy conversion losses, resulting in a significant decrease in the overall efficiency of the energy storage system. Therefore, in order to balance the stability of the DC bus voltage and the efficient operation of the system, how to reduce the power conversion losses at the point where energy storage is connected to the DC bus has become an urgent problem to be solved in this field. Summary of the Invention

[0005] This invention provides a solid-state transformer and a control signal generation method to solve the technical problem of how to reduce power conversion losses in the connection of energy storage to the DC bus.

[0006] This invention provides a solid-state transformer, comprising: a grid-side filter reactor, an AC / DC conversion submodule, an energy storage unit, a first DC conversion unit, and a second DC conversion unit; the AC ports of multiple AC / DC conversion submodules are connected in series to form a power bridge arm, and three power bridge arms are connected in series with the grid-side filter reactor and then connected to a three-phase power grid; the DC port of each AC / DC conversion submodule is respectively connected to one of the energy storage units, one of the first DC conversion units, and one of the second DC conversion units; the primary DC port of the first DC conversion unit is connected to the DC port of the AC / DC conversion submodule, and the secondary DC ports of multiple first DC conversion units converge to form a total DC port; the negative port of the energy storage unit is simultaneously connected to the negative bus of the DC port of the AC / DC conversion submodule and the negative bus of the primary DC port of the second DC conversion unit, and the positive port of the energy storage unit is simultaneously connected to the positive bus of the primary DC port of the second DC conversion unit and the negative bus of the secondary DC port of the second DC conversion unit, and the positive bus of the secondary DC port of the second DC conversion unit is connected to the positive bus of the DC port of the AC / DC conversion submodule.

[0007] According to a solid-state transformer provided by the present invention, the second DC-DC converter unit includes a primary-side full-bridge circuit, a secondary-side full-bridge circuit, a high-frequency isolation transformer, and a high-frequency reactor; the AC port of the primary-side full-bridge circuit and the high-frequency reactor together with the primary winding of the high-frequency isolation transformer form a primary-side circuit; the AC port of the secondary-side full-bridge circuit and the secondary winding of the high-frequency isolation transformer form a secondary-side circuit; the DC ports of the primary-side full-bridge circuit and the secondary-side full-bridge circuit are respectively connected to the primary-side DC port and the secondary-side DC port of the second DC-DC converter unit.

[0008] According to a solid-state transformer provided by the present invention, the second DC-DC converter unit includes an isolated DC-DC converter and a non-isolated DC-DC converter; the isolated DC-DC converter is used to realize electrical isolation and voltage transformation between the primary and secondary circuits; the non-isolated DC-DC converter is connected to the isolated DC-DC converter and is used to realize voltage regulation.

[0009] The present invention also provides a control signal generation method, applied to any of the solid-state transformers described herein, comprising: determining an intermediate DC bus voltage command value based on the total DC port voltage; generating a secondary DC port voltage command for each second DC-DC converter unit based on the intermediate DC bus voltage command value and the energy storage voltage of each energy storage unit; and generating a control signal for the solid-state transformer based on the secondary DC port voltage command, the grid-connected current, and the low-voltage DC bus voltage.

[0010] According to a control signal generation method provided by the present invention, a control signal for a solid-state transformer is generated based on the secondary-side DC port voltage command, the grid-connected current, and the low-voltage DC bus voltage. The method includes: generating a corresponding control signal for a second DC-DC converter unit based on the secondary-side DC port voltage command; generating a control signal for a first DC-DC converter unit based on the control signal for the second DC-DC converter unit and the low-voltage DC bus voltage; generating a control signal for an AC / DC converter submodule based on the control signal for the first DC-DC converter unit and the grid-connected current; and using the control signal for the second DC-DC converter unit, the control signal for the first DC-DC converter unit, and the control signal for the AC / DC converter submodule as the control signal for the solid-state transformer.

[0011] According to a control signal generation method provided by the present invention, a control signal for an AC / DC conversion submodule is generated based on the control signal of the first DC-DC converter unit and the grid-connected current, comprising: generating a grid-connected current command based on an active power reference value, a reactive power reference value and a grid-connected point voltage reference value; and generating the control signal for the AC / DC conversion submodule based on the grid-connected current command, the grid-connected current and the control signal of the first DC-DC converter unit.

[0012] According to a control signal generation method provided by the present invention, a grid-connected current command is generated based on an active power reference value, a reactive power reference value, and a grid connection point voltage reference value. The method includes: determining the power angle based on the active power reference value; determining the internal potential amplitude based on the power angle, the reactive power reference value, and the grid connection point voltage reference value; simulating virtual impedance characteristics based on the internal potential amplitude to generate a pre-limiting current command; and limiting the pre-limiting current command based on the current grid connection point voltage amplitude to generate the grid-connected current command.

[0013] According to a control signal generation method provided by the present invention, the method limits the current command before limiting based on the current grid connection point voltage amplitude to generate the grid connection current command, comprising: determining the vector amplitude of the current command before limiting; and, if the vector amplitude is greater than a preset limit value, scaling the current command before limiting based on the current grid connection point voltage amplitude to generate the grid connection current command.

[0014] According to a control signal generation method provided by the present invention, a control signal for an AC / DC converter submodule is generated based on a grid-connected current command, the grid-connected current, and a control signal from a first DC-DC converter unit. The method includes: generating an initial modulation voltage for the AC / DC converter submodule based on the grid-connected current command and the grid-connected current; generating a first modulation voltage superimposed with a zero-sequence voltage based on the initial modulation voltage and the state of charge of the energy storage units in the three phases; generating a second modulation voltage based on the first modulation voltage, the state of charge of the energy storage units corresponding to each AC / DC converter submodule in the power bridge arm, and the average state of charge; and generating the control signal for the AC / DC converter submodule based on the second modulation voltage and the control signal from the first DC-DC converter unit.

[0015] According to a control signal generation method provided by the present invention, a first modulation voltage superimposed with a zero-sequence voltage is generated based on an initial modulation voltage and the state of charge of the three-phase energy storage units. The method includes: determining the direct coordinate axis components and quadrature axis components of the state of charge of the three-phase energy storage units; determining the direct coordinate axis reference value and quadrature axis reference value of the zero-sequence voltage based on the target state of charge and the direct coordinate axis components and the quadrature axis components; generating a zero-sequence voltage through coordinate transformation based on the direct coordinate axis reference value and the quadrature axis reference value of the zero-sequence voltage; and superimposing the zero-sequence voltage onto the initial modulation voltage to generate the first modulation voltage superimposed with the zero-sequence voltage.

[0016] The solid-state transformer and control signal generation method provided by this invention connect the negative port of the energy storage unit to the negative bus, and the positive port to the primary positive bus and the secondary negative bus. The secondary positive bus is connected to the positive bus of the AC / DC submodule, so that the secondary DC port of the second DC conversion unit only bears the difference between the intermediate DC bus voltage and the energy storage voltage. The second DC conversion unit no longer needs to process all the charging and discharging power of the energy storage unit, effectively reducing the power conversion loss of the energy storage access DC bus link. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0018] Figure 1 This is a schematic diagram of the solid-state transformer provided by the present invention.

[0019] Figure 2 This is a schematic diagram of the structure of the second DC-DC converter unit provided by the present invention.

[0020] Figure 3 This is a flowchart illustrating the control signal generation method provided by the present invention. Detailed Implementation

[0021] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0022] The following is combined Figures 1 to 3 The present invention describes a solid-state transformer and a control signal generation method.

[0023] Figure 1 This is a schematic diagram of the solid-state transformer provided by the present invention. The solid-state transformer includes: a grid-side filter reactor, an AC / DC conversion submodule, an energy storage unit, a first DC conversion unit, and a second DC conversion unit. The AC ports of multiple AC / DC conversion submodules are connected in series to form a power bridge arm. The three power bridge arms are connected in series with the grid-side filter reactor and then connected to a three-phase power grid. The DC port of each AC / DC conversion submodule is connected to an energy storage unit, a first DC conversion unit, and a second DC conversion unit. The primary DC port of the first DC conversion unit is connected to the DC port of the AC / DC conversion submodule. The secondary DC ports of multiple first DC conversion units converge to form a total DC port. The negative port of the energy storage unit is simultaneously connected to the negative bus of the DC port of the AC / DC conversion submodule and the negative bus of the primary DC port of the second DC conversion unit. The positive port of the energy storage unit is simultaneously connected to the positive bus of the primary DC port of the second DC conversion unit and the negative bus of the secondary DC port of the second DC conversion unit. The positive bus of the secondary DC port of the second DC conversion unit is connected to the positive bus of the DC port of the AC / DC conversion submodule.

[0024] The grid-side filter reactor can be an air-core reactor, an iron-core reactor, or a filter inductor used to suppress switching ripple in the grid-connected current, and is used to filter out high-frequency harmonics to ensure the power quality of the grid connection.

[0025] The AC / DC conversion submodule may include a full-bridge topology circuit, a half-bridge topology circuit, a three-level full-bridge topology circuit, or a three-level half-bridge topology circuit, etc., to realize bidirectional power conversion between AC and DC.

[0026] Energy storage units can include energy storage carriers such as lithium batteries, supercapacitors, lead-acid batteries, or lithium supercapacitors, for storing and releasing electrical energy.

[0027] The first DC-DC converter unit may include topologies such as dual active bridge converters, LLC resonant converters, or CLLC resonant converters.

[0028] The second DC-DC converter unit can be an isolated or non-isolated DC / DC converter used to handle the power difference between the energy storage voltage and the bus voltage.

[0029] Multiple AC / DC conversion submodules have their AC ports connected in series to form a power arm, such as an A-phase, B-phase, or C-phase arm. Each of the three power arms is connected in series with a grid-side filter reactor and then connected to the three-phase power grid via a star or delta connection. The DC port of each AC / DC conversion submodule (connected in parallel with a DC capacitor to form an intermediate DC bus) is connected to an energy storage unit, a first DC conversion unit, and a second DC conversion unit.

[0030] The primary DC port of the first DC-DC converter unit is connected to the DC port of the AC-DC converter submodule. The secondary DC ports of multiple first DC-DC converter units are connected in parallel to form a total DC port, which is used to connect DC loads such as computing power loads. This total DC port is also called the low-voltage DC bus.

[0031] The negative port of the energy storage unit is connected to both the negative busbar of the DC port of the AC / DC converter submodule and the negative busbar of the primary DC port of the second DC converter unit. The positive port of the energy storage unit is connected to both the positive busbar of the primary DC port of the second DC converter unit and the negative busbar of the secondary DC port of the second DC converter unit. The positive busbar of the secondary DC port of the second DC converter unit is connected to the positive busbar of the DC port of the AC / DC converter submodule. For example, when the intermediate DC bus voltage is 800V and the energy storage unit voltage is 600V, due to the specific series-parallel connection described above, the primary side of the second DC converter unit bears 600V, while the secondary side only bears the difference between the intermediate bus voltage and the energy storage voltage, i.e., 800V - 600V = 200V.

[0032] A second DC-DC converter unit enables bidirectional power flow between the energy storage unit and the intermediate DC bus. This avoids directly connecting the energy storage unit in parallel to the DC bus, which would cause forced voltage clamping, and allows for flexible control of energy storage power and voltage matching by adjusting the second DC-DC converter unit. When the energy storage unit needs charging, energy is drawn from the intermediate DC bus, stepped down or stepped up by the second DC-DC converter unit, and then fed into the energy storage unit. When the energy storage unit needs to release energy to computing loads or the grid, its voltage is converted by the second DC-DC converter unit and then superimposed in series with its own voltage to jointly support the intermediate DC bus. This makes it more flexible and efficient for the first DC-DC converter unit to draw energy from the bus. This structure retains the bidirectional support capability of connecting the energy storage unit to the intermediate DC bus while avoiding the need for additional power conversion stages for voltage matching, significantly reducing conduction losses and conversion stages.

[0033] As can be seen from the above, the negative terminal of the solid-state transformer energy storage unit of the present invention is connected to the negative bus, the positive terminal is connected to the primary positive bus and the secondary negative bus, and the secondary positive bus is connected to the AC / DC submodule positive bus. This allows the secondary DC port of the second DC conversion unit to only bear the difference between the intermediate DC bus voltage and the energy storage voltage. The second DC conversion unit no longer needs to handle all the charging and discharging power of the energy storage unit, effectively reducing the power conversion loss of the energy storage access DC bus link.

[0034] In one embodiment, the second DC-DC converter unit may include a primary-side full-bridge circuit, a secondary-side full-bridge circuit, a high-frequency isolation transformer, and a high-frequency reactance; the AC port of the primary-side full-bridge circuit and the high-frequency reactance together with the primary winding of the high-frequency isolation transformer form a primary-side loop; the AC port of the secondary-side full-bridge circuit and the secondary winding of the high-frequency isolation transformer form a secondary-side loop; the DC ports of the primary-side full-bridge circuit and the secondary-side full-bridge circuit are respectively connected to the primary-side DC port and the secondary-side DC port of the second DC-DC converter unit.

[0035] The primary-side full-bridge circuit and the secondary-side full-bridge circuit can each include an H-bridge structure composed of four insulated-gate bipolar transistors (IGBTs) or metal-oxide-semiconductor field-effect transistors (MOSFETs), which are used to realize bidirectional power conversion between DC and AC.

[0036] A high-frequency isolation transformer can be a high-frequency transformer with a ferrite core, used to achieve electrical isolation between the primary and secondary sides and voltage proportional conversion.

[0037] High-frequency reactance Figure 2 Ls in the figure can be the leakage inductance of the transformer or an external high-frequency inductor, used to achieve energy transfer and resonance.

[0038] Combination Figure 2 The primary-side full-bridge circuit can be composed of switching transistors S1, S2, S3, and S4, with a first capacitor C1 connected in parallel to its DC port. The AC port of the primary-side full-bridge circuit and the high-frequency reactance Ls are connected in series with the primary winding of the high-frequency isolation transformer via wires to form the primary-side loop. The secondary-side full-bridge circuit is specifically composed of switching transistors S5, S6, S7, and S8, with a second capacitor C2 connected in parallel to its DC port. The AC port of the secondary-side full-bridge circuit and the secondary winding of the high-frequency isolation transformer are directly connected to form the secondary-side loop. The DC ports of the primary-side and secondary-side full-bridge circuits are respectively connected to the primary and secondary DC ports of the second DC-DC converter unit via cables. For example, the control chip sends a control signal to alternately turn on S1 / S4 and S2 / S3 to convert DC to high-frequency AC, which is coupled to the secondary side via the high-frequency isolation transformer, and then rectified back to DC by S5~S8. The purpose of this process is to flexibly and bidirectionally adjust the transmitted power.

[0039] This invention employs a structure comprising a primary-side full-bridge circuit, a secondary-side full-bridge circuit, and a high-frequency isolation transformer. This structure enables physical and electrical isolation between the primary and secondary circuits using the high-frequency isolation transformer, thereby improving the system's safety performance.

[0040] In one embodiment, the second DC-DC converter of the present invention may include an isolated DC-DC converter and a non-isolated DC-DC converter; the isolated DC-DC converter is used to realize electrical isolation between the primary and secondary circuits and voltage conversion; the non-isolated DC-DC converter is connected to the isolated DC-DC converter and is used to realize voltage regulation.

[0041] Isolated DC-DC converters may include power electronic conversion circuits with high-frequency transformers, such as LLC resonant circuits, CLLC resonant circuits, or dual active bridge circuits.

[0042] Non-isolated DC-DC converters can include transformerless conversion circuits such as multiphase Buck (step-down) interleaved circuits, Boost (step-up) circuits, or Buck-Boost (step-up / step-down) circuits.

[0043] Isolated DC-DC converters are configured to achieve electrical isolation and voltage transformation between the primary and secondary circuits. Non-isolated DC-DC converters are cascaded with isolated DC-DC converters via a DC bus and are configured to achieve voltage regulation. For example, when the energy storage battery voltage fluctuates within a wide range of 400V to 700V, a non-isolated DC-DC converter (such as a Buck circuit) operates to stabilize its wide-range voltage to 500V, after which the isolated DC-DC converter performs isolation and voltage transformation at a fixed ratio.

[0044] This invention divides the second DC-DC converter unit into an isolated DC-DC converter and a non-isolated DC-DC converter, so that the voltage conversion and voltage regulation functions are separated in the circuit. The voltage regulation is specifically implemented through the non-isolated DC-DC converter, which can better adapt to and smooth the wide range of terminal voltage changes during the charging and discharging of the energy storage unit.

[0045] This invention also provides a control signal generation method, applicable to any of the solid-state transformers described above, such as... Figure 3 As shown, it includes steps S01, S02 and S03.

[0046] Step S01: Determine the intermediate DC bus voltage command value based on the total DC port voltage.

[0047] The total DC port voltage is the actual measured voltage value of the low-voltage DC bus of the equipment. The intermediate DC bus voltage command value is the target voltage reference value that the intermediate DC capacitor is expected to maintain.

[0048] The intermediate DC bus voltage command value can be calculated by executing program instructions through a central processing unit or digital signal processor, by looking up a preset efficiency voltage mapping table or running a maximum efficiency point tracking optimization algorithm. For example, if the CPU reads that the total DC port voltage is 400V, it can calculate through an optimization algorithm that the loss of the first DC-DC converter unit is lowest when the intermediate DC bus voltage is 750V, thus determining the command value to be 750V.

[0049] Step S02: Generate the secondary DC port voltage command for each second DC converter unit based on the intermediate DC bus voltage command value and the energy storage voltage of each energy storage unit.

[0050] The energy storage voltage is the sampled value of the battery terminal voltage measured by a sensor. The secondary-side DC port voltage command is the target voltage applied to the secondary output of the second DC-DC converter unit.

[0051] For example, if the controller obtains an energy storage voltage of 600V, it subtracts 600V from the command value of 750V obtained in step S01 to generate a secondary DC port voltage command of 150V.

[0052] Step S03: Generate control signals for the solid-state transformer based on the secondary DC port voltage command, grid current, and low-voltage DC bus voltage.

[0053] The grid-connected current is the alternating current exchanged between the transformer and the power grid. Control signals may include pulse width modulation (PWM) signals used to drive the switching transistors.

[0054] Error amplification and compensation closed-loop calculations can be performed using multi-stage proportional-integral regulators, followed by the output of a PWM signal. For example, the system can collect the aforementioned electrical quantities in parallel and input them to each closed-loop control loop, jointly generating the switching signals for the underlying power devices.

[0055] As can be seen from the above, the control signal generation method of the present invention first determines the intermediate bus command based on the total DC port voltage, then generates the secondary port command in combination with the energy storage voltage, and finally generates the control signal in combination with the aforementioned command and grid current. This layer-by-layer recursive signal generation logic ensures the pertinence of the voltage command and improves the coordination of the overall control of the solid-state transformer.

[0056] According to a control signal generation method provided by the present invention, a control signal for a solid-state transformer is generated based on a secondary DC port voltage command, a grid-connected current, and a low-voltage DC bus voltage. The method includes: generating a control signal for a corresponding second DC-DC converter unit based on the secondary DC port voltage command; generating a control signal for a first DC-DC converter unit based on the control signal for the second DC-DC converter unit and the low-voltage DC bus voltage; generating a control signal for an AC / DC converter submodule based on the control signal for the first DC-DC converter unit and the grid-connected current; and using the control signals for the second DC-DC converter unit, the first DC-DC converter unit, and the AC / DC converter submodule as the control signals for the solid-state transformer.

[0057] The intermediate DC bus voltage control module can compare the secondary DC port voltage command with the actual sampled voltage and calculate the difference. This difference is then input to a PI controller, which outputs a phase shift angle or duty cycle to generate a control signal. For example, if the command is 150V but the actual voltage is 145V, the controller increases the duty cycle to generate a corresponding PWM signal.

[0058] The low-voltage DC bus voltage can be closed-loop controlled by the low-voltage DC bus voltage control module to maintain the load-side voltage stability, and the control state of the second conversion unit can be extracted as a feedforward quantity for compensation calculation to generate the corresponding control signal.

[0059] The grid-connected current can be controlled in a closed loop by the grid-side power control module, and a power decoupling operation can be performed to generate a switching signal by combining the extracted control signal from the first conversion unit.

[0060] This invention achieves independent adjustment of three different power conversion stages by generating control signals for the second DC-DC converter, the first DC-DC converter, and the AC-DC converter submodule respectively and summing them into an overall control signal, thus avoiding the mutual coupling of control variables.

[0061] In one embodiment, generating the control signal for the AC / DC conversion submodule based on the control signal of the first DC-DC converter unit and the grid-connected current may include: generating a grid-connected current command based on the active power reference value, the reactive power reference value, and the grid-connected point voltage reference value; and generating the control signal for the AC / DC conversion submodule based on the grid-connected current command, the grid-connected current, and the control signal of the first DC-DC converter unit.

[0062] The active power reference value is the target value of active power to be injected into the grid as issued by the superior dispatching system, for example, 1 MW. The reactive power reference value is the target value of reactive power used to support the grid voltage, for example, 0.2 Mvar. The grid connection point voltage reference value is the rated AC voltage amplitude of the grid, for example, 10 kV. The grid connection current command can be a reference quantity of the sinusoidal trajectory that the grid current is expected to follow.

[0063] The desired grid-connected current trajectory curve can be obtained by executing the swing equation and droop control equation calculations through the grid control module (an algorithm module that simulates the characteristics of a traditional generator). For example, when a step increase in the active power reference value is detected, the algorithm generates a current command with increased amplitude and leading phase through differential and integral operations. The solid-state transformer used in the grid control module, which provides bidirectional support for both the power grid and computing power, exhibits voltage source characteristics on the AC side, enabling the AC port to possess synchronous generator characteristics such as inertia support, primary frequency regulation, and damped oscillation.

[0064] The error between the grid-connected current command and the actual grid-connected current can be calculated through the current inner loop controller. The control signal of the first DC-DC converter unit is added to the control loop as a power feedforward to eliminate the influence of DC-side fluctuations on the AC side. After PI calculation, the output modulation voltage generates the final control signal.

[0065] This invention first generates a grid-connected current command based on active power, reactive power, and grid connection point voltage reference values. Then, it combines the grid-connected current and the control signal of the first DC-DC converter unit to generate a control signal for the AC-DC converter submodule. This enables the control signal on the AC side to respond to both the power scheduling command and the power status on the DC side, thereby improving the linkage between power regulation on the grid side and the computing power side.

[0066] In one embodiment, generating a grid-connected current command based on active power reference values, reactive power reference values, and grid connection point voltage reference values ​​may include: determining the power angle based on the active power reference values; determining the internal potential amplitude based on the power angle, reactive power reference values, and grid connection point voltage reference values; simulating virtual impedance characteristics based on the internal potential amplitude to generate a pre-limiting current command; and limiting the pre-limiting current command based on the current grid connection point voltage amplitude to generate a grid-connected current command.

[0067] The power angle is the angle between the internal electromotive force of the virtual generator and the grid voltage. The internal electromotive force amplitude is the magnitude of the electromotive force induced in the stator windings of the simulated synchronous generator. The current command before limiting is the original current reference vector before it has been clipped by the overcurrent protection algorithm.

[0068] The power angle can be obtained by integral calculation using the active power-frequency characteristic control module, which includes a primary frequency regulation stage, a virtual inertia stage, and a damping control stage. This allows for the adjustment of the primary frequency regulation characteristic, inertia response characteristic, and frequency damping characteristic of the solid-state transformer used for bidirectional support of the power grid and computing power. The specific swing equation formula is as follows:

[0069] ;

[0070] ;

[0071] In the above formula, P refP is the reference value for active power, and D is the instantaneous active power output. p ω is the droop coefficient of active power-frequency, used to adjust the primary frequency regulation characteristics; J is the inertia time constant of the active power loop, used to adjust the virtual inertia; ω is the angular frequency generated by the active power loop, and ωn is the rated angular frequency; θ is the power angle determined by integral calculation.

[0072] The internal potential amplitude can be calculated by executing the reactive power-voltage characteristic control module using the reactive voltage droop and internal potential adjustment equations. The specific formula is:

[0073] ;

[0074] In the above formula, Q ref and V ref These are the reference values ​​for reactive power and grid connection point voltage, respectively; Q and V are the instantaneous output reactive power and grid connection point voltage, respectively; D q It is the droop coefficient of reactive power-voltage; K is the integral coefficient of the reactive power loop; E is the calculated internal potential amplitude.

[0075] The virtual admittance control module can be used to establish a virtual resistance and inductance model between the internal potential and the grid connection point to perform calculations and obtain the current command before limiting. The specific circuit equation matrix formula is as follows:

[0076] ;

[0077] In the above formula, Rv and Lv represent virtual resistance and virtual inductance, respectively. and For the corresponding voltage command, vd and vq are the grid-connected voltages, and s is the Laplace operator. and These represent the d-axis (direct coordinate axis) and q-axis (intersecting axis) components of the output pre-limiting current command, respectively.

[0078] Excessively large command values ​​can be truncated or scaled using the limiting and clipping circuit or threshold judgment software algorithm in the adaptive current limiting control module to generate grid-connected current commands.

[0079] This invention determines the power angle based on the active power reference value, then determines the internal potential amplitude based on the power angle, and generates current commands by simulating virtual impedance characteristics, giving the equipment inertia and impedance characteristics similar to a synchronous generator, thereby enhancing the equipment's ability to actively support the AC power grid.

[0080] In one embodiment, limiting the current command before limiting based on the current grid connection point voltage amplitude to generate a grid-connected current command may include: determining the vector amplitude of the current command before limiting; and if the vector amplitude is greater than a preset limit value, scaling the current command before limiting by the same amplitude based on the current grid connection point voltage amplitude to generate a grid-connected current command.

[0081] The vector magnitude of the current command before limiting is the magnitude of the composite current vector in the dq rotating coordinate system. The preset limit value is a maximum safe current threshold set in advance to protect semiconductor switching devices from burnout. Equal amplitude scaling means keeping the angle of the complex vector unchanged, only reducing its length proportionally to the threshold boundary.

[0082] The vector magnitude can be obtained by extracting the square root of the sum of the squares of the d-axis and q-axis current commands. For example, the previously calculated... and The calculated vector magnitude is .

[0083] An adaptive ring current limiting algorithm can be used to divide the d-axis and q-axis commands proportionally by the magnitude and multiply by the threshold when the current exceeds the threshold Imax. Specifically, this generates the grid-connected current command (positive sequence current reference value i). dref and i qref The algorithm formula is:

[0084] ;

[0085] Among them, i dqref It covers the d-axis positive sequence current reference value i dref and q-axis positive sequence current reference value i qref When calculating the d-axis current reference value i after limiting... dref hour, It will be specifically substituted as When calculating the q-axis current reference value i after limiting... qref hour, It will be specifically substituted as .

[0086] This invention performs equal scaling on the current command when the vector amplitude is greater than a preset limit value. This can limit the maximum current peak value to protect the device hardware while maintaining the original vector phase angle of the current command, thus improving the stability of the system under overload conditions.

[0087] In one embodiment, generating control signals for the AC / DC conversion submodule based on the grid-connected current command, the grid-connected current, and the control signal of the first DC-DC converter unit may include: generating an initial modulation voltage for the AC / DC conversion submodule based on the grid-connected current command and the grid-connected current; generating a first modulation voltage superimposed with a zero-sequence voltage based on the initial modulation voltage and the state of charge of the three-phase energy storage units; generating a second modulation voltage based on the first modulation voltage, the state of charge of the energy storage units corresponding to each AC / DC conversion submodule in the power bridge arm, and the average state of charge; and generating control signals for the AC / DC conversion submodule based on the second modulation voltage and the control signal of the first DC-DC converter unit.

[0088] The initial modulation voltage is a drive reference waveform generated purely to track the grid-connected current. The first modulation voltage is a waveform with a bias component added to address the uneven battery charge between phases (A, B, and C). The second modulation voltage is the final waveform after further correction to address the uneven charge charge between multiple sub-modules connected in series within the same bridge arm.

[0089] The reference voltage dq-axis component can be output via a PI regulator through a current inner loop control module, based on Kirchhoff's voltage law equation. The generation formula is:

[0090] ;

[0091] In the formula, , This is the initial modulation voltage (inner loop output reference voltage); v gd v gq dq-axis component of the grid voltage; w is the grid angular frequency; L is the filter inductance; K p K i These are the proportional and integral coefficients, respectively; s is the Laplace operator; and For grid-connected current command; i d i q This is the actual value of the grid-connected current. , After the inverse Parker transformation, it becomes the initial AC modulation wave.

[0092] A set of common-mode zero-sequence voltage signals calculated based on the three-phase state-of-charge differences can be added to the initial modulation wave to obtain the first modulation voltage. For example, the calculated 10V zero-sequence voltage can be simultaneously superimposed on the three-phase modulation waves A, B, and C.

[0093] The second modulation voltage can be obtained by using the deviation between the individual SOC of a submodule and the average SOC of the bridge arm through the energy storage phase equalization controller, and then superimposing this value onto the first modulation voltage. Taking the k-th submodule of the j-phase bridge arm as an example, the formula for its superimposed equalization voltage reference value is as follows:

[0094] ;

[0095] Where, Δu pjk For equalization voltage; K is the equalization coefficient; SOC pj The average SOC of phase j bridge arm; SOC pjk For this individual's SOC; i pj This represents the bridge arm current.

[0096] The second modulation voltage can be compared with the triangular carrier wave using modulation algorithms such as carrier phase shifting (CPS-PWM) to generate the final drive pulse for the switching transistors of each submodule.

[0097] This invention combines the three-phase energy storage state of charge and the average state of charge of each sub-module within the bridge arm, and sequentially superimposes the first and second modulation voltages on the initial modulation voltage to achieve fine-grained equalization control of the state of charge between the overall multiphase and between the modules within the same local bridge arm.

[0098] In one embodiment, generating a first modulation voltage superimposed with a zero-sequence voltage based on an initial modulation voltage and the state of charge of the three-phase energy storage units may include: determining the direct coordinate axis components and cross-axis components of the state of charge of the three-phase energy storage units; determining the direct coordinate axis reference value and cross-axis reference value of the zero-sequence voltage based on the target state of charge and the direct coordinate axis components and cross-axis components; generating a zero-sequence voltage through coordinate transformation based on the direct coordinate axis reference value and cross-axis reference value of the zero-sequence voltage; and superimposing the zero-sequence voltage onto the initial modulation voltage to generate the first modulation voltage superimposed with the zero-sequence voltage.

[0099] The direct coordinate axis components and the quadrature axis components are the two DC projection components representing the original three-phase AC physical quantities in a two-phase rotating orthogonal coordinate system (dq coordinate system). The zero-sequence voltage is the three-phase common-mode voltage component in a three-phase system with identical amplitude and phase, used to inject neutral point movement.

[0100] The Park coordinate transformation algorithm can be used to map the collected static three-phase abc scalar of charge state into the direct coordinate axis components (d-axis component) in a rotating coordinate system. ) and cross-axis components (q-axis components) ).

[0101] The direct axis reference value and the quadrature axis reference value can be calculated by subtracting the target reference value from the feedback component and inputting this result into a PI proportional-integral controller configured with a specific gain. The specific calculation formula is as follows:

[0102] ;

[0103] In the formula, and That is, the determined direct coordinate axis reference value and quadrature axis reference value of the zero-sequence voltage; SOC ref The desired target state of charge.

[0104] This can be achieved by performing the inverse Park coordinate transformation matrix operation. and Reconverted to a three-phase time-domain AC signal in a stationary coordinate system .

[0105] The addition instruction within the digital signal controller can be used to... It is added to the three-phase initial modulation waveform output by the inner loop controller.

[0106] This invention determines the direct and quadrature axes of the three-phase state of charge in a rotating coordinate system, generates corresponding reference values ​​based on these, and finally generates and superimposes the zero-sequence voltage through coordinate transformation. This method of converting the three-phase AC state into DC quadrature components for processing effectively reduces the computational complexity of zero-sequence voltage signal processing and closed-loop control.

[0107] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention, and not to limit them; although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features; and these modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A solid-state transformer, characterized in that, include: Grid-side filter reactor, AC / DC conversion submodule, energy storage unit, first DC conversion unit and second DC conversion unit; The AC ports of multiple AC / DC conversion submodules are connected in series to form a power bridge arm, and the three power bridge arms are connected in series with the grid-side filter reactor and then connected to the three-phase power grid. The DC port of each of the AC / DC conversion submodules is connected to one of the energy storage units, one of the first DC conversion units, and one of the second DC conversion units, respectively. The primary DC port of the first DC-DC converter unit is connected to the DC port of the AC-DC converter submodule, and the secondary DC ports of multiple first DC-DC converter units are combined to form a total DC port. The negative port of the energy storage unit is connected to both the negative bus of the DC port of the AC / DC converter submodule and the negative bus of the primary DC port of the second DC converter unit. The positive port of the energy storage unit is connected to both the positive bus of the primary DC port of the second DC converter unit and the negative bus of the secondary DC port of the second DC converter unit. The positive bus of the secondary DC port of the second DC converter unit is connected to the positive bus of the DC port of the AC / DC converter submodule.

2. The solid-state transformer according to claim 1, characterized in that, The second DC-DC converter unit includes a primary-side full-bridge circuit, a secondary-side full-bridge circuit, a high-frequency isolation transformer, and a high-frequency reactor; The AC port of the primary-side full-bridge circuit and the high-frequency reactance together with the primary-side winding of the high-frequency isolation transformer form a primary-side circuit. The AC port of the secondary full-bridge circuit and the secondary winding of the high-frequency isolation transformer constitute the secondary circuit. The DC ports of the primary-side full-bridge circuit and the secondary-side full-bridge circuit are respectively connected to the primary-side DC port and the secondary-side DC port of the second DC-DC converter unit.

3. The solid-state transformer according to claim 1, characterized in that, The second DC-DC converter unit includes an isolated DC-DC converter and a non-isolated DC-DC converter; The isolated DC-DC converter is used to achieve electrical isolation between the primary and secondary circuits, as well as voltage transformation. The non-isolated DC-DC converter is connected to the isolated DC-DC converter to achieve voltage regulation.

4. A control signal generation method, applied to a solid-state transformer as described in any one of claims 1 to 3, characterized in that, include: The intermediate DC bus voltage command value is determined based on the total DC port voltage. Based on the intermediate DC bus voltage command value and the energy storage voltage of each energy storage unit, the secondary DC port voltage command of each second DC converter unit is generated. The control signal for the solid-state transformer is generated based on the secondary DC port voltage command, grid current, and low-voltage DC bus voltage.

5. The control signal generation method according to claim 4, characterized in that, Based on the secondary DC port voltage command, grid current, and low-voltage DC bus voltage, control signals for the solid-state transformer are generated, including: The control signal for the second DC-DC converter unit is generated according to the secondary DC port voltage command. Based on the control signal of the second DC-DC converter unit and the low-voltage DC bus voltage, a control signal for the first DC-DC converter unit is generated. Based on the control signal of the first DC-DC converter unit and the grid-connected current, the control signal of the AC-DC converter submodule is generated; The control signals of the second DC-DC converter, the first DC-DC converter, and the AC-DC converter submodule are used as the control signals of the solid-state transformer.

6. The control signal generation method according to claim 5, characterized in that, Based on the control signal of the first DC-DC converter unit and the grid-connected current, control signals for the AC-DC converter submodule are generated, including: The grid connection current command is generated based on the active power reference value, reactive power reference value and grid connection point voltage reference value; The control signal for the AC / DC conversion submodule is generated based on the grid-connected current command, the grid-connected current, and the control signal of the first DC-DC converter unit.

7. The control signal generation method according to claim 6, characterized in that, The grid-connected current command is generated based on the active power reference value, reactive power reference value, and grid connection point voltage reference value, including: The power angle is determined based on the aforementioned active power reference value; The internal potential amplitude is determined based on the power angle, the reactive power reference value, and the grid connection point voltage reference value. Based on the internal potential amplitude, simulate the virtual impedance characteristics to generate a current command before limiting. The current command before limiting is limited based on the current voltage amplitude at the current grid connection point, and the grid connection current command is generated.

8. The control signal generation method according to claim 7, characterized in that, The current command before limiting is limited based on the current grid connection point voltage amplitude, and the grid connection current command is generated, including: Determine the vector magnitude of the current command before limiting; If the vector amplitude is greater than the preset limit value, the current command before the limit is scaled by the same amplitude according to the current grid connection point voltage amplitude to generate the grid connection current command.

9. The control signal generation method according to claim 6, characterized in that, Based on the grid-connected current command, the grid-connected current, and the control signal of the first DC-DC converter unit, the control signal for the AC-DC converter submodule is generated, including: The initial modulation voltage of the AC / DC converter submodule is generated based on the grid-connected current command and the grid-connected current. Based on the initial modulation voltage and the state of charge of the three-phase energy storage units, a first modulation voltage superimposed with the zero-sequence voltage is generated; A second modulation voltage is generated based on the first modulation voltage, the state of charge and average state of charge of the energy storage unit corresponding to each AC / DC conversion submodule in the power bridge arm; The control signal for the AC / DC conversion submodule is generated based on the second modulation voltage and the control signal of the first DC-DC converter unit.

10. The control signal generation method according to claim 9, characterized in that, Based on the initial modulation voltage and the state of charge of the three-phase energy storage units, a first modulation voltage superimposed with the zero-sequence voltage is generated, including: Determine the direct coordinate axis components and quadrature axis components of the state of charge of the three-phase energy storage unit; Based on the target state of charge and the direct coordinate axis components and the cross-axis components, determine the direct coordinate axis reference value and the cross-axis reference value of the zero-sequence voltage; The zero-sequence voltage is generated by coordinate transformation based on the direct coordinate axis reference value and the cross-axis reference value of the zero-sequence voltage. The zero-sequence voltage is superimposed on the initial modulation voltage to generate the first modulation voltage after superimposing the zero-sequence voltage.

Images