A Deeply Grid-Based DC Transmission System Based on Hybrid Energy Storage

Abstract

This invention discloses a deep-grid DC transmission system based on hybrid energy storage, belonging to the field of DC transmission technology for power systems. It includes multiple deep-grid converter stations, multiple DC lines, and a coordination control module. The deep-grid converter stations adopt a modular multilevel converter topology. The hybrid energy storage units within their sub-modules are connected to the converter arms via passive-active co-coupling circuits. A dual-port decoupling control strategy is employed, utilizing the energy storage in the sub-modules to process the unbalanced power on both the AC and DC sides, enabling each converter station to simultaneously possess AC-side virtual synchronous machine characteristics and DC-side grid voltage support capabilities. The coordination control module controls key parameters of the grid-connected converters to achieve system-level coordinated grid construction. Combined with a superconducting current limiter and circuit breaker connected in series on the DC side, the system achieves fault current limiting, non-blocking ride-through, and rapid isolation. This invention significantly improves the transient stability and battery life of high-proportion renewable energy grids.

Description

Technical Field

[0001] This invention relates to the field of DC power transmission technology, specifically to a deep grid-type DC power transmission system based on hybrid energy storage. Background Technology

[0002] Currently, the power system is transforming into a new type of power system dominated by new energy sources. High-voltage direct current (HVDC) transmission technology based on voltage source converter stations has become the preferred choice for offshore wind power grid connection and inter-regional interconnection due to its flexible control and lack of commutation failure risk. However, traditional HVDC transmission systems mostly employ grid-following control, relying on phase-locked loops (PLLs) to follow the grid voltage phase. Under weak grid conditions (short-circuit ratio SCR < 1.5), the nonlinear dynamic characteristics of the PLL can easily induce system oscillations. Furthermore, traditional HVDC transmission systems cannot provide physical inertia, resulting in excessively large frequency changes under disturbances, threatening system safety. To address this technical problem, grid-based control technology has emerged.

[0003] However, existing grid-based technologies face the following technical challenges in engineering applications: 1. AC / DC dynamic coupling risk: Traditional grid control typically utilizes the electrostatic energy of DC capacitors to simulate inertia, meaning that frequency fluctuations on the AC side are directly converted into voltage fluctuations on the DC side. In multi-terminal DC systems, this strong coupling can easily trigger voltage oscillations throughout the system, and even cause overvoltage / undervoltage protection malfunctions at non-faulty sites. 2. Lifespan challenges of integrated energy storage: To provide continuous inertial support, energy storage must be integrated. Existing solutions attempt to directly connect batteries in parallel within converter station submodules. However, the converter station arm current contains significant second-harmonic power ripples. If batteries are directly connected, this ripple current will flow through the battery's internal resistance, leading to severe micro-charge-discharge cycles and heat generation, drastically shortening battery life. Existing active filtering solutions (adding additional DC / DC converters or switching devices) are effective, but significantly increase cost and control complexity. 3. DC fault protection dilemma: Current protection strategies largely rely on converter station blocking and AC-side circuit breaker tripping. This would cause the AC system to lose voltage support momentarily during a fault, which could easily lead to large-scale disconnection of wind turbines from the grid, failing to meet the requirements for fault ride-through.

[0004] To address the aforementioned technical problems, this invention proposes a "deep networking" architecture that deeply integrates physical topology and control strategy. This architecture solves the problems by using passive-active cooperative coupling circuits and dual-port decoupling control. Summary of the Invention

[0005] Therefore, the present invention provides a deep grid-type DC transmission system based on hybrid energy storage to solve the problems existing in the prior art.

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

[0007] A deep-grid DC transmission system based on hybrid energy storage is disclosed. The DC transmission system is a multi-terminal DC grid used to transmit electrical energy from multiple AC grids to multiple user devices. The DC transmission system includes multiple deep-grid converter stations, multiple DC lines, and a coordination control module. The multiple deep-grid converter stations are connected between the multiple AC grids and the multiple user devices, and the multiple deep-grid converter stations are connected to each other through multiple DC lines. The multiple deep-grid converter stations communicate with each other through the coordination control module.

[0008] The deep grid-type converter stations all adopt a modular multilevel converter topology, and their bridge arms are composed of multiple cascaded sub-modules. The sub-modules include a hybrid energy storage unit and a passive-active co-coupling circuit. The hybrid energy storage unit is connected to the converter station bridge arm through the passive-active co-coupling circuit. The deep grid-type converter station is equipped with a dual-port decoupling controller, which is configured on the AC port and DC port of the converter station. The dual-port decoupling controller enables the AC port of the converter station to perform virtual synchronous generator control to provide voltage source support, and enables the DC port of the converter station to independently perform DC voltage or power control. The instantaneous power difference between the AC port and the DC port is processed through the hybrid energy storage unit.

[0009] Each of the aforementioned DC lines is equipped with a superconducting fault current limiter (SFCL) and a DC circuit breaker, which are connected in series.

[0010] Furthermore, the hybrid energy storage unit includes a first energy storage unit and a second energy storage unit. The power density of the first energy storage unit is higher than that of the second energy storage unit, and the energy density of the second energy storage unit is higher than that of the first energy storage unit. The first energy storage unit is connected to the converter station arm through a half-bridge or full-bridge converter, and the second energy storage unit is connected in parallel with the first energy storage unit through a unidirectional conducting device.

[0011] The polarity configuration of the unidirectional conducting device is such that the unidirectional conducting device is in a conducting state only when the terminal voltage of the first energy storage unit is lower than the terminal voltage of the second energy storage unit and the difference exceeds the conduction voltage drop of the unidirectional conducting device, allowing the second energy storage unit to discharge to the outside.

[0012] Furthermore, the first energy storage unit is a supercapacitor module, the second energy storage unit is a battery module, the unidirectional conducting device is a power diode, and the half-bridge or full-bridge converter controls the voltage of the supercapacitor module so that it is higher than the open-circuit voltage of the battery module under normal operating conditions, so that the power diode is in a reverse cutoff state.

[0013] When the AC power grid experiences a frequency drop or inertial response demand, the supercapacitor module is controlled to discharge rapidly, causing its voltage to drop and naturally triggering the power diode to conduct.

[0014] Furthermore, the dual-port decoupling controller includes an AC-side grid construction control module, a DC-side voltage control module, and an energy storage management module. The AC-side grid construction control module uses an AC grid construction strategy to calculate the desired output voltage, the DC-side voltage control module uses a DC grid construction strategy to calculate the DC voltage command value, and the energy storage management module uses an energy storage control strategy to calculate the supercapacitor voltage reference value.

[0015] Based on the expected output voltage, DC voltage command value, supercapacitor voltage reference value, and voltage obtained from converter station circulating current suppression, combined with the converter station's nearest level approximation strategy, the number of sub-modules that are turned on in the upper and lower bridge arms is obtained to ensure that the capacitor voltage of the sub-modules is balanced.

[0016] Furthermore, the superconducting current limiter and DC circuit breaker configured within the DC line are used for protection and recovery of DC transmission network faults. The protection and recovery strategy for DC transmission network faults is as follows:

[0017] Fault current limiting stage: When a short circuit fault occurs in the DC line, and the DC grid of the deep grid-type converter station causes the fault current rise rate to exceed the threshold, the physical characteristics of the superconducting current limiter change from the superconducting state to the high-resistivity state, thus limiting the peak fault current.

[0018] Fault ride-through phase: During the fault clearing process, the deep grid-type converter station is not blocked, but maintains the AC side voltage source characteristics and controls the hybrid energy storage unit to absorb the active power that should have been delivered to the DC side, thereby preventing the connected wind farm or AC grid from disconnecting from the grid.

[0019] Fault isolation phase: After the fault current is limited by the superconducting current limiter, the DC circuit breaker is controlled to disconnect the faulty line;

[0020] Recovery phase: After the fault is cleared, the hybrid energy storage unit releases energy to assist in the establishment of DC voltage, while waiting for the superconducting current limiter to cool down and return to the superconducting state.

[0021] Furthermore, the coordination control module is configured to execute an adaptive parameter adjustment strategy based on the state of charge and small disturbance stability margin, wherein the adaptive parameter adjustment strategy is as follows:

[0022] Real-time acquisition of the state of charge and small disturbance stability margin of the hybrid energy storage units in each converter station;

[0023] Based on the state of charge and small disturbance stability margin, the virtual inertial time constant and damping coefficient of each converter station are dynamically adjusted through a preset nonlinear function.

[0024] When the state of charge of a converter station falls below the preset lower limit warning value, the coordination control module sends a command to the converter station to smoothly switch its control mode from grid-based to grid-following mode and reduce its active power reference value to restore energy storage.

[0025] Furthermore, the nonlinear function is configured as follows:

[0026] When the state of charge is within the normal range, the virtual inertial time constant is positively correlated with the state of charge, allowing converter stations with sufficient energy storage to undertake more inertial support tasks; when the small disturbance stability margin reaches the lower limit, the damping coefficient is increased to suppress power oscillations.

[0027] Furthermore, the AC network construction strategy adopted by the AC-side network construction control module is as follows:

[0028] Based on the active power deviation of the deep grid-type converter station, and combined with the active power control of the virtual synchronous generator, the phase angle of the AC bus voltage of the sending-end AC grid is determined.

[0029] Based on the reactive power deviation of the deep grid converter station, and in conjunction with reactive power control, the reference value of the control voltage is determined.

[0030] Based on the reference value of the control voltage, combined with AC voltage control and preset current control, the d-axis output voltage and q-axis output voltage of the converter station are generated;

[0031] Based on the d-axis output voltage, q-axis output voltage, and AC bus voltage phase angle of the converter station, an inverse Park transformation is performed to obtain the desired output voltage of the converter station.

[0032] Furthermore, the DC grid construction strategy adopted by the DC-side voltage control module is as follows: based on the deviation between the measured DC voltage value and the DC voltage reference value, a DC current command value is generated, and a DC voltage command value is generated through the DC current control inner loop.

[0033] Furthermore, the energy storage control strategy adopted by the energy storage management module is as follows: generate a supercapacitor voltage reference value based on the deviation between the energy storage energy of the converter station and the reference value.

[0034] The present invention has the following advantages:

[0035] This invention proposes a deep-grid DC transmission system based on hybrid energy storage. It integrates supercapacitors and battery energy storage units within the sub-modules of a deep-grid converter station, and achieves differentiated and coordinated responses between the two types of energy storage through passive components such as unidirectional diodes. This deeply embeds the energy storage units into the converter topology and control system, enabling each converter station to simultaneously possess AC-side virtual synchronous machine characteristics and DC-side grid voltage support capabilities. Through a dual-port grid control strategy and power balancing involving energy storage, effective decoupling of AC and DC dynamic processes is achieved. By configuring superconducting current limiters and DC circuit breakers in series on the DC side and coordinating them with grid control, rapid isolation of DC faults and rapid system recovery are achieved under the condition of limiting fault current. Through a coordinated control module, the virtual inertia, damping, and other parameters of multiple converter stations are uniformly adjusted, enabling coordinated grid operation of multi-terminal DC systems under conditions of high-proportion renewable energy access and weak grids. In summary, this invention, while maintaining the advantages of large-capacity and long-distance transmission of HVDC, significantly improves the transient stability and operational reliability of AC / DC hybrid power grids, and has promising engineering application prospects. Attached Figure Description

[0036] To more clearly illustrate the embodiments of the present invention or the technical solutions in 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 merely exemplary, and those skilled in the art can derive other embodiments based on the provided drawings without creative effort.

[0037] The structures, proportions, sizes, etc. illustrated in this specification are only for the purpose of assisting those skilled in the art in understanding and reading the content disclosed herein, and are not intended to limit the conditions under which the present invention can be implemented. Therefore, they have no substantial technical significance. Any modifications to the structure, changes in the proportions, or adjustments to the size, without affecting the effects and objectives that the present invention can produce, should still fall within the scope of the technical content disclosed in the present invention.

[0038] Figure 1 A schematic diagram of the topology of a deep grid-type DC transmission system provided by the present invention;

[0039] Figure 2 A schematic diagram of the topology of the deep grid-type converter station provided by the present invention;

[0040] Figure 3 The control strategy block diagram of the dual-port decoupling controller provided by the present invention;

[0041] Figure 4 A block diagram of the adaptive parameter adjustment strategy for the coordination control module provided by the present invention;

[0042] In the picture:

[0043] 1 First AC power grid; 2 Second AC power grid; 3 DC power grid; 31 First converter station; 32 Second converter station; 33 Third converter station; 34 Fourth converter station; 36 Superconducting current limiter; 37 DC circuit breaker; 38 DC line; 4 First wind farm; 5 Second wind farm; 6 Energy island. Detailed Implementation

[0044] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some, not all, of the embodiments of the present invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0045] Example 1

[0046] A deep-grid DC transmission system based on hybrid energy storage is disclosed. The DC transmission system is a multi-terminal DC grid used to transmit electrical energy from multiple AC grids to multiple user devices. The DC transmission system includes multiple deep-grid converter stations, multiple DC lines, and a coordination control module. The multiple deep-grid converter stations are connected between multiple AC grids and multiple user devices, and the multiple deep-grid converter stations are connected to each other through multiple DC lines. The multiple deep-grid converter stations communicate with each other through the coordination control module.

[0047] User facilities include wind farms and energy islands, with the energy islands comprising photovoltaic power generation units and hydrogen production loads. Deep grid-type converter stations are configured to establish the system's frequency and voltage as the main voltage source in the energy island's isolated operation mode, and to balance the random fluctuations of wind and solar power generation with the electricity demand of hydrogen production loads using hybrid energy storage units.

[0048] Multiple DC lines are equipped with superconducting current limiters and DC circuit breakers, which are connected in series to limit fault current at different fault locations.

[0049] like Figure 1 The diagram illustrates two AC power grids: a first AC power grid 1 and a second AC power grid 2. The first AC power grid 1 can be a large power grid or a regional power grid connected to a grid-connected wind farm, while the second AC power grid 2 can be another regional power grid or a load center power grid. This invention is not limited to the case of two AC power grids; in other embodiments, it may also include three or more AC power grids.

[0050] DC grid 3 is a multi-terminal DC transmission network, which includes multiple deep-networked converter stations, namely the first converter station 31, the second converter station 32, the third converter station 33 and the fourth converter station 34, several DC lines 38, and superconducting current limiters 36 and DC circuit breakers 37 configured on each DC line 38. Figure 1 In the process, the first converter station 31, the second converter station 32, the third converter station 33 and the fourth converter station 34 are connected to the corresponding AC power grid or wind farm through transformers, and their DC sides are interconnected through DC lines 38 and superconducting current limiters 36 to form a three-terminal or four-terminal DC network.

[0051] The first converter station 31 and the second converter station 32 are connected to the first AC power grid 1 and the second AC power grid 2 respectively via transformers; the third converter station 33 is connected to the first wind farm 4; the fourth converter station 34 is connected to the second wind farm 5 and the energy island 6. The energy island 6 may include photovoltaic power generation (PV), hydrogen production units (H2), and data center loads, thus forming a multi-terminal DC transmission system structure integrating "multiple power sources, multiple loads, and multiple DC terminals".

[0052] On the DC side, the first converter station 31 is connected to the third converter station 33 via the upper DC line 38. A superconducting current limiter 36 and a DC circuit breaker 37 are connected in series in this branch. The second converter station 32 is connected to the fourth converter station 34 via the lower DC line 38. A superconducting current limiter 36 and a DC circuit breaker 37 are also connected in series in this branch. The third converter station 33 and the fourth converter station 34 are also connected by an oblique DC line 38. Multiple superconducting current limiters 36 are installed along this branch to limit the fault current at different fault locations.

[0053] With the above structural arrangement, the system of the present invention can realize power transmission between multiple AC power grids and multiple wind farms, and realize flexible power flow distribution and fault detour paths through a multi-branch DC structure.

[0054] Example 2

[0055] Deeply networked converter stations all adopt a modular multilevel converter (MMC) topology. Its bridge arm consists of multiple cascaded sub-modules. The sub-modules include hybrid energy storage units and passive-active co-coupling circuits. The hybrid energy storage units are connected to the converter station bridge arm through the passive-active co-coupling circuits. The deeply networked converter station is equipped with a dual-port decoupling controller, which is configured on the AC port and DC port of the converter station. The dual-port decoupling controller enables the AC port of the converter station to perform virtual synchronous generator control to provide voltage source support, and enables the DC port of the converter station to independently perform DC voltage or power control. It also absorbs and transmits the instantaneous power difference between the AC port and the DC port through the hybrid energy storage unit.

[0056] The hybrid energy storage unit includes a first energy storage unit and a second energy storage unit. The power density of the first energy storage unit is higher than that of the second energy storage unit, and the energy density of the second energy storage unit is higher than that of the first energy storage unit. The first energy storage unit is connected to the converter station arm through a half-bridge or full-bridge converter, and the second energy storage unit is connected in parallel with the first energy storage unit through a unidirectional conducting device.

[0057] The polarity configuration of the unidirectional conducting device is as follows: the unidirectional conducting device is in the conducting state only when the terminal voltage of the first energy storage unit is lower than the terminal voltage of the second energy storage unit and the difference exceeds the conduction voltage drop of the unidirectional conducting device, allowing the second energy storage unit to discharge to the outside.

[0058] Specifically, the first energy storage unit is a supercapacitor module, the second energy storage unit is a battery module, the unidirectional conducting device is a power diode, and a half-bridge or full-bridge converter controls the voltage of the supercapacitor module so that it is higher than the open-circuit voltage of the battery module under normal operating conditions, so that the power diode is in the reverse cutoff state.

[0059] When the AC power grid experiences a frequency drop or inertial response demand, the supercapacitor module is controlled to discharge rapidly, causing its voltage to drop and naturally triggering the power diode to conduct.

[0060] like Figure 2 As shown, the deep grid-type converter station in this invention adopts a modular multilevel converter topology. Each phase arm of each converter station consists of multiple sub-modules SM1…SM2. N The sub-modules are connected in series and integrate supercapacitors and lithium battery energy storage units to form an integrated energy storage converter station structure.

[0061] The converter station submodule has a half-bridge or full-bridge structure. The two ends of the supercapacitor are connected to the half-bridge or full-bridge structure to realize bidirectional power control of the supercapacitor. The battery is connected in parallel with the supercapacitor through a unidirectional diode (DIO).

[0062] To fully leverage the synergistic effect of the two energy storage units, the supercapacitor's normal operating voltage is set higher than the battery's nominal voltage in the design, ensuring that the battery remains inactive under normal circumstances. For example, if the battery pack's nominal voltage is V... bat_nom The forward voltage drop of the diode is V. D Then the reference voltage V of the supercapacitor can be... SCref Set to approximately 1.1V bat_nom At this time, there is V. SCref >V bat_nom -V DThe diode is reverse-biased and cut off for a long time. Under steady-state and small disturbance conditions, the battery cell is always isolated from the circuit. The voltage fluctuations caused by the dual-frequency (2ω) power pulsation on the AC side inside the converter station submodule are completely buffered by the supercapacitor, and the battery current is approximately zero. This avoids the problem of the battery overheating and accelerating aging due to participation in high-frequency power pulsation. When a large power disturbance occurs that requires additional energy support (for example, a sudden increase in AC load causes the virtual synchronous generator control to require the converter station to output additional power instantaneously), the submodule capacitor and the supercapacitor first rapidly release the stored energy. The voltage at the supercapacitor terminal follows the discharge process according to E=0.5CV. 2 The trend is downward. When V... SCref Reduce to near the battery terminal voltage (approximately V) bat_nom -V D When the battery cell outputs current, the DIO diode naturally conducts in the forward direction, and the battery cell outputs current to support it. Its function is like a voltage clamp, preventing the supercapacitor voltage from dropping further rapidly, thereby providing continuous subsequent energy support (similar to the energy support of a frequency modulation process).

[0063] Compared to active switching control of battery access, this design, which passively switches batteries via unidirectional diodes, boasts nanosecond-level response speeds, eliminates the need for complex state detection algorithms and communication processes, and offers extremely high reliability. In summary, by leveraging the characteristics of both supercapacitors and batteries and combining them with the aforementioned control logic, a balance between rapid power response and continuous support is achieved: under normal conditions, the battery does not participate in high-frequency power fluctuations, avoiding excessive stress; when needed, the battery can seamlessly connect within milliseconds, ensuring the stability of the converter station's output.

[0064] Example 3

[0065] To ensure that each converter station has voltage source and inertia support capabilities on the AC side, and voltage support and fault decoupling capabilities on the DC side, this invention employs a dual-port decoupling controller in each converter station. For example... Figure 3 As shown, the dual-port decoupling controller includes an AC-side grid construction control module, a DC-side voltage control module, and an energy storage management module. The AC-side grid construction control module uses an AC grid construction strategy to calculate the desired output voltage, the DC-side voltage control module uses a DC grid construction strategy to calculate the DC voltage command value, and the energy storage management module uses an energy storage control strategy to calculate the supercapacitor voltage reference value.

[0066] Based on the expected output voltage, DC voltage command value, supercapacitor voltage reference value, and voltage obtained from converter station circulating current suppression, combined with the converter station's nearest level approximation strategy, the number of sub-modules that are turned on in the upper and lower bridge arms is obtained to ensure that the capacitor voltage of the sub-modules is balanced.

[0067] The AC-side grid control module employs either virtual synchronous generator control or Pf / QV droop control. Specifically, to simulate the rotor dynamics equations of a virtual synchronous generator, virtual moment of inertia and damping coefficients can be introduced into the active power control, allowing the frequency deviation to satisfy a power angle swing equation similar to that of a synchronous machine. The active power control of the virtual synchronous generator is as follows:

[0068] ;

[0069] In the formula, J represents the virtual inertia time constant (moment of inertia), with units of W·s² / rad; D represents the damping coefficient, with units of W·s / rad; and These represent the rotor angular velocity and the rotor angular velocity reference value, respectively, in rad / s; This represents the active power command value transmitted by the sending-end converter station, in watts (W). This indicates the transmitted active power of the sending-end converter station; This represents the phase angle of the AC bus voltage of the sending-end AC grid, in rad. By properly setting parameters such as J and D, the converter station on the AC side can behave as a voltage source unit with virtual inertia and damping characteristics, actively providing inertial support and damping adjustment when the grid frequency changes.

[0070] The AC-side grid control module also includes reactive power control, such as QV droop control:

[0071] ;

[0072] in, This indicates the reference value of the control voltage, in units of V; and These represent the reactive power and reactive power reference value of the converter station, respectively, in MVar. This represents the control factor, with units of V / MVar; This represents the feedforward value of the voltage, in units of V.

[0073] The AC-side grid control module also includes AC voltage control: the output current of the converter station. Perform the Park transformation to obtain the corresponding d-axis voltage value. and q-axis voltage value According to voltage After determining the outer loop reference value of the d-axis voltage, and assuming the outer loop reference value of the q-axis voltage is 0, the d-axis active current reference value is obtained through the following control method. and q-axis reactive current reference value :

[0074] ;

[0075] In the formula, and These represent the proportional and integral coefficients of the preset voltage controller, respectively. The unit is A / V. The unit is A / (V·s); This represents the Laplace operator, with units of reciprocal time (s). -1 ); and These represent the output current of the converter station. Perform a Park transformation to obtain the corresponding d-axis and q-axis voltage values, in V; and Reference values ​​for d-axis active current and q-axis reactive current of the converter station, respectively, in A.

[0076] The output current of the converter station Perform the Park transformation to obtain the corresponding d-axis current value. and q-axis current value And based on the d-axis active current reference value With d-axis current value The difference, and the q-axis reactive current reference value With the q-axis current value The difference is used to generate the d-axis output voltage of the converter station based on the preset current controller. and q-axis output voltage The control equation for the preset current controller is expressed as follows:

[0077] ;

[0078] in, and These represent the proportional and integral coefficients of the preset current controller, respectively. The unit is V / A. The unit is V / (A·s); This represents the Laplace operator, with units of reciprocal time (s). -1 ); and These represent the output current of the converter station. Perform a Park transformation to obtain the corresponding d-axis and q-axis voltage values, in V; and The output current of the converter station Perform Park transformation to obtain the corresponding d-axis and q-axis current values, in A; L represents the equivalent inductance of the deep grid-connected converter station when connected to the grid, in H or V·s / (A·rad); This represents the rotor angular velocity, with units of rad / s; and These represent the d-axis output voltage and q-axis output voltage of the converter station, respectively, in V.

[0079] The d-axis output voltage of the converter station and q-axis output voltage Perform the inverse Park transform to obtain the desired output voltage of the converter station. .

[0080] The DC-side voltage control module generates a DC current command value based on the deviation between the measured DC voltage value and the DC voltage reference value, thereby controlling the DC voltage. For example, a PI regulator can be used to calculate the DC current command value based on the error; the specific adjustment equation is as follows:

[0081] ;

[0082] In the formula, and These represent the DC voltage reference value and the DC voltage measurement value, respectively, both in V; and These represent the proportional and integral coefficients of the DC voltage proportional-integral controller, respectively. The unit is A / V. The unit is A / (V·s); This represents the Laplace operator, with units of reciprocal time (s). -1 ); This indicates the rated DC current of the converter station, in amperes (A). This indicates the DC current command value of the sending-end converter station, in amperes (A).

[0083] The DC-side voltage control module also includes a DC current control inner loop, which generates the DC voltage command value required by the converter station's nearest-level strategy. The specific control equation is as follows:

[0084] ;

[0085] In the formula, and These represent the reference value and the measured value of DC current, respectively, both in amperes (A). and Let represent the proportional coefficient and integral coefficient of the DC current proportional-integral controller, respectively. The unit is V / A. The unit is V / (A·s); This represents the Laplace operator, with units of reciprocal time (s). -1 ); This indicates the DC voltage command value required by the converter station's closest level approximation strategy, in units of V.

[0086] The energy storage management module generates a supercapacitor voltage reference value based on the deviation between the converter station's stored energy and a reference value, thereby controlling the supercapacitor voltage. For example, a PI controller can be used to calculate the supercapacitor voltage reference value based on the error; the specific adjustment equation is as follows:

[0087] ;

[0088] in, and These represent the reference and measured values ​​of the supercapacitor energy storage in the converter station, respectively, in J. and Let represent the proportional coefficient and integral coefficient of the proportional-integral controller for energy storage control, respectively. The unit is A -1 ·s -1 , The unit is A -1 ·s -2 ; This represents the Laplace operator, with units of reciprocal time (s). -1 ); This indicates the reference value for the supercapacitor voltage at the converter station, typically in volts (V). It is worth noting that... It can be adjusted under external disturbances.

[0089] The converter station employs the nearest-level approximation (NLM) strategy to determine the number of submodules conducting in both upper and lower bridge arms, ensuring balanced capacitor voltages in the submodules. The number of submodules conducting in both upper and lower bridge arms is:

[0090] ;

[0091] in, and These represent the number of conductors connected in the upper and lower arms of the converter station, respectively, with dimensionless units; This is the voltage obtained from the circulating current suppression control of the converter station, and the unit is V; This indicates the reference value for the supercapacitor voltage at the converter station; This indicates the DC voltage command value required by the converter station's closest level approximation strategy. This represents the desired output voltage of the converter station.

[0092] It is important to emphasize that the AC-side voltage frequency control and the DC-side voltage control are structurally decoupled: the AC-side control does not require detection of the DC-side voltage U. dcThe control system primarily adjusts based on AC measurement values ​​at the grid connection point; the coupling between the two is mainly coordinated by the energy storage unit through energy balance. When AC-side frequency fluctuations or DC-side voltage fluctuations occur, the control system prioritizes balancing the instantaneous power difference by adjusting the charging and discharging power of the supercapacitor in the submodule, and then adjusts the charging and discharging of the battery when necessary. This "dual-port" control mode, combined with the rapid response of energy storage, achieves decoupling of AC-side grid control and DC-side voltage control on a time scale: in other words, rapid power disturbances on the AC side are absorbed or released locally by the supercapacitor in the submodule, and the DC side only experiences the buffered, slowly changing power, thus preventing AC disturbances from directly propagating to the DC side; similarly, transient impacts on the DC side (such as voltage drops caused by faults) will not be immediately transmitted to the AC side through the converter station. Through the above mechanism, effective isolation of AC and DC dynamic processes is achieved, significantly improving the system's stability in response to transient events.

[0093] Example 4

[0094] The coordinated control module is configured to execute an adaptive parameter adjustment strategy based on the state of charge and small disturbance stability margin. The adaptive parameter adjustment strategy is as follows:

[0095] Real-time acquisition of the state of charge and small disturbance stability margin of the hybrid energy storage units in each converter station;

[0096] The virtual inertial time constant and damping coefficient of each converter station are dynamically adjusted through a preset nonlinear function based on the state of charge and small disturbance stability margin.

[0097] When the state of charge of a converter station falls below a preset lower warning value, the coordination control module sends a command to the converter station to smoothly switch its control mode from grid-forming (GFM) to grid-following (GFL) and reduce its active power reference value to restore energy storage.

[0098] The nonlinear function is configured as follows:

[0099] When the state of charge is within the normal range, the virtual inertial time constant is positively correlated with the state of charge, allowing converter stations with sufficient energy storage to undertake more inertial support tasks; when the small disturbance stability margin reaches the lower limit, the damping coefficient is increased to suppress power oscillations.

[0100] like Figure 4As shown, in another embodiment of the present invention, a coordination control module is provided to coordinate the grid-connection capabilities of multiple converter stations in a multi-terminal system. This module interacts with the control systems of each converter station through a communication network to obtain real-time operating information such as voltage, frequency, and power of the AC power grid where each converter station is located, as well as parameters such as the state of charge (SOC) of each internal energy storage unit, its own broadband impedance, and the voltage level of the DC power grid.

[0101] Based on the above information, the coordination control module runs an optimization algorithm to calculate and distribute network control parameters such as the virtual inertia time constant and damping coefficient to each converter station. The optimization objective can be set to balance the energy storage utilization of each converter station while ensuring stability; for example, this can be achieved by minimizing the sum of squares of the deviations of the SOC from the average value of each station. The specific adaptive parameter adjustment strategy control mode is as follows:

[0102] When the power output of a wind farm in a certain region fluctuates significantly and the requirements for frequency stability are higher, the coordinated control module can appropriately increase the network control parameters such as the virtual inertia time constant and damping coefficient of the converter station connected to that region, so that it can assume more inertial support responsibility during local disturbances, thereby smoothing out frequency fluctuations in that region.

[0103] When the State of Charge (SOC) of an energy storage unit at a converter station approaches its upper or lower limit, the coordination control module will reduce the virtual inertia and damping coefficient of that converter station, as well as other grid control parameters. Simultaneously, it will increase the inertia and support parameters of other converter stations with larger energy storage margins to optimize the allocation of grid capacity across the entire system, preventing excessive over-extraction or prolonged idleness of energy storage at a single converter station. For example, when a station's SOC is detected to be above 80% (sufficient energy storage), its virtual inertia coefficient can be increased, prompting it to output more power when frequency support is available. Conversely, when the SOC is below 20% (insufficient energy storage), its inertia coefficient will be reduced to decrease its power response, allowing its energy storage to recover without over-discharge.

[0104] When the state of charge (SOC) of any energy storage unit at a converter station exceeds a preset safety threshold, the coordination control module automatically switches the control mode of that converter station from grid-based control to grid-following control (from active voltage source mode to conventional droop or power control mode). In grid-following mode, the converter station only maintains power output according to dispatch instructions and no longer actively supports system voltage or frequency, thus avoiding instability caused by energy storage depletion or overcharging. Once the SOC of the converter station recovers to the preset range, the coordination control module switches its control mode back to grid-based control, allowing the converter station to re-participate in system inertial support and voltage maintenance. Through adaptive switching between grid-based and grid-following modes, the coordination control module can maintain the overall sustainability of multi-converter station grid-based capabilities while ensuring the safe operation of energy storage units.

[0105] Example 5

[0106] Superconducting current limiters and DC circuit breakers configured within DC lines are used for the protection and recovery of DC transmission network faults. The protection and recovery strategies for DC transmission network faults are as follows:

[0107] Fault current limiting stage: When a short circuit fault occurs in the DC line, and the DC grid structure of the deep grid converter station causes the fault current rise rate to exceed the threshold, the physical characteristics of the superconducting current limiter change from the superconducting state to the high resistance state, thus limiting the peak fault current.

[0108] Fault ride-through phase: During the fault clearing process, the deep grid-type converter station does not shut down, but maintains the AC side voltage source characteristics and controls the hybrid energy storage unit to absorb the active power that should have been delivered to the DC side, thereby preventing the connected wind farm or AC grid from disconnecting from the grid.

[0109] Fault isolation phase: After the fault current is limited by the superconducting current limiter, the DC circuit breaker is controlled to operate and disconnect the faulty line;

[0110] Recovery phase: After the fault is cleared, the hybrid energy storage unit releases energy to assist in the establishment of DC voltage, while waiting for the superconducting current limiter to cool down and return to the superconducting state.

[0111] In one embodiment, when a severe fault such as a DC line short circuit occurs in a branch of a DC power grid, this invention achieves rapid fault isolation and system self-recovery by utilizing the synergistic effect of a superconducting current limiter and a DC circuit breaker, while limiting the fault current. The process can be described in chronological order as follows:

[0112] Time T0 (fault occurrence): After a short-circuit fault occurs in the DC line, the DC side current I... dc The voltage rose sharply. At this instant, the converter station continued to perform grid control on the AC side to maintain the stability of AC side voltage and frequency, and did not shut down the converter station.

[0113] Time T1 (Current limiting begins): When the DC fault current rises above the critical current I of the superconducting current limiter. c Subsequently, the superconducting material in the superconducting current limiter instantly loses its superconducting properties, and the superconducting current limiter switches from a zero-resistance state to a resistive state, with an equivalent resistance value R. SFCL The current rapidly increases to tens of ohms (e.g., 50-90 Ω). At this point, the superconducting current limiter confines the fault current to I. lim =U dc / R SFCL The current limiter effectively curbed the continuous rise of the fault current. Simultaneously, as the superconducting current limiter entered the current-limiting state, the DC circuit breaker detected that the fault current had been limited to a low level and began preparing to execute the interruption operation according to the preset triggering logic.

[0114] T1–T2 (Energy Buffering Stage): During the brief period following successful current limiting by the superconducting current limiter (from T1 until the actual opening of the DC circuit breaker), the DC fault branch cannot transmit power, while AC energy sources (such as wind farms) may continue to generate electricity. Without mitigation, excess energy will cause a sudden surge in the capacitor voltage of the converter station submodules, posing a risk of damage or explosion. To avoid DC bus overvoltage, the control system immediately instructs each submodule to rapidly absorb excess energy from the bridge arm and store it in the supercapacitor (causing the supercapacitor voltage to rise). Because the hybrid energy storage system composed of the supercapacitor and battery configured in this invention has an MJ-level energy throughput capacity, it can absorb a large amount of excess energy within a timescale of hundreds of milliseconds. Therefore, the accumulated fault energy in the system is effectively buffered before the DC circuit breaker opens, preventing a significant rise in the submodule capacitor voltage.

[0115] At time T2 (fault isolation): The DC circuit breaker reliably operates under current-limiting conditions according to its predetermined delay, completing the interruption of the faulty branch and electrically isolating the faulty line from the DC power grid. Since the fault current is limited by the superconducting current limiter during interruption, compared to interrupting tens of thousands of amperes of current without current-limiting measures, the interruption difficulty and cost of the DC circuit breaker are greatly reduced.

[0116] At time T3 (system recovery): After the fault is cleared, the superconductor in the superconducting current limiter gradually cools and returns to the superconducting state, ready to be put back into operation. The converter station, based on the DC-side grid control strategy, utilizes the energy buffer previously stored in supercapacitors and batteries to smoothly raise the DC grid voltage from its post-fault decrease back to the predetermined reference value, ultimately restoring the DC grid to normal operation. During this process, the rapid discharge of the supercapacitors suppresses potential oscillations during DC voltage recovery, and batteries also participate in energy release to balance DC power when necessary.

[0117] Through the above process, the DC short-circuit fault is quickly isolated, and the energy during the fault is effectively absorbed inside the converter station, ensuring that the DC fault will not drag down the voltage stability of the AC grid; at the same time, the DC voltage of other unfaulted branches also returns to normal levels in a short time after the fault is cleared, greatly improving the safety and stability of the entire system.

[0118] In summary, this invention deeply integrates supercapacitors and battery energy storage units within the sub-modules of a deeply grid-connected converter station, and utilizes unidirectional diodes to achieve differentiated responses for the two types of energy storage units. This embeds the energy storage components into the converter station's topology and control system, enabling the converter station to simultaneously possess AC-side virtual synchronous machine characteristics and DC-side grid voltage support capabilities. Through a dual-port grid control strategy and power balancing involving energy storage, effective decoupling and non-interference of AC / DC dynamic processes are achieved. By configuring superconducting current limiters and DC circuit breakers in series on the DC side and coordinating them with grid control, rapid fault isolation and system recovery under fault current-limited conditions are achieved. Furthermore, by coordinating the unified scheduling of virtual inertia, damping, and other parameters of multiple converter stations through a coordinated control module, collaborative grid operation of multi-terminal DC systems under conditions of high-proportion renewable energy access and weak grids is realized. In conclusion, this invention significantly improves the transient stability and operational reliability of AC / DC hybrid power grids while maintaining the advantages of large-capacity and long-distance HVDC transmission, demonstrating outstanding technological advancement and engineering application value.

[0119] Although the present invention has been described in detail above with general descriptions and specific embodiments, modifications or improvements can be made to it, which will be obvious to those skilled in the art. Therefore, all such modifications or improvements made without departing from the spirit of the present invention fall within the scope of protection claimed by the present invention.

Claims

1. A deep-grid DC transmission system based on hybrid energy storage, characterized in that, The DC transmission system is a multi-terminal DC grid used to transmit electrical energy from multiple AC grids to multiple user devices. The DC transmission system includes multiple deep-grid converter stations, multiple DC lines, and a coordination control module. The multiple deep-grid converter stations are connected between the multiple AC grids and the multiple user devices, and the multiple deep-grid converter stations are connected to each other through multiple DC lines. The multiple deep-grid converter stations communicate with each other through the coordination control module. The deep grid-type converter stations all adopt a modular multilevel converter topology, and their bridge arms are composed of multiple cascaded sub-modules. The sub-modules include a hybrid energy storage unit and a passive-active co-coupling circuit. The hybrid energy storage unit is connected to the converter station bridge arm through the passive-active co-coupling circuit. The deep grid-type converter station is equipped with a dual-port decoupling controller, which is configured on the AC port and DC port of the converter station. The dual-port decoupling controller enables the AC port of the converter station to perform virtual synchronous generator control to provide voltage source support, and enables the DC port of the converter station to independently perform DC voltage or power control. The instantaneous power difference between the AC port and the DC port is processed through the hybrid energy storage unit. Each of the aforementioned DC lines is equipped with a superconducting current limiter and a DC circuit breaker, which are connected in series.

2. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 1, characterized in that, The hybrid energy storage unit includes a first energy storage unit and a second energy storage unit. The power density of the first energy storage unit is higher than that of the second energy storage unit, and the energy density of the second energy storage unit is higher than that of the first energy storage unit. The first energy storage unit is connected to the converter station arm through a half-bridge or full-bridge converter, and the second energy storage unit is connected in parallel with the first energy storage unit through a unidirectional conducting device. The polarity configuration of the unidirectional conducting device is such that the unidirectional conducting device is in a conducting state only when the terminal voltage of the first energy storage unit is lower than the terminal voltage of the second energy storage unit and the difference exceeds the conduction voltage drop of the unidirectional conducting device, allowing the second energy storage unit to discharge to the outside.

3. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 2, characterized in that, The first energy storage unit is a supercapacitor module, the second energy storage unit is a battery module, the unidirectional conducting device is a power diode, and the half-bridge or full-bridge converter controls the voltage of the supercapacitor module so that it is higher than the open-circuit voltage of the battery module under normal operating conditions, so that the power diode is in the reverse cutoff state. When the AC power grid experiences a frequency drop or inertial response demand, the supercapacitor module is controlled to discharge rapidly, causing its voltage to drop and naturally triggering the power diode to conduct.

4. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 1, characterized in that, The dual-port decoupling controller includes an AC-side grid construction control module, a DC-side voltage control module, and an energy storage management module. The AC-side grid construction control module uses an AC grid construction strategy to calculate the desired output voltage, the DC-side voltage control module uses a DC grid construction strategy to calculate the DC voltage command value, and the energy storage management module uses an energy storage control strategy to calculate the supercapacitor voltage reference value. Based on the expected output voltage, DC voltage command value, supercapacitor voltage reference value, and voltage obtained from converter station circulating current suppression, combined with the converter station's nearest level approximation strategy, the number of sub-modules that are turned on in the upper and lower bridge arms is obtained to ensure that the capacitor voltage of the sub-modules is balanced.

5. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 1, characterized in that, The superconducting current limiter and DC circuit breaker configured within the DC line are used for protection and recovery of DC transmission network faults. The protection and recovery strategy for DC transmission network faults is as follows: Fault current limiting stage: When a short circuit fault occurs in the DC line, and the DC grid of the deep grid-type converter station causes the fault current rise rate to exceed the threshold, the physical characteristics of the superconducting current limiter change from the superconducting state to the high-resistivity state, thus limiting the peak fault current. Fault ride-through phase: During the fault clearing process, the deep grid-type converter station is not blocked, the AC side voltage source characteristics are maintained, and the hybrid energy storage unit is controlled to absorb the active power that should have been delivered to the DC side. Fault isolation phase: After the fault current is limited by the superconducting current limiter, the DC circuit breaker is controlled to disconnect the faulty line; Recovery phase: After the fault is cleared, the hybrid energy storage unit releases energy to assist in the establishment of DC voltage, while waiting for the superconducting current limiter to cool down and return to the superconducting state.

6. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 1, characterized in that, The coordination control module is configured to execute an adaptive parameter adjustment strategy based on the state of charge and a small disturbance stability margin. The adaptive parameter adjustment strategy is as follows: Real-time acquisition of the state of charge and small disturbance stability margin of the hybrid energy storage units in each converter station; Based on the state of charge and small disturbance stability margin, the virtual inertial time constant and damping coefficient of each converter station are dynamically adjusted through a preset nonlinear function. When the state of charge of a deep grid-connected converter station falls below a preset lower warning value, the coordination control module sends a command to the converter station to smoothly switch its control mode from grid-connected to grid-following mode and reduce its active power reference value to restore energy storage.

7. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 6, characterized in that, The nonlinear function is configured as follows: When the state of charge is within the normal range, the virtual inertial time constant is positively correlated with the state of charge; when the stability margin for small disturbances reaches the lower limit, the damping coefficient is increased to suppress power oscillations.

8. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 4, characterized in that, The AC network construction strategy adopted by the AC-side network construction control module is as follows: Based on the active power deviation of the deep grid-type converter station, and combined with the active power control of the virtual synchronous generator, the phase angle of the AC bus voltage of the sending-end AC grid is determined. Based on the reactive power deviation of the deep grid converter station, and in conjunction with reactive power control, the reference value of the control voltage is determined. Based on the reference value of the control voltage, combined with AC voltage control and preset current control, the d-axis output voltage and q-axis output voltage of the converter station are generated; Based on the d-axis output voltage, q-axis output voltage, and AC bus voltage phase angle of the converter station, an inverse Park transformation is performed to obtain the desired output voltage of the converter station.

9. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 4, characterized in that, The DC grid strategy adopted by the DC-side voltage control module is as follows: based on the deviation between the measured DC voltage value and the reference DC voltage value, a DC current command value is generated, and a DC voltage command value is generated through the DC current control inner loop.

10. The deep grid-type DC transmission system based on hybrid energy storage as described in claim 4, characterized in that, The energy storage management module adopts the following energy storage control strategy: based on the deviation between the energy storage energy of the converter station and the reference value, a supercapacitor voltage reference value is generated.

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