Unloading control method and system for hybrid direct current load device

Abstract

The invention discloses an unloading control method and system for a hybrid direct current load device, and belongs to the field of power system control. The method is suitable for an energy consumption valve controller of the hybrid direct current load device, the device further comprises an energy consumption valve module connected with the controller, and the energy consumption valve module comprises a plurality of connected submodules. When a power grid breaks down, the energy consumption valve module is controlled to be switched to an input state according to an energy consumption valve switching signal on the power grid side; the input priority is determined according to the arrangement sequence of the current average capacitor voltage of the sub-modules in a preset control period, the sub-modules are controlled to be switched to an input mode, and at the moment, thyristors of the sub-modules are in a conducting state; when the power grid is recovered to be in a non-fault state, the energy consumption valve module is controlled to be switched to a cut-off state according to the energy consumption valve cut-off signal, and the sub-modules are controlled to be switched to a cut-off mode according to the current average capacitor voltage arrangement sequence; and after all the sub-modules are switched to the cut-off mode, the energy consumption valve module is controlled to be switched to the locked state. The problems that in the prior art, the control cost is high, and the efficiency is low can be solved.

Description

Technical Field

[0001] This invention relates to the field of power system control technology, and in particular to a method and system for unloading control of a hybrid DC load device. Background Technology

[0002] Offshore wind power, as an important component of renewable energy, is gradually being developed in deep-sea areas. Offshore wind power transmission and grid connection systems based on VSC-HVDC (Voltage Source Converter-based Flexible DC Transmission System) are widely used due to their low loss and long-distance transmission capabilities. This system consists of an offshore wind farm, a sending-end converter station, a receiving-end converter station, and an AC main grid. When a ground fault or short-circuit fault occurs on the AC side of the receiving-end station's grid, the grid voltage drops rapidly, limiting the receiving-end station's output current and blocking the power transmission path. However, the sending-end converter station continues to inject power, resulting in a power surplus in the system. This power imbalance causes the system's DC capacitors to charge continuously, leading to a rapid rise in DC voltage, triggering overvoltage protection, and even causing the DC line to trip.

[0003] Currently, addressing surplus power primarily involves reducing wind farm output through coordinated control and installing unloading devices. However, coordinated control suffers from slow response times, making DC unloading equipment (DCC) a more effective method. DC unloading equipment is typically installed on the DC side of onshore converter stations, requiring a large number of power devices for control of existing hybrid topologies, leading to excessively high equipment costs. Therefore, improving the response speed of coordinated unloading control of DC load devices while optimizing the number of devices is a pressing issue. Summary of the Invention

[0004] This invention provides a method and system for unloading control of a hybrid DC load device, which can effectively solve the problems of high unloading control cost and low control response rate in the prior art.

[0005] An embodiment of the present invention provides an unloading control method for a hybrid DC load cell, comprising: Furthermore, the hybrid DC load device also includes a centralized dissipation resistor module and an unloading reactor; the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in sequence to form an unloading branch; The energy dissipation valve controller is also used to control the energy dissipation valve module to switch to the working state, drive the energy dissipation valve module to output an adjustable voltage so that the unloading branch forms a branch current; control the branch current to flow through the centralized dissipation resistor module so that the centralized dissipation resistor module dissipates surplus power; and control the unloading reactance to suppress sudden changes in the unloading branch current.

[0006] Furthermore, the submodule also includes a first capacitor, a thyristor, a first diode, a bipolar transistor module, a distributed energy-dissipating resistor, and a second capacitor; one end of the first capacitor is connected to the input port of the submodule and the anode of the thyristor, and the other end is connected to the anode of the first diode; one end of the second capacitor is connected to the cathode of the first diode, and the other end is connected to the output port of the submodule and one end of the bipolar transistor module; one end of the distributed energy-dissipating resistor is connected to the cathode of the thyristor and the second capacitor, and the other end is connected to the bipolar transistor module. The energy-consuming valve controller is also used to control the branch current to flow through the thyristor, the distributed energy-consuming resistor and the bipolar transistor module after the submodule is switched to the working state. After the submodule is switched to the cut-off state, the bipolar transistor module is controlled to turn on to form an equipotential point, and a reverse voltage is applied to the thyristor so that the duration of the reverse voltage is greater than the turn-off time of the thyristor.

[0007] Furthermore, the average capacitor voltages are arranged in order to determine the priority of sub-module deployment, including: The average capacitor voltages are arranged in ascending order. Based on this order, the input priority of each submodule is determined from high to low, so that the submodules can be switched to the input mode sequentially according to their input priority from high to low.

[0008] Furthermore, the acquisition of the current average capacitor voltage for each submodule includes: Within a preset control cycle, acquisition delays are added to the rising and falling edges of the energy dissipation valve switching signal, and the capacitor voltages of the first capacitor and the second capacitor are obtained. The average value of the capacitor voltage of the first capacitor and the capacitor voltage of the second capacitor is used as the current average capacitor voltage of the submodule. The acquisition delay is calculated based on the preset minimum sampling interval, the number of sub-modules, and the current adjacent sub-module switching delay; the initial adjacent sub-module switching delay is a preset value.

[0009] Furthermore, the control submodule switches to the cut-off mode according to the current average capacitor voltage order, including: Arrange the current average capacitor voltage in descending order, determine the cut-off priority of the sub-module from high to low, and control the sub-modules to switch to the cut-off mode in descending order of cut-off priority.

[0010] Furthermore, determining whether the power grid has returned to a fault-free state includes: Obtain the real-time voltage and rated voltage of the power grid, the DC voltage and rated voltage of the hybrid DC load device; If the real-time voltage recovers to a preset threshold of the grid-side rated voltage and the DC voltage is less than the device's rated voltage, the grid is determined to have recovered to a fault-free state.

[0011] As an improvement to the above solution, another embodiment of the present invention provides a load unloading control system for a hybrid DC load device, including a hybrid DC load device; the hybrid DC load device is connected to the power grid side and includes an energy dissipation valve controller and an energy dissipation valve module; the energy dissipation valve module is connected to the energy dissipation valve controller, and the energy dissipation valve module includes several sub-modules, each sub-module including a thyristor, and the input port of the sub-module is connected to the output port of the adjacent sub-module; The energy-consuming valve controller is used to acquire the energy-consuming valve switching signal from the power grid side in the event of a power grid fault, and control the energy-consuming valve module to switch to the active state according to the energy-consuming valve switching signal; wherein, in the event of no power grid fault, the controller controls the energy-consuming valve module to remain in the locked state. After the energy dissipation valve module switches to the active state, the current average capacitor voltage of each sub-module is acquired within a preset control cycle. The average capacitor voltages are arranged in order to determine the active priority of the sub-modules. Based on the active priority, the sub-modules are controlled to switch to the active mode. In this mode, the thyristors are in the conducting state. When the power grid is restored to a fault-free state, the power consumption valve disconnection signal on the power grid side is obtained. Based on the power consumption valve disconnection signal, the power consumption valve module is controlled to switch to the disconnection state, and the sub-module is controlled to switch to the disconnection mode according to the current average capacitor voltage arrangement order. After all submodules have switched to the cut-off mode, a preset DC voltage is output and the energy dissipation valve module is controlled to switch to the locked state.

[0012] Furthermore, the hybrid DC load device also includes a centralized dissipation resistor module and an unloading reactor; the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in sequence to form an unloading branch; The energy dissipation valve controller is also used to control the energy dissipation valve module to switch to the working state, drive the energy dissipation valve module to output an adjustable voltage so that the unloading branch forms a branch current; control the branch current to flow through the centralized dissipation resistor module so that the centralized dissipation resistor module dissipates surplus power; and control the unloading reactance to suppress sudden changes in the unloading branch current.

[0013] Furthermore, the submodule also includes a first capacitor, a thyristor, a first diode, a bipolar transistor module, a distributed energy-dissipating resistor, and a second capacitor; one end of the first capacitor is connected to the input port of the submodule and the anode of the thyristor, and the other end is connected to the anode of the first diode; one end of the second capacitor is connected to the cathode of the first diode, and the other end is connected to the output port of the submodule and one end of the bipolar transistor module; one end of the distributed energy-dissipating resistor is connected to the cathode of the thyristor and the second capacitor, and the other end is connected to the bipolar transistor module. The energy-consuming valve controller is also used to control the branch current to flow through the thyristor, the distributed energy-consuming resistor and the bipolar transistor module after the submodule is switched to the working state. After the submodule is switched to the cut-off state, the bipolar transistor module is controlled to turn on to form an equipotential point, and a reverse voltage is applied to the thyristor so that the duration of the reverse voltage is greater than the turn-off time of the thyristor.

[0014] By implementing this invention, at least the following beneficial effects are achieved: This invention provides a method and system for unloading control of a hybrid DC load device. The method is applicable to the energy dissipation valve controller of a hybrid DC load device. The hybrid DC load device is connected to the power grid. The hybrid DC load device also includes an energy dissipation valve module, which is connected to the energy dissipation valve controller. The energy dissipation valve module includes several sub-modules, each including a thyristor. The input port of the sub-module is connected to the output port of the adjacent sub-module. The sub-module uses a thyristor as its core switch. Its characteristic of naturally turning on when the voltage changes from negative to positive eliminates the need for complex drive timing. The controller only needs to control the core actions of the sub-module's activation and deactivation, avoiding multi-dimensional drive control delays of the entire control device. It also eliminates the need for additional complex drive protection circuits, directly reducing the cost of a single module. The energy dissipation valve module adopts a structure where the input port of the sub-module is connected to the output port of the adjacent sub-module. The flexible configuration of the number of sub-modules adapts to different voltage levels, eliminating the need for parallel redundant sub-modules or additional power units, further avoiding an increase in the number of devices. During grid faults, the energy dissipation valve controller directly acquires the energy dissipation valve switching signal from the grid side and controls the energy dissipation valve module to switch to the active state. In the active state, the energy dissipation valve controller determines the sub-module's activation priority by sampling the average capacitor voltage of the sub-modules within a preset control cycle, and then controls the sub-module to switch to the active mode, thereby driving the thyristor to conduct and the sub-module to participate in unloading. The entire process has no additional intermediate coordination links, with a short response path and low latency, which can quickly absorb surplus power to suppress DC voltage over-limit, meeting the millisecond-level response requirements of offshore wind power flexible DC systems. With a hardware architecture using thyristors and several sub-modules, efficient control logic achieves rapid absorption of surplus power, meeting the requirement of improving unloading control response speed under the premise of optimizing the number of devices. Attached Figure Description

[0015] Figure 1 This is a schematic flowchart of an unloading control method for a hybrid DC load device provided in an embodiment of the present invention; Figure 2 This is a topology diagram of a TDS-DCC submodule provided in an embodiment of the present invention; Figure 3 This is a topology diagram of the hybrid sea breeze flexible direct unloading device TDS-DCC provided in an embodiment of the present invention; Figure 4 This is a timing waveform diagram of the TDS-DCC submodule switching provided in an embodiment of the present invention; Figure 5 This is a schematic diagram of the voltage and current distribution of a TDS-DCC submodule operating in different modes according to an embodiment of the present invention; Figure 6 This is a simulation verification waveform diagram of the thyristor turn-off control of the TDS-DCC submodule provided in an embodiment of the present invention; Figure 7 This is a system control strategy block diagram of a TDS-DCC device provided in an embodiment of the present invention; Figure 8 This is a specific block diagram of the overall energy dissipation control provided in an embodiment of the present invention; Figure 9 This is a block diagram of capacitor voltage equalization control for a TDS-DCC submodule provided in an embodiment of the present invention; Figure 10 This is a schematic diagram of the voltage fluctuation waveform of a submodule capacitor provided in an embodiment of the present invention; Figure 11 This is a comparison of simulation waveforms showing the effect of a submodule capacitor voltage equalization control strategy provided in an embodiment of the present invention. Figure 12 This is a schematic diagram of the overall topology with the power grid side provided in an embodiment of the present invention; Figure 13 This is a schematic diagram of the simulation waveform when the surplus power factor is 0.5 according to an embodiment of the present invention; Figure 14 This is a schematic diagram of the simulation waveform when the surplus power factor is 0.8 according to an embodiment of the present invention; Figure 15 This is a schematic diagram of the simulation waveform when the surplus power factor is 0.3 according to an embodiment of the present invention; Figure 16 This is a schematic diagram of the unloading control system of a hybrid DC load device provided in an embodiment of the present invention. Detailed Implementation

[0016] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. 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.

[0017] See Figure 1 To address the problems of high cost and low response rate in existing unloading control technologies, an embodiment of the present invention provides a flowchart illustrating an unloading control method for a hybrid DC load device, comprising: S1. In the event of a power grid failure, obtain the power consumption valve switching signal from the power grid side, and control the power consumption valve module to switch to the active state according to the power consumption valve switching signal; wherein, in the event of no power grid failure, control the power consumption valve module to remain in the locked state. Specifically, the unloading control method for a hybrid DC load device in this embodiment is applicable to the energy dissipation valve controller of the hybrid DC load device; the hybrid DC load device is connected to the power grid side, and the hybrid DC load device also includes an energy dissipation valve module (energy dissipation valve) U. sm The energy-consuming valve module is connected to the energy-consuming valve controller, such as... Figure 2 As shown, the energy-consuming valve module includes several sub-modules SM, each sub-module including a thyristor T1, and the input port of the sub-module is connected to the output port of the adjacent sub-module.

[0018] Specifically, the hybrid DC load cell (TDS-DCC) device is a type of equipment used to absorb surplus power from flexible DC systems, such as... Figure 3 As shown, the system mainly includes an energy dissipation valve controller and an energy dissipation valve module, which are connected in parallel to the DC side of the receiving-end converter station. The energy dissipation valve controller receives signals from the grid side, collects the voltage of the sub-modules, and generates control commands to control the state switching and mode adjustment of the energy dissipation valve module. The energy dissipation valve module is the core of the hybrid DC load device, consisting of several sub-modules connected in series. It can be equivalent to a controlled voltage source, outputting an adjustable voltage to drive the unloading branch through the switching of the sub-modules' activation and deactivation modes. The sub-module is the functional unit of the energy dissipation valve module, and adjacent sub-modules are connected in series through input and output ports. The activated state is one of the operating states of the energy dissipation valve module. At this time, the sub-module can switch to the activated mode to participate in surplus power dissipation, corresponding to the state of the energy dissipation valve switching signal PWM=1. The locked state is the standby state of the energy dissipation valve module. At this time, all sub-module devices are turned off and do not participate in power dissipation, corresponding to the hot standby state when the grid is fault-free. The sub-module output capacitor series voltage is used to match the system DC voltage.

[0019] In a preferred embodiment of the present invention, the energy dissipation valve controller acquires the DC voltage of the receiving-end converter station. If the DC voltage is >1.05 pu, or directly receives the energy dissipation valve switching signal (PWM modulation signal) generated by the grid side, the energy dissipation valve controller sends a device turn-on permission command to the energy dissipation valve module, enabling the sub-module to switch to the input mode. When the grid is not faulty, if the DC voltage is ≤1.05 pu, the energy dissipation valve controller sends a device turn-off command to the energy dissipation valve module, and all thyristors and bipolar transistor modules in the sub-module are turned off. The capacitors in the sub-module are connected in series, and the energy dissipation valve outputs a voltage that matches the system and does not participate in dissipation.

[0020] Preferably, the hybrid DC load device further includes a centralized dissipation resistor module U. R and unloading reactance L dcc The energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in sequence to form an unloading branch. The energy dissipation valve controller is also used to control the energy dissipation valve module to switch to the working state, drive the energy dissipation valve module to output an adjustable voltage so that the unloading branch forms a branch current; control the branch current to flow through the centralized dissipation resistor module so that the centralized dissipation resistor module dissipates surplus power; and control the unloading reactance to suppress sudden changes in the unloading branch current.

[0021] Specifically, the centralized dissipation resistor module U R It is the core dissipation component of the hybrid DC load cell, connected in series with the energy dissipation valve module and the unloading reactor. It is used to dissipate some of the surplus power and supplement the dissipation capacity of the submodule's distributed resistance. Unloading reactor L dcc This is a current suppression component in the hybrid DC load cell. Utilizing the characteristic of inductance to impede sudden current changes, it reduces the rate of change of current (di / dt) in the unloading branch, preventing impact on the concentrated dissipation resistor. The adjustable voltage is the output voltage of the energy dissipation valve module, adjusted by the number of sub-modules switched on; more switches result in a higher voltage, used to drive the unloading branch to generate current. The branch current is the current in the unloading branch, serving as the carrier of power dissipation. The "D" below the unloading reactor represents a diode. During current changes in the unloading branch, it provides a freewheeling path for the inductor energy stored in the unloading reactor, preventing reverse voltage generated by sudden inductor current changes from damaging power devices in the circuit. It also suppresses voltage spikes in the circuit, ensuring the stable operation of the entire hybrid DC load cell.

[0022] The centralized dissipation resistor module supplements the dissipation capacity of the distributed resistor in the sub-module, especially in the event of a severe fault, where it can dissipate all the input power of the wind farm together with the distributed resistor. The unloading reactance suppresses di / dt, preventing damage to the centralized resistor due to a sudden current surge, while also reducing the system's DC voltage ripple. The unloading branch is connected in parallel to the DC side of the onshore converter station, eliminating the need to occupy space on the offshore platform and improving practicality.

[0023] Preferably, the submodule further includes a first capacitor. thyristor First diode Bipolar transistor module, distributed power dissipation resistor and the second capacitor One end of the first capacitor is connected to the input port of the submodule and the anode of the thyristor, and the other end is connected to the anode of the first diode; one end of the second capacitor is connected to the cathode of the first diode, and the other end is connected to the output port of the submodule and one end of the bipolar transistor module; one end of the distributed power dissipation resistor is connected to the cathode of the thyristor and the second capacitor, and the other end is connected to the bipolar transistor module; the bipolar transistor module includes an IGBT module. and the second diode .

[0024] The energy-consuming valve controller is also used to control the branch current to flow through the thyristor, the distributed energy-consuming resistor and the bipolar transistor module after the submodule is switched to the working state. After the submodule is switched to the cut-off state, the bipolar transistor module is controlled to turn on to form an equipotential point, and a reverse voltage is applied to the thyristor so that the duration of the reverse voltage is greater than the turn-off time of the thyristor.

[0025] Specifically, such as Figure 2 and Figure 3 As shown, the first capacitor It is one of the energy storage capacitors in the submodule. Its positive terminal is connected to the submodule input port and the thyristor anode, and its negative terminal is connected to the anode of the first diode. It is used for voltage balancing and power dissipation. (First diode) It is the reverse-feedback protection diode of the submodule, with the anode connected to... The negative terminal and the cathode are connected to the second capacitor. Positive electrode, used to activate in the locked state , Series connection prevents capacitor reverse charging. A bipolar junction transistor (IGBT) module, i.e., an IGBT module with an anti-parallel diode, has its collector connected to the distributed resistance and... Positive electrode, emitter connected The negative terminal and submodule output port are the core fully controlled devices for controlling the submodule potential and turning off the thyristor; the anti-parallel diode is used for freewheeling to avoid voltage spikes when the IGBT is turned off. The distributed energy dissipation resistor is a local energy dissipation component of the submodule, with one end connected to the thyristor cathode and... The positive terminal is connected to the IGBT collector, and the other end is used to dissipate excess power at the submodule level. Second capacitor. It is another energy storage capacitor in the submodule, with the positive terminal connected to... The cathode and distributed resistance, the negative terminal is connected to the IGBT emitter and the submodule output port, and... Voltage balancing is achieved collaboratively. The equipotential point is formed by the IGBTs being turned on. The equipotential state between the negative terminal, the IGBT emitter, and the system's DC negative terminal is used to change the voltage polarity across the thyristor, i.e., from reverse voltage to positive voltage, or from positive voltage to reverse voltage. The thyristor turn-off time is the minimum time required for the thyristor to transition from conduction to turn-off. The duration of the reverse voltage must be greater than the thyristor turn-off time to ensure reliable turn-off. Utilizing the equipotential point formed by the IGBT's conduction, and through appropriate mode allocation, a reverse voltage is applied to thyristor T1 to turn it off.

[0026] S2. After the energy dissipation valve module switches to the active state, the current average capacitor voltage of each sub-module is obtained within a preset control cycle. The average capacitor voltages are arranged in order to determine the active priority of the sub-modules. According to the active priority, the sub-modules are controlled to switch to the active mode. The thyristors are in the conducting state. Specifically, the preset control cycle is the core control cycle of the energy dissipation valve controller, used for synchronous voltage sampling, submodule sorting, and generation of switching commands. A typical value is 5-20ms, preferably 10ms, to match the rhythm of changes in the system's surplus power. The average capacitor voltage is the average value of the capacitor voltages within the submodule, used to determine the voltage balance of the submodules and is the core basis for determining the activation priority. Activation priority indicates the order in which submodules are activated, determined by the average capacitor voltage sorting to ensure voltage balance. The activation mode is one of the operating modes of the submodule. In this mode, the thyristor is turned on, the submodule is connected to the unloading branch to participate in dissipation, and current flows through the thyristor, distributed resistor, and other paths.

[0027] In a preferred embodiment of the present invention, the energy dissipation valve controller collects the capacitor voltage of each submodule through a voltage sensor within a preset control cycle and calculates the average capacitor voltage. The energy dissipation valve controller arranges the average capacitor voltage of all submodules in order, and the sorting result corresponds to the switching priority. The earlier the sorting, the higher the priority. The energy dissipation valve controller controls the submodules to be put into operation in order of priority from high to low. After the capacitor voltage is balanced, the voltage across the thyristor changes from negative to positive and naturally conducts. The submodule enters the input mode and is connected to the unloading branch.

[0028] In a preferred embodiment of the present invention, in high-voltage, high-power applications, thyristors are superior to fully controlled devices such as IGBTs in both cost and reliability. The reason they haven't been more widely used in hybrid DC unloading devices is because they cannot actively turn off the current. Therefore, achieving thyristor turn-on and turn-off control is one of the key steps to the feasibility of TDS-DCC. During fault ride-through, to achieve flexible power absorption under different surplus power coefficients, TDS-DCC requires effective turn-off control of the thyristors in the device sub-modules, thereby achieving sub-module state switching and capacitor voltage balance. (Thyristor) The shutdown method is in On, capacitor and When the negative electrode potentials are equal, control The voltage is greater than , Both ends are shut off under reverse pressure. For example... Figure 5 As shown, the TDS-DCC submodule has four modes: cut-off mode, input mode, transition mode 1, and transition mode 2. Figure 4 The control signals of each device and the changes in the voltage of two capacitors within the module are shown during the periodic activation and deactivation of the TDS-DCC submodule during operation. This cycle is divided into four processes and five sub-intervals. Figure 4 middle, This is the switching control signal for the corresponding submodule, with a duty cycle of d. This indicates that the submodule group number is Duty cycle. This is the gate trigger signal for the thyristor within the module. This is the gate drive signal for the IGBT. The time period is the transition phase before the submodule is deployed, referred to as Process I; The time period is the submodule implementation phase, referred to as Process II; The time period is the transition phase of submodule removal, referred to as Process III; The time period is the submodule removal phase, referred to as process IV.

[0029] Before the specified time, the power consumption valve control signal PWM is 0, and the submodule is in the disconnected state. and All are off, capacitors and Through the anti-reverse diode In series, the submodule output voltage is and The sum of the capacitor voltages results in no current flowing through the distributed resistance. The equivalent circuit for this stage is as follows: Figure 5 As shown in (a).

[0030] Time period: At that moment, the submodule's power consumption control signal is set to 1, and unloading begins. Switch and When both are given a conduction signal, the lower IGBT is turned on. and The negative electrode potentials are equal. However, since at this time... The voltage across the terminals is greater than With the voltage across its terminals, the thyristor experiences reverse voltage, which does not meet the conduction condition. It remains in the off state. At this time, the submodule enters transition state 2, and the unloaded branch current flows through the IGBT to supply... As the capacitor charges, its voltage rises. Then through the distributed resistance within the module Discharge occurs, and the capacitor voltage drops. The submodule output voltage is... Voltage drop across the terminals. The equivalent circuit for this stage is as follows: Figure 5 As shown in (b). The duration of this interval is... .

[0031] Time period: At time, capacitor and When the voltages are equal, the voltage across the thyristor changes from negative to positive. and With all drive signals still at 1, the thyristor naturally turns on, and the submodule enters the input mode. At this time, the switch... and All are conducting. and In parallel connection, the capacitor voltages are equal, and the submodule outputs the voltage of a single capacitor. The unloading current flows through the thyristor, distributed resistor, and IGBT; the submodule participates in energy dissipation. The equivalent circuit for this stage is as follows: Figure 5 As shown in (c). The duration of this interval is... .

[0032] Time period: At this moment, the IGBT drive signal is set to 0, the lower transistor is turned off, and the submodule enters transition state 1. At this time, the capacitor... and The negative electrode potentials are no longer equal, and the upper thyristor continues to conduct because it cannot turn off autonomously. As the capacitor charges, its voltage rises. Discharge causes the capacitor voltage to drop. The voltage across the terminals gradually increases The voltage across the terminals is such that the discharge current flows through the distributed resistor to unload the load, and the module output voltage is the capacitor voltage. Voltage drop across the terminals. The equivalent circuit for this stage is as follows: Figure 5 As shown in (d), the duration of this interval is... .

[0033] Time period: At a certain moment, the thyristor drive signal is set to 0. The IGBT drive signal is then set to 1 again and remains so for a period of time. Conduction, and With the negative electrode potentials equal, the thyristor is turned off under reverse voltage, and the submodule enters transition 2. The unloaded branch current flows through the IGBT to... As the capacitor charges, its voltage rises. Then through the distributed resistance within the module Discharge occurs, and the capacitor voltage drops. The submodule output voltage is... Voltage drop across the terminals. The equivalent circuit for this stage is as follows: Figure 5 As shown in (b). The duration of this interval is... .

[0034] Time period: At that moment, the IGBT drive signal was set to 0 again. When shut down, the submodule enters the disconnected state. At this time, neither the thyristor nor the IGBT is conducting. and In series, the submodule output voltage is and The sum of the capacitor voltages results in no current flowing through the distributed resistance. The equivalent circuit for this stage is as follows: Figure 5 As shown in (a). The above process is then repeated until the fault is cleared, the system DC bus voltage drops, and the TDS-DCC stops operating. This interval lasts for [duration missing]. .

[0035] When the thyristor is turned off, the duration of the reverse voltage applied across the thyristor must be greater than the turn-off time of the thyristor. And when the thyristor is turned on, It cannot withstand reverse voltage. Considering that in transition states 1 and 2, the magnitude of the capacitor charging and discharging current is equal to the magnitude of the unloading branch current. Therefore, the timing of the submodule switching action must satisfy: ,make: In the formula, It is greater than the turn-off time of the selected thyristor. The constant. Solving for: In the formula, Submodule switching control signal The duty cycle of the switch and The gate trigger signal satisfies: In the formula, For TDS-DCC valve control cycle, This is the first high-level signal of the IGBT gate control signal in the k-th submodule. This is the second high-level signal of the IGBT gate control signal in the k-th submodule. Let be the drive pulse for the thyristor in the k-th submodule. Based on the gate trigger signal satisfying the condition, to ensure the complete timing of the submodule's operation and the normal turn-off of the thyristor, Must meet: .

[0036] Based on the above analysis, a TDS-DCC submodule switching simulation was performed on the MATLAB / Simulink platform to verify the effectiveness of the proposed thyristor turn-off strategy and submodule switching timing design. Figure 6 As shown in the figure, For submodule capacitors The capacitor voltage, For submodule capacitors The capacitor voltage, This refers to the general term for the capacitor voltage of the TDS-DCC submodule, including... and .

[0037] Preferably, the average capacitor voltages are arranged in order to determine the priority of sub-module deployment, including: The average capacitor voltages are arranged in ascending order. Based on this order, the input priority of each submodule is determined from high to low, so that the submodules can be switched to the input mode sequentially according to their input priority from high to low.

[0038] Preferably, obtaining the current average capacitor voltage of each submodule includes: Within a preset control cycle, acquisition delays are added to the rising and falling edges of the energy dissipation valve switching signal, and the capacitor voltages of the first capacitor and the second capacitor are obtained. The average value of the capacitor voltage of the first capacitor and the capacitor voltage of the second capacitor is used as the current average capacitor voltage of the submodule. The acquisition delay is calculated based on the preset minimum sampling interval, the number of sub-modules, and the current adjacent sub-module switching delay; the initial adjacent sub-module switching delay is a preset value.

[0039] Specifically, the acquisition delay represents the delay time added after the rising / falling edge of the energy dissipation valve switching signal to avoid transient fluctuations during submodule mode switching, ensuring stable submodule voltage during sampling. The minimum sampling interval represents the minimum time to avoid transient fluctuations in the submodule, ensuring no spikes or drops in capacitor voltage during sampling. The adjacent submodule switching delay represents the switching time difference between adjacent priority submodules, used to achieve stepped switching and reduce du / dt. The initial adjacent submodule switching delay is the initial setting value of the switching delay, which can be dynamically adjusted according to the voltage balancing effect.

[0040] In this embodiment, to achieve voltage balancing of the submodule capacitors in the TDS-DCC device, the capacitor voltages of each submodule need to be periodically sorted. The average capacitor voltage of the submodule is defined as... The specific method for capacitor voltage equalization control is as follows: Within one valve control cycle, the average capacitor voltage of all submodules is sampled twice and these voltages are sorted. During the energy dissipation valve activation process, the group containing the submodule with the largest average capacitor voltage is activated last. During the energy dissipation valve deactivation process, the group containing the submodule with the largest average capacitor voltage is deactivated first. The average capacitor voltage sampling time is set to the rising and falling edges of the energy dissipation valve activation / deactivation signal plus a sampling delay. , The calculation method is as follows: In the formula, Minimum sampling interval. Acquisition delay. This ensures that all submodules enter the cut-off mode during sampling. At this time, the average capacitor voltage of each submodule is stable, which is beneficial for precise control of the submodule capacitor voltage. The module switching signal of the first group of activated submodules is in phase with the energy dissipation valve switching signal. The module switching signal of the second group of activated submodules is increased by a certain amount compared to the first group. The delay is calculated, and the remaining submodules are switched in a similar manner, thus achieving a stepped switching of submodules, reducing device du / dt, and lowering system DC voltage ripple. Simulation verification results are as follows: Figure 11 As shown. Figure 11 (a) indicates capacitorless voltage equalization control. Figure 11 (b) indicates that with capacitor voltage equalization control, the DC voltage and power waveforms fluctuate violently without equalization control, exhibiting obvious spikes and irregular oscillations. This suggests that surplus power absorption is unstable, and the DC voltage is prone to exceeding limits. With equalization control, the DC voltage and power waveforms are smooth and stable, with significantly reduced fluctuation amplitude. This proves that equalization control makes surplus power absorption more precise, and the DC voltage is maintained within a safe range. Without equalization control, the capacitor voltage of the submodule fluctuates greatly and has poor consistency. The AC current also oscillates irregularly due to voltage imbalance, which can easily lead to overload of power devices. With equalization control, the capacitor voltage of the submodule fluctuates less and has strong consistency. The AC current waveform is more regular, indicating that equalization control matches the voltages of each submodule, stabilizes the system current, and makes the device operation more reliable.

[0041] S3. When the power grid is restored to a fault-free state, obtain the energy dissipation valve cut-off signal on the power grid side, control the energy dissipation valve module to switch to the cut-off state according to the energy dissipation valve cut-off signal, and control the sub-module to switch to the cut-off mode according to the current average capacitor voltage arrangement order. Specifically, the cut-off state represents the intermediate state of the energy dissipation valve module transitioning from being engaged to being locked. At this time, the submodule switches to the cut-off mode, stops power dissipation, and corresponds to the state of the energy dissipation valve switching signal PWM=0.

[0042] In a preferred embodiment of the present invention, the energy dissipation valve controller collects the real-time voltage on the grid side. For example, if the AC voltage at the receiving end recovers to more than 90% of the rated value, and the DC voltage of the system drops below the rated value, or directly receives the energy dissipation valve cut-off signal on the grid side, the energy dissipation valve controller sends a cut-off command to the energy dissipation valve module and controls the sub-modules to switch to the cut-off mode in sequence according to the current average capacitor voltage arrangement. First, the bipolar transistor module is turned off, and then a reverse voltage is applied to the thyristor to turn it off.

[0043] Preferably, the control submodule switches to the cut-off mode according to the current average capacitor voltage arrangement order, including: Arrange the current average capacitor voltage in descending order, determine the cut-off priority of the sub-module from high to low, and control the sub-modules to switch to the cut-off mode in descending order of cut-off priority.

[0044] In a preferred embodiment of the present invention, in actual engineering, the device parameters within each submodule will always differ, and factors such as the magnitude of distributed resistance will affect the static operating point of the submodule capacitor voltage. Simultaneously, the grouped, stepped switching of submodules will also lead to different voltage fluctuations in the submodule capacitors with different switching sequences. Therefore, TDS-DCC needs to incorporate voltage equalization control into its submodule switching control strategy. Taking the energy dissipation valve disconnection process as an example, the voltage fluctuation of the submodule capacitor during this period consists of three parts, such as... Figure 10 As shown. The capacitor voltage fluctuation caused by the thyristor turn-off process within the submodule. And the inductance in the circuit after the energy dissipation valve is removed The same applies to all submodules; during the TDS-DCC unloading process, the overall module capacitor voltage balancing control is mainly related to the charging and discharging of submodule capacitors caused by the transition time of the energy dissipation valve switching. related. The occurrence of this phenomenon is due to the non-zero current in the unloading branch during the transition time of the energy dissipation valve switching. During this time, the capacitors of the submodules in the disconnected state charge, causing their voltages to rise. Taking the capacitor voltage of the first disconnected submodule group as an example, let its fluctuation value be... During the transition period when the energy dissipation valve is disconnected, the output voltage of the energy dissipation valve is: In the formula, This represents the number of submodule groups that have been removed. The number of TDS-DCC energy-consuming valve sub-modules. The rated voltage of the submodule capacitor. This represents the number of switching groups for the submodule. The current on the unloading branch at this time is: In the formula This is the system's rated current. Therefore, the submodule that has been disconnected experiences an increase in current across its distributed resistance, affecting the submodule's capacitance. and They will charge simultaneously. and The increase values ​​are the same. The solution can be obtained from the formula for the current in the unloading branch: In the formula, This refers to the cut-off order of the corresponding submodules. This is the second part of the capacitor voltage fluctuation in the k-th submodule. This refers to the time interval between actions in two adjacent groups of submodules. Simultaneously, it is... It can be seen that the earlier a submodule is removed, the greater the fluctuation in the second part of the capacitor. Similarly, it can be deduced that the later a submodule is added, the greater the fluctuation in the second part of the capacitor. The larger.

[0045] Preferably, determining that the power grid has returned to a fault-free state includes: Obtain the real-time voltage and rated voltage of the power grid, the DC voltage and rated voltage of the hybrid DC load device; If the real-time voltage recovers to a preset threshold of the grid-side rated voltage and the DC voltage is less than the device's rated voltage, the grid is determined to have recovered to a fault-free state.

[0046] Specifically, the real-time voltage on the grid side is the actual voltage on the AC side of the receiving-end converter station, used to determine the grid recovery status. The rated voltage on the grid side is the rated voltage on the AC side of the receiving-end converter station. The preset threshold is the voltage criterion for determining grid recovery. The DC voltage of the hybrid DC load device is the DC voltage at the parallel terminal of the unloading branch.

[0047] S4. After all sub-modules have switched to the cut-off mode, output a preset DC voltage and control the energy dissipation valve module to switch to the locked state.

[0048] Specifically, the energy dissipation valve controller detects the current (unloading branch current drops to zero) and voltage (series voltage of submodule output capacitor) of all submodules, confirming that they have all entered the cut-off mode; the energy dissipation valve controller sends a lockout command to the energy dissipation valve module, all submodule devices remain off, the energy dissipation valve outputs a preset DC voltage to match the system voltage, and enters hot standby.

[0049] The controller directly receives signals from the grid and drives the energy-consuming valves, resulting in a short response path. Compared to the long process of coordinating wind farm output reduction, this significantly improves the speed of surplus power absorption and avoids DC voltage exceeding limits. The submodules are based on thyristors, offering lower cost and higher reliability compared to fully controlled devices. The submodules are series-reused, eliminating the need for redundant parallel connections, reducing the total number of devices and controlling hardware costs. Adaptable to different voltage levels and fault conditions, it is suitable for deep-sea wind power flexible DC systems.

[0050] In a preferred embodiment of the present invention, the energy-consuming valve has three main states: a locked state, an engaged state, and a deactivated state. A detailed analysis follows: Locked-out state: When the onshore AC grid is fault-free, the unloading device is in hot standby mode. All switching devices are turned off, the two capacitors in the submodule are connected in series, and the energy dissipation valve outputs the rated DC voltage. Engaged state: When a fault occurs in the AC grid of the receiving-end converter station, the HVDC system initiates fault ride-through, and the TDS-DCC is put into operation. At this time, the voltage at the onshore converter station's grid connection point drops to... , 0 <k<1。 When the voltage is too high (above 1.05 pu), the energy dissipation valve is activated, and all sub-modules are activated in a tiered manner, meaning that each sub-module group is activated sequentially according to the capacitor voltage ranking. Figure 5 (c) shows the input mode, which generates a stepped output voltage for the energy-consuming valve. The submodule group with the highest capacitor voltage enters this mode first. At this time, both centralized dissipation resistance and distributed resistance contribute to energy consumption, and the system's equivalent capacitance increases. Discharge, Decline. Resection status: When the voltage is low, the energy dissipation valve is disconnected, all submodules are disconnected in a stepped manner, the energy dissipation valve outputs its rated DC voltage, there is no current in the unloaded branch, and the system's equivalent capacitor is charged. rise.

[0051] The unloading device regulates the power dissipation of the device by periodically switching multiple groups of sub-modules, reducing the du / dt of the energy-consuming valve switching. When the energy-consuming valve control signal PWM is set to 1, the sub-modules are engaged in a step-by-step manner, the unloading device consumes energy, and the system DC voltage decreases; when the energy-consuming valve control signal is set to 0, the sub-modules are disengaged in a step-by-step manner, the wind farm inputs surplus power, and the system DC voltage increases.

[0052] In a preferred embodiment of the present invention, the rated input power of the wind farm is defined as follows: The rated DC voltage of the HVDC system is The number of TDS-DCC energy-consuming valve sub-modules is: The output voltage is The concentrated dissipation resistance is The voltage across its terminals is The magnitude of the unloading branch current is The system is operating without faults, and the device is locked. , All submodules are in the cut-off state. and All are off. and The submodule outputs the sum of the voltages of the two capacitors via a series anti-reverse diode. The rated voltage of the submodule capacitors is... satisfy: The most severe fault occurs on the AC grid side of the receiving end of the system: the onshore grid voltage drops to 0 p.u., and the receiving-end converter station completely loses its power transmission capacity. Under this condition, the unit dissipates all the power input from the wind farm, and all sub-modules are activated. and Closed, the capacitors within the module are connected in parallel. The expressions for the voltage and power of the lumped resistor and the energy dissipation valve are: ; ; When the unloading device is engaged, the current in the TDS-DCC branch satisfies: ; When a grid fault occurs on the AC side of the system, the surplus power factor is between 0 and 1, and the device enters a dynamic unloading state, with the energy dissipation valve periodically switching between on and off states. During the unloading valve control cycle, the average charging current and average discharging current of the system's equivalent capacitor satisfy the following: The rise and fall values ​​of the system DC voltage within one cycle should be the same. The duty cycle of the PWM control signal for the dissipation valve switching of the TDS-DCC should satisfy the following relationship: Solving for The energy-consuming valve control cycle (referred to as valve control cycle) of TDS-DCC. for: .

[0053] The specific block diagram of overall energy dissipation control is as follows: Figure 8 As shown, its working principle is as follows: Given a system DC voltage reference value, it is compared with the actual DC voltage on the DC side of the system's receiving-end converter station. This deviation is controlled by a PI controller to obtain the duty cycle of the energy dissipation valve control signal. This duty cycle is then modulated by PWM to generate corresponding switching actions, thereby achieving system DC voltage control and ensuring system power balance. The overall energy dissipation controller is used to quickly track the system DC reference voltage. All control is ultimately achieved through the action of switching devices. By changing the modulation index of the PWM modulation wave, the DC current in the unloading branch is adjusted, achieving DC injection control in the unloading branch, thus controlling the energy dissipation power of the centralized dissipation resistor and the distributed resistor within the submodule under the corresponding operating conditions. The control strategy of the TDS-DCC device is as follows: Figure 7 As shown, the reference DC voltage U ref (e.g., the system's rated DC voltage) and the actual DC voltage U dc The voltage deviation is obtained by comparison. The voltage deviation is divided by the rated DC voltage u. dcNThe absolute voltage deviation is converted to a per-unit value (dimensionless) for easier controller design. Proportional-integral (PI) control is applied to the deviation, and the output duty cycle signal D represents the proportion of the submodule's engagement time. A limiter restricts the range of D to prevent the duty cycle from exceeding the physically feasible range (e.g., 0 ≤ D ≤ 1). A triangular wave generator produces a carrier wave; the duty cycle signal D is compared with the triangular wave to generate a PWM signal for controlling the engagement / disengagement mode switching of the submodule. The delay (including the thyristor turn-off time t) is collected. Q Switching period T s Generate trigger signals ST2 for different power devices. a ST2 b The IGBT module T2 is controlled to turn on / off, establishing and breaking the equipotential point to provide reverse voltage for thyristor turn-off. Controlling the conduction of thyristor T1 requires triggering after the IGBT has established the equipotential point and the capacitor voltage is balanced to avoid device conflicts. A delay design ensures sufficient thyristor turn-off time to prevent short-circuit risks.

[0054] In a preferred embodiment of the present invention, such as Figure 9 The diagram showing the submodule average capacitor voltage control strategy uses the average capacitor voltage of each submodule as input. These voltage inputs are sorted and prioritized according to their average capacitor voltage magnitude. The purpose is to: prioritize the activation of submodules with lower voltages when activating a mode; prioritize the deactivation of submodules with higher voltages when deactivating a mode; ultimately achieving capacitor voltage balance across all submodules and preventing device damage due to excessive voltage deviation. The sorted signals generate PWM signals and trigger signals (ST1, ST2) for the power devices, and a delay is added to ensure safe device triggering timing.

[0055] In a preferred embodiment of the present invention, based on Figure 2 The TDS-DCC device structure shown is constructed using MATLAB / Simulink software as follows: Figure 12 shown A simulation model of an 800kV / 1100MW symmetrical monopolar offshore wind-powered flexible DC power transmission and grid-connected system was used to verify the topology of the device. The simulation parameters are shown in Table 1 below. Table 1 In the constructed model, the BTB-MMC in the offshore wind-flexible DC grid-connected system uses an average value model. REC-MMC employs voltage outer-loop control, and SEC-MMC uses PQ control to simulate the operation mode of the converter station on the wind farm side. According to the fault ride-through requirements of the offshore wind-flexible DC system, the peak DC bus voltage during TDS-DCC unloading should be less than 1.1 pu, and the ripple should be less than 10%. The peak voltage of the submodule capacitors should be less than 1.8 kV. Simulations verified the proposed PWM control strategy and the submodule capacitor voltage balancing strategy that incorporates the capacitor average voltage control component, and applied the proposed semi-controlled device shutdown control strategy. To accelerate the simulation, the number of submodule switching groups was reduced to 4 groups, and a single submodule structure was used to equivalently represent all series-connected submodules within a group. For TDS-DCC submodules within the same group, their submodule switching control signal SM-PWM is the same, and the action timing is the same; therefore, their distributed resistance and submodule capacitors can be considered as series-connected. The simulation uses one submodule equivalent to 70 series-connected submodules, with a distributed resistance of... The capacitance is To verify the effectiveness of the proposed topology and its control under all operating conditions, the grid connection point voltage was considered. The typical operating conditions of dropping to 0.5 PU, 0.8 PU and 0.3 PU were simulated respectively.

[0056] Under the operating conditions shown in the table above, the system operates normally before simulation time t=0.7s. After t=0.7s, the effective value of the equivalent power supply output voltage on the AC side of the onshore converter station MMC is modified to 0.5pu (208.21kV) to simulate a three-phase short-circuit fault in the onshore AC power grid, with surplus power factor... The system DC voltage was detected after 0.025 seconds. TDS-DCC is enabled, the unloading device is put into operation, the surplus power is absorbed, and the DC voltage of the system is kept stable. At t=1.3s, the effective value of the equivalent power supply output voltage on the AC side of the MMC of the shore converter station recovers to 1p.u., the fault of the offshore flexible DC system is recovered, and TDS-DCC is locked out of operation.

[0057] Simulation results are as follows Figure 13 , Figure 14 and Figure 15 As shown. Figure 13 (a) shows the waveform when k=0.5. Figure 13 (b) is a detailed waveform diagram of the result when k=0.5. Figure 14 The simulated waveform is when k=0.8. Figure 15 In the simulation waveform diagram when k=0.3, For the power transmitted from the MMC AC side of the receiving station, For the AC input power of the MMC at the sending station, This refers to the grid connection point voltage on the AC side of the REC-MMC. This refers to the grid-connected current on the AC side of the REC-MMC. The system DC voltage, To unload the branch current, This refers to the output voltage of the TDS-DCC energy-consuming valve. This represents the capacitor voltage of the TDS-DCC submodule. To make the waveform more intuitive, all physical quantities use standard values.

[0058] During fault ride-through, the TDS-DCC device consumes the system's surplus power to control the DC bus voltage to fluctuate around its rated value. The TDS-DCC valve control period is 100Hz. Regarding the surplus power factor... Under these operating conditions, the DC voltage ripple reaches its maximum value among all operating conditions, and the result is as follows: Figure 13 As shown in the figure, during fault ride-through, the TDS-DCC starts up, and the instantaneous current caused by the periodic switching on and off of the energy dissipation valves periodically impacts the system's equivalent inductance. Due to the presence of the unloading reactor and the adoption of a submodule grouping and step-by-step switching strategy, the DC voltage ripple caused by this impact is limited, making it smaller than the DC bus voltage ripple caused by the periodic charging and discharging of the system's equivalent capacitance. Overall ripple Within 3%. Meanwhile, the output voltage of the energy-consuming valve is within... arrive The current exhibits approximately two levels of rising and falling, with the unloading branch current ranging from 0 to... The intervals rise and fall in a stepped manner, with a duty cycle of [missing information]. =0.5. Submodules are centrally grouped and stepped in operation, with capacitor voltage fluctuations around 11%.

[0059] for The working conditions and the results are as follows Figure 14 As shown, during the startup of the TDS-DCC, the energy-consuming current causes a significant impact on the system's equivalent inductance. However, due to the presence of the unloading inductor and the adoption of a step-by-step switching strategy for submodule groups, the system DC voltage ripple caused by this impact is limited to the standard requirements, less than 3% in steady state. The power consumption drops to 0.2 pu, at which point the TDS-DCC power dissipation is 0.8 pu, and the PWM duty cycle of the power dissipation valve switching control signal... Approaching 1, all sub-modules are engaged, and the unloading branch current is... The unloading branch is equivalent to a voltage divider formed by a series connection of a concentrated dissipation resistor and a distributed resistor. and Each consumes half of the rated power, and the energy-consuming valve outputs 0.5 times the rated power. During fault ride-through, the DC voltage ripple was close to 0%, while the submodule capacitor voltage ripple was approximately 11%. Once the fault on the REC AC side was cleared, When restored to the rated value, the PWM duty cycle of the energy dissipation valve switching control signal... Rapidly dropped to the minimum value , At this point, each submodule begins periodic switching, causing instantaneous changes in the unloading branch current and resulting in a significant impact on the system's equivalent inductance. However, due to the presence of the unloading inductance and the adoption of a grouped, stepped switching strategy for submodules, the system's DC voltage ripple caused by this impact is still limited to the standard requirements. After the unloading device enable signal is set to 0, the IGBT control signals of each module are set to 1, waiting... The IGBT control signal is reset, and the unloading device is disconnected.

[0060] for The working conditions and the results are as follows Figure 15 As shown, the system DC voltage ripple is less than 3% during device operation.

[0061] This embodiment provides a power dissipation valve controller applicable to hybrid DC load devices. The hybrid DC load device is connected to the power grid and includes a power dissipation valve module connected to the power dissipation valve controller. The power dissipation valve module comprises several sub-modules, each including a thyristor. The input port of each sub-module is connected to the output port of the adjacent sub-module. The sub-module uses a thyristor as its core switch; its ability to naturally conduct upon voltage change from negative to positive eliminates the need for complex drive timing. The controller only needs to control the core actions of sub-module activation and deactivation, avoiding multi-dimensional drive control delays in all controlled devices and eliminating the need for complex drive protection circuits, thus directly reducing the cost of a single module. The power dissipation valve module uses a structure where the input port of the sub-module is connected to the output port of the adjacent sub-module. Flexible configuration of the number of sub-modules allows for adaptation to different voltage levels, eliminating the need for parallel redundant sub-modules or additional power units, further avoiding increased device redundancy. During grid faults, the energy dissipation valve controller directly acquires the energy dissipation valve switching signal from the grid side and controls the energy dissipation valve module to switch to the active state. In the active state, the energy dissipation valve controller determines the sub-module's activation priority by sampling the average capacitor voltage of the sub-modules within a preset control cycle, and then controls the sub-module to switch to the active mode, thereby driving the thyristor to conduct and the sub-module to participate in unloading. The entire process has no additional intermediate coordination links, with a short response path and low latency, which can quickly absorb surplus power to suppress DC voltage over-limit, meeting the millisecond-level response requirements of offshore wind power flexible DC systems. With a hardware architecture using thyristors and several sub-modules, efficient control logic achieves rapid absorption of surplus power, meeting the requirement of improving unloading control response speed under the premise of optimizing the number of devices.

[0062] See Figure 16 This is a schematic diagram of the unloading control system of a hybrid DC load device according to an embodiment of the present invention, including: a hybrid DC load device; the hybrid DC load device is connected to the power grid side, including an energy dissipation valve controller and an energy dissipation valve module; the energy dissipation valve module is connected to the energy dissipation valve controller, the energy dissipation valve module includes several sub-modules, each sub-module includes a thyristor, and the input port of the sub-module is connected to the output port of the adjacent sub-module; The energy-consuming valve controller is used to acquire the energy-consuming valve switching signal from the power grid side in the event of a power grid fault, and control the energy-consuming valve module to switch to the active state according to the energy-consuming valve switching signal; wherein, in the event of no power grid fault, the controller controls the energy-consuming valve module to remain in the locked state. After the energy dissipation valve module switches to the active state, the current average capacitor voltage of each sub-module is acquired within a preset control cycle. The average capacitor voltages are arranged in order to determine the active priority of the sub-modules. Based on the active priority, the sub-modules are controlled to switch to the active mode. In this mode, the thyristors are in the conducting state. When the power grid is restored to a fault-free state, the power consumption valve disconnection signal on the power grid side is obtained. Based on the power consumption valve disconnection signal, the power consumption valve module is controlled to switch to the disconnection state, and the sub-module is controlled to switch to the disconnection mode according to the current average capacitor voltage arrangement order. After all submodules have switched to the cut-off mode, a preset DC voltage is output and the energy dissipation valve module is controlled to switch to the locked state.

[0063] Specifically, the hybrid DC load device also includes a centralized dissipation resistor module and an unloading reactor; the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in sequence to form an unloading branch; The energy dissipation valve controller is also used to control the energy dissipation valve module to switch to the working state, drive the energy dissipation valve module to output an adjustable voltage so that the unloading branch forms a branch current; control the branch current to flow through the centralized dissipation resistor module so that the centralized dissipation resistor module dissipates surplus power; and control the unloading reactance to suppress sudden changes in the unloading branch current.

[0064] Preferably, the submodule further includes a first capacitor, a thyristor, a first diode, a bipolar transistor module, a distributed energy-dissipating resistor, and a second capacitor; one end of the first capacitor is connected to the input port of the submodule and the anode of the thyristor, and the other end is connected to the anode of the first diode; one end of the second capacitor is connected to the cathode of the first diode, and the other end is connected to the output port of the submodule and one end of the bipolar transistor module; one end of the distributed energy-dissipating resistor is connected to the cathode of the thyristor and the second capacitor, and the other end is connected to the bipolar transistor module. The energy-consuming valve controller is also used to control the branch current to flow through the thyristor, the distributed energy-consuming resistor and the bipolar transistor module after the submodule is switched to the working state. After the submodule is switched to the cut-off state, the bipolar transistor module is controlled to turn on to form an equipotential point, and a reverse voltage is applied to the thyristor so that the duration of the reverse voltage is greater than the turn-off time of the thyristor.

[0065] This invention provides a load shedding control system for a hybrid DC load device, including a hybrid DC load device connected to the power grid. The hybrid DC load device also includes an energy dissipation valve module connected to an energy dissipation valve controller. The energy dissipation valve module comprises several sub-modules, each including a thyristor. The input port of each sub-module is connected to the output port of the adjacent sub-module. The sub-module uses a thyristor as its core switch; its characteristic of automatically turning on when the voltage changes from negative to positive eliminates the need for complex drive timing. The controller only needs to control the core actions of sub-module activation and deactivation, avoiding multi-dimensional drive control delays of the entire control device and eliminating the need for additional complex drive protection circuits, directly reducing the cost of a single module. The energy dissipation valve module adopts a structure where the input port of the sub-module is connected to the output port of the adjacent sub-module. The flexible configuration of the number of sub-modules adapts to different voltage levels, eliminating the need for parallel redundant sub-modules or additional power units, further avoiding increased redundancy in the number of devices. During grid faults, the energy dissipation valve controller directly acquires the energy dissipation valve switching signal from the grid side and controls the energy dissipation valve module to switch to the active state. In the active state, the energy dissipation valve controller determines the sub-module's activation priority by sampling the average capacitor voltage of the sub-modules within a preset control cycle, and then controls the sub-module to switch to the active mode, thereby driving the thyristor to conduct and the sub-module to participate in unloading. The entire process has no additional intermediate coordination links, with a short response path and low latency, which can quickly absorb surplus power to suppress DC voltage over-limit, meeting the millisecond-level response requirements of offshore wind power flexible DC systems. With a hardware architecture using thyristors and several sub-modules, efficient control logic achieves rapid absorption of surplus power, meeting the requirement of improving unloading control response speed under the premise of optimizing the number of devices.

[0066] The above description represents the preferred embodiments of the present invention. It should be noted that those skilled in the art can make various improvements and modifications without departing from the principles of the present invention, and these improvements and modifications are also considered to be within the scope of protection of the present invention.

Claims

1. A method for unloading control of a hybrid DC load cell, characterized in that, A power dissipation valve controller is applicable to a hybrid DC load device. The hybrid DC load device is connected to the power grid side. The hybrid DC load device also includes a power dissipation valve module, which is connected to the power dissipation valve controller. The power dissipation valve module includes several sub-modules, each of which includes a thyristor. The input port of the sub-module is connected to the output port of the adjacent sub-module. The unloading control method for the hybrid DC load device includes: In the event of a power grid failure, the power consumption valve switching signal on the power grid side is acquired, and the power consumption valve module is switched to the active state according to the power consumption valve switching signal; wherein, in the event of no power grid failure, the power consumption valve module is controlled to remain in the locked state. After the energy dissipation valve module switches to the active state, the current average capacitor voltage of each sub-module is acquired within a preset control cycle. The average capacitor voltages are arranged in order to determine the active priority of the sub-modules. Based on the active priority, the sub-modules are controlled to switch to the active mode. In this mode, the thyristors are in the conducting state. When the power grid is restored to a fault-free state, the power consumption valve disconnection signal on the power grid side is obtained. Based on the power consumption valve disconnection signal, the power consumption valve module is controlled to switch to the disconnection state, and the sub-module is controlled to switch to the disconnection mode according to the current average capacitor voltage arrangement order. After all submodules have switched to the cut-off mode, a preset DC voltage is output and the energy dissipation valve module is controlled to switch to the locked state.

2. The unloading control method for a hybrid DC load device as described in claim 1, characterized in that, The hybrid DC load device also includes a centralized dissipation resistor module and an unloading reactor; the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in sequence to form an unloading branch; The energy dissipation valve controller is also used to control the energy dissipation valve module to switch to the working state, drive the energy dissipation valve module to output an adjustable voltage so that the unloading branch forms a branch current; control the branch current to flow through the centralized dissipation resistor module so that the centralized dissipation resistor module dissipates surplus power; and control the unloading reactance to suppress sudden changes in the unloading branch current.

3. The unloading control method for a hybrid DC load device as described in claim 2, characterized in that, The submodule also includes a first capacitor, a thyristor, a first diode, a bipolar transistor module, a distributed energy-dissipating resistor, and a second capacitor; one end of the first capacitor is connected to the input port of the submodule and the anode of the thyristor, and the other end is connected to the anode of the first diode; one end of the second capacitor is connected to the cathode of the first diode, and the other end is connected to the output port of the submodule and one end of the bipolar transistor module; one end of the distributed energy-dissipating resistor is connected to the cathode of the thyristor and the second capacitor, and the other end is connected to the bipolar transistor module. The energy-consuming valve controller is also used to control the branch current to flow through the thyristor, the distributed energy-consuming resistor and the bipolar transistor module after the submodule is switched to the working state. After the submodule is switched to the cut-off state, the bipolar transistor module is turned on to form an equipotential point, and a reverse voltage is applied to the thyristor so that the duration of the reverse voltage is greater than the turn-off time of the thyristor.

4. The unloading control method for a hybrid DC load device as described in claim 3, characterized in that, Arrange the average capacitor voltages in order to determine the priority of sub-module deployment, including: The average capacitor voltages are arranged in ascending order. Based on this order, the input priority of each submodule is determined from high to low, so that the submodules can be switched to the input mode sequentially according to their input priority from high to low.

5. The unloading control method for a hybrid DC load device as described in claim 4, characterized in that, The acquisition of the current average capacitor voltage for each submodule includes: Within a preset control cycle, acquisition delays are added to the rising and falling edges of the energy dissipation valve switching signal, and the capacitor voltages of the first capacitor and the second capacitor are obtained. The average value of the capacitor voltage of the first capacitor and the capacitor voltage of the second capacitor is used as the current average capacitor voltage of the submodule. The acquisition delay is calculated based on the preset minimum sampling interval, the number of sub-modules, and the current adjacent sub-module switching delay; the initial adjacent sub-module switching delay is a preset value.

6. The unloading control method for a hybrid DC load device as described in claim 5, characterized in that, The control submodule switches to the cut-off mode according to the current average capacitor voltage order, including: Arrange the current average capacitor voltage in descending order, determine the cut-off priority of the sub-module from high to low, and control the sub-modules to switch to the cut-off mode in descending order of cut-off priority.

7. The unloading control method for a hybrid DC load device as described in claim 6, characterized in that, Determining whether the power grid has returned to a fault-free state includes: Obtain the real-time voltage and rated voltage of the power grid, the DC voltage and rated voltage of the hybrid DC load device; If the real-time voltage recovers to a preset threshold of the grid-side rated voltage and the DC voltage is less than the device's rated voltage, the grid is determined to have recovered to a fault-free state.

8. A load unloading control system for a hybrid DC load cell, characterized in that, The device includes a hybrid DC load device; the hybrid DC load device is connected to the grid side and includes an energy dissipation valve controller and an energy dissipation valve module; the energy dissipation valve module is connected to the energy dissipation valve controller, and the energy dissipation valve module includes several sub-modules, each sub-module including a thyristor, and the input port of the sub-module is connected to the output port of the adjacent sub-module. The energy-consuming valve controller is used to acquire the energy-consuming valve switching signal from the power grid side in the event of a power grid fault, and control the energy-consuming valve module to switch to the active state according to the energy-consuming valve switching signal; wherein, in the event of no power grid fault, the controller controls the energy-consuming valve module to remain in the locked state. After the energy dissipation valve module switches to the active state, the current average capacitor voltage of each sub-module is acquired within a preset control cycle. The average capacitor voltages are arranged in order to determine the active priority of the sub-modules. Based on the active priority, the sub-modules are controlled to switch to the active mode. In this mode, the thyristors are in the conducting state. When the power grid is restored to a fault-free state, the power consumption valve disconnection signal on the power grid side is obtained. Based on the power consumption valve disconnection signal, the power consumption valve module is controlled to switch to the disconnection state, and the sub-module is controlled to switch to the disconnection mode according to the current average capacitor voltage arrangement order. After all submodules have switched to the cut-off mode, a preset DC voltage is output and the energy dissipation valve module is controlled to switch to the locked state.

9. The unloading control system for a hybrid DC load cell as described in claim 8, characterized in that, The hybrid DC load device also includes a centralized dissipation resistor module and an unloading reactor; the energy dissipation valve module, the centralized dissipation resistor module, and the unloading reactor are connected in sequence to form an unloading branch; The energy dissipation valve controller is also used to control the energy dissipation valve module to switch to the working state, drive the energy dissipation valve module to output an adjustable voltage so that the unloading branch forms a branch current; control the branch current to flow through the centralized dissipation resistor module so that the centralized dissipation resistor module dissipates surplus power; and control the unloading reactance to suppress sudden changes in the unloading branch current.

10. The unloading control system for a hybrid DC load cell as described in claim 9, characterized in that, The submodule also includes a first capacitor, a thyristor, a first diode, a bipolar transistor module, a distributed energy-dissipating resistor, and a second capacitor; one end of the first capacitor is connected to the input port of the submodule and the anode of the thyristor, and the other end is connected to the anode of the first diode; one end of the second capacitor is connected to the cathode of the first diode, and the other end is connected to the output port of the submodule and one end of the bipolar transistor module; one end of the distributed energy-dissipating resistor is connected to the cathode of the thyristor and the second capacitor, and the other end is connected to the bipolar transistor module. The energy-consuming valve controller is also used to control the branch current to flow through the thyristor, the distributed energy-consuming resistor and the bipolar transistor module after the submodule is switched to the working state. After the submodule is switched to the cut-off state, the bipolar transistor module is turned on to form an equipotential point, and a reverse voltage is applied to the thyristor so that the duration of the reverse voltage is greater than the turn-off time of the thyristor.

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