AC low voltage unbalanced fault ride-through method based on electromotive force reconstruction of energy-consuming submodules

Abstract

The present invention relates to high-voltage direct current (HVDC) transmission technology, and aims to provide a method for crossing an AC low-voltage unbalanced fault by reconstructing the electromotive force of an energy-consuming submodule. The method comprises: connecting a plurality of identical energy-consuming submodules in series between the bridge arm inductance of each bridge arm and the midpoint of the upper and lower bridge arms in the MMC converter valve; collecting the voltage and current signals on the AC side of the converter valve in real time and extracting the fault characteristic quantity, and calculating the difference between the output voltage amplitude of each bridge arm and the normal operation; controlling the on and off of the switching devices in the energy-consuming submodule with the goal of reconstructing the electromotive force balance between each bridge arm; changing the damping of the energy-consuming resistor in each bridge arm by switching the energy-consuming submodule on or off, so that each bridge arm in the MMC converter valve achieves electromotive force balance while utilizing the energy-consuming resistor to achieve power balance. The present invention is based on the control of the bridge arm electromotive force balance, which can fundamentally solve the problem of single-phase voltage drop; makes the switching operation of the device simple, the control logic easy to implement, and can maintain the stability of the DC bus voltage.

Description

Technical Field

[0001] The present invention belongs to the field of high-voltage direct current (HVDC) transmission, and in particular is an AC low-voltage unbalanced fault ride-through method for reconstructing the electromotive force of an energy-consuming submodule in a high-voltage flexible HVDC transmission system. Background Art

[0002] With the booming development of high-voltage flexible direct current (HVDC) transmission systems, modular multilevel converters (MMCs) are widely used in numerous power transmission scenarios due to their unique advantages. Composed of numerous submodules connected in a cascaded fashion, MMCs possess numerous remarkable characteristics, including extremely high-quality output waveforms with minimal harmonic content, significantly reducing grid pollution. Their low switching frequency effectively reduces switching losses and electromagnetic interference, improving system stability. Furthermore, MMCs are easily scalable, allowing for flexible adjustments based on actual transmission needs. These characteristics enable efficient, large-capacity, long-distance power transmission. In the area of ​​renewable energy grid integration, MMCs facilitate the stable integration of clean energy sources such as wind and solar power into the grid, promoting a green transition in the energy mix. In urban power grids, MMCs ensure a reliable power supply to meet growing urban electricity demand, playing an indispensable role in the normal operation and development of cities.

[0003] Under complex real-world operating conditions, various AC-side imbalance faults can severely impact the safe and stable operation of DC transmission systems. Faults such as single-phase grounding and two-phase short circuits not only cause overcurrent and overvoltage, leading to equipment damage and uneven energy distribution in the system, resulting in energy surpluses, but also generate significant harmonics. Harmonics can severely distort the current and voltage waveforms of converter valves, subjecting the power electronics within them to additional thermal and electrical stress. Long-term exposure to these stresses significantly shortens the lifespan of key converter valve components, such as thyristors and IGBTs, increasing maintenance costs and replacement frequency. Furthermore, the additional losses caused by harmonics can increase the temperature of converter valves, affecting their electrical performance and reducing operational reliability. They can even cause malfunctions, potentially leading to the failure of the entire DC transmission system.

[0004] While modular multilevel converters (MMCs) offer excellent performance, they still require MMC control strategies for normal operation, especially maintaining transmission system stability under various fault conditions. However, when faced with AC low-voltage imbalance faults (typically when the AC voltage falls below 70% to 85% of the rated voltage during normal operation), traditional MMC control strategies still have limitations: they lack effective measures for handling excess energy and are even more difficult to mitigate against harmonics that can damage converter valves.

[0005] For example, the traditional positive-negative sequence separation control matrix commonly used in MMC control strategies is as follows:

[0006] (1)

[0007] Where P represents active power, Q represents reactive power, the subscript dc represents the constant components of active and reactive power, the subscript sd represents the double-frequency cosine ripple components of active and reactive power, and the subscript sq represents the double-frequency sine ripple components of active and reactive power. There are four controllable current quantities in formula (1), but there are six required control power quantities. Therefore, a trade-off must be made. In order to ensure the stability of the DC grid under unbalanced grid conditions, it is usually necessary to give P dc , Q dc and P sd =P sq = 0 to suppress the double frequency fluctuation of DC bus voltage. This control strategy also means that the double frequency fluctuation of reactive power (Q sd , Q sq ) will not be controlled, resulting in the presence of negative sequence current in the AC current, distorting the overall waveform and further subjecting the power electronic devices inside the converter valve to additional thermal and electrical stress. In addition, in the event of an unbalanced AC grid fault, power cannot be transmitted according to the rated power. Therefore, surplus power will be generated between the sending and receiving ends of the flexible high-voltage DC grid, causing the DC bus voltage to increase. When the voltage rises to the protection threshold of the transmission system, the transmission system will trigger the protection threshold and cause the system to shut down. Therefore, when adopting the traditional positive and negative sequence separation control strategy, it is necessary to additionally configure a dedicated harmonic compensation device or use an energy dissipation device to dissipate the surplus power in the event of an AC short circuit fault. This will obviously lead to increased hardware costs and more complex control schemes.

[0008] When using energy dissipation devices, existing technologies often employ DC energy dissipation devices (connected in parallel with the DC bus) or MMC energy dissipation devices (connected in parallel with the MMC converter valve submodule). These solutions primarily address fault ride-through from the perspective of energy surplus. However, due to the decoupling of AC and DC grids by MMC, energy dissipation devices can only dissipate energy on the DC bus, and their effectiveness in suppressing harmonics on the AC side is limited.

[0009] For example, patent publication CN102820646A proposes a device and method for controlling grid fault ride-through in a flexible DC transmission system. This device describes a three-stage power release structure consisting of a switchable braking resistor unit, a power voltage regulator unit, and a full-bridge braking unit. This solution detects the DC bus voltage and AC voltage, sequentially switching on the three-stage braking resistors in the event of a fault, and dynamically adjusting the power release. The core technical concept is to achieve DC-side overvoltage suppression through "passive energy dissipation" through a multi-stage resistor network, a traditional energy balancing strategy.

[0010] Patent publication CN117439394A proposes an energy-balancing flexible DC converter valve and control method, describing the integration of an energy-balancing circuit (power electronic switch + energy-draining resistor) within the MMC submodule. This solution monitors the submodule capacitor voltage and, when overvoltage occurs, switches on the resistor to discharge energy, collaborating with an AC energy dissipation device at the sending end to assist in energy dissipation. The core technical concept is to achieve capacitor voltage balancing through "passive energy dissipation" at the submodule level.

[0011] Patent publication CN119341066A proposes a series-connected distributed DC energy consumption device and offshore wind power flexible DC system. The document describes a topology consisting of a cascaded centralized and distributed energy consumption module. In the event of a fault, a controllable switch is turned on to allow current to flow through a network of energy-consuming resistors. This solution uses pulse-width modulation (PWM) to control the switching of distributed energy-consuming units, achieving smooth consumption of surplus power. The core technical concept is to distribute energy consumption through a "distributed resistor network" to reduce thermal stress on centralized resistors. This represents a hardware optimization strategy for DC-side power balancing.

[0012] Patent publication CN118739280A proposes a DC energy dissipation device and DC power transmission system. It describes a hybrid structure combining centralized and distributed energy dissipation modules in parallel, with controllable switching units controlling the switching of distributed energy dissipation resistors. This solution switches on a resistor network in energy dissipation mode, combining it with PWM modulation to suppress DC power fluctuations. The core technical concept is to achieve power dissipation smoothing through a "hybrid energy dissipation structure." This strategy is a DC-side performance optimization strategy and does not include an electromotive force control mechanism for AC-side imbalance faults.

[0013] The applicant previously filed a patent application (publication number CN119315614A) describing an MMC converter valve and method for AC / DC fault ride-through based on energy-dissipating submodules. This document, based on the topology of a conventional MMC converter valve, features M identical energy-dissipating submodules connected in series between the arm inductor and the midpoint of each upper and lower arm. While this solution represents a novel technology compared to previous energy-dissipating device solutions, the utilization and control strategy for the series-connected energy-dissipating submodules remains based on traditional damping. Once an AC unbalanced fault causes a single-phase voltage drop, the traditional positive-negative sequence separation control strategy remains irreversible and incapable of suppressing AC-side harmonics. This control strategy shuts off the main current-carrying branch in the event of an AC / DC short-circuit fault or an AC-side short-circuit fault, diverting the fault current to the energy-dissipating branch. The energy-dissipating resistor Rd is then used to limit the inrush current amplitude or dissipate excess power. Therefore, its core concept remains energy dissipation, addressing faults from the perspective of energy surplus. For example, in paragraph 0014 of its specification, it is stated that: “When an AC / DC short circuit fault or an AC side short circuit fault occurs, the switch is switched to only the energy dissipation branch connected in series in the MMC bridge arm circuit, and the energy dissipation resistor R d Energy consumption", and emphasizes "using energy dissipation resistor R d Limit the impact current amplitude after a DC short circuit fault occurs, or use energy dissipation resistor R d Consume the surplus power of the transmission system caused by the AC side ground fault." This shows that the core idea of ​​its technical implementation is to directly consume energy through energy-dissipating resistors, which is a typical traditional "energy balance" thinking. In paragraph 0053 of its manual, it is stated: "For DC faults, the energy-dissipating resistor Rd limits the amplitude of the DC short-circuit fault impact current; for AC faults, the energy-dissipating resistor Rd consumes the surplus power between the sending and receiving ends to maintain the stability of the high-voltage flexible DC transmission system." After the fault is cleared, "the high-voltage flexible DC transmission system is restarted and the switching devices (i.e., the thyristor T1 and IGBT T2) are turned on, and the MMC converter valve is put back into normal operation." The content here further highlights that its control logic always revolves around "energy consumption" and the essence of fault ride-through is the passive consumption of surplus energy.

[0014] Therefore, the present invention proposes a new solution to solve the system stability problem under AC imbalance fault through a new control method, especially the hidden dangers of energy surplus and the damage of harmonics to the converter valve. Summary of the Invention

[0015] The technical problem to be solved by the present invention is to overcome the deficiencies in the prior art and propose a flexible direct current transmission AC unbalanced fault ride-through method based on an energy-consuming submodule.

[0016] To solve the technical problem, the solution of the present invention is:

[0017] Provided is an AC low voltage unbalanced fault ride-through method for reconstructing the electromotive force of an energy-consuming submodule, comprising:

[0018] In the MMC converter valve, multiple identical energy-consuming submodules are connected in series between the bridge arm inductance and the midpoints of the upper and lower bridge arms of each bridge arm. The energy-consuming submodules include a parallel main flow branch, an energy-consuming branch, and a bypass loop. A switching device is provided in the main flow branch, and an energy-consuming resistor is provided in the energy-consuming branch.

[0019] Real-time acquisition of voltage and current signals on the AC side of the converter valve, extraction of fault characteristics, and calculation of the difference between the output voltage amplitude of each bridge arm and that during normal operation;

[0020] With the goal of reconstructing the electromotive force balance between each bridge arm, the on-off of the switching devices in the energy-consuming submodule is controlled, and the energy-consuming resistor is used as the electromotive force regulating device. By switching on or off the energy-consuming submodule, the damping of the energy-consuming resistor in each bridge arm is changed, so that each bridge arm in the MMC converter valve can achieve electromotive force balance while using the energy-consuming resistor to achieve power balance.

[0021] As a preferred solution of the present invention, in the case of an AC imbalance fault, the number of energy-consuming submodules required to be put into operation or removed in each bridge arm of the MMC converter valve is calculated according to the following formula:

[0022] ;

[0023] In the above formulas, N dissx is the number of energy-consuming submodules in each bridge arm; R dissx Represents the total damping of the energy-consuming submodules in each bridge arm; R smdiss Indicates the damping size of a single energy-consuming submodule; ΔU sx Indicates the difference between the three-phase voltage output by the converter valve in the case of AC unbalanced fault and the normal operating state; u sx Indicates the three-phase voltage output by the converter valve under normal operating conditions; u sfx Indicates the three-phase voltage output by the converter valve in the event of a fault; i ox Indicates the three-phase current output by the converter valve; x=a, b, c in the above symbols respectively refer to the three bridge arms in the converter valve or the output side of the converter valve corresponding to each bridge arm.

[0024] As a preferred solution of the present invention, by changing the damping of the energy-consuming resistor in each bridge arm, the output voltage of each bridge arm in the MMC converter valve is made close to the AC side voltage during normal operation.

[0025] As a preferred solution of the present invention, a voltage sensor and a current sensor are set on the AC side bus of the MMC converter valve to respectively collect the voltage signal and current signal on the AC side; the collected signals are transmitted to the signal processing and analysis circuit, and the signals are filtered, amplified and analog-to-digital converted to obtain the fault characteristic quantity.

[0026] As a preferred solution of the present invention, the fault characteristic quantity includes the amplitude components and phase quantity of the voltage and current on the AC side.

[0027] As a preferred solution of the present invention, the main control unit compares the fault characteristic quantity with the preset fault threshold, and determines whether an AC unbalance fault has occurred based on the voltage and current positive and negative sequence separation algorithm; if the judgment result is that a fault has occurred, the calculation result is further compared with the preset conditions to confirm the fault type and fault degree, and then a warning message is issued in the human-computer dialogue interface.

[0028] As a preferred solution of the present invention, the main control unit calculates the energy-consuming resistor damping required to be switched on or off in each bridge arm of the MMC converter valve in response to the current fault, generates a corresponding control signal, and transmits it to the drive circuit; the latter controls the switching devices of the energy-consuming submodules in each bridge arm based on the signal, switching on or off the corresponding number of energy-consuming resistors.

[0029] As a preferred solution of the present invention, the main control unit is a microprocessor or a programmable logic device.

[0030] The present invention also provides a computer device, comprising: at least one processor, and a memory communicatively connected to the at least one processor, wherein the memory stores instructions executed by the at least one processor, and the instructions are executed by the at least one processor to enable the at least one processor to execute the aforementioned AC low voltage unbalanced fault ride-through method for reconstructing the electromotive force of the energy-consuming sub-module.

[0031] The present invention also provides a computer-readable storage medium, characterized in that the computer-readable storage medium stores computer instructions, and the computer instructions are used to enable the computer to execute the aforementioned AC low voltage unbalanced fault ride-through method for reconstructing the electromotive force of the energy-consuming sub-module.

[0032] Compared with the prior art, the present invention has the following beneficial effects:

[0033] 1. The present invention breaks through the traditional technical thinking in the existing technology, and changes the energy-consuming damper that was originally used as a power consumption device into an electromotive force regulating device; the original control target of power consumption balance is changed to the control target of the electromotive force balance of the three bridge arms in the MMC converter valve. By controlling the input or removal of the energy-consuming resistor, a new balanced electromotive force is rebuilt between the three bridge arms inside the MMC under the condition of AC imbalance, thereby suppressing overcurrent and harmonics under AC imbalance fault. Since the electromotive force of each bridge arm remains balanced, in fact, the power balance of each bridge arm is also achieved at the same time. More importantly, the present invention fundamentally solves the problem of single-phase voltage drop based on the control of the electromotive force balance of the bridge arm. Therefore, there is no need to give up part of the power control amount, which further causes the negative sequence current to cause the overall waveform to be distorted, as in the traditional positive and negative sequence separation control strategy.

[0034] 2. Compared to existing control strategies for various energy-consuming devices, the method of the present invention effectively suppresses grid harmonics under unbalanced grid conditions, reducing the harmful effects of harmonics on converter valves. By suppressing harmonics, the additional thermal and electrical stresses on the power electronic components within the converter valves are reduced, extending the service life of key components such as thyristors and IGBTs, and lowering equipment maintenance costs and replacement frequency. Furthermore, by reducing the additional losses caused by harmonics, the method prevents temperature rise in the converter valves, improves valve operation reliability, and reduces the risk of DC transmission system failure due to converter valve failure.

[0035] 3. This invention efficiently dissipates excess power generated during AC short-circuit faults, preventing the generation of excess power between the transmitting and receiving ends of the flexible HVDC grid from causing an increase in the DC bus voltage, which in turn triggers the transmission system protection threshold and causes the system to shut down. This helps maintain DC bus voltage stability, ensures the continuous and stable operation of the DC transmission system, and improves the system's power supply reliability.

[0036] 4. The control and calculation scheme of the present invention is concise and has clear functions. Compared with the traditional positive-negative sequence separation control strategy, the amount of calculation is greatly reduced. In addition, the scheme of the present invention can directly determine the stable operating point of the system under the fault scenario through theoretical analysis combined with the open-loop control strategy. Compared with the closed-loop control method adopted by other schemes, it does not need to rely on the real-time feedback adjustment process, avoiding the response delay of the dynamic adjustment of the closed-loop system, thereby more efficiently prompting the device to quickly reach a steady-state operating state during the fault ride-through process. The device switching operation is simple and the control logic is easy to implement. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 This is the topology of the MMC converter valve used to achieve AC / DC fault ride-through in the present invention.

[0038] Figure 2This is the AC side circuit of the MMC converter valve in normal and fault conditions.

[0039] Figure 3 The waveform is a simulation of a two-phase voltage drop (80% drop) on the AC side.

[0040] Figure 4 The DC bus voltage of the fault ride-through under AC unbalanced fault conditions is compared between the conventional solution using a DC energy dissipation device and the method of the present invention.

[0041] Figure 5 The active and reactive powers of the conventional scheme using DC energy dissipation devices and the method of the present invention are compared in the case of AC unbalanced fault.

[0042] Figure 6 The fault-crossing bridge arm current is compared between the conventional solution using a DC energy dissipation device and the method of the present invention under the condition of an AC unbalanced fault.

[0043] Figure 7 The voltage of the fault ride-through submodule is compared between the conventional solution using a DC energy dissipation device and the method of the present invention under the condition of an AC unbalanced fault. DETAILED DESCRIPTION

[0044] The present invention will be described in further detail below with reference to the accompanying drawings.

[0045] 1. Description of the topological structure of the MMC converter valve

[0046] As an example, in Figure 1 The topology of the MMC converter valve used in the fault ride-through method of the present invention is shown in FIG. This converter valve topology is substantially the same as that described in patent application document CN119315614A and will not be further described in the present invention.

[0047] In each bridge arm of the MMC converter valve, the number of MMC sub-modules and energy-consuming sub-modules connected in series, as well as the selection of capacitors and switching devices, are all determined by technical personnel based on the actual situation of the high-voltage flexible direct current transmission system.

[0048] It should be pointed out that although the present invention adopts a converter valve topology structure that is basically the same as the prior art, the various circuit structures in the energy-consuming submodule actually play a role that is different from the prior art but perfectly matches the specific control strategy of the present invention. Among them, the main flow branch carries the main current of the bridge arm during normal operation. When an imbalance of the electromotive force on the AC side is detected, the main flow branch is shut down and the current is transferred to the energy-consuming branch; the energy-consuming branch is composed of energy-consuming resistors. While the fault current flows through the energy-consuming resistors to dissipate energy, the resistor module on the energy-consuming branch is regarded as an equivalent voltage source, which assists the MMC converter valve in building a stable and reliable three-phase electromotive force, further suppresses the harmonic components in the AC current, and improves the symmetry of the electromotive force waveform. The bypass circuit is composed of a bypass switch. When an energy-consuming submodule fails or needs to exit operation, the bypass switch is closed to isolate it to prevent the fault from spreading and affecting the overall electromotive force balance.

[0049] 2. Description of AC Unbalanced Fault Ride-Through Method Using Coordinated Reconstruction of the Electromotive Force of Energy-consuming Submodules

[0050] 1. Based on the aforementioned converter valve topology, the present invention proposes an AC low-voltage unbalanced fault ride-through method that reconstructs the electromotive force of energy-consuming submodules. Its essence lies in balancing power and achieving fault ride-through by building a reliable electromotive force. This method leverages the additional electromotive force generated by the energy-consuming submodules to compensate for the missing electromotive force during an AC-side unbalanced fault. This method, in conjunction with the MMC converter valve body, builds a balanced electromotive force in the event of an AC low-voltage unbalanced fault, further eliminating the effects of harmonics.

[0051] 2. The fault ride-through method specifically includes:

[0052] (1) Real-time acquisition of voltage and current signals on the AC side of the converter valve, extraction of fault characteristics, and calculation of the difference between the output voltage amplitude of each bridge arm and that during normal operation;

[0053] (a) Specifically, it is manifested in the signal acquisition link: voltage sensors and current sensors are set on the AC side busbar of the MMC converter valve to collect the voltage signal on the AC side of the converter valve in real time. U a,b,c and current signal I a,b,c The collected signal is sent to the signal processing and analysis circuit, which filters, amplifies, and converts the signal into analog-to-digital data to obtain the fault characteristic quantity. The fault characteristic quantity includes the amplitude component and phase quantity of the voltage and current on the AC side.

[0054] (b) Specifically, it involves signal processing and fault diagnosis: The main control unit compares the fault signature with a preset fault threshold and determines whether an AC imbalance fault has occurred based on a voltage and current positive- and negative-sequence separation algorithm. If a fault is detected, the calculated result is further compared with preset conditions to confirm the fault type and severity, and a warning message is then issued on the human-computer interface. The main control unit can be a microprocessor or a programmable logic device.

[0055] (2) With the goal of reconstructing the electromotive force balance between each bridge arm, the on-off of the switching devices in the energy-consuming submodule is controlled, and the energy-consuming resistor is used as the electromotive force regulating device; by switching on or off the energy-consuming submodule, the damping of the energy-consuming resistor in each bridge arm is changed, so that each bridge arm in the MMC converter valve can achieve power balance by using the energy-consuming resistor while achieving electromotive force balance.

[0056] (c) Specifically, it is manifested in the signal generation and control link: the main control unit calculates the energy-consuming resistor damping of each bridge arm in the MMC converter valve required to respond to the current fault, generates the corresponding control signal and transmits it to the drive circuit.

[0057] In the AC side low voltage unbalance fault state, the three-phase voltage output by each bridge arm of the MMC converter valve is different from that in the normal operating state, as shown in the following formula:

[0058] (2)

[0059] In order to restore the unbalanced electromotive force on the AC side to a normal balanced state as much as possible under the fault condition, the present invention innovatively proposes to use the energy-consuming damping R diss The additional electromotive force brought by the energy dissipation damping makes the output voltage of each bridge arm approach u sx .

[0060] Based on the above principles, the required energy dissipation damping can be obtained by solving the following formula:

[0061] (3)

[0062] The number of energy-consuming submodules required is further calculated based on the damping of the energy-consuming resistor configured in the MMC converter valve:

[0063] (4)

[0064] (d) Specifically, this is manifested as a damping adjustment step: The drive circuit, based on control signals, controls the on / off switching of the energy-consuming submodules in each bridge arm, switching in or out the corresponding number of energy-consuming resistors. This allows the MMC converter valve to reestablish a new equilibrium electromotive force in the event of an unbalanced fault, thereby suppressing overcurrent and harmonics caused by AC unbalanced faults, ultimately bringing the output voltage of each bridge arm of the MMC converter valve closer to the AC side voltage during normal operation.

[0065] Figure 2 The diagram shows the electromotive force diagram of the MMC AC side in normal state and fault condition.

[0066] exist Figure 2 In the above formulas, u diffx (x=a,b,c) represents the differential mode voltage of the MMC three-phase bridge arm, L ac Indicates the AC side reactance; L arm Represents the bridge arm reactance; R eq Represents the stray resistance inside the converter valve bridge arm; N dissx is the number of energy-consuming submodules in each bridge arm; R dissx Represents the total damping of the energy-consuming submodules in each bridge arm; R smdiss Indicates the damping size of a single energy-consuming submodule; ΔU sx Indicates the difference between the three-phase voltage output by the converter valve in the case of AC unbalanced fault and the normal operating state; u sx Indicates the three-phase voltage output by the converter valve under normal operating conditions; u sfx Indicates the three-phase voltage output by the converter valve in the event of a fault; i ox represents the three-phase current output by the converter valve. (Due to damped EMF reconstruction technology, the difference between the current under fault conditions and normal operation is negligible.) In the aforementioned symbols, x = a, b, and c refer to the three bridge arms of the converter valve or the output side of the converter valve corresponding to each bridge arm, respectively.

[0067] 3. Although the publicly available document CN119315614A also employs a fundamentally similar converter valve topology, its approach to utilizing the series-connected energy-consuming submodules and its control strategy differ substantially from the present invention. This document's control strategy remains based on traditional damping and dissipation. Its technical approach does not address the control of the electromotive force balance of each bridge arm of the MMC converter valve, nor does it achieve active balancing by adjusting the bridge arm electromotive force. Once an AC imbalance fault occurs, resulting in a single-phase voltage drop, the traditional positive-negative sequence separation control strategy remains irreversible and incapable of suppressing AC-side harmonics. This document never mentions the goals of "electromotive force balance" or "harmonic suppression," and the embodiments provided to address the technical issues only utilize "power dissipation through energy-consuming resistors" as the sole means. Therefore, in the event of an AC-side short-circuit fault, this solution can only dissipate excess power between the transmitter and receiver through energy-consuming resistors to maintain system stability, but its effectiveness in suppressing harmonics generated by AC-side imbalance faults is limited.

[0068] This invention significantly differs from the solution in the public document CN119315614A. It innovatively proposes transforming the energy-consuming resistor from an "energy consumer" into an "electromotive force regulator" by establishing a bridge-arm electromotive force compensation model. This fundamentally changes the control logic and breaks through conventional technical thinking. Therefore, the corresponding control strategy targets the electromotive force balance of the three bridge arms of the MMC converter valve. By controlling the activation and deactivation of the energy-consuming resistor, each bridge arm within the MMC reestablishes a new balanced electromotive force under AC imbalance conditions. This approach suppresses overcurrent and harmonics during AC imbalance faults, fundamentally resolving the waveform distortion caused by single-phase voltage sags and negative-sequence currents, and effectively suppressing AC-side harmonics.

[0069] 4. The solution in the public document CN119315614A mentions that the switching of MMC submodules during the control process will cause different voltage offsets, which require balanced control. It should be noted that this control of voltage fluctuations is completely different from the "control of the electromotive force balance of each bridge arm of the MMC converter valve" described in this invention.

[0070] The control of voltage fluctuation in this document is to eliminate the DC bus voltage fluctuation caused by switching on and off the energy-consuming submodule. Specifically, when the energy-consuming submodule is switched on due to an AC short circuit fault, the bridge arm inductor will introduce a DC bias voltage U dc , it is necessary to adjust the voltage by cutting off some MMC submodules; when the current is reversed, the energy-consuming submodule will generate a reverse bias voltage U biasTo balance this bias, the number of MMC submodules must be increased. The core objective is to maintain DC voltage stability and prevent voltage fluctuations from triggering protection thresholds and causing system lockout. Therefore, this technical solution can only smooth DC bus voltage fluctuations and prevent system lockout due to overvoltage or undervoltage, but it cannot address issues such as harmonic distortion and negative sequence current caused by AC-side imbalance. Its application is limited to energy dissipation in AC / DC short-circuit faults and cannot address complex unbalanced operating conditions such as single-phase or two-phase voltage drops.

[0071] Different from the literature, the goal of the present invention is to actively reconstruct the electromotive force balance of the three-phase bridge arm on the AC side. Specifically, by calculating the difference ΔU between the output voltage of each bridge arm and the normal operating value in real time sx , dynamically switching energy-consuming submodules to change the bridge arm damping and compensate for the missing electromotive force. Its core is to suppress overcurrent, harmonics, and negative-sequence current caused by unbalanced faults on the AC side, and solve the waveform distortion problem caused by the traditional positive and negative sequence control strategy abandoning part of the power control amount. The present invention can not only dissipate surplus power, but also suppress AC side harmonics through electromotive force reconstruction to achieve three-phase bridge arm power balance. The applicable scenarios of the present invention are extended to AC low voltage unbalanced faults, which can fill the gaps in harmonic suppression and electromotive force control of traditional strategies.

[0072] 3. A specific verification example

[0073] This embodiment is constructed in PSCAD simulation software Figure 1 The topology and control model of the MMC converter valve is shown in Figure 1. The specific parameters are shown in Table 1.

[0074] Table 1 Simulation parameters of HVDC system based on energy consumption submodule

[0075] .

[0076] The simulation results of the control process are as follows Figures 3 to 7 shown.

[0077] like Figure 3 As shown in the figure, the simulation is set to occur at 0.5s when an AC asymmetric fault occurs. The fault voltage of phases A and B drops to 0.2 times of the original value and lasts for 1s. After the fault occurs, the protection system detects the voltage drop and collects the AC side voltage u sx With current i ox The collected voltage and current signals are transmitted to the signal processing and analysis circuit, where they are filtered, amplified, and converted into analog-to-digital signals before the fault signature is extracted. The main control unit compares the fault signature with the preset fault threshold and, based on the voltage and current positive and negative sequence separation algorithm, determines whether an AC imbalance fault has occurred and confirms the fault type and severity.

[0078] For example, the voltage amplitude differences between phases A and B under the current unbalanced fault condition and the normal operating condition are calculated as follows:

[0079] .

[0080] Further calculation shows that the total energy-consuming resistor damping required in the three-phase bridge arm of the MMC converter valve under the current asymmetric fault condition is:

[0081] .

[0082] Then, based on the damping of the energy-consuming submodules configured in the MMC converter valve, the number of energy-consuming submodules required for phases A and B is further calculated as:

[0083] .

[0084] The main control unit generates a control signal and transmits it to the drive circuit to control the on / off of the switch devices of the energy-consuming submodules in each bridge arm, and to switch on or off a corresponding number of energy-consuming resistors, so that each bridge arm in the MMC converter valve achieves electromotive force balance while using energy-consuming resistors to achieve power balance.

[0085] Figures 4 to 7 The waveform comparison of the traditional DC energy dissipation device and the present invention under AC unbalanced fault conditions is shown. Based on the comparison of the various figures, it can be seen that compared with the traditional solution, the present invention can effectively maintain the stability of the DC bus voltage, reduce the power fluctuation during the AC unbalanced fault ride-through process, effectively suppress the internal bridge arm current harmonics of the MMC converter valve under unbalanced grid working conditions, reduce the harm of harmonics to the converter valve, improve the reliability of the converter valve operation, and reduce the risk of DC transmission system paralysis due to converter valve failure. In addition, during the AC low voltage unbalanced fault ride-through process, the submodule voltage of the present invention can be maintained near the rated value, and the fluctuation is small, which is conducive to rapid recovery after the AC fault is cleared.

[0086] Therefore, the AC low-voltage unbalanced fault ride-through method for reconstructing the electromotive force of the energy-consuming sub-module provided by the present invention can effectively dissipate surplus power while suppressing grid harmonics under unbalanced grid working conditions, thereby improving the safety and stability of the high-voltage flexible direct current transmission system under AC unbalanced fault ride-through conditions, which is of great significance for promoting the rapid development and safety assurance of the flexible direct current transmission system.

Claims

1. A method for AC low voltage unbalanced fault ride-through based on electromotive force reconstruction of energy-consuming submodules, characterized in that: include: In the MMC converter valve, multiple identical energy-consuming submodules are connected in series between the bridge arm inductance and the midpoints of the upper and lower bridge arms of each bridge arm. The energy-consuming submodules include a parallel main flow branch, an energy-consuming branch, and a bypass loop. A switching device is provided in the main flow branch, and an energy-consuming resistor is provided in the energy-consuming branch. Real-time acquisition of voltage and current signals on the AC side of the converter valve, extraction of fault characteristics, and calculation of the difference between the output voltage amplitude of each bridge arm and that during normal operation; With the goal of re-establishing the electromotive force balance between each bridge arm, the on-off of the switching devices in the energy-consuming submodule is controlled, and the energy-consuming resistor is used as the electromotive force regulating device; By switching on or off the energy-consuming submodule, the damping of the energy-consuming resistor in each bridge arm is changed, so that each bridge arm in the MMC converter valve can achieve power balance by using the energy-consuming resistor while achieving electromotive force balance.

2. The method according to claim 1, characterized in that In the case of an AC imbalance fault, the number of energy-consuming submodules that need to be switched on or off in each bridge arm of the MMC converter valve is calculated according to the following formula: ； In the above formulas, N dissx is the number of energy-consuming submodules put into each bridge arm; R dissx Represents the total damping of the energy-consuming submodules in each bridge arm; R smdiss Indicates the damping size of a single energy-consuming submodule; ΔU sx It indicates the difference between the three-phase voltage output by the converter valve in the case of AC unbalance fault and that in normal operation; u sx Indicates the three-phase voltage output by the converter valve under normal operating conditions; u sfx Indicates the three-phase voltage output by the converter valve in the event of a fault; I ox Indicates the three-phase current output by the converter valve; x =a, b, c, respectively referring to the three bridge arms in the converter valve or the output side of the converter valve corresponding to each bridge arm.

3. The method according to claim 1, characterized in that By changing the damping of the energy-consuming resistor in each bridge arm, the output voltage of each bridge arm in the MMC converter valve is made close to the AC side voltage during normal operation.

4. The method according to claim 1, wherein A voltage sensor and a current sensor are installed on the AC side busbar of the MMC converter valve to collect the voltage signal and current signal on the AC side respectively. The collected signals are transmitted to the signal processing and analysis circuit, and the fault characteristic quantity is obtained after filtering, amplification and analog-to-digital conversion of the signals.

5. The method according to claim 1, wherein The fault characteristic quantities include the amplitude components and phase quantities of the voltage and current on the AC side.

6. The method according to claim 1, characterized in that The main control unit compares the fault characteristic quantity with the preset fault threshold and determines whether an AC imbalance fault has occurred based on the voltage and current positive and negative sequence separation algorithm. If the judgment result is that a fault has occurred, the calculation result is further compared with the preset conditions to confirm the fault type and fault severity, and then a warning message is issued in the human-computer dialogue interface.

7. The method according to claim 1, characterized in that The main control unit calculates the energy-consuming resistor damping required to switch on or off in each bridge arm of the MMC converter valve to cope with the current fault, generates a corresponding control signal and transmits it to the drive circuit; the latter controls the switching devices of the energy-consuming sub-modules in each bridge arm based on the signal, switching on or off the corresponding number of energy-consuming resistors.

8. The method according to claim 6 or 7, characterized in that The main control unit is a microprocessor or a programmable logic device.

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