Multi-feeder interconnection voltage sag flexible supporting device and control method thereof

Abstract

The invention provides a multi-feeder interconnection voltage sag flexible supporting device and a control method thereof, and relates to the technical field of power electronics and electric energy quality control. The device comprises a series compensation unit and a parallel energy supply unit which are connected back to back through a common direct current bus. The series compensation unit is used for voltage compensation, and the parallel energy supply unit is used for power exchange. A master-slave cooperative control strategy is adopted, a parallel energy supply unit serves as a master control unit and is in unified charge of common direct current bus voltage stability control of the whole device and voltage balance control of all H-bridge sub-modules, and a series compensation unit serves as a slave control unit and accurately tracks a compensation current instruction issued by the master control unit. According to the invention, by using the electric energy of a healthy feeder line, the non-energy-storage and long-time voltage sag support of a sensitive load on a fault feeder line is realized, and multiple functions of active mutual aid between feeder lines, dynamic reactive compensation and the like are integrated.

Description

Technical Field

[0001] This invention relates to the fields of power electronics and power quality control technology, specifically to a flexible support device for voltage sag across multiple feeders and its control method. Background Technology

[0002] In modern power distribution networks, with the increasing penetration of distributed power sources and the large-scale integration of nonlinear and impulsive loads, power quality issues are becoming increasingly prominent. Among these, voltage sag, as one of the most frequent and widespread power quality disturbances, has become a key bottleneck restricting the power supply reliability of voltage-sensitive users such as high-end manufacturing, continuous chemical processes, and information technology industries. Voltage sag is generally defined as the phenomenon where the effective value of the supply voltage drops suddenly to 10% to 90% of the rated value within a short period of time (defined by the International Electrotechnical Commission (IEC) standard as 10 milliseconds to 1 minute). Its main causes include power system short-circuit faults, the starting of high-power induction motors, or natural disturbances such as lightning strikes.

[0003] To address the hazards posed by voltage dips, various mitigation solutions have been developed. Among them, the Dynamic Voltage Restorer (DVR) is widely recognized as a mainstream and effective technology for protecting sensitive loads in medium-voltage distribution networks. A DVR is a series compensation device based on a Voltage Source Converter (VSC), connected to the grid via a series injection transformer. When a voltage dip is detected in the grid, the DVR's control system responds quickly, synthesizing a precisely controllable compensation voltage in amplitude, phase, and frequency through its internal converter. This compensation voltage is then injected into the line via the transformer, superimposed on the dipped grid voltage, thereby maintaining a stable, high-quality supply voltage on the load side.

[0004] In the process of voltage compensation, a DVR not only needs to compensate for reactive power but also provide active power to meet the active power demands of sensitive loads during compensation and the operating losses of its control method itself. The active power source of a traditional DVR fundamentally relies on its internally integrated energy storage unit. Commonly used energy storage technologies in engineering include supercapacitors, valve-regulated lead-acid batteries, lithium-ion batteries, and flywheel energy storage systems. This strong reliance on local energy storage leads to the following inherent and unavoidable defects in the practical application of existing DVR technology: 1. Limited support capacity and reliability shortcomings: The upper limit of the energy storage unit's capacity directly determines the total energy that the DVR can compensate, thus strictly limiting its maximum support time at a specific sag depth. Once the upstream grid fault clearing or reclosing operation time exceeds the energy storage unit's support limit, the energy will be exhausted, the DVR will be forced to stop compensation, ultimately leading to governance failure and the inability to provide reliable protection for critical loads throughout the entire process.

[0005] 2. High initial investment and land costs: To extend the support time, larger capacity energy storage systems must be configured. Large-capacity energy storage units, especially battery energy storage systems, account for a very high proportion of the cost of the entire DVR installation, significantly increasing the initial investment of the project. At the same time, these energy storage devices are typically large and heavy, posing a significant challenge to the physical space and load-bearing capacity of the installation site, further increasing the overall cost of the project.

[0006] 3. High total lifecycle cost, complex operation and maintenance, and potential safety hazards: Energy storage units, especially electrochemical batteries, are consumable components in the system with limited cycle life and calendar life, requiring regular maintenance and even replacement, resulting in high total lifecycle costs. Furthermore, large-scale energy storage systems require complex battery management systems and constant monitoring of thermal management and fire safety, increasing the complexity of system operation and maintenance and potential safety risks.

[0007] Based on the above analysis, the problems and shortcomings of the existing technology are as follows: Traditional solutions rely heavily on internal energy storage units, making it impossible to effectively compensate for long-term voltage dips; initial investment and land costs are high; and energy storage systems result in high total lifecycle costs and make it difficult to guarantee long-term operational reliability. Summary of the Invention

[0008] The purpose of this invention is to provide a flexible voltage sag support device and its control method for multi-feeder interconnection, which solves the problems of existing voltage sag mitigation devices heavily relying on large-capacity energy storage units, resulting in limited support time, large equipment costs and footprint, complex operation and maintenance, and safety hazards. This invention provides energy to the series compensation unit through a common DC bus, thus completely eliminating the dependence on large-capacity local energy storage. Furthermore, during normal grid operation, it can also function as a flexible interconnection device to achieve power flow scheduling between feeders, or as a STATCOM for reactive power compensation and other comprehensive power quality management. The use of a two-port collaborative control topology enables long-term voltage sag support without energy storage, improving the system's economy and reliability.

[0009] To achieve the above objectives, in a first aspect, the present invention provides a flexible support device for multi-feeder interconnection voltage sag, comprising: A series compensation unit is provided, with its AC side connected in series with the first feeder via a fast switching switch, for voltage compensation of sensitive loads on the first feeder. The parallel power supply unit has its AC side connected in parallel with the second feeder via a multi-winding isolation transformer for power exchange with the second feeder. A common DC bus connects the DC side of the series compensation unit and the DC side of the parallel power supply unit, and is used to transmit active power between the series compensation unit and the parallel power supply unit.

[0010] According to the present invention, a multi-feeder interconnected voltage sag flexible support device is provided, wherein both the series compensation unit and the parallel power supply unit adopt modular multilevel converters. According to the present invention, a multi-feeder interconnected voltage sag flexible support device includes a modular multilevel converter comprising three identical phase units corresponding to phases A, B, and C, respectively, with the AC output terminals of the three phase units connected in a star configuration; the AC output terminals of the three phase units of the series compensation unit together constitute its three-phase AC side, which is connected in series with the first feeder via a fast switching switch; the AC output terminals of the three phase units of the parallel power supply unit together constitute its three-phase AC side, which is connected to the secondary winding of a multi-winding isolation transformer; the primary winding of the multi-winding isolation transformer is connected in parallel with the second feeder. According to the present invention, a multi-feeder interconnection voltage sag flexible support device is provided, wherein each phase unit includes two bridge arms, and each bridge arm is composed of N cascaded H-bridge sub-modules and a bridge arm reactor connected in series, where N is a positive integer. According to the present invention, a multi-feeder interconnection voltage sag flexible support device is provided, wherein N H-bridge sub-modules in each bridge arm are connected in a cascade manner, specifically: the AC input terminal of the first H-bridge sub-module is connected to the bridge arm reactor, the AC output terminal of the k-th H-bridge module is connected to the AC input terminal of the (k+1)-th H-bridge module, and so on, with the AC output terminal of the N-th H-bridge module serving as the AC output terminal of that phase bridge arm; wherein 1≤k<N, and k is an integer; each H-bridge module includes a DC-side energy storage capacitor and a full-bridge circuit composed of four switching transistors and their anti-parallel diodes. According to the present invention, a multi-feeder interconnected voltage sag flexible support device is provided, wherein the multi-winding isolation transformer further includes a low-voltage three-phase winding. The low-voltage three-phase winding can be used for soft-start pre-charging when the multi-feeder interconnected voltage sag flexible support device is powered on, and can also be used as a low-voltage energy storage interface. In a second aspect, the present invention provides a control method for a multi-feeder interconnected voltage sag flexible support device as described in the first aspect, employing a master-slave cooperative control strategy, including: The parallel power supply unit, as the main control unit, performs common DC bus voltage stabilization control and voltage equalization control of all H-bridge sub-modules, and generates current reference commands for the parallel power supply unit and the series compensation unit. The series compensation unit acts as a slave control unit, performing precise current tracking control based on current reference commands.

[0011] The control method for the multi-feeder interconnected voltage sag flexible support device provided by the present invention includes common DC bus voltage stabilization control and voltage equalization control of all H-bridge submodules, comprising: The three-phase AC voltage of the grid connection point of the parallel power supply unit, namely the second feeder, is sampled in real time, and the sampled value is sent into the phase-locked loop to extract the synchronous phase angle of the grid voltage. The DC voltage of all H-bridge submodules in each phase is collected, summed, and then averaged to obtain the total average voltage of each phase; the average of the three-phase total average voltage is then calculated to obtain the total DC voltage average value; the difference between the total DC voltage average value and a given reference value is calculated, and the difference is used to generate the d-axis active current reference value through a PI controller; The average DC voltage of the H-bridge submodule in the A, B, and C three-phase bridge arms of the parallel power supply unit is calculated to obtain the three-phase average voltage. The average value of the three-phase average voltage is then calculated to obtain the total average voltage. The deviation between the three-phase average voltage and the total average voltage is input into the respective phase-to-phase voltage equalization PI controller. The output of the phase-to-phase voltage equalization PI controller is then processed with an orthogonal signal based on the synchronization phase angle to synthesize a phase-to-phase voltage equalization modulation signal for compensation. The d-axis active current reference value and the given q-axis reactive current reference value are compared with the dq-axis current after Park transformation of the actual grid-connected three-phase current of the parallel power supply unit. The dq-axis current deviation obtained by comparison is adjusted by PI controller and cross decoupling term respectively, and after inverse Park transformation, the grid-side voltage feedforward is superimposed to generate dq-axis modulation voltage command. The dq-axis modulation voltage command is subjected to inverse Park transformation to obtain the three-phase fundamental modulation voltage. This three-phase fundamental modulation voltage is then vector-superimposed with the inter-phase equalization modulation signal and the fine-tuning signal used to achieve intra-phase equalization within the parallel power supply unit to ensure voltage balance of all H-bridge submodules. The resulting modulation signal is then sent to the carrier phase-shift modulation stage to generate PWM control signals that drive the H-bridge submodules in the A, B, and C three-phase bridge arms of the parallel power supply unit.

[0012] The control method for a multi-feeder interconnected voltage sag flexible support device provided by the present invention performs precise current tracking control based on a current reference command, including: The actual three-phase output current injected into the first feeder by the series compensation unit is sampled in real time. Using the synchronous phase angle, the actual three-phase output current is decomposed into fundamental positive sequence component and fundamental negative sequence component through positive and negative sequence separation algorithm. After the fundamental positive sequence component is transformed by Park, the actual positive sequence current of the dq axis is obtained. The actual positive sequence current of the dq axis is compared with the reference value of the positive sequence current. The deviation obtained from the comparison is adjusted by the PI controller and the cross decoupling term to generate the positive sequence dq axis modulation voltage. After the fundamental negative sequence component is transformed by Park, the actual negative sequence current of the dq axis is obtained. The actual negative sequence current of the dq axis is compared with the negative sequence current reference value. The deviation obtained by the comparison is adjusted by the PI controller and the cross decoupling term to generate the negative sequence dq axis modulation voltage. The positive-sequence dq-axis modulation voltage and the negative-sequence dq-axis modulation voltage are respectively subjected to inverse Park transformation to obtain three-phase positive-sequence modulation voltage and three-phase negative-sequence modulation voltage. The three-phase positive-sequence modulation voltage and the three-phase negative-sequence modulation voltage are vector superimposed to synthesize a three-phase modulation voltage command. The three-phase modulation voltage command is sent to the carrier phase-shift modulation stage to generate the PWM control signal for driving each H-bridge submodule in the series compensation unit.

[0013] According to the control method of the multi-feeder interconnected voltage sag flexible support device provided by the present invention, the positive and negative sequence separation algorithm is based on the 1 / 4 cycle delay principle, including: transforming the three-phase AC line voltage by abc / αβ and constructing orthogonal components by using the delay element, thereby decomposing the actual three-phase output current into fundamental positive sequence component and fundamental negative sequence component.

[0014] Compared with the prior art, the present invention has at least the following technical effects: This invention provides a multi-feeder interconnected voltage sag flexible support device and its control method. The device includes a series compensation unit and a parallel power supply unit, which are connected back-to-back via a common DC bus. The series compensation unit is connected in series with the first feeder via a fast-switching switch for voltage compensation; the parallel power supply unit is connected in parallel with the second feeder via a multi-winding isolation transformer for power exchange. This invention employs a master-slave cooperative control strategy, with the parallel power supply unit acting as the master control unit, uniformly responsible for the common DC bus voltage stability control and the voltage balance control of all H-bridge submodules. The series compensation unit acts as the slave control unit, accurately tracking the compensation current commands issued by the master control unit. This invention utilizes the energy of healthy feeders to achieve long-term voltage sag support for sensitive loads on faulty feeders without energy storage, and integrates multiple functions such as inter-feeder active power exchange and dynamic reactive power compensation. Attached Figure Description

[0015] To more clearly illustrate the technical solutions in this invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are some embodiments of this invention. For those skilled in the art, other drawings can be obtained from these drawings without creative effort.

[0016] In the attached diagram: Figure 1 This is a schematic diagram of the structure of the multi-feeder interconnection voltage sag flexible support device according to an embodiment of the present invention; Figure 2 This is a schematic diagram of the topology of a single-phase cascaded multilevel converter link of a multi-feeder interconnected voltage sag flexible support device according to an embodiment of the present invention; Figure 3 This is a schematic diagram of four typical operating modes of the multi-feeder interconnected voltage sag flexible support device under different working conditions according to an embodiment of the present invention; Figure 4 This is a control block diagram of the series compensation unit in an embodiment of the present invention; Figure 5 This is a control block diagram of the parallel power supply unit according to an embodiment of the present invention; Figure 6 This is a block diagram of the phase-to-phase voltage balancing control used in the parallel power supply unit of this invention. Figure 7 This is a block diagram of the total DC voltage control used in the parallel power supply unit of this invention embodiment; Figure 8 This is a schematic diagram of the positive and negative order separation algorithm based on 1 / 4 period delay used in an embodiment of the present invention; Figure 9 This is a block diagram of the phase-locked loop control used in an embodiment of the present invention. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some, not all, of the embodiments of this invention. All other embodiments obtained by those skilled in the art based on the embodiments of this invention without creative effort are within the scope of protection of this invention.

[0018] The following detailed description of some embodiments of the present invention will be provided in conjunction with the accompanying drawings. Unless otherwise specified, the following embodiments and features can be combined with each other.

[0019] Please see Figure 1This invention provides a flexible voltage sag support device for multi-feeder interconnection, applicable to medium-voltage (e.g., 10kV) distribution networks. It aims to achieve energy balance and comprehensive power quality management between two different feeders, enabling flexible interconnection of multiple feeders and providing voltage sag support for sensitive loads. The flexible voltage sag support device for multi-feeder interconnection includes: The series compensation unit has its AC side connected in series with the first feeder (feeder I) via a fast switching switch (referred to as fast switch) for voltage compensation of sensitive loads on the first feeder. The parallel power supply unit is connected in parallel with the second feeder (feeder II) via a multi-winding isolation transformer on its AC side, and is used to exchange power with the second feeder. A common DC bus connects the DC side of the series compensation unit and the DC side of the parallel power supply unit, and is used to transmit active power between the series compensation unit and the parallel power supply unit.

[0020] It should be noted that the DC side of the series compensation unit and the DC side of the parallel power supply unit of this invention are connected to a common DC bus, thus forming a back-to-back power electronic converter that allows power exchange between the series compensation unit and the parallel power supply unit. Specifically, the series compensation unit connected to the first feeder containing sensitive loads uses a series connection method, isolating the grid and providing high-quality voltage support for sensitive loads during voltage dips. The parallel power supply unit connected to another healthy second feeder uses a parallel connection method; its core function is to obtain active power from the healthy feeder and provide energy to the series compensation unit through the common DC bus, thereby completely eliminating the dependence on large-capacity local energy storage. During normal grid operation, this device can also function as a flexible interconnection device to achieve power flow scheduling between feeders, or as a STATCOM (Static Synchronous Compensator) for reactive power compensation and other comprehensive power quality management. This invention employs a two-port collaborative control topology, achieving long-term voltage dip support without energy storage, thus improving the system's economy and reliability.

[0021] Specifically, the series compensation unit can be a DVR, and the parallel power supply unit can be an SVG (Static Var Generator). A fast-switching switch is connected in series with the first feeder to quickly connect and disconnect sensitive loads on the first feeder from the upstream power grid. The primary winding of the multi-winding isolation transformer is connected in parallel with the second feeder, and the secondary winding is connected to the three-phase AC side of the SVG.

[0022] Specifically, to improve the device's startup safety, operational flexibility, and power supply reliability under extreme conditions, the multi-winding isolation transformer, on the side of the parallel power supply unit, includes a low-voltage three-phase winding in addition to the high-voltage working winding connected to the parallel power supply unit. This low-voltage three-phase winding serves as a dual-function port, functioning both as the device's startup winding for soft-start pre-charging when the device is powered on, and as an optional low-voltage energy storage interface.

[0023] Furthermore, the rated voltage of the low-voltage three-phase winding is 0.38kV, and it adopts a star connection. This low-voltage three-phase winding is connected to an external low-voltage auxiliary power supply through a dedicated starting switch. Its core function is to provide a controllable pre-charge path for the entire system during the initial power-on or "black start" of the device.

[0024] Specifically, before the main circuit breaker of the device is closed to the 10kV feeder, the 0.38kV starting winding and the converter of the parallel power supply unit are used to perform soft start and pre-charge of the common DC bus of the entire device (i.e. the DC side capacitors of all H-bridge sub-modules) with a small, controllable current.

[0025] This process avoids the huge inrush current that could be caused by directly switching to 10kV high voltage when the internal capacitor voltage is zero, thus ensuring the smoothness and safety of the device's power-on process and improving the equipment's engineering practicality and operational reliability.

[0026] The 0.38kV low-voltage three-phase winding is also designed as an optional low-voltage energy storage interface. Users can add an external low-voltage energy storage system, such as a supercapacitor module, through this interface to meet the most stringent requirements for power supply reliability. When configured with an energy storage system, this interface connects to the supercapacitor or other energy storage unit via a bidirectional DC / DC or DC / AC converter. Its functions include: Short-term ride-through capability under extreme faults. In rare extreme cases where two 10kV feeders fail or lose power simultaneously, the energy storage unit can instantly provide short-term active power to the series compensation unit through the parallel power supply unit and the common DC bus, providing a last line of defense for the most critical sensitive loads and ensuring power supply.

[0027] Enhanced dynamic performance. When dealing with extremely rapid power surges, this energy storage unit can act as a power buffer, further improving the device's dynamic response speed and stability.

[0028] Therefore, in addition to the 10kV / Δ-connected main working winding, the multi-winding isolation transformer also includes a 0.38kV / Y-connected starting winding (low-voltage three-phase winding). This starting winding can be used for safe pre-charging of the device, or as an optional low-voltage energy storage interface, such as connecting a supercapacitor module to cope with extreme operating conditions.

[0029] It should be noted that the DVR is connected in series to feeder I via a fast switching switch, located between the feeder circuit breaker and the sensitive load. Its function is to inject compensating voltage into the line when the grid voltage is abnormal. The SVG, on the other hand, is connected in parallel to feeder II via a multi-winding isolation transformer. This feeder typically supplies power to non-sensitive loads, and the SVG's main function is to perform power exchange and power quality regulation from this feeder.

[0030] In some embodiments, both the DVR and SVG employ a modular multilevel converter (MMC), which consists of three identical phase units (corresponding to phases A, B, and C, respectively), with their AC output terminals connected in a star configuration. The AC output terminals of the three phase units of the DVR (i.e., the midpoint between the two arms of each phase unit) collectively constitute its three-phase AC side, which is connected in series with the first feeder via a fast-switching switch. Similarly, the AC output terminals of the three phase units of the SVG collectively constitute its three-phase AC side, which is connected to the secondary winding of a multi-winding isolation transformer.

[0031] Each phase unit includes two bridge arms, and each bridge arm consists of N cascaded H-bridge submodules and one bridge arm reactor (i.e., Figure 1 The grid-connected inductor L1 is connected in series, where N is a positive integer, and its specific value can be selected according to the actual voltage level and design requirements. For example... Figure 1 As shown, the left-hand bridge arm is a DVR, and its N cascaded H-bridge submodules are denoted as LH. C1 LH C2 ... LH CN The bridge arm on the right is an SVG, and its N cascaded H-bridge submodules are denoted as RH. C1 RH C2 , ..., RH CN .

[0032] The N H-bridge submodules within each bridge arm are connected in a cascaded manner, specifically: the AC input terminal of the first H-bridge submodule HM1 is connected to the bridge arm reactor, and the k-th H-bridge submodule HM... k The AC output terminal is connected to the (k+1)th H-bridge submodule HM. k+1 The AC input terminal, and so on, the Nth H-bridge submodule HM N The AC output terminal of the phase bridge arm is used as the AC output terminal of that phase bridge arm. Where 1≤k<N, and k is an integer.

[0033] In some embodiments, the circuit structure of the H-bridge submodule includes a DC-side energy storage capacitor Cdc and a full-bridge circuit consisting of four IGBT switches and their anti-parallel diodes. The four IGBT switches are divided into two sub-bridge arms: one sub-bridge arm consists of switches VT1 and VT2 connected in series, and the other sub-bridge arm consists of switches VT3 and VT4 connected in series. The two sub-bridge arms are connected in parallel, and the DC-side capacitor Cdc is connected in parallel across the two sub-bridge arms. The AC output terminal of the full-bridge circuit is located between the midpoints of the two sub-bridge arms. By controlling the on / off combinations of the four switches within the H-bridge submodule, the output voltage of the H-bridge submodule can be adjusted, generating three voltage levels at the output terminal: +Uc, 0, and -Uc.

[0034] like Figure 2 The diagram shown illustrates a single-phase link topology of the device provided in this embodiment of the invention. Its core logic lies in constructing an efficient energy path between sensitive and non-sensitive feeders. This link employs a modular cascaded multi-level architecture. Figure 2 The image shows a single-phase system consisting of 12 independent power modules (links 1 to 12) cascaded together. Each power module comprises two back-to-back H-bridge submodules, one connected to the sensitive feeder side (i.e., the DVR, i.e., the inverter circuit unit (LH)). C )) and the SVG (i.e., rectifier circuit unit (RH)) connected to the non-sensitive feeder side C These two units share a single DC bus capacitor, and through a built-in capacitor voltage equalization control strategy, they achieve dynamic balance and stable energy flow among the various H-bridge submodules.

[0035] Among them, LH C The AC ports are cascaded into the sensitive feeder circuit for high-precision voltage compensation or power regulation; RH C It is connected to the non-sensitive feeder through a multi-winding isolation transformer, and is mainly responsible for obtaining energy from the power grid to support the common DC bus voltage.

[0036] Reference Figure 3 This paper demonstrates four control strategies and operating modes of the voltage sag flexible support device for multi-feeder interconnection under different grid boundary conditions in the embodiments of the present invention: First, in the flexible support mode, when a voltage dip occurs in feeder I or feeder II, the device acts as a flexible power interface to achieve dynamic voltage recovery and support for the faulty feeder through rapid bidirectional active / reactive power throughput.

[0037] Subsequently, in the harsh operating condition protection mode, for extreme conditions such as power outages, the device uses the starting winding of the secondary side of the multi-winding isolation transformer as a DC source interface to connect to energy storage units such as batteries. At this time, the device switches to off-grid / UPS (uninterruptible power supply) mode, using energy storage units to continuously supply power to critical loads, ensuring power supply reliability.

[0038] Secondly, in the active power balance mode, when the distribution network is operating normally, the device controls the flow of active power from the light load side to the heavy load side according to the load rate difference between the two feeders, so as to achieve load balance and asset utilization optimization between feeders.

[0039] Finally, the harmonic mitigation mode addresses power quality issues caused by nonlinear loads. The device injects a reverse-phase compensation current through high-frequency modulation, functioning as an active power filter to actively suppress and purify specific harmonics.

[0040] Based on the same inventive concept, another embodiment of the present invention provides a control method for the 10kV multi-feeder interconnection voltage sag flexible support device of the aforementioned embodiment. The control method adopted by the present invention is a master-slave cooperative control strategy, including: The parallel power supply unit acts as the main control unit, performing common DC bus voltage stabilization control and voltage balancing control of all H-bridge submodules (i.e., all H-bridge submodules containing both SVG and DVR), and generating current reference commands for the parallel power supply unit and the series compensation unit. The series compensation unit acts as the slave control unit, responsible only for accurately tracking the current reference commands issued by the main control unit, i.e., performing precise current tracking control based on the current reference commands. The specific implementation steps of this method are as follows: I. Control Strategy for Parallel Power Supply Units (SVG). (Refer to...) Figure 5 The control strategy of the parallel power supply unit (SVG) is shown in detail. As the main control core of the device, this strategy adopts a dual closed-loop structure with DC voltage control as the outer loop and AC current control as the inner loop, and integrates multi-level voltage equalization control.

[0041] (1) Synchronization of the second feeder grid status: The three-phase AC voltage of the grid connection point of the parallel power supply unit (SVG terminal), namely the second feeder, is sampled in real time, and the sampled value is sent to the phase-locked loop (PLL) to accurately and quickly extract the synchronous phase angle θ of the grid voltage.

[0042] (2) DC bus global voltage control: The outer voltage loop (its core is shown in the image) Figure 7The core task of the system is to stabilize the common DC bus voltage. It is the highest level of control in the entire device, responsible for maintaining the global energy balance of the common DC bus. Specifically, it involves: real-time acquisition of the DC-side capacitor voltages (udcA1~udcA12, etc.) of all H-bridge submodules within the device (i.e., all H-bridge submodules containing both SVG and DVR, for example, 12 modules); calculating the average of all voltage values ​​(multiplying by 1 / 3) to obtain the total average DC voltage udc of all H-bridge submodules; and comparing this total average DC voltage udc with a given reference value. The difference is calculated, and the resulting deviation is input to the PI controller of the outer loop of the voltage. The output of this PI controller is the total active current command required to maintain the DC voltage stability of the entire device, and serves as the d-axis active current reference value for the SVG. .

[0043] (3) SVG interphase voltage equalization control: To ensure energy balance among the three phase arms of the SVG, independent phase-to-phase voltage equalization control is implemented. Specifically: (e.g.) Figure 6 As shown, the average DC voltages of the H-bridge submodules in the A, B, and C arms of the SVG are calculated respectively to obtain the three-phase average voltages udcA, udcB, and udcC. Then, the average of these three-phase average voltages udcA, udcB, and udcC is calculated to obtain the total average voltage. The deviations between the three-phase average voltages udcA, udcB, and udcC and the total average voltage are input to their respective phase-to-phase voltage equalization PI controllers. The outputs of these controllers are then processed with orthogonal signals based on the synchronization phase angle θ=ωt (-sin(ωt), -sin(ωt-2π / 3), -sin(ωt+2π / 3)) to synthesize a phase-to-phase voltage equalization modulation signal for compensation.

[0044] (4) SVG AC current decoupling control: The inner current loop section. The d-axis active current reference value output from the outer voltage loop. and the externally given q-axis reactive current reference value The dq-axis current i, obtained by Park transformation of the actual grid-connected three-phase currents Ia_svg, Ib_svg, and Ic_svg of the SVG. d i q The current deviations of the dq axes are compared. The current deviations of the dq axes are adjusted by independent PI controllers and cross-decoupling terms (ωL1), and after inverse Park transformation and superimposed with grid-side voltage feedforward us_svg, the dq axis modulation voltage command is generated.

[0045] (5) SVG-end modulated signal synthesis: The dq-axis modulation voltage command is subjected to an inverse Park transform to obtain the three-phase fundamental modulation voltage. This three-phase fundamental modulation voltage is then vector-superimposed with the inter-phase voltage equalization modulation signal and a fine-tuning signal (not detailed in the figure, but a standard technique) used to achieve intra-phase voltage equalization within the SVG to ensure voltage balance across all H-bridge submodules. The final synthesized modulation signal is the final modulation command driving each H-bridge submodule of the SVG. This command is then modulated by a carrier phase-shifting PWM (CPS-PWM) stage to generate the PWM control signal driving each H-bridge submodule of the SVG.

[0046] Figure 8 This paper details the positive and negative sequence separation algorithm based on the 1 / 4-cycle delay principle. By performing an abc / αβ transformation on the three-phase AC line voltages usab, usbc, and usca, and constructing orthogonal components using a delay element, the actual three-phase output current can be decomposed into fundamental positive-sequence and fundamental negative-sequence components in real time and accurately. Figure 4 The dual decoupling control in the system provides the input signal. Figure 9 This paper details a phase-locked loop (PLL) based on a synchronous rotating coordinate system (dq). The three-phase AC line voltages usab, usbc, and usca are transformed to the dq coordinate system, and a PI controller is used to make the q-axis component (u... sq As the phase angle approaches zero, this loop can quickly and accurately lock the phase angle θ of the grid voltage, providing a precise synchronization reference for all Park and inverse Park transformations in the system.

[0047] II. Control Strategy for Series Compensation Unit (DVR). (Refer to...) Figure 4 The paper details the control strategy of the series compensation unit (DVR), which is based on the fundamental positive and negative sequence dual decoupled current inner loop and aims to provide fast and accurate compensation for symmetrical and asymmetrical voltage sags.

[0048] (6) Signal conversion of DVR output current: The actual three-phase output current i injected into the first feeder of the series compensation unit (DVR) a i b i c Real-time sampling is performed. Utilizing the synchronization phase angle θ uniformly provided by the phase-locked loop of the main control unit, the three-phase output current i is sampled using a positive-negative sequence separation algorithm. a i b i c Perform positive and negative sequence separation, decomposing it into fundamental positive sequence component i a p i b p i c p and fundamental negative sequence component i an i b n i c n .

[0049] (7) DVR AC current tracking control: The control is divided into two parallel branches. In the fundamental positive sequence current loop, the fundamental positive sequence component i a p i b p i c p After Park transformation (rotation angle θ), the actual positive sequence current i along the dq axis is obtained. d p i q p This value is related to the positive sequence current reference value issued by the main control unit of the parallel power supply unit. The deviation signal is compared and then adjusted by a PI controller and a cross-decoupling term (ωL1) to generate a positive-sequence dq-axis modulated voltage. .

[0050] In the fundamental negative sequence current loop, the fundamental negative sequence component i a n i b n i c n After Park transformation (rotation angle -θ), the actual negative sequence current i along the dq axis is obtained. d n i q n And compared with the negative sequence current reference value i issued by the main control unit of the parallel power supply unit. Ld n i Lq n After comparison and adjustment by an independent PI controller and decoupling term, a negative-sequence dq-axis modulation voltage is generated. .

[0051] (8) DVR modulation signal synthesis and PWM generation: Finally, the positive-sequence and negative-sequence dq-axis modulated voltages are subjected to inverse Park transforms to obtain the three-phase positive-sequence modulated voltage u. a p , u b p , u c p and three-phase negative sequence modulation voltage u a n , ub n , u c n These two sets of voltage signals are vector-superimposed with the grid-side voltage feedforward us_dvr and the sum of the phase capacitors (feedback signals used for phase-to-phase voltage equalization) to synthesize the final three-phase modulated voltage command. This command is sent to the carrier phase-shifting PWM modulation stage to generate the PWM control signal driving each H-bridge submodule of the DVR.

[0052] It should be noted that since the voltage equalization task of the entire device is now handled by the main control unit (SVG), the modulation signal synthesis stage of the DVR is greatly simplified, and in principle, there is no need to superimpose complex inter-phase and intra-phase voltage equalization control signals. The final synthesized modulation voltage is then modulated by a carrier phase-shifting (CPS-PWM) modulation stage to generate the PWM control signal driving each H-bridge submodule of the DVR.

[0053] In summary, the advantages and positive effects of this invention are as follows: 1. This invention provides an innovative, highly integrated 10kV multi-feeder interconnected voltage sag flexible support device and its control method. This invention uniquely constructs a "series-parallel" hybrid topology by connecting the series voltage compensation unit (DVR) responsible for series voltage compensation and the parallel power supply unit (SVG) responsible for parallel power exchange back-to-back via a common DC bus. This topology can draw active power from a healthy second feeder in real time through the SVG to provide energy to the DVR, thereby achieving long-term voltage sag support for sensitive loads without energy storage. This design fundamentally solves the core problems of traditional DVR solutions, such as limited support time, high cost, large footprint, and complex operation and maintenance, caused by reliance on energy storage. The modular multilevel converter (MMC) and asymmetric port function design adopted in this invention make the device not only a voltage sag mitigation device but also a multifunctional flexible node in the distribution network, making it highly suitable for modern urban distribution networks with high requirements for power supply reliability and power quality.

[0054] 2. The significant technological advancements brought about by the collaborative control method proposed in this invention include: This invention achieves decoupling and simplification of control. It innovatively adopts a master-slave cooperative control architecture of "SVG-led, DVR-following". The SVG is uniformly responsible for the stability of the common DC bus voltage and the complex voltage balancing control of all H-bridge submodules, while the DVR acts only as a pure current command tracker. This clear separation of control responsibilities greatly simplifies the DVR's control algorithm, making the control logic clearer and the decoupling stronger.

[0055] This method improves the dynamic response and stability of the system. Through master-slave collaboration, it ensures rapid and stable energy balance on the DC bus during severe transient processes such as voltage sags and topology switching, avoiding conflicts and oscillations between the two port control strategies. The positive and negative sequence dual decoupled current inner loop employed in the DVR can quickly and accurately compensate for unbalanced sags, jointly enhancing the overall dynamic response speed and operational stability of the device.

[0056] The system's safety and reliability have been enhanced. The control method integrates "black start" and pre-charge control logic for the 0.38kV starting winding, effectively avoiding the huge inrush current caused by direct high-voltage energization. This solves a key safety issue in the engineering application of large-scale power electronic equipment and improves the system's inherent safety and engineering reliability.

[0057] 3. The present invention is also reflected in the following important aspects: The technical solution of this invention fills a technological gap in the industry both domestically and internationally: in existing technologies, whether it is a DVR, IDVR (Inter-line Dynamic Voltage Restorer), or a flexible interconnection device (SOP), the functions are relatively simple. This invention proposes and implements a single, compact power electronic device that efficiently integrates multiple functions such as long-term voltage sag support, inter-feeder active power balance, and dynamic reactive power compensation. This "series-parallel" hybrid topology and its master-slave cooperative control strategy provide a system-level solution for the comprehensive management and flexible upgrading of power quality in medium-voltage distribution networks.

[0058] The technical solution of this invention solves a long-standing technical problem that has been desired but not yet successfully addressed in the prior art: how to eliminate reliance on large-capacity energy storage while simultaneously solving voltage sag issues and meeting multiple needs such as power flow regulation and reactive power compensation in the power distribution network field—a long-term goal pursued by the technology sector. The traditional "series-series" structure of IDVRs limits their versatility, while the parallel structure of SOPs cannot provide voltage compensation. This invention cleverly solves this problem through an asymmetric hybrid topology, enabling a single device to simultaneously achieve "protection" (DVR) and "routing" (SVG), providing a practical technical path for realizing "multi-functional" equipment at key nodes in the power distribution network and maximizing asset value.

[0059] 4. The technical problems solved by this invention and the significant technical progress achieved are mainly reflected in the following aspects: (1) The technical problem of the prior art to be solved: Functional fragmentation and equipment redundancy issues: This solution addresses the problem of needing to install multiple sets of equipment such as DVR, SOP, and STATCOM separately to achieve various functions in traditional solutions, thus avoiding investment waste and site space occupation.

[0060] Energy storage dependence and limited support time: It solves the fundamental defects of traditional DVRs caused by their dependence on energy storage, such as limited support time, high cost, complex operation and maintenance, and safety risks.

[0061] Control complexity and stability issues: It solves the problems of complex control strategies and easy dynamic conflicts between two series units in traditional symmetric IDVR, and achieves better decoupling and stability through master-slave control.

[0062] (2) Significant technological advancements achieved: High integration of functions: It achieves seamless integration of multiple core functions such as voltage sag support under transient conditions and power flow regulation and reactive power compensation under steady conditions in a single device, which greatly improves the practicality and efficiency of the device.

[0063] Fundamental improvement in economic efficiency: By eliminating the need for large-capacity energy storage systems and achieving "one machine, multiple functions", significant optimizations have been achieved in both initial investment and total life cycle cost of equipment, making advanced power quality and flexible interconnection technologies more economical and conducive to widespread application.

[0064] Enhanced grid adaptability: By providing an optional low-voltage energy storage interface, the device has flexible functional upgrade capabilities, enabling it to adapt to different application scenarios from conventional to extremely high reliability requirements, demonstrating excellent design foresight and adaptability to the future development needs of the power grid.

[0065] Other embodiments of the invention will readily occur to those skilled in the art upon consideration of the specification and practice of the embodiments disclosed herein. This invention is intended to cover any variations, uses, or adaptations of the invention that follow the general principles of the invention and include common knowledge or customary techniques in the art not disclosed herein. It should be understood that the invention is not limited to the precise structures described above and shown in the accompanying drawings, and various modifications and changes can be made without departing from its scope. The scope of the invention is limited only by the appended claims.

Claims

1. A flexible support device for voltage sag across multiple feeders, characterized in that, include: A series compensation unit, wherein the AC side of the series compensation unit is connected in series with the first feeder via a fast switching switch, and is used to compensate the voltage of the sensitive load on the first feeder. A parallel power supply unit, wherein the AC side of the parallel power supply unit is connected in parallel with the second feeder through a multi-winding isolation transformer for power exchange with the second feeder; A common DC bus connects the DC side of the series compensation unit and the DC side of the parallel power supply unit, and is used for active power transmission between the series compensation unit and the parallel power supply unit.

2. The multi-feeder interconnection voltage sag flexible support device according to claim 1, characterized in that, Both the series compensation unit and the parallel power supply unit adopt modular multilevel converters.

3. The multi-feeder interconnected voltage sag flexible support device according to claim 2, characterized in that, The modular multilevel converter includes three identical phase units corresponding to phases A, B, and C, respectively, with the AC output terminals of the three phase units connected in a star configuration. The AC output terminals of the three phase units of the series compensation unit together constitute its three-phase AC side, which is connected in series with the first feeder via a fast switching switch. The AC output terminals of the three phase units of the parallel power supply unit together constitute its three-phase AC side, which is connected to the secondary winding of the multi-winding isolation transformer. The primary winding of the multi-winding isolation transformer is connected in parallel with the second feeder.

4. The multi-feeder interconnection voltage sag flexible support device according to claim 3, characterized in that, Each phase unit includes two bridge arms, and each bridge arm consists of N cascaded H-bridge submodules and a bridge arm reactor connected in series, where N is a positive integer.

5. The multi-feeder interconnection voltage sag flexible support device according to claim 4, characterized in that, The N H-bridge submodules within each bridge arm are connected in a cascaded manner, specifically: the AC input terminal of the first H-bridge submodule is connected to the bridge arm reactor, the AC output terminal of the k-th H-bridge module is connected to the AC input terminal of the (k+1)-th H-bridge module, and so on, with the AC output terminal of the N-th H-bridge module serving as the AC output terminal of that phase bridge arm; where 1≤k<N, and k is an integer; each H-bridge module includes a DC-side energy storage capacitor and a full-bridge circuit consisting of four switching transistors and their anti-parallel diodes.

6. The multi-feeder interconnected voltage sag flexible support device according to claim 4, characterized in that, The multi-winding isolation transformer also includes a low-voltage three-phase winding, which can be used for soft-start pre-charging when the multi-feeder interconnected voltage sag flexible support device is powered on, and can also be used as a low-voltage energy storage interface.

7. A control method for a multi-feeder interconnected voltage sag flexible support device as described in any one of claims 4 to 6, characterized in that, A master-slave collaborative control strategy is adopted, including: The parallel power supply unit, as the main control unit, performs common DC bus voltage stabilization control and voltage equalization control of all H-bridge submodules, and generates current reference commands for the parallel power supply unit and the series compensation unit. The series compensation unit acts as a slave control unit, performing precise current tracking control based on the current reference command.

8. The control method for the multi-feeder interconnected voltage sag flexible support device according to claim 7, characterized in that, The common DC bus voltage stabilization control and the voltage equalization control of all H-bridge submodules include: The three-phase AC voltage of the grid connection point of the parallel power supply unit, namely the second feeder, is sampled in real time, and the sampled value is sent into the phase-locked loop to extract the synchronous phase angle of the grid voltage. Collect the DC voltage of all H-bridge submodules in each phase, sum them separately and then take the average value to obtain the total average voltage of each phase; take the average value of the total average voltage of the three phases to obtain the total average DC voltage; subtract the total average DC voltage from the given reference value, and use the difference to generate the d-axis active current reference value through the PI controller; The average DC voltage of the H-bridge submodule in the A, B, and C three-phase bridge arms of the parallel power supply unit is calculated to obtain the three-phase average voltage. The average value of the three-phase average voltage is then calculated to obtain the total average voltage. The deviation between the three-phase average voltage and the total average voltage is input into the respective phase-to-phase voltage equalization PI controller. The output of the phase-to-phase voltage equalization PI controller is then processed with an orthogonal signal based on the synchronization phase angle to synthesize a phase-to-phase voltage equalization modulation signal for compensation. The d-axis active current reference value and the given q-axis reactive current reference value are compared with the dq-axis current after Park transformation of the actual grid-connected three-phase current of the parallel power supply unit. The dq-axis current deviation obtained by comparison is adjusted by PI controller and cross decoupling term respectively, and after inverse Park transformation, the grid-side voltage feedforward is superimposed to generate dq-axis modulation voltage command. The dq-axis modulation voltage command is subjected to inverse Park transformation to obtain the three-phase fundamental modulation voltage. This three-phase fundamental modulation voltage is then vector-superimposed with the inter-phase equalization modulation signal and the fine-tuning signal used to achieve intra-phase equalization within the parallel power supply unit to ensure voltage balance of all H-bridge submodules. The resulting modulation signal is then sent to the carrier phase-shift modulation stage to generate PWM control signals that drive the H-bridge submodules in the A, B, and C three-phase bridge arms of the parallel power supply unit.

9. The control method for the multi-feeder interconnected voltage sag flexible support device according to claim 7, characterized in that, Performing precise current tracking control based on the current reference command includes: The actual three-phase output current injected into the first feeder by the series compensation unit is sampled in real time. Using the synchronous phase angle, the actual three-phase output current is decomposed into fundamental positive sequence component and fundamental negative sequence component through positive and negative sequence separation algorithm. After the fundamental positive sequence component is transformed by Park, the actual positive sequence current of the dq axis is obtained. The actual positive sequence current of the dq axis is compared with the reference value of the positive sequence current. The deviation obtained from the comparison is adjusted by the PI controller and the cross decoupling term to generate the positive sequence dq axis modulation voltage. After the fundamental negative sequence component is transformed by Park, the actual negative sequence current of the dq axis is obtained. The actual negative sequence current of the dq axis is compared with the negative sequence current reference value. The deviation obtained by the comparison is adjusted by the PI controller and the cross decoupling term to generate the negative sequence dq axis modulation voltage. The positive-sequence dq-axis modulation voltage and the negative-sequence dq-axis modulation voltage are respectively subjected to inverse Park transformation to obtain three-phase positive-sequence modulation voltage and three-phase negative-sequence modulation voltage. The three-phase positive-sequence modulation voltage and the three-phase negative-sequence modulation voltage are vector superimposed to synthesize a three-phase modulation voltage command. The three-phase modulation voltage command is sent to the carrier phase-shift modulation stage to generate the PWM control signal for driving each H-bridge submodule in the series compensation unit.

10. The control method for the multi-feeder interconnected voltage sag flexible support device according to claim 9, characterized in that, The positive and negative sequence separation algorithm is based on the 1 / 4-cycle delay principle, which includes: transforming the three-phase AC line voltages by abc / αβ and constructing orthogonal components using a delay element, thereby decomposing the actual three-phase output current into fundamental positive sequence components and fundamental negative sequence components.

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