Analysis method for transient overload operation characteristics of cascade H-bridge SVG

Abstract

The application belongs to the technical field of power electronics, and particularly relates to a kind of analysis method for transient overload operation characteristics of cascaded H-bridge SVG.The method comprises: establishing a cascaded H-bridge SVG simulation model;setting a first fault condition group and a second fault condition group based on the simulation model, the first fault condition group corresponding to a fixed overload multiple and changing a remaining value of a grid-connected point voltage, the second fault condition group corresponding to a fixed grid-connected point voltage remaining value and changing an overload multiple;recording output current, support post-grid-connected point voltage, voltage uplift and DC side voltage transient minimum value under each transient fault condition;determining voltage support capability upper limit under current limiting, relationship between overload multiple and voltage support effect, fault severity and relationship between overload multiple and DC side voltage transient stress according to voltage uplift and DC side voltage transient minimum value, and outputting characteristic analysis results.The application can quantitatively analyze transient overload operation boundary.

Description

Technical Field

[0001] This invention relates to the field of power electronics technology, and specifically to a method for analyzing the transient overload operation characteristics of a cascaded H-bridge SVG. Background Technology

[0002] Cascaded H-bridge SVG is commonly used in high-voltage, high-capacity reactive power compensation scenarios. Its transient operating capability directly affects the reactive power support capability and device protection parameter settings during grid voltage dips. Existing analysis methods typically verify the SVG's output current, grid connection voltage support results, and DC-side voltage changes based on steady-state control models, single fault conditions, or product-specified overload capacity indicators. While this approach can demonstrate whether a specific device meets operational requirements under specific conditions, it lacks sufficient breakdown of the influencing factors during transient overload operation.

[0003] During transient voltage drops, there is a coupling relationship between overload factor, residual grid voltage, current limiting, and DC-side capacitor voltage changes. If only a single operating condition is changed or only the final support voltage is recorded, it is difficult to determine the upper limit of voltage support capacity after current limiting, and it is also difficult to distinguish the impact of fault severity and overload factor on transient stress of the DC-side voltage. In engineering projects, without the aforementioned quantitative relationships, overload capacity design, protection setting, and DC-side capacitor parameter verification easily rely on empirical values ​​or single simulation results, leading to insufficient comparability of analysis results and making it difficult to provide a stable basis for operating boundaries under different fault depths and overload capacities. Summary of the Invention

[0004] This invention provides a method for analyzing the transient overload operating characteristics of a cascaded H-bridge SVG, which at least addresses the problem of how to quantitatively analyze the upper limit of the voltage support capability and the transient stress relationship of the DC side voltage of the cascaded H-bridge SVG under transient voltage drop and overload current limiting conditions, and output the characteristic analysis results.

[0005] This invention provides a method for analyzing the transient overload operating characteristics of a cascaded H-bridge SVG, the method comprising: Establish a cascaded H-bridge SVG simulation model; Based on the simulation model, a first fault condition group and a second fault condition group are set. The first fault condition group corresponds to a fixed overload multiple and changes the residual value of the grid connection point voltage. The second fault condition group corresponds to a fixed residual value of the grid connection point voltage and changes the overload multiple. Record the output current, the voltage at the grid connection point after support, the voltage rise, and the minimum transient value of the DC side voltage under each transient fault condition. The upper limit of voltage support capability under current limiting, the relationship between overload multiple and voltage support effect, the fault severity and the relationship between overload multiple and DC-side voltage transient stress are determined based on the voltage rise and the minimum transient value of DC-side voltage, and the output characteristic analysis results are presented.

[0006] In one possible implementation, a cascaded H-bridge SVG simulation model is established, including: establishing a star-connected cascaded H-bridge topology model, a three-phase stationary coordinate coefficient mathematical model, a dq synchronous rotating coordinate coefficient mathematical model, and a DC-side capacitor voltage dynamic model; establishing a control model including a phase-locked loop, voltage and current dual closed-loop decoupling control, overall voltage equalization control, inter-phase voltage equalization control, and intra-phase voltage equalization control, and establishing a unipolar carrier phase-shifting sinusoidal pulse width modulation model; based on the star-connected cascaded H-bridge topology model, the three-phase stationary coordinate coefficient mathematical model, the dq synchronous rotating coordinate coefficient mathematical model, the DC-side capacitor voltage dynamic model, the control model, and the unipolar carrier phase-shifting sinusoidal pulse width modulation model, the cascaded H-bridge SVG simulation model is obtained.

[0007] In one possible implementation, the cascaded H-bridge SVG simulation model is built based on input parameters, including grid rated voltage, device rated capacity, grid frequency, connection inductance, equivalent resistance, line short-circuit ratio, DC-side capacitor voltage, DC-side capacitor, number of links per phase, carrier frequency, fault injection parameters, and overload current limiting parameters.

[0008] In one possible implementation, the first fault condition group includes at least three transient fault conditions, and the second fault condition group includes at least three transient fault conditions.

[0009] In one possible implementation, each transient fault condition in the first fault condition group has the same overload current limit value, and each transient fault condition in the second fault condition group has the same grid connection point voltage residual value.

[0010] In one possible implementation, the output current, the voltage at the grid connection point after support, the voltage rise, and the transient minimum value of the DC-side voltage are recorded under each transient fault condition. This includes: recording the output current and the voltage at the grid connection point after support during the fault duration; determining the voltage rise based on the difference between the voltage at the grid connection point after support and the remaining value of the grid connection point voltage under the same transient fault condition; and recording the minimum value of the DC-side capacitor voltage within a preset observation window after the fault occurs to obtain the transient minimum value of the DC-side voltage.

[0011] In one possible implementation, the upper limit of voltage support capability under current limiting is determined as follows: under the same overload multiple, the voltage rise corresponding to different grid connection point voltage residual values ​​is compared; if the difference in voltage rise between adjacent transient fault conditions is less than a preset rise change threshold, the larger value among the voltage rises under adjacent transient fault conditions is determined as the upper limit of voltage support capability under the current overload multiple.

[0012] In one possible implementation, the voltage rise is determined by the product of the reactive current component and the line equivalent reactance; the overload current limit is determined by the overload multiple and the rated output current; and the upper limit of the voltage support capability under the current limit is determined as follows: when the output current reaches the overload current limit, the upper limit of the voltage support capability is determined based on the voltage rise.

[0013] In one possible implementation, the relationship between fault severity, overload factor, and transient stress of DC-side voltage is determined as follows: under the same overload factor, the relationship between fault severity and transient stress of DC-side voltage is determined based on the minimum transient value of DC-side voltage corresponding to different residual values ​​of grid connection point voltage; under the same residual value of grid connection point voltage, the relationship between overload factor and transient stress of DC-side voltage is determined based on the minimum transient value of DC-side voltage corresponding to different overload factors.

[0014] In one possible implementation, the characteristic analysis results include the relationship between the upper limit of voltage support capability under current limiting determined by voltage rise and the relationship between overload multiple and voltage support effect, as well as the relationship between fault severity determined by DC-side voltage transient minimum value and DC-side voltage transient stress, and the relationship between overload multiple and DC-side voltage transient stress.

[0015] Compared with the prior art, the advantages and beneficial effects of the present invention are as follows: By establishing a cascaded H-bridge SVG simulation model, a unified description of the main circuit topology, control loop, voltage equalization control, and modulation process is achieved in the same simulation link, so that the output current, grid connection point voltage, and DC side voltage data under transient fault conditions have a unified source.

[0016] By setting a first fault condition group with a fixed overload multiple and changing the residual value of the grid connection point voltage, the impact of fault severity on voltage support capability and DC side voltage transient stress can be analyzed under the condition that the current limiting condition remains unchanged.

[0017] By setting a second fault condition group with a fixed grid connection point voltage residual value and changing the overload factor, the influence of overload capacity changes on voltage support effect and DC side voltage transient stress can be analyzed under the condition that the fault benchmark is consistent.

[0018] By recording the voltage rise and the minimum transient value of the DC side voltage, the relationship between the upper limit of voltage support capacity, the overload multiple and the voltage support effect, the severity of the fault, and the relationship between the overload multiple and the transient stress of the DC side voltage can be output in the same analysis result, providing a quantitative basis for overload capacity design, protection parameter setting, and DC side capacitor parameter verification. Attached Figure Description

[0019] Figure 1 This is a schematic flowchart of the method of the present invention; Figure 2 This is a graph showing the influence of the residual value of the grid connection point voltage on the minimum transient value of the DC side voltage in an embodiment of the present invention. Figure 3 This is a graph showing the effect of overload factor on voltage support in an embodiment of the present invention. Detailed Implementation

[0020] To enable those skilled in the art to better understand the technical solutions of this invention, the technical solutions of this invention will be clearly and completely described below with reference to the accompanying drawings of one or more embodiments. Obviously, the described embodiments are only a part of the embodiments of this invention, and not all of them. Based on the embodiments of this invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the protection scope of this invention.

[0021] Cascaded H-bridge SVG is a type of static var generator that uses multiple H-bridge power units connected in series to form the AC output voltage. It typically uses DC-side capacitors in each power unit to provide energy buffering and utilizes carrier phase-shift modulation, dual closed-loop voltage and current control, and hierarchical voltage equalization control to achieve reactive current output. Compared to two-level or three-level commutator structures, the cascaded H-bridge structure can reduce the voltage stress on individual power devices and improve the output voltage waveform quality at higher voltage levels through multi-level output, making it suitable for constructing high-voltage, high-capacity reactive power support devices. When a transient voltage drop occurs in the grid, the cascaded H-bridge SVG needs to increase the reactive current output in a short time to support the grid connection voltage; simultaneously, the DC-side capacitor voltage of each link is affected by the output current, fault depth, modulation state, and voltage equalization control response. Based on these operating characteristics, analyzing the voltage support capability and DC-side voltage transient stress of the cascaded H-bridge SVG under transient overload conditions is an important foundation for determining the device's overload boundary, protection setting basis, and DC-side capacitor configuration margin.

[0022] like Figure 1 As shown, a method for analyzing the transient overload operation characteristics of a cascaded H-bridge SVG includes the following processing steps.

[0023] The simulation modeling unit receives the main circuit parameters, control parameters, and fault condition parameters of the cascaded H-bridge SVG and establishes a simulation model for transient overload operation analysis in the electromagnetic transient simulation environment. The cascaded H-bridge SVG adopts a star connection structure, with each phase consisting of multiple H-bridge power units connected in series to form a bridge arm, which is connected to the grid connection point via a connecting inductor and equivalent resistance. The simulation modeling unit configures the three-phase AC side voltage and current relationships, the control variable relationships in the synchronous rotating coordinate system, and the DC side capacitor voltage change relationships into the model computation layer, and connects the phase-locked loop, voltage and current dual closed-loop decoupling control, overall voltage equalization control, inter-phase voltage equalization control, intra-phase voltage equalization control, and unipolar carrier phase-shifted sinusoidal pulse width modulation (CPS-SPWM) to the same simulation link. After the model completes steady-state verification, a cascaded H-bridge SVG simulation model that can be loaded with transient fault conditions is generated. Subsequent condition group settings, data recording, and transient characteristic analysis are all performed on this model.

[0024] In one embodiment, the cascaded H-bridge SVG simulation model is built according to the main circuit layer, mathematical operation layer, control layer, and modulation layer. The main circuit layer is based on a star-connected cascaded H-bridge topology, with each phase having the same number of H-bridge power units, which are connected in series to form a phase arm. Each H-bridge power unit is equipped with a DC-side parallel capacitor and a fully controlled power switch. The DC-side parallel capacitor is used to reflect the voltage change caused by energy exchange of the power unit during transient faults. The AC terminal of the arm is connected to the grid connection point via a connecting inductor and an equivalent resistance. The connecting inductor is used to reflect the coupling relationship between the SVG and the grid, and the equivalent resistance is used to reflect the equivalent loss in the connecting branch. The star-connected neutral point participates in the electrical relationship calculation as the common connection point of the three-phase arms in the simulation, but is not recorded separately as an external output index.

[0025] The mathematical operation layer consists of a three-phase static coordinate coefficient mathematical model, a dq synchronous rotating coordinate coefficient mathematical model, and a DC-side capacitor voltage dynamic model. The three-phase static coordinate coefficient mathematical model receives the three-phase voltage at the grid connection point, the bridge arm output voltage, the connecting inductance, and the equivalent resistance. It is used to calculate the transient changes in the three-phase output current and transmits the three-phase output current to the current control and data recording stages. The dq synchronous rotating coordinate coefficient mathematical model receives the synchronous phase angle output from the phase-locked loop and converts the three-phase voltage and three-phase current into d-axis and q-axis components, allowing DC voltage control and reactive current support to be handled separately in the two control components. The DC-side capacitor voltage dynamic model is established for each H-bridge power unit, receiving the bridge arm current, the power unit's operational status, and capacitor parameters. It calculates the change process of the DC-side capacitor voltage for each power unit and transmits the calculation results to the voltage equalization control and DC-side voltage recording stages.

[0026] The control layer receives the current component, DC-side capacitor voltage, and grid-connected point voltage phase angle from the mathematical operation layer to generate modulation reference quantities for each phase. A phase-locked loop (PLL) extracts the synchronous phase of the grid-connected point voltage, providing a phase angle reference for dq synchronous rotating coordinate system transformation and current decoupling control. A voltage-current dual-closed-loop decoupling control receives the DC-side voltage deviation, reactive current command, and actual output current to generate control quantities for modulation. Overall voltage equalization control adjusts the overall level of the DC-side voltage of all power units; inter-phase voltage equalization control adjusts the DC-side voltage difference between the three phases; and intra-phase voltage equalization control adjusts the DC-side voltage difference between H-bridge power units within the same phase. The modulation layer employs unipolar carrier phase-shifted sinusoidal pulse width modulation. Each power unit is configured with a triangular carrier with sequentially staggered phases. The modulation reference quantity is compared with the phase-shifted carrier to generate a switching drive signal. The switching drive signal returns to the main circuit layer to change the on / off state of each H-bridge power unit, forming a closed-loop operation relationship between the main circuit layer, mathematical operation layer, control layer, and modulation layer.

[0027] In one embodiment, the cascaded H-bridge SVG simulation model is initialized based on unified input parameters. These input parameters consist of grid parameters, device parameters, connection branch parameters, power unit parameters, modulation parameters, fault condition parameters, and overload limiting parameters. The grid rated voltage is used to set the grid connection point reference voltage and per-unit conversion benchmark; the device rated capacity is used to determine the rated output current and subsequent overload multiple conversion basis; and the grid frequency is used to set the frequency benchmark and carrier synchronization benchmark for the synchronous rotating coordinate system. The connecting inductance and equivalent resistance are used to construct the equivalent connection branch between the SVG and the grid connection point; their values ​​can be determined based on the device's input reactance, connecting cable impedance, or equivalent engineering parameters. The line short-circuit ratio characterizes the strength of the grid connection, and the simulation modeling unit configures the equivalent support capacity on the grid side accordingly, making the voltage changes at the grid connection point comparable during transient faults.

[0028] The DC-side capacitor voltage and DC-side capacitor are used to establish the DC-side energy storage stage of the power unit. The DC-side capacitor voltage serves as a benchmark for comparing the steady-state target value and the transient minimum value, while the DC-side capacitor acts as a constraint parameter for the rate of change of the DC-side voltage. The number of links per phase is used to determine the number of power units in the series H-bridge per phase and simultaneously determine the phase shift of each carrier in the unipolar carrier phase-shifted sinusoidal pulse width modulation. The carrier frequency is used to set the triangular carrier period in the modulation model. The simulation step size should be matched with the carrier frequency to avoid missing switching states during sampling. If the simulation step size is larger than the resolution required for switching state changes, the simulation modeling unit adjusts the simulation step size until the switching drive signal, output current, and DC-side voltage data can be recorded on the same time axis.

[0029] Fault injection parameters are used to set the transient fault occurrence time, fault duration, fault clearing time, and grid connection point voltage residual value. The grid connection point voltage residual value represents the proportion of the grid connection point voltage retained relative to the rated voltage during the fault, used to characterize the fault severity, and serves as a variable parameter in the subsequent first fault condition group. Overload current limiting parameters are used to limit the maximum allowable output current of the current control loop, and their setting is based on the device's allowable overload capacity, the current margin of the power switching devices, and protection setting requirements. The simulation modeling unit reads the device's rated capacity and the grid's rated voltage to determine the rated output current, and then combines this with the overload multiple to obtain the overload current limiting value under each condition. After the input parameters are written into the model, the simulation modeling unit performs a steady-state pre-run to check whether the output current, grid connection point voltage, and DC-side capacitor voltage have reached the rated operating state. If there are missing parameters, inconsistent units, or the operating state does not converge, the current model does not enter the fault condition loading stage, and a parameter verification mark is output; after the parameter verification passes, the steady-state data serves as the reference data for subsequent transient fault simulation, and the cascaded H-bridge SVG simulation model enters the condition group setting stage.

[0030] After completing the steady-state pre-run of the cascaded H-bridge SVG simulation model, the operating condition configuration unit loads two types of transient fault conditions onto the same simulation model. The first fault condition group maintains the overload current limiting capability unchanged but changes the residual value of the grid connection point voltage to observe changes in voltage support and DC-side voltage transient stress as the fault severity changes. The second fault condition group maintains the residual value of the grid connection point voltage unchanged but changes the overload factor to observe changes in voltage support and DC-side voltage transient stress as the overload capability changes. Both fault condition groups use the same model parameters, fault injection time, fault duration, and data sampling rules, and are then transferred to the transient simulation execution phase after generation.

[0031] In one embodiment, both the first fault condition group and the second fault condition group are configured with no fewer than three transient fault conditions. A transient fault condition refers to a set of simulation conditions based on rated steady-state operation, where a transient drop in the grid-connected point voltage is caused according to specified fault injection parameters, and the changes in SVG output current, grid-connected point voltage after support, DC-side voltage, and the recovery process are recorded during the fault duration. Each set of transient fault conditions is defined by the fault occurrence time, fault duration, remaining grid-connected point voltage, overload multiple, overload current limit value, and data recording window. Setting the number of conditions to no fewer than three ensures that at least three control points (low, medium, and high) are formed when the same variable changes, avoiding linearization or random judgments based on only two conditions. The number of conditions can be determined based on the device design capacity, protection setting accuracy, and simulation calculation resources; if a denser relationship curve is required, the number of conditions can be increased without changing the control principle.

[0032] In the first fault condition group, the residual grid connection voltage is set according to the severity of the fault, from mild to severe, and the value should be within the transient voltage range allowed by the power grid operation regulations and simulation model. The residual grid connection voltage represents the proportion of the grid connection voltage retained relative to the rated voltage during the fault, used to distinguish the severity of the transient fault. Before writing each residual grid connection voltage value, the condition configuration unit performs a range check on the value. If the value is not within the allowable range, the current condition will not enter the simulation queue, and a parameter check flag will be generated. All conditions that have completed the check are loaded at the same fault injection time, so that the output current and DC side voltage data under different residual grid connection voltage values ​​have a comparable basis.

[0033] In the second fault condition group, the overload multiple is set according to the overload capacity range that the device can withstand, covering at least three levels: low overload, intermediate overload, and high overload. The overload multiple is used to determine the maximum allowable output current of the current control loop during a fault. The value of the overload multiple is determined based on the device's rated capacity, the current margin of the power switching devices, heat dissipation capacity, and protection setting requirements. After reading the overload multiple, the condition configuration unit transmits it to the current limiting circuit and saves the correspondence between the overload multiple and the transient fault condition in the condition table. If the overload multiple exceeds the device's allowable boundary, the current condition is marked as an unexecutable condition and is not included in subsequent transient index statistics. Through the above settings, the two types of condition groups form simulation inputs for the fault severity dimension and the overload capacity dimension, respectively, and enter the same transient simulation execution process.

[0034] In one embodiment, all transient fault conditions in the first fault condition group maintain the same overload current limit value. The overload current limit value is determined by the device's rated output current and a fixed overload multiple, and is incorporated into the current limiting element of the voltage-current dual closed-loop decoupling control. When generating the first fault condition group, the condition configuration unit copies the same overload current limit value to each transient fault condition within the group, ensuring that differences within the group arise only from the residual voltage at the grid connection point. With this setting, when the output current reaches the limit during the fault duration, the difference in voltage rise under different conditions can reflect the impact of fault severity on voltage support performance and DC-side voltage transient stress, without being affected by changes in the overload current limit value.

[0035] To ensure data comparability for the first fault case group, all simulation model parameters, fault occurrence time, fault duration, fault clearance time, initial DC-side capacitor voltage, carrier frequency, and sampling window are kept consistent, except for the residual value of the grid connection point voltage. Before starting the simulation, the case configuration unit checks whether the overload current limit values ​​of each transient fault case within the group are consistent. If the overload current limit value in any case is inconsistent with the group's baseline value, the current case group will not enter batch simulation, and a case consistency verification flag will be returned. After successful verification, each transient fault case is sequentially loaded into the same simulation model, and the obtained output current, supported grid connection point voltage, voltage rise, and the lowest transient value of the DC-side voltage are recorded for subsequent analysis of the upper limit of voltage support capability.

[0036] In the second fault condition group, all transient fault conditions maintain the same residual grid-connected voltage value. This residual grid-connected voltage value serves as a fixed fault severity parameter within the group, ensuring that the voltage support effect and DC-side voltage transient stress under different overload ratios have the same fault benchmark. When generating the second fault condition group, the condition configuration unit writes the same residual grid-connected voltage value into each transient fault condition within the group, and then writes it into different overload ratios. During simulation, different overload ratios change the maximum allowable output current of the current limiting circuit, and the minimum transient values ​​of the grid-connected voltage and DC-side voltage after support change with the overload capacity to form comparative data.

[0037] The second fault condition group also requires consistency verification. The condition configuration unit checks whether the residual grid-connected voltage values ​​of each transient fault condition within the group are the same, and also checks whether different overload ratios are within the device's allowable overload range. If the residual grid-connected voltage values ​​are inconsistent, the data within the group cannot be used for overload ratio dimension analysis; if the overload ratio exceeds the limit, the out-of-limit condition is excluded and a parameter verification mark is recorded. The second fault condition group that has completed verification is written into the transient simulation queue. The voltage rise obtained subsequently is used to determine the relationship between the overload ratio and the voltage support effect, and the minimum transient DC-side voltage value is used to determine the relationship between the overload ratio and the transient stress of the DC-side voltage. The output data of the two types of condition groups are saved using a unified time axis and unified recording fields, providing directly comparable transient operating data for subsequent analysis stages.

[0038] The transient simulation execution unit receives the verified first and second fault condition groups and sequentially loads each transient fault condition into the cascaded H-bridge SVG simulation model. Before the fault occurs, the model maintains rated steady-state operation and saves baseline data; after the fault occurs, the simulation execution unit records the changes in output current, grid connection point voltage, and DC-side capacitor voltage on a unified time axis. After completing the simulation of each transient fault condition, the data processing unit generates the voltage rise based on the voltage records during the fault period and extracts the transient minimum value of the DC-side voltage from the DC-side capacitor voltage records. The above data is written into the transient operation dataset according to the condition number, and then proceeds to the subsequent voltage support capability and DC-side voltage transient stress analysis stages.

[0039] In one embodiment, the transient operation dataset is organized according to transient fault conditions, sampling times, and index types. Transient fault conditions are used to distinguish different simulation conditions in the first and second fault condition groups. Sampling times are used to store continuous data before the fault occurs, during the fault duration, and after the fault is cleared. Index types are used to distinguish between output current, grid-connected voltage, DC-side capacitor voltage, voltage rise, and the transient minimum value of DC-side voltage. Before each transient fault condition begins, the transient simulation execution unit reads the steady-state pre-operation results to confirm that the grid-connected voltage, output current, and DC-side capacitor voltage are near their rated operating conditions. If the steady-state data does not meet the set operating conditions, the current condition does not enter the fault injection phase and a steady-state verification flag is generated to prevent transient index deviations caused by non-fault reasons.

[0040] The output current during the fault duration phase is formed by sampling the AC side current value of the cascaded H-bridge SVG. The recorded data can be the effective value of the three-phase current, the positive-sequence fundamental current, or the per-unit value of the output current set by the simulation model. The specific recording criteria should remain consistent within the same batch of operating conditions. The grid connection point voltage after support is formed by the stable recorded value of the grid connection point voltage during the fault duration phase, reflecting the level to which the grid connection point voltage is supported after the SVG outputs reactive current. To avoid the impact of spikes or switching ripples at the moment of fault injection on the recording results, the data processing unit selects a stable sampling interval during the fault duration phase. The stable sampling interval can be determined based on the fault duration, control loop response time, and sampling step size. In principle, it avoids the instantaneous jump interval at the moment the fault occurs and retains the time period when the current limiting and voltage support have entered a comparable state.

[0041] The voltage rise is determined by the difference between the supported grid connection point voltage and the residual grid connection point voltage under the same transient fault condition. The residual grid connection point voltage is the voltage retention ratio during the fault period written during the condition configuration phase, while the supported grid connection point voltage is the actual support result recorded during the simulation operation phase. Both originate from the same transient fault condition, and the data processing unit performs matching under the same condition number to avoid voltage rise distortion caused by cross-condition value taking. If no valid supported grid connection point voltage is recorded in the same condition, the voltage rise of the current condition is not included in subsequent comparisons, and a data missing marker is output.

[0042] The minimum transient value of the DC-side voltage is extracted from the DC-side capacitor voltage record. The preset observation window starts from the moment the fault occurs, covering the fault duration and the initial stage of the control system restoring the DC-side voltage. The window length can be determined based on the fault duration, the DC voltage outer loop response time, and the protection setting observation cycle. The data processing unit retrieves the DC-side capacitor voltage records of each power unit within the preset observation window and extracts the minimum value according to the statistical caliber preset in the model. The statistical caliber can use the lowest voltage among all power units or the lowest value among the average DC-side capacitor voltages of each phase, but it must remain consistent within the same analysis batch. The extracted minimum transient value of the DC-side voltage is bound to the transient fault condition number and saved, and is subsequently used to compare the residual voltage values ​​at different grid connection points and the transient stress of the DC-side voltage under different overload ratios.

[0043] The analysis and processing unit receives the output current, grid-connected voltage after support, voltage rise, and DC-side voltage transient minimum value generated under various transient fault conditions, and groups the data according to the source of the fault condition group. For the first fault condition group, the analysis and processing unit compares the voltage rise and DC-side voltage transient minimum value corresponding to different grid-connected voltage residual values ​​under the same overload multiple to determine the upper limit of voltage support capability under current limiting conditions and the impact of fault severity on DC-side voltage transient stress. For the second fault condition group, the analysis and processing unit compares the voltage rise and DC-side voltage transient minimum value corresponding to different overload multiples under the same grid-connected voltage residual value to determine the relationship between overload multiple, voltage support effect, and DC-side voltage transient stress. The analyzed data is compiled into characteristic analysis results, which serve as the basis for overload capacity design, protection parameter setting, and DC-side capacitor parameter verification.

[0044] In one embodiment, the analysis and processing unit determines the upper limit of voltage support capability under current limiting conditions based on a first fault condition group. Each transient fault condition in the first fault condition group has the same overload current limiting value and different grid connection point voltage residual values. Therefore, the change in voltage rise within the group mainly reflects the change in voltage support that the cascaded H-bridge SVG can provide under the same current limiting capability after the fault severity changes. The analysis and processing unit arranges each transient fault condition in descending or ascending order of grid connection point voltage residual value and extracts the voltage rise under each transient fault condition. The lower the grid connection point voltage residual value, the more severe the transient fault; if the output current is close to or reaches the limiting state while the overload multiple remains constant, the voltage rise will generally not continue to increase as the fault deepens.

[0045] To avoid judging the upper limit of voltage support capacity based on a single operating condition, the analysis and processing unit judges the difference in voltage rise between adjacent transient fault conditions. A preset voltage rise change threshold is used to distinguish whether the voltage rise has entered a state of slowing change. This threshold can be determined based on voltage sampling accuracy, simulation step size, grid connection point voltage allowable deviation, and protection setting accuracy, and is used to constrain the difference in voltage rise between adjacent operating conditions. When the difference in voltage rise between adjacent transient fault conditions is less than this threshold, it indicates that deepening the fault under the current overload multiple will not significantly increase the grid connection point voltage rise. The analysis and processing unit takes the larger value of the voltage rise under adjacent transient fault conditions as the upper limit of voltage support capacity under the current overload multiple. If the difference in voltage rise between adjacent transient fault conditions is still greater than this threshold, the analysis and processing unit retains this set of data and marks it as not having reached the support saturation state, and continues to compare after adding deeper fault conditions or more dense grid connection point voltage remaining values. Through the above processing, the upper limit of voltage support capability is determined by multi-point operating condition data under the same overload multiple, without relying on the results of a single simulation.

[0046] In one embodiment, the voltage rise can also be verified for consistency by combining the reactive current component and the line equivalent reactance. During the fault duration, the cascaded H-bridge SVG forms voltage support by injecting the reactive current component into the grid connection point. The relationship between the voltage rise, the reactive current component, and the line equivalent reactance is expressed by the following expression:

[0047] in, This is the voltage rise at the grid connection point. This refers to the reactive current component in the output current of the cascaded H-bridge SVG. The line equivalent reactance is determined by the connecting inductance and the grid frequency. This expression illustrates that, when the equivalent grid connection conditions remain unchanged, the reactive current component is the main control variable affecting the voltage rise at the grid connection point. The analysis and processing unit reads the output current during the fault duration in each transient fault condition and obtains the reactive current component by combining the phase-locked loop and synchronous rotating coordinate system transformation. The line equivalent reactance remains consistent across the same batch of operating conditions to ensure that the voltage rise comparisons are based on identical network conditions.

[0048] The overload current limit is determined by the overload multiple and the rated output current. The rated output current is determined by the device's rated capacity and rated voltage, and serves as the reference value for the current limiting circuit. The overload multiple is set based on the device's allowable overload capacity, the current margin of the power switching devices, and protection setting requirements. When determining the upper limit of voltage support capability, the analysis and processing unit simultaneously checks whether the output current reaches the overload current limit. If the output current does not reach the limit, the voltage rise may still change with the increase of the reactive current component, and the current operating condition is not used as the basis for determining the upper limit of support under current limiting. If the output current reaches the overload current limit, and the change in voltage rise between adjacent fault conditions is less than the preset rise change threshold, the upper limit of voltage support capability under the current overload multiple is determined and written into the characteristic analysis results. If the output current reaches the limit but the voltage rise still changes significantly, the analysis and processing unit retains the record of this operating condition and marks the need to add more fault severity sampling points in the results to avoid misjudging the support result that has not yet saturated as the upper limit.

[0049] In one embodiment, the analysis and processing unit determines the relationship between fault severity and DC-side voltage transient stress based on the minimum transient value of the DC-side voltage. A first fault condition group maintains the same overload factor but varies the residual value of the grid connection point voltage, making it suitable for analyzing the impact of fault severity on DC-side voltage transient stress. The analysis and processing unit sorts the data within the first fault condition group according to the residual value of the grid connection point voltage and extracts the minimum transient value of the DC-side voltage corresponding to each transient fault condition. The lower the minimum transient value of the DC-side voltage, the more significant the transient voltage drop experienced by the DC-side capacitor during the fault, and the higher the DC-side voltage transient stress. If the minimum transient value of the DC-side voltage corresponding to different residual values ​​of the grid connection point voltage decreases as the fault worsens, the analysis and processing unit records this as a change in the relationship between fault severity and DC-side voltage transient stress.

[0050] The second fault condition group maintains the same residual grid-connected voltage but changes the overload factor, making it suitable for analyzing the impact of the overload factor on the transient stress of the DC-side voltage. The analysis and processing unit sorts the data within the second fault condition group according to the overload factor and extracts the minimum transient value of the DC-side voltage for each transient fault condition. As the overload factor increases, the upper limit of the current allowed by the current control loop increases, and the modulation requirements of the bridge arm output voltage and the instantaneous losses of power devices may change. The DC-side capacitor bears short-term energy fluctuations before the voltage outer loop completes regulation. Therefore, the minimum transient value of the DC-side voltage can be used to reflect the transient stress of the DC-side voltage under different overload capabilities. If the minimum transient value of the DC-side voltage under a certain overload factor is lower than the device's allowable lower limit, the analysis and processing unit marks this condition as a DC-side voltage stress over-limit condition and retains this mark in the characteristic analysis results. The device's allowable lower limit can be determined based on the rated voltage of the DC-side capacitor, the undervoltage protection setting of the power unit, and the allowable recovery margin of the control system, and is used to constrain the engineering applicability range of the overload factor. The above treatment enables the fault severity dimension and overload multiple dimension to form independent DC-side voltage transient stress relationships.

[0051] In one embodiment, the characteristic analysis results consist of voltage support results and DC-side voltage transient stress results. Voltage support results include the upper limit of voltage support capability under current limiting conditions, and the relationship between voltage support effects formed by different overload factors under the same grid connection point voltage residual value. The upper limit of voltage support capability represents the maximum grid connection point voltage rise that a cascaded H-bridge SVG can generate under the current overload factor and line equivalent reactance conditions; the relationship between overload factor and voltage support effect represents the changing trend of grid connection point voltage support value and voltage rise after increasing the overload factor. The analysis and processing unit saves the above results along with the corresponding overload factor, grid connection point voltage residual value, output current state, and limiting state to prevent the output results from deviating from the operating conditions.

[0052] The DC-side voltage transient stress results include the relationship between fault severity and DC-side voltage transient stress, as well as the relationship between overload factor and DC-side voltage transient stress. The former comes from the lowest DC-side voltage transient stress corresponding to different grid connection point voltage residual values ​​under the same overload factor, while the latter comes from the lowest DC-side voltage transient stress corresponding to different overload factors under the same grid connection point voltage residual value. When outputting results, the analysis and processing unit organizes data according to operating condition group, operating condition number, overload factor, grid connection point voltage residual value, voltage rise, and lowest DC-side voltage transient stress, ensuring that each relationship can be traced back to the corresponding transient fault operating condition. If there are missing data, unsuccessful steady-state verification, output current not reaching the limit, or DC-side voltage stress exceeding the limit, the analysis and processing unit writes a status flag in the corresponding operating condition record, preventing abnormal operating conditions from being directly included in the capacity limit judgment. The processed characteristic analysis results can be output as data tables, relationship curves, or parameter tuning records for use in overload capacity design, protection parameter tuning, and DC-side capacitor parameter verification.

[0053] In one embodiment, the simulation platform uses MATLAB / Simulink, and the analysis object is a star-connected cascaded H-bridge SVG with a rated voltage of ±10kV and a rated capacity of ±1Mvar. The grid rated voltage in the simulation model is set to 10kV, the grid frequency to 50Hz, the connecting inductance to 15.92mH, the equivalent resistance to 2.67Ω, and the line short-circuit ratio to 20. The number of links per phase is set to 12, the DC-side capacitor voltage is set to 850V, the DC-side capacitor to 1950μF, and the carrier frequency to 500Hz. After these parameters are written into the simulation model, the simulation platform first performs rated steady-state operation to stabilize the grid connection point voltage, output current, and DC-side capacitor voltage, and then loads transient fault conditions. The connecting inductance and equivalent resistance are used to construct the equivalent connection branch between the SVG and the grid connection point, the line short-circuit ratio is used to constrain the grid-side support capacity, and the DC-side capacitor voltage and DC-side capacitor are used to record transient changes in the DC-side voltage. If parameters are missing, units are inconsistent, or the steady-state DC side voltage cannot converge before the simulation runs, the current operating condition will not enter the transient fault simulation. The steady-state pre-run will be re-executed after the parameters are corrected.

[0054] In the fixed overload multiple operating condition group, the overload current limit is maintained at 2.0 times, and the residual grid connection point voltage is set to 0.8pu, 0.5pu, and 0.2pu respectively. Under the condition of a residual grid connection point voltage of 0.8pu, the SVG output current is 2.0pu, the supported grid connection point voltage is 0.86pu, the voltage rise is 0.06pu, and the minimum transient DC-side voltage is 812V. Under the condition of a residual grid connection point voltage of 0.5pu, the SVG output current is 2.0pu, the supported grid connection point voltage is 0.57pu, the voltage rise is 0.07pu, and the minimum transient DC-side voltage is 798V. Under the condition of a residual grid connection point voltage of 0.2pu, the SVG output current is 2.0pu, the supported grid connection point voltage is 0.27pu, the voltage rise is 0.07pu, and the minimum transient DC-side voltage is 768V. The three sets of data show that, under the same overload current limit, as the residual value of the grid connection point voltage decreases, the minimum transient value of the DC side voltage decreases synchronously, indicating that an increase in the severity of the fault will increase the transient stress on the DC side voltage; at the same time, the voltage rise tends to stabilize around 0.07 pu, indicating that the voltage support capability under the current overload multiple is constrained by the current limit.

[0055] In the operating condition group with a fixed grid connection point voltage residual value, the grid connection point voltage residual value is maintained at 0.8 pu, and overload multiples are set to 1.2, 1.5, and 2.0 times respectively. Under the condition of 1.2 times overload multiple, the SVG output current is 1.2 pu, the grid connection point voltage after support is 0.82 pu, the voltage rise is 0.02 pu, and the minimum transient DC-side voltage is 846V. Under the condition of 1.5 times overload multiple, the SVG output current is 1.5 pu, the grid connection point voltage after support is 0.83 pu, the voltage rise is 0.03 pu, and the minimum transient DC-side voltage is 830V. Under the condition of 2.0 times overload multiple, the SVG output current is 2.0 pu, the grid connection point voltage after support is 0.86 pu, the voltage rise is 0.06 pu, and the minimum transient DC-side voltage is 817V. The voltage rise is calculated according to the expression... To explain, among which, This is the voltage rise at the grid connection point. This is the reactive current component. This represents the line's equivalent reactance. As the overload factor increases, the upper limit of the current allowed by the current control loop increases, the reactive current component increases, and the voltage at the grid connection point rises. The minimum transient value of the DC-side voltage decreases as the overload factor increases, indicating that higher overload output will subject the DC-side voltage to greater transient stress.

[0056] After processing the data for the two types of operating conditions, the simulation platform generates characteristic analysis results based on the operating condition number, overload factor, residual grid connection voltage, output current, grid connection voltage after support, voltage rise, and minimum transient DC-side voltage. The operating condition group with a fixed overload factor is used to determine the relationship between the upper limit of voltage support capability under output current limiting, fault severity, and transient DC-side voltage stress. The operating condition group with a fixed residual grid connection voltage is used to determine the relationship between the output overload factor and voltage support effect, as well as the relationship between the overload factor and transient DC-side voltage stress. If a certain operating condition fails to reach the steady-state reference, the output current fails to reach the set limit, or the DC-side voltage is lower than the device's allowable lower limit, the simulation platform adds a status marker to the corresponding record. This operating condition is not used as the basis for determining the upper limit of voltage support capability, but is retained as reference data for protection parameter setting and DC-side capacitor parameter verification.

[0057] It should also be noted that the terms "comprising," "including," or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Without further limitation, an element defined by the phrase "comprising one..." does not exclude the presence of other identical elements in the process, method, article, or apparatus that includes said element.

[0058] The various embodiments in this invention are described in a progressive manner. Similar or identical parts between embodiments can be referred to mutually. Each embodiment focuses on describing the differences from other embodiments. In particular, the system embodiments are basically similar to the method embodiments, so the description is relatively simple; relevant parts can be referred to the descriptions in the method embodiments.

Claims

1. A method for analyzing the transient overload operating characteristics of a cascaded H-bridge SVG, characterized in that, The method includes: Establish a cascaded H-bridge SVG simulation model; Based on the simulation model, a first fault condition group and a second fault condition group are set. The first fault condition group corresponds to a fixed overload multiple and changes the residual value of the grid connection point voltage. The second fault condition group corresponds to a fixed residual value of the grid connection point voltage and changes the overload multiple. Record the output current, the voltage at the grid connection point after support, the voltage rise, and the minimum transient value of the DC side voltage under each transient fault condition. Based on the voltage rise and the minimum transient value of the DC-side voltage, determine the upper limit of voltage support capability under current limiting, the relationship between overload multiple and voltage support effect, fault severity, and the relationship between overload multiple and transient stress of DC-side voltage, and output the characteristic analysis results.

2. The method according to claim 1, characterized in that, The establishment of the cascaded H-bridge SVG simulation model includes: Establish a star-connected cascaded H-bridge topology model, a three-phase static coordinate coefficient mathematical model, a dq synchronous rotating coordinate coefficient mathematical model, and a dynamic model of the DC-side capacitor voltage; A control model including phase-locked loop, voltage and current dual closed-loop decoupling control, overall voltage equalization control, inter-phase voltage equalization control and intra-phase voltage equalization control is established, and a unipolar carrier phase-shifting sinusoidal pulse width modulation model is also established. Based on the star-connected cascaded H-bridge topology model, the three-phase stationary coordinate coefficient mathematical model, the dq synchronous rotating coordinate coefficient mathematical model, the DC-side capacitor voltage dynamic model, the control model, and the unipolar carrier phase-shifting sinusoidal pulse width modulation model, the cascaded H-bridge SVG simulation model is obtained.

3. The method according to claim 2, characterized in that, The cascaded H-bridge SVG simulation model is established based on input parameters, which include grid rated voltage, device rated capacity, grid frequency, connection inductance, equivalent resistance, line short-circuit ratio, DC-side capacitor voltage, DC-side capacitor, number of links per phase, carrier frequency, fault injection parameters, and overload current limiting parameters.

4. The method according to claim 1, characterized in that, The first fault condition group includes at least three transient fault conditions, and the second fault condition group includes at least three transient fault conditions.

5. The method according to claim 4, characterized in that, Each transient fault condition in the first fault condition group has the same overload current limit value, and each transient fault condition in the second fault condition group has the same grid connection point voltage residual value.

6. The method according to claim 1, characterized in that, The recording of output current, grid connection point voltage, voltage rise, and DC side voltage transient minimum value under various transient fault conditions includes: Record the output current and the voltage at the grid connection point after the support during the fault duration phase; The voltage rise is determined based on the difference between the voltage at the grid connection point after the support and the remaining voltage at the grid connection point under the same transient fault condition. Record the lowest DC-side capacitor voltage within a preset observation window after the fault occurs to obtain the transient lowest DC-side voltage value.

7. The method according to claim 6, characterized in that, The upper limit of voltage support capability under current limiting is determined in the following manner: Under the same overload factor, the voltage rise corresponding to different residual values ​​of the grid connection point voltage is compared; If the difference in voltage rise between adjacent transient fault conditions is less than a preset voltage rise change threshold, the larger of the voltage rises under the adjacent transient fault conditions is determined as the upper limit of the voltage support capacity under the current overload multiple.

8. The method according to claim 1, characterized in that, The voltage rise is determined by the product of the reactive current component and the line equivalent reactance; The overload current limit is determined by the overload multiple and the rated output current; The upper limit of voltage support capability under current limiting is determined in the following manner: When the output current reaches the overload current limit, the upper limit of the voltage support capability is determined based on the voltage rise.

9. The method according to claim 1, characterized in that, The relationship between the severity of the fault, the overload factor, and the transient stress of the DC-side voltage is determined as follows: Under the same overload factor, the relationship between the severity of the fault and the transient stress of the DC side voltage is determined based on the minimum transient value of the DC side voltage corresponding to different residual values ​​of the grid connection point voltage. Under the same residual value of the grid connection point voltage, the relationship between the overload factor and the transient stress of the DC side voltage is determined based on the minimum transient value of the DC side voltage corresponding to different overload factors.

10. The method according to claim 1, characterized in that, The characteristic analysis results include the relationship between the upper limit of voltage support capability under current limiting and the overload multiple and voltage support effect determined based on the voltage rise amount, as well as the relationship between the fault severity and DC-side voltage transient stress determined based on the minimum DC-side voltage transient value and the relationship between the overload multiple and the DC-side voltage transient stress.

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