Weak power grid full load test method and system of double sampling closed loop frequency converter based on SVG reactive compensation and orthogonal decomposition

Abstract

The application provides a weak power grid full load test method and system for a dual sampling closed loop frequency converter based on SVG reactive compensation and orthogonal decomposition, and relates to the technical field of frequency converter testing. The method constructs a frequency converter full load circulating test loop, sets a first sampling point and a second sampling point respectively, establishes an equivalent control model, decomposes the frequency converter to be tested, uses a phase-locked loop to establish a synchronous rotating coordinate system, extracts the second sampling point feedback value and the first sampling point observation value through orthogonal decomposition. A dual closed loop control logic is used, a voltage reference value generated by a reactive loop is used to control the static reactive generator to output reactive current, the reactive loss generated by the internal phase-shifting transformer leakage is compensated, the system input side power factor is tracked to a preset target value, and load testing is realized in combination with an active loop. The application solves the coupling oscillation problem caused by leakage reactive under a weak power grid, realizes long time stable full load testing of a super large power frequency converter with a minimum power grid capacity.

Description

Technical Field

[0001] This invention relates to the field of frequency converter testing technology, and more specifically, to a method and system for full-load testing of a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition. Background Technology

[0002] In the manufacturing process of high-voltage, high-power frequency converters, full-load testing before shipment is a crucial step in verifying the product's output performance, electrical reliability, and thermal design rationality. Currently, the mainstream full-load testing method in the industry typically employs circulating current testing. The core advantage of this method is its ability to significantly reduce the grid capacity requirements of the testing system. For example, when testing a frequency converter with a rated capacity of 20MW, only approximately 500kVA of grid capacity is usually required to achieve full-load operation testing.

[0003] The basic control principle of conventional circulating current testing is as follows: by controlling the phase and amplitude of the output voltage of the inverter under test, the voltage vector across the output reactor is kept orthogonal to the grid voltage vector, thereby ensuring that the inverter's output current is in phase with the grid voltage. Ideally, the input current absorbed by the test system from the external grid should approach zero. For example... Figure 1 As shown, the voltage vector relationship in conventional control methods typically follows the formula:

[0004] in, The output voltage vector of the frequency converter under test; The grid voltage vector is the reference voltage; This is the voltage vector across the output reactor.

[0005] However, in practical industrial applications, the aforementioned conventional testing methods have significant limitations. High-voltage frequency converters typically use phase-shifting transformers as input-side devices. The inherent leakage flux characteristics of phase-shifting transformers generate specific inductive reactive currents during operation. The amplitude of this reactive current is generally about 10% of the frequency converter's rated output current. Conventional circulating current control strategies only adjust the frequency converter's output voltage to match the orthogonality between the output reactor and the grid voltage. While this can ideally eliminate the extraction of active current from the grid side, it cannot completely eliminate the inductive reactive current caused by the leakage flux of the phase-shifting transformer.

[0006] With the rapid advancement of modern major equipment construction projects, the capacity of single high-voltage frequency converters required for drive systems is constantly increasing, with single-unit power gradually breaking through from the conventional 20MW to 60MW or even higher levels. In this ultra-high power testing scenario, the negative effects caused by the aforementioned unavoidable reactive current problem become increasingly prominent. When the operating power required by the test equipment approaches the capacity limit of the plant's power grid, even slight changes in the current within the circulating current loop can cause significant grid voltage fluctuations and waveform distortions, easily leading to coupled oscillations between the frequency converter and the power grid, severely compromising the stability of the testing process. Simultaneously, the test sites of frequency converter manufacturers are typically "weak grid environments" where line impedance cannot be ignored and voltage amplitude and phase are easily affected by interference, making it difficult to provide the "rigid" ideal voltage reference required for conventional circulating current testing.

[0007] Furthermore, under long-term full-load testing conditions, the starting, stopping, and fluctuation of other grid-connected loads on the same distribution bus often cause dynamic voltage deviations in the grid, requiring the testing system to have the ability to dynamically output reactive current for real-time compensation. However, conventional circulating current testing methods cannot achieve dynamic reactive power compensation closed-loop and effective control of the grid-side power factor. In addition, some users' testing conditions require the frequency converter to simultaneously achieve true full-load active power output at rated current and rated voltage, further exposing the shortcomings of traditional methods, such as high grid capacity requirements and poor disturbance rejection capabilities. Therefore, there is an urgent need in this field for a technical solution that can effectively suppress internal reactive power disturbances and avoid coupled oscillations in weak grid environments, and support the stable long-term full-load testing of ultra-high power frequency converters with minimal grid capacity. Summary of the Invention

[0008] The present invention aims to solve at least one of the aforementioned technical problems existing in the prior art.

[0009] Therefore, the first aspect of the present invention provides a method for full-load testing of a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition.

[0010] The second aspect of this invention provides a full-load test system for a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition.

[0011] This invention provides a method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition, including: Construct a full-load circulating current test circuit for the frequency converter, set a first sampling point between the output terminal of the frequency converter under test and the output reactor, and set a second sampling point at the power input terminal of the test system connected to the external power grid. Based on the circuit superposition theorem, an equivalent control model is established, and the frequency converter under test is equivalently decomposed into a static var generator for reactive power compensation and a grid-connected inverter for outputting active power. The leakage flux characteristics of the phase-shifting transformer inside the frequency converter under test are equivalent to inductive electrical appliances. The control objective of the equivalent control model is set such that the output active current component of the grid-connected inverter and the output reactive current component of the static var generator are orthogonal in phase, and the output active current component is controlled to be in phase with the grid voltage through a phase-locked loop. Acquire the system input current signal collected at the second sampling point, and extract the system input reactive current feedback value and the system input active current feedback value respectively; The inverter output current signal collected at the first sampling point is acquired and orthogonally decomposed to synchronously observe the active and reactive components of the actual output of the inverter, ensuring the observability of the system. The reactive current feedback value input to the system is compared with the preset reactive current reference value to obtain a reactive error signal. After adjustment and calculation, a reactive voltage reference value is output to control the static var generator to output a reactive current of corresponding amplitude, compensate for the inductive reactive power generated by the inductive electrical appliance, and make the power factor on the input side of the test system track the preset target value. The active current feedback value input to the system is compared with the active current reference value set according to the full load test requirements to obtain the active error signal, and the active voltage reference value is output after adjustment and calculation. The reactive voltage reference value and the active voltage reference value are inversely transformed to generate a three-phase voltage reference signal, which is then pulse-width modulated. The output drive signal is sent to the inverter under test to achieve dual closed-loop control of the power factor and current on the input side of the test system.

[0012] The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition according to the above-mentioned technical solution of the present invention may also have the following additional technical features: In the above technical solution, the method for extracting feedback values ​​based on the system input current signal acquired at the second sampling point includes: Using the grid voltage phase extracted by the phase-locked loop as a rotation reference, the system input current signal is projected onto a synchronous rotating coordinate system. The d-axis component that is in phase with the grid voltage is extracted as the system input active current feedback value, and the q-axis component that is orthogonal to the grid voltage is extracted as the system input reactive current feedback value. The methods for extracting observation values ​​based on the inverter output current signal acquired at the first sampling point include: The inverter output current signal is mapped onto the synchronous rotating coordinate system, and the d-axis component is extracted as the output active current observation value, and the q-axis component is extracted as the output reactive current observation value. During the full-load test, the system input current signal at the second sampling point is monitored in real time and its q-axis component is calculated. The output voltage vector of the inverter under test is finely adjusted to make the q-axis component approach zero in order to compensate for the reactive power changes caused by environmental fluctuations during long-term testing.

[0013] In the above technical solution, in the reactive power closed-loop control, the preset reactive current reference value is set to 0, and the calculation formula for the reactive power error signal is:

[0014] in, This indicates a reactive power error signal; This indicates the reference value for reactive current. This indicates the system input reactive current feedback value.

[0015] In the above technical solution, in the active power closed-loop control, the active power current reference value is set as the equivalent active power current of the load, and the calculation formula for the active power error signal is:

[0016] in, This indicates the active power error signal; This indicates the reference value for active current; This indicates the system output reactive current feedback value.

[0017] In the above technical solution, the generation process of the reactive voltage reference value and the active voltage reference value adopts proportional-integral (PI) regulation calculation. By dynamically adjusting the dq axis component of the output voltage of the inverter under test, the coupling relationship between the output amplitude of the inverter and the power factor on the system input side is decoupled.

[0018] In the above technical solution, the phase-locked loop is used to extract the phase and frequency of the grid voltage in real time, and use it as the synchronization reference for the inverse coordinate transformation and pulse width modulation, so as to ensure that the active component and reactive component of the output current of the inverter under test are phase aligned in the synchronous rotating coordinate system.

[0019] In the above technical solution, the frequency obtained by the phase-locked loop is used as a reference to adjust the output frequency of the inverter under test, so that the test system keeps the inverter output current frequency synchronized with the grid voltage frequency in a weak power grid environment.

[0020] This invention provides a full-load testing system for a dual-sampling closed-loop frequency converter in a weak-voltage power grid based on SVG reactive power compensation and orthogonal decomposition, characterized in that it is applied to the method described in any one of the above technical solutions, and the system comprises: The inverter under test has its output terminal connected to the circulating current test circuit through an output reactor, and its input terminal connected to the power grid interface through a phase-shifting transformer. The dual sampling device includes a first sampling point located at the output terminal of the inverter under test and a second sampling point located at the system power input terminal; The control unit is used to receive signals from the first sampling point and the second sampling point and execute the dual closed-loop control logic to output drive signals to the inverter under test.

[0021] In the above technical solution, the power grid connection interface is equipped with a voltage transformer for collecting power grid voltage signals, which is used to provide a synchronization reference signal for the phase-locked loop in the control unit; the output reactor is a step-up transformer, which is used to stabilize the output current of the frequency converter.

[0022] In the above technical solution, the control unit is provided with: The observer module is used to extract the phase and frequency of the grid voltage in real time through a built-in phase-locked loop to provide a synchronization reference signal; receive the current signal collected by the first sampling point and perform orthogonal decomposition to extract the output active and reactive components; and receive the current signal collected by the second sampling point and perform projection calculation to extract the system input active current feedback value and the system input reactive current feedback value, so as to realize the synchronous observation of the operating status of the equivalent reconstructed grid-connected inverter module and static var generator module.

[0023] In summary, due to the adoption of the above-mentioned technical features, the beneficial effects of the present invention are: This invention innovatively establishes an equivalent control model based on the circuit superposition theorem, virtually reconstructing the inverter under test into a coordinated combination of a static var generator (SVM) and a grid-connected inverter. This design cleverly utilizes the inverter's own output capacity to dynamically compensate for the inductive reactive power generated by the leakage flux of its internal phase-shifting transformer, achieving self-generation and self-consumption of reactive power within the test system. This mechanism completely breaks through the physical bottleneck of inevitable reactive power extraction from the grid side in traditional circulating current testing, reducing the demand on external grid capacity for full-load testing to an extremely low level, and greatly alleviating the grid power supply pressure faced by ultra-high-power inverters with capacities of 20 MW or even larger during factory testing.

[0024] This invention constructs a unique dual-sampling, dual-closed-loop feedback architecture. By setting sampling points at the power input terminal of the test system connected to the grid, and using an orthogonal decomposition algorithm to accurately extract the grid-side reactive current feedback value for closed-loop adjustment, the system can accurately track the preset power factor target value. Combined with synchronous observation at the inverter output terminal, this method completely decouples the strong relationship between the inverter output voltage amplitude and the grid-side power factor at the control algorithm level. This means that no matter how drastically the output voltage or active current conditions are adjusted during the full-load circulating current test, the control system can ensure that the input-side power factor remains stable at the optimal state, fundamentally eliminating reactive power pollution caused by the test equipment to the plant's power distribution network.

[0025] The technical solution of this invention significantly improves the anti-disturbance capability and long-term operational stability of the testing system in weak power grid environments. During full-load testing lasting tens of hours, the control unit can calculate the orthogonal component of the total input current on the grid side in real time and dynamically offset reactive power fluctuations caused by the start-up and shutdown of other electrical loads on the same bus by fine-tuning the inverter's output voltage vector. This active suppression and dynamic compensation strategy effectively mitigates the coupling oscillation risks between the high-voltage, high-power inverter and the non-rigid power grid caused by line impedance and voltage deviation, ensuring that the inverter can still smoothly and safely complete the full-load performance verification process even under weak power grid conditions with high grid voltage distortion rates or harsh external conditions.

[0026] The control method of this invention not only considers the adaptability to the external power grid but also greatly improves the accuracy and hardware security of the testing system. Through parallel processing and coordinate transformation of dual-sampled data, the system achieves macroscopic control of the grid-side power of the entire unit while ensuring the complete observability of the virtually reconstructed active power output components and reactive power compensation components. This not only provides a precise phase-locked synchronization reference for pulse width modulation but also allows testers and underlying protection logic to monitor the real current stress of the power devices inside the inverter in real time. This provides robust data support and technical assurance for the inverter's ultimate load capacity calibration, thermal design verification, and high-reliability factory release.

[0027] Additional aspects and advantages of the invention will become apparent in the following description or may be learned by practice of the invention. Attached Figure Description

[0028] The above and / or additional aspects and advantages of the present invention will become apparent and readily understood from the description of the embodiments taken in conjunction with the following drawings, in which: Figure 1 This is a vector diagram of a conventional circulating current control voltage; Figure 2 This is a flowchart illustrating a method for full-load testing of a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition, according to an embodiment of the present invention. Figure 3 This is an electrical connection architecture diagram of a dual-sampling closed-loop frequency converter weak grid full-load test system according to an embodiment of the present invention; Figure 4 This is an equivalent model diagram of a dual-sampling closed-loop frequency converter weak grid full-load test system according to an embodiment of the present invention; Figure 5 This is a schematic diagram of the control execution of a dual-sampling closed-loop frequency converter full-load test method for weak grid based on SVG reactive power compensation and orthogonal decomposition according to an embodiment of the present invention; Figure 6 This is a diagram showing the directional relationship between voltage vectors and current vectors in the control loop of one embodiment of the present invention; Figure 7 This is an example system architecture diagram of a dual-sampling closed-loop frequency converter full-load test system according to an embodiment of the present invention. Detailed Implementation

[0029] To better understand the above-mentioned objectives, features, and advantages of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings and specific embodiments. It should be noted that, unless otherwise specified, the embodiments and features described in these embodiments can be combined with each other.

[0030] Many specific details are set forth in the following description in order to provide a full understanding of the invention. However, the invention may also be practiced in other ways different from those described herein, and therefore the scope of protection of the invention is not limited to the specific embodiments disclosed below.

[0031] The following reference Figures 2 to 7 This invention describes a method and system for full-load testing of a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition, according to some embodiments of the present invention.

[0032] Some embodiments of this application provide a method for full-load testing of a dual-sampling closed-loop frequency converter in a weak power grid based on SVG reactive power compensation and orthogonal decomposition.

[0033] like Figure 2 As shown, the first embodiment of the present invention proposes a method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition, including the following steps S1 to S8. It should be noted that the order of the above steps S1-S8 is only an illustrative representation of the present disclosure. Unless otherwise specified, the order between different steps can be adjusted as needed, and different steps can also be executed simultaneously.

[0034] S1. Construct a full-load circulating current test circuit for the frequency converter. Set a first sampling point between the output terminal of the frequency converter under test and the output reactor, and set a second sampling point at the power input terminal of the test system connected to the external power grid.

[0035] Combination Figure 3 The electrical connection architecture diagram of the dual-sampling closed-loop inverter weak current full-load test system shown can intuitively illustrate the physical topology of the full-load circulating current test circuit. When constructing this test circuit, the inverter under test (VFD) is used as the core test object. Its output terminal is connected in series with the output reactor L1, which is then connected in parallel to the circulating current circuit. Its input terminal is connected to the power grid through necessary electrical switches.

[0036] In one specific embodiment, to meet the requirements of safety control and step-by-step start-up and shutdown during the testing process, the circuit is configured with multi-level switching devices, specifically including an input-side circuit breaker QF1 located on the grid side to control the connection and disconnection between the main circuit of the testing system and the grid; an output-side contactor K1 located between the output terminal of the frequency converter and the input terminal of the output reactor to control the frequency converter to enter the no-load or load-bearing operation state; and a grid-connected contactor KM1 located on the grid feedback side, which can realize the step-by-step grid connection operation from the output reactor to the grid feedback circuit by controlling the opening and closing of KM1.

[0037] Based on this hardware topology, this invention breaks through the conventional design of single sampling points and introduces a dual sampling mechanism. The first sampling point is... Figure 3 The sample point marked Sample1 is physically installed close to the output side of the inverter under test, located between the inverter and the output reactor. This sampling point is not directly affected by the grid-side feedback loop, and the inverter output current signal it collects accurately reflects the current stress that the inverter body is experiencing. This provides basic data for subsequent steps to synchronously observe the operating status of the inverter's internal virtual modules and ensure the physical safety of the equipment. The second sampling point is... Figure 3 Sample2, as marked, is physically installed at the main power input terminal connecting the entire test system to the external plant power grid. Since the high-voltage frequency converter system contains components prone to leakage flux, such as phase-shifting transformers, the second sampling point can macroscopically and in real-time acquire the total input current signal requested by the entire test system from the grid. This signal directly reflects the actual reactive power consumption of the test system in a weak grid environment and serves as the reference signal source for subsequent closed-loop feedback regulation of the entire system's power factor. In specific engineering implementation, to ensure the accuracy and response speed of the dual-loop regulation in a weak grid environment, both the first and second sampling points use high-precision Hall current sensors, with a sampling accuracy requirement of no less than ±0.5%. This ensures that the acquired dual-channel current vector data has high fidelity, laying a solid hardware measurement foundation for stable full-load operation for more than 72 hours.

[0038] S2. Based on the circuit superposition theorem, an equivalent control model is established, and the inverter under test is equivalently decomposed into a static var generator for reactive power compensation and a grid-connected inverter for outputting active power. The leakage flux characteristics of the phase-shifting transformer inside the inverter under test are equivalent to inductive electrical appliances.

[0039] Reference Figure 4 The diagram shown is an equivalent model of a dual-sampling closed-loop inverter under full load testing in a weak power grid environment. This step is fundamental to the entire control algorithm logic. Because ultra-high power high-voltage inverter systems physically exhibit as highly coupled and complex wholes, direct single-vector closed-loop control is insufficient to simultaneously ensure output accuracy and grid-side power factor in a weak power grid environment. Therefore, this invention applies the circuit superposition theorem at the control algorithm level, treating the inverter under test as a linear superposition of multiple functional sources in terms of its electrical characteristics. For example... Figure 4 As shown in the equivalent logic, the system decouples the functions of the inverter under test: one part of the function is abstracted as a static var generator (SVG), which does not generate active power exchange and is specifically responsible for dynamically generating reactive current in the four quadrants; the other part of the function is abstracted as a grid-connected inverter, which focuses on outputting stable active power to the circuit according to the test requirements.

[0040] Meanwhile, the core innovation of this step lies in the explicit modeling of internal disturbance terms. Because the phase-shifting transformer on the input side of the high-voltage frequency converter generates non-negligible inductive reactive power losses during full-load operation, these losses are often considered as systematic errors in traditional models and are difficult to eliminate accurately. In the equivalent model established in this invention, the leakage flux characteristics of the phase-shifting transformer inside the frequency converter under test are independently and equivalently represented as an inductive electrical appliance connected in parallel in the test circuit. This equivalent treatment transforms the originally elusive transformer losses into a clear "compensation term" in the model.

[0041] At this point, the first sampling point Sample1 is abstracted and differentiated into SampleA and SampleB. The current at SampleA is the reactive current component, while the current at SampleB is the active current component.

[0042] Through this modular logical decomposition, the originally complex inverter test control problem is transformed into three relatively independent sub-problems: how to control the grid-connected inverter to achieve the load power required for the test, how to use the reactive power generated by the SVG to offset the reactive power gap caused by inductive appliances (i.e., leakage flux of the phase-shifting transformer) in real time, and how to ensure that these two are orthogonally non-interfering during physical execution. This equivalent model based on the superposition theorem provides a clear control object and theoretical criterion for performing orthogonal decomposition and dual closed-loop current regulation in the synchronous rotating coordinate system in subsequent steps, thereby ensuring that even under weak grid conditions with limited grid capacity, the test system can still maintain extremely high operational stability through its internal "self-compensation" mechanism.

[0043] S3. Set the control objective of the equivalent control model so that the output active current component of the grid-connected inverter and the output reactive current component of the static var generator are orthogonal in phase, and control the output active current component to be in phase with the grid voltage through a phase-locked loop.

[0044] Combination Figure 5 The diagram shows the control execution of a dual-sampling closed-loop frequency converter for full-load testing in weak grid environments based on SVG reactive power compensation and orthogonal decomposition. Figure 6 The diagram showing the directional relationship between voltage and current vectors in the control loop clearly illustrates the underlying vector logic of this step. In traditional circulating current testing, the amplitude of the inverter's output active current and the reactive power factor on the system input side are usually strongly coupled. Adjusting one often causes drastic fluctuations in the other, which can easily lead to system instability or even coupled oscillations in weak power grid environments such as factory areas where interference is likely. To fundamentally eliminate this coupling, this invention introduces a synchronous rotating coordinate system (dq coordinate system) as the underlying adjustment reference in the control algorithm, transforming the AC control problem into a DC orthogonal control problem.

[0045] Specifically, the observer module within the control unit plays a crucial role. Its built-in high-precision phase-locked loop (PLL) reads the grid voltage signal collected by the voltage transformer at the grid connection interface in real time, dynamically extracting the pure grid voltage phase and frequency information. This extracted grid voltage phase is strictly set by the system to the d-axis direction of the synchronous rotating coordinate system, serving as the absolute phase reference for the entire control system. Subsequently, the system assigns distinct and independent vector control objectives to the equivalently reconstructed grid-connected inverter module and static var generator module. For the grid-connected inverter module, which undertakes the full-load power assessment task, the system strictly anchors its output current component to the d-axis, corresponding to... Figure 6 The active current component I marked in the middle sample1B (Right now This ensures that it always maintains absolute synchronization with the grid voltage vector, while its amplitude is left to be dynamically determined in subsequent steps based on the specific test load requirements.

[0046] Meanwhile, for the static var generator module designed to counteract the leakage flux characteristics of phase-shifting transformers, the system strictly anchors its output current component to the q-axis, which is perpendicular to the d-axis. Figure 6 The reactive current component I marked in the middle sample1A (Right now This involves forcing the phase of the reactive power compensation loop to maintain a 90° orthogonal relationship with the grid voltage vector, either leading or lagging behind. Through this control strategy of mapping complex AC physical quantities to an orthogonal coordinate system and forcibly calibrating the phases separately, the test system ensures that active power flow and reactive power compensation do not interfere with each other at both the mathematical theory and physical execution levels. This means that during a demanding full-load test lasting over 72 hours, regardless of how much the system significantly increases the output amplitude of the active current to simulate any extreme rated load conditions, it will not cause any interference to the phase of the reactive power compensation loop. This establishes the most critical directional coordinate criteria and synchronization time reference for the independent calculation and precise issuance of dual closed-loop regulation commands in subsequent steps.

[0047] S4. Obtain the system input current signal collected at the second sampling point, and extract the system input reactive current feedback value and the system input active current feedback value respectively.

[0048] S5. Acquire the inverter output current signal collected at the first sampling point and perform orthogonal decomposition to synchronously observe the active and reactive components of the actual output of the inverter, ensuring the observability of the system.

[0049] After establishing the phase reference for the control target, this embodiment of the invention proceeds to the synchronous processing stage of the dual-sampling-point signals, namely, executing steps S4 and S5. These two steps together constitute the basis for data acquisition and state observation of the dual-closed-loop control system.

[0050] Combination Figure 4 The equivalent model control loop shown is as follows: Figure 5 The control execution diagram shown illustrates that in step S4, the control unit acquires the system input current signal collected at the second sampling point located at the power input terminal. In some embodiments, to convert this AC physical signal into a feedback signal usable for DC closed-loop regulation, the vector decomposition module inside the control unit uses the grid voltage phase extracted by the phase-locked loop in step S3 as a rotation reference to project the system input current signal onto a synchronous rotating coordinate system (dq coordinate system). Specifically, the d-axis component in phase with the grid voltage is extracted as the system input active current feedback value. The q-axis component orthogonal to the grid voltage is extracted as the system input reactive current feedback value. (i.e. I sample2A The physical significance of this step is that the signal obtained at the second sampling point includes all grid-side losses, such as inductive reactive power generated by the leakage flux of the phase-shifting transformer. Through precise coordinate projection extraction, the system can quantify in real time the actual active power and reactive power gap requested by the entire test loop from the grid in a weak grid environment, thereby providing an accurate source of error calculation for subsequent outer loop feedback regulation.

[0051] In parallel with step S4, the control unit executes step S5, acquiring the inverter output current signal collected at the first sampling point set at the output terminal of the inverter under test, and performing orthogonal decomposition on it. Similarly, using... Figure 5 The observer module shown maps the inverter's single output current vector to the same synchronous rotating coordinate system and extracts the d-axis component as the output active current observation value. Extract the q-axis component as the output reactive current observation value. Since the feedback value at the second sampling point can only reflect the macroscopic power factor on the grid side and cannot reflect the current stress that the inverter itself is experiencing, the execution of step S5 is crucial. Through orthogonal decomposition of the signal at the first sampling point, the system not only achieves equivalent synchronous observation of the operating status of the internally virtual reconstructed grid-connected inverter module and static var generator module, establishing the complete observability of the system, but also allows the underlying protection logic to monitor the actual output load of the internal power devices in real time, ensuring that active power output and reactive power compensation are truly decoupled at the physical level during long-term full-load testing.

[0052] like Figure 5 As shown, in the specific signal acquisition process, this embodiment uses a two-phase current sampling architecture for both the first and second sampling points. Since the inverter under test and the full-load circulating current test circuit adopt a three-phase three-wire structure, according to Kirchhoff's current law, the system satisfies the characteristic that the algebraic sum of the instantaneous values ​​of the three-phase currents is zero. Therefore, only two current sensors are needed at the first sampling point to acquire the A-phase current output by the inverter. and C-phase current The second sampling point only needs to collect the A-phase current input by the system. and C-phase current After receiving the aforementioned two-phase current signals, the observer module within the control unit reconstructs the missing B-phase current signal in real time at the algorithm's underlying layer. This two-phase sampling method effectively reduces the number of high-cost, high-power current sensors used, improving the economy and computational efficiency of the hardware system, while ensuring complete observability of the system current state and without affecting the accuracy of orthogonal decomposition and dual closed-loop control.

[0053] S6. The reactive current feedback value input to the system is compared with the preset reactive current reference value to obtain a reactive error signal. After adjustment and calculation, a reactive voltage reference value is output to control the static var generator to output a reactive current of corresponding amplitude, to compensate for the inductive reactive power generated by the inductive electrical appliance, so that the power factor on the input side of the test system tracks the preset target value.

[0054] S7. The active current feedback value input to the system is compared with the active current reference value set according to the full load test requirements to obtain the active error signal, and the active voltage reference value is output after adjustment and calculation.

[0055] After acquiring and separating the precise feedback signal and the observation signal, the embodiment of the present invention then enters the core dual closed-loop feedback regulation stage, that is, steps S6 and S7 are executed in parallel to achieve complete decoupling control of active power and reactive power.

[0056] Combination Figure 5 The diagram shown illustrates the control execution of a dual-sampling closed-loop frequency converter for full-load testing in a weak power grid based on SVG reactive power compensation and orthogonal decomposition. The system first executes step S6, reactive power closed-loop control. In the static var generator control loop (i.e.,...) Figure 5 In the SVG Loop (as indicated in the diagram), the primary goal of the system is to eliminate reactive power disturbances in weak grid environments and achieve unity power factor grid connection. To this end, the control unit will use a preset reactive current reference value... Setting it to 0 indicates that the reactive current the test system is expected to draw from the external power grid is zero. Subsequently, the system will use the system input reactive current feedback value extracted in step S4. The reactive power error signal is calculated by subtracting it from the reference value. The calculation formula is as follows:

[0057] in, This indicates a reactive power error signal; This indicates the reference value for reactive current. This indicates the system input reactive current feedback value.

[0058] The reactive power error signal The voltage is then fed into the built-in SVG current regulator (which uses a PI proportional-integral regulator, such as an SVG Current Regulator). After precise proportional-integral calculations, it outputs a reactive voltage reference value for controlling the reactive component of the frequency converter. Through this outer loop feedback regulation, the system continuously corrects the voltage command using the error, forcibly driving the inverter under test to actively output reactive current of the corresponding amplitude. This self-generated reactive current can precisely offset the inductive reactive power gap generated by the inductive electrical appliances (i.e., the phase-shifting transformer) in the equivalent model, thereby enabling the total power factor on the input side of the test system to accurately track and stabilize at the preset target value.

[0059] Simultaneously, to meet the actual performance requirements of the frequency converter reaching its rated load, the system synchronously executes the active power closed-loop control in step S7. In the frequency converter control loop (i.e.... Figure 5 In the VFD Loop (marked in the diagram), the control unit sets an active current reference value based on pre-planned full-load test requirements. This reference value is equal to the load's equivalent active current. This represents the actual active power required for the inverter to maintain operation under full load conditions. Next, the control unit will use the system input active current feedback value extracted in step S4. With the set reference value By comparison, the active power error signal is calculated, and its mathematical expression is:

[0060] in, This indicates the active power error signal; This indicates the reference value for active current; This indicates the system output reactive current feedback value.

[0061] The active power error signal Similarly, the current is fed into an independent VFD current regulator (PI regulator, such as a VFD Loop Current Regulator), and after proportional-integral dynamic calculation, the output controls the active voltage reference value of the grid-connected inverter's output active component. .

[0062] By performing steps S6 and S7 independently on two orthogonal axes in a synchronous rotating coordinate system, this invention completely eliminates the strong physical coupling between the inverter output amplitude and the system input power factor in traditional testing methods from the underlying algorithm control level. This means that during subsequent rigorous long-term full-load tests, regardless of how the system is adjusted... Significantly altering the active power output conditions to test hardware limits will not affect or disrupt the reactive power balance on the grid side, thus providing dual protection for grid security and test stability in weak grid environments.

[0063] S8. Perform inverse coordinate transformation on the reactive voltage reference value and the active voltage reference value to generate a three-phase voltage reference signal and perform pulse width modulation. Output the drive signal to the inverter under test to realize dual closed-loop control of the power factor and current on the input side of the test system.

[0064] Combination Figure 5 The diagram shown illustrates the control execution of a dual-sampling closed-loop frequency converter for full-load testing in a weak-grid system based on SVG reactive power compensation and orthogonal decomposition. The control unit includes a voltage reconstruction and coordinate transformation module (i.e., the VR&2 / 3 module). This module simultaneously receives the reactive voltage reference value output in step S6. and the active voltage reference value output in step S7 Subsequently, the system uses the grid voltage phase extracted in real time by the phase-locked loop in the observer module. As a reverse rotation reference, the two DC quantities mentioned above undergo a coordinate inverse transformation from dq to abc. This mathematical process accurately restores the mutually orthogonal DC control commands in the synchronous rotating coordinate system to a three-phase AC voltage reference signal that is strictly synchronized with the grid voltage in the stationary coordinate system.

[0065] Understandably, the observer module receives the three-phase grid voltage signals (including phase A voltage) collected in real time by the grid-side voltage transformers. Phase B voltage C-phase voltage It processes the input three-phase AC voltage signal and extracts the phase of the grid voltage in real time. With angular frequency The extracted phase is used as a global rotation reference and provided to the vector decomposition module and the coordinate inverse transformation module to ensure the orientation accuracy of the dual closed-loop operation; while the extracted angular frequency is used as the system's time and frequency reference to ensure that the inverter under test always maintains strict frequency synchronization and phase alignment with the external power grid in a fluctuating weak power grid environment.

[0066] Next, the three-phase voltage reference signal is sent to the SPWM generation module of the control unit. Within this module, the three-phase voltage reference signal, acting as a modulation wave, is compared in real-time with the system's built-in high-frequency triangular carrier signal to generate PWM pulse drive signals for controlling the on / off state of power devices. These drive signals are directly sent to the core switching devices such as IGBTs inside the inverter under test, forcing the inverter bridge of the inverter under test to accurately generate waveforms according to the desired dual-closed-loop regulation.

[0067] Thus, the system has completed a full data closed loop, from front-end signal acquisition, decomposition observation, decoupling calculation to end-end pulse width modulation. Through this dual-sampling closed-loop control mechanism, the inverter under test can not only stably output full-load active power to meet extreme performance requirements, but also, under the command of the algorithm's underlying layer, function as a static var generator, dynamically and without delay outputting inductive reactive power compensation current to fill the reactive power gap caused by the phase-shifting transformer. This method completely suppresses power fluctuations between the test circuit and the external bus, enabling the entire ultra-high power test system to maintain a strictly 1 total power factor on the input side even with extremely low grid capacity support. This fundamentally overcomes the technical challenge of coupled oscillations easily caused by long-term full-load circulating current testing in weak grid environments.

[0068] In some embodiments, based on the closed-loop control logic constructed in steps S1 to S8 above, the full-load test method of this embodiment further includes a dynamic compensation stage for fluctuations in a weak power grid environment during actual long-term operation. Specifically, after setting the output current amplitude of the grid-connected inverter to the rated output current of the inverter under test (e.g., 1950A) and entering the full-load test condition, the system will run continuously for 72 hours or longer. During this long-term test, the start-up, shutdown, and fluctuations of other electrical loads on the same distribution bus will cause reactive power changes in the external power grid environment. The control unit monitors the system input current signal at the second sampling point in real time and calculates its q-axis component (i.e., the system input reactive current feedback value). Once the q-axis component is found to deviate from zero, the control unit will dynamically fine-tune the reactive voltage reference value output by the inverter under test according to the reactive power closed-loop adjustment logic described in step S6, thereby changing the reactive current amplitude output by the static var generator module. This real-time fine-tuning mechanism can immediately offset reactive power disturbances caused by environmental fluctuations, forcing the q-axis component of the total system input current to always approach zero, thereby continuously maintaining the power factor of the test system input side at 1 in extreme weak grid environments, effectively avoiding the coupling oscillation phenomenon between the frequency converter and the non-rigid grid that is easily caused in conventional tests.

[0069] In a specific test example, the method described in the embodiments of the present invention was used to conduct a full-load test on a high-voltage frequency converter with a rated capacity of 65MVA before its factory shipment.

[0070] Reference Figure 7The diagram shows an example system architecture of a dual-sampling closed-loop inverter full-load test system. This test system is built in a typical factory weak power grid environment. The power for the entire test system comes from the 6kV factory distribution bus, connected through the 6kV switchgear QF1 on the incoming side. To suppress the instantaneous impact when high-power equipment is powered on, a soft starter cabinet is configured in the system. After the system is pre-magnetized, a bypass contactor is used to achieve stable operation. Subsequently, the system steps up the voltage from 6kV to 35kV through the incoming transformer T1, and provides input power to the inverter under test via the 35kV incoming switchgear QF11. In this embodiment, the inverter under test (VFD) is a high-voltage high-power inverter with a rated capacity of 65MVA. Its internal core architecture includes a phase-shifting transformer T3 with a rated capacity of 75MVA and an inverter matrix composed of power units. The main side of the phase-shifting transformer T3 is connected to a 35kV power supply, and the voltage is reduced to 1.35kV through multi-winding phase-shifting technology to drive the subsequent power units.

[0071] At the inverter output, to simulate real motor load characteristics and construct a circulating current loop, a 65MVA load transformer T2 is connected in series as an output reactor. The primary voltage of the load transformer T2 is matched to the inverter's rated output of 18kV, while the secondary voltage is boosted to 35kV. Figure 3 The grid-connected contactor KM1 shown has its output terminal of the load transformer T2 fed back to the 35kV busbar side, thus forming an energy cycle. Under this physical architecture, the dual-sampling device described in this invention is deployed as follows: the first sampling point, Sample1, is deployed between the output terminal of the inverter power unit and the load transformer T2, directly collecting the three-phase output current of the inverter; the second sampling point, Sample2, is deployed on the incoming side of the 35kV main incoming switchgear QF11, used to collect the total system input current, including the losses of the phase-shifting transformer T3.

[0072] The control unit, acting as the nerve center of the entire system, is built on a hardware platform based on a high-performance digital signal processor (DSP) and a field-programmable gate array (FPGA). The control unit receives current signals from Sample1 and Sample2, as well as a synchronization signal from the 35kV bus voltage transformer, in real time through signal conditioning circuitry. Figure 7 The hardware connection shown demonstrates that the control unit internally decomposes the inverter body into an SVG compensation module and a grid-connected inverter module, and issues SPWM commands according to the control strategies described in steps S1 to S8. This hardware layout ensures that even in a weak grid environment with limited 6kV bus capacity and high line impedance, the system can still maintain good regularity of the grid-side current waveform at QF11 by internally offsetting the leakage flux reactive power of T3, and the total harmonic distortion rate meets national standard requirements.

[0073] Under the condition of a weak power grid with a capacity of only 2000kVA in the factory area, the input current of the test system was significantly reduced by more than 10% compared with the conventional circulating current test through the above-mentioned orthogonal decomposition and dual closed-loop control. During the continuous uninterrupted test lasting 72 hours, the inverter output current reached the real full-load condition of 1950A, the output voltage remained stable at 18kV±5%, and the output frequency was precisely locked at 50Hz. The system operated smoothly throughout the test, without any coupling oscillations or voltage distortion exceeding the standard, and the total harmonic distortion rate of the grid voltage was strictly controlled within 2%. In addition, when performing short-time overload tests, under extreme load conditions where the inverter under test had a maximum output current of 2245A and continued to operate for 150 seconds, the dual closed-loop system was still able to maintain the stability of the grid-side power factor and the safety monitoring of internal components, fully verifying the extremely high technical reliability and engineering practical value of this invention in the factory testing of ultra-high power equipment.

[0074] Other embodiments of the present invention provide a dual-sampling closed-loop frequency converter full-load test system for weak grid based on SVG reactive power compensation and orthogonal decomposition, which is applied to the method described in any of the above embodiments.

[0075] based on Figure 3 , Figure 6 and Figure 7 The system architecture shown in this embodiment of the invention, which provides a dual-sampling closed-loop inverter full-load test system for weak grids, mainly consists of five core parts: the inverter under test (DUT), the output reactor, the dual-sampling device, the control unit, and the grid connection interface. The DUT, as the core test object, is physically connected in a typical configuration where the input end is connected to the grid and the output end is connected to the test circuit. In a specific example, the DUT is selected as a high-voltage, high-power model with a rated capacity of 65MVA. It integrates a 75MVA phase-shifting transformer, which not only handles electrical isolation and multi-pulse rectification, but its inherent leakage flux characteristics also serve as the physical source of the inductive electrical components that need to be compensated for in the equivalent control model.

[0076] The system's hardware topology interacts with the plant's power distribution network via a power grid connection interface. (Refer to...) Figure 7 The interface consists of a 6kV switchgear, a soft starter, an incoming transformer, and a 35kV high-voltage switchgear. The incoming transformer steps up the 6kV plant voltage to 35kV, providing the necessary voltage level for high-power testing. To achieve energy recycling and simulate a real load, the output reactor is implemented as a 65MVA step-up load transformer. The 18kV output voltage from the frequency converter is stepped up by this load transformer and then fed back to the 35kV bus via the grid-connected contactor KM1. This allows for maintaining a full-load circulating current of up to 65MVA with minimal external power input even in weak grid environments.

[0077] The dual sampling device is the fundamental sensing basis for achieving accurate closed-loop control in this system. The system constructs a comprehensive information feedback chain through two sampling points in distinct physical locations. The first sampling point (Sample1) is deployed on the physical connection line between the inverter under test and the load transformer, used to directly acquire the three-phase output current signal of the inverter's power unit. The second sampling point (Sample2) is deployed at the 35kV incoming line connecting the system to the external power grid, responsible for monitoring the total input current signal, including transformer losses. Both sampling points utilize high-precision Hall current sensors with a sampling accuracy of ±0.5%, ensuring signal fidelity even in complex electromagnetic interference environments.

[0078] The control unit integrates multiple logic function modules through a high-performance computing platform, realizing a complete chain from signal sensing to power drive. The observer module is the core, responsible not only for receiving the current signal from two sampling points but also for extracting the phase and frequency of the grid voltage in real time through a built-in phase-locked loop (PLL) module. Within this architecture, two independent PI regulators are deployed within the control unit, forming the SVG reactive power control loop and the VFD active power control loop, respectively. Based on the feedback signal after orthogonal decomposition, these two regulators calculate the reactive voltage reference value for compensating for leakage flux and the active voltage reference value for meeting load requirements, respectively.

[0079] Finally, the control unit uses the voltage reconstruction and coordinate transformation module (i.e., the VR&2 / 3 module) to convert the aforementioned orthogonal DC reference command into a three-phase AC signal, and the SPWM generation module outputs high-frequency drive pulses to the power unit of the inverter. This highly coordinated hardware and software system architecture enables the inverter under test to transcend the functional scope of a single inverter and possess dynamic compensation capabilities similar to SVG in weak grid environments. By fine-tuning the output voltage vector in real time, the system can not only maintain ultra-long-term full-load operation stability for more than 72 hours, but also ensure that the voltage distortion rate on the grid incoming side remains at an extremely low level of less than 2%, thereby guaranteeing the test safety and data accuracy in weak grid environments with frequent fluctuations in bus load.

[0080] In this specification, the illustrative expressions of the terms used do not necessarily refer to the same embodiments or examples. Moreover, the specific features, structures, materials, or characteristics described may be combined in any suitable manner in one or more embodiments or examples.

[0081] Any modifications, equivalent substitutions, or improvements made within the spirit and principles of this invention shall be included within the scope of protection of this invention.

Claims

1. A method for full-load testing of a dual-sampling closed-loop frequency converter in a weak-grid system based on SVG reactive power compensation and orthogonal decomposition, characterized in that, include: Construct a full-load circulating current test circuit for the frequency converter, set a first sampling point between the output terminal of the frequency converter under test and the output reactor, and set a second sampling point at the power input terminal of the test system connected to the external power grid. Based on the circuit superposition theorem, an equivalent control model is established, and the frequency converter under test is equivalently decomposed into a static var generator for reactive power compensation and a grid-connected inverter for outputting active power. The leakage flux characteristics of the phase-shifting transformer inside the frequency converter under test are equivalent to inductive electrical appliances. The control objective of the equivalent control model is set such that the output active current component of the grid-connected inverter and the output reactive current component of the static var generator are orthogonal in phase, and the output active current component is controlled to be in phase with the grid voltage through a phase-locked loop. Acquire the system input current signal collected at the second sampling point, and extract the system input reactive current feedback value and the system input active current feedback value respectively; The inverter output current signal collected at the first sampling point is acquired and orthogonally decomposed to synchronously observe the active and reactive components of the actual output of the inverter, ensuring the observability of the system. The reactive current feedback value input to the system is compared with the preset reactive current reference value to obtain a reactive error signal. After adjustment and calculation, a reactive voltage reference value is output to control the static var generator to output a reactive current of corresponding amplitude, compensate for the inductive reactive power generated by the inductive electrical appliance, and make the power factor on the input side of the test system track the preset target value. The active current feedback value input to the system is compared with the active current reference value set according to the full load test requirements to obtain the active error signal, and the active voltage reference value is output after adjustment and calculation. The reactive voltage reference value and the active voltage reference value are inversely transformed to generate a three-phase voltage reference signal, which is then pulse-width modulated. The output drive signal is sent to the inverter under test to achieve dual closed-loop control of the power factor and current on the input side of the test system.

2. The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 1, characterized in that, The methods for extracting feedback values ​​based on the system input current signal acquired at the second sampling point include: Using the grid voltage phase extracted by the phase-locked loop as a rotation reference, the system input current signal is projected onto a synchronous rotating coordinate system. The d-axis component that is in phase with the grid voltage is extracted as the system input active current feedback value, and the q-axis component that is orthogonal to the grid voltage is extracted as the system input reactive current feedback value. The methods for extracting observation values ​​based on the inverter output current signal acquired at the first sampling point include: The inverter output current signal is mapped onto the synchronous rotating coordinate system, and the d-axis component is extracted as the output active current observation value, and the q-axis component is extracted as the output reactive current observation value. During the full-load test, the system input current signal at the second sampling point is monitored in real time and its q-axis component is calculated. The output voltage vector of the inverter under test is finely adjusted to make the q-axis component approach zero in order to compensate for the reactive power changes caused by environmental fluctuations during long-term testing.

3. The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 2, is characterized in that... In reactive power closed-loop control, the preset reactive current reference value is set to 0, and the formula for calculating the reactive power error signal is: in, This indicates a reactive power error signal; This indicates the reference value for reactive current. This indicates the system input reactive current feedback value.

4. The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 2, is characterized in that... In active power closed-loop control, the active current reference value is set as the equivalent active current of the load, and the calculation formula for the active power error signal is: in, This indicates the active power error signal; This indicates the reference value for active current; This represents the system input active current feedback value.

5. The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 2, characterized in that, The generation process of the reactive voltage reference value and the active voltage reference value adopts proportional-integral (PI) regulation calculation. By dynamically adjusting the dq axis component of the output voltage of the inverter under test, the coupling relationship between the inverter output amplitude and the power factor on the system input side is decoupled.

6. The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 1, characterized in that, The phase-locked loop is used to extract the grid voltage phase and frequency in real time, and uses them as the synchronization reference for the inverse coordinate transformation and pulse width modulation, so as to ensure that the active and reactive components of the output current of the inverter under test are phase aligned in the synchronous rotating coordinate system.

7. The method for full-load testing of a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 1, characterized in that, Using the frequency obtained by the phase-locked loop as a reference, the output frequency of the inverter under test is adjusted so that the test system keeps the inverter output current frequency synchronized with the grid voltage frequency in a weak power grid environment.

8. A full-load test system for a weak grid using a dual-sampling closed-loop frequency converter based on SVG reactive power compensation and orthogonal decomposition, characterized in that, The system, applied to the method of any one of claims 1-7, comprises: The inverter under test has its output terminal connected to the circulating current test circuit through an output reactor, and its input terminal connected to the power grid interface through a phase-shifting transformer. The dual sampling device includes a first sampling point located at the output terminal of the inverter under test and a second sampling point located at the system power input terminal; The control unit is used to receive signals from the first sampling point and the second sampling point and execute dual closed-loop control logic to output drive signals to the inverter under test.

9. The full-load test system for a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 8, characterized in that, The power grid connection interface is equipped with a voltage transformer for collecting power grid voltage signals, which is used to provide a synchronization reference signal for the phase-locked loop in the control unit; the output reactor is a step-up transformer, which is used to stabilize the output current of the frequency converter.

10. The full-load test system for a dual-sampling closed-loop frequency converter in a weak grid based on SVG reactive power compensation and orthogonal decomposition according to claim 8, characterized in that, The control unit is equipped with: The observer module is used to extract the phase and frequency of the grid voltage in real time through the built-in phase-locked loop to provide a synchronization reference signal; it receives the current signal collected by the first sampling point and performs orthogonal decomposition to extract the output active component and the output reactive component; It also receives the current signal collected by the second sampling point and performs projection calculation to extract the system input active current feedback value and the system input reactive current feedback value, so as to realize the synchronous observation of the operating status of the equivalent reconstructed grid-connected inverter module and static var generator module.

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