Grid reactive power compensation method and device based on single-stage ac-ac converter

Abstract

The application provides a grid reactive power compensation method and device based on a single-stage AC-AC converter. The input end is connected in parallel with an AC grid, and the output end is connected in parallel with a fixed-capacity compensation capacitor. The method comprises the following steps: controlling the high-frequency on-off of each switch tube in the converter to PWM modulate the output end voltage with a duty cycle D, so that the converter generates a voltage gain A that continuously changes with the duty cycle D for the compensation capacitor v , and the capacitive current flowing through the compensation capacitor is mapped to the grid side through the converter; and the duty cycle D is adjusted to continuously change the equivalent capacity of the capacitive adjustable virtual capacitor equivalently presented on the grid side, so as to continuously adjust the reactive power injected into the grid. The application utilizes the nonlinear voltage gain characteristic of the single-stage AC-AC converter to map the fixed capacitor on the output end to the continuously adjustable virtual capacitor on the grid side, realizes the continuous stepless adjustment of the reactive power, and eliminates the compensation blind area and inrush current impact of the traditional switched capacitor.

Description

Technical Field

[0001] This invention relates to the field of reactive power compensation technology for power grids, specifically to a method and apparatus for reactive power compensation for power grids based on a single-stage AC-AC converter. Background Technology

[0002] During the operation of AC power grids, inductive loads such as motors, transformers, and electric arc furnaces absorb a large amount of reactive power, leading to a decrease in the power factor, an increase in line losses, and a decline in voltage quality. Reactive power compensation, as a key technology for improving power quality, increasing the power factor, and reducing line losses, plays an important role in the operation and control of power systems. With the continuous development of power electronics technology, industrial sites are placing increasingly higher demands on the compensation accuracy, response speed, size, weight, and operational reliability of reactive power compensation devices.

[0003] Currently, common reactive power compensation technologies mainly fall into two categories: The first category is based on grouped capacitor banks using mechanical or solid-state switches. This type of device connects multiple fixed-capacity physical film capacitors in parallel to the AC grid via mechanical contactors or solid-state switches. The system adjusts the number of capacitors connected to the grid in a stepped manner by opening or closing the switches according to changes in the grid's power factor, thereby providing reactive power to the grid. The second category is static var generators based on voltage source converters. This technology uses fully controlled power switches to construct a bridge converter circuit and connects a large-capacity DC energy storage electrolytic capacitor in parallel on its DC side to maintain DC bus voltage stability. The system is equivalent to an AC voltage source with controllable amplitude and phase, directly generating or absorbing the required capacitive or inductive reactive current on the AC side through high-frequency PWM modulation.

[0004] In reactive power compensation technology, the capacitor bank switching in groups suffers from problems such as stepped discrete adjustment of compensation capacity, large compensation blind zone, slow dynamic response, and severe inrush current impact at the moment of switching. These issues make it impossible to match the continuously fluctuating reactive power demand of the load, which can easily lead to undercompensation or overcompensation, and cause voltage jumps in the power grid and accelerated aging of components. On the other hand, static var generators based on voltage source converters rely on large DC electrolytic capacitors to maintain the bus voltage, resulting in large device size and low power density. Moreover, the electrolytic capacitors are prone to aging and failure at high temperatures, becoming a bottleneck restricting the lifespan of the entire unit. At the same time, high-frequency chopping is still required to maintain the bus voltage in standby mode, resulting in high standby losses. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a power grid reactive power compensation method and device based on a single-stage AC-AC converter, which solves the problems of stepped discrete adjustment of compensation capacity, large compensation blind zone, slow dynamic response, and severe inrush current impact at the moment of switching in reactive power compensation technology.

[0006] To achieve the above objectives, the present invention provides the following technical solution: This invention provides a power grid reactive power compensation method based on a single-stage AC-AC converter. The single-stage AC-AC converter includes an inductor, a capacitor, and a switching transistor. The input terminal of the single-stage AC-AC converter is connected in parallel to the AC power grid, and the output terminal of the single-stage AC-AC converter is connected in parallel to a compensation capacitor. The method includes the following steps: The high-frequency switching of each transistor in the AC-AC converter is controlled to perform PWM modulation on the output voltage with a duty cycle D, so that the AC-AC converter generates a voltage gain A for the compensation capacitor that varies continuously with the duty cycle D. v And the capacitive current flowing through the compensation capacitor is mapped to the grid side via the AC-AC converter; By adjusting the duty cycle D, the equivalent capacity of the capacitively adjustable virtual capacitor presented on the grid side is continuously changed, thereby continuously adjusting the reactive power injected into the grid.

[0007] In one embodiment of the present invention, voltage gain satisfy: in, This refers to the output voltage of the AC-AC converter. The input voltage is the grid-side voltage, and D is the duty cycle; voltage gain. The absolute value of the voltage gain increases nonlinearly as the duty cycle D decreases. When the duty cycle D approaches zero, the voltage gain... The absolute value of approaches infinity.

[0008] In one embodiment of the present invention, the equivalent capacitance of the virtual capacitor satisfy: in, To compensate for the physical capacitance of the capacitor; the equivalent capacitance of the virtual capacitor. It increases continuously as the duty cycle D decreases.

[0009] In one embodiment of the present invention, the AC-AC converter includes four pairs of bidirectional switch groups consisting of a first switch to an eighth switch, a first inductor, a second inductor, a first capacitor, and an output capacitor. When the mains voltage is in the positive half-cycle, the drive signals of the second, fourth, sixth, and eighth switches are high and remain on. The first and fifth switches are turned on and off at the same time, and the third and seventh switches are turned on and off at the same time. The two groups are switched in a high-frequency complementary manner with a duty cycle D. When the mains voltage is in the negative half-cycle, the drive signals of the first, third, fifth, and seventh switches are high and remain on. The second and sixth switches are turned on and off simultaneously, and the fourth and eighth switches are turned on and off simultaneously. The two groups are switched in a high-frequency complementary manner with a duty cycle D.

[0010] In one embodiment of the present invention, the step of adjusting the duty cycle D includes: Real-time monitoring of grid voltage and current, calculation of actual reactive power deficit on the grid side, and generation of target reactive power command Q. ref ; Obtain the current actual compensated reactive power Q eq The target reactive power command Q will be executed. ref The actual compensated reactive power Q eq The difference is calculated to obtain the reactive power error e; The reactive power error e is input to the PI controller, which dynamically outputs the duty cycle D based on the reactive power error e to change the voltage gain. This adjusts the voltage amplitude and current flowing through the compensation capacitor, thereby changing the equivalent reactive power mapped to the grid side until the reactive power error e converges.

[0011] In one embodiment of the present invention, when large-capacity reactive power compensation is required, the duty cycle D output by the PI controller operates in the low duty cycle range, thereby increasing the voltage gain. The absolute value of the capacitor increases nonlinearly as the duty cycle D decreases, thus creating a large-capacity virtual capacitor on the grid side.

[0012] In one embodiment of the present invention, the reactive power injected into the power grid satisfy: Where ω is the angular frequency of the power grid, V in The effective value of the input voltage on the grid side; the voltage gain is changed by adjusting the duty cycle D. To achieve reactive power injected into the power grid Continuous adjustment.

[0013] The present invention also provides a power grid reactive power compensation device based on a single-stage AC-AC converter, comprising: A single-stage AC-AC converter circuit has its input terminal connected in parallel to the AC power grid and its output terminal connected in parallel to a compensation capacitor of fixed capacity. The single-stage AC-AC converter circuit consists of a switching transistor, an inductor, and a capacitor. It is used to perform direct AC-AC energy conversion between the AC power grid and the compensation capacitor. There is no intermediate DC bus or DC energy storage capacitor between the input and output terminals. The controller is used to detect the reactive power status of the power grid, calculate the reactive power error, generate the duty cycle D based on the reactive power error, and control the on / off state of each switch in the single-stage AC-AC converter circuit according to the polarity of the power grid voltage. It also performs PWM modulation on the output voltage, causing the single-stage AC-AC converter circuit to generate a voltage gain for the compensation capacitor that continuously varies with the duty cycle D. The capacitive current flowing through the compensation capacitor is mapped to the grid side, making the grid side equivalent to a virtual capacitor with adjustable capacitiveness, and continuously adjusting the reactive power injected into the grid.

[0014] In one embodiment of the present invention, a single-stage AC-AC converter circuit includes a first to an eighth switching transistor, a first inductor, a second inductor, a first capacitor, and an output capacitor. The source of the first switch is connected to the source of the second switch to form a first bidirectional switch group; the source of the third switch is connected to the source of the fourth switch to form a second bidirectional switch group; the source of the fifth switch is connected to the source of the sixth switch to form a third bidirectional switch group; and the source of the seventh switch is connected to the source of the eighth switch to form a fourth bidirectional switch group. The first terminal of the input port is connected to the drain of the seventh switch and the drain of the first switch; the drain of the eighth switch is connected to the first terminal of the second inductor and the drain of the fifth switch; the drain of the fourth switch and the second terminal of the second inductor are connected to the second terminal of the input port. The drain of the second switch is connected to the drain of the third switch and the first terminal of the first capacitor, respectively; the drain of the sixth switch is connected to the second terminal of the first capacitor. The common node formed by connecting the drain of the sixth switching transistor to the second terminal of the first capacitor is connected to the first terminal of the first inductor; the second terminal of the first inductor is connected to one terminal of the output capacitor, and the other terminal of the output capacitor is connected to the second terminal of the input port; the two terminals of the output capacitor constitute the output port, and the output port is connected in parallel with a compensation capacitor.

[0015] In one embodiment of the present invention, the controller includes a reactive power detection unit, an error calculation unit, and a PI controller; The reactive power detection unit is used to detect grid voltage and grid current in real time, calculate the reactive power deficit on the grid side, and generate the target reactive power command Q. ref ; The error calculation unit is used to calculate the target reactive power command Q. ref The actual compensated reactive power Q eq The difference is calculated to obtain the reactive power error e; The PI controller is used to dynamically output the duty cycle D based on the reactive power error e. The duty cycle D is adjusted by changing the voltage gain A. v This is used to adjust the voltage amplitude and current flowing through the compensation capacitor, thereby changing the equivalent reactive power mapped to the grid side.

[0016] This invention provides a method and apparatus for reactive power compensation in power grids based on a single-stage AC-AC converter. Compared with existing technologies, it has the following advantages: By connecting a fixed-capacity compensation capacitor in parallel at the output of a single-stage AC-AC converter and utilizing high-frequency PWM modulation to generate a voltage gain on the capacitor that continuously varies with the duty cycle, an equivalent virtual capacitor is formed on the grid side. This architecture achieves continuous and smooth reactive power regulation, completely eliminating the stepped compensation blind spot caused by traditional group switching methods. Simultaneously, since the capacitor is always connected to the grid through the converter in a high-frequency modulation manner, direct switching by mechanical or solid-state switches is avoided, fundamentally eliminating transient inrush current and operational overvoltage, and protecting the insulation between the switch contacts and the capacitor. Furthermore, the single-stage structure eliminates the intermediate DC bus and large-capacity DC energy storage capacitor, not only reducing the size and weight of the device and increasing power density, but also avoiding system failures caused by high-temperature aging of electrolytic capacitors, enhancing environmental adaptability. Attached Figure Description

[0017] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0018] Figure 1 The overall structure diagram of nonlinear gain single-stage AC-AC reactive power compensation provided for the embodiments of this application is shown.

[0019] Figure 2 A circuit diagram of a nonlinear gain single-stage AC-AC topology provided in an embodiment of this application.

[0020] Figure 3 The circuit diagram of the nonlinear gain single-stage AC-AC topology provided in the embodiments of this application in operating mode 1 is shown.

[0021] Figure 4 The circuit diagram of the nonlinear gain single-stage AC-AC topology provided in the embodiments of this application in operating mode 2 is shown.

[0022] Figure 5 A flowchart illustrating the reactive power compensation method provided in this application embodiment. Detailed Implementation

[0023] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions in the embodiments of the present invention are described clearly and completely. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0024] This application provides a power grid reactive power compensation method and device based on a single-stage AC-AC converter. It solves the problems of large compensation level blind zone, slow dynamic response, and severe inrush current impact during switching of capacitor banks in reactive power compensation technology, as well as the problems of large device size, high standby loss, easy aging and failure of electrolytic capacitors, and complex control structure caused by the reliance on intermediate DC electrolytic capacitors in static var generators. It realizes continuous stepless adjustment of reactive power, eliminates intermediate DC energy storage links, simplifies the control architecture, and significantly improves the dynamic response speed and operational reliability of the compensation device.

[0025] The technical solution in this application is to solve the above-mentioned technical problems, and the general idea is as follows: This embodiment provides a power grid reactive power compensation device based on a single-stage AC-AC converter, such as... Figure 1 As shown, it includes a single-stage AC-AC converter circuit and a controller. The input of the single-stage AC-AC converter circuit is connected in parallel to the AC power grid, and the output is connected in parallel to a fixed-capacity compensation capacitor. .

[0026] A single-stage AC-AC converter circuit consists of a switching transistor, an inductor, and a capacitor, and is used to connect the AC mains to a compensation capacitor. Direct AC-AC energy conversion is performed between the input and output terminals. There is no intermediate DC bus or DC energy storage capacitor between the input and output terminals of this circuit, meaning there is no DC electrolytic capacitor commonly used in traditional static var generators. This topological feature eliminates the shortest-lived component from a physical source, significantly improving the device's mean time between failures (MTBF) while also substantially reducing the device's size and weight.

[0027] The controller detects the reactive power status of the power grid, calculates the reactive power error, generates the duty cycle D based on the reactive power error, and controls the on / off state of each switch in the single-stage AC-AC converter circuit according to the grid voltage polarity, performing PWM modulation on the output voltage. Through PWM modulation, the converter circuit adjusts the compensation capacitor... Generates a voltage gain that varies continuously with the duty cycle D. Using this voltage gain, the capacitive current flowing through the compensation capacitor is mapped to the grid side, making the grid side equivalent to a virtual capacitor with adjustable capacitiveness, thereby continuously adjusting the reactive power injected into the grid.

[0028] The basic principle of this invention can be summarized as follows: Although the physical compensation capacitor connected in parallel at the output terminal... The capacitance is fixed, but by changing the amplitude of its terminal voltage and the current flowing through it through high-frequency PWM modulation, and then through the impedance mapping effect of the converter, an equivalent virtual capacitor with continuously adjustable capacitance as the duty cycle D is synthesized on the grid side. By adjusting the duty cycle D, the virtual capacitor can be continuously changed. The equivalent capacity is thus used to achieve smooth and stepless adjustment of reactive power.

[0029] To better understand the above technical solutions, the following will provide a detailed explanation of the technical solutions in conjunction with the accompanying drawings and specific implementation methods.

[0030] like Figure 2 As shown, the single-stage AC-AC converter circuit includes first to eighth switching transistors S1 to S8, a first inductor L1, a second inductor L2, a first capacitor C1, and an output capacitor Co. Each switching transistor S1 to S8 is a fully controlled power switching device with a body diode, such as a MOSFET.

[0031] The bidirectional switching groups of each switching transistor are configured as follows: the source of the first switching transistor S1 is connected to the source of the second switching transistor S2 to form the first bidirectional switching group; the source of the third switching transistor S3 is connected to the source of the fourth switching transistor S4 to form the second bidirectional switching group; the source of the fifth switching transistor S5 is connected to the source of the sixth switching transistor S6 to form the third bidirectional switching group; and the source of the seventh switching transistor S7 is connected to the source of the eighth switching transistor S8 to form the fourth bidirectional switching group.

[0032] The specific connection relationships of each component are as follows: The first end of the input port (i.e., the grid-side port V) in The first terminal of the inductor L2 is connected to the drain of the seventh switch S7 and the drain of the first switch S1; the source of the seventh switch S7 is connected to the source of the eighth switch S8; the source of the first switch S1 is connected to the source of the second switch S2; the source of the third switch S3 is connected to the source of the fourth switch S4; the drain of the fourth switch S4 and the second terminal of the second inductor L2 are respectively connected to the second terminal of the input port, which is simultaneously connected to the AC power supply V. in The other end is connected to the N line.

[0033] The drain of the eighth switch S8 is connected to the first terminal of the second inductor L2 and the drain of the fifth switch S5, respectively; the source of the fifth switch S5 is connected to the source of the sixth switch S6; the drain of the second switch S2 is connected to the drain of the third switch S3 and the first terminal of the first capacitor C1, respectively; and the drain of the sixth switch S6 is connected to the second terminal of the first capacitor C1.

[0034] The common node formed by connecting the drain of the sixth switch S6 and the second terminal of the first capacitor C1 is connected to the first terminal of the first inductor L1; the second terminal of the first inductor L1 is connected to the output capacitor C o One end is connected to the output capacitor C. o The other end is connected to the second end of the input port. Output capacitor C o The two ends form the output port, and a compensation capacitor is connected in parallel to the output port. .

[0035] By employing four pairs of bidirectional switch groups, bidirectional direct conversion of AC voltage can be achieved, and the switching logic of the positive and negative half cycles is symmetrical, ensuring stable operation of the grid voltage throughout the entire cycle.

[0036] In one embodiment, the controller implements the following driving logic for each switch transistor based on the polarity of the mains voltage: When the mains voltage is in the positive half-cycle, the drive signals for the second switch S2, the fourth switch S4, the sixth switch S6, and the eighth switch S8 are high, maintaining a normally on state. The first switch S1 and the fifth switch S5 are simultaneously on and off, and the third switch S3 and the seventh switch S7 are simultaneously on and off. The two groups perform high-frequency complementary switching with a duty cycle D. That is, within one high-frequency switching cycle, when S1 and S5 are on, S3 and S7 are off; when S3 and S7 are on, S1 and S5 are off. The ratio of the on-time of the two groups of switches is D:(1-D).

[0037] When the grid voltage is in the negative half-cycle, the drive signals of the first switch S1, the third switch S3, the fifth switch S5, and the seventh switch S7 are high level, maintaining the normally on state; the second switch S2 and the sixth switch S6 are turned on and off at the same time, and the fourth switch S4 and the eighth switch S8 are turned on and off at the same time. The two groups are switched in a high-frequency complementary manner with a duty cycle D, and the ratio of the conduction time is also D:(1-D).

[0038] It can be seen that the high-frequency switching transistor group and the normally-conducting transistor group in the positive and negative half-cycles exactly interchange roles. This symmetrical design enables the conversion circuit to achieve bidirectional energy flow and bidirectional conversion throughout the entire cycle of the AC power grid.

[0039] In one embodiment, the two high-frequency operating modes of the converter circuit are analyzed in detail below, taking the positive half-cycle of the grid voltage as an example.

[0040] Operating mode 1 (S1 and S5 are on, S3 and S7 are off): like Figure 3 As shown, there are two current flow paths in the circuit at this time: Path 1: "Grid-side port V" in →S1→S2→C1→L1→C o→V in ”; Path 2: "Grid-side port V" in →S1→S2→C1→S6→S5→L2→V in ".

[0041] In this mode, the input voltage is applied directly to the two ends of the first inductor L1 through the through-circuit switching transistors S1 and S2, causing the current inside L1 to rise linearly, converting the electrical energy from the power supply into magnetic field energy and storing it. At the same time, due to the switching of the circuit state, the first capacitor C1, which has been charged in the early stage, forms a closed loop with the second inductor L2 and the load (output port V0), and C1 begins to discharge.

[0042] Operating mode 2 (S3 and S7 are on, S1 and S5 are off): like Figure 4 As shown, there are also two current flow paths in the circuit at this time: Path 3: "Grid-side port V" in →S7→S8→L2→V in ”; Path 4: "S3→S4→C" o →L1→C1→S3.

[0043] In this mode, the input voltage is directly applied across the second inductor L2, charging L2. The electrical energy provided by the power supply is continuously converted into magnetic field energy and stored in L2, accumulating power for subsequent energy transfer. At the same time, the first inductor L1 enters the demagnetization stage, acting as a temporary power source, converting the previously stored magnetic field energy back into electrical energy and releasing it to the downstream load. Its internal current exhibits a stable linear decreasing trend.

[0044] It should be noted that, due to the symmetry of the circuit topology and switch drive logic, the operation is similar to the positive half-cycle during the negative half-cycle of the mains voltage, except that the current direction is reversed and the charging and discharging roles are interchanged. At this time, the high-frequency switching transistor groups become S2 / S6 and S4 / S8, and the normally conducting transistors are S1, S3, S5, and S7. The current path during the negative half-cycle can be directly obtained by mirroring the corresponding path during the positive half-cycle. Those skilled in the art can directly derive the specific current path during the negative half-cycle based on the above description of the positive half-cycle mode and the symmetrical characteristics of the bidirectional switching group; therefore, it will not be elaborated upon here.

[0045] In one embodiment, when the duty cycle of switches S1, S2, S5, and S6 is D, and the duty cycle of switches S3, S4, S7, and S8 is 1-D, by performing inductor volt-second balance analysis on the two operating modes of the positive half-cycle, the voltage gain relationship of the converter can be derived as follows: in, This is the output voltage. Where is the grid-side input voltage, D is the duty cycle, and 0 <D<1。

[0046] As can be seen from this formula, the voltage gain It is a non-linear function of the duty cycle D. When D=1, =0; when D starts decreasing from 1, | |It begins to increase gradually; as D continues to decrease,| The growth rate of | is accelerating, exhibiting a significant non-linear upward trend; when D approaches zero, | The voltage gain tends towards infinity. This nonlinear voltage gain characteristic provides the physical basis for the virtual capacitance mapping technique in this invention.

[0047] In one embodiment, a compensation capacitor of fixed capacitance is connected in parallel at the output of the conversion circuit. At the grid angular frequency ω, the output current of the output capacitor is... for: From voltage gain This allows us to obtain the mapping relationship between the input current and the output current: Will Substituting the expression We can obtain: According to the law of conservation of energy, the reactive power on the input side is equal to the reactive power on the output side, that is: From the power grid input end Looking inside, the current absorbed by the system It still leads the grid voltage by 90°, exhibiting a purely capacitive current. At this point, the equivalent output capacitance felt by the grid is... for: Therefore, it can be seen that although the compensation capacitor at the output end The capacity remains constant, but the voltage gain can be changed by adjusting the duty cycle D using a high-frequency PWM signal. This changes the capacity of the equivalent virtual capacitor on the grid side. .because and Proportional to the square of the value, a relatively small physical compensation capacitor can be equivalent to a large reactive power capacity on the grid side. Adjusting the duty cycle D is equivalent to creating a virtual capacitor with continuously adjustable capacity on the grid side.

[0048] Based on the above equivalent virtual capacitor model, the reactive power injected into the power grid satisfy: Where ω is the angular frequency of the power grid. This represents the effective value of the input voltage from the grid side. The voltage gain is changed by adjusting the duty cycle D. This enables the injection of reactive power into the power grid. Continuous adjustment. As the duty cycle D decreases from 1, the voltage gain... As the absolute value increases, the converter applies pressure to the compensation capacitor. As the effective value of the high-frequency voltage increases, the reactive current flowing through the capacitor increases, which in turn increases the reactive power reflected back to the grid side.

[0049] Thus, based on the mathematical analysis of the gain function, it can be seen that when the duty cycle D approaches 0, the voltage gain... The absolute value of diverges exponentially. Therefore, under high-capacity reactive power compensation conditions, this invention only requires the converter to operate in a low duty cycle range to achieve extremely high equivalent impedance amplification. For example, when D=0.2, | |≈5, ≈25· That is, the equivalent capacitance of the physical capacitor is magnified by about 25 times.

[0050] This feature avoids the gain collapse and hardware dead zone problems caused by traditional converters approaching the limit of high duty cycle, ensuring that the converter always operates within a wide safe operating range. At the same time, the low duty cycle operation mechanism significantly reduces the effective conduction time of the main power switch in a single cycle, significantly reducing the concentrated thermal stress on the device. Combined with a reasonable heat dissipation design, it can fundamentally eliminate the risk of thermal runaway caused by a single power device being continuously turned on for a long time, thereby greatly improving the long-term operational reliability of the equipment.

[0051] In one embodiment, the controller includes a reactive power detection unit, an error calculation unit, and a PI controller, as shown in the schematic diagram below. Figure 5 As shown.

[0052] The reactive power detection unit is used to detect grid voltage and grid current in real time, calculate the reactive power deficit on the grid side, and generate the target reactive power command Q. ref .

[0053] The error calculation unit is used to calculate the target reactive power command Q. ref The actual compensated reactive power Q eq The difference is calculated to obtain the reactive power error e.

[0054] The PI controller dynamically outputs the duty cycle D based on the reactive power error e. The duty cycle D is adjusted by changing the voltage gain A. v To adjust the compensation capacitor C p The voltage amplitude and current flowing through it change the equivalent reactive power mapped to the grid side.

[0055] Specifically, the reactive power compensation closed-loop control process in this embodiment is as follows: The first step is to monitor the grid voltage and current in real time to determine the reactive power status of the grid; when it is determined that there is a reactive power deficit in the grid, the target reactive power command Q is calculated. ref .

[0056] The second step is to detect or calculate the current actual reactive power Q being compensated. eq .

[0057] The third step is to assign the target reactive power command Qr. ef The actual compensated reactive power Q eq The difference is calculated to obtain the reactive power error e.

[0058] The fourth step is to input the reactive power error e into the PI controller, which then dynamically outputs the duty cycle D based on the reactive power error e.

[0059] Fifth, based on the polarity of the mains voltage, select the corresponding switching drive logic (positive half-cycle logic or negative half-cycle logic), and apply PWM modulation to the conversion circuit with a duty cycle D to change the voltage gain A. v .

[0060] Step 6, Voltage Gain A v Adjustment of the compensation capacitor C p The voltage amplitude and current flowing through it change the equivalent reactive power mapped to the grid side.

[0061] Step 7: Repeat the above process until the reactive power error e converges to zero, thus achieving closed-loop tracking compensation of reactive power.

[0062] This control strategy directly targets reactive power error, outputting the duty cycle D directly through a PI controller, thus abandoning the traditional dual-loop decoupled control structure of "DC voltage outer loop + AC current inner loop". Due to the extremely short closed-loop path, the system's response speed to sudden reactive power changes in the power grid can reach the millisecond level, meeting the rapid reactive power support requirements for impulsive nonlinear loads.

[0063] In one embodiment, when the power grid requires large-capacity reactive power compensation, the duty cycle D of the PI controller output operates in a low duty cycle range (e.g., D < 0.5). Within this range, the voltage gain A... vThe absolute value of the voltage increases nonlinearly as the duty cycle D decreases, thus effectively synthesizing a large-capacity virtual capacitor on the grid side. The advantage of this operating mechanism is that the converter never needs to approach the high duty cycle limit, thereby eliminating the risk of gain collapse and hardware dead-time limitations in principle. This keeps the effective conduction time of the main power switch in a short single cycle, significantly reducing device thermal stress.

[0064] The above embodiments are illustrated using a single-phase system as an example, but the present invention is equally applicable to three-phase power grids. Specifically, three sets of the above-described single-stage AC-AC conversion circuits can be used, connected in parallel to each phase (phase A, phase B, and phase C) of the three-phase power grid. Each phase independently performs reactive power detection and duty cycle control to achieve reactive power compensation for the three-phase power grid. The topology, switch drive logic, and control strategy of each phase conversion circuit are exactly the same as those in the single-phase embodiment; only the parameters such as input voltage and grid voltage polarity discrimination need to be replaced with the corresponding quantities for the corresponding phases. This modular expansion method has a simple structure and is particularly suitable for independent phase compensation in three-phase unbalanced load scenarios.

[0065] To prevent surges during startup, the reactive power compensation device of this invention employs a soft-start strategy: during the startup phase, the duty cycle D is first set to be close to 1 (at which point the voltage gain A...). v The equivalent capacitance of the virtual capacitor is approximately zero (≈0). Then, the duty cycle D is gradually decreased according to a certain slope, so that the equivalent capacitance increases smoothly from zero until the target compensation capacity is reached. This soft-start process makes full use of the characteristic that the gain function is zero at D=1. It is a direct application of the technical means of "continuously changing the equivalent capacitance of the virtual capacitor by adjusting the duty cycle D" in this invention, which can effectively suppress the starting inrush current and ensure the safe operation of the equipment.

[0066] In this embodiment, each switch S1 to S8 can be a MOSFET or other fully controlled power switching device. The switching frequency can be selected from tens of kilohertz to hundreds of kilohertz depending on the actual power level, with typical values ​​ranging from 50kHz to 200kHz. Higher switching frequencies are beneficial for reducing the physical size of inductors L1 and L2, but will increase switching losses; in practical applications, a trade-off can be made based on power level and efficiency requirements. The inductance values ​​of the first inductor L1 and the second inductor L2 are designed based on the switching frequency, input voltage range, and desired current ripple; the first capacitor C1 and the output capacitor C... o The capacitance value is selected based on the operating frequency and output voltage ripple requirements; compensation capacitor C p The capacity is designed based on the required maximum compensation capacity and maximum voltage gain. Due to the equivalent capacitance amplification effect (C... eq = A v 2 ·C p ), Cp The actual capacity can be much smaller than the required equivalent compensation capacity, thereby effectively reducing the physical capacity requirement and cost of the capacitor.

[0067] In summary, the power grid reactive power compensation method and apparatus based on a single-stage AC-AC converter described in this invention utilizes a single-stage AC-AC converter to achieve direct AC-AC power conversion between the AC power grid and a fixed compensation capacitor. By adjusting the duty cycle D using high-frequency PWM, the converter generates a non-linearly adjustable voltage gain A on the fixed capacitor at the output terminal. v Based on this gain relationship, the fixed capacitor at the output end, after impedance mapping by the converter, is equivalent to a virtual capacitor C with continuously adjustable capacitance on the grid side. eq Furthermore, by adjusting the duty cycle D, the equivalent reactive power capacity on the grid side can be smoothly changed, achieving stepless dynamic compensation for the reactive power of the grid. Simultaneously, this scheme uses reactive power error as the control target, employing a PI controller to directly output the duty cycle D, forming a simplified control architecture with a short closed-loop path and fast response speed. Moreover, by utilizing the nonlinear divergence characteristic of the gain function in the low duty cycle range, the converter operates in a safe and low-thermal-stress operating range under high-capacity compensation conditions.

[0068] In summary, compared with existing technologies, it has the following beneficial effects: 1. Achieve continuous stepless adjustment of reactive power and eliminate compensation blind spots.

[0069] By connecting a fixed-capacity compensation capacitor in parallel at the output of a single-stage AC-AC converter, and utilizing the continuously adjustable voltage gain of the converter, the fixed capacitor is impedance-mapped onto the grid side and effectively transformed into a virtual capacitor with continuously adjustable capacity. Adjusting the duty cycle allows for continuous change of the virtual capacitor capacity, fundamentally avoiding the inherent limitation of traditional group-switching capacitor banks which can only be adjusted discretely in fixed steps. This eliminates undercompensation and overcompensation, achieving smooth, stepless regulation of reactive power.

[0070] 2. Eliminate intermediate DC energy storage links to fundamentally improve reliability and power density.

[0071] Employing a single-stage AC-AC direct conversion architecture, this design eliminates the need for an intermediate DC bus and DC energy storage capacitor between the input and output terminals. Energy is directly and frequently transferred between the grid and the physical capacitor at the output. This design completely removes the DC electrolytic capacitor, which is the shortest-lived, largest, and most prone to aging component in traditional static var generators. This not only improves the system's mean time between failures (MTBF) by orders of magnitude but also reduces the device's size and weight, while enhancing power density and environmental adaptability.

[0072] 3. The gain function exhibits significant nonlinear characteristics, breaking through the bottleneck of extreme duty cycle operation.

[0073] Converter voltage gain A v Satisfy A v =(D 2 The absolute value of the gain is -1) / D, which increases nonlinearly as the duty cycle D decreases, and approaches infinity when D approaches zero. Based on this nonlinear gain characteristic, the system can achieve extremely high equivalent impedance amplification by operating only in the low duty cycle range under high-capacity reactive power compensation conditions. This completely avoids the gain collapse problem caused by hardware dead-time limitations when traditional converters approach the limit of high duty cycle, ensuring that the converter always operates within a wide safe operating range.

[0074] 4. The equivalent capacitance square amplification effect is significant, reducing the physical capacitance requirement.

[0075] Since the equivalent capacitance of the virtual capacitor satisfies C eq = A v 2 ·C p The equivalent capacity is proportional to the square of the voltage gain. Therefore, a smaller capacity physical compensation capacitor can achieve a large capacity reactive power compensation effect on the grid side, effectively reducing the physical capacity requirement and cost of the compensation capacitor, while also reducing the weight and volume of the device.

[0076] 5. Low duty cycle operation significantly reduces thermal stress and ensures long-term operational reliability.

[0077] During high-capacity compensation, the system operates in a low duty cycle range, significantly compressing the effective on-time of the main power switch within a single switching cycle. This extremely narrow pulse operating mechanism effectively prevents severe heat accumulation caused by prolonged continuous conduction of a single switching device, significantly reducing concentrated thermal stress and temperature rise of the device, fundamentally reducing the risk of thermal runaway, and greatly improving the energy conversion efficiency and long-term operational reliability of the device under heavy load conditions.

[0078] 6. It adopts single-loop PI direct closed-loop control, which has a simplified structure and fast dynamic response.

[0079] This design abandons the traditional dual-loop decoupled control of "DC voltage outer loop + AC current inner loop" and adopts a direct PI closed-loop tracking strategy based on reactive power error. The controller calculates the reactive power deficit Q of the power grid in real time. ref With actual compensation amount Q eq The difference is directly output as the duty cycle D by the PI regulator, eliminating the need for coordinate transformation and decoupling calculations, thus effectively reducing the computational load on the microprocessor. Due to the extremely short closed-loop path, the system's response speed to sudden reactive power changes in the power grid can reach the millisecond level, making it capable of meeting the rapid reactive power support requirements of impulsive nonlinear loads.

[0080] 7. Full-cycle symmetrical switching logic ensures bidirectional power flow.

[0081] Based on four pairs of bidirectional switch groups and symmetrical switching drive logic for positive and negative half-cycles, the converter circuit can achieve symmetrical energy storage-discharge alternation in the positive and negative half-cycles of the AC grid voltage, ensuring that the grid voltage can stably provide or absorb capacitive reactive current to the grid throughout the entire cycle, thus avoiding the inherent half-cycle failure problem of unidirectional converters in AC applications.

[0082] 8. The parallel connection method is flexible and simple, and has strong scalability.

[0083] The converter input is connected to the grid only in parallel, eliminating the need for series transformers or complex coupling structures, facilitating modular deployment at existing grid nodes. For three-phase grids, three single-phase conversion circuits can be used to connect to each phase independently, naturally adapting to three-phase imbalance compensation scenarios and offering excellent engineering scalability and flexibility.

[0084] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit it. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some of the technical features. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the spirit and scope of the technical solutions of the embodiments of the present invention.

Claims

1. A power grid reactive power compensation method based on a single-stage AC-AC converter, characterized in that, The single-stage AC-AC converter includes an inductor, a capacitor, and a switching transistor. The input terminal of the single-stage AC-AC converter is connected in parallel to the AC power grid, and the output terminal of the single-stage AC-AC converter is connected in parallel to a compensation capacitor. The method includes the following steps: The high-frequency switching of each switch in the AC-AC converter is controlled to perform PWM modulation on the output voltage with a duty cycle D, so that the AC-AC converter generates a voltage gain A for the compensation capacitor that continuously varies with the duty cycle D. v And the capacitive current flowing through the compensation capacitor is mapped to the grid side through the AC-AC converter; The duty cycle D is adjusted to continuously change the equivalent capacity of the capacitively adjustable virtual capacitor presented on the grid side, thereby continuously adjusting the reactive power injected into the grid.

2. The power grid reactive power compensation method according to claim 1, characterized in that, The voltage gain satisfy: in, The output voltage of the AC-AC converter is... The voltage is the grid-side input voltage, D is the duty cycle, and the voltage gain is... The absolute value of the voltage gain increases non-linearly as the duty cycle D decreases. When the duty cycle D approaches zero, the voltage gain... The absolute value of approaches infinity.

3. The power grid reactive power compensation method according to claim 1, characterized in that, The equivalent capacitance of the virtual capacitor satisfy: in, The physical capacitance of the compensation capacitor; the equivalent capacitance of the virtual capacitor. It increases continuously as the duty cycle D decreases.

4. The power grid reactive power compensation method according to claim 1, characterized in that, The AC-AC converter includes four pairs of bidirectional switches consisting of the first switch (S1) to the eighth switch (S8), a first inductor (L1), a second inductor (L2), a first capacitor (C1), and an output capacitor (C). o ); When the grid voltage is in the positive half-cycle, the drive signals of the second switch (S2), the fourth switch (S4), the sixth switch (S6), and the eighth switch (S8) are high and remain on. The first switch (S1) and the fifth switch (S5) are turned on and off at the same time, and the third switch (S3) and the seventh switch (S7) are turned on and off at the same time. The two groups are switched in a high-frequency complementary manner with the duty cycle D. When the grid voltage is in the negative half-cycle, the drive signals of the first switch (S1), the third switch (S3), the fifth switch (S5), and the seventh switch (S7) are high and remain on. The second switch (S2) and the sixth switch (S6) are turned on and off at the same time, and the fourth switch (S4) and the eighth switch (S8) are turned on and off at the same time. The two groups are switched in a high-frequency complementary manner with the duty cycle D.

5. The power grid reactive power compensation method according to any one of claims 1 to 4, characterized in that, The steps for adjusting the duty cycle D include: Real-time monitoring of grid voltage and current, calculation of actual reactive power deficit on the grid side, and generation of target reactive power command Q. ref ; Obtain the current actual compensated reactive power Q eq The target reactive power command Q ref The actual compensated reactive power Q eq The difference is calculated to obtain the reactive power error e; The reactive power error e is input to the PI controller, which dynamically outputs the duty cycle D based on the reactive power error e to change the voltage gain. This adjusts the voltage amplitude and current flowing through the compensation capacitor, thereby changing the equivalent reactive power mapped to the grid side until the reactive power error e converges.

6. The power grid reactive power compensation method according to claim 5, characterized in that, When large-capacity reactive power compensation is required, the duty cycle D output by the PI controller operates in the low duty cycle range, thus increasing the voltage gain. The absolute value of the capacitor increases nonlinearly as the duty cycle D decreases, thereby effectively synthesizing a large-capacity virtual capacitor on the grid side.

7. The power grid reactive power compensation method according to claim 1, characterized in that, reactive power injected into the power grid satisfy: Where ω is the angular frequency of the power grid, V in The effective value of the input voltage on the grid side; the voltage gain is changed by adjusting the duty cycle D. To achieve reactive power injected into the power grid Continuous adjustment.

8. A power grid reactive power compensation device based on a single-stage AC-AC converter, characterized in that, include: A single-stage AC-AC converter circuit has its input terminal connected in parallel to the AC power grid and its output terminal connected in parallel to a compensation capacitor of fixed capacity. The single-stage AC-AC converter circuit consists of a switching transistor, an inductor, and a capacitor, and is used to perform direct AC-AC energy conversion between the AC power grid and the compensation capacitor. There is no intermediate DC bus or DC energy storage capacitor between the input terminal and the output terminal. The controller is used to detect the reactive power status of the power grid, calculate the reactive power error, generate a duty cycle D based on the reactive power error, and control the on / off state of each switch in the single-stage AC-AC converter circuit according to the polarity of the power grid voltage. It also performs PWM modulation on the output voltage, causing the single-stage AC-AC converter circuit to generate a voltage gain for the compensation capacitor that continuously varies with the duty cycle D. The capacitive current flowing through the compensation capacitor is mapped to the grid side, so that the grid side is equivalent to a virtual capacitor with adjustable capacitiveness, and the reactive power injected into the grid is continuously adjusted.

9. The power grid reactive power compensation device according to claim 8, characterized in that, The single-stage AC-AC converter circuit includes a first switch (S1) to an eighth switch (S8), a first inductor (L1), a second inductor (L2), a first capacitor (C1), and an output capacitor (C). o ); The source of the first switch (S1) is connected to the source of the second switch (S2) to form a first bidirectional switch group; the source of the third switch (S3) is connected to the source of the fourth switch (S4) to form a second bidirectional switch group; the source of the fifth switch (S5) is connected to the source of the sixth switch (S6) to form a third bidirectional switch group; and the source of the seventh switch (S7) is connected to the source of the eighth switch (S8) to form a fourth bidirectional switch group. The first terminal of the input port is connected to the drain of the seventh switch (S7) and the drain of the first switch (S1); the drain of the eighth switch (S8) is connected to the first terminal of the second inductor (L2) and the drain of the fifth switch (S5); the drain of the fourth switch (S4) and the second terminal of the second inductor (L2) are connected to the second terminal of the input port. The drain of the second switch (S2) is connected to the drain of the third switch (S3) and the first terminal of the first capacitor (C1); the drain of the sixth switch (S6) is connected to the second terminal of the first capacitor (C1). The common node formed by connecting the drain of the sixth switch (S6) and the second terminal of the first capacitor (C1) is connected to the first terminal of the first inductor (L1); the second terminal of the first inductor (L1) is connected to the output capacitor (C1). o Connect one end of the capacitor to the output capacitor (C). o The other end of the capacitor is connected to the second end of the input port; the output capacitor (C) o The two ends of the capacitor form the output port, and the output port is connected in parallel to the compensation capacitor. ).

10. The power grid reactive power compensation device according to claim 8, characterized in that, The controller includes a reactive power detection unit, an error calculation unit, and a PI controller; The reactive power detection unit is used to detect grid voltage and grid current in real time, calculate the reactive power deficit on the grid side, and generate the target reactive power command Q. ref ; The error calculation unit is used to calculate the target reactive power command Q. ref The actual compensated reactive power Q eq The difference is calculated to obtain the reactive power error e; The PI controller is used to dynamically output the duty cycle D based on the reactive power error e, and the duty cycle D is adjusted by changing the voltage gain A. v This is used to adjust the voltage amplitude and current flowing through the compensation capacitor, thereby changing the equivalent reactive power mapped to the grid side.

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