Flexible AC interconnection device based on non-fundamental harmonic energy and control method thereof

Abstract

The present invention relates to the field of flexible interconnection technology, and in particular to a flexible AC interconnection device based on non-fundamental harmonic energy and a control method thereof, comprising a single-phase converter module, a transformer neutral point, and a single-phase full-bridge voltage source converter module; the single-phase converter module is configured to output non-fundamental harmonic currents through a self-energy extraction strategy, and direct the non-fundamental harmonic currents to the single-phase full-bridge voltage source converter module through a closed-loop energy exchange circuit formed by the transformer neutral point; the single-phase full-bridge voltage source converter module is configured to extract active power support from the non-fundamental harmonic currents, so that the flexible AC interconnection device outputs a series compensation voltage with controllable amplitude and phase angle according to the active power support, compensates for voltage amplitude and phase angle differences between interconnected nodes, and completes flexible interconnection between different power supply areas. The present invention controls the closed-loop energy exchange of non-fundamental harmonic currents and extracts active power through the transformer neutral point, thereby achieving flexible interconnection between different power supply areas.

Description

Technical Field

[0001] The present invention relates to the field of flexible interconnection technology, and in particular to a flexible AC interconnection device based on non-fundamental harmonic energy and a control method thereof. Background Art

[0002] With the large-scale access of renewable energy, the source-load characteristics of the distribution network have shown a high degree of uncertainty and volatility. On the power supply side, a large number of distributed photovoltaic, wind power and other fluctuating power sources are decentralized and connected through medium and low voltage distribution networks; on the load side, flexible loads with temporal and spatial random characteristics such as electric vehicle charging facilities are connected to the grid on a large scale. This two-way dynamic characteristic poses unprecedented challenges to the flexible regulation and control capabilities of the distribution network, requiring the system to have stronger adaptability and intelligence.

[0003] Interconnected microgrids have become an important direction for building new distribution networks due to their flexible operation and control, efficient absorption of renewable energy, and excellent performance in matching AC and DC multi-loads. In this context, the importance of flexible distribution network interconnection devices, as key equipment for achieving flexible interconnection between different feeders, has become increasingly prominent. However, the current mainstream flexible distribution network interconnection devices mainly adopt a back-to-back multi-port converter architecture, which has some problems in engineering practice. At the topology level, traditional solutions generally adopt an AC / DC / AC multi-stage conversion structure, in which each power module requires an independent DC bus capacitor and filtering link. Taking a typical three-port back-to-back flexible DC device as an example, its complex topology not only increases the size and weight of the device, but also leads to strong electromagnetic coupling between the power units. This coupling relationship can cause harmonic interaction problems under dynamic conditions. Especially when multiple modules work together, complex decoupling control algorithms must be designed to maintain system stability, significantly increasing the difficulty of control system design. On the other hand, these devices generally rely on transformers for energy extraction, which not only increases the initial construction cost but also introduces additional operating losses and reduces overall energy efficiency.

[0004] Therefore, in view of the shortcomings of existing flexible interconnection devices, there is an increasingly urgent demand for a new flexible AC interconnection device topology that can overcome the above-mentioned defects in order to better adapt to the development needs of modern distribution networks. Summary of the Invention

[0005] In order to solve the above technical problems, the present invention provides a flexible AC interconnection device based on non-fundamental harmonic energy and a control method thereof.

[0006] In a first aspect, the present invention provides a flexible AC interconnection device based on non-fundamental harmonic energy, comprising a single-phase converter module, a transformer neutral point, and a single-phase full-bridge voltage source converter module; the AC output end of the single-phase converter module is connected to the transformer neutral point through a filter circuit to form a closed-loop energy exchange circuit for non-fundamental harmonic current; the input end of the single-phase full-bridge voltage source converter module is connected to the single-phase converter module through the transformer neutral point, and the output end of the single-phase full-bridge voltage source converter module is connected in series between transmission lines in different power supply areas;

[0007] The single-phase converter module is configured to modulate and output non-fundamental harmonic currents through a self-energy extraction strategy, and guide the non-fundamental harmonic currents to the single-phase full-bridge voltage source converter module through the closed-loop energy exchange circuit;

[0008] The neutral point of the transformer serves as a channel for energy exchange between the single-phase converter module and the single-phase full-bridge voltage source converter module, and is used to control the normal flow of non-fundamental harmonic currents between the single-phase converter module and the single-phase full-bridge voltage source converter module, and to suppress the leakage of non-fundamental harmonic currents to non-target transmission lines;

[0009] The single-phase full-bridge voltage source converter module is used to extract the required active power support from the received non-fundamental harmonic current, so that the flexible AC interconnection device outputs a series compensation voltage with controllable amplitude and phase angle according to the active power support, and compensates for the voltage amplitude and phase angle differences between interconnected nodes by injecting the series compensation voltage, thereby completing flexible interconnection between different power supply areas.

[0010] In a further embodiment, the self-energy extraction strategy is specifically:

[0011] Determining a target sinusoidal wave frequency and amplitude based on a target output characteristic of the single-phase converter module, and generating a sinusoidal wave modulation signal according to the target sinusoidal wave frequency and amplitude;

[0012] Generate a high-frequency triangular wave carrier signal according to preset carrier frequency data; wherein the frequency of the high-frequency triangular wave carrier signal is greater than the frequency of the sine wave modulation signal;

[0013] Comparing the sinusoidal wave modulation signal with the high-frequency triangular wave carrier signal point by point, and generating a complementary pulse width modulation pulse signal according to the point by point comparison result;

[0014] The complementary pulse width modulation pulse signal is used to control the on-off state of the switch of the single-phase converter module, and drive the single-phase converter module to output non-fundamental harmonic current.

[0015] In a further embodiment, the process of generating a complementary pulse width modulated pulse signal according to the point-by-point comparison result includes:

[0016] When the instantaneous value of the sine wave modulation signal is higher than the instantaneous value of the high-frequency triangular wave carrier signal, a high-level pulse width modulation pulse signal is output;

[0017] When the instantaneous value of the sine wave modulation signal is lower than the high-frequency triangular wave carrier signal, a low-level pulse width modulation pulse signal is output;

[0018] A complementary pulse width modulated pulse signal is generated according to the high level pulse width modulated pulse signal and the low level pulse width modulated pulse signal.

[0019] In a further embodiment, the single-phase converter module includes a single-phase inverter, and the single-phase inverter includes a first switching device, a single-phase DC link capacitor, and a filter circuit;

[0020] The single-phase DC side capacitor is connected in parallel to the DC side input terminal of the first switching device, and is used to stabilize the DC side voltage of the single-phase converter module and provide voltage support for the normal operation of the single-phase inverter;

[0021] The AC side output terminal of the first switching device is connected to the neutral point of the transformer through the filter circuit, and is used to control its on and off state based on the complementary pulse width modulation pulse signal;

[0022] The filter circuit is connected in series between the AC side output terminal of the first switching device and the neutral point of the transformer, and is used to filter out high-frequency switching harmonic components and output non-fundamental harmonic current.

[0023] In a further embodiment, the single-phase full-bridge voltage source converter module includes a single-phase full-bridge inverter unit, a DC side energy storage capacitor and an AC filter network, and the AC side of the single-phase full-bridge voltage source converter module is connected to the transmission line through the AC filter network; each of the single-phase full-bridge inverter units includes a full-bridge topology structure composed of multiple second switching devices;

[0024] The DC side energy storage capacitor is connected in parallel to the DC side input end of the single-phase full-bridge inverter unit to stabilize the DC bus voltage;

[0025] The AC filter network is connected in series between the AC side output terminal of the second switching device and the output line, and is used to suppress switching harmonics and output a controllable series compensation voltage.

[0026] In a further embodiment, the single-phase full-bridge voltage source converter module further includes a control unit connected to the AC filter network;

[0027] The control unit is used to decompose the series compensation voltage into a component perpendicular to the line current and a component parallel to the line current, and to achieve bidirectional independent regulation of the line active power flow and reactive power flow by collaboratively adjusting the amplitude and phase of the component perpendicular to the line current and the component parallel to the line current.

[0028] In a further embodiment, the control unit is further configured to:

[0029] Calculating the active power and reactive power injected into the AC power grid by the single-phase full-bridge voltage source converter module based on the AC bus voltage phasor, the converter port voltage of the single-phase full-bridge voltage source converter module, and the lag angle between the AC bus voltage phasor and the converter port voltage;

[0030] According to the active power and the reactive power, the magnitude and direction of the active power and reactive power exchanged between the single-phase full-bridge voltage source converter module and the AC power grid are regulated.

[0031] In a further embodiment, the process of obtaining the active power and the reactive power is specifically as follows:

[0032] Multiply the AC bus voltage phasor, the converter port voltage, and the sine value of the lag angle to obtain the intermediate value of active power;

[0033] The equivalent reactance is obtained by multiplying the sum of the commutation inductance and the converter transformer inductance by the angular frequency;

[0034] Calculating the active power injected into the AC power grid by the single-phase full-bridge voltage source converter module according to the ratio of the active power intermediate value to the equivalent reactance;

[0035] Calculating the voltage component of the converter port voltage in the direction of the AC bus voltage, and obtaining a reactive power control parameter based on the difference between the AC bus voltage phasor and the voltage component;

[0036] Multiplying the reactive power control parameter by the AC bus voltage phasor to obtain a reactive power intermediate value;

[0037] The reactive power injected into the AC power grid by the single-phase full-bridge voltage source converter module is calculated according to the ratio of the reactive power intermediate value to the equivalent reactance.

[0038] In a further embodiment, the control unit is further configured to switch the rectification-inversion mode of the single-phase full-bridge voltage source converter module, specifically:

[0039] When the hysteresis angle is a positive value, controlling the single-phase full-bridge voltage source converter module to operate in a rectification state to absorb active power;

[0040] When the hysteresis angle is a negative value, controlling the single-phase full-bridge voltage source converter module to operate in an inverter state and output active power;

[0041] When the reactive power control parameter is greater than zero, controlling the single-phase full-bridge voltage source converter module to absorb reactive power;

[0042] When the reactive power control parameter is less than zero, the single-phase full-bridge voltage source converter module is controlled to generate reactive power.

[0043] In a second aspect, the present invention provides a control method for a flexible AC interconnection device based on non-fundamental harmonic energy. The control method comprises the following steps:

[0044] The output non-fundamental harmonic current is modulated by the self-energy extraction strategy;

[0045] Utilizing the non-fundamental harmonic current to extract the required active power support, and outputting a series compensation voltage with controllable amplitude and phase angle according to the active power support;

[0046] By injecting the series compensation voltage to compensate for the voltage amplitude difference and phase angle difference between interconnected nodes, flexible interconnection between different power supply areas is achieved.

[0047] The present invention provides a flexible AC interconnection device based on non-fundamental harmonic energy and a control method thereof, wherein the device comprises a single-phase converter module, a transformer neutral point and a single-phase full-bridge voltage source converter module; the AC output end of the single-phase converter module is connected to the transformer neutral point through a filter circuit to form a closed-loop energy exchange circuit for non-fundamental harmonic current; the input end of the single-phase full-bridge voltage source converter module is connected to the single-phase converter module through the transformer neutral point, and the output end of the single-phase full-bridge voltage source converter module is connected in series between power transmission lines in different power supply areas; the single-phase converter module is used to pass The self-energy extraction strategy modulates the output of non-fundamental harmonic currents and directs them to a single-phase full-bridge voltage source converter module through a closed-loop energy exchange circuit. The transformer neutral point is used to suppress the leakage of non-fundamental harmonic currents to non-target transmission lines. The single-phase full-bridge voltage source converter module is used to extract the required active power support from the non-fundamental harmonic currents, enabling the flexible AC interconnection device to output a series compensation voltage with controllable amplitude and phase angle based on the active power support. The series compensation voltage is injected to compensate for the voltage amplitude and phase angle differences between interconnected nodes, thus achieving flexible interconnection between different power supply areas. Compared with the existing technology, the device achieves closed-loop energy exchange of non-fundamental harmonic currents and extracts active power through the coordinated operation of the single-phase converter module, the transformer neutral point, and the single-phase full-bridge voltage source converter module, achieving flexible interconnection between different power supply areas. This improves the control capability and energy utilization efficiency of the flexible AC interconnection device, reduces the structural complexity and cost of the device, and improves the stability and reliability of the device. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a structural diagram of a flexible AC interconnection device based on non-fundamental harmonic energy provided by an embodiment of the present invention;

[0049] Figure 2 This is a schematic diagram of the phase relationship of the series side control based on self-energy extraction provided by an embodiment of the present invention;

[0050] Figure 3 This is a schematic diagram of the phasor relationship of series side regulation based on transformer neutral point closed-loop energy exchange provided by an embodiment of the present invention;

[0051] Figure 4 Schematic diagram of the steady-state vector relationship of the AC side of the converter provided by an embodiment of the present invention;

[0052] Figure 5 Schematic diagram of the fundamental and non-fundamental harmonic components of the receiving-end line voltage provided by an embodiment of the present invention;

[0053] Figure 6 Schematic diagram of the fundamental and non-fundamental components of the receiving-end line current provided by an embodiment of the present invention;

[0054] Figure 7 Schematic diagram of active power of receiving-end line provided by an embodiment of the present invention;

[0055] Figure 8 It is a flow chart of a control method for a flexible AC interconnection device based on non-fundamental harmonic energy provided by an embodiment of the present invention. DETAILED DESCRIPTION

[0056] The following describes the embodiments of the present invention in detail with reference to the accompanying drawings. The embodiments are provided for illustrative purposes only and are not to be construed as limiting the present invention. The accompanying drawings are provided for reference and illustration only and do not constitute a limitation on the scope of protection of the present invention. Many changes may be made to the present invention without departing from the spirit and scope of the present invention.

[0057] refer to Figure 1 , the embodiment of the present invention provides a flexible AC interconnection device based on non-fundamental harmonic energy, such as Figure 1 As shown, the flexible AC interconnection device includes a single-phase converter module, a transformer neutral point and a single-phase full-bridge voltage source converter module; the AC output end of the single-phase converter module is connected to the transformer neutral point through a filter circuit to form a closed-loop energy exchange circuit for non-fundamental harmonic current; the input end of the single-phase full-bridge voltage source converter module is connected to the single-phase converter module through the transformer neutral point, and the output end of the single-phase full-bridge voltage source converter module is connected in series between the transmission lines of different power supply areas.

[0058] In some embodiments, the single-phase converter module includes a single-phase inverter, which includes a first switching device, a single-phase DC-side capacitor and a filter circuit, wherein the first switching device can be an IGBT (insulated gate bipolar transistor) switching device, and the filter circuit can be an LC low-pass filter. The single-phase DC-side capacitor is connected in parallel to the DC-side input end of the first switching device. The single-phase DC-side capacitor is used to stabilize the DC-side voltage of the single-phase converter module, suppress voltage fluctuations, and provide voltage support for the normal operation of the single-phase inverter; the AC-side output end of the first switching device is connected to the neutral point of the transformer through the filter circuit, and the first switching device is electrically connected to the neutral point of the transformer. The switching device is used to control its on-off state based on the complementary pulse width modulation pulse signal; the filter circuit is connected in series between the AC side output terminal of the first switching device and the neutral point of the transformer, and the filter circuit is used to filter out the high-frequency switching harmonic components output by the inverter and retain the non-fundamental harmonic current. In this embodiment, the single-phase converter module is used to modulate the output of the non-fundamental harmonic current through a self-energy extraction strategy, and provide a low-impedance path for the non-fundamental harmonic current through the closed-loop energy exchange circuit (so that the neutral point of the transformer presents a low impedance to the non-fundamental harmonic), and guide the non-fundamental harmonic current to the single-phase full-bridge voltage source converter module. In some embodiments, the self-energy extraction strategy is specifically as follows:

[0059] Determining a target sinusoidal wave frequency and amplitude based on a target output characteristic of the single-phase converter module, and generating a sinusoidal wave modulation signal according to the target sinusoidal wave frequency and amplitude;

[0060] Generate a high-frequency triangular wave carrier signal according to preset carrier frequency data; wherein the frequency of the high-frequency triangular wave carrier signal is greater than the frequency of the sine wave modulation signal;

[0061] Comparing the sinusoidal wave modulation signal with the high-frequency triangular wave carrier signal point by point, and generating a complementary pulse width modulation pulse signal according to the point by point comparison result;

[0062] The complementary pulse width modulation pulse signal is used to control the on-off state of the switch of the single-phase converter module, and drive the single-phase converter module to output non-fundamental harmonic current.

[0063] Specifically, the flexible AC interconnection device based on non-fundamental harmonic energy proposed in this embodiment is mainly composed of VSC1 (single-phase converter module) and a group of VSC2 (single-phase full-bridge voltage source converter modules). Among them, the single-phase converter module VSC1 not only maintains the stability of its own capacitor voltage, but also provides non-fundamental harmonic currents for the single-phase full-bridge voltage source converter module VSC2. The single-phase converter module VSC1 modulates and outputs non-fundamental harmonic currents through a self-energy extraction strategy, and guides these currents to the single-phase full-bridge voltage source converter module V through a closed-loop energy exchange circuit. SC2, thereby reducing the energy acquisition device and reducing the device cost. At the same time, the coordinated operation of the single-phase converter module VSC1 and the single-phase full-bridge voltage source converter module VSC2 increases the control range of the flexible AC interconnection device and improves the overall performance of the power system. For the self-energy acquisition strategy, this embodiment determines the frequency and amplitude of the target sinusoidal wave according to the power demand of the single-phase converter module VSC1 and the system harmonic suppression requirements, and uses a signal generator to generate a sinusoidal wave modulation signal based on the determined target sinusoidal wave frequency and amplitude. The frequency of the sinusoidal wave modulation signal is At the same time, this embodiment selects a set of triangular waves as high-frequency triangular wave carrier signals according to the preset carrier frequency data. The frequency of the high-frequency triangular wave carrier signal is , the frequency of the high-frequency triangle wave carrier signal Usually much higher than the frequency of the sine wave modulating signal , thus forming the modulation ratio M, which is expressed as:

[0064]

[0065] Next, this embodiment compares the sinusoidal wave modulation signal with the high-frequency triangular wave carrier signal point by point, and generates a complementary pulse width modulated pulse signal based on the point-by-point comparison result. Specifically, when the instantaneous value of the sinusoidal wave modulation signal is higher than the instantaneous value of the high-frequency triangular wave carrier signal, a high-level pulse width modulated pulse signal is output; otherwise, a low-level pulse width modulated pulse signal is output, thereby generating two sets of complementary pulse width modulated pulse signals to ensure that the upper and lower switches of the same bridge arm are not turned on at the same time, thereby avoiding a short circuit on the DC side. The generated complementary pulse width modulated pulse signal is distributed to the first switching device of the single-phase inverter, and the complementary pulse width modulated pulse signal is used to control the first switching device of the single-phase converter module. The on-off state of the component is determined, and after passing through the filtering circuit, the target waveform (i.e., non-fundamental harmonic current) is obtained. These non-fundamental harmonic currents flow into the single-phase full-bridge voltage source converter module VSC2 through the neutral point of the transformer, providing it with the active power required for the output series voltage phase angle, thereby realizing multiple control functions of the single-phase full-bridge voltage source converter module VSC2, such as power flow control, three-phase imbalance compensation, phase shifting, and harmonic control. The traditional solution requires an additional energy acquisition module to power the single-phase full-bridge voltage source converter module VSC2. However, this embodiment directly provides active power to the VSC2 through non-fundamental harmonic energy exchange, saving the energy acquisition transformer and rectifier circuit, and reducing hardware costs.

[0066] In some embodiments, the neutral point of the transformer serves as a channel for energy exchange between the single-phase converter module and the single-phase full-bridge voltage source converter module, and is used to control the normal flow of non-fundamental harmonic currents between the single-phase converter module and the single-phase full-bridge voltage source converter module, and to suppress the leakage of non-fundamental harmonic currents to non-target transmission lines.

[0067] In a specific embodiment, in order to form a loop of harmonic current and prevent it from leaking to other lines, this embodiment proposes to connect the non-fundamental harmonic current generated by the single-phase converter module VSC1 to the neutral point of the transformer, and by injecting non-fundamental harmonics at the neutral point of the transformer, it is ensured that the non-fundamental current can form a closed loop through the neutral point of the transformer. This not only simplifies the device structure, but also reduces the cost, and realizes energy exchange with the single-phase full-bridge voltage source converter module VSC2. Specifically, the single-phase converter module VSC1 is connected to the neutral point of the transformer, and by connecting to the neutral point of the transformer, the non-fundamental harmonic current can form a closed loop of non-fundamental harmonic current through the neutral point of the transformer, and in this closed loop, the non-fundamental harmonic current is guided to the single-phase full-bridge voltage source converter module VSC2 through the neutral point of the transformer, providing the single-phase full-bridge voltage source converter module VSC2 with a Active power support is provided, thereby realizing energy exchange of non-fundamental current between the single-phase converter module VSC1 and the single-phase full-bridge voltage source converter module VSC2, increasing the control range, and preventing harmonics from spreading to other lines, ensuring the effective transmission of non-fundamental harmonic currents. This embodiment not only ensures that non-fundamental harmonic currents flow smoothly between VSC1 and VSC2 through the transformer neutral point circuit, but also effectively suppresses the leakage of non-fundamental harmonic currents to non-target power lines through its special electrical characteristics, limits the propagation range of non-fundamental harmonic currents, and avoids interference with other parts of the power grid. Therefore, the flexible AC interconnection device proposed in this embodiment realizes flexible AC interconnection based on non-fundamental harmonic energy support through the organic combination of the VSC1 single-phase converter module, the transformer neutral point, and the VSC2 single-phase full-bridge voltage source converter module, thereby improving the control capability and operation efficiency of the power grid.

[0068] In some embodiments, the single-phase full-bridge voltage source converter module includes a single-phase full-bridge inverter unit, a DC side energy storage capacitor, and an AC filter network, wherein each of the single-phase full-bridge inverter units includes a full-bridge topology structure composed of multiple second switching devices, and the AC side of the single-phase full-bridge voltage source converter module is connected to the transmission line through the AC filter network; the DC side input end of the single-phase full-bridge inverter unit is connected in parallel with the DC side energy storage capacitor, and the single-phase full-bridge inverter unit is used to stabilize the DC bus voltage; the AC filter network is connected in series between the AC side output end of the second switching device and the output line, and the AC filter network is used to suppress switching harmonics and output a controllable series compensation voltage. In this embodiment, the single-phase full-bridge voltage source converter module is used to use the received non-fundamental harmonic current to extract the required active power support, so that the flexible AC interconnection device outputs a series compensation voltage with controllable amplitude and phase angle according to the active power support, and compensates for the voltage amplitude difference and phase angle difference between the interconnected nodes by injecting the series compensation voltage, thereby completing flexible interconnection between different power supply areas.

[0069] In a traditional flexible AC interconnection system, if the single-phase full-bridge voltage source converter module VSC2 stabilizes the DC side capacitor voltage by self-sampling, the line flow can only be regulated by injecting a voltage at 90° to the line current. The voltage regulation range of this regulation method is limited to one line, such as Figure 2 As shown, in Figure 2 middle, The output voltage of the single-phase full-bridge voltage source converter module VSC2 in capacitive compensation mode is perpendicular to the line current and is used to provide capacitive reactive power compensation. The output voltage of the single-phase full-bridge voltage source converter module VSC2 under inductive compensation is perpendicular to the line current and is used to provide inductive reactive power compensation. is the receiving end voltage under capacitive compensation, that is, the sending end voltage Add capacitive compensation voltage The result after is the receiving end voltage under inductive compensation, that is, the sending end voltage Add inductive compensation voltage The result after is the line sending end voltage; is the line current; is the phase angle difference between the sending end voltage and the receiving end voltage under capacitive compensation; It is the phase angle difference between the sending-end voltage and the receiving-end voltage under inductive compensation. The voltage output by the series power flow controller based on the voltage source converter is 90° to the line current. This control method has a high voltage phase angle limitation and limited power flow regulation capability. In order to overcome this limitation and better meet the needs of the flexible interconnection system, this embodiment proposes to output non-fundamental harmonic current through the single-phase converter module VSC1 to expand the adjustment range of the line voltage injected by the single-phase full-bridge voltage source converter module VSC2, thereby enhancing the power flow regulation capability of the flexible AC interconnection device.

[0070] Specifically, in this embodiment, the single-phase converter module VSC1 is configured to output non-fundamental harmonic currents through self-energy modulation, ensuring that VSC1 can obtain energy in a self-sufficient manner to maintain its stable operation. These non-fundamental harmonic currents will exchange energy between the single-phase converter module VSC1 and the single-phase full-bridge voltage source converter module VSC2 through the power line. The single-phase full-bridge voltage source converter module VSC2 monitors the non-fundamental harmonic currents in the power line and adjusts its output voltage accordingly to exchange energy with the single-phase converter module VSC1. Through the adjustment of the single-phase full-bridge voltage source converter module VSC2, the adjustment range of the line voltage injected by the single-phase full-bridge voltage source converter module VSC2 is increased, so that the flexible AC interconnection device can The output amplitude and phase angle of the equivalent voltage are controllable. The single-phase full-bridge voltage source converter module VSC2 is no longer limited to injecting a voltage at 90° to the line current, but can generate an equivalent voltage with controllable amplitude and phase angle. By using the controllable equivalent voltage output by the single-phase full-bridge voltage source converter module VSC2, the flexible AC interconnection device can compensate for the voltage amplitude difference and phase angle difference between the interconnected nodes to achieve the effect of rapid phase shift. At the same time, by adjusting the amplitude and phase angle of the output voltage of the single-phase full-bridge voltage source converter module VSC2, the flexible AC interconnection device can realize the simultaneous regulation of the active power flow and reactive power flow of the line, which improves the flexibility and stability of the system and meets the needs of the flexible interconnection system. The control phasor relationship based on non-fundamental energy exchange is as follows: Figure 3 As shown, in Figure 3 middle, is the lag angle of the converter port voltage relative to the AC bus voltage phasor; is the line current; is the receiving end voltage of the transmission line; is the voltage at the sending end of the transmission line; is the current component perpendicular to the line; is the current component parallel to the line.

[0071] Therefore, the single-phase converter module VSC1 outputs non-fundamental harmonics by self-powered modulation, effectively expanding the adjustment range of the line voltage injected by the single-phase full-bridge voltage source converter module VSC2 and enhancing the power flow regulation capability of the flexible AC interconnection device. This method not only meets the needs of the flexible interconnection system but also reduces the device cost.

[0072] The flexible AC interconnection device provided in this embodiment can change the output voltage of the single-phase full-bridge voltage source converter module VSC2 in the traditional solution from the original state of being only perpendicular to the line current to only containing the component perpendicular to the line current. and the component of the current parallel to the line In the form of two independent components, in some embodiments, the single-phase full-bridge voltage source converter module further includes a control unit connected to the AC filter network, the control unit being configured to decompose the series compensation voltage into components perpendicular to the line current. and parallel to the line current component By collaboratively adjusting the amplitude and phase of the current component perpendicular to the line and the current component parallel to the line, the bidirectional independent regulation of the active power flow and the reactive power flow of the line is completed. It should be noted that, in this embodiment, the component perpendicular to the line current and the component of the current parallel to the line The magnitude and direction of both components can be changed. This embodiment can achieve precise regulation of the amplitude and phase angle of the output voltage of the single-phase full-bridge voltage source converter module VSC2 by controlling the magnitude and direction of the two components. Specifically, by adjusting the amplitude and phase of the component perpendicular to the line current and the component parallel to the line current, independent control of active power and reactive power can be achieved.

[0073] In some embodiments, the control unit is further configured to calculate the active power and reactive power injected into the AC grid by the single-phase full-bridge voltage source converter module based on the AC bus voltage phasor, the converter port voltage of the single-phase full-bridge voltage source converter module, and the lag angle between the AC bus voltage phasor and the converter port voltage; and to regulate the magnitude and direction of the active power and reactive power exchanged between the single-phase full-bridge voltage source converter module and the AC grid according to the active power and the reactive power, wherein the process of obtaining the active power and the reactive power is specifically as follows:

[0074] Multiply the AC bus voltage phasor, the converter port voltage, and the sine value of the lag angle to obtain the intermediate value of active power;

[0075] The equivalent reactance is obtained by multiplying the sum of the commutation inductance and the converter transformer inductance by the angular frequency;

[0076] Calculating the active power injected into the AC power grid by the single-phase full-bridge voltage source converter module according to the ratio of the active power intermediate value to the equivalent reactance;

[0077] Calculating the voltage component of the converter port voltage in the direction of the AC bus voltage, and obtaining a reactive power control parameter based on the difference between the AC bus voltage phasor and the voltage component;

[0078] Multiplying the reactive power control parameter by the AC bus voltage phasor to obtain a reactive power intermediate value;

[0079] The reactive power injected into the AC power grid by the single-phase full-bridge voltage source converter module is calculated according to the ratio of the reactive power intermediate value to the equivalent reactance.

[0080] Specifically, in this embodiment, ignoring the power electronic switch loss, the active power injected into the AC grid by the single-phase full-bridge voltage source converter module VSC2 is and reactive power It can be calculated by the following formula:

[0081]

[0082] Where, is the AC bus voltage phasor; is the converter port voltage of the single-phase full-bridge voltage source converter module VSC2; is the lag angle of the converter port voltage relative to the AC bus voltage phasor, that is, the lag angle between the AC bus voltage phasor and the converter port voltage; X is the equivalent reactance of the converter. In this embodiment, the equivalent reactance of the converter is calculated as follows:

[0083]

[0084] Where, is the commutation inductance; is the inductance of the converter transformer; is the angular frequency; L is the total inductance.

[0085] It can be seen that the working mode of the single-phase full-bridge voltage source converter module VSC2 is mainly related to Related, that is, the control unit is further used to switch the rectification-inversion mode of the single-phase full-bridge voltage source converter module, specifically:

[0086] When the hysteresis angle is a positive value, controlling the single-phase full-bridge voltage source converter module to operate in a rectification state to absorb active power;

[0087] When the hysteresis angle is a negative value, controlling the single-phase full-bridge voltage source converter module to operate in an inverter state and output active power;

[0088] When the reactive power control parameter is greater than zero, controlling the single-phase full-bridge voltage source converter module to absorb reactive power;

[0089] When the reactive power control parameter is less than zero, the single-phase full-bridge voltage source converter module is controlled to generate reactive power.

[0090] In a specific embodiment, Figure 4 As shown, when When , VSC2 works in the rectification state and absorbs active power; when When VSC2 works in the inverter state, it outputs active power; while reactive power transmission mainly depends on ,when When VSC2 absorbs reactive power; when When VSC2 generates reactive power, the present embodiment can control the hysteresis angle The size of the active power exchanged between VSC2 and the grid is regulated by the size and direction of the active power. The size of the reactive power exchanged between VSC2 and the grid is regulated by the size and direction of the reactive power. Through the above regulation method, VSC2 can not only realize the independent control of active power and reactive power, but also flexibly adjust the amplitude and phase angle of the output voltage according to the needs of the grid. This regulation method greatly improves the flexibility and adaptability of VSC2, enabling it to better meet the needs of flexible interconnected systems. At the same time, by precisely controlling the component perpendicular to the line current and the component of the current parallel to the line By adjusting the size and direction of the power grid, it is possible to achieve precise control of the power grid flow and improve the stability and reliability of the power grid.

[0091] To verify the effectiveness of this embodiment, the simulation test process for the flexible AC interconnection device in this embodiment is as follows: the total simulation test run time is set to 10 seconds. 1.0 seconds after the start of the test, the parallel-side converter is started and put into operation. Subsequently, at 1.5 seconds, the VSC1 converter begins to inject non-fundamental current into the system, and VSC2 is connected to the line. At 2.5 seconds, VSC2 begins to perform power flow control tasks to ensure that the single-phase active power of the line is stable at 0.005MW. This state lasts until 5.5 seconds, at which time VSC2 adjusts the control target to make the single-phase active power of the line flip to -0.005MW. Figure 5 The data shows the changes in the fundamental and non-fundamental components of the receiving-end line voltage. Before VSC1 injected non-fundamental current into the system, the non-fundamental voltage and non-fundamental current at the receiving end of the line were 5.4e-7kV and 1.59e-5kA, respectively. 1.5 seconds after VSC1 injected non-fundamental current (about 0.74kA), the non-fundamental voltage at the receiving end of the line rose to 0.0156kV. When the system reached steady state again, the non-fundamental content of the receiving-end line voltage dropped to 0.007kV.

[0092] Figure 6 The waveforms of the fundamental and non-fundamental harmonic components of the receiving-end line current are shown. At the 1.5-second moment when VSC1 injects the non-fundamental current (about 0.74kA), the non-fundamental current at the receiving end of the line is 0.218kA. After the system reaches steady state again, the non-fundamental current content of the receiving-end line decreases to 0.008kA. Figure 7The active power waveform of the receiving-end line is shown. At 2.5 seconds and 5.5 seconds when VSC2 is performing power flow control, the system's active power reaches the command values ​​of 0.015MW and -0.015MW, respectively. Compared with the existing technology, the flexible AC interconnection device topology based on non-fundamental harmonic energy support proposed in this embodiment can simultaneously achieve multiple functions such as power flow control, reactive power compensation, harmonic suppression, and three-phase imbalance compensation according to the needs of grid control, while reducing the structural complexity and cost of the device. Secondly, this embodiment provides active power support through the non-fundamental current modulated by the VSC1 single-phase converter, realizing energy exchange between the VSC2 and VSC1 modules. After the device is flexibly closed, the flexible AC interconnection device can flexibly respond to grid control needs and perform multiple control functions. Finally, this embodiment injects non-fundamental harmonics into the neutral point of the transformer to ensure that the non-fundamental current can form a loop through the neutral point of the transformer, effectively preventing it from leaking to other lines, thereby improving the stability and safety of the system.

[0093] In summary, in the flexible AC interconnection device proposed in this embodiment, the VSC1 converter can not only perform reactive compensation for the system, but also generate harmonics. These harmonics form a closed loop through the neutral point of the transformer, thereby providing active power support for the series side. At the same time, the VSC2 converter achieves flexible interconnection between different power supply areas by precisely controlling its output voltage. While avoiding the need to set up additional energy acquisition modules, it significantly expands the control range of the flexible AC interconnection device. After the device successfully completes the flexible interconnection, this embodiment can also flexibly perform various control functions such as power flow control, reactive compensation, harmonic suppression, and three-phase imbalance compensation according to the actual control needs of the power grid. This not only improves the stability and efficiency of the power grid, but also provides strong technical support for the future construction of smart grids.

[0094] An embodiment of the present invention provides a flexible AC interconnection device based on non-fundamental harmonic energy, the device comprising a single-phase converter module, a transformer neutral point and a single-phase full-bridge voltage source converter module; the AC output end of the single-phase converter module is connected to the transformer neutral point through a filter circuit to form a closed-loop energy exchange circuit for non-fundamental harmonic current; the input end of the single-phase full-bridge voltage source converter module is connected to the single-phase converter module through the transformer neutral point, and the output end of the single-phase full-bridge voltage source converter module is connected in series between transmission lines in different power supply areas; the single-phase converter module is used to The self-energy extraction strategy modulates the output of non-fundamental harmonic currents and directs them to a single-phase full-bridge voltage source converter module through a closed-loop energy exchange circuit. The transformer neutral point is used to suppress the leakage of non-fundamental harmonic currents to non-target transmission lines. The single-phase full-bridge voltage source converter module is used to extract the required active power support from the non-fundamental harmonic currents, enabling the flexible AC interconnection device to output a series compensation voltage with controllable amplitude and phase angle based on the active power support. The series compensation voltage is injected to compensate for the voltage amplitude and phase angle differences between interconnected nodes, thus achieving flexible interconnection between different power supply areas. Compared with the existing technology, the device achieves closed-loop energy exchange of non-fundamental harmonic currents and extracts active power through the coordinated operation of the single-phase converter module, the transformer neutral point, and the single-phase full-bridge voltage source converter module, achieving flexible interconnection between different power supply areas. This improves the control capability and energy utilization efficiency of the flexible AC interconnection device, reduces the structural complexity and cost of the device, and improves the stability and reliability of the device.

[0095] In one embodiment, Figure 8 As shown, an embodiment of the present invention provides a control method for a flexible AC interconnection device based on non-fundamental harmonic energy. Applying the flexible AC interconnection device based on non-fundamental harmonic energy as described above, the control method includes the following steps:

[0096] S1. Modulate the output of non-fundamental harmonic current through self-energy extraction strategy.

[0097] S2. Utilize the non-fundamental harmonic current to extract the required active power support, and output a series compensation voltage with controllable amplitude and phase angle according to the active power support.

[0098] S3. By injecting the series compensation voltage to compensate for the voltage amplitude difference and phase angle difference between the interconnected nodes, flexible interconnection between different power supply areas is completed.

[0099] It should be noted that the size of the serial numbers of the above-mentioned processes does not mean the order of execution. The execution order of each process should be determined by its function and internal logic, and should not constitute any limitation on the implementation process of the embodiment of this application.

[0100] For the specific definition of a control method for a flexible AC interconnection device based on non-fundamental harmonic energy, please refer to the above-mentioned definition of a flexible AC interconnection device based on non-fundamental harmonic energy, which will not be repeated here. A person of ordinary skill in the art will appreciate that the various modules and steps described in conjunction with the embodiments disclosed in this application can be implemented in hardware, software, or a combination of both. Whether these functions are performed in hardware or software depends on the specific application and design constraints of the technical solution. Professional and technical personnel can use different methods to implement the described functions for each specific application, but such implementation should not be considered to be beyond the scope of this application.

[0101] An embodiment of the present invention provides a control method for a flexible AC interconnection device based on non-fundamental harmonic energy. The control method includes modulating the output of non-fundamental harmonic currents through a self-energy extraction strategy; extracting the required active power support from the non-fundamental harmonic currents, and outputting a series compensation voltage with controllable amplitude and phase angle based on the active power support; and compensating for voltage amplitude and phase angle differences between interconnected nodes by injecting the series compensation voltage, thereby achieving flexible interconnection between different power supply areas. Compared with existing technologies, this control method controls the closed-loop energy exchange of non-fundamental harmonic currents and extracts active power through the transformer neutral point, achieving flexible interconnection between different power supply areas.

[0102] The above-described embodiments merely represent several preferred implementations of the present application. While the descriptions are relatively specific and detailed, they should not be construed as limiting the scope of the patent. It should be noted that a person skilled in the art could make several improvements and substitutions without departing from the technical principles of the present invention, and these improvements and substitutions should also be considered within the scope of protection of the present application. Therefore, the scope of protection of the present patent application shall be based on the scope of protection of the claims.

Claims

1. A flexible AC interconnection device based on non-fundamental harmonic energy, characterized by: The invention comprises a single-phase converter module, a transformer neutral point, and a single-phase full-bridge voltage source converter module; the AC output end of the single-phase converter module is connected to the transformer neutral point through a filter circuit to form a closed-loop energy exchange circuit for non-fundamental harmonic currents; the input end of the single-phase full-bridge voltage source converter module is connected to the single-phase converter module through the transformer neutral point, and the output end of the single-phase full-bridge voltage source converter module is connected in series between transmission lines in different power supply areas. The single-phase full-bridge voltage source converter module also includes a control unit connected to the AC filter network; The single-phase converter module is used to modulate and output non-fundamental harmonic currents through a self-energy extraction strategy and maintain DC side voltage stability, and guide the non-fundamental harmonic currents to the single-phase full-bridge voltage source converter module through the closed-loop energy exchange circuit; The neutral point of the transformer serves as a channel for energy exchange between the single-phase converter module and the single-phase full-bridge voltage source converter module, forming a closed loop connecting only the single-phase converter module and the single-phase full-bridge voltage source converter module, and is used to control the normal flow of non-fundamental harmonic currents between the single-phase converter module and the single-phase full-bridge voltage source converter module, and suppress the leakage of non-fundamental harmonic currents to non-target transmission lines; The single-phase full-bridge voltage source converter module is used to extract the required active power support using the received non-fundamental harmonic current, so that the flexible AC interconnection device outputs a series compensation voltage with controllable amplitude and phase angle according to the active power support, decomposes the series compensation voltage into a component perpendicular to the line current and a component parallel to the line current, and completes bidirectional independent regulation of the active power flow and reactive power flow of the line by collaboratively adjusting the amplitude and phase of the component perpendicular to the line current and the component parallel to the line current. The flexible interconnection between different power supply areas is completed by injecting the series compensation voltage to compensate for the voltage amplitude difference and phase angle difference between the interconnected nodes.

2. A flexible AC interconnection device based on non-fundamental harmonic energy according to claim 1, characterized in that: The self-energy acquisition strategy is specifically as follows: Determining a target sinusoidal wave frequency and amplitude based on a target output characteristic of the single-phase converter module, and generating a sinusoidal wave modulation signal according to the target sinusoidal wave frequency and amplitude; Generate a high-frequency triangular wave carrier signal according to preset carrier frequency data; wherein the frequency of the high-frequency triangular wave carrier signal is greater than the frequency of the sine wave modulation signal; Comparing the sinusoidal wave modulation signal with the high-frequency triangular wave carrier signal point by point, and generating a complementary pulse width modulation pulse signal according to the point by point comparison result; The complementary pulse width modulation pulse signal is used to control the on-off state of the switch of the single-phase converter module, and drive the single-phase converter module to output non-fundamental harmonic current.

3. A flexible AC interconnection device based on non-fundamental harmonic energy according to claim 2, characterized in that: The process of generating a complementary pulse width modulation pulse signal according to the point-by-point comparison result includes: When the instantaneous value of the sine wave modulation signal is higher than the instantaneous value of the high-frequency triangular wave carrier signal, a high-level pulse width modulation pulse signal is output; When the instantaneous value of the sine wave modulation signal is lower than the high-frequency triangular wave carrier signal, a low-level pulse width modulation pulse signal is output; A complementary pulse width modulated pulse signal is generated according to the high level pulse width modulated pulse signal and the low level pulse width modulated pulse signal.

4. A flexible AC interconnection device based on non-fundamental harmonic energy according to claim 2, characterized in that: The single-phase converter module includes a single-phase inverter, and the single-phase inverter includes a first switching device, a single-phase DC side capacitor and a filter circuit; The single-phase DC side capacitor is connected in parallel to the DC side input terminal of the first switching device, and is used to stabilize the DC side voltage of the single-phase converter module and provide voltage support for the normal operation of the single-phase inverter; The AC side output terminal of the first switching device is connected to the neutral point of the transformer through the filter circuit, and is used to control its on and off state based on the complementary pulse width modulation pulse signal; The filter circuit is connected in series between the AC side output terminal of the first switching device and the neutral point of the transformer, and is used to filter out high-frequency switching harmonic components and output non-fundamental harmonic current.

5. The flexible AC interconnection device based on non-fundamental harmonic energy according to claim 1, characterized in that: The single-phase full-bridge voltage source converter module includes a single-phase full-bridge inverter unit, a DC side energy storage capacitor and an AC filter network. The AC side of the single-phase full-bridge voltage source converter module is connected to the transmission line through the AC filter network. Each of the single-phase full-bridge inverter units includes a full-bridge topology structure composed of a plurality of second switching devices. The DC side energy storage capacitor is connected in parallel to the DC side input end of the single-phase full-bridge inverter unit to stabilize the DC bus voltage; The AC filter network is connected in series between the AC side output terminal of the second switching device and the output line, and is used to suppress switching harmonics and output a controllable series compensation voltage.

6. A flexible AC interconnection device based on non-fundamental harmonic energy according to claim 5, characterized in that: The control unit is further configured to: Calculating the active power and reactive power injected into the AC power grid by the single-phase full-bridge voltage source converter module based on the AC bus voltage phasor, the converter port voltage of the single-phase full-bridge voltage source converter module, and the lag angle between the AC bus voltage phasor and the converter port voltage; According to the active power and the reactive power, the magnitude and direction of the active power and reactive power exchanged between the single-phase full-bridge voltage source converter module and the AC power grid are regulated.

7. A flexible AC interconnection device based on non-fundamental harmonic energy according to claim 6, characterized in that: The process of obtaining the active power and the reactive power is specifically as follows: Multiply the AC bus voltage phasor, the converter port voltage, and the sine value of the lag angle to obtain the intermediate value of active power; The equivalent reactance is obtained by multiplying the sum of the commutation inductance and the converter transformer inductance by the angular frequency; Calculating the active power injected into the AC power grid by the single-phase full-bridge voltage source converter module according to the ratio of the active power intermediate value to the equivalent reactance; Calculating the voltage component of the converter port voltage in the direction of the AC bus voltage, and obtaining a reactive power control parameter based on the difference between the AC bus voltage phasor and the voltage component; Multiplying the reactive power control parameter by the AC bus voltage phasor to obtain a reactive power intermediate value; The reactive power injected into the AC power grid by the single-phase full-bridge voltage source converter module is calculated according to the ratio of the reactive power intermediate value to the equivalent reactance.

8. The flexible AC interconnection device based on non-fundamental harmonic energy according to claim 7, characterized in that: The control unit is further configured to switch the rectification-inversion mode of the single-phase full-bridge voltage source converter module, specifically: When the hysteresis angle is a positive value, controlling the single-phase full-bridge voltage source converter module to operate in a rectification state to absorb active power; When the hysteresis angle is a negative value, controlling the single-phase full-bridge voltage source converter module to operate in an inverter state and output active power; When the reactive power control parameter is greater than zero, controlling the single-phase full-bridge voltage source converter module to absorb reactive power; When the reactive power control parameter is less than zero, the single-phase full-bridge voltage source converter module is controlled to generate reactive power.

9. A control method for a flexible AC interconnection device based on non-fundamental harmonic energy, characterized in that: Applying the flexible AC interconnection device based on non-fundamental harmonic energy according to any one of claims 1 to 8, the control method comprises the following steps: The output non-fundamental harmonic current is modulated by the self-energy extraction strategy; Utilizing the non-fundamental harmonic current to extract the required active power support, and outputting a series compensation voltage with controllable amplitude and phase angle according to the active power support; By injecting the series compensation voltage to compensate for the voltage amplitude difference and phase angle difference between interconnected nodes, flexible interconnection between different power supply areas is achieved.

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