A series flexible switching converter supporting full autonomous control of microgrid and control method

Abstract

This invention discloses a series-type flexible switching converter and its control method that support fully autonomous control of a microgrid. The two ends of the series-type flexible switching converter are connected to a first power grid and a second power grid. The proposed control method enables the device to pre-charge and start automatically without relying on energy storage or additional control circuits; it can achieve asynchronous networking without directly controlling each distributed power source in the grid in real time, and can softly switch to direct networking via a bypass switch after pre-synchronization to reduce losses; it can transmit active power to the grid that needs to be supported when the grid needs emergency power support. The above functions only require controlling the series-type flexible switching converter rather than each distributed power source in the microgrid, thereby supporting fully autonomous control of the microgrid. Moreover, the device cost and operating losses are significantly reduced compared to flexible switching converters based on back-to-back converters.

Description

Technical Field

[0001] This invention belongs to the field of AC microgrid control, specifically relating to a series-type flexible switching converter and control method that supports fully autonomous control of microgrids. Background Technology

[0002] A microgrid is a small-scale power system that can integrate both renewable and traditional power generation, improving power supply reliability while avoiding losses associated with long-distance power transmission. A microgrid typically contains numerous distributed power sources, most of which are connected to the microgrid bus via power electronic converter interfaces, such as inverters. To ensure the normal and efficient operation of a microgrid, it is necessary to coordinate and control the converters within the microgrid, and for the microgrid as a whole to interact effectively with the main power grid, cooperating to fully leverage its advantages. The main power grid refers to the traditional power grid infrastructure. If each key device in a microgrid achieves autonomous control based solely on detecting its own main circuit signals, without relying on communication, this is called fully autonomous control of the microgrid. If fully autonomous control of a microgrid can be achieved in both of these aspects, it will greatly improve system reliability while enabling plug-and-play functionality, significantly simplifying equipment installation, operation, and maintenance.

[0003] The geographically dispersed distributed power sources within a microgrid often rely on communication to achieve coordinated control objectives, resulting in high costs and low reliability. This is especially true when there are numerous distributed power sources and the microgrid needs to interact with the grid as a whole; existing methods relying on direct communication between the central controller and the distributed power sources are insufficient for achieving fully autonomous control of the microgrid.

[0004] Regarding coordinated control strategies among converters within a microgrid, the fully autonomous control method, specifically vertical control, has garnered significant attention. This approach can essentially guarantee the rational power allocation and coordinated operation among parallel distributed generation sources without relying on communication. In research on the interaction between a microgrid as a whole and the main grid, the interface equipment and control strategies between the grids are core technologies. Early interface equipment was primarily grid-connected switches, with simple functions limited to on / off control. With the opening and closing of these switches, the microgrid switches between islanded and grid-connected modes. The safe operation of the microgrid depends on the smoothness of the transition between these two modes, and the microgrid has different requirements for the control characteristics and objectives of distributed generation sources in each mode. After transitioning from grid-connected to islanded, distributed generation sources in the microgrid need voltage support capabilities; before transitioning from islanded to grid-connected, it is necessary to coordinate distributed generation sources to achieve voltage pre-synchronization between the microgrid bus and the main grid connection point; and after grid connection, continuous regulation of grid-connected power depends on the scheduling of each distributed generation source. When the interface equipment is a grid-connected switch, the microgrid and the main grid can interact effectively. This generally relies on the direct control of distributed power sources, adjusting their control strategies and objectives to adapt to the needs of both grid-connected and islanded modes.

[0005] Therefore, in terms of effective interaction between microgrids and the main grid, the core issue of achieving fully autonomous control of microgrids can be summarized as: how to achieve power transmission between the microgrid and the main grid according to instructions from the higher level or its own needs, while avoiding direct real-time control of each distributed power source within the microgrid, and expecting that the power transmission process has the characteristics of continuous adjustability, low loss and high efficiency.

[0006] When the interface equipment is a grid-connected switch, the prerequisite for power transmission between the actual power grids is the closing of the grid-connected switch. To prevent the impact caused by the direct interconnection of two AC power grids with different voltage amplitudes and phase angles, pre-synchronization is required. The passive pre-synchronization method closes the switch after detecting that the phase angle difference between the two ends is zero, which is time-consuming when the frequency difference is small. The active pre-synchronization method requires high-bandwidth communication with each distributed power source in the microgrid to synchronously adjust the output voltage, which is unacceptable in terms of system cost and reliability when there are many power sources. In recent years, based on existing power grid interconnection equipment such as smart transformers, power electronic transformers, frequency converters, and power routers, researchers have proposed using flexible switching converters with power electronic equipment as the core as the interface equipment between the microgrid and the power grid to achieve fully autonomous control of the microgrid. The proposed general-purpose flexible switching converter consists of a voltage source back-to-back converter and a controllable parallel switch, combining the advantages of high controllability of power electronic converters and low loss of parallel switches. Power electronic converters can achieve continuous and adjustable power between power grids. When it is desired to reduce grid interconnection losses, pre-synchronization can be achieved by adjusting the power exchange between power grids, thereby switching to a switching grid interconnection mode. However, there are problems such as high power flow losses in two-stage converters, a large number of semiconductor switching devices in the equipment, and high cost. Summary of the Invention

[0007] To overcome the shortcomings of the existing technology, the present invention aims to provide a series-type flexible switching converter and control method that supports fully autonomous control of microgrids. On the one hand, it can achieve pre-charge self-starting control without relying on additional energy storage units. On the other hand, it can achieve asynchronous networking without directly controlling each distributed power source in the microgrid in real time, and can soft switch to direct networking through a bypass switch to reduce losses as needed. At the same time, when the grid needs emergency power support, active power can be transferred from another grid to the grid that needs to be supported through the series-type flexible switching converter. The device cost and operating losses are significantly reduced compared with general-purpose flexible switching converters based on back-to-back converters.

[0008] This invention is achieved through the following technical solution:

[0009] A series-type flexible switching converter supporting fully autonomous control of a microgrid includes a main circuit and a control circuit for controlling the operation of the main circuit. The main circuit includes a pre-charge switch SW1, a phase selection switch SW3, a bypass switch SW4, a line cutout CUT, and a power electronic converter for converting DC to AC. The line cutout CUT and the bypass switch SW4 are connected in parallel. The AC side of the power electronic converter is connected to the line cutout CUT. One end of the parallel structure of the line cutout CUT and the bypass switch SW4 is connected in parallel with the pre-charge switch SW1 and then in series with the phase selection switch SW3.

[0010] DC side capacitor C of power electronic converter dc A buffer resistor R is connected in series with either the positive or negative terminal. dc DC side capacitor C dc On the positive or negative terminal of the buffer resistor R dc A bypass switch SW2 with a buffer resistor connected in parallel at both ends; the number of phases of the pre-charge switch SW1, the phase selection switch SW3, the bypass switch SW4, and the power electronic converter is the same as the number of phases of the power grid.

[0011] Preferably, a series transformer T is provided between the power electronic converter and the line cut-out CUT. s Series transformer T s The two ends of the primary side are connected to the line cut-out CUT, and the series transformer T is connected. s The secondary side is connected to the AC side of the power electronic converter.

[0012] Preferably, the bypass switch SW4 is connected in series with a series reactor X. ins .

[0013] Preferably, the power electronic converter has an energy storage and additional control circuit on its DC side.

[0014] This invention provides a control method for a series-type flexible switching converter that supports fully autonomous microgrid control as described above. This control method is used to control the pre-charge self-starting of the series-type flexible switching converter supporting fully autonomous microgrid control. The control method is implemented through the control of the main circuit by a control circuit, and includes the following processes:

[0015] Before the pre-charge self-start process starts, the pre-charge switch SW1, buffer resistor bypass switch SW2, phase selection switch SW3 and bypass switch SW4 are all in the open state.

[0016] When the power grid is a single-phase grid, one end of the pre-charge switch SW1 is connected to the phase line of the grid, and the other end is connected to the neutral line. Upon receiving the pre-charge command, the pre-charge switch SW1 closes, and all power switches in the power electronic converter are locked out. The grid voltage charges the DC-side capacitor through the uncontrolled rectifier. After the peak inrush current of the charging phase disappears, the buffer resistor bypass switch SW2 is closed, and the buffer resistor R... dc bypass;

[0017] When the power grid is a three-phase grid, the three terminals of the pre-charge switch SW1 at one end are connected to the three phases of the grid, and the three terminals at the other end are electrically connected together. Upon receiving the pre-charge command, the pre-charge switch SW1 closes, short-circuiting the three-phase terminals connected to the grid at one end. All power switches in the power electronic converter are then locked out. The grid voltage charges the DC-side capacitor through the uncontrolled rectifier. After the peak inrush current of the pre-charge operation disappears, the buffer resistor bypass switch SW2 is closed, and the buffer resistor R... dc bypass;

[0018] The control circuit starts working by drawing power from the DC-side capacitor, and then controls the power electronic converter to lock the voltage of its own AC port in phase-locked mode. It operates in a unity power factor PWM rectifier mode, absorbs active power from the grid, and adjusts the DC-side capacitor voltage to the reference value.

[0019] This invention also provides another control method for the series-type flexible switching converter that supports fully autonomous control of microgrids. This control method is used to control the pre-synchronization between the first grid and the second grid, wherein the phase selection switch SW3 is connected to the first grid, and the other end of the parallel structure of the line cut-out CUT and the bypass switch SW4 is connected to the second grid. This control method is implemented by controlling the main circuit through a control circuit, and the control method includes the following process:

[0020] When the DC side of the power electronic converter has energy storage and additional control circuits, the reactive power output of its AC port can be directly controlled by the power electronic converter to regulate the active power transmitted between the first grid and the second grid, so that the phase angle of the voltage of the first grid and the phase angle of the voltage of the second grid are pre-synchronized. Then the bypass switch SW4 is closed, and the pre-synchronization control between the first grid and the second grid is completed.

[0021] When the DC side of the power electronic converter does not have energy storage and additional control circuits, the series-type flexible switching converter supporting full autonomous control of the microgrid is pre-charged and self-started according to the above self-starting method; then, the reactive power output of the AC port of the power electronic converter is controlled to adjust the active power transmitted between the first grid and the second grid, so that the phase angle of the voltage of the first grid and the phase angle of the voltage of the second grid are pre-synchronized, and then the bypass switch SW4 is closed, and the pre-synchronization control between the first grid and the second grid is completed.

[0022] Preferably, the above control method specifically includes the following steps:

[0023] Step 1) Start the series-type flexible switching converter that supports fully autonomous control of the microgrid, control the DC side capacitor voltage to the reference value, and disconnect the pre-charge switch SW1 after receiving the pre-synchronization command.

[0024] Step 2), select the closing phase sequence of phase selection switch SW3; phase selection switch SW3 only needs to select the closing phase sequence when the first power grid and the second power grid are three-phase power grids; the control circuit selects the closing phase sequence of SW3 according to the relative relationship between the voltage phasors of the first power grid and the voltage phasors of the second power grid; the selection criterion for the closing phase sequence is that after phase selection switch SW3 is closed under the selected phase sequence, the phase angle difference of the power grid voltage phasors on both sides of the line cut CUT is the smallest; when both the first power grid and the second power grid are single-phase power grids, phase selection switch SW3 does not need to be performed, and phase selection switch SW3 can be closed directly;

[0025] Step 3): The control circuit enables the power electronic converter to operate in voltage control mode. In the case of a single-phase power grid, the instantaneous voltage difference between the first and second power grids is sampled and calculated as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed. Then, the phase selection switch SW3 is closed to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid.

[0026] In a three-phase power grid, in voltage control mode, the instantaneous voltage difference between the first and second power grids is sampled and calculated based on the phase sequence selected in step 2) as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed under the selected phase sequence. Then, the phase selection switch SW3 is closed according to the selected phase sequence to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid.

[0027] Step 4): After the phase selection switch SW3 is closed, the power electronic converter begins to transition from voltage control mode to current control mode. After the transition is completed, the power electronic converter operates in current control mode.

[0028] Step 5): After the power electronic converter enters the current control mode, the difference between the DC voltage reference value and the actual DC voltage value of the power electronic converter is sent to the DC voltage controller of the control circuit to obtain the active current command; the phase angle difference between the voltages at both ends of the parallel structure of the line cut-out CUT and the bypass switch SW4 is processed by the phase angle jump and then sent to the phase angle difference controller of the control circuit to obtain the reactive current command of the power electronic converter.

[0029] Step 6): Based on the voltage command obtained in Step 3) or the current command obtained in Step 5), the control circuit combines the actual voltage and current, uses voltage closed-loop control or current closed-loop control to generate a modulation wave, and then generates a drive signal through pulse width modulation. The drive circuit of each power switch in the power electronic converter controls the power switch to turn on or off according to the obtained drive signal.

[0030] Step 7), repeat steps 5) to 6) to make the frequency and phase angle of the voltage at both ends of the parallel structure of the line cut-out CUT and the bypass switch SW4 approach synchronization. When the frequency difference and phase angle difference of the voltage at both ends of the parallel structure of the line cut-out CUT and the bypass switch SW4 are both less than their respective thresholds, switch to connect the first power grid to the second power grid through the bypass switch SW4 branch.

[0031] Preferably, the active current command and reactive current command of the power electronic converter are as follows:

[0032]

[0033] in, and These are the active current command and reactive current command for the power electronic converter, G p and G q These represent the DC voltage controller and the phase angle difference controller, respectively. dc_ref and u dcThese are the reference DC voltage value and the actual DC voltage value of the power electronic converter, respectively. θ1 and θ2 are the phase angles of the fundamental voltage components of the first power grid and the second power grid, respectively.

[0034] Preferably, the first power grid and the second power grid have the same voltage level, or have different voltage levels but are converted to the same voltage level after passing through an additional step-up or step-down transformer;

[0035] The first power grid is a three-phase or single-phase AC microgrid, and the second power grid is a three-phase or single-phase large power grid;

[0036] Alternatively, the first power grid may be a three-phase or single-phase large power grid, and the second power grid may be a three-phase or single-phase AC microgrid;

[0037] Alternatively, the first power grid and the second power grid can be two three-phase or single-phase AC microgrids;

[0038] At least a portion of the distributed power sources in the microgrid adopts a grid-type control with active power-frequency droop.

[0039] This invention also provides another control method for the series-type flexible switching converter that supports fully autonomous control of microgrids. This control method is used to realize emergency power support for the power grid. The phase selection switch SW3 is connected to the first power grid, and the other end of the parallel structure of the line cut-out CUT and bypass switch SW4 is connected to the second power grid. This control method is implemented by controlling the main circuit through a control circuit. The control method includes the following process:

[0040] When the DC side of the power electronic converter has energy storage and additional control circuits, the reactive power output from its AC port is controlled by the power electronic converter to regulate the active power transmitted between the first and second power grids, thus providing emergency power support to a certain power grid. When the DC side of the power electronic converter does not have energy storage and additional control circuits, the series-type flexible switching converter supporting full autonomous control of the microgrid is pre-charged and self-started according to the above self-starting method. The reactive power output from its AC port is controlled by the power electronic converter to regulate the active power transmitted between the first and second power grids, thus providing emergency power support to a certain power grid.

[0041] The control method includes the following steps:

[0042] Step 1), start the series-type flexible switching converter that supports fully autonomous control of the microgrid, and control the DC side capacitor voltage to the reference value;

[0043] Step 2), detect the frequencies of the first and second power grids, determine whether the first or second power grid needs emergency power support based on the set power grid frequency threshold, and identify the power grid that needs to be supported; or identify the power grid that needs to be supported based on the instructions from the superior; after identifying the power grid that needs to be supported, disconnect the pre-charge switch SW1;

[0044] Step 3) Select the closing phase sequence of phase selection switch SW3; phase selection switch SW3 only needs to select the closing phase sequence when the first power grid and the second power grid are three-phase power grids; the control circuit selects the closing phase sequence of SW3 according to the relative relationship between the voltage phasors of the first power grid and the voltage phasors of the second power grid; the selection criterion for the closing phase sequence is that after phase selection switch SW3 is closed under the selected phase sequence, the phase angle difference of the power grid voltage phasors on both sides of the line cut CUT is the smallest; when both the first power grid and the second power grid are single-phase power grids, phase selection switch SW3 does not need to perform phase selection, and phase selection switch SW3 can be directly closed in subsequent steps;

[0045] Step 4): The control circuit enables the power electronic converter to operate in voltage control mode. In the case of a single-phase power grid, the instantaneous voltage difference between the first and second power grids is sampled and calculated as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed. Then, the phase selection switch SW3 is closed to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid.

[0046] In a three-phase power grid, in voltage control mode, the instantaneous voltage difference between the first and second power grids is sampled and calculated based on the phase sequence selected in step 3) as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed under the selected phase sequence. Then, the phase selection switch SW3 is closed according to the selected phase sequence to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid.

[0047] Step 5): After the phase selection switch SW3 is closed, the power electronic converter begins to transition from voltage control mode to current control mode. After the transition is complete, the power electronic converter operates in current control mode. Step 6): After the power electronic converter enters current control mode, the difference between the DC voltage reference value and the actual DC voltage value of the power electronic converter is sent to the DC voltage controller of the control circuit to obtain the active current command of the power electronic converter; the difference between the frequency reference value and the actual frequency value of the grid to be supported is sent to the emergency power support controller in the control circuit to obtain the reactive current command of the power electronic converter.

[0048] The active current command and reactive current command of the power electronic converter are as follows:

[0049]

[0050] in, and These are the active current command and reactive current command for the power electronic converter, G p and G f These represent the DC voltage controller and the emergency power support controller, respectively. dc_ref and u dc These are the reference DC voltage value and the actual DC voltage value of the power electronic converter, respectively, ω ref and ω 12 These are the frequency reference value and the actual frequency value of the power grid to be supported, respectively.

[0051] Step 7): Based on the current command obtained in Step 6), the control circuit uses current closed-loop control to generate a modulation wave, and then generates a drive signal through pulse width modulation. The drive circuit of each power switch in the power electronic converter controls the power switch to turn on or off according to the obtained drive signal; thereby realizing the transmission of active power to the grid that needs to be supported, and realizing emergency power support for the grid.

[0052] Compared with the prior art, the present invention has the following beneficial technical effects:

[0053] This invention relates to a series-type flexible switching converter and its control method that support fully autonomous control of microgrids. It aims to solve the problem of achieving fully autonomous control of microgrids when connected to or interconnected with a large power grid. Compared to existing general-purpose flexible switching converters based on back-to-back converters, it further reduces costs and power losses. It achieves pre-charge self-starting control of the series-type flexible switching converter without requiring additional energy storage devices. Compared to connecting to the grid via static switches or other interface converters, this invention not only enables asynchronous grid connection operation in real time, ensuring a certain power exchange between the two grids, but also allows for soft switching to direct grid connection operation via power flow control when long-term grid connection operation is required, reducing overall losses. Furthermore, when the grid needs emergency power support, active power can be transferred from another grid to the grid requiring support via the series-type flexible switching converter. The entire operation of this invention only requires control of the series-type flexible switching converter, avoiding direct real-time control of each distributed power source within the AC microgrid connected to the series-type flexible switching converter, thus supporting fully autonomous control of the microgrid and showing promising engineering application prospects. Attached Figure Description

[0054] 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 some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0055] Figure 1 A schematic diagram of the system structure of the series-type flexible switching converter provided by the present invention;

[0056] Figure 2 This is a typical topology diagram of a series-type flexible switching converter in an embodiment of the present invention;

[0057] Figure 3 This is a detailed structural diagram of the voltage and current phasors and the phase selection switch SW3 of the system containing a series flexible switching converter in an embodiment of the present invention.

[0058] Figure 4 This is a block diagram of the control system for a series-type flexible switching converter in an embodiment of the present invention, wherein, Figure 4 (a) is a block diagram of the phase-locked loop and voltage-current Parker conversion control of the series flexible switching converter of the present invention; Figure 4 (b) is a block diagram of the DC voltage control of the series-type flexible switching converter of the present invention; Figure 4 (c) is a pre-synchronization control block diagram of the series-type flexible switching converter of the present invention; Figure 4 (d) is the control mode switching logic diagram of the series flexible switching converter of the present invention; Figure 4 (e) is a block diagram of the emergency power support control of the series flexible switching converter of the present invention; Figure 4 (f) is a block diagram of the voltage and current control loop of the series-type flexible switching converter of the present invention;

[0059] Figure 5 This is a timing diagram of the control signals and switching states of the series-type flexible switching converter in an embodiment of the present invention;

[0060] Figure 6 The above are waveforms of relevant electrical quantities from the simulation results of the series-type flexible switching converter completing pre-charge self-start and subsequent phase angle pre-synchronization between the microgrid and the main grid in an embodiment of the present invention. Figure 6 (a) is a waveform diagram of the active power output of the series flexible switching converter and the microgrid; Figure 6 (b) is a series transformer T s Waveform diagram of the phase angle difference of the grid voltage at both ends of the primary side; Figure 6 (c) is a waveform diagram of reactive power output from a series-type flexible switching converter and a microgrid; Figure 6(d) shows the reference DC voltage value and the actual DC voltage waveform of the power electronic converter; Figure 6 (e) is a series transformer T s Voltage waveform on the secondary side; Figure 6 (f) shows the current waveform on the bypass switch SW4; Figure 6 (g) and Figure 6 (h) represents different time periods Figure 6 (e) is an enlarged view of the voltage waveform; Figure 6 (i) is Figure 6 (f) is an enlarged view of the current waveform.

[0061] Figure 7 The above are waveforms of relevant electrical quantities from the simulation results of the series-type flexible switching converter providing emergency power support for the microgrid in this embodiment of the invention. Figure 7 (a) is a waveform diagram of the active power output of the series flexible switching converter and the microgrid; Figure 7 (b) Series transformer T s Waveform diagram of the phase angle difference of the grid voltage at both ends of the primary side; Figure 7 (c) is a waveform diagram of reactive power output from a series-type flexible switching converter and a microgrid; Figure 7 (d) shows the frequency waveform of the microgrid; Figure 7 (e) is a series transformer T s Voltage waveform on the secondary side; Figure 7 (f) represents the current flowing through the series transformer T s Waveform of the primary current; Detailed Implementation

[0062] This invention proposes a series-type flexible switching converter for realizing fully autonomous control of microgrids. To make the objectives and technical solutions of this invention clearer and easier to understand, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0063] The technical solution of the present invention will be clearly and completely described below with reference to the accompanying drawings and specific embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and 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 the present invention.

[0064] Reference Figure 1The series-type flexible switching converter supporting fully autonomous control of microgrids proposed in this invention includes a main circuit and a control circuit for controlling the operation of the main circuit. The main circuit includes a pre-charge switch SW1, a phase selection switch SW3, a bypass switch SW4, a line cutout CUT, and a power electronic converter for converting DC to AC. The line cutout CUT and the bypass switch SW4 are connected in parallel. The AC side of the power electronic converter is connected to the line cutout CUT. One end of the parallel structure of the line cutout CUT and the bypass switch SW4 is connected in parallel with the pre-charge switch SW1 and then in series with the phase selection switch SW3.

[0065] DC side capacitor C of power electronic converter dc A buffer resistor R is connected in series with either the positive or negative terminal. dc DC side capacitor C dc On the positive or negative terminal of the buffer resistor R dc A bypass switch SW2 with a buffer resistor connected in parallel at both ends; the number of phases of the pre-charge switch SW1, the phase selection switch SW3, the bypass switch SW4, and the power electronic converter is the same as the number of phases of the power grid.

[0066] Reference Figure 1 The series transformer T s It can provide isolation and adapt power electronic converters to grid voltage levels, etc. It should be understood that T... s It is optional, not mandatory.

[0067] If the power electronic converter contains a series transformer T between the line cut-out CUT and the power electronic converter s Series transformer T s The two ends of the primary side are connected to the line cut-out CUT, and the series transformer T is connected. s The secondary side is connected to the AC side of the power electronic converter.

[0068] In a three-phase power grid, the series transformer T s The connection method of the secondary winding can be adjusted according to the voltage level and actual needs; depending on actual needs, it can also be used for series transformer T. s Add auxiliary winding; series transformer T s The turns ratio between windings can be set as needed, and its rated operating frequency is determined according to the actual power grid frequency, such as 50Hz or 60Hz.

[0069] For those without the series transformer T s In this case, the power electronic converter is directly connected in series to the cut-out section of the line, and the power electronic converter is selected with an appropriate topology, such as a cascaded H-bridge topology.

[0070] The bypass switch SW4 can increase the series reactance X. ins X insIt is optional, not mandatory, and its reactance value can be determined according to the actual operating conditions; the bypass switch SW4 and the series reactance X ins The branch formed by series connection is called the bypass branch SWX; the line cut section and the two ends of the bypass branch SWX are connected in parallel and are called the parallel structure part PESW of the main circuit.

[0071] One end of the parallel structure PESW is connected in parallel to the pre-charge switch SW1 and then in series with the phase selection switch SW3, and then connected to the first power grid through the phase selection switch SW3; then the other end of the parallel structure PESW is connected to the second power grid; in the above way, the main circuit of the series flexible switching converter is connected to the first power grid and the second power grid.

[0072] The first power grid and the second power grid have the same voltage level, or have different voltage levels but are converted to the same voltage level after being stepped up or stepped down by an additional transformer; the first power grid is a three-phase or single-phase AC microgrid, and the second power grid is a three-phase or single-phase large power grid; or the first power grid is a three-phase or single-phase large power grid, and the second power grid is a three-phase or single-phase AC microgrid; or the first power grid and the second power grid are two three-phase or single-phase AC microgrids.

[0073] At least a portion of the distributed power sources in the microgrid adopts a grid-type control with active power-frequency droop.

[0074] Depending on whether the power grid is single-phase or three-phase, and whether it contains the aforementioned series transformer T s The power electronic converter may adopt a single-phase half-bridge circuit or a single-phase full-bridge circuit, a three-phase half-bridge circuit or a three-phase full-bridge circuit, a single-phase or three-phase cascaded H-bridge circuit, or other DC-to-AC power electronic converter circuit topology that can generate a sinusoidal fundamental voltage at the AC port; the power electronic converter may have two-level, three-level or multi-level voltages.

[0075] The filter circuit of the power electronic converter is a single inductor L filter circuit or an inductor-capacitor LC filter circuit.

[0076] The power switching transistors in the power electronic converter are MOSFETs, IGBTs, IGCTs, or other fully controlled power electronic switching devices. Each power switching transistor contains an anti-parallel body diode or an external anti-parallel diode.

[0077] Reference Figure 2 This is a typical topology of a specific embodiment of the series-type flexible switching converter applied in three-phase AC microgrids and three-phase large power grids; in this specific embodiment, referring to... Figure 1 The system structure, Figure 1 The first power grid is Figure 2 The middle section is a three-phase AC microgrid. Figure 1 The second power grid is Figure 2 The example described is a three-phase large power grid, but it should be noted that this is an example of the invention and not a limitation; see reference. Figure 3 The series-type flexible switching converter is applied in Figure 2 The system shown includes the relevant voltage and current phasor diagrams, as well as the detailed structure of the phase selection switch SW3.

[0078] Reference Figure 2 V g ∠θ1 and V mg ∠θ2 represents the equivalent internal potential of the large power grid and the microgrid, respectively, U g ∠γ1 and U mg ∠γ2 represents the voltage phasors at the grid connection point of the large power grid and the bus of the microgrid, respectively. g and X mg The equivalent grid impedances of the microgrid and the main grid are respectively; the current phasor flowing into the microgrid bus is I. FTC ∠φ, if the active power absorbed by the series flexible switching converter is ignored, the current phasor I FTC ∠φ is perpendicular to the series transformer T s Primary voltage phasor U FTC ∠(φ+π / 2); P mg and Q mg P g and Q g These represent the active power and reactive power output by the microgrid and the main grid, respectively. Figure 2 Medium series transformer T s Pre-charge switch SW1, buffer resistor bypass switch SW2, phase selection switch SW3, bypass switch SW4, and series reactor X ins Connection methods and Figure 1 The basic principles are the same, so I will not repeat them here. Figure 2 In the middle, series transformer T s The secondary side is connected by a triangle, and the turn ratio of the primary and secondary sides is 1:1.

[0079] Reference Figure 2 The characteristics of the three-phase AC microgrid in this specific embodiment are explained below:

[0080] In this microgrid, the equivalent impedance of the transmission lines between each distributed power source and the point of common coupling is inductive, and the voltage at the inverter output voltage E∠φ and the voltage at the point of common coupling U∠φ are also inductive. L The active power P and reactive power Q transmitted between ∠0 are defined as follows:

[0081]

[0082] Among them, E and U LThese are the inverter output voltage and the voltage amplitude at the point of common coupling, respectively; φ is the phase angle difference between the power supply and the point of common coupling; and Z is the equivalent impedance of the transmission line.

[0083] In this scenario, at least a portion of the distributed power sources in the AC microgrid are voltage sources controlled using a droop control method. The specific droop control formula is as follows:

[0084]

[0085] Where, ω * and E * These are the frequency and voltage control commands generated by the droop control loop, respectively. P and Q are the active and reactive power output detected by the inverter, respectively. P0 and Q0 are the active and reactive power output by the inverter at frequency ω0 and voltage E0, respectively, determined by the inverter based on the generation status of the distributed power source itself. k p and k q Defined as positive, these are the slopes of the droop control lines for frequency and voltage, respectively.

[0086] It should be understood that the above description of the characteristics of microgrids is not intended to limit the present invention. In addition to droop control, some power sources in a microgrid may also employ improved droop control strategies such as droop control with dead zone, as well as other grid-type control strategies such as virtual synchronous generator control, where the microgrid frequency changes with the output power of the power sources.

[0087] Reference Figure 2 The three terminals on one side of the precharge switch SW1 and the series transformer T s The three terminals SW1-a, SW1-b, and SW1-c on the primary side are connected, while the three terminals on the other side of SW1 are electrically shorted together. Therefore, after the pre-charge switch SW1 is closed, the three terminals SW1-a, SW1-b, and SW1-c are electrically connected together, at which point the series transformer T... s Its structure is equivalent to that of a regular transformer, through T s The secondary power electronic converter is connected in parallel to the main power grid.

[0088] In a three-phase power grid scenario, the internal structure of the phase selection switch SW3 can be referenced. Figure 2 and Figure 3 ; Figure 3 When SW3 is closed, there are three phase sequences to choose from: phase sequence 1, phase sequence 2, and phase sequence 3. The phase sequence differs by 120 degrees. The selection criterion for the closing phase sequence is that the phase angle difference between the grid voltage phasors on both sides of the line cut-out is minimized after SW3 is closed in the selected phase sequence. For example, refer to... Figure 3 By selecting a suitable closed phase sequence, the microgrid bus is connected to the series transformer T via SW3.s After the microgrid side, the microgrid bus voltage phasor V mg ∠θ2 and the voltage phasor V at the grid connection point of the large power grid g ∠θ1 is located within the same sector I, thus connecting the series transformer T s The voltage stress borne by the primary side is limited to one times the rated voltage of the power grid.

[0089] Reference Figure 2 The power electronic converter's circuit topology is a two-level three-phase half-bridge circuit, including six power switches S1, S2, S3, S4, S5, and S6. Each power switch includes an anti-parallel body diode or an externally added anti-parallel diode. Power switches S1 and S4 are connected end-to-end to form the first bridge arm, S3 and S6 are connected end-to-end to form the second bridge arm, and S5 and S2 are connected end-to-end to form the third bridge arm. The connection point of the switches is the midpoint of the bridge arm. The midpoints of the first, second, and third bridge arms are connected to the series transformer T via a three-phase LC filter circuit. s The secondary side, the upper ends of the first bridge arm, the second bridge arm and the third bridge arm are connected to each other to form a common upper end, and the lower ends are connected to each other to form a common lower end;

[0090] Reference Figure 2 The structure shown does not have energy storage or additional control circuitry on the DC side, and the DC side buffer resistor R... dc It is connected in parallel with the buffer resistor bypass switch SW2, and the common upper terminal is connected in series with this part, and then connected to the DC side capacitor C. dc The positive or negative terminal, and the common lower terminal are connected to the DC side capacitor C. dc The lower end.

[0091] It should be understood that this structure is not intended to limit the invention; as previously stated, the series transformer T s The specifications and structure of the power electronic converter, the filter circuit structure of the power electronic converter, and the fully controlled power electronic switching devices used in the power switching tubes can be of different types. Energy storage and additional control circuits can also be added to the DC side as needed.

[0092] The control method for the series-type flexible switching converter includes pre-charge self-starting control, inter-grid pre-synchronization and soft switching control, and emergency power support control. To verify the feasibility of this invention, a simulation model was built in simulation software for this specific embodiment for simulation verification; the simulation model includes, for example... Figure 2The series-type flexible switching converter shown represents the entire system of a three-phase AC microgrid and a mains grid. Three inverters, acting as distributed power sources, are connected to the microgrid bus via their respective line impedances to supply power to active and reactive loads, forming the three-phase AC microgrid. The voltage and current phasor relationships and the structure of the phase selection switch SW3 in this embodiment are as follows: Figure 3 As shown, the control system block diagram of the series flexible switching converter is as follows: Figure 4 As shown, the timing diagram of control signals and switch states is as follows: Figure 5 As shown, the waveforms of the relevant electrical quantities in the simulation results are as follows: Figure 6 and Figure 7 As shown.

[0093] The following describes a control method for a series-type flexible switching converter supporting fully autonomous control of a microgrid. This control method is used to control the pre-charge self-starting of the series-type flexible switching converter supporting fully autonomous control of a microgrid. The system structure is as follows: Figure 2 and Figure 3 The control system block diagram is referenced. Figure 4 (a) Figure 4 (b) Figure 4 (d) and Figure 4 (f) Timing reference for control signals and switch states Figure 5 The waveforms of each signal within the time period from 0s to 1.9s are shown in the simulation results, along with the relevant electrical quantity waveforms. Figure 6 (a) to Figure 6 (g) shows the relevant waveforms during the time period from 0s to 1.9s.

[0094] Before pre-charge self-start, switches SW1, SW2, SW3, and SW4 are all in the open state. After pre-charge self-start is completed, the DC voltage of the power electronic converter is controlled to the DC voltage reference value. It should be noted that for switches SW1, SW2, SW3, and SW4, in Figure 4 and Figure 5 The state representation method in SW is as follows: when the switch is closed, SW i =0 (i=1,2,3,4), SW when the switch is open i =1(i=1,2,3,4); in Figure 5 In this context, VM stands for Voltage Mode and CM stands for Current Mode.

[0095] The specific steps are as follows:

[0096] Upon receiving the start command, the pre-charge switch SW1 closes at t=0s, and the series transformer T... s A short circuit occurs at the three-phase terminals on the microgrid side, at which point T sThe actual structure is equivalent to a conventional transformer, with the primary side connected to the main power grid and the secondary side connected to the AC port of the power electronic converter. Because all power switches S1-S6 are blocked, the mains voltage is rectified through a three-phase diode bridge to the DC-side capacitor C using uncontrolled rectification. dc Charging; such as Figure 6 As shown in (d), after the peak inrush current to be charged passes, the DC side capacitor voltage stabilizes near the amplitude of the AC side line voltage of the power electronic converter. At t = 1.1s, the switch SW2 is closed, and the buffer resistor R... dc bypass;

[0097] The control circuit draws from the DC side capacitor C dc Self-reliant can start working, such as Figure 5 As shown, CS is set at t = 1.2s. PWM =1, and set CS at the same time dcc =1, enable DC voltage control, such as Figure 4 As shown in (a), the power electronic converter is connected to the series transformer T. s The secondary voltage phase-locked loop obtains the phase angle θ of the main grid voltage. g Therefore, it operates in unity power factor PWM rectifier mode to absorb energy from the mains grid, such as... Figure 6 (a) shows the DC voltage being adjusted to the DC voltage reference value of 800V;

[0098] It should be understood that this control method is not intended to limit the present invention; if the DC side of the power electronic converter has energy storage and additional control circuitry, it is also possible to realize the DC voltage control of the series flexible switching converter so that it follows the DC voltage reference value, in which case the above-mentioned pre-charge self-starting circuit is not required.

[0099] The following describes a control method for a series-type flexible switching converter that supports fully autonomous control of a microgrid. This control method is used to control the pre-synchronization between the microgrid and the main grid. The system structure is described below. Figure 2 and Figure 3 The control system block diagram is referenced. Figure 4 (a) to Figure 4 (d) and Figure 4 (f) Timing reference for control signals and switch states Figure 5 The waveforms of each signal after the 1.9s interval are shown in the simulation results. The relevant electrical quantity waveforms are also shown in the simulation results. Figure 6 (a) to Figure 6 (i) shows the relevant waveforms after the 1.9s time period.

[0100] The DC-side capacitor voltage of the power electronic converter is controlled as a reference value according to the above pre-charge self-starting control method. Then, the reactive power output of the AC port of the power electronic converter is controlled to adjust the active power transmitted between the microgrid and the main grid, so that the phase angle of the microgrid voltage and the phase angle of the main grid voltage are pre-synchronized. Then, the bypass switch SW4 is closed, and the pre-synchronization control between the microgrid and the main grid is completed.

[0101] The specific steps are as follows:

[0102] Step 1), start the series-type flexible switching converter that supports fully autonomous microgrid control, control the DC-side capacitor voltage to the reference value, and upon receiving the pre-synchronization command, as follows: Figure 5 As shown, the pre-charge switch SW1 is disconnected at t = 1.9s;

[0103] Step 2), select the closing phase sequence of phase selection switch SW3; the control circuit selects the closing phase sequence of SW3 based on the relative relationship between the microgrid voltage phasors and the main grid voltage phasors; the selection criterion for the closing phase sequence is that after phase selection switch SW3 closes under the selected phase sequence, the phase angle difference of the grid voltage phasors on both sides of the line cut-out is minimized, referring to... Figure 2 That is, series transformer T s The voltage difference across the primary side is minimal, not exceeding one times the rated voltage of the power grid; in the simulation of this specific embodiment, when the closed phase sequence is phase sequence 1, such as Figure 3 As shown, the phase angle difference between the grid voltage phasor and the microgrid voltage phasor is the smallest, and both are located within sector I.

[0104] Step 3), according to the phase sequence selected in Step 2), such as Figure 5 As shown, at t=1.9s, CS1=1 is set, and the phase-locked voltage target of the power electronic converter switches from the main grid voltage to the voltage difference between the microgrid and the main grid, as follows. Figure 4 As shown in (a), the phase angle θ FTC From θ g Switch to θ mg-g The power electronic converter operates in Voltage Mode (VM). In this mode, the instantaneous voltage difference between the two grids under the selected phase sequence in step 2) is used as a voltage command to control the power electronic converter to... Figure 1 The middle line cut, i.e. Figure 2 Medium series transformer T s The primary voltage follows the voltage command; thus, under the selected phase sequence, the voltage amplitude difference across the primary side is zero before the phase selection switch SW3 is closed; the series transformer T controlled by the power electronic converter... s The voltage waveform on the primary side is as follows Figure 6As shown in (e), it varies with the voltage difference between the microgrid voltage and the main grid voltage; it should be noted that in step 3), the power electronic converter operates in voltage control mode and cannot absorb active power from its AC port to maintain DC voltage stability. Furthermore, due to losses, therefore... Figure 6 As shown in (d), the DC voltage drops slightly, but the drop is small and does not affect the normal operation of the power electronic converter; Figure 5 As shown, at t=2s, SW3 is closed according to the selected phase sequence to establish a microgrid, and transformer T is connected in series. s and the electrical connection between the connected devices and the main power grid;

[0105] Step 4), after the phase selection switch SW3 is closed, as follows: Figure 5 As shown, with CS2=1 set at t=2s, the power electronic converter begins the transition from Voltage Mode (VM) to Current Mode (CM), as follows. Figure 4 As shown in (d), Sig line =1, then Sig i =1; for example Figure 4 As shown in (f), the outputs of the d-axis and q-axis of the voltage control loop gradually decrease to 0 from t=2s under the control of CS2, because Sig line =Sig i =1, so the current control loop starts, allowing direct control of the dq-axis current; the transition process ends after 1 second. Figure 4 The voltage control loop in (f) is deactivated, and the system operates entirely in current control mode;

[0106] Step 5), as Figure 5 As shown, CS is set at t=4s. psc =1, enable pre-synchronization control, such as Figure 4 As shown in (b), the difference between the DC voltage reference value and the actual DC voltage value of the power electronic converter is sent to the DC voltage controller to obtain the active current command and maintain DC voltage stability; Figure 4 As shown in (c), series transformer T s The phase angle difference of the grid voltage at both ends of the primary side is processed by phase angle switching and then sent to the phase angle difference controller to obtain the reactive current command of the power electronic converter.

[0107] Step 6): Based on the voltage command obtained in Step 3) or the current command obtained in Step 5), the control circuit combines the actual voltage and current and adopts... Figure 4(f) Voltage closed-loop control or current closed-loop control generates a modulation wave, and then a drive signal is generated through a pulse width modulation process. The drive circuit of each power switch in the power electronic converter controls the power switch to turn on or off according to the obtained drive signal.

[0108] Step 7), repeat steps 5) and 6), as follows Figure 6 (b) shows how the frequency and phase angle of the voltages at both ends gradually become synchronized, when the frequency difference and phase angle difference of the voltages at both ends are simultaneously less than Figure 4 (d) After their respective thresholds, such as Figure 5 As shown, at t=6s, the bypass switch SW4 closes, and the microgrid soft switching is achieved through the bypass switch SW4 and the series reactor X. ins Connected to the main power grid, the grid-connected current is as follows Figure 6 (f) and Figure 6 (i) shows the flow passing through SW4 and quickly reaching a steady state.

[0109] like Figure 4 (b) and Figure 4 As shown in (c), the active and reactive current commands of the power electronic converter are as follows:

[0110]

[0111] in, and These are the active current command and reactive current command for the power electronic converter, G p and G q These represent the DC voltage controller and the phase angle difference controller, respectively. Figure 4 The DC voltage controller and phase angle difference controller described herein are both proportional-integral (PI) controllers. dc_ref and u dc These are the reference DC voltage value and the actual DC voltage value of the power electronic converter, respectively, θ mg and θ g These are the phase angles of the fundamental voltage components on the microgrid side and the main grid side, respectively.

[0112] Reference Figure 5 and Figure 6 Each sub-diagram provides a systematic explanation of the simulation results for the aforementioned pre-charge self-starting and pre-synchronization soft switching control, arranged according to the timing sequence.

[0113] At t = 0.5s, such as Figure 5 As shown, the pre-charge switch SW1 is closed, and the series transformer T is shown. s The secondary voltage is rectified to the DC capacitor C in an uncontrolled rectification manner. dc Charging, such as Figure 6 The DC voltage shown in (a) rises to approximately 300V in 0.75s, as... Figure 5As shown, at t = 1.1s, after the peak impact current, the buffer resistor bypass switch SW2 closes, closing the buffer resistor R. dc Bypass; such as Figure 5 As shown, at t = 1.2s, CS is set... PWM and CS dcc =1, Figure 4 (b) The DC voltage control loop of the power electronic converter is started, and the DC voltage is controlled to the DC voltage reference value of 800V in 1.4s, and the pre-charge self-starting process is completed.

[0114] Upon receiving the pre-synchronization command, at t=1.9s, the pre-charge switch SW1 opens; according to the aforementioned phase sequence selection criteria for the phase selection switch SW3, the phase sequence of SW3 is selected based on the relative relationship between the microgrid voltage phasors and the main grid voltage phasors; CS1 is set to 1, and the power electronic converter operates in voltage control mode, such as... Figure 6 (e) shows the power electronic converter controlling the series transformer T. s The primary voltage follows the instantaneous value of the voltage difference between the two grid sides under the selected closed phase sequence. During this process, the power electronic converter cannot absorb active power from the line. Figure 6 (a) The DC voltage drops slightly; at t = 2s, according to the aforementioned closed phase sequence selection method, the phase selection switch SW3 closes, and the microgrid is connected to the series transformer T. s On one side of the primary side, CS2 = 1, and the transition from voltage control mode to current control mode begins, as shown below. Figure 6 As shown in (a), the DC voltage quickly recovers to 800V, and the transition is completed after 1 second. The power electronic converter operates in current control mode.

[0115] like Figure 5 As shown, at t=4s, CS pcc =1, pre-synchronization control starts; such as Figure 6 (a) and Figure 6 As shown in (c), the series-type flexible switching converter regulates the active power transmitted between the microgrid and the main grid by controlling the reactive power output, thereby regulating the frequency of the microgrid, as follows: Figure 6 As shown in (b), under the selected closed phase sequence, the phase angle difference between the voltages of the two grids gradually decreases to zero, while the series transformer T... s The voltage across the primary side gradually decreases during the pre-synchronization process, therefore... Figure 6 As shown in (d), the DC voltage cannot maintain the reference value of 800V after t=6s and drops rapidly. However, pre-synchronization has been completed at this time, and the bypass switch SW4 is closed. The microgrid is directly connected to the main grid through the bypass branch SWX. Figure 6 As shown in (f), the grid-connected current flows through the bypass switch SW4 and quickly reaches a steady state.

[0116] The following describes another control method for a series-type flexible switching converter that supports fully autonomous control of a microgrid. This control method is used to provide emergency power support for the power grid. The system structure is described in reference [reference needed]. Figure 2 and Figure 3 The control system block diagram is referenced. Figure 4 (a) Figure 4 (b) Figure 4 (d) Figure 4 (e) and Figure 4 (f) The waveform diagrams of the relevant electrical quantities in the simulation results are shown in reference. Figure 7 .

[0117] The control method includes the following steps:

[0118] Step 1), start the series-type flexible switching converter that supports fully autonomous control of the microgrid, and control the DC side capacitor voltage to the reference value;

[0119] Step 2) Detect the frequencies of the microgrid and the main grid, and determine whether the microgrid or the main grid requires emergency power support based on the set grid frequency threshold, thus identifying the grid that needs support; or determine the grid that needs support based on instructions from higher authorities; according to Figure 4 The judgment condition in (d) is... Figure 7 (d) After detecting that the microgrid frequency is below 49.95Hz, the microgrid is determined to be a grid that needs to be supported, and the pre-charge switch SW1 is disconnected;

[0120] Step 3) Select the closing phase sequence of phase selection switch SW3; the control circuit selects the closing phase sequence of SW3 based on the relative relationship between the microgrid voltage phasors and the main grid voltage phasors; the selection criterion for the closing phase sequence is that after phase selection switch SW3 closes under the selected phase sequence, the phase angle difference of the grid voltage phasors on both sides of the line cut-out is minimized, referring to... Figure 2 That is, series transformer T s The voltage difference across the primary side is minimal, not exceeding one times the rated voltage of the power grid; in the simulation of this specific embodiment, when the closed phase sequence is phase sequence 1, such as Figure 3 As shown, the phase angle difference between the grid voltage phasor and the microgrid voltage phasor is the smallest, and both are located within sector I.

[0121] Step 4): Based on the phase sequence selected in Step 3), set CS1 = 1 at t = 1.9s. The phase-locked voltage target of the power electronic converter switches from the main grid voltage to the voltage difference between the microgrid and the main grid, as follows: Figure 4 As shown in (a), the phase angle θ FTC From θ g Switch to θ mg-gThe power electronic converter operates in Voltage Mode (VM). In this mode, the instantaneous voltage difference between the two grids under the selected phase sequence in step 3) is used as a voltage command to control the power electronic converter to... Figure 2 Medium series transformer T s The primary voltage follows the voltage command; thus, under the selected phase sequence, the voltage amplitude difference across the primary side is zero before the phase selection switch SW3 is closed; the series transformer T controlled by the power electronic converter... s The voltage waveform on the primary side is as follows Figure 7 As shown in (e), it changes with the voltage difference between the microgrid and the main grid; at t=2s, SW3 is closed according to the selected phase sequence to establish the microgrid, and the transformer T is connected in series. s and the electrical connection between the connected devices and the main power grid;

[0122] Step 5): After the phase selection switch SW3 is closed, CS2 is set to 1 at t = 2s. The power electronic converter then begins the transition from voltage control mode (VM) to current control mode (CM). Figure 4 As shown in (d), Sig line =1, then Sig i =1; for example Figure 4 As shown in (f), the outputs of the d-axis and q-axis of the voltage control loop gradually decrease to 0 from t=2s under the control of CS2, because Sig line =Sig i =1, so the current control loop starts, allowing direct control of the dq-axis current; the transition process ends after 1 second. Figure 4 The voltage control loop in (f) is deactivated, and the system operates entirely in current control mode;

[0123] Step 6), after the power electronic converter enters the current control mode, as follows: Figure 4 As shown in (b), the difference between the DC voltage reference value and the actual DC voltage value of the power electronic converter is sent to the DC voltage controller of the control circuit to obtain the active current command of the power electronic converter; as Figure 4 As shown in (e), the difference between the frequency reference value and the actual frequency value of the grid to be supported is sent to the emergency power support controller in the control circuit to obtain the reactive current command of the power electronic converter.

[0124] The active current command and reactive current command of the power electronic converter are as follows:

[0125]

[0126] in, and These are the active current command and reactive current command for the power electronic converter, G p and G f These represent the DC voltage controller and the emergency power support controller, respectively. dc_ref and u dc These are the reference DC voltage value and the actual DC voltage value of the power electronic converter, respectively, ω ref and ω 12 These are the frequency reference value and the actual frequency value of the power grid to be supported, respectively.

[0127] Step 7), based on the current command obtained in step 6), such as Figure 4 (f) The control circuit uses current closed-loop control to generate a modulated wave, and then generates a drive signal through pulse width modulation. The drive circuit of each power switch in the power electronic converter controls the power switch to turn on or off according to the obtained drive signal; such as Figure 7 As shown in (f), the current used for emergency power support flows through the series transformer T. s The original edge, such as Figure 7 As shown in (a), the large power grid transmits active power to the microgrid that needs to be supported, thus providing emergency power support to the microgrid; Figure 7 As shown in (d), after receiving emergency power support, the microgrid frequency recovers to the frequency reference value ω of the grid that needs to be supported. ref .

[0128] The above are merely preferred embodiments of the present invention and are not intended to limit the present invention in any way. Any simple modifications, alterations, or equivalent structural changes made to the above embodiments based on the technical essence of the present invention shall still fall within the protection scope of the present invention.

[0129] The above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art can still make modifications or equivalent substitutions to the specific implementation schemes of the present invention, and these modifications or equivalent substitutions do not depart from the spirit and scope of the present invention, and are all within the protection scope of the claims of the present invention.

Claims

1. A series flexible switching converter supporting full autonomous control of microgrid, characterized in that, It includes a main circuit and a control circuit for controlling the operation of the main circuit. The main circuit includes a pre-charge switch SW1, a phase selection switch SW3, a bypass switch SW4, a line cut-out CUT, and a power electronic converter for converting DC to AC. The line cut-out CUT and the bypass switch SW4 are connected in parallel. The AC side of the power electronic converter is connected to the line cut-out CUT. One end of the parallel structure of the line cut-out CUT and the bypass switch SW4 is connected in parallel with the pre-charge switch SW1 and then in series with the phase selection switch SW3. DC-side capacitor of power electronic converter C dc A buffer resistor is connected in series with either the positive or negative terminal. R dc DC side capacitor C dc On the positive or negative terminal of the buffer resistor R dc A bypass switch SW2 with a buffer resistor connected in parallel at both ends; the number of phases of the pre-charge switch SW1, the phase selection switch SW3, the bypass switch SW4, and the power electronic converter is the same as the number of phases of the power grid; The control method for the series-type flexible switching converter that supports fully autonomous control of microgrids includes: Before the pre-charge self-start process begins, the pre-charge switch SW1, the buffer resistor bypass switch SW2, the phase selection switch SW3, and the bypass switch SW4 are all in the open state. When the power grid is a single-phase grid, one end of the pre-charge switch SW1 is connected to the phase line of the grid, and the other end is connected to the neutral line. Upon receiving the pre-charge command, the pre-charge switch SW1 closes, and all power switches in the power electronic converter are locked out. The grid voltage charges the DC-side capacitor through the uncontrolled rectifier. After the peak inrush current disappears, the buffer resistor bypass switch SW2 is closed, and the buffer resistor... R dc bypass; When the power grid is a three-phase grid, the three terminals of the pre-charge switch SW1 at one end are connected to the three phases of the grid, and the three terminals at the other end are electrically connected together. Upon receiving the pre-charge command, the pre-charge switch SW1 closes, short-circuiting the three-phase terminals connected to the grid at one end. All power switches in the power electronic converter are then locked out. The grid voltage charges the DC-side capacitor through the uncontrolled rectifier. After the peak inrush current of the pre-charge operation disappears, the buffer resistor bypass switch SW2 closes, opening the buffer resistor... R dc bypass; The control circuit starts working by drawing power from the DC-side capacitor, and then controls the power electronic converter to lock the voltage of its own AC port in phase-locked mode. It operates in a unity power factor PWM rectifier mode, absorbs active power from the grid, and adjusts the DC-side capacitor voltage to the reference value.

2. The series flexible switching converter supporting full autonomous control of microgrid according to claim 1, characterized in that, The power electronic converter is provided with a series transformer T between the line cut CUT s The primary side of the series transformer T s is connected to the line cut CUT, and the secondary side of the series transformer T s is connected to the AC side of the power electronic converter.

3. A series-type flexible switching converter supporting fully autonomous control of a microgrid according to claim 1, characterized in that, The bypass switch SW4 is in series with a series reactance X ins .

4. The series flexible switching converter of any one of claims 1-3, wherein, The power electronic converter has energy storage and additional control circuitry on its DC side.

5. The control method of the series flexible switching converter supporting full autonomous control of microgrid according to any one of claims 1-3, which is used for pre-charging and self-starting of the series flexible switching converter supporting full autonomous control of microgrid, and is realized by control of the main circuit through the control circuit, characterized in that, The control method includes the following process: Before the pre-charge self-start process begins, the pre-charge switch SW1, the buffer resistor bypass switch SW2, the phase selection switch SW3, and the bypass switch SW4 are all in the open state. When the power grid is a single-phase power grid, one end of the pre-charge switch SW1 is connected to the phase line of the power grid, and the other end is connected to the neutral line of the power grid; after receiving a pre-charge instruction, the pre-charge switch SW1 is closed, all power switch tubes in the power electronic converter are locked at this time, and the power grid voltage charges the DC side capacitor through uncontrolled rectification; after the peak impact current of the charge disappears, the buffer resistance bypass switch SW2 is closed to bypass the buffer resistance R dc bypass When the power grid is a three-phase grid, the three terminals of the pre-charge switch SW1 at one end are connected to the three phases of the grid, and the three terminals at the other end are electrically connected together. Upon receiving the pre-charge command, the pre-charge switch SW1 closes, short-circuiting the three-phase terminals connected to the grid at one end. All power switches in the power electronic converter are then locked out. The grid voltage charges the DC-side capacitor through the uncontrolled rectifier. After the peak inrush current of the pre-charge operation disappears, the buffer resistor bypass switch SW2 closes, opening the buffer resistor... R dc bypass; The control circuit starts working by drawing power from the DC-side capacitor, and then controls the power electronic converter to lock the voltage of its own AC port in phase-locked mode. It operates in a unity power factor PWM rectifier mode, absorbs active power from the grid, and adjusts the DC-side capacitor voltage to the reference value.

6. A control method for a series-type flexible switching converter supporting fully autonomous control of a microgrid, as described in claim 5, wherein the control method is used to control the pre-synchronization between a first power grid and a second power grid, wherein the phase selection switch SW3 is connected to the first power grid, and the other end of the parallel structure of the line cut-out CUT and the bypass switch SW4 is connected to the second power grid, and the control method is implemented by controlling the main circuit through a control circuit, characterized in that... The control method includes the following process: When the DC side of the power electronic converter has energy storage and additional control circuits, the reactive power output of its AC port can be directly controlled by the power electronic converter to regulate the active power transmitted between the first grid and the second grid, so that the phase angle of the voltage of the first grid and the phase angle of the voltage of the second grid are pre-synchronized. Then the bypass switch SW4 is closed, and the pre-synchronization control between the first grid and the second grid is completed. When the DC side of the power electronic converter does not have energy storage and additional control circuits, the series-type flexible switching converter supporting full autonomous control of the microgrid is pre-charged and self-started according to the method of claim 5; then, the reactive power output of the AC port of the power electronic converter is controlled to adjust the active power transmitted between the first grid and the second grid, so that the phase angle of the voltage of the first grid and the phase angle of the voltage of the second grid are pre-synchronized, and then the bypass switch SW4 is closed, and the pre-synchronization control between the first grid and the second grid is completed.

7. The control method for a series-type flexible switching converter supporting fully autonomous control of a microgrid according to claim 6, characterized in that, The control method specifically includes the following steps: Step 1), start the series-type flexible switching converter that supports fully autonomous control of the microgrid, control the DC side capacitor voltage to the reference value, and disconnect the pre-charge switch SW1 after receiving the pre-synchronization command; Step 2), select the closing phase sequence of the phase selection switch SW3; the phase selection switch SW3 only needs to select the closing phase sequence when the first grid and the second grid are three-phase grids; the control circuit selects the closing phase sequence of SW3 according to the relative relationship between the voltage phasors of the first grid and the voltage phasors of the second grid; the selection criterion for the closing phase sequence is that after the phase selection switch SW3 is closed under the selected phase sequence, the phase angle difference of the grid voltage phasors on both sides of the line cut CUT is the smallest; Step 3): The control circuit enables the power electronic converter to operate in voltage control mode. In the case of a single-phase power grid, the instantaneous voltage difference between the first and second power grids is sampled and calculated as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed. Then, the phase selection switch SW3 is closed to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid. In a three-phase power grid, in voltage control mode, the instantaneous voltage difference between the first and second power grids is sampled and calculated based on the phase sequence selected in step 2) as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed under the selected phase sequence. Then, the phase selection switch SW3 is closed according to the selected phase sequence to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid. Step 4): After the phase selection switch SW3 is closed, the power electronic converter begins to transition from voltage control mode to current control mode. After the transition is completed, the power electronic converter operates in current control mode. Step 5): After the power electronic converter enters the current control mode, the difference between the DC voltage reference value and the actual DC voltage value of the power electronic converter is sent to the DC voltage controller of the control circuit to obtain the active current command. The phase angle difference between the voltages at both ends of the parallel structure of the line cut-out CUT and the bypass switch SW4 is processed by the phase angle jumper and then sent to the phase angle difference controller of the control circuit to obtain the reactive current command of the power electronic converter. Step 6): Based on the voltage command obtained in Step 3) or the current command obtained in Step 5), the control circuit combines the actual voltage and current, uses voltage closed-loop control or current closed-loop control to generate a modulation wave, and then generates a drive signal through pulse width modulation. The drive circuit of each power switch in the power electronic converter controls the power switch to turn on or off according to the obtained drive signal. Step 7), repeat steps 5) to 6), so that the frequency and phase angle of the voltages at both ends of the parallel structure of the line cut-out CUT and the bypass switch SW4 tend to be synchronized. When the frequency difference and phase angle difference of the voltages at both ends of the parallel structure of the line cut-out CUT and the bypass switch SW4 are both less than their respective thresholds, switch to connect the first power grid to the second power grid through the bypass switch SW4 branch.

8. The control method for a series-type flexible switching converter supporting fully autonomous control of a microgrid according to claim 7, characterized in that, The active current command and reactive current command of the power electronic converter are as follows: in, i* FTCd and i* FTCq are the active current command and reactive current command for the power electronic converter, respectively. G p and G q These represent the DC voltage controller and the phase angle difference controller, respectively. u dc_ref and u dc These are the reference DC voltage value and the actual DC voltage value of the power electronic converter, respectively. θ 1 and θ 2 represents the phase angle of the fundamental voltage components of the first power grid and the second power grid, respectively.

9. The control method for a series-type flexible switching converter supporting fully autonomous control of a microgrid according to claim 6, characterized in that, The first power grid and the second power grid have the same voltage level, or have different voltage levels but become the same voltage level after being converted by an additional step-up or step-down transformer; The first power grid is a three-phase or single-phase AC microgrid, and the second power grid is a three-phase or single-phase large power grid; Alternatively, the first power grid may be a three-phase or single-phase large power grid, and the second power grid may be a three-phase or single-phase AC microgrid; Alternatively, the first power grid and the second power grid can be two three-phase or single-phase AC microgrids; At least a portion of the distributed power sources in the microgrid adopts a grid-type control with active power-frequency droop.

10. A control method for a series-type flexible switching converter supporting fully autonomous control of a microgrid, as described in claim 5, wherein the control method is used to realize emergency power support for the power grid, wherein the phase selection switch SW3 is connected to the first power grid, and the other end of the parallel structure of the line cut-out CUT and the bypass switch SW4 is connected to the second power grid, and the control method is realized by controlling the main circuit through a control circuit, characterized in that... The control method includes the following process: When the DC side of the power electronic converter has energy storage and additional control circuits, the reactive power output from its AC port is controlled by the power electronic converter to regulate the active power transmitted between the first and second power grids, thus providing emergency power support for a certain power grid. When the DC side of the power electronic converter does not have energy storage and additional control circuits, the series-type flexible switching converter supporting full autonomous control of the microgrid is pre-charged and self-started according to the method described in claim 5. The reactive power output from its AC port is controlled by the power electronic converter to regulate the active power transmitted between the first and second power grids, thus providing emergency power support for a certain power grid. The control method includes the following steps: Step 1), start the series-type flexible switching converter that supports fully autonomous control of the microgrid, and control the DC side capacitor voltage to the reference value; Step 2), detect the frequencies of the first and second power grids, and determine whether the first or second power grid needs emergency power support based on the set power grid frequency threshold, and determine the power grid that needs to be supported; or determine the power grid that needs to be supported based on the instructions from the superior. After determining the power grid that needs to be supported, disconnect the pre-charge switch SW1; Step 3), select the closing phase sequence of the phase selection switch SW3; the phase selection switch SW3 only needs to select the closing phase sequence when the first grid and the second grid are three-phase grids; the control circuit selects the closing phase sequence of SW3 according to the relative relationship between the voltage phasors of the first grid and the voltage phasors of the second grid; the selection criterion for the closing phase sequence is that after the phase selection switch SW3 is closed under the selected phase sequence, the phase angle difference of the grid voltage phasors on both sides of the line cut CUT is the smallest; Step 4): The control circuit enables the power electronic converter to operate in voltage control mode. In the case of a single-phase power grid, the instantaneous voltage difference between the first and second power grids is sampled and calculated as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed. Then, the phase selection switch SW3 is closed to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid. In a three-phase power grid, in voltage control mode, the instantaneous voltage difference between the first and second power grids is sampled and calculated based on the phase sequence selected in step 3) as a voltage command. Then, the power electronic converter is controlled to make the voltage across the line cut-out CUT follow the voltage command. This ensures that the voltage amplitude difference across the line cut-out CUT is zero before the phase selection switch SW3 is closed under the selected phase sequence. Then, the phase selection switch SW3 is closed according to the selected phase sequence to establish the electrical connection between the first power grid, the line cut-out CUT, the device connected to the line cut-out CUT, and the second power grid. Step 5): After the phase selection switch SW3 is closed, the power electronic converter begins to transition from voltage control mode to current control mode. After the transition is completed, the power electronic converter operates in current control mode. Step 6): After the power electronic converter enters the current control mode, the difference between the DC voltage reference value and the actual DC voltage value of the power electronic converter is sent to the DC voltage controller of the control circuit to obtain the active current command of the power electronic converter; the difference between the frequency reference value and the actual frequency value of the grid to be supported is sent to the emergency power support controller in the control circuit to obtain the reactive current command of the power electronic converter. The active current command and reactive current command of the power electronic converter are as follows: in, i* FTCd and i* FTCq are the active current command and reactive current command for the power electronic converter, respectively. G p and G f These represent the DC voltage controller and the emergency power support controller, respectively. u dc_ref and u dc These are the reference DC voltage value and the actual DC voltage value of the power electronic converter, respectively. ω ref and ω 12 These are the frequency reference value and the actual frequency value of the power grid to be supported, respectively. Step 7): Based on the current command obtained in Step 6), the control circuit uses current closed-loop control to generate a modulation wave, and then generates a drive signal through pulse width modulation. The drive circuit of each power switch in the power electronic converter controls the power switch to turn on or off according to the obtained drive signal; thereby realizing the transmission of active power to the grid that needs to be supported, and realizing emergency power support for the grid.

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