Island operation smooth switching method and device for AC / DC hybrid system

Abstract

The invention relates to the technical field of flexible direct-current power transmission, and discloses an isolated island operation smooth switching method and device for an alternating-current and direct-current hybrid system, and the method comprises the steps: dynamically adjusting a power outer ring given value before system switching, enabling the power of an alternating-current parallel branch to return to zero, and eliminating the power impact at the switching moment; before switching, the V / F controls the amplitude and phase of the outer ring reference voltage and the amplitude and phase of the power grid voltage to be kept synchronous, voltage jump at the switching moment is avoided, and system fluctuation in the switching process is reduced; and after the switching is completed, the phase in the control loop is automatically generated according to the standard frequency of the power grid. Therefore, smooth switching of island operation is realized.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of flexible direct current transmission, in particular to a method and device for smooth switching of island operation of an AC / DC hybrid system. BACKGROUND

[0002] In recent years, with the expansion of new energy installed capacity year by year, its grid-connection and accommodation scheme and the resulting power grid stability problem have become the focus of industry attention. For onshore wind farms, photovoltaic power stations and new energy bases, according to the transmission distance and capacity, high-voltage alternating current transmission or flexible direct current transmission is usually selected for collection and transmission.

[0003] High-voltage alternating current transmission can be connected to the alternating current network around the station, and the structure is relatively simple, with lower engineering construction cost, but it is difficult to meet the demand of large capacity and long distance transmission due to the limitation of transmission capacity and distance, and in the weak area of alternating current network, it will further aggravate the risk of power grid instability. Compared with high-voltage alternating current transmission, flexible direct current transmission has higher efficiency and can meet the demand of large capacity and long distance transmission, but it has problems such as higher initial investment and difficulty in solving the problem of power supply point in the middle.

[0004] In order to comprehensively exert the advantages of alternating current transmission and flexible direct current transmission, part of the newly built new energy stations adopt AC / DC hybrid transmission scheme. For this system, the redundant configuration of AC / DC transmission channel improves the system stability and reduces the risk of new energy station off-grid caused by single system failure. The introduction of flexible direct current system enhances the control ability of the original alternating current system, which can not only provide reactive power support, but also can improve the network strength through network type control. In addition, the AC / DC hybrid system can also meet the needs of local power supply and long distance transmission of new energy stations.

[0005] For AC / DC hybrid transmission system, in response to the needs of power grid dispatching and equipment maintenance, system operation state switching is required, and when the island operation switching is carried out in the current project, the controller of the flexible direct current system only collects the operation state before the switching as the initial value of the control loop after the system switching. Due to the simple processing strategy, there is still a certain degree of power fluctuation during system switching, which causes the system switching to be not smooth enough. SUMMARY

[0006] Therefore, the present application provides a method and device for smooth switching of island operation of an AC / DC hybrid system to solve the problem of how to realize the smooth switching of new energy station from AC / DC parallel operation state to island operation state.

[0007] In a first aspect, the present application provides a method for smooth switching of island operation of an AC-DC hybrid system, the AC-DC hybrid system comprising: an AC parallel branch and a DC transmission system in parallel, the method comprising: before receiving an island operation switching instruction, controlling the DC converter by using a power control outer loop and a current control inner loop, the phase in the control loop being generated according to the three-phase grid voltage by using a phase-locked loop; after receiving the island operation switching instruction, controlling the AC parallel branch power to be 0 by dynamically adjusting the given value of the power control outer loop, and synchronizing the amplitude and phase of the reference voltage of the V / F control outer loop with the grid voltage; during execution of the island operation switching instruction, disconnecting the bypass switch of the AC parallel branch, switching the power control outer loop to the V / F control outer loop, wherein the phase in the control loop is synchronized with the phase of the grid voltage, and the initial value of the current reference instruction output by the V / F control outer loop is determined according to the output value of the power control outer loop before execution of the switching instruction; after completion of the switching, controlling the DC converter by using the V / F control outer loop and the current control inner loop, wherein the phase in the control loop is generated by the control loop according to the standard frequency of the grid, and the amplitude of the reference voltage of the V / F control outer loop is a preset voltage constant value.

[0008] In an optional embodiment, the method for smooth switching of island operation of an AC-DC hybrid system further comprises: after receiving the island operation switching instruction and during execution of the island operation switching instruction, obtaining the amplitude of the reference voltage of the V / F control outer loop by using AC voltage synchronization control based on the three-phase grid voltage; after completion of the switching, taking the preset voltage constant value as the amplitude of the reference voltage of the V / F control outer loop.

[0009] In an optional embodiment, the method for smooth switching of island operation of an AC-DC hybrid system further comprises: obtaining an active power compensation value by using a PI regulator based on the active power of the AC bypass branch; obtaining a reactive power compensation value by using a PI regulator based on the reactive power of the AC bypass branch; obtaining an active power reference value of the power control outer loop by compensating a preset active power constant value by using the active power compensation value; obtaining a reactive power reference value of the power control outer loop by compensating a preset reactive power constant value by using the reactive power compensation value.

[0010] In an optional embodiment, the process of determining the initial value of the current reference instruction output by the V / F control outer loop comprises: before execution of the switching instruction, continuously recording the power control outer loop output values of the MMC of the DC converter in multiple control periods; obtaining the initial value of the current reference instruction output by the V / F control outer loop by moving average processing of the power control outer loop output values in multiple control periods.

[0011] In an alternative embodiment, before receiving the island operation switching instruction, the process of generating the phase of the coordinate transformation based on the three-phase grid voltage using a phase-locked loop comprises: obtaining an angular frequency compensation value using a PI regulator based on the q-axis three-phase grid voltage; and integrating the grid standard angular frequency compensation value to obtain the phase of the coordinate transformation.

[0012] In an alternative embodiment, after receiving the island operation switching instruction, the process of controlling the phase of the reference voltage of the V / F control outer loop to be synchronized with the grid voltage comprises: integrating the grid standard angular frequency compensation value to obtain the phase of the reference voltage.

[0013] In an alternative embodiment, after the switching is completed, the process of generating the phase in the control loop comprises: integrating the grid standard angular frequency compensation value to obtain the phase of the control loop.

[0014] In a second aspect, the present application provides an island operation smooth switching device for an AC-DC hybrid system, the AC-DC hybrid system comprising: an AC parallel branch and a DC converter connected in parallel, comprising: a first control module for controlling the DC converter using a power control outer loop and a current control inner loop before receiving an island operation switching instruction, the phase in the control loop being generated based on the three-phase grid voltage using a phase-locked loop; a second control module for controlling the AC parallel branch power to be 0 by dynamically adjusting the given value of the power control outer loop, and controlling the amplitude and phase of the reference voltage of the V / F control outer loop to be synchronized with the grid voltage after receiving the island operation switching instruction; a third control module for disconnecting the bypass switch of the AC parallel branch and switching the power control outer loop to the V / F control outer loop during execution of the island operation switching instruction, wherein the phase in the control loop is synchronized with the phase of the grid voltage, and the initial value of the current reference instruction output by the V / F control outer loop is determined according to the output value of the power control outer loop before the switching command is executed; and a fourth control module for controlling the DC converter using the V / F control outer loop and the current control inner loop after the switching is completed, wherein the phase in the control loop is generated based on the grid standard frequency, and the amplitude of the reference voltage of the V / F control outer loop is a preset voltage constant value.

[0015] In a third aspect, the present application provides a computer device, comprising: a memory and a processor, which are communicatively connected to each other, and the memory stores computer instructions, and the processor executes the computer instructions to perform the island operation smooth switching method for an AC-DC hybrid system according to the first aspect or any one of the corresponding embodiments thereof.

[0016] In a fourth aspect, the present application provides a computer readable storage medium, which stores computer instructions for causing a computer to execute the islanding operation smooth switching method for an AC-DC hybrid system according to the first aspect or any one of the corresponding embodiments thereof.

[0017] In a fifth aspect, the present application provides a computer program product comprising computer instructions for causing a computer to execute the islanding operation smooth switching method for an AC-DC hybrid system according to the first aspect or any one of the corresponding embodiments thereof.

[0018] The present application makes the AC parallel branch power zero by dynamically adjusting the power control outer ring given value, eliminates the power impact at the switching moment, and at the same time, the V / F control outer ring reference voltage is synchronized with the grid voltage, avoiding the mutation of voltage amplitude and phase, greatly reducing the influence of transient disturbance on the system. The phase is always synchronized with the grid voltage phase during the switching process, ensuring the consistency of the control coordinate system; after the switching is completed, the phase in the control loop is generated automatically according to the grid standard frequency phase. Thus, the islanding operation smooth switching is realized.

[0019] The present application realizes the smooth switching of the new energy station from AC-DC parallel operation to islanding operation state by synchronizing, state acquisition and processing of the electrical quantities before and after the system switching. Compared with the switching scheme used in the existing project, the voltage, current and power fluctuations in the switching process can be obviously reduced, and the impact on the system can be reduced.

[0020] The smooth switching of the AC-DC hybrid system can reduce the impact of the switching process on the primary equipment such as switches and converter valves, and prolong the service life of the equipment. For the areas with weak AC grid, the system switching can also prevent the grid instability, reduce the risk of new energy station off-grid caused by switching, and improve the stability, reliability and security of the system. BRIEF DESCRIPTION OF DRAWINGS

[0021] In order to more clearly illustrate the specific embodiments of the present application or the technical solutions in the prior art, the following will briefly introduce the drawings needed to be used in the specific embodiments or prior art description. Obviously, the drawings described below are some embodiments of the present application, and those skilled in the art can also obtain other drawings according to these drawings without creative labor.

[0022] Figure 1 is a flowchart of the islanding operation smooth switching method for an AC-DC hybrid system according to an embodiment of the present application;

[0023] Figure 2 is a topology diagram of an AC-DC hybrid system according to an embodiment of the present application;

[0024] Figure 3 is a control block diagram of an AC parallel branch power zero algorithm according to an embodiment of the present application;

[0025] Figure 4 is a control block diagram of AC voltage loop given synchronization according to an embodiment of the present application;

[0026] Figure 5 is a control block diagram of a control loop self-generating grid synchronization phase and control loop phase switching according to an embodiment of the present application;

[0027] Figure 6 is a flowchart of a method for smooth switching of island operation of an AC-DC hybrid system according to an embodiment of the present application;

[0028] Figure 7 is a control block diagram of a current loop given value synchronization algorithm according to an embodiment of the present application;

[0029] Figure 8 is a hardware structure schematic diagram of a computer device according to an embodiment of the present application. DETAILED DESCRIPTION

[0030] In order to make the objectives, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are some but not all of the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those skilled in the art without creative work fall within the protection scope of the present application.

[0031] According to an embodiment of the present application, a method for smooth switching of island operation of an AC-DC hybrid system is provided. It should be noted that the steps shown in the flowchart of the drawings can be executed in a computer system such as a set of computer executable instructions, and although a logical order is shown in the flowchart, in some cases, the steps shown or described can be executed in an order different from that shown here.

[0032] In the present embodiment, a method for smooth switching of island operation of an AC-DC hybrid system is provided, which can be used in the mobile terminal described above, Figure 1 is a flowchart of a method for smooth switching of island operation of an AC-DC hybrid system according to an embodiment of the present application, as shown in Figure 2 , the flowchart includes the following steps: Figure 1

[0033] ​Step S1: Before receiving the island operation switching instruction, the DC converter is controlled by a power control outer loop and a current control inner loop, and the phase in the control loop is generated according to the three-phase grid voltage based on a phase-locked loop.

[0034] Specifically, in the initial operation stage of the system (when the island operation switching instruction is not received), the DC converter adopts a double-layer control structure of "power control outer loop + current control inner loop". Among them, the power control outer loop is responsible for generating the reference current value of the current control inner loop according to the grid scheduling instruction or the preset power target; the current control inner loop realizes accurate regulation of the converter output current by tracking the reference current in real time, and ensures stable power transmission from the DC side to the AC system. In this stage, the phase information used for coordinate transformation (such as converting the three-phase stationary coordinate system into the two-phase rotating coordinate system) in the control loop is generated in real time by a phase-locked loop (PLL) based on the three-phase grid voltage signal. The phase-locked loop dynamically tracks the phase change of the grid voltage to provide a synchronous reference for coordinate transformation, ensures that the entire control loop is strictly synchronized with the grid voltage, and thus realizes the grid-connected operation control of the converter.

[0035] Optionally, as shown in Figure 3 , the active power compensation value (i.e. P ac ) is obtained based on the active power (i.e. P ref_comp ) of the AC bypass branch by using a PI regulator; the reactive power compensation value (i.e. Q ac ) is obtained based on the reactive power (i.e. Q ref_comp ) of the AC bypass branch by using a PI regulator; the active power reference value (i.e. P ref_i ) of the power control outer loop is obtained by compensating the preset active power set value (i.e. P ref ) using the active power compensation value; the reactive power reference value (i.e. Q ref_i ) of the power control outer loop is obtained by compensating the preset reactive power set value (i.e. Q ref ) using the reactive power compensation value.

[0036] Step S2: After receiving the island operation switching instruction, the given value of the power control outer loop is dynamically adjusted to control the AC parallel branch power to be 0, and the amplitude and phase of the reference voltage of the V / F control outer loop are synchronized with the grid voltage.

[0037] Specifically, when the system receives the island operation switching instruction, it enters the switching preparation stage. At this time, the control system first adjusts the power given value of the power control outer ring by dynamic adjustment (such as using a ramp function or an exponential function to adjust step by step), so that the transmission power of the AC parallel branch is smoothly reduced to 0, avoiding system oscillation caused by power mutation. At the same time, the reference voltage amplitude and phase of the V / F control outer ring (voltage / frequency control outer ring) are adjusted to be completely synchronized with the current grid voltage, that is, the reference voltage amplitude tracks the real-time amplitude change of the grid voltage, and the phase is consistent with the phase of the grid voltage output by the phase-locked loop. The core purpose of this operation is to build a "no difference transition reference" for subsequent control mode switching, ensuring that there is no amplitude difference and phase difference between the output voltage of the converter and the grid voltage at the switching moment, laying the foundation for smooth switching.

[0038] Step S3: During the execution of the island operation switching instruction, the bypass switch of the AC parallel branch is turned off, and the power control outer ring is switched to the V / F control outer ring, wherein the phase in the control loop is synchronized with the phase of the grid voltage, and the initial value of the current reference instruction output by the V / F control outer ring is determined according to the output value of the power control outer ring before the execution of the switching command.

[0039] Specifically, during the transition process of executing the island operation switching instruction, the control system completes the mode switching from the "power control outer ring" to the "V / F control outer ring". In this stage, the phase in the control loop is synchronized with the phase of the grid voltage to avoid current shock caused by sudden change of the phase reference during the switching process. At the same time, the initial current reference instruction output by the V / F control outer ring is strictly determined according to the output current value of the power control outer ring before the execution of the switching command (such as directly inheriting the instantaneous value as the initial value). This design effectively suppresses the current fluctuation that may occur at the moment of control mode switching by maintaining the continuity of the current instruction, ensuring smooth transition of the converter output characteristics.

[0040] Optionally, the process of determining the initial value of the current reference instruction output by the V / F control outer ring includes: continuously recording the power control outer ring output value of the DC converter for multiple control cycles of the sending end MMC before the execution of the switching command; and obtaining the initial value of the current reference instruction output by the V / F control outer ring by moving average processing of the power control outer ring output values of multiple control cycles.

[0041] Specifically, within a preset time window before the switching command is executed, the control system initiates a real-time recording of the power control outer loop output value of the sending end modular multilevel converter (MMC) of the direct current transmission system. This recording process is in units of the basic control period of the converter, continuously collecting the current reference value of the power control outer loop output in each period, which directly reflects the real-time control amount output by the converter in the grid-connected operation state to maintain power balance, and its dynamic change is closely related to disturbance factors such as grid load fluctuation, new energy output change, etc. By continuously recording the output values of multiple control periods, the dynamic characteristics of the power control outer loop at the moment before switching can be completely captured, providing comprehensive raw data support for subsequent initial value determination.

[0042] After completing the multi-cycle data acquisition, the recorded power control outer loop output values are subjected to moving average processing. Specifically, a sliding window algorithm is used to smooth the time series data: in chronological order, the output values of the continuous N control periods are taken as a calculation window, and by calculating the arithmetic mean of all data in the window, a smoothed intermediate value is obtained; then the window is moved by a time step, and the above calculation process is repeated until all collected data are covered. Finally, the smoothed value corresponding to the switching command triggering time is determined as the current reference instruction initial value of the V / F control outer loop output.

[0043] Step S4: After completing the switching, the direct current converter is controlled using the V / F control outer loop and the current control inner loop, wherein the phase in the control loop is generated according to the grid standard frequency, and the amplitude of the reference voltage of the V / F control outer loop is a preset voltage constant value.

[0044] Specifically, after completing the control mode switching, the system enters the stable island operation phase. At this time, the direct current converter officially adopts the control structure of "V / F control outer loop + current control inner loop": the V / F control outer loop maintains the stability of the output voltage amplitude and frequency (replaces the power regulation function of the original power control outer loop), providing voltage and frequency support for the island system; the current control inner loop continues to play the role of current tracking, ensuring the dynamic response performance of the converter output current. At the same time, the control system sends a command to disconnect the bypass switch of the AC parallel branch, forming an independent island operation network. In this phase, the phase information used for coordinate transformation in the control loop is generated based on the grid standard frequency, and the reference voltage amplitude of the V / F control outer loop is switched to a preset rated voltage constant value, ensuring that the voltage level of the AC system during island operation remains within the rated range, providing stable power supply for local loads.

[0045] In some alternative embodiments, the islanding operation smooth switching method for the AC-DC hybrid system further comprises: after receiving the islanding operation switching instruction and during the execution of the islanding operation switching instruction, obtaining the amplitude of the reference voltage of the V / F control outer loop based on the three-phase grid voltage by using the AC voltage control loop; and after the switching is completed, taking the preset voltage constant as the amplitude of the reference voltage of the V / F control outer loop.

[0046] Specifically, during the transition period from when the system receives the islanding operation switching instruction to when the entire switching operation is completed, the reference voltage amplitude of the V / F control outer loop is not directly taken as a fixed value, but is dynamically generated in real time by the AC voltage control loop. As shown in Figure 4 , the three-phase grid voltage (i.e. V g_abc ) is taken as the core input signal, and the reference instruction strictly matched with the current grid voltage amplitude is generated through real-time detection, filtering and closed-loop adjustment of the voltage amplitude. Specifically, the control loop continuously acquires the instantaneous value of the three-phase voltage on the AC side and obtains the d-axis voltage component; then, the d-axis voltage component is taken as the reference voltage amplitude of the V / F control outer loop. The core purpose of this design is to make the voltage reference of the V / F control consistent with the actual grid voltage at all times during the switching transition period, avoiding voltage impact caused by the deviation between the reference value and the actual value, and providing a no-difference voltage transition condition for subsequent control mode switching.

[0047] After the islanding operation switching is completed, the system formally enters the independent operation state, and at this time the reference voltage amplitude of the V / F control outer loop is switched to the preset voltage constant (i.e. V gd_ref_i ). The preset value is the rated voltage level preset according to the system design specification and load characteristics, which is usually stored in the parameter configuration module of the control system. After switching to the fixed value, the V / F control loop will take the constant as the reference to ensure that the amplitude of the inverter output voltage is stable within the rated range and is not affected by external grid fluctuations.

[0048] In some alternative embodiments, before receiving the islanding operation switching instruction, the process of generating the phase of coordinate transformation based on the three-phase grid voltage by using the phase-locked loop comprises: obtaining an angular frequency compensation value by using a PI regulator based on the q-axis three-phase grid voltage; and integrating the grid standard angular frequency after compensation by using the angular frequency compensation value to obtain the phase of coordinate transformation.

[0049] Specifically, as shown in Figure 5 , the q-axis component (i.e. V gq) as the key input signal of the phase-locked loop. In the transformation from the three-phase stationary coordinate system to the two-phase rotating coordinate system, the q-axis grid voltage is introduced into a PI regulator (proportional-integral regulator) to quickly respond to transient deviations through the proportional link and eliminate steady-state errors through the integral link, and finally output an angular frequency compensation value. The physical meaning of the compensation value is that, in order to lock the grid phase, a dynamic adjustment amount needs to be superimposed on the basis of the standard angular frequency of the grid, and the value is positively correlated with the deviation degree of the q-axis voltage, and the sign is determined by the deviation direction (leading or lagging).

[0050] Subsequently, the angular frequency compensation value is superimposed on the standard angular frequency of the grid (i.e., ω N ) to obtain a real-time angular frequency (i.e., ω PLL ) actually used for phase calculation. Integrating the compensated real-time angular frequency can obtain the phase information (i.e., θ PLL ) required for coordinate transformation.

[0051] In some optional embodiments, after receiving the island operation switching instruction, the process of synchronizing the phase of the reference voltage controlled by the V / F control outer ring with the grid voltage, and the process of generating the phase for coordinate transformation according to the grid voltage phase during the execution of the island operation switching instruction, include: obtaining an angular frequency compensation value by using a PI regulator based on the difference between the grid voltage phase and the phase of the reference voltage; and compensating the standard angular frequency of the grid by using the angular frequency compensation value and then integrating to obtain the phase of the reference voltage.

[0052] Specifically, referring to Figure 5 , the actual phase (i.e., θ grid ) of the grid voltage and the phase (θ vf ) of the reference voltage output by the V / F control outer ring are collected in real time, and the difference between the two is calculated. This phase difference directly reflects the deviation degree of the reference voltage from the grid voltage in phase, and is the core feedback signal for realizing synchronous control. Subsequently, the phase difference is input into a PI regulator for processing: the proportional link can quickly respond to transient phase deviations, so that the phase of the reference voltage quickly approaches the phase of the grid voltage; and the integral link is responsible for eliminating the residual phase deviation in the steady state, so that the two ultimately achieve error-free synchronization. The output obtained after the PI regulator operation is the angular frequency compensation value, which is superimposed on the standard angular frequency of the grid (i.e., ω N ) to obtain a real-time adjustment angular frequency (ω vf ) for phase synchronization.

[0053] In an actual application scenario, after receiving the island operation switching instruction, in order to realize the accurate synchronization of the reference voltage phase of the V / F control outer loop and the grid voltage phase, and to generate the phase required for coordinate transformation according to the grid voltage phase during the switching execution process, the system adopts the control logic based on phase deviation closed-loop regulation. The specific system switching process and the synchronization and switching logic of the current reference are described with reference to Figure 6 and Figure 7 respectively, and the specific process is as follows:

[0054] When the island operation switching instruction is received, the process of synchronizing the reference voltage phase of the V / F control outer loop with the grid voltage is started. At this time, the system will collect the actual phase of the grid voltage and the reference voltage phase output by the V / F control outer loop in real time, and calculate the difference between the two. This phase difference intuitively reflects the degree of deviation of the reference voltage from the grid voltage in phase, and is the core feedback signal for realizing synchronization control. Subsequently, the phase difference is input into the PI regulator for processing: the proportional element can quickly respond to the instantaneous phase deviation, so that the reference voltage phase quickly approaches the grid voltage phase; the integral element is responsible for eliminating the residual phase deviation in the steady state, ensuring that the two eventually achieve zero-error synchronization. The output obtained after the PI regulator operation is the angular frequency compensation value. After obtaining the angular frequency compensation value, it is superimposed with the standard angular frequency of the grid to obtain the real-time adjustment angular frequency used for phase calculation. Integrating the real-time adjustment angular frequency can obtain the phase of the V / F control outer loop reference voltage. Through such a dynamic adjustment mechanism, the phase of the reference voltage can track the change of the grid voltage phase in real time, ensuring that the two are always strictly synchronized during the switching preparation stage, creating conditions for the subsequent control mode switching without phase impact.

[0055] During the execution of the island operation switching instruction, the process of generating the coordinate transformation phase according to the grid voltage phase is consistent with the above-mentioned synchronization control logic of the reference voltage phase. At this time, the system still takes the grid voltage phase as the reference, generates the phase required for coordinate transformation through the same phase deviation detection, PI regulation to generate the angular frequency compensation value, superimposes the standard angular frequency and integrates.

[0056] In the embodiment, an island operation smooth switching device for an AC / DC hybrid system is also provided, which is used to realize the above-mentioned embodiments and preferred embodiments, and will not be described again. As used below, the term "module" can be a combination of software and / or hardware that implements a predetermined function. Although the device described in the following embodiments is preferably implemented in software, hardware, or a combination of software and hardware is also possible and contemplated.

[0057] This embodiment provides a smooth switching device for islanded operation in an AC / DC hybrid system. The AC / DC hybrid system includes: parallel AC branches and a DC converter, including:

[0058] The first control module is used to control the DC converter using an outer power control loop and an inner current control loop before receiving an islanding operation switching command. The phase in the control loop is generated using a phase-locked loop based on the three-phase grid voltage.

[0059] The second control module is used to receive the islanded operation switching command, dynamically adjust the power control outer loop setpoint, control the AC parallel branch power to 0, and control the amplitude and phase of the reference voltage of the V / F control outer loop to synchronize with the grid voltage.

[0060] The third control module is used to disconnect the bypass switch of the AC parallel branch during the execution of the islanding operation switching command, and switch the power control outer loop to the V / F control outer loop. The phase in the control loop is synchronized with the grid voltage phase, and the current reference command initial value output by the V / F control outer loop is determined according to the power control outer loop output value before the switching command is executed.

[0061] The fourth control module is used to control the DC converter using a V / F control outer loop and a current control inner loop after the switching is completed. The phase in the control loop is automatically generated according to the standard frequency of the power grid, and the amplitude of the reference voltage of the V / F control outer loop is a preset voltage setpoint.

[0062] Further functional descriptions of the above modules and units are the same as those in the corresponding embodiments described above, and will not be repeated here.

[0063] In this embodiment, the islanding smooth switching device for AC / DC hybrid systems is presented in the form of a functional unit. Here, a unit refers to an ASIC (Application Specific Integrated Circuit) circuit, a processor and memory that execute one or more software or fixed programs, and / or other devices that can provide the above functions.

[0064] This invention also provides a computer device having the above-described smooth switching device for islanded operation of an AC / DC hybrid system.

[0065] Please see Figure 8 , Figure 8 This is a schematic diagram of the structure of a computer device provided in an optional embodiment of the present invention, such as... Figure 8As shown, the computer device includes one or more processors 10, memory 20, and interfaces for connecting the components, including high-speed interfaces and low-speed interfaces. The components communicate with each other via different buses and can be mounted on a common motherboard or otherwise installed as needed. The processors can process instructions executed within the computer device, including instructions stored in or on memory to display graphical information of a GUI on external input / output devices (such as display devices coupled to the interfaces). In some alternative implementations, multiple processors and / or multiple buses can be used with multiple memories and multiple memory modules, if desired. Similarly, multiple computer devices can be connected, each providing some of the necessary operations (e.g., as a server array, a group of blade servers, or a multiprocessor system). Figure 8 Take a processor 10 as an example.

[0066] Processor 10 may be a central processing unit, a network processor, or a combination thereof. Processor 10 may further include a hardware chip. The hardware chip may be an application-specific integrated circuit (ASIC), a programmable logic device (PLD), or a combination thereof. The programmable logic device may be a complex programmable logic device (CAMP), a field-programmable gate array (FPGA), a general-purpose array logic (GDA), or any combination thereof.

[0067] The memory 20 stores instructions executable by at least one processor 10 to cause at least one processor 10 to perform the method shown in the above embodiments.

[0068] The memory 20 may include a program storage area and a data storage area. The program storage area may store the operating system and applications required for at least one function; the data storage area may store data created based on the use of the computer device. Furthermore, the memory 20 may include high-speed random access memory and may also include non-transitory memory, such as at least one disk storage device, flash memory device, or other non-transitory solid-state storage device. In some alternative embodiments, the memory 20 may optionally include memory remotely located relative to the processor 10, and these remote memories may be connected to the computer device via a network. Examples of such networks include, but are not limited to, the Internet, intranets, local area networks, mobile communication networks, and combinations thereof.

[0069] The memory 20 may include volatile memory, such as random access memory; the memory may also include non-volatile memory, such as flash memory, hard disk or solid-state drive; the memory 20 may also include a combination of the above types of memory.

[0070] The computer device also includes an input device 30 and an output device 40. The processor 10, memory 20, input device 30, and output device 40 can be connected via a bus or other means.Figure 8 Taking the example of a connection between China and Israel via a bus.

[0071] Input device 30 can receive input numerical or character information, and generate key signal inputs related to user settings and function control of the computer device, such as a touchscreen, keypad, mouse, trackpad, touchpad, joystick, one or more mouse buttons, trackball, joystick, etc. Output device 40 may include display devices, auxiliary lighting devices (e.g., LEDs), and haptic feedback devices (e.g., vibration motors). The aforementioned display devices include, but are not limited to, liquid crystal displays, light-emitting diodes, displays, and plasma displays. In some alternative embodiments, the display device may be a touchscreen.

[0072] This invention also provides a computer-readable storage medium. The methods described above according to embodiments of the invention can be implemented in hardware or firmware, or implemented as computer code that can be recorded on a storage medium, or implemented as computer code downloaded via a network and originally stored on a remote storage medium or a non-transitory machine-readable storage medium and then stored on a local storage medium. Thus, the methods described herein can be processed by software stored on a storage medium using a general-purpose computer, a dedicated processor, or programmable or dedicated hardware. The storage medium can be a magnetic disk, optical disk, read-only memory, random access memory, flash memory, hard disk, or solid-state drive, etc.; further, the storage medium can also include combinations of the above types of memory. It is understood that computers, processors, microprocessor controllers, or programmable hardware include storage components capable of storing or receiving software or computer code, which, when accessed and executed by the computer, processor, or hardware, implements the methods shown in the above embodiments.

[0073] A portion of this invention can be applied as a computer program product, such as computer program instructions, which, when executed by a computer, can invoke or provide the methods and / or technical solutions according to the invention through the operation of the computer. Those skilled in the art will understand that the forms in which computer program instructions exist in a computer-readable medium include, but are not limited to, source files, executable files, installation package files, etc. Correspondingly, the ways in which computer program instructions are executed by a computer include, but are not limited to: the computer directly executing the instructions, or the computer compiling the instructions and then executing the corresponding compiled program, or the computer reading and executing the instructions, or the computer reading and installing the instructions and then executing the corresponding installed program. Here, the computer-readable medium can be any available computer-readable storage medium or communication medium accessible to a computer.

[0074] Although embodiments of the invention have been described in conjunction with the accompanying drawings, those skilled in the art can make various modifications and variations without departing from the spirit and scope of the invention, and such modifications and variations all fall within the scope defined by the appended claims.

Claims

1. A method for smooth switching of islanded operation in an AC / DC hybrid system, characterized in that, The AC / DC hybrid system includes: parallel AC branches and a DC transmission system, and the method includes: Before receiving the islanding operation switching command, the DC converter at the sending end of the DC transmission system is controlled by the power control outer loop and the current control inner loop. The phase in the control loop is generated by the phase-locked loop based on the three-phase grid voltage. Upon receiving the islanded operation switching command, the system dynamically adjusts the power control outer loop setpoint to control the AC parallel branch power to 0, and controls the amplitude and phase of the reference voltage of the V / F control outer loop to synchronize with the grid voltage. During the execution of the islanded operation switching command, the bypass switch of the AC parallel branch is disconnected, and the power control outer loop is switched to the V / F control outer loop. The phase in the control loop is synchronized with the grid voltage phase, and the current output of the V / F control outer loop is determined based on the power control outer loop output value before the switching command is executed. After the switching is completed, the sending-end DC converter is controlled by the V / F control outer loop and the current control inner loop. The phase in the control loop is automatically generated according to the grid standard frequency, and the amplitude of the reference voltage of the V / F control outer loop is a preset voltage setpoint.

2. The method for smooth switching of islanded operation in an AC / DC hybrid system according to claim 1, characterized in that, Also includes: Upon receiving the islanding operation switching command and during the execution of the islanding operation switching command, the amplitude of the reference voltage of the V / F control outer loop is obtained based on the three-phase grid voltage and using AC voltage synchronization control. After the switching is completed, the preset voltage setpoint will be used as the amplitude of the reference voltage for the V / F control outer loop.

3. The method for smooth switching of islanded operation in an AC / DC hybrid system according to claim 1, characterized in that, Also includes: Based on the active power of the AC bypass branch, the active power compensation value is obtained using a PI regulator. Based on the reactive power of the AC bypass branch, the reactive power compensation value is obtained using a PI regulator. After compensating the preset active power setpoint with the active power compensation value, the active power reference value of the power control outer loop is obtained; After compensating the preset reactive power setpoint with the reactive power compensation value, the reactive power reference value of the power control outer loop is obtained.

4. The method for smooth switching of islanded operation in an AC / DC hybrid system according to claim 1, characterized in that, The process of determining the initial value of the current reference command output of the V / F control outer loop includes: Before executing the switching command, continuously record the power control outer loop output value of the DC converter at the sending end of the DC transmission system for multiple control cycles; After performing a moving average on the power control outer loop output values ​​over multiple control cycles, the initial value of the current reference command output of the V / F control outer loop is obtained.

5. The method for smooth switching of islanded operation in an AC / DC hybrid system according to claim 1, characterized in that, Before receiving the islanding operation switching command, the process of generating the phase of the coordinate transformation using a phase-locked loop based on the three-phase grid voltage includes: Based on the q-axis three-phase grid voltage, the angular frequency compensation value is obtained using a PI regulator; The phase of the coordinate transformation is obtained by integrating the angular frequency compensation value with the standard angular frequency of the power grid.

6. The method for smooth switching of islanded operation in an AC / DC hybrid system according to claim 1, characterized in that, Upon receiving the islanding operation switching command, the process of synchronizing the phase of the reference voltage of the outer loop of the V / F control system with the grid voltage includes: The angular frequency compensation value is obtained by using a PI regulator based on the difference between the phase of the grid voltage and the phase of the reference voltage. The phase of the reference voltage is obtained by integrating the angular frequency compensation value with the standard angular frequency of the power grid.

7. The method for smooth switching of islanded operation in an AC / DC hybrid system according to claim 1, characterized in that, After the switch is completed, the phase generation process in the control loop includes: The phase in the control loop is obtained by integrating after compensating for the standard angular frequency of the power grid.

8. A device for smooth switching of islanded operation in an AC / DC hybrid system, characterized in that, The AC / DC hybrid system includes: parallel AC branches and a DC output system, and the device includes: The first control module is used to control the DC converter at the sending end of the DC transmission system using a power control outer loop and a current control inner loop before receiving the islanding operation switching command. The phase in the control loop is generated by a phase-locked loop based on the three-phase grid voltage. The second control module is used to receive the islanded operation switching command, dynamically adjust the power control outer loop setpoint, control the AC parallel branch power to 0, and control the amplitude and phase of the reference voltage of the V / F control outer loop to synchronize with the grid voltage. The third control module is used to disconnect the bypass switch of the AC parallel branch during the execution of the islanded operation switching command, and switch the power control outer loop to the V / F control outer loop. The phase in the control loop is synchronized with the grid voltage phase, and the current reference command initial value output by the V / F control outer loop is determined according to the power control outer loop output value before the switching command is executed. The fourth control module is used to control the DC converter using a V / F control outer loop and a current control inner loop after the switching is completed. The phase in the control loop is generated automatically by the control loop according to the standard frequency of the power grid, and the amplitude of the reference voltage of the V / F control outer loop is a preset voltage setpoint.

9. A computer device, characterized in that, include: A memory and a processor are communicatively connected, the memory stores computer instructions, and the processor executes the computer instructions to perform the islanding smooth switching method for an AC / DC hybrid system as described in any one of claims 1 to 7.

10. A computer-readable storage medium, characterized in that, The computer-readable storage medium stores computer instructions for causing the computer to execute the islanding smooth switching method for an AC / DC hybrid system as described in any one of claims 1 to 7.

11. A computer program product, characterized in that, Includes computer instructions for causing a computer to execute any one of claims 1 to 7 of the method for smooth switching of islanded operation in an AC / DC hybrid system.

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