A hostless black start method and microgrid system

Abstract

This invention relates to the field of new energy inverter microgrid technology, and in particular to a hostless black start method and microgrid system. By eliminating the need for a host, each inverter autonomously generates a random delay duration after power-on and enters the grid connection preparation state during off-peak hours. Each inverter operates either pre-synchronous grid connection or fixed voltage zero-crossing point grid connection based on the voltage state on the microgrid bus. No host is required throughout the process, and even if a single inverter malfunctions, it will not hinder the normal startup of other devices. This completely eliminates dependence on a host, and the entire process can automatically complete the microgrid black start without manual intervention to reconfigure the host, effectively improving the automation level and operational stability of the microgrid black start.

Description

Technical Field

[0001] This invention relates to the field of new energy inverter microgrid technology, and in particular to a hostless black start method and microgrid system. Background Technology

[0002] With the use of new energy power generation such as photovoltaic, wind power and hydropower, many users are using new energy to form their own microgrid system to operate independently without relying on the municipal power grid. In a microgrid system, there are usually multiple inverters that output power together. The startup process of a microgrid system is called black start.

[0003] Black start requires strict adherence to the start-up procedure, which is generally as follows: In a microgrid system, there is one inverter as the main unit. Before starting, all loads should generally be turned off to ensure that the load power is less than the rated output power of the main unit when it starts up. The main unit starts first, and after the main unit starts up, the other inverters are connected to the grid synchronously. Only after all inverters have started up can the loads be started, thus completing the entire black start process.

[0004] Currently, during black start, an inverter must be designated as the primary starter. Therefore, the startup of the microgrid system depends on the primary starter. If the primary starter fails to start for any reason, another primary starter needs to be manually designated to complete the black start. In this startup method, the microgrid system startup depends on a single primary starter. When the primary starter fails to start for any reason, the entire microgrid system cannot be established and needs to be reconfigured before it can start.

[0005] Therefore, overcoming the shortcomings of the existing technology is an urgent problem to be solved in this technical field. Summary of the Invention

[0006] The technical problem that this invention aims to solve is that existing microgrid black start relies on a designated host to start first. If the host fails, the entire network will fail to start, and a host needs to be manually redesignated, resulting in low reliability and automation of the startup process.

[0007] The present invention adopts the following technical solution: Firstly, a hostless black-start method is provided, applied to a microgrid including multiple inverters, the method comprising: After the microgrid is powered on, each inverter generates a random delay duration and waits for the corresponding random delay duration to end. After the corresponding random delay period ends, the inverter enters the grid connection preparation state and detects the voltage on the microgrid bus according to the preset control cycle; When a voltage is detected on the microgrid bus, a pre-synchronization operation is performed to complete grid connection; When no voltage is detected on the microgrid bus, grid connection is triggered at a fixed voltage zero-crossing point. After all inverters have completed grid connection, the microgrid achieves black start without a master unit.

[0008] Preferably, after the microgrid is powered on, each inverter generates a random delay duration and waits for the corresponding delay to end, specifically including: After the microgrid is powered on and all inverters have completed their device initialization and self-test simultaneously, each inverter independently generates its own random delay duration. During the timing of their respective random delay periods, all inverters remain in standby locked state and do not perform voltage detection, synchronization adjustment, or grid connection actions. Once the corresponding random delay period of any inverter ends, they exit the standby locked state.

[0009] Preferably, when a voltage is detected on the microgrid bus, performing a pre-synchronization operation to complete grid connection specifically includes: When the inverter detects a voltage on the microgrid bus, it continuously collects the voltage amplitude, power frequency and phase of the voltage on the microgrid bus at a preset control cycle, and also collects the voltage amplitude, power frequency and phase of its own output voltage in real time. The system performs difference calculations on data of the same type and dynamically adjusts its own output voltage through an internal closed-loop control circuit to gradually reduce the voltage amplitude difference, power frequency difference, and phase difference between the voltage and the voltage on the microgrid bus. Once the voltage amplitude difference, power frequency difference, and phase difference all fall within the preset safety threshold range, the pre-synchronization operation is determined to be complete. The grid-connected switching device is then driven to close the output circuit, enabling the inverter to stably connect to the microgrid bus, thereby completing the grid-connected operation.

[0010] Preferably, the preset safety threshold range includes voltage amplitude difference ≤3%, power frequency difference ≤0.2Hz, and phase difference ≤5°. When the real-time detected parameter differences all meet the threshold requirements, the synchronization operation is determined to be completed and grid connection is executed.

[0011] Preferably, the step of triggering grid connection at a fixed voltage zero-crossing point when no voltage is detected on the microgrid bus specifically includes: When the inverter does not detect a valid voltage signal on the microgrid bus for several consecutive preset control cycles, the inverter continuously monitors the voltage waveform timing and waits for the voltage to reach the zero-crossing point to drive the grid-connected switching device to close the output circuit, so that the inverter can stably connect to the microgrid bus and complete the grid-connected operation.

[0012] Preferably, the grid-connected switching device is a thyristor or a high-speed power device.

[0013] Preferably, the preset control cycle ranges from 50µs to 200µs.

[0014] Preferably, the voltage zero-crossing point is a voltage drop zero-crossing point in the power frequency sinusoidal AC voltage waveform where the instantaneous voltage value switches from positive to negative potential, or a voltage rise zero-crossing point where the voltage switches from negative to positive potential. Each inverter pre-selects one type of zero-crossing point as a fixed grid-connection trigger point, and all inverters in the microgrid use the same type of voltage zero-crossing point to perform grid-connection.

[0015] In a second aspect, a microgrid system is provided for implementing the hostless black-start method as described in the first aspect, comprising: multiple inverters with their output terminals connected in parallel to the microgrid bus, and each inverter having a grid-connected switching device connected in series with the microgrid bus.

[0016] Preferably, each inverter integrates a local control unit, a random delay generation unit, a voltage sampling unit, and a pre-synchronization adjustment unit. The local control unit is used to drive the random delay generation unit to generate a random delay duration after the inverter is powered on, and to complete the delay timing. After the delay timer ends, the voltage sampling unit is used to continuously collect the microgrid bus voltage signal according to the preset control cycle, determine whether the microgrid bus is in a state of having voltage or no voltage, and upload the detection result to the local control unit in real time. The pre-synchronization adjustment unit is activated when the voltage sampling unit detects voltage on the microgrid bus. It compares its own output voltage parameters with the microgrid bus voltage parameters in real time and dynamically adjusts the amplitude, frequency, and phase of its own output voltage until the conditions for synchronous grid connection are met.

[0017] Compared with the prior art, the beneficial effects of the present invention are as follows: This invention eliminates the need for a host computer. After power-on, each inverter automatically generates a random delay duration and enters the grid connection preparation state during off-peak hours. Each inverter operates either pre-synchronous grid connection or fixed voltage zero-crossing point grid connection based on the voltage status on the microgrid bus. No host computer is required throughout the process. Even if a single inverter malfunctions, it will not prevent the normal startup of other devices. This completely eliminates the dependence on a host computer. The entire process can automatically complete the microgrid black start without manual intervention to reconfigure the host computer, effectively improving the automation level and operational stability of the microgrid black start. Attached Figure Description

[0018] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0019] Figure 1 This is a schematic diagram of the structure of a microgrid system provided in an embodiment of the present invention; Figure 2 This is a schematic flowchart of a hostless black boot method provided in an embodiment of the present invention; Figure 3 This is another schematic flowchart of a hostless black boot method provided in an embodiment of the present invention; Figure 4 This is a waveform diagram of a voltage zero-crossing point provided in an embodiment of the present invention. Detailed Implementation

[0020] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0021] Unless the context otherwise requires, throughout the specification and claims, the term "comprising" is interpreted as openly inclusive, meaning "including, but not limited to." In the description of the specification, terms such as "one embodiment," "some embodiments," "exemplary embodiment," "example," "specific example," or "some examples" are intended to indicate that a particular feature, structure, material, or characteristic associated with that embodiment or example is included in at least one embodiment or example of this disclosure. The illustrative representations of the above terms do not necessarily refer to the same embodiment or example. Furthermore, the specific features, structures, materials, or characteristics mentioned may be included in any suitable manner in any one or more embodiments or examples; that is, although they may be incorporated into embodiments or examples using the above terms for reasons such as order and position, it does not limit them to be incorporated in combination by a single embodiment or example.

[0022] In the description of this invention, the terms "first" and "second" are used for descriptive purposes only and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features. Thus, a feature defined with "first" or "second" may explicitly or implicitly include one or more of that feature. In the description of embodiments of this disclosure, unless otherwise stated, "a plurality of" means two or more. Furthermore, for example, the description may use the prefix "A" or "B" to describe the same type of nouns as two independent entities. In this case, the corresponding features defined with "A" and "B" are used only to distinguish between similar entities and should not be construed as indicating or implying relative importance or implicitly specifying the number of indicated technical features.

[0023] In describing some embodiments, the terms "coupled," "coupled," and "connected," and their derivative expressions, may be used. For example, the term "connected" may be used in describing some embodiments to indicate that two or more components have direct physical or electrical contact with each other. Similarly, the term "coupled" may be used in describing some embodiments to indicate that two or more components have direct physical or electrical contact. However, the terms "connected" or "coupled" may also refer to two or more components that do not have direct contact with each other but still cooperate or interact with each other, such as "optical coupling," "wireless connection," etc. The embodiments disclosed herein are not necessarily limited to the scope of this invention.

[0024] Furthermore, the technical features involved in the various embodiments of the present invention described below can be combined with each other as long as they do not conflict with each other.

[0025] Example 1: In a home photovoltaic system using an inverter, one photovoltaic panel is paired with one inverter, and multiple inverters form a microgrid. The microgrid is connected to the mains grid via an anti-reverse current controller and a circuit breaker. For example... Figure 1 The image shows a typical connection method.

[0026] Under normal circumstances, the mains grid and the microgrid operate in parallel. When the load power is less than or equal to P (i.e., the total power of the three inverters), the anti-reverse current controller detects the sign of the power flow (positive means the mains grid is supplying power to the microgrid, and negative means the microgrid is supplying power to the mains grid). It controls the output power Pi of each inverter through wireless or wired communication, and controls the power at the anti-reverse current controller to 0, that is, the inverter output power P = the load power. The mains grid does not supply power to the microgrid, and the microgrid will not supply power to the grid in reverse. This is the ideal state.

[0027] In this complex operating scenario, the normal startup of a microgrid or its startup under abnormal conditions, such as a sudden power outage in the mains grid, requires a strict startup sequence. When the anti-reverse current controller communicates normally with each inverter, it can designate one inverter as the master inverter and start it first, then notify the other inverters to start up in grid-connected mode. However, this startup method relies on normal communication and all inverters functioning correctly. If the anti-reverse current controller malfunctions or communication is interfered with, safe startup of the microgrid becomes very difficult.

[0028] To overcome the technical shortcomings of traditional microgrid black start methods, such as high dependence on a dedicated master unit, master unit failure leading to grid-wide startup failure, the need for manual reassignment of a master unit, and poor automation and fault tolerance, this embodiment proposes a masterless black start method. This method is applied to microgrids with multiple inverters, where all inverters are equal in status, eliminating the hierarchical division of master and slave units. The entire distributed startup and grid connection is completed through the local autonomous logic of each inverter, requiring no upper-level controller scheduling or manual configuration intervention, effectively improving the reliability and stability of islanded black start in microgrids. In one implementation, such as... Figure 2 As shown, the hostless black boot method includes: Step 101: After the microgrid is powered on, each inverter generates a random delay duration and waits for the corresponding random delay duration to end.

[0029] The structure of the microgrid used in this embodiment can also be as follows: Figure 1 As shown. After the microgrid completes the overall power-on wake-up, all inverters synchronously complete equipment initialization and operation self-test. Each inverter independently generates a random delay duration within a preset range (e.g., 2s-5s) based on its local control logic. There is no fixed start priority or preset start order among the inverters, and they rely entirely on the random delay duration generated by themselves.

[0030] Meanwhile, each inverter independently performs delay timing, maintaining a standby locked state with the output disconnected during the delay waiting phase after power-on, without prematurely performing voltage detection and grid connection actions. By using each inverter's autonomous random delay to stagger peak waiting, grid disturbances caused by the simultaneous centralized startup of multiple inverters can be effectively avoided. At the same time, relying on the disordered nature of the random delay duration, any inverter has the possibility of priority startup and voltage build-up, eliminating the dependence on a dedicated startup host in traditional solutions from the architectural level.

[0031] Step 102: After the corresponding random delay period ends, the inverter enters the grid connection preparation state and detects the voltage on the microgrid bus according to the preset control cycle.

[0032] Once any inverter completes its random delay timeout period and enters the grid-connection preparation state, it immediately exits the standby lockout mode and enters the grid-connection preparation state. Inverters in the grid-connection preparation state continuously collect real-time voltage signals from the microgrid bus at a uniform preset control cycle and determine the energized status of the microgrid bus in real time.

[0033] Step 103: When voltage is detected on the microgrid bus, perform pre-synchronization operation and complete grid connection; when no voltage is detected on the microgrid bus, trigger grid connection at a fixed voltage zero crossing point. After all inverters are connected to the grid, realize black start of the microgrid without a master unit.

[0034] The inverter adaptively matches the grid connection strategy based on the real-time voltage detection results collected on the microgrid bus. The grid connection strategy includes: In one implementation, if a stable voltage is detected on the microgrid bus, it indicates that one or more inverters have already completed voltage building and grid connection in advance. At this time, the inverter starts pre-synchronization operation to actively calibrate the amplitude, frequency and phase of its own output voltage. After its own output voltage parameters are synchronized and matched with the bus voltage parameters, it can smoothly complete grid connection. This can effectively avoid grid connection inrush current and internal circulating current problems.

[0035] In one implementation, if no voltage is detected on the microgrid bus, the microgrid is determined to be in an initial empty state. The inverter locks a preset fixed voltage zero-crossing point as the trigger timing and instantaneously executes the grid connection action. Relying on the timing constraints of a unified control cycle, the phase deviation is ensured to be within a safe range when multiple inverters are synchronously connected to the grid. Multiple inverters work together to complete the initial voltage build-up of the microgrid. After all inverters in the microgrid have completed the adaptive grid connection action, the microgrid stably establishes its operating voltage, and finally, the entire process of masterless black start is completed fully automatically.

[0036] In one implementation, before grid connection, each inverter needs to continuously detect the microgrid voltage in each preset control cycle (e.g., 100µs). The grid connection point is selected at the voltage zero-crossing point before the voltage increases (or decreases) in a cycle. If no voltage is detected in the microgrid during the entire preset control cycle before the voltage zero-crossing point, the grid connection switching device is activated at the voltage zero-crossing point. For all inverters, all inverters adopt a bus voltage detection cycle with the same duration as the preset control cycle. Taking 100µs as an example, the inverter can only update the voltage and zero-crossing point judgment information according to this fixed time interval. When two inverters trigger the zero-crossing grid connection action synchronously, the maximum time deviation between the two closing times will not exceed 100µs. The corresponding phase deviation can be controlled within the same range as the preset safety threshold for pre-synchronous grid connection. The output voltage waveforms of the two inverters are almost synchronized. When the grid connection circuit is closed together at the zero-crossing point where the instantaneous voltage value is zero, no significant potential difference will be generated, and therefore no large inrush current will be formed.

[0037] Among them, the delay of the grid-connected switching device is <100us, and thyristors can be used to realize grid-connected control. The conduction time of thyristors is generally tens of microseconds, so it fully meets the requirements.

[0038] The steps of the hostless black boot method will be explained in detail below.

[0039] In one implementation, after the microgrid is powered on, each inverter generates a random delay duration and waits for the corresponding delay to end. Specifically, after the microgrid is powered on and all inverters synchronously complete device initialization and self-test, each inverter independently generates its corresponding random delay duration. During the timing of its respective random delay duration, all inverters remain in a standby locked state with their output disconnected, and do not perform voltage detection, synchronization adjustment, or grid connection actions. After the corresponding random delay duration of any inverter ends, the inverter exits the standby locked state. The random delay duration is estimated based on an AC cycle of 20ms. There are 50 cycles within 1 second, meaning there are 50 in-phase zero-crossing points. If 10 inverters are connected to the grid, ensuring that the number of grid-connectable zero-crossing points is >= 10 times the number of inverters, the shortest random time can be 2 seconds. This inverter calculates the time of 100 zero-crossing points within 2 seconds based on its own phase, and randomly selects one point from these 100 points as the random delay duration.

[0040] Once the microgrid is fully powered on, the system power supply circuit is completely connected. All inverters in the microgrid are powered on synchronously and start the underlying equipment initialization and fault self-test process, such as completing pre-operations like register reset, sampling calibration, drive self-test, and fault signal clearing.

[0041] Under the premise that all inverters complete initialization and self-test synchronously and there are no fault alarms, each inverter independently calculates and autonomously generates the corresponding random delay duration based on its local built-in random algorithm. The entire random delay duration generation process does not rely on the microgrid upper-level master control command, does not require communication interaction between devices, and does not require manual preset sorting. The random delay duration generated by each inverter is independent of each other, realizing fully distributed autonomous timing configuration.

[0042] During the entire timing process of each inverter's corresponding random delay duration, all inverters uniformly enter the standby lockout state with the output disconnected. The system locks the grid connection triggering authority and voltage detection start authority, prohibiting the premature execution of all grid connection pre-processing and grid connection actions such as bus voltage detection, output parameter synchronous adjustment, and grid connection switch closure. This eliminates the problem of premature, erroneous, or disordered inverter startup from the control logic level, ensuring that all inverters follow fixed timing logic.

[0043] Once the random delay timer of any inverter is completely cleared and the random delay time has completely ended, the inverter will immediately release from standby lockout, unlock subsequent voltage detection and grid connection control permissions, and officially enter the grid connection preparation stage.

[0044] To address the issues of inability of post-start inverters to adaptively connect to the grid in hostless microgrids, the potential for inrush currents and internal circulating currents in multi-unit parallel operation, and poor grid stability, one implementation method is as follows: Figure 3 As shown, when voltage is detected on the microgrid bus, the pre-synchronization operation is performed to complete grid connection, which specifically includes: Step 1031: When the inverter detects a voltage on the microgrid bus, it continuously collects the voltage amplitude, power frequency and phase of the voltage on the microgrid bus at a preset control cycle, and collects the voltage amplitude, power frequency and phase of its own output voltage in real time.

[0045] The preset control cycle ranges from 50µs to 200µs.

[0046] After the random delay period ends and the inverter enters the grid connection preparation state, it continuously monitors the status of the microgrid bus through the built-in voltage sampling unit. If a stable AC voltage is detected on the bus for several consecutive control cycles, it is determined that the current microgrid has completed voltage building by the inverter that started earlier, forming a stable operating reference grid.

[0047] At this time, the inverter starts the pre-synchronization sampling process, continuously collecting the core electrical parameters of the bus side voltage with a unified preset control cycle across the entire network. At the same time, it synchronously samples the amplitude, power frequency and phase parameters of the output voltage of the inverter, and obtains the original parameter data of the output voltage of the inverter and the bus reference voltage in real time, providing accurate data support for subsequent synchronous adjustment.

[0048] Step 1032: Perform difference calculation on data of the same type, and dynamically adjust its own output voltage through the internal closed-loop control circuit to gradually reduce the voltage amplitude difference, power frequency difference and phase difference between the voltage and the voltage on the microgrid bus.

[0049] The inverter performs difference calculations between the real-time acquired bus voltage parameters and its own output voltage parameters to determine the current voltage amplitude deviation, power frequency deviation, and phase deviation. Based on the calculated deviation data, the inverter performs dynamic adaptive adjustment through its internal closed-loop control circuit. Using the microgrid bus voltage parameters as the sole reference, it corrects the amplitude, operating frequency, and output phase of its own output voltage in real time, gradually reducing the differences in various parameters. This avoids voltage oscillations caused by sudden parameter adjustments, ensuring a smooth and continuous adjustment process, and gradually achieving dynamic matching between the own output voltage and the bus reference voltage.

[0050] Step 1033: When the voltage amplitude difference, power frequency difference and phase difference all fall within the preset safety threshold range, it is determined that the pre-synchronization operation is completed, and the grid-connected switching device is driven to close the output circuit, so that the inverter is stably connected to the microgrid bus, thereby completing the grid-connected operation.

[0051] The preset safety threshold range includes voltage amplitude difference ≤3%, power frequency difference ≤0.2Hz, and phase difference ≤5°. When the real-time detected parameter differences all meet the threshold requirements, the synchronization operation is deemed complete and grid connection is executed. The grid connection switching device is a thyristor or a high-speed power device.

[0052] The sole core reason for the inrush current generated when a power inverter is connected to the grid is that there are amplitude, frequency, and phase differences between the inverter's output voltage and the grid bus voltage at the moment of grid connection. Among these, the phase difference is the dominant factor in the inrush current of low-voltage microgrids. The AC voltage of a microgrid is a sine wave, and the instantaneous voltage value of the sine wave at any given moment is uniquely determined by its phase. If two inverters are connected to the grid with inconsistent phases, their instantaneous output voltage values ​​will deviate. After parallel connection, a circulating voltage difference will be formed between the inverters and between the inverter and the microgrid bus. The larger the voltage difference, the larger the inrush current.

[0053] Therefore, this embodiment compares the deviations of various parameters with preset safety thresholds in real time throughout the inverter process. Only when the three core deviation indicators—voltage amplitude difference, power frequency difference, and phase difference—simultaneously meet the grid connection safety conditions can the pre-synchronization adjustment be determined to be complete, completely eliminating the impact risks caused by grid voltage difference, frequency difference, and phase difference. After the synchronization verification is passed, the inverter immediately drives the thyristor or high-speed grid-connected switching device to quickly close the connection path between the local output circuit and the microgrid bus, enabling the inverter to smoothly connect to the already stable microgrid without any impact or circulating current disturbance, ultimately achieving safe and reliable grid-connected operation of the inverter.

[0054] Whether an inverter in an AC microgrid can safely connect to the grid depends crucially on whether the AC parameters of the inverter to be connected are consistent with those of the microgrid bus. If there are deviations in amplitude, frequency, and phase between the two, inrush current and internal circulating current will be generated due to voltage difference at the moment of grid connection, causing grid disturbances or even equipment failure. This embodiment achieves safe grid connection through complete pre-synchronization control logic. When the inverter detects that the bus has a stable voltage, it means that the microgrid has formed a fixed grid operation benchmark. The inverter no longer uses the zero-crossing grid connection logic, but instead continuously samples and compares its own voltage core parameters with those of the bus to accurately capture the amplitude, frequency, and phase deviations between the two. Then, relying on the internal closed-loop control circuit, it slightly and smoothly corrects its own output state, gradually aligning with the bus grid parameters.

[0055] Once the deviations of the three key parameters have all converged to within the safe threshold, the inverter output waveform and the bus grid waveform will be almost completely overlapped. At this point, grid connection will not generate significant voltage difference and current surge, allowing the inverter to be smoothly connected to the microgrid bus and reliably complete grid connection operation, thus meeting the grid connection requirements of subsequent startup equipment in a hostless microgrid.

[0056] In a scenario where the microgrid bus is completely de-voltaged, the grid lacks a fixed amplitude, frequency, and phase reference, eliminating the need for parameter comparison and pre-synchronization adjustment, thus rendering pre-synchronization adjustment inapplicable. Correspondingly, the step of triggering grid connection at a fixed voltage zero-crossing point when no voltage is detected on the microgrid bus specifically includes: when the inverter fails to detect a valid voltage signal on the microgrid bus for multiple consecutive preset control cycles, the inverter continuously monitors the voltage waveform timing, waiting for the voltage to reach the voltage zero-crossing point to drive the grid-connected switching device to close its output circuit, thereby enabling the inverter to stably connect to the microgrid bus and completing grid-connected operation.

[0057] Among them, such as Figure 4 As shown, the voltage zero-crossing point is either the voltage zero-crossing point where the instantaneous voltage value switches from positive to negative potential in the power frequency sinusoidal AC voltage waveform, or the voltage zero-crossing point where the voltage switches from negative to positive potential; wherein, each inverter pre-selects one of the zero-crossing points as a fixed grid-connection trigger point, and all inverters in the microgrid use the same type of voltage zero-crossing point to perform grid-connection operation.

[0058] In one implementation, the inverter continuously monitors the waveform timing of its output voltage in real time, tracking the trajectory of the sinusoidal voltage waveform throughout, and waiting for the voltage to reach a preset fixed zero-crossing point. Since the instantaneous voltage value at the zero-crossing point is zero, the grid-connected switching devices are driven to quickly close the output circuit, eliminating the voltage difference at the moment of grid connection from the root cause and avoiding voltage drop shocks and instantaneous circulating current problems. Relying on the unified preset control cycle timing constraints of the entire grid, the phase deviation between multiple inverters can be strictly controlled within a safe range. Multiple inverters synchronously and smoothly connect to the microgrid bus, collaboratively establish the initial operating voltage of the microgrid, successfully complete the safe grid connection under no-grid conditions, and adapt to the startup requirements of the initial voltage build-up of the hostless microgrid.

[0059] Because the sinusoidal waveform of AC power is constantly changing, the magnitude of the impact generated by grid connection at any given moment depends entirely on the instantaneous voltage difference between the inverter output voltage and the grid voltage at the instant of closing the circuit. In the initial black start phase of a microgrid with no grid voltage, the bus lacks a well-established grid reference, making precise pre-synchronization calibration of amplitude, frequency, and phase impossible. This is the core reason why conventional pre-synchronization grid connection cannot be used under this condition. The core principle of triggering grid connection at the voltage zero-crossing point is to utilize the waveform characteristic of the sinusoidal voltage instantaneous value returning to zero at the zero-crossing point, directly eliminating the instantaneous voltage difference at the physical level. When the voltage waveform passes through the zero-crossing point, the instantaneous value of the inverter output voltage is zero, and there is no potential difference. Closing the grid connection switching device at this time will not create an inrush current, completely avoiding the device surge, voltage oscillation, and equipment tripping problems caused by excessive instantaneous voltage differences during grid connection at non-zero-crossing moments.

[0060] In summary, this embodiment eliminates the need for a host computer. After power-on, each inverter automatically generates a random delay duration and enters the grid connection preparation state during off-peak hours. Each inverter operates using either pre-synchronous grid connection or fixed voltage zero-crossing point grid connection, depending on the voltage status on the microgrid bus. No host computer is required throughout the process. Even if a single inverter malfunctions, it will not prevent the normal startup of other devices, completely eliminating dependence on a host computer. The entire process can automatically complete the microgrid black start without manual intervention to reconfigure the host computer, effectively improving the automation level and operational stability of the microgrid black start.

[0061] Example 2: To further illustrate the hostless black-start method proposed in Example 1, this example proposes a microgrid system. In one embodiment, the microgrid system includes: multiple inverters with their output terminals connected in parallel to the microgrid bus, and each inverter is connected in series with a grid-connected switching device between itself and the microgrid bus.

[0062] In one implementation, each inverter integrates a local control unit, a random delay generation unit, a voltage sampling unit, and a pre-synchronization adjustment unit. The local control unit drives the random delay generation unit to generate a random delay duration and complete the delay timing after the inverter is powered on. After the delay timing ends, the voltage sampling unit continuously collects the microgrid bus voltage signal according to a preset control cycle, determines whether the microgrid bus is in a voltage-on or voltage-off state, and uploads the detection results to the local control unit in real time. The pre-synchronization adjustment unit is activated when the voltage sampling unit detects voltage on the microgrid bus, compares its own output voltage parameters with the microgrid bus voltage parameters in real time, and dynamically adjusts the amplitude, frequency, and phase of its own output voltage until the synchronization grid connection conditions are met.

[0063] In summary, this embodiment eliminates the need for a host computer. After power-on, each inverter automatically generates a random delay duration and enters the grid connection preparation state during off-peak hours. Each inverter operates using either pre-synchronous grid connection or fixed voltage zero-crossing point grid connection, depending on the voltage status on the microgrid bus. No host computer is required throughout the process. Even if a single inverter malfunctions, it will not prevent the normal startup of other devices, completely eliminating dependence on a host computer. The entire process can automatically complete the microgrid black start without manual intervention to reconfigure the host computer, effectively improving the automation level and operational stability of the microgrid black start.

[0064] The specific process of the hostless black boot method is described in Example 1, and will not be repeated in this example.

[0065] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A hostless black boot method, characterized in that, Applied to a microgrid including multiple inverters, the method includes: After the microgrid is powered on, each inverter generates a random delay duration and waits for the corresponding random delay duration to end. After the corresponding random delay period ends, the inverter enters the grid connection preparation state and detects the voltage on the microgrid bus according to the preset control cycle; When a voltage is detected on the microgrid bus, a pre-synchronization operation is performed to complete grid connection; When no voltage is detected on the microgrid bus, grid connection is triggered at a fixed voltage zero-crossing point. After all inverters have completed grid connection, the microgrid achieves black start without a master unit.

2. The hostless black boot method according to claim 1, characterized in that, After the microgrid is powered on, each inverter generates a random delay duration and waits for the corresponding delay to end, specifically including: After the microgrid is powered on and all inverters have completed their device initialization and self-test simultaneously, each inverter independently generates its own random delay duration. During the timing of their respective random delay periods, all inverters remain in standby locked state and do not perform voltage detection, synchronization adjustment, or grid connection actions. Once the corresponding random delay period of any inverter ends, they exit the standby locked state.

3. The hostless black boot method according to claim 1, characterized in that, When a voltage is detected on the microgrid bus, a pre-synchronization operation is performed to complete grid connection, specifically including: When the inverter detects a voltage on the microgrid bus, it continuously collects the voltage amplitude, power frequency and phase of the voltage on the microgrid bus at a preset control cycle, and also collects the voltage amplitude, power frequency and phase of its own output voltage in real time. The system performs difference calculations on data of the same type and dynamically adjusts its own output voltage through an internal closed-loop control circuit to gradually reduce the voltage amplitude difference, power frequency difference, and phase difference between the voltage and the voltage on the microgrid bus. Once the voltage amplitude difference, power frequency difference, and phase difference all fall within the preset safety threshold range, the pre-synchronization operation is determined to be complete. The grid-connected switching device is then driven to close the output circuit, enabling the inverter to stably connect to the microgrid bus, thereby completing the grid-connected operation.

4. The hostless black boot method according to claim 3, characterized in that, The preset safety threshold range includes voltage amplitude difference ≤3%, power frequency difference ≤0.2Hz, and phase difference ≤5°. When the real-time detected parameter differences all meet the threshold requirements, the synchronization operation is determined to be completed and grid connection is executed.

5. The hostless black boot method according to claim 1, characterized in that, The step of triggering grid connection at a fixed voltage zero-crossing point when no voltage is detected on the microgrid bus specifically includes: When the inverter does not detect a valid voltage signal on the microgrid bus for several consecutive preset control cycles, the inverter continuously monitors the voltage waveform timing and waits for the voltage to reach the zero-crossing point to drive the grid-connected switching device to close the output circuit, so that the inverter can stably connect to the microgrid bus and complete the grid-connected operation.

6. The hostless black boot method according to claim 3 or 5, characterized in that, The grid-connected switching device is a thyristor or a high-speed power device.

7. The hostless black boot method according to claim 1, characterized in that, The preset control cycle range is 50µs to 200µs.

8. The hostless black boot method according to claim 1, characterized in that, The voltage zero-crossing point is either the voltage drop zero-crossing point where the instantaneous voltage value switches from positive to negative potential in the power frequency sinusoidal AC voltage waveform, or the voltage rise zero-crossing point where the instantaneous voltage value switches from negative to positive potential. Each inverter pre-selects one type of zero-crossing point as a fixed grid-connection trigger point, and all inverters in the microgrid use the same type of voltage zero-crossing point to perform grid-connection.

9. A microgrid system, characterized in that, The method for implementing the hostless black start method as described in any one of claims 1-8 includes: multiple inverters with their output terminals connected in parallel to the microgrid bus, and each inverter having a grid-connected switching device connected in series with the microgrid bus.

10. The microgrid system according to claim 9, characterized in that, Each inverter integrates a local control unit, a random delay generation unit, a voltage sampling unit, and a pre-synchronization adjustment unit. The local control unit is used to drive the random delay generation unit to generate a random delay duration after the inverter is powered on, and to complete the delay timing. After the delay timer ends, the voltage sampling unit is used to continuously collect the microgrid bus voltage signal according to the preset control cycle, determine whether the microgrid bus is in a state of having voltage or no voltage, and upload the detection result to the local control unit in real time. The pre-synchronization adjustment unit is activated when the voltage sampling unit detects voltage on the microgrid bus. It compares its own output voltage parameters with the microgrid bus voltage parameters in real time and dynamically adjusts the amplitude, frequency, and phase of its own output voltage until the conditions for synchronous grid connection are met.

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