Grid switching system and method based on microgrid

Abstract

The application discloses a micro-grid-based power grid switching system and method, wherein the method comprises the following steps: acquiring an alternating voltage of an external power grid and a direct-current bus voltage of an internal power grid in real time; when the alternating voltage is lower than a first preset value, performing active isolated grid; in the isolated grid state, when the current direct-current bus voltage is lower than a second preset value, not performing a relay protection shutdown action. When the pumping unit is monitored to be in a downstroke, the pumping unit inverter is controlled to generate power to the direct-current bus. The application will not directly perform a shutdown operation when the isolated grid fails, but when the pumping unit is in a downstroke, the pumping unit inverter is opened to generate power to the direct-current bus, so that the voltage of the direct-current bus is increased until the voltage is increased to a voltage value required for rising to a highest power working point in a photovoltaic power supply characteristic curve. Thus, the isolated grid failure is rescued, the failure probability of the active isolated grid is reduced, and the abnormal shutdown of oil production operation is reduced.

Description

Technical Field

[0001] This invention relates to the field of power systems, and in particular to a grid switching system and method based on microgrids. Background Technology

[0002] In the process of promoting the integrated development of new energy and traditional energy, as the penetration rate of new energy continues to increase, a large number of enterprises' power systems are being converted into new power systems with a high proportion of new energy and a high proportion of power electronic equipment connected.

[0003] Currently, the power sources in the new power systems of oilfield production systems can include: external power grid connected to the internal power grid, and photovoltaic power connected to the internal power grid through photovoltaic inverters for coordinated power supply.

[0004] New energy power generation equipment (such as photovoltaic power sources) and electrical equipment (such as frequency converters) in the internal power grid all use power electronic components as energy conversion elements. These power electronic components are susceptible to voltage dips (i.e., voltage sags). When voltage waveform distortions such as voltage sags occur, in order to avoid equipment damage, the existing technology generally adopts the method of cutting off the fault and shutting down the machine, and then reconnecting to the grid after power supply is restored.

[0005] The inventors discovered through research that existing technologies still have at least the following shortcomings:

[0006] A shutdown due to a malfunction results in losses in oil production, and restarting the equipment after a shutdown places a significant impact on the equipment.

[0007] The information disclosed in this background section is intended only to enhance the understanding of the overall background of the invention and should not be construed as an admission or in any way implying that the information constitutes prior art known to those skilled in the art. Summary of the Invention

[0008] The purpose of this invention is to reduce the failure rate of active islanding and reduce abnormal downtime in oil production operations.

[0009] This invention provides a grid switching method based on a microgrid, comprising the following steps:

[0010] S11. Set the response time of the islanded grid shutdown relay protection to be greater than the time required for the pumping unit's downstroke.

[0011] S12. Real-time acquisition of AC voltage from the external power grid and DC bus voltage from the internal power grid;

[0012] S13. When the AC voltage is lower than the first preset value, active network isolation is performed;

[0013] S14. In islanded mode, when the current DC bus voltage is lower than the second preset value and the pumping unit is detected to switch to the downstroke, control the pumping unit inverter to generate electricity back to the DC bus.

[0014] S15. When the duration of the DC voltage being lower than the second preset value exceeds the response time of the islanded grid shutdown relay protection, or when the DC bus voltage is still lower than the second preset value after the downstroke is completed, the islanded grid shutdown relay protection action is executed.

[0015] Preferably, in this invention, it further includes:

[0016] S16. When the AC voltage is higher than the first preset value, coordinated power supply is executed; the coordinated power supply includes: the photovoltaic power source and the external power grid jointly supply power to the oil pumping unit through the internal power grid.

[0017] S17. In the coordinated power supply state, when the pumping unit is working in the downstroke, the operating voltage of the DC bus is regulated to the preset low power operating point through the controllable rectifier, and the reverse power generation of the pumping unit is reduced by increasing the output frequency of the pumping unit inverter.

[0018] Preferably, in this invention, it further includes:

[0019] S18. In the coordinated power supply state, when the pumping unit is working in the upstroke, the working voltage of the DC bus is regulated to the preset high power operating point through the pumping unit inverter, and the output frequency to the pumping unit is increased through the controllable rectifier to increase the load of the pumping unit.

[0020] In another aspect of the invention, a microgrid-based grid switching system is also provided, comprising a DC bus, a photovoltaic power source, multiple pumping units equipped with pumping unit frequency converters, a controllable rectifier, a monitoring device, and a control device, wherein:

[0021] The pumping unit inverter and photovoltaic power supply are respectively connected to the DC bus circuit; the DC bus can output power to the external power grid or input power from the external power grid through a controllable rectifier; the monitoring device is used to acquire the AC voltage of the connected external power grid and the DC bus voltage of the internal power grid in real time; the control device is used to generate control commands according to preset rules; the control commands include the frequency control command of the pumping unit inverter and the voltage control command of the controllable rectifier;

[0022] The preset rules include:

[0023] Set the response time of the islanded grid shutdown relay protection to be no less than the time required for the downstroke of the pumping unit;

[0024] In islanded mode, when the current DC bus voltage is lower than the second preset value and the pumping unit is detected to switch to the downstroke, the pumping unit inverter is controlled to generate electricity back to the DC bus.

[0025] If the duration for which the DC voltage of the grid is lower than the second preset value exceeds the response time of the islanded grid shutdown relay protection, or if the DC bus voltage is still lower than the second preset value after the downstroke is completed, the islanded grid shutdown relay protection will be activated.

[0026] Preferably, in this invention, the preset rule further includes:

[0027] In the coordinated power supply mode, when the pumping unit is working in the upstroke, the operating voltage of the DC bus is regulated to the preset high power operating point through the controllable rectifier, and the load of the pumping unit is increased by increasing the output frequency of the pumping unit inverter.

[0028] Compared with the prior art, the present invention has the following beneficial effects:

[0029] The inventive concept of this invention includes: when an external power grid fails, oil production operations are not interrupted; instead, an active grid islanding is implemented to form an independent DC microgrid. Then, when the DC bus voltage falls below the warning value for islanding failure (i.e., the second preset value), the invention does not directly perform islanding shutdown relay. Instead, when the pumping unit switches to its downstroke, the pumping unit inverter is activated to generate electricity back to the DC bus, increasing the DC bus voltage until it exceeds the voltage value required for the highest power operating point in the photovoltaic power supply characteristic curve. This achieves recovery from islanding failure, reducing the probability of active islanding failure and minimizing abnormal downtime in oil production operations.

[0030] Furthermore, after waiting during islanded operation, when the external power grid returns to normal, this invention can further optimize control under coordinated power supply conditions. Specifically:

[0031] To avoid the load impact on the controllable rectifier caused by the combined output of the pumping unit's reverse power generation and the photovoltaic power, this invention appropriately reduces the reverse power generation and the output of the photovoltaic power during the reverse power generation, thereby reducing the capacity requirement of the controllable rectifier. In addition, this invention eliminates the need for a rectifier for the photovoltaic power under the power grid control system, thus effectively reducing equipment costs.

[0032] Furthermore, the present invention also improves the energy utilization rate of photovoltaic power by increasing the output of photovoltaic power and the load of the pumping unit during the upstroke of the pumping unit.

[0033] Furthermore, by controlling the up and down strokes of the pumping unit differently, this invention can significantly reduce the load fluctuation of the controllable rectifier, thereby effectively reducing the operating losses of the controllable rectifier, extending its service life, and saving maintenance costs.

[0034] The above description is merely an overview of the technical solution of the present invention. In order to better understand the technical means of the present invention and to implement it according to the contents of the specification, and to make the above and other objects, technical features and advantages of the present invention easier to understand, one or more preferred embodiments are listed below and described in detail with reference to the accompanying drawings. Attached Figure Description

[0035] To more clearly illustrate the technical solution of the present invention, the accompanying drawings used in the embodiments 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.

[0036] Figure 1 This is a flowchart illustrating the steps of the microgrid-based grid switching method described in this embodiment of the invention.

[0037] Figure 2 This is a schematic diagram of the structure of the microgrid-based power grid switching system described in this embodiment of the invention;

[0038] Figure 3 This is a schematic diagram of the photovoltaic power supply characteristic curve described in an embodiment of the present invention. Detailed Implementation

[0039] The specific embodiments of the present invention will now be described in detail with reference to the accompanying drawings, but it should be understood that the scope of protection of the present invention is not limited to the specific embodiments.

[0040] Unless otherwise expressly stated, throughout the specification and claims, the term "comprising" or its variations such as "including" or "comprising" shall be understood to include the stated elements or components without excluding other elements or other components.

[0041] In this document, the terms "first," "second," etc., are used to distinguish two different elements or parts, and are not used to define specific positions or relative relationships. In other words, in some embodiments, the terms "first," "second," etc., can also be used interchangeably.

[0042] Example 1

[0043] To reduce the failure rate of active grid isolation and minimize abnormal downtime in oil production operations, such as... Figure 1 As shown, this embodiment of the invention provides a grid switching method based on a microgrid, including the following steps:

[0044] S11. Set the response time of the islanded grid shutdown relay protection to be no less than the time required for the downstroke of the pumping unit;

[0045] like Figure 2 As shown, the microgrid in this embodiment of the invention can be specifically described as an oilfield power system. Its components may include: a DC bus 01, a photovoltaic power source 02, multiple pumping units 03 equipped with pumping unit inverters 31, a controllable rectifier 04, a monitoring device 05, and a control device 06. Specifically, the pumping unit inverters 31 and the photovoltaic power source 02 are respectively connected to the DC bus 01. The DC bus 01 can output power to or receive power from an external power grid 07 via the controllable rectifier 04. The monitoring device 05 is used to acquire in real time the AC voltage of the connected external power grid 07 and the DC bus voltage of the internal power grid. The control device 06 is used to generate control commands according to preset rules. The control commands include frequency control commands for the pumping unit inverters 31 and voltage control commands for the controllable rectifier 04.

[0046] The control device 06 in this embodiment of the invention can be a device with data processing capabilities, such as a computer or PLC; it can generate various required control instructions through preset programs and algorithms.

[0047] The inventive concept of this invention includes that when the islanded grid fails, instead of directly performing islanded grid shutdown relay protection, the islanded grid is salvaged by using the electrical energy generated by the reverse generator of the pumping unit 03. In view of this, in this embodiment of the invention, the response time of the islanded grid shutdown relay protection will be appropriately extended to allow time to enter the subsequent islanded grid salvage process.

[0048] In this embodiment of the invention, the timing for islanding failure recovery occurs during the downstroke of the pumping unit. If the DC bus voltage still cannot return to the normal operating level (second preset value) after the pumping unit 03 completes its downstroke, the equipment needs to be protected by the islanding shutdown relay protection. Therefore, in practical applications, the response time of the islanding shutdown relay protection can be set to be equal to or slightly greater than the downstroke time required by the pumping unit, based on the downstroke time setting.

[0049] S12. Real-time acquisition of the AC voltage of the power grid and the DC bus voltage of the internal power grid;

[0050] In this embodiment of the invention, the external power grid 07 is connected to the DC bus 01 of the internal power grid as a power supplier through a controllable rectifier 04. In addition, the photovoltaic power source 02 is also connected to the DC bus 01 of the internal power grid as another power supplier.

[0051] The monitoring device 05 can obtain the AC voltage of the external power grid 07 and the current DC bus voltage of the DC bus 01 in real time.

[0052] S13. When the AC voltage is lower than the first preset value, active network isolation is performed;

[0053] In this embodiment of the invention, the presence or absence of a fault in the external power grid 07 is determined by detecting the AC voltage of the external power grid 07. Specifically, when the AC voltage is lower than a first preset value, the external power grid 07 is considered to have faulted. In this case, the oil extraction operation is not interrupted, but active islanding is implemented to form an independent DC microgrid. The voltage of the internal power grid is stabilized by utilizing the principle that the output power of the photovoltaic power source 02 fluctuates with the DC bus voltage, thereby achieving automatic tracking of the bus voltage value by the photovoltaic output power. Specifically, based on the photovoltaic power source characteristic curve (refer to...), Figure 3 As can be seen, in the monotonically decreasing range on the right side of the photovoltaic power supply characteristic curve, after islanding, the output power of photovoltaic power supply 02 decreases as the DC bus voltage increases and increases as the DC bus voltage decreases. Thus, when pumping unit 03 is working in the downstroke, it is in reverse generation mode, the DC bus voltage increases, the output power of photovoltaic power supply 02 decreases, and the DC bus voltage decreases. When pumping unit 03 is working in the upstroke, it is in power consumption mode, the power consumption is greater than the photovoltaic output power, and the DC bus voltage will decrease. When the DC bus voltage decreases, the output power of photovoltaic power supply 02 increases as the DC bus voltage decreases, that is, the output of photovoltaic power supply 02 increases, which will increase the DC bus voltage again, thus maintaining the DC bus voltage balance (self-balancing).

[0054] It should be noted that, in this embodiment of the invention, the range of the first preset value can be 340V-360V, such as 350V.

[0055] S14. In islanded mode, when the current DC bus voltage is lower than the second preset value and the pumping unit is detected to switch to the downstroke, control the pumping unit inverter to generate electricity back to the DC bus.

[0056] During the oil production operation of pumping unit 03, the current stroke status of pumping unit 03 can be monitored in real time, that is, it can be determined whether pumping unit 03 is currently in the upstroke or downstroke.

[0057] During active islanding or after entering islanding mode, islanding failure may occur due to excessively low voltage on the DC bus 01. Generally, excessively low DC bus voltage often occurs during the upstroke of the pumping unit 03. When the voltage is too low, the pumping unit 03 lacks sufficient power, leading to overload of the pumping unit inverter 31. When the DC bus voltage is too low, the photovoltaic power supply 02 is in the non-operating region, causing system instability. In this embodiment of the invention, the position of the second preset value used for early warning of islanding failure can be set as the peak position of the photovoltaic power supply characteristic curve, which is the maximum output point of the photovoltaic power supply 02. When the voltage of the DC bus 01 is lower than the second preset value, it will be in the monotonically increasing range on the left side of the photovoltaic power supply characteristic curve. An increase in the voltage of the DC bus 01 will cause an increase in the output of the photovoltaic power supply 02.

[0058] In existing technologies, islanding failure triggers rapid islanding shutdown relay protection. However, in this embodiment of the invention, the electrical energy generated by the pumping unit 03 under gravity after an islanding failure is cleverly utilized to boost the voltage of the DC bus 01. During an islanding failure, the pumping unit 03 is in its upstroke phase, at which point the pumping unit inverter 31 blocks its output to prevent further load pull on the DC voltage, which could lead to a DC voltage collapse. After the pumping unit 03 transitions from its upstroke to its downstroke phase, it triggers a pulse in the pumping unit inverter 31, which then begins charging the DC bus 01. During an islanding failure, the pumping unit 03 is in its downstroke phase, allowing it to directly begin charging the DC bus 01. To prevent the reverse power generation from rising too quickly and burning out related equipment, in this embodiment of the invention, the output frequency of the pumping unit inverter 31 is calculated based on the pumping unit speed. Specifically, the output frequency of the pumping unit inverter 31 is k% of the pumping unit speed electrical frequency. The value of k can be: 70 < k < 90. The smaller the value of k, the more reverse power generation; the larger the value of k, the less reverse power generation.

[0059] When the grid fails, the voltage of DC bus 01 is in the monotonically increasing range to the left of the photovoltaic power supply characteristic curve (i.e., to the left of the peak of the photovoltaic power supply characteristic curve). At this time, the increase in the voltage of DC bus 01 will increase the output of photovoltaic power supply 02. In this embodiment of the invention, the photovoltaic panels and the pumping unit's reverse generator work together to charge the DC bus voltage. When the voltage of DC bus 01 rises to the monotonically decreasing range to the right of the photovoltaic power supply characteristic curve, the microgrid after the grid failure will return to its normal operating voltage range, meaning the grid failure is successfully resolved. After successful grid failure, there is no need for grid shutdown relay protection, and the pumping unit 03 will also return to normal operation.

[0060] S15. When the duration of the DC voltage being lower than the second preset value exceeds the response time of the islanded grid shutdown relay protection, or when the DC bus voltage is still lower than the second preset value after the downstroke is completed, the islanded grid shutdown relay protection action is executed.

[0061] If the recovery from the islanding failure is not completed during the downstroke of pumping unit 03, it indicates that the DC bus voltage will drop further. In order to avoid damage to the equipment due to the sudden voltage drop, the equipment needs to be protected by the islanding shutdown relay protection.

[0062] In summary, in this embodiment of the invention, oil production is not interrupted when the external power grid fails. Instead, an active islanding process is implemented to form an independent DC microgrid. Then, when the DC bus voltage falls below the warning value for islanding failure (i.e., the second preset value), this embodiment does not directly implement immediate islanding shutdown relay protection. Instead, when the pumping unit enters its downstroke phase, the pumping unit inverter is activated to generate electricity back to the DC bus, raising the DC bus voltage until it exceeds the voltage value required for the highest power operating point in the photovoltaic power supply characteristic curve. This achieves recovery from islanding failure, reducing the probability of active islanding failure and minimizing abnormal shutdowns in oil production operations.

[0063] Example 2

[0064] Based on Embodiment 1, further embodiments of the present invention may include the following steps:

[0065] S16. When the AC voltage is higher than the first preset value, coordinated power supply is executed; the coordinated power supply includes: the photovoltaic power source and the external power grid jointly supply power to the oil pumping unit through the internal power grid.

[0066] S17. In the coordinated power supply state, when the pumping unit is working in the downstroke, the operating voltage of the DC bus is regulated to the preset low power operating point through the controllable rectifier, and the reverse power generation of the pumping unit is reduced by increasing the output frequency of the pumping unit inverter.

[0067] The inventive concept of this invention includes: In a coordinated power supply state, firstly, this invention receives electrical energy from the photovoltaic power source through the DC bus, thus eliminating the need for a photovoltaic power rectifier; secondly, in this invention, by reducing the reverse power generation of the pumping unit during its downstroke and the output of the photovoltaic power source, the peak current of the DC bus transmitting power to the external power grid is reduced, thus eliminating the need for a large-capacity controllable rectifier and saving equipment costs.

[0068] In a specific example, the specific working process of this embodiment of the invention can be as follows:

[0069] The control device in this embodiment of the invention can be a device with data processing and computing functions. Through preset programs and algorithms, it can generate control commands that can control components such as pumping unit inverters and controllable rectifiers.

[0070] In practical applications, the stroke status of the pumping unit can be obtained through the operating condition monitoring unit, that is, the pumping unit is currently in the upstroke or downstroke through real-time monitoring.

[0071] When the control device detects that the pumping unit is about to enter the downstroke, it generates voltage control commands and frequency control commands corresponding to the downstroke.

[0072] During the downstroke of the pumping unit, reverse power generation occurs. At this time, the internal power grid switches to power output mode. The photovoltaic power and the power generated by the reverse power generation of the pumping unit are rectified by a controllable rectifier and then transmitted to the external power grid. During this time, the load on the controllable rectifier is at its peak. In order to reduce the load on the controllable rectifier when the pumping unit enters the downstroke, in this embodiment of the invention, on the one hand, the operating voltage of the DC bus is adjusted to a preset low-power operating point by the controllable rectifier to reduce the output of the photovoltaic power; on the other hand, the output frequency of the pumping unit to the DC bus is increased by the pumping unit inverter, thereby reducing the reverse power generation of the pumping unit. This effectively reduces the overall load on the controllable rectifier when the pumping unit enters the downstroke.

[0073] It should be noted that, in the embodiments of the present invention, the reduction in photovoltaic power output and the reduction in reverse power generation of the oil pump can be determined by those skilled in the art based on actual operating conditions (e.g., the current capacity of the controllable rectifier and the actual output power of the photovoltaic power source), and no specific limitation is made here.

[0074] In practical applications, the voltage value of the low-power operating point in the embodiments of the invention can preferably be in the range of 605-615V.

[0075] In this embodiment of the invention, the operating range of the DC bus operating voltage is located within the photovoltaic power supply characteristic curve (reference). Figure 3 The monotonically decreasing interval in the curve indicates that the smaller the DC bus operating voltage, the greater the output of the photovoltaic power supply. The peak of the curve is the position of the extreme value of the photovoltaic power supply output. Then, as the operating voltage increases, the output of the photovoltaic power supply will decrease accordingly. Therefore, the operating voltage value corresponding to the low power operating point can be determined based on the set decrease range of the photovoltaic power supply output.

[0076] Furthermore, in this embodiment of the invention, the efficiency of electrical energy utilization can be improved by performing corresponding control when the pumping unit is operating in the upstroke phase. Specifically:

[0077] S18. In the coordinated power supply state, when the pumping unit is working in the upstroke, the working voltage of the DC bus is regulated to the preset high power operating point through the pumping unit inverter, and the load of the pumping unit is increased by increasing the output frequency of the pumping unit inverter.

[0078] Within the monotonically decreasing range, the higher the operating voltage of the DC bus, the lower the output of the photovoltaic power supply. The peak of the curve represents the extreme value of the photovoltaic power supply's output. Subsequently, as the operating voltage increases, the output of the photovoltaic power supply will decrease accordingly. Therefore, the position near the peak of the curve can be determined as the operating voltage value corresponding to the high-power operating point. In practical applications, the voltage range of the high-power operating point in the embodiments of the invention is preferably 560-570V.

[0079] When the pumping unit enters its upstroke, it switches to energy-consuming mode. At this time, the combined power from the photovoltaic (PV) power source and the external power grid supplies energy to the pumping unit via its inverter. To fully utilize the capabilities of the PV power source, in this embodiment, on one hand, a controllable rectifier adjusts the DC bus voltage to a preset high-power operating point to fully utilize the PV power output; on the other hand, the pumping unit inverter increases the output frequency to the pumping unit. This allows for a corresponding increase in the PV power output by increasing the pumping unit's load. Through these controls, the maximum capacity of the PV power source can be fully utilized during the pumping unit's upstroke, thereby improving its efficiency.

[0080] In summary, in this embodiment of the invention, oil production is not interrupted when the external power grid fails. Instead, an active grid isolating is implemented to form an independent DC microgrid. Under normal circumstances, this embodiment utilizes the principle that the output power of the photovoltaic power source fluctuates with the DC bus voltage to automatically track the bus voltage value and stabilize the voltage of the internal power grid. To prevent the grid from becoming isolated due to excessively low DC bus voltage, this embodiment also utilizes the reverse generator of the pumping unit to salvage the grid before determining to execute the grid shutdown relay protection. Since this embodiment can effectively improve the stability of the grid in the internal power grid, it can reduce the interruption of oil production operations caused by power fluctuations, thereby reducing economic losses such as equipment wear and operational losses caused by the interruption of oil production operations.

[0081] Furthermore, to avoid the load impact on the controllable rectifier caused by the superposition of the pumping unit's reverse power generation and the photovoltaic power output, in this embodiment of the invention, the reverse power generation and the photovoltaic power output are appropriately reduced during the pumping unit's reverse power generation, thereby reducing the capacity requirement of the controllable rectifier. In addition, in this embodiment of the invention, there is no need to set up a converter for the photovoltaic power under the power grid control system, so the invention can effectively reduce equipment costs.

[0082] Furthermore, embodiments of the present invention also improve the energy utilization rate of photovoltaic power by increasing the output of photovoltaic power and the load of the pumping unit during the upstroke of the pumping unit.

[0083] Furthermore, by controlling the up and down strokes of the pumping unit differently, the embodiments of the present invention can significantly reduce the load fluctuation of the controllable rectifier, thereby effectively reducing the working loss of the controllable rectifier, extending its service life, and saving maintenance costs.

[0084] Example 3

[0085] Corresponding to the above-described embodiment of the microgrid-based grid switching method, another aspect of this invention provides a microgrid-based grid switching system, such as... Figure 2 As shown, it includes a DC bus 01, a photovoltaic power supply 02, multiple oil pumps equipped with pumping unit frequency converters, a controllable rectifier, a monitoring device, and a control device, wherein:

[0086] like Figure 2 As shown, the microgrid in this embodiment of the invention can be specifically described as an oilfield power system. Its components may include: a DC bus 01, a photovoltaic power source 02, multiple pumping units 03 equipped with pumping unit inverters 31, a controllable rectifier 04, a monitoring device 05, and a control device 06. Specifically, the pumping unit inverters 31 and the photovoltaic power source 02 are respectively connected to the DC bus 01. The DC bus 01 can output power to or receive power from an external power grid 07 via the controllable rectifier 04. The monitoring device 05 is used to acquire in real time the AC voltage of the connected external power grid 07 and the DC bus voltage of the internal power grid. The control device 06 is used to generate control commands according to preset rules. The control commands include frequency control commands for the pumping unit inverters 31 and voltage control commands for the controllable rectifier 04.

[0087] The control device 06 in this embodiment of the invention can be a device with data processing capabilities, such as a computer or PLC; it can generate various required control instructions through preset programs and algorithms.

[0088] The preset rules in this embodiment of the invention may specifically include:

[0089] Set the response time of the islanded grid shutdown relay protection to be no less than the time required for the downstroke of the pumping unit;

[0090] In islanded mode, when the current DC bus voltage is lower than the second preset value and the pumping unit 03 is detected to switch to the downstroke, the pumping unit inverter 31 is controlled to generate electricity back to the DC bus 01.

[0091] If the duration for which the DC voltage of the grid is lower than the second preset value exceeds the response time of the islanded grid shutdown relay protection, or if the DC bus voltage is still lower than the second preset value after the downstroke is completed, the islanded grid shutdown relay protection will be activated.

[0092] The inventive concept of this invention includes that when the islanded grid fails, instead of directly performing islanded grid shutdown relay protection, the islanded grid is salvaged by using the electrical energy generated by the reverse generator of the pumping unit 03. In view of this, in this embodiment of the invention, the response time of the islanded grid shutdown relay protection will be appropriately extended to allow time to enter the subsequent islanded grid salvage process.

[0093] In this embodiment of the invention, the timing for islanding failure recovery occurs during the downstroke of the pumping unit. If the DC bus voltage still cannot return to the normal operating level (second preset value) after the pumping unit 03 completes its downstroke, the equipment needs to be protected by the islanding shutdown relay protection. Therefore, in practical applications, the response time of the islanding shutdown relay protection can be set to be equal to or slightly greater than the downstroke time required by the pumping unit, based on the downstroke time setting.

[0094] In this embodiment of the invention, after an isolated grid failure, the pumping unit will switch to a downstroke under the action of gravity. By using its reverse power generation, the voltage of the DC bus will be increased by turning on the pumping unit inverter, thereby rescuing the isolated grid from failure.

[0095] Preferably, in the embodiments of the present invention, under the cooperative power supply state, when the pumping unit is working in the upstroke, the operating voltage of the DC bus is regulated to a preset high power operating point by a controllable rectifier, and the load of the pumping unit is increased by increasing the output frequency of the pumping unit inverter.

[0096] It should be noted that, since the grid switching method based on microgrid in the embodiments of the present invention is a specific usage of the grid switching system based on microgrid in Embodiments 1 and 2, its specific implementation and beneficial effects can be referred to the grid switching system based on microgrid in Embodiments 1 and 2, and will not be repeated here.

[0097] Those skilled in the art will recognize that the units and algorithm steps of the various examples described in conjunction with the embodiments disclosed herein can be implemented in electronic hardware, or a combination of computer software and electronic hardware. Whether these functions are implemented in hardware or software depends on the specific application and design constraints of the technical solution. Those skilled in the art can use different methods to implement the described functions for each specific application, but such implementation should not be considered beyond the scope of this application.

[0098] In the several embodiments provided in this application, it should be understood that the disclosed systems, apparatuses, and methods can be implemented in other ways. Furthermore, the couplings or direct couplings or communication connections shown or discussed may be indirect couplings or communication connections through interfaces, devices, or units, and may be electrical, mechanical, or other forms.

[0099] The units described as separate components may or may not be physically separate. The components shown as units may or may not be physical units; that is, they may be located in one place or distributed across multiple network units. Some or all of the units can be selected to achieve the purpose of this embodiment according to actual needs.

[0100] In addition, the functional units in the various embodiments of this application can be integrated into one processing unit, or each unit can exist physically separately, or two or more units can be integrated into one unit.

[0101] It should be understood that in the embodiments of this application, the claims, various embodiments, and features can be combined with each other to solve the aforementioned technical problems.

[0102] If the aforementioned functions are implemented as software functional units and sold or used as independent products, they can be stored in a computer-readable storage medium. Based on this understanding, the technical solution of this application, in essence, or the part that contributes to the prior art, or a portion of the technical solution, can be embodied in the form of a software product. This computer software product is stored in a storage medium and includes several instructions to cause a computer device (which may be a personal computer, server, or network device, etc.) to execute all or part of the steps of the methods described in the various embodiments of this application. The aforementioned storage medium includes various media capable of storing program code, such as USB flash drives, portable hard drives, read-only memory (ROM), random access memory (RAM), magnetic disks, or optical disks.

[0103] The above description of the disclosed embodiments enables those skilled in the art to make or use this application. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of this application. Therefore, this application is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features disclosed herein.

Claims

1. A grid switching method based on a microgrid, characterized in that, Including the following steps: S11. Set the response time of the islanded grid shutdown relay protection to be no less than the time required for the downstroke of the pumping unit; S12. Real-time acquisition of the AC voltage of the external power grid and the DC bus voltage of the internal power grid; S13. When the AC voltage is lower than the first preset value, active network isolation is performed; S14. In islanded mode, when the current DC bus voltage is lower than the second preset value and the pumping unit is detected to switch to the downstroke, control the pumping unit inverter to generate electricity back to the DC bus. S15. When the duration of the DC voltage being lower than the second preset value exceeds the response time of the islanded shutdown relay protection, or when the DC bus voltage is still lower than the second preset value after the downstroke is completed, the islanded shutdown relay protection action is executed.

2. The microgrid-based grid switching method according to claim 2, characterized in that, Also includes: S16. When the AC voltage is higher than the first preset value, perform coordinated power supply; The coordinated power supply includes: photovoltaic power and external power grid jointly supplying power to the oil pumping unit through internal power grid; S17. In the coordinated power supply state, when the pumping unit is working in the downstroke, the operating voltage of the DC bus is regulated to the preset low power operating point through the controllable rectifier, and the reverse power generation of the pumping unit is reduced by increasing the output frequency of the pumping unit inverter.

3. The microgrid-based grid switching method according to claim 2, characterized in that, Also includes: S18. In the coordinated power supply state, when the pumping unit is working in the upstroke, the working voltage of the DC bus is regulated to the preset high power operating point through the pumping unit inverter, and the output frequency to the pumping unit is increased through the controllable rectifier to increase the load of the pumping unit.

4. The microgrid-based grid switching method according to claim 3, characterized in that, The control of the pumping unit inverter to generate electricity from the DC bus includes: The output frequency of the pumping unit inverter is determined according to a preset algorithm, which includes: The inverter output frequency is k% of the pumping unit's rotational speed electrical frequency; where 70 < k < 90.

5. The microgrid-based grid switching method according to claim 4, characterized in that, The positions of the high-power operating point and the low-power operating point are both in the monotonically decreasing range of the photovoltaic power supply characteristic curve. The position of the second preset value is the vertex position of the photovoltaic power supply characteristic curve.

6. The microgrid-based grid switching method according to claim 5, characterized in that, The location of the high-power operating point includes: The position near the peak in the monotonically decreasing range of the photovoltaic power source characteristic curve.

7. The microgrid-based grid switching method according to claim 6, characterized in that, The location of the low-power operating point is: The position below the high-power operating point in the monotonically decreasing range of the photovoltaic power supply characteristic curve.

8. A grid switching system based on a microgrid, characterized in that, This includes a DC bus, photovoltaic power supply, multiple oil pumping units equipped with pumping unit frequency converters, a controllable rectifier, monitoring devices, and control devices, wherein: The pumping unit inverter and photovoltaic power supply are respectively connected to the DC bus circuit; the DC bus can output power to the external power grid or input power from the external power grid through a controllable rectifier; the monitoring device is used to acquire the AC voltage of the connected external power grid and the DC bus voltage of the internal power grid in real time; the control device is used to generate control commands according to preset rules; the control commands include the frequency control command of the pumping unit inverter and the voltage control command of the controllable rectifier; The preset rules include: Set the response time of the islanded grid shutdown relay protection to be no less than the time required for the downstroke of the pumping unit; In islanded mode, when the current DC bus voltage is lower than the second preset value and the pumping unit is detected to switch to the downstroke, the pumping unit inverter is controlled to generate electricity back to the DC bus. If the duration for which the DC voltage of the grid is lower than the second preset value exceeds the response time of the islanded grid shutdown relay protection, or if the DC bus voltage is still lower than the second preset value after the downstroke is completed, the islanded grid shutdown relay protection will be activated.

9. The microgrid-based grid switching system according to claim 8, characterized in that, The preset rules also include: In the coordinated power supply mode, when the pumping unit is working in the upstroke, the operating voltage of the DC bus is regulated to the preset high power operating point through the controllable rectifier, and the load of the pumping unit is increased by increasing the output frequency of the pumping unit inverter.

10. The microgrid-based grid switching system according to claim 9, characterized in that, The control of the pumping unit inverter to generate electricity from the DC bus includes: The output frequency of the pumping unit inverter is determined according to a preset algorithm, which includes: The inverter output frequency is k% of the pumping unit's rotational speed electrical frequency; where 70 < k < 90.

11. The microgrid-based grid switching system according to claim 10, characterized in that, The positions of the high-power operating point and the low-power operating point are both in the monotonically decreasing range of the photovoltaic power supply characteristic curve. The position of the second preset value is the vertex position of the photovoltaic power supply characteristic curve.

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