Multi-source micro-grid fault positioning, isolating and self-healing method and device

Abstract

The invention discloses a multi-source micro-grid fault positioning, isolating and self-healing method and device, and the method comprises the steps: judging that a fault point is located at the downstream of a power reference positive direction of a demarcation switch when the demarcation switch detects an overcurrent signal, and executing a demarcation positioning and isolating program; and when the outlet switch or the section switch detects an overcurrent signal, judging that a fault point is located at the downstream of the positive power reference direction of the outlet switch or the section switch, and executing a section positioning isolation program. According to the invention, the accuracy of fault positioning and isolation of the multi-source micro-grid is improved, the power failure range is narrowed, and the power supply reliability of the micro-grid is improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid fault self-healing considering the dynamic and rapid support response of distributed energy resources (DER), and specifically relates to a multi-source microgrid fault location and isolation, and self-healing method and device. Background Art

[0002] With the influx of DERs into distribution networks, a microgrid consisting of DERs and loads has gradually emerged. However, when a distribution network fault occurs, existing fault location and isolation technologies are difficult to adapt to microgrids operating with multiple power sources. Furthermore, existing fault-handling methods do not consider the capacity support provided by DERs to microgrids in grid-connected operation or the power restoration role of regional microgrids in off-grid operation. A detailed analysis is as follows: When a distribution network line fails, in order to avoid a complete power outage, intelligent terminals, namely FTU switches and DTU ring network cabinets, are often configured in 10kV overhead lines and cable lines to shorten the power outage range and improve the power supply reliability of the distribution network. However, when applied to feeder-type microgrid systems, this approach presents the following challenges: ① The proximity of sectionalizers in the line and the high fault current make it easy for multiple switches to trip simultaneously during a fault, leading to unexpected power outages in non-fault areas. ② When a fault occurs, both the substation and the DER provide short-circuit current to the fault point. This prevents centralized feeder automation (FA) technology based on fault current signals from locating and isolating the fault, resulting in a power outage along the entire line. ③ When a microgrid fault occurs and is isolated, for a grid-connected microgrid, upstream of the fault point will remain connected to the grid. However, downstream grid-connected inverters, such as distributed generation (DGs) and energy storage, will lose their grid clamping function, causing voltage and frequency limits to exceed in isolated islands, rendering them unable to operate off-grid. ④ For off-grid microgrids, even after the fault area is isolated, the grid-connected inverters in the isolated island area will still lose their grid-connected function, causing voltage and frequency limits to exceed in isolated islands, making off-grid operation impossible.

[0003] Currently, there are two main control methods for fault self-healing in traditional distribution networks: centralized and distributed. The details are as follows: (1) For the centralized control method, the fault section is located and isolated by comparing the switch telesignaling position change information and protection action information transmitted back to the automation master system by different intelligent terminal switches of the 10kV line. The DER will exit operation through its own anti-islanding protection action, and after the distribution network fault defect is eliminated or the power supply to its grid connection point is restored through the tie switch, the DER will resume grid connection operation according to the dispatching instruction. (2) For the distributed control method, the basic area is randomly divided and Kirchhoff's current law is used to determine whether the current flowing into the basic area at the same time is zero, so as to achieve fault location and isolation; the DER will automatically switch to the off-grid operation mode and restore the power supply of some important loads. The conditional planned island will switch to the grid-connected operation mode through the transfer of power by the tie switch.

[0004] Existing fault isolation and power restoration methods have the following disadvantages: (1) For centralized FA, it is impossible to determine whether the fault current is provided by the DER in the microgrid. That is, the access of the microgrid will change the fault characteristics of the 10kV distribution network, which may easily lead to the error of the fault interval judgment of the centralized FA. (2) For distributed FA, this method requires accurate current values and times, which is actually line current differential protection. Due to the rapid change of current during faults and the influence of wireless network communication, it is easy to make mistakes in fault interval judgment, resulting in unexpected power outages in non-fault areas. (3) For microgrids that adopt a centralized power restoration control method, when a fault occurs, DER does not participate in the power restoration process. When the capacity of the substation and feeder to which it is connected is slightly smaller than the load capacity of the power supply area to be restored, the power supply to the non-fault power outage area cannot be restored, and this type of method takes a long time. (4) For on-grid / off-grid microgrids that adopt distributed power restoration control methods, after isolating the fault section, the problem of grid-following DER being unable to operate off-grid may occur; in addition, when the grid-forming DER is restored to grid-connected operation after a long period of off-grid operation, the existing methods do not study the adjustment strategy of the island phase, which is easy to cause asynchronous closing. Summary of the Invention

[0005] To address the deficiencies in the prior art, the present invention provides a multi-source microgrid fault location, isolation, and self-healing method and device, which improves the accuracy of fault location and isolation in the multi-source microgrid, reduces the scope of power outages, and improves the power supply reliability of the microgrid.

[0006] In order to achieve the above object, the technical solution adopted by the present invention is: In a first aspect, a multi-source microgrid fault location and isolation method is provided, in which the switches on the line are divided into outgoing line switches, section switches, boundary switches and tie switches; the method includes: when the boundary switch detects an overcurrent signal, determining that the fault point is located downstream of the positive direction of the power reference of the boundary switch, and executing a boundary location and isolation procedure; when the outgoing line switch or the section switch detects an overcurrent signal, determining that the fault point is located downstream of the positive direction of the power reference of the outgoing line switch or the section switch, and executing a section location and isolation procedure.

[0007] Furthermore, a section switch refers to a switch with power supplies on both sides; an outgoing line switch refers to a switch with power supplies on both sides and one side directly connected to the power bus; a boundary switch refers to a switch with power supplies on only one side; and a connecting switch refers to a switch used to connect different power supplies.

[0008] Furthermore, the power reference positive direction of the outgoing line switch, the section switch and the boundary switch is the positive direction of the load flow of the passive distribution network.

[0009] Furthermore, the boundary positioning isolation procedure is specifically as follows: when the boundary switch detects an overcurrent signal, the boundary switch is tripped without delay and a locking signal is sent to the adjacent switch.

[0010] Furthermore, the segmented positioning isolation procedure is specifically as follows: for outgoing line switches and segment switches, only the locking signals issued by adjacent outgoing line switches or segment switches and consistent with the positive direction of their power reference are received; the overcurrent signals of the outgoing line switches and segment switches themselves are used as starting signals, otherwise, locking signals and export tripping signals are not allowed to be sent; for substation outgoing line switches with adjacent switches on one side and segment switches at the end of the line, and segment switches with adjacent switches on both sides of the line, when the number of locking signals received is 0 and 1 respectively, and it has a starting signal, the export tripping signal is sent, otherwise, it remains in the closed state.

[0011] In a second aspect, a multi-source microgrid fault location and isolation device is provided, comprising a storage medium and a processor; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the multi-source microgrid fault location and isolation method described in the first aspect.

[0012] In a third aspect, a multi-source microgrid fault self-healing method is provided, which adopts the multi-source microgrid fault location and isolation method described in the first aspect to locate and isolate the fault; after the fault is located and isolated, the off-grid control strategy of the grid-following inverter power supply is adopted to restore power supply to the load in the non-fault area downstream of the fault point; after the fault point defect is eliminated, the grid-connected control strategy of the grid-connected inverter power supply is adopted to realize the non-stop grid-connected operation of the microgrid running in the off-grid state.

[0013] Furthermore, the off-grid control strategy for the grid-following inverter power supply is as follows: when the DER is grid-connected, reactive power negative feedback control is used to ensure that the DER output frequency and phase are consistent with the distribution network, that is, the DER operates in grid-following mode. When the DER switches from grid-connected operation to off-grid operation, the grid clamping effect is lost, resulting in the reactive power required by the load being provided by the DER. The phase shift angle adjusted by the reactive power negative feedback control gradually reaches a maximum value and remains unchanged, thereby controlling the DER output frequency to fluctuate steadily within a 50Hz range. In addition, voltage negative feedback control is used to stabilize the DER output voltage within the rated voltage range, thus achieving automatic switching of the DER from grid-following mode to grid-forming mode, thereby continuously supplying power to loads in non-fault areas. The DER refers to distributed energy resources.

[0014] Furthermore, the grid-connected control strategy for the grid-connected inverter power supply is as follows: when the DER switches from off-grid operation to grid-connected operation, the output parameters of the DER are adjusted according to the voltage amplitude, phase, and frequency of the distribution network. When these parameters remain consistent, the microgrid grid-connected switch is switched from open to closed to achieve grid-connected operation. Afterwards, the DER will again be clamped by the distribution network, causing its reactive power negative feedback regulation phase shift angle to gradually decrease, that is, controlling the DER to operate at a unity power factor state, thereby achieving the transition of the DER control mode from grid-connected to grid-following.

[0015] In a fourth aspect, a multi-source microgrid fault self-healing device is provided, comprising a storage medium and a processor; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the multi-source microgrid fault self-healing method described in the third aspect.

[0016] Compared with the prior art, the present invention has the following beneficial effects: (1) In order to solve the problem that both the grid side and the DER side provide fault current to the fault point after a distribution or microgrid fault occurs, which makes the existing centralized feeder automation technology unable to realize fault location, the switches on the line are divided into outgoing switches, section switches, boundary switches and tie switches; when the boundary switch detects an overcurrent signal, it is judged that the fault point is located downstream of the positive direction of the power reference of the boundary switch, and the boundary location isolation procedure is executed; when the outgoing switch or section switch detects an overcurrent signal, it is judged that the fault point is located downstream of the positive direction of the power reference of the outgoing switch or section switch, and the section location isolation procedure is executed; the accuracy of fault location and isolation of the multi-source microgrid is improved, the scope of power outage is reduced, and the power supply reliability of the microgrid is improved; (2) In view of the power outage of loads in non-fault areas after a fault, the multi-level coordination of substation-feeder-substation microgrid, the dynamic rapid power restoration with DER participation, and the support of substation-type microgrid in the downstream area of the fault point have not been considered. A control method for automatically switching the grid-connected DER to the off-grid operation mode is proposed to realize the support role of DER in the process of load power restoration; (3) In order to solve the problem that DERs operating off-grid cannot be connected to the grid without power outage after the fault defect is eliminated, a grid-connected control strategy for grid-connected inverter power supply is proposed to achieve power outage in the non-fault area throughout the entire process, thereby improving the power supply reliability of the distribution network. BRIEF DESCRIPTION OF THE DRAWINGS

[0017] Figure 1 is a topological diagram of a multi-source microgrid in an embodiment of the present invention; Figure 2 This is a block diagram of the inverter control principle in an embodiment of the present invention; Figure 3 is a waveform diagram of the main inverter in grid-connected synchronous operation according to an embodiment of the present invention; Figure 4 3 is a diagram showing the relationship between Q′ and the offset θ in an embodiment of the present invention. DETAILED DESCRIPTION

[0018] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solutions of the present invention and are not intended to limit the scope of protection of the present invention.

[0019] Example 1 A multi-source microgrid fault location and isolation method divides switches on a line into outgoing switches, section switches, boundary switches, and tie switches. The method includes: when the boundary switch detects an overcurrent signal, determining that the fault point is located downstream of the boundary switch in a positive power reference direction, and executing a boundary location and isolation procedure; when the outgoing switch or section switch detects an overcurrent signal, determining that the fault point is located downstream of the outgoing switch or section switch in a positive power reference direction, and executing a section location and isolation procedure.

[0020] 1. Fault location and isolation of multi-source microgrids With the large-scale access of DERs, the fault characteristics of multi-source microgrids have changed significantly compared to traditional passive distribution networks, resulting in the traditional three-stage current protection being unable to meet the selectivity requirements.

[0021] The present invention proposes a distributed feeder automation technology method based on communication. First, the 10kV switches are divided into four categories: (station) outgoing line switches CX, (line) section switches FD, boundary switches FJ and tie switches LL. Figure 1As shown in the figure; secondly, the power reference positive direction of the switch and the blocking signal channel between adjacent switches are automatically configured based on the line topology structure; finally, whether each switch receives the blocking signal and the number of blocking signals during the fault is used to determine whether its protection action signal is output tripping, thereby realizing fault location and isolation of multi-source microgrids containing DERs.

[0022] 1.1 Fault Characteristics Analysis The topology of the microgrid in on-grid / off-grid operation mode is as follows: Figure 1 As shown in the figure, the dotted box is a microgrid composed of multiple DERs and loads. When the FD3 switch or the LL2 switch is in the closed position, the microgrid is connected to the grid; when the FD3 switch and the LL2 switch are both in the open position, the microgrid is off-grid.

[0023] Depend on Figure 1 As can be seen, the switches in the figure can be divided into two categories according to the number of power sources on both sides: (1) switches with power on only one side are named as boundary switches, such as FJ1 and FJ2; (2) switches with power on both sides are named as substation outgoing line switches or section switches, such as outgoing line switch CX2 (referring to a switch with power on both sides and one side directly connected to the power bus), section switches FD1, FD2, FD3, FD4, and FD5. Tie switches are switches used to connect different power sources.

[0024] When faults occur at points k1, k2, k3, and k4, the DERs within the microgrid will supply short-circuit current to the fault point. If this short-circuit current is less than the protection action threshold of the downstream switch, only the upstream switch will detect the overcurrent signal. This means that while a traditional centralized FA can locate and isolate the grid-side fault, load-side fault isolation requires the DER's anti-islanding protection to shut down the load. If the short-circuit current exceeds the action threshold of the downstream switch, switches CX2, FD1, FD3, FD4, and FD5 will all detect the overcurrent signal. Consequently, the traditional centralized FA will mistakenly believe a fault has occurred downstream of FD5, leading to failure in fault location and isolation.

[0025] The above analysis shows that with the large-scale connection of DERs, the short-circuit current provided by DERs will increasingly reach the action threshold of the switch overcurrent protection when a fault occurs, causing the centralized FA to fail in fault location, thereby expanding the scope of the power outage and affecting power supply reliability.

[0026] 1.2 Fault Location and Isolation Strategy To address the above issues, the present invention proposes a communication-based distributed feeder automation technology method. This method uses the power direction before and after the fault to determine whether the blocking signal of the adjacent switch is valid. The specific analysis is as follows.

[0027] 1.2.1. Specified power reference positive direction In order to ensure that the method proposed in the present invention is applicable to the open / closed loop operation mode of the traditional passive distribution network and the grid-connected / off-grid operation mode of the multi-source microgrid, the present invention sets the power reference positive direction of the outgoing line, segment and boundary switches as the load flow positive direction of the passive distribution network by default, and the tie switch does not specify the power positive direction. Figure 1 As shown by the black arrows at each switch.

[0028] 1.2.2 Action Logic According to the different fault intervals, the present invention divides the fault range into the following two types: ① Fault downstream of the boundary switch; ② Fault between the outgoing line switch and the section switch, or between the section switches. The specific analysis is as follows.

[0029] (1) Fault downstream of the boundary switch As shown in the fault characteristic analysis in Section 1.1, when the boundary switch detects an overcurrent signal, it determines that the fault point is downstream of the boundary switch's positive power reference and executes the boundary location and isolation procedure. Specifically, the boundary location and isolation procedure requires the boundary switch to be tripped without delay and a blocking signal to adjacent switches to prevent tripping other switches and expanding the power outage, thereby achieving fault location and isolation.

[0030] (2) Fault between outgoing line switch and section switch, or between section switches As shown in the fault characteristic analysis in Section 1.1, when an outgoing switch and / or sectionalizer detects an overcurrent signal, the fault point is determined to be downstream of the outgoing switch or sectionalizer in the positive direction of the power reference (downstream of the actual power flow direction), and the sectionalizer location and isolation procedure is executed. Specifically, the following steps are performed: ① For outgoing lines and section switches, only the blocking signals sent by adjacent outgoing lines or section switches and with the same power direction are received; ② The overcurrent signal of the switch itself is used as the starting signal, otherwise it is not allowed to send the locking signal and the output trip signal; ③ For the substation outgoing line switch and line end sectionalizing switch with adjacent switches on one side, and the sectionalizing switch with adjacent switches on both sides of the line, when the number of blocking signals received is 0 or 1 respectively, and it has a start signal, the output trip signal is issued; otherwise, the switch remains in the closed state.

[0031] From the above analysis and Figure 1 It can be seen that when faults occur at points k1~k6 respectively, the number of blocking signals received by each switch is shown in Table 1.

[0032] Table 1 Number of blocking signals received by each outgoing switch, section switch and tie switch

[0033] Note: x means the switch has no overcurrent signal; y means the switch has an overcurrent signal.

[0034] In summary, from Table 1 and Figure 1 It can be seen that when faults occur at points k1, k2, k3 and k4 respectively, the switches on both sides of the fault points can trip reliably; when faults occur at points k5 and k6 respectively, the upstream boundary switch of the fault point that detects the fault current can trip reliably, realizing fault location and isolation.

[0035] 2. Restoring power to non-fault areas considering the support role of DER The present invention proposes a control method for grid-connected / off-grid adaptive switching of grid-connected / connected inverters. First, the method is implemented using sinusoidal pulse width modulation (SPWM) technology and a reactive power negative feedback control strategy. Based on the impedance characteristics of the distribution network and microgrid lines, it can be seen that the DER grid-connected point voltage negative feedback can be used to regulate its output active power, and the output reactive power negative feedback can be used to regulate the phase of the output voltage. Second, when applied to a grid-connected microgrid, if the DER and part of the load form an island due to the isolation of the fault area, the method can control the DER to automatically switch to an off-grid operation mode, ensuring continuous and reliable power supply to the partial load. When the power required by the island load is large, the present invention can prioritize the electrical connection between the DER and the tie switch, thus providing a prerequisite for the subsequent grid-connected operation of the island. Furthermore, when applied to an off-grid microgrid, consistent with the application to the grid-connected microgrid, DER that is separated from the grid-connected inverter power supply clamping control can still automatically switch to off-grid operation, ensuring continuous and reliable power supply to part of the load. Finally, when the DER When the isolated island area composed of loads switches from off-grid operation to grid-connected operation, the present invention can adjust the phase of the microgrid according to the phase of the distribution network feeder voltage to achieve quasi-synchronous grid-connected operation of the off-grid microgrid without power outage.

[0036] The present invention can realize fault location and isolation of a multi-source microgrid and restore power supply to non-fault areas taking into account the supporting role of DERs, thereby reducing the scope of power outages and improving the power supply reliability of the microgrid.

[0037] When a distribution line fault occurs, power can be restored to downstream areas not experiencing power outages, where tie switches are available and the opposite substation and line capacity are sufficient. Otherwise, power is lost, reducing the reliability of the distribution network. To address this issue, this paper proposes a strategy for restoring power to non-fault areas that considers the support provided by DERs. Specifically, it proposes a control method for grid-connected / connected inverters with adaptive on-grid / off-grid switching.

[0038] 2.1. Control Principle of On-Grid / Off-Grid Adaptive Switching Inverter When the microgrid is connected to the grid, the DER, which is a grid-following inverter, only emits active power, and its output frequency, phase, and voltage are consistent with those of the grid. When the microgrid is off-grid, the grid-following inverter loses the grid's clamping effect, causing its output voltage and frequency to shift, i.e., it cannot operate off-grid. To solve this problem, the present invention proposes an inverter control method, which uses sinusoidal pulse width modulation (SPWM) technology to provide the inverter with the original signal of the output voltage phase. According to the characteristics of the distribution and microgrid line impedance, the reactive power negative feedback strategy can be used to correct the actual output voltage phase of the inverter to ensure that the inverter output voltage phase is consistent with the grid during grid-connected operation, and can automatically switch to the rated frequency of 50Hz for stable operation during off-grid operation. In addition, the inverter output voltage amplitude negative feedback strategy is used to adjust its output active power during off-grid operation to ensure its output voltage quality and meet the load's requirements for power quality. The inverter control principle block diagram is shown below. Figure 2 shown.

[0039] Figure 2 The "measuring device" is installed between the filter output terminal and the grid-connected switch of the main circuit. The collected data include voltage U DER (voltage at DER), current I DER (current at DER), active power P DER (active power output by DER), reactive power Q DER (reactive power output by DER), frequency f DER The actual value of the frequency at the grid connection point is converted into a digital signal after preliminary signal processing and then transmitted to the SPWM control, reactive power negative feedback control, voltage negative feedback control and grid connection control in the control circuit.

[0040] (1) SPWM control Depend on Figure 2 It can be seen that due to the existence of the filter, the phase of the inverter output side voltage leads the phase of the grid connection point voltage during normal operation. d For ease of description, assume that the normal operation d =0.

[0041] During grid-connected operation, when the phase of the grid-connected point voltage suddenly changes in the second cycle, such as Figure 3 As shown, the phase of the inverter output voltage will be inconsistent with the phase of the grid voltage, making the inverter DQ Deviating from zero value, the reactive power negative feedback control will produce an offset of leading mutation θ , and then control the phase of the sawtooth wave, thereby controlling the phase of the sinusoidal modulation wave, and realizing the operation of the fixed frequency controlled inverter in the unity power factor state; similarly, when the phase of the grid-connected point voltage lags and mutates in the 5th cycle, the reactive power negative feedback control will also produce an offset of the lag mutation. θ , causing the phase of the inverter output voltage to change suddenly, realizing SPWM control of the inverter.

[0042] In actual operation, due to D Q ' changes continuously, and D Q ′, offset θ The phase mutation points of the sawtooth wave correspond one to one, so the phase mutation of the sawtooth wave has the characteristics of many mutation points and small mutation amount. Figure 3 Only two mutation points are magnified and shown in the figure.

[0043] (2) Reactive power negative feedback control In order to improve the response speed of the system and make the phase of the inverter output voltage quickly follow the phase of the grid voltage, D Q (Reactive power setting value Q ref With actual value Q DER The difference) is obtained after PI adjustment Q ′, then the offset θ =arcsin(D Q ′ / 3 UI ),in U and I The output voltage of the main inverter is U DER and output current I DER The effective value of the offset θ The curve is as Figure 4 shown.

[0044] Depend on Figure 4 It can be seen that when D Q When non-zero, it causes |D Q '| continues to increase, the offset θ The inverter output voltage phase is controlled to be consistent with the grid connection point voltage phase by changing in real time, thus realizing the reactive power negative feedback control of the inverter.

[0045] (3) Voltage negative feedback control Consistent with reactive power negative feedback control, voltage negative feedback control will D U (Voltage setting value Uref With actual value U DER The difference) is obtained after PI adjustment U ′, to adjust the amplitude of the modulation wave, thereby realizing the voltage negative feedback control of the inverter.

[0046] (4) Grid connection control In order to achieve quasi-synchronous grid connection of the inverter, the grid connection control will compare the distribution network side frequency received by the communication unit. f grid and voltage U grid The frequency of the signal and the inverter output voltage f DER and amplitude U DER , and then adjust the frequency and amplitude of the inverter output voltage to meet the requirements of voltage consistency, frequency consistency and phase consistency, thereby realizing quasi-synchronous grid connection of the inverter.

[0047] 2.1.1 Frequency Adjustment Strategy Depend on Figure 2 It can be seen that there are three factors that affect the output frequency and phase of DER: (1) Standard sawtooth wave: provided by a triangular sawtooth wave with a frequency of 50 Hz and an amplitude of 2π. When the DER is connected to the grid, its output current is controlled to have a small harmonic distortion rate. When the DER is off-grid, its output frequency (50 Hz) is controlled to meet the load requirements. (2) Reactive power negative feedback control: When the DER is connected to the grid, its output reactive power is controlled to zero by adjusting the sawtooth wave phase. When the DER is off-grid, as the time of DER outputting reactive power increases, this adjustment will gradually exceed the limit and remain unchanged, that is, its output frequency of 50 Hz is restored. (3) When the DER switches from off-grid operation to grid operation, its quasi-synchronous grid connection is achieved by adjusting the sawtooth wave phase.

[0048] 2.1.2 Voltage Adjustment Strategy Depend on Figure 2 It can be seen that the DER output voltage amplitude is affected by only one factor: the power quality required by the microgrid. When DERs are connected to the grid, their connection point voltage is controlled by the distribution network's clamping action. Therefore, DERs can operate at full power within unit power. When operating off-grid, they utilize negative feedback from their output voltage to regulate their output active power, thus ensuring that their output voltage remains within the specified limits.

[0049] 2.2 Off-grid control strategy for grid-following inverter power supply As shown in the first and second sections, when a microgrid stops operating due to a line fault, DERs in the faulted area and downstream areas will cease operation, rendering off-grid operation impossible, leading to power loss in non-faulty areas. Furthermore, for substations and lines with insufficient capacity, the DERs cannot initiate islanded operation, increasing the microgrid's power demand on these substations and lines. To address this issue, a control strategy is proposed to automatically switch from grid-connected to grid-forming inverters, enabling DERs to automatically switch from grid-connected to off-grid operation.

[0050] Depend on Figure 2 As can be seen from Section 2.1.1, when DERs are grid-connected, reactive power negative feedback control ensures that the DER output frequency and phase are consistent with those of the distribution network, meaning that the DERs operate in grid-following mode. When DERs transition from grid-connected to off-grid operation, the grid's clamping effect is lost, causing the reactive power required by the load to be provided by the DERs. This causes the phase shift angle adjusted by reactive power negative feedback control to gradually reach its maximum value and remain constant, thereby stabilizing the DER's output frequency within a 50Hz range. Furthermore, voltage negative feedback control stabilizes the DER's output voltage within the rated voltage range, enabling automatic switching of the DER's operating mode from grid-following to grid-forming, thereby providing continuous power to loads in non-fault areas.

[0051] 2.3. Grid-connected control strategy for grid-connected inverter power supply As shown in Section 2.2, for grid-connected DERs, if a microgrid fault occurs, they will automatically switch to off-grid, grid-forming control mode. Once the fault is resolved, power must be restored to the faulted area and to non-faulty areas downstream of the fault. However, the voltage, frequency, and phase of islands operating for extended periods of off-grid operation are difficult to maintain consistency with the distribution network. To address this issue, this paper proposes a grid-connected control strategy for grid-forming inverters to automatically switch DERs from off-grid to grid-connected operation.

[0052] Depend on Figure 2 As can be seen from Section 2.1.1, when the DER transitions from off-grid operation to grid-connected operation, the control strategy adjusts the DER's output parameters based on the distribution network's voltage amplitude, phase, and frequency. When these parameters remain consistent, the microgrid's grid-connected switch is switched from open to closed, achieving grid-connected operation. Thereafter, the DER is again clamped by the distribution network, causing its reactive power negative feedback regulation phase shift angle to gradually decrease. This controls the DER to operate at unity power factor, thereby transforming the DER's control mode from a grid-forming to a grid-following mode.

[0053] In summary, the specific steps of the microgrid fault self-healing method based on distributed energy proposed in the present invention are as follows: Step 1: When a fault occurs, the proposed communication-based distributed feeder automation technology is used to locate and isolate the fault. Step 2: After fault location and isolation, the proposed off-grid control strategy for the grid-following inverter power supply is used to ensure continuous power supply to the loads in the non-fault area downstream of the fault point, while reducing the load capacity to be restored in this area. This avoids the disadvantage of difficulty in restoring power supply due to insufficient capacity of the substation and line with the tie switch. Step 3: After the fault point defect is eliminated, the proposed grid-connected control strategy of the grid-connected inverter power supply is used to realize the non-stop grid-connected operation of the microgrid running in the off-grid state, thereby improving the power supply reliability.

[0054] Example 2 Based on the multi-source microgrid fault location and isolation method described in Example 1, this embodiment provides a multi-source microgrid fault location and isolation device, including a storage medium and a processor; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the multi-source microgrid fault location and isolation method described in Example 1.

[0055] Example 3 Based on the multi-source microgrid fault location and isolation method described in Example 1, this embodiment provides a multi-source microgrid fault self-healing method, which uses the multi-source microgrid fault location and isolation method described in Example 1 to perform fault location and isolation.

[0056] After the fault is located and isolated, an off-grid control strategy for a grid-following inverter is used to restore power to loads in non-fault areas downstream of the fault point. Specifically, when the DER is grid-connected, reactive power negative feedback control is used to ensure that the DER's output frequency and phase are consistent with those of the distribution network, effectively placing the DER in grid-following mode. When the DER transitions from grid-connected to off-grid operation, the grid's clamping function is lost, causing the reactive power required by the load to be provided by the DER. This causes the phase shift angle adjusted by the reactive power negative feedback control to gradually reach its maximum value and remain constant, thereby stabilizing the DER's output frequency within a 50Hz range. Furthermore, voltage negative feedback control is used to stabilize the DER's output voltage within the rated voltage range, enabling automatic switching of the DER from grid-following mode to grid-forming mode, thereby continuously supplying power to loads in non-fault areas. DER refers to distributed energy resources.

[0057] After the fault point defect is eliminated, the grid-connected control strategy of a grid-connected inverter is used to achieve non-stop grid-connected operation of the microgrid operating in an off-grid state. Specifically, the grid-connected control strategy of a grid-connected inverter is as follows: During the transition from off-grid operation to grid-connected operation, the DER's output parameters are adjusted based on the voltage amplitude, phase, and frequency of the distribution network. When these parameters remain consistent, the microgrid's grid-connected switch is switched from open to closed, achieving grid-connected operation. Thereafter, the DER is again clamped by the distribution network, causing its reactive power negative feedback regulation phase shift angle to gradually decrease, effectively controlling the DER to operate at unity power factor, thereby transitioning the DER from a grid-connected to a grid-following control mode.

[0058] Example 4 Based on the multi-source microgrid fault self-healing method described in Example 3, this embodiment provides a multi-source microgrid fault self-healing device, including a storage medium and a processor; the storage medium is used to store instructions; the processor is used to operate according to the instructions to execute the multi-source microgrid fault self-healing method described in Example 3.

[0059] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the technical principles of the present invention. These improvements and modifications should also be regarded as the scope of protection of the present invention.

Claims

1. A multi-source microgrid fault location and isolation method, characterized in that: The switches on the line are divided into outgoing switches, section switches, boundary switches and tie switches; the method includes: When the boundary switch detects an overcurrent signal, it determines that the fault point is located downstream of the boundary switch power reference in the positive direction, and executes the boundary positioning isolation procedure; When the outgoing line switch or the section switch detects an overcurrent signal, it is determined that the fault point is located downstream of the outgoing line switch or the section switch in the positive direction of the power reference, and the section positioning isolation procedure is executed.

2. The multi-source microgrid fault location and isolation method according to claim 1, characterized in that: A section switch is one where power is present on both sides; An outgoing switch is a switch with power on both sides and one side directly connected to the power bus; A demarcation switch is a switch with power on only one side; A tie switch is a switch used to connect different power sources.

3. The multi-source microgrid fault location and isolation method according to claim 2, characterized in that: The positive direction of the power reference of the outgoing line switch, section switch and boundary switch is the positive direction of the load flow of the passive distribution network.

4. The multi-source microgrid fault location and isolation method according to claim 3, characterized in that: The boundary positioning isolation procedure is as follows: when the boundary switch detects an overcurrent signal, it will trip the boundary switch without delay and send a locking signal to the adjacent switch.

5. The multi-source microgrid fault location and isolation method according to claim 3, characterized in that: The segmented positioning isolation procedure is as follows: For outgoing line switches and section switches, only the blocking signals sent by adjacent outgoing line switches or section switches and in the same direction as their power reference are received; The overcurrent signal of the outgoing line switch and the section switch itself is used as the starting signal. Otherwise, the blocking signal and the outlet tripping signal are not allowed to be sent; For the substation outgoing line switch and line end sectionalizing switch with adjacent switches on one side, and the sectionalizing switch with adjacent switches on both sides of the line, when the number of blocking signals received is 0 or 1 respectively, and it has a start signal, it will output a tripping signal; otherwise, it will remain in the closed state.

6. A multi-source microgrid fault location and isolation device, characterized in that: including storage media and processors; The storage medium is used to store instructions; The processor is configured to operate according to the instruction to execute the multi-source microgrid fault location and isolation method according to any one of claims 1 to 5.

7. A multi-source microgrid fault self-healing method, characterized in that: Fault location and isolation are performed using the multi-source microgrid fault location and isolation method according to any one of claims 1 to 5; After the fault is located and isolated, the off-grid control strategy of the grid-following inverter power supply is used to restore power supply to the loads in the non-fault area downstream of the fault point; After the fault point defect is eliminated, the grid-connected control strategy of the grid-connected inverter power supply is adopted to realize the non-stop grid-connected operation of the microgrid running in the off-grid state.

8. The multi-source microgrid fault self-healing method according to claim 7, characterized in that: The off-grid control strategy of the grid-following inverter power supply is as follows: When DERs are grid-connected, reactive power negative feedback control is used to ensure that their output frequency and phase are consistent with those of the distribution network, effectively placing them in grid-following mode. When DERs transition from grid-connected to off-grid operation, the grid's clamping function is lost, causing the reactive power required by the load to be provided by the DERs. This causes the phase shift angle adjusted by the reactive power negative feedback control to gradually reach its maximum value and remain constant, thereby stabilizing the DER's output frequency within a 50Hz range. Furthermore, voltage negative feedback control is used to stabilize the DER's output voltage within the rated voltage range, enabling automatic switching from grid-following to grid-forming mode, thereby continuously supplying power to loads in non-fault areas. DERs are distributed energy resources.

9. The multi-source microgrid fault self-healing method according to claim 8, characterized in that: The grid-connected control strategy of the grid-connected inverter power supply is as follows: During the transition from off-grid to grid-connected operation, the DER's output parameters are adjusted based on the distribution network's voltage amplitude, phase, and frequency. When these parameters remain consistent, the microgrid's grid-connected switch is switched from open to closed, achieving grid-connected operation. Thereafter, the DER is again clamped by the distribution network, causing its reactive power negative feedback regulation phase shift angle to gradually decrease. This controls the DER's operation at unity power factor, thereby transitioning the DER's control mode from grid-forming to grid-following.

10. A multi-source microgrid fault self-healing device, characterized in that: including storage media and processors; The storage medium is used to store instructions; The processor is configured to operate according to the instruction to execute the multi-source microgrid fault self-healing method according to any one of claims 7 to 9.

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