Micro-grid island detection and pre-synchronization control method and device

Abstract

The present application belongs to the field of micro-grid control strategy, and particularly relates to a micro-grid island detection and pre-synchronization control method and device. The micro-grid island detection method uses the island detection method of intermittent use under different conditions through the AFD detection method and the AFD PF detection method. When the frequency difference is large, a small disturbance current is applied. When the frequency difference is small, a larger disturbance current is added to improve the detection speed and avoid the influence of long-time injection of the interference current on the power quality. The pre-synchronization control method adds a compensation amount to the micro-grid voltage frequency or amplitude through the pre-synchronization method before the micro-grid is in the island state and starts to be connected to the grid, so that the micro-grid voltage frequency and amplitude can be consistent with the large grid. Therefore, the impact voltage and impact current caused by the merging of voltages in different states can be eliminated when the grid is connected, the influence on the power quality of the micro-grid can be avoided, and the grid connection efficiency and success rate can be improved.

Description

Technical Field

[0001] This invention belongs to the field of microgrid control strategies, specifically relating to a microgrid islanding detection and pre-synchronization control method and device. Background Technology

[0002] In today's society, energy issues are receiving increasing attention. Distributed power generation technologies, represented by clean energy power generation, can effectively solve energy problems, but they also bring new challenges such as the stable operation and security of the power grid. According to data from the National Bureau of Statistics in 2020, all generating units in my country generated a total of 7.5 trillion kilowatt-hours of electricity in 2020. Among them, the proportion of new energy power generation steadily increased, with wind power and solar power generation increasing by 15.1% year-on-year, accounting for 9.5% of total power generation. With the country's vigorous development of distributed power generation, my country's utilization of new energy sources is becoming increasingly widespread, and the proportion of new energy power generation is gradually increasing.

[0003] Distributed power generation suffers from extremely unstable output power, requiring regulation before integration into the main power grid. Otherwise, it negatively impacts the safety, reliability, and economic viability of microgrids, affecting grid stability. Microgrids have emerged to address this issue, resolving the poor reliability of distributed generation, improving its stability, and providing a solution for reliable grid connection of distributed power sources. Furthermore, the increasing maturity of photovoltaic power generation and other new energy power generation technologies has ensured the rise of microgrids.

[0004] Microgrids must not only achieve grid-connected and islanded operation, but also seamlessly switch between these two modes. During grid-connected operation, the microgrid and the main grid coordinate and optimize with each other. When the main grid experiences an anomaly or requires maintenance, the microgrid must quickly detect and disconnect switches, switching to islanded or outage mode. When the main grid recovers, the microgrid can reconnect and resume grid-connected operation. Transient process control is also crucial during mode switching. If no measures are taken after the operating state of distributed generation changes, a series of transient problems can occur, potentially threatening the safety of loads within the microgrid and, in severe cases, causing microgrid collapse.

[0005] Currently, common islanding detection methods for DC microgrids include passive detection methods and active detection methods. Active detection methods include frequency drift detection (AFD), sliding mode drift detection (SMS), and periodic current interference detection (ACD). The principle of AFD detection is as follows: Figure 1As shown, a current with a frequency difference from the microgrid's voltage is injected into the microgrid through the converter. When the microgrid is connected to the grid, its impact on the system is negligible due to the clamping effect of the main grid. However, when the microgrid is islanded, the impact of the disturbance current on the microgrid increases over time until it exceeds the detection threshold set by the AFD (Automatic Discharge Function), at which point the microgrid can be successfully detected as being in an islanded state. If the cutoff factor corresponding to the AFD is too large, the injected current distortion will be too large, which will seriously affect the power quality of the microgrid. However, if the cutoff factor is too small, the frequency offset will be small, which will not only reduce the detection speed but also lead to too many injection currents or too long a duration of disturbance current application to the microgrid, which will also seriously affect the power quality. Furthermore, as the microgrid operates in an islanded state for a long time, if it is directly connected to the grid while in an islanded state, the merging of voltages from different states will result in inrush voltages and inrush currents, which will cause serious damage to the power quality of the microgrid and may even lead to grid connection failure in severe cases. Summary of the Invention

[0006] The purpose of this invention is to provide a microgrid islanding detection and pre-synchronization control method and apparatus to solve the problem that microgrid power quality is affected during microgrid islanding detection in the prior art.

[0007] To achieve the above objectives, the present invention provides a microgrid islanding detection method, comprising the following steps:

[0008] 1) Detect the voltage frequency of the microgrid and the voltage frequency of the main grid, and obtain the frequency difference between the two;

[0009] 2) Determine whether the frequency difference is greater than the set frequency difference threshold. If it is greater, use the AFD detection method to detect whether the microgrid is in an islanded state; if it is not greater, use the AFDPF detection method to detect whether the microgrid is in an islanded state.

[0010] 3) Determine whether the voltage frequency of the microgrid exceeds the frequency threshold corresponding to the detection method used. If it exceeds the corresponding frequency threshold, the microgrid is determined to be in an islanded state; if it does not exceed the corresponding frequency threshold, the microgrid is determined not to be in an islanded state.

[0011] The beneficial effects of the above technical solution are as follows: The island detection method uses AFD detection method and AFDPF detection method intermittently under different conditions. When the frequency difference is large, a small disturbance current is applied, and when the frequency difference is small, a large disturbance current is added to improve the detection speed. This can avoid the impact of long-term injection of interference current on power quality.

[0012] Furthermore, the expression for the AFD detection method is:

[0013]

[0014] The expression for the AFDPF detection method is:

[0015] c f =cf0+k(f-f0)=cf0+Δf

[0016] Where cf is the cutoff coefficient, t Z denoted as the dead time between the positive and negative half-cycles of the microgrid current waveform, T is the period of the microgrid current waveform, cf0 is the initial cutoff coefficient, k(f-f0) is the positive feedback coefficient of the AFDPF algorithm, f is the microgrid voltage frequency, f0 is the mains power frequency voltage frequency, and Δf is the voltage frequency difference between the microgrid and the mains power.

[0017] Furthermore, the set frequency difference threshold is 0.2Hz.

[0018] This invention also provides a microgrid pre-synchronization control method. First, the voltage frequency of the microgrid and the voltage frequency of the main grid are detected, and the frequency difference between them is obtained. Then, it is determined whether the frequency difference is greater than a set frequency difference threshold. If it is greater, the AFD detection method is used to detect whether the microgrid is in an islanded state. If it is not greater, the AFDPF detection method is used to detect whether the microgrid is in an islanded state. Next, it is determined whether the microgrid voltage frequency exceeds the frequency threshold corresponding to the adopted detection method. If it exceeds the corresponding frequency threshold, the microgrid is determined to be in an islanded state. If it does not exceed the corresponding frequency threshold, the microgrid is determined not to be in an islanded state and needs to be connected to the grid.

[0019] When it is determined that the microgrid is not in an islanded state and needs to be connected to the grid, a compensation amount is added to the droop equation of the microgrid control system to keep the voltage frequency of the microgrid consistent with the voltage frequency of the main grid.

[0020] If the control method for islanded operation of a microgrid is frequency-active power control, then the compensation amount added to the active power droop equation is:

[0021]

[0022]

[0023] In the formula, Δω and Δf are the compensation amounts for the voltage angular frequency and voltage frequency, respectively; K pθ K iθ These represent the proportional-integral coefficients in the voltage phase angle pre-synchronization PI controller; θ is the phase angle of the microgrid output voltage in islanded mode. o The phase angle of the main power grid voltage;

[0024] If the control method for islanded operation of a microgrid is reactive power-voltage control, then the compensation amount added to the reactive power droop equation is:

[0025]

[0026] Where ΔE is the voltage amplitude compensation amount; K pu K iu These are respectively represented as the proportional-integral coefficients in the PI controller; u od u d These are the d-axis voltage components of the large power grid and the microgrid in a two-phase rotating coordinate system, obtained by coordinate transformation.

[0027] The beneficial effects of the above technical solution are as follows: Before the microgrid is in an islanded state and begins grid connection, the pre-synchronization control method adds a targeted compensation amount to the voltage frequency or amplitude of the microgrid through the pre-synchronization method, so that the voltage frequency and amplitude of the microgrid can be consistent with the large grid. Therefore, it can eliminate the inrush voltage and inrush current caused by the merging of voltages in different states during grid connection, avoid the impact on the power quality of the microgrid, and improve grid connection efficiency and success rate.

[0028] Furthermore, the expression for the AFD detection method is:

[0029]

[0030] The expression for the AFDPF detection method is:

[0031] c f =cf0+k(f-f0)=cf0+Δf

[0032] Where cf is the cutoff coefficient, t Z denoted as the dead time between the positive and negative half-cycles of the microgrid current waveform, T is the period of the microgrid current waveform, cf0 is the initial cutoff coefficient, k(f-f0) is the positive feedback coefficient of the AFDPF algorithm, f is the microgrid voltage frequency, f0 is the mains power frequency voltage frequency, and Δf is the voltage frequency difference between the microgrid and the mains power.

[0033] Furthermore, the set frequency difference threshold is 0.2Hz.

[0034] The present invention also provides a microgrid islanding detection device for implementing the above-described microgrid islanding detection method.

[0035] This island detection device can achieve the same beneficial effects as the island detection method described above.

[0036] The present invention also provides a microgrid pre-synchronization control device for implementing the above-described microgrid pre-synchronization control method.

[0037] This pre-synchronization control device can achieve the same beneficial effects as the pre-synchronization control method described above. Attached Figure Description

[0038] Figure 1 This is a schematic diagram illustrating the principle of the AFD detection method in the background art of this invention;

[0039] Figure 2 This is a power transmission diagram of a microgrid when connected to the grid in an embodiment of the islanding detection method of the present invention;

[0040] Figure 3 This is a flowchart illustrating the island detection method in an embodiment of the island detection method of the present invention.

[0041] Figure 4 This is a schematic diagram of the voltage phase angle pre-synchronization control structure in an embodiment of the islanding detection method of the present invention;

[0042] Figure 5 This is a schematic diagram of the voltage amplitude pre-synchronization control structure in an embodiment of the island detection method of the present invention;

[0043] Figure 6 This is a flowchart illustrating the microgrid operation mode switching process in an embodiment of the pre-synchronization control method of the present invention.

[0044] Figure 7 This is a schematic diagram illustrating the application of the island detection device and the pre-synchronization control device in the embodiments of the present invention. Detailed Implementation

[0045] To make the objectives, technical solutions, and advantages of the present invention clearer, the present invention will be further described in detail below with reference to the accompanying drawings and embodiments.

[0046] Examples of Microgrid Islanding Detection Methods

[0047] This embodiment provides a technical solution for a microgrid islanding detection method, the specific steps of which are as follows:

[0048] 1) Detect the voltage frequency of the microgrid and the voltage frequency of the main grid, and obtain the frequency difference between the two.

[0049] 2) Determine whether the frequency difference is less than the set frequency difference threshold. If it is greater than the threshold, use the AFD detection method to detect whether the microgrid is in an islanded state, which is equivalent to applying a small frequency disturbance current to the microgrid. If it is less than the threshold, use the AFDPF detection method to detect whether the microgrid is in an islanded state, which is equivalent to applying a large disturbance current to the main grid.

[0050] The principle of this island detection method is as follows:

[0051] like Figure 2 The power transmission diagram for a microgrid connected to the grid is shown below, and the specific power relationships of the loads are as follows:

[0052]

[0053] Among them, P L Q L For the power required by the load, P and Q represent the power generated by the microgrid, while Po and Qo represent the power supplied by the main grid. As for the load power:

[0054]

[0055] Where R, L, and C are the equivalent resistance, inductance, and capacitance of the load, respectively, and U is the voltage value at PCC. From equation (2), the formulas for the amplitude and frequency of the PCC voltage can be derived:

[0056]

[0057] f0 is the resonant frequency of the load; Q f The quality factor of the load can be expressed as:

[0058]

[0059] As shown in the above equation, the voltage amplitude at PCC is related to the active power flowing into the load. Frequency and both active and reactive power are also relevant. In islanded operation of a microgrid, if its distributed power sources cannot meet the power requirements of the load, the power P flowing into the load... L Q L As the voltage decreases, the voltage amplitude and frequency at the PCC point become inconsistent with those of the main grid, thus enabling the detection that the microgrid is operating in an islanded manner.

[0060] In the ADF detection method, the cutoff factor for the applied disturbance current is defined as c. f The strength of the disturbance is characterized by the following expression:

[0061]

[0062] The sensitivity of AFD detection mainly depends on the cutoff coefficient c. f Size, c f The larger the value, the better the detection effect. However, the long-term applied disturbance current can affect the power quality of the microgrid. Therefore, the AFD detection method improves upon the AFD detection method by introducing a positive feedback coefficient k(f-f0). The expression for the cutoff coefficient of the AFD detection method is then:

[0063] c f =cf0+k(f-f0)=cf0+Δf;

[0064] Where cf is the cutoff coefficient, t Zdenoted as the dead time between the positive and negative half-cycles of the microgrid current waveform, T is the period of the microgrid current waveform, cf0 is the initial cutoff coefficient, k(f-f0) is the positive feedback coefficient of the AFDPF algorithm, f is the microgrid voltage frequency, f0 is the mains power frequency voltage frequency, and Δf is the voltage frequency difference between the microgrid and the mains power frequency, which is the absolute value of the difference between the voltage frequencies of the microgrid and the mains power frequency.

[0065] The AFDPF detection method can adjust the cutoff coefficient in real time according to the operating conditions of the microgrid system. That is, when there is a large difference between the voltage frequency of the microgrid and the voltage frequency of the main grid, the cutoff coefficient is increased accordingly, thereby accelerating the rate at which the voltage frequency of the microgrid deviates from the set threshold range. Therefore, it can complete islanding detection more quickly and shorten the application time of disturbance current, thus reducing the impact of islanding detection on the power quality of the microgrid.

[0066] This embodiment of the islanding detection method combines the AFD (Automatic Frequency Difference) and AFDPF (Automatic Frequency Difference Factor) detection methods. It utilizes phase-locked loop (PLL) technology to acquire the voltage frequencies of the microgrid and the mains grid. If the frequency difference between the two does not exceed a set frequency difference threshold, a large disturbance current is applied; if the frequency difference is large, exceeding the set threshold, a small disturbance current is applied. When the microgrid is in islanded operation, the voltage frequency at the PCC (Power Control Center) will continuously accumulate until it exceeds the detection threshold set by the AFDPF method, thus completing the change of the control strategy for the energy storage converter. In this embodiment, the set frequency difference threshold is 0.2Hz, and the specific detection equation for the islanding detection method is as follows:

[0067]

[0068] Where cf is the cutoff coefficient, cf0 is the initial cutoff coefficient, and Δf is the voltage-frequency difference between the microgrid and the main grid.

[0069] 3) Determine whether the microgrid voltage frequency exceeds the frequency threshold corresponding to the adopted detection method. If it exceeds the corresponding frequency threshold, the microgrid is determined to be in an islanded state; if it does not exceed the corresponding frequency threshold, the microgrid is determined not to be in an islanded state and needs to be connected to the grid. In this embodiment, the frequency thresholds corresponding to the AFD detection method and the AFDPF detection method are the same, both being ±0.5Hz of the power frequency specified by the national standard. That is, the microgrid voltage frequency being lower than the lower limit of the frequency threshold or higher than the upper limit of the frequency threshold is considered to exceed the frequency threshold. In the AFD detection method, if the cutoff coefficient cf is too small, it will result in a small frequency offset, thereby reducing the detection speed and ultimately leading to too many injection currents, which will seriously affect the power quality; however, if the value of cf is too large, it will result in excessive current distortion, which will also seriously affect the power quality of the microgrid. Similarly, this is its contradiction; the AFDPF detection method, due to the introduction of positive feedback, can ensure that its detection speed continuously increases with the increase of disturbance current. Therefore, in this embodiment, before using the AFD or AFDPF algorithm, the difference between the microgrid and the power frequency is detected. If it is less than the frequency difference threshold, the AFDPF detection method is used; if it is greater than the frequency difference threshold, the AFD detection method is used. Thus, it ensures the detection speed when the current distortion is small, avoiding the cumulative effect of multiple injections, and controls the single injection amount of disturbance current when the current distortion is large, avoiding excessive single disturbance impacting the microgrid, achieving an automatic balance between detection speed and power quality. Since the final goal is to compare the microgrid voltage frequency with the thresholds of 49.5Hz and 50.5Hz, the smaller the difference, the larger the frequency difference between it and 49.5Hz and 50.5Hz.

[0070] In summary, the island detection method in this embodiment follows the detection process as follows: Figure 3 As shown.

[0071] Examples of Microgrid Pre-synchronization Control Methods

[0072] This embodiment provides a technical solution for a microgrid pre-synchronization control method. First, the voltage frequency of the microgrid and the voltage frequency of the main grid are detected, and the frequency difference between them is obtained. It is then determined whether the frequency difference is greater than a set frequency difference threshold. If it is, the AFD detection method is used to detect whether the microgrid is in an islanded state. If it is not, the AFDPF detection method is used to detect whether the microgrid is in an islanded state. Next, it is determined whether the microgrid voltage frequency exceeds the frequency threshold corresponding to the adopted detection method. If it exceeds the frequency threshold, the microgrid is determined to be in an islanded state. If it does not exceed the frequency threshold, the microgrid is determined not to be in an islanded state and needs to be connected to the grid. In this embodiment, the frequency thresholds corresponding to the AFD detection method and the AFDPF detection method are the same, both being ±0.5Hz of the national standard power frequency.

[0073] In the ADF detection method, the cutoff factor for the applied disturbance current is defined as c. f The strength of the disturbance is characterized by the following expression:

[0074]

[0075] The sensitivity of AFD detection mainly depends on the cutoff coefficient c. f Size, c f The larger the value, the better the detection effect. However, the long-term applied disturbance current can affect the power quality of the microgrid. Therefore, the AFD detection method improves upon the AFD detection method by introducing a positive feedback coefficient k(f-f0). The expression for the cutoff coefficient of the AFD detection method is then:

[0076] c f =cf0+k(f-f0)=cf0+Δf;

[0077] In this embodiment, the frequency difference threshold is set to 0.2Hz, and the detection equation of the island detection method is as follows:

[0078]

[0079] Where Δf is the voltage-frequency difference between the microgrid and the main grid, cf is the cutoff coefficient, and t Z , where is the dead time between the positive and negative half-cycles of the microgrid current waveform, T is the period of the microgrid current waveform, cf0 is the initial cutoff coefficient, k(f-f0) is the positive feedback coefficient of the AFDPF algorithm, f is the microgrid voltage frequency, and f0 is the mains power frequency voltage frequency.

[0080] As microgrids operate in islanded mode for extended periods, direct grid connection can lead to inrush voltages and currents due to the merging of voltages from different states. This can severely impact the power quality of the microgrid, potentially causing grid connection failure. Taking phase a as an example, when the microgrid is operating in islanded mode, its AC side voltage and grid voltage are as follows:

[0081]

[0082] Among them, u a u oa Let ω represent the phase a voltage of the microgrid in islanded operation and the phase a voltage of the grid, respectively. ω is the angular frequency of the microgrid in islanded operation. o Let θ be the angular frequency of the grid voltage, and θ be the phase angle of the microgrid voltage. o The phase angle of the grid voltage. When a microgrid is directly connected to the main grid without pre-synchronization control, an instantaneous voltage difference will occur:

[0083] Δu=u a -uoa =U a cos(ωt+θ)-U oa cos(ω o t+θ a )

[0084] As can be seen from the above formula, if no pre-synchronization control is applied at the moment of grid connection of a microgrid, the voltage difference generated can reach up to u. a +u oa This can generate a very large inrush current, leading to switching failure. Therefore, it is essential to pre-synchronize the voltage of the microgrid and the main grid before grid connection. After pre-synchronization, ensuring synchronization between the microgrid and the main grid before grid connection guarantees successful connection. Therefore, when a microgrid needs to be connected to the grid, it must first undergo pre-synchronization control. Once the voltage amplitude, frequency, and phase meet the requirements, a closing signal is issued to complete the grid connection.

[0085] When it is determined that the microgrid is not in an islanded state and needs to be connected to the grid, a compensation amount is added to the droop equation of the microgrid control system to keep the voltage frequency of the microgrid consistent with the voltage frequency of the main grid.

[0086] When a microgrid needs to be connected to the grid, it must first undergo pre-synchronization control. Once the voltage amplitude, frequency, and phase meet the requirements, a closing signal is issued to complete the grid connection.

[0087] When a microgrid operates in islanded mode using VCG control, the active power droop characteristic refers to the property that the generator output active power and the system frequency change in opposite directions, as simulated by the VCG. For example, when the system frequency decreases, the generator needs to generate more active power. The active power droop characteristic curve shows that when the active power of the islanded microgrid increases, it indicates that the system frequency has decreased (i.e., more active power is needed to support the frequency). Simultaneously, the voltage phase angle will also change accordingly. When the microgrid system reaches equilibrium again, the frequency will differ from the original equilibrium point by Δf. To ensure that the microgrid frequency matches the main grid frequency, a frequency compensation amount Δf can be added to the frequency-active power control loop of the virtual synchronization control. This shifts the active power droop characteristic curve upwards, thereby performing voltage phase angle pre-synchronization control, i.e., controlling the voltage frequency of the microgrid to be the same as the voltage frequency of the main grid.

[0088] The equation for active power drop is as follows:

[0089] P m -P ref =K p (w0-w)

[0090] Where Pm is the generated active power, Pref is the active power reference value, w0 is the angular velocity reference value, w is the actual angular velocity value, and Kp is the droop coefficient.

[0091] Therefore, if the control method for islanded operation of a microgrid is frequency-active power control, then the compensation amount added to the active power droop equation is:

[0092]

[0093]

[0094] In the formula, Δω and Δf are the compensation amounts for the voltage angular frequency and voltage frequency, respectively; K pθ K iθ These represent the proportional-integral coefficients in the voltage phase angle pre-synchronization PI controller; θ is the phase angle of the microgrid output voltage in islanded mode. o Let be the phase angle of the main grid voltage; the voltage frequency and phase can be obtained from the phase-locked loop module. Based on the above formula, a voltage phase angle pre-synchronization control structure, constructed by adding frequency compensation, that ensures the microgrid frequency matches the main grid frequency is as follows: Figure 4 As shown, the essence of this voltage phase angle pre-synchronization control structure is a structure for calculating the compensation amount, and the added frequency compensation amount is added as an increment to the original VSG control structure.

[0095] In the control strategies for islanded operation of microgrids, in addition to frequency-active power control, some control strategies also employ reactive power-voltage control; the reactive power droop characteristic equation is as follows:

[0096] U0-U ref =K Q (Q ref -Q)

[0097] U0 is the voltage of the microgrid, U ref Q is the voltage reference value. ref Q is the reactive power reference value, and K is the actual reactive power value. Q This represents the reactive power droop coefficient. Similarly, based on the reactive power droop characteristic curve, when the voltage amplitude of the microgrid differs from that of the main grid, a voltage amplitude compensation amount ΔE can be added to the reactive power droop equation of the control system. This shifts the reactive power droop characteristic curve upward, thus bringing the voltage amplitude of the microgrid and the main grid into agreement. Therefore, if the control method under islanded operation of the microgrid is reactive power-voltage control, the compensation amount added to the reactive power droop equation is:

[0098]

[0099] Where ΔE is the voltage amplitude compensation amount; K pu K iu These are respectively represented as the proportional-integral coefficients in the PI controller; u od u dThese represent the d-axis voltage components of the large power grid and the microgrid in a two-phase rotating coordinate system, respectively, which can be obtained through coordinate transformation. Based on the above formula, a voltage amplitude pre-synchronization control structure, constructed by adding amplitude compensation, that ensures the frequency of the microgrid remains consistent with the frequency of the large power grid is as follows: Figure 5 As shown; similar to the frequency-active power control method, the essence of this voltage amplitude pre-synchronization control structure is a structure for calculating the compensation amount, and the added amplitude compensation amount is added as an increment to the original VSG control structure.

[0100] In this embodiment, refer to Figure 6 When the above-mentioned pre-synchronization control method is specifically applied to the microgrid operation mode switching process, the steps are as follows:

[0101] 1) Monitor the microgrid's operating mode in real time to determine whether the microgrid is in islanded operation mode. If it is in islanded operation mode, proceed to step 2); if it is in grid-connected operation mode, proceed to step 5.

[0102] 2) Use the islanding detection method to detect whether the microgrid is in an islanded state; if the detection result is that it is in an islanded state, continue to maintain the islanded operation mode; if the detection result is that it is not in an islanded state, proceed to step 3).

[0103] 3) Employ a pre-synchronization control method to simultaneously determine the synchronization status of the microgrid and the main grid. If they are out of sync, return to step 2); if they are in sync, proceed to step 4.

[0104] 4) Issue grid connection command, and the microgrid switches to grid-connected operation mode;

[0105] 5) Use an islanding detection method to detect whether the microgrid is in an islanded state; if the detection result is that it is in an islanded state, switch to islanded operation mode; if the detection result is that it is not in an islanded state, maintain grid-connected operation mode.

[0106] Example of a microgrid islanding detection device

[0107] This embodiment provides a technical solution for a microgrid islanding detection device. Figure 7 This is a schematic diagram illustrating the application of the island detection device under actual working conditions. Figure 7 The pre-synchronization control / islanding detection unit in this embodiment is the microgrid islanding detection device, which is used to implement the microgrid islanding detection method as described in the above-described islanding detection method embodiment.

[0108] Since the specific principle and detection process of the microgrid islanding detection device in this embodiment have been described in detail in the above-described islanding detection method embodiment, they will not be repeated here.

[0109] Example of a microgrid pre-synchronization control device

[0110] This embodiment provides a technical solution for a microgrid pre-synchronization control device. Figure 7 This is a schematic diagram illustrating the application of the pre-synchronization control device under actual operating conditions. Figure 7 The pre-synchronization control / islanding detection unit in this embodiment is the microgrid pre-synchronization control device, which is used to implement the microgrid pre-synchronization control method as described in the above-described pre-synchronization control method embodiment.

[0111] Since the specific principles and processes of the microgrid pre-synchronization control device in this embodiment have been described in detail in the above-described pre-synchronization control method embodiments, they will not be repeated here.

[0112] This invention employs an islanding detection method that uses AFD and AFDPF detection methods intermittently under different conditions. When the frequency difference is large, a smaller disturbance current is applied; when the frequency difference is small, a larger disturbance current is added to increase the detection speed, thus avoiding the impact of long-term injected interference current on power quality. Furthermore, before the microgrid is in an islanded state and begins grid connection, a pre-synchronization method is used to add compensation to the microgrid voltage frequency or amplitude, ensuring that the microgrid voltage frequency and amplitude are consistent with the main grid. This eliminates the inrush voltage and inrush current caused by the merging of voltages in different states during grid connection, preventing the impact on the power quality of the microgrid and improving grid connection efficiency and success rate.

[0113] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the scope of protection of the claims of the present invention.

Claims

1. A method for detecting islanding in a microgrid, characterized in that, The steps are as follows: 1) Detect the voltage frequency of the microgrid and the voltage frequency of the main grid, and obtain the frequency difference between the two; 2) Determine whether the frequency difference is greater than the set frequency difference threshold. If it is greater, use the AFD detection method with an initial cutoff coefficient that does not change with the microgrid voltage frequency to detect whether the microgrid is in an islanded state, so as to ensure the detection speed when the current distortion is small and avoid the cumulative effect caused by multiple injections. If it is not greater than, the AFDPF detection method with an initial cutoff coefficient that does not change with the microgrid voltage frequency is used to detect whether the microgrid is in an islanded state, so as to control the single injection amount of disturbance current when the current distortion is large, and achieve an automatic balance between detection speed and power quality. 3) Determine whether the microgrid voltage frequency exceeds the frequency threshold corresponding to the detection method used. If it exceeds the corresponding frequency threshold, the microgrid is determined to be in an islanded state; if it does not exceed the corresponding frequency threshold, the microgrid is determined to be not in an islanded state.

2. The microgrid islanding detection method according to claim 1, characterized in that, The expression for the AFD detection method is: ； The expression for the AFDPF detection method is: Where cf is the cutoff coefficient, t Z Let cf0 be the dead time between the positive and negative half-cycles of the microgrid current waveform, T be the period of the microgrid current waveform, cf0 be the initial cutoff coefficient, k(f-f0) be the positive feedback coefficient of the AFDPF algorithm, f be the microgrid voltage frequency, and f0 be the mains power frequency voltage frequency. This represents the voltage-frequency difference between the microgrid and the main power grid.

3. The microgrid islanding detection method according to claim 1 or 2, characterized in that, The set frequency difference threshold is 0.2Hz.

4. A microgrid pre-synchronization control method, characterized in that, First, the voltage frequency of the microgrid and the voltage frequency of the main grid are detected, and the frequency difference between the two is obtained. It is then determined whether the frequency difference is greater than the set frequency difference threshold. If it is greater, the AFD detection method with an initial cutoff coefficient that does not change with the voltage frequency of the microgrid is used to detect whether the microgrid is in an islanded state, so as to ensure the detection speed when the current distortion is small and avoid the cumulative effect caused by multiple injections. If the initial cutoff coefficient is not greater than the specified value, the AFDPF detection method, which does not change with the microgrid voltage frequency, is used to detect whether the microgrid is in an islanded state. This is to control the single injection amount of disturbance current when the current distortion is large, so as to achieve an automatic balance between detection speed and power quality. It is then determined whether the microgrid voltage frequency exceeds the frequency threshold corresponding to the detection method used. If it exceeds the corresponding frequency threshold, the microgrid is determined to be in an islanded state. If it does not exceed the corresponding frequency threshold, the microgrid is determined not to be in an islanded state and needs to be connected to the grid. When it is determined that the microgrid is not in an islanded state and needs to be connected to the grid, a compensation amount is added to the droop equation of the microgrid control system to keep the voltage frequency of the microgrid consistent with the voltage frequency of the main grid. If the control method for islanded operation of a microgrid is frequency-active power control, then the compensation amount added to the active power droop equation is: In the formula, and These are the compensation amounts for the voltage angular frequency and voltage frequency, respectively; K pθ K iθ These represent the proportional-integral coefficients in the voltage phase angle pre-synchronization PI controller; θ is the phase angle of the microgrid output voltage in islanded mode. o The phase angle of the main power grid voltage; If the control method for islanded operation of a microgrid is reactive power-voltage control, then the compensation amount added to the reactive power droop equation is: in, K represents the voltage amplitude compensation amount. pu K iu These are respectively represented as the proportional-integral coefficients in the PI controller; u od u d These are the d-axis voltage components of the large power grid and the microgrid in a two-phase rotating coordinate system, obtained by coordinate transformation.

5. The microgrid pre-synchronization control method according to claim 4, characterized in that, The expression for the AFD detection method is: ； The expression for the AFDPF detection method is: Where cf is the cutoff coefficient, t Z Let cf0 be the dead time between the positive and negative half-cycles of the microgrid current waveform, T be the period of the microgrid current waveform, cf0 be the initial cutoff coefficient, k(f-f0) be the positive feedback coefficient of the AFDPF algorithm, f be the microgrid voltage frequency, and f0 be the mains power frequency voltage frequency. This represents the voltage-frequency difference between the microgrid and the main power grid.

6. The microgrid pre-synchronization control method according to claim 4 or 5, characterized in that, The set frequency difference threshold is 0.2Hz.

7. A microgrid islanding detection device, characterized in that, This device is used to implement the microgrid islanding detection method as described in any one of claims 1-3.

8. A microgrid pre-synchronization control device, characterized in that, This device is used to implement the microgrid pre-synchronization control method as described in any one of claims 4-6.

Images