Fault detection method applied to mesh direct current micro-grid

Abstract

The invention discloses a fault detection method applied to a mesh DC micro-grid, and the method specifically comprises the steps: carrying out the analysis of a fault current according to an equivalent circuit model when the fault of the mesh DC micro-grid occurs, and deducing a calculation formula of the fault current and a first derivative and a second derivative of the fault current; after the fault of the DC micro-grid occurs, determining parameters according to the change of the fault current, and designing a threshold value to realize fault judgment; the FCLO is used for carrying out offset on the fault current; different FCLO values are applied according to different lines, and event classification is carried out by analyzing the change of fault current first-order derivatives of different lines; and finally, performing fault isolation according to the event type of each unit. According to the method, fault clearing can be completed without any communication channel or system shutdown, the time delay of fault recovery is greatly reduced, and the stability and response speed of the system are improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of DC microgrid fault detection, and in particular relates to a fault detection method applied to a meshed DC microgrid. Background Art

[0002] With the advancement of renewable clean energy and smart grid construction, flexible DC transmission technology is becoming more and more mature. As an important part of smart grid, DC microgrid has gradually become a key infrastructure of the future energy Internet. DC microgrid is usually composed of distributed micro-sources, energy storage systems, protection and monitoring devices, and AC and DC loads. It has significant advantages such as high efficiency, simple control, flexible operation, and easy expansion. However, the presence of a large number of power electronic equipment in the system makes the operating conditions complicated, and the short-circuit current impact is large and lacks zero crossing points, which makes DC microgrid fault detection and removal extremely difficult. Therefore, fast and reliable detection and removal of faults has become one of the key technologies to promote the widespread application of DC microgrids.

[0003] In multi-busbar meshed DC microgrids, line short-circuit faults are the most common type of faults, and their causes may include physical damage, environmental stress, electrical stress, and cable aging. At present, differential protection technology is mostly used to deal with this problem. However, traditional differential protection technology relies on communication channels between device terminals for real-time data transmission, which may result in communication failures or data loss, resulting in protection failure. Therefore, differential protection systems usually require additional backup protection solutions, which not only increases the cost and scale of the system, but also limits its application in microgrids. In addition, this method's over-reliance on communication may introduce delays, thereby extending the protection response time and reducing the operational reliability of the DC microgrid. Summary of the invention

[0004] The present invention aims to provide a fault detection method for a meshed DC microgrid, which can complete fault clearing without any communication channel or system shutdown, greatly reducing the time delay of fault recovery.

[0005] The technical solution adopted by the present invention is a fault detection method for a meshed DC microgrid, which is specifically implemented according to the following steps:

[0006] Step 1: Analyze the fault current according to the equivalent circuit model of the meshed DC microgrid when a fault occurs, and derive the calculation formula of the fault current and its first-order derivative and second-order derivative;

[0007] Step 2: After a DC microgrid fault occurs, parameters are determined according to the change of the fault current, and a low-resistance first-order threshold, a high-resistance first-order threshold, and a high-resistance second-order threshold are designed to realize fault determination;

[0008] Step 3: Use FCLO to offset the fault current. To determine the fault line, apply different FCLO values ​​to different lines, and classify events by analyzing the changes in the first-order derivatives of the fault currents of different lines. Finally, isolate the fault according to the event type of each unit.

[0009] The present invention is also characterized in that:

[0010] In step 1, specifically:

[0011] Step 1.1: After a fault occurs, the frequency domain fault current i(s) is calculated based on the equivalent circuit, as shown in formula (1):

[0012]

[0013] In formula (1), i L (0) and V C (0) are the current through the inductor and the voltage through the capacitor before the fault occurs, L and r are the equivalent series inductance and resistance of the cable to the fault point, R F is the fault resistance, C is the equivalent capacitance; s is the Laplace operator;

[0014] In the time domain, the fault current i(t) can be expressed as formula (2);

[0015]

[0016] Where t is time in milliseconds; p1 and p2 are the poles of i(s), as shown in equation (3);

[0017]

[0018] Step 1.2, calculate the first-order derivative of the fault current; the time derivative of equation (2) is shown in equation (4);

[0019]

[0020] After the fault occurs, at t = 0 + When , the magnitude of the current derivative is as shown in formula (5);

[0021]

[0022] Substituting the values ​​of p1 and p2 in equation (3) into equation (5), we get equation (6):

[0023]

[0024] Step 1.3, calculate the second-order derivative of the fault current; the second-order derivative of the line current can be obtained by derivation of equation (5), as shown in equation (7);

[0025]

[0026] After a fault occurs, at t = 0 + When , the magnitude of the second-order derivative of current can be given by equation (8):

[0027]

[0028] Substituting the values ​​of p1 and p2 in equation (3) into equation (8), we can obtain equation (9):

[0029]

[0030] In step 2, specifically:

[0031] Step 2.1: Substitute the sampling time into the first-order derivative, and use the finite difference approximation to calculate di / dt, which can be expressed as formula (10):

[0032]

[0033] Δi can be written as formula (11):

[0034] Δi=i k -i k-1 (11);

[0035] In the formula, k is the sampling moment, i k and i k-1 are the current and previously sampled line currents respectively;

[0036] Step 2.2, second-order derivative calculation: d 2 i / dt 2 At the time interval dt, each line is calculated using the current value of di / dt and the previous calculated value, as shown in equation (12);

[0037]

[0038] Among them, Δ 2 i can be written as formula (13) or formula (14):

[0039] Δ 2 i=Δi k -Δi k-1 (13);

[0040] Δ 2 i=i k -2i k-1 +i k-2 (14);

[0041] When the fault occurs at k = 0, k increases by 1 at every sampling time; when k = 1, since i0 = i -1 =iL (0), so Δi=Δ 2 i; Therefore, when k = 2, Δ is calculated by equation (13) or equation (14) 2 The actual value of i;

[0042] Step 2.3, Δi threshold setting; Δi threshold setting is divided into low resistance first-order threshold and low resistance first-order threshold; low resistance first-order threshold Δi max According to the given DC microgrid parameters, the high-resistance first-order threshold Δi min Depends on the operation of the DC microgrid;

[0043] Step 2.4, set the high impedance second-order threshold Δ 2 i min ;

[0044] Step 2.5, fault determination: Compare the calculated Δi with Δi max Compare and judge whether a fault occurs. If it is greater than, directly judge that a fault occurs. Otherwise, continue to the next step for judgment. When Δi is less than Δi min It is directly judged that no fault has occurred, otherwise continue to the next step to calculate the second-order derivative of the real-time fault current Δ 2 i, when it exceeds Δi 2 min If the fault occurs, it is judged that the fault occurs; otherwise, the fault does not occur.

[0045] In step 2.3, when a low resistance fault occurs, i L The amplitude of the first-order derivative of the (0) component is very small, approximately 0. According to formula (6), the low-resistance first-order threshold Δi can be obtained. max The calculation formula is shown in formula (15);

[0046]

[0047] Among them, Leq depends on the line inductance of each unit from the busbar and the line inductance of its parallel branch. For example, when the cable line 1-2 is close to the busbar 1, the polar fault (such as Figure 3a ), the equivalent inductance L eq , as shown in formula (16);

[0048]

[0049] If it is an inter-pole fault (such as Figure 3b ) Its equivalent inductance L eq , as shown in formula (17);

[0050]

[0051] In step 2.3, the high-resistance first-order threshold Δi minThe calculation formula is (18);

[0052]

[0053] In step 2.4, Δ 2 i min The calculation process is shown in formula (19);

[0054]

[0055] In step 3, specifically: after receiving the fault signal in step 2, FCLO is applied to each line to offset the fault current. Since the inductance of FCLO of different lines is different, the first-order derivative of the fault current will change differently; by analyzing the changes in the first-order derivative of the current of the photovoltaic unit, energy storage unit, AC power grid and load unit, the fault is divided into three event types: 1) bus fault; 2) interconnected feeder fault; 3) adjacent feeder or adjacent bus fault, and finally the fault is isolated according to the event type.

[0056] The beneficial effect of the present invention is that the method of the present invention determines the fault by analyzing the fault current and its first-order derivative and second-order derivative, and realizes fault isolation through event classification, and the process is simple, fast, efficient and reliable. At the same time, during the fault detection process, the system only relies on the parameters measured locally, and can complete the fault clearing without any communication channel or system shutdown, which greatly reduces the time delay of fault recovery. In addition, since the fault isolation part does not need to transmit real-time data such as voltage and current in traditional protection schemes, it only needs to transmit and receive event types, thereby greatly reducing the dependence on communication and improving the stability and response speed of the system. BRIEF DESCRIPTION OF THE DRAWINGS

[0057] Figure 1 is a model structure diagram of a meshed DC microgrid in the method of the present invention;

[0058] Figure 2a is the short-circuit fault equivalent circuit diagram (I) in the method of the present invention;

[0059] Figure 2b is the short-circuit fault equivalent circuit diagram (II) in the method of the present invention;

[0060] Figure 2c is the short-circuit fault equivalent circuit diagram (III) in the method of the present invention;

[0061] Figure 3a is a polar fault and its RLC equivalent circuit diagram in the method of the present invention;

[0062] Figure 3b is an inter-pole fault and its RLC equivalent circuit diagram in the method of the present invention;

[0063] Figure 4 is a flow chart of fault judgment in the method of the present invention;

[0064] Figure 5a It is the fault current diagram when the fault F23 in the method of the present invention occurs at a distance of 35m from the busbar;

[0065] Figure 5b It is the fault current diagram when the fault F34 in the method of the present invention occurs at a distance of 35m from the busbar;

[0066] Figure 6 It is a flow chart of event classification and isolation decision in the method of the present invention;

[0067] Figure 7 is a diagram showing the effect of different FCLOs on current offset in the method of the present invention;

[0068] Figure 8a It is a fault current and its first-order derivative diagram of a FCLO of 0.6 mH applied to feeder 2-3 in the method of the present invention;

[0069] Figure 8b It is a fault current with FCLO of 1 mH applied to feeder 3-4 and its first-order derivative diagram in the method of the present invention;

[0070] Figure 9a is the detection result diagram when bus 3 fails;

[0071] Figure 9b This is the current simulation result diagram of bus 3;

[0072] Fig.9c This is the voltage simulation result diagram of bus 3;

[0073] Fig.10a This is the detection result diagram when the feeder 2-3 fails;

[0074] Fig.10b This is the current simulation result diagram of feeder 2-3;

[0075] Fig.10c This is the voltage simulation result diagram of feeder 2-3;

[0076] Fig.11a This is the detection result diagram when the feeder 3-4 fails;

[0077] Fig.11b This is the current simulation result diagram of feeder 3-4;

[0078] Fig.11c This is the voltage simulation result diagram of feeder 3-4. DETAILED DESCRIPTION

[0079] The present invention is described in detail below with reference to the accompanying drawings and specific embodiments.

[0080] Example 1

[0081] The present invention is applied to a fault detection method of a meshed DC microgrid, wherein the structure of the meshed DC microgrid is as follows: Figure 1 As shown, it consists of four parts: photovoltaic unit, energy storage unit, AC grid unit and load unit. Figure 1 The possible bus fault and feeder fault locations are also marked, where F1 to F4 represent bus faults, and F12, F14, F23, F24, and F34 represent feeder faults. DC short circuit fault is the most serious fault type in DC microgrids. No matter where the short circuit occurs, it can be Figure 2a A short circuit fault in a DC circuit can result in a high fault current due to the low impedance provided by the charged capacitors and cables.

[0082] In order to comprehensively analyze the response of nonlinear circuits, the present invention performs fault analysis in two different stages.

[0083] Phase 1: Capacitor discharge. When a short circuit occurs, the voltage stabilizing capacitor will quickly discharge along the fault circuit, causing the voltage to drop to zero quickly and the current to rise rapidly. At this time, the equivalent circuit consists of the equivalent resistance (r), the fault resistance (R F ), equivalent capacitance (C) and equivalent inductance (L), forming a second-order discharge circuit, as shown in the following figure: Figure 2b As shown;

[0084] Stage 2: Diode freewheeling stage. When the DC bus voltage drops to zero and becomes negative, the system enters the second stage. Figure 2c As shown. At this time, the voltage reversal affects the voltage at the conversion end and causes the diode to conduct. The figure shows the equivalent model of two diodes conducting in any converter branch. The diode provides an alternative path for the current that is independent of the IGBT conduction state, thereby changing the current response. At this stage, the current that the diode bears is almost ten times the nominal current, and the current rises sharply, which may cause the diode to be damaged.

[0085] Therefore, the present invention aims to timely identify DC network faults through fault detection before the system enters the second stage, to prevent the power electronic converter and its components from being damaged by high fault currents, and to ensure the stability and safety of the system.

[0086] Example 2

[0087] The present invention is applied to the fault detection method of the meshed DC microgrid, which is specifically implemented according to the following steps:

[0088] Step 1: Analyze the fault current according to the equivalent circuit model of the meshed DC microgrid when a fault occurs, and derive the calculation formula of the fault current and its first-order derivative and second-order derivative;

[0089] Step 1.1: After a fault occurs, according to the equivalent circuit Figure 2b As shown, the frequency domain fault current i(s) is calculated as shown in formula (1);

[0090]

[0091] In formula (1), i L (0) and V C (0) are the current through the inductor and the voltage through the capacitor before the fault occurs, L and r are the equivalent series inductance and resistance of the cable to the fault point, R F is the fault resistance, C is the equivalent capacitance; s is the Laplace operator;

[0092] In the time domain, the fault current i(t) can be expressed as formula (2);

[0093]

[0094] Where t is time in milliseconds;

[0095] p1 and p2 are the poles of i(s), as shown in equation (3);

[0096]

[0097] Step 1.2, find the first-order derivative of the fault current; In equation (2), the fault current i(t) depends on V C (0), for a given DC microgrid line current i before fault L , capacitor voltage V C (0) is fixed, so the time derivative of equation (2) is shown in equation (4);

[0098]

[0099] After the fault occurs, at t = 0 + When , the magnitude of the current derivative is as shown in formula (5);

[0100]

[0101] Substituting the values ​​of p1 and p2 in equation (3), we obtain equation (6):

[0102]

[0103] When R F= 0, the equivalent cable resistance before the fault point is r. For short-circuit fault current, relative to V C (0),i L The contribution of the (0) component to the first-order derivative amplitude is very small and can be ignored. Therefore, when determining the main protection threshold, only V C (0)Quantity.

[0104] Step 1.3, calculate the second-order derivative of the fault current; the second-order derivative of the line current can be obtained by taking the time derivative of equation (5), as shown in equation (7);

[0105]

[0106] At t = 0 + After a fault occurs, the magnitude of the second-order derivative can be given by equation (8):

[0107]

[0108] Substituting the values ​​of p1 and p2 into the circuit parameters, we can obtain equation (9):

[0109]

[0110] From formula (9), we can get that the size of the second-order derivative depends on the equivalent resistance R = R F +r. For R = R c , the initial line current contribution to the second-order derivative will be zero, where R c =L / C. So when 0 <R F <R c This is a low impedance fault.

[0111] Step 2: After a DC microgrid fault occurs, parameters are determined according to the change of the fault current, and appropriate setting values ​​are designed to achieve fault determination;

[0112] Step 2.1: Substitute the sampling time into the first-order derivative, and use the finite difference approximation to calculate di / dt, which can be expressed as formula (10):

[0113]

[0114] Δi can be written as formula (11):

[0115] Δi=i k -i k-1 (11);

[0116] In the formula, k is the sampling moment, i k and i k-1are the line currents sampled currently and previously, respectively. Since the sampling time is the same for all measurements, only the current difference is measured at consecutive sampling moments to calculate the time derivative.

[0117] Step 2.2, second-order derivative calculation: d 2 i / dt 2 At the time interval dt, each line is calculated using the current value of di / dt and the previous calculated value, as shown in equation (12);

[0118]

[0119] Among them, Δ 2 i can be written as formula (13) or formula (14):

[0120] Δ 2 i=Δi k -Δi k-1 (13);

[0121] Δ 2 i=i k -2i k-1 +i k-2 (14);

[0122] The fault occurs when k = 0, and k increases by 1 at every sampling time. When k = 1, since i0 = i -1 =i L (0), so Δi=Δ 2 Therefore, when k = 2, Δ is calculated by equation (13) or equation (14): 2 The actual value of i is used for fault detection. 2 i is compared with the respective threshold setting. As soon as the calculated value exceeds the threshold, a fault signal is issued.

[0123] Step 2.3, Δi threshold setting; Δi threshold setting is divided into low resistance first-order threshold and low resistance first-order threshold. Low resistance first-order threshold (Δi max ) is calculated based on the given DC microgrid parameters, the high resistance first-order threshold (Δi min ) depends on the operation of the DC microgrid.

[0124] 1) Low resistance first-order threshold: The magnitude of Δi depends on the fault location and its impedance. For a direct short-circuit fault occurring at the end of the line, the threshold of Δi is calculated. The equivalent circuit considered in this case is 0≤R F ≤R c When a fault occurs, the time interval Δ t The expression of Δi can be derived from formula (7), Δi max The calculation process of is shown in formula (15);

[0125]

[0126] In the formula, if it is a polar fault, the equivalent inductance L eq , as shown in formula (16);

[0127]

[0128] If it is an inter-pole fault, the equivalent inductance L eq , as shown in formula (17);

[0129]

[0130] 2) High impedance first order threshold: For high impedance faults, line load also plays an important role in determining the magnitude of Δi. To calculate the fault threshold Δi min , considering the case where the fault impedance is 25Ω, the Δi value of different line loads is calculated using formula (7). F >>r,Δi min The calculation formula is (18);

[0131]

[0132] Substitute the equivalent inductance calculated from equation (16) to the fault point into the above equation, Δi min The value depends on the line current i when the fault occurs L (0), the threshold setting of Δi changes with the operation of the DC microgrid. min The settings are self-adjusted according to the different operation modes of the microgrid;

[0133] Step 2.4, high resistance second-order threshold Δ 2 i min Setting Δ 2 i min Similar to the calculation of Δi, Δ 2 i is also derived analytically. 2 i min The value is based on the worst case R F The goal is to detect faults in the grid before starting the second phase. Substituting R = 25Ω into equation (9), we can calculate Δ 2 i min , as shown in formula (19);

[0134]

[0135] Due to R F >>R c, the initial line current effect cannot be ignored. In this mode, the current is overdamped because (R / L eq ) 2 >>1 / L eq C, therefore, i L (0) will also contribute to Δ 2 i min Since the line current depends on the working mode of the DC microgrid, Δ 2 i min The calculation should be self-adjusting.

[0136] Step 2.5, fault determination. Compare the calculated Δi with Δi max Compare and judge whether a fault occurs. If it is greater than, directly judge that a fault occurs. Otherwise, continue to the next step for judgment. When Δi is less than Δi min It is directly judged that no fault has occurred, otherwise continue to the next step to calculate the second-order derivative of the real-time fault current Δ 2 i, when the variable exceeds Δi 2 min If the fault occurs, it is judged that the fault occurs; otherwise, the fault does not occur.

[0137] If it is determined that a fault has occurred, the event classification module begins to determine the specific line where the fault occurred and outputs a fault isolation plan to perform fault isolation.

[0138] Step 3: Use FCLO to offset the fault current; apply different FCLO values ​​to different lines, and classify events by analyzing the changes in the first-order derivatives of the fault currents of different lines. Then, the event types of each unit are transmitted to the central processing unit through data communication, and finally the fault is isolated according to the event type; specifically:

[0139] Step 3.1: The fault signal is received through step 2, but the line where the fault occurs cannot be accurately determined and isolated. Therefore, FCLO is applied to each line to offset the fault current, thereby improving the accuracy of fault location.

[0140] Step 3.2: Due to the different inductances of the FCLO of different lines, the current offset varies, so the first-order derivative of the fault current changes differently. By analyzing the first-order derivatives of different currents, the fault is divided into three event types: 1) bus fault; 2) interconnected feeder fault; 3) adjacent feeder or adjacent bus fault;

[0141] Step 3.3, the event type of each unit (including photovoltaic unit, energy storage unit, AC power grid and load unit) is transmitted to the central processing unit through data communication, and finally the fault is isolated according to the event type; as shown in Table 1;

[0142] Table 1 Event types and their isolation decisions

[0143]

[0144] Example 3

[0145] From formula (2), we can see that the fault current increases rapidly after the fault occurs, so it is necessary to quickly detect the fault. According to formula (7), the low impedance pole-to-ground short circuit fault and the pole-to-pole short circuit fault are 0 + The first-order derivative of the fault current at the moment is consistent, so the pole-to-pole fault and low-impedance pole-to-ground fault can be judged by the first-order derivative of the fault current. However, when a high-impedance pole-to-ground short-circuit fault occurs, it will be missed because its first-order derivative is small. Therefore, it can be obtained from the analysis of formula (9) that the size of the second-order derivative of the fault current is not only affected by V C Influence, line equivalent impedance R (R = r + R F ) also has a great influence on it. Therefore, the high impedance polar short circuit fault can be detected by the second-order derivative of the fault current. According to formula (16), the polar short circuit equivalent inductance L is calculated. eq ,like Figure 3a As shown, according to formula (17), the inter-pole fault equivalent inductance L is calculated eq ,like Figure 3b As shown. Figure 4 As shown, Δi is calculated according to formula (15), formula (18), and formula (19) respectively. max , Δi min and Δi 2 min , calculate the real-time first-order derivative Δi through the local real-time current data, and compare the calculated Δi with Δi max Compare and judge whether a fault occurs. If it is greater than, directly judge that a fault occurs. Otherwise, proceed to the next step. When Δi is less than Δi min It is directly determined that no fault has occurred, otherwise proceed to the next step to calculate the second-order derivative of the real-time fault current Δ 2 i, when the variable exceeds Δi 2 min If the fault is less than , the fault does not occur. If the fault is judged to occur, the event classification module starts to determine the specific line where the fault occurs and outputs a fault isolation plan to isolate the fault.

[0146] In order to optimize and refine the fault isolation process of the microgrid, it is first necessary to solve the problem of overlapping of the fault current and its first-order derivative when the fault occurs near the busbar. Figure 5a and Figure 5bIt shows that fault F23 and fault F34 occurred at a distance of 35 meters from the busbar, which made the fault current determination vague and the fault type unable to be accurately distinguished. This would result in an excessively large isolation range, thus causing unnecessary power outages and affecting the stability and reliability of the microgrid.

[0147] In order to solve this problem, the following optimization method is proposed:

[0148] 1. Fault current offset: By introducing different FCLOs into the line, the fault current can be effectively separated from the current near the busbar to avoid current overlap. This offset makes the fault current more clearly distinguishable in the first-order derivative, thereby improving the accuracy of fault detection.

[0149] 2. Fault event classification: Based on the offset fault current and its first-order derivative, the event classification module can autonomously classify each fault event into one of three types:

[0150] Busbar failure (event 1)

[0151] Interconnection feeder failure (Event 2)

[0152] Adjacent feeder or adjacent busbar fault (event 3)

[0153] 3. Fault isolation decision: By autonomously classifying the fault type in each unit and combining it with high-level data from other interconnected units, the system can make more accurate fault isolation decisions. Figure 6 The decision-making process of fault isolation is demonstrated: after identifying the specific fault type, the area to be isolated is determined based on the different fault types and location information to reduce the scope of fault propagation and power outage.

[0154] By introducing FCLO for fault current offset, combined with accurate event classification and high-level data communication, the system can more effectively identify fault types and make reasonable isolation decisions. This method not only improves the accuracy of fault diagnosis, but also optimizes isolation strategies and avoids over-isolation, thereby ensuring the stable operation of the microgrid and reducing unnecessary power outages.

[0155] Example 4

[0156] Figure 7 The short-circuit current (i) of fault F34 occurring at a distance of 35 meters from the busbar is shown when the FCLO is 0.3mH, 1mH, and 3mH respectively. ACG ). By analyzing Figure 7The following conclusions can be drawn: when FCLO is 0.3mH, the slope of the fault current decreases, but the maximum current value does not change much; when FCLO increases to 1mH and 3mH, the maximum value of the fault current decreases as the slope decreases. The results show that the introduction of FCLO and the use of current derivatives can significantly improve the accuracy of fault classification.

[0157] It should be noted that the increase of FCLO will lead to an increase in power loss. In this study, when the FCLO is selected as 1mH, the efficiency of the system under full load operation only decreases by 0.25%. Therefore, in order to take into account the effect of fault classification and the operating efficiency of the system, it is recommended that the selection range of FCLO be between 0.3mH and 1mH. Specifically, the FCLO of feeder 2-3 is set to 0.6mH, and the FCLO of feeder 3-4 is set to 1mH. When faults F23 and F34 occur at a distance of 35 meters from the busbar, the short-circuit current (i ACG ) and its first-order derivative as Figure 8a and Figure 8b As shown;

[0158] Example 5

[0159] In this system, when a fault occurs, the occurrence of the fault is first determined. However, when the fault occurs close to the busbar, it may not be possible to accurately give an isolation solution. Therefore, the fault current is first limited and offset by applying different FCLOs, and then the event is classified based on the first-order derivatives of different fault currents. Each unit sends and receives event types to each other in order to collaboratively complete fault isolation decisions. Through the optimization solution, the system can quickly respond and isolate faults in different types of fault situations, as shown below:

[0160] When the fault occurs at bus 3;

[0161] like Figure 9a As shown, detection and isolation time: when a fault occurs on bus 3, the present invention can complete fault detection and isolation within 0.2ms.

[0162] Current and voltage recovery: After the fault is isolated, the current i of bus 3 is ACG With voltage V ACG like Figure 9b and Fig.9c As shown, return to normal.

[0163] Isolation solution: Because the fault occurred on bus 3, the system isolated the entire AC grid unit to ensure that the fault was resolved quickly.

[0164] The fault occurred on feeder 2-3, about 5 m away from bus 3;

[0165] Detection and isolation time: When the fault occurs in feeder 2-3 and is about 5m away from bus 3, the present invention can complete fault detection and isolation within 0.7ms.

[0166] Example 6

[0167] like Fig.10a As shown, the detection and isolation time is: when the fault occurs in feeder 2-3 and is about 5m away from bus 3, the present invention can complete fault detection and isolation within 0.7ms.

[0168] Current and voltage recovery: After fault isolation, the current i of feeder 2-3 ACG and voltage V ACG Restore to normal. The specific recovery process is as follows: Fig.10b and Fig.10c shown.

[0169] Isolation solution: Since the fault occurs on feeder 2-3, the system will only isolate this feeder.

[0170] The fault occurred on feeder 3-4, about 5 m away from bus 3;

[0171] like Fig.11a As shown, the detection and isolation time is: when the fault occurs in feeder 3-4 and is about 5m away from bus 3, the present invention can complete fault detection and isolation within 0.85ms.

[0172] Current and voltage recovery: After fault isolation, the current i of feeder 3-4 ACG and voltage V ACG Return to normal, the recovery process is as follows Fig.11b and Fig.11c shown.

[0173] Isolation solution: Similarly, the system will only isolate feeder 3-4 where the fault is located, ensuring minimal impact on the grid.

[0174] The present invention first uses the fault current of each unit and its first-order and second-order derivatives to perform fault discrimination, then outputs the fault type according to the different effects of FCLO on the first-order derivative of the fault current, and makes a fault isolation decision in combination with the output information of other interconnected units. To verify the proposed protection scheme, the present invention establishes an accurate model of the microgrid in the MATLAB / Simulink simulation environment and performs simulation verification.

Claims

1. A fault detection method for a meshed DC microgrid, characterized in that: Follow the steps below to implement it: Step 1: Analyze the fault current according to the equivalent circuit model of the meshed DC microgrid when a fault occurs, and derive the calculation formula of the fault current and its first-order derivative and second-order derivative; Step 2: After a DC microgrid fault occurs, parameters are determined according to the change of the fault current, and a low-resistance first-order threshold, a high-resistance first-order threshold, and a high-resistance second-order threshold are designed to realize fault determination; Step 3: Use FCLO to offset the fault current; Different FCLO values ​​are applied to different lines, and event classification is performed by analyzing the changes in the first-order derivatives of fault currents of different lines; finally, fault isolation is performed based on the event type of each unit.

2. The fault detection method for a meshed DC microgrid according to claim 1, characterized in that: In the step 1, specifically: Step 1.1, after a fault occurs, calculate the frequency domain fault current i(s), as shown in formula (1); In formula (1), i L (0) and V C (0) are the current through the inductor and the voltage through the capacitor before the fault occurs, L and r are the equivalent series inductance and resistance of the cable to the fault point, R F is the fault resistance, C is the equivalent capacitance; s is the Laplace operator; In the time domain, the fault current i(t) can be expressed as formula (2); Where t is time in milliseconds; p1 and p2 are the poles of i(s), as shown in equation (3); Step 1.2, calculate the first-order derivative of the fault current; the time derivative of equation (2) is shown in equation (4); After the fault occurs, at t = 0 + When , the magnitude of the current derivative is as shown in formula (5); Substituting the values ​​of p1 and p2 in equation (3) into equation (5), we get equation (6): Step 1.3, calculate the second-order derivative of the fault current; the second-order derivative of the line current can be obtained by taking the time derivative of equation (5), as shown in equation (7); After a fault occurs, at t = 0 + When , the magnitude of the second-order derivative of current can be given by equation (8): Substituting the values ​​of p1 and p2 in equation (3) into equation (8), we can obtain equation (9):

3. The fault detection method applied to a meshed DC microgrid as claimed in claim 2, characterized in that: In the step 2, specifically: Step 2.1: Substitute the sampling time into the first-order derivative, and use the finite difference approximation to calculate di / dt, which can be expressed as formula (10): Δi can be written as formula (11): Δi=i k -i k-1 (11); In the formula, k is the sampling moment, i k and i k-1 are the current and previously sampled line currents respectively; Step 2.2, second-order derivative calculation: d 2 i / dt 2 At the time interval dt, each line is calculated using the current value of di / dt and the previous calculated value, as shown in equation (12); Among them, Δ 2 i can be written as formula (13) or formula (14): Δ 2 i=Δi k -Δi k-1 (13); Δ 2 and=and k -2i k-1 +and k-2 (14); When the fault occurs at k = 0, k increases by 1 at every sampling time; when k = 1, since i0 = i -1 =i L (0), so Δi=Δ 2 i; Therefore, when k = 2, Δ is calculated by equation (13) or equation (14) 2 The actual value of i; Step 2.3, Δi threshold setting; Δi threshold setting is divided into low resistance first-order threshold and low resistance first-order threshold; low resistance first-order threshold Δi max According to the given DC microgrid parameters, the high-resistance first-order threshold Δi min Depends on the operation of the DC microgrid; Step 2.4: Set the high-resistance second-order threshold Δ 2 i min ; Step 2.5, fault determination: Compare the calculated Δi with Δi max Compare and judge whether a fault occurs. If it is greater than, directly judge that a fault occurs. Otherwise, continue to the next step for judgment. When Δi is less than Δi min It is directly judged that no fault has occurred, otherwise continue to the next step to calculate the second-order derivative of the real-time fault current Δ 2 i, when it exceeds Δi 2 min If the fault occurs, it is judged that the fault occurs; otherwise, the fault does not occur.

4. The fault detection method for a meshed DC microgrid as claimed in claim 3, characterized in that: In step 2.3, the low resistance first-order threshold Δi max The calculation process of is shown in formula (15); 5. The fault detection method for a meshed DC microgrid as claimed in claim 4, characterized in that: If it is a polar fault, the equivalent inductance L eq , as shown in formula (16); If it is an inter-pole fault, the equivalent inductance L eq , as shown in formula (17); 6. The fault detection method for a meshed DC microgrid as claimed in claim 5, characterized in that: In step 2.3, the high-resistance first-order threshold Δi min The calculation formula is (18); 7. The fault detection method for a meshed DC microgrid as claimed in claim 6, characterized in that: In step 2.4, Δ 2 i min The calculation process is shown in formula (19); 8. The fault detection method for a meshed DC microgrid as claimed in claim 7, characterized in that: In the step 3, specifically: after receiving the fault signal in step 2, FCLO is applied to each line to offset the fault current. Since the inductance of FCLO of different lines is different, the current offset is different, so the first-order derivative of the fault current will change differently; by analyzing different first-order derivatives of current, the fault is divided into three event types: 1) bus fault; 2) interconnected feeder fault; 3) adjacent feeder or adjacent bus fault; through the event type judgment of photovoltaic unit, energy storage unit, AC power grid and load unit, the fault is finally isolated according to the event type.

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