A Microgrid Fault Detection Method Based on Equivalent Impedance Drop

Abstract

The present invention belongs to the technical field of microgrid protection, and particularly relates to a microgrid fault detection method based on equivalent impedance drop. The method includes: calculating the positive, negative, and zero-sequence impedance parameters of the doubly-fed line in the microgrid and storing them in a database; measuring the voltage and current values at the head and tail of the doubly-fed line, and calculating the positive-sequence, negative-sequence, and zero-sequence voltage and current at the head of the line and the magnitude of the positive-sequence voltage at the tail; calling the corresponding line parameters in the database, and substituting the positive-sequence, negative-sequence, and zero-sequence voltage and current at the head of the line and the magnitude of the positive-sequence voltage at the tail into the corresponding equations under different fault types to obtain the equivalent impedance Z Ⅰ,i ; substituting the value of the equivalent impedance Z Ⅰ,i into the fault detection index calculation formula and comparing it with the set value to determine whether the protection of this line does not operate or operates. The present invention has the advantages of network reconfiguration independence, short response time, low sampling frequency, and small data communication, etc. Its effectiveness is not affected by load changes, distributed power uncertainty, fault initial phase angle, and fault location.

Description

Technical Field

[0001] The present invention belongs to the technical field of microgrid protection, and particularly relates to a microgrid fault detection method based on equivalent impedance drop. Background Art

[0002] The wide application of distributed power sources has led to the large-scale development and application of microgrid technology. With the large-scale access of distributed power sources, the possibility of microgrid line faults has also been increasing.

[0003] Due to the bidirectional power flow in the feeder, the existence of loop feeders, and the significant reduction in the magnitude of fault current in the island mode, especially for inverter-type distributed power sources, traditional protection strategies cannot be applied to microgrids. Microgrid fault detection methods based on network modification generally change the network behavior during a fault to correct conventional protection measures. By switching between the microgrid photovoltaic grid-connected mode and the island mode, the fault current level will change significantly. Microgrid fault detection methods based on protection strategies generally consist of several microgrid fault detection methods. Among them, in the adaptive scheme, when switching between the microgrid grid-connected mode and the island mode, the settings of the protection device will be automatically readjusted. The independence of the fault level change in the grid-connected mode and the island mode is the most significant advantage of these schemes. However, the fault resistance and feed-in caused by distributed power sources may affect the measured impedance, and the transient current may also lead to inaccurate measurement. There are high requirements for impedance-based fault detection methods, that is, the influence of power supply, fault resistance, and transient conditions on their performance is minimized.

[0004] Therefore, studying a new type of microgrid fault detection method with strong independence, short response time, low sampling frequency, and small data communication not only has theoretical research value but also has important practical significance for improving the reliability and rapidity of microgrid protection. Summary of the Invention

[0005] In order to solve the problems mentioned in the above background art, the present invention proposes a microgrid fault detection method based on equivalent impedance drop.

[0006] In order to achieve the above technical objectives and reach the above technical effects, the present invention is realized through the following technical solutions:

[0007] A microgrid fault detection method based on equivalent impedance drop, which includes the following steps:

[0008] Step 1: Based on the line structure and electrical characteristics of the microgrid, calculate the positive, negative, and zero-sequence impedance parameters of the double-fed line in the microgrid and store them in the database;

[0009] Step 2: Measure the voltage and current values at the head and tail ends of the double-fed line, and calculate the positive-sequence, negative-sequence, and zero-sequence voltage and current at the head end of the line and the positive-sequence voltage modulus at the tail end.

[0010] Step 3: Call the corresponding line parameters in the database described in Step 1, and substitute the positive-sequence, negative-sequence, zero-sequence voltages at the head of the line and the magnitude of the positive-sequence voltage at the end of the line into the equation for the equivalent impedance Z under the corresponding fault type Ⅰ,i to obtain the equivalent impedance Z Ⅰ,i , where i can be replaced by three-phase (3P), line-to-line (LL), single-line-to-ground (SLG);

[0011] The specific steps to obtain the equivalent impedance Z Ⅰ,i are as follows:

[0012] Step 3-1: If it is a three-phase short-circuit fault, call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, zero-sequence voltage and current values at the head of the line and the magnitude of the positive-sequence voltage at the end of the line into the equation for the equivalent impedance Z Ⅰ,3P to obtain Z Ⅰ,3P ;

[0013] The equation expression for obtaining the equivalent impedance Z Ⅰ,3P is:

[0014]

[0015] In the formula, Z Ⅰ,3P is the equivalent impedance under a three-phase short-circuit fault, U A1 represents the positive-sequence voltage at end A of the line, U B1 represents the positive-sequence voltage at end B of the line, I A1 represents the positive-sequence current at end A of the line, Z 1 is the positive-sequence impedance of the line.

[0016] Step 3-2: If it is a line-to-line short-circuit fault, call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, zero-sequence voltage and current values at the head of the line and the magnitude of the positive-sequence voltage at the end of the line into the equation for the equivalent impedance Z Ⅰ,LL to obtain Z Ⅰ,LL ;

[0017] The equation expression for obtaining the equivalent impedance Z Ⅰ,LL is:

[0018]

[0019] In the formula, Z Ⅰ,LL is the equivalent impedance under a line-to-line short-circuit fault, U A1 , U A2 respectively represent the positive-sequence voltage and negative-sequence voltage at end A of the line, U B1 represents the positive-sequence voltage at end B of the line, U B2 represents the negative-sequence voltage at end B of the line, I A1 、IA2 respectively represent the positive-sequence current and negative-sequence current at the A end of the line, Z 1 is the positive-sequence impedance of the line, R f is the fault resistance, and K represents the proportion of the distance from the fault point to the beginning of the line in the total length of the line (0 < K < 1).

[0020] Step 3-3: If it is a single-phase grounding fault, call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, and zero-sequence voltage and current values at the beginning of the line and the positive-sequence voltage modulus value at the end of the line into the equation for the equivalent impedance Z Ⅰ,SLG to obtain Z Ⅰ,SLG ;

[0021] Obtain the equivalent impedance Z Ⅰ,SLG The equation expression is:

[0022]

[0023] In the formula, Z Ⅰ,SLG is the equivalent impedance under single-phase grounding fault, U A1 , U A2 , U A0 respectively represent the positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage at the A end of the line, U B1 represents the positive-sequence voltage at the B end of the line, U B2 , U B0 respectively represent the negative-sequence voltage and zero-sequence voltage at the B end of the line, I A1 , I A2 , I A0 respectively represent the positive-sequence current, negative-sequence current, and zero-sequence current at the A end of the line, Z 1 , Z 0 are the positive-sequence impedance and zero-sequence impedance of the line respectively, R f is the fault resistance, and K represents the proportion of the distance from the fault point to the beginning of the line in the total length of the line (0 < K < 1).

[0024] Step 4: Substitute the equivalent impedance Z Ⅰ,i into the fault detection index calculation formula, and its calculation formula is:

[0025]

[0026] Among them, FDI i is the fault detection index, C i is a constant coefficient, which is generally slightly larger than the equivalent impedance Z I,i in the high-resistance fault near the B end;

[0027] If the number of positive samples of the fault detection index collected continuously is less than the set value N, it is determined that the line protection of this line does not operate, and the new fault detection index is calculated continuously; otherwise, it is determined that the line protection of this line should operate. The set value N needs to consider various fault types under different fault resistances at different positions, and is generally greater than or equal to 6.

[0028] The specific calculation and determination steps of step 4 are as follows:

[0029] Step 4-1: Let the initial sample number n = 0, and calculate the fault detection index FDI at the current moment i ;

[0030] Step 4-2: Compare the magnitude relationship between the fault detection index FDI i and 0;

[0031] Step 4-3: If FDI i < 0, then n = 0; if FDI i > 0, then n = n + 1;

[0032] Step 4-4: If n ≠ N, it is determined that the line protection does not operate, and the fault detection index FDI at the new moment is calculated i ;

[0033] Step 4-5: If n = N, it is determined that the line protection should operate.

[0034] Compared with the prior art, the beneficial effects of the present invention are:

[0035] This method proposes the FDI index for fault detection in grid-connected and island modes. The change of fault level or load caused by the switching of the microgrid operation mode does not affect the performance of the microgrid fault detection method based on the equivalent impedance drop, and the average response time is relatively fast. This scheme uses the sequence voltage and sequence current at the head of the line and the modulus value of the positive sequence voltage sent from the end of the line, which can release at least 50% of the communication volume, has a lower sampling frequency, can detect faults under different fault resistances, and is robust to network reconfiguration, load change, distributed power uncertainty, and fault initial phase angle change. Description of the Drawings

[0036] Figure 1 It is a microgrid fault detection method based on equivalent impedance drop;

[0037] Figure 2 It is the topology diagram of the microgrid system simulation model. Detailed Embodiments

[0038] The present invention will be further described in detail below with reference to the drawings and specific embodiments.

[0039] The present invention proposes a microgrid fault detection method based on equivalent impedance drop, and its overall process is as Figure 1 shown, including the following steps:

[0040] (1) Based on the line structure and electrical characteristics of the microgrid, calculate the positive, negative, and zero-sequence impedance parameters of the doubly-fed lines in the microgrid, and store them in the database.

[0041] (2) Measure the voltage and current values at the head and tail of the doubly-fed line, and calculate the positive-sequence, negative-sequence, and zero-sequence voltage and current at the head of the line and the magnitude of the positive-sequence voltage at the tail.

[0042] (3) Call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, and zero-sequence voltage and current at the head of the line and the magnitude of the positive-sequence voltage at the tail into the equation about the equivalent impedance Z Ⅰ,i under the corresponding fault type to obtain the equivalent impedance Z Ⅰ,i , where i can be replaced by three-phase (3P), line-to-line (LL), single-line-to-ground (SLG);

[0043] (4) Substitute the value of the equivalent impedance Z Ⅰ,i into the fault detection index calculation formula. If the number of positive samples of the continuously collected fault detection index is less than the set value N, it is determined that the protection of this line does not operate, and continue to jump to step 3) to calculate a new fault detection index; otherwise, it is determined that the protection of this line should operate.

[0044] Simulation verification

[0045] To verify the effectiveness of the present invention, the present invention is built based on PSCAD / EMTDC as Figure 2Topological diagram of the 50Hz microgrid simulation model shown. This network consists of two subnets of 0.4kV and 10kV connected to the 35kV main network through a transformer. The fault level at the point of common coupling is 500MVA, and the ratio of resistance R to impedance X is 0.1. DG1 is a battery energy storage system with a rated capacity of 200kVA, DG2 is a combined cooling, heating and power with a rated capacity of 100kVA, and DG3 is a photovoltaic cell with a rated capacity of 50kVA. The positive and negative sequence resistances and inductances of the 0.4kV subnet are 0.32Ω / km and 0.261mH / km respectively, and the zero sequence resistance and inductance are 1.1Ω / km and 0.955mH / km respectively. The capacities of loads 1 to 6 are 40kVA, 20kVA, 40kVA, 40kVA, 5kVA and 25kVA respectively, with a power factor of 0.9, all being inductive loads, and the capacity of the shunt capacitor is 20kVAr. The positive sequence resistance and inductance of the 10kV subnet are 0.38Ω / km and 1.432mH / km respectively, and the zero sequence resistance and inductance are 0.76Ω / km and 4.2mH / km respectively. There is a 600kVA diesel generator DG4 at the end of this subnet, and the capacities of loads 7 - 9 are 100kVA, 500kVA and 500kVA respectively, with a power factor of 0.85, all being inductive loads. To maintain voltage and frequency, the control of the battery energy storage system or the combined cooling, heating and power system will switch from P-Q to v-f, and the photovoltaic system can maintain P-Q control regardless of the grid-connected mode or island mode. The maximum output current of the distributed power source under fault conditions is limited to 1.5 times the rated current.

[0046] This invention studies the performance of line 4 under different fault resistances in various fault types, as well as the performance of lines 3 and 5 in external faults. In the simulation, the fault is set to occur at t = 10s, and the sampling frequency is 200Hz. Set C i To make the highest expected fault resistance at the end of the line positive, when a fault occurs at 90% of line 4 and the fault resistance is 50Ω, the constant coefficient C 3P under three-phase short-circuit fault, the constant coefficient C LL under phase-to-phase short-circuit fault, and the constant coefficient C SLG under single-phase ground fault are 550, 550 and 1640 respectively.

[0047] Table 1 Effect of the fault detection method in 3P faults

[0048]

[0049]

[0050] Limited by space, only the fault detection index FDI of the protection at the head of line 4 after 6 consecutive positive samples under three-phase short-circuit fault in Table 1 is shown. iThe value, as well as the response time of the fault resistance from 0.01 Ω to 50 Ω, where TP represents the transient positive value, verifies the effectiveness of the proposed scheme.

[0051] Table 2 Performance of the fault detection method under the new pre-fault conditions

[0052]

[0053] To study the influence of the load and distributed generation currents on the performance of the microgrid fault detection method, another pre-fault condition is considered, in which the loads in the microgrid, namely Load 1 to 6, are considered to be of double size, and DG3 is in the off state. In addition, to study the influence of the fault initial phase angle, the fault starting time is considered to be 10.005 s and the fault initial phase angle is 90°. The results in Table 2 show that the proposed scheme is robust to load variations, uncertainties of distributed generations, and changes in the fault initial phase angle.

Claims

1. A microgrid fault detection method based on equivalent impedance drop, characterized in that, the method comprises the following steps: Step 1: Based on the line structure and electrical characteristics of the microgrid, calculate the positive, negative, and zero-sequence impedance parameters of the doubly-fed line in the microgrid, and store them in the database; Step 2: Measure the voltage and current values at the head and tail ends of the doubly-fed line, and calculate the positive-sequence, negative-sequence, and zero-sequence voltage and current at the head end of the line and the positive-sequence voltage modulus at the tail end; Step 3: Call the corresponding line parameters in the database described in Step 1, and substitute the positive sequence, negative sequence, zero sequence voltages at the head end of the line and the magnitude of the positive sequence voltage at the tail end into the equations for the equivalent impedance under the corresponding fault types Z Ⅰ,i to obtain the equivalent impedance Z Ⅰ,i , where i is three-phase 3P, phase-to-phase LL or single-phase grounding SLG; Step 4: Substitute the equivalent impedance Z Ⅰ,i into the fault detection index calculation formula. If the number of positive samples of the continuously collected fault detection index is less than the set value N, it is determined that the line protection of this strip does not operate, and the new fault detection index is calculated continuously; Otherwise, it is determined that the protection of this line should act; Fault Detection Index FDI i The calculation formula is as follows: ； Among them, FDI i is the fault detection index; C i is a constant coefficient, and C i is greater than that in the high-resistance fault near the B end Z I,i .

2. The microgrid fault detection method based on equivalent impedance drop according to claim 1, characterized in that, the set value N described in Step 4 is greater than or equal to 6.

3. The microgrid fault detection method based on equivalent impedance drop according to claim 1, characterized in that, In step 3), the equivalent impedance is obtained. Z Ⅰ,i The specific steps are as follows: Step 3-1: If it is a three-phase short-circuit fault, call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, and zero-sequence voltage and current values at the head of the line and the positive-sequence voltage magnitude at the end of the line into the equation for the equivalent impedance Z Ⅰ,3P to obtain Z Ⅰ,3P ; Obtain the equivalent impedance Z Ⅰ,3P The equation expression is ； Wherein, Z Ⅰ,3P is the equivalent impedance under three-phase short-circuit fault, U A1 represents the positive-sequence voltage at the A end of the line, U B1 represents the positive-sequence voltage at the B end of the line, I A1 represents the positive-sequence current at the A end of the line, Z 1 is the positive-sequence impedance of the line; Step 3-2: If it is an interphase short-circuit fault, call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, and zero-sequence voltage and current values at the head of the line and the positive-sequence voltage magnitude at the end of the line into the equation for the equivalent impedance Z Ⅰ,LL to obtain Z Ⅰ,LL ; Obtain the equivalent impedance Z Ⅰ,LL The equation expression is as follows: ； Wherein, Z Ⅰ,LL is the equivalent impedance under the interphase short - circuit fault, U A1 and U A2 respectively represent the positive - sequence voltage and negative - sequence voltage at the A - end of the line, U B1 represents the positive - sequence voltage at the B - end of the line, U B2 represents the negative - sequence voltage at the B - end of the line, I A1、 I A2 respectively represent the positive - sequence current and negative - sequence current at the A - end of the line, Z 1 is the positive - sequence impedance of the line, R f is the fault resistance, K represents the proportion of the distance from the fault point to the line head to the total length of the line, and 0 < K < 1; Step 3-3: If it is a single-phase ground fault, call the corresponding line parameters in the database, and substitute the positive-sequence, negative-sequence, and zero-sequence voltage and current values at the head of the line and the positive-sequence voltage modulus value at the end of the line into the equation for the equivalent impedance Z Ⅰ,SLG to obtain Z Ⅰ,SLG ; Obtain the equivalent impedance Z Ⅰ,SLG The equation expression is as follows: ； Wherein, Z Ⅰ, SLG is the equivalent impedance under single-phase grounding fault, U A1 , U A2 , U A0 respectively represent the positive-sequence voltage, negative-sequence voltage, and zero-sequence voltage at the A end of the line, U B1 represents the positive-sequence voltage at the B end of the line, U B2、 U B0 respectively represent the negative-sequence voltage and zero-sequence voltage at the B end of the line, I A1 , I A2 , I A0 respectively represent the positive-sequence current, negative-sequence current, and zero-sequence current at the A end of the line, Z 1 , Z 0 are respectively the positive-sequence impedance and zero-sequence impedance of the line, R f is the fault resistance, K represents the ratio of the distance from the fault point to the line head to the total length of the line, and 0 < K < 1.

4. The microgrid fault detection method based on equivalent impedance drop according to claim 1, characterized in that, the specific calculation and determination steps of Step 4 are as follows: Step 4-1: Let the initial sample number n = 0, and calculate the Fault Detection Index (FDI) at the current moment i ; Step 4-2, compare the fault detection index FDI i with 0 to determine their magnitude relationship; Step 4-3: If FDI i < 0, then n = 0; if FDI i > 0, then n = n + 1; Step 4-4: If n≠N, it is determined that the line protection does not operate, and the fault detection index FDI at the new moment is calculated i ; Step 4-5: If n = N, it is determined that the protection of this line should act.

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