Distribution transformer and its low voltage lead zero sequence impedance online monitoring method and terminal

Abstract

The present invention discloses an online monitoring method and terminal for zero-sequence impedance of a distribution transformer and its low-voltage lead. A power supply end measurement point and a load end measurement point are selected on a running distribution transformer and its low-voltage lead. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point. According to the total zero-sequence voltage and the total zero-sequence current, the total zero-sequence impedance of the distribution transformer and its low-voltage lead is calculated. According to the relationship between the value of the total zero-sequence impedance and the first preset normal value, the operation condition of the zero line of the distribution transformer and its low-voltage lead is judged. The present invention effectively prevents and reduces the occurrence of zero-line failure, and improves the power supply reliability and voltage qualification rate of the distribution transformer and the low-voltage power grid.

Description

Technical Field

[0001] The present invention relates to the field of power distribution technology, and in particular to a distribution transformer and a method and a terminal for online monitoring of zero-sequence impedance of a low-voltage lead thereof. Background Art

[0002] Domestic low-voltage power distribution systems generally use a three-phase four-wire system for power supply. The zero line (also known as the neutral line) broken fault is one of the common faults. In the case of unbalanced three-phase load, this type of fault will cause a serious shift in the neutral point of the power supply system, which will lead to a serious high or low supply voltage problem. In severe cases, it may cause insulation breakdown of large-scale electrical equipment and damage to user electrical appliances, and may even cause fires and electric shock accidents. There are a lot of literature on the identification of zero line broken (abbreviated as "zero line broken") faults, and the judgment criteria used include zero-sequence current judgment method, phase voltage overvoltage judgment method, displacement voltage judgment method, etc. The above methods mainly realize fault judgment by analyzing the voltage and current signal data after the zero line broken fault occurs.

[0003] However, the occurrence of a large number of zero-break faults in distribution transformers and lines is a process of gradual development and deterioration to the occurrence of line break faults. As the distribution transformer and its low-voltage lead are affected by factors such as poor conductor contact and electrochemical corrosion, the intermittent and fluctuating increase of its zero-sequence loop impedance eventually leads to the occurrence of zero-break faults. In the gradual development of zero-break faults, the increase in neutral point displacement voltage leads to an increase in the imbalance of the three-phase voltage at the outlet point of the distribution transformer or the trunk line, thereby causing a significant decrease in the qualified rate of the power supply voltage. Therefore, how to prevent and reduce the occurrence of zero-break faults is very necessary. Summary of the invention

[0004] The technical problem to be solved by the present invention is to propose an online monitoring method and terminal for zero-sequence impedance of a distribution transformer and its low-voltage lead, so as to effectively prevent and reduce the occurrence of zero-sequence faults and improve the power supply reliability and voltage qualification rate of the distribution transformer and the low-voltage power grid.

[0005] In order to solve the above technical problems, the technical solution adopted by the present invention is:

[0006] The method for online monitoring of zero-sequence impedance of a distribution transformer and its low-voltage lead wire comprises the following steps:

[0007] S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead;

[0008] S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point;

[0009] S3. Calculating the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current;

[0010] S4. Determine the operating condition of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the total zero-sequence impedance and the first preset normal value.

[0011] In order to solve the above technical problems, another technical solution adopted by the present invention is:

[0012] A distribution transformer and its low-voltage lead zero-sequence impedance online monitoring terminal comprises a memory, a processor and a computer program stored in the memory and executable on the processor. When the processor executes the computer program, the following steps are implemented:

[0013] S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead;

[0014] S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point;

[0015] S3. Calculate the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current;

[0016] S4. Determine the operating condition of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the total zero-sequence impedance and the first preset normal value.

[0017] The beneficial effects of the present invention are as follows: an online monitoring method and terminal for zero-sequence impedance of a distribution transformer and its low-voltage lead are proposed. During the operation of the distribution transformer and its low-voltage lead, when a three-phase load imbalance occurs, the total zero-sequence current and the total zero-sequence voltage are measured at the measuring point to accurately calculate the total zero-sequence impedance of the distribution transformer and its low-voltage lead. According to the change of the total zero-sequence impedance, the operating condition of the zero line of the distribution transformer and its low-voltage lead is analyzed, and it is judged whether the zero line has poor contact or has failed, etc., so as to timely discover and deal with the abnormality, effectively prevent and reduce the occurrence of zero-break faults, and improve the power supply reliability and voltage qualification rate of the distribution transformer and the low-voltage power grid. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 It is a schematic diagram of the steps of the method for online monitoring of zero-sequence impedance of a distribution transformer and its low-voltage lead wire of the present invention;

[0019] Figure 2A single-phase zero-sequence impedance equivalent circuit of a Dyn11 distribution transformer of the present invention for the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring method thereof;

[0020] Figure 3 The total zero-sequence impedance equivalent circuit of the Dyn11 distribution transformer of the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring method of the present invention;

[0021] Figure 4 A single-phase zero-sequence impedance equivalent circuit of a Yyn0 distribution transformer of the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring method of the present invention;

[0022] Figure 5 The total zero-sequence impedance equivalent circuit of the Yyn0 distribution transformer of the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring method of the present invention;

[0023] Figure 6 A simplified equivalent circuit of a low-voltage line of a distribution transformer for a distribution transformer and a method for online monitoring the zero-sequence impedance of a low-voltage lead wire thereof of the present invention;

[0024] Figure 7 The total zero-sequence impedance equivalent circuit of the distribution transformer and its low-voltage lead wire of the distribution transformer and its low-voltage lead wire zero-sequence impedance online monitoring method of the present invention;

[0025] Figure 8 A fault equivalent circuit of a distribution transformer and its low-voltage lead wires in the online monitoring method for zero-sequence impedance of the distribution transformer and its low-voltage lead wires of the present invention;

[0026] Fig. 9 It is the first zero-sequence impedance equivalent circuit of the power supply end measurement point of the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring method of the present invention;

[0027] Fig.10 It is a second zero-sequence impedance equivalent circuit of the load-end measurement point of the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring method of the present invention;

[0028] Fig.11 A Simulink simulation model for implementing the method for online monitoring the zero-sequence impedance of a distribution transformer and its low-voltage lead wires of the present invention on the distribution transformer and its low-voltage lead wires;

[0029] Fig.12 The invention relates to a distribution transformer and a method for online monitoring of the zero-sequence impedance of a low-voltage lead wire thereof. a1 The waveform diagram of zero-sequence voltage and current of Dyn11 distribution transformer during the change;

[0030] Fig.13 The invention relates to a distribution transformer and a method for online monitoring of the zero-sequence impedance of a low-voltage lead wire thereof.a1 The waveform diagram of zero-sequence voltage and current of Yyn0 distribution transformer during the change;

[0031] Fig.14 The invention relates to a distribution transformer and a method for online monitoring of the zero-sequence impedance of a low-voltage lead wire thereof. n1 The zero-sequence voltage and current waveforms of Dyn11 distribution transformer during changes;

[0032] Fig.15 The system block diagram of the distribution transformer and the low-voltage lead zero-sequence impedance online monitoring terminal of the present invention.

[0033] Description of labels:

[0034] 1. On-line monitoring terminal for zero-sequence impedance of distribution transformer and its low-voltage lead; 2. Memory; 3. Processor. DETAILED DESCRIPTION

[0035] In order to explain the technical content, achieved objectives and effects of the present invention in detail, the following is an explanation in conjunction with the implementation modes and the accompanying drawings.

[0036] Please refer to Figures 1 to 14 , a method for online monitoring of zero-sequence impedance of a distribution transformer and its low-voltage lead wire comprises the following steps:

[0037] S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead;

[0038] S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point;

[0039] S3. Calculating the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current;

[0040] S4. Determine the operating condition of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the total zero-sequence impedance and the first preset normal value.

[0041] From the above description, it can be seen that the beneficial effects of the present invention are: during the operation of the distribution transformer and its low-voltage lead, when a three-phase load imbalance occurs, the total zero-sequence current and the total zero-sequence voltage are measured at the measuring point to accurately calculate the total zero-sequence impedance of the distribution transformer and its low-voltage lead, and the operating status of the zero line of the distribution transformer and its low-voltage lead is analyzed according to the change of the total zero-sequence impedance, and it is determined whether the zero line has poor contact or has failed, etc., so as to promptly discover and deal with the abnormality, effectively prevent and reduce the occurrence of zero-break faults, and improve the power supply reliability and voltage qualification rate of the distribution transformer and the low-voltage power grid.

[0042] Furthermore, the step S2 comprises:

[0043] S21, obtaining a first zero-sequence impedance equivalent circuit of the power supply end measurement point and a second zero-sequence impedance equivalent circuit of the load end measurement point according to a fault simulation equivalent circuit of the distribution transformer and its low-voltage lead when a zero-sequence fault occurs;

[0044] S22. Calculate the distribution transformer zero-sequence current and distribution transformer zero-sequence voltage according to the first zero-sequence impedance equivalent circuit;

[0045] S23. Calculate the total zero-sequence current and the total zero-sequence voltage according to the second zero-sequence impedance equivalent circuit.

[0046] From the above description, it can be seen that by applying the fault simulation equivalent circuit of the distribution transformer and its low-voltage lead when a zero-sequence fault occurs, the first zero-sequence impedance equivalent circuit of the power supply end measurement point and the second zero-sequence impedance equivalent circuit of the load end measurement point are derived, so that the zero-sequence impedance inside the distribution and the total zero-sequence impedance of the distribution transformer and its low-voltage lead can be analyzed respectively.

[0047] Furthermore, the step S22 specifically includes:

[0048] According to the first zero-sequence impedance equivalent circuit, three-phase current and voltage vector addition method, the distribution transformer zero-sequence current and the distribution transformer zero-sequence voltage are calculated, and the expressions are as follows:

[0049]

[0050] in, represents the zero-sequence voltage of the distribution transformer, represents the zero-sequence current of the distribution transformer, Expressed as the three-phase voltage of the distribution transformer; Expressed as the three-phase current of the distribution transformer;

[0051] The step S3 further comprises:

[0052] According to the zero-sequence current and the zero-sequence voltage of the distribution transformer, the total zero-sequence impedance of the distribution transformer is calculated, and its expression is as follows:

[0053]

[0054] Among them, Z 0t Indicates the total zero-sequence impedance of the distribution transformer;

[0055] The step S4 further comprises:

[0056] The operation status inside the distribution transformer is judged according to the magnitude relationship between the value of the total zero-sequence impedance of the distribution transformer and the second preset normal value.

[0057] From the above description, it can be seen that at the measurement point on the power supply end, by calculating the zero-sequence current and zero-sequence voltage of the distribution transformer, the total zero-sequence impedance of the distribution transformer can be further obtained. Therefore, by comparing the values, the internal operation of the distribution transformer can be analyzed to determine whether there is a poor contact or a broken line fault inside the distribution transformer.

[0058] Further, step S23 specifically includes:

[0059] According to the second zero-sequence impedance equivalent circuit, three-phase current and voltage vector addition method, the total zero-sequence current and the total zero-sequence voltage are calculated, which are expressed as follows:

[0060]

[0061] in, Represents the total zero-sequence voltage, represents the total zero-sequence current, Expressed as the three-phase voltage at the load end, Expressed as the three-phase current at the load end;

[0062] The total zero-sequence impedance of the distribution transformer and its low-voltage lead in step 3 is expressed as:

[0063]

[0064] Among them, r1 represents the zero line resistance.

[0065] From the above description, it can be seen that the above is a calculation expression for the total zero-sequence impedance, which is specifically based on the calculation of the total zero-sequence current and the total zero-sequence voltage, combined with the neutral line resistance.

[0066] Furthermore, the step S2 further includes:

[0067] S24, measuring at the power supply end measurement point and the load end measurement point at the same time, and obtaining the lead zero-sequence impedance of the distribution transformer and its low-voltage lead according to the obtained total zero-sequence impedance, which is expressed as:

[0068]

[0069] Simplifying the expression of the lead zero-sequence impedance, we get:

[0070]

[0071] or

[0072]

[0073] From the above description, it can be seen that in addition to monitoring the total zero-sequence impedance of the distribution transformer and its low-voltage lead, it is also possible to monitor the lead zero-sequence impedance of the low-voltage lead in real time based on this, so as to analyze the operation of the lead part.

[0074] Please refer to Fig.15 The terminal 1 for online monitoring of zero-sequence impedance of distribution transformer and its low-voltage lead wire comprises a memory 2, a processor 3 and a computer program stored in the memory 2 and executable on the processor 3. When the processor 3 executes the computer program, the following steps are implemented:

[0075] S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead;

[0076] S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point;

[0077] S3. Calculating the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current;

[0078] S4. Determine the operating condition of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the total zero-sequence impedance and the first preset normal value.

[0079] From the above description, it can be seen that the beneficial effects of the present invention are: during the operation of the distribution transformer and its low-voltage lead, when a three-phase load imbalance occurs, the total zero-sequence current and the total zero-sequence voltage are measured at the measuring point to accurately calculate the total zero-sequence impedance of the distribution transformer and its low-voltage lead, and the operating status of the zero line of the distribution transformer and its low-voltage lead is analyzed according to the change of the total zero-sequence impedance, and it is determined whether the zero line has poor contact or has failed, etc., so as to promptly discover and deal with the abnormality, effectively prevent and reduce the occurrence of zero-break faults, and improve the power supply reliability and voltage qualification rate of the distribution transformer and the low-voltage power grid.

[0080] Please refer to Figures 1 to 14 , Embodiment 1 of the present invention is:

[0081] Before introducing the technical solution of this embodiment, the relationship between the total zero-sequence impedance of the distribution transformer and its low-voltage lead and the zero-line fault is analyzed as follows:

[0082] First, according to the different wiring methods, the zero-sequence circuits of the two distribution transformers (Dyn11 and Yyn0) are analyzed as follows:

[0083] Reference Figure 2 and Figure 3As shown, take any phase of Dyn11 distribution transformer for analysis, the primary side (high voltage side) is connected in triangle, and the secondary side (low voltage side) is connected in star. From the secondary side, the primary side port of the distribution transformer is equivalent to a zero-sequence loop short circuit, and the zero-sequence current flows through the primary and secondary windings and the neutral line. The impedance Z t1 , Z t2 With excitation impedance Z m To form a "T" shaped circuit, such as Figure 2 shown.

[0084] According to the characteristics of the distribution transformer, due to the excitation impedance Z m is much larger than the winding impedance, so Z m Ignore, then the single-phase zero-sequence impedance Z of the distribution transformer t It can be approximated as the winding resistance Z of the distribution transformer t1 , secondary winding impedance Z t2 In addition, under normal circumstances, the impedance parameter characteristics of the three-phase windings and core of the distribution transformer are basically the same, that is, Z ta =Z tb =Z tc =Z t , in zero-sequence circuit analysis, it can be equivalent to three-phase windings in parallel, such as Figure 3 As shown. Then the total zero-sequence impedance of the Dyn11 distribution transformer can be expressed as:

[0085]

[0086] Reference Figure 4 and Figure 5 , the primary and secondary sides of the Yyn0 distribution transformer are both star-connected. Since there is no neutral line on the primary side, the zero-sequence current cannot pass through the primary side; from the secondary side, the primary side port of the distribution transformer is equivalent to an open zero-sequence circuit, and the zero-sequence current flows through the secondary winding, excitation impedance and neutral line. Figure 4 shown.

[0087] According to the distribution transformer characteristics and circuit analysis, since there is no zero-sequence current passing through the primary side, the primary winding impedance Z t1 Ignore, the single-phase impedance Z of the distribution transformer t is the transformer excitation impedance Z m and the secondary winding impedance Z t2 In series, it can be approximated as the distribution transformer excitation impedance Z m , secondary winding impedance Z t2 Similarly, the impedance parameter characteristics of the three-phase winding and core of the Yyn0 distribution transformer under normal conditions are basically the same, that is, Z ta =Z tb =Z tc =Z t , which is equivalent to the three-phase excitation impedance and winding impedance in parallel, such as Figure 5 As shown. Then the total zero-sequence impedance of the three-phase Dyn11 distribution transformer can be expressed as:

[0088]

[0089] Secondly, based on the above, the zero-sequence circuit analysis of the low-voltage lead of the distribution transformer is as follows:

[0090] Reference Figure 6 As shown in the figure, U an , U bn , U cn U is the three-phase equivalent voltage source of A, B, and C on the low-voltage side of the distribution transformer. a , U b , U c is the voltage of the three-phase low-voltage terminal of the distribution transformer to the neutral terminal n0, Ua1, Ub1, Uc1 are the voltages of the three-phase low-voltage lead wire of the distribution transformer to the neutral point n1, Z ta , Z tb , Z tc is the three-phase equivalent internal impedance of the distribution transformer, r la 、r lb 、r l c is the equivalent impedance of the low voltage phase line lead, r l is the zero line impedance, Z a , Z b , Z c is the calculated three-phase equivalent load impedance. Since low-voltage lines usually use four-core cables or wires with equal diameters, the resistance of the neutral line is consistent with that of the phase line, that is, r la =r lb =r lc =r l , can be drawn Figure 7 Equivalent circuit of total zero-sequence impedance of distribution transformer and low-voltage lead. Figure 7 In, Z 0t is the equivalent three-phase total zero-sequence impedance of the distribution transformer. Therefore, the expression of line zero-sequence impedance can be obtained as follows:

[0091]

[0092] The expression of the total zero-sequence impedance of the distribution transformer and its low-voltage leads is as follows:

[0093] Z0=Z 0t +r 0l (4)

[0094] Combined with formula (1), for the Dyn11 distribution transformer, the total zero-sequence impedance of the distribution transformer and its low-voltage lead is:

[0095]

[0096] Combined with formula (2), for the Yyn0 distribution transformer, the total zero-sequence impedance of the distribution transformer and its low-voltage lead is:

[0097]

[0098] Then, in Figure 6 Based on this, the fault simulation equivalent circuit of the distribution transformer and its low-voltage lead in the neutral line fault state is as follows: Figure 8 As shown in the figure, it can be seen that when the lead neutral line has poor contact or a broken line fault, it is equivalent to a resistor r in series in the line zero sequence circuit. d ; Therefore, the line zero-sequence impedance at this time becomes:

[0099]

[0100] Combined with formula (4), the total zero-sequence impedance of the distribution transformer and its low-voltage lead when the zero line fails is:

[0101]

[0102] By comparing the r of equations (3) and (7) 0l 'with r 0l It can be seen that the total zero-sequence impedance value change rate of the lead loop is 0.75r d / r l The impedance of the conductor is usually r l < <r d Therefore, as long as the neutral line fails, the total zero-sequence impedance Z0 will change significantly.

[0103] Finally, based on the above analysis, the following conclusions can be drawn:

[0104] For Dyn11 distribution transformer, its zero-sequence impedance can be approximated as a series connection of the primary and secondary winding impedances, which is basically not affected by the excitation impedance Z. m Therefore, in the case of symmetrical three-phase winding impedance, the total zero-sequence impedance Z 0d Considered as a constant.

[0105] For Yyn0 distribution transformer, its zero sequence impedance can be approximated as the series connection of excitation impedance and secondary winding impedance. m It is not a constant, but a nonlinear parameter. When the transformer is operating at rated state, if the primary voltage drops, its iron loss resistance and excitation reactance will increase. Conversely, if the primary voltage rises, its iron loss resistance and excitation reactance will decrease. This means that the zero-sequence impedance of the Yyn0 distribution transformer is not a constant, but changes with the change of the primary voltage. However, under normal operating conditions, the primary voltage change range is extremely limited. Therefore, the total zero-sequence impedance Z can be 0YConsidered as a constant domain with a limited range of variation. Iron loss resistance R m , excitation reactance X m The relationship curve between Z and the transformer primary side voltage U1 can be used to estimate Z 0Y The range of variation is [0.7,1.5]*Z 0Yn , where Z 0Yn Y yn0 The total zero-sequence impedance of the distribution transformer at rated voltage.

[0106] Moreover, for three-phase four-wire low-voltage lines, the impedance of the line conductor is mainly related to factors such as wire type, wire diameter, and length. At the same time, affected by the inherent temperature properties of metal conductors, their resistivity generally increases with the increase of operating temperature. Therefore, under allowable operating conditions, the zero-sequence impedance of the low-voltage line will also change slightly with changes in temperature.

[0107] Based on the above analysis, this embodiment proposes an online monitoring method for zero-sequence impedance of a distribution transformer and its low-voltage lead, including the following steps:

[0108] S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead;

[0109] In this embodiment, the power supply end measurement point can be specifically selected as the distribution transformer low-voltage terminal as the measurement point, and the load end measurement point is selected as the lower end of the distribution transformer low-voltage lead, and the lead impedance cannot be ignored.

[0110] S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point;

[0111] S3. Calculate the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current;

[0112] In this embodiment, step S2 specifically includes:

[0113] S21, obtaining a first zero-sequence impedance equivalent circuit at a power supply end measurement point and a second zero-sequence impedance equivalent circuit at a load end measurement point according to a fault simulation equivalent circuit of a distribution transformer and a low-voltage lead thereof when a zero-sequence fault occurs;

[0114] exist Figure 8 Based on this, we can draw Fig. 9 The first zero-sequence impedance equivalent circuit of the power supply terminal measurement point shown in FIG. Fig.10 The second zero-sequence impedance equivalent circuit of the load end measurement point is shown.

[0115] For distribution transformers, in zero-sequence circuit analysis, according to the circuit substitution theorem, the distribution transformer can be regarded as a passive element Z 0t , replace the zero-sequence voltage formed by the unbalanced load with the voltage source U0 / 3. Since the zero-sequence voltage and zero-sequence current are both known, it is easy to solve the zero-sequence impedance of the circuit according to Ohm's law. The specific process is as follows:

[0116] S22. Calculate the distribution transformer zero-sequence current and distribution transformer zero-sequence voltage according to the first zero-sequence impedance equivalent circuit;

[0117] Combined with the above content, for the power supply end measurement point, the distribution transformer zero-sequence voltage at the power supply end measurement point is Zero sequence current of distribution transformer It can be obtained by adding the three-phase voltage and three-phase current phase quantities:

[0118]

[0119] in, It is expressed as the three-phase voltage of the measuring point at the power supply end; Expressed as the three-phase current at the measurement point at the power supply end.

[0120] In addition, 3U0 (denoted as U 30 ) and 3I0 (denoted as I 30 ), usually, the residual current (or ground leakage current) of the low-voltage power grid is much smaller than the neutral current, so the neutral current (denoted as I n ) is regarded as the approximate value of the zero-sequence current. The zero-sequence voltage and zero-sequence current obtained at the measuring point are:

[0121]

[0122] Furthermore, in step S3, the zero-sequence impedance of the total zero-sequence impedance of the distribution transformer at the power supply end measurement point is expressed as:

[0123]

[0124] Based on this, in the subsequent step S4, the operation status inside the distribution transformer can be determined according to the magnitude relationship between the value of the total zero-sequence impedance of the distribution transformer and the second preset normal value.

[0125] S23. Calculate the total zero-sequence current and total zero-sequence voltage according to the second zero-sequence impedance equivalent circuit.

[0126] For the load end measurement point. The total zero sequence voltage at the load end measurement point and total zero sequence current They are:

[0127]

[0128] in, It is expressed as the three-phase voltage at the load end measurement point, Expressed as the three-phase current at the load end measurement point.

[0129] In step S3, combining Fig. 9 and Fig.10 As well as equations (3) and (4), the total zero-sequence impedance equation of the distribution transformer and its low-voltage lead is obtained, that is, the total zero-sequence impedance:

[0130]

[0131] In addition, step S2 further comprises:

[0132] S24. Measure at the power supply end measurement point and the load end measurement point at the same time. According to the total zero-sequence impedance obtained, the lead zero-sequence impedance of the distribution transformer and its low-voltage lead is obtained. The expression is:

[0133]

[0134] because Formula (14) can be further simplified as:

[0135]

[0136] or

[0137]

[0138] S4. Determine the operating condition of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the first total zero-sequence impedance and the first preset normal value.

[0139] In this embodiment, according to the total zero-sequence impedance, the load-end measurement point is calculated when the distribution transformer and its low-voltage lead are not faulty (r d =0) is the zero-sequence impedance equation:

[0140]

[0141] The zero-sequence impedance equation when a fault occurs in the distribution transformer and its low-voltage lead (rd≠0):

[0142]

[0143] Combining equations (17) and (18), the neutral line fault resistance of the corresponding distribution transformer and its low-voltage lead is calculated as follows:

[0144]

[0145] In addition to the above calculation method, two-terminal calculation can also be used. When the zero line of the lead fails, that is, rd≠0, the expression of the zero-sequence impedance equation between the power supply end measurement point and the load end measurement point is as follows:

[0146]

[0147] Therefore, the resistance expression of the neutral fault resistance of the distribution transformer and its low-voltage lead is as follows:

[0148]

[0149] Combining equation (19) or equation (21), it can be seen that under the premise that the three-phase impedance of the distribution transformer and its lead is symmetrical, if r d >0, it can be determined that the zero-sequence impedance change must occur in the zero-sequence circuit. d >>0, it can be determined that the neutral line is most likely to have poor contact or disconnection. Of course, it is also possible that the impedance of the three-phase line loop changes at the same time.

[0150] In summary, the implementation process of the method of this embodiment and the benefits achieved are summarized as follows:

[0151] 1. When the length and model parameters of the distribution transformer and its low-voltage side leads are clear, if the three-phase impedance of the distribution transformer and its leads are symmetrical, the total zero-sequence impedance theoretical value of the entire circuit can be obtained by calculation. In order to realize the online monitoring of the zero-sequence impedance of the distribution transformer and its leads, the zero-sequence voltage and current data can be measured directly or indirectly, and then the actual total zero-sequence impedance of the distribution transformer and its lead circuit can be calculated.

[0152] 2. By comparing the calculated actual operating zero-sequence impedance value with the theoretical value or historical data, the changes in the zero-sequence impedance inside the distribution transformer and the zero-sequence impedance of the lead can be monitored.

[0153] 3. If the zero-sequence impedance change value measured and calculated above is much greater than zero or significantly greater than the theoretical value, it can be determined that the neutral line of the distribution transformer or its lead is likely to have a fault such as poor contact or broken line.

[0154] Finally, in order to verify the above conclusions, the experimental process of the online monitoring method for zero-sequence impedance of the distribution transformer and its low-voltage lead wire of this embodiment is introduced as follows:

[0155] (I) Simulation model

[0156] like Fig.11 As shown, Fig.11It is a simulation model of Simulink, including 10kV and 0.4kV lines and components. The 10kV part includes power supply, distribution network lines, and two distribution transformers with a capacity of 500kVA, whose connection groups are Dyn11 and Yyn0 respectively. The distribution transformer parameters are shown in Table 1:

[0157]

[0158] Table 1 Distribution transformer parameters

[0159] According to formula (1), the total zero-sequence impedance Z of Dyn11 distribution transformer can be calculated: 0t ND=0.0044∠75.96°Ω, according to formula (2), the total zero-sequence impedance Z of Yyn0 distribution transformer at rated voltage can be calculated 0tNY =9.6970∠77.82°Ω.

[0160] 0.4kV part of Dyn11 distribution transformer: a1 、r b1 、r c1 、r n1 The impedance is 0.01Ω for the distribution transformer low voltage lead; a 、z b 、z c is the load impedance, where z a is a variable resistor (including 0.64Ω, 6.4Ω, and infinity), z b =0.64Ω,z c =0.704Ω. M0 and M1 are the three-phase phase voltage and phase current measuring devices at the beginning and end of the low-voltage lead. According to formula (3), the total zero-sequence impedance of the low-voltage lead of Dyn11 distribution transformer can be calculated as Z 01D =0.0133Ω.

[0161] The 0.4kV part of Yyn0 distribution transformer includes: a3 、r b3 、r c3 、r n3 The impedance is 0.02Ω for the distribution transformer low voltage lead; a1 、z b1 、z c1 is the load impedance, and its impedance value is the same as the low-voltage lead of the Dyn11 distribution transformer. M2 and M3 are the three-phase phase voltage and phase current measuring devices at the beginning and end of the low-voltage lead. According to formula (3), the total zero-sequence impedance of the low-voltage lead of the Yyn0 distribution transformer can be calculated as Z 23Y =0.0267Ω

[0162] (II) Distribution transformer and lead wire total zero-sequence impedance measurement experiment

[0163] 1. Measurement of total zero-sequence impedance of distribution transformer when load impedance changes

[0164] Simulation period setting: During the 0-0.04s period, the load impedance of phase A of the distribution transformer is 0.64Ω; during the 0.04-0.08s period, the load impedance of phase A of the distribution transformer is 6.4Ω; during the 0.08-0.12s period, the load of phase A of the distribution transformer is disconnected and the load impedance is infinite. The load impedances of phases B and C are 0.64Ω and 0.704Ω respectively, and remain unchanged.

[0165] Through simulation experiments, the phase voltage and phase current data measured on the low-voltage side of the two types of distribution transformers Dyn11 and Yyn0 are added and calculated to obtain Fig.12 and 13 The zero-sequence voltage and current waveforms on the low-voltage side when the distribution transformer load changes, as well as the corresponding zero-sequence voltage and current data in Tables 2 and 3

[0166] Data\Time 0.04s 0.08s 0.12s u0 / V 0.414∠-168° 4.059∠79° 4.5476∠78° i0 / A 31.4662∠-64° 308.0046∠-177° 345.077∠-178° u1 / V 1.4158∠133° 13.8588∠19° 15.5269∠19° i1 / A 31.4662∠-64° 308.0046∠-177° 345.077∠-178° z0 / Ω 0.0044∠76° 0.0044∠76° 0.0044∠76° z1 / Ω 0.015∠17° 0.015∠17° 0.015∠17° z01 / Ω 0.0133∠0° 0.0133∠0° 0.0133∠0°

[0167] Table 2z a1 Simulation data of total zero-sequence impedance of Dyn11 distribution transformer and its leads during change

[0168] In Table 2, u0, i0, u1, i1 are the zero-sequence voltage and zero-sequence current data calculated from the data of the M0 and M1 measurement points under different load impedance conditions, z0 is the total zero-sequence impedance of the Dyn11 distribution transformer calculated according to formula (11) based on the zero-sequence voltage and zero-sequence current, z1 is the total zero-sequence impedance of the Dyn11 distribution transformer and its leads calculated according to formula (13), z 01 is the zero-sequence impedance of the low-voltage lead calculated according to formula (15). Comparing the experimental calculation data with the model parameter data, it can be seen that z0 = Z 0tND 、z 01 =Z 01D 、z1=Z 0tND +Z 01D .

[0169]

[0170]

[0171] Table 3z a1 Simulation data of total zero-sequence impedance of Yyn0 distribution transformer and its leads during change

[0172] In Table 3, u2, i2, u3, i3 are the zero-sequence voltage and zero-sequence current data calculated according to the data of the M2 and M3 measurement points under different load impedance conditions, z2 is the total zero-sequence impedance of the Yyn0 distribution transformer calculated according to formula (11) based on the zero-sequence voltage and zero-sequence current, z3 is the total zero-sequence impedance of the Yyn0 distribution transformer and its leads calculated according to formula (13), z23 is the lead zero-sequence impedance calculated according to formula (15). Comparing the experimental calculation data with the model parameter data, we can see that z2=Z 0tNY 、z 23 =Z 23Y 、z3=Z 0tNY +Z 23Y .

[0173] The following conclusions can be drawn from the experiments: for the Dyn11 and Yyn0 distribution transformers in operation, when the three-phase load is unbalanced, the total zero-sequence impedance of the distribution transformer, the total zero-sequence impedance of the distribution transformer and its leads, and the total zero-sequence impedance of the leads can be calculated according to formulas (11), (13), and (15) respectively. When the load impedance of the distribution transformer changes, it will not affect the measurement and calculation results of the total zero-sequence impedance of the distribution transformer and its leads.

[0174] 2. Load impedance remains unchanged, neutral line impedance changes

[0175] The Dyn11 distribution transformer is taken as the research object. The impedance of the three-phase lead wires A, B, and C of the distribution transformer is 0.01Ω, and the three-phase load impedance is 0.64Ω, 0.64Ω, and 0.704Ω respectively, and remains unchanged.

[0176] In the three periods of 0-0.04s, 0.04-0.08s, and 0.08-0.12s, the impedance rn1 of the lead neutral line is switched to 0.01Ω, 0.1Ω, and 1Ω respectively. Through the simulation experiment, the obtained phase voltage and phase current data are added and calculated. Fig.14 The zero-sequence voltage and zero-sequence current waveforms shown in the figure, as well as the zero-sequence voltage and zero-sequence current data in Table 4:

[0177] Data\Time 0.04s 0.08s 0.12s u0 / V 0.415∠-168° 0.2991∠-167° 0.0792∠-166° i0 / A 31.4912∠-64° 22.6956∠-63° 6.0103∠-62° u1 / V 1.4171∠133° 7.1128∠119° 18.11∠118° i1 / A 31.4912∠-64° 22.6956∠-63° 6.0103∠-62° z0 / Ω 0.0044∠76° 0.0044∠76° 0.0044∠76° z1 / Ω 0.015∠17° 0.1045∠2° 1.0044∠0° z01 / Ω 0.0133∠0° 0.1033∠0° 1.0033∠0°

[0178] Table 4r n1 Simulation data of total zero-sequence impedance of Dyn11 distribution transformer and its leads during change

[0179] It can be seen that in the 0-0.04s period, the data are consistent with the previous experiment, and in the 0.04-0.08s period, r n1 From 0.01Ω to 0.1Ω, the calculated z 01 The value changes to 0.1033Ω, and the lead zero-sequence impedance increases by 0.09Ω compared with the previous period; during the 0.08-0.12s period, z 01 The value becomes 0.1033Ω, and the lead zero-sequence impedance increases by 0.9Ω compared with the previous period. The above calculation shows that the change in zero-sequence impedance satisfies the formula r d =Z 10 '-Z 10 Moreover, in each period, by substituting data into the calculation, it can be verified that z 01 =z1-z0.

[0180] Experiments show that when the neutral line impedance changes, the corresponding zero-sequence impedance can be calculated by measuring the zero-sequence voltage and zero-sequence current of the distribution transformer and the lead line multiple times. The difference between the zero-sequence impedance calculated from two measurements is the change in zero-sequence impedance.

[0181] The above simulation experiments show that if the three-phase impedance of the distribution transformer and the line is consistent, when the three-phase load is unbalanced, by measuring the zero-sequence voltage and zero-sequence current of the distribution transformer and its leads, the zero-sequence impedance of the corresponding object can be directly and accurately calculated using the measured data; by comparing the changes in zero-sequence impedance, poor zero-line contact and broken line faults of the distribution transformer and its leads can be accurately identified.

[0182] Please refer to Fig.15 , Embodiment 2 of the present invention is:

[0183] The terminal 1 for online monitoring of zero-sequence impedance of a distribution transformer and its low-voltage lead wire comprises a memory 2, a processor 3 and a computer program stored in the memory 2 and executable on the processor 3. When the processor 3 executes the computer program, the method for online monitoring of zero-sequence impedance of a distribution transformer and its low-voltage lead wire of the first embodiment is implemented.

[0184] Finally, the present invention aims at the current situation that the zero-sequence impedance of the distribution transformer and its lead in operation cannot be measured in real time online by simple technical means. Based on the analysis of the equivalent zero-sequence circuits of two types of distribution transformers and their leads, Dyn11 and Yyn0, a method for measuring and calculating the zero-sequence impedance of the distribution transformer and its lead is proposed. The simulation experiment proves that the method is feasible. The present invention can be applied to the real-time online monitoring of the distribution transformer and its lead, and provides technical means for grasping the changes and analysis of the zero-sequence loop impedance of the distribution transformer and its line, and provides a basis for timely discovering the poor contact of the zero line of the distribution transformer area line, preventing and reducing the zero-break fault. The application of the present invention can improve the power supply reliability and voltage qualification rate of the low-voltage power supply system. The method is made into a special terminal device, or integrated into the distribution transformer terminal, electric meter, low-voltage circuit breaker and other instrument and electrical equipment, which can realize the fault identification of the poor contact of the distribution transformer and the zero line, and can also be used as an auxiliary for the low-voltage distribution system to improve the voltage qualification rate and regulation, and can also be used for zero-break fault protection.

[0185] The above descriptions are merely embodiments of the present invention and are not intended to limit the patent scope of the present invention. Any equivalent transformations made using the contents of the present invention's specification and drawings, or directly or indirectly applied in related technical fields, are also included in the patent protection scope of the present invention.

Claims

1. A method for online monitoring of zero-sequence impedance of a distribution transformer and its low-voltage lead, characterized in that: The steps include: S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead; S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point; S3. Calculating the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current; S4. judging the operation status of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the total zero-sequence impedance and the first preset normal value; The step S2 comprises: S21, obtaining a first zero-sequence impedance equivalent circuit of the power supply end measurement point and a second zero-sequence impedance equivalent circuit of the load end measurement point according to a fault simulation equivalent circuit of the distribution transformer and its low-voltage lead when a zero-sequence fault occurs; S22. Calculate the distribution transformer zero-sequence current and distribution transformer zero-sequence voltage according to the first zero-sequence impedance equivalent circuit; S23, calculating the total zero-sequence current and the total zero-sequence voltage according to the second zero-sequence impedance equivalent circuit; The step S22 specifically includes: According to the first zero-sequence impedance equivalent circuit, three-phase current and voltage vector addition method, the distribution transformer zero-sequence current and the distribution transformer zero-sequence voltage are calculated, and the expressions are as follows: ； in, represents the zero-sequence voltage of the distribution transformer, represents the zero-sequence current of the distribution transformer, , , Expressed as the three-phase voltage of the distribution transformer; , , Expressed as the three-phase current of the distribution transformer; The step S3 further comprises: According to the zero-sequence current and the zero-sequence voltage of the distribution transformer, the total zero-sequence impedance of the distribution transformer is calculated, and its expression is as follows: ； in, Indicates the total zero-sequence impedance of the distribution transformer; The step S4 further comprises: According to the magnitude relationship between the total zero-sequence impedance of the distribution transformer and a second preset normal value, judging the internal operation status of the distribution transformer; Step S23 specifically includes: According to the second zero-sequence impedance equivalent circuit, three-phase current and voltage vector addition method, the total zero-sequence current and the total zero-sequence voltage are calculated, which are expressed as follows: ； in, Represents the total zero-sequence voltage, represents the total zero-sequence current, , , Expressed as the three-phase voltage at the load end, , , Expressed as the three-phase current at the load end; The total zero-sequence impedance of the distribution transformer and its low-voltage lead in step 3 is expressed as: ； Among them, r1 represents the zero line resistance; The step S2 further comprises: S24, measuring at the power supply end measurement point and the load end measurement point at the same time, and obtaining the lead zero-sequence impedance of the distribution transformer and its low-voltage lead according to the obtained total zero-sequence impedance, which is expressed as: ； Simplifying the expression of the lead zero-sequence impedance, we get: ； or 。 2. Distribution transformer and its low voltage lead zero sequence impedance online monitoring terminal, characterized in that: The invention comprises a memory, a processor, and a computer program stored in the memory and executable on the processor, wherein the processor implements the following steps when executing the computer program: S1. Select the power supply end measurement point and the load end measurement point on the running distribution transformer and its low-voltage lead; S2. When the three-phase load current of the distribution transformer and its low-voltage lead is unbalanced, the total zero-sequence voltage and total zero-sequence current of the distribution transformer and its low-voltage lead are calculated through the power supply end measurement point and the load end measurement point; S3. Calculating the total zero-sequence impedance of the distribution transformer and its low-voltage lead according to the total zero-sequence voltage and the total zero-sequence current; S4. judging the operation status of the zero line of the distribution transformer and its low-voltage lead according to the magnitude relationship between the total zero-sequence impedance and the first preset normal value; The step S2 comprises: S21, obtaining a first zero-sequence impedance equivalent circuit of the power supply end measurement point and a second zero-sequence impedance equivalent circuit of the load end measurement point according to a fault simulation equivalent circuit of the distribution transformer and its low-voltage lead when a zero-sequence fault occurs; S22. Calculate the distribution transformer zero-sequence current and distribution transformer zero-sequence voltage according to the first zero-sequence impedance equivalent circuit; S23, calculating the total zero-sequence current and the total zero-sequence voltage according to the second zero-sequence impedance equivalent circuit; The step S22 specifically includes: According to the first zero-sequence impedance equivalent circuit, three-phase current and voltage vector addition method, the distribution transformer zero-sequence current and the distribution transformer zero-sequence voltage are calculated, and the expressions are as follows: ； in, represents the zero-sequence voltage of the distribution transformer, represents the zero-sequence current of the distribution transformer, , , Expressed as the three-phase voltage of the distribution transformer; , , Expressed as the three-phase current of the distribution transformer; The step S3 further comprises: According to the zero-sequence current and the zero-sequence voltage of the distribution transformer, the total zero-sequence impedance of the distribution transformer is calculated, and its expression is as follows: ； in, Indicates the total zero-sequence impedance of the distribution transformer; The step S4 further comprises: According to the magnitude relationship between the total zero-sequence impedance of the distribution transformer and a second preset normal value, judging the internal operation status of the distribution transformer; Step S23 specifically includes: According to the second zero-sequence impedance equivalent circuit, three-phase current and voltage vector addition method, the total zero-sequence current and the total zero-sequence voltage are calculated, which are expressed as follows: ； in, Represents the total zero-sequence voltage, represents the total zero-sequence current, , , Expressed as the three-phase voltage at the load end, , , Expressed as the three-phase current at the load end; The total zero-sequence impedance of the distribution transformer and its low-voltage lead in step 3 is expressed as: ； Among them, r1 represents the zero line resistance; The step S2 further comprises: S24, measuring at the power supply end measurement point and the load end measurement point at the same time, and obtaining the lead zero-sequence impedance of the distribution transformer and its low-voltage lead according to the obtained total zero-sequence impedance, which is expressed as: ； Simplifying the expression of the lead zero-sequence impedance, we get: ； or 。

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