A method for measuring DC bias current of urban power grid transformer

Abstract

The present invention provides a method for measuring a DC bias current of a city power grid transformer, comprising the following steps: A, installing a current source, a voltmeter and a corresponding switch at a grounding flat steel of the city power grid transformer; B, based on the measurement circuit installed in step A, performing segmented measurement by opening and closing corresponding switches; C, calculating the size of a sampling resistor according to the segmented measurement result of step B; D, calculating the size of a DC bias current flowing through the grounding flat steel according to the size of the sampling resistor calculated in step C; the method for measuring a DC bias current of a city power grid transformer provided by the present invention can solve the error problem existing in traditional methods by setting different sections on the same flat steel for multiple measurements and calculating the size of the sampling resistor.

Description

Technical Field

[0001] The invention relates to the technical field of rail transit and transformer DC bias magnetic technology, in particular to a method for measuring DC bias magnetic current of a city power grid transformer. Background Art

[0002] The rail transit traction power supply is a DC current, which is relatively large and can reach thousands of amperes. Since the rail transit track is not completely insulated from the ground, part of the traction DC current does not return to the negative pole of the power supply along the return rail. This part of the current is called stray current. When the neutral point of the grid transformer is directly grounded, the stray current may flow into the neutral point of the grid transformer, causing DC bias in the transformer and affecting the normal operation of the transformer. In order to analyze and evaluate the impact of rail transit stray current on the DC bias of the grid transformer, it is necessary to synchronously monitor the DC bias current of the urban grid transformer.

[0003] The size and distribution of DC bias magnetic field caused by stray current of urban rail transit power traction power supply system are affected by multiple factors such as specific regional conditions, tracks, power supply, power grid, etc., and have the characteristics of small scale, small range, high frequency, periodicity, superposition, etc. The traditional DC bias magnetic field current of transformer has the characteristics of large scale, large range, slow variation, etc.

[0004] The Chinese utility model patent application number 201920903714.1, "A current measuring device for DC bias of neutral point of power transformer", proposes a current measuring device for DC bias of neutral point of power transformer. Although it can be used to monitor DC bias of neutral point of power transformer, the device does not consider the influence of DC of neutral point when calculating flat steel sampling resistance, and each sampling resistance flat steel of the device needs to be connected to a current source. The device is relatively complex and difficult to use on site. The Chinese invention patent application number 202111205923.7, "A DC bias current measuring method for multi-point underground power transformer with neutral point", provides a DC bias current measuring method for multi-point underground power transformer with neutral point. Different currents are applied twice by a current source, and each sampling flat steel extracts voltage signals under two current conditions respectively. Through the known current and voltage signals, the actual resistance value of the sampling resistor is converted under the condition of the inherent underground current of the neutral point, and then the DC bias current signal flowing through each flat steel is measured. However, this method needs to ensure that the length cut from each flat steel is exactly the same when measuring DC bias current, which is difficult in actual operation. In addition, during field testing, since the cross-sectional area and resistivity of each flat steel may be different, even if the length cut from each flat steel is exactly the same, the resistance of the two cut sections of flat steel may be inconsistent, resulting in measurement errors. Summary of the invention

[0005] In view of the above technical problems existing in the prior art, the present invention provides a method for measuring the DC bias current of a city power grid transformer. By setting different sections on the same flat steel for multiple measurements and calculating the size of the sampling resistance, the error problem existing in the traditional method can be solved.

[0006] The technical solution adopted by the present invention is:

[0007] A method for measuring direct current bias current of a transformer in a city power grid comprises the following steps:

[0008] A. Install current source, voltmeter and corresponding switch at the grounding flat steel of the city power grid transformer;

[0009] B. Based on the measurement circuit installed in step A, perform segment measurement by opening and closing the corresponding switches;

[0010] C. Calculate the size of the sampling resistor based on the segmented measurement results in step B;

[0011] D. Based on the size of the sampling resistance calculated in step C, calculate the size of the DC bias current flowing through the grounding flat steel.

[0012] Furthermore, in step A, a current source, a voltmeter and corresponding switches are installed at the grounding flat steel of the city power grid transformer, specifically:

[0013] One of the two parallel transformer grounding flat steels is divided into four sections. The resistances of the four sections are R1, R2, R3, and R4 respectively. A current source I1, a voltmeter and switches K1, K2, K3, K4, K5, K6, K7, and K8 are installed between resistors R2 and R3. The measured section resistance is switched between R2, R3, R2 and R3 by switching the switches. The DC bias current is I2, U1 is the indication of the voltmeter, and the resistance of the other grounding flat steel is R5.

[0014] Furthermore, in step B, based on the measurement circuit installed in step A, segment measurement is performed by opening and closing corresponding switches, specifically:

[0015] 1) Select the period when rail transit trains are out of service for testing;

[0016] 2) Close switches K1, K2, K7, K8, and open switches K3, K4, K5, K6. At this time, there are two branches for the current source I1, one is R2, R3, and the shunt coefficient is recorded as k1, and the other is R1, R4, R5, and the shunt coefficient is recorded as k2; there are also two branches for the current source I2, one is R5, and the shunt coefficient is recorded as k3, and the other is R1, R2, R3, R4, and the shunt coefficient is recorded as k4; Assume that R2 = kR3; the current source I1 applies different currents twice, which are recorded as I 11,I 12 The voltages measured by the voltmeter twice are recorded as U 11 ,U 12 , according to the superposition principle of the circuit, there is the following relationship:

[0017] U 11 =(I2k4+I 11 k1)(R2+R3)

[0018] U 12 =(I2k4+I 12 k1)(R2+R3)

[0019] Combining the above two equations, we get in:

[0020]

[0021] Let R1+R4+R5=x, Substituting R2=kR3 and simplifying, we get:

[0022]

[0023] 3) Close switches K1, K2, K5, K6, and open switches K3, K4, K7, K8. At this time, there are two branches for the current source I1, one is R2, the shunt coefficient is recorded as k5, and the other is R1, R3, R4, R5, and the shunt coefficient is recorded as k6; there are also two branches for the current source I2, one is R5, the shunt coefficient is recorded as k7, and the other is R1, R2, R3, R4, and the shunt coefficient is recorded as k8; Assume that R2 = kR3; the current source I1 applies different currents twice, recorded as I 13 ,I 14 The voltages measured by the voltmeter twice are recorded as U 13 ,U 14 , according to the superposition principle of the circuit, there is the following relationship:

[0024] U 13 =(I2k8+I 13 k5)R2

[0025] U 14 =(I2k8+I 14 k5)R2

[0026] Combining the above two equations, we can get in:

[0027]

[0028] Let R1+R4+R5=x, Substituting R2=kR3 into the formula and simplifying it, we can get:

[0029]

[0030] 4) Close switches K3, K4, K7, and K8, and open switches K1, K2, K5, and K6. At this time, there are two branches for the current source I1, one is R3, and the shunt coefficient is recorded as k9, and the other is R1, R2, R4, and R5, and the shunt coefficient is recorded as k 10 ; For the current source I2, there are also two branches, one is R5, and the shunt coefficient is k 11 , one is R1, R2, R3, R4, and the shunt coefficient is k 12 ; Assume R2 = kR3; the current source I1 applies different currents twice, denoted as I 15 ,I 16 The voltages measured by the voltmeter twice are recorded as U 15 ,U 16 , according to the superposition principle of the circuit, there is the following relationship:

[0031] U 15 =(I2k 12 +I 15 k9)R3

[0032] U 16 =(I2k 12 +I 16 k9)R2

[0033] Combining the above two equations, we get in:

[0034]

[0035] Let R1+R4+R5=x, Substituting R2=kR3 into the formula, we can get:

[0036]

[0037] Furthermore, in step C, the size of the sampling resistor is calculated according to the segmented measurement results in step B, specifically:

[0038] The three-partition test results are expressed in the form of equations, as shown below:

[0039]

[0040]

[0041]

[0042] Solve the equations f1, f2, f3 using the solve function of MATLAB, where Z1, Z2, Z3 are constants, x, k, R3 are unknowns, and R3 is the sampling resistor. The solution is:

[0043] x=(0.5*(-1.0*Z1 2 +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2 2 +2.0*Z2*Z3-1.0*Z3 2 )) / (Z2-1.0*Z1+Z3)

[0044] k=(Z1+Z2-1.0*Z3) / (Z1-1.0*Z2+Z3)

[0045] R3=(-1.0*Z1 2 +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2 2 +2.0*Z2*Z3-1.0*Z3 2 ) / (2.0*Z1+2.0*Z2-2.0*Z3).

[0046] Furthermore, in step D, the size of the sampling resistor is calculated according to step C, and the size of the DC bias current flowing through the ground flat steel is calculated, specifically:

[0047] Set the current source I1 to zero, close switches K3, K4, K7, K8, and open switches K1, K2, K5, K6. The voltage at time t is represented by U 1Z,t , then the DC bias current flowing through the grounded flat steel at time t is I Z,t for:

[0048] I Z,t =U 1Z,t / R3.

[0049] The present invention provides a method for measuring the DC bias current of a transformer in a city power grid. By setting different sections on the same flat steel for multiple measurements, the size of the sampling resistance is calculated. The method is not affected by the sampling length, the resistivity of the flat steel, and the cross-sectional area. The method can solve the problem of inconsistent sampling resistance sizes on two flat steels caused by different sampling lengths, inconsistent resistivity and cross-sectional areas of the flat steel during measurement using the traditional method. The error problem existing in the traditional method can be theoretically solved. BRIEF DESCRIPTION OF THE DRAWINGS

[0050] Figure 1 It is a schematic diagram of one embodiment of a method for measuring DC bias current of a transformer in a city power grid according to the present invention;

[0051] Figure 2It is a schematic diagram of a method for measuring DC bias current of a city power grid transformer in the present invention, tested in sections R2 and R3;

[0052] Figure 3 It is a schematic diagram of a method for measuring DC bias current of a transformer in a city power grid according to the present invention, tested in section R2;

[0053] Figure 4 It is a schematic diagram of a method for measuring DC bias current of a transformer in a city power grid according to the present invention, tested in section R3; DETAILED DESCRIPTION

[0054] The technical solution of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the present invention.

[0055] See also Figure 1 , is a schematic diagram of one embodiment of a method for measuring DC bias current of a transformer in a city power grid according to the present invention, the method comprising the following steps:

[0056] A. Install the current source, voltmeter and corresponding switches at the grounding flat steel of the urban power grid transformer, specifically:

[0057] like Figure 1 As shown, one of the two parallel transformer grounding flat steels is divided into four sections, and the resistances of the four sections are R1, R2, R3, and R4 respectively. A current source I1, a voltmeter V, and switches K1, K2, K3, K4, K5, K6, K7, and K8 are installed between the resistors R2 and R3, wherein switches K1 and K2 are connected in series between the node between the resistors R1 and R2 and one end of the current source I1; switches K3 and K4 are connected in series between the node between the resistors R2 and R3 and one end of the current source I1; switches K5 and K6 are connected in series between the node between the resistors R2 and R3 and the other end of the current source I1; and switches K6 and K7 are connected in series between the node between the resistors R3 and R4. The node and the other end of the current source I1 are connected in series with switches K8, K8; the measured section is controlled by the closing of the eight switches, for example, when switches K1, K2, K7, K8 are closed and switches K3, K4, K5, K6 are opened, the measured section is R2, R3; when switches K1, K2, K5, K6 are closed and switches K3, K4, K7, K8 are opened, the measured section is R2; when switches K3, K4, K7, K8 are closed and switches K1, K2, K5, K6 are opened, the measured section is R3; the magnitude of the DC bias current is I2, U1 is the indication of the voltmeter, and the resistance of another flat steel is R5.

[0058] B. Based on the measurement circuit installed in step A, perform segmented measurement by opening and closing the corresponding switches, specifically:

[0059] 1) Select the rail transit train suspension period for testing, such as 3:00-4:00 in the morning. During this period, the DC bias current I2 can be considered to be a constant value;

[0060] 2) If Figure 2 As shown, switch K1, K2, K7, K8 are closed, and switch K3, K4, K5, K6 are opened. At this time, there are two branches for current source I1, one is R2, R3, and the shunt coefficient is recorded as k1, and the other is R1, R4, R5, and the shunt coefficient is recorded as k2; there are also two branches for current source I2, one is R5, and the shunt coefficient is recorded as k3, and the other is R1, R2, R3, R4, and the shunt coefficient is recorded as k4; assuming R2 = kR3; current source I1 applies different currents twice, respectively recorded as I 11 ,I 12 The voltages measured by the voltmeter twice are recorded as U 11 ,U 12 , according to the superposition principle of the circuit, there is the following relationship:

[0061] U 11 =(I2k4+I 11 k1)(R2+R3)

[0062] U 12 =(I2k4+I 12 k1)(R2+R3)

[0063] Combining the above two equations, we can get in:

[0064]

[0065] Let R1+R4+R5=x, Substituting R2=kR3 into the formula and simplifying it, we can get:

[0066]

[0067] 3) If Figure 3 As shown, switch K1, K2, K5, K6 are closed, and switch K3, K4, K7, K8 are opened. At this time, there are two branches for the current source I1, one is R2, the shunt coefficient is recorded as k5, and the other is R1, R3, R4, R5, and the shunt coefficient is recorded as k6; there are also two branches for the current source I2, one is R5, the shunt coefficient is recorded as k7, and the other is R1, R2, R3, R4, and the shunt coefficient is recorded as k8; Assume that R2 = kR3; the current source I1 applies different currents twice, respectively recorded as I 13 ,I 14 The voltages measured by the voltmeter twice are recorded as U 13 ,U 14, according to the superposition principle of the circuit, there is the following relationship:

[0068] U 13 =(I2k8+I 13 k5)R2

[0069] U 14 =(I2k8+I 14 k5)R2

[0070] Combining the above two equations, we can get in:

[0071]

[0072] Let R1+R4+R5=x, Substituting R2=kR3 into the formula and simplifying it, we can get:

[0073]

[0074] 4) If Figure 4 As shown, switch K3, K4, K7, K8 are closed, and switch K1, K2, K5, K6 are opened. At this time, there are two branches for the current source I1, one is R3, and the shunt coefficient is recorded as k9, and the other is R1, R2, R4, R5, and the shunt coefficient is recorded as k 10 ; For the current source I2, there are also two branches, one is R5, and the shunt coefficient is k 11 , one is R1, R2, R3, R4, and the shunt coefficient is k 12 ; Assume R2 = kR3; the current source I1 applies different currents twice, denoted as I 15 ,I 16 The voltages measured by the voltmeter twice are recorded as U 15 ,U 16 , according to the superposition principle of the circuit, there is the following relationship:

[0075] U 15 =(I2k 12 +I 15 k9)R3

[0076] U 16 =(I2k 12 +I 16 k9)R2

[0077] Combining the above two equations, we can get in:

[0078]

[0079] Let R1+R4+R5=x, Substituting R2=kR3 into the formula and simplifying it, we can get:

[0080]

[0081] C. Calculate the size of the sampling resistor based on the segmented measurement results in step B, specifically:

[0082] The three-partition test results are expressed in the form of equations, as shown below:

[0083]

[0084]

[0085]

[0086] Solve the equations f1, f2, f3 using the solve function of MATLAB, where Z1, Z2, Z3 are constants, x, k, R3 are unknowns, and R3 is the sampling resistor. The solution is:

[0087] x=(0.5*(-1.0*Z1 2 +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2 2 +2.0*Z2*Z3-1.0*Z3 2 )) / (Z2-1.0*Z1+Z3)

[0088] k=(Z1+Z2-1.0*Z3) / (Z1-1.0*Z2+Z3)

[0089] R3=(-1.0*Z1 2 +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2 2 +2.0*Z2*Z3-1.0*Z3 2 ) / (2.0*Z1+2.0*Z2-2.0*Z3).

[0090] D. According to the size of the sampling resistance calculated in step C, calculate the size of the DC bias current flowing through the grounding flat steel, specifically:

[0091] Set the current source I1 to zero, close switches K3, K4, K7, K8, and open switches K1, K2, K5, K6. The voltage at time t is represented by U 1Z,t , then the DC bias current flowing through the grounded flat steel at time t is I Z,t for:

[0092] I Z,t =U 1Z,t / R3

[0093] The technical solution and effects of the present invention are described in detail below with a specific embodiment:

[0094] In step A, press Figure 1 The circuits are arranged in a manner that includes current sources, voltmeters, switches, etc.

[0095] In step B, press Figure 2 , Figure 3 and Figure 4 Perform a segment test and record the readings of the current source I1 and the voltmeter to obtain the values ​​of Z1, Z2 and Z3 as shown in Table 1 below:

[0096] Table 1

[0097] <![CDATA[Z1]]> 0.06mΩ <![CDATA[Z2]]> 0.3mΩ <![CDATA[Z3]]> 0.15mΩ

[0098] In step C, according to the test result of step B, the sampling resistance R3=(-1.0*Z1 2 +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2 2 +2.0*Z2*Z3-1.0*Z3 2 ) / (2.0*Z1+2.0*Z2-2.0*Z3)=0.0835mΩ

[0099] In step D, the DC bias current is calculated based on the value of the sampling resistor R3 obtained in step C. The voltage meter reading at time t is 1.2 mV, so the DC bias current I at time t is Z,t =U 1Z,t / R3=1.2 / 0.0835=14.35A. The above test is a laboratory simulation test result. At this time, the DC bias current actually injected into the flat steel is 14.3A, which verifies the correctness of the method proposed in the present invention.

[0100] The invention provides a method for measuring direct current bias current of a transformer in a city power grid. By setting different sections on the same flat steel for multiple measurements and calculating the size of a sampling resistor, the error problem existing in the traditional method can be solved.

[0101] The above description is only a specific implementation mode of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily thought of by technicians in this technical field within the technical scope disclosed by the present invention should be covered by the protection scope of the present invention.

Claims

1. A method for measuring DC bias current of a city power grid transformer, characterized in that The following steps are involved: A. Install current source, voltmeter and corresponding switch at the grounding flat steel of the city power grid transformer; B. Based on the measurement circuit installed in step A, perform segment measurement by opening and closing the corresponding switches; C. Calculate the size of the sampling resistor based on the segmented measurement results in step B; D. Calculate the magnitude of the DC bias current flowing through the grounded flat steel according to the magnitude of the sampling resistance calculated in step C; In step A, a current source, a voltmeter and corresponding switches are installed at the grounding flat steel of the city power grid transformer, specifically: One of the two parallel transformer grounding flat steels is divided into four sections. The resistances of the four sections are R1, R2, R3, and R4 respectively. A current source I1, a voltmeter, and switches K1, K2, K3, K4, K5, K6, K7, and K8 are installed between resistors R2 and R3. The measured section resistance is switched between R2, R3, R2, and R3 by switching the switches. The DC bias current is I2, U1 is the indication of the voltmeter, and the resistance of the other grounding flat steel is R5.

2. A method for measuring DC bias current of a city power grid transformer as claimed in claim 1, characterized in that: In step B, based on the measurement circuit installed in step A, the corresponding switches are opened and closed to perform segmented measurement, specifically: 1) Select the period when rail transit trains are out of service for testing; 2) Close switches K1, K2, K7, K8, and open switches K3, K4, K5, K6. At this time, there are two branches for the current source I1, one is R2, R3, and the shunt coefficient is recorded as k1, and the other is R1, R4, R5, and the shunt coefficient is recorded as k2; there are also two branches for the current source I2, one is R5, and the shunt coefficient is recorded as k3, and the other is R1, R2, R3, R4, and the shunt coefficient is recorded as k4; Assume that R2 = kR3; the current source I1 applies different currents twice, which are recorded as I 11 ,I 12 The voltages measured by the voltmeter twice are recorded as U 11 ,U 12 , according to the superposition principle of the circuit, there is the following relationship: U 11 =(I2k4+I 11 k1)(R2+R3) U 12 =(I2k4+I 12 k1)(R2+R3) Combining the above two equations, we get in: Let R1+R4+R5=x, Substituting R2=kR3 and simplifying, we get: 3) Close switches K1, K2, K5, K6, and open switches K3, K4, K7, K8. At this time, there are two branches for the current source I1, one is R2, the shunt coefficient is recorded as k5, and the other is R1, R3, R4, R5, and the shunt coefficient is recorded as k6; there are also two branches for the current source I2, one is R5, the shunt coefficient is recorded as k7, and the other is R1, R2, R3, R4, and the shunt coefficient is recorded as k8; Assume that R2 = kR3; the current source I1 applies different currents twice, recorded as I 13 ,I 14 The voltages measured by the voltmeter twice are recorded as U 13 ,U 14 , according to the superposition principle of the circuit, there is the following relationship: U 13 =(I2k8+I 13 k5)R2 U 14 =(I2k8+I 14 k5)R2 Combining the above two equations, we can get in: Let R1+R4+R5=x, Substituting R2=kR3 into the formula and simplifying it, we can get: 4) Close switches K3, K4, K7, and K8, and open switches K1, K2, K5, and K6. At this time, there are two branches for the current source I1, one is R3, and the shunt coefficient is recorded as k9, and the other is R1, R2, R4, and R5, and the shunt coefficient is recorded as k 10 ; For the current source I2, there are also two branches, one is R5, and the shunt coefficient is k 11 , one is R1, R2, R3, R4, and the shunt coefficient is k 12 ; Assume R2 = kR3; the current source I1 applies different currents twice, denoted as I 15 ,I 16 The voltages measured by the voltmeter twice are recorded as U 15 ,U 16 , according to the superposition principle of the circuit, there is the following relationship: U 15 =(I2k 12 +I 15 k9)R3 U 16 =(I2k 12 +I 16 k9)R2 Combining the above two equations, we get in: Let R1+R4+R5=x, Substituting R2=kR3 into the formula, we can get:

3. A method for measuring DC bias current of a city power grid transformer as claimed in claim 2, characterized in that: In step C, the size of the sampling resistor R3 is calculated according to the segmented measurement results of step B, specifically: The three-partition test results are expressed in the form of equations, as shown below: Solve the equations f1, f2, f3 using the solve function of MATLAB, where Z1, Z2, Z3 are constants, x, k, R3 are unknowns, and R3 is the sampling resistor. The solution is: x=(0.5*(-1.0*Z1 2 +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2 2 +2.0*Z2*Z3-1.0*Z3 2 )) / (Z2-1.0*Z1+Z3) k=(Z1+Z2-1.0*Z3) / (Z1-1.0*Z2+Z3) <h2 style=";text-align:left;direction:ltr">R3 = (-1.0*Z1<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> +2.0*Z1*Z2+2.0*Z1*Z3-1.0*Z2<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> +2.0*Z2*Z3-1.0*Z3<h2 style=";text-align:left;direction:ltr"> 2 <h2 style=";text-align:left;direction:ltr"> ) / (2.0*Z1+2.0*Z2-2.0*Z3)。 4. A method for measuring DC bias current of a city power grid transformer as claimed in claim 3, characterized in that: In step D, the size of the sampling resistor is calculated according to step C, and the size of the DC bias current flowing through the grounded flat steel is calculated, specifically: Set the current source I1 to zero, close switches K3, K4, K7, K8, and open switches K1, K2, K5, K6. The voltage at time t is represented by U 1Z,t , then the DC bias current flowing through the grounded flat steel at time t is I Z,t for: I Z,t =U 1Z,t / R3。

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