Method for carrying out zero sequence loop impedance calculation by using frequency conversion signal

Abstract

The invention discloses a method for carrying out zero sequence loop impedance calculation by using a frequency conversion signal, the calculation method is applied to a power distribution network arc suppression coil grounding system, and the power distribution network arc suppression coil grounding system comprises a control device, a grounding transformer, an injection transformer and a variable signal source; according to the method, the signal source is adopted to inject signals with different frequencies to calculate the capacitive current, the impedance parameter of the arc suppression coil and the capacitive current of the system during parallel operation are calculated by establishing a loop equation according to different system zero sequence loop impedances under different frequencies, and then the capacitive current needing to be compensated by the system is accurately calculated. The method solves the problems of the calculation of the capacitance current of the system and the automatic tracking of the arc suppression coil during the parallel operation of the two machines, is not affected by the unbalance of the system, and provides a reliable basis for the precise control of the parallel operation of the two machines of the arc suppression coil.

Description

Technical Field

[0001] The present invention relates to the technical field of automatic tracking of resonant grounding systems in power system distribution networks, and particularly to a method for calculating the impedance of a zero-sequence loop. Background Art

[0002] In distribution networks, the grounding method of grounding the neutral point through an arc suppression coil is commonly used. The automatic control device calculates the capacitive current of the system and adjusts the inductive reactance of the arc suppression coil to be approximately equal to the capacitive reactance of the system. When a ground fault occurs, the current at the grounding point can be compensated to achieve the purpose of extinguishing the arc. Therefore, the accurate calculation of the capacitive current is crucial.

[0003] With the increase in the capacity of the distribution network, the overhead lines are decreasing, the use of cables is increasing, the capacitive current of the system is getting larger, the system unbalance is getting smaller, and there are often cases where two arc suppression coils are operating in parallel. This undoubtedly increases the difficulty of calculating the capacitive current by the control device.

[0004] Conventional methods for calculating capacitive current generally adopt the method of adjusting the gear of the arc suppression coil and calculating the capacitive current through the tuning curve method and the incremental method before and after the gear adjustment. When the system unbalance is very small, the calculation error of the above methods is very large, and it is difficult to correctly track and compensate; when operating in parallel, due to the influence of the damping resistance, the calculation error will be even larger, and sometimes the arc suppression parameters of the parallel operation cannot be obtained in real time. The conventional algorithm cannot accurately calculate the compensation situation of the slave arc suppression, resulting in incorrect calculation of the capacitive current and inability to achieve precise control of the parallel operation of two machines.

[0005] In addition, there is also a method of calculating capacitive current by injecting signals through a PT. However, due to the limitation of the PT capacity, the injected signals are very small, and generally the PT harmonic eliminator needs to be withdrawn. The method of injecting signals through a PT is commonly used to evaluate the total capacitive current range of the system and is not suitable for application to the automatic tracking calculation of arc suppression coils. Summary of the Invention

[0006] The technical problem to be solved by the present invention is to provide a method for calculating the impedance of a zero-sequence loop using a variable-frequency signal, and calculate the capacitive current of the system by outputting voltage signals of different frequencies, so as to solve the problems of capacitive current calculation and automatic tracking of arc suppression coils during parallel operation of two machines.

[0007] To solve the above technical problems, the technical solutions adopted by the present invention are as follows.

[0008] A method for calculating the zero-sequence loop impedance using a variable-frequency signal. The calculation method is applied to an arc suppression coil grounding system of a distribution network. The arc suppression coil grounding system of the distribution network includes a control device, a grounding transformer, an injection transformer, and a variable signal source. The controlled end of the variable signal source is connected to the output end of the control device, the output end of the variable signal source is connected to the secondary side of the injection transformer, one end of the primary side of the injection transformer is grounded, the other end of the primary side is connected to the neutral point of the grounding transformer, and the secondary side of the grounding transformer is grounded through an arc suppression coil; The specific calculation method includes the following steps: S1. The control device controls the variable signal source to inject a voltage signal into the system; S2. Collect the current injected into the system and the current of the arc suppression coil through a current transformer, and calculate the zero-sequence loop current on the system side; S3. Calculate the series impedance of the system to the ground, and calculate the parallel impedance of the system through impedance transformation; S4. The parallel impedance measured at the injection frequency is transformed into the impedance at the power frequency, which is the actual impedance of the zero-sequence loop of the system.

[0009] Preferably, in step S3, when two or more arc suppression coils are arranged in parallel in the system, calculate the vector difference on the output side according to the voltage signals with different frequencies but the same amplitude injected into the system by the variable signal source, calculate the reactance value of the slave arc suppression coil put into operation according to the vector difference, and then calculate the series impedance of the system to the ground according to the impedance relationship.

[0010] Preferably, the parallel impedance parameters described in step S3 include the system capacitive reactance and the system resistance to the ground.

[0011] Preferably, the secondary side of the grounding transformer is connected to the power supply end of the variable signal source to supply power to the variable signal source.

[0012] Preferably, a damping resistor is connected in series between the arc suppression coil and the ground.

[0013] Due to the adoption of the above technical solutions, the technical progress obtained by the present invention is as follows.

[0014] The present invention uses a signal source to inject signals of different frequencies to calculate the capacitive current, utilizes the different zero-sequence loop impedances of the system at different frequencies, calculates the impedance parameters of the arc suppression coil during parallel operation and the system capacitive current by establishing a loop equation, and then accurately calculates the capacitive current that needs to be compensated by the system, solves the calculation of the system capacitive current and the automatic tracking problem of the arc suppression coil during dual-machine parallel operation, is not affected by the magnitude of system imbalance, and provides a reliable basis for realizing the precise control of dual-machine parallel operation of the arc suppression coil. Description of the Drawings

[0015] Figure 1This is the system wiring diagram of the signal source injected when two arc suppression coils of the present invention operate in parallel; Figure 2 This is the schematic diagram of the zero-sequence circuit on the injection source side when there is a single arc suppression coil; Figure 3 This is the schematic diagram of the zero-sequence circuit on the injection source side when there are two arc suppression coils; Figure 4 Schematic diagram of the mutual conversion of impedance in series and parallel. Specific embodiments

[0016] The present invention will be further described in detail below in conjunction with the drawings and specific embodiments.

[0017] A method for calculating the impedance of the zero-sequence circuit using a variable-frequency signal, which is applied to the arc suppression coil grounding system of the distribution network, calculates the actual parallel impedance parameters of the zero-sequence circuit, and further calculates the capacitive current of the system, providing a reliable basis for the automatic tracking compensation of the arc suppression coil in the system.

[0018] The arc suppression coil grounding system of the distribution network includes a control device, a grounding transformer, an injection transformer, a damping resistor and a variable signal source. The controlled end of the variable signal source is connected to the output end of the control device, the output end of the variable signal source is connected to the secondary side of the injection transformer, one end of the primary side of the injection transformer is grounded, the other end of the primary side is connected to the neutral point of the grounding transformer, and the secondary side of the grounding transformer is grounded through the arc suppression coil and the damping resistor in sequence; in the present invention, the secondary side of the grounding transformer is connected to the power supply end of the variable signal source to supply power to the variable signal source.

[0019] The method for calculating the impedance of the zero-sequence circuit using a variable-frequency signal in the present invention specifically includes the following steps.

[0020] S1. The control device controls the variable signal source to inject a voltage signal into the system.

[0021] S2. The current transformer is used to collect the current injected into the system and the current of the arc suppression coil, and calculate the zero-sequence circuit current on the system side.

[0022] S3. Calculate the series impedance of the system to the ground, and calculate the parallel impedance of the system through impedance transformation.

[0023] When two or more arc suppression coils are arranged in parallel in the system, according to the voltage signals with different frequencies but the same amplitude injected into the system by the variable signal source, calculate the vector difference on the output side, calculate the reactance value of the slave arc suppression coil input according to the vector difference, and then calculate the series impedance of the system to the ground according to the impedance relationship.

[0024] The parallel impedance parameter includes the system capacitive reactance X c and the system resistance R to the ground sys .

[0025] S4. The shunt impedance measured at the injection frequency is transformed into the impedance at power frequency, which is the actual impedance of the zero-sequence loop of the system. Embodiment 1

[0026] When the present invention is applied to the arc suppression coil grounding system of a distribution network with only one arc suppression coil in operation, its principle is as Figure 2 shown.

[0027] Figure 2 Among them: U s is the variable signal source voltage; XL0 is the reactance of the arc suppression coil; R z 0 is the series damping resistance configured for the arc suppression coil; IL is the current flowing through the arc suppression coil collected by the current transformer; X c is the capacitive reactance to ground of the system to be solved; R sys is the resistance to ground of the system to be solved; U0 is the voltage injected by the signal source into the system side; I0 is the current injected by the signal source into the system side.

[0028] During the normal operation of the system, the grounding transformer inputs the working voltage Us to the variable signal source, and the control device controls the variable signal source to inject a voltage signal with a fixed frequency f1 (corresponding angular frequency is ) and an amplitude of U0 into the neutral point of the system. The current I s injected into the system and the current IL of the arc suppression coil are collected through the current transformer; from the principle Figure 2 , the zero-sequence loop current I0 on the system side can be calculated, that is, the current injected by the variable signal source into the system side.

[0029]

[0030] According to the voltage and current on the system side, the series impedance parameters of the system can be directly calculated.

[0031]

[0032] Then, through impedance transformation, the actual system shunt impedance parameters - the system capacitive reactance X c and the system resistance to ground R sys can be obtained. The schematic diagram of impedance transformation is as Figure 4 shown.

[0033] 1 Among them: is the complex impedance on the system side calculated when the signal source outputs at frequency f1; is the complex impedance on the system side of the real part; is the complex impedance on the system side The imaginary part of; is a unit vector.

[0034] 2 Where: is the system's capacitive reactance to the ground at the f1 frequency; is the complex impedance on the system side The real part of; is the complex impedance on the system side The imaginary part of.

[0035] 3 Where is the system's resistance to the ground; is the complex impedance on the system side The real part of; is the complex impedance on the system side The imaginary part of.

[0036] Frequency transformation is performed to obtain the system's capacitive reactance to the ground at power frequency: 4 Where: is the system's capacitive reactance to the ground at the f1 frequency; is the system's capacitive reactance to the ground at a frequency of 50 Hz.

[0037] The system capacitive current can be obtained by dividing the system bus phase voltage by Obtained. Embodiment 2

[0038] When the present invention is applied to an arc suppression coil grounding system in a distribution network with two arc suppression coils operating in parallel, its system wiring diagram is as shown in Figure 1 Shown, the principle is as shown in Figure 3 Shown.

[0039] Figure 3 Among them: U s is the variable signal source voltage; XL1 is the reactance of the main arc suppression coil; R z 1 is the series damping resistance configured for the main arc suppression coil; IL is the current flowing through the main arc suppression coil collected by the current transformer; XL2 is the reactance of the slave arc suppression coil to be solved; R z 2 is the series damping resistance configured for the slave arc suppression coil; X c is the system's capacitive reactance to the ground to be solved; R sys$R_0$ is the system-to-ground resistance to be solved; $U_0$ is the voltage injected by the variable signal source into the system side; $I_0$ is the current injected by the variable signal source into the system side.

[0040] In this embodiment, since a slave arc suppression coil is introduced into the system, the operating state of the slave arc suppression coil is an uncertain state, that is, the reactance $X_{L2}$ of the slave arc suppression coil is unknown, and the damping resistance $R$ z 2 of the slave arc suppression coil is a fixed value. Therefore, to obtain the system impedance parameters and the reactance value of the slave arc suppression coil, the variable signal source needs to inject at least two frequencies of voltage signals into the system.

[0041] When the system is operating normally, the control device controls the variable signal source to inject a voltage signal with a frequency $f_1$ and an amplitude of $U_0$ into the neutral point of the system. Taking the $U_0$ voltage as the reference vector, the current $I$ injected into the system at the $f_1$ frequency is collected through a current transformer s1 and the current $I$ L1 of the main arc suppression coil.

[0042]

[0043] From Figure 3 the admittance equation of the parallel connection of the system side and the slave arc suppression coil can be obtained.

[0044] 5 where $f_1$ is the output frequency of the variable signal source; is the vector value of the output current when the variable signal source outputs the $f_1$ frequency; is the vector value of the current of the main arc suppression coil measured when the variable signal source outputs the $f_1$ frequency; is the difference between the output current of the variable signal source and the current vector of the main arc suppression coil at the same moment when the variable signal source outputs the $f_1$ frequency; is the neutral point voltage vector value when the variable signal source outputs the $f_1$ frequency, and is the reference vector; is the neutral point voltage vector when the variable signal source outputs the $f_1$ frequency and is the value of the damping resistance in series with the slave arc suppression coil; is the reactance value of the slave arc suppression coil at the $f_1$ frequency; is the capacitive reactance value of the system at the $f_1$ frequency; is the system-to-ground resistance value.

[0045] Replace the signal of the variable signal source injection system. To simplify the calculation, the voltage signal injected by the variable signal source into the neutral point of the system is a voltage signal with a frequency of nf1 (i.e., n times the frequency of f1) and an amplitude of U0. The voltage amplitude and phase of the injected signal are the same before and after changing the frequency. The current I at the frequency of nf1 injected into the system is collected through a current transformer. sn and the current I of the main arc suppression coil Ln .

[0046]

[0047] From the principle Figure 3 the admittance equation of the system side in parallel with the slave arc suppression coil can be obtained, and then the zero-sequence loop current I of the system side in parallel with the slave arc suppression coil system can be calculated. 0n .

[0048] Method 1: Inject two different-frequency signals for solution According to two different frequencies, two impedance relationships can be obtained. Combining the total voltage and current of the system after the system side is in parallel with the slave arc suppression coil, the impedance parameters of the system and the reactance of the slave arc suppression coil can be accurately calculated by solving its equation.

[0049] 6 where: f1 is the output frequency of the signal source, n is an integer multiple of the frequency of f1, n > 1; is the vector value of the output current when the variable signal source outputs at the frequency of nf1; is the vector value of the current of the main arc suppression coil measured when the variable signal source outputs at the frequency of nf1; is the difference between the output current of the variable signal source and the vector of the current of the main arc suppression coil at the same moment when the variable signal source outputs at the frequency of nf1; is the neutral point voltage vector value when the variable signal source outputs at the frequency of nf1, and is the reference vector; is the neutral point voltage vector when the variable signal source outputs at the frequency of nf1 and ratio; is the value of the damping resistance in series with the slave arc suppression coil; is the reactance value of the slave arc suppression coil at the frequency of f1; is the capacitive reactance value of the system at the frequency of f1; Is the ground resistance value of the system.

[0050] The admittance is obtained from the above formula 6-5 , Let , then 7 Where: a and b are measured and calculated values; n is the frequency multiple of f1; Is the damping resistance value in series with the arc suppression coil of the slave unit; Is the reactance value of the arc suppression coil of the slave unit at the f1 frequency; Is the capacitive reactance value of the system at the f1 frequency.

[0051] Simplify the above formula to get:

[0052] Therefore: 8 9 Decompose formula 8 to get:

[0053] Further organize to get: Let: 10 The above formula 8 has only one unknown , so it can be calculated to get : 11 Take the square root of the above formula to get: 12 Discard the invalid solution to get the actual reactance value of the arc suppression coil of the slave unit.

[0054] 13 Substitute the calculation result 13 into formula 9 to calculate the system capacitive reactance .

[0055] 14 Then the system capacitance C0 can be directly calculated as:

[0056] Substitute the calculated , Substituting it into Equation 5, the system-to-ground damping can be calculated .

[0057] Simplify Equation 5 and let the calculation result ,

[0058] Further simplify:

[0059]

[0060] 15 where c is the measured and calculated value, and the system-to-ground resistance to be solved is the real part component of the calculated impedance, and the imaginary part component d is not used here.

[0061] is the value of the damping resistance in series with the arc suppression coil of the slave unit; is the reactance value of the arc suppression coil of the slave unit at frequency f1; is the system-to-ground resistance value.

[0062] Perform a frequency transformation on the calculation result 14 to obtain the system-to-ground capacitive reactance at power frequency : 1 16 The system capacitive current can be obtained by dividing the system bus phase voltage by the system-to-ground capacitive reactance .

[0063] Then the system capacitance C0 can be directly calculated as follows:

[0064] Perform a frequency transformation on the calculation result 13 to obtain the inductive reactance of the arc suppression coil of the slave unit at power frequency: 17 Then the inductance L2 of the arc suppression coil of the slave unit can be directly calculated as follows:

[0065] where is the angular frequency corresponding to the frequency of f1; is the inductive reactance value of the arc suppression coil of the slave unit at frequency f1; is the inductive reactance value of the arc suppression coil of the slave unit at 50 Hz frequency.

[0066] Method 2: Inject three different frequency signals for solution When the three-frequency injection method is used for solving, there is no need to set the damping resistance parameters of the slave arc suppression coil. The inductance, damping resistance and system capacitance of the slave arc suppression coil can all be obtained through calculation, which makes the calculation more flexible and simple.

[0067] We will not repeat the derivation here, and directly quote the calculation formula 8 in the above method 1:

[0068] Where n is a frequency multiple of f1; taking n=8 and n=16 as examples, the equations are established as follows, substituting n=8 and n=16 into the above formulas respectively.

[0069]

[0070]

[0071] After simplifying the two equations, we get the following equation.

[0072]

[0073]

[0074] Where: f1 is the output frequency of the signal source, n is an integer multiple of the frequency of f1, n>1; Calculated values ​​for n=8 measurements; Calculated values ​​for n=16 measurements; is the damping resistance value connected in series with the arc suppression coil of the slave machine; is the inductive reactance value of the slave arc suppression coil at frequency f1.

[0075] Defining intermediate variables , and as follows:

[0076]

[0077]

[0078] Will , Substitute into the above formula and expand:

[0079]

[0080]

[0081]

[0082]

[0083] 18 Wherein: is the measured value when n = 8; is the measured value when n = 16; n is an integer multiple of the frequency of f1, n > 1; is the value of the damping resistance connected in series with the slave arc suppression coil; is the inductive reactance value of the slave arc suppression coil at the frequency of f1.

[0084] Substitute Equation 18 into the above Equation 8 to calculate the inductive reactance value of the slave arc suppression coil.

[0085]

[0086]

[0087]

[0088] 19 Then the inductance L2 of the slave arc suppression coil can be directly calculated as:

[0089] Wherein: is the measured value when n = 8; is the measured value when n = 16; n is an integer multiple of the frequency of f1, n > 1; is the value of the damping resistance connected in series with the slave arc suppression coil; is the inductive reactance value of the slave arc suppression coil at the frequency of f1.

[0090] is the inductance value of the slave arc suppression coil.

[0091] After calculation, Substituting into Equation 18, it can be calculated that .

[0092] Substituting the calculated and into Equation 14, it can be calculated that .

[0093]

[0094] Then the system capacitance C0 can be directly converted to obtain:

[0095] For the calculation result perform frequency transformation to obtain the system capacitance reactance to ground at power frequency : 1

[0096] The system capacitance current can be obtained by dividing the system bus phase voltage by the system capacitance reactance to ground .

[0097] The present invention injects signals of different frequencies from a signal source for capacitance current calculation. By utilizing the different impedances of the system zero-sequence loop at different frequencies, the impedance parameters of the arc suppression coil during parallel operation and the system capacitance current are calculated by establishing a loop equation, and then the capacitive current that needs to be compensated by the system is accurately calculated, providing a reliable basis for the precise control of the parallel operation of the arc suppression coil.

Claims

1. A method for calculating the zero-sequence loop impedance using a variable-frequency signal, characterized in that, The calculation method is applied to the arc suppression coil grounding system of the distribution network. The arc suppression coil grounding system of the distribution network includes a control device, a grounding transformer, an injection transformer, and a variable signal source. The controlled end of the variable signal source is connected to the output end of the control device, the output end of the variable signal source is connected to the secondary side of the injection transformer, one end of the primary side of the injection transformer is grounded, the other end of the primary side is connected to the neutral point of the grounding transformer, and the secondary side of the grounding transformer is grounded through the arc suppression coil; The specific steps of the calculation method are as follows: S1. The control device controls the variable signal source to inject a voltage signal into the system; S2. The current transformer is used to collect the current injected into the system and the current of the arc suppression coil, and calculate the zero-sequence loop current on the system side; S3. Calculate the series impedance of the system to the ground, and calculate the parallel impedance of the system through impedance transformation; S4. The parallel impedance measured at the injection frequency is transformed into the impedance at the power frequency, which is the actual impedance of the zero-sequence loop of the system.

2. The method for calculating the zero-sequence loop impedance using a variable-frequency signal according to claim 1, wherein In step S3, when two or more arc suppression coils are arranged in parallel in the system, the vector difference at the output side is calculated according to the voltage signals with different frequencies but the same amplitude injected into the system by the variable signal source. The reactance value of the slave arc suppression coil put into operation is calculated according to the vector difference, and then the series impedance of the system to the ground is calculated according to the impedance relationship.

3. A method for calculating the zero-sequence loop impedance using a variable-frequency signal according to claim 1, characterized in that The parallel impedance parameters described in step S3 include the system capacitive reactance and the system resistance to the ground.

4. A method for calculating the zero-sequence loop impedance using a variable-frequency signal according to claim 1, characterized in that, The secondary side of the grounding transformer is connected to the power supply end of the variable signal source to supply power to the variable signal source.

5. A method for calculating the zero-sequence loop impedance using a variable-frequency signal according to claim 1, characterized in that, A damping resistor is connected in series between the arc suppression coil and the ground.

Images