Capacitance current calculation method under distributed compensation

Abstract

The invention provides a capacitance current calculation method under distributed compensation. The method comprises the following steps: S1, fitting a zero-sequence transient voltage of a bus into a sine function of which a rectangular coordinate is a time function; s2, setting a line terminal zero sequence voltage initial value; s3, calculating bus zero-sequence voltage and line outgoing line zero-sequence current according to the line distribution parameter circuit model; s4, comparing the calculated value of the bus voltage with an actual value, and calculating an error; s5, judging whether the error meets the precision requirement or not, and if yes, outputting line outgoing line zero-sequence current; and if not, correcting the initial value of the zero-sequence voltage of the line terminal and repeating the steps S3-S5. According to the method, the calculation error can meet the requirement of capacitance current compensation, the calculation time of the capacitance current is short, and the requirement of online adjustment of the arc suppression coil can be met.

Description

Technical Field

[0001] This invention relates to the field of power system protection and control technology, and more specifically, to a method for calculating capacitor current under distributed compensation. Background Technology

[0002] With the acceleration of urbanization and the increasing urban population, land scarcity coupled with high loads has led to traditional overhead lines being unable to meet the growing electricity demand. Introducing cable lines can reduce the land area occupied by power lines, but this also causes a surge in capacitive current. Existing substation arc suppression coils have limited compensation capacity, and when capacitive current is not effectively compensated, the distribution network will experience safety hazards such as resonant overvoltage. Therefore, it is necessary to upgrade the compensation devices in substations, one method being the use of distributed arc suppression coils to compensate for the surge in capacitive current. After the distributed compensation device is installed, its priority is: first, fixed compensation is performed by the distributed arc suppression coils, and the remaining current is compensated by online adjustment of the substation's arc suppression coils. When a single-phase ground fault occurs in the system, in order to safely adjust the arc suppression coils under distributed compensation online, it is necessary to calculate the capacitive current under the single-phase ground fault in the system online. Existing methods for calculating capacitive current usually involve measuring it using current transformers on the outgoing side. For distribution network resonant grounding systems with distributed compensation devices, measuring the capacitive current using current transformers becomes difficult, causing uncertainties in the online adjustment of the arc suppression coils and affecting the safe operation of the system. Summary of the Invention

[0003] The purpose of this invention is to overcome at least one of the aforementioned shortcomings of the prior art. For example, one objective of this invention is to provide reliable compensation for arc suppression coils, thereby improving the safety of the power supply system.

[0004] To achieve the above objectives, the present invention provides a method for calculating capacitive current under distributed compensation.

[0005] The method includes the following steps: S1, converting the zero-sequence transient voltage of the bus... The fitted rectangular coordinates are a sine function of time; S2, based on the sine function fitted in S1, set the zero-sequence voltage at the line terminal. Initial value; S3, based on the zero-sequence current at the line terminal. and zero sequence voltage Initial values ​​are used to calculate the zero-sequence voltage of the bus based on the circuit model with distributed line parameters. and line outgoing zero-sequence current S4. Calculate the bus voltage. Actual bus voltage Compare and calculate the error e; S5, determine the error. Does it meet the accuracy requirements? If so, output the zero-sequence current of the line. and / or line termination zero-sequence voltage If the condition is not met, then the zero-sequence voltage at the line terminal should be... The initial value is corrected, and steps S3 to S5 are repeated.

[0006] Optionally, the method further includes: S6, calculating the zero-sequence current of all outgoing lines connected to the substation busbar, and then summing them to obtain the capacitive current of the distribution network system.

[0007] Optionally, in S1, the expression for the sine function is: ; in, , These are the constant terms of the fitting function for the x and y coordinates, respectively. For fitting function polynomial Order term coefficients, This represents the polynomial order of the fitted function. for Hilbert transform, s For exponential functions The attenuation constant, ω Let t be the angular frequency and t be the time.

[0008] Optionally, in S2, the zero-sequence voltage of the line terminal The expression for calculating the initial value is: ; The coefficients in the table are: ; Where n is a set constant.

[0009] Optionally, in S3, the circuit model for the distributed line parameters includes: ; in, and Represented as ; in, R The resistance per unit length of the line, in Ω / km; L Where H is the inductance per unit length of the line, H / km; C is the capacitance per unit length of the line, F / km; and G is the conductance per unit length of the line, S / km. L The length of the line is in km; Indicates from Take from different elements A combination of elements; , Each expressed Current and voltage at point N at time N The first derivative.

[0010] Optionally, in S4, the error e is calculated using the following formula: .

[0011] Optionally, in S5, the accuracy requirement is within 5%.

[0012] Alternatively, in S5, the correction is performed using the following formula: .

[0013] Compared with the prior art, the beneficial effects of the present invention include: (1) This invention is not affected by distributed compensation devices and arc suppression coil devices, and can effectively calculate the capacitive current of the power distribution network system, providing a basis for online adjustment of the arc suppression coil.

[0014] (2) The error between the calculated capacitance current value and the measured value is within the allowable range. The error can meet the requirements of capacitance current compensation. Moreover, the calculation time of capacitance current is short, which can meet the requirements of online adjustment of arc suppression coil, so that the arc suppression coil can be reliably compensated and the safety of power supply system can be improved.

[0015] (3) The method of the present invention is simple, the calculation results are accurate and reliable, and it has high value for promotion and application and strong applicability. Attached Figure Description

[0016] The above and other objects and / or features of the present invention will become clearer from the following description taken in conjunction with the accompanying drawings, in which: Figure 1 A power distribution network diagram is shown.

[0017] Figure 2 A simulation network diagram of a 10kV distribution network is shown.

[0018] Figure 3 The diagram shows a comparison of zero-sequence current waveforms in non-faulty circuits.

[0019] Figure 4A It shows The waveform of the capacitor current.

[0020] Figure 4B It shows The waveform of the capacitor current.

[0021] Figure 5A It shows The waveform of the capacitor current.

[0022] Figure 5B It shows The waveform of the capacitor current.

[0023] Figure 6A It shows Time capacitor current waveform diagram Figure 6B It shows The waveform of the capacitor current. Detailed Implementation

[0024] In the following sections, the method for calculating capacitor current under distributed compensation according to the present invention will be described in detail with reference to exemplary embodiments.

[0025] Due to the increasing proportion of cabled distribution networks, capacitive current surges during single-phase ground faults. When the existing arc suppression coil compensation capacity is insufficient, using distributed compensation devices to compensate for capacitive current is an effective measure. However, the addition of distributed compensation devices introduces errors into existing methods for measuring capacitive current, leading to uncertainties in the online adjustment of the original arc suppression coil and affecting the operational safety of the distribution network. To address this, this invention provides an online calculation method for capacitive current under distributed compensation: based on the zero-sequence voltage collected from the substation bus (e.g., a 10kV bus), assuming the zero-sequence current at the line terminal is zero, the zero-sequence current of the outgoing line is calculated using the LMS algorithm based on the distributed parameter circuit model of the line. This calculated zero-sequence current of the outgoing line is the capacitive current of the line. Summing the capacitive currents of all lines on the bus yields the capacitive current of the distribution network system. Furthermore, simulation results show that the method of this invention is unaffected by the distributed compensation device and the arc suppression coil device, and can effectively calculate the capacitive current of the distribution network system, providing a basis for online adjustment of the arc suppression coil.

[0026] Exemplary Example 1 This example provides a method for calculating capacitive current under distributed compensation. The method includes the following steps: S1, the zero-sequence transient voltage of the busbar The fitted rectangular coordinates are the sine functions of the time function.

[0027] In this embodiment, the expression for the sine function is: ; in, , These are the constant terms of the fitting function for the x and y coordinates, respectively. For fitting function polynomial Order term coefficients, This represents the polynomial order of the fitted function. for Hilbert transform, For exponential functions The attenuation constant, ω Let t be the angular frequency and t be the time.

[0028] S2. Based on the sinusoidal function fitted in S1, set the zero-sequence voltage at the line terminal. Initial value.

[0029] In this embodiment, considering the similarity of zero-sequence voltage waveforms at different points in the distribution network, the initial value of the line terminal voltage is set to be... , To set a constant, substituting the sine function of S1 into the equation yields the zero-sequence voltage at the line terminal. The expression for calculating the initial value is: ; The coefficients in the table are: ; Where n is a set constant.

[0030] S3, Based on zero-sequence current at line terminal and zero sequence voltage Initial values ​​are used to calculate the zero-sequence voltage at the bus terminals based on the circuit model with distributed line parameters. and line outgoing zero-sequence current .

[0031] In this embodiment, the circuit model for distributed line parameters includes: ; in, i‘ ON ( t The meaning of ) is the zero-sequence current at the line terminal. Derivative.

[0032] in, and Represented as ; in, R The resistance per unit length of the line, in Ω / km; L Inductance per unit length of the line, H / km; C is the capacitance per unit length of the line, expressed in F / km; G is the conductivity per unit length of the line, in S / km; L The length of the line is in km; Indicates from Take from different elements A combination of elements; , Each expressed Current and voltage at point N at time N The first derivative.

[0033] S4. The calculated bus zero-sequence voltage Actual bus voltage Compare the results and calculate the error e.

[0034] In this embodiment, the error e is calculated using the following formula: .

[0035] S5, Judgment Error Does it meet the accuracy requirements? If so, output the zero-sequence current of the line. and line terminal zero sequence voltage If the condition is not met, then the zero-sequence voltage at the line terminal should be... The initial value is corrected, and steps S3 to S5 are repeated.

[0036] In this embodiment, the correction is performed using the following formula: .

[0037] In the above formula, e represents the error.

[0038] Exemplary Example 2 Based on Exemplary Example 1, the method further includes: For all lines connected to the substation busbar (e.g., 10kV busbar), use steps S3 to S5 as described above to calculate the zero-sequence current of the outgoing lines. Sum all the calculated zero-sequence currents of the outgoing lines to obtain the capacitance current of the distribution network system.

[0039] Exemplary Example 3 When a single-phase ground fault occurs in a distribution network system, for distribution networks equipped with distributed compensation, the reliability of relying solely on line-based capacitance current detection is reduced because the current flowing through the faulty line or the line with distributed compensation is a superposition of capacitive current and inductive compensation current. Under steady-state conditions of a single-phase ground fault, the capacitance current can be approximately calculated based on the bus zero-sequence voltage and the line-to-ground capacitance. However, during the transient phase after a ground fault, the arc suppression coil needs to provide reliable compensation as quickly as possible. When calculating the capacitance current value, since the bus zero-sequence voltage is still in a transient phase, the approximate calculation method using the line-to-ground capacitance and bus zero-sequence voltage is clearly unreliable.

[0040] To address the difficulty in calculating capacitor current, this invention proposes a method for calculating capacitor current under distributed compensation.

[0041] Figure 1The diagram illustrates a distribution network line, with the M-end being the busbar and the N-end being the line terminal. Typically, the zero-sequence voltage of the M-end busbar is detectable. The transformer at the line terminal uses a delta connection, and its terminal zero-sequence current is approximately zero. The following section will utilize the busbar zero-sequence voltage and the terminal zero-sequence current to explain the technical solution of this invention for calculating the line outgoing zero-sequence current (line capacitance current).

[0042] First, the zero-sequence transient voltage of the bus... Based on equation (1), the fitted rectangular coordinates are the sine function of the time function. (1) In the formula , These are the constant terms of the fitting function for the x and y coordinates, respectively. For fitting function polynomial Order term coefficients, This represents the polynomial order of the fitted function. for Hilbert transform, For exponential functions The attenuation constant, ω Let t be the angular frequency and t be the time.

[0043] Considering the similarity of zero-sequence voltage waveforms at different points in the distribution network, let the initial value of the line terminal voltage be... (2) To set a constant, substituting equation (2) into equation (1) yields... (3) The coefficients in the text are (4) Utilizing the zero-sequence current at the line terminal and setting zero-sequence voltage Based on the circuit model of the distributed line parameters given in equations (5) and (6), the zero-sequence voltage of the bus is calculated. and line outgoing zero-sequence current : (5) In equation (5) and It can be represented as (6) and It includes the calculation of current and voltage for each infinitesimal element, which involves intermediate terms of the derivatives of inductance and capacitance, and finally integrates them through iteration and merging to obtain equation (6). Each is the resistance per unit length of the line ( ),inductance( ),capacitance( ), conductivity ( ), Indicates the length of the line ( ). Indicates from Take from different elements A combination of elements, , Each expressed Current and voltage at point N at time N The first derivative.

[0044] The calculated bus voltage Actual bus voltage Compare and calculate the error (7) Determine the error in bus voltage calculation If the accuracy requirements are met, output the zero-sequence current of the line output and the zero-sequence voltage of the line terminal; otherwise, proceed to the correction step of equation (8). (8) It is the convergence factor.

[0045] After correction, return to equation (3) to calculate the zero-sequence voltage of the bus and the zero-sequence current of the line outlet until the error condition is met and the zero-sequence current of the line outlet is output.

[0046] Based on the LMS algorithm derived above, the capacitive current of non-faulted lines can be accurately calculated and the capacitive current of faulted lines can be approximately calculated using the bus zero-sequence voltage and the zero-sequence current at the end of the line. The sum of the calculated capacitive currents of all lines is the capacitive current under a single-phase ground fault in the distribution network system.

[0047] To better understand the exemplary embodiments described above, further explanations will be provided below with specific examples.

[0048] An example of this invention is simulation analysis using Matlab / Simulink software, building such a... Figure 2 The simulation model shown is for a 10kV distribution network. The main transformer has a turns ratio of 115 / 10.5 and a rated capacity of 50MVA. The four outgoing lines adopt a distributed parameter line model. The relevant line parameter settings are shown in Table 1. The load is a 1MW constant impedance model.

[0049] Table 1 Simulation Network Line Parameters

[0050] Example 1 - Simulation Calculation and Analysis of Capacitive Current in Non-Faulty Lines for Figure 2 For non-faulty lines, the capacitive current can be calculated using the bus zero-sequence voltage and the zero-sequence current at the end of the line, based on the LMS calculation algorithm for the line capacitive current.

[0051] A comparison of the waveforms of the calculated capacitance current and the measured capacitance current is shown below. Figure 3 As can be seen from the figure, the waveforms almost overlap, and their average relative error is 0.3%.

[0052] Example 2 - Simulation Calculation and Analysis of Capacitive Current in Faulted Lines for Figure 2 For a single-phase ground fault line in phase A, the fault points are selected at distances of 0km, 3km, and 10km on the line, with transition resistances set to 10Ω and 100Ω, respectively. The zero-sequence current is sampled within one power frequency cycle (20ms) after the fault occurs, and the sampling frequency is 1.6Hz.

[0053] 1) Simulation analysis of capacitor current when the fault point is 0km away Using the bus zero-sequence voltage and the zero-sequence current at the end of the line, the capacitive current of the faulted line is calculated based on the LMS algorithm for line capacitive current. A waveform comparison between the calculated capacitive current and the measured capacitive current is shown below. Figure 4A and Figure 4B As can be seen from the figure, the waveforms almost overlap, and the average relative errors of the two figures are 0.2% and 0.22% respectively. In this case, the increase in the transition resistance has little impact on the calculation of the capacitor current.

[0054] 2) Simulation analysis of capacitor current at a fault distance of 3km Using the bus zero-sequence voltage and the zero-sequence current at the end of the line, and based on the LMS calculation algorithm for line capacitive current, the capacitive current of the faulty line is calculated. A waveform comparison between the calculated capacitive current and the measured capacitive current is shown below. Figure 5A and Figure 5B As can be seen from the figure, the waveforms almost overlap, and the average relative errors of the two figures are 1.5% and 0.5% respectively. In this case, the increase of the transition resistance reduces the error in the capacitor current calculation.

[0055] 3) Simulation analysis of capacitive current at the fault point 10km away Using the bus zero-sequence voltage and the zero-sequence current at the end of the line, and based on the LMS calculation algorithm for line capacitive current, the capacitive current of the faulty line is calculated. A waveform comparison between the calculated capacitive current and the measured capacitive current is shown below. Figure 6A and Figure 6BAs can be seen from the figure, the waveforms almost overlap, and the average relative errors of the two figures are 2.8% and 1.3% respectively. In this case, the increase of the transition resistance reduces the error of the capacitor current calculation.

[0056] The above simulation calculation and analysis of the capacitor current shows that the calculation error increases with the distance of the fault point from the starting point. Considering the total calculated capacitor current of the distribution network system, the error can meet the requirements of capacitor current compensation (within 5%).

[0057] This invention proposes a method for calculating capacitive current under distributed compensation. When the distance to the fault point is unknown, the system's capacitive current to ground is calculated by approximating the zero-sequence voltage of the bus to the zero-sequence voltage of the fault point. Through simulation calculation and analysis, it is clear that the calculation error of the method proposed in this invention can meet the requirements of capacitive current compensation, and the calculation time of the capacitive current is short, which can meet the needs of online adjustment of the arc suppression coil.

[0058] Although the present invention has been described above in conjunction with exemplary embodiments and accompanying drawings, those skilled in the art should understand that various modifications can be made to the above embodiments without departing from the spirit and scope of the claims.

Claims

1. A method for calculating capacitive current under distributed compensation, characterized in that, The method includes the following steps: S1, the zero-sequence transient voltage of the busbar The fitted rectangular coordinates are the sine function of the time function; S2. Based on the sinusoidal function fitted in S1, set the zero-sequence voltage at the line terminal. Initial value; S3, Based on zero-sequence current at line terminal and zero sequence voltage Initial values ​​are used to calculate the zero-sequence voltage of the bus based on the circuit model with distributed line parameters. and line outgoing zero-sequence current ; S4. Calculate the bus voltage Actual bus voltage Compare the results and calculate the error e; S5, Judgment Error Does it meet the accuracy requirements? If so, output the zero-sequence current of the line. and / or line termination zero-sequence voltage If the condition is not met, then the zero-sequence voltage at the line terminal should be... The initial value is corrected, and steps S3 to S5 are repeated.

2. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, The method further includes; S6. Calculate the zero-sequence current of all outgoing lines connected to the substation busbar, and then sum them to obtain the capacitive current of the distribution network system.

3. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, In S1, the expression for the sine function is: ； in, , These are the constant terms of the fitting function for the x and y coordinates, respectively. For fitting function polynomial Order term coefficients, This represents the polynomial order of the fitted function. for Hilbert transform, For exponential functions The attenuation constant, ω Let t be the angular frequency and t be the time.

4. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, In S2, the zero-sequence voltage of the line terminal The expression for calculating the initial value is: ； The coefficients in the table are: ； Where n is a set constant.

5. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, In S3, the circuit model for distributed line parameters includes: ； in, and Represented as ； in, R The resistance per unit length of the line, in Ω / km; L The inductance per unit length of the line is expressed in H / km. C is the capacitance per unit length of the line, expressed in F / km; G is the conductivity per unit length of the line, in S / km; L The length of the line is in km; Indicates from Take from different elements A combination of elements; , Each expressed Current and voltage at point N at time N The first derivative.

6. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, In step S4, the error e is calculated using the following formula: 。 7. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, In S5, the accuracy requirement is within 5%.

8. The method for calculating capacitive current under distributed compensation according to claim 1, characterized in that, In S5, the correction is performed using the following formula: 。

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