A method for detecting the capacitance current to the ground based on the transient process

Abstract

A method for detecting the capacitance current to the ground based on the transient process, which serves for the arc suppression coil tracking tuning. By disconnecting the neutral point tuning circuit, measuring the transient process time constant and the electrical quantity amplitude after the circuit stabilizes, the capacitance current to the ground is calculated. The present invention provides a method for calculating the capacitance to the ground by measuring the zero-sequence current before the neutral point disconnector is disconnected and the steady-state voltage value and the time constant after the neutral point disconnector is disconnected. This method will not introduce harmonics into the system, does not need to consider the non-linear distortion caused by the relatively low voltage at the arc suppression coil end, does not need to adjust the tap of the arc suppression coil, and needs to disconnect the neutral point disconnector, with less impact on the system. The measurement method does not need to set a grounding fault, avoiding the harm of the grounding fault to the power grid equipment; does not need to introduce harmonics, avoiding the harmonic pollution of the circuit; does not need to repeatedly adjust the arc suppression coil, avoiding the problem of affecting the service life of the arc suppression coil.

Description

Technical Field

[0001] The present invention relates to the technical field of medium-voltage power transmission and distribution, and particularly to a method for detecting the capacitance current to the ground based on a transient process. Background Art

[0002] In China's power grid system mainly composed of overhead lines of 35 kV and below, the neutral point is mostly grounded through an arc suppression coil. For such a resonant grounding system, the measurement of the capacitance to the ground is particularly important. In this type of system, when a single-phase grounding fault (the most common fault type) occurs, the arc suppression coil compensates for the capacitance current to the ground, reduces the recovery voltage, is easy to extinguish the arc, and reduces the circuit hazard. During normal operation, a shunt damper is connected in parallel at the neutral point to suppress the neutral point voltage offset. When a fault occurs, the damper is removed, and the arc suppression coil is adjusted to the full compensation state according to the calculation result of the capacitance current to the ground. If the calculation of the capacitance to the ground is incorrect and the arc suppression coil and the detuning degree are not reasonably configured, it may lead to situations such as difficult arc extinguishing, resonance, overvoltage, etc., which endanger the operation of the power grid. At the same time, the measurement of the capacitance to the ground also affects aspects such as relay protection, fault detection, reactive power compensation, and harmonics.

[0003] Currently, the commonly used methods for detecting the capacitance current to the ground include the extreme value method, the injection signal method, the impedance triangle method, the bias method, etc. The injection method injects harmonics on the secondary side of the TV to measure the capacitance current to the ground. According to the injected signal, it can be divided into the single-frequency method, the double-frequency method, the triple-frequency method, and the frequency sweep method. This method will cause harmonic pollution to the system, the measurement is complex, the measurement result depends on the selection of the harmonic frequency, and the measurement process is also relatively complex. The extreme value method, the impedance method, etc. require multiple parameter adjustments to reach the full compensation state. On the one hand, the repeated adjustment of the arc suppression coil affects the use of the equipment; on the other hand, by adjusting to make the system reach full compensation, some components resonate, which is not good for the equipment; and the measurement accuracy of these methods is related to various factors, and there are many restrictive factors. In addition, if the system unbalance degree is low and the voltage at the arc suppression coil end is small, there is a problem of non-linear distortion in the measured value. Summary of the Invention

[0004] To solve the above problems, the present invention proposes a method for detecting the capacitance current to the ground based on a transient process, which serves for the tracking tuning of the arc suppression coil. By disconnecting the neutral point tuning circuit, measuring the transient process time constant, and the amplitude of the electrical quantity after the circuit stabilizes, the capacitance current to the ground is calculated.

[0005] To achieve the above object, the technical solution adopted by the present invention is as follows: A method for detecting the capacitance current to the ground based on a transient process, the steps are as follows:

[0006] Step ①, construct a circuit with the neutral point grounded through an arc suppression coil

[0007] The conductance G and the capacitance L, that is, the admittance branch, are connected in parallel and then connected in series with the neutral point disconnecting switch QF in the line grounding circuit;

[0008] Among them, QF is the neutral point disconnector, C A , C B , C C are respectively the capacitance to ground of the A-phase, B-phase, and C-phase of the power grid; G A , G B , G C are respectively the conductance to ground of the A-phase, B-phase, and C-phase of the power grid; L is the arc suppression coil inductance, G is the adjustable conductance in parallel with the arc suppression coil, are respectively the phase voltages of the power supplies of the A-phase, B-phase, and C-phase;

[0009] Step ②, Transformation from voltage source to current source

[0010] Convert the series connection of the voltage source and admittance into a parallel connection of the current source and admittance;

[0011] Among them, are respectively the current source currents after the equivalence of the voltage sources of the A-phase, B-phase, and C-phase, G A , G B , G C , C A , C B , C C are the conductance to ground and capacitance values of the A-phase, B-phase, and C-phases of the power grid after equivalence, is the current flowing from the neutral point to the damping winding and into the ground, and its value is 3 times the zero-sequence current;

[0012] During normal operation, the three-phase power supply and three-phase load are symmetrical, the potential of the neutral point is zero, and the current flowing through the neutral point is zero. When the three-phase load is unbalanced or an asymmetrical short circuit occurs, the potential of the neutral point is not zero, and the neutral point current enters the ground through the admittance branch;

[0013] Step ③, Construct a simplified circuit

[0014] Obtain as the current on the capacitor, G ∑ , C ∑ meet the conditions of formula (1):

[0015]

[0016] Step ④, Closing and opening of the neutral point disconnector QF

[0017] The closing position of QF during normal operation, measure the voltage of the neutral point to the ground 3 times the zero-sequence current Then the three-phase unbalanced current is:

[0018]

[0019] In Equation (2), Y L is the susceptance of L; It can be easily obtained from Equation (2) that and are directly proportional;

[0020] Then, disconnect QF and write the KCL equation for the circuit on the right side of QF to obtain:

[0021]

[0022] In Equation (3), I m and φ i are the amplitude and initial phase angle of the current source respectively;

[0023] Step ⑤, Calculation method considering the transient process

[0024] The general solution of Equation (3) is U C = U'+U", where the free component is the time constant; U' is the particular solution of the differential equation. Assume the particular solution is U' = U m cos(ωt + θ) and substitute it into the differential equation to obtain:

[0025]

[0026] Introduce Let

[0027] Equation (4) can be transformed into:

[0028]

[0029] Obtain:

[0030] The particular solution of the equation is:

[0031]

[0032] The general solution of the equation is:

[0033]

[0034] Substitute the initial conditions, U C(0+) = U C(0-)

[0035]

[0036]

[0037] Thus, the general solution of the equation is obtained as:

[0038]

[0039] Equation (10) is the voltage of the neutral point voltage after the switch is disconnected;

[0040] wherein, the decay time constant After the transient process is completed, the steady-state voltage is:

[0041]

[0042] Substitute into (11), and we get:

[0043]

[0044] G G is much smaller than Y L and can be ignored; taking the modulus length on both sides of Equation (2), we get:

[0045]

[0046] Substitute Equation (12) into Equation (13), and substitute Eliminate G ∑ , and we get:

[0047]

[0048] Let and we get:

[0049]

[0050] Take the positive real root among them.

[0051] The present invention aims at a power system with a neutral point grounded through an arc suppression coil, and proposes a method for detecting the capacitance current to the ground based on the transient process. The beneficial effects of the detection method of the present invention are as follows:

[0052] (1) The present invention provides a method for calculating the capacitance to the ground by measuring the zero-sequence current before the neutral point disconnector is disconnected and the steady-state voltage value and the time constant after the neutral point disconnector is disconnected. This method will not introduce harmonics into the system, does not need to consider the non-linear distortion caused by the low voltage at the arc suppression coil end, does not need to adjust the tap of the arc suppression coil, and needs to disconnect the neutral point disconnector, and has little impact on the system.

[0053] (2) It is not necessary to set a ground fault, avoiding the harm of the ground fault to the power grid equipment; it is not necessary to introduce harmonics, avoiding the harmonic pollution of the circuit; it is not necessary to repeatedly adjust the arc suppression coil, avoiding the problem of affecting the service life of the arc suppression coil.

[0054] (3) The measurement signal uses a current signal. Unlike the traditional voltage signal, it does not require repeated adjustment of the arc suppression coil, does not need to set up ground faults, and does not need to add harmonic signals or bias equipment. This avoids harmonic pollution and artificially setting circuit faults. The measurement circuit is simple and only requires adjusting the resistance once to accurately measure the capacitance to ground value, which can be used in practical engineering.

[0055] (4) The parallel damping at the neutral point has the problem of reducing the detection sensitivity, and the reflected transition impedance will also be greatly reduced. This measurement method does not need to utilize the damping connected in parallel at the transformer neutral point, and whether there is parallel damping at the neutral point has no influence on the measurement result.

[0056] (5) The method is verified by building a model in Matlab. The simulation data verifies the effectiveness of the method. The measurement method is simple, the calculation is easy, the measurement time is short, and the calculation accuracy meets the requirements, providing a theoretical basis for the arc suppression coil tracking tuning based on measuring the capacitance during the transient process. Description of the Drawings

[0057] The following further details the method for detecting the capacitance current to ground based on the transient process of the present invention in combination with embodiments and drawings.

[0058] Figure 1 It is a circuit diagram of the neutral point grounded through an arc suppression coil with damping.

[0059] Figure 2 It is for Figure 1 The transformation diagram of the grounding circuit from a voltage source to a current source.

[0060] Figure 3 It is for Figure 2 The simplified circuit diagram.

[0061] Figure 4 It is the experimental simulation circuit diagram. Detailed Embodiment

[0062] Embodiment 1

[0063] A method for detecting the capacitance current to ground based on the transient process proposed by the present invention mainly includes the following steps:

[0064] Step ①: Construct a grounding circuit with the neutral point grounded through an arc suppression coil

[0065] As Figure 1 shown, the conductance G and the capacitance L, that is, the admittance branch, are connected in parallel and then connected in series with the neutral point disconnecting switch QF in the line grounding loop.

[0066] Among them, QF is the neutral point disconnecting switch, C A , C B , C C are the capacitances to ground of the A-phase, B-phase, and C-phase of the power grid respectively; GA , G B , G C are the ground conductances of the A, B, and C phases of the power grid respectively; L is the arc suppression coil inductance, and G is the adjustable conductance in parallel with the arc suppression coil. are the phase voltages of the power supplies of the A, B, and C phases respectively.

[0067] Step ②: Transformation from voltage source to current source

[0068] As Figure 2 shown, the series connection of the voltage source and the admittance is transformed into the parallel connection of the current source and the admittance.

[0069] Among them, are respectively Figure 1 the current source currents after the equivalence of the voltage sources of the A, B, and C phases in A , G B , G C , C A , C B , C C are the ground conductance and capacitance values of the A, B, and C phases of the power grid after equivalence, which are equal to the corresponding conductance and capacitance values in Figure 1 , is the current flowing from the neutral point to the ground through the damping winding, and its value is 3 times the zero-sequence current.

[0070] During normal operation, the three-phase power supply and the three-phase load are symmetrical, the potential of the neutral point is zero, and the current flowing through the neutral point is zero. When the three-phase load is unbalanced or an asymmetrical short circuit occurs, the potential of the neutral point is not zero, and the neutral point current enters the ground through the admittance branch.

[0071] For Figure 2 simplification, the circuit diagram shown in Figure 3 is obtained.

[0072] Among them is the current on the capacitor, G ∑ , C ∑ meet the conditions of formula (1).

[0073]

[0074] The closing position of QF during normal operation, measure the voltage of the neutral point to the ground 3 times the zero-sequence current Then the three-phase unbalanced current, that is, the current source in the figure is:

[0075]

[0076] In formula (2), YL The susceptance is L, It can be easily obtained from equation (2) that and are directly proportional.

[0077] Then, disconnect QF and write the KCL equation for the circuit on the right side of QF to obtain:

[0078]

[0079] In equation (3), I m and φ i are the amplitude and initial phase angle of the current source respectively.

[0080] The general solution of the equation is U C = U'+U”, where the free component is the time constant. U' is the particular solution of the differential equation. Assume the particular solution is U' = U m cos(ωt + θ) and substitute it into the differential equation to obtain:

[0081]

[0082] Introduce Let

[0083] Equation (4) can be transformed into:

[0084]

[0085] Obtain:

[0086] The particular solution of the equation is:

[0087]

[0088] The general solution of the equation is:

[0089]

[0090] Substitute the initial conditions, U C(0+) = U C(0-)

[0091]

[0092]

[0093] Thus, the general solution of the equation is obtained as:

[0094]

[0095] Equation (10) is the voltage of the neutral point after the switch is disconnected.

[0096] Among them, the decay time constant After the transient process is completed, the steady-state voltage is:

[0097]

[0098] Substitute into (11), and we get:

[0099]

[0100] G G is much smaller than Y L and can be ignored; taking the modulus length on both sides of equation (2), we get:

[0101]

[0102] Substitute equation (12) into equation (13), and substitute Eliminate G ∑ , and we get:

[0103]

[0104] Let and we get:

[0105]

[0106] Take the positive real root among them.

[0107] According to the basic principle of measuring capacitance mentioned above, disconnect QF, and calculate the size of the system-to-ground capacitance according to the current, voltage at the neutral point before and after disconnection, and the decay time of the DC component in the transient process. In this method, it is necessary to select to disconnect the resonant grounding damping circuit. Disconnecting the neutral point disconnector has a certain impact on the circuit. The condition for disconnecting the neutral point disconnector is that the neutral point displacement is less than 20%.

[0108] Analyze that the free component of equation (10) is related to the closing time of the switch.

[0109] If when the switch closes,

[0110] then there is no transient process.

[0111] Therefore, this measurement method needs to avoid this point.

[0112] Example 2 Matlab Simulation

[0113] Build an experimental circuit in Matlab, as Figure 4As shown in the figure. The circuit is a 10 kV system of the distribution network in our country. There are four outgoing lines from the transformer, namely overhead lines L1 and L2, and cable lines L3 and L4. The lengths of the four lines are 15 km, 20 km, 25 km, and 10 km in sequence. Due to insufficient transposition, the A-phase of L1 is 2 km longer. The parameters of the transmission line are shown in Table 1. The total capacitance value of the system is 1.52×10 -5 F, and the resonant inductor is set to 0.67 H. If tracking compensation is required, a fault can be set on L1. In the calculation, the π-type equivalent circuit is selected for the line. In the simulation, the control switch B2 is used to read the voltage and current amplitudes before disconnection, the voltage amplitude after disconnection, and the waveform of the voltage after the switch is disconnected from the oscilloscope Sope3, and the time constant τ is calculated through the waveform. The amplitudes and phase angles of the neutral point current before and after changing the resistance are read from the oscilloscope 3, and the results are substituted into Equation (15) to calculate the value of the capacitance to the ground. As shown in Table 2, it is consistent with the actual total capacitance value of 1.52×10 -5 F, meeting the measurement requirements.

[0114] Table 1 Parameters of the distribution system

[0115]

[0116] Table 2 Results of the example

[0117]

[0118] It can be seen from this that the feasibility of this method is verified by simulation using Matlab software. The measurement signal of this method is easy to obtain, the method is simple, the measurement time is short, the impact on the system is small, the practicability is high, and the accuracy is high, which has practical reference significance for engineering measurement.

[0119] The above content is only an example and explanation of the concept of the present invention. Those skilled in the art of this technology can make various modifications or supplements to the described specific embodiments or use similar methods to replace them, as long as they do not deviate from the concept of the invention or exceed the scope defined by this claim book, they should all fall within the protection scope of the present invention.

Claims

1. A method for detecting the capacitance current to the ground based on the transient process, characterized in that: The steps are as follows: Step ①: Construct a circuit with the neutral point grounded through an arc suppression coil The conductance G and the capacitance L, i.e., the admittance branch, are connected in parallel and then connected in series with the neutral point disconnector QF in the line grounding loop; Among them, QF is the neutral point disconnector, C A , C B , C C are the capacitance to ground of phases A, B, and C of the power grid respectively; G A , G B , G C are the conductance to ground of phases A, B, and C of the power grid respectively; L is the arc suppression coil inductance, G is the adjustable conductance in parallel with the arc suppression coil, are the phase voltages of the power supplies of phases A, B, and C respectively; Step ②: Transformation from voltage source to current source The series connection of the voltage source and the admittance is transformed into the parallel connection of the current source and the admittance; Among them, are the current source currents after the equivalent of the phase-A, phase-B, and phase-C voltage sources, respectively, G A , G B , G C , C A , C B , C C are the conductance and capacitance values of the phase-A, phase-B, and phase-C of the power grid to the ground after equivalent, is the current flowing from the neutral point to the ground through the damping winding, and its value is 3 times the zero-sequence current; During normal operation, the three-phase power supply and the three-phase load are symmetric, the potential of the neutral point is zero, and the current flowing through the neutral point is zero. When the three-phase load is unbalanced or an asymmetric short circuit occurs, the potential of the neutral point is not zero, and the neutral point current enters the ground through the admittance branch; Step ③: Construct a simplified circuit Obtained is the current on the capacitor, G ∑ , C ∑ meet the conditions of formula (1): Step ④: Closing and opening of the neutral point disconnector QF When QF is in the closing position during normal operation, measure the voltage of the neutral point to the ground 3 times the zero-sequence current Then the three-phase unbalanced current is as follows: In formula (2), Y L is the susceptance of L, It is easy to obtain from formula (2) that and are proportional to each other; Then disconnect QF, write the KCL equation for the circuit on the right side of QF, and obtain: In Equation (3), I m and φ i are respectively the amplitude and the initial phase angle of the current source; Step ⑤: Calculation method considering the transient process The general solution of Equation (3) is U C = U'+U”, where the free component is the time constant; U' is the particular solution of the differential equation. Let the particular solution be U' = U m cos(ωt + θ) is substituted into the differential equation, and we get: Introduction Let Equation (4) can be transformed into: Obtained: The particular solution of the equation is: The general solution of the equation is: Substitute the initial conditions, U C(0+) = U C(0-) Thus, the general solution of the equation is obtained as: Equation (10) is the voltage of the neutral point after the switch is disconnected; wherein, the decay time constant After the transient process is completed, the steady-state voltage is: Substitute into (11), we get: G G Much smaller than Y and can be ignored; taking the modulus length on both sides of Equation (2), we get: L ​ Substitute Equation (12) into Equation (13), and substitute Eliminate G ∑ , to obtain: Let Obtain: Take the positive real root among them.

Images