A real-time measurement algorithm for ground parameters after single-phase ground fault in distribution network

Abstract

The present invention discloses a real-time measurement algorithm for ground parameters after a single-phase grounding fault in a distribution network. The scheme is as follows: 1. Real-time sampling of the three-phase power supply voltage and zero-sequence voltage of the distribution network is performed to monitor the occurrence of a single-phase grounding fault, and a phase selection operation is performed after the fault. 2. After a single-phase grounding fault in the distribution network occurs, a non-power frequency characteristic signal is injected into the power grid, a time domain differential equation is written, and the power frequency components in the voltage and current signals are filtered out using trigonometric function formulas, a differential equation containing only characteristic components is constructed, and the three-phase ground capacitance and transition resistance of the distribution network after the fault are calculated by fitting. The ground parameter measurement algorithm described in the present invention is not affected by changes in the distribution network structure and whether the arc extinguishing coil branch parameters are known or not. It can reflect and track changes in the distribution network structure after the fault in real time, and can provide real-time and accurate compensation current instruction references for active current arc extinguishing, thereby achieving rapid arc extinguishing of single-phase grounding faults in the distribution network.

Description

Technical Field

[0001] The invention belongs to the field of active arc extinguishing for single-phase grounding fault in distribution network, and in particular relates to a real-time measurement algorithm for ground parameters after single-phase grounding fault in distribution network. Background Art

[0002] my country's medium and low voltage distribution networks generally use low current grounding methods such as neutral point grounding through arc suppression coils. However, with the large-scale access of cables and nonlinear loads in my country's medium and low voltage distribution networks, the active component of single-phase grounding fault current has increased significantly. However, the neutral point grounding method grounded through arc suppression coils cannot compensate for the active component in the grounding fault current, resulting in the arc at the fault point being unable to extinguish itself, which will produce serious consequences such as arc grounding overvoltage, which not only threatens the personal safety of workers, but also may cause large-scale power outages and other accidents, seriously affecting the safe and stable operation of the power grid. In view of the defect that the traditional passive arc extinguishing method cannot compensate for the active current, scholars at home and abroad have proposed the concepts of active voltage arc extinguishing and active current arc extinguishing respectively. Among them, the active voltage arc extinguishing method generally adopts the method of limiting the voltage of the fault phase bus to zero. However, for low-resistance grounding faults, when the load current is large and the fault point is far away from the bus, the voltage at the fault point is not equal to the voltage of the fault phase bus, which makes the active voltage arc extinguishing method have poor arc extinguishing effect when facing low-resistance grounding faults; while the active current arc extinguishing method calculates the compensation current command of the inverter by calculating the ground parameters of the distribution network, and can also achieve full compensation of the fault current in the case of low-resistance faults, but it is very dependent on the speed and accuracy of the ground parameter measurement. The existing ground parameter measurement scheme is generally selected before the fault, and the ground parameters are calculated by injecting power frequency or non-power frequency current signals at the neutral point and detecting the zero-sequence voltage and current at the corresponding frequency. This scheme applies disturbance to the power grid before the fault, which not only affects the normal operation of the power grid, but also cannot reflect the changes in the structure and parameters of the distribution network after the fault, which restricts the application of active current arc extinguishing. Summary of the invention

[0003] The present invention proposes a real-time measurement algorithm for ground parameters after a single-phase grounding fault in a distribution network, so as to overcome the problems in the prior art that the speed and accuracy of active current arc extinguishing ground parameter measurement are low and the changes in the distribution network structure and parameters after the fault cannot be reflected.

[0004] To achieve the above object, the present invention adopts the following technical solution: a real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network, comprising the following steps:

[0005] Step 1: Sample the three-phase power supply voltage and zero-sequence voltage to monitor whether a single-phase grounding fault occurs;

[0006] Step 2: After a single-phase grounding fault is detected, a non-power frequency characteristic signal is actively injected into the neutral point of the distribution network through an effective arc extinguishing device, and the inverter injection current, arc extinguishing coil branch current and neutral point voltage are measured. A time domain differential equation is written according to the distribution network's equivalent circuit to ground, and a digital filtering algorithm is used to filter out the power frequency component in the measured electrical quantity, and a differential equation containing only the characteristic component is constructed. The three-phase to ground capacitance and transition resistance of the distribution network after the fault are calculated by the least squares fitting algorithm;

[0007] Step 3: According to the distribution network grounding parameters and phase selection results obtained in step 2, the power frequency current command value injected by the active inverter into the neutral point is calculated to achieve full compensation of the grounding fault current of the distribution network.

[0008] Furthermore, in the above step 2, after a single-phase grounding fault is detected, a non-power frequency characteristic signal is actively injected into the neutral point of the distribution network through an active arc extinguishing device, and a time domain differential equation is written according to the ground equivalent circuit of the distribution network. The specific expression is as follows:

[0009]

[0010] In the above formula, i0 is the system zero-sequence current, u0 is the voltage at the neutral point of the distribution network, and u f is the system equivalent zero-sequence voltage source, C ∑ is the sum of the capacitance of each feeder to ground in the distribution network, R f is the transition resistance of the fault branch;

[0011] Furthermore, in the above step 2, the specific method of constructing a differential equation containing only non-power frequency characteristic components is as follows:

[0012] The differential equation for the kth sampling point is as follows:

[0013]

[0014] The differential equation for the k+1th sampling point is as follows:

[0015]

[0016] The differential equation for the k+1th sampling point is as follows:

[0017]

[0018] Among them, i0(k), u0(k) are the measurement results of the current and voltage transformer at the kth sampling point, U fis the amplitude of the system equivalent zero-sequence voltage source, ω is the angular frequency corresponding to the power frequency, and Δt is the time interval between adjacent sampling points. i0(k+1), u0(k+1) are the measurement results of the voltage and current transformer at the k+1th sampling point, i0(k-1), u0(k-1) are the measurement results of the voltage and current transformer at the k-1th sampling point;

[0019] Adding the differential equations of the k-1th and k+1th sampling points and dividing by 2cos(ωΔt) yields:

[0020]

[0021] Subtract the above equation from the differential equation of the kth sampling point to obtain the differential equation containing only non-industrial frequency components:

[0022] Furthermore, in the above step 2, the five-point numerical differential formula is used to calculate the derivative value at the sampling point, that is:

[0023] Furthermore, in the above step 3, the compensation current command value I m The calculation method is:

[0024]

[0025] In the above formula, E i is the fault phase power supply voltage, i=A,B,C,R h is the damping resistance of the arc suppression coil branch, L p is the inductance of the arc suppression coil, C ∑ is the sum of the capacitance of the distribution network to ground,

[0026] R h , L p The calculation method is as follows:

[0027] (2) Write the time domain differential equation;

[0028]

[0029] (2) According to the method of constructing a differential equation containing only non-power frequency components in step 2, a differential equation containing only damping resistance and arc suppression coil inductance is constructed, and the arc suppression coil inductance L is obtained by function fitting. p With arc suppression coil damping resistor R h .

[0030] Compared with the existing distribution network active arc extinguishing technology, the present invention has the following beneficial effects:

[0031] 1. The present invention adopts the idea of ​​parameter identification and constructs a differential equation containing only injected characteristic components in the time domain, thereby identifying the ground capacitance and transition resistance of the distribution network after the fault. When the inductance and damping resistance of the arc suppression coil branch are unknown, they can also be calculated.

[0032] 2. The present invention actively injects detection signals and measures the ground parameters of the distribution network after a fault, which not only avoids the impact of the traditional solution of injecting signals before a fault on the system, but also can reflect and track the changes in the distribution network structure after the fault in real time.

[0033] 3. The present invention only needs half-cycle or even quarter-cycle sampling point data to realize accurate acquisition of the system's ground parameters. It is not affected by changes in the distribution network structure and whether the arc extinguishing coil branch parameters are known or not. It can quickly and accurately track changes in distribution network parameters after a fault, provide a reference for the inverter's compensation current command, and thus realize rapid arc extinguishing of single-phase grounding faults in the distribution network.

[0034] 4. In step 2, the five-point numerical differential formula is used to calculate the derivative value at the sampling point. This is because the differential equation contains the derivative term of the voltage sampling value, so the partial derivative value of the zero-sequence voltage at the sampling point needs to be calculated when fitting the multivariate function. This formula is a five-point numerical differential formula for the first-order derivative. Compared with the three-point numerical differential formula, it has a smaller truncation error and higher calculation accuracy. BRIEF DESCRIPTION OF THE DRAWINGS

[0035] Figure 1 This is a topological structure diagram of active arc suppression in a distribution network according to an embodiment of the present invention;

[0036] Figure 2 It is an equivalent circuit diagram to ground after a single-phase grounding fault in the distribution network according to an embodiment of the present invention;

[0037] Figure 3 This is a flow chart of the algorithm described in the embodiment of the present invention;

[0038] Figure 4 It is a visualization diagram of the result of the fitting algorithm in the present invention when the transition resistance is 50Ω;

[0039] Figure 5 The current waveform of the fault point after compensation by the algorithm of the method of the present invention when the transition resistance is 50Ω. DETAILED DESCRIPTION

[0040] The present invention will be further described below in conjunction with the accompanying drawings. The following embodiments are only used to more clearly illustrate the technical solution of the present invention, and cannot be used to limit the protection scope of the present invention.

[0041] The present invention discloses a real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network. After the fault, the method injects a non-power frequency characteristic signal into the distribution network through the neutral point of the distribution network, eliminates the power frequency signal component using a digital filtering algorithm to obtain a differential equation containing only the injection signal frequency, obtains the transition resistance and ground capacitance parameters of the distribution network through multivariate function fitting, and then obtains the compensation current instruction for active current arc extinguishing, thereby achieving full compensation for single-phase ground faults in the distribution network. The algorithm can quickly measure the ground parameters of the distribution network after the fault, provide real-time and accurate compensation current instructions for active current arc extinguishing, and then achieve reliable extinguishing of the arc at the fault point.

[0042] See also Figure 3 The present invention proposes a real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network, comprising the following steps:

[0043] Step 1: Sample the three-phase power supply voltage and zero-sequence voltage in real time to monitor whether a single-phase grounding fault occurs: If a single-phase grounding fault is detected in the distribution network, determine the fault phase based on the relationship between the three-phase voltage and the neutral point voltage.

[0044] The judgment condition for a single-phase grounding fault is: when the neutral point voltage of the distribution network exceeds 15% of the phase voltage, a single-phase grounding fault is determined to have occurred, and the phase with the lowest voltage is determined to be the faulty phase.

[0045] Step 2: Actively inject non-power frequency characteristic signals into the neutral point of the distribution network through an effective arc extinguishing device, measure the inverter injection current, arc extinguishing coil branch current and neutral point voltage, write a time domain differential equation based on the distribution network's equivalent circuit to ground, use a digital filtering algorithm to filter out the power frequency components in the measured electrical quantities, construct a differential equation containing only characteristic components, and calculate the three-phase ground capacitance and transition resistance of the distribution network after the fault through a least squares fitting algorithm;

[0046] Step 3: According to the distribution network grounding parameters and phase selection results obtained in step 2, the power frequency current command value injected by the active inverter into the neutral point is calculated to achieve full compensation of the grounding fault current of the distribution network.

[0047] Embodiment: A real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network comprises the following steps:

[0048] Step 1: See Figure 1 , we get a single-phase grounding fault in phase C of the 10kV distribution network, E A 、E B 、E C are the three-phase power supply voltages of the distribution network; U0 is the neutral point voltage of the distribution network; C is the single-phase to ground capacitance of the distribution network, and the sum of the three-phase to ground capacitance C ∑ =5.8uF; R f is the transition resistance of the fault point; the arc suppression coil inductance Lp =1H, arc suppression coil damping resistance R h =10Ω; I m Injects current into the neutral point for active arc suppression devices.

[0049] Step 2: After a single-phase grounding fault is detected, the active arc extinguishing device is put into operation to inject an arbitrary non-power frequency characteristic signal into the neutral point of the distribution network. The present invention injects a detection current with a frequency of 75Hz and an amplitude of 1A. After a delay of one cycle, the inverter injection current, arc extinguishing coil branch current and neutral point voltage are sampled at a sampling frequency of 2kHz and a sampling interval of 500us.

[0050] See also Figure 2 , write Kirchhoff's current equation according to the equivalent circuit of the distribution network to the ground after the fault. The specific expression is:

[0051]

[0052] In the formula, i0 is the system zero-sequence current, u0 is the voltage at the neutral point of the distribution network, and u f is the system equivalent zero-sequence voltage source, C ∑ is the sum of the capacitance of each feeder to ground in the distribution network, R f is the transition resistance of the fault branch;

[0053] During the process of injecting non-power frequency signals into the inverter, power frequency components and non-power frequency components coexist in the equivalent circuit of the distribution network to the ground. Aiming at the shortcomings of using the traditional Fourier algorithm to extract non-power frequency components, the present invention proposes an algorithm for eliminating 50Hz signals in the time domain to construct a differential equation containing only non-power frequency components. The specific process is as follows:

[0054] The Kirchhoff current equation satisfied by three consecutive sampling points is:

[0055]

[0056]

[0057]

[0058] Among them, i0(k), u0(k) are the measurement results of the current and voltage transformer at the kth sampling point, U f is the amplitude of the system equivalent zero-sequence voltage source, ω is the angular frequency corresponding to the power frequency, 100πrad / s, Δt is the time interval between adjacent sampling points, which is 500us, i0(k+1), u0(k+1) are the measurement results of the voltage, current and mutual inductance at the k+1th sampling point, i0(k-1), u0(k-1) are the measurement results of the voltage, current and mutual inductance at the k-1th sampling point.

[0059] Adding the differential equations of the k-1th and k+1th sampling points and dividing by 2cos(ωΔt) yields:

[0060]

[0061] Subtracting the above equation from the differential equation of the kth sampling point will yield a differential equation containing only the 75Hz component:

[0062] The above formula only contains the transition resistance R f The sum of the capacitance to ground C ∑ The equation group, in which i0 and u0 only contain 75Hz components, can obtain the ground capacitance and transition resistance values ​​of the distribution network by fitting based on the data of several sampling points. The present invention uses a quarter cycle, i.e., 10 sampling points, for fitting.

[0063] It should be noted that when the three-point numerical differential formula is used to calculate the derivative value at the sampling point, the introduction of truncation error will lead to a large error in the ground parameter obtained by fitting. In order to improve the accuracy of numerical calculation, this algorithm uses a five-point numerical differential formula to calculate the derivative value at the sampling point, that is:

[0064]

[0065] Use MATLAB's fitting toolbox to fit and visualize i m -i L The term is projected onto the z-axis, and the derivative term The projected value is projected onto the x-axis and the fault current term is projected onto the y-axis. Figure 4 It is a visualization diagram of the result of the fitting algorithm in the present invention when the transition resistance is 50Ω, where:

[0066] i c For the formula Calculation results at each sampling point;

[0067] i f For the formula Calculation results at each sampling point;

[0068] i n For the formula Calculation results at each sampling point.

[0069] The results show that the plane drawn according to the fitting results is consistent with the fault current term i f (x-axis), neutral point voltage derivative term i c (y-axis) and the neutral point current i nThe intermediate variable data points formed by the term (z axis) are highly overlapped, and the goodness of fit index R is used to evaluate the degree of fit of the regression equation to the sample observations. 2 is 1, indicating that the method of characterizing the system ground parameters by fitting the obtained parameters is credible and reasonable.

[0070] After setting the C phase to ground through a 50Ω transition resistor in the PSCAD electromagnetic transient simulation software, the transition resistance is obtained through the MATLAB fitting function to be 51.63Ω with an error of 3.26%, and the ground capacitance is obtained to be 5.826uF with an error of 0.48%, indicating that the algorithm described in the present invention can accurately identify the distribution network parameters after a fault.

[0071] If the inductance and damping resistance of the arc suppression coil are unknown or there is an error in the nominal inductance of the arc suppression coil, the parameters of the arc suppression coil branch can also be calculated by fitting after eliminating the power frequency component. The specific process is as follows:

[0072]

[0073]

[0074]

[0075] Adding the differential equations of the k-1th and k+1th sampling points and dividing by 2cos(ωΔt) yields:

[0076]

[0077] Subtracting the above equation from the differential equation of the kth sampling point will yield a differential equation containing only the 75Hz component:

[0078]

[0079] The arc suppression coil inductance L can be obtained by function fitting p With arc suppression coil damping resistor R h , the arc suppression coil damping resistance obtained by MATLAB fitting function is 10Ω, and the arc suppression coil inductance is 0.9998H, which has almost no error compared with the simulation setting parameters.

[0080] Step 3: After obtaining the ground capacitance and transition resistance of the distribution network after the fault and the inductance and resistance of the arc suppression coil branch, Figure 2 From the equivalent circuit of the distribution network to the ground and the phase selection results shown, it can be seen that the command value of the inverter compensation current is:

[0081]

[0082] In the formula, E i is the fault phase power supply voltage, i=A,B,C,Rh is the damping resistance of the arc suppression coil branch, L p is the inductance of the arc suppression coil, C ∑ is the sum of the capacitance of the distribution network to the ground;

[0083] R h , L p The calculation method is as follows:

[0084] (1) Write the time domain differential equation;

[0085]

[0086] (2) According to the method of constructing a differential equation containing only non-power frequency components in step 2, a differential equation containing only damping resistance and arc suppression coil inductance is constructed, and the arc suppression coil inductance L is obtained by function fitting. p With arc suppression coil damping resistor R h .

[0087] A single-phase grounding fault occurs in the system at 0.2s. After the fault is detected, the active arc extinguishing device injects the current characteristic component into the neutral point of the distribution network for 80ms. The inverter outputs the corresponding current according to the calculated inverter compensation current command. The fault current waveform at the fault point is as follows: Figure 5 As shown. Figure 5 It can be seen that when a fault occurs, the fault current amplitude at the fault point reaches 11A before the active arc extinguishing device is compensated. At this time, the arc at the fault point cannot be extinguished. After the active arc extinguishing device is compensated, the fault current amplitude at the fault point is reduced to 0.07A. The arc extinguishing effect is excellent, which can ensure that the arc is effectively extinguished.

[0088] The above contents are only for explaining the technical idea of ​​the present invention and cannot be used to limit the protection scope of the present invention. Any changes made on the basis of the technical solution in accordance with the technical idea proposed by the present invention shall fall within the protection scope of the claims of the present invention.

Claims

1. A real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network, characterized in that: The following steps are involved: Step 1: Sample the three-phase power supply voltage and zero-sequence voltage to monitor whether a single-phase grounding fault occurs; Step 2: After a single-phase grounding fault is detected, a non-power frequency characteristic signal is actively injected into the neutral point of the distribution network through an effective arc extinguishing device, and the inverter injection current, arc extinguishing coil branch current and neutral point voltage are measured. A time domain differential equation is written according to the distribution network's equivalent circuit to ground, and a digital filtering algorithm is used to filter out the power frequency component in the measured electrical quantity, and a differential equation containing only the characteristic component is constructed. The three-phase to ground capacitance and transition resistance of the distribution network after the fault are calculated by the least squares fitting algorithm; Step 3: The ground parameters of the distribution network obtained in step 2: the sum of the ground capacitance of each feeder in the distribution network C ∑ , the fault branch Resistance R f and a phase selection algorithm to calculate the power frequency current command value injected into the neutral point by the active inverter to achieve full compensation of the ground fault current in the distribution network; In step 2, after a single-phase grounding fault is detected, a non-power frequency characteristic signal is actively injected into the neutral point of the distribution network through an active arc extinguishing device, and a time domain differential equation is written according to the ground equivalent circuit of the distribution network. The specific expression is as follows: In the above formula, i0 is the system zero-sequence current, u0 is the voltage at the neutral point of the distribution network, and u f is the system equivalent zero-sequence voltage source, C ∑ is the sum of the capacitance of each feeder to ground in the distribution network, R f is the transition resistance of the fault branch; In step 2, the specific method of constructing a differential equation containing only non-power frequency characteristic components is as follows: The differential equation for the kth sampling point is as follows: The differential equation for the k+1th sampling point is as follows: The differential equation for the k-1th sampling point is as follows: Among them, i0(k), u0(k) are the measurement results of the current and voltage transformer at the kth sampling point, U f is the amplitude of the system equivalent zero-sequence voltage source, ω is the angular frequency corresponding to the power frequency, and Δt is the time interval between adjacent sampling points; i0(k+1), u0(k+1) are the measurement results of the voltage and current transformer at the k+1th sampling point, i0(k-1), u0(k-1) are the measurement results of the voltage and current transformer at the k-1th sampling point; Adding the differential equations of the k-1th and k+1th sampling points and dividing by 2cos(ωΔt) yields: Subtract the above equation from the differential equation of the kth sampling point to obtain the differential equation containing only non-industrial frequency components:

2. According to claim 1, a real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network is characterized in that: In step 2, the five-point numerical differential formula is used to calculate the derivative value at the sampling point, that is:

3. A real-time measurement algorithm for ground parameters after a single-phase ground fault in a distribution network according to claim 1 or 2, characterized in that: In step 3, the compensation current command value I m The calculation method is: In the above formula, E i is the fault phase power supply voltage, i=A,B,C,R h is the damping resistance of the arc suppression coil branch, L p is the inductance of the arc suppression coil, C ∑ is the sum of the capacitance of the distribution network to ground, R h , L p The calculation method is as follows: (1) Write the time domain differential equation; (2) According to the method of constructing a differential equation containing only non-power frequency components in step 2, a differential equation containing only damping resistance and arc suppression coil inductance is constructed, and the arc suppression coil inductance L is obtained by function fitting. p With arc suppression coil damping resistor R h .

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