A method for on-line identification of distribution network grounding parameters and grounding transient conductance

Abstract

The present application relates to a kind of power distribution network ground parameter and grounding transition conductance online identification method, this method utilizes the neutral point voltage during the single-phase ground fault of power distribution network and the compensation current information injected by active inverter (active inverter in abstract drawing is taken as an example with cascaded H bridge), combined with recursive least square method and limit approximation idea, obtain the online measurement formula of power distribution network ground capacitance, ground resistance, grounding transition conductance, simultaneously introduce variable forgetting factor strategy on the basis of recursive least square method, improve the anti-interference ability of algorithm.The method of the present application can realize the integration of active inverter parameter measurement and arc extinction, using the ground parameter and grounding transition conductance information of online identification, adapt to line operating condition change and improve the arc extinction effect under the ground fault of power distribution line change, quickly judge the moment when ground fault disappears, reduce the overvoltage risk of non-fault phase equipment caused by long time input of active inverter during arc extinction.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of distribution network line, in particular to a method for on-line identification of distribution network ground parameters and grounding transient electric conductance. BACKGROUND

[0002] Random faults are prone to occur in distribution network lines, among which single-phase grounding faults are the most common, accounting for about 80% of the total faults. With the increasing proportion of cable lines in the distribution network and the installation of a large number of nonlinear power electronic devices, the grounding fault current value is rising. Arc suppression devices are often used to suppress single-phase grounding fault current and fault phase voltage, thereby achieving arc suppression. Arc suppression devices are divided into passive arc suppression devices, active arc suppression devices and hybrid arc suppression devices, and their operation all requires accurate measurement of the ground parameters of the distribution network.

[0003] Distribution network ground parameter measurement can be divided into offline measurement and online measurement. The traditional distribution network ground parameter measurement method uses offline measurement, which assumes that the ground parameters of the distribution network remain unchanged during arc suppression, which is difficult to adapt to the changes in the operating conditions of the distribution line during single-phase grounding fault arc suppression in the distribution network. Online parameter measurement can sense system changes and update measurement results in real time when the system ground parameters change, which can effectively prevent the increase of fault residual current.

[0004] In the online measurement method, the least squares method is a method that aims to minimize the sum of squares of errors to obtain optimal parameters. Parameter estimation is convenient, but in practical applications, the dimension of the operation matrix will exceed the operation capacity of the controller due to the increase of the sample size. SUMMARY

[0005] Therefore, the purpose of the present application is to provide a method for on-line identification of distribution network ground parameters and grounding transient electric conductance, which realizes the integration of active inverter parameter measurement and arc suppression, uses the ground parameter and grounding transient electric conductance information obtained by on-line identification to adapt to changes in line operating conditions and improve the arc suppression effect under the changes of distribution line, quickly identifies the moment when the grounding fault disappears, and reduces the overvoltage risk of non-fault phase equipment caused by long-term investment of active inverters during arc suppression.

[0006] To achieve the above purpose, the present application adopts the following technical solution: a method for on-line identification of distribution network ground parameters and grounding transient electric conductance, comprising the following steps:

[0007] Step S1: Relying on the neutral point voltage and active inverter injected current information during single-phase grounding fault of the distribution network, without the need for additional injected current equipment; according to the zero sequence equivalent circuit during normal operation of the distribution network, combining the KCL current law and the mathematical limit approximation idea, deriving the on-line measurement formula of the ground parameters of the distribution network based on the recursive least squares method RLS;

[0008] Step S2: According to the superposition theorem, the zero sequence equivalent circuit of the distribution network during single-phase grounding fault is divided into the equivalent circuit under the action of flexible arc-extinguishing device only and the equivalent circuit under the action of fault phase power supply voltage only, and the on-line measurement formula of the grounding parameter and the grounding transient conductance of the distribution network during single-phase grounding fault is derived;

[0009] Step S3: Considering that the on-line measurement is affected by the change of line operation condition, the on-line measurement formula of the grounding parameter and the grounding transient conductance of the distribution network during single-phase grounding fault is rewritten, and the variable forgetting factor VFF strategy is introduced to enhance the anti-interference performance of the RLS algorithm.

[0010] In a preferred embodiment, the specific derivation of the on-line measurement formula of the grounding parameter of the distribution network during normal operation in step S1 is as follows:

[0011] According to the KCL current law, the circuit equation is written as follows:

[0012]

[0013] The Laplace transform of formula (1) can be obtained as

[0014]

[0015] Wherein, the system input u(s) = i H (s), the system output y(s) = u N (s); therefore, the transfer function of the system is

[0016]

[0017] The open-loop system pulse transfer function with zero-order holder is

[0018]

[0019] Wherein, T c is the sampling period; from formula (4), it can be obtained that

[0020]

[0021] Simplify the coefficients of the injected compensation current and the neutral point voltage in formula (5), and let Then formula (5) can be arranged as

[0022] u N (k) = ai H (k-1) - bu N (k-1) (6)

[0023] Since T c is the sampling period, the value is very small and close to 0; by using the equivalent infinitesimal formula, it can be known that

[0024]

[0025] Therefore, according to the parameter identification results a and b of the recursive least square method, the identification formula of the leakage conductance and the capacitance of the distribution line to the ground in the normal operation condition of the system can be obtained as follows

[0026] g0= (1+b) / a, C0≈T c / a (8).

[0027] In a preferred embodiment, the specific derivation of the on-line measurement formula of the distribution network parameters to the ground and the grounding transition conductance during the single-phase ground fault in the step S2 is as follows:

[0028] Similar to the step S1, the circuit equations under the action of only the flexible arc extinguishing device and only the fault phase power voltage are arranged, and the system output u N (k)

[0029]

[0030] Similarly, since T c is the sampling period, the value is very small and close to 0; by using the equivalent infinitesimal formula, it can be known that

[0031]

[0032] According to the parameter identification results a1, a2, b1 and b2 of the recursive least square method, the identification formula of the leakage conductance, the capacitance to the ground and the grounding transition resistance of the distribution line in the fault operation condition of the system can be obtained as follows

[0033]

[0034] In a preferred embodiment, the specific process of rewriting the on-line measurement formula of the distribution network parameters to the ground and the grounding transition conductance during the single-phase ground fault and introducing the VFF strategy to enhance the anti-interference performance of the RLS algorithm in the step S3 is as follows:

[0035] The system output y(k), the system input and the system parameter θ are written as:

[0036]

[0037] The formula of the recursive least square method with the forgetting factor can be written as:

[0038]

[0039] wherein, For the estimated value of the system parameter, K(k) is a gain vector, λ is a forgetting factor, P(k) is a covariance matrix, and E(k) is a priori error;

[0040] In the scenario where the system is disturbed, a smaller value of λ is often needed to make the improved algorithm quickly perceive the parameter change of the system and have a faster convergence speed.

[0041] Based on the analysis of the forgetting factor, the following correction function is proposed as the basis for changing the forgetting factor of the patent:

[0042]

[0043] Wherein, S1 represents the case that round(E 3 (k))=0, S2 represents the case that round(E 3 (k))≠0, round(E 3 (k)) represents the integer closest to E 3 (k); when round(E 3 (k))=0, it means that the error is within the acceptable range, and λ(k)=λ0 at this time; when round(E 3 (k))≠0, it means that the error exceeds the specified threshold, and λ(k) is taken at this time

[0044] Compared with the prior art, the present application has the following beneficial effects:

[0045] 1. The present application uses the information of the injected current during arc extinction of the active inverter and the neutral point voltage in the power distribution network, and combines the algorithm to online identify the power distribution network ground capacitance value without additional ground parameter measurement device, so that the identified parameter is fed back to the arc extinction device in real time, the compensation current value is corrected in real time, and fast arc extinction is realized.

[0046] 2. The present application uses the algorithm to identify the ground transient conductance, which can accurately identify the size of the ground transient conductance, so as to realize fault tracking. According to the ground transient conductance identification value, the fault nature is judged, the arc extinction device is exited in time, and the overvoltage risk of non-fault phase equipment is reduced.

[0047] 3. The present application improves the performance of RLS algorithm by VFF strategy, which can adapt to the change of power distribution line operation condition, and the VFF-RLS algorithm has faster parameter identification convergence speed and better anti-interference performance than the RLS algorithm.

[0048] 4. The present application realizes the integration of parameter measurement and arc extinction by using VFF-RLS algorithm combined with existing information of power grid, improves the utilization rate of equipment, and reduces the cost and interference problem caused by additional measurement equipment. BRIEF DESCRIPTION OF DRAWINGS

[0049] Figure 1 Zero sequence equivalent circuit diagram during normal operation of distribution network.

[0050] Figure 2 Zero sequence equivalent circuit diagram during single-phase grounding fault of distribution network.

[0051] Figure 3 10kV distribution network simulation model containing source inverter (the source inverter in the figure is taken as an example of cascaded H-bridge).

[0052] Figure 4 Structure schematic diagram of a method for online identification of distribution network grounding parameters and grounding transition conductance. DETAILED DESCRIPTION

[0053] The application will be further described below in conjunction with the drawings and examples.

[0054] It should be noted that the following detailed description is exemplary and is intended to provide further explanation of the present application. Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs.

[0055] It should be noted that the terms used herein are only for the purpose of describing specific embodiments and are not intended to limit the exemplary embodiments according to the present application; as used herein, the singular form is intended to include the plural form unless the context clearly indicates otherwise, and furthermore, it should be understood that when the terms "comprise" and / or "include" are used in the specification, there is a presence of a feature, step, operation, device, component and / or combination thereof.

[0056] The present application is a method for online identification of distribution network grounding parameters and grounding transition conductance, referring to Figures 1-4 , comprising the following steps:

[0057] Step S1: the zero sequence equivalent circuit diagram during normal operation of the distribution network is as shown in Figure 1 , according to Figure 1 , the online measurement formula of the grounding parameters during normal operation of the distribution network is derived, and this step specifically comprises the following steps:

[0058] According to the KCL current law, the circuit equation is written as follows:

[0059]

[0060] Taking Laplace transform of formula (15), we can obtain

[0061]

[0062] wherein, the system input u(s) = iH (s), system output y(s) = u N (s). Therefore, the transfer function of the system is

[0063]

[0064] The open-loop system pulse transfer function with zero-order hold is

[0065]

[0066] where T c is the sampling period. From equation (18), we have

[0067]

[0068] Simplify the coefficients of the injected compensation current and the neutral point voltage in equation (19), let Then equation (19) can be rearranged as

[0069] u N (k) = ai H (k-1) - bu N (k-1) (20)

[0070] Since T c is the sampling period, the value is very small and close to 0. Using the equivalent infinitesimal formula, we have

[0071]

[0072] Therefore, according to the parameter identification results a and b of the recursive least squares method, the identification formula of the leakage conductance and the ground capacitance of the distribution line under normal operating conditions of the system can be obtained as follows

[0073] g0= (1+b) / a, C0≈T c / a (22)

[0074] Step S2: The zero sequence equivalent circuit during single-phase ground fault of the distribution network is shown in Figure 2 The zero sequence equivalent circuit during single-phase ground fault of the distribution network is divided into an equivalent circuit under the action of the flexible arc extinguishing device only and an equivalent circuit under the action of the fault phase power voltage only, and the online measurement formula of the ground parameters and the grounding transition conductance of the distribution network during single-phase ground fault is derived. This step specifically includes the following steps:

[0075] Similar to step S1, rearrange the circuit equations under the action of the flexible arc extinguishing device only and the action of the fault phase power voltage only, and the system output u N (k)

[0076]

[0077] Similarly, since T c is a sampling period, its value is very small and close to 0. Using the equivalent infinitesimal formula, it can be known that

[0078]

[0079] According to the parameter identification results a1, a2, b1 and b2 of the recursive least square method, the identification formula of the leakage conductance of the distribution line to the ground, the capacitance to the ground and the grounding transition resistance under the fault operation condition of the system can be obtained as follows

[0080]

[0081] Step S3: rewriting the online measurement formula of the distribution network ground parameter and the grounding transition resistance during the single-phase ground fault, introducing the VFF strategy to enhance the anti-interference performance of the RLS algorithm, and the specific steps of this step include the following steps:

[0082] The system output y(k), the system input and the system parameter θ are written as:

[0083]

[0084] The formula of the recursive least square method with a forgetting factor can be written as:

[0085]

[0086] wherein, is the estimated value of the system parameter, K(k) is the gain vector, λ is the forgetting factor, P(k) is the covariance matrix, and E(k) is the prior error.

[0087] In the scene where the system is disturbed, it is often necessary to find a smaller value of λ, so that the improved algorithm can quickly perceive the parameter change of the system and has a faster convergence speed. When the system is in a steady state, it is hoped that the value of λ can be appropriately increased, so as to reduce the estimation error in the parameter identification process of the distribution line.

[0088] Based on the analysis of the forgetting factor, the following correction function is proposed in the patent as the basis for changing the forgetting factor of the patent.

[0089]

[0090] wherein, S1 represents the case that round(E 3 (k))=0, S2 represents the case that round(E 3 (k))≠0, and round(E 3 (k)) represents the integer closest to E 3 (k). When round(E3 When (k))=0, it means the error is within an acceptable range, and at this time λ(k)=λ0. When round(E 3 When (k))≠0, it indicates that the error exceeds the specified threshold, and at this time, take

[0091] Replacing λ(k) in formula (27) with λ(k) in formula (28), the recursive least squares method with a fixed forgetting factor is transformed into the recursive least squares method with a variable forgetting factor. The formula for the recursive least squares method with a variable forgetting factor can be written as:

[0092]

[0093] This enables online identification of distribution network ground parameters and ground transition conductance without the need for additional injection equipment.

[0094] In this embodiment, to verify the feasibility of the online measurement method, a software simulation model was built using MATLAB / SIMULINK software. The line parameters are centralized, and the 10kV distribution network simulation model built according to the topology is as follows: Figure 3 As shown, the active inverter and filter inductor are connected to the busbar via grounding transformer ZT. Three feeders are drawn from the busbar, with a single-phase ground fault occurring on phase A of feeder OL1. Considering that the accuracy of the identification of ground leakage resistance in the ground parameters has a negligible impact on the calculation accuracy of the compensation current, this secondary factor is ignored, and only the online identification of ground capacitance is considered. A single-phase ground fault is simulated in the 10kV distribution network at simulation time t1 = 0.01s, and the arc suppression device is activated at t2 = 0.07s. Simultaneously with the injection of current, the recursive least squares algorithm is started to identify the ground capacitance online and update the compensation current value in real time. Based on the online identified ground capacitance value, the arc suppression device can correct the compensation current value in real time under different ground transition resistances. After the compensation current is injected, the current at the fault point is significantly reduced.

[0095] Meanwhile, a recursive least squares algorithm based on a variable forgetting factor can be used to identify ground transition conductance. By identifying the magnitude of ground transition conductance online, fault tracking can be achieved. The nature of the fault is determined based on the identified ground transition conductance value, and the arc suppression device can be shut down in a timely manner, reducing the risk of overvoltage in non-faulty phase equipment caused by prolonged operation of the active inverter during arc suppression.

[0096] The application discloses an online parameter identification method of recursive least square method, which can update operation results only by using new data once, and can improve the online identification precision and anti-interference performance of the algorithm by setting the size of the forgetting factor according to requirements. The method uses neutral point voltage and active inverter injected current information during single-phase ground fault to realize real-time identification of the ground parameter and ground transient conductance of the distribution line, effectively suppresses the fault current, and realizes rapid discrimination of the disappearance of the ground fault.

[0097] The above merely describes preferred embodiments of the present application, and any equivalent changes and modifications made within the scope of the present application should be included in the scope of the present application.

Claims

1. A method for on-line identification of power distribution network earth parameters and ground transition conductance, characterized in that, Includes the following steps: Step S1: Based on the neutral point voltage and active inverter injection current information during a single-phase ground fault in the distribution network, no additional current injection equipment is required; Based on the zero-sequence equivalent circuit during normal operation of the distribution network, combined with KCL current law and mathematical limit approximation, the formula for online measurement of distribution network ground parameters based on recursive least squares (RLS) is derived. Step S2: Based on the superposition theorem, the zero-sequence equivalent circuit during a single-phase ground fault in the distribution network is divided into equivalent circuits under the action of the flexible arc suppression device only and under the action of the fault phase power supply voltage only. The online measurement formulas for the distribution network ground parameters and ground transition conductance during a single-phase ground fault are derived. Step S3: Considering that online measurements are affected by changes in line operating conditions, the online measurement formulas for distribution network ground parameters and ground transition conductance during single-phase ground faults are rewritten, and a variable forgetting factor (VFF) strategy is introduced to enhance the anti-interference performance of the RLS algorithm. The specific derivation of the formula for online measurement of ground parameters during normal operation of the distribution network in step S1 is as follows: According to Kirchhoff's current law, the circuit equations are as follows: The formula Taking Laplace transform, we have Among them, system input System output Therefore, the system's transfer function is The pulse transfer function of an open-loop system with a zero-order hold is: in, The sampling period is given by the formula. achievable Simplified The coefficients of the injected compensation current and neutral point voltage are set as follows: , , then the formula It can be organized into because The sampling period is very small and close to 0; using the equivalent infinitesimal formula, we can know... Therefore, based on the parameter identification results of the recursive least squares method and The identification formulas for the leakage conductance to ground and capacitance to ground of the distribution line under normal operating conditions can be obtained as follows: 。 2. The method for online identification of distribution network ground parameters and grounding transition conductance according to claim 1, characterized in that, The specific derivation of the online measurement formulas for the distribution network ground parameters and ground transition conductance during a single-phase ground fault in step S2 is as follows: Similar to step S1, by refining the circuit equations under the action of the flexible arc suppression device and under the action of the fault phase power supply voltage, the system output can be obtained. Similarly, due to The sampling period is very small and close to 0; Using the formula for equivalent infinitesimals, we can know Based on the parameter identification results of the recursive least squares method , , and The identification formulas for the leakage conductance to ground, capacitance to ground, and grounding transition resistance of the distribution line under system fault operating conditions can be obtained as follows: 。 3. The method for online identification of distribution network ground parameters and grounding transition conductance according to claim 1, characterized in that, The specific process of rewriting the online measurement formulas for the distribution network ground parameters and ground transition conductance during a single-phase ground fault in step S3, and introducing the VFF strategy to enhance the anti-interference performance of the RLS algorithm, is as follows: System output System Input and system parameters Written as: The formula for recursive least squares with a forgetting factor can be written as: in, These are estimated values ​​for the system parameters. For the gain vector, Forgetting factor, Let covariance matrix be the variance matrix. This is the prior error; In scenarios where the system is disturbed, it is often necessary to find a smaller value. This allows the improved algorithm to detect changes in system parameters more quickly and achieve a faster convergence speed. Based on the analysis of the forgetting factor, the following correction function is proposed as the basis for changing the forgetting factor in this patent. Where S1 represents In this case, S2 represents In this situation, Indicates and The closest integer; when When the error is within an acceptable range, it means that the error is within acceptable limits. ;when When the error exceeds the specified threshold, it indicates that the error is taken as follows. .

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