A distance protection method suitable for differential electronic voltage transformer

Abstract

The present invention discloses a distance protection method applicable to a differential electronic voltage transformer, the method comprising the steps of: analyzing the circuit of the differential electronic voltage transformer and establishing a model; constructing a virtual digital electronic voltage transformer based on the model; using the virtual digital electronic voltage transformer to process the current sampling signal to obtain a current signal in the differential domain; using a low-pass filter to process the voltage signal and the current signal output by the differential electronic voltage transformer; using the voltage signal and the current signal processed by the low-pass filter to calculate the reactance, and comparing the calculated reactance with the distance protection reactance constant, so as to determine whether the distance protection is activated. The method is applicable to an electronic voltage transformer whose output is a differential signal, and can effectively avoid the influence of the integral loop on the protection accuracy, thereby improving the accuracy of the protection.

Description

Technical Field

[0001] The invention relates to the technical field of relay protection of power systems, and in particular to a distance protection method suitable for a differential electronic voltage transformer. Background Art

[0002] With the increase of voltage level and capacity of power system, the maturity of modern sensor technology, digital technology and computer technology, and the continuous improvement of measurement and protection requirements, the shortcomings of traditional electromagnetic voltage transformers have become increasingly prominent. Capacitor voltage-dividing electronic voltage transformer (EVT) has become the leading research direction of the new generation of voltage transformers and has been widely used in substations. The output voltage of the secondary side of the EVT based on capacitor voltage division is the differential of the input voltage signal, and its output is a differential signal, so it must be restored to a signal proportional to the primary voltage through an integrator. The accurate restoration of the signal by the integrator is one of the prerequisites for ensuring the accuracy of the entire electronic transformer. Its performance determines the spectrum range and response time of the signal, and directly affects the steady-state and transient performance of the transformer. Since actual devices are not ideal devices, the offset of the op amp, the leakage and loss of the capacitor, the time drift and temperature drift of the op amp and other analog devices will affect the results and cause integration errors. Therefore, in actual applications, the ideal analog circuit integrator will produce zero drift due to non-ideal factors such as the offset current, bias current and temperature change of the operational amplifier, resulting in failure to work stably for a long time. For the integrator in the EVT, it is required to work stably for a long time, but it is very difficult to completely overcome the errors caused by these factors. Summary of the invention

[0003] The purpose of the embodiment of the present invention is to provide a distance protection method applicable to a differential electronic voltage transformer, so as to solve the adverse effect of the integral link error adopted by the electronic voltage transformer on the distance protection.

[0004] The embodiment of the present invention provides a distance protection method applicable to a differential electronic voltage transformer, the method comprising the following steps:

[0005] 1) Analyze the circuit of differential electronic voltage transformer and establish a model;

[0006] 2) constructing a virtual digital electronic voltage transformer based on the above model;

[0007] 3) using the virtual digital electronic voltage transformer to process the current sampling signal to obtain a current signal in the differential domain;

[0008] 4) using a low-pass filter to process the voltage signal output by the differential electronic voltage transformer and the current signal;

[0009] 5) Calculate the reactance using the voltage and current signals processed by the low-pass filter;

[0010] 6) Compare the calculated reactance with the distance protection reactance setting to determine whether the distance protection is activated.

[0011] Furthermore, in step 1), the circuit of the differential electronic voltage transformer is analyzed and a model is established. The specific model established is as follows:

[0012]

[0013] Where: s is a complex variable, C1 is the high voltage arm capacitance, C2 is the low voltage arm capacitance, R is the low voltage arm parallel resistance, R a is the load resistance.

[0014] Furthermore, the step 2) of constructing a virtual digital electronic voltage transformer based on the above model specifically includes: step 2.1) discretizing the model of the differential electronic voltage transformer using a pre-corrected bilinear transformation to obtain a discretized z transfer function; and step 2.2) obtaining a virtual digital electronic voltage transformer based on the discretized z transfer function.

[0015] Furthermore, in step 2.1), the model of the differential electronic voltage transformer is discretized by using a pre-corrected bilinear transformation, and the discretized z transfer function is obtained as follows:

[0016]

[0017] Wherein, z is the z-transformation operator, a1, a2, b1, b2 are the coefficients of the model of the differential electronic voltage transformer discretized by pre-corrected bilinear transformation;

[0018] Furthermore, the step 2.2) of obtaining a virtual digital electronic voltage transformer based on the discretized z transfer function specifically includes the following steps:

[0019] The form of the discretized z transfer function for the sampling point is used as a virtual digital electronic voltage transformer:

[0020]

[0021] Among them, X is the input signal of the virtual digital electronic voltage transformer, Y is the output signal of the virtual digital electronic voltage transformer, n is the sampling number, a1, a2, b1, b2 are the coefficients of the model of the differential electronic voltage transformer discretized by pre-corrected bilinear transformation.

[0022] Furthermore, before step 4), the method further includes the step of selecting low-pass filters with different cut-off frequencies according to the line length.

[0023] Furthermore, low-pass filters with different cutoff frequencies are selected according to the line length. The specific method includes: when the line length is greater than or equal to 100 km, a first low-pass filter with a cutoff frequency of 400 Hz is selected; when the line length is less than 100 km, a second low-pass filter with a cutoff frequency of 700 Hz is selected.

[0024] Furthermore, the expression of the low-pass filter in step 4) is as follows:

[0025]

[0026] Among them, X is the sampling data, Y is the data output after the sampling data passes through the low-pass filter, n is the sampling sequence number, b is the coefficient of the low-pass filter, and m is the number of coefficients of the low-pass filter.

[0027] Furthermore, the step 5) calculating the reactance using the voltage signal and the current signal processed by the low-pass filter includes: using a full-cycle Fourier algorithm to calculate the phasor values ​​of the voltage signal and the current signal processed by the low-pass filter, and using the phasor values ​​to calculate the reactance.

[0028] Furthermore, the step 6) of comparing the calculated reactance with the distance protection reactance constant to determine whether the distance protection is activated includes: if the calculated reactance is less than the distance protection reactance constant, the distance protection is activated; otherwise, the distance protection is not activated.

[0029] Furthermore, in step 2.1), the model of the differential electronic voltage transformer is discretized by using a pre-corrected bilinear transformation, and the amplitude and phase response of the transfer function before and after the discretization remain unchanged at a specific frequency, which is 50 Hz.

[0030] The above technical solution of the embodiment of the present invention has the following beneficial technical effects:

[0031] The present invention constructs a virtual EVT to perform equal-transformation processing on voltage and current signals. The distance protection constructed based on equal-transformation processing can omit the integration link of the electronic mutual inductor in field engineering applications, effectively avoid the influence of parameter discreteness, temperature drift, time drift and other characteristics on the performance of the integrator, and avoid the influence of integrator damage on the protection, while reducing the cost and the delay of the intermediate link, ensuring that the protection can reliably operate in the fault zone and reliably not operate in the fault zone. When filtering the voltage and current signals in the differential domain, the present invention designs two low-pass filters with different cutoff frequencies according to the line length, while taking into account the filtering speed and filtering effect. BRIEF DESCRIPTION OF THE DRAWINGS

[0032] Figure 1 This is a flow chart of a distance protection method applicable to a differential electronic voltage transformer according to an embodiment of the present invention;

[0033] Figure 2 It is a schematic diagram of a data processing link of a distance protection method applicable to a differential electronic voltage transformer according to an embodiment of the present invention;

[0034] Figure 3 It is a circuit diagram of a differential electronic voltage transformer applicable to a distance protection method of a differential electronic voltage transformer according to an embodiment of the present invention;

[0035] Figure 4 The diagram is an amplitude-frequency response diagram of a low-pass filter of a distance protection method suitable for a differential electronic voltage transformer according to an embodiment of the present invention. DETAILED DESCRIPTION

[0036] In order to make the purpose, technical scheme and advantages of the present invention clearer, the present invention is further described in detail below in conjunction with specific embodiments and with reference to the accompanying drawings. It should be understood that these descriptions are only exemplary and are not intended to limit the scope of the present invention. In addition, in the following description, the description of well-known structures and technologies is omitted to avoid unnecessary confusion of the concept of the present invention.

[0037] The equal transformation theory of transmission lines points out that the relationship between the distributed voltage and current of the transmission line will not change after being transformed through the same linear circuit, and the original transmission line distributed parameter model is still satisfied. Considering that the reactance value of the calculation result of the distance protection is usually the ratio of voltage to current, the present invention is based on the equal transformation theory of transmission lines, and designs that the voltage and current signals used in the distance protection all pass through the transmission link of the differential electronic voltage transformer, and the relationship between the voltage and current signals does not change, that is, it does not affect the result of the distance protection calculation based on the reactance value. Among them, for the current signal, in order to make it pass through the same transmission model as the voltage signal, a virtual differential electronic voltage transformer is designed, and the distance protection is constructed using the output voltage of the actual differential electronic voltage transformer and the current through the virtual differential electronic voltage transformer, thereby achieving a technical effect of not affecting the action behavior of the distance protection.

[0038] The present invention proposes a distance protection method suitable for differential electronic voltage transformers. The method is based on the differential voltage signal output by the electronic voltage transformer and does not require integration of the differential voltage signal, thus avoiding the error effect of the integration link on the protection. Figure 1 The steps of a distance protection method suitable for a differential electronic voltage transformer proposed by the present invention are as follows: Figure 1 As shown, the specific steps include:

[0039] Step 1) Analyze the circuit of the differential electronic voltage transformer and establish a model;

[0040] The equivalent circuit of differential electronic voltage transformer is as follows Figure 3 As shown, the circuit is analyzed and the model is established as follows:

[0041]

[0042] Where: s is a complex variable, C1 is the high voltage arm capacitance, C2 is the low voltage arm capacitance, R is the low voltage arm parallel resistance, R a is the load resistance.

[0043] After the model of the actual differential electronic voltage transformer is obtained, step 2) is adopted to construct a virtual digital electronic voltage transformer based on the model.

[0044] When constructing a virtual digital electronic voltage transformer, the actual model is first discretized. The pre-corrected bilinear transformation method can ensure that the amplitude and phase response at a certain specific frequency before and after discretization remain unchanged. The present invention uses this feature to discretize the actual model using the pre-corrected bilinear transformation, where the specific frequency is 50Hz.

[0045] The process of constructing a virtual digital electronic voltage transformer includes: step 2.1) discretizing the model of the differential electronic voltage transformer using a pre-corrected bilinear transformation to obtain a discretized z transfer function;

[0046] The model of the electronic voltage transformer is a continuous transfer function. After the pre-corrected bilinear transformation, the form of the z transfer function is discretized as follows:

[0047]

[0048] Wherein, z is the z-transform operator, and a1, a2, b1, and b2 are the coefficients obtained by discretizing the continuous transfer function of the differential EVT through a pre-corrected bilinear transformation.

[0049] After obtaining the discretized z transfer function, step 2.2) is performed to perform z inverse transformation on the discretized z transfer function to obtain a virtual digital electronic voltage transformer, which is expressed as:

[0050]

[0051] Wherein, X is the input signal of the virtual digital EVT, Y is the output signal of the virtual digital EVT, n is the sampling number, and a1, a2, b1, and b2 are the coefficients of the continuous transfer function of the EVT discretized by pre-corrected bilinear transformation.

[0052] After obtaining the virtual digital electronic voltage transformer, step 3) uses the virtual digital electronic voltage transformer to process the current sampling signal, so that the current signal is also converted into a current signal in the differential domain;

[0053] The voltage signal output by the actual electronic voltage transformer is a voltage signal in the differential domain. Since both the voltage signal and the current signal are in the differential domain, the high-order harmonic components in the voltage and current will be amplified during the differential process, thereby affecting the accuracy of the protection calculation. Therefore, the voltage signal and current signal in the differential domain must be filtered.

[0054] In step 4), the voltage signal output by the electronic voltage transformer and the current signal output by the virtual digital electronic voltage transformer are filtered by low-pass filters respectively.

[0055] The present invention fully considers the influence of different frequencies of harmonic components contained in the fault voltage signal and current signal caused by different line lengths. The longer the line, the lower the frequency of the lowest high frequency in the harmonics, so a filter with a lower cutoff frequency is required. However, for the filter, the lower the cutoff frequency, the longer the data window. Therefore, in order to take into account both the filtering speed and the filtering effect, the present invention designs two low-pass filters with different cutoff frequencies. When the line length is greater than a certain value, a low-pass filter with a low cutoff frequency is selected, otherwise a low-pass filter with a high cutoff frequency is selected.

[0056] In the present invention, the line length is set to 100km. If the line length is greater than or equal to 100km, a filter with a cutoff frequency of 400Hz is selected; if the line length is less than 100km, a filter with a cutoff frequency of 700Hz is selected.

[0057] At the same time, the low-pass filter selected by the present invention is a FIR type low-pass filter, and the specific expression is:

[0058]

[0059] Where X is the sampled data, Y is the data output after the sampled data passes through the low-pass filter, n is the sampling number, b is the coefficient of the low-pass filter, and m is the number of coefficients of the filter.

[0060] The cut-off frequency of low-pass filter 1 is 400 Hz, and the filter coefficient, that is, b0~b m-1 The values ​​are: 0.0100, 0.016, 0.0272, 0.0403, 0.0548, 0.0692, 0.0819, 0.0914, 0.0965, 0.0965, 0.0914, 0.0819, 0.0692, 0.0548, 0.0403, 0.02712, 0.0163, 0.0100.

[0061] The cut-off frequency of low-pass filter 2 is 700 Hz, and the filter coefficient, that is, b0~b m-1 The values ​​are: 0.0147, 0.0401, 0.0784, 0.1207, 0.1541, 0.1668, 0.1541, 0.1207, 0.0784, 0.0401, 0.0147.

[0062] The amplitude-frequency response of the two low-pass filters is as follows: Figure 4 shown.

[0063] Step 5) Calculate the reactance using the voltage signal and current signal processed by the low-pass filter;

[0064] After the differential voltage signal output by the EVT and the differential current signal output by the virtual EVT are passed through a low-pass filter, the phasor values ​​of the differential voltage signal and the differential current signal are calculated using the full-cycle Fourier algorithm, and the impedance is calculated according to the fault type, where:

[0065] In case of single-phase fault, the impedance is calculated using the phase voltage and the phase current that compensates for the zero-sequence current. In case of single-phase fault, the impedance calculation formula for each phase is:

[0066]

[0067] in: is a single-phase voltage, is the corresponding single-phase current, is the zero sequence current.

[0068] Z0 and Z1 are the zero-sequence impedance and positive-sequence impedance per unit length of the line respectively.

[0069] When there is a phase-to-phase fault, the impedance is calculated using the phase-to-phase voltage and phase-to-phase current. When there is a phase-to-phase fault, the impedance calculation formula for each phase is:

[0070]

[0071] in: is the phase-to-phase voltage, is the corresponding phase-to-phase current.

[0072] The imaginary part of the calculated impedance is the calculated reactance.

[0073] 6) Compare the calculated reactance with the distance protection reactance setting value to determine whether the distance protection is activated. If the calculated reactance is less than the distance protection reactance setting value, the distance protection is activated, otherwise the distance protection is not activated.

[0074] In summary, the present invention provides a distance protection method suitable for differential electronic voltage transformers. The method first analyzes and models the circuit of the differential EVT, and discretizes the transfer function of its model, thereby obtaining its digital transfer model to construct a virtual digital EVT; secondly, the current signal is subjected to equal transfer processing through the virtual digital EVT; thirdly, the voltage signal in the differential domain output by the EVT and the current signal in the differential domain output by the virtual EVT are subjected to a low-pass filter to filter out high-order harmonics; finally, the voltage signal and current signal in the differential domain after the low-pass filter are used to construct distance protection. A distance protection method suitable for differential electronic voltage transformers of the present invention, the sampling loop of the electronic voltage transformer used does not contain an integral link, avoiding the influence of the integrator link on the accuracy. The invention is applicable to the circuit where the electronic voltage transformer outputs a differential signal, which can effectively avoid the adverse effects of the integrator performance, and ensure that the protection is reliably operated for faults within the zone and reliably inactive for faults outside the zone.

[0075] It should be understood that the above specific embodiments of the present invention are only used to illustrate or explain the principles of the present invention, and do not constitute a limitation of the present invention. Therefore, any modifications, equivalent substitutions, improvements, etc. made without departing from the spirit and scope of the present invention should be included in the protection scope of the present invention. In addition, the appended claims of the present invention are intended to cover all changes and modifications that fall within the scope and boundaries of the appended claims, or the equivalent forms of such scope and boundaries.

Claims

1. A distance protection method suitable for a differential electronic voltage transformer, characterized in that: The method comprises the following steps: 1) Analyze the circuit of differential electronic voltage transformer and establish a model; 2) constructing a virtual digital electronic voltage transformer based on the above model; 3) using the virtual digital electronic voltage transformer to process the current sampling signal to obtain a current signal in the differential domain; 4) using a low-pass filter to process the voltage signal and current signal output by the differential electronic voltage transformer; 5) Calculate the reactance using the voltage and current signals processed by the low-pass filter; 6) Compare the calculated reactance with the distance protection reactance setting value to determine whether the distance protection is activated; In the step 1), the circuit of the differential electronic voltage transformer is analyzed and a model is established. The specific model established is as follows: Where: s is a complex variable, C1 is the high voltage arm capacitance, C2 is the low voltage arm capacitance, R is the low voltage arm parallel resistance, R a is the load resistance; The step 2) of constructing a virtual digital electronic voltage transformer based on the above model specifically comprises: step 2.1) discretizing the model of the differential electronic voltage transformer using a pre-corrected bilinear transformation to obtain a discretized z transfer function; and step 2.2) obtaining a virtual digital electronic voltage transformer based on the discretized z transfer function; The step 4) also includes the following steps: selecting low-pass filters with different cut-off frequencies according to the line length, and the specific method includes: when the line length is greater than or equal to 100 km, selecting a first low-pass filter with a cut-off frequency of 400 Hz; when the line length is less than 100 km, selecting a second low-pass filter with a cut-off frequency of 700 Hz; The step 5) calculating the reactance using the voltage signal and the current signal processed by the low-pass filter includes: using a full-cycle Fourier algorithm to calculate the phasor values ​​of the voltage signal and the current signal processed by the low-pass filter, and using the phasor values ​​to calculate the reactance.

2. A distance protection method suitable for a differential electronic voltage transformer according to claim 1, characterized in that: Step 2.1) The model of the differential electronic voltage transformer is discretized by using a pre-corrected bilinear transformation, and the discretized z transfer function is obtained as follows: Wherein, z is the z-transformation operator, and a1, a2, b1, and b2 are coefficients obtained by discretizing the model of the differential electronic voltage transformer through a pre-corrected bilinear transformation.

3. A distance protection method suitable for a differential electronic voltage transformer according to claim 2, characterized in that: Step 2.2) obtaining a virtual digital electronic voltage transformer based on the discretized z transfer function specifically comprises the following steps: The form of the discretized z transfer function for the sampling point is used as a virtual digital electronic voltage transformer: Among them, X is the input signal of the virtual digital electronic voltage transformer, Y is the output signal of the virtual digital electronic voltage transformer, n is the sampling number, a1, a2, b1, b2 are the coefficients of the model of the differential electronic voltage transformer discretized by pre-corrected bilinear transformation.

4. A distance protection method applicable to a differential electronic voltage transformer according to any one of claims 1 to 3, characterized in that: The expression of the low-pass filter in step 4) is as follows: Among them, X is the sampling data, Y is the data output after the sampling data passes through the low-pass filter, n is the sampling sequence number, b is the coefficient of the low-pass filter, and m is the number of coefficients of the low-pass filter.

5. The distance protection method applicable to a differential electronic voltage transformer according to claim 1, characterized in that: The step 6) of comparing the calculated reactance with the distance protection reactance constant to determine whether the distance protection is activated includes: if the calculated reactance is less than the distance protection reactance constant, the distance protection is activated; otherwise, the distance protection is not activated.

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