阅读说明：本技术 一种计及变电站现场电磁干扰状况的接地网磁场检测系统及其检测方法 (Grounding grid magnetic field detection system considering electromagnetic interference condition of transformer substation site and detection method thereof ) 是由 范丽君 张翔 仇式鹍 袁辉 陈冠 朱洪志 王哲斐 贺润平 李亮亮 于 2021-09-28 设计创作，主要内容包括：本发明公开了一种计及变电站现场电磁干扰状况的接地网磁场检测系统,包括励磁电源、探测线圈、前置放大电路、-(50Hz)工频陷波电路、带通滤波器、后置放大电路、数据采集卡和上位机,励磁电源用于对接地网注入激励电流,产生激励磁场；探测线圈、前置放大电路、-(50Hz)工频陷波电路、带通滤波器、后置放大电路、数据采集卡和上位机依次相连；探测线圈、前置放大电路、-(50Hz)工频陷波电路和带通滤波器通过直流电源供电,直流电源和带通滤波器之间设置有-(STM32)微控制器。该检测系统具备较强的抗电磁干扰能力,能够在电磁环境复杂的变电站有效采集到测量所需的磁场信号。本发明还公开了一种计及变电站现场电磁干扰状况的接地网磁场检测系统的检测方法。(The invention discloses a grounding grid magnetic field detection system considering the on-site electromagnetic interference condition of a transformer substation, which comprises an excitation power supply, a detection coil, a preamplifier circuit, 50Hz The device comprises a power frequency trap circuit, a band-pass filter, a post-amplification circuit, a data acquisition card and an upper computer, wherein an excitation power supply is used for injecting excitation current into a grounding network to generate an excitation magnetic field; a detection coil, a pre-amplification circuit, 50Hz The power frequency trap circuit, the band-pass filter, the post-amplification circuit, the data acquisition card and the upper computer are sequentially connected; a detection coil, a pre-amplification circuit, 50Hz The power frequency trap circuit and the band-pass filter are powered by a direct current power supply, and a part between the direct current power supply and the band-pass filter is provided with STM32 A microcontroller. The detection system has strong anti-electromagnetic interference capability, and can effectively acquire magnetic field signals required by measurement in a transformer substation with a complex electromagnetic environment. The invention also discloses a detection method of the grounding grid magnetic field detection system considering the electromagnetic interference condition of the transformer substation site。)

1. The utility model provides a take into account ground net magnetic field detecting system of on-spot electromagnetic interference situation of transformer substation, its characterized in that includes excitation power, detection coil, preamplifier circuit, 50Hz power frequency trap circuit, band-pass filter, post amplifier circuit, data acquisition card and host computer, wherein:

the excitation power supply is connected with the grounding grid and used for injecting excitation current into the grounding grid to generate an excitation magnetic field;

the detection coil, the pre-amplification circuit, the 50Hz power frequency trap circuit, the band-pass filter, the post-amplification circuit, the data acquisition card and the upper computer are sequentially connected;

the detection coil, the pre-amplification circuit, the 50Hz power frequency trap circuit and the band-pass filter are powered by a direct current power supply, and an STM32 microcontroller is arranged between the direct current power supply and the band-pass filter;

the detection coil is used for receiving a magnetic field signal of an excitation magnetic field and transmitting the magnetic field signal to the pre-amplification circuit;

the preamplifier circuit is used for amplifying the received magnetic field signal and transmitting the amplified magnetic field signal to the 50Hz power frequency trap circuit;

the 50Hz power frequency trap circuit is used for adding 50Hz power frequency trap to the magnetic field signal processed by the pre-amplification circuit, completely filtering the 50Hz signal in the magnetic field signal after amplification processing, and transmitting the magnetic field signal after filtering processing to the band-pass filter;

the band-pass filter is used for filtering the magnetic field signal processed by the 50Hz power frequency trap circuit and transmitting the processed magnetic field signal to the post-amplification circuit;

the post-amplification circuit is used for performing compensation amplification on the magnetic field signal processed by the band-pass filter and transmitting the magnetic field signal subjected to compensation amplification to the data acquisition card;

the data acquisition card is used for transmitting the collected magnetic field signals to the upper computer through the USB interface;

the upper computer is used for observing the waveform of the magnetic field signal in real time, recording the magnetic field signal data and exporting the magnetic field signal data.

2. The grounding grid magnetic field detection system considering electromagnetic interference conditions of a substation site as claimed in claim 1, wherein the excitation power supply adopts a 33220A type function signal generator, the frequency interval is 300 Hz-1 kHz, and odd harmonic points of power frequencies of 350Hz, 450Hz and 550Hz are avoided.

3. The system for detecting the magnetic field of the grounding grid considering the electromagnetic interference condition of the substation field according to claim 1, wherein the detection coil is a close-wound planar rectangular coil; according to Faraday' S law of electromagnetic induction, the relationship between the magnitude of the induced electromotive force epsilon at the two ends of the detection coil and the magnitude of the magnetic induction intensity B of the magnetic field, the number of turns N of the detection coil and the cross-sectional area S of the cutting magnetic field of the detection coil is as follows:

wherein k is the product of N and S;is a magnetic flux;^tis time; b (t) is a function of the magnitude of magnetic induction B over time;

the pre-amplifying circuit adopts an AD620 amplifying circuit, and the gain and the R of the pre-amplifying circuit_GThe numerical calculation formula is as follows:

wherein k Ω is the unit of resistance; g is the gain value of the preamplifier circuit; r_GIs an external variable resistance value.

4. The system of claim 1, wherein the 50Hz power frequency notch circuit is a double T-notch circuit comprising an electrical power sourceContainer C₁Capacitor C₂Capacitor C₃Resistance R₁Resistance R₂Resistance R₃Resistance R₄Resistance R₅Operational amplifier A₁And operational amplifier A₂Wherein:

the capacitor C₁One terminal of (1), a capacitor C₂Resistance R₄Resistance R₃And a capacitor C₁Are connected in sequence, the capacitor C₁And a capacitor C₂Connected terminal, resistor R₅Capacitor C₃And the resistance R₄And a resistance R₃The connecting ends of the two are connected in sequence;

the capacitor C₂Resistance R₄And the operational amplifier A₂The positive phase input ends of the two are connected;

the operational amplifier A₂Output terminal of (1), resistor R₁And a resistance R₂Are connected in turn, the operational amplifier A₂Output terminal and resistor R₁And the operational amplifier A₂Is connected to the negative phase input terminal of the resistor R₂The other end of the first and second electrodes is grounded;

the resistor R₁And a resistance R₂And the operational amplifier A₁Is connected with the positive input end of the resistor R₅And a capacitor C₃And the operational amplifier A₁Is connected with the operational amplifier A₁The negative phase input end of the transformer is connected;

the capacitor C₁And a capacitor C₂Is equal in capacitance, said capacitor C₃The capacitance of (C) is₁Twice the capacitance of (c);

the resistor R₃And a resistance R₄Is equal to the resistance value of (A), the resistance R₃Is a resistance R₅Twice the resistance value of (c);

the capacitor C₁And a resistance R₃The connected end of the operational amplifier A forms the input end of the 50Hz power frequency trap circuit₂Output terminal of (1), resistor R₁And operational amplifier A₂The connected end of the negative phase input end of the voltage regulator forms the output end of the 50Hz power frequency trap circuit.

5. The system for detecting the magnetic field of the grounding grid considering the electromagnetic interference condition of the substation site as claimed in claim 1, wherein the band-pass filter adopts an LTC1068 type digital capacitor filter chip with an adjustable center frequency, and the center frequency of the band-pass filter is controlled by an external clock signal;

the post-amplification circuit adopts an LM358 amplification circuit;

the data acquisition card adopts a virtual oscilloscope with the model number of TLACTE-7104;

and ChengTecTool software is installed on the upper computer, and is used for realizing real-time observation of the waveform of the magnetic field signal, recording of magnetic field signal data and derivation of the magnetic field signal data.

6. The method for detecting the grounding grid magnetic field detection system considering the electromagnetic interference condition of the substation site in claim 1, comprising the following steps:

s1, injecting exciting current between the port and the node of the area above the grounding grid by the exciting power supply to generate an exciting magnetic field;

s2, the detection coil receives the magnetic field signal of the excitation magnetic field and transmits the magnetic field signal to the preamplification circuit;

s3, the pre-amplifier circuit amplifies the received magnetic field signal and transmits the amplified magnetic field signal to the 50Hz power frequency trap circuit;

s4, adding a 50Hz power frequency notch circuit into the magnetic field signal processed by the pre-amplification circuit, completely filtering the 50Hz signal in the magnetic field signal after amplification, and transmitting the magnetic field signal after filtering to a band-pass filter;

s5, the band-pass filter carries out filtering processing on the magnetic field signal processed by the 50Hz power frequency trap circuit, and transmits the processed magnetic field signal to the post-amplification circuit;

s6, the post-amplification circuit performs compensation amplification on the magnetic field signal processed by the band-pass filter and transmits the magnetic field signal after compensation amplification to the data acquisition card;

s7, the data acquisition card transmits the collected magnetic field signal to an upper computer through a USB interface;

and S8, observing the waveform of the magnetic field signal in real time through the upper computer, recording the magnetic field signal data and deriving the magnetic field signal data.

Technical Field

The invention relates to a grounding grid magnetic field detection system and a detection method thereof, wherein the detection system takes the electromagnetic interference condition of a transformer substation site into consideration.

Background

Interpretation of terms:

electromagnetic interference: a large amount of power equipment which can generate electromagnetic interference exists in the transformer substation, and the electromagnetic interference can affect the reliability and stability of the operation of a secondary system.

Magnetic field diagnosis: the electromagnetic field is induced on the earth surface by injecting exciting current between two grounding leads of the grounding grid, and the exciting electric field and the exciting magnetic field on the earth surface are calculated and analyzed by a moment method. When the grounding conductor buried underground fails, the electromagnetic field distribution on the earth surface above the grounding conductor is obviously different, so that the corrosion of the grounding network is diagnosed.

The grounding grid is generally buried in the position of about 0.5-2 m deep under the transformer substation, and plays an important role in guaranteeing the unification of reference potential of power equipment, evacuating leakage current and lightning current, and guaranteeing the personal safety and the equipment safety of the transformer substation. However, as the grounding grid in China is generally made of galvanized steel, the material is buried underground and is easy to corrode and fail under the action of moist soil and air, so that the grounding performance is reduced, and the system safety is endangered.

At present, the method for overhauling the grounding grid in practice in the power grid in China still mainly adopts regular large-area excavation, and on one hand, the method consumes a large amount of manpower and material resources and needs to be carried out for power-off overhauling, and is contrary to the economy and stability of long-term construction of a power system. On the other hand, due to different climatic conditions, soil environments and burying depths of various regions, the corrosion rates of the grounding conductors can have large differences, so that it is difficult to define a universal excavation and maintenance time.

Therefore, in recent years, researchers at home and abroad have been working on an excavation-free and online fault diagnosis method for a grounding grid. The basic principle of the method is to equate the grounding conductor to a multi-port pure resistive electric network, and the resistance value of the grounding conductor is solved by establishing a diagnostic equation for the electric network and solving a node voltage equation. The method has the advantages that the resistance value is only required to be measured between the grounding downlead, and the operation is convenient; the disadvantage is that the diagnostic result is highly dependent on the number and position of the down conductor, and once the number is small, the diagnostic equation is highly underdetermined.

And because the distribution of the magnetic field is not limited by the circuit topology, a magnetic field detection method can be introduced to realize accurate and intuitive fault diagnosis on the grounding network. The canadian scholars f.p.dawalibi first proposed this concept and the system proposed a magnetic field detection method based on this concept. The principle of this conventional magnetic field detection method can be summarized as that a detection magnetic field is formed on the ground surface above the ground conductor by injecting an excitation current from the ground down conductor, and the health of the ground conductor is reversely deduced by detecting the actual distribution of the magnetic induction intensity of the magnetic field. The method has the advantages of intuitive detection and no limitation on the number and position distribution of accessible nodes, but has the defects that the field magnetic field detection of a transformer substation is difficult and is easy to be interfered by electromagnetism, and the traditional magnetic field detection device cannot work in the environment.

Disclosure of Invention

The invention aims to overcome the defects of the prior art, and provides a grounding grid magnetic field detection system for calculating the electromagnetic interference condition of a transformer substation site.

Another object of the present invention is to provide a method for detecting a grounding grid magnetic field detection system, which takes into account electromagnetic interference conditions in a substation site.

One technical scheme for achieving the purpose is as follows: the utility model provides a take into account ground net magnetic field detecting system of on-spot electromagnetic interference situation of transformer substation, includes excitation power, detection coil, preamplifier circuit, 50Hz power frequency trap circuit, band-pass filter, post amplifier circuit, data acquisition card and host computer, wherein:

the excitation power supply is connected with the grounding grid and used for injecting excitation current into the grounding grid to generate an excitation magnetic field;

the detection coil, the pre-amplification circuit, the 50Hz power frequency trap circuit, the band-pass filter, the post-amplification circuit, the data acquisition card and the upper computer are sequentially connected;

the detection coil, the pre-amplification circuit, the 50Hz power frequency trap circuit and the band-pass filter are powered by a direct current power supply, and an STM32 microcontroller is arranged between the direct current power supply and the band-pass filter;

the detection coil is used for receiving a magnetic field signal of an excitation magnetic field and transmitting the magnetic field signal to the pre-amplification circuit;

the preamplifier circuit is used for amplifying the received magnetic field signal and transmitting the amplified magnetic field signal to the 50Hz power frequency trap circuit;

the 50Hz power frequency trap circuit is used for adding 50Hz power frequency trap to the magnetic field signal processed by the pre-amplification circuit, completely filtering the 50Hz signal in the magnetic field signal after amplification processing, and transmitting the magnetic field signal after filtering processing to the band-pass filter;

the band-pass filter is used for filtering the magnetic field signal processed by the 50Hz power frequency trap circuit and transmitting the processed magnetic field signal to the post-amplification circuit;

the post-amplification circuit is used for performing compensation amplification on the magnetic field signal processed by the band-pass filter and transmitting the magnetic field signal subjected to compensation amplification to the data acquisition card;

the data acquisition card is used for transmitting the collected magnetic field signals to the upper computer through the USB interface;

the upper computer is used for observing the waveform of the magnetic field signal in real time, recording the magnetic field signal data and exporting the magnetic field signal data.

The grounding grid magnetic field detection system considering the electromagnetic interference condition of the transformer substation site is characterized in that the excitation power supply adopts a 33220A type function signal generator, the frequency interval is 300 Hz-1 kHz, and odd harmonic points of power frequencies of 350Hz, 450Hz and 550Hz are avoided.

The above ground grid magnetic field detection system considering the electromagnetic interference condition of the substation site, wherein the detection coil is a close-wound planar rectangular coil; according to Faraday' S law of electromagnetic induction, the relationship between the magnitude of the induced electromotive force epsilon at the two ends of the detection coil and the magnitude of the magnetic induction intensity B of the magnetic field, the number of turns N of the detection coil and the cross-sectional area S of the cutting magnetic field of the detection coil is as follows:

wherein k is the product of N and S;is a magnetic flux; t is time; b (t) is a function of the magnitude of magnetic induction B over time;

the pre-amplifying circuit adopts an AD620 amplifying circuit, and the gain and the R of the pre-amplifying circuit_GThe numerical calculation formula is as follows:

wherein k Ω is the unit of resistance; g is the gain value of the preamplifier circuit;R_Gis an external variable resistance value.

In the above ground grid magnetic field detection system considering the electromagnetic interference condition on site of the transformer substation, the 50Hz power frequency trap circuit adopts a double T-type trap circuit, and includes a capacitor C₁Capacitor C₂Capacitor C₃Resistance R₁Resistance R₂Resistance R₃Resistance R₄Resistance R₅Operational amplifier A₁And operational amplifier A₂Wherein:

the capacitor C₁One terminal of (1), a capacitor C₂Resistance R₄Resistance R₃And a capacitor C₁Are connected in sequence, the capacitor C₁And a capacitor C₂Connected terminal, resistor R₅Capacitor C₃And the resistance R₄And a resistance R₃The connecting ends of the two are connected in sequence;

the capacitor C₂Resistance R₄And the operational amplifier A₂The positive phase input ends of the two are connected;

the operational amplifier A₂Output terminal of (1), resistor R₁And a resistance R₂Are connected in turn, the operational amplifier A₂Output terminal and resistor R₁And the operational amplifier A₂Is connected to the negative phase input terminal of the resistor R₂The other end of the first and second electrodes is grounded;

the resistor R₁And a resistance R₂And the operational amplifier A₁Is connected with the positive input end of the resistor R₅And a capacitor C₃And the operational amplifier A₁Is connected with the operational amplifier A₁The negative phase input end of the transformer is connected;

the capacitor C₁And a capacitor C₂Is equal in capacitance, said capacitor C₃The capacitance of (C) is₁Twice the capacitance of (c);

the resistor R₃And a resistance R₄Is equal to the resistance value of (A), the resistance R₃Is a resistance R₅Resistance ofTwice the value;

the capacitor C₁And a resistance R₃The connected end of the operational amplifier A forms the input end of the 50Hz power frequency trap circuit₂Output terminal of (1), resistor R₁And operational amplifier A₂The connected end of the negative phase input end of the voltage regulator forms the output end of the 50Hz power frequency trap circuit.

The above ground grid magnetic field detection system considering the electromagnetic interference condition of the substation site, wherein the band-pass filter adopts an LTC1068 type digital capacitive filter chip with an adjustable center frequency, and the center frequency of the band-pass filter is controlled by an external clock signal;

the post-amplification circuit adopts an LM358 amplification circuit;

the data acquisition card adopts a virtual oscilloscope with the model number being TLA CTE-7104;

and ChengTecTool (digital-analog mixed pocket instrument) software is installed on the upper computer, and the ChengTecTool software is used for realizing real-time observation of the waveform of the magnetic field signal, recording of the magnetic field signal data and derivation of the magnetic field signal data.

The invention also provides a detection method of the grounding grid magnetic field detection system considering the electromagnetic interference condition of the transformer substation site, which comprises the following steps:

s1, injecting exciting current between the port and the node of the area above the grounding grid by the exciting power supply to generate an exciting magnetic field;

s2, the detection coil receives the magnetic field signal of the excitation magnetic field and transmits the magnetic field signal to the preamplification circuit;

s3, the pre-amplifier circuit amplifies the received magnetic field signal and transmits the amplified magnetic field signal to the 50Hz power frequency trap circuit;

s4, adding a 50Hz power frequency notch circuit into the magnetic field signal processed by the pre-amplification circuit, completely filtering the 50Hz signal in the magnetic field signal after amplification, and transmitting the magnetic field signal after filtering to a band-pass filter;

s5, the band-pass filter carries out filtering processing on the magnetic field signal processed by the 50Hz power frequency trap circuit, and transmits the processed magnetic field signal to the post-amplification circuit;

s6, the post-amplification circuit performs compensation amplification on the magnetic field signal processed by the band-pass filter and transmits the magnetic field signal after compensation amplification to the data acquisition card;

s7, the data acquisition card transmits the collected magnetic field signal to an upper computer through a USB interface;

and S8, observing the waveform of the magnetic field signal in real time through the upper computer, recording the magnetic field signal data and deriving the magnetic field signal data.

The grounding grid magnetic field detection system and the grounding grid magnetic field detection method have strong anti-electromagnetic interference capability, can effectively acquire magnetic field signals required by measurement in a transformer substation with a complex electromagnetic environment, can realize the magnetic field detection system capable of detecting effective data in the transformer substation by selecting an excitation power supply with proper frequency and matching with proper trapped waves, band-pass circuits and amplifying circuits, can effectively overcome the influence of electromagnetic interference on the magnetic field detection method, and can realize fault diagnosis with higher precision.

Drawings

FIG. 1 is an equivalent model diagram of a grounding grid;

FIG. 2 is a schematic diagram of the magnetic field distribution in the vertical direction;

FIG. 3 is a diagram illustrating the results of a magnetic field detection fault branch;

FIG. 4 is a block diagram of a grid magnetic field detection system of the present invention that accounts for electromagnetic interference conditions at a substation site;

FIG. 5 is a topology diagram of a double T notch circuit;

fig. 6 is a graph of the frequency response characteristic of the filter circuit.

Detailed Description

In order that those skilled in the art will better understand the technical solution of the present invention, the following detailed description is given with reference to the accompanying drawings:

principle of magnetic field detection method for grounding grid

Referring to fig. 1, the grounding grid in our country is composed of a galvanized steel conductor and a grounding down conductor which are horizontally laid underground. Considering that the resistivity of soil is much greater than the conductor itself, soil is considered as an insulating medium. The down conductor is the only measurement port from the ground that can contact the grounded screen. The entire earth grid can therefore be regarded as a multi-port purely resistive electrical network whose basic unit is the resistance of the individual conductors.

In such an electrical network, the node corresponding to the ground down conductor is referred to as the reachable node, and the remaining nodes are referred to as unreachable nodes. From the characteristics of the electrical network, the port resistance of the network is a function of the resistances of all the branches, so that by measuring the port resistance of the network between the ground down-leads, the branch resistance can be inferred. However, in engineering practice, unreachable nodes are generally ubiquitous, and therefore the column-written port resistance diagnostic equation is underdetermined. At the moment, the method of the electric network cannot further accurately solve the branch resistance value, and the less the accessible nodes are, the lower the diagnostic reliability is.

In this case, the magnetic field detection method can be applied to break through the constraints imposed on the electric network method, considering that the distribution of the magnetic field is not constrained by the topology of the circuit.

The basic idea of the magnetic field detection method is to inject exciting current between the accessible nodes of the ports in the area above the grounding grid, so that the exciting current flows through all grounding conductor branches as much as possible, at the moment, the magnetic induction intensity distribution condition of the exciting magnetic field is depicted on the earth surface, and the magnetic induction intensity distribution condition is compared with the theoretical magnetic field magnetic induction intensity distribution, so that specific pathological branches can be found out. According to the biot law, the following can be obtained:

in the formula, B_ijThe magnetic induction intensity vector of the j section of conductor in the original grounding grid induced at the ith earth surface point; μ is the permeability in soil; i is_jIs the current vector in the j section conductor; dl (dl)_jIs a current infinitesimal in the conductor; e.g. of the type_rThe direction vector from the current infinitesimal to the ith earth surface point is obtained; z is a radical of_ijIs jthThe distance from the conductor element to the ith surface measurement point. Then the magnetic induction intensity of the ith earth surface measuring point is the vector sum of the magnetic induction intensities induced by all the b conductors, that is:

in the formula, B_iThe magnetic induction intensity induced for the ith earth surface measuring point; m_ijAnd (4) a magnetic induction operator of the j section conductor current at the ith earth surface measuring point.

Referring to fig. 2, a lot of simulation experiments show that the influence of the ground conductor fault on the earth surface magnetic field is summarized as follows: when a ground conductor fails, the magnetic induction component perpendicular to its excitation current direction is mainly affected. I.e. when a ground conductor P (conductor parallel to the X-axis) distributed along the X-axis fails, the magnitude of the current I flowing through the conductor_sWhen changed, the magnetic induction intensity B of the earth's surface_YThe components will vary greatly.

According to the rule of the influence of the grounding conductor fault on the earth surface magnetic field, the earth surface magnetic field of the grounding grid can be measured in large quantity, and a magnetic field distribution image can be described. And comparing the magnetic field with the calculated magnetic field of the earth surface of the intact grounding grid, thereby judging whether the grounding grid has a fault or not and determining the position of the fault grounding conductor. Referring to fig. 3, for example, in a certain grounding grid model, an exciting current is injected between the grounding down conductors at two ends of the grounding grid to respectively simulate the corrosion fault and the breakage fault of the branch R28, and the measured magnetic induction intensity perpendicular to the direction of R28.

Second, electromagnetic interference analysis of transformer substation

The grounding grid magnetic field detection method has clear principle and more intuitive detection result, but the main factor limiting the application of the method is the difficulty of field magnetic field measurement. Magnetic field detection in a laboratory environment can be generally realized by adopting a teslameter, and the teslameter is generally provided with a three-dimensional Hall probe to realize measurement of a space magnetic field. However, the electromagnetic environment of the substation site is complex, and the measurement method relying only on the teslameter has no signal selectivity, so that the method is seriously interfered, and the detection magnetic field to be measured is difficult to measure. Therefore, in order to design a hardware device capable of effectively acquiring a magnetic field signal, the electromagnetic environment of the substation is analyzed first.

Electromagnetic interference in a power system substation can be classified into interference generated by a primary system and interference generated by a secondary system according to the source. The electromagnetic interference generated at the primary side comprises steady-state electromagnetic interference and transient electromagnetic interference, the steady-state interference is generated by high voltage and large current of power frequency 50Hz, and comprises an electric field and a magnetic field which are widely present in the range near the equipment, and the frequency is 50Hz of the power frequency and harmonic waves of each odd multiple of the power frequency; the transient electromagnetic interference is mainly generated by lightning stroke, faults and changes of voltage and current at the moment of switching action, has complex components and generally contains a large amount of high-frequency interference. Electromagnetic interference generated by devices on the secondary side is mainly caused by high-frequency electromagnetic fields of several thousand Hz.

In the electromagnetic interference signals, the amplitude of the power frequency interference signal is the largest, and the amplitude of the odd harmonics of the power frequency interference signal is rapidly reduced along with the increase of the frequency. The interference signals exist all the time as long as the power system works normally and stably, and transient interference signals only appear in large quantities at the moment of lightning stroke, short circuit, overload and switching action, and have smaller amplitude under normal conditions.

Therefore, the corresponding magnetic field detection device should satisfy: (1) the capability of matching the frequency of an excitation power supply is required; (2) the method has the advantages that the method has very strong power frequency signal suppression capability; (3) the capability of suppressing high-frequency signal interference is required to be certain; (4) the measurement frequency of the system should not change the electrical network model of the ground conductor.

Referring to fig. 4, 5 and 6, in an embodiment of the present invention, a system for detecting a magnetic field of a ground grid in consideration of electromagnetic interference conditions in a transformer substation site includes an excitation power supply 1, a detection coil 2, a pre-amplification circuit 3, a 50Hz power frequency notch circuit 4, a band-pass filter 5, a post-amplification circuit 6, a data acquisition card 7 and an upper computer 8.

The excitation power supply 1 is connected with the grounding grid 100, and the excitation power supply 1 is used for injecting excitation current into the grounding grid 100 to generate an excitation magnetic field; the detection coil 2, the pre-amplification circuit 3, the 50Hz power frequency trap circuit 4, the band-pass filter 5, the post-amplification circuit 6, the data acquisition card 7 and the upper computer 8 are connected in sequence. The detection coil 2, the pre-amplification circuit 3, the 50Hz power frequency trap circuit 4 and the band-pass filter 5 are powered by a direct current power supply 9, and an STM32 microcontroller 10 is arranged between the direct current power supply 9 and the band-pass filter 5.

The detection coil 2 is used for receiving a magnetic field signal of the excitation magnetic field and transmitting the magnetic field signal to the preamplification circuit 3; the preamplification circuit 3 is used for amplifying the received magnetic field signal and transmitting the amplified magnetic field signal to the 50Hz power frequency trap circuit 4; the 50Hz power frequency trap circuit 4 is used for adding 50Hz power frequency trap to the magnetic field signal processed by the preamplifier circuit, completely filtering the 50Hz signal in the amplified magnetic field signal, and transmitting the magnetic field signal after filtering to the band-pass filter 5; the band-pass filter 5 is used for filtering the magnetic field signal processed by the 50Hz power frequency trap circuit and transmitting the processed magnetic field signal to the post-amplification circuit 6; the post-amplification circuit 6 is used for performing compensation amplification on the magnetic field signal processed by the band-pass filter and transmitting the magnetic field signal subjected to compensation amplification to the data acquisition card 7; the data acquisition card 7 is used for transmitting the collected magnetic field signals to the upper computer 8 through a USB interface; the upper computer 8 is used for observing the waveform of the magnetic field signal in real time, recording the magnetic field signal data and exporting the magnetic field signal data.

The invention also provides a detection method of the grounding grid magnetic field detection system considering the electromagnetic interference condition of the transformer substation site, which comprises the following steps:

s1, injecting exciting current between the port and the node of the area above the grounding grid by the exciting power supply to generate an exciting magnetic field;

s2, the detection coil receives the magnetic field signal of the excitation magnetic field and transmits the magnetic field signal to the preamplification circuit;

s3, the pre-amplifier circuit amplifies the received magnetic field signal and transmits the amplified magnetic field signal to the 50Hz power frequency trap circuit;

s4, adding a 50Hz power frequency notch circuit into the magnetic field signal processed by the pre-amplification circuit, completely filtering the 50Hz signal in the magnetic field signal after amplification, and transmitting the magnetic field signal after filtering to a band-pass filter;

s5, the band-pass filter carries out filtering processing on the magnetic field signal processed by the 50Hz power frequency trap circuit, and transmits the processed magnetic field signal to the post-amplification circuit;

s6, the post-amplification circuit performs compensation amplification on the magnetic field signal processed by the band-pass filter and transmits the magnetic field signal after compensation amplification to the data acquisition card;

s7, the data acquisition card transmits the collected magnetic field signal to an upper computer through a USB interface;

s8, observing the waveform of the magnetic field signal in real time through the upper computer, recording the magnetic field signal data and deriving the magnetic field signal data

1. Frequency selection of the excitation power supply:

the first step of the magnetic field detection link is the generation of a magnetic field, and the excitation of the magnetic field is realized by an external excitation power supply. According to the analysis of the electromagnetic environment of the transformer substation, the selection principle of the excitation power supply comprises the following steps:

(1) the frequency is different frequency, namely the excitation frequency avoids the power frequency of the transformer substation and each odd harmonic frequency point thereof, namely 50Hz, 150Hz, 250Hz, 350Hz and the like, so as to ensure that the measurement signal keeps the resolution ratio in the interference signals of various frequencies;

(2) the device can output stable alternating current under specific frequency, can ensure that the current output in a safe and reasonable range can generate a magnetic field with stronger magnetic induction intensity on the ground above a grounding grid, and can penetrate through soil, sand and other media with the thickness of 0.5-2 meters above a grounding conductor so as to be convenient for measurement;

(3) considering that when the excitation frequency is too high, the self-inductance, mutual inductance between the grounding conductors and the equivalent capacitance to the ground are no longer negligible, the excitation frequency is not too high for the purpose of not deviating from the detection of the resistance value of the conductor.

By combining the above analysis, the excitation power supply 1 adopts a 33220A type function signal generator, the frequency interval is 300 Hz-1 kHz, and odd harmonic points of power frequencies such as 350Hz, 450Hz, 550Hz and the like are avoided.

2. Designing a signal conditioning circuit:

the receiving probe of the magnetic field is realized by a special coil. Considering that the close-wound coil has higher coil coupling degree, smaller leakage inductance and smaller occupied volume, the detection coil adopts a close-wound planar rectangular coil and adopts the close-wound planar rectangular coil to realize magnetic field receiving. According to Faraday' S law of electromagnetic induction, the relationship between the magnitude of the induced electromotive force epsilon at the two ends of the detection coil and the magnitude of the magnetic induction intensity B of the magnetic field, the number of turns N of the detection coil and the cross-sectional area S of the cutting magnetic field of the detection coil is as follows:

wherein k is the product of N and S;is a magnetic flux; t is time; b (t) is a function of the magnitude of the magnetic induction B as a function of time.

The initial signal received by the detection coil 2 is generally weak, so that it is necessary to access the pre-amplification circuit 3 after the detection coil 2. In consideration of the requirements of high precision and low noise of magnetic field detection, the preamplifier circuit 3 is realized by an AD620 amplifier circuit. Gain and R of the entire system_GThe numerical calculation formula is as follows:

wherein k Ω is the unit of resistance; g is the gain value of the preamplifier circuit; r_GIs an external variable resistance value.

Considering that the electromagnetic field interference on the site of the transformer substation is most obvious as the interference of power frequency 50Hz, the 50Hz power frequency trap circuit is added in the echo receiving link to completely filter and process 50Hz signals, which is effective and necessary.

Referring to FIG. 5, the 50Hz power frequency trap circuit 4 is a double T-type trap circuit including a capacitor C₁Capacitor C₂Capacitor C₃Resistance R₁Resistance R₂Resistance R₃Resistance R₄Resistance R₅Operational amplifier A₁And operational amplifier A₂。

Capacitor C₁One terminal of (1), a capacitor C₂Resistance R₄Resistance R₃And a capacitor C₁Are connected in turn to a capacitor C₁And a capacitor C₂Connected terminal, resistor R₅Capacitor C₃And a resistance R₄And a resistance R₃The connecting ends of the two are connected in sequence; capacitor C₂Resistance R₄And the operational amplifier A₂The positive phase input ends of the two are connected; operational amplifier A₂Output terminal of (1), resistor R₁And a resistance R₂Are connected in turn, an operational amplifier A₂Output terminal and resistor R₁And the operational amplifier A₂Is connected to the negative phase input terminal of the resistor R₂The other end of the first and second electrodes is grounded; resistance R₁And a resistance R₂And the operational amplifier A₁Is connected with the positive input end of the resistor R₅And a capacitor C₃And the operational amplifier A₁Is connected with the operational amplifier A₁The negative phase input end of the transformer is connected; capacitor C₁And a capacitor C₂Equal in capacitance, capacitance C₃The capacitance of (C) is₁Twice the capacitance of (c); resistance R₃And a resistance R₄Are equal in resistance value of, the resistance R₃Is a resistance R₅Twice the resistance value of (c); capacitor C₁And a resistance R₃The connected end of the input end u forms the input end u of the 50Hz power frequency trap circuit_iAn operational amplifier A₂Output terminal of (1), resistor R₁And operational amplifier A₂The negative phase input end of the negative phase filter forms the output end u of the 50Hz power frequency trap circuit₀。

Capacitor C₁Capacitor C₂Resistance R₅And operational amplifierAmplifier A₂Forming a second-order high-pass filter circuit, a resistor R₃Resistance R₄Capacitor C₃And operational amplifier A₂A second-order low-pass filter circuit is formed, the second-order high-pass filter circuit and the second-order low-pass filter circuit are connected in parallel to form a double-T-shaped band-resistance filter circuit, and a resistor R₁Resistance R₂And operational amplifier A₁To form a negative feedback amplifying circuit. The basic principle of the double-T type notch circuit is that a double-T type band-stop filter is added with a negative feedback amplifying circuit, input voltage is simultaneously acted on a low-pass filter and a high-pass filter, and then output voltages of the two circuits are summed to obtain the band-stop filter. The cut-off frequency of the low-pass filter is smaller than that of the high-pass filter, so that the stop band of the circuit is the difference value of the cut-off frequencies of the high-pass filter and the low-pass filter. When the stopband of the bandstop filter is narrow, it is called a notch filter. The frequency response of the notch filter is to be equal to zero at the point where the 50Hz signal is removed; and at other frequencies, its value is equal to 1. And the voltage control negative feedback is added, so that the output plays a role opposite to the input, the error between the output of the double-T type trap circuit and the target is reduced, and the system tends to be stable.

The magnetic field signal is processed by the 50Hz power frequency trap circuit 4, the power frequency 50Hz signal is almost filtered, but the interference signals of other frequencies are not subjected to any inhibition treatment at present. Because harmonic interference signals of the transformer substation except for power frequency are complex in components and weak in amplitude, it is necessary to design a band-pass filter 5 with adjustable center frequency (the center frequency is the frequency of the excitation power supply). In consideration of the complexity of field measurement and the high requirement on the signal-to-noise ratio, the band-pass filter 5 is realized by using a digital capacitive filter chip of the LTC1068 type with better performance. The center frequency of the band-pass filter circuit designed by the chip is controlled by an external clock signal.

Referring to fig. 6, the frequency response curve of the filter circuit can be measured by setting the center frequency of the band-pass filter 5 to 1000 Hz.

Finally, in order to compensate the energy loss of the magnetic field signal in the above-mentioned link, an LM358 amplifying circuit is connected as a post-amplifying circuit 6 after the band-pass filter 5. The post-amplification circuit 6 is also designed to have high signal-to-noise ratio and adjustable amplification amplitude.

3. Signal recording and displaying:

in consideration of convenience of field measurement and data recording and processing, the data acquisition card 7 and the virtual oscilloscope with the model number of TLA CTE-7104 are used for transmitting the collected magnetic field signals to the PC end of the upper computer 8 through the USB interface, and the waveforms of the magnetic field signals can be observed in real time, the magnetic field signal data can be recorded and the magnetic field signal data can be derived by matching with corresponding upper computer ChengTeTool (digital-analog mixed pocket instrument) software.

The grounding grid magnetic field detection system considering the electromagnetic interference condition of the transformer substation on site completes the design of the whole magnetic field detection hardware circuit, has stronger anti-electromagnetic interference capability, and can effectively acquire the magnetic field signal required by measurement in the transformer substation with a complex electromagnetic environment.

The invention also provides a grounding grid magnetic field detection method considering the electromagnetic interference condition of the transformer substation site and provides a hardware design scheme. Transient interference comes from switching action or secondary equipment when a magnetic field detection method detects a grounding grid fault, high-frequency components are taken as main components, and transient interference signals in the magnetic field detection method are obtained by analyzing on-site steady electromagnetic interference of a transformer substation and power frequency interference and odd harmonic components thereof. By selecting an excitation power supply with proper frequency and matching with proper notch, band-pass and amplifying circuits, the magnetic field detection hardware design capable of detecting effective data on the site of a transformer substation can be realized, the influence of electromagnetic interference on the magnetic field detection method can be effectively overcome, and higher-precision fault diagnosis is realized.

On the basis of a magnetic field detection method, the electromagnetic interference distribution condition of a transformer substation site is analyzed, a magnetic field detection device capable of receiving an inverted magnetic field with specific frequency and suppressing an interference electromagnetic field is designed and manufactured, and a barrier of the magnetic field detection method in the actual field application is broken through. Firstly, modeling analysis is carried out on a grounding network, the basic principle of a magnetic field detection method is researched, and the theoretical superiority and the biggest problem in the application level of the magnetic field detection method are analyzed, namely the problem that the electromagnetic interference on a transformer substation site is serious and the traditional magnetic field detection equipment cannot work effectively is solved. Secondly, the generation reason, the distribution characteristics and the influence caused by the on-site electromagnetic field interference of the transformer substation are analyzed, and the design requirements on the hardware detection device are obtained according to the characteristics. And then, designing and manufacturing the whole grounding grid magnetic field detection system considering the electromagnetic interference condition of the transformer substation site according to the design requirements, wherein the grounding grid magnetic field detection system comprises a proper excitation power supply, a proper signal receiving and conditioning circuit, a display circuit and the like. Finally, experiments are carried out by utilizing the grounding network model, and the effectiveness and the accuracy of the whole detection method and system are verified.

According to the grounding grid magnetic field detection system and the detection method thereof considering the electromagnetic interference condition of the transformer substation site, the influence of leakage current and excitation source frequency on conductor impedance can be considered to calculate the grounding grid surface magnetic field, and the key point of the development of the method is that the number of measurement points is greatly reduced while the diagnosis precision is ensured.

In summary, according to the grounding grid magnetic field detection system and the detection method thereof considering the electromagnetic interference condition of the transformer substation site, transient interference signals in a magnetic field detection method are obtained by analyzing the steady-state electromagnetic interference of the transformer substation site and power frequency interference and odd harmonic components thereof, and by selecting an excitation power supply with a proper frequency and matching with a proper trap, band pass and amplifying circuit, the magnetic field detection system capable of detecting effective data on the transformer substation site can be realized, the influence of the electromagnetic interference on the magnetic field detection method can be effectively overcome, and fault diagnosis with higher precision can be realized.

It should be understood by those skilled in the art that the above embodiments are only for illustrating the present invention and are not to be used as a limitation of the present invention, and that changes and modifications to the above described embodiments are within the scope of the claims of the present invention as long as they are within the spirit and scope of the present invention.