CN103197178A - Device and method of on-line monitoring of transformer direct current magnetic bias - Google Patents

Abstract

The invention belongs to the field of transformer on-line monitoring, and relates to an intelligent transformer DC bias on-line monitoring device and a monitoring method thereof. The present invention is a core board in which a through-hole Hall current sensor is arranged outside the device casing, and a filter circuit, a voltage signal conditioning circuit, a gear switching circuit, a real-time clock system, a FLASH storage system and a controller system are arranged in the device casing above; the display screen is set on the device shell, and the display screen and the FLASH storage system are connected to the controller system. The invention can be used for on-line monitoring of transformer neutral point grounding current, wherein the monitoring device has the functions of real-time monitoring of transformer neutral point grounding current data and over-limit alarm function, and the alarm threshold can be defined by the user. When an abnormal situation occurs, the device's Both the display screen and the remote monitoring computer will send out alarm signals, so that the staff can take timely measures to avoid transformer failures and ensure the stable operation of the power system.

Description

Transformer DC Bias Online Monitoring Device and Monitoring Method

technical field

The invention belongs to the field of transformer on-line monitoring, and relates to an intelligent transformer DC bias on-line monitoring device and a monitoring method thereof.

Background technique

With the development of economy, high voltage direct current (HVDC) transmission system is getting more and more widely used in our country. Compared with the AC transmission system, the DC transmission has the characteristics of low cost, small loss, high withstand voltage, large capacity, and can realize asynchronous networking of the power system. In order to achieve cross-regional, long-distance and large-capacity power transmission, more ultra-high voltage and even ultra-high voltage DC projects will be built and put into operation.

However, the grid structure of AC and DC mixed operation makes the interaction between AC and DC systems more obvious. When the DC transmission system runs unbalanced on one pole or two poles, the DC current will be injected into the ground through the ground electrode, and the DC current in the ground will invade the AC system through the neutral point ground wire of the transformer, which will have an impact on the AC system. The most significant impact is Cause transformer DC bias phenomenon. In addition, the geomagnetic "storm" generated by the interaction between the dynamic changes of the solar plasma wind and the geomagnetic field will cause a geomagnetic induction current, and the geomagnetic induction current will also generate a DC bias in the center point grounding transformer.

The DC current flows through the transformer winding, and a certain DC magnetic flux is generated inside the transformer core, which makes the positive and negative half cycles of the excitation current obviously asymmetrical, and the magnetic flux shifts, resulting in a DC bias. DC bias will lead to a sharp increase in the excitation current of the transformer, causing overheating of the magnetic circuit and increased noise and vibration. In the case of DC bias, the frequency spectrum of the transformer noise has obvious odd wave noise, and the problems caused by the high vibration may cause the loosening of the relevant parts in the transformer, and then threaten the safe operation of the transformer.

Therefore, it is necessary to monitor the DC bias to provide a basis for future control to eliminate the bias. Install a neutral point DC current monitoring device on the neutral point lead of the relevant main transformer to monitor the DC current caused by factors such as DC transmission and solar magnetic storms, so as to grasp the occurrence and current value of the DC bias current in time, and provide Take measures to limit the DC bias current to create the necessary conditions.

Contents of the invention

Aiming at the problems existing in the above-mentioned prior art, the present invention provides a transformer DC bias on-line monitoring device and its monitoring method. The occurrence status and current value of the phenomenon are convenient to grasp the law of the occurrence of the DC bias phenomenon, and create necessary conditions for taking measures to limit the DC bias current.

The present invention is achieved through the following technical solutions:

The transformer DC bias online monitoring device is set outside the device shell by a through-hole Hall current sensor, and the filter circuit, voltage signal conditioning circuit, gear switching circuit, real-time clock system, FLASH storage system and controller system are set on the The core board in the device shell; the display screen is set on the device shell, and the display screen and FLASH storage system are connected to the controller system;

The through-hole Hall current sensor is connected to the device shell through a cable, and then connected to the gear switching circuit inside the device shell through the device shell, and the gear switching circuit is connected to the filter circuit and the voltage signal conditioning circuit, and then connected to the real-time clock The system is connected to the controller system.

The through-hole Hall current sensor is connected with the aviation plug on the device shell through a cable.

The controller system is also provided with a data communication system.

The through-hole Hall current sensor is in the form of opening installation.

There are three switching ranges of the gear switching circuit, which are 100mA-1A, 1A-10A and 10A-50A.

The through-hole Hall current sensor is fastened by a sensor opening fastening screw to fasten two half-block-structured sensors.

The transformer DC bias online monitoring method is to reflect the current flowing through the transformer winding through online monitoring of the current flowing through the transformer neutral point grounding lead without changing the neutral point grounding mode of the transformer, and then reflect the DC bias of the transformer. Magnetic degree, the specific steps are as follows:

(1) Under the premise of not disconnecting the neutral point grounding lead of the transformer, use a hole-through Hall current sensor set on the neutral point grounding lead of the transformer to obtain the current signal flowing through the neutral point grounding lead of the transformer ;

(2) Transform the neutral point grounding current signal of the transformer into a voltage signal through the through-hole Hall current sensor;

(3) After the voltage signal is processed by the filter circuit, conditioning circuit and gear switching circuit, it is sent to the analog-to-digital conversion circuit of the controller for calculation and processing, and the digital transformer neutral point grounding current is obtained;

(4) Display the digital signal in step (3) on the display screen of the device, and compare it with the allowable value of the transformer neutral point grounding current set in advance, if it is greater than the allowable value, an alarm signal will be displayed on the display screen at the same time ;

(5) At the same time as step (4), save the current measurement result and time to the FLASH memory of the device, so as to facilitate subsequent analysis of the DC bias status of the transformer;

(6) Upload the real-time data or historical data calculated in step (3) to the background monitoring computer or comprehensive monitoring unit or IED through the RS485 bus, and the computer or IED configures various related parameters of the monitoring device through the communication protocol to meet different On-line monitoring requirements for type transformers.

The method for measuring the DC current flowing through the neutral point grounding lead of the transformer is as follows:

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; K</span>}}\mathrm{<span\; class="notranslate">\; R</span>}$

Among them, I ₁ is the DC current on the grounding lead of the neutral point of the transformer;

K is the built-in amplification factor of the through-hole Hall current sensor;

R is the built-in sampling resistor of the through-hole Hall current sensor;

U ₁ samples the output voltage signal for the through-hole Hall current sensor.

K and R can take different values by customizing feed-through Hall current sensors to meet different types of transformers to meet various needs.

The specific implementation method of obtaining the digital transformer neutral point grounding current is as follows:

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

根据U₂的大小，档位切换电路自动选择不同的量程，N的数值随之变化；Among them, U ₂ is the voltage value amplified by N times after U ₁ passes through the filter circuit and conditioning circuit,

According to the size of _U2 , the gear switching circuit automatically selects different ranges, and the value of N changes accordingly;

Set the reference voltage of the AD sampling circuit of the controller as U _ref , and switch between different ranges by adjusting the value of N to ensure that U ₂ always falls within the interval [0, U _ref ];

Through AD conversion, the value of voltage U ₂ is obtained, and through multiple average processing and reverse derivation, the value of the neutral point grounding current of the transformer is obtained, that is

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; KR</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; R</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}$

The gear switching circuit can realize 100mA-1A, 1A-10A and 10A-50A.

Advantage of the present invention and beneficial effect are:

The invention can be used for on-line monitoring of transformer neutral point grounding current, wherein the monitoring device has the functions of real-time monitoring of transformer neutral point grounding current data and over-limit alarm function, and the alarm threshold can be defined by the user. When an abnormal situation occurs, the device's Both the display screen and the remote monitoring computer will send out alarm signals, so that the staff can take timely measures to avoid transformer failures and ensure the stable operation of the power system.

Through the implementation of the technical scheme of the invention, accurate and stable real-time measurement of the neutral point grounding current of the transformer can be truly realized. The design method of the present invention has the following innovations:

1. Innovation of measurement methods:

1. This device measures the current signal on the neutral point grounding wire by using a high-precision, open-mounted through-hole Hall current sensor set on the neutral point grounding lead of the transformer. This method can truly reflect the neutral point of the transformer. The generation and value of the DC current on the point grounding lead, the device is not electrically connected to the transformer, and there is no need to disassemble the neutral point grounding lead, which will not have any impact on the operation of the transformer;

2. The electrical signal output by the through-hole Hall current sensor is directly connected to the measuring device, and the obtained electrical signal is analyzed and calculated by digital signal processing technology, and the calculation result is stored and displayed. Operators can view measurement data and alarm information on the spot.

3. When the transformer is working normally, the grounding current at the neutral point is very small. Different transformers allow different grounding currents to flow through the neutral point according to the core material, design value of magnetic flux density, and transformer structure. It is about one ampere to several amperes, but when the DC bias phenomenon occurs, the neutral point grounding current of the transformer will increase, and it will become several amperes to more than ten amperes, and the range of variation is very large.

The device is also designed with a current range shift switch. According to the range of the current change, the device can automatically adjust the appropriate gear to measure the current, ensuring the accuracy of the measurement and the safety of the measurement device. The maximum current measurement range of this device is 50A, and the measurement accuracy is 1%, which fully meets the requirements of different types of transformer neutral point grounding current measurement range and accuracy.

2. Device structure and installation method: The measurement, display and storage parts of this device are installed in a device shell made of cast aluminum material, and a display window is opened on the top cover of the device shell for installing a display screen, which is convenient for on-site observation of current data. The shell part of the device is grounded separately for shielding external electromagnetic interference.

The device has a compact structure design and a beautiful appearance. In terms of installation method, it adopts the method of directly installing near the neutral point grounding lead of the transformer through the pendant of the device shell. The installation method is simple and safe, and it is convenient for operators to install, use and maintain.

1. The device has its own data display function, which displays time information and real-time current data on the display screen, which is convenient for operators to observe during inspections. When the monitoring data exceeds the limit, the display will display an alarm message to remind the inspectors to pay attention.

2. The device has a data storage function, which saves time information and real-time current data in the FLASH storage system that comes with the device. The stored data will not be lost when the power is turned off, and it can continuously save the measurement data for more than one year. It provides a basis for subsequent analysis and calculation of the historical data of the neutral point grounding current of the transformer, and understanding of the change of the neutral point grounding current.

3. The device supports RS-232/485 communication interface, which can realize multi-point monitoring based on RS485 bus. The MODBUS-RTU communication protocol with good versatility in the industrial field is adopted, and the monitoring data and monitoring time information can be uploaded to the computer or the integrated monitoring unit (IED) through the communication protocol.

4. The device parameters (such as device address, baud rate, parity, sampling time interval, over-limit alarm value, clock time, etc.) can be flexibly configured through the computer or the integrated monitoring unit (IED), which is convenient for the device to access other online Monitoring system to meet the requirements of different sites.

5. The device has the function of time synchronization with the computer or integrated monitoring unit (IED), which ensures the accuracy and consistency of the device monitoring time and the actual running time of the equipment.

6. Electromagnetic compatibility and protection circuit design. Because the electromagnetic environment of the substation site is very harsh, high voltage and strong magnetic field can easily make the monitoring equipment work abnormally, so the electromagnetic compatibility of the device has been carefully designed. Through the verification of the electromagnetic compatibility test, the device fully complies with the State Grid Corporation's standard requirements for equipment electromagnetic compatibility, including electrostatic discharge, electrical fast transient bursts, surge impact, etc., which improves the safety and reliability of the device.

7. The device of the present invention has beautiful appearance, simple operation, convenient installation, and stable operation. The Hall current sensor can be customized according to the shape of the neutral point lead, and is suitable for installation and use on different sites.

The invention has the functions of real-time on-line monitoring of transformer neutral point grounding current, data storage, local display and remote transmission, over-limit alarm, system time synchronization and parameter configuration. With the popularization of direct current transmission, the application of the present invention is more and more extensive, applicable to many different types of transformers, providing basis and certain guarantee for the normal operation of the neutral point grounding transformer, and providing a basis for the analysis of the neutral point grounding transformer DC The generation law and numerical value of the magnetic bias phenomenon provide data support.

Description of drawings

The present invention will be described in further detail below in conjunction with the accompanying drawings and specific embodiments.

Fig. 1 is a schematic diagram of the working principle of the present invention;

Fig. 2 is a functional block diagram of the present invention;

Fig. 3 is a schematic diagram of the networking structure of the present invention;

Fig. 4 is a schematic diagram of the internal structure of the present invention;

Fig. 5 is a schematic diagram of the external structure of the present invention;

Fig. 6 is a schematic diagram of the external three-dimensional structure of the present invention;

Fig. 7 is a schematic structural diagram of a through-hole Hall current sensor in the present invention.

In the figure: device shell 1, aviation plug 2, core board 3, display screen 4, through-hole Hall current sensor 5, sensor opening fastening screw 6.

Detailed ways

The present invention is a transformer DC bias online monitoring device and its monitoring method, as shown in Figure 2, which is a functional structural block diagram of the line monitoring device of the present invention; and as shown in Figures 4-7, Figures 4 and 5 , FIG. 6 and FIG. 7 are respectively a schematic diagram of the internal structure, a schematic diagram of the external structure, a schematic diagram of the external three-dimensional structure and a schematic diagram of the structure of the through-hole Hall current sensor of the online monitoring device of the present invention.

The online monitoring device of the present invention includes an openable through-hole type Hall current sensor 5, a filter circuit, a gear switching circuit, a voltage signal conditioning circuit, a real-time clock system, a FLASH storage system, an EEPROM storage system, a controller system, and a display screen and data communication systems.

The monitoring device of the present invention is provided with an openable through-hole type Hall current sensor installed on the outside of the device housing 1, a filter circuit, a voltage signal conditioning circuit, a gear switching circuit, a real-time clock system, a FLASH storage system and a controller. The system is set on the core board 3 inside the device shell 1 ; the display screen 4 is set on the device shell 1 . The display screen and the FLASH storage system are connected to the controller system; wherein the through-hole Hall current sensor 5 is connected to the aviation plug 2 on the device shell 1 through a cable, and the aviation plug 2 is connected to the gear switching circuit inside the device shell 1, The gear switching circuit is connected to the filter circuit and the voltage signal conditioning circuit, and then connected to the controller system with the real-time clock system. The controller system is also provided with a data communication system. There are three switching ranges of the gear switching circuit, which are 100mA-1A, 1A-10A and 10A-50A.

In addition, the power supply cable and communication cable of the device are connected to the device through the aviation plug 2, which on the one hand ensures reliable power supply and communication, and on the other hand realizes waterproof and dustproof protection for the inside of the device. The through-hole Hall current sensor 5 can be opened, and the through-hole Hall current sensor 5 can be set on the neutral point grounding lead of the transformer without disconnecting the transformer neutral point grounding lead, which is convenient for installation. The measurement accuracy of the through-hole Hall current sensor needs to use the sensor opening fastening screw 6 to fasten the two half-block sensors.

When the on-line monitoring device is specifically implemented, the open hole-through Hall current sensor is set on the neutral point grounding lead of the transformer, and the secondary signal output by the through-hole Hall current sensor is connected to the measurement circuit of the device. The measurement circuit consists of It is composed of a filter circuit, a gear switching control circuit and a voltage signal conditioning circuit. The measurement circuit sends the adjusted analog signal to the controller for calculation and processing, and the calculation result is displayed on the display screen and stored in the FLASH memory system to provide data support for the subsequent analysis of the law of the DC bias phenomenon of the transformer. In addition, this online monitoring device can be connected with the online monitoring integrated monitoring unit (IED) or a computer, and upload the measurement results through the MODBUS communication protocol. parameters to configure.

As shown in Figure 3, Figure 3 is a schematic diagram of the network structure of the present invention, a plurality of transformer DC bias online monitoring devices at the bottom realize data communication with the online monitoring IED and the online monitoring computer through the RS-485 bus, the online monitoring The device can upload real-time data and alarm information to the online monitoring IED and the online monitoring computer. At the same time, the online monitoring IED and the online monitoring computer can read the measurement data of the underlying device and configure the underlying device.

As shown in Figure 1, Figure 1 is a schematic diagram of the working principle of the present invention. The DC bias current value of the transformer is obtained by putting the through-hole Hall current sensor on the neutral point grounding lead of the transformer, and the device collects the current signal, Computational processing, local storage and display, and communication with the online monitoring IED or monitoring computer through the data communication system.

The specific working process of the present invention is as follows: the through-hole Hall current sensor sends the collected neutral point current signal to the filter circuit and the gear switching circuit, and then transmits it to the voltage signal conditioning circuit after being adjusted to a suitable range, and then through the control The analog-to-digital conversion circuit of the device calculates and processes this signal. On the one hand, the calculated results are displayed on the display screen of the device, and on the other hand, they are stored in the FLASH storage system together with the acquisition time generated by the real-time clock system for subsequent analysis. It provides a basis for the change of neutral ground current. The device can preset the alarm value of the neutral point grounding current. When the calculation result exceeds the preset alarm value, the alarm information will be displayed locally on the device on the one hand, and transmitted to the background monitoring computer on the other hand to remind the operator to take measures to deal with it. Prevent transformer failures from occurring. In addition to uploading the alarm information to the monitoring computer or integrated monitoring unit or IED through the RS485 bus, the data communication system will also upload the real-time data calculated by the controller or the historical data stored in FLASH, and the computer or IED can communicate The protocol configures various parameters of the monitoring device to meet the needs of transformers with different specifications.

The transformer DC bias online monitoring method is to reflect the current flowing through the transformer winding through online monitoring of the current flowing through the neutral point grounding lead of the transformer without changing the neutral point grounding mode of the transformer, and then reflect the DC current of the transformer. The degree of magnetic bias, the specific steps are as follows:

(1) Under the premise of not disconnecting the neutral point grounding lead of the transformer, use a hole-through Hall current sensor set on the neutral point grounding lead of the transformer to obtain the current signal flowing through the neutral point grounding lead of the transformer ;

(2) Transform the neutral point grounding current signal of the transformer into a voltage signal through the through-hole Hall current sensor;

(3) After the voltage signal is processed by the filter circuit, conditioning circuit and gear switching circuit, it is sent to the analog-to-digital conversion circuit of the controller for calculation and processing, and the digital transformer neutral point grounding current is obtained;

(4) Display the digital signal in step (3) on the display screen of the device, and compare it with the allowable value of the transformer neutral point grounding current set in advance, if it is greater than the allowable value, an alarm signal will be displayed on the display screen at the same time ;

(5) At the same time as step (4), save the current measurement result and time to the FLASH memory of the device, which is convenient for subsequent analysis of the DC bias status of the transformer.

(6) Upload the real-time data or historical data calculated in step (3) to the background monitoring computer or comprehensive monitoring unit or IED through the RS485 bus, and the computer or IED configures various related parameters of the monitoring device through the communication protocol to meet different On-line monitoring requirements for type transformers.

The method described therein for measuring the DC current flowing through the neutral ground lead of a transformer is as follows:

${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; K</span>}}\mathrm{<span\; class="notranslate">\; R</span>}$

Among them, I ₁ is the DC current on the grounding lead of the neutral point of the transformer;

K is the built-in amplification factor of the through-hole Hall current sensor;

R is the built-in sampling resistor of the through-hole Hall current sensor;

U ₁ samples the output voltage signal for the through-hole Hall current sensor.

K and R can take different values by customizing feed-through Hall current sensors to meet different types of transformers to meet various needs.

The specific implementation method of obtaining the digital transformer neutral point grounding current described therein is as follows:

${\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="u"\; filenames="130321104101.png"\; state="\{\{state\}\}">u</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="R"\; filenames="130321104101.png"\; state="\{\{state\}\}">R</figure-callout></span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}$

Among them, U ₂ is the voltage value amplified by N times after U ₁ passes through the filter circuit and conditioning circuit, According to the size of _U2 , the gear switching circuit automatically selects different ranges, and the value of N changes accordingly;

Set the reference voltage of the AD sampling circuit of the controller as U _ref , and switch between different ranges by adjusting the value of N to ensure that U ₂ always falls within the interval [0, U _ref ];

Through AD conversion, the value of voltage U ₂ is obtained, and through multiple average processing and reverse derivation, the value of the neutral point grounding current of the transformer is obtained, that is

${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; KR</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; <figure-callout\; id="2"\; label="R\; u"\; filenames="130321104101.png"\; state="\{\{state\}\}">R</figure-callout></span><figure-callout\; id="2"\; label="R\; u"\; filenames="130321104101.png"\; state="\{\{state\}\}">\; </figure-callout>}}{\mathrm{<span\; class="notranslate">\; </span>}}_{}{\mathrm{<span\; class="notranslate">u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}$

The gear switching circuit described in the present invention can realize 100mA-1A, 1A-10A and 10A-50A.

In the laboratory environment, the measured data are shown in Table 1.

Table 1 Laboratory measurement data

Once applied current source (A) Device display result (A) device display time 0.50 0.50 2012-9-1610:08 1.00 1.01 2012-9-1610:10 2.00 1.99 2012-9-1610:12 5.00 5.03 2012-9-1610:14 8.00 7.95 2012-9-1610:16 10.00 9.97 2012-9-1610:18 15.00 15.12 2012-9-1610:20

Claims (10)

1. Transformer DC bias online monitoring device, characterized in that: The through-hole Hall current sensor (5) is set outside the device shell (1), and the filter circuit, voltage signal conditioning circuit, gear switching circuit, real-time clock system, FLASH storage system and controller system are set on the device shell (1 ) on the core board (3); the display screen (4) is set on the device casing (1), and the display screen and the FLASH storage system are connected to the controller system; The through-hole Hall current sensor is connected to the device shell (1) through a cable, and then connected to the gear switching circuit inside the device shell (1) through the device shell (1), and the gear switching circuit is connected to the filter circuit and The voltage signal conditioning circuit is then connected to the controller system with the real time clock system. 2. The transformer DC bias online monitoring device according to claim 1, characterized in that: the through-hole Hall current sensor (5) is connected to the aviation plug (2) on the device casing (1) through a cable . 3. The transformer DC bias online monitoring device according to claim 1, characterized in that: said controller system is also provided with a data communication system. 4. The transformer DC bias online monitoring device according to claim 1, characterized in that: the through-hole Hall current sensor (5) is installed in an opening form. 5 . The transformer DC bias online monitoring device according to claim 1 , characterized in that there are three switching ranges of the gear switching circuit, namely 100mA-1A, 1A-10A and 10A-50A. 6. The transformer DC bias on-line monitoring device according to claim 1, characterized in that: said through-hole Hall current sensor (5) is composed of two halves of a fastening screw (6) at the sensor opening The sensor is tightly connected. 7. The transformer DC bias online monitoring method is characterized in that: under the premise of not changing the neutral point grounding mode of the transformer, the current flowing through the transformer winding is reflected by online monitoring of the current flowing through the neutral point grounding lead of the transformer, and then To reflect the DC bias degree of the transformer, the specific steps are as follows: (1) Under the premise of not disconnecting the neutral point grounding lead of the transformer, use a hole-through Hall current sensor set on the neutral point grounding lead of the transformer to obtain the current signal flowing through the neutral point grounding lead of the transformer ; (2) Transform the neutral point grounding current signal of the transformer into a voltage signal through the through-hole Hall current sensor; (3) After the voltage signal is processed by the filter circuit, conditioning circuit and gear switching circuit, it is sent to the analog-to-digital conversion circuit of the controller for calculation and processing, and the digital transformer neutral point grounding current is obtained; (4) Display the digital signal in step (3) on the display screen of the device, and compare it with the allowable value of the transformer neutral point grounding current set in advance, if it is greater than the allowable value, an alarm signal will be displayed on the display screen at the same time ; (5) At the same time as step (4), save the current measurement result and time to the FLASH memory of the device, so as to facilitate subsequent analysis of the DC bias status of the transformer; (6) Upload the real-time data or historical data calculated in step (3) to the background monitoring computer or comprehensive monitoring unit or IED through the RS485 bus, and the computer or IED configures various related parameters of the monitoring device through the communication protocol to meet different On-line monitoring requirements for type transformers. 8. The transformer DC bias online monitoring method according to claim 7, characterized in that: The method for measuring the DC current flowing through the neutral point grounding lead of the transformer is as follows: ${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{\mathrm{<span\; class="notranslate">\; K</span>}}\mathrm{<span\; class="notranslate">\; R</span>}$ Among them, I ₁ is the DC current on the grounding lead of the neutral point of the transformer; K is the built-in amplification factor of the through-hole Hall current sensor; R is the built-in sampling resistor of the through-hole Hall current sensor; U ₁ samples the output voltage signal for the through-hole Hall current sensor. K and R can take different values by customizing feed-through Hall current sensors to meet different types of transformers to meet various needs. 9. The transformer DC bias online monitoring method according to claim 7, characterized in that: The specific implementation method of obtaining the digital transformer neutral point grounding current is as follows: ${\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}<span\; class="notranslate">\; =</span>{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\frac{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}$ Among them, U ₂ is the voltage value amplified by N times after U ₁ passes through the filter circuit and conditioning circuit,

According to the size of _U2 , the gear switching circuit automatically selects different ranges, and the value of N changes accordingly; Set the reference voltage of the AD sampling circuit of the controller as U _ref , and switch between different ranges by adjusting the value of N to ensure that U ₂ always falls within the interval [0, U _ref ]; Through AD conversion, the value of voltage U ₂ is obtained, and through multiple average processing and reverse derivation, the value of the neutral point grounding current of the transformer is obtained, that is ${\mathrm{<span\; class="notranslate">\; I</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}<span\; class="notranslate">\; =</span>\frac{{\mathrm{<span\; class="notranslate">\; KR</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}}{{\mathrm{<span\; class="notranslate">\; R</span>}}_{\mathrm{<span\; class="notranslate">\; 1</span>}}\mathrm{<span\; class="notranslate">\; R</span>}}{\mathrm{<span\; class="notranslate">\; u</span>}}_{\mathrm{<span\; class="notranslate">\; 2</span>}}$ 10. The method for on-line monitoring of transformer DC bias according to claim 7, characterized in that: said gear switching circuit can realize 100mA-1A, 1A-10A and 10A-50A.

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