Feasibility Study Report of Transformer Core Online Monitoring System

Manufacturer: Sen Electric Co., Ltd. of Zhuhai market: Deer wave

Keywords: power transformer excessive current transformer core online monitoring transformer explosion transformer core online monitoring JTX transformer core ground current online monitoring system

First, introduce

Power transformers are the most important electrical equipment in power systems. Once a fault occurs in operation, it will have serious consequences for the power system. The normally running transformer core is grounded at one point. If there are two or more points at the same time, a current loop will form between the core and the earth. The maximum current can reach several tens of amps, which will cause the core to overheat and even burn. At present, there are three methods for judging whether the transformer core has multiple grounding points: the electrical method of the clamp-type ammeter to periodically monitor the grounding current of the core, the method of measuring the insulation resistance of the core to the ground, and the gas chromatographic method of monitoring the characteristic gas of the insulating oil of the transformer. The insufficiency of the above methods is that the multi-point earth fault of the iron core cannot be discovered in time, and in the event of a fault, corresponding measures cannot be taken in time. Therefore, real-time online monitoring of transformer core ground current is necessary.

At present, on-site personnel mostly use clamp ammeters to clamp the core ground wire to monitor its current. However, due to the strong magnetic field of the transformer, the measured value is inaccurate, and even if the measured value of the same measurement point is different, the measurement is different. The reference value is doubtful; while the automatic monitoring device developed by some domestic units can detect the multi-point ground fault in time, it lacks the real-time monitoring function after the fault. For this reason, an on-line monitoring device based on GSM communication is designed to solve the problem of online monitoring of transformer core ground current. In order to solve the problem that the grounding current in the normal state is very small, the on-site interference is rather serious.

1. Purpose The change in the core current of a large power transformer can directly reflect the fault condition of the transformer - whether there is a multipoint earthing of the core. In the past, monitoring methods were carried out regularly by substation personnel on duty using hand-held instruments. The biggest drawback of this method is that it cannot immediately respond to faults, and there are serious hidden dangers that further increase the accident. With the improvement of the power automation level and the increase of unattended substations, there is an urgent need for a stable, reliable, high-precision, and powerful power transformer core current online monitoring device. The device described in this article is designed for this purpose and has a very high application value.

2. Future development

At present, the country is developing a smart grid, and the transformer core online monitoring system constitutes an important part of the smart grid. As early as the 1980s, foreign countries have begun to realize the research and realization of the transformer core online monitoring system. Substations can save a lot of manpower and material resources, and can timely and effectively find the transformer core grounding point, and through a series of data can analyze the transformer if there is more danger of grounding, and better guarantee the normal operation of transformer equipment. At present, the national requirements for transformer core monitoring have been put forward in the safety regulations. In the next five years, all transformer cores will be deployed online. Foreign scientific and technical workers and relevant departments have long been aware of the shortcomings of this type of regular maintenance and preventive testing, and have been committed to the on-line monitoring and condition-based maintenance of electrical equipment. Its advantage lies in timely detection of early failure symptoms, so that operation and maintenance personnel can eliminate hidden dangers early through the detection means when the failure is in the bud, so as to avoid the occurrence of malignant accidents and improve the quality and efficiency of maintenance. With the development and application of sensor technology, signal processing technology and computer technology, the on-line monitoring technology of electrical equipment, which is the basis of state-based maintenance, has been rapidly developed and has become an important part of insulation testing. It will make up for it in many ways. The shortcomings of regular preventive testing. Through on-line monitoring of the core ground current, accurately determine the working condition of the core, and thus targeted to maintain the core in a timely manner before the failure, not only improves the reliability of power supply, but also reduces the operating costs of the power system. It is of great significance to ensure the safe operation of power transformers.

3, the application of the results and promotion

1. Solved the outage and overhaul to make the power department pay a huge amount of manpower and material costs, which not only brings inconvenience to the power users, but also has the problem of excess maintenance.

2. Solve the long interval between two preventive tests. It is difficult to find out about sudden insulation faults and there is a problem of insufficient maintenance.

3. The differences between test conditions and operating conditions are solved. Some off-line tests cannot fully reflect the insulation conditions of the equipment under operating conditions.

4. The phenomenon of artificially causing new equipment failures during overhaul and power outage tests has also been resolved.

Second, the goal

In order to meet the needs of the power system for transformer core condition maintenance, a design scheme for transformer core multi-point grounding on-line monitoring system was proposed. The monitoring system should meet the following functions: (1) The input and use of the monitoring system does not change, does not affect The normal operation of the transformer (2) Can continuously monitor, record and process data, timely alarm and fault diagnosis (3) Has good anti-jamming capability and reasonable monitoring sensitivity (4) The system itself is highly reliable, easy to maintain, suitable for long-term Operation (5) can effectively manage data, and the human-machine interface is good.

2, content

Based on the above considerations, we need to complete the following research and work:

1. Theoretical analysis of iron core grounding current

According to the actual structure of the transformer core, establish a suitable mathematical model of the core, calculate the point current and multi-point ground fault current value of the core, and analyze the variation law of the fault current with the fault point, to provide theoretical support for system design.

2, system hardware circuit design

According to the requirements of the iron core grounding online monitoring system, data acquisition circuits including current sensors, filtering, amplification, A/D conversion, data storage and control are designed to build a hardware platform for monitoring and extracting the ground current signals.

3, system software design

Programming and communication based on the RS232 serial port of the host computer, so that the host computer can control the lower machine running and can receive the next crew to collect the core ground current online monitoring data, the establishment of grounding current online monitoring database, data storage, query and other functions , and use LabVIEW to develop a transformer core ground current analysis software.

4, system structure

The system consists of leakage current sensors, data acquisition cabinets, local display instruments, serial network servers, routers, and system servers:

The serial networking server provides TCP/UDP socket working modes, including server and client, support for status monitoring and Web publishing. Each serial network server has 4 RS-485 ports, which can make each serial device occupy a serial port separately to reduce the interference of different serial devices. The data acquisition cabinet collects leakage currents from the cores and clamps of transformers or reactors and connects to the networked servers via serial ports.

Remote display: The system's data acquisition module can collect leakage current parameters of transformers and other electrical equipment in real time, and can be transmitted to the serial network server through RS-485, and then transmitted to the power LAN. The serial network server has self-connection function, even if the system Power off, when the power is restored when the serial network server can automatically connect to the LAN.

Local display: The local display instrument can display leakage current parameters of transformers and other electrical equipment in real time and has an audible alarm function.

5, workflow

The iron cores of transformers and reactors and their holders under the action of the electric field of the windings will form different potential or charge accumulations at various positions above them. These potentials and charges will discharge when they reach a certain level, posing a threat to the operation of the transformer. In order to eliminate this discharge phenomenon, the iron core and the clamp must be grounded reliably so that they always maintain the ground potential during the operation of the transformer. If more than one point is grounded, a short circuit will be formed in the core. The flux or leakage flux in the area covered by the short circuit will produce a great first-class in the reverberation, and the more contacts, the shorter the contact. The more circuit connections, the greater the circulation. The presence of the circulating current will burn the grounding plate and cause discharge or even the burning of the iron core, so the iron core and the clamp must be grounded at one point. The working flow of the transformer core ground current online monitoring system is described as follows:

Leakage current sensors are installed on the transformer core, the core of the reactor, and the ground bar of the clamp to monitor the leakage current of the iron core and the clamps. The data acquisition cabinet is installed on the site of the transformer and connected to the serial network server through the RS485 cable. The serial network server eventually connects to the server monitoring terminal via the router.

The data acquisition cabinet converts the analog signal collected by the leakage current sensor into a digital signal and displays it on the digital display instrument. At the same time, the digital signal is transmitted to the serial port network server through the RS485 cable, and the monitoring data is displayed in real time through the serial network server and the router to the server of the monitoring center.

Background management software receives all data records, and provides alarm, historical data query, trend analysis, report printing and other functions.

After the project has been running for a period of time, through the trend analysis, it can be seen that the transformer core grounding conditions, and find some potential equipment failure, provide maintenance basis for maintenance personnel of electrical equipment.

6, system features

The leakage current sensor adopts the fixing method of clamping copper bars without destroying any structure of the original system. The use of the metal mold greatly reduces the influence of the external magnetic field on the collection accuracy of the leakage current.

Leakage current sensor adopts the principle of active zero-flux design, which can not only meet the acquisition of mA-level current signal, but also has strong anti-interference ability.

The analog input chip of the data acquisition device adopts 16-bit high-precision resolution, the sampling rate is 10HZ, and the isolation voltage is 3000VDC.

The alarm value of the leakage current can be flexibly set by the user. The system not only has an overall sound alarm in place, but also has a separate relay alarm output to the substation control center for each core or clip.

The system uses a stable and reliable serial network server to share the core and current data of transformers, reactors and other electrical equipment in the substation.

The system uses the local and remote display modes simultaneously. The high-precision digital table display is used in the local area. The remote display uses the web publishing function of the software to enable access to authorized personnel in the power network.

The software design adopts convenient and flexible power version configuration software, which makes the software have very powerful functions.

The report printing function can print the current data in real time, and the daily report function saves a report for the daily display current data.

The system has the display function of the current alarm status and historical alarm status, displaying the current alarm status and historical alarm status.