The non-electrical protection device of power transformer can eliminate all kinds of faults

1 Introduction

Power transformers are widely used and very important electrical equipment in power systems. They are relatively reliable and have a low probability of failure. However, during operation, internal faults, external faults, and abnormal working conditions may still occur. Among them, internal faults are very dangerous because the arc generated by the short-circuit current will not only destroy the insulation of the winding and burn the iron core, but also heat the insulating material and transformer oil to produce a large amount of gas, causing the transformer oil tank to explode. Once a fault occurs, it will have serious consequences for the operation of the power system.

In order to ensure the safe operation of the transformer and prevent the expansion of accidents, it is extremely important to install various special protection devices that are sensitive, fast, reliable and selective according to the capacity and importance of the transformer.

The protection of transformers is mainly divided into electrical quantity type relay protection and non-electrical quantity type protection.

The electric quantity type relay protection of power transformer mainly includes: differential protection, current quick-break protection, zero-sequence current protection, etc., but these protections are not sensitive to internal faults of transformer. Non-electric quantity protection is that the non-electric quantity contacts of the transformer body (such as gas, temperature, etc.) directly enter the protection device to send a signal and trip after re-operation.

Non-electrical protection mainly includes gas, temperature, pressure, cold control power failure protection, etc. Among them, the main protection for transformer internal fault is gas protection, which can instantly cut off the faulty equipment, but the sensitivity of the gas relay depends on the identification value (flow rate). For temperature protection, the current new transformers mostly use high temperature direct tripping, while the early transformers use high temperature and cold control power failure tripping.

2 Overall system design

The measurement and control device designed in this paper integrates non-electrical quantity monitoring, control and alarm functions. Its advantages are relatively concentrated functions, full digital communication, and only one single-chip microcomputer is needed to realize all functions, which greatly saves cost and space.

Functions that need to be realized: Using a single-chip microcomputer as the control core to achieve high-speed calculation and control; to realize temperature monitoring and alarm of the main transformer; to realize monitoring and alarm of the voltage regulation gear; to realize monitoring and alarm of the gas signal; with Chinese LCD display and keyboard input, parameters can be input in the Chinese environment, and the data can also be displayed in the Chinese environment, with a humanized display style; with RS-485 bus communication interface and CAN bus communication interface; with simple status signal output; with relay output function.

According to the functions that need to be realized in the scheme, the author constructs the system into a signal input → signal processing → signal output mode, and its system block diagram is shown in Figure 1. The left side is the signal input part, which can be divided into several small modules for design; the middle is the signal processing part, which is the minimum system of C8051F041; the right side is the signal output part, which can be divided into several small modules for design.

The keyboard input and LCD display module is also called the human-machine interface module, which is mainly responsible for parameter input and status display. Here, a 2×2 keypad input and a 128×64 LCD module are used.

The temperature monitoring module uses Maxim's MAX6674. The gear monitoring module, gas monitoring module and other additional switch quantities all detect the status of the relay. Therefore, the relay signal is directly collected and isolated using an optocoupler and collected into the I/O pin of C8051F041. The high-speed communication optocoupler 6N137 is used to isolate the C8051F-041 from the RS-485 communication device SN75HVD10D to prevent external interference from directly entering the microcontroller through the communication line and SN75HVD10D, causing the program to run away. Similarly, in the connection between the CAN bus communication device PCA82C250 and the microcontroller, a high-speed optocoupler 6N137 is also used to isolate the microcontroller and PCA82C250 to prevent external interference from entering. In terms of output relay control, a low-speed optocoupler TLP521 is used to isolate the microcontroller I/O port and the transistor TIP42 that controls the relay. 3 Design of system modules

Generally speaking, the system can be divided into the following modules: CPU module, temperature signal processing module, gas signal processing module, voltage regulation gear monitoring and output module, power module, and communication module. Considering the length of the article, this article will introduce the CPU module, temperature signal processing module, and gas signal processing module in detail.

3.1 CPU module

The microprogram controller (MCU) adopts Silabs' SoC (System on Chip) CAN mixed signal MCU, which has a series of characteristics: 51 cores, very powerful functions, high instruction running speed, I/O port functions are configured by software, and there are multiple reset methods.

This design uses the MCU model C8051F041, which has powerful analog and digital processing capabilities. In this system, only one C8051F41 and a little expansion of peripheral signal conditioning circuits are needed to achieve all system functions.

The minimum system hardware schematic diagram of the MCU is shown in Figure 2, which includes the reset circuit, crystal oscillator circuit and JATG debugging circuit.

3.2 Temperature signal processing module

The temperature signal processing circuit is shown in Figure 3. The MAX6674 is provided with 3.3 V voltage by analog power supply, and the ground point is connected to the analog ground AG-ND. AGND, digital ground DGND and communication ground CGND are grounded with the system shell through a single point, and finally connected to the real earth. The reason for using analog voltage power supply and separate access to analog ground AGND is to prevent the signal noise of the digital circuit from interfering with the analog signal acquisition circuit, resulting in errors in the acquisition of analog signals. The communication between MAX6674 and the digital system adopts high-speed communication optocoupler isolation.

3.3 Gas signal processing module

In order to send the +24V detection signal to the I/O terminal of the microcontroller, the voltage signal needs to be converted. In order to improve the anti-interference ability of the hardware system, an optocoupler isolator is used to collect the signal.

In terms of software, the port status can be read in real time. The hardware measures are as follows: resistors are used to limit the current of the front-end photodiode. When any gas relay is activated, its port will be short-circuited. The circuit schematic diagram of the gas signal processing module is shown in Figure 4. The MCU module only needs to collect data in real time at regular intervals to monitor the status of the gas relay.

3.4 Voltage regulation position monitoring and output

The output of the voltage regulation gear status signal uses a single-pole normally open relay. The voltage regulation gear status signal is output from the controlled relay in the form of BCD code. The single-chip microcomputer outputs the digital signal to the low-speed optocoupler TLP521, and after optical isolation, it controls the conduction of the transistor TIP42 to control the on and off of the relay.

3.5 Power Module

In order to make the whole system stable and reliable, the power module is a crucial part. In the power design of this system, multi-level filtering and anti-interference measures are adopted. In view of the existence of high-voltage environment at the transformer site and the problem of lightning induction at the transformer site, multi-level lightning protection function is also added to the system to prevent the high voltage caused by induced lightning on the power line from entering the control system and causing damage to the control system.

3.6 Communication Module

In order to prevent external interference, high-speed optocouplers are first used to isolate the UART port of the microcontroller and the RS-485 communication device. The microcontroller and RS-485 communication device are powered separately. In this way, the interference introduced from the outside world by the communication line will stop at the high-speed optocoupler and will not enter the microcontroller from the optocoupler, which greatly improves the stability of the microcontroller.

At the same time, the C8051F04x series devices have a controller area network (CAN) controller, which uses the CAN protocol for serial communication. Based on this controller, two high-speed communication optocouplers are added for isolation, and then connected to the CAN bus physical layer transceiver.

3.7 System Anti-interference Technology

This system mainly uses series mode interference and its suppression, common mode interference and its suppression, and CPU anti-interference technology to enhance the stability and reliability of the system. 4 Software Design

4.1 Selection of Real-time Operating System and Development Environment

Taking into account the high real-time performance, multi-tasking and multi-threading of the main transformer non-electrical quantity protection system, the hardware conditions of C8051F041 and the actual operability, it is finally decided to use the 51 series μC/OS-II small real-time operating system as the operating platform for system control, and on its basis, carry out intelligent real-time measurement, control and protection of the main transformer non-electrical quantity.

Taking into account the functions that need to be realized in this system, ten major tasks are summarized for the system to call: gas detection task, temperature detection task, on-load tap detection task, switch output task, information display task, keyboard detection task, CAN bus communication task, RS-485 bus communication task, constant current source output adjustment task and constant voltage source output adjustment task.

Among these tasks, the gas detection task, temperature detection task, on-load tap detection task, switch output task, information display task, constant current source output adjustment task, and constant voltage source output adjustment task have no external interrupt start conditions, so these tasks are all used as clock real-time timing tasks. The detection of keyboard input is also completed by the clock real-time timing task. The CAN bus communication task and RS-485 bus communication task are triggered by external interrupts or actively called by internal transactions.

The development and compilation environment of this system adopts KeilμVision3, which is the latest 51 series compatible microcontroller C language software development system produced by Keil Software. KeilμVision3 was originally designed as a development environment for ARM, and ARM generally runs operating systems.

4.2 Software Algorithm

The software algorithms of this system mainly involve temperature monitoring, gas monitoring, on-load tap changer monitoring, keyboard display control, communication control, and system dispatching control.

Temperature monitoring and gas monitoring will be used as a system clock interrupt driven task, which can be completed in real time; on-load tap changer monitoring will also be used as a system clock interrupt driven task, which can be completed in real time; it can be called by display tasks and communication tasks; there are 4 buttons in the keyboard display control, which are first scanned and output by 2 I/O ports, and then scanned and input by 2 I/O ports to determine whether the button is pressed; the communication control algorithm mainly solves the startup method of the communication task, which is divided into two methods: real-time interrupt startup and random interrupt startup; the role of the system scheduling control is to schedule and allocate the above tasks, and also undertakes the modification, control and detection of various control parameters of the system.

5 Conclusion

The new main transformer integrated non-electric quantity intelligent measurement and control device realizes "integration" and "intelligence". Its hardware circuit uses C8051F041 mixed signal MCU as the core of the controller, which greatly reduces the power consumption and volume of the entire measurement and control device and improves the real-time control capability. In terms of software design, it takes the real-time operating system as the platform, and runs various detection and display tasks quickly, well and in real time on its basis, achieving the purpose of intelligence and real-time performance.