Design of a Cathodic Protection Power Supply-EEWORLD

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1 Introduction

At present, buried pipelines such as urban gas, oil, water, power cables, and communication optical fibers are becoming more and more dense and complex. The anti-corrosion layer of pipelines ages and falls off due to long burial time, causing pipeline corrosion and perforation, leakage, explosion, etc., resulting in inestimable losses.
The traditional pipeline anti-corrosion method is to paint the pipeline or wrap composite materials as an anti-corrosion insulation layer. The pipelines anti-corrosion in this way have a short service life, cumbersome construction, and poor anti-corrosion effect because the anti-corrosion layer is easy to fall off and corrode. Modern pipeline anti-corrosion generally adopts the electrochemical principle and implements the method of cathodic protection. Appropriate direct current is passed into the protected metal pipeline, making it a large cathode for the anode grounding device, forming a protective potential for the protected pipeline, thereby reducing corrosion to a minimum and extending the service life of the pipeline.
With the continuous development of microelectronics technology, computer technology and information technology, as well as the improvement of system reliability and fault-tolerant technology, it is very necessary and urgent to develop a new generation of buried pipeline anti-corrosion power supply with good safety, high reliability, uninterrupted power supply, low maintenance or even maintenance-free, intelligent, and suitable for remote centralized control! This is also the key to this article.

2 Impressed current cathodic protection system

From the principle of cathodic protection, it is known that the cathodic polarization can be caused by applying cathodic current to the protected metal structure by an external DC power supply, so as to reduce or even completely inhibit metal corrosion. This determines the three components of the impressed current cathodic protection system: DC power supply, auxiliary anode and protected cathode. Figure 1 shows the impressed current cathodic protection system for buried pipelines.

[page]3 Selection of Half-bridge Voltage-Type PWM Control Scheme

3.1 Introduction to PWM Switching Conversion Technology

With the full control of power switching devices, power electronics technology has entered the inverter era. Power switching conversion technology has always been an important part of power electronics technology and has always been a key research direction. Currently, the more representative power switching conversion technologies include pulse width modulation (PWM), pulse frequency modulation (PFM) and hybrid modulation (a mixture of PWM and PFM).

PWM is the abbreviation of Pulse Width Modulation. This method uses a constant switching frequency (i.e., a constant switching period) and changes the conduction pulse width, that is, changes the conduction ratio by changing Ton or Toff. The PWM control method has been widely used in DC/DC and DC/AC circuits due to its simple circuit and convenient control. The basic structure of the pulse width modulation control circuit is shown in Figure 2.

3.2 Analysis of the topological circuit of the power switching converter

PWM power switching converters are usually divided into two categories. One is the basic DC-DC converter, which is usually called the transformerless time ratio control (TRC) converter; the other is a DC converter composed of a transformer and a switch. It embeds a transformer in various basic DC-DC converters to achieve different functional requirements.
According to the conversion function, DC-DC converters can be divided into two types. One is a voltage-voltage converter, that is, the input of this converter is a voltage source and the output is also a voltage source. This type includes Buck converter, Boost converter and Buck-Boost converter; one is current-current converter, that is, the input of this converter is a current source and the output is also a current source. This type includes Cuk converter, Buck converter with input filter and Boost converter with output filter.

3.3 Working principle of half-bridge power converter

This paper selects the topological structure of the half-bridge circuit and adopts the voltage feedback control mode. The schematic diagram of the half-bridge circuit is shown in Figure 3.

[page]4 Main Circuit Design

4.1 Input Stage Structure Design

In the switch rectifier module, the structure of the input stage is shown in Figure 4. As can be seen from the figure, this part is mainly composed of fuses, surge voltage suppression circuits, automatic air switches, closing control circuits, anti-EMI circuits, etc.

4.2 Selection of main switch devices

At present, the power electronic switch devices widely used in inverter circuits are mainly SCR, TRIS, GTO, GTR, VMOSFET and IGBT. Because their current capacity and switching speed are different, their application range in inverter circuits is also different. In the design, the power output power is 500W and the inverter frequency is 100KHz. According to the above principles, the main switch device is VMOSFET, and the VMOSFET model is selected as IRFP450.

4.3 Selection of drive circuit

In high-power applications, the drive circuit of VMOSFET, whether it is a discrete device or an integrated circuit, mainly adopts optocoupler or transformer coupling. The optocoupler drive circuit has the disadvantages of poor anti-interference ability and low signal transmission frequency (generally not exceeding 20KHz); when the switching frequency in this circuit is 100kHz, the transformer coupling drive has the advantages of economy and practicality.
In this design, the transformer drive mode is selected, and the transformer is EI-28 ferrite core (width 28mm×height 20.5mm×thickness 11.0mm). Since the supply voltage of SG3525A is 15V, the secondary winding of the driving transformer needs to be slightly reduced, and a clamping diode is connected in parallel between the gate and the source to prevent the gate surge voltage from damaging the IRFP450 power switch tube, as shown in Figure 5.

Figure 5 Half-bridge converter drive circuit

5 Control Circuit Design

5.1 Introduction to Voltage Mode PWM Controller SG3525A

The pulse width modulator in this design adjusts the pulse width according to the feedback voltage. The signal flowing through the output inductor coil voltage is directly compared with the error amplifier output signal at the input of the pulse width comparator, thereby adjusting the duty cycle so that the output voltage changes with the error voltage. The controller SG1525/7A is currently a relatively ideal new controller. The 3-digit number (such as 3525/7) is a civilian product and was first manufactured by Silicon Communication Corporation of the United States. The corresponding models in China are CW1525A and CW1327. SG3525A is a standard 16-pin DIP package chip.

5.2 SG3525A control circuit

The connection method of SG3525A is shown in Figure 6.

[page]6 Monitoring system design

6.1 Hardware design of monitoring system

The hardware structure diagram of the monitoring system is shown in Figure 7:

Figure 7 Hardware structure diagram
The monitoring system uses SCB-31-5 computer board and universal keyboard display board.
The resources of SCB-31-5 include 8031, 74LS373, 2764, 6264, ADC0809, DAC0832, and the clock is 6MHz; the universal keyboard display uses 8279 (universal keyboard/display interface chip). 8279 can be directly connected to the data bus of 8031, does not occupy CPU time, and can automatically complete the scanning display.

6.2 Monitoring system software design

The task of this monitoring system is mainly to complete the following functions:
(1) Realize the setting of the output voltage given signal of the anti-corrosion power supply;
(2) Display the sampling voltage;
(3) Display the sampling current;
(4) Display the current power value.

7 Conclusion

According to the requirements of the cathodic protection process, this design studies the basic principles of high-frequency switching power supply, and combines computer control technology to design a new type of anti-corrosion power supply system to replace the old phase-controlled power supply system. The design is simple, reliable, and low-cost, and can be widely used in buried pipeline anti-corrosion projects.

References:

1 Li Aiwen Modern Inverter Technology and Its Application, Science Press, 2000.9
2 Liu Shengli Modern High Frequency Switching Power Supply Practical Technology, Electronic Industry Press, 2001.9
3 Ma Zhongmei et al. C Language Application Design for Single Chip Microcomputer (Revised Edition), Beijing University of Aeronautics and Astronautics Press, 2001.2

Reference address：Design of a Cathodic Protection Power Supply

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