Power Supply System Configuration for Preventing Corrosion on Pipeline using a Low-cost SMPS Chip

Abstract

As a wide range of industries using iron, such as water and sewerage pipes, gas pipelines, heat pipes, electric engines, communication pipes, and oil pipelines, rapidly become active, there is a demand for reliability and low cost of DC power supplies that can prevent corrosion of pipe networks. In particular, high-efficiency corrosion prevention systems due to changes in the perception of carbon emissions and energy saving are essential elements. Therefore, the construction of a switching-type corrosion current controller is being activated. Also, in such systems, DC/DC converters capable of multi-channel current control are demanded for corrosion prevention functions and uniform consumption of sacrificial anodes. This paper proposes a new current supply system for preventing pipeline corrosion using a low-cost SMPS dedicated chip. The proposed method can maintain excellent parallel operation function, protection function, and response speed by configuring a current controller using a hybrid method using analog and digital. The proposed method verified its superiority through simulations and experiments.

1. Introduction

Demand for quality water services is increasing rapidly in the water industry market as the importance of environmental management becomes prominent.

As a countermeasure, research is being actively carried out on the need to replace aging water infrastructure, digitalization in the utility sector, and smart water management solutions. In particular, a major factor driving the growth of the water industry is increasing awareness about water discarded due to pipe corrosion and leaks. Therefore, there is a demand for customized anti-corrosion DC/DC converters that can prevent corrosion of pipe networks. In the manual setting method, the inspector periodically sets the method current by hand, but it is difficult to reflect immediate changes in the environment. In particular, when the corrosion current is uneven, the consumption of the anode is also uneven, and when the method current is excessive, there is a problem that maintenance costs of the electrical system increase and operating costs due to energy waste increase as the anode consumption increases faster. An active current control corrosion prevention system can automatically set the amount of current injected to prevent corrosion in underground pipelines [1-3].

Therefore, this controller has various advantages because it is controlled by an optimal corrosion current. Recently, there has been a growing demand for active current control corrosion prevention systems. In these systems, a DC/DC converter capable of multi-channel current control for uniform consumption of the sacrificial anode is required along with a corrosion prevention function [4 - 6].

This paper proposes a new current supply system for preventing pipeline corrosion using a low-cost SMPS dedicated chip. The proposed method can maintain excellent parallel operation function, protection function, and response speed by configuring a current controller in a hybrid method. T he proposed method verified its superiority through simulations and experiments.

2. Power Supply System for Corrosion Resistant

2.1 Method System

Corrosion is electro-chemically oxidizing a metal by reacting with an oxidizing agent such as air or water. This is caused by the movement of electrons (metal) and ions (aqueous solution) or on the surface of the anode metal during polarization formation, causing metal aging.

Anti-corrosion is a method of preventing rust from forming, and it is the opposite concept of corrosion. In particular, the electric method artificially injects current from the outside into the cathode part of the metal to make the potential of the cathode part equal to the potential of the anode part. It is broadly divided into sacrificial anode method and external power method for stopping corrosion by eliminating corrosion currents formed on metal surfaces [7-9].

The sacrificial anode method is anticorrosion based on the potential difference between the anode itself and the ferrous metal itself. When a metal with a lower potential than the prevent corrosion is connected to the prevent corrosion with a wire, a battery reaction is formed between the two metals. As a result, metal ions are eluted from the metal with the lower potential, and current flows through prevent corrosion.

Low-potential metals are consumed instead of materials to prevent corrosion, It has the advantage of not requiring an external power source. But there is a disadvantage in that the range of corrosion effects is narrow and the corrosion current cannot be adjusted. The external power supply method installs an anode in electrolytes (seawater, fresh water, soil, etc.) with a prevent corrosion. It is a method of supplying corrosion current by connecting the (+) pole of a DC power source supplied separately from the outside to the (-) pole to prevent corrosion.

This method requires an external power source, but it has a wide range of corrosion effects and is preferred as a device that can control corrosion current [10 - 13].

Fig. 1 shows a pipe corrosion prevention system. The system mainly consists of a sensing unit TB, a DC power supply device, and multiple DC/DC converters to control system current Generally, the sensing unit Tb and the corrosion current controller are position separated. According to the control method, it is divided into a passive corrosion prevention system that sets the method current by a person, and an active corrosion prevention system that automatically controls the type current to detect the TB voltage of the sensing unit and control the set voltage.

Fig. 1 Configuration in anti-corrosion system

Recently, the importance of water management has been highlighted, and the supply of active corrosion prevention systems is gradually increasing as digital and smart water management is being promoted.

Active corrosion prevention systems recommend maintaining the potential value of the pipeline between the appropriate prevent corrosion potential (—0.85 V or less) and the over prevent corrosion limit potential (-2.5 V) in the same way as existing passive electrical products.

Fig. 2 shows an active corrosion prevention system. A junction box with a built-in anti-corrosion current controller receives date, location, and time information through an internally configured GPS module. The pipeline reference potential value is measured using wireless communication from multiple TB at a fixed time (1 to 2 times per day), and multiple anti-corrosion current controllers using the output voltage of the rectifier are controlled. In this active corrosion prevention system, a digitized DC/DC converter is essential to optimally control multiple corrosion currents.

Fig. 2 Active corrosion protection systems

2.2 DC/DC Converter Analysis

Fig. 3 shows a DC/DC converter commonly used for a type anti-corrosion current controller. The harmonic voltage generated by switching is removed by the LC filter, which is the passive low frequency filter in Fig. 3, to generate a prevoltage. However, in order to reduce the LC filter size due to economic problems, the harmonic current of the output current is reduced by high-frequency PWM.

Fig. 3 DC/DC converters for anti-corrosion current controllers

The output voltage according to the time ratio of the switch of the reduced voltage converter is defined as follows.

V_R = dV_in (1)

A voltage-reducing converter can change the output voltage at a duty ratio as shown (1). When configuring a voltage-reducing converter as shown in Fig. 3, there is a major weakness in economic feasibility due to the control power supply circuit, the power circuit for driving the switch, the PWM generating circuit, and the control circuit. In particular, it is difficult to guarantee the excellent operation of the controller settings of the control circuit and the protection circuit. Currently, many companies produce small-capacity SMPS chips, and they also have the advantage of being very inexpensive. In addition, due to its excellent design and protection characteristics from many years of use, the controller is becoming widespread in various industries. However, most SMPS dedicated chips are configured as constant voltage controllers, so adding a special external circuit is essential to use them as current controllers. T herefore, the economic disadvantage can be overcome by appropriately selecting a small-capacity SMPS dedicated chip with a built-in driving circuit. In this paper, through various analyses, the SMPS dedicated chip suitable for the current controller for the method was selected as the LM2576 chip, which integrates the controller and power converter.

Fig. 4 shows the internal configuration of the LM2576 chip, a small-capacity step-down DC/DC converter widely used in industry. The features of the LM2576 chip are that it is very inexpensive and the output voltage value can be set by setting the feedback resistor.

Fig. 4 Internal configuration of LM2576 chip controllers

In particular, this chip has the advantage of operating at a high switching frequency of 52 [kHz] and causing no problems even when the output is short-circuited due to the current limiting circuit.

Recently, the IoT of large-capacity method control systems for energy management is being promoted, and in order to respond to this IoT, it is essential to install a microcomputer in the method current controller driving circuit. In these systems, information on overcurrent and overcurrent protection, on/off status, and failure status is required to be transmitted to the upper controller.

2.3 DC/DC Converter for Hybrid Type Method

The LM2576 chip is a typical low-cost, small-sized constant-voltage control SMPS chip.

This chip is classified into a type with a built-in resistor for voltage detection and a type with a constant output voltage and a type that can be set variably using an external voltage divider resistor.

In this paper, an external voltage divider resistor type is used. The characteristics of LN2576 for control are designed so that the controller operates so that the feedback voltage is 1.235 [V]. In this paper, we propose an interface circuit, as shown in Fig. 5, to configure a wide range of current controllers by utilizing the characteristics of LM2576, which is designed as a voltage controller. The proposed interface circuit can be largely divided into an analog part consisting of an OP-Amp and a digital part consisting of a DSP. T he analog operation largely consists of an LPF section that allows the PWM signal to be used as an analog current reference value and an offset setting section to remove the voltage reference value of 1.235 [V] of the LM2576.

Fig. 5 Proposed interface diagram

The digital section, which consists of DSP, has 6 PWM signal generators that can generate current command values for six LM2576s and a digital output for On/Off, and has six output voltages and currents, one input voltage, and one input signal for sensing temperature. It also has a Modbus 485 communication function for information exchange with other devices. Therefore, various functions such as constant current control, voltage control, and power control are possible in DSP.

3. Simulation and Experiment

3.1 Simulation Results

Fig. 6 is a Psim simulation circuit diagram to verify the validity of the proposed topology. It is composed of an analog controller that operates similarly to the characteristics of LM2576, and the analog control part is composed using an OP-Amp that takes into account the actual configuration. Additionally, the digital part consists of dll files written in C language.

Fig. 6 Simulation diagram

At this time, the sampling and PWM frequency were set to 10 [kHz], and the parallel operation characteristics were analyzed along with the converter characteristics through a two-stage current controller.

Fig. 7 shows the step response characteristics for a command value of 2.5 [A] assuming a resistance of 10 [Ω] in a current controller using a 60 [V] input voltage. As can be seen in the figure, the corrosion command current and the corrosion current have some time delay in the transient state, but they match after 0.1 [sec].

Fig. 7 30[A] Step response characteristics for command values

At this time, it can be seen that the inductor current ripple rate of the LM2576 output filter is approximately 13 [%]. It can be seen that the LM2576 feedback voltage of the analog interface circuit converges to the internal reference voltage of 1.235 [V] of LM2576 in the steady state, and the duty ratio of the internal controller is controlled to operate at 0.417 and the output voltage is 25 [V].

Fig. 8 is a simulation result for analyzing the parallel control characteristics for expanding the current capacity of the corrosion current controller, assuming that the inductors of the two converters are 20 [%], when the current command changes from 1 [A] to 3 [A]. As can be seen from the simulation results, the currents of the two converters are almost identical for the same current command. Therefore, it can be seen that this topology is easy to expand the current capacity.

Fig. 8 Parallel control characteristics

3.2 Experimental Results

Fig. 9 shows a prototype of a 6-CH corrosion current control system. T his corrosion current control board contains 6 LN2576H chips and is equipped with the proposed analog interface circuit suitable for them. A 6-channel current controller with a rated current of 3 [A] was configured.

Fig. 9 Prototype of 6-CH corrosion current control system

Fig. 10 shows the response characteristics for the 2.0 [A] / 1.0 [A] command under the same conditions as Fig. 7. As can be seen in the figure, the corrosion current has some transient states, but good estimation characteristics were confirmed.

Fig. 10 2.0 [A] Step response characteristics for command values

Fig. 11 shows the parallel characteristics of four types of current controllers. As can be seen in the figure, it can be confirmed that the four types of corrosion current controllers have good dynamic characteristics in the same form.

Fig. 11 4-CH parallel operation characteristics

Fig. 12 shows the experimental results for analyzing the characteristics when a short circuit occurs in the output of the corrosion current controller. As shown in the figure, when an output short circuit occurs, the output voltage becomes zero voltage and the current value remains constant without any change. When the short circuit is released, the output voltage is instantaneously restored to maintain the constant current.

Fig. 12 Output short circuit characteristics

Fig. 12 shows the experimental results for analyzing the characteristics when a short circuit occurs in the output of the corrosion current controller. As shown in the figure, when an output short circuit occurs, the output voltage becomes zero voltage and the current value remains constant without any change. When the short circuit is released, the output voltage is instantaneously restored to maintain the constant current.

Fig. 13 shows a photo of the configuration of a simulated active current control corrosion prevention system. The system is divided into two tanks, with 10 L of salt water in each tank and 5 L in each tank, and the salt water creates conditions to induce corrosion of steel pipe specimens (pipes). In the left tank, a steel pipe specimen (pipe) without active current control corrosion prevention was placed, and in the right tank, a steel pipe specimen (pipe) with an active current control corrosion prevention system was placed. In the steel pipe specimen, a reference potential of -2,000 [mV] was maintained through current control using the active current control corrosion prevention system converter, and an ionized current was injected into the steel pipe specimen through the MMO anode (1 unit) to prevent corrosion. After a 4 week testing of results, showed a clear difference in the degree of corrosion between the steel pipe specimens (pipes) to which the active current control corrosion prevention system was applied and those without.

Fig. 13 Current control corrosion protection system of simulation active

4. Conclusion

This proposes a new analog and digital hybrid interface for the current controller of a typical low-cost constant voltage control SMPS chip LM2576 suitable for a customized anti-corrosion DC/DC converter that can prevent corrosion of the water pipe network. To verify the validity of the proposed method, a simulation was performed on a 2-channel current controller, and a 6-channel corossion current controller was actually configured and tested. In addition, a simulated active current control corrosion prevention system was configured to verify the corrosion prevention function. The hybrid interface concept using low-cost SMPS is expected to be applicable to various fields requiring DC power and is expected to be suitable for practical power supplies.