For critical infrastructures such as highways, bridges and pipelines, cathodic corrosion protection (CCP) is an important tool for protecting systems from corrosion, which can cause significant structural damage. If the degree of corrosion is not controlled, there is a risk that the systems will become unsafe and have to be shut down. Steve Hughes, Managing Director of power quality specialist REO UK, explains how harmonic currents affect the effectiveness of CP systems and how to protect against them.
Corrosion is a natural, electrochemical process in which metals are gradually destroyed as part of two simultaneous chemical reactions. A common example of this is rust, in which iron is electrochemically converted into hydrated iron oxides by oxygen and water. Two reactions occur simultaneously: an anodic oxidation that destroys the iron and a cathodic reduction with electron transfer between the anodic and cathodic cell through an electrolyte.
CP systems are used to protect metal structures from corrosion by making them the cathode of an electrochemical cell. This is achieved by applying an external electrical current to the structure, which drives electrons into the metal, so that the electrochemical potential of the metal is more negative than that of the environment. This prevents the formation of a protective layer of corrosion products on the metal surface.
Typically, CP systems use rectifiers to convert AC to DC. The switching from AC to DC and back through the rectifiers generates harmonic current waveforms that are multiples of the mains frequency. These harmonics have several negative effects on the grid, including increased heating of the rectifiers and other equipment, increased voltage drop in the power lines, interference with other electrical equipment, and reduced efficiency of the CP system.
Control of harmonic interference
There are numerous ways to reduce harmonic problems in CP systems, such as oversizing the rectifiers, using parallel rectifiers, and using a power factor correction device. Oversizing the rectifiers can help reduce the amount of harmonics generated. This is because the larger the rectifiers, the less likely they are to introduce harmonics into the power grid.
The use of parallel rectifiers can also help reduce harmonic emissions. This is because the current flowing through each rectifier is reduced, which reduces the likelihood of harmonics being generated. The use of a PFC device can help improve the power factor of the CP system. This is important because a high power factor can help reduce the amount of harmonics generated. While oversizing rectifiers and using parallel rectifiers are effective, they incur significant operating costs, so these solutions are not always practical.
The most cost-effective method of reducing the negative effects of harmonic currents is generally the use of harmonic filters. These are devices that filter harmonic currents from the grid.
In addition, there are a number of other measures that companies can take to mitigate harmonic problems in CP systems. These include ensuring proper grounding of the system, using high-quality rectifiers that allow efficient switching from AC to DC, regularly monitoring the CP system to identify any problems, and performing regular maintenance, especially if problems are detected during monitoring.
By following these tips, you can help ensure that your CP system operates safely and efficiently and does not cause harmonic problems.
