Steel has been used as a substitute material for grounding grid material in many countries including INDIA, but the problem with iron is that the oxide forward by oxidation does not firmly adhere to the surface of the metal and flakes off easily. It should not be noted, however that certain metals such as aluminum and copper, form a very tough oxide coating which strongly bonds to the surface of the metal preventing the surface from further exposure to oxygen and corrosion. Thus it is very relevant to study the corrosion processes in electric (substation) grounding which helps in designing grounding grid with steel as grounding material. Steel corrosion (rust) has been with us since the very dawn of the Iron Age but still poses significant problems. Though, there are several measures available to control corrosion and prevent the formation of rust, not every protection can be used in case of electric grounding electrodes. Most of the corrosion controlling on the surface of the metal whereas the grounding electrodes are supposed to be conductive so as to pass the current to the ground in contact. Thus it is very relevant to study the corrosion processes in electric grounding which helps in designing grounding grid with steel as grounding material. GROUNDING SYSTEM: In all types of electrical equipment including in power houses and large substations, it is necessary to install an effective grounding system to safeguard both lives and equipment from dangerous fault currents, surges, transients and lightning strikes. GROUNDING MATERIALS: Copper commands supremacy over other metals so far as the choice of material for grounding is concerned. IEEE standard No. 80-2000: “IEEE Guide for safety in AC Substation Grounding” discusses, in detail, the ground grid design procedure for AC substations with copper as the primary grid material. STEEL as substitute for COPPER Literature reviews reveal that the steel grounding system has been readily accepted in many countries worldwide for the following reasons: - Steel is much cheaper than copper
- Reduction of galvanic action between dissimilar metals (particularly with copper)
- Ability to provide protection t steel structures
- Higher mechanical strength. Steel is approximately twice as mechanically strong compared to copper for the same cross-sectional area
- Steel absorbs approximately 1.36 times as much heat as an equal volume of copper, and can withstand higher temperatures before melting (1510°C compared to 1083°C for copper)
- Copper as earthing material forms a “Galvanic Cell” with buried steel pipes, conduits, rebar’s, etc. in the vicinity and corrodes the steel.
Corrosion: A) General In terms of ISO 8044-1986, metallic corrosion is defined as physicochemical interaction between a metal and its environment – which results in charges in the properties of the metal, and which may often lead to impairment of the function of the metal, the environment, or the technical system of which these form a part. Corrosion generally involves the loss of metal at a particular location on the outer surface. B) Corrosion Process The metal is exposed. Metallic Corrosion is an electrolytic phenomenon. The corroding metal is a t higher potential than its surroundings and accordingly, there will be a flow of electricity away from the metal living it in an ionized (oxidized) state. This flow of current causes the loss of metal at the point where current discharges into the environment. The rate of corrosion is proportional to the magnitude of the corrosion current. One ampere of direct current removes approximately twenty pounds of steel in one year. Where corrosion occurs, and to what extent, depends upon the environment to which C) Simple Corrosion Cell The substation grounding grid corrosion is caused by an electrochemical reaction in the soil environment called a “galvanic cell” or “corrosion cell”. The galvanic cell will have three essential constituents – an anode, a cathode and a electrolyte (soil). In simple terms, electrochemical reaction due to potential difference causes loss of metal ions at the Anode (current) flows out and enters the cathode (metal surface) through the electrolyte (soil) or through the metal itself - and the cathode is not consumed in the corrosion process because of the deposition of hydrogen or other ions that carry current. Usually, corrosion cells will be much smaller and more numerous, occurring at different points on the surface of the component. Anodes and Cathodes may arise from differences in the constituent phase if the metal itself, from Variations in surface deposits or coatings on the metal, or from variations in the electrolyte. D) Types of Corrosion - In Air
- Underground
In Air, corrosion is normally caused by either dust particles from industrial processes or due to chemical reaction with rainwater solutions which have dissolved airborne gases. There are TWO sources of underground corrosion – which are bimetallic corrosion and chemical corrosion. E) Forms of Corrosion Corrosion exhibits itself in a number of ways. Such as - Uniform Corrosion
- Pitting Corrosion
- Crevice Corrosion
- Galvanic Corrosion
F) Magnitude of Corrosion There are FOUR basic factors that determine the magnitude of electrochemical corrosion activity - Electrical continuity between sections of the underground structures
- Magnitude of voltage developed between dissimilar materials ( like steel structure and copper ground grid)
- Ratio of exposed anodic area to that of anodic are ( steel structure to that of copper ground grid) Electrical resistance of the electrolyte (soil) between the steel structure and copper ground grid
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