The electrical earthing is the most important element for safe and reliable operation of power system. Earthing is vital to achieve equipment and personnel protection. It is of two types i.e. system earthing and equipment earthing. In system earthing current carrying conductor is connected to earth for protection of system and in equipment earthing noncurrent carrying parts or body of apparatus is connected to earth for protection of human life and equipment. Moreover, the stability of any electrical or electronic system is greatly affected by the condition of earthing. Earthing is not a new concept. It has started in a way back at end of 19th century. It has been studied and standardised since then. Earthing has been studied by various researchers in mainly four aspects viz.:
- Earthing pertaining to substations, industrial plants
- Earthing requirement and standardisation related to it
- Safety aspects of earthing
- Earthing methodology.
Out of these, earthing methodology is the most neglected aspect in recent past. Earthing is considered as matured field of electrical engineering. However, the natural and manmade changes are influencing earthing design and associated parameters. Urbanisation and new life style has changed soil contents over the years. Rain fall, seasons’ patterns have changed over the last few decades. It changed moisture contents in soil. The natural soil layer is depleted because of crowding over places. Population has increased at such a rate that available land is almost occupied. Dense localities leave a very little space for earthing. Soil pollution is bringing lot of changes in earthing methodology. Electrical generation capacities have grown up rapidly. With increase in non-linear load and uncertain renewable energy sources feeding grid, lot of harmonics are added in system. Harmonics and neutral earthing affect quality of power. Advances in digital electronics and automation lead to use of sensitive instruments. For such instruments earthing is must. Due to all these changes there is a need to reinvestigate earthing methodology and various aspects of earthing right from design to commissioning of earthing.
Major objective in designing earthing system is to achieve lowest possible earth resistance value. Factors which affect earth resistance are earthing electrode and soil conditions around earth electrode are shown in Fig.1. The earth resistance is reduced by varying depth, diameter and material of earth electrode. Deep driven electrodes give less earth resistance. As the diameter of earth electrode increases, contact area with soil increases which lowers earth resistance. The most preferred option between these two is deep driven electrode from practical concerns. The parallel connection lowers resistance so multiple electrodes connected in parallel can be an option but should not have overlapping zone. Composition, moisture and temperature of soil affect the earth resistance value. More salts in soil around electrode helps in easy passage of current. Earth resistance is less in presence of moisture. Increase in temperature results in rise in soil resistance. The resistance of earth electrodes itself do not have major contribution to overall earth resistance. The significant factor of earth resistance is resistivity of soil around electrode.
Fig. 1: Important Factors affecting Earth Resistance
Various factors and their effects have been studied over a time witnessing all revolutionary technology developments. These factors have been investigated since 19th century. Now, to know how much earthing is enough, effects of all above parameters need to be considered while designing earthing system. This is done through modelling of earth system. It has started with simple practical based equation technique which is now turned into multifaceted technique.
Determination of adequacy of earthing system:
The need of accurate modelling of earthing system is of great importance as they are tool to define adequacy of earthing system. Fault conditions can be simulated and system behaviour can be studied. The modelling can be categorised in three families of methods viz;
- Circuit Theory based methods
- Electromagnetic model-based methods
- Hybrid methods
1. Circuit Theory based methods
In these models earthing electrodes or rods are modelled with lumped or distributed passive components. Early models were only of resistances and hence frequency independent. Later developments included frequency dependence of segments’ self and mutual inductance, capacitance, conductance and internal resistance. A circuit simulator is then used to calculate the voltages and currents on the electrodes. These methods provide an easy model for earthing systems. They work directly in time domain. The main drawback is that they cannot predict surge propagation delay. These models can be extended to transmission line models. This technique cannot accurately model other than straight conductors.
- Electromagnetic theory-based methods:
This is the most rigorous method for modelling of earthing system, because it solves full Maxwell’s equations with minimum approximations. The physical phenomena that underline the fault current dissipation into the ground can be studied by means of Maxwell’s Electromagnetic Theory. The realistic prediction of the currents distributed in the aerial parts and in the parts embedded in the soil is of basic importance. The methods employ numerical techniques such as the Finite Element Method, the Method of Moments or other numerical methods. It is believed to be very accurate. However, this model is too complex to be implemented. Computation time is more for large structure. Another disadvantage of electromagnetic field approach is that, because of its frequency domain solution procedure, it cannot be easily modified to include non-linearity due to soil ionisation, and combine other non- linear devices that have time domain models.
- Hybrid methods:
There exist hybrid methods which try to combine the circuit theory methods and the electromagnetic methods in order to profit from the advantages of both. The goal of the hybrid method is to obtain a frequency-dependent equivalent electric circuit for the earthing system where all the inductive, capacitive and conductive couplings among different conductor elements are accounted for and their analysis can be carried out in the frequency domain. These models are summarised in following Table1.
Advent of computers and their development can solve complex mathematical expression and hence more realistic earthing design model can be made. Modelling has evolved with technology from simple electric circuits to complex electromagnetic circuits to give more accurate real scenario. The accurate, realistic model can be obtained when all aspects of earthing stemming from various disciplines like soil science, metallurgy, chemistry, civil, mechanical, computer and electrical are considered. All sciences, right from nanotechnology that dealing materials at nano level to geology dealing with earth and whole have evolved over period of time. This development readdresses need of finding innovative practices in earthing system design by carrying out multidisciplinary research. The influence of various disciplines on earthing system design and their progression is explained in following section.
Interdisciplinary Relevance:
Complex and real-world problems are driving forces for interdisciplinary research. Innovations are result of integration of knowledge from various research areas. Different perspectives from various fields are studied to chalk out best solution. Earthing is related to many various fields like soil science, chemistry, and metallurgy, mechanical, civil and electrical. Optimal solution for earthing design can be found out by amalgam of research from these fields. Fig.2 depicts prominent fields influencing earthing.
Fig.2 Disciplines influencing earthing
Soil around earth electrode is decisive in earth resistance. The effective earth resistance depends on the soil resistivity. The determination of its value is often a complicated task for two main reasons: 1) the soil does not have a homogenous structure. It is formed by layers of various materials making it multi-layered in nature. Layers can be horizontal, vertical or mix of both.2) The electrical resistivity of a given type of soil varies widely. It is affected by parameters like the nature of the solid constituents, particle size distribution, arrangement of voids, porosity, degree of water saturation, electrical resistivity of the fluid and temperature. Change of seasons, temperature and moisture has effect on soil resistance. Thus, soil science plays an important role. Soil properties are changing due to natural and manmade changes. Their effects need to be investigated.
Various chemicals are used as additives in soil to reduce earth resistance. Soil improvers such as salts, charcoal powder and bentonite powder are mixed with soil. Lack of proper understanding and application of additives usually lead to poor performance of earthing systems. Nature friendly additives are required to reduce cost of earthing. Chemical soil conditioners are costly and have prolonged secondary effect on soil causing soil pollution. They might lead to contamination of water table nearby. There is a need for maintenance free, cost effective and nature friendly option for soil improvers to reduce the earth resistance over long periods of time and in order to avoid the expensive cost of elements and their secondary effects. Thus, chemical engineering plays vital role to find out such options. Nanotechnology opens up doors to unexplored world of material, their undiscovered potentials to be utilised to earthing practices. Newer materials can be invented as soil improver by changing properties using nanotechnology. Ball milling operation can be used to change particle size of soil additives and their effects needs to be investigated.
Earth electrodes are main bridge between system and ground. Electrodes must have low electrical resistance as per soil, strong, reliable, long life, less maintenance. Various plate sizes, shapes, pipe structures of conductors, various joint types and their mechanical strength are fields to be investigated related to mechanical engineering. The grid is welded at various joints which make welding process important to decide strength, durability of joints under fault condition and their effect on resistance.
Metallurgy plays an important role as electrodes’ composition will play significant role in earthing. Electrodes are buried in soil. Various chemical reactions take place like oxidation, reduction. Even some buried metals in soil near to earthing are deciding factors. With advent of new technologies new materials can be used as electrodes. Various materials for coating can be thought of to prevent corrosion and increase life of earthing system.
Excavation of soil to commissioning of earthing system, civil engineering aspects needs to be considered. Pits construction needs to be relooked in terms of its design and material used. Funnel placement around pits can be investigated. Multiple water entries in each pit needs to be designed. Conductive cement can be used as soil improver. Due to concretisation original soil has almost replaced so concrete itself can help in earthing is to be investigated. Thus there are many civil engineering aspects in earthing system.
Computers are influencing every aspect of our day today life and so is true in earthing. Various simulations of practical scenarios need to be considered to get most accurate results so various computational techniques and advanced algorithms are used with computers. Computer software packages can be used to assist in earthing grid design by modelling and simulation of various earthing grid configurations.
Electrical engineering is involved as major contribution for safety to personnel and equipment. Load types and patterns have changed over the years. Lot of harmonics are added in system affecting power quality due to nonlinear load. Providing earthing for various conditions of load and fault types is an important task. Simple design and complexity of power system has to be matched by electrical engineer in current situation.
Earthing is complex because of inclusion of earlier discussed factors from various disciplines. It’s much more than a rod or plate buried in soil and connected to electrical system. All disciplines’ involvement from earthing point of view underlines the necessity of multidisciplinary research in current scenario.
Conclusion
Earthing has been in practice from many years. The natural and manmade changes are influencing earthing. Due to various developments in sciences and technology, lot of design and practical approaches has been changed in earthing system. All fields of sciences and technology should work together to get best earthing practice in current situation. Effective yet simple solution for earthing system is the need of time.
A lot of research is carried out to lower earthing resistance either by improving earth electrode properties or by soil conditioning. There is further scope to reduce earth resistance by use of advances in material sciences. Complex design process in earthing can still be improved by taking best of both conventional and computational techniques. Advances in material science and improvement of computation techniques will surely give optimised and cost-effective earthing system. Multidisciplinary research needs more time, efforts, finance than single discipline research but the results are substantial.
