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Electric Power Transmission and Distribution

Question:

Discuss about the Electric Power Transmission and Distribution.

Answer:

Introduction:

A grounding system of the earthing system is a circuit which connects sections of the electrical circuit with the ground, hence defining the electrical potential of the conductors with respect to the conductive surface of the earth. The choice of the system is determined by the compatibility of the electromagnetic and also the safety of the power system. Earthing system specifically affects the distribution and magnitude of the current of a short circuit through the system, and the impacts created by the people and equipment in the circuit proximity. In case of a fault as a result of an electronic device connecting a conductor supply that is life to a conductive surface exposed, any person coming into contact with it when connected electrically to the earth will complete the circuit back to the supply conductor earthed and experience an electric shock.

In the neutral earthing system, the neural of the transformer or rotating system or any other system is connected to the ground. The neutral earthing system is a significant power system design aspect of the system performance concerning protection, stability, or short circuit is importantly affected by the neutral condition. A system of three phase can be operated in two different ways namely with a neutral grounded and with ungrounded neutral. In the grounded neutral system, the system’s neutral is connected to the ground. In this system, there is no internal connection between the earth and the conductor (Bakshi, 2009).

Some of the benefits associated with the system above of grounded neutral include improving service reliability, greater safety to equipment and operators, discharge of overvoltage due to lightning,  elimination of surge voltages due to arcing grounds, and also there is a limitation of phase voltages to the line-to-ground voltages. Due to the issues related to the systems of ungrounded neutral, there are grounded neutrals in the majority of the system of high voltage. In the system with ungrounded neutral, the neutral is connected to the ground, hence this system is always referred to as a free neutral system or isolated neutral system since the neutral is isolated from the ground (Erkki, 2010).

In this system of networks distribution, there is the distribution of electric power to the numerous class of final consumers, the major anxiety for this system of earthing design consumer safety who use the electrical equipment and their protection from electric shock. This system of earthing together with devices of protection like residual current and fuses devices should eventually ensure that an individual is not in contact with an object that is metal whose relative potential is more than the safe threshold, normally set at approximately 50V. In case the earth is absence, equipment that requires a connection of earthing normally use the neutral of the supply. Some use ground rods that are dedicated (Gouda, 2017).

Numerous appliances rated 110V have plugs that are polarized so as to maintain a difference between neutral and live, however, using the neutral supply since the earthing of the equipment can be troublesome. In case there is accidental energization of the fault path and the connection supply has low impedance, the current of the fault will be huge such that the circuit over the device of current protection like circuit breaker or fuse, will open to clear the ground fault. In the case where the system if earthing does not supply a metallic conductor of low impedance between the supply return and equipment enclosures, the fault current is minute, and will not operate necessarily over the device of current protection (Heathcote, 2011).

Methods of Low-voltage System

There are three different earthing arrangement of the low-voltage system under the International Standard IEC 60364. These include IT, TT, and TN networks. The initial letter denoted the connection between equipment of power supply and earth. The equipment of power supply can either be transformer or generator. Where T denotes the direct connection on an earth point and I denotes that there is no connection with the earth, excluding possibly through a high impedance. The second letter denotes the connection between the electronic device being supplied and the earth. The letter N denotes a direct connection to the neutral at the beginning of the installation which is coupled to the earth. The letter T denotes the direct connection of a point with earth (Hewitson, 2012).

In this type of network, the system of electrical distribution has no connection to the earth in any way, or it possesses specifically a connection of high impedance. In this system, there is an application of insulation monitoring device for impedance monitoring.

This system is characterized by the application of earth electrode at the site, high fault loop impedance, cheap, continuity of operation in case of a fault, double fault overvoltage, low electromagnetic interference, less safe, no risk of broken neutral, and low PE conductor cost.

The term TT is an abbreviation of Terra-Terra, and in this system of earth, the earth connection for protection of the users is issued by an earth local electrode, and also there is an extra generator installed independently. There is an absence of earth wire between the earth electrode and the generator installed. The impedance of the fault loop is higher, and the installation of the TT earthing system should always have a GFCI (RCD) as its initial isolator unless the impedance of the electrode is very low. The major shortcoming of this earthing system is a reduction in the conducted interference from other connected devices of the users. This system has always been favoured for applications that are special such as in sites of telecommunication which benefit from the earth that is free from interference (Jain, 2009).

This earthing system does not have dangers caused by broken neutral. In areas where there is the overhead distribution of power and this earthing system is applied, earth conductors installation is not at any risk of changing to live in case any conductor of overhead distribution is fractured as a result of fallen branch or tree. This earthing system was not attractive for normal usage in the pre-RCD era due to the trouble of arranging automatic disconnections that are reliable in case short circuit caused by live-to-PE (Kiank, 2012).

The TT earthing system is characterized by high loop impedance, safe and reliable, low electromagnetic interference, no risk of broken neutral, low PE conductor cost, need of earth electrode at the site, and also high earth fault loop impedance.

In this earthing system, a single point in the transformer or generator is coupled with the earth. This point is normally the star point in the system of three-phase. The electrical equipment body is coupled with the earth through the earth connection at the generator. The conductor that couples the uncovered metallic section the electrical installation of the earth are known as the protective earth (PE). The conductor that conveys the return current in a system of single-phase, or that connects the star point in a system of three-phase is known as neutral (N). The TN earthing system can be categorized into TN-C-S, TN-C, and TN-S (Pabla, 2009).

TN-C-S: A part of this system uses PEN conductor combined, which is a particular section split up into different lines of N and PE. The PEN conductor combined normally takes place between the point of entry into the structure and the substation and separated in the head service. 

TN-C: This earthing system is made up of combined neutral (N) and protective earth (PE) conductor all the way to the consuming equipment and transformer. A combined PEN conductor satisfies the work of both N and PE conductor. This earthing system is characterized by low earth loop impedance, high PE conductor cost, high risk of broken neutral, safe, and high electromagnetic inference (Rajasekar, 2018).

TN-S: The Neutral (N) and PE are different conductors that are joined together specifically near the source of power. This earthing system is the present standard for the majority of the electrical systems in industrial and residential sectors. This earthing system is characterized by low earth fault loop, high PE conductor cost, high risk of broken neutral, safest, and low electromagnetic interference (Ravindranath, 2011).

There are separate neutral and protective earth conductors to consuming device from the transformers, which are not coupled together at any section after the point of distribution of the building. This earthing system is characterized by low earth loop impedance, high PE conductor cost, high risk of broken neutral, safe, and low electromagnetic inference (Strauss, 2011).

This earthing system operates within the voltage range of 1kV to 72.5kV, which are far less accessible to the overall public, the goal of this design of the earthing system is less on safety and greatly on effects on the devices in the presence of short circuit, protection reliability, and supply reliability. Specifically, the phase-to-ground short circuit magnitude is greatly affected by the selection of the system of earthing since the current path is generally closed through the earth (Tleis, 2010).

Methods of Medium-voltage System

There are five medium-voltage earthing methods, these include earthing transformer system, resonant neutral earthing, resistance to neutral earthing, solid neutral earthing, and unearthed neutral systems. These earthing systems are discussed below:

In neutral directly earthed, the star point of the transformer is coupled directly to the ground. In this system, a path of low impedance is supplied from the ground fault current to close, consequently, their magnitudes are equivalent with the fault currents in three-phase. The voltages are the phases unaffected remain at levels same to the ones of pre-fault since the neutral remains at the potential near to the ground. This is the major reason why this earthing system is normally applied in transmission networks involving high voltage where the cost of insulation is high (Erkki, 2010).

This earthing system is also referred to as floating or isolated system. There is no direct coupling on the ground and the star point. Therefore, a current of ground faults have no path to be closed, hence have magnitudes that are negligible, practically, the fault current is not equal to zero since the underground cables, particularly have an inherent capacitance towards the earth, which gives a path of high impedance. Isolated neutral systems may proceed in their operation and supply and supply power without interruption even in the incidence of a ground fault (Gouda, 2017). In case of a fault, the potential of the other two phases reaches relative to the ground of the normal voltage of operation, producing extra stress for the insulation. The failure of insulation may impose extra ground faults in the system, but with much greater currents. In the distribution network in the urban areas with numerous feeders underground, the capacitive current may attain high current, imposing a great risk for the devices (Rajput, 2010).

The solid neutral earth system is normally used in the applications of low voltage at 600V or less. In this system, the neutral point is connected to the earth. This earthing system minimizes the issue of transient overvoltages found on the pat provided and ungrounded system for the current of a ground fault is in the range of 25% to 100% of the three-phase fault current system. In case the transformer or generator reactance is too huge, the issue of transient overvoltages will not be resolved. To maintain the safety and health of the system, the neutral of the transformer is grounded and the grounding conductor must be extended from the source to the furthest point of system so that a high fault current can flow hence making sure that fuses and circuit breakers will quickly clear the fault and reduce the damage (Strauss, 2011).

The current magnitude depends on the fault resistance and fault location. The main advantage of this system is that it involves low overvoltages, which makes the design of earthing common at levels of high voltage. Some of the drawbacks of this system include high dangers for operators due to high voltages created, o service continuity on the faulted feeder, and also maximum disturbances and damage caused by high earth fault current. The solid neutral earthed system is normally applied when there is low source short-circuit power, in three-phase neutral distribution and in distributed neutral conductor (Strauss, 2011).

Resistance Neutral Earthing System

The resistance neutral; the earthing system is also known as earthing through a resistor and it is where the neutral is connected to the earth through a single resistor. There are two categories of the earthing system, these include low resistance earthing and high resistance earthing. These two types of the resistance neutral earthing system are differentiated by the ground fault level allowed to flow without any recognized standards for the earth fault current level which defines these categories (Prévé, 2013).

The current of the fault is limited to the selected value. Some of the reasons for limiting the current by the use of resistor include to minimize the hazards of electric shock, to minimize the mechanical stresses in the apparatus and circuits conveying fault current, to minimize the melting and burning effects of electrical devices faulted. The low resistance value normally uses levels of ground fault current between 10A and 3000A while the high resistance value normally uses the level of ground fault current of less than 10A (Gouda, 2017).

These systems protect the operators from death caused by electric shock by providing an alternative path for the flow of fault current so that it does not endanger the human beings. The appliances, machinery, and buildings which are under conditions of fault can also be protected, from the excessive flow of current in case of faults. These systems also ensure that the all the conductive parts exposed do not reach a critical potential by providing an alternative path for the flow of fault current so that is the devices and operators are not affected (Erkki, 2010).

These earthing systems also provide a safe path for dissipation of short circuit and lightning current. In case of short circuit, the fault current is channelled to the ground to protect the equipment being used and also the operator. These systems also provide a stable platform for sensitive electronic devices operation through maintaining the voltage at any section of the electrical system at a specific value so as to prevent excessive voltage or overcurrent on the equipment or appliances (Ravindranath, 2011).

Unintentional contacts with higher voltage lines, line surges, or lightning can result in hazardous high voltages to the systems of electrical distribution. Earthing ensures that there is a provision of alternative paths through the electrical system to reduce the damages within the system.  These damages may be deaths and destruction of properties and buildings (Sivanagaraju, 2012).

Each transformer can be considered to be a separate source since there are numerous sources of electricity. It would be very difficult to evaluate the relationship between each source of electricity in case there was no common point of reference for entire sources of voltages (Rajput, 2010).

Conclusion

A grounding system of the earthing system is a circuit which connects sections of the electrical circuit with the ground, hence defining the electrical potential of the conductors with respect to the conductive surface of the earth. This research paper evaluates the importance of neutral earthing of medium-voltage and low-voltage systems and also the various methods involved in each system. In the low-voltage system, there is the distribution of electric power to the numerous class of final consumers, the major anxiety for this system of earthing design consumer safety who use the electrical equipment and their protection from electric shock. In the medium-voltage earthing system, it operates within the voltage range of 1kV to 72.5kV, which are far less accessible to the overall public, the goal of this design of the earthing system is less on safety and greatly on effects on the devices in the presence of short circuit, protection reliability, and supply reliability.

References

Bakshi, U., 2009. Transmission And Distribution. New York: Technical Publications.

Bayliss, C., 2009. Transmission and Distribution Electrical Engineering. New Zealand: Elsevier.

Erkki, L., 2010. Electricity Distribution Network Design. New Zealand: Institution of Electrical Engineers.

Flurscheim, C., 2011. Power Circuit Breaker Theory and Design. Colorado: Institution of Electrical Engineers.

Gouda, E.-S., 2017. Design Parameters of Electrical Network Grounding Systems. Perth: IGI Global.

Heathcote, M., 2011. J & P Transformer Book. Melbourne: Elsevier.

Hewitson, G., 2012. Practical Power Systems Protection. Sydney: Newnes.

Internationale, C., 2010. Electrical Installation Guide: According to IEC International Standards. Berlin: Schneider Electric.

Jain, K., 2009. A Text Book of Design of Electrical Installations. Toledo: Firewall Media.

Kiank, H., 2012. Planning Guide for Power Distribution Plants: Design, Implementation and Operation of Industrial Networks. New Zealand: John Wiley & Sons.

Lehtonen, M., 2013. Neutral Earthing and Power System Protection: Earthing Solutions and Protective Relaying in Medium Voltage Distribution Networks. Toledo: ABB Transmit.

Pablo, A., 2009. Electric Power Distribution. London: Tata McGraw-Hill Education.

Prévé, C., 2013. Protection of Electrical Networks. New York: John Wiley & Sons.

Rajasekar, S., 2018. Practices in Power System Management in India. Michigan: Springer.

Rajput, R., 2010. Power System Engineering. Michigan: Firewall Media.

Ravindranath, B., 2011. Power System Protection and Switchgear. New Zealand: New Age International.

Sivanagaraju, S., 2012. Electric Power Transmission and Distribution. Mumbai: Pearson Education India.

Strauss, C., 2011. Practical Power Distribution for Industry. New Zealand: Elsevier.

Tleis, N., 2010. Power Systems Modelling and Fault Analysis: Theory and Practice. Colorado: Elsevier.

Willheim, R., 2009. Neutral Grounding in High-voltage Transmission. Perth: Elsevier Publishing Company.

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