Restricted Earth Fault Protection System - Case Study Example

 Restricted Earth Fault Protection System Introduction Restricted earth fault is a part of electrical power engineering that bases its core role in ensuring that the electrical power by isolating some of the faulted parts from the rest of electrical power. The core objective of the restricted earth faulted is mainly for keeping the power system in a stable condition through isolating only the parts that are at fault, while leaving the rest of the network under operation. In order to remove the fault system, it must apply a pessimistic and a very pragmatic approach (Yeshwant G. Paithankar, 2010) Restricted earth fault or differential Earth protection is a restricted earth fault protection. It is termed to be “restricted” because it detects earth fault for a portion of the network. The position is also referred to as unit protection zone. As per US Standards Restricted Earth Fault Protection corresponds to the Impedance Grounded Fault Protection. The impedance may be a low resistance, high resistance or by combining resistance and induction. Restricted earth fault protection is provided in a transformer for sensing internal fault of the transformer. However, the part needs to coordinate with any other protection thus; there need be no Intentional delay for operation. However, protection is reliable if and only accurate connection of CT selection is properly done. However, it is more advisable to avoid using CTs for REF and O/L in series. (Harker, 1998) Circuit Diagram and Operation The protection system contains five major parts that are the batteries that can quickly provide power in the case of power disconnection within the system. Protective relays that performs the function of sensing and identifying the fault and respond by initiating a trip. Circuit breakers are mainly responsible for closing and opening the system through the commands of the relay and the commands through the autorecloser. (Harker, 1998) Current and voltage transformers help in regulating the voltage and the current flowage to offer an efficient condition for the relay action. Lastly, communication network that enables the analysis of voltage and current at remote terminals of a path and allow remote tripping of equipment. The circuit only acts on the F=K1I2 – K2 where F= a force conversion is constant I=the R.M.S magnitude of the current in the actual coil k2= the restraining force (Including friction) As the relay reaches, the verge of picking up the net force is zero K112=K2 Using Kirchhoff’s laws, the addition of currents of both current transformers (CTs) is equal to zero under normal conditions. If earth fault exists, between the CTs, some current bypasses the CTs and the sum of currents cannot be zero thus by measuring the imbalance of the current faults between the faults is easily identified and can quickly be cleared. Since the fault detection is concentrated at the two CTs thus the name ’Restricted Earth Fault’. Restricted Earth Fault being faster it can isolate winding faults is a quick way hence limiting the damage and the consequent cost of repairing. However, if the CTs are only placed on the transformer terminals, the winding is the only that will be protected from the damages. However, the secondary CT is always set in the distribution switchboard thus widening the protection zone to the main cable. (L. G. Hewitson, 2005) However, reflected currents on the primary of the transformer can detect the faults in the transformer star winding in the absence of Restricted Earth Fault. Hence, as the winding fault position starts moving in the direction of the neutral terminals, the magnitude of the current as on the primary quickly reduces and may not be detected. As the magnitude of the current is seen to be large on the secondary, nearly Restricted Earth Fault may protect the whole winding. Generally, the protection covers only the secondary of the transformer. Nevertheless, in times restricted earth fault is added to the primary. Historically restricted earth fault as been put in place using High Impedance Relays although as per the moment the current in the CTs is balanced only during normal conditions and always via faults. However, impendence wires and interconnecting cables have been specified as matched pairs and have a significant impact on the normal functioning of the relay. (Jan De Kock, 2004) For it operation contains the CT secondary of each phase of the electrical power transformer which are connected are connected together. The common terminal of the transformer is also connected to the secondary of a neutral current NCT. The Restricted Earth Fault acts in that whenever it senses some inequality or imbalance between the three phases of the electrical power transformer, it responds by a resultant unbalance current flow along the close route connected to the common terminals to the secondary CT. (L. G. Hewitson, 2005) As a result, there will be a secondary current in the Neutral Current CT due to an unbalance neutral current that is of a resultant flow of an unbalance current along the neutral of the electrical power transformer. Always the common terminals of stage CTs are attached /connected to the secondary of neutral CT in a way that secondary unbalance current of phase CTs, and the secondary current of neutral CT will automatically oppose each other. (RAY, 2006) Restricted earth fault relay is considered the most protective units of the transformer digital protection system it is also considered the best due to its fastness in response and hence is prone to maloperation. However, in the existence of equal amplitudes in both currents there will be no resultant current circuit along the close path as seen from above. Hence, even if there is an unbalancing in the phase current of the power transformer the restricted Earth fault relay will not respond. Power system protection is a very fundamental feature of all forms of electrical power systems. However there are different types of protection associated with restricted earth fault system that include high voltage network that perform two main function of protecting the employees and the plant .basically protection involves disconnecting the equipment that has experienced a shot to earth. In addition, secondarily protecting some items in substation like transformers in terms of regulating the gas pressure and temperature. As the distance, relay receives its power through an input and test switch transformers. The transformer converts the identified amount to a level recommendable to the electronic units. On the protected line in the event of under imbalance timing element will start and eventually initiate the phase selector unit that ensures that always the required current and voltage measuring and correct direction is used. (L. G. Hewitson, 2005). The start indicators and the signals relays of the different and responsible phases are also energized and in the case of single and two-phase faults, the relay uses healthy phase polarization. Each measuring zone is determined by the settings on the voltage, current setting unit’s hence four signals for measuring are given out, and they are filtered and converted to digital signals. The signals generated are then used in measuring unit that determine the position and the direction of the fault. However, the different zone behave differently, in the case the fault lies in the first zone the output is simultaneously activated. In case the fault lies outside the first zone, the reach is increased to successively according to the selective times T2, T3 and T4 by reconnection in the voltage circuits (Uday A. Bakshi, 2010) As per the programming the function of the distance relay can be enhanced through input relays, and the information from the distance relays relating to e.g. relations with communication equipment and tripping of the breaker is achieved through the output relays. In case the distance relay incorporates over current starting against the under impedance starting the starting signals will largely influence the timing circuit and the phase selection unit in the same way as the under impedance unit. The protective device contains three parts, the relay, circuit breaker and instrument transformer that contain potential or current. Practical Applications In practical, protective relaying is majorly associated with security of supply; faulted plants are reduced of damage, reduce the cost of energy and most substantially help to reduce all aspects of safety. Thus the system protection must be of help to the organisation or company that is using it in that it should be reliable i.e. consistency in functioning of the machine in occurrence of fault condition, in absence of reliability the device may lead to high cost of damages., the REF should at any time avoid false and unwarranted trips. Restricted earth fault protection system is of high importance in regulating the temperature of the working machine and enhancing the functioning of the machine through reducing the risk of current wastage hence ensuring effective and efficient running of the machines. For instance in the sewage plant connection should be done using 1.omm twin capable from the control box to the alarm panel. The main supply must always be RCD protected then the alarm capable connections are not polarised even if they are of only extra low voltage data signal, the cables must be rated to 240v 750C in order to comply with AS 3000 standards of electrical wiring. The Bio Treat Alarm Controller provides power distribution and it helps in monitoring and remote alarms for the Bio Treat Waste treatment system. The alarm contains two principal parts, which are remote alarm plate and control unit. The controller unit provides electrons required to monitor the alarm conditions and then send the information to the remote alarm plate. It also contains 240 distributions to the air blower and pump via 240V sockets on the lower side of the unit.240V flexible lead supplies power to the unit .the alarm plate typically consists of three LEDs in normal conditions the green power led will be lit. The red led will lit in the case of high water level in the tank and the alarm sound to attract attention. The Air Fault LED will remain off under proper functioning of the aerator and associated air piping but if the air pressure drops below the required threshold for not less than 30 seconds, the Air Fault LED will light and the alarm will sound. (Judd, 2011) The remote alarm plate connects through the two –core insulated cable, the cables have a minimum cross-sectional area of 0.8 mm per. The connection is polarity independent hence; no matter the wire connected to the terminal on the remote alarm, the plate does not affect the connection. The alarm connections are a safe low voltage, but a lot of caution need to be taken and joints be well insulated as poor connection will interfere with the alarm signal and may cause problems with the remote alarm plate. Value and the benefits gained from the project. Also to reduce equipment damage and the duration of fault the machine need to be quick in responding, in any organisation the core function is making profit that it should provide maximum protection at a very minimal cost,. Other than reducing minimal cost also minimisation protection equipment and circuit. Zero sequence differential/Restricted earth fault approach to most organisation is of high beneficial in transformer application since it has no response to load current, thus offering a significant improvement in sensitivity over the outdated differential protection. In determine if a fault is internal to the transformer the ground current in a transformer neutral is used to look on and is used to compare to zero sequence current at the transformer terminals. (Judd, 2011) However, with the measurement errors accompanied with mismatched CTs, the different resistance of leads and saturations have been responsible for nuisance tripping. With the modern power system protection to ensure accurate, effective and efficient operation of the high impedance, Restricted Earth Fault an accurate transformation of the primary short circuit is critical. Restricted earth fault protection system is of great help tho most company users in many ways s it helps the day o day running of the operational machines that ensure the normal running of the company. Thus for better functioning of organisations, the machine helps in regulating the electricity and the current flowage which is translated to a better functioning of the machine that lead to better performance and high production rate of the organisation. (Uday A. Bakshi, 2010) For instance, the restricted earth fault system improves the safety within the machine by creating an electrical isolation from the power hence creating a favourable working condition for personnel under relays work. As observed from above the rate of accuracy generated by the REF ,the fastness created in responding to relay and making the required responses, and establishing safety for personnel working under the relay translates all these to an improved level of economy since relay are highly small, cheap and simpler given lower-level relay inputs. Bibliography Harker, K. (1998). Power System Commissioning and Maintenance Practice. IET. Jan De Kock, C. S. (2004). Practical Power Distribution for Industry. Elsevier. Judd, S. (2011 ). The MBR Book: Principles and Applications of Membrane Bioreactors for Water and Wastewater Treatment. Elsevier. L. G. Hewitson, M. B. (2005). Practical Power System Protection. Newnes. RAY, S. (2006). ELECTRICAL POWER SYSTEMS: CONCEPTS, THEORY AND PRACTICE. PHI Learning Pvt. Ltd. Uday A. Bakshi, A. (2010). Protection of Power System. Technical Publications. Yeshwant G. Paithankar, S. R. (2010). Fundamentals of Power System Protection. PHI Learning Pvt. Ltd. Read More