Problems with Protection Coordination Using Current Magnitudes - Research Paper Example

Problem with protection co-ordination using current magnitude along feeders Introduction There are different types of protective coordination systems used to safeguard appliances. In our case we are going to consider a system for controlling high current than the predetermined one. This is called the overcurrent relay. This system is designed to operate when there is excess flow of current than the desired one into a particular portion of the power system. When services have been impaired to a distribution feeder for over twenty minutes or more, it may end up being difficult to re-energize the load without resulting to operation of the relay systems. The reason for such is often linked to an abnormal inrush of current that is caused by motor starting current, current to raise the temperatures of the heater elements and the lamp filaments as well as the magnetizing inrush of current to motors and transformers (Gers & Holmes, 2008). Circuit diagram and operation There are two types of overcurrent relay: the instantaneous type and the time delay type. The instantaneous relay operates in a way that there is no time delay when there has been excess current detected. Its operating time varies from as low as 0.016 seconds to as high as 0.1 seconds. While for the time overcurrent delay time varies inversely to the current flowing in the relay. The time-overcurrent exhibits three commonly applied characteristics. This are: inverse, very inverse and extremely inverse. The three characteristics differ by the rate at which current increases inversely with time. The time-delay and the instantaneous delay are both non selective because they detect the overcurrent conditions even on other adjoined equipment. Selectivity of protecting different systems by the overcurrent system works on the basis of sensitivity or operating time (Gers & Holmes, 2008). A circuit diagram showing the use of a system protection device With the increasing voltage, load and short-circuit duty of distribution substation feeders, the protection of distribution overcurrent has become more essential than it used to be ten years ago. The abilities of the equipments utilized in protection to reduce damage when failure takes place as well as reduce interruption time of the services is demanded by the public for economic and reliable services. The various ways that equipments are protected include ground fault detection, cold pick up, line burndown and the current transformer connections. The various characteristics that is essential for the protective equipments to work well entails speed and reliability, sensitivity and selectivity. This is often important especially for relay services. Sensitivity is important for the relay system to operate under normal conditions that give rise to reduced operating tendencies (Gers & Holmes, 2008). For example, a time-overcurrent relay has to operate under fault current conditions that are minimal. On a distribution feeder, the relay must be sensitive in order to perform under the circumstances of minimal generation when a short circuit under protection draws a minimum current through the relay system. On many systems of distribution, the magnitude of the fault current does not differ very much for maximum and minimum generation conditions as a result of the system impedance being in the lines and transformers rather than generators themselves. On issues of selectivity must be able to recognize fault on their own protective equipments while at the same time ignoring all the faults located outside their protected areas. The relay system has to be selective with an intention of ensuring minimum number of devices operates to interrupt and isolate services o fewest of the customers as much as possible (Nash, 2011). Differential relying is an example of inherently selective scheme. Selectivity should be established in situations where the devices are of diverse operating characteristics as compared to the full range of short-circuit current magnitudes. Speed is also essential in clearing the faults as a result of its direct bearing on the damage that is effectively done in the short circuit current. The most important aim of protective equipment is to ensure that fault equipment is disconnected as fast as possible when an error takes place. One of the basic requirements of a protective device is its reliability. The protective relaying equipment should be able to function appropriately. The essential application of the protective device entails the correct choice of the both the associated apparatus and relaying equipments. Absence of suitable sources of voltage and current for energizing the relay may jeopardize or compromise the protection. The over-current relay is considered the simplest example of protective relay. The relay is often made to operate when more than the required amount of current is flows into a certain power system portion. There are two types of overcurrent relays; the time-delay type and the instantaneous type. The instantaneous types are often designed to function without intentional time delays in cases where the current exceeds the setting of the relay. Moreover, the operating time may change significantly. It may be as high as 0.1 seconds or as low as 0.016 seconds. On the other hand, the time-overcurrent relay has a functional feature such that its time of operation varies inversely as the current that is flowing in the relay. The three types of overcurrent relays that are often utilized includes extremely inverse, very inverse and inverse. Both overcurrent relay types are often non selective in the forms in which they detect the overcurrent conditions in their own protected equipment and adjoining equipments (Nash, 2011). Selectivity between overcurrent relays can be acquired on the basis of pickup, operating time, or a combinations of both. The operation of overcurrent relays is often influenced by variations in short-circuit-current magnitude that is formed by variations in the operation systems as well as figurations. Some of the generators are often shut down during light loads. Under delayed or instantaneous overcurrent relaying is utilized only for primary relaying to help supplement inverse-time that is working in most of the utilities. The curves are of time against multiple pickup of current. The multiple pickups are used so that same curves can be used for any value of current. This is used in induction type, which makes use of coil taps, because the ampere-turns at each pickup are the same for each tap. Hence for a given multiple of current, the coil ampere-turns, causes a torque which is the same for all taps. This time–current curves are used to determine the time it will take relay to close its contacts at a given multiple of currents (pickups) in any time adjustment. It also helps to determine how far the disc will move towards the contact-closed position at any time interval. For example, if the no.5 time-dial adjustment is used with multiple pickup of no.3. It will take time relay of 2.5 seconds for it to close its contacts. That is to say that the disc will move a distance corresponding to 3 time-dial divisions. The minimum value of current for which the relay must operate must be 1.5 times pickup and below (Chen et al, 2030). When there is an increase in the minimum tripping time then faults occur nearer to the distribution substation. It also shows that inverse time characteristic reduces an increase of inherent overcurrent relaying. It also shows the effect of inverse-time relaying in reducing this increase. In any case the more the line sections there are in a series the greater the tripping time at the source. For locations where inverse overcurrent should be selective it is good to use curves which have the same degree of inverseness for easier determination of short-circuit current over wide ranges. Examples of practical applications Numerous institutions and companies such as cement factories, TRANSCO and AADC have installed effective system protection in place to protect damage to machines and other components by power or electric current. Power system protection deals with prevention of electric power systems from faults by isolating the parts with faults from the rest of the electrical networks. The aim of protection is to ensure the systems remains stables by isolating only those components that are under faults while ensuring the other part is still under control (AIEE Committee Report, 2007). The protection schemes of any institution should therefore apply a pessimistic or pragmatic approach to help clear the faults in the systems. For such a reason, then the philosophies and the technology used in protection schemes can be well established and old since they are supposed to be reliable. Protective devices installed by institutions that utilizes power in most of their operations includes protective relays that prevents the tripping of the circuit breaker that surrounds the parts which are faulted in the system, monitoring equipments which collects data from the system for post event analysis and the automatic operations such as auto-re-closing or system restart. While the protective analysis and operating qualities of such devices may be considered critical, different strategies are often employed in order to protect the various parts of the systems. Very essential protective devices may have completely independent or redundant protective systems while a minor branch line of distribution may present low cost protection. There are three parts of a protective device which include the relay, the circuit breaker and the instrument transformer that is used in an institution that utilizes electric current in most of its operations. The utilization of such protective devices with the three basic components entails accuracy, economy and safety. Companies such as the cement factor that utilizes electrical current in part or almost all their operation have to install such components to prevent damage that may lead to loss of data and reduced performance since it may take a long time to rectify or replace the protective devices (AIEE Committee Report, 2007). In terms of safety the instrument transformer develops electrical isolations from the systems of power and thereby establishes an environment that is safer for individual working with relays. In terms of economy for the company, relays are able to be smaller, cheaper, and simpler given lower level relay inputs. In terms of accuracy, current and power system is currently reproduced by instruments transformers over operational ranges that are considered larger. In the company, the generator sets are often used to control damage to transformers or alternators in causes of abnormal conditions as a result of failures as well as regulating malfunctions or insulation failures (Elrefaie & Irving, 2009). The failures are often unusual and therefore the operator relays have to operate rarely. In cases where the protective relays fails to detect the fault, the resultant damage to machines and other company’s sophisticated equipments may be so worse and will require a lot of capital to help replace it. Protection on the distribution and transmission tend to serve two functions, protection of the employees and the plant or the company. Cases of an electric fault have been associated with a lot of damage that include triggering fires that results to a lot of damage to the equipment including loss of lives and properties. Companies should therefore ensure that effective protective devices are put in place to prevent damage. Protection of overload often requires a current transformer that measures the current that is already in the system. The two types of overload protection include time overcurrent and instantaneous overcurrent. Instantaneous overcurrent is used when the current exceeds a level that is already predetermined for the operation of the circuit breaker. Time overcurrent on the other hand operates on a time versus current curve. Based on the curve, the circuit breaker or fuse only operates when the current exceeds a given level for the preset time amount. Distance protection in a company can help detect both current and voltage. A fault in an electric circuit will often develop a sag in the voltage level. The circuit breaker installed in the power system only operates when the ratio of the voltage to current is within the predetermined level. The fault is often within the zone of protection when the relay setting is below the apparent impedance. Distance protection is often difficult to coordinate in situations where the length is too short and less than ten miles. The best choice for an institution in such a case will be current differential protection. The objective of system protection is always to remove the portion that has been affected by the failure of the plant and nothing else. In some situations, both the protection relay and circuit breaker may fail to operate. A failure of primary protection may result to operations that are aimed at backing up the protections. Remote back up protection may help remove both the unaffected and the affected items of plant with an intention of clearing up the fault. The low voltage networks often rely on low voltage circuit breakers to remove both faults and overloads. The installation of the disturbance-monitoring equipment by the company will be significant in protecting the system through model validation, disturbance investigation as well as assessment of the system protection performance (Nash, 2011). Reflections and feedback Design criteria and performance for system protection includes selectivity, speed, simplicity and selectivity, it is important for power plants to ensure effective and reliable protective devices are installed to prevent damage that can result to loss of lives and properties. Devices must minimize protection equipments and circuit; provide maximum protection at minimum costs, function quickly to reduce damage to equipments, avoid unwarranted false trips and function consistently when the fault condition takes place. The magnitude of the incoming current is often related to the diversity of the load, but often difficult to be determined as a result of variations between the loads and the feeders. Since the inception of the electrical systems in industries, coordination tasks were often performed to ensure the protection systems are operated under necessary security and reliabilities (Madani & Rijanto, 2010). The tools for performance of such actions have changed or evolved from the utilization of the glass table with log-log curve sheet into programs in computers. Protective devices have also undergone advancements from electromechanical devices to numerical, multifunctional devices. Throughout the evolution in protective devices and coordination tools, a good number of principles linked to protection coordination have remained with us (Nash, 2011). Having protective system operation in place will help deal with issues such as failure of voltage or current signals to the relays, DC supply failures, failure of relay itself targeting power supply failure, software failure and relay hardware components, failure of the fuse, miscoordiation and failure of the circuit breaker. We recommend that institutions and other organizations especially the ones that utilize power in most of their operation to ensure reliable and effective systems of protections are put in place to prevent damage that may lead to loss of lives and properties. References AIEE Committee Report,. (2007). Bibliography of industrial system co-ordination and protection literature. IEEE Trans. Appl. Ind., 82(65), 1-2. doi:10.1109/tai.1963.5407853 Chen, Y., Li, S., Sheng, J., Jin, Z., Hong, Z., & Gu, J. (2013). Experimental and Numerical Study of Co-ordination of Resistive-Type Superconductor Fault Current Limiter and Relay Protection. Journal Of Superconductivity And Novel Magnetism, 26(11), 3225-3230. doi:10.1007/s10948-013-2181-9 Elrefaie, H., & Irving, M. (2009). Determination of minimum break point set for protection co-ordination using a functional dependency concept. International Journal Of Electrical Power & Energy Systems, 15(6), 371-375. doi:10.1016/0142-0615(93)90005-8 Gers, J. M., & Holmes, E. J. (2004). Protection of electricity distribution networks (Vol. 47). IET. Madani, S., & Rijanto, H. (2010). Protection co-ordination: determination of the break point set. IEE Proceedings - Generation, Transmission And Distribution, 145(6), 717. doi:10.1049/ip-gtd:19982365 Nash, H. (2011). Ground-fault protection and the problem of nuisance tripping of critical feeders. IEEE Transactions On Industry Applications, 26(3), 563-579. doi:10.1109/28.55951 Read More