Protection Scheme for Power Transformers Using Digital Relays - Capstone Project Example

Transformer Protection Name Institution Table of contents 1.Introduction 3 1.1.Motivation 3 1.2.Objectives 8 1.3.Significance 8 2.Proposed Approach 8 3.Project Timeline 8 4.Risk assessment 9 5.Progress to date 9 6.Conclusion 9 7.References 10 Abstract Power transformers find a lot of applications today, especially in transmission lines and other high voltage applications. In a bid to enhance the efficiency of operation of these devices, various technologies have been put in place to protect the transformers both from mechanical damage as well as other electrical faults that may cause the transformer to malfunction. Despite the great role that these technologies have played as far as transformer protection is concerned, there are still so many flaws that exist even in situations where these traditional methods of protection are apply. The purpose of this project is to come up with a protection scheme for power transformers using digital relays. These should be able to offer advantages over what traditional protection schemes are not able to do. The project will make use of the MATLAB/SIMULINK environment to prove the functionality of a power transformer protected using the digital relay system. A number of algorithms will be implemented to determine the behavior of the transformer, especially when it comes to the avoidance of false tripping incidences. 1. Introduction 1.1. Motivation Various approaches have been made in the past regarding the protection of power transformers. These have helped a great deal but have, however, failed in a number of areas. There have been cases of false tripping for some of these transformers, occasioned by the likes of magnetizing inrush currents, overexcitation currents and harmonic restraints among others. With the advent of digital relays, however, it is possible to implement algorithms that may be useful in guarding against false tripping, as long as the conditions leading to these situations are known. This becomes the basis of this project as far as the implementation of digital relays in the protection of power transformers is concerned. Project Background Differential protection is a protection mode that makes use of the fact that the difference between input and output currents can only be high in instances when internal faults exists in a particular zone of a device. These differences of current, also known as the differential current, play an important role in the protection of such a device using differential means. There are also cases when substantial differential currents occur, though an internal fault may not be evident in such cases. As is with the case of current transformers, such situations arise from certain characteristics of such devices, and these include the saturation levels and nonlinearities that may occur within the current transformer. These characteristics directly affect the output currents of the specific power transformer that needs protection. Apart from overexcitation current, and a few instances of inrush currents, most solutions to problems of a transformer are solvable using a percent differential relay (Tripathy, Maheshwari & Verma, 2010). This works on the principle of adding two differential coils to the normal differential relay. These are fed to the relay by means of the zone-through current, and usually requires a carefully considered selection of resultant percent differential characteristics. There is always the need to connect current transformers on each side properly, as well. Transformer Differential Protection For quite some time now, percentage restraint differential relays have been in use for protection purposes of transformers. The figure below shows a typical connection of one such relay. The working principle is such that the differential elements in the protection circuit analyze and do comparison of current by means of a restraining current. The operation current, Id, which also doubles up as the differential current, is gotten by adding together the phasors of currents that get into the protected element: The differential current is usually proportional to the fault currents in the event that internal faults arise, hence it is expected that this current will always tend towards zero when the conditions under which the transformer operates are ideal (Eissa, 2005). Various options are available for the process of obtaining the restraining current from in the case above, which may be any of the following: This is such that k is the compensation factor, which may be either 1 or 0.5. In the event that the differential current goes above a specified percentage of the restraining current, the differential relay releases a trip signal. Provided the primary currents produced by the current transformers are correct, differential relays will always work well for the case of external faults. Once any to the current transformers saturates at different levels, there may be a case of false triggering, which results from the indication of false operating current. There are a number of differential relays that make use of harmonics resulting from current transformers in the case of added restraint to avoid such false triggering of the trip signal. At the same time, percentage differential relays have a slope characteristic that offers considerable security for instances of external faults with current transformer saturation. This may be further advanced using a dual-slope characteristic or a variable-percentage for instances of heavy current transformer saturation. FALSE DIFFERENTIAL CURRENT There are several instances when the false differential current may be produces. This is based on a number of occurrences, such that differential current may flow, when no fault exists in the system, such that the magnitude of these currents is sufficient to cause the differential relay to send a trip signal. During these occasions, ideally, the system should not be disconnected by the differential relay since such are not resulting from the internal fault of the transformer. In most cases, such happenings emanate from the nonlinearities occurring in the core of the transformer, and include inrush currents, over excitation conditions, current- transformer saturation and harmonic restraint currents. Magnetizing inrush currents These currents arise from sudden changes in the magnetizing voltage within the transformer. Much as they are associated with the energization of the transformer, these currents may also be caused by other factors such as the change of a fault character (Sortomme, Venkata & Mitra 2010). This may be occasioned by situations such as the development of a phase-to-phase-to-ground fault from a phase to ground fault. In addition, another leading cause of inrush currents is as a result of an external fault. After occurrence of these external faults, there is also a possibility of inrush currents arising from voltage recovery in such incidences. When a generator is connected, and its synchronization is out of phase, it may be another cause of inrush currents. The magnetizing branch, which in most cases represents the core, is seen as a shunt element when the equivalent circuit of the transformer is obtained. This magnetizing branch is responsible for the upsetting of currents at the different terminals of the transformer, which appear as false differential currents to the differential relay. When inrush conditions arise, the differential relay is supposed to remain stable for proper functioning. At the same time, putting consideration into the lifetime of the transformer, inrush conditions should not cause the transformer to trip out (El-Amin & Al-Abbas, 2006). This is due to the fact that the process of breaking the current of a purely inductive system puts at great risk. The risk in this case arises because of the increase in chances of the insulation of the transformer being at stake, consequently leading to an internal fault indirectly (Zhang & He 2006). The inrush currents that occur within the transformer have a set of notable characteristics. First, they usually have both odd and even harmonics in addition to the dc offset. In addition, they exhibit various unipolar and bipolar pulses, which usually have separations of extremely low current values (Kang, Lim, Kang & Crossley, 2005). Another common characteristic of these currents is that the peak values of unipolar inrush currents usually decrease at a very slow rate, and they have a relatively higher time constant in comparison to that of the dc offset, whose decay is usually exponential in nature. Another feature of these currents is that their second harmonic component begins as a low value and increases with the decrease of the inrush current. Overexcitation conditions When a transformer is overexcited, it may result in the unnecessary operation of its differential relays, and this is common in most generating plants. The most common cause in such cases is a situation when the unit-connected generator is separated in the process of VAR exportation (Herman, 2011). This leads in an abrupt rise in voltage on the windings of the transformer. This change in voltage may lead to a volts per hertz condition that is far much than the nominal case leading to overexcitation (Barbosa, Netto, Coury & Oleskovicz, 2011). There are times when such a situation arises within transmission systems in cases where huge reactive loads are removed from transformers when the energized state of the primary winding remains constant. In situations when the primary winding of a transformer goes into saturation when overexcited, more power is seen to flow to the primary in comparison to that coming out of the secondary (Saleh, Scaplen & Rahman, 2011). With a proper selection of current transformers for the differential relays, such a situation may be salvaged by ensuring that they can stand possible shifts in ratios and phases. These will help to view such currents as differential currents between the primary and secondary windings, leading to an unwanted operation. Such an operation is not supposed to be, for the mere fact that there is no internal fault, considering that a current imbalance emanates from the overexcitation situation (Tripathy, Maheshwari, & Verma, 2005). Considering the fact that overexcitation is greatly associated with the generation of odd harmonics, also putting in mind that there is a possibility of cancelling third harmonic factors in a delta transformer winding, the fifth harmonic may act as the restraining/blocking factor in the differential relay to judge between the case of faults and that of overexcitation. Current Transformer saturation Current transformers have different effects on the functioning of the system. It is evident that the percentage restraint usually leads to a reduction in the consequence of the unbalanced differential current (Phadke & Thorp 2009). However, when external faults exist, such a differential current that arises may be very high-valued causing false triggering of the differential relay. In the event of internal faults, the harmonics resulting in this case may lead to a time lag in the operation of these differential relays when harmonic restraint exists. Harmonics restrain This usually arises from the large second-harmonic component associated with the differential current such that it is usually of a higher magnitude when there is inrush in comparison to a situation of a fault (Djekic, Portillo & Kezunovic, 2008). There is also a fifth harmonic component for the excitation current, which is also relatively high. These harmonics are all useful in the restraining of the relay from releasing a false trip signal in both situations. As opposed to odd harmonics generated by current transformer saturation, even harmonics openly indicate presence of magnetizing inrush. These even harmonics, which result from dc current transformer saturation may also be used to distinguish inrush currents and internal fault currents. 1.2. Objectives 1) To protect the transformer using digital relays 2) To implement an algorithm using a simulation software to protect the relay against false tripping 1.3. Significance It is expected that upon implementation, this project will minimize the problems associated with transformers in high voltage applications. At the same time, the operation of transformers using this approach will improve in terms of efficiency because cases of false tripping will be generally limited. 2. Proposed Approach The scope of the project will allow for the simulation of the proposed project using a suitable program such as the MATLAB/SIMULINK environment. A number of algorithms are available for the digital relay simulation technique, namely the Discrete Fourier transform technique, Correlation techniques, Least error square approach, and the Kalman filtering approach. The proposed solution will be based on one or a combination of these algorithms to come up with the most viable solution to the problem. Graphs of different current values under different conditions will be generated from the system, and these will be used to analyze the effectiveness of the digital relay in comparison with the traditional approaches to the protection of the transformer. 3. Project Timeline Mar Apr May June July Aug Sep Oct Nov Dec Documentation Research Project Design Circuit building Simulation Circuit assembly and documentation 4. Risk assessment Much as there are no direct risk factors associated with this project since its implementation is basically a simulation, a number of factors come into play. When it comes to the actual implementation of the system in the real environment, digital relays will imply a microprocessor based system, which requires a lot of caution when it comes to handling due to its delicate nature. The implementation of the simulation, a number of challenges arise, such as the lack of the current transformer in the MATLAB Simpower toolbox. This may be solved by either using a regular single phase, amended to fit the specification of the transformer in use or adopting current measurement, which has the flipside of not simulating the problems of current transformers. 5. Progress to date Presently, the project is at the initiatory stage whereby sufficient materials that are needed have been gathered. Such include reading materials such as books and journals as well as relevant websites. Other materials include simulation software such as MATLAB which has already been purchased. The research process is also ongoing as well as the initial stages of documentation. 6. Conclusion This is a viable project, and its significance is clearly evident due to the inconveniences experienced when using large power transformers. The approach used, is sufficient to determine the operational characteristics of the transformer, and through the graphs produced by MATLAB it is possible to analyze the performance of the transformer when digital relays are used for different conditions that may exist, especially with regard to the changes in the differential currents. 7. References Barbosa, D., Netto, U. C., Coury, D. V., & Oleskovicz, M. (2011). Power transformer differential protection based on Clarke's transform and fuzzy systems. Power Delivery, IEEE Transactions on, 26(2), 1212-1220. Djekic, Z., Portillo, L., & Kezunovic, M. (2008, July). Compatibility and interoperability evaluation of all-digital protection systems based on IEC 61850-9-2 communication standard. In Power and Energy Society General Meeting-Conversion and Delivery of Electrical Energy in the 21st Century, 2008 IEEE (pp. 1-5). IEEE. Eissa, M. M. (2005). A novel digital directional transformer protection technique based on wavelet packet. Power Delivery, IEEE Transactions on, 20(3), 1830-1836. El-Amin, I. M., & Al-Abbas, N. H. (2006, August). Saturation of current transformers and its impact on digital overcurrent relays. In Transmission & Distribution Conference and Exposition: Latin America, 2006. TDC'06. IEEE/PES (pp. 1-6). IEEE. Herman, S. L. (2011). Electrical Transformers and Rotating Machines. New York: Delmar Cengage Learning. Kang, Y. C., Lim, U. J., Kang, S. H., & Crossley, P. A. (2005). A busbar differential protection relay suitable for use with measurement type current transformers. Power Delivery, IEEE Transactions on, 20(2), 1291-1298. Liu, Q., WANG, Z., Xu, Y., & JIAO, Y. (2005). Interface of Digital Relay Protection and Electrical Transducers [J]. High Voltage Engineering, 4, 001. Phadke, A. G., & Thorp, J. S. (2009). Computer relaying for power systems. John Wiley & Sons. Saleh, S. A., Scaplen, B., & Rahman, M. A. (2011). A new implementation method of wavelet-packet-transform differential protection for power transformers. Industry Applications, IEEE Transactions on, 47(2), 1003-1012. Sortomme, E., Venkata, S. S., & Mitra, J. (2010). Microgrid protection using communication-assisted digital relays. Power Delivery, IEEE Transactions on, 25(4), 2789-2796. Tripathy, M., Maheshwari, R. P., & Verma, H. K. (2005). Advances in transformer protection: a review. Electric Power Components and Systems, 33(11), 1203-1209. Tripathy, M., Maheshwari, R. P., & Verma, H. K. (2010). Power transformer differential protection based on optimal probabilistic neural network. Power Delivery, IEEE Transactions on, 25(1), 102-112. Zhang, X. S., & He, B. T. (2006, July). Influence of Sympathetic Interaction between Transformers on Relay Protection. In Zhongguo Dianji Gongcheng Xuebao (Proceedings of the Chinese Society of Electrical Engineering) (Vol. 26, No. 14, pp. 12-17). Read More