"Protection Scheme for Power Transformers Using Digital Relays"

  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. I