Hayavadana

1. INTRODUCTION

          A century has passed since the application of the first electro chemical over current relays in power system protection. The majority of protection principles where developed with in the first three decades of century .a rough guide to there development is shown in fig1

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          Distance protection has played an important role in power line protection since it was first introduced in the early part of the century. it has many advantages over the power line protection techniques and can be adopted for fault location and back up protection. however , like other power frequency based protection   techniques   it suffers from limitation due to power system frequency wave form , fault path resistance , line loading and source parameter variations. In particular , the response speed of the relay cannot meet the reqirements when very high speed fault   clearance is required .

              With the continuous development of modern technology, protection relays have advanced with the development of electromechanical, semiconductor, integrated circuits and microprocessor technologies. Al tough decades of research have been put in to the continued development and perfection of the relay technology , many of the basic relaying principles of protection have not been changed and are still playing a dominant role today. the introduction of computer technology have been an important milestone in the history of power system protection .since the concept was first raised in the late 60’s relay technology   has gone through rapid development. digital techniques for transmission line protection   have been quickly developed and have included various digital and   numeric impedance algorithm for distance protection.

          Modern development   for power system network , the demand for fast fault clearance to improve system stability and the need for alternative protection principles have resulted in the search for methods to increase the...