This paper is a tutorial on the protection of mutually coupled transmission lines. It discusses how mutual coupling affects the polarizing quantities of ground directional elements, the reach of ground distance elements, and the accuracy of single-ended fault locating algorithms. The paper provides settings guidelines for instantaneous directional overcurrent and ground distance elements. It discusses in detail how transmission line mutual coupling causes overreaching or underreaching of ground distance elements. It also discusses the impact on these elements of grounding the mutually coupled line at both line ends during maintenance. The paper analyzes whether mutual coupling compensation offers any benefits to line protection. The ease and benefit of line current differential schemes are contrasted in the discussion. Lastly, the paper examines a case when a double-circuit transmission line is operated as a single circuit with jumpers placed across similar phases along the line. This situation typically arises when the utility company needs to free one of the bays to bring an additional line into the substation. The protection engineer needs to decide where to install jumpers to parallel the two circuits in order to avoid distance element underreaching. The paper provides an analysis of this problem and offers suggestions on how to address it.

/pdf/protecting-mutually-coupled-transmission-lines-challenges-4iofvjs2rl.pdf

Protecting mutually coupled transmission lines: Challenges and solutions

In this paper, a novel adaptive stator-ground fault protection scheme based on third-harmonic differential voltages is implemented on a lab scale synchronous generator. The lab scale generator is used to emulate the fault characteristics of industrial generators along the stator windings. Simulations based on finite element analysis methods were used to design and tune the laboratory testbed. Experimental results prove the expected fault characteristics as well as fault detection capability of the adaptive stator-ground fault protection scheme.

Modeling and Experimental Verification of a 100% Stator Ground Fault Protection Based on Adaptive Third-Harmonic Differential Voltage Scheme for Synchronous Generators

The IEEE Working Group on Generator Protection with Digital Computers surveyed power industry engineers to determine what problems are generally encountered and for which adequate protective devices are not available. Two surveys were conducted: one to identify the problems; and the other to determine how prevalent they are. The industry responses are briefly reported, the problems identified are described, and the feasibility of using digital processors to alleviate them is examined. >

Survey of experience with generator protection and prospects for improvements using digital computers

The fundamentals of inverse time overcurrent relaying, recent changes in transformer characteristic curves, and a flexible microprocessor-based relay that has wide application possibilities are described Insight into the process of selecting a particular time-current operating characteristic shape for a given application is provided Guidelines are presented for the selection of a setting for an instantaneous-trip device based upon its transient reach tendencies It is concluded that the availability of a microprocessor-based inverse time overcurrent relay that has selectable characteristics built in will greatly relieve the application difficulties that have been associated historically with their fixed-characteristic electrochemical counterparts >

Flexible inverse overcurrent relaying using a microprocessor

The authors report on and analyze a survey of practices and experiences with time-delay back-up protection associated with unit-connected generators, predominantly rated 100 MVA and above. In addition, application and setting considerations are discussed. The survey covered large unit-connected installations, involving 47 respondents and 6698 unit-years of experience. Phase back-up, negative-sequence overcurrent, and high-side neutral overcurrent protection were considered. The majority of respondents were found to apply all three classes of back-up protection: phase, negative-sequence, and high-side neutral overcurrent. >

A survey of generator back-up protection practices: IEEE committee report

This paper describes the protective relaying schemes employed by Georgia Power Company to protect synchronous generators from single-phase-to-ground faults. Three types of relays are connected in the secondary of a distribution grounding transformer. These include a conventional electro-mechanical overcurrent relay with time overcurrent unit and instantaneous overcurrent unit, a solid-state overvoltage relay (with timing module) tuned to reject frequencies near 180 hertz, and a solid-state undervoltage relay (with timer) tuned to reject frequencies near 60 hertz present at the generator neutral. The proper method of selecting the generator grounding components and protective relays is described, and detailed setting instructions for all relays are also included.

Technical Council Generator Ground Fault Protection Using Overcurrent, Overvoltage, and Undervoltage Relays

Generator station service transformers, also called unit auxiliary transformers, are subjected to very rigorous short circuit duty. These faults fall generally into three areas: Faults within the transformers themselves, low-side faults between the transformer and the low-side bank breakers, and faults on the station service supply buses. The purpose of this paper is to show how station service transformers should be protected, and why any protective scheme can only be effective if these transformers are properly specified for UAT service.

Relay Protection of Generator Station Service Transformers

This paper is a tutorial discussion of the basic principles of ground relaying for high voltage and extra high voltage transmission lines. Three different HV configurations are considered: Long lines, lines with a weak mid-point station, and mutually-coupled lines. Application criteria for EHV circuits are also discussed, and specific setting calculations are included where appropriate.

Principles of Ground Relaying for High Voltage and Extra High Voltage Transmission Lines

After five years of effort, a working group of the IEEE Transformers Committee completed work on one of the most important new electrical standards to be developed in recent years, the Transformer Through Fault Current Duration Guide. This document defines the exposure limits of liquid-immersed transformers to short circuit currents, and establishes fault withstand guidelines for all conditions of service. This paper provides a description of these new standards and a brief history of how and why they were developed.

C. H. Griffin

Papers

Technical Council Generator Ground Fault Protection Using Overcurrent, Overvoltage, and Undervoltage Relays

Relay Protection of Generator Station Service Transformers

Principles of Ground Relaying for High Voltage and Extra High Voltage Transmission Lines

Development and Application of Fault-Withstand Standards for Power Transformers