Protective Relaying and System Protection

A governing principle at T&D is a concern for safety and reliability in the power delivery systems it designs and supports.  To that end, our engineers are well-versed in system protection and the selection, programming, and use of protective relaying devices for transmission and distribution lines.  T&D acts not only to protect system infrastructure and minimize outages but also to prevent potential injuries to clients’ personnel and the general public.

A protection system is used to detect defective power system elements or conditions of an abnormal or dangerous nature, to initiate the appropriate control circuit action, and to isolate the appropriate system components.  A protective relay device senses an abnormal condition in an electrical circuit and proactively closes the contact.


Substation protection is designed to protect substation equipment, the lines supplying the substation’s power, and the lines leaving the substation.  If a fault is not detected and isolated, it may quickly spread to a system-wide disturbance causing unexpected outages.  The fault may also present a hazard to utility personnel working in the area and the public plus cause damage to other equipment.  T&D is adept in the use and programming of digital (microprocessor based) relays to improve system performance and monitoring as a replacement of more limited electromechanical relays.  As a single digital relay can be programmed to perform the functions of multiple electromechanical relays, it is a flexible technology and the use of electronics, rather than moving parts, results in greater reliability.  Digital relays are also capable of providing metering and recording of pre- and post-fault data to assist in determining the cause of a fault and whether a protective device functioned as expected.  Additionally, digital relays can provide a relatively accurate fault location to direct crews where to begin line inspection to quickly restore service.

Important factors for transmission lines are reliability that a protective relaying device will correctly operate correctly and security that it will not malfunction.  Transmission protection systems proactively identify the location of faults and isolate the faulted section.  Weather occurrences such as lightning, rain storms, snow storms, or high winds may affect just a small section of the distribution system or a single distribution circuit.  Protective relaying and system protection react so that temporary faults can be cleared and the line kept in service.  The physical construction of the transmission line is also a factor in protection system application.  Dependability in system protection is obtained by the provision of redundant relays, fail-safe designs, and thorough testing of protection schemes.  Security is enhanced by the use of high-quality equipment, self-checking relays, and avoiding overly complex protection schemes.

T&D also designs system protection for industrial facilities and manufacturing plants.  Protective relaying devices minimize arc flash hazards and protect the integrity of motors and switch gears.  System protection for our industrial clients minimizes the risk of potential harm to personnel and protects expensive processing equipment and controls instrumentation.