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The P&C Information presented here is compatible with both the ANSI/IEEE Standards

About Protection Relays

Protective relays are used to detect defective lines or apparatus and to initiate the operation of circuit-interrupting devices to isolate the defective equipment. Relays are also used to detect abnormal or undesirable operating conditions other than those caused by defective equipment and either operate an alarm or initiate operation ofcircuit-interrupting devices.

Protective relays protect the electrical system by causing the defective apparatus or lines to be disconnected to minimize damage and maintain service continuity to the rest of the system. Dependability is the certainty of correct operation in response to system troubles.

Dependability includes the reliable operation of the relay system operating when it is supposed to and selectivity of the relay system operating to isolate the minimum amount of the system necessary to provide continuity of service.

Security is the ability to avoid misoperations between faults. Every relay system has to be designed to either operate or not operate selectively with other systems. Speed means clearing all faults in the shortest possible time with all due regard to dependability and security. A relaying system should be no more complex than is required for any given application. Adding more equipment into a scheme than is necessary for good coverage adds to the possibility of equipment failure and misoperation.

A short circuit is an abnormal connection of relatively low resistance between two or more points of differing potential in a circuit. If one of these points is at ground potential, it is referred to as a ground fault. If ground potential is not involved, it is referred to as a phase fault. Phase faults cause excessive currents and low voltages. Ground faults may or may not cause excessive currents or abnormal voltages, depending on whether the system is normally ungrounded, high- or low-resistance grounded, or effectively grounded.

The design objectives of a protective relaying are to minimize the effects of a system disturbance and to minimize the possible damage to power system equipment. A good protective relaying system will address dependability, security, speed, and simplicity.

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Protection & Control IEEE Relay and Device Designations

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Relay Designations

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Device Designations

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Microprocessor Relays

Protective relays play a critical role in the operation of the electrical power system. The protective relays are designed to take action when abnonnal conditions occur on the power system. These abnonnal conditions may be short circuits, overload conditions, and loss of system synchronism.

Elaborate protection schemes have been developed to detect these various conditions using trial and error and system operating experience. The protection schemes have typically been made up of discrete components such as overcurrent relays, diStance relays, auxiliary relays, and reclosing relays. All of the devices must be wired together to have a complete, functional scheme, which means time and money in the design, development, and installation process.

Microprocessor relays provide many functions that were not available in electromechanical or solid-state designs. Relay logic is very important to understand the microprocessor-based relay. Microprocessor relays can accommodate more complex system operation because of the inputs provided to the relay and the programming features included with the relay. In many cases, numerous functions are provided in the relay so that the number of discrete relays required for protection may be reduced substantially. In many cases, additional functions may be provided to protect a system from abnormal conditions that might not otherwise be considered based on the low occurrence of such conditions or because of other considerations.

Due to the number of components that make up these protection schemes, detailed installation tests, and routine maintenance programs must be performed to ensure that the schemes are functioning correctly. Again, this requires a significant investment in time, money, and manpower. For example, a typical step time distance transmission line protection scheme must be maintained every one to three years to ensure that it is performing within specific guidelines.

Microprocessor-based relays offer many advantages over schemes using discrete components. The overall scheme takes up less panel space. The number of components is greatly reduced. The design and wiring is simpler and less costly to implement. Installation testing and maintenance testing can be greatly reduced. Microprocessor-based relays also offer many features and functions in addition to the base protection functions.

This information can also be used to implement the transmittal of data remotely through SCADA or other data retrieval means. In terms of the number of functions, the microprocessor relay will often provide more protective functions for the dollar spent. This type of relay may be slightly slower to operate than some of the electromechanical or static relays that are available. The operating speeds of microprocessor relays are typically in the range of 1.5 to 2 cycles. Some static relays may operate in less than 1 cycle.

Microprocessor relays usually include internal self-checking functions for the majority of their functions. As noted, the relay engineer also needs to be aware of the functions and features that may not be covered by an alarm, such as the capability of the output relays to function.