Tag: current protection

An important type of “accessory” relay, especially for legacy electromechanical protective relays, is the so-called auxiliary or lockout relay, designated by the ANSI/IEEE number code 86. The purpose of an 86 relay is to serve as an intermediary element between one or more protective relays and one or more control devices, both expanding the number of control elements actuated…

A form of protection against faults on long-distance power lines is called distance relaying, so named because it is actually able to estimate the physical distance between the relay’s sensing transformers (PTs and CTs) and the location of the fault. In this way, it is a more sophisticated form of fault detection than simple overcurrent (e.g. 50…

While 50 and 51 (instantaneous and time overcurrent) relay functions monitor line current magnitude and guard against excesses, there are applications where the direction of line current is just as relevant as the magnitude. In such cases, we need a protective relay function able to discriminate between current in one direction versus current in the other direction.…

One of the fundamental laws of electric circuits is Kirchhoff’s Current Law, which states the algebraic sum of all currents at a circuit node (junction) must be zero. A simpler way of stating this is to say “what goes in must come out.” We may exploit this principle to provide another form of protection against certain…

Perhaps the most basic and necessary protective relay function is overcurrent: commanding a circuit breaker to trip when the line current becomes excessive. The purpose of overcurrent protection is to guard against power distribution equipment damage, due to the fact that excessive current in a power system dissipates excessive heat in the metal conductors comprising that…

In the United States, the ANSI and IEEE organizations have standardized a set of numerical codes referring to different types of power system devices and functions (IEEE C 37.2). Some of these codes refer to specific pieces of equipment (e.g. circuit breakers) while other codes refer to abstract functions (e.g. overcurrent protection). Two partial listings…

Circuit breakers used in residential, commercial, and light industrial service are self-tripping devices: they internally sense the amount of electric current going through them, automatically opening when that current exceeds a pre-determined level. Circuit breakers used in medium-voltage (2.4 kV to 35 kV) and higher-voltage applications, however, must be triggered to trip by external devices. This remote-control…

Electrical power grids primarily consist of three-phase AC circuits. This means most power lines (transmission and distribution) have at least three conductors, and power transformers are either three-phase units or banks of single-phase transformers connected in Delta and/or Wye primary and secondary winding configurations. Of course, diagrams must be drawn to document how all these conductors and…

Any power grid large enough to meet the demand of an entire nation must have multiple generators drawing from multiple energy sources supplying that power. Connecting these generators together so as to equitably share loads in the grid is no trivial task. The basic concepts involved with interconnecting generators, however, are independent of the distance…