# Tag: Control measurements

• ## 25.13 Auxiliary and lockout (86) relays

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…

• ## 25.12 Distance (21) protection

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…

• ## 25.11 Directional overcurrent (67) protection

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.…

• ## 25.10 Differential (87) current protection

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…

• ## 25.9 Instantaneous and time-overcurrent (50/51) protection

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…

• ## 25.8 ANSI/IEEE function number codes

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…

• ## 25.7 Introduction to protective relaying

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…

• ## 25.6 Electrical Sensors: Potential Transformers (PTs) and Current Transformers (CTs)

The two “process variables” we rely on most heavily in the field of electrical measurement and control are voltage and current. From these primary variables we may determine impedance, reactance, resistance, as well as the reciprocals of those quantities (admittance, susceptance, and conductance). Other sensors more common to general process measurement such as temperature, pressure, level, and flow…

• ## 25.5 Circuit breakers and disconnects

Circuit breakers are the “final control elements” of the electric power industry, akin to control valves in the process industries. They are strictly on/off devices, used to make and break connections under load in power systems. Circuit breakers automatically open when dangerous circuit conditions are detected. Some low-voltage circuit breakers are strictly local-controlled devices, but larger…

• ## 25.4 Single-line electrical diagrams

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…

• ## 25.3 Interconnected generators

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…

• ## 25.2 Understanding Electrical Power Grids

The term “grid” refers to the conductors and equipment interconnecting power sources to power loads in a wide-spread electrical system. Generating stations (i.e. “power plants”) convert various forms of energy such as fossil fuel, solar, wind, elevated water, and nuclear into electrical power; which is then sent through step-up transformers to raise the voltage and…

• ## Chapter 25 Electric Power Measurement and Control Systems

Electrical power is a commodity in the modern world, bought and sold on the open market like any other. Thus, it is important to be able to measure and control electricity, not only for reasons of efficiency but also for sale, taxation, safety, equipment protection, and reliability of service. As with any other quantity we…