![Protective relay](https://upload.wikimedia.org/wikipedia/commons/c/c2/Protective_relay.jpg)
In utility and industrial
electric power transmission
Electric power transmission is the bulk movement of electrical energy from a generating site, such as a power plant, to an electrical substation. The interconnected lines that facilitate this movement form a ''transmission network''. This is d ...
and
distribution Distribution may refer to:
Mathematics
* Distribution (mathematics), generalized functions used to formulate solutions of partial differential equations
*Probability distribution, the probability of a particular value or value range of a vari ...
systems, a numerical relay is a computer-based system with software-based protection
algorithm
In mathematics and computer science, an algorithm () is a finite sequence of rigorous instructions, typically used to solve a class of specific problems or to perform a computation. Algorithms are used as specifications for performing ...
s for the detection of electrical
faults. Such relays are also termed as
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
type protective relays. They are functional replacements for electro-mechanical
protective relay
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detecti ...
s and may include many protection functions in one unit, as well as providing metering, communication, and self-test functions.
Description and definition
The digital protective
relay
A relay
Electromechanical relay schematic showing a control coil, four pairs of normally open and one pair of normally closed contacts
An automotive-style miniature relay with the dust cover taken off
A relay is an electrically operated switch ...
is a
protective relay
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detecti ...
that uses a
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
to analyze power system voltages, currents or other process quantities for the purpose of detection of faults in an electric power system or industrial process system. A digital protective relay may also be called a "numeric protective relay". It is also called numerical relay.
Input processing
Low
voltage
Voltage, also known as electric pressure, electric tension, or (electric) potential difference, is the difference in electric potential between two points. In a static electric field, it corresponds to the work needed per unit of charge to ...
and low
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (stre ...
signals (i.e., at the secondary of a
voltage transformers and
current transformers) are brought into a
low pass filter
A low-pass filter is a filter that passes signals with a frequency lower than a selected cutoff frequency and attenuates signals with frequencies higher than the cutoff frequency. The exact frequency response of the filter depends on the filter ...
that removes
frequency
Frequency is the number of occurrences of a repeating event per unit of time. It is also occasionally referred to as ''temporal frequency'' for clarity, and is distinct from ''angular frequency''. Frequency is measured in hertz (Hz) which is eq ...
content above about 1/3 of the
sampling frequency
In signal processing, sampling is the reduction of a continuous-time signal to a discrete-time signal. A common example is the conversion of a sound wave to a sequence of "samples".
A sample is a value of the signal at a point in time and/or s ...
(a relay
A/D converter needs to sample faster than twice per cycle of the highest frequency that it is to monitor). The
AC signal is then sampled by the relay's
analog-to-digital converter
In electronics, an analog-to-digital converter (ADC, A/D, or A-to-D) is a system that converts an analog signal, such as a sound picked up by a microphone or light entering a digital camera, into a digital signal. An ADC may also provide ...
from 4 to 64 (varies by relay) samples per power system cycle. As a minimum, magnitude of the incoming quantity, commonly using
Fourier transform
A Fourier transform (FT) is a mathematical transform that decomposes functions into frequency components, which are represented by the output of the transform as a function of frequency. Most commonly functions of time or space are transformed ...
concepts (
RMS and some form of averaging) would be used in a simple relay function. More advanced analysis can be used to determine
phase angles,
power
Power most often refers to:
* Power (physics), meaning "rate of doing work"
** Engine power, the power put out by an engine
** Electric power
* Power (social and political), the ability to influence people or events
** Abusive power
Power may a ...
,
reactive power
Reactive may refer to:
*Generally, capable of having a reaction (disambiguation)
*An adjective abbreviation denoting a bowling ball coverstock made of reactive resin
*Reactivity (chemistry)
*Reactive mind
*Reactive programming
See also
*Reactanc ...
,
impedance,
waveform
In electronics, acoustics, and related fields, the waveform of a signal is the shape of its graph as a function of time, independent of its time and magnitude scales and of any displacement in time.David Crecraft, David Gorham, ''Electronic ...
distortion
In signal processing, distortion is the alteration of the original shape (or other characteristic) of a signal. In communications and electronics it means the alteration of the waveform of an information-bearing signal, such as an audio signal ...
, and other
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
quantities.
Only the fundamental component is needed for most protection algorithms, unless a high speed algorithm is used that uses subcycle data to monitor for fast changing issues. The sampled data is then passed through a low pass filter that numerically removes the frequency content that is above the
fundamental frequency
The fundamental frequency, often referred to simply as the ''fundamental'', is defined as the lowest frequency of a periodic waveform. In music, the fundamental is the musical pitch of a note that is perceived as the lowest partial present. In ...
of interest (i.e., nominal system frequency), and uses Fourier transform algorithms to extract the fundamental frequency magnitude and angle.
Logic processing
The relay analyzes the resultant A/D converter outputs to determine if action is required under its protection algorithm(s). Protection algorithms are a set of logic equations in part designed by the protection engineer, and in part designed by the relay manufacturer.
The relay is capable of applying advanced logic. It is capable of analyzing whether the relay should trip or restrain from tripping based on parameters set by the user, compared against many functions of its
analogue inputs, relay contact inputs, timing and order of event sequences.
If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).
Parameter setting
The logic is user-configurable and can vary from simply changing front panel switches or moving of
circuit board
A printed circuit board (PCB; also printed wiring board or PWB) is a medium used in Electrical engineering, electrical and electronic engineering to connect electronic components to one another in a controlled manner. It takes the form of a L ...
jumpers to accessing the relay's internal parameter setting webpage via communications link on another computer hundreds of kilometers away.
The relay may have an extensive collection of settings, beyond what can be entered via front panel knobs and dials, and these settings are transferred to the relay via an interface with a PC (
personal computer
A personal computer (PC) is a multi-purpose microcomputer whose size, capabilities, and price make it feasible for individual use. Personal computers are intended to be operated directly by an end user, rather than by a computer expert or tec ...
), and this same PC interface may be used to collect event reports from the relay.
Event recording
In some relays, a short history of the entire sampled data is kept for oscillographic records. The event recording would include some means for the user to see the timing of key logic decisions, relay I/O (input/output) changes, and see, in an
oscillograph
An oscilloscope (informally a scope) is a type of electronic test instrument that graphically displays varying electrical voltages as a two-dimensional plot of one or more signals as a function of time. The main purposes are to display repetiti ...
ic fashion, at least the fundamental component of the incoming analogue parameters.
Data display
Digital/numerical relays provide a front panel display, or display on a terminal through a communication interface. This is used to display relay settings and real-time current/voltage values, etc.
More complex digital relays will have metering and communication protocol ports, allowing the relay to become an element in a
SCADA
Supervisory control and data acquisition (SCADA) is a control system architecture comprising computers, networked data communications and graphical user interfaces for high-level supervision of machines and processes. It also covers sensors and ...
system. Communication ports may include
RS-232
In telecommunications, RS-232 or Recommended Standard 232 is a standard originally introduced in 1960 for serial communication transmission of data. It formally defines signals connecting between a ''DTE'' (''data terminal equipment'') such a ...
/
RS-485
RS-485, also known as TIA-485(-A) or EIA-485, is a standard defining the electrical characteristics of drivers and receivers for use in serial communications systems. Electrical signaling is balanced, and multipoint systems are supported. The st ...
or
Ethernet
Ethernet () is a family of wired computer networking technologies commonly used in local area networks (LAN), metropolitan area networks (MAN) and wide area networks (WAN). It was commercially introduced in 1980 and first standardized in 198 ...
(copper or fibre-optic). Communication languages may include
Modbus
Modbus is a data communications protocol originally published by Modicon (now Schneider Electric) in 1979 for use with its programmable logic controllers (PLCs). Modbus has become a ''de facto'' standard communication protocol and is now a commonl ...
,
DNP3
Distributed Network Protocol 3 (DNP3) is a set of communications protocols used between components in process automation systems. Its main use is in utilities such as electric and water companies. Usage in other industries is not common. It was ...
or
IEC61850
IEC 61850 is an international standard defining communication protocols for intelligent electronic devices at electrical substations. It is a part of the International Electrotechnical Commission's (IEC) Technical Committee 57 reference archit ...
protocols.
Comparison with other types
![Protective Relays Hydroelectric Station](https://upload.wikimedia.org/wikipedia/commons/c/c1/Protective_Relays_Hydroelectric_Station.JPG)
By contrast, an electromechanical
protective relay
In electrical engineering, a protective relay is a relay device designed to trip a circuit breaker when a fault is detected. The first protective relays were electromagnetic devices, relying on coils operating on moving parts to provide detecti ...
converts the voltages and currents to magnetic and electric forces and torques that press against spring tensions in the relay. The tension of the spring and taps on the electromagnetic coils in the relay are the main processes by which a user sets such a relay.
In a
solid-state relay
Solid state contactor
PCB mount solid-state DIL relay
A solid state relay (SSR) is an electronic switching device that switches on or off when an external voltage (AC or DC) is applied across its control terminals. They serve the same function ...
, the incoming voltage and current wave-forms are monitored by analog circuits, not recorded or digitized. The analog values are compared to settings made by the user via
potentiometer
A potentiometer is a three-terminal resistor with a sliding or rotating contact that forms an adjustable voltage divider. If only two terminals are used, one end and the wiper, it acts as a variable resistor or rheostat.
The measuring instrume ...
s in the relay, and in some case, taps on transformers.
In some solid-state relays, a simple
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
does some of the relay logic, but the logic is fixed and simple. For instance, in some time overcurrent solid state relays, the incoming AC current is first converted into a small signal AC value, then the AC is fed into a
rectifier
A rectifier is an electrical device that converts alternating current (AC), which periodically reverses direction, to direct current (DC), which flows in only one direction. The reverse operation (converting DC to AC) is performed by an Power ...
and filter that converts the
AC to a
DC value proportionate to the AC waveform. An
op-amp
An operational amplifier (often op amp or opamp) is a DC-coupled high-gain electronic voltage amplifier with a differential input and, usually, a single-ended output. In this configuration, an op amp produces an output potential (relative to c ...
and comparator is used to create a DC that rises when a trip point is reached. Then a relatively simple microprocessor does a slow speed A/D conversion of the DC signal, integrates the results to create the time-overcurrent curve response, and trips when the integration rises above a set-point. Though this relay has a microprocessor, it lacks the attributes of a digital/numeric relay, and hence the term "microprocessor relay" is not a clear term.
History
The digital/numeric relay was invented by George Rockefeller. George conceived of it in his Master's Thesis in 1967–68 at Newark College of Engineering. He published his seminal paper ''Fault Protection with a Digital Computer'' in 1969. Westinghouse developed the first digital relay with the Prodar 70 being developed between 1969 and 1971. It was commissioned in service on a 230kV transmission line at PG&E's Tesla substation in February 1971 and was in service for six years. In 2017, George received the IEEE Halperin Electric Transmission and Distribution Award. The award was for "pioneering development and practical demonstration of protective relaying of electric power systems with real-time digital computer techniques." George was chairman of the IEEE Power System Relaying and Contro
(PSRC)committee (1981-1982) as well as a member of the "Computer Relaying Subcommittee" which was created by the PSRC in 1971 and disbanded in 1978. He wrote the foreword for the PSR
tutorial on Computer Relayingproduced in 1979.
In 1971 M. Ramamoorty was the first to describe
calculation of impedance for distance protection using discrete
Fourier analysis.
The first practical commercially available microprocessor based digital/numeric relay was made by
Edmund O. Schweitzer, III in the early 1980s.
SEL
Sel is a municipality in Innlandet county, Norway. It is located in the traditional district of Gudbrandsdal. The administrative centre of the municipality is the town of Otta. The municipality also includes several notable villages including ...
,
AREVA
Areva S.A. is a French multinational group specializing in nuclear power headquartered in Courbevoie, France. Before its 2016 corporate restructuring, Areva was majority-owned by the French state through the French Alternative Energies and Atom ...
, and
ABB Group
ABB Ltd. is a Swedish-Swiss multinational corporation headquartered in Zürich, Switzerland. The company was formed in 1988 when Sweden's Allmänna Svenska Elektriska Aktiebolaget (ASEA) and Switzerland's Brown, Boveri & Cie merged to create ...
's were early forerunners making some of the early market advances in the arena, but the arena has become crowded today with many manufacturers. In transmission line and generator protection, by the mid-1990s the digital relay had nearly replaced the solid state and electro-mechanical relay in new construction. In distribution applications, the replacement by the digital relay proceeded a bit more slowly. While the great majority of feeder relays in new applications today are digital, the solid state relay still sees some use where simplicity of the application allows for simpler relays, which allows one to avoid the complexity of digital relays.
Protective element types
Protective elements refer to the overall logic surrounding the electrical condition that is being monitored. For instance, a differential element refers to the logic required to monitor two (or more) currents, find their difference, and trip if the difference is beyond certain parameters. The term element and function are quite interchangeable in many instances.
For simplicity on one-line diagrams, the protection function is usually identified by an
ANSI
The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
device number. In the era of electromechanical and solid state relays, any one relay could implement only one or two protective functions, so a complete protection system may have many relays on its panel. In a digital/numeric relay, many functions are implemented by the microprocessor programming. Any one numeric relay may implement one or all of these functions.
A listing of device numbers is found at
ANSI Device Numbers.
A summary of some common device numbers seen in digital relays is:
* 11 – Multi-function Device
* 21 – Distance
* 24 – Volts/Hz
* 25 – Synchronizing
* 27 – Under Voltage
* 32 – Directional Power Element
* 46 – Negative Sequence Current
* 40 – Loss of Excitation
* 47 – Negative Sequence Voltage
* 50 – Instantaneous Overcurrent (N for neutral, G for ground current)
* 51 – Inverse Time Overcurrent (N for neutral, G from ground current)
* 59 – Over Voltage
* 62 – Timer
* 64 – Ground Fault (64F = Field Ground, 64G = Generator Ground)
* 67 – Directional Over Current (typically controls a 50/51 element)
* 79 – Reclosing Relay
* 81 – Under/Over Frequency
* 86 – Lockout Relay / Trip Circuit Supervision
* 87 – Current Differential (87L=transmission line diff; 87T=transformer diff; 87G=generator diff)
See also
*
Polyphase system
A polyphase system is a means of distributing alternating-current (AC) electrical power where the power transfer is constant during each electrical cycle. AC phase refers to the phase offset value (in degrees) between AC in multiple conducting ...
*
Overhead power line
An overhead power line is a structure used in electric power transmission and distribution to transmit electrical energy across large distances. It consists of one or more uninsulated electrical cables (commonly multiples of three for three-p ...
*
Power outage
A power outage (also called a powercut, a power out, a power failure, a power blackout, a power loss, or a blackout) is the loss of the electrical power network supply to an end user.
There are many causes of power failures in an electricit ...
*
Three-phase electric power
Three-phase electric power (abbreviated 3φ) is a common type of alternating current used in electricity generation, transmission, and distribution. It is a type of polyphase system employing three wires (or four including an optional neutral ...
References
External links
"Understanding Microprocessor-Based Technology Applied to Relaying" Working Group Report IEEE Power System Relaying Committee
*
* http://www.scielo.org.za/pdf/jesa/v25n2/10.pdf A review on protective relays' developments and trends
* Video that explains overcurrent protection testing: https://www.youtube.com/watch?v=bQ6fZrrP0H4
{{DEFAULTSORT:Digital Protective Relay, R
Electric power infrastructure
Electric power distribution
Relays