Digital protective relay
Encyclopedia
In electrical engineering
of power systems, a digital protective relay uses a microcontroller
with software-based protection algorithms for the detection of electrical faults
. Such relays are also termed as microprocessor
type protective relays.
that uses a microprocessor to analyze power system voltages and currents for the purpose of detection of faults in an electric power system. General characteristics of a digital protective relay include:
By contrast, an electromechanical protective relay
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
, the incoming voltage and current waveforms are monitored by analog circuits, not recorded or digitized. The analog values are compared to settings made by the user via potentiometer
s in the relay, and in some case, taps on transformers.
In some solid state relays, a simple microprocessor 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
and filter that converts the AC to a DC value proportionate to the AC waveform. An op-amp 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 setpoint. 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.
The digital/numeric relay was introduced in the early 1980s, with AREVA
and ABB Group's forerunners and SEL
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 electromechanical 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, and which allows one to avoid the complexity of digital relays.
s) are brought into a low pass filter that removes frequency content above about 1/3 of the sampling frequency (a relay A/D converter needs to sample faster than 2x 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 at anywhere from about 4 to 64 (varies by relay) samples per power system cycle. In some relays, the entire sampled data is kept for oscillographic records, but in the relay, 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 of interest (i.e., nominal system frequency), and uses Fourier transform algorithms to extract the fundamental frequency magnitude and angle. Next the microprocessor passes the data into a set of protection algorithms, which are a set of logic equations in part designed by the protection engineer, and in part designed by the relay manufacturer, that monitor for abnormal conditions that indicate a fault. If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).
For simplicity on one-line diagrams, the protection function is usually identified by an ANSI 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:
Electrical engineering
Electrical engineering is a field of engineering that generally deals with the study and application of electricity, electronics and electromagnetism. The field first became an identifiable occupation in the late nineteenth century after commercialization of the electric telegraph and electrical...
of power systems, a digital protective relay uses a microcontroller
Microcontroller
A microcontroller is a small computer on a single integrated circuit containing a processor core, memory, and programmable input/output peripherals. Program memory in the form of NOR flash or OTP ROM is also often included on chip, as well as a typically small amount of RAM...
with software-based protection algorithms for the detection of electrical faults
Fault (power engineering)
In an electric power system, a fault is any abnormal flow of electric current. For example, a short circuit is a fault in which current flow bypasses the normal load. An open-circuit fault occurs if a circuit is interrupted by some failure. In three-phase systems, a fault may involve one or more...
. Such relays are also termed as microprocessor
Microprocessor
A microprocessor incorporates the functions of a computer's central processing unit on a single integrated circuit, or at most a few integrated circuits. It is a multipurpose, programmable device that accepts digital data as input, processes it according to instructions stored in its memory, and...
type protective relays.
Description and definition
The digital protective relay, or numeric relay , is a protective relayProtective relay
In electrical engineering, a protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected...
that uses a microprocessor to analyze power system voltages and currents for the purpose of detection of faults in an electric power system. General characteristics of a digital protective relay include:
- The relay applies A/D (analog-to-digital) conversion processes to the incoming voltages and currents.
- The relay analyzes the A/D converter output to extract, as a minimum, magnitude of the incoming quantity, commonly using Fourier transformFourier transformIn mathematics, Fourier analysis is a subject area which grew from the study of Fourier series. The subject began with the study of the way general functions may be represented by sums of simpler trigonometric functions...
concepts (RMSRMS-Science and technology:* Root mean square, a concept encapsulating the "average", in some sense, of a quantity. Frequently encountered in statistics, the physical sciences and electronics...
and some form of averaging are used in basic products). Further, the Fourier transform is commonly used to extract the signal's phase anglePhase angleIn the context of vectors and phasors, the term phase angle refers to the angular component of the polar coordinate representation. The notation A\ang \!\ \theta, for a vector with magnitude A and phase angle θ, is called angle notation.In the context of periodic phenomena, such as a wave,...
relative to some reference, except in the most basic applications. - The relay is capable of applying advanced logic. It is capable of analyzing whether the relay should trip or restrain from tripping based on current and/or voltage magnitude (and angle in some applications), parameters set by the user, relay contact inputs, and in some applications, the timing and order of event sequences.
- The logic is user-configurable at a level well beyond simply changing front panel switches or moving of jumpers on a circuit board.
- The relay has some form of event recording. 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 oscillographic fashion at least the fundamental frequency component of the incoming AC waveform.
- The relay has 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), and this same PC interface is used to collect event reports from the relay.
- Digital/Numerical relays also provides LCD Display, or display on a terminal through serial interface. This is used to display current/voltage values in real-time, and relay settings etc.
- More complex digital relays will have metering and communication protocol ports, allowing the relay to become an element in a SCADASCADASCADA generally refers to industrial control systems : computer systems that monitor and control industrial, infrastructure, or facility-based processes, as described below:...
system. - Communications protocol ports may include MODBUS interface, RS232/RS485 interface, or IEC61850 based substation automation interface on high-end models.
By contrast, an electromechanical protective relay
Protective relay
In electrical engineering, a protective relay is a complex electromechanical apparatus, often with more than one coil, designed to calculate operating conditions on an electrical circuit and trip circuit breakers when a fault is detected...
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 relay
A solid state relay is an electronic switching device in which a small control signal controls a larger load current or voltage. It comprises a voltage or current sensor which responds to an appropriate input , a solid-state electronic switching device of some kind which switches power to the load...
, the incoming voltage and current waveforms are monitored by analog circuits, not recorded or digitized. The analog values are compared to settings made by the user via potentiometer
Potentiometer
A potentiometer , informally, a pot, is a three-terminal resistor with a sliding contact that forms an adjustable voltage divider. If only two terminals are used , it acts as a variable resistor or rheostat. Potentiometers are commonly used to control electrical devices such as volume controls on...
s in the relay, and in some case, taps on transformers.
In some solid state relays, a simple microprocessor 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
Rectifier
A rectifier is an electrical device that converts alternating current , which periodically reverses direction, to direct current , which flows in only one direction. The process is known as rectification...
and filter that converts the AC to a DC value proportionate to the AC waveform. An op-amp 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 setpoint. 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.
The digital/numeric relay was introduced in the early 1980s, with AREVA
Areva
AREVA is a French public multinational industrial conglomerate headquartered in the Tour Areva in Courbevoie, Paris. AREVA is mainly known for nuclear power; it also has interests in other energy projects. It was created on 3 September 2001, by the merger of Framatome , Cogema and...
and ABB Group's forerunners and SEL
Schweitzer Engineering Laboratories
Schweitzer Engineering Laboratories, Inc. designs, manufactures, and supports products and services ranging from generator and transmission protection to distribution automation and control systems. Founded in 1982 by Edmund O. Schweitzer III, SEL shipped the world's first digital protective relay...
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 electromechanical 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, and which allows one to avoid the complexity of digital relays.
Basic principles
Low voltage and low current signals (i.e., at the secondary of a Voltage transformers and Current transformerCurrent transformer
In electrical engineering, a current transformer is used for measurement of electric currents. Current transformers, together with voltage transformers , are known as instrument transformers...
s) are brought into a low pass filter that removes frequency content above about 1/3 of the sampling frequency (a relay A/D converter needs to sample faster than 2x 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 at anywhere from about 4 to 64 (varies by relay) samples per power system cycle. In some relays, the entire sampled data is kept for oscillographic records, but in the relay, 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 of interest (i.e., nominal system frequency), and uses Fourier transform algorithms to extract the fundamental frequency magnitude and angle. Next the microprocessor passes the data into a set of protection algorithms, which are a set of logic equations in part designed by the protection engineer, and in part designed by the relay manufacturer, that monitor for abnormal conditions that indicate a fault. If a fault condition is detected, output contacts operate to trip the associated circuit breaker(s).
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 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
ANSI Device Numbers
In the design of electrical power systems, the ANSI Standard Device Numbers denote what features a protective device supports . These types of devices protect electrical systems and components from damage when an unwanted event occurs, such as an electrical fault...
.
A summary of some common device numbers seen in digital relays is:
- 11 - Multifunction device.
- 21 - Impedance
- 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
- Electric power transmissionElectric power transmissionElectric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
- Polyphase systemPolyphase systemA polyphase system is a means of distributing alternating current electrical power. Polyphase systems have three or more energized electrical conductors carrying alternating currents with a definite time offset between the voltage waves in each conductor. Polyphase systems are particularly useful...
- Electricity distributionElectricity distributionFile:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumers...
- Overhead powerlineOverhead powerlineAn overhead power line is an electric power transmission line suspended by towers or utility poles. Since most of the insulation is provided by air, overhead power lines are generally the lowest-cost method of transmission for large quantities of electric energy...
- Power outagePower outageA power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
- Three-phase electric powerThree-phase electric powerThree-phase electric power is a common method of alternating-current electric power generation, transmission, and distribution. It is a type of polyphase system and is the most common method used by grids worldwide to transfer power. It is also used to power large motors and other heavy loads...