A
digital protective relay uses a
microcontrollerA microcontroller is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog timer, serial and analog I/O etc. Program memory in the form of NOR flash or OTP ROM is also often included on...
with software-based protection algorithms for the detection of electrical faults.
The digital protective relay, also called a numeric relay by some manufacturers and resources, refers to a protective relay that uses an advanced microprocessor to analyze power system voltages and currents for the purpose of detection of faults in an electric power system.
A
digital protective relay uses a
microcontrollerA microcontroller is a small computer on a single integrated circuit consisting of a relatively simple CPU combined with support functions such as a crystal oscillator, timers, watchdog timer, serial and analog I/O etc. Program memory in the form of NOR flash or OTP ROM is also often included on...
with software-based protection algorithms for the detection of electrical faults.
Description and definition
The digital protective relay, also called a numeric relay by some manufacturers and resources, refers to a protective relay that uses an advanced microprocessor to analyze power system voltages and currents for the purpose of detection of faults in an electric power system. There are gray areas on what constitutes a digital/numeric relay, but most engineers will recognize the design as having the majority of these attributes:
- The relay applies A/D (analog/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, most commonly using Fourier transform
In mathematics, Fourier analysis is a subject area which grew out of the study of Fourier series. The subject began with trying to understand when it was possible to represent general functions by sums of simpler trigonometric functions...
concepts (RMS and some form of averaging are used in basic products). Further, the Fourier transform is commonly used to extract the signal's phase angle 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), complex 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 advanced 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.
- The more modern versions of the digital relay will contain advanced metering and communication protocol ports, allowing the relay to become a focal point in a SCADA
SCADA stands for supervisory control and data acquisition. It generally refers to an industrial control system: a computer system monitoring and controlling a process...
system.
As a point of comparison, an
electromechanical relayA relay is an electrically operated switch. Electric current through the coil of the relay creates a magnetic field which attracts a lever and changes the switch contacts...
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 relayA solid state relay is an electronic switch, which, unlike an electromechanical relay, contains no moving parts. The types of SSR are photo-coupled SSR, transformer-coupled SSR, and hybrid SSR. A photo-coupled SSR is controlled by a low voltage signal which is isolated optically from 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 potentiometers 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
rectifierA rectifier is an electrical device that converts alternating current to direct current , a process known as rectification. Rectifiers have many uses including as components of power supplies and as detectors of radio signals...
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'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 1990's 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 VT and CT) 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-lines, the element/function is usually identified by what is referred to as an ANSI device number, and hence there are three terms (element, function, device number) in use for approximately the same concept. In the era of electromechanical and solid state relays, any one relay could implement only one or two protective elements/functions, so a complete protection system may have many relays on its panel. In a digital/numeric relay, many functions/elements are implemented by the microprocessor programming. Any one digital/numeric relay may implement one or all of these device numbers/functions/elements.
A listing of device numbers is found at the site
ANSI Device NumbersIn 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:
- 21 - Impedance (21G implies ground impedance)
- 27 - Under Voltage (27LL = line to line, 27LN = line to neutral/ground)
- 32 - Directional Power Element
- 46 - Negative sequence current
- 47 - Negative sequence voltage
- 50 - Instantaneous OverCurrent (subscript N or G implies Ground)
- 51 - Inverse Time Overcurrent (subscript N or G implies Ground)
- 59 - Over Voltage (59LL = line to line, 59LN = line to neutral/ground)
- 67 - Directional Over Current (typically controls a 50/51 element)
- 79 - Auto-reclosure
- 81 - Under/Over Frequency
- 87 - Current Differential (87L=transmission line diff; 87T=transformer diff; 87G=generator diff)
- 86 - Trip Circuit Supervision
See also
- Power Transmission
Power transmission is the movement of energy from its place of generation to a location where it is applied to performing useful work.Power is defined formally as units of energy per unit time...
- Polyphase system
A 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 distribution
File:Electricity grid simple- North America.svg|thumb|380px|right|Simplified diagram of AC electricity distribution from generation stations to consumersrect 2 243 235 438 Power stationrect 276 317 412 556 Transformer...
- Overhead powerline
An overhead power line is an electric power transmission line suspended by towers or 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 power...
- Power outage
A 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 power
Three-phase electric power is a common method of alternating-current electric power transmission. It is a type of polyphase system, and is the most common method used by electric power distribution grids worldwide to distribute power. It is also used to power large motors and other large loads...
External links / manufacturers