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A digital protective relay utilizes a microcontroller 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 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 system.
As a point of comparison, an electromechanical 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 stay within analog circuits that use transformers, resistor, capacitors, inductors, transistors, op amps, comparators, etc. The incoming waveform is not recorded or sent into an A/D circuit. 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 relatively simple microprocessor does some of the relay logic, but the logic is relatively 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 tripping 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 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 relatively complete listing of device numbers is found at the site ANSI Device Numbers. 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)
Manufacturers
There are many more than listed here. This especially becomes true when one includes relays manufactured for niche or regional markets, and manufactures that offer relays in part hidden and buried within a larger product mix.
- GE Multilin
- ABB
- AREVA T&D
- Basler
- Bresler
- Beckwith
- Cooper
- Cutler Hammer
- DEIF
- General Electric
- RFL
- Schneider Electric
- Schweitzer
- Siemens
- Orion Italia
- VAMP
- ZIV
- NARI
See also
References
See the various mfr web sites. Most have libraries of technical data, including both commercial/product oriented, and non-commercial technical data.
External links
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