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Digital protective relay

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Protective relay

In utility and industrial electric power transmission and distribution systems, a digital protective relay uses

a microcontroller  with software-based protection algorithms for the detection of electrical or process

faults.[1] Such relays are also termed as microprocessor  type protective relays. They are functional

replacements for electromechanical  protective relays and may include many protection functions in one unit,

as well as providing metering, communication, and self-test functions.

Contents

  1 Description and definition 

o  1.1 Input processing 

o 

1.2 Logic processing 

o  1.3 Parameter setting 

o 

1.4 Event recording 

o 

1.5 Data display 

  2 Comparison with other types 

 

3 History   4 Protective element types 

  5 See also 

  6 References 

  7 External links 

Description and definition

The digital protective relay is the most, or numeric relay, is a  protective relay that uses a microprocessor  to

analyze power system voltages, currents or other process quantities for the purpose of detection of faults in

an industrial process system.

Input processing

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Low voltage and low current signals (i.e., at the secondary of a voltage transformers and current

transformers) 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 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 from

4 to 64 (varies by relay) samples per power system cycle. As a minimum, magnitude of the incoming

quantity, commonly using Fourier transform 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 , reactive

 power , impedance, waveform distortion, and other  complex 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 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  jumpers to accessing the relay's internal parameter setting webpage via communications link onanother computer hundreds of kilometres 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 ), 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 oscillographic 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 system. Communication ports may include RS232/RS485 or  Ethernet 

(copper or fibre-optic). Communication languages may include Modbus, DNP3 or  IEC61850  protocols.

Comparison with other types

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 Electromechanical protective relays at a hydroelectric generation station

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  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 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.

History

The digital/numeric relay was introduced in the early 1980s, with AREVA, 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, 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 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:

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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

  Polyphase system 

  Overhead powerline 

 

Power outage 

  Three-phase electric power  

References

1.  "Schweitzer Programmable Automation Controller". Schweitzer Engineering Laboratories. Retrieved

21 November 2012.

External links

  List of manufacturers of protective relays 

Categories: 

 

Electric power infrastructure   Electric power distribution 

  Electrical engineering