PROTECTION RELAY | PROTECTIVE RELAY ON POWER SYSTEM

It is one of the six functional categories of relays. Protection relays detect defective lines, defective apparatus, or other dangerous or intolerable conditions. These relays generally trip one or more circuit breakers or isolators (circuit), but may also be used to sound an alarm.
Types of Protection Relays | Protective Relays and their respective ANSI designation numbers are found below.

1 - Master Element
2 - Time Delay Starting or Closing Relay
3 - Checking or Interlocking Relay
4 - Master Contactor
5 - Stopping Device
6 - Starting Circuit Breaker
7 - Rate of Change Relay
8 - Control Power Disconnecting Device
9 - Reversing Device
10 - Unit Sequence Switch
11 - Multi-function Device
12 - Overspeed Device
13 - Synchronous-speed Device
14 - Underspeed Device
15 - Speed - or Frequency, Matching Device
16 - Data Communications Device
17 - Shunting or Discharge Switch
18 - Accelerating or Decelerating Device
19 - Starting to Running Transition Contactor
20 - Electrically Operated Valve
21 - Distance Relay
22 - Equalizer Circuit Breaker
23 - Temperature Control Device
24 - Volts Per Hertz Relay
25 - Synchronizing or Synchronism-Check Device
26 - Apparatus Thermal Device
27 - Undervoltage Relay
28 - Flame detector
29 - Isolating Contactor or Switch
30 - Annunciator Relay
31 - Separate Excitation Device
32 - Directional Power Relay
33 - Position Switch
34 - Master Sequence Device
35 - Brush-Operating or Slip-Ring Short-Circuiting Device
36 - Polarity or Polarizing Voltage Devices
37 - Undercurrent or Underpower Relay
38 - Bearing Protective Device
39 - Mechanical Condition Monitor
40 - Field (over/under excitation) Relay
41 - Field Circuit Breaker
42 - Running Circuit Breaker
43 - Manual Transfer or Selector Device
44 - Unit Sequence Starting Relay
45 - Abnormal Atmospheric Condition Monitor
46 - Reverse-phase or Phase-Balance Current Relay
47 - Phase-Sequence or Phase-Balance Voltage Relay
48 - Incomplete Sequence Relay
49 - Machine or Transformer, Thermal Relay
50 - Instantaneous Overcurrent Relay
51 - AC Inverse Time Overcurrent Relay
52 - AC Circuit Breaker
53 - Exciter or DC Generator Relay
54 - Turning Gear Engaging Device
55 - Power Factor Relay
56 - Field Application Relay
57 - Short-Circuiting or Grounding Device
58 - Rectification Failure Relay
59 - Overvoltage Relay
60 - Voltage or Current Balance Relay
61 - Density Switch or Sensor
62 - Time-Delay Stopping or Opening Relay
63 - Pressure Switch
64 - Ground Detector Relay
65 - Governor
66 - Notching or Jogging Device
67 - AC Directional Overcurrent Relay
68 - Blocking or "Out-of-Step" Relay
69 - Permissive Control Device
70 - Rheostat
71 - Liquid Level Switch
72 - DC Circuit Breaker
73 - Load-Resistor Contactor
74 - Alarm Relay
75 - Position Changing Mechanism
76 - DC Overcurrent Relay
77 - Telemetering Device
78 - Phase-Angle Measuring Relay
79 - AC Reclosing Relay
80 - Flow Switch
81 - Frequency Relay
82 - DC Reclosing Relay
83 - Automatic Selective Control or Transfer Relay
84 - Operating Mechanism
85 - Communications,Carrier or Pilot-Wire Relay
86 - Lockout Relay
87 - Differential Protective Relay
88 - Auxiliary Motor or Motor Generator
89 - Line Switch
90 - Regulating Device
91 - Voltage Directional Relay
92 - Voltage and Power Directional Relay
93 - Field Changing Contactor
94 - Tripping or Trip-Free Relay

In a power system where expensive power equipment are on hand, it is important to protect them from damage caused by abnormal conditions, such as faults, over voltage, and others. Another apparatus or equipment does the actual isolation from the system, but the protective relays are the ones that gives signal to those equipment. In short, protective relays or relays in general are the brains of the protection system.

Protective relays monitor large AC currents or voltage and utilized them into manageable levels through instrument transformers. Instrument transformers step down the monitored current or voltage to a secondary (output) to power the protective relay. These output are used as signal to power its internal mechanism, closing a contact to switch the DC power isolating or breaking apparatus.

Protective relays are not required to function during normal conditions, but must immediately be made  available to handle intolerable system conditions to avoid serious outages and damage. The true operating life of these relays can be in the order of few seconds.

Protection relays or relays in general can either be analog or digital.

Analog relays are those in which the measured quantities are converted into lower voltage but similar signals, which are then combined or compared directly to reference values in level detectors to produce the desired outputs.

Digital relays are those in which the measured ac quantities are manipulated in analog form and subsequently converted into square wave (binary) voltages. Logic circuits or microprocessors compare the phase relationships of the square waves to make a trip decision.

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