Relay Coordination Basics | Principles and Objectives

Relay Coordination Basics. Knowing the principles are very important in power system protection and coordination. First things first, one must understand these principles by heart.

Prevent human injury arising from short circuits or equipment failure

The most important objective of power system protection is preventing human injury. Therefore, it is important that short circuit protection devices such as protection relays are properly configured. Likewise, all circuit breakers, fuses, and other fault interrupting devices should be selected and sized correctly.

Limit the damage to equipment as a result of short circuits in the power system or failure of adjacent equipment

The protection system should be designed to limit the damage to equipment. This is in order to minimize the cost of repair or replacement including equipment downtime.

Limit the extent of service interruption to ensure a very high level of continuity of service.

While limiting the damage to equipment is very important, ensuring that service interruption is kept at a minimum is equally important. In most cases, the design of the protection system should be the best compromise between these two.

Relay Coordination Basics

The principles of relay coordination can be translated into the following objectives.

Reliability

IEEE specifically defined reliability as

“the degree of certainty that a relay or relay system will operate correctly” – dependability

IEEE C37.2, 2008

and

“the degree of certainty that a relay or relay system will not operate incorrectly” – security

IEEE C37.2, 2008

In other words, we can define reliability as the measure of the soundness of the protection system.

Selectivity

This the measure of how well the protection system limits service interruptions by isolating the smallest portion of the affected area. This is to ensure a very high level of continuity of service.

Speed of Operation

In order to preserve system stability and limit the damage caused by short circuits, the protection system should isolate the faulted areas as fast a possible.

Simplicity

A good practice in any protection system is to keep things simple. This way, troubleshooting issues can be done quickly.

Sensitivity

A good measure of a well-designed protection system is how well this detects a fault. This is especially important in the case of high-impedance faults and dispersed generation.

Economics

Ensuring a well-protected power system involves cost and there is no better way than achieving maximum protection at a minimal cost.

States of a Power System

From one of our previous discussions, we presented the following power system states: normal, abnormal, and faulted state.

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Normal State

In a normal state, the system is operating within the designed limits and maximum equipment life expectancy is anticipated.

Abnormal State

An abnormal state is when the system operates outside the designed limits for short periods that may accelerate equipment aging.

Faulted State

A faulted state is when the system is subjected to severe stress and where equipment failure is very likely. It may be caused by natural events, accidents, deterioration of insulation, and other causes that are impossible or impractical to avoid. Moreover, it can be distinguished generally by a sudden and significant increase in current. It is because of this high current that the system is subjected to high levels of stress given that mechanical and thermal stress are functions of the square of the current.

Power System Faults

Most faults in the power system, typically on overhead lines, is caused by lightning-induced transients, and vegetation. Based on statistical data, single line-to-ground occurs more frequently than other types of faults. This accounts for 70% to 80% of all fault occurrences.

So how are power system faults detected?

The detection of faults in the power system is basically done by monitoring changes in the system operating quantities such as:

• Current
• Voltage
• Power Factor
• Power Flow
• Impedance
• Frequency
• Temperature
• Pressure
• Mechanical movements

Of these operating quantities, changes in current are the easiest to detect and is the most common fault indicator. That is why overcurrent protection is widely used.

Protective Relaying

The IEEE defines a protective relay as

“a relay whose function is to detect defective lines or apparatus or other power system conditions of an abnormal or dangerous nature and to initiate appropriate control circuit action”

IEEE 100, 2000

Protective relaying is, therefore, the craft of configuring protection relays to identify an intolerable system condition, initiate an alarm, and/or circuit isolation at the right instant. This is relay coordination basics!

From the book, Protective Relaying by J. Blackburn and T. Domin, protective relaying is defined as

“A nonprofit, nonrevenue-producing item that is not necessary in the normal operation of an electric power system until a fault, an abnormal, intolerable situation, occurs.”

J. Blackburn and T. Domin

Sounds about right, right?

Protective Devices

The following are the most common protective devices on many power systems around the world.

• Fuses
• Automatic reclosers
• Sectionalizers
• Circuit breakers
• Protective relays

Other Relay Classifications

Aside from protective relays, relays can be classified according to their function. The following are the most common relay classifications which are present many power systems.

• Protective
• Regulating
• Reclosing and synchronism check
• Monitoring
• Auxiliary

ANSI Device Numbers

ANSI /IEEE Standard C37.2 Standard for Electrical Power System Device Function Numbers, Acronyms, and Contact Designations ) lists Device numbers which used to identify the functions of devices shown on a protection schematic diagram.

List of device numbers and acronyms

1 - Master Element
2 - Time-delay Starting or Closing Relay
3 - Checking or Interlocking Relay, complete Sequence
4 - Master Protective
5 - Stopping Device, Emergency Stop Switch
6 - Starting Circuit Breaker
7 - Rate of Change Relay
8 - Control Power Disconnecting Device
9 - Reversing Device
10 - Unit Sequence Switch
11 - Multifunction 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 ( Solenoid Valve )
21 - Distance Relay
21G - Ground Distance
21P - Phase Distance
22 – Equalizer circuit breaker
23 – Temperature control device, Heater
24 – Volts per hertz relay
25 – Synchronizing or synchronism-check device
26 – Apparatus thermal device, Temperature Switch
27 – Undervoltage relay
27P - Phase Undervoltage
27S - DC undervoltage relay
27TN - Third Harmonic Neutral Undervoltage
27TN/59N - 100% Stator Earth Fault
27X - Auxiliary Undervoltage
27 AUX - Undervoltage Auxiliary Input
27/27X - Bus/Line Undervoltage
27/50 - Accidental Generator Energization
28 - Flame Detector
29 - Isolating Contactor
30 - Annunciator Relay
31 - Separate Excitation Device
32 - Directional Power Relay
32L - Low Forward Power
32H - High Directional Power
32N - Wattmetric Zero-Sequence Directional
32P - Directional Power
32R - Reverse Power
33 - Position Switch
34 - Master Sequence Device
35 - Brush-Operating or Slip-ring Short Circuiting Device
36 - Polarity or Polarizing Voltage Device
37 - Undercurrent or Underpower Relay
37P - Underpower
38 - Bearing Protective Device / Bearing Rtd
39 - Mechanical Condition Monitor ( Vibration )
40 - Field Relay / Loss of Excitation
41 - Field Circuit Breaker
42 - Running Circuit Breaker
43 - Manual Transfer or Selector Device
44 - Unit Sequence Starting Relay
45 - Fire Detector
46 - Reverse-Phase or Phase Balance Current Relay or Stator Current Unbalance
47 - Phase-Sequence or Phase Balance Voltage Relay
48 - Incomplete Sequence Relay / Blocked Rotor
49 - Machine or Transformer Thermal Relay / Thermal Overload
49RTD - RTD Biased Thermal Overload
50 - Instantaneous Overcurrent Relay
50BF - Breaker Failure
50DD - Current Disturbance Detector
50EF - End Fault Protection
50G - Ground Instantaneous Overcurrent
50IG - Isolated Ground Instantaneous Overcurrent
50LR - Acceleration Time
50N - Neutral Instantaneous Overcurrent
50NBF - Neutral Instantaneous Breaker Failure
50P - Phase Instantaneous Overcurrent
50SG - Sensitive Ground Instantaneous Overcurrent
50SP - Split Phase Instantaneous Current
50Q - Negative Sequence Instantaneous Overcurrent
50/27 - Accidental Energization
50/51 - Instantaneous / Time-delay Overcurrent relay
50Ns/51Ns - Sensitive earth-fault protection
50/74 - Ct Trouble
50/87 - Instantaneous Differential
51 - AC Time Overcurrent Relay
51G - Ground Time Overcurrent
51LR - AC inverse time overcurrent (locked rotor) protection relay
51N - Neutral Time Overcurrent
51P - Phase Time Overcurrent
51R - Locked / Stalled Rotor
51V - Voltage Restrained Time Overcurrent
51Q - Negative Sequence Time Overcurrent
52 – AC circuit breaker
52a - AC circuit breaker position (contact open when circuit breaker open)
52b - AC circuit breaker position (contact closed when circuit breaker open)
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
59B - Bank Phase Overvoltage
59P - Phase Overvoltage
59N - Neutral Overvoltage
59NU - Neutral Voltage Unbalance
59P - Phase Overvoltage
59X - Auxiliary Overvoltage
59Q - Negative Sequence Overvoltage
60 - Voltage or Current Balance Relay
60N - Neutral Current Unbalance
60P - Phase Current Unbalance
61 - Density Switch or Sensor
62 - Time-Delay Stopping or Opening Relay
63 - Pressure Switch Detector
64 - Ground Protective Relay
64F - Field Ground Protection
64R – Rotor earth fault
64REF – Restricted earth fault differential
64S – Stator earth fault
64S - Sub-harmonic Stator Ground Protection
64TN - 100% Stator Ground
65 - Governor
66 - Notching or Jogging Device/Maximum Starting Rate/Starts Per Hour/Time Between Starts
67 - AC Directional Overcurrent Relay
67G - Ground Directional Overcurrent
67N - Neutral Directional Overcurrent
67Ns – Earth fault directional
67P - Phase Directional Overcurrent
67SG - Sensitive Ground Directional Overcurrent
67Q - Negative Sequence Directional Overcurrent
68 - Blocking Relay / Power Swing Blocking
69 - Permissive Control Device
70 - Rheostat
71 - Liquid Switch, Level Switch
72 - DC Circuit Breaker
73 - Load-Resistor Contactor
74 - Alarm Relay
75 - Position Changing Mechanism
76 - DC Overcurrent Relay
77 - Telemetering Device, Speed Sensor
78 - Phase Angle Measuring or Out-of-Step Protective Relay
78V - Loss of Mains
79 - AC Reclosing Relay / Auto Reclose
80 - Liquid or Gas Flow Relay
81 - Frequency Relay
81O - Over Frequency
81R - Rate-of-Change Frequency
81U - Under Frequency
82 - DC Reclosing Relay
83 - Automatic Selective Control or Transfer Relay
84 - Operating Mechanism
85 - Pilot Communications, Carrier or Pilot-Wire Relay
86 - Lock-Out Relay, Master Trip Relay
87 - Differential Protective Relay
87B - Bus Differential
87G - Generator Differential
87GT - Generator/Transformer Differential
87L - Segregated Line Current Differential
87LG - Ground Line Current Differential
87M - Motor Differential
87O - Overall Differential
87PC - Phase Comparison
87RGF - Restricted Ground Fault
87S - Stator Differential
87S - Percent Differential
87T - Transformer Differential
87V - Voltage Differential
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
95 – For specific applications where other numbers are not suitable
96 – Transmitter
97 – For specific applications where other numbers are not suitable
98 – For specific applications where other numbers are not suitable
99 – For specific applications where other numbers are not suitable

Acronyms Description

AFD - Arc Flash Detector
CLK - Clock or Timing Source
CLP - Cold Load Pickup
DDR – Dynamic Disturbance Recorder
DFR – Digital Fault Recorder
DME – Disturbance Monitor Equipment
ENV – Environmental data
HIZ – High Impedance Fault Detector
HMI – Human Machine Interface
HST – Historian
LGC – Scheme Logic
MET – Substation Metering
PDC – Phasor Data Concentrator
PMU – Phasor Measurement Unit
PQM – Power Quality Monitor
RIO – Remote Input/Output Device
RTD - Resistance Temperature Detector
RTU – Remote Terminal Unit/Data Concentrator
SER – Sequence of Events Recorder
TCM – Trip Circuit Monitor
LRSS – Local/Remote selector switch
VTFF - Vt Fuse Fail

Suffixes Description

_1 - Positive-Sequence
_2 - Negative-Sequence
A - Alarm, Auxiliary Power
AC - Alternating Current
AN - Anode
B - Bus, Battery, or Blower
BF - Breaker Failure
BK - Brake
BL - Block (Valve)
BP - Bypass
BT - Bus Tie
BU - Backup
C - Capacitor, Condenser, Compensator, Carrier Current, Case or Compressor
CA - Cathode
CH - Check (Valve)
D - Discharge (Valve)
DC - Direct Current
DCB - Directional Comparison Blocking
DCUB - Directional Comparison Unblocking
DD - Disturbance Detector
DUTT - Direct Underreaching Transfer Trip
E - Exciter
F - Feeder, Field, Filament, Filter, or Fan
G - Ground or Generator
GC - Ground Check
H - Heater or Housing
L - Line or Logic
M - Motor or Metering
MOC - Mechanism Operated Contact
N - Neutral or Network
O - Over
P - Phase or Pump
PC - Phase Comparison
POTT - Pott: Permissive Overreaching Transfer Trip
PUTT - Putt: Permissive Underreaching Transfer Trip
R - Reactor, Rectifier, or Room
S - Synchronizing, Secondary, Strainer, Sump, or Suction (Valve)
SOTF - Switch On To Fault
T - Transformer or Thyratron
TD - Time Delay
TDC - Time-Delay Closing Contact
TDDO - Time Delayed Relay Coil Drop-Out
TDO - Time-Delay Opening Contact
TDPU - Time Delayed Relay Coil Pickup
THD - Total Harmonic Distortion
TH - Transformer (High-Voltage Side)
TL - Transformer (Low-Voltage Side)
TM - Telemeter
TT - Transformer (Tertiary-Voltage Side)
Q - Lube Oil
W - Water
F - Fuel
G - Gas
U - Under or Unit
X - Auxiliary
Z - Impedance

And that’s it for relay coordination principles.

References:

IEEE Std 242-2001 [The Buff Book]: IEEE Recommended Practice for Protection and Coordination of Industrial and Commercial Power Systems.(2001). S.I.: IEEE.

Blackburn, J. (2014). Protective Relaying Principles and Application, 4th ed. Boca Raton, FL: CRC Press.

G. Pradeep Kumar (2006), Power System Protection Design, notes on Power System Protection Training, Visayan Electric Company, Cebu City, Philippines.

Schweitzer Engineering Laboratories (2013). Protection Basics.

ANSI Device Numbers, https://en.wikipedia.org.