PROTECTIVE
RELAYING
Principles &
Philosophies
FORTUNATO C. LEYNES, FIIEE
Chairman
Board of Electrical Engineering
Professional Regulation Commission
Vice President
Manila Electric Company
15th IIEE Region 8 Conference
June 26, 2010
Protective Relaying
The branch of electric power engineering concerned
with the principles of design, construction/
installation, operation and maintenance of
equipment (called “relays or protective relays”)
which detect abnormal power system conditions,
and initiate corrective action as quickly as possible
in order to return the power system to its normal
state.
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ARE PROTECTIVE RELAYING PRACTICES
BASED ON THE PROBABILITY OF FAILURE
• protective relaying practices are based on the probability of
failure to the extent that present-day practices are the result
of years of experience in which the frequency of failure
undoubtedly has played a part;
• the probability of failure, seldom if ever, enters directly into
the choice of a particular type of relaying equipment except
when, for one reason or another, one finds it most difficult to
apply the type that otherwise would be used;
• more importantly, the probability of failure should be
considered only together with the consequences of failure
should it occur;
• the justification for a given practice equals the likelihood of
trouble times the cost of the trouble;
• regardless of the probability of failure, no portion of a
system should be entirely without protection, even if it is
only back-up relaying.
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EVALUATION OF
PROTECTIVE RELAYING
• the cost of repairing the damage.
• the likelihood that the trouble may spread and
involve other equipment.
• the time that the equipment is out of service.
• the loss in revenue and the strained public
relations while the equipment is out of service.
By expediting the equipment’s return to
service, protective relaying helps to
minimize the amount of equipment reserve
required, since there is less likelihood of
another failure before the first failure can be
repaired.
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PROTECTION SYSTEM
OBJECTIVES
1. To remove the faulty device from the power system to
prevent or minimize hazards to people, equipment damage,
and adverse effect upon the normal operation of the
remaining system.
2. To provide alternate means for removing the faulty device, for
the same reason as in 1, when there is a protective
equipment failure such as a breaker or any primary
protection.
3. Prevent operation of protective system for heavy load surges
and power swings or other conditions that will not cause
damage or adversely affect operation of the system.
4. Recognize when a catastrophic system failure is imminent or
has occurred and take necessary steps to minimize the
disturbance and facilitate the speedy restoration to normal
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FACTORS AFFECTING THE
PROTECTION SYSTEM
•
•
•
•
Economics
‘‘Personality’’ of the relay engineer and
the characteristics of the power system
Location and availability of
disconnecting and isolating devices
[circuit breakers, switches, and input
devices (CTs and VTs)]
Available fault indicators (fault studies
and such)
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HOW DO PROTECTIVE
RELAYS OPERATE?
These are the parameters that may cause
the protective relays to operate:
–
–
–
–
–
–
–
magnitude (voltage, current, power)
frequency
phase angle
duration
rate of change
direction or order of change
harmonics or wave shape
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RELAY CLASSIFICATIONS
BY FUNCTION
1. Protective relays
2. Regulating relays
3. Reclosing, synchronism check, and
synchronizing relays
4. Monitoring relays
5. Auxiliary relays
6. Other relay classifications
• by operating principles
• by performance characteristics
• etc.
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RELAY CLASSIFICATIONS
BY SPEED OF OPERATION
1. Instantaneous. These relays operate as soon as
a secure decision is made. No intentional time
delay is introduced to slow down the relay
response.
2. Time delay. An intentional time delay is inserted
between the relay decision time and the initiation
of the trip action.
3. High speed. A relay that operates in less than a
specified time. The specified time in present
practice is 50 milliseconds (3 cycles on a 60 Hz
system).
4. Ultra high speed. This term is not included in the
Relay Standards but is commonly considered to be
operation in 4 milliseconds or less.
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CLASSIFICATION OF
RELAY OPERATION
• Correct
• Correct but undesired
• Incorrect
• No conclusion
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CLASSIFICATION OF
RELAY OPERATION
CORRECT TRIPPING
CORRECT TRIPPING BUT UNDESIRED
F
INCORRECT TRIPPING
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PRIMARY AND BACK-UP
PROTECTION
Primary Protection - Schemes that are designed to
specifically protect one equipment zone. In any locations, this
primary relaying may overlap into other zone of protection,
providing additional protection for those zones.
Primary
A. Limited
B. Overlap
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BACK-UP PROTECTION
Schemes that are designed to operate in place of or in
parallel with the primary protection. Back-up protection probably
will sense faults in more that one zone, is usually slower in
operation, and may isolate a larger portion of the system. Backup protection for a specific zone may be provided by a local
scheme or one located remotely.
Back-up
A.
B.
C.
D.
E.
In Place of Primary
Overlap
Slower
Increase Coverage in Isolation
Local/Remote
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THREE MEMBERS OF
PROTECTIVE SYSTEM
1. Sensor - Feeds system information to the relay,
e.g., currents and voltages
2. Relay - Makes a decision as to the need for
action, e.g., overcurrent relay, etc.
3. Switching or Controlling Device - Physically
isolates or control the problem, e.g.,
circuit breaker
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THREE MEMBERS OF
PROTECTIVE SYSTEM
Sensor
Feedback
Signals
Relay
Power Circuit Breaker
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FUNCTIONAL DIAGRAM
OF RELAYING
Decides whether system
quantities are normal or
abnormal
Power
System
Voltage
and
current
transformer
Relay
These devices Change
Electrical Quantities
to a Level low enough
for the relay to use i.e.
5A, 110 V
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Circuit
Breaker
Opens and isolate
a faulty section of
the system as sent
by the relay
ELECTRICAL DIAGRAM OF
RELAYING
CB
CT
Transmission
Line
Trip
Coil
Station
Battery
Relay Contacts
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TYPICAL CONTROL
CIRCUIT
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DEFINITION OF OPERATION
Mechanical movement of the
operating mechanism is imparted to
a contact structure to close or to
open contacts
– we say that a relay "operates," we
mean that it either closes or opens its
contacts - whichever is the required
action under the circumstances.
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RELAY CONTACTS
“a” contact - normally open
contact, it closes when the
relay operates and opens
when the relay resets
“b” contact - normally closed
contact, it opens when the
relay operates and closes
when the relay resets
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INSTRUMENT TRANSFORMERS
(Transducers)
Change the magnitudes, but not the nature of the
measured quantities
Provide isolation from the hostile environment of the
power system
Types
Current Transformers
- CTs
Potential Transformers
- PTs
Voltage Transformers
- VTs
Coupling Capacitor Voltage Transformers - CCVT’s
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CURRENT TRANSFORMERS
Secondary Winding
Iron Core
Primary Conductor
Secondary
Terminals
Rating:
Specify continuous rating of secondary winding (1A, 5A)
Specify primary current which will nominally produce rated
secondary current (e.g., 800A, 1,000A)
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CURRENT TRANSFORMERS
Current Ratio
Polarity:
- Indicated by dots (dot or
square) on drawings
- Indicates instantaneous
relationship in the directions of
primary and secondary currents.
Is
Ip
Current entering the polarity mark on the primary
will cause a current to instantaneously leave the
polarity mark on the secondary
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100/5
200/5
400/5
500/5
600/5
800/5
1000/5
1200/5
2000/5
100/1
200/1
400/1
500/1
600/1
800/1
1000/1
1200/1
2000/1
MOST COMMON TYPES OF
CURRENT TRANSFORMERS
Core-Balanced or Ring type or Doughnut Type
Bushing or the Bar-Type
Wound Primary Type
Rogowsky Coil - Optical CT
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CT EQUIVALENT CIRCUIT
Rct
Ip/N
Rw
Is
Ie
Zm
Ve
Vs
Ip
Zb
Rw
N-turns
Rct - CT Winding
resistance in
ohms/turn
Rw - Lead (wiring)
Resistance
Zb - Burden Impedance
Zm - Magnetizing
Impedance
Ve
Is = Ip/N - Ie
Vs = Is * (Zb + 2Rw)
Ve = Vs + Is*Rct
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Ie
CT EQUIVALENT CIRCUIT
Rct
Ip/N
Rw
Is
Ie
Zm
Ve
Ip
Vs
Zb
Rw
N-turns
Rct - CT Winding
resistance in
ohms/turn
Rw - Lead (wiring)
Resistance
Zb - Burden Impedance
Zm - Magnetizing
Impedance
N - is the “nominal” ratio
of CT
At Saturation point: Is = Ip/N
Ve
Zm will be small which
result in Ie being large
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Ie
CT ERROR CALCULATION
Given:
• Primary Current , Ip
• Total impedance burden on the CT, including lead
wire resistance
• CT Secondary Excitation Characteristics
Neglected Factor: CT transient characteristic
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CT ERROR CALCULATION
Rct
Ip/N
Rw
Is
Ve
Ie
Zm
Ve
Vs
Zb
Rw
Ip
N-turns
Ie
Given :
Is, Zb, Secondary Excitation Characteristic curve
Steps :
1.
2.
3.
4.
5.
From the Burden and Is, cal. Vs
From Vs, Rct and Is, cal. Ve
From Ve and Sec. Excitation curve, determine Ie
From Is and Ie, determine Ip/N
From Ip/N and N, determine Ip
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CT ERROR CALCULATION
Rct
Ip/N
Rw
Is
Ve
Ie
Zm
Ve
Vs
Zb
Rw
Ip
N-turns
Ie
Given :
Ip, Zb, Secondary Excitation Characteristic curve
Steps :
1.
2.
3.
4.
From Ip and N, det Ip/N
Calculate Ve to determine Ie from curve
From Ie, calculate Is, Vs and Ve
From Secondary Excitation curve, determine
new value of Ie
5. Repeat step 3 and 4 until successive
iterations yields insignificant changes in Ie
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CT CONNECTION
Delta Connection
For balance 3 - phase fault :
Relay
Ip
Is = Ip/N * 3
For phase - to - phase fault :
Is = Ip/N * 3 / 2 in two lines
Is
Is = Ip/N * 3
in remaning line
Is is 30 degrees phase shifted relative to Ip.
Delta-connected CT will not produce Zero-sequence currents.
Zero-sequence currents will be “trapped” inside the delta
and cannot be measured by the relays in the CT secondary.
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CT CONNECTION
Wye Connection
Is1
Is2
Is3
Ir
Ip1 I
p2
Relay
Is1 = Ip1/N
Ir = Is1 + Is2 + Is3
Ip3
Is is in phase with Ip
Wye connection will detect all kinds of fault and loads
With the saturation of any one CT, a fake residual
current will be produced
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CT CONNECTION
Core Balance CT
Induced Current is a function of:
Ia + Ib + Ic = 3Io
Will not respond to 3-phase and
phase-to-phase faults
Power Cables Normally used for low voltage ground
fault applications
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CT ACCURACY CLASS
Secondary Terminal Voltage
ANSI C57.13
800
700
600
500
Errors will not exceed 10%
for secondary voltage equal
to or less than value described
by curve
8Ω
C400
400
300
200
100
C800
4Ω
C200
2Ω
1Ω
C100
Class C - Indicates that
the transformer ratio can
be calculated
Class T - Indicates that
the transformer ratio
must be determine by
test
10 20 30 40 50 60 70 80 90 100
Secondary Amperes
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CT SATURATION CURVE
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PHILOSOPHY OF
PROTECTIVE RELAYING
A critical factor in the success of any nation is electric
power. Providing, operating and maintaining an effective
power system is an important challenge. One key element
to be considered in power system design is system
protection.
System Protection is accomplished via the coordinated
application of protective devices including fuses, circuit
breakers, reclosers, sectionalizers and other relays.
Protective relays are devices which monitor power
system conditions and operate to quickly and
accurately isolate faults or dangerous conditions. A
well designed protective system can limit damage to
equipment, as well as minimize the extent of
associated service interruption.
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PHILOSOPHY OF
PROTECTIVE RELAYING
Factors Which Influence Design of a Protective System
•Sensitivity
•Selectivity
•Reliability
•Dependability
•Security
•Speed
•Economics
•Experience
•Industry Standards
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PHILOSOPHY OF
PROTECTIVE RELAYING
Sensitivity - the minimum signal required to produce an
output. A more sensitive relay will be able to
discern a “smaller” condition. Sensitivity is
very important when the input quantities
are very small
Selectivity - the ability of the relay to recognize a fault or
abnormal system condition, and to discriminate
between those upon which it should and
should not operate or at a slightly delayed
manner
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PHILOSOPHY OF
PROTECTIVE RELAYING
Reliability -
the level of assurance that the relay will
function as intended. Reliability is
considered in two parts, dependability and
security
Dependability - the ability of the relay to trip for all faults
and conditions for which operation
“tripping” is desired.
Security -
the ability of the relay to not operate “trip”
for any fault or condition for which tripping
is undesired.
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PHILOSOPHY OF
PROTECTIVE RELAYING
Speed -
The ability of the relay to operate in the required
time period. The ultimate goal of the protective
equipment is to isolate the fault as quickly as
possible.
Economics - The cost of installation, operation, and maintenance
of the protection system which must be weighted
against potential losses due to equipment damage
or service interruption.
Experience - Those problems which experience has shown to be
most likely are given highest priority. Larger,
critical systems are protected from less probable
events.
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PROTECTIVE RELAYING
Industry Standards
The Institute of Electrical and Electronic Engineers (IEEE)
and other organizations provide industry standards through
ANSI or IEC. These include specific standards for many
applications.
ANSI-C37.90-1989 - Relays and Relay System
Associated with Electric Power
Apparatus
IEEE STD 242-1975 - Recommended Practice for
Protection and Coordination of
Industrial and Commercial Power
System
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FAULTS VERSUS
ABNORMAL CONDITIONS
One important concept in protective relaying is the
difference between faults and abnormal conditions. Faults
are short circuits or arcs, actual system failures. Abnormal
conditions are such as overvoltage, undervoltage, or
overexcitation. Abnormal conditions are undesirable
events, and can often lead to faults or equipment failure.
Most relays are applied to protect the system or equipment
from either faults or abnormal conditions. This will govern
the philosophy of protection.
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ZONE OF PROTECTION
Relay schemes are designed to protect specific areas or
equipment. The electric grid is divided into zones which can be
isolated via circuit breakers, fuses or sectionalizers. Each zone is
individually protected, and is defined as a ZONE of Protection.
Protective relay schemes are designed to isolate a given zone for
any tripping condition. This minimizes or prevents equipment
damage, thus, permitting more rapid restoration of the system,
and, minimizes the extent and duration of the interference with the
operation of the whole system (overtrip).
Zones are established encompassing certain system elements
such as generators, busses, transformers, and lines. This allows
protective relaying schemes to be tailored to the equipment of a
specific element. When a fault occurs, the zone including the
failed equipment is isolated from the rest of the system.
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ZONE OF PROTECTION
The boundaries of the zone of protection are defined by the
current and voltage transformers, which provide the system
information to the relays.
• Each zone of protection includes the isolating circuit
breakers, as well as the protected equipment.
• Each zone overlaps the adjacent zone, and the circuit
breaker will be in two zones. This is necessary to ensure
that “blind spots” cannot exist, and that all the portions of
the power system are protected.
• A fault in the overlap area will trip both zones. This
especially desirable in the case of a circuit breaker failure.
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ZONE OF PROTECTION
3
6
Zone of Protection
5
1
52
87B
50/51
2
4
G
CT REQUIREMENTS FOR
OVERLAPPING ZONES
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PROTECTION
COORDINATION
In order to increase dependability, and insure that all faults will
be cleared, protective relays from a given zone of protection
will usually operate as backup devices for faults in the
adjacent zones. Utilities generally design their systems for
single contingency, meaning, that the system can survive the
loss of any single device (including protective relays). In order
to provide this backup function while still isolating the minimum
amount of equipment, the protective relays must be
coordinated. That is, if the relays in the faulted zone fail to
operate (single contingency), the relays in the adjacent
zone(s), will operate after a time delay. In this means,
dependability is increased with only a small risk to security.
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PROTECTION
COORDINATION
51
LOADS
50/51
TO SOURCE
R
51
LOADS
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DEVELOPMENT OF
PROTECTIVE RELAYS
•Electro-mechanical relay
•Solid-state relay
•Digital relay
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ELECTRO-MECHANICAL
RELAYS
• The most commonly
used
• Uses the induction disc
principle
(watthour meter)
• Provides individual phase
protection
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SOLID-STATE RELAYS
• Characteristic curve is
obtained through use of RC
timing circuits
• No moving parts
• Used to retrofit electromechanical relays
• Fast reset
• Less maintenance
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DIGITAL RELAYS
• Selectable characteristic
curves and protection
functions
• Metering and control
functions
• Event and/or disturbance
recording
• Remote communication
• Self-monitoring
• “All in”
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DIGITAL RELAYS
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DEVICE FUNCTION
NUMBERS
1
2
21
24
25
27
30
32
37
46
47
50
51
Device
master element
Description
A device, such as a control switch, etc., that serves, either directly or through such permissive
devices as protective and time-delay relays, to place equipment in or out of operation.
time delay starting or
A device that functions to give a desired amount of time delay before or after any point of
closing relay
operation in a switching sequence or protective relay system, except as specifically provided
distance relay
A device that functions when the circuit admittance, impedance, or reactance increases or
decreases beyond a predetermined value.
volts per hertz relay
A device that operates when the ratio of voltage to frequency is above a preset value or is
below a different preset value. The relay may have any combination of instantaneous or time
synchronizing or
A synchronizing device produces an output that causes closure at zero-phase angle
synchronism-check relay difference between two circuits. It may or may not include voltage and speed control. A
synchronism-check relay permits the paralleling of two circuits that are within presc
undervoltage relay
A device that operates when its input voltage is less than a predetermined value.
annunciator relay
A nonautomatically reset device that gives a number of separate visual indications upon the
functioning of protective devices and that may also be arranged to perform a lockout function.
directional power relay
A device that operates on a predetermined value of power flow in a given direction such as
reverse power flow resulting from the motoring of a generator upon loss of its prime mover.
undercurrent or
A device that functions when the current or power flow decreases below a predetermined
underpower relay
value.
reverse-phase or phase- A device in a polyphase circuit that operates when the polyphase currents are of reversebalance current relay
phase sequence or when the polyphase currents are unbalanced or when the negative phasephase-sequence or
A device in a polyphase circuit that functions upon a predetermined value of polyphase
phase-balance voltage
voltage in the desired phase sequence, when the polyphase voltages are unbalanced, or
relay
when the negative phase-sequence voltage exceeds a preset value.
instantaneous
A device that operates with no intentional time delay when the current exceeds a preset
overcurrent relay
value.
ac time overcurrent relay A device that functions when the ac input current exceeds a predetermined value, and in
which the input cur-rent and operating time are inversely related through a substantial portion
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DEVICE FUNCTION
NUMBERS
Description
A device that is used to close and interrupt an ac power circuit under normal conditions or to
interrupt this circuit under fault or emergency conditions.
59 overvoltage relay
A device that operates when its input voltage exceeds a predetermined value.
64 ground detector relay
A device that operates upon failure of machine or other apparatus insulation to ground.
NOTE This function is not applied to a device connected in the secondary circuit of current
transformers in a normally grounded power system where other overcurrent device numbers
with the suffix G or N should be used; for example, 51N for an ac time over
67 ac directional overcurrent A device that functions at a desired value of ac overcurrent flowing in a predetermined
relay
direction.
68 blocking or "out-of-step" A device that initiates a pilot signal for blocking of tripping on external faults in a transmission
line or in other apparatus under predetermined conditions, or cooperates with other devices to
relay
block
tripping
or reclosing onthat
an in
out-of-step
condition
69 permissive control device A
device
with two-positions
one position
permitsorthe closing of a circuit breaker, or the
placing of a piece of equipment into operation, and in the other position, prevents the circuit
breaker or the equipment from being operated.
79 reclosing relay
A device that controls the automatic reclosing and locking out of an ac circuit interrupter.
81 frequency relay
A device that responds to the frequency of an electrical quantity, operating when the
frequency or rate of change of frequency exceeds or is less than a predetermined value.
86 lockout relay
A device that trips and maintains the associated equipment or devices inoperative until it is
reset by an operator, either locally or remotely.
87 differential protective
A device that operates on a percentage, phase angle, or other quantitative difference of two
relay
or more currents or other electrical quantities.
94 tripping or trip-free relay A device that functions to trip a circuit breaker, contactor, or equipment; to permit immediate
tripping by other devices; or to prevent immediate reclosing of a circuit interrupter if it should
open
even
though
itsindividual
closing circuit
is maint
95-99 used only for specific
Theseautomatically,
device numbers
are
used in
specific
installations if none of the functions
applications
assigned to the numbers from 1 through 94 are suitable.
52
Device
ac circuit breaker
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DEVICE FUNCTION
NUMBERS
(Suffixes)
Suffix
Letter
A
B
G
GS
L
M
N
T
V
U
X
Y
Z
Relay Application
Alarm only or automatic
Bus protection
Ground -fault or generator
Ground -fault protection
Line protection
Motor protection
Ground -fault protection
Transformer protection
Voltage
Unit protection
Auxiliary relay
Auxiliary relay
Auxiliary relay
Amplifying Information
System neutral type protection
Toroidal or ground sensor type
Relay coil connected in residual CT circuit
Generator and transformer
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BASIC STEPS FOR RELAY
SETTING &
COORDINATION STUDY
•
•
•
•
•
Data collection
Fault current calculation
Equipment performance
Special requirements
Selection and plotting of preliminary
settings
• Check final settings
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SETTING & COORDINATION
• Organized time-current study of
all devices in series from the
utilization device to the source.
• Comparison of the time it takes
the individual devices to operate.
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SETTING & COORDINATION
• Determine the characteristics, ratings
and settings of overcurrent protective
devices against a fault
• Provide protection against overloads
on equipment
• Data useful for selection of instrument
transformer ratios, fuse ratings, CB
ratings and settings
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QUESTIONS?
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PROTECTIVE
RELAYING
Principles &
Philosophies
FORTUNATO C. LEYNES, FIIEE
Chairman
Board of Electrical Engineering
Professional Regulation Commission
Vice President
Manila Electric Company
15th IIEE Region 8 Conference
June 26, 2010