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7/30/2019 Protective Relaying - An Overview
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PROTECTIVERELAYING
Principles &Philosophies
FORTUNATO C. LEYNES, FIIEE
Chairman
Board of Electrical Engineering
Professional Regulation CommissionVice President
Manila Electric Company
15th IIEE Region 8 Conference
June 26, 2010
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The branch of electric power engineering concernedwith 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.
Protective Relaying
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ARE PROTECTIVE RELAYING PRACTICES
BASED ON THE PROBABILITY OF FAILURE
protective relaying practices are based on the probability offailure to the extent that present-day practices are the resultof years of experience in which the frequency of failureundoubtedly has played a part;
the probability of failure, seldom if ever, enters directly intothe choice of a particular type of relaying equipment exceptwhen, 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 failureshould 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 isonly back-up relaying.
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EVALUATION OF
PROTECTIVE RELAYING
the cost of repairing the damage.
the likelihood that the trouble may spread andinvolve 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 equipments return toservice, protective relaying helps tominimize the amount of equipment reserve
required, since there is less likelihood ofanother failure before the first failure can berepaired.
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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 primaryprotection.
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
PROTECTION SYSTEM
OBJECTIVES
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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 ofdisconnecting 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, andsynchronizing 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 timedelay is introduced to slow down the relayresponse.
2. Time delay. An intentional time delay is insertedbetween the relay decision time and the initiationof the trip action.
3. High speed. A relay that operates in less than aspecified time. The specified time in presentpractice 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 beoperation in 4 milliseconds or less.
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CLASSIFICATION OF
RELAY OPERATION
CORRECT TRIPPING
CORRECT TRIPPING BUT UNDESIRED
INCORRECT TRIPPING
F
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Primary Protection - Schemes that are designed tospecifically protect one equipment zone. In any locations, thisprimary relaying may overlap into other zone of protection,
providing additional protection for those zones.
Primary
A. Limited
B. Overlap
PRIMARY AND BACK-UP
PROTECTION
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Schemes that are designed to operate in place of or in
parallel with the primary protection. Back-up protection probablywill sense faults in more that one zone, is usually slower in
operation, and may isolate a larger portion of the system. Back-
up protection for a specific zone may be provided by a local
scheme or one located remotely.
Back-up
A. In Place of Primary
B. Overlap
C. Slower
D. Increase Coverage in IsolationE. Local/Remote
BACK-UP PROTECTION
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1. Sensor - Feeds system information to the relay,
e.g., currents and voltages
2. Relay - Makes a decision as to the need foraction, e.g., overcurrent relay, etc.
3. Switching or Controlling Device - Physically
isolates or control the problem, e.g.,
circuit breaker
THREE MEMBERS OF
PROTECTIVE SYSTEM
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Power Circuit Breaker
Relay
Feedback
Signals
Sensor
THREE MEMBERS OF
PROTECTIVE SYSTEM
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Power
System
Voltage
and
current
transformer
RelayCircuit
Breaker
These devices ChangeElectrical Quantities
to a Level low enough
for the relay to use i.e.
5A, 110 V
Decides whether system
quantities are normal or
abnormal
Opens and isolate
a faulty section ofthe system as sent
by the relay
FUNCTIONAL DIAGRAM
OF RELAYING
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Station
Battery
Transmission
Line
TripCoil
Relay Contacts
CB
CT
ELECTRICAL DIAGRAM OF
RELAYING
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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 requiredaction 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 closeswhen the relay resets
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Change the magnitudes, but not the nature of the
measured quantities
Provide isolation from the hostile environment of the
power system
Types
Current Transformers - CTsPotential Transformers - PTs
Voltage Transformers - VTsCoupling Capacitor Voltage Transformers - CCVTs
INSTRUMENT TRANSFORMERS
(Transducers)
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Secondary
Terminals
Iron Core
Secondary Winding
Primary Conductor
Rating:Specify continuous rating of secondary winding (1A, 5A)
Specify primary current which will nominally produce ratedsecondary current (e.g., 800A, 1,000A)
CURRENT TRANSFORMERS
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Current Ratio
100/5200/5
400/5500/5
600/5
800/51000/5
1200/52000/5
100/1200/1
400/1500/1
600/1
800/11000/1
1200/12000/1
Polarity:
- Indicated by dots (dot or
square) on drawings- Indicates instantaneous
relationship in the directions ofprimary and secondary currents.
Current entering the polarity mark on the primary
will cause a current to instantaneously leave thepolarity mark on the secondary
Ip
Is
CURRENT TRANSFORMERS
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Core-Balanced or Ring type or Doughnut Type
Bushing or the Bar-Type
Wound Primary Type
Rogowsky Coil - Optical CT
MOST COMMON TYPES OF
CURRENT TRANSFORMERS
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Is = Ip/N - Ie
Vs = Is * (Zb + 2Rw)
Ve = Vs + Is*Rct
Ip
IsIp/N
Ie
Rct Rw
Rw
ZbZm Ve Vs
N-turns
Ve
Ie
Rct - CT Winding
resistance in
ohms/turn
Rw - Lead (wiring)
Resistance
Zb - Burden Impedance
Zm - MagnetizingImpedance
CT EQUIVALENT CIRCUIT
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Ip
IsIp/N
Ie
Rct Rw
Rw
ZbZm Ve Vs
N-turns
Rct - CT Winding
resistance inohms/turn
Rw - Lead (wiring)
Resistance
Zb - Burden Impedance
Zm - Magnetizing
ImpedanceN - is the nominal ratio
of CT
CT EQUIVALENT CIRCUIT
At Saturation point: Is = Ip/NVe
Ie
Zm will be small which
result in Ie being large
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Given:
Primary Current , Ip
Total impedance burden on the CT, including leadwire resistance
CT Secondary Excitation Characteristics
Neglected Factor: CT transient characteristic
CT ERROR CALCULATION
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CT ERROR CALCULATION
Ip
IsIp/N
Ie
Rct Rw
Rw
ZbZm Ve Vs
N-turns
Given :
Is, Zb, Secondary Excitation Characteristic curve
Steps :
1. From the Burden and Is, cal. Vs2. From Vs, Rct and Is, cal. Ve3. From Ve and Sec. Excitation curve, determine Ie
4. From Is and Ie, determine Ip/N5. From Ip/N and N, determine Ip
Ve
Ie
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CT ERROR CALCULATION
Ip
IsIp/N
Ie
Rct Rw
Rw
ZbZm Ve Vs
N-turns
Ve
Ie
Given : Ip, Zb, Secondary Excitation Characteristic curve
Steps :
1. From Ip and N, det Ip/N
2. Calculate Ve to determine Ie from curve
3. From Ie, calculate Is, Vs and Ve4. 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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RelayIp
Is
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.
CT CONNECTIONDelta Connection
lineremaningin3*Ip/NIslinesin two2/3*Ip/NIs
:faultphase-to-phaseFor
3*Ip/NIs
:faultphase-3balanceFor
=
=
=
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Relay
Ip1
Is1
Ir
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 residualcurrent will be produced
Is1 = Ip1/N
Ir= Is1 + Is2 + Is3
CT CONNECTIONWye Connection
Is2Is3
Ip2Ip3
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Power Cables
Induced Current is a function of:
Ia + Ib + Ic = 3Io
Will not respond to 3-phase and
phase-to-phase faults
Normally used for low voltage groundfault applications
CT CONNECTIONCore Balance CT
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100
200
300
400
500
600
700800
10010 20 30 40 50 60 70 80 90
8
4
2
1
C400
C800
C200
C100
Secondary Amperes
S
econdaryTerminalVoltage
ANSI C57.13
Class C - Indicates thatthe transformer ratio can
be calculated
Class T - Indicates thatthe transformer ratio
must be determine bytest
Errors will not exceed 10%
for secondary voltage equal
to or less than value described
by curve
CT ACCURACY CLASS
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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 elementto be considered in power system design is system
protection.
System Protection is accomplished via the coordinated
application of protective devices including fuses, circuitbreakers, 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. Awell designed protective system can limit damage to
equipment, as well as minimize the extent ofassociated service interruption.
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Factors Which Influence Design of a Protective System
Sensitivity
Selectivity
Reliability
Dependability
Security
Speed
Economics
Experience
Industry Standards
PHILOSOPHY OF
PROTECTIVE RELAYING
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Sensitivity - the minimum signal required to produce an
output. A more sensitive relay will be able todiscern 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
PHILOSOPHY OF
PROTECTIVE RELAYING
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Reliability - the level of assurance that the relay will
function as intended. Reliability is
considered in two parts, dependability andsecurity
Dependability - the ability of the relay to trip for all faults
and conditions for which operationtripping is desired.
Security - the ability of the relay to not operate trip
for any fault or condition for which tripping
is undesired.
PHILOSOPHY OF
PROTECTIVE RELAYING
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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 aspossible.
Economics - The cost of installation, operation, and maintenance
of the protection system which must be weighted
against potential losses due to equipment damageor service interruption.
Experience - Those problems which experience has shown to bemost likely are given highest priority. Larger,
critical systems are protected from less probableevents.
PHILOSOPHY OF
PROTECTIVE RELAYING
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The Institute of Electrical and Electronic Engineers (IEEE)
and other organizations provide industry standards throughANSI or IEC. These include specific standards for many
applications.
ANSI-C37.90-1989 - Relays and Relay System
Associated with Electric PowerApparatus
IEEE STD 242-1975 - Recommended Practice for
Protection and Coordination of
Industrial and Commercial PowerSystem
PROTECTIVE RELAYING
Industry Standards
FAULTS VERSUS
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One important concept in protective relaying is thedifference 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 undesirableevents, 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.
FAULTS VERSUS
ABNORMAL CONDITIONS
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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 isindividually 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 equipmentdamage, thus, permitting more rapid restoration of the system,
and, minimizes the extent and duration of the interference with theoperation 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 aspecific element. When a fault occurs, the zone including thefailed equipment is isolated from the rest of the system.
ZONE OF PROTECTION
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The boundaries of the zone of protection are defined by thecurrent 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 circuitbreaker will be in two zones. This is necessary to ensure
that blind spots cannot exist, and that all the portions ofthe 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.
ZONE OF PROTECTION
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Zone of Protection
52
87B
50/51
CT REQUIREMENTS FOR
OVERLAPPING ZONES
ZONE OF PROTECTION
G
1
3
5
6
42
PROTECTION
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In order to increase dependability, and insure that all faults will
be cleared, protective relays from a given zone of protectionwill 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 minimumamount 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.
PROTECTION
COORDINATION
PROTECTION
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50/51
51
51
LOADS
LOADS
TO SOURCE
R
PROTECTION
COORDINATION
DEVELOPMENT OF
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Electro-mechanical relay
Solid-state relay
Digital relay
DEVELOPMENT OF
PROTECTIVE RELAYS
ELECTRO MECHANICAL
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ELECTRO-MECHANICAL
RELAYS
The most commonlyused
Uses the induction disc
principle(watthour meter)
Provides individual phase
protection
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SOLID-STATE RELAYS
Characteristic curve is
obtained through use ofRC
timing circuits
No moving parts
Used to retrofit electro-
mechanical relays Fast reset
Less maintenance
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DIGITAL RELAYS
Selectable characteristic
curves and protectionfunctions Metering and control
functions
Event and/or disturbancerecording
Remote communication
Self-monitoring
All in
DIGITAL RELAYS
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DIGITAL RELAYS
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DEVICE FUNCTION
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DEVICE FUNCTION
NUMBERSDevice Description
52 ac circuit breaker 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 over67 ac directional overcurrent
relayA device that functions at a desired value of ac overcurrent flowing in a predetermineddirection.
68 blocking or "out-of-step"relay
A device that initiates a pilot signal for blocking of tripping on external faults in a transmissionline or in other apparatus under predetermined conditions, or cooperates with other devices to
69 permissive control device A device with two-positions that in one position permits the closing of a circuit breaker, or the
placing of a piece of equipment into operation, and in the other position, prevents the circuitbreaker or the e ui ment from bein o erated.
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
rela
A device that operates on a percentage, phase angle, or other quantitative difference of two
or more currents or other electrical uantities.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
95-99 used only for specificapplications
These device numbers are used in individual specific installations if none of the functionsassigned to the numbers from 1 through 94 are suitable.
DEVICE FUNCTION
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NUMBERS(Suffixes)
Suffix
Letter
Relay Application Amplifying Information
A Alarm only or automaticB Bus protection
G Ground -fault or generator System neutral type protect ion
GS Ground -fault protection Toroidal or ground sensor type
L Line protection
M Motor protectionN Ground -fault protection Relay coil connected in residual CT circuit
T Transformer protection
V Voltage
U Unit protection Generator and transformer
X Auxiliary relay
Y Auxiliary relayZ Auxiliary relay
BASIC STEPS FOR RELAY
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SETTING &
COORDINATION STUDY
Data collection
Fault current calculation
Equipment performance
Special requirements
Selection and plotting of preliminary
settings Check final settings
SETTING & COORDINATION
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SETTING & COORDINATION
Organized time-current study of
all devices in series from theutilization device to the source.
Comparison of the time it takesthe individual devices to operate.
SETTING & COORDINATION
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SETTING & COORDINATION
Determine the characteristics, ratings
and settings of overcurrent protectivedevices against a fault
Provide protection against overloadson equipment
Data useful for selection of instrument
transformer ratios, fuse ratings, CBratings and settings
QUESTIONS?
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QUESTIONS?
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PROTECTIVERELAYING
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