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Both failure to oprate and incorrect operation
can result in major system upsets involving
increased equipment damage, increased
personnel hazards, and possible long
interruption of service. These stringent
requirements with serious potential
consequences tend to make protection engineers
somewhat conservative. !ne of the advantages
of the modern solidstate relays is that they can
check and monitor themselves to minimize
equipment problems as well as to provide
information on the events that resulted in
triggering their operation.
"roblems can and do occur in protective
equipment# nothing is perfect. To minimize the
potential catastrophic problems that can result in
the power system from a protection failure, thepractice is to use several relays or relay systems
operating in parallel. These can be at the same
location $primary backup%, at the same station
$local backup%, or at various remote stations
$remote backup%. &ll three are used together in
many applications. 'n highervoltage power
systems this concept is e(tended by providing
sep)rate current or voltage, or both measuring
devices, sep)rate trip coils on the circuit
breakers, and sep)rate tripping battery sources.
The various protective devices must beproperly coordinated such that the primary relays
assigned to oprate at the first sign of trouble in
their assigned protective zone oprate first. *hould
they fail, various backup systems must be available
and able to oprate to clear the trouble. &n
adequate, highprotection redundancy capability is
very important. &dditional redundancy, however,
does have a negative impact on security. &s more
systems are added to enhance dependability, an
increased probability of incorrect operations
results +ood judgment must be utilized when
Integrati
ens g timing,
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results +ood judgment must be utilized when
and applied. &nalog type electronic relays using
discreet electronic components were introduced
in the 345s. 'n recent years, microprocessor
based electronic relays have been developed and
are being applied at an increasing rale.
6icroprocessorbased relays are sometimesreferred to as numerical type relays since the
analog inputs are converted to digital numbers
that are then processed within the relay. 2ven
with this trend toward the utilization of
microprocessorbased relays, however, it may be a
long time before electromechanical devices are
completely replaced.
7ith electronic relays, the protection
principies and fundament)is are essentially
unchanged as are the issues regarding protection
reliability. 6icroprocessor type relays doprovide many benefits such as higher accuracy,
reduced space, lower equipment and
installation costs, wider application and setting
capabilities, plus various other desirable
supplemental features. These include control
logic, remote and peertopeer 8ommunications,
data acquisition, event recording, fault location,
remote setting, and self monitoring and
checking. The specifics of these features will vary
between different types of relays and relay
manufacturers. 6icroprocessorbased relays wil lbe discussed in greater detail in 8hapter 9.
:arious types of protective relays and relay
assemblies are illustrated in -igure .; through
-igure .elay settings can also be changed
through the 8= interface without the need for a
data terminal. Target information is typically
displayed on microprocessor relays with the useof 2=s that identify the protective functions
that had operated to initiate tripping along with
other information such as the type of fault that
had been detected $i.e., &phasetoground%,
recloser status, etc. Terminal blocks are
nonnally provided on the back of the relay
for connecting the various inputs that are
required and outputs that are provided by the
relay. 8ommunication ports are provided for
transmitting digital data.
The fundamental characteristics of relaydesigns necessary to understand the applications
are outlined in 8hapter < and are augmented as
required in subsequent chaplers.
1.3 TYPICAL POWER CIRCUIT
BREAKERS
"rotective relays provide the ?brains? to sense
trouble, but as lowenergy devices, they are not
able to open and isolate the problem )rea of the
power system. 8ircuit breakers and various typesof circuit interrupters, including motor
contactors and motor controllers, are used
for this and provide the ?muscle? for fault
isolation. Thus, protective relays and circuit
breakerin terrupt ing dev ice s work toget her #
bot h a re nec essary for the prompt
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S 2!! #$ %aylor & 'runcs (.roup, LL(. ) 2!! #y %aylor & 'tancis *roup, LL(.
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