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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(.