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INTRODUCTIONDigital relays that uses a microprocessor to analyze
power system voltages, currents or other process quantities for
the
purpose of detection of faults in an industrial process
system.The digital relay is a multifunctional device using
numerica
algorithms that can easily duplicate any of the ANSI protection
functions with simple software modifications. Digital
relays are those in which the measured ac quantities are
manipulated in analog form and subsequently converted into
square-wave (binary) voltages. Logic circuits or microprocessors
compare the phase relation-ships of the square waves
to make a trip decision. A multifunction digital relay is a
microprocessor-based unit that uses Digital Signal Processing
technology in order to provide multiple protective relaying
functions for generator protection in one unit. Since many
functions are incorporated into one package, much less panel
space and wiring is required than would be if individualrelays were
used.
PARTS OF DIGITAL RELAY BASED IN OPERATION
Low voltage and low current signals (i.e., at the secondary of
avoltage transformersandcurrent transformers) are brought intoalow
pass filterthat removes frequency content above about 1/3 of the
sampling frequency (a relay A/D converterneeds to sample
faster than 2x per cycle of the highest frequency that it is to
monitor). The AC signal is then sampled by the relay's analog to
digita
converter from 4 to 64 (varies by relay) samples per power
system cycle. In some relays, a short history of the entire sampled
data is
kept for oscillographic records. Only the fundamental component
is needed for most protection algorithms, unless a high speed
algorithm is used that uses subcycle data to monitor for fast
changing issues. The sampled data is then passed through a low
pass
filter that numerically removes the frequency content that is
above the fundamental frequency of interest (i.e., nominal
system
frequency), and uses Fourier transform algorithms to extract the
fundamental frequency magnitude and angle. Next the
microprocessor passes the data into a set of protection
algorithms, which are a set of logic equations in part designed by
the
protection engineer, and in part designed by the relay
manufacturer, that monitor for abnormal conditions that indicate a
fault. If a
fault condition is detected, output contacts operate to trip the
associated circuit breaker(s).
PARTS OF DIGITAL RELAY BASED IN CONSTRUCTIONFRONT PANEL
OPERATIONENERGIZING: When initially energized or re-energized, the
digital relay automatically restartsin the following sequence: the
green on LED and red self-diagnostic LED light the red
self-diagnostic LED goes out the watchdog relay contacts reset the
digital relay displays the version name followed by the version
letter, forexample, DR LXS01 KNote: To test the indicators and the
display, press simultaneously on the meters anddevice buttons. All
indicators and LCDs will activate.
MODES OF OPERATION:The digital relay has two basic modes:
Standard and Setup. An overview of thesemodes follows. Setup
ModeThe digital relays settings are defined and modified in the
Setup mode. While in this mode, you can: scroll through all
display pages use the value buttons (, +, and enter) to define or
modify the relaysconfiguration Standard ModeThis is the digital
relays basic operating mode. While in this mode, you can: scroll
through all display pages use the meter, device, and settings
buttons to display metered values andsettings (the value buttons
are inoperable in the Standard mode)Note: Settings cannot be
altered in the Standard mode. Settings must be defined orchanged in
the Setup mode. See Setting Parameters, page 35.After Settings And
Metered Values, which explains how to access the settingsand
metered values available on the digital relay, the Setup and
Standard modes
http://en.wikipedia.org/wiki/Transformer_types#Voltage_transformershttp://en.wikipedia.org/wiki/Transformer_types#Voltage_transformershttp://en.wikipedia.org/wiki/Transformer_types#Voltage_transformershttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Low_pass_filterhttp://en.wikipedia.org/wiki/Low_pass_filterhttp://en.wikipedia.org/wiki/Low_pass_filterhttp://en.wikipedia.org/wiki/A/D_converterhttp://en.wikipedia.org/wiki/A/D_converterhttp://en.wikipedia.org/wiki/A/D_converterhttp://en.wikipedia.org/wiki/A/D_converterhttp://en.wikipedia.org/wiki/Low_pass_filterhttp://en.wikipedia.org/wiki/Current_transformerhttp://en.wikipedia.org/wiki/Transformer_types#Voltage_transformers
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are detailed more fully.
REAR PANEL OPERATIONProtective Output : The digital relay has
three output relay contacts (1), which operate when a phaseRelay
Contacts fault or ground fault is detected: output 1terminals 8 and
7 (normally open) output 2terminals 6 and 5 (normally open)
output 3terminals 4 and 3 (normally closed)These relays connect
to the trip circuit.Control Power: Both ac and dc models of the
digital relay are available. The dc model of thedigital relay
supports 48125 Vdc power supplies; the ac model supports100127 Vac
power supplies.The control power is connected to the two control
power terminals and thegrounding screw (2).Parameter Setup Mode
Pressing the button located in the parameter setup mode access hole
P (3)Access Hole activates the parameter setup mode. The digital
relay settings are entered inthis mode using the value + and value
buttons.Watchdog Relay The digital relay continually performs a
self-diagnostics check. If the unit detectsan internal failure, the
protective output relay contacts are then inhibited and awatchdog
relay releases. The watchdog relay consists of two output
relaycontacts (4):
output 4terminals 4 and 3 (normally open position when digital
relay de-energized) output 5terminals 2 and 1 (normally closed
position when digital relay de-energized)The watchdog output relay
contacts can be connected to an alarm (e.g., a light ora bell) to
indicate when an internal failure occursSensing Connections The
line currents are sensed by three external 1 A or 5 A standard
phase CTs. TheCT secondary leads are connected to the CCA 660 phase
current sensor module,which contains the digital relay current
sensors and attaches to the rear of thedigital relay by the DB-9
connector (7).There are two methods for sensing the ground fault
current: internal summation of the three phase currents (default)
an optional method using an external CSH core balance CT connected
to theback of the digital relay (8); the CSH core balance CT is
purchased separatelyCommunications The POWERLOGIC communicating
version of the digital relay has the followingcapabilities:
POWERLOGIC software compatibility can share RS-485 communications
link (9) with circuit monitors operates at a baud rate from 1,200
to 19,200 bps can be daisy-chained with up to 31 additional
POWERLOGIC devices over a10,000 foot (3,048 m) spanCommunication
Indicator Lights There are three communication indicator lights
(11), which show the status of theoptional communications card as
follows: red CPU LED alternates one second on/one second off when
thecommunications card central processing unit (CPU) is operating
normally yellow RX LED flashes when a master device is sending a
message to a deviceon the network green TX LED flashes when the
digital relay is acknowledging a message received
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CIRCUIT BREAKER PROTECTION
This implies a current flow that keeps on feeding the fault
which can be used to detect the breaker failure itself.
In those applications which even though the mechanical failure
exist, the current could not be high enough to be detected, the
opening must also
be verified by means of breaker auxiliary contacts.
OPERATION: A tripping order for the circuit breaker initiates
the time delay count down for the protection.Once the time delay is
over , if the
breaker is not yet open, the protection sends a tripping order
to all the adjacent breakers, including those at the end of the
lines if necessary.
Sometimes two time delays are used, the first one to repeat the
tripping order for the breaker itself, and the second for the other
breakers.
