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HISTORY OF PROTECTIVE RELAYS
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DISCOVERY OF DIGITAL RELAY
INTRODUCTION
Digital 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 numerical 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 tomake 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 individual relays were used.
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PARTS OF DIGITAL RELAY BASED IN OPERATION
Low voltage and low current signals (i.e., at the secondary of a voltage transformers and current transformers) are
brought into a low pass filter that removes frequency content above about 1/3 of the sampling frequency (a
relay A/D converter needs 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 digital 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 CONSTRUCTION
FRONT PANEL REAR PANEL
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FRONT PANEL OPERATION
ENERGIZING: When initially energized or re-energized, the digital relay automatically restarts
in 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, for
example, DR LXS01 KNote: To test the indicators and the display, press simultaneously on the meters and
device buttons. All indicators and LCDs will activate.
MODES OF OPERATION:The digital relay has two basic modes: Standard and Setup. An overview of these
modes 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 relays
configuration
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 and
settings (the value buttons are inoperable in the Standard mode)
Note: Settings cannot be altered in the Standard mode. Settings must be defined or
changed in the Setup mode. See Setting Parameters, page 35.
After Settings And Metered Values, which explains how to access the settings
and metered values available on the digital relay, the Setup and Standard modes
are detailed more fully.
REAR PANEL OPERATIONProtective Output : The digital relay has three output relay contacts (1), which operate when a phase
Relay 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 the
digital relay supports 48125 Vdc power supplies; the ac model supports
100127 Vac power supplies.
The control power is connected to the two control power terminals and the
grounding 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 in
this mode using the value + and value buttons.
Watchdog Relay The digital relay continually performs a self-diagnostics check. If the unit detects
an internal failure, the protective output relay contacts are then inhibited and a
watchdog relay releases. The watchdog relay consists of two output relay
contacts (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 or
a bell) to indicate when an internal failure occurs
Sensing Connections The line currents are sensed by three external 1 A or 5 A standard phase CTs. The
CT 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 the
digital 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 the
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back of the digital relay (8); the CSH core balance CT is purchased separately
Communications The POWERLOGIC communicating version of the digital relay has the following
capabilities:
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 a
10,000 foot (3,048 m) spanCommunication Indicator Lights There are three communication indicator lights (11), which show the status of the
optional communications card as follows:
red CPU LED alternates one second on/one second off when the
communications card central processing unit (CPU) is operating normally
yellow RX LED flashes when a master device is sending a message to a device
on the network
green TX LED flashes when the digital relay is acknowledging a message received
REASONS TO TEST DIGITAL RELAY
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DIGITAL RELAY AREA OF USE
DISTRIBUTION FEEDER PROTECTION
CIRCUIT BREAKER PROTECTIONThis 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.
GENERATOR PROTECTION
REAL TIME ABILITY OF DIGITAL RELAY
FAULT LOCATION AND FAULT TYPE
PRE-FAULT,FAULT, AND POST-FAULT CURRENT AND VOLTAGES
RELAY INTERNAL ELEMENT STATUS
RELAY CONTROL INPUT AND OUTPUT STATUS
INSTANTANEOUS AND DEMAND METERING
BREAKER OPERATION DATA
RELAY SELF STATUS
EVOLUTION OF DIGITAL RELAY COMMUNICATION INTERFACE
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Conclusion
Digital relays are presently available that provide highly reliable protection for generators. These units
utilize the latest digital signal processing technology to accomplish many relaying functions
simultaneously. By appropriate use of multiple multifunction digital relays, or by using multifunction
digital relays in combination with single-function relays, protection engineers can provide the reliabilityand security level needed for their specific applications.
Digital relays provide many extra features that are quickly becoming priority items. These include self-
checking, digital fault recording, metering and remote communications. The self calibration and special
test programs included with the relays simplify test and checkout of installations using these devices.
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TECHNOLOGICAL UNIVERSITY OF THE PHILIPPINES
AYALA BLVD. ERMITA MANILA
COLLEGE OF ENGINEERING
ELECTRICAL ENGINEERING DEPARTMENT
DIGITAL RELAY
SUBMITTED BY:
BENEDICT C. FORTEZA
JEFFREY D. FRIO
BSEE 4C
SUBMITTED TO:
ENGR.GILBERT B. HOLLMAN
PEE 3009