1
General Information
1.1
Description
The Siemens Communicating Overcurrent Relay (SCOR) is a
microprocessor -based , time overcurrent relay designed for easy
incorporation into a computer-monitored power system. It is
available in a number of styles to supply single-phase, two
phase-with-ground, three-phase, and three-phase-with-ground
protection for 60
Hz
power systems.
The relay provides for the incorporation of an optional plug-in
communications board to interface with the Siemens Power
Monitor™ display and monitoring unit. The communications
interface, when fully implemented, allows remote monitoring of
real-time system and circuit breaker information and the trans
mission of event and historical data, as well as remote configu
ration of operating parameters.
1.2
Application
The SCOR relay is utility grade and is provided in a draw-out case
with built -in test facilities. It is used for the protection of medium
voltage electrical power systems. It is designed to monitor the
outputs of standard (5 A secondary) current transformers and,
when it operates, to close an output contact that may be used
to trip a circuit breaker.
The relay requires control power for its internal circuits. A number
of ac and de voltage options are available for this purpose that
match the usual ac or de control power used for tripping the
circuit breaker.
1.3
The Time Overcurrent Function
1 .3.1
Pickup
A coarse incremental selection of overcurrent pickup tap is
provided by front panel rotary switches. One switch simulta
neously sets the pickup tap for all the monitored phases. If ground
is also monitored, a second rotary switch independently sets the
ground overcurrent pickup tap.
A fine incremental adjustment that provides 99 intermediate
pickup points between adjacent positions of the rotary switches
is provided by entering data into the memory of the internal
microcomputer tap calibration registers.
1 .3.2
Timing
A time delay is initiated when a pickup point is exceeded. When
the current drops below pickup, the timing circuit is reset
immediately. The amount of delay required before trip is a
function of the overcurrent magnitude.
Page 1
One of 1 6 families of time overcurrent characteristics may be
selected for the monitored phases. These families are graphi
cally illustrated in
Figures A.1
through
A.1 6
in
Appendix
A.
If ground current is monitored, its timing characteristic is
independently selected from the 16 families.
Selection of the timing characteristics is made at the front panel
or via one of the two communications links. After a characteristic
is selected , it is adjusted to specific requirements by choosing
the TIME DIAL number. (These are the numbers in a vertical row
along the right hand margin of
Figures A.1
through
A.16.)
This
TIME DIALnumber (Oto 99) selects oneofthe 1 OOcharacteristic
curves available for each characteristic. (Only 1 4 of the 1 00
curves in the relay's memory are shown on each graph because
of space limitations.)
The selected TIME DIAL number is entered into the relay's
memory, again using either the frontpanel data entry controls,
or one of the two communications links. The available charac
teristic curves include one definite time, six inverse time, and
nine IZT curves. (Refer to
Table
4.)
1 .3.3 Trip and Reset
When the monitored current exceeds the overcurrent pickup
point, the TMG LED illuminates as timing begins. The timing
process continues until the interval calculated by the selected
time overcurrent characteristic is completed (thereby tripping
the associated output contact and target indicators}, or until the
sensed overcurrent drops below the pickup setting (which
causes the timer to reset). In either case (trip or reset), the timing
process is terminated. The TMG LED extinguishes at reset, but
remains on at trip as an indication of contact closure.
If a relay output is closed, it is immediately reset when the
monitored current drops below the pickup setting. Targets,
however, remain tripped until manually reset at the front panel.
(Control power is required to reset the targets.)
1.4
RMS Sensing
·:ff
The SCOR protective relay uses RMS Sensing, a technology first
introduced by Siemens in 1 985, to sample the current wave
shape and quickly calculate the effective heating value of the
current. SCOR relays evaluate the impact of harmonics and
provide accurate circuit protection. The SCOR relay uses a sum
of squares algorithm for both determining trip level and for
calculating metered values of the relay current level. The input
waveform is sampled several times to determine instantaneous
values. These instananeous values are processed to obtain the
true RMS value of the input current.
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