Appendix
B
Functional Description of Operation
Page
37
8.1
General
The SCOR relay is a microprocessor-based overcurrent relay
that can optionally be equipped to communicate with a remote
computer. The following text is referenced to
Figure 8.1 ,
which is a functional block d iagram that illustrates the overall
operation of a fully implemented version of the relay.
8.2
Current Sensing
The SCOR relay can be provided for single-phase, two-phase
with-ground, three-phase, or three-phase-with-ground current
monitoring. The illustrated relay is of the three-phase-with
ground style.
The internal current sensing transformers are designed to receive
their input frorn the 5 A nominal secondaries of standard current
transformers (CTs). Note the dual primaries of the internal
transformers. One or the other is connected by the factory to
obtain a LOW or a HIGH current sensing range (0.50 to 5.0 A, or
2 .00 to 1 1 .00 A) in accordance with the specified Style Number
of the relay.
The outputs of the internal transformers are applied to scaling
circuits (one for each of the phases, one for ground) that convert
each of the input currents to a voltage level that can be utilized
within the relay.
A "coarse" incremental adjustment for the current pickup setting
is provided by the TAP switches. These are front-panel rotary
switches; one for the phases, and another for ground, if ground
current is monitored. The current setting represented by each of
the 1 0 switch positions is given for each range (HIGH and LOW)
in the table printed on the front panel. (Reference
Figure 3.)
The "fine" adjustment of the current pickup point is determined
by the value (0 to 99) stored in the relay's Tap Cal registers (again,
one setting for the phases, one for ground). When the Tap Cal
register contains the value 0, the pickup setting of the relay is the
indicated TAP setting.
The Tap Cal registers provide a means of adjusting the timed trip
pickup between the selected position of the TAP switch and the
next higher position. For example, when the TAP switch is set to
position A, the Tap Cal register provides an adjustment from 0. 50
to 1 .00A forthe LOW range, and from 2.00to3.00Aforthe HIGH
range. EXCEPTION: At the highest TAP position (i.e. , J), the
setting of the Tap Cal register is ignored. Therefore the
J
values
of 5.0 and 1 1 .00 represent the upper limits ofthe LOW and HIGH
ranges, respectively.
8.3
Power Supply
B.3.1 General
The solid state power supply is a low burden, flyback switching
design which delivers a n5 and +1-1 2 Vdc to the relay's
internal circuits; also an is5 Vdc for the communications
circuits.
The input terminals of the power supply are not polarity sensitive.
A red POWER LED illuminates to indicate that the power supply
is functioning.
B.3.2 Loss-of-Power Sensing
A loss-of-power circuit monitors the power supply. If power is
interrupted, a Power Fail signal is delivered to the computer in
time for it to store all vital information in non-volatile registers. Also
at this time, the Relay Disabled output contact closes as ex
plained below under "Outputs."
8.4
Multiplexer
A multiplexer is utilized to select each current sensing input.
These inputs are then supplied to the sample-and-hold circuit,
and then to the level shifter. The level of each input sample is
strobed into the microprocessor, where analog-to-digital con
version is implemented.
8.5
Internal Microcomputer
The onboard computer fulfills many of the logic and signal
processing functions described in the following paragraphs, and
performs the time delay computations.
B.5.1 Analog-to-Digital Converter
The voltage representing a component of the actual sensed
current is presented to the analog-to-digital (NO) converter. The
NO converter supplies the equivalent binary value of the sensed
current. The accummulator stores a number of these squared
samples and presents this sum to the trip computer. The
microcomputer then calculates the true RMS value ofthe current
at each input.
B.5.2 RMS Sensing
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
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