Sel 751A - Analog Inputs; Analog Input Calibration Process; Analog Input Setting Example; Figure 4.55 Analog Input Card Adaptive Name

4.100
SEL-751A Relay Instruction Manual Date Code 20100129
Protection and Logic Functions
Analog Inputs
Analog Inputs
The SEL-751A tracks the power system frequency and samples the analog
inputs four times per power system cycle. For analog inputs, set the following
parameters for each input:
➤ Analog type
➤ High and low input levels
➤ Engineering units
Because of the flexibility to install different cards in the rear-panel slots on the
device, the setting prompt adapts to the x and y variables shown in
Figure 4.55. Variable x displays the slot position (3 through 5), and variable y
displays the transducer (analog) input number (1 through 4 or 8).
Figure 4.55 Analog Input Card Adaptive Name
Analog Input
Calibration Process
In the analog input circuit, the dominant error is signal offset. To minimize the
signal offset, we adjust each of the device analog input channels by a
compensation factor. These compensation factors correct the signal offset
errors to within ±1 uA or ±1 mV.
Signal offset compensation factor calculation procedure:
Step 1. Turn the SEL-751A on and allow it to warm up for a few
minutes.
Step 2. Set the analog inputs for each analog channel to the desired
range using the AIxxxTYP, AIxxxL, AIxxxH, AIxxxEL, and
AIxxxEH settings (for example, ±1 mA).
Step 3. Short each analog input in turn at the device terminals using
short, low resistance leads with solid connections.
Step 4. Issue the command MET AI 10 to obtain 10 measurements for
each channel.
Step 5. Record these 10 measurements, then calculate the average of
the 10 measurements by adding the 10 values algebraically, and
dividing the sum by 10. This is the average offset error in
engineering units at zero input (for example, –0.014 mA).
Step 6. Negate this value (flip the sign) and add the result to each of the
AIxxxEL and AIxxxEH quantities. For this example, the new
AIxxxEL and AIxxxEH values are –0.986 mA and 1.014 mA.
Analog Input Setting
Example
Assume we installed an analog card in Slot 3. On Input 1 of this analog card,
we connect a 4–20 mA transducer driven from a device that measures
temperature on a transformer load tap changer mechanism. For this
temperature transducer, 4 mA corresponds to –50°C, and 20 mA corresponds
to 150°C. You have already installed the correct hardware jumper (see
Figure 2.3 for more information) for Input 1 to operate as a current input. At
power up, allow approximately five seconds for the SEL-751A to boot up,
perform self-diagnostics, and detect installed cards.
Inputs Number (1 through 8)
Slot Position (3 through 5)
Analo
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In
p
ut
A
Ix0y