4.181
Date Code 20170927 Instruction Manual SEL-751 Relay
Protection and Logic Functions
Global Settings (SET G Command)
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 compen-
sation factor. These compensation factors correct the signal offset errors to
within ±1 mA or ±1 mV.
Signal offset compensation factor calculation procedure:
Step 1. Turn the SEL-751 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 by 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 by 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 tem-
perature 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-751 to boot up, perform self-diagnos-
tics, and detect installed cards.
Table 4.76 summarizes the steps and describes the settings we carry out in this
example.
Ta b l e 4 .76 S u m m a r y o f Step s (Sheet 1 of 2)
Step Activity Terse Description
General
1
SET G
AI301NAM
Access settings for INPUT 1
2
TX_TEMP Enter a Tag name
3
I Select type of analog input; “I” for current
Transducer
High/Low
Output
4
4 Enter transducer low output (LOW IN VAL)
5
20 Enter transducer high output (HI IN VAL)
Level
6
Degrees C Enter Engineering unit
7
–50 Enter Engineering unit value LOW
8
150 Enter Engineering unit value HIGH
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