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Scaling of Measured Values
3.1 Measurement conversion
3.1
Measurement conversion
Measured values are transferred as integer values (2 bytes) via Modbus TCP between the SIPROTEC 4
device and the Modbus TCP master. However, the measured values are in general available in the
SIPROTEC 4 device in floating-point format as a percentage referred to the parameterized nominal values of
the primary equipment.
Before transmission of a measured value via Modbus TCP a measurement conversion (scaling) must be
carried out in the SIPROTEC 4 device.
Scaling
Scaling of a measured value to the format for the transmission via Modbus TCP means the definition of:
•
Type,
•
Scaling factor,
•
Zero offset.
Type
Decision, whether the measured value is transmitted via Modbus TCP as percentage value, primary value or
secondary value.
The transmission possibilities depend on the the measured value, for example, no secondary values possible
for power values.
Scaling Factor
The measured value in the SIPROTEC 4 device (floating-point format) is multiplied by the scaling factor
before transformation to an integer value (for Modbus TCP).
It is possible to transfer fractional digits by multiplication by a multiple of 10 to an integer value with that.
Zero Offset
The Zero offset is added to the result of the multiplication of the measured value in the SIPROTEC 4 device
(floating-point format) by the scaling factor.
Formula
The measured-value in integer format for transmission via Modbus TCP is calculated according to the
following formula:
Measured value(Integer) = Measured value(Float) ⋅ Scaling factor + Zero offset
Measured value(Float): percentage value or primary value or secondary value (according to Type).
Table 3-1
Scaling Index
0
1
2
3
4
5
6
7
8
9
42
Conversion Table for Measured Values with Default Entries
Scaling Factor
Measured-Value Type
1.0
Primary value
10.0
Primary value
100.0
Primary value
1000.0
Primary value
10000.0
Primary value
1000.0
Secondary value
100.0
Percentage value
1.0
Secondary value
10.0
Secondary value
100.0
Secondary value
Zero offset
Default Usage
0.0
-
0.0
AC currents
0.0
AC voltages,
Power, Frequency
0.0
-
0.0
Power factor
0.0
-
0.0
-
0.0
-
0.0
-
0.0
-
SIPROTEC 4, Modbus TCP Profile, Manual
C53000-L1840-C582-1, Edition 07.2015
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