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IEC 61850

Communication networks and systems in substations ... and beyond

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Question and Answers related to Information and Service modelling

1. Is there a brief example to learn about logical nodes, data, and common data classes?

Answer 1

An excerpt of the logical nodes defined in IEC 61850-7-4 (XCBR - circuit breaker) shows the three levels of classes: logical node, data, and data attributes (derived from common data classes). Just click on the data name in the logical node and check the definition of the semantic of the data, ...

The above shown pages are not up-to-date. SCC has provided a comprehensive tool that provides the complete models defined in IEC 61850-7-4 and IEC 61850-7-3 and an introduction to IEC 61850-7-2.

With the purchase of all four (4) parts: IEC 61850-7-4, IEC 61850-7-3, IEC 61850-7-2 and IEC 61850-7-1 you get the complete browsable information model!! Order information.

Answer 2

A small subset of the browsable tool is available here [pdf, 2.5 MByte].

2. What is a Logical Node (LN)?


A brief introduction of the approach of logical nodes can be downloaded here.

3. What is the scaling of a measured value?


Introduction to Floating Point to Integer conversion in IEC 61850-7-3

The formula for the conversion between the floating point value and an integer value is given as follows:

f * 10**(units.multiplier) = (i * scalefactor) + offset

The following describes the basisc of the scaling of values ("." is the decimal point).

Supose the following five values to be communicated or stored. The objective of scaling is to find a minimum integer represen-tation (INT) that can be used instead of a floating point (FP):

FP1 = 104.561
FP2 = 99.459
FP3 = 100.545
FP4 = 105.644

The smallest number is 99.459; we can now substract this value from all 4 values (this is the offset):

This results in:
FP1 = 5.102
FP2 = 0
FP3 = 1.086
FP4 = 6.185

Now we can multiply with 100 (Scale) to get a reasonable value we can convert to an Integer value:

5.102*100 = 510.2
0*100 = 0
1.086*100 = 108.6
6.185*100 = 618.5

These numbers can be rounded to get Integer values (which are quite small and close to the original values):

INT1 = 510
INT2 = 0
INT3 = 109
INT4 = 619

These values can be stored and communicated. Integer values communicated from a device that provides just Integer proces-sing can be converted to Floating Point values at the receiving side as follows:

The received Integer values have to be devided by 100 (Scale) and the offset value 99.459 has to be added:

FP1 = 510/100 + 99.459 = 104.559
FP2 = 0/100 + 99.459 = 99.459
FP3 = 109/100 + 99.459 = 100.549
FP4 = 619/100 + 99.459 = 105.649

These Floating Point values are very close to the original values.

These explanation helps to better understand the definitions in IEC 61850-7-3 (Common data classes):

The instCVal is avlaue that uses Integer and Floating Point representation – one of the two must be provided by any device thathas values of instCVal:


instCVal.mag.i INT32
instCVal.mag.f FLOAT32
magSVC.offset FLOAT32
magSVC.scaleFactor FLOAT32

offset and scaleFactor are applied to mag.i and mag.f


instCVal.ang.i INT32
instCVal.ang.f FLOAT32

angSVC.offset FLOAT32
angSVC.scaleFactor FLOAT32

offset and scaleFactor are applied to ang.i und ang.f


units.multiplier ENUMERATED [2=>100] is the same for mag.f and ang.f

The formula

f * 10**(units.multiplier) = (i * scalefactor) + offset

can be applied for "ang.i" in "ang.f" and independently for "mag.f" in "mag.f"

The fact that there is only one Multiplier (e.g. 103) means that the value "mag" is given 103 (Kilo) as well as the value for the angle "ang" (with a multiplier of 103) a value of the angle of "0.00175 rad" communicated means 100 Grad (1.75 rad).



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