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P746/EN SC/G31

SCADA Communications

(SC) 13-

MiCOM

P746

Since the range of the data type is only 100 years, the century must be deduced. The
century is calculated as the one that will produce the nearest time value to the current date.
For example: 30-12-99 is 30-12-1999 when received in 1999 & 2000, but is 30-12-2099
when received in 2050. This technique allows 2 digit years to be accurately converted to 4
digits in a ±50 year window around the current datum.

The invalid bit has two applications:

It can indicate that the date-time information is considered inaccurate, but is the best
information

available.

Date-time information is not available.

The summertime bit is used to indicate that summertime (day light saving) is being used and,
more importantly, to resolve the alias and time discontinuity which occurs when summertime
starts and ends. This is important for the correct time correlation of time stamped records.

The day of the week field is optional and if not calculated will be set to zero.

The concept of time zone is not catered for by this data type and hence by the relay. It is up
to the end user to determine the time zone utilized by the relay. Normal practice is to use
UTC (universal co-ordinated time), which avoids the complications with day light saving
time-stamp correlation’s.

3.9

Power & energy measurement data formats (G29 & G125)

The power and energy measurements are available in two data formats; G29 integer format
and G125 IEEE754 floating point format. For historical reasons the registers listed in the
main part of the “Measurements 2” column of the menu database (see P746/EN MD) are of
the G29 format. The floating point, G125, versions appear at the end of the column.

3.9.1

Data type G29

Data type G29 consists of three registers. The first register is the per unit power or energy
measurement and is of type G28, which is a signed 16 bit quantity. The second and third
registers contain a multiplier to convert the per unit value to a real value. The multiplier is of
type G27, which is an unsigned 32-bit quantity. Thus, the overall value conveyed by the G29
data type must be calculated as G29 = G28

G27.

The relay calculates the G28 per unit power or energy value as G28 = ((measured
secondary quantity)/(CT secondary)

(110V/(VT secondary)). Since data type G28 is a

signed 16-bit integer, its dynamic range is constrained to ±32768. This limitation should be
borne in mind for the energy measurements, as the G29 value will saturate a long time
before the equivalent G125 does.

The associated G27 multiplier is calculated as G27 = (CT primary)

(VT primary/110V)

when primary value measurements are selected, and as G27 = (CT secondary)

(VT

secondary/110V) when secondary value measurements are selected.

Due to the required truncations from floating point values to integer values in the calculations
of the G29 component parts and its limited dynamic range, the use of the G29 values is only
recommended when the MODBUS master cannot deal with the G125 IEEE754 floating point
equivalents.

Note that the G29 values must be read in whole multiples of three registers. It is not
possible to read the G28 and G27 parts with separate read commands.

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