Figure 962: The 10-minute interval with time synchronization and free running 3
seconds
The 10-minute mean value is shown in the monitored data until the next 10-minute
mean value has been cumulated. Another 10-minute calculation might be started
when time synchronization is used to change the protection relay's internal clock.
The first calculation continues for 10 minutes and is then seen in the monitored data
with a time stamp. An event is sent every time a 10-minute value calculation is ready
in the monitored data.
See the Time synchronization chapter in this document.
When the protection relay is powered on, the shown value is zero until the first
10- minute mean value has been cumulated. During this time, the 10-minute output
quality is marked questionable in the IEC 61850 data.
When the settings
Reference Cur Sel
or
Demand current
are changed, older data
is not cleared. Although older data is calculated based on different setting values,
those can still be used as they are used to calculate mean values. The influence
might be seen in mean values. These are considered as minor adjustments to the
reference current, similar to varying fundamental current.
Changing setting
Sliding interval
has influence on the seen mean values. Sliding
values change slowly towards the input value during the time interval. When mean
values have not been calculated for a full interval, quality is marked questionable in
the IEC 61850 data. Questionable quality is also indicated for a full interval if the
protection relay is reset.
10.1.6 Application
Typically, the measured short-time 3-second mean values are used with a suitable
logic to give alarms. If a more stabilized value is needed, a 1-minute or 5-minute
mean value should be selected.
One example of using short-time mean values is to compare the level of THD and
some harmonics, for example, 5
th
and 7
th
. The values to be compared to the same
limit can be combined with MAX3R, and MAPGAPC can be used to test if the settable
limit value is exceeded. Outputs of MAPGAPC can be connected to the needed logic
and, for example, an event can be sent with MVGAPC.
The 10-minute mean values can be collected to an external substation monitoring
system. Collected data can be used in weekly reporting. If longer mean values are
needed, the collected 10-minute mean values can be used to calculate, for example,
two-hour mean values.
Power quality measurement functions
1MRS759142 F
1812 REX640
Technical Manual
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