Figure 966: The 10-minute interval with time synchronization and free running 3
seconds
The 10-minute mean value is shown in monitored data until the next 10-minute mean
value has been cumulated. Another 10-minute calculation can 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 also 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 as bad in the IEC 61850 data.
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 as bad in the IEC
61850 data. Bad quality is also indicated after the protection relay has been reset.
10.2.6
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.
Typically, 10-minute THD or TDD mean values are collected from all three phases.
Individual harmonics and the fundamental frequency component can also be
collected.
Power quality measurement functions
1MRS759142 F
1824
REX640
Technical Manual