Wednesday, November 20, 2019

IEC 61850 Deadband Reporting and Logging - What Does It Provide?

IEC 61850 Reporting and Logging are very useful services to keep the needed bandwidth for messaging low. It is a bit tricky to understand how it works ... due to the fact that there are two documents that need to be studied to understand the function (IEC 61850-7-2 and IEC 61850-7-3). And: You need to know that some changes in the operation of Reporting and Logging happened from Edition 2 to Edition 2.1 (soon to be published as International Standard).

Deadbanding in IEC 61850 provides a filter mechanism that defines, when an analogue value will be reported by the IED (Server side) to another system (Client side).

Edition 2 of IEC 61850-7-3 defines:

"Deadband. Shall represent a configuration parameter used to calculate all deadbanded attributes (for example mag attribute in the CDC MV). The value shall represent the percentage of difference between max. and min. in units of 0,001 %.
If an integral calculation is used to determine the deadbanded value, the value shall be represented as 0,001 % s.
A db value of 0 shall suppress reporting events on the analog value, so that only changes of the range value will lead to events."

Edition 2.1 of IEC 61850-7-3 will define:

"Deadband is a configuration parameter used to calculate deadbanded value 'mag'. The value of 'db' shall represent the percentage of 'dbRef' in units of 0.001 %. Therefore, 'db' = [0...100000], corresponding to [0 %...100 %], respectively. If an integral calculation is used to determine the deadbanded value, the value of 'db' shall be represented as 0.001 %s.
With a 'db' = 0 the attribute 'mag' follows the instantaneous value.
If 'db' is not present in the model, then the deadband calculation is a local issue."

See Tissue Data Base on deadbanding.

"db" = 0 is a dangerous configuration value!! It is like divide by "0". Any (small) change of "instMag" will issue a report or log entry. I am a little bit confused ... but finally it may help with the following use case: Output of the Schedule "ValMV" that is of CDC "MV". See the following figures for some details:

"db" >= 0 could reduce the number of events (reports/log entries) for analogue values. Usually an analogue value is a measurement or a calculated value.

In case of modeling schedules (FSCH in IEC 61850-90-10 / IEC 61850-7-4 Ed2.1) it is required to report/log a scheduled value - which is similar to a measurement ... but it is a non-continuous value (has "jumps" only) ... OK. CDC MV is used for the Output of a Schedule. What is the impact of the "db" value?!? Be careful!
The following figure discusses the impact of trigger option "data change" and "db"=0 as well as "data update":

Finally: If you need to see (as a client) changes only, then "db"=0 is the right configuration. If you want to receive/log a "confirmation" of any new value at the beginning of a new Schedule Interval (SchdIntv) then "data update" is the right configuration.

You need to understand your needs before you decide how to configure deadbanding.

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