Quality Bits - Clarifications with Edition 2.1

Author: Christoph Brunner, it4power, Switzerland

Based on some recent discussions, I realized that apparently, there is still some confusion related to some basic concepts of IEC 61850. Those should be clarified with Edition 2.1 of the core parts of IEC 61850.
First, it should be noted that there were some reallocations of specifications between IEC 61850-7-2 and -7-3. With the rearrangement, all types of the definitions are now in part 7-2. Types are either basic types (INT32) or domain types (Timestamp).
Basic types are never structured; domain types can be structured and include many domain specific enumerations as well. Quality, which was introduced before in part 7-3 is now included as a domain type in 7-2. On the other hand, all common data classes (CDC) are now defined in 7-3. This includes the CDC for service tracking, which were originally introduced in part 7-2.
A topic that is always a source of confusion is the relation between the mode / behavior of a logical node and the values in the quality. While validity, source and detail quality were more or less clear all the time, test and operator blocked were always a source of confusion.

Regarding operator blocked, unfortunately, Ed 2 of part 7-4 introduced an error indicating that in the mode blocked, the quality element ?operatorBlocked? will be set to TRUE.
This was never the intention. The mode blocked or test/blocked were intended to block physical outputs to the process with the purpose to support testing without a physical operation of process equipment. It is a logical replacement of a physical test switch that interrupts the wire to e.g. a trip coil of a circuit breaker. Ed 2.1 of part 7-4 will correct that error based on a TISSUE that had been reported.
The quality operatorBlocked is related to the communication and the update of a status or measurement value. It is linked with the quality being questionable or invalid and the detailed quality indicating oldData.

As it is now explained in Ed 2.1 of part 7-2, communication updating as well as input updating may be frozen both by an operator as well as an automatic function. If the operator wants to freeze the updating, this is possible through the data attribute blkEna (introduced in Ed 2) and is indicated with the additional quality ?operatorBlocked.?

The second clarification I would like to mention is the difference between test mode and the activation to receive simulated messages. While those features are related regarding when to use them, they are impacting a completely different level. In IEC 61850, GOOSE messaging has been introduced to allow virtual wiring: wires with typically binary signals between IEDs are replaced with a signal in a GOOSE message. The GOOSE message is like a multi-wire cable that you plug in with a connector or through a terminal block to the IED. When you now switch to receive simulated messages instead of the real ones, this is like plugging in a test connector instead of the real signals in the traditional world.

In the IEC 61850 world, this happens typically at the level of the communication interface. On the other side, the test mode and the test bit in the quality of a data object happen at the (application) logic level. As an IED typically receives simulated messages while it is in test mode, the quality of the DOs in a simulated message will normally have the test bit activated. By the way, this differentiation between communication level message flow and application level signal flow exists as well with regard to the design in the configuration file based on IEC 61850-6.

So, summarizing - reception of simulated message is used to control the input to the IED during a test; the test bit in the quality is important to avoid messages produced during a test to impact the rest of the system that is live. As it is only possible to switch the whole message from the input perspective, it is important to ensure the testability already during the design. Depending on future planned live test scenarios, segregation of signals in different GOOSE messages may be required.
At the time this column is being prepared, part 7-2, 7-3 and 7-4 Ed 2.1 mentioned are in preparation for FDIS circulation.

Biography:

Christoph Brunner is the President of his own independent consulting company it4power LLC based in Switzerland. He has over 25 years of experience with knowledge across several areas within the Utility Industry and of technologies from the Automation Industry. He has worked as a project manager at ABB Switzerland Ltd in the area of Power Technology Products in Zurich / Switzerland where he was responsible for the process close communication architecture of the automation system. He is Convener of WG 10 of the IEC TC57 and is a member of WG 17, 18 and 19 of IEC TC57. He is senior member of IEEE-PES and IEEE-SA. He is an IEEE Fellow and he is active in several working groups of the IEEE-PSRC and a member of the PSRC main committee and the subcommittee H. He is international advisor to the board of the UCA international users group. 

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