IEC 61850-10 Performance Testing: Latency and Time

What the standard says and what it is for

The performance clause of IEC 61850-10 (edition 1, 2005) takes the requirements of part 5 and defines which quantities are measured inside a device so that the declarations of one manufacturer can be compared with those of another. It does not impose pass values of its own: the tests check what is declared against the times of part 5. With those numbers, the integrator can calculate the performance of a system and choose devices for each application. This guide complements the one on transfer time and the one on conformance testing; here only the performance part is covered.

Where latency fits

Part 5 defines the transfer time as the time needed to deliver a process value from a sending physical device to the process logic of the receiver. It is counted from when the sender places the content on its transmit stack until the receiver extracts it from its own, and it is made up of three intervals.

The interval tb depends on the network infrastructure and is not an attribute of the IED. From the point of view of testing a device, only the output latency and the input latency can be measured; ta and tc are estimated from them. Part 5 also notes that verification of the complete transfer time must be done in the site acceptance test, because the devices and the network may come from different manufacturers.

Two rules of part 10 divide responsibilities:

• the device manufacturer declares and documents the part of the latency that corresponds to the estimated processing times: input signal conditioning (debounce, sampling) and output activation (contact delay, input and output scan cycle);
• the manufacturer of the network equipment, such as switches, declares the latency due to its processing for all the priorities it supports.

What is measured

The standard lists five delivery mechanisms and measures the output latency for each:

The test system generates a sequence of physical triggers on the IED inputs and measures the delay until the corresponding message. Seen as a black box, each mechanism offers two metrics, output and input: from a change in the message to a change in the physical output.

Repetition and result. The mean and the standard deviation are calculated over 1000 triggers. What is documented for each latency is the measured value (mean and deviation) and the two estimated values, that is, the input and output processing times declared by the manufacturer.

Acceptance criterion in edition 1

The measured output latency (and the input latency) must be less than or equal to 40% of the total transmission time defined for that message type in part 5. With 40% at each end, more than 20% is left for the network. The standard specifies that this limit applies mainly to type 1 (fast) and type 4 (raw data) messages, which use the priorities of the network equipment of parts 8-1 and 9-2, and that type 2 messages could be assigned high priority.

With the total times of edition 1 of part 5, the limit works out as follows. These are direct calculations on those times, not values that part 10 lists.

Clarification: classes P1 to P3 and TT0 to TT6

In the text of edition 1 of part 5, performance is expressed with classes P1, P2 and P3 for control and protection (P1 typical of a distribution bay, P2 of a transmission bay and P3 of a transmission bay with synchronization and the most demanding breaker differential) and M1 to M3 for metering and power quality, together with message types 1, 1A, 1B, 2, 3, 4, 5 and 6. The TT0 to TT6 classes, which appear in more recent documents, are not in that text and belong to later editions of part 5; the guide on transfer time covers them. When comparing a device with a specification, confirm which edition it was written against.

Time accuracy

The objective of this test is to verify that the IED correctly reports the time stamp of an event. An accurate time stamp depends on several functions: that the clock decodes the received signal correctly, that it synchronizes to it, that the device detects the change of state in time, and that it uses its clock properly to stamp the data. With synchronization over the substation network, an IED with a precision time source acts as master and another of the same type can be the backup.

The test system has a change generator and a time master function, both connected to the same external clock source, such as a radio or satellite clock. The generator causes physical events on the IED and records their exact time; an analyzer retrieves the stamp of each event and compares it.

Two measurements are made:

1. With an external source: the IED receives time from the same source the test system uses. After synchronizing, 1000 changes are generated and the mean and standard deviation of the differences are calculated.
2. With the synchronization protocol: the IED synchronizes with the test system master. The sequence of 1000 events starts right after the IED requests synchronization, and if another request is made during the sequence, the sequence is interrupted until the exchange is complete.

Accuracy is evaluated against UTC, and the IED is tested for the accuracy class it is declared for under part 5. Switches are not considered a significant source of variation, and the IED manufacturer must declare the drift of its internal clock, which is independent of synchronization.

Accuracy classes in edition 1

Meeting them requires both the synchronization and the stamping mechanism. The IED clock must have a finer resolution than the class in order to receive the source and synchronize to it. To see how time is delivered in a substation, read the guide on time synchronization and the one on the PTP power profile.

Additional tests

Part 10 leaves out of its scope the other tests of part 4 (system, type, routine, FAT and SAT), which must be defined in other specifications. In a substation, the numbers from part 10 feed those tests: the declared times are compared with what is measured in the project. For instruments and methods, see IEC 61850 test tools.

How to apply it

• Specification: ask the manufacturer for the output and input latencies of each mechanism, with mean, deviation and estimated times, and the clock drift.
• Network design: add the latency of each switch by priority to the ta and tc budget.
• FAT and SAT: measure the complete end-to-end chain, which is where the transfer time is verified.
• Time quality: check the stamp accuracy against the required class.

We can help you measure your times

At Tiza Group we measure the latency of your GOOSE messages and the time accuracy of your devices, end to end, and document the result in your test report. We do it in our FAT and SAT testing service and with our IEC 61850 engineering. Field work is done in Colombia, or elsewhere on request, and document and configuration reviews can be done remotely, worldwide.

Related guides

• Transfer time in IEC 61850: classes and measurement
• IEC 61850-10 conformance testing: what it verifies
• IEC 61850 GOOSE
• Time synchronization in substations
• PTP IEEE 1588: power profile
• IEC 61850 test tools

References

• IEC 61850-10, edition 1 (2005): clauses 7 and 8.
• IEC 61850-5, edition 1 (2003): clauses 13.4 to 13.7.

Frequently asked questions