Latency Measurements in Digital grids

by Fred Steinhauser, OMICRON electronics GmbH, Austria

What is latency?

As it has become good practice, let us first check what Wikipedia says:
Latency is a time interval between the stimulation and response, or, from a more general point of view, a time delay between the cause and the effect of some physical change in the system being observed.
Not everything written in Wikipedia is unquestionably reliable, but this general definition actually describes very well also our case in the digital grid. The stimulus may be a data change to be communicated and the response may be an action taken at the receiving end. If we really wanted to bring in a physical change this could be a disconnector switch changing position or a power system fault.
We will use the term latency for the entire overall time delay in the following and state that it is made up from several contributing times, that we will call delays.

Example: the round-trip test
This test, also called "ping-pong test," is proposed in the GOOSE performance test procedures published by the UCA International User Group. Other than most of the tests explained below, it does not at all focus on network latencies, but it serves as an example of how distinct delays can be extracted from a latency measurement. This test is performed to determine the GOOSE processing delay of the IED under test from a latency measurement, in this case the round-trip time. (Figure 1).
Several preconditions must be met to make a useful evaluation. The network delay must be negligible, the test set delay and the application delay must be known, and symmetry of the input/output delays must apply. Of course, the measurement is repeated many times to allow sensible statistical evaluation. So, the measurement of the latency may not be the actual goal of the test, but a mean to deduct other results.
The test laid out above is one that can be performed with a finished product without special instrumentation for the measurement within the product.
 This was the reason that it was included in that form in the GOOSE performance test specifications. It is also a test where the measurements are entirely done with network messages for the stimulus and the response. In a development lab, such tests would be done with additional test pins to get access to intermediate events to isolate the individual delays more specifically. Likewise, latency measurements in Smart Grids may not be only done with network messages, but also with hard wired signals such a breaker auxiliary contacts.

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