Instrumentation & Measurement Magazine 25-6 - 12

possible. Nevertheless, different requirements can be mapped
to different existing technologies. For instance, the traditional
Supervisory Control and Data Acquisition (SCADA) systems
are suitable to detect, record and handle " slow " events, i.e.,
with sampling periods ranging from a few seconds to a few
minutes. In this case, a time synchronization accuracy in the
order of hundreds of ms is sufficient to merge and to correctly
process the data related to the same event (e.g., a severe fault
or service interruption).
For power quality measurements, higher time synchronization
accuracy is needed because events such as voltage dips
and sags may be as short as one power line cycle. So, higher
temporal resolution and 10-ms synchronization accuracy are
needed to investigate the correlation between multiple events
detected in different points of the grid.
The coordination of protection devices installed in primary
and secondary substations for the realization of logic
selectivity systems is even more challenging. In fact, a time
synchronization accuracy in the order of hundreds of μs is
required to: identify the source of the anomaly; estimate its
propagation through the grid; and perform a corrective action
prior to service interruption.
Moreover, the increasing penetration of renewable-based
and distributed energy resources is expected to cause not
only sudden variations of power supply and demand, but
also voltage amplitude and frequency oscillations that require
the implementation of fast detection and stabilization techniques.
To address these problems, the so-called Wide Area
Monitoring, Protection, and Control (WAMPAC) systems are
used. Such systems strongly rely on Phasor Measurement
Units (PMUs), i.e., instruments able to measure amplitude,
phase, frequency and rate of change of frequency (ROCOF) of
ac voltage or current waveforms at times synchronized to the
Universal Coordinated Time (UTC). In this case, sub-microsecond
time synchronization accuracy as well as immunity to
cyberattacks must be achieved. However, the security issues
are generally not related to the technique employed to provide
time synchronization as they depend instead on the communication
protocol and on the technology used to transfer the
temporal information.
Overview of Time Synchronization
Techniques
Electronic devices are typically equipped with a clock whose
purpose is to provide a local time reference to the rest of the
system. Unfortunately, the time values measured by different
clocks tend to drift away from one another due to oscillator
tolerances and a variety of phase and frequency power-law
noises. Thus, the time synchronization techniques are needed
to keep the time values measured by different clocks aligned
on the same time scale. In particular, " two clocks are synchronized
to a specified uncertainty if they have the same epoch
(i.e., the timescale origin is the same) and their measurements
of the time of a single event differ by no more than that uncertainty "
[2].
Traditional Time Synchronization Solutions
The series of Standards IEC 61850 defines the architecture and
the requirements that a Substation Automation Systems (SAS)
must fulfill, including those related to time synchronization, as
Fig. 1. (a) Traditional (wired) and (b) network-based time synchronization in primary SAS.
12
IEEE Instrumentation & Measurement Magazine
September 2022

Instrumentation & Measurement Magazine 25-6

Table of Contents for the Digital Edition of Instrumentation & Measurement Magazine 25-6