Instrumentation & Measurement Magazine 25-6 - 11

Time Synchronization for Smart
Grids Applications: Requirements
and Uncertainty Issues
David Macii and Stefano Rinaldi
S
mart grid implementation requires that multiple
distributed systems are synchronized within and between
substations. This is needed to coordinate the
Intelligent Electronic Devices (IEDs) as well as to align in time
the measurement data associated with the same event, but collected
in different points of a broad geographical area. The
time synchronization accuracy requirements in power systems
are application-specific and are classified in the IEC Standard
61850-5:2013. Depending on such requirements, alternative
solutions can be used. For instance, sub-microsecond time synchronization
is essential for the implementation of Wide Area
Monitoring, Protection, and Control (WAMPAC) systems.
In fact, time synchronization directly and indirectly affects
Phasor Measurement Units (PMUs) accuracy. The PMUs can
measure the magnitude, the phase, the fundamental frequency
and the Rate of Change of Frequency (ROCOF) of ac voltage
and current waveforms at times synchronized to the Universal
Coordinated Time (UTC). Among such quantities, the phase
data are those most strongly affected by time synchronization.
If time synchronization accuracy is within ±1 μs, its impact on
synchrophasor phase measurement is in the order of a fraction
of mrad, i.e., low enough to have a negligible impact on the
state estimation uncertainty of most of transmission and distribution
systems. However, the need to interpolate the missing
phasor values when data from PMUs with a different reporting
rate are collected and aligned in time may unexpectedly
boost synchrophasor phase estimation uncertainty.
Time Synchronization Requirements for
Power Systems
In smart grids, transmission and distribution systems operators
have to improve energy efficiency, prosumers'
management flexibility, safety, security and more in general
resilience. Services such as distributed generators scheduling,
optimal energy dispatching (e.g., for electrical vehicles
charging), volt/var optimization and advanced network protection
schemes require widespread measurement and control
devices as well as a ubiquitous information and communication
infrastructure to improve situational awareness. A
September 2022
function that is very useful to support smart grid operation is
time synchronization. Usually, the role of time synchronization
is twofold. On the measurement side, time synchronization
is needed to perform the time alignment of multiple and heterogeneous
data sets collected by different instruments and
sensors spread over the grid. Data alignment is particularly
important if such data refer to a common event (e.g., a fault)
and/or for system state estimation. From a control perspective,
instead, time synchronization is essential to coordinate
the actions of different agents and Intelligent Electronic Devices
(IEDs). Of course, the time synchronization accuracy
requirements depend on the specific measurement and control
application considered. The IEC Standard 61850-5:2013
provides a classification of time synchronization requirements
for different kinds of applications and services [1], like those
listed in Table 1. The Standard specifies six synchronization accuracy
classes, ranging from Class A (accuracy in the order of
±1μs) to Class F (greater than 1 s). It is important to emphasize
that the IEC Standard 61850-5:2013 was purposely conceived
to be independent of technology to keep its scope as broad as
Table 1 - Time synchronization accuracy classes for
power systems applications specified in the IEC
Standard 61850-5:2013
Class
Class A
Class B
Class C
Class D
Class E
Class F
Synchronization
accuracy
requirements
1 μs
100 μs
1 ms
10 ms
100 ms
>1 s
IEEE Instrumentation & Measurement Magazine
1094-6969/22/$25.00©2022IEEE
Applications
Phasor and distributed
measurements
Automated fault recording
Time tagging of fast/
transient events
Power quality monitoring
Slow event monitoring and
logging
Other no time-critical
monitoring applications
11
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