Instrumentation & Measurement Magazine 25-6 - 14

Table 2 - Time synchronization solutions for smart grid applications
Time
Solutions
NTP
PTPv2 (Soft. TS)
IRIG-B
Pros
Network-based, well-known
Network-based, good accuracy
Indoor and outdoor
PTPv2 (Hard. TS) Network-based, good accuracy
GPS
PTPv2.1-HA
Limited accuracy
Accuracy affected by the network traffic
Dedicated synchronization line, cable
calibration
Dedicated devices
Available worldwide, No-cable required Not for indoor applications
Network-based, extremely high
accuracy
can achieve sub-microsecond accuracy even when up to 15
TCs are cascaded.
The High Accuracy (HA) PTP profile (also called White
Rabbit) has instead the ambitious goal to reach sub-nanosecond
accuracy through a variety of additional features such as
[4]: the use of synchronous Ethernet for low-level reference
frequency transfer, the estimation and correction of the delays
due to optical fiber cables and transceiver asymmetry, and the
compensation of possible phase errors at the physical layer.
Thus, the HA profile requires a much more complex and costly
implementation. Table 2 summarizes the main features of the
various synchronization techniques described above.
Time Synchronization and
Synchrophasor Measurements
Synchrophasor Angle Accuracy Requirements
The state variables of power transmission or distribution
systems (namely the quantities that should be measured to
Dedicated hardware and wiring,
expensive calibration
Cons
Synchronization
Accuracy
±1 ms
±1 μs - ±10 μs
±1 μs
±100 ns
±100 ns
<±1 ns
monitor the operation of a grid) are usually the bus nodal voltage
phasors and/or the line current phasors [5]. To provide
direct observability of such state variables, the PMUs can be
used. The phasors of the ac voltage or current signals acquired
by different PMUs, when referred to the same UTC reference
time, can provide a snapshot of the actual state of a grid
[6]. The measurement results of multiple PMUs are usually
downsampled and transferred to a Phasor Data Concentrator
(PDC) at rates ranging from 10 frame/s up to 100 frame/s or
120 frame/s depending on whether the power system nominal
frequency f0
is 50 Hz or 60 Hz [7]. The role of a PDC is to aggregate
and to align data with identical timestamps over time (if
available) or even to reconstruct some missing samples if the
reporting rates of the input PMU streams of data are different
or when some data are lost [6]. A qualitative overview of the
role of time synchronization in PMUs and PDCs is sketched in
the block diagram of Fig. 2. Focusing on the highlighted blocks
only, time synchronization is essential to:
◗ Timestamp PMU values;
Fig. 2. Role of time synchronization functions for PMU signal processing and PDC data aggregation.
14
IEEE Instrumentation & Measurement Magazine
September 2022

Instrumentation & Measurement Magazine 25-6

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