IEEE Power & Energy Magazine - September/October 2015 - 57

An adequate communication infrastructure
was one of the biggest challenges in executing
the synchrophasors projects.

The PDC installed at RLDC works in data sorting based
on absolute time and expects an accuracy of 1 μs or better in
the PMU time stamps. If the accuracy in time stamp is under
1 μs, the corresponding PMU data will be flagged as "data
sort on arrival" instead of "data sort on time stamp," as per
IEEE Standard C37.118. This might be what has caused the
quality of the data arrived to be flagged as unreliable.

Communication Challenges in
Integrating PMU
An adequate communication infrastructure was one of the
biggest challenges in executing the synchrophasors projects. In India, the availability of communication between
the extra-high voltage substation and the RLDC was one
of the deciding factors for identifying the PMU locations.
Due to this, some of the PMUs either had to be relocated
to a different substation having communication feasibility
or a corporate-wide area network had to be used where
the communication path is shared with other applications,
thus compromising data reliability/latency. The data
update rate adopted for all the projects is 25 samples/s
(40 ms/sample). For such a high rate of reporting, in general, a dedicated fiber channel is required for reliability of
real-time data.

Synchrophasor Data in Multicast
In existing projects, synchrophasor streams are transmitted
using unicast packets to only one PDC. However, in cases
where PMU measurement data is also required to transmit to
multiple PDCs simultaneously, unicast may overload communication channels with multiple packets of same data. For
such scenarios, multicasting is a more suitable method of
data transmission.

Reliability of Synchrophasor Data
Even with a healthy PMU in place, there had been instances
of data losses due to communication link issues between
PMUs and the control center and synchronization loss at
PMUs. Due to communication loss, there is a complete loss
of data from one or more location or sometimes intermittent data loss for shorter period. For example, an application
running all reported synchrophasor data like an oscillations
monitoring system (OMS) and angle monitoring applications need to detect such scenarios and have the capability
to either discard unreliable data or handle these situations to
avoid wrong interpretations from results.
september/october 2015

Calculation of Sequence
Components at PDC Level
Some PMUs are reporting only three-phase voltages and
currents, and some are reporting three phases as well
as positive sequence quantities but not reporting other
sequence components to the PDC. One of the options is
to calculate the sequence components of voltages and currents at PDCs. As long as the number of PMUs is not more,
this option is acceptable, but with the deployment of a large
number of PMUs, calculating components at PMUs will be
preferable option.

Synchrophasor Angle
Measurement Unwrapping
To assess stress on the grid, an angular difference between two
distinct nodes is used. In similar lines, angle measurements
from PMUs are used for monitoring grid condition. Synchrophasor-supporting IEEE C37.118 standards are reported over
a range of ±π rad or ±180˚; hence they "wrap" at the end of the
range limits, which is a discontinuous signal and needs to be
addressed properly. These discontinuities result in the faulty
analysis of grid instabilities, especially when considering the
angle difference between two separate locations on the grid.
While dealing with only four to five PMUs, calculating
the angle difference using commonly available unwrapping algorithms can solve the problem, but the problem may
become significant while dealing with missing data frames
and handling thousands of PMUs.

Synchrophasor Data Storage Related
Experience and Challenges
A large volume of synchrophasor data accumulates at the
control center over a period of time. Presently installed historian storage capacity is in the range of 4-5 Tb at the control
center level. With more PMUs to be installed, the capacity of
the historian needs to be increased and proper mechanisms
need to be devised for data storage. Big data and data center
type technologies may be needed, for which further feasibility studies are required. However, event-based data storage
is one of the options.

Integration with SCADA State Estimator/EMS
Some of the important characteristics of WAMS compared
with SCADA are
✔ high precision
✔ synchronized measurements
ieee power & energy magazine

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Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - September/October 2015

IEEE Power & Energy Magazine - September/October 2015 - Cover1
IEEE Power & Energy Magazine - September/October 2015 - Cover2
IEEE Power & Energy Magazine - September/October 2015 - 1
IEEE Power & Energy Magazine - September/October 2015 - 2
IEEE Power & Energy Magazine - September/October 2015 - 3
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IEEE Power & Energy Magazine - September/October 2015 - 96
IEEE Power & Energy Magazine - September/October 2015 - Cover3
IEEE Power & Energy Magazine - September/October 2015 - Cover4
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