Designing a Smart Grid
Communication System to
Achieve 99.999% Link Availability
Abstract
To upgrade today’s mature electrical infrastructure
for the Smart Grid, utilities are investing in initiatives
such as automatic restoration, Volt-Var optimization,
and advanced SCADA. High-reliability, high-capacity
communication systems are needed to handle the
requirements of the switching and protection devices
associated with these initiatives.
This paper discusses the issues to consider in
implementing a communication system that can
provide the “five nines” availability needed for such
Smart Grid applications—equivalent to 5.26 minutes
of the downtime annually. It considers point-topoint, point-to-multipoint, and mesh communication
systems. And it explains why mesh communication
systems provide the best overall performance for
Smart Grid applications.
Changing Control and Communication Needs
As today’s electrical infrastructure transitions to the
Smart Grid, grid functions are moving from centralized
control to peer-to-peer control, which can perform
the necessary decision-making more quickly and
efficiently. There’s an ever-increasing need for realtime distribution system data, and thus the high-
Availability, %
Downtime per Year
availability, high-capacity mesh communication
systems that can provide this data.
But what does “high availability” mean? To the
uninitiated, it might seem that 99% or 99.9% availability
would satisfy the communication system performance
needs of the Smart Grid.
It doesn’t.
Let’s take a closer look to the implications of a few
“nines” on availability.
Understanding Availability
As shown in the table below, a communication
system offering 99% (two nines) availability means
that it isn’t available 3.65 days per year. By most
utility customers’ standards, such availability is not
acceptable.
Industries such as oil and gas have availability
requirements similar to electric utilities and typically
utilize communication systems designed for a
minimum of three nines availability for low-priority
traffic and five nines availability for critical traffic.
Not many kinds of communication systems offer the
high level of availability—plus the bandwidth and low
latency—required for Smart Grid applications.
Downtime per Month
Downtime per Week
99.9999% (“six nines”)
31.5 seconds
2.59 seconds
0.605 seconds
99.999% (“five nines”)
5.26 minutes
25.9 seconds
6.05 seconds
99.99% (“four nines”)
52.56 minutes
4.32 minutes
1.01 minutes
99.95%
4.38 hours
21.56 minutes
5.04 minutes
99.9% (“three nines”)
8.76 hours
43.2 minutes
10.1 minutes
99.8%
17.52 hours
86.23 minutes
20.16 minutes
99.5%
1.83 days
3.60 hours
50.4 minutes
99% (“two nines”)
3.65 days
7.20 hours
1.68 hours
90% (“one nines”)
36.5 days
72 hours
16.8 hours
White Paper
Network Topologies
Point-to-Point Communication Systems
Point-to-point communication systems provide
dedicated access between two locations. They
offer full bandwidth plus latency of less than
0.15 millisecond. But these systems usually require
line of sight between the antennas at the endpoints, so
they can be difficult to deploy in rugged terrains.
Point-to-point systems are susceptible to a single point
of failure too. Since it can potentially take hours to
get back up and running after such a failure, point-topoint systems are not generally suited to Smart Grid
applications.
Figure 1: Point-to-Point Topology.
Point-to-Multipoint Communication Systems
Point-to-multipoint communication systems
connect multiple remote sites to a master site location.
These systems usually require line of sight between
the antennas, so they too can be difficult to deploy in
rugged terrain. Bandwidth is shared between users at
each site, and is typically limited to 50 to100 Mbps per
site. Latency can exceed 50 milliseconds.
Point-to-multipoint systems are also susceptible to a
single point of failure at the master site. Since it can
potentially take hours to get back up and running
after such a failure, point-to-multipoint systems are
not well suited for Smart Grid applications other than
AMI backhaul.
Mesh Communication Systems
Mesh communication systems eliminate single-point
congestion and provide the highest-possible reliability,
as there is no single point of failure. These systems
typically offer latency of 1 to 100 milliseconds.
Mesh systems continually evaluate and select the
best path between points, for optimal performance
and reliability. Since they provide the best overall
performance, mesh systems are preferred for most
Smart Grid applications.
Figure 3: Mesh Topology.
Maximizing Link Availability
To maximize the availability of a communication
system, the performance of each link needs to be
maximized. Figure 4 points out factors to consider.
The transmitter, on the left, produces a specific radio
frequency (RF) output power. This output power is
released into the transmission line—typically coaxial
cable—which connects the radio to the transmitter
antenna. The transmission line has a specific signal
loss which is based on frequency.
The transmitter antenna provides signal gain which
is proportional to antenna size and beam width.
Generally speaking, the larger the antenna, the higher
the gain and the narrower the beam width.
The signal leaves the transmitter antenna and travels
through air, which is known as “maximum tolerated
path loss” based on distance, frequency,
and environment.
Figure 2: Point-to-Multipoint Topology.
2
The receiver antenna, on the right, picks up the signal
and provides gain. The transmission line connected to
the receiver antenna carries the signal to the receiver.
Again, the transmission line has a specific signal loss
which is based on frequency.
Designing a Smart Grid Communication System to Achieve 99.999% Link Availability
The receiver requires a minimum signal strength to
maintain error-free communication. This parameter
is called the “radio receiver threshold.” In designing
a communication system, the actual receive signal
strength should be 20 to 40 dB above this threshold.
Antenna
(Gain)
Best practice dictates that antennas with the
narrowest-practical beam be used, to minimize
interference and maximize signal gain . . . and thus
achieve the highest-possible link availability.
Maximum
Tolerated
Path Loss
Antenna
(Gain)
Transmission
Line (Loss)
Transmission
Line (Loss)
Radio
(Output Power)
Radio
(Threshold)
Figure 4: Link Path Factors.
Designing a Communication System
To design the communication system, you’ll need to
add up the bandwidth requirements of all existing and
proposed switching and protection device radios. This
ensures that the design will have sufficient capacity.
Then obtain the GPS coordinates for all communicating device sites, and enter them in Google Earth™
mapping service, as shown in Figure 5 below.
Figure 5: Picking GPS Coordinates.
Designing a Smart Grid Communication System to Achieve 99.999% Link Availability 3
Copy the site information into the path planning tool. Select the appropriate frequency band, product, antennas,
and various adjustable parameters so the design meets your requirements. Figure 6 shows the final performance
model.
Figure 6: Link Performance Model.
Data collected from a field survey can be used to
update the model in the planning tool so a final design
can be created. Please refer to Figure 7.
Once the communication system is installed, a field
acceptance test should be performed to ensure that
each link in the system is performing to the design
specifications.
Summary
Follow this checklist to achieve the highest level
of availability—plus the bandwidth and low latency—
required for your Smart Grid communication system:
• Engineer every link in the system to the desired
throughput and availability.
• Verify design specifications are met in actual
deployment.
• Monitor performance regularly and perform
maintenance as needed to maintain expected
performance.
Printed in U.S.A.
Wireless engineering is a science. Follow good
practices, make accurate measurements, and your
communication system will perform with high
availability for many years.
Figure 7: Google Earth Final Link Design.
April 8, 2013 ©
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1070-T103