Subject: [NEXTEL-1] Con Edison the "Other IDEN Carrier" GPS and Power Grid Intermesh

From: [email-masked]
Date: Sun, 14 Dec 2003 22:58:40 -0500

Con Edison the "Other IDEN Carrier" GPS and Power Grid Intermesh

GPS and Power Grid Intermesh
November 1, 2003
By: Paul Stergiou, David Kalokitis
GPS World

Sometimes it takes a disaster of the magnitude of the terrorist attacks on the
World Trade Center to bring home the need for new systems. In the wake of that
event and its elimination of two key substation resources in lower Manhattan,
Consolidated Edison (Con Edison) of New York found that the switch to digital
communications made it impossible to use conventional, proven means of
responding to a desperate need for new power. The traditional intermesh method
for bringing substations on line required analog phone lines, and these no
longer existed.

Con Edison teamed with Sarnoff Corporation, a Princeton, New Jersey-based
research and development company formerly known as RCA Labs, to find a new
approach to intermesh, the intermediary period when power load is transferred
between area substations. Ultimately they solved the problem with the same
digital technology that had made the old method unworkable.

The new methodology has provided a valuable tool for safely and effectively
adding resources to the grid, and an energy industry global honors program
recognized it as an outside-the-box solution.

Destruction and Resurrection
Con Edison lost two lower-Manhattan
substations during the 9/11 World Trade
Center disaster. As quickly as possible,
Con Edison temporarily transferred load to
other nearby substations, so that
customers were unaffected. This formed a
workable solution for September through the
spring because the peak of the summer cooling season had passed.  But the area
urgently required a new substation to come into service before the following
summer, when the cooling season would expose that section of the city to power
shortages.

Con Edison swung into action, designing and constructing a new substation,
Seaport No. 1.
Bringing It Online. Connecting a new substation to an operating, loaded network
such as the one on Manhattan is a complicated task.  Engineers must carefully
establish power flow from the new substation while reducing power flow from the
existing substation. Con Edison needed to transfer the power load from the
Leonard Street substation to Seaport without interrupting the load and causing a
power outage.

To avoid any complications or a power outage during an intermesh, it is
necessary to precisely measure the phase displacement and voltage magnitude
difference between the two substations. The phase displacement is the difference
between the phases of the 60Hz sinusoidal waves at both stations.

The phase displacement between any two substations can vary widely due to a
variety of factors such as station loading, alternate transmission source, and
varying generation output. When connecting a new substation or transferring load
between existing stations, the conditions must be adjusted to avoid a large
circulating current flow that could trip breakers or even damage equipment and
interrupt service. For Con Edison, because of the low impedance, the phase
displacement between the two stations needed to remain almost perfectly matched;
the difference could not exceed 3 degrees.

Because the load at Seaport would increase and the load at Leonard Street would
decrease, Seaport No. 1's 13.6-kV bus would have to be leading in phase between
0-3 degrees and remain 200 volts higher than Leonard Street No. 1's. This would
ensure that the network protectors (NWP) and station breakers would close in and
remain closed for the duration of the intermesh.

Simple instrumentation routinely accomplishes relative phase measurements
between two points.  Connect a phase meter across two phases in a power system
and the phase can be read directly.  However, the Seaport and Leonard Street
substations lie approximately 1.2 miles apart, with no direct path to connect
the instrumentation.  Traditionally, Con Edison measured phase displacement over
copper phone lines. Leased from the phone company, the copper lines connected
each substation to a central control center. The phone lines provided the direct
path for relative phase measurement. Engineers understood the process well.
Using these lines to monitor phase angle required the use of a cumbersome,
time-consuming method to calibrate (zero-out) the phase delay caused by the long
copper wires. The latency factor involved in this method varies by length of
copper wire. In any event, the engineers who developed this method have all
since retired.

Time Marched On. During the past decade, telecom companies had replaced their
century-old copper lines with fiber optic cables. Fiber optics provided enhanced
bandwidth to users, a necessity for companies in New York's financial district
that plow through endless amounts of data.

The increased bandwidth is made possible because fiber transmits information as
light instead of electricity. As a result of this fundamental difference, power
companies like Con Edison cannot measure phase displacement via fiber optics
because phase and phase displacement are characteristics of electrical current
(electrons) but not of optical transmission (photons) technologies. Photons
cannot travel on copper lines - no longer available anyway - and electrons
cannot travel on fiber.  The process would require digitizing the alternating
current (AC) waveform and transmitting it with careful accounting of timing
delays and offsets, making it a difficult system to calibrate.
GPS Candidate. New York's daunting infrastructure made it economically
prohibitive to lay down copper wire between the two stations, forcing Con Edison
to look for a wireless solution. Loran C and GPS appeared to be the only viable
candidates.

While Loran-C is based on accurately timed transmissions from a group of base
stations, developing an accurate time stamp at a given location in a large city
is not easy. Con Edison turned to the Sarnoff Corporation to develop a GPS
solution.

Sarnoff staff determined that GPS timing could serve as an independent reference
for measuring the absolute phase displacement. In essence, a digital signal
processor could sample the 60Hz power waveform and associate an accurate time
stamp with the voltage and phase of the waveform. The information would go to a
control location to determine the difference in phase angle.

Typically, a GPS clock provides a 1-pulse-per-second (PPS) output, accurate to 1
millisecond. This still did not guarantee the accuracy necessary for a
successful intermesh. In the course of a millisecond, the phase of a 60Hz signal
advances 21.6 degrees, far in excess of the accuracy requirements for an
intermesh. The 1-PPS output was not suitable for triggering a phase measurement.

The Sarnoff team solved this problem by using the higher accuracy timing
available at the internal processor of the GPS clock. The GPS-controlled clock
can be outfitted with a digital signal processor triggered by the internal
timing, resulting in a time stamp accurate to 1 microsecond. The phase of a 60Hz
waveform only advances about one fiftieth of a degree in 1 microsecond. The
digital signal processor can sample the AC waveform and measure phase accurately
to about 0.3 degrees. Add it all up and the system can provide sub-degree
accuracy where it is needed.

The system measures absolute phase of the AC signal, where zero degrees
corresponds to the positive maximum of a cosine wave coincident with a reference
synchronized with the 1-microsecond-accurate clock. What does that look like?
The instrument reports a measurement each second. The reported data includes a
Coordinated Universal Time (UTC) time stamp, a phase measurement, and a voltage
measurement. Each second, the UTC time advances one full second, and new phase
and voltage values are reported.

With GPS time acting as an independent precision reference, absolute phase can
be measured anywhere in the world. More importantly, it can be measured between
Seaport and Leonard Street.

Relative Phase Control. Power engineers must control the relative phase between
the two substations, using some simple communications and calculations. The
absolute phase is measured and timestamped at the two substations, and the data
goes to a remote-control room using telephone modems. The relative phase
difference between the two substations is determined by subtracting the absolute
phase at one substation from the absolute phase at the other and normalizing to
+/-180 degrees (see Figure 1). The reference (GPS time) cancels leaving the
relative phase between the two locations.

Because the power engineers would have their hands full controlling the
intermesh process, the Phase Measurement System (PMS) developed by Sarnoff had
to be robust and user friendly. The procedure had to account in advance for all
the vagaries of modem connections, automatic redialing, data dropouts, and
potential operator error. Sarnoff built a graphical user interface operating
with a single screen for system control and data display. A single screen
centralizes control of all operation and displays data in multiple formats, as
shown in Figure 2.

The top of the screen shows controls for the modems and instruments located at
the two stations. The operator can enter the phone number and command the
instruments to begin data acquisition and reporting.  When both substations are
reporting data, the central portion of the screen will report key information in
multiple formats. Fine and coarse resolution analog meter displays provide a
visual reference for operators as they adjust the power distribution system. A
digital display (indicating +13.0 in the screen shot) displays the relative
phase difference between the two substations. Additional controls allow logging
data to disk and visual and audible alarms if the phase should move outside
settable limits.

This display, running on a laptop computer sitting on an equipment cart in the
middle of a large Energy Control Center, some miles away elsewhere in Manhattan,
became the focus of attention during the intermesh operation.

On the morning of April 27, 2002, Con Edison positioned Ted Maffetone at the
West End Avenue Energy Control Center and author Stergiou at the Seaport
station, just in case. The control center gradually started to adjust the phase
angle between the two substations by manipulating the output power of specific
generators and removing specific transformers from service. This aspect of the
process was controlled through separate instrumentation.

The GPS phase angle monitoring system provided data updates every second,
allowing Con Edison to make crucial adjustments to the generators and
transformers during the intermesh. Because of its precision and ease of use,
Sarnoff's GPS solution allowed Con Edison to make the necessary transfer
flawlessly. Downtown Manhattan residents did not lose power during the
intermesh.The entire process, which would have taken 72 hours in the old
copper-wire environment, took just four hours. Con Edison has continued to use
Sarnoff's GPS solution for all additional intermesh transfers.

In addition to planned intermesh transfers, this technique can be used to check
for cross-phase and check the phase rotation between any two geographically
separated voltage sources 60Hz or 50Hz. In a modified form it can be used in the
field to accurately identify phases on high-voltage feeder cables (provided the
feeder is single conductor cable) while the feeder is in service and carrying
load.

Acknowledgments

Con Edison department manager Ted Maffetone supported and guided this effort.
Joseph E. Brown at Sarnoff developed the software for the PMS system.
RaderEnergy's "50 Key Information Technology Players in Energy" 2002 global
honors program recognized the authors as "Outside-the-Box Thinkers."

Paul V. Stergiou holds a B.E. in electrical engineering from City College of New
York and is an IEEE member. He is currently manager of power quality at
Consolidated Edison of New York. In 16 years with Con Edison, he has worked on a
variety of projects, including Unit Substation Automation (USA), Secondary
Underground Network Distribution Automation System (SUNDAS), and the design and
development of power line carrier test equipment. He has designed both overhead
and underground distribution systems, including networks, auto loops, radial,
multibank, step-down, and URD. His recent work has focused on expanding power
quality monitoring to include all the substations.

David Kalokitis holds a BSEET and MSEE degree and is a member of the technical
staff at Sarnoff Corporation, Princeton, New Jersey. In 20 years at Sarnoff, he
has worked on a variety of microwave and communications systems, including
telemetry systems, superconductor applications, and satellite systems. His
recent work has focused on radio frequency identification and location systems.
He holds three U.S. patents.
Manufacturers

The Phase Measurement System used a Model 1093B GPS controlled clock from
Arbiter Systems, Inc. (Paso Robles, California), that incorporates a Motorola
(Tempe, Arizona) Oncore GPS receiver and a Trimble (Sunnyvale, California)
Bullet III antenna.

http://www.gpsworld.com/gpsworld/article/articleDetail.jsp?id=75734

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