Why Synchrophasors?

Because mainly they enable us to measure and track the power system frequency 60 times a second.

REMEMBER:

To measure is to know... If you cannot measure it,you cannot improve it ~ LORD KELVIN

PMUs, PMUs & PMUs everywhere.....

"Here is some fresh news about Midswest ISO and ISO-NE PMU Installations"

Electric transmission systems got a relatively small slice of the $3.4 billion in Smart Grid Investment Grants awarded last week: $148 million, or 4% of the total in 10 grants. Yet grant awardees in that category that we spoke to yesterday said the work they plan is essential to the smart grid concept.

Most said they will spend much of their awards on phasor measurement units (PMUs), also known as synchrophasors. These GPS clock-equipped devices monitor transmission line voltage and current at key spots between 10 and 30 times per-second -- far faster than most of the devices they will replace.

That rapid feedback lets ISOs and utilities more economically accommodate intermittent power sources such as wind and solar, backing off other generators such as coal plants as loads from alternate sources grow. And it avoids overloading the lines, keeping them safe from automatic tripping and from overheating and consequent sagging, some awardees said.

In earlier stories, we described two other transmission-system recipients, Midwest ISO (MISO) and PJM Interconnection (SGT, Oct-28) and looked at Duke Energy's $3.9 million transmission award (SGT, Oct-30). We begin our continued coverage with details of MISO's SGIG award.

MISO adds 150 PMUs

MISO is based in Carmel, Ind, and won the full $17.2 million it sought from the DOE. Wayne Schug, executive director of planning & performance management, told us yesterday the ISO will match that with the same amount raised through rates paid by its 30 transmission-owning members. Those members collectively own 95,600 miles of transmission lines over 750,000 square miles.

“We definitely believe what we're doing is part of the smart grid,” Schug said. “It can improve the reliability and economic aspects of the grid, so that makes it smart grid in our view.”

Schug said MISO will collect data from about 150 new PMUs to be installed by its members. A variety of PMU suppliers will likely be used since each transmission-owning firm will make its own vendor choices, Schug reported. That said, vendors have not yet been selected, he added.

Under the grant, MISO will do a study of how many phasor data concentrators (PDCs) are needed to collect data from the PMUs. A regional PDC will gather data for use by members of the Eastern Interconnect, Schug said, referring to a conglomeration of East Coast transmission owners that coordinate power transmission across a large section of the eastern US.

The University of Tennessee and North Dakota State University will research how to optimally present visually for use by operators the “vast amounts of data” and how best to use the data for long-term planning, he added.

The SGIG was the first federal grant Midwest ISO applied for.

“We learned we need to react very quickly and address the DOE's concerns very directly,” Schug said.

Challenges ahead include meeting reporting requirements imposed by the grant, he added.

ISO-NE adds 30 PMUs

ISO New England (ISO-NE), of Holyoke, Mass, plans to use its $3.7 million grant, supplemented by $4.8 million of its own, to install 30 PMUs, each serving a substation, spokeswoman Erin O'Brien told us yesterday. The added money will come from all seven of the ISO's transmission-line owning members and the ISO.

“Today we can take a snapshot of system conditions every four seconds,” O'Brien said. “With the PMUs, we'll be able to receive data 30 times a second. With our SCADA already in place, that will give us better ability to monitor and to detect problems on the system.”

An added benefit of the PMUs, she said, is that they will help the system better accommodate power variability when the increasingly numerous local sources of wind and solar power add energy to the grid.

ISO-NE, serving 14 million people in six states, plans to start the work in 2010. Meanwhile, it is “seeking clarification” from DOE as to why it received $3.7 million rather than the $9 million it asked for, O'Brien said.

“If we get only the $3.7 million, we'll still work to move forward,” she added.

American switches to fiber

American Transmission Co, of Waukesha, Wisc, won $11.4 million to build a fiber-optic network for its PMUs, to be called an enhanced SCADA communications backbone. The firm's matching funding will be raised through a combination of debt and equity, Steve Dykstra, director of shared services for the utility, told us yesterday.

Along roughly 60 miles of transmission line, the firm plans to replace the ground wire -- the topmost wire on the tower -- with optical ground wire, a similar technology that adds a fiber-optic networking core.

“It's almost identical in weight and diameter and it's pretty efficient,” Dykstra reported.

The firm has been making such replacements incrementally, creating “a tapestry of optical ground wire,” he said. The grant will help the firm make ground wire replacements in key areas, closing gaps between its two operations centers and adding connections to 48 more substations than the 44 of the firm's 500 substations now connected to an operations center via optical ground wire or leased fiber optic cable.

The firm won a $1.3 million grant, matched with its own equal funding through debt and equity, to expand its collection of real-time data by installing another 48 PMUs in Wisconsin. It has eight of the reporting units now. The PMU data flows to a device called a state estimator that Dykstra said helps gauge the system's performance in Wisconsin.

“I can't say how many PMUs we'd like to have eventually,” Dykstra said. “You add some, see how it helps the state estimator, then analyze whether more would help you or just be additional cost.”

A vendor for the PMUs has been chosen but Dykstra declined to name it since the contract isn't final. American Transmission has used SCADA for over a decade and does not plan changing it.

The firm applied for a third grant, for $10 million to install power-flow control equipment on lines to Michigan's Upper Peninsula, but it was denied. “We will continue to study the project and evaluate the feasibility, cost and timeline for proceeding,” said spokeswoman Anne Spaltholz. “We were lucky to have two of our three proposals granted.”

The application process “flowed well,” Dykstra noted. “We have good processes in place for specifying plans because we've built out so much transmission in the past. Now we're looking forward to building out.”

WECC

TVA currently has around 1.5 trillion points of time-series data in 15TB of PMU archive files.

There are currently 103 active PMU devices placed around the Eastern United States that actively send TVA data while new PMU devices come online regularly. PMU devices sample high voltage electric system busses and transmission lines at a substation several thousand times a second which is then reported for collection and aggregation.

The rate of incoming PMU data is growing very quickly with more and more PMU devices coming online regularly. They expect to have around 40TB of PMU data by the end of 2010 with 5 years worth of PMU data estimated to be at half a petabyte (500TB)

The amount of this time-series data created by even a regional area of PMU devices provides a unique architectural demand on the TVA infrastructure. The flow of data from measurement device to TVA is as follows:

1. A measurement device located at the substation (the PMU) samples various data values, timestamps them via a GPS clock, and sends them over fiber or other suitable lines to a central location.
2. For some participant companies this may be a local concentrator or it may be a direct connection to TVA itself. Communication between TVA and these participants is commonly a VPN tunnel over a LAN-to-LAN connection but several partners utilize a MPLS connection for more remote regions.
3. After a few network hops the data is sent to a TVA developed data concentrator termed the Super Phasor Concentrator (or SPDC) which accepts these PMUs’ input, ordering them into the correct time-aligned sequence - compensating for any missing data or delay introduced by network congestion or latency.
4. Once organized by the SPDC, its modular architecture allows this data to be operated on by third party algorithms via a simple plug-in layer.
5. The entirety of the stream, currently involving 19 companies, 10 different manufacturers of PMU devices, and 103 PMUs - each reporting an average of 16 measured values at a rate of 30 samples a second - with a possibility of 9 different encodings (and this only from the Eastern United States), is passed to one of three servers running an archiving application which writes the data to a size optimized fixed length binary file to disk.
6. A real-time data stream is simultaneously forwarded to a server program hosted by TVA which passes the conditioned data in a standard phasor data protocol (IEEE C37.118-2005) to client visualization tools for use at participant companies.
7. An agent moves PMU archive files into the Hadoop cluster via an FTP interface
8. Alternatively, regulators such as NERC or approved researchers can directly request this data over secure VPN tunnels for operation at their remote location.

F R E Q U E N C Y......

Q: How do we calculate the power system frequency in a PMU? What is the ideal measurement that needs to be chosen for calculating the frequency?

A: In a Phasor Measurement unit, the positive sequence voltage measurement is the ideal measurement for power system frequency determination.

Basic things to remember while installing a PMU

1. All voltage inputs to the PMU should be line to neutral (not line to line) and all current inputs should be line currents (not delta currents).

Symptoms for wrong connections:

a) Voltage magnitudes 70% larger than expected
b) Referenced voltage angles appear to be displaced by +30 degrees
c) Current magnitudes 40% smaller than expected
d) Referenced current angles appear to be displaced by +30 degrees

FACTS:
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V (L-L) = 1.732 V(L-N)
Delta I = 0.577 Line I
V (L-L) and Delta I have their angle displaced by +30 degrees wrt V(L-N) and Line Currents
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2. PMUs are set to compute positive sequence phasors out of three phase signals normally labeled "A", "B" and "C". The positive sequence is obtained by rotating the phase "B" phasor by 120 positive degrees, rotating the phase "C" phasor by 120 negative degrees, adding the two results to the phase "A" phasor, and dividing by three.

Symptoms for incorrect phase sequence:

a) Zero or close to zero magnitude of the positive sequence phasor while the magnitude of the individual phases is not zero.

FACTS:
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In a perfectly balanced system, and for a correctly wired unit the result of these operations is a phasor equal to the phase "A" phasor. Any error in the identification of the system phases will cause two of the phases to be connected incorrectly. If this happens the three phase system will be wired as a negative sequence to the PMU and the signal will be filtered by the positive sequence algorithm resulting in a zero or close to zero magnitude.

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TYPICAL PMU INSTALLATION AT A SUBSTATION