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