Background

Synchrophasor Definition

Alternating Current (AC) is mathematically represented by a cosine wave,
x = A cos(2πfAC t + φ) (2.1)

where fAC = 60Hz in North America. Using a technique proposed by Charles Proteus Steinmetz in, AC can be represented as a simplified quantity called a phasor. When representing a cosine as a phasor, it is assumed that the frequency of the signal remains the same. Therefore, the variable quantities are magnitude and phase. For AC, magnitude is commonly defined as the root mean square of voltage. Equation (2.1) becomes

X=√ φ (2.2)

Establishing phase requires either a signal or time reference. Synchrophasors calculate phase using an absolute time reference, commonly Coordinated Universal Time (UTC). Figure 2.1 shows a cosine superimposed on a UTC time pulse. The synchrophasor is defined to be 0◦ if the cosine has a maximum during the pulse and 90◦ if the cosine has a zero crossing at the pulse. Values between 0◦ and 90◦ are calculated according to the selected phasor estimation algorithm. Previously, phasor measurement at generators and nodes in the transmission network was impractical due to geographic separation between the two. Implementation of synchrophasors allows for easy calculation of magnitude and phase differences between nodes based off a shared standard time.

Previous Work

The concept of a synchrophasor was first introduced in the 1980s and has since generated a large body of commercial and academic research. It is impossible to address all work on synchrophasors and their applications in the scope of this project so emphasis will be placed on development of inexpensive PMUs.

K. Kirihara, B. Pinte, and A. Yoon designed and tested a relatively low cost (approx- imately $1050) PMU as part of an undergraduate senior project described in. Their project utilized a National Instruments sbRIO for digital filtering and calculation of syn- chrophasors. Global Positioning System (GPS) was used to generate the time reference. The project was able to successfully measure phasors, but utilized only the National Elec- trical Code residential voltage standards to test the PMU, ignoring IEEE C37.118.1. The group also did not address the transmission of synchrophasors to a centralized server or phasor data concentrator (PDC).

In Brian Miller’s Masters thesis, alternatives for conventional current transducers are considered. Miller also examines the use of wireless networks for time synchronization under the IEEE 1588 standard. Use of wireless networks is found to provide a viable alternative to GPS synchronization, useful in areas where signal strength is diminished. It would also provide a cost reduction due to the elimination of the GPS module. These proposed changes were found to be viable improvements while remaining compliant with the IEEE C37.118.1 standard.

In order to lessen the restrictions of proprietary hardware and algorithms on the progress of PMU development, the OpenPMU group was formed, dedicated to de- signing ”open source platform for synchrophasor applications and research.” The group utilizes a standard data acquisition device (DAQ) from National Instruments and a GPS receiver from Garmin as the basis for the OpenPMU. A PIC from Microchip synchronizes the DAQ to the GPS timecode. The OpenPMU uses the Python scripting language run- ning on Microsoft Windows. It is currently able to measure synchrophasors, but has yet to achieve full compliance with IEEE C37.118.1.

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