GPS-Disciplined Analog-to-Digital Converter for Phasor Measurement Applications
This paper presents a data acquisition unit which synchronously samples multiple channels in a manner such that the time of day at which each sample is taken is known. This allows measurements taken at multiple locations to be compared with confidence. The intended application is wide area electrical power system measurements, in particular phasor measurement units (PMUs). The novelty of the authors’ design is the application of an open hardware development platform to discipline a commodity analog-to-digital converter (ADC) to a broadcast time signal, usually but not exclusively GPS. The methodology used creates a driver layer for the ADC to achieve real-time sampling in a nonpreemptive Linux environment. The use of open hardware and software addresses the need for a transparent instrument for use in research and development of PMU technology. Through a choice of either a software or hardware phase-locked loop, the ADC is controlled to acquire exactly 256 samples per nominal power system cycle (i.e., 50/60 Hz), precisely time synchronized to GPS, at 16-b resolution and 94.2-dB SNR. The design of a printed circuit board expansion board featuring all necessary components is provided. The performance of the system is evaluated. Interoperability and data exchange with other systems is achieved by use of open schemas and communication protocols. This allows rapid integration with popular numerical simulation environments.
In previous work, the OpenPMU  used a commercially available data acquisition (DAQ) unit. Although the overall performance of the OpenPMU was satisfactory, its future development in the form described could not be further improved upon. One of the principal limitations was that the sampling clock of the DAQ employed utilized a local crystal oscillator and provided no means to discipline this to an external source. Instead, a trigger interrupt was used to window the sample data according to a time signal. The impact of sampling time error is well described in , which notes that a “saw tooth” error is observed on the outputs of susceptible PMUs. While DAQs which do allow for externally-disciplined sampling clocks are commercially available, these tend to be expensive, complex and proprietary. The latter, namely, that the means by which the sampling is performed is not known to the end user, is an important motivation for this paper. An approach in  uses similar equipment to the authors’ technique, however, the methodology is not open sourced and thus is not transparent to the end user.
This paper describes a GPS-disciplined analog-to-digital converter (ADC) that has been developed by the authors to provide a low cost, yet accurate and precise, alternative to commercial offerings. The main benefit of this approach is that by carefully disciplining the sampling clock to an external time signal; the timing related errors described in  are eliminated. Furthermore, this approach will reduce development time for instruments requiring precision timing and wide area coordination. An added benefit is the use of an open source development platform, the BeagleBone Black. The architecture of the device is described and the performance of the time keeping is evaluated. The open source phasor measurement system planned by the authors, OpenPMU, modularizes the functions of a PMU into four distinct components. These are shown in Fig. 1, and are based upon functional descriptions of PMU described in . The main functions are DAQ, signal processing and data representation. These are governed by a time signal, disciplining the system to Coordinated Universal Time (UTC). This paper is primarily concerned with the DAQ module. In the OpenPMU system, information is exchanged between modules by user datagram protocol (UDP) datagrams, with the data arranged in an XML schema. The correct function of the PMU is predicated on the availability of good quality time synchronized waveform sample data, from which phasor parameters can be estimated. PMUs are normally synchronized to a broadcast time signal that is itself disciplined to the UTC time base. Due to the widespread availability of the NAVSTAR GPS time signals distributed by satellites, many commercial PMUs synchronize to this time signal by means of an integrated GPS receiver. Often an input compatible with IRIG-B is fit for use with substation master clocks and other time services.
It is anticipated that PMU adoption will increase across the electrical utility sector in both traditional monitoring, and novel protection and control applications. Myriad commercial devices are in existence but many lack certifications against dynamic testing requirements. The “closed” nature of the hardware and software of these devices makes them of limited use for research and development work. This paper has presented a method to discipline the sampling clock of an ADC to a broadcast time signal for the purpose of waveform DAQ for phasor estimation. Of particular importance to this application is that the sampled data is taken at a consistent sampling rate, in synchronism with the time signal. This allows synchrophasors to be computed and compared over a wide area. The DAQ unit is capable of synchronously acquiring eight analog waveforms at a precise rate of 12.8 kHz (for 50-Hz systems) or 15.36 kHz (for 60 Hz systems), with 16-b resolution and 94.2-dB SNR. The sampling clock is achieved by PLL, using either a commercially available IC, or a software PLL of the authors’ design. Both have been evaluated and are shown to be suited to the application of phasor measurement. The software PLL described by the authors is implemented on the popular open source Beaglebone Black development board. The external components for the DAQ unit have been constructed into a “cape” for the BeagleBone Black, yielding a highly affordable instrument. Data is expressed in base64 and communicated along with the time at which it was acquired and other metadata in an XML formatted datagram using UDP/IP. This allows the data to be exchanged with a wide variety of platforms, software languages, and environments. Due to the flexibility of the data representation, in addition to phasor estimation, this system could have hardware-in-the-loop application in providing sampled data to power system simulation environments. This system is of use to any application which requires real-time sampling on a nonpreemptive operating system. It is proposed that this design can be used as the “engine” for smart grid appliances, reducing development times, and costs for the DAQ stage.
 E. O. Schweitzer, D. Whitehead, G. Zweigle, K. G. Ravikumar, and G. Rzepka, “Synchrophasor-based power system protection and control applications,” in Proc. Int. Symp. Modern Electr. Power Syst. (MEPS), Sep. 2010, pp. 1–10.
 M. Prasad, K. N. Satish, K. S. Meena, and R. Sodhi, “A synchrophasor measurements based adaptive underfrequency load shedding scheme,” in Proc. IEEE Innov. Smart Grid Technol. Asia (ISGT Asia), May 2014, pp. 424–428.
 R. J. Best, D. J. Morrow, D. M. Laverty, and P. A. Crossley, “Synchrophasor broadcast over Internet protocol for distributed generator synchronization,” IEEE Trans. Power Del., vol. 25, no. 4, pp. 2835–2841, Oct. 2010.
 D. M. Laverty, R. J. Best, and D. J. Morrow, “Loss-of-mains protection system by application of phasor measurement unit technology with experimentally assessed threshold settings,” IET Generat. Transmiss. Distrib., vol. 9, no. 2, pp. 146–153, 2015.
 IEEE Standard for Synchrophasor Measurements for Power Systems— Amendment 1: Modification of Selected Performance Requirements, IEEE Standard C37.118.1a-2014, Amendment to IEEE Standard C37.118.1-2011, Apr. 2014, pp. 1–25.
 D. M. Laverty, R. J. Best, P. Brogan, I. Al Khatib, L. Vanfretti, and D. J. Morrow, “The OpenPMU platform for open-source phasor measurements,” IEEE Trans. Instrum. Meas., vol. 62, no. 4, pp. 701–709, Apr. 2013.
 W. Yao et al., “A novel method for phasor measurement unit sampling time error compensation,” IEEE Trans. Smart Grid, to be published, doi 10.1109/TSG.2016.2574946.
 J. A. Culliss, “A 3rd generation frequency disturbance recorder: A secure, low cost synchophasor measurement device,” Ph.D. dissertation, Dept. Elect. Eng. Comput. Sci., Univ. Tennessee, Knoxville, TN, USA, 2015.
 A. G. Phadke and J. S. Thorp, Synchronized Phasor Measurements and Their Applications. New York, NY, USA: Springer, 2008.
 BeagleBoard.Org—Black—What is BeagleBone Black?. accessed on Jan. 12, 2016. [Online]. Available: http://beagleboard.org/BLACK
 Z. He and Y. Liao, “The design of analog acquisition system in distribution automation,” in Proc. China Int. Conf. Electr. Distrib. (CICED), Sep. 2012, pp. 1–4.