OpenPMU: The Open-Source Phasor Measurement Unit

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"With LabVIEW, we can rapidly prototype and test ideas as well as develop highly detailed user interfaces with ease. The dataflow architecture is ideally suited to the time-critical nature of phasor measurement technology, and the built-in signal analysis libraries are invaluable in the design and development of custom phase-estimation techniques."

- David Laverty, Queen's University Belfast

The Challenge:
Creating a flexible, open-source phasor measurement unit (PMU) system for academic research.

The Solution:
Using NI LabVIEW and a low-cost NI USB-6009 OEM device to measure the voltage and current waveforms from an electrical power system.

Author(s):
David Laverty - Queen's University Belfast

Introduction

Established in 1912, the Queen’s University Belfast (QUB) electrical engineering department has historically been at the forefront of electrical power systems research. The recent worldwide growth in the renewable energy sector opened new opportunities for QUB, given its location on the island of Ireland. Ireland is experiencing rapid growth in installed capacity of wind energy, and can operate with as much as 50 percent of its power supplied by wind turbines. However, in our research, we encountered a unique challenge known as generator islanding. In this scenario, a small generator, such as a wind turbine, is isolated from the rest of the power system. There are many dangers to the plant and personnel when unknowingly operating a generator in a power island, so it is important to detect when a generator is islanded and take appropriate action.

Traditional islanding detection technologies were designed for an electrical power system supplied mainly by large bulk generators in thermal plants (fossil/nuclear). These technologies do not always behave appropriately in power systems such as Ireland’s that feature large numbers of small, distributed generators.

The solution we developed uses synchrophasor technology (with time-coded phasor measurements) to determine whether a distributed generator is operating with a connection to the utility grid, or if it is operating in isolation. For a successful system, we needed to acquire a faster synchrophasor than commercial units could offer. Thus, we built a custom solution based on National Instruments products.

Phasors

A phasor is a mathematical representation that describes the amplitude, frequency, and phase angle of an electrical waveform (voltage or current). PMUs are devices that estimate phasor values from electrical waveforms. To make comparisons of phasors from PMUs installed at different geographical locations useful, we needed to synchronize all measurements (called synchrophasors) to a common time base. This is typically achieved using a GPS satellite navigation system.

Today’s synchrophasor technology emerged from pioneer efforts by Arun Phadke, et al., at Virginia Tech. Phadke demonstrated the first synchronized PMU in 1988, and in 1991, Macrodyne, Inc., launched the first commercial PMU product. Due to the cost of early PMU devices, PMU technology historically has been limited to transmission system applications, where business justifies expensive phasor analysis equipment.

System Description

Our solution addresses the closed architecture of commercial PMU products. It also considers cost. Because this solution protects distributed generators, it needed to be priced competitively and offer equivalent forms of protection.

The core of the OpenPMU is an NI USB-6009 OEM. This low-cost data acquisition module captures the voltage and current waveforms of the electrical power system, which are then processed in LabVIEW on a PC. We selected LabVIEW because we had experience using the software in other projects. We augmented the USB-6009 with a circuit comprising a GPS receiver and a PIC microcontroller running firmware we designed. The microcontroller operates a phase-locked loop (PLL), which generates sampling triggers disciplined to the GPS receiver’s one pulse per second. USB-6009 acquisition is aligned to UTC time, based on wherever the OpenPMU is in the world. The microcontroller also provides GPS time transfer to LabVIEW for time coding the resulting phasor measurements.

LabVIEW is a fast, intuitive environment for developing synchrophasor technology. With LabVIEW, we can rapidly prototype and test ideas as well as develop highly detailed user interfaces with ease. The dataflow architecture is ideally suited to the time-critical nature of phasor measurement technology, and the built-in signal analysis libraries are invaluable in the design and development of custom phase-estimation techniques. Additionally, National Instruments staff helped us debug code errors and offered advice for optimization.

Conclusion

We expanded the project to include the KTH Royal Institute of Technology in Stockholm, and we would like to accumulate even more academic and commercial partners. We recently released an open-source interface for commercial PMU products. It supplies sample data to LabVIEW to test innovative phase-estimation algorithms with both National Instruments hardware and commercial PMU hardware. We look forward to the release of National Instruments GPS synchronization units on the NI CompactRIO platform, which will benefit OpenPMU.

To learn more about OpenPMU, visit openpmu.org.

Author Information:
David Laverty
Queen's University Belfast
Ashby Building, Stranmillis Road
Belfast
United Kingdom
david.laverty@qub.ac.uk

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