Rapidly Prototyping a Measurement System to Detect High-Frequency Transients in a Power System with LabVIEW and CompactRIO

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"The prototype system based on CompactRIO and LabVIEW demonstrates how modern communication techniques combined with a cost-efficient, flexible development platform create many opportunities for control and measurement in various parts of the power system. "

- Christoffer Örndal, Department of Measurement Technology and Industrial Electrical Engineering, Lunds University

The Challenge:
Creating a portable measuring device with flexible communication that can record high-frequency transients in power systems and present the data online to multiple users.

The Solution:
Using the NI CompactRIO platform and LabVIEW software for rapid prototyping of a highly flexible measurement system offering fast sampling and large bandwidth.

Author(s):
Christoffer Örndal - Department of Measurement Technology and Industrial Electrical Engineering, Lunds University
Andreas Andreas Jönsson - DLAB – Lund University
Magnus Akke - DLAB – Lund University

A major shortcoming of most electric grid protection systems is the inability to accurately detect the origin of an earth fault. As a result, large parts of the power grid must be disconnected after such an occurrence. Many customers lose their power supply, resulting in customer frustration as well as fines for the power supplier. The underlying problem in many protection units is low sampling frequency and low-pass filtering. The lack of data from the high-frequency content of the measured signals is a fundamental limitation in existing protection systems.

The faculty of the electrical engineering department at Lund University in Sweden worked closely with energy company E.ON Elnät to build a laboratory setup representing a distribution system for an electric grid. The project’s aim is to simulate and accurately detect events, such as earth faults, that can adversely affect the power system.

High-frequency transients are a common occurrence in the power grid. In addition to earth faults, transients also appear in a healthy system, usually due to switching transients when lines are energized. Analysis of transient signals shows that the frequency content may reach several kilohertz, (Figure 1).

Transient

Figure 1. In the diagram of the recorded transients during an earth fault, the solid line is the original signal sampled at 50 kHz with an NI 9239 analog input module, and the dashed line is the same signal, but digitally low-pass filtered in a second order Butterworth filter with crossover frequency of 300 Hz and then decimated to a 1,000 Hz sample frequency.

Traditionally, fault recorders used in the power grid are either stand-alone instruments or integrated into modern digital relay protection. While a stand-alone fault recorder has a high sampling frequency (up to 20 kHz) and bandwidth suitable for harmonic analysis, it can be prohibitively expensive. A cheaper alternative is to use a fault recorder integrated in modern digital relays.

Modern relay protections typically employ a sampling rate of around 1 kHz and low-pass filtering, resulting in an effective bandwidth of approximately 300 Hz. As illustrated in Figure 1, units with this bandwidth lack the ability to capture high-frequency transients. This deficiency demonstrates why existing relay protection performs so poorly at intermittent earth faults. Without high-frequency data, it is extremely difficult, if not impossible, to design a reliable algorithm for intermittent earth faults.

System Development

During the development process, it became clear that the ability for a stand-alone unit was critical because the equipment would be positioned inside a power station. To meet this requirement, the development team selected NI CompactRIO hardware with LabVIEW graphical system design software. This system offers the mobility, flexibility, and expandability to fit the application needs. In addition, the expansive software library available for LabVIEW, coupled with preconstructed hardware modules, significantly reduced development time.

The NI 9239 analog input module has built-in antialiasing filters, which enable fast sampling with high effective bandwidth. A 50 kHz sampling frequency and optimized built-in filters offer an efficient bandwidth of 22 kHz. This feature, coupled with channel isolation of up to 250 V, provides a ready-to-use measuring system without the need for customization. A portable 3G modem and a small router ensure communication with users and offer the ability to perform development remotely.  

The ability to perform time-critical programming in the CompactRIO field-programmable gate array (FPGA), along with the data logging and communication capabilities of the LabVIEW Real-Time Module, made this platform a great foundation on which to build a high-quality measurement system. In addition, the complete solution is suitable for rapid prototyping.

Communication

Because the power equipment includes a stand-alone system inside a power station, it is critical that operators maintain reliable remote communication. To ensure mobility of the measuring unit, we chose a solution with a 3G modem and a small router. A host PC handles communication with the operators (Figure 2). In addition, the team used the LabVIEW application builder to create an easy-to-use interface because most of the operators lacked LabVIEW experience.

 

 

Projekt_dataflow_diagram_FW

 

Figure 2. Communication

Future Plans

The prototype system based on CompactRIO and LabVIEW demonstrates how modern communication techniques combined with a cost-efficient, flexible development platform create many opportunities for control and measurement in various parts of the power system. An opportunity that could benefit power suppliers and their customers is a reliable, high-performance relay protection system coordinated for an entire transformer station with an integrated fault recorder. Such a solution would allow for accurate fault detection without needlessly cutting power to numerous customers.

Author Information:
Christoffer Örndal
Department of Measurement Technology and Industrial Electrical Engineering, Lunds University
PO Box 118
SE-221 00 Lund
Sweden
Tel: +46 46 222 92 90
christoffer.orndal@iea.lth.se

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