Detection of High Frequency Transients in Power Systems Based on LabVIEW

Develop a communications-flexible portable measurement device that can record high-frequency transients in power systems and display the data online to multiple users.

Solution:

A highly flexible measurement system prototype was quickly developed using the NI CompactRIO platform and LabVIEW software, providing fast sampling and high bandwidth.

"The prototype system is based on CompactRIO and LabVIEW, demonstrating how a low-cost and flexible development platform can be combined with modern communication technology to achieve control and measurement of various parts of the power system." The main disadvantage of most power grid protection systems is that they cannot accurately detect the root cause of ground faults. As a result, large parts of the power grid have to be forcibly disconnected after a ground fault occurs, and many customers will lose power supply. This not only leads to customer disappointment in power service, but also to fines for power suppliers. The reason behind this is mostly due to the low sampling frequency of the protection unit and the use of low-pass filtering. The basic limitation of existing protection systems comes from the lack of data on the high-frequency components of the measurement signal.

The Department of Electrical Engineering at Lund University in Sweden has collaborated with energy company E.ON Elnät to establish a laboratory to study the distribution system of the power grid. The project aims to simulate and accurately detect adverse factors that affect the normal operation of the power system, such as ground faults. High-frequency transients are common in power grids. In addition to ground faults, transients also appear in healthy systems, usually due to transients generated when line switches are energized. Analysis of transient signals shows that their frequency components can reach several thousand hertz (Figure 1).

Figure 1 The table in the figure records the transients during the ground fault. The solid line is the original signal collected by the NI 9239 analog input module at a sampling rate of 50kHz, and the dashed line is the same signal after passing through a second-order Butterworth low-pass filter with a cutoff frequency of 300Hz and collecting it at a sampling rate of 1000Hz. [page]

Traditionally, fault recorders used in power grids are either stand-alone instruments or integrated into modern digital relay protection devices. Although stand-alone fault recorders have high sampling rates (up to 20kHz) and bandwidths suitable for harmonic analysis, they can be extremely expensive. A lower-cost alternative is to use a fault recorder integrated into a digital relay. Modern relay protection devices typically use a sampling rate of approximately 1 kHz and low-pass filtering, achieving an effective bandwidth of approximately 300Hz. As shown in Figure 1, units with this bandwidth lack the ability to detect high-frequency transient signals. The lack of bandwidth explains why existing relay protection devices perform so poorly in detecting intermittent ground faults. Without high-frequency data, it is extremely difficult to design a reliable algorithm for intermittent ground faults. [page]

System Development

During the development process, the performance of a stand-alone unit was critical because the device would be placed in a power station. To meet this requirement, the development team selected NI CompactRIO hardware and graphical system design software LabVIEW. This system provides mobility, flexibility, and scalability to adapt to the needs of the application. In addition, LabVIEW's large number of readily available software libraries, coupled with commercial off-the-shelf hardware modules, greatly shortened the development time.

The NI 9239 analog input module has a built-in anti-aliasing filter that enables fast sampling at an efficient bandwidth. The 50kHz sampling rate and optimized built-in filter provide an effective bandwidth of 22 kHz. It also supports channel isolation characteristics up to 250 volts, providing a ready-to-use measurement system without customization. A portable 3G modem and a small router ensure smooth communication with the user and provide remote development capabilities. The execution of programs on the CompactRIO FPGA is highly deterministic, and the LabVIEW Real-Time Module has data logging and communication capabilities. This platform provides a good foundation for building high-performance measurement systems. In addition, the complete solution is suitable for rapid prototyping applications.

Communications

Since the power equipment comprises an independent system within the power station, it is crucial for the operators to maintain reliable remote communication. To ensure the mobility of the measurement unit, we chose a solution using a 3G modem and a small router. A host PC handles the communication with the operator, as shown in Figure 2. In addition, the team designed an easy-to-use interface using LabVIEW Application Builder, mainly because most operators lack experience with LabVIEW.

Figure 2 Communication

Future plans

This prototype system based on CompactRIO and LabVIEW demonstrates how modern communication technology can be combined with a low-cost and flexible development platform to achieve control and measurement of various parts of the power system. Power suppliers and customers benefit from this reliable, high-performance relay protection system that coordinates the entire substation by integrating a full-featured fault recorder. This solution supports accurate fault detection without shutting down power to many customers.