Detecting high-frequency transients in power systems

Challenge:

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

Solution:

Use the NI CompactRIO platform and LabVIEW software to quickly develop a highly flexible measurement system prototype that provides fast sampling and high bandwidth.

"The prototype system, based on CompactRIO and LabVIEW, demonstrates how a low-cost, flexible development platform can be combined with modern communication technology to control and measure various parts of the power system."

The main drawback of most power grid protection systems is that they cannot accurately detect the source 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. 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 cooperated with the 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 occur 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 a ground fault, where 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 about 1kHz and low-pass filtering to achieve an effective bandwidth of about 300Hz. As shown in Figure 1, units with this bandwidth lack the ability to capture 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.

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. The 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, which enables fast sampling at an efficient bandwidth. The 50kHz sampling rate and optimized built-in filters provide an effective bandwidth of 22 kHz. At the same time, supporting channel isolation characteristics up to 250 volts provides a ready-to-use measurement system without customization. A portable 3G modem and a small router ensure smooth communication with users and provide remote development capabilities.

The execution of programs on the CompactRIO FPGA has a high degree of time determinism, and the LabVIEW Real-Time Module has data logging and communication capabilities. This platform provides a good foundation for building a high-performance measurement system. 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 communications. To ensure the mobility of the measurement unit, a solution using a 3G modem and a small router was chosen. 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 technologies can be combined with a low-cost and flexible development platform to enable 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 fully functional fault recorder. This solution allows accurate fault detection without disconnecting power to many customers.