Digital earthquake monitoring system for nuclear power plants based on LabVIEW and PXI technology

In order to ensure the safe operation of nuclear power plants and evaluate their safety performance after an earthquake, according to the relevant regulations on nuclear power plants, nuclear power plants must be equipped with seismic instruments to detect ground motion and the response motion of seismic Class I structures.

The digital earthquake monitoring system for nuclear power plants based on virtual instruments was successfully developed by applying the LabVIEW system development platform (including signal processing software), advanced PXI technology and seismic measurement sensors of the American NI company. Due to the powerful data processing capabilities, rich data expression methods and high efficiency of LabVIEW, the development speed of the system was strongly supported and accelerated. In a relatively short period of time, the digital earthquake monitoring system for nuclear power plants was successfully developed. The monitoring system has powerful seismic signal processing functions, rich and diverse information expression, and a friendly human-computer interface. Its main functions are to monitor the change of earthquake acceleration over time in real time and distinguish true and false earthquakes. When the earthquake signal exceeds the alarm threshold, it displays and records the peak value of earthquake acceleration and sends an alarm signal to the main control room; collects and records earthquake acceleration response time history data, calculates the acceleration response spectrum and design response spectrum of basic ground equipment, and compares them with the theoretical design response spectrum to verify whether the seismic design of seismic class I structures and equipment is reasonable, or to determine whether it is necessary to implement post-earthquake inspections on certain items.

Principle of ground motion measurement using basic ground equipment

Assume a single degree of freedom model of basic ground equipment as shown in Figure 1:

The two-order Newton forward interpolation formula is used to perform numerical calculations on the above equations (7), (8) and (9), and finally the acceleration response spectrum curve of the ground equipment is obtained.

According to the provisions of the ASME code, based on the calculated acceleration response spectrum curve, the peak points of the acceleration response spectrum are broadened by ± 15% of the frequency, and then connected in parallel to generate the corresponding design response spectrum curve.

Digital seismic monitoring system for nuclear power plants

The main functions of the digital earthquake monitoring system for nuclear power plants are:

(1) Real-time monitoring of the change of earthquake acceleration over time and identification of true and false earthquake signals; when the earthquake signal exceeds the alarm threshold, the peak value of earthquake acceleration is displayed and recorded, and an audible and visual alarm signal is sent to the main control room for the duty personnel to decide whether to issue an alarm or shut down the reactor;

(2) Collect and record the time history data of the ground earthquake acceleration response, calculate the acceleration response spectrum and design response spectrum of the ground equipment, so as to understand and verify whether the seismic design of Class I seismic-resistant structures and equipment is reasonable, or determine whether it is necessary to carry out post-earthquake inspection on certain items;

(3) Display and print the collected seismic acceleration signals and the data and graphics obtained through analysis, and store them for archiving.

System Configuration

The earthquake monitoring system is composed of LabVIEW6i, PXI-1010, PXIPCI18335, SCX1102, PXI3031E, MXI-3, Signal Processing Suite, triaxial acceleration sensor, industrial control computer and laser printer from NI Corporation of the United States (Figure 3).

Digital earthquake monitoring system platform

● Digital earthquake monitoring system panel

The digital earthquake monitoring system platform panel is shown in Figures 4 and 5. The system platform uses the tab page turning function of LabVIEW. The functions of the monitoring system are clear at a glance, and the relationship between the functions is clear and easy to select. For example, as shown in the panel of Figure 4, the monitoring system is in the earthquake monitoring function. At this time, the acceleration time history in the three directions of the free field, -15m and 11m foundation ground is measured and displayed, and the maximum and minimum values ​​of the acceleration are displayed at the same time. Figure 5 shows the calculated earthquake acceleration response spectrum of the equipment on the free field, -15m and 11m foundation ground when the damping ξ is 2% (the earthquake signal is taken from the simulated earthquake signal of the earthquake test bench of Tongji University).

The main functions of the seismic signal analysis software completed by LabVIEW and signal processing package are: seismic signal conditioning, acquisition, storage, judgment of true and false earthquakes and alarm; generation of equipment calculation response spectrum and design response spectrum, and comparison of design response spectrum with theoretical response spectrum, and finally output of complete seismic data file.

The following example illustrates the application of joint time-frequency analysis and wavelet analysis in seismic signal analysis software.

● Joint time-frequency analysis

For the analysis of earthquake signals, in addition to knowing the amplitude and frequency of earthquake acceleration when an earthquake occurs, people also need to know the change of earthquake frequency with time during the earthquake period, that is, it is necessary to conduct joint time-frequency analysis on earthquake signals to obtain the relationship between earthquake frequency and time. We applied the joint time-frequency analysis software package of LabVIEW and used the earthquake test bench of Tongji University to simulate earthquake signals and obtained the relationship between earthquake frequency and time. Figures 6 and 7 are the analysis results obtained by using the STFT and ConeShaped algorithms. It can be seen from the figure that in the time range of about 0-20 seconds, the center frequency of earthquake motion is about 1.5HZ, and at about 8 seconds, the earthquake frequency is about 3.0Hz.

● Wavelet analysis

Since 1986, wavelet analysis has developed rapidly, and its application has gradually become more and more extensive. It has also achieved extraordinary results in noise elimination, weak signal extraction and image processing. Obviously, wavelet analysis is also a very useful tool for seismic signal analysis. We use LabVIEW's "Wavelet and Filter BankDesign" software package to decompose the above seismic signal by high-pass and low-pass filtering. Figure 8 (a) is the original data, Figure 8 (b) is the reconstructed signal, (c) is the signal after high-pass filtering, and (d) is the seismic signal after low-pass filtering. Using wavelet analysis technology, the high-frequency and low-frequency parts of the signal are "refined and amplified", and the seismic signal (d) after low-pass filtering is clearer.

in conclusion

This paper uses advanced virtual instrument technologies such as LabVIEW and PXI from NI of the United States to successfully develop a digital earthquake monitoring system for nuclear power plants based on virtual instruments in a relatively short period of time. The digital earthquake monitoring system for nuclear power plants integrates computer data acquisition and processing of earthquake signals, has a large signal processing capacity, and intuitive and diversified data expression. The successful development of the digital earthquake monitoring system for nuclear power plants has strongly supported the development of China's nuclear power plants.