Transformer Test System Based on LabVIEW and PCI-1712L

0Introduction

At present, the performance test of power transformers in China is still basically in the stage of manual measurement. Each performance index is measured by a dedicated instrument. Many test items still use some pointer instruments, which have problems such as large reading errors, labor-intensive and time-consuming, simple functions, and poor measurement repeatability and reliability. The emergence of virtual instruments effectively solves this problem. The so-called virtual instrument uses existing computers, specially designed instrument hardware and dedicated software to form a high-end and low-priced new instrument that has both the basic functions of ordinary instruments and special functions that ordinary instruments do not have. In the virtual instrument system, the hardware is only to solve the input and output of signals, and the software is the key to the entire instrument system. The software processes and analyzes the digital signals collected by the hardware to obtain various required information. This paper uses Advantech's PCI-1712L high-speed data acquisition card and its Advantech DA&C toolkit customized for Labview, combined with LabVIEW, to successfully develop a transformer test system.

1Transformer test system

1.1 System Principle and Function

It is used to test the input/output voltage and current of the transformer under no-load/load conditions, and can complete the transformer conventional experimental system, and can realize the functions of self-voltage regulation, data measurement, analysis, real-time display, storage and printing. The current and voltage signals are converted into standard signals after passing through the transformer. After being collected by the data acquisition card, the system determines the range of the interval, and communicates with the CPLD through the serial port to select the current and voltage transformer with the appropriate range, thereby improving the measurement accuracy of the system.

1.2 System Structure

The structural block diagram of the test system is shown in Figure 1.

2 Hardware Design of Test System

Voltage transformer: VSM series Hall voltage sensor is used to measure AC, pulse and various irregular waveform voltages under electrical isolation conditions.

Current transformer: Use 6 CSM series Hall voltage sensors with different ranges, CSM025B, CSM050B, CSM100B, CSM200B, CSM300B.

Data acquisition card: PCI-1712L is a powerful, high-speed, multifunctional PCI bus data acquisition card from Advantech. It has a 12-bit A/D converter with a conversion speed of 1M, and a FIFO buffer on the card (which can store 1K A/D sample values ​​and 32K D/A conversion data). PCI-1712L provides 16 single-ended or 8 differential analog inputs (single-ended and differential can also be mixed), 2 12-bit D/A analog output channels, 16 digital output channels, and 3 16-bit multifunctional counter channels with a clock of 10 MHz. The acquisition modes include input pre-trigger, over-trigger, delay trigger and other data acquisition modes. The analog input channels can be independently programmed for gain control, and the terminal board uses the ADAM-3968 from Advantech.

CPLD: EPM1270CPLD of Altera's MAXII series is used, which contains 1270 LEs equivalent to 40K gates, 980 equivalent macro units, and 8K user-available Flash bits. [page]

3. Software Design of Test System

3.1 Software Features

The software structure diagram of the self-voltage test system is shown in Figure 2.

3.2 Data Collection

Advantech not only develops a dedicated VI toolkit for LabVIEW programming, but also provides a wealth of example programs. Using its Advantech DA&C toolkit, you can quickly and efficiently develop data acquisition modules.

The setting parameters are as follows: trigger source, trigger mode, clock source, trigger edge, signal source type, sampling channel, sampling start channel, number of samples, sampling frequency, channel gain; the above can be set according to the actual test hardware configuration and requirements.

3.3 Data measurement and spectrum analysis

LabVIEW has powerful array analysis and data processing functions. The measured parameter values ​​include AC RMS, DC component, RMS, duty cycle, rise time, fall time, overshoot, peak-to-peak value, maximum value, and minimum value. Using the signal processing toolkit provided by LabVIEW, you can quickly perform FFT power spectrum density analysis on the collected waveform data.

3.4 Data judgment

The collected current and voltage values ​​are judged in intervals, and control signals are sent to CPLD to select the corresponding current and voltage transformers to improve the measurement accuracy of the system. To further improve the accuracy, the collected current and voltage data are filtered using the principle of pulse interference average filtering before judgment. A for loop is used to convert the data collected by a certain channel into an array. The length of the array can be defined by yourself. Assuming that the system is defined as N, the N data collected by the channel are judged, the maximum and minimum values ​​are removed, and the remaining N-2 data are arithmetic averaged to obtain a valid sampling value. The software filtering of the collected data can be completed by using Array Max & Min, Delete From Array and mean.vi in ​​the LabVIEW array toolkit.

Since there are many intervals for current and voltage judgment, it is a bit cumbersome to implement it with LabVIEW. Therefore, this paper adopts the method of combining C language and Lab-VIEW. First, write the judgment program in Visual C++ and generate a dll file, and then call it through the code interface point (Code Interface Node) in Lab-VIEW to implement this function. It turns out that this is an efficient and easy method. The principle of the C language part of the program code is as follows:

Singal is the input parameter of the CIN interface and the current signal after collection and filtering, and pattern is the output parameter of the CIN interface and the control signal, which is directly written to the CPLD through the serial port to select the corresponding current or voltage transformer. [page]

3.5 Serial Communication

Labview not only provides a special serial port communication VI in the instrument I/O, but also provides serial port examples to speed up the development. First, use VISA Configure Serial Port.vi to configure the corresponding serial port, use VISA write.vi to write the control signal to the serial port to communicate with CPLD, and finally use VISA dose.vi to close the serial port.

3.6 Display, print and save

The most attractive feature of LabVIEW is its rich support for graphical display of data. The powerful graphical display function enhances the expressiveness of the user interface. Through these rich icon controls, engineers no longer need to spend a lot of energy on complex interface programming. The Graph control processes the collected data and organizes the data into the required graphics according to actual requirements. The waveform display of the front panel of the test system software uses the Graph control, as shown in Figure 3. The waveform in the front panel can be printed out by setting the front panel menu file-VI properties-category print option.

The powerful file I/O functions provided by LabVIEW are a set of powerful file processing tools that can meet different file operation requirements. In order to meet the storage format and performance requirements of different data, LabVIEW provides a variety of file types. Users can use text files (text files), spreadsheet files (spreadsheet), binary files (binary files), data log files (data-log files), waveform files, LabVIEW test files and other formats to store the collected data, and users can use spreadsheets or text editors to open and view them. This test system uses spreadsheet files, which can be opened with Microsoft Excel.

4 Conclusion

The test system uses the outstanding features of LabVIEW in test instrument development and cooperates with third-party boards to make the application of LabVIEW broader and more flexible, verifying the purpose of virtual instruments, "software is instrument". The prototype of the test system has been put into actual production testing with good results.