Design of power quality analysis and test system for medium frequency power supply based on LabVIEW

With the widespread application and development of power electronics technology, a large number of nonlinear loads have been added to the power supply system, which will cause the distortion of the grid current and voltage waveforms, causing harmonic "pollution" of the grid. In addition, the operation of impact and fluctuating loads will not only generate a large number of high-order harmonics, but also make voltage fluctuations, flickers, and three-phase imbalance increasingly serious. These adverse effects on the power grid will not only reduce the safety of the power supply and use equipment itself, but also seriously weaken and interfere with the economic operation of the power grid, causing "public hazards" to the power grid. 400 Hz medium frequency power supply is mainly used in special occasions such as aircraft onboard equipment, radar, navigation and other aviation and military electronic equipment such as ships. It is precisely because of the particularity of the medium frequency power supply system that there are not many power quality analyzers suitable for medium frequency power supply on the market.

In order to accurately test the power quality of the medium frequency power supply system, a portable medium frequency power supply quality analysis and test system (hereinafter referred to as the "test system") based on LabVIEW was proposed and designed on the basis of demand analysis. The system can accurately test the power quality of the medium frequency power supply system.

1 Test system requirements analysis

According to the system requirements, 6-channel analog input detection and analysis are realized, that is, the three-phase current and three-phase line voltage are detected respectively. Various electric energy parameters such as voltage, current, apparent power, active power and reactive power are displayed in real time. The most important is the harmonic analysis test function, which can perform 31 harmonic analysis and display each single harmonic in a bar graph and percentage table. Other functions include saving current data, report generation, log writing, data management and other functions.

The report function should include a bar chart or table of the three-phase line voltage, average voltage, three-phase current, average current, three-phase active power, total active power, three-phase reactive power, total reactive power, apparent power, total apparent power, three-phase power factor, total power factor, and the three-phase line voltage and three-phase current, which are contained in the single saved record. The final report should also include the logs of the detection object, location, personnel, and each channel recorded in the log function.

2 Test System Hardware Design

The main hardware of this test system includes a portable computer, a data acquisition card, and a voltage and current sensor . The voltage and current of the medium frequency power supply system are converted into low voltage signals by the sensor, which are collected at high speed by the data acquisition card and sent to the computer for calculation and analysis by the LabVIEW virtual instrument software.

The portable computer is ACME-850. The chassis is made of impact-resistant ABS engineering plastics, and the inner box is made of aluminum-magnesium alloy, which improves the anti-vibration performance. The metal shielding net at the front end of the fan and the special EMI processing technology of the side baffles are used.

The electromagnetic compatibility of the whole machine is improved , and the surface of the chassis is coated with a layer of ceramic paint. Its main configuration is N-852 industrial dual-core motherboard, Core 2.2G processor, DDR2-2G memory, SATA-500G hard disk, high-brightness and high-resolution 15-inch XGA LCD screen and equipped with an aviation trolley case, which fully meets the functional needs.

The data acquisition card used is the DAQ-2502 data acquisition card from ADLINK. It has 4 analog voltage outputs, with an output range of 0 to ±10 V, a maximum update rate of 1MS/s, 12-bit resolution, and 8K onboard FIFO; it also has 8 single-ended analog inputs, with an input range of 0 to ±10 V, 14-bit error-free resolution, and a maximum sampling frequency of 400 kS/s. In addition, there are a variety of digital interfaces, counters/timers, and other applications.

The voltage and current sensors are all industrial high-performance sensors from Swiss LEM Company, with high test accuracy and stable performance.

3 Test system status determination

The system has five states: initialization, idle, run, hold, and exit. Its state transition is shown in Figure 1. When the system starts, it enters the initialization state, and after loading the last user's setting parameters, it enters the idle state and waits for the user's button operation. In the idle state, the system's auxiliary functions "data management" and "fault diagnosis" can be used, and user parameters can be configured at the same time. Click the "Run" button in the idle state to enter the running state. At this time, the data acquisition card starts to collect data, and the system processes and calculates and displays it in the window. Click "Stop" in the running state and the system returns to the idle state. If you click the "Hold" state, the system stops collecting data, and you can save the data to save all the data and settings before the stop of collection. The system background can further calculate and analyze the saved data.

 

4 Test system software design

The power quality analysis of medium frequency power supply mainly focuses on harmonic analysis of the detected voltage and current signals. Spectrum measurement and analysis are the core of the calculation and analysis of the software part. The purpose of harmonic analysis is to find the amplitude and phase angle of each harmonic. There are corresponding analysis methods for different types of harmonics. For steady-state harmonics, the FFT algorithm is usually used. In addition, there are fast Hartley transform (FHT) algorithm, discrete W transform, etc.; for transient harmonics, there are improved FFT analysis, wavelet transform and other methods. The general program of the extended library function and the professional mathematical analysis package of LabVIEW can meet the requirements of complex engineering calculations and analysis. The harmonic distortion analysis function can quickly and accurately calculate the reference frequency and the amplitude of each single harmonic component, reducing the workload of software development.

The following focuses on the design of the test system software system.

4.1 LabVIEW Software

LabVIEW  (Laboratory Virtual Instrument Engineering Workbench) is a graphical programming language that uses icons instead of text lines to create applications. Its appearance and operation imitate real instruments. This design greatly facilitates traditional instrument engineers and is also an important factor in its success. The core technical idea of ​​virtual instruments is "software is the instrument", which highlights the importance of software in virtual instrument systems. Before the birth of Microsoft's Windows, the American NI company had already launched versions before LabVIEW 2.0 on Macintosh computers. Through long-term, systematic and effective research and development, LabVIEW has gradually established its leading position in virtual instrument programming software. Its appearance finally liberated people, especially traditional instrument engineers and scientists, from the complicated programming work, allowing them to truly concentrate on what they are concerned about.

4.2 Functional module division of main modules of software system

When programming in LabVIEW, modular and structured programming ideas should be adopted. LabVIEW's data flow control corresponds to the information flow in structured programming. It is undoubtedly a better choice to use structured methods to implement a complex system like a debugger.

According to the functional requirements of the test system, the system software can be divided into five functional modules, namely IO setting module, data display module, data processing module, data management module and report management module, as shown in Figure 2. After determining the functions of the test system and dividing the main functional modules of the software system, the main program structure and program can be planned.

 

4.3 Software System Hierarchy

We adopt a top-down design approach and divide the system into three layers: the main interface layer, the logic function layer and the bottom layer. The "main interface layer" mainly includes the main machine interface and the overall system control. The "logic function layer" is responsible for various logical functions of the system, such as I/O settings, data acquisition, data processing, data management and data query. The "bottom layer" is divided into subVIs according to basic functions, which are called by high-level VIs. The calling and association between modules are realized by calling subVIs. When calling a subVI, the system needs to spend very little overhead, about tens of milliseconds. The software hierarchy of the test system is shown in Figure 3.

 

4.4 Design of priority of each task in software system

The medium frequency power supply detection system based on LabVIEW is a multi-functional multi-task system that runs in a multi-task parallel manner. In order to optimize the use of system resources and ensure that each functional module accurately completes its tasks and responds to user needs, the main task priorities of the test system are allocated as follows:

The signal transmission acquisition task has the highest priority. Once the acquisition is started, it will only stop when it enters the hold or stop state, which is the first level. The data processing task has the second priority. After the data is collected, software filtering and power quality parameters calculation are required, which is the second level. The data display task displays the collected data in the chart in real time, and the task priority is also the second level. The data management task and the report generation task can only be started when the system is not sending or receiving data. Therefore, it is listed as the third level.

It is very convenient to write multi-task programs on LabVIEW. LabVIEW is an automatic multi-threaded programming language. As long as the VI code can be executed in parallel, LabVIEW will allocate them to multiple execution threads to run simultaneously. Generally speaking, when writing programs, you should follow this principle: modules that can run simultaneously should be placed side by side, and never use wires, sequence boxes, etc. to force them to execute in sequence. When executing in parallel, LabVIEW will automatically arrange them to run simultaneously in different threads to increase the execution speed of the program and save the running time of the program.

4.5 Overall Software Architecture

Figure 4 shows the overall architecture of the test system software. The main points are: using while loops to implement the system state machine, using event structures to monitor interface button events (users click various buttons) and user-defined events, the above two are synchronized by events, and in the "running substate" of the state machine, queues are used to synchronize data collection and data processing display.

 

4.6 Main User Interface Programming

As shown in Figure 5, the test system software user interface is divided into 6 parts: 1) is the detection object selection control; 2) is the voltage and current waveform display channel selection control; 3) is the voltage and current waveform display; 4) is the voltage, current, power and other power quality analysis display; 5) is the software control menu control; 6) is the harmonic analysis display.

 

5 Conclusion

The test system uses a portable test computer with a high-speed data acquisition card as the hardware platform. The software design uses the modular and structured programming ideas compiled in LabVIEW to improve the reliability and maintainability of the system. The test system has been equipped in the medium-frequency power supply system of a certain type of ship. The actual application shows that the test system has the characteristics of accurate testing, stable and reliable, and friendly human-computer interface, meeting the design requirements.