Improved dq conversion voltage sag analysis based on LabVIEW

Since the 1980s, with the rapid development of high-tech technologies such as computer application technology, automation control technology and high-power power electronics technology, a large number of new electrical equipment and various power electronic equipment have been put into use in the power system. They are very sensitive to system interference and have much more stringent requirements on power quality than traditional electrical equipment. Even a short voltage disturbance will affect the normal operation of these devices and cause huge losses. Therefore, the detection and analysis of power quality disturbances has become the focus of current international research.

Voltage sag or drop refers to a phenomenon in which the root mean square value of the power supply voltage drops to 90% to 10% of the rated voltage amplitude in a short period of time, with a typical duration of 0.5 to 30 cycles. There is no consensus on the definition of voltage sag internationally. The Institute of Electrical and Electronics Engineers (IEEE) defines voltage sag as the sudden drop of the effective value of the power frequency voltage at a certain point in the power supply system to 10% to 90% of the rated value, and then returns to normal after a short duration of 10ms to 1min. The International Electrotechnical Commission (IEC) defines it as a drop to 1% to 90% of the rated value, with a duration of 10ms to 1min. In China, at the review meeting of the national standard "Operational Terms in the Field of Power Generation, Transmission and Distribution" organized by the Electrical Terminology Standardization Committee, "voltage sag" was defined as: "At a certain power supply point in the system, the voltage suddenly drops and recovers within a few cycles to a few seconds." This definition only roughly describes the external characteristics of the voltage sag, but does not specifically point out the amplitude range of the voltage drop.

Comparison of voltage sag characteristic detection methods

Voltage sags can be caused by short-circuit faults, induction motor starting, lightning strikes, switch operations, transformer and capacitor bank switching, etc.

In the analysis of voltage sag, three characteristic quantities are usually considered: voltage sag amplitude, voltage sag duration and phase jump. In order to control the impact of voltage sag as much as possible, it is necessary to measure the three characteristic quantities of voltage sag in real time, and then take corresponding measures to control it. This requires efficient analysis and calculation methods to extract the voltage disturbance signal of the system. The commonly used analysis methods are mainly the following:

(1) Root mean square value calculation method: According to the definition of voltage sag, the root mean square value calculation method can be used to measure the degree of voltage sag. However, the voltage sag duration obtained by the root mean square value calculation method has a significant error from the actual duration. At the same time, it cannot clearly give the start and end time of the voltage sag and the size of the phase jump that may occur when the voltage sag occurs;

(2) Single voltage conversion average method: This algorithm can obtain the sag amplitude and phase jump size, and has good real-time performance, but it cannot detect the start and end time of the voltage sag;

(3) αβ coordinate transformation detection method: This method transforms the voltage in the αβ stationary coordinate system first, and then changes it to the dq coordinate system, which makes it easy to obtain the characteristic quantities of voltage sag amplitude change and phase jump. Compared with the abc-dq transformation detection algorithm, this algorithm greatly reduces the amount of calculation, but the data used in this algorithm is not simultaneous, which leads to a long detection time and poor real-time performance. In addition, the asynchronicity of the data often causes short-term disturbances in the detection waveform, affecting the detection accuracy.

(4) Instantaneous voltage dq decomposition method: The idea of ​​this algorithm is derived from the instantaneous reactive power of the three-phase circuit. The detection algorithm using the abc-dq transformation can instantly determine the voltage effective value and the size of the phase jump. It has good dynamic responsiveness and is suitable for use in real-time compensation devices for voltage sags.

In addition to the detection methods described above, voltage sag can also be detected using peak voltage method, fundamental component method, wavelet transform and other methods.