- Definition of SCR polyphase phase controlled power frequency rectifier
The so-called SCR polyphase phase controlled power frequency rectifier refers to the use of polyphase rectifier transformer to convert the three-phase power supply of mains power input into an even multiple polyphase power supply of 6-phase, 12-phase, 18-phase and 24, and then connect the corresponding even multiple SCR phase controlled three-phase bridge rectifiers for multiple superposition rectification. This has two purposes: one is to adjust the DC output voltage of the rectifier; The second is to minimize the waveform distortion rate thdii of the input current of the mains power, so that the input power factor of the mains power is close to 1. Because this rectifier adopts SCR thyristor phase controlled rectifier and works in the power frequency state of commercial power, it is usually referred to as power frequency rectifier.
- Classification of SCR polyphase phase controlled power frequency rectifier
SCR polyphase phase controlled power frequency rectifier (power frequency rectifier) is generally divided into equal amplitude rectifier and variable amplitude rectifier. The so-called equal amplitude rectifier means that the output voltage of polyphase rectifier transformer is equal, that is, the transformation ratio of output voltage of each phase is the same; The so-called variable amplitude rectifier refers to that the output voltage of multiphase rectifier transformer is not equal, and the output voltage of each phase is determined by eliminating the harmonic content of mains input current to the greatest extent, that is, the transformation ratio of output voltage of each phase is calculated by the number of eliminated harmonics through simultaneous equations. Therefore, the variable amplitude rectifier has stronger harmonic elimination ability than the equal amplitude rectifier, which can reduce the waveform distortion rate of the input current of the commercial power more, improve the input power factor of the commercial power more, and make it close to 1.
For the SCR polyphase phase controlled power frequency rectifier for UPS, of course, the more the number of phases, the stronger the harmonic elimination ability of the rectifier, the less the harmonic content of the municipal power transmission current, and the closer the input power factor of the municipal power to 1. However, the more the number of phases, the more complex the structure and manufacturing process of rectifier transformer. Generally, the maximum number of phases of polyphase power frequency rectifier for UPS is limited to 30 phases. There are three commonly used types: 12 phase, 18 phase and 24 phase. When the control angle of thyristor SCR α= 0, displacement factor K φ= Cos0 ° = 1, the input power factor of mains power can reach pf = 0.9886 ~ 0.9971, which can meet the strict requirements of power industry departments of various countries for UPS harmonics.
- Characteristics of SCR polyphase phase controlled power frequency rectifier
SCR polyphase phase controlled power frequency rectifier is mainly used in traditional three-phase high-power power power frequency ups. This rectifier has been used for 20 years and has accumulated rich production and maintenance experience. It is the most used UPS rectifier.
The characteristics of SCR polyphase phase controlled power frequency rectifier are as follows.
(1) Advantages: mature technology, simple production and maintenance, low failure rate and high reliability. The input rectifier transformer can isolate the ups load from the mains and reduce the distortion rate of the mains input current.
(2) Disadvantages: the most obvious disadvantages are large volume and mass. The improvement of municipal power input power factor is limited by the structure of polyphase rectifier transformer. Since SCR phase controlled rectifier is adopted, the mains input power factor of this rectifier is greatly affected by distortion factor, displacement factor and load change rate caused by phase control. For example, for a 12 phase rectifier, at full load, the THDi of the mains input current is less than 16%, and the mains input power factor PF is > 0.98; However, at half load or lower load, the THDi of the mains input current and the displacement factor K φ、 The input power factor of mains power will decrease significantly. In addition, the linearity of phase controlled voltage regulation is poor.
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