Dual rectification 12-pulse IF power rectifier principle:

The rectifier transformer is a primary side delta connection method, the secondary side is a star connection method, and the double output of a delta connection method is respectively connected to two rectifier bridges. According to the transformer principle, the delta connection output hysteresis star connection method is 30 degrees, so The six-pulse current of the star-shaped rectifier bridge leads the six-pulse 30 degrees of the six-pulse rectifier bridge, which are staggered from each other to form 12 pulses. The advantage of 12 pulses is that the voltage is stable, requiring less filter capacitors and higher power factor.

12 pulses means that on the basis of the original 6-pulse rectification, after adding a phase shifting transformer at the input end, a set of 6-pulse rectifiers is added, so that the DC bus current is completed by 12 thyristors, so it is also called 12-pulse rectification. .

The two three-phase rectifier circuits I and II shown in the figure below are 12-phase rectifier circuits through different joints of the transformer.

12-pulse rectifier schematic (composed of 2 6-pulse parallel)

The grid side current Fourier series of bridge 1 is expanded to:

        (1-2)

The voltage of the side line of the bridge II network is 30 times higher than that of the bridge I. Because the current of the side line of the network is 30 times higher than that of the bridge I?

        (1-3)

Synthetic network side line current

        (1-4)

It can be seen that the 5th, 7th, 17th, 19th, and...th harmonics generated by the two rectifier bridges cancel each other out, and only 12k?1 (k is a positive integer) subharmonics are injected into the power grid, that is, 11, 13, 23, 25 Each harmonic is equal, and its effective value is inversely proportional to the harmonic order, and the ratio to the fundamental effective value is the reciprocal of the harmonic order.

Figure 1.2 Input voltage and current waveform of a 12-pulse UPS phase A computer simulation

  A 12-pulse rectifier + 11th harmonic can achieve a current harmonic index of less than 4.5%. The single harmonic and total harmonic content meet the requirements of IEC61000-3-4.

Table 1: Input current harmonic total content data under different load conditions

Schedule 2: A model 400KVA 12 pulse + 11 filters UPS input current total harmonic content table

What affects the grid is the absolute value of the harmonic current, not the percentage of the harmonic current. It can be seen from the above table that the absolute value of the harmonic current is the largest under full load conditions; in the case of half load and light load, the absolute value of the harmonic current does not exceed the absolute value of 100% of the full load harmonic current. The 12-pulse + 11th harmonic filter has the smallest total input current total harmonics, while avoiding the shortcomings of the active filter "false compensation" and low system efficiency, due to the 12-pulse + 11th harmonic filter. The grid has the lowest pollution and is suitable for use in applications where reliability is high.