Most of the SCR polyphase phase-controlled rectifiers used in three-phase high-power power frequency UPS use 12-phase rectifiers or **12-phase rectification** + 11th harmonic passive filters, which have become a typical power frequency UPS circuit mode, which can Meet the requirements of the national standard GB/T 7260.3 for the harmonic content of the mains input current. However, due to the birth of the new international harmonic standard IEC 61000-3-4, 12-phase rectification or 12-phase rectification + 11th harmonic passive filter can no longer meet the basic requirements of this standard. Driven by the development of active filter technology in power electronics technology, the power frequency rectification method of 6-phase rectifier + power active filter has been applied in high-power power frequency UPS. The international standard IEC61000-3-4 harmonic standard is shown in Table 1.

The total current harmonic distortion of the 12-phase phase-controlled rectifier is 10%, which meets the standard, but the single harmonics such as the 11th harmonic (H11) and the 13th harmonic (H13) are more than twice the standard value, even if Using 12-phase rectification + 11th harmonic (H11) passive filter method can make the mains input power factor reach 0.90, but the effect is not ideal. However, the method of 6-phase rectification + power active filter can well meet the standard requirements shown in Table 1. The principle circuit of the series power active filter plus passive filter is shown in Figure 1. Since the 6-phase rectifier used in UPS is a capacitor filter and a harmonic source with the nature of a voltage source, in order to reduce the capacity of the active filter and reduce the cost, a series of active filters and LC passive filters are generally used. filter method to use. In this way of use, the capacity of the series power active filter is only 2% to 3% of the UPS capacity. A 20kV·A UPS can only use a 0.45kV·A series-type power active filter. The series-type power active filter has the following three functions in the circuit:

①It is equivalent to a harmonic isolator, which can isolate the harmonic voltage of the mains power supply from the load;

②It can prevent the harmonic current caused by the nonlinear load from flowing to the mains power supply, so that the power factor of the mains input is close to 1;

③Improve the performance of the passive filter, thereby avoiding the “harmonic resonance amplification phenomenon” that may be formed with the mains impedance. The main function of the LC passive filter in the circuit is to eliminate the harmonics in the input current of the mains and compensate for the reactive power required by the load.

The circuit shown in Figure 1 is tested, and the waveforms shown in Figure 2 to Figure 4 are obtained. Figure 2 shows the waveform obtained from the test when the mains voltage contains 1% harmonics and no other loads are connected at the same time. This test shows that when the active filter does not work, the mains input current contains 10% harmonic components. When the active filter is started and put into operation, the input current of the mains becomes a sine wave (a test performed by replacing the mains with a diesel generator set), and the terminal voltage waveform ual of the passive filter is still a sine wave. Therefore, The waveform of the current iF flowing into the passive filter is naturally also a sine wave. The waveform shown in Figure 3 indicates that if the series active filter does not start, the mains input current contains harmonic components; when the series active filter is started and put into operation, the mains input current becomes a sine wave.

Figure 4 is the test waveform when considering the harmonics in the mains voltage. At this time, when the series active filter is started and put into operation, the voltage on the load no longer contains harmonic components, and the mains input current is also a sine wave, while avoiding the resonance amplification of the fourth harmonic.

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