For large-capacity three-phase UPS, a high-power three-phase SPWM inverter needs to be selected. Due to the substantial improvement of the power level, new requirements are put forward for the structure of the three-phase SPWM inverter. Using a single-phase full-bridge inverter (2H bridge) as a bridge arm of a three-phase SPWM inverter is one of the options. Theoretically, the three-phase full-bridge SPWM inverter can achieve twice the power of the three-phase half-bridge SPWM inverter under the condition of selecting the same power switching device.

The three-phase full-bridge SPWM inverter requires half the DC supply voltage to be used than the three-phase half-bridge SPWM inverter. For the UPS inverter, only the voltage value > 230V×√2=325.3V is required, so the three-phase current type Buck SPWM rectifier can be used as the DC power supply.

**1. Composition and working principle**The three-phase full-bridge SPWM inverter is shown in Figure 1, in which Figure (a) is the main circuit, and Figures (b) and (c) are its two working waveforms. It can be seen from Figure 1 that the three-phase full-bridge SPWM inverter is composed of three single-phase full-bridge SPWM inverters (2H bridges) as shown in Figure 2(a) as a bridge arm. The single-phase full-bridge SPWM inverter (2H bridge) is a single-phase half-bridge two-level SPWM inverter obtained by two phase-shifting and superposition methods: one is to phase-shift the sinusoidal modulation wave by 2π/2 =180°, use the sine modulated wave before and after the phase shift to perform SPWM control respectively with the same carrier triangular wave, and superimpose (subtract) the obtained voltages uA1 and uA2; the other is to phase-shift the carrier triangular wave uC 2π/2=180°, use the carrier triangular wave before and after the phase shift and the same sine modulation wave to carry out SPWM control respectively, and superimpose (add) the obtained voltages uA1 and uA2, as shown in Figure 1(b) and (c) shown. The double Fourier series expression of the phase output voltage of the three-phase full-bridge SPWM inverter can be derived from equation (1), that is,

It can be seen from formula (2): due to the existence of cos (mπ) in the formula, m=1, 3, 5, … all odd-numbered carriers, carrier harmonics and the harmonics of n=even in the upper and lower side frequencies are equal. Eliminated, so that the harmonic content of the output phase voltage is greatly reduced.

The advantages of the three-phase full-bridge SPWM inverter: the harmonic content of the output voltage is small; since it requires a DC power supply voltage of ≥325V, the three-phase current-type Buck SPWM rectifier can also be directly used as the DC power supply. The disadvantage is that the three-phase four-wire symmetrical voltage with zero line cannot be directly output, and the output transformer with three-phase multi-winding △/Y connection method must be used to output the three-phase four-wire symmetrical voltage. The primary of the transformer adopts the △ connection method to facilitate the connection of three independent windings; the secondary adopts the Y connection method to facilitate the output of the three-phase four-wire system. The reason for using this three-phase output transformer is to facilitate control, but this increases the cost of the transformer. Due to the above shortcomings of the three-phase full-bridge SPWM inverter, it is rarely used in UPS power supplies.

**2. Comparison with the parallel stacking method of two three-phase half-bridge inverters**The parallel superimposed inverter circuit of two three-phase half-bridge SPWM inverters is shown in Figure 3, in which Figure (a) is the main circuit and Figure (b) is the working waveform.

Since one bridge arm of the three-phase full-bridge SPWM inverter is composed of the single-phase full-bridge SPWM inverter (2H bridge) shown in Figure 2(a), and the single-phase full-bridge SPWM inverter (2H bridge) is also composed of a two-level SPWM inverter by shifting the phase of the carrier triangular wave by 2π/2=180° and then superimposing it in parallel, so it can be seen that the three-phase full-bridge SPWM inverter is composed of two three-phase half-bridges. The bridge-type SPWM inverter is formed by parallel stacking after the carrier triangular wave is phase-shifted by 180°, which only doubles the DC power supply voltage. The double Fourier series expression of the phase output voltage superimposed in parallel by two three-phase half-bridge SPWM inverters can be derived from equation (3), that is,

One point to note here is: Equation (2) is obtained according to the unipolar carrier triangular wave SPWM control, while Equation (4) is obtained according to the bipolar carrier triangular wave SWPM control. If you want to change Equation (4) If it is represented by unipolar carrier triangular wave SPWM control, the parameters in formula (4) need to be converted as follows: F single = 2F double, m single = number m double. The converted formula (4) becomes

Comparing Equation (5) and Equation (2), it can be seen that they are exactly the same, but the DC power supply voltage is doubled. That is to say, the effect of eliminating harmonics of the three-phase full-bridge SPWM inverter and the effect of eliminating harmonics of the inverter circuit obtained by the phase-shifting and superposition of two three-phase half-bridge SPWM inverters by the carrier triangular wave is Exactly the same, the number of components used is also the same. The only difference is that the parameters of the components and the way they are used in the UPS circuit are different. The voltage of the components used in the three-phase full-bridge SPWM inverter is half the voltage and the current is twice as large. The DC power supply voltage is ≥325V, and the current-type Buck SPWM can be used. rectifier, but the output of the three-phase inverter must use a three-phase △/Y transformer to obtain a three-phase symmetrical voltage output with a zero line; and when two three-phase half-bridge SPWM inverters are stacked in parallel, the device used The voltage is twice as large, the current is half as small, the DC power supply voltage is ≥650V, only the voltage type Boost SPWM rectifier can be used, and the output of the three-phase inverter circuit does not need a three-phase △/Y transformer, only 6 balanced inductors. . Therefore, in the UPS system, if the load does not need to be electrically isolated from the mains power supply, it is better to use the parallel superposition of two three-phase half-bridge inverters rather than a three-phase full-bridge SPWM inverter. Because the three-phase full-bridge SPWM inverter must use a three-phase △/Y transformer to output a three-phase four-wire symmetrical voltage, the volume and quality are large and the cost is high.

In addition, it can also be deduced from this that the parallel superposition of phase-shifting with N three-phase full-bridge SPWM inverters will never compare to the phase-shifting with 2N three-phase half-bridge SPWM inverters. Superimposed in parallel. Because the former needs to use a large-capacity power frequency transformer with a complex structure (composed of 3N primary windings and 3N secondary windings), high cost, large volume and large quality three-phase △/Y connection.