A perfect harmonic-free multilevel ZVS UPS (Part 2)

3 Perfect harmonic-free multi-level ZVS UPS circuit

The principle circuit of the perfect harmonic-free multi-level ZVS UPS is shown in Figure 5 (the AC bypass switch is not drawn). It is composed of an 18-phase triple superimposed AC input rectifier circuit, a DC power supply SPWM cascade ZVS soft switch multi-level inverter. In the UPS circuit shown in Figure 5, no input or output AC filter is used, which is a feature of this UPS. For the three-phase DC power supply SPWM cascade ZVS soft switch multi-level inverter in Figure 5, when it is controlled by the control circuit shown in Figure 6 (the voltage stabilization control circuit part is not drawn), a three-phase 7-level SPWM sinusoidal AC voltage output can be obtained, and the waveform of the output voltage is shown in Figure 7. This is a new type of ZVS 7-level UPS with input and output waveforms closest to sine waves, the least switching devices, and the smallest switching loss.

Figure 5 Principle circuit of three-phase perfect harmonic-free multilevel ZVS UPS (phases B and C are the same as phase A, and the mains bypass switch is not shown)

Figure 6 Control circuit of the UPS circuit shown in Figure 5 (voltage stabilization control circuit is not shown)

3.1 Derivation of the output voltage equation

The three-phase perfect harmonic-free multi-level ZVS UPS adopts the carrier triangle wave phase-shift SPWM control method, and its working waveform is shown in Figure 7. The initial phase angles of the three carrier triangle waves uc1~uc3 lag by 2π/3 respectively. Assuming that the initial phase angle α1 of uc1 is 0°, the initial phase angle α2 of uc2 is 2π/3, and the initial phase angle α3 of uc3 is 4π/3. uc1~uc3 share a sinusoidal modulation wave uS4. Compare the carrier triangle wave uc1 with the sinusoidal modulation wave usA, and the SPWM control pulse generated in the part of usA>uc1 is used to control the switch SA1. Make the output voltage of 2HA inverter up1; compare uc2 with usA, and use the SPWM control pulse generated in the part usA>uc2 to control switch SA2, so that the output voltage of 2HA is up2; compare uc3 with usA, and use the SPWM control pulse generated in the part usA>uc3 to control switch SA3, so that the output voltage of 2HA is up3, then the output voltage uA of A-phase UPS is up1+up2+up3.

The expression of the carrier triangle wave uc1 is:

k=0, ±1, ±2, …

The sinusoidal modulation wave expression is:

Assumed carrier ratio Modulation

Then the double Fourier series expression of the output voltage up1 of the DC power supply E1 through 2HA can be obtained from reference [2]:

Since sinm(π-0)+sinm(π-2π/3)+sinm(π-4π/3)=0, cosm(π-0)+cosm(π-2π/3)+cosm(π-4π/3)=±3 or 0. When m is an odd multiple of 3, it is equal to (-3), when m is an even multiple of 3, it is equal to (+3), and when m is a number other than an integer multiple of 3, it is equal to 0, so:

From the expression of uA, we can know that in the A-phase output voltage uA, harmonics below 3F±1 and carrier harmonics below m<3 will be eliminated, and only harmonics above 3F±1 and carrier harmonics above m>3 will be included. When the carrier ratio F=120, harmonics below 3×120±1=360±1 can be eliminated, so it can be called a perfect harmonic-free UPS.

Figure 7 UPS AC output voltage waveform

3.2 Derived circuits and their parameters

The three-phase perfect harmonic-free ZVS UPS circuit shown in Figure 5 is a UPS circuit with N=3 independent DC power supplies per phase. This is set for the convenience of description. When N=2, 3, 4, and 5, the number of independent DC power supplies used by the UPS, the number of superimposed switch IGBTs , the number of GTOs or SCRs used in the 2H bridge, the type of AC input rectifier power supply, the general expression of uA output voltage, and the number of output voltage levels are shown in Table 2. Designers can choose according to different requirements. Generally speaking, the larger N is, the closer the waveform of the output voltage is to the sine wave, but the circuit is also the most complex and the cost is higher. Usually, N=3 is sufficient. When N=5, the simulated waveform of the output voltage of phase A is shown in Figure 8. It can be seen that it is very close to the sine wave.

Table 2 Relationship between UPS parameter structure and N

Figure 8 When N=5, the simulation waveform of the output voltage uA

4 Conclusion

The perfect harmonic-free multi-level ZVS UPS introduced in this article adopts three latest technologies: In the inverter part, we use the DC power supply SPWM cascade multi-level inverter technology independently developed by us. It separates the cascade superposition and SPWM control from the inverter and moves them to the superposition control switch of the DC power supply to achieve the purpose of reducing the number of components (especially the number of switching devices) and reducing switching losses, and makes the switch tubes in the 2H bridge inverter naturally work in the ZVS state, creating conditions for the application of low-speed switching devices GTO or SCR. For the DC power supply superposition control switches SA1~SA3 in the inverter, quasi-resonant ZVS soft switching technology is used to further reduce the switching loss. The mains input rectifier power supply adopts the corresponding 18-phase three-phase superposition rectifier power supply to improve the input power factor. In summary, through the application of the above three new technologies, the UPS introduced in this article has the following characteristics:

(1) With perfect input and output waveforms, it can meet the strict requirements of power supply departments in various countries on harmonics without the need for any additional filtering devices, thus realizing the green revolution of UPS.

(2) ZVS zero voltage soft switching is fully realized, which reduces switching loss and EMI, maximizes the inverter efficiency, and enables the total efficiency to reach more than 97.5%.

(3) Excellent surge resistance.

(4) The AC input power factor can reach above 0.98.

(5) Compared with other multi-level inverters, it uses the least number of components, reducing manufacturing costs.

(6) Simple control, fast dynamic response, small size and weight.

(7) It is suitable for medium and high power UPS applications and is the current development direction of medium and high power green UPS.