There are two control methods for the three-phase half-bridge SPWM inverter: one is to use the complementary switches of the same bridge arm to perform SPWM control according to the waveforms of the phase voltages uA, uB, and uC. This control method is derived from the early inverter control method using the half-controlled switching device SCR. Because SCR is a half-controlled switching device, the turn-on and turn-off of the two switches on the same bridge arm are mutually dependent, that is, the turn-off of one switch must be realized by the turn-on of the other switch, so The two switches on the same bridge arm must work in a complementary state, and the phase voltage waveform must be used for two-level SPWM control; the other is to use the positive and negative half cycles to separate the SPWM control according to the line voltage uAB, uBC, uCA waveforms. . This control method is a new control method based on the fully controlled switching device IGBT. The turn-on and turn-off of this switching device is realized by the external SPWM control circuit, and there is no dependence between the turn-on and turn-off of the two switches on the same bridge arm, so it is not necessary to adopt the complementary switching method, but Three-level SPWM control can be performed directly using the line voltage waveform.
1. Two-level SPWM control according to the phase voltage waveform
This is a traditional control method, as shown in Figure 1, where Figure (a) is the main circuit, Figure (b) is the control circuit, and Figure 2 is the working waveform. In Figure (b), uA is the output voltage feedback signal, and uref is the given reference voltage. uA is compared with uref, and the error signal is outputted by the voltage regulator to adjust the amplitude Us of the three-phase sine modulation wave. The three-phase sinusoidal modulation waves uSA, uSB, and uSC are compared with the bipolar carrier triangular wave uc, and the driving signals of the switches Sap, San; Sbp, Sbn; Scp and Scn are generated to control the switches Sap, San; Sbp, Sbn; Scp , Scn is turned on and off, resulting in phase voltages uA, uB, uC, as shown in the upper part of Figure 2. Comparing uSA with uC, when uSA>uC part of the switch Sap is turned on, a positive pulse of uA is generated; when uA<uC part of the switch San is turned on, a negative pulse of uA is generated, and the amplitude of uA is E/2 ,the waveform of uA is shown in the upper part of Figure 2 . The waveforms of uB and uC can be obtained in the same way. The magnitude of the fundamental wave of uA is proportional to the modulation degree M=USA/UC. When uA＜uref, the voltage deviation Δu=uref-uA>0, the output signal of the voltage regulator increases, so that the amplitude USA of uSA increases, M=USA/UC, which increases accordingly, so that uA increases; when When uA>uref, the voltage deviation Δu=uref-uA<0, M decreases, so that uA decreases. If the voltage regulator is a PI regulator, uA=uref in the steady state, that is, when the power supply voltage E or the load changes, the output voltage changes, and the output voltage can be kept stable through the closed-loop control of the voltage. The double Fourier series representation of uA is
In uA, there are carrier, carrier harmonics, upper and lower side frequencies of carrier and carrier harmonics. When M<0.8, the amplitude of the carrier wave is larger than that of the fundamental wave.
Through the above analysis, it can be known that the two-level SPWM control according to the phase voltage waveform has the following congenital shortcomings.
①The harmonic content in the output voltage has many types and large amplitudes.
② Since the two switches on the same bridge arm work in a complementary state, the probability of collusion failure is high.
③ The utilization rate of the DC power supply voltage is low.
④ Since the sum of the instantaneous values of the three-phase output phase voltages is not equal to zero, that is, uA+uB+uC≠0, energy-saving SPWM control cannot be realized.
2. Three-level SPWM control according to line voltage
This is a new energy-saving SPWM control method that can save more than 30% switching loss and harmonic loss. The three-level SPWM waveform of the line voltage is shown in the lower part of Figure 1(c). The three line voltages uAB, uBC, and uCA are obtained from the three phase voltages uA, uB, and uC, that is, uAB=uA-uB; uBC=uB-uC; uCA=uC-uA, so uAB+uBC+uCA= uA-uB+uB-uC+uC-uA=0, this is a very useful characteristic, according to which the method of energy saving control can be found.
For the three-phase half-bridge SPWM inverter, uAB+uBC+uCA=0, so when the three-level SPWM control of the line voltage is used, it is only necessary to perform SPWM control on two of the phases, which can reduce the 1/3 switching losses.
In practical application, in order to further reduce the switching loss, SPWM control is adopted for the middle 60° part of a phase with the largest absolute value of the voltage (because the current in the middle 60° region of the maximum value is the largest, and the switching loss is also the largest), and only The SPWM control method for the other two phases has the advantage of reducing the switching times by 1/3 when the switching loss is the largest, making the reduction of the switching loss close to 1/2. The corresponding control circuit block diagram is shown in Figure 3. This figure is formed by adding uA-uB, uB-uC, uC-uA links in Figure 1(b), and injecting a 60° square wave pulse with an amplitude greater than or equal to the carrier triangular wave amplitude Uc into the middle of the driving pulse. of.
The driving signals, input DC current waveform, input DC voltage waveform, and three-phase output voltage uA, uB, uC waveforms of the switches Sap, San; Sbp, Sbn; Scp, Scn obtained according to the three-level SPWM control of the line voltage are shown in the figure 4 shown.
where the double Fourier series representation of the phase voltage uA is
It can be seen from this formula that the carrier component with a larger amplitude in the voltage uA is eliminated, the harmonic component of the carrier wave is also eliminated, and the integral multiple of 3 components in the upper and lower side frequencies of the carrier harmonic does not exist, Thus, the distortion rate of the output voltage waveform is greatly reduced.
Compared with the phase voltage two-level SPWM control, the line voltage three-level SPWM control has the following advantages.
①The harmonic content in the output voltage is greatly reduced, so that the volume and quality of the AC output filter of the UPS are also reduced accordingly.
② Eliminates the possibility of colluding short-circuit faults between the two switches on the same bridge arm. This is because the two switches on the same bridge arm no longer work in a complementary state, but commutate under ZVS conditions, so it will not happen. Collusion faults, thereby increasing the reliability of the inverter.
③ Since the top of the sinusoidal modulation wave is flattened and no longer participates in the SPWM control, the switching loss can be reduced by nearly 1/2, and the harmonic content of the output voltage is reduced (harmonics are also a kind of consumption), Therefore, this control method is called the energy saving control method.
④Since the top of the sinusoidal modulation wave is flattened, the utilization rate of the DC voltage has also been improved accordingly.
3. Influence of unbalanced load on DC input current fluctuation
No matter whether the phase voltage waveform is used for two-level SPWM control, or the line voltage waveform is used for three-level SPWM control, when the load is asymmetric or unbalanced or nonlinear, the DC input current will be twice as high as that of the inverter. The pulsating component of the operating frequency of the device. The cause and waveform are shown in Figure 5 when used as a graphical representation. This mapping method is consistent with the previous analytical calculation method.