1) Topological structure of three-level inverter
Three-level photovoltaic grid-connected inverters have a variety of topologies. The three main topologies are diode-clamped, independent DC power cascades and flying capacitors. The most widely used in practice is diode-clamped multi-electric Flat topology. The main circuit structure is shown in Figure 1.
The diode-clamped three-level photovoltaic inverter is the simplest and most economical multi-level structure in practice. The so-called three-level is because each phase can output three levels relative to the midpoint n, that is, each phase can obtain three potentials of +Udc/2, 0, and one Udc/2, and Udc is the DC bus voltage.
It can be seen from Figure 1 that the potential of the neutral point is clamped by a diode, so it is also called a diode clamp structure. Phase series capacitors C1 and C2 are set on its DC input terminal to balance Udc, and C1=C2, that is, the voltage drops on C1, C, and 2 are all Udc/2, and n is the neutral point. Each phase bridge arm in the inverter circuit topology has four series-connected power tube IGBTs, and each power tube has an independent diode connected in parallel with it to complete the freewheeling. When the two middle switching power tubes are turned on, the two clamping diodes on each phase can clamp the potential at zero level. In addition, the reverse cut-off characteristic can be used when the corresponding bridge arm power tube is turned on. Prevent C1 and C2 from being short-circuited.
2) Three-level circuit PWM modulation method
The principle of three-phase SPWM modulation is the same as that of single-phase. It can be simply considered that the three-phase is a combination of three single-phases with a phase difference of 120°, but this combination has the problem of low DC voltage utilization. In practical applications, a sine voltage superimposed with a triangular wave or a third spectrum wave is generally used to improve the voltage utilization rate. This section only gives the waveforms of several commonly used modulation waves, and will not elaborate on them, as shown in Figure 2.
The principle of three-level and two-level space vector modulation is basically the same. Both are combined with a waveform equivalent to a sine wave through the action of the regional switch vector. The two-level SVPWM has been explained in the centralized inverter. The part is mainly about three-level
① State distribution diagram of SVPWM
②Analysis of three-level SVPWM
a. Reference voltage vector correction
b. SVPWM calculation
c. SVPWM action sequence determination
3) Three-level midpoint control
Compared with the traditional two-level three-level power consumption, the sine of the voltage waveform is good, and the harmonic content is low, so the grid-connected performance is superior, but it also has its own shortcomings, that is, the midpoint potential is unbalanced. It is precisely because of the fluctuation of the DC side voltage that the medium and low harmonic content of the inverter output signal is larger than before, which causes the quality of the waveform to decrease. The unbalance of the midpoint voltage will increase the current spectrum wave rate of the inverter output, resulting in increased loss and decreased utilization. If it is severely unbalanced, a capacitor with a large voltage drop will be damaged due to excessive voltage, and the power tube connected in parallel with it will also be damaged.
4) Simulation and result analysis
Based on the above control strategy, the SIMULINK simulation model as shown in Figure 3 is built, and the voltage and current waveforms shown in Figure 4 are obtained. The PWM waveform is shown in Figure 5, and the waveform of the midpoint potential is shown in Figure 6.
If you also know other knowledge about three-phase photovoltaic grid-connected inverters, you can read other articles on this site——three-phase photovoltaic grid-connected inverter control system and software design of string three phase inverter, etc.