A complete photovoltaic power generation system is mainly composed of solar cells, batteries, inverters, controllers and loads. The main circuit of the inverter includes a DC/DC circuit, a DC/AC circuit, a filter circuit and an isolation transformer. The controller circuit generally uses DSP as the main control unit, which also includes the inverter’s SPWM signal generation, closed-loop control and maximum power point tracking circuit, as shown in Figure 1.
The photovoltaic inverter is generally composed of a boost circuit and an inverter bridge circuit. The boost circuit boosts the DC voltage of the solar cell to the DC voltage required for inverter output control; the inverter bridge circuit converts the boosted DC voltage into an AC voltage of common frequency equivalently. The inverter is mainly composed of switching devices such as transistors, and the switching devices are regularly turned on/off (ON/OFF) to turn the DC input into an AC output. Of course, such an inverter output waveform generated solely by the on and off loop is not practical. Generally, high-frequency pulse width modulation (SPWM) is required to narrow the voltage width near the two ends of the pseudo-sine wave, widen the voltage width at the center of the pseudo-sine wave, and always make the switching device move in one direction at a certain frequency within a half cycle In this way, a pulse wave train (quasi-sine wave) is formed, and then the pulse wave is passed through a simple filter to form a sine wave.
The core of the inverter device is the inverter switch circuit, referred to as the inverter circuit for short. This circuit completes the function of inverter by turning on and off the power electronic switch. Its simple principle is shown in Figure 2.
The inverter bridge circuit and the corresponding waveform diagram are shown in Figure 3.
Its working principle is as follows:
●S1, S4 are closed, S2, S3 are disconnected, and the output uo is positive; on the contrary, S1, S4 are open, S2 and S3 are closed, and the output uo is negative, so that the direct current is converted into alternating current.
●Changing the switching frequency of the two sets of switches can change the frequency of the output AC power.
●In the case of resistive load, the waveform frequency and initial phase of the current and voltage are the same. In the case of an inductive load, the initial phases of the current and voltage waveforms are different, and the current lags the voltage by a certain angle.
Figure 3 Schematic diagram and waveform diagram of the inverter circuit
(1) Circuit topology of the inverter main circuit Generally, single-phase voltage inverters are mainly divided into push-pull, half-bridge and full-bridge three types. These three methods are applied to different occasions according to their different characteristics.
The main circuit of the push-pull inverter circuit is simple, as shown in Figure 4. However, the switch tube needs to withstand twice the peak line voltage, so it is suitable for applications with low input voltage. At the same time, there is a phenomenon of magnetic bias in the transformer, the primary winding has a central beat, the effective value of the current flowing and the copper loss are relatively large, the two parts of the primary winding should be tightly coupled, and the winding process is complicated.
Compared with push-pull inverter circuits, half-bridge inverters have lower withstand voltage requirements for the power switch transistors used in the circuit, and their withstand voltage will not exceed the peak voltage of the line. Secondly, the saturation voltage drop of the transistor It is also reduced to a minimum and is no longer an important influencing factor. Furthermore, the voltage requirement for the input filter capacitor is relatively low. The circuit diagram of the half-bridge inverter circuit is shown in Figure 5. There is no DC bias problem, and it can be widely used in switching power supplies of hundreds to thousands of watts. However, when the transistor is turned on, the collector current doubles, and the current The increase of φ will not affect medium and small power switching power supplies, but for high-power switching power supplies, transistors that can withstand high voltage and large current are expensive, which is difficult to achieve.
The full-bridge inverter circuit not only maintains the voltage properties of the half-bridge inverter circuit, but also has the push-pull current properties. In the inverter circuit, when switching devices of the same voltage and current capacity are used, the full-bridge inverter circuit can reach the maximum power output. Therefore, the circuit is often used in medium and high power power supplies. The circuit structure is shown in Figure 6. Compared with the half-bridge inverter circuit, it has a better inverter output waveform.
(2) Maximum power point tracking control function The output of the solar cell module changes with the solar radiation intensity and the solar cell module’s own temperature (chip temperature). In addition, because the solar cell module has the characteristic that the voltage decreases with the increase of the current, there is an optimal operating point that can obtain the maximum power. The intensity of solar radiation is changing, and obviously the best operating point is also changing. Relative to these changes, the operating point of the solar cell module is always kept at the maximum power point, and the system always obtains the maximum power output from the solar cell module. This kind of control is the maximum power tracking control. The biggest feature of the inverter used in the solar power generation system is that it includes the function of Maximum Power Point Tracking (MPPT). The relationship between the maximum power point of photovoltaic modules and the voltage is shown in Figure 7.
It can be seen from Figure 7 that when the solar cell works on the left side of the maximum power point voltage Umax, the output power increases with the rise of the battery terminal voltage; when the solar cell works on the right side of the maximum power point voltage Umax, the output power increases with the battery The terminal voltage rises and decreases.
The most important thing in the implementation of MPPT technology is to find a suitable MPPT control algorithm. MPPT implementation methods include Constant Voltage Tracking (CVT), Perturbation and Observation Method (P&O), and Incremental Conductance (Incremental). Conductance Method, Inc method) and fuzzy logic control method.