1. The circuit principle of the three-phase grid-connected inverter
The output voltage of the three-phase grid-connected inverter is generally AC 380V or higher, and the frequency is 50 Hz/60 Hz, of which 50 Hz is the Chinese and European standards, and 60 Hz is the American and Japanese standards. Three-phase grid-connected inverters are mostly used in large-capacity photovoltaic power generation systems. The output waveform is a standard sine wave, and the power factor is close to 1.0.
The circuit principle of the three-phase grid-connected inverter is shown in Figure 1, which is divided into two parts: the main circuit and the microprocessor circuit. Among them, the main circuit mainly completes the conversion and inversion process of DC-DC-AC, and the microprocessor circuit mainly completes the control process of system grid connection.
The purpose of system grid-connected control is to maintain the AC voltage value, waveform, phase, etc. output by the inverter within the specified range. Therefore, the microprocessor control circuit needs to complete the real-time detection of the grid voltage phase, the current phase feedback control, the maximum power tracking of the photovoltaic square array, and the generation of real-time sine wave pulse width modulation signals. Its specific working process is as follows: the voltage and phase of the utility grid are sent to the A/D converter of the microprocessor through the Hall voltage sensor, and the microprocessor compares the phase of the feedback current with the voltage phase of the utility grid, and the error signal is adjusted by the PID operator and then sent to the pulse width modulator (PWM), which completes the power feedback process with a power factor of 1. Another major job done by the microprocessor is to achieve the maximum power output of the photovoltaic array. The output voltage and current of the photovoltaic square array are detected and multiplied by the voltage and current sensors respectively to obtain the output power of the square array, and then the PWM output duty cycle is adjusted. The adjustment of this duty cycle is essentially to adjust the size of the feedback voltage, so as to achieve maximum power optimization. When the amplitude of U changes, the phase angle Φ between the feedback current and the grid voltage will also change to some extent. Since the feedback control of the current phase has been realized, the decoupling control of the phase and the amplitude is naturally realized, which makes the processing process of the microprocessor easier.
2. Circuit principle of single-phase grid-connected inverter
The output voltage of the single-phase grid-connected inverter is AC 220V or 110V, the frequency is 50Hz, and the waveform is sine wave, which is mostly used in small household systems. The circuit principle of single-phase grid-connected inverter is shown in Figure 2. Its inverter and control process are basically similar to the three-phase grid-connected inverter.
3. Switch structure type of grid-connected inverter
Generally speaking, the cost of grid-connected inverter accounts for 10%~15% of the total cost of the entire photovoltaic power generation system, and the cost of grid-connected inverter mainly depends on its internal switch structure type and power electronic components. The current grid-connected inverters generally have the following three types of switch structures.
①Inverter with power frequency transformer. This type of switch is usually composed of a single-phase inverter bridge composed of power transistors (such as MOSFETs) and a post-power frequency transformer. The power frequency transformer can not only easily achieve the matching with the grid voltage, but also play the role of DC-AC isolation. The inverter using the power frequency transformer technology works stably and reliably, and has good economy in the low power range. The disadvantage of this structure is that it is bulky and bulky, and the inverter efficiency is relatively low.
②Inverter with high frequency transformer. The use of high frequency electronic switching circuits can significantly reduce the size and weight of the inverter.
This type of switch structure consists of a DC converter that boosts the DC voltage to more than 300 volts and a bridge inverter circuit composed of IGBTs. The high-frequency transformer is much smaller in volume and weight than the power-frequency transformer. For example, the power-frequency transformer of a 2.5KW inverter weighs about 20kg, while the high-frequency inverter of the same power inverter is only about 0.5kg. The circuit of this type of structure has high working efficiency, but the disadvantage is that the cost of high-frequency switching circuits and components is also high, and even depends on imports. However, the overall cost disadvantage is not obvious, especially for high-power applications, which have relatively good economics.
③ Transformerless inverter. This switching structure has the relatively highest conversion efficiency because the loss caused by the transformer link is reduced, but the cost of anti-interference and safety measures will increase.
Introductory articles on related principles:
1. Circuit principle of off-grid single-phase inverter.