The Principle and Application of Inverter in Photovoltaic Power Generation System

1. Requires higher efficiency. As the price of solar cells is relatively high, in order to maximize the use of solar cells and improve system efficiency, it is necessary to find ways to improve the efficiency of the inverter.

2. Requires high reliability. At present, photovoltaic power generation systems are mainly used in remote areas, and many power stations are unattended and unmaintained. This requires the inverter to have a reasonable circuit structure, strict component screening, and various protection functions, such as input DC polarity reverse protection, AC output short circuit protection, overheating, overload protection, etc.

3. The DC input voltage is required to have a wide adaptability range. Since the terminal voltage of the solar cell varies with the load and sunlight intensity, the battery plays an important role in the voltage of the solar cell. However, the battery voltage fluctuates with the change of the remaining capacity and internal resistance of the battery. Especially when the battery is aged, the terminal voltage of the battery varies greatly. For example, the terminal voltage of a 12V battery can vary between 10V and 16V. This requires the inverter to ensure normal operation within a larger DC input voltage range and ensure the stability of the AC output voltage.

4. In medium and large-capacity photovoltaic power generation systems, the output of the inverter power supply should be a sine wave with less distortion. This is because in medium and large-capacity systems, if square wave power supply is used, the output will contain more harmonic components, and high-order harmonics will generate additional losses. The loads of many photovoltaic power generation systems are communication or instrumentation equipment, which have high requirements for the quality of the power grid. When medium and large-capacity photovoltaic power generation systems are connected to the grid, in order to avoid power pollution from the public power grid, the inverter is also required to output a sine wave current. The inverter converts DC power into AC power. If the DC voltage is low, it is boosted through an AC transformer to obtain a standard AC voltage and frequency. For large-capacity inverters, due to the high DC bus voltage, the AC output generally does not require a transformer to boost the voltage to reach 220V. In medium and small-capacity inverters, due to the low DC voltage, such as 12V and 24V, a boost circuit must be designed. Medium and small capacity inverters generally have three types: push-pull inverter circuit, full-bridge inverter circuit and high-frequency boost inverter circuit. In the push-pull circuit, the neutral plug of the boost transformer is connected to the positive power supply, and the two power tubes work alternately to output AC power. Since the power transistors are connected to the common ground, the drive and control circuits are simple. In addition, since the transformer has a certain leakage inductance, the short-circuit current can be limited, thereby improving the reliability of the circuit. Its disadvantage is that the transformer utilization rate is low and the ability to drive the inductive load is poor. The full-bridge inverter circuit overcomes the disadvantages of the push-pull circuit. The power transistor adjusts the output pulse width, and the effective value of the output AC voltage changes accordingly. Since the circuit has a freewheeling circuit, the output voltage waveform will not be distorted even for inductive loads. The disadvantage of this circuit is that the power transistors of the upper and lower bridge arms are not connected to the common ground, so a special drive circuit or an isolated power supply must be used.

In addition, in order to prevent the upper and lower bridge arms from being turned on together, a circuit that turns off first and then turns on must be designed, that is, a dead time must be set, and the circuit structure is relatively complex. The output of both the push-pull circuit and the full-bridge circuit must be added with a step-up transformer. Since the step-up transformer is large in size, low in efficiency, and expensive, with the development of power electronics and microelectronics technology, high-frequency step-up conversion technology is used to achieve inversion, which can achieve high power density inversion. The front-stage step-up circuit of this inverter circuit adopts a push-pull structure, but the operating frequency is above 20KHz. The step-up transformer uses high-frequency magnetic core materials, so it is small in size and light in weight. After high-frequency inversion, it is converted into high-frequency alternating current through a high-frequency transformer, and then high-voltage direct current (generally above 300V) is obtained through a high-frequency rectifier and filter circuit, and then inverted through an industrial frequency inverter circuit. The use of this circuit structure greatly increases the power of the inverter, reduces the no-load loss of the inverter accordingly, and improves efficiency. The disadvantage of this circuit is that the circuit is complex and the reliability is lower than the above two circuits.