1. High efficiency is required. As the current price of solar cells is high, in order to maximize the use of solar cells and improve the efficiency of the system, we must try to improve the efficiency of the inverter.
2. Requirement of high reliability. At present, PV power generation system is mainly used in remote areas, and many power stations are unattended and maintained, which requires the inverter to have a reasonable circuit structure, strict component selection, and requires the inverter to have various protection functions, such as input DC polarity reversal protection, AC output short circuit protection, overheating, overload protection, etc.
3. DC input voltage is required to have a wide range of adaptation, because the terminal voltage of the solar cell changes with the load and sunlight intensity, although the battery has an important role in the voltage of the solar cell, but because the voltage of the battery fluctuates with the remaining capacity and internal resistance of the battery, especially when the battery is aging, its terminal voltage varies greatly, such as 12V battery, its terminal voltage can vary from 10V to 16V, which requires the inverter must be in a larger DC input voltage range to ensure normal operation, and to ensure the stability of the AC output voltage.
4. In medium and large capacity PV power generation system, the output of inverter power supply should be a sine wave with less distortion. This is because in the medium and large capacity system, if the square wave power supply, the output will contain more harmonic components, high harmonics will produce additional losses, many photovoltaic power generation system load for communication or instrumentation equipment, these devices have high requirements for the quality of the power grid, in the medium and large capacity of the photovoltaic power generation system grid operation, in order to avoid power pollution with the public grid, also requires the inverter output sine wave current The inverter is also required to output sinusoidal current to avoid power pollution with the public grid.
How does a PV grid-connected inverter work?
The inverter converts DC power into AC power, and if the DC voltage is low, it is stepped up through an AC transformer to get the standard AC voltage and frequency. For high-capacity inverters, due to the high DC bus voltage, the AC output can generally reach 220V without transformer step-up. In small and medium-capacity inverters, due to the low DC voltage, such as 12V or 14V, it is necessary to design a step-up circuit.
There are three kinds of inverters: push-pull inverter circuit, full-bridge inverter circuit and high-frequency step-up inverter circuit. In the push-pull circuit, the neutral plug of the step-up transformer is connected to the positive power supply, and the two power tubes work alternately to get AC power output. reliability of the circuit. The disadvantage is that the transformer utilization rate is low and the ability to drive inductive loads is poor.
The full-bridge inverter circuit overcomes the disadvantages of the push-pull circuit by adjusting the output pulse width with a power transistor, which changes the RMS value of the output AC voltage. Since the circuit has a current-continuation circuit, the output voltage waveform is not distorted even for inductive loads. The disadvantage of this circuit is that the power crystals of the upper and lower bridge arms do not share a common ground, so a special drive circuit or an isolated power supply must be used. In addition, to prevent common conduction of the upper and lower bridge arms, the circuit must be designed to turn off before conduction, i.e., the dead time must be set, and the circuit structure is more complicated.
The control circuit of the inverter circuit of PV grid-connected inverter:
The main circuits of the above-mentioned inverters all need to have control circuits to achieve this, and there are generally two types of control methods, square wave and positive weak wave. The inverter power circuit with square wave output is simple and low cost, but the efficiency is low and the harmonic component is large. With the development of microelectronics technology, microprocessors with PWM function have been introduced, so the inverter technology with sine wave output is mature.
1. Square wave output inverter at present mostly uses pulse width modulation product circuit, such as SG3525, TL494, etc.. The SG3525 has the ability to directly drive the power field effect tube and has internal reference source and operational amplifier and undervoltage protection function, so its peripheral circuit is very simple.
2. Inverter control of sine wave output integral circuit, sine wave output inverter, its control circuit can be controlled by microprocessors, such as INTEL 80C196MC, Motorola MP16 and MI-CROCHIP PIC16C73, these microcontrollers have multiple PWM generators, and can set the upper and upper bridge arm The dead time between the upper and upper bridge arms, the circuit of sine wave output is realized by using 80C196MC from INTEL. 80C196MC completes the generation of sine wave signal and detects the AC output voltage to realize voltage regulation.
