China’s photovoltaic power generation system is mainly a DC system, which charges the electric energy generated by the solar battery, and the battery directly supplies power to the load. For example, the solar household lighting system in Northwest China and the microwave-station power supply system far away from the grid are all DC systems. This type of system has a simple structure and low cost. However, due to the different load DC voltages (such as 12V, 24V, 48V, etc.), it is challenging to achieve standardization and compatibility of the system, especially for civilian power, as technicians use most AC loads with DC power. It is difficult for the photovoltaic power supply to supply electricity to enter the market as a commodity. In addition, photovoltaic power generation will eventually achieve grid-connected operation, which must adopt a mature market model. In the future, AC photovoltaic power generation systems will become the mainstream of photovoltaic power generation.
requirements of photovoltaic power generation system for inverter power supply
The photovoltaic power generation system using AC power output consists of four parts: photovoltaic array, charge and discharge controller, battery, and inverter (the grid-connected power generation system can generally save the battery), and the inverter is the critical component. Photovoltaic has higher requirements for inverters:
1. High efficiency is required. Due to the high price of solar cells at present, it is necessary to improve the inverter’s efficiency to maximize the use of solar cells and improve system efficiency.
2. High reliability is required. Technicians mainly use photovoltaic power generation systems in remote areas, and many power stations are unattended and maintained. That requires the inverter to have a reasonable circuit structure and strict component selection and requires the inverter to have various protection functions, such as input DC Polarity connection protection, AC output short circuit protection, overheating, and overload protection, etc.
3. The DC input voltage must have a wide range of adaptation. Since the terminal voltage of the battery changes with the load and the intensity of sunlight, although the battery has an essential effect on the battery voltage, the battery voltage fluctuates with the modification of the battery’s remaining capacity and internal resistance. Especially when the battery is aging, its terminal voltage varies widely. For example, the terminal voltage of a 12 V battery can vary from 10 V to 16 V. This requires the inverter to operate at a more extensive DC to ensure regular operation within the 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. If technicians use square wave power in medium and large-capacity systems, the result will contain more harmonic components, and higher harmonics will generate additional losses. Technicians load many photovoltaic power generation systems with communication or instrumentation equipment. The equipment has higher requirements on the quality of the power grid. When technicians connect the medium and large-capacity photovoltaic power generation systems to the grid, the inverter must also output a sine wave current to avoid power pollution with the public grid.
PRinciple of Solar Inverters
The inverter converts direct current into alternating current. If the direct current voltage is low, an alternating current transformer boosts it to obtain a standard voltage and frequency. For large-capacity inverters, due to the high DC bus voltage, the AC output generally does not need a transformer to boost the voltage to 220V. The DC voltage is relatively low in the medium and small-capacity inverters, such as 12V. For 24V, manufacturers must design a boost circuit. Medium and small-capacity inverters generally include push-pull circuits, full-bridge inverter circuits, and high-frequency boost inverter circuits. Push-pull circuits connect the neutral plug of the boost transformer to the positive power supply. Two power tubes Alternate work, output AC power. The drive and control circuits are simple because the power transistors are connected to the common ground. Because the transformer has a specific leakage inductance, it can limit the short-circuit current, thus improving the reliability of the circuit. The disadvantage is that the transformer utilization is low and the ability to drive inductive loads is poor.
The full-bridge inverter circuit overcomes the shortcomings of the push-pull circuit. The power transistor adjusts the output pulse width, and the practical value of the output AC voltage changes accordingly. Because the circuit has a freewheeling loop, even for inductive loads, the output voltage waveform will not be distorted. The disadvantage of this circuit is that the power transistors of the upper and lower arms do not share the ground, so technicians must use a dedicated drive circuit or an isolated power supply. In addition, to prevent the ordinary conduction of the upper and lower bridge arms, manufacturers must design a circuit to be turned off and then turned on, technicians must set a dead time, and the circuit structure is more complicated.
The output of the push-pull circuit and the full-bridge circuit must add a step-up transformer. Because the step-up transformer is large, low inefficiency, and more expensive, with the development of power electronics and microelectronics technology, high-frequency step-up conversion technology is used to achieve the reverse. It can realize a high power density inverter. The front-stage boost circuit of this inverter circuit adopts a push-pull structure, but the working frequency is above 20KHz. The boost transformer adopts high-frequency magnetic core material, which is small and lightweight. After high-frequency inversion, it is converted into a high-frequency alternating current through a high-frequency transformer. Then high-voltage direct-current (generally above 300V) is obtained through a high-frequency rectifier filter circuit and then inverted through a power frequency inverter circuit.
This circuit structure considerably improves the power of the inverter, reduces the no-load loss of the inverter, and improves efficiency. The disadvantage of the circuit is that it is complicated, and the reliability is lower than the above two circuits.
control circuit of the inverter circuit
The primary circuits of the inverters mentioned above all need to be realized by a control circuit. Generally, there are two control methods: square wave and positive and weak wave. The inverter power supply circuit with square wave output is simple, low in cost, low efficiency, and has significant inharmonic components. So sine wave output is the development trend of inverters. With the development of microelectronics technology, microprocessors with PWM functions have also come out. Therefore, the inverter technology for sine wave output has matured.
1. Inverters with square wave output currently use pulse-width modulation integrated circuits, such as SG3525, TL 494, etc. The practice has proved that using SG3525 integrated circuits and power FETs as switching power components can achieve relatively high performance and price inverters. ITS PERIPHERAL CIRCUIT IS ELEMENTARY because SG3525 can directly drive power FETs capability and has an internal reference source, operational amplifier, and Undervoltage protection function.
2. The inverter controls the integrated circuit with sine wave output. A microprocessor can control the control circuit of the inverter with sine wave output. Such as 80C 196 MC produced by INTEL Corporation and produced by Motorola Company, MP 16 and PIC 16 C 73 produced by MICRO-CHIP Company, etc. These single-chip computers have multiple PWM generators. They can set the upper and upper bridge arms. During the dead time, use the INTEL company’s 80 C 196 MC to realize the sine wave output circuit, 80 C 196 MC to complete the sine wave signal generation, and detect the AC output voltage to achieve Voltage stabilization.
Selection of Power Devices in the Main Circuit of the Inverter
The choice of the main power components of the inverter is essential. Currently, the most used power components include Darlington power transistors (BJT), power field-effect transistors (MOS-F ET), insulated gate transistors (IGB), and turn-off thyristor (GTO), etc. The most used devices in small-capacity low-voltage systems are MOS FET because MOS FET has a lower on-state voltage drop and higher. Technicians generally use the switching frequency of IG BT in high-voltage and large-capacity systems. That is because the on-state resistance of MOS FET increases with the increase of voltage, and IG BT is in medium-capacity systems occupies a more significant advantage. In contrast, technicians generally use GTOs as power components in super-large-capacity (above 100 kVA) systems.