Introduction to Inverter

(1)Classification and characteristic parameters of inverter

Solar cell photovoltaic power generation is a DC system, that is, solar cell power generation can charge the battery, and the battery directly to the load; when the load is AC power, it is necessary to change the DC power to AC power, then it is necessary to use the inverter. The function of the inverter is to convert DC power to AC power, which is a "reverse" rectification process, so it is called "inverter". Depending on the inverter line inverter principle, there are self-excited oscillating inverters, step wave superposition inverters and pulse width modulation (PWM) inverters. According to the different inverter main circuit topology, it can be divided into half-bridge structure, full-bridge structure, push-pull structure, etc. The inverter protection functions include: output short circuit protection, output overcurrent protection, output overvoltage protection, output undervoltage protection, output phase loss protection, output reverse protection, power circuit overload protection, etc.

Output reverse protection, power circuit overheating protection and automatic voltage stabilization, etc.

Since the voltage of PV cells is usually lower than the AC voltage, a DC step-up converter is needed in the PV inverter system, and the DC step-up voltage needs to be converted into AC energy through the inverter. The core of the PV inverter system is the DC booster circuit and the inverter switch circuit. Both the DC boost circuit and the inverter switch circuit are used to perform the corresponding DC boost and inverter functions by turning the power electronic switch devices on and off. These pulses can be regulated by changing a voltage signal. The circuit that generates and regulates the pulses is usually called a control circuit. Inverter conversion is the opposite of quadrature conversion, it uses a fully controlled power device with switching characteristics, through a certain control logic, the main control circuit periodically sends a switching control signal to the power device, and then coupled by the transformer step-up (step-down), shaping and filtering to get the required AC power. General small and medium power inverters use power field effect tubes, insulated gate transistors, high-power inverters are used to turn off thyristor devices.

The choice of inverter affects the performance reliability and cost of the PV system. The following are inverter characteristics parameters: output waveform, power conversion efficiency, nominal power, input voltage, voltage regulation, voltage protection, frequency, modulating power factor, reactive current, size and weight, audio and RF noise, meter head and switch; some inverters also have battery charging remote operation, load changeover switch, and parallel operation. Stand-alone inverters generally produce 120V or 240V AC power at 50Hz or 60Hz frequency at DC 12V, 24V, 48V or 120V voltage input.

Inverters are usually classified according to their output waveforms: (i) square wave; (ii) sine wave-like; and (iii) sine wave. Square wave inverters are relatively inexpensive, with efficiencies up to 90% or more, high harmonics, and small output voltage adjustments; they are suitable for impedance-type loads and incandescent lamps. Sine wave inverters are available in the output pulse width to improve voltage regulation, efficiency up to 90%, they can be used to drive a variety of loads such as lamps, electronic equipment and most motors, however, they are driven by motors due to harmonic energy loss and lower efficiency than sine wave inverters driven. Sine wave inverters produce AC waveforms that are as good as those produced by most electronic devices. They can drive any AC load within the power range. Typically, inverter specifications can be increased by 25% from the calculated value, which both

This increases the reliability of the component's operation and also allows for a modest increase in load. For small load demands, the efficiency of all inverters is relatively low: when the load demand exceeds more than 50% of the nominal load, the inverter efficiency reaches the nominal efficiency (about 90%).

Here are some explanatory notes on some parameters: ①Power conversion efficiency: Its value is equal to the inverter output power divided by the input power, and the efficiency of the inverter will vary greatly depending on the load. ②Input voltage: It is determined by the power and voltage required by the AC (direct) current load. Generally, the larger the load, the higher the required inverter input voltage. ③ Anti-surge capability: Most inverters can exceed its power rating for a limited time (a few seconds), some transformers and AC motors require several times higher than normal operation of the start-up current (usually also lasts only a few seconds), the surge requirements for these special loads should be measured. ④ quiescent current: this is the inverter

current used by itself when it is not carrying a load (no power consumption), this parameter is important when carrying a small load for a long time, when the load is not large, the efficiency of the inverter is extremely low. ⑤ Voltage regulation: This means the diversity of the output voltage. More systems have a root mean square output voltage close to a constant over a large load range. ⑥Voltage protection: The inverter will be damaged when the DC voltage is too high, and the DC input voltage of the inverter will exceed the nominal value when the inverter's front stage a battery is overcharged, for example, a 12V battery may reach 16V or higher after overcharging, which may damage the inverter connected to the rear stage. So it is very necessary to control the charging status of the battery with a controller. The inverter shall have a check test protection circuit when there is no controller. When the battery voltage is higher than the set value, the protection circuit will disconnect the inverter. (7) Frequency: Our AC load is working at 50Hz frequency. High quality equipment requires precise frequency adjustment because frequency deviation can cause degradation of meter and electronic timer performance. ⑧ Modulation: It is very advantageous to use multiple inverters in some systems, which can be connected in parallel to drive different loads. Sometimes a manual load switch is used to make one inverter available to meet a circuit's specific load requirements in order to prevent a failure. Adding this switch improves the reliability of the system. Power factor: The cosine of the phase difference between the current and voltage generated by the inverter is the power factor, which is 1 for impedance type loads, but decreases for inductive loads (commonly used in residential systems) and may sometimes be less than 0.5. The power factor is determined by the load and not by the inverter.

It should be noted that; the positive and negative poles of the inverter should not be reversed, otherwise it will burn the electrical appliances concerned; the maximum input voltage should not exceed the upper limit of the rated input voltage; because the inverter has a certain no-load current, the input power should be cut off when it is not in use; the ambient temperature for use is generally a 10~40℃, therefore, do not spill water on top of the inverter, try to avoid direct sunlight, do not place other objects on top of the inverter or cover the working inverter, do not use it near flammable materials, and do not use it in places where flammable gases gather.

From the technical aspect, there is still a gap between domestic enterprises and foreign advanced level in terms of conversion efficiency, structure process, degree of intelligence, stability, etc. At present, China is at the same level with foreign countries in small power inverter technology, but in high power grid-connected inverter, it still needs further improvement and development.

The development of inverter is highly dependent on power electronics and microelectronics technology. The power conversion technology based on semiconductor technology and signal processing technology enables different power facilities (renewable energy generation, energy storage, flexible transmission and controllable load) to achieve efficient and flexible interconnection with the power system, while the grid-connected inverter, as a power conversion device, will play a future power system based on smart grid technology As a power conversion device, the grid-connected inverter will play a major role in the future power system based on smart grid technology and is a key element of the grid-connected PV power system. The primary function of the PV inverter is to convert the DC power from the PV panels into AC power synchronized with the grid. As one of the most important forms of distributed power generation, the efficiency and power quality of photovoltaic

The topology and grid-connected current control method of the inverter, as the channel of energy transmission, are the hot spots of attention and research in the industry.

(3) Technical requirements of inverter in PV grid-connected system

As the interface device between PV modules and the grid, the PV grid-connected inverter should not only convert the DC power emitted by the solar panel into AC power, but also control the voltage, current, frequency and phase of the output AC power, and solve the technical problems of electromagnetic interference to the grid, self-protection, separate operation and maximum power tracking, and transmit it to the public grid. Therefore, the grid-connected operation of PV plants places high demands on the inverter.

①Inverters have high efficiency. The efficiency of the conversion will directly affect the amount of power generated by the solar power system during its life cycle. The current price of solar cells is still high, so in order to maximize the efficiency of solar modules and maximize the output of the system, the efficiency of the system must be improved, and the efficiency of the inverter must be improved. Depending on the model, the conversion efficiency of international first-class brand products is up to

can reach more than 98%. High power PV inverters can reach 98.7% conversion efficiency, and the maximum power tracker (MPPT) efficiency can reach 99.9%.

②Inverters with high reliability. China's PV distributed power plants are mostly used in remote unattended areas, which makes it more difficult to check and maintain the inverter, which requires the PV inverter to have a reasonable circuit structure, high-quality and strict components, and various automatic protection functions, such as AC output short-circuit protection, DC input polarity reversal protection, overheating and overload protection, etc.

③The inverter has a wide range of DC input voltage and is guaranteed to meet the grid requirements. As photovoltaic power generation is subject to weather conditions, its output terminal voltage varies with the load and sunlight intensity and temperature. Although the battery has an important role in the voltage of solar cells, but because the battery voltage fluctuates with the remaining capacity and internal resistance of the battery, especially the battery end voltage changes more with the growth of service life, which requires the inverter must be able to work normally within a wide DC input voltage and ensure the stability of the output voltage, while the output current can not cause impact on the grid, in line with the grid Grid-connected requirements.

④ In medium and large capacity PV power generation systems, the inverter should output a sine wave with less distortion. This is because in medium and large photovoltaic power plants, if the square wave power supply is used, the output will contain more harmonic components, and the high harmonics will bring additional losses. Many photovoltaic power generation systems are loaded with communication equipment or instrumentation equipment, which has high requirements for wave quality. When medium and large capacity photovoltaic power plants are running on the grid, the inverter must also be required to output sinusoidal current in order to avoid power pollution to the public grid.