Working principle of photovoltaic inverter
working principle:
The core of the inverter device is the inverter switching circuit, which is called inverter circuit for short. The circuit completes the function of inverter by turning on and off the power electronic switch.
characteristic:
(1) High efficiency is required.
Due to the high price of solar cells at present, in order to maximize the use of solar cells and improve system efficiency, we must try to improve the efficiency of inverter.
(2) High reliability is required.
At present, the photovoltaic power station 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 screening, and various protection functions, such as input DC polarity reversal protection, AC output short circuit protection, overheating, overload protection, etc.
(3) The input voltage is required to have a wide adaptive range.
Because the terminal voltage of the solar cell changes with the load and sunshine intensity. Especially when the battery is aged, its terminal voltage changes greatly. For example, for a 12V battery, its terminal voltage may change between 10V and 16V, which requires the inverter to ensure normal operation within a large DC input voltage range.
Photovoltaic inverter classification
There are many ways to classify inverters. For example, according to the number of phases of inverter output AC voltage, it can be divided into single-phase inverter and three-phase inverter; According to the different types of semiconductor devices used in the inverter, it can be divided into transistor inverter, thyristor inverter and turn off thyristor inverter. According to the different circuit principles of the inverter, it can also be divided into self-excited oscillation inverter, step wave superposition inverter and pulse width modulation inverter. According to the application in grid connected system or off grid system, it can be divided into grid connected inverter and off grid inverter. In order to facilitate photoelectric users to select inverters, they are only classified according to the different application occasions of inverters.
1. Centralized inverter
Centralized inverter technology is that several parallel photovoltaic strings are connected to the DC input of the same centralized inverter. Generally, three-phase IGBT power module is used for high power and field effect transistor is used for low power. At the same time, DSP conversion controller is used to improve the quality of generated power, making it very close to sine wave current, It is generally used in the system of large photovoltaic power station (> 10kW). The biggest feature is the high power and low cost of the system. However, due to the incomplete matching of output voltage and current of different photovoltaic strings (especially when the photovoltaic strings are partially blocked due to cloudy, shade, stains and other reasons), the centralized inverter will reduce the efficiency of the inverter process and the power consumption. At the same time, the power generation reliability of the whole photovoltaic system is affected by the poor working state of a photovoltaic unit group. The latest research direction is to use space vector modulation control and develop new inverter topology connection to obtain high efficiency under partial load.
2. Series inverter
Series inverter is based on the modular concept. Each photovoltaic series (1-5kw) passes through an inverter, which has maximum power peak tracking at the DC end and parallel grid connection at the AC end. It has become the most popular inverter in the international market.
Many large photovoltaic power plants use series inverters. The advantage is that it is not affected by the module difference and shading between strings, and reduces the mismatch between the optimal working point of photovoltaic module and inverter, so as to increase the power generation. These technical advantages not only reduce the system cost, but also increase the reliability of the system. At the same time, the concept of "master-slave" is introduced between the strings, so that the system can connect several groups of photovoltaic strings together and make one or several of them work under the condition that a single string of electric energy can not make a single inverter work, so as to produce more electric energy.
The latest concept is that several inverters form a "team" to replace the "master-slave" concept, which further improves the reliability of the system. At present, transformerless series inverter has occupied a dominant position.
3. Micro inverter
In the traditional PV system, the DC input of each series inverter will be connected in series by about 10 photovoltaic panels. If one of the 10 battery boards in series does not work well, the string will be affected. If multiple inputs of the inverter use the same MPPT, all inputs will also be affected, greatly reducing the power generation efficiency. In practical application, clouds, trees, chimneys, animals, dust, ice and snow and other shielding factors will cause the above factors, which is very common. In the PV system of micro inverter, each battery board is connected to a micro inverter respectively. When one of the battery boards fails to work well, only this one will be affected. Other photovoltaic panels will operate in the best working state, making the overall efficiency of the system higher and the power generation greater. In practical application, if the series inverter fails, thousands of watts of battery board will not work, and the impact of micro inverter failure is quite small.
4. Power optimizer
The installation of power optimizer in solar power generation system can greatly improve the conversion efficiency, simplify the functions of inverter and reduce the cost. In order to realize the intelligent solar power generation system, the power optimizer can ensure the best performance of each solar cell and monitor the battery consumption state at any time. Power optimizer is a device between power generation system and inverter. Its main task is to replace the original optimal power point tracking function of inverter. By simplifying the circuit and a single solar cell corresponding to a power optimizer, the power optimizer analogically performs extremely fast optimal power point tracking scanning, so that each solar cell can indeed achieve optimal power point tracking. In addition, it can monitor the battery status anytime and anywhere by placing a communication chip, Immediately report the problem and let relevant personnel repair it as soon as possible.
Functions of photovoltaic inverter
The inverter not only has the function of DC-AC conversion, but also has the function of maximizing the performance of solar cells and system fault protection. To sum up, it has automatic operation and shutdown function, maximum power tracking control function, anti separate operation function (for grid connected system), automatic voltage adjustment function (for grid connected system), DC detection function (for grid connected system) and DC grounding detection function (for grid connected system). Here, the automatic operation and shutdown functions and the maximum power tracking control function are briefly introduced.
(1) Automatic operation and shutdown function
After sunrise in the morning, the solar radiation intensity gradually increases, and the output of the solar cell also increases. When the output power required by the inverter is reached, the inverter will automatically start running. After entering operation, the inverter will monitor the output of the solar cell module at all times. As long as the output power of the solar cell module is greater than the output power required for the operation of the inverter, the inverter will continue to operate; The inverter can operate even in cloudy and rainy days until the shutdown at sunset. When the output of the solar cell module becomes smaller and the output of the inverter approaches 0, the inverter will form a standby state.
(2) Maximum power tracking control function
The output of the solar cell module changes with the solar radiation intensity and the temperature of the solar cell module itself (chip temperature). In addition, because the solar cell module has the characteristic that the voltage decreases with the increase of current, there is the best working point to obtain the maximum power. The intensity of solar radiation is changing, and obviously the best working point is also changing. Relative to these changes, the working point of the solar cell module is always at the maximum power point, and the system always obtains the maximum power output from the solar cell module. This control is the maximum power tracking control. The biggest feature of the inverter used in solar power generation system is that it includes the function of maximum power point tracking (MPPT).
Main technical indexes of photovoltaic inverter
1. Stability of output voltage
In the photovoltaic system, the electric energy generated by the solar cell is first stored by the battery, and then inverted into 220V or 380V AC through the inverter. However, due to the influence of its own charge and discharge, the output voltage of the battery varies widely. For example, the voltage value of the nominal 12V battery can vary between 10.8 ~ 14.4V (exceeding this range may cause damage to the battery). For a qualified inverter, when the input voltage changes within this range, the change of its steady-state output voltage shall not exceed plusmn of the rated value; At the same time, when the load changes suddenly, the output voltage deviation shall not exceed ± 10% of the rated value.
2. Waveform distortion of output voltage
For sine wave inverter, the maximum allowable waveform distortion (or harmonic content) shall be specified. It is usually expressed by the total waveform distortion of the output voltage, and its value shall not exceed 5% (l0% is allowed for single-phase output). Because the high-order harmonic current output by the inverter will produce eddy current and other additional losses on the inductive load, if the waveform distortion of the inverter is too large, it will lead to serious heating of the load components, which is not conducive to the safety of electrical equipment, and seriously affect the operation efficiency of the system.
3. Rated output frequency
For loads including motors, such as washing machines and refrigerators, since the optimal frequency operating point of the motor is 50Hz, too high or too low frequency will cause equipment heating and reduce the operation efficiency and service life of the system, the output frequency of the inverter should be a relatively stable value, usually 50Hz, and its deviation should be plusmn under normal working conditions; Within 1%.
4. Load power factor
Characterize the ability of the inverter to carry inductive load or capacitive load. The load power factor of sine wave inverter is 0.7 ~ 0.9, and the rated value is 0.9. When the load power is certain, if the power factor of the inverter is low, the capacity of the required inverter will increase. On the one hand, it will increase the cost. At the same time, the apparent power of the AC circuit of the photovoltaic system will increase, the circuit current will increase, the loss will inevitably increase, and the system efficiency will also decrease.
5. Inverter efficiency
The efficiency of inverter refers to the ratio of output power to input power under specified working conditions, expressed as a percentage. Generally, the nominal efficiency of photovoltaic inverter refers to the efficiency under pure resistance load and 80% load. Due to the high overall cost of photovoltaic system, we should maximize the efficiency of photovoltaic inverter, reduce system cost and improve the cost performance of photovoltaic system. At present, the nominal efficiency of mainstream inverter is between 80% ~ 95%, and the efficiency of low-power inverter is required to be no less than 85%. In the actual design process of photovoltaic system, we should not only select high-efficiency inverter, but also make the photovoltaic system load work near the best efficiency point through reasonable system configuration.
6. Rated output current (or rated output capacity)
Indicates the rated output current of the inverter within the specified load power factor range. Some inverter products give the rated output capacity, which is expressed in VA or kVA. The rated capacity of the inverter is the product of the rated output voltage and the rated output current when the output power factor is 1 (i.e. pure resistive load).
7. Protective measures
An inverter with excellent performance shall also have complete protection functions or measures to deal with various abnormal conditions during actual use, so as to protect the inverter itself and other parts of the system from damage.
(1) Input under voltage insured:
When the input voltage is lower than 85% of the rated voltage, the inverter shall be protected and displayed.
(2) Input overvoltage insured:
When the input voltage is higher than 130% of the rated voltage, the inverter shall be protected and displayed.
(3) Overcurrent protection:
The over-current protection of inverter shall be able to ensure timely action in case of short circuit of load or current exceeding the allowable value, so as to prevent it from being damaged by surge current. When the working current exceeds 150% of the rated, the inverter shall be able to protect automatically.
(4) Insured output short circuit
The action time of inverter short-circuit protection shall not exceed 0.5s.
(5) Input reverse connection protection:
When the positive and negative terminals of the input terminals are connected reversely, the inverter shall have protection function and display.
(6) Lightning protection:
The inverter shall be provided with lightning protection.
(7) Over temperature protection, etc.
In addition, for the inverter without voltage stability measures, the inverter shall also have output overvoltage protection measures to prevent the load from overvoltage damage.
8. Starting characteristics
Characterize the starting ability of the inverter with load and the performance during dynamic operation. The inverter shall ensure reliable starting under rated load.
9. Noise
Transformers, filter inductors, electromagnetic switches and fans in power electronic equipment will produce noise. During normal operation of the inverter, the noise shall not exceed 80dB, and the noise of small inverter shall not exceed 65dB.