1. Working principle and characteristics:

Working principle: The core of the inverter device is the inverter switching circuit, referred to simply as the inverter circuit. The circuit completes the inverter function by turning on and off the power electronic switch.

Features:

(1) High efficiency is required.

Due to the current high price of solar cells, in order to maximize the use of solar cells and improve system efficiency, we must try to improve the efficiency of the inverter.

(2) High reliability is required.

At present, photovoltaic power station systems are mainly used in remote areas. Many power stations are unattended and maintained. This requires the inverter to have a reasonable circuit structure, strict component screening, and requires the inverter to have various protection functions, such as: input DC polarity reverse connection protection, AC output short circuit protection, overheating, overload protection, etc.

(3) The input voltage is required to have a wider adaptation range.

Because the terminal voltage of the solar cell changes with the load and the intensity of sunshine. Especially when the battery ages, its terminal voltage varies greatly, such as 12V battery, its terminal voltage may vary between 10V ~ 16V, which requires the inverter to ensure normal operation in a large DC input voltage range .

2. Classification of photovoltaic inverters

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 inverters and three-phase inverters; depending on the type of semiconductor device used by the inverter, it can be Divided into transistor inverters, thyristor inverters and thyristor inverters that can be turned off. According to the different circuit principle of the inverter, it can be divided into self-excited oscillation type inverter, step wave superposition type inverter and pulse width modulation type 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 the optoelectronic users to choose the inverter, we only classify the inverter according to the different occasions.

1. Centralized inverter

In the centralized inverter technology, several parallel photovoltaic strings are connected to the DC input terminal of the same centralized inverter. Generally, the three-phase IGBT power module is used for large power, the field effect transistor is used for low power, and the DSP is used at the same time. Converting the controller to improve the quality of the electrical energy produced, making it very close to the sine wave current, is generally used in the system of large-scale photovoltaic power plants (> 10kW). The biggest feature is that the system has high power and low cost, but because the output voltage and current of different photovoltaic strings are often not completely matched (especially when the photovoltaic strings are partially blocked due to cloudy, tree shade, stains, etc.), centralized inversion is used Changing the way will lead to a decrease in the efficiency of the inverter process and a decrease in the energy of the electricity consumers. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working state of a photovoltaic unit group. The latest research direction is the use of space vector modulation control and the development of new inverter topological connections to achieve high efficiency under partial load conditions.

2. String inverter

The string inverter is based on the modular concept. Each photovoltaic string (1-5kw) passes an inverter, which has the maximum power peak tracking at the DC terminal, and is connected to the grid in parallel at the AC terminal. The most popular inverter on the market.

Many large photovoltaic power plants use string inverters. The advantage is that it is not affected by module differences and shadows between strings, and at the same time reduces the mismatch between the optimal working point of the photovoltaic module and the inverter, thereby increasing the amount of power generation. These technical advantages not only reduce system cost, but also increase system reliability. At the same time, the concept of "master-slave" is introduced between the strings, so that the system can connect several sets of photovoltaic strings together and allow one or more of them to work when a single string of energy cannot make a single inverter work. To produce more electrical energy.

3. Micro inverter

In a traditional PV system, the DC input of each string of string inverters is connected in series by about 10 photovoltaic panels. When one of the ten panels connected in series does not work well, this string will be affected. If the same MPPT is used for multiple inputs of the inverter, each input will also be affected, greatly reducing the efficiency of power generation. In practical applications, various blocking factors such as clouds, trees, chimneys, animals, dust, ice and snow will cause the above factors, and the situation is very common. In the PV system of a micro-inverter, each panel is connected to a micro-inverter. When one of the panels does not work well, only this one will be affected. All other photovoltaic panels will operate in the best working state, making the overall efficiency of the system higher and generating more power. In practical applications, if the string-type inverter fails, it will cause a few kilowatts of solar panels to fail to function, and the impact of the micro-inverter failure is quite small.

4. Power optimizer

The installation of a power optimizer (OptimizEr) in the solar power generation system can greatly improve the conversion efficiency, and simplify the function of the inverter (Inverter) to reduce costs. In order to realize a smart solar power generation system, the device power optimizer can make sure that each solar cell exerts its best performance, and monitor the battery wear status at any time. The power optimizer is a device between the power generation system and the inverter, and its main task is to replace the original best power point tracking function of the inverter. The power optimizer can simplify the circuit and a single solar cell corresponds to a power optimizer, etc., to perform an extremely fast optimal power point tracking scan by analogy, so that each solar cell can indeed achieve the optimal power point tracking In addition, it can also borrow a communication chip to monitor the battery status anytime, anywhere, and report the problem in real time so that the relevant personnel can repair it as soon as possible.

3. The function of photovoltaic inverter

The inverter not only has the function of direct AC conversion, but also has the function of maximizing the performance of the solar cell and the system fault protection function. In summary, there are automatic operation and shutdown functions, 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), DC ground detection Function (for grid-connected system). Here is a brief introduction to the automatic operation and shutdown functions and the maximum power tracking control function.

(1) Automatic operation and stop function

After sunrise in the morning, the solar radiation intensity gradually increases, and the output of the solar cell also increases accordingly. When the output power required for the inverter to work is reached, the inverter automatically starts to operate. After entering operation, the inverter monitors the output of the solar cell module all the time. As long as the output power of the solar cell module is greater than the output power required by the inverter, the inverter will continue to operate; until the sunset, even if it is rainy The inverter can also run. When the output of the solar cell module becomes smaller and the output of the inverter is close to 0, the inverter forms 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, since the solar cell module has a characteristic that the voltage decreases with increasing current, there is an optimal operating point that can obtain maximum power. The intensity of solar radiation is changing, and obviously the optimal operating point is also changing. Relative to these changes, the operating 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 most important feature of the inverter used in the solar power generation system is that it includes the function of maximum power point tracking (MPPT).

Fourth, the main technical indicators of photovoltaic inverter

1. Output voltage stability

In the photovoltaic system, the electrical energy generated by the solar cell is first stored by the storage battery, and then converted into 220V or 380V alternating current through the inverter. However, the storage battery is affected by its own charge and discharge, and its output voltage has a large range of variation. For example, a nominal 12V battery can have a voltage value ranging from 10.8 to 14.4V (beyond this range may cause damage to the battery) . For a qualified inverter, when the input terminal voltage changes within this range, its steady-state output voltage should not change by more than 5% of the rated value. At the same time, when the load changes suddenly, its output voltage deviation should not More than ± 10% of rated value.

2. Waveform distortion of output voltage

For sine wave inverters, the maximum allowable waveform distortion (or harmonic content) should be specified. Usually expressed in terms of the total waveform distortion of the output voltage, its value should not exceed 5% (single-phase output allows l0%). The higher harmonic current output by the inverter will cause additional losses such as eddy current on the inductive load. If the waveform distortion of the inverter is too large, it will cause the load components to heat up seriously, which is not conducive to the safety of electrical equipment and seriously affects the system Operating efficiency.

3. Rated output frequency

For loads including motors, such as washing machines, refrigerators, etc., because the optimal frequency operating point of the motor is 50Hz, the frequency is too high or too low will cause the device to heat, reduce the system operating efficiency and service life, so the inverter The output frequency should be a relatively stable value, usually 50Hz, and the deviation under normal operating conditions should be within 1%.

4. Load power factor

Characterize the inverter's ability to carry inductive or capacitive loads. The load power factor of the sine wave inverter is 0.7 to 0.9, and the rated value is 0.9. In the case of a certain load power, if the power factor of the inverter is low, the capacity of the required inverter will increase, on the one hand, the cost will increase, and the apparent power of the AC circuit of the photovoltaic system will increase. As current increases, losses will inevitably increase, and system efficiency will also decrease.

5. Inverter efficiency

The efficiency of the inverter refers to the ratio of its output power to the input power under specified working conditions, expressed as a percentage. Under normal circumstances, the nominal efficiency of the photovoltaic inverter refers to pure resistance load, 80% load s efficiency. Since the overall cost of the photovoltaic system is relatively high, the efficiency of the photovoltaic inverter should be maximized, the system cost should be reduced, and the cost-effectiveness of the photovoltaic system should be improved. At present, the nominal efficiency of mainstream inverters is between 80% and 95%, and the efficiency of low-power inverters is required to be not less than 85%. In the actual design process of the photovoltaic system, not only the high-efficiency inverter should be selected, but also the reasonable configuration of the system should be used to try to make the photovoltaic system load work near the optimal efficiency point.

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 are rated output capacity, the unit is expressed in VA or kVA. The rated capacity of the inverter is when the output power factor is 1 (that is, purely resistive load), the rated output voltage is the product of the rated output current.

7. Protection measures

An inverter with excellent performance should also have complete protection functions or measures to cope with various abnormal situations that occur during actual use, so as to protect the inverter itself and other system components from damage.

(1) Enter under-voltage protection account:

When the input voltage is lower than 85% of the rated voltage, the inverter should have protection and display.

(2) Enter the overvoltage protection account:

When the input voltage is higher than 130% of the rated voltage, the inverter should have protection and display.

(3) Overcurrent protection:

The overcurrent protection of the inverter should ensure that it can act in time when the load short-circuits or the current exceeds the allowable value, so as to protect it from the damage of surge current. When the working current exceeds 150% of the rated value, the inverter should be able to automatically protect.

(4) Output short circuit protector

Inverter short-circuit protection action time should not exceed 0.5s.

(5) Input reverse connection protection:

When the positive and negative poles of the input terminal are connected in reverse, the inverter should have protection function and display.

(6) Lightning protection:

The inverter should have lightning protection.

(7) Over-temperature protection, etc.

In addition, for inverters without voltage stabilization measures, the inverter should also have output overvoltage protection measures to protect the load from overvoltage damage.

8. Starting characteristics

Characterize the inverter's ability to start under load and its performance during dynamic operation. The inverter should ensure reliable start under rated load.

9. noise

Transformers, filter inductors, electromagnetic switches and fans in power electronic equipment will generate noise. When the inverter is operating normally, its noise should not exceed 80dB, and the noise of a small inverter should not exceed 65dB.

V. Selection techniques

In the selection of the inverter, we must first consider having sufficient rated capacity to meet the electrical power requirements of the equipment under maximum load. For an inverter with a single device as its load, the selection of its rated capacity is relatively simple.

When the electrical equipment is a purely resistive load or the power factor is greater than 0.9, the rated capacity of the inverter is selected to be 1.1 to 1.15 times the capacity of the electrical equipment. At the same time, the inverter should also have the ability to resist capacitive and inductive load shocks.

For general inductive loads, such as motors, refrigerators, air conditioners, washing machines, high-power water pumps, etc., when starting, the instantaneous power may be 5 to 6 times its rated power, at this time, the inverter will withstand a large instant surge. For such systems, the rated capacity of the inverter should be left with sufficient margin to ensure that the load can be reliably started, and the high-performance inverter can achieve multiple full-load starts without damaging the power devices. For their own safety, small inverters sometimes need to use soft start or current limiting start.

6. Installation precautions and maintenance

1. Before installation, first check whether the inverter is damaged during transportation.

2. When choosing the installation site, you should ensure that there is no interference from any other power electronic equipment in the surrounding area.

3. Before making electrical connections, be sure to use opaque materials to cover the photovoltaic panel or disconnect the DC side circuit breaker. Exposure to sunlight, photovoltaic arrays will generate dangerous voltages.

4. All installation operations must be completed by professional technicians only.

5. The cables used in the photovoltaic system power generation system must be connected firmly, with good insulation and appropriate specifications.

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