Inverter working principle

The core of the inverter is the inverter switch circuit, referred to as the inverter circuit. The circuit through the power electronic switch on and off, to complete the inverter function.

Features

1. High efficiency is required

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

2. High reliability is required

At present, 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 selection of components, and requires the inverter to have a variety of protection functions, such as: input DC polarity reverse protection, AC output short-circuit protection, overheating, overload protection, etc..

3. the input voltage is required to have a wide range of adaptation

Because the terminal voltage of the solar cell changes with the load and sunshine intensity. In particular, when the battery is aging, its terminal voltage varies widely, such as the 12V battery, its terminal voltage may vary between 10V and 16V, which requires the inverter to ensure normal operation within a larger DC input voltage range.

Photovoltaic Inverter Classification

There are many ways to classify inverters, such as: according to the phase number of the output AC voltage of the inverter, it can be divided into single-phase inverter and three-phase inverter; According to the different types of semiconductor devices used by the inverter, it can be divided into transistor inverter, thyristor inverter and switch-off thyristor inverter. According to the different circuit principles of inverters, they can also be divided into self-excited oscillation type inverters, step wave stacking type inverters and pulse width modulation type inverters. According to the application in grid-connected system or off-grid system can be divided into grid-connected inverter and off-grid inverter. In order to make it easy for photoelectric users to choose inverter, the inverter is only classified by different application occasions.

1. Centralized inverter

Concentration is several parallel photovoltaic inverter technology group of string is connected to the same concentration of inverter dc input, the use of the general power three-phase IGB T power module, the use of smaller power field effect transistor, at the same time using DSP conversion controller to improve the quality of the output power, making it very close to the sine wave current, Generally used in large photovoltaic power stations (& GT; 10kW) system. The biggest characteristic is the high power and low cost of the system. However, because the output voltage and current of different photovoltaic arrays are often not completely matched (especially when the photovoltaic arrays are partially blocked due to cloudy, shady trees, stains and other reasons), the centralized inverter mode will lead to the reduction of the efficiency of the inverter process and the decline of power consumption. At the same time, the power generation reliability of the whole photovoltaic system is affected by the bad working state of a certain photovoltaic cell group. The latest research direction is to use space vector modulation control and develop a new inverter topology connection to obtain high efficiency under partial load.

2. Group series inverter

Cluster inverter is based on the modular concept, each pv cluster (1-5kW) through an inverter, at the DC end has the maximum power peak tracking, at the AC end parallel grid, has become the most popular inverter in the international market.

Many large photovoltaic power plants use cluster inverters. The advantage is that it is not affected by module difference and shadow between groups, and at the same time it reduces the mismatch between the best working point of photovoltaic modules and the inverter, thus increasing the power generation. These technological advantages not only reduce the cost of the system, but also increase the reliability of the system. At the same time, the concept of “master-slave” is introduced between the series, so that the system can not make a single inverter work in a single series of electric energy, several photovoltaic series connected together, so that one or more of them work, so as to produce more electric energy.

The latest concept takes the reliability of the system a step further by grouping several inverters into a “team” instead of a “master-slave” system. At present, transformerless series inverters have been dominant.

3. Micro inverter

In a traditional PV system, the DC input terminal of each series inverter is connected in series by about 10 PHOTOVOLTAIC panels. When one of the 10 panels in series doesn’t work well, all of them will be affected. If the inverter multichannel input uses the same MPPT, then all inputs will be affected, greatly reducing the power generation efficiency. In practical applications, clouds, trees, chimneys, animals, dust, snow and ice and other shielding factors can cause the above factors, the situation is very common. In the PV system of micro inverter, each panel is connected to a micro inverter separately. When one of the battery panels does not work well, only this one will be affected. All other photovoltaic panels will operate at optimum operating conditions, resulting in higher overall efficiency and greater power generation. In practical applications, if the cluster inverter failure, will cause several kilowatts of panels can not play a role, while the micro inverter failure caused by relatively small impact.

4. Power optimizer

The solar power system with power optimizer can greatly improve conversion efficiency and simplify the functions of the inverter to reduce costs. To achieve smart solar power generation system, the installation of power optimizer can ensure the best performance of each solar cell, and monitor the cell depletion status 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 power of inverter. Power optimizer to simplify the circuit and by which corresponds to a single solar cells power optimizer, by analogy and very quickly the most beautiful power point tracking scanning, and let each solar cells can indeed maximum power point tracking, in addition, can also through the placement when communication chip with anywhere to monitor battery status, Immediate report of the problem to the relevant personnel as soon as possible repair.

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 solar cells and system failure 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), DC grounding detection function (for grid-connected system). The functions of automatic running and shutdown and maximum power tracking control are briefly introduced here.

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 automatically starts to run. After entering the operation, the inverter will always monitor the output of the solar cell module, 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 run; Until the sunset shutdown, the inverter can operate even on rainy days. When the solar cell module output becomes smaller and the inverter output approaches 0, the inverter will form standby state.

2. Maximum power tracking control function

The output of the solar cell module varies with the solar radiation intensity and the temperature of the solar cell module itself (chip temperature). In addition, the voltage of solar cell modules decreases with the increase of current, so there is an optimal working point to obtain the maximum power. The intensity of solar radiation is changing, so is the optimal working point. 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 solar power system inverter is to include maximum power point tracking (MPPT) this function.

Main Technical Indicators of Photovoltaic Inverter

1. Stability of output voltage

In a photovoltaic system, the energy generated by solar cells is stored in a battery and then converted by an inverter into 220V or 380V alternating current. However, the battery is affected by its own charging and discharging, and its output voltage varies widely. For example, the nominal 12V battery, its voltage value can vary from 10.8 V to 14.4V (beyond this range, the battery may be damaged). For a qualified inverter, when the input voltage changes within this range, the steady-state output voltage should not change by more than ± 5% of the rated value, and when the load changes suddenly, the output voltage deviation should not exceed ± 10% of the rated value.

2. Waveform distortion of output voltage

The maximum allowable waveform distortion (or harmonic content) shall be specified for sine wave inverters. It is usually expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (l0% for single-phase output). Because the high order harmonic current output by the inverter will produce additional losses such as eddy current 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 affects the operating efficiency of the system.

3. Rated output frequency

Such as contain motor load, such as washing machines, refrigerators, etc., due to its best working point frequency 50 hz motor, frequency is too high or too low will cause heating equipment, reduce the system efficiency and service life, so the inverter output frequency should be a relatively stable value, usually for power frequency 50 hz, under normal working conditions the deviation should be within plus or minus l %.

4. Load power factor

Represents the capacity of the inverter with inductive or capacitive load. The load power factor of the sinusoidal inverter is 0.7 ~ 0.9 with a rating of 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 PHOTOVOLTAIC system AC loop increases, the loop current increases, the loss is bound to increase, and the system efficiency will also be reduced.

5. Inverter efficiency

The efficiency of the inverter refers to the ratio of its output power to its input power under specified working conditions, expressed as a percentage. In general, the nominal efficiency of the photovoltaic inverter refers to the pure resistance load, the efficiency under 80% load. Since the overall cost of photovoltaic system is high, the efficiency of photovoltaic inverter should be maximized to reduce the system cost and improve the cost performance of photovoltaic system. At present, the nominal efficiency of mainstream inverters is between 80% and 95%, and the efficiency of low-power inverters is required to be no less than 85%. In the actual design process of photovoltaic system, not only high efficiency inverter should be selected, but also reasonable system configuration should be adopted to make photovoltaic system load work near the optimal efficiency point.

6. Rated output current (or rated output capacity)

Represents the rated output current of the inverter within the specified load power factor range. Some inverter products give rated output capacity, expressed in VA or kVA. The rated capacity of an inverter is the product of the rated output current when the output power factor is 1 (i.e. the purely resistive load).

7. Protective measures

A good performance of the inverter, but also should have complete protection function or measures, in order to cope with the actual use of various abnormal conditions, so that the inverter itself and other parts of the system from damage.

(1) Input under voltage insured

When the input voltage is below 85% of the rated voltage, the inverter should be protected and displayed.

(2) Input over voltage protector

When the input voltage is higher than 130% of the rated voltage, the inverter should be protected and displayed.

(3) Over current protection

The inverter over current protection, should be able to ensure that the load short-circuit or current exceeds the allowable value when the timely action, so that it is not damaged by the surge current. When the operating current exceeds 150% of the rated, the inverter should be able to automatically protect.

(4) Output short circuit insured

The short circuit protection operation time of the inverter should not exceed 0.5s.

(5) Input reverse connection protection

When the input correct, negative pole is connected, the inverter should be protective function and display.

(6) Lightning protection

The inverter must be protected against lightning.

(7) Over temperature protection, etc.

In addition, the inverter without voltage stability measures, the inverter should also output overvoltage protection measures, in order to make the load from overvoltage damage.

8. Starting characteristic

Characterizes the ability of the inverter to start with load and the performance of dynamic operation. The inverter should be guaranteed to start reliably under rated load.

9. Noise

Transformers, filter inductors, electromagnetic switches and fans in power electronic equipment all produce noise. When the inverter runs normally, its noise should not exceed 80dB, and the noise of small inverter should not exceed 65dB.

Selection Technique

The selection of inverter, first of all to consider enough rated capacity, in order to meet the maximum load equipment on the electrical power requirements. For the inverter with a single device as the load, the selection of rated capacity is relatively simple.

When the electrical equipment is pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter is 1.1 to 1.15 times of the capacity of the electrical equipment. At the same time, the inverter should also have the capacity of resistance to capacitive and inductive load impact.

For general inductive load, such as motor, refrigerator, air conditioner, washing machine, high-power water pump, etc., when starting, its instantaneous power may be 5 ~ 6 times of its rated power, at this time, the inverter will bear a large instantaneous surge. For this kind of system, the rated capacity of the inverter should have sufficient allowance to ensure that the load can be started reliably. The high-performance inverter can be started continuously for many times at full load without damaging the power device. For its own safety, small inverters sometimes need to adopt soft start or current limiting start.

Installation Precautions and Maintenance

1. Before installation, the inverter should be checked whether there is any damage in the transportation process.

2, in the selection of installation site, should ensure that there is no interference from other power electronic equipment in the surrounding area.

3. Before electrical connection, be sure to use opaque material to cover the photovoltaic panel or disconnect the DC side circuit breaker. Exposed to sunlight, the photovoltaic array will generate dangerous voltages.

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

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

The Development Trend

For solar inverters, improve the power conversion efficiency is an eternal topic, but when the system efficiency is higher and higher, almost close to 100%, will further improve efficiency along with price lower, therefore, how to maintain a high efficiency, and can maintain good price competitiveness is the current important topic.

Compared with the efforts to improve the efficiency of the inverter, how to improve the efficiency of the whole inverter system is gradually becoming another important topic of solar energy system. In a solar array, when the local 2~3% area of the shadow appears, for the use of an MPPT function of the inverter, the system output power bad at this time will even appear about 20% power decline! In order to better adapt to such a situation for a single or part of the solar module, the use of one-to-one MPPT or multiple MPPT control function is a very effective method.

Because the inverter system is connected to the grid, the leakage of the system to the ground will cause serious security problems. In addition, to improve the efficiency of the system, solar arrays are mostly used in series with high DC output voltage. Therefore, due to the occurrence of abnormal conditions between the electrodes, it is easy to produce DC arc. Because of the high DC voltage, it is very difficult to extinguish the arc, and it is very easy to cause fire. With the widespread use of solar inverter system, system security will be an important part of the inverter technology.

In addition, the power system is embracing the rapid development and popularization of smart grid technology. A large number of solar and other new energy power systems are connected to the grid, which poses new technical challenges to the stability of smart grid system. To design an inverter system that can be more quickly, accurately and intelligently compatible with smart grid will become a necessary condition for the solar inverter system in the future.

In general, the development of inverter technology is developed with the development of power electronics technology, microelectronics technology and modern control theory. As time goes by, inverter technology is developing towards higher frequency, greater power, higher efficiency and smaller volume.