What Is a Solar Inverter?

Inverters, also known as power regulators and power regulators, are an essential part of photovoltaic systems. The most important function of a photovoltaic inverter is to convert the direct current generated by a solar panel into alternating current used by household appliances. All the power generated by a solar panel must be processed by the inverter before being output. ^[1] Through a full-bridge circuit, an SPWM processor is generally used to modulate, filter, boost, etc. to obtain a sinusoidal AC power that matches the lighting load frequency and rated voltage for system end users. With an inverter, you can use a DC battery to provide AC power to your appliances.

Chinese name: Solar inverters
Foreign name: Solar Inverter
Make up: Solar panels, charging controllers, etc.

The main function: Invert the battery's DC power to AC power
Classification of incentive methods: Self-excited oscillation inverter and other-excited oscillation inverter
Core: Inverter circuit

Introduction of solar inverter

The solar AC power generation system is composed of solar panels, charge controllers, inverters and batteries; the solar DC power generation system does not include inverters. The process of converting AC power into DC power is called rectification, the circuit that performs the rectification function is called a rectification circuit, and the device that implements the rectification process is called a rectification device or a rectifier. Correspondingly, the process of converting DC power into AC power is called inverter, the circuit that completes the inverter function is called the inverter circuit, and the device that implements the inverter process is called the inverter device or inverter. ^[2]

The core of the inverter device is an inverter switching circuit, referred to as an inverter circuit for short. This circuit completes the inverter function by turning on and off the power electronic switch. The switching of power electronic switching devices requires certain driving pulses. These pulses may be adjusted by changing a voltage signal. The circuit that generates and regulates the pulse is often called a control circuit or control loop. The basic structure of the inverter device includes a protection circuit, an output circuit, an input circuit, an output circuit, etc. in addition to the inverter circuit and the control circuit described above. ^[2]

Solar inverter features

Due to the diversity of buildings, it is bound to lead to the diversity of solar panel installation. In order to maximize the efficiency of solar energy conversion

Solar inverters At the same time, taking into account the beautiful appearance of the building, this requires the diversification of our inverters to achieve the best way of solar energy conversion.

Centralized solar inverter

Centralized inverters are generally used in large photovoltaic power station (> 10kW) systems. Many parallel photovoltaic strings are connected to the DC input of the same centralized inverter. Generally, three-phase IGBT power modules are used for high power. The smaller power uses a field effect transistor, and a DSP conversion controller is used to improve the quality of the generated power, making it very close to a sine wave current. The biggest feature is the high power and low cost of the system. However, due to the matching of photovoltaic strings and partial shading, it leads to the efficiency and electrical capacity of the entire photovoltaic system. At the same time, the power generation reliability of the entire photovoltaic system is affected by the poor working condition of a certain photovoltaic unit group. The latest research directions are the use of space vector modulation control and the development of new inverter topology connections to achieve high efficiency under partial load conditions. On the SolarMax centralized inverter, a photovoltaic array interface box can be attached to monitor each string of solar panel strings. If one of the strings is not working properly, the system will The information is transmitted to the remote controller, and at the same time, the string can be stopped by remote control, so that the failure of a string of photovoltaic strings will not reduce and affect the work and energy output of the entire photovoltaic system.

Solar inverter string inverter

The string inverter has become the most popular inverter in the international market. The string inverter is based on the modular concept. Each photovoltaic string (1kW-5kW) passes an inverter with maximum power peak tracking at the DC end and is connected to the grid in parallel at the AC end. Many large photovoltaic power plants use string inverters. The advantage is that it is not affected by module differences and shading between strings, while reducing the optimal operating point of photovoltaic modules.

Mismatch with the inverter, which increases 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 in the case that a single string of energy cannot make a single inverter work, several groups of photovoltaic strings are linked together, and one or more of them work To produce more electricity. The latest concept is that several inverters form a "team" with each other instead of the "master-slave" concept, which makes the reliability of the system a step further.

Solar inverter multi-string inverter

Multi-string inverters take the advantages of centralized inverters and string inverters, avoiding their shortcomings, and can be applied to photovoltaic power stations of several kilowatts. Multi-string inverters include different individual power peak tracking and DC-to-DC converters. These DCs are converted into AC power by a common DC-to-AC inverter and connected to the grid. Different ratings of photovoltaic strings (such as: different rated power, different number of modules per string, different manufacturers of modules, etc.), photovoltaic modules of different sizes or different technologies, strings of different directions (such as (East, South, and West), different inclination angles or shading, can be connected to a common inverter, while each string works at their respective maximum power peaks. At the same time, the length of the DC cable is reduced, the shadowing effect between strings and the loss caused by the differences between strings are minimized.

Solar inverter module inverter

The module inverter connects each photovoltaic module with an inverter, and each module has a separate maximum power peak tracking, so that the cooperation between the module and the inverter is better. Generally used for 50W to 400W photovoltaic power stations, the total efficiency is lower than string inverters. Because it is connected in parallel at the AC, this increases the complexity of the wiring on the AC side and makes maintenance difficult. Another problem that needs to be solved is how to connect to the grid more effectively. The simple way is to directly connect to the grid through ordinary AC outlets. This can reduce costs and equipment installation. In doing so, the power company may object to the direct connection of the power generating device to the ordinary socket of the ordinary household user. Another safety-related factor is whether an isolation transformer (high or low frequency) is needed, or whether a transformerless inverter is allowed. This inverter is most widely used in glass curtain walls.

Solar inverter efficiency

The efficiency of solar inverters refers to the growing market of solar inverters (photoelectric inverters) due to the demand for renewable energy. And these inverters need extremely high efficiency and reliability. The power circuits used in these inverters are reviewed and the best choices for switches and rectifiers are recommended. The general structure of a photovoltaic inverter is shown in Figure 1. There are three different inverters to choose from. Sunlight shines on solar modules connected in series, and each module contains a series of solar cell units connected in series. The direct current (DC) voltage generated by a solar module is in the order of several hundred volts, and the specific value depends on the lighting conditions of the module array, the temperature of the battery, and the number of modules connected in series.

The primary function of this type of inverter is to convert the input DC voltage to a stable value. This function is implemented by a boost converter and requires a boost switch and a boost diode. In the first configuration, the boost stage is followed by an isolated full-bridge converter. The role of a full-bridge transformer is to provide isolation. The second full-bridge converter on the output is used to convert the direct-current DC of the first-stage full-bridge converter into an alternating current (AC) voltage. Its output is filtered before being connected to the AC grid network via an additional two-contact relay switch.

Solar inverter pictures (4 photos)

The purpose is to provide safe isolation in the event of a fault and isolation from the power grid at night. The second structure is a non-isolated scheme. The AC alternating voltage is directly generated by the DC voltage output by the boost stage. The third structure uses the innovative topology of power switches and power diodes to integrate the functions of the boost and AC AC generation parts in a dedicated topology. Although the conversion efficiency of the solar panel is very low, the inverter efficiency is as much as possible Close to 100% is very important. In Germany, a 3kW series module installed on a south-facing roof is expected to generate 2550 kWh per year. If the inverter efficiency is increased from 95% to 96%, an additional 25kWh can be generated each year. The cost of generating this 25kWh with an additional solar module is comparable to adding an inverter. Since increasing the efficiency from 95% to 96% will not double the cost of the inverter, investing in a more efficient inverter is an inevitable choice. For emerging designs, the most cost-effective way to improve inverter efficiency is a key design criterion. As for the reliability and cost of the inverter, there are two other design criteria. Higher efficiencies can reduce temperature fluctuations over the load cycle, thereby increasing reliability, so these guidelines are actually related. The use of modules also improves reliability.

Solar inverter boost switch and diode

All topologies shown in Figure 1 require fast-switching power switches. The boost stage and the full-bridge conversion stage require fast switching diodes. In addition, switches optimized for low-frequency (100Hz) conversion are also useful for these topologies. For any particular silicon technology, switches optimized for fast conversion have higher conduction losses than switches optimized for low-frequency conversion applications.

The boost stage is generally designed as a continuous current mode converter. According to the number of solar modules in the array used by the inverter, choose whether to use 600V or 1200V devices. Two options for power switches are MOSFET and IGBT. In general, MOSFETs can operate at higher switching frequencies than IGBTs. In addition, the effect of the body diode must always be considered: in the case of the boost stage, there is no problem, because the body diode does not conduct in the normal operating mode. The MOSFET conduction loss can be calculated based on the on-resistance RDS (ON), which is proportional to the effective die area for a given MOSFET family. When the rated voltage is changed from 600V to 1200V, the conduction loss of the MOSFET will be greatly increased. Therefore, even if the rated RDS (ON) is equivalent, the 1200V MOSFET is not available or the price is too high.

For boost switches rated at 600V, super-junction MOSFETs can be used. For high frequency switching applications, this technique has the best conduction loss. MOSFETs with TO-220 package and RDS (ON) value less than 100 milliohms and MOSFETs with TO-247 package and RDS (ON) value of less than 50 milliohms. For solar inverters that require a 1200V power switch, IGBTs are the appropriate choice. More advanced IGBT technologies, such as NPT Trench and NPT Field Stop, are optimized to reduce conduction losses, but at the cost of higher switching losses, which makes them less suitable for boost applications at high frequencies.

Based on the old NPT plane technology, a device that can improve the efficiency of the boost circuit of the high switching frequency, FGL40N120AND, has an EOFF of 43uJ / A. Compared with the more advanced technology, the EOFF is 80uJ / A. This performance is very difficult. The disadvantage of the FGL40N120AND device is that the saturation voltage drop VCE (SAT) (3.0V compared to 125 & ordm; C's 2.1V) is high, but the advantage of low switching loss at high boost switching frequency is enough to make up for it. The device also integrates anti-parallel diodes. This diode will not conduct during normal boost operation. However, during start-up or transient conditions, the boost circuit may be driven into working mode, and the anti-parallel diode will be turned on. Since the IGBT does not have an inherent body diode, such a co-packaged diode is required to ensure reliable operation. For boost diodes, fast recovery diodes such as Stealth or carbon silicon diodes are required. Carbon silicon diodes have very low forward voltage and losses. When selecting a boost diode, the effect of the reverse recovery current (or the junction capacitance of the carbon-silicon diode) on the boost switch must be considered, as this will cause additional losses. Here, the newly introduced Stealth II diode FFP08S60S can provide higher performance. When VDD = 390V, ID = 8A, di / dt = 200A / us, and the case temperature is 100 & ordm; C, the calculated switching loss is lower than the parameter 205mJ of FFP08S60S. With the ISL9R860P2 Stealth diode, this value is 225mJ. Therefore, this also improves the efficiency of the inverter at high switching frequencies.

Solar inverter bridge switch and diode

After the MOSFET full-bridge filtering, the output bridge generates a 50Hz sinusoidal voltage and current signal. A common implementation is to use a standard full bridge structure (Figure 2). In the figure, if the upper left and lower right switches are turned on, a positive voltage is applied between the left and right terminals; when the upper right and lower left switches are turned on, a negative voltage is loaded between the left and right terminals. For this application, only one switch is on at a time. One switch can be switched to PWM high frequency, the other switch is at 50Hz low frequency. Because the bootstrap circuit relies on the conversion of the low-end device, the low-end device is switched to the PWM high frequency, and the high-end device is switched to the 50Hz low frequency. This application uses a 600V power switch, so a 600V super-junction MOSFET is very suitable for this high-speed switching device. Because these switching devices will withstand the full reverse recovery current of other devices when the switch is on, fast recovery superjunction devices such as 600V FCH47N60F are ideal. Its RDS (ON) is 73 milliohms, and its conduction loss is very low compared to other similar fast recovery devices. When this device is converted at 50Hz, there is no need to use the fast recovery feature. These devices have excellent dv / dt and di / dt characteristics, which improves system reliability compared to standard super-junction MOSFETs.

Another option worth exploring is the FGH30N60LSD device. It is a 30A / 600V IGBT with a saturated voltage VCE (SAT) of only 1.1V. Its turn-off loss EOFF is very high, reaching 10mJ, so it is only suitable for low-frequency conversion. A 50 milliohm MOSFET has an on-resistance RDS (ON) of 100 milliohms at operating temperature. So at 11A, it has the same VDS as the VCE (SAT) of the IGBT. Because this IGBT is based on older breakdown technology, VCE (SAT) does not change much with temperature. Therefore, such an IGBT can reduce the overall loss in the output bridge, thereby improving the overall efficiency of the inverter. The FGH30N60LSD IGBT is also useful for switching from one power conversion technology to another dedicated topology every half cycle. IGBTs are used here as topology switches. For faster conversions, conventional and fast recovery superjunction devices are used. For the dedicated topology of 1200V and the full-bridge structure, the aforementioned FGL40N120AND is a very suitable switch for new high-frequency solar inverters. When proprietary technology requires diodes, Stealth II, Hyperfast II diodes and carbon-silicon diodes are good solutions.

Solar inverter function

The inverter not only has the function of direct AC conversion, but also has the function of carrying out the solar battery function and the system fault maintenance function to the maximum extent. In summary, there are active operation and shutdown functions, maximum power tracking control function, anti-standalone operation function (for grid connection system), active voltage adjustment function (for grid connection system), DC detection function (for grid connection system), and DC ground detection. Function (for grid-connected system). Here we briefly introduce the active running and stopping functions and the maximum power tracking control function.

1. Active operation and shutdown functions: 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 for the inverter task is reached, the inverter starts to operate actively. After entering the operation, the inverter will take care of the output of the solar cell module at any time. As long as the output power of the solar cell module is greater than the output power required for the inverter task, the inverter will continue to run; The inverter can also work in rainy days. When the output of the solar cell module becomes small and the inverter output approaches 0, the inverter will form a standby mode.

2. Maximum power tracking control function: The output of the solar cell module is changed 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 increasing current, there is an optimal task point for obtaining the maximum power. The intensity of solar radiation is changing, and obviously the best task point is also changing. Related to these changes, the task point of the solar cell module has been at the maximum power point, and the system has always obtained the maximum power output from the solar cell module. This kind of control is the maximum power tracking control. The biggest feature of the inverter used in solar power generation system is to include the function of maximum power point tracking (MPPT).

Solar inverter type

Classification of solar inverter applications

(1) Ordinary inverter

Solar inverter (2 photos)

DC 12V or 24V input, AC 220V, 50Hz output, power from 75W to 5000W, some models have AC and DC conversion, that is, UPS function.

(2) Inverter / charging machine

In this type of inverter, the user can use various forms of power to power the AC load: when AC power is available, the inverter uses AC power to power the load, or charge the battery; when there is no AC power, the battery is used to power the AC load . It can be used with a variety of power sources: batteries, generators, solar panels and wind turbines.

(3) Special inverter for post and telecommunications

Provide high-quality 48V inverter for post and telecommunications, communication, its product quality is good, high reliability, modular (1KW module) inverter, and has N + 1 redundancy function, expandable (power from 2KW to 20KW ).

(4) Aviation and military inverters

This type of inverter has 28Vdc input and can provide the following AC output: 26Vac, 115Vac, 230Vac, its output frequency can be: 50Hz, 60Hz and 400Hz, the output power ranges from 30VA to 3500VA. There are also DC-DC converters and inverters for aviation.

Classification of solar inverter output waveforms

(1) Square wave inverter

The AC voltage waveform output by the square wave inverter is a square wave. The inverter lines used in this type of inverter are not exactly the same, but the common feature is that the lines are relatively simple and the number of power switch tubes used is small. Design power is generally between 100 watts and kilowatts. The advantages of square wave inverter are: simple circuit, cheap price and easy maintenance. The disadvantage is that because the square wave voltage contains a large number of higher harmonics, additional losses will be generated in the load electrical appliances with core inductors or transformers, which will interfere with the radio and some communication equipment. In addition, this type of inverter has the disadvantages that the voltage regulation range is not wide enough, the protection function is not perfect, and the noise is relatively large.

(2) Step wave inverter

The AC voltage waveform output by this type of inverter is a step wave. There are also many different lines for the inverter to achieve a step wave output, and the number of steps in the output waveform varies greatly. The advantages of the step wave inverter are that the output waveform is significantly improved than the square wave, and the higher harmonic content is reduced. When the step reaches more than 17, the output waveform can realize a quasi-sine wave. When no transformer output is used, the efficiency of the whole machine is very high. The disadvantage is that there are more power switch tubes used in the step wave superposition line, and some of the line forms also require multiple sets of DC power input. This brings trouble to the grouping and wiring of solar cell squares and the balanced charging of storage batteries. In addition, the step wave voltage still has some high-frequency interference on the radio and some communication equipment.

(3) Sine wave inverter

The AC voltage waveform output by the sine wave inverter is a sine wave. The sine wave inverter has the advantages of good output waveform, low distortion, low interference to the radio and equipment, and low noise. In addition, it has complete protection functions and high efficiency. The disadvantages are: the circuit is relatively complicated, the maintenance technology is high, and the price is expensive.

The classification of the above three types of inverters is helpful for designers and users of photovoltaic systems and wind power generation systems to identify and select inverters. In fact, inverters with the same waveform still have great differences in terms of line principle, use of devices and control methods.

Other classification of solar inverter

1. According to the output AC power frequency classification, it can be divided into industrial frequency inverter, intermediate frequency inverter and high frequency inverter. Power frequency inverters are inverters with a frequency of 50 to 60 Hz; intermediate frequency inverters generally have a frequency of 400 Hz to more than ten kHz; high frequency inverters generally have a frequency of more than ten kHz to MHz.

2. According to the number of phases output by the inverter, it can be divided into single-phase inverter, three-phase inverter and multi-phase inverter.

3. According to the whereabouts of the inverter's output power, it can be divided into active inverters and passive inverters. An inverter that transmits the power output from the inverter to the industrial grid is called an active inverter; an inverter that outputs the power output from the inverter to a certain power load is called a passive inverter Device.

4. According to the form of the inverter's main circuit, it can be divided into single-ended inverters, push-pull inverters, half-bridge inverters and full-bridge inverters.

5. According to the type of the inverter's main switching device, it can be divided into thyristor inverters, transistor inverters, field effect inverters, and insulated gate bipolar transistor (IGBT) inverters. It can be summarized into two categories: "semi-controlled" inverter and "full-control" inverter. The former does not have self-shutdown capability, and the components lose control after they are turned on. Therefore, it is called a "semi-controlled" ordinary thyristor. Both on and off can be controlled by the control electrode, so it is called "full control type". Power field effect transistors and insulated gate double-weight transistors (IGBT) belong to this category.

6. According to DC power supply, it can be divided into voltage source inverter (VSI) and current source inverter (CSI). In the former, the DC voltage is nearly constant and the output voltage is an alternating square wave; in the latter, the DC current is nearly constant and the input current is an alternating square wave.

7. According to the inverter control mode, it can be divided into frequency modulation (PFM) inverter and pulse width modulation (PWM) inverter.

8. According to the working mode of the inverter switching circuit, it can be divided into resonant inverters, fixed-frequency hard-switching inverters and fixed-frequency soft-switching inverters.

9. According to the inverter commutation method, it can be divided into load commutated inverter and self-converted inverter. ^[2]

Solar inverter performance parameters

There are many parameters and technical conditions for describing the performance of the inverter. Here we only briefly explain the technical parameters commonly used in evaluating the inverter.

1. The environmental conditions of the inverter. The normal operating conditions of the inverter: the altitude does not exceed 1000m, and the air temperature is 0 + 40 .

2. DC input power supply conditions, input DC voltage fluctuation range: ± 15% of the rated voltage of the battery pack.

3. The rated output voltage is within the allowed fluctuation range of the specified input DC voltage, which indicates the rated voltage value that the inverter should be able to output. The stability accuracy of the output rated voltage value is generally as follows:

(1) During steady-state operation, the voltage fluctuation range should be limited, for example, its deviation should not exceed ± 3% or ± 5% of the rated value.

(2) The output voltage deviation should not exceed ± 8% or ± 10% of the rated value in a dynamic situation with sudden load changes or other interference factors.

4. Rated output frequency. The frequency of inverter output AC voltage should be a relatively stable value, usually 50Hz. The deviation should be within ± 1% under normal working conditions.

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

6. Rated output efficiency. The efficiency of the inverter is the ratio of its output power to input power under specified operating conditions, expressed in%. The efficiency of the inverter at rated output capacity is full load efficiency, and the efficiency at 10% of rated output capacity is low load efficiency.

7. The maximum harmonic content of the inverter. For a sine wave inverter, under resistive load, the maximum harmonic content of the output voltage should be 10%.

8. The overload capacity of the inverter, under specified conditions, the ability of the inverter to output more than the rated current in a short period of time. The overload capacity of the inverter should meet certain requirements under the specified load power factor.

9, the efficiency of the inverter, under the rated output voltage, output current and the specified load power factor, the inverter output active power and input active power (or DC power) ratio.

10. Load power factor, which characterizes the inverter's ability to carry inductive or capacitive loads. Under sine wave conditions, the load power factor is 0.7 to 0.9 (lag), and the rated value is 0.9.

11. Asymmetry of the load. Under a 10% asymmetric load, the asymmetry of the output voltage of a three-phase inverter with a fixed frequency should be 10%.

12. Unbalance of output voltage. Under normal operating conditions, the unbalance of three-phase voltage (ratio of reverse sequence component to positive sequence component) output by the inverter should not exceed a specified value, generally expressed in%, such as 5 % Or 8%.

13. Starting characteristics: Under normal operating conditions, the inverter should be able to start normally for 5 consecutive times under full-load and no-load operating conditions.

14, protection function, the inverter should be set: short circuit protection, over current protection, over temperature protection, over voltage protection, under voltage protection and phase loss protection. Among them, overvoltage protection means that for inverters without voltage stabilization measures, there should be output overvoltage protection measures to prevent the negative cutoff from being damaged by the output overvoltage. Over-current protection refers to the over-current protection of the inverter, which should be able to act in time when the load is short-circuited or the current exceeds the allowable value, so that it is protected from the surge current.

15. Interference and anti-interference. The inverter should be able to withstand electromagnetic interference in the general environment under the specified normal working conditions. The anti-interference performance and electromagnetic compatibility of the inverter should meet the requirements of relevant standards.

16. Inverters that are not frequently operated, monitored and maintained should be 95db; inverters that are frequently operated, monitored and maintained should be 80db.

17. Display: The inverter should be provided with data display of parameters such as AC output voltage, output current and output frequency, as well as signal display of input live, energized and fault status.

18. Communication function, remote communication function enables users to check the machine's running status and stored data without having to go to the scene.

19. The waveform distortion of the output voltage. When the inverter output voltage is sinusoidal, the maximum allowed waveform distortion (or harmonic content) should be specified. It is usually expressed as the total waveform distortion of the output voltage, and its value should not exceed 5% (single-phase output allows 10%).

20. Starting characteristics, characterizing the inverter's ability to start with load and performance during dynamic operation. The inverter should ensure reliable starting under the rated load.

21. Noise. Transformers, filter inductors, electromagnetic switches, and fans in power electronics equipment can cause noise. When the inverter is running normally, its noise should not exceed 80dB, and the noise of small inverter should not exceed 65dB. ^[2]

Solar inverter battery characteristics

PV Solar inverter PV cell

To develop a solar inverter system, it is important to first understand the different characteristics of solar cells (PV cells) . Rp and Rs are parasitic resistances, which are ideally infinite and zero, respectively.

Light intensity and temperature can greatly affect the operating characteristics of PV cells. The current is directly proportional to the intensity of the light, but changes in light have little effect on the operating voltage. However, the operating voltage is affected by temperature. Increasing battery temperature reduces the operating voltage, but has little effect on the generated current. The figure below illustrates the effects of temperature and light on PV modules.

The effect of light intensity changes on battery output power is greater than the effect of temperature changes. This applies to all common PV materials. An important result of combining these two effects is that the power of a PV cell decreases with decreasing light intensity and / or increasing temperature.

(MPP) Solar inverter maximum power point (MPP)

Solar cells can operate over a wide range of voltages and currents. By continuously increasing the resistive load on the irradiated battery from zero (short-circuit event) to a very high value (open-circuit event), it can be determined that MPP.MPP is the operating point where V x I reaches its maximum value, and at this intensity Achieve maximum power. The output power is zero in the event of a short circuit (PV voltage equal to zero) or an open circuit (PV current equal to zero).

High-quality monocrystalline silicon solar cells can generate an open-circuit voltage of 0.60 volts at a temperature of 25 ° C. In the case of sufficient light and air temperature of 25 ° C, the temperature of a given battery may be close to 45 ° C, which will reduce the open circuit voltage to about 0.55V. As the temperature increases, the open circuit voltage continues to decrease until PV Module is shorted.

The maximum power at a battery temperature of 45 ° C is usually generated at 80% open circuit voltage and 90% short circuit current. The short-circuit current of the battery is almost proportional to the illuminance, and the open-circuit voltage may only decrease by 10% when the illuminance is reduced by 80%. A lower-quality battery will decrease the voltage faster when the current increases, thereby reducing the available power The output dropped from 70% to 50%, or even only 25%.

The figure above shows the output current and output power of PV panels as a function of operating voltage at a given illuminance.

Solar micro-inverters must ensure that the PV module is operating in the MPP at any given time in order to obtain maximum energy from the PV module. This can be achieved using a maximum power point control loop, which is also known as a Maximum Power Point Tracker (MPPT). Achieving a high percentage of MPP tracking also requires the PV output voltage ripple to be small enough so that its PV current does not change much when it is operating near its maximum power point.

The MPP voltage range of PV modules can usually be defined in the range of 25V to 45V, the power generation is about 250W, and the open circuit voltage is lower than 50V. ^[3]

Use and maintenance of solar inverter

Use of solar inverter

1. Connect and install the equipment in strict accordance with the requirements of the inverter's operation and maintenance instructions. When installing, you should carefully check: whether the wire diameter meets the requirements; whether the components and terminals are loose during transportation; whether the insulation should be good; whether the system grounding meets the requirements.

2. Operate strictly in accordance with the instructions of the inverter maintenance manual. In particular: pay attention to whether the input voltage is normal before turning on the power; when operating, pay attention to whether the sequence of switching on and off is correct, and whether the indicators of the meters and indicators are normal.

3. The inverter generally has automatic protection for items such as open circuit, over-current, over-voltage, and over-heating, so when these phenomena occur, no manual shutdown is required; the protection points for automatic protection are generally set at the factory, Make adjustments again.

4. There is high voltage in the inverter cabinet. Operators are generally not allowed to open the cabinet door. The cabinet door should be locked normally.

5. When the room temperature exceeds 30 ° C, heat dissipation and cooling measures should be taken to prevent equipment failure and prolong the service life of the equipment.

Solar inverter maintenance

1. The wiring of each part of the inverter should be checked regularly for looseness and looseness. In particular, the fans, power modules, input terminals, output terminals, and ground should be carefully checked.

2. Once the alarm is shut down, it is not allowed to start immediately. The cause should be found and repaired before starting. The inspection should be carried out strictly in accordance with the steps in the inverter maintenance manual.

3. The operator must be specially trained to be able to judge the cause of general faults and be able to eliminate them, such as being able to replace fuses, components and damaged circuit boards skillfully. Untrained personnel are not allowed to work or use equipment.

4. If an accident that is difficult to rule out or the cause of the accident is unclear, a detailed record of the accident should be made and the inverter manufacturer should be notified in time to resolve it. ^[2]