what is a solar inverter?

Views: 67     Author: Site Editor     Publish Time: 2023-05-20      Origin: Site

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As the world marches forward in its pursuit of sustainable and clean energy solutions, solar power has emerged as a frontrunner in the race towards a greener future. Harnessing the abundant and renewable energy of the sun, solar photovoltaic (PV) systems have gained widespread popularity, paving the way for a remarkable transformation in the way we generate electricity.


what is a solar inverter


At the heart of every solar PV system lies a crucial component that enables the conversion of sunlight into usable energy: the solar inverter. Acting as the bridge between the solar panels and the electrical grid, solar inverters play a vital role in the efficient utilization of solar power. Understanding their working principle and exploring their various types is key to comprehending the fascinating mechanics behind solar energy conversion.


How Does A Solar Inverter Work?


A solar inverter is an electronic device that converts the direct current (DC) electricity produced by solar panels into alternating current (AC) electricity that can be used to power household appliances and be fed into the electrical grid. The working principle of a solar inverter can be divided into three main stages: conversion, control, and output.


Conversion:


The solar inverter first receives the DC electricity generated by the solar panels. This DC electricity is typically in the form of a fluctuating voltage that varies with the intensity of sunlight. The inverter's primary task is to convert this variable DC voltage into a stable AC voltage suitable for consumption.


The conversion process involves two key components: a set of power electronic switches (usually insulated-gate bipolar transistors or IGBTs) and a high-frequency transformer. The switches are responsible for rapidly switching the DC voltage on and off, creating a high-frequency pulse signal. The transformer then steps up the voltage to the desired AC voltage level.


Control:


The control stage of a solar inverter ensures that the conversion process operates efficiently and safely. It involves the use of sophisticated control algorithms and sensors to monitor and regulate various parameters. Some important control functions include:


a. Maximum Power Point Tracking (MPPT): Solar panels have an optimal operating point called the maximum power point (MPP), where they produce the maximum power for a given sunlight intensity. The MPPT algorithm continuously adjusts the operating point of the solar panels to maximize the power output by tracking the MPP.


b. Voltage and Frequency Regulation: The inverter's control system maintains a stable AC output voltage and frequency, typically following the standards of the utility grid. This ensures compatibility with other electrical devices and allows seamless integration with the grid.


c. Grid Synchronization: Grid-connected solar inverters synchronize the phase and frequency of the AC output with the utility grid. This synchronization enables the inverter to feed excess power back into the grid or draw power from the grid when solar production is insufficient.


Output:


In the final stage, the solar inverter delivers the converted AC electricity to the electrical loads or the grid. The output can be utilized in two ways:


a. On-Grid or Grid-Tied Systems: In grid-tied systems, the solar inverter feeds the AC electricity directly into the utility grid. This reduces dependence on fossil fuel-based power plants and allows for net metering, where excess electricity generated during the day can be credited and used during low solar production periods.


b. Off-Grid Systems: In off-grid systems, the solar inverter charges a battery bank in addition to supplying power to the electrical loads. The batteries store excess solar energy, which can be utilized during times of low solar production or at night when the solar panels are not generating electricity.


Characteristics of Solar Inverters:


Efficiency: 


Solar inverters are designed to operate with high efficiency to maximize the energy yield of the solar PV system. Higher efficiency results in less energy loss during the conversion process, ensuring that a larger proportion of the solar energy is effectively utilized.


Power Output: 


Solar inverters are available in various power ratings, ranging from small residential systems to large-scale commercial installations. The power output of an inverter should be appropriately matched with the capacity of the solar panels to achieve optimal performance.


Durability and Reliability: 


Solar inverters are exposed to varying environmental conditions, including temperature fluctuations, humidity, and potential electrical surges. Therefore, inverters should be built with robust materials and designed to withstand these conditions, ensuring long-term reliability.


Monitoring and Communication: 


Many modern solar inverters come equipped with monitoring systems that allow users to track the performance of their solar PV system. Some inverters can also communicate with external devices and software platforms, providing real-time data and enabling remote monitoring and control.


Safety Features: 


Solar inverters incorporate various safety features to protect both the system and the individuals working with it. These features include overvoltage protection, overcurrent protection, ground fault detection, and anti-islanding protection, which prevents the inverter from feeding power into the grid during power outages.


Solar Inverter Classification by Power Rating


PV inverters, also known as solar inverters, can be classified into different types based on their design, functionality, and application. Understanding these classifications can help in selecting the most suitable inverter for a specific solar PV system. The following are the main types of PV inverters classified by power level:


Inverter according to power level: mainly divided into distributed inverter (string inverter & micro inverter), centralized inverter


String Inverters:


String inverters are the most commonly used type of PV inverters in residential and commercial solar installations, they are designed to handle multiple solar panels connected in series, forming a "string." The PV string (1-5kw) has become the most popular inverter in the international market nowadays through an inverter with maximum power peak tracking at the DC side and parallel grid connection at the AC side.


The DC electricity generated by the solar panels is fed into the string inverter, which converts it into AC electricity for immediate use or for export to the grid. String inverters are known for their simplicity, cost-effectiveness, and ease of installation. However, the performance of the entire string is dependent on the lowest-performing panel, which can impact the overall system efficiency.


Micro inverters:


Micro inverters are small inverters that are installed on each individual solar panel in a PV system. Unlike string inverters, micro inverters convert the DC electricity to AC right at the panel level. This design allows each panel to operate independently, optimizing the system's overall energy output. Micro inverters offer several advantages, including panel-level maximum power point tracking (MPPT), improved system performance in shaded or mismatched panels, increased safety due to lower DC voltages, and detailed monitoring of individual panel performance. However, the higher upfront cost and potential complexity of installation are factors to consider.


Centralized Inverters:


Centralized inverters, also known as large or utility-scale (>10kW) inverters, are commonly used in large-scale solar PV installations, such as solar farms or commercial solar projects. These inverters are designed to handle high DC power inputs from multiple strings or arrays of solar panels and convert them into AC power for grid connection.


The biggest feature is the high power and low cost of the system, but since the output voltage and current of different PV strings are often not exactly matched (especially when the PV strings are partially shaded due to cloudiness, shade, stains, etc.), the use of centralized inverter will lead to lower efficiency of the inverting process and lower electric household energy.


Centralized inverters typically have a higher power capacity compared to other types, ranging from several kilowatts to several megawatts. They are installed in a central location or inverter station, and multiple strings or arrays of solar panels are connected to them in parallel.


What Does A Solar Inverter Do?


Photovoltaic inverters serve multiple functions, including AC conversion, optimizing solar cell performance, and system protection. These functions encompass automatic operation and shutdown, maximum power tracking control, anti-islanding (for grid-connected systems), automatic voltage adjustment (for grid-connected systems), DC detection (for grid-connected systems), and DC ground detection (for grid-connected systems). Let's briefly explore the automatic operation and shutdown function and the maximum power tracking control function.


1) Automatic operation and shutdown function


After sunrise in the morning, the intensity of solar radiation increases gradually, and the output of solar cells increases accordingly. When the output power required by the inverter is reached, the inverter starts to run automatically. After entering the operation, the inverter will monitor the output of the solar cell components all the time, as long as the output power of the solar cell components is greater than the output power required by the inverter, the inverter will continue to run; until the sunset stops, even if it is rainy The inverter also works. When the output of the solar cell module becomes smaller and the output of the inverter is close to 0, the inverter will form a standby state.


2) Maximum power tracking control function


The output of the solar cell module varies with the intensity of solar radiation 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 the current, so there is an optimal operating point that can obtain the maximum power. The intensity of solar radiation is changing, obviously the best working 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 kind of control is the maximum power tracking control. The biggest feature of the inverter used in the solar power generation system is the function of maximum power point tracking (MPPT).


The Main Technical Indicators 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 converted into 220V or 380V alternating current through the inverter. However, the battery is affected by its own charge and discharge, and its output voltage varies in a large range. For example, the nominal 12V battery has a voltage value that can vary between 10.8 and 14.4V (beyond this range may cause damage to the battery). For a qualified inverter, when the input terminal voltage changes within this range, the variation of its steady-state output voltage should not exceed Plusmn; 5% of the rated value. At the same time, when the load changes suddenly, its output voltage deviation should not exceed ±10% over rated value.


2. Waveform distortion of output voltage


For sine wave inverters, the maximum allowable waveform distortion (or harmonic content) should be specified. It is usually expressed by the total waveform distortion of the output voltage, and its value should not exceed 5% (10% is allowed for single-phase output). Since the high-order harmonic current output by the inverter will generate additional losses such as eddy currents on the inductive load, if the waveform distortion of the inverter is too large, it will cause serious heating of the load components, 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., since the optimal frequency operating point of the motors is 50Hz, too high or too low frequencies will cause the equipment to heat up, reducing the system's operating efficiency and service life, so the inverter's The output frequency should be a relatively stable value, usually power frequency 50Hz, and its deviation should be within Plusmn;l% under normal working conditions.


4. Load power factor


Characterize the ability of the inverter with inductive load or capacitive load. The load power factor of the sine wave inverter is 0.7~0.9, 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 at the same time, the apparent power of the AC circuit of the photovoltaic system will increase. As the current increases, the loss will inevitably increase, and the 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. In general, the nominal efficiency of a photovoltaic inverter refers to a pure resistance load. Under the condition of 80% load s efficiency. Since the overall cost of the photovoltaic system is high, the efficiency of the photovoltaic inverter should be maximized to reduce the system cost and improve the cost performance of the 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 a photovoltaic system, not only should a high-efficiency inverter be selected, but also a reasonable configuration of the system should be used to make the load of the photovoltaic system work near the best efficiency point as much as possible.


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, and its unit 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 (that is, purely resistive load).


7. Protection measures


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


1) Enter the undervoltage insurance account:


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


2) Input overvoltage protector:


When the input terminal 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 be able to ensure timely action when the load is short-circuited or the current exceeds the allowable value, so as to prevent it from being damaged by the surge current. When the working current exceeds 150% of the rated value, the inverter should be able to automatically protect.


4) output short circuit protection


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


5) Input reverse polarity protection:


When the positive and negative poles of the input terminal are reversed, 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


To characterize the ability of the inverter to start with load and the performance during dynamic operation. The inverter should ensure reliable starting under rated load.


9. Noise


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


Selection Skills of Solar Inverters


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


When the electrical equipment is a pure resistive load or the power factor is greater than 0.9, the rated capacity of the inverter can be selected as 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 impacts.


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 the rated power. At this time, the inverter will bear a large instantaneous power surge. For this type of system, the rated capacity of the inverter should have a sufficient margin to ensure that the load can be started reliably, and a high-performance inverter can be started at full load for many times in a row without damaging the power device. Small inverters sometimes need soft start or current-limited start for their own safety.


Solar Inverter Installation Precautions and Maintenance


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


2. When selecting the installation site, it should be ensured that there is no interference from any other power electronic equipment in the surrounding area.


3. Before making electrical connections, be sure to cover the photovoltaic panels with opaque materials or disconnect the DC side circuit breaker. Photovoltaic arrays will generate dangerous voltages when exposed to sunlight.


4. All installation operations must be done by professional and technical personnel only.


5. The cables used in the photovoltaic system power generation system must be firmly connected, well insulated and suitable in size.


The Development Trend of Solar Inverter


For solar inverters, improving the conversion efficiency of power is an eternal subject, but when the efficiency of the system is getting higher and higher, almost close to 100%, further efficiency improvement will be accompanied by low cost performance. Therefore, how to maintain A very high efficiency, while maintaining a good price competitiveness will be an important issue at present.


Compared with efforts to improve inverter efficiency, how to improve the efficiency of the entire inverter system is gradually becoming another important issue in solar energy systems. In a solar array, when a partial shadow of 2~3% of the area appears, for an inverter using an MPPT function, the output power of the system at this time may even drop by about 20% when the output power of the system is bad! In order to Better adapt to situations like this For single or partial solar modules, it is very effective to use one-to-one MPPT or multiple MPPT control functions.


Since the inverter system is in grid-connected operation, the leakage of the system to the ground will cause serious safety problems; in addition, in order to improve the efficiency of the system, most of the solar arrays will be connected in series to use a high DC output voltage; for this reason, in Due to the occurrence of abnormal conditions between the electrodes, it is easy to generate a DC arc. Due to the high DC voltage, it is very difficult to extinguish the arc, and it is very easy to cause a fire. With the widespread adoption of solar inverter systems, the issue of system security will also be an important part of inverter technology.


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


Generally speaking, the development of inverter technology is developed along with the development of power electronics technology, microelectronics technology and modern control theory. With the passage of time, inverter technology is developing towards higher frequency, higher power, higher efficiency and smaller size.