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A newer approach to optimizing the efficiency and reliability of solar systems is to use micro-inverters connected to each individual solar panel. Each solar panel is equipped with its own micro-inverter, allowing the system to adapt to its changing load and air environment, providing optimal conversion efficiency for individual solar panels and the entire system. The micro-inverter architecture also enables simpler wiring, resulting in lower installation costs. By improving the efficiency of the user's solar energy system, the initial technology investment return time of the system can be shortened.
Power inverters are the key electronic components in solar power systems. In some commercial applications, these components connect a photovoltaic (PV) board, a battery that stores charge, and a local power distribution system or a public power grid. Figure 1 shows a typical solar inverter that derives a very low voltage from the PV array DC output and then converts it to some combination of DC battery voltage, AC line voltage, and distribution network voltage.
In a typical solar energy harvesting system, multiple solar panels are connected in parallel to a single inverter that converts the variable DC output of multiple PV units into a clean sinusoidal 50Hz or 60Hz voltage source. .
The main design goal is to maximize conversion efficiency. This is a complex, iterative process that involves algorithms (Maximum Power Point Tracking Algorithm, MPPT) and real-time controllers that execute these algorithms.
Maximize power conversion
Inverters that do not use the MPPT algorithm simply connect the modules directly to the battery, forcing them to operate at battery voltage. Almost without exception, the battery voltage is not an ideal value for collecting the maximum available solar energy.
After implementing the MPPT algorithm, the situation is very different. In this example, the voltage at which the module reaches its maximum power is 17V. Therefore, the function of the MPPT algorithm is to operate the module at 17V to obtain a full 75W power, which is independent of the battery voltage.
The high efficiency DC/DC power converter converts the 17V module voltage at the controller input to the battery voltage at the output. Since the DC/DC converter gradually reduces the 17V voltage to 12V, the battery charging current of the MPPT system in this example is (VMODULE/VBATTERY)×IMODULE or (17V/12V)×4.45A=6.30A.
Assuming that the DC/DC converter is 100% efficient, the 1.85A charge current will increase, and it will reach 42%.
Although this example assumes that the inverter is processing energy from a single solar panel, conventional systems typically have many solar panels connected to a single inverter. This kind of topological structure has some disadvantages at the same time, depending on the application.
MPPT algorithm
There are mainly three MPPT algorithms: perturbation observation method, conductance increment method and constant voltage method. The first two methods are often referred to as "climbing" because they take advantage of the fact that the MPP curve on the left continues to rise (dP/dV>0) and the curve on the right of the MPP continues to decline (dP/dV < 0).
Perturbation observation (P&O) is the most common. The algorithm perturbs the operating voltage in a specific direction and then samples the dP/dV. If dP/dV is positive, the algorithm knows that it has adjusted the voltage toward MPP. Then, continue to adjust the voltage in this direction until dP/dV is negative.
P&O algorithms are easy to implement, but sometimes they can cause oscillations around the MPP that are operating in steady state. In addition, in the rapidly changing air conditions, their response time is longer and they may even track in the wrong direction.
The Conductance Delta (INC) method uses the delta conductance dI/dV of the PV array to calculate the dP/dV sign. Compared to P&O, INC quickly tracks changing lighting conditions more accurately. However, like P&O, it oscillates and becomes confusing under the influence of rapidly changing air conditions. Another disadvantage is that its high complexity increases the calculation time and reduces the sampling frequency. The third method is the constant voltage method, which utilizes the fact that, in general, the VMPP/VOC ratio is approximately equal to 0.76. The problem with this approach is that it requires that the PV array current be set to zero immediately to measure the open circuit voltage of the array. In this way, the array's operating voltage is set to 76% of this measurement. However, during this period, the array was disconnected, wasting effective energy. At the same time it was also found that 76% of the open circuit voltage is a very close value, but it is not always consistent with the MPP.
Since there is no MPPT algorithm that successfully satisfies all common scenarios, many designers will take some detours. They evaluate the system's environmental conditions and select the best algorithm. In fact, there are many MPPT algorithms available, and it is common for solar panel manufacturers to provide their own algorithms.
For some inexpensive controllers, implementing MPPT algorithms can be a difficult task. Because, in addition to the normal control functions of the MCU, the algorithm also requires these controllers have high-performance computing capabilities. Advanced 32-bit real-time microcontrollers (eg, some microcontrollers in the TIC2000 platform) are suitable for many solar applications.
Power inverter
The use of a single inverter has many advantages, of which the most prominent is simplicity and low cost. Using the MPPT algorithm and other techniques can increase the efficiency of a single inverter system, but only to a certain extent. The downtrend of the single inverter topology is obvious, but it depends on the application. What people are most concerned about is the reliability problem: If an inverter malfunctions, it will lose all the solar panel energy until it is repaired or replaced.
Single inverter topology can have a negative effect on system efficiency even when it is perfectly running. In most cases, each solar panel has a different maximum efficiency control requirement. Some of the factors that determine the efficiency of each solar panel include differences in the manufacturing of its PV cells, ambient temperature differences, and varying degrees of sunlight (primary energy received from the sun) due to sunlight shading and direction.
The overall system conversion efficiency can be further improved by installing a micro-inverter for each individual solar panel rather than a single inverter for the entire system. The main benefit of the micro-inverter topology is that energy is continuously converted even in the event of an inverter failure.
Other benefits of the micro-inverter approach include the ability to use high-precision PWM to adjust the conversion parameters of each solar panel. Since clouds, shadows, and shading all change the output of a single solar panel, installing a micro-inverter for each solar panel allows the system to adapt to changing loads. This will provide the best conversion efficiency for a single solar panel and the entire system.
The micro-inverter architecture requires a dedicated MCU so that each solar panel can manage energy conversion. However, these additional MCUs can also be used to improve system and solar panel monitoring capabilities. For example, large-scale solar panel power plants benefit from solar-to-board communication, which helps maintain load balancing and allows system administrators to plan ahead for the amount of solar energy available—and the measures that should be taken. However, to take advantage of these benefits of system monitoring, MCUs must integrate on-chip communication peripherals (CAN, SPI, UART, etc.) to simplify the connection to the other micro-inverters in the solar array.
In many applications, using a micro-inverter topology can greatly increase overall system efficiency. At the solar panel level, 30% efficiency gain is expected. However, due to the wide variety of applications, the "average" system-level increase percentage does not make much sense.
Application Analysis
When evaluating the micro-inverter values ​​for an application, several aspects of the topology should be considered. In some small installations, solar panels may receive almost the same light, temperature, and shadow conditions. In this way, micro-inverters may have only a small efficiency advantage. Maximizing the efficiency of each solar panel by operating solar panels at different voltages requires that each output voltage be normalized to the battery voltage through a DC/DC converter. In order to minimize manufacturing costs, the DC/DC converters and inverters are integrated into a single module. DC/AC converters for local line power or access to the distribution network will also be part of the module.
Solar panels must communicate with each other, which adds wiring and complexity. This is another controversy for creating a module that includes both inverters, DC/DC converters, and solar panels. Each inverter's MCU functionality must still be powerful enough to run multiple MPPT algorithms to adapt to different operating conditions. Having multiple MCUs increases the total system BOM cost. As long as the changes in the architecture are considered, the cost is a problem. To achieve the system cost goal, installing a controller for each solar panel means that the chips must have competitive costs, have a relatively small size, and still be able to handle all control, communication, and computing tasks simultaneously.
The control of peripheral devices on the integrated positive hybrid and the high degree of analog integration are fundamental factors in keeping the system low cost. High-performance algorithms are also critical. These algorithms are developed for the optimization of the efficiency of each step in the execution of optimized transformations, system monitoring, and stored procedures. By selecting an MCU that can meet most of the total system requirements, the high cost of using multiple MCUs can be reduced. In addition to the micro-inverter's own needs, these requirements include AC/DC conversion, DC/DC conversion, and communication between solar panels.
MCU features
Careful study of these advanced requirements is the best way to determine what functionality is needed for the MCU. For example, load balancing control is required when solar panels are connected in parallel. The MCU must be able to detect the load current and then increase or decrease the output voltage by turning off the output MOSFET. This requires a fast on-chip ADC to sample the voltage and current.
There is no "cookie cutter" (universal) design for a micro-inverter. This means that designers must be smart and innovative in finding new skills and methods, especially in solar panels and systems. The selected MCU should support various protocols, including some special protocols such as Power Line Communication (PLC) and Controller Area Network (CAN). In particular, power line communication can reduce system costs by removing dedicated lines for communications. However, this requires high-performance PWM functions, fast ADCs, and high-performance CPUs that are integrated into the MCU.
An unexpected but highly valuable feature of solar inverter application-specific MCUs is the on-chip oscillator, which can be used to enhance reliability of clock fault detection. The ability to run two system clocks at the same time also helps reduce problems during solar panel installation. Since solar micro-inverter designs are destined for so many innovations, perhaps the most important feature for MCUs is software programmability. This feature provides maximum flexibility in power circuit design and control.
The C2000 microcontroller has been widely used in many traditional solar panel inverter topologies due to the combination of an on-chip peripheral device with an advanced digital core that can effectively handle arithmetic calculations and some power conversion control. A more cost-effective option is the Piccolo Series C2000 microcontroller. It has a minimum 38-pin package size, functional architectural improvements, and enhanced peripherals to bring 32-bit real-time control benefits to applications such as micro-inverters that require lower total system cost.
In addition, the Piccolo MCU family integrates a 10-chip dual-chip oscillator for clock comparison, an on-chip VREG with power-on reset and shoot-through protection, multiple high-accuracy 150-psPWM, a 12-bit, and 4.6-megasample. /sec ADC, and some interfaces for I2C (PMBus), CAN, SPI, and UART communication protocols. Figure 3 shows a computer system configuration that works with a micro-inverter-based PV system.
For micro-inverters, performance is a key feature. Although Piccolo devices are cheaper and smaller in size than other C2000 MCU products, this device has many improvements, such as: Programmable floating-point control law accelerator (CLA) designed to ease complex high-speed control algorithms This allows the CPU to allocate resources for handling I/O and feedback loop metrics, resulting in up to 5x performance gains in some closed-loop applications.
PV challenge
One of the disadvantages of solar power systems is their conversion efficiency. The solar panel collects about 1 mW of average power per 100 mm2 PV cell. The general efficiency is about 10%. The power generation utilization PV source (ie, the ratio of the average generated power to the amount of power that can be generated when the sun is always illuminated) is about 15% to 20%. There are many reasons for this result, including the unpredictable changes in the sun itself, that is, the disappearance at night and the weakening of the shadows and weather conditions during the day.
PV conversion introduces more variables into the efficiency equation, including the solar panel temperature and its theoretical peak efficiency. Another problem for design engineers is that the PV cell will produce a voltage that varies about 0.5V irregularly. When choosing a power conversion topology, this change can have a serious impact. For example, poor power conversion implementation may consume a large amount of acquired PV power.
In order to adapt to the fact that the sun does not illuminate this situation 24 hours a day, the solar energy system includes some batteries and the complex electronic components needed to efficiently charge these batteries. After the battery is integrated into the system, additional DC/DC conversion must be added to charge the battery, and battery management and monitoring are also required.
Many solar systems also connect to the grid, requiring phase synchronization and power factor correction. In addition, there are several use cases that require complex controls. For example, fault prediction must be built in to prevent events such as limiting the use of electricity and power outages in the public grid. This is just an important issue that some design engineers must consider.
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