Renewable energy through microinverters

A newer way to optimize the efficiency and reliability of solar energy systems is to use microinverters connected to each individual solar panel. Installing its own microinverter for each solar panel allows the system to adapt to its changing load and air environment, providing the best conversion efficiency for the individual solar panel and the entire system. The microinverter architecture also enables simpler wiring, resulting in lower installation costs. By improving the efficiency of the user's solar energy system, the initial technical investment payback time of the system can be shortened.

Figure 1. The traditional power converter architecture consists of a solar inverter that receives low DC output voltage from a PV array and generates AC line voltage.

Power inverters are key electronic components in solar power systems. In some commercial applications, these components connect photovoltaic (PV) panels, batteries that store charge, and local distribution systems or utility grids. Figure 1 shows a typical solar inverter, which takes a very low voltage from the PV array DC output and converts it to some combination of DC battery voltage, AC line voltage, and distribution grid voltage.

In a typical solar energy harvesting system, multiple solar panels are connected in parallel to a single inverter, which converts the variable DC output of the multiple PV cells into a clean sinusoidal 50-Hz or 60-Hz voltage source.

In addition, it should be noted that the microcontroller (MCU) module in Figure 1, the TMS320C2000 or MSP430 microcontroller generally includes key on-chip peripheral devices such as pulse width modulation (PWM) modules and A/D converters.

The main design goal is to maximize conversion efficiency. This is a complex, iterative process involving algorithms (maximum power point tracking, MPPT) and real-time controllers that execute them.

Maximizing Power Conversion

Inverters that do not use an MPPT algorithm simply connect the modules directly to the battery, forcing them to operate at the battery voltage. Almost without exception, the battery voltage is not ideal for harvesting the maximum amount of available solar energy.

Figure 2. The maximum power point tracking (MPPT) algorithm achieves 75W PV output compared to 53W for a non-MPPT system.

Figure 2 The maximum power point tracking (MPPT) algorithm achieves 75W PV output compared to 53W for a non-MPPT system.

Figure 2 depicts the conventional current/voltage characteristics for a typical 75W module and 25°C cell temperature. The dashed line represents the relationship between voltage (PV volts) and power (PV watts). The solid line represents the relationship between voltage and current (PV amps). As shown in Figure 2, at 12V, the output power is approximately 53W. In other words, after forcing the PV module to operate at 12V, the power is limited to approximately 53W.

After implementing the MPPT algorithm, the situation is very different. In this case, the voltage at which the module reaches maximum power is 17V. Therefore, the MPPT algorithm works by operating the module at 17V to obtain the 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 steps down 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 the DC/DC converter has 100% conversion efficiency, the 1.85A charging current is increased by 42%.

Although this example assumes that the inverter is processing energy from a single solar panel, a traditional system typically has many solar panels connected to a single inverter. This topology has many advantages as well as some disadvantages, depending on the application.

MPPT Algorithm

There are three main MPPT algorithms: perturbation and observation, conductance increment, and constant voltage. The first two methods are often called "hill climbing" methods because they take advantage of the fact that the left side of the MPP curve is constantly rising (dP/dV>0) and the right side of the MPP curve is constantly falling (dP/dV<0).

The most common is the perturb and observe (P&O) algorithm. This algorithm perturbs the operating voltage in a specific direction and then samples the dP/dV. If the dP/dV is positive, the algorithm knows that it has adjusted the voltage toward the MPP. It then continues to adjust the voltage in that direction until the dP/dV is negative.

P&O algorithms are easy to implement, but sometimes they can cause oscillations around the MPP in steady-state operation. Also, they have a long response time under rapidly changing air conditions and can even track in the wrong direction.

The incremental conductance (INC) method uses the incremental conductance dI/dV of the PV array to calculate the sign of dP/dV. Compared to P&O, INC tracks changing light conditions more accurately and quickly. However, like P&O, it can produce oscillations and become confused by rapidly changing air conditions. Another disadvantage is that its high complexity increases calculation time and reduces sampling frequency.

The third method is the constant voltage method, which uses the fact that, in general, the ratio VMPP/VOC is approximately equal to 0.76. The problem with this method is that it requires the PV array current to be set to zero immediately to measure the array's open circuit voltage. Thus, the array's operating voltage is set to 76% of this measured value. However, during this period, the array is disconnected, wasting useful energy. It was also found that while the 76% open circuit voltage is a very close value, it is not always consistent with the MPP.

Since there is no single MPPT algorithm that can successfully meet the requirements of all common scenarios, many designers take a detour and evaluate the environmental conditions of the system and then 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.

Implementing the MPPT algorithm can be a difficult task for some inexpensive controllers because, in addition to the normal control functions of the MCU, the algorithm requires these controllers to have high-performance computing capabilities. Advanced 32-bit real-time microcontrollers (such as some of the microcontrollers in the TI C2000 platform) are suitable for many solar applications.

Power Inverter

Using a single inverter has many advantages, the most prominent of which are simplicity and low cost. Using MPPT algorithms and other techniques can improve the efficiency of a single-inverter system, but only to a certain extent. The downside of a single-inverter topology is obvious, but it depends on the application. The biggest concern is reliability: if an inverter fails, all the energy generated by the solar panels will be lost until the inverter is repaired or replaced.

Even when it works perfectly, a single inverter topology can negatively impact system efficiency. In most cases, each solar panel has different control requirements to achieve maximum efficiency. Some of the factors that determine the efficiency of each solar panel include manufacturing differences in the PV cells that make up its components, differences in ambient temperature, and varying degrees of insolation (the amount of raw energy received from the sun) due to shade and direction of sunlight.

The overall system conversion efficiency can be further improved by installing a microinverter for each individual solar panel instead of using a single inverter for the entire system. The main benefit of the microinverter topology is that energy will continue to be converted even in the event of an inverter failure.

Some other benefits of the microinverter approach include the ability to adjust the conversion parameters of each solar panel using high-precision PWM. Since clouds, shadows, and shading can change the output of individual solar panels, installing microinverters for each solar panel allows the system to adapt to changing loads. Doing so provides the best conversion efficiency for individual solar panels as well as the entire system.

Microinverter architectures require a dedicated MCU to enable each solar panel to manage energy conversion. However, these additional MCUs can also be used to improve system and panel monitoring capabilities. For example, large solar panel power plants benefit from inter-panel communication, which helps maintain load balance and allows system managers to plan ahead how much solar energy will be available - and what actions should be taken. However, to take advantage of these benefits of system monitoring, the MCU must integrate on-chip communication peripherals (CAN, SPI, UART, etc.) to simplify connections with other microinverters in the solar array.

In many applications, using a microinverter topology can greatly improve total system efficiency. At the panel level, 30% efficiency improvements are expected. However, because applications vary widely, an “average” system-level improvement percentage is not very meaningful.

Application Analysis

When evaluating the value of a microinverter for an application, several aspects of the topology should be considered.

In some small installations, the panels may experience nearly identical light, temperature, and shade conditions, so microinverters may offer only a small efficiency advantage.

Maximizing the efficiency of each solar panel by operating the panels at different voltages requires that each output voltage be normalized to the battery voltage via a DC/DC converter. To minimize manufacturing costs, the DC/DC converter and inverter are integrated into a single module. A DC/AC converter for local line power or access to the distribution grid will also be part of the module.

The panels must communicate with each other, which adds wiring and complexity. This is another point of contention in creating a module that contains the inverter, DC/DC converter and solar panels at the same time.

Each inverter’s MCU must still be powerful enough to run multiple MPPT algorithms to accommodate different operating conditions.

Having multiple MCUs increases the total system bill of materials cost.

Cost is an issue whenever architectural changes are considered. Meeting system cost targets by installing a controller for each solar panel means that the chip must be competitively priced, relatively small in size, and still be able to handle all control, communication, and computation tasks simultaneously.

Integration of on-chip control peripherals and a high degree of analog integration are essential factors in keeping the system cost low. Also critical are high-performance algorithms that are developed to optimize efficiency at each step of the execution optimization conversion, system monitoring and storage process.

The high cost of using multiple MCUs can be reduced by selecting an MCU that meets most of the total system requirements. These requirements include AC/DC conversion, DC/DC conversion, and communication between solar panels, in addition to some requirements of the microinverter itself.

MCU Features

Taking a closer look at these high-level requirements is the best way to determine what functions are needed from 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" (one size fits all) design for microinverters. This means that designers must be creative and innovative in finding new tricks and methods, especially for inter-panel and inter-system communication. The selected MCU should support a variety of protocols, including some special protocols such as power line communication (PLC) and controller area network (CAN). Power line communication in particular can reduce system cost by removing dedicated lines for communication. However, this requires high-performance PWM functions, fast ADCs, and high-performance CPUs integrated into the MCU.

An unexpected but valuable feature in MCUs dedicated to solar inverter applications is dual on-chip oscillators, which can be used for clock failure detection to enhance reliability. The ability to run two system clocks simultaneously also helps reduce problems during solar panel installation.

With so much innovation set to take place in solar microinverter designs, perhaps the most important feature for MCUs is software programmability, which allows for maximum flexibility in power circuit design and control.

With an advanced digital computing core that can effectively handle algorithmic calculations and a combination of on-chip peripheral devices for power conversion control, C2000 microcontrollers have been widely used in many traditional solar panel inverter topologies. A lower-cost option is the Piccolo series C2000 microcontroller. It has a minimum 38-pin package size, functional architecture improvements, and enhanced peripheral devices to bring the advantages of 32-bit real-time control to applications such as micro inverters that require lower total system costs.

Figure 3. The MCU system configuration of a microinverter-based PV system includes the CPU, memory, power supply and clock, and some peripheral devices.

In addition, the various members of the Piccolo MCU family integrate dual on-chip 10-MHz oscillators for clock comparison, on-chip VREG with power-on reset and shoot-through protection, multiple high-precision 150-ps PWMs, a 12-bit and 4.6-Msample/s ADC, and interfaces for I ² C (PMBus), CAN, SPI, and UART communication protocols. Figure 3 shows a computer system configuration that works with a microinverter-based PV system.

Performance is a key feature for micro inverters. Although Piccolo devices are cheaper and smaller than other C2000 MCU products, they have many improvements, such as the programmable floating-point control law accelerator (CLA) designed to offload complex high-speed control algorithms, allowing the CPU to allocate resources to handle I/O and feedback loop metrics, resulting in up to 5 times performance improvement in some closed-loop applications.

PV Challenge

One of the drawbacks of solar power systems is conversion efficiency. Solar panels collect about 1mW of average power from each 100-mm2 ^PV cell. Typical efficiencies are around 10%. The power utilization of a PV source (i.e., the average power generated compared to the amount of power that could be generated if the sun was always shining) is about 15% to 20%. There are many reasons for this, including the variability of sunlight itself, which disappears completely at night and is often reduced during the day by shadows and weather conditions.

PV conversion introduces more variables into the efficiency equation, including the temperature of the solar panel and its theoretical peak efficiency. Another problem for design engineers is that the PV cell produces a voltage that varies erratically by about 0.5V. This variation can have serious consequences when choosing a power conversion topology. For example, a poor implementation of power conversion technology can consume a large amount of the harvested PV power.

To accommodate the fact that the sun does not shine 24 hours a day, solar energy systems include batteries and the complex electronics required to efficiently charge these batteries. Once the batteries are integrated into the system, additional DC/DC conversion must be added for battery charging, while battery management and monitoring are also required.

Many solar systems are also grid connected, requiring phase synchronization and power factor correction. In addition, there are several use cases that require complex control. For example, fault prediction must be built in to prevent events such as brownouts and blackouts on the utility grid. These are just some of the important issues that design engineers must consider.