Optimizing Solar Systems with Microinverters

A relatively new approach to optimizing the efficiency and reliability of solar energy systems is to use micro-inverters connected to each solar panel. Equipping each solar panel with a separate micro-inverter allows the system to adapt to changing load and weather conditions, thereby providing the best conversion efficiency for the individual panel and the entire system.

Microinverter architecture also simplifies wiring, which means lower installation costs. By making consumers' solar power systems more efficient, the time it takes for the system to "pay back" the initial investment in solar technology is shortened.

Power inverters are key electronic components in solar power systems. In commercial applications, these components connect photovoltaic (PV) panels, batteries that store electrical energy , and the local power distribution system or utility grid. Figure 1 shows a typical solar inverter, which converts the very low DC voltage from the output of the PV array into several voltages, including battery DC voltage, AC line voltage, and distribution grid voltage.

In a typical solar harvesting system, multiple solar panels are connected in parallel to an inverter that converts the variable DC output from multiple photovoltaic cells into a clean 50Hz or 60Hz sine wave inverter power.

Additionally, it should be noted that the microcontroller (MCU) module TMS320C2000 or MSP430 in Figure 1 typically includes key on-chip peripherals such as a pulse width modulation (PWM) module and an A/D converter.

Figure 1: Traditional power conversion architecture consists of a solar inverter that receives the low DC output voltage from the PV array and generates AC line voltage.

The main goal of the design is to maximize the conversion efficiency. This is a complex and iterative process that involves a maximum power point tracking algorithm (MPPT) and a real-time controller that executes the algorithm.

Maximizing power conversion efficiency

Inverters that do not use an MPPT algorithm simply connect the PV modules directly to the battery, forcing the PV modules to operate at the battery voltage. Almost without exception, the battery voltage is not ideal for collecting the most available solar energy.

Figure 2 illustrates the conventional current/voltage characteristics of a typical 75W PV module at a cell temperature of 25°C. The dashed line shows the ratio of voltage (PV VOLTS) to power (PV WATTS).

The solid line represents the ratio of voltage to current (PV AMPS). As shown in Figure 2, at 12V, the output power is approximately 53W. In other words, by forcing the PV module to operate at 12V, the output power is limited to approximately 53W.

But with the MPPT algorithm, the situation changes radically. In this case, the voltage at which the module can achieve maximum output power is 17V. Therefore, the MPPT algorithm's job is to operate the module at 17V, so that the full 75W of power can be obtained from the module regardless of the battery voltage.

The high-efficiency DC/DC power converter converts the 17V voltage at the controller input to the battery voltage at the output. Since the DC/DC converter steps down the voltage from 17V to 12V, the battery charging current in the MPPT-enabled system in this example is: (VMODULE/VBATTERY) × IMODULE, or (17V/12V) × 4.45A = 6.30A.

Assuming the conversion efficiency of the DC/DC converter is 100%, the charging current will increase by 1.85A (or 42%).

While this example assumes that the inverter is processing energy from a single solar panel, a traditional system typically has one inverter connected to multiple panels. This topology has both advantages and disadvantages, depending on the application.

MPPT Algorithm

There are three main types of MPPT algorithms: perturb-and-observe, conductance increment, and constant voltage. The first two methods are often called “hill climbing” methods because they are based on the fact that to the left of the MPP, the curve is rising (dP/dV>0), while to the right of the MPP, the curve is falling (dP/dV<0).

The perturb-and-observe (P&O) method is the most commonly used. The algorithm perturbs the operating voltage in a given direction and samples the dP/dV. If the dP/dV is positive, the algorithm "understands" that it has just adjusted the voltage toward the MPP. It will then continue to adjust the voltage in this direction until the dP/dV becomes negative.

P&O algorithms are easy to implement, but they can sometimes oscillate around the MPP in steady-state operation. They are also slow to respond and can even go in the wrong direction in rapidly changing weather conditions.

The incremental conductance (INC) method uses the incremental conductance dI/dV of the PV array to calculate the sign of dP/dV. INC can track rapidly changing light irradiance conditions more accurately than P&O. But like P&O, it can also oscillate and be "fooled" by rapidly changing atmospheric conditions. Another disadvantage is that the added complexity increases the calculation time and reduces the sampling frequency.

The third method, the "constant voltage method", is based on the fact that, in general, VMPP/VOC≈0.76. The problem with this method is that it requires the current of the PV array to be instantly adjusted to zero to measure the open-circuit voltage of the array. Then, the operating voltage of the array is set to 76% of this measured value. However, during the period when the array is disconnected, the available energy is wasted. It has also been found that although 76% of the open-circuit voltage is a good approximation, it is not always consistent with the MPP.

Since no MPPT algorithm can successfully meet the requirements of all common usage environments, many design engineers let the system first estimate the environmental conditions and then choose the algorithm that best suits the environmental conditions at that time. In fact, there are many MPPT algorithms available, and it is not uncommon for solar panel manufacturers to provide their own algorithms.

For cheap controllers, it is not easy to implement the MPPT algorithm in addition to the normal control functions of the MCU itself, which requires these controllers to have superb computing power. Advanced 32-bit real-time microcontrollers such as the Texas Instruments C2000 platform series are suitable for various solar applications.

Power Inverter

There are many benefits to using a single inverter, the most prominent of which are simplicity and low cost. The use of MPPT algorithms and other techniques improves the efficiency of single-inverter systems, but only to a certain extent. Depending on the application, the disadvantages of a single inverter topology can be significant. The most prominent is the reliability issue: as long as this inverter fails, all the energy produced by the panels is wasted until the inverter is repaired or replaced.

Even if the inverter is working properly, a single inverter topology can have a negative impact on system efficiency. In most cases, each solar panel has different control requirements to achieve maximum efficiency. Factors that determine the efficiency of each panel include manufacturing differences in the photovoltaic cell components contained in the panel, different ambient temperatures, different light intensities (the amount of raw energy received from the sun) caused by shadows and orientation.

Compared to using one inverter for the entire system, equipping each solar panel in the system with a microinverter will once again increase the conversion efficiency of the entire system. The main benefit of the microinverter topology is that even if one of the inverters fails, energy conversion can still be carried out.

Other benefits of using microinverters include the ability to adjust the conversion parameters of each solar panel using high-resolution PWM. Since clouds, shadows, and shading change the output of each panel, equipping each panel with a unique microinverter allows the system to adapt to changing load conditions. This provides optimal conversion efficiency for each panel and the entire system.

Microinverter architectures require each panel to have a dedicated MCU to manage the energy conversion. However, these additional MCUs can also be used to improve system and panel monitoring.

For example, large solar farms benefit from inter-panel communication to help maintain load balance and allow system managers to plan ahead how much energy is available and what to do with it. However, to fully benefit from system monitoring, the MCU must integrate on-chip communication peripherals (CAN, SPI, UART, etc.) to simplify interfacing with other microinverters in the solar array .

In many applications, using a microinverter topology can significantly improve overall system efficiency. At the panel level, efficiency improvements of 30% are expected. However, because each application varies greatly, the "average" percentage of system-level improvement is not very meaningful.

Application Analysis

When evaluating the value of a micro inverter in a specific application, several aspects of the topology should be considered.

In small applications, each panel is likely to experience essentially the same conditions of light, temperature, and shadow, so microinverters can only do so much to improve efficiency.

In order to make each panel work at different voltages to achieve the highest energy efficiency, a DC/DC converter is required to unify the output voltage of each panel to the working voltage of the energy storage battery. To minimize manufacturing costs, the DC/DC converter and inverter can be designed into one module. The DC/AC converter for local power lines or connecting to the distribution network can also be integrated into the module.

Solar panels have to communicate with each other, which adds wiring and complexity. This is another argument for including inverters, DC/DC converters and solar panels in modules.

Each inverter’s MCU must still have sufficient power to run multiple MPPT algorithms to accommodate different operating environments.

Using multiple MCUs will increase the material cost of the overall system.

Whenever a change in architecture is considered, the focus is on cost. To meet the price target of the system, having a controller for each panel means that the controller must be cost competitive, small in size, but still capable of handling all control, communication and computing tasks simultaneously.

The integration of appropriate control peripherals on-chip and high analog integration are two essential elements to ensure low system cost. High performance is also required to execute algorithms developed to optimize efficiency in various aspects of conversion, system monitoring and energy storage.

Using an MCU that can handle most of the requirements of the entire system, including AC/DC conversion, DC/DC conversion, and inter-panel communication , in addition to the requirements of the micro-inverter itself , can reduce the cost increase caused by using multiple MCUs.

MCU Features

Carefully weighing these high-level requirements is the best way to determine what features are needed from the MCU. For example, load balancing control is required when paralleling panels. The selected MCU must be able to sense the load current and be able to increase or decrease the output voltage by turning the output MOSFET on or off. This requires a high-speed on-chip ADC to sample the voltage and current.

There is no "one size fits all" model for micro inverter design. This means that designers must have the ability and innovative spirit to adopt new techniques and technologies, especially in terms of communication between panels and systems. The most suitable MCU should support a variety of protocols, including some that you would not normally think of, such as power line communication (PLC) and controller area network (CAN). Power line communication, in particular, can reduce system costs because it no longer requires dedicated communication lines. However, this requires the MCU to have built-in high-performance PWM, high-speed ADC and high-performance CPU.

An unexpected but valuable feature of MCUs designed for solar inverter applications is the dual on-chip oscillators that can be used for clock failure detection to improve reliability. The ability to run two system clocks simultaneously also helps reduce problems when installing solar panels.

With so much innovation going into solar microinverter design, perhaps the most important feature of an MCU is its software programmability, which allows for maximum flexibility in power circuit design and control.

The C2000 microcontroller is equipped with an advanced digital computing core that can efficiently handle algorithm operations and an on-chip peripheral set for energy conversion control, which has been widely used in traditional solar panel inverter topologies. The newly launched Piccolo series C2000 series microcontroller is an economical model. The smallest package of the series has only 38 pins, but its architecture is more advanced and the peripherals are enhanced, which can bring the benefits of 32-bit real-time control to applications such as micro inverters that require low overall system cost.

In addition, each product in the Piccolo MCU series integrates two on-chip 10MHz oscillators for clock comparison, on-chip VREG with power-on reset and brownout protection, multiple high-resolution 150ps PWMs, a 12-bit 4.6 megasample/second ADC, and communication protocol interfaces such as I2C (PMBus), CAN, SPI, and UART. Figure 3 shows a computer system configuration used with a micro-inverter-based photovoltaic system.

Figure 3: The MCU system for a microinverter PV-based system includes the CPU, memory, power and clock, and peripherals.

Performance is a key feature of micro inverters. Although the Piccolo family of devices is smaller and cheaper than other C2000 MCU products, its functions have been improved, such as it has a programmable floating-point control law accelerator (CLA) that can offload the CPU from processing complex high-speed control algorithms, thereby freeing the CPU from processing I/O and feedback loops, which can improve performance by 5 times in closed-loop applications.

The challenges of photovoltaic cells

One of the drawbacks of solar-based power systems is conversion efficiency. Solar panels can extract about 1mW of average power from every 100mm2 of photovoltaic cells. Typical efficiency is about 10%. The power factor of photovoltaic power (that is, the ratio of the average power actually generated by solar cells to the theoretical power that can be generated under the condition of constant sunlight) is about 15% to 20%. There are many reasons for this result, including the variability of sunlight itself, such as disappearing completely at night, and even during the day, shadows and weather conditions often lead to reduced light.

The photovoltaic conversion introduces more variables into the efficiency calculation, including the temperature of the solar panel and its theoretical peak efficiency. Another problem for design engineers is that the voltage generated by the photovoltaic cell varies irregularly by about 0.5V. This variation can have serious consequences when choosing an energy conversion topology. For example, for an inefficient energy conversion technology, it is possible to consume a large portion of the collected photovoltaic power.

To accommodate the fact that the sun does not shine 24 hours a day, solar-powered systems include batteries and the complex electronics required to charge them efficiently. When batteries are integrated into the system, battery charging requires additional DC/DC conversion circuitry, as well as battery management and monitoring.

Many solar-powered systems also interface with the grid, requiring phase synchronization and power factor correction. There are also many usage scenarios that require complex control. For example, fault warning mechanisms must be built in to prevent events such as power outages on the public grid. These are just the top issues that design engineers must consider.