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.

In addition, it should be pointed out that the microcontroller (MCU) module TMS320C2000 or MSP430 in Figure 1 usually contains key on-chip peripherals such as pulse width modulation (PWM) modules and A/D converters.

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 25°C cell temperature . 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 the 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 single MPPT algorithm can successfully meet all common usage environment requirements, many design engineers will let the system first estimate the environmental conditions and then choose the algorithm that best suits the current environmental conditions. 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 Inversion

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.