Renewable Energy Through Micro Inverter

Fig. 1. A conventional power converter architecture includes a solar inverter that receives a low DC output voltage from a PV array and then generates an AC line voltage.

Power inverter is a key electronic component in solar power generation system. In some commercial applications, these modules connect photovoltaic (PV) panels, charge storage cells, and local distribution systems or public grids. Figure 1 shows a typical solar inverter, which obtains a very low voltage from the DC output of PV array, and then converts it into some combination of DC cell voltage, AC line voltage and distribution network voltage.

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

In addition, it should be noted that the microcontroller (MCU) module, tms320c2000 or MSP430 microcontroller in Figure 1 generally includes key on-chip peripheral devices such as pulse width modulation (PWM) module and a / D converter.

The main design objective is to maximize conversion efficiency. This is a complex and iterative process, involving 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 modules directly to the battery, forcing them to work at battery voltage. Almost without exception, the battery voltage is not an ideal value to collect and maximize the available solar energy.

Figure 2. Maximum power point tracking (MPPT) algorithm achieves 75W PV output compared to 53W in non MPPT system.

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

After the implementation of MPPT algorithm, the situation is very different. In this example, the voltage when the module reaches maximum power is 17V. Therefore, the function of MPPT algorithm is to make the module work at 17V to obtain full 75W power, which is independent of battery voltage.

The high-efficiency DC / DC power converter converts the 17V module voltage at the input of the controller into the battery voltage at the output. Since the DC / DC converter gradually reduces the voltage of 17V to 12V, the battery charging current of MPPT system in this example is:

Assuming that the DC / DC converter has 100% conversion efficiency, the 1.85a charging current increases to 42%.

Although this example assumes that the inverter is processing energy from a single solar panel, traditional systems generally have many solar panels connected to a single inverter. While this topology has many advantages, it also has some disadvantages. The specific situation depends on the application.

MPPT algorithm

There are three main MPPT algorithms: disturbance observation method, conductance increment method and constant voltage method. The first two methods are often called "mountain climbing" methods because they take advantage of the fact that the left curve of MPP keeps rising (DP / DV > 0) and the right curve of MPP keeps falling (DP / DV Disturbance observation (P & O) is the most common method. The algorithm perturbs the working voltage in a specific direction, and then samples DP / DV. If DP / DV is positive, the algorithm knows that it has adjusted the voltage in the MPP direction. Then, continue to adjust the voltage in this direction until DP / DV is negative.

P & O algorithms are easy to implement, but sometimes they lead to oscillations around MPP running in steady state. In addition, under rapidly changing air conditions, their response time is long, and they will 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 with P & O, Inc can quickly track changing lighting conditions more accurately. However, like P & O, it oscillates and becomes chaotic 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 takes advantage of the fact that in general, the ratio of vmpp / VOC is approximately equal to 0.76. The problem with this method is that it requires to immediately set the PV array current to 0 to measure the open circuit voltage of the array. In this way, the operating voltage of the array is set to 76% of this measured value. However, during this period, the array was disconnected, wasting effective energy. It is also found that the 76% open circuit voltage is very close to the value, but it is not always consistent with the MPP.

Since there is no MPPT algorithm that can successfully meet the requirements of all common scenarios, many designers will take some detours. They evaluate the environmental conditions of the system and then select the best algorithm. In fact, there are many MPPT algorithms that can be used, and it is common for solar panel manufacturers to provide their own algorithms.

For some cheap controllers, the implementation of MPPT algorithm will be a difficult task. Because, in addition to the normal control function of MCU, the algorithm also requires these controllers to have high-performance computing power. Advanced 32-bit real-time microcontrollers (such as some Microcontrollers in Ti C2000 platform) are suitable for many solar applications.