PV Power Integrated Converters

Solar energy is one of the most employed renewable energy sources due to its ubiquity and sustainability. Photo-voltaic (PV) system is the most commonly used system to extract the clean solar energy due to its feature of generating electrical power with relatively simple interfacing components. Consequently, the research on the PV systems has taken on an accelerated pace and the PV modules prices are continuously falling due to the mass production and the technological improvements.

In order to maximize the energy captured from the PV sources, they are desired to operate at their maximum power points (MPPs). PV panels are preferably connected in series to generate a relatively high DC bus voltage that is capable of driving a grid or a microgrid inverter at a high system efficiency. 

However, a series string of PV elements is highly sensitive to the mismatch in the output power of the series connected PV elements. These mismatch conditions appear due to partial shading, aging of the PV panels, silicon impurities, and dust accumulation.

For illustration, a partial shading case study example is shown in Fig. 1. In this example 5 PV modules are connected in series to form a PV string. However, the 5 modules are having different I-V characteristics as shown in Fig. 2 due to the shading applied differently across them. 

Fig. 1. PV Shading Conditions Example
Fig. 2. The I-V characteristics of the 5 PV elements under partial shading conditions.

The P-I characteristics of the integrated system is shown in Fig. 3 when every PV module is equipped with a bypassing diode. Although the total available power is 428.95 w, but the global MPP of the system is at 331.5 w.

 Moreover, local MPPs will be created in the system that will occur due to the operation of the bypass diodes as illustrated in the Fig. For example, if the string current exceeds PV-2 and PV-5 short circuit current of 3.15 A, then these modules will be shorted leading to a significant drop in the system output power. The same phenomena is repeated at the short circuit current values of PV-4, and PV-3 respectively.

 The creation of local MPPs adds to the complexity of the MPP tracking topology that needs to be applied in the studied system in order to track the global MPP and avoid the local ones. The results of the illustrating example shows that for the given shading conditions, the use of DICs would lead to a minimum of 30% increment in the captured energy.

Fig. 3. The P-V Characteristics of the overall system using a centralized MPP tracker

The centralized technologies are incapable of dealing with the power mismatch challenge. Thus, distributed integrated converters (DICs) for PV systems are being developed on an accelerated path. Also known as submodule integrated converters (SMICs), the DICs are used to individually track the MPP of the PV elements in order to capture more solar energy by allowing the PV elements to operate at different current and voltage levels. Different structures of the DICs are studied in the literature and applied in the solar PV industry as shown in Fig. 4. 

Fig. 4. PV Power Electronic Interfacing Systems

Although the mismatch current between the series connected PV elements is a fraction of their total power, but the configurations shown in Fig.4 process the total PV elements power through their interfacing power converters for individual MPP tracking.

 Thus, differential power processing (DPP) is proposed in this project to allow for fractional power processing while tracking the PV elements MPPs. The proposed DPP configuration increases the system efficiency by the means of decreasing the processed current through the DICs to only a fraction of the PV panel current values.

 The deliverables of this project are:

1- An evaluation of the current available power electronic solutions of the PV power mismatch problem (including to micro-invert & to power optimize).
2- A newly enhanced power electronic configuration to process the PV mismatch power based on the differential power processing concept.
3- A working prototype converter with feedback control tested capacilities.
4- An IEEE publication.