IRRADIANCE-ADAPTIVE PV MODULE INTEGRATED CONVERTER FOR HIGH EFFICIENCY AND POWER QUALITY IN STANDALONE AND DC MICROGRID APPLICATIONS

Abstract

The strive for efficient and cost-effective photovoltaic systems motivated the power electronic design developed here. The work resulted in a DC-DC converter for module integration and distributed maximum power point tracking (MPPT) with a novel adaptive control scheme. The latter is essential for the combined features of high energy efficiency and high power quality over a wide range of operating conditions. The switching frequency is optimally modulated as a function of solar irradiance for power conversion efficiency maximization. With the rise of irradiance, the frequency is reduced to reach the conversion efficiency target. A search algorithm is developed to determine the optimal switching frequency step. Reducing the switching frequency may, however, compromise MPPT efficiency. Furthermore, it leads to increased ripple content. Therefore, to achieve a uniform high power quality at all conditions, interleaved converter cells are adaptively activated. The overall cost is kept low by selecting components that allow for implementing the functions at low cost. Simulation results show the high value of the module integrated converter for DC standalone and microgrid applications.

Existing System

Switching frequency modulation (SFM) utilizing fixed switching frequencies are used in controlling DC-DC converters.

Proposed System

In the proposed system Employing optimal SFM in PV systems is introduced in three main parts: the irradiance-adaptive SFM; the optimization of the SFM scheme and MIC topology; and the MPPT algorithm. The PV current increases in strength with the solar irradiance. At high irradiance, the PV-fed MIC can operate in CCM for a wide load range. Low switching frequencies in that case can contribute to a high efficiency without altering the converter mode of operation to discontinuous conduction mode (DCM). At low irradiance and due to the low supply current, the converter may move to DCM. The instantaneous power drawn from input sources is zero at the moment when the inductor current is zero in DCM. Increasing the switching frequency can keep operation in CCM. The proposed switching frequency modulation (SFM) selects an irradiance adapted switching frequency that is always high enough to avoid operation in discontinuous conduction mode. At a high irradiance, the switching frequency modulation sets a lower value for the frequency, guided by the strive for high efficiency through low switching losses. The proposed automated procedure has shown to be effective in searching for the optimal number and values of switching frequencies. Furthermore, an interleaved boost cell is activated at high irradiance to retain a high level of power quality. Hysteresis functions support the transitions between different discrete switching frequencies as the irradiance changes. The adaptive MIC control scheme is complemented by an MPPT designed for fast tracking. Thus, by combining the SFM with the adaptive usage of the boost converter interleaved cells and a fast MPPT, targets of efficiency and power quality can be reached.

Architecture

PV module integrated converter in DC microgrid application