ABSTRACT:

This paper presents a novel single-stage buckboost transformer less inverter (BBTI) topology for singlephase grid-connected solar PV applications. In this topology, the input PV source shares the common ground with neutral of the grid which eliminates the leakage currents. Further, the proposed topology has the buck-boost ability which tracks the maximum power point even under the wide variation of input PV voltage. Another feature of the proposed topology is that it uses only one energy storage inductor which provides symmetric operation during both half cycles of the grid. In addition, the two out of five switches of the proposed topology operate at a line frequency, thereby, it exhibits low switching losses and the other three switches conduct in any mode of operation which incurs low conductions losses. A simple sinetriangle pulse width modulation strategy is proposed to control the proposed inverter topology is analyzed at all operating modes The simulation work is carried out using MATLAB/SIMULINK.

INTRODUCTION

Generally, the PV fed transformerless inverters suffer from leakage currents. To overcome the leakage currents the researchers have come up with numerous PV fed transformerless inverter topologies and control strategies. For example, grid-connected central or string inverter configurations consist of strings of PV panels which doesn’t require boost stage. However, the low voltage PV source requires a boost stage which reduces the efficiency of the system. Several researches have come up with the buck derived transformerless inverters which may not work during the low voltage PV source or PV source with shaded conditions. It is advisable to have transformerless inverter topologies with the buck-boost capability to have a wide operational range of PV sources. In this context, it can be understood that nowadays researchers have been showing more interest in proposing buck-boost based transformerless topologies. The authors proposed a buck-boost derived transformerless inverter topology which suits for wide range operation of the PV system. But the disadvantage of this topology is that it requires two separate PV sources for each half cycle of the output voltage. A buck-boost based transformerless topology is also proposed, which uses only four power switches and two input inductors. In this topology, each input inductor operates in either positive or negative half cycles which may lead to DC current injection. Another disadvantage of this topology is that the THD in current is more than 5% which is well beyond IEEE limits. The authors also proposed a buck-boost derived topology with a single input inductor and 5 switches. But this topology requires three extra diodes. Even though this topology has one single input inductor it requires a large input capacitor to track the maximum power from the PV source. Another disadvantage of this topology is that it has low voltage gain. The topology in can operate for a wide range of PV system. But it requires eight power switches and one single inductor. The higher switch’s count reduces the efficiency, reliability and increases the cost of the system. The proposed buck-boost derived topology reduces the switch count (i.e five switches). However, this topology requires larger input capacitance to track maximum point of solar PV. The topology also works for a wide range of PV system. In this topology, three switches conduct in every switching cycle which increases the conduction losses. Another disadvantage of this system is that it requires high current capability inductor which is large in size at the input which increases the system size, cost and reduces the efficiency. Further to reduce the switch’s count, researchers proposed a buck-boost topology with only two power switches. But this topology doesn’t have a symmetrical operation in both positive and negative half cycles of the output voltage. Another disadvantage of this topology is that the voltage across input PV should be greater than the required output voltage. Another topology was proposed in by using coupled inductor. This topology can provide high voltage gain at the output but in this topology also three power switches conduct during one switching cycle which increases the conduction losses and reduces the efficiency of the system.

PROBLEM STATEMENT

In this, a buck-boost transformerless inverter topology is proposed with only five power switches and a single input inductor at the input. The major advantages of the proposed topology are as follows: 1. Zero leakage current due to the common terminal is shared between PV and grid neutral. 2. Negligible DC current injection due to the symmetry of operation in both positive and negative half-cycles. 3. Lesser number of controllable switches which makes the system more reliable and highly efficient. 4. A wide range of PV power tracking is possible due to the presence of buck-boost operation.