The traditional grid -connected PV inverter includes either a line frequency or a high frequency transformer between  the inverter and grid acting as isolation transformer.

In order to increase the efficiency, to reduce the size and cost, the effective solution is to remove the isolation transformer. It leads to appearance of common mode (CM) ground leakage current due to parasitic capacitance between the PV panels and the ground. The common mode current reduces the efficiency of power conversion stage,affects the quality of grid current, deteriorate the electric magnetic

compatibility and give rise to the safety threats. In order to eliminate the common mode leakage current in transformerless PV system, the concept of virtual DC bus is proposed by connecting the grid neutral line directly to the negative pole of the DC bus, the stray capacitance between the PV panels and the ground is bypassed. The CM ground leakage current can be suppressed completely. Virtual DC bus is created to provide the negative voltage level for the negative AC grid current generation.

This concept, a novel transformerless inverter topology is derived, in which the virtual dc bus is realized with the switched capacitor technology. It consists of only five power switches, two capacitors, and a single filter inductor. Therefore, the power electronics cost can be curtailed. This advanced topology can be modulated with the unipolar sinusoidal pulse width modulation (SPWM) and the double frequency SPWM to reduce the output current ripple. As a result, a smaller filter inductor can be used to reduce the size and magnetic losses.

   The following references may be useful.

1.   http://ijeit.com/vol%202/Issue%2012/IJEIT1412201306_02.pdf

An Improved Transformerless Inverter For Grid -Connected Photovoltaic Power Systems

              V.Abinaya,       B.Prabakaran,       R.B.Selvakumar

2.   http://www.ijrde.com/attachments/article/116/11.pdf

A Novel Inverter with Virtual DC Bus for Non Linear Load PV Power System

                     E.Nirmal kumar , S.kamalakkannan

3.    http://www.iraj.in/up_proc/pdf/85-1403613104170-177.pdf

AN IMPROVED TRANSFORMER LESS INVERTER TOPOLOGY FOR COST   EFFECTIVE PV SYSTEMS

                GEETHU CHACKO,             RIYA SCARIA

4.  http://ieeexplore.ieee.org/xpl/articleDetails.jsp?arnumber=6213561

IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 28, NO. 2, FEBRUARY 2013

Transformerless Inverter With Virtual DC Bus Concept for Cost-Effective Grid-Connected PV Power Systems

    Yunke Gu et al

The advantageous circuit performances of the proposed transformer-

less topology are analyzed in detail, with the results verified by a 500-W prototype.

P.S.

Dear Vivekanandan Kandiannan

in contrast to what  Pisupati Sadasiva Subramanyam stated, there is a further interpretation:

If you consider the converter to be an AC/AC converter instead of a PV-inverter you end up with a so-called matrix converter. This topology does not feature a DC-link at all but as described by Venturini et.al., there is a possibility to understand this converter by introducing a virtual DC-link.

A transformer is not necessary in this sort of device as long as the application driven can operate at AC voltages slightly lower than the input voltage to the converter and galvanic isolation is not demanded.

Mabe this paper:

https://www.researchgate.net/publication/260712330_Design_Challenges_in_the_Use_of_Silicon_Carbide_JFETs_in_Matrix_Converter_Applications?ev=prf_pub

can help to understand matrix converters.

Article Design Challenges in the Use of Silicon Carbide JFETs in Mat...

Thanks Pisupati Sadasiva Subramanyam and Martin Schulz. Now i got a clear idea about it.