# Advancing Grid-Connected Renewable Generation Systems
## 3 Part 2: Power Converters  
The next important step towards an energy-efficient world is the use of new materials, such as wide bandgap (WBG) semiconductors like silicon carbide (SiC) and gallium nitride (GaN), the advantages of which are increased energy efficiency, higher power density, and cooling density, thanks to an improved electrical and thermal conductivities. WBG transistors are expected to be applied to a wide range of power conversion applications, which nowadays are dominated by the traditional silicon (Si)-based power devices. In [7] the authors present a comparative analysis using a high-power converter with and without a light-load upgrade circuit and the paper includes the calculation of total (switching and conduction) losses of a high-power Si-IGBT converter and a low power SiC-MOSFET converter. In addition, the utilization of GaN transistors in such applications is considered.  
In [8], the authors discuss the dilemma between a transformer and transformerless grid-connected photovoltaic system. On one side, the transformer makes the system bulky, heavy, and increases the cost and losses, which in turn also reduces the efficiency by 1% or 2%. Correspondingly, transformerless systems are used to avoid the mentioneddisadvantages. Nonetheless, the main drawback of transformerless topologies is the presence of a leakage current between the terminals of the photovoltaic cell and the physical ground of the grid. The main efforts in [8] are leakage current reduction in transformerless inverters. Description and analysis of the common-mode voltage and its influence on the value of the leakage current are performed in the paper. At last, it is also shown that the modulation strategy is a critical factor in reducing the leakage current.  
The modular multilevel converter (MMC) is one of the promising candidates from the family of multilevel converters to be applied in PV system power conversion. Its promising advantages are quality of output performance, high modularity, simple scalability, and low voltage and current rating demand for the power switches. In [9] the authors investigate the MMC with an advanced submodule (SM) capacitor, which is a key element in the MMC. A practical method for sizing the capacitors is proposed, which takes into account the grid-fault-ride-through operation of the MMC, impact on the MMC control system, and also the aging mechanism of capacitors.  
One of the options to reduce leakage currents in grid-connected transformerless photovoltaic (PV) systems is the implementation of a proper common-mode voltage (CMV) control; in turn, CMV depends on the converter topology and modulation strategy. In [10] the authors explore a new split-input quasi-Z-source inverter (qZSI) inductor which is divided into two parts for CMV reduction. On one side, all CMV reduction strategies for voltage source inverter (VSI) can be used directly here with an appropriate shoot-through (ST) state usage. On the other side, the proposed split-inductor qZSI shows the same operating features as the original qZSI. Finally, the proposed method can be extended to other impedance source converters of a similar design.  
Understanding the thermal-mechanical stress in semiconductors is becoming very important. As semiconductors heat up, thermal expansion is induced, mechanical stresses can cause either failure or a change in the performance due to changes in electron transport in the material. In [11] the authors experimentally examine loss and thermal models of power semiconductors, shown by an example of an H-bridge circuit for emulating various loading conditions taking into consideration influential factors like power factor, current amplitude, and fundamental and switching frequencies for the loading condition in order to obtain the desired thermal stress.