# Common-Ground Photovoltaic Inverters for Leakage Current Mitigation: Comparative Review
## 1 Introduction  
Among various renewable energy sources, photovoltaic (PV) is currently one of the most extensively used in the world. This is because of its abundance, easy availability, and pollution-free operation. In addition, with the rapid advancements in the material of manufacturing techniques, the cost of PV systems is continuously decreased. Therefore, it is expected to be the cheapest energy source for massive deployment in the future [1, 2, 3]. This will further increase the adoption and integration of PV systems into the utility grid. Single-phase connections are usually adopted for distribution grid connections where the power rating of PV systems is up to 10 kw [3, 4, 5, 6]. A typical PV single-phase grid-connected inverter is illustrated in Figure 1, where Q is the negative terminal of the PV panel and represents a common reference point for the output inverter voltages, \(v_{g}\) is the grid voltage at the point of common coupling (PCC), \(C_{\text{QG}}\) is the parasitic capacitance of the PV panel, and \(L_{1}\) and \(L_{2}\) are the lumped inductances from the output inverter terminals to PCC; the value of these impedancesinclude the harmonic filter impedance, the equivalent grid impedance and impedances of other stray elements. The parasitic capacitance of the PV panel (\(C_{\text{QG}}\)) is mainly measured between the PV terminals and the metal frame where PV panels are mounted, and its value depends on many factors such as the manufacturing methods of PV panels and cells, the mounting structure, weather conditions, and dust covering the PV panel. Thus, with varying atmospheric conditions, this parasitic capacitance could reach a high value [6,7]. Due to the switching nature of PV converters, a high-frequency voltage component (known as common-mode voltage) may be produced over the parasitic capacitance of the PV panel. According to the used PV converter along with the adopted switching mechanism, the frequency of this voltage component can be identified. For instance, using of H-bridge converter with unipolar switching mechanism will produce common-mode voltage with a frequency equal to the switching frequency [1,6]. This, in turn, can generate common-mode current (known as leakage current), which can badly reach a high value if resonance is excited through the circuit formed by the PV stray capacitance and the circuit inductances. With equal line inductances (\(L_{1}=L_{2}=L/2\)) of the above-shown single-phase PV converter; this resonance frequency can be determined as follow [1]:  
\[f_{res}=\frac{1}{2\pi\sqrt{LC_{\text{QG}}}} \tag{1}\]  
With the increased penetration of PV systems into the utility grid, the common-mode voltage (CMV) and the resultant ground leakage current are becoming matters of great concern for both electric utility companies and PV systems owners; this is because of the following problems, which can be arisen due to these issues [8, 9, 10, 11]:  
* Undesirable tripping of residual current protection system;
* Deterioration of electromagnetic compatibility (EMC);
* Decreasing of PV modules lifetime;
* Harmonic distortion of the current injected into the grid;
* An electric shock for persons touching the PV module. Galvanic isolation, by using of bulky line frequency (LF) transformer in the AC grid side or compact high-frequency transformer in power electronics interface, is usually adopted to deal with these safety issues. This galvanic isolation can also offer two other advantages for PV systems:  
* It can eliminate the injection of DC current into the grid. Such DC current might result in saturation of the distribution transformers and electric motors along over the grid [12,13]. In addition, it can degrade power cables over time and affect normal load operation [14,15].