## 6 Various Criteria for Assessing the Power Quality and Efficiency of the Studied System
### Power Quality
#### 6.1.3 Zero-Sequence Voltage (ZSV)  
It is widely recognized that unbalanced three-phase voltages can be decomposed into three distinct sets of voltage components [126]. These components, known as \"symmetrical components,\" are classified into positive sequence, negative sequence, and zero sequence. Their schematic representation is shown in Figure 17a. The positive and negative sequence components represent three-phase, rotationally balanced phases, while the zero sequence components correspond to a phase with a zero-phase angle. To provide a clearer understanding of this decomposition, Figure 17b illustrates how an unbalanced three-phase voltage breaks down into its symmetrical voltage components. Unlike positive and negative sequence currents, zero-sequence currents present a unique challenge. Unlike the former, which compensate for each other, zero-sequence currents add arithmetically at the neutral point of a three-phase, four-wire system. This accumulation can lead to the overloading of the neutral conductor or cause a higher voltage between neutral and earth. Additionally, harmonic currents of any sequence flowing through an AC drive can increase the RMS current, resulting in higher system losses, increased THD for current or voltage, and potential overheating and vibration issues in the machine [135]. \begin{table}
\begin{tabular}{c c c} \hline \hline \(\mathbf{N}^{\circ}\) & **Vectors** & **CMV Levels** \\ \hline \hline \(1\) & \(V_{8}V_{78}\) & \(-V_{dc}/2\) \\ \hline \(2\) & \(V_{11}V_{8,V}\)\(V_{3}V_{8,V}\)\(V_{5}V_{8,V}\)\(V_  
In a DTLI-powered configuration with a single DCB source, zero-sequence currents (ZSCs) can occur due to ZSV. This voltage arises from the asymmetry of the instantaneous pulse width modulated phase voltages applied to the machine windings, which results from the use of spatial voltage vectors. The corresponding equivalent diagram of the zero-sequence loop is shown in Figure 18. In this diagram, \((L_{0})\) represents the inductance of the homopolar sequence, \((R)\) is the resistance of the phase winding, and \(V_{cmvj}\) and \(V_{cmvj}\) are the CMVs generated by the two inverters in the system, respectively. The ZSV can be expressed in terms of the CMV of each inverter \(\left(V_{cmvj},\ V_{cmvj}\right)\) as follows [62]:  
\[V_{zsv}=V_{cmvj}-V_{cmvj} \tag{20}\]  
\[V_{zsv}=\frac{\left(S_{i1}-S_{j1}\right)+\left(S_{i3}-S_{j3}\right)+\left(S_{i5 }-S_{j5}\right)}{3}V_{dc} \tag{21}\]  
In the DTLI configuration, the ZSV resulting from the 64 space vector combinations can be calculated using Equation (21), as shown in Table 4. From Table 3, it is clear that the zero vectors \((V_{i7}V_{i7},\ V_{8}V_{8})\) and the intermediate vectors \((V_{i1}V_{j3},\ V_{i2}V_{j4},\ V_{i3}V_{j5},\ V_{i4}V_{j6},\ V_{i5}V_{j1},\ V_{i6}V_{j2},\ V_{i1}V_{j5},\ V_{i2}V_{j6},\ V_{i3}V_{j1},\ V_{i4}V_{j2},\ V_{i5}V_{j3},\ V_{i6}V_{j4})\) eliminate the ZSV. Conversely, large vectors \((V_{i1}V_{A},\ V_{i2}V_{i5},\ V_{i3}V_{j6},\ V_{i4}V_{j1},\ V_{i5}V_{j2},\ V_{i6}V_{j3})\) and zero vectors \((V_{8}V_{7},\ V_{7}V_{8})\) generate various levels of ZSV. Tables 3 and 4 demonstrate that vectors generating ZSV effectively eliminate CMV. In contrast, vectors designed to eliminate ZSV inevitably introduce CMV. These results highlight the complex interplay between the vectors responsible for both generating and reducing the CMV and ZSV components. * ZSV reduction The ZSV issue generates ZSCs, leading to the presence of triple current harmonics [137]. This current can be harmful to motor windings, resulting in increased copper losses and  
Figure 17: (a) Symmetrical components. (b) Decomposition of an unbalanced three-phase voltage into symmetrical components. Figure 18: ZSV equivalent circuit of an OEWM. potential circuit saturation. Several methods have been proposed to eliminate this voltage in DTLI configurations.