## 2 Establishment of the Steady-State Power Quality Comprehensive Evaluation System  
### Introduction to Power Quality Indicators  
The integration of a photovoltaic power generation system will affect the power quality in frequency, voltage fluctuation, flicker, voltage total harmonic distortion, and so on. Based on the content of China's national standard for power quality [20; 21; 22; 23; 24], we choose six indicators as the steady-state power quality indicators: frequency deviation, voltage deviation, three-phase voltage unbalance, voltage fluctuation, voltage total harmonic distortion, and voltage flicker. The allowable limits of these indicators in China's national standards are shown in Table 1[25]. The power quality indicators are introduced below [26]. In power systems, the frequency deviation can be determined using Equation (1),  
\[\Delta f=f_{s}-f_{N} \tag{1}\]  
where \(f_{s}\) is the real-time frequency monitored and \(f_{N}\) is the standard frequency value. Voltage total harmonic distortion is a commonly used parameter to measure the level of harmonics, and its calculation is shown in Equation (2). \[THD_{u}=\frac{\sqrt{\sum\limits_{h=1}^{\infty}U_{h}^{2}}}{U_{1}} \tag{2}\]  
where \(THD_{u}\) represents the voltage total harmonic distortion, \(U_{1}\) represents the fundamental voltage root mean square (RMS) value and \(U_{h}\) represents each harmonic voltage RMS value. The voltage fluctuation value can be determined using Equation (3). \[d=\frac{U_{\max}-U_{\min}}{U_{N}}\times 100\% \tag{3}\]  
where \(U_{\max}\) and \(U_{\min}\) represent two adjacent extreme values of the voltage RMS value, respectively, and \(U_{N}\) represents the rated voltage value. \begin{table}
\begin{tabular}{c c c} \hline \hline
**Standard Number** & **Standard Name** & **Allowed Limit Values** \\ \hline \multirow{3}{*}{GB/T 15945-2008} & Power quality—Frequency deviation & The allowable deviation under normal conditions is \(\pm\)0.2 \(H\)z; it can also be changed to \(\pm\)0.5 \(H\)z according to the system capacity. \\  & & Quality of electric energy supply & The allowable upper distortion rate is 4\% for the rated voltage of 6–10 kV, and 5\% for 380 V. \\  & & & For the point of common coupling, normal allowable unbalance is 2\%; short time allowable unbalance is 4\%. For each user, normal allowable unbalance is 1.3\%; short time allowable unbalance is 2.6\%. \\  & & & The allowable deviation is \(\pm\)5\% for the rated voltage of \(\geq\)35 kV, and \(\pm\)7\% for \(\leq\)10 kV. \\  & & & The allowable voltage variation varies with its frequency, and, if the variation is random and irregular, the allowable voltage variation is 2.5\% for high voltage and 3\% for others \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The allowable limits of steady-state power quality indicators in China’s national standards. The shortterm and long-term flicker values are two quantitative parameters used to quantify voltage flicker. The measurement period of the short-term flicker value is 10 min, and it can be determined from Equation (4). The measurement period of the long-term flicker value is 2 h, and the value is calculated from the short-term flicker value within 2 h, as shown in Equation (5). \[P_{st}=\sqrt{0.0314P_{0.1}+0.525P_{1}+0.0657P_{3}+0.28P_{10}+0.08P_{50}} \tag{4}\]  
\[P_{lt}=\sqrt[3]{\frac{1}{12}{\sum_{j=2}^{n}}\Big{(}P_{stj}\Big{)}^{3}} \tag{5}\]  
where \(P_{st}\) represents the short-term flicker value; \(P_{0.1}\),\(P_{1}\),\(P_{3}\),\(P_{10}\), and \(P_{50}\) represent instantaneous visual sensitivity exceeding 0.1%, 1%, 3%, 10%, and 50% of the sensing units within 10 min, respectively; \(P_{lt}\) represents the long-term flicker value; and \(P_{stj}\) represents the short-term flicker value in the \(j\)th 10-min interval. The voltage deviation can be determined using Equation (6). \[\Delta U=\frac{U_{s}-U_{N}}{U_{N}} \tag{6}\]  
where \(U_{s}\) represents the measured voltage value and \(U_{N}\) represents the rated voltage value. The three-phase imbalance can be determined using Equation (7).