## 2 Overview of the Relevant Standards  
### IEEE 1547-2018 Standard  
The \"IEEE Standard for Interconnection and Interoperability of Distributed Energy Resources with Related Power System Interfaces\", labeled as IEEE 1547-2018, specifies criteria for connecting and integrating distributed energy resources (DERs) such as PV installations containing inverters, modules, and optional battery storage to the power grid [26]. It focuses on the requirements necessary to ensure that DERs, when connected to the utility grid, operate safely, reliably, and efficiently. This section describes parts relevant to the research, specifically focusing on the limitations of DC injection and the limitations of current distortion [26].  
The standard IEEE 1547-2018 defines current distortion limits based on the standard IEEE 519-2022 [29]. However, there are notable differences between these two standards. Firstly, there has been a change from using total harmonic distortion (THD) limits, as seen in the previous IEEE 1547-2003 standard, to total rated current distortion (TRD) [26]. This change aims to include inter-harmonics in the total distortion calculation.  
Secondly, there is a change concerning the limits on even harmonics. Tighter limits--precisely 25% of the odd harmonic limits--should apply to even harmonics due to concerns about the DC offset effect. This effect can lead to the misoperation of electronic switches by affecting zero crossing and other control logic. The main contributor to this DC offset effect is the 2 nd harmonic, while the influence of higher-order harmonics decreases rapidly with increasing harmonic order [26].  
Moreover, despite the findings regarding tighter limits for higher-order harmonics, the standard IEEE 1547-2018 has relaxed these limits. Specifically, the limits for the 8 th harmonic and higher are now the same as those for their corresponding odd harmonics. The limits for the 2 nd harmonic remain unchanged. A stepped approach has been taken for the 4 th and 6 th harmonics, which have been adjusted to 50% and 75% of the corresponding odd harmonic limits, respectively. Voltage distortion limits for DERs are not defined, and current distortion limits are intended to exclude from consideration current distortions caused by the harmonic voltage distortion of the power system that exists when the DER is not connected [26].  
The limitations related to current distortion, i.e., the maximum values of individual current harmonics and the DC component, are summarized in Table 1. The total current of all inter-harmonics with frequencies within \(\pm 5\) Hz of the frequency of a given harmonic is limited to the maximum magnitude of that harmonic. The total current of inter-harmonics, with frequencies ranging from \(\mathrm{f_{h}}+5\) Hz to \(\mathrm{f_{h+1}}-5\) Hz, is limited by a lower limit for order h and h + 1 harmonics [26].  
The TRD is calculated using the following formula:  
\[TRD=\sqrt{\frac{I_{\mathrm{rms}}^{2}-I_{1}^{2}}{I_{\mathrm{r}}^{2}}}\psi[], \tag{1}\]  
\begin{table}
\begin{tabular}{c c c c} \hline
**Odd Harmonic** & **\%** & **Even Harmonic** & **\%** \\ \hline DC & 0.5 & h = 2 & 1.0 \\ h \(<11\) & 4.0 & h = 4 & 2.0 \\ \(11\leq h<17\) & 2.0 & h = 6 & 3.0 \\ \(17\leq h<23\) & 1.5 & \(8\leq h<24\) & 1.5 \\ \(23\leq h<35\) & 0.6 & \(24\leq h<36\) & 0.6 \\ \(35\leq h<50\) & 0.3 & \(36\leq h<50\) & 0.3 \\ \hline \end{tabular}
\end{table}
Table 1: Maximum harmonic current distortion, given in percent of the rated current capacity, \(I_{\mathrm{r}}\).  
\(I_{\rm rms}\) represents the RMS value of generated current in A, \(I_{1}\) is the RMS value of the fundamental harmonic in A, and \(I_{\rm r}\) stands for the rated current capacity of DER in A. The standard limits the TRD to 5%.