\subsection{Grid Support Services}


To study the use of coordinated inverters for grid support services (GSS) we populate the 114-node LV distribution network with a number of coordinated inverters and passive inverters replicating one of the voltage disturbance-free PV layouts at 60\% penetration. The provision of GSS is illustrated in Fig.~\ref{fig:DR_Presponse} with $\mathbf{UR}$ and $\mathbf{DR}$ requests received at random time intervals over the daylight hours. After receiving a GSS request (Fig.~\ref{fig:DR_Presponse}, top), coordinated inverters respond to it by adjusting the power output (Fig.~\ref{fig:DR_Presponse}, middle). The VPP aggregator's capability to provide GSS during peak sun-hours is significantly reduced due to the additional curtailment required for keeping voltages below the tripping threshold. The deployment of GSS leads to voltage drop/rise along the distribution lines causing autonomous inverters to increase/decrease their power injection according to droop curve settings (Fig.~\ref{fig:DR_Presponse}, bottom). 

\begin{figure}[tb] 
\centerline{\includegraphics[width=\linewidth]{FiguresSec42/partB_profile2.png}}
\caption{The change in active power output from autonomous and coordinated inverters following a GSS request.} 
\label{fig:DR_Presponse} 
\end{figure}

\begin{figure}[b] 
\centerline{\includegraphics[width=6cm]{FiguresSec42/fig_PVoutput_vs_GSS.png}}
\caption{The response rates of autonomous inverters at various generation to load ratios, following a GSS request.} 
\label{fig:GSS} 
\end{figure}

\begin{figure}[b] 
\centerline{\includegraphics[width=6cm]{FiguresSec42/fig_AUTresponse_vs_GSS.png}}
\caption{The response rates of autonomous inverters at various generation to load ratios, following a GSS request.} 
\label{fig:GSS2} 
\end{figure}

\begin{figure}[b] 
\centerline{\includegraphics[width= 6cm]{FiguresSec42/fig_puResponse_vs_GSS.png}}
\caption{The autonomous inverter response rate per unit of the regulation request value between 50-90\% PV output.} 
\label{fig:GSSperunit} 
\end{figure}

\subsubsection{Passive inverter response}

The operation of legacy inverters is not affected by the VPP aggregator providing GSS. The VPP aggregator prioritises local voltage regulation through own curtailment optimisation in order to maintain all PV resources online. Since the maximum $\mathit{UR}$ offer capacity is calculated for the optimum operation of coordinated inverters at full output, the delivery of any $\mathit{UR}$ request will keep voltages below the inverter disconnection threshold allowing legacy inverters to operate at unity power factor. Therefore, a legacy inverter would only shut off once all coordinated inverters have reduced power injection to zero and have disconnected from the grid. 


\subsubsection{Autonomous inverter response}

The autonomous inverters will adjust operating points according to the voltage difference prior to and after a GSS event. At each time interval, the initial voltage level is driven by the amount of excess solar output that is being pushed back to the transformer, whereas the GSS dispatch capacity will determine the level at which voltage settles. Combining these two factors, the autonomous inverter response is illustrated in Fig.~\ref{fig:GSS}. PV output levels are varied between 40-100\% with constant customer load. The total width of each $\mathbf{UR}$ and $\mathbf{DR}$ band corresponds to the VPP feasible operating region (FOR) at the given PV output. The size of FOR follows the increase in solar generation, peaking at 55\%. Above this level, the VPP aggregator is required to curtail additional power from coordinated inverters (can be observed in Fig.~\ref{fig:DR_Presponse}) in order to maintain voltages below the disconnection threshold across the network. 


The $\mathbf{DR}$ delivery affects autonomous inverters when the PV output is between 60-90\%. Voltage drop induced by coordinated inverters allows autonomous inverters to either move up the Volt/Watt generation curve or deactivates them completely. Autonomous inverters recover up to 5\;kW when a $\mathbf{DR}$ request is between 30-45\;kW. It is important to note that the total response rate of autonomous inverters is limited by the curtailment occurring before a GSS event. Figure~\ref{fig:GSS2} illustrates the response rate against the regulation value for each PV output level. Depending on the PV output level, autonomous inverter response becomes constant for increasing $\mathbf{DR}$ values when inverters have recovered to full output. Then, the flat segment of each curve sets the upper bound of the $\mathbf{DR}$ response rate.

The impact of the $\mathbf{UR}$ request on autonomous inverters stretches across a wider range of PV output than during $\mathbf{DR}$. By delivering the $\mathbf{UR}$ request, CIC lifts the initial voltages to within the Volt/Watt response region. This causes autonomous inverter setpoints to move away from full output or increases the pre-existing curtailment. The total response rate from autonomous inverters increases together with $\mathbf{UR}$ request capacity. Since the voltage range of the Volt/Watt response mode exceeds the CIC tripping threshold, a larger $\mathbf{UR}$ request than the available reserve capacity would further reduce the output from autonomous inverters. However, it would also compromise the ability of CIC to maintain voltages below the average tripping level.  

It may be argued whether the idea of providing up-regulation from PV inverters is reasonable. Operating coordinated inverters below the optimal output in the expectation of an $\mathbf{UR}$ request leads to unnecessary energy curtailment. However, without an option to consume or store excess PV output locally, it may have low or no market value anyway. The energy in wholesale electricity and ancillary markets is frequently traded at zero price during peak-sun hours. With the number of solar PV installations far exceeding the uptake of battery storage for a foreseeable future, the price received for $\mathbf{UR}$ is likely to be greater than for the $\mathbf{DR}$ capacity. The fact that operating coordinated inverters below the optimum level may relax the voltage constraints also supports the idea of operating coordinated inverters with reserve capacity. Further research should focus on introducing a dynamic ratio between the $\mathit{UR}$ and $\mathit{DR}$ offer. This would enable the VPP aggregator to maximise the revenue while operating coordinated inverters and DER within the network limits. 

The autonomous inverter response rate per unit of GSS request is illustrated in Fig.~\ref{fig:GSSperunit}. For each kW of the $\mathbf{DR}$ request, autonomous inverters may increase the output by up to 0.25\;kW when operating at 85\% of PV output. The response rate increases up to 0.45\;kW during up-regulation. The difference between $\mathbf{UR}$ and $\mathbf{DR}$ response levels is related to the response being bounded by the total autonomous inverter curtailment and near-zero regulation requests. Although occurring at specific PV output levels, the extent of autonomous inverter response is large enough to derail the GSS delivery. The VPP aggregator, likely the only entity with controllable assets in the distribution network, should consider this counter-effect when operating in a network with a significant share of autonomous inverters. As shown before, at higher penetration levels, distribution networks will operate within the Volt/Watt mode more frequently and for longer time periods. If the actual load/output change seen by the system operator is only 55-75\% of the requested capacity, this undermines the utilisation of DER for grid support services.
{"config": "arxiv", "file": "2006.12644/Sections/Section5_2.tex"}