# Research Needs for Realization of Zero-Carbon Power Grids with Selected Case Studies
## 3 Selected Case Studies and Numerical Results
### HVDC Applications and Optimal Control  
Offshore wind farms (OWFs) have attracted significant attention; these reduce fossil-fuel emissions and mitigate climate change [50]. HVDC technology is a promising means by which remote, large-scale OWFs can be connected to a main power grid. HVDC technologies include both line-commutated converters (LCCs) and voltage source converters (VSCs); these differ. Specifically, LCC control strategies are mature and trusted. An LCC can transmit bulk power with little loss, and it is inexpensive to install [51]. Several ongoing projects worldwide use LCC-HVDC systems ranging from 500 to 1500 MW to facilitate the connection of large-scale OWFs in remote host regions to distant load centers [52]. However, an LCC operates only as a rectifier or an inverter, and thus controls either the DC voltage or the DC current [53]. In contrast, a VSC can operate as both a rectifier and an inverter. In other words, a VSC can change the direction of DC current and independently control both active and reactive power. A VSC improves reactive power compensation in the absence of additional compensators, rendering the footprint smaller. The power ratings and power conversion efficiencies of VSCs have continued to increase, particularly when VSCs are implemented as modular multi-level converters (MMCs), although the power ratings are lower, and the power losses are higher than those of an LCC [54]. Recently, a hybrid technology using an LCC and a VSC as interfaces with two remote grids was developed to combine the advantages of the two converter types. A hybrid HVDC system can be further developed as a multi-terminal DC (MTDC) system [55]; for example, LCCs, grid-side VSCs (GSVSCs), and OWF-side VSCs (WFVSCs) are linked via a DC network to connect regional AC grids with OWFs, as shown in Figure 1. This enhances the flexibility of inter-grid power delivery and sharing, facilitating the integration of large-scale OWFs with regional load centers.  
\begin{table}
\begin{tabular}{c c c} \hline \hline \multicolumn{1}{c}{**Challenges**} & \multicolumn{2}{c}{**Research Questions and Requirements**} \\ \hline Frequency and voltage stabilities & \(\bullet\) & _Which parameters best indicate the supply-and-demand balance and how the grid frequency should be controlled?_ \\  & \(\bullet\) & It is necessary to _understand the effects of system harmonics and sub-synchronous oscillations from VRE inverters on grids and to design mitigation method_, such as shaping of the inverter harmonic impedances at specific frequencies. \\ \hline Short-circuit currents & \(\bullet\) & Traditional protection schemes, designed for synchronous generators should be replaced by _new protection schemes for a wide variety of VRE inverters_. \\  & \(\bullet\) & New methods are needed for the _restoration and black start of VRE-dominated grids_. \\ \hline Behind-the-meter units & \(\bullet\) & Detailed planning and operating applications are needed to _integrate various DC devices into an AC power grid_. \\ \hline \hline \end{tabular}
\end{table}
Table 2: Cont.