# HVdc Circuit Breakers: Prospects and Challenges
## 2 High Voltage Direct Current Networks
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{**Project Name**} & \multicolumn{5}{c}{**Specifications**} \\ \cline{3-6}  & & **Year** & **Length (km)** & **Rating (MW)** & **Dc-Link (kV)** \\ \hline China & Jinpin Sunan & 2012 & 2093 & 7200 & \(\pm\)800 \\ Brazil & Rio Madeira & 2013 & 2375 & 2 \(\times\) 3150 & \(\pm\)600 \\ India & Biswanath Agra & 2014 & 1728 & 6000 & \(\pm\)800 \\ China & Xiloudu Zhejiang & 2014 & 1688 & 8000 & \(\pm\)800 \\ China & Zhundong Sichuan & 2015 & 2600 & 10,000 & \(\pm\)1100 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Point–point LCC–HVdc transmission link. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Country**} & \multirow{2}{*}{**Project Name**} & \multicolumn{5}{c}{**Specifications**} \\ \cline{3-6}  & & **Year** & **Length (km)** & **Rating (MW)** & **Dc-Link (kV)** \\ \hline China & Nan’aco & 2013 & 09 subsea & 200 & \(\pm\)160 \\ China & Dalian city & 2013 & 43 subsea & 1000 & \(\pm\)320 \\ Namibia & Caprivi link & 2010 & 951 & 300 & \(\pm\)350 \\ Norway & Skagerrak 4 & 2014 & 244 & 700 & \(\pm\)500 \\ France & Inelfe & 2013 & 65 & 1000 & \(\pm\)320 \\ \hline \hline \end{tabular}
\end{table} 
Table 2: VSC–HVdc transmission systems. \begin{table}
\begin{tabular}{c c c} \hline \hline \multirow{2}{*}{**Property**} & \multirow{2}{*}{**LCC–HVdc**} & \multirow{2}{*}{**VSC–HVdc**} \\ \hline Basic building block & & & \\ Control of dc grid in the event of dc fault & Control phase angle & Converter control is lost due to diodes \\ Harmonics & Strong low order harmonics & Weak high order harmonics \\ P/Q & Unidirectional, consume a large amount & Fully controlled P and Q in both \\  & of reactive power & directions \\ Available power ratings & 800 kV, 6400 MW & 400–800 MW, 300 kV \\ Power flow direction & Reversal of poles voltage & Stiff voltage achieved by adjusting PWM \\ Black start & No & Yes \\ The dc inductors & Large & Small \\ The dc capacitors & Small & Large \\ The dc fault current & The rate of increasing a fault current is & The rate of growth of a fault current is & very high and uncontrollable \\ \hline \hline \end{tabular}
\end{table} 
Table 3: An evaluation between VSC–HVdc and LCC–HVdc Transmission Systems. realized by blocking the flow of current through the switches during the dc fault [45; 48; 49] at the expense of high switching losses, which develops the need for fast-acting DCCB. Most of today's HVdc interconnections are two terminals. A factual M-HVdc system consists of several onshore and offshore VSC's positioned in different topographical locations. Thus, control, operation, and energy management are more complex and challenging. In the event of dc fault, to ensure the reliable and smooth operation of M-HVdc systems, it must be possible to isolate the faulty portion/system from a healthy system. The ac breakers can interrupt full/complete short circuit current, while HVdc circuit breakers have only limited ratings. The DCCBs are expensive and larger than ac CBs of the same rating. Thus, new methods are developed to extinguish the fault from today's point-point dc-link based on; control [50] or by installing the ac CBs on the ac side to de-energize the system [51]. Such a protection scheme operation may take a few 100 ms [52; 53; 54] to a few seconds [53], but only suited to two terminals HVdc systems. De-energization of complete HVdc system is also suggested in [40; 55; 56; 57] but limited to three to four terminals [58]. For reliable operation of the M-HVdc grid, the availability of dc circuit breakers would be vital. The first parallel M- HVdc system was anticipated, and a debate started in 1963 [59; 60; 61], whereas the series M-HVdc grid was discussed in 1965 [62]. However, major interest in M-HVdc emerges in 1980s [58; 22; 56], and [63; 64; 65]. More recent studies suggest VSC-based multi-terminal HVdc systems for integration of offshore wind farms [64; 65; 66; 67].