## 7 Black Start Capabilities through HVDC  
With further developments on renewable energy inclusion on electric grids, system operators have to look for efficient ways to provide a reliable and secure service to every user on these new low-inertia grids. One of the key aspects of this goal is to improve the overall robustness and controllability of electric systems, reducing both the frequency and the duration of unplanned events like blackouts. This is exactly where HVDC-based technologies can support with ancillary services beyond those primarily intended for them (energy transmission). The inclusion of single and multi-terminal HVDC links on power grids, with appropriate control systems, can help to achieve a robust network that responds better than conventional AC grids to power outages. The tools and advantages on which VSC-based HVDC systems can support in restoration events, due to their advantages over LCC in HVDC systems, include proposals that address issues like low-inertia mitigation and ease of control, and their increased use in today's electric grids. These ideas are summarized in the Table 2.  
Method for soft-energizing major power systems through VSC-HVDC. Overview process similar to the previous paper, for using VSC HVDC terminals for soft starting transmission lines, later energizing assistance equipment for a generator and switching it on for restoring load. A method for supplying passive AC networks through VSC HVDC is proposed, using constant AC voltage control for enabling the system to operate normally under disturbance conditions. This paper explores the possibility to black start a passive AC grid without the need of telecommunication systems or external power electronics, controlling the AC voltage locally. Method for damping power oscillation both through active and reactive power supplied by VSC. Through independent P and Q injections from VSC, a method is proposed for increasing stability based on frequency deviations. A control method is proposed and tested for supplying the grid with fast frequency assistance, without risking the operation of the VSC. This proposed method for providing frequency support for a system through the use of MMS VSC HVDC. \begin{table}
\begin{tabular}{p{14.2 pt} p{142.3 pt} p{142.3 pt}} \hline \hline
**Paper** & **Problem** & **Solution** & **Results** \\ \hline
[29] & Traditional VSC frequency control techniques are not appropriate for high harmonic distortion conditions. Conventional black start strategies require periodical tests to the equipment. Harmful phenomena related to transmission lines and transformers occur during abrupt start strategies. & A controller based on frequency self-adaptation tracking algorithm (FATA) is proposed for replacing Phase-Locked Loop (PLL) methods. & Better harmonic suppression, even for severe operating conditions, appropriate for both dynamic and steady state. \\
[31] & Overall black start strategies require periodical tests to the equipment. Harmful phenomena related to transmission lines and transformers occur during abrupt start strategies. & This paper includes field test results of VSC HVDC performance as a black start source, with an overview of its control mechanisms and characteristics. & A comparison between traditional and VSC black start show that the latter offers reduced restoration time, safer and smoother restoration, and lower investment and maintenance costs. \\
[31] & Inrush currents and transient over-voltages can cause failure in a black start restoration. Traditional black start problems such as: generator’s self-excitation, switching surge and over-voltage while powering long no-load lines and ferromagnetic resonance while charging transformers. Traditional black start strategies are limited due to small power supply capacity, slow recovery process, slow recovery process, and weak power adjustment. & Method for soft-energizing major power systems through VSC-HVDC.