The energy sector is currently undergoing a rapid transformation with the integration of power electronic converter (PEC)-interfaced renewable energy sources (RES), such as wind and solar photovoltaic (PV) systems, at both the transmission and distribution networks. Power system stability has been significantly influenced by this power grid transformation. This paper comprehensively reviews major power system stability issues affected due to large-scale integration of PEC-interfaced RES in power grids, with some example case studies relevant for each stability category. According to the review, stability issues are mainly originating from reduction in synchronous inertia, reduction in reactive power reserve, low short-circuit strength of the power network, and fault ride-through (FRT) strategy/capability of the PEC-interfaced RES. Decrease in synchronous inertia could affect both the rotor angle stability and the frequency stability, while decrease in short-circuit strength and reactive power reserve could cause voltage stability and rotor angle stability issues in power networks. Sub-synchronous control interactions are also receiving a lot of attention by the power industry due to increasing oscillatory stability incidents reported in power networks with PEC-interfaced RES. FRT capabilities/strategies of PEC-interfaced RES are also playing a pivotal role in power grid stability due to its influence on active and reactive power, hence more emphasis should be placed on FRT schemes of PEC-interfaced RES, since future power grids are expected to operate with 100% PEC-interfaced generation sources. Stability improvement strategies could be implemented to address multiple stability issues in PEC-interfaced power networks; however, rigorous stability studies are required to identify the optimal conditions to implement these improvement strategies. Furthermore, ongoing structural changes in power grids to accommodate remotely sited PEC-interfaced RES are also influencing the stability of power grids. Therefore, all these factors must be carefully considered by system operators when planning and operating power grids in a secure and stable manner with high penetration levels of PEC-interfaced RES.

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Electric vehicles (EVs) are penetrating rapidly into the transport sector while making profound implications on the electricity and energy sectors. Although EVs have many benefits, it poses several challenges on power grid operators. Uncoordinated EV charging is one of the critical issues that need to be addressed to mitigate the potential adverse effects on power grids. Smart charging and vehicleto-grid (V2G) technologies could alleviate these adverse effects but requires policies and regulatory frameworks to increase the uptake of these technologies. This paper presents a critical review on the effects of uncoordinated charging of EVs, and the benefits of smart charging and V2G considering the published research studies and real-world field trials. Simulation case studies also demonstrate the adverse effects of uncoordinated charging and the benefits of smart charging. According to this study, uncoordinated charging increases the peak load, which in turn causes high power losses, voltage violations, voltage unbalance, reduction of transformer lifespan and harmonic distortion. This study has established that smart charging alleviates these network issues and brings a wide range of economic, social, and environmental benefits. In particular, role of smart charging as a mandatory requirement for attaining the net-zero decarbonization target of the transport sector is highlighted. Finally, the paper sheds light on the policy, standards and regulatory frameworks that need to be implemented to promote smart charging and V2G technologies among EV owners and charging infrastructure developers.INDEX TERMS Electric vehicle (EV), renewable energy, smart charging, uncoordinated charging, voltage violations, voltage unbalance, vehicle to grid (VG).

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The voltage-source converter (VSC) based multi-terminal direct-current (MTDC) networks are currently being developed in large-scale power grids for efficient and economical transmission of electrical energy generated from remotely sited renewable energy sources (RESs). As the MTDC network is developed alongside the conventional AC grid, it has transformed the power grid to a hybrid AC/MTDC power network. However, since the MTDC network is controlled via the power electronic converters (PECs), it significantly affects the AC power grid damping and synchronising performance as it decouples the natural dynamics between various electro-mechanical systems in the power grid. This paper proposes a supplementary control scheme to improve the damping performance of the entire hybrid AC/MTDC power grid. The damping torque analysis (DTA) technique is used as the primary technique to develop the supplementary control scheme. The preliminary analysis is carried out using a two-terminal hybrid AC/DC power grid and has suggested a supplementary control loop based on the rotor speed deviation for the DC voltage controller of the VSC to improve the damping performance. With the state-space model, the synchronising and damping torque coefficients are calculated and accordingly, the feedback gain is determined to provide optimal synchronising and damping torque components. Subsequently, the fidelity of the supplementary controller is verified using a four-terminal hybrid AC/MTDC grid. Simulation studies prove that the proposed supplementary controller can improve the hybrid AC/MTDC network damping performance, and it performs very effectively with the master-slave control and the conventional droop control scheme. INDEX TERMS Damping torque analysis (DTA), high-voltage direct-current (HVDC), multi-terminal DC (MTDC), rotor angle stability, small-signal analysis, voltage-source converter (VSC).