Abstract
The rapid growth of utility-scale photovoltaic (PV) generation is changing the needs for transmission and creating new stability problems. Less rotational inertia, faster power fluctuations, and more complex interactions between power-electronic converters primarily cause these problems. A new way to solve these problems is to use hybrid systems that combine the long-distance, bulk-transfer capabilities of high-voltage direct current (HVDC) transmission with the fast, localized power flow and voltage control of Unified Power Flow Controllers (UPFCs). This review synthesizes the literature from the past decade concerning the modelling, control, and case study evaluation of PV-integrated with HVDC and UPFC systems, emphasizing the improvement of small-signal and transient stability. Unlike prior reviews that separately discuss PV integration, HVDC transmission, or FACTS devices, this review focuses on the coordinated stability role of PV-interfaced converters, LCC/VSC-HVDC systems, UPFC control, ESS/V2G support, and modern GFM/GFL control strategies. This study compares both a wide variety of converter and network modeling methodologies (such as EMT models versus small signal models used to perform modal analysis) and various coordinated control methods. The control strategies are grouped by type as follows; droop, dc voltage/active power coordinated control, supplementary damping loops, model predictive control, and adaptive control. In addition, this study presents the results of several studies that compared different systems using bench mark studies and hardware in the loop experiments. This synthesis finds that PLL- and converter-interaction oscillations, DC-side contingencies, and insufficient reactive support commonly cause failures. It reveals the weaknesses in multi-terminal coordination and protection schemes and the scarcity of large-scale field validations. The review ends with a suggestion for a research plan. Standardized multi-scale models, strong coordination between HVDC, UPFC, and PV controls, integrated storage strategies for synthetic inertia, and experimental testbeds for validation are all important parts that need to be in place to make HVDC and UPFC systems architectures work well with a high PV penetration.