In this article, we focus on the development and scope of virtual power plants (VPPs) as a strategy to facilitate the integration of distributed energy resources (DERs) in the power system. Firstly, the concepts about VPPs and their scope and limitations are introduced. Secondly, smart management systems for the integration of DERs are considered and a scheme of DER management through a bottom-up strategy is proposed. Then, we analyze the coordination of VPPs with the system operators and their commercial integration in the electricity markets. Finally, the challenges that must be overcome to achieve the large-scale implementation of VPPs in the power system are identified and discussed.

Local-measurement-based algorithms are relevant as primary protection for high voltage direct current (HVdc) grids due to the restriction in operation speed. The communication time delay significantly affects communication-based algorithms, therefore, they are mostly implemented as backup protection or for high-resistance fault detection. This article proposes a local derivative-based protection scheme for HVdc grids. Primary protection is based on the voltage derivative. Conversely, the backup and busbar protections are based on the current derivative. The proposed system is evaluated against low- and high-resistance fault case scenarios via simulation in PSCAD software. The voltage derivative-based algorithm provides a fast and accurate primary protection. On the other hand, the current derivative-based algorithm is implemented to operate when a failure is produced in the primary protection. The parallel operation of these two algorithms saves time when it comes to backup operation after primary protection's failure. Likewise, the busbar protection operates rapidly and selectively. Influence of the limiting inductor's size, sampling frequency, fault resistance, and noise disturbance on the performance of the proposed protection scheme are also analyzed. The presented protection scheme is capable of rapidly and selectively detecting faults up to 200 Ω while covering primary, backup and busbar protections. Moreover, only common local derivative-based algorithms are employed in addition to typical limiting inductor's sizes and relatively low sampling frequencies.

Nowadays, High Voltage Direct Current (HVDC) transmissions are gaining relevance in long distance and renewable energy integration projects over the conventional Alternating Current (AC) transmissions due to several advantages as improved flexibility and independent active and reactive power controllability. Despite that, the high currents and voltage collapses generated by fault conditions in HVDC systems imply some unresolved technical challenges regarding the detection, location and clearance of faults. Faults have to be cleared in a very short time range in order to minimize their impact on the system. For this objective, very fast protection algorithms and reliable HVDC circuit breakers are required. This paper focuses on the technical challenges and limitations of HVDC protection systems. This way, protection requirements and different types of measurement devices are considered. Protection algorithms are classified into local-measurement-based and communication-based, highlighting the most important characteristics. In addition, the different technologies of HVDC circuit breakers are compared. Finally, the characteristics and effects of the different available fault-clearing strategies are presented.

One of the most important challenges of developing multi-terminal (MT) high voltage direct current (HVDC) grids is the system performance under fault conditions. It must be highlighted that the operating time of the protection system needs to be shorter than a few milliseconds. Due to this restrictive requirement of speed, local measurement based algorithms are mostly used as primary protection since they present an appropriate operation speed. This paper focuses on the analysis of local measurement based algorithms, specifically overcurrent, undervoltage, rate-of-change-of-current, and rate-of-change-of-voltage algorithms. A review of these fault detection algorithms is presented. Furthermore, these algorithms are applied to a multi-terminal grid, where the influence of fault location and fault resistance is assessed. Then, their performances are compared in terms of detection speed and maximum current interrupted by the HVDC circuit breakers. This analysis aims to enhance the protection systems by facilitating the selection of the most suitable algorithm for primary or backup protection systems. In addition, two new fault type identification algorithms based on the rate-of-change-of-voltage and rate-of-change-of-current are proposed and analyzed. The paper finally includes a comparison between the previously reviewed local measurement based algorithms found in the literature and the simulation results of the present work.

HVDC grids demand the fast and reliable operation of the protection system. The failure of any protection element should initialize a backup protection almost immediately in order to assure the system’s stability. This paper proposes a novel backup strategy that covers the failure of the primary protection including the malfunctioning of the HVDC circuit breaker. Only local voltage measurements are employed in the proposed backup protection and the voltage derivative is calculated at both sides of the limiting inductor. Consequently, the speed and reliability of the protection system are enhanced, since no communication channel is needed. This paper contains a thorough specification of the proposed protection strategy. This strategy is validated in a four-terminal HVDC grid with various fault case scenarios, including high-resistance fault cases. The operation of the backup protection is reliable and remarkably fast.