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Abstract:The modern power system is progressing from a synchronous machine-based system towards an inverter-dominated system, with large-scale penetration of renewable energy sources (RESs) like wind and photovoltaics. RES units today represent a major share of the generation, and the traditional approach of integrating them as grid following units can lead to frequency instability. Many researchers have pointed towards using inverters with virtual inertia control algorithms so that they appear as synchronous generators to the grid, maintaining and enhancing system stability. This paper presents a literature review of the current state-of-the-art of virtual inertia implementation techniques, and explores potential research directions and challenges. The major virtual inertia topologies are compared and classified. Through literature review and simulations of some selected topologies it has been shown that similar inertial response can be achieved by relating the parameters of these topologies through time constants and inertia constants, although the exact frequency dynamics may vary slightly. The suitability of a topology depends on system control architecture and desired level of detail in replication of the dynamics of synchronous generators. A discussion on the challenges and research directions points out several research needs, especially for systems level integration of virtual inertia systems.

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In this paper, we review the emerging challenges and research opportunities for voltage control in smart grids. For transmission grids, the voltage control for accommodating wind and solar power, fault-induced delayed voltage recovery (FIDVR), and measurement-based Thévenin equivalent for voltage stability analysis are reviewed. For distribution grids, the impact of high penetration of distributed energy resources (DER) is analyzed, typical control strategies are reviewed, and the challenges for local inverter Volt-Var control is discussed. In addition, the motivation, state-of-art, and future directions of the coordination of transmission system operators (TSO) and distribution system operators (DSO) are also thoroughly discussed.

Big data has potential to unlock novel groundbreaking opportunities in power grid that enhances a multitude of technical, social, and economic gains. As power grid technologies evolve in conjunction with measurement and communication technologies, this results in unprecedented amount of heterogeneous big data. In particular, computational complexity, data security, and operational integration of big data into power system planning and operational frameworks are the key challenges to transform the heterogeneous large dataset into actionable outcomes. In this context, suitable big data analytics combined with visualization can lead to better situational awareness and predictive decisions. This paper presents a comprehensive stateof-the-art review of big data analytics and its applications in power grids, and also identifies challenges and opportunities from utility, industry, and research perspectives. The paper analyzes research gaps and presents insights on future research directions to integrate big data analytics into power system planning and operational frameworks. Detailed information for utilities looking to apply big data analytics and insights on how utilities can enhance revenue streams and bring disruptive innovation are discussed. General guidelines for utilities to make the right investment in the adoption of big data analytics by unveiling interdependencies among critical infrastructures and operations are also provided.

As non-controllable power sources, photovoltaics (PV) can create overvoltage in low voltage (LV) distribution feeders during periods of high generation and low load. This is usually prevented passively by limiting the penetration level of PV to very conservative values, even if the critical periods rarely occur. Alternatively, one can use active power curtailment (APC) techniques, reducing the amount of active power injected by the PV inverters, as the voltage at their buses increase above a certain value. In this way, it is possible to increase the installed PV capacity and energy yield while preventing overvoltage. This paper investigates a number of approaches for sizing and controlling the PV power generated by 12 net-zero energy houses equipped with large roof-top PV systems in a typical 240V/75kVA Canadian suburban radial distribution feeder. Simulations of a one year period with typical solar irradiance and load profiles are conducted with PSCAD to assess the performance of the different approaches in terms of overvoltage occurrence, sharing of the burden for overvoltage prevention per house and total energy yield of the residential PV feeder.Keywords--power distribution, overvoltages, solar power generation, power systems, power quality and voltage control. INTRODUCTIONDistribution systems have been designed and operated under the premise that power flows from the distribution substation to the end users, which only consume power. However, with the addition of intermittent, consumer-owned and non-dispatchable distributed generation (DG) units, current standard procedures for meeting power quality and reliability (PQR) requirements might not be as effective as they are without DG. This has led many electricity utilities to adopt conservative limits regarding the amount of DG that can be installed in distribution networks without an impact assessment study.Overvoltage is one of the main reasons for limiting the capacity (active power) of non-dispatchable DG units, such as photovoltaic (PV), that can be connected to a low voltage (LV) distribution system [1]. During high PV generation and low load periods, there is a possibility of reverse power flow, and consequently voltage rise, in the LV feeder [1][2][3][4][5][6][7][8]. This problem can be avoided, without conservatively limiting the capacity of the DG units, by using inverters with active power curtailment (APC) schemes. These allow the inverters to inject maximum available power from the dc source, as long as the ac bus voltage is below a certain value. Above this value, the injected power is reduced (curtailed) linearly with the ac bus voltage increase. The option of active power curtailment for overvoltage prevention looks very attractive because it requires minor modifications in the DG's inverter control logic. Besides, it is only activated when needed, thus minimizing the amount of curtailed active power also known as output power losses (OPL) [1].The use of droop based APC techniques for overvoltage prevention in a LV radial distribution feeders ...

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