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Selecting a battery energy storage technology for application on offshore platforms or marine vessels can be a challenging task. Offshore oil and gas platforms (OOGPs) require battery energy storage systems (BESSs) with high volumetric density, high gravimetric density, high safety, a long life span, low maintenance, and good operational experience, amongst other BESS properties. No single battery chemistry can satisfy all these factors perfectly, which implies that there is a need for a method that determines the most suitable battery chemistry for a given application. To this end, this paper proposes an improved version of a 7-step procedure proposed in the literature to systematically and logically determine the most suitable BESS for high-energy applications on OOGPs. In order to implement the 7-step procedure, a review of the state-of-the-art of consolidated and emerging battery chemistry is done. As part of the 7-step procedure, the operational experience of the battery chemistry was also reviewed. The 7-step procedure was then applied to a case study (with two test cases) of a real OOGP in the North Sea. The first test case considers BESS for peak shaving, for which six battery chemistries were assessed in detail. A technology suitability assessment (TSA) weighted score is calculated, which is based on five attributes critical for the energy storage choice in the considered application, which are weight, space, safety, life cycle cost, and operational experience. Of the six battery chemistries assessed, lithium iron phosphate (LFP) has the highest technology suitability assessment (TSA) weighted score and is therefore deemed the most suitable battery chemistry for peak shaving. The second test case considers BESS for spinning reserve. Since this is a high C-rate application, only battery chemistry capable of high C-rate was evaluated. From the TSA evaluation, LFP and lithium nickel manganese cobalt have the joint highest TSA weighted score and are therefore deemed the most suitable battery chemistry for spinning reserve. [ABSTRACT FROM AUTHOR]

Different control strategies for microgrid applications have been developed in the last decade. In order to enhance flexibility, scalability and reliability, special attention has been given to control organisations based on distributed communication infrastructures. Among these strategies, the implementation of consensus protocol stands out to cooperatively steer multi-agent systems (i.e., distributed generators), which is justified by its benefits, such as plug and play capability and enhanced resilience against communication failures. However, as the consensus protocol has a long trajectory of development in different areas of knowledge including multidisciplinary subjects, it may be a challenge to collect all the relevant information for its application in an emerging field. Therefore, the main goal of this paper is to provide the fundamentals of multi-agent systems and consensus protocol to the electrical engineering community, and an overview of its application to control systems for microgrids. The fundamentals of consensus protocol herein cover the concepts, formulations, steady-state and stability analysis for leaderless and leader-following consensus problems, in both continuous- and discrete-time. The overview of the applications summarises the main contributions achieved with this technique in the literature concerning microgrids, as well as the associated challenges and trends. [ABSTRACT FROM AUTHOR]

Low-voltage grid-connected microgrids rely on the exploitation of inverter-interfaced distributed energy resources (DERs) in order to feed loads and to achieve bidirectional power flow controllability at their point of common coupling (PCC) with the upstream grid. However, adverse operational conditions, such as the existence of DERs of different operation natures, DERs of non-equal power ratings, as well as the occurrence of non-steady and non-sinusoidal grid voltage scenarios, bring complications to microgrid energy management. Consequently, control strategies employed to coordinate DERs in dispatchable microgrids need to be resilient to such non-ideal conditions. Hence, this paper demonstrates that a multi-purpose strategy, so-called the Generalized Current-Based Control (GCBC) approach, is capable of steering DERs under such adverse operational scenarios, ensuring proportional current sharing among them while also regulating the microgrid power dispatchability at the PCC. The discussions are supported by an extensive experimental validation on a laboratory-scale single-phase microgrid prototype, demonstrating that the GCBC approach allows DERs of different operational natures to be coordinated, respecting their power ratings, and allowing the single-controllable microgrid to endure operation under distorted voltages and support voltage ride-through conditions. [ABSTRACT FROM AUTHOR]

Beyond the impacting presence of nonlinear loads, low-voltage microgrids also experience low energy efficiency and resonance phenomena when operating interconnected to a distribution grid that suffers from distorted voltages. This article proposes a model-free control strategy capable of coordinating inverters existing within a dispatchable microgrid, allowing to operate it as a single-controllable entity that behaves like a resistor at selected harmonic frequencies. Such resistive shaping uses a centralized control architecture to steer inverters to distributively compensate reactive and harmonic currents, supporting active current sharing. Consequently, the microgrid point-of-common-coupling operates with a high power factor when the grid imposes distorted voltages. Additionally, if resonant components exist, the strategy supports harmonic resonance damping, which minimizes deterioration of voltage quality. For instance, comparative results show that, for the considered scenario, the proposed resistive shaping damps resonances up to 50% better than a previous approach that compensates harmonics using sinusoidal current synthesis. Simulation results carried out on a three-phase low-voltage microgrid testbench, considering three inverters, demonstrate the abovementioned capabilities of the proposed approach. Experimental results based on a single-phase microgrid prototype comprising two inverters with two linear loads and one nonlinear load validate the applicability of the method to real-life implementations. [ABSTRACT FROM AUTHOR]

Interconnection and expansion of AC networks through high‐voltage direct current grids based on modular multilevel converters to form a multiterminal hybrid AC/DC grid can pose stability issues. These challenges can arise from dynamic interactions between/within AC and DC subgrids due to poorly damped modes that are potential sources of persistent and disruptive oscillations. This paper aims to ensure the stability of multiterminal hybrid AC/DC grids via a decentralized optimal controller. The proposed methodology analytically and simultaneously identifies both the decentralized optimal controller and the worst‐case perturbation scenario under the grid control inputs' and state variables' constraints, without the need for detailed and time‐consuming dynamic simulations of all possible scenarios. Eigenvalue stability analysis and time‐domain simulations show that the proposed controller can efficiently enhance the test grid stability margins and reduce the oscillations, not only under the worst‐case perturbation scenario (increasing damping ratios of the two pairs of least damped modes by 2.34 and 4.05 times) but also under other critical fault conditions. Furthermore, the controller's superior performance is validated through comparison with the power system stabilizer and modular multilevel converter droop controller under small and large disturbances, and its robustness is assessed against parametric uncertainties. [ABSTRACT FROM AUTHOR]

This paper presents a multiobjective optimization technique for the submarine cables of high-power remotely operated vehicles (ROVs). A detailed design flowchart with integrated electrical and mechanical designs of the ROV submarine cables is provided to quantitatively compare AC and DC-based power distribution systems and identify the optimal design from multiple standpoints, prioritizing electrical loss and cable weight. The electromechanical cable design optimization plays a vital role in the ROV industry and in all those applications where the power distribution system is employed vertically from the support platform/vessel. The entire procedure is exemplified by carrying out a particular case study; for an ROV power hub with rated power of 5 MW and operational depth of 6000 m, the DC submarine cable design with distribution voltage of 33 kV results in an electrical loss reduction of 54.3% compared to the AC submarine cable design with distribution voltage of 22 kV at similar cable weights of 63 ton. Additionally, the proposed method can be widely applied to any type of AC-DC comparison to achieve the optimal ROV submarine cable design with a considerably short process time in comparison to the approaches with finite element analysis. [ABSTRACT FROM AUTHOR]

Most of the distributed control strategies for grid-connected power converters are droop-based approaches composed of converters driven in voltage-control mode, based on local and shared data with adjacent units. They are usually combined with consensus protocols to deal with the trade-off between power sharing accuracy and voltage/frequency regulation. To achieve the desired results these control systems usually incorporate other techniques and need to take into account details of primary control dynamic. Additionally, power flow control and current unbalance compensation at the PCC are rarely addressed in such approaches. Contrariwise, the centralized control strategy power-based control has been successful in achieving these functionalities. It is oriented to a set point selection to the whole system, considering converters driven in current-control mode and a central converter in voltage-control mode. However, the dependence on centralized communication network in this method still requires improvement. Thereby, the complementary features of both strategies are combined herein in the consensus power-based control, based on a master/slave peer-to-peer integration using sparse communication. This model-free approach provides all aforementioned benefits to the grid without any other technique. Implementation complexity and costs are decreased, while the flexibility and reliability are enhanced. All these achievements are demonstrated by simulation results under different operational conditions and compared to previous works. [ABSTRACT FROM AUTHOR]

This paper analyses the frequency stability of ac grids in the presence of non-dispatchable generation and stochastic loads. Its main goal is to evaluate conditions in which the system is robust to large, persistent active power disturbances without recurring to time-domain simulations. Considering the ongoing energy transition to more renewable sources, defining robustness boundaries is a key topic for power system planning and operation. However, much of the research on long-term studies has not dealt with robust dynamic constraints, while short-term analyses usually depend on time-consuming simulations to evaluate nonlinearities. To bridge this gap, the authors derive an algebraic equation that provides sufficient conditions for robust frequency stability in ac power systems and a relationship among four key quantities: the maximum active power perturbation, the minimum system damping, the steady-state and the transient frequency limits. To achieve this goal, it uses a nonlinear average-model of the ac grid and Lyapunov's direct method extended by perturbation analysis requiring only limited knowledge of the system parameters. The algebraic calculations are validated using time-domain simulations of the IEEE 39-bus test system and results are compared to the traditional Swing Equation model. [ABSTRACT FROM AUTHOR]

This letter proposes a new representation of electrical circuit quantities based on three-phase dimensional space, in contrast to the conventional two-dimensional representation. It contributes to an alternative mathematical approach to display the voltage and current waveforms in three-dimensional frame, in which an unbalance displacement angle becomes evident, and can be used as an unbalance metric. It is outside the scope of this letter to propose a new power theory, or state new definitions for the power terms. In fact, such electric power representation can be applied to different power theories. Finally, this letter exemplifies the three-dimensional representation through numerical and illustrative case studies. [ABSTRACT FROM AUTHOR]

Power oscillation damping (POD) is one of the ancillary services expected from high‐voltage direct current (HVDC) converters. When providing POD to the ac side, converters draw power from the dc side, which can cause distortion to the dc voltage especially in the case of limited dc capacitance. In meshed/multi‐terminal HVDC grids, where dc voltage regulation is distributed using a dc voltage droop control strategy, the distortion due to the POD controller action is propagated to other connected ac grids because of the droop action. This propagation can be reduced by using the inherent energy storage capability of the modular multilevel converter (MMC), which is a common converter topology for HVDC. Different methods to utilise the energy stored in the MMC for the purpose of POD have been proposed in the literature. This study presents a detailed analysis and experimental validation of one of these methods, referred to as virtual capacitance support, which increases the effective dc grid capacitance by using the stored energy of multiple MMCs connected in the same grid. The experiments, which were carried out using power hardware in the loop setup, demonstrated the effectiveness of the method. [ABSTRACT FROM AUTHOR]

The presence of single-phase distributed generators unevenly injecting active power in three-phase microgrids may create undesired upstream current unbalance. Consequently, voltage asymmetry and even active power curtailment may occur in such networks with negative economic impact. Thus, this paper proposes an optimal multiobjective approach to regulate the active and reactive power delivered by distributed generators driven by a three-layer hierarchical control technique in low-voltage microgrids. This method does not require previous knowledge of network parameters. The multiobjective algorithm is implemented in the secondary level achieving optimal dispatch in terms of maximizing the active power generation, as well as minimizing the reactive power circulation and current unbalance. By the existence of a utility interface three-phase converter placed at the point-of-common-coupling, the proposed control can regulate the power circulating among the microgrid phases, and the microgrid structure can withstand grid-connected and islanded operating modes. The path for interphase power circulation through the DC-link of the utility interface allows the multiobjective algorithm to achieve better results in terms of generation and compensation compared to the system without utility interface. The proposed method is assessed herein by computational simulations in a three-phase four-wire microgrid under realistic operational conditions. [ABSTRACT FROM AUTHOR]

The method used to calculate insertion indexes plays an important role in determining the overall performance of the modular multilevel converter. Direct voltage control, which is the simplest option, results in a large circulating current ripple because this modulation technique does not account for the arm voltage ripples. This led to the development of compensated modulation techniques that compensate for the arm voltage ripples thereby eliminating the circulating current ripple. There are two variants of compensated modulation—closed loop and open loop. The closed-loop version requires measurement of the arm voltages without distortion and delay, which is difficult to achieve in practice. The open-loop method overcomes this challenge by using estimated arm voltages. However, accurate knowledge of the system parameters is needed for effective removal of the circulating current ripples. This is a limitation because the parameters change with time and operating conditions. This paper presents a modified version of the open-loop method, which includes a scheme for correcting parameter errors online. The method estimates the parameters, arm capacitances, and time delay, by using feedback controllers acting on the circulating current ripples. Mathematical derivation of the method, together with its validation using simulation and experimental tests, is presented in detail. [ABSTRACT FROM AUTHOR]

This paper investigates the existence of an equilibrium point in multiterminal HVdc grids, assesses its uniqueness, and defines conditions to ensure its stability. An offshore multiterminal HVdc system including two wind farms is selected as an application test case. At first, a generalized dynamic model of the network is proposed, using multigraph theory. Such model captures the frequency dependence of transmission lines and cables; it is nonlinear due to the constant power behavior of the converter terminals using droop regulation, and presents a suitable degree of simplifications of the modular multilevel converters, under given conditions, to allow system level studies over potentially large networks. Based on this model, the existence and uniqueness of the equilibrium point are demonstrated by returning the analysis to a load-flow problem and using the Banach fixed point theorem. Additionally, the stability of the equilibrium is analyzed by obtaining a Lyapunov function by the Krasovskii's theorem. Computational results obtained for the selected four-terminal HVdc grid corroborate the requirement for the existence and stability of the equilibrium point. [ABSTRACT FROM AUTHOR]

Over the past decade, modular multilevel converters have evolved as the preferred choice for high voltage conversion systems. The design of the converter poses a computational challenge to the classical electromagnetic transient simulation techniques, for which the existing literature proposes numerous simplified and computationally-efficient equivalent representations. Despite the extensive collection of proposed models, the literature lacks a systematic comparison of these representations, which could enable their appropriate selection based on the specific simulation objectives. In light of these requirements, this article presents a comprehensive review of MMC models and discusses their applicability for parallel and real-time simulation. Using PSCAD/EMTDC simulations, the models are objectively compared with regards to computational efficiency and accuracy for various transient conditions. In addition, the paper also presents a brief account of MMC operation and the associated control and modulation system. [ABSTRACT FROM AUTHOR]

The pollution coming from the use of gas turbines (GTs) in oil and gas (O&G) platforms requires greener solutions. One option is to use wind turbines (WTs) to supply the water injection systems (WISs) and explore smart energy management to make the O&G sector more sustainable. The effect of WT integration, and their the coordination with the local GTs, needs to be carefully analysed since the intermittency of the wind can jeopardise the stability of the offshore grid. The existence of flexible loads (FLEX) interfaced by variable speed drives (VSD), such as WISs, can help overcome the challenges related to wind intermittency and rotor angle stability: suitable control of such non‐essential loads can be implemented to reduce the effect of wind power fluctuations, balance power generation and consumption and contribute to the optimal efficiency of the GT adjusting its loading conditions. This work investigates the system dynamics under short‐term wind‐induced power fluctuations, analysing the evolution of electrical variables, such as active power and generator rotor speed, to evaluate the possible arise of low‐frequency oscillations. The impact of an adequate WIS load control to increase the system damping is also explored and an algorithm for system damping estimation is proposed. [ABSTRACT FROM AUTHOR]

The application of energy storage in offshore renewable generation systems allows managing the intrinsic uncertainty of the resources and improving the utilization factor of the electrical network. Optimal storage design algorithms generally have to evaluate the behavior of the whole system thousands times before converging to the optimal solution and the reliability of the results obviously depends on the quality of input data. On the other hand, the utilization of simplified storage models in the design stage can reduce the simulation time drastically, while still providing useful information. The goal of this paper is to evaluate the applicability of a methodology for sizing the energy storage system in a hybrid wind and wave farm, which is based on fluid models. The description and performance of thismodeling approachwill be introduced and compared to standard design procedures based on extensive simulations. Advantages and limitations of each approach will be underlined and the impact of input data quality will be discussed. [ABSTRACT FROM AUTHOR]