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In the quest of improving the frequency response capability of wind turbines, control solutions trying to emulate synchronous machines have been proposed. While their performance in terms of frequency support is relatively good, they may trigger torsional oscillations, cause fatigue and damage in the drivetrain, and reduce its lifetime. In this paper, the torsional oscillations caused by a virtual synchronous machine (VSM)-based frequency controller are illustrated, and methods for damping them are introduced. Two torsional oscillation damping methods are compared and combined to derive an improved method. Dynamic simulation results show better damping performance from the combined damping method.

An optimization framework for global optimization of the cable layout topology for Offshore Wind Farm (OWF) is presented. The framework designs and compares closed-loop and radial layouts for the collection system of OWFs. For the former, a two-stage stochastic optimization program based on a Mixed Integer Linear Programming (MILP) model is developed, while for the latter, a hop-indexed full binary model is used. The purpose of the framework is to provide a common base for assessing both designs economically, using the same underlying contingency treatment. A discrete Markov model is implemented for calculating the cable failure probability, useful for estimating the time under contingency for multiple power generation scenarios. The objective function supports simultaneous optimization of: (i) initial investment (network topology and cable sizing), (ii) total electrical power losses costs, and (iii) operation costs due to energy curtailment from cables failures. Constraints are added accounting for common engineering aspects. The applicability of the full method is demonstrated by tackling three differently sized real-world OWFs. Results show that: (i) the profitability of either topology type depends strongly on the project size and wind turbine rating. Closed-loop may be a competitive solution for large-scale projects where large amounts of energy are potentially curtailed. (ii) The stochastic model presents low tractability to tackle large-scale instances, increasing the required computing time and memory resources. (iii) Strategies must be adopted in order to apply stochastic optimization for modern OWFs, intending analytically or numerically simplification of mathematical models.

Two models and a heuristic algorithm to address the wind farm layout optimization problem are presented. The models are linear integer programming formulations where candidate locations of wind turbines are described by binary variables. One formulation considers an approximation of the power curve by means of a step-wise constant function. The other model is based on a power-curve-free model where minimization of a measure closely related to total wind speed deficit is aimed. A special-purpose neighborhood search heuristic wraps the formulations in order to increase tractability and effectiveness compared to the full model. The heuristic iteratively searches neighborhoods around the incumbent using a branch-and-cut algorithm. The number of candidate locations and neighborhood sizes are adjusted adaptively. Numerical results on a set of publicly available benchmark problems indicate that a proxy for total velocity deficit as objective is a functional approach, since high-quality solutions of an annual energy production metric are found. Furthermore, the proposed heuristic is able to match and in some cases improve the results obtained when considering the turbine positions as continuous variables.

The demand for low carbon energy calls for close to 100% renewable power systems, with decarbonization of other energy sectors adding to the anticipated paradigm shift. Rising levels of variable inverter-based renewable energy sources (VIBRES) are prompting questions about how such systems will be planned and operated when variable renewable generation becomes the dominant technology. Here, we examine the implications of this paradigm shift with respect to planning, operation and system stability, also addressing the need for integration with other energy vectors, including heat, transport and Power-to-X. We highlight the knowledge gaps and provide recommendations for improved methods and models needed as power systems transform towards 100% VIBRES. Keywords: Power system operation , variable inverter based renewables , power electronics , energy systems integration

Large-scale integration of renewable energy sources with power-electronic converters is pushing the power system closer to its dynamic stability limit. This has increased the risk of wide-area blackouts. Thus, the changing generation profile in the power system necessitates the use of alternate sources of energy such as wind power plants, to provide black-start services in the future. However, this requires grid-forming and not the traditionally prevalent grid-following wind turbines. This paper introduces the general working principle of grid-forming control and examines four of such control schemes. To compare their performance, a simulation study has been carried out for the different stages of energization of onshore load by a high-voltage direct-current (HVDC)-connected wind power plant. Their transient behaviour during transformer inrush, converter pre-charging and de-blocking, and onshore block-load pickup has been compared and analysed qualitatively to highlight the advantages and disadvantages of each control strategy.

The operation of transmission systems with large share of wind power is specially challenging under storm conditions. Under the stormy wind speed conditions, wind turbine protection system is designed to shut down the turbine to avoid excessive mechanical load. The sudden loss of wind power from large offshore plants is difficult to forecast accurately, which results in a large amount of power imbalance. The severity of such a wind power imbalance towards frequency stability needs to be studied for the future power systems. In addition, the overhead transmission lines can also be affected during storms, thereby increasing their probability of failure in the operation of power system under the islanded conditions. This paper investigates how the stormy weather can threaten the frequency stability of future Danish power system with large share of wind power and how to avoid the frequency instability through proper control and defence strategies such as high-voltage direct current (HVDC) control and load shedding. Sensitivity studies are performed for ramp rates of HVDC control, loadshedding strategies, inertia of the system with different volumes of disturbances to understand their impact on frequency stability.

Fast frequency response from wind turbines and plants has been playing an increasingly important role in our modern power system, which has a large share of wind energy integration. Frequency control from grid-following wind turbines and plants has been extensively investigated in the literature, whereas grid-forming frequency control methods have aroused people’s attention in recent years, with relatively little comparison available. This paper provides such a comparison, illustrating the differences between the two methods of providing frequency support in a power system. The comparison shows the superiority of grid-forming frequency control methods over grid-following ones, and gives a glance into power system frequency stability in the transition from grid-following to grid-forming frequency support supplied by wind power. The results are elaborated in a benchmark WSCC 9-bus system, which features detailed electromagnetic transient modeling of the components from a single converter, a wind turbine, and a wind plant to the whole power system.

Grid codes mandate that wind turbines (WTs) remain connected even during severe faults. This requirement can lead to WTs' loss of synchronisation (LoS) with the AC grid under severe symmetrical faults. In this paper, the LoS is illustrated by the power balance principle. Then, a solution with improved performance for avoiding LoS during severe faults is proposed, implemented and verified by simulations. The results show that the solution eliminates the steady-state frequency deviation during faults, resulting in a smoother WT transient during and post-fault.

As the size and distance from shore of new offshore wind power plants (OWPPs) increase, connection to shore using high-voltage (HV) direct-current (DC) technology becomes more cost-effective. Currently, every offshore wind power plant has a collection system based on medium-voltage alternating-current technology. Such systems rely on heavy and bulky low-frequency (i.e., 50 or 60 Hz) transformers: a drawback offshore, where equipment weight and space are restricted. Consequently, there is growing interest in medium-voltage direct-current collection systems, in which low-frequency transformers are replaced with DC/DC converters equipped with lighter and smaller medium-frequency transformers. However, the deployment of all-DC OWPPs still faces several challenges. Based on a very comprehensive and critical literature review, three of them are identified and discussed in this paper. The first challenge is the technological gap at component level. In this work, the DC/DC converter topologies most suitable for application to OWPPs are described and compared. The second challenge is the controllability of DC collection systems. Parallel, series and hybrid DC collection system layouts are presented and discussed. The third challenge is the compliance of all-DC OWPPs with current requirements for their connection to the onshore grids. The three challenges are discussed to highlight current research gaps and potential future directions.

An optimization framework for simultaneous design of wind turbines (WTs) and cable layout for a collection system of offshore wind farms (OWFs) is presented in this paper. The typical approach used in both research and practical design is sequential, with an initial annual energy production (AEP) maximization, followed then by the collection system design. The sequential approach is robust and effective. However it fails to exploit the synergies between optimization blocks. Intuitively, one of the strongest trade-offs is between the WTs and cable layout, as they generally compete; i.e. spreading out WTs mitigates wake losses for larger AEP but also results in longer submarine cables in the collection system and higher costs. The proposed optimization framework implements a gradient-free optimization algorithm to smartly move the WTs within the project area subject to minimum distance constraint, while a fast heuristic algorithm is called in every function evaluation in order to calculate a cost estimation of the cable layout. In a final stage, a refined cable layout design is obtained by iteratively solving a mixed integer linear programme (MILP), modelling all typical engineering constraints of this particular problem. A comprehensive performance analysis of the cost estimation from the fast heuristic algorithm with respect to the exact model is carried out. The applicability of the method is illustrated through a large-scale real-world case study. Results shows that (i) the quality of the cable layout estimation is strongly dependent on the separation between WTs, where dense WT layouts present better performance parameters in terms of error, correlation, and computing time, and (ii) the proposed simultaneous design approach provides up to 6 % of improvement on the quality of fully feasible wind farm designs, and broadly, a statistically significant enhancement is ensured in spite of the stochasticity of the optimization algorithm.

A methodology for optimum sizing of offshore wind farms high voltage AC export cables is presented. The method uses as main input site-dependent time series of wind power generation and seabed temperature, in order to apply Dynamic Temperature Estimation (DTE) analysis using a Thermo-Electrical Equivalent model (TEE), followed by an estimation of loss-of-life fractions in the insulation materials through a probabilistic lifetime model, known as Arrhenius-IPM. The lifetime estimation takes into consideration the effects of cable total length: the high capacitive currents and the statistical volume enlargement law. Furthermore, the cable lifetime is inferred based on the accumulated ageing effects previously quantified. The methodology is embedded in an optimization framework which provides a transparent, flexible, and scalable formulation. Finally, the applicability of the method is illustrated through a case study, complemented with a sensitivity analysis targeting the main parameters. Results show that a reduction of the objective function of around 5 % is achieved when using the proposed methodology.

Frequency support capability is becoming an important requirement for wind turbines, as wind power is increasingly integrated into power systems. In this paper, a frequency controller is implemented and validated. Such a controller allows wind turbines to help regulate the system frequency automatically and includes virtual inertia to help limit the rate of change of frequency. Compared with other methods, the controller achieves satisfactory frequency support capability with considerable simplicity. The controller is added to the grid-side converter controls, together with cascaded inner loops, which enables wind turbines to operate in grid-forming mode with overcurrent protection. The influence of the controller parameters on the frequency response is investigated.

Diode rectifiers (DRs) have been recently suggested as a viable alternative for connecting offshore wind farms (OWFs) to HVdc, eliciting growing interest from both industry and academia. However, energisation of DR-connected OWFs is not straightforward. The present study constitutes a proof of concept of a novel energisation method for DR-connected OWFs, in which auxiliary power is provided from the shore through the HVdc link and the dc bus bar of one or more WTs. The proposed method provides an alternative with minimal additional hardware, which can be easily extended to more WTs in the OWF, increasing reliability by providing redundancy. The study includes coinciding auxiliary loads with active and reactive power components and a semi-aggregated OWF model, in which every WT is individually represented in the string containing the energising WT. An additional sequence of simulation events is considered following the initial energisation sequence. Such sequence comprises wind power taking over the provision of the auxiliary power. The simulation results indicate that the proposed method is a suitable alternative for energising OWFs connected to HVdc via DRs.The manuscript has been submitted to the CIGRÉ Symposium 2021.

Diode-rectifier-based high voltage dc (DR-HVdc) systems can be a promising low-cost solution for exporting wind power from remote offshore wind farms to onshore power systems. Industrializing the offshore DR-HVdc requires technical maturation, achievable through in-depth studies and pilot experiments. Deployment of offshore DR-HVdc systems may entail a fundamental change of control philosophy in wind turbine (WT) converters from grid-following control to grid-forming. This article proposes a new grid-forming control for DR-connected offshore WT converters. The proposed controller uses two sequential control loops to regulate WTs’ active power, and maintain the frequency and voltage of the offshore ac network. The first control regulates the active-power mismatch of each WT into a voltage angle deviation, which leads to a frequency change. The second control adjusts the WT's alternating voltage magnitude to counteract the frequency change. An internal current control loop is used to limit the fault current and eliminate high-frequency resonances in the system. The proposed control is verified by electromagnetic transient simulations, including fault-ride through, WTs power change, reactive power disturbance, and WTs outage.

Before diode rectifier (DR) technology for connecting offshore wind farms (OWFs) to HVdc is deployed, in-depth studies are needed to assess the actual capabilities of DR-connected OWFs to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide communication-less frequency support (CLFS) to an onshore ac network. It is shown that the HVdc link’s offshore terminal direct voltage can be estimated from measurements at the OWF’s point of connection with the DR platform. Two different methods are proposed for implementing CLFS in the OWF active power controls. In Method 1, the estimated offshore terminal direct voltage is used for estimating the onshore frequency deviation. In Method 2, the actual offshore terminal direct voltage measurement is used instead. Unique features of the provision of CLFS from OWFs connected to HVdc via DRs are highlighted, and the dynamic and static performance of the CLFS control scheme is compared to that of the communication-based frequency support scheme. To assess the impact of parameter estimation errors on the provision of CLFS, a parametric sensitivity study is presented as well, and recommendations are given to increase accuracy.The manuscript has been accepted for publication in IEEE Transactions on Sustainable Energy.

Large-scale integration of renewable energy sources with power-electronic converters is pushing the power system closer to its dynamic stability limit. This has increased the risk of wide-area blackouts. Thus, the changing generation profile in the power system necessitates the use of alternate sources of energy such as wind power plants, to provide blackstart services in the future. This however, requires grid-forming and not the traditionally prevalent grid-following wind turbines. In this paper, four different grid-forming control strategies have been implemented in an HVDC-connected wind farm. A simulation study has been carried out to test the different control schemes for the different stages of energization of onshore load by the wind farm. Their transient behaviour during transformer inrush, converter pre-charge and de-blocking, and onshore block-load pickup, has been compared to demonstrate the blackstart capabilities of grid-forming wind power plants for early participation in power system restoration.

Offshore multi-terminal high-voltage dc (HVDC) grids are emerging as a technical reliable and economical solution to transfer more offshore wind energy to inland power grids. It is also envisaged that the offshore HVDC grids pave the way for both offshore wind participation and sharing inland frequency reserves for efficient power systems’ frequency control. The frequency control mechanism in an HVDC grid can be either centralized or decentralized. An autonomous frequency control (AFC) is a decentralized control, which does not require communication links between dc grid terminals. In the AFC, the dc-link voltage is used as a medium to reflect the inland frequency change to other terminals. The AFC has some technical and non-technical challenges, especially when the dc grid is connected to more than one shore ac systems. Among challenges of the AFC are the deficiency in meeting grid code requirements, adverse reaction of converters, dc voltage variations, difficulties of offshore wind participation in power markets. This paper proposes a new methodology of frequency control which uses both centralized control and AFC simultaneously. The proposed methodology shall improve system security and mitigate the listed problems of the AFC. A four-terminal dc grid is described and used for analysis and demonstration of the proposed methodology.

A MILP program for integrated global optimization of electrical cables systems in Offshore Wind Farms (OWFs) is presented. Electrical cables encompass the cable layout in collection systems to interconnect Wind Turbines (WTs), and transmission systems to couple Offshore Substations (OSSs) to the Onshore Connection Point (OCP). The program is solved through a modern branch-and-cut solver, demonstrating the ability to tackle large-scale instances with hundreds of WTs and several OSSs. The model supports as objective function the initial investment plus economic losses due to total electrical power losses. The importance and functionality of incorporating electrical losses is demonstrated, along with the need to simultaneously optimize the cable layout, OSSs location, and transmission cables. The method is tested for three case studies. The results show that (i) points near the global optimum, with an imposed maximum tolerance, are calculable within reasonable computational time and effort, and (ii) the integrated model can be much more efficient than a benchmark approach based on enumeration, i.e., exhaustive evaluation of all possible optimization problems derived from unique OSSs locations.

At present, wind farms control their power production by using a closed-loop feedback control approach, which distributes the total power to the wind turbines. However, the total power is distributed according to the turbines’ available power only. The use of model-predictive control allows considering multiple objectives, nonetheless, since it is open-loop, it can result in poor tracking of the total power reference. This work is the first to combine the standard, closed-loop feedback controller with model-predictive optimization (MPO) in order to yield the benefits of both approaches. As such, we developed an optimization-based dispatch function employed in a closed-loop feedback controller. The dispatch function uses model-predictive, multi-objective optimization to determine the distribution of the total power to the wind turbines. The model employed in the developed dispatch function is the Dynamic Flow Predictor, which uses Kalman-filter-driven feedback to correct the wind farm flow model dynamically. The developed optimization-based dispatch function is compared to dispatch functions commonly employed in present wind farms in a secondary regulation scenario in dynamic simulation. The comparison is carried out on an 80-turbine, large-scale wind farm. The newly developed, optimization-based dispatch function yields a reduction of the mean error and the normalized root-mean-square (NRMS) error by 43% and 36% with respect to the best-performing, commonly used dispatch function. Furthermore, for the large-scale wind farm, the duration of the MPO is only 0.21 s, which is two orders of magnitude faster than comparable approaches in the literature.

A two-stage stochastic optimization model for the design of the closed-loop cable layout of an Offshore Wind Farm (OWF) is presented. The model consists on a Mixed Integer Linear Program (MILP) with scenario numeration incorporation to account for both wind power and cable failure stochasticity. The objective function supports simultaneous optimization of: (i) Initial investment (network topology and cable sizing), (ii) Total electrical power losses costs, and (iii) Reliability costs due to energy curtailment from cables failures. The mathematical optimization program is embedded in an iterative framework called PCI (Progressive Contingency Incorporation), in order to simplify the full problem while still including its global optimum. The applicability of the method is demonstrated by application to a real-world instance. The results show the functionality of the model in quantifying the economic profitability when applying stochastic optimization compared to a deterministic approach, given certain values of cables failure parameters.

As installed wind generation capacities increase, there is a need to model variability in wind generation in detail to analyse its impacts on power systems. Utilization of meteorological reanalysis data and stochastic simulation are possible approaches for modelling this variability. In this paper, a combination of these two approaches is used to model wind generation variability. Parameters for the model are determined based on measured wind speed data. The model is used to simulate wind generation from the level of a single offshore wind power plant to the aggregate onshore wind generation of western Denmark. The simulations are compared to two years of generation measurements on 15 min resolution. The results indicate that the model, combining reanalysis data and stochastic simulation, can successfully model wind generation variability on different geographical aggregation levels on sub-hourly resolution. It is shown that the addition of stochastic simulation to reanalysis data is required when modelling offshore wind generation and when analysing onshore wind in small geographical regions.

The uncertainty arising from high levels of solar photovoltaic (PV) penetration can have a substantial impact on power system operation. Therefore, there is a need to develop models capable of representing PV generation in a rigorous manner. This paper introduces a novel transformation-based methodology to generate stochastic solar area power forecast scenarios; easy to apply to new locations. We present a simulation study comparing day-ahead solar forecast errors covering regions with different geographical sizes, total installed capacities, and climatic characteristics. The results show that our model can capture the spatio-temporal properties and match the long-term statistical properties of actual data. Hence, it can be used to characterize the PV input uncertainty in power system studies.

Weather-driven renewable generation is characterized by being uncertain and geographically dependent. In this regard, the recent deployment of wind and solar power has had a significant impact on the operation and planning of modern electricity grids; justifying the need to model high dimensional dependence. It is a relevant topic which is starting to have a significant importance in power systems. This paper presents a general overview on different multivariate dependence modeling techniques, namely parametric, non-parametric and copula functions. In addition, approximated methods based on limited information e.g. some statistical measures or a predefined dependence structure are presented. Autoregressive moving average (ARMA) and Markov models are discussed as general frameworks to reproduce spatio-temporal processes. Moreover, different applications in power systems are discussed in detail, along with a case study exemplifying the importance of a correct dependence modeling of wind generation.

The deployment of Renewable Energy Sources (RES) is driving modern power systems towards a fundamental green transition. In this regard, there is a need to develop models to accurately capture the variability of wind and solar photovoltaic (PV) power, at different geographical and temporal scales. This paper presents a general methodology based on meteorological reanalysis techniques allowing to simulate aggregated RES time series over large geographical areas. It also introduces a novel PV conversion approach based on aggregated power curves in order to capture the uncertainty associated to the technical characteristics of individual installations spread across large regions. The proposed methodology is validated using actual power data in Europe and can be applied to represent intermittent generation in network development plans, reliability and market studies, as well as operational guidelines.

A synchronous generator is directly coupled to grid in the novel wind turbine drive train concept based on electromagnetic coupler (WT-EMC). Similarly to conventional power plants, WT-EMC has inherent (inertial) grid frequency support capability, albeit rather limited due to its configuration. Additional power should be generated in response to a grid frequency drop in order to improve the dynamic characteristic of the grid frequency. In this paper, a novel control strategy for WT-EMC to improve the dynamic characteristic of grid frequency is proposed. The principle is to detect active power imbalance in the grid and then rapidly regulate the output power of WT-EMC. Based on the calculated electromagnetic torque of the synchronous generator in WT-EMC—acquired faster than the calculated grid frequency—the synchronous generator mechanical torque is controlled to track its electromagnetic torque to stabilize the rotor speed, therefore directly improving the grid frequency. The proposed control strategy effectiveness is firstly tested through simulations and then validated on a specially built experimental platform.

Diode rectifiers (DRs) have been recently suggested as a viable alternative for connecting offshore wind farms (OWFs) to HVdc, eliciting growing interest from both industry and academia. However, energisation of DR-connected OWFs is not straightforward. The present study constitutes a proof of concept of a novel energisation method for DR-connected OWFs, in which auxiliary power is provided from the shore through the HVdc link and the dc bus bar of one or more WTs. The proposed method provides an alternative with minimal additional hardware, which can be easily extended to more WTs in the OWF, increasing reliability by providing redundancy. The study includes coinciding auxiliary loads with active and reactive power components and a semi-aggregated OWF model, in which every WT is individually represented in the string containing the energising WT. Two additional sequences of simulation events are considered following the initial energisation sequence. Such sequences comprise wind power taking over the provision of the auxiliary power and the run-up to maximum (available) power production. The simulation results indicate that the proposed method is a suitable alternative for energising OWFs connected to HVdc via DRs.The manuscript has been submitted to IEEE Transactions on Power Delivery.

Nowadays, wind is considered as a remarkable renewable energy source to be implemented in power systems. Most wind power plant experiences have been based on onshore installations, as they are considered as a mature technological solution by the electricity sector. However, future power scenarios and roadmaps promote offshore power plants as an alternative and additional power generation source, especially in some regions such as the North and Baltic seas. According to this framework, the present paper discusses and reviews trends and perspectives of offshore wind power plants for massive offshore wind power integration into future power systems. Different offshore trends, including turbine capacity, wind power plant capacity as well as water depth and distance from the shore, are discussed. In addition, electrical transmission high voltage alternating current (HVAC) and high voltage direct current (HVDC) solutions are described by considering the advantages and technical limitations of these alternatives. Several future advancements focused on increasing the offshore wind energy capacity currently under analysis are also included in the paper. This work was supported by ‘Ministerio de Educación, Cultura y Deporte’ of Spain (grant numbers FPU16/04282 and EST18/00738). This research received no external funding.

Recently put forward as a feasible alternative for connecting OWFs to HVdc networks, diode rectifiers (DRs) have elicited growing interest from both academia and industry. Energisation of such OWFs is, however, not straightforward. The present study constitutes a proof of concept for the application of a novel energisation method to the case of an OWF connected to HVdc via DRs. The proposed method provides a robust and reliable alternative with minimal additional hardware, i.e. short additional dc cables connecting the dc bus bar of the energising WT to the HVdc link, and corresponding dc disconnectors at the cable terminals and at the DR dc terminals. This can be easily extended to more WTs in the OWF, increasing reliability by providing redundancy. The simulation results indicate that the proposed method is a suitable alternative for energising OWFs connected to HVdc via DRs.The manuscript has been accepted for presentation in the 45th Annual Conference of the IEEE Industrial Electronics Society (IECON 2019).

A mathematical program for global optimization of the cable layout of Offshore Wind Farms (OWFs) is presented. The model consists on a Mixed Integer Linear Program (MILP). Modern branch-and-cut solvers are able to solve large-scale instances, defined by more than hundred Wind Turbines (WTs), and a reasonable number of Offshore Substations (OSSs). In addition to the MILP model to optimize total cable length or initial investment, a pre-processing strategy is proposed in order to incorporate total electrical power losses into the objective function. High fidelity models are adapted to calculate cables current capacities, spatial currents. The MILP model is embedded in an iterative algorithmic framework, consisting in solving a sequence of problems with increasing size of the search space. The search space is defined as a set of underlying candidate arcs. The applicability of the method is illustrated through 10 case studies of real-world large-scale wind farms. Results show that: (i) feasible points can quickly be obtained in seconds, (ii) points near the global optimum with an imposed maximum tolerance, are calculable in reasonable computational time in the order of hours, and (iii) the proposed method compares favorably against a state-of-the art method available in literature.

An optimization framework for automated design of offshore wind farms collection systems is proposed in this paper. The core of the framework consists of a metaheuristic algorithm, namely a Genetic Algorithm (GA). The GA is designed for searching high-quality feasible solutions in terms of the capital expenditure (CAPEXcs); a subsequent step runs a power flow in order to calculate electrical power losses for estimating the collection systems share on the Levelized Cost of Energy (LCOEcs). Finally, after several executions of the full framework, the feasible solution bringing the cheapest LCOEcs is selected. The main inputs are the coordinate’s location of the wind turbines and the offshore substation (OSS), wind power production time series, and the set of considered cables for the collection system design. The proposed approach offers a full search space exploration for feasible solutions, while taking into account cables capacities and disallowing for cable crossings. The results show that this framework can find feasible solutions improving benchmark methods by 8%.

Diode rectifiers (DRs) have elicited increasing interest from both industry and academiaas a feasible alternative for connecting offshore wind farms (OWFs) to HVDC networks. However,before such technology is deployed, more studies are needed to assess the actual capabilities ofDR-connected OWFs to contribute to the secure operation of the networks linked to them. This studyassessed the capability of such an OWF to provide support to an onshore AC network by means of(active) power oscillation damping (POD). A semi-aggregated OWF representation was considered inorder to examine the dynamics of each grid-forming wind turbine (WT) within a string whenproviding POD, while achieving reasonable simulation times. Simulation results corroboratethat such an OWF can provide POD by means of OWF active power controls similar to thosedeveloped for OWFs connected to HVDC via voltage source converters, while its grid-forming WTsshare the reactive power consumption/production and keep the offshore voltage frequency andmagnitude within their normal operating ranges. Open-loop test results show that such capabilitycan, however, be restricted at operating points corresponding to the lowest and highest values ofactive power output.

Diode rectifiers have been gaining traction as a viable alternative for connecting offshore wind farms (OWFs) to HVdc networks. However, before technical connection requirements compatible with such solutions can be determined, more studies are needed to assess their capabilities to contribute to the secure operation of the networks linked to them. This study assesses the capability of such an OWF to provide support to an onshore ac network by means of primary frequency response (PFR). A semi-aggregated OWF representation is considered in order to examine the dynamics of each grid-forming wind turbine (WT) within a string when providing PFR. Simulation results corroborate that such an OWF can indeed provide PFR, while its grid-forming WTs share the reactive power and keep the offshore frequency and voltages within their normal operating ranges.

Contribution to the power systems’ frequency support is expected to be one of the essential ancillary services that wind power plants (WPPs) shall provide. The high-voltage DC (HVDC) connected offshore WPPs may provide this service with and without using fast communication links between onshore and offshore. In the case of offshore HVDC grid, implementing the communication-less frequency support is challenging. Although it increases the reliability of the frequency control, among other challenges, it is not straightforward to comply with relevant grid code requirements. In this paper, this issue is mathematically described and a static model is developed to calculate the deviation of various electrical parameters of an HVDC grid in case of frequency drop on the land ac systems. A solution for the aforementioned problem is presented and its associated concerns are addressed. The study is verified by simulations of a four terminals dc grid with two offshore WPPs and two inland ac systems.

This paper introduces a virtual synchronous machine (VSM) control scheme which is based on a minimized emulation of the model of a synchronous generator (SG) for a grid-forming wind turbine (WT) inverter. Based on this VSM control scheme, state-space representation and block diagram of the small-signal model for the VSM-controlled grid-forming inverter connected to an infinite bus are derived, which are used for small-signal stability analysis of the power angle in the VSM control scheme. This power angle small-signal stability (PAS) analysis is proposed as a simpler way for analyzing the influence on the rotor angle stability of power system from large share of VSM-controlled grid-forming inverters. Influence of parameters in the power loop of the control scheme on the PAS is investigated by analyzing the eigenvalue trajectories of the state matrix.

The development of efficient and reliable offshore electrical transmission infrastructure is a key factor in the proliferation of offshore wind farms (OWFs). Traditionally, high-voltage AC (HVAC) transmission has been used for OWFs. Recently, voltage-source-converter-based (VSC-based) high-voltage DC (VSC-HVDC) transmission technologies have also been considered due to their grid-forming capabilities. Diode-rectifier-based (DR-based) HVDC (DR-HVDC) transmission is also getting attention due to its increased reliability and reduced offshore platform footprint. Parallel operation of transmission systems using such technologies can be expected in the near future as new OWFs are planned in the vicinity of existing ones, with connections to more than one onshore AC system. This work addresses the control and parallel operation of three transmission links: VSC-HVDC, DR-HVDC, and HVAC, connecting a large OWF (cluster) to three different onshore AC systems. The HVAC link forms the offshore AC grid, while the diode rectifier and the wind farm are synchronized to this grid voltage. The offshore HVDC converter can operate in grid-following or grid-forming mode, depending on the requirement. The contributions of this paper are threefold. (1) Novel DR- and VSC-HVDC control methods are proposed for the parallel operation of the three transmission systems. (2) An effective control method for the offshore converter of VSC-HVDC is proposed such that it can effectively operate as either a grid-following or a grid-forming converter. (3) A novel phase-locked loop (PLL) control for VSC-HVDC is proposed for the easy transition from the grid-following to the grid-forming converter in case the HVAC link trips. Dynamic simulations in PSCAD validate the ability of the proposed controllers to ride through faults and transition between grid-following and grid-forming operation.

The current practice of power system restoration mainly relies on conventional power plants, which can provide black start in case of a black out using fossil fuels. HVdc-connected offshore wind power plants can, on the other hand, provide fast and environmentally friendly solutions for power system restoration, once their state of the art wind turbines are equipped with the grid-forming capability. In this paper, the background and existing solutions for wind turbine and wind power plant self-energization and onshore grid black start are presented, together with simulation results of an offshore wind power plant sequentially energizing the offshore ac network, offshore HVdc terminal, HVdc link, onshore HVdc terminal, and onshore ac terminal and load.

A state-of-the-art review of the optimization of electrical cables in Offshore Wind Farms (OWFs) is presented in this paper. One of the main contributions of this work is to propose a general classification of this problem, framed in the general context of the Offshore Wind Farms Design and Optimization (OWiFDO). The classification encompasses two complementary aspects. First, the optimum sizing of electrical cables, with the three main approaches used nowadays: static rated sizing, dynamic load cycle profile, and dynamic full time series, being conceptually analyzed and compared. The latest techniques and advances are described, along with the presentation of potential research areas not thoroughly addressed today, such as Dynamic Cable Rating, and cable’s lifetime estimation under time-varying conditions. Second, the network optimization of large OWFs is thoroughly presented, dividing the problem with a bottom-top approach: cable layout of collection system, Wind Turbines (WTs) allocation to Offshore Substations (OSSs), number and location of OSSs, and interconnection between OSSs and Onshore Connection Points (OCPs). A comparison among different methods is performed, taking into consideration the main engineering constraints. Global optimization, specifically, Binary Programming (BIP) or Mixed Integer Linear Programming (MILP), is envisaged as the best way to tackle this topic. The full combinatorial problem is found to be better addressed following a Top-Bottom approach, combining exact formulations with high level heuristics, or holistically with evolutionary algorithms.

As variable renewable energy (VRE) shares are growing around the world, power systems are becoming more weather dependent. The weather driven variability in VRE generation can cause challenges to the operation and planning of power systems. This paper investigates how the expected technology development of wind power affects VRE generation variability over a large geographical area. A case study of Northern Europe is presented, where a mixture of offshore wind, onshore wind and solar photovoltaic generation is considered. Different scenarios with a doubling of today’s annual VRE energy generation is modelled. The results show that modern wind turbine technology can significantly decrease the variability in aggregate VRE generation. When considering also an optimal mixture of different VRE sources, standard deviation of the aggregate VRE generation is estimated to be 31 % lower compared to simply doubling existing installations.

Virtual synchronous machine (VSM) control has been put up and developing for the past ten years, but not much work has been done specially focusing on its application on wind turbines (WTs). This paper gives a thorough review of the work that has been done until now about application of VSM control on WTs. Aspects on control schemes, energy storage (ES), frequency and voltage control capabilities, different grid conditions etc. are summarized separately. Further research work in this field is emphasized.

Lower costs and higher reliability make diode rectifiers (DRs) a promising alternative for connecting offshore wind power plants to HVdc networks. However, such offshore substations lack the grid-forming capability provided by voltage source converters. Hence, the grid-side converters of type-4 (fullconverter) wind turbines (WTs) have been proposed as viable candidates to take over such responsibility, apart from controlling the WT output power. In this work, a common synchronization reference (CSR) is proposed for the synchronization of such converters. Such reference is generated in the offshore HVdc substation and transmitted to the converters through the optical fibers embedded in the power cables. The proposed CSR enables the use of conventional current control for grid-forming WTs. Additionally, a scheme which combines communication-less and communication-based onshore frequency support is proposed and implemented. The proposed controls are validated by simulations under different operational scenarios.

A coordinated fast primary frequency control scheme from offshore wind power plants (OWPPs) integrated to a three terminal high voltage DC (HVDC) system is proposed in this study. The impact of wind speed variation on the OWPP active power output and thus on the AC grid frequency and DC grid voltage is analysed. The removal of active power support from OWPP after the frequency control action may result in second frequency (and DC voltage) dips. Three different methods to mitigate these secondary effects are proposed, such as, (i) Varying the droop gains of the HVDC converter (ii) Releasing the active power support from OWPP with a ramp rate limiter and (iii) An alternative method for the wind turbine overloading considering rotor speed. The effectiveness of the proposed control scheme is demonstrated on a wind power plant integrated into a three terminal HVDC system developed in DIgSILIENT PowerFactory. The results show that the proposed coordinated frequency control method performs effectively at different wind speeds and minimises the secondary effects on frequency and DC voltage.