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Offshore wind farms (OWF) are being rapidly integrated to the power systems, which can be primarily attributed to saturation of the mainland onshore area, and high wind speed in offshore sites. However, to maintain stable grid operation, OWFs are required to comply with Low Voltage Ride Through (LVRT) capability. OWF during LVRT period should not only stay grid connected, but also support the grid during the disturbance. In this paper, a new communication-independent LVRT control scheme for HVDC connected OWF has been proposed. Under the proposed strategy, OWFs not only successfully ride through a fault induced voltage dip, but also comply with active power recovery ramp rate during the fault recovery period. Besides complying with LVRT requirement and ramp rate limit, the proposed strategy improves voltage dip induced frequency (VDIF) issue by increasing active power injection from the OWF during LVRT period. The proposed LVRT strategy also exploits the stored energy in HVDC link capacitors in order to reduce the impact of VDIF issue. The proposed and the conventional LVRT strategies for HVDC connected OWF have been implemented and evaluated on IEEE 39 bus benchmark system.

This paper proposes a new and comprehensive approach for optimal sizing and placement of a wind farm (WF) in a radial distribution network (RDN) while considering several factors associated with its reliability, operation, economics and environment impact. The consideration of the reliability aspect is essential to correctly assess the impacts of forced outages of wind turbines in the field. In this context, a sequential Monte Carlo simulation-based technique has been formulated to evaluate the performance of the WF with varying capacities at different locations in the RDN. Additionally, the capability of the WF to perform Volt-Var and Volt-Watt control has been capitalized on to provide voltage support to the grid in compliance with international standards (IEEE 1547-2018). Finally, a combination of Analytic Hierarchy Process and Technique for Order Preference by Similarity to Ideal Solution has been used to assess and determine the optimal size and location of the WF, based on a selected set of performance criteria, namely cumulative violations of the steady-state voltage range, cumulative losses in RDN, cumulative violations of the line thermal rating, levelized cost of energy, and reduction in carbon emission. The proposed approach has been validated considering different modes of voltage control on the standard 33-bus RDN system.

This paper proposes a comprehensive method to estimate non-synchronous inertia in an AC microgrid system by analyzing measured frequency transients. Non-synchronous inertia includes inertial support from various inverter-interfaced renewable energy sources and energy storage devices in the form of virtual or emulated inertia. In the proposed method, a multi-step approach is used to identify a matrix of parameters that estimates the overall non-synchronous inertial response. Firstly, motivations are drawn from the fundamental system requirements of adequacy in RoCoF and maximum deviation in frequency, for any credible contingency. A systematic classification of various contributors to the non-synchronous inertial response is also carried out. System-identification studies are then performed to identify various component-models that contribute to the non-synchronous inertia. The component models include a faster RoCoF-based response model, a relatively slower frequency deviation based model, and in some cases, a slow, integral-of-frequency based model. Lastly, the concept of releasable kinetic energy and non-linear estimation is used to identify the capability of the virtual inertia sources and improve the accuracy of the overall estimate. The developed methodology is tested in a microgrid network hosting a mix of renewable energy generations and battery energy storage systems. The estimate of non-synchronous inertia will help system operators to accurately estimate the maximum frequency deviation and the time to reach the maximum deviation for any credible contingency. The estimate can also help in deciding the type of virtual inertia that will best maintain frequency stability in the microgrid system.

This paper proposes a novel multi- objective planning framework to determine optimal capacity of battery energy storage system (BESS) for coordinated operation of large scale offshore wind farm (OWF) and BESS. Multiple objectives such as battery cost and lifetime, availability of wind turbines (WTs), expected energy not supplied (EENS), loss of load hour (LOLH) and curtailment of wind energy have been considered for determining optimal capacity of BESS for OWF. Wake effect loss in OWF and uncertainty of wind speed have been also incorporated the optimal BESS framework for determining aforementioned objectives. Moreover, as WTs are susceptible to random failure, availability of WTs is an important issue which has been also considered in the planning stage for determining optimal size of BESS in this study. One of the main features of the proposed approach is that it provides flexibility to decision makers in selecting best available alternative for BESS (among different types) to minimize mismatch between forecasted generation schedule and actual generation of OWF, while taking into account economical and operational reliability issues. The effectivene ss of the proposed planning strategy has been validated through different case studies conducted on Horns Rev- 1 OWF situated in Denmark.

Low-voltage ride-through (LVRT) compliance of wind energy conversion systems is one of the significant concerns for both the wind industry and the system operator. In this study, power electronic-assisted on load tap changer (OLTC) is proposed as a pragmatic solution to enhance LVRT performance of wind turbine generator (WTG). Furthermore, a coordinated control strategy between OLTC and WTG controller is proposed to mitigate post-fault over-voltage issues that are encountered due to the relatively slow dynamics of hybrid OLTC. The results suggest that employing hybrid OLTC does not only improve LVRT compliance of WTG but also significantly improve voltage dip-induced delayed active power recovery of WTG.

Large-scale renewable energy integration and consequent displacement of synchronous machines result in low system inertia. Low system inertia leads to high rate of change of frequency, higher frequency nadir, and potentially require higher primary frequency reserve to stabilise system frequency following a sudden generation shortfall. To address such frequency stability issue in wind energy integrated system, this paper has proposed a new and realistic control strategy for inertial and primary frequency support from HVDC connected Offshore Wind Farms (OWF). The proposed control strategy, which is based on offshore AC voltage control is not only addressing limitations of high rate of frequency change in the offshore grid, but also does not result in any frequency deviation in the offshore grid while providing frequency support to the main grid. The proposed control strategy is robust for any frequency deviation in the main grid, which utilises HVDC converters to map onshore frequency variation in to voltage variation in the offshore grid, thus enabling adequate inertial and primary frequency support from the OWF. The proposed control strategy has been tested on two test systems and the results are compared with conventional method to demonstrate the robustness, effectiveness and reliability of the proposed frequency support strategy.

This paper presents an improved method to estimate the available inertia in an islanded AC microgrid. Inertia estimation is carried out based on measured frequency response for any arbitrary disturbance that occurs in the system. Modifications are made to the conventional swing equation-based curve-fitting method to obtain an accurate estimate for a system with high penetration of renewable generations. A polynomial curve fit over the total power generation is introduced to estimate the size of the disturbance accurately. Additionally, a variable order polynomial fit is carried out over the measured frequency, which not only improves the estimate of inertia but also helps to refrain the influence of network topology and size/location of the disturbance. The test microgrid system considered is a modified Standard IEEE distribution network, which consists of radial feeders and distributed generations. Firstly, the proposed method is tested on a system with only synchronous generations to assess the accuracy of the estimate. This is followed by the integration of Type 3 and Type 4 wind turbines, and a PV array within the microgrid system. Virtual inertia control is then implemented in the wind turbines to obtain inertial support. Estimation study of the microgrid system with virtual inertia is then carried out. The developed estimation method can accurately estimate the inertia provided by the synchronous sources within the generation mix. Finally, from all the results and observations, the inertia estimation process in a microgrid system is segregated into synchronous and nonsynchronous inertia estimation.

In this paper, a novel wind farm control strategy is proposed to mitigate voltage dip induced frequency excursion in wind integrated power systems. In the proposed control strategy, conventional fixed reactive/active current injection priority is replaced by an optimum adaptive priority based reactive and active current injection. The control problem is formulated as a binary integer optimization problem, which simultaneously minimizes active power shortfall from wind generation during fault and complies with grid code requirements. Detailed wind farm model and wake effect are taken into consideration to achieve realistic response from the wind farm, while IEC 61400-27-1 wind turbine generator model is adopted for wind turbine modeling. Dynamic model of the test system has been developed in DIgSILENT PowerFactory, while the static model is also developed in MATLAB, and cosimulation platform has been used to test and validate the proposed control strategy. The proposed control strategy is applicable to variable speed wind turbine-based wind farm.

In conventional power systems, synchronous generators have been the main source of dynamic reactive power. However, displacement of conventional power plants by non-synchronous generation resources, such as wind power plants (WPPs) and other renewable energy sources, result in diminished dynamic reactive power reserve at system level. System operators in various countries are planning to tackle this issue by introducing dynamic reactive power reserve as a new ancillary service product. In this paper, reactive power capability of Type-4 wind turbine based WPP considering practical constraints is explored. A new and effective centralized WPP capability based control strategy is proposed to supply dynamic reactive power from a WPP, particularly, beyond grid code requirement. An adaptive method to dispatch reactive power from wind turbine generators (WTGs) in a WPP, is also proposed. In the presented study, wake effect and detailed WPP model are taken into consideration. The proposed strategy is implemented on three wind integrated benchmark power systems developed in DIgSILENT PowerFactory platform using IEC 61400–27–1 standard model of WTG.

The last few decades have seen renewable energy source (RES) integration growing at a rapid pace and this trend is likely to continue. The displacement of conventional synchronous plant, particularly at higher RES shares, is resulting in a ‘lighter’ system with low system inertia and reduced governor droop stiffness, leading to increased vulnerability to fast frequency stability. This paper presents a review of literature from the research and industry communities on possible sources of non-synchronous fast frequency reserve, considered as a potential solution to address diminishing synchronous reserve in RES integrated systems. The wide range of potential sources includes wind turbines, solar photovoltaic plant, energy storage, high voltage direct current (HVDC) systems, and demand response. The challenges and limitations associated with each approach are highlighted, and alternative strategies to maintain a minimum volume of security constrained fast frequency reserve are discussed based on the recent industry trends.

This paper presents an accurate and realistic estimation of reactive power capability of solar photovoltaic (PV) inverters considering ambient temperature, solar irradiance, and terminal voltage. Based on the accurate estimation of reactive power capability, a self-adaptive voltage controller is proposed to enable solar PV power plant participation in voltage control ancillary service. The proposed accurate and realistic estimation has revealed the possibility of solar PV power plant failing to comply with grid code requirements under extreme weather conditions. On the other hand, the proposed control strategy has shown significantly better effectiveness to utilise solar PV inverter capability, and provide better voltage control support service to the grid.

Wind generation in India is an up-rising power sector in recent times, which is nearly 10% of its total installed generation capacity. India is also exploring the potential of offshore wind energy near the coastal area of Gujarat. This paper presents an optimal and economic methodology to connect wind turbines (WTs) of 1 GW offshore wind farm (OWF) to be constructed near the Gujarat coast. The OWF consists of 500 WTs, with each having capacity of 2 MW. A segment of the proposed layout (about 25% of the total capacity) of the OWF has been identified for this study. Sweep algorithm is used to determine an appropriate number of WT clusters, followed by optimal cabling by minimizing the length of submarine cables of each WT cluster using Clarke and Wright saving algorithm. Each WT cluster is connected to an offshore substation (OS). The optimal placement of OS for the segment of OWF has been determined considering the lowest capital cost for the submarine cable in this study.

Renewable energy-driven displacement of synchronous generators leads to low system inertia which creates frequency stability issues in power system, thus necessitating system inertia estimation for secure and stable grid operation. However, one of the main challenges in system inertia estimation is frequency monitoring owing to the high cost associated with placing adequate number of measurement devices across the system. In this backdrop, a new approach for system inertia estimation with reduced number of frequency monitoring, however, without compromising estimation accuracy has been proposed in this paper. The frequency control areas depending on modal analysis based generator coherency are formed and single frequency measurement at each area determine system frequency response during grid disturbance, which in turn is used for system inertia estimation. The proposed approach of system inertia estimation has been implemented and validated on IEEE 39 bus system and Gujarat State grid developed in DIgSILENT PowerFactory simulation platform.

The penetration of offshore wind farms (OWF) connected to the electric utility grid through voltage source converter-based high-voltage DC (VSC-HVDC) increased year by year in some countries. However, penetration of HVDC-connected OWF affects electric power system voltage and frequency stability during a grid disturbance, which enforces system regulators to update their grid code regulations for secure and stable operation. Low voltage ride through capability (LVRT) for OWF ensures that OWF stays grid connected during a voltage disturbance and supports the electric grid during such low-voltage fault events instead of direct tripping. Several LVRT strategies for HVDC connected OWF are reported in the literature, however, without considering a critical factor of active power recovery (APR) ramp rate requirements. In this paper, three LVRT strategies have been investigated to comply LVRT and APR requirements. The different LVRT strategies for OWF considered in this study were tested under different scenarios in IEEE 39 bus benchmark system, and a comparative analysis of these LVRT strategies has been reported in this paper.

Large scale deployment of various Renewable Energy Sources (RES) due to climatic issues and depletion of fossil fuels has led to significant RES generation share in power systems especially from Solar Photovoltaic (PV). Among the various challenges of higher solar PV integration, voltage and frequency stability issues are of major concern to the system operator. Voltage dip induced frequency excursions due to delayed active power recovery from the solar PV systems following a network fault is one such critical issue that has not been investigated enough and reported in the literature. This paper examines potential impact of such delayed active power recovery from utility scale solar PV systems, and its effect on the system frequency stability under increasing PV penetration levels. The studies have been carried out on modified New England Benchmark system and Gujarat state power system in India for various system scenarios and cases.

This paper presents a new approach to select a suitable wind turbine (WT) for a wind farm (WF) site. In this approach, economy, reliability, resilience, and environmental aspects have been taken into account, as all the four aspects have a great deal of impact on the selection of a WT for a given site. Six commercially available WTs have been considered for this study. The energy output from a WF is significantly affected by wake, hence, in the proposed approach, placement of WTs in a WF is optimized using particle swarm optimization to reduce wake effect while increasing the energy production. A new algorithm has been proposed to find the reliability indices of the WF considering forced outage of WTs. In this context, two important reliability indices, such as expected energy not supplied and availability have been adopted to study the reliable viability of the optimized WF with all the six types of WTs. Power system resilience study has been gaining great deal of importance in recent times, but individual WF resilience study has not been addressed till now. This paper incorporates resilience as one of the criteria to select a suitable WT for any site. A new approach has been described in this regard to quantify the WF operational resilience by proposing an ideal resilience pentagon. The impact of all the six WTs on environment in terms of reduction of greenhouse gas emission has also been considered. Two different shaped WFs (same area) with same wind profile have been considered to study the proposed approach. A priority table has been formulated according to the ranking of WTs for different indices. The results show that the choice of WTs for WFs of different geometrical shape, however, with the same wind profile and the area may be different and largely depend on the choice of decision maker.

In this paper, a two-level planning framework has been proposed to assess the techno-economic feasibility of hybrid offshore wind-solar PV power plants. In the first level, the optimal layout design of hybrid offshore wind-solar PV plants is determined to maximize the generation considering wake effect and shadow loss for wind turbines (WTs) and solar PV panels, respectively. Particle Swarm Optimization (PSO) is used to determine the optimal layout of hybrid power plants. In the second level, economic analysis has been conducted to determine the feasibility of the hybrid offshore power plants for any particular site. In this context, two scenarios have been investigated. In scenario-1, the optimal share of wind and solar generation for an offshore hybrid plant of given capacity is determined from a techno-economic point of view. On the other hand, in scenario-2, the optimal share of solar PV panels utilizing the existing electrical infrastructure of offshore wind plants has been determined. Both cases are compared on the basis of the Levelized cost of energy (LCOE), and the best alternative has been highlighted for a site in this study. Yearly data of wind speed and solar irradiation of a site near the coast of Gujarat in India has been considered to investigate the proposed approach.

This paper presents a study on the impact of grid code requirements for grid connection of wind power generator on power system stability. Particularly, the interaction between voltage stability and frequency stability for grid code regulations of various countries has been studied. Fault current prioritization, post fault active power recovery and load model are the major aspects considered in this study. The results under different grid code regulations are compared with conventional generation to assess the impact of the high wind power penetration on the system voltage and frequency stability. IEEE 39 bus benchmark system, developed in DIgSILENT PowerFactory has been used as test system for this study. Wind turbine generators are modelled as per IEC 61400-27-1 standard.

This paper estimates reliability of an Offshore Wind Farm (OWF) considering wake effect as well as the failure probability of wind turbines. To calculate wake effect in the OWF, Jensen’s wake model has been considered while OWF reliability analysis has been conducted considering random failure of wind turbines (WTs). Using the data of probability of WT failure, a sequential Monte Carlo method has been developed to generate the failure matrix. Two different scenarios, viz; without any wind speed forecast error and with ± 20% error in wind speed forecast, have been studied to justify the effectiveness of the proposed approach. The proposed methodology developed in MATLAB® has been used to estimate reliability of Horns Rev-1 OWF.

Summary Effective phasor measurement unit (PMU) placement is a key to the implementation of efficient and economically feasible wide area measurement systems in modern power systems. This paper proposes a pragmatic approach for cost-effective stage-wise deployment of PMUs while considering realistic constraints. Inspired from a real world experience, the proposed approach optimally allocates PMU placement in a stage-wise manner. The proposed approach also considers large-scale wind integration for effective grid state monitoring of wind generation dynamics. The proposed approach is implemented on the Danish power system projected for the year 2040. Furthermore, practical experience learnt from an optimal PMU placement project aimed at PMU placement in the Danish power system is presented, which is expected to provide insight of practical challenges at ground level that could be considered by PMU placement software developers as well as for future research in this field. Copyright © 2016 John Wiley & Sons, Ltd.

Due to increasing demand for electricity, fast depletion of conventional fossil fuels and adverse environmental impact of fossil fuel use, there is a paradigm shift towards renewable energy (RE) sources, with solar photovoltaic (PV) and wind being the frontrunners. To address diminishing inertia issue in RE integrated system, this paper presents a control strategy to exploit fast frequency support from hybrid solar PV and wind power plant (HSWP). The fast frequency support from HSWP has been studied under different scenarios including inertial support without any de-loading, inertial support along with primary frequency support from de-loaded solar PV system, inertial support along with primary frequency support from de-loaded wind farm, and inertial support along with primary frequency support under de-loaded operation of both the Solar PV and wind farm in the HSWP. A comparative analysis of results under different scenarios is also presented.

This paper proposes a new approach for optimal placement of wind farm (WF) in a radial distribution network (RDN) considering reliability aspect of the WF. Since Wind turbines (WTs) are prone to failure, forced outage (or random failure) of WTs are considered to find reliability of a WF while selecting its optimal placement in a RDN in this study. In this context one year wind profile data of a real life site, and time series demand data from a real distribution system have been considered. A non-sequential sampling strategy has been applied to sample the state (fail or active) of WTs considering 95% availability over a year for individual WT in the WF and a Monte Carlo simulation is developed to assess reliability of the WF for each position (bus) in the RDN. Multi-stage power flow (PF) with one hour time resolution for entire year is performed for each position of WF in the RDN. The best position of WF is selected based on placement of WF in the RDN for which minimum number of cases of bus voltage limit violation (< 0.93 p.u.) occur and the aggregated electrical losses are least over 8760 hours (one year). The proposed approach is validated on IEEE 33 bus RDN system.