Optimization modeling of frequency reserves and inertia in the transition to a climate-neutral electricity system

The ongoing transition towards electricity production systems that are dominated by wind and solar power challenge both the traditional strategy for meeting a varying electricity demand and the traditional way of controlling the AC frequency of the electricity grid. This work investigates how frequency reserves (FR) and inertia, as well as inter-hourly variation management interact in the transition to a climate-neutral electricity system. For this purpose, a linear optimization model is developed to co-optimize investments in and operation of generation capacity and storage, as well as the supply of inertia and FR. The model is applied to three European geographic contexts, northern Europe, the British Isles, and the Iberian Peninsula, with different availability levels of wind, solar and hydro power resources. In addition, the model is applied to four separate indicative years, representing the current system and near-, mid- and long-term futures.\ua0The results indicate that while FR and inertia may increase the total system cost and investments, this will not decrease the cost-optimal share of renewable energy as the electricity supply-side transitions away from fossil fuels. Instead, the modeling shows that double-use of battery investments for FR and inter-hourly variations slightly increases the share of electricity supplied by wind and solar power. It is also shown that an electrified car fleet has the potential to eliminate all system costs associated with FR and inertia if a sufficient share of vehicles (30%) participates at no cost.\ua0The importance of specific technologies used for FR and inertia is investigated by excluding one-by-one the batteries, power-to-heat, and wind and solar power from the inertia and frequency reserve supply. The findings indicate that batteries confer the greatest reduction in the cost of FR and inertia, with wind and solar power and power-to-heat having system cost impacts only in the northern Europe case

Spatially-resolved and temporally-explicit global wind energy potentials as inputs to assessment models

Several decarbonisation scenarios indicate that renewable energy will be a key supply route to mitigate carbon emissions this century. To better represent the implications of such an energy transition, it is important that energy systems models (ESMs) can realistically characterise the technical and economic potential of renewable energy resources. This thesis presents a temporally-explicit and geospatially-resolved methodology for estimating the global wind energy potential, i.e. the annual terawatt-hour (TWh/yr) production potential of wind farms, assuming that capacity could be built across the viable onshore and offshore areas of each country, globally. Further, a geospatially-resolved levelised cost of electricity (LCOE) model is developed to characterise the offshore cost potential, accounting for non-resource related cost factors. Capacity potential is produced in tranches according to the average annual capacity factor and the capacity factor in each time slice. For offshore wind, capacity potential is also disaggregated by the distance to shore and water depth, which are the main cost drivers. A technology-rich description of fixed and floating foundation types allows LCOEs to be calculated for each grid cell across the globe, relative to location-specific annual energy production (AEP). Results show that the global wind energy potential is vast, but severely diminished if areas far from electricity infrastructure are discounted. Nevertheless, for onshore wind the capacity potential for capacity factors above 15% is 267 TW, with a generation potential of 580,000 TWh/yr. The offshore potential is 329,600 TWh/yr with a relatively smaller deployment capacity of 85.6 TW, reflecting the access to higher capacity factors in offshore areas. Deployment potential is favourable for countries with large shallow water areas. However, recent cost developments have made access to transitional and deep water locations much more feasible as long as turbine size increases continue to offset the relatively higher foundation costs.Open Acces

Frequency control and synthetic inertia in energy systems modelling

This study investigates how inclusion of frequency control constraints in electricity system modelling impacts the investment and dispatch in electricity generation and storage technologies for high-VRE futures. This is done using a linear cost-minimizing investment and dispatch model using historical load, wind and solar power conditions from Spain, Ireland, Sweden and Hungary for the year 2050. With an hourly time-resolution, constraints are added to ensure that, within each hour, sufficient inertial power and reserves are available to control the frequency. Comparing the results with and without these constraints show that nearly all impact on the results is in battery investments and operation. Furthermore, it is found that reserve requirements have a higher impact on system composition and operation than inertial power requirements

Inclusion of frequency control constraints in energy system investment modeling

This study investigates how the inclusion of frequency control constraints in electricity system modeling impacts the levels of investment and dispatch in electricity generation and storage technologies for futures that include high-level penetration of variable renewable energy. This is achieved using a linear cost-minimizing investment and dispatch model using historic load, wind and solar conditions from Spain, Ireland, Sweden and Hungary for Year 2050. With an hourly time-resolution, constraints are added so as to ensure that, within each hour, sufficient inertial power and reserves are available to control the frequency of the power grid. Comparing the results obtained with and without these constraints reveals that the main impacts on the results are from battery investments and operation. Furthermore, it is found that the reserve requirements exert a greater impact on system composition and operation than do the inertial power requirements

Adaptation of VSC-HVDC Connected DFIG Based Offshore Wind Farm to Grid Codes: A Comparative Analysis

Lack of synchronism between VSC-HVDC (Voltage Source Converter - High Voltage Direct Current) connected offshore wind farm and onshore grid leads to immunity of wind turbines to grid contingencies. Focusing on DFIG (Doubly Fed Induction Generator) based wind farms; this paper has presented a univalent control structure based on inertial and primary frequency response in which DC link voltage is utilized as synchronization interface. Based on the presented structure, four approaches based on the communication system, frequency, voltage and combined frequency and voltage modulation are utilized and compared to inform the onshore grid status to individual wind turbines. Considering Kondurs two area power system, results have revealed that all four approaches have similar ability (with negligible error) in offering inertial and primary frequency response to improve slow network oscillations. On the other hand, voltage and combined frequency and voltage modulation approaches have the ability to satisfy Fault Ride Through (FRT) requirements thanks to superior dynamics. However, communication and frequency modulation approaches lose that ability as communication and frequency measurement delays increase respectively. It has been concluded that combined frequency and voltage modulation, as the superior approach, has advantages like minimum FRT DC voltage profile increase and deviation from operating point after the fault, the minimum imposition of electrical and mechanical stress on DFIG and preservation of prevalent control structure thanks to appropriate dissociation between slow and fast dynamics. ©2019. CBIORE-IJRED. All rights reserved Article History: Received Dec 8th 2017; Received in revised form July 16th 2018; Accepted December 15th 2018; Available online How to Cite This Article: Yazdi, S.S.H., Milimonfared, J. and Fathi, S.H. (2019). Adaptation of VSC-HVDC Connected DFIG Based Offshore Wind Farm to Grid Codes: A Comparative Analysis. Int. Journal of Renewable Energy Development, 8(1), 91-101. https://doi.org/10.14710/ijred.8.1.91-10

Frequency control studies: A review of power system, conventional and renewable generation unit modeling

Over the last decades, renewable energy sources have increased considerably their generation share in power systems. As a consequence, in terms of frequency deviations, both grid reliability and stability have raised interest. By considering the absence of a consensual set of models for frequency control analysis, both for the different generation units (conventional and renewables) and the power system itself, this paper provides extensive and significant information focused on the models and parameters for studies about frequency control and grid stability. An extensive analysis of supply-side and power system modeling for frequency stability studies over the last decade is presented and reviewed. Parameters commonly used and assumed in the specific literature for such simulations are also given and compared. Modeling of generation units are described as well, including both conventional and renewable power plants.The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper

Active power control response from large offshore wind farms

This thesis was submitted for the degree of Doctor of Engineering and awarded by Brunel University LondonThe GB power system will see huge growth in transmission connected wind farms over the next decade, driven by European clean energy targets. The majority of the UK’s wind development is likely to be offshore and many of these wind farms will be interfaced to the grid through power converters. This will lead to a loss of intrinsic inertia and an increasing challenge for the system operator to keep grid frequency stable. Given this challenge, there is increasing interest in understanding the capabilities of converter control systems to provide a synthesised response to grid transients. It is interesting to consider whether this response should be demanded of wind turbines, with a consequential reduction in their output, or if advanced energy storage can provide a viable solution. In order to investigate how large offshore wind farms could contribute to securing the power system, wind turbine and wind farm models have been developed. These have been used to design a patented method of protecting permanent magnet generator’s converters under grid faults. Furthermore, these models have enabled investigation of methods by which a wind turbine can provide inertial and frequency response. Conventionally inertial response relies on the derivative of a filtered measurement of system frequency; this introduces either noise, delay or both. This research proposes alternative methods, without these shortcomings, which are shown to have fast response. Overall, wind farms are shown to be technically capable of providing both high and low frequency response; however, holding reserves for low frequency response inevitably requires spilling wind. Wind’s intermittency and full output operation are in tension with the need of the power system for reliable frequency response reserves. This means that whilst wind farms can meet the technical requirements to hold reserves, they bid uncompetitive prices in the market. This research shows that frequency response market prices are likely to rise in future suggesting that the Vanadium Redox Flow Battery is one technology which could enter this market and also complement wind power. Novel control incorporating fuzzy logic to manage the battery is developed to allow a hybrid wind and storage system to aggregate the benefits of frequency response and daily price arbitrage. However, the research finds that the costs of smoothing wind power output are a burden on the store’s revenue, leading to a method of optimising the combined response from an energy store and generator that is the subject of a patent application. Furthermore, whilst positive present value may be derived from this application, the long payback periods do not represent attractive investments without a small storage subsidy.The Engineering and Physical Sciences Research Council (EPSRC) and GE Energ