Comparison of electrical collection topologies for multi-rotor wind turbines

Multi-rotor wind turbines (MRWTs) have been suggested in the literature as a solution to achieving wind turbine systems with capacities greater than 10 MW. MRWTs utilize a large number of small rotors connected to one support structure instead of one large rotor with the aim of circumventing the square cube law. Potential benefits of MRWTs include cost and material savings, standardization of parts, increased control possibilities, and improved logistics for assembly and maintenance. Almost all previous work has focused on mechanical and aerodynamic feasibility, with almost no attention being paid to the electrical systems. In this research eight different topologies of the electrical collection network for MRWTs are analysed to assess which are the most economically and practically viable options. AC and DC collection networks are presented in radial, star, cluster and DC series topologies. Mass, capital cost and losses are estimated based on scaling relationships from the academic literature and up-to-date commercial data. The focus of this study is the assessment of the type of electrical collector topology, so component type and voltage level are kept consistent between topology designs in order to facilitate a fair comparison. Topologies are compared in terms of four main criteria: capital cost, cost effectiveness, total mass and reliability. A comparison table is presented to summarize the findings of the research in a convenient way. It is found that the most cost-effective solutions are the AC radial and AC star topologies, with the least cost-effective being the DC series-parallel and DC cluster topologies. This is due to the high cost of DC-DC converters and DC switchgear along with the lower efficiency of DC converters. Radial designs perform best in terms of efficiency and annual energy capture. DC systems achieve a slightly lower nacelle mass compared to their equivalent AC systems. DC topologies are generally found to be more expensive when compared to their AC counterparts due to the high cost of DC-DC converters and DC switchgear. Star topologies are considered to have the best reliability due to having no shared equipment. The most suitable collection topology for MRWTs is shown to be of the star type, in which each turbine is connected to the step-up transformer via its own cable

Voltage control ancillary services for low voltage distributed generation

This paper sheds light on the provision of voltage control ancillary services for low voltage distributed generation, DG, the need for the creation of the microgrid ancillary services market, since the ancillary service network operation does not include small size energy generation. The limitations facing the participation of DG in the provision of ancillary services, the basis for the creation of a microgrid ancillary service market, the gap between the large power plants and smaller sources, as well as useful recommendations to help proffer solution to the problem were considered. This work also embraces the types of ancillary services, microgrid, distributed generations, voltage control ancillary services, the various types of voltage control service and their usefulness to power system network. The various techniques for voltage control and the relationship between reactive power and voltage control, power factor and power factor corrections and its useful to power system was also illustrated

Dynamic wind power plant control for system integration using the generator response following concept

In this paper, a novel concept to integrate High Voltage Direct Current (HVDC)-connected offshore wind power plants with the onshore grid is presented. The concept makes use of a holistic wind farm controller along with a fully instrumented conventional synchronous generator at the point of common coupling. In our approach, the wind farm is able to replicate the natural response of the generator to a system, even enabling the wind farm to reproduce, in a scaled up manner, a range of ancillary services without having to rely on indirect frequency measurements which are prone to noise and delays. Simulation results are presented to validate the proposed solution

DC-link control filtering options for torque ripple reduction in low power wind turbines

Small Wind Energy Conversion Systems (WECSs) are becoming an attractive option for distributed energy generation. WECSs use permanent magnet synchronous generators (PMSGs) directly coupled to the wind turbine and connected to the grid through a single-phase grid-tie converter. The loading produced on the DC-link is characterized by large ripple currents at twice the grid frequency. These ripple currents are reflected through the DC bus into the PMSG, causing increased heating and ripple torque. In this paper, the PMSG inverter is used to control the DC link voltage. In order to avoid reflecting the ripple currents into the PMSG, the feedback DC-link voltage is passed through a filter. The Butterworth filters, notch filters, antiresonant filter (ARF) and moving average filter (MAF) are considered. For a fair comparison, formulas are provided to tune the filter parameters so that DC-link voltage control will achieve the selected bandwidth. The different filtering options produce different levels of torque ripple reduction. Notch Filter, ARF and MAF obtain the best results and there is a trade-off between the filter implementation complexity, bandwidth, overshoot and the torque ripple reduction. Simulations and experiments using a 2.5 kW PMSG turbine generator validate the proposals

Modelling stability improvement in Kazakhstan's power system by using battery energy storage

Kazakhstan is going to increase the share of RES up to 10% until 2030 and up to 50% until 2050. The current share of RES is 3% and BESSs are not used. This paper analyzes the simplified national power grid and the ability of BESS participation in frequency regulation in accident loss of generation on one of the stations. The results show that BESS only is not enough to keep frequency in desirable restrictions

Co-located battery energy storage optimisation for dynamic containment under the UK frequency response market reforms

The accelerated development of battery technologies heightens an interest in co-locating battery energy storage systems (BESSs) with renewable power plants for the stacking of multiple revenue streams such as frequency response services to AC grids. The frequency response market reforms in the UK introduce new end-state services and require evaluating the techno-economic feasibility of co-location projects in new circumstances. This paper develops a BESS optimisation method to optimise the capacity and operating strategy of a co-located BESS for providing the latest Dynamic Containment (DC) services based on the UK perspective. The BESS optimisation method simulates the BESS delivering DC responses and following operational baselines for state of energy (SoE) restoration as well as coordinating with its co-located power plant. Then the net present value of the BESS co-location project is estimated from power flows across the system and maximised to suggest the optimal BESS capacity, target energy footroom and/or headroom levels for baseline estimation, and possible SoE ranges suitable for energy interchange with its co-located power plant. The BESS optimisation method is tested based on a particular transmission-level wind farm in the UK and discussed alongside the operation and profitability of a BESS co-location project under frequency response market reforms

The co-development of offshore wind and hydrogen in the UK – a case study of Milford Haven South Wales

Green hydrogen produced from renewable energy resources can not only contribute to the decarbonisation of different energy sectors, but also serve as a carrier for long-distance delivery of renewable generation, offering a cost-effective way to exploit the renewables far from electrical grids. To facilitate the co-development of offshore wind and hydrogen, the paper develops a modelling framework to dispatch power and hydrogen flows across dedicated offshore wind hydrogen production systems to meet onshore hydrogen demands while keeping similar state of charge levels between multiple systems. Then the hydrogen supply to shore and the system investments and ongoing costs are discounted to their present values to calculate the levelised cost of hydrogen, which is minimised by the particle swarm optimsiation algorithm to suggest the best capacities of hydrogen system components including converters, desalination devices, electrolysers, compressors and storage assets. The proposed modelling framework is tested based on a case study at Milford Haven South Wales which is evaluated to have massive offshore wind resources in the Celtic Sea and comparable demands for hydrogen by 2040. The optimisation results are presented based on the techno-economic input parameters projected for 2030 and 2050 scenarios and discussed alongside the influences of technology advances on the system optimisation and resulting metrics including the levelised costs of hydrogen, net present values and potential levels of green hydrogen supply to Milford Haven

Control design of a neutral point clamped converter based active power filter for the selective harmonic compensation

This paper presents the control of an active power filter (APF) based on a 3-phase, 3-level neutral point clamped (NPC) converter with selective harmonic compensation. To achieve the selective harmonic compensation, the APF use several synchronous rotatory frames, which are rotating at the angular frequency and sequence of their respective harmonics, to detect and control the magnitude and angle of each individual harmonic using d and q variables. A three dimensional space vector modulator (3D-SVPWM) is used to generate the compensation currents. Due to its multilevel topology, the proposed active power filter can be used in high voltage power quality applications, such as sub-transmission and distribution levels. Simulation results are shown to validate the proposed solution and corroborate the proper function of the multilevel active power filter

Mathematical modelling of reduced order induction machines for VFT applications

This paper presents a reduced order approach to the modelling of induction machines for the purposes of power transfer between asynchronous networks. The method of interconnection between the networks under consideration is a variable frequency transformer (VFT) whose technology is rooted in that of an induction machine. Attempts to apply a high-order model to VFT studies have been met with numerical instabilities and unacceptably long computation times. A reduced order approach therefore serves to reduce the complexity of the simulation by neglecting transient behaviour. The classical fifth order method is presented, upon which a third order and first order model are derived. These differ from the traditional reduced order models due to the inclusion of an otherwise neglected frequency differential term. Care has also been taken to avoid algebraic loops during simulation so that additional delays need not be introduced. The performance of the models is then analysed when they are operating as an induction machine and then as a VFT for asynchronous power transfer simulations. The result is a stable, low-order model that performs correctly under a wide range of VFT operative scenarios, including asynchronous and different-frequency networks connected to its terminals

Dynamic wind power plant control for system integration

This project presents a holistic wind farm control approach that enables wind power plant to provide the full range of ancillary services including synthetic inertia at the wind farm level rather than single turbine level. In order to detect a power system event and select the magnitude of the service provision from the wind farm, a fully instrumented small/medium generator is used. By slaving the wind farm output to the generator natural response during power system events, the wind farm is able to provide a stable scaled-up range of ancillary services without relying in delayed or noisy grid frequency measurements