Real Time Hardware-In-The-Loop Comparison of Frequency Estimation Techniques in Application to ROCOF Based Islanding Detection

© 2018 IEEE. Rate of change of frequency (ROCOF) is one of the most common islanding detection techniques because of its simplicity and economical benefits. However, it does need thorough considerations to avoid mis-operations of ROCOF relays during islanding. These relays operates based on frequency estimation, which is performed either from the phase voltages or by obtaining the speed measurement from generator. With the penetration of distributed generation (DG), there is high probability for unavailability of frequency measurement from a synchronous generator. This calls for frequency estimation based on network voltages and this has a significant influence on ROCOF operation performance. This paper discusses the most commonly used frequency estimation methods and summarizes the usability of these methods through real-time hardware-in-the-loop (RT-HIL) simulations for ROCOF relay

Comparison of wind farm topologies for offshore applications

Increasing energy demand and environmental factors are driving the need for the green energy sources. The trend, in general, with respect to wind farms is to increase the number and the size of wind farms. The wind farms are also being located offshore with the prospect of more consistent and higher energy capture. The offshore wind farms are likely to move farther off from the shores to reduce visual impacts and increase the size. But, this has implications in terms of design of the collection grid and grid interconnection. Farms of 1 GW size and at distances of about 100 km are envisaged [1]. The design of collection system and turbine interconnection will become very important as the farms move farther offshore. Proper choice of collection system topology is important from the point of view of maximum energy capture while ensuring a high reliability of the design. Different collection system topologies have been proposed by researchers before, with the radial system being most popular. One of the key factors in selection would be the losses in the farm. In order to select the most suitable topology a comparison of different topologies with respect to losses, reliability and costs has to be done. Comparisons of calculations indicate that the DC series and series-parallel wind farm design may be options for future wind farm designs. The DC series and series-parallel design have lower reliability, but can be improved by providing redundancies. The designs have equipment costs almost equal to the AC wind farm costs. The losses in DC series-parallel wind farm are higher by about 12 % when compared to AC wind farms. The DC series design is also very attractive design, but has restrictions with respect to insulation. Also, the required turbine ratings may be significantly and unrealistically high when it comes to designing large wind farms. It can also be concluded that the novel designs require significant amount of work before these can be used in real wind farms. © 2012 IEEE

Reliability assessment of DC wind farms

New offshore wind farms are being proposed to be located at distances of 50-100 km from the shore. These large distances make HVDC connection to grid a feasible option. DC collection grids have been proposed to eliminate offshore platforms in wind farms. Two main topologies previously proposed for DC offshore wind farms with series connection of wind turbines to eliminate offshore platforms have been studied. The aspects of reliability of wind farm collection system are important in order to attract investors and make the designs physically realizable. Wind farms are large economic investments and in order to make good economic decisions a very important factor is the amount of additional income that can be generated over the life time of the wind farm. Thus the reliability calculations have to be included as a part of design process. The collection system reliability for DC Series and Series-Parallel topologies is mainly addressed in this paper. Various reliability indices like the interruption frequency, interruption duration and expected energy not supplied are calculated using an analytical method. The largest contributors to unreliability are identified. Alternative designs with added redundancy are proposed and evaluated. The important factor of possible additional income that can be generated is calculated to select and improve designs to be implemented. The evaluation results indicate that the DC Series-Parallel wind farms offer a great promise in terms of feasibility and reliability. It is observed that the DC Series design can be used as base system for calculations and understanding of the important factors in reliability. © 2012 IEEE