Adaptive droop control strategy for Flywheel Energy Storage Systems: A Power Hardware-in-the-Loop validation

Low-inertia power systems can suffer from high rates of change of frequency during imbalances between the generation and the demand. Fast-reacting storage systems such as a Flywheel Energy Storage System (FESS) can help maintain the frequency by quickly reacting to frequency disturbances, with no concern over its lifetime. While a modern high-speed FESS has a significantly higher energy density than the conventional low-speed ones, the capacity of this storage technology is still limited. Therefore, this paper proposes a new adaptive droop controller for a FESS, considering the practical advantages and also limitations of this storage technology. The proposed controller increases the contribution of the FESS for frequency support during the first instances of a disturbance, while it reduces its output when the frequency is recovering. To verify the advantages of the proposed control strategy, the controller is implemented on a real 60 kW high-speed FESS using the concept of rapid control prototyping. Next, the performance of the FESS with the new controller is tested using Power Hardware-in-the-Loop simulations in a low-voltage microgrid. The PHIL simulation results show that the proposed adaptive controller improves the performance of the FESS in terms of limiting the frequency deviations, while preserving more energy in the FESS

Cooling Requirements of Superconducting Power Cables

Superconducting cables have been built and tested for voltages larger than 100 kV, currents up to 5 kA and a length of up to 1 km. Within long term field tests, superconducting power cables have shown that they can fulfill all technical and operational requirements. This resulted in an increased interest for more applications and in contrast to the voltage and current design parameters, the cooling design parameters of a superconducting cable depend mainly on the length of the cable and the heat input. These parameters have to be adapted to each cable to fulfil the specification. This presentation gives an overview on cable specifications and requirements and points out specific cooling requirements. For two different examples of a medium and high voltage cable, design options are presented and discussed. It can be seen that the different loss contributions differ very much between medium and high voltage cables and that for example the interconnection between specification and maximum length before re-cooling needs a careful investigation. Finally, a design procedure is proposed that considers main specification and cooling requirements

Sizing of Hybrid Energy Storage Systems using Recurring Daily Patterns

A hybrid Energy Storage Systems (ESS) consists of two or more energy storage technologies, with different power and energy characteristics. Using a hybrid ESS, both high-frequency and low-frequency power variations can be addressed at the same time. For an accurate sizing of a hybrid ESS, the use of high-resolution data is required. However, high-resolution data over long periods leads to large data sets, which are difficult to handle. In this paper, an improved motif discovery algorithm is introduced to find the most recurring daily consumption patterns within the time series of interest. The most recurring pattern is selected as the representative of the time series for sizing the hybrid ESS. Next, a simple optimization framework is proposed for selecting the cut-off frequency of a low-pass filter, used for allocating the power to different storage technologies. Finally, the proposed sizing approach is applied for sizing a hybrid battery-flywheel ESS at four different low voltage distribution grids in southern Germany using real measurement data. It is demonstrated that a hybrid ESS, with the characteristics derived from the most recurring patterns only, can effectively provide their intended grid services for most of the days during the whole period of the time series

Sizing of Hybrid Energy Storage Systems using Recurring Daily Patterns

A hybrid Energy Storage Systems (ESS) consists of two or more energy storage technologies, with different power and energy characteristics. Using a hybrid ESS, both high-frequency and low-frequency power variations can be addressed at the same time. For an accurate sizing of a hybrid ESS, the use of high-resolution data is required. However, high-resolution data over long periods leads to large data sets, which are difficult to handle. In this paper, an improved motif discovery algorithm is introduced to find the most recurring daily consumption patterns within the time series of interest. The most recurring pattern is selected as the representative of the time series for sizing the hybrid ESS. Next, a simple optimization framework is proposed for selecting the cut-off frequency of a low-pass filter, used for allocating the power to different storage technologies. Finally, the proposed sizing approach is applied for sizing a hybrid battery-flywheel ESS at four different low voltage distribution grids in southern Germany using real measurement data. It is demonstrated that a hybrid ESS, with the characteristics derived from the most recurring patterns only, can effectively provide their intended grid services for most of the days during the whole period of the time series

Adaptive inertia emulation control for high-speed flywheel energy storage systems

Low-inertia power systems suffer from a high rate of change of frequency (ROCOF) during a sudden imbalance in supply and demand. Inertia emulation techniques using storage systems, such as flywheel energy storage systems (FESSs), can elp to reduce the ROCOF by rapidly providing the needed power to balance the grid. In this work, a new adaptive ontroller for inertia emulation using high-speed FESS is proposed. The controller inertia and damping coefficients vary using a combination of bang–bang control approaches and self-adaptive ones, to simultaneously improve both the ROCOF and the frequency nadir. The performance of the proposed adaptive controller has been initially validated and compared with several existing adaptive controllers by means of offline simulations, and then validated with experimental results. The proposed controller has been implemented on a real 60 kW high-speed FESS, and its performance has been evaluated by means of power hardware-in-the-loop (PHIL) testing of the FESS in realistic grid conditions. Both Simulations and PHIL testing results confirm that the proposed inertia emulation control for the FESS outperforms several previously reported controllers, in terms of reducing the maximum ROCOF and improving the frequency nadir during large disturbances

Project FASTGRID - Tests on 2G HTS for its Application in DC Resistive SC FCL

HVDC (High Voltage Direct Current) super-grids could become a future solution for the long-distance power-transmission. The Superconducting Fault Current Limiter (SCFCL) is a necessary facility to protect such transmission lines. In the framework of the project FASTGRID dedicated HTS wires for resistive type DC-SCFCL are under development. To reduce the cost per switching capacity: Reduction of the amount of HTS shall be achieved by increasing the allowed electrical field and the critical current density at operating conditions. A wire with an additional laminated 500 µm Hastelloy® shunt is the basic solution for FASTGRID. This work shows experiments on this prospective solution, compared with tests on bare coated conductors at lower E-field, once applied in ECCOFLOW SCFCL. The goal of this work is the validation of the HTS conductor for an electric field higher than 130 V/m for a fault clearing time of 50 ms

Predesign Considerations for the DC Link Voltage Level of the CENTRELINE Fuselage Fan Drive Unit

Electric propulsion (EP) systems offer considerably more degrees of freedom (DOFs) within the design process of aircraft compared to conventional aircraft engines. This requires large, computationally expensive design space explorations (DSE) with coupled models of the single components to incorporate interdependencies during optimization. The purpose of this paper is to exemplarily study these interdependencies of system key performance parameters (KPIs), e.g., system mass and efficiency, for a varying DC link voltage level of the power transmission system considering the example of the propulsion system of the CENTRELINE project, including an electric motor, a DC/AC inverter, and the DC power transmission cables. Each component is described by a physically derived, analytical model linking specific subdomains, e.g., electromagnetics, structural mechanics and thermal analysis, which are used for a coupled system model. This approach strongly enhances model accuracy and simultaneously keeps the computational effort at a low level. The results of the DSE reveal that the system KPIs improve for higher DC link voltage despite slightly inferior performance of motor and inverter as the mass of the DC power transmission cable has a major share for a an aircraft of the size as in the CENTRELINE project. Modeling of further components and implementation of optimization strategies will be part of future work