• Energy Research

The energy balancing problem is the main challenge for the effective application of micro combined heat and power (m-CHP) in a residential context. Due to its high energy density and relative robustness, the lead-acid battery is widely used for power demand management to compensate the mismatch between the m-CHP electrical output and domestic demand. However, batteries are not suited to respond effectively to high frequency power fluctuations, but when coupled to the m-CHP, they experience frequent short-term charge/discharge cycles and abrupt power changes, which significantly decreases their lifetime. This paper addresses this problem by hybridising the lead-acid battery storage with superconducting magnetic energy storage (SMES) to form a hybrid energy storage system (HESS) that is coordinated by a novel sizing based droop control method. The control method for the first time considers both the capacity sizing of the HESS technologies and the droop control method of the battery and the SMES. A hardware in the loop test circuit is developed coupling with the real time digital simulator (RTDS) to verify the performance of the HESS with the new control algorithm. The experimental results show that control method is able to exploit the different characteristics of the SMES and the battery to meet the mismatch of m-CHP power generation and domestic demand. In addition, the lifetime analysis is implemented in this paper to quantify the battery lifetime extension in the HESS, which further proves the validity of the proposed control strategy.

Due to the increased fault-level currents, superconducting fault current limiter (SFCL) is more likely to penetrate into a low voltage and medium voltage transmission network to improve their stability and lower the electric devices capacity. Therefore it is important to model a SFCL in power system to analyze its performance and study its characteristics. In this paper, a simulation model for a resistive type SFCL consisted of YBCO tapes is developed using Matlab/Simulink software. This model will take into account SFCL's internal electromagnetic behavior by coupling its internal resistance and the current density characteristics based on the E-J power law. Finally, the SFCL simulation model is applied in a transmission and a wind farm power grid, respectively. Different fault limiting scenarios are investigated and the results show that the SFCL is effective in limiting fault currents with a maximum of 50% in transmission lines, particularly for wind farm networks.

This paper presents a preliminary study of Superconducting Magnetic Energy Storage (SMES) system design and cost analysis for power grid application. A brief introduction of SMES systems is presented in three aspects, history of development, structure and application. Several SMES systems are designed using the state of art superconductors and have taken into account their electromagnetic properties. The magnets of the SMES systems are designed using a global optimization algorithm. Both solenoid and toroid configurations are designed and compared. The capacities of SMES systems are designed on different scales including kJ-, MJ- and GJ-class for different applications. Finally the costs are calculated based on the commercial price of superconductors. The result indicates that the solenoid configuration is suitable for kJ-scale SMES, while the toroid configuration is suitable for GJ-scale SMES.

Current is no longer sinusoidal in modern electric networks because of widespread use of power electronic-based equipments and nonlinear loads. Usually AC loss is calculated for pure sinusoidal current, while it is no longer accurate when current is nonsinusoidal. On the other hand, efficiency of cooling system in large scale power devices is dependent on accurate estimation and prediction of the heat load caused by AC loss in design stage. Therefore, estimation of nonsinusoidal AC loss of high temperature superconducting (HTS) material would be of great interest for designers of large-scale superconducting devices. In this paper, at first nonsinusoidal AC loss of a typical HTS tape was calculated under distorted currents using H-formulation finite element method. Then, a range of artificial intelligence (AI) models were implemented to predict AC loss of a typical HTS tape. In order to find the best and more adaptive AI model for nonsinusoidal AC loss prediction, different regression models are evaluated using Support Vector Machine regression model, Generalized Linear regression model, Decision Tree regression model, Feed Forward Neural Network based model, Adaptive Neuro Fuzzy Inference System based model, and Radial Basis Function Neural Network (RBFNN) based model. In order to evaluate robustness of developed models cross-validation technique is implemented on experimental data. To compare the performance of different AI models, four prediction measures were used: Theil's U coefficients (UAccuracy and UQuality), Root Mean Square Error (RMSE) and Regression value (R-value). Obtained results show that best performance belongs to RBFNN based model and then ANFIS based model. U coefficients and RMSE values are obtained less than 0.005 and R-Value is become close to one by using RBFNN based model for testing data, which indicates high accuracy prediction performance.

Abstract Wind energy is seen as one of the main pillars of renewable energy. However, the intermittent nature of these sources still poses as a major challenge. Moreover, sensitivity to grid faults and response to load changes are also main concerns. Superconducting devices have been introduced to solve grid faults and energy storage problems associated with renewable energy sources. Nevertheless, the cost of superconducting materials was still a major drawback for their application in power grids. In this paper, a novel power electronics circuit is used to connect the superconducting magnetic energy storage (SMES) to a DC system based on a doubly fed induction generator wind turbine. The proposed system merges energy storage function and the fault current limiting function into one device which is referred to as SMES-FCL in this paper. The role played by the SMES-FCL is studied under various scenarios that may affect the whole system. The study of the system is carried in MATLAB/SIMULINK where the system is simulated in standalone and grid-connected modes. In the end, the proposed SMES-FCL control circuit is tested in a small-scale DC system experimentally.

This paper presents an SMES coil which has been designed and tested by University of Cambridge. The design gives the maximum stored energy in the coil which has been wound by a certain length of second-generation high-temperature superconductors (2G HTS). A numerical model has been developed to analyse the current density and magnetic field distribution and calculate the AC losses during the charge and discharge process of the coil. A cryostat has been designed and a test of the I-V curve measurement of the coil has been accomplished. In addition, the power electronics control of the SMES coil has been simulated.

Because of the booming development of offshore wind power around the world, a stable transmission system which is used for the connection between the offshore wind farms and the onshore grid is required. For the offshore wind farms not far from the coast, high voltage alternating current (HVAC) transmission system is the best choice. Aiming to study the transient problems caused by cable operation, a 60 km submarine cable is modeled in this paper using ATP-EMTP. The larger capacitance effect of HVAC submarine cables will cause more severe transient problems. Also, the variable wind power generated by offshore wind farm will bring undesired impact on the onshore power grid. This paper proposes a superconducting magnetic energy storage (SMES) system which can mitigate both the high frequency fluctuation of wind power and the transient over voltage of the HVAC cable system. In addition, SMES sizing study has been done to achieve the proposed functions.

Abstract Since high temperature superconducting magnetic energy storage system (HT SMES) has attracted significant attention for their fast response in milliseconds, high efficiency (cyclic efficiency over 95%) and unlimited times of charging and discharging cycles, it can be used for system stabilizing – damping out low frequency power oscillations. A voltage source converter (VSC) based HTS SMES has been optimal designed for achieving a high efficiency and has been constructed by China Electric Power Research Institute (CEPRI). This SMES can store the maximum energy, while for the first time used two states of art high temperature superconductors, YBCO and BSCCO tapes. It has been tested in a 110 kV transmission power system by a dynamic power fluctuation compensation experiment using three different controlling strategies in CEPRI. The experimental output powers with these three strategies are compared and the results show that the SMES can trace the power variation and provide the required power to restrain the power fluctuation in milliseconds successfully. Finally, the application planning of SMES with the equivalent capacity in a practical renewable power system at Zhangbei wind power test base is evaluated by a case study based on the PSCAD/EMTDC simulation. An optimal switch time of the SMES in wind power system is presented using the real transmission parameters of Zhangbei power grid. This study can provide a reference for the demonstration of large-scale SMES systems in renewable power system.