• Energy Research

Frequency dynamics, occurring due to the high penetration of the renewable energy in the microgrid (MG) are of great concern to the system dynamic stability. The battery energy storage systems are reported to have a good frequency regulating ability in the off-grid microgrid systems. However, to compensate the power irregularities, the battery is needed to charge and discharge at a high frequency, which degrades its lifetime significantly. In addition, in the primary frequency control (PFC) the battery needs to deal with the abrupt power changes, which will also accelerate the battery degradation process. In this regard, this paper presents a new concept of primary frequency control by integrating the superconducting magnetic energy storage (SMES) with battery, thus achieving the ability of not only performing a good frequency regulating function but also extending the battery service time. A novel power sharing method using the dynamic droop factors to control charge/discharge prioritization between the SMES and the battery is proposed and has been proved to have a better operation than the preceding droop control. A microgrid system based on the case of Uligam Island of Maldives is developed in the PSCAD, verifying the performance of PFC with the hybrid energy storage system (HESS) using the dynamic droop control. The results show that the HESSs have a better frequency regulating ability and the proposed dynamic droop control is capable of exploiting the different characteristics of both SMES and battery, forming a kind of complementary hybrid energy storage system. Moreover, the battery in the new control scheme is better protected from the short-term frequent cycles and abrupt currents, hence has been proved to have a longer lifetime extension.

Abstract This study proposes a hybrid energy storage system (HESS) composed of the superconducting energy storage system (SMES) and the battery. The system is designed to compensate power fluctuations within a microgrid. A novel control method is developed to share the instantaneous power between the SMES and the battery. The new control scheme takes into account the characteristics of the components of the HESS, and the battery charges and discharges as a function of the SMES current rather than directly to the power disturbances. In this way, the battery is protected from the abrupt power changes and works as an energy buffer to the SMES. An new hardware-in-loop experiment approach is introduced by integrating a real-time digital simulator (RTDS) with a control circuit to verify the proposed hybrid scheme and the new control method. This paper also presents a battery lifetime prediction method to quantify the benefits of the HESS in the microgrid. A much better power sharing between the SMES and the battery can be observed from the experimental results with the new control method. Moreover, compared to the battery only system the battery lifetime is quantifiably increased from 6.38 years to 9.21 years.

Abstract Electric Vehicles (EVs) have been suggested as alternatives to conventional vehicles for reducing petrol consumption and carbon dioxide (CO 2 ) emissions. When a large number of EVs connect to the grid, they can cause a large amount of power loss. Where to install multiple charge stations in the grid, so as to mitigate losses caused by EVs when providing energy to those EVs, is becoming vitally important. In this paper, a distribution test-network model is described. A new analytical method is proposed, using the stations’ cooperation in terms of optimal active and reactive power dispatch as well as power flow analysis for locating the optimal placement of charge stations, so as to reduce power losses. This method is compared with the previously developed current density method for single charge stations using system simulation results. It was demonstrated that the methods proposed in this paper are more accurate than the current density method, and that 17% of the average active power loss can be saved for three different types of load profile. In addition, 27% of the average active power loss was saved by installing two charge stations rather than no charge stations in the test-line. It is shown that this could represent a 2.6% annual yield above inflation for investing in installing and running such charge stations.

It is of great industrial interest and academic importance to investigate current harmonics impacts on AC losses of superconductors, especially in large-scale power devices. However, only effect of amplitude of in-phase current harmonics on AC loss has been studied in the works of literature. We numerically characterized nonsinusoidal AC loss of superconducting tape carrying harmonic currents with orders below 20th versus phase angles. A drastic AC loss variation was found when phase angle was considered for harmonic components. We observed that different harmonic orders show different AC loss profile versus phase angle.

Abstract In off-grid wind energy systems, batteries often undergo frequent charge/discharge cycles, which reduce battery service life. In addition, due to motor start and other high ‘inrush current’ loads batteries undergo high rates of discharge which also degrade battery life. In this paper, a superconducting magnetic energy storage and battery hybrid energy storage system is proposed, which is beneficial in reducing battery short term power cycling and high discharge currents. To demonstrate system performance, a representative off-grid wind power system model is described in detail which incorporates turbulent wind variations, load variations and energy storage systems. To estimate battery lifetime improvement, a novel battery lifetime model is described, which quantifies the impact of both the number of charge/discharge cycles and also the effect rate of discharge. The model is validated using previously reported data. This work advances previous studies by describing the estimated improvement in terms of battery life in a wind energy conversion application by use of superconducting energy storage and by presenting a novel method for doing so. In addition, the proposed battery lifetime model can be potentially used in other applications.

Linear wave energy converters generate intrinsically intermittent power with variable frequency and amplitude. A composite energy storage system consisting of batteries and super capacitors has been developed and controlled by buck-boost converters. The purpose of the composite energy storage system is to handle the fluctuations and intermittent characteristics of the renewable source, and hence provide a steady output power. Linear wave energy converters working in conjunction with a system composed of various energy storage devices, is considered as a microsystem, which can function in a stand-alone or a grid connected mode. Simulation results have shown that by applying a boost H-bridge and a composite energy storage system more power could be extracted from linear wave energy converters. Simulation results have shown that the super capacitors charge and discharge often to handle the frequent power fluctuations, and the batteries charge and discharge slowly for handling the intermittent power of wave energy converters. Hardware systems have been constructed to control the linear wave energy converter and the composite energy storage system. The performance of the composite energy storage system has been verified in experiments by using electronics-based wave energy emulators.

With DC grids, the usage of PMSG becomes more feasible due to the saving of inverter stage. However, integration of PMSG with DC grids faces some technical problems. The most critical problem is voltage stabilization. Any sudden change in wind power is suddenly reflected on the output voltage and power. So, this paper aims to use Superconducting Magnetic Energy Storage (SMES) for voltage stabilization of PMSG connected to DC grids. The proposed system was modeled using MATLAB software. The control scheme has been built so that SMES can be operated in charging mode, discharging mode, or freewheeling mode. A proportional integral (PI) controller is used to provide accurate pulse width modulation (PWM) for the switches to control SMES output voltage. Before using SMES, the output power of PMSG was fluctuated with an impact on voltage profile of the DC bus. After using SMES, a smooth power has been delivered to the load.