Employment of a battery energy storage system to compensate for the generation-consumption mismatch is a vital element for a resilient microgrid. However, the frequent (i.e., short-term) cyclic activity and the abrupt current variations (i.e., high di/dt) have adverse effects on the energy-type battery’s lifespan. On the other hand, the power-type supercapacitor energy storage system (SCES) has almost zero lifetime degradation when it is subjected to irregular charging/discharging patterns. Therefore, the hybridization between them can reduce battery stress levels. This study proposes a droop control algorithm with multiple segments for a SCES/battery hybrid energy storage system (HESS) that is employed in a solar/wind DC microgrid. The presented control scheme is decentralized since it avoids the central controller (CC) and the communication links between the controllers of SCES and battery units. To eliminate the short-term cyclic operation of the battery, the SCES regulates the bus voltage inside a narrow zone with a droop curve that exploits all its available energy capacity. Inside this zone, the battery is kept inactive. As soon as the bus voltage deviates from this band, the SCES enters the idle mode, and the battery is triggered following a droop curve different for charging or discharging mode to stabilize the bus voltage. To evaluate the effectiveness of the proposed system over a battery-only system and a hybrid SCES/battery system controlled with the classical droop method, a comparative analysis under different scenarios is presented.

The volatility of grid-coupled photovoltaics can cause local voltage deviations, while the impact on frequency becomes obvious in isolated weak grids. Thus, a standalone battery is usually proposed for smoothing purposes. However, the frequent cycles and abrupt power variations shrink its life and impair control performance. To this end, this study introduces a controller for a hybrid system composed of a superconducting magnetic energy storage (SMES) and a battery. The proposed method establishes an idling zone for battery to eliminate its short-term activity, while SMES handles any power mismatch. The zone limits are dynamically adjusted in case of power balance detection, while an adaptive saturation is applied to them for maximal SMES utilization and minimal battery degradation. When SMES current deviates from this zone, battery operates with an adaptive ramp rate (i.e., di/dt) depending on the state of charge of SMES, to further optimize its life. Additionally, to prevent unnecessary power circulation among SMES and battery, supervisory control loops are implemented. Finally, to evaluate this scheme against preceding controllers regarding battery life extension, a real-time approach is followed using a dedicated simulator, while a hardware-in-the-loop verification is presented using an actual controller.

Robust operation of DC microgrids is seriously jeopardized by renewable energy sources fluctuations and intermittency, abrupt load changes and unscheduled disconnections from the utility grid. These disturbances can cause DC bus voltage instability and power quality issues. Therefore, utilization of properly controlled energy storage systems, plays a substantial role for a smooth and secure operation, especially in islanded operation mode. However, frequent cycling operation and high charging/discharging rates render the (energy-type) battery storage systems unsustainable solution due to their quick lifetime reduction. On the other hand, (power-type) superconducting magnetic energy storage (SMES) systems have almost zero lifetime degradation under the aforementioned conditions. This study proposes a PI-droop controlled SMES-battery hybrid energy storage system to enrich the stability of an islanded DC microgrid and extend battery service time, simultaneously. Simulations conducted in PSIM software, demonstrate the effectiveness of the adopted control scheme, in terms of microgrid power quality and battery lifetime extension.

Resilient operation of DC microgrids is seriously endangered by the unpredictable and volatile renewable generation, arbitrary load variations and unplanned disconnections from the utility grid. These instabilities can cause DC bus voltage fluctuations and significant power quality issues. Therefore, employing a robust, quick-responsive energy storage system with reduced lifetime degradation under frequent cycling operation, to maintain the power equilibrium, is a prominent solution in such circumstances. This study investigates the integration of a PI controlled supercapacitor energy storage (SCES) system into an islanded solar-wind DC microgrid. The proposed system is examined under normal and extreme climatic activity and under abrupt loading events. Simulations conducted in PSIM software demonstrate the efficacy of SCES employment, in terms of overall system stability and power quality.