Aitor Etxeberria

Hybrid Energy Storage Systems for renewable Energy Sources Integration in microgrids: A review

The increasing use of the Renewable Energy Sources (RES) and the intermittency of the power generated by them create stability, reliability and power quality problems in the main electrical grid. The microgrid is called to be a feasible alternative to solve these issues. As it is a weak electrical grid, the microgrid is very sensitive to load or generation changes. To reduce the effect of these variations and to better harness the energy generated by RES, the Energy Storage Systems (ESS) are used. As the different ESS technologies that are currently available are not enough to satisfy the wide frequency spectrum of the generated energy, the use of a Hybrid Energy Storage System (HESS) is necessary. A HESS is usually formed by two complementary storage devices that can be associated in many different topologies. Of course, the two devices have to be coordinated by an Energy Management System (EMS). In this paper the different topologies and energy management algorithms that have been applied in the RES and microgrid contexts have been analysed and compared. A review of the latest papers related to the use of HESS to facilitate the integration of the RES in the microgrid context is carried out. Some examples of the (hybrid) electric vehicles are also addressed, as the use of the HESS is more extended in this field than in the field of the microgrids.

Control of a hybrid Energy Storage System using a three level neutral point clamped converter

The increasing penetration level of the Renewable Energy Sources (RES) and their variability can compromise the proper operation of an electrical grid. The microgrid is being analysed as a solution to obtain a high penetration of RES in a controlled way. Due to the stochastic nature of the RES, the Energy Storage Systems must be used to maintain the energy and power balance between generation and demand. The limits of the storage technologies that are nowadays available make it necessary to associate more than one storage technology creating a Hybrid Energy Storage System (HESS) in order to satisfy the specifications of a microgrid application: high energy autonomy, power capacity and fast response. This paper analyses by means of simulations and experimentally the feasibility of a promising topology based on a Three Level Neutral Point Clamped (3LNPC) converter. The reduced power losses, improved THD and the fact of being an integrated solution are the advantages of this system. An islanding operation case study where a HESS formed by a SuperCapacitor (SC) and a Vanadium Redox Battery (VRB) supplies power to a resistive AC load that is suddenly reduced is analysed by means of simulations and experimentally. The simulation and experimental results prove the feasibility of the presented topology and of the control system to divide the power between the SC (fast power variations) and the VRB (low frequency part of the power).

Control of a microgrid-connected hybrid energy storage system

A promising solution to face the problems related to the integration of the Distributed Generation (DG) with a high renewable energy penetration level is to redesign the distribution grid using the concept of MicroGrids (MGs). Due to its constitutive components, production capacities, controllable loads and storage systems, a MG can contribute to the market either as load or production and its status can change according with the renewable energy resources availability and the needs of the main grid. The use of storage system in a MG plays an essential role in order to ensure energy balance and also allowing maintaining high power quality. In order to satisfy these two constraints, this paper proposes the association of a Vanadium Redox Battery (VRB) and SuperCapacitor (SC) bank in a Hybrid Energy Storage System (HESS). A Three-level Neutral Point Clamped (3LNPC) Inverter is used as a unique interface between the HESS and the MG. The paper focuses on the modelling and validation of the HESS, the power division limits using the 3LNPC inverter and its operation integrated in the MG. The validity of the proposed solution has been confirmed using both simulation and experimental tests.

Power quality improvement using an advanced control of a four-leg multilevel converter

In this paper, the transient operation of a Three-Level Neutral Point Clamped (3LNPC) four-leg inverter equipped with an innovative control strategy able to improve the power quality in renewable energy based weak grids is investigated. The 3LNPC four-leg inverter is controlled to manage a Hybrid Energy Storage System in order to satisfy the energy/power demand in a weak grid and simultaneously to balance the AC voltage during unbalanced 3-phase loads. The proposed control strategy uses a special designed offset for the DC power/energy division and the decomposition of the supply three-phase voltage and current into symmetrical components using phasor representation for voltage balance. Real time simulations and experimental tests have been performed using a RT-LAB simulator, a 3LNPC converter prototype and an experimental MicroGrid in order to prove the effectiveness of the proposed system in a weak grid context.

Hybrid Energy Storage System Microgrids Integration for Power Quality Improvement Using Four-Leg Three-Level NPC Inverter and Second-Order Sliding Mode Control

Rising demand for distributed generation based on renewable energy sources (RES) has led to several issues in the operation of utility grids. The microgrid is a promising solution to solve these problems. A dedicated energy storage system could contribute to a better integration of RES into the microgrid by smoothing the renewable resources intermittency, improving the quality of the injected power and enabling additional services like voltage and frequency regulation. However, due to energy/power technological limitations, it is often necessary to use hybrid energy storage systems (HESS). In this paper, a second-order sliding mode controller is proposed for the power flow control of a HESS, using a four-leg three-level neutral-point-clamped (4-Leg 3L-NPC) inverter as the only interface between the RES/HESS and the microgrid. A 3-D space vector modulation and a sequence-decomposition-based ac-side control allow the inverter to work in unbalanced load conditions while maintaining a balanced ac voltage at the point of common coupling. DC current harmonics caused by unbalanced load and the NPC floating middle point voltage, together with the power division limits, are carefully addressed in this paper. The effectiveness of the proposed technique for the HESS power flow control is compared to a classical PI control scheme and is proven through simulations and experimentally using a 4-Leg 3L-NPC prototype on a test bench.

Hybrid energy storage system with unique power electronic interface for microgrids

The increasing penetration of Distributed Generation systems based on Renewable Energy Sources is introducing new challenges in the current centralized electric grid. Microgrids are one of the most useful and efficient ways to integrate the renewable energy technologies. As the stability of a microgrid is highly sensitive, an energy storage system is essential and it must satisfy two criteria: to have a high storage capacity and to be able to supply fast power variations. In order to satisfy these two constraints, this paper proposes the association of a Vanadium Redox Battery (VRB) and Supercapacitor (SC) bank in a Hybrid Energy Storage System (HESS). A Three-level Neutral Point Clamped (3LNPC) Inverter is used as a unique interface between the HESS and the microgrid. The paper focuses on the dynamic modelling and validation of the HESS, the power division and the modulation strategies used with the 3LNPC to mitigate the neutral point voltage unbalance effect.

Operational limits of four wires three levels NPC topology for power quality improvement in weak grids

In this paper, the limits of a Four-Wires Three-Level Neutral Point Clamped (4W-3L-NPC) inverter used as the only interface between a Hybrid Energy Storage System (HESS) supplied by a renewable energy system and a MicroGrid are under investigation. The investigation focus on the power division and power exchange capabilities among a Redox Flow Battery (VRB) and a Lithium-Ion (Li-Ion) battery, the two energy storage systems investigated in the context of a MicroGrid. The final objective of this work is to define the limits of the investigated topology and prove its capacity to manage the power flow exchange between the HESS and a weak grid, as well as inside of the HESS.