Ionel Vechiu

Transient Operation of a Four-Leg Inverter for Autonomous Applications With Unbalanced Load

In this paper, the transient operation of a four-leg inverter equipped with an innovative control strategy under unbalanced load conditions is investigated. The inverter is proposed for transformerless hybrid power system applications, in order to provide simultaneous supply of three-phase and single-phase AC loads with balanced voltage and constant frequency. The four-leg inverter is controlled to ensure balanced voltage by means of a control strategy based on the decomposition of the supply three-phase voltage and current into instantaneous positive, negative, and homopolar sequence components using phasor representation. These three sequences are controlled independently in their own reference frames as DC signals. The implementation derived for the controller design is also described. The transient operation performance of the proposed control strategy has been tested in simulations with an average model and experimentally using a laboratory prototype.

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.

Comparison of wind turbine LQG controllers using Individual Pitch Control to alleviate fatigue loads

This paper focuses on the design of Linear Quadratic Gaussian (LQG) controllers for variable-speed horizontal axis Wind Turbines (WT). These turbines use blade pitch angle and electromagnetic torque control variables to meet specified objectives for Full Load (FL) zone. The main control objectives are to reduce structural dynamic loads and to regulate the power of the WT. The controllers are designed in order to optimize a trade-off between several control objectives. Four different LQG using Individual Pitch Control (IPC) are designed, with Wireless-Sensors (WS) placed at the end of the blades for the last one. Their control model is progressively more complex. The first one takes into account a rigid simple behavior, the second control model considers the first mode of the drive-train flexibility, the third model takes into account the drive-train and tower flexibilities and the fourth that of the blades. Likewise, their optimization criteria consider for each controller a new control objective to alleviate fatigue loads in the drive-train, then, also in the tower and finally also in the blades. The evaluation of the fatigue loads affecting the WT components are based on a Rainflow Counting Algorithm (RFC) and the Miners rule. The results indicate a significant reduction of fatigue loads especially in the drive-train and the blades when its flexibility is taken into account in the control models.

Control of power converters for microgrids

Purpose – Nowadays, distributed generation and microgrids (MGs) are becoming an important research line because of their peculiar characteristics. MGs are composed of small power sources which can be renewable, placed near customer sites. Moreover, they have the inherent property of islanding: the disconnection of either the MG from the main grid or a part of a MG from the rest of the MG. The purpose of this paper is to study two different control strategies allowing grid connected and islanding operation of the MG.Design/methodology/approach – In this paper, the behaviour of a particular MG during grid connected and islanding operation is investigated. The studied MG is based on different energy sources: a wind turbine, a photovoltaic array, a backup diesel generator and a storage system. The renewable sources and the storage system are connected on a DC bus which is interconnected with a main grid through a voltage source inverter (VSI). The attention focuses on the control technique of the VSI during g...

Digital control of a three-phase four-leg inverter under unbalanced voltage conditions

In this paper, the implementation of an innovative inverter control strategy based on the symmetrical components is presented. Symmetrical components of the inverter output voltage and current are decomposed into dq DC quantities to control a three-phase four-leg inverter. This kind of inverter can be a good choice for the applications with hybrid power system based on renewable energy used to supply unbalanced loads. The addition of a fourth leg provides an extra degree of freedom, making possible to handle the neutral current caused by the unbalanced load. Simulation results are compared with experimental results for a prototype inverter to verify the validity of the proposed control strategy.

Three-level Neutral Point Clamped Inverter Interface for flow battery/supercapacitor Energy Storage System used for microgrids

Microgrids can potentially improve security, quality, reliability and availability of electricity supply for many applications. In order to achieve these benefits when a microgrid with a substantial amount of stochastic generation operates in grid connected mode but also during islanding mode, the use of storage systems is essential. The main goal of the storage is to perform power balance during islanding mode and to avoid the effect of the renewable energy sources intermittency and load fluctuations to the grid. However, none of the currently available Energy Storage Systems (ESS) has the necessary high power and energy densities to face the effect of different disturbances coming from the load or renewable energy sources. Thus, the association of more than one storage technology in a Hybrid Energy Storage System (HESS) can be used to satisfy the above mentioned requirements. Generally, separate DC/DC converters are required to interconnect the different ESS with an inverter in order to supply the distributed system in a microgrid. The present work proposes the association of a Vanadium-Redox flow battery (VRB) with supercapacitors and a Three-level Neutral Point Clamped Inverter Interface for energy management in a microgrid. The power converter control and the energy management are also detailed. Simulation results are presented to prove the feasibility of the proposed system.

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).

Comparison of wind turbine LQG controllers designed to alleviate fatigue loads

This paper focuses on the design of Linear Quadratic Gaussian (LQG) controllers for variable-speed horizontal axis Wind Turbines (WTs). Those turbines use blades pitch angle and electromagnetic torque control variables to meet specified objectives for Full Load (FL) zone. The main control objectives are to reduce the structural dynamic loads and to regulate the power of the WT. The controllers are designed in order to optimize a trade-off between several control objectives. Four different LQG controllers are designed. Their control model is progressively more complex. The first one takes into account a rigid simple behavior, the second control model considers the first mode of flexibility of the drive train, the third model takes into account the tower flexibilities and the fourth that of the blades. In the same manner, their optimization criteria considers for each controller a new control objective to alleviate fatigue loads in the drive train, then, also in the tower and finally also in the blades. The evaluation of the fatigue loads affecting the WT components are based on a Rainflow Counting Algorithm and the Miners rule (RFC). The results indicate a significant reduction of fatigue loads in the drive-train when its flexibility is taken into account in the control model.

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.