Power Quality Enhancement in Hybrid Photovoltaic-Battery System based on three–Level Inverter associated with DC bus Voltage Control

This modest paper presents a study on the energy quality produced by a hybrid system consisting of a Photovoltaic (PV) power source connected to a battery. A three-level inverter was used in the system studied for the purpose of improving the quality of energy injected into the grid and decreasing the Total Harmonic Distortion (THD). A Maximum Power Point Tracking (MPPT) algorithm based on a Fuzzy Logic Controller (FLC) is used for the purpose of ensuring optimal production of photovoltaic energy. In addition, another FLC controller is used to ensure DC bus stabilization. The considered system was implemented in the Matlab /SimPowerSystems environment. The results show the effectiveness of the proposed inverter at three levels in improving the quality of energy injected from the system into the grid.Peer reviewedFinal Published versio

Single-phase inverter with active ripple energy storage

It is well known that conventional energy sources such as coal, oil, and natural gas are decreasing and a growing problem of environmental pollution. The renewable energy sources are becoming the best alternative for a clean and inexhaustible energy source, and solar energy is one of the most popular energy sources. Solar energy has gained more and more attention because of its advantages such as abundance, pollution free, renewability and low maintenance. The solar energy is usually obtained from photovoltaic (PV) cell which transform the solar irradiance into direct current (DC), that is electric energy. Since the majority of the electric devices and the main grid, require AC (alternate current) a power converter is needed to convert the DC electricity coming from the PV cell into AC electricity. The most used electronic converter for that is an inverter. Inverters contains semiconductor switches that are often controlled using the pulse width modulation technique, which yields second-order harmonic currents and corresponding ripple voltages on the DC bus. This double line frequency on the DC bus affect the performance of the photovoltaic system. Bulky DC link electrolytic capacitors are typically employed as transient energy buffer to decouple, or smooth out, the pulsating ac power from constant dc power. However, the use of electrolytic capacitor leads to temperature and aging concerns, and this also result in a low power density. A novel active power decoupling method proposed to add a bidirectional buck and boost converter that can store the ripple energy in its inductor and capacitor. This method can effectively reduce the energy storage in the DC link capacitor. This thesis deals with the design of such as bidirectional DC-DC converter and an inverter. The theoretical work mode of the bidirectional converter together with an inverter is studied. The power stages, inverter and bidirectional converter are studied in steady state to dimension the components. These stages are also modelled in their small signal equivalent model to find their transfer functions need to design the control loops. Different control strategies are studied and implemented to achieve the independent controls of the inverter and DC-DC converter. By using LTspice, the simulation results have verified the proposed power decoupling method.Es bien conocido que las fuentes de energía convencionales como el carbón, petróleo y gas natural están disminuyendo y volviéndose un problema de contaminación ambiental. Las fuentes de energías renovables están llegando a ser la mejor alternativa para a una fuente de energía limpia e inagotable y la energía solar es una de la más popular fuente de energía. La energía solar ha ganado más y más atención por sus ventajas, tales como, abundancia, libre de polución, renovabilidad y poco mantenimiento. La energía solar es normalmente obtenida de una célula fotovoltaica (FV) la cual transforma la irradiancia solar en corriente continua (CC), es decir, en energía eléctrica. Como la mayoría de los dispositivos electrónicos y la red requieren corriente alterna (CA) un convertidor de potencia es necesitado para convertir la electricidad continua proveniente de la célula fotovoltaica en electricidad alterna. El dispositivo más usado para esto es un inversor. Los inversores contienen conmutadores semiconductores que son a menudo controlados usando la técnica de modulación por ancho de pulso la cual produce un armónico de segundo orden en la corriente que da a lugar un rizado en el voltaje del bus de continua. Esta frecuencia de dos veces la frecuencia de línea en el bus de continua afecta el rendimiento del sistema fotovoltaico. Grandes condensadores electrolíticos son típicamente usados como buffer de energía transitoria para desacoplar, o suavizar, la potencia alterna de la potencia continua. Sin embargo, el uso de condensadores electrolíticos da lugar a problemas de temperatura y degeneración y estos además resultan en una baja densidad de potencia. Un método novedoso propone añadir un convertidor elevador reductor, bidireccional, que almacene la energía de rizado en sus inductor y capacitor. Este método puede reducir eficazmente la energía almacenada en el condensador usado en el DC link. Esta tesis trata sobre el diseño de un convertidor CC-CC bidireccional y un inversor. El modo de operación teórico del convertidor bidireccional junto con un inversor es estudiado. Las etapas de potencia, inversor y convertidor bidireccional son estudiadas en estado estacionario para dimensionar los componentes. Estas etapas son también modeladas en su modelo equivalente en pequeña señal para encontrar sus funciones de transferencia necesarias para el diseño de los lazos de control. Diferentes estrategias de control son estudiadas e implementadas para conseguir el control del inversor y del convertidor de continua. Usando LTspice, los resultados de las simulaciones han verificado el método propuesto de desacoplo de potencia.Sutil Ortiz, AM. (2018). Inversor monofásico con corrección activa de rizado. Universitat Politècnica de València. http://hdl.handle.net/10251/103423TFG

Single-Phase Bi-directional Ćuk Inverter for Battery Applications

Bidirectional inverters are widely applied in photovoltaic and wind systems that require battery power backup. They are advantageous over unidirectional inverters because of their ability to convert DC power into AC power and then AC power back into DC power to recharge for storage purposes. Generally, bidirectional inverters are designed to have multiple power stages and/or make use of transformers for isolation and voltage/current gain. This usually increases the cost of production and oftentimes reduces the efficiency of the system. At the same time, attempts at eliminating usage of transformers and reduction in the number of power stages limits the range of bidirectional inverters’ capabilities. This is because battery applications today require low voltage DC-AC inverters with AC-DC power flow capability to store energy for later use. As such, only buck-boost based topologies are majorly being proposed and used for this functionality. The buck boost converter is the most widely used in such applications because of its higher efficiency, low component count and simple structure. It has drawbacks, however, such as: pulsating input and output currents - this leads to lower high electromagnetic interference; lower power factor during AC-DC power flow rectification when the batteries are being recharged; and external filter is also required during this power flow to keep the charging voltage constant. This research proposes a bidirectional inverter that attempts to overcome the drawbacks of the widely used buck-boost converter-based topology. The bidirectional inverter proposed in this work is based on a bidirectional Ćuk converter. The Ćuk converter has both continuous input and output currents. A galvanic isolation option on a Ćuk converter is simpler than a buck boost converter - this is important for grid tied systems. The inverter is based on a pseudo DC-link architecture - it uses a front end Ćuk converter cascaded with an unfolding bridge to convert DC power into AC power. The switches in the converter stage are switched at high frequency, while the switches in the unfolding stage are switched slower at the grid frequency. This configuration is desirable over the two-stage topologies because the switching losses in the unfolding bridge are lower because of this low switching frequency used. This configuration also ensures good switch utilization at the unfolding stage by lowering the parasitic effects on the power transfer. The proposed inverter has 4 modes of operation: during modes I and II the power is positive, and it converts DC power into AC power; during modes III and IV the power is negative, and it converts AC power back into DC power. The inverter is designed such that during DC-AC power flow, the input and output inductor currents and coupling capacitor voltage are continuous for improved efficiency. During the AC-DC power flow, the coupling capacitor voltage is discontinuous to achieve a higher input power factor by improving the AC line current, thereby simultaneously increasing the efficiency. The inverter was analysed in terms of: the dead time inserted into the switches to avoid shoot through and shortcircuiting switches; the parasitic effects on the power transfer ratio. Because the Cúk inverter is a high order system, several robust control strategies, such as sliding mode and current control have been proposed. These control methods require complex theory and present practical challenges to be reviewed. As such a new nested loop control strategy was proposed based on the dynamics of the coupling capacitor as the primary energy storage in the Cúk inverter. The control strategy uses 2 loops: an inner current loop and an outer voltage loop. Lead compensators were designed for both the current and voltage loops to achieve good dynamic response at a high bandwidth. Both simulated and experimental results showed that the bidirectional inverter was able to meet the design specifications. The control strategy showed good dynamic response and disturbance rejection under several inverter variations. Although the efficiency during simulations was above 96%, the experimental efficiency dropped significantly because the inverter was built on a Vero board for easy manipulation. The AC input power factor was > 0.95 for both simulated and experimental results

Review of Various Power Conversion converter for battery Energy Storage Systems

The demand for safe and reliable electricity increases, our infrastructure continues to evolve and innovate in order to accommodate such growth. The advantages of energy storage can traverse power age, through transmission and dissemination, and right to clients. An energy storage framework is essential for pay of the active-power change; it can alleviate the unsettling influence and keep up the dependability of voltage and recurrence. Power conversion framework (PCS), as an interface between storage framework and open network, assumes an extraordinary job in accomplishing the power move between storage framework and open matrix. This paper summarize the different research dependent on power conversion converter for battery energy storage systems ebb and flow topologies and the control strategies ordinarily utilized in building under various working circumstances and prerequisites, and analyze their disparities and characters, which will helps in picking the PCS structures and control strategies

Improving the Capacity Factor and Stability of Multi-MW Grid Connected PV Systems with Results from a 1MW/2MWh Battery Demonstrator

Conventional PV systems integrated with a battery connect the array and the energy storage unit to a dc-link through individual dc-dc converters for maximum power point tracking (MPPT) and battery charge control. This paper proposes a new system configuration, which connects the PV array and battery unit to the dc-link of the system inverter via a single dc-dc converter capable of simultaneously operating as a charge controller and MPPT device. This dc-dc converter is controlled such that it charges/discharges the battery with the amount of power required to maintain the PV array at its MPPT reference voltage. The proposed system ensures that the PV array operates at its MPP for all irradiance conditions, therefore increasing the PV system capacity factor as well as ensuring MPPT stability for all irradiance conditions. Also, this configuration may also be adopted for PV power smoothing, where the curtailed power may be used to smooth the PV inverter output without sacrificing battery state of charge (SOC). The behavior of the proposed system is studied and simulated in PSCAD TM /EMTDC TM . The computations are compared with experimental data retrieved from the LG&E and KU E.W. Brown universal solar facility, which houses a 10MW(ac) PV farm and a 1MW/2MWh battery energy storage system (BESS). The results show that for the examples considered, and allowing curtailment adapted to the current power ratings of the system, an increase in the capacity factor of up to 20% is possible

Hybrid micro-grids exploiting renewables sources, battery energy storages and bi-directional converters

publishedVersio

Multilevel Converter Topologies for Utility Scale Solar Photovoltaic Power Systems

Renewable energy technologies have been growing in their installed capacity rapidly over the past few years. This growth in solar, wind and other technologies is fueled by state incentives, renewable energy mandates, increased fossil fuel prices and environmental consciousness. Utility scale systems form a substantial portion of electricity capacity addition in modern times. This sets the stage for research activity to explore new efficient, compact and alternative power electronic topologies to integrate sources like photovoltaics (PV) to the utility grid, some of which are multilevel topologies. Multilevel topologies allow for use of lower voltage semiconductor devices than two-level converters. They also produce lower distortion output voltage waveforms. This dissertation proposes a cascaded multilevel converter with medium frequency AC link which reduces the size of DC bus capacitor and also eliminates power imbalance between the three phases. A control strategy which modulates the output voltage magnitude and phase angle of the inverter cells is proposed. This improves differential power processing amongst cells while keeping the voltage and current ratings of the devices low. A battery energy storage system for the multilevel PV converter has also been proposed. Renewable technologies such as PV and wind suffer from varying degrees of intermittency, depending on the geographical location. With increased installation of these sources, management of intermittency is critical to the stability of the grid. The proposed battery system is rated at 10% of the plant it is designed to support. Energy is stored and extracted by means of a bidirectional DC-DC converter connected to the PV DC bus. Different battery chemistries available for this application are also discussed. In this dissertation, the analyses of common mode voltages and currents in various PV topologies are detailed. The grid integration of PV power employs a combination of pulse width modulation (PWM) DC-DC converters and inverters. Due to their fast switching nature a common mode voltage is generated with respect to the ground, inducing a circulating current through the ground capacitance. Common mode voltages lead to increased voltage stress, electromagnetic interference and malfunctioning of ground fault protection systems. Common mode voltages and currents present in high and low power PV systems are analyzed and mitigation strategies such as common mode filter and transformer shielding are proposed to minimize them