Yuval Beck

Optimal Power Flow in Microgrids With Energy Storage

Energy storage may improve power management in microgrids that include renewable energy sources. The storage devices match energy generation to consumption, facilitating a smooth and robust energy balance within the microgrid. This paper addresses the optimal control of the microgrids energy storage devices. Stored energy is controlled to balance power generation of renewable sources to optimize overall power consumption at the microgrid point of common coupling. Recent works emphasize constraints imposed by the storage device itself, such as limited capacity and internal losses. However, these works assume flat, highly simplified network models, which overlook the physical connectivity. This work proposes an optimal power flow solution that considers the entire system: the storage device limits, voltages limits, currents limits, and power limits. The power network may be arbitrarily complex, and the proposed solver obtains a globally optimal solution.

Capacitive Transposed Series-Parallel Topology With Fine Tuning Capabilities

A general transposed series-parallel topology of a switched-capacitor converter is presented and analyzed in this paper. This topology evolved from the conventional series-parallel through rectangular matrices based topologies to partial arbitrary matrices which are constructed of different size strings of capacitors to arbitrary matrices, based on a bank of equally valued capacitors. Theoretically, these topologies have a large number of dc/dc voltage ratios, which lead to the capability to achieve very accurate fixed dc/dc voltage ratios. These topologies also have the capability for “fine tuning” when voltage regulation is necessary. For the general transposed series-parallel topology, it is shown that the number of possible dc/dc voltage transfer ratios escalates exponentially with an addition of each capacitor as the sum of partition functions. Therefore, relatively fewer components are required for an assumed accurate voltage ratio. Dispersion of parameters in the capacitance value is considered. Small ripple analysis, losses calculations and efficiency considerations are also discussed. Simulations are performed for 2.2 and 2.25 voltage ratios. The simulations are in good agreement with the theoretical calculations.

Modular Realization of Capacitive Converters Based on General Transposed Series–Parallel and Derived Topologies

The realization of capacitive converters based on a modular approach is presented and analyzed in this paper. The converters are based on topologies with identical capacitors. First, the switching network (SN) of a basic series-parallel topology is presented. The analysis reveals the assembly and growth rules for expanding the topology to any number of capacitors. Subsequently, the SN for the more versatile and complex general transposed series-parallel (GTSP) topology is presented. The topology is expanded by defining a basic cell comprising a capacitor and five peripheral switches. The expansion of this converter improves its performance by ameliorating accuracy and regulation in the dc/dc voltage ratio. The control schemes of these converters are also presented with emphasis on the GTSP topology, whose switches are not assigned in advance. The control scheme for achieving the desired voltage ratio is described. A five-capacitor converter with its control is built to illustrate the theory in open-loop mode, which provides experimental results that are in full agreement with the analytical predictions.

Connecting an alternative energy source to the power grid by a DSP controlled DC/AC inverter

In times when environmental issues such as global warming and air pollution are in focus, there are more objections to the use of conventional and nuclear power stations. The use of alternative energy sources such as water turbines, solar cells and wind turbines becomes essential for producing clean energy. In this paper, a novel design of a 1 kW DC/AC inverter is presented. The inverter is versatile, allowing for the connection of any DC alternative power source such as solar cells, wind turbines, etc, to the AC grid. Therefore, in peak load times, every household can perform as a clean micro power station. The inverter is implemented as a phase shift synchronous generator and it is controlled by means of a DSP. Sampling the grid voltage makes it possible for the inverter to synchronize with the grid at a zero angle. Then the DSP synthesizes the AC voltage waveform to be shifted, with respect to the grid, with a software controllable leading angle. This configuration makes it possible to transfer the clean energy omitted by the alternative power source to the grid. Energy contribution to the grid is possible by connecting synchronous generators that have 80% efficiency. In addition, the inverter is designed for low cost and its parameters are easy to control via computer software

Capacitive Matrix converters

This paper describes three ideal topologies of switched-capacitor converters, taking into account integration considerations and multiple DC voltage ratios. First, the ideal continuous capacitor idea is described, and then a Matrix of Capacitors configuration is proposed. This configuration is based on rectangular matrices of capacitors charging and discharging in transposed configuration for achieving the input/output DC voltage ratio. The third topology is The General Transposed Series-Parallel configuration. This is a modification with a discrete number of capacitors. The configuration is based on parallel brunches of series capacitors in the charging state and series elements of parallel capacitors in the discharging state. This topology is suitable for fine tuning in the DC voltage ratios. The Matrix and the Transposed Series-Parallel topologies are compared for an input/output ratio of 1.1. In the later topology, fewer components are required for the assumed ratio. Simulation is performed for 3 elements in the Transposed Series-Parallel topology, where each element consists of two dual identical capacitors for the elimination of external large capacitors.

On Loss Mechanisms of Complex Switched Capacitor Converters

In this paper the power losses of efficient capacitive converters with multitude transfer ratio values are discussed. The loss mechanism circuits, based on general transposed series parallel (GTSP) topology, is studied and the losses are calculated, which in turn enables the “bottlenecks” to be identified and overcome. The calculations show that for a given number of switching cells the power losses of such converters are mainly dependent on the voltage conversion ratio, the topology function, the resistances of the switches in on and off states, the ESR of the capacitors, the switching frequency, the voltage drop, and the load. The theory explains the fact that was observed in experiments, that for low voltage conversion ratio values, efficiency decrease was measured. It is shown that for these low voltage ratio values the mechanism associated to the off resistance of the switches is more dominant and reduces the efficiency. This power loss estimation can be used as a powerful tool for the performance optimization of capacitive converters. The theoretical results are verified experimentally on a 10-cell GTSP converter prototype. The experimental results show good agreement with the proposed theory.

Experimental Verification and Comparative Study of Various MPPT Algorithms

Choosing and implementing a Maximum Power Point Tracking (MPPT) algorithm in a photovoltaic (PV) system, along with choosing the power converter, constitutes the fundamental basic capabilities of a photovoltaic system. MPPT techniques are implemented in photovoltaic systems to achieve full utilization of PV array output power. Today there is a wide variety of MPPT algorithms, each one has its advantages and disadvantages. This paper presents theoretical, as well as experimental comparison results, in several aspects regarding four MPPT methods based on the basic two MPPT algorithms (Perturb and Observe, Incremental Conductance), implemented on a single DC/DC converter, under the same experimental conditions. The theoretical and experimental comparison is performed for characteristics of ripple around the MPP and the convergence time. The experimental results are provided and supported by theoretical analysis and show that gradient based methods have better convergence time as well as ripple values in comparison to fixed step methods.

Photovoltaic power conditioning and maximum power point tracking by means of a self commutated inverter

A novel design of a photovoltaic grid connected inverter is presented. The inverter is realized so as to attain current source characteristics for simple connectivity to the grid. The advantage of this realization is a simplified system due to the fact that no voltage amplitude matching or modulating is required and the wave shape of the grids voltage has no influence. The inverters current polarity must be taken care of in order to match the voltage polarity of the grid. The simple realization yields simplicity in the switching circuitry and control. The design includes an H-bridge topology that is switched by the AC grid itself; therefore no special synchronization scheme is required. Consequently, this simple switching scheme inherently suits all wave shapes and all frequency changes of the power grid. The system is combined with a maximum power point tracker (MPPT) with only output current control.

A Matlab-Simulink Model of AC grid with a FC-TCR and invariant control system for reactive power compensation

Matlab-Simulink Model of a three-phase grid with a system Fixed Capacitor - Thyristor Controlled Reactor (FC-TCR) for compensation of reactive power and for maintaining the power factor in the grid close to unity is presented. The specific features of the proposed model are applying the method of instantaneous powers for determining the load reactive power, and using a control system, whose structure as such ensures the constancy of the power factor, which is kept close to unit (the invariant character) without involving an automatic control system. The model of the control system is made maximally close to its hardware implementation. A detailed description of the system blocks is given. The results of the model tests under various loads - including the results of harmonic composition of voltage and currents - are given.

Suitability of capacitive converters to photovoltaic systems

In this paper the connectivity of capacitive converters in general and General Transposed Series-Parallel converter in particular to photovoltaic sources is discussed. The paper describes the compatibility of capacitive converters to photovoltaic as soft sources since the voltage source can adjust itself to the change in the capacitors voltage and therefore reduces the inherent pulsating currents of switched capacitors converters when they are connected to rigid sources. The theory is demonstrated by simulation and the simulation results are in full agreement with the presented theory. The results show that if not better at least capacitive converters can be adequate interface with soft sources and their ability to be integrated makes this technology attractive.