Power Flow Management Research Articles

Power management of PV/battery hybrid power source via passivity-based control

In this paper, the power electronic interface between a DC hybrid power source with a photovoltaic main source and a Li-ion battery storage as the secondary power source is modelled based on the Euler–Lagrange framework. Subsequently, passivity-based controllers are synthesised using the energy shaping and damping injection techniques. Local asymptotic stability is ensured as well. In addition, the power management system is designed to manage power flow between components. Evaluation of the proposed system and simulation of the hybrid system are accomplished using MATLAB/Simulink. The results show that the outputs of the hybrid system have good tracking response, low overshoot, short settling time and zero steady-state error. Afterwards, linear PI controllers are provided to compare the results with those of passivity-based controller responses. This comparison demonstrates the robustness of the proposed controllers for reference DC voltage and load resistance changes.

SIMULATION AND DESIGN OF IPFC SYSTEM FOR VOLTAGE SAG COMPENSATION

Inter line power flow controller is a new concept for the compensation and effective power flow management of multi-line transmission systems. The IPFC employs a number of Voltage Sourced Converters (VSCs) linked at the same DC terminal, each of which can provide series compensation for the selected line of the transmission system. Through common dc link, any inverters within the IPFC is able to transfer real power to any other and thereby facilitate real power transfer among the line. In this work the IPFC system used for compensation of voltage sag in two identical lines like 11kv and 10kv systems. The prototype IPFC system was developed and verified with hardware results. When the voltage sag is compensated, reactive power is controlled and transmission line efficiency is improved.

Robust PLL Algorithm for Three-Phase Grid-Connected Converters

The estimation of the grid angle is a key factor in the control of many power electronics applications using grid-connected three-phase converters. The knowledge of the grid angle is required for different goals such as the synchronization of the voltage or current waveforms produced by the converter to the utility grid, the power flow management and the reduction of the transients generated on grid failures or on the reconnection of the converter.Generally, the grid voltages are polluted with harmonics and affected by amplitude unbalances; thus, oscillation on the grid angle signal can lead to an improper synchronization between the converter outputs and the grid voltages. To improve the synchronization, the paper proposes a robust PLL structure employing a prediction-correction filter based on a second-order linear and time-invariant observation model. Such filter is able to provide an accurate tracking of the grid voltage angle also in critical operating conditions. The paper describes also the implementation on a low cost fixed-point DSP controller and a comparison, obtained by simulation, with traditional filters. Finally, some significant experimental results validate the performances of the proposed approach.

An application of the successive shortest path algorithm to manage power in multi‐agent system based active networks

AbstractIn order to handle a high penetration of distributed generation (DG), the existing passive distribution systems need to become more active. In the so‐called active network (AN), power flow management has developed to cope with bidirectional power flow and the intermittence of DGs. This paper proposes a method to better manage active power flows based on graph theory application and multi‐agent system (MAS). While the power system is represented as a directed graph, the optimal power flow (OPF) is understood as a minimum cost flow problem that can be solved by the successive shortest path algorithm. The algorithm is implemented in a distributed way and is supported by MAS. Simulations using a five sub‐areas network show that the method is able to reach the optimal operation state while restraining power flows within defined bounds. Furthermore, the method can also response changes in power generation cost, network configuration and load demand increase. Copyright © 2009 John Wiley & Sons, Ltd.

Power Flow and Stability Control Using an Integrated HV Bundle-Controlled Line-Impedance Modulator

This paper presents a flexible ac transmission system device under development for the management of power flow under steady-state and dynamic conditions, the bundle-controlled line-impedance modulator (LIM). In its simplest form, an LIM is made of switching modules connected in series with transmission-line segments whose bundles have subconductors insulated from each other. A module contains one switch in series with each subconductor of a bundle and the modules are anchored to dead-end towers in place of yoke plates. Together, with the measurement of sensors and controls, these modules can be used to change series impedances of transmission lines and implement functions, such as deicing, power-flow control, stability control, and line monitoring. In order to illustrate the LIM capabilities, two load-flow studies and one stability study are presented in this paper. Finally, the LIM performance is qualitatively compared to the thyristor-controlled series compensator technology.

Evaluating the benefits of an electrical energy storage system in a future smart grid

Interest in electrical energy storage systems is increasing as the opportunities for their application become more compelling in an industry with a back-drop of ageing assets, increasing distributed generation and a desire to transform networks into Smart Grids. A field trial of an energy storage system designed and built by ABB is taking place on a section of 11 kV distribution network operated by EDF Energy Networks in Great Britain. This paper reports on the findings from simulation software developed at Durham University that evaluates the benefits brought by operating an energy storage system in response to multiple events on multiple networks. The tool manages the allocation of a finite energy resource to achieve the most beneficial shared operation across two adjacent areas of distribution network. Simulations account for the key energy storage system parameters of capacity and power rating. Results for events requiring voltage control and power flow management show how the choice of operating strategy influences the benefits achieved. The wider implications of these results are discussed to provide an assessment of the role of electrical energy storage systems in future Smart Grids.

On-line fuzzy energy management for hybrid fuel cell systems

Fuel Cell Hybrid Vehicles (FCHV) can reach near zero emission by removing the conventional internal combustion from the vehicle powertrain. Nevertheless, before seeing competitive and efficient FCHV on the market, at market prices, different technical, economic, and social challenges should be overcome. A typical hybrid fuel cell powertrain combines a fuel cell stack and a dedicated energy storage system along with their necessary power converters. Energy storage systems are used in order to enhance the well-to-wheel efficiency and thus reducing the hydrogen consumption. An efficient management of power flows on the vehicle, allows optimizing the recovery of energy braking. Moreover, working in the fuel cell maximum efficiency leads to reduced thermal losses and thus to the downsizing of the heat exchangers. This paper presents an enhanced control of the power flows on a FCHV in order to reduce the hydrogen consumption, by generating and storing the electrical energy only at the most suitable moments on a given driving cycle. While the off-line optimization-based on dynamic programming algorithm offers the necessary optimal comparison reference on a known demand, the proposed strategy which can be implemented on-line, is based on a fuzzy logic decision system. The fine tuning of the fuzzy system parameters (mainly the membership functions and the gains), is made using a genetic algorithm and the fuzzy supervisor shows performing results for different load profiles.

Online energy management applied to fuel cell hybrid electric vehicles

Hybrid Electric Vehicles (HEVs) are equipped with multiple power sources for improving the efficiency and performance of their power supply system. Hence, an Energy Management (EM) strategy is needed to optimise the internal power flows and to satisfy the driver's power demand. This paper deals with power flow management within a hybrid fuel cell-powered vehicle during real-time operation. The aim is real-time control of the power distribution between the fuel cell and battery to optimise the global hydrogen consumption while the vehicle performs an online driving cycle. To do so, two strategies are proposed based on fuzzy controllers. Both controllers determine the optimised operation points for the vehicle energy sources during online driving cycles and guarantee soft dynamic transition in switching between operating modes. The simulation results confirm the feasibility of both proposed strategies and encourage more research towards an actual application.

Optimization and Techno-Economic Analysis of Autonomous Photovoltaic/Fuel Cell Energy System

This paper introduces a method to unit sizing hybrid Photovoltaic/Fuel Cell generation system for a typical domestic load that is not located near the electric grid. In this con¯guration the combination of a battery, an electrolyser, and a hydrogen storage tank are used as the energy storage system. The aim of this design is finding the configuration, among a set of systems components, which meets the desired system reliability requirements, with the lowest value of levelized cost of energy over 20 years of operation. An energy based modelling has been developed using Matlab/Simulink to observe evolution of the system during a typical day, and the results are reported and discussed in the paper. An overall power management strategy is designed for the proposed system to manage power flows among the different energy sources and the storage unit in the system. The results show that a system composed with a photovoltaic generator, a fuel cell, an electrolyser and a battery can deliver energy in a stand-alone installation with an acceptable cost.

Hybrid buses: defining the power flow management strategy and energy storage system needs

The following article will analyze the benefits of retrofitting a diesel-electric bus by hybridizing its drive train with a supercapacitor based energy storage system (ESS). Several power flow management strategies will be proposed and compared; and their respective ESSs will be sized and designed accordingly. The energy savings achieved range between 32 % and 40%, due to braking energy recovery and an improved ICE efficiency, and show the importance of the power flow strategy. Results are obtained by means of a quasi-static ‘backwards/forward looking’ simulation program, developed for this purpose, which includes a dynamic model of the ICE. Performance of the conventional bus and the hybrid bus is also compared.

Distributed generation output control for network power flow management

The development stages in the output control of distributed generation (DG) for network power flow management are illustrated. The first stage requires an assessment of the location of thermally vulnerable components within the distribution network. This is achieved through the offline calculation of thermal vulnerability factors that relate component power flow sensitivity factors to component thermal limits. This directly informs Stage 2 – the installation of meteorological stations and component temperature monitoring equipment for network thermal characterisation. In Stage 3, steady-state component rating models are populated with real-time environmental information from the meteorological stations to generate component real-time thermal ratings. In Stage 4, the power flow sensitivity factors calculated in Stage 1 are embedded within a network power flow management system which, together with the component real-time thermal ratings calculated in Stage 3, is used to control the power output of DG schemes.

An Isolated Three-Port Bidirectional DC-DC Converter With Decoupled Power Flow Management

An isolated three-port bidirectional dc-dc converter composed of three full-bridge cells and a high-frequency transformer is proposed in this paper. Besides the phase shift control managing the power flow between the ports, utilization of the duty cycle control for optimizing the system behavior is discussed and the control laws ensuring the minimum overall system losses are studied. Furthermore, the dynamic analysis and associated control design are presented. A control-oriented converter model is developed and the Bode plots of the control-output transfer functions are given. A control strategy with the decoupled power flow management is implemented to obtain fast dynamic response. Finally, a 1.5 kW prototype has been built to verify all theoretical considerations. The proposed topology and control is particularly relevant to multiple voltage electrical systems in hybrid electric vehicles and renewable energy generation systems.

Power Management of a Stand-Alone Wind/Photovoltaic/Fuel Cell Energy System

This paper proposes an AC-linked hybrid wind/photovoltaic (PV)/fuel cell (FC) alternative energy system for stand-alone applications. Wind and PV are the primary power sources of the system, and an FC-electrolyzer combination is used as a backup and a long-term storage system. An overall power management strategy is designed for the proposed system to manage power flows among the different energy sources and the storage unit in the system. A simulation model for the hybrid energy system has been developed using MATLAB/Simulink. The system performance under different scenarios has been verified by carrying out simulation studies using a practical load demand profile and real weather data.

Composite System Reliability Assessment Incorporating an Interline Power-Flow Controller

The impact of an interline power-flow controller (IPFC) on composite system delivery point and overall system reliability indices is examined in this paper. In this application, the IPFC with dc transmission lines is employed in a system for coordinated control of line impedances of two, distance away, transmission lines with the objective of managing power flows on the two lines. It is also used to balance the real power being transferred between the compensated lines. For this purpose, the reliability model associated with a converter station has been developed and incorporated in an IPFC reliability model. A number of reliability indices are calculated at both the loadpoint and system levels to illustrate the impact of employing IPFC and examine the effects of different failure modes on the system performance. The technique is applied to the IEEE reliability test system and the results are presented. The results indicate that IPFC improves system reliability.

Transformer-Coupled Multiport ZVS Bidirectional DC–DC Converter With Wide Input Range

Multiport dc-dc converters are particularly interesting for sustainable energy generation systems where diverse sources and storage elements are to be integrated. This paper presents a zero-voltage switching (ZVS) three-port bidirectional dc-dc converter. A simple and effective duty ratio control method is proposed to extend the ZVS operating range when input voltages vary widely. Soft-switching conditions over the full operating range are achievable by adjusting the duty ratio of the voltage applied to the transformer winding in response to the dc voltage variations at the port. Keeping the volt-second product (half-cycle voltage-time integral) equal for all the windings leads to ZVS conditions over the entire operating range. A detailed analysis is provided for both the two-port and the three-port converters. Furthermore, for the three-port converter a dual-PI-loop based control strategy is proposed to achieve constant output voltage, power flow management, and soft-switching. The three-port converter is implemented and tested for a fuel cell and supercapacitor system.

System Integration and Power-Flow Management for a Series Hybrid Electric Vehicle Using Supercapacitors and Batteries

In this paper, system integration and power-flow management algorithms for a four-wheel-driven series hybrid electric vehicle (HEV) having multiple power sources composed of a diesel-engine-based generator, lead acid battery bank, and supercapacitor bank are presented. The super-capacitor is utilized as a short-term energy storage device to meet the dynamic performance of the vehicle, while the battery is utilized as a mid-term energy storage for the electric vehicle (EV) mode operation due to its higher energy density. The generator based on an interior permanent magnet machine (IPMM), run by a diesel engine, provides the average power for the normal operation of the vehicle. Thanks to the proposed power-flow management algorithm, each of the energy sources is controlled appropriately and also the dynamic performance of the vehicle has been improved. The proposed power-flow management algorithm has been experimentally verified with a full-scale prototype vehicle.

Modeling of Induction Machines Using a Voltage-Behind-Reactance Formulation

This paper proposes an AC-linked hybrid wind/photovoltaic (PV)/fuel cell (FC) alternative energy system for stand-alone applications. Wind and PV are the primary power sources of the system, and an FC-electrolyzer combination is used as a backup and a long-term storage system. An overall power management strategy is designed for the proposed system to manage power flows among the different energy sources and the storage unit in the system. A simulation model for the hybrid energy system has been developed using MATLAB/Simulink. The system performance under different scenarios has been verified by carrying out simulation studies using a practical load demand profile and real weather data.

Analysis of SSR With Three-Level Twelve-Pulse VSC-Based Interline Power-Flow Controller

The interline power-flow controller (IPFC) is a voltage-source-converter (VSC)-based flexible ac transmission system (FACTS) controller for series compensation with the unique capability of power-flow management among the multiline transmission systems of a substation. The reactive voltage injected by individual VSCs can be maintained constant or controlled to regulate active power flow in the respective line. While one VSC regulates the dc voltage, the others control the reactive power flows in the lines by injecting series active voltage. This paper presents the modelling of IPFC with 12-pulse, three-level converters and investigates the subsynchronous-resonance (SSR) characteristics of IPFC for different operating modes. The analysis of SSR is carried out based on eigenvalue analysis and transient simulation of the detailed system. It is illustrated with the help of a case study on a system adapted from the IEEE Second Benchmark Model. The analysis uses both D-Q model (neglecting harmonics in the output voltages of VSCs) and the three-phase model of VSCs using switching functions. While the eigenvalue analysis and controller design is based on the D-Q model, the transient simulation considers both models.

POWER FLOW MANAGEMENT WITH PREDICTIVE CAPABILITIES FOR A HYBRID FUEL CELL VEHICLE

The hybridization of fuel cell vehicles not only leads to an improved efficiency, but also results in a significant lessening of the dynamic control requirements for automotive fuel cell systems. However, a proper power flow management plays a fundamental role in order to guarantee the optimal performances. In this paper, a two level control architecture is proposed for vehicles equipped with a fuel cell system and a battery. The lower level scheme controls the fuel cell acting on the compressor command and on the back pressure valves of anode and cathode. On the other hand the higher control level is devoted to manage the power flows, that is the power absorbed by the motor and the one provided by the fuel cell. In view of its capability to manage constraints, such as those inherently related to the level of the battery charge, the Model Predictive Control (MPC) approach is used to design the controller acting at the higher level.

A Multilevel Modular Capacitor-Clamped DC–DC Converter

A novel topology of a multilevel modular capacitor-clamped dc-dc converter will be presented in this paper. In contrast to the conventional flying capacitor multilevel dc-dc converter, this new topology is completely modular and requires a simpler gate drive circuit. Moreover, the new topology has many advantageous features such as high frequency operation capability, low input/output current ripple, low on-state voltage drop, and bidirectional power flow management. This paper discusses the construction and operation of the new converter along with a comparison to a conventional converter. Finally, simulation and experimental results are used to validate the concept of this new topology.