Showing papers on "Electrical network published in 2009"

Evaluation of Current Controllers for Distributed Power Generation Systems

Abstract: This paper discusses the evaluation of different current controllers employed for grid-connected distributed power generation systems having variable input power, such as wind turbines and photovoltaic systems. The focus is mainly set on linear controllers such as proportional-integral, proportional-resonant, and deadbeat (DB) controllers. Additionally, an improved DB controller robust against grid impedance variation is also presented. Since the paper discusses the implementation of these controllers for grid-connected applications, their evaluation is made in three operating conditions. First, in steady-state conditions, the contribution of controllers to the total harmonic distortion of the grid current is pursued. Further on, the behavior of controllers in the case of transient conditions like input power variations and grid voltage faults is also examined. Experimental results in each case are presented in order to evaluate the performance of the controllers.

Protection of Low-Voltage DC Microgrids

Abstract: In this paper, a low-voltage (LV) DC microgrid protection system design is proposed. The LV DC microgrid is used to interconnect distributed resources and sensitive electronic loads. When designing an LV DC microgrid protection system, knowledge from existing DC power systems can be used. However, in most cases, these systems use grid-connected rectifiers with current-limiting capability during DC faults. In contrast, an LV DC microgrid must be connected to an AC grid through converters with bidirectional power flow and, therefore, a different protection-system design is needed. In this paper, the operating principles and technical data of LV DC protection devices, both available and in the research stage, are presented. Furthermore, different fault-detection and grounding methods are discussed. The influence of the selected protection devices and grounding method on an LV DC microgrid is studied through simulations. The results show that it is possible to use available devices to protect such a system. Problems may arise with high-impedance ground faults which can be difficult to detect.

Small-Signal Methods for AC Distributed Power Systems–A Review

Abstract: AC distributed power systems (DPS) can be found in several new and emerging applications. Similar to dc distributed power systems, an ac DPS relies on power electronics and control to realize its functions and achieve the required performance. System stability and power quality are important issues in both types of systems due to the complex system behavior resulted from active control at both the source and the load side. Traditional small-signal analysis methods cannot be directly applied to an ac DPS because of the periodically time-varying system operation trajectory. Possible solutions to this problem include transformation into a rotating (dq) reference frame, modeling using dynamic phasors, reduced-order modeling, and harmonic linearization. This paper reviews these small-signal methods and discusses their utilities as well as limitations. Compatibility of each type of models with state-space and impedance-based system analysis approaches will also be discussed. Problems related to the linearization of phasor-based models and their use in impedance-based system analysis are highlighted in particular.

A Control Methodology and Characterization of Dynamics for a Photovoltaic (PV) System Interfaced With a Distribution Network

Abstract: This paper proposes a control strategy for a single-stage, three-phase, photovoltaic (PV) system that is connected to a distribution network. The control is based on an inner current-control loop and an outer DC-link voltage regulator. The current-control mechanism decouples the PV system dynamics from those of the network and the loads. The DC-link voltage-control scheme enables control and maximization of the real power output. Proper feedforward actions are proposed for the current-control loop to make its dynamics independent of those of the rest of the system. Further, a feedforward compensation mechanism is proposed for the DC-link voltage-control loop, to make the PV system dynamics immune to the PV array nonlinear characteristic. This, in turn, permits the design and optimization of the PV system controllers for a wide range of operating conditions. A modal/sensitivity analysis is also conducted on a linearized model of the overall system, to characterize dynamic properties of the system, to evaluate robustness of the controllers, and to identify the nature of interactions between the PV system and the network/loads. The results of the modal analysis confirm that under the proposed control strategy, dynamics of the PV system are decoupled from those of the distribution network and, therefore, the PV system does not destabilize the distribution network. It is also shown that the PV system dynamics are not influenced by those of the network (i.e., the PV system maintains its stability and dynamic properties despite major variations in the line length, line X/R ratio, load type, and load distance from the PV system).

A General Photo-Electro-Thermal Theory for Light Emitting Diode (LED) Systems

Abstract: The photometric, electrical, and thermal features of LED systems are highly dependent on one another. By considering all these factors together, it is possible to optimize the design of LED systems. This paper presents a general theory that links the photometric, electrical, and thermal behaviors of an LED system together. The theory shows that the thermal design is an indispensable part of the electrical circuit design and will strongly influence the peak luminous output of LED systems. It can be used to explain why the optimal operating power, at which maximum luminous flux is generated, may not occur at the rated power of the LEDs. This theory can be used to determine the optimal operating point for an LED system so that the maximum luminous flux can be achieved for a given thermal design. The general theory has been verified favorably by experiments using high-brightness LEDs.

Extended Fault-Location Formulation for Power Distribution Systems

Abstract: In this paper, an extended impedance-based fault-location formulation for generalized distribution systems is presented. The majority of distribution feeders are characterized by having several laterals, nonsymmetrical lines, highly unbalanced operation, and time-varying loads. These characteristics compromise traditional fault-location methods performance. The proposed method uses only local voltages and currents as input data. The current load profile is obtained through these measurements. The formulation considers load variation effects and different fault types. Results are obtained from numerical simulations by using a real distribution system from the Electrical Energy Distribution State Company of Rio Grande do Sul (CEEE-D), Southern Brazil. Comparative results show the technique robustness with respect to fault type and traditional fault-location problems, such as fault distance, resistance, inception angle, and load variation. The formulation was implemented as embedded software and is currently used at CEEE-D's distribution operation center.

Improved Direct Power Control of Grid-Connected DC/AC Converters

Abstract: This paper proposes new direct power control (DPC) strategies for three-phase DC/AC converters with improved dynamic response and steady-state performance. As with an electrical machine, source and converter flux, which equal the integration of the respective source and converter voltage, are used to define active and reactive power flow. Optimization of the lookup table used in conventional DPC is outlined first so as to improve power control and reduce current distortion. Then, constant switching frequency DPC is developed where the required converter voltage vector within a fixed half switching period is calculated directly from the active and reactive power errors. Detailed angle compensation due to the finite sampling frequency and the use of an integral controller to further improve the power control accuracy are described. Both simulation and experimental results are used to compare conventional DPC and vector control, and to demonstrate the effectiveness and robustness of the proposed control strategies during active and reactive power steps, and line inductance variations.

Electrical Circuit Sharing for Electric Vehicle Charging Stations

Abstract: Electric vehicle charging stations are coupled with a circuit sharing controller. Multiple electric vehicle charging stations are wired on the same electrical circuit. The circuit sharing controller implements a circuit sharing process that dynamically allocates electric current to charging stations on the same electrical circuit such that the capacity of the electrical circuit is not exceeded while permitting each of those charging stations to draw electric current through that electrical circuit for at least some amount of time.

System and method of detecting and locating intermittent electrical faults in electrical systems

Abstract: Signals are transmitted from at least one transmitter that is positioned in an electrical network. The signals that have been transmitted are received by the at least one single receiver at a single receiver positioned within the electrical network. At the single receiver, the received signals are analyzed and a determination from the analyzing the received signals is made as to whether a fault has occurred in the electrical network.

Vibrations of an elastic structure with shunted piezoelectric patches: efficient finite element formulation and electromechanical coupling coefficients

Abstract: This article is devoted to the numerical simulation of the vibrations of an elastic mechanical structure equipped with several piezoelectric patches, with applications for the control, sensing and reduction of vibrations. At first, a finite element formulation of the coupled electromechanical problem is introduced, whose originality is that provided a set of non-restrictive assumptions, the system's electrical state is fully described by very few global discrete unknowns: only a couple of variables per piezoelectric patches, namely (1) the electric charge contained in the electrodes and (2) the voltage between the electrodes. The main advantages are (1) since the electrical state is fully discretized at the weak formulation step, any standard (elastic only) finite element formulation can be easily modified to include the piezoelectric patches (2) realistic electrical boundary conditions such that equipotentiality on the electrodes and prescribed global charges naturally appear (3) the global charge/voltage variables are intrinsically adapted to include any external electrical circuit into the electromechanical problem and to simulate shunted piezoelectric patches. The second part of the article is devoted to the introduction of a reduced-order model (ROM) of the problem, by means of a modal expansion. This leads to show that the classical efficient electromechanical coupling factors (EEMCF) naturally appear as the main parameters that master the electromechanical coupling in the ROM. Finally, all the above results are applied in the case of a cantilever beam whose vibrations are reduced by means of a resonant shunt. A finite element formulation of this problem is described. It enables to compute the system EEMCF as well as its frequency response, which are compared with experimental results, showing an excellent agreement.

Reactive Power Control Design in Doubly Fed Induction Generators for Wind Turbines

Abstract: The high penetration level of wind power generation in an interconnected electrical network carries new technical challenges regarding system stability. Despite the stochastic nature of the wind, grid reliability has to be guaranteed. In compliance with power companies' new requirements on wind turbines as regards contribution to voltage regulation, this paper proposes a reactive power controller design for doubly fed induction generator drives. This drive topology enables the control of reactive power independently of the active power flow using both rotor and mains-side inverters. This degree of freedom is used in order to share the reactive powers circulating in the drive system and resulting from the reactive power required in the network. The controllers are designed using well-known linear control techniques in order to present the same dynamics. Furthermore, the proper reactive power share can be used to enhance the drive efficiency. Experimental results show the performance of the proposed reactive power control.

Generalized serial dilution module for monotonic and arbitrary microfluidic gradient generators

Abstract: In this paper, we propose a generalized serial dilution module for universal microfluidic concentration gradient generators including N cascaded-mixing stages in a stepwise manner. Desired concentrations were generated by means of controlled volumetric mixing ratios of two merging solutions in each stage. The flow rates were adjusted by controlling channel length, which is proportional to fluidic resistance in each channel. A generalized mathematical model for generating any complex concentration and output flow rate gradients is presented based on the fact that there is an analogy between microfluidic circuits and electrical circuits. The pressure drop corresponds to a voltage drop, the flow rate to an electrical current, and the flow resistance to an electrical resistance. A simple equivalent electrical circuit model was generalized, and in the model each channel segment was represented by an electrical resistance. As a result of the mathematical modelling, the only variable parameter in the generalized serial dilution module was the channel length. By the use of the generalized serial dilution module with N = 4, three types of microfluidic gradient generators for linear, logarithmic and Gaussian gradients were successfully designed and tested. The proposed strategy is capable of generating universal monotonic gradients with a single module or arbitrary gradients with multiple modules ranging from linear to complex non-linear shapes of concentration gradients as well as arbitrary output flow rate gradients in a stepwise manner. The simple universal gradient generation technology using the generalized serial dilution module will find widespread use in the greater chemical and biological community, and address many challenges of gradient-dependent phenomena.

Microgenetic multiobjective reconfiguration algorithm considering power losses and reliability indices for medium voltage distribution network

Abstract: This study proposes and applies an evolutionary-based approach for multiobjective reconfiguration in electrical power distribution networks. In this model, two types of indicators of power quality are minimised: (i) power system's losses and (ii) reliability indices. Four types of reliability indices are considered. A microgenetic algorithm ('GA) is used to handle the reconfiguration problem as a multiobjective optimisation problem with competing and non-commensurable objectives. In this context, experiments have been conducted on two standard test systems and a real network. Such problems characterise typical distribution systems taking into consideration several factors associated with the practical operation of medium voltage electrical power networks. The results show the ability of the proposed approach to generate well-distributed Pareto optimal solutions to the multiobjective reconfiguration problem. In the systems adopted for assessment purposes, our proposed approach was able to find the entire Pareto front. Furthermore, better performance indexes were found in comparison to the Pareto envelope-based selection algorithm 2 (PESA 2) technique, which is another well-known multiobjective evolutionary algorithm available in the specialised literature. From a practical point of view, the results established, in general, that a compact trade-off region exists between the power losses and the reliability indices. This means that the proposed approach can recommend to the decision maker a small set of possible solutions in order to select from them the most suitable radial topology.

Nonsuperconducting Fault Current Limiter With Controlling the Magnitudes of Fault Currents

Abstract: In this paper, a diode-bridge-type nonsuperconductor fault current limiter (NSFCL) is proposed. The structure has the capability of controlling the DC reactor current that yields to control the magnitude of fault current. In order to control the magnitude of DC reactor current, a discharging resistor is used in the proposed structure. By controlling the magnitude of DC reactor current, it is possible to reduce the current rating and inductance of DC reactor. In addition, a series connection of DC voltage source with DC reactor is used to compensate the power loss of both nonsuperconducting dc reactor and diodes of fault current limiters (FCLs). By means of proposed NSFCL, it is possible to eliminate all disconnecting switches in the distribution network. The simulation and experimental results are presented to clarify the theory and possibility of implementation of the proposed NSFCL.

Compact modelling of Through-Silicon Vias (TSVs) in three-dimensional (3-D) integrated circuits

Abstract: Modeling parasitic parameters of Through-Silicon-Via (TSV) structures is essential in exploring electrical characteristics such as delay and signal integrity (SI) of circuits and interconnections in three-dimensional (3-D) Integrated Circuits (ICs). This paper presents a complete set of self-consistent equations including self and coupling terms for resistance, capacitance and inductance of various TSV structures. Further, a reduced-order electrical circuit model is proposed for isolated TSVs as well as bundled structures for delay and SI analysis, and extracted TSV parasitics are employed in Spectre simulations for performance evaluations. Critical issues in the performance modeling for design space exploration of 3-D ICs such as cross-talk induced switching pattern dependent delay variation and cross-talk on noise are discussed. The error in these metrics when using the proposed models as compared to a field solver is contained to a few percentage points.

Propulsion Drive Models for Full Electric Marine Propulsion Systems

Abstract: Integrated full electric propulsion systems are being introduced across both civil and military marine sectors. Standard power system analysis packages cover electrical and electromagnetic components but have limited models of mechanical subsystems and their controllers. Hence, electromechanical system interactions between the prime movers, power network, and driven loads are poorly understood. This paper reviews available models of the propulsion drive system components: the power converter, motor, propeller, and ship. Due to the wide range of time constants in the system, reduced-order models of the power converter are required. A new model using state-averaged models of the inverter and a hybrid model of the rectifier is developed to give an effective solution combining accuracy with speed of simulation and an appropriate interface to the electrical network model. Simulation results for a typical ship maneuver are presented.

Materials, structures and power interfaces for efficient piezoelectric energy harvesting

Abstract: The possibility of recycling ambient energies with miniature electrical generators instead of using batteries with limited lifespan has stimulated important research efforts over the past years. Integration of such miniature generators is mainly envisioned into low power autonomous systems, for various industrial or domestic applications. This paper focuses on the use of piezoelectric materials for generating electrical energy from ambient mechanical vibrations. A review of the piezoelectric materials and the electromechanical structures which have been proposed in this field is first presented. Electrical circuits with one-stage, two-stage and three-stage interfaces which have been developed for optimizing the electrical power flow from piezoelectric devices to energy storage elements are then compared to a novel technique for controlling the energy converted by piezoelectric materials. This novel approach is derived from Ericsson thermodynamic cycle. A solution for practical implementation is proposed, theoretical predictions and experimental results are compared and discussed.

Optimal VAR Control Considering Wind Farms Using Probabilistic Load-Flow and Gray-Based Genetic Algorithms

Abstract: Techniques for distributed generations (DGs) have attracted increasing attention due to considerations of environmental sustainability. Wind farms are one of the DGs, and they have intermittent characteristics. This paper presents a method using wind generator voltages, static compensators, and transformer taps as controllers to regulate the voltage profile for operation planning in a distribution system. Wind power generations and bus loads are modeled with random variables. Through gray-based genetic algorithms (GAs), the MW loss in the system is minimized and the operational constraints are fulfilled. Moreover, this paper uses the cumulant method to calculate the bus voltage fluctuation by using the algebraic addition and multiplication to avoid convolution. The probability density function of the voltage fluctuation can be further expressed by the Gram-Charlier series expansion. Applicability of the proposed method is verified through simulation by using a 17-bus system and an autonomous 24-bus (Penghu) system.

Electrical anomaly detection method and system

Abstract: There are provided a method and a system for real time monitoring of a powered-on electrical network in a facility in order to detect an electrical anomaly while the electrical network is in use. The system comprises a sensor data receiver for receiving real time sensor data including at least one measured value measured in real time by at least one sensor installed on a conductor of the electrical network in the facility while the electrical network is in use; an anomaly detector for retrieving an anomaly detection rule from an anomaly detection rule database, the rule having an identification of a required input, a formula, and a threshold reference value, for a detection of an anomaly; receiving the real time sensor data from the sensor data receiver and extracting at least one relevant measured value from the at least one measured value using the identification of the required input; comparing the at least one relevant measured value to the threshold reference value according to the formula to determine one of a presence and an absence of the anomaly in the real time sensor data; an event generator controlled by the processor for retrieving and providing anomaly monitoring data if the anomaly is determined to be present by the processor, the anomaly monitoring data including an indication of a monitoring course of action to be carried out to address the anomaly determined to be present.

Charge storage devices containing carbon nanotube films as electrodes and charge collectors

Abstract: An energy storage device includes a nanostructured network and an electrolyte in contact with the nanostructured network. The nanostructured network is an electrically conducting nanostructured network that provides combined functions of an electrode and a charge collector of the energy storage device. An electrical device includes an energy storage device that includes a nanostructured network and an electrolyte in contact with the nanostructured network, and a load-bearing electrical circuit electrically connected to the electrical energy storage device. The energy storage device is suitable to power the electrical device while in operation.

Simple Modeling and Identification Procedures for “Black-Box” Behavioral Modeling of Power Converters Based on Transient Response Analysis

Abstract: Today, "black-box" behavioral models of power converters are becoming interesting for system level simulation of power electronics systems. These models can be used to evaluate the response of systems that are composed of commercial converters, since they can be fully parameterized by analyzing the actual converter response. To optimize the required computational resources, these models should be as simple as possible. Furthermore, the identification of the parameters should be carried out easily, looking for simple experiments and straightforward adjusting algorithms. Easy modeling and identification procedures, based on a transient response analysis, are proposed in this paper. Using this method, a large-signal "black-box" behavioral model of a power converter is composed of reduced-order transfer functions, which are identified by analyzing the step response of the converter. Both an actual commercial dc-dc converter and a line-commutated rectifier have been modeled and identified by means of this approach, in order to validate the proposed procedures.

An Efficient Mixed-Integer Linear Formulation for Long-Term Overhead Lines Maintenance Scheduling in Power Distribution Systems

Abstract: This paper presents an efficient mixed-integer linear formulation for long term maintenance schedule of overhead lines. The proposed formulation is based on risk management approach and utilizes the model of decoupled risk factors. The proposed methodology yields a significant computational saving comparing to the previously reported dynamic programming. Risk management approach enables the asset managers to consider the actual condition of electrical equipment and expected consequence of their failures. Furthermore, the decoupled risk strategy in conjunction with the mixed-integer linear programming formulation establishes a precise description of time-dependent deterioration failure rate and provides the ability to determine the most cost-effective maintenance scenario while satisfying the reliability constraints. The proposed approach is tested on the Roy Billinton Test System distribution feeders and a typical real size case study. The results presented show the accuracy and efficiency of the proposed approach.

A Recursive Park Transformation to Improve the Performance of Synchronous Reference Frame Controllers in Shunt Active Power Filters

Abstract: Load harmonic currents and load unbalances reduce power quality (PQ) supplied by electrical networks. Shunt active power filters (SAPFs) are a well-known solution that can be employed to enhance electrical PQ by injecting a compensation current at the point of common coupling (PCC) of the SAPF, the load, and the electrical grid. Hence, SAPF controllers must determine the instantaneous values of the compensation reference current, including nondesirable components of the load current. A family of SAPF controllers, which evaluates the compensation reference current using synchronous rotating frames (SRFs), employs a structure based on Park transformations: direct transform, low- pass filtering (LPF), and inverse transform. The cutoff frequency and the filter order of the LPF stage must be designed properly in order to obtain an accurate reference current and a fast dynamic response of these SAPF controllers. This paper proposes a recursive implementation of the direct Park transformation that avoids the filtering stage and allows accurate SRF controllers to be designed. Moreover, the proposed implementation is not dependent on PCC conditions. The proposed implementation is evaluated using a three-phase, three-wire SAPF and compared with LPF-based controllers by simulation and experiment.

Multivariable Dynamic Model and Robust Control of a Voltage-Source Converter for Power System Applications

Abstract: This paper proposes a new multivariable dynamic model and a control approach for a voltage-source converter (VSC) based on adopting instantaneous real- and reactive-power components as the VSC dynamic variable. Using power components as the dynamic variables reduces the degree of nonlinearities of the VSC model in comparison with the conventional VSC model that uses qd current components as variables. Furthermore, since waveforms of power components are independent of the selected qd coordinates, the proposed control is more robust to the conventionally unmodelled dynamics such as dynamics of the VSC phase-locked loop system. The proposed control system regulates the VSC AC-side real- and reactive-power components, and the DC-side voltage. This paper also introduces an overcurrent limiting mechanism, based on limiting the VSC power exchange during abnormal conditions, for the proposed control system. The proposed model and control are applied to a VSC-based reactive power compensator and simulation results based on PSCAD/EMTDC software are presented.

Designing optimal controllers for doubly fed induction generators using a genetic algorithm

Abstract: This work presents a design procedure based on evolutionary computation, more specifically on a genetic algorithm combined with the formal pole placement project, to obtain optimal controllers to the rotor-side converter of doubly fed induction generators (DFIGs), in variable-speed wind generation systems connected to the electrical grid. With this procedure it is intended to improve the global system dynamic behaviour during and after the fault period, also increasing the transient stability margin of the power system and the fault ride-through capability. The control action of the DFIG converters is accomplished by proportional and integral controllers, whose gains' adjustment is not a trivial task, because of the high complexity of the system. The results obtained confirm the efficiency of the proposed control design procedure.