Showing papers on "Voltage regulation published in 2004"

Control and testing of a dynamic voltage restorer (DVR) at medium voltage level

Abstract: The dynamic voltage restorer (DVR) has become popular as a cost effective solution for the protection of sensitive loads from voltage sags. Implementations of the DVR have been proposed at both a low voltage (LV) level, as well as a medium voltage (MV) level; and give an opportunity to protect high power sensitive loads from voltage sags. This paper reports practical test results obtained on a medium voltage (10 kV) level using a DVR at a distribution test facility in Kyndby, Denmark. The DVR was designed to protect a 400-kVA load from a 0.5-p.u. maximum voltage sag. The reported DVR verifies the use of a combined feed-forward and feed-back technique of the controller and it obtains both good transient and steady-state responses. The effect of the DVR on the system is experimentally investigated under both faulted and nonfaulted system states, for a variety of linear and nonlinear loads. Variable duration voltage sags were created using a controllable LV breaker fed by a 630 kVA distribution transformer placed upstream of the sensitive load. The fault currents in excess of 12 kA were designed and created to obtain the required voltage sags. It is concluded the DVR works well in all operating conditions.

Analysis and design of STATCOM-based voltage regulator for self-excited induction generators

Abstract: This paper deals with the design of static compensator (STATCOM)-based voltage regulator for self-excited induction generators (SEIGs). SEIG has poor voltage regulation and it requires adjustable reactive power source with varying load to maintain constant terminal voltage. The required reactive power can be provided by a STATCOM consisting of ac inductors, a dc bus capacitor, and solid-state self-commutating devices. Selection and ratings of these components are quite important for design and control of STATCOM to regulate the terminal voltage of SEIG. The analysis, design, and selection of these STATCOM components are presented for five different rating machines to operate at varying power factor loads. Two criteria (full and reduced rating of STATCOM) are considered while designing STATCOM-SEIG systems.

Renewable Energy Systems: Design and Analysis with Induction Generators

Abstract: This book is written for the reader whose goal is to understand the technology of induction generators. Topics such as the process of self-excitation, numerical analysis of stand-alone and multiple-induction generators, requirements for optimized laboratory experimentation, application of modern vector control, optimization of power transference, use of doubly fed induction generators, computer-based simulations, and the social and economic impact of induction generators are presented in order to take the subject from the academic realm to the desks of practicing engineers and undergraduate, and graduate students. This book is organized in 13 chapters. Chapter 1 presents, some definitions and the characteristics of primary sources and industrial, commercial, residential, and remote sites and of rural loads, with highlights for the selection of the suitable electric generator. Chapter 2 presents the steady-state model of the induction generator with classical steady-state representation, parameter measurements, and peculiarities of the interconnection to the distribution grid. Chapter 3 expounds on transient modeling of induction generators with a novel numerical representation of state space modeling that permits generalization of the aggregation of generators in parallel, an important subject for wind farms. Chapter 4 introduces in detail the performance of self-exited induction generators, voltage regulation, and the mathematical description of self-excitation. Chapter 5 presents some general characteristics of the induction generator with regard to torque vs. speed, power vs. output current, and the relationship of air-gap voltage to magnetizing current. Chapter 6 discusses the construction features of induction machines, particularly generator sizing, design, and manufacturing aspects. Chapter 7 presents power electronic devices, requirements for injection of power into the grid, interfacing with renewable energy systems, and AC-DC, DC-DC, DC-AC, and AC-AC conversion topologies as they apply to the control of inuction macines used for motoring and generation purposes. Chapter 8 describes the fundamental principles of scalar control of induction motors/generators and how control of the magnitude of voltage and frequency achieves suitable torque and speed with impressed slip. Chapter 9 presents vector control techniques in order to calculate stator current components for decoupling of torque and flux for fast-transient closed-loop response. Chapter 10 presents contemporary optimization techniques for peak-power tracking control of induction generators. In particular, hill-climbing control and fuzzy control are emphasized. Chapter 11 covers wound-rotor induction generators as applied to high-power renewable energy systems with important pumped-hydro and grid-tied applications. Chapter 12 describes simulation approaches to transient response of self-excited induction generators in several environments, steady-state analysis, and vector-control-based induction monitoring/generating systems.

A voltage and frequency droop control method for parallel inverters

Abstract: In this paper, a new control method for the parallel operation of one or several inverters in an island grid or the mains is described. Frequency and voltage control, including mitigation of voltage harmonics, are achieved without the need for any common control circuitry or communication between the inverters. Each inverter supplies a current that is the result of the voltage difference between a reference AC voltage source and the grid voltage across a virtual impedance with real and/or imaginary parts. The reference AC voltage source is synchronised with the grid, with a phase shift, depending on the difference between nominal and real grid frequency. A detailed analysis show that this approach has superior behaviour in comparison with the existing droop control methods, considering the mitigation of voltage harmonics, short-circuit behaviour and, in the case of a non-negligible line resistance, the 'efficient' control of frequency and voltage. Experiments show the behaviour of the method for an inverter feeding a highly distorted load and during the connection of two parallel inverters in operation.

Low cost fuel cell converter system for residential power generation

Abstract: The high installation cost is the major obstacle of the commercialization of the solid oxide fuel cell for distributed power generation. This paper presents a new low cost 10-kW converter system to overcome this obstacle. The proposed system consists of an isolated dc-dc converter to boost the fuel cell voltage to 400 V dc and a pulse-width modulated inverter with filter to convert the dc voltage to two split-phase 120-V ac. The dc-dc converter uses phase shifting to control power flow through a transformer with a metal oxide semiconductor field effect transistor full bridge on the low voltage side and a voltage doubler on the high voltage side. One IPM is used to realize the voltage doubler and the dc-ac inverter. Compared to the existing fuel cell converter systems, the proposed circuit has low cost, less component count, smaller size, and reduced dc-dc converter peak current. Simulation and experimental results are demonstrated.

Lagrangian modeling and passivity-based control of three-phase AC/DC voltage-source converters

Abstract: In this paper, we investigate the dc-bus voltage regulation problem for a three-phase boost-type pulsewidth-modulated (PWM) ac/dc converter using passivity-based control theory of Euler-Lagrange (EL) systems. The three-phase PWM ac/dc converters modeled in the a-b-c reference frame are first shown to be EL systems whose EL parameters are explicitly identified. The energy-dissipative properties of this model are fully retained under the d-q-axis transformation. Based on the transformed d-q EL model, passivity-based controllers are then synthesized using the techniques of energy shaping and damping injection. Two possible passivity-based feedback designs are discussed, leading to a feasible dynamic current-loop controller. Motivated from the usual power electronics control schemes and the study of Lee, the internal dc-bus voltage dynamics are regulated via an outer loop proportional plus integral (PI) controller cascaded to the d-axis current loop. Nonlinear PI control results of Desoer and Lin are applied to theoretically validate the proposed outer loop control scheme. The PWM ac/dc converter controlled by the proposed passivity-based current control scheme with outer loop PI compensation has the features of enhanced robustness under model uncertainties, decoupled current-loop dynamics, guaranteed zero steady-state error, and asymptotic rejection of constant load disturbance. Experimental results on a 1.5-kVA PC-based controlled prototype provide verification of these salient features. The experimental responses of a classical linear PI scheme are also included for comparative study.

Closure of "The coordinated automatic voltage control of the Italian transmission grid - part I: reasons of the choice and overview of the consolidated hierarchical system"

Abstract: Transmission network voltage regulation through coordinated automatic control of reactive powers is now becoming a challenging objective for many utilities and system operators (ISO or TSO) to achieve significant improvements in the security, quality, and efficiency of power system operation. This survey paper is concerned with application of Secondary and Tertiary Voltage Regulations (SVR and TVR), already operating in the Italian transmission network. The basic concepts behind SVR and TVR and the related hierarchical control system are first described in terms of system characteristics, main functions and design requirements. Cost/benefit analysis and comparison with alternative voltage control solutions are also provided in relation to the influence of power system restructuring process on the voltage ancillary service. More details about control system apparatuses and their field test results are provided in Part II.

Localized reactive power markets using the concept of voltage control areas

Abstract: In this paper, we present the design of a localized competitive market for reactive power ancillary services at the level of individual voltage-control areas. The concept of electrical distance has been used to identify the different voltage-control areas within a power system. The proposed reactive power market is settled on uniform price auction, using a modified optimal power-flow model. Uniform prices for various components of reactive power service are obtained for each voltage-control area. In the study cases described in the paper, we examine whether such a localized reactive power market is more desirable than a common system-wide reactive power market.

Memory sensing circuit and method for low voltage operation

Abstract: A sensing module operates with a sense amplifier sensing a conduction current of a memory cell via a coupled bit line under constant voltage condition in order to minimize bit-line to bit-line coupling. The rate of discharge of a dedicated capacitor as measured by a change in the voltage drop thereacross in a predetermined period is used to indicate the magnitude of the conduction current. The voltage cannot drop below a minimum level imposed by a circuit for maintaining the constant voltage condition on the bit line. A voltage shifter is used to boost the voltage during the discharge and to unboost the voltage after the discharge, so that the change in voltage drop properly reflects the rate of discharge without running into the minimum level.

Design of a capacitor-supported dynamic voltage restorer (DVR) for unbalanced and distorted loads

Abstract: The paper discusses the operating principles and control characteristics of a dynamic voltage restorer (DVR) that protects sensitive but unbalanced and/or distorted loads. The main aim of the DVR is to regulate the voltage at the load terminal irrespective of sag/swell, distortion, or unbalance in the supply voltage. In this paper, the DVR is operated in such a fashion that it does not supply or absorb any active power during the steady-state operation. Hence, a DC capacitor rather than a DC source can supply the voltage source inverter realizing the DVR. The proposed DVR operation is verified through extensive digital computer simulation studies.

High voltage generation and regulation system for digital multilevel nonvolatile memory

Abstract: A high voltage generator provides high voltage signals with different regulated voltage levels. A charge pump generates the high voltage, and includes a quadrature phase forward and backward Vt-canceling high-voltage self-biasing charge pump with a powerup-assist diode. A high voltage series regulator generates the high voltage supply levels, and includes slew rate enhancement and trimmable diode regulation. A nested loop regulator eliminates shunt regulation.

Switching power converter and method of controlling output voltage thereof using predictive sensing of magnetic flux

Abstract: A switching power converter and method of controlling an output voltage thereof using predictive sensing of magnetic flux provides a low-cost switching power converter via primary-side control using a primary-side winding. An integrator generates a voltage that represents flux within a magnetic element by integrating a primary-side winding voltage. A detection circuit detects the end of a half-cycle of post-conduction resonance that occurs in the power magnetic element subsequent to zero energy level in the power magnetic element. The integrator voltage is stored at the end of the half-cycle and is used to determine a sampling point prior to or equal to the start of post-conduction resonance in a subsequent switching cycle of the power converter. The primary-side winding voltage is then sampled at the sampling point, providing an indication of the output voltage of the power converter by which the output voltage of the converter can be controlled.

An innovative digital control architecture for low-Voltage, high-current DC-DC converters with tight voltage regulation

Abstract: This paper describes an innovative digital control architecture for low-voltage, high-current dc-dc converters, based on a combination of current-programmed control and variable frequency operation. The key feature of the proposed architecture is the low complexity: only two digital-to-analog converters (DACs) with low resolution (7-b) are used for control. An original control algorithm is used to reduce quantization effects to negligible levels, in spite of the low resolution of the DACs. Thanks to this algorithm, both static and dynamic output voltage regulation are improved with respect to traditional digital solutions. Adaptive voltage positioning and active current sharing are inherently provided by the new architecture. A detailed description of the control strategy is given with reference to a single-phase buck converter. Extension to multiphase converters is straightforward. The digital control architecture is experimentally verified on a FPGA-based four-phase prototype buck converter operating at 350 kHz/phase. Output voltage tolerance within /spl plusmn/0.5% is experimentally demonstrated, along with negligible quantization effects and fast transient response. The features and the performance of the proposed architecture make it a valuable candidate for the control of next generation voltage regulator modules.

New two-inductor boost converter with auxiliary transformer

Abstract: A new, two-inductor, two-switch boost converter topology and its variations suitable for applications with a large difference between the input and output voltage are described. The output voltage regulation of the proposed converters is achieved in a wide load and input-voltage range with constant-frequency control by employing an auxiliary transformer that couples the current paths of the two boost inductors.

Terminal voltage regulation characteristics by static var compensator for a three-phase self-excited induction generator

Abstract: In this paper, the practical impedance approach steady-state analysis in the frequency domain for the three-phase self-excited induction generator (SEIG) with squirrel-cage rotor is presented along with its operating performance evaluations. The three-phase SEIG is driven by a variable-speed prime mover(VSPM) in addition to a constant-speed prime mover (CSPM) such as a wind turbine and a micro gas turbine for clean alternative renewable energy in rural areas. The basic steady-state characteristics of the VSPM are considered in the three-phase SEIG approximate equivalent circuit and the operating performance of the three-phase SEIG coupled with a VSPM and/or a CSPM are evaluated and discussed online under the conditions related to the speed changes of the prime mover and the electrical inductive load power variations with simple computation processing procedures. A three-phase SEIG prototype setup with a VSPM is implemented for small-scale clean renewable and alternative energy utilizations. The experimental performance results give good agreement with those obtained from the simulation results. Furthermore, a proportional-integral (PI) closed-loop feedback voltage regulation of the three-phase SEIG driven by the VSPM on the basis of the static var compensator (SVC) composed of the thyristor phase-controlled reactor in parallel with the thyristor switched capacitor and the fixed-excitation capacitor bank is designed and considered for the wind generation as a renewable power conditioner. The simulation analysis and experimental results obtained from the three-phase SEIG with SVC for its voltage regulation prove the practical effectiveness of the additional SVC with the PI-controller-based feedback loop in steady-state operation in terms of high performance with low cost.

Voltage regulation and power losses minimization in automated distribution networks by an evolutionary multiobjective approach

Abstract: In this paper, the problem of voltage regulation and power losses minimization for automated distribution systems is dealt with. The classical formulation of the problem of optimal control of shunt capacitor banks and Under Load Tap Changers located at HV/MV substations has been coupled with the optimal control of tie-switches and capacitor banks on the feeders of a large radially operated meshed distribution system with the aim of attaining minimum power losses and the flattening of the voltage profile. The considered formulation requires the optimization of two different objectives; therefore the use of adequate multiobjective heuristic optimization methods is needed. The heuristic strategy used for the optimization is based on fuzzy sets theory. After a brief description of the general problem of optimal control of voltage and power losses in automated distribution networks, the most recent papers on the topic are reported and commented. Then the problem formulation and the solution algorithm are described in detail. Finally, numerical results on a large distribution system demonstrate that the proposed formulation and approach are effective and feasible for finding an optimal generalized dispatching schedule.

A novel control algorithm for the DG interface to mitigate power quality problems

Abstract: Distributed Generation (DG) exists in distribution systems and is installed by either the utility or the customers. This paper proposes a novel utilization of the existing DG nonlinear interface not only to control the active power flow, but also to mitigate unbalance and harmonics, and to manage the reactive power of the system. The proposed Flexible Distributed Generation (FDG) is similar in functionality to FACTS, but works at the distribution level. Moreover, a novel ADAptive LINEar neuron (ADALINE) structure is presented. The new structure is applied to multi output (MO) systems for parameter tracking/estimation, and is called MO-ADALINE. It is dedicated to symmetrical components estimation. The control loop combines a Fuzzy Logic Controller (FLC) for voltage regulation, and a processing unit-based ADALINE to deal with unbalance, harmonics and reactive power compensation. One advantage of the proposed control system is its insensitivity to parameter variation, a necessity for distribution system applications. Simulations of the suggested FDG based control algorithm are conducted to evaluate the performance of the novel system.

Real and reactive power coordination for a unified power flow controller

Abstract: This paper proposes a new real and reactive power coordination controller for a unified power flow controller (UPFC). The basic control for the UPFC is such that the series converter of the UPFC controls the transmission line real/reactive power flow and the shunt converter of the UPFC controls the UPFC bus voltage/shunt reactive power and the DC link capacitor voltage. In steady state, the real power demand of the series converter is supplied by the shunt converter of the UPFC. To avoid instability/loss of DC link capacitor voltage during transient conditions, a new real power coordination controller has been designed. The need for reactive power coordination controller for UPFC arises from the fact that excessive bus voltage (the bus to which the shunt converter is connected) excursions occur during reactive power transfers. A new reactive power coordination controller has been designed to limit excessive voltage excursions during reactive power transfers. PSCAD-EMTDC simulation results have been presented to show the improvement in the performance of the UPFC control with the proposed real power and reactive power coordination controller.

Control of series active power filters compensating for source voltage unbalance and current harmonics

Abstract: In this paper, a novel control scheme compensating for source voltage unbalance and current harmonics in series-type active power filter systems combined with shunt passive filters is proposed, which focuses on reducing the delay time effect required to generate the reference voltage. Using digital all-pass filters, the positive voltage sequence component out of the unbalanced source voltage is derived. The all-pass filter can give a desired phase shift and no magnitude reduction, unlike conventional low-or high-pass filters. Based on this positive-sequence component, the source phase angle and the reference voltage for compensation are derived. This method is easier to implement and to tune controller gains. In order to reduce the delay time effect in the voltage control loop, the reference voltage is predicted a sampling period ahead. The validity of the proposed control scheme has been verified by experimental results.

Analysis of series compensation and DC-link voltage controls of a transformerless self-charging dynamic voltage restorer

Abstract: This paper describes a transformerless self-charging dynamic voltage restorer (DVR) series compensation device used to mitigate voltage sags. A detailed analysis on the control of the restorer for voltage sag mitigation and dc-link voltage regulation are presented. A nonlinear element is shown to exist in the regulator, the activation of which can adversely affect its stability. Active cancellation for this element is recommended. Simulation and experimental results are presented for a 1-kVA prototype to validate the analysis as well as demonstrate the DVR's performance under both voltage restoration and self-charging operating conditions.

A new concept of voltage-collapse protection based on local phasors

Abstract: A new algorithm for protection against voltage collapse is proposed. The algorithm makes use of the magnitudes and angles of the local phasors (i.e., bus voltages and load currents). The change in an apparent power-line flow during a time interval is exploited for computing the voltage-collapse criterion. The criterion is based on the fact that the line losses in the vicinity of the voltage collapse increase faster than the delivery of the apparent power and, at the voltage-collapse point, the losses consume all of the increased power. The selected criterion equals 0 when a voltage collapse occurs. The proposed algorithm could be easily implemented in a numerical relay. The information obtained by the relay can be used at two levels-for the coordinated system-wide control action or for automatic action on the local level. The algorithm is simple and computationally very fast. It was tested on the IEEE 118-bus test system.

STATCOM-Based Damping Stabilizers for Power System Stability Enhancement

Abstract: Power system stability enhancement via STATCOM-bas ed stabilizers is thoroughly investigated in this paper. This study presents a singular value decomposition (SVD) based approach to assess and measure the controllability of the poorly damped electromechanical modes by different control inputs of a STATCOM. The Coordination among the proposed damping stabilizers and the STATCOM internal AC and DC voltage controllers has been taken into consideration. The design problem of STATCOM-based stabilizers is formulated as an optimization problem. For coordination purposes, a time domain-based multiobjective function to improve the system stability as well as AC and DC voltage regulation is proposed. Then, a real-coded genetic algorithm (RCGA) is employed to search for optimal stabilizer parameters. This aims to enhance both rotor angle stability and voltage regulation of the power system. The proposed stabilizers are tested on a weakly connected power system with different loading conditions. The nonlinear simulation results show the effectiveness and robustness of the proposed control schemes over a wide range of loading conditions.

A carrier-based PWM scheme for neutral-point voltage balancing in three-level inverters

Abstract: A significant problem with neutral-point-clamped three-level inverters is the fluctuation in the neutral point voltage. In this paper, a capacitor voltage balancing technique for carrier-based three-level PWM is developed, with improved voltage control capability at high modulation index. The method incorporates a novel scheme that requires measurements of only the motor currents and capacitor voltages to implement voltage balancing control, and does not need to determine the direction of power flow between the motor and the inverter.

Closed loop control of excitation parameters for high speed switched-reluctance generators

Abstract: This paper presents a new approach to the automatic control of excitation parameters for the switched-reluctance generator (SRG) where the SRG system operates at sufficiently high speed that it operates in the single pulse mode. The turn-on and turn-off angles are the two parameters through which we can control the electric power generation. The objective of the work is to develop an easily implementable control algorithm that automatically maintains the most efficient excitation angles in producing the required amount of electric power. The work is focused on finding the most efficient excitation angles and characterizing them for easy implemention under closed loop control. Through modeling of an experimental SRG and extensive simulation, it can be seen that the optimal-efficiency turn-off angles can be characterized as a function of power and speed level. Within the closed-loop power controller, the optimal-efficiency turn-off angle is determined from an analytic curve fit. The turn-on angle is then used as the degree of freedom necessary to regulate the power produced by the SRG. Given that the turn-off angle is associated with optimal-efficiency at each speed and power point, overall operation is achieved at optimal-efficiency. The SRG, inverter and control system are modeled in Simulink to demonstrate the operation of the system when implemented within a voltage regulation system. The control technique is then applied to an experimental SRG system. Experimental operation documents that the technique provides for efficient operation of the SRG system through tuning the controller at only four operating points.

Load driving device and portable apparatus utilizing such driving device

Abstract: A load such as an LED and a constant-current source are connected in series with each other between the node of a dc-dc conversion type power supply circuit providing an output voltage and the ground. The constant-current source provides a constant current Io, the magnitude of which can be adjusted. The power supply circuit controls the output voltage such that the voltage drop across the constant-current source serving as a detection voltage becomes equal to a reference voltage. Thus, the load current can be varied within a predetermined range while avoiding the power loss due to an increase in the load current, thereby always permitting efficient operation of the load.

Adaptive voltage position design for voltage regulators

Abstract: This paper proposes a general design guideline for the voltage regulator (VR) to achieve adaptive voltage position (AVP). All existing control methods are covered for different kinds of output filter capacitors. Based on the small-signal model analysis, the output impedance and system control bandwidth are discussed. Following the proposed design guidelines, simulation and experimental results demonstrate very good VR transient response.

Maximising energy capture from distributed generators in weak networks

Abstract: The paper discusses the implications of the increasing capacity of synchronous generators at the remote ends of rural distribution networks where the line resistances are high and the X/R ratios are low. Local voltage variation is specifically examined and two methods of compensation are proposed. The first of them is a deterministic system that uses a set of rules to switch intelligently between voltage and power factor control modes, while the second is based on a fuzzy inference system that adjusts the reference setting of the automatic power factor controller in response to the terminal voltage. Extensive simulations have verified that the proposed approaches may increase the export of real power while maintaining voltage within the statutory limits.

Capacitor placement for conservative voltage reduction on distribution feeders

Abstract: This paper proposes a strategy for placing capacitors at multiple locations on a distribution feeder to allow: 1) deeper levels of substation voltage reduction for peak load reduction; 2) power factor correction; and 3) power loss reduction. By reducing peak demand, a utility can avoid paying high prices when purchasing power or it may sell excess generation at high prices. By minimizing system losses, savings are obtained through reduced demand and energy charges. Besides a positive economic response, load reduction associated with improved power factor at the substation has a beneficial effect on voltage stability by increasing the system stability limit margin.

Induction voltage adders and the induction accelerator family

Abstract: Induction voltage adders (IVA) and induction accelerators of various types are described and their principles and advantages are discussed. The designs and technologies used in the various subsections and components of high-current IVAs are described. Some features of the pulse power that drives IVAs are discussed. Two representative high-current IVAs are briefly described, and characteristics of other IVAs with different features are mentioned. Some other IVA and induction accelerator variants that could be considered are also suggested.