Showing papers on "AC power published in 2001"

Dynamic power management in wireless sensor networks

Abstract: We propose an OS-directed power management technique to improve the energy efficiency of sensor nodes. Dynamic power management (DPM) is an effective tool in reducing system power consumption without significantly degrading performance. The basic idea is to shut down devices when not needed and wake them up when necessary. DPM, in general, is not a trivial problem. If the energy and performance overheads in sleep-state transition were negligible, then a simple greedy algorithm that makes the system enter the deepest sleep state when idling would be perfect. However, in reality, sleep-state transitioning has the overhead of storing processor state and turning off power. Waking up also takes a finite amount of time. Therefore, implementing the correct policy for sleep-state transitioning is critical for DPM success. It is argued that power-aware methodology uses an embedded microoperating system to reduce node energy consumption by exploiting both sleep state and active power management.

A closed-loop selective harmonic compensation for active filters

Abstract: This paper proposes a control algorithm for parallel active power filters, based on current-controlled pulsewidth-modulated converters, which allows precise compensation of selected harmonic currents produced by distorting loads. The approach is based on the measurement of line currents and performs the compensation of the selected harmonics using closed-loop synchronous frame controllers. Thanks to the closed-loop operation, full compensation of the desired harmonics is achieved even in the presence of a significant delay in the current control. Thanks to the selective approach, active filter interactions with possible dynamic components of the load are minimized. Moreover, the complexity of the synchronous frame controllers is overcome using equivalent stationary frame controllers. Experimental results confirm the theoretical expectations.

A topology survey of single-stage power factor corrector with a boost type input-current-shaper

Abstract: A topological review of the single stage power factor corrected (PFC) rectifiers is presented in this paper. Most reported single-stage PFC rectifiers cascade a boost-type converter with a forward or a flyback DC-DC converter so that input current shaping, isolation, and fast output voltage regulation are performed in one single stage. The cost and performance of single-stage PFC converters depend greatly on how its input current shaper (ICS) and the DC-DC converter are integrated together. For the cascade connected single-stage PFC rectifiers, the energy storage capacitor is found in either series or parallel path of energy flow. The second group appears to represent the main stream. Therefore, the focus of this paper is on the second group. It is found that many of these topologies can be implemented by combining a two-terminal or three-terminal boost ICS cell with DC-DC converter along with an energy storage capacitor in between. A general rule is observed that translates a three-terminal ICS cell to a two-terminal ICS cell using an additional winding from the transformer and vice verse. According to the translation rule, many of the reported single-stage PFC topologies can be viewed as electrically equivalent to one another. Several new PFC converters were derived from some existing topologies using the translation rule.

Direct power control of grid-connected wound rotor induction machine without rotor position sensors

Abstract: A method is presented for fast response control of the torque and flux of a grid connected wound rotor induction machine fed by back to back connected voltage source inverters on the rotor side. It is based on the measurement of active and reactive power on the grid side where voltages and currents are alternating at fixed frequency. The active and reactive powers are made to track references using hysteresis controllers. The method eliminates the need for rotor position sensing and gives excellent dynamic performance, as shown by simulation and experimental results from a variable speed constant frequency induction generator system. It is also capable of starting on the fly. It is thus an attractive sensorless control method for drive as well as generator applications.

Reactive Power as an Ancillary Service

Abstract: This paper addresses the problem of reactive power procurement by an independent system operator (ISO) in deregulated electricity markets. A reactive bid structure is proposed in the context of a reactive power market. Based on the reactive power price offers and technical constraints involved in reactive power planning, a two-tier approach is developed to determine the most beneficial reactive power contracts for the ISO. The reactive capability of a generator and, therefore, the opportunity costs in providing reactive power is also included in the model. Uncertainty in reactive demand and in reactive bids of participating parties is incorporated through Monte Carlo simulations and the expected reactive power procurement plan for the ISO is hence determined. The CIGRE 32-bus network, approximately representing the Swedish system, is used for the studies.

Synchronous-frame harmonic control for high-performance AC power supplies

Abstract: In order to achieve the reduction of voltage distortion in AC power supplies (ACPSs), this paper describes an implementation of synchronous-frame control for selected frequencies in the output voltage. The regulation of the fundamental output voltage, as well as that of some low-order harmonics, is achieved using a synchronous-frame controller for each selected frequency in addition to a conventional control. The conventional part conserves good dynamic performance under load changes, while rotating-frame controllers allow a slow, but very precise compensation of the residual errors within the assumption that the harmonics produced by distorting load are slowly varying. Moreover, motivated by a fixed-point implementation, a set of refinements and modifications of the original scheme is proposed, allowing a reduction of signal processing requirements and a new control algorithm structure less sensitive to quantization and rounding errors. This solution is particularly effective for high-power fully digitally controlled ACPSs, where the voltage loop bandwidth is usually not large enough to provide regulation at harmonic frequencies. The proposed control scheme has been implemented using a fixed-point single-chip digital signal processor (ADMC401 by Analog Devices). Experimental results on a 3-kVA three-phase converter prototype show the effectiveness of the proposed approach.

Integration of a StatCom and Battery Energy Storage

Abstract: The integration of an energy storage system, such as battery energy storage (BESS), into a FACTS device can provide dynamic decentralized active power capabilities and much-needed flexibility for mitigating transmission level power flow problems. This paper will introduce an integrated static synchronous compensator (StatCom)/BESS for the improvement of dynamic and transient stability and transmission capability, compare the performance of the different FACTS/BESS combinations, and provide experimental verification of the proposed controls on a scaled StatCom/BESS system.

Compensation of distribution system voltage sag by DVR and D-STATCOM

Abstract: This paper describes the techniques of correcting the supply voltage sag in a distribution system by two power electronics based devices called DVR and D-STATCOM. A DVR injects a voltage in series with the system voltage and a D-STATCOM injects a current into the system to correct the voltage sag. The steady state performance of both DVR and D-STATCOM is determined and compared for various values of voltage sag, system fault level and load level. The minimum apparent power injection required to correct a given voltage sag by these devices is also determined and compared. The maximum voltage sag that can be corrected without injecting any active power into the system is also determined. Simulation results indicated that a DVR can correct a voltage sag with much less injected apparent power compared to that of a D-STATCOM.

Unified constant-frequency integration control of active power filters-steady-state and dynamics

Abstract: An active power filter (APF) is a device that is connected in parallel to and cancels the reactive and harmonic currents from a group of nonlinear loads so that the resulting total current drawn from the AC mains is sinusoidal. This paper presents a unified constant-frequency integration (UCI) APF control method based on one-cycle control. This method employs an integrator with reset as its core component to control the pulse width of an AC-DC converter so that its current draw is precisely opposite to the reactive and harmonic current draw of the nonlinear loads. In contrast to previously proposed methods, there is no need to generate a current reference for the control of the converter current, thus no need for a multiplier and no need to sense the AC line voltage, the APF current, or the nonlinear load current. Only one AC current sensor is used to sense the AC main current and one DC voltage sensor is used to sense the DC capacitor voltage. The control method features constant switching frequency operation, minimum reactive and harmonic current generation, and simple analog circuitry. It provides a low cost and high performance solution for power quality control. Steady-state and dynamic study is presented in this paper. Design example is given using a two-level AC-DC boost topology. A prototype was developed to demonstrate the performance of the proposed APF. This control method is generalized to control a family of converters that are suitable for APF applications. All findings are supported by experiments and simulation.

Impact of renewable distributed generation on power systems

Abstract: The traditional approach in electric power generation is to have centralized plants distributing electricity through an extensive transmission & distribution network. Distributed generation (DG) provides electric power at a site closer to the customer, eliminating the unnecessary transmission and distribution costs. In addition, it can reduce fossil fuel emissions, defer capital cost, reduce maintenance investments and improve the distribution feeder voltage conditions. In the case of small residential photovoltaic (PV) and wind systems, the actual generator locations and DG penetration level are usually not apriori known. The following study attempts to calculate the boundaries of the impact of randomly placed distributed generators on a distribution feeder. Monte Carlo simulations are performed, and boundaries for overall improvements are determined. The study shows that the knowledge of total penetration of small PV systems is sufficient to estimate the effects of DG on the feeder.

Optimal Dispatch of Generating Units of the Itaipu Hydroelectric Plant

Abstract: This article is concemed with the optimal dispatch of generating units of Itaipu, the world's largest hydroelectric plant in operation. Itaipi is a 12.6 GW hydro plant, located on the Parana river in South America, composed of 18 identical 700 MW generating units. A dynamic programming model has been developed to optimize the number of generating units in operation at each hour of the day in order to attain the total generation scheduling of the plant in the most economic way. The model highlights the trade-off between start-up/shut-down of generating units and hydro power efficiency, taking into account variations in tailrace elevation, penstock head losses, and turbine-generator efficiencies. The methodology has been tested for a typical generation scheduling, and the results show that the number of the turbine-generator sets dispatched has a major influence on the overall hydroplant efficiency, and therefore it is a key aspect to be considered in the dispatch of hydro generating units. In the case of Itaipu, the economic benefits, in terms of greater power efficiency with respect to actual operation, are in the range of millions of dollars per year.

A particle swarm optimization for reactive power and voltage control in electric power systems

Abstract: This paper presents a particle swarm optimization (PSO) for reactive power and voltage control (RPVC) in electric power systems. RPVC can be formulated as a mixed-integer nonlinear optimization problem (MINLP). The proposed method expands the original PSO to handle a MINLP and determines an RPVC strategy with continuous and discrete control variables such as automatic voltage regulator (AVR) operating values of generators, tap positions of on-load tap changer (OLTC) of transformers, and the amount of reactive power compensation equipment (RPCE). The feasibility of the proposed method is demonstrated and compared with reactive tabu search (RTS) and the enumeration method on practical power system models with promising results.

A three-phase line-interactive UPS system implementation with series-parallel active power-line conditioning capabilities

Abstract: This paper presents a three-phase line-interactive uninterruptible power supply (UPS) system with series-parallel active power-line conditioning capabilities, using a synchronous reference frame (SRF) based controller, which allows an effective power factor correction, load harmonic current suppression and output voltage regulation. The three-phase UPS system is composed of two active power filter topologies. The first one is a series active power filter, which works as a sinusoidal current source in phase with the input voltage. The other is a parallel active power filter, which works as a sinusoidal voltage source in phase with the input voltage, providing to the load a regulated and sinusoidal voltage with low total harmonic distortion (THD). Operation of a three-phase phase-locked loop (PLL) structure, used in the proposed line-interactive UPS implementation, is presented and experimentally verified under distorted utility conditions. The control algorithm using SRF method and the active power flow through the UPS system are described and analytically studied. Design procedures, digital simulations and experimental results for a prototype are presented to verify the good performance of the proposed three-phase line-interactive UPS system.

Dispatch of Main Transformer ULTC and Capacitors in a Distribution System

Abstract: This paper presents a dynamic programming method for solving reactive power / voltage control problems in a distribution system. The objective of this paper is to properly dispatch main transformers under load tap changers, substation capacitors, and feeder capacitors based on hourly forecast loads of each feeder section and primary bus voltage such that the total feeder loss can be minimized, voltage profile can be improved, and the reactive power flow into the main transformer can be restrained. The constraints that must be considered include the maximum allowable number of switching operations in a day for under load tap changer and each capacitor, and the voltage limit on the feeder and secondary bus voltage is limited. To demonstrate the usefulness of the proposed approach, reactive power /voltage control in a distribution system within the service area of the Yunlin District Office of Taiwan Power Company is performed. It is found that a proper dispatching schedule for each capacitor and under load tap changer can be reached by the presented method.

A Robust Control Strategy for Shunt and Series Reactive Compensators to Dame Electromechanical Oscillations

Abstract: This paper examines the enhancement of power system stability properties by use of thyristor controlled series capacitors (TCSCs) and static VAr systems (SVCs). Models suitable for incorporation in dynamic simulation programs used to study angle stability are analyzed. A control strategy for damping of electromechanical power oscillations using an energy function method is derived. Using this control strategy each device (TCSC and SVC) will contribute to the damping of power swings without deteriorating the effect of the other power oscillation damping (POD) devices. The damping effect is robust with respect to loading condition, fault location and network structure. Furthermore, the control inputs are based on local signals. The effectiveness of the controls are demonstrated for model power systems.

A multimodule parallelable series-connected PWM voltage regulator

Abstract: This paper presents the analysis and design of a single-phase voltage regulator (VR) and its multinodule parallel control. The VR employs the pulsewidth modulation three-arm rectifier-inverter topology. The inverter side adjusts the load voltage with the series regulating structure aiming to minimize converter capacity and attain higher efficiency. The rectifier side regenerates the load power and executes the active power filter function to achieve unity power factor. Based on such high-performance VR, a resistive droop method combined with the P-V droop and Q-/spl delta/ shift scheme is then proposed to control the current sharing such that multiple VRs can be paralleled directly without any control interconnection. The proposed parallel control technique possesses the features of fast response, precise voltage regulation, equal fundamental and harmonic current sharing, tolerance for parameter mismatch, and so on. Two prototype 1 KVA VRs are implemented, and the effectiveness is demonstrated by some simulation and experimental results.

Modeling and simulation of a distribution STATCOM using Simulink's Power System Blockset

Abstract: This paper presents a study on the modeling of a STATCOM (Static Synchronous Compensator) used for reactive power compensation on a distribution network. The power circuits of the D-STATCOM and the distribution network are modeled by specific blocks from the Power System Blockset while the control system is modeled by Simulink blocks. Static and dynamic performance of a /spl plusmn/3 MVar D-STATCOM on a 25-kV network is evaluated. An "average modeling" approach is proposed to simplify the PWM inverter operation and to accelerate the simulation for control parameters adjusting purpose. Simulation performance obtained with both modeling approaches are presented and compared.

Solar generation system

Abstract: A solar generation system includes a standard solar cell string and a substandard solar cell string. A DC voltage output from the substandard solar cell string is boosted by a booster unit to the level of the DC voltage output from the standard solar cell string, and the DC voltage from the standard solar cell string and the boosted DC voltage are applied to a DC/AC inverter, whereby an AC power is obtained, which is supplied to a utility power supply.

Unified constant-frequency integration control of three-phase power factor corrected rectifiers, active power filters, and grid-connected inverters

Abstract: An unified constant-frequency integration (UCI) control method based on one-cycle control employs an integrator with reset as its core component along with a few logic and linear components to control the pulse width of a three-phase recitifier, active power filter, or grid-connected inverter so that the all three phase current draw from or the current output to the utility line is sinusoidal. No multipliers and reference calculation circuitry are needed for controlling active power filters. The UCI control employs constant switching frequency and operates in continuous conduction mode (CCM). If in some cases a DSP is desired for some other purposes, the Unified Constant-frequency Control function can be realized by a low cost DSP with a high reliability, because no high speed calcutation, high speed A/D converter, and mutipliers are required.

Power factor correction: a survey

Abstract: New recommendations and future standards have increased the interest in power factor correction circuits. There are multiple solutions to this problem to obtain sinusoidal line current and in addition, a great number of circuits have been proposed with nonsinusoidal line current. In this paper, a review of the most interesting solutions for single phase applications is carried out. They are classified attending to the line current waveform, energy processing, number of switches, control loops, etc. The major advantages and disadvantages are highlighted and the field of application is found.

The effect of voltage distortion on ageing acceleration of insulation systems under partial discharge activity

Abstract: This paper focuses on the features of harmonic distortion which may affect significantly the reliability of typical ac-power network equipment, such as low-voltage self-healing capacitors used for reactive power and harmonic compensation. Moreover, the effect of high-frequency pulse-like voltage generated by Adjustable Speed Drives (ASD) on electrical machine insulation is also investigated, resorting to life tests carried out on different insulating materials of the standard and "corona resistant" type, at electrical field levels able to incept partial discharges (PD).

Analysis of a robust DC-bus voltage control system for a VSC transmission scheme

Abstract: This paper proposes a high dynamic digital control scheme for IGBT based VSC HVDC power transmission, ie VSC-HVDC system, based on equivalent continuous-time state-space models for the rectifier/inverter actuators, using a nonlinear approach In this paper, both tracking and regulation problems are presented in order to perform a robust DC-bus voltage control Results illustrate the good behaviour of the system variables in response to positive and negative active power steps

Power converting apparatus, control method therefor, and solar power generation apparatus

Abstract: The object is to reliably detect a ground fault of a solar battery. To detect a ground fault position to take an efficient measure against the ground fault, DC power input from a solar battery is converted into AC power and supplied to a system. In a system interconnection inverter (utility connected inverter) having non-insulated input and output, the input voltage of a converter circuit and/or the intermediate voltage between the converter circuit and an inverter circuit are varied to control the potential to ground at each portion of the solar battery to a value other than a value close to zero.

Experience with testing and modeling of gas turbines

Abstract: The North American western grid experienced two major power disturbances on July 2 and August 10, 1996. The postmortem studies of the incidents indicated that some of the generating units did not respond as predicted by system studies. Consequently, the Western System Coordinating Council (WSCC) mandated that all units (generator, excitation, governor and turbine systems) in the western grid greater than 10 MVA be tested to verify the generator reactive power limits as well as the dynamic model data being used for system studies. This paper presents field experiences of the authors in testing and modeling of gas turbines and their associated governors during the WSCC recommended test program.

A cage induction generator using back to back PWM converters for variable speed grid connected wind energy system

Abstract: A new control scheme of a variable speed grid connected wind energy generation system is presented. The scheme uses a cage induction generator driven by an emulated wind turbine with two back-to-back voltage-fed PWM inverters to interface the generator and the grid. The machine currents are controlled using an indirect vector control technique. The generator torque is controlled to drive the machine to the speed for maximum wind turbine aerodynamic efficiency. The supply side converter currents are also controlled using a vector approach using a reference frame aligned with stator voltage vector. The DC link voltage of the power electronics interface is controlled acting upon the supply active power current component using a nonlinear control and a fuzzy based interpolation of linear PI controllers to improve the disturbance rejection and meet noise considerations in steady state. Experimental validation of the proposed control scheme on an emulated 3.2 kW wind energy system is shown.

Low power design challenges for the decade

Abstract: Technology scaling will become difficult beyond 0.18 micron. For continued growth in performance, transistor density, and reduced energy per computation, circuit design will have to employ a new set of design techniques, with adequate design automation tools support. This paper discusses a few such techniques that reduce active and leakage power, and deliver higher performance. It concludes by pointing out some of the potential paradigm shifts.

Design and implementation of a modified Fourier analysis harmonic current computation technique for power active filters using DSPs

Abstract: The design and implementation of a harmonic current computation technique based on a modified Fourier analysis, suitable for active power filters incorporating DSPs is presented. The proposed technique is suitable for the monitoring and control of load current harmonics for real-time applications. The derivation of the basic equations based on the proposed technique and the system implementation using the Analogue Devices SHARC processor are presented. The steady state and dynamic performance of the system are evaluated for a range of loading conditions.

Digital control with improved performance for boost power factor correction circuits

Abstract: In this paper, an approach for the design of a digital controller for a PFC pre-regulator is proposed. The controller is modified to account for large control periods and computational delays, and can therefore be implemented on processors with few available computational resources. Results show that, even with a very low control rate, system specifications can be met using the proposed technique.

Real-Time Digital Simulation of Power Electronic Apparatus Interfaced with Digital Controllers

Abstract: This paper presents a new approach for the real-time digital simulation of power electronic controllers in power systems. Digital controllers for power electronic systems present a problem when testing in real-time using a digital simulator due to the discrete nature of their outputs, which are not necessarily in synchronism with the time step of the simulator. The proposed algorithm combines the variable step-size numerical integration method with linear interpolation for the synchronization of a real-time digital simulator and a digital controller. It is shown that lack of such synchronization leads to inaccurate simulation results, specifically with regard to the fundamental and harmonics of the voltage and current signals. Sampling theory is used to model the interaction between a digital simulator and a digital controller. A pulse width modulated (PWM) voltage source converter (VSC) based reactive power compensator system is used as an illustrative example for the simulation.