Showing papers on "AC power published in 2003"

Modeling and control of a wind turbine driven doubly fed induction generator

Abstract: This paper presents the simulation results of a grid-connected wind driven doubly fed induction machine (DFIM) together with some real machine performance results. The modeling of the machine considers operating conditions below and above synchronous speed, which are actually achieved by means of a double-sided PWM converter joining the machine rotor to the grid. In order to decouple the active and reactive powers generated by the machine, stator-flux-oriented vector control is applied. The wind generator mathematical model developed in this paper is used to show how such a control strategy offers the possibility of controlling the power factor of the energy to be generated.

Single phase power factor correction: a survey

Abstract: New recommendations and future standards have increased the interest in power factor correction circuits. There are multiple solutions in which line current is sinusoidal. 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 and low power 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.

Constant-voltage switching power supply provided with overvoltage output protecting circuit, and electronic apparatus provided with overvoltage protecting circuit

Abstract: An overvoltage output protector is electrically connected to a constant-voltage switching power supply which includes a switching transistor converting a DC voltage obtained by smoothing an AC voltage supplied from an AC power source into a cyclic pulse signal. In the overvoltage output protector, an overvoltage monitor whether a potential of the cyclic pulse signal is a predetermined value or more. A deactivator turns off the switching transistor in a case where the overvoltage monitor judges that the potential of the cyclic pulse signal is the predetermined value or more.

System and method for data retrieval in ac power tools via an ac line cord

Abstract: An apparatus (10) and method for obtaining data stored in an electronic module (30) within a power tool (18) over a power line cord (16) of the tool (18), and without any disassembly of the tool (18) being required. An interface subsystem (12) applies a high frequency, low power AC input signal to the power cord (16) of the tool (18). A controller (28) within the tool (18) recognizes this signal as an indication that the tool (18) is to be placed in a communications mode. The controller (28) modulates a power switching device used for turning on and off the motor of the tool (18) in such a fashion that current pulses are transmitted over the power line cord (16) back to the interface subsystem (12). The current pulses correspond to tool (18) usage/performance/identification information stored in the electronic module (30) of the tool (18). This information is interpreted by a current reader circuit of the interface subsystem (12) as binary information which is then transmitted to an external computer (13) data logging device for analysis and/or recording.

Control of a hybrid asymmetric multi-level inverter for competitive medium-voltage industrial drives

Abstract: In symmetric multilevel inverters, there is a tradeoff between the output quality and the reliability and efficiency of the converter. New asymmetric and hybrid solutions, using different voltages and devices in various parts of the inverter, promise significant improvements for medium-voltage industrial drives. This paper investigates such a hybrid asymmetric nine-level inverter. It consists of a three-phase three-level integrated gate-commutated thyristor inverter (main inverter), with a two-level insulated-gate bipolar transistor H-bridge (subinverter) in series with each phase. To keep the power part simple and the efficiency high, the subinverters have no feeding from the net and can only supply reactive power. This is a very interesting solution in terms of power quality, efficiency, reliability, and cost. But the nonsupplied intermediate-circuit capacitors form an unstable system. This paper proposes a control method to stabilize their voltages. Power balancing is guaranteed by varying the common-mode voltage, using an online nonlinear model-predictive controller. The controller predicts the system evolution as a function of the control inputs. A cost function of system and control quantities is iteratively minimized in real time, to find the optimal control to apply to the system. Simulations and measurements demonstrate stable behavior in steady state and during transients. Precharging of the nonsupplied capacitors is also an issue to consider. This paper proposes a startup method that charges them in parallel with the supplied ones, without any additional equipment. Measurements show its successful application in the proposed drive system.

Dynamic-sleep transistor and body bias for active leakage power control of microprocessors

Abstract: In order to manage the active power consumption of high-performance digital designs, active leakage control techniques are required to provide significant leakage power savings coupled with fast time constants for entering and exiting idle mode. In this paper, dynamic sleep transistors and body bias are used in conjunction with clock gating to control active leakage for a 32-bit integer execution core in 130-nm CMOS technology. Measurements on pMOS sleep transistor reveal that lowest-leakage state is reached in less than 1 /spl mu/s, resulting in 37/spl times/ reduction in leakage power, while reactivation of block is achieved in less than two clock cycles. PMOS body bias reduces leakage power by 2/spl times/ with no performance penalty, and similar reactivation time. Power measurements at 4 GHz, 1.3 V, 75/spl deg/C demonstrate 8% total power reduction using dynamic body bias and 15% power reduction using a pMOS sleep transistor, for a typical activity profile.

Implementation and control of grid connected AC-DC-AC power converter for variable speed wind energy conversion system

Abstract: 30 kW electrical power conversion system is developed for a variable speed wind turbine system. In the wind energy conversion system (WECS) a synchronous generator converts the mechanical energy into electrical energy. As the voltage and frequency of generator output vary along the wind speed change, a DC-DC boosting chopper is utilized to maintain constant DC link voltage. The input DC current is regulated to follow the optimized current reference for maximum power point operation of turbine system. Line side PWM inverter supply currents into the utility line by regulating the DC link voltage. The active power is controlled by q-axis current whereas the reactive power can be controlled by d-axis current. The phase angle of utility voltage is detected using software PLL (phased locked loop) in d-q synchronous reference frame. Proposed scheme gives a low cost and high quality power conversion solution for variable speed WECS.

Analysis and design of direct power control (DPC) for a three phase synchronous rectifier via output regulation subspaces

Abstract: In this paper, the authors present a controller that directly regulates the active and instantaneous reactive power in a synchronous three-phase boost-type rectifier. The controller ensures a good regulation of the output voltage, and guarantees the power factor close to one. The controller builds upon the ideas of the well known direct torque control (DTC) for induction motors. In their case, the active and reactive powers replace the torque and flux amplitude used as the controlled outputs in DTC, thus motivating the name DPC-control. They show that a simple modification to the original algorithm makes the selection of the control inputs more accurate. To formalize this technique, they utilize the concept of output regulation subspaces. A modification is added to the basic controller to deal with disturbances such as unbalance and distortion in the source voltage. Finally, the proposed controller was tested both in simulations and experimentally, and illustrative results are presented.

Multiobjective optimal power flows to evaluate voltage security costs in power networks

Abstract: In this paper, new optimal power flow (OPF) techniques are proposed based on multiobjective methodologies to optimize active and reactive power dispatch while maximizing voltage security in power systems. The use of interior point methods together with goal programming and linearly combined objective functions as the basic optimization techniques are explained in detail. The effects of minimizing operating costs, minimizing reactive power generation, and/or maximizing loading margins are then compared in both a 57-bus system and a 118-bus system, which are based on IEEE test systems and modeled using standard power flow models. The results obtained using the proposed mixed OPFs are compared and analyzed to suggest possible ways of costing voltage security in power systems.

A comprehensive approach to transient stability control. I. Near optimal preventive control

Abstract: A general approach to real-time transient stability control is proposed, and two complementary techniques are devised: one for preventive, the other for emergency control. In this paper, the general transient stability control approach is first revisited then applied to real-time preventive control. The technique consists of shifting active power generation. The amount of power and the machines from which to shift it are methodically determined, and various patterns of generation decrease/increase are considered. Further, a standard OPF algorithm is combined with this control technique to get a real-time transient stability-constrained OPF software, able to meet power system security and electricity market requirements. Simulations conducted on the 88-machine EPRI system and the Mexican interconnected power system illustrate the various techniques, highlight their specifics, and assess their performance.

Synchronous and bi-directional variable frequency power conversion systems

Abstract: A synchronous bi-directional active power conditioning system (11) suitable for wide variable frequency systems or active loads such as adjustable speed drives which require variable voltage variable frequency power management systems is disclosed. Common power electronics building blocks (100, 200) (both hardware and software modular blocks) are presented which can be used for AC-DC, DC-AC individually or cascaded together for AC-DC-AC power conversion suitable for variable voltage and/or wide variable frequency power management systems. A common control software building block (2, 5) includes a digital control strategy/algorithm and digital phase lock loop method and apparatus which are developed and implemented in a digital environment to provide gating patterns for the switching elements (3, 6) of the common power-pass modular power electronics building blocks (100, 200).

Impact of distributed generation on volt/Var control in distribution networks

Abstract: As power system in many countries is going to be restructured and deregulated. After deregulation, because of numerous advantageous of distributed generation (DG), the number of this kind of generators are going to be increased. DGs can affect entire system and especially distribution networks. One of the important control schemes at distribution system that DGs can change it is volt/Var control. This paper presents an efficient approach for volt/Var control in radial distribution networks takes DGs performance into consideration. In general distributed generations can be considered as PV or PQ nodes. In this paper DGs are modeled as PV nodes. The goal of this approach is to minimize power losses at distribution system through controlling the tap of load tap changer (LTC), size of substation capacitor, local controller settings and voltage amplitude of DGs. DGs, voltage regulators, local controllers, and load tap changer (LTC) are modeled completely and the optimization problem has been solved by using genetic algorithm. Finally the method is tested on IEEE 34 bus radial distribution feeders.

Active power filter control using neural network technologies

Abstract: A method for controlling an active power filter using neural networks is presented. Currently, there is an increase of voltage and current harmonics in power systems, caused by nonlinear loads. The active power filters (APFs) are used to compensate the generated harmonics and to correct the load power factor. The proposed control design is a pulse width modulation control (PWM) with two blocks that include neural networks. Adaptive networks estimate the reference compensation currents. On the other hand, a multilayer perceptron feedforward network (trained by a backpropagation algorithm) that works as a hysteresis band comparator is used. Two practical cases with Matlab-Simulink are presented to check the proposed control performance.

Continuous reactive power support for wind turbine generators

Abstract: Real and reactive power control for wind turbine generator systems. The technique described herein provides the potential to utilize the total capacity of a wind turbine generator system (e.g., a wind farm) to provide dynamic VAR (reactive power support). The VAR support provided by individual wind turbine generators in a system can be dynamically varied to suit application parameters.

Advanced modeling of the multicontrol functional static synchronous series compensator (SSSC) in Newton power flow

Abstract: The static synchronous series compensator (SSSC) is one of the recently developed flexible AC transmission system (FACTS) controllers. The SSSC coupled with a transformer is connected in series with a transmission line. This paper describes a multicontrol functional model of the SSSC for power flow analysis, which can be used for steady state control of one of the following parameters: (1) the active power flow on the transmission line; (2) the reactive power flow on the transmission line; (3) the voltage at the bus; and (4) the impedance (precisely reactance) of the transmission line. Furthermore, the model can also take into account the voltage and current constraints of the SSSC. The detailed implementation of such a multicontrol functional model in Newton power flow algorithm is presented. A special consideration of the initialization of the variables of the SSSC in power flow analysis is also proposed. Numerical examples on the IEEE 30-bus system, IEEE 118-bus system, and IEEE 300-bus system are used to illustrate the feasibility of the SSSC model and performance of the Newton power flow algorithm.

Three-phase unity-power-factor star-connected switch (VIENNA) rectifier with unified constant-frequency integration control

Abstract: A unified constant-frequency integration (UCI) controller for a three-phase star-connected switch three-level rectifier (VIENNA) with unity-power-factor-correction is proposed. One of advantages of this rectifier is that the switch voltage stress is one half of the total output voltage. The proposed control approach is based on one-cycle control and features great simplicity and reliability: all three phases will be power factor corrected using one or two integrators with reset along with several flips-flops, comparators and logic and linear components. It does not require multipliers to scale the current reference according to the output power level as used in many other control approaches. In addition, the input voltage sensor is eliminated. It employs constant switching frequency modulation that is desirable for industrial applications. The proposed controller can operate by sensing either the inductor currents or the switching currents. If the switching currents are sensed, the cost is further reduced because switching currents are easier to sense comparing with inductor currents. The proposed approach is supported by experimental results.

Modeling, control and implementation of three-phase PWM converters

Abstract: Three-phase voltage- and current-source converters are the building blocks of a great number of power electronic systems. The origin of difficulties in the control of the above converters is in their nonlinear nature. In this paper, a novel modeling technique is introduced to derive the linear models of the converters from the nonlinear transformations of the conventional nonlinear models. Then, based on the derived linear models, a high-performance linear controller with satisfactory performances is designed. The bold feature of the new model is the independence of the controller design from the operating point. A DSP-based control system has been built in the lab to verify the performance of the new models and the control algorithm. The simulation and experimental results are in close agreement. The results show that the DC term and the AC-side reactive power can be controlled independently in less than one cycle.

Low voltage electrified furniture unit

Abstract: Low voltage DC electrification of a furniture unit is provided by a power supply having a rechargeable battery and an automobile passenger compartment power outlet which is electrically connected to the rechargeable battery. In one embodiment, the power supply is mounted to a furniture unit having a height adjustable worksurface, the lift motors of the furniture unit also being powered by the power supply. Additionally, the power supply may include a control device prioritizing recharging of the battery to times of off-peak or low-cost AC power available. The low-voltage DC power outlet advantageously receives adapter plugs that are readily available for a wide range of portable appliances and other devices. Another embodiment of a furniture unit having a low-voltage DC power supply includes a computing device for controlling the recharging of the power supply and for controlling other accessories such as furniture unit lift motors, lighting, HVAC components, audiovisual components, or other such devices.

Led lighting apparatus

Abstract: A light comprising a housing, a plurality of LED lights coupled in an array inside of the housing, and a reflective protrusion coupled to the housing wherein the reflective protrusion is for reflecting light from the LED lights out of the housing. The housing can be of any shape such as tubular, bowl shaped, or having an oval cross section. In addition, the reflective protrusion can be of any shape such as dome shaped or pyramidal shaped. The circuitry relating to this LED light array can include a power source such as a connection to an AC or DC input. If the connection is to an AC input, the device can also include an AC/DC converter coupled to the power source for receiving an input from the AC power source. In this way the LED array receives a consistent flow of DC current that will not result in the LED lights burning out. To prevent this LED array from burning out there is also a current regulator for controlling a current flowing through this LED array.

Design Simulation and Experimental Investigations, on a Shunt Active Power Filter for Harmonics, and Reactive Power Compensation

Abstract: This paper presents complete design, simulation, and experimental investigations on a 3-phase shunt active power filter to compensate harmonics and the reactive power requirement of nonlinear loads. The paper describes the complete design aspects of power circuit elements and control circuit parameters. The compensation process is based on sensing line currents only, an approach different from conventional methods that require the harmonics and reactive volt-ampere requirement of the load. Various simulation results are presented to study the performance during steady-state and transient conditions to validate the design. A laboratory prototype has been developed to verify the simulation results. The control scheme is realized on a dedicated micro-controller-based system. PWM pattern generation is based on carrierless hysteresis-based current control to obtain the switching signals. Based on simulation and experimental results it can be concluded that the compensation process is simple and easy to impleme...

A theoretical investigation of original and modified instantaneous power theory applied to four-wire systems

Abstract: This paper investigates whether the instantaneous reactive currents calculated by the instantaneous power theories contain all currents which do not contribute to the instantaneous power. In 1962, Depenbrock published a power theory generally valid for N-wire systems. In 1983, independently, Akagi, Kanazawa, and Nabae introduced the original "theory of instantaneous power," valid with few restrictions for N=3. Because of more problems at N=4, Nabae et al. introduced a modified version of the instantaneous power theory in 1995. By a discussion of this modified power theory, Akagi et al. in 1999 showed that, under certain conditions, the modified theory leads to compensation results slightly inferior to those resulting from the original theory. This paper gives the compensation rules based on the older generally valid FBD power theory and analyzes the differences with regard to the other theories.

Application study of a STATCOM with energy storage

Abstract: With advances in energy storage technology the application of STATCOMs with energy storage for utility applications, such as active- and reactive-power compensation of loads, network-voltage control and mitigation of power system disturbances, is increasingly feasible. As it is more expensive to produce active power than reactive power, it is important to consider methods which can be adopted to minimise the use of the energy store. An application study of a STATCOM with energy storage giving special emphasis to control strategies which minimise the use of the stored energy is reported. Calculation techniques to determine the current rating of the IGBTs, diodes and connecting transformer as well as the losses associated with the switches when the compensator is operated under space-vector modulation are demonstrated. Application studies of the compensator with energy storage for load compensation, steady-state voltage control, mitigation of voltage sags and elimination of power oscillations are described. The analytical studies of each of these applications are supplemented by simulation results carried out in PSCAD/EMTDC and by experimental results obtained from a laboratory prototype.

Hybrid variable speed generator/uninterruptible power supply power converter

Abstract: The invention in the simplest form is an improved hybrid power converter with uninterruptible power supply (UPS) and power quality capabilities as well as an integrated variable speed power source and control method that is used to generate high quality, uninterruptible AC power utilizing fewer batteries and operating at optimum fuel efficiency and with reduced emissions. The variable speed generator control scheme allows for load adaptive speed control of a power source such as an engine (20) and generator (10). The transformerless hybrid power converter topology and control method provides the necessary output frequency, voltage and/or current waveform regulation, harmonic distortion rejection, and provides for single-phase or unbalanced loading. The transformerless hybrid power converter also provides inline or offline UPS capability, line voltage and/or frequency sag and surge compensation, peak shaving capability, VAR compensation and active harmonic filtering.

p-q Theory power components calculations

Abstract: The "generalized theory of the instantaneous reactive power in three-phase circuits", proposed by Akagi et al., and also known as the p-q theory, is an interesting tool to apply to the control of active power filters, or even to analyze three-phase power system in order to detect problems related to harmonics, reactive power and unbalance. In this paper it will be shown that in three phase electrical systems the instantaneous power waveform presents symmetries if 1/6, 1/3, 1/2 or 1 cycle of the power system fundamental frequency, depending on the system being balanced or not, and having or not even harmonics (interharmonics and subharmonics are not considered in this analysis). These symmetries can be exploited to accelerate the calculations for active filters controllers based on the p-q theory. In the case of the conventional reactive power or zero-sequence compensation, it is shown that the theoretical control system dynamic response delay is zero.

Variation of distribution factors with loading

Abstract: Power transfer distribution factors depend on the operating point and topology of an electric power system. However, it is known empirically that, for a fixed topology, the power transfer distribution factors are relatively insensitive to the operating point. We demonstrate this result theoretically for systems of arbitrary topology with losses, but only for the special case of having reactive compensation sufficient to keep voltages constant at all busses. We also analyze a power to current distribution factor that more closely relates to thermal constraints. We provide empirical corroboration for the theoretical result.

Voltage control for wind generators

Abstract: A wind turbine generator control system includes relatively fast regulation of voltage near the individual generators with relatively slower overall reactive power regulation at a substation or wind farm level. The setpoint of the relatively fast voltage regulator is adjusted by the relatively slow reactive power regulator. The fast voltage regulation can be at the generator terminals or at a synthesized remote point.

A reactive power management proposal for transmission operators

Abstract: The management of reactive resources, in particular the generation facilities under control of transmission operators, plays an important role in maintaining voltage stability and system reliability. The current practice of reactive management relies on heuristics and operators' judgments to procure reactive services. Consequently, it lacks a transparent market process and assurance that the procurement costs are the lowest. This paper presents a practical, market-based solution for managing reactive services by transmission operators. Three distinct features for the proposed solution are: (1) obligating generation facilities to provide reactive services in proportion to their active power output; (2) optimizing and integrating the reactive procurement with market operation for least-cost solution; and (3) taking into account the interactions of active and reactive powers for accurate calculation of the lost opportunity costs of generators. The proposed solution further simplifies the modeling and settlement aspects of reactive management. A model process of reactive power management is presented for use by independent transmission operators. A numerical example is also given to illustrate the application of the proposed solution.

A high-efficiency single-phase three-wire photovoltaic energy conversion system

Abstract: A single-phase three-wire photovoltaic energy conversion system with single-stage structure using a novel maximum power-point tracking (MPPT) algorithm is presented. An equivalent model of the proposed system is derived to analyze the characteristics of the system and to design the controller. Owing to the linear relation of the PV array parameters versus insolation, the model is easy to analyze. The proposed system employs a three-leg inverter to control the MPPT process, the line current, and neutral line current. A current-controlled MPPT algorithm controls the MPPT. A neutral line-mode controller maintains a utility neutral line current of zero. A line-mode controller controls the line current so as to provide power to the utility with a unity power factor. The proposed system acts as a solar generator on sunny days and functions as an active power filter on rainy days. Computer simulation and experimental results demonstrate the accuracy and the superior performance of the proposed technique.

A three-phase AC/AC high-frequency link matrix converter for VSCF applications

Abstract: In this paper, a new three-phase high-frequency link matrix converter is discussed and the topology of conventional matrix converter is modified with the addition of a transformer for a variable speed constant frequency (VSCF) application. The proposed approach accomplishes voltage transfer ratio more than unity, galvanic isolation between both voltage sources and higher power density by employing a high-frequency transformer into the intermediate stage of the dual bridge matrix converter. It has a bidirectional power flow capability, controllable displacement power factor and lower harmonic distortion at both variable speed source and fixed frequency utility. Further, asynchronous or synchronous PWM can be employed depending on the frequency modulation ratio in the primary side converter and it guarantees full input voltage utilization for DC-link and near symmetric square wave pulse trains applied to high frequency transformer regardless of varying input frequency. The proposed approach is a competitive solution for VSCF distributed generating application such as wind-turbine and micro-turbine application. Simulation results are shown to demonstrate the advantages of the proposed system. Experimental results on a 230V, 3kVA 400 Hz to 60 Hz VSCF system based on DSP controller are presented.