Showing papers on "AC power published in 1998"

The unified power quality conditioner: the integration of series- and shunt-active filters

Abstract: This paper deals with unified power quality conditioners (UPQCs), which aim at the integration of series-active and shunt-active power filters. The main purpose of a UPQC is to compensate for voltage flicker/imbalance, reactive power, negative-sequence current and harmonics. In other words, the UPQC has the capability of improving power quality at the point of installation on power distribution systems or industrial power systems. This paper discusses the control strategy of the UPQC, with a focus on the how of instantaneous active and reactive powers inside the UPQC. Experimental results obtained from a laboratory model of 20 kVA, along with a theoretical analysis, are shown to verify the viability and effectiveness of the UPQC.

Application issues of active power filters

Abstract: In this article, common nonlinear loads have been characterized into two types of harmonic sources, current-source type of harmonic source and voltage-source type of harmonic source. Compensation characteristics of both parallel active filters and series active filters have been discussed analytically and experimentally for these two types of harmonic sources. The corresponding required operation conditions, features, application issues, and adaptive harmonic sources of both filters have been presented. The fact that the traditional active filter, the parallel active filter, is not a panacea to harmonic compensation, and that one cannot use it blindly, has been clearly addressed. The parallel active filter will increase harmonic current and may cause overcurrent of the load when the load is a harmonic voltage source. Instead, it has been verified that the series active filter is better suited for compensation of a harmonic voltage source such as a diode rectifier with smoothing DC capacitor. The conclusions of this article also imply that when a parallel active filter is installed in a power system network such as at a point of common coupling, the network impedance and main harmonic sources downstream from the installation point should be investigated in order to get good performance and to minimize influence to the loads downstream. In some cases, a combined system of parallel active filter and series active filter may be necessary by utilizing the harmonic isolation function of the series active filters. No doubt active filters are superior to passive filters if used in their niche applications.

Harmonic and reactive power compensation based on the generalized instantaneous reactive power theory for three-phase four-wire systems

Abstract: This paper presents harmonic and reactive power compensation based on a generalized theory of instantaneous reactive power for three-phase power systems. This new theory gives a generalized definition of instantaneous reactive power, which is valid for sinusoidal or nonsinusoidal and balanced or unbalanced three-phase power systems with or without zero-sequence currents and/or voltages. The properties and physical meanings of the newly defined instantaneous reactive power are discussed in detail. A harmonic and reactive power compensator based on the new theory for a three-phase harmonic-distorted power system with zero-sequence components in the load current and/or source voltage is then used as an example to show harmonic and reactive power measurement and compensation using the new theory. Simulation and experimental results are presented.

Residential photovoltaic energy storage system

Abstract: This paper introduces a residential photovoltaic (PV) energy storage system, in which the PV power is controlled by a DC-DC power converter and transferred to a small battery energy storage system (BESS). For managing the power, a pattern of daily operation considering the load characteristic of the homeowner, the generation characteristic of the PV power, and the power-leveling demand of the electric utility is prescribed. The system looks up the pattern to select the operation mode, so that powers from the PV array, the batteries and the utility are utilized in a cost-effective manner. As for the control of the system, a novel control technique for the maximum power-point tracking (MPPT) of the PV array is proposed, in which the state-averaged model of the DC-DC power converter, including the dynamic model of the PV array, is derived. Accordingly, a high-performance discrete MPPT controller that tracks the maximum power point with zero-slope regulation and current-mode control is presented. With proposed arrangements on the control of the BESS and the current-to-power scaling factor setting, the DC-DC power converter is capable of combining with the BESS for performing the functions of power conditioning and active power filtering. An experimental 600 W system is implemented, and some simulation and experimental results are provided to demonstrate the effectiveness of the proposed system.

A new type of STATCOM based on cascading voltage source inverters with phase-shifted unipolar SPWM

Abstract: In this paper, a new type of static compensator (STATCOM) is proposed. This new STATCOM is constructed by cascading several identical full-bridge (H bridge) voltage-source inverters (VSIs). A so-called phase-shifted sinusoidal pulsewidth modulation (SPWM) unipolar voltage switching scheme is applied to control the switching devices of each VSI. The harmonics in STATCOM current caused by the DC voltage ripple is rejected by a new method developed in this paper. As a result, the size of inductor and DC capacitors can be further reduced. A very effective startup procedure is proposed to start up the STATCOM. The proposed STATCOM has the advantage of a fewer number of VSIs, the VSIs being identical and extremely fast in response to reactive power change.

An universal active power line conditioner

Abstract: The power circuit of a general active power line conditioner (APLC) is based on series and shunt power converters that share a single DC link. In the present paper, a generic control concept for these series and shunt converters is proposed. It is based on the instantaneous real and imaginary power theory. In fact, the resulting equipment deals with the custom power and FACTS concepts. This equipment incorporates not only the compensation functions at the fundamental frequency like a unified power flow controller (UPFC), but also provides active harmonic mitigation capabilities. For these reasons, the compensator proposed here is called the universal active power line conditioner (UPLC). Simulation and experimental results are presented to confirm that the new approach has better performance than those obtained by controllers based on traditional concepts of active and reactive power.

Available transfer capability calculations

Abstract: This paper reports on the features and implementation of a program for available (transmission) transfer capability (ATC) calculations. A novel formulation of the ATC problem has been adopted based on full AC power flow solution to incorporate the effects of reactive power flows, voltage limits and voltage collapse as well as the traditional line flow (thermal loading) effects. An efficient continuation power flow approach with adaptive localization enhances speed in processing a large number of contingencies to determine ATC for each specified transfer. Test and performance results for practical power system models are presented.

Reactive power optimization using successive quadratic programming method

Abstract: This paper presents a reactive power optimization model that is based on successive quadratic programming (SQP) methods. Mathematical formulation and unified algorithm suppose different objective functions (OF) of reactive power optimization, depending on type and purposes of current reactive power control or planning problem. A bicriterion reactive power optimization model, that represents compromise between economical and security objective functions, is proposed. An efficient algorithm for approximation of initial problem by quadratic programming problem is described. The quadratic programming problem (QP) is solved on the basis of the Newton type quadratic programming method. A modified successive quadratic programming method was developed, that provides reliable convergence of the SQP method.

Performance optimization for doubly-fed wind power generation systems

Abstract: Significant variation of the resource kinetic energy, in the form of wind speed, results in substantially reduced energy capture in a fixed speed wind turbine. In order to increase the wind energy capture in the turbine, variable speed generation (VSG) strategies have been proposed and implemented. However, that requires an expensive AC/AC power converter which increases the capital investment significantly. Consequently doubly-fed systems have been proposed to reduce the size of the power converter and thereby the associated cost. Additionally, in doubly-fed systems, at a fixed operating point (power and speed), power flow can be regulated between the two winding systems on the machine. This feature can be utilized to essentially minimize losses in the machine associated with the given operating point or achieve other desired performance enhancements. In this paper, a brushless doubly-fed machine (BDFM) is utilized to develop a VSG wind power generator. The VSG controller employs a wind speed estimation based maximum power point tracker (MPPT) and a heuristic model based maximum efficiency point tracker (MEPT) to optimize the power output of the system. The controller has been verified for efficacy on a 1.5 kW laboratory VSG wind generator. The strategy is applicable to all doubly-fed configurations, including conventional wound rotor induction machines, Scherbius cascades, brushless doubly fed machines, and doubly-fed reluctance machines.

Anti-islanding method and apparatus for distributed power generation

Abstract: The invention in the simplest form is a method and apparatus for reliably protecting against island situations with one or multiple power sources connected to an electric distribution grid. The method and apparatus detects variations in the voltage and frequency of the grid. An observed change in grid voltage causes a change in output power that is sufficient to cause an even larger change in grid voltage when the utility AC power source is disconnected. An observed change in grid frequency causes a change in phase or reactive output power that is sufficient to cause an even larger change in grid frequency. If several shifts in voltage or frequency happen in the same direction, the response to the change is increased in an accelerating manner.

A new control approach to three-phase active filter for harmonics and reactive power compensation

Abstract: This paper deals with a new control scheme for a parallel 3-phase active filter to eliminate harmonics and to compensate the reactive power of the nonlinear loads. A 3-phase voltage source inverter bridge with a DC bus capacitor is used as an active filter (AF). A hysteresis based carrierless PWM current control is employed to derive the switching signals to the AF. Source reference currents are derived using load currents, DC bus voltage and source voltage. The command currents of the AF are derived using source reference and load currents. A 3-phase diode rectifier with capacitive loading is employed as the nonlinear load. The AF is found effective to meet IEEE-519 standard recommendations on the harmonics level.

UPFC-unified power flow controller: theory, modeling, and applications

Abstract: This paper describes the theory and the modeling technique of a flexible alternating current transmission systems (FACTS) device, namely, unified power flow controller (UPFC) using an Electromagnetic Transients Program (EMTP) simulation package. The UPFC, in this paper, consists of two solid-state voltage source inverters which are connected through a common DC link capacitor. Each inverter is coupled with a transformer at its output. The first voltage source inverter, known as a static synchronous compensator (STATCOM), injects an almost sinusoidal current, of variable magnitude, at the point of connection. The second voltage source inverter, known as a static synchronous series compensator (SSSC) injects an almost sinusoidal voltage, of variable magnitude, in series with the transmission line. This injected voltage can be at any angle with respect to the line current. The exchanged real power at the terminals of one inverter with the line flows to the terminals of the other inverter through the common DC link capacitor. In addition, each inverter can exchange reactive power at its terminals independently. The functionalities of the models have been verified.

Application of STATCOMs to wind farms

Abstract: Effective reactive power control in distribution networks can have an important influence on the level of embedded generation that is acceptable and on the costs of its connection. A comprehensive study was undertaken to investigate how STATCOMs could be used with fixed-speed wind turbines, which use induction generators, to improve both the steady-state and dynamic impact of a wind farm on the network. An optimal power flow model based on loss minimisation was developed and used to show that operation of a wind farm at unity power factor is unlikely to allow maximum penetration of wind energy into a weak distribution circuit. The results of the electromagnetic simulations showed that the use of a STATCOM improves the steady-state stability limits of the network. They also showed that the use of a STATCOM with an appropriately designed control strategy can prevent damaging overvoltages that may occur under islanding conditions. The results also demonstrated that a STATCOM can be used to mitigate voltage fluctuations at blade passing frequency, which may occur if the rotors of a number of wind turbines fall into synchronism.

Two-loop controller for maximizing performance of a grid-connected photovoltaic-fuel cell hybrid power plant

Abstract: Maximizing performance of a grid-connected photovoltaic (PV)-fuel cell hybrid system by use of a two-loop controller is discussed. One loop is a neural network controller for maximum power point tracking, which extracts maximum available solar power from PV arrays under varying conditions of insolation, temperature, and system load. A real/reactive power controller (RRPC) is the other loop. The RRPC achieves the system's requirements for real and reactive powers by controlling incoming fuel to fuel cell stacks as well as switching control signals to a power conditioning subsystem. Results of time-domain simulations prove not only the effectiveness of the proposed computer models of the two-loop controller but also its applicability for use in stability analysis of the hybrid power plant.

Double-fed induction machine: converter optimisation and field oriented control without position sensor

Abstract: The authors present a new system for variable speed using a double-fed induction machine. With a special operating mode, the apparent power of the inverter can reach only 20% of the maximum mechanical power, compared to a classical solution which needs 120%. Control laws are also studied with and without a sensor and they are particularly robust. The approach is validated by simulations and experimental results.

Comparison of basic converter topologies for power factor correction

Abstract: Basic types of DC-DC converters, when operating in discontinuous conduction mode, have self power factor correction (PFC) property, that is, if these converters are connected to the rectified AC line, they have the capability to give higher power factor by the nature of their topologies. Input current feedback is unnecessary when these converters are employed to improve power factor. In this paper, basic types of DC-DC converter topologies are studied to investigate their self-PFC capabilities. Their input characteristics are compared and their input line current waveforms are predicted.

Control and analysis of a unified power flow controller

Abstract: This paper presents a control scheme and comprehensive analysis for a unified power flow controller (UPFC) on the basis of theory, computer simulation and experiment. This developed theoretical analysis reveals that a conventional power feedback control scheme makes the UPFC induce power fluctuation in transient states. The conventional control scheme cannot attenuate the power fluctuation, and so the time constant of damping is independent of active and reactive power feedback gains integrated in its control circuit. This paper proposes an advanced control scheme which has the function of successfully damping out the power fluctuation. A UPFC rated at 10 kVA is designed and constructed, which is a combination of a series device consisting of three single-phase pulsewidth modulation (PWM) converters and a shunt device consisting of a three-phase diode rectifier. Although the dynamics of the shunt device are not included, it is possible to confirm and demonstrate the performance of the series device. Experimental results agree well with both analytical and simulated results and show viability and effectiveness of the proposed control scheme.

Control of parallel inverters in distributed AC power systems with consideration of the line impedance effect

Abstract: The authors have developed a new control technique which allows paralleled inverters to share linear or nonlinear load in a distributed AC power supply system. This technique does not require control interconnections and automatically compensates for inverter parameter variations and line impedance imbalances. Simulation results are provided in the paper to prove the concept.

A unity power factor converter using half-bridge boost topology

Abstract: A single-phase high-efficiency near-unity power-factor (PF) half-bridge boost converter circuit, which has been proposed earlier by other researchers, is presented with detailed analysis. This converter is capable of operating under variable PF. However, the focus of this paper is in achieving unity PF operation only. The efficiency of this circuit is high because there is only one series semiconductor on-state voltage drop at any instant. The existence of an imbalance in the voltages of the two DC-link capacitors, which was noted before, is confirmed here. The cause for the imbalance is analyzed using appropriate models, and a control method to eliminate it is discussed in detail. Analysis and design considerations for the power circuit using the fixed-band hysteresis current control (HCC) technique are provided. The analytical results are verified through simulation using switched and averaged circuit models of the scheme and also through experimental work. At 90-V AC input and 300-W 300-V output, the experimental prototype demonstrates an efficiency of 96.23% and a PF of 0.998. This converter, with its relatively high DC-output voltage, is well suited for the 110-V utility supply system. A circuit modification for universal input voltage range operation is also suggested.

Control design and simulation of unified power flow controller

Abstract: The unified power flow controller (UPFC) is a solid-state controller which can be used to control active and reactive power flows in a power transmission line. In this paper, the authors propose a control strategy for UPFC in which they control real power flow through the line, while regulating magnitudes of the voltages at its two ports. They design a controller for this purpose which uses only local measurements. The control strategy is evaluated using digital simulation for a case study.

A new scheme for generating and measuring active, reactive, and apparent power at power frequencies with uncertainties of 2.5/spl times/10/sup -6/

Abstract: A new method for generating and measuring active, reactive, and apparent power at power frequencies has been devised. It makes use of digital signal synthesis and discrete Fourier transform (DFT) evaluation based on a single master clock. This results in a significant reduction of synchronizing errors and thus in an uncertainty of only 2.5/spl times/10/sup -6/ (k=1).

Power bar with remote control

Abstract: The Power Bar with Remote Control allows a user to control from a distance the power of each outlet contained in the attached Power Bar. A master switch also allows the power to all outlets to be turned on or off in unison. Therefore, a user can selectively turn on or off electrical devices attached to the Power Bar. For example, while sitting at a desk a person could use the Remote Control to turn on their personal computer, monitor, speakers, external modem, printer and desk lamp. Also, by integrating the Power Bar with a USB (or Firewire) hub, only a single composite cable, with either integrated AC power/USB plugs or separate AC power and USB plugs at each end is required to supply both power and data to a PC or peripheral device. Also, by integrating an under-the-monitor type power box with a USB (or Firewire) hub, only a single composite cable, with either integrated AC power/USB plugs or separate AC power and USB plugs at each end is required to supply both power and data to a PC or peripheral device.

Allocation of transmission supplementary charge to real and reactive loads

Abstract: This paper has applied the MW-MILE methodology to allocate the transmission supplementary charge to real and reactive power loads. The charge for usage of an individual transmission asset is split into a nonlocational component, due to the unused capacity of the asset, and a locational component, due to the actually used capacity of the asset. The latter is allocated, using the previously proposed electricity tracing method, to individual real and reactive loads in the network.

Energy-conserving power-supply system

Abstract: An energy-conserving power-supply system utilizes a keep-alive DC power to consistently power a control circuit so as to render a main power supply readily actuatable for either distributing AC power or supplying regulated DC power. The energy-conserving power-supply system allows any one of modern information-processing or entertainment electronic products to be readily operable manually, remotely, or automatically from a standby state; while, it totally eliminates any unnecessary energy waste because AC power is neither transmitted nor wasted not only within but outside the power-supply system. The latter allows an external line-operated device to be concurrently operable from a no-power state, i.e., without consuming any standby energy at all. Not only can the energy-conserving power-supply system conserve energy usage to the greatest extent, it will also correspondingly extend the life spans of electronic circuitry and electromechanical components.

HVDC Light: A New Technology for a Better Environment

Abstract: A new transmission and distribution technology, HVDC Light, makes it economically feasible to connect smallscale, renewable power generation plants to the main AC grid. Vice versa, using the very same technology, remote locations as islands, mining districts and drilling platforms can be supplied with power from the main grid, thereby eliminating the need for inefficient, polluting local generation such as diesel units. The voltage, frequency, active and reactive power can be controlled precisely and independently of each other. This technology also relies on a new type of underground cable which can replace overhead lines at no cost penalty.

Grid interface options for variable-speed, permanent-magnet generators

Abstract: The authors survey the many forms of DC/AC converter which may be applied to permanent magnet generators, examine their potential to meet the waveform, power factor, cost and efficiency requirements and consider their features in relation to the special environment of a wind turbine system. Feasible options are identified and compared. The control needed to achieve maximum energy capture from the wind is described. Inverters have been selected which are able to effect the necessary power control whilst providing controllable reactive power and causing minimal harmonic pollution of the network. Device cost and overall efficiency have been assessed for each option.

Pitfalls of electric power quality indices

Abstract: Several indices are in common use for the quantification of electric power quality. These indices are convenient for condensing complex time and frequency domain waveform phenomena into a number. However, a variety of commonly encountered circumstances are not readily accommodated (e.g. quasiperiodic and aperiodic signal components and noninteger multiples of the power frequency). Also, the power acceptability curves have been used as convenient measures of power quality and these curves do not capture three-phase detail of power quality problems. In this paper, the general subject of pitfalls of power quality indices is discussed and suggestions for alleviating these problems are made.

Design and performance of active power filters

Abstract: This article has demonstrated the feasibility of a three phase active filter based on a half-bridge topology. Design guidelines for the power circuit have been derived and applied to a 5 kVA IGBT laboratory prototype. In the single phase mode, the active filter can reduce the low frequency harmonic content in the AC line to below 1% excluding the harmonics due to switching action of the converter. In the three phase mode, line currents are corrected under balanced and unbalanced conditions. The neutral current is reduced significantly.