Showing papers on "AC power published in 1999"

A review of active filters for power quality improvement

Abstract: Active filtering of electric power has now become a mature technology for harmonic and reactive power compensation in two-wire (single phase), three-wire (three phase without neutral), and four-wire (three phase with neutral) AC power networks with nonlinear loads. This paper presents a comprehensive review of active filter (AF) configurations, control strategies, selection of components, other related economic and technical considerations, and their selection for specific applications. It is aimed at providing a broad perspective on the status of AF technology to researchers and application engineers dealing with power quality issues. A list of more than 200 research publications on the subject is also appended for a quick reference.

Dual current control scheme for PWM converter under unbalanced input voltage conditions

Abstract: Voltage unbalance in a three-phase system causes performance deterioration of PWM power converters by producing 120 Hz voltage ripples in the DC link and by increasing the reactive power. To eliminate the DC link voltage ripple and the DC component of the reactive power, both positive- and negative-sequence currents should be controlled simultaneously, according to the paper by Rioual et al (1996). The authors used two synchronous reference frames: a positive-sequence current regulated by a proportional integral (PI) controller in a positive synchronous reference frame (SRF); and a negative-sequence current regulated by a PI controller in a negative SRF. In the positive SRF, which rotates counterclockwise, the positive sequence appears as DC, while the negative sequence appears as 120 Hz. In contrast, in the negative SRF, which rotates clockwise, the negative sequence appears as DC, while the positive sequence appears as 120 Hz. By deleting 120 Hz components using a notch filter in each SRF, one can measure positive- and negative-sequence currents separately, and use them for constructing two feedback controllers. Since the negative-sequence current is also controlled in its own SRF by a DC command, this approach yields better performance without increasing the control gain. Note that, since the controller is implemented by a software routine in the digital signal professor chip, using two SRFs does not require additional hardware. The authors demonstrated the effectiveness of the proposed control scheme by using computer simulation and experiments.

The interline power flow controller concept: a new approach to power flow management in transmission systems

Abstract: The interline power flow controller (IPFC) proposed is a new concept for the compensation and effective power flow management of multi-line transmission systems. In its general form, the IPFC employs a number of inverters with a common DC link, each to provide series compensation for a selected line of the transmission system. Because of the common DC link, any inverter within the IPFC is able to transfer real power to any other and thereby facilitate real power transfer among the lines of the transmission system. Since each inverter is also able to provide reactive compensation, the IPFC is able to carry out an overall real and reactive power compensation of the total transmission system. This capability makes it possible to equalize both real and reactive power flow between the lines, transfer power from overloaded to underloaded lines, compensate against reactive voltage drops and the corresponding reactive line power, and to increase the effectiveness of the compensating system against dynamic disturbances. The paper explains the basic theory and operating characteristics of the IPFC with phasor diagrams, P-Q plots and simulated waveforms.

Design and optimization of dual-threshold circuits for low-voltage low-power applications

Abstract: Reduction in leakage power has become an important concern in low-voltage, low-power, and high-performance applications. In this paper, we use the dual-threshold technique to reduce leakage power by assigning a high-threshold voltage to some transistors in noncritical paths, and using low-threshold transistors in critical path(s). In order to achieve the best leakage power saving under target performance constraints, an algorithm is presented for selecting and assigning an optimal high-threshold voltage. A general leakage current model which has been verified by HSPICE simulations is used to estimate leakage power. Results show that the dual-threshold technique is good for leakage power reduction during both standby and active modes. For some ISCAS benchmark circuits, the leakage power can be reduced by more than 80%. The total active power saving can be around 50% and 20% at low- and high-switching activities, respectively.

Cost analysis of reactive power support

Abstract: The transfer of real power cannot be realized without reactive power/voltage support. However, compared with real power, the idea of reactive power costing and pricing is still not well defined. In this paper, the economic cost of reactive power is first analyzed. This cost includes the explicit and opportunity cost from various generation sources, such as generators, and the explicit cost from various transmission sources, such as reactive compensators, tap-changing transformers and the cost of transmission losses. Different from previous objectives functions, in this paper a cost-based reactive power dispatch is presented, which minimizes the total cost of reactive power support and determines the resultant reactive profile. This cost-based reactive power dispatch methodology provides information on the costing of reactive power and the information may be further expanded to include the pricing of reactive power.

The theory of instantaneous power in three-phase four-wire systems: a comprehensive approach

Abstract: This paper describes a holistic approach to the theory of instantaneous power in three-phase four-wire systems, focusing on the original theory created in 1983 and a modified theory presented in 1994. The two theories are perfectly identical if no zero-sequence voltage is included in a three-phase three-wire system. However, they are different in the instantaneous active and reactive power in each phase if a zero-sequence voltage and current are included in a three-phase four-wire system. Theory and computer simulations in this paper lead to the following conclusions: an active filter without energy storage components can fully compensate for the neutral current even in a three-phase four-wire system including a zero-sequence voltage and current, when a proposed control strategy based on the original theory is applied. However, the active filter cannot compensate for the neutral current fully, when an already-proposed control strategy based on the modified theory is applied.

AEP unified power flow controller performance

Abstract: The installation of the world's first unified power flow controller (UPFC) has been completed and a series of commissioning tests were conducted at the Inez Substation of American Electric Power (AEP) in eastern Kentucky. The project is a collaborative effort between AEP, the Westinghouse Electric Corporation, and the Electric Power Research Institute (EPRI). Comprising two /spl plusmn/160 MVA voltage-sourced GTO-thyristor-based inverters, this installation is the first large-scale practical demonstration of the UPFC concept, and its completion is a significant milestone in the progress of power electronics technology for flexible AC transmission system (FACTS). This paper briefly reviews the main features of the Inez installation and discusses the operation of the equipment. A collection of measured performance characteristics is presented to illustrate the unique capabilities of the UPFC. These dynamic measurements were made in the course of commissioning, and they graphically illustrate the ability of the UPFC to independently control the real and reactive power on a transmission line, while also regulating the local bus voltage. The results include a demonstration of the series inverter at Inez, operating in stand-alone mode. This test constitutes the first practical large-scale demonstration of the static synchronous series compensator (SSSC) concept.

Tracing active and reactive power between generators and loads using real and imaginary currents

Abstract: In a competitive environment, usage allocation questions must be answered clearly and unequivocally To help answer such questions, this paper proposes a method for determining how much of the active and reactive power output of each generator is contributed by each load This method takes as its starting point a solved power flow solution All power injections are translated into real and imaginary currents to avoid the problems arising from the nonlinear coupling between active and reactive power flows caused by losses The method then traces these currents to determine how much current each source supplies to each sink These current contributions can then be translated into contributions to the active and reactive power output of the generators It is also shown that the global contribution of a load can be decomposed into contributions from its active and reactive parts This decomposition is reasonably accurate for the reactive power generation To determine the contributions to active power generation, the previously-described method based on the active power flows is recommended

Uninterruptible power supply

Abstract: An electronic circuit, such as a UPS, interfaces a main ac power source and at least one secondary power source to a load. The secondary power source(s) may include one or more auxiliary generators, a flywheel motor generator or microturbine with high speed motor generator, and/or any of a variety of dc storage devices. The electronic circuit includes a dc bus, a first uncontrolled rectifier in combination with a first filter for coupling the main ac power supply to the dc bus, one or more additional uncontrolled rectifier(s) and filter(s) for coupling the auxiliary generator(s) to the dc bus, and a dc-to-ac inverter (between the dc bus and the load) for providing ac output power to the load. The advantages of reduced parts count, increased compatibility between the generator(s) and the electronic circuit, and a simpler method for paralleling many storage and generation devices with a very high power factor to the sources, regardless of the load power factor, are provided.

Improving an interior-point-based OPF by dynamic adjustments of step sizes and tolerances

Abstract: This paper presents an efficient interior point algorithm for optimal power flow (OPF) problems, in particular, the real power dispatch and the reactive power dispatch problems. The nonlinear OPF problem is solved by a predictor-corrector primal-dual log-barrier (PCPDLB) method as a sequence of linearized sub-problems. Besides discussing the problem formulation, the paper offers a detailed description of the PCPDLB algorithm; it also addresses several implementation issues such as the determination of barrier parameter and the customization of initial points for OPF problems. In addition, practical issues on how to choose linear step sizes and convergence criteria are investigated to evaluate their impact on the performance of the algorithm. Some heuristics of dynamically adjusting step sizes and tolerance are proposed which significantly improve OPF solution speed. Computational results on power systems of 118 and 1062 buses are presented and discussed. Comparisons with other variants of primal-dual log-barrier methods are also provided to demonstrate the superiority of the proposed predictor-corrector interior point algorithm.

The instantaneous power theory on the rotating p-q-r reference frames

Abstract: This paper proposed the rotating p-q-r reference frames where one instantaneous active power p, and two instantaneous reactive powers q/sub q/, q/sub r/ were defined in 3-phase 4-wire systems. The three power components are linearly independent, so that compensating for the two instantaneous reactive powers leads to control the two components of the current space vector independently. With the theory, this paper shows that the neutral current of a 3-phase 4-wire system can be eliminated by only compensating for the instantaneous reactive power using no energy storage element. Simulation results verify the theory very well.

Balancing batteries, power, and performance: system issues in cpu speed-setting for mobile computing

Abstract: This thesis studies the problem of balancing power and performance in mobile computers, specifically, trading off power for performance by CPU speed-setting. The traditional approach to power-performance trade-offs assumes that batteries and memory bandwidth are ideal and focuses on lowering the energy per operation. This research, however, shows that non-ideal battery and performance behavior must be considered to properly balance power and performance, and that computations per battery life is a better metric for powerperformance trade-offs than energy per operation. The thesis begins with a description of non-ideal battery properties that can affect powerperformance trade-offs and then presents models for those properties. The models delineate regions where batteries can be treated ideally and where their non-ideal behavior must be considered. Furthermore, the models show that peak power rather than average power determines the available battery capacity. Thus, the first major result is that decreasing a mobile computer’s active power will increase the battery life more than decreasing its idle power, even if both reduce the average power by the same amount. The thesis then shows that the memory system also has an impact on CPU speed-setting. Because of limits in memory bandwidth, code performance will not scale with CPU speed when there are a considerable number of accesses to main memory. The second major result is to show that, because of non-ideal memory performance and non-ideal battery capacity, the results of some experiments are nearly a factor of four less for a real system than what would be expected using the ideal assumptions. For those experiments, the computations per discharge is expected to increase by 230%, but instead the measured results show a 37% decrease. Consequently, a system-level approach to CPU speed-setting should account for the nonidealities of both the memory and the battery. The final major result is an outline of a realistic method for CPU speed-setting, one that accounts for non-ideal memory and battery behavior by using performance-monitoring registers and battery “gas gauge” integrated circuits.

A new phase detecting method for power conversion systems considering distorted conditions in power system

Abstract: In this paper, a new phase detecting method is proposed for PWM rectifiers and active filters considering distorted utility conditions. The accurate phase angle information of utility voltage is essential to the active power conversion systems tied to the utility line. The angle information can be easily corrupted by the distorted utility voltage such as voltage unbalance and harmonics in the waveform. Thus, large harmonics may be caused in the current or voltage reference signal whose phase angle is based on the detected voltage phase angle. First, focusing on the distorted utility condition, effects of the several phase angle detecting methods on the operation of PWM rectifiers and active filters are investigated. Then a new voltage phase angle detector is proposed using voltage sequence detector and PI controller. The proposed method does not cause harmonics in the reference signal by adequately eliminating unbalance components and high-order harmonic terms in the utility voltage. The basic principle of the proposed method is described in detail. Experimental results prove the feasibility of the proposed strategy compared with the conventional method.

Adaptive repetitive control of PWM inverters for very low THD AC-voltage regulation with unknown loads

Abstract: An adaptive repetitive control scheme is proposed and applied to the control of a pulsewidth-modulated (PWM) inverter used in a high-performance AC power supply. The proposed control scheme can adaptively eliminate periodic distortions caused by unknown periodic load disturbances in an AC power supply. The proposed adaptive repetitive controller consists of a voltage regulator using state feedback control, a repetitive controller with tuning parameters and an adaptive controller with a recursive least-squares estimator (LSE). This adaptive repetitive controller designed for AC voltage regulation has been realized using a single-chip digital signal processor (DSP) TMS320C14 from Texas Instruments. Experimental verification has been carried out on a 2 kVA PWM inverter. Simulation and experimental results show that the DSP-based adaptive repetitive controller can achieve both good dynamic response and low total harmonic distortion (THD) under large-load disturbances and uncertainties.

Integrated photovoltaic system

Abstract: An integrated photovoltaic system includes a plurality of individual photovoltaic modules that each convert solar energy into DC power. Each of the photovoltaic modules is coupled to a busway that combines the DC power generated by each photovoltaic module into a DC power output. The connection between each photovoltaic module and the busway is environmentally sealed, as is the electrical wiring contained within the busway. The busway is coupled to a DC to AC power inverter that provides a supply of AC power. The coupling between the busway and the power inverter is environmentally protected, such that the entire DC wiring for the photovoltaic system is environmentally sealed and the system functions as an “AC photovoltaic system” having an AC power output.

Power line conditioner using cascade multilevel inverters for voltage regulation, reactive power correction, and harmonic filtering

Abstract: A power line conditioner using cascade multilevel inverter used for voltage regulation, reactive power (var) compensation and harmonic filtering, including the control schemes for operating the cascade inverter for voltage regulation and harmonic filtering in distribution systems. The cascade M-level inverter consists of (M-1)/2 H-bridges in which each bridge has its own separate DC source. This new inverter (1) can generate almost sinusoidal waveform voltage with only one time switching per line cycle, (2) can eliminate transformers of multipulse inverters used in the conventional static VAR compensators, and (3) makes possible direct connection to the 13.8 kV power distribution system in parallel and series without any transformer. In other words, the power line conditioner is much more efficient and more suitable to VAR compensation and harmonic filtering of distribution systems than traditional multipulse and pulse width modulation (PWM) inverters. It has been shown that the new inverter is specially suited for simultaneous VAR compensation and harmonic filtering.

WSCC voltage stability criteria, undervoltage load shedding strategy, and reactive power reserve monitoring methodology

Abstract: Engineers have long been struggling with developing voltage stability criteria for their systems. The development of the criteria becomes even more difficult if one attempts to apply the criteria to an entire region such as the one within Western Systems Coordinating Council (WSCC). The WSCC which is comprised of 86 member systems from the western region of North America (Canada, Mexico, and US) has developed strict voltage stability standards which must be complied with by all member systems. The purpose of this paper is to describe the WSCC system-wide voltage stability criteria, which are based on V-Q and P-V methodologies. A methodology for determining the proper mixture of static and dynamic reactive power source, an undervoltage load shedding strategy, and a reactive power reserve monitoring methodology are also discussed.

Volt/VAr control at distribution substations

Abstract: This paper proposes new methods to improve the performance of the current volt/VAr control schemes at distribution substations. The approach makes use of the new measurement and computer resources that are now available at the substations. The paper shows that new supervisory type control schemes can be developed to improve the performance of both the voltage regulator controllers and also the substation capacitor controllers that are used at the substations for volt/VAr control. The new voltage regulator controller makes use of the voltage drop characteristics of the feeders it regulates. The capacitor controller uses local real and reactive power measurements for power factor control at the substation.

A new configuration of single-phase symmetrical PWM AC chopper voltage controller

Abstract: With the increased availability of power MOSFETs and insulated gate bipolar transistors, a new generation of simple choppers for AC inductive loads is foreseen. These new power semiconductors ease the use of forced commutations of thyristor switches to improve the supply power factor, even with highly inductive loads. The AC controllers with thyristor technology can be replaced by pulsewidth modulation (PWM) AC chopper controllers which have important advantages. In this paper, a symmetrical PWM AC chopper designed to operate with single-phase inductive loads with a reduced number of controlled switches is described. The operation as a variable voltage source of this converter is evaluated. This includes the conversion characteristics, harmonic generation, harmonic distortion factor, and input power factor. By digital simulation, these characteristics are investigated theoretically, and to correlate the measurements with theory, an experimental setup is presented to confirm the analytical analysis.

Small signal stability for single phase inverter connected to stiff AC system

Abstract: A control technique based on frequency and voltage droop is used to control a single phase inverter connected to a stiff AC system. The small signal analysis of the system is presented, which allows the behavior of the system to be obtained for the functions of frequency and voltage droop. Simulation results and experimental results for the different frequency and voltage droop are presented.

A control algorithm for three-phase three-wired active power filters under nonideal mains voltages

Abstract: In this paper, a control algorithm for the performance improvement of three-phase three-wired active power filters under nonideal mains voltages is proposed and analyzed. It begins with an analysis of an instantaneous reactive power algorithm that was previously applied to the three-phase active power filter design. In that design, the circuit performance was found unsatisfactory under nonideal test scenarios. Our proposed design was, therefore, motivated in order to solve such problems. In the proposed scheme, not only was the control circuit simplified, but it also served as a potential candidate for the performance improvement of active filter design. The proposed algorithm has been implemented as a prototype. Results of experimental verification under various scenarios are presented.

Optimal active power flow incorporating power flow control needs in flexible AC transmission systems

Abstract: This paper presents a new method to incorporate the power flow control needs of flexible AC transmission system (FACTS) in studying the optimal active power flow problem. The linearized (DC) network model is used in this paper. Three main types of FACTS devices, namely thyristor controlled series compensators (TCSC), thyristor controlled phase shifters (TCPS) and unified power flow controllers (UPFC), are considered. The proposed method decomposes the solution of such modified OPF problem into two problems' iteration. The first problem is a power flow control subproblem while the second problem is a normal OPF problem. It is shown that the modified active power OFF can be solved by a new iterative strategy where a conventional LP-based technique may be used. Digital simulation studies on the IEEE 30 bus test system are presented to show the effectiveness of the proposed method.

A fuzzy-controlled active front-end rectifier with current harmonic filtering characteristics and minimum sensing variables

Abstract: A control strategy which allows conventional voltage-source current-controlled (VSCC) pulsewidth modulation (PWM) rectifiers to work simultaneously as active power filters is presented. The proposed control strategy also allows compensating the system power factor and compensating unbalanced loads. The measurement and/or calculation of the harmonics and reactive power are not required, making the proposed control scheme very simple. The active front-end rectifier acts directly on the mains line currents, forcing them to be sinusoidal and in phase with the mains voltage supply. To improve the dynamic of the system, the amplitude of the current is controlled by a fuzzy system, which adjusts the DC-link voltage of the PWM rectifier. The strategy is based on connecting all the polluting loads between the PWM rectifier and their input current sensors. The main advantages of this approach are the following: (1) there is no need to install a specially dedicated active power filter; (2) it also works simultaneously as a power factor compensator; and (3) no special and complicated calculations are required for harmonic elimination. The viability of the proposed active front-end rectifier is proved by simulation and with experimental results obtained from a 2 kVA PWM prototype.

A new approach for single-phase harmonic current detecting and its application in a hybrid active power filter

Abstract: In this paper, a new approach for single-phase harmonic current detection is presented. The approach was obtained through extending ideas of three-phase instantaneous reactive power theory and constructing a two-phase system from the existing single-phase circuit. By theoretical and simulation analysis, it is shown to be a precise approach, which can be easily realized and has the merits of better steady state and dynamic performance than conventional approaches that could be used in single-phase circuits. The approach was applied into a hybrid active power filter, which combines a series active filter and a shunt passive filter together and aims at solving the harmonics problem originated by high-power single-phase nonlinear load. Following the system configuration and basic principles of the hybrid active power filter, the overall detection and control algorithm, the PWM generating technique and the DC voltage stabilizing method are introduced in detail. The detection and control algorithm were then realized by a digital control circuit with DSP processor, and successfully employed in a prototype hybrid active power filter. Experimental results on the prototype verified the effectiveness of the new detecting approach, the performance of the control circuit and the filtering characteristics of the hybrid active power filter.

Decoupled control of active and reactive power for a grid-connected doubly-fed wound rotor induction machine without position sensors

Abstract: In this paper field-oriented control of a doubly-fed wound rotor induction machine without shaft position sensors is presented. The stator is directly connected to the constant frequency three phase grid and the rotor currents are appropriately controlled to vary the slip power thereby enabling variable speed constant frequency operation. Instead of using dynamic angle controllers for rotor position estimation, a direct method based on simple transformations is proposed. The control algorithms are implemented on a DSP-based hardware platform and tested on a laboratory motor-generator set. Details of the algorithms, implementation, and relevant experimental results are presented.

SMES for power utility applications: a review of technical and cost considerations

Abstract: Advances in both superconducting technologies and the necessary power electronics interface have made SMES a viable technology for high power utility and defense applications. The power industry's demands for more flexible, reliable and fast active power compensation devices make the ideal opportunity for SMES applications. However, in order to make this technology attractive to the deregulated utility market, it is necessary for industry to provide cost-effective systems. The information presented herein is taken from results to date of a DARPA technology Reinvestment Program SMES Commercialization Demonstration. This program is currently in the design and risk reduction phase. Completion is expected in 2001. This system will provide +/- 100 MW peak and +/- 50 MW oscillatory power with 100 MJ of stored energy. The base line for the coil design assumes a cable-in-conduit conductor (CICC), with rated voltage of 24 kV, and operating at nominal temperature of 4.5 K. This paper reviews the possible utility industry applications and discusses a number of technical issues and trade-offs resulting from the design optimization process for SMES utility applications. The conductor design options, system configuration, current/voltage levels and insulation issues for a low temperature superconducting coil are discussed. The power electronics interfaces (system configuration, circuit topology and devices and switching technologies) are also discussed. Finally, consideration is given to the impact of the new business environment, potential markets and overall cost.

A general technique for derivation of average current mode control laws for single-phase power-factor-correction circuits without input voltage sensing

Abstract: This paper presents a general technique to derive average current mode control (CMC) laws without input voltage sensing to achieve high power factor for single-phase topologies operating in continuous conduction mode (CCM). The control laws are derived based on the steady-state input-output voltage relationships and the CCM large-signal averaged pulsewidth modulation (PWM)-switch model. Using this methodology, average CMC laws with linear PWM waveforms are discovered for commonly used single-phase power stage topologies such as boost, flyback, SEPIC, and buck/boost. Conventional three-loop-controlled average CMC converters can now be controlled with a two-loop architecture. Hardware results for a boost power factor correction (PFC) and simulation results for flyback, SEPIC, and buck/boost topologies verify operation. The small-signal models of the current loop and voltage loop are derived for the boost topology and are used for control loop design. Input current harmonic distortion measurements demonstrate improved performance compared to the conventional three-loop control technique.

Uninterruptible power supplies with dual-sourcing capability and methods of operation thereof

Abstract: An uninterruptible power supply (UPS) includes an AC source port configured to connect to an AC power source, a DC source port configured to connect to a DC power source and a load port configured to connect to a load. A rectifier circuit is operative to selectively couple the AC source port to first and second voltage busses through a first inductance. An inverter circuit is operative to selectively couple the load port to the first and second voltage busses through a second inductance. A bidirectional DC source coupling circuit is operative to couple the DC source port to the first and second voltage busses to provide bidirectional power transfer therebetween, preferably such that a voltage at the DC source port is maintained in a substantially fixed proportion to first and second DC voltages at respective ones of the first and second voltage busses. In one embodiment, the bidirectional DC source coupling circuit includes a transformer having first and second inductively coupled windings, the first winding having first and second end taps and a center tap coupled to the neutral bus. A switching circuit is operative to selectively couple the first end tap of the first winding to the first and second voltage busses. A rectifying circuit couples the second end tap of the first winding to the first and second voltage busses. A bidirectional DC-AC converter circuit is coupled between the second winding and the DC source port. Related operating methods are also discussed.

A robust estimation method for topology error identification

Abstract: A pre-processing method that identifies both multiple topology errors and bad measurements is described. The method determines the branch statuses by testing the real and reactive power flow estimates of all the branches of the network, irrespective of their assumed statuses. The power flows are the state variables of two decoupled real and reactive power models that stem from both a detailed substation representation and a super-node modeling. They are estimated by means of the iteratively reweighted least-squares algorithm that implements the Huber M-estimator. The procedure is not prone to divergence problems, which is of great value in a real-time environment. The performance of the method is demonstrated on the IEEE-118 bus system.