Showing papers on "AC power published in 2000"

A particle swarm optimization for reactive power and voltage control considering voltage security assessment

Abstract: Summary form only given, as follows. This paper presents a particle swarm optimization (PSO) for reactive power and voltage control (volt/VAr control: VVC) considering voltage security assessment (VSA). VVC 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 online VVC 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 number of reactive power compensation equipment. The method considers voltage security using a continuation power now and a contingency analysis technique. 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.

Stationary-frame generalized integrators for current control of active power filters with zero steady-state error for current harmonics of concern under unbalanced and distorted operating conditions

Abstract: The paper proposes several concepts of integrators for sinusoidal signals. Parallel and series associations of the basic PI units using the stationary frame generalized integrators are used for current control of active power filters. Zero steady state error for the concerned current harmonics are realized, with reduced computation, under unbalanced utility or load conditions. Designing of the PI constants, digital realization of the generalized integrators, as well as compensation of the computation delay etc. are studied. Extensive test results from a 10 kW active power filter prototype are demonstrated.

Virtual flux based direct power control of three-phase PWM rectifiers

Abstract: In this paper, direct power control (DPC) of three-phase pulsewidth-modulated rectifiers without line voltage sensors is presented. The new system is based on virtual flux (VF) estimation. Theoretical principles of this method are discussed. The steady-state and dynamic behavior of VF-DPC are presented that illustrate the operation and performance of the proposed system compared to a conventional DPC method. Both strategies are also investigated under unbalance and predistorted grid. It is shown that the VF-DPC exhibits several advantages, particularly providing sinusoidal line current when the supply voltage is not ideal. Test results show the excellent performance of the proposed system.

Active power filters: a review

Abstract: There has been considerable interest in the development and applications of active filters because of the increasing concern over power quality, at both distribution and consumer levels, and the need to control reactive power and voltage stability at transmission levels. The existing approaches are classified and assessed to provide a framework of references for both researchers in this field and for generators, suppliers and consumers of electrical power who are, or may be, concerned about the problems associated with power quality and are considering installing active filters for their particular sets of problems.

Control of parallel inverters in distributed AC power systems with consideration of 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. Experimental results are provided in the paper to prove the concept.

An instantaneous active and reactive current component method for active filters

Abstract: A shunt active filter based on the instantaneous active and reactive current component i/sub d/-i/sub q/ method is proposed. This new control method aims to compensate harmonics and first harmonic unbalance. To evaluate its relative performance, it is compared with the instantaneous active and reactive power p-q method under various mains voltage conditions and for different harmonic injection high-pass filters. Both methods are completely frequency-independent, however under distorted mains voltages the proposed method presents a better harmonic compensation performance. The system synthesis and implementation are performed. Simulation and experimental results are presented.

The potential of distributed generation to provide ancillary services

Abstract: The growing concerns regarding electric power quality and availability have led to the installation of more and more distributed generation. In parallel and in the context of an accelerating trend towards deregulation of the electric industry, the unbundling of services, many grouped under ancillary services, should create a market for some of these services. This paper discusses the potential of distributed generation (DG) to provide some of these services. In particular, DG can serve locally as the equivalent of a spinning reserve and voltage support of the AC bus. The main types of distributed generation with emphasis on the power electronic interface and the configurations appropriate to provide ancillary services are reviewed. The flexibility and features provided by the power electronic interface are illustrated. In addition to control of the real power, other functions can be incorporated into the design of the interface to provide services, such as reactive power, and resources associated with power quality. These include voltage sag compensation and harmonic filtering. The implications on the design of the power converter interface are discussed.

A method for determining customer and utility harmonic contributions at the point of common coupling

Abstract: A new method is proposed in this paper to determine the harmonic contributions of a customer at the point of common coupling. The method can quantify customer and utility responsibilities for limit violations caused by either harmonic source changes or harmonic impedance changes. It can be implemented in current power quality monitors and digital revenue meters. The method makes it possible to develop fair and consistent billing schemes for harmonic distortion control.

Power transfer allocation for open access using graph theory-fundamentals and applications in systems without loopflow

Abstract: In this paper, graph theory is used to calculate the contributions of individual generators and loads to line flows and the real power transfer between individual generators and loads that are significant to transmission open access. Related lemmas are proved which present necessary conditions required by the method. Based on AC load flow solution a novel method is suggested which can decide downstream and upstream power flow tracing paths very fast and can calculate the contribution factors of generations and loads to the line flows efficiently. The power transfer between generators and loads can also be determined. The suggested method is suitable for both active and reactive power tracings of real power systems.

Multilevel PWM methods at low modulation indices

Abstract: When utilized at low amplitude modulation indices, existing multilevel carrier-based PWM strategies have no special provisions for this operating region, and several levels of the inverter go unused. This paper proposes some novel multilevel PWM strategies to take advantage of the multiple levels in both a diode-clamped inverter and a cascaded H-bridges inverter by utilizing all of the levels in the inverter even at low modulation indices. Simulation results show what effects the different strategies have on the active device utilization. A prototype 6-level diode-clamped inverter and an 11-level cascaded H-bridges inverter have been built and controlled with the novel PWM strategies proposed in this paper.

A new optimal reactive power scheduling method for loss minimization and voltage stability margin maximization using successive multi-objective fuzzy LP technique

Abstract: Insufficient reactive power support in a power grid results in under voltages at load centres and also limits the real power transfer capacities of the transmission systems leading to voltage collapse. An optimal reactive power scheduling method is sought which minimizes real power transmission loss and maximizes voltage stability margin (VSM) subject to the utmost satisfaction of all the violated load bus voltage constraints. A multi-objective fuzzy LP (MFLP) method of solution in the successive LP (SLP) framework is proposed to solve the problem. A set of least singular values of the load flow Jacobian is used as VSM indicator. This set is expressed in terms of the control vector and is maximized in the proposed formulation to maximize VSM. Results of the tests of the proposed method on modified IEEE 6 bus and 57 bus systems are presented. The changes in the load flow Jacobian singular values during scheduling are studied.

A particle swarm optimization for reactive power and voltage control considering voltage stability

Abstract: This paper presents a particle swarm optimization for reactive power and voltage control considering voltage stability. The proposed method determines a control strategy with continuous and discrete control variables such as AVR operating values, OLTC tap positions, and the amount of reactive power compensation equipment. The method also considers voltage stability using a continuation power flow technique. The feasibility of the proposed method is demonstrated on model power systems with promising results.

Modular multi-level adjustable supply with parallel connected active inputs

Abstract: An adjustable frequency multiphase power supply providing low harmonics to the line and load. The power supply having a plurality of power cells in each phase output thereof, each cell having an input side, and an output side. The input side is capable of converting single-phase AC into DC and for converting DC into AC to said source. The output side is capable of converting DC into a controlled AC output and for converting AC from said load to feed said DC bus. The respective outputs and respective inputs of the cells are series connected. Each cell can have a single-phase transformer connected either to the input side or the output side. The output side of each cell can be controlled to limit the harmonic content of the output AC to the load, and the input side of each cell can be controllable to limit harmonic content of the AC power feed to the source. The input and/or output sides can be interdigitated to limit harmonic components to the source and/or to reduce the harmonic content of the AC supplied to the load.

Reactive power pricing: a conceptual framework for remuneration and charging procedures

Abstract: A new electrical sector regulation is being implemented in different countries all over the world. The new regulation stresses the role of competitive markets for the procurement and remuneration of ancillary services. Among these services stand the ones associated with reactive power supply and transmission network voltage control in order to maintain the required system security levels. The object of this paper is twofold. On one hand, reactive power supply and voltage control services, which today are bundled, are decomposed in two types: (i) voltage profile management and reactive dispatch, and (ii) voltage regulation. A theoretical approach based on marginal pricing is proposed in order to clarify the principles to remunerate the suppliers and to charge the consumers of these services. On the other hand, a practical organization of reactive supply and voltage service markets is presented to be implemented in a competitive environment.

AC/AC power conversion based on matrix converter topology with unidirectional switches

Abstract: In this paper, a new type of AC/AC power converter is proposed. The proposed converter is capable of direct AC/AC power conversion and, except for a few small snubber elements, it does not require the use of any input inductors or a DC-link capacitor. In contrast to the matrix converter, which requires bidirectional switches, the proposed converter consists of only unidirectional switches such as insulated gate bipolar transistors. The converter has a unity input displacement power factor, and its input line current waveform is similar to that of a diode rectifier with a DC-link inductor. The proposed converter has been validated experimentally using an induction motor driven with field-oriented control.

A radial basis function neural network controller for UPFC

Abstract: This paper presents the design of radial basis function neural network controllers (RBFNN) for UPFC to improve the transient stability performance of a power system. The RBFNN uses either a single neuron or multi-neuron architecture and the parameters are dynamically adjusted using an error surface derived from active or reactive power/voltage deviations at the UPFC injection bus. The performance of the new single neuron controller is evaluated using both single-machine infinite-bus and three-machine power systems subjected to various transient disturbances. In the case of three-machine 8-bus power system, the performance of the single neuron RBF controller is compared with a BP (backpropagation) algorithm based multi-layered ANN controller. Further it is seen that by using a multi-input multi-neuron RBF controller, instead of a single neuron one, the critical clearing time and damping performance are improved. The new RBFNN controller for UPFC exhibits a superior damping performance in comparison to the existing PI controllers. Its simple architecture reduces the computational burden thereby making it attractive for real-time implementation.

Transformerless 3 phase power inverter

Abstract: The invention in the simplest form is an improved transformerless 3 phase power inverter topology and control method for power distribution systems. This 3 phase power inverter system provides the necessary output voltage and current waveform regulation, harmonic distortion rejection, very low output impedance, and ultra low DC offset of the output AC power, even when exposed to a high magnitude of the non-linear, single phase, or the unbalanced loading.

A multilevel converter-based universal power conditioner

Abstract: This paper presents the development of a control scheme for a multilevel diode-clamped converter connected in a series-parallel fashion to the electrical system such that it can compensate for deviations in utility voltage (sag, surge, and unbalance) and act as a harmonic and/or reactive current source for a load. New carrier-based multilevel pulsewidth modulation techniques are identified to maximize switch utilization of the two back-to-back diode-clamped inverters that constitute the universal power conditioner. An experimental verification for a six-level power conditioner is given.

Sensorless control strategies for PWM rectifier

Abstract: In this paper two different control strategies for a PWM rectifier without line voltage sensors are compared. The direct power control (DPC) which has no need for line voltage measurements is compared to the conventional voltage oriented control (VOC) strategy in rotating coordinates with a novel line voltage estimator. The steady-state performance of both strategies is compared with regards to voltage unbalance and pre-distorted grid. Furthermore, the use of discontinuous modulation is motivated in the classical control strategy and is analyzed along with the novel line voltage estimator. It is shown that the VOC strategy with line voltage estimator exhibits several advantages compared to DPC. Some simulations and experimental results verifying the comparison are presented.

Secondary coordinated voltage control system: feedback of EDF

Abstract: With power systems operating under tighter constraints, and closer to their actual operating limits, optimization of available margins becomes an increasingly critical concern, especially with regard to voltage stability and the management of static and dynamic reactive-power compensation systems. With this aim, Electricite de France has developed and experimented a new secondary voltage control system, capable of coordinating compensation systems on a large scale. This report presents the results of this new secondary voltage control system, which has been operating for more than two years in the west of France.

Energy flow and power phenomena in electrical circuits: illusions and reality

Abstract: Common opinions respective to the nature of the reactive power, energy flow and oscillations, as well as the notion of the apparent power in single- and in three-phase systems are discussed in this paper. It is shown that some interpretations of powers and energy flow in linear, single-phase circuits are often generalized for more complex situations where these interpretations are not longer valid. Consequently, power phenomena in electrical systems are often misinterpreted. This relates to the reactive power which occurs in three-phase systems without energy oscillation between the supply source and the load, as well as it occurs in time-variant systems without energy storage capability. Also, it was demonstrated in the paper that the arithmetic and geometric apparent powers, commonly used in three-phase systems, do not characterize the supply loading correctly when the load is unbalanced.

Induction-generator-based system providing regulated voltage with constant frequency

Abstract: The electrical characteristics of an isolated induction-generator-based system are improved through the association with a voltage-source pulsewidth modulation (PWM) inverter The electronic converter allows the achievement of a better system behavior in many aspects: voltage regulation, frequency stabilization, and reactive power compensation The system operation strategy consists of maintaining constant synchronous frequency at the induction generator via an association with a PWM inverter The system power balance and the generator voltage regulation may be accomplished by two different means: through the rotor speed regulation, or by sending part of the energy stored in the inverter DC side to the grid through a single-phase line, in case the rotor speed is not regulated and a single-phase grid connection is available The obtained results demonstrated the system is stable, robust, and an effective source of regulated three-phase voltages

Reactive power measurement using the wavelet transform

Abstract: This paper provides the theoretical basis for the measurement of reactive and distortion powers from the wavelet transforms. The measurement of reactive power relies on the use of broad-band phase-shift networks to create concurrent in-phase currents and quadrature voltages. The wavelet real power computation resulting from these 90/spl deg/ phase-shift networks yields the reactive power associated with each wavelet frequency level or subband. The distortion power at each wavelet subband is then derived from the real, reactive and apparent powers of the subband, where the apparent power is the product of the v, i element pair's subband rms voltage and current. The advantage of viewing the real and reactive powers. In the wavelet domain is that the domain preserves both the frequency and time relationship of these powers. In addition, the reactive power associated with each wavelet subband is a signed quantity and thus has a direction associated with it. This permits tracking the reactive power flow in each subband through the power system.

Adjustable speed generators for wind turbines based on doubly-fed induction machines and 4-quadrant IGBT converters linked to the rotor

Abstract: Wind turbines are being built at power levels above 1.5 MW. Higher power levels are being anticipated for offshore applications. To limit mechanical stresses and power surges in these high power systems, speed control is necessary. The doubly-fed induction generator (DFIG) system is investigated as a viable alternative to adjust speed over a wide range while keeping the cost of the power converters minimal. A four-quadrant IGBT AC-to-AC converter is used to feed power bi-directionally to the rotor circuit. Decoupled control of active and reactive power can be realized using the dynamic model of the DFIG. Simulations and measurements confirm the validity of the model and the viability of the DFIG wind turbine.

Voltage stability analysis: V-Q power flow simulation versus dynamic simulation

Abstract: Summary form only given as follows. Several analysis methods are available for long-term voltage stability. The V-Q curve power flow method is widely used by Western Systems Coordinating Council utilities, and has some advantages. Long-term dynamic simulation with proper modeling, however, is clearly the most accurate simulation method. We compare the two methods for wintertime voltage stability problems in the Portland, Oregon USA load area. Results from the V-Q method can be misleading. The same is true of other power flow program based analysis employing conventional modeling. Results from these power flow methods may be pessimistic, causing overdesign or overly conservative operation.

Control system for a PWM-based STATCOM

Abstract: The always-increasing switching frequency of modern solid-state power switches, together with the application of multi-converter topologies, make it possible to use pulse width modulation (PWM) in high power applications of STATCOMs (static synchronous compensators). This paper investigates the control system for a PWM-based STATCOM. First of all, a discrete-time model of the STATCOM is derived to take into account the discrete-time implementation of the controller. Secondly, the control algorithm is detailed. It ensures decoupled control of the real and reactive power exchanged between the power converter and the electric-energy system. This is necessary to control the DC capacitor voltage during transients of the exchanged reactive power. Finally, the control of the capacitor voltage is explained in detail. The controller is tailored to keep the capacitor voltage almost constant in spite of the fast control of the reactive power. This helps to reduce the capacitor size significantly. The main contributions are illustrated using a 15 kVA laboratory prototype.

A vector controlled current-source PWM rectifier with a novel current damping method

Abstract: Three-phase current-type pulse width modulation (PWM) rectifiers are becoming increasingly popular as the front-end converter unit in power electronic systems due to tighter electromagnetic compatibility (EMC) regulations. In this paper the control of the current source PWM rectifier in the synchronously rotating reference frame is discussed. A control system is presented in which the active and reactive power are independently controlled with real and imaginary axis components of the supply current vector. A new damping method for supply current oscillations is introduced. The method operates in an open-loop manner and is very suitable for microcontroller implementation since the calculation power demand is low. Furthermore, it is shown that in the synchronously rotating coordinates, where the sinusoidal variables appear as DC quantities, the compensation of the reactive power drawn by the supply filter can be done very easily. The proposed control methods are realized using a single-chip Motorola MC68HC916Y1 microcontroller. The experimental tests show excellent performance in both steady state and transient conditions.

Power reception circuits for a device receiving an AC power signal

Abstract: Power reception circuits employable in portable data devices (e.g., smart cards) to derive power and/or data from an input AC power signal (e.g., an ASK modulated carrier signal). In one embodiment, the power reception circuit comprises a power rectifier (120), a shunt rectifier (142) and a shunting element (132). The power rectifier (120) is adapted to rectify the input power signal, yielding a rectified output waveform. The shunt rectifier (142) is connected in parallel with the power rectifier (120). The shunting element (132) is connected to the shunt rectifier (142) and is operable to regulate an output voltage or current waveform produced at the output of the power rectifier (120). In another embodiment, the power reception circuit includes an analog circuit (610) for recovering data from a modulated carrier signal. A decoupling device (630) isolates the analog circuit (610) from impedance variations of a load. A shunt device (640) diverts undesired current from the load.

Method of operating a power supply system having parallel-connected inverters, and power converting system

Abstract: A method whereby a plurality of inverters (14) for converting DC power outputted from a DC power supply, such as solar cells (1) or fuel cells, to AC power are operated efficiently without being biased to particular inverters. The number of inverters (14) to be run is determined in correspondence with at least one output value of the DC output or AC output, and the determined number of inverters are selected and made to run from among the plurality of inverters (14) on the basis of a predetermined rule. In addition, a parallel-connected system is disclosed for enabling efficient and appropriate parallel-in operation by the use of inverters, with one of the inverters controlling the remaining inverters and effecting system interconnection protection.

Islanding detection method of distributed generation units connected to power distribution system

Abstract: In this paper, as an active technique for detecting the islanding, voltage magnitude variation method of distributed generation (DG) unit is proposed. The proposed new algorithm detects the islanding by distinguishing the differences in the system characteristic between those experienced when the DG unit is running in parallel with the utility supply and when it is operating independently. In the parallel operation mode, due to the high capacity of the utility system, any change in the voltage magnitude of DG unit produces smaller change in the voltage at the inter-tie point than that in islanding operation mode, and larger change in the injecting reactive power at the inter-tie point than that in islanding operation mode. That is, when the small change on the voltage magnitude of DG unit is made within 0.01 per unit, if the voltage at the inter-tie point changes as much as the small change and the injecting reactive power at the inter-tie point has almost no change, that operation state can be decided to be islanding operation mode. This proposed method is proved through computer simulations.