Showing papers on "AC power published in 1996"

Doubly fed induction generator using back-to-back PWM converters and its application to variable-speed wind-energy generation

Abstract: The paper describes the engineering and design of a doubly fed induction generator (DFIG), using back-to-back PWM voltage-source converters in the rotor circuit. A vector-control scheme for the supply-side PWM converter results in independent control of active and reactive power drawn the supply, while ensuring sinusoidal supply currents. Vector control of the rotor-connected converter provides for wide speed-range operation; the vector scheme is embedded in control loops which enable optimal speed tracking for maximum energy capture from the wind. An experimental rig, which represents a 7.5 kW variable speed wind-energy generation system is described, and experimental results are given that illustrate the excellent performance characteristics of the system. The paper considers a grid-connected system; a further paper will describe a stand-alone system.

New trends in active filters for power conditioning

Abstract: Attention has been paid to active filters for power conditioning which provide the following multifunctions: reactive power compensation; harmonic compensation; flicker/imbalance compensation; and voltage regulation. Active filters in a range of 50 kVA-60 MVA have been practically installed in Japan. In the near future, the term "active filters" will have a much wider meaning than it did in the 1970s. For instance, active filters intended for harmonic solutions are expanding their functions from harmonic compensation of nonlinear loads into harmonic isolation between utilities and consumers, and harmonic damping throughout power distribution systems. This paper presents the present status of active filters based on state-of-the-art power electronics technology, and their future prospects and directions toward the 21st Century, including the personal views and expectations of the author.

Direct power control of PWM converter without power source voltage sensors

Abstract: This paper proposes a novel control strategy of a pulsewidth modulation (PWM) converter with no power-source voltage sensors. The strategy has two main features to improve a total power factor and efficiency, taking harmonic components into account without detecting the voltage waveforms. One feature is a direct instantaneous power control technique for the converter, which has been developed to control the instantaneous active and reactive power directly by selecting the optimum switching state of the converter. The other feature is an estimation technique of the power-source voltages, which can be performed by calculating the active and reactive power for each switching state of the converter from the line currents. A digital-signal-processor-based experimental system was developed, and experimental tests were conducted to examine the controllability. As a result, it was confirmed that the total power factor and efficiency were more than 97% and 93% over the load power range from 200 to 1400 W, respectively. These results have proven the excellent performance of the proposed system.

Tracing the flow of electricity

Abstract: Continuing trend towards deregulation and unbundling of transmission services has resulted in the need to assess what the impact of a particular generator or load is on the power system. A new method of tracing the flow of electricity in meshed electrical networks is proposed which may be applied to both real and reactive power flows. The method allows assessment of how much of the real and reactive power output from a particular station goes to a particular load. It also allows the assessment of contributions of individual generators (or loads) to individual line flows. A loss-apportioning algorithm has also been introduced which allows the break down of the total transmission loss into components to be allocated to individual loads or generators. The method can be useful in providing additional insight into power system operation and can be used to modify existing tariffs of charging for transmission loss, reactive power and transmission services.

Generalized instantaneous reactive power theory for three-phase power systems

Abstract: A generalized theory of instantaneous reactive power for three-phase power systems is proposed in this paper. This theory gives a generalized definition of instantaneous reactive power, which is valid for sinusoidal or nonsinusoidal, 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 three-phase harmonic distorted power system with zero-sequence components is then used as an example to show reactive power measurement and compensation using the proposed theory.

Damping controller design for power system oscillations using global signals

Abstract: This paper describes a new power system stabilizer (PSS) design for damping power system oscillations focusing on interarea modes. The input to the PSS consists of two signals. The first signal is mainly to damp the local mode in the area where PSS is located using the generator rotor speed as an input signal. The second is an additional global signal for damping interarea modes. Two global signals are suggested; the tie-line active power and speed difference signals. The choice of PSS location, input signals and tuning is based on modal analysis and frequency response information. These two signals can also be used to enhance damping of interarea modes using SVC located in the middle of the transmission circuit connecting the two oscillating groups. The effectiveness and robustness of the new design are tested on a 19-generator system having characteristics and structure similar to the Western North American grid.

Practical definitions for powers in systems with nonsinusoidal waveforms and unbalanced loads: a discussion

Abstract: Existing definitions for power terms in alternating current systems work well for single-phase and three-phase systems where both voltages and currents are sinusoidal with respect to time. This paper clarifies and proposes definitions for power terms that are practical and effective when voltage and/or currents are distorted and/or unbalanced. It also suggests definitions for measurable values that may be used to indicate the level of distortion and unbalance.

A procedure for designing EMI filters for AC line applications

Abstract: A procedure for designing AC power line EMI filters is presented. This procedure is based on the analysis of conducted EMI problems and the use of a noise separator. Design examples are given, and results are experimentally verified.

A high-performance single-phase rectifier with input power factor correction

Abstract: In this paper, a high-performance single-phase AC-to-DC rectifier with input power factor correction is proposed. The proposed approach has many advantages, including fewer semiconductor components, simplified control, and high-performance features, and satisfies IEC 555 harmonic current standards. Simulation and experimental results obtained on a laboratory prototype are discussed. A hybrid power module of the proposed approach is also shown.

Dynamic performance and control of a static VAr generator using cascade multilevel inverters

Abstract: A cascade multilevel inverter is proposed for static VAr compensation/generation applications. The new cascade M-level inverter consists of (M-1)/2 single-phase full bridges in which each bridge has its own separate DC source. This inverter can generate an almost sinusoidal waveform voltage with only one time switching per cycle. It can eliminate the need for transformers in multipulse inverters. A prototype static VAr generator (SVG) system using an 11-level cascade inverter (21-level line-to-line voltage waveform) has been built. The output voltage waveform is equivalent to that of a 60-pulse inverter. This paper focuses on the dynamic performance of the cascade inverter based SVG system. Control schemes are proposed to achieve a fast response which is impossible for a conventional static VAr compensator (SVC). Analytical, simulation and experimental results show the superiority of the proposed SVG system.

Performance optimization controller and control method for doubly-fed machines

Abstract: A variable speed, constant frequency (VSCF) system utilizes a doubly-fed machine (DFM) to maximize the output power of the system. The system includes a power converter that provides a frequency signal and a current signal to the DFM. The power converter is controlled by an adaptive controller. The controller signals the converter to vary its frequency signal and thereby the rotor speed of the DFM until a maximum power output is sensed. The controller also signals the converter to vary its current signal and thereby the portions of power carried by the respective windings until a maximum power output is sensed. The control can be augmented to not only maximize power and efficiency, but also provide for harmonic and reactive power compensation.

Application of interior point methods to power flow unsolvability

Abstract: This paper describes an application of an optimal power flow, solved by a direct interior point (IP) method, to restore system solvability. Using the P-Q load representation, power flow unsolvability occurs when, for a given set of active and reactive bus injections, the power flow equations have no real solution. The set of control actions in the IP algorithm includes rescheduling of active power of generators, adjustments on terminal voltage of generators, tap changes on LTC transformers, and as a last resort, minimum load shedding. The IP formulation allows observation of the impact of each control optimization in system solvability. Also described is a framework to calculate probabilistic indicators of solvability problems taking into account the probability of contingencies. The role of control optimization is illustrated in a real 11-bus power system and the probabilistic approach is applied to a 1600-bus power system derived from the Brazilian South/Southeast/Central West system.

A new single-phase active power filter with reduced energy-storage capacity

Abstract: The paper presents an active power filter (APF) circuit which employs a new control method, using an integration and sampling technique, to simplify the calculation algorithm for the real fundamental component of load current. In addition, a new simple control scheme, based on the energy balance concept, is proposed to control the voltage of an energy-storage capacitor. Since the energy change in the energy-storage capacitor can be compensated in the next cycle, and a larger DC bus voltage ripple can be tolerated if a sampling technique is used, a relatively smaller energy storage capacitor is required. The advantages of this APF circuit are simplicity of control circuits, low cost (a smaller energy-storage capacitor) and good transient response. In theory, the time delay for the compensation of reactive power and harmonic currents is zero. The feasibility of this theory is veriffied by using a PSPICE simulation and experimental results.

A new definition of instantaneous active-reactive current and power based on instantaneous space vectors on polar coordinates in three-phase circuits

Abstract: This paper proposes a new definition of the instantaneous active-reactive current and power based directly on instantaneous space vectors on polar coordinates, and presents its application. The definition is applicable in three-phase three-wire systems. The instantaneous active-reactive power and current on /spl alpha/-/spl beta/ orthogonal coordinates have been defined by the so-called p-q theory. In comparison with the p-q theory, our new definition offers a lucid physical concept for the active-reactive current and power in three-phase circuits. The new method, thus, can decompose current into the instantaneous active and reactive currents without calculating the instantaneous active and reactive powers. An application example is presented to confirm the validity and practicability of the new definition using digital simulation.

A novel control to actively damp resonance in input LC filter of a three phase voltage source converter

Abstract: The voltage source power converter (VSC) provides constant/controllable DC bus voltage, regenerating capability, controllable power factor and nearly sinusoidal input current. If an input LC filter is added, it prevents pollution of the utility with current ripple due to the PWM switching. A novel control strategy for actively damping high frequency resonances (above 1.5 kHz) in the input LC filter is presented. No additional sensors to the standard VSC are required. Theoretical results of the analysis and simulation are experimentally verified.

Optimal PWM based on real-time solution of harmonic elimination equations

Abstract: Pulse-width modulation of DC/AC power converters (PWM) based on the elimination of low-order harmonics necessitates solving systems of nonlinear equations. Conventional implementations of this technique based on storing off-line calculated solutions have the common problem that the system flexibility is very limited, especially for applications that require both amplitude and frequency control. A new implementation scheme based on real-time solution of nonlinear harmonic elimination equations using a digital signal processor DSP56001 is reported in this paper. With this digital signal processor (DSP), optimal pulse patterns having 15 switching angles in each quarter fundamental period can be determined within 2.15 ms. Details of the system hardware and software are described. New theoretical results concerning the solvability of harmonic elimination equations are also presented.

A simple frequency-independent method for calculating the reactive and harmonic current in a nonlinear load

Abstract: A basic criterion that determines the behavior of an active power filter is the method of calculating the reference current. There are many ways of generating this reference, but the methods are generally complex and hard to tune. This paper describes a simple and effective method for calculating the reference current necessary to feed a shunt active power filter to compensate the power factor and harmonic currents generated by a nonlinear load. Simulations and experimental results are presented, showing that the proposed circuit may operate at frequencies ranging from 40 to 65 Hz without adjustment.

Linear peak current mode control: a simple active power factor correction control technique for continuous conduction mode

Abstract: A simplified power factor correction (PFC) technique for continuous conduction mode (CCM) operated power converters is presented which has the following advantages over conventional peak and average current mode PFC techniques: elimination of the controller multiplier and input voltage sensing circuits; unconditional stability of the current loop; and ease of implementation using low cost standard PWM control ICs (e.g. UC2843). Simulation and experimental results verify the viability of the new controller.

Input current ripple cancellation in synchronized, parallel connected critically continuous boost converters

Abstract: In high power factor AC-to-DC applications, boost power converters operating on the boundary of continuous mode and discontinuous mode switch with variable frequency and draw high peak input currents. A method is presented to parallel two or more of these power converters to reduce the high peak input currents. Each power converter continues to operate on the boundary of continuous mode and discontinuous mode and maintains the benefits of zero-voltage switching.

Cellular communication system with remote power source for providing power to access points

Abstract: A cellular communication system which eliminates high costs and difficulties associated with providing electrical power to the access points. The cellular communication system includes a remote power source which obviates the need to install an AC power outlet in close proximity to each access point. The remote power source transforms AC power to DC power at a central remote location, and provides as its output one or more low voltage DC power lines. By performing the AC/DC power transformation at a central location, only the low voltage DC power lines need to be fed to each access point. The remote power source also includes a backup power supply feature and an alarm to draw attention to system malfunctions.

Principle and characteristics of a fault current limiter with series compensation

Abstract: A power system fault current limiter with series compensation, which is composed of a compensation capacitor and a limiting reactor in series, is proposed. A solid-state switch connected in parallel with the capacitor controls either the ordinal series compensation or fault current limitation. A feasibility study of the current limiter by simulation analysis is presented and the effectiveness of the current limiter is evaluated from the viewpoints of transient stability improvement and device capacity. The current limiter is a useful protection device for large, high power transmission systems.

New trends in active filters for improving power quality

Abstract: Since their basic compensation principles were proposed around 1970, active filters have been studied by many researchers and engineers aiming to put them into practical applications. Shunt active filters for harmonic compensation with or without reactive power compensation, flicker compensation or voltage regulation have been put on a commercial base in Japan, and their rating or capacity has ranged from 50 kVA to 60 MVA at present. In near future, the term of active filters will cover a much wider sense than that of active filters in the 1970s did. The function of active filters will be expanded from voltage flicker compensation or voltage regulation into power quality improvement for power distribution systems as the capacity of active filters becomes larger. This paper describes present states of the active filters based on state-of-the-art power electronics technology, and their future prospects toward the 21st century, including the personal view and expectation of the author.

A new fuzzy-reasoning approach to optimum capacitor allocation for primary distribution systems

Abstract: This paper presents a fuzzy-reasoning method to optimum shunt capacitor placement and sizing for the radial distribution systems. In the method, capacitor allocation is applied to correct voltage and reduce power loss for a given load pattern. The problem of capacitor allocation includes the location, type (fixed or switched), and size of capacitor. Hence it is a combinatorial optimization problem with multiple objectives. A simple algorithm is presented for optimal capacitor placement and sizing problem of distribution systems having concentrated loads. Two membership functions are defined for voltage and power loss, respectively. Results are obtained through applying a simple min-operation. The method proposed is simple, straightforward, and effective.

An adaptive digital controller for a unity power factor converter

Abstract: This paper describes an adaptive digital controller for a unity power factor AC-DC power converter. The controller is based on a linear large-signal model of the boost power converter. A hardware design is presented and analyzed, followed by the software implementation of the control algorithm. Issues in digital control of power converters, such as quantization effects and fixed-point representation of system parameters, are examined in the context of this system. Experimental results are presented and compared with simulations.

Uninterruptible power system with a flywheel-driven source of standby power

Abstract: An UPS (uninterruptible power system) includes an UPS power conditioning unit that provides conditioned AC power to a critical load. The UPS power conditioning unit includes a variable speed drive that operates in response to AC utility power or to a standby DC input by providing a motor drive signal. The UPS power conditioning unit further includes a motor-generator that operates in response to the motor drive output by providing the conditioned AC power to the critical load. In response to an outage in the utility AC power, standby DC power is provided by a standby DC power source that includes a variable speed drive and a flywheel motor-generator connected to the variable speed drive. Both the UPS power conditioning unit and the standby DC power source are initially operated in response to the utility AC power, the flywheel motor-generator storing kinetic energy in a rotating flywheel. When an outage occurs, the rotating flywheel continues to operate the flywheel motor-generator of the standby DC power source, causing the production of AC power which is rectified and provided as standby DC power to operate the variable speed drive of the UPS power conditioning unit until either the utility AC power outage is over or a standby emergency generator is brought on line.

Compact and efficient transformerless power conversion system

Abstract: The power conversion system (PCS) developed by the author is a new technology that efficiently transforms power between AC and DC. The conversion process permits the input voltage level to be stepped up or down without the use of magnetic core transformers. The transformation is accomplished using solid state switching devices, capacitors, air-core inductors and an intelligent control system. The technology is based on the property of resonance charging, thereby permitting the use of self commutating switches. The current through the switches, being sinusoidal in nature, has a low dI/dt. This permits the use of slower, less expensive thyristors, similar to those used in high voltage phase control applications. Using these components, a highly efficient inverter can be constructed for a variety of large utility applications.

Field orientation control of a doubly excited brushless reluctance machine

Abstract: In this paper, the concept and implementation of field-orientation control (FOC) of a doubly excited brushless reluctance machine (DEBRM) for variable-speed drives and generating systems are presented. A stator-flux-orientation scheme is employed to achieve decoupled control of torque and reactive power. A 2 HP DEBRM prototype system with a digital signal processor (DSP)-based controller and a bidirectional power converter is built to verify the theoretical analysis. Computer simulation and laboratory experimental results are shown in excellent agreement.

Analysis and controller design of static VAr compensator using three-level GTO inverter

Abstract: A static VAr compensator (SVC) using a three-level GTO voltage source inverter (VSI) is presented for high-voltage, high-power applications. The three-level VSI has lower harmonic components and higher DC-link voltage than the two-level VSI and thus can be operated at lower switching frequency (f/sub sw/<500 Hz) without excessive harmonic contents. From the DQ-transformed equivalent circuit of the presented SVC system, DC and AC analyses are carried out to find the steady-state and the dynamic characteristics of the system. Based on the open-loop transfer function of the system, a controller is designed to achieve fast dynamic response. The experimental results confirm the theoretical analyses and controller design.

A single stage fluorescent lamp ballast with high power factor

Abstract: The steady state operation modes and the commutation zones of a single stage, high power factor ballast are studied. The design criteria to allow the ballast to operate at the optimum soft switching conditions are given. A control circuit is also proposed. High efficiency is achieved by soft switching. Experimental results verify the theoretical study.