Showing papers on "Voltage regulation published in 1998"

Voltage Stability of Electric Power Systems

Abstract: Foreword. Preface. Part I: Components and Phenomena. 1. Introduction. 2. Transmission System Aspects. 3. Generation Aspects. 4. Load Aspects. Part II: Instability Mechanisms and Analysis Methods. 5. Mathematical Background. 6. Modelling: System Perspective. 7. Loadability, Sensitivity and Bifurcation Analysis. 8. Instability Mechanisms and Countermeasures. 9. Criteria and Methods for Voltage Security Assessment. References. Index.

Statcom controls for operation with unbalanced voltages

Abstract: Voltage sourced static VAr compensators such as the Statcom need to be able to handle unbalanced voltages. Mild imbalance can be caused by unbalanced loads while severe short-term imbalance can be caused by power system faults. A synchronous frame voltage regulator is presented that works even when three phase symmetry is lost. This regulator addresses voltage imbalance by using separate regulation loops for the positive and negative sequence components of the voltage. The proposed regulator allows the Statcom to ride through severe transient imbalance without disconnecting from the power system and, further, to assist in rebalancing voltages. The regulator maintains sufficient bandwidth to perform flicker compensation. The controller's performance is simulated for a Statcom in a model distribution system where it is subjected to a severe single line to ground fault and a rapidly varying three phase load.

Single-stage single-switch power-factor-correction AC/DC converters with DC-bus voltage feedback for universal line applications

Abstract: Single-stage single-switch power-factor-correction (S/sup 4/-PFC) AC/DC converters with DC-bus voltage feedback are proposed. The DC-bus voltage stress at light load for the S/sup 4/-PFC converter is first analyzed. The DC-bus voltage negative feedback concept in the power stage to suppress the DC-bus voltage stress is then proposed. Design considerations of total harmonic distortion (THD), power factor (PF), DC-bus voltage stress, and the boost inductor are discussed. A S/sup 4/-PFC converter is analyzed and implemented with 5 V 12 A output as an example. It is shown that the DC-bus voltage is limited within 405 V from no load to full load, and the measured line input-current harmonics satisfy the IEC 1000-3-2 Class D requirements with universal line input. The measured efficiency and PF are about 71% and 0.93, respectively. The experimental results show that using the DC-bus voltage feedback to reduce the DC-bus voltage is effective.

Laboratory measurements and models of modern loads and their effect on voltage stability studies

Abstract: Accurate load models are important in determining the status of a power system when subjected to sudden or gradual changes in operating conditions. This paper describes laboratory measurements and derived models of modern loads subjected to large voltage changes and their effect on voltage stability studies. Low-voltage, long-time models of loads such as modern air conditioners, discharge lighting and devices containing electronic-regulated power supplies were developed. One means used by utilities to avert voltage collapse has been system voltage reduction. The characteristics of modern loads and controls reduces the effectiveness of this voltage reduction. The results indicate the importance of using accurate load models, especially for large industrial loads where the load composition can be identified.

Dynamic Voltage Scaling and the Design of a Low-Power Microprocessor System

Abstract: This paper describes the design of a low-power microprocessor system that can run between 8Mhz at 1.1V and 100MHz at 3.3V. The ramifications of Dynamic Voltage Scaling, which allows the processor to dynamically alter its operating voltage at run-time, will be presented along with a description of the system design and an approach to benchmarking. In addition, a more in-depth discussion of the cache memory system will be

Analysis of a novel solid state voltage regulator for a self-excited induction generator

Abstract: An analysis of a solid state voltage regulator for a self-excited induction generator (SEIG) using a static condenser (STATCON) is presented. The power circuit of the STATCON-based voltage regulator employs a three-phase current controlled voltage source inverter (CC-VSI) with an electrolytic capacitor in its DC bus. The control scheme consists of two PI voltage controllers and one hysteresis current controller. The mathematical model of the proposed scheme is developed to simulate the improved performance of the SEIG with a STATCON-based voltage regulator. The voltage regulator provides the SEIG with the desirable feature of a synchronous condenser, and is capable of operating in capacitive and inductive modes. The use of STATCON saves the SEIG from de-excitation during the severe condition of load perturbation. It has a fast dynamic response for regulating the voltage when the SEIG faces sudden application/removal of load. The voltage regulating scheme is adaptive to the changing load condition and hence it is possible to operate the SEIG at almost constant voltage from no load to full load.

A combined artificial neural network-fuzzy dynamic programming approach to reactive power/voltage control in a distribution substation

Abstract: Reactive power/voltage control in a distribution substation is investigated in this work. The purpose is to determine proper capacitor on/off status and suitable load tap changer (LTC) positions for the 24 hours in the next day. To reach this goal, an artificial neural network (ANN) is designed to reach a preliminary dispatch schedule for the capacitor and LTC. The inputs to the ANN are main transformer real power and reactive power and primary and secondary bus voltages and the outputs are the desired capacitor on/off status and LTC tap positions. The preliminary dispatch schedule is further refined by fuzzy dynamic programming in order to reach the final schedule. To demonstrate the effectiveness of the proposed method, reactive power/voltage control is performed on a distribution substation in Taipei, Taiwan. Results from the example show that a proper dispatch schedule for capacitor and LTC can be reached by the proposed method in a very short period.

Method and apparatus for charging a rechargeable battery

Abstract: In a system and method for charging a rechargeable battery having a protection circuit, the rechargeable battery operates according to a model that includes an ideal battery and an internal resistance. In the model, an electrode voltage develops across the ideal battery. An internal voltage drop equal to the difference between this electrode voltage and the battery voltage is developed across the internal resistance. The battery is charged with a charging voltage that may exceed the threshold voltage and the electrode voltage. The charging system and method ensures that (1) the voltage at the protection circuit is kept below the threshold voltage and (2) the electrode voltage is kept below the end-of-charge voltage.

DRAM having a power supply voltage lowering circuit

Abstract: A DRAM includes first to third voltage lowering circuits for lowering a power supply voltage supplied from the exterior and supplying the lowered voltage to an internal circuit. The first to third voltage lowering circuits are separately provided. The first voltage lowering circuit is a feedback type circuit having a P-channel MOS transistor. The first voltage lowering circuit is an exclusive circuit for creating a first potential by lowering the power supply voltage supplied from the exterior and supplying a thus created lowered power supply voltage to a RAS signal input buffer, CAS signal input buffer and WE signal input buffer. The second voltage lowering circuit is a feedback type circuit having a P-channel MOS transistor or source follower type circuit having an N-channel MOS transistor. The second voltage lowering circuit is an exclusive circuit for creating a second potential by lowering the power supply voltage supplied from the exterior and supplying a thus created lowered power supply voltage to a V BL generating circuit for generating a bit line precharge potential and a V PL generating circuit for generating a cell plate potential. The third voltage lowering circuit is a source follower type circuit having an N-channel MOS transistor. The third voltage lowering circuit is a circuit for creating a third potential by lowering the power supply voltage supplied from the exterior and supplying a thus created lowered power supply voltage to all of the other internal circuits except the above circuits.

Step-up/down AC voltage regulator using transformer with tap changer and PWM AC chopper

Abstract: A step-up/down AC voltage regulator is proposed, in which a tap-changing transformer and a pulsewidth modulation (PWM) AC chopper are combined. The proposed regulator can step up or down the output voltage to input voltage. Also, the proposed regulator restrains more harmonics of output voltage compared to the conventional PWM regulator. The input current flows continuously in the proposed regulator, while it flows discontinuously in the conventional PWM regulator. Through digital simulation, several characteristics are investigated theoretically and then compared with those of conventional schemes. Practical verification of the theoretical predictions is presented to confirm the capabilities of the proposed regulator.

Constant-current driving circuit

Abstract: PROBLEM TO BE SOLVED: To enable a constant-current driving circuit, which drives a load to be subjected to current/voltage conversion and a capacitor connected in parallel with the load by a constant current, to shorten the rising time of the current flowing to the load. SOLUTION: A capacitor 15 is charged at the time constant determined by a constant-current value from a transistor 8 and the capacitance of the capacitor 15 until its potential reaches a specified voltage which is obtained by converting the constant current of the transistor 8 into a voltage through a load 14. In this case, the reference voltage at the reference voltage terminal 10 of a reference voltage source 9 is set at a value which is smaller than the sum of the specified voltage and the gate-source voltage of a transistor 13. Since a switch 12 is set in continuity synchronously with a control terminal 2 similarly to a switch 11, the capacitor 15 connected in parallel with the load 14 is quickly charged to the voltage through the transistor 13. When the capacitor 15 reaches the voltage, the transistor 13 is disconnected, thereafter, the capacitor 15 is charged to a specified voltage with the constant current of the transistor 8. COPYRIGHT: (C)2000,JPO

Control design and dynamic performance analysis of a wind turbine-induction generator unit

Abstract: This paper presents the modeling and control design for a wind energy conversion scheme using induction generators. The scheme consists of a three-phase induction generator driven by a horizontal axis wind turbine and interfaced to the utility through a double overhead transmission line. A static VAr compensator was connected at the induction generator terminals to regulate its voltage. The mechanical power input was controlled using the blade pitch-angle. Both state and output feedback controllers are designed using MATLAB software to regulate the generator output. From the simulation results, the response of closed loop system exhibited a good damping and fast recovery under different type of large disturbances.

Charge/discharge control circuit and chargeable electric power source apparatus

Abstract: A charge/discharge control circuit is provided for an electric power source apparatus in which a service life is prolonged. A voltage dividing circuit, an overcharge voltage detection circuit, an overdischarge voltage detection circuit and a control circuit are connected in parallel to a secondary cell which is an electric power source, wherein the control circuit detects a condition of the secondary cell from the overcharge/overdischarge voltage detection circuits and outputs a signal Vs for controlling a power supply to an external equipment and a charge by an external power source and controls a switching element provided in series with the voltage dividing circuit and reduces a current which flows through the voltage dividing circuit.

Single-stage single-switch isolated PFC regulator with unity power factor, fast transient response and low voltage stress

Abstract: In this paper, a simple control method is presented for a single-stage single-switch isolated power-factor-correction (PFC) regulator that can simultaneously achieve unity power factor and fast output voltage regulation while keeping the voltage stress of the storage capacitor low. The converter topology comprises essentially a cascade combination of a discontinuous-mode boost converter and a continuous-mode forward converter. The proposed control utilizes variation of both duty cycle and frequency. The role of varying the duty cycle is mainly to regulate the output voltage. Changing the frequency, moreover, can achieve unity power factor as well as low-voltage stress. Basically, the switching frequency is controlled such that it has a time periodic component superposed on top of a static value. While the time periodic component removes the harmonic contents of the input current, the static value is adjusted according to the load condition so as to maintain a sufficiently low-voltage stress across the storage capacitor. The theory is first presented which shows the possibility of meeting all three requirements using a combined duty cycle and frequency control. An experimental prototype circuit is presented to verify the controller's functions.

An analysis of midpoint balance for the neutral-point-clamped three-level VSI

Abstract: The three level voltage source inverter (VSI) is a good topology for high voltage and high power applications where no semiconductor devices are available. However, it has an inherent problem of midpoint charge balance. The problem becomes critical when the load is nonlinear and/or has large phase angle and harmonics. If it exceeds the critical points, there is no way to control the midpoint voltage by itself. The midpoint balancing problem is analyzed with respect to the reactive loads with harmonic currents and a way to solve the problem is suggested.

Luo-converters, voltage lift technique

Abstract: The voltage lift technique is a popular method widely applied in electronic circuit design. Since the effect of parasitic elements limits the output voltage and power transfer efficiency of DC-DC converters, the voltage lift technique opens a good way to improve circuit characteristics. After long term research, this technique has been successfully applied for DC-DC converters. Luo-converters are a series of new DC-DC step-up (boost) converters, which were developed from prototypes using voltage lift technique. These converters perform positive to positive DC-DC voltage increasing conversion with high power density, high efficiency and cheap topology in simple structure. They possess high output voltage with small ripples. Therefore, these converters will be widely used in computer peripheral equipment and industrial applications, especially for high output voltage projects.

Controlled shunt capacitor self-excited induction generator

Abstract: This paper proposes a new self-excited induction generator (SEIG) voltage regulation scheme. The proposed SEIG scheme utilizes the concept of a continuously controlled capacitor and is called the controlled shunt capacitor SEIG. Anti-parallel IGBT switches are used across the fixed excitation capacitors to regulate the voltage across a 7.5 hp induction generator. The experimental results and those obtained by simulation are similar. The experimental results prove that the controlled shunt capacitor SEIG maintains a constant terminal voltage over wide variety of loads and changes in speed, and hence is a reliable and cost-effective electric generator control system.

DC power supply apparatus

Abstract: DC power supply apparatus includes an input-side rectifier for converting an AC signal from an AC power supply into a DC signal. A DC-to-high-frequency converter converts the DC signal into a high frequency signal in response to a control signal from a control unit. An output-side rectifier converts the high frequency signal into a DC signal and applies it to a load. An input current detector detects an input current flowing through the input-side rectifier and generates a signal representative of the input current. A load current detector detects a load current flowing through the load and generates a signal representative of the load current. The control unit receives the input-current and load-current representative signals, a reference input current signal and a reference load current signal, and provides the DC-to-high-frequency converter with the control signal such that the input-current representative signal can be equal to the reference input current signal and the load-current representative signal can be equal to the reference load current signal.

Characteristic of voltage dips (sags) in power systems

Abstract: This paper analyses the characteristic of voltage dips in power systems with special emphasis on unbalanced dips and symmetrical components analysis. A generalized dip model is built for all unbalanced dips taking into account the fault types, transformer types and load connections, which shows the transformation for different types of unbalanced dips. The characterization results in a so-called "remaining complex voltage" (a generalized magnitude and phase angle shift) for balanced and unbalanced dips and a factor, related to the rotating machine contribution to the source impedance for unbalanced dips. It is shown that the factor is close to unity, so that both balanced and unbalanced dips can be characterized through one phasor.

Full bridge dc-dc converters

Abstract: A full bridge DC-DC converter has four switching devices in two switching legs each of two devices connected in series betwen two supply voltage terminals, junction points of the legs being connected to a primary winding of a transformer, from a secondary winding of which an output voltage of the converter is derived by rectifying and filtering. The output voltage is regulated by phase shift control of the switching devices. Zero voltage switching (ZVS) of the switching devices under potentially all converter load conditions is provided by snubber capacitors, connected in parallel with the switching devices, in conjunction with two inductors each connected between the junction point of a respective one of the switching legs and a point at a voltage midway between the voltages of the supply voltage terminals, which can be provided by a capacitive voltage divider between the supply voltage terminals. The inductors typically have different inductances, and one of the inductors may be omitted.

Voltage down converter for multiple voltage levels

Abstract: A voltage regulator circuit in an integrated circuit (IC) device such as a Complex Programmable Logic Device (CPLD) comprises a reference voltage generator, a tuning circuit, and an output driver circuit. The reference voltage generator converts an external supply voltage provided to the IC device into a stable reference voltage. The tuning circuit converts the stable reference voltage into a desired internal supply voltage, such as the reduced voltage required by deep sub-micron transistors. The output driver circuit provides the desired internal supply voltage with sufficient current to properly power the circuits of the IC device. The tuning circuit includes an op-amp and resistive elements configured in a voltage divider configuration in the negative feedback loop of the op-amp. The output of the op-amp can be set to the desired internal supply voltage by properly sizing the resistive elements. By making at least one of the resistive elements adjustable, a variable internal supply voltage can be provided by the voltage regulator circuit.

An integrated maximum power point tracker for photovoltaic panels

Abstract: This paper proposes a maximum power point tracker (MPPT) for a photovoltaic panel, that is to be integrated with the panel during manufacturing. The MPPT is inexpensive, efficient and has few components that serve to increase the MPPT's mean time between failures (MTBF). The MPPT uses an inexpensive microcontroller to perform all of its functions. This includes maximum power point tracking, series battery voltage regulation, sensorless short circuit protection of the MPPT's converter and intelligent shutdown and wakeup at dusk and dawn. The MPPT can source 10 A to a 6-36 V lead-acid storage battery and can be connected in parallel or series with other MPPTs. The MPPT may be easily configured to perform output voltage regulation on passive and water pumping loads. It could also control the actuation of a diesel generator in a hybrid remote area power supply (RAPS). Energy transfer enhancements of up to 26%, compared to solar panels without MPPTs, have been measured, The complete component and materials cost of the MPPT is approximately 2.8% of the cost of photovoltaic panels with a peak power rating of 154 W. The integrated MPPT also consumes no stored energy at night.

Unified approach to power quality mitigation

Abstract: A unified approach to the compensation of multiple power quality phenomenon is described in this paper. It is shown that the converter-based power quality mitigation device is a viable solution to power quality problems on power networks since more than one power quality problem can be compensated by the same hardware. These devices should be capable of harmonic compensation or isolation, improving dynamic voltage regulation and flicker simultaneously, while capable of controlling power factor and power flow through the network. These functions can also be implemented independently. These power quality mitigating devices can be hybrid devices comprising passive components and converters. The drive is to minimize the converter power rating while maximizing the compensating performance. Several projects where this approach has been implemented are discussed in this paper.

Voltage booster for enabling the power factor controller of a LED lamp upon low ac or dc supply

Abstract: The light-emitting-diode lamp comprises a set of light-emitting diodes, a rectifier and power converter circuit, and a power factor controller. The rectifier and power converter circuit converts ac or dc voltage and current from a power source to dc voltage and current supplied to the set of light emitting diodes. The rectifier and power converter circuit includes an electronic switch through which it is supplied with ac or dc voltage and current from the power source, and an inductor device including windings adapted to charge a capacitor with a voltage signal representative of the amplitude of the ac or dc voltage from the power source. The power factor controller is responsive to the voltage signal across the capacitor for controlling the electronic switch of the rectifier and power converter circuit in view of supplying dc voltage and current to the set of light emitting diodes while maintaining the power factor of the light-emitting-diode lamp equal to or close to unity. The power factor controller comprises a voltage comparator supplied with the voltage signal across the capacitor for enabling operation of the power factor controller as long as the ac or dc voltage from the power source has an amplitude higher than a first predetermined voltage threshold. A voltage boosting circuit raises the amplitude of the voltage signal across the capacitor when the ac or dc voltage from the power source is lower than the first voltage threshold to keep operation of the power factor controller enabled as long as the ac or dc voltage is higher than a second predetermined voltage threshold lower than the first predetermined voltage threshold.

Redundant voltage supply for motor vehicle electrical load

Abstract: The redundant voltage supply has a generator (10) supplying two separate circuits, each containing a voltage storage device, eg a battery (15a,b), connected to the generator and to the supplied load (16) via respective switches (14a,b,17a,b) which are closed in the normal condition and opened to allow a defective circuit to be separated from the generator and the load

On influence of load modelling for undervoltage load shedding studies

Abstract: This paper explores the influence of load models on decisions of undervoltage load shedding in power systems. A controlled load rejection can be used as an emergency countermeasure to avoid widespread blackout when system voltages are unstable. In this paper, dynamic simulations of a small power system using both static and dynamic load models are presented. When using a static load model, the system includes an explicit model of a transformer with load tap changer. The aim is to demonstrate how different load models influence the analysis and calculation of the amount of load needed to be shed to stabilise the system voltage.

A generalized structure of multilevel power converter

Abstract: Multilevel inverters are becoming popular in the past few years for high voltage and high power applications Various topologies and modulation strategies have been investigated for utility and drive applications in the recent literature However, most of the reported topologies for multilevel structures employ DC voltage sources of equal magnitudes This paper is devoted to the investigation of a generalized structure of multilevel power converter where these discrete voltage sources are not necessarily equal The topological structures, operating principles and performance characteristics resulting from this general approach are presented Various design criteria and issues like spectral structure are discussed A comparative evaluation of various alternatives obtained from the generalized structure is presented

Internal voltage generator with reduced power consumption

Abstract: A first periodic pulse is rectified to generate an internal voltage by a charge pump circuit. A level detector is provided for detecting whether or not the internal voltage reaches a desired level. The charge pump circuit is controlled by a controller in accordance with the detection signal so that the internal voltage may take the desired level. A switch element switched by a second periodic pulse is provided in the current path of the level detector. A leakage current path for allowing a lower electric current than the electric current to flow through the former current path is provided between the output terminal of the charge pump circuit and a predetermined power supply terminal.