Showing papers on "Voltage regulation published in 2002"

Voltage rise: the big issue when connecting embedded generation to long 11 kV overhead lines

Abstract: There has been much interest in embedding small generators deep within distribution systems. The steady-state voltage rise resulting from the connection of these generators can be a major obstacle to their connection at the lower voltage levels. This article summarises the results of some generic studies, explaining this voltage rise issue and how it may be overcome. Methods discussed to counteract voltage rise are primary substation voltage reduction, reactive power import, autotransformers installation, conductor upgrading, and generation constraints.

A novel droop method for converter parallel operation

Abstract: For the converter parallel operation, the current sharing between modules is important for the reliability of the system. Among several current sharing schemes, the droop method needs no interconnection between modules, which implies true redundancy. But the droop method has poor voltage regulation and poor current sharing characteristics. In this paper, a novel droop method is proposed for the converter parallel operation, which adaptively controls the reference voltage of each module. This greatly improves the output voltage regulation and the current sharing of the conventional droop method. The analysis of the proposed method and design procedure are provided and experimental results verify the excellent performance of the proposed method.

Voltage sag detection technique for a dynamic voltage restorer

Abstract: Dynamic voltage restorers (DVRs) are used to protect sensitive loads from the effects of voltage sags on the distribution feeder. This paper presents and verifies a novel voltage sag detection technique for use in conjunction with the main control system of a DVR. In all cases it is necessary for the DVR control system to not only detect the start and end of a voltage sag but also to determine the sag depth and any associated phase shift. The DVR, which is placed in series with a sensitive load, must be able to respond quickly to a voltage sag if end users of sensitive equipment are to experience no voltage sags. A problem arises when fast evaluation of the sag depth and phase shift is required, as this information is normally embedded within the core of a main DVR control scheme and is not readily available to either users monitoring the state of the grid or parallel controllers. Previous research presented an additional controller, which required phase and sag depth information to manipulate the injection voltage vector returned by the main controller in order to prevent the DVR injection transformers from saturating. Typical standard information tracking or detection methods such as the Fourier transform or phase-locked loop (PLL) are too slow in returning this information, when either applied to the injection voltage vector, or to the supply voltages directly. As a result of this the voltage sag detection method in this paper proposes a new matrix method, which is able to compute the phase shift and voltage reduction of the supply voltage much quicker than the Fourier transform or a PLL. The paper also illustrates that the matrix method returns results that can be directly interpreted, whereas other methods such as the wavelet transform return results that can be difficult to interpret.

Compensation of distribution system voltage using DVR

Abstract: A dynamic voltage restorer (DVR) is a power-electronic controller that can protect sensitive loads from disturbances in the supply system. In this paper, it is demonstrated that this device can tightly regulate the voltage at the load terminal against imbalance or harmonic in the source side. The behavior of the device is studied through steady-state analysis, and limits to achievable performance are found. This analysis is extended to the study of transient operation where the generation of the reference voltage of the DVR is discussed. Once the reference signals are generated, they are tracked using a switching band scheme. A suitable structure in which the DVR is realized by voltage-source inverters (VSIs) is also discussed. Particular emphasis to the rating of this device is provided. Extensive simulation results are included to illustrate the operating principles of a DVR.

Novel fast voltage stability index (FVSI) for voltage stability analysis in power transmission system

Abstract: Since a couple of decades ago, voltage stability assessment has received increasing attention due to the complexity of power systems. With the increase in power demand and limited power sources has caused the system to operate at its maximum capacity. Therefore, a study that is able to determine the maximum capacity limit before voltage collapse must be carried out so that necessary precaution can be taken to avoid system capacity violation. This paper presents a novel fast voltage stability index (FVSI) simplified from a pre-developed voltage stability index referred to a line initiated from the voltage quadratic equation at the sending end of a representation of a 2-bus system. The line index in the interconnected system in which the value that is closed to 1.00 indicates that the line has reached its instability limit which could cause sudden voltage drop to the corresponding bus caused by the reactive load variation. The formulated index was tested on the IEEE reliability test system in order to verify the performance of the proposed indicator. Results showed that the proposed technique is indicative in predicting the occurrence of system collapse and hence necessary action can be taken to avoid such incident.

Performance improvement of the dynamic voltage restorer with closed-loop load voltage and current-mode control

Abstract: The performance of a dynamic voltage restorer (DVR) in improving the quality of power supply is examined. It is shown that the existing open-loop control strategy used in the DVR to regulate load voltage can produce poorly damped response due to the presence of the switching harmonic filter in the restorer. Damping is shown to be improved if the proposed multiloop controller is used. Furthermore, the new control scheme permits a closer tracking of the reference load voltage under varied load conditions. The analysis of the proposed method is then validated by simulation studies and laboratory experimental tests which show the efficacy of the new control scheme.

Performance improvement of the dynamic voltage restorer with closed-loop load voltage and current-mode control

Abstract: The performance of a dynamic voltage restorer (DVR) in improving the quality of power supply is examined. It is shown that the existing open-loop control strategy used in the DVR to regulate load voltage can produce poorly damped response due to the presence of the switching harmonic filter in the restorer. Damping is shown to be improved if the proposed multiloop controller is used. Furthermore, the new control scheme permits a closer tracking of the reference load voltage under varied load conditions. The analysis of the proposed method is then validated by simulation studies and laboratory experimental tests which show the efficacy of the new control scheme.

UPS inverter design using discrete-time sliding-mode control scheme

Abstract: This paper presents a novel discrete-time sliding-mode control algorithm for an uninterruptible power supply (UPS) inverter design. The approach offers a dual-loop design, in which a current predictor utilizes the tracking error of output voltage to estimate the desired inductor current, while a current controller is adopted to regulate the inductor current and, thus, produces a control command to the pulsewidth modulation inverter. An explicit condition for stable controller design is derived. The efficacy of this scheme is validated via a successful implementation on a digital-signal-processor-based UPS inverter. The proposed scheme has shown its robustness on low output voltage distortion, excellent voltage regulation, and it is insensitive to load variation, even under nonlinear loads. Experimental studies were performed to further validate the effectiveness of this scheme.

High-frequency digital controller IC for DC/DC converters

Abstract: This paper describes a complete digital controller IC for high-frequency switching converters. Novel architecture and configurations of the key building blocks: A/D converter, compensator and digital pulse-width modulator, are introduced to meet the requirements of tight output voltage regulation, highspeed dynamic response, and programmability without external passive components. The implementation techniques are experimentally verified on a prototype chip that takes less than 1 mm/sup 2/ of silicon area in a standard 0.5 /spl mu/ digital CMOS process and operates at the switching frequency of 1 MHz.

Use of load control to regulate voltage on distribution networks with embedded generation

Abstract: Since 1994 Econnect has, in conjunction with Northern Electric PLC, investigated the use of consumer load control as a new and innovative method to actively regulate distribution system voltage when affected by the operation of embedded generators [1],[2]. There are a number of issues that can limit the installed capacity of embedded generators; these are often voltage related, and the most common is steady state voltage rise. A number of techniques can be applied to limit steady state voltage rise, some of which are static in time (e.g., network reinforcement) and some dynamic (e.g., power factor control). This paper discusses the issue of excess steady state voltage rise and the methods of limitation that can be applied with specific reference to wind generation. The new and innovative approach using consumer load control is discussed and compared with the existing methods using a simulation case study.

Fuzzy-based reactive power and voltage control in a distribution system

Abstract: This paper presents fuzzy-based reactive power and voltage control in a distribution system. The main purpose is to find the combination of main transformer load tap changer (LTC) positions and capacitors on/off switching operations in a day, such that the voltage deviations at the secondary bus of main transformer become as small as possible, while the reactive power flows through the main transformer and the real power losses at feeders become as little as possible. To minimize system repair cost, the total number of switching operations of LTC and capacitors in a day must be kept as few as possible. From the descriptions above, the linguistic expressions such as "as small as possible," "as little as possible," and "as few as possible" are not clear. So in this paper, the reactive power and voltage control problem is first formulated with fuzzy sets then an annealing searching technique is used to find a proper combination of LTC positions and capacitors on/off switching operations in a day. To demonstrate the effectiveness of the proposed method, reactive power and voltage control in a distribution system within the service area of Yunlin District Office of Taiwan Power Company (TPC) are analyzed. It is found that a proper dispatching schedule for LTC positions and capacitors switching operations can be reached by the proposed method.

Operation of a DSTATCOM in voltage control mode

Abstract: This paper presents the operating principles of a distribution static compensator (DSTATCOM) that is used to maintain the voltage of a distribution bus. A three-phase, four-wire distribution system is assumed in this study. A three-phase bridge inverter circuit that is supplied by two neutral-clamped dc storage capacitors realizes the DSTATCOM. Three filter capacitors, one for each phase, are connected in parallel with the DSTATCOM to eliminate high-frequency switching components. The voltage across the filter capacitor is controlled by a dead-beat controller to maintain the ac bus voltage. The magnitude of the bus voltage is chosen as a nominal value, i.e., 1.0 per unit, while its phase angle is obtained through a feedback loop that maintains the voltage across the dc storage capacitors. Through detailed simulation and experimental results it has been shown that the DSTATCOM can maintain the voltage against any unbalance and distortion in either the load or supply side.

Control method and circuit to provide voltage and current regulation for multiphase DC/DC converters

Abstract: A multiphase DC/DC converter with voltage-mode hysteretic control and instantaneous current sharing functionality method and circuit. The disclosed method and circuit provides independent output voltage regulation and phases current regulation for the converter in two separate loops; one of which is concerned with voltage regulation preferably hysteretic regulation and the other which is concerned with phase current regulation. In one embodiment, each phase in the DC/DC converter is driven by a frequency divided signal derived directly from the output voltage ripple so that each phase has a switching frequency equal to the output voltage ripple frequency divided by the number of paralleled phase. In another embodiment, only the phase that carries the smallest current among a set of phases will be turned ON in each switching cycle to supply power to the output, avoiding multiple switching. Current sharing and regulation is achieved without shortening the ON time and with no multiple switching in any cycle.

Stability and dynamic performance of current-sharing control for paralleled voltage regulator modules

Abstract: The parallel operation of voltage regulator modules (VRMs) for high-end microprocessors requires a current-sharing (CS) circuit to provide a uniform load distribution among the modules. A good dynamic performance of the CS circuit is very important since the microprocessors present highly dynamic loads to the VRMs. Stability and dynamic performance of the CS control are considered. To assess these issues, a comprehensive small-signal model of the paralleled VRMs was developed and verified.

Understanding and solving short-term voltage stability problems

Abstract: Based on actual incidents, short-term voltage instability is an increasing, but often overlooked, industry concern. A common scenario is a large disturbance such as a multi-phase fault near a load center that decelerate motor loads. Following fault clearing with transmission outages, motors raw very high current while simultaneously attempting to reaccelerate, and may stall if the power system is weak. Massive loss of load and possibly area instability and voltage collapse may follow. The authors describe actual incidents. Fast-acting generator excitation controls, fast-acting reactive power support devices (SVC, STATCOM, SMES), or fast load shedding can prevent voltage collapse. Proper analysis requires dynamic modeling of aggregated motor loads, with equivalents for distribution feeders. Power electronic based voltage support devices must be realistically modeled to determine required size, location, number, and type. Based on simulations, they conclude that voltage-sourced converter devices (STATCOM, SMES) are attractive countermeasures against load loss and voltage collapse. Factory built distribution-connected distributed devices may be cost-effective compared to larger transmission-connected devices.

Vector current controlled voltage source converter-deadbeat control and saturation strategies

Abstract: This paper presents a voltage source converter connected to a grid with software specially designed for limited control voltage. The voltage source converter uses a deadbeat vector current controller. The paper deals with limiting reference voltage, integrator windup and delay time compensation. Simulations and experimental verifications of the proposed controller are included.

System, device and method for providing voltage regulation to a microelectronic device

Abstract: The present invention provides a power regulation system and method with high speed signal settling capabilities for providing rapid active transient response to a microelectronic device. An active transient response system includes a power supply configured to receive external and/or internal signals indicating the occurrence of transient load conditions and to respond to the transient load conditions based on one or more of these signals. The system may further include a transient suppressor configured for early detection of transients, assisting in transient suppression, and early signaling of transient activity to the power supply. The system provides rapid recovery to steady state operation from the active transient response mode by using a digital compensator to quickly modifying the duty cycle and provide a voltage offset proportional to the transient microprocessor load step. Recovery is further improved by current rephasing techniques.

Adaptive off-time control for variable-frequency, soft-switched flyback converter at light loads

Abstract: The soft switching of a flyback converter can be achieved by operating the circuit in the critical conduction mode. However, the critical-mode operation at light loads cannot be maintained due to a very high switching frequency and the loss of the output voltage regulation. A control which regulates the output down to the zero load and maintains soft switching at light loads is proposed. The proposed control scheme was implemented in the 380 V/19 V, 65 W flyback DC/DC converter.

Voltage recovery after unbalanced and balanced voltage dips in three-phase systems

Abstract: This paper studies the recovery of the voltage after a voltage dip due to a fault in a three-phase system. The instant of voltage recovery corresponds to the instant of fault clearing. For single-phase and phase-to-phase faults, a single point-on-wave of voltage recovery can be defined. For two-phase-to-ground and three-phase faults, the recovery takes place in two or three steps. The voltage recovery is described in a systematic way by using a classification of three-phase unbalanced voltage dips. The voltage recovery needs to be modeled correctly for studies of equipment immunity against voltage dips.

Resonant frequency tracking system and method for use in a radio frequency (RF) power supply

Abstract: An RF power supply that is capable of tracking rapid changes in the resonant frequency of a load and capable of quickly responding to varying load conditions so as to deliver the desired amount of power. The present invention also provides an RF power supply capable of delivering a wide range of power over a broad frequency range to a load that is remotely located from the power supply. According to one embodiment, the RF power supply includes a direct current (DC) voltage source that provides a DC voltage within a predetermined voltage range; an amplifier, coupled to the DC voltage source, that provides an alternating voltage to a tank circuit connected to an output of the RF power supply; a frequency controller, coupled to the amplifier, to set the frequency of the alternating voltage produced by the amplifier; and a sensor, coupled to the load, to provide a signal to the frequency controller, where the frequency controller sets the frequency of the alternating voltage based on the signal received from the sensor.

Boost-buck cascade converter for pulsating loads

Abstract: A power converter including a first stage, a reservoir capacitor and a second stage. The first stage converts the voltage of a primary energy source, such as a battery, to a voltage on the reservoir capacitor, which stores a large amount of energy in the form of a voltage substantially larger than the voltage of the primary energy source. The second stage converts the voltage on the reservoir capacitor to a substantially constant voltage for a load device that demands current having the form of large, short, current pulses. The cascaded converter prevents the pulsating load currents of the load device, such as a GSM power amplifier, from causing a severe voltage loss at the battery. This increases the power available from the battery and reduces loses from the internal resistance of the battery.

Global Transient Stability and Volt age Regulation for Power Systems

Abstract: This paper concerns the global control of power systems. It arises from the practical concern that transient stability and voltage regulation are both important properties of power system control, but they are ascribed to different model descriptions and relate to different stages of system operation (i.e. transient period and post-transient period respectively). Earlier control results deal with the two problems separately, or employ a switching strategy of two different kinds of controllers, which causes a discontinuity of system behavior. We design in this paper a global controller to co-ordinate the transient stabilizer and voltage regulator. The designed controller is smooth and robust with respect to different transient faults. Simulations on a single-machine infinite bus power system have demonstrated better performances compared with existing controllers.

Multiphase clamp coupled-buck converter and magnetic integration

Abstract: Voltage regulation, transient response and efficiency of a voltage regulator module (VRM) is improved where short duty cycles are necessitated by large differentials of input and output voltage by including at least one clamping of a tap of an inductance in series with an output of each of a plurality of parallel branches or phases which are switched in a complementary fashion or providing coupling between inductors of respective phases. Such coupling between inductors is achieved in a small module with an integrated magnetic structure. Reduced component counts are achieved while deriving built-in input and output filters. Principals of the invention can be extended to isolation applications and push-pull forward converts, in particular. A lossless clamping circuit is also provided allowing spike currents to be suppressed while returning power to the output of the VRM.

Switched charge voltage driver and method for applying voltage to tunable dielectric devices

Abstract: A circuit for providing a control voltage for tunable dielectric devices, the circuit comprising an input for receiving a voltage command signal, a charging circuit for establishing a desired voltage level on a first capacitor in response to the voltage command signal, and a switch for switching voltage on the first capacitor to a tunable dielectric device to control a dielectric constant of tunable dielectric material in the tunable dielectric device. A second capacitor, second charging circuit, and second switch can be used to provide voltage to the tunable dielectric device during charging or discharging of the first capacitor. The method for providing a control voltage for tunable dielectric devices performed by these circuits is also included.

Analysis and design of a new voltage sag compensator for critical loads in electrical power distribution systems

Abstract: In this paper, a new voltage sag compensator for powering critical loads in electric distribution systems is discussed. The proposed scheme employs a pulsewidth-modulation AC-AC converter (four insulated gate bipolar transistors per phase) along with an autotransformer. During a disturbance such as voltage sag, the proposed scheme supplies the missing voltage and helps in maintaining rated voltage at the terminals of the critical load. The approach does not employ any energy storage components such as bulk capacitors/inductors and provides fast response at low cost. Under normal conditions the approach works in bypass mode, delivering utility power directly to the load; this method of control allows the transformer to be rated only for transient conditions, thus reducing its required size. A four-step switching technique to drive the AC-AC converter is employed to realize snubberless operation. A design example is presented, and simulation results are shown for a three-phase 230-V 5-kVA system. Experimental results on a single-phase unit are discussed. The proposed approach can be easily integrated into a distribution transformer supplying critical loads.

Microgrids and sensitive loads

Abstract: This paper discusses the issues related to the operation of a microgrid being called to feed reliable electric power to sensitive electrical loads. Challenges and solutions involved in terms maintaining adequate voltage quality and load tracking capabilities are discussed in the paper.

Compensation of Distribution System Voltage Using Dynamic Voltage Restorer (DVR)

Abstract: A dynamic voltage restorer (DVR) is a power electronic controller that can protect sensitive loads from disturbances in the supply system. This device can tightly regulate the voltage at the load terminal against imbalance or harmonic in the source side. The behavior of the device is studied through steady-state analysis, and limits to achievable performance are found. This analysis is extended to the study of transient operation where the generation of the reference voltage of the DVR is discussed. Once the reference signals are generated, they are tracked using a switching band scheme. A suitable structure in which the DVR is realized by voltage source inverters is also discussed. Particular emphasis on the rating of this device is given. Extensive simulation results are included to illustrate the operating principles of a DVR.

On-line voltage stability based contingency ranking using fast voltage stability index (FVSI)

Abstract: Voltage instability is one phenomenon that could happen in a power system due to its stressed condition. The result would be the occurrence of voltage collapse which leads to total blackout to the whole system. Therefore voltage collapse prediction is important in power system planning and operation so that the occurrence of voltage collapse due to voltage instability could be avoided. Line outage in a power system could also lead to the event of voltage collapse which implies the contingency in the system. Line outage contingencies are ranked so that the line which highly affects voltage stability of the system when there is an outage occurs in this line could be identified. The contingency ranking process can be conducted by computing the line stability index of each line for a particular line outage and sort them in descending order. The contingency which is ranked highest implies that it contributes to system instability. This paper presents a new voltage stability index refers to a line namely fast voltage stability index (FVSI). The values of the line indices indicate the voltage stability condition in the system and it is used to rank the line outage contingency. The information from the contingency ranking denotes the severity of the voltage stability condition in a power system due to line outage. The proposed contingency ranking technique was tested on an IEEE reliability test system.

Method and apparatus for maintaining a constant load current with line voltage in a switch mode power supply

Abstract: A power supply including a regulation circuit that maintains an approximately constant load current with line voltage. In one embodiment, a regulation circuit includes a semiconductor switch and current sense circuitry to sense the current in the semiconductor switch. The current sense circuitry has a current limit threshold. The regulation circuit current limit threshold is varied from a first level to a second level during the time when the semiconductor switch is on. One embodiment of the regulation circuit is used in a power supply having an output characteristic having an approximately constant output voltage below an output current threshold and an approximately constant output current below an output voltage threshold.

New two-inductor boost converter with auxiliary transformer

Abstract: A new, two-inductor, two-switch boost converter topology and its variations suitable for applications with a large difference between the input and output voltage are described. The output voltage regulation of the proposed converters is achieved in a wide load and input-voltage range with constant-frequency control by employing an auxiliary transformer that couples the current paths of the two boost inductors.