Showing papers on "AC power published in 1988"

Power electronics in electric utilities: static VAR compensators

Abstract: The author deals with dynamic VAR compensation of electric power systems, applying power electronics for reactive power generation and control. After an overview of the emergence and status of modern, solid-state VAR compensators in utility and industrial applications, it is shown how dynamic VAR compensation increased transmittable power by providing voltage support, transient stability improvement, and power oscillation damping in electric power transmission systems. Methods of reactive power generation and control using thyristor-controlled reactors, with fixed and thyristor-switched capacitors or modern gate-turn-off (GTO) power converters that can function without AC capacitors or reactors, are described. A summary is included of the control structure and operation to provide the desired characteristics and performance in power systems applications. >

Application of superconducting magnet energy storage to improve power system dynamic performance

Abstract: The application of superconducting magnet energy storage (SMES) to the stabilization of a power system with long-distance bulk power transmission lines which has the problem of poorly damped power oscillations is presented. Control schemes for stabilization using SMES capable of controlling active and reactive power simultaneously in four quadrant ranges are proposed. The effective locations and the necessary capacities of SMES for power-system-stabilizing control are discussed in detail. Results of numerical analysis and experiments in an artificial power-transmission system demonstrate the significant effect of the control by SMES on the improvement of power-system oscillatory performance. >

Intrinsic amplitude limits and optimum design of 9-switches direct PWM AC-AC converters

Abstract: The maximum input-output transformer ratio, or output voltage ability, of direct AC-AC PWM converters is explored. An intrinsic limit, independent of the control algorithm, is found. A novel converter control algorithm is discussed that achieves the maximum output amplitude and displays some interesting features. The implementation of AC-AC converter control using feedback techniques is considered, and a feedback-based control algorithm is proposed. >

10 MW GTO converter for battery peaking service

Abstract: A bidirectional 18-pulse voltage-source converter utilizing gate-turnoff thyristors (GTOs) is described. The converter, which is rated 10 MVA, was placed in service in early 1988 to connect an energy storage battery to a utility grid. The converter is rated and controlled to operate in all four quadrants (discharge, charge, leading VAr, or lagging VAr) at the full 10 MVA rating. It is capable of independent rapid control of real and reactive power with a transient response of 16 ms to changes in commanded value of real or reactive power. Thus it is usable as a reactive power controller (static VAr control), voltage control, frequency control, power system stabilizer, or real power peaking station. For use as a reactive power controller only, no battery would be needed. The design, construction, control, and application of the converter are described, and performance data taken at factory power test and at the installation are given. >

A single-phase controlled-current PWM rectifier

Abstract: The analysis and test results are given for an experimental single-phase controlled-current PWM (pulse-width-modulated) rectifier that operates at unity power factor with near sinusoidal current waveform and that has power reversal capability. The twice-line-frequency AC power is identified as a source of voltage harmonics in the DC link. The harmonics enter into the voltage regulation feedback loop to distort the AC current waveform. These undesirable harmonics can be removed by a low-pass filter. It is noted that the overall design must address the possibility of instability due to the low-pass filter in the feedback path. >

MW/voltage control in a linear programming based optimal power flow

Abstract: A functional improvement to the linear-programming-based optimal power flow technique is reported. The new feature allows the rescheduling of the active power controls to correct voltage-magnitude-constraint violations. It preserves the reliability and speed characteristics of the traditional linear programming approach. Results demonstrating the effectiveness of the method on a small and a large power system are presented. >

Practical reactive power allocation/operation planning using successive linear programming

Abstract: A method for reactive power planning is presented that it finds an optimal solution for both allocation and operation planning in large systems using linear programming (LP). The method utilizes calculated linear sensitivities including active power and voltage phase angle in the formulation. Although the overall method includes these relations, the number of constraints and variables are not augmented in its first procedure, APPROACH-1. Its second procedure, APPROACH-2, overcomes numerical problems caused by a dense constraint matrix. This is achieved by retaining untouched sparse sensitivities in the constraint matrix and by eliminating any calculations related to the inverse matrix. The results of applying this method to a practical 224-bus system and the IEEE-30 bus system verify its robustness and fast convergence. >

High-frequency series-resonant DC link power conversion

Abstract: An AC/DC conversion scheme is presented which eliminates the need for self-commutated devices and requires only 12 thyristors for full double bridge AC to AC power conversion. The system utilizes a series resonant DC link between the AC/DC and DC/AC converters. This series resonant scheme is, in effect, the dual of the parallel DC resonant converter. The DC resonant circuit can be essentially considered as a commutating circuit which ensures turn-off of all twelve thyristors by providing the necessary zero-current instants. A significantly improved sinusoidal current waveform can be obtained at both the input and output compared to conventional high-power converters by the use of high-frequency pulse density modulation. >

Efficient bounding method for linear contingency analysis

Abstract: A very efficient contingency analysis method for detecting branch megawatt flow violation is presented. The efficiency stems from the use of a bounding criterion that drastically reduces the number of branch-flow computations and limits checking, and the use of state-of-the-art compensation and sparse matrix/vector methods. The method requires no offline setup, is highly efficient, and can handle contingencies with any time of network topology and load/generation changes. The method is based on the linear incremental-power-flow model and consequently does not consider reactive power. >

Harmonic current and reactive power compensation with an active filter

Abstract: A static VAr compensator (SVC) using an active filter has been developed that compensates reactive power, harmonic current, negative-phase current, and voltage fluctuations. The system configuration is described, and five types of control scheme for the filter, which are based on practical applications for various loads, and the performance characteristics for each type of control are analyzed. The active filter is shown by simulation to be more effective for suppressing arc-furnace flicker than the TCR (thyristor-controlled-reactor) SVC. >

Static power conversion method and apparatus having essentially zero switching losses and clamped voltage levels

Abstract: A high efficiency power converter is achieved utilizing a resonant DC link between a DC source, such as a converter rectifying power from an AC power system, to a variable frequency voltage source inverter. A resonant circuit composed of an inductor and capacitor is connected to the DC power supply and to a DC bus supplying the inverter and is caused to oscillate stably at a high frequency to provide a uni-directional voltage across the DC bus which reaches zero volts during each cycle of oscillation of the resonant circuit. The switching devices of the inverter are controlled to switch on and off only at times when the DC bus voltage is zero, thereby eliminating switching losses in the inverter. The resonant circuit can be caused to oscillate utilizing pairs of switching devices in the inverter or a separate switching device across the capacitor, which again are caused to switch on and off only at times of zero voltage on the DC bus. For AC to AC conversion, enabling bi-directional power flow, the switching devices of the power source which converts AC power to DC power may have switching devices which are also switched only at the times of zero voltage so that switching losses in these devices is also minimized. A clamp limits the maximum voltage applied to the switching devices, thereby reducing voltage stresses on the devices, and preferably returns energy to the resonant circuit during each cycle of oscillation.

Secondary voltage control using pilot point information

Abstract: The authors study the minimal information structure needed for monitoring and control of the voltage profile and the reactive power flow of a power system. Emphasis is on the minimum amount of information (data) required for reliable decision-making. An effort is made to work with fewer voltage data and therefore make the real-time monitoring and control more manageable. An annealing algorithm for selecting pilot points is presented. These are the load buses at which the voltage is to be measured in real time. Simulation results obtained with the algorithm for the Central Illinois Light Company system are presented. >

Determination of voltage stability limit in multimachine power systems

Abstract: A method of determining the load limit and the critical state of a general multimachine power system is presented. In the method, the search for the load limit is formulated as an optimization problem. With this formulation, difficulties related to singularity of the load-flow-equations Jacobian matrix, and convergence of the load-flow solution around the voltage stability limit, are avoided. Modification to the basic formulation to allow consideration of load-voltage characteristics is also shown. A voltage stability margin is defined which may serve as a measure of the security of a given operating condition from voltage instability or collapse. Results of application of the proposed method to the AEP 14-bus network are presented. >

An expert system for voltage and reactive power control of a power system

Abstract: A methodology called the sensitivity tree, which can be easily used to form an expert system for real-time control, is proposed. Based on this methodology, an expert system for the control of voltage and reactive power of a power system is developed. The main objective of this expert system is to help the operator detect buses experiencing abnormal conditions, select the most effective control measures, and calculate the control actions required to overcome the voltage violation. The control measures used to alleviate the voltage problem are capacitor compensation, transformer tap changes, and generator terminal voltage changes. By keeping the bus voltage in the entire system within limits, system security is increased. The expert system is written in the PROLOG language. Simulation studies with this expert system applied to a 30-bus power system show satisfactory results. >

A novel harmonic power filter

Abstract: A novel harmonic power filter system consisting of passive LC filters and voltage source pulsewidth-modulated (PWM) converters is described. This filter system has the merits of both conventional passive LC filters and conventional active power filters. The operating principle and a control scheme for the system are described. Compensation characteristics are discussed from both a theoretical and experimental point of view. The practicability and validity of compensation for large-capacity thyristor converters is experimentally demonstrated. >

Backup electrical system for lamps

Abstract: A backup power system is provided for fluorescent or other gas discharge lamps which energizes one of the lamps when normal AC mains power is not available The circuit operates with a standard lamp fixture which contains two or more fluorescent lamps When AC mains current is available, both lamps are operational so that the lighting fixture produces a maximum brightness The AC mains current also trickle-charges a low-voltage battery contained in the lighting fixture When mains current is not available, the battery maintains only one of the lamps lighted Consequently, although the lighting fixture produces a reduced output, the battery life is extended In one embodiment both fluorescent lamps are powered directly from the AC line When AC power is not available, a relay disconnects the AC line from both lamps and connects one lamp to a DC/AC inverter which is powered by the internal battery In another embodiment, one lamp is powered directly from the AC line and the other lamp is powered by a DC/AC inverter which receives power either from a DC power supply operating off the AC line or from the internal low-voltage battery In a third embodiment, during normal operating conditions, both lamps are powered by separated DC/AC inverters driven from a power supply that operates off the AC line When the AC voltage is not present, the DC/AC inverter for one lamp is powered by the internal low-voltage battery

Engine start type VSCF generating system

Abstract: An AC exciter and a main AC generator are mounted on a common rotating shaft of an engine, and a power rectifier and a power inverter are connected in series to the output of the main AC generator. In starter mode, an external AC power source is connected by a switch to the input of the power rectifier, and the output of the power inverter is applied to an armature winding of the main AC generator to drive it as a nocommutator motor to start the engine. A permanent magnet AC generator is also mounted on the common shaft of the engine to provide DC excitation to the AC exciter after converting the output of the permanent magnet AC generator into DC current, and the output of the permanent magnet AC generator is further utilized to detect the rotational speed of the engine to control the switch.

Emergency lighting system

Abstract: This is an emergency lighting system which permits at least one gas discharge lamp to be operated from an AC power source when AC current is present and from a battery when AC signal is not present. The circuit also provides the capability for turning the lamp on under other selected emergency conditions such as in response to an intruder detection, or in response to detection of smoke, heat, water, or some other emergency. In order to converse battery power in the DC mode, controls are also provided for turning off the light or reducing its output level in response to such control inputs as an ambient light detector or manually operated dimmer control. The system also provides a number of other improved features which cause the system to operate more efficiently than prior systems for performing the same function, thus providing greater energy to the lamp with the same source energy input, and reducing the heat generated by the system. Features are also provided for protecting the battery and other components against damage.

Electrostatically activated gating mechanism

Abstract: An electromechanical gating mechanism comprises an electrical energy generating system for generating electrical energy in response to and utilizing energy derived from mechanical motion consciously generated by an intelligent agency. An electronic decision making apparatus is solely powered by the electrical energy generated by the electrical energy generating device. The decision making apparatus, which may include a non-volatile memory, is adapted to receive information, to make one of a set of possible decisions based on the information and on its logic and/or data in its memory and to generate a specific low power electrical output in response to a selected one of the set of possible decisions being made. A mechanical gate has at least two positions. An electrical to mechanical energy conversion device serves for converting electrostatically the low power electrical output into a minute mechanical force/movement and for applying the minute mechanical force/movement to position the gate in a selected one of its positions. An apparatus as set forth above does not require a separate power source, be it a battery or access to AC power. Thus wiring and installation costs are significantly reduced.

Power generating system

Abstract: A power supply system for developing electrical power from variable-speed motive power produced by a prime mover includes a differential speed summer having a first shaft coupled to the prime mover and further having second and third shafts coupled to first and second generators, respectively, a power conditioner coupled to the second generator for conditioning the power developed thereby and means for regulating the conditioned power developed by the second generator so that the first generator develops constant-frequency AC power.

Aircraft engine unducted fan blade pitch control system

Abstract: An electric motor driven blade pitch varying system for the fan blades of an unducted fan type aircraft gas turbine engine utilizes permanent magnet alternating current motors located in the oil sump region of the engine. An alternator driven by the engine generates unregulated variable frequency, variable amplitude power which is rectified and placed on a DC bus. Controlled inverters convert the DC power on the bus to appropriate AC power for the motors. The system incorporates redundancy without significant weight penalty by providing alternators and motors dividing into two independent operating sections on common shafts. Separate electronic circuits are provided to supply power for each sectionalized motor or to rectify power from each alternator section. A common power bus may be used to couple the multiple motors and alternators. The common bus may alternatively receive power from tbe aircraft 400 Hz system or from on-board batteries.

Modular AC output battery load levelling system

Abstract: An electric load levelling system includes a plurality of individual modules for storing and supplying AC electrical power. Each module is preferably contained within a separate enclosure and includes a plurality of individual electrochemical storage cells for storing DC electrical energy. Each enclosure includes a power conversion mechanism for converting input AC power for storage in the enclosed DC cells, and output DC power from the cells to AC for supply to an input/output line. The system is capable of storing electrical energy during periods of off-peak demand, and outputting such stored energy during periods of peak demand. The individual modules function as an "AC battery" having an AC input and output, and are adapted for mass production and ease of interchangeability.

Power system for inductively coupled plasma torch

Abstract: An induction plasma system comprises a torch (10) and an induction coil (14). A sample substance is injected into the plasma at an axial position that is adjustable while the plasma is being energized. The plasma-forming gas flows through the induction coil prior to passing through the plasma torch. A piezoelectric crystal (98) is used for initiating the plasma. An osciallator network generates radio frequency power at a first frequency, and an output LC network (206) that includes the induction coil is tuned to a second frequency higher than the first frequency. Means for maintaining constant power to the plasma includes an AC circuit for duty cycling AC power input to a DC power supply in response to a feedback signal relative to the rectified voltage. Thus a change in the rectified voltage effects an inverse change in the duty cycling such as to nullify the change in the rectified voltage.

Asynchronous multiphase switching gear

Abstract: An asynchronous multiphase switching method and apparatus are disclosed which transfers a system load between two asynchronous AC power sources in an improved manner. It operates as a make-before-break switch to provide uninterrupted power to the system load during the transfer while minimizing voltage and current fluctuations. The control circuitry allows energy to be supplied to the load during the transition without allowing current to flow between the power sources. A matrix of SCRs is used to transfer the load between the two power sources. The switches to be gated are determined by the relative timing relationships of the existing and takeover voltage waveforms. By altering the pattern of the switches which are gated and by controlling the direction of energy flow, the transition can be made between the power sources without interruption of power.

A new control philosophy for power electronic converters as fictitious power compensators

Abstract: A novel control approach that utilizes both thyristor-controlled-reactive sources (TCR), and dynamic power filters (DPF), is proposed for applications in fictitious power compensation. The network parameters for the control loops are calculated from digital-time-domain, cross-correlation signal processing techniques, implemented with the aid of a microcomputer. A measurement system for fictitious power and a converter for dynamic power filters are described. Experimental results obtained with a 150 kVA three-phase passive dynamic power filter are presented. >

Optimal active power dispatch by network flow approach

Abstract: An optimal active power dispatch problem is formulated as a nonlinear capacitated network flow problem with additional linear constraints. Transmission flow limits and both Kirchhoff laws are taken into account. The problem is solved by a generalized upper bounding technique that takes advantage of the network flow structure of the problem. The proposed approach has potential applications to power system problems such as economic dispatch, load supplying capability, minimum load shedding and generation-transmission reliability. The authors also review the use of transportation models for power system analysis. A detailed illustrative example is presented. >

Experiences with implementing optimal power flow for reactive scheduling in the Taiwan power system

Abstract: An OPF (optimal power flow) program based on the Newton methodology has been implemented, as the first step in the development of a VAr (reactive-volt-ampere) planning program for the Taiwan Power Company. The strength of the Newton method was demonstrated by extensive system tests. Of particular importance is the demonstration that binding inequality constraints could be selected and enforced reliably and efficiently. Different levels of functional capabilities regarding infeasible solutions have been identified; and test results showed that they could be satisfied effectively. Detailed test results on the selection and enforcement of binding inequality constraints are presented in an effort to clarify some uncertainties that have persisted in this key aspect of the Newton-OPF methodology. >

Device for reducing the leakage noise of a power converter

Abstract: A device for reducing the leakage noise of a power converter having a rectifier circuit connected to an alternating current power source and an inverter circuit connected to the rectifier circuit has a first noise filter, with a plurality of inductors and capacitors, and which is coupled between the alternating current power source and the rectifier circuit; and a second noise filter, with a plurality of inductors and capacitors, and which is coupled between the rectifier circuit and the inverter circuit.

System performance improvement provided by a power conditioning subsystem for central station photovoltaic-fuel cell power plant

Abstract: An augmented power conditioning subsystem (APCS), which can improve the power system performance besides performing DC/AC power conversion, is proposed. With the benefits provided by the APCS, connecting photovoltaic cells, fuel cells, or their combination to a power system would provide not only additional sources and control of real power, but also additional economic reactive power control as well. The basic principle of the APCS, its operation and control, economic considerations, and the results of a preliminary study are presented. >