Showing papers on "Electrical network published in 1993"

Vector analysis and control of advanced static VAr compensators

Abstract: The advanced static Var compensator (now widely known as the static condenser or STATCON) uses a high power self-commutating inverter to draw reactive current from a transmission line. Two fundamentally different types of invertor can be used for this purpose, one providing control of output voltage magnitude and phase angle, and the other having only phase angle control. For each of these types, the governing equations are derived, and frequency domain analysis is used to obtain the relevant transfer functions for control system synthesis. Further analysis is provided to determine the response of the STATCON to negative sequence and harmonic voltage components on the transmission line. The results are illustrated with measured waveforms obtained from a scaled analogue model of an 80 MVAr STATCON.< >

Point of collapse and continuation methods for large AC/DC systems

Abstract: The implementation of both point of collapse (PoC) methods and continuation methods for the computation of voltage collapse points (saddle-node bifurcations) in large AC/DC power systems is described. The performance of these methods is compared for real systems of up to 2158 buses. Computational details of the implementation of the PoC and continuation methods are detailed, and the unique problems encountered due to the presence of high-voltage direct-current (HVDC) transmission, area interchange power control, regulating transformers, and voltage and reactive power limits are discussed. The characteristics of a robust PoC power flow program are presented, and its application to detection and solution of voltage stability problems is demonstrated. >

New concepts of instantaneous active and reactive powers in electrical systems with generic loads

Abstract: Conventional active and reactive power theory, valid for the steady-state analysis is reviewed. The instantaneous power theory, introduced by H. Akagi et al. (1983, 1984) is also presented. This instantaneous theory is valid for steady and transient states and for generic voltage and current waveforms. Some examples explaining the physical meaning of the new concepts are presented. By using the concepts of symmetrical components together with the new theory, the powers in an unbalanced system are analyzed, including the zero-sequence instantaneous power. An example showing how this theory can be used to design and control an active power filter is presented. Some simulation results are presented and discussed. >

State estimation for electric power distribution systems in quasi real-time conditions

Abstract: This paper proposes a new method for the state estimation of electric power distribution system conditions oriented towards working with minimum number of remote measurements available in the network. This approach will require information concerning the network reconfiguration, remote measurements of voltages, real and reactive power as well as feeder currents in distribution substations. All kinds of statistical information pertaining to distribution transformer loads are used together with real-time remote measurements. In the proposed method, the existing scheme configuration is examined by a configuration pre-screening process based on the addressed references in an oriented graph. The permissible intervals for remote measurement errors and the reliability of statistical information are discussed. A practical example is presented and the exploitation of results has proved its accuracy and efficiency. >

Power flow control by use of controllable series components

Abstract: Power flow control in electric power systems by use of controllable series power capacitors and phase shifters is discussed. Models suitable for incorporation in power flow programs are developed and analyzed. The power flow control problem is defined in a stringent way, and a method for solving the power flow control problem is proposed based on decomposition. This method is applied on test systems and the convergence rate of the algorithm is discussed. Simplified models of the components are also derived. These simplified models are shown to be quite accurate in most situations, and are used to construct regions of feasible power flows of the controlled lines. >

Time varying load analysis to reduce distribution losses through reconfiguration

Abstract: An electrical distribution system reconfiguration algorithm for reducing losses is presented. The algorithm calculates switching patterns as a function of time. Either seasonal or daily time studies may be performed. A load estimation algorithm, which provides load information for each time point to be analyzed, can incorporate any or all of the following: spot loads, circuit measurements, and customer time-varying diversified load characteristics. Voltage dependence of loads is considered at the circuit level. It is shown that switching at the system peak can reduce losses, but may cause a marginal increase in system peak. Data structures used to model loads and to store switch configurations as a function of time are described. Example problems are provided to illustrate results. >

Utilizing HVDC to damp power oscillations

Abstract: Damping of slow oscillations with active and reactive power modulation of high-voltage direct current (HVDC)-links is analyzed with the aim of gaining a physical insight into the problem. The analysis shows that active power modulation is efficient when applied a short mass-scaled electrical distance from one of the swinging machines, and reactive power modulation is most efficient when a well-defined power flow direction exists and the modulation is made at a point close to the electrical midpoint between the swinging machines. It is shown that the intuitively appealing feedback signal frequency and derivative of the voltage are appropriate for active and reactive power modulation. The impact of the constraints imposed by the HVDC equations is analyzed, and it is determined when the implicit reactive power modulation resulting from constant gamma control may be detrimental for the damping. >

Optimal short-term scheduling for a large-scale cascaded hydro system

Abstract: This paper describes a short term hydro generation optimization program that has been developed by the Hydro Electric Commission (HEC) to determine optimal generation schedules and to investigate export and import capabilities of the Tasmanian system under a proposed DC interconnection with mainland Australia. The optimal hydro scheduling problem is formulated as a large scale linear programming algorithm and is solved using a commercially-available linear programming package. The selected objective function requires minimization of the value of energy used by turbines and spilled during the study period. Alternative formulations of the objective function are also discussed. The system model incorporates the following elements: hydro station (turbine efficiency, turbine flow limits, penstock head losses, tailrace elevation and generator losses), hydro system (reservoirs and hydro network: active volume, spillway flow, flow between reservoirs and travel time), and other models including thermal plant and DC link. A valuable by-product of the linear programming solution is system and unit incremental costs which may be used for interchange scheduling and short-term generation dispatch. >

Simulating the dynamic electrothermal behavior of power electronic circuits and systems

Abstract: The simulator solves for the temperature distribution within the semiconductor devices, packages, and heat sinks (thermal network) as well as the currents and voltages within the electrical network. The thermal network is coupled to the electrical network through the electrothermal models for the semiconductor devices. The electrothermal semiconductor device models calculate the electrical characteristics based on the instantaneous value of the device silicon chip surface temperature and calculate the instantaneous power dissipated as heat within the device. The thermal network describes the flow of heat from the chip surface through the package and heat sink and thus determines the evolution of the chip surface temperature used by the semiconductor device models. The thermal component models for the device silicon chip, packages, and heat sinks are developed by discretizing the nonlinear heat diffusion equation and are represented in component form so that the thermal component models for various packages and heat sinks can be readily connected to one another to form the thermal network. >

A simplified network approach to the VAr control problem for radial distribution systems

Abstract: A simplified network approach to the VAr (volt-ampere reactive) control problem in a distribution system with lateral branches is presented. According to this method, the power capacitors are assumed to be located optimally at the feeder branches. The optimal compensation levels (capacitor size) are represented by dependent current sources located at the branch connected buses. The solution of the equivalent circuit for the distribution system yields the values of the voltage at any bus. The actual compensation level is then determined by substituting the bus voltage in the dependent current source formula. The method can be used as an online controller and in the planning stage. It can be easily adapted in an expert system configuration. >

Thermal component models for electro-thermal network simulations

Abstract: A procedure is given for developing thermal component models for electrothermal network simulation. In the new electrothermal network simulation methodology, the simulator solves for the temperature distribution within the semiconductor devices, packages, and heat sinks (thermal network) as well as the currents and voltages within the electrical network. The thermal network is represented as an interconnection of compact thermal component models so that the system designer can readily interchange different thermal components and examine different configurations of the thermal network. To facilitate electrothermal network design, the interconnection of the thermal component models is specified by the user in the same way that the interconnection of the electrical network components is specified. The thermal component models are also parameterized in terms of structural and material parameters so that the details of the heat transport physics are transparent to the user. Examples of electrothermal network simulations are given, and the temperature measurement methods used to validate the thermal component models are described. >

A comparative evaluation of harmonic reduction techniques in three-phase utility interface of power electronic loads

Abstract: Power electronic loads inject harmonic currents into the utility system. This paper presents a comparative evaluation of harmonic reduction techniques which satisfy the current harmonic limits specified by the IEEE Standard 519, and at the same time provide a regulated DC output voltage. The techniques considered include active and hybrid filters, and various current waveshaping approaches for a three-phase utility interface. These techniques are compared in terms of their complexity (number of switches) and their component ratings. Based on the application requirements and the cost of active and passive components, this paper enables the estimation of the minimum cost topology. >

Stability constrained optimal rescheduling of generation

Abstract: An effective approach, based on the transient energy function (TEF) method of power system transient stability analysis, is proposed for stability-constrained rescheduling of the generation and critical line flows for a given initial operating condition and designated contingency. The sensitivities of the energy margin with respect to changes in generation are used in the transient stability constraints, and distribution factors are used to monitor and constrain the critical line flows. The problem is formulated as a multiobjective optimization problem, which is solved using a goal programming algorithm that incorporates an explicit knowledge base. The approach has been successfully tested on two power systems: a 17-generator and a 50-generator test network. >

New synchronization method for thyristor power converters to weak AC-systems

Abstract: An improved approach to obtaining good zero-voltage-crossing signals is presented. These signals are subsequently used as synchronization signals for a phase-controlled thyristor power converter. Detection of accurate zero crossings is possible even when there are large frequency changes, sudden load changes, or large commutation overlap angles. The improved accuracy in the integrity of the zero crossing is obtained by reconstructing a voltage representing the AC source voltage. This voltage is determined from the distorted thyristor converter input voltage, the converter input current, and an online identification of the source impedance using a microcontroller-based adaptive algorithm. The improvement provided by the new zero crossing detection scheme is verified experimentally. >

Refinement of conventional PSS design in multimachine system by modal analysis

Abstract: The design of the lead/lag network in a conventional power system stabilizer (PSS) is intended to provide the correct compensation in order to obtain an electrical torque component in phase with the speed variation. It is shown that, for a multimachine system, the conventional design analysis and synthesis tend to oversimplify the system representation and hence the interaction effects. A more rigorous approach that considers four torque (or power) components instead of the conventional single component is discussed. Using a modal analysis, the significance of these other components is revealed, the more comprehensive treatment using a generalized multimachine representation is justified and a reliable means for optimizing the PSS parameters is presented. >

A new approach for optimal VAr sources planning in large scale electric power systems

Abstract: A new approach is presented for contingency-constrained optimal volt ampere reactive (VAr) source planning in large-scale power systems. Features distinguishing the proposed approach from many of the existing methods include that it allows a more realistic problem formulation, and that it can find the (global) optimal solution. The new problem formulation takes into consideration practical aspects of VAr sources and the load constraints and operational constraints at different load levels. This methodology based on simulated annealing determines the location to install VAr sources; the types and sizes of VAr sources to be installed; and the settings of VAr sources at different loading conditions. To speed up the solution algorithm, the authors make a slight modification of the fast decoupled load flow and incorporate it into the solution algorithm. This method is suitable for large-scale power systems and has been tested on several power systems with promising results. Simulation results on the IEEE 30-bus system and the Tai-power 358-bus system are presented. >

A minimal realization approach to reduced-order modelling and modal analysis for power system response signals

Abstract: The authors report on a numerical scheme for compressing in parametric form small signal electromechanical responses of multimachine power systems, originating from transient stability programs (TSPs) or actual field testing. The result is achieved by using a multi-input multi-output (MIMO) minimal realization algorithm based on singular value decomposition (SVD), which can explicitly take into account the critical impact of the input interactions. The resulting parametric model is a reduced order representation of the underlying complex system, yet it is optimal (in the least-squares sense). Besides compact storage of damping information, the balanced state-space realization as such retains the principal components of the response signals, and could thus be useful for the tuning of static volt-ampere reactive (VAr) systems (SVSs) and power system stabilizers (PSSs). When it is transformed in the modal space, the model also provides insight into modal interaction mechanisms. Several examples are included for illustration purposes and other applications and improvements are also discussed. >

A coordinated control of voltage and reactive power by heuristic modeling and approximate reasoning

Abstract: A flexible approximate reasoning approach to coordinated control of voltage and reactive power in order to enhance the voltage security of an electric power system is presented. The control strategy is expressed by simple rules, which measure the proximity of system state to certain operating conditions, and utilize linear equations to obtain the effective control models. The desired control actions are determined by considering several criteria at the same time. The procedure has been applied to a model system in order to verify its effectiveness. The simulation results show the advantages of this fuzzy modeling approach over conventional expert systems for voltage-reactive-power control. >

The impact of modeling short circuit branches in state estimation

Abstract: The authors discuss the modeling of zero impedance branches (short circuits) in power system state estimation. The concept of state variable in power system state estimation is generalized, and an extended version of the numerical observability method is proposed to cope with complex situations involving multiple short circuit branches, as they occur in some station arrangements. The impact of short circuit exact representation on bad data identification is also discussed. >

Harmonic power flow for unbalanced systems

Abstract: A harmonic power flow method that analyzes harmonics in unbalanced power systems is presented. The developed algorithm has two steps which are executed successively: the first is a fundamental frequency power flow for the AC linear network in which nonlinear loads are represented by current sources; the second is a frequency domain iterative Newton-Raphson method to calculate the harmonics generated by nonlinear loads. In this second step, the AC linear network is represented by a generalized Thevenin equivalent with respect to the nonlinear loads, obtained from the power flow solution. Both linear and nonlinear loads are considered in terms of power. >

A digital method for the identification of the source of distortion in electric power systems

Abstract: The correct identification of the source of distortion in voltages and currents in the electric power systems is an important factor to attain an effective compensation of the non-active components of power. The authors illustrate a method based on the evaluation of the sign of the harmonic active powers that can identify the source of distortion in three-phase systems. The practical application of this method requires the accurate determination of the frequency-domain components of voltages and currents both in amplitude and phase. An instrument is also described based on digital signal processing techniques, specially realized to perform such a frequency-domain analysis in real-time and to indicate whether the current distortion is caused by load nonlinearity or by nonsinusoidal supply voltages. The results of some experimental work are reported and the instrument accuracy is discussed. >

Dynamic simulation of voltage collapses

Abstract: The authors show some examples of dynamic simulations of voltage phenomena using a new general purpose stability program (EUROSTAG) covering in a continuous way the classical fields of transient, mid-term and long-term stability, and also the quasi steady state conditions of a power system. The three examples show the ability of dynamic simulations to study in depth all the phenomena taking part in the voltage collapses. The first example considers the case of an infinite bus feeding a reactive load through a purely reactive line. With this simple example, it is possible to show the basics of voltage instability. The second example illustrates the operation of a system on the lower part of the V, P curve. A two bus system is considered. The third example shows transient phenomena merged with mid-term voltage dynamics. >

Method and apparatus for testing vehicle engine sensors

Abstract: A multi-test, hand-held instrument and method for testing manifold absolute pressure (MAP) and mass air flow (MAF) sensors includes a housing enclosing a battery and an electrical circuit. The electrical circuit can test d.c. voltage output signals and frequency output signals from MAP/MAF sensors and can test continuity and battery strength. Electrical leads extend outwardly from the housing and are connectable to the sensor being tested. A pair of light indicators, comprising LEDs and resistors connected in parallel, are selectively illuminated to allow the test instrument to be used to test both MAP and MAF sensors, and to test these components on different makes and models of vehicles.

Precharge for non-disruptive bus live insertion

Abstract: The invention is related to hot plugging of an electrical circuit (1) into a separate non-quiesced signal net (6) in an active system (7), such as a digital or analog bus. The inventive solution proposes the addition of a preconditioning network (4) to precondition the electrical circuit (1) to be hot plugged by partially precharging the parasitic input capacitances (Cc, Ce, Cm) of the electrical circuit (1) before hot plugging. The precharging of the parasitic input capacitances serves to minimize transient effects on the active system (7).

Prediction of power system frequency response after generator outages using neural nets

Abstract: A new methodology is presented for estimating the frequency behaviour of power systems necessary for an indication of under-frequency load shedding in steady-state security assessment. It is well known that large structural disturbances such as generator tripping or load outages can initiate cascading outages, system separation into islands, and even the complete breakup. The underfrequency load shedding takes place during the beginning phase of a dynamic change of the system frequency initiated by these distrubances. In this context the authors examine the ability of neural nets to properly interpolate among training data sets and to accurately predict the system frequency variations. The neutral-net approach provides a fairly accurate method of estimating the system average frequency response without making simplifications or neglecting non-linearities and small time constants in the equations of generating units, voltage regulators and turbines. The understanding and selection of the input features comes from the developed, simple low-order system frequency response model. Additional features are defined in terms of the centre of inertia acceleration and admittance distances. The efficiency of the new procedure is demonstrated using the New England power system model for a series of characteristic perturbations. The validity of the proposed approach is verified by comparison with the simulation of short-term dynamics including effects of control and automatic devices.

Calculation of harmonics and interharmonics in HVDC schemes with low DC side impedance

Abstract: When HVDC schemes are used to join AC systems of different frequencies, frequencies other than the well known characteristic harmonics are produced in the AC systems, and these are often called interharmonics. This paper examines these interharmonics and shows that, for HVDC schemes with low DC side reactance, another set of interharmonics will exist only on the DC side. These are a function of the impedance of the convertor seen from the DC side. It is shown that this impedance is time varying and, by modulation theory, will produce DC side harmonic frequencies some of which may be at low frequency. It is also shown that the amplitudes of characteristic harmonics and interharmonics will be changed by this effect. The method based upon modulation theory is compared to the results of a time domain computer simulation. The results shown will also be applicable to variable frequency motor drives.

Multi-infeed HVDC transient response and recovery strategies

Abstract: Should two or more DC transmission systems terminate in close electrical proximity in the same AC system, there is the potential for degradation of DC transient performance due to interaction in response to common AC system disturbances. This paper presents the digital simulation of a test system and shows the DC responses to AC faults at various locations. Control strategies are evaluated to enhance the recovery of the DC systems from the disturbances. >

Quality assurance and upgrade of analog characteristics by fast mismatch analysis option in network analysis environment

Abstract: In contrast to digital circuits, the fabrication tolerance of electrical characteristics of analog integrated circuits depends highly on the local device matching accuracy. Especially for scaled structures down to the submicrometer range, the local statistical device parameter mismatching increases rapidly. As in network analysis programs (i.e., SPICE), statistical mismatch effects are not represented within the implemented device modeling; consequently no analysis options are available to compute their influence on electrical circuit characteristics in production