Two-machine Model Research Articles

Evaluation of stochastic flow lines with provisioning of auxiliary material

Flow lines are often used to perform assembly operations in multi-stage processes. During these assembly operations, components that are relatively small, compared to the work pieces travelling down the flow line, are mounted to the work pieces at a given stage. Those components, or more generally, any kind of auxiliary material, are provisioned to the corresponding production stage in a repetitive but not necessarily deterministic manner using a certain delivery frequency, each time filling the local storage up to a predetermined order-up-to level. Just like random processing times, machine failures, and repairs, the randomness of the provisioning process can impact the long-term throughput of such a flow line. In this paper, we develop a fast and accurate analytical performance evaluation method to estimate the long-term throughput of a Markovian flow line of this type for the practically important case of limited buffer capacities between the production stages. We first give an exact characterization of a two-machine line of that type and show how to determine system state probabilities and aggregate performance measures. Furthermore, we show how to use this two-machine model as the building block of an approximate decomposition approach for longer flow lines. As opposed to previous decomposition approaches, even the state space of the two-machine lines can become so large that an exact solution of the Markov chains can become impractical. We hence show how to set up, train, and use an artificial neural network to replace the Markov chain solver embedded in the decomposition approach, which then leads to an accurate and extremely fast flow line evaluation tool. The proposed methodology is evaluated by a comparison with simulation results and used to characterize the structural patterns describing the behaviour of flow lines of this type. The method can be used to systematically consider the combined impact of the delivery frequency and the local order-up-to levels for the auxiliary material when designing a flow line of this type.

Fast Algorithms for Estimating the Disturbance Inception Time in Power Systems Based on Time Series of Instantaneous Values of Current and Voltage with a High Sampling Rate

The study examines the development and testing of algorithms for disturbance inception time estimation in a power system using instantaneous values of current and voltage with a high sampling rate. The algorithms were tested on both modeled and physical data. The error of signal extremum forecast, the error of signal form forecast, and the signal value at the so-called joint point provided the basis for the suggested algorithms. The method of tuning for each algorithm was described. The time delay and accuracy of the algorithms were evaluated with varying tuning parameters. The algorithms were tested on the two-machine model of a power system in Matlab/Simulink. Signals from emergency event recorders installed on real power facilities were used in testing procedures. The results of this study indicated a possible and promising application of the suggested methods in the emergency control of power systems.

An optimal transmission line power control under non‐sinusoidal conditions using static synchronous series compensator

In view of the growing load demand in developing countries, overloading in existing transmission lines is a major concern. In addition to the voltage drop due to overloading, the proliferation of non-linear loads in recent years distort the shape of sinusoidal voltage. Stable transmission of power through a transmission line under such condition is a challenge. In the presence of harmonics, control of transmittable power using static synchronous series compensator needs additional efforts, where increased power demand can be fulfilled by taking into consideration various other parameters. If an attempt is made to increase transmittable power through a transmission line using SSSC, power loss, the magnitude of voltage and its shape deteriorate in the presence of non-linear loads. Thus, the trade-off between transmittable power, transmission loss and voltage quality is tackled by optimal control of the compensator. In this study, a novel optimal control of SSSC is proposed with the objective of minimising transmission loss by maintaining the voltage within the limits while enhancing the transmittable power under non-sinusoidal conditions. A two-machine model of power system with 400 kV transmission line with parameters as per standards issued by the Central Electricity Authority is considered for validation of the algorithm using MATLAB platform and results are presented.

Entire stability region estimation using the invariant manifolds and singularities at infinity

This study provides a new perspective to study the formation and wholeness of the stability region of power systems. The main idea of the approach proposed in this study is to project the system into a closed area and then investigate the behaviours of the field in a neighbourhood of infinity on a sphere that is topologically equivalent to the original system. The invariant manifolds and singularities at infinity are described in detail to indicate the boundary of the entire stability region, based on the concept of the basin of attraction. The proposed method is applied to the single-machine model, two-machine model and multi-machine system to illustrate the complex geometry structure of the transient stability region. A singularity variation index is presented to estimate the size of the entire region and indicate the transient stability degree of the power system. Simulation results verify the correctness and effectiveness of the theoretical analysis.

Parallel Operation of Three-phase Self-excited Induction Generators Driven by Renewable Energy Sources

Squirrel-cage induction machines, because of their low prices and rare demand for maintenance, can work as self-excited induction generators (SEIGs) in remote areas to convert wind or pico-hydro power to electricity, enhancing the use of different forms of renewable energy. However, with the increase of load, a single SEIG may not provide enough capacity to meet the demand, so parallel operation of several SEIGs is often required, necessitating a technique to analyze the characteristics of this kind of operation. This paper presents steady-state analysis of two SEIGs based on the equivalent circuit of the induction generator. The two-machine model allows the performance and load sharing features to be evaluated when the two generators undergo different rotating speeds.

Fundamental Performance Limitations in Utilizing HVDC to Damp Interarea Modes

This paper considers power oscillation damping (POD) using active power modulation of high-voltage dc transmissions. An analytical study of how the proximity between interarea modal frequencies in two interconnected asynchronous grids puts a fundamental limit to the achievable performance is presented. It is shown that the ratio between the modal frequencies is the sole factor determining the achievable nominal performance. To illustrate the inherent limitations, simulations using a proportional controller tuned to optimize performance in terms of POD are done on a simplified two-machine model. The influence of limited system information and unmodeled dynamics is shown. The analytical result is then further validated on a realistic model with two interconnected 32-bus networks.

Comparisons of two-machine line models in throughput analysis

Throughput analysis is important for the design, operation and management of manufacturing systems. Due to the large state space, exact analytical results only exist for two-machine serial lines. To analyse longer lines or more complex systems, the two-machine line model becomes the building block for further development. Therefore, many two-machine line models have been presented in the literature. A thorough understanding of the nature and differences of the various two-machine line models is critical for future analysis and development. In this paper, we study eight different two-machine models, categorized by synchronous and asynchronous lines with both time-dependent and operation-dependent failures, respectively. For each model, we introduce and compare the assumptions, calculation formulae, and their performance in system throughput. The results show that all the models exhibit similar performance with small differences, comparable to (or less than) the accuracy of data collection on the factory floor.

Control scheme for emergency state control of interconnected power systems with multistage recursive least-squares identification

An on-line emergency state control technique for the stabilization of an electric power system after major disturbances is proposed in this paper. Based on the multistage recursive least-squares identification of the equivalent two-machine model of an interconnected power system through local measurements, the proposed optimal-aim strategy, having an explicit solution, is particularly suited for making on-line control decisions to track the estimated stable equilibrium point — the target of control. The proposed technique appears feasible for application in control schemes for transient stability augmentation.

Estimation of equivalent models for emergency state control of interconnected power-systems based on multistage recursive least-squares identification

Multistage recursive least-squares (MRLS) identification of the equivalent two-machine model of interconnected electric power systems in emergency state is presented in the paper. Requiring only locally measured variables, the MRLS identification allows the estimation of the reactance of interties and the parameters of the equivalent two-machine model, including inertia constant, damping coefficient, internal voltage, relative rotor angle and relative speed. In other words, on-line estimation of the equilibrium point and the system states of interconnected electric power systems in emergency state is possible. The estimated model can be utilised in various emergency control strategies or other applications Studies demonstrate the effectiveness of the MRLS identification technique. >

Torsional interaction between nonidentical turbine generators

Torsional interaction between nonidentical generating units with a coincident torsional mode is investigated. The generators are electrically identical but have differences in their mechanical parameters. The mechanical system of each generator in the two-machine model is represented by the modal mode; this permits independent variation of mode damping, inertia, and frequency, which is not possible with the conventional spring-mass representation. It is shown that in a system without series capacitance, differences in mode damping affect the phase deviation but not the relative amplitude of torsional oscillations, the opposite being true for mode inertia and frequency differences. The amplitude and phase characteristics are both greatly affected by series capacitance for a difference in any mechanical parameter.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>