Off-line Computer Simulation Research Articles

PMU-Based Wide-Area Security Assessment: Concept, Method, and Implementation

This paper presents a concept, method, and implementation of utilizing phasor measurement unit (PMU) information to monitor the wide-area security of a power system. The close dependency of major transmission paths requires an approach that takes that interaction into account while establishing operational transfer capability, and evaluates grid reliability and security on a system-wide basis. Thus, the concept of wide-area security region, which considers all essential constraints, including thermal, voltage stability, transient stability, and small signal stability, is proposed. This approach expands the idea of traditional transmission system nomograms to a multidimensional case, involving multiple system limits and parameters such as transmission path constraints, zonal generation or load, etc., considered concurrently. In this paper, the security region boundary is represented using piecewise approximation with the help of linear inequalities (so called hyperplanes) in a multidimensional space, consisting of system parameters that are critical for security analysis. The goal of this approximation is to find a minimum set of hyperplanes that describe the boundary with a given accuracy. Offline computer simulations are conducted to build the security region and the hyperplanes can be applied in real time with phasor information for on-line security assessment. Numerical simulations have been performed for the full size Western Electricity Coordinating Council (WECC) system model, which comprises 15 126 buses and 3034 generators. Simulation results demonstrated the feasibility and effectiveness of this approach, and proved that the proposed approach can significantly enhance the wide-area situation awareness for a bulk power system like WECC.

Predicting Pilot Task Demand Load During Final Approach

This research aims at developing a method to assess the safety of airport approaches from the perspective of airline pilots. The main hypothesis is that safety is inversely proportional to the task demand load (TDL) experienced by the pilots conducting it. The TDL is the mental workload imposed by the system to be controlled, and is not to be mistaken for the mental workload experienced by the operator, referred to as mental load. This article presents the results of preliminary research that focused on how some of the environmental factors that determine the approach, such as the type of aircraft and the meteorological conditions, affect the TDL. A paper pilot model, consisting of a scanning model, mental model, and control model, was used to quantify how hard an average pilot has to work to conduct the specified approach. Offline computer simulations with this model revealed some clear trends in pilot scanning rate, task error, and control activity, as a function of the environmental factors, which will eventually allow for a prediction and quantification of the TDL. Depending on the approach performance requirements on the one hand, and the pilot scanning capacity on the other, increasing turbulence intensity showed a change in trend for all TDL-related parameters at a particular turbulence intensity. Results from a pilot-in-the-loop evaluation, conducted in a fixed-base flight simulator, supported the main trends of the offline simulation trials. They also indicated that through adopting different scanning strategies, pilots can leverage their workload; more research is necessary to clarify this result, however.

Fed-batch cultivation of Wautersia eutropha

Batch kinetics of polyhydroxybutyrate (PHB) synthesis in a bioreactor under controlled conditions of pH and dissolved oxygen gave a biomass of 14 g l −1 with a PHB concentration of 6.1 g l −1 in 60 h. The data of the batch kinetics was used to develop a mathematical model, which was then extrapolated to fed-batch by incorporating the dilution due to substrate feeding. Offline computer simulation of the fed-batch model was done to develop the nutrient feeding strategies in the fed-batch cultivation. Fed-batch strategies with constant feeding of only nitrogen and constant feeding of both nitrogen and fructose were tried. Constant feeding strategy for nitrogen and fructose gave a better PHB production rate of 0.56 g h −1 over the value obtained in batch cultivation (PHB production rate – 0.4 g h −1).

Evaluation accuracy for homing system of ground-air missile

In this paper approach of evaluation accuracy for homing system of ground - air missile is showed by OFF-LINE computer simulations. Two ranges of action under different angles of approach are appraised in the case of the presence of the seeker noise. The probability density function and the root-mean-square estimation of the miss distance are realised by Monte - Carlo methods, using linearized model only of the missiles flight and main nonlinearity of guidance and control systems. .

Real-time assessment of power system transient stability using rate of change of kinetic energy method

The effectiveness of the rate of change of kinetic energy (RACKE) in defining power system stability was studied previously using off-line computer simulation. Theoretical analyses indicated the effectiveness of this method in deciding transient stability of power systems, and its effectiveness in increasing the stability margin when a dynamic brake element is used. The practical implementation of the RACKE method with a resistive dynamic brake is presented. The brake insertion and removal instants are decided on-line by a microcomputer controller. The hardware and software design considerations of the proposed controller are described. The observed behaviour of the power system prototype during fault conditions is found to be in agreement with that obtained from simulation tests.

Exponential stabilization of a class of unstable bilinear systems

Considers the control design of bilinear systems with multiplicative control inputs. Previous control designs for such systems normally assume that the open-loop bilinear system is (neutrally) stable. In this paper, a nonlinear control design is proposed for open-loop unstable bilinear systems. The new control stabilizes the bilinear system globally and exponentially if a sufficient stability condition, which can be checked by off-line computer simulations in advance of the control, is satisfied.

An off-line computer simulation for controlling the repulp section in a plant

An off-line computer simulation program is proposed to control a repulping section of a plant that has previously been controlled solely by an operator. The off-line computer program can be used as a tool by the operator to improve the control of repulping a certain material before leaching commences. Controlling the repulping section is of special importance, especially to processes employing unit operations after the repulping section with fast reaction kinetics. The fast reaction kinetics make it essential to control the conditions of the incoming pulp to the leaching vessel. The simulation uses dynamic mass balance equations to simulate the physical properties of the system. It then solves the set of differential equations with an iteration procedure that uses the fourth order Runge-Kutta(RK) procedure and the Golden Section Search(GSS) optimisation procedure, thus obtaining optimum settings so that the desired conditions in the process could be maintained or reached as quickly as possible. The simulation was tested on a plant and indicated that considerable improvement in the stability of the operation could be achieved. Two calculation methods were also investigated for the mathematical models, i.e. the Runge-Kutta procedure mentioned above and the Laplace transform solution. Comparing the two methods it was evident that the Laplacian solution was not as accurate as the Runge-Kutta solution due to the inherent linearisation step employed. This was especially evident for large deviations from the desired conditions.

Direct estimation of voltage phasor, frequency and its rate of change using Newton's iterative method

A new approach to the design of a digital algorithm for direct estimation of voltage phasor, frequency and its rate of change is presented. The algorithm derived is based on Newton's iterative method, very commonly used in the field of unconstrained optimization studies. A five-parameter voltage model was assumed, so the result of the estimation was a parameter vector, consisting of the following unknown parameters of the voltage signal processed: its DC component, magnitude, phase angle, frequency and its rate of change. To demonstrate the performance of the algorithm, off-line computer simulation results are presented. The algorithm showed high measurement accuracy over a wide range of frequency changes. The algorithm ‘order two convergence’ provided a very good dynamic response as well as a fast algorithm adaptability. The new algorithm seems to be a particularly useful tool in the field of frequency relaying as well as in various aspects of power engineering applications.

Non-Linear Dynamic Power System State Estimation

This paper describes the application of the invariant imbedding non-linear dynamic estimation technique to determine the transient state of a power system. The estimator handles system and measurement non-linearities effectively and works in the presence of system input disturbance and measurement noise. Off-line computer simulations of the estimator applied to a single machine infinite busbar power system successfully estimated the states of the machine. The results indicated that successful estimation is achieved by a careful choice of the input disturbance noise covariance matrix, Q. High Q values are recommended for the stepout region and low Q values for the hunting region.