OpenUCT :: Browsing by Author "Petroianu, Alexander"

Browsing by Author "Petroianu, Alexander"

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Open Access
Artificial neural networks for state estimation of electric power systems
(1996) Zivanovic, Rastko; Petroianu, Alexander
This thesis deals with the application of Artificial Neural Network (ANN) technology in power system state estimation. It addresses the following developments: the fundamentals of the state estimation based on ANN technology; the feasible ANN state estimation methods; use of voltage phasor angle measurements in ANN state estimation; and bad data processing for ANN state estimation. The power system state estimation problem is formulated as an optimization problem applied to dynamic ANN model. Two types of dynamic ANN models are used: ANN model with steepest descent dynamic; and ANN model with Hopfield-style dynamic. The complexity of an ANN State Estimator (ANN SE) is reduced by using the following techniques: a special algebraic transformation of the ANN objective function; and the incorporation of zero-injection measurements by the using variable reduction technique. At the same time, these two techniques improve the filtering performance of the ANN SE. Two methods for designing the ANN SE for a specific power system are developed: an analytical method: it maps the structure and the parameters of a power system into the ANN SE structure and parameters; and a synthetic method: it is based on the Real Time Recurrent Learning (RTRL) technique (used in training dynamic ANN), where the ANN SE structure and parameters are determined through learning from available input/output (measurements/state variables) data. In continuation of the thesis feasible ANN SE methods are developed.
Open Access
Damping of electromechanical oscillations using power system stabilizers
(1995) Ahmed, Shaheen Sooleman; Petroianu, Alexander
This thesis deals with the damping of electromechanical oscillations using Power System Stabilizers (PSS). The thesis focuses on three problems associated with the damping of these oscillations, namely the determination of the optimal locations of the PSS, the determination of the best control structure of the PSS and the design of robust PSS. We develop two new methods for determining the optimal locations of the PSS. These two methods are based on Total Modified Coupling Factors (TMC) and optimization by Simulated Annealing (SA). The TMC is a measure of the damping influence of each machine pair on several power system modes. The TMC incorporates the effect of the performance and the type of excitation system of the generator. The method based on TMC is tested on a nine-bus benchmark network. In the method based on SA, we formulate the PSS placement problem as a discrete nonlinear optimization problem. The objective function corresponds to the damping of the electromechanical modes of the system. In this method, the placement is performed simultaneously for all PSS. Using SA, we obtain a placement scheme which guarantees that the undesired poles can be controlled with finite control energy. As a result of the optimization formulation, the method based on SA is computationally more intensive than the method based on TMC. We demonstrate the method based on SA on two networks namely, a seven-bus network and a 35-bus equivalent of the Eskom network. The problem of determining the control structure for damping of the electromechanical oscillations is composed of three aspects namely, the type of feedback, the type of signal and the type of control.
Open Access
Development and implementation of an enhanced design for automatic generation control
(1997) Hartman, Robert C; Petroianu, Alexander
Automatic generation control (AGC) is the centralised short-term closed-loop control (regulation) of generating units in a control area. The main objectives of AGC are to maintain the system frequency at 50 Hz; to obtain correct tie-line interchange; and to operate each generating unit at its most economic value. The cost of operation of a regulating (moving) generating unit is, however, higher than that of the same unit being used at a fixed output. Primary energy cost as well as longer-term maintenance cost and life expectancy is affected by regulating generating units. With the continuous drive to reduce costs, the question was raised whether the amount of regulation done by AGC is justified. The original AGC system could not be set up to achieve the required quality of supply while using the minimum control expected. An enhanced design of load following and distinctive ACE regulation was developed and implemented. Although the load-following component of the original controller could be applied with minor modifications, the short-term ACE regulation component had to be improved significantly. The main alterations involved the addition of a derivative component by means of fuzzy logic and the elimination of nonlinearities from the control. This research describes the enhanced design developed to obtain the required performance while minimising the control effort.
Open Access
An investigation into the costs of active power transmission transactions and methods for evaluation of costs
(1998) Gorodkova, Elena; Petroianu, Alexander
The power supply industry of South Africa is currently engaged in the process of restructuring, in a move from traditional vertically integrated and regulated structures to the unbundled, deregulated marketing of electric energy and power. Since this is a new process for South Africa, the investigation was commissioned with the aim of introducing this new area of research, to get an international perspective on the topic, and to build up a theoretical background for further studies. In this new deregulated environment, the transmission segment of the supply industry is seen as an independent business, which can provide services at the appropriate terms and conditions. A transmission utility offers services at costs that can be evaluated by various methods. In this context, the thesis has three objectives. The first objective of the thesis is to consider different types of transmission services and costs associated with these services. The second objective is to analyse existing methods for evaluation of costs of transmission transactions and to define advantages and drawbacks of these methods. The third objective of the thesis is to test selected methods for illustration by means of case studies.
Open Access
An investigation into the effects of load modeling of transient stability and analysis of voltage collapse
(1992) Khumalo, Joseph; Petroianu, Alexander
The aim of this thesis is to investigate the effects of load modeling on transient stability studies and to analyze the phenomenon of voltage collapse. In addition, the different generator models are compared and the effects of voltage dips on induction motor performance are investigated. The modeling of loads dates back to the late forties when network analyzers were still in use. The prohibitive computational requirements resulted in many approximations being made to the load models. In turn, this resulted in the use of simple models which did not provide sufficient information about the dynamic behavior of loads. With the advent of digital computers, more accurate load models could be used in dynamic simulations. Despite this improvement in computational tools, the problem of load modeling for stability studies is still very complex. The load composition changes with the time of the day, the consumer's lifestyle, weather, state of the economy and other factors. The accurate load model would include amongst other things, the effects of the abovementioned factors. Since these factors are unpredictable, accurate load modeling becomes very complex indeed. It is mainly for these reasons that the approximate are still widely in use. Ideally, the response of these approximate models should be compared to the actual loads under similar disturbances. A further concern of the thesis is the study of voltage stability. The voltage stability problem has become a matter of growing concern amongst bulk transmission utilities worldwide over the last decade. For long, the stability of a power system was related exclusively to the synchronous stability of the generators.
Open Access
A new method of utilising stability study results in applying out-of-step protection to allow observability of rotor angle unstable conditions
(1999) Van Eyssen, Juanita; Petroianu, Alexander
Power system rotor-angle unstable conditions may cause undesired operation of distance relays, resulting in the unnecessary switching of lines. To prevent the possible undesired operation of distance relays, a protection type known as out-of-step blocking is used. This protection type is complemented by another type of protection, known as out-of-step tripping protection. The tripping protection's main functions are to detect total loss of synchronism and send tripping signals to selected locations in the system to initiate islanding. In this research the application of out-of-step blocking and tripping protection was investigated. The motivation for the research was provided by system unstable incidents that occurred in Eskom, the electricity utility of South Africa. Detailed out-of-step protection investigations indicated that the out-of-step blocking and tripping protection applied according to a method which is based on an existing approach failed to detect the unstable conditions. In summary, the detailed investigations showed that the existing approach of applying out-of-step blocking and tripping protection is not adequate as it does not allow the necessary observability1 and correct detection of rotor-angle unstable conditions.
Open Access
A novel method for power system stabilizer design
(2003) Chen, Lian; Petroianu, Alexander
Power system stability is defined as the condition of a power system that enables it to remain in a state of operating equilibrium under normal operating conditions and to regain an acceptable state of equilibrium after being subjected to a finite disturbance. In the evaluation of stability, the focus is on the behavior of the power system when subjected to both large and small disturbances. Large disturbances are caused by severe changes in the power system, e.g. a short-circuit on a transmission line, loss of a large generator or load, loss of a tie-line between two systems. Small disturbances in the form of load changes take place continuously requiring the system to adjust to the changing conditions. The system should be capable of operating satisfactorily under these conditions and successfully supplying the maximum amount ofload. This dissertation deals with the use of Power System Stabilizers (PSS) to damp electromechanical oscillations arising from small disturbances. In particular, it focuses on three issues associated with the damping of these oscillations. These include ensuring robustness of PSS under changing operating conditions, maintaining or selecting the structure of the PSS and coordinating multiple PSS to ensure global power system robustness.
Open Access
Optimal capacitor placement to minimise harmonics in power systems and software tools
(1996) Hitzeroth, Helmuth Victor; Petroianu, Alexander
Harmonics in power systems is a relatively new area of research. In view of this and the growing awareness of the quality of the electricity supply, the theory of harmonics in power systems is reviewed. The sources and the effects of harmonics are investigated. The algorithms that are used for the frequency analysis of power systems are investigated and compared. These algorithms comprise the companion circuit method, the Gauss-Seidel method, the Newton-Raphson method and the current injection method. In addition various freely and commercially available software packages for the harmonic analysis of power systems are studied and compared. For this purpose a questionnaire was sent out to software developers and suppliers. This questionnaire as well as the results of the comparative investigation are presented. A power system has many configurations due to the switching of power capacitors on to and off the power grid. Some of these configurations can result in unacceptable distortion levels. An existing state space method is investigated to analyse these configurations and an example is worked through, to illustrate how this method works. However, this state space model is only applicable to radial power systems and there have to be power capacitors at the end of every feeder amongst others. Because of these significant disadvantages of this method, a new analytical approach or theoretical foundation for the analysis of power capacitors in radial as well as meshed power systems is developed in this thesis. For this purpose the branch current and nodal voltage equations are determined. Redundant nodal voltages are eliminated from the set of branch current equations. The remaining equations and the nodal voltage equations are then combined to form a system realisation. This system realisation is still overspecified and a further reduction is done to obtain a minimal realisation of the power system. This approach is demonstrated analytically and numerically by way of five case studies. This approach is also verified by comparing it with the current injection method. Identical results are obtained with the state space approach and with the current injection method, demonstrating that the state space approach is indeed valid.
Open Access
Power system harmonic field measurements PC simulations and standards
(1996) Atkinson-Hope, Gary; Petroianu, Alexander
The research covered three main objectives: Application of PC based harmonic penetration packages and a comparison of their simulation results. Investigation and comparison of the IEEE, IEC and ESKOM power system harmonic standards. Measurement of harmonics in two industrial networks and the application of the results to the regulatory limits provided by the standards. The research makes use of two packages and uses parameters from an industrial network. Case studies were conducted using the same parameters so that the results obtained from each package could be compared. The standards have been individually investigated to determine each method for controlling harmonic distortion. The three standards have also been compared. Harmonic field measurements were conducted in industrial networks so that the limits imposed by the standards could be compared with real life operating conditions.
Open Access
Stabilising an islanded nuclear power plant with a high- energy resistor
(2009) Lilje, Peter; Petroianu, Alexander
Open Access
Web based distance learning for power system engineering
(2000) Lehr, Rainer Helmut; Petroianu, Alexander
Bibliography: leaves 81-84.

Browsing by Author "Petroianu, Alexander"

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