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ABSTRACT: Transients caused by shunt reactor switching have been an important parameter in the design of the relevant equipment (reactor, circuit breaker, insulation) of the Hellenic-Interconnected Power System. Next to the well-known damaging overvoltages produced by shunt reactor de-energization, high dc components of inrush currents with low damping factor have been observed during energization, resulting in excessive electromagnetic stresses to the windings and undesired activation of zero-sequence current relays. In the present study, inrush currents produced by shunt reactor energization are analyzed and the performance of the techniques used for their limitation is investigated.
IEEE Transactions on Power Delivery 05/2005; · 1.35 Impact Factor
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ABSTRACT: Lightning protection studies require estimation of grounding systems dynamic behavior. This paper presents the results of a new methodology for calculating the lightning response of the basic component of any grounding system, the grounding electrode. Lightning strike is modeled using a double exponential time function. Closed-form mathematical formulae are used to describe current and voltage distribution along the electrode. The effect of soil ionization can be also taken into account. The proposed methodology is validated by comparison of the obtained results with experimental and simulated waveforms found in literature.
IEEE Transactions on Power Delivery 05/2003; · 1.35 Impact Factor
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ABSTRACT: Lightning protection studies require estimation of grounding systems' dynamic behavior. This paper presents the results of a new methodology for calculating the lightning response of the basic component of any grounding system, the grounding electrode. Lightning strike is modeled using a double exponential time function. Closed-form mathematical formulae are used to describe current and voltage distribution along the electrode. The effect of soil ionization can be also taken into account. The proposed methodology is validated by comparison of the obtained results with experimental and simulated waveforms found in literature.
IEEE Power Engineering Review 11/2002; 22(10):61-61.
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ABSTRACT: Switching transients occur quite frequently in medium-sized
independent power systems including wind farms. However, the
characteristics and the consequences of these phenomena have not been
thoroughly investigated, taking into account the complexity of the
network configuration. In this paper a contribution to that direction is
made, focusing on the power system of Greek island Crete
Power Tech Proceedings, 2001 IEEE Porto; 02/2001
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ABSTRACT: Suggested network reinforcements to increase the available
transfer capability of an interconnected power system include series
compensation of transmission lines. In this paper, a methodology for the
identification of the critical transmission lines with respect to
voltage stability issues is proposed. The methodology is applied on the
CIGRE 33-bus test system
Power Tech Proceedings, 2001 IEEE Porto; 02/2001
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ABSTRACT: In this paper, the response of wind turbine grounding systems is calculated under fault conditions or when they are hit by lightning. The objective is their effective design in terms of dispersion of imposed currents and minimization of raised potentials. Locally raised potentials are calculated in terms of steady state GPR. Transferred potentials to neighboring wind turbines are also computed. Measures taken to reduce potentials are presented and commented upon. For the purposes of this paper the well-known software packages EMTP (Electromagnetic Transients Program) and CYMGRD (CYMe's grounding) have been used.
Electrotechnical Conference, 2000. MELECON 2000. 10th Mediterranean; 06/2000
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ABSTRACT: In this paper, the effect of soil ionization due to lightning strike on wind turbine grounding electrodes is investigated. The developed ionization model is validated and subsequently used to investigate the influence of several parameters on the phenomenon studied, namely, the soil type, the electrode type and the crest value of the impulse current. All simulations are performed using EMTP computer program.
Electrotechnical Conference, 2000. MELECON 2000. 10th Mediterranean; 06/2000
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ABSTRACT: Thyristor controlled equipment is one of the major sources of
harmonics in power systems impacting power quality. In this paper the
thyristor controlled series capacitor (TCSC) installed by power
utilities for increase of power transfer capability of transmission
lines is investigated in terms of harmonic distortion in the power
system. In addition, series compensation may cause subsynchronous
oscillations leading to subsynchronous resonance (SSR) phenomena. The
investigation of a TCSC control strategy that ensures sufficient SSR
damping is presented
Harmonics and Quality of Power Proceedings, 1998. Proceedings. 8th International Conference On; 11/1998
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ABSTRACT: An efficient algorithm for the implementation of hysteresis effect
in three phase transformer models for electromagnetic transient studies
is presented. It is based on a simplification of the well-known Preisach
model requiring few data, easily available, while it provides results
close to the experimental ones. The model is validated by comparing
measured and calculated results obtained from tests on a magnetic
material used for the construction of transformer cores and a laboratory
three-phase transformer
IEE Proceedings - Electric Power Applications 08/1996; · 0.55 Impact Factor
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ABSTRACT: In this paper the energization of a transmission line, a
phenomenon particularly related to black start studies, is simulated
using the Electromagnetic Transient Program (EMTP). The effect of
several parameters, such as the line models used in EMTP, the line
transposition, the existence of a surge arrester and the transient
behaviour of the synchronous generator are investigated
Electrotechnical Conference, 1994. Proceedings., 7th Mediterranean; 05/1994
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ABSTRACT: In this paper the overvoltages produced by switching the primary
side of reactor loaded transformers are simulated using the
Electromagnetic Transients Program (EMTP). Attention is focused on
transformer modelling. Five general three-phase transformer models are
used and from the results obtained, and comparisons with field tests,
positive conclusions concerning the reliability and the accuracy of
these models in the study of switching fast electromagnetic transients
are drawn
IEEE Transactions on Power Delivery 05/1994; · 1.35 Impact Factor
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ABSTRACT: This paper presents a unified approach to solve the damping problem of electromechanical oscillations in a multimachine power system. First, an easy and reliable method for determining the most suitable generator to damp out each mode of oscillation is presented. The advantage of this method is that it is based only on small dimension eigenvector computations. The method involves the participation factors of the electromechanical oscillation modes as is customary in this type of studies. Next, a parameter optimisation method is applied to the problem of tuning the power system stabilisers. This method is based on a minimax problem formulation, which is solved using the necessary optimality conditions in a sequential algorithm. The overall stability can be greatly enhanced when the AVR parameters are tuned together with the PSS.
Control, 1994. Control '94. International Conference on; 04/1994
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Athens Power Tech, 1993. APT 93. Proceedings. Joint International Power Conference; 10/1993
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Athens Power Tech, 1993. APT 93. Proceedings. Joint International Power Conference; 10/1993
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ABSTRACT: A new transformer model, named `geometrical', is developed, based
on the circuit analysis of its magnetic core. The method is general and
can be used for any type of multiphase multiwinding transformer. The
model simulates the transformer as a set of coupled branches and can be
incorporated into the widely known electromagnetic transients program
(EMTP). A basic advantage of the model is that it can represent any
asymmetry of the transformer magnetic circuit. As a study case, the
model of an actual 280 MVA autotransformer is derived and used for the
simulation of various steady state and transient conditions
Generation, Transmission and Distribution [see also IEE Proceedings-Generation, Transmission and Distribution], IEE Proceedings C 08/1993;
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ABSTRACT: Short-circuit currents have by nature a multi-variable stochastic
character affected by probabilistic conditions, like the fault location
in the network, the type of the fault, the fault impedance and the
prefault state of the system. The proposed method takes into account the
random nature of these conditions using conditional probabilities and
provides an analytical expression for the short-circuit current
distribution with a minimum of assumptions. Thus, not only the worst
case probability but also the probability of each occurrence is
obtained. This increased amount of information can be extremely valuable
for an objective decision-making. As a result, not only considerable
savings in capital investment can be achieved, but more important
designs with known risks, reliability and with sufficient margins of
safety can be produced
Probabilistic Methods Applied to Electric Power Systems, 1991., Third International Conference on; 08/1991
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ABSTRACT: The authors describe SUBES, a rule-based expert system approach
for distribution substations troubleshooting and maintenance scheduling.
These complex decision tasks are currently performed by highly
experienced engineers, whose knowledge can be expressed as a set of
production rules. As the inference mechanism for the knowledge
processing, first order logic is natural. Therefore the system has been
implemented in Turbo-Prolog. SUBES works interactively: a very
user-friendly environment for the interaction between computer and end
user is achieved by a menu-driven dialog and a multi-window display. The
transfer of new knowledge-experience gained is very easy since the
modification/expansion of the knowledge base can be carried out in a
very simple way. The provided explanation module enables the operator
under training to understand the reasoning process followed by the
system. The system can be used for educational as well as engineering
purposes
IEEE Transactions on Power Delivery 08/1991; · 1.35 Impact Factor
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ABSTRACT: The origin of forced, low frequency oscillations introduced in
power systems by low-speed diesel generators is investigated. The linear
models of a multimachine power system are presented for both
interconnected and autonomous systems. Particular emphasis is given to
the calculation of the electromechanical oscillation frequencies. A
three-machine test system exhibiting a very strong resonance is
analysed. The method of analysis is based upon the computation of the
frequency response of a multivariable system. Finally, the
electromechanical oscillation analysis and the frequency response
computation are applied to the electric power system of the island of
Crete, where a number of large low-speed diesel engine driven generators
have recently been installed
Control 1991. Control '91., International Conference on; 04/1991
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ABSTRACT: The feasibility of power system dynamic analysis using qualitative
reasonings with deep-level power system structure models is
demonstrated. State transition models of the power system network
components are developed to encode deep-level power system expert
knowledge into the computer. The dynamic behavior of the system is then
analyzed using this knowledge. The developed computational method is
used to analyze the dynamic behavior of the Hellenic transmission
system. Both the transient stability data and the acquisition of
knowledge relevant to reasons behind problems affecting system stability
can be ascertained
IEEE Transactions on Systems Man and Cybernetics 06/1990;
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ABSTRACT: The authors describe a rule-based expert system for
troubleshooting and repairing of electrical machines. Empirical rules
are used to help the user to find and repair any known fault for any
electrical machine. An interactive methodology is implemented for fast
and safe data interchange between computer and user, using tree menus.
The expert system, which has been developed and tested at the National
Technical University of Athens, can be used by industries and
universities for engineering and educational purposes
IEEE Transactions on Energy Conversion 04/1990; · 2.27 Impact Factor
