Abstract: MicroGrids are attracting substantial interest because they have the potential to increase the use of renewable generation and micro-CHP. They can also defer in-vestment in distribution capital plant and can improve local power quality. How-ever the primary operational requirement of power systems is that they must oper-ate safely from a user point of view, even during contingencies. Yet electrical safety of MicroGrids has received little attention to date. This paper addresses this important area. The fault current distribution in a generic MicroGrid is investigated for different fault contingencies during grid-connected and islanded operation. Based on an extensive investigation of earthing systems, a grounding electrode system is then developed for the MicroGrid study-case so that safe step and touch potentials are obtained
Abstract: Soil ionization occurs around a grounding electrode
when current density in the soil exceeds a critical value and reduces
grounding resistance. This paper proposes a current- dependent
grounding resistance model considering the soil ionization.
The proposed model is derived on the basis of energy balance of
the soil ionization. The resistivity of the ionization zone depends
on energy stored in the zone. Analytical expressions of the model
are proposed to estimate the zone resistivity. The model is verified
by comparing it with experimental results.
Abstract: In this paper, network constrained setting of voltage control variables based on probabilistic load flow techniques is presented. The method determines constraint violations for a whole planning period together with the probability of each violation and leads to the satisfaction of these constraints with a minimum number of control corrective actions in a desired order. The method is applied to define fixed positions of tap-changers and reactive compensation capacitors for voltage control of a realistic study case network with increased wind power penetration. Results show that the proposed method can be effectively applied within the available control means for the limitation of voltages within desired limits at all load buses for various degrees of wind power penetration.
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.
Abstract: Grounding systems have to be effectively designed in order to prevent electrical installation from excessive overvoltages and potential gradients when lightning occurs or in case of short circuit. Effective design of extended grounding systems as in case of windfarms grounding is investigated in terms of combining good performance in high and in low frequencies.
Abstract: This paper investigates the performance of
grounding electrodes buried in the close vieihty of small soil
heterogeneity, by using 2D and 3D finite element modelling.
The proposed methodology enables consideration of electrode
geometry effects which are not taken intp account in standard
analytical grounding system representation, The results are
compared to those found in the literature or obtained by
standard programs.
Abstract: This paper examines the use of computer modelling to assess the suitability of windfarm grounding systems for protection against power system faults and lightning strikes. The transient behaviour of practical earthing system designs are presented using the general purpose EMTP program and the specialist CDEGS grounding analysis software
Abstract: Grounding electrodes, being the basic component of any grounding system, need to be accurately modeled in
transient analysis studies. An easy-to-apply methodology for calculation of voltage and current distribution along the electrode has been presented and validated in [1]. It is based on a distributed parameters transmission line model of the electrode. In the middle stages, closedÂform mathematical expressions are used for the solution of telegraphy equations that describe the propagation of voltage and current waves along the electrode. The final expressions include few terms, so they are easy to apply. In this paper per unit length parameters of the electrode are comparatively calculated using various approaches. Calculation of the impulse response of grounding
electrodes shows that there is a limit in their length that seriously contributes in lowering raised potentials. This limit is the Âeffective length of grounding electrodes and is calculated in the paper.
Notes: Objective of the design of grounding systems is the prevention of the development of overvoltages in the power transmission system as well as dangerous potentials in the surface of the earth, by effective dispersion into the ground of the fault current. In this paper a short description of the most important methods used for grounding systems design is done, as they are presented in the international standards. The most important differences between the standard 2000 IEEE "Guide for Safety in AC Substation Grounding" (Standard 80 - 2000) with respect to the version 1986 (Standard 80 - 1986) and also with the CENELEC Harmonization Document 637 S1 "Power Installations exceeding 1kV A.C." - 1999 considering the proposed practices for design of grounding systems are presented and commented. Referring to the IEEE standards, practical case studies, where the changes and modifications of calculations are obvious, are comparatively commented. Also the safety margin for existing installations that could be achieved is also taken into consideration.
Abstract: Wind turbines (W/T) and Wind farms (W/F) require a grounding system for the protection of human life and the installed equipment, in case of short-ciruits or lightning strikes. In particular, there is an increased probability of lightning striking a W/T, because good wind potential normally exists in places of high altitude. An effective grounding system means that the resistance of W/T and W/F grounding systems remains low. The objective of
this paper is to present a methodology and a practical study case concerning the optimal design of individual W/T and W/F grounding systems. In this design the minimization of the grounding resistance and the satisfaction of the safety criteria (step and touch voltage) are of major concern [1].
Abstract: In this paper a review of major AC substation grounding practices given in international standards is presented with special reference to important considerations, differences and modifications. More especially the major changes in the 2000 version of IEEE Guide for Safety in AC Substation Grounding (Standard 80-2000)
with respect to the 1986 version, (Standard 80- 1986) that affect the grounding design and analysis are discussed. Comparisons are made for the portions where major changes occur. Examples are presented to show the effects of the changes in the design and analysis of power system grounding.
Abstract: In this paper, the response of windturbine 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 windturbines are also computed. Measures taken to reduce potentials are presented and commented. For the purposes of this paper the well-known software packages EMTP (Electro Magnetic Transients Program) and CYMGRD (CYMeâs GRounDing) have been used.
Abstract: Grounding systems have to be effectively designed in order to prevent electrical installation from excessive overvoltages and potential gradients when lightning occurs or in case of short circuit. In general, extended grounding systems improve their behavior under short circuit currents and in case of lightning. However in the second case there is a limit in the dimensions that seriously contribute in lowering
raised potentials and grounding resistance. Effective design of large grounding systems as in case of windfarms grounding is done in terms of combining
good performance in high and in low frequencies.
Abstract: This paper addresses the problem of modelling grounding conductors whose lengths exceed 1km or more using the Electromagnetics Transients Program (EMTP). For such lengths existing techniques are commented and a new technique is presented based on the pi-equivalent circuits representation of grounding conductors. The parameters of each circuit are suitably modified to fit the impedance in the frequency domain. A suitably small integration step is chosen in order to consider the whole frequency range of the response.
Abstract: In this paper, the capabilities of the Electromagnetic Transients Program (EMTP) are investigated so as to examine the transient behaviour of grounding electrodes under lightning current excitation. Two methods are used, to model the transient response of grounding electrodes, based upon the transmission line approach. The results from the two methods are compares, and general comments regarding the propagation of voltage wave form along the electrode are drawn.
Abstract: This paper addresses grounding systems analysis using EMTP. Grounding systems usually
made of conductors embedded in or laid on the surface of the ground, can be modelled using
two different methods that simulate the behavior of their components under fast and low
frequency excitation. These methods are briefly described in the paper and results obtained
from simulations of various grounding arrangements are compared. Comparison with other
software packages or experimental data found in literature, are also presented.
Abstract: Lightning has been reported to cause a large number of wind turbine failures and the grounding system of the wind turbine plays an important part in protection against this lightning damage. The design of the grounding system serves two main purposes. Firstly, the safety of personnel who are in the vicinity of the grounding system at the time of a lightning strike or power system fault must be ensured. Secondly, the lightning or power system currents must be dispersed into the ground while preventing large overvoltages. In this paper the transient behaviour of wind turbine grounding systems are studied. The modelling and analysis capabilities of the general purpose program EMTP (Electro Magnetics Transient Program) have been investigated with necessary supporting calculations being developed. Simulation results are presented using both EMTP and a specialist grounding software, CDEGS (Current Distribution, Electromagnetic fields, Grounding and Soil structure analysis). Practical examples of grounding system designs are given.
