Powersys has discussed the LIOV Toolbox with Mr Carlo Alberto Nucci, Full Professor, Coordinator of the Power Systems Laboratory of the Dpt of Electrical, Electronic and Information Engineering ‘Guglielmo Marconi’ at the University of Bologna.
He is author/co-author of over 300 scientific papers published on peer-reviewed journals or on proceedings of international conferences. He also wrote six book chapters edited by IEE, IET (two), Kluwer, Rumanian Academy of Science and WIT press and of a couple of IEEE Standards as well as some CIGRE technical brochures.
What is the phenomenon of Lightning Induced Over Voltage?
It is a complex phenomenon, which took a somewhat long time to be studied and understood in its main aspects.
According to the first interpretation given by Wagner (K.W) at the beginning of the 20th century, the induced lightning over-voltage is produced basically via electrostatic induction by a charged cloud. When the lighting discharge occurs, according to K.W. Wagner, the charge bond in the line is released in form of travelling waves of voltage and currents. However, in his study, he did not take into account the field radiated by the lightning return stroke current.
Some 40 years later, C.F. Wagner and McCann, based on Schonland's investigations on the nature of the lighting discharge, published a landmark paper on the AIEE Transactions (now IEEE Transactions) in which the overvoltage was assumed to be due mostly to the return stroke phase, an assumption that was accepted in practically all following papers dealing with the subject and that was more recently verified by means of important experimental campaigns in Florida and Brazil, respectively on real scale and reduced scale models.
The analysis of lightning-induced voltages is much more complex than the one of direct lightning originated ones, as the surge propagation is the result, not of an inject current, but of the interaction between the current along the lighting channel during the return stroke phase and the line conductors. It therefore involves electromagnetic field and electromagnetic coupling calculations.
The effect of the return strokes velocity, of the channel base current wave shape (peak amplitude and maximum time derivative), of the value of the soli resistivity, just to name a few of the most influential parameters involved, play a role in determining the amplitude and wave shape of the induced voltages and requires advanced models to be properly evaluated. The problem has received growing attention in the last decades due to the increasing requirements for electric power quality and the parallel diffusion of sensitive devices.
How can we study this phenomenon? By using what methods?
This phenomenon can be studied both theoretically and experimentally. Experimental results can serve excellently to validate the developed theories/models. In this respect it is worth noting that the inherent theoretical complexity is enhanced by the specific requirements of the experimental installations, which need to be designed and realized to measure contemporary channel base currents, radiated electromagnetic fields and induced currents and voltages.
From the theoretical point of view, in most cases the use of the electromagnetic coupling models based on the transmission lines approximation, such as the model by Agrawal, Price and Gurbaxani, can be successfully used for this purpose. Available lightning return-stroke current models can be also successfully employed to appraise the electromagnetic field radiated by the lighting channel by means of Maxwell’s equations integration.
For what reasons has the LIOV code been developed?
The main reason is that with the LIOV code, the assessment of the lightning performance of a distribution line becomes possible in a much more accurate way than when using previously available models/codes.
The effects of the soil resistivity, shield wires, stroke location (and observation point alone the line), can be dealt with the accurate models implemented in the LIOV code, which represent the state of the art for this field.
Have you been working with others universities?
The LIOV code has been developed within the framework of a joint research cooperation involving the University of Bologna, the Swiss EPFL (the EMC Group of Prof. Farhad Rachidi), and the University of Rome ‘La Sapienza’ (Prof. Carlo Mazzetti). Its experimental validation has been achieved also thanks to the cooperation with of University of Florida and the University of Sao Paolo.
Some specific issues, such as the taking into account of corona effect has been achieved in cooperation of the Instituto Superior Tecnico of Lisboa and Bucharest Polytechnic.
The first interface with EMTP was presented in 1994 at the Budapest International Conference on Lighting Protection, but the core of the code was developed some years before.
What are the benefits of this tool?
Distribution systems have configurations that largely differ from the one consisting of a single-conductor overhead straight line with no laterals, usually assumed even in modern standards.
The LIOV-EMTP code instead allows to compute lightning-induced over voltages on distribution networks having complex, realistic configurations and structure: the presence of power components and surge protective devices can indeed be taken into account. This is accomplished in short calculation time thanks to the analytical expressions implemented within the code for the appraisal of the electromagnetic field.
There are a number of problems that can be solved with LIOV-EMTP which power distribution utilities have now to face. Think for instance at the distribution system operators interested in changing the earthing method from solidly grounded to resonant one: this makes it crucial to distinguish single-phase to ground faults from other types of faults. In addition to that, the increasing deployment of distributed generation is leading towards more complex network structure and dynamic behaviors. Only with a module such as LIOV, suitably interfaced with EMTP-RV, can these issues be addressed.
The LIOV module can be mainly used by University Researchers, Distribution and Transmission Systems Operators, Power Utilities, Docents, Technical Bureaus.
What major improvements could be made in future releases?
- Possibility to exploit the multi-conductor model included in LIOV, useful for instance to deal with the case of double circuit overhead lines
- Possibility to exploit statistical events generation
- Inclusion of advanced expressions for taking into account lossy soil effects
- Possibility to include steady-state initialization (utility voltage in addition to lightning induced voltage)
- Possibility to take into account the corona effect
- Possibility to take into account the leader phase prior to the return-stroke phase
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