Distribution systems - Smart Grids

Topics of the research area

The activity focuses on the analysis of:

- electromagnetic transients, overvoltage protections and insulation coordination: particularly referred to overvoltages
  induced by indirect lightning; 

- fault location procedures in distribution lines through voltage/current fault transient analysis;

- distributed generation from renewable resources and active microgrids: effects analysis and algorithm of optimum
  management of generation, storage and compensation resources connected with voltage adjustment, overload lines
  and minimization of losses, islanding operation;

- development of Phasor Measurement for measuring voltage syncrophasors in grid's nodes and state estimation;

- development of measuring automated system for power quality analysis;

- distributed measurement systems for power quality monitoring;

- lightning-induced voltages

- communication networks for distribution systems

The description of some research projects is reported here below. 

• Fault location procedures in distribution lines through voltage/current fault transient analysis
  This research activity, initially carried out within a collaboration with CESI-Italy, is aimed at developing an effective and reliable fault location procedures in power distribution systems in order to reduce the subsequent restoration times and increase the service continuity. In particular, the activity has developed a fault location procedure based on the analysis of the electromagnetic transients originated by the fault and associated to the travelling waves triggered by the fault itself. The procedure uses the continuous wavelet analysis that allows the extraction of characteristic frequencies from the fault transient to permit the identification of both the faulted branch and the fault location. The fault location accuracy has been improved by a specific mathematical algorithm based on the identification of the part of the fault transient necessary to build the specific so-called mother wavelet. Several experimental tests on both reduced scale and full-scale setups have been proved the robustness and the reduced uncertainties of the procedure in the identification of the fault location which has been than implemented into an embedded microcontroller.
   

• Distributed generation from renewable resources and active microgrids
  This research activity focuses on the optimal operation of active distribution network in presence of distributed generation and storage devices. The aim of the activity is the development of algorithms and relevant procedures, that minimize specific object functions (e.g. feeder voltage profiles, network losses, branches transmitted power) by properly changing the set points of the various distributed energy resources (active-reactive power, position of transformers OLTC, etc.) taking into account both technical/economical constraints, reservoir limits, the non-dispatchable availability of renewable resources and the possible operation of the distribution network in islanding conditions. Concerning this last aspect, a specific algorithm has been developed in order to optimally manage the intentional, or unintentional, islanding maneuver of active distribution networks. These algorithms have been implemented into a short-term (15 min.) scheduler which performances have been experimentally validated by means of specific tests carried out into an experimental microgrid facility installed at CESI, Milan, Italy.
   

• Development of Phasor Measurement for measuring voltage syncrophasors in grid's nodes and state estimation
  The measurement of synchronized phasors is one of the key tasks for the operation of modern power transmission networks and represents one of the main functions to be implemented in the future active distribution systems in order to evolve these networks into smart grids.As discussed in the literature, the control functions implemented in the so-called Distribution Management Systems (DMSs) of active distribution networks present some similarities with those of transmission networks. In this respect, the extension of the concept of Wide Area Monitoring Systems (WAMSs) from transmission to active distribution networks can be certainly considered as one of these functions.Within this context, the peculiar characteristics of distribution networks compared to transmission ones (e.g. different feeder impedances, reduced power flows and high levels of harmonic distortion) do not allow for a straightforward implementation of these PMUs. Therefore this research activity was aimed at develop specific algorithms to improve the accuracy of the synchrophasors estimation. In particular, it has proposed a time-domain tone-reconstruction algorithm, based on the use of the Discrete Fourier Transform (DFT), implemented into a specific real-time microcontroller in order to develop a PMU prototype, based on a DSP-FPGA architecture, which characteristics have been shown to be adequate for its use in active distribution networks. The developed prototype has been then used to monitor the steady state and islanding maneuver of an active distribution network and provide to the network operator the relevant feedback.

   

• Distributed measurement systems for power quality monitoring
  This activity is carried out in close cooperation with the Electrical and Electronic measurement group, within the framework of a research program supported by the University of Bologna (Funds for Selected Research Topics).The research activity is developed within the research project entitled "Study, project and realization of an automatic measurement system prototype with innovative characteristics for the power quality analysis". Aim of this research is the realization of a distributed measurement system able to perform measurements, relevant to the power quality, in the nodes of a distribution power system. In particular the LISEP is involved in the analysis and characterization of a-periocal disturbances. Within the research activity the LISEP has collaborate to the development of the hardware relevant to the instrumentation having complex and distributed architecture for the analysis of the power quality. It is analyzed the possibility of use of virtual instrumentation with complex architecture in which the measurement hardware can be composed by different commercial instruments and data acquisition systems suitable interfaced and managed by a central unit which provide the on-line and off-line elaboration and analysis of the acquired signals. In the research activity is presented a method aimed at the simultaneous use of data provided by distributed measurement system (having the above mentioned complex and distributed architecture) and data provided by lightning location systems for correlating lightning events with distribution systems a-periodical disturbances.
   

• Lightning-induced voltages

The LISEP activity is coordinated with that of CIGRE Working Group C4.1 "Lightning", with that of the JWG CIGRE C4 –CIRED 05 "Protection of MV and LV networks against lightning" and with the IEEE WG "Lightning performance of distribution lines" and is carried out within the framework of an international research collaboration among the University of Bologna (Department of Electrical Engineering), the Swiss Federal Institute of Technology (Laboratoire de Reseauxd’énergie électrique) and the University of Roma "La Sapienza" (Department of Electrical Engineering). In the last years,LISEP has extended the collaboration to other Italian and foreign universities, research centers and companies: University of Florida, Lisbon Polytechnic, University of Uppsala, ENEL-CESI, University of Armed Forces in Munich, University of Arizona, EdF-Electricité de France, University of Sao Paulo, University of Pisa and University of Naples. 

The research develop in the following point:

1. Lightning Electromagnetic Pulse (LEMP) characterization
   The research activity concerns either the frequency domain and the time-domain LEMP characterization. One of the goals is the development of engineering lightning return-stroke models for the spatial-temporal description of the lightning current along the channel during the return-stroke phase. The “Modified Transmission Line (MTL) model” is one of the results of such an activity. Such a model has been adopted either by ENEL and by EdF for the LEMP calculations. It is compared with other ones presented in the literature and with available experimental results. The research has been aimed also at gathering new experimental results for LEMP characterization and model validation: they have been obtained in occasion of some experimental campaigns carried out in collaboration with the Swiss Federal Institute of Technology and with the University of Florida. The MTL model has been extended to take into account the decrease of the return stroke velocity during the return-stroke phase, and used for the calculation of the horizontal component of the electric field. It has been further extended to the case of an elevated strike object, representing instrumented towers; its validation has been achieved by means of experimental results gathered at the Munich University of Armed Forces.
    
2. Critical analysis of LEMP-to-transmission line coupling models and their extension to overhead distribution lines above a lossy ground
   The research activity is aimed at:
   - Comparing the existing coupling models for the estimation of LEMP-to-transmission line coupling (models which  appear formulated in different ways), an issue which has been and still is of great interest in the power/lightning literature.
   - Analyzing the role played by the soil resistivity: depending on the stroke location and on the observation point the soil resistivity can result in an enhancement of the induced voltage amplitude with respect of the case of a lossless ground, due essentially to the its effect on the exciting LEMP. A simplified method is proposed, aimed at the evaluation of the induced voltages for multiconductor lines above a lossy soil. The critical analysis of the multi-conductor line resistance matrix expressions proposed in the literature is carried out and a new analytical expression for time-domain solution of the coupling equations is proposed.
    
3. Analysis of the lightning- induced voltages and development of computer codes for their evaluation on power networks with realistic configuration
   
   LIOV code
   The main effort of the activity is devoted to the development of a computer code, LIOV (“Lightning-inducedovervoltages”), which allows for the calculation of lighting-induced voltages on multiconductor lines above a lossy soil as a function of line geometry, lightning current wave shape, return-stroke velocity, soil resistivity, etc.. For the LEMP calculation, the developed MTL model is used; concerning the electromagnetic coupling, the model implemented, based on the transmission line theory and on the formulation by Agrawal et al. is adopted. Experimental validation of the LIOV code is performed. The effectiveness of shielding wires (assumed grounded only at the line terminations) as well as some theoretical aspects of the problem are discussed and clarified. Leader induction effects are discussed and the influence on corona effect is analyzed. Both leader and corona effect are shown to be important for particularly close or severe strokes and both can result in amplitude enhancement of the induced surges. The LIOV code has been suitably modified to deal with the above cases. A further enlargement of the code which allows for the treatment of non linear and/or capacitive line terminations has been accomplished in cooperation with CESI. The LIOV code has been employed, in cooperation with ENEL, to perform a first analysis of the response of MV overhead lines to lightning induced overvoltages. A further improvement of the LIOV code has been achieved by implementing a modification to the model by Agrawal et al., which allows for the treatment of LEMP-illuminated multiconductor overhead lines above alossy ground with multiple groundings of the shielding wire or the neutral conductor (if any) or of the phase conductors through surge arresters or spark gaps. The experimental validation of such an improved LIOV version, based on a 2nd order finite difference integration scheme, has been accomplished by means of tests carried out on a reduced scale model illuminated by the NEMP simulator of the Swiss federal institute of technology 'SEMIRAMIS'.
   
   LIOV-EMTP code
   Although the LIOV code can be in principle enlarged and adapted to deal with any type of network configuration, it has been considered it more straightforward to link it with the Electromagnetic Transient Program (EMTP), making thus it possible to deal with distribution networks any complex. This research, initiated in cooperation with ENEL-CESI, has brought to the development of the so-called LIOV-EMTP code, which has been used within the framework of an international Cigré-Cired Task force for the analysis of the LEMP-induced disturbances on LV distribution networks having complex topology and, in cooperation also with the University of Pisa, for the estimation of the overvoltages transferred from MV to LV through distribution transformers. The LIOV-EMTP code, has been then improved, still in cooperation with ENEL-CESI, to take into account the presence of the stationary industrial-frequency voltage, and of such an activity it has been referred at the Cigré general session in Paris, in 1996. The LIOV-EMTP code has been compared with a similar version of it, subsequently developed at EDF. The new version of the LIOV-EMTP code (LIOV-EMTP96) has been realized at the University of Bologna, in cooperation with the EPFL. Compared to the previous version of the LIOV-EMTP, the new code is provided with a more stable integration routine, does not imply any modification to the source code to be realized, and allows for a more straightforward construction of the simulation case. A MATLAB version of it has been realized too (Mat-LIOV) which exploits the capabilities of the Power System Blockset. These codes have been tested versus experimental results obtained through an experimental campaign carried out for such a purpose in Switzerland. LISEP participates to the experimental activity carried out at the ICLRT (International Center on Lightning Research Tests), Camp Blanding, Florida, each year. During the two summers of 2002 and 2003, additional experimental results useful for code validation have been gathered on a unenergized distribution line, and some tests on underground cables have been accomplished too.
   
   The results of the research activity have been presented also at invited lectures and tutorials at various international conferences.
   
    
4. Insulation and protection coordination of overhead distribution lines with reference to indirect lightning for power quality improvement
   This research activity is aimed at developing a procedure, based on previously mentioned models and on the Monte Carlo method, which allows the estimation of the lightning performance of a distribution line, expressed in terms of number of flashover per year VS the line critical flashover voltage, and the choice of the most appropriate means for reducing the number of flashovers (use of shielding wires, surge arresters, etc.). The procedure developed allows for the estimation of the lightning performance of a distribution line as a function of several parameters, such as the soil resistivity, the lateral distance expression, the influence of the return stroke speed, taking into account possible correlations between such a parameter and the current amplitude. The proposed method is compared with others presented in the literature and with the one proposed by IEEE (Std. 1410) showing its more general capabilities. The influence of the line grounding electrodes modeling on the line performance has been analyzed. Clearly, the activity takes advantage of the availability of the LIOV-EMTP96 code, described above. Another topic faced within this research activity is the determination of the correlation between voltage sags and lighting events in power distribution networks, which is carried out also in cooperation with CESI. There are a number of reasons for voltage sags in distribution networks: there is some evidence, however, that in electrical systems located in regions with high value of isoceraunic level, lightning can cause the majority of voltage sags. Data from Lightning Location Systems, which provide an estimation of both lightning flash location and return-stroke current amplitude, can then be used to understand whether lightning is indeed the real cause of circuit breaker operation during thunderstorms – which means, in turn, of voltage sags – or not. Due to the complexity of the problem, the information coming from LLS (lightning location and estimate of lightning current amplitude) are, in general, not enough to infer the origin for voltage sags. It is necessary to suitably integrate them with data from system monitoring, e.g. relevant to the intervention of circuit breakers in primary substations, and with simulation results from of accurate models for computing lightning–induced voltages on complex power networks.
   
    
5. Critical reassessment of statistical distributions of lightning current parameters
   Statistical distributions of lightning current parameters are one of the fundamental inputs needed for the statistical assessment of the lightning performance of transmission and distribution lines. Several researchers have devoted many efforts in the past years to obtain meaningful statistics, all based on direct current measurement data gathered by means of instrumented towers. They still serve as conventional input for insulation coordination studies (e.g.: Berger et al., Electra, 41, 1975; R.B. Anderson and Eriksson., Electra, 69, 1980). Nowadays, some questions concerning the need for revisiting these statistical distributions, especially concerning the current amplitude, appear to exist, and this for a number of reasons, among which:
   - the large amount of data gathered by means of lightning location systems provides statistical distributions of the current peak amplitude that do not agree, in general, with the above mentioned ones; - current waveforms directly measured using instrumented towers could have been ‘altered’ in some cases by current reflections occurring at both tower top and bottom;
   - "conventional" statistical distributions of lightning current peak amplitude have been gathered using tall-instrumented towers relative to flat ground or so, and are therefore ‘biased’ towards higher amplitudes. A procedure based on the Monte Carlo method, which allows to obtain the statistical distributions of lightning current parameters at ground level starting from those inferred from experimental data recorded by means of tall instrumented towers, has been developed. The proposed procedure is more general than others presented in the literature for the same purpose, in that it can be applied whatever model is adopted to represent the exposure of the tower to direct strokes, and because it allows to quantify the tower effect on the statistical distributions of all lightning current parameters of interest, and not only of the peak value one. The statistical distributions at ground, calculated with the proposed procedure, have been applied for the assessment of the lighting performance of distribution lines. The obtained results find also a useful application for the assessment of current statistical distributions based on data from lightning location systems. 
   
   Also for this subject the results of the research activity have been presented also at invited lectures and tutorials at various international conferences.