Fabio Massimo Gatta

Modelling and computer simulation of dispersed generation in distribution networks. Measures to prevent disconnection during system disturbances

Dispersed generation (DG) from renewable resources and mini-cogeneration in public MV and LV distribution networks has so far been a small part of total installed capacity of power systems. This situation has justified the requirement in the technical standards in force in various countries that DG must be automatically disconnected at the occurrence of faults or abnormal operation conditions of the public network. A massive installation of DG is encouraged in many countries. If this target will be achieved, the automatic disconnection of large amounts of DG in an area initiated by network short circuits would drastically reduce the expected benefits of DG. The paper presents models for simulation of DG and investigates the possibility of keeping DG in service during network disturbances, in particular: assumptions are made on possible future extensive applications of DG in regional networks. Dynamic simulations of distribution networks with adequate models of DG using various interfacings (synchronous generators (SGs); asynchronous generators (AGs); static power converters (SPCs)) are set up. Results of parametric analyses are presented for significant case studies and conditions to be fulfilled are identified for avoiding or minimizing disconnection of DG during short circuits in HV, MV and LV networks. Simulations are made with PSAF program for the comprehensive study of DG interfaced with SGs, AGs, and SPCs. The ATP-EMTP is used for the analysis of special transients. Results show that it is possible to ride through the network faults by keeping in service DG and loads except SGs connected to faulty distribution lines.

Ground Fault Temporary Overvoltages in MV Networks: Evaluation and Experimental Tests

Single-phase-to-ground faults may cause substantial temporary overvoltages (TOVs) in large radial medium-voltage networks with isolated neutral, even over 3-p.u. phase to ground. Resonant neutral earthing limits these overvoltages to 1.8 p.u. but credible earthing apparatus failures might trigger TOVs up to 2.4 p.u. This paper presents the ground fault study of an Italian 20-kV ENEL Distribuzione network. Analytical evaluations in a wide parametric range of neutral earthing arrangements, include isolated neutral and ENEL resonant earthing with parallel resistance, as evidence of 2.4-p.u. TOVs with isolated neutral, 1.8 p.u. with resonant earthing, and more than 2.0 p.u. with partial compensation. Recordings of ground faults staged in the same network are presented, showing excellent agreement between analytical predictions and experimental test. The tests confirm TOVs of more than 2.3 p.u. with isolated neutral, sometimes evolving into cross-country faults (possibly explaining unforeseen cable fault rates), and the effectiveness of the ENEL neutral earthing practices in suppressing these TOVs.

Very long EHV cables and mixed overhead-cable lines. Steady-state operation

The paper deals with very long EHV cables (say, over 30 km) and their application in EHV mixed lines, i.e. those formed by a cable line between two sections of overhead lines. Such very special lines could be built for instance at the northern borders of Italy, laying the cable in the service gallery of the extra-long high-speed railway tunnels planned for construction under the Alps. EHV cable lines over 25-30 km long require the compensation of the reactive power generated by the cable, by means of fixed or variable shunt devices. After presenting expressions for the calculation of the maximum permissible length of EHV cable lines, the paper shows that 420 kV-50 Hz cable lines long up to 90-100 km, transmitting an active power up to 90% of the cable thermal limit, could be operated by using only two shunt reactors at the cable terminals. Fixed shunt reactors are adequate if the mixed line is practically symmetrical, i.e. the two stretches of the overhead line are of same length. On the other hand, if one of the overhead line sections is much longer than the other, or the terminal voltages of the mixed line are rather different, variable compensation allows to fully exploit the carrying capacity of the cable. The paper presents an algorithm to find the optimal taps of the variable shunt reactor. Results of the steady-state simulation of two hypothetical 420 kV-50 Hz mixed lines are finally reported, also showing that shunt reactors contain efficiently temporary overvoltages due to load rejection and no load energization.

Steady-state operation of very long EHV AC cable lines

Technical improvements in the construction of EHV cables have made possible the installation of very long EHV AC cable lines; with a sufficient degree of voltage control such lines can retain a large part of their theoretical power transmission capability. Quick, accurate expressions for the evaluation of cable length-loading relationships are given to that purpose. Appropriate choice of shunt compensation by standard, fixed-type shunt reactors, aimed at solving most steady-state constraints on the operation of very long EHV cable lines as well as temporary overvoltages, is discussed. Analysis of the operating envelopes of very long EHV AC cable lines shows that line losses play a very limited role. A simple criterion for optimal utilization of real, lossy cable lines is also proposed.

TRVs across circuit breakers of series compensated lines-status with present technology and analysis for the Turkish 420 kV grid

The analysis of transient recovery voltages (TRVs) across circuit breakers of EHV lines, compensated with modern series capacitors protected by varistors, is presented. The applicable measures for limiting TRVs with present station equipment technology are updated. A physical explanation of the phenomenon is presented. This is useful for classifying the series compensation transmission schemes on the basis of the TRV magnification they can cause. Series capacitors protected by varistors result in higher TRVs than capacitors protected by spark-gaps, because trapped charge voltage remains on the capacitors following clearing of faults of any type and location. Simplified formulas are provided for a quick approximate estimation of TRVs. Means for limiting TRVs are reviewed. A solution, namely varistors branched across line breaker contacts, is explored. Results are presented of EMTP TRV analysis performed for three groups of series compensated 420 kV lines in Turkey, and measures for limiting TRVs are described. It is shown that line outage contingency cases and line fault clearing by breaker failure protection may cause a magnification of TRVs, to which attention must be paid. >

Temporary overvoltages due to ground faults in MV networks

The paper deals with the temporary overvoltages that build up in radial MV distribution networks following the inception of a 1-phase-to-ground fault (1-Φ -to-Gr). For extended cable/overhead MV distribution networks with ungrounded neutral, in case of low resistance faults at critical stretch of overhead lines, in [1] it has been evidenced that the temporary overvoltages on healthy phases can be very large, much higher than √3 p.u. (up to 3.5 p.u.). Fault currents can reach twice the value calculated with simplified methods, i.e. neglecting series impedances. In this paper the study is extended to MV networks with neutral grounded by both Petersen coil and compensating impedance (Petersen coil with a resistance in parallel), in normal operation and under contingency, i. e. in case of whole or partial loss of the compensating impedance. It is demonstrated that the presence of Petersen coil, stand alone or in parallel with a grounding resistance, drastically reduces the above temporary overvoltages at values not greater than 1.7-1.8 p.u. Application of simple derived formulas to the case of partial loss of the compensating neutral impedance show that overvoltages can be reduced at 1.8-5÷2.2 p.u., also in case of MV network having very high capacitive fault current (e.g. ≥300 A) and long overhead lines. An ATP case study on an existing 20kV large Enel-Distribuzione network reported in the paper confirm that the theorical predicted overvoltages are in the above mentioned range, and that the technical solutions adopted by Enel-Distribuzione [9-15] are able to limit in most cases the overvoltages at values not greater than 1.85 p.u.

An ATP-EMTP Monte Carlo procedure for backflashover rate evaluation

The paper presents the ATP-EMTP implementation of a Monte Carlo procedure aimed at evaluating the backflashover rate (BFOR) of an HV overhead line (OHL). The ATP-EMTP circuit model of the OHL includes detailed line insulation and lightning representation; spatially extended and/or involved grounding systems are represented by a new, simplified model which reproduces the effects of propagation and soil ionization phenomena; statistical input data concerning lightning polarity, lightning stroke parameters (peak current, front and tail times), lightning location, line insulation and phase angle of the supply voltage. An external software engine generates all the required statistically-oriented ATP-EMTP input data, sequentially launches and manages ATP simulations and post-processes all results. An application on a 150 kV - 50 Hz typical Italian OHL is reported and discussed.

Design and operation of EHV transmission lines including long insulated cable and overhead sections

Application of EHV lines formed by series connected overhead sections and XLPE-insulated cables sections is planned in various projects in Europe. At first the maximum feasible length of 380 kV-50 Hz and of 500 kV-60 Hz XLPE cable lines is calculated as a function of the power carrying capacity derating due to charging current. Then shunt compensation and optimal voltage-reactive power control are analysed with use of shunt reactors with tapped windings and on-load tap changer for regulation of Mvar output. The authors ATP-EMTP electromagnetic transient analysis of the long mixed EHV lines has revealed the risk of sustained overvoltages due to resonance on 3rd harmonics. The phenomenon is described and countermeasures are proposed. The feasibility of the single-pole high speed reclosure of mixed EHV lines is analysed, and means for limitation of the secondary arc current are examined. A protection scheme is proposed for fast selective detection of faults and for implementation of the single-pole reclosure only in the cases of 1-Phi-to-Gr faults in the overhead sections of mixed EHV lines

Hybrid and pi-circuit approaches for grounding system lightning response

Prediction of the potential threats due to direct lightning strokes of sensitive targets, e.g., overhead power-line towers, substations or buildings, requires accurate modeling of the transient impedance of grounding systems. When large-scale simulations are needed, detailed and computationally intensive models are hardly viable, although they usually offer great accuracies. In this work we investigate the suitability and compare the accuracies of two different approaches. In the first we adopt a hybrid circuit-field approach to predict the behavior of grounding systems at any frequency of interest. This approach is based on circuit theory and Method of Moments, and is capable to fully account for resistive, inductive and capacitive couplings between elements of the grounding system. Since the analysis is carried out in the frequency domain, the actual transient behavior can be obtained by means of an Inverse Fourier Transform. In the second approach we consider a fully circuital approach. It is a genetic algorithm-based procedure able to synthesize a generalized equivalent pi-circuit model that is used to simulate the impulse response of complex grounding systems, including possible nonlinear soil ionization, working directly in the time domain.

Battery energy storage efficiency calculation including auxiliary losses: Technology comparison and operating strategies

The overall efficiency of battery electrical storage systems (BESSs) strongly depends on auxiliary loads, usually disregarded in studies concerning BESS integration in power systems. In this paper, detailed electrical-thermal battery models have been developed and implemented in order to assess a realistic evaluation of the efficiency of NaS and Li-ion batteries. BESSs have been sized in order to operate on a real low voltage distribution network, based on load and photovoltaic generation measurements during an 8-month period. The performance of NaS and Li-ion batteries have been evaluated for two different operating strategies. Results show that, considering auxiliary losses, overall efficiencies of both technologies are very low with respect to the charge/discharge efficiency. Finally, two simplified formulas, able to evaluate the efficiency and the auxiliary losses of a NaS BESS, are presented.