M. Maccioni

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.

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.

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.

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.

Tower Grounding Improvement Versus Line Surge Arresters: Comparison of Remedial Measures for High-BFOR Subtransmission Lines

This paper presents a technical/economic comparison between remedial measures aimed at improving the lightning performance of an existing Italian three-phase 150-kV overhead line. The line is characterized by a very high back-flashover rate (BFOR), due to large grounding resistance values. Two countermeasures are proposed: grounding system improvement with additional vertical rods and line metal oxide surge arrester (MOSA) installation on one or all phases. A Monte Carlo ATP-EMTP procedure developed by the authors, which takes into account both the tower grounding nonlinear transient response due to soil ionization and MOSA nonlinear response, has been applied to evaluate and compare the effectiveness of the proposed countermeasures. The installation of MOSA on all phases is technically the best option, but it is relatively expensive. Tower grounding improvement and MOSA installation on the lower phase yield very similar BFORs: the economic comparison strongly depends on towers accessibility and soil nature.

Power system adequacy: An efficient procedure based on genetic algorithms

The paper presents a methodology which can be used to improve the static adequacy of high voltage (HV) transmission systems under contingency. The most suitable corrective actions for bringing the power system back to acceptable operation conditions are identified by means of a power system management software. The proposed procedure combines a micro genetic algorithm (μβΑ) optimization procedure with a load-flow program. The foreseen control actions consist in change of network configuration, generation re-dispatching, transformer tap setting, insertion and/or regulation (if variable) of shunt reactor and capacitor banks, load shedding. Several case studies, including an application to the Italian EHV/ HV transmission grid are presented and discussed in order to evaluate its possible use by a Transmission System Operator (TSO).

Equivalent lumped parameter Π-network of typical grounding systems for linear and non-linear transient analysis

In this paper, the authors propose and validate a simple lumped parameter model based on a π-type network. This model is able to simulate the ground potential rise of typical tower grounding systems under surge conditions. The lumped parameter values are estimated by means of an optimization procedure based the micro-genetic algorithm (μGA). This procedure minimizes the standard deviation between target values (obtained using a full circuit model able to perform frequency domain analyses and time domain analysis under linear and non-linear conditions) and simulation results (obtained using the equivalent π-network model). The proposed model has been validated by means of massive simulations of typical Italian 380 HV transmission line grounding systems under surge conditions (varying impulsive current wave shapes, grounding system geometrical configurations, soil characteristics and also accounting for non-linear ionization phenomena). The simulation results have always revealed a very good agreement between target values and model results. In addition, the proposed π-network model drastically reduces the computational resources required for linear and non-linear transient analyses of typical tower grounding systems.

Cross-country fault protection in ENEL Distribuzione's experimental MV loop lines

In the last years, high penetration of distributed generation (DG) has strongly affected public MV networks. In order to improve their reliability and availability, loop operation is being tested. This solution requires the installation of new line protection devices and the identification of appropriate settings against phase-to-phase and ground faults. This study refers to an experimental loop line operated by ENEL Distribuzione, the largest Italian Distribution System Operator (DSO). The paper deals with the behavior of the currently installed line protection during cross-country faults (CCFs), occurring both in radial and loop lines. MV network response has been preliminary evaluated by approximated formula (which allow a fast check of line protection performances), and then verified by accurate simulations based on a detailed Simulink network model. CCFs occurring inside and outside the loop line have been analyzed. Obtained results show that the zero-sequence and phase directional overcurrent (Blocking-Scheme supported) are generally able to selectively identify and clear CCFs. Nevertheless, if CCFs occur in the same overhead stretch of the loop line, the adopted protection scheme may fail.