MV Networks Research Articles

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.

Self-Extinguishing Faults in MV Cable Networks - Feasibility Study of Fault Prediction

The Dutch grid operator Alliander started an ongoing substation refurbishment in 2006 with SASensor, a measurement, protection and control system, including fault localization of persistent faults. Around 20 percent of the recorded measurements comprise the phenomena of 1-phase self-extinguishing faults. In order to assess the usability of these events to predict faults, the phenomena and their behaviour in time are described in MV networks with isolated neutral and low-impedance earthing. Furthermore the feasibility of fault localization using the information included in those events is discussed.

Maximal wind power penetration in radial MV networks

The increment of Distributed Generation (DG), especially wind power, in distribution networks has motivated the development of methodologies to evaluate their limits and effect in the power system. The distribution network is already characterized by variable parameters such as: loads, line parameters; the interconnection of renewable energy sources adds more uncertainties in planning studies. In this paper, a new approach is applied in Load Flow (LF) and Short-Circuit studies based on Monte-Carlo simulation (MCS) which takes into account network uncertainties to determine the maximal wind power penetration and its dispersion in the network.

Synthesis of LV load models for MV networks fast transient studies

This paper presents two techniques to obtain LV load models adapted for fast transient studies in MV networks. The first LV load model is based on CIGRE recommendations for harmonic impedance calculus while the second LV load model follows the analysis of the recorded waveforms in time and frequency domains. The results of the proposed models in ATP-EMTP environment are verified against the measured voltage and current waveforms.

A three phase photovoltaic power system connected to the MV network: behaviour during voltage dips

The presence of Distributed Generation (DG) in t he distribution network is increasing and represents one of the most important research areas. Several reasons have forced this increase like the possibility to use renewable resources reducing the fossil fuels use with environmental and energy benefits. The paper deals with the results of a study performed by RSE, in the frame of the Research Fund for the Italian Electrical System, focused on the behaviour during voltage dips of a DG system, consisting in a Photovoltaic generator (PV) connected to the network with three phase Voltage Source Inverter (VSI).

Current Transformers Effects on the Measurement of Harmonic Active Power in LV and MV Networks

The harmonic active power is used to determine the location of polluting loads, the direction of harmonic power flow and to estimate how consequential a certain current harmonic source is. The highest uncertainties, when measuring harmonic active power, are introduced by the current and voltage transducers. This paper presents experimental results which show the impact of current transformers (CTs) on the accuracy of the harmonic power measurements. An expression for the evaluation of the power error in distorted conditions was developed which can be easily utilized in the case of linear current transducers. It is shown how CTs cannot be considered linear transducers and how the higher the absolute value of the CT phase angle error is, the higher becomes the harmonic power error. The harmonic power error reaches maximum value when the phase shift between the voltage and current harmonic nears . Even high accuracy class CTs may not be adequate enough to assist on harmonic power measurement, while linear current transducers will perform satisfactorily.

Technological driving forces of LUCC: Conceptualization, quantification, and the example of urban power distribution networks

Consideration of technology is important for understanding the driving forces of human induced land-use and land-cover changes (LUCC). However, technological driving forces (TechDF) are seldom considered in quantitative analyses of LUCC because technology is difficult to quantify. In this paper we suggest addressing this difficulty by deducing quantification rationales from conceptualizations. We use the example of a comparative study of urban medium-voltage networks (MV networks) and urban land-use distribution to illustrate our approach. First, we outline how the effect of TechDF on LUCC can be conceptualized based on ‘contextualism’, a position concerning the role of technology in social and environmental change, and contrast this with the more common assumption of technological determinism. We outline further how MV networks can be conceptualized based on network genealogy, deduce a quantification rationale based on contextualism and network genealogy, and quantify MV networks. As a result of our case study we find that based on contextualism and network genealogy, the Tech DF of MV networks can be conceptualized as relational graphs of supplier–consumer interactions. Thus, the supplier–consumer-land-use interactions are specific for each context, for example for each city. A number of graph-theoretic measures of network topology provide a corresponding quantification rationale. Second, we use the results of a statistical analysis to discuss whether the quantification and quantification rationale for MV networks correspond to the conceptualization of MV networks based on contextualism and network genealogy. We answer this question broadly in the affirmative. Third, we discuss the issues that remain ambiguous for a conclusive answer. We also discuss the results of the statistical analysis in the light of comparative studies of urban land-use distribution. In conclusion, we regard combining conceptualization, quantification rationale, and quantification as promising in overcoming difficulties with the quantification of Tech DF in general.

Procedures for fault location and isolation to solve protection selectivity problems in MV distribution networks with dispersed generation

Present planning and operation criteria used for MV passive radial distribution networks are, in general, not suitable to cope with the presence of a significant distributed generation (DG) capacity. In fact, many technical problems are still to be solved in order to increase DG penetration and to provide, at the same time, high service quality levels to customers. In this context, the present paper illustrates the results of the first part of a research project, conducted in collaboration with ENEL Distribuzione S.p.A. (Italy), which analyses possible solutions for various protection issues arising in distribution networks in presence of DG. The paper deals, in particular, with the problem of incorrect operation of traditional protection schemes for multi-phase and phase-to-ground faults in MV networks, and proposes a protection philosophy based on innovative technical solutions to solve the problem of lack of protective devices coordination in presence of DG to improve service continuity.

Fast fault isolation and restoration of looped MV network

A new fault isolation and restoration technique employing local Ethernet network for the looped MV circuit is presented. It can isolate all short circuit faults in the loop within 300 ms and restore the supply of healthy sections in less than 1.2 s. A reliable fault location method of isolating the single phase-to-ground fault (SPGF) in the non-effectively earthed network, which makes use of the fault generated trnasient signals, is also proposed. And the method locates the SPGF by examining transient zero-mode current correlation, which is not influenced by the neutral grounding mode and need no additional device. Laboratory test and simulation results show the proposed techniques are feasible, and significantly improve the supply reliability.

DWT-Based Detection and Transient Power Direction-Based Location of High-Impedance Faults Due to Leaning Trees in Unearthed MV Networks

Electrical faults due to leaning trees are common in Nordic countries. This fault type has been studied in and it was found that the initial transients in the electrical network due to the associated arc reignitions are behavioral traits. In this paper, these features are extracted using the discrete wavelet transform (DWT) to localize this fault event. Wireless sensors are considered for processing the DWTs on a residual voltage of different measuring nodes that are distributed in the network. Therefore, the fault detection is confirmed by numerous DWT processors over a wide area of the network. The detection security is also enhanced because the DWT responded to a periodicity of the initial transients. The term for locating the faulty section is based on the polarity of a specific frequency bandpower computed by multiplying the DWT detail coefficient of the residual current and voltage at each measuring node. The fault due to a leaning tree occurring at different locations in an unearthed 20-kV network is simulated by the alternate transients program/electromagnetic transients program and the arc model is implemented using the universal arc representation. Test cases provide evidence of the efficacy of the proposed technique.

Mitigating Ferroresonance in Voltage Transformers in Ungrounded MV Networks

In this paper, it is shown that ferroresonant oscillations involving voltage transformers (VTs) may be initiated by switching transients in ungrounded medium-voltage networks. It was demonstrated that various modes of ferroresonant oscillations may exist for the same VT type depending on the initial conditions and on the network capacitance. As the ferroresonant oscillations pose a risk to the VTs due to large overcurrents in the primary windings, the VTs operating in ungrounded networks should be protected against this phenomenon. The analysis of the classic method of protecting the VTs by means of connecting the damping resistor to the open-delta-arranged auxiliary winding has shown that often a very low ohmic value of the resistor is required to provide reliable protection. A new method of protecting the VTs against ferroresonance with a compact active load was thus developed.

Voltage regulation in MV networks with dispersed generations by a neural-based multiobjective methodology

This paper puts forward the role of learning techniques in addressing the problem of an efficient and optimal centralized voltage control in distribution networks equipped with dispersed generation systems (DGSs). The proposed methodology employs a radial basis function network (RBFN) to identify the multidimensional nonlinear mapping between a vector of observable variables describing the network operating point and the optimal set points of the voltage regulating devices. The RBFN is trained by numerical data generated by solving the voltage regulation problem for a set of network operating points by a rigorous multiobjective solution methodology. The RBFN performance is continuously monitored by a supervisor process that notifies when there is the need of a more accurate solution of the voltage regulation problem if nonoptimal network operating conditions ( ex post monitoring) or excessive distances between the actual network state and the neuron's centres ( ex ante monitoring) are detected. A more rigorous problem solution, if required, can be obtained by solving the voltage regulation problem by a conventional multiobjective optimization technique. This new solution, in conjunction with the corresponding input vector, is then adopted as a new train data sample to adapt the RBFN. This online training process allows RBFN to (i) adaptively learn the more representative domain space regions of the input/output mapping without needing a prior knowledge of a complete and representative training set, and (ii) manage effectively any time varying phenomena affecting this mapping. The results obtained by simulating the regulation policy in the case of a medium-voltage network are very promising.

A novel selectivity technique for high impedance arcing fault detection in compensated MV networks

AbstractIn this paper, the initial transients due to arc reignitions associated with high impedance faults caused by leaning trees are extracted using discrete wavelet transform (DWT). In this way, the fault occurrence is localized. The feature extraction is carried out for the phase quantities corresponding to a band frequency 12.5–6.25 kHz. The detection security is enhanced because the DWT corresponds to the periodicity of these transients. The selectivity term of the faulty feeder is based on a novel technique, in which the power polarity is examined. This power is mathematically processed by multiplying the DWT detail coefficients of the phase voltage and current for each feeder. Its polarity identifies the faulty feeder. In order to reduce the computational burden of the technique, the extraction of the fault features from the residual components is examined. The same methodology of computing the power is considered by taking into account the residual voltage and current detail coefficients where the proposed algorithm performs best. Test cases provide evidence of the efficacy of the proposed technique. Copyright © 2007 John Wiley & Sons, Ltd.

Voltage and reactive power control in MV networks integrating MicroGrids

The main objective of this paper is to describe a strategy to deal with the voltage/reactive power problem for a MV distribution network integrating microgrids. The global problem, concerning all voltage levels, is detailed here and will imply the optimization of operating conditions by using the control capabilities of power electronic interfaces from DG sources, OLTCs and microgrids, through the application of an EPSO optimization algorithm.

Analytical prediction of abnormal 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. It will be demonstrated that, for extended cable/overhead MV distribution networks with ungrounded neutral, in case of low resistance faults at critical locations along overhead lines, the neutral voltage can reach very large values, significantly higher than 1 p.u. (up to 2.5 p.u.). The attendant temporary overvoltages on healthy phases can be very large, much higher than √3 p.u. (up to 3.5 p.u.). Fault currents are also affected, reaching twice the value calculated with simplified methods, i.e. neglecting series impedances. Quick yet accurate analytical formulas for the prediction of maximum overvoltages, fault currents and critical fault distances are presented, together with their validation by detailed ATP-EMTP simulations. The results of a study on an existing Italian 15 kV–50 Hz extended mixed cable/overhead distribution network are finally presented and discussed.

Regular paper section: Model for the apportionment of the total voltage drop in combined medium and low voltage distribution feeders

The maximum allowable voltage variation is a major and often primary constraint in the planning and design of distribution networks. In South Africa the apportionment of the allowable voltage drop between the MV and LV networks is not standardised and often not managed. The best voltage apportionment depends on network and load characteristics, including distribution transformer tap settings. A model is developed to calculate MV and LV voltage and voltage drop limits based on selected network and load characteristics. Comparing the results of the model with load flow simulations show the model to be suitably accurate for application to feeder calculations.

A new sensitive detection algorithm for low and high impedance earth faults in compensated MV networks based on the admittance method

Since the majority of all electrical faults in distribution networks are single line to earth faults, these events have a significant impact on the overall quality of power supply. The resonant earthing approach allows to limit the fault current in case of single pole earth faults (ground faults) and leads to a significant increased rise time of the recovery voltage. Consequently, most of the arcing ground faults will self extinguish and the quality of power supply is greatly increased. As a result a general trend towards the use of resonance earthing can be observed all over Europe. Due to the minimised fault current continuous network operation during sustained earth fault is possible. On the other hand, sensitive and highly reliable earth fault protection schemes are required to identify the faulty feeder without unwanted interruption of supply. In this paper, a new detection algorithm for low and high impedance earth faults based on the admittance method is presented.

Fuzzy-Heuristic Methodology to Estimate the Load Restoration Time in MV Networks

A marked feature of electrical markets in Latin America is the high level of penalizations imposed on the utilities on account of nonsupplied energy (NSE). These economical sanctions are intended as a preventive measure to keep the Electric Service Quality (ESQ) as high as possible. NSE is computed and controlled during a fixed period individually for each group of clients (INSE). When the INSE is violated, the utilities must pay a cost of penalization for the entire INSE. The total cost of penalizations will depend on the NSE allocation between different groups of clients. In case of a fault or simultaneous faults in MV networks, there are several alternatives for radial configuration operation to restore the service. They can be classified according to the switches that have to be operated for such an end, and each alternative has an associated INSE penalty cost that depends mainly on the restoration time implied. This work develops a fuzzy-heuristic methodology to estimate the times involved in the restoration times, and it is a part of a larger optimization method for solving the load restoration problem. The results from testing the methodology in a real 290-node network have proven the feasibility of this proposed tool.

Evaluation of the Operational Condition and Reliability of Surge Arresters Used on Medium Voltage Networks

Laboratory studies were conducted on gapped silicon-carbide arresters and gapless metal-oxide surge arresters (MOAs) gathered from 24 kV networks in Finland. A total of 410 arresters of 21 different types from eight different manufacturers were investigated. A total of 246 silicon-carbide arresters (age from 14 to 38 years) and 164 MOAs (age from 4 to 15 years) were studied at the High Voltage Laboratory of Tampere University of Technology. The results of the measurements on MOAs were compared to the measurements conducted earlier on 16 new, unused MOAs of the same types as those taken from the field. A portion of 16.7% (41 specimens) of the silicon-carbide arresters failed in the ac voltage withstand test and 34.5% (85 specimens) did not pass the lightning impulse sparkover test. The current impulse test of the silicon-carbide arresters resulted in a lot of damage. Due to the damage and failures in the electrical tests the authors recommend the electricity companies in Finland to exchange 7 types of silicon-carbide arresters to new MOA types in order to reduce the risk of arrester failures and to improve the protection levels in MV networks. All the ten MOA types studied were generally in good condition after being used 4-15 years in networks. Only three specimens (out of 164) could be evaluated to be faulty. Comparisons between the MOAs gathered from the networks and the unused MOAs showed that the protection levels of the MOA types studied had remained stable and the ac durability of these MOA types had not remarkably weakened during the use in networks.

Multiobjective design of distributed reactive power production in a deregulated electric market

Reactive flows compensation optimal design for losses reduction in MV automated distribution systems is a problem deeply studied in literature. In this paper, the design problem is formulated considering the new deregulated energy market. These new operational conditions allow the MV grid operator to consider the generation of reactive energy also as a service that can be sold to the transmission system above. In this case, the transmission system would be helped in all the working conditions in which a strong modulation of the reactive flow through the HV/MV node is required. Then, for the MV grid operator, the compensation system optimal design problem is therefore that of the search for the configuration (number, location and rated power of the capacitor banks) attaining the following main objectives: (i) to minimise power losses; (ii) to maximise the return on investment for the compensation system. Since results are strongly influenced by the parameter ‘economic value given to the reactive power’, RPEV, flowing from the MV network through the HV node, the optimisation problem has been parameterised for different values of RPEV and the total installed reactive power has been observed. A threshold value of this parameter has been identified. For values greater than the threshold, the proposed investment is economically profitable for the distribution grid operator, DGO.