PERFORMANCE OF PARALLEL SURGE ARRESTERS

The operation of parallel surge arresters can improve energy absorption capability if the arresters are similar and are installed close to each other. However, it has been reported that any small difference in the individual V-I characteristics can lead to unbalance in current sharing. When the arresters are installed some distance away from each other, travelling wave effects can modify the effectiveness of parallel arresters for surge overvoltage protection and, in this case, detailed simulations are required to ascertain the level of protection. Such a situation occurs in practice with large substations or short underground cable connections. Various studies have shown that a requirement for two-arrester protection is closely dependent upon the type and length of cable used. In the case of overhead lines, the distances are much bigger and the main objective of line arresters is to reduce the flashover rates due to surge overvoltages. This is especially relevant to lines located in regions of high lightning activity, lines with compact/uprated design where the phase-to-phase and phase-to-earth air clearances are reduced. In this paper, we present a study of parallel arresters considering the separation distance and their application to overhead lines. Various scenarios of overhead line configurations were considered and the overvoltage levels were calculated for each case. Assessment of flashover performance is also conducted for a number of conditions. A number of calculation techniques were used and compared

Modeling and experimental investigation of lightning arcs and overvoltages for medium voltage distribution lines

In this dissertation, lightning overvoltages in Medium Voltage (MV) lines are thoroughly investigated. The other goal is to propose new protection schemes for the designs. The lines consist of overhead lines, underground cables and covered conductors. These overvoltage problems range from direct and indirect strokes to lightning arcs. All the models and simulations are developed using the Electromagnetic Transient Program (EMTP) and Finite Element Method (FEM), while MATLAB is used for post-processing the results and identification of the model parameters. Improvement in the surge protection of MV overhead lines is demonstrated with a combination of surge arresters and a shield wire. Using the IEEE 34-node feeder injected with multiple lightning strokes, the feeder is simulated using EMTP. The response of the line is modeled both with and without the surge protection devices. The simulation study extends to the performance of a MV underground cable due to a nearby lightning discharge using FEM. The use of shield wire for limiting the overvoltage stress in the cables is proposed. A numerical analysis and simulations are performed to determine the outage rate of MV covered conductors due to lightning strokes of different characteristics. The optimum distance for surge protective devices on the conductors is also assessed. An enhancement in the surge analysis of distribution lines with the shielding effect of trees is proposed. An experimental study shows that a tree can intercept a lightning stroke in the vicinity of a distribution line. This study also analyzes experimental results of the shielding effectiveness of a tree and the induced voltages existing between the tree and the distribution line. The study is extended to evaluate the induced voltage on a distribution line for larger clearances using a Rusck model. This work investigates the lightning arc between an overhead line and a nearby tree under artificial rainfall. A full-scale laboratory experiment confirms that a direct stroke to a tree can cause severe damage to nearby power lines by initiating an arc channel through air to the conductors. A complete model of this phenomenon is developed by combining the existing static and dynamic arc equations. The model is accomplished by the bilateral interaction between the EMTP and Transient Analysis Control System (TACS) field. The experimental results have been reproduced by the computer simulations. The performance of the arc phenomenon is examined using a typical Finnish distribution network design. Using the modified arc model, the lightning arc performance of the MV/ LV network under the influence of nearby trees and the network characteristics is evaluated

Lightning-induced overvoltages in medium voltage distribution systems and customer experienced voltage spikes

In Finland, distribution transformers are frequently subjected to lightning strokes for which they are continuously protected by spark-gaps. So, the breakdown probability of medium voltage (MV) spark-gaps is modeled using the Gaussian distribution function under an impulse voltage test in accordance with the IEC 60060-1 standard. The model is presented in the form of the well-known Gaussian tail probability. Accordingly, a modified probabilistic model is proposed to study the effect of impulse voltage superimposed on the ac voltage on the breakdown probability of MV spark-gaps. The modified model is verified using experimental data, where the experimental setup is arranged to generate a range of impulse voltages superimposed on the ac voltages. The experimental verification shows evidence of the efficacy of the proposed probabilistic model. Furthermore, the proposed model is used to evaluate single-phase, two-phase and three-phase spark-gap breakdown probabilities in the case of lightning induced overvoltages. These breakdown probabilities are used along with the simplified Rusck expression to evaluate the performance of MV overhead lines above a perfectly conducting ground under lightning-induced overvoltages using a statistical approach. In order to study the overvoltages propagating through the transformer to its low voltage side, the high frequency model of the transformer is investigated. First, the investigation is carried out using model introduced by Piantini at no-load condition. This model is modified to take more than one resonance frequency into consideration. Therefore, the frequency response of the simulated transient voltage is improved. A verification of the modified model is carried out through the comparison between the experimental and simulation results, in which the time domain simulation is carried out using ATP/EMTP while MATLAB is used to identify the model parameters. As this model is found suitable only for unloaded transformer, an accurate and simplified model is proposed concerning unloaded and loaded conditions as well. The proposed high frequency transformer model is experimentally verified under different balanced load conditions considering two different practical distribution transformers. Then the impact of low voltage (LV) network feeder numbers, lengths, types and loads on the lightning-induced overvoltage reached at the service entrance point is investigated with and without MV spark-gap operation. The high frequency model representation of the distribution transformer and low voltage network are combined in a single arrangement in the environment of ATP/EMTP. A simplified low voltage surge arrester model is represented and verified. Finally, a study is carried out to mitigate the overvoltages by allocating the surge arrester at secondary side of the distribution transformer with concerning MV spark-gap operation

A New Cost Effective Approach to Suppress Very Fast Transients on Power Transformers Connected to Gas Insulated Substations

This thesis investigates the feasibility of a new method to prevent internal resonance in transformers connected to gas insulated substation due to very fast transient overvoltage in the GIS. The possibility of utilizing a cost effective RC surge suppressor to prevent internal transformer resonance is analyzed and the optimum suppressor in terms of capability to suppress high risk frequencies of VFTO is proposed. In comparison with other existing methods, the proposed method is a simple and cost effective method

Electrical transmission systems for large offshore wind farms

Simulations of switching transients were carried out in EMTP-RV. Overvoltages in offshore wind farms ranged from temporary over voltages to very fast front transients. Transient Recovery Voltages of the offshore circuit breakers exceeded IEC 62271 requirements in some situations. The disconnection of an array produced the most severe overvoltages, exceeding IEC 60071 requirements.EThOS - Electronic Theses Online ServiceGBUnited Kingdo

Electrical transmission systems for large offshore wind farms

Simulations of switching transients were carried out in EMTP-RV. Overvoltages in offshore wind farms ranged from temporary over voltages to very fast front transients. Transient Recovery Voltages of the offshore circuit breakers exceeded IEC 62271 requirements in some situations. The disconnection of an array produced the most severe overvoltages, exceeding IEC 60071 requirements

Estimation of the Developed Overvoltages at the Entrance of a HV/MV Substation

External overvoltages can cause several damages to a HV/MV substation, leading to insulation breakdowns. The appropriate protection of the substation against external overvoltages is critical, in order to ensure the efficient, reliable and safe operation of the system. Shielding wires, mast and surge arresters are the main parts of a lightning protection system. The current PhD project deals with the effective lightning protection of HV/MV substations, examining the impact of various factors on the magnitude of the developed overvoltages at different positions of the system. The influence of the equivalent circuit model of the various components to the calculated overvoltages is also examined. Moreover, the substation outage rate due to lightning strokes is calculated, considering shielding failures and back-flashovers. In addition, special issues are discussed, i.e. induced overvoltages and installation of arresters in parallel. In comparison with other studies, main contribution of the thesis is that does not focus only on the role of the grounding resistance, but examines the dominant influence of other parameters; the improvement of the lightning performance of the substation can be achieved by the appropriate adjustment of various factors of the system. The current work highlights also the advantages and drawbacks of each equivalent circuit model, providing a guide to other researchers in order to select the appropriate models. As far as the risk assessment analysis is concerned, innovation of the performed study constitutes the inclusion of the arresters‟ failure rate to the total substation failure rate, since their possible failure results in malfunction of the nominal operation of the system. The induced overvoltages arriving at the entrance of the substation are also calculated, examining the role of the lightning hit position, the waveform of the lightning current, emphasizing to the role of the installation position of the arresters. Finally, the need for good matching of the voltage – current characteristics of the arresters is revealed, otherwise, the expected equal sharing of the injected lightning current will not be achieved. The current Thesis indicates that the combination of the arresters in parallel does not influence the expected overvoltages, but has to do mainly with the absorbed energy by the arresters