Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Adaptive Single-Phase Reclosing in Transmission Lines

This research work is mainly concerned about dealing with temporary short circuit faults in power system transmission lines. In fact, there are two types of electrical faults in power systems, namely temporary and permanent. When a fault is permanent, the only way to clear it is to de-energize the transmission line by opening the associated circuit breakers. However, in many cases the fault is not solid and is caused by objects such as flying birds or broken branches of trees. For these cases, electrical arc plays a major role. For such fault cases, it is also possible to de-energize the faulted phase, temporarily, and re-energize it after a short delay by reclosing the opened circuit breakers. This operation is called single-phase reclosing. There is a chance that the fault becomes clear by natural extinction of the arc after the faulted phase isolation in case the fault is temporary. There are two considerable challenges regarding traditional single-phase reclosing in transmission lines. The first challenge is the determination of the fault type, i.e., permanent or temporary, as there is no guarantee that the fault is temporary. This is crucially important as reclosing-on-fault, i.e., reclosing the opened breakers while the fault still stands, is harmful for both power system stability and power system equipment. The second challenge which is regarding temporary faults only, is that there is still no guarantee that the arc is extinguished by the moment of reclosing. In such cases, reclosing leads in re-striking of arc and therefore, an unsuccessful reclosing. This research work is conducted in two phases. At the first phase, two adaptive methods are developed to improve the traditional reclosing method upon the two challenges mentioned in the second paragraph. The developed methods are capable of recognition of the fault type in a reasonable amount of time after single-phase isolation of the line. Therefore, the protection system will be able to block the reclosing function in case the fault is recognized as permanent and to issue three-phase-trip signal as the next action. For temporary faults, re-energizing of the isolated phase by reclosing the opened breakers is the next action which has to be performed after the arc extinction. The developed methods also have the capability of detection of the arc extinction and therefore, a better performance for temporary fault cases is guaranteed. This is the second feature required for an adaptive reclosing method. The second phase of the research project is to estimate the arc extinction time well in advance in case the fault is temporary. The idea is that three-phase tripping could be the right action if the arc extinction time is too long as working under unbalanced conditions for an unnecessarily long time duration is harmful for the power system. Both of the proposed adaptive single-phase reclosing methods in this research work employ local voltage information. Therefore, communication facilities are not needed for implementation of the proposed methods. It is shown in the thesis that the proposed methods are able to quickly detect the fault type and also the arc extinction if the fault is temporary. Also, the two proposed arc extinction time prediction methods are capable of prediction of the arc extinction time well in advance and with acceptable precision. All four proposed methods are effective for various system configurations including ideally-transposed, untransposed and partially-transposed transmission lines and also for transmission lines with different compensation conditions including with and without shunt reactor. Superior performance of the proposed methods have been verified using 550 case studies simulated in PSCAD and Matlab, and also a field recorded temporary fault case associated with a 765 kV transmission line. The 550 simulated case studies include 100 ideally-transposed, 240 untransposed and 210 partially-transposed line cases. The performances of the two proposed reclosing methods are also compared with two of the existing adaptive reclosing methods where considerable improvements are observed

Single phase earth faults in high impedance grounded networks : characteristics, indication and location

The subject of this thesis is the single phase earth fault in medium voltage distribution networks that are high impedance grounded. Networks are normally radially operated but partially meshed. First, the basic properties of high impedance grounded networks are discussed. Following this, the characteristics of earth faults in distribution networks are determined based on real case recordings. Exploiting these characteristics, new applications for earth fault indication and location are then developed. The characteristics discussed are the clearing of earth faults, arc extinction, arcing faults, fault resistances and transients. Arcing faults made up at least half of all the disturbances, and they were especially predominant in the unearthed network. In the case of arcing faults, typical fault durations are outlined, and the overvoltages measured in different systems are analysed. In the unearthed systems, the maximum currents that allowed for autoextinction were small. Transients appeared in nearly all fault occurrences that caused the action of the circuit breaker. Fault resistances fell into two major categories, one where the fault resistances were below a few hundred ohms and the other where they were of the order of thousands of ohms. Some faults can evolve gradually, for example faults caused by broken pin insulators, snow burden, downed conductor or tree contact. Using a novel application based on the neutral voltage and residual current analysis with the probabilistic method, it is possible to detect and locate resistive earth faults up to a resistance of 220 kΩ. The main results were also to develop new applications of the transient based differential equation, wavelet and neural network methods for fault distance estimation. The performance of the artificial neural network methods was comparable to that of the conventional algorithms. It was also shown that the neural network, trained by the harmonic components of the neutral voltage transients, is applicable for earth fault distance computation. The benefit of this method is that only one measurement per primary transformer is needed. Regarding only the earth faults with very low fault resistance, the mean error in absolute terms was about 1.0 km for neural network methods and about 2.0 km for the conventional algorithms in staged field tests. The restriction of neural network methods is the huge training process needed because so many different parameters affect the amplitude and frequency of the transient signal. For practical use the conventional methods based on the faulty line impedance calculation proved to be more promising.reviewe

High voltage covered conductor overhead lines: detection of incipient tree faults

The aim of this thesis is the study of a new type of high voltage overhead power line, made by means of an insulation layer located around the conductor: these are the covered conductor lines. In particular, the work focuses on the study of the behaviour of these conductors when they get in contact with the vegetation around, which can touch or fall on the line.ope

Failure analysis informing intelligent asset management

With increasing demands on the UK’s power grid it has become increasingly important to reform the methods of asset management used to maintain it. The science of Prognostics and Health Management (PHM) presents interesting possibilities by allowing the online diagnosis of faults in a component and the dynamic trending of its remaining useful life (RUL). Before a PHM system can be developed an extensive failure analysis must be conducted on the asset in question to determine the mechanisms of failure and their associated data precursors that precede them. In order to gain experience in the development of prognostic systems we have conducted a study of commercial power relays, using a data capture regime that revealed precursors to relay failure. We were able to determine important failure precursors for both stuck open failures caused by contact erosion and stuck closed failures caused by material transfer and are in a position to develop a more detailed prognostic system from this base. This research when expanded and applied to a system such as the power grid, presents an opportunity for more efficient asset management when compared to maintenance based upon time to replacement or purely on condition

Outdoor Insulation and Gas Insulated Switchgears

This book focuses on theoretical and practical developments in the performance of high-voltage transmission line against atmospheric pollution and icing. Modifications using suitable fillers are also pinpointed to improve silicone rubber insulation materials. Very fast transient overvoltage (VFTO) mitigation techniques, along with some suggestions for reliable partial discharge measurements under DC voltage stresses inside gas-insulated switchgears, are addressed. The application of an inductor-based filter for the protective performance of surge arresters against indirect lightning strikes is also discussed

Capacitive current interruption with high voltage air-break disconnectors

Disconnectors are low-cost switching devices in high voltage electrical power supply systems that basically have an insulation function only. Nevertheless, they have a very limited capability to interrupt current (below one Ampere), e.g. from unloaded busbars or short overhead lines. The present study is a search for possibilities to increase the current interruption capability with auxiliary devices interacting with the switching arc. In this project the state of the art of disconnector switching is investigated and an inventory is presented of models of the free burning arc in air. A series of experiments were arranged at different laboratories. The switching arc and the interruption process are studied in detail through electrical and optical measurements during the switching process for a disconnector with (without) auxiliary devices under high voltage (300 kV) conditions. Three options for auxiliary devices were investigated: (i) arc cooling by forced air flow; (ii) fast interrupting by high-velocity opening contacts; (iii) reduction of arc energy by added resistive elements. Finally, a qualitative description is provided on the physical nature of the arc and how the evaluated methods affect the arc characteristics. All results are obtained by analysis of highresolution measurement of arc current (including all relevant transients), voltages across the disconnector and high-speed video observation. It was found that, depending on the current to be interrupted, the interruption process is governed by dielectric and/or thermal processes. In the dielectric regime, the interrupted current is low (roughly below 1A) and the switching arc is characterized by a high rate of repetition of interruptions and restrikes that only cease after a sufficient gap spacing has been reached. The restrikes interact severely with the circuitry in which the disconnector is embedded, exciting transients in current and voltage with frequencies up to the megahertz range. High overvoltages can be generated. Their magnitudes can be limited by a proper choice of the capacitance at supply side of the disconnector. The arc-circuit interaction has been studied and relevant processes have been modelled and verified by experiments in full-power test-circuits. In the thermal regime, the switching arc behaves less vehemently, interrupting and re-igniting basically occur at every power frequency current zero. Because of the presence of sufficient thermal energy in the switching gap along the arc path, the voltage to re-ignite the arc is limited, and the arc-circuit interaction is less pronounced. Though not producing very severe overvoltages, the arc duration is longer and the current may not be interrupted at every current zero crossing. The ultimate thermal regime is reached when the arc continues to exist after power frequency current zero without any appreciable voltage to re-ignite. This situation must be avoided because arcing goes on until a higher level breaker interrupts the current. Before this, the arc can reach far away from its roots and can greatly reduce insulation clearance. The main factors influencing the interruption performance are the level of current to be interrupted, the system voltage, the ratio of capacitances at both sides of the disconnector and the gap length. These factors influence the energy supplied to the arc upon re-strike. This energy extends the arcing time by lowering the breakdown voltage. It has been observed that the arc in its thermal mode always re-ignites in its former trajectory. Key to the interruption process is the reduction of breakdown voltage in this path, created by hot gases remaining from the former arc. The existing breakdown models are reviewed in order to understand the influence of high temperature air on the breakdown process. Based on the observed arc behaviour, various methods have been researched to increase the interruption capability. The most successful methods are those that remove the residual (partially) ionized air from the arc path. Experiments were carried out to demonstrate the effectiveness of air flow directed into the arc’s foot point. A substantial gain in interruption capability is demonstrated, but at the cost of generating re-ignition transients at a very rapid succession. Specifically, the experiments showed that 7.5 A could be interrupted successfully at 90 kVrms voltage with a shorter arcing duration (a factor of 0.5 was observed) than without air flow. With application of air flow, the frequency of re-ignitions occurring, and the breakdown voltage are much higher than without air flow. Another method, the assistance of an auxiliary switch able to produce a very fast opening, was also successful. Herein, the arc is forced mechanically into ambient cool air, thus avoiding accumulation of thermal energy in the arc path. Specifically, it can interrupt currents up to 7 A at 100 kVrms safely and 9 A at 90 kVrms in the experiments with arcing time only a few tens of milliseconds instead of a few seconds. The arc exhibits a "stiff" (linear) character instead of the "erratic" (randomly moving) arc mode with a disconnector alone. This method reduces the number of re-strikes. The possible influence of energy absorbing elements (resistors) is investigated through circuit modelling, supported by some laboratory experiments. Other methods, such as the application of series auxiliary interrupting elements (vacuum, SF6 interrupter and ablation assisted approaches) have been evaluated. From the practical point of view, the auxiliary fast-opening interrupter is recommended due to its economic, simple and effective merits. Other approaches have certain disadvantages. The method with air flow needs a complex construction in order to introduce the compressed air flow into the disconnector, and the hazard for nearby equipments from the overvoltages caused by the interruption is greater. The method of inserted resistor requires very expensive arrangement. Regarding the application of auxiliary interrupters, vacuum interrupters have to be applied in considerable numbers in series and SF6 interrupters have good performance but at very high cost. An ablation assisted approach seems less promising because the level of the interrupted current is too low to be effective

Pintapurkausilmiöt keskijännitekaapelipäätteissä

Medium voltage terminations are used in the underground cable network to connect the cables to other accessories or equipment. Secondary outdoor substations are one common place where the terminations are used in. The operating conditions inside these unheated outdoor enclosures can become extremely harsh, because of airborne contaminants and condensed moisture. The contaminants and moisture generate a conducting layer over the termination surface which can eventually lead to electrical discharges on the surface. The aim of this thesis was to study the behavior of these discharges and to compare the performance of different terminations types. The work was done by analyzing field inspection results of existing medium voltage terminations and by performing laboratory tests.Keskijännitekaapelipäätteitä käytetään maakaapeliverkossa kaapeleiden liittämiseen muihin varusteisiin tai laitteistoihin. Puistomuuntamo on yksi yleinen kaapelipäätteen käyttökohde. Puistomuuntamon kaltaisessa ulkona sijaitsevassa lämmittämättömässä kotelossa käyttöolosuhteet voivat olla hyvinkin haastavat saasteiden ja kondensoituvan kosteuden takia. Saasteet ja kosteus luovat kaapelipäätteen pinnalle johtavan kerroksen joka voi johtaa sähköisiin purkauksiin päätteen pinnalla. Tämän diplomityön tarkoituksena oli tutkia tätä pintapurkausilmiötä ja vertailla erilaisten kaapelipäätteiden suoriutumista haastavissa olosuhteissa. Työ toteutettiin analysoimalla kenttätarkastuksista saatuja kaapelipäätteiden tuloksia ja tekemällä erilaisia laboratoriokokeita