Overvoltage Protection System Research Articles

Comparative Performance of DFIG and PMSG Wind Turbines during Transient State in Weak and Strong Grid Conditions Considering Series Dynamic Braking Resistor

The recently stipulated grid codes require wind generators to re-initiate normal power production after grid voltage sag. This paper presents a comparative performance of two commonly employed variable speed wind turbines in today’s electricity market, the doubly fed induction generator (DFIG) and the permanent magnet synchronous generator (PMSG) wind turbines. The evaluation of both wind turbines was performed for weak, normal and strong grids, considering the same machine ratings of the wind turbines. Because of the critical situations of the wind turbines during faulty conditions in the weak grids, an analysis was done considering the use of effective series dynamic braking resistor (SDBR) for both wind turbines. The grid voltage variable was employed as the signal for switching the SDBR in both wind turbines during transient state. Additionally, an overvoltage protection system was considered for both wind turbines using the DC chopper in the DC-link excitation circuitry of both wind turbines. Furthermore, a combination of the SDBR over-voltage protection scheme (OVPS) was employed in both wind turbines at weak grid condition in order to improve the performance of the variable speed wind turbines and keep the operation of the power converters within their permissible limits. Furthermore, the performance of the DFIG and PMSG wind turbines in weak grids were further investigated, considering the combination of 75% and 50% effectively sized SDBR and OVPS. It was observed that, even with a 50% reduction in SDBR or OVPS, the performance of both wind turbines is still satisfactory with faulty conditions. Therefore, it is recommended to use a combination of the SDBR and OVPS with DFIG- or PMSG-based variable speed wind turbines to achieve a superior fault ride through performance, especially in weak grids. The system performance was evaluated using the power system computer design and electromagnetic transient including DC (PSCAD/EMTDC) platform.

Lightning Impulse Overvoltage Propagation in HVDC Meshed Grid

This paper reports on the propagation of lightning overvoltage in a high-voltage direct current (HVDC) meshed grid. Since several topologies of meshed grids have been elaborated in the last decade, we used a common comprehensive reference test platform. The lightning impulse propagation was investigated with regard to the impact of surge arresters and the polarity of the lightning stroke concerning the DC line polarity (±500 kV). Various scenarios were considered, including a direct lightning strike to the DC+ conductor, to the tower, and to the shielding wire in the middle of the span, including backflash on the insulators. The influence of tower footing impedance on overvoltage levels at various nodes was assessed, depicting the critical value. A description of the models used in the simulations was provided. The main focus of the paper was on the wide-area propagation of the overvoltages in the meshed grid, at distant terminals and inside the feeders. An interesting observation was the effects of lightning at the far end of the analyzed grid, propagating through multiterminal and long-distance connections. The presented analysis, based on an exemplary meshed HVDC grid, underlines the importance of the insulation coordination studies and system security studies with respect to the localization of overvoltage protection systems.

Grounding System Modeling and Evaluation Using Integrated Circuit Based Fast Relaxed Vector Fitting Approach, Considering Soil Ionization

Since high voltage transmission line towers or wind turbines structures are installed in high-altitude areas, it is essential to achieve a high overvoltage protection system against direct and indirect lightning strikes collisions. The lightning current must be discharged quickly into the protective earth, to prevent the dangerous over-voltages formation and define a reference voltage node. This paper presents a novel model to assess the behavior of the grounding system, based on Pocklington integral equations under lightning magnetic fields and variations in soil ionization, in which an explicit circuit-based vector fitting RLC admittance branches are proposed. The frequency-dependent behavior of grounding system frequency response and soil ionization effect is modeled in time domain, straightly to implement into the electro-magnetic transient program (EMTP). The model verification contains horizontal, vertical, and their combinations of grounding grids to represent the complete investigations under lightning strikes. The harmonic impedance mathematical formulations and principles are derived based on a rational function, that could be applicable on ground potential rise (GPR) investigation.

Analysis of Internal Overvoltages in Transformer Windings during Transients in Electrical Networks

Due to the increasing requirements for the reliability of electrical power supply and associated apparatus, it is necessary to provide a detailed analysis of the overvoltage risk of power transformer insulation systems and equipment connected to their terminals. Exposure of transformer windings to overvoltages is the result of the propagation condition of electromagnetic waves in electrical networks and transformer windings. An analysis of transformer winding responses to transients in power systems is of particular importance, especially when protection against surges by typical overvoltage protection systems is applied. The analysis of internal overvoltages in transformers during a typical transient related to switching operations and selected failures is of great importance, particularly to assess the overvoltage exposure of insulation systems in operating conditions. The random nature of overvoltage phenomena in electrical networks implies the usage of computer simulations for the analysis of overvoltage exposures of electrical devices in operation. This article presents the analysis of the impact of transient phenomena in a model of a medium-voltage electrical network during switching operations and ground faults on overvoltages in the internal insulation systems of transformer windings. The basis of the analysis is simulations of overvoltages in the windings, made in the Electromagnetic Transients Program/Alternative Transients Program (EMTP/ATP) using a model with lumped parameters of transformer windings. The analysis covers the impact of the cable line length and the ground fault resistance value on internal overvoltage distributions.

Comparison of transformer winding responses to standard lightning impulses and operational overvoltages

During operation transformers are typically subjected to overvoltages with different wave shapes. Even though overvoltage protection systems, such as metal oxide surge arresters, are often employed, the overvoltages generated in power systems can still have a significant impact on the insulation systems of the windings of a power transformer. There are numerous examples of transformer failures as a result of overvoltages occurring during operation, even in cases that standard impulse voltage tests show no indication of potential problems and the transformers are protected by surge arresters. Comparisons of the internal overvoltages generated in transformers during voltage tests as well as those generated during standard operation of electrical networks provides fundamental insights into these overvoltages; insights which in turn can be used to improve the construction of insulation systems and overvoltage protection approaches and ultimately increase the reliability of transformers. In this paper, internal overvoltages occurring in the windings of selected transformer during standard voltage tests are compared with those which occur during typical operation of a transformer in an electrical power system. Overvoltages measured in the windings of the transformer form the basis of this comparison. Measurements were conducted with the transformer being subjected to voltage impulse tests, as well as transient voltages with waveforms simulating overvoltages generated in electrical power systems. In order to investigate the response of the windings to voltage strokes, full and chopped lightning voltage impulses were applied to the transformer. The waveforms of overvoltages generated in the electrical power systems were modelled as a rectangular voltage stroke, sinusoidal voltage, voltage with shape of “sweep sinus” also often referred to as a chirp signal and voltage with shape of “sweep ramp”.

A simulation approach on rotor blade electrostatic charging and its effect on the lightning overvoltages in wind parks

This study is an attempt to present a theoretical approach of the hypothesis of electrostatic charging of rotor blades during storms and its effects on the lightning protection and overvoltage protection systems during lightning strokes.Overvoltage and overcurrent in form of surges may be the result of electromagnetic traveling waves caused by lightning strokes, especially if the grounding (earthing) system of the wind turbines (WTs) is galvanically connected between each other in order to ensure a common earth potential in the wind park (WP).These effects may impose additional electrical issues to the low and medium voltage electrical installation including the surge protection devices.

Modeling and simulation of surge arresters for lightning protection of distribution systems

Overvoltage protection systems are necessary to protect power system components against several types of overvoltages. In this paper, protection against overvoltage caused by lightning in 69 kV distribution feeders is addressed. The idea is to describe a methodology for which is possible to model and simulate the performance of a protection system which is composed by surge arresters when exposed mainly to direct lightning incidence. Moreover, typical overvoltage ranges were considered in this paper for the purpose of evaluating the developed protection system. Thus the major contribution of this paper relies upon the assessment of the lightning protection system performance, under different configurations.

Overvoltage protection for magnetic system during disruption in tokamak

During a plasma disruption the magnetic flux in the tokamak changes rapidly, which in most cases will cause high-voltage surges among the magnetic systems and may bring severe damage to the components if there is no overvoltage protection. This paper investigates the way to limit the plasma disruption overvoltage and absorb the energy with the use of metal oxide varistors (MOVs). An overvoltage model of plasma disruption is introduced which can be used for the simulation of plasma disruption and the analysis of the overvoltage. The effectiveness of the overvoltage protection system is validated with disruption experiments. It shows that by optimizing the varistors voltage, the overvoltage during plasma disruption can be limited to an ideal low value. Now the overvoltage protection system has been deployed in J-TEXT tokamak and serves well for daily experiments.

Overvoltage Protection of Large Power Transformers—A Real-Life Study Case

This paper demonstrates the results from a detailed study of the overvoltage protection of a particular 400/150-kV 400-MVA power transformer. The work presented here is based on a real-life power system substation design and data and initiated by Danish TSO Energinet.dk as a consequence of serious transformer overvoltage damage. A simulation model for the entire system consisting of overhead line, transformer, surge arrester, and earth grid has been created in PSCAD/EMTDC. The main focus has been put on the earth grid, which has been submodeled in detail in MATLAB using an electromagnetic transient approach based on the thin-wire program made by J. H. Richmond for NASA in 1974. The earth grid model is verified with excellent agreement compared to already published results. The overvoltage performance of the particular case is analyzed, and it shows that the transformers LIWL have probably been exceeded. It is clearly illustrated that the transient performance of the earth grid plays an important role in the overall overvoltage protection system design.

Overvoltage Protection Using a Gas Discharge Arrester Within the MATLAB Program Tool

This paper deals with the modeling of a low-voltage gas discharge arrester using the MATLAB/Simulink program tool. The model is designed to enable a combination with other overvoltage protection system elements (metal-oxide varistor, supressor diode) in order to put a model of an integrated overvoltage protection system together. This model comprises all phases that may appear during the operation of a gas discharge arrester. Therefore, it consists of five main submodels, each of them representing one physical phenomenon. The results obtained by performing model analyses within the MATLAB/Simulink program tool were verified by laboratory tests with various voltage sources (lightning surge voltage, ac voltage, and combinations of them), performed on real gas discharge arresters.

Microstructure and non-ohmic properties of ZPCCT-based ceramics

Zinc oxide doped with several different metal oxides are smart ceramic semiconductors possessing nonohmic properties, which exhibit abruptly increasing current in accordance with increasing voltage. This non-ohmicity of current–voltage properties is because of the presence of a double Schottky barrier (DSB) formed at active grain boundaries containing many trap states. Owing to highly non-ohmicity, these ceramic devices are used widely in the field of overvoltage protection systems from electronic circuits to electric power systems [1, 2]. ZnO non-ohmic ceramics are generally divided into two categories, called Bi2O3-based and Pr6O11-based ceramics, in terms of non-ohmicity-forming oxides. ZnO–Bi2O3based ceramics have been mainly studied in various aspects since ZnO non-ohmic ceramics were discovered. Although ZnO–Bi2O3-based ceramics show excellent non-ohmic properties, Bi2O3 reacts easily with some or many, but not all, of the metals used in preparing multilayer chip non-ohmic ceramics, and it destroys the multilayer structure [3]. And it is reported to have an additional insulating spinel phase, which does not play any role in electrical conduction [3]. Recently, ZnO Pr6O11-based ceramics have been studied in order to improve a few drawbacks [3] associated with Bi2O3 [4–12]. Nahm et al. reported that Zn–Pr–Co–Cr–R oxide (R = Er, Y, Dy, La)-based ceramics have highly non-ohmic properties [6, 8, 11, 12]. To develop the non-ohmic ceramics of high performance, it is very important to comprehend the effects of the additives on non-ohmic properties. No study of the effects of Tb4O7 (terbium oxide) on the non-ohmic properties has been reported. In the present study, the effect of Tb4O7 on the microstructure and non-ohmic properties of ZPCCT (ZnO–Pr6O11–CoO–Cr2O3–Tb4O7)-based ceramics was examined. Reagent-grade raw materials were prepared for ZnO non-ohmic ceramics with composition expression, such as (98.0-x) mol% ZnO + 0.5 mol% Pr6O11 + 1.0 mol% CoO + 0.5 mol% Cr2O3 + x mol% Tb4O7 (x = 0.0, 0.25, 0.5, 0.75, 1.0). Raw materials were mixed by ball milling with zirconia balls and acetone in a polypropylene bottle for 24 h. The mixture was calcined in air at 750 C for 2 h. The powder was pressed into discs of 10 mm diameter and 2 mm thickness at a pressure of 80 MPa. The discs were sintered at 1350 C for 1 h. The final samples were 8 mm in diameter and 1.0mm in thickness. Silver paste was coated on both faces of samples and the electrode was formed by heating at 600 C for 10 min. The area of electrodes was approximately 0.196 cm. The surface microstructure was examined by scanning electron microscopy (SEM, Model S2400, Hitachi, Japan). The average grain size (d) was determined by the lineal intercept method [13]. The compositional analysis of the selected areas was determined by an attached energy dispersion X-ray analysis (EDX) system. The crystalline phases were identified by powder X-ray diffraction (XRD, Model D/max 2100, Rigaku, Japan) with CuKa radiation. The sintered density (q) was measured by the Archimedes method. The current– voltage (I–V) characteristics of ZPCCT-based ceramics C.-W. Nahm (&) Department of Electrical Engineering, Dongeui University, Busan 614-714, Korea e-mail: cwnahm@deu.ac.kr J Mater Sci (2006) 41:8382–8385 DOI 10.1007/s10853-006-0775-3

A Field-to-Line Coupling Model for Overvoltage Analysis in Light-Rail Transit DC Traction Power Systems

This paper faces the problem of the protection against overvoltages of atmospheric origin for modern electric mass-transit systems such as light-rail transportation systems (LRTS) and proposes a transmission-line (TL) model for the field-to-line coupling, which allows to evaluate the response of the supply system of a typical LRTS to a generic electromagnetic field. The incident-field excitation has been modeled in the time domain by means of a plane wave and the method of characteristics has been applied in order to solve the resulting partial differential equation (PDE) system. The developed model has been implemented into the electromagnetic code PSCAD-EMTDC and several simulations have been performed in order to determine the "optimal" configuration in terms of locations of the surge arresters for the overvoltage protection system.

Thyristor crowbar system for the high current power supplies of ASDEX upgrade

The ohmic heating system and the poloidal field coils of ASDEX upgrade are supplied by 15 thyristor converter units with an installed apparent power of 600 MVA. To protect the thyristor converters against dc overvoltage arising from abnormal operations and resulting damages caused by the large energy stored in the AUG magnet coils an overvoltage protection system was required. The paper describes the motivation for—and the design and testing of the thyristor crowbar system representing the thyristor converter overvoltage protection system. It will present the layout, analyse the results of measurements obtained during commissioning, compare them to the calculated (design) values and report on the first experience of operation on the AUG coils improving the safety of the equipment.

Study on overvoltage protection in HVDC LTT valve

The light-triggered thyristor (LTT), which can be directly fired by light, does not need gate electronics on the high-voltage side and is thus a near-ideal semiconductor device for a HVDC valve. In providing overvoltage protection for an LTT without losing its advantage of simple circuits associated with the thyristor, the overvoltage protection system adopted in the conventional electrically triggered thyristor is not necessarily appropriate. The authors have shown that high-speed protective gate pulses generated from the valve base electronics together with the valve arrester are sufficient for the LTT valve overvoltage protection. The adequacy of this protection strategy is demonstrated by factory type tests and also excellent service experiences.

Investigation of overvoltages in a gas insulated substation (G.I.S.) caused by a lightning stroke

A back flashover for a shielding failure can be caused by a lightning stroke, which will damage equipment and devices in a gas insulated substation if the devices' maximum voltages are exceeded. In this study, the branch timetable method, which is implemented from the lattice diagram method, is used in a computer program for the calculation of overvoltages in the gas insulated substation, as a result of a shielding failure, so that the gas insulated substation performance under lightning stroke can be determined. The study of the lightning performance of a gas insulated substation is very important for the purpose of designing lightning overvoltage protection systems.

The varistor protected series capacitors at the 500 kV Broadview substation

Two 235 MVAR series capacitors have been successfully applied at the 500 kV Broadview substation of the Colstrip Transmission Project. The series capacitors have an overvoltage protection system based on metal oxide varistors. The system provides instantaneous bypass and reinsertion of the capacitors following external line section faults. The excellent performance of the equipment was demonstrated during staged fault tests performed in October 1985. The banks have been in successful operation since January 1986. >