Diaa-Eldin A. Mansour

Improving Fault Ride-Through Capability of DFIG-Based Wind Turbine Using Superconducting Fault Current Limiter

With increased penetration of wind energy as a renewable energy source, there is a need to keep wind turbines connected to the grid during different disturbances such as grid faults. In this paper, the use of superconducting fault current limiter (SFCL) is proposed to reduce fault current level at the stator side and improve the fault ride-through (FRT) capability of the system. To highlight the proposed technique, a doubly fed induction generator (DFIG) is considered as a wind-turbine generator, where the whole system is simulated using PSCAD/EMTDC software. Detailed simulation results are obtained with and without SFCL considering stator and rotor currents. In addition, the voltage profile at the generator terminals is analyzed. The effect of limiting resistance value is also investigated. The obtained results ensure that the SFCL is effective in decreasing the fault current. Moreover, both the voltage dip at the generator terminals and the reactive power consumption from the grid are decreased during the fault. The voltage dip characteristics are discussed in accordance with international grid codes for wind turbines.

Surface charge accumulation and partial discharge activity for small gaps of electrode/epoxy interface in sf 6 gas

The electrical insulation reliability of solid spacers in gas insulated switchgears (GISs) is an important issue to achieve a safe operation of such equipment. Among different phenomena, charge accumulation represents the most important matter that can degrade the overall performance of these insulation systems. For this respect, this paper discusses the contribution of partial discharge (PD) activity by ac voltage application to charge accumulation in the small gap at the electrode/epoxy interface as one of the weakest points in GIS solid spacers. The partial discharge inception voltages for non-accumulated charge case (PDIV0) and after exposing to PD activity (PDIVn) are measured among different gap lengths, simulating delamination at the electrode/epoxy interface. The PD activity is generated using applied voltage with 1.2×PDIV0 for all gap lengths examined in this study (50~500 ¿m). In these measurements, PDIV increased with increasing the number of PD pulses as a result of accumulated charges. The accumulated surface charge density is estimated using the boundary equations and is compared for the different gap lengths. The accumulated charge density was larger for the smaller gap lengths. Comparing PD parameters with accumulated charge density enabled us to identify that the number of negative and positive PD pulses is the main parameter that corresponds to charge accumulation process.

Dispersion behavior and breakdown strength of transformer oil filled with TiO 2 nanoparticles

Recently, the study of transformer oil-based nanofluids became of great interest, due to their prospective properties as a dielectric and cooling medium. However, agglomeration of nanoparticles limits the beneficial properties that can be obtained from nanofluids. So, the work presented in this paper aims to get enhanced dispersion behavior of nanoparticles within transformer oil-based nanofluids. Then, breakdown strength with the enhanced dispersion behavior is evaluated. In order to get enhanced dispersion behavior, nanofluids were prepared using nanoparticles with a surfactant. The surfactant plays a role in the stabilization of nanoparticles and maintaining suspension stability. The considered type of nanoparticles is TiO2 nanoparticles. Two series of nanofluid samples were prepared. Through the first series, the effect of surfactant concentration on dispersion behavior and agglomerate size was studied. The dispersion of nanoparticles was characterized using three different techniques. These techniques are optical microscope analysis, transmission electron microscope (TEM) analysis and zeta potential measurements. Based on these techniques, the suitable concentration of surfactant was identified. The second series of nanofluid samples was prepared with different weight percents of nanoparticles for assessing the breakdown strength. Weibull distribution was used to calculate the breakdown probability for the base oil as well as nanofluid samples. The results showed an enhancement in the breakdown strength by about 27% in comparison to the base oil. Based on the obtained results, mechanisms behind the dispersion behavior and breakdown strength were proposed and discussed.

Effect of titania nanoparticles on the dielectric properties of transformer oil-based nanofluids

In this paper, a study on the dielectric properties of transformer oil-based nanofluids is presented. Titanium oxide nano ceramic particles, known as titania nanofillers, was used in the investigation process. The nanofluid samples were prepared using two-step method, in which, nanoparticles were mixed with the base oil and then dispersed using magnetic stirrer, dispersants and ultrasonic processor. Different amounts of dispersants were considered and the best amount of dispersant was determined according to the observed images obtained by an optical microscope. Different nanoparticle volume fractions were considered, starting from 0.1 g/L to 0.3 g/L. Breakdown strength was measured for the prepared samples using a liquid tester and then compared to that of the base oil. The breakdown strength increased significantly with the addition of nanofillers. The obtained results will enable to develop a new class of liquid dielectrics with a visibility for the application into power transformers.

Development of a new graphical technique for dissolved gas analysis in power transformers based on the five combustible gases

Several methods have been proposed for dissolved gas analysis (DGA) of mineral oil in power transformers. One of the simple and most widely used methods is Duval triangle. However, Duval triangle does not consider the concentrations of two combustible gases, namely ethane (C2H6) and hydrogen (H2). As a result, Duval triangle exhibits a lower accuracy in diagnosing certain fault types, for which these gases are the key gases, such as low overheating and corona discharge. Accordingly, this paper proposes a novel graphical technique for DGA based on all the five combustible gases. The proposed graphical technique is developed in the form of a pentagon shape. The pentagon heads represents the percentage concentration of each individual gas to the total combustible gases. The corresponding point for a certain faulty case is determined by the center of mass of all pentagon heads. The knowledge extracted from previous DGA methods and field experiences are used to estimate the preliminary fault regions within the pentagon. The exact boundaries between fault regions are then specified using actual DGA data with corresponding fault types collected from the Egyptian Electricity Network and published cases. The overall performance of the proposed pentagon has been evaluated using a set of fault cases and it is revealed that the proposed pentagon has higher diagnostic accuracy compared to other methods including Duval triangles and IEC standard 60599.

Charge accumulation effects on time transition of partial discharge activity at GIS spacer defects

The partial discharge (PD) measurements are considered as the most important tool for condition monitoring of Gas Insulated Switchgears (GISs). However, if spacer surface is involved in PD activity, charge accumulation process greatly affects the time transition of PD characteristics. This paper investigates this effect considering the defect types of delamination at the electrode/spacer interface and metallic particles adhered to the spacer surface as the most serious defects that can lead to major failure. Two different electrode configurations were built to simulate the two defect types. The PD characteristics were measured and analyzed from immediately after voltage application up to several minutes where steady state characteristics have been obtained. As a result, it was found that the generation rate of PD pulses changed considerably with time for both defect types. This tendency of PD characteristics has been discussed considering the effect of charge accumulation. Comparing these effects for negative and positive PD enabled to clarify the similarities and differences in the charge accumulation effect for both defect types which can contribute to the defect type identification in GIS. It was found that the effects of charge accumulation on the electric field strength for the delamination defect case as well as on the ionization volume for the metallic particle case were the responsible parameters for the different PD tendencies.

Partial discharges and associated mechanisms for micro gap delamination at epoxy spacer in GIS

For accurate detection and diagnosis of Partial Discharges (PDs) in Gas Insulated Switchgears (GISs), there is a need for better understanding of the physical mechanisms for the PD activity. Accordingly, we have been investigating the PD characteristics and associated mechanisms for electrode/epoxy delamination of GIS spacers as one of the severest defects in GIS. The gas pressure inside delamination usually changes from sub-atmospheric pressure, just after delamination initiates, to filled SF6 gas pressure in the GIS tank, after SF6 gas infiltrates into the delamination area. Therefore, in this paper, the PD activity at different gas pressures, from 0.02 to 0.4 MPa, is acquired. Delamination gap length at the level of 50 μm is considered, simulating possible size of actual delamination in a GIS spacer. The relative PD generation rate is analyzed regarding the phase characteristics at different pressures and then discussed from the viewpoint of PD mechanism in order to assess the possibility of delamination diagnosis in GIS spacers. Experimental results show that, PD parameters and phase characteristics change significantly with infiltrating SF6 gas from the GIS tank into the delamination. The obtained results give a characterized feature for the delamination defect and can be useful for delamination diagnosis.

A new graphical technique for the interpretation of dissolved gas analysis in power transformers

Duval triangle for dissolved gas analysis (DGA) in power transformers was frequently used due to its simplicity. However, Duval triangle did not consider the concentrations of Ethane (C2H6) and Hydrogen (H2) in spite of their importance in diagnosing certain types of faults. Accordingly, in this paper, a new graphical technique for DGA was developed considering all five combustible gases. The proposed graphical technique was built using a pentagon shape with its heads representing the percentage concentration of each individual gas to the total combustible gases. The corresponding point for each case was determined by the center of mass of these percentage concentrations. First, the knowledge related to each fault type was extracted from previous DGA methods as well as from field experience. Then, this knowledge was used to estimate the preliminary positions of boundaries between fault regions within the pentagon. Finally, the exact boundaries between fault regions were proposed by plotting actual DGA data with corresponding fault types collected from the Egyptian Electricity Network. The comparison with Duval triangle showed that the proposed pentagon shape has good diagnosis accuracy.

The role of interfacial zone in dielectric properties of transformer oil-based nanofluids

Transformer oil-based nanofluids have unexpected and distinctive dielectric properties that are different from properties of constituting components. This is believed to be attributed to the effect of interfacial zone between oil and nanoparticles. Accordingly, this paper aims to investigate the role through which the interfacial zone affects the dielectric properties of transformer oil-based nanofluids. Oil-based nanofluids are prepared with two different types of nanoparticles having different surface charging polarity. These types are alumina (Al2O3) nanoparticles with cationic surface and titania (TiO2) nanoparticles with anionic surface. For each type of nanoparticles, two groups are prepared with and without a surface modifier, which is called surfactant. Each group consists of four samples having nanoparticles weight fractions of 0.01, 0.04, 0.07 and 0.1 g/L. Surfactant is chosen with charge opposite to that of nanoparticle surface charge to be adsorbed on the nanoparticle surface. For each nanofluid sample, surfactant is distributed through the oil using magnetic stirrer, and then, nanoparticles are mixed and homogenized using magnetic stirrer and ultrasonic processing. After preparation, dielectric properties including breakdown strength and dielectric constant are measured. Based on the obtained results, the role of interfacial zone could be clarified in conjunction to that developed in solid nanodielectrics.

Influence of accumulated surface charges on partial discharge activity at micro gap delamination in epoxy GIS spacer

In epoxy spacers in Gas Insulated Switchgear (GIS) under operating thermal or mechanical stresses, delamination would be initiated at electrode/epoxy interface leading to partial discharge (PD) activity. In this paper, PD activity for delamination at electrode/epoxy interface in SF6 gas is measured and analyzed to clarify the effect of accumulated charges on the epoxy surface. The delamination gap length considered in this paper is 50 μm. The moment of delamination initiation in GIS is successfully simulated by applying sudden voltage to the electrode setup. The main results show that the charge accumulation has great effect on the PD generation rate. This effect is different between negative and positive PD as a result of the difference in initial electron generation mechanism. The measurement of surface potential and the change in PD inception voltage after PD activity enable to estimate the accumulated charge polarity. Consequently, the role of charge accumulation in PD activity is proposed based on the change in local electric field inside the delamination gap.