Condition monitoring in New Zealand power transformers

Transpower owns and operates New Zealand’s high voltage electricity grid which includes approximately 725 in service power transformers [1]. Presently, condition monitoring of these units is routinely carried out by oil testing (moisture, acidity and dielectric breakdown) and using dissolved gas analysis (DGA), (every year), and winding resistance, insulation resistance, and bushing power factor tests (every four years). However, since the average age of a power transformer in New Zealand is nearly 40 years [1], it is considered that online condition monitoring of important transformers or transformers that have known issues is carried out to identify any incipient faults. The online condition monitoring in existing power transformers is hoped to minimize the risk of sudden failures and thereby prolong the in service life. It is equally important to decide on what to monitor in a power transformer and how to monitor, and these are also governed by the budgetary constraints. Transpower is in the process of acquiring online condition monitoring units for some of the new large power transformers it plans to purchase and will also retrofit such units to some old transformers as required. This paper presents the condition monitoring techniques currently used by Transpower on power transformers, and the online condition monitoring techniques for new and existing power transformers

Preferred growth direction of III-V nanowires on differently oriented Si substrates

One of the nanowire characteristics is its preferred elongation direction. Here, we investigated the impact of Si substrate crystal orientation on the growth direction of GaAs nanowires. We first studied the self-catalyzed GaAs nanowire growth on Si (111) and Si (001) substrates. SEM observations show GaAs nanowires on Si (001) are grown along four directions without preference on one or some of them. This non-preferential nanowire growth on Si (001) is morphologically in contrast to the extensively reported vertical preferred GaAs nanowire growth on Si (111) substrates. We propose a model based on the initial condition of an ideal Ga droplet formation on Si substrates and the surface free energy calculation which takes into account the dangling bond surface density for different facets. This model provides further understanding of the different preferences in the growth of GaAs nanowires along selected directions depending on the Si substrate orientation. To verify the prevalence of the model, nanowires were grown on Si (311) substrates. The results are in good agreement with the three-dimensional mapping of surface free energy by our model. This general model can also be applied to predictions of nanowire preferred growth directions by the vapor-liquid-solid growth mode on other group IV and III-V substrates

Self-formed quantum wires and dots in GaAsP-GaAsP core-shell nanowires

Quantum structures designed using nanowires as a basis are excellent candidates to achieve novel design architectures. Here, triplets of quantum wires (QWRs) that form at the core–shell interface of GaAsP–GaAsP nanowires are reported. Their formation, on only three of the six vertices of the hexagonal nanowire, is governed by the three-fold symmetry of the cubic crystal on the (111) plane. In twinned nanowires, the QWRs are segmented, to alternating vertices, forming quantum dots (QDs). Simulations confirm the possibility of QWR and QD-like behavior from the respective regions. Optical measurements confirm the presence of two different types of quantum emitters in the twinned individual nanowires. The possibility to control the relative formation of QWRs or QDs, and resulting emission wavelengths of the QDs, by controlling the twinning of the nanowire core, opens up new possibilities for designing nanowire devices

Long-term stability and optoelectronic performance enhancement of InAsP nanowires with an ultrathin InP passivation layer

The influence of nanowire (NW) surface states increases rapidly with the reduction of diameter and hence severely degrades the optoelectronic performance of narrow-diameter NWs. Surface passivation is therefore critical, but it is challenging to achieve long-term effective passivation without significantly affecting other qualities. Here, we demonstrate that an ultrathin InP passivation layer of 2–3 nm can effectively solve these challenges. For InAsP nanowires with small diameters of 30–40 nm, the ultrathin passivation layer reduces the surface recombination velocity by at least 70% and increases the charge carrier lifetime by a factor of 3. These improvements are maintained even after storing the samples in ambient atmosphere for over 3 years. This passivation also greatly improves the performance thermal tolerance of these thin NWs and extends their operating temperature from <150 K to room temperature. This study provides a new route toward high-performance room-temperature narrow-diameter NW devices with long-term stability

Highly strained III-V-V coaxial nanowire quantum wells with strong carrier confinement

Coaxial quantum wells (QWs) are ideal candidates for nanowire (NW) lasers, providing strong carrier confinement and allowing close matching of the cavity mode and gain medium. We report a detailed structural and optical study and the observation of lasing for a mixed group-V GaAsP NW with GaAs QWs. This system offers a number of potential advantages in comparison to previously studied common group-V structures (e.g., AlGaAs/GaAs) including highly strained binary GaAs QWs, the absence of a lower band gap core region, and deep carrier potential wells. Despite the large lattice mismatch (∼1.7%), it is possible to grow defect-free GaAs coaxial QWs with high optical quality. The large band gap difference results in strong carrier confinement, and the ability to apply a high degree of compressive strain to the GaAs QWs is also expected to be beneficial for laser performance. For a non-fully optimized structure containing three QWs, we achieve low-temperature lasing with a low external (internal) threshold of 20 (0.9) μJ/cm2/pulse. In addition, a very narrow lasing line width of ∼0.15 nm is observed. These results extend the NW laser structure to coaxial III–V–V QWs, which are highly suitable as the platform for NW emitters

A novel visualisation technique for dissolved gas analysis datasets – A case study

Dissolved gas analysis is the most widely used diagnostic test in power transformers. There are established methods used in industry for interpreting DGA results. Among these are the IEEE Key Gas Method, Rogers’ Ratios and the Duval Triangle. However, collectively these methods can lead to conflicting results or unclassifiable measurements. This paper presents a visualization technique for interpreting DGA results to mitigate these effects, based on Kernel Principal Component Analysis. DGA measurements from more than 200 power transformers are used to validate the approach

Magneto-optical properties of wurtzite-phase inp nanowires

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Magneto-Optical Properties of Wurtzite-Phase InP Nanowires

The possibility to grow in zincblende (ZB) and/or wurtzite (WZ) crystal phase widens the potential applications of semiconductor nanowires (NWs). This is particularly true in technologically relevant III-V compounds, such as GaAs, InAs, and InP, for which WZ is not available in bulk form. The WZ band structure of many III-V NWs has been widely studied. Yet, transport (that is, carrier effective mass) and spin (that is, carrier g-factor) properties are almost experimentally unknown. We address these issues in a well-characterized material: WZ indium phosphide. The value and anisotropy of the reduced mass (μexc) and g-factor (gexc) of the band gap exciton are determined by photoluminescence measurements under intense magnetic fields (B, up to 28 T) applied along different crystallographic directions. μexc is 14% greater in WZ NWs than in a ZB bulk reference and it is 6% greater in a plane containing the WZ ĉ axis than in a plane orthogonal to ĉ. The Zeeman splitting is markedly anisotropic with g exc = |ge| = 1.4 for Bĉ (where ge is the electron g-factor) and gexc = |ge - gh,//| = 3.5 for B//ĉ (where gh,// is the hole g-factor). A noticeable B-induced circular dichroism of the emitted photons is found only for B//ĉ, as expected in WZ-phase materials. © 2014 American Chemical Society