Wade Enright

HTS Transformer: Construction Details, Test Results, and Noted Failure Mechanisms

An experimental high temperature superconducting transformer has been designed and built using Bi2223 HTS tape. The transformer is unique in that the magnetic circuit is comprised of air and a silicon steel partial core. Electrical tests were performed on the transformer and it was found to be 98.6% efficient at full load. The transformer failed during a full load endurance run and an investigation was carried out to determine the cause of the failure. The cause was believed to be from operating the HTS windings close to critical conditions. Presentation of the failure details will be of use to other researchers who are building HTS transformers.

A resonant transformer for high voltage testing of generator stators

Abstract A partial core transformer has been designed such that its magnetising reactance has been matched to a generator stator insulation capacitance. The reactive current drawn by the stator insulation is provided by the transformer magnetisation This means that the supply only has to provide the real power losses of the transformer and in practice any mismatch between the magnetisation current and the stator capacitance. A high voltage inductor was designed and used in the testing of a generator at a New Zealand power station. It supplied 115kVAr of reactive power compensation at 23kV. The inductor was turned into a resonant transformer by the addition of a low voltage primary. A laboratory test showed that the required high voltage of 23kV could be obtained from energising the primary at 285V at 60A or at a rating of about 1/7th the load. A further resonant transformer was then designed for a 334kVAr capacitor load to test generator stators at 31.5kV. This transformer was supplied from a nominal 400V supply and gave a gain in kVA of 16. The transformer has a finished weight of approximately 300kg.

A 15-kVA High-Temperature Superconducting Partial-Core Transformer—Part 1: Transformer Modeling

A new 15-kVA, 230-230 V, high-temperature superconducting (HTS), partial-core transformer has been designed, built, and tested. The transformer utilizes a unique core design termed partial core, consisting of a central laminated slug of core steel only. The windings are layer wound with first-generation Bi2223 HTS. A model was developed to predict the performance of the transformer as well as the ac losses of the HTS and is presented in this paper. Part 2 of this paper presents the construction details and experimental results.

Observations of the long distance exploding wire restrike mechanism

An exploding wire restrike mechanism is applied to create plasma paths up to 9 m in length. The mechanism uses enameled copper wires in a 5 to 10 kV/m region of average electric field (AEF). This relatively low AEF restrike mechanism appears to be linked to the formation of plasma beads along the wire’s length. Voltage traces, measurement of relative emitted light intensity and photographs are presented at AEFs below, inside and above the identified restrike region.

Generating extra long arcs using exploding wires

Long electrical arcs that are created using high voltage impulses in air gaps require average electric fields (AEFs) in the order of 100 to 1000 kV/m. In this paper, a method is presented where arcs can be created using AEFs of just 4.5 kV/m. The results from experiments up to 60 m long are used to form a mathematical model that predicts the occurrence of arc formation. The parameters of the energy supply, namely the voltage and capacitance, are found to determine the length of the arc that can be obtained. This model can be used by other research groups to reliably create long arcs; it is shown that many high voltage laboratories already have equipment capable of producing arcs several hundred meters long by using this method.

Investigating Long-Distance Exploding-Wire Restrike

An experimental setup was built using commonly available high-voltage laboratory equipment to investigate the creation of exploding-wire restrikes up to 9 m long. Sets of voltage traces are presented with varying applied average electric fields. Restrikes (formation of plasma paths) have been found to occur in a region of average electric field between 5.6 and 15 kV/m. The average electric field of this region of restrikes is relatively low and will assist in investigation of novel plasma conductor configurations.

Harmonic measurements from a group connected generator HVDC converter scheme

A recent CIGRE document published in ELECTRA has described the potential benefits of a direct connection of generators to HVDC power converters. While many theoretical contributions have been made, no practical test data has become available so far. This paper reports on harmonic tests carried out at the Benmore end of the New Zealand HVDC link operating as a group connected scheme. It was found that the measured harmonic current levels were well below specified generator ratings. Dynamic simulation accurately predicted the harmonic currents whereas the results of a steady state formulation were less-reliable. >

A 15-kVA High-Temperature Superconducting Partial-Core Transformer—Part II: Construction Details and Experimental Testing

A new 15-kVA, 230-230 V, high-temperature superconducting, partial-core transformer has been designed, built, and tested. The transformer utilizes a unique core design called partial core, consisting of a central laminated slug of core steel only. The windings are layer wound with first-generation Bi2223 HTS. In part 1 of this paper, a model is used to predict the performance of the transformer as well as the ac losses of the HTS. In this part, a series of electrical tests was performed on the transformer, including open circuit, short circuit, resistive load, overload, ac withstand voltage, and fault ridethrough tests. The test results are compared with the model. The transformer was found to be 98.2% efficient at rated power with 2.86% voltage regulation.

Wire Explosion by Electromagnetic Induction

A conductive plasma channel in the shape of a 360^ helical arc has been realized via the mechanisms of electromagnetic induction and exploding wire (EW). This paper is focused on documenting the restrike (RS) mechanism for a wire exploded via electromagnetic induction. The apparatus is a pair of mutually coupled helical coils, which are arranged on cylindrical polyvinylchloride formers, with a capacitor bank as the energy source. Voltage and current waveforms are presented. The wire explosion by induction exhibits key features that exist in conventional wire explosion by conduction, namely, wire fragmentation, a dwell period, and RS phenomena. Exponential damping envelopes were graphically applied to distinguish between RS and non-RS outcomes of the wire explosions. Photography, visual observation, and copper oxide residue patterns also helped to determine whether an RS took place. Lichtenburg figures in the form of dust trees were found to have formed at some time during the induced EW discharge.