Space charge behaviour of thick oil and pressboard in a HVDC converter transformer

Abstract

High Voltage Direct Current (HVDC) power transmission is essential for the development of long-distance power transmission. Within the HVDC conversion system, the most significant apparatus is a HVDC converter transformer. In converter transformers, on the one hand, the high operational electric field is desirable to decrease the dimension, weight, and material usage in converter transformers; on the other hand, the electric field must be lower compared to the dielectric strength to avoid the surface discharge or destructive breakdown of insulation materials. In addition the operation voltage, the electric field is also affected by space charge dynamics in insulation materials. For this reason, extensive efforts have been made to investigate space charge dynamics in the insulation material of converter transformers, such as the oil-impregnated paper. However, the space charge characteristics are not well understood for the thick oil and pressboard (PB) materials under a temperature gradient. Hence, a new purpose-built pulsed electroacoustic (PEA) system has been set up to measure the thick oil and PB space charge behaviour under a temperature gradient. This dissertation comprises the quantitative analysis of the space charge dynamics in thick oil and PB insulation materials under a temperature gradient through an experimental approach.

With the purpose-built PEA system, the influential factor of temperature gradient on dynamics of space charge in thick oil and PB has been investigated. The space charge measured from
the PEA system depends on the acoustic wave propagation in the insulation materials. The temperature can affect the acoustic velocity and thus space charge distribution. Therefore, for a one-layer sample, the space charge recovery method under a temperature gradient is important to ensure that the correct space charge pattern is acquired. Furthermore, in multilayer samples, an acoustic wave may experience different generation and transmission coefficients that lead to attenuation and dispersion after the propagation through the dispersive materials. Therefore, the space charge recovery method of multilayer samples under the temperature gradient is proposed.

With the proposed space charge recovery method under a temperature gradient, the space charge influential factors, such as temperature gradient and multilayer oil and PB, are investigated. Furthermore, the polarity reversal (PR) effect on dynamics of space charge in multilayer oil and PB under a temperature gradient is presented using an experimental approach. The measured space charge is further interpolated into the COMOSOL model to quantitatively evaluate the electric field oil and PB after PR. The difference between the electric field after PR calculated by the Maxwell-Wagner theory and space charge density is compared. The thickness effect on the space charge is investigated on two-layer oil and PB
materials. The simulation model is also expanded into four and six layers for the electric field calculation based on the estimated space charge results.

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Identifiers

ORCID for Zhiqiang Xu: orcid.org/0000-0002-6640-7335

ORCID for Paul Lewin: orcid.org/0000-0002-3299-2556

ORCID for Matthew Praeger: orcid.org/0000-0002-5814-6155

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