Space charge behaviour in thick oil pressboard insulation systems for converter transformers

Abstract

With increasing desire for renewable energy integration and international power trade, the development and utilization of the high voltage direct current (HVDC) technologies for the long distance and massive power transmission have been boosted in the recent years. The reliability of the converter transformer, which is one the most important components in HVDC transmission, has become a general concern, because of its complex structure and operating conditions. It is well known that the presence of space charge can distort the electric field distribution within the dielectrics, which can potentially influence the reliability of the converter transformer by accelerating the insulation ageing or failure. Therefore, investigation and evaluation of the space charge behaviours within the insulation system similar to those in converter transformers is paramount for delivery of a reliable HVDC transmission system.

Unfortunately, the space charge behaviours in thick oil pressboard insulation systems used in converter transformers have rarely been studied due to its complex solid and liquid mixed insulation structure and severe attenuated signal that cannot be easily measured. Therefore, in this work a purpose built pulsed electroacoustic system (PEA) has been developed to allow the measurement of space charge in a thick oil-gap and pressboard combined insulation system with a total thickness of 2 mm. In order to have a better understanding of the space charge characteristics in converter transformers, three different sample configurations have been used and they are single layer pressboard, oil gap combined with single layer pressboard and oil gap sandwiched between two pressboard layers. Investigations of space charge dynamics in thick oil pressboard insulation systems under various DC stresses, polarity reversal voltages and AC/DC superimposed stresses have been successfully conducted. In addition, the impact of aged oil on the space charge behaviour has been analysed by extracting space charge features using numerical calculations to evaluate the insulation performance of the long time served converter transformer

The results identified that the space charge dependent electric field distribution is significantly distorted in the pressboard bulk or at the oil/pressboard interface under the DC stress. The peak electric stress could be much more severe than the predicted by calculations based on Maxwell-Wagner theory, particularly in the aged oil samples. After polarity reversal, the electric field increase across the oil gap can be significant due to the residual space charge in the pressboard. A method to estimate the maximum electric field enhancement immediately after polarity reversal by using the DC space charge characteristics is proposed in this work, and its effectiveness and accuracy have been experimentally validated. The space charge behaviours under AC/DC superimposed stress, in oil-pressboard insulation system are investigated for the first time in this research. The results revealed the non-linear charge injection behaviour under the superimposed stress in the oil pressboard insulation, which has been evidenced experimentally by the accelerated space charge movement and the increased charge amount when compared with under AC or DC stress separately.

This research demonstrates the severe electric field distortion caused by the space charge accumulation in the thick oil pressboard insulation system under real operating conditions of the converter transformer. This fundamental study paves the way for further improving the reliability of HVDC transmission system, leading to the realization of new rules of design, testing, operation, and maintenance are needed for converter transformers in the power industry.

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