Chương 1. Giới thiệu1.1 Động lực th

Chương 1. Giới thiệu
1.1 Động lực thực hiện đề tài
The transformer is generally considered to be one of the most important power system components. If it fails or breaks down, the consumer is deprived of electrical energy and can no longer function properly in today’s society. Oil-immersed transformers have commonly and economically been used for a wide range of voltages and power ratings for many decades, i.e. from distribution to transmission levels or from medium high voltage to ultrahigh voltage applications [1]. In the oil insulated power transformers, the insulating system consists of paper wrapped conductors in the transformer windings plus mineral oil and pressboard to insulate the windings from ground. In this oil-solid composite insulation system, two types of failures may occur. The first type involves a complete failure between two electrodes (which can be bulk-oil breakdown, creepage breakdown along oil-solid interface or combination of both). The second one is a local failure (partial discharge), which may not immediately lead to failure between two electrodes. Sustained partial discharges lead to deterioration of the insulation system eventually leading to a failure. It was found that about 60-80% of failure of in-service transformers occurs at on-load-tap changer, windings and bushing [1][2]. One of the main origins of failure is the ageing/weakness of the oil-solid insulation system. Thus, the reliability of a power transformer is largely determined by its insulation condition, and the insulation of transformer is therefore an important issue.
It was known that the breakdown in insulating oil is preceded by the pre-breakdown phenomena, called streamers. It means that the breakdown will not occur without the presence of streamers. Thus, many studies have been performed during the last few decades to understand the mechanisms forstreamer initiation and propagation. Nevertheless, we still do not completely understand several streamer phenomena. The need to understand these phenomena comes fromthe three following reasons. First, presently, most of the designing criteria for transformer insulation are empirically based and derived on long term experience and not through basic scientific understanding. Second, the present standards for insulation tests on liquids do not reflect the functional needs of an insulating liquid. Third, the needs to improve the existing insulating liquids and design new insulating liquids that are environmentally friendly are indispensable.
Streamer initiation and propagation have been investigated under various gaps with various liquids such as mineral transformer oil, white oil , hydrocarbon liquid and ester oil and many papers published. Various experimental conditions were examined such as voltage magnitude, voltage polarity, voltage waveform, electrodetip radius, gap distance, hydrostatic pressure, viscosity and additives. With the huge number of those published papers, our knowledge on streamer has greatly expanded. Streamer phenomenon was well reviewed in [3][4]. The main findings can be briefly presented as follows. Streamer characteristics (structure, current (charge), velocity, light emission…) were well observed and depended on experiment conditions. Streamers were classified according to specificpropagation modes. A special phenomenon was observed that beyond a threshold applied voltage, i.e. acceleration voltage, streamer velocity jumps from 2 km/s to 100 km/s[5][6][7][8][9]. Low ionization potential additives were proved to increase the acceleration voltage of cyclohexane under short and long gaps[7][10]. Streamer initiation is controlled by the electric field of the needle tip while the macroscopic field of streamer envelopegoverns its propagation. Gaseous plasma is possibly present inside streamer channels. Both gaseous and electronic processes are involved in streamer propagation. The mechanism for initiation of negative streamer is quite well known, but this is not the case of positive streamer. Several hypotheses for streamer propagation have been suggested. Although many results were obtained, there are still numerous problems that have not yet been addressed. To improve the qualitative understanding of streamers, this study will address the following problems;the correlation between branching and velocity of streamers, the effect of experiment conditions on streamers (carbon particles, dissolved gases/air, low hydrostatic pressure), theinfluence of additives on streamer characteristics, properties of positive streamer channel and mechanism behind the fast mode.
The excellent property to withstand very high voltage, i.e. high acceleration voltage [6][8], is one of the major reasons why mineral oilis the unique choice for power transformers. It is known that mineral oil mainly contains paraffinic, naphthenic and aromatic/polyaromatic compounds. The aromatic/polyaromatic compounds have two important electronic properties: low ionization potentials and large electron-trapping cross sections. It was found that low ionization potential additives strongly increases the acceleration voltage of positive streamers[7][10]. Large electron trapping cross section additives (electron scavengers) drastically increase the velocity and largely reduce the breakdown voltage of negative streamers [7][11]. In addition, it was observed that streamer behaviour strongly depends on the chemical composition of liquids[12]. However, the complex chemical compositions of mineral oil make it difficult to identify the effect of each chemical compound on streamer behaviour and acceleration voltage.Therefore, the streamer characteristics should be investigated in model oil. The model oil is either only white oilorwhite oil containing a small amount of N,N-dimethylaniline (DMA)or tricloroethylene (TCE). DMA stands for the low ionization potential property of aromatic compounds while TCE represents their properties of electron trapping.
The breakdown of streamers is electrode gap dependent. For short electrode gaps, the electric field distribution is quasi-uniform and thebreakdown largely depends on streamer initiation[13][14]. However, for long gaps the electric field becomes non-uniform and the breakdown is dominated bystreamer propagation[14]. In addition, breakdown in long gaps is more relevant to that of oil-solid insulation system for practical high voltage transformers. Thus, the investigation of streamer characteristics should be performed with long gaps.
Increased hydrostatic pressure suppressed streamer propagation in pentane and cyclohexane in short gaps, thus the propagation of streamer may relate to gaseous processes [15][16]. However, the effect of increased or reduced pressures on streamers has not yet been investigated in long gaps.

1.2 Mục tiêu của đề tài
The main purposes of this study are to better describe the characteristics of streamers and develop the understanding of streamer propagation in large scale. To achieve these main purposes, the following specific objectives are set out.
• Investigate the effect of experiment conditions (carbon particles, dissolved gases/air, low hydrostatic pressure, and voltage polarity) on streamers.
• Investigate the effect of low ionization potential and electron scavenging additives on streamers.
• Investigate the impact of chemical composition on streamers
• Investigate the properties of the positive streamer channels
• Suggest the mechanism responsible for fast mode streamers

1.3 Đóng góp của đề tài

The main contributions of this thesis are given as follows:
• Streamer characteristics of white oil (Exxsol-D140 and Marcol-52) in non-uniform field in a long gap are achieved. Streamers generally behave in similar way in these types of oilwith increasing applied voltage. When the applied voltage is raised, velocity of streamers increases in “step” and they become more branched at both the 2nd and 4th modes. Positive streamers are about ten times faster than negative streamers, and the breakdown and acceleration voltagesof positive streamers are about half of negative streamers.The process of shape shifting from multi-channels to few channels is associated with the transition from the 2nd to the 4thmodes of streamers. Finally, it is found that more branching results in lower speed and vice versa.

• This thesis presents the effect of various experiment conditions on streamers. Dissolved gases/air has no effect on streamer propagation while carbon particles significantly influence it especially for negative streamers.With the presence of a small amount of carbon particles, velocity of the 2nd mode negative streamers is increased by a factor of about ten. However, these particles do not seem to affect the 3rdor4thmode streamers. Carbon particles significantlyreduce both inception and breakdown voltages of both polarities. In addition, reduced pressure significantly facilitates streamer propagation of both polarities leading to a large reduction in breakdown voltage. Although reduced pressure markedly increases the number of streamer branches, the streamer velocity of positive polarity is still not reduced.However, reduced pressure decreases the velocity of non-breakdown negative streamers but raises that of breakdown streamers.

• The effect of additives on streamer propagation in a long gap is recorded. The propagation of streamers in white oil (Exxsol-D140 and Marcol-52) is significantly affected with the presence of a small amount of low ionization potential additive (DMA) or an electron scavenger (TCE).DMA can either increase or decrease the breakdown voltage of positive streamersdepending upon whether more filamentary channels or more branching is the dominating effect in the actual liquid. With more filamentary channels, DMA will speed up streamers whereas with higher number of branches, it will slow down streamers. However, DMA always makes breakdown streamers of positive polarity more branch