Christophe Perrier

Investigations on streamers phenomena in mineral, synthetic and natural ester oils under lightning impulse voltage

This paper presents the experimental results of a comparative study of mineral, synthetic and natural ester oils issued from different origins through the analyze of the initiation threshold voltage of streamers, their pattern and their stopping length (final length) Lf as well as the associated current and electrical charge, in a point - plane electrodes arrangement submitted to a standard lightning impulse voltage (1.2/50 μs). Its shown that the streamers are filamentary in the different tested oils whatever the polarity of voltage. The stopping lengths Lf are longer and the conductivity higher when the point is positive than when its negative. Lf is generally longer in ester oils (natural and synthetic) than in some mineral oils. When the point is negative, Lf can be ten times higher in ester oils than in mineral ones. Different modes of propagation of streamers are also observed; the transition between these modes of propagation depends on the experimental conditions and the molecular structure of liquid. This transition is well pronounced with some mineral oils when the point is negative.

Aging behavior of cellulosic materials in presence of mineral oil and ester liquids under various conditions

This paper is aimed at the study of aging behavior of cellulosic materials in different dielectric liquids under air and nitrogen atmospheres. Paper for copper conductor and high density transformer board were aged at 130°C up to 1128 h in sealed vessels in presence of mineral oil (MO), natural ester (VO) and synthetic ester (SE). The influence of thermal aging on cellulose and oil characteristics are investigated and compared. Its observed that the paper degradation is associated with the generation of CO2 and CO gases. Whatever the type of oil, this generation is more important with vessel under air atmosphere than that under nitrogen gap indicating a protective effect of nitrogen. In mineral oil, the degree of polymerization (DPv) reaches the end of life criteria at around 200 whereas it remains around 400 with ester oils. This is mainly due to the higher water solubility of esters compared to mineral oil. The measured degree of polymerization in MO is comparable to that calculated through 2-FAL (Furanic compounds) content using Vuarchex equation whereas for esters oil, there is no agreement between the measured and calculated DPv through 2-FAL content. This can constitute a limitation in the use of 2-FAL for the diagnosis of ester-filled transformers. Overall, the insulation cellulose/oil behavior seems better under nitrogen atmosphere.

Breakdown strength of vegetable-based oils under AC and lightning impulse voltages

The breakdown strength of vegetable-based oil (VBO) and mineral oil is compared by means of experimental determination of breakdown voltage. These oils were tested under AC and positive lightning impulse voltages for uniform and non-uniform electrical field. The oil gap varied from 2 mm up to 85 mm. The results show same or higher AC breakdown strength of VBO compared to mineral oil for small gaps and uniform electrical field, whereas for lightning impulses and non-uniform field breakdown strength of VBO is reduced.

Electrical conductivity measurement and determination of ion mobility in insulating oil

The insulation system of power transformers is a key topic which has to be taken into account in order to achieve an optimized design and safe longterm operation. The design of feasible insulation for high voltage direct current transformers requires the understanding of field distributions in insulating materials in both AC and DC operation because the transient stresses are dependent on both conductivities and permittivities and geometry. The steady state DC field stresses are determined by conductivity of insulating material which is normally a combination of oil and paper. Therefore knowledge about the conductivity of the oil is of great importance for design and safe operation in DC case. Moreover the space and surface charges will occur in field space and also will influence the electric field. If such charges are present in a noteworthy amount, models depending on material properties only will probably be an approximation. The conductivity of insulating material is dependent on several parameters, e.g. time, temperature and electrical field strength and the geometry of the arrangement. In this contribution, a new measuring cell with plate type electrodes is presented for analysis of oil conductivity. The measurement accuracy of the set-up is investigated both analytically and experimentally. Consequently, measurements are performed for investigating the time behavior of conductivity. The results show that the determination of ion mobility using polarity reversal test represents to a suitable approach to investigate the transit times more precisely.

Investigation on creeping discharges propagating over pressboard immersed in mineral and vegetable oils under AC, DC and lightning impulse voltages

This paper deals with the experimental characterization of discharges (namely the shape and stopping length) propagating over pressboard of different thicknesses immersed in mineral and vegetable oils, under AC, DC and lightning impulse voltages (1.2/50 μs) using a pointplane electrode arrangement; the point being perpendicular to pressboard. Its shown that the thickness of pressboard and the type of oil as well as the magnitude and polarity of voltage significantly influence the characteristics of discharges. For a given thickness, the stopping length of discharges Lf increases quasi-linearly with the voltage (U); and it is longer with a positive point than with a negative point whatever the type of voltage. Lf decreases when the thickness increases. The thinner the pressboard, the more branched are the discharge structures. The density of discharge branches increases when the pressboard thickness is reduced. On the other hand, for a given voltage and thickness of pressboard, Lf is longer under AC than under DC whatever the type of oil. Also, the characteristics Lf(U) in presence of processed vegetable oil and mineral oil are very close except for thin pressboard. The influence of electric field and capacitive effects on the propagation mechanism is also evidenced.

Ester oils and flow electrification hazards in power transformers

In large power transformers where forced convection of the insulating liquid is the predominant heat transfer mean, the electrification risk can appear. This phenomenon has been suspected to be responsible for several failures. Studies have shown, in addition to the transformer design (shell or core type), that the type of liquid used is quite important in limiting these hazards. The goal of this paper is to present an analysis of ester oil behavior versus electrostatic hazard due to flow electrification. Two experimental studies were conducted on different ester oils: the first one is for measuring the Electrostatic Charging Tendency (ECT) by using ministatic charge tester, and the second one is made by mean of a test loop simulating the oil flow as in a real transformer and equipped with a capacitive sensor. This sensor is able to measure both the charge accumulation at the pressboard surface and the generating current. All these results are compared with those of mineral oils which are actually used for large power transformers. It was shown that even if ester oils increase the charge generation in comparison with mineral oil, the charge accumulation on the solid surface is not excessive thus limiting the potential at the solid/liquid interface and associated electrification hazards. One can assume that the higher conductivity of esters would enable a better evacuation of generated charges.

Study on flow electrification hazards with ester oils

Mineral oils are commonly used for cooling and electrical insulation purposes in electrical power transformers. Nevertheless, these oils are especially problematic for the environment in the case of burning and leakage. Thanks to their higher fire point and excellent biodegradability, ester oils could be a good candidate to replace mineral oils. Thus, some transformer manufacturers have launched investigations by filling medium power transformers with ester oils and especially natural ester oils.

Behavior of the charge accumulation at the pressboard/oil interface under DC external electric field stress

Mineral oils are commonly used for cooling and electrical insulation purposes in electrical power transformers. The physicochemical process appearing at the Pressboard/Oil interface, leads to the formation of an Electrical Double Layer (EDL). Therefore the interface is polarized by the EDL, the charge of one sign is generated within the solid medium and the opposite charge remains in the liquid medium. The process of flow electrification is due to the convection of the diffuse layer inside the liquid. The test loop was recently optimized with the addition of an energized wrapped copper electrode in the sensor. This modification allows studying the effect of an external electric field on the charge production and the distribution of the EDL : in the liquid via the streaming current (global charge production) and in the pressboard via the capacitive current. This capacitive current allows quantifying the accumulated charge within the pressboard plate (local charge production). Therefore, the dynamics of the accumulated charge refer to two EDL affected in the same way by the DC electric field. This paper proposes to show the influence of an external DC electric field on the accumulated charge within the pressboard as well as on the streaming current.

DGA comparison between ester and mineral oils

This paper reports on a comparative study between mineral and ester oils basing on DGA (Dissolved Gas Analysis). Electrical (breakdown of low energy) and thermal (stray gassing) faults are realized at laboratory scale and gas formation as well as diagnosis methods are compared. Two extraction methods described in IEC 60567 were used: total vacuum degassing and stripping method. Three different diagnosis methods were used: IEC 60599 gas ratios, Duvals triangle and key gases. It appears that for electrical faults and especially for low energy discharges, the same gases (Hydrogen and Acetylene), in relatively same ratios, are created. It is shown that classic Duvals triangle is well applicable for this type of fault confirming thus the results reported in literature. For thermal faults and especially for stray gassing issue (low thermal faults), Ethane (associated with Hydrogen) constitutes the key gas of natural esters and the new Duvals triangle can be applied for mineral oil whereas some arrangements need to be introduced for ester oils.

Space charge formation in insulating liquids under DC stresses

The occurrence of space charges has to be understood, in order to study the field distortion which is due to space charges. Hence, the investigation of space charge behavior and its origin in insulating liquids is necessary for design of optimal insulation system. It is important to know about the type of charge carriers present in insulating gap. Applying different DC stresses with various polarities can be helpful to determine the thickness of space charge regions near the electrodes and consequently, to achieve knowledge of space charge distribution both in insulating liquid and along the electrode surfaces. If an electrostatic field is applied onto the insulation between two electrodes, the charge carriers will begin to move towards the electrodes and neutralize themselves on metal electrodes. To determine the velocity of charge carriers during the measurements, a polarity reversal test is recommended which can be performed after long-duration conductivity measurements. In this investigation, the polarization current is measured using a plate-shaped test cell with stainless steel electrodes which allows a variation of gap between 1 to 5mm. In this case, the measurement set-up is based on low level current measurement with high sensitive ammeters because the current flowing through an insulation system is some pAs only. Finally, all these input parameters are used to describe the physical phenomena which occur during the conduction process.