L. Calcara

Scatter reduction of the 50-60 Hz breakdown voltage test for insulating liquids

There are several reasons to test the breakdown voltage of insulating liquids: acceptance tests on new deliveries, testing of treated liquids prior to or during filling into electrical equipment, and monitoring and maintaining oil-filled apparatus in service. Among others, the test for the determination of breakdown voltage at 50 or 60 Hz is one of the most important. In fact, it is worldwide accepted that this test gives quickly very useful information even if mainly related with moisture and particle content than the intrinsic properties of the insulating liquids. This test is standardized by IEC and ASTM but has some limitations in terms of the statistical scatter of mean value of the breakdown voltage. The present paper presents an improvement on the scatter of the mean values of the breakdown voltage by increasing the number of requested shots and discarding the most disperse results. In addition, it is presented as a revised test cell having stirring device on the top and round edges at the bottom may further reduce the standard deviation of the same breakdown voltage. Others experimental parameters which may still influence the scatter of the same standardized test are also examined and discussed.

Standard evolution of Partial Discharge detection in dielectric liquids

Partial Discharges (PD) detection in HV components has shown to be a very powerful diagnostic tool. Anyway, also the acquisition of the same signals in simple insulations, like the dielectric liquids, may give additional information for maintenance of HV components in which they are employed, as in the case of liquid insulated transformers. Among the standardized electrical tests suggested for the insulating liquids, the breakdown voltage (IEC 60156) and partial discharges determination (IEC 61294TR) at power frequency are not basic material properties but test procedures intended to indicate the presence of contaminants such as water and solid suspended matter and the advisability of carrying out drying and filtration treatment. Nevertheless, the PD detection method for insulating liquids actually standardized is based on superseded circuitry and is only addressed to measure the PD Inception Voltages (PDIV). The present paper shows the state of art in PD measurements in dielectric liquids to be taken into account when revising the actual IEC 61294TR or preparing new international Standards (ASTM).

Natural esters distribution transformers: A solution for environmental and fire risk prevention

The electrical transformers insulated and cooled with mineral oils remain, still today, the most widespread and effective solution for converting electrical energy. Mineral oils are used for their excellent dielectric and thermal properties, but have a low flash/fire points and biodegradability and these factors may negatively contribute in cases of fire and toxic release with contamination of soil and surface water and groundwater. For these reasons, innovative insulating fluids, such as natural esters (vegetable oils), have been suggested worldwide.

Risk prevention for HV transformers: Beyond the health index

An innovative tool to prevent the negative consequences of unexpected failure of HV transformers may be based on the risk evaluation. This risk is formed by three partial components, which are: causes of failure, probability that a failure may provoke a damage and type and entity of damages (magnitude). This approach may be very useful for supporting decisions and addressing periodical investments of electrical operators and utilities having in service HV transformers. One of the more important parameter forming the total risk is the Health Index (HI) of the transformers which is normally based on periodical inspection of the units. HI is related with the capability of transformer to withstand to a cause of failure. The present paper described as the application of this practice, able to evaluate the Health Index, has allowed to prevent a possible unexpected failure of a HV transformer, avoiding negative impacts to electric system, environment and operators.

Thermal behavior of distribution MV underground cables

The present paper reports an experimental investigation on the thermal behavior of medium voltage underground cables laid in different types of soils and under different conditions of the ambient temperatures. The same paper shows as thermal degradation of the insulating system of the cables and their joints can become more consistent and faster due to the effect of the continuously overheating due to the ambient temperatures and to the thermal resistivity of the soil of higher values. An other source of thermal degradation of the cables and their joints may be indicated in the fault currents affecting the metallic shields in systems operating with a compensated neutral connection to ground. In particular, in these systems the fault current is lasted for a time of some tens of seconds to allow a faster localization of the failure, through the automatic sectioning switches. During this time, currents flow through the metallic shields of the cables, overheating the same shields especially in correspondence of pour connections which may be present inside the joints. In case that the single-fault-to-ground evolves in a double-fault-to-ground the same shield will be interested by much higher current (short-circuit) which will create a deeper degradation of the semiconductive compounds and the insulation located nearby the metallic shields. The paper also presents the results of visual inspections of failed cable joints, due to thermal causes. Based on these considerations, important solutions may be indicated to reduce the failure rate of the MV electrical system allowing improvements in the overall power quality of the entire electrical systems.

Insulating liquids breakdown voltage determination: Test method efficiency

The 50–60 Hz breakdown voltage of insulating liquids is not an intrinsic property of the dielectric but strongly depends on the presence of contaminants, such as particles and suspended water. Both IEC and ASTM give tests procedures finalized to determine the 50–60 Hz breakdown voltages. This test is adopted as acceptance on new deliveries, testing of treated liquids prior to or during filling into electrical equipment, and monitoring and maintaining oil-filled apparatus in service. Following the IEC Standards, this test has to be performed following the IEC 60156, Ed. 2.0 published in 1995 which is presently under revision. The main reason for revising the standard for the 50–60 Hz breakdown voltage determination of the insulating liquids is the attempt to reduce the scatter in the results, the quantities of the sampling oils and the testing times. The future new edition of IEC 60156 is on the way to reconfirm the present standardized procedure but will recommend the use of a stirring system. Also in an informative annex a new advanced procedure will be included, claiming to be able to reduce the scatter in the determination of the 50–60 Hz breakdown voltages. With the aim to evaluate the mean value and related scatter of power frequency breakdown voltages of insulating liquids, testing time and quantities of samples, this paper presents a comparison of adopted test methods recommended by IEC 60156 and other methods, like the Up-and-Down- and Multiple-Level methods.