LEDT

Low Energy Degradation Triangle (LEDT)

Introduction

Power transformers remain one of the key components of any power network and although passive in nature internally it hosts a dynamic environment of magnetic forces, chemical reactions and electrical activity which has to maintain the finest equilibrium to ensure long term sustainability.The proposed model, Low Energy Degradation Triangle, is composed from the three dissolved gases hydrogen, methane and carbon monoxide. These three dissolved gases generally start to be formed from low energy degradation processes within the power transformer. The three gases are plotted as a triangular plot on a XY plane allowing for creation of specific regions and trends as the transformer insulation degrades under different operating conditions. The nature of the model is that it is sensitive to both degradation of the cellulose and oil insulation and the amount of energy that may be present.The Low Energy Degradation Triangle provides an early detection of a change in transformer health from normal to defective state. This model is potentially effective when applied to on-line dissolved gas samples were trending of dissolved gases play a key role in detecting incipient changes in the level of insulation degradation. The advantage of this method is that it allows for the identification of a change in transformer health status caused by degradation mechanisms developing from low energy sources. The Low Energy Degradation Triangle has been successfully applied to the GSU transformer fleet within a large Power Utility where significant defective transformer health statuses have been identified and highlighted as a warning for intense monitoring.

Energy Triangle Concept

It is found that H₂ increases with increasing energy. CH₄ starts to develop early from oil degradation. CH₄ however, due to chemical bonding, at higher temperatures start to decrease in percentage formation. CO starts to develop early from paper degradation. CO also, due to chemical bonding, starts to decrease with higher temperatures.

In a power transformer due to the breakdown of oil and paper carbon oxides and hydrocarbons are produced. Due to the paper breaking down at much lower temperatures than the oil, the CO levels tend to be higher than the H₂ and CH₄ at these temperatures (usually at operating temperatures below 110°C). The hypothesis that these three dissolved gases can provide some indication of the insulation degradation is tested. Thus the combination of these three gases in a triangular plot enables the interdependent relationship to exist. For lower temperatures the CO values are high and the H₂ and CH₄ are low. For moderate temperatures both H₂ and CH₄ tend to be higher than CO. For high temperatures H₂ tends to be the dominant gas. As long as there is oil and paper insulation with elevated energy levels (temperature) this theory should hold true due to the chemical nature of cellulose and hydrocarbon oil.The proposed three dissolved gases (hydrogen, methane and carbon monoxide) are plotted as a triangular plot on an XY plane similar to Duval triangle [Moodley2]. These three dissolved gases form the sides of the triangle and are represented as percentages having a summation of 100%. Each side of the triangle has a zero starting point vertex reaching 100 % on the far side. Movement along the triangular plot is clockwise for each of the three parameters. Figure 1 provides an indication of the general layout of the triangular plot where for further reference the starting vertex on the left is denoted as point O, top apex is denoted as point M and the right vertex is denoted as point H.For example if hydrogen is 40 ppm, methane 10 ppm and carbon monoxide is 150 ppm the sum is 200 ppm which is equivalent to 100%. The composition of hydrogen is 20%, methane 5% and carbon monoxide 75%. This is plotted as point X in figure 1.

Figure 1: LEDT

Hydrogen levels increase steadily over the fault range implying that the level of energy also increases with increasing hydrogen. With increasing fault energy, and depending on the involvement of oil and paper, the levels of methane and carbon monoxide also increase but then start to decrease for higher energy levels. For medium energy conditions, all three parameters are in the range 30-40% thus the point of intersection is located in the region at the centre of the triangle. Low energy conditions mean low levels of methane and hydrogen with some carbon monoxide thus the values are focused on the lower left side of the triangle. Extremely high energy conditions as a result of arcing conditions usually imply high levels of hydrogen with decreased levels of methane and carbon monoxide such that the intersect points are focused on the lower right hand side of the triangle. More specific conditions and locations are explored further with empirical evidence.

R-VALUE TREND

One of the key aspects of the LEDT is early and clear identification of insulation degradation. This is made possible by utilising the R-value trend which is a vector derived from polar plot coordinates for each sample plotted on the LEDT [3]. Figure 2A provides a polar representation of each sample point and figure 2B displays the trend of the R-value. The Normal state within the LEDT starts from the left bottom vertex (having a R value of 0) with a R value of 0.13 is considered the outer limits of normal according to empirical analysis. Values greater 0.13 is considered abnormal insulation degradation and a trigger for further investigation.

                                                     Figure 2: R-Value

REGIONS WITHIN LEDT

One advantage of the LEDT is that it provides a visual trend of potential insulation degradation in relation to the gas production rates of hydrogen, methane and carbon monoxide. The Normal region is in the green region which is on the lower left vertex of the LEDT. Movement upward along the %CH₄ axis is characterised by thermal heating with elevated %CH₄ and relatively constant %H₂. This region is composed of both T1 (thermal fault, t<300 °C) and T2 (thermal fault, 300 °C < t < 700 °C) type faults and may vary accordingly.

Figure 3: Regions within LEDT

The fault progression then leads further into the triangle which can enter the regions of D1 (discharges of low energy), D2 (discharges of high energy) and T3 (thermal faults, t > 700 °C). Progression into these regions is characterised by significant increase in the percentage hydrogen and methane in differing proportions. Partial discharges are noted in the region PD which is for percentage hydrogen levels greater than 90%. These regions have been verified with the IEC TC 10 Database [5].

Use the following link to the "Analysis" section to get the Rogers Ratio diagnosis of the oil samples. Enter the oil sample under "Sample 5" to get the diagnosis.