Alternative Liquid Dielectrics for High Voltage Transformer Insulation Systems. Группа авторов

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Figure 2.11 NMR analysis of (a) new FR3, (b) aged FR3 for 2000 hours, (c) new JAT, and (d) aged JAT for 2000 hours.

      Source: Baruah et al. [73] / with permission of IEEE.

2.6 Dissolved Gas Analysis in Natural Esters

Occurrence of faults in the electrical network is very much likely, but proper monitoring can help avert any catastrophic incidence. The two main categories of faults arising in the transformers are thermal and electrical. These faults lead to breakdown of insulation, and release gases in the interior of the transformer, which are detrimental to the overall functioning of the apparatus. Thus, it is very much important to measure the health of the transformer at fixed durations. DGA is an indispensable method to assess the condition of a transformer to gauge the severity of the incipient faults. The key gases developing in the transformer as a result of fault and aging are: (i) hydrogen and hydrocarbons – hydrogen (H₂), ethane (C₂H₆), methane (CH₄), ethylene (C₂H₄), acetylene (C₂H₂), (ii) Carbon oxides – carbon monoxide (CO) and carbon dioxide (CO₂), and (iii) propylene (C₃H₆) and propane (C₃H₈). The increase in temperature leads to the evolution of these gases and with progressing time, the concentration of these gases change. Detecting the individual concentration and applying the various standard methods can help identify the incipient faults that might occur inside the transformer.

      The standard IEEE C57‐104 is dedicated to MO whereas the standard IEEE C57‐155 is about Interpretation of Gases Generated in Natural Ester and Synthetic Ester‐Immersed Transformers. As all the gas ratio techniques are elaborated in IEEE C57‐104, it is used for analysis. The Duval Triangle method is mentioned as per IEEE C57‐155 and it is used with regards to the stray gassing phenomenon in natural esters under the effect of the thermal stress.

      2.6.1 Standard Gas Ratios

      There are several methods to infer the DGA results of a transformer, with some of them listed below. The most important hydrocarbon gases are methane (CH₄), ethane (C₂H₆), hydrogen (H₂), ethylene (C₂H₄), and acetylene (C₂H₂). These gases are taken into consideration when analyzing the gas ratios in IEC, Rogers, Doernenburg, and Duval’s triangle methods. All these gas ratio methods indicate the types of faults likely to occur in a transformer after the oils are subjected to thermal or electrical stress. The five gas ratios according to standard are: Ratio 1 (R1) = CH₄/H₂, Ratio 2 (R2) = C₂H₂/C₂H₄, Ratio 3 (R3) = C₂H₂/CH₄, Ratio 4 (R4) = C₂H₆/C₂H₂, and Ratio 5 (R5) = C₂H₄/C₂H₆.

      2.6.1.1 IEC Gas Ratios

The IEC 60599 standard is one of the prevalent approaches for an elucidation of the faults occurring in a transformer, which is based on ratios of five key gases: CH₄, H₂, C₂H₄, C₂H₆, and C₂H₂. In this method, the ratios R1, R2, and R5 are measured to make the interpretation of the faults as per Tables 2.5 and 2.6. A grouping of the individual codes of R1, R2, and R5 indicates the type of incipient fault. However, this method does not give very accurate results for all fault types.

      2.6.1.2 Doernenburg Ratio Method

This technique uses the four ratios R1, R2, R3, and R4 for diagnostics of faults in the transformer. In this method, it is primarily determined whether a fault exists in the transformer by examining the quantity of each gas associated to a minimum concentration limit L1 as given in Table 2.7. There is a fault in the transformer if any of the gases from H₂, CH₄, C₂H₂, and C₂H₄ exceeds double the recommended limit L1 and the quantity (ppm) of any one of the other two gases (C₂H₆ and CO) exceeds this limit L1. This procedure is reliable only if the quantity of at least one of the gases in each ratio exceeds the limit value, otherwise oil samples may be again collected for repeated analysis. If the ratio analysis is valid, then each successive ratio is compared in the order of R1, R2, R3, R4 and the fault type is ascertained as given in Table 2.8.

Table 2.5 IEC gas ratios.

Table 2.6 Types of faults.