The Health Index in Transformer Management and Monitoring

Asset and maintenance managers are routinely faced with the challenge of planning their actions based on restricted budgets or time. This challenge creates the need for an effective prioritization strategy in determining which assets are the most critical. Usually more evident attributes are taken into consideration, such as power, position in the system (energy input), defects detected in the preventive maintenance and etc. However, a more in-depth analysis can deliver more favorable results in determining the criticality of assets and prioritizing. An industry can have a fleet of transformers with an average quantity between 10 and 20 units, being an even more challenging scenario for the maintenance manager. Transformer monitoring can be carried out either offline (oil sampling) or online (sensors), delivering intelligence for decision-making. Thus, the optimization of these monitoring methods should use techniques such as the Health Index to help determine the criticality of each asset and enable the generation of a ranking, through a score determined from the data provided by the laboratory reports of the oil samples collected in routine preventive maintenance.

Figure 1: HV Assets Care Platform, Health Index.

Preventive maintenance usually includes oil sampling from all transformers in the fleet, comprising a physicochemical analysis (oil quality analysis) and chromatographic analysis (disolved gas analysis). These data from the samples are processed and analyzed by algorithms that have parameters of the average limits detected by the market that are acceptable for each variable (for example, for acetylene it is determined that above 1 ppm is classified as above the limit). Then, when a variable above the established limit is detected, advanced analyzes are applied, such as the Duval Triangle and Duval Pentagon, to determine the diagnosis of the defect. Finally, each defect has its weight (or score) listed in the Health Index score matrix, thus generating a final score that is the sum of all the weights of each critically analyzed variable and its weight determined.

A score generated from preventive maintenance oil sampling data can be a valuable primary indicator of the condition of the transformer, as it can consolidate all the variables into a single number that can easily be classified according to the needs of the maintenance/assets manager. However, it is important to note that for an in-depth assessment of the condition of an asset, it is necessary to compile as much data as possible from the analysis of all the main components of the transformer, such as the OLTC, cooling system, tank inspection and others. Therefore, a score based on the analysis of oil quality and dissolved gases delivers a valuable initial indicator, but which should be deepened in critical or abnormal cases during the evaluation of the Health Index.

In addition, other indices can be derived from the initial score generated by the Health Index, such as the Criticality Index, which uses weights determined by the asset manager to improve the classification of transformers in the ranking. Some examples are the classification of assets by value (aquisition cost) range and impact on production. That is, to determine a higher weight, for example, for a transformer that is directly connected to a production line and has a high acquisition cost value, compared to a transformer that is connected to an auxiliary building, such as a warehouse, and with a low acquisition cost value. Thus, a transformer that was apparently evaluated as critical due to its power and condition, can be classified as less critical when these weights are applied in relation to its condition, determining its real criticality for the business as a whole.

Optimize your asset management with the Health Index

The Health Index is part of the package of solutions that HV Assets offers to the market and can be contracted by subscribing to the HV Assets Care platform. When hiring, our team of experts will assist in processing the data of each asset to prepare the platform for customer use. Thus, when the data upload is carried out by the customer through the platform, the team will process the data within 48 hours. The data of the entire history of each transformer, including physicochemical (oil quality analysis) and chromatographic (dissolved gas analysis) analyzes, can be viewed and analyzed by the maintenance/assets manager on the platform. In addition, all new oil collections may in the future be updated on the platform for the processing of automated analyzes and diagnoses, optimizing the management of the entire fleet. The Health Index is automatically updated after the insertion of new data from collected samples and the history of the last 5 years of the score can be viewed by the customer. The Health Index technique, developed by HV Assets, is consolidated in the market and recommended by CIGRE in its Technical Brochure on the evaluation of the condition of transformers[1].

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1. CIGRE, Condition assessment of power transformers - Reference: 761 - 2019.