The health indices are an objective and quantitative representation of the health of the asset incorporating data obtained by operating observations, field inspections, site, and laboratory testing. This is a powerful tool for asset management and identifying and prioritizing investment needs. This is clearly set out in budget planning and capital application. The maintenance program compilation and practical maintenance activities can also be stipulated. 
 
Unfortunately, there is no standard available to quantify the condition of the asset considering all the data. We will strive to compile a practical HI calculation method using available data by considering some recommendations. 

Parameters and Testing

In the CIGRE WG 12 report, it stipulates that there are 4 main subsystems in a transformer that are exposed to degradation
 
  • Dielectric – major/minor insulation, leads, and windings
  • Magnetic circuit – core and clamping
  • Tap Changer – Load Tap Changers
  • Mechanical Parts – Bushings, Tank, Cooling, etc
Routine testing or monitoring technique which is most widely used consist of DGA (Dissolved Gas Analysis), Oil Quality, Furfural, Power Factor, Tap Changer monitoring, Load History, and Maintenance Data. 
 
A) Dissolved Gas Analysis
IEEE Std C57.104 introduced four-level criteria to classify the risks to a transformer, ensuring continued operation at various combustible gas levels. Theoretically, using dissolved gas analysis (DGA), it is possible to distinguish internal faults such as arcing, partial discharge, low-energy sparking, severe overloading, and overheating in the insulation system. [1]
 
The DGA data does not always provide sufficient information to evaluate the integrity of the transformer system. It is important to also have a history of the maintenance, loading, previous faults, manufacturer data, etc to ensure enough data to make an evaluation. 
Scoring and weight factors for Gas Levels (ppm) Table I
Transformer rating based on DGA Factor Table II
Gas production rates are extremely important in determining the severity of an electrical fault. If an elevated production rate of any of the key gases is present the DGA analysis intervals should be reconsidered to ensure safe operation of the unit and to have a timeous repair or replacement if required in order to avoid any loss of equipment of production time. An intervention in the HI classification is needed if there is a larger than 30%  increase for three consecutive samples or a 20% increase for five consecutive oil samples. Special precautions need to be taken in the case of rapid Acetylene increase, then an immediate action is recommended. 
 
DGAF = Sum of (S1) x  Sum of (W1)/ Sum of (W1)
 
S1 = the score for each gas based on Table 1 values and  W1 =  the weighting factor for the corresponding gas
 
Part III – soon
 
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INDEPENDENT TRANSFORMER CONSULTANTS

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