TRANSFORMER ASSET MANAGEMENT-AN OVERVIEW

Presented By: S.M.G. MUZAFFAR HUSSAIN

SUBSTATION ENGINEER

Unrestricted

CONTENTS INTRODUCTION TAM TAMP TRANSFORMER AGING TRANSFORMER FAILURES CONDITION MONITORING / TESTING ASSET MANAGEMENT TOOLS FACTORS DECIDING PROCUREMENT OF SPARE

TRANSFORMER CONCLUSION REFERENCES AND ACKNOWLEDGEMENTS

INTRODUCTION

Asset Management is defined as an efficient way of utilizing and maintaining a group of assets over their entire life cycle assuring return on investments, ensuring defined service, reliability and safety as per standards.

An asset is the one which is, used in production and supply of goods and services. utilized maximum throughout its life cycle. A vital link in the network and has a specified life

period.

TRANSFORMER ASSET MANAGEMENT (TAM) IN ELECTRICAL UTILITIESA power transformer being a significant asset in the process of energy

transmission, Transformer Asset Management (TAM) becomes really necessary that focuses on Plant reliability, Managing risks and Optimizing life cycle costs.

In simple managing transformer assets for value and risk implies Balancing investors, customers and interested parties. Aiming uninterrupted long term service to customers. Cost effective quality energy supply to avoid customer conflict. Evaluating massive risk implications i.e. Cost of transformer Vs

Loss of revenue due to its failure at site Using management tools to calculate the life of a transformer to

avoid extending its use beyond planned to avoid windfall. Managing redundancy usually built in the network. Building teams and developing asset management cultures. Encouraging both internal and external interested parties to invest. 

TRANSFORMER ASSET MANAGEMENT PLAN (TAMP)

A Transformer Asset Management Plan (TAMP) is made with the business objective of providing a comprehensive

a) Qualitative assessment of condition. i.e. Healthiness of the transformer

b) Risk Exposed to the business or extended usage of transformers beyond its lifetime. (Power interruption due to continued usage of defective transformers without redundancy till the time it fails and not thinking of repair, refurbishment or replacement).

c) Developing an effective inventory management strategy to minimize the life cycle costs / Inventory costs.

a. QUALITATIVE ASSESSMENT OF CONDITION

Healthiness of the transformers is assessed during the various stages of its life cycle through electrical tests , dielectric tests on oil and insulation and other diagnostics methods.

Preventive maintenance and Quality inspections are carried out to detect faults and fix it in time to ensure and acceptable level of transformer reliability. However the spare parts and cost of inventory should be mitigated.

b. RISK EXPOSED TO THE BUSINESS

A life extension beyond that planned is a significant windfall for the utility. Any failure by the transformer has massive risk implications.

Redundancy built in the network results in power interruptions to customers and loss of revenue which is many times its cost to the utility.

c. DEVELOPING AN EFFECTIVE INVENTORY MANAGEMENT STRATEGY

The components of the Power transformer are classified into three categories.

i. Most Expensive Items (OLTC, Bushings Arrestors)ii. Ordinary Items (Insulating Oil)iii. Cheap Items (Nuts, Bolts and Gaskets)The most expensive items that need close control and which

in cults usually high inventory cost are usually not kept in huge numbers as stock due to capital investment limitations. Hence the number of spare parts is based on Minimum Safety Stock (MSS) with acceptable stock availability.

Statistical distribution techniques such as Poisson and Normal distribution is used to determine the optimum number of spare parts after analyzing the failure rate on Power transformer.

c. DEVELOPING AN EFFECTIVE INVENTORY MANAGEMENT STRATEGY (Continued..)

The Ordinary items that need standard care, the method of Economic Order Quantity (EOQ) is proposed to minimize inventory cost.

The cheap items that need little care are managed by KANBAN System.

With this effective inventory management the electric utilities not only reduce the inventory costs but also significantly reduce the hidden capital costs due to power cuts.

4. TRANSFORMER AGING & END OF LIFEAging of a transformer refers to the thermal degradation

of the insulation system during its normal life cycle. End-of-life refers to a point where it can no longer withstand the stresses coming on to it during fault conditions.

The primary aging factors are temperature, time, over loads and frequent faults.

The environmental conditions such as corrosive atmosphere, excessive vibration may affect actual service life.

Agents of deterioration like moisture and tracking accelerates aging.

Degree of Polarization (DP test) on cellulose insulation material sample taken from the hottest-spot region is recommended as a supplementary test to determine its correlation with transformer aging.

5. TRANSFORMER FAILURESFailure is described as an insulation breakdown and that a

service outage would result if it occurs in the field. Main causes of transformer failure: Internal insulation breakdown caused by hot spots, partial

discharges and arcing. Oil Contamination caused by oil leaks. Voltage spikes caused by lightning surges. Movement of coils and weakening of cellulose caused by

external short circuits. Moisture content Internal flash over of tap changer External flash over of insulators Bursting out of tank due to enormous pressure created

during short circuits and failure of operation of pressure relief vent.

Operating an Off load tap changer without de-energizing the transformer.

6. CONDITION MONITORING/TESTING Since it is not possible to predict when a final failure will

occur, it is necessary that a number of parameters such as dielectric strength, thermal/mechanical strength and electrical strength are to be monitored to avoid breakdown.

Maintenance Engineers are required to conduct analysis, tests, and inspections and review previous data to assess the condition (Healthiness) of the power transformer.

INSULATION SYSTEMDissolved Gas Analysis (DGA) Acidity Water Content (ppm test)Insulation power factor (Tan ∂ test)Furan AnalysisDegree of Polymerization (DP test)

6. CONDITION MONITORING/TESTING (Continued..)

ELECTRICAL SYSTEM Ratio (TTR test)DC measurement of Winding Resistance

MECHANICAL SYSTEM Sweep Frequency Response AnalysisTightness test for Oil leakageCore to earth resistance

BENCH MARKING Previous resultsSignaturesBench mark set by Electric utility companies

7. ASSET MANAGEMENT TOOLSIn practice as necessary tools are not available to determine the

end-of-life of a power transformer, the minimum life expectancy (elapsed time) is calculated.

Minimum life expectancy calculation as per IEEE C57.100.1999

7. ASSET MANAGEMENT TOOLS (Continued..)

The elapsed time in actual for a transformer in service will be very much greater than the life determined by the continuous loading. Therefore it is indicative of the fact that a transformer operated continuously at rated hottest-spot temperature should give a satisfactory life expectancy of 180 000 h approximately 20.5yrs.

If a transformer fails before its life expectancy due to material defect or poor workmanship the issue has to be taken up with the manufacturer for immediate replacement.

If the transformer fails in the end of life tests (series of high voltage and high current tests), a service outage occurs in the field and to refurbish with spare transformer.

8. FACTORS DECIDING PROCUREMENT OF SPARE TRANSFORMERS

Transformer is a costly equipment and incurs huge investment and stock build up.

Even though its failure rate is low it plays an important role in transmission network. Therefore a wise decision has to be taken on the procurement of spare transformers considering system reliability and risk assessment.

Before procuring the spare transformers, the following has to be evaluated

Whether the aged transformers can be repaired. Value of the spare transformers V/s Repair cost Overloading of aged transformers or its Replacement. 

8. FACTORS DECIDING PROCUREMENT OF SPARE TRANSFORMERS (Continued..)

 The other factors deciding on the procurement of spare transformer are

Total number of transformers available in the system.Load demand on the service transformersCondition of aged transformersTotal years in serviceRisk Management Strategy

◦Shifting aged transformers to under loaded substations to minimize failures.

◦Availability of identical spare transformers to mitigate the risk of transformer failures and outage at substation.

9. CONCLUSIONSince it is impracticable to determine thecondition of aged power transformers and itsend of life, the following are the conclusions.

◦The aged transformers are to be identified in the system to avoid overloading and failures at site during its revenue earning period.

◦Spare transformers to be procured as per the inventory management strategy to prevent outage and loss of revenue.

◦Load demand and Load flow study to be done to operate transformers in parallel.

◦Mobile transformers identical to the existing ones have many advantages in shifting and restoration.

◦Avoid un-served customers and energy loss.◦Quality supply as per customer demand to avoid customer

conflict.

9. CONCLUSION (Continued..)◦Increasing transformer life through condition

monitoring and Preventive maintenance.◦Lessen Maintenance Costs. ◦Depreciation on the value of the asset to be

calculated every year to decide on replacement.◦When the depreciated value of the asset becomes

equal to the maintenance cost it is the right time for replacement.

◦The Residual / Fair value of the asset at the end of its service shall be an appreciable one.

◦As such in the whole substation the asset value of transformers are less compared to its cost of failure. Hence avoid risk implications.

◦Data Collection is a must for analysis

9. References and Acknowledgements Asset Management in Power Transformers by G. Kalinde Asset Management Plan of transformer & Factors affecting the decision on

procurement of spare transformer by Nahar S. Al Mutawah and Navin Rego Asset Management for Power transformer in High Voltage Substation by Thanapong

Suwanasri, Rattanakorn Phadungthin and Cattareeya Suwanasri

END OF PAPER

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