Age

Con

ditio

nal p

roba

bilit

y of

failu

re

Infant mortality Useful life period Phase out period

Classical Maintenance Thinking

Design,Reliability and manufacturing

Preventive and Condition basedmaintenance

RLA

Bath Tub Curve

Indirect Costs in Maintenance

COST OF MAINTENANCE

Types of Faults

Random Time Induced

Non-observable Non-observableobservable

Maintenance Strategies for combating the Above Faul t Types

• Accept breakdowns• Readiness to repair• Improve reliability• Design out maintenance

Condition Monitoring• Continuous monitoring• Periodic monitoring

• Periodic overhauls• Periodic replacements• Reconditioning

Swedish Standard SS-EN 13306, 2001

MAINTENANCE TERMINOLOGIES

Figure-1: Impact of maintenance philosophy on failu re rate

Maintenance of Transformer• Any equipment in service, specially electrical equipment needs regular maintenance.

• Electrical equipments in service are subject to electrical, mechanical and thermal stresses.

• Periodical check – necessary to monitor condition.

• Measure to be taken to rectify defects to avoid total failure.

• This process of checking and taking corrective action is known as maintenance.

• Types of maintenance:

a.Breakdown maintenance – though not best – inevitable.

b Periodical/preventive maintenance – based on periodical tests.

c. Proactive maintenance – The very cause which affects the transformer is anticipated and avoided.

• Predictive maintenance – modern technique – consisting of tests done using special equipments

• These tests can be termed as condition monitoring tests.

• No single test can confirm the fault in the transformer. Hence integrated approach is required taking into account results of various tests.

• Expertise and experience required to interpret the test results.

BREAKDOWN MAINTENANCE

• Equipment Productivity– High downtime– Frequent failures– Poor quality– Reduced safety

• Employee Productivity– High work time– High waiting time– Low morale

PREVENTIVE MAINTENANCE

• Planned Maintenance based on periodicity

• Advantages– Reduction in failures– Enables planning

• Disadvantages– Over maintenance– Human intervention in equipment

technology

PREDICTIVE MAINTENANCE

• Maintenance based on machine condition

• Advantages– Better availability– Reduced maintenance costs– Safety and quality– Operation and maintenance planning– Design Improvements

CONDITION MONITORING

CONDITION MONITORING

OBJECTIVE (30%)SUBJECTIVE (70%)

SIMPLE AIDS INSTRUMENTAL TECHNIQUES

CONDITION ASSESSMENT OF POWER TRANSFORMER

DEGRADATION OF DIELECTRIC SYSTEMAgeing of Oil and Solid insulation� Degradation of oil by oxidation� Oxidation - Acids, sludge and water

�DGA - method for detecting incipient faults�Degree of Polymerization (DP) test is another method for cellulose insulation health

�Advanced diagnostic tests - FRA and Di-electric Loss Angle (Tan Delta)measurement

CONDITION ASSESSMENT OF POWER TRANSFORMER

Ageing of winding conductor insulation� Dielectric, thermal, electromechanical

stresses� DP of new insulation >1100� DP of degraded insulation is <=200� Knowledge about water content in

insulation and oil both important

CONDITION ASSESSMENT OF POWER TRANSFORMER

� Decrease in DP - reduction in mechanical strength

� Drying of old insulation - brittleness increases

� Pressing of aged insulation - insulation damage

CONDITION ASSESSMENT OF POWER TRANSFORMER

BUSHINGSi. Typical defects and failure mode ii. Test tap iii. Danger due to tap connection not properly earthediv. Corrosion of threads on tap connection cap causes

high resistance arcingv. Moisture absorption by test tapvi. thermal overloads

CONDITION ASSESSMENT OF POWER TRANSFORMER

Condition Based Maintenance (CBM)

1. CBM is based on Failure Mode and Effects analysis (FMEA)

2. FMEA gives a structured and systematic analysis of critical failure modes.

3. CBM applies diagnostic techniques that link to FEMA like DCRM, DGA, Capacitance and Tan Delta

4. Some utilities claim considerable reduction in O&M costs after implementation of CBM.

Failure Modes and Effect Analysis

1. Understanding the Equipment i.e. its design weaknesses and likelihood of associated malfunctions –listing all known malfunctions where they actually originate in each equipment like Transformer Core, winding, oil, tank, bushing etc.

2. Identifying the effects of the weaknesses, particularly the rate at which a weakness can develop into a fault.

3. Relating the developing fault to diagnostics, particularly to diagnostics that can reflect the rate of fault development

4. Developing and applying diagnostics, using the outcomes to trigger next activities.

Reliability Centered Maintenance (RCM)(a) RCM methodology requires an intimate

knowledge of equipment design and manufacturing standards, various condition assessment parameters, maintenance functions and their impact on equipment performance.

(b) RCM applies combination of all techniques including timed based maintenance & breakdown maintenance.

(c) RCM may show that breakdown maintenance may be the best method for a particular asset / equipment but for other equipment, predictive maintenance may be the best fitted technique.

CBM and RCM concepts1. Almost all utilities in India carry out Time Based

Maintenance (Preventive Maintenance)2. Need for increased use of Condition Based

Maintenance3. Implementation of Off line and Online diagnostic

techniques for assessing deteriorating performance or condition of the Equipment for necessary correction / rectifications before forced outage / failure.

4. CBM and RCM emphasizes need to identify the reasons of forced outages for initiating preventive measures in advance

1. Preventive / Routine Maintenance may be over doing Equipment maintenance.

2. But anything less than required maintenance may lead to unplanned outages

3. Many utilities have stopped doing Time Based Maintenance and introduced Focused Maintenance

4. RCM and CBM – both rely on knowledge of reliability & condition to indicate what to do and when.

Reliability Centered Maintenance (RCM)

Benefits of Condition Monitoring

1. Economic

� Adaptive Maintenance

� Reduction of Maintenance personnel

� Minimum period of outage

� Minimum cost of outage

2. Safety

� Reduction of risk by early warning

� Timely technical input

� Less stress on service personnel

3. Technical

� Optimum use of operation

� Optimum use of systems

� Registration of system problems for future action

� Better correlation of testing & symptoms

� Upgradation of standards for tests

� Life extension & planned replacement

POWER TRANSFORMERSDIAGNOSTIC TESTS

Detection of incipient faultsPartial Discharge Test (HFCT Technique)

Loose or bad conductor jointsWinding resistance

Level of deterioration of Paper and Oil

Dielectric Spectroscopy (Frequency domain analysis)

Inter turn shortsTurns ratio

Healthiness of coreMagnetic balance

In-homogeneities & Moisture level in paper insulation

Recovery Voltage Measurement (RVM)

Dielectric lossesTan delta & cap.

Index drynessIR/PI

Detection capabilityTests

Continued…

Inter turn faultsSurge comparison

• Detection and location of Partial Discharge site

• Detection of Hot spots

• Healthiness oil* Internal condition

•By-products of paper ageing

ONLINE TESTING• Acoustic Technique

• Infrared Thermo-Vision

•Tests on oil* DGA

* Furan analysis

Mechanical condition, Winding deformation & displacement

Frequency Response Analysis (FRA)

Detection capabilityTests

RECOVERY VOLTAGE MEASUREMENT

• Paper is a cellulosic material consisting chains of cellulose molecules

• Ageing causes breaking up of cellulose chains• Main byproduct of paper/pressboard ageing is water• Paper being hygroscopic retains major part of the water

• RVM• Provides polarisation spectrum of the insulation• Polarisation spectrum indicates homogeneity/ in-

homogeneity• Provides information on moisture content

RVM ….Continued

• Principle is based on Polarization & De-polarization processes• Methodology involves four steps

1. Charging of the insulation by applying a DC voltage (tc)2. Discharging for a limited time interval (td)… (tc/ td=2)3. Opening of the short circuit & allowing remaining

polarization to build up recovery voltage (Vr)4. Measurement of the recovery voltage

• Change tc in a time range from 0.02 Sec. To 10,000 Sec.• Repeat steps (1) to (4) to get series of Vr values• Plot Vr versus tc to get polarization spectrum

FRA capable of detecting

�Core movement

�Winding deformation and displacement

�Faulty core grounds

�Partial winding collapse

�Hoop buckling

�Broken or loosened clamping structures

�Shorted turns and open windings

:

* Mechanical Stresses cause

⇒Winding displacement or deformation

⇒Winding collapse in extreme cases

⇒ Such mechanical defects eventually lead to

dielectric faults in the winding

Frequency Response Analysis

* Reliable tool for mechanical condition assessment of the windings

* Transformers subjected to mechanical stresses during

⇒ transportation

⇒ short circuit faults near the transformer

⇒ Transient over voltages such as switching, lightening etc.

* Transfer function (Vo/Vi) is measured for three frequency ranges

# Low frequency range 50 Hz to 2 kHz

# Medium frequency range 50Hz to 20 kHz

# High frequency range 5 kHz to 2 MHz

Typical Test Circuit

Test method:

* Consists of application of a sinusoidal signal (2V) to one endof the winding

* Output voltage is measured at the other end of the winding

* Other windings are left open

* Each winding turn is linked to the other inductively or capacitively

•Each winding exhibits a characteristic frequency response which

•acts as the finger print

•Any winding movement results in substantial changes in the

•values of L & C at the local level

•Any winding movement causes changes in the characteristic

frequency response

Principle:

•Transformer is a complicated network of distributed inductance, capacitance & resistance (LCR network)

Deteriorating Factors

Bushing insulation integrity degrades due to :

Internal Moisture

• Internal PD & tracking

• External corona, flashover and tracking

• Ageing

• Physical damage

• Strategically placed conducting wrappings orlayers to equalise axial andradial voltage stresses

Typical condenser bushing design* Insulation

HV Bushings

Condenser Type

• Strategically placed conducting wrappings orlayers to equalise axial andradial voltage stresses

Typical condenser bushing design* Insulation

HV Bushings

Condenser Type

Voids conducting particleswet fibers

gas bubblessharp conductors

Tracking

Partial DischargesChemical decomposition

of oil and paper

Oxidation of paper and oil

Increased Dielectric Losses

Degrading factors

Moisture level, Local over stressing

The gases generated inside the transformers are hydrogen and hydrocarbon gases.

The causes are:

a. Thermal decomposition.b. Electrical stress.c. Electrolysis.d. Vapourisatoin.e. Chemical reaction

DGA FOR NEW TRANSFORMERS

• Useful quality control test for new transformers

• Gas analysis before & after factory tests (heat run test, H.V. Test, temperature raise test) can reveal the internal condition

• For New Transformers Gas Concentration Are Very Low – 2 PPM

Advantage of DGA technique

• Avoidance of unplanned outage as transformer defects are detected at incipient stages itself so that timely remedial measures can be undertaken to prevent damage or total loss of equipment

• Status of health check for transformer periodically

• Is a quality test for new transformer / repaired transformer before dispatch, installation & commissioning

• Several cases where transformers have been saved from total destruction, the confidence in DGA technique is so high that the transformers are sent to repairs by no other evidence other than that of DGA

Standards applicable

•Sampling of oil - IEC 567-971

IS 9434-1992

• Extraction of gases - ASTM-831-41

•Analysis of gases - IS 9434-1992

By gas Chromograph

•Interpretation of - IS 10593-1992

Data

Assembly & erectionAssembly & erection

� Measurement of winding resistance

� Measurement of voltage ratio� Polarity test

� Measurement of no-load loss & no-load current� Measurement of load-loss & short-circuit

impedance

� Measurement of insulation resistance� Dielectric tests

� Measurement of winding resistance

� Measurement of voltage ratio� Polarity test

� Measurement of no-load loss & no-load current

� Measurement of load-loss & short-circuit impedance

� Measurement of insulation resistance

� Dielectric tests� Temperature-rise test

� Dielectric test� Short-circuit test� Measurement of zero-sequence

impedance of three phase transformers� Measurement of acoustic noise level� Measurement of harmonics of the no-

load current� Measurement of power taken by the

fans & oil pumps

• Separate source voltage withstand test

• Induced over voltage withstand test

• Impulse voltage withstand test

• Prior to the short-circuit, transformer is subjected to routine tests.Asymmetrical current• The peak current that transformer is required to withstand = Isc(peak)=K Isc

Value of K :x/r 1 1.5 2 3 4 5 6 8 10 >14

K 1.51 1.64 1.76 1.95 2.09 2.19 2.27 2.38 2.46 2.55

This is to ascertain that transformer & its cooling arrangements are effectively designed so that temp. rise of winding & cooling medium

does not exceed the permissible limits.

Temp. rise limits for Dry type transformersClass of insulation Temp. rise °C

A 50E 65B 70F 90H 115

C 140

tests to prove the capability of the CB to withsta nd the dynamic and thermal stresses due to short circuits.

• The thermal stresses are determined by the duration and magnitude of the current. The short time current specified is equal to at least the breaking capacity of the Circuit Breaker with time duration ranging from 1.0 to 3.0 seconds.

• The rated peak withstand current is equal to 2.5 ti mes the ratedshort time withstand current (i.e., equal to rated short circuitmaking current of the breaker).

Short Time Withstand Current Test

Partial Discharge Tests

100pC

500pC

AN

IEC 76-1 (1993) Power Transformers, GeneralIEC 76-2 (1993) Power Transformers, Temperature riseIEC 76-3 (1980) Power Transformers, Insulation levels & Dielectric testsIEC 76-3-1 (1980) Power Transformers, Insulation levels & Dielectric tests,

External Clearances in Air,IEC 76-4 (1976) Power Transformers, Tapping & ConnectionsIEC 76-5 (1976) Power Transformers, Ability to withstand short circuitIEC 606 (1978) Application Guide for Power TransformersIEC 616 (1978) Terminal & tapping markings for Power TransformersIEC 214 (1976) On-load tap changersIEC 722 (1982) Guide to the Lighting Impulse & Switching Impulse

Testing of Power Transformers & ReactorsIEC 726 (1982) Dry type Power TransformersIEC 742 (1983) Isolating transformers & Safety isolating transformers

Requirements

IEEE C57.12.00-1993 IEEE Standard General Requirements for LiquidImmersed Distribution, Power & Regulating Transformers

Standards

IEC 76-1 (1993) Power Transformers, GeneralIEC 76-2 (1993) Power Transformers, Temperature riseIEC 76-3 (1980) Power Transformers, Insulation levels & Dielectric testsIEC 76-3-1 (1980) Power Transformers, Insulation levels & Dielectric tests,

External Clearances in Air,IEC 76-4 (1976) Power Transformers, Tapping & ConnectionsIEC 76-5 (1976) Power Transformers, Ability to withstand short circuitIEC 606 (1978) Application Guide for Power TransformersIEC 616 (1978) Terminal & tapping markings for Power TransformersIEC 214 (1976) On-load tap changersIEC 722 (1982) Guide to the Lighting Impulse & Switching Impulse

Testing of Power Transformers & ReactorsIEC 726 (1982) Dry type Power TransformersIEC 742 (1983) Isolating transformers & Safety isolating transformers

Requirements

IEEE C57.12.00-1993 IEEE Standard General Requirements for LiquidImmersed Distribution, Power & Regulating Transformers

Standards