1. INTRODUCTION

This section briefly describes about the probable causes of failure of major sub-station equipment viz. Power Transformers, Reactors, Circuit Breaker, Instrument Transformers, Surge Arrestors etc. and remedial measures taken to prevent such failures.

2. POWER TRANSFORMERS

Power Transformers are vital links in the chain of components constituting a power system, the failure of which affect the supply of electric power to the consumers. Internationally, the transformers are found to be very reliable but in our country the failure rates are quite high. Failure analysis quotes a host of reasons behind the failure of power transformers. These may include abused operations inept maintenance, substandard techniques adopted during manufacturing, testing and commissioning, substandard input materials, inconsistency environment, design deficiencies, abnormal operating conditions, over voltages, system short circuits etc.

The main causes of failures of transformers in service (CIGRE Survey) is given below:

However the main causes of failure as pertaining to our country are given below:

Table

However, The failures in power transformers can be broadly classified as:

• Weakness in specification, design / manufacturing deficiency
  

• Installation / operation / maintenance deficiency
  
• Adverse operating conditions
  
• Aging

2.1 Weakness in specifications

Many a times, a customer specification is silent on the various aspects of site conditions such as loading pattern, over fluxing, over voltage conditions, various system parameters, environmental conditions. These are some of the aspects where care has to be taken at the time of drafting of technical specifications.

2.2 Failure due to defective design

Some of the failures due to defective design are listed in the table given below.

Cause	Effect	Remedial Measures
Failure of yoke bolt in insulation	Causes local short circuit in the lamination resulting in intense local eddy currents	Insulated yoke bands preferred or yoke bolt insulation should be class ‘B’ insulation or higher.
High flux density in core	Causes large amount of force at time of switching and repeated switching damage winding insulation	Flux density should not exceed 1.9 Tesla at maximum operating voltage
Narrow oil duct in winding	Results in improper cooling and damages insulation	Adequate duct from point of effective cooling
Improper transpositions	Results in more loss and more heating	Adjust the transpositions so that all conductors should have equal reactance
Inadequate clearance between phases	May result in short circuit	Provide adequate clearance as per the voltage class
Clamping ring not properly designed	May fail during short circuit condition	Thickness of clamping ring should be designed such as to withstand short circuit forces
Insufficient bracing of leads	May fail during short circuit condition	Strong supports are required for bracing of leads
Radiators not properly designed	Result in improper cooling causing higher temperature for oil/windings	Proper calculation of radiators is necessary

2.3 Failures due to manufacturing deficiencies

Transformer manufacturing is more a craftsmanship rather than the machine work. The reliability of the transformer depends on the quality of raw materials and the workmanship. There are certain steps to be taken at manufacturing stage so that apparently minus slips-ups at that stage do not get amplified in major defects later on in service. Some of the failures due to manufacturing deficiencies are listed in Table given below.

Cause	Effect	Remedial Measure
Loose winding and improper sizing	Result in interturn or interdisc short circuit	Proper sizing for keeping winding under clamping condition
Burrs on lamination	Result in local short circuit and result in heating	Burr free condition to be ensured by good manufacturing facility
Burrs on spacers and blocks	Result in damaging conductor insulation	Burr free condition to be ensured by good manufacturing facility
Bad brazed joints	Damage the conductor insulation and winding may fail	Adopt good brazing procedures
Metallic parts left over during manufacture	May cause partial discharge	Better house keeping to ensured
Insulation surface contamination	Results in insulation failure	Cleanliness to be ensured
All metal components not earthed	Partial discharge may start and oil quality may get affected	All metal components are to be properly earthed and this is to be added in check-list
Bad and porous welding of transformer tank	Result in oil leakage	Surface cleanliness to be ensured and adopt good welding procedures
Improper drying process	Winding and insulation are not fully stabilized due to moisture leading to failure	Extensive drying and oil impregnation process should be strictly followed as per voltage class

2.4 Failures due to deffective materials

The quality of material used also reflects on the life of the transformers. A rigid control of quality at all the successive stages of manufacture right from raw material to finished product will avoid the failure in transformers. Some of the failures due to defective material is listed in Table given below:

Cause	Effect	Remedial measure
Sharp edges in copper conductors	Produce partial discharge and damage the conductor insulation	The surface finish should be smooth
Improper conductor insulation	Deteriorate under influence of high voltage stress and damage insulation	Check the incoming conductor insulation and also no. of layers for conductor covering
Poor oil quality	Insulation failure	Maintain BDV & PPM as per manufacturer’s recommendations
Particles in oil held in suspension	Temporary breakdown	Maintain oil cleanliness
Bare copper for connection	Formation of oxidation and sludges	Provide enamel coating or paper covering on bare copper
Defective accessories OLTC   Bushings Buchholz relay Protective equipment	Results in transformer failure	These accessories to be procured from well established supplier in view of high service reliability

 2.5 Adverse operating conditions

The life of a transformer is normally dependent on the life of insulation. During the normal operation of the transformer, the ageing process is also at normal rate. The rate of ageing is related to temperature, moisture content and duration of loading conditions. At temperatures more than 140ºC, the gas bubbles are formed as a result of insulation deterioration. These bubbles are of potential danger in the vicinity of high voltage stress zone. This can initiate electrical damage leading to breakdown.

Life expectancy of transformer will get diminished through inadequate protection while operating in the abnormal conditions such as:

1. Sustained overload conditions
   

2. Switching surges
   
3. Lightning surges
   
4. Transferred surges

2.6 Improper maintenance practices

Poor/inadequate maintenance in the areas of oil leakage, oil quality, critical accessories such as tapchangers, bushings, protective instruments etc. will cause trouble in transformer. In addition to this, there are various trouble-shooting problems encountered in the field, such as moisture, oxidation, solid contamination, gas bubbles, overcurrent, overvoltage (transient or dynamic), over temperatures, short circuit (mechanical forces) etc., for which sufficient care should be taken to safeguard the transformer. Preventive maintenance is strongly recommended to improve the reliability of transformer.

Some of the reported failures for transformers have been attributed due to either of the following causes:

1. Failure of the winding insulation due to short circuit stresses
   

2. Failure of winding insulation due to surge voltages and transient surges
   
3. Failure of magnetic circuit
   
4. Failure of OLTC
   
5. Failure of bushings and other accessories
   
6. Failure due to poor insulation and poor cooling arrangements

Some of these failures are briefly described below:

2.7 Failure of HV, LV and Tertiary Winding due to Short Circuit and Surge Voltages

Safety margin of transformer with reference to short circuit withstand capability has been reduced widely. ISS stipulates a time of 2 seconds. The World Bank Specifications recommends a time duration of 1 sec. On account of the graded time discremination provided on the protective relays, which are essentially over current and earth fault relays. Bus faults are cleared after a time delay and many of the transformers have been found failing for bus faults or a nearby fault on feeder converting into a bus fault due to failure of feeder breaker to trip.

The tertiary winding provided on the power transformer are not adequately related to provide insulation to withstand surge voltages as also not rated for adequate short circuit stresses. Tertiary winding inter-turn insulation failures have been found to be due to transferred surges also. Based on investigations of failure of tertiary winding CBIP has already brought out research paper providing guidelines for protections to be provided on loaded tertiary. As per CBIP manual on transformers, provision of tertiary winding has now been deleted upto 100 MVA, 3 phase 3 limbed core type construction. Special precaution for protection of tertiary is necessary particularly in case of capcitive/reactive loading. Frequency of switching on/off of capacitor/reactor, distance of source from the transformer, design and location of gapless arresters are some of the important factors which have to be considered before loading of the tertiary. Failure of tertiary windings generally have been experienced because of:

• Overstressing and inadequate cooling
  

• Improper implementation of protective schemes
  
• Frequent switching ON/OFF of capacitive reactive load
  
• Improper short circuit withstand capability

CBIP’s technical report on causes of failure of tertiary windings and BHEL’s recommendations for protection of tertiary winding (Journal Vol. 3 No.1 of 1978) provides required guidelines on the subject.

The clamping arrangement provided on the transformer to contain the short circuit forces were not found to be adequate. In some cases, the interphase and phase to ground clearance of the leads were found to be less which resulted into flash over and damage of insulation due to vibration and displacement under short circuit current. Damage has taken place in some cases due to failure of insulating components eg., insulating cylinders, supports, permalli wood etc. The failure of joints have also been reported while handling short circuit current. A few cases of failure of transformer on lighting impulse have been reported inspite of protection provided by lightning arrestors.

Failure of the transformers have been reported on switching surges. The transformer failed, when it was being energized after a supply failure form upper substation on tripping of transformer on external faults. Operation of differential and bucholz protection took place tripping the transformer breaker and isolating the transformer. In a few cases, the transformers have failed where Polarisation Index (P1) of winding insulation had deteriorated to 1.1 or less inspite of moisture content in transformer oil remaining within limit upto 35 PPM & BDV 50 KV. Deterioration of P1 Index on sustained temperature on load needs to be specified and examined. In one or two cases, the substation earth resistence was found to be higher. This resulted into high voltages to be impressed at neutral end of the winding during phase to ground short circuit on feeder/bus. This caused failure of interturn insulation at neutral end.

2.8 Failure of magnetic circuits

There have been failure transformer due to overheating of core and core burning, failure of core insulation and core assembly getting used, slipping of stampings and coming in contact with tank bottom. To overcome the above problems separate provision for core earthing and core fixture earthing through bushings provided by transformer is being resorted to. This facilitates monitoring of core leakage current, if any, and in ascertaining that core is not getting multiple earth and also healthiness of core board insulation.

2.9 Failure of on-load tap changers (oltc)

On –load tap changers are the second largest reason for trouble in power transformers after short circuit. The defects in OLTC are of the following type:

1. Burning of transition resistance
   

2. Burning and damage of rollers and fixed contacts
   
3. Misalignment of the tap changer assembly
   
4. Error in time sequence operation
   
5. Defect in tap changing driving gear i.e. mis operation of limit switches and step-by-step contractors etc.

Some of the common problems noticed in the OLTC compartment, selector/debetor switch are:

• It appears proper care for selecting current rating of the OLTC is not exercised by the manufacturer. Factors for efficiency of operation and over loading capability of transformer have to be accounted for to arrive at design current rating. The selected current rating normally should be one step higher than the calculated value. It would be advisable if purchaser’s technical specifications do not leave this option to the manufacturer and current/voltage ratings are specifically stipulated.
   

• Quality and rating of transition resistors have been one of the main source of problem in OLTC. Repeated incidences of burning of transition resistors is an area which calls for serious attention from OLTC manufactures.
  

• Open circuiting or burning of transition resistors leading to selector switch spark over and fire in tap switch resulted into bursting of pressure relief diaphram in MR type tap changer.
  
• Failure of limit switch to stop operation at extreme position of tap changer have led to severe arcing, pressure build-up and bursting of OLTC compartment.
  
• In sealed breathing transformers, defective oil seals and ‘O’ rings have led to transfer of oil under pressure from main takn to diverter switch and leakages through silicagel breather resulting into fall in main tank oil level which is an operational hazard.
  
• Crack in barrier board has also been a cause of failure owing to non-equalization of pressure between main tank and OLTC, while applying vacuum at the time of first erection and drying out.

2.10 Failure of bushings

Condenser type bushings are sent with tip portion sealed and covered by porcelain rain shade. The bottom condenser portion is sent covered with wax coated cotton tape. In some of the transformers these transit tapes were found to have not been removed wqhile hoisting the bushings on the transformer. Over a period of service, the wax melted on contact with hot oil inside the tank and the cotton tape opened out and caused discharge inside the tank. It is better to dispatch bushings from works with bottom portion sealed in oil filled tanks to be removed at site at the time of erection to avoid moisture ingress.

2.11 Suggestions to reduce failures

From the foregoing discussion reasons for transformer failure could be attributed to various causes. Some of the possible corrective steps are enlisted herein, to reduce such casualty.

2.12 Improved design and manufacturing practice

By adopting CAD and better shop floor management, more reliable units could be manufactured to eliminate:

• Poor short circuit withstand capability
  

• Manufacturing defects including cooling system
  
• Problems associated with bushings
  
• OLTC including selector/diverter switch
  
• Tertiary winding failure wherever provided

2.13 Improved testing method

Transformer should be simulated to actual service condition first by sequential testing and then passing necessary current which could result into temperature rise. Thereafter conduction of all high voltage application tests could bring out insulation weakness. Simulated short circuit test if necessary on scaled model and measurement of magnetic balance and magnetization current could reveal abnormality. A chart is included at Annesure 1, depicting testing sequence. Finally oil parameter could be recorded after completion of all the tests and compared with initial values.

2.14 Erection at site

By adopting strict pre-commissioning test and checks possible erection mistakes and omissions could be avoided. This list is enclosed at Annexure 2 and could be based on site experience.

2.15 Problems external to transformer

Load management

By adopting efficient load frequency management systems could be better controlled avoiding damages and transformer failure due to

• Overloading
  

• Over fluxing and over voltage
  
• Hot joints and spark over
  
• Frequent feeder tripping due to reflected faults

After every tripping of transformer whether manually or through protective relay, before recharging the tap switch should be manually operated to bring the same to No.1 position. After loading, the transformer tap could be changed to suit bus voltage requirement.

Failure of switchgear and battery

This could be avoided by periodic testing and using proper duty switch-gear and battery.

2.16 Sub-station layout

Layout

Whenever single phase units are installed it is essential to provide partition valves of adequate heights and strength to prevent collapse. This will minimize chances of fire extending to the other units.

Soak pit and drain pit

Provision may be made for the necessary soak pit and drain pit in the substation layout.

 3. FAILURE OF CIRCUIT BARKERS

A circuit breaker is considered to have failed, when the breaker fails to operates after a command is given or unable to interrupt the arc or withstand a system voltage. CB failures have resulted in blasting of one or more of the following components:

1. interrupting chambers
   

2. pre-insertion resistor chambers
   
3. grading capacitors
   
4. support column including tie/operating rods

Besides the above, the failure of breakers could also be attributed due to following reasons:

• Mechanical failure of operating lever
  

• Shearing off of the locking pin of pull rod
  
• Grading capacitor failure
  
• Embedding of PIR fixed contact assembly into the moving contact housing due to loosening of grub screw.
  
• Dielectric failure inside interrupting chamber due to high moisture in SF₆ gas.
  
• Insulation failure (live to earth) due to accumulation of moisture on tie rod during storage.
  
• Failure of mechanical coupling between tie rod and operating mechanism.
  
• Failure of actuating valves in operating mechanism.
  
• Failure due to foreign particles (eye pieces) inside interrupter.
  
• Failure due to high transient recovery voltage (TRV). In case of circuit breakers switching HVDC filter banks, the normal duty of CB is opening, the resulting TRV can be severely distorted due to presence of filters and can severely depart from the 1 – cos wave shape which may cause high TRV and leading to internal breakdown in grading capacitor or interrupter.
  
• Dielectric failure in Air Blast CBs.
  
• Others.

3.1 Preventive Measures for Avoiding CB Failures

The type of failures that have taken place reveals that there is urgent need to improve manufacturing quality of various components besides, if effective condition monitoring checks are also carried out, failures could be identified at the incipient stage and corrective actions can be taken accordingly. However following areas are suggested for preventive measures.

Design/technical specification modification

The modification required to be done in the technical specifications are as follows:

• ‘Mechanical Close Interlock’ wherever provided should be identical for CB with PIR or without PIR in order to avoid mixing of operating drives during erection and during O&M.
  

• Interconnecting piping between different poles of CBs have been removed and individual pole density monitor has been provided.
  
• SF₆ gas cylinder to be tested for dew point, air content etc. as per IEC-376 and test certificate alone will not be sufficient.
  
• Minimum time for PIR contacts open prior to opening of minimum contacts should be 5 msec.
  
• Routine tests shall include measurement of dynamic contact resistance measurement and tan delta measurement of grading capacitor in order to have base values.
  
• Upto 200 kilometers transmission line length pre insertion resistance need not be provided with the barkers. This is based on study carried by a leading power utility.

Stringent quality checks during manufacturing

To avoid failures of CBs due to manufacturing defects, it is required to introduce a stringent quality checks in the standard manufacturing plans. Following quality checks are suggested:

• All operating levers to be tested for ultrasonic and radiography.
  

• PIR contact gap adjustment during assembly was made as 100% CIP.
  
• Mechanical endurance test for 10000 operations conducted on 400 kV CBs.
  
• Dynamic contact resistance measurement and tan delta measurement of grading capacitor made as part of routine tests.
  
• Tensile test on operating levers made as customer inspection point
  
• Microstructure analysis of operating levers
  
• HV test on operating rod made as 100% customer inspection point (CIP)
  
• Testing SF₆ gas foe dew point measurement before supply
  
• Modification of PIR Pull rod (PTFE) having better tensile strength.
  
• Improvement in machining of contact surface of pilot valves
  
• Nitrogen accumulators
  
• Gaskets: Gaskets can sometime fail to do their job of forming a gas or liquid seal but care must be exercised against excessive or unevenly applied greasing of the gasket. Positioning of the gasket is important.
  
• SF₆ gas tightness: The possible origins and causes are various for SF₆ gas leakage, for example:
  1. Corrosion near a seal can be avoided by controlling moisture content in SF₆ gas.
     
  2. Damage of a seal
     
  3. Impurity under a seal
     
  4. Porosity of metal component (casting, brazing).

Introduction of state-of-the-art condition monitoring checks during service

Following condition assessment techniques which have been discussed in detail in the section dealing with the maintenance techniques may be adopted.

1. Dynamic contact resistance measurement
   

2. Dew point measurement of SF₆ gas.
   
3. Contact travel measurement
   
4. Operating timings
   
5. Tan delta measurement of grading capacitors
   
6. Trip/close coil currents measurement
   
7. SF₆ gas/hydraulic oil/air leakage monitoring.

4. FAILURES OF INSTRUMENT TRANSFORMERS

4.1 Failure of CTs

Preliminary failure analysis of failed CTs have revealed that most of the CTs have failed due to pre-mature ageing of primary insulation. Besides, other probable reasons of failure have been attributed to high system parameters i.e. voltage and frequency, switching over voltages, lighting over voltages. To minimize the failure of CTs following tests/checks are suggested for carrying out at site.

• Measurement of tangent delta and capacitance
  

• Recovery voltage measurement
  
• DGA monitoring
  
• Furan analysis

4.2 Failure of CVTs

Preliminary failure analysis of failed CVTs have revealed that main reasons of CVT failures are:

• High value of tan delta
  

• Secondary voltage abnormal
  
• High value of capacitance
  
• Oil leakage
  
• Humming sound

Besides the above following problems were also observed in one make of CVT:

• Snapping of bellow connection
  

• Burning of damping resistor
  
• Burning/failure of lightning arrestor
  
• Failures of capacitor stacks
  
• Blackening of insulating oil in EMU tank

The only remedial measures suggested to avoid failures is to do the proper condition monitoring checks of CVTs at site.

5. FAILURES OF SURGE ARRESTORS

Analysis of failure of surge arrestors have revealed failures mostly due to premature degradation of ZnO discs. To minimize the failure of surge arrestors it is suggested that surge arrestors are monitored online for presence of third harmonic resistive current in the leakage current flowing through surge arrestors. Leakage current upto 500 micro Amp is generally considered within acceptable limits.

 Annexure 1

Testing Sequence for Power Transformers

S.No	Test	AT Manufactuer’s Work	While Commissioning	During Maintenance
1	Ratio	Yes	Yes	Yes
2	Winding resistance measurement (at all taps)	Yes	Yes	Yes
3	Insulation resistance and Polarisation Index	Yes	Yes	Yes
4	Polarity, vector group	Yes	Yes	---
5	Separate source withstand voltage	Yes	---	---
6	Measurement of No. load losses	Yes	Yes	---
7	Load losses and measurement of impedance (at all taps)	Yes	Yes	---
8	Temperature rise test	Yes	Yes	---
9	Impulse withstand test	Yes	---	---
10	Switching surge withstand test	Yes	---	---
11	Induced voltage withstand and partial discharge measurement	Yes	---	---
12	Measurement of iron losses (after all type tests)	Yes	Yes	---
13	Measurement of insulation resistance and polarization index	Yes	Yes	Yes
14	Measurement of capacitance and tan delta of windings	Yes	Yes	Yes

 Annexure 2

Check List for Transformer Assembly

Maintenance Management Part II Maintenance of Substation Equipment Preventive Maintenance Schedules