ABSTRACT :
In view of high failure rate of Distribution transformers , this paper intends to discuss causes of failure & accidents , some facts on manufacturing & usage, some suggestions for incorporating in IS 1180 Part 1- 2014 and way forward. Reducing the failure rate of distribution transformers is the responsibility of all the stake holders- Manufacturer, Utility, Standardization bodies, Regulator, Consumer and Govt. agencies.
1.0 INTRODUCTION
• Distribution Transformers (DTs) are key assets for any distribution network. Their reliable and efficient operation can result in long-term benefits for the Indian power distribution utilities.
• The total size of the domestic distribution transformer market stood at Rs 64.78 billion in 2017-18
• The market recorded a growth index of 11.3 per cent in 2017-18 over 2016-17.
• With the increase in distributed generation and smart grid, requirement of smart transformers is the need of the hour
• But the DT failure rate in India is very high in the order of 12-15% (in State Utilities), as against global average of 1- 2%.
• Reduction in failure rate of distribution transformers is a big challenge to overcome.
• In this paper we will see,
Causes of failure of Distribution transformers
Causes of accidents in Distribution transformers
Hard Facts on manufacturing and Usage of Distribution transformers
Suggestions for incorporating in IS 1180 part 1 – 2014
Way forward
2.0 CAUSES OF FAILURE OF DISTRIBUTION TRANSORMERS
Causes of failure can be put into 7 categories
2.1 Quality
2.2 Installation & Commissioning
2.3 Protection
2.4 Maintenance/Condition Monitoring
2.5 Lack of Know how/skill
2.6 Theft.
2.7 People/Civic Authorities
2.1 Quality
• The distribution transformer industry is dominated by small and medium enterprise units across the country.
• They mainly supply to the state utilities, as SEB’s follow L1 vendor selection criteria , which has led to proliferation of many small players.
• Small scale industries, compromises for small financial gains by using poor quality of material, poor workmanship, adopting non standard practices resulting in poor quality of transformers.
• Design defect such as- Tank size , clearance between HV/LV leads with
• channel & tank, LV bus bar with channel & tank , coils with core & tank, radial gap between HT & LT and Core & LT etc.
• Such transformers are failing as early as three years, where as operational life of transformer is between 25 – 30 yrs.
2.2 Installation & Commissioning
• In case of distribution transformers, mostly small contractors with unskilled labors are involved in loading, unloading, transportation and installation of transformers.
• It had been also observed that people down the line, of the utility have become vendor dependent and lacks in knowledge and duty consciousness.
• During Installation & Commissioning, neglection in following, leads to problem in successful operation of distribution transformers.
Handling of transformer during loading, unloading, transportation .
Level and BDV of oil in transformer.
Insulation resistance values
Body and Neutral Earthing and its resistance value.
Locking of transformer on plinth/Mounting structure
Condition of breather and silica gel.
Locking of Tap changer at rated tap position
Substation Size/Area,
Fencing, it’s locking & Earthing,
Water logging, Tree , bushes, grass inside/near the substation.
No. of Earth pits, Earth resistance, Integration of all Earth pits
Size , Condition/Quality and Level of Plinth/Mounting structure and it’s Earthing
HT & LT cable support frame and cleat condition/alignment
HT & LT cable /Jumpher socket crimping
HT & LT cable Earthing
LT bus bar, its size/length
Bimetallic strips
Size of nut, bolts, washers
LT side Main ACB/MCCB/MCB rating, Relay settings and tripping
No. of LT feeders and feeders ACB/MCCB/MCB rating, relay settings and trippings
HT side RMU/VCB rating , Relay settings and tripping
GO Switch and its Earthing
DD fuse size
Surge arrestor and its Earthing
2.3 Protection
• Protection plays major role in the life of transformers.
• In case of distribution transformers and substations, LT side protection is still neglected.
• In most of the cases ACB’s, MCCB’s and MCB’s are either bypassed or faulty.
• HT side protection is mostly intact in view of concern that tripping should not go upstream
• But neglecting LT side protection leads to weakening of transformers with every fault travelling through it, reducing its life.
• Transformers less then and upto 200 kva needs more attention even in IS 1180 part 1 – 2014 also.
• Neglection in following, leads to failure or reducing life of transformers;
MOG, Pressure relief valve, Explosion vent pipe and its diaphragm, OTI on transformers
LT side Main & Feeders ACB/MCCB/MCB rating, Relay settings and tripping.
No. of LT feeders
HT side RMU/VCB rating , relay settings and tripping
GO Switch and its Earthing
DD fuse size
Surge arrestor and its Earthing
2.4 Maintenance/Condition Monitoring
• Maintenance /Conditioning monitoring is another very important activity affecting life of transformer.
• Transformer having hygroscopic solid and liquid insulation, rubberized cork gaskets, oil seals etc require monitoring as frequent as possible.
• Now a days Contractors are getting involved in maintenance activities and skill man power with them is an issue.
• It had been also observed that people down the line, of the utility who are responsible for maintenance activities have become vendor dependent and lacks in knowledge and duty consciousness .
• Neglection in following cases, leads to failure or reducing life of transformers;
Oil leakages from bushings, top cover etc.
Condition of oil
Oil level and its BDV.
Oil topping with filtered oil.
Cracked/Flashed bushings
Insulation resistance values
Body and Neutral Earthing and it’s value at all points
Locking of transformer on plinth/Mounting structure
Color of Silica Gel and condition of breather
Locking of Tap changer at rated tap position
Fencing, it’s locking & Earthing.
Water logging, Tree , bushes, grass inside/near the substation.
Condition of Plinth/Mounting structure and it’s Earthing
HT & LT cable support frame and cleat condition/alignment
HT & LT cable /Jumpher socket crimping
HT & LT cable Earthing
LT bus bar, its size/length
Bimetallic strips
Size of nut, bolts, washers
LT side Main ACB/MCCB/MCB Relay settings and tripping
No. of LT feeders and feeders ACB/MCCB/MCB relay settings and trippings
HT side RMU/VCB, Relay settings and tripping
GO Switch and its Earthing
DD fuse size
Surge arrestor and its Earthing
• There are system constraints such as overloading and unbalancing which required timey action.
• There is material management issue such as non availability of material on time which hampers maintenance.
2.5 Lack of Know how/skill
• The man power of Contractors as well as people down the line of utilities, lacks in knowledge and do not have awareness on modern trends and practices.
• Situation is such that the people are not having know how on IR values and its measurements, precautions to be taken in sample collections for BDV tests , which are one of the basic tests.
• They are not confident in replacing bushings, gaskets at sites and many a times because of lack of knowledge even healthy transformers are returned from site as defective.
2.6 Theft
• There are cases of theft of core coil assemblies from rural areas .
• Cases of complete theft of lower rating transformers had also been reported from rural areas .
• Oil theft from plinth mounted transformers in urban areas is also becoming a challenging issue.
• There are cases of petty theft of metal part, terminal connectors, bus bars, breather etc.
2.7 People/Civic Authorities
• There are cases of transformer failure due to overloading because of non availability of alternate space for new addition/upgradation.
• There are cases of Tress passing in S/Stn.’s resulting into accident.
• Residents of multi storey buildings near S/Stn.’s throwing garbage inside it resulting into faults and even fire .
• Unauthorized colonies and load growth due to them, theft of electricity resulting into overloading and unbalancing of distribution transformers.
3.0 CAUSES OF ACCIDENTS IN DISTRIBUTION TRASFORMERS
• In case of distribution transformers, many a times failure results into accidents also, due to following reasons;
3.1 Protection device not operating due to non availability/ setting inappropriate/ by passed, in case of ;
Inter turn Insulation failure due to mechanical damage during manufacture, long term overload and over voltages, deteriorated oil.
Thermal fault caused by overloading of transformer, poor connections at bolted connections with cables or draw rod of bushings, deteriorated oil.
Partial discharges may occur due to incomplete oil impregnation leaving cavities , high humidity in paper, arcing between bad connections, adjacent disks, broken brazing
Eddy current heating in magnetic core due to malfunction of the magnetic circuit. Large negative or zero sequence currents, or circulating currents in the tank and core lead to heating
Pressure builds up inside tank during internal fault.
3.2 Sub Standard tank fabrication, In appropriate Welding and Pressure Testing
Low thickness of tank wall sheet used then specified
Pressure builds up inside tank during internal fault leading to transformer tank or cover being blown out.
3.3 Absence of strict maintenance practices
Oil spillage due to leakage/seepages resulting into fire.
Uncleaned S/Stn.’s with grass , bushes etc.
3.4 Site condition
Trees near S/Stn,’s having branches over transformers.
3.5 People
Tress passing in S/Stn.’s resulting into accident.
Residents of multi storey buildings near S/Stn.’s throwing garbage inside it.
3.6 Civic deptt.
Unauthorized colonies and load growth.
4.0 HARD FACTS ON MANUFACTURING AND USAGE OF DISTRIBUTION TRANSFORMERS
4.1 Few hard facts on distribution transformers manufactured at small scale industries
New transformers found Low IR after delivery at utility stores.
Use of low thickness of tank wall MS Sheet then what is specified.
Quality of welding in fabrication of tank.
Less clearance between core coil assembly and tank.
Length of pipe inside conservator connecting to main tank too long.
Drain & filter valves welded from outside but no hole inside the tank.
Air release socket welded on top cover but no hole inside .
PRV installed but inside there is no hole.
No provision of standard safety device.
Poor quality of HV and LV metal parts.
Poor quality of gaskets, washers, oil seals.
No primer, no surface treatment on tank resulting into rusting at very early stage.
4.2 Few hard facts on distribution transformer in field
Transformer running at low oil level
Transformer running with Low IR, Low BDV and deteriorated oil
Oil leakage from bushings
Breather not Installed/Sold/Theft .
Breather pipe is plugged .
Line man using higher size of fuse wire to avoid tripping.
LT side protection such as ACB, MCCB, MCB by passed, faulty/not Installed/Theft .
HV and LV terminal connectors sold /theft.
Cable connection by twisting the conductor on poor quality metal part.
Earth resistance too high
Integration/Mess Earthing not in practice.
Surge arrestor damaged/not Earthed.
S/Stn.’s not fenced
Uncleaned S/Stn.’s
4.3 Some Pics of distribution transformers in the System
In appropriate handling of D.T.
Space Constraint
Trf. Installed on wooden sleepers
990 kva D.T. without LT main / feeder ACB’s
In appropriate crimping
Oil leakage from New transformer
LV bushing cracked of New transformer
MCB burnt
Leads connected to cable through wire
Lead connected to cable without switch gear
HV busing damage
PVC tap on HV terminal
Transformer Tank Bursting
Transformer Tank Bursting
5.0 SUGGESTIONS FOR INCORPORATING IN IS 1180 PART 1 – 2014
DMCR relay for rating 1 Mva as part of standard accessory instead of optional.
MOG/Oil level sensor for rating 400 kva and above.
VCB on 11 kv side and ACB on 433 v side mandatory for transformers 400 kva and above.
Making MCCB/ MCB on 433 v side mandatory for transformers less then 400 kva as per rating of trf.
Pressure relief device or explosion vent upto 200 kva , as part of standard accessory instead of optional.
Review of standard pressure required to test distribution transformer tank.
Thickness of tank sheet should be standardized.
Minimum Gap between core coil assembly and tank wall should be standardized.
6.0 WAY FORWARD
Certification of small scale transformer manufacturing units by competent authority for Standard manufacturing practices inorder to have quality product.
Utilities to ensure that vendor selection L1 criteria should be clubbed with procurement of quality transformers from standard manufacturers only.
Availability of sufficient funds with utility for operation, maintenance and timely upgradation of distribution network .
Manufacturing and procurement of transformers with new technologies such as , (a) High temperature material, (b) Hybrid Insulation system, (c) Esters, (d) Amorphous core and (e) Dry type.
Adoption of smart technologies for real time monitoring of (a) Oil/Wdg. temperature, (b) Oil level, (c)Loading & (d) Voltage of transformer along with consumer end voltage, current and energy.
To begin with, off line monitoring of all the important parameters and taking prompt action can also be very useful.
Condition monitoring and Maintenance of distribution transformers and substation, should be round the year basis instead of annual preventive maintenance.
Along with maintenance of transformers, concept of servicing of distribution transformers should also be adopted by utilities.
Utilities to ensure not to compromise with LT side protection of distribution transformers.
Failure analysis of each and every transformers at site and repair workshops.
Double decker substations in cities to meet challenge of space crunch.
Certification Training of contractor and utility people on, transformer maintenance, servicing and repairing, along with Motivational and behavioral sessions also, for development of skill and in house competency . Power Sector Skill Council is going to launch the program soon.
Author Profile: Neeraj Khare
B.E.(Electrical), PGDM, FIE, CE, FIV
Managing Director – Adishaktyai India
Member : CIGRE, IEEE, CBIP, SESI, QCFI
Fellow : IEI , IOV
Association : NPTI , PSSC
