Title Of The Study:

Title of the study:

Site address:

Period of study: Rating:

Reduction in Distribution Transformer (DTR) failure rate at Doddaballapur Sub Division Doddaballapur Sub Division, Bangalore Electricity Supply Company Limited June 2005 to June 09 ****

Contents

Page Nos

1. Introduction

2

2. Analysis of reasons for failure of DTRs

6

3. Implementation

9

4. Results

10

5. Cost Benefit analysis

14

6. Conclusions

15

7. Guide lines for repeatability in other distribution areas

16

a. Benefits b. Challenges c. Key Pitfalls/Precautions

1

1. Introduction Distribution Transformer is costly and critical equipment in electricity distribution network. Its outage due to failure causes immense inconvenience in network management and involves high expenditure on account of repair/replacement. Any Distribution Utility, therefore, takes all possible actions to reduce downtime / failure of transformers to a minimum and to enlarge their lives, at the most economic cost. BESCOM has about 1.25 lakh distribution transformers of different capacities and the DTRs failure rate over the years has come down from 13% in FY 04 to less than 9% in FY 09. On an average BESCOM spends Rs. 14-15 crs. annually on repairs to distribution transformers alone, constituting about 40-45% of the R&M expenses of the Company.

Distribution Transformer is a static Electrical device which steps down 11KV (Primary) voltage to 440 Volts between Phases & 230 Volts between Phase & Neutral (secondary) through Delta–Star Windings by electromagnetic induction, which feeds different types of loads such as Domestic, Commercial, Agriculture, Industrial etc. Hence Distribution Transformers form the essential link between power utility & large number of consumers. This can also be called as Distribution Transformer sub-station or Distribution Transformer centre. Studies have shown that the salient parts of Distribution Transformers which are likely to affect damage & failures when exposed to abnormal conditions are :-

•

Laminated iron core for magnetic circuit.

•

Primary & Secondary windings for electric circuit.

•

Main

tank

to

house

the

core

&

windings

with

cooling

arrangement, conservator tank, Breather & Explosion vent.

2

•

Transformer oil in the tank acts as an insulation and cooling media.

•

HV & LV Bushings to bring out the terminal connections of the windings.

The protective devices provided to the Distribution Transformer substation are:Table -1 Sl.No.

Protective Device

Protection given

1 2

Against over current on Primary side Against over current on Secondary side.

5

Horn-gap (H-G) fuse Open type fuses/ LT protection kit/ Distribution box 11KV Lightning Arresters Explosion vent with a thin diaphragm Breather with slicagel

6

Earthing system

3 4

Against lightning strokes or surges on Primary side protect the main tank from bursting by itself acting as a relief valve Absorbs moisture in the Transformer Transformer neutral earthing provides a path for the earth fault current

Reasons for the failure of Distribution Transformers are due to :•

Ageing

•

Manufacturing defects

•

Utility side defects

•

Natural calamities

In order to improve the performance efficiency, Utilities throughout the world are now forced to drastically reduce their expenditure and as a consequence of this, budget for fresh investment is often reduced to a bare minimum. It, therefore, becomes imperative to extract as much life as possible from the existing capital-intensive equipments. Transformers being one of the costliest equipment in a Distribution network, it is a common practice, amongst the Utilities, to maximize their service lives 3

by

taking

timely

precautionary

measures

and

adopting

good

maintenance practices. Hence in the pilot site at Doddaballapur, Distribution Transformers failure rate has been considered as one of the parameters of KPIs (Key Performance Indicator) and is aimed to be brought to Zero.

In Doddaballapura Sub-division, 3245 Distribution Transformers are existing as at the end of June-2009. The number of Distribution Transformers failed & percentage of failure from 2004-05 to June-2009 is shown in the Table below. Table -2 Sl.No. 1 2 3 4 5 6

Year

Transformers Existing 1615 1746 2017 2329 3194 3245

2004-2005 2005-2006 2006-2007 2007-2008 2008-2009 Apl./09-Jun/09

Trs. failed

% of failure 16.78 9.85 7.19 4.94 2.91 0.74

271 172 145 115 93 24

Annualized % 17.46 10.24 7.71 5.29 3.37 2.91

Month wise % failure of DTRs at Doddaballapur sub-division is shown in the following graph, which shows a gradual decline in the failure %

DT Failure Rate at BESCOM's Pilot Site % Age

Target

Linear (% Age)

16.0 14.0 14.0

13.6 12.6

12.0

Trend 11.8

8.0 7.3

8.3

8.5 7.4

7.0

7.0 6.8

9.5

9.2

8.7

7.9

8.2 6.8

5.4

7.0 5.6 5.0

4.0

4.9

5.0 4.5

4.7

3.9 2.0

4.3

4.0 3.9

3.2

4.3 3.4

3.0

3.4 3.0 3.2

3.7 3.7 3.3

3.4

2.9 2.7

3.5 2.4 1.8

0.0

Ju A u l' 05 g S e ' 05 p Oc ' 05 N o t '0 5 v D e ' 05 c Ja ' 05 n '0 Fe 6 b M '06 ar A p '06 M r' 0 6 ay J u '0 6 n' Ju 06 A u l' 06 g S e ' 06 p Oc ' 06 N o t '0 6 D e v' 06 c Ja ' 06 n F e '0 7 b M '07 ar A p '07 M r' 0 7 ay J u '0 7 n Ju '0 7 l A u y' 07 S e g' 07 pt O c '0 7 N o t '0 7 D e v' 07 c Ja ' 07 n F e '0 8 b M '08 a A p r '08 r M il '0 8 ay J u '0 8 n' Ju 08 A u l' 08 S e g' 08 pt O c '0 8 N o t '0 8 D e v' 08 c Ja ' 08 n F e '0 9 b M '09 ar A p '09 M r' 0 9 a J u y '0 9 ne J u '0 9 l- 0 9

DT % F ailu re

9.7

8.2

6.0

11.4

9.3

10.0

Month

4

Details of age wise failure of Distribution Transformers are shown in the Table below.

Table -3 <1Yr

Sl.no. Year

1-2Yr

2-5Yr

5-10Yr 10-15Yr 15-20Yr 20-25Yr 25-30Yr >30Yr

Total

1

04-05

85

95

44

40

2

1

2

1

1

271

2

05-06

36

61

45

20

6

2

1

1

0

172

3

06-07

31

42

36

23

7

4

1

1

0

145

4

07-08

8

36

40

24

5

1

0

1

0

115

5

08-09

19

34

19

16

4

0

0

1

0

93

2

8

6

7

0

1

0

0

0

24

6

4/09 -6/09

No.s of DTRs failed

Age wise analysis of DTRs failure at DRUM Project site

100 75 50 25 0

< 1 year

2004-05

1 to 2 Yrs

2005-06

2 to 5 Yrs

2006-07

5 to 10Yrs

2007-08

10 to 15 Yrs

2008-09

> 15 years

04/09 to 06/09

Analysis shows that out of the DTR’s failed in each year more than 90% of the DTR’s have failed with in the first 10 years of their commission and more than 50% in the first two year of their commission. For the purpose of providing depreciation in books of accounts the average life of a DTRs is considered as 25-35 years, where as most of the transformers are failing before 10 years of their commissioning, thereby putting severe strain on the cash flows of the company. This could be

5

due to the reason that earlier the DTR manufacturers were allowing more factor of safety in design, where as in recent years manufacturers are adopting cost-benefit ratio in the design by just satisfying the requirements of IS specification. Hence many transformers are not serving for the expected full life period. It is learnt that in the pilot site still there are a few transformers (copper wound) that are in service for over 40 years, thus justifying the argument that old DTRs are more sturdy and durable.

2. Analysis of reasons for failure of DTRs As per the analysis carried out, the main reasons for the Transformers failure are: 1. Over loading:- Most of the Transformers have failed due to over loading ie., adding more loads or due to unauthorized connection of loads. The overloading can be avoided by proactively identifying the transformer loading by taking readings periodically during peak hours using Tong Tester.

2. LT line loose span:- This is also the main reason for failure of large number of Transformers. Because of loose spans (more sag) there are always shorting of LT lines & also conductor snapping since proper size of fuses are not used. This problem can be overcome by adopting proper maintenance techniques and practices as part of maintenance plan.

3. Usage of improper size of fuse wire:reliable

protection

for

Distribution

H-G fuse is the only

Transformers

under

fault

condition in LT distribution. Almost all Transformers are provided with heavy size fuses (Some times ACSR conductor strands are used) in the LT side, the chances of blowing of LT fuses in short time is less. If higher size H-G fuse is used, the fault will sustain for

6

large period until heavy size fuse blows causing failure of Distribution Transformers. Good engineering practices always yield good results. Awareness, availability of proper material

and

responsiveness and accountability are key drivers to address this problem.

4. Tree branches touching LT lines:-

Tree branches touching LT

lines continuously will result snapping of conductor causes heavy earth fault current which leads to failure of Transformers. Timely tree pruning mechanism followed based on the results of periodic field survey and line patrolling report.

5. Improper maintenance of Breathers:-

If it is not maintained

properly moistured air will enter in to the Transformer tank causes failure of Transformers.

6. Oil leakage:-Oil leakage is mainly due to excessive heat or pressure developed in the bushings, this will bring down the oil level in the tank & also moisture enter in to the tank through the oil oozing gap causing contamination of transformer oil resulting in deterioration of HV/LV insulation (low IR value) ultimately

to

Transformers failure.

7. Non topping of oil:- There will be leakage of oil from the transformers due to heating of the oil especially during peak load conditions and results in decrease in oil level over a period of time, which is not being watched and set right regularly by topping up the level.

8. Improper earthing:- Condition of the earthing electrode and the loose connections of the earthing conductor are not being checked regularly. As a result, due to improper earthing the flow of fault 7

current is obstructed affecting windings, which causes failure of DTRs. Besides, improper earthing will result in electrical accidents due to development of high voltages in the earthing conductor which is in open condition and accessible to passers by.

9. Not connecting the Lightning Arresters:-

Because of not

properly fixing & not adequately earthing the Lightning Arresters , they fails to divert the direct lightning stroke or surges due to discontinuity in earthing system results either failure of HV winding due to surge voltage or bursting of importance

of

earthing

becomes

Lightning Arrester itself. The imperative

under

such

circumstances and moreover for safety aspects perspective too.

10. Using Two Phase supply in Rural areas:- During roastering period IP set consumers have invented so many methods to run the motor under Three phase conditions contributing unbalanced overload in Two phases. This will also cause failure of Transformer.

11. Un-standard methods adopted:-

a) Use of Aluminum ACSR conductor either bare or enclosed in PVC pipes from the Transformer bushing instead of PVC insulated cables.

b) Use of open type fuse in the LT side of the Transformers instead of standard porcelain cutouts or LT protection kits.

c) Use of Aluminum ACSR conductor strands as fuses in LT side & HG fuses instead of Tinned copper fuse wire.

8

3. Implementation To reduce the failure rate of Distribution Transformers, following implementation action was considered.

a) In the DPR approved for DRUM works at pilot site, more focus was given towards refurbishment of DTCs, so that all the inadequacies are addressed. The activities include •

Rectification of oil leakages

•

Replacement of damaged bushings/ busing rods

•

Proper alignment of the GOS

•

Providing DOLO/HG fuse units on the HT side of the DTC

•

Replacement of breathers

•

Reconditioning

of

LT

connections

by

replacing

deteriorated LT PVC wires and providing proper lugs •

Improvement of earthing system wherever the earth resistance is more than 10 Ohms

•

Renewal of HT jumps

•

Providing LT protection kit/distribution box

•

Providing surge arrestors

b) Refurbishment works on Distribution Transformers was entrusted to the outside agencies on RC rates & Turn key basis during the year 2007. In the beginning out of 55 feeders 24 feeders were selected for Refurbishment works on Distribution Transformers. Till now the works for only eight feeders are completed and the the number of DTRs covered is shown in the table below, for which the cost incurred is Rs. 0.52 Crs.

9

Table -4 Sl.no.

1 2 3 4 5 6 7 8

Feeder No. & name

No. of DTRs R&M works carried out

DBF-1(B) Nicholus DBF-2 Doddabelavangala DBF-5 Hulikunte DBF-6 S.G.Hally KLF-8 Kolala TBF-8 G.Hosahally SDLF-4 Soladevanahally KF-8 Semi Urban TOTAL

82 52 59 70 23 68 03 8 365

For the remaining feeders i.e. 47 (55 numbers-8 already completed) tenders have been invited & it is under process.

c)

Trimming of tree branches touching the LT and HT lines on a regular basis and tightening of loose spans were taken up as per the maintenance schedule drawn up

d) Tong tester readings are being taken regularly and load balancing works carried out. Weekly targets were fixed O&M unit wise for each of the section officers and monitored closely at the level of Superintending Engineer.

e) Requirement of additional transformers to release the load on the existing overloaded transformers was reviewed regularly and the approval for providing additional DTCs was communicated from time to time along with the necessary budgetary support.

4. Result After

carrying

out

the

Refurbishment

works

on

Distribution

Transformers, Load balancing, providing the additional Distribution Transformers to avoid over loading based on the Tong Tester readings

10

taken during peak hours & after providing 665 Nos. 25KVA Distribution Transformers on 4 Rural 11KV feeders under HVDS, the failure of Distribution Transformers has drastically reduced, from 17.46% in 2004-05 to 3.37% in 2008-09, as could be seen from Table -2 above.

The following graph shows how the year on year reduction has been achieved in the project area bettering the reduction achieved by the Company as a whole.

Photo of a 63 KVA DTC before refurbishment

Photo of a 63 KVA DTC after refurbishment

Com parative analysis of DTRs failure rate in BESCOM vis-a-vis DRUM Project site 20.00%

% failure

16.78%

12.76% 9.39%

8.45%

8.72%

13.27% 9.85% 7.19%

4.94%

0.00%

2004-05

2005-06

BESCOM

2006-07

2007-08

2.91% 2008-08

Doddaballapur sub-dvn.

11

The failure rate can be further reduced if the remaining Refurbishment works are carried out on all the Distribution Transformers & proper action as mentioned below are taken.

1. DOLO / H-G fuse units are to be provided compulsorily with proper size Tinned copper fuse wire depending upon the capacity of Distribution Transformers as H-G fuse is the only main reliable protection for the faults in LT side.

2. Lightning Arresters are to be provided & they have to be replaced immediately if found flashed over.

3. Transformer oil to be topped up if it is found less in the conservator tank & oil leakage to be arrested.

4. Earthing (Grounding) should be kept in good condition and Guy wire to be used with lugs on both ends for neutral earthing.

5. Drawing LT line beneath the 11KV line on the same support should be avoided.

6. Overloading of Transformers should be avoided (Transferring the load to the nearby Transformer if it is Under loaded or new/additional Transformer to be proposed)

7. Tree branches which are touching LT lines are to be trimmed in routine manner

8. Adequate

protection

Transformer

by

to

be

providing

provided

either

LT

on

the

protection

LT kit

side

of

or

LT

Distribution box with MCB switch facility 12

9. Sag on LT line to be avoided by providing intermediate poles, restringing of loose spans and also providing LT spacers where ever it is required

10. Tong Tester readings during peak hours are to be taken to know the load condition of Transformer & load to be balanced on all the phases by redistributing among phases to avoid over loading in one phase

11. Additional Transformers are to be installed where ever the existing Transformers are found over loaded

12. First span of LT line on each circuit of the Transformer should be made small

13. First 10 spans of LT lines on each circuit should be provided with Rabbit ACSR conductor to avoid snapping of conductor

14. IP

set consumers are to be advised not to run the motors by

providing condensers during rostering

15. Bimetallic

clamps with proper size GI bolts nuts & Washers are

to be provided (connected) to the LT bushings to reduce excessive heat & damage to bushing rods due to loose contacts.

13

5. Cost Benefit Analysis Investments made in the project area towards achieving reduction transformer failure rate include the following:-

Table -5 Sl No.

Area of Investment

1

Additional Transformer cost: Providing additional DTRs for releasing the load on overloaded DTRs (Assumed to be about 30 DTCs per annum sanctioned under Extension and Improvement Head at an approximate cost of Rs. 1.0 lakhs/ DTC) Refurbishment and renovation cost: Expenditure incurred towards DTR refurbishment works (based on sectioned estimates) Total Direct Investment Made

Amount (Rs in crs)

(a) Direct Expenses

2

0.30

0.52

0.82

(b) Indirect Expenses 1

1

DTR maintenance, Peak load monitoring and load balancing cost: Taking tong tester readings and load balancing for overloaded DTRs, Topping up of oil, Replacement of burnt out PVC cable etc. (Assumed at Rs 25,000/employee/ month for six months for fifteen persons at 25% of the working time in a day) Total of Indirect expenses made

Grand Total of (a+b) Cost benefit analysis Savings in dismantling and erection cost

0.06

0.06

0.88 0.02

of DTRs including transportation at the rate of 3.5% per annum i.e. about 75 DTRs at an approximate cost of Rs 3,000/DTR

2

(a) Savings in DTR repair cost considering 40% of the failed DTRs are repairable i.e.

0.05

30 DTRs (40% of 75) at an approximate cost of Rs 15,000/DTR

(b) Savings in purchase of new DTRs 60% of the failed DTRs are not repairable i.e. 45

0.27

DTRs (60% of 75) at an approximate cost of Rs 60,000/DTR (net of salvage value)

Total Benefit Payback Period ( in years)

0.32 2.75 years

14

6. Conclusions At the field it shall be the endeavour of every technical and maintenance staff to focus on three key areas of distribution transformers maintenance i.e.

(a)

To find out the load on each DTR through tong tester readings and

taking

action

to

provide

additional

DTCs

if

found

overloaded or transferring some of the loads to the nearby transformer if it is feasible

(b)

Based on tong tester readings to take action for load balancing by redistribution of loads among the phases

(c)

Carrying out of regular maintenance works as per the schedule prescribed

At the management level in the Corporate office, reasons for distribution transformer failures make wise, age-wise, capacity-wise needs to be analyzed more scientifically so that transformers with higher degree of safety factor are procured.

Currently the transformer load details and LT net work details are manually entered in the “Transformers Maintenance Register” kept at the O&M unit offices. Accuracy of the information maintained lacks authenticity and does not provide full information about the live status of the network, DTRs and meters. This can be over come by implementation of GIS mapping of the net work in the pilot area. This enables capturing of accurate data of HT/LT network and easy updation regularly. Obtaining of metering data can also be built in as an additional feature. Transformer Management system (TMS) can also

15

be introduced to track the asset details and to plan the load management at the distribution level.

To enable the technical and maintenance staff to carry out minor repair works of DTRs, BESCOM should consider imparting proper training. This will help the field staff to repair the transformer at the site itself which would in turn avoid unnecessary expenditure on transportation of DTRs to the repair centre. This will also significantly reduce the interruptions to consumers. BESCOM should consider providing necessary spare parts viz., HV/LV bushing shells, HV/LV bushing rods, Bimetallic clamps/ copper plates for connecting to the LV bushing rods, PG clamps, Slicagel, transformer oil for topping up, earthing materials for neutral earthing, lightning arrestors, standard size fuses etc., to the section officers for routine maintenance and emergency use.

7. Guide lines for repeatability in other distribution areas a) Benefits By better management of fixed assets in general and DTRs in particular there will be huge cost savings to the utilities besides helping in improved quality of supply and service to consumers.

b) Challenges Following are some of the challenges experienced at project site:•

Resistance from consumers/general public from passing through their land/street/building for drawing the 11 KV lines to install additional DTRs.

•

Selection of suitable location to install additional DTRs especially in the town area as there is no right of way since the streets are very narrow 16

•

As

the

work

of

providing

of

additional

DTRs

and

refurbishment works are being done through contract agency, there will be more interruptions to consumers (in addition to the scheduled outage), that too during day time, at the time of execution of work. Consumer generally resist for such interruptions.

c) Key pitfalls/precautions Though maintenance of DTRs is a very important work as for as the O&M units are concerned, due to other priorities and shortage of maintenance staff, routine maintenance of DTRs are not being done as per the schedule prescribed. As a result, DTRs get overloaded, unbalanced, reduction of oil level due to leakage, entry of moisture through oil oozing gap, formation of heat in the LT busing rods etc., If these aspects are strictly ensured, both the utility and the consumers would gain significantly in terms of cost savings and quality of supply and service.

17