Distribution Transformer Failures and

Counter Measures

Dr. Katsutoshi Toda Executive Chairman,

Toshiba Transmission & Distribution Systems (India) Pvt. Ltd.

Performance, Customer Satisfaction & Profitability of any

Distribution Network completely depends on functional

performance of Distribution Transformer

Role of Distribution Transformer In Network

o A simple Internal (or) External fault which obstructs functioning of transformer in

network.

o Transformer failure causes black-out of power to the whole area.

o It can also lead to risk of fire and explosions, as the transformers contains

substantial quantity of oil.

o Though it looks simple, transformer failure will lead to chain of adverse effects

on:

End User

Distribution Utility Company

--------------------------------------------------------------------------------------------------------------------------What is Distribution Transformer Failure

Types & Sources of Distribution Transformer Failure

Improper

installation

Improper

terminations

Faulty earth

connections

Bypassing of

Protection

systems

Inadequate

maintenance

Prolonged Over

loading &

Unbalancing

Faulty design

Quality of Raw

Materials

Poor workmanship

Improper Mfg.

Process

Improper

transportation

Chances for Under-

rated Transformers

Improper

Technical Specifications

Improper

Inspection

Process

Source -2

Network OperatorSource -3

Manufacturer

Source -4

Procurement Stage

Unauthorized

Tapping

Vandalism

Oil Leakages

Faults on LV

System

Lightening

Strikes

External short-

circuit

Source -1

External Conditions

Few Examples indicating Improper Installation and Operating

Conditions

Unauthorized Tapping (Power Theft)--------------------------------------------------------------------------------------------------------------------------

Improper Termination--------------------------------------------------------------------------------------------------------------------------

Improper Earthing--------------------------------------------------------------------------------------------------------------------------

Bypassing of Protection SystemsLow voltage Terminal Connection provided directly bypassing MCCB

Failure due to Overloading--------------------------------------------------------------------------------------------------------------------------

Arcing Horns - Improper Assembly/Not used--------------------------------------------------------------------------------------------------------------------------

Failures due to Low Oil Level resulted from Oil Leakage--------------------------------------------------------------------------------------------------------------------------

0.00%

5.00%

10.00%

15.00%

20.00%

25.00%

30.00%

35.00%

Ligh

tnin

g St

rike

s

Van

dal

ism

Fau

lt o

n L

V S

yste

m

Bro

ken

Bu

shin

gs

Bu

rnt/

Bro

ken

Ro

ds

Oil

Leak

age

Acc

ide

nta

l Dam

age

Ove

rlo

ad

Faile

d o

n C

om

mis

sio

nin

g

Inte

rnal

De

fect

s

13.88%10.59%

32.19%

3.55%

9.52%11.33%

0.93%

5.42%

1.51%

11.08%

Dist. Trfr Failure Statistics in Kenya(Aug-2015 to Dec-2016)

--------------------------------------------------------------------------------------------------------------------------

Kenya Power is the main distributor of electricity in Kenya.

It has Installed around 59000 Nos distribution transformers in network

The approx. failure rate is 7-8%

(Sh

ort

Cir

cuit

)

Source: M/s. KPLC, Kenya.

What Next?

Can we really control these failures?

Counter Measures to overcome Transformer failures

Source: External Conditions

Pro

ble

m

Training on

Installation,

Operation &

Maintenance

All Technical

measures to be

incorporated in

Specifications.

Way ForwardSolution

Unauthorized

Tapping

Vandalism

Oil Leakages

Faults on LV `System

Lightening Strikes

External short-circuit

Proper Maintenance

and Checks to be

done.

Transformer should

be manufactured with

Anti-Theft

arrangements.

Aluminium Winding

may be one of the

Solutions to prevent

Vandalism

Sources of Failures & Counter Measures

Source: Network Operator

Know the Right

Practices of

Installation and

Maintenance

Improve Skill Levels

of Operation and

Maintenance Staff.

Pro

ble

m

Training for the field Engineers

and Staff.

Way ForwardSolution

Improper installation

Improper terminations

Faulty earth

connections

Bypassing of

Protection systems

Inadequate

maintenance

Prolonged Over

loading & Unbalancing

Someof the counter

measures

Sources of Failures & Counter Measures

Example Measure-1: LV Termination through MCCB for Overload Protection

Don’t

bypass

MCCB

Cables connected to transformer

LT cables towards Customer end

LT cables connected to MCCB through rubber grommet

LT cable passing through MCCB (bottom view)

LT terminals should connect through MCCB only

()

()

Do’s:

Always connect LT terminals through MCCB to

protect transformer from overloading & external

faults on LT side.

Don’ts:

Do not bypass MCCB to avoid transformer

tripping in fault conditions

()

()

Do’s Don'ts

Example Measure-2: Arcing Horns Assembling Procedures

85mm

Do’s : Ensure the assembling of top and bottom arcing horns alignment with 85mm gap for 11KV.

Don’ts: Never assemble arcing horns in misaligned or missing condition as shown in above

photos.

© 2015 Toshiba Corporation

()

Connect HT lead and terminal stem through Fuse link

Don’t Bypass Fuse link

Don’t Fix HT lead directly to terminal stem without fuse link

Do’s Don'ts

Example Measure-3: HT Fuse link connecting procedures

Do’s:Always connect HT phase leads through fuse links to avoid transformer failures due to overload or fault

currents.

Don’ts : Never Bypass or connect fuse links directly as shown in above figures.

Source:Manufacturer

Pro

ble

m

Technical Evaluation

of Manufacturing

facilities & processes

by a Competent Team

Proto Testing in 3rd

Party independent

laboratory.

Adopt Stringent FAT

Conduct Post Landing

Inspection

Way ForwardSolution

Faulty design

Quality of Raw

Materials

Poor

workmanship

Improper Mfg.

Process

Improper

transportation

Chances for

Under-rated

Transformers

Select Reliable

Manufacturer

Ensure, transformers

are manufactured as

per the Specifications.

Sources of Failures & Counter Measures

Mini Test Set-up (Upto 500kVA)(Tests: No Load & Load Loss, Impedance Voltage, Winding Resistance and Oil BDV)

(This is an example test setup for explanatory purposes only)

CONCLUSIONS

In Procurement Stage, all the Counter measures

should be taken in to Consideration

o Vendor Pre-Qualification Processes

o Product Qualification Processes

o Factory Acceptance Test

o Post Landing Inspection

o Training for field Engineers and Staff

Thank you !