Differences between OVDT

and CRT Technologies

2

What is the difference in the manufacturing process ?

Cast coilCast coil OVDTOVDT

Preheat resinPreheat resin

Weigh resinWeigh resin

Mix resinMix resin

pouringpouring

Assemble moldAssemble mold

curingcuring

Disassemble Disassemble

moldmold

--

--

Impregnation(VPI)Impregnation(VPI)

dryingdrying

--

--

3

What is the difference in the manufacturing process ?

As compared to cast coil transformers, OVDT have simpler manufacturing process as,

1. there is no process requirements for moulding, pre-heating and mixing of resins.

2. Pouring of resins is critical activity in cast coil manufacturing – if not done carefully,

air bubbles can be trapped in the casted coils, which later lead to partial discharge.

3. there is need for maintaining moulds for all the various types of ratings. This means

that there is less flexibility for making non- regular ratings and designs

4

How is heat dissipation different in cast coil and OVDT Technologies ?

In OVDT transformers, air is the dielectric medium.

In cast resin transformers resins are the dielectric medium.

So, this means that

In OVDT, the heat generated in the coils is passed on to external

environment in a single stage process.

conductor – insulation – resin – air

Insulation is Nomex® paper (0.3mm) + resin (0.05mm)

In CRT, the heat generated in the coils is passed on in two stage

process

conductor – insulation – resins – air

Insulation is normally glass fiber (0.5 mm) + resin capsule (15- 20 mm)

So, heat dissipation in cast coil transformers depends on the

ability of resins to transfer effectively from conductors to

external environment.

In OVDT the heat is directly dissipated from conductors

insulation to external environment. There is no medium in

between, Also, OVDT using Nomex® can dissipate heat very

effectively as thermal conductivity of Nomex® paper is quite

good at temperature of 100 Deg C.

The performance needs to be sustained over long period.

5

How are Overloading capabilities different between CRT and OVDT Technologies ?

Based on IS/ IEC standard, the average winding rise is 115C and max. hottest temperature is (115 + ambient

(50) + hot-spot(15)) = 180 Dec C

In this case there is temperature margin which is 40C differential between thermal capability of

Nomex(220C) and max. hottest temperature(180C). This contributes to higher overloading capability.

For certain cases, one can put dual ratings on the name plate for flexible operation either with lower

load loss or higher capacity.

Steady State Loading on the Basis of test temperature rise

(Quoted from ANSI C59.96-IEEE Guide for Loading Dry type distribution and power transformer

Rated average winding

temperature rise

For each degree C below the

rated temperature rise,

increase rated load by

80C

115C

150C

0.57%

0.43%

0.35%

If a 1000KVA is 100C average winding temperature rise, the additional rating for over loading can be

calculated. (115C - 100C) x 0.43% + (150C - 115C) x 0.35% = 18.7%

===> 1000KVA x 1.187 = 1187KVA

* Note: In general, ReliatraN® provides about 30% additional capability for

overloading based on practical design.

6

How are Overloading capabilities different between CRT and OVDT Technologies ?

Cast Resin Transformers can also follow the same principle but,

The additional heat generated during overloads needs to be dissipated faster.

As heat dissipation of OVDT is better as compared to CRT, ability of OVDT to handle overloads is

better.

Also, if thermal co-efficient of expansion of windings and resins do not match, then there is

possibility of cracks on the cast coils.

7

How are final sizes different between CRT and OVDT Technologies ?

Size of the transformer depends on the temperature index of the transformers.

Normally OVDT transformers are rated as Class H (average winding temperature rise of 115 Dec C)

and cast resin transformers are rate das class F (average winding temperature rise of 90 Deg C)

Thermal Size Difference

Class Difference (in %)

Class B (130) 130%

Class F (155) 115%

Class H (180) 100%

For the same temperature index, there is very small difference in size of OVDT and CRT

transformers.

However, class H cast resin transformers normally need silica fillers in the resins, which

make them expensive as compared to OVDT.

8

How are maintenance practices differ for OVDT and CRT technologies ?

Both the transformers are practically maintenance free.

Both technologies require basic minimum operations like blowers etc for regular maintenance

OVDT has no problem for working with deposited dust etc.

9

Impulse strength – Cast Resin and OVDT technologies ?

Impulse strength is function of design, especially winding design.

For the same winding design, OVDT technology would have lesser damage to insulation in case

of impulse.

Impulse causes electrical stresses, at times leads to partial discharge.

With OVDT technology, as air is used as dielectric medium, the issue of partial discharge is

taken care at design level.

In CRT technology, as resins are uses, PD is not a design parameter. If air bubbles are trapped

in the coils, then there can be a PD activity during impulse and that can damage insulation.

10

What is short circuit strength ?

Ability to withstand mechanical forces generated in case of

short circuit.

So, if we say that a transformer has 3 sec short circuit

strength, that means that the transformer can withstand

the mechanical forces for 3 seconds.

Also, short circuit strength cannot be looked in isolation.

A 5 sec short circuit strength for a transformer does not

make sense if system capability is 3 sec.

So, short circuit strength is again a design parameter.

With OVDT technology, we can use disc type design for

windings and use combs as separators. These combs

give superior mechanical strength to windings and also

distribute the stresses across the windings.

VPI process with varnish enhances mechanical strength

of coils to prevent failure by short circuit accident.

CRT technology normally would not uses mechanical

supports for windings.

Short Circuit Strength – Cast Resin and OVDT technologies ?

11

What does partial discharge mean to dry type

transformer ?

OVDT – use air as dielectric medium . PD is taken care

of at design level/

CRT – use resin as dielectric medium. PD is function of

manufacturing process. Its not taken into consideration

at design level.

Cast coil transformers assume the epoxy

resin fill the gaps and therefore design for

epoxy dielectric, assuming no air –

If any air bubbles appear, there will be high

stress on the air and partial discharge will

develop. It may cause insulation

breakdown resulting in transformer failure .

ReliatraN®/ OVDT has a thinner layer of varnish without

any possible air bubble inside and is designed to have

low voltage stress far below corona inception voltage.

The construction process is simple and VPI process takes care for air voids.

Design takes care that continuous working stress

(voltage) is kept below 1.6KV/mm in general and in special cases its 1.2KV/mm.

IEC has made PD tests mandatory for CRT, but has kept optional for OVDT.

Partial Discharge – Cast Resin and OVDT technologies ?

10610510410310210110010-110-210-310-410-510-610-710-80

8

16

24

32

40

48

56

64

72

Voltage Stress vs. Time 0,25 mm aramid paper

Time to Breakdown (HRS)

Bre

akdow

n S

tren

gth

(kV

/mm

)

12

Partial DischargePartial Discharge

13

Reparability Reparability –– CRT and OVDT ?CRT and OVDT ?

• Fault diagnosis

• In OVDT its easy and simple to detect where the fault has happened.

• IN CRT its not possible to locate the fault, so entire coil needs to be replaced.

• Terminal plates

• Most occurring maintenance issue is change of the terminal plates

• In OVDT transformers, the plates are outside the windings

• In case of CRT – terminal plates are inside.

• for repairing, we would need to completely burn the coil.

14

15

Note) In general, life span of transformers in most of standards (ANSI/IEEE, IEC, UL)

depends on insulation around conductors. So, we use data of insulation materials

to predict life span of dry type transformers based on ANSI/IEEE C57.96

16

17

In case of a fire – which is the major cause of casualties ?

Fire Burns ?

Stampede ?

Smoke ?

18

0

50

100

150

200

250

300

350

400

450

500

0 2 4 6 8 101214 1618 202224 2628 30323436 3840 424446 4850 52545658 6062

Time (min.)

K

0

10

20

30

40

50

60

70

80

90

100

%

Temperature rise OTFMaximum Standard Temperature Rise

Minimum Standard OTF

Alcohol + heating pannel Heating pannel only

Fire Test Behavior According to IEC60076Fire Test Behavior According to IEC60076--1111

Temperature Rise and Optical Transmission Factor [OTF]Temperature Rise and Optical Transmission Factor [OTF]

[Deg C] [%]

19

CRTCRT

• Pass with minimum margin

• Hard to make balance between climatic and fire behavior test

• High levels of smoke (35% OTF) and temperature of off-gasses (400 K)

• Long burn times after removal of flame (>50 min.)

OVDTOVDT

• Highest level of performance

• Same coils for all tests

• Low levels of smoke (65% OTF) and low temperature of off-gasses (200 K)

• Immediate fire extinction after removal of flame

20

Fire Behavior TestFire Behavior Test

21

Flame Resistance ComparisonFlame Resistance Comparison

4003002001000

10

20

30

40

0.13 mm T-410

0.13 mm PET Film

6 mm Epoxy/Filler

Limiting Oxygen Index

vs. Temperature

Temperature (°C)

Lim

itin

g O

xygen

Index

(%

)

Nomex® is a DuPont Registered Trademark

22

Transformer type VDT Cast coil Silicon Ugilec T

Class H(Aramid) Class B

Rating(KVA):

mass tank/vessel (kg):

630 400 250 250

- - 190/40 338/60

First fire occurrence (min):

Fire end (min):

30 3 30 20

40 70 110 55

Total energy measured at 12 m from the

fire (MJ after 40/60 min):

Energy emitted by radiating panels (MJ):

Transformer part in total energy (%):

Combustible destruction part

(%)

Released carbon dioxide

mass(kg):

Released HC1 mass(kg):

Released soot And dust mass

(kg):

225/225 600/640 700/4900 400/500

336 143 ** 336 336

<1 80 95 55

20 20100 100

10 100 300 75

- - - 25

0.15 0.5 180 1.5

** Panels shut off owing to fire evolution

Quoted from “Development of Insulating Systems in Distribution Transformers -

Improvement of Fire Resistant Characteristic” by G.LE ROY and F.SANDOZ

23

EnvironmentEnvironment

What are the environmental concerns

• Emissions of toxic gasses

• While in case of fire ?

• During manufacturing ?

• Environmental foot-print

• Use of active materials

• End of life – material disposal/ reclaim

• Reclaim of copper in windings

• Disposal of coils/ core/ other materials

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