OVDT vs CRT - General

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Differences between OVDT and CRT Technologies

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What is the difference in the manufacturing process ?

Cast coilAssemble mold Preheat resin Weigh resin Mix resin pouring curing Disassemble mold -

OVDT

Impregnation(VPI) drying

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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

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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 airInsulation 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 airInsulation 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.

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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 80C 115C 150C

For each degree C below the rated temperature rise, increase rated load by

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

In general, ReliatraN provides about 30% additional capability for overloading based on practical design.

* Note:

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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.

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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 Class Class B (130) Class F (155) Class H (180)

Size Difference Difference (in %) 130% 115% 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.

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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.

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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.

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Short Circuit Strength Cast Resin and OVDT technologies ?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.

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Partial Discharge Cast Resin and OVDT technologies ?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.Breakdown Strength (kV/mm)

Voltage Stress vs. Time 0,25 mm aramid paper72 64 56 48 40 32 24 16 8 0 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 100 101 102 103 104 105 106

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.

Time to Breakdown (HRS)

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Partial Discharge

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Reparability 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.

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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

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In case of a fire which is the major cause of casualties ? Fire Burns ? Stampede ? Smoke ?

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Fire Test Behavior According to IEC60076-11 Temperature Rise and Optical Transmission Factor [OTF]500 450 400 350 300 Maximum Standard Temperature Rise Temperature rise OTF 100 90 80 70 60 50 40 30 20 Minimum Standard OTF 50Alcohol + heating pannel Heating pannel only

[Deg C]K 250 200 150 100

[%]%

10 00 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42 44 46 48 50 52 54 56 58 60 62

0 Time (min.)

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CRT 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.)

OVDT 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

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Fire Behavior Test

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Flame Resistance Comparison

Limiting Oxygen Index vs. Temperature40 0.13 mm T-410 0.13 mm PET Film

Limiting Oxygen Index (%)

6 mm Epoxy/Filler 30

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10 0 100 200 300 400

Temperature (C)

Nomex is a DuPont Registered Trademark

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Quoted from Development of Insulating Systems in Distribution Transformers Improvement of Fire Resistant Characteristic by G.LE ROY and F.SANDOZTransformer type VDT Class H(Aramid) Rating(KVA): mass tank/vessel (kg): First fire occurrence (min): Fire end (min): Total energy measured at 12 m f