Transformer oil Tests

Mineral insulating oil Petroleum origin

Other type Oil with PCB content, Synthetic oil

USES Used in HV equipments like

Transformers, Reactors , CBs, Switchgear,Cables,Capacitors.

TRANSFORMER:

An equipment to Transform power from one circuit (primary) to another circuit (secondary) normally with change in voltage and current but with no change in frequency through mutual induction;

BASIC COMPONENTS IN A TRANSFORMER

Core

Winding

Insulation Life of transformer Life of insulation

TRANSFORMER OILAn essential part of oil filled

Transformers (there are other types of Transformers like dry

type , SF6 etc)

OIL IN TRANSFORMER Acts as an insulant (paper is the

other major insulation and is termed the solid insulant).

Acts as a coolant (heat from the core and

windings dissipates through oil)

Preserves core and coil assembly

Impregnates winding Insulation

Replaces air cavities

Improves PD behaviour

DEPENDS ON CRUDE NATURE (ORIGIN OF CRUDE )

DEPENDS ON REFINING PROCESS. Hydrocracking Hydrogenation Acid treatment Clay treatment.

NATURE AND CHARACTERISTICS OF

TRANSFORMER OIL

CrudeDistillation

TOBSRefining/Treatment

(acid, hydrotreatment, solvent extrn) Clay treatment

FiltrationTransformer oil

MANUFACTURING PROCESS

TRANSFORMER OIL TYPES

PARAFFINIC TYPE

NAPTHENIC TYPE

MIXED TYPE

NATURE AND COMPOSITION

OF OIL CHAIN OF HYDRO CARBONS (CH) 84% carbons,14% hydrogen ,1-3

% sulphur, 1% N2,O2 metals &salts

MAIN CONSTITUENT TYPES Napthenic Paraffinic Aromatic

PARAFFINIC TYPE

Derived From Crudes Having Substantial

Contents of Naturally Occurring N Paraffinic (Wax ) – require Dewaxing

Chain Structure

Most Commonly Used in India

High Viscosity, Low Resistivity

NAPTHENIC TYPE

Derived From Crudes Having very Low Naturally Occurring N

Paraffinic(wax)contents

Has Low Natural Pour Points,no Dewaxing Requirement

Cyclic Structure and Hence Stronger Bonding

Used Abroad &Now Also in India

Low Viscosity, High Quality, Costlier

IDEAL TRANSFORMER OIL

Low Density & Low Viscosity - Better Heat Transfer

Low Pour Point- Avoid Solidification

High Flash Point - Fire Resistance

Low Dissipation Factor - Less power loss

High BDV -High Voltage Application

High Resistivity - Better Insulation Resistance

STANDARDS

Important Indian Standards for Acceptance, Limit Values and

Interpretation

IS 335/1993 for New Oil IS 1866/2000 for Oil in Service IS 10593 for DGA

There Are Other Standards for Testing Methods.

OIL TESTS

PHYSICAL TESTS

Appearance Density Viscosity Pour point Flash point Inter Facial Tension

CHEMICAL TESTS

Acidity Oxidation stability Water content SK value Inhibitor content Corrosive sulphur Sludge

ELECTRICAL TESTS

Electric Strength(BDV)

Resistivity

DDF

SIGNIFICANCE AND MEANING OF TESTS

APPEARANCE

Clear, Transparent : No suspended matters and

sediments

COLOUR OF OIL:Clear pale yellow : GoodBright yellow : O.K.

Amber,Brown: Start of deterioration/Sludge

Green :Dissolved Cu or metallic contaminants;

Investigate and Repalce oil

Bad smell oil : Investigate and replace oil

Applicable Standards: IS 335, 1866

DENSITY

Mass per unit volume;

Higher Density Means Higher Viscosity

Which Will Reduce Oil Flow and

Hence Affects Heat Dissipation.

Transformer Oil Density Max =0.89g/cm3

Applicable Standards : IS 1448 ,335, 1866/2000

VISCOSITY

Resistance to Flow; High Viscosity Means Higher

Resistance to Flow.

Hence Low Viscosity Oils to Be Preferred.More heat transfer;Value-27 cstApplicable Standards : IS 335, 1866

POUR POINT

Temperature at Which Oil Begins to

Solidify

Lowest temperature at which oil flows

Cold weather application

Value : -6°c

Applicable Standards : IS 1448 , 1866

FLASH POINTFLASH POINT

The minimum temperature at which oil when heated gives off enough vapour to form a flammable mixture with air.

This test assumes significance after an arcing fault inside a transformer.

If FP is low -- Fire risk

Flash point can reveal presence of volatile and flammable materials

in oil

APPARATUS :

Fully Automatic Flash Point Test Kit has Been Procured Recently for the Chennai R&D Lab.

LIMITS:

New oil: 140 C (Min)

Transformer New Before commissioning 140 C

(Min)

In service 125C (Min) and should not to decrease by

more than 15 C from previous value.

Applicable Standards : IS 1448 ,335, 1866

Prepare the brass cup of the FP kit for filling the oil ; 75 ml of oil is taken;

Prepare the kit with appropriate settings

The flame is automatically dipped for every 3 deg(settable value), starting from 105 deg till the flash point occurs. The flash point is displayed automatically.

IFTInterfacial tension between oil and water surface.

It is the force necessary to detach a planar ring. IFT is a good indicator of oil quality and it should

be high. Very low IFT oil indicates sludge and deterioration. IFT helps in the detection of polar contaminants . Apparatus: KRUSS / German makeLimits: New oil :0.040 N/m Transformer New before commissioning : 0.035

N/m For in service Transformer : min 0.015 N/m Applicable Standards : IS 6104,335,1866

CORROSIVE SULPHUR

To detects corrosive and free sulphur compounds in oil.

If present they are hazardous and deteriorates metal parts of transformer .

Requirement – Non corrosive

Applicable Standards : IS 335

AcidityThe acid content in the oil and plays a major part in oil contamination.

High Acidity: Metal parts corrosion, Insulation deterioration,Sludge formation.

Limits: New oil:0.03 mg KOH / gm(Max) Transformer New before commissioning : 0.03 mg KOH / gm (Max) For in service Transformer : 0.3 mg KOH /

gm(Max) Applicable Standards: IS 1448, 335, 1866

OXIDATION STABILITY

Stability of oil under oxidation .

Acidity & Sludge are tested after subjecting oil to ageing .

Acidity max: 0.4 mg KOH/g

Total Sludge : Max 0.1%

Applicable Standard : IS 335

WATER CONTENTAmount of dissolved water in oil.

The test is carried out using automatic

coulomatric titration and expressed in PPM. If high, affects BDV,IR value/winding PF.

Apparatus

Automatic water content test kit swiss make

TEST PROCEDURE

Take 2ml of the oil in a Syringe;

Weigh it in an accurate digital balance;

Inject the oil in the syringe into the Automatic Titrator kept ready; Titration is started and goes on until end point is detected;

Weigh the empty syringe to find the weight of oil alone; The weight of oil is automatically transferred to the titrator for calculation;

The software does the necessary calculations and the water content is displayed automatically in ppm.

Limits : New oil 50 (max) New Tr. In

service Tr.

For V >170KV 10ppm 20 ppm For V between 72.5 to 170 KV 15 ppm 40 ppm For V < 72.5 KV 20 ppm 50 ppm Applicable Standards : IS 13567, 335, 1866

Before commg

SK VALUE

Degree of refining of oil .Optimum H2SO4 dosage during refining

results in good quality oil.No limits prescribed in standards Preferable value 2 to 4%Very low over refining, natural anti

oxidation are killed.Very high under refined.Applicable Standards: IS 335

BDV Why oil should have high BDV? Transformer insulation composite insulation --

Paper and oil Electrical stress = 1/Dielectric constant Dielectric constant of cellulose = 4.5 Dielectric constant of Transformer oil : 2.1 – 2.2 Electrical stress on Transformer oil is double for

the same thickness. Hence oil should have high BDV .

The Breakdown Voltage value of oil in KV measured across 2.5mm gap electrodes.

It is the min. voltage at which the oil breaks down when subjected to an electric field with a continuously increasing voltage, contained in a specified apparatus.

Low BDV indicate presence of contaminants such as water , dirt, cellulose fiber or conducting particles

Limit value :New oil 30 KV , min (un filtered) 60 KV min (filtered) Minimum requirement New Tr. Before

commg Inservice Tr.For V >170KV 60 KV 50 KV

For V between 72.5 to 170 KV 50 KV 40 KV

For V < 72.5 KV 40 KV 30KV

Applicable Standards: IS 6792, 335, 1866

DDF

Quality of oil

Measure of dielectric loss in oil and hence the amount of energy dissipated as heat.

Low DDF – Good oil, as it indicates low dielectric losses and and low level of soluble polar ionic or colloidal contaminants.

The dielectric dissipation factor and is a measure of the quality of oil. It is the tangent of the angle (delta) by which the phase difference between applied voltage and resulting current deviate from 90° when the dielectric of the capacitor consists exclusively of the insulating oil. Will be high if oil has conducting impurities, moisture, ageing products like acid and sludge.

LIMITS:

New oil 0.002 (max) at 90°C (filtered oil) Transformer new before commissioning Max : 0.010, for v > 170 KV Max: 0.015, for v <170 KV In service Transformer (max) : 0.2, v > 170 KV (Max): 1, for v < 170

KV Applicable Standards: IS 6262, 335, 1866

RESISTIVITY

RESISTANCE PER UNIT VOLUME RESISTANCE BETWEEN OPPOSITE FACES OF A CENTIMETRE CUBE OF OIL - - CM.MEASURE OF ELECTRICAL INSULATING PROPERTY , HIGHRESISTIVITY – LOW FREE ION AND CONDUCTING CONTAMINANTS. WILL BE LOW IF OIL HAS CONDUCTING IMPURITIES, MOISTURE AND FREE IONS. AFFECTS IR VALUE.

LIMITS:

For new oil : 35x1012 Ohm--cm (min) at 90 °C

1500x1012 Ohm-cm (min) at 27 °C

For Transformers New before commissioning : 6x1012 Ohm-cm

For In service Transformers : 0.1x1012 Ohm-cm

Applicable Standards: IS 6103, 335, 1866

SLUDGE

For all voltages should be Nil

About 40ml of oil is taken in the oil cell and the entire test is carried out as per the IS with the help of built in software ;

The results are displayed as shown;

TEST PROCEDURE

BUCHHOLZ GAS COLLECTION TEST

Test with Silver Nitrate –solution1(AgNo3) COLOR: No color – Air – investigate air

collection WHITE BLACK : Presence of CO –

Deterioration of solid insulation – DGA , Furan tests .

WHITE : Presence of Acetylene – Arcing – Hi temp oil degradation – DGA – Internal Inspection.

TEST WITH AMMONIACAL SILVER NITRATE SOLUTION

Color : No color – air Red brown to black with Silver gray

intermediate – CO Green Yellow – Acetylene Combustibility Test: Collect portion of Gas in a test tube and apply

a lighted match stick to check combustibility of the gas.

If not combustible -- Air

Most commonly carried out test on oil Test Equipment Modern Fully Automatic KitSTEP BY STEP PROCEDURE 1. Gap between electrode of 2.5mm +/- 0.01

2. Take sample oil, Rinse twice the electrode and container

3. Take oil in the container sufficiently to immerse the

electrodes4. Switch on the BDV equipment, keeping in place the

oil container.

BDV TEST

5. Allow 10 minutes initial time (as per IS)

6. Raise voltage at 2KV/sec rate and the oil breaks at a voltage.

7. After the first break (it may be a low value but still has to be taken), stir oil using magnetic stirrer for 2 minutes and allow stand time of two minutes. Then raise voltage to get second trip (breakdown) value.

8. Repeat the above for 6 readings. Take the average

of 6 readings for the BDV value of the oil

Note:

1. Oil for BDV test from the transformer should be taken from the bottom sampling valve with due precautions.2. Care should be taken to avoid air bubbles3. BDV test should always be done at ambient temperature.

4. Humidity should not be more than 50%

5. Spheres must be cleaned of deposits preferably by steel wool. Never use cotton, fibrous materials.

6. Sampling vessel to be dry, clean; Hands also to be dry and clean; atm. to be dry;

7. Sample to be protected from light, air and moisture

8. Sample to be drawn very quickly and tested within a few hours

TEST PROCEDURE

Observing proper precautions, take about 400ml of sample oil in the test vessel;

The gap set is 2.5mm

• The oil is kept in the test kit and the preset test procedure as per Indian Standard is activated;

• The BDV value is displayed automatically at the end of the test;

OIL QUALITY TEST (all the

above tests determine oil quality)

OIL TEST

DGA TEST (determines

transformer healthiness)

AGEING AND DETERIORATION OF TRANSFORMER OIL

CAUSES OF DETERIORATION Thermal stress Electrical stress Chemical contamination Gaseous contamination

ARRESTING DETERIORATION Procuring good quality oil Proper maintenance Prevention of moisture/air entry through

leak points, sealing and breather etc.

How to know oil condition and level of deterioration?

by periodical testing and comparing the characteristic with prescribed limit values

Why take action if oil is deteriorated? Deteriorated oil

can transfer moisture to paper and lower IR value (resistivity, water content)

can reduce heat dissipation (IFT, sludge) can affect other components of the

transformer (water content, acidity) can contribute to transformer failure

What action to be taken if oil is deteriorated? --- depends on degree of deterioration

Corrective actions available ---- Reconditioning, Reclamation and Replacement.

Reconditioning---- When the following characteristic becomes bad

on a sustained basis: BDV, water content, IFT and sludge.Also for degassing oil.

Reconditioning normally cannot improve acidity,

resistivity and tan delta significantly.

Reclamation Promising area

Currently carried out only in Distribution Transformers

Re refining

some apply refining and filtering to used oil as is done in the same way transformer oil is manufactured from crude oil. This involves no addition of inhibitors.

Replacement

Resorted to when Acidity, Resistivity, Tan delta and IFT are very

poor Served more than 10 years After confirming deteriorating trend Resistivity less than 0.03*1012 with very low

IR/PI value or high winding tan delta value

On line Filtration techniques Moisture absorbing cartridges

Oil quality summary

We have so far seen

Type and composition of oil

Oil quality parameters

Significance

Why we should preserve the oil characteristics throughout the life of the transformer

How to preserve these characteristics

Why these parameters deteriorate

What to do in deteriorated condition

Periodicity and limit values for these oil test parameters

.