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Transformer & OLTC Maintenance


Life of transformer

Residual Life Assessment

According to IS 10028 Part III Transformer more than 5 years old should be assessed for residual life and steps should be taken for life extension.

 

Dissolved Gas Analysis

Insulating materials within transformers and electrical equipment break down to liberate gases within the unit. The distribution of these gases can be related to the type of electrical fault, and the rate of gas generation can indicate the severity of the fault. The identity of the gases being generated by a particular unit can be very useful information in any preventative maintenance program.

 

Furan Analysis (Remaining Life of Transformer)

The decomposition of paper is caused by the processes of hydrolysis, pyrolysis and oxidation. The degree of polymerization (DP-value) of paper was defined by IEC 60450 and it counts the number of polymerized glucose rings. During the paper decomposition, the DP-value is reduced and the tensile strength decreases. New cellulose has a DP-value of 1000-1100, aged cellulose on the other hand has a value of only 150-200, which means the end of the transformer’s lifespan.

As it is not possible to take paper samples during running operation, the condition of the solid insulation is estimated based on the cellulose decomposition products (2- Furfural). For this purpose a regular trend analysis is necessary.

 

Type test of Transformer as follows:

  1. Transformer winding resistance measurement
  2. Transformer ratio test.
  3. Transformer vector group test.
  4. Measurement of impedance voltage/short circuit impedance (principal tap) and load loss (Short circuit test).
  5. Measurement of no load loss and current (Open circuit test).
  6. Measurement of insulation resistance.
  7. Dielectric tests of transformer.
  8. Temperature rise test of transformer.
  9. Tests on on-load tap-changer.
  10. Vacuum tests on tank and radiator.

 

 

Life Extension Program

Lifting of coil with sludge removal from coil and tank

Sludge forms from the degradation of the oil and paper insulation due to stressors such as oxygen and temperature and catalysts such as copper. Excessive sludge deposition reduces life of transformer and poses various safety hazards. The presence of solid particles reduces electric strength of the oil and in addition deposit may hinder heat transfer, thus promoting further deterioration of insulation. The process of lifting the coil is for sludge removal from coil and tank which is not possible with Transformer oil filtration and dehydration

 

Use of Ester Based transformer oil to increase the life of transformers.

Oil Dehydration & Filtration System

On-Site Transformer Oil Dehydration, Filtration, Acidity Removal and IR value Improvement

 

The principal function of Transformer oil is to provide Electrical Insulation and to cool transformer. Transformers require insulating oil of a superior quality in respect to freedom from contamination in order to insure high dielectric strength and a long time service under extreme stresses.

There are many types of contamination, such as water, solid particles of dust, metals, colloidal carbon, products of corrosion, acids, resins, gums, tar, sludge, organic contaminants and solvent, air and gas which adversely affect the insulating properties of oil.

Contaminants and products of oil deterioration reduce dielectric strength, resistivity and loss factor of oil by forming a path of low resistance for electric discharge through the oil. Protection of electrical apparatus by continuous or frequent purification of insulating oils will eliminate costly breakdowns and assure a safe and dependable operation.

 

Double Stage Ultra High Vacuum Transformer Oil Purification System

 

For achieving the desired results, the Oil Filtration Plant is designed for Ultra High Vacuum and Low temperature of oil as high temperature can result in thermo-oxidative degradation of oil. The machine is equipped with a 2-stage vacuum pump (High Vacuum Pump and Booster Pump) operating down to 0.5 Torr vacuum. For effectively removing of dissolved gases and moisture content from the oil under High vacuum.

 

The Ultra High Vacuum Double Stage type oil treatment plant is ideal for treating new/used Insulating oil of high voltage and super high voltage transformers.

Transformer Insulation Resistance (IR) Value Improvement (Evacuation System):

In addition the plant has inbuilt transformer evacuation system, it can pull high vacuum on empty transformers to remove moisture from the paper windings.

OLTC Maintenance and servicing

On-load tap changers (or circuit tap changers) as the name suggests, permit tap changing and hence voltage regulation with the transformer on-load. Tap changing is usually done on the HV winding for two reasons:

  1. Because the currents are lower, the tap changer contacts, leads, etc., can be smaller.
  2. As the HV winding is wound outside the LV winding, it is easier to get the tapping connections out to the tap changer.

 

Due to this tap changing carbon is formed in the OLTC tank and diverter switch contacts gets worn out. The maintenance primarily consists servicing of

  1. Diverter switch contacts
  2. Checking the oil level in the diverter switch chamber
  3. Replacement of diverter switch oil when the same becomes unsuitable for further service.
  4. Checking motor driving mechanism
  5. Removing carbon & sludge from OLTC tank
  6. Filtration & Dehydration of transformer oil

Onsite testing of moisture in oil

On Site Hand Held Moisture Meter for Checking Water Level In Terms Of PPM (Parts Per Million) For Better Predictive Maintenance of Transformer

 

Moisture in Oil analysis or better known as Water Content in oil is one of the most critical and important parameter for Transformer.

 

It is one of the major factor which determines the life of transformer and excessive moisture content can accelerated the aging of transformer.

 

In the extreme case, transformers can fail because of excessive water in the insulation. The dielectric breakdown voltage of the oil is also affected by the relative saturation (RS) of water in oil. The maximum loading that is possible while retaining reliable operation (i.e., preventing the formation of water vapor bubbles) is a function of the insulation water content.

 

Traditional method is very vulnerable & susceptible to contamination of oil sample which is taken to lab due to transportation involved and high risk of getting in contact with air which might alter the result drastically also the scope of improvement is very less in this case

 

While in the case of on the spot checking of Water level in oil using Hand Held Moisture meter we can act on it on real time basis and take corrective step to minimize the moisture using Dehydration Machine.

 

Apart from PPM calculation of water content it also tells about WATER ACTIVITY (aw) is a measurement which indicates moisture (water) content in a fluid based on a scale from 0…1

 

“aw” is a parameter that will always indicate how close a fluid is to reaching its saturation point which is the maximum water holding capacity.

OTI, WTI, Buchholz relay, Insulation resistance testing

OTI & WTI Meter Calibration and Tripping Testing

These are generally precision instruments. A temperature indicator of power transformer is specially designed for protection of transformer in addition to its temperature indication and cooling control features. That means, this device performs three functions

1) These instruments indicate instantaneous temperature of oil and windings of transformer.

2) These also record maximum temperature rise of oil and windings.

3) These instruments operate high temperature alarm at a predetermined value of allowable temperature limit.

4) Temperature indicators of transformer can also trip the circuit breakers associated with the power transformer when the temperature of oil or winding reaches a predetermined limit.

 

Buchholz Relay testing with tripping

In the field of electric power distribution and transmission, a Buchholz relay is a safety device mounted on oil-filled power transformers and reactors, equipped with an external overhead oil reservoir called a conservator. The Buchholz relay is used as a protective device sensitive to the effects of dielectric failure inside the equipment which cause production of gas.

This switch energized the trip circuit of the circuit breakers associated with the transformer immediately isolate the faulty transformer from the rest of the electrical power system by inter tripping the circuit breakers associated with both LV and HV sides of the transformer. This is how Buchholz relay functions.

 

Insulation Resistance

Insulation resistance test of transformer is essential test. This test is carried out to ensure the healthiness of overall insulation system of an electrical power transformer

 

Transformer Insulation Oil Testing

Laboratory Tests
The complete examination includes all the tests listed below along with the evaluation of the general condition of the oil, consideration of all results together often enables the cause of degradation or the source of a contaminant to be recognized, so that the appropriate action can be taken to ensure the reliable operation of the equipment.

  • Dielectric strength(Break Down Voltage)
  • Water Content
  • Dielectric Dissipation Factor (tan delta)
  • Specific Resistance (Resistivity)
  • Neutralisation Value (total acidity)
  • Sediment/Perceptible sludge
  • Interfacial Tension & Flash point
  • Dissolved Gas Analysis

Terminolgy

Electric Strength (Break down Voltage)

The voltage at which the oil breaks down when subjected to an ac electric field with a continuously

increasing voltage contained in specified apparatus. The voltage is expressed in KV.

Water Content

Flash Point

The temperature at which the oil gives off so much of vapor that this vapor, when mixed with air,

forms a ignitable mixture and gives a momentary flash on application of pilot flame under the

prescribed conditions.

Precipitable Sludge

Oil deterioration products or contaminants or both, Which are insoluble after dilution of the oil

with n-heptane but are soluble in the solvent mixture of equal parts of toluene, acetone and

alcohol under prescribed conditions.

Sediment

Any substance which is insoluble after dilution of the oil with n-heptane and also insoluble in the

solvent mixture of equal parts of toluene, acetone and alcohol Under prescribed conditions.

Specific Resistance (Resistivity)

It is the ratio of the dc potential gradient in volts per centimeter paralleling the current flow within the specimens to the

current density in amperes per square Centimeters at a given instant of time and under prescribed conditions. This is

numerically equal to the Resistance between opposite faces of a centimeter cube of the liquid. It is expressed in ohm-

centimeter.

Dielectric Dissipation Factor (Tangent Delta)

It is the tangent of the angle (delta) by which the phase difference between applied voltage and resulting current deviates

from ^/2 radian when the dielectric of the Capacitor consists exclusively of the insulating oil.

Interfacial Tension

It is a force necessary to detach a planar ring of platinum wire from the surface of the liquid of higher surface tension that

is upward from the water-oil surface. It is expressed in N/m.

Dissolved Gas Analysis

Insulating materials within transformers and electrical equipment break down to liberate gases within the unit. The

distribution of these gases can be related to the type of electrical fault, and the rate of gas generation can indicate the

severity of the fault. The identity of the gases being generated by a particular unit can be very useful information in any

preventative maintenance program.

Govt. of Haryana approved chartered electrical safety engineer Bhanu Bansal for self-certification of electrical installation.
Govt. approved ‘A Class’ Electrical Contractor up to EHT, 132 KV

Address

1376, Sector-15
Faridabad-121002
Haryana

Sanjiv Bansal

sanjiv@dewenergy.com

+91 9810513849


Deepak Bansal

deepak@dewenergy.com

+91 9971690541


Bhanu Bansal

bhanu@dewenergy.com

+91 9999150666