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Power Transformer Maintenance

Maintenance Practices & Procedures for Substation

Class Distribution Power Transformers


Why should special maintenance practices be

applied to power transformers?

• Power transformers utilize active cooling systems

• Equipped with additional monitoring and

protection equipment

• Represent a large capital investment

• Often serve critical customers who suffer very

large financial losses in the case of transformer

failure

• Spare power transformers are typically not

available and replacements can take several

months




• IEEE/ANSI consider average transformer life to be

20-25 years based on the assumption that

adequate maintenance is performed over their

service life.

• IEEE life based on a continual ambient

temperature of 40º C and temperature rise of 65º C

• Due to lower ambient temperatures and lower

temperature rises in typical distribution

conditions, transformer life can be extended to 40

years or more with proper monitoring and

maintenance.


A transformer’s 2 worst enemies? HEAT


… and WATER


• Operation at only 10% above the transformer

temperature rating will cut transformer life by 50%

• Transformer heating by:

• Internal losses due to loading

• High ambient temperature

• Solar radiation

• FortisAlberta makes use of ONAN/ONAF/ONAF

cooling systems on power transformers.

• ONAN – Oil Natural/Air Natural

• ONAF – Oil Natural/Air Forced


Typical Oil Flow of ONAF cooling system


Inspection of ONAF Equipped Power Transformers

• Performed after 1 month of service and yearly

• Visible leaks and/or corrosion

• Shutoff valve position on radiators

• Fans

• Radiators

• Bushings

• Gaskets

• Top oil thermometer

• Winding temperature thermometer

• Thermometer testing every 3-5 years



• Inspection after 1 month and every 3-5 years

• Oil Level Indicator



• Inspected after 1 month of service and yearly

• Pressure relief device



• Inspected after 1 month of service and yearly

• Sudden Pressure Relay


Sealed Breathing System

• Allows ‘breathing’ to the atmosphere when pre-set

pressure gradients are surpassed

• Can lead to oil contamination


Electrical Testing of Power Transformers

• Performed before energizing and every 3-5 years

• Factory testing records should be kept as

baseline records

• Testing done before energizing to ensure no

damage occurred during shipping and as

confirmation of factory testing

• Test throughout the life of the transformer

when customer shutdowns or outages allow

• Test when a known fault has occurred


Insulating Power Factor Test

• Provides overall test of the insulating properties

of a transformer

• Measures the power losses due to leakage current

through the insulation

Turns Ratio Test

• Can detect shorted turns in the windings or a

defective tap changer

• Early indication of transformer failure


Winding Resistance Test

• Direct current evaluation test of the insulation

• Reliable indication of the presence of moisture,

contamination, or breakdown in the insulation

Core Insulation Resistance Test

• Measured by standard ‘megger’ test

• Core ground must be disconnected prior to test

• Detects deteriorating insulation between the core

and ground


Transformer Oils

• Transformer oils perform at least three functions

for the transformer:

• Provides Insulation

• Provides Cooling

• Helps extinguish arcs

• Oils also dissolve elements that can be measured

for analysis from:

• Oil degradation

• Moisture and gas from cellulose insulation

degradation

• Gases and moisture from whatever

atmosphere the oil is exposed to


Dissolved Gases in Transformer Oil

• Close observation of dissolved gases in the oil,

and other oil properties, PROVIDE THE MOST

VALUABLE INFORMATION REGARDING

TRANSFORMER HEALTH

• Oil samples should be taken and sent to a

qualified lab for analysis prior to energizing, one

month into service and yearly thereafter

• Great care must be taken to ensure that the

sample does not become contaminated and is not

exposed to the atmosphere


Dissolved Gases Analysis (DGA) is the most

valuable diagnostic tool available as part of a

transformer maintenance schedule.

•It is paramount that accurate records of DGA for

each individual transformer are kept as trends in gas

concentration changes are far more valuable than

individual gas concentrations.

• A sudden increase in key gases and the rate

of gas production is more important in

evaluating a transformer than the amount of

gas present in a DGA

• The exception to this is the presence of

Acetylene (C2H2)


IEEE C57-104 outlines a method of interpreting DGA

known as the ‘Key Gas Method’

•This method sets conditions that coincide with

given concentrations of gases and outlines

recommended actions based on these

concentrations and their generation rates

•Key Gases are:

• Hydrogen (H2)

• Methane (CH4)

• Ethane (C2H6)

• Ethylene (C2H4)

• Acetylene (C2H2)

• Carbon Monoxide (CO)

• Carbon Dioxide (CO2)

• Oxygen (O2)


IEEE C57-104 DCG Conditions


IEEE C57-104 Recommended Actions for DCG


Large concentrations of Nitrogen (N2), near

saturation, can also be discovered through DGA, but

present no concern as this is due to the nitrogen gas

blanket that is injected in the tank above the oil by

the manufacturer.

Large concentrations of atmospheric gasses (N2,

CO2, O2) can be very valuable in DGA in revealing a

possible leak.

The presence of Acetylene (C2H2), and to a lesser

extent Ethylene (C2H4), in concentrations higher than

trace levels should be further investigated as these

gases typically are only produced under arching

conditions.


IEC 60599- Fault Types indicated by DGA


Moisture Problems

• Each time the moisture is doubled in paper

insulation, the life of the transformer is cut in half.

• The life of the transformer IS the life of the paper!

• Keeping the water and oxygen levels in a

transformer to a minimum will greatly extend the

life of a transformer.

• The sealed breathing tank system creates the

possibility of moisture and oxygen problems as

the external atmosphere will be allowed to enter

the tank under conditions where the tank is in

vacuum.


Moisture Problems

• Moisture, especially in the presence of oxygen, is

extremely hazardous to cellulose transformer

insulation

• Each DGA or Insulating Power Factor Test should

be analyzed to determine the moisture by dry

weight (M/DW) that is present in the paper

insulation

• New transformers typically have a M/DW

under 0.5%

• Plans should be made to dry out the

transformer insulation once a M/DW of 2%

has been reached.

• An M/DW value of 4% presents a great

danger of flashover


Moisture Problems

• Moisture causes insulation to decay, which forms

more moisture, acids and sludge

• The additional moisture and acids further

increase the rate of insulation decay

• Sludge coats the coils and radiators,

leading to a slowly rising operating

temperature for a given load and ambient

temperature

• Both of these conditions lead to

exponentially increasing insulation decay if

steps are not taken to dry out the

transformer.


Where does the water come from?

•Some moisture is in the insulation when it is

delivered from the factory

•Atmospheric moisture can enter the transformer

when it is opened for inspection or through the

breather system when the tank is in vacuum

•Rain or atmospheric moisture can enter the tank

through leaky gaskets, especially during times of

rapid transformer cooling when a vacuum is created

inside the tank


Water Distribution in Oil and Paper


Additional Oil Tests the be performed with DGA

• Dielectric Strength Test

• Measures the voltage

at which the oil

electrically breaks down.

• Gives a good indication

of contaminants in the oil

• As per ASTM Test Method D-877 or D-1816

• Interfacial Tension Test (IFT)

• Determines interfacial tension between

distilled water and oil

• Measures concentration of particles in the

oil


Additional Oil Tests the be performed with DGA

•Acid Number Test

• Measures the amount of potassium

hydroxide required to neutralize acid in the

oil.

•Test for Oxygen Inhibitor (every 3-5 years)

• Determines level of inhibitor present

• The inhibitor acts as a sacrificial substance

that the oxygen will react with rather than

with metal or paper within the transformer.

•Power Factor Test

• Indicates dielectric loss (leakage current) if

‘Doble Test’ is not available

• Power factor should be maintained at less

than 0.5% at 25° C


Questions?

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