Power Transformer Maintenance Best Practices

Introduction

Transformers are critical components in power systems, responsible for the efficient transmission and distribution of electrical energy. Proper maintenance of transformers is essential to ensure their reliability, longevity, and safe operation. Regular maintenance helps identify potential issues early on and allows for timely corrective actions, preventing costly downtime and major failures.

The manufacturer's instruction manual should serve as the primary reference for transformer inspection, installation, and maintenance, particularly during the equipment's warranty period. To enhance this guidance, the IEEE publication C57.93-2019, titled "Guide for Installation and Maintenance of Liquid-Immersed Power Transformers," can be employed in tandem with the manufacturer's manual. In cases where the manufacturer's manual is unavailable, the IEEE publication can also serve as a suitable substitute. In this article, we will discuss transformer maintenance best practices that GENCOs, NTDC, DISCOs and industries should follow to keep their transformers in optimal condition.

Regular Inspection and Testing?

While numerous tests are available to assess the condition of power transformers, practical experience demonstrates that visual inspection remains highly effective and occasionally the sole method for detecting specific power transformer issues. Regularly scheduled inspections form the cornerstone of transformer maintenance practices. These inspections involve monitoring the transformer for indications of oil leaks, compromised cooling systems, loose connections, and external corrosion.

In addition to visual assessments, periodic tests such as insulation resistance tests, Dissolved Gas Analysis (DGA), Dielectric Frequency Response (DFR), and Infrared Thermographic inspections should be conducted to identify abnormalities.

Regular Scheduled Maintenance

Establish a comprehensive maintenance schedule and adhere to it strictly. This schedule should include routine inspections, oil analysis, and electrical tests. The frequency of maintenance may vary depending on the transformer's age, loading conditions, and environment. Older transformers or those operating in harsh conditions may require more frequent maintenance.

Training and Skill Development

Ensure that the maintenance team responsible for transformer upkeep is well-trained and knowledgeable about transformer systems and maintenance practices. Regularly organize training sessions to update their skills and keep them abreast of the latest industry practices and advancements in transformer technology.

Proper Oil Analysis

All power transformer fluids are subject to deterioration, and the main pollutants are air, moisture, and heat. These pollutants react with transformer fluid and generate acids and sludge. In turn, acid attacks the winding insulation, and sludge deposits tend to decrease cooling. Transformer fluid moisture tends to decrease the fluid dielectric strength, which combined with sludge, will decrease the flashover value of insulators and terminal boards inside the transformer tank.

Transformer oil analysis is a key aspect of transformer maintenance for maintaining it in a contaminant-free state. It involves sampling and testing the oil to assess its condition and detect any potential issues. Routine oil analysis can reveal the presence of harmful gases generated due to internal faults, which helps in diagnosing problems early on.

On-Line Fault Gas and Moisture Analyzer

One of the parameters most commonly used to assess the health status of power transformers is DGA. This parameter enables convenient online monitoring of an energized transformer. While much of the DGA is still conducted in laboratories, there is a growing trend toward online DGA monitoring due to its ability to detect and diagnose faults throughout the lifespan of power transformers. For more information, refer to the Saudi Electricity Company (SEC), Transmission Material Standard Specifications 53-TMSS-01, 53-TMSS-02, and 53-TMSS-03.

On-Load oil Treatment and Cellulose Dry-Out Systems

Maintaining a low level of moisture within the insulating system maximizes the lifespan of insulation. Decreasing moisture within insulation enhances its dielectric characteristics, subsequently improving the safety and longevity of power transformers.

To achieve this objective, performing oil treatments on energized transformers is needed. This practice aids in preserving dielectric strength and improving cooling efficiency. Additionally, on-load cellulose insulation dry-out systems are available for efficiently drying a transformer's cellulose insulation. These systems are installed permanently on transformers, enabling continuous monitoring and reconditioning of both the transformer's cellulose insulation and its oil.

Maintaining Adequate Cooling

Efficient cooling is vital for a transformer's proper functioning and longevity. Overheating can lead to insulation degradation and contributing to transformer failure. ?

• Cooling Pumps (if provided)

?Visual inspection of the transformer oil cooling circuit components should be performed regularly. Oil Pumps should be?manually energized to ensure proper operation. Any significant noises (grinding, rubbing, scraping, and oil flow cavitation) should be noted, investigated further, and corrected. Flow gauges should indicate full flow without fluctuation. All areas including piping, valves and the surrounding ground area must not show evidence of oil leakage.

Pumps equipped with a bearing condition monitor should have readings taken annually to ensure the bearing integrity. Pumps with ball bearings are particularly vulnerable to in-service failure. Therefore, their replacement by pumps with sleeve bearings should be considered.

Therefore, their replacement by pumps with sleeve bearings should be considered.

• Radiators and Fans

Fans should be manually energized to ensure proper operation. Any significant fan motor noise or fan imbalance shudder should be noted, investigated further, and corrected. Fan blade guards must be maintained for the safety of personnel. Obvious obstructions to the through airflow caused by debris should be noted and cleaned.

Measuring the current of fan and pump motors and comparing it with the values indicated on the nameplates allows us to identify any deviations that could lead to defects. Periodic infrared imaging of the radiators should be made to ensure proper oil flow and heat transfer. All areas including piping, valves, and the surrounding ground area must not show evidence of oil leakage.

Typical oil leakage points include pipe work flange joints, valve stems, oil pump electrical connections, radiator headers and air bleed plugs.

Transformer Cabinets

As most accessories are connected through the Transformer Cabinet, this component requires special attention. The following inspection tasks are suggested:

• Check functionality of anti-condensation heaters
• Tighten screws and clean contacts on all current-carrying parts
• Check service hour counter, if installed
• Record oil and winding temperature drag hand readings and reset

Monitoring Load and Overloading

Transformers should be operated within their rated load capacity. Continuous overloading can cause overheating and insulation damage. Install load monitoring systems to keep track of the transformer's operating conditions and identify any load irregularities. Consider load sharing or adding additional transformers if the demand exceeds the capacity of a single unit.

Infrared Thermography

Infrared thermography is a technique that provides?an image of invisible infrared light?emitted by objects due to their thermal condition without any direct contact with the scanned object. This technique can be used to detect a defective connection on transformer bushings, hot spots on surge arresters, blocked cooling systems, and circulating currents affecting localized overheating of the tank wall, or to confirm oil level.

For compartment type OLTCs, infrared thermography can sometimes detect abnormal heating of degraded contacts (coking, low pressure) [Coking of oil means changing oil to low molecular weight gases, naphtha, and gas oil. Coking is a process in which raw materials are decomposed into products with lower boiling point.], or other malfunctions that cause temperature rise on the compartment.

Such problems are easily discerned on these OLTC designs because the OLTC compartment is normally cooler than the main tank, and any IR scan that shows to the contrary should trigger an investigation.

Protection Systems

Implement a robust protection system for the transformer to safeguard it against various faults, such as overcurrent, overvoltage, and short circuits. The protection system should be regularly tested and calibrated to ensure its proper functionality. A well-designed protection scheme can prevent catastrophic failures and minimize downtime.

Controlling Moisture

Moisture constitutes a significant threat to transformer insulation integrity. It is essential to effectively seal the transformer to thwart the infiltration of moisture. Consistent monitoring and maintenance of the breather unit are imperative to verify its proper functionality. The implementation of desiccants is crucial for managing moisture levels within the transformer. Furthermore, periodic controlled heating to dry the transformer can facilitate the elimination of accumulated moisture.

Cleaning and Maintenance of Bushing

Transformer bushings provide the necessary insulation and support for electrical connections. Regular cleaning and inspection of the bushings for signs of aging, cracking, or oil leaks to be done. Dirty surface of bushing will cause leakage current. Defective bushings can lead to partial discharges and may compromise the transformer's reliability.

They play a crucial dual role in insulating and securing the oil tank while facilitating the transmission of current to the exterior. They must be adaptable to varying environmental conditions and possess specific mechanical strength. Various types of bushings exist, including pure porcelain bushings, oil-filled bushings, gas-filled bushings, and capacitive bushings. Capacitive bushings can be categorized into two types: resin paper capacitive and oil paper capacitive. In this discussion, we will focus on the maintenance and potential faults associated with oil paper capacitive bushings.

One major cause of transformer bushing faults is the accumulation of dirt on the bushing's surface, leading to the absorption of water and a subsequent reduction in insulation resistance. This reduction can result in an increase in surface leakage current, which in turn raises the risk of flashovers. These flashovers have the potential to inflict damage upon the bushing's surface. Furthermore, the absorption of water leads to an increase in conductivity, driving up leakage current. This increased current flow has the potential to damage the porcelain surface, possibly leading to breakdown, especially in humid weather conditions. The author has personally observed instances of such breakdowns occurring in high-voltage equipment.

Another significant cause of poor bushing sealing can be attributed to two main factors. Firstly, insufficient maintenance experience among staff can lead to loosely tightened screws. Secondly, the use of subpar rubber sheets can undermine the quality of the seal. Other contributing factors include the presence of poorly designed or defective bushing structures, the usage of inadequately qualified insulation oil, water ingress due to dampness, breakdown of grounding of bushings, and the generation of floating potential by bushing shields, resulting in partial discharge. Additionally, a dirty oil pointer can obscure the clear visibility of the oil level, further complicating the issue.?

Main Tank and Conservator

The main tank contains the transformer active element (core and coil assembly) and its insulating fluid. An on-load tap changer compartment can also be included in the assessment of tank condition. The integrity of the tank is dictated primarily by its mechanical characteristics. The tank must hold pressure and should not leak.

The integrity of the tank depends primarily on the condition of:

• ?the gasket containing surfaces,
• the hand hole, manhole, and tap changer door gaskets, and
• the integrity of tank welds.

?Deficiencies are detected?by observing oil leakage. The inability to adequately seal a tank may render a tank environmentally unacceptable for its current installation. External sealant repair systems, involving specialty sealants and unique hardware containing gasket provisions, may also prove effective in stopping leaks.

The tank should also be inspected for?indications of deformation. Tank deformation may result from extreme pressure or electrical arc?experienced during a fault, improper foundation support, and for spare transformers, frequent installation and removal.

The integrity of a bladder may be checked in service by inserting a swab stick with a cotton cloth on the end through the bladder access port in the top of the conservator tank and gently swabbing the inside of the bladder. If the cotton swab becomes saturated with oil, this indicates that there is a leak in the bladder, and it must be replaced.

Transducers that can detect rupture include:

• ?A capacitive transducer that detects contact with oil
• A gas collection relay mounted in the head of the conservator to detect air

The oil level in the conservator system should be checked regularly to make sure that?it is at the proper level and to verify there is no undetected oil leak in the transformer. There is normally a 25°C gradation mark on the oil level gauge to use in adjusting the proper oil level.

In the past, conservator tanks were not necessarily designed to withstand full vacuum. In these designs, whenever a vacuum is applied to the main tank, the valve between the main tank and the conservator tank must be closed. However, most modern conservator tanks are designed?for full vacuum and the valve should be left open. In all cases, the transformer manual or manufacturer should be consulted.

The desiccant for the bladder type conservator system should be checked regularly and replaced when it reaches the end of its moisture removal capability. Operating such a system with a non-functioning desiccant, (especially in a humid environment) can allow moisture to enter the tank, or cause the bladder to age prematurely.

The following actions can be performed on the main tank:

• Check for oil leaks, re-seal if necessary
• Check for paint damage and corrosion, repair if necessary
• Check oil levels in all compartments
• Check air drier and main conservator bladder – refill or replace if necessary
• Check grounding of the main tank and neutral terminals
• Check electrical insulation for tank base where fitted
• ?Check the status of all valves before putting back into service

Where silica gel is used for air-drying,?the color change in the silica gel?indicates that the silica gel is saturated with water and that the air passing through the breather will remain humid. During the inspection, silica gel replacement should be done when more than 50% of the material has changed color.

Moreover, the color change should be observed to start from the fresh air input side only. If the silica gel changes color on the conservator side of the breather, or if the color of the silica gel remains unchanged after several months, then this indicates there is air leakage in the connection piping allowing air to bypass the breather.

On-Load Tap Changer

Since all on-load tap changers are not of the same design and construction, special instructions of manufacturer should be followed. However, a few points are enumerated:

a)???Diverter Switch - The maintenance primarily consists of servicing of diverter switch contacts, checking the oil level in the diverter switch chamber, and replacement of diverter switch oil when the same becomes unsuitable for further service.

b)???Motor Driving Mechanism.

1.???Do not allow dirt to accumulate between contact rings of notching controller.

2.???Do not use oil/grease on contacts rings on notching controller.

3.???Check the operation of anti-condensation heater.

4.???If the contacts of contactors are silver faced, no touching up shall be done, but should be replaced when they are worn out. Copper contacts may be lightly touched up with a file when they become rough. The pole faces of electromagnet shall be kept clean.

5.???Do not Oil/grease the contact surface of radial multi-contact switches, unless a special contact lubricant is used. The space between the rings should be cleaned occasionally. If necessary, a few drops of benzine be used.

c)???Selector Switch - The contacts do not make/break current. As such, the wear is only due to mechanical movement of moving contacts. These may be inspected when internal inspection is carried out.

External Connection

• All connections should be tight. If they appear blackened or corroded, undo the connection and clean down to bright metal with emery paper. Remake the connection and give it a heavy coating of grease. It is particularly important that heavy-current carrying connections should be properly maintained. If the metal has the characteristic bluish tinge which indicates that it has been hot, then in most cases the connection shall not be considered satisfactory. Either it has become loose or dirty, or the conductor is not suitable for carrying the current.
• The earth connections shall be properly maintained. A small copper loop to bridge the top cover of the transformer and the tank may also be provided to avoid earth fault current passing through the fastening bolts when there is a lightning surge, high-voltage surge or failure of bushings.

Internal inspection

This verification involves the tank and core internal check. On liquid-filled power transformers, the covers can be removed to check for evidence of moisture or rust around the bushing supports and transformer top cover. To check the tank and core, the liquid can be drained out. Core examination needs to be done to check for sludge deposits, loose connections, and any damage to the transformer elements. Presence of carbon may indicate internal issues. The winding inspection needs to be checked for damage to terminal panels, barriers, loose connections, and overall winding connections. Apparently, such things as untanking the power transformer for internal inspection would have to be judiciously made and would depend on transformer age, its overloading and trouble history. Inspection frequency needs to be 5 to 10 years or more.

Spares

It is a healthy practice to have essential spares like one member of each type of bushings, one spare limb winding, one thermometer, one cooling fan, etc., for each group of similar transformers. Suppliers' recommendations may be followed in this connection.

Conclusions

To achieve the desired level of performance and prevent failures, utilities and industries need to follow best practices for the maintenance of transformer.

With this in mind, following these transformer maintenance best practices can significantly improve the performance, reliability, and lifespan of transformers in power systems. Implementing a well-structured maintenance program can reduce downtime, enhance safety, and minimize overall operating costs. A proactive approach to transformer maintenance is an investment in the reliability and stability of power supply, benefiting both utilities and consumers alike.

Transformers that are approaching their end-of-service life need to be monitored more closely and actions need to be taken in advance for their replacement in a phased manner.?

REFERENCES

[1] Transmission Materials Standard Specification, 53-TMSS-01, Revision no. 0, “Power Transformers, Rated up toe 100 MVA, Saudi Electric Company.

[2] Transmission Materials Standard Specification, 53-TMSS-02, Revision no. 0, “Power Transformers, Rated Above 100 MVA”, Saudi Electric Company.

[3] Transmission Materials Standard Specification, 53-TMSS-03, Revision no. 0, “Online Gas Analyzer”, Saudi Electric Company.

[4] CIGRE TB 445, “Guide for Transformer Maintenance”, Working Group A2.34, 2011.

[5] IEEE Publication C57.93-2019, "Guide for Installation and Maintenance of Liquid-Immersed Power Transformers.

[6] IS: 10028-3 (1981), Part III, (Reaffirmed 2003), Indian Standard Code of Practice for Selection, Installation and Maintenance of Transformers, Part III - Maintenance, Bureau of Indian Standards, New Delhi.

[7] IEC 60076-7:2018, Power Transformers - Part 7: Loading guide for mineral-oil-immersed power transformers.

[8] Velimir Lackovic, CED Engineering, Continuing Education and Development, Inc., “Power Transformer Maintenance”.

[9]https://transorfilter.se/en/oil-filter-transformer-oil-online-dry-out-system/#:~:text=The%20purpose%20of%20a%20transformer,of%20about%2070%20degrees%20Celsius.

[10]https://electrical-engineering-portal.com/download-center/books-and-guides/power-substations/transformer-maintenance, "Best practice guide for power transformer maintenance".

[11] https://www.mdpi.com/1424-8220/19/19/4057, Dissolved Gas Analysis Equipment for Online Monitoring of Transformer Oil: A Review.

[12] Katherine Feng, High Voltage Test System – Himalaya.com, “Faults and Maintenance of Transformer Bushing”, 2017.

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