Maintenance Matters - Guide to Transformer Inspection and Maintenance


Introduction

Transformers are vital components in our electrical networks, serving a crucial role in power generation, transmission, and distribution, as well as within power-consuming equipment, machinery, and portable devices. To ensure the uninterrupted operation of these transformers and the stability of our power systems, proper maintenance practices are essential. This article explores the methods and importance of transformer inspection and maintenance, emphasizing routine preventive measures that help extend their lifespan and maintain their optimal performance.

The Significance of Transformer Maintenance

Transformers play a pivotal role in electrical systems, facilitating the efficient transfer of electrical energy from one voltage level to another. They can be found at power generation plants, substations, and even within various electrical appliances we use daily. Due to their widespread application, it is imperative to maintain transformers to ensure their reliability and safety.

Periodic inspections and routine maintenance are indispensable for transformers, as they help detect and address potential issues before they escalate, thereby extending the operational lifespan of these critical components. By conducting regular maintenance, transformers can operate in a safe and reliable condition, minimizing the risk of costly downtime and equipment failure.

Visual Inspection (Cost effective Maintenance)

A comprehensive visual inspection plays a crucial role in transformer maintenance. Conducting this inspection can be both straightforward and cost-effective if you know what to focus on. Initially, when we observe a transformer, we perceive it as a substantial metallic enclosure with various components attached, such as radiators, fans, bushings, tap changers, breathers, and gauges. However, delving deeper into the examination of these components allows us to proactively identify potential issues, prevent the emergence of problems, and ensure the equipment's uninterrupted and efficient performance.

Conducting a visual inspection of a transformer is as simple as physically inspecting key areas. This involves walking around the unit to assess critical aspects such as the cooling system, condition of the paint, presence of leaks, state of bushings, and the status of breathers and gauges.

When to Perform a Visual Inspection

Among the maintenance options available for an energized transformer (namely, oil analysis, infrared scanning, and visual inspection), visual inspection is the sole procedure that incurs no cost. The frequency of oil testing and infrared scanning should be determined based on the transformer's size and criticality, typically necessitating annual assessments.

Visual inspections can be conducted every six months or as frequently as needed. Nevertheless, for critical equipment like generator step-up transformers, transmission and distribution transformers or furnace transformers that are crucial for operations, it is advisable to schedule thorough visual inspections more frequently, ideally every three months.

Key Areas for Routine Transformer Inspection and Maintenance

Cooling System:

  • Check the fan by operating it on Auto/Manual using the control switch

  • Check and ensure that the fan rotates freely and at full speed within approximately 5 seconds

  • They should rotate smoothly with minimal vibration

  • Check to see if there are any defects

  • Check for debris and scratch on the fan paint and physical defects

  • Listen for noise as the fan runs

  • If there is noise, check the bearings of the fan

  • If the cooling system is equipped with the pumps as well then examine the pump valves for evidence of leaking around the gland seals

  • Tighten the gland nut if necessary.

                   

Figure 1: Power transformer cooling system (photo credit: Bureau of Reclamation)

Gauges:

  • Check the temperature gauge readings on the transformer and record them

  • Check the ambient temperature and record it. Confirm that the temperature is within the normal operating temperature, but if it is normally high, carry out further investigations to find out why the high temperature

  • Check the KVA or MVA load on the transformer and record it. Ensure that the transformer is not loaded beyond its capacity

  • Check the oil level readings to ensure that there is no possible leak. If there are signs of a leak, carry out a pressure test. Transformer leaks must be repaired immediately to prevent further leaks, ingress of moisture and damage to the transformer

  • When adding oil if needed, add only the same type of oil that is in the transformer

Figure 2: Oil and winding temperature gauges (photo credit: Electrical Engineering Portal)

Paint Condition:

  • Inspect the exterior paint for peeling, cracking, or corrosion

  • Repaint as needed to protect against environmental factors

  • Properly inspected and maintained transformer coating ensures an increased lifespan of a transformer as the tank metal will not be exposed to weather conditions for it to degrade, which could cause a leak of the oil

  • Note the paint conditions as good, fair, or bad

 Bushing Terminals:

  • If the transformer is energized and on load, check the temperature of the bushing using a thermal imager (e.g. Fluke TiX590 Infrared Camera). If the temperature reading is high, then there is a loose or dirty connection at the terminals

  • If the transformer is not energized, use a torque measuring tool to make sure terminal connections are tight

  • If connections are not tight, maintenance need will include cleaning, applying contact grease and tightening connectors

  • Record your findings

 

Figure 3: 240 kV breaker bushing connection with connector hot spot (photo credit:

Gaskets:

  • Inspect visually gaskets for wear and tear, ensuring a proper seal

  • Replace gaskets as necessary to prevent oil leakage

  • When replacing a gasket, carefully clean mating surfaces to remove any rust, dirt, transformer oil, or other contamination that might prevent a good seal. Use an appropriate gasket cement when installing new gaskets

  • Do not reuse old gaskets

  • Six months after replacing a gasket, check and retighten if necessary

Figure 4: Transformer seal gasket to check for cracking or other signs (photo credit: Electrical Engineering Portal)

Rust Prevention:

During inspections, check all metallic parts of the transformer tank body for rust and degradation, starting from the tank to the radiator fins to the fans to the chassis and all parts of the tank body. Transformer should be regularly inspected for the rust.

Cover, Joints, and Bolts:

  • Inspect the cover and joints for tightness and integrity

  • Ensure that there is no rust on the bolts, joints, and the cover of the transformer

  • In order to keep the rust away, make use of petroleum jelly to rub all over the bolts

  • Check back every six months to see the condition of the bolts and joints, and if necessary, add more petroleum jelly on joints with little or no petroleum jelly left on them

Figure 5: Oil seepage from transformer top cover joint

Bushing and Surge Arrester Insulators:

  • Bushings and surge arresters should be clean. If the surfaces are dirty, they should be cleaned while the transformer is not energized.

  • Examine insulators for cracks or signs of electrical tracking and take necessary action

  • Replace defective insulators if found to maintain insulation resistance

 Transformer Oil Level:

  • Monitor transformer oil levels for the main tank and OLTC to ensure they remain within the recommended range. The transformer oil gauge is shown in Figure 6 and its location is shown in Figure 6.

  • Top up oil as needed

Figure 6: Transformer magnetic oil gauge installed on the conservator tank (photo credit: EEP)
Figure 7: Location of MOG on transformer (photo credit: SAVREE)

 Control Wiring:

  • The control wiring used to control the transformer should always be checked for functionality

  • The control cabinet and associated conduit should be inspected to ensure that weather seals are intact.

  • Inspect control wiring for damage or deterioration

  • Replace or repair wiring as required to maintain operational control

 Oil Dielectric Test:

  • It is advisable to annually test the dielectric strength of the transformer oil. To carry out the oil dielectric test on a transformer, oil samples should be drawn from the bottom of the tank

  • It is important to make use of proper sampling and standard operating procedures to sample the oil from the transformer

  • Conduct periodic oil dielectric tests to assess the condition of the insulating oil 

Figure 8: Common safe oil sampling practice avoiding travel of air bubble in the tank (photo credit: TechCon North America)

Silica gel Breather:

  • During this breathing cycle, there is a need to prevent moisture from the atmosphere from entering the transformer, which can contaminate the oil inside and lead to a breakdown of the dielectric strength.

  • Regularly check the breather by looking at its color

  • A bright crystal with a blue tint shows that the silica gel is still fit for use, while a pink color shows that the breather has absorbed enough moisture and is due for a change 

Figure 9: Silica gel breather (Photo Credit: Qualitrol Corporation)

On Load Tap Changer

  • If the transformer is equipped with a load tap changer, inspect the tap changer for proper operation

  • Detailed information for the inspection procedures and the frequency of inspection for the tap changer is usually supplied by the manufacturers

Figure 10: Power Transformer with OLTC (photo credit: Polywater)

Conclusions

Maintaining transformers is crucial for ensuring their reliability and long lifespan as vital power system components. To achieve this, regular inspections and preventive actions in areas like cooling systems, monitoring oil levels and temperature, keeping the breather moisture-free, maintaining insulation, checking terminals for hot spot, and ensuring structural integrity are essential for peak performance. Adhering to maintenance guidelines discussed, allows power system to operate safely and efficiently, reducing downtime and costly repairs. In the end, a well-maintained transformer plays a significant role in strengthening the stability and resilience of our electrical networks, supporting the daily functioning of our society.


Courtesy :

Muhammad Hanif

Former Quality Manager, ABB Electrical Industries Co. Ltd., Riyadh, SA. Currently working with EPESOL Lahore, Pakistan as Senior Technical Manager.


Transformer - HT Distribution

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