Maintenance of Transformer

Maintenance of Transformer . Transformers are critical components in electrical systems, enabling the efficient transmission and distribution of electricity. Their reliability ensures the smooth functioning of industrial, commercial, and residential applications. However, like all mechanical and electrical equipment, transformers require regular maintenance to prevent failures, optimize performance, and extend their lifespan. This article dives into the essential practices for transformer maintenance, ensuring safety, efficiency, and durability.

Basics of Transformer Maintenance

What is Transformer Maintenance?
Transformer maintenance involves systematic procedures to ensure the transformer operates efficiently and safely. It includes inspections, testing, cleaning, and repairs to detect and address potential issues before they escalate.

Key Components to Maintain:

Core: Maintains magnetic flux and efficiency.
Windings: Ensures proper current flow.
Cooling System: Prevents overheating.
Insulating Oil: Provides insulation and dissipates heat.

Objectives of Maintenance:

Enhance transformer performance.
Minimize downtime.
Reduce repair costs.

Visual Inspection of Power Transformer

The most frequent attention given to power transformers is visual inspection, which involves mainly checking the transformer’s general exterior condition and cooling system.

Power transformers must be inspected regularly so that problems can be detected early and corrected before major repairs are needed.

Inspections are done routinely, usually once a week, although the frequency can vary from company to company and among transformers. For example, a transformer may be checked more often if there is reason to believe that a problem is developing.

Transformer Diagnostic and Monitoring

Transformer monitoring refers to on-line measurement techniques, where the emphasis is on collecting pertinent data on transformer integrity and not on interpretation of data.

Transformer monitoring techniques vary with respect to the sensor used, transformer parameters measured, and measurement techniques applied. Since monitoring equipment is usually permanently mounted on a transformer, it must also be reliable and inexpensive.

Winding and On-Load Tap Changers (OLTC) failures dominate; consequently, the focus of most monitoring techniques is to collect data from parameters that can be used to assess the condition of winding and tap changers.

Dissolved gases in oil and partial discharges (PD) are common parameters monitored related to winding and insulation condition.

Temperature and vibration monitoring are commonly used for assessing OLTC condition.

Statistics-distribution-of-failures-in-an-oil-immersed-transformer

Common parameters used to monitor windings and insulation status is PD and dissolved gases in the oil; in what concerns monitoring of OLTC temperature and vibration are used.

Main monitoring units used for transformers diagnostics are:

Oil temperature monitoring unit.
Oil level monitoring sensor.
Gas-in-oil monitoring unit.
OLTC operation monitoring sensor.
Overload monitoring unit.

To reduce the risk of missing incipient faults due to long sampling intervals, monitoring techniques are being developed to provide warnings with respect to changes in gas types and concentrations observed within a transformer. Conventional dissolved gas-in-oil analysis is performed after a warning is issued.

By extracting the gas dissolved in the insulation oil of the main transformer and measuring the amounts of the six gas components at their low level, it is possible to detect local overheating or partial electric discharge in the unit depending on the analyzer data and to prevent any accidents before they occur.

Schedule of Preventive Maintenance Actions and Inspection of Transformer

Frequency of maintenance shall be established taking into account equipment reliability requirements and the manufacturers’ manuals and recommendations.

Maintenance activities may be planned for each segment of the installation at different periods, but major industries usually once or twice a year have a global shut-down for maintenance purposes.

NETA^[1] Standard MTS-2007 Appendix B presents the time-based maintenance schedule and matrix shown in Table 3. The application of the matrix is recognized as a guide only.

Specific condition, criticality, and reliability must be determined to correctly apply the matrix. Application of the matrix, along with the culmination of historical testing data and trending, should provide a quality electrical preventive maintenance program.

Data from monitoring sensors and units are transformed into digital and analogue signals and establish a real-time basis communication with a man-machine interface and data recording.

Dissolved gas-in-oil analysis is an effective diagnostic tool for determining problems in transformer operation.

Preventive Maintenance

Preventive measures form the backbone of transformer care. Regularly scheduled inspections ensure minor issues are resolved before they grow.

Key Activities:

Cleaning: Dust and debris can cause insulation breakdown.
Visual Inspections: Check for physical damages like cracks or oil leaks.
Oil Management: Monitor and replace oil to prevent contamination.

Recommended Frequency:
Preventive maintenance should be performed quarterly or semi-annually, depending on the environment and transformer usage.

Predictive Maintenance

Predictive techniques use data-driven insights to predict when maintenance is required.

Diagnostic Tools:

Dissolved Gas Analysis (DGA): Identifies gases produced by oil degradation.
Infrared Scanning: Detects overheating components.
Partial Discharge Testing: Monitors electrical discharges within insulation.

By adopting predictive maintenance, companies can schedule repairs strategically, avoiding unplanned outages.

Corrective Maintenance

Corrective maintenance deals with resolving faults after detection.

Common Repairs:

Rewinding Coils: Addresses winding damage due to short circuits.
Oil Reconditioning: Restores oil properties by removing contaminants.
Leak Repairs: Fixes seals and gaskets to prevent oil loss.

Challenges:
Delays in corrective actions can lead to extensive damage and costly downtime.

Inspection Checklist

Routine inspections ensure transformers operate efficiently.

Key Inspection Points:

Bushings: Look for cracks or leaks.
Cooling System: Ensure fans and radiators are functional.
Core and Windings: Check for abnormal noise or overheating.
Connections: Tighten loose bolts and terminals.

Documentation:
Maintain detailed records of inspections for tracking performance trends.

Cooling System Maintenance

Transformers generate significant heat, making cooling systems vital.

Maintenance Tips:

Inspect and clean radiators.
Ensure oil pumps and fans are operational.
Check for blockages in cooling ducts.

Proper cooling maintenance prevents overheating, which can damage insulation and windings.

Safety Protocols

Transformer maintenance involves high voltage and flammable materials, making safety paramount.

Essential Guidelines:

De-energize the transformer before maintenance.
Use appropriate personal protective equipment (PPE).
Follow lockout/tagout (LOTO) procedures.
Ensure proper grounding.

Safety training for maintenance personnel is critical to prevent accidents.

Read More : What Is an Ideal Transformer?

Common Issues and Solutions

Transformers often face challenges that affect performance.

Frequent Problems:

Overheating: Caused by poor cooling or overloading.
- Solution: Clean cooling systems, reduce load.
Oil Leaks: Result from damaged seals or gaskets.
- Solution: Replace faulty components.
Winding Damage: Arises from electrical surges or overheating.
- Solution: Conduct regular electrical tests and repairs.

Benefits of Proper Maintenance

Proper transformer maintenance offers numerous advantages:

Extended Lifespan: Reduces wear and tear.
Enhanced Efficiency: Ensures optimal performance.
Cost Savings: Minimizes expensive repairs and downtime.
Improved Safety: Prevents accidents and equipment damage.

Organizations that prioritize maintenance achieve reliable and cost-effective operations.

Transformer Tests for Maintenance & Diagnoses purposes

Read More : Transformer vs. Induction Motor

Bushing Test

For bushings that have a potential tap, both the capacitance between the top of the bushing and the bottom tap (normally called C1) and the capacitance between the tap and ground (normally called C2) are measured.

To determine bushing losses, power factor tests are also performed. C2 capacitance is much greater than C1 capacitance.

Bushings without a potential tap are normally tested from the bushing top conductor to ground.

This test results are compared with factory tests and/or prior tests to determine deterioration.

About 90% of bushing failures may be attributed to moisture ingress evidenced by an increasing power factor.

Sweep Frequency Response Analysis Test

Frequency Response Analysis (SFRA)^[6] consists of measuring the impedance of transformer windings over a wide range of frequencies and comparing the results of these measurements to a reference set.

Differences may indicate damage to the transformer, which can be investigated further using other techniques or by an internal examination. The swept frequency method for SFRA requires the use of a network analyzer to generate the signal, take the measurements and manipulate the results.

Ultrasonic and Sonic Fault Detection

This test should be applied when hydrogen is increasing markedly in the DGA.

High hydrogen generation indicates partial discharge occurring inside the transformer. Other gases such as methane, ethane, and ethylene may also be increasing. Acetylene may also be present if arcing is occurring and may also be increasing.

Vibration Analysis

Vibration analysis by itself cannot predict many faults associated with transformers, but it is another useful tool to help determine transformer condition.

Vibration can result from loose transformer core segments, loose windings, shield problems, loose parts, or bad bearings on oil cooling pumps or fans. Extreme care must be exercised in evaluating the source of vibration. Many times, a loose panel cover, door, or bolts/screws lying in control panels, or loose on the outside have been misdiagnosed as problems inside the tank.

Read More : What is Potential Transformer (PT)?

Core Insulation Resistance

To do this test, the intentional core ground must be disconnected.

This may be difficult, and some oil may have to be drained to accomplish this.

On some transformers, core grounds are brought outside through insulated bushings and are easily accessed.

Expected values of insulation resistance are:

New transformers: > 1000 MΩ
Service-aged transformer: > 100 MΩ

Values between 10 and 100 MΩ reveal possible damage of insulation between the core and the ground and values lower than 10 MΩ can origin destructive circulating currents and must be further investigated.

Infrared Thermography

Infrared thermography (IR) is a noncontact and nondestructive way to detect problems in electrical systems.

All electrical and mechanical equipment emits heat in the form of electromagnetic radiation. Infrared cameras, which are sensitive to thermal radiation, can detect and measure the temperature differences between surfaces.

Abnormal or unexpected thermal patterns can be indicative of a problem with the equipment problems that could lead to a breakdown or failure, or cause a fire.

Commonly Infrared Analysis is done each 2 or 3 years, while equipment is energized and under full load, if possible, but special functioning and environment conditions may require to conduct IR annually.

IR analysis should also be conducted after any maintenance or testing to see if connections that were broken were remade properly. Also, if IR is done during factory heat run, the results can be used as a baseline for later comparison.

The following components of transformers are usually subjected to IR analysis:

Tank
Radiators and cooling system
Bushings
OLTC

Tank

Unusually high external temperatures or unusual thermal patterns of transformer tanks indicate problems inside the transformer such as low oil level, circulating stray currents, blocked cooling, loose shields, tap changer problems, etc.

Abnormally high temperatures can damage or destroy transformer insulation and, thus, reduce life expectancy.

An IR inspection can find over-heating conditions or incorrect thermal patterns. IR scanning and analysis requires trained staff experienced in these techniques.

Radiators and Cooling System

Radiators must be examined with an IR camera and compare them with each other.

A cool radiator or segment indicates that a valve is closed or the radiator or segment is plugged.

If visual inspection shows the valves are open, the radiator or segment must be isolated, drained, and removed and the blockage cleared.

A transformer operating with reduced cooling will have its useful life is drastically shortened (an increased operating temperature of only 8 to 10^oC will reduce transformer life by one-half).

Bushings and Insulators

a) Oil level

IR scans of bushings can show low oil levels, which would call for immediate De-energizing and replacement.

In general the reason for this is that the seal in the bushing bottom has failed, leaking oil into the transformer. The top seal has probably failed, also allowing air and moisture to enter the top.

Too high oil level in bushings generally means the seal in the bottom of the bushing has failed and oil head from the conservator, or nitrogen pressure, has pushed transformer oil up the bushing.

Another reason a bushing can exhibit high oil level is the top seal leaking, allowing water to enter. The water migrates to the bushing bottom displacing the oil upward.

Over 90% of bushing failures are attributed to water entrance through the top seal.

Bushings commonly fail catastrophically, many times destroying the host transformer and nearby equipment and causing hazards to workers. Previous IR scans of the same bushing must be compared with the current scan.

b) Bushing connections

Bushings have two internal connections, one in the head and another much deeper inside connected to the transformer coils.

Both will show up externally, but the head connection will be on the upper part of the bushing while the coil connection will be at the base of the bushing.

Problems with cracks have been found in certain insulators that affect the electrical and mechanical strength of the insulator.

When surface moisture is present, a very small discharge current flows over the surface of the insulator raising the temperature by one or two degrees. When an insulator is cracked the discharge current flows down the crack and not over the surface and the insulator shows up slightly colder.

When the crack becomes severe enough a temperature increase may become evident.

**OLTC (On-Load Tap Changers)**

The temperature of the OLTC cover should be the same temperature as the transformer itself.

The source of the heat is inside the OLTC casing and is considerably hotter than the indicated temperature.

An external OLTC compartment should be no warmer than the body of the transformer. If it is warmer, it indicates probable heating of the internal tap connections.

One difficulty with tap inspections is that all taps are not connected at the time of the inspection so results may not be conclusive.

FAQs

1. Why is transformer maintenance important?
Transformer maintenance prevents failures, enhances efficiency, and extends equipment lifespan.

2. What is the difference between preventive and predictive maintenance?
Preventive maintenance involves routine inspections, while predictive maintenance uses diagnostic tools to forecast potential issues.

3. How often should transformer oil be tested?
Oil should be tested annually, or more frequently for critical transformers.

4. What are common causes of transformer overheating?
Overheating is often due to overloading, inadequate cooling, or poor insulation.