Assessment of Transformer Condition

Introduction

Power transformers are efficient and reliable pieces of equipment that typically have a lifespan ranging from 30 to 50 years, or even longer, under normal operating conditions. Their longevity largely depends on factors such as design and loading.

The transformer is the costliest and most critical component in power plants and utilities, playing a pivotal role in the transmission and distribution of electrical energy. Common failure modes in a transformer include dielectric failures, which can be caused by partial discharge, internal arcing, tracking, static electrification, and various other factors. Another frequent issue is tap changer failures, often resulting from contact arcing or coking that occurs over time. Furthermore, transformers can fail due to overloads, through-faults, over-voltage conditions, and over-fluxing, all of which can stress the transformer's components beyond their limits. These failures are not merely technical issues; they can lead to significant outages, disrupting the supply of electricity and potentially causing extensive downtime and financial losses. Therefore, maintaining the health of transformers is essential for the reliable operation of power systems.

The failures of power transformers can be classified conventionally as

• Slowly developing faults in power transformers are often attributed to the gradual deterioration of insulation due to aging. Over time, the insulation materials within the transformer can degrade, leading to various defects that compromise the transformer's performance and reliability. This aging process can result in a host of issues, such as decreased dielectric strength, increased vulnerability to partial discharges, and a heightened risk of internal arcing. Addressing these slowly developing faults is crucial for maintaining the overall health and longevity of power transformers, as neglected insulation defects can escalate into more severe problems, potentially causing significant disruptions and financial losses.

and

• Random faults that occur suddenly in the active part of the transformer and develop rapidly, often within a matter of days or even hours, can pose significant challenges. These faults can arise without warning and escalate quickly, potentially leading to severe damage and operational disruptions. The swift progression of these faults necessitates immediate attention and prompt remedial actions to mitigate the impact on the transformer's performance and reliability.

Insulation paper

Each transformer winding needs insulation between turns and coils, typically using cellulosic kraft paper in power transformers.

As the transformer ages, the mechanical strength of the paper diminishes, impacting the rigidity of the winding. The primary failure scenario involves winding movement due to high mechanical stresses during short circuits and potentially multiple inrush currents.

Transformer End Of Life (EOL) depends not only on the solid insulation (paper) EOL but also on most transformer components that can be replaced (bushings, OLTC, etc.) if it is economical. However, replacing solid insulation (paper) is a significant task that is, in most cases, neither technically nor economically feasible. Therefore, transformer paper EOL is often considered as transformer EOL.

Insulating paper ageing

Assessing transformer paper ageing is essential.

• During the life of a transformer, we achieve a more realistic monitoring of the state of insulation, a superior transformer health assessment ranking (replacement), and a confident validation or rejection of refurbishment decisions.
• During post-mortem investigations, we gain a clear understanding of the ageing state of decommissioned units and can confidently predict end-of-life scenarios for entire families of transformers.

There are several factors that influence paper aging, including oil acidity, oxygen, moisture, temperature, and both electrical and mechanical stresses. To understand the mechanisms of degradation, it's important to consider these factors specifically.

• Hydrolysis

At the start of a transformer's life, the kraft insulation typically contains less than 0.5% water, and the oil used within the transformer is thoroughly dried to ensure optimal performance and longevity. This initial state is crucial for the efficient functioning of the transformer. However, over the course of its operational lifetime, the water content within the transformer's paper insulation can gradually increase, potentially reaching up to 5% as documented by Fallou in 1970. This gradual absorption of moisture can have detrimental effects.

The presence of water within the insulation is not merely a minor inconvenience; it significantly accelerates the degradation of cellulose, which is a key component of the transformer's insulation system. Extensive research has demonstrated that paper with an initial water content of 4% degrades at a rate that is 20 times faster than paper containing only 0.5% water. This stark difference underscores the critical importance of maintaining low moisture levels.