How Transformer Bushings Work: Tipi, Failure Causes and Maintenance Guide

A transformer is designed to operate reliably for decades, but some components require more attention than others. Among all transformer accessories, transformer bushings​ are among the most critical components because they provide the connections between the internal windings and the external power network.

Although a bushing looks like a simple insulating component, it actually performs several important functions:

• Carrying high-voltage current safely;
• Preventing electrical breakdown between conductor and transformer tank;
• Maintaining insulation performance under temperature, umidità, and pollution conditions;
• Ensuring reliable operation of the transformer throughout its service life.

A damaged transformer bushing can lead to partial discharge, insulation failure, oil leakage, and even unexpected transformer shutdown.

For power utilities, impianti industriali, e progetti di energia rinnovabile, understanding transformer bushings is essential for proper selection and maintenance.

1. Cos'è una boccola del trasformatore?

A transformer bushing is an insulated device installed on the transformer tank that allows an electrical conductor to pass safely through the grounded enclosure.

Inside an oil immersed transformer, the winding is surrounded by transformer oil for insulation and cooling. Tuttavia, the electrical connection must extend outside the transformer to connect with cables, sbarre, or switchgear.

The transformer bushing creates a safe transition point:

Transformer winding → Internal conductor → Transformer bushing → External power connection

Without a suitable bushing, the high-voltage conductor would create excessive electrical stress and could cause insulation failure.

2. How Does a Transformer Bushing Work?

The main challenge of a transformer bushing is controlling the electric field around the high-voltage conductor.

High voltage creates strong electrical stress. If the stress is concentrated in one area, the insulation material may deteriorate over time.

Modern transformer bushings use multiple insulation layers and grading structures to distribute the electric field evenly.

The main components include:

A high-quality bushing must maintain stable insulation performance during:

• Normal operation;
• Short circuit conditions;
• Temperature changes;
• Environmental exposure.

3. Main Types of Transformer Bushings

Different transformer applications require different bushing designs.

3.1 Oil Impregnated Paper (OIP) Boccole

Oil Impregnated Paper (OIP) bushings are widely used in traditional power transformers.

The insulation system consists of:

• Special insulating paper;
• Conductive grading layers;
• Olio per trasformatori.

Vantaggi:

• Proven technology;
• Lunga storia operativa;
• Suitable for medium and high voltage transformers.

OIP bushings are commonly used in:

• Trasformatori di distribuzione;
• Trasformatori di potenza;
• Substation equipment.

Tuttavia, they require regular inspection because oil aging and moisture can reduce insulation performance.

3.2 Resin Impregnated Paper (RIP) Boccole

RIP bushings use epoxy resin instead of oil as the main insulation material.

Vantaggi:

• Nessun rischio di perdite d'olio;
• Lower maintenance requirement;
• Better environmental performance.

They are increasingly used in:

• Renewable energy substations;
• Modern power networks;
• High-voltage applications.

3.3 Porcelain vs Composite Bushings

The external insulation material is another important consideration.

Composite bushings are becoming popular in coastal areas and regions with heavy pollution because of their excellent resistance to environmental conditions.

4. Common Transformer Bushing Failure Causes

Although transformer bushings are designed for long service life, several factors can cause failure.

4.1 Ingresso di umidità

Moisture is one of the biggest threats to transformer insulation.

Cause comuni:

• Aging seals;
• Damaged gasket;
• Improper storage;
• Poor installation.

Moisture reduces dielectric strength and accelerates insulation aging.

4.2 Invecchiamento dell'isolamento

After years of operation, insulation materials naturally deteriorate.

Typical signs include:

• Increased dielectric loss;
• Reduced insulation resistance;
• Partial discharge activity.

Regular testing helps identify problems before failure occurs.

4.3 Oil Leakage

Oil leakage is one of the easiest problems to identify visually.

Possible reasons:

• Aging sealing gasket;
• Loose flange connection;
• Temperature expansion;
• Mechanical damage.

Even minor oil leakage should be inspected because it may allow moisture to enter the insulation system.

4.4 Surriscaldamento

Excessive temperature can damage the bushing insulation.

Possible causes:

• Loose electrical connections;
• High contact resistance;
• Transformer overload.

Long-term overheating significantly reduces bushing lifespan.

5. How to Select the Right Transformer Bushings?

When purchasing or replacing transformer bushings, engineers should consider several parameters.

1. Tensione nominale

The bushing voltage rating must match the transformer system voltage.

Esempio:

• 11kV transformer → 12kV class bushing
• 33kV transformer → 36kV class bushing

2. Corrente nominale

The bushing current rating must be higher than the transformer operating current.

Incorrect selection may cause overheating.

3. Dimensioni di installazione

Before ordering replacement bushings, always confirm:

• Mounting hole dimensions;
• Flange size;
• Conductor connection type;
• Overall height;
• Distanza di dispersione.

Many replacement problems occur because the electrical parameters are correct but the mechanical dimensions are different.

4. Applicable Standards

Common transformer bushing standards include:

• CEI 60137: Insulated bushings for AC voltages above 1000V;
• IEEE standards for power equipment applications.

6. Transformer Bushings Maintenance Checklist

Regular inspection can significantly extend transformer service life.

7. Common Questions About Transformer Bushings

How long does a transformer bushing last?

A high-quality transformer bushing can typically operate for 20–30 years or longer under proper maintenance.

What causes transformer bushing failure?

The main causes include moisture, insulation aging, surriscaldamento, oil leakage, and mechanical damage.

Can a transformer bushing be replaced?

SÌ. Replacement is possible if the new bushing matches:

• Valutazione della tensione;
• Current rating;
• Mechanical dimensions;
• Installation requirements.

Why do transformer bushings crack?

Cracking may result from mechanical stress, temperature changes, aging, or external impact.

Manufacturer Insight

As a transformer manufacturer and transformer accessories supplier, PDDN understands that reliable transformer operation depends not only on the transformer body but also on every connected component.

For transformer bushing replacement or OEM requirements, customers should provide:

• Transformer rated voltage;
• Current rating;
• Installation drawing;
• Existing bushing model;
• Required standard.

This information helps ensure electrical compatibility and reliable installation.

Informazioni su PDDN

PDDN è la piattaforma online diLuoyang Datang Energy Technology Co., Ltd., un produttore affidabile e fornitore globale ditrasformatori di potenza, accessori per trasformatori, componenti del quadro, sistemi di accumulo dell’energia a batteria (BESS), e soluzioni di ricarica per veicoli elettrici. Da 2015, abbiamo supportato le società di servizi pubblici, Appaltatori EPC, e clienti industriali in tutto il mondo con prodotti affidabili, Servizi OEM/ODM, e competenza tecnica.