General

Many developing countries experience either a lack of dependable power or suffer through countless periods of blackouts. The reasons vary from vandalism to theft of insulating oil, but stem most often from poor or non-existent maintenance and security of the existing power grid. The consequence is often the complete destruction of transformers. A comprehensive maintenance and security program for transformers can prevent this.

The availability of dependable electric power is necessary for economic development. The Allen Group devised a plan for a country in Africa that would allow the National Electric Authority to improve their maintenance procedures. It includes the design of a comprehensive, centrally-monitored security and data transmission system. The objective was to clearly show the cost-effectiveness and return on investment (ROI), a direct consequence of implementation of the plan.

This article is the first in a series that will outline the approach any power provider can take to maximize transformer useful service life. Sustainable power is now changing lives and changing countries in Africa.

The Plan

The master plan covers the following subjects:

• Diagnostic Methods
• Transformer Maintenance
• Security
• Remote Data Monitoring

Diagnostic Methods

Diagnostic methods are either reactive or preventive. In practice, transformer maintenance procedures involve a combination of both.

The diagnostic part of this master plan has three elements:

1. Obtaining the basic diagnostic equipment and acquiring the skills to operate it
2. Obtaining baseline operating characteristics (signature) of every transformer in the grid
3. Determining the schedule for periodic re-testing and monitoring

The Basic Equipment Defined

Infrared Thermography

A comprehensive infrared thermography scan shows areas of unusually high temperature, which could present a danger of fire or explosions. A scan should be performed at least once a year for every transformer. A typical scan is shown below. The bright yellow indicates a hot spot that requires attention.

Image courtesy of Fluke

Dielectric Tester

A dielectric tester indicates the insulating strength of the transformer oil. A low dielectric reading indicates a decrease in insulating strength and an accumulation of contaminants such as moisture. A typical portable tester is shown below.

Image courtesy of Hipotronics

Moisture Detection with a Moisture Analyzer

Moisture is one of the most harmful contaminants in transformer insulating oil. If the oil is sampled for moisture at all, the resulting value is often meaningless because it does not reflect the relative saturation of the moisture in the oil at the time the sample was taken. The relative saturation must be recorded along with the oil temperature.

Typical moisture meters are shown below. The instrument measures the ppm of the water in the oil, relative saturation, and temperature of the oil or the dew point and temperature of the gas. It takes only 2 to 3 minutes to attach the adapters and just a few more minutes to obtain a stable reading once oil or gas is flowing.

Images courtesy Doble Engineering

Fault Gas Detection with Dissolved Gas Analyzers

Several organic gases are generated in transformer insulating oil. The types, quantities, and distribution of the gases are indicative of the internal faults. A valuable tool to detect these gases is the dissolved gas analyzer. Depending on results, readings should be taken at least twice a year. More frequent analysis is needed if the results indicate it. A typical permanently installed dissolved gas analyzer is shown below.

Image courtesy of Serveron Co.

The Serveron Transformer Monitor offers accurate and repeatable measurements of eight critical fault gases and other key parameters.

Acidity Detection with a Test Kit

Moisture reacts with the sulfur in oil and creates sulfuric acid. The indication is a rise in the total acid number (TAN). Acid will promote sludge formation and will ultimately destroy the cellulose insulation.

A simple test will yield the level of acidity.

Image courtesy of Dexsil Co.

A Transformer’s Signature

Any diagnostic program must begin by establishing the current state of the transformers in the grid. Data collected on each transformer will provide a baseline reference from which future trends can be determined. Such data includes:

• Dissolved gas analysis
• Oil power factor
• Insulation resistance
• Insulation power factor
• FRA
• Winding resistance
• Turns ratio

Preventive measures can then be taken before major damage occurs.

Determining the Schedule of Diagnostic Testing

From the results of the baseline data, a schedule can be put together for repeat testing. The transformers that indicated borderline problems in any of the measured parameters must be tested more frequently until the problem has been corrected by any of the maintenance procedures.

In my next article, I will describe transformer maintenance methods that will extend a transformer’s useful service life to between 40 to 50 years.

Further Reading

Water Activity in Oil white paper

The Aging of Cellulose white paper