This month, I’m sharing my insights based on years of inspection experience and my evaluation of transformers along with their most common faults, their causes, and their impact.
Even with new technology, improved materials, and increased factor of safety built into the manufacture of transformers, their failure rate has remained largely unchanged. However, the cost of new transformers has doubled and tripled, with a similar increase in the rewind cost.
In considering the cost of repair and replacement of transformers, the need for preventive diagnostic testing of transformers is clear.
Transformers’ breakdowns are primarily the result of one or more of the following:
- Insulation breakdown
- Core failure
- Protection system failure
- Bushing failure
- Tap changer failure
- Cooling system failure
- High Operating Temperature
- Lightning or Line Surges
- Overloading
Many failures result from lightening and line surges, which are often caused by a lack of surge protection or inadequate testing of the existing surge protection. The other main factor is insulation breakdown resulting from poor maintenance and operating conditions.
Through proper maintenance and electrical testing programs, the life of transformers can be extended and, in many cases, failures can be prevented. Also, contingency plans can be put in place to repair/replace the transformer in a timely manner. This reduces the possibility of sudden breakdown which can cause extended property damage and loss of business.
There are several methods of electrical testing, and the scope of testing depends on the size of the transformer, voltages, application, and if the transformer is critical for the continuation of plant operations.
Testing procedures include:
- Overall Power Factor test – Measures the leakage/loss current of electrical insulation (Dielectric loss)
- Exciting Current test – Detects the condition of insulation integrity, developing core defects, connections faults within the transformer.
- Turns-Ratio (TTR) – Low voltage test that determines the number of turns in one winding in relation to the number of turns in the other windings of the same phase of the transformer. It establishes shorted turns.
- Sweep Frequency Response Analysis (SFRA) – Diagnostic test used to assess the mechanical integrity of a power transformer. It verifies the designed characteristic of the transformer such as displacement of transformer windings from their position. This is a comparative test and useful if baseline results are available and is especially useful if the transformer has been moved and transported.
- Dielectric Absorption Testing (Polarization Index) – Time-resistance method of testing the insulation resistance and voltage which is applied to the equipment over a period, usually ten minutes. The ratio of the ten-minute reading to the one-minute reading is called the polarization index and is used as a guide to the condition of insulation. The acceptable value should be > 1.
- Dissolved gas analysis (DGA) – Detects the presence of gases within the transformer. (Described below)
- Furan analysis – Establishes insulation deterioration. (Described below)
- Presence of corrosive Sulphur – Completed for large transformers. Sulfur and some sulfur-containing compounds in oil will react with metals in the transformer, particularly copper and silver, which leads to corrosion of conductor, connections, and soldered or braised joints.
- Particulate metals analysis in oil – Dissolved copper and other metals act as catalysts to promote oxidation and to elevate liquid power factor to unacceptable levels. Dissolved metals, in sufficient quantity, that promote aging of the oil can be removed by reclamation.
- DGA – The majority of main power transformers are oil filled type. The dielectric fluid used in transformers is quite stable, and the condition can be easily monitored by taking periodic oil samples and performing ASTM-approved tests such as dissolved gas analysis (DGA). By detecting the presence of combustible gasses, the presence of each gas identifies a developing fault inside the transformer. Detection and taking corrective actions in a timely manner can prevent a sudden failure. Other than combustible gasses, the impurities and decomposition products found in the oil – such as acid, sludge and moisture – can be removed by centrifuging the oil and other means.
- Furan analysis – Verifies the integrity of insulation. Though a very important test, it’s often not completed. The solid insulation on the windings is usually a cellulose-based material due to its excellent insulation. Cellulose is thermodynamically unstable, however, and deterioration could occur even at ambient temperatures. The rate of deterioration increases dramatically with increasing temperature, therefore shortening the life of the transformers. In addition, the presence of moisture and oxidation in the oil impregnates the cellulose leads and decreases the mechanical strength and eventual breakdown of the cellulose. Furan analysis is highly recommended to evaluate the condition of cellulose insulation with an oil sample. Essentially, an aromatic compound is produced during this degradation, called furan, whose results are used to determine the average expected degree of polymerization for the paper. The calculated degree of polymerization is then used to estimate the percentage of solid insulation life remaining inside the transformer.
The solid insulation in a transformer is made up of kraft paper (Cellulose). New kraft paper has an average cellulose polymer chain that is 1,000 glucose molecules to 1,200 glucose molecules long, and over time, a natural and steady breakdown of the polymer chains occurs, reducing the mechanical strength of the paper.
The degree of paper polymerization has a direct correlation to the paper’s tensile strength, and when the degree of polymerization has fallen to around 200, any stress to the weakened paper will lead to failure. When the cellulose chain splits and two shorter chains are formed, the breakdown process forces one or more of the glucose molecules out. The breakdown also creates water, carbon monoxide, and carbon dioxide. During this event, the glucose molecule chemically changes and creates a compound containing a furan ring – furans are measured in parts per billion. Thus, the life of the paper insulation is typically the life of the transformer.