Energized and De-Energized Hot Oil Cleaning of Transformers

Reliability and efficiency of modern electrical grids depends on power transformers. Transformer failures are down time can be very costly. All things considered (the cost of a new transformer, disposal of the failed transformer, transportation, installation etc), the expenses may run into millions of dollars.

The primary cause of transformer failure is damaged insulation, which is the weakest link in this equipment. The insulation system of modern transformer is comprised of two components: liquid insulation (transformer oil) and solid insulation (cellulose).

Aging of the paper insulation depends on the following factors:

• long influence of elevated temperatures;
• the environment (oil, air etc);
• the influence of moisture, oxygen and the products of oil and cellulose oxidation.

In this article we look into the removal of oxidation products, and hot oil cleaning of transformers in particular.

Transformer oil change

The oil in the transformer must comply with certain standards. Its quality gradually drops during operation under the influence of various factors (high temperature, moisture, oxygen etc) to the point when the decision must be made to change the oil entirely or regenerate it for continued use.

A regular oil change usually cannot solve all the problems. Even when old oil is drained thoroughly from the transformer, some old oil remains in the cellulose insulation and as a film on most of the winding surfaces and the internal surface of the transformer tank. This oil contaminates new oil and drastically reduce its quality. For instance, a -/14/12 (ISO 4406) oil rapidly degrades to -/16/13. This is because the new oil is contaminated by particulate matter and oxidation products from the cellulose. Therefore, the optimal process must include not only filling the transformer with new or regenerated oil, but also cleaning of transformer solid insulation.

GlobeCore suggests a modern approach to transformer lifetime extension, which includes both regeneration of oil and circulation of hot oil through the transformer with continuous oil regeneration. The circulation of hot oil in the transformer cleans the cellulose insulation and improves transformer reliability. These processes are performed by the CMM-R type regeneration units.

Hot oil cleaning of transformers

Hot oil cleaning of transformers is performed as follows. The oil is heated to a temperature when the sediment accumulated in the windings can be dissolved, normally about 80°С. The oil is taken from the bottom of the transformer into the CMM-R unit, where it is degassed by heat and vacuum, filtered and regenerated by special Fuller’s earth sorbent. From there, the oil is supplied to the top of the transformer. The oil circulates in this manner, dissolving the sediment from the windings. The CMM-R unit can reactivate saturated adsorbent automatically when this becomes necessary. After reactivation, Fuller’s earth can be used again, as many as 300 times, the equivalent of 1.5-2 years of operation.

The GlobeCore hot oil cleaning process can be applied to both energized and offline transformers. If the transformer is offline, there is no vibrations in the windings, which reduces the efficiency of sediment solution; in this case the cleaning process takes longer.

Safety

Before performing hot oil cleaning of transformer, the operator must take oil samples to determine if online (energized) operation is safe, making sure that gas content is below threshold and the transformer is not in critical condition.

The process requires the use of the TSS (Transformer Safety System). The system is equipped with oil level sensors. The operator sets the critical levels, and if the level of oil exceeds those levels, the system activates automatic valves, closing off the top and bottom gates, cutting off the transformer from the unit and preventing potentially dangerous situations.

The benefits of hot oil cleaning of transformers

GlobeCore hot oil cleaning of transformers offers the following benefits:

• simultaneous oil regeneration and removal of sediment from transformer windings;
• improved transformer reliability;
• reduced transformer downtime;
• decreased transformer maintenance costs and the costs of insulation oil disposal;
• 20-25 year extension of transformer life.