Transformer handling and transport

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• Damages that may arise and how to identify them

Severe damage may be caused to transformers when they are subjected to challenging transport conditions such as troubled during sea transportation, or a rough road during road transportation. Although it is known that shipping under these conditions will cause vibrations and small shocks in packages and equipment, it is also known that if the packaging and fixing of the material are not suitably performed, the amplitude of those vibrations and shocks can cause non-visible damages which can further lead to a failure of the equipment.

It must be emphasized that this paper does not intend to be an in-depth document on these problems because they are generally known; however, as it is unfortunately still a frequent scenario that transformers arrive at the site with visible or concealed damages caused by improper packing, handling or transportation procedures; the article sets out to reiterate these issues and raise awareness of the procedures aimed at preventing them from happening.

For this reason, it cannot be stressed enough how important it is to draw the attention of manufacturers, logistics and transportation companies, contractors, and transformer owners to this issue and the respective consequences. To prevent the causes that lead to damage and destruction, it is necessary to perform some special tests during Factory Acceptance Tests (FAT). Such as those defined in IEC Standard 60076-1 [3]. IEEE C57.12.90 [4]. IEEE C57.152 [5] and IEC 60137 [6], and then repeat them during Site Acceptance Tests (SAT) whenever the conditions of handling and transportation indicate any potential damage.

Recommended special tests for this purpose include the following:

• Measurement of frequency response analysis (SFRA)
• Measurement of 8 (dielectric losses) of the bushings
• Determination of capacitances between windings and windings to earth
• Measurement of DC insulation resistance between windings and windings to earth
• Measurement of DC insulation resistance core to earth and core frame to earth
• Single-phase excitation current tests
• Magnetic balancing tests

Transformer handling and transportation may result in internal damages to the transformer and its components that sometimes may even lead to transformer failure

The test results obtained during FAT and SAT must be compared, and conclusions about possible damage to the transformers must be drawn. Although a reference has been made to both IEC and IEEE standards, IEC standards make the basis for this article considering they are international standards used worldwide; compared to IEEE standards which are U.S. standards used only in a few countries.

• Problems and damages

Transformers with high rated power and for voltages above 123 kV are usually transported without the oil (their tank being filled with nitrogen or dry air), and without the bushings, the conservator and the cooling equipment. Mechanical shocks above design limits or about 3 g (g standing for the gravitational acceleration of approx. 9.8 m/s2, and 3 g being the force equivalent to three times the gravitational acceleration) may cause visible and/or concealed damages to transformers such as the following:

• Visible damages include, but are not limited to, scratches in the surface protective coating and finishing of the tank, whether they are just hot-dip galvanization or painting, which sooner or later will lead to corrosion (see Figure 1), leaks of nitrogen, and external cracks and chips, even contamination, in the bushings.
• Common concealed damages inside the transformer, which can negatively impact the reliability of the transformer and whose consequences may become apparent only after an indefinite time

upon energizing, include:

• Geometrical distortion of the winding/ core. Due to the movement of the active part, the insulation between the turns can be abraded, causing a short circuit and damage to the windings that may occur later during operation;
• Loss of coil clamping pressure. Mechanical vibrations may cause the windings to lose their clamping pressure, eventually leading to the collapse of the windings during electrical faults
• Contamination of the oil (resulting from the degradation of windings insulation);
• A safe clearance between the tank and the active part may be compromised;
• The unintentional grounding of core or core frame that can cause gassing during operation.

Apart from physical damage, incorrect packing and specific transportation procedures can cause other types of damage, namely the contamination of oil or the insulation of the windings with water, moisture, dust, and other contaminants. These contaminations will result in premature aging of insulation materials, meaning that their dielectric strength will be reduced with the corresponding decrease of the useful life of the transformer and/or severe failures.

In order to investigate whether transformers have been subjected to excessive mechanical impacts, it is recommended to use impact (or shock) recorders during
transformer transportation to evaluate the magnitude of these mechanical shocks (Figure 2).

• Tests for detecting mechanical damages

IEC standard 60076-1 [3] defines special tests that can be used to identify potential damages resulting from excessive mechanical impacts or some contamination. These are the following:

• Measurement of frequency response analysis (SFRA)
• Measurement of tan (dielectric losses) of the bushings
• Determination of capacitances windings to earth and between windings
• Determination of DC insulation resistance between windings and windings to earth
• This standard also defines testing procedures and acceptance criteria.

To evaluate the possibility of internal cracks or contamination in transformer bushings, tan six tests must be performed to detect the dielectric losses of the insulation material, also known as the dissipation factor. However, while the current passing through an ideal insulator is capacitive (JC), real insulators do not have 100 % purity. This means that the current passing through the insulation also has a resistive component (IR), indicating that the insulation has losses which are represented by an 8, c5 being the angle shown in Figure 3.

Resistive current results from impurities or damages in the insulator and the dielectric strength is inversely proportional
to this current.

Hence, with the increase of it, the losses will increase as well, meaning that the dielectric strength of the insulator will decrease.

 

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