4 common defects of liquid immersed transformers
In this post we present some of the common defects appearing in liquid immersed transformers. As stated at the end of this article always adhere to all applicable safety rules and regulations.
Leaks of insulating liquid
Despite high quality standards and best practices a leakage of insulting liquid belongs to common issues. In most cases the leak is small so that it can be detected early enough before larger amount of insulating liquid is lost. Due to environmental concerns there shall be a system collecting the insulating liquid in order to avoid contamination of soil etc.
Leaks at threaded joints must not be allowed to continue for too long otherwise the gasket material may be destroyed. The joint shall be tightened first, and if this brings no improvement, the gasket shall be replaced.
→ It is recommended to always have a spare set of gaskets available on stock.
Other leaks, e.g. in weld seams, should be reported back to the manufacturer or nearest service shop (make sure to state the size and position of the leak in order to speed up the process).
Dysfunction of dehydrating breather
Dehydrating breather shall prevent air moisture from contaminating the insulating liquid. Conventional dehydrating breathers contain silica gel or similar substances that absorb moisture and air humidity. The silica gel needs to be changed after some time. A rule of thumb recommends to replace the silica gel when approx. 2/3 of its volume has changed the color.
The fact that the dehydrating breather is not working properly may be noticed if there are no air bubbles visible in the oil seal. It is recommended to tighten loose bolts to the conservator or breather and replace the gaskets if necessary.
Too high oil temperature
Too high oil temperature may trigger an alarm. There are several possible root causes for oil temperature exceeding the alarm level.
(a) Ambient temperature exceeds the technical specification that the transformer was designed accordingly (e.g. exposure to direct sunlight).
(b) There is an issue with the cooling system. The transformer may be installed too close to a wall or an object preventing adequate air circulation. In case of forced cooling the cooling devices shall be checked.
(c) Transformer is overloaded / operates outside of design load.
(d) Temperature monitoring device has an error (issue with the thermometer).
(e) Other reason.
Check to ensure that the transformer is not overloaded, eventually measure the currents. If multiple transformers operate in parallel check the parallel operation requirements carefully. Inspect the transformer cooling.
Activation of Buchholz relay
Take a gas sample of the air which has accumulated in the Buchholz relay (do not release any gas without taking a sample first!). Analyse the sample with a gas tester or contact the manufacture or a certified service provider.
— The presence of combustible gases indicates transformer internal damage due to local overheating.
— The presence of air suggests oil loss from the conservator or incomplete venting of the transformer.
— No gas at all points to a possible malfunction of the alarm float and contacts.
If the trip contacts operate: It means that an electric flashover inside the oil has caused a damage and resulted in the accumulation of gas in the Buchholz relay. Check the insulation resistance, conductor resistance and voltage ratio of the transformer.
Disclaimer
This article provides general information and does not replace safety instructions, internal guidelines or any kind of rules and regulations applicable to you or your company. When troubleshooting a transformer issue always refer to the Operation and Maintenance (O&M) manual of your product. If you have any doubts contact the service department of the manufacturer. Follow all safety rules and regulations with the highest priority.
[1] ABB User’s Manual: Operation and Maintenance for Power Transformers, https://www.hitachiabb-powergrids.com/offering/product-and-system/transformers
[2] Trafo Union: Oil immersed distribution transformers – Operating instructions
