Power Transformer Maintenance

Introduction to Power Transformer

Power transformer is a main component of the power generation station. They are generator step up transformer, station service transformer, unit auxiliary transformer, power distribution transformer and many more.Range from low voltage transformer 400 Volt, medium voltage transfomer 16 kV, High voltage transformer 150 kV, Extra High voltage transformer 500 kV, etc.

Power Transformer component,failure mode, preventive, and predictive maintenance.

1. Transformer LV & HV Bushing

Transformer porcelain bushing is used to take the high voltage and low voltage down-lead within the transformer out of the oil tank. It is not only the insulation to the earth for the down-lead but also a supporting and fixing part. Therefore, the transformer hollow insulator must meet the electrical and mechnical strength shown in the manufacturing standard.

Middle voltage transformer can use ceramic/porcelain bushing insulator; high voltage transformer normally uses oil immersed porcelain bushing and capacitive ceramic bushing insulator.

Failure Modes

It has long been recognized that bushing failures represent a large, if not the greatest, source of transformer and breaker failures. The bushings of the greatest concern are those greater than 25kV, which are of the condenser or capacitive type. These bushings are wound with capacitive layers which distribute the electric field strength within the bushing equalizing it from top to bottom. These capacitive layers are sealed under oil to protect against moisture and act as a dielectric and coolant. There are just a few failure modes typical to condenser bushings.

Porcelain damage
Chips or cracks from vandalism may not cause immediate failure but result in eventual failure due to leakage current tracking over the sheds or through the porcelain.

Pollution Contamination
Usually affecting all bushings equally, heavy contamination can result in creep or tracking over the porcelain sheds and eventually, flashover.

Failure within capacitive layers
The most common bushing failure occurs within these capacitive layers as the insulation weakens and current leaks through. This current causes further weakening which results in greater leakage current and the bushing
eventually fails.


Gasket leakage For Oil filled and oil immersed Bushing

 Gasket could be nitrile material and for aged bushing the gasket britle. Moisture can enter the bushing when gasket leak and may cause low dielectric strength of the oil.

The good news is that these failures usually occurslowly over time, allowing for the failure to be captured and the bushing replaced prior to the destruction of the transformer. Costeffective on-line bushing monitoring has finally arrived which can be implemented to protect historically the most common failure of power transformers for less than 1% of the transformer purchase price 


How to keep bushing reliable ;
- Perform routine test when the transformer deenergized. The test include Tan delta test on C1 and C2 (EHV bushing). C1 & C2 value can be found on the bushing name plate. The value is as a standart of the measurement. Tan delta result indicates wheter the bushing surface cracked or not,and  dielectric strength in the inner conductor insulation.
- Inspect oil level at oil level indicator
- Perform bushing cleaning periodically when the transformer deenergized. I used Sakaphen cleaning pasta for cleaning for best result both cleaness and tan delta.
- Perform thermography monitoring periodically,at least once a month. By this manner you can see leakage current through bushing insulator surface and lead connection condition.

- When you have spare money, install bushing on line monitoring system that measures bushing power factor. This is more practically and no need transformer deenergized when you monitor. Instalation is very easy. Deenergized one time then Plug the receiver socket onto bushing tap and connect to the analyzer monitor.


Complete failure mode descibed on following table ;

 

2. High Voltage & Low Voltage Winding 

The windings belong to the active part of a transformer, and their function is to carry current. The windings are arranged as cylindrical shells around the core limb, where each strand is wrapped with insulation paper. Copper is today the primary choice as winding material. In addition to dielectric stresses and thermal requirements the windings have to withstand mechanical forces that may cause windings replacement. Such forces can appear during short circuits, lightning, short circuits in the net or during a movement of the transformer. A fault in the windings can occur due to material faults in the cellulose isolation,

The solid insulation in a transformer is cellulose based products such as press board and paper. Its function is to provide dielectric and mechanical isolation to the windings. Cellulose consists of long chains of glucose rings. When degradation of the cellulose occurs these chains get shorter. Degree of polymerization (DP) is the average number of these rings in the chain and indicates the condition of the paper. New paper has an average DP number of 1200-1400. A DP less than 200 means that the paper has a poor mechanical strength and may no longer withstand short circuit and other mechanical forces that appear. The solid insulation is the weakest link in the transformer insulation system, due to the degradation of the cellulose is irreversible and it is often not economically defensible to replace it. The aging of cellulose is accelerated by water, oxygen and heat. An analysis of the paper can tell the moisture content and the DP number of the paper insulation. One method of maintenance like degassing/oil filtering process of the oil also reduces the gas-moisture content in the paper as a new equilibrium between the gas-moisture in the oil and in the paper is reached. Remember, Comparison number 1:100. That means 1 ppm water content in the oil, 100 ppm water content in the cellulose insulation.

How to manage above Phenomena ;

During transformer operation ;

-    - Perform insulating oil monitoring such as ; water content, acid number, dissolved gas analysis (DGA).The  interval of sampling as manufacturer recommendation, but we do this every 6 month. If combustible gas rate high, shorten the interval for example every 3 months.

-      -  Perform top oil tank temperature and winding temperature by transformer on-line monitoring system (We used Early Warning System, General Electric). Because oil and winding temperature data will help full for analyzing combustible gas occurred.

During Shutdown :

-       Perform winding diagnostic test. They are ; Tan delta test, Megger, Rdc Measurement for each step OLTC, winding ratio test.

-         Perform Sweep frequency analysis test to obtain winding – core signature.

The next action ;

- Finding ; combustible gas rate always high for 3 samplings with 3 month interval that means you have serius problem with your transformer insulation. Analyze the cause; overloading, thermal hot spot, low energy discharge, spark, or arching.You can analyse with some methodes,Roger chart, key gas ratio, or Duval triangle. And then Plan to buy new transformer because repairing transformer required more time and you will loose longer time to produce electric power rather than replace with new one.

- Finding ; Water content high i.e >30 ppm for my experience and accumulated combustible gas exceeds IEC standart, you have to make oil filtering/degassing as soon as possible. Perform degassing until you find <10 ppm moisture in oil & combustible gas in traceable value.
- Finding ; Oil characteristic is bad i.e. high acidity, bad color you have to make oil purifying with degassing & clay treatment.

- Finding ; The result of winding diagnostic shows poor measurement value, contact transformer vendor or plan to buy new transformer.I don’t want to play a game with bad insulation winding.