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The power transformer is the most expensive equipment in a transmission grid substation and it does not require any exaggeration to understand the importance of operating and maintaining it properly for a longer service and for a reliable power supply. There are several theoretical formulae developed by various standard groups to calculate the life span of a power transformer. However in order to predict an accurate life span of a transformer, it should be operated under certain loading conditions which will never be the case when it comes to commercial operation. Whatever the standard used, if the transformer is loaded continuously on its rated value with a constant ambient temperature (which will never be the case) the average theoretical life span of a power transformer is about 20-25 years.
Since the utilities never operate the power transformers at its rated load continuously and the ambient temperature varies throughout the year, the average life span of the transformer in practice would be more than 25 years; may be around 35-40 years (with no major overhauls) in countries like Sri Lanka. This figure is much more in colder countries like Canada, Norway and Russia where it is around 50 years or even more.
Recently there were a couple of transformer failures that occurred in Biyagama and Kotugoda substations. These power transformers were installed with the introduction of Mahaweli Hydroelectric project; hence the transformers are more than 30 years old now. The entire fleet of transformers installed during that era is operating in the fourth quarter of their life, hence need replacement plans or major overhauls for the future. This includes the transformers in generating stations like Victoria, Kotmale and Randenigala too. It would be advisable to analyze the condition and the remaining life of these transformers thoroughly, if there are no plans in place for replacing them in next five to ten years.
Although the power transformers can serve up to 35-40 years in the Sri Lankan climate, they may fail prematurely if the transformers are not maintained properly. There are several ways in which the transformer can fail. Transformer failure can usually be attributed to the failure of a component. These failures can occur mostly in the tap changer (as suspected in Biyagama and Kotugoda transformer failures) and the HV bushings. The other failures may be in the core of the transformer, winding of the transformer, insulation oil of the transformer and on tank or pipes of the transformer. However, these latter failure rates are considerably low compared to tap changer and HV bushing failures.
This article is written mainly on an understanding of how to maintain this vital component of the transformer, namely the Tap Changer. The purpose of the tap changer in a transformer is to regulate the voltage of the system according to the stipulated value. As customers, we all know that the electricity supply provided by the utility shall have a proper voltage value and that value shall be at least within +, - 6% of the rated value in Sri Lanka. There are a few methods available in any utility to maintain this voltage value at the required range. One method is by using the tap changers installed in the transformers. This device is used to maintain the output voltage within the required limit by adjusting the taps of the transformer windings.
There are two types of Tap changers namely; De-Energize Tap changers (DETC) in which taps can be changed when the transformer is de-energized only and, On Load Tap Changers (OLTC)- in which the Taps of the transformer can be changed while the transformer is in operation. OLTCs are usually installed by the transformer manufacturers mainly in larger transformers eg., Grid Transformers; On the contrary, DETCs are installed in small distribution transformers.
As mentioned earlier, Transformers with OLTCs are more vulnerable for failures unless the tap changer is properly maintained regularly. The main supplier of the OLTCs for almost all transformers in the world is MR – “Meschinenfabrik Reinhausen”. There are ABB, Hyundai and old CGE type tap changers used in some of the transformers in some utilities but their contribution for making this device is around 25% compared to 75% by MR.
In an OLTC power transformer, there are two oil compartments in it. One compartment is for the Tap changer and the other compartment is for the main windings and the core. The oil conservator at the top of the transformer too has two compartments to keep the “oil compression/expansion” controlled accordingly. The volume of oil in the tap changer compartment is very little compared to the oil volume in the main tank and oil in the two compartments do not mix with each other. The MR tap changer consists of two components; the diverter switch and the tap selector switch. Depending on the design type, the diverter switch can be in the tap changing compartment leaving the tap selector switch in the main winding compartment, or both could be in the same tap changer compartment. These two devices operate at the same time when there is a tap changing operation.
Once the tap changer is being called for its operations, there are some switching activities taking place in the tap changer compartment since the current is switched when changing the taps from one to the other. MR invented two main design types for its tap changers based on the medium where the switching action is taking place for its main contacts.
The switching medium of the tap changers manufactured before the year 2001 was oil and the newer tap changers manufactured after 2001, the switching is taking place in vacuum bottles. When switching occurs in oil, there will be some carbon deposits on switching contacts due to arcing in oil. These carbon deposits would react negatively on the switching contacts further by creating more carbon residuals on the switching contacts and in oil, which would reduce the insulation properties of oil drastically. Hence it is understandable that the oil in this type of tap changer compartments will get soiled with impurities very frequently compared to the oil in the main tank since the oil capacity in the tap changer is less and it does some switching activities.
This impure oil will lead to overheating of the entire tap changer and unwanted flash-overs during operations and finally it will explode. Although there are Bucholz type relays installed in transformer tap changer compartments to identify these faults, it may be too late to avoid the explosion as these relays are generally slow in operation.
In case of switching in vacuum type tap changers there are no carbon deposits since the arcing does not occur in insulation oil but in vacuum bottles. Hence, these tap changers will last longer than the switching in oil type tap changers. Therefore, newer vacuum type tap changers do not require any maintenance work for a longer time compared to the switching in oil type tap changers.
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