A transformer is a static device used either for raising or lowering the voltage of an AC supply with a corresponding decrease or increase in current. By electromagnetic induction, an alternating current of one voltage is transformed to another voltage, without change of frequency. It essentially consists of two windings, the primary and secondary, wound on a common laminated magnetic core. Depending on the number of turns on each winding, transformers are classified as a step-up transformer or a step-down transformer. Stepup transformer has more turnings on the secondary coil than on the primary coil and therefore the voltage induced in the secondary coil is larger than the primary coil voltage. Transformers are widely used in electrical apparatus of all kinds and in particular in power transmission and distribution.
Power Transformers
A Power Transformer transfers energy between high voltage and very high voltage systems, i.e. between generators and transmission systems and between transmission systems and distribution systems. Also, they are used in transmission network for stepping up or down the voltage level. It operates mainly during high or peak loads and has maximum efficiency at or near full load. A single-phase transformer is basically made out of two separate windings that are inserted into each other into a closed loop of magnetic core. Moreover, the Joules heat effect is proportional to the square of the current transmitted into any ordinary conductor like transformer windings or transmission lines. Both these effects combined at constant power of elevating voltage reduce heat dissipation accordingly by the square of the current, and enable the transmission of power of alternating current and voltage over very long distances from the energy producer to the energy consumer while limiting the power losses in the grid. This is possible due to a key grid component the power transformer. Most of them are the three-phase transformers or the three single-phase transformers. Thus from these electromagnetic principles, with the voltage increase of an electrical network, the Joule losses are reduced and the two main constraints of power transformers are high voltage and high current, depending on whether the HV or LV is observed.
Windings
The active part of a transformer is made of the elements that are in contact with the voltage and the current, and are mainly composed of windings, core, and tap changer bushings. The windings are handmade out of copper coils insulated mainly with several layers of paper between the turns. The two main winding designs and technologies have been developed over time with many variations: the core type and the shell type windings. The electromagnetic basis remains the same in both cases but the mechanical construction is different. In the core type design, the winding is “enclosing” the magnetic core legs, while in the shell type the core is “enclosing” the windings. Every transformer manufacturer has its own experience with these technologies, neither of which is automated. The manufacturing of windings involves a lot of human labour and requires significant experience as well as application of the highest quality standards. This is because winding conductors are covered by a type of insulation such as varnish or insulating paper with a limited mechanical and thermal stability. Nevertheless, this insulation type provides protection from high over-voltages, high over-currents, short-term overheating, and high mechanical stresses in order to prevent reduction of the insulation paper durability. It must be taken into account that the winding insulation cannot be easily repaired or replaced during the service life of a transformer and rewinding has to be performed only in a specialised workshop.
Core
The core is an important part of a transformer and generally the heaviest one. Produced from steel, it has high magnetic permeability and provides low magnetic resistance to the magnetic flux. In power transformer, the flux density is higher than the distribution transformer. It is made from thin steel sheets with the thickness of a few tenths of a mm in order to reduce losses and magnetising current. The main way to produce a core is to stack the sheets, cut to desired size, onto the automatic machines, and then manually stack them to build a core. The main core parts are the legs i.e. vertical parts, and yokes i.e. horizontal parts. The legs are mainly situated in a same plain. Most transformers have additional turns added to the HV windings and some of those turns are linked to a device called the “Tap Changer”. It enables a specific range of the voltage variation during the transformer service life. The electric circuit of the windings and the tap changer has some movable contacts. The two main types of tap changers are the De-Energised Tap Changer (DETC), mechanically quite simple type that changes the voltage while the transformer is not loaded; and the On Load Tap Changer (OLTC), a more complex type which operates when the transformer supplies the load. It should be noted that the tap changers, the OLTCs in particular, are contributing to an increasing transformer failure rate, mainly due to the movable contacts wearing over the years i.e. hot spots, aging mechanisms.
Bushings
The bushings are the components that link the windings to a network through the grounded tank. High voltage bushings can be technically complex and, in some cases, their failure can lead to a transformer explosion quite rapidly. This is because one of the highest voltage gradients is between the HV bushing central part at full potential, and the grounded tank at the distance of just a few centimetres. The insulating oil just below is very flammable and if the bushing is sparking, it could generate a lot of energy, open the tank slightly and then ignite the oil, which could lead to an explosion. For this reason, the HV bushing is manufactured to withstand very high voltages within a small space filled with paper and oil between the bushing and transformer tank.
Insulating materials
The three major insulating materials for the power transformers are: mineral oil and paper and pressboard in different forms. The mineral insulating oil is weighted in tons within the tank and can be used to assess many essential points about the condition of a transformer and some critical incipient faults. The paper insulates the winding turns, while the pressboard strengthens the electrical insulation and provides dielectric distance at specific locations, for example in the main duct between the windings. Insulating materials, such as paper, pressboard and mineral oil are organic materials subject to aging. As the solid insulation cannot be repaired or replaced easily like other transformer parts and components, it limits the transformer service lifetime. Therefore, the solid insulation lifetime is the lifetime of a transformer.
Trends in Power Transformers
Dryformers
Recently, HV cross-linked polyethylene (XLPE) cables are used in these transformers. Dryformer is an oil free HV transformer based on cable technology used in Powerformer. Forced-air cooled, it has innovative windings made from XLPE cables with circular conductors. The absence of oil means that there is no risk of ground or water pollution in the event of damage and a less risk of fire or explosion. Therefore, Dryformer can be sited closer to the consumer, for example below ground and in urban or ecologically sensitive locations. As the electric field is fully contained within the XLPE cable and the cable surface is at ground potential, Dryformer offers unique opportunities for optimising power transformer design. By using the state-of-the-art of cable technology, XLPE cable can have electric field strengths up to 15kV/mm. From a manufacturing perspective, the Dryformer has the considerable advantage of having the insulation system built up at the cable factory.
Gas-insulated transformers (GITs)
In GITs, SF6 gas is used as insulation media with relatively low gas pressure. The principal solid insulation material for the GIT winding is polyethylene terephtalate (PET) and polyphenylene sulphide (PPS) films which are defined as class E insulating material with a temperature limit of 120 C. It has been initially specified to operate GITs, especially the gas-natural air-natural (GNAN) distribution GIT, with top gas temperature limit of 110 C instead of the maximum conventional 95 C top oil temperature for oil-immersed transformers. There have been a number of such interface problems for those heavily loaded GIT caused by gas temperatures higher than 100 C. Corresponding counter measures, such as oversizing the first section of LV bus bar, adoption of higher temperature class bushing material, and modifying clamping design to absorb higher temperature fluctuation have to be introduced. GIT have the following advantageous:
- Non-inflammable and nonexplosive, hence they are usable for multi-storeyed buildings, underground markets and other overpopulated places.
- Moisture resistant and dust resistant, therefore, they are unaffected by open air moisture, dust and other ambient conditions since the windings and core of these transformers are fully enclosed in mild steel box and sealed with SF6 gas. In addition, they have easy maintenance and check because these transformers are hermetic sealed with an inert SF6 gas and materials are scarcely deteriorated.
- Clean as there are no contaminations to surroundings since these transformers are sealed with non-poisonous, odourless SF6 gas, even if the SF6 gas leaks unlike mineral oil-immersed transformers.
- Higher reliability with simple internal structure.
- Better compatibility with gas-insulated switchgear (GIS).
GIT with onload tap changer (OLTC) used to be the most vulnerable part of any power transformer from electrical and mechanical points of view. In line with the use of SF6 gas as insulation media, vacuum switch type OLTC is installed for transmission GIT at 30 MVA and above. These vacuum switches housed inside the gas chamber are used as diverter switches and no arcing product can be possibly produced. Such OLTC is basically maintenance free. In the extreme case when OLTC malfunction due to mechanical defect or connection problem, the damage will be minimal. GIT turn out to be cheaper than oil immersed transformers when maintenance costs are considered. Power-distribution transformers have a high recycling value because they can be easily disassembled and their chief constituents, which are high-purity steel, aluminium and copper, can be recycled indefinitely. GIT are far more easily recycled than oil-immersed types.
Distribution Transformers
Distribution transformers are units of electric power systems, in which electricity is transformed form the voltage level 1 – 50 kV to the voltage level 120 V + 1 kV, in dependence on consumer’s needs. Energy efficiency of distribution transformers is very high, typically ranging between 96 per cent and 99 per cent. However, due to a large number of distribution transformers in electric power system and their long lifetime (30 – 40 years), even small improvement in the efficiency of these units could result in significant energy savings. These issues are important both from economic and ecological viewpoints. Increase of energy efficiency of distribution transformers could be obtained reducing three types of transformer losses:
- No-load loss (iron or core loss) can be reduced by improvement in design and assembling processes or in magnetic properties of material core,
- Load loss (copper loss) can be reduced increasing the cross-section of the windings,
- Cooling loss can be reduced by decrease of other types of transformer losses.
Further increase in transformer efficiency is possible to reach by replacement silicon steel cores with new types of magnetic core materials, e.g. amorphous ribbons. These materials are produced by rapid solidification of a liquid alloy, what gives specific magnetic properties, especially very low energy loss. However, these materials have quite low saturation induction and they are thermal unstable.
Trends in Distribution Transformers
Amorphous core transformers
Amorphous cores are usually produced as wounded, one-side cutting ones, due to mechanical properties of amorphous ribbons. This solution ensures the correct location of air gaps inside a core and simplifies electric windings assembling as well. Amorphous transformers are produced as 1-phase or 3-phase units, with 3-limbs or 5-limbs core construction. The capacity of currently produced amorphous transformers is limited up to 10 MVA. The cross-section of amorphous cores is larger in comparison to silicon steel ones, due to lower saturation induction of amorphous ribbons. It results in the increase of transformer dimensions and weight. High efficiency distribution transformers with amorphous core become more and more popular. The energy savings from amorphous transformers have a great influence on the scope of electricity production and consumption.
High temperature materials
A new technology for the transformer industry involves the use of high temperature materials to provide a variety of economic, environmental and safety benefits to the user, including: lighter weight, smaller size, reduction in fluids, improved safety, less flammable, more capacity and lower energy losses. A new IEEE Standard has been published to provide guidance to manufacturers and users regarding the production and application of these transformers. The high temperature materials and boards can be used with conventional fluids in an economic way for power and distribution transformers for increased capacity and improved reliability. They can also be used with less flammable or higher temperature fluids for dramatic reduction in size and weight, with greater safety and environmental reliability.
Hybrid Insulation Systems
Thermal aging studies have given rise to, hybrid insulation systems. It is found that these systems reduced evolution of gases and much longer life expectancies with the use of “hybrid” insulation systems, which use paper and board in the hot winding area and cellulose materials in the cooler, bulk insulation areas of power transformers. These systems also eliminate the furan compounds, which evolve from the degradation of cellulose in the hot windings. The cells were designed to duplicate the material ratios of a 25 MVA transformer, including hot winding insulation, cool bulk insulation, copper and core steel. The hybrid systems include paper on the conductor and spacers of the winding, with cellulose board in the cooler sections of the cells.
- Power transformers are used in transmission network so they do not directly connect to the consumers, so load fluctuations are very less. These are loaded fully during 24 hours a day, so copper losses and iron losses takes place throughout day, the specific weight i.e. (iron weight)/ (cu weight) is very less. The average loads are nearer to full loaded or full load and these are designed in such a way that maximum efficiency at full load condition. These are independent of time so in calculating the efficiency only power basis is enough.
- Distribution transformers are used in distribution network so directly connected to the consumer so load fluctuations are very high. these are not loaded fully at all time so iron losses take place 24-hour a day and copper losses takes place based on load cycle. The specific weight is more i.e. (iron weight)/ (cu weight). Average loads are about only 75 per cent of full load and these are designed in such a way that max efficiency occurs at 75 per cent of full load. As these are time dependent the all-day efficiency is defined in order to calculate the efficiency.
- Power transformers are used for transmission as a step up devices so that the I2r loss can be minimised for a given power flow. These transformers are designed to utilise the core to maximum and will operate very much near to the knee point of B-H curve (slightly above the knee point value). This brings down the mass of the core enormously. Naturally these transformers have the matched iron losses and copper losses at peak load (i.e. the maximum efficiency point where both the losses match).
- Distribution transformers obviously cannot be designed like this. Hence the all-day-efficiency comes into picture while designing it. It depends on the typical load cycle for which it has to supply. Definitely core design will be done to take care of peak load and as well as all-day-efficiency.
- Power transformer generally operated at full load. Hence, it is designed such that copper losses are minimal. However, a distribution transformer is always online and operated at loads less than full load for most of time. Hence, it is designed such that core losses are minimal.
- The main difference between power and distribution transformer is distribution transformer is designed for maximum efficiency at 60 per cent to 70 per cent load as normally doesn’t operate at full load all the time. Its load depends on distribution demand. Whereas power transformer is designed for maximum efficiency at 100 per cent load as it always runs at 100 per cent load being near to generating station.
