Technological advances in distribution transformers

Electrical energy has been used as a driving force in industrial units and power stations due to its versatility, usability and eco-friendliness. The electrical energy is easily transferred from one voltage level circuit to another voltage level circuit by transformers which are static electrical equipment. Transformers are robust and sturdy without any moving parts. These also require least maintenance compared to other electrical equipment.
Amorphous core transformersThe iron losses in transformers are about 0.15 per cent of total power transferred and 11.3 per cent of transformer losses. The iron losses account for 1.77 – 3.85 per cent of total input energy in an industry. These losses are mainly due to eddy current and hysteresis losses. The increased iron losses are due to idle charging, over sizing, use of inferior core material, and harmonics in the distribution system, to name a few.
The core losses have two components: eddy current losses and hysteresis losses. The hysteresis losses are directly proportional to frequency and more for non-linear loads.
In order to reduce these both iron losses, new amorphous cores are developed for transformers.
The amorphous core is made from a non-crystal substance created by rapidly freezing liquids of high temperature. Random molecular structure of amorphous metal core, there is no rule of atomic arrangement causes less (Fig. 1 and 2) friction than conventional silicon steel core when a magnetic field is applied. This property of amorphous core allows ease of magnetisation and demagnetisation reduces the hysteresis losses significantly.
As the eddy current losses are proportional to square of thickness of laminations, the thickness of laminations of amorphous cores will be of the about 0.025 mm which is about 1/10th of laminations of silicon steel. This causes reduction of eddy current losses considerably.
The overall reduction in iron losses (combination of both hysteresis and eddy current losses) in amorphous core transformer will reduced by 1/5th that of iron losses in silicon steel cores. At a flux density of 1.3 tesla, iron loss of amorphous core is about 0.27 W/kg where in silicon steel it is about 0.71 W/kg.
The amorphous core has higher electrical resistance, i.e. 130 μΩ-cm which is almost triple of silicon steel cores (50 μΩ‑cm). This higher resistance of amorphous core helps in reduction of eddy losses in core, i.e. when magnetic flux flows, eddy current flows to negate the flux.
Advantages of amorphous core transformers

Easy magnetisation due to low coercivity, low hysteresis loss and high permeability
Fast flux reversal due to low magnetic loss
Easy to construct and less maintenance
Single coil construction reduces the axial component whereas interlayer bonding by use of epoxy paper reduces radial component of short-circuit forces
Reduced iron losses reduce the heat generation considerably
Considerable reduction in losses in harmonic prone areas
Fast, easy repair due to its modular construction
Use of single quality (grade) core material
Easy lacing and unlacing of core for coil installation
Reduces the CO2 emission
completely self-protected (CSP) transformersThe failure rate of distribution transformers in rural areas is higher compared to urban areas. Wide variation in ambient conditions and loading pattern cause undesirable breathing and ingress of moisture which allow oxygen to come in contact with transformer oil. The moisture weakens the dielectric strength of oil that forms sludge which will restrict the natural circulation of oil inside transformer and also this sludge deposit of winding. Due to restriction in oil circulation cause increase in oil temperature and final winding temperature that may lead to failure of windings.
The CSP transformers consist of mainly three components:

Primary fuse: Internal expulsion fuse for system protection
Secondary circuit breaker: for over-load and secondary fault protection
Surge arrester: for lightning protection.
The CSP system protects the transformer from lightning, secondary faults and severe overloads. The system also provides visual warning of uneconomical loading conditions and protects the distribution system from line “lock-out” during transformer failure. For secondary fault and overload protection, the circuit breaker is mounted inside of the transformer and connected between the secondary winding and the secondary bushings in such a way that the load current and fault current flows through the circuit breaker. For line “lock-out”, it is connected between the incoming high-voltage lead from the bushing and the high-voltage line.
Primary fuseIn a CSP transformer, the primary fuse is mounted inside the transformer and the rating of the fuse is determined on the basis that it should not for surge. As per the British Electricity Authority’s experience, the rating of fuse will be selected for 12 times the full load rated current for 10 ms. This fuse is connected in series with primary winding. This fuse is housed inside of the primary bushing and connected to the high-voltage winding through terminal block.
The purpose of this expulsion fuse is to protect the part of the electrical distribution system, which is ahead of the transformer from faults which occur inside of the distribution transformer. If a fault occurs in the windings or some other part of the transformer, it will cause abnormally large currents to flow and the flow of these currents will cause the fuse to melt, open and clear the circuit. When this type of fault exists, the transformer must be removed from service for repair. Any fault ahead of the transformer will not be seen by any of the transformer’s internal protective devices and has to be cleared by other protective devices upstream from the transformer.
Secondary circuit breakerIn the case of cyclic loads (containing peaks and valleys), where peak load is of relatively shorter, transformers smaller than the peak loads can be safely installed without any rapid loss of transformer’s life due to overload. The CSP circuit breaker will permit the transformer to function within cyclic loads up to the point where the amount and duration of the peak load begins to cause significant loss of transformer life. When this point is approached, the signal light will light with the first indication that the loads on this particular transformer have grown to the point when significant insulation deterioration can occur.
Surge arresterThe surge arrester is installed to protect the transformer from lightning strikes. As the mounting of surge arrester closer to the transformer, the shorter will be the ground lead connection between the arrester and the transformer. The shorter connection will reduce the surge induced voltage stress on transformer winding. The mounting of surge arrester on the transformer tank, the ground lead length is effectively zero that help maximum protection.
Dry type of transformersIn dry type transformers, air is used as the cooling and dielectric medium. Most are manufactured with vacuum pressure impregnation in polyester or silicone varnish. In oil-filled transformers, the cooling and dielectric media are a fluid which remains as a liquid during the operation of the transformer. Oil type transformers are considered a potential fire and safety hazard. Oil type transformers require the development and maintenance of reliable fire safety and extinction procedures.
Dry type transformers can be located closer to the load unlike oil transformers which require special location and civil construction for safety reasons. Locating the transformers near the loads may lead to savings in cable costs and reduced electrical losses. Oil type transformers may require periodic sampling of the oil and more exhaustive maintenance procedures. However, though dry type transformers are advantageous, they are limited by size and voltage rating. Higher MVA ratings and voltage ratings may require the use of oil transformers alone. For outdoor applications, oil-filled transformers are cheaper than dry types.Transformers play a major role in power transmission and distribution system because the generator terminal voltage will be less due to constraints in insulation cost, and transmission voltage will be high to curtail T&D loss.
As transformers are in the circuit all the time and iron losses will be continuously occurring, use of amorphous core transformers will reduce the iron losses by about 70 – 75 per cent which helps in reduction of T&D losses considerably.
CSP system protects the transformer from lightning, secondary faults and severe overloads. It also provides visual warning of uneconomical loading conditions and protects the distribution system from line “lock-out” during a transformer failure.
Use of dry type transformer is most suitable where fire hazards (i.e. multi-storied buildings, chemical plants) are most important and oil-filled transformers are economical for open area. 
Authored by—Rajashekar P. Mandi,                Energy Efficiency & Renewable Energy Division,       Central Power Research Institute, Bangalore
 
Dr Udaykumar R. Yaragatti,Dept. of Electrical & Electronic Engg. National Institute of Technology, Surathkal