Distribution of electricity starts from the sub-transmission level in some countries. It starts from a voltage level lower than 66KV. Distribution of electricity is often undertaken by discoms. The distribution of electricity plays a very critical role in shaping any economy.
Distribution transformer plays an important role in the transformation of electricity from sub-transmission voltages to distribution voltages. 33KV, 11KV, and 0.440 KV are among the most popular distribution voltage levels.
Table of Contents
What is a Distribution Transformer?
A Distribution Transformer is a voltage transformation unit that is taken into use by discoms to step down the voltage of electricity to distribution voltage. The standard ratings are 16, 25, 63, 100,160, 200, 250, 315, 400, 500, 630, 1000, 1250, 1600, 2000, and 2500 kVA for 11/.440 kV distribution transformers and up to 10000 kVA for 33/11 kV distribution transformers.
Since the distribution transformers are subjected to a wide range of varying load conditions, the design philosophy involves limiting the losses to a bare minimum. By the normal efficiency calculation, the output power/input power * 100% cannot judge the performance of the equipment therefore the all-day efficiency of the distribution transformer is introduced.
Where Output power and the constant no-load losses, independent of loading conditions are computed for 24 hours using the corresponding load cycle
Different types of distribution transformers.
Distribution Transformers can be classified into different categories based on factors:
Mounting Location:
Pad Mounted:
DTs above 500 KVA have necessarily to be mounted on a plinth, however, lower capacity DTs may also be mounted on a plinth as per the practices being followed in Discoms.
The Pad, which is a low-height platform/plinth, is normally made of concrete structure. It can also be prefabricated by fiber blocks on which the transformers can be mounted, however, the strength of the fiber block must be ensured by the Discoms before installation. The Pad should be capable of carrying the weight of the DT and should also have the facility for cable entry and exit at two sides as per the terminals available at the Transformers. The height of the Pad/plinth should be designed by considering factors such as flood level & topography of the locality etc. and should be adequately protected by fencing to prevent access by any unauthorized persons.
Pole Mounted:
It is the most common type of outdoor type substation, designed by Discoms/power department conveniently at load centers. Normally, single-phase DTs up to 25 KVA capacities are installed on single pole/2 pole structures, and 3-phase DTs up to 500 KVA capacities are mounted on 2-pole or 4-pole structures or on a plinth.
The two-pole structure is made of poles with channels and associated accessories creating an H-type pole configuration to locate the DT at a certain minimum height from the ground level to meet the ground clearance. This arrangement of pole type S/S needs about 3 meters by 2 meters space (on the ground) around the H Pole structure to locate the Distribution box and other accessibility. This area is also to be provided with suitable fencing and lockable doors to prevent unauthorized access to the Distribution box. The structures should also be provided with anti-climbing devices and a danger board.
Insulation used
Oil immersed Distribution Transformer:
These DTs are the ones in which cooling of the windings is achieved by utilizing the oil cooling method. The body of the transformer is immersed in a welded steel tank filled with insulation oil.
Dry-type Distribution Transformer:
These types of transformers are typically best suited for distribution up to 1600 KVA and voltage ratings of up to 11 KV. Here the cooling is achieved by air.
No. of phases
Single-phase distribution transformers:
Single-phase transformers are used for small loads such as to supply single-domestic loads like pumps and lightings, etc.,
Three-phase distribution transformers:
These have higher power handling capabilities than single-phase transformers. They are also more efficient and economical in the long run.
Components of a Distribution Transformer
Core components:
The Transformer core for the power frequency application is made of highly permeable material. High permeability helps in providing a low reluctance for the path of the flux and the flux lines mostly confine themselves to the iron. Relative permeability μr well over 1000 is achieved by the present-day materials. Silicon steel in the form of thin lamination is used for the core material.
Earlier, better magnetic properties were obtained by going in for hot rolled non-oriented to hot rolled grain-oriented steel. However, now better Cold Rolled Grain Oriented (CRGO) lamination in the form of High permeability grade- has become available. Lamination thickness has progressively reduced from over 0.5mm to the present 0.23mm or upto0.18mm per lamination. These are coated with a thin layer of insulating varnish, oxide, or phosphate.
Broadly classifying, the core construction can be separated into two types:
(i) Core type and
(ii) Shell type
In a core-type construction, the winding surrounds the core, on the other hand, the iron surrounds the winding.
Windings:
Windings, made out of mainly Copper and or Aluminum wires, form another important part of the transformer. In a two-winding transformer, two windings would be present. The one which is connected to the voltage source and creates the flux is called a primary winding. The second winding where the voltage is induced by induction is called a secondary winding. If the secondary voltage is less than that of the primary, the transformer is called a step-down transformer. If the secondary voltage is higher, it is a step-up transformer.
A step-down transformer can be made to operate as a step-up transformer by making the low voltage winding its primary. Hence it may be more appropriate to designate the windings as High Voltage (HV) and Low Voltage (LV). The winding with more turns will be an HV winding. The current on the HV side will be lower as the V-I product is constant and given as the VA rating of the transformer. Also, the HV winding needs to be insulated more to withstand the higher voltage across it. HV also needs more clearance to the core, yoke, or body. These aspects influence the type of winding used for the HV or LV windings.
The rectangular conductor is used for windings above 250KVA. This is because the use of rectangular conductors has the benefit of providing a higher surface area for better heat dissipation and winding such conductors in a rectangular form is easier compared to a circular one.
Insulation materials:
The insulation used in the case of electrical conductors in a transformer is varnish or enamel in a dry type transformer. In larger transformers to improve the heat transfer characteristics the conductors are insulated using un-impregnated paper or cloth and the whole core winding assembly is immersed in a tank containing transformer oil. The transformer oil thus has a dual role. It is an insulator and also a coolant.
The porous insulation around the conductor helps the oil reach the conductor surface and extract the heat. The conductor insulation may be called minor insulation as the voltage required to be withstood is not high. The major insulation is between the windings. Annular Bakelite cylinders serve this purpose. Oil ducts are also used as part of insulation between windings. The oil used in the transformer tank should be free from moisture or other contamination to be of any use as an insulator.
Common Failures of Distribution Transformer
| Type of Failure | Operational Defects | Manufacturing Defects |
| Winding failures | ||
| a) Turn-turn faults, phase faults | 1. Overheating of the transformer. 2. Overload. | Use of improper insulation material for winding Use of low-quality material Bad insulation covering |
| b) Open winding | 1. Design shortcomings 2. Poor clamping of laminations at the yoke area | Improper brazing/connection |
| c) Opening at core joints | Short Circuit | Improper varnishing of the stamping Improper formation of the core |
| Core faults | ||
| a) Core insulation failure | Continuous overloading and overheating of the transformer | Improper varnishing of the stamping Improper formation of core |
| b) Shorted laminations | Short terminal circuit at the transformer | Improper insulation Improper formation of core Burr to the lamination blades Improper core bolt insulation |
| Terminal failures | ||
| a) Open leads | — | Bad insulation covering Improper brazing of joints |
| b) Loose connections | — | Improper joining terminals |
| c) Short circuits | — | Loose connection Improper use of insulating material Bad insulation covering |
| Off-load tap changer failures | ||
| a) Mechanical | Improper handling of tap changer switch | — |
| b) Electrical | Improper handling of the tap changer switch | — |
| c) Short circuit | Sparking at the contacts | — |
| d) Overheating | Due to operation problems like no load in the night hours Oversizing of the capacitor bank | — |
| Abnormal operating conditions | ||
| a) Over fluxing | Overvoltage in system | — |
| b) Overvoltage | Improper varnishing of the stamping Improper formation of the core | — |
| c) Overheating | Failure of the cooling Improper maintenance/checking of oil level Not topping up oil to required level Unbalanced load conditions | Insufficient cooling ducts in the winding |
| d) Overloading | Continuous overloading of transformer beyond rated capacity Improper contact of fuses Over capacity fuses | — |
