The Distribution transformer is one of the important link between 11kV or 22kV Transmission system and Distribution system of Electrical power network. As the majority of the consumers are served on the 440V supply level, the, distribution transformer has vital importance and failure of which will immediately affect the agriculture and industrial production in the state as well as life of common public and huge monetary loss to the SEB. The voltage ratio of the distribution transformer used in the distribution system are 11/0.433kV, 22/0.433kV, 33/0.433kV as per HV incoming system voltage, with capacity 10kVA, 16kVA, 25kVA, 63kVA, 100kVA, 200kVA, 315kVA, 630kVA, 1000kVA etc. The distribution transformers are available in following types: 1. Mineral Oil filled distribution transformer 2. Dry type distribution transformer 3. Ester oil filled distribution transformer.
1. Mineral Oil filled distribution transformer:- These are the conventional types of transformers. The main purpose of mineral oil is for insulation as well as cooling.
• It is not biodegradable & hence not environment friendly.
• It can cause accidents due to less flash(1400C) and fire point (1500C) which may result in risk to human life.
• Periodical maintenance (filtration) is required due to sludge formation.
Pole mounted Distribution Transformer center (DTC):
It is the most common type of outdoor type substation, designed by Discoms. Normally, single phase DTs up to 25 KVA capacities [Fig 5.2] are installed on single pole structure and 3 Phase DTs up to 200 KVA capacities may be mounted on 2 pole ref. [Fig 5.3]. All DTs having capacities 315 KVA and above shall be mounted on plinth only [ Fig 5.5]. The two poles structure is made of RSJ poles with channels and associated accessories shall maintain minimum height from the ground level to meet the ground clearance. This area also to be provided with suitable fencing and lockable doors to prevent unauthorized access to Distribution box. The structures should also be provided with anti-climbing devices and danger board.
For Installation of the distribution transformer :-
1. In urban as well as rural area, RSJ supports are to be used of suitable length.
2. HD Galvanized Nuts of 16 mm dia, bolts with plain/spring washers shall be used for tying of overhead structure items like top & base channel, HG & AB switch channel, clamps, bracing, strain plates etc.
3. Stay set shall be used at all Dist. Transformer Centre location to nullify the tension of conductor/cable/transformer on the supports. Stay shall be properly earthed.
4. ACSR / AAAC conductor is to be used for connection between overhead lines to transformer bushing. All these connectors/jumpers shall be insulated with Alkathene Pipe.
5. Two numbers Earthing bolts of 10mm dia on the distribution box should be provided. The distribution boxes are to be earthed using 8 SWG GI wire / G.I Strip with direct connection to the earthing.
6. The single core power cables should be terminated with proper size lugs and metal gland. Necessary tagging, identification of cores and dressing of cables with nylon cable ties shall be in the scope of work.
7. The unutilized holes in the Distribution box provided for cable entry needs to be plugged /sealed properly in a manner that, it must stop access to reptiles, dust and water ingress.
8. Distribution Transformer Earthing shall be provided with 5 Nos. earthing. Earthing should be provided with GI earth pipe or Chemical Earthing depending of strength of soil in the location.
9. Out of 5 earth pits, two shall be reserved for transformer neutral and lightening arresters each.
10. Metering system of DTC shall be provided. The meter shall be installed in a metallic enclosure. Enclosure shall be earthed at two locations.
11. Anti-climbing device and danger boards with clamps shall be provided to each pole of DTC.