WHAT ARE DISTRIBUTION TRANSFORMERS?
- Distribution Transformer is an electrical isolation transformer which convert high-voltage electricity in transmission lines to lower voltage levels acceptable for use in Domestic and household applications.
- Transmission network they are generally operated at a voltage level greater than 33 kV while?distribution transformers?are generally connected at a voltage level of less than 33 kV. Due to the higher?voltage rating?the insulation?level?of the power?transformers?is very high compared to the?insulation?level of distribution transformers?
- The Domestic usage?voltage?in?India?is 220 volts, alternating at 50 cycles (Hertz) per second.
WHY ARE THEY CONNECTED IN DELTA-STAR ARRANGEMENT?
Delta-Star transformers are preferred on distribution side primarily because of the following reasons:
- Electrical loading on the three phases is often unbalanced in nature. These gives rise to zero sequence current in the system. We don’t want these zero sequence currents to perpetuate to the other parts of the power system. A delta-star transformer acts as an isolation between the distribution system and the upstream power system by making the zero sequence current to circulate in the delta winding and not in the line/phases.
- With the development in power electronics, the distribution side loads largely comprises of power electronics based variable speed drives, rectifiers, inverter, and UPS. These loads are the major sources of triplen harmonics (having frequency as multiple of three) in the power system. These current just constitute to the losses in the system, hence needs to be handled at the distribution level itself. A delta-star transformer provides the isolation by making the triplen harmonic currents to circulate in the delta winding and not in the phases.
- Delta HV & Star LV transformers are used for distribution because you can get the HV power to the transformer with 3 conductors instead of 4. Saves a 1/4 of the cost in cable alone. Also,?for the same power transmission P, Delta schema gets higher voltage (ideally sqrt(3) of the Star voltage one),this means less current flows in the line and hence reducing the conductor cost. The equation being I = P/V.
STANDARD RATINGS AVAILABLE IN THE MARKET
The standard ratings shall be 16, 25, 63, 100,160, 200, 250, 315, 400, 500, 630, 1000, 1250, 1600, 2000 and 2500 kVA for 11 kV distribution transformers and 100, 160, 200, 315, 400, 500, 630, 1000, 1250, 1600,2000, 2500 kVA for 33 kV distribution transformers.
