A generating station located in Larkana is supplying power to a
load center (with four types of
loads) in the Sukkur region through a transmission line (line
impedance: 0.15+ j0.32 ?/km). A pair
of three-phase transformers (0.4/11 kV and 11/0.4 kV) are utilized
at either end to reduce line
losses. The three-phase transformer at each end is designed using a
bank of three single phase
transformers. A short circuit test has been executed at HV side of
any single-phase transformer
to determine the following values of transformer impedances:
• Equivalent series resistance: 3.2 ? (series elements referred to
the HV side)
• Equivalent series reactance: 5.7 ? (series elements referred to
the HV side)
As for the load center with multiple types of loads:
• The first load is a flour milling company, which absorbs an
average active power of 270
kW, and the company has maintained the power factor to 0.82
Lagging. The time of
operation for this company is 08-00 am to 4-00 pm.
• The second load is a small textile mill, working from 10-00 am to
06-00 pm while
maintaining the power factor of 0.86 lagging and consuming 250
kVAs.
• On the other hand, the third load consists of a group of laundry
shops (where clothes
irons are used for pressing clothes on large scale), absorbing real
power of 25 kW and it
is operational from 12-00 pm to 08-00 pm.
• Similarly, the last load is a toy manufacturing unit, consuming
the apparent power of 350
kVA while maintaining a power factor of 0.85 lagging for a time
duration of 8 hours per
day, from 02-00 pm to 10-00 pm.
• The electrical frequency of all the loads is fixed to be 50
Hertz.
You are hired as a design engineer to propose
• The VA ratings of the transformers.
• The three-phase transformer connections in terms of Wye and
Delta.
Also, justify the type of transformer that you are selecting for
the said scenario; it may be an
autotransformer or an ordinary transformer keeping in view the
financial perspective of your
selection along with technical ones. While designing the
transformer, also take the following
factors into account:
• Total demand for the load
• Overall weather of the region
• Height relative to sea level
Note: National Electric Code or NFPA 70 standards states:
transformer kilovolt-ampere should be
derated by 8% for each 10°C above 40°C (when air-cooled for a
dry-type transformer) and by
0.3% for every 330 feet over 3,300 feet altitude).