- Power transformers are used in transmission
network of higher voltages for step-up and step down application (400
kV, 200 kV, 110 kV, 66 kV, 33kV) and are generally rated above 200MVA.
transformers are used for lower voltage distribution networks as a
means to end user connectivity. (11kV, 6.6 kV, 3.3 kV, 440V, 230V) and
are generally rated less than 200 MVA.
Transformer Size / Insulation Level:
- Power transformer is used for the transmission purpose at heavy load, high voltage
greater than 33 KV & 100% efficiency. It also having a big in size
as compare to distribution transformer, it used in generating station
and Transmission substation .high insulation level.
distribution transformer is used for the distribution of electrical
energy at low voltage as less than 33KV in industrial purpose and
440v-220v in domestic purpose. It work at low efficiency at 50-70%,
small size, easy in installation, having low magnetic losses & it is
not always fully loaded.
Iron Loss & Copper Loss:
Transformers are used in Transmission network so they do not directly
connect to the consumers, so load fluctuations are very less. These are
loaded fully during 24 hr’s a day, so Cu losses & Fe losses
takes place throughout day the specific weight i.e. (iron weight)/(cu
weight) is very less .the average loads are nearer to full loaded or
full load and these are designed in such a way that maximum efficiency
at full load condition. These are independent of time so in calculating
the efficiency only power basis is enough.
Transformers are used in Distribution Network so directly connected to
the consumer so load fluctuations are very high. these are not loaded
fully at all time so iron losses takes place 24hr a day and cu losses
takes place based on load cycle. the specific weight is more i.e. (iron
weight)/(cu weight).average loads are about only 75% of full load and
these are designed in such a way that max efficiency occurs at 75% of
full load. As these are time dependent the all day efficiency is defined
in order to calculate the efficiency.
transformers are used for transmission as a step up devices so that the
I2r loss can be minimized for a given power flow. These transformers
are designed to utilize the core to maximum and will operate very much
near to the knee point of B-H curve (slightly above the knee point
value).This brings down the mass of the core enormously. Naturally these
transformers have the matched iron losses and copper losses at peak
load (i.e. the maximum efficiency point where both the losses match).
transformers obviously cannot be designed like this. Hence the
all-day-efficiency comes into picture while designing it. It depends on
the typical load cycle for which it has to supply. Definitely Core
design will be done to take care of peak load and as well as
all-day-efficiency. It is a bargain between these two points.
transformer generally operated at full load. Hence, it is designed such
that copper losses are minimal. However, a distribution transformer is
always online and operated at loads less than full load for most of
time. Hence, it is designed such that core losses are minimal.
- In Power Transformer the flux density is higher than the distribution transformer.
main difference between power and distribution transformer is
distribution transformer is designed for maximum efficiency at 60% to
70% load as normally doesn’t operate at full load all the time. Its load
depends on distribution demand. Whereas power transformer is designed
for maximum efficiency at 100% load as it always runs at 100% load being
near to generating station.
Transformer is used at the distribution level where voltages tend to be
lower .The secondary voltage is almost always the voltage delivered to
the end consumer. Because of voltage drop limitations, it is usually not
possible to deliver that secondary voltage over great distances. As a
result, most distribution systems tend to involve many ‘clusters’ of
loads fed from distribution transformers, and this in turn means that
the thermal rating of distribution transformers doesn’t have to be very
high to support the loads that they have to serve.
- All day efficiency = (Output in KWhr) / (Input in KWhr) in 24 hrs which is always less than power efficiency.