Notesale: Turn your study into money

Document Preview

Extracts from the notes are below, to see the PDF you'll receive please use the links above

---

Three – phase transformers
Previously we discussed about single phase transformers, and now we will discuss about
three phase transformers
...
They are generally grouped together in banks
...
There are
four standard ways of connecting a three-transformer bank, namely wye-wye (Y-Y), delta-delta (ΔΔ), delta-wye (Δ-y) and wye-delta (Y-Δ)
...
Scott connection (T - T) is seldom used in some system
...
When transformers are group together, it is customary to have
similar kilovolt-ampere and voltage ratings and identical polarity markings
...

The construction of 3 phase transformer is similar to the single phase transformers
...

In core type three-phase transformer has a core with 3 legs or limbs
...
Each limb contain both the primary and secondary winding
...
This construction is used for balanced load
...
There is another
construction of core type3 transformer whereas a leg is common to each phase

In shell type three phase transformers looks like three single phase shell type transformers
that are vertically stacked to each other
...
The magnetic circuit of the transformer is connected in parallel
...
This type of construction is rarely
used in practice
...
First we
will discuss the Y- Y connection
...
If the ratio of transformation of each transformer is a,
the same ratio will exist between the line voltages on the primary and secondary sides
...
When unbalanced load is connected, the electrical neutral
will shift from its exact center to a point that will make the three line-neutral voltages unequal
...
The advantage of Y – Y connection
is that the insulation is stressed only to the line to neutral voltage, which is 58 percent of the line to
line voltage
...

The connection of both the primary and secondary windings are in delta connection
...
When one transformer is
removed it is called open delta
...
The load voltage will be equal even though there is an unbalanced in the load
...
The greater the number of turns in the windings, it will need larger amount of copper for
winding which will be expensive and larger space will be needed
...
Just like the
wye to wye connection, the insulation is only stressed by 58 percent compared to delta connection
...
One of the disadvantages of this connection is that the ratio of
transformation of the line voltages will no longer be the same as the ratio of transformation of each
transformer
...
Below are the equivalent
schematic diagram and the formula for the ratio of transformation
...
73𝑎
𝑉𝐿
𝑉 𝐻 = 𝐻𝑖𝑔ℎ𝑒𝑟 𝑉𝑜𝑙𝑡𝑎𝑔𝑒 𝑉 𝐿 = 𝐿𝑜𝑤𝑒𝑟 𝑉𝑜𝑙𝑡𝑎𝑔𝑒
If there is a delta – wye connection, wye – delta connection is also possible
...
The formula VH/VL=1
...
It is employed in moderately lowvoltage distribution circuits for stepping down 4000 or 8000 volts to 230 and 115 volts
...


In two transformer banks V – V or open delta is very common
...
The open delta is used:
1
...
When one transformer in a Δ – Δ bank fails, so that service may be continued until the faulty
transformer is repaired or a good one is substituted
3
...
This is a reduction of about 15 percent from its normal rating
...
6 percent of the balanced load power factor
...
One of the disadvantages of V – V connection is that the secondary terminals
become more unbalanced as the load increased, even though the load is balanced
...
The
formula and the equivalent schematic diagram for open delta connection
...
A two
transformer bank connection that can be used to transform three phase power from one voltage to
another
...

Since it can transform from three phase power system to two phase power system or vice versa, it
has wide applications where such a change is necessary
...
One of the
transformers is the one that they call the main transformers
...
The other
one is the one that they call the teaser transformer
...
6 per cent of the respective turns of the main
transformer
...
The
secondary and primary phasor voltages form Ts (it the reason why scott connection was also name
T – T connection)
...
6 percent of that across the main
transformer, the reason is that two halves of the main transformer are out of phase by 60 electrical
degrees, the sum equals the kilovolt – ampere load on the transformer
...
Each transformer in a T – T
connection must have a rating of 15 per cent greater than each of the transformers in a three –
transformer bank
...
The two phase side makes use of the teaser windings as well as the main
winding
...
With transformers, transmission of
power on distant places became more possible and with these innovations it became more efficient
and cost effective
...


References:

Valenzona, M
...
Textbook-Reviewer in Electrical Engineering
...
Cebu City: MG
Siskind, C
...
Electrical Machines
...
Manila, Philippines: Meriam & Webster
Bookstore, Inc
...
(2003)
...
Cebu City
...
d
...
Retrieved December
http://www
...
ws/transformer/three-phase-transformer
...
d

---