19-03-2012, 02:16 PM
ENERGY CONVERSION ONE
25471_ENERGY_CONVERSION_7.ppt (Size: 183 KB / Downloads: 95)
Three Phase Transformers
Almost all major generation & Distribution Systems in the world are three phase ac systems
Three phase transformers play an important role in these systems
Transformer for 3 phase cct.s is either:
(a) constructed from 3 single phase transformers, or
(b) another approach is to employ a common core for
the three sets of windings of the three phases
The construction of a single three phase transformer is the preferred today, it is lighter, smaller, cheaper and slightly more efficient
There is an advantage that each unit in the bank could be replaced individually in the event of a fault, however this does not outweigh the other advantages of combined 3 ph. unit
Three Phase Transformers
3 phase transformer connections
The windings of primary and secondary (in any construction) can be connected in either a wye (Y) or delta (Δ)
This provides a total of 4 possible connections for 3
phase transformer (if Neutral is not grounded):
(a) Wye-wye Y-Y
(b) Wye-delta Y-Δ
© Delta-wye Δ-Y
(d) Delta-Delta Δ-Δ
To analyze a 3-phase transformer, each single transformer in the bank should be analyzed
Any single phase in bank behaves exactly like 1 phase transformer just studied
impedance, V.R., efficiency, & similar calculations for 3 ph. are done on per phase basis, using the same technique already used in single phase Transformer
The applications, advantages and disadvantages of each type of three phase connections will be discussed next
Three Phase TransformersY-Δ Connection
The phase angles of secondaries must be equal if they are to be paralleled, it means that direction of phase shifts also should be the same
In figure shown here, secondary lags primary if abc phase sequence applied,
However secondary leads primary when acb phase sequence applied
DELTA-WYE CONNECTION
In Δ-Y primary line voltage is equal to primary phase voltage VLP=VφP, in secondary VLS=√3VφS
Line to line voltage ratio ;
VLP/ VLS = VφP/ [√3 VφS ]=a/√3 Δ-Y
This connection has the same advantages & phase shifts as Y- Δ
And Secondary voltage lags primary voltage by 30◦ with abc phase sequence