The
windings of three phase transformers may be connected in by Y
or Δ in the same manner as for three single phase transformer.
Since the
secondary’s may be connected either in Y or Δ regardless of which
connection is used on the primaries, there must be four ways of connecting the
windings of a 3-phase transformer for transformation of 3-phase voltages,
namely Y-y,Δ -Δ, Y-Δ, and Δ -y.
The inter-connections are made
inside of the case so that only the terminal leads need to be brought outside
the case:
1. Star – Star Transformer (Yy0 or Yy6)
2. Delta – Delta Transformer (Dd0 or Dd6)
3. Delta – Star Transformer (Dy)
4. Star – Delta Transformer (Yd)
(Grounding Transformer)
5. Zig-zag Transformer (Yz, Dz)
(Grounding Transformer)
6. Scott (“T” Type) Transformer
(Grounding Transformer)
1. Star-Star (Y-y) Connection
Star-Star (Y-y) Transformer
Connection
In Primary Winding Each Phase
is120°electrical degrees out of phase with the other two phases.
In Secondary
Winding Each Phase is120°electrical degrees out of phase with the other two
phases.
Each primary
winding is magnetically linked to one secondary winding through a common core
leg. Sets of windings that are magnetically linked are drawn parallel to each
other in the vector diagram. In the Y-Y connection, each primary and secondary
winding is connected to a neutral point.
The neutral
point may or may not be brought out to an external physical connection and the
neutral may or may not be grounded.
Neutral
connection
Transformer
magnetizing currents are not purely sinusoidal, even if the exciting voltages
are sinusoidal. The magnetizing currents have significant quantities of
odd-harmonic components. If three identical transformers are connected to each
phase and are excited by 60 Hz voltages of equal magnitude, the 60 Hz fundamental
components of the exciting currents cancel out each other at the neutral.
This is because
the 60 Hz fundamental currents of A, B, and C phase are 120° out of phase with
one another and the vector sum of these currents is zero.
The third,
ninth, fifteenth and other so-called zero-sequence harmonic currents are in
phase with each other; therefore, these components do not cancel out each other
at the neutral but add in phase with one another to produce a zero-sequence
neutral current, provided there is a path for the neutral current to flow.
Due to the
nonlinear shape of the B-H curve, odd-harmonic magnetizing currents are
required to support sinusoidal induced voltages. If some of
the magnetizing current harmonics are not present, then the induced
voltages cannot be sinusoidal.
Y-Y Connection with Grounded Neutral
Figure below
shows the situation where the primary neutral is returned to the voltage source
in a four-wire three-phase circuit. Each of the magnetizing currents labeled
IR, IY, and IB contain the 60 Hz fundamental current and all of the odd
harmonic currents necessary to support sinusoidal induced voltages.
Transformer Y-Y Connection with
Grounded Neutral
The
zero-sequence magnetizing currents combine to form the neutral current IN,
which returns these odd harmonics to the voltage source. Assuming that the
primary voltage is sinusoidal, the induced voltages VR, VY and VB (in both the
primary and secondary) are sinusoidal as well.
The connection
of primary neutral to the neutral of generator has an add advantage that it
eliminates distortion in the secondary phase voltages. If the flux in the core
has sinusoidal waveform then it will give sinusoidal waveform for the
voltage. But due to characteristic of iron, a sinusoidal waveform of flux
requires a third harmonic component in the exciting current. As the frequency
of this component is thrice the frequency of circuit at any given constant. It
will try to flow either towards or away from the neutral point in the
transformer windings.
With isolated
neutral, the triple frequency current cannot flow so the flux in the core will
not be a sine wave and the voltages are distorted. If primary neutral is
connected to generator neutral the triple frequency currents get the path to
solve the difficulty. The alternative way of overcoming with this difficulty is
the use of tertiary winding of low KVA rating. These windings are connected in
delta and provide a circuit in which triple frequency currents can flow. Thus
sinusoidal voltage on primary will give sinusoidal voltage on secondary side.
This situation
changes if the neutrals of both sets of the primary and secondary windings are
not grounded.
Y-Y Connection without Grounded Neutral
If the neutrals
of both the primary and the secondary are open-circuited and so there is no
path for the zero-sequence harmonic currents to flow and the induced
voltages will not be sinusoidal.
Transformer Y-Y Connection without
Grounded Neutral
V’R, V’Y, and
V’B will not be sinusoidal. This results in distortions of the secondary
voltages. The resulting voltage distortion is equivalent to a Y-Y transformer
with zero-sequence currents allowed to flow in the primary neutral with an
imaginary superimposed primary winding carrying only the zero-sequence currents
180° out of phase with the normal zero-sequence currents.
Analysis of the
voltages induced by the ‘‘primary windings’’ is greatly complicated by the fact
that the core is highly nonlinear so that each of the individual zero-sequence
harmonics currents carried by the phantom primary windings will induce even
higher-order harmonic voltages as well.
Fourier analysis
can be used to arrive at an approximation of the secondary voltages with an
open primary neutral. Taking one phase at a time, the normal magnetizing
current for a sinusoidal exciting voltage is plotted from the B-H curve of the
transformer. The normal magnetizing current is converted to a Fourier series
and then it is reconstructed by removing all of the zero-sequence harmonics.
The resulting
exciting current will have a shape different from the normal exciting current,
which is then used to construct an induced voltage using the B-H curve in there
verse manner that was used to construct the original exciting current.
This process is
rather laborious, so suffice it to say that if a Y-Y transformer does not have
a neutral path for zero-sequence exciting currents, there will be harmonic
voltages induced in the secondary even if the exciting voltage is purely
sinusoidal.
Advantages of Y-Y connection
No Phase Displacement
The primary and
secondary circuits are in phase; i.e., there are no phase angle displacements
introduced by the Y-Y connection. This is an important advantage when
transformers are used to interconnect systems of different voltages in a
cascading manner. For example, suppose there are four systems operating at 800,
440, 220, and 66 kV that need to be interconnected. Substations can be
constructed using Y-Y transformer connections to interconnect any two of these
voltages. The 800 kV systems can be tied with the 66 kV systems through a
single 800 to 66 kV transformation or through a series of cascading
transformations at 440,220 and 66 kV.
Required Few Turns for winding
Due to star
connection, phase voltages is (1/√3) times the line voltage. Hence less number
of turns is required. Also the stress on insulation is less. This makes the
connection economical for small high voltage purposes.
Required Less Insulation Level
If the neutral
end of a Y-connected winding is grounded, then there is an opportunity to use
reduced levels of insulation at the neutral end of the winding. A winding that
is connected across the phases requires full insulation throughout the winding.
Handle Heavy Load
Due to star
connection, phase current is same as line current. Hence windings have to carry
high currents. This makes cross section of the windings high. Thus the windings
are mechanically strong and windings can bear heavy loads and short circuit
current.
Use for Three phases Four Wires
System
As neutral is
available, suitable for three phases four wire system.
Eliminate Distortion in
Secondary Phase Voltage
The connection
of primary neutral to the neutral of generator eliminates distortion in the
secondary phase voltages by giving path to triple frequency currents toward to
generator.
Sinusoidal voltage on secondary
side
Neutral give
path to flow Triple frequency current to flow Generator side thus sinusoidal
voltage on primary will give sinusoidal voltage on secondary side.
Used as Auto Transformer
A Y-Y transformer
may be constructed as an autotransformer, with the possibility of great cost
savings compared to the two-winding transformer construction.
Better Protective Relaying
The protective
relay settings will be protecting better on the line to ground faults when the
Y-Y transformer connections with solidly grounded neutrals are applied.
Disadvantages of Y-Y connection
The Third harmonic issue
The voltages in
any phase of a Y-Y transformer are 1200 apart from the voltages in any other
phase. However, the third-harmonic components of each phase will be in phase
with each other. Nonlinearities in the transformer core always lead to
generation of third harmonic.
These components
will add up resulting in large (can be even larger than the fundamental
component) third harmonic component.
Overvoltage at Lighting Load
The presence of
third (and other zero-sequence) harmonics at an ungrounded neutral can cause
overvoltage conditions at light load. When constructing a Y-Y transformer using
single-phase transformers connected in a bank, the measured line-to-neutral
voltages are not 57.7% of the system phase-to-phase voltage at no load but are
about 68% and diminish very rapidly as the bank is loaded.
The effective
values of voltages at different frequencies combine by taking the square root
of the sum of the voltages squared. With sinusoidal phase-to-phase voltage, the
third-harmonic component of the phase-to-neutral voltage is about 60%.
Voltage drop at Unbalance Load
There can be a
large voltage drop for unbalanced phase-to-neutral loads. This is caused by the
fact that phase-to-phase loads cause a voltage drop through the leakage
reactance of the transformer whereas phase-to-neutral loads cause a voltage
drop through the magnetizing reactance, which is 100 to 1000 times larger than
the leakage reactance.
Overheated Transformer Tank
Under certain
circumstances, a Y-Y connected three-phase trans- can produce severe tank
overheating that can quickly destroy the transformer. This usually occurs with
an open phase on the primary circuit and load on the secondary.
Over Excitation of Core in
Fault Condition
If a
phase-to-ground fault occurs on the primary circuit with the primary neutral
grounded, then the phase-to-neutral voltage on the un faulted phases increases
to 173% of the normal voltage. This would almost certainly result in over
excitation of the core, with greatly increased magnetizing currents and core
losses
If the neutrals
of the primary and secondary are both brought out, then a phase-to-ground fault
on the secondary circuit causes neutral fault current to flow in the primary
circuit. Ground protection re- laying in the neutral of the primary circuit may
then operate for faults on the secondary circuit
Neutral Shifting
If the load on
the secondary side unbalanced then the performance of this connection is not
satisfactory then the shifting of neutral point is possible. To prevent
this, star point of the primary is required to be connected to the star point
of the generator.
Distortion of Secondary voltage
Even though the
star or neutral point of the primary is earthed, the third harmonic present in
the alternator voltage may appear on the secondary side. This causes distortion
in the secondary phase voltages.
Over Voltage at Light Load
The presence of
third (and other zero-sequence) harmonics at an ungrounded neutral can cause
overvoltage conditions at light load.
Difficulty in coordination of
Ground Protection
In Y-Y
Transformer, a low-side ground fault causes primary ground fault current,
making coordination more difficult.
Increase Healthy Phase Voltage
under Phase to ground Fault
If a
phase-to-ground fault occurs on the primary circuit with the primary neutral
grounded, then the phase-to-neutral voltage on the UN faulted phase’s increases
to 173% of the normal voltage.
If the neutrals
of the primary and secondary are both brought out, then a phase-to-ground fault
on the secondary circuit causes neutral fault current to flow in the primary
circuit.
Trip the T/C in Line-Ground
Fault
All harmonics
will propagate through the transformer, zero-sequence current path is
continuous through the transformer, one line-to-ground fault will trip the
transformer.
Suitable for Core Type
Transformer
The third
harmonic voltage and current is absent in such type of connection with three
phase wire system. or shell type of three phase units, the third harmonic phase
voltage may be high. This type of connection is more suitable for core type
transformers.
Application
This Type of
Transformer is rarely used due to problems with unbalanced loads.
It is
economical for small high voltage transformers as the number
of turns per phase and the amount of insulation required is less.
