Download Same Voltage Ratio

DC Machine & Transformer
• Parallel Operation of
Transformer
• Excitation Phenomenon in
Transformer
• Harmonics in Transformer
Made By:
Name
Enrollment No.
Abhishek Chovatia
1304201009508
Nehal Desai
1304201009509
Farzan Todiwala
1304201009510
Jay Kheni
1304201009513
Khushbu Naik
1304201009522
Content
•
•
•
•
•
•
•
Parallel Operation of 1-Phase Transformer
Need for Parallel Connection
Condition for Parallel Connection
Parallel Connection of 3 Phase Transformer
Advantage of Parallel Connection
Excitation Phenomenon in Transformer
Harmonics in Transformer
Introduction
• Parallel Operation of 1-Phase Transformer
By parallel operation we mean two or more transformers are
connected to the same supply bus bars on the primary side
and to a common bus bar/load on the secondary side. Such
requirement is frequently encountered in practice.
Need for Parallel Connection
• Non-availability of a single large transformer to meet the total
load requirement.
• The power demand might have increased over a time
necessitating augmentation of the capacity. More
transformers connected in parallel will then be pressed into
service.
• To ensure improved reliability. Even if one of the transformers
gets into a fault or is taken out for maintenance/repair the
load can continued to be serviced.
Need for Parallel Connection
• To reduce the spare capacity. If many smaller size
transformers are used one machine can be used as spare. If
only one large machine is feeding the load, a spare of similar
rating has to be available. The problem of spares becomes
more acute with fewer machines in service at a location.
• When transportation problems limit installation of large
transformers at site, it may be easier to transport smaller ones
to site and work them in parallel.
Condition for Parallel Connection
• The voltage ratio must be the same.
• The per unit impedance of each machine on its own base
must be the same.
• The polarity must be the same, so that there is no circulating
current between the transformers.
• The phase sequence must be the same and no phase
difference must exist between the voltages of the two
transformers.
Same Voltage Ratio:
If two transformers of different voltage ratio are connected in parallel
with same primary supply voltage, there will be a difference in secondary
voltages. Now say the secondary of these transformers are connected to
same bus, there will be a circulating current between secondaries and
therefore between primaries also. As the internal impedance of
transformer is small, a small voltage difference may cause sufficiently
high circulating current causing unnecessary extra I2R loss.
Same Percentage Impedance:
The current shared by two transformers running in parallel should be
proportional to their MVA ratings. Again, current carried by these
transformers are inversely proportional to their internal impedance.
From these two statements it can be said that, impedance of
transformer running in parallel are inversely proportional to their MVA
ratings. In other words, percentage impedance or per unit values of
impedance should be identical for all the transformers that run in
parallel.
Same Polarity:
Polarity of all transformers that run in parallel, should be the same
otherwise huge circulating current that flows in the transformer but no
load will be fed from these transformers. Polarity of transformer means the
instantaneous direction of induced emf in secondary. If the instantaneous
directions of induced secondary emf in two transformers are opposite to
each other when same input power is fed to both of the transformers, the
transformers are said to be in opposite polarity. If the instantaneous
directions of induced secondary emf in two transformers are same when
same input power is fed to the both of the transformers, the transformers
are said to be in same polarity.
Same Phase Sequence:
The phase sequence or the order in which the phases reach their maximum
positive voltage, must be identical for two parallel transformers. Otherwise,
during the cycle, each pair of phases will be short circuited.
Parallel Connection of 3 Phase
Transformer
• Single-phase transformers can be connected to form 3-phase
transformer banks for 3-phase power systems. Four common
methods of connecting three transformers for 3-phase circuits
are ∆- ∆, Y-Y, Y-∆, and ∆-Y connections.
Parallel Connection of 3 Phase
Transformer
• An advantage of ∆-∆ connection is that if one of the
transformers fails or is removed from the circuit, the
remaining two can operate in the open-∆ or Y connection.
This way, the bank still delivers 3-phase currents and voltages
in their correct phase relationship. However, the capacity of
the bank is reduced to 57.7 % of its original value.
• In the Y-Y connection, only 57.7% of the line voltage is applied
to each winding but full line current flows in each winding.
The Y-Y connection is rarely used
• The ∆-Y connection is used for stepping up voltages since the
voltage is increased by the transformer ratio multiplied by 3 .
The Y-∆ connection may be used for stepping down voltages.
Advantage of Parallel Connection
• It enables an existing transformer to be upgraded to meet an
increase in load, without having to remove that transformer
and replace it with a 'larger' (expressed in volt amperes) one.
It provides an useful way of using up a stock of smaller
transformers.
• Ex. We are currently using 55kVA transformer but there is a
need of 110kVA in future then we don’t need to replace it by
a bigger one, but we can connect another 55kVA transformer
and we can get the 110kVA output.
Excitation Phenomenon
It is known that though secondary is open, the transformer draws
current from supply, when primary is excited by rated voltage. This
current is not load current and is basically required to produce core
flux. But due to non-linearity of core material such as hysteresis and
saturation, the no load current is not sinusoidal in nature. Let us
study the effect of hysteresis and saturation on the waveform of no
load current which is also called exciting current.
Current inrush phenomenon ( Switching transient )
In the steady state operation, both V1 and Φ are sinusoidal and Φ lags V1 by
90°as shown in the Figure
When the primary voltage V1 is switched on to the transformer, the
core flux and the exciting current undergo a transient before
achieving the steady state. They pass through a transient period.
The effect of transient is severe when voltage wave pass through
origin.
In the inductive circuit flux can start with zero value. But the steady
state value of flux at start is -Φm , as shown in the Fig. 2, at t = 0. Thus
during transients a transient flux called off-set flux, Φt = Φm originates
such that at t = 0, Φt +Φss is zero at the instant of switching. This
transient flux Φt then decays according to circuit constants i.e. ratio
L/R. This ratio is generally very small for transformers.
Thus during transients, the total flux goes through a maximum
value of 2Φm. Such effect is called doubling effect. This is shown in the
Fig.3.
Due to the doubling effect, core flux achieves a value of 2Φm due
to which transformer draws a large exciting current. This is due to the
fact that core goes into deep saturation region of magnetisation. Such
a large exiting current can be as large as 100 times the normal exiting
current. To withstand electromagnetic forces developed due to large
current, the windings of transformer must be strongly braced. This
large current drawn during transient is called inrush phenomenon
Harmonics in Transformer
Harmonics increase both load and no-load losses due to increased
skin effect, eddy current, stray and hysteresis losses. The most
important of these losses is that due to eddy current losses in the
winding; it can be very large and consequently most calculation
models ignore the other harmonic induced losses. The precise
impact of a harmonic current on load loss depends on the
harmonic frequency and the way the transformer is designed. In a
transformer that is heavily loaded with harmonic currents, the
excess loss can cause high temperature at some locations in the
windings. This can seriously reduce the life span of the transformer
and even cause immediate damage and sometimes fire. Reducing
the maximum apparent power transferred by the transformer,
often called de-rating.
Problems Caused By Harmonics
•
•
•
•
•
•
Excessive heating in distribution transformer.
Increased cooling load on buildings.
Increased heating in motors and generators.
Increased heating in cables.
Nuisance tripping of breakers.
Malfunction or failure of communication and data processing
equipments.
Benefits
•
•
•
•
•
•
•
•
•
•
Reduces the K-Factor ratio of transformer.
Saves energy.
Prevents distribution system apparatus from overheating.
Saves Money.
Balances Phase currents and voltages on primary.
Reduces voltage distortion.
Increases system capacity and reliability.
Reduces apparatus vibration and noise.
Prevents electronic circuit breaker malfunction.
Provides healthier environment for the loads.