Voltage Regulation

Ammiel G.
Buising December 28, 2017

EE-4B
Topic: Voltage Regulation and Transformer Efficiency
Voltage Regulation
Modern power systems operate at some standard voltages. The equipment’s working on these
systems are therefore given input voltages at these standard values, within certain agreed tolerance
limits. In many applications this voltage itself may not be good enough for obtaining the best
operating condition for the loads. A transformer is interposed in between the load and the supply
terminals in such cases. There are additional drops inside the transformer due to the load currents.
While input voltage is the responsibility of the supply provider, the voltage at the load is the one
which the user has to worry about. If undue voltage drop is permitted to occur inside the transformer
the load voltage becomes too low and affects its performance. It is therefore necessary to quantify
the drop that takes place inside a transformer when certain load current, at any power factor, is
drawn from its output leads.
The voltage regulation can be defined in two ways - Regulation Down and Regulation up. These two
definitions differ only in the reference voltage as can be seen below. Regulation down: This is
defined as “the change in terminal voltage when a load current at any power factor is applied,
expressed as a fraction of the no-load terminal voltage”.
Vnl and Vl are no-load and load terminal voltages. This is the definition normally used in the case of
the transformers, the no-load voltage being the one given by the power supply provider on which
the user has no say. Hence no-load voltage is taken as the reference. Regulation up: Here again the
regulation is expressed as the ratio of the change in the terminal voltage when a load at a given
power factor is thrown off, and the on load voltage.
He has to generate proper no-load voltage at the generating station to provide the user the voltage
he has asked for. In the expressions for the regulation, only the numerical differences of the voltages
are taken and not vector differences. In the case of transformers both definitions result in more or
less the same value for the regulation as the transformer impedance is very low and the power factor
of operation is quite high.
The power factor of the load is defined with respect to the terminal voltage on load. Hence a
convenient starting point is the load voltage. Also the full load output voltage is taken from the name
plate. Hence regulation up has some advantage when it comes to its application.
1
A low value of the short circuit impedance /reactance results in a large short circuit current in case
of a short circuit. This in turn results in large mechanical forces on the winding. So, in large
transformers the short circuit impedance is made high to give better short circuit protection to the
transformer which results in poorer regulation performance. In the case of transformers provided
with taps on windings, so that the turn’s ratio can be changed, the voltage regulation is not a serious
issue.
The output voltage in a transformer will not be maintained constant from no load to the full load
condition, for a fixed input voltage in the primary. This is because there will be internal voltage drop
in the series leakage impedance of the transformer the magnitude of which will depend upon the
degree of loading as well as on the power factor of the load. The knowledge of regulation gives us
idea about change in the magnitude of the secondary voltage from no load to full load condition at a
given power factor.
This can be determined experimentally by direct loading of the transformer. To do this, primary is
energized with rated voltage and the secondary terminal voltage is recorded in absence of any load
and also in presence of full load. Suppose the readings of the voltmeters are respectively V20 and V2.
Therefore change in the magnitudes of the secondary voltage is V20 – V2. This change is expressed
as a percentage of the no load secondary voltage to express regulation. Lower value of regulation will
ensure lesser fluctuation of the voltage across the loads. If the transformer were ideal regulation
would have been zero.
Efficiency of transformer
In a practical transformer we have seen mainly two types of major losses namely core and copper
losses occur. These losses are wasted as heat and temperature of the transformer rises. Therefore
output power of the transformer will be always less than the input power drawn by the primary
from the source and efficiency is defined as
Pcore remains practically constant since the level of flux remains practically same. On the other hand
we know that the winding currents depend upon the degree of loading and copper loss directly
depends upon the square of the current and not a constant from no load to full load condition. We
shall write a general expression for efficiency for the transformer operating at x per unit loading and
delivering power to a known power factor load.
2
Therefore efficiency of the transformer for general loading will become:
If the power factor of the load (i.e., cos θ) is kept constant and degree of loading of the transformer is
varied we get the efficiency Vs degree of loading curve as shown in the figure 25.5. For a given load
power factor, transformer can operate at maximum efficiency at some unique value of loading i.e., x.
To find out the condition for maximum efficiency, the above equation for η can be differentiated with
respect to x and the result is set to 0. Alternatively, the right hand side of the above equation can be
simplified to, by dividing the numerator and the denominator by x. the expression for η then
becomes:
Reference:
L-27TBET-EENPTEL.pdf
1_9.pdf

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Ammiel G.

Buising December 28, 2017

Topic: Voltage Regulation and Transformer Efficiency

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