Potential Transformer
Potential transformers
are a type of instrument transformer and these transformers are used to
measure the voltages of high transmission lines or any electrical
circuits. Potential transformers are used to step down the high voltages.
Their primary coils are connected to high voltage side and their secondary
coils are connected to various instruments such as voltmeters, wattmeters,
power factor meters etc. These potential transformers provide
100v to 150 v on their secondary winding so that the measuring instruments are
connected easily according to their input voltages and these are sufficient to
operate in this range of voltage. Measuring instruments such as wattmeters,
voltmeters which are connected to the secondary winding of potential
transformer provide low readings according to their input voltage. The exact
readings are obtained from these measuring instruments by dividing the
transformation ratio. So that we can easily obtain the exact amount of voltage
with the help of potential transformer. Figure shows the potential transformer
is connected with the voltmeter and how it is used when connected to the high
voltage side below:
The potential transformers and the power transformers are
same to each other only the difference between these is that potential
transformers have special requirements for the measurement of various
electrical parameters. These requirements are as follows:
- Attenuation ratio must be accurately maintained in a potential transformer for measurement of electrical parameters.
- Voltage drops should be reduced as possible and these are reduced by using proper design of core and by using large conductors.
- The load voltage / secondary voltage should be minimum and of few volt-amperes so that the measuring instruments are connected easily on the secondary of potential transformers.
Construction
A potential transformer consists of its so many parts in its
construction and these parts of potential transformer (PT) are discussed
below:
Core
Potential transformers are made with its two types of core
construction and these two types are core type PT and shell type PT.
The core type PT’s are used for low voltage while the shell type PT’s are used
for high voltage. The core laminations of both shell type and core type PT’s
are assembled with so many care to avoid from air gap between the joints.
Winding
The both windings (primary and secondary) are coaxial cable type
so that the leakage reactance is minimized. The secondary winding is placed
nearer to the core and the primary winding is twisted on the secondary winding.
The primary winding is made of a single coil when PT is required for low
voltage use otherwise the primary winding is of double coil for high voltage
uses.
Insulation
The insulation is required to separate the primary and secondary
winding so cotton tape and varnish are most widely used as an insulation in
PT’s. PT’s are filled with solid compounds for low voltage otherwise the
insulation is oil immersed.
Bushings
The bushings of PT are consisting of various types some are oil
filed and these bushings are used for oil filled potential transformers.
Some PT’s have two bushings for high voltage and some has only one bushing for
high voltage and some have two bushings and these are grounded and these not
require any neutral connections.
Working
Potential transformers are commonly used for the measurement of
voltage and power of a distribution lines or transmission lines and it is used
for the measurement of high voltage and power between the high power cable and
mainly used in grid stations, power stations and on those places where power is
generated and controlled for transmission and distribution for power. The
working of a potential transformer is that it steps down the high voltage and
provide the power according to the reading of a measuring instrument which is
installed for the measurement of voltage or power. So, its main purpose is to
provide voltage according to the measuring instrument so that the power
is controlled according to its rating.
Theory of Potential Transformer
Figure shows the equivalent circuit of potential transformer and
the theory of potential transformer is as under:
Vp = primary voltage
Ep = induced voltage in primary winding
Vs = secondary voltage
Es = induced voltage in secondary winding
Ip = primary current
Is = secondary current
I0 = no load current
Ic = core loss component of current
Im = magnetizing component of current
Rp = resistance of primary winding
Xp = reactance of primary winding
Rs = secondary winding resistance
Xs = secondary winding reactance
Rc = imaginary resistance / core loss
Xm = magnetizing reactance
Re = resistance of external load
Xe = reactance of external load
Np = primary winding turns
Ns = secondary winding turns
N = turn ratio
Φ = flux of potential transformer
δ = Phase angle between secondary winding voltage and secondary
winding
β = Phase angle between primary current and Secondary current
α = Phase angle between no-load current I0 and flux φ
The flux is conspired along the x axis. Im is
in phase with flux. Ic leads by Im 900. The
sum of Ic and Im produces no load current Io. Ep
is in the phase with the core loss component of current Ic. Es
is 180o out of phase with the primary winding voltage Ep.
Secondary voltage Vs are obtained by subtracting the IsRs
and IsXs from secondary voltage.
Phasor Diagram of Potential Transformer (PT)
The phasor diagram is displayed below:
The phase angle between the primary voltage and the secondary
voltage is called the phase angle of PT. This phase angle is ideally equal to
zero because these two phasors are in same phase.
From the phasor diagram we have,
But in reality, the phase angle is very small and the primary and
secondary voltage are perpendicular to the flux and then;
Eq pt 2
Where Rs is equivalent resistance of PT and Xs
equivalent reactance of PT.
Phase Angle of Potential Transformer (PT)
From the phasor diagram the terms Ip and Is are less compared to
large voltage and these terms are neglected thus we get:
Errors in Potential Transformers (PT)
There are two types of errors occurred in potential
transformers. These errors also launch in measurement of voltage. These errors
are occurred in terms of magnitude of the measured values. These errors are
discussed below:
Ratio Errors
The ratio of the potential transformer is the difference between
the minor and actual transformation ratio. This error is occurred in
measurements of voltage and these errors are occurred due to the less ratio
between the transformation ratio.
Phase Angle Errors
Phase angle errors are occurred during the measurements of
power. In these errors, the primary circuit of the potential transformer cannot
be attained by multiplying the voltage which are measured with voltmeter. These
errors are determined by the resistance and reactance and on the no-load
current of the transformer.
Reducing Errors in Potential Transformer
(PT)
The following points are used for the reduction of errors in
potential transformer:
To reduce the length of magnetic path in the core. With this, the
no load primary current is reduced.
By using thick conductors and to reduce the length of mean turn
of the windings.
By keeping close primary and secondary windings to each other.
This reduces the leakage flux and the leakage reactance.
By reducing the length of wound winding over the core. This will
reduce resistance of winding and provide high flux densities in the core.
Characteristics of ON Load Potential Transformer
(PT)
Characteristics of potential transformer are determined through
the phasor diagram which is shown above and determination between the ratio
errors and phase angle errors of potential transformers. The following are the
effects of various electrical parameters through which we will know the
characteristics of practical potential transformer.
Effects of Changing in Secondary Voltage
of Potential Transformer
When we increase the secondary voltage of the potential
transformer then the secondary current also increases and this secondary
current also increases the primary current of the potential transformer. Thus
the voltage drops are produced in both primary and secondary winding and
increases with respect to increase in primary and secondary currents. The
secondary voltage are reduced according to the primary supply voltage. So, this
effect increases the transformation ratio (Vp/Vs where Vp
are the primary voltage and Vs are the secondary voltage) and it
also increases the ratio error and phase angle error and these errors remain
linear in position as shown in the figure below:
Effects of Changing in Power Factor of Secondary
Voltage
According to the phasor diagram of potential transformer, the
secondary current lags in arrears to the secondary voltage and the phase angle
difference remains positive. When we lower the power factor, this phase angle
increases and it moves absent from secondary voltage. The phasor diagram
represents that the primary current closes to no load current and primary and
secondary voltage are in phase with induced voltage in primary and secondary
voltage. The primary voltage remains same and the transformation ratio
increases due to reduction in power factor. The figure below identifies the
phase angle and ratio errors in potential transformer according to changes in
power factor of secondary voltage.
