Safety being the major concern for working in the high voltage laboratory, the alternate is to simulate the hardware system using various circuit design software’s. Thus, in this paper, broad overview of various methodologies for the generation of both AC/DC impulse voltage, the method used to si...
High Voltage1 Practical 2021
High voltage AC for testing an 88 kV XLPE cable
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Abstract:
Safety being the major concern for working in the high voltage laboratory, the alternate is to simulate
the hardware system using various circuit design software’s. Thus, in this paper, broad overview of
various methodologies for the generation of both AC/DC impulse voltage, the method used to
simulate circuits for the 2-stage cascade transformer and series resonant transformer is MATLAB
Simulink where analysis and results obtained by analysing graphs after running the simulation.
Comparison of results was also observed graphical from MATLAB and calculations of output current
and voltage where validated from MATLAB after simulating the circuits accurately as per given
specification of the Capacitors, Resistance and length of the cable.
, 1.INTRODUCTION
The practical is on the generation of high voltage AC for testing
an 88 kV XLPE cable and will involve simulating two high
voltage AC generators and defining what the best generator to
use on site is. The first is a 2-stage cascade transformer and the
second is a series resonant generator. The simulations where
done on MATLAB Simulink and obtaining results graphical
from the 2-stage cascade transformer and series resonant
generator. Comparison was obtained by analysis the results
produced by the circuits output current and voltage and power
which should correspond with the calculated values. We where
given the capacitor to work on which is capacitance per unit
length is 0.2557 µF/km. The length of the cable is the last 2
digits of your student number multiplied by 10. After
performing calculations then we compare results obtained from
MATLAB simulation. Figure 1. Typical circuit of a Cascade Transformer
1.1 PREREQUISITES B) Series Resonant Transformer
A resonant transformer is one that has a capacitor through one
1.1.1. Two Types of Ac Generators used: or both windings and acts as a tuned circuit. Resonant
2-stage cascade transformer transformers can be used as high Q factor bandpass filters at
Series Resonant transformer radio frequencies.
The theory of a series resonant circuit is used to generate the
high voltages and currents needed for equipment testing (such
A) 2-stage cascade transformer as cables).
A source of high voltage that is comprised of an assortment of The following are some of the benefits of using a series
step-up transformers; auxiliary windings are in arrangement, resonant circuit:
and primary windings, with the exception of the first, are A sine wave with a frequency of 50 Hz and no
provided from a couple of taps on the secondary winding of the harmonics.
former transformer. Resonance ceases upon loss (i.e., the capacitance
Advantages of Cascading: does not stay the same, causing the voltage to drop)
Cascading has the following benefits over a single unit testing There is the possibility of cascading
transformer: It is simple and compact
1) As compared to a single transformer, the cost of a cascaded There are no repeated flashovers (Charging time)
transformer is less for a comparable power.
2) The individual units are shorter, and the bushings are smaller. Disadvantage of Resonant transformer:
Although the units can be stacked one on top of the other, this Very Low Frequency Transformers and cables are tested at 0.1
decreases the amount of space required. Hz or 0.01 Hz because the necessary charging current is much
3) Since the units are similar, there is a higher level of reliability lower.
and interchangeability. Equation of voltage across the capacitance is given by:
Disadvantages of Cascading:
The following are some of the limitations of transformer
cascading:
1) As the impedance increases exponentially, roughly as
n2, where n is the number of cascaded units, the
current in a short circuit state decrease. As a result,
the number of cascaded units is limited to 3 or 4
cannot exceed to 5.
2) Iron and copper losses are high since the first unit
must supply the iron and copper losses of all
subsequent units in the sequence, while the second
provides for the third and the fourth unit.
Figure 2. Series Resonant transformer
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