HVE M1 Ktunotes - in
HVE M1 Ktunotes - in
HVE M1 Ktunotes - in
• Test voltages generated using rectifiers are never constant in magnitude. These
deviate from the mean value periodically and this deviation is known as ripple.
The magnitude of the ripple voltage denoted by δV is defined as half the
difference between the maximum and minimum values of voltage i.e.,
• and ripple factor is defined as the ratio of ripple magnitude to the mean value Vd
i.e., δV/Vd.
• The test voltages should not have ripple factor more than 5% or as specified in a
specific standard for a particular equipment as the requirement on voltage shape
may differ for different applications.
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HALF-WAVE RECTIFIER CIRCUIT
• If the capacitor is not connected, pulsating d.c. voltage is obtained at the output
terminals whereas with the capacitance C, the pulsation at the output terminal are
reduced.
• Assuming the ideal transformer and small internal resistance of the diode during
conduction the capacitor C is charged to the maximum voltage Vmax during
conduction of the diode D.
• Assuming that there is no load connected, the d.c. voltage across capacitance
remains constant at Vmax whereas the supply voltage oscillates between ±Vmax
and during negative half cycle the potential of point A becomes – Vmax and hence
the diode must be rated for 2Vmax.
• This would also be the case if the transformer is grounded at A instead of B as
shown in Fig.
where I is the mean value of the d.c output iL(t) and VRL(t) the d.c. voltage which
includes a ripple
• This charge is supplied by the capacitor over the period T when the voltage
changes from Vmax to Vmin over approximately period T neglecting the
conduction period of the diode.
• The product fC is, therefore, an important design factor for the rectifiers. The
higher the frequency of supply and larger the value of filtering capacitor the
smaller will be the ripple in the d.c. output.
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• The single phase half-wave rectifier circuits have the following disadvantages:
i. The size of the circuits is very large if high and pure d.c. output voltages are
desired.
ii. The h.t. transformer may get saturated if the amplitude of direct current is
comparable with the nominal alternating current of the transformer.
It is to be noted that all the circuits considered here are able to supply relatively
low currents and therefore are not suitable for high current applications such as
HVDC transmission.
equals the discharge current which will include the load current and the leakage
and corona loss currents.
• The moving belt system also distorts the electric field and, therefore, it is
placed within properly shaped field grading rings. The grading is provided by
resistors and additional corona discharge elements.
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• The collector needle system is placed near the point where the belt enters the h.t.
terminal. A second point system excited by a self-inducing arrangement enables
the down going belt to be charged to the polarity opposite to that of the terminal
and thus the rate of charging of the latter, for a givens speed, is doubled.
• The self inducing arrangement requires insulating the upper pulley and
maintaining it at a potential higher than that of the h.t. terminal by connecting
the pulley to the collector needle system.
• The arrangement also consists of a row of points (shown as upper spray points in
Fig)connected to the inside of the h.t. terminal and directed towards the pulley
above its points of entry into the terminal. As the pulley is at a higher potential
(positive), the negative charges due to corona discharge at the upper spray points
are collected by the belt.
• This neutralises any remaining positive charge on the belt and leaves an excess
of negative charges on the down going belt to be neutralised by the lower spray
points. Since these negative charges leave the h.t. terminal, the potential of the
h.t. terminal is raised by the corresponding amount.
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The advantages of the generator are:
• Very high voltages can be easily generated
• Ripple free output
• Precision and flexibility of control
The disadvantages are:
• Low current output
• Limitations on belt velocity due to its tendency for vibration. The vibrations may
make it difficult to have an accurate grading of electric fields
These generators are used in nuclear physics laboratories for particle acceleration
and other processes in research work.
where Q is the quality factor of the inductor which usually varies between 40 and
80. This means that with Q = 40, the output voltage is 40 times the supply
voltage. Downloaded from Ktunotes.in
The following are the advantages of series resonance circuit.
i. The power requirements in KW of the feed circuit are (kVA)/Q where kVA is
the reactive power requirements of the load and Q is the quality factor of
variable reactor usually greater than 40. Hence, the requirement is very small.
ii. The series resonance circuit suppresses harmonics and interference to a large
extent. The near sinusoidal wave helps accurate partial discharge measurements
and is also desirable for measuring loss angle and capacitance of insulating
materials using Schering Bridge.
iii. In case of a flashover or breakdown of a test specimen during testing on high
voltage side,the resonant circuit is detuned and the test voltage collapses
immediately. The short circuit current is limited by the reactance of the variable
reactor. It has proved to be of great value as the weak part of the isolation of the
specimen does not get destroyed. In fact, since the arc flash over has very small
energy, it is easier to observe where exactly the flashover is occurring by
delaying the tripping of supply and allowing the recurrence of flashover.
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The following are the advantages of series resonance circuit.
(iv) No separate compensating reactors (just as we have in case of test transformers)
are required.This results in a lower overall weight.
(v) When testing SF6 switchgear, multiple breakdowns do not result in high
transients. Hence, no special protection against transients is required.
(vi) Series or parallel connections of several units is not at all a problem. Any
number of units can be connected in series without bothering for the
impedance problem which is very severely associated with a cascaded test
transformer. In case the test specimen requires large current for testing, units
may be connected in parallel without any problem.