Essay: Generation of High DC, AC, impulse voltages and currents

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  • Generation of High DC, AC, impulse voltages and currents
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High frequency AC voltage are required for rectifier circuits. It is used for testing electrical apparatus used in case of switching surges.

Advantages of high frequency transformer (High Frequency Transformer are used for producing high-frequency, high voltage damped oscillation):

(i) Absence of iron core results in less cost & size.

(ii) Pure sine wave output without any ripples.

(iii) Increase in voltage is slow hence no damage due to surges.

Commonly used high frequency resonant transformer is the Tesla coil. It is nothing but a doubly tuned resonant circuit.

(i) Power frequency voltage and current (AC)

In an AC network ,the equipment is continuously subjected to full power frequency voltage. The equipment should be able to withstand power frequency voltage allowing for some over voltage.

In a high voltage laboratory, the test transformers steps up the voltage from a lower voltage (220 V or 11 kV) to the desired voltage level. All laboratory tests are single phase and the low voltage side of the transformer is supplied via a regulating transformer to be able to adjust the magnitude of the output high voltage. A typical AC high voltage set-up is shown in figure.

The following features should be noted:

• Ground Plane: The high voltage is generated with respect to the laboratory ground, a low impedance sheet connected to an earth electrode.

• Voltage Divider: The voltage is measured with a resistive or capacitive voltage divider.

Schematic diagram of a typical AC test transformer and its connections

In the design, an insulated tank (a resin impregnated paper cylinder) is used and a bushing is not required.

Test transformers can be used in cascade connections as shown in figure. Each unit has 3 windings: a primary (low voltage), a secondary (high voltage) and a tertiary (low voltage) winding. The tertiary has the same rating as the primary winding, however it is insulated for high voltage.

The tertiary winding is used to supply the primary of the next unit. The tanks of the second and third units are insulated for high voltage and are mounted on insulators.

Cascade Connected Test Transformers

The method when performing AC tests is to increase the voltage gradually until flashover occurs. The voltage just before flashover is the flashover voltage.

(ii) Resonant Transformers

The equivalent circuit of a high voltage testing transformer consist of the leakage reactances of the windings, the windings resistances, the magnetizing reactance and the shunt capacitance across the output terminal due to the bushing of the high voltage terminal and also that of the test object. This is shown in figure.

Resonant Transforme- Equivalent Circuit.

T – Testing transformer

L – Choke

C – Capacitance of a h.v. terminal and test object

L0 – Magnetizing inductance

L1, L2 – Leakage inductance of the transformer

r1, r2 – Resistance of the windings

R0 – Resistance due to core loss

It may be seen that it is possible to have series resonance at power frequency

With this condition, the current in the test object is very large and is limited only by the resistance of the circuit. The waveform of the voltage across the test object will be purely sinusoidal.

The magnitude of the voltage across the capacitance C of the test object will be,

Where R is the total series resistances of the circuit.

The factor XC /R is the Q factor of the circuit and gives the magnitude of the voltage multiplication across the test object under resonance conditions.

Therefore the input voltage required for excitation is reduced by a factor 1/Q and the output kVA required is also reduced by a factor 1/Q. The secondary power factor of the circuit is unity.

This principle is utilized in testing at very high voltage and on occasions requiring large current outputs such as cable testing, dielectric loss measurements, partial discharge measurements, etc.

A transformer with 50 to 100 kV voltage rating and a relatively large current rating is connected together with an additional choke, if necessary.

The test condition is set such that

Where Le is the total equivalent leakage inductance of the transformer including its regulating transformer.

The chief advantages of this principle are:

a) It gives an output of pure sine wave.

b) Power requirements are less (5 to 10% of total kVA required).

c) No high power arcing and heavy current surges occur if the test object failed as resonance ceases at the failure of the test object.

d) Cascading is also possible for very high voltage.

e) Simple and compact test arrangement.

f) No repeated flashovers occur in case of partial failures of the test object and insulation recovery.

It can be shown that the supply source takes Q number of cycles atleast to charge the test specimen to the full voltage.

The disadvantages are the requirements of additional variable chokes capable of withstanding the full test voltage and the full current rating.


High DC voltages are needed in insulation tests on cables and capacitors. Impulse generator charging units also require high DC voltages of about 100 to 200 kV. Normally for the generation of DC voltages of upto 100 kV, electronic valve rectifiers are used and the output currents are about 100 mA.

The rectifier valves require special construction for cathode and filaments since a high electrostatic field of several kV/cm exist between the anode and the cathode in the non-conduction period. The AC supply to the rectifier tubes may be of power frequency or may be of audio frequency from an oscillator.

The latter is used when a ripple of very small magnitude is required without the use of costly filters to smoothen the ripple.

Circuits to generate high DC voltages:

(i) Half & full wave rectifier.

(ii) Voltage doubler.

(iii) Voltage multiplier.

(iv) Van de Graaff generator.

(i) Rectifier Circuits:

One of the simplest methods of producing high direct voltages for testing is to use either a half-wave for full-wave rectifier circuit with a high alternating voltage source.

The rectifiers used must be high voltage rectifiers with a peak inverse voltage of at least twice the peak value of the alternating voltage supply. In theory, a low pass filter may be used to smooth the output, however when the test device is highly capacitive, no smoothing is required.

Half-wave and Full-wave Rectifier circuits

Only a capacitance may be used across the test device for smoothing. Figure shows the half-wave and the full-wave arrangements.

In testing with high voltage direct current, care must be taken to discharge any capacitors that may be present before changing connections. In certain test sets, automatic discharging is provided which discharges the capacitors to earth.

(ii) Voltage Doubler Circuit

The voltage doubler circuit makes use of the positive and the negative half cycles to charge two different capacitors. These are then connected in series aiding to obtain double the direct voltage output. Figure shows a voltage doubler circuit.

Voltage Doubler Circuit

In this case, the transformer will be of small rating than for the same direct voltage rating with only simple rectification. Further for the same direct voltage output, the peak inverse voltage of the diodes will be halved.

(iii) Electrostatic generators

Electrostatic generators using the principle of charge transfer can give very high direct voltages.

The basic principle involved is that the charge is placed on a carrier either insulating or an isolated conductor and raised to the required potential by being mechanically moved through the electrostatic field.

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