In the fields of electrical engineering and applied physics, high voltages (d.c., a.c., and impulse) are required for several applications. For example, electron microscopes and x-ray units require high d.c. voltages of the order of 100 kV or more. Electrostatic precipitators, particle accelerators in nuclear physics, etc. require high voltages (d.c.) of several kilovolts and even megavolts.

High a.c. voltages of one million volts or even more are required for testing power apparatus rated for extra high transmission voltages (400 kV system and above). High impulse voltages are required for testing purposes to simulate overvoltages that occur in power systems due to lightning or switching surges.

For electrical engineers, the main concern of high voltages is for the insulation testing of various components in power systems for different types of voltages, namely, power frequency a.c., high frequency, switching or lightning impulses.

Hence, generation of high voltages in laboratories for testing purposes is essential and is presented here.

Different forms of high voltages mentioned are classified as(i)high d.c. voltages(ii) high a.c. voltages of power frequency.(iii) high a.c. voltages of high frequency.(iv) high transient or impulse voltages of very short duration such

as lightning overvoltages, and(v) transient voltages of longer duration such as switching surges.Normally, in high voltage testing, the current under conditions of

failure is limited to a small value (less than an ampere in the case of d.c. or a.c. voltages and few amperes in the case of impulse or transient voltages). But in certain cases, like the testing of surge diverters or the short circuit testing of switchgear, high current testing with several hundreds of amperes is of importance. Tests on surge diverters require high surge currents of the order of several kiloamperes.

Therefore, test facilities require high voltage and high current generators. High impulse current generation is also required along with voltage generation for testing purposes.

Normally, in high voltage testing, the current under conditions of failure is limited to a small value (less than an ampere in the case of d.c. or a.c. voltages and few amperes in the case of impulse or transient voltages).

But in certain cases, like the testing of surge diverters or the short circuit testing of switchgear, high current testing with several hundreds of amperes is of importance.

Tests on surge diverters require high surge currents of the order of several kiloamperes. Therefore, test facilities require high voltage and high current generators.

High impulse current generation is also required along with voltage generation for testing purposes.

Generation of High Alternating VoltagesSingle transformer test units are made for high alternating voltages up to

about 200 kV.However, for high voltages to reduce the cost (insulation cost increases

rapidly with voltage) and make transportation easier, a cascade arrangement of several transformers is used.

Cascade arrangement of transformersFigure shows a typical cascade arrangement of transformers used to

obtain up to 300 kV from three units each rated at 100 kV insulation. The low voltage winding is connected to the primary of the first transformer,and this is connected to the transformer tank which is earthed. One end of the high voltage winding is also earthed through the tank. The high voltage end and a tapping near this end is taken out at the top of the transformer through a bushing, and forms the primary of the second transformer.

One end of this winding is connected to the tank of the second transformer to maintain the tank at high voltage. The secondary of this transformer too has one end connected to the tank and at the other end the next cascaded transformer is fed.

This cascade arrangement can be continued further if a still higher voltage is required.

Figure - Cascade arrangement of transformers

In the cascade arrangement shown, each transformer needs only to be insulated for 100 kV, and hence the transformer can be relatively small.

If a 300 kV transformer had to be used instead, the size would be massive.

High voltage transformers for testing purposes are designed purposely to have a poor regulation.

This is to ensure that when the secondary of the transformer is short circuited (as will commonly happen in flash-over tests of insulation), the current would not increase to too high a value and to reduce the cost. In practice, an additional series resistance (commonly a water resistance) is also used in such cases to limit the current and prevent possible damage to the transformer.

Figure : resonant transformer

For certain applications, particularly when the final requirement is a direct voltage, it is an advantage to select a frequency higher than power frequency (50 Hz). This would result in a smaller transformer having fewer turns, and also simplifies the smoothing after rectification.

High frequency high voltages

High frequency (few kHz to Mhz) high voltages are required in testing apparatus for behaviour with switching surges, insulation flashover etc. The importance of testing with high frequency is that high frequency

oscillations cause failure of insulator at comparatively low voltage due to high dielectric loss and consequent heating. Thus it is necessary to produce damped high frequency voltages.

A high frequency resonant transformer is the Tesla coil, which is a doubly tuned resonant circuit. The primary voltage rating is 10 kV and the secondary may be rated to as high as 500 to 1000 kV.The primary is fed from a d.c. or a.c. supply through the condenser C1. A spark gap G connected across the primary is triggered at the desired voltage V1 which induces a high self-excitation in the secondary.

The primary and the secondary windings (L1and L2) are wound on an insulated former with

no core (air-cored) and are immersed in oil. The windings are tuned to a frequency of 10 to 100 kHz by means of the condensers C1 and C2.

The output voltage V2 is a function of the parameters L1, L2,C1, C2 and the mutual

inductance M. Usually, the winding resistances will be small and contribute only for damping of the oscillations.

GENERATION OF IMPULSE VOLTAGETransient overvoltages due to lightning and switching

surges cause steep build-up of voltage on transmission lines and other electrical apparatus.

Experimental investigations showed that these waves have a rise time of 0.5 to. 10 micro seconds and decay time to 50% of the peak value of the order of 30 to 200 micro seconds. The waveshapes are arbitrary, but mostly unidirectional.

Impulse waves are specified by defining their rise or front time, fall or tail time to 50% peak value, and the value of the peak voltage.

Circuits for Producing Impulse WavesA double exponential impulse waveform may be produced in the laboratory with a combination of a series R-L-C circuit under over damped conditions or by the combination of two R-C circuits.Circuit shown in Fig.a is limited to model generators only, and commercial generators employ circuits shown in Figs.b.A capacitor (C1 or C previously charged to a particular d.c. voltage is suddenlydischarged into the wave shaping network (LR, R1R2C2 or other combination) by closing the switch S. The discharge voltage V0 gives rise to the desired double exponential impulse wave shape.

The wave front and the wave tail times are controlled by changing the values of R and L simultaneously with a given generator capacitance C; choosing a suitable value for L. ß or the wave front time is determined and a or the wave tail time is controlled by the value of R in the circuit.

The advantage of this circuit is its simplicity. But the wave shape control is not flexible and

independent.

GENERATION OF HIGH d.C. VOLTAGES

Generation of high d.c. voltages is mainly required in research work in the areas of pure and applied physics.

Sometimes, high direct voltages are needed in insulation tests on cables and capacitors. Impulse generator charging units also require high d.c.voltages of about 100 to 200 kV.

Half and Full Wave Rectifier CircuitsRectifier circuits for producing high d.c. voltages

from a.c. sources may be (a) half wave, (b) full wave, or (c) voltage doubler type rectifiers

In the half wave rectifier the capacitor is charged to Vmax, the maximum a.c. voltage of the secondary of the high voltage transformer in the conducting half cycle.In the other half cycle, the capacitor is discharged into the load. The value of thecapacitor C is chosen such that the time constant CR^ is at least 10 times that of theperiod of the a.c. supply. The rectifier valve must have a peak inverse rating of at least2 Vmax-

In full wave rectifier circuit in the positive half cycle, the rectifier A conducts and charges the capacitor C, while in the negative half cycle the rectifier B conducts and charges the capacitor. The source transformer requires a centre tapped secondary with a rating of 2V.

Both full wave and half wave rectifiers produce d.c. voltages less than the a.c. maximum voltage. Also, ripple or the voltage fluctuation will be present, and this has to be kept within a reasonable limit by means of filters.

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. Even otherwise only a capacitance may be used across the test device for smoothing

Voltage Multiplier Circuits

Both full wave and half wave rectifier circuits produce a d.c. voltage less than the a.c.

maximum voltage. When higher voltages are required voltage

multiplier circuits are used. The common circuits are the

voltage double circuit and the Cockroft-Walton Circuit.

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.

In this case, the transformer will be of small rating that 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.

Cockroft-Walton CircuitWhen more than doubling of the voltage is required, the Cockroft-Walton voltage multiplier circuit is commonly used.

Let Vmax be the peak value of the secondary voltage of the high voltage transformer.

To analyze the behaviour,let us consider that charging of capacitors actually takes place stage by stage rather than somewhat simultaneously.

Consider the first part of the circuit containing the diode D1, the capacitor C1, and the secondary winding.

During the first negative half cycle of the applied voltage, the capacitor C1 charge up to voltage Vmax. Since during the positive half cycle which follows, the diode D1 is reverse biased, the capacitor C1 will not discharge (or will not charge up in the other direction) and the peak of this half cycle, the point a will be at 2 Vmax.

During the following cycles, the potential at a will vary between 0 and 2 Vmax, depending on whether the secondary voltage and the capacitor voltage are opposing or assisting.

Initially, capacitor C2 would be uncharged, and the voltage at b would be zero.

Thus as the voltage at a varies between 0 and 2 Vmax, the diode D2 is forward biassed, and the capacitor C2 would charge to 2 Vmax.

Once the voltage at b has reached 2 Vmax, the voltage at a would be less than or equal to the voltage at b. Thus once C2 has charged up, this diode too would be reverse biased and the capacitor C2 would not discharge.

The voltage at b would now remain constant at 2 Vmax. C3 is also initially assumed uncharged.

Since the voltage at a varies between 0 and 2 Vmax, the diode D3 would initially be forward biased for almost the whole cycle.

Thus the capacitor C3 charges until it reaches 2 Vmax when b is 2 Vmax and a is 0.

As the voltage at a again increases to 2 Vmax, the voltage at c increases, and thus the diode D3 is reverse biased and C3 would not discharge.

Now as a reaches 2 Vmax the voltage at c rises to 4 Vmax, as C3 has not discharged.

Thus after charging up has taken place, the voltage at c varies between 2 Vmax and 4 Vmax.

Assuming C4 also to be initially uncharged, since the voltage at b is a constant at 2 Vmax and the voltage at c varies between 2 Vmax and 4 Vmax initially, during most of the cycle, the diode D4 is forward biased and C4 charges up to the maximum difference between d and b (i.e. to 2 Vmax).

This occurs when the voltage at c is 4 Vmax and the voltage at d would now be 4 Vmax.

As the voltage at c falls from 4 Vmax to 2 Vmax, since the capacitor C4 has charged up it would not discharge, since there is no discharge path.

Thus once the capacitors are charged up the voltage at d remains constant at 4 Vmax

Example

A Cockcroft-Walton type voltage multiplier has eight stages with capacitances, all equal to 0.05 µF. The supply transformer secondary voltage is 125 kV at a frequency of 150 Hz. If the load current to be supplied is 5 mA, find (a) the percentage ripple, (b) the regulation, and (c) the optimum number of stages for minimum regulation or voltage drop

Electrostatic Machines: Basic Principle

In electromagnetic machines, current carrying conductors are moved in a magnetic field, so that the mechanical energy is converted into electrical energy. In electrostatic machines charged bodies are moved in an electric field against an electrostatic field in order that mechanical energy is converted into electrical energy.

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.

The Van de Graaff generator is one such electrostatic machine which generates very high voltages, with small output current.

Van de Graeff generatorThe Van de Graeff generator uses an insulating belt as the carrier of charge. The generator consists of a low direct voltage source, with corona discharge taking place at the positive end of the source. The corona formation (spray) is caused by a core like structure with sharp points (corona spray device). Charge is sprayed onto the belt at the bottom by corona discharges at a potential of 10 to 100 kV above earth and carried to the top of the column and deposited at a collector. The upper electrode at which the charge is collected has a high radius of curvature and the edges should be curved so as to have no loss. The generator is usually enclosed in an earthed metallic cylindrical vessel and is operated under pressure or in vacuum.

The higher voltage of the upper electrode arises from the fact that for the same charge, a smaller capacitance gives a larger voltage. The upper electrode has a smaller capacitance to earth on account of the larger spacing involved.V = Q/CThe potential of the high voltage electrode rises at a rate of where I is the net charging currentA steady potential will be reached by the high voltage electrode when the leakage currents na the load current are equal to the charging current. The edges of the upper electrode are so rounded as to avoid corona and other local discharges.With a single source at the lower end, the belt moves upwards with a positive charge and returns uncharged.Charging can be made more effective by having an additional charge of opposite polarity sprayed onto the belt by a self inducing arrangement (negative corona spray). using an ingenious method. this arrangement effectively doubles the charging rate.

GENERATION OF IMPULSE CURRENTS

Lightening discharges involve both high voltage impulses and high current impulses on transmission lines. Protective gear like surge diverters have to discharge the lightning currents without damage. Therefore, generation of impulse current waveforms of high magnitude (100 kA peak) find application in testing work as well as in basic research on non-linear resistors, electric arc studies, and studies relating to electric plasmas in high current discharges.

The essential parts of an impulse current generator are:(i) a d.c. charging unit giving a variable voltage to the capacitor bank,(ii) capacitors of high value (0.5 to 5 µF) each with very low self-inductance,capable of giving high short circuit currents,(iii) an additional air cored inductor of high current value,(jv) proper shunts and oscillograph for measurement purposes, and(v) a triggering unit and spark gap for the initiation of the current generator

The maximum value of im is normally independent of the value of V and C for a given energy ,and the effective inductance L. It is also clear) that a low inductance is needed in order to get high current magnitudes for a given charging voltage V.