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High Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringHigh Voltage EngineeringUnitUnitUnitUnit----IIIIIIIIIIII

Deependra SinghDeependra SinghDeependra SinghDeependra Singh57

UNIT-III

GENERATION OF HV AC DC IMPULSE VOLTAGE & CURRENT

22. INTRODUCTION

A fundamental knowledge about generators and circuits which are in use for the

generation of high voltages belongs to the background of work on H.V. technology.

Generally commercially available H.V. generators are applied in routine testing

laboratories; they are used for testing equipment such as transformers, bushings, cables,

capacitors, switchgear, etc. The tests should confirm the efficiency and reliability of the

products and therefore the H.V. testing equipment is required to study the insulation behavior

under all conditions which the apparatus is likely to encounter. The amplitudes and types of

the test voltages, which are always higher than the normal or rated voltages of the apparatus

under test, are in general prescribed by national or international standards or

recommendations, and therefore there is not much freedom in the selection of the H.V.

testing equipment. Quite often, however, routine testing laboratories are also used for the

development of new products. Then even higher voltages might be necessary to determine the

factor of safety over the prospective working conditions and to ensure that the working

margin is neither too high nor too low. Most of the H.V. generator circuits can be changed to

increase the output voltage levels, if the original circuit was properly designed. Therefore,

even the selection of routine testing equipment should always consider a future extension of

the testing capabilities.

The work carried out in research laboratories varies considerably from one

establishment to another, and the type of equipment needed varies accordingly. As there are

always some interactions between the H.V. generating circuits used and the test results, the

layout of these circuits has to be done very carefully. The classes of tests may differ from the

routine tests, and therefore specially designed circuits are often necessary for such

laboratories. The knowledge about some fundamental circuits treated in this chapter will also

support the development of new test circuits.

Finally, high voltages are used in many branches of natural sciences or other technical

applications. The generating circuits are often the same or similar to those treated in the

following sections. It is not the aim, however, of this introductory text to treat the broad

variations of possible circuits, due to space limitation. Not taken into account are also the

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Deependra SinghDeependra SinghDeependra SinghDeependra Singh 58

differing problems of electrical power generation and transmission with high voltages of A.C.

or D.C., or the pure testing technique of H.V. equipment, the procedures of which may be

found in relevant standards of the individual equipment. Power generation and transmission

problems are treated in many modern books, some of which are listed within the bibliography

of an earlier report.

This chapter discusses the generation of the following main classes of voltages: direct

voltages, alternating voltages, and transient voltages.

REFERENCE:REFERENCE:REFERENCE:REFERENCE:

HVE FUNDAMENTALS: KUFFEL, ZAENGL & KUFFEL: P-8-9

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23. DIRECT VOLTAGES

In H.V. technology direct voltages are mainly used for pure scientific research work

and for testing equipment related to HVDC transmission systems. There is still a main

application in tests on HVAC power cables of long length, as the large capacitance of those

cables would take too large a current if tested with A.C. voltages Although such D.C. tests on

A.C. cables are more economical and convenient, the validity of this test suffers from the

experimentally obtained stress distribution within the insulating material, which may

considerably be different from the normal working conditions where the cable is transmitting

power at low-frequency alternating voltages. For the testing of polyethylene H.V. cables, in

use now for some time, D.C. tests are no longer used, as such tests may not confirm the

quality of the insulation.

High D.C. voltages are even more extensively used in applied physics (accelerators,

electron microscopy, etc.), electro-medical equipment (X-rays), industrial applications

(precipitation and filtering of exhaust gases in thermal power stations and the cement

industry; electrostatic painting and powder coating, etc.), or communications electronics (TV,

broadcasting stations). Therefore, the requirements on voltage shape, voltage level, and

current rating, short- or long-term stability for every HVDC generating system may differ

strongly from each other. With the knowledge of the fundamental generating principles it will

be possible, however, to select proper circuits for a special application.

In the International Standard IEC 60-1 or IEEE Standard 4-1995 the value of a direct

test voltage is defined by its arithmetic mean value, which will be designated asV .

Therefore, this value may be derived from:

=T

dttVT

V0

)(1

2.1

Where T equals a certain period of time if the voltage V (t)is not constant, but periodically

oscillating with a frequency of f = 1/T. Test voltages as applied to test objects then deviate

periodically from the mean value. This means that a ripple is present. The amplitude of the

ripple, V, is defined as half the difference between the maximum and minimum values, or

V = 0.5(Vmax-Vmin) 2.2

The ripple factor is the ratio of the ripple amplitude to the arithmetic mean value, or V/V .

For test voltages this ripple factor should not exceed 3 percent unless otherwise specified by

the appropriate apparatus standard or be necessary for fundamental investigations. The D.C.

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voltages are generally obtained by means of rectifying circuits applied to A.C. voltages or by

electrostatic generation. A treatment of the generation principles according to this subdivision

is appropriate.


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24. HVAC HV TRANSFORMER

Most of the present day transmission and distribution networks are operating on A.C.

voltages and hence most of the testing equipments relate to high A.C. voltages. Even though

most of the equipments on the system are 3-phase systems, a single phase transformer

operating at power frequency is the most common from of HVAC testing equipment.

Test transformers normally used for the purpose have low power rating but high

voltage ratings. These transformers are mainly used for short time tests on high voltage

equipments. The currents required for these tests on various equipments are given below:

Insulators, C.B., bushings, Instrument transformers = 0.1 0.5 A

Power transformers, h.v. capacitors. = 0.51 A

Cables = 1 A and above

The design of a test transformer is similar to a potential transformer used for the

measurement of voltage and power in transmission lines. The flux density chosen is low so

that it does not draw large magnetizing current which would otherwise saturate the core and

produce higher harmonics.


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25. NEED FOR CASCADE CONNECTION & WORKING OF

TRANSFORMERS UNITS CONNECTED IN CASCADE

For voltages higher than 400 KV, it is desired to cascade two or more transformers

depending upon the voltage requirements. With this, the weight of the whole unit is

subdivided into single units and, therefore, transport and erection becomes easier. Also, with

this, the transformer cost for a given voltage may be reduced, since cascaded units need not

individually possess the expensive and heavy insulation required in single stage transformers

for high voltages exceeding 345 kV. It is found that the cost of insulation for such voltages

for a single unit becomes proportional to square of operating voltage.

Fig. 2.9 shows a basic scheme for cascading three transformers. The primary of the

first stage transformer is connected to a low voltage supply. A voltage is available across the

secondary of this transformer. The tertiary winding (excitation winding) of first stage has the

same number of turns as the primary winding, and feeds the primary of the second stage

transformer. The potential of the tertiary is fixed to the potential V of the secondary winding

as shown in Fig. 2.9. The secondary winding of the second stage transformer is connected in

series with the secondary winding of the first stage transformer, so that a voltage of 2V is

available between the ground and the terminal of secondary of the second stage transformer.

Similarly, the stage-III transformer is connected in series with the second stage transformer.

With this the output voltage between ground and the third stage transformer, secondary is 3V.

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It is to be noted that the individual stages except the upper most must have three-winding

transformers. The upper most, however, will be a two winding transformer.

Fig. 2.9 shows metal tank construction of transformers and the secondary winding is

not divided. Here the low voltage terminal of the secondary winding is connected to the tank.

The tank of stage-I transformer is earthed. The tanks of stage-II and stage-III transformers

have potentials of V and 2V, respectively above earth and, therefore, these must be insulated

from the earth with suitable solid insulation. Through H.T. bushings, the leads from the

tertiary winding and the H.V. winding are brought out to be connected to the next stage

transformer.

However, if the high voltage windings are of mid-point potential type, the tanks are

held at 0.5 V, 1.5 V and 2.5 V, respectively. This connection results in a cheaper construction

and the high voltage insulation now needs to be designed for V/2 from its tank potential.

The main disadvantage of cascading the transformers is that the lower stages of the

primaries of the transformers are loaded more as compared with the upper stages.

The loading of various windings is indicated by P in Fig. 2.9. For the three-stage

transformer, the total output VA will be 3VI = 3P and, therefore, each of the secondary

winding of the transformer would carry a current of I = P/V. The primary winding of stage-III

transformer is loaded with P and so also the tertiary winding of second stage transformer.

Therefore, the primary of the second stage transformer would be loaded with 2P. Extending

the same logic, it is found that the first stage primary would be loaded with P. Therefore,

while designing the primaries and tertiaries of these transformers, this factor must be taken

into consideration.

The total short circuit impedance of a cascaded transformer from data for individual

stages can be obtained. The equivalent circuit of an individual stage is shown in Fig. 2.10.

Here Zp, Zs, and Zt, are the impedances associated with each winding. The

impedances are shown in series with an ideal 3-winding transformer with corresponding

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number of turns Np, Ns and Nt. The impedances are obtained either from calculated or

experimentally-derived results of the three short-circuit tests between any two windings taken

at a time.

Let Zps= leakage impedance measured on primary side with secondary short circuited and

tertiary open.

Zpt= leakage impedance measured on primary side with tertiary short circuited and

secondary open.

Zst= leakage impedance on secondary side with tertiary short circuited and primary

open.

If these measured impedances are referred to primary side then

Zps= Zp+ Zs, Zpt = Zp+ Zt and Zst= Zs + Zt

Solving these equations, we have

)(2

1stptpsp ZZZZ += , )(

2

1ptstpss ZZZZ += and )(

2

1psstptt ZZZZ += 2.19

Assuming negligible magnetizing current, the sum of the ampere turns of all the windings

must be zero.

0= ttsspp INININ

Assuming lossless transformer, we have,

Zp= jXp, Zs= jXs, and Zt= jXt

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Also letNp= Ntfor all stages, the equivalent circuit for a 3-stage transformer would

be given as in Fig.2.11.

Fig. 2.11 can be further reduced to a very simplified circuit as shown in Fig. 2.12. The

resulting short circuit reactance Xresis obtained from the condition that the power rating of

the two circuits be the same. Here currents have been shown corresponding to high voltage

side.

ttsssppres XIXIXIXIXIXIXIXI 22222222 )2()2()3( ++++++=

tspres XXXX 5314 ++= 2.20

Instead of 3(Xp+ Xs+ Xt)as might be expected. Equation (2.20) can be generalized for an n-

stage transformer as follows:

=

++++=

n

i

tisipi XiXXinXres1

22 )1()1[(

REFERENCREFERENCREFERENCREFERENCE:E:E:E:

HVE: C. L. WADHWA, 2.4; P-69

HVE: C. L. WADHWA, 2.4; P-70-72

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26. SERIES RESONANT CIRCUIT

(PRINCIPLE OF OPERATION AND ADVANTAGES)

The equivalent circuit of a single-stage-test transformer along with its capacitive load

is shown in Fig. 2.15. Here L1 represents the inductance of the voltage regulator and the

transformer primary, L the exciting inductance

of the transformer, L2 the inductance of the

transformer secondary and C the capacitance

of the load. Normally inductance L is very

large as compared to L1and L2and hence its

shunting effect can be neglected. Usually the

load capacitance is variable and it is possible

that for certain loading, resonance may occur in the circuit suddenly and the current will then

only be limited by the resistance of the circuit and the voltage across the test specimen may

go up as high as 20 to 40 times the desired value.

Similarly, presence of harmonics due to saturation of iron core of transformer may

also result in resonance. Third harmonic frequencies have been found to be quite disastrous.

With series resonance, the resonance is controlled at fundamental frequency and

hence no unwanted resonance occurs.

The development of series resonance circuit for testing purpose has been very widely

welcome by the cable industry as they faced resonance problem with test transformer while

testing short lengths of cables.

In the initial stages, it was difficult to

manufacture continuously variable high

voltage and high value reactors to be used in

the series circuit and therefore, indirect

methods to achieve this objective were

employed. Fig. 2.16 shows a continuously

variable reactor connected in the low voltage

winding of the step up transformer whose

secondary is rated for the full test voltage. C2

represents the load capacitance.

If N is the transformation ratio and L is the inductance on the low voltage side of the

transformer, then it is reflected with N2L value on the secondary side (load side) of the

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transformer. For certain setting of the reactor, the inductive reactance may equal the

capacitive reactance of the circuit, hence resonance will take place. Thus, the reactive power

requirement of the supply becomes zero and it has to supply only the losses of the circuit.

However, the transformer has to carry the full load current on the high voltage side. This is a

disadvantage of the method. The inductor are designed for high quality factors Q = L/R.

The feed transformer, therefore, injects the losses of the circuit only.

It has now been possible to manufacture high voltage continuously variable reactors

300 kV per unit using a new technique with split iron core. With this, the testing step up

transformer can be omitted as shown in Fig. 2.17. The inductance of these inductors can be

varied over a wide range depending upon the capacitance of the load to produce resonance.

Fig. 2.17 (b) represents an equivalent circuit for series resonance circuit. Here R is

usually of low value. After the resonance condition is achieved, the output voltage can be

increased by increasing the input voltage. The feed transformers are rated for nominal current

ratings of the reactor. Under resonance, the output voltage will be

2

0

1

CR

VV

=

Where V is the supply voltage.

Since at resonance2

1

CL

=

Therefore, VQLR

VV == 0

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. It also means

that the reactive power requirements of the load capacitance in kVA is 40 times the power to

be provided by the feed transformer in KW. This results in a relatively small power rating for

the feed transformer.

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ADVANTAGES:

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 of 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.

(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.

RRRREFERENCE:EFERENCE:EFERENCE:EFERENCE:

HVE: C. L. WADHWA, 2.4; P-73-76

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2. TESLA COIL

High frequency high voltages are required for rectifier D.C. power supplies. Also, for

testing electrical apparatus for switching surges, high frequency high voltage damped

oscillations are needed which need high voltage high frequency transformers. The advantages

of these high frequency transformers are:

(i) The absence of iron core in transformers and hence saving in cost and size,

(ii) Pure sine wave output,

(iii) Slow build-up of voltage over a few cycles and hence no damage due to switching

surges, and

(iv) Uniform distribution of voltage across the winding coils due to subdivision of coil

stack into a number of units.

The commonly used high frequency resonant transformer is the Tesla coil, which is a

doubly tuned resonant circuit shown schematically in Fig. 6.13a. 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 (L1 and 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 C1and C2.

Fig. 6.13 Tesla Coil equivalent circuit and its output waveform

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.

The analysis of the output waveform can be done in a simple manner neglecting the

winding resistances. Let the condenser C1be charged to a voltage V1when the spark gap is

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triggered. Let a current i1 flow through the primary winding L1 and produce a current i2

through L2and C2.

Then,

dtdiM

dtdiLdti

CV

t

2

0

111

1

1 1 ++= (6.9)

And,dt

diM

dt

diLdti

C

t

1

0

222

2

10 ++=

The Laplace transformed equations for the above are,

21

1

11 1 MsII

sCsL

s

V+

+= (6.10)

And, [ ] 22

22

10 I

sCsLIMs

++=

WhereI1&I2are the Laplace transformed values, of i1and i2.

The output voltage V2across the condenser C2 is

;1

0

2

2

2 dtiC

V

t

= Or its transformed equation is

( )sC

sV2

2

1= (6.11)

Where V2(s)is the Laplace transform of V2.

The solution for V2from the above equations will be

[ ]ttCLL

MVV 212

1

2

2121

12 coscos

1

= (6.12)

Where, 2

21

22 11 K

LL

M==

K = coefficient of coupling between the windingsL1andL2

( )2222

1

2

2

2

1

2

2

2

12,1 1

22K

+

+=

11

1

1

CL= And

22

2

1

CL=

The output waveform is shown in Fig. 6.13(b).

The peak amplitude of the secondary voltage V2can be expressed as,

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1

21max2

L

LeVV = (6.13)

Where,

aa

e

4)1(

)1(2

2

+

=

2

2

2

1

11

22

W

W

CL

CLa ==

A more simplified analysis for the Tesla coil may be presented by considering that the

energy stored in the primary circuit in the capacitance C1is transferred to C2via the magnetic

coupling. If W1is the energy stored in C1and W2is the energy transferred to C2 and if the

efficiency of the transformer is , then

== 222

2

11121

21 VCVCW (6.14)

From which2

112

C

CVV = (6.14a)

It can be shown that if the coefficient of coupling K is large the oscillation frequency

is less, and for large values of the winding resistances and AT, the waveform may become a

unidirectional impulse. This is shown in the next sections while dealing with the generation

of switching surges.


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Cascaded voltage doublers are used when larger output voltages are needed without

changing the input transformer voltage level. A typical voltage doubler is shown in Fig. 6.3b

and its input and output waveforms are shown in Fig. 6.3(c). The rectifiersR1 and R2with

transformer T1and condensers C1and C2produce an output voltage of 2V in the same way as

described above. This circuit is duplicated and connected in series or cascade to obtain a

further voltage doubling to 4V. Tis an isolating transformer to give insulation for 2Vmaxsince

the transformer T2is at a potential of 2Vmaxabove the ground. The voltage distribution along

the rectifier string R1, R2, R3and R4is made uniform by having condensers C1, C2, C3and C4

of equal values. The arrangement may be extended to give 6V, 8V, and so on by repeating

further stages with suitable isolating transformers. In all the voltage doubler circuits, if valves

are used, the filament transformers have to be suitably designed and insulated, as all the

cathodes will not be at the same potential from ground. The arrangement becomes

cumbersome if more than 4V is needed with cascaded steps.

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(1) A.C. input voltage waveform, (2) A.C. output voltage waveform without condenser filter,(3) A.C. output voltage waveform with condenser filter

Fig. 6.3 Voltage doubler circuits

T1, T2- H.V. transformers; R1, R2, R3, R4- rectifiers;

C1, C2, C3- condensers; RL- Load resistance; T- Isolating transformer


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2#. COCK CROFT- WALTON T$PE HIGH VOLTAGE DC SET

In 1932, Cockroft and Walton suggested an improvement over the circuit developed by

Greinacher for producing high D.C. voltages. Fig. 2.3 shows a multistage single phase

cascade circuit of the Cockroft-Walton type.

No Load Operation: The portion

ABMMA is exactly indentical to Greinarcher

voltage doubler circuit and the voltage across

C becomes 2Vmax when M attains a voltage

2Vmax. During the next half cycle when B

becomes positive with respect to A, potential

of M falls and, therefore, potential of N also

falls becoming less than potential at M

hence C2 is charged through D2. Finally all

the capacitors C1, C2, C3, C1, C2, and C3 are

charged. The voltage across the column of

capacitors consisting of C1, C2, C3, keeps on

oscillating as the supply voltage alternates.

This column, therefore, is known as oscillating column. However, the voltage across the

capacitances C1, C2, C3, remains constant and is known as smoothening column. The

voltages at M, N, and O are 2Vmax, 4Vmax

and 6Vmax. Therefore, voltage across all the

capacitors is 2Vmaxexcept for C1where it is

Vmax only. The total output voltage is

2nVmax where n is the number of stages.Thus, the use of multistages arranged in the

manner shown enables very high voltage to

be obtained. The equal stress of the elements

(both capacitors and diodes) used is very

helpful and promotes a modular design of

such generators.

Generator Loaded: When the

generator is loaded, the output voltage will

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never reach the value 2nVmax. Also, the output wave will consist of ripples on the voltage.

Thus, we have to deal with two quantities, the voltage drop V and the ripple V.

Suppose a charge q is transferred to the load per cycle. This charge is q = 1/f = 1T.

The charge comes from the smoothening column, the series connection of C1, C2, and C3. If

no charge were transferred during T from this stack via D1, D2, D3, to the oscillating column,

the peak to peak ripple would merely be

=

=

n

n iCTV

0 '

112 (2.6)

But in practice charges are transferred.

Cascade generators of Cockcroft Walton type are used and manufactured today

worldwide. The D.C. voltages produced with this circuit may range from some 10 kV up to

more than 2 MV, with current ratings from some 10 A up to some 100 mA. Supply

frequencies of 50/60 Hz are heavily limiting the efficiency, and therefore higher frequencies

up to about 1000 Hz (produced by single-phase alternators) or some 10 kHz (produced by

electronic circuits) are dominating.


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3%. INTRODUCTION TO STANDARD LIGHTNING AND SWITCHING

IMPULSE VOLTAGES

Disturbances of electric power transmission and distribution systems are frequently

caused by two kinds of transient voltages whose amplitudes may greatly exceed the peak

values of the normal A.C. operating voltage.

The first kind is lightning overvoltages, originated by lightning strokes hitting the

phase wires of overhead lines or the busbars

of outdoor substations. The amplitudes are

very high, usually in the order of 1000 kV or

more, as every stroke may inject lightning

currents up to about 100 kA and even more

into the transmission line; each stroke is

then followed by traveling waves, whose

amplitude is often limited by the maximum

insulation strength of the overhead line. The rate of voltage rise of such a traveling wave is at

its origin directly proportional to the steepness of the lightning current, which may exceed

100 kA/sec, and the voltage levels may simply be calculated by the current multiplied by

the effective surge impedance of the line. Too high voltage levels are immediately chopped

by the breakdown of the insulation and therefore traveling waves with steep wave fronts and

even steeper wave tails may stress the insulation of power transformers or other H.V.

equipment severely. Lightning protection systems, surge arresters and the different kinds of

losses will damp and distort the traveling waves, and therefore lightning overvoltages with

very different waveshapes are present within the transmission system.

The second kind is caused by switching phenomena. Their amplitudes are always

related to the operating voltage and the shape is

influenced by the impedances of the system as

well as by the switching conditions. The rate of

voltage rise is usually slower, but it is well known

that the waveshape can also be very dangerous to

different insulation systems, especially to

atmospheric air insulation in transmission systems

with voltage levels higher than 245 kV.

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Both types of overvoltages are also effective in the L.V. distribution systems, where

they are either produced by the usual, sometimes current-limiting, switches or where they

have been transmitted from the H.V. distribution systems. Here they may often cause a

breakdown of electronic equipment, as they can reach amplitudes of several kilovolts, and it

should be mentioned that the testing of certain L.V. apparatus with transient voltages or

currents is a need today.


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3. ANAL$SIS OF SINGLE STAGE IMPULSE GENERATOR

E'PRESSION FOR OUTPUT IMPULSE VOLTAGE

Two basic circuits for single-stage impulse generators are shown in Fig. 2.25. The

capacitor C1is slowly charged from a D.C. source until the spark gap G breaks down. This

spark gap acts as a voltage-limiting and voltage-sensitive switch, whose ignition time (time to

voltage breakdown) is very short in comparison to T1.

As such single-stage generators may be used for charging voltages from some kV up

to about 1MV; the sphere gaps will offer proper operating conditions. An economic limit of

the charging voltage V0is, however, a value of about 200 to 250 kV, as too large diameters

of the spheres would otherwise be required to avoid excessive inhomogeneous field

distributions between the spheres. The resistors R1, R2and the capacitance C2form the wave

shaping network. R1will primarily damp the circuit and control the front time T1. R2will

discharge the capacitors and therefore essentially control the wave tail. The capacitance C2

represents the full load, i.e. the object under test as well as all other capacitive elements

which are in parallel to the test object (measuring devices; additional load capacitor to avoid

large variations of T1/T2, if the test objects are changed). No inductances are assumed so far,

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and are neglected in the first fundamental analysis, which is also necessary to understand

multistage generators. In general this approximation is permissible, as the inductance of all

circuit elements has to be kept as low as possible.

Before starting the analysis, we should mention the most significant parameter of

impulse generators. This is the maximum stored energy:

2

01 )(2

1MAX

VCW = (2.22)

Within the discharge capacitance C1.As C1is always much larger than C2, this figure

determines mainly the cost of a generator.

For the analysis we may use the Laplace transform circuit sketched in Fig. 2.25(c),

which simulates the boundary condition, that for t 0, C1is charged to V0and for t> 0 this

capacitor is directly connected to the wave-shaping network. For the circuit Fig. 2.25(a) the

output voltage is thus given by the expression

21

20)(ZZ

Z

s

VsV

+= ,

Where 11

1

1R

sCZ += ;

sCR

sCRZ

22

222

/1

/

+

= .

By Substitution we findbassk

VsV

++=

2

0 1)( (2.23)

Where ;111

222111

++=

CRCRCRa

;1

2121

=

CCRRb

21CRk = . (2.24)

For circuit Fig. 2.25(b) one finds the same general expression (eqn (2.23)), with the following

constants; however,

;111

122111

++=

CRCRCRa

as

CRk

CCRRb

=

=

21

2121

;1

Above (2.25)

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For both circuits, therefore, we obtain from the transform tables the same expression in the

time domain:

[ ]ttk

VtV 21

21

0 exp()exp()(

1)(

+= (2.26)

Where 1and 2are the roots of the equation s2+ab+b = 0, or

baa

=

2

2122

, m (2.27)

The output voltage V(t) is therefore the superposition of two exponential functions of

different signs. According to eqn (2.27), the negative root leads to a larger time constant 1/ 1

than the positive one, which are 1/ 2. A graph of the expression (eqn (2.26)) is shown in Fig.

2.26, and a comparison with Figs 2.23 and 2.24 demonstrates the possibility to generate bothtypes of impulse voltages with these circuits.


HVE FUNDAMENTALS: KUFFEL, ZAENGL & KUFFEL; 2.3.1; P-52-55

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32. MULTISTAGE IMPULSE GENERATOR

COMPONENTS OF MULTISTAGE IMPULSE GENERATOR

In order to obtain higher and higher impulse voltage, a single stage circuit is inconvenient for

the following reasons:

(i). The physical size of the circuit elements becomes very large.

(ii). High D.C. charging voltage is required.

(iii). Suppression of corona discharges from the structure and leads during the charging

period is difficult.

(iv). Switching of vary high voltages with spark gaps is difficult.

In 1923 E. Marx suggested a multiplier circuit which is commonly used to obtain impulse

voltages with as high a peak value as possible for a given D.C. charging voltage.

Depending upon the charging voltage available and the output voltage required a

number of identical impulse capacitors are charged in parallel and then discharged in series,

thus obtaining a multiplied total charging voltage corresponding to the number of stages. Fig.

3.7 shows a 3-stage impulse generator circuit due to Marx employing b circuit connections.

The impulse capacitors C1are charged to the charging voltage V0through the high charging

resistors RCin parallel. When all the gaps G break down, the C1capacitances are connected

in series so that C2is charged through the series connection of all the wave front resistances

R1and finally all C1and C2will discharge through the resistors R2and R1. Usually RC

>> R2>> R1.

If in Fig. 3.7 the wave tail resistors R2in each stage are connected in parallel to the

series combination of R1, G and C1, an impulse generator of type circuit a is obtained.

In order that the Marx circuit operates consistently it is essential to adjust the

distances between various sphere gaps such that the first gap G1is only slightly less than thatof G2and so on. If is also necessary that the axes of the gaps G be in the same vertical plane

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so that the ultraviolet radiations due to spark in the first gap G, will irradiate the other gaps.

This ensures a supply of electrons released from the gap electrons to initiate breakdown

during the short period when the gaps are subjected to overvoltages.

The wave front control resistance can have three possible locations (i) entirely within

the generator (ii) entirely outside the generator (iii) partly within and partly outside the

generator.

The first arrangement is unsatisfactory as the inductance and capacitance of the

external leads and the load form an oscillatory circuit which requires to be damped by an

external resistance. The second arrangement is also unsatisfactory as a single external front

resistance will have to withstand, even though for a very short time, the full rated voltage and

therefore, will turn out to be inconveniently long and would occupy much space. A

compromise between the two is the third arrangement as shown in Fig. 3.7 and thus both the

space economy and damping of oscillations are taken care of.

It can be seen that Fig. 3.7 can be reduced to the single stage impulse generator of Fig.

3.4 (b).

After the generator has fired, the total discharge capacitance C1may be given as:

n

CC '

11

11

the equivalent front resistance +=n

RRR "' 111

and the equivalent tail control resistance =n

RR '22

where n is the number of stages.

CONSTRUCTION OF IMPULSE GENERATOR:

An impulse generator requires a D.C. power supply for charging the impulse

capacitance C1of the generator. The supply consists of a step-up transformer and rectifier.

The charging times should not be less than 3 to 10 seconds as every application of voltage

will leave behind pre-ionizing effects, so a time should be allowed between two successive

applications so that the pre-stressing effects within the insulations do not influence the

breakdown strength. These days thyristor controlled D.C. supply for charging is available.

The resistors may be made up of wire, liquid or composite (carbon etc.) resistive

materials. The liquid and carbon resistors even though have high heat capacity, these are

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HVE FUNDAMENTALS: C L WADHWA; 3.5; P-94-98

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33. TRIGGERING OF IMPULSE GENERATOR "$ THREE ELECTRODE

GAP ARRANGEMENT

Impulse generators are normally operated in conjunction with cathode ray

oscillographs for measurement and for studying the effect of impulse waves on the

performance of the insulations of the equipments. Since the impulse waves are of shorter

duration, it is necessary that the operation of the generator and the oscillograph should be

synchronised accurately and if the wave front of the wave is to be recorded accurately, the

time sweep circuit of the oscillograph should be initiated at a time slightly before the impulse

wave reaches the deflecting plates.

If the impulse generator itself initiates the sweep circuit of the oscillograph, it is then

necessary to connect a delay cable between the generator or the potential divider and the

deflecting plates of the oscilloscope so that the impulse wave reaches the plates at a

controlled time after the sweep has been tripped. However, the use of delay cable leads to

inaccuracies in measurement. For this reason, some tripping circuits have been developed

where the sweep circuit is operated first and then after a time of about 0.1 to 0.5 sec. the

generator is triggered.

Fig. 3.11 shows a trigatron gap which is used as the first gap of the impulse generator

and consists essentially of a three-electrode gap. The high voltage electrode is a sphere and

the earthed electrode may be a sphere, a semi-sphere or any other configuration which gives

homogeneous electric field. A small hole is drilled into the earthed electrode into which a

metal rod projects. The annular gap between the rod and the surrounding hemisphere is about

1 mm. A glass tube is fitted over the rod electrode and is surrounded by a metal foil which is

connected to the earthed hemisphere. The metal rod or trigger electrode forms the third

electrode, being essentially at the same potential as the drilled electrode, as it is connected to

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it through a high resistance, so that the control or tripping pulse can be applied between these

two electrodes. When a tripping pulse is applied to the rod, the field is distorted in the main

gap and the latter breaks down at a voltage appreciably lower than that required to cause its

breakdown in the absence of the tripping pulse. The function of the glass tube is to promote

corona discharge round the rod as this causes photo-ionization in the annular gap and the

main gap and consequently facilitates their rapid breakdown.


HVE FUNDAMENTALS: C L WADHWA; 3.7; P-98-99

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34. TRIGGERING GAP AND OSCILLOGRAPH TIME SWEEP CIRCUITS

For single stage or multi-stage impulse generators the trigatron gaps have been found

quite satisfactory and these require a tripping voltage of about 5 kV of either polarity. The

tripping circuits used today are commercially available and provide in general two or three

tripping pulses of lower amplitudes. Fig. 3.12 shows a typical tripping circuit. The capacitor

C1is charged through a high resistance R1. As the remotely controlled switch S is closed, a

pulse is applied to the sweep circuit of the oscillograph through the capacitor C3. At the same

time the capacitor C2is charged up and a triggering pulse is applied to the trigger electrode of

the trigatron. The requisite delay in triggering the generator can be provided by suitably

adjusting the values of R2and C2. The residual charge on C2can be discharged through a high

resistance R3. Nowadays lasers are also used for tripping the spark gap.

The trigatron also has a phase shifting circuit associated with it so as to synchronise

the initiation time with an external alternating voltage. Thus, it is possible to combine high

alternating voltage tests with a superimposed impulse wave of adjustable phase angle.

The trigatron is designed so as to prevent the overcharging of the impulse capacitors

in case of an accidental failure of triggering. An indicating device shows whether the

generator is going to fire correctly or not. An additional feedback circuit provides for a safe

wave chopping and oscillograph release, independent of the emitted control pulse.


HVE FUNDAMENTALS: C L WADHWA; 3.7; P-100

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35. GENERATION OF SWITCHING IMPULSE VOLTAGE

The common impulse generator circuits discussed so far are well capable of

producing standard switching impulses with adequate voltage efficiency , if the circuit is

well designed and the ratio C2/C1is kept adequately small.

Other methods, however, have taken advantage of utilizing testing transformers to

step up the amplitudes from impulse voltages also. One such circuit is shown in Fig. 2.31. An

initially charged capacitor C1is discharged into the waveshaping circuit R1, C2as well as into

the L.V. winding of the transformer. The elements R1and C2or other suitable components, in

the dotted rectangle, may be used to control the waveshape. The wave tail is not only

controlled by the resistive voltage divider included, but also from the main inductance of thetransformer equivalent circuit. The time to crest Tp is even without R1, C2 limited by the

series inductance of the transformer, LS, which forms a series resonant circuit in combination

with C1 and the load

capacitance C2. Neglecting

any losses within the circuit,

the voltage across the test

object would therefore start

with a (1-cost) function and

as Tp T/2 = /fr, frbeing the

resonance frequency, the time

to crest is approximately

CLTscr

where, neglecting transformer ratio,

''

21

21

CCCCC+

=

In general, low values of Tp are difficult to achieve, as Ls is quite large and also the

capacitance of the H.V. winding of the transformer contributes to the load C 2. Further

problems arise with transient oscillations within the transformer windings, mainly if cascaded

testing transformers are used.


HVE FUNDAMENTALS: KUFFEL, ZAENGL & KUFFEL; 2.3.1; P-64-65

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36. ENERATION OF HIGH IMPULSE CURRENT

For producing impulse currents of large

value, a bank of capacitors connected in parallel

are charged to a specified value and are

discharged through a series R-L circuit as

shown in Fig. 6.20. C represents a bank of

capacitors connected in parallel which are

charged from a D.C. source to a voltage up to

200 kV. R represents the dynamic resistance of

the test object and the resistance of the circuit and the shunt L is an air cored high currentinductor, usually a spiral tube of a few turns. If the capacitor is charged to a voltage V and

discharged when the spark gap is triggered, the current imwill be given by the equation

dtiCdt

diLRiV

t

mm

m ++=0

1 (6.24)

The circuit is usually underdamped, so that

CLR

UNIT-3 of HVE

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