Detection and location of discharges: In partuicular in plastic-insulated high-voltage cables

DETECTION AND

LOCATION OF DISCHARGES

IN PARTICULAR IN PLASTlC-INSULATED HIGH-VOLTAGE CABLES

!

PROEFSCHRIFT

TER VERKRIJGING VAN DE GRAAD VAN DOCTOR IN DE TECHNISCHE WETENSCHAP AAN DE TECHNISCHE HOGESCHOOL TE DELFT OP GEZAG VAN DE RECTOR MAGNIFICUS DR. R. KRO NlG, HOOGLERAAR IN DE AFDELING DER TECHNISCHE NATUURKUNDE, VOOR EEN COMMISSIE UIT DE SENAAT TE VERDEDIGEN OP WOENSDAGS FEBRUARI 1961 DES NAMIDDAGS TE 4 UUR

DOOR

FREDERIK HENDRIK

KREUOER

ELEKTROTECHNISCH INGENIEUR GEBOREN TE AMSTERDAM

DER

T

.

H.

16 NOV.

1961

DIT PROEFSCHRIFT IS GOEDGEKEURD DOOR DE PROMOTOR PROF. DR. IR. F. A. HEYN

The research work described in this thesis was carried out during the years 1957 - 1960 in the H.V.research labor-atory of the N.V. Nederlandsche Kabel-fabrieken. The author is indebted to the directors of the Nederlandsche Kabel-fabrieken for the permission to pUblish the results in the present form.

•

CON'lENTS

PAR T 1

Survey of l i teraturp.. study of. discharges and discharge detection

CHAPTER 1 CHAPTER 2 CHAPTER 3 CHAPTER 4 CHAPTER 5 . CHAPTER 6 APPEND I X SUMMARY SAMENVATT ING REFERENCES GENERAL INTRODUCTION Object and scope ANALYSI S

Internal discharges and discharge detection

Survey of detection methods

PAR T 2

New methods for the detection of d.ischarges

THE DIFFERENTlAL METHOD Conclusions

SCANNING SYSTEMS Conclusions

DISCHARGE STANDARD Conclusions

THE TRAVELLING WAVE METHOD Conclusions

NON ElECTRICAL DETECTION Conclusions (Noise detection) Conclusions (Light detection)

for. summary in english see section 2

1 3 7 29 33 69 71 92 95 111 113 148 149 .151 153 3· 155 157

PAR T I

Survey of literature, study of

CHAPTER 1

General introduction

1 POWER TRANSMISSION AND HIGH-VOLTAGE

ELECTRIC POWER TRANSMISSION

Energy obtained fr om various sources can be supplied to the consumers in many forms, such as gas, oil, electricity or steam. Among these

forms electricity is an important one, as about one fifth of the

world' s energy is transformed into electric energy [A3]. Electricity

is so widely in use because it is easy to handle; it can be generated

and transported in large quantities, distributed in arbitrarily small or large quantities and transformed in a simple way into other forms Qf energy.

Electric power usely is generated in large quantities, which must be transported to the consumer. Forreasons of reliability and economy most power plants nowadays are linked up so that in case of a break-down of one of the plants the others are able to take over, whereas during normal operation the more economical plants can provide the greater part of the load. This also entails transmission of electric power.

Eiectric power transmission takes place by means of transmission lines and cables. The voltage between the conductors and the current in the conductors determine the amount of transported power. In order to in-crease the power, both the voltage and the current can be raised,

ac-cording to the well-known law P

=

I V cos~. The increase of the

cur-rent is limited by several causes, the main cause being that the loss in the conductors increases at least proportional to the square of the

current, W

=

R 12• The resultant 10ss per unit of transmitted power is

W

P I V cos ~

I R V cos ~

It follows that the percentual loss increases if the current is in-creased and decreases if the voltage is inin-creased, consequentlY the application of a higher voltage makes a bettèr economy in transmission possible. In the case of transmission over long distances the resis-tance of the conductors R tends to be relativelY large in which case V certainly must be high in order to keep the percentage of the loss within reasonable limits.

Nowadays in several countries supergrids have been built, operating at 380 kV and it is expected that the transmission voltages will go up to 500 to 700 kV as far as it concerns the customary a.c. power trans-mission [A4]. This limit is caused among other things by the. in-creasing dielectric losses in the insulation of the cables and the excessive corona on the overhead lines at higher voltages. For trans-mission at still higher voltages d.c. systems are used [A3]. There is a growing tendency to increase the voltage not only in large power systems but also in local distribution networks. Distribution voltages up to 10 kV have been in general use so far, higher voltages are ga in-ing ground.

The equipment in all these systems, the alternators, the transformers, the circuit-breakers, the overhead lines and the cables have to be insulated for these high voltages. The construct ion of the insulation entails many problems. In order to ensure that the insulation is reli-able, thorough tests are necessary. The testing of high-voltage insu-lation has become a more and more important part of the high-voltage engineering of today.

HIGH-VOLTAGE INSULATION

High voltage can be insulated in several ways. The insulating proper-ties of the ambient air can be used. It is a relatively simple method as only a few solid insulators need to be used for mechanical support. This applies to overhead lines and open rail systems. But asolid, liquid or compound insulation is required if high voltage has to be insulated within a small space, which for instance is the case in al-ternators, transformers and cables. The latter will be considered here in particular.

In all these cases special.precautions against breakdown are required. For instance field concentrations, which can of ten be disastrous, must be prevented. For this reason the conductors must not present'sharp edges and their surface must be smooth. Thermal breakdown must be

vented. For that reason the dielectric losses must. be low and the in-sulation resistance high. Voltage surges of short duration and many times the working voltage may occur. The breakdown strength of the in-sulation must be high enough to withstand these surges.

A further requirement for the satisfactory performance of a dielectric is the limitation of the internal discharges which may take place in voids in a dielectric and injure the dielectric. The importance of ,these discharges has been recognized long ago. but only in the last decennia special methods for detecting and measuring internal dis-charges have entered the test plants of the manufacturers of high-voltage equipment. Particularly the introduction of pl~stic dielec-trics, which are more liable to suffer from discharges than the con-ventional ones, has rendered this necessary.

2 OBJECT AND SCOPE OF THE PRESENT STUDY

OBJECT

The object of this study is to add some tools to the existing outfit for detecting internal discharges. Although some of the described methods can be used for the detection of discharges in other equip-ment as weIl, the methods and the instruequip-ments have been developed in particular with a view to the application to plastic-insulated high-voltage cables. Two subjects which are of particular importance for

the testing of cable have received special attention: in the first place the discrimination of discharges in the cable from those in the outer circuit or in the terminals of the cabIe; in the second place the location of discharges in unfinished cores and complete cables with a dielectric of extruded pla'stic.

SURVEY OF THE STUDY

In part I a study has been made of internal discharges and the detec-tion of discharges, based on what is known from literature. The origin and the properties of internal discharges are described. A study is made of the best way to de fine the magnitude of discharges. The exis-tance of a discharge-magnitude below which practically no deteriora-tion of the dielectric takes place, is discussed. Furthermore a survey is given of the eXisting methods for the detection of discharges; a classification of the systems is made. based on the phenomena which

are used for detection, such as light, heat, noise, electric impulses, dielectric losses, etc.

In part 11 a description is given of the research-work carried out. In chapter 3 a new method is described which enables to differentiate between inner and outer discharges and by means of which internal dis-charges can be detected in the presence of outer disturbances. The sensitivity of the system is dealt with. A calibration-circuit is des-cribed. The resolution, that is to say the maximum number of dis-charges per unit of time which can be distinguished by means of the detector, is discussed.

In chapter 4 a description is given of the scanning of cores, that is to say the testing of a small part of a core in an electrode system while the core is moving through this system. In the first place the scanning of unscreened cores is dealt with. In the second place a description is given of a newly developed method for the scanning of cores with a screen of semi-conducting material. The second method is of importance, since the cores are nearly completed so that the chance of introducing faults when finishing the cable is smaller than with unscreened cores. In order to obtain a continuous recording of the magnitude of the discharges when scanning, a recording system has been developed which is described in the last section of the chapter. In chapter 5 the development is described of a source of constant discharges. It makes use of the corona-discharges around a sharp point at high-voltage, faced by a counter electrode. This' device can be used for comparing the sensitivity and the resolution of discharge detectors. A detector without calibration circuit can be calibrated with the aid of the device. Moreover it was necessary to develop this source of discharges before the studY discussed in chapter 6 could be made, as it was an indispensable device for feeding discharges of known magni-tude in a long length of cable.

In chapter 6 the detection and location of discharges in large lengths of cable are dealt with. Use is made of the travelling waves which occur in a cable. Whereas with the scanning methods described in chapter 4 the discharges in an unfinished product can be located with a accuracy of some millimeters and if necessary at a great number of places, with the new detection system described in chapter 6 the dis-charges in a finished cable can be located with a accuracy of a few meters only and if many discharges are present only a few ofthe largest ones can be'separated. The sensitivity which can be obtained with this method is dealt with. Different circuits for locating the

discharges are discussed. The determination of the discharge magnitude is described. In the last section the results of measurements are sented which support the theory which has been developed in the pre-ceding sections.

In the appendix two non-electrical detection methods are discussed: deteçtion by means of noise and by means of light. Both methods are not generally applicable but can sometimes be' used as an additional method to detect or locate discharges. Noise-detection is used for locating discharges in a cable, light-detection for detecting and 10-cating discharges at cable-terminals. The study of these subjects is only superficial; the work has mainly been carried out in order to obtain an idea of the sensitivity which can be obtained.

TERMINOLOGY

A few words must be said gbout the terminology used. In many public-ations the words ionization, discharge and corona are mixed up. Of late a more uniform terminology has been developed which is used in this study.

The word 'ionization' refers to any process in which an electron is separated from an atom or a molecule.

The term 'gas-discharge' or 'discharge' for short is used for the passage of electric current through gas or vapour; it involves ioniza-tion of the gas. The term 'internal discharge' is reserved for a Bas-discharge in a void in a iielectric.

The word 'corona' is used for the intermittent ionization of the air in a strongly inhomogeneous field such as the field around a sharp point.

CHAPTER 2

Analysis

3 INTERNAL DISCHARGES

ORIGIN

Internal discharges in a dielectric occur in inclusions of low die

lec-tric strength. The material in the inclus.ion breaks down at a field

intensity which is low as compared with the breakdown strength of the dielectric. Moreover, the dielectric constant of the material in the inclusion is of ten lower than the dielectric constant of the

insula-ting material, which causes the field intensity in the inclusion to be

higher than in the dielectric. Accordingly the field intensity in the dielectric at which the inclusion breaks down is lower again than the breakdown strength of the dielectric. Two examples are gas-filled voids in asolid dielectric and fluid-filled voids in an impregnated dielectric.

GAS-FILLED VOIDS

The field strength at which the discharges start depends on the actual field intensity in the inclusion and the breakdown strength of the in-clusion. In case of gaseous voids the field strength in the void

de-pends on the form of the cavity and on the dielectric constant E of

the solid insulating material. In some cases the field intensity in

the void can be calculated. A frequently occurring case is a cavity

which is flat and small as compared with the thickness of the

dielec-tric; then the field intensity in the cavity is E times as large as in

the dielectric. If the cavity is spherical the field intensity is

3 E

i

+ 2 E times the field intensity in the dielectric tending to 1,5

times i f E is large [C2]. I f the longi tudinal axis of the cavi ty is

parallel to the field, the field intensity in the cavity tends to be

equal to that in the dielectric.

20·:) torr

I

10~

L_L--::k-~--:3~0~

\

~40~5~0)i,60D

-

18ïCO)

.

1C

. 02 03 0,4 Q5 Q6 Q8 I mm

15 20 Olmm " . . .

' ~Iectrode spacmg Fig I

The dielectric strength of a gas-filled void depends on the kind of gas in the cavity, the gas pressure and the dimensions of the void. According to several investigators [S3, Cl] the discharge in a void bounded by insulation mater~al occurs at approximatelY the same volt-age as between equally spaced metal electrodes. The effict of the spacing of metal electrodes and the gas pressure on the breakdown of

a void is given for a certain gas by the Paschen curve for this gas.

The measurements of Hall and Russek [S3] on cavities in several kinds of plastics are consistent wi th Paschen' s law. Mason

[cd

found th at the discharge inception voltage in nonventilated voids in polythene is about 10 - 20% lower than the value derived from the Paschen curve. The Paschen curve can be modified to represent the breakdown field strength in the void instead of the voltage. If the gas pressure is taken as a parameter a set of curves restilts [G2], in figure 1 such a set of curves is given for air; the field intensity is given in kV/mm r.m.s. The location of the minimum in the Paschen curve is indicated by a dotted line.

OIL-FILLED VOID

ALG.

H

,~D· !BL'OTHEEK

S:"';

;1: 'UNDE

Or::U'T

The field intensity in·an oil-filled void can be calculated similar to the field in a gas-filled void. The field intensity in a void, which is flat in a direction perpendicular to the direction of the electri-cal field, is ~~ times as large as the field strength in the rest of

El

the dielectric, if El is the dielectric constant of the fluid and E2 the dielectric constant of the solid dielectric. If the gap is spheri-cal the ratio is ___ 3 __ E_2 ___ and if the gap is lon~ and flat, parallel

El + 2 E2 to the field, the ratio is 1.

The oil breaks down at

à

certain field intensity, gasbubbles are prod-uced and gas discharges occur in these bubbles. Fluids are able to abs orb gases and a balance is reached between the formation of gas by the discharges and the absorption of gas by the fluid. As a consequence the discharges exting~ish, increase or become stabIe.

RECURRENCE OF DISCHARGES AT A.C. VOLTAGE

Af ter a discharge in a void has occurred once, it will recur. The mechanism of this recurrence depends on the kind of voltage, a.c., d.c., or impulse voltage.

In the case of a.c. the dielectric can be represented by the circuit of figure 2 [B1, B2, Al]; In this conventional analogue circuit

capac-ity

c

corresponds to the cavity or that part of the cavity which is discharged, a corresponds to the capac.i ty of the sample and b cor-responds to the capacity of the dielectric in ~eries with c.

Fig 2

In figure 3 the high-voltage across the dielectric is denoted by va and the voltage across the void by v . A discharge takes place in the

c +

void when the voltage V_{c reaches the breakdown voltage U . As stated}

before for the case of a gasfilled void, U+ is given by the Paschen curve. The voltage then drops to V+, where the discharge extinguisnes. According to Mason the value of V+ is not more than about 100 V.

The voltage drop takes place in a time shorter than 10- 7 sec. This has be~n inferred among other things from the fact that the frequency spectrum. is found to extent to the region of 100 Mc/sec [Al]. Conse-quently this time is extremely short compared with the duration of a cycle of the 50 c/sec sine wave and the voltage drop in the void may be regarded as a step function.

Af ter the discharge has occurred the voltage across the void increases again, this voltage is det~rmined by the surface charge in the cavity and the growing voltage across the dielectric, which counteract one another. In this case leakage of charge through the dielectric is neg-lected.

When the voltage across the void reaches U+ a new discharge occurs. This happens several times af ter which the high voltage over the sample decreases'and the voltage across the void must drop to U- before a new discharge can take place. In this way groups of discharges originating from one void will be found which cause current impulses in one direc-tion at rising voltage and current impulses in the other direcdirec-tion at decreasing voltage. The voltage at which the discharges occur when the voltage across the sample is increased, is cal led the inception volt-age.

"

_ t

Fig 3

Austen and Whitehead [BIJ'have shown that if the voltage-drops in both

h~lf cycle~ are equal, thus t:J.V+

=

t:J.V-,the discharges will give a ~ta­ tlonary picture on a 50 cis time-baseonan oscilloscope-screen. If the

, A +

voltage-drop~ differ (uV 1 t:J.v-) which of ten occurs in asymmetrical voids or in voids bounded at one side by metal, the impulses seem to

be in motion whenmade visible on an oscilloscope-screen.

Of ten a discharge-picture recurs periodicallY. In the case of an asym-metrical void an intermittent discharge can arise [Al]; as is shown in figure 4. The voltage across the void vc reaches the breakdown voltage U+ at point A, which results in a discharge in positive direction at A. Due to the surface charge and the vol tage v c the void breaks down again at B, giving a discharge in negative direction. The next dis-charge occurs at C, earlier in the cyclus than A. The voltage in the next half-cycle passes 'through its minimum without reaching the value U-, so that no discharge can occur. The surface charge persists during manl-cycles and no more discharges occur until this charge has leaked

;;;-~y~.

~

A:-:n~o

:-:

t~h

~

e

:-:

r

:':::

s

:::

e

:-:

q

~

u

~

e

~

n

~

c

~

e

""'':

t

~

h

~

a

:

n

~

s

:

t

::'

a

=

r

::

t

'::

s

"'::

an

'::

~

d

e

:'

x

-

t

::

l

::-'

n

::'

g

~

U

~

i

~

s

::

h

:;

e

:':

s

:""':

a

:':

g

::

a

:;

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~

n

::

,

:"':

s

:':

o

::':

t

';'

h

;::

a

:;

t

::'::

an

ermittent discharge is observed. At a higher voltage, namely if v_c exceeds U-, the discharge becomes constant.

.- - - -- - - - U- 6V+.,6V-Fig 4

Once a discharge has started, the di: charges can persist at a voltage much lower than the inception vol tage, theoretically as low as half the inception voltage. For the curve given in figure 5 it is assumed that the first discharge starts due to a short overvoltage at A, or on ac-count of the fact that a voltage higher than the inception vol tage was present before point A was reached. The voltage across the void,

orig-inally smaller than the breakdown voltage U+ or U-, reaches the break-down voltage each halfcycle due to the surface charges which are pre-sent af ter the first discharge. It is because of this phenomenon that the extinction voltage, i.e. the voltage at which the discharges dis-appear when the vol tage across the samp Ie is diminished, is of ten lower than the inception voltage.

Combinations of the mechanisms of figure 4 and figure 5 are possible.

A , 12 , _,

"

\ \ \ \

,

-

-- --\ \

\

Fig 5

MECHANISM OF THE DISCHARGES

The description of the phenomena by means of the model of figure 2· is very simp1e. It has been assumed that the void or part thereof behaves 1ike·a capacitor with metallic e1ectrodes. But in rea1ity the phenome-na are far more comp1icated. Fried1ander and Reed [B7] and Mason [B5, 86] have shown that at the p1ace where the discharge reaches the di-e1ectric, Lichtenberg-figures are formed. It is be1ieved that when the ~h e which takes p1ace in a re1ative1y narrow channe1 rea~~p

d tric rong tangentia1 fie1ds arise. This leads to dischar3es e-charges

be-~ ~er the dischar es have been extinguished a pat.:ern_o~~e­

charges is 1eft which neutra1izes the e rtc field who11y at the

-p-1-c-e- o-f-ilie discharge and partial1y- in t e surroundings'-T~ch­

ten~b-e-r-g--~f~i-g-u-r-e-s~h-a-v-e~t~y-p-i~C-a~l--shapes, th~i~i~g star-shaped with many branches, the negative being circu1ar in shape and diffuse. Due to this difference in shape the charge-figures 1eft by consecutive discharges of different po1arity cannot cover each other. According t o Morris Thomas 6BS] the surface-charges hard1y can move as they ar!

trapped at the surface of the die1ectric, so that af ter discharges of ëfifferent po1arity at some p1aces positi~e surface-charges can occur when no charge or negative surface-charges wi11 be expected and vice

VërSä

.

Mäson-has shown [85, 86] that this can lead to comp1ete1y dif

--

ferent discharge patterns than those of figu.re 3. Different patterns in the positive and negative ha1f-cyc1e of the app1ied voltage can appear even if the void is symmetrie; the time between successive dis-charges can be different and an unequa1 number of disdis-charges can be found in opposite ha1f-cyc1es.

In voids exceeding a certain size, severa1 discharges can occur af ter each other at different p1aces in the same void. The first discharge occurs somewhere in the cavity. TQe next discharge wi11 occur at the other end due to the screening effect of the surface charge and further d1scharges fi11 up the as ected areas as indicated in figure 6. Mason Cl

Fig 6

When a cavity at one side is adjacent to a conductor a large dis-charge will occur during one half-cycle of the high-voltage, whereas several small discharges will be discernible during the other half-cycle. Mason (Cl] explains this as follows: in the case of a dielectric

and a parallel metal surface, the discharge ~en

the metal is negative than when it~ itive, because in the latter

c~e

mechanism intensifies the

discha

~

~---DISCHARGE PATTERNS

As a rule a sample has several voids and these in turn have usually a number of places at which discharges occur; again at one place several discharges may occur. Consequently if the discharges are made visible on anoscilloscope-screen a very complex picture will appear. Constant,

intermittent and wandering discharges can appear, separatelY or super-imposed. Sometimes it is possible to discern pictures which are char-acteristic for certain types of discharges. With the aid of the fol-lowing table some of these pictures can be interpreted.

DISCHARGE PATTERNs

intermittent discharges (fig.4)

discharges at the negative crest of the applied voltage

ibid. at positive crest

discharges, fairly symmetrical in the ascending and descending half-cycle of the applied voltage

(fig.3)

one large discharge in the de-scending half-cycle, several smaller discharges' in the ascending half-cycle

one large discharge in the ascend-ing half-cycle, several smaller discharges in the descending half-cycle

14

ORIGINE OF DISCHARGE

asymmetric cavity at about the inception-voltage

corona or surface discharges at the high-voltage conductor

ibid. at the low-voltage conductor

symmetric cavity

cavity adjacent to the conductor at high-voltage L. ( j

cavity adjacent to th~ conductor at earth potential

DISCHARGES AT OTHER THAN A.C. VOLTAGES

When applying d.c. voltage, discharges will occur during the rise of the voltage, as in the case of a.c. voltage. Af ter the voltage has become constant discharges occur only infrequently. The dielectric can

be represented by the circuit of figure 7. The capacity of the void c

is continually charged by the conductivity g, due to the leak in the

dielectric; c discharges when the voltage has reached the

breakdown-voltage of the void.

c

o

b

Fig 7

The number of discharges per unit of time increases with the voltage across the sample; the higher this voltage the larger the leakage cur-rent which charges the void-capacity c. The number of discharges de-pends also on the resistivity of the dielectric, the greater the re-sistivity the larger the time-interval between the discharges will be.

In the ordinary dielectrics the resistivity is so high that at field intensi ties at which a sample usually shows discharges at a. c. voltage, discharges at d.c. occur at intervals which range from a few hours to several weeks. In such a case it will be difficult to detect discharg-es. In a dielectric of impregnated paper at d.c. voltage, the recur-rence-frequency of the discharge reaches a value, comparable to that at a.c. voltage, if the voltage is 5 to 10 times higher than under a.c. conditions [figure 21 and figure 22 of reference GIJ.

Tests with surge voltages are performed at so high voltage-levels that it must be expected that internal discharges will occur. The test methods, based on the observation of the capacitive current in the sample, usually are too insensitive for the detection of internal

dis-charges in voids. However surge-voltages occur very infrequently so

that discharges in voids due to surges will not damage the dielectric. Discharges or discharge detection at other than a.c. voltage will not be discussed.

MAGNITUDE OF THE DISCHARGES

The magnitude of the discharges can be defined in several ways. In the !irst place c 6 V (see figure 8) can be taken as a measure. It

cor-responds to the charge which is neutralized in the void. But it is not a practicabie choice as c

6

V is doomed to remain unknown. No dischar-ge detector reveals the magnitude of the chardischar-ge c 6

v.

Fig 8

In the second placb b 6 V can be taken. It corresponds to the dis-placement of charge in the sample due to a discharge in avoid. The corresponding instantaneous change in voltage across the sample is equal to b 6 V. Most discharge detectors respond to this voltage-drop

a+b

and are. thus capable of determining b 6 V • However, it does not give a correct measure of the discharge in the vold, as the capacity b is not completely determined by the dimensions of the void. It is just by accident that b 6 V can be used as a measure for discharges, as will be explained below. The displacement of charge b 6 V is de-noted by Cl.

Thirdly ~ c

6

v

2 can be taken as a measure for the discharges. It is nearly equal to the energy which is dissipated in the void and which may cause a deterioration of the dielectric. This energy in the void which is denoted by p can be related to the displacement of charge

Cl, measured by means of a detector, as follows.

The voltage across the void drops froni u+ to V+ (notations as in fig. 3). The energy dissipated amounts to

If V+ is neglected

p

=

J.2 c AV but this value is too low.

At 'the time of the first breakdown of the void no surface charges are present in the void and the ratio between u+ and the crest of the high-voltage

V

is

U+ = _ b _

_{V ,}

b + c

As

V

is the voltage at which the discharge appears for thê first time,

V

is identical to the inception voltage and can be denoted by

V

_{inC '}

thus U+

=

b

_~inc

' p b + c J.2 c 6 V __ b __

V

inc b + c

If b is neglected with respect t~ c the value of p in this ex-pression is too high. This counteracts the effect of the former ne-glection. Consequently

p~J.2 b 6 V

v

_inc

or p~J.2 q <J_inc

Expressed in volts r. m. s.

p ~ 0,7 q Vinc (1)

Thus the energy p of the discharge is related in a simple way to the displacement of charge q. This is the reason why the displacement of charge q is a relatively good measure for the magnitude of a dis-charge. Moreover most experience about discharges has been obtained in the relatively small voltage range between a few kV and 20 kV. Conse- '

quently the energy-contents of the investigated discharges were of the same order of magnitude if the discharge-displacements were equal. In the fourth place the total energy of the discharges in a sample can be measured: ~ ~ L c 6

v

2 . This is accomplished by means of the Schering bridge and some special discharge-detectors. In certain

samples where it can be expected that all discharges are of about the same magnitude, this total energy can be taken as a measure. It is used for instance for coils in H. V. machines and bushings.

The author prefers to measure the largest dischàrge present in a sample as this gives a bet ter idea of the damage which the discharges cao

in-flict on the dielectric. The displacement of charge q = b'~V is 'taken as a measure for the magnitude of the discharge. The word 'discharge magni tude' will be used for q. I t must be emphasized that this dis-charge magnitude is not equal to the charge which is neutralized in the void but is a practlcál measure with the dimension of charge.

OAMAGE TO THE OIELECTRIC

Internal discharges are known to be injurious to a dielectric. ,The damage can be ,due to several causes

- 'the ion- bombardment, causing'

- the electron-bombardment, causing

- chemical products which are formed in the gas of the void; such as nitric-acid and ozone

- uI tra violet 1 igh t.

heating of the cathode erósion of the cathode

heating of the anode

-chemica 1 processes in the anode (polymerisation, cracking,

gas-forDII~.tion)

The causes differ from case to case and are stronglY dependent on the type of dielectric material. Masón [cll has shown that for polythene thermal degradation is paramount. In the case of mica insulated wind-ings of alternators the deterioration of the mica results from the ion bombardment, [E2J. With increasing field intensity the mechanism of dé-terioration can change and therefore the voltage life iS the more de-pendent on the fiel~ intensity in the sample. Mason [Cl]has shown that at a voltage twice the inception voltage the mechanism of the deterioration of polythene is changing: narrow carbonized channels are formed at the top of which the intrinsic breakdown strength is reached. These channels propagate quickly through the dielectric.

Little is known of the deterioration of the dielectric in relation to the discharge magnitude. The study of this relation is made difficult by the tendency of the discharges to extinguish af ter some time. Rogers [C4] has shown for several"_{kinds of plastics that this} self-extinction is caused"_{by a conducting path which is formed along the} walls of the void. Depending on the shape and the position of the void the discharge extinguishes completely, intermittently or not at all. As the shape of the void can not be determined from outside it is usually impossible to predict the time to breakdown for a certain discharge. The deterioration increases also with growing repetition-frequency of

the discharges and the repetition-frequency in turn increases if the frequency of the applied voltage is raised. It also increases with growing voltage, _{measured from peak-to-peak.}

The ra te of deterioration depends to a great extent on the kind of di-electric. Tests [Cl] have shown for instance that under equal conditi-ons polytetrafluorethylene and perspex are less resistant to dis-charges than polythene.

_On

_{the other hand glass and mica are far more} resistant. Many investigations have been carried out on this subject and attempts have been made to draw up tables in whi"ch the insulating materials are placed in order of succession of their resistance to

discharges. But these tables do not prove to be valid under all circ-umstances.

DETERIORATION LIMIT

It is believed that below a certain discharge magnitude the deteriora-tion of a certain dielectric becomes so slight that a practically in-finite life of the dielectric can be expected. This magnitude will be called the 'deterioration limit'. It is an important value for prac-tical discharge detection. The discharge detector used must be able to detect discharges of at least that magnitude and preferably smaller. From the investigated electrical equipment it must ~~required _{that at} least at working voltage no discharges occur which are larger than the deterioration limit.

The de~erioration _{limit of a given insulation material depends on the} frequency of the applied voltage. A discharge magnitude which can be tolerated at 50 cjsis likely not to be acceptable at 100 kc. It is probable that the deterioration limit depends also on the field in-tensity in the dielectric as the discharges concentrate at higher field intensity to form a small number of deep pits. Mason has shown this for polyth~ne _{[C3]; Thus at a higher field intensity a lower} de-terioration limit must be expected and taken into account.

The deterioration limit is not a' sharp limit as there is a consider-able variation in the results of breakdown tests of long duration. Reproduceable results are difficult to obtain. Only a few estimations

of the deterioration limit have become known. Davis and· otiïers [CS]·

have infered that dischargesin polythene-cables of a magnitude smaller than 2 pC are harmless; this applies for a field intensity of about 3 kV/mmo Mason [C3] doubts if this magnitude is correct and in any case limits the validity to industrial frequency. Mildner and Humph-ries [12] consider it amply sufficient to scan pvc-insulated cable at l,S times the working voltage and with such a sensitivity that dis-charges larger than 3 pC are detected. The maximum field strength in these cables is 3 kV/mmo

4 DISCHARGE DETECTION

PR INe IPLES

The detection of discharges is based on the energy exchanges which take place during the discharges. The dischargesgive rise to many phenomena which can be used for detection.

- dielectric losses ( electric phenomena

( - electric impulses

Internal ~ischarges ---+ (electro-magnetic radiation - light

( heat ( noise

( gas pressure

( chemica 1 transformations

The most frequently used methods are the electrical ones, viz. mea-surement of dielectric· losses and detection of electric impulses. These will be dealt with in the next section.

The non-electrical methods are not u,.sed very of ten. They ·are in many

cases not as sensitive as the electrical methods. They are dealt with in the next sub-section.

In all these cases attent ion has to be paid to the three fOllowing aspects.

20

DETECTION The determination of the presence of discharges. The

voltage at which the discharges appear is determined. The elec-trical methods are the most reliable for this purpose.

LOCATION The determination of the place of the discharges. This

is somewhat ~ore diffi~ult. With the aid of noise it is possible

to locate discharges to some extent. In the case of plastic.

in-~ulated cores an electrical scanning method is possible. In

spec-ial cases light produced by discharges can 'be used.

EVALUATION The question of the evaluation is the most difficult

one. With several electrical methods the

~agn

i

tude

of the

dis-charges can be determined, but the value that can be attached to this magnitude is not yet weIl known. Among other things a know-ledge has to be gained o( the deterioration limit of various di-electrics.

NON-ELECTRICAL DISCHARGE DETECTION

LIGHT The application of light for detection is limited. There are

two possibilities: the detection of edge-effects and that of

discharg-es in tranSparent mediums. Figure 58 in the book o~ Whitehead [Al]

gives an example for polythene. In part II some experiments will be described.

HEAT The temperature rise caused by discharges, or more likely caus-ed by a bad dielectric destroycaus-ed by discharges, can be measurcaus-ed. Scanning by hand of a cable or a bushing af ter an overvoltage test is

a common method. Robinson [A2] in his book about cables describes a

method with thermo-couples. The method is not sensitive.

NOISE The .presence of noise due to discharges has be.en known from

the beginning of high-voltage engineering. The 'hissing test' for

cables and bushings has long been in use. Austen and Racket [82]' give

all estimation of the sensitivity of this aural method, which they con-sider to be low. Anderson [Kl] and Beldi make use of an ultrasonic

transducer immersed in the oil of H.V.transformers. The experime~ts

have mainly been conducted with the object of detecting discharges at impulse voltage. When using more then one transducer at the time even location is possible. The sensitivity of the method is limited by the ultrasonic magnetostriction noise which is produced in the steel core

of the transformer by __ tpe surges. Discharges at power frequency

can be detected in this way as weIl but location is more difficult.

UnfortunatelY Anderson does not give an estimation of the' sensitivity.

When cables' are tested good usw can ~e made of the relatively small

distance between a discharge and the surface of ,the cabie. Experiments

regarding detection and location will be described in part 11.

m in both casu ia equivalent to

A+c

a+k 11 C «a and k

<.l

-...!.-V~ fI

C)

eRm CO$(.&)t ~+k a+C

û)...,V_I-

I

~Lm

=

Fig 9< Fig 9

GAS-PRESSURE Kitchin and Pratt [K2] detect the discharges_ in the air gaps between a cable conductor and the dielectric by means of a mano-meter. Due to the chemical reaction of activated oxygen with the in-sulation the internal gas pressure decreases as soon as discharges appear. A refined method is obtained by measuring the internal gas-pressure of two samples by means of a differential manometer, one sam-ple is given high-voltage the other is not. The method is limited to cables which have stranded conductors without filling. No indication is given about the sensitivity of the method, but it appears from the diagrams th at the discharge inception voltage of the air between con-ductor and insulation can be clearly determined.

CHEMICAL TRANSFORMATIONS The presence of chemical products caused by

the discharges, can be used for detection and location.

Robinson [A2] determines the presence of wax in mass-impregnated cables, formed by the discharge~ by means of dying the paper tapes. He uses magenta dye which gives the tapes a bright colour except at

those places where wax has been formed. In this way a good location of the discharges is possible. Robinson shows several examples of dis-charges in butt gaps, between strands, etc. As the sample must be de-molished for inspection, the application of the method is restricted.

5 ELECTRICAL DISCHARGE DETECTION

PRINCIPLES

The discharges in a cavity cause current impulses in the circuit in series with the dielectric. The phenomena which occur can be analyzed with the aid of the analogue circuit of figure 2. In series with this circuit a detection-impedance is placed as shown in figure 9. In gen-eral two types of impedances are in use, a resistance R as shown in figure 9a-" or a resonance circuit LCR as shown in figure 9b. The circuit is completed by the capacitance k which corresponds to the capacity of the high voltage source. Of ten an actual capacitor is in-serted in this place in order to facilitate the passage of the current impulses.

The voltage impulses which appear across the impedance as a result of the discharges in the insulation can be calculated with the aid of Laplace-transformations. The shapes of the impulses are shown in figure 9 and are also given in a formula. In the circuit with the resistor

-t

(figure 9a) the impulse decays by a curve e-r in the resonance

circuit (figure 9b) it foIlows an attenuated oscillation. 'In the cal-culations and in figure 9 the initial rise of the voltage is taken as a step. In reality the steepness of this rise is limited by the self-induction and the resistance of the circuit, but in the circuits com-monly in use the resistance can be neglected and the self-induction is so low that the voltage-rise takes place, in less than a microsecond. Consequently in most applications the voltage-rise can safely be

re-garded as a step-function~

The impulses are amplified. As the frequency-spectrum of the impulses in figure 9a is large, usually a wide-band amplifier will be used there. The resolution, that is to say the number of impulses per unit of time which canbe separated, will be large. With a resonance circuit as in figure 9b a band-pass amplifier tuned to the frequency of the resonance circuit usually will be applied. Owing to the narrow band the amplification can be larger but the resolution will be smaller. starting from these principles many different discharge detectors have been devised. They can be distinguished into three main types. The first type is based on the amplification of the impulses across the detection-impedance. The method will hereafter be called the straight method. The impulses are usually made visible on an oscillo-scope screen.

The second type, the balanced detector, is intended to diminish the effect of external disturbances. Whereas with the straight methods disturbances, such as discharges in the high-voltage source and edge effects are shown, these can be suppressed with the balanced methods and only internal discharges appear on the screen.

With the third type the total power ~

*

L c

6

v

2 is measured which is dissipated by the discharges in the dielectric.

STRAIGHT METHOOS

A method which also is available as a commercial equipment is that of

,Mole [Fl, F2J. He makes use of a resonance circuit. A coupling conden-sor is placed in parallel with the object, see figure 10. The reson-ance frequency of the circuit A is tuned to the midband of the band-pass amplifier for obtaining maximum sensitivity. The midband frequen-cy is 500 kc. This frequenfrequen-cy is chosen because of the absence of in-terferences from broadcast transmissions, 500 kc being the centre of a rescue-band. According to Mole the instrument is able to resolve 35 discharges per quadrant of a 50 cis wave.

The magnitude of the individual discharges can be determined with the

aid of a calibration circuit. The discharge-magnitude q

=

b 6 V is expressed in pico-coulombs. The sensitivity of the detector is good. In an object of for instance 1000 pF a discharge of 0,02 pC can be de-tected [Fl], but it must be said that in practice this sensitivity usuallY can not be reached due to pick-up and disturbances •.

As. is the case with all straight methods, signals due to external dis-charges are not rejected. The high voltage source must be free from discharges. A filter is provided in order to block interferences from the mains. dischorge_ free HV tronsformer filter A ~ Fig 10 oscillosco' t

[>

@

Quinn [F6] and Viale [F3] use a coil in series with the object, simi-lar to figure 9b. Quinn employs a broad-band amplifier, Viale accord-ing to a later publication [El] makes use of a band-pass amplifier with an adjustable midband. The midband-frequency is tuned to the re-sonance frequency of circuit A for maximum response and forms in that way the counterpart of the system of Mo~e. The sensitivity of the set-up of Quinn is low, it enables to detect a discharge of about 20 pC in an object of 1000pF. The equipment of Viale enables to detect a dis-charge of about 0,1 pC under the same circumstances [F3]. Graham [F4] , Adolphson [F5] and many others [Al, Gl, B2] have worked along the same lines.

An other form of straight detection corresponds to figure 9a and makes use of a resistor as a detection-element. Renaudin [F7, E2] " makes use of a simple device; he amplifies the voltage impulses across the re-sistor and makes them audible in a loudspeaker. The sensitivity is not given, an estimation leads to 5 to 25 pC in an object of 1000 pF. By

means of a q~adratic detector and a voltmeter Renaudin measures the

ener~y content of the frequency spectrum of the discharges.

A specialized equipment fQr straight detection is that of the

Compag-nie Electro M~canique. Laverlochere [E2] describes that af ter

detect-ion on aresistor and af ter amplificatdetect-ion,the impulses are electronic-ally classified according to their amplitude and counted thereafter. In this way a diagram is obtained, which shows the number of discharges per unit of time in relation to their magnitude. In order to do this, full advantage is taken of the abilities of resistance detection by

using a braad band. ~ bandwidth of ROO kc is provided.

The sensitivity is not stated, but it is certainly of the order of 1 pC

in a sample of 1000 pF. This apparatus provides a good means for

evaluating the discharges. Unfortunately the equipment is rather ex-tensive and will be too laborious for normal use.

Blanchardt" {FSJ e~ploys a sphere-gap as a couple condensor and uses a resistor. The sensitivity is likely to be low.

In part II a description is given of a method for detecting and

10-cating discharges which isbased on the observation of travelling waves in long lengths of cabIe. A resistor is used for detection. The method is akin to the straight methods.

BALANCED METHOOS

In the ~ormer methods signals due to outer disturbances can not be

rej~cted. They can be suppressed a little if the detection impedance is placed in series with the sample as in figure 9; the coupling

con-densor k is made larger than the sample a, outer disturbances as

weIl as edge-effects in a will be reduced in the ratio ~. If the

a

existing outer disturbances can not be sufficiently controlled in this way, one of the balanced systems must be used.

An of ten applied method makes use of the Schering-bridge, which usual-ly is present in a high-voltage laboratory. It is in fact a method with a resistance as a detection-element, as shown in figure 11. A discharge in the sample causes an impulse across resistor R3. The im-pulse is supplied to the oscilloscope via a filter and an amplifier. A disturbance from outside causes an impulse on either side of the

bridge, el and e2 in figure 11. The difference between these impulses.

H.V. source Schering-bridge filter Çlmplifier oscilloscope

Fig II

measured between the bridge points 1 and 2 is smaller than the im-pulses themselves. In this way external disturbances can be reduced, a minimum being adjusted by varying R3 and C₄. Graham [F4] states that the reduction usually is about 30 times.

Arman and Starr [Gl] 'used this method and measured the voltage due to the discharges with a galvanometer. Greenfield [G4] displayed the dis-charges on an oscilloscope. Both employed a high-pass filter in order to suppress the 50 els-voltage. Austen and Hacket [B2] employed the Schering-~ridge without a filter. Hagenguth a~d Liao [G5] used a simi-lar set-up, without filter, for measurements at impulse voltage. By using a narrow-band amplifier the rejection of the disturbances can be increased; the Schering-bridge is adjusted so that the frequency at which the bridge is in balance is equal to the mid-band frequency of the amplifier. But the resolution of the impulses decreases, due to the narrow band of frequencies which is observed.

An interesting method, based on a total other principle, is that of Hashimoto [G3]. The impulses due to internal and external discharges

have opposite directions, 1 and 11 in figure ga. By means of an elec-tronic device, only the impulses in direct ion 1 are amplified.

Although the method seems to be excellent it shows some disadvantages whi ch make i t 1 ess useful. In th is sys tem only discharges of one half-cycle of the 50 cis SIne-wave can be shown, as in the other half

-cycle the discharges from outside have changed sign and should become visible. This can cause the neglecting or misinterpreting of an asym-metrical discharge, such as a discharge in a void at one side bounded by metal. Furtheron a practical system can not be made without over -shoot of th e impulsp.s. The overshoot becomes v.isi bIe as an impulse in the same direction as the wanted impulses. Consequently the overshoot limits the rejection of the outer discharges. From an oscillogram of ·Hashimoto i t appeared tha t an impulse of 22,5 mm height was accompanied by an overshoot of 0,5 mlO, consequently a signal due to outer dis-charges was reduced by a factor 45 only.

The differential method, as will be described in part Il, belongs to the balanced methods as weIl.

LOSS DETECTION

Detection of the dielectric losses by means of the Schering-bridge is an old and well-known method. A sudden increase of the losses daring increase of the voltage is attributed to internal discharges, the start of the increase being taken as the inception voltage. But not every increase of the losses coincides with discharges and the incep-tion voltage is of ten difficult to determine.

Gelez [HIJ makes use of a variant of the Schering bridge and separates the normal losses and the losses due to discharges by means of two different adjustments of the balance.

Veverka and Chladeck [H2J use an electronic device which integrates the impulses due to discharges and measure them together with the voltage across the sample in a wattmeter. In this way they obtain a direct reading of the totalloss caused by discharges.

SCANNING

A special application of the foregoing methods to cable-engineering is the scanning of cable cores. The core is passed through a hollow cYlindrical electrode, which is immersed in an insulating fluid [12, F4, G4J. Either the conductor or the electrode is maintained at

high voltage and discharges are datected by one of the straight meth-ods.

Gooding and Slade [11] immerse the core in a water container. Tha close-fitting water serves as anelectrode. By using a long tube filled with distilled water it is possible to earth the extremities of the pipe and apply high voltage to the centre. In this way a gradual in-crease of the field strength in the core is obtained and edge-effects are avoided.

In part 11 experiments with scanning methods will be described.

SURVEY OF DETECTION METHOOS

In the table 'Detection Methods' a survey is given of the various methods used for the detection of discharges. In the column 'sensitiv-ity' the minimum detectable discharge is stated. Where necessary it is assumed that a discharge of the stated magnitude occurs twice during one· cycle of the 50 cis vol tage. The sensi t i vi ty of the electrical methods depends on the capacity of the sample. If the effect of

para-sitic capacities is neglected, the minimum detectable discharge is q

= a

V

_{min , where} a is the capacity of the sample and

V

_min the minimum peak voltage of an impulse which can be discerned with the aid of the detector. In the table a is taken as 1000 pF. Thischoice of a is quite arbitrarily but it is assumed that at this capacity of the sample parasitic capacities can be neglected,

V

_{min is inferredor} esti-mated from thy consulted literature .

. With the Schering bridge and the double balanced bridge of Gelez the minimum detectable discharge is

q

=

{2 7T a V /::, tg 0

The minimum variation in tgo that can be detected is assumed to be 10- 6 at V

=

10 kV and a

=

1000 pF, then q ~ 50 pC.

For the methods by means of which the high frequent impulses are ob-served the resolution of the impulses is stated. By resolution is meant the maximum number of impulses·which can be separated in one quadrant of the 50 cis wave. A survey of the cable scanning systems will be given in part 11 af ter the desciiption of the experiments with these systems.

DETECTION METHODS

DETECTION LOCATION EVALUATION

detected .ethod au thor &: year rete-I ) Sensitivity2} Sens! ti v ity distinction3 ) locatlon3) 8ultable 3 ) measured kind ot

phenomenon rence amon, others or reJ Be tien in tbe tor magot tude Object

atfected by ot Quter object lIeasurlng

dlscharges

vlsual tommon [82) 20 pC

ractice transparent

light Gosden 1950 Al

·

_. + + ± light abjects and

photographlc lo J 1 pC intens! ty edge ettects

manual cammon

practice - very thlcknes8 of + + - cables

heat 10_ the lnsula- °and

tbermocouples Robinson 1936 [A2] tien bushings

-aurd cammon [82] 40 pC thicknes8 ot ± ±

-ractice the lnaula- neise all Obj ects

neise microphone Anderson 1956 lKlJ ·

.

tien and + + ± intens! ty in special

in 011 ambian tooise trans

tor-liers

contact- [Gil + +

-microphone [0 ] 50 pC

gas-pressure manometer K! toh!n 1957 [K2] _{· .} +

-

_- voide be-tween con-ductor acd insula.t1on

of cables

mony objects

possibllities which are

chemical _{· .} + + - allowed to

transformations d.ying paper Robinson 1936 [A2] be

de-.olished

Quin 1940 (P6] 20 pC resol ution in

(2)

iapulses per

Austen 1944 3 pC Quadrant

detection _{Male 1952 [pil 0,02 pC ca.paci ty of}

,.

" wi th LCR the samples ~ 36 _"

~

... circui t _{IF3] - +} u Viale 1954 0.1 pC _" e1ectric "140 :<I

impulses many others _{[Al F4J} .. :~ 0

~

(straight _{P5 G1 piek-up of 0"}

..

"

..

de~ect1on) _ambient • u

'" w

detection Laverlochere lE2) _-s: 1 pC disturbances .~ 3000

... '" with ij resistor Renaudin 1954 [P7) 5 - 25 pC S

..

deteetion of

I

long lengths travelling [0 ] 1 pC + + + _{of cable} wa.ves

-DETECTION METHODS

DETECTION LOCATION

detected method author &: year rete-I ) senslt1vity2) senslt1vlt.Y distlnction 3 ) location 3)

phenomenon rence amonl others or rej ection 1n the

a!fected by ot outer object

d1scharges

.

Arman 1935 [G1]

..

rejec-tian

Austen 1944 [82] ;:: 10 pC capacl ty of rÀt1o:

the sample ±

h. t. Schering Greenfleld [G4]

. .

"" 30 x .

bridge 1946

electrlc piek-up of

impulses impulse bridge Hagenguth 1952 [G5] 25 pC achten t ± . .

(balanced dlsturbances

detectioD) polari ty Hashlmoto 1958 [G3] 5 pC ± ... 45 x

discrimin!ltor

di fferen tial [G2] 1/4 pC

•

up to

method [0 ] 4000 x

Schering bridge Comman practice :::: 50 pC capaci ty ot the

:::: 50· pC

object and 1088

double balanced Gelez 1956 [Hll variation due . -dielectric bridae to other causes

lOBses

electron ic Veverka 1958 [H2] .. caJ>Î.ci ty ot

watt meter thé samples

electros ta tic Fabre 1958 [El] sensit1vityof

probe the scanning the limitations ±

sys tem can be of the applied

brought at detector

e1ectric See table of about 1 pC

impulses scanning systems

.

.

(scanning)

1) [0] refers to the present study

2) tor electrlc systems tbe sensi~1vity is stated tor a capacity of the sample ot 1000 pF.

3) + denotes that the methad ls suitable tor the purpose,

• denotes that the method ls unsui tabie, ± denotes a suitalrlilty in between.

EVALUATION

sui table 3 ) measured kind of for malIll tude object meaeuring resa- ma.gnl-I iutio, tUde

•

of a

r.-:-:-:-single

r-:-:-

all dis- objects Charge

..

-15

-up to 1200

power dissipation all

•

of the joint obj ects

impulses

coUs H. V.

machines depends on

the applied extruded

·

detector plastic cores

-PART I.I

New methods for the detection of discharges

C " A P TER 3

The differential method

6 INTRODUCTION

PRINCIPLES

Discharges in a sample can be detected by means of the simple circuit shown in figure 12. An impedance, for instanee aresistanee R, is placed in series with the sample a and the high voltage souree. For high-frequency the circuit is closed by the capacity k. Voltage im-pulses which occur across R due to discharges in the sample are ob-served as has been discussed in chapter 2 when dealing with the

'straight' detection systems.

To eliminate the signals due to disturbances and discharges in the high-voltage souree or in the capacity k the circuit must be im-prbved. It is common practice to place in parallel with the original circuit a second capacity in series with an impedance, in such a way that a Schering-bridge is formed, as has been discussed in chapter 2 when dealing with the 'balanced' detection systems. The circuit is shown in figure 13. The voltage between the points 1 and 2is observed. In this way the disturbances from outside the sample can be eliminated. A disadvantage of the circuit is that to obtain a sufficient rejection of disturbances only a narrow frequency-band around a certain frequen-cy Wo must be observed as the bridge is in balance for a frequency

tan S _only. C' _R

r

~In the circuit developed by the author a capacity a which has the same dielectric losses as the sample frequency-band. The circuit is shown in figure 14.

is chosen for Cn' a within a wide A sufficient re-jection of disturbances and discharges can be obtained even if a wide frequency-bandisobserved, as the bridgecan be adjusted for balance for all frequencies at whicn tan 0

=

tan

S'.

The capacity a need notre

c:JI----.---4~1----wVM---!i

Fig 12 Ic Fig 13 777) //7 Fig 14 0,0' capocity of 0 sample

r, r' representation of lhe dielectrIc losses in 0 and a' R,R' ruistancu ocross which the voltage impulses

are formed

C,C' capacitiu in the low voltage side of the bridge, mainly parasitic

C. capacit y (usualy 0 standard condensor) k copocity betwun H.I/. ond earlh