h.voltage (chapter#05(b))

7/22/2019 h.voltage (Chapter#05(b))

http://slidepdf.com/reader/full/hvoltage-chapter05b 1/64

"Prof Dr. Suhail Aftab Qureshi" 1

Advance High Voltage Eng ineer ing

LECTURE – 8 (PART-II)

Prof Dr. Suhail A. Qureshi.Elect. Engg. Deptt. UET LHR

Measurement of High-Voltages




For the measurement of A.C, D.C and Impulse

voltage.

Sphere gap, though extensively used for high-voltage

measurements, produces a field which is uniform over only a

very limited region of the gap namely along the axis of the

gap and it is not possible to ensure that sparking would

always take place along the uniform field region.

Rogowski presented designs for profiles for electrodes

giving uniform field for voltages up to 600 KV.

3. Unifo rm Field Gap s





Where A and B are constants, and V is the sparkover

voltage for a gap length S.

This limitation of the sphere gap led to the development

of uniform-field gap and one of the early designs was putforward by Stephenson.

Fig:16 shows the design of the electrodes used, for

measurement of 50 c/s alternating voltages up to 400 kV

(peak).The portion AC is plane and is of a diameter equal to or

greater than the maximum spacing S. The radius from A

to B and C to D should be greater than 10S.

3. Unifo rm Field Gaps







Fig: 16.




To avoid sparking at the edges, the curvature from B to Eand D to F should be of continuously increasing

magnitude. Under this arrangement, it was found

experimentally that sparking always occurred in the

uniform-field region of the gap for all spacings up to 16

cm under normal atmospheric conditions.

From a theoretical analysis of the experimental results,

Stephenson has shown that the breakdown voltage and

spacing are related, at 25°C and 760 mm Hg by S is thegap length in centimetres.

v = 24.4S+7.50 S kV(peak)






It has also been shown that the sparking potential is a

function of both the spacing S and the gas density ℓ, so

that the density correction factor at a given density varies

with the spacing, and the above relation is accordinglymodified as:

V = 24.4ℓS + 7.50 (ℓS)1/2 KW (Peak) [Stephenson)

Where e is the gas density which is unity at normal

atmospheric conditions.






Bruce has also studied the characteristics of uniform-fieldelectrode for power frequency alternating voltages up to

315 kV. A half contour of one of the electrodes used by

Bruce is shown in Fig.17.

The flat portion AB is of a diameter not less than the

maximum gap length to be used. BC is a sine curve,

based on the axes OB and OC, such that XY = OC sin

(BX/ BOXπ/ 2). CD forms the arc of circle with centre at O.








Fig: 17




In order to maintain the uniformity of the field at different gap

lengths, different pairs of electrodes were used.

The diameters of the flat surfaces were 2.25, 4.4 and 7.8 in.

for the measurement of voltages up to 140, 280 and 420 kV

respectively. The corresponding overall diameters of theelectrodes were 4.5, 9.0 and 15.0 in.

Using these electrodes for voltages from 9 to 315 kV(peak),

Bruce has shown that the breakdown voltage of a gap of

length S cm in air at 25°C and 760 mm Hg is within ± 0.2 %of the value given by the relation

V = 24.22S + 6.08 S kV (peak). Bruce






The influence of nearby objects on the breakdown has

also been studied by Bruce and he recommended that

the clearance between the edges of the electrodes and

the nearest earthed conductor in the plane of the gap

should be more than four times the maximum gap length.The clearance between the edges of the electrodes and a

discharging conductor should' be not less than ten times

the maximum spacing.

To account for the air density the above expression

becomes

V = 24.22ℓS + 6.08 (ℓS)1/2






The effect of humidity on uniform field gaps was studiedby Ritz who observed an increase of about 2 % in the

breakdown of a 1 cm air gap when the water vapour

pressure varied from 10 to 25 mm Hg. He suggested a

relationship of the following form.






Kohrmann observed a greater humidity effect and found

that the breakdown voltage of 0.5-cm gap increased by

2.4 % when the water vapour pressure was increased

from zero to 10 mm Hg.

Kuffel has studied the effect of humidity for a number of

gap lengths. Fig: 18 illustrates the percentage increase

in breakdown voltage when humidity was varied from 0 to

17 mm Hg, and it is seen that the breakdown voltage for

gaps up to 2 cm increases by 4-5.5% at 17 mm Hg.The change in voltage is not linearly related to either the

humidity for a given gap length, or the gap length for a

given humidity.






Fig: 18






The voltage change is, however, nearly linearly related to

the humidity between the range 4-17 mm Hg when the

gap length is constant. Between these humidity limits the

voltage increase is greater for longer gaps, giving 0.19

%/mm Hg for a 0.5-cm spacing and 0.27 %/mm Hg for a

2-cm gap.

The increase in breakdown voltage with the quantity of

water vapour may be explained partly by considering thehigher electron attachment in moist air.






The problem has been discussed by Kuffel and an

estimation of the increase in the breakdown voltage gave

a reasonable agreement with the observed values over a

range of gaps from 0.5 to 2.0 cm.

An increase with gap length in the voltage change for agiven humidity change also follows from electron

attachment considerations. The ionization coefficient

varies more rapidly with the field gradient than does the

attachment coefficient.The field gradient at breakdown decreases with

increasing gap length and consequently the influence of

the attachment coefficient on the ionization efficiency will

be greater for longer gaps.






Prasad and Craggs have recently shown that the rate of

increase of the attachment coefficient is considerably

greater than that of the ionization coefficient as the partialpressure of water vapour is increased.

In addition, the secondary ionization coefficient was

observed to decrease with increasing water vapour

pressure. In consequence the breakdown voltage increases with increasing humidity.






Holzer and Ritz studied uniform field breakdown under

alternating voltages using the electrodes with the profile

suggested by Rogowski.

The relation between the breakdown voltage and gaplength 8 in cm, at atmospheric conditions, obtained by

Holzer is

V = 23.85S+ 7.85 S kV (peak), Holzer and that given by Ritz is

V = 24.558+6.66 S kV (peak). Ritz






Results obtained by various workers are given in Table:7

and the values obtained by sphere-gap measurements are

also inserted for comparison.

The discrepancies observed in the results of these

measurements are likely to be due to different test

conditions and the accuracy with which the voltages were

measured.



M t f Hi h V lt




Fig: 17.






From the experimental results available, it may be

assumed that within the limits of experimental accuracy

no difference has been detected between the d.c. anda.c. breakdown voltages.






The impulse breakdown characteristics of the uniform-

field gap has not yet been studied extensively.

Holzer made a study with impulse voltages of differentrate of rise and observed an increase of about 4% in the

breakdown voltage for a 12-cm gap above the static

breakdown voltage when an impulse voltage of a rate of

rise of 9.3X 108

kV /s was used.






At smaller gaps the difference became less. Also when

the rate of rise was reduced to 1.6 X 106 kV/s the impulse

values became equal to static breakdown values.

Cooper made measurements by applying recurrently a

voltage impulse of 1 μsec front duration and studied the

time intervals between the application of the impulse and

the instant of breakdown.






Rectangular impulses of negative polarity were applied toa parallel-plate electrode gap at a rate of 400 impulses/so

The maximum amplitude of the voltage was about 25 kV.

The impulse voltages applied to the gap were measured

by means of a cathode-ray oscillograph in conjunctionwith a resistance potential divider.

The results of his finding are summarized in Table. 8

which compares the effect of irradiation on breakdown

voltage.

Irradiation of the gap was obtained by inserting a metal

capsule containing 0.2 mg of radium inside the high-

voltage electrode.






Table: 8.






The results in Table 5.8 show that for gaps greater than

0.2 cm long, irradiation produced practically no effect onthe breakdown voltage.






Comparing the uniform-field geometry and the

sphere gap as a standard method of measuring

high voltages, it is readily seen that the former

has certain advantages over the latter. The

breakdown voltage of the uniform-field electrode

can be calculated accurately over a wide range of

gap lengths while there is no accurate

expression by means of which the sparking

voltage is calculable for sphere gaps for all

conditions to an accuracy better than several per

cent.






The air-density correction factor is a mean valuefor the whole range of spacing of the sphere gap

while in the case of uniform-field electrodes it is

a function of both the gas density and the gap

length.

The sphere gap calibration depends on whether

the arrangement is symmetrical or with one

sphere earthed. The uniform-field, however, does

not show any such effect and the breakdown

voltage remains the same whether both

electrodes are insulated or one electrode is

earthed.






For uniform-field gaps the clearance to nearby

objects is much smaller than is the case of sphere

gaps. The field in the central portion of the uniform-

field gap is well shielded against the effect of nearby

objects as compared with sphere gap.






In spite of a superior performance of the

uniform-field gap, it has not yet

replaced the sphere gap as a standard

method of voltage measurement. The

serious practical disadvantages are the

need for very accurate mechanical

finish of the electrodes and extremelycareful parallel alignment.






The effective value of high voltages can be obtained bymeasuring the current flowing through a circuit containing

a high series resistance. Neglecting the impedance of the

instrument, the product of the current and the series

resistance gives the unknown voltage.

4. Ammeter in Series w ith HighImpedance

H.V

R

A μA

V =IR

I





A milliammeter of the dynamometer or thermal type is

commonly used in such measurements; however,

moving-coil instruments can be used for direct voltages

provided the superimposed a.c. ripple is less than 10 %.The accuracy of measurements depends upon the design

of the high-voltage resistor.

These resistors should have negligible resistance

temperature coefficient and should be free from corona

discharges. Also leakage along the supporting structures

should be small. With alternating voltages, the stray

Capacitance of the resistor sections to earth has to be

considered.)

4. Ammeter in Ser ies w ith High Impedance


M f Hi h V l




Wire-wound resistances have often been used for high-

voltage measurements and one such design is due to

Taylor.

One hundred units ( MΩ, 1W) made of Ni-Cr wire were

arranged in twenty sets of 5 units each. Each set was

mounted in a spun aluminium corona shield with an

insulating lid made of hard rubber or pyrex glass. A springcontact was provided between each resistance set and

the aluminium cover ofthe next set.



M f Hi h V l




The resistance units were wound noninductively and the

temperature coefficient was 0.0152 %/C. The

temperature and resistance characteristics of theassembly were determined under the operating

conditions for voltages up to 1.3 kV / resistor and the

results are reproduced in Fig. 19. The accuracy of

measurement was claimed to be 0.01%.



M f Hi h V l






Fig: 19.

M t f Hi h V lt




The length of the wire required becomes very

considerable for resistances of 100 MΩ and above-evenfor the finest gauge of the coil remained constant within

0.1 % over long periods. The coil formers are mounted on

an ebonite rod to form a stack shown in Fig.20, and

placed in a container 18 in. high X10½ in. diameter.



M t f Hi h V lt






Fig: 20.

M t f Hi h V lt




Corona formation on the surface of the resistance coil is

thereby prevented. A 100-MΩ unit suitable for 100 kV

working voltage is shown in Fig.21. The high-voltage endof the resistor is fitted with a large "hat" which, together

with the vertical helical configuration of the resistor unit,

prevents concentration of electric field and corona

formation.



M t f Hi h V lt






Fig: 21

Measurement of High Voltages




A method suitable for determining the peak and r.m.S.value of an alternating voltage is to measure the

current flowing through a capacitor connected to the

high-voltage source.

The current is measured by a rectifier milliammeter

circuit. The circuit of a peak voltmeter described by

Chubb and Fortescue is given in Fig. 22.

Neglecting the rectifier impedance, the instantaneous

current is: i = C(de/dt) where e is the instantaneous

voltage and C is the capacitance. The total charge

through each rectifier per cycle is:

5. Series Capacitance Vo ltmeters








Fig: 22.





1/4 f

3/4 f

1

43

4

, , sin ,

2 cos 2 2 f

f

dQ dvi i c e E wt de wE Costwt dt dt

Q idt C de fCE fdt CE







Where e = E sin 2 πft .

The charge per second or the mean current through the

instrument is then 2CEf, where f is the frequency of the

test voltage. The peak voltmeter, therefore, measures the

total voltage swing from positive to negative peak of an

alternating voltage. The chief source of error is the

imperfect rectifier characteristic.







In the above treatment, the integration is taken betweenthe time limits when de/dt has one sign only (positive for

one rectifier and negative for the other), the peak voltage

cannot be measured if there are subsidiary peaks in the

voltage wave.

This method is on ly appl icable when the + ve and - ve

peak heigh ts are equal.

This method is used extensively for measuring the peak

values of power frequency voltages up to about 1000 kV

(r.m.s.), and accuracies of the order of 1 or 2 parts in

1000 can be achieved.







The Chubb-Fortescue peak voltmeter described earlier

suffers from two serious drawbacks. These are frequencydependence of the indication and the error introduced by

heavy discharges on the h.v. system; a less important

shortcoming is the error introduced by multiple peaks in

the voltage waveform.







The maximum voltage range of such a voltmeter is limited

by, the peak inverse voltage of the rectifier. Baker has

presented the design of an instrument which includes

rectifiers in a feedback loop, and is to a great extentindependent of the rectifier characteristic.

The instrument is particularly applicable in the

measurement of h.v. in the presence of corona discharge.







Dividers for Direct and Alternating Voltages.

Potential divider is, basically, a series combination of ahigh and low impedance. The voltage to be measured is

applied across the combination and the drop across the

low-impedance section is measured by means of an

indicating instrument.

6. Potential Div iders

VZ1

Z2 V2

Low impedance






The design of a potential divider is, essentially, the design

of the high-impedance section and high-voltageresistances described in Section 5.4 (Ammeter in series

with High-Impedance) can be used as a resistance

potential divider.

6. Potential Div iders






A resistance potential divider for the measurement of upto 1.25 million volts d.c. was described by Kunkte. The

resistance 1500 M-ohms was made from 2000 units of

carbon resistances 0.75 M-ohms each. Across a tap on

the low voltage end was connected a 1500-V electrostaticvoltmeter. The divider ratio was adjusted at 1:1000 and

the voltmeter scale was marked to read directly 1500 kV.

The resistor was immersed in oil and a pump was used to

maintain a continuous circulation of oil. Under full-load

conditions the resistance value was within 1.5 %. The

accuracy of measurement was about 1 %.

6. Poten tial Divid ers






A very precise potential divider for measurement of directcurrent voltages of about 50 kV with an accuracy of

0.01% has been designed by Rymer and Wright. The

effect of leakage currents was eliminated partly by the

use of a comprehensive system of shielding circuits

supplied by an auxilliary network across the main h.t.supply and partly by the use of a Wheatstone bridge

circuit which enables the potentiometer ratio to be

measured under operating conditions, while the high

potential is applied. The apparatus has been used forprecise measurement of the voltage in experiments on

electron diffraction.







The elaborations required in the proper design of an a.c.

high-voltage resistor have led to the development of

capacitors which can be used as a high impedance

element, either in a bridge circuit or in a potential divider.The advantages of a standard air-capacitor over a high-

voltage resistor are the relative simplicity of its

construction, the ease with which it may be shielded from

stray capacitance effects, and the absence of heating.







A divider for recording high transient voltages may consist

of resistors or capacitors or combinations of both. The

essential requirement is that the wave shape of the

voltage to by measured should be faithfully reproduced

on the oscillograph with a reduction ratio which can be

accurately known.

7. Dividers fo r Impu lse Vo ltages






The chief sources of error common to all types of dividers

are:

1) Residual inductance in any resistance or

capacitive element.2) Stray capacitance: (a) from any section of the

divider to the high voltage lead, (b) from any

section of the divider to ground and (c) between

sections of the divider.3) Impedance drop in the connecting lead between

the divider and the test object.


7. Dividers for Impu lse Voltages





4) Impedance drop in the ground return lead from

the divider resulting from extraneous ground

currents flowing in this lead.

5) Oscillations in the divider circuit caused by

capacitance from divider high-voltage terminal to

ground and lead inductance.







The simplest type is a resistor divider which is often

acceptable for recording the standard impulse of 1/50

μsec wave. When the duration of the surge is less than Iμsec a resistor divider may give large errors due to stray

capacitance. The error is a function of the product of the

resistance of the high-voltage arm and the earth

capacitance.







One of the simplest types of a divider is a plain resistancepotentiometer connected to a cable as shown in Fig. 24.

If the high-voltage resistance has no distributed capacity

and no self-inductance, the above arrangement will

respond satisfactorily to all transients.The possible sources of error in a cable type divider such

as shown in Fig. 24 are as follows: (1) the terminal

resistor R ≠ Z (surge impedance), due both to errors in

adjustment and to the fact that the concentric cable is not

a true surge impedance invariant to changes of wave

shape; (2) capacitance of the measuring circuit;

7. Dividers fo r Impu lse Voltages








Fig: 24.





It can take the form of either parallel or series

arrangement of capacitors and resistors. The equivalent

network of the parallel arrangement is that in which C: is

infinite and its response can be obtained by neglectingthe self-inductance L’ . From the response terms, too

lengthy to be included here, it can be seen that the

arrangement behaves as a capacitor divider with fast

transients and as a resistor divider with slow transients.

8. Mixed Divider







8. Mixed Divid er

Fig: 25.





Fig: 26.

8. Mixed Divid er

g g





The low-voltage arm of a potential divider is connected to

the oscillograph deflection plates by means of a coaxial

cable.

9. Delay Cable

Delay Cable

H.V

Z1

Z2 CRO

g g





The length of the cable varies between a few meters and

100-200 m, depending upon the time delay required

between the operation of the generator and the arrival of

the signal to the plates.

The two main types are the air-cored cable and the

polythene cable. In air-cored cable, the surge voltage

travels approximately at the speed of light and a cable of

100 m length gives a delay of, 0.333 μsec which is

sufficient for the C.R.O. time base to be triggered.

g g

9. Delay Cable





In a polythene cable, the velocity of propagation would be

slower, being inversely proportional to the square root of

the permittivity, and therefore. a shorter length of cable

can be used to provide the same time delay.

The delay cable causes distortion and attenuation in the

recorded wave and a study of the response of air-spaced

and polythene cable shows that, in order to avoid

appreciable errors, air-spaced cable should be used on

fast surges.

9. Delay Cable

g g




An ideal cable is one which has no losses and whose

surge impedance does not vary with frequency. However,

all cables have some attenuation and their surgeimpedance varies with frequency. The sources of cable

losses are the resistance of the central conductor and

outer sheath and conductance and dielectric hysteresis in

the insulating medium used.

g g

9. Delay Cable

h.voltage (chapter#05(b))

Documents