Copy of 3 - High Voltage Testing Techqunies

8/14/2019 Copy of 3 - High Voltage Testing Techqunies

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INPT-R&DAgenda

Introduction

Definitions, Significance of insulation, Why testing?

High voltage testing techniques

Definitions, General requirement, Generation and measurement, Test

procedure, Methods of evaluation, Uncertainty in tests and

measurement

High voltage tests on high voltage products

Test objects and various high voltage tests, General safety and

precautions, Test objects standard requirement, Evaluation of test

results


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TechniquesTraining Module/CTR


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INPT-R&DTopics

Definitions and general standards requirements

Generation of high voltages Measurement of high voltages

Test procedures

Uncertainty


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INPT-R&DStandards

IEC 60060-1 High-voltage testing techniques

Part 1 General definitions and test requirements

Describes general definitions and test requirements in high voltage

testing techniques

IEC 60060-2 High-voltage testing techniques

Part 2 Measuring devices Describes general definitions and measuring systems requirement in

high voltage testing techniques


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INPT-R&DDefinitions

Disruptive discharge (also referred as electrical breakdown)

Failure of insulation under electrical stress, in which the discharge completely bridges theinsulation under test, reducing the voltage between the electrodes practically to zero

It applies to electrical breakdown in solid, liquid and gaseous dielectrics and combinations ofthese

Non-disruptive discharge (also referred as partial discharges)

A discharge that does not completely bridge the insulation between electrodes, the voltagebetween the electrodes does not drop to zero

Sparkover

Disruptive discharge occurs in a gaseous or liquid medium

Flashover

Disruptive discharge occurs over the surface of a dielectric in a gaseous or liquid medium

Puncture

Disruptive discharge occurs through the solid dielectrics


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INPT-R&DDefinitions

External insulation

Is the air insulation and the exposed surfaces of the solid insulation of the

equipment

Internal insulation

Is the internal solid, liquid or gaseous elements of the insulation of equipment

Self-restoring insulation

Is the insulation which completely recovers its insulating properties after adisruptive discharge caused by the application of test voltage

Non-self-restoring insulation

Is the insulation which losses its insulating properties, or does not recover them

completely, after a disruptive discharge caused by the application of test voltage


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INPT-R&DDefinitions

Impulses

Is an intentionally applied aperiodic transient voltage or current which

usually rises rapidly to a peak value and then falls slowly to zero

Lightning and switching impulses

Impulses with front duration up to 20 s are defined lightning impulsesand those with longer fronts are defined as switching impulses


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INPT-R&DGeneral requirements for test procedures

The requirements of test procedures are dependent on the follwing

factors

Required accuracy of tests results

Random nature of observed phenomena / polarity dependence of

measurement

Progressive deterioration with repeated voltage application


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INPT-R&DRequirements for test objects

Test object should be complete in all respects;

Complete in all details

Should be processed in normal manner for similar equipment

General arrangement in terms of clearances to other live /

grounded parts should be taken care of

A clearance of not less than 1.5 times of the shortest possible

discharge path should be maintained from extraneous structures


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INPT-R&DDry tests

Test object should be dry and clean

Test should be made at ambient temeprature The procedure for voltage application is as per IEC 60060 1

The voltage value and various configuration to be tested is as per

relevant product standard


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INPT-R&DWet tests

Intended to simulate performance of test object when overvoltages occur in raincondition

The procedure for voltage application is as per IEC 60060 1 The voltage value and various configuration to be tested is as per relevant product

standard

Precipitation condition for standard procedure

Water resistivity 100 15 m

Precipitation rate 1 to 2 mm/min in horizontal and vertical

Water temperature ambient temperature 15 C

Test specimen should be wetted for minimum15 minutes before start of testing

Reproducibility of wet test results is low

Adequate precautions on collecting vessel and method precipitation measurement aretaken to minimize this dispersion

The test object may be cleaned with a surface-active detergent. This is to be removed

before beginning of wetting


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INPT-R&DAtmospheric conditions

The standard reference atmosphere is;

Temperature to

20 C

Pressure bo 101.3 kPa

Absolute humidity 11 g/m3

As products are tested under exsisting ambient conditions the

applied voltage has to be corrected for the prevailing conditions

Refer to relevant product standard for applicability of correction factors


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INPT-R&DCorrection factors

The total correction factor Kt is the product of

air density correction factor k1 and humidity correction factor k2

The applied voltage is calculated as

U = Uo Kt where Uo is the specified test voltage

Air density correction factor at temperature t and pressure b is

figurefromtakenism

t273

t273

b

bwhere

k

o

o

m

1

=

+

+=

=


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Humidity correction factor

No humidity correction shall be applied for wet

tests

metresinpathdischargeminimumtheisL

voltage)testtimes1.1assumeavailablenot(if

voltagedischargeedistruptiv50%theisUwhere

kL500

Ug

slide)previousingraph(refergofvalueonbasedisw

hratioandvoltagetestoftypeondependentiskwhere

kk

B

B

w2

=

=

=

=


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Conflicting requirments for testing internal and external insulation

Due to laboratory altitude and or extreme climatic conditions, the

correction factor results in withstand level for internal insulation in

excess to discharge voltages of external insulation

In such condition the test object may be immersed in oil or

compressed gas so that there are no discharges in external

insulation during test

Reverse may happen in some cases, where external insulation is to be

tested at significantly higher voltages. In order to assess the external

insulation,

Either the internal insulation is reinforced for test purpose or

The test is made with dummies


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INPT-R&DAltitude Correction factors

For installation at an altitude higher than 1 000 m, the insulation level of external insulation isdetermined by multiplying the insulation withstand voltages required at the service location by afactor Ka in accordance with figure (see next slide)

Also see product standard for specific requirements

Ka is also give by following formula;

Ka = em(H- 1000)/8150

where

H is the altitude in metres

m is taken as fixed value in each case for simplification as follows:

m = 1 for power-frequency, lightning impulse and phase-to-phase switching impulse voltages

m = 0,9 for longitudinal switching impulse voltage

m = 0,75 for phase-to-earth switching impulse voltage.


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INPT-R&DAltitude Correction factors

For internal insulation, the

dielectric characteristics are

identical at any altitude andno special precautions need

to be taken. For external and

internal insulation, see IEC

60071-2

For low-voltage auxiliary and

control equipment, no special

precautions need to be taken

if the altitude is lower than

2000 m. For higher altitude,

see IEC 60664-1


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INPT-R&D


Generation of high voltages Measurement of high voltages

Test procedures

Uncertainty


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INPT-R&DTypes of high voltage waveshapes

High ac voltage of power frequency

High ac voltage of higher frequency

Impulse voltage

Lightning impulse ( High transient or

impulse voltage of very short duration)

Switching impulse ( Transient voltage

of longer duration)

High dc voltage


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INPT-R&DAlternating voltages

A sinusoid in the range 45 to 65 Hz

Value of peak to rms ratio should be 2 5% For test durations upto 60 s the measured value of test voltage shall

be maintained within 1%

For test durations exceeding 60 s the measured value of test

voltage shall be maintained within 3%

The total uncertainty of measurement of test value should be no

more than 3%


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INPT-R&DGeneration Power frequency voltages

Single phase testing transformer

Cascaded transformer

Resonance test transformer

Testing transformers are generally designed to withstand frequent short circuit

failures.

Short circuit current rating is 10 to 20 times rated current.

Three Phase test transformer


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INPT-R&DTest circuit requirement

The voltage in the test circuit should be stable enough to be practicallyunaffected by varying leakage currents

Non-sustained disruptive discharges may cause over-voltages in the testcircuit due to uncontrolled resonance conditions produced by theinteraction of leakage inductance of the source and the varyingimpedance of the high-voltage circuit

This can be eliminated

by providing sufficient damping resistance in the high-voltage circuit or

short-circuiting the primary voltage to the high-voltage test transformerimmediately following a disruptive discharge

Controlled high-voltage resonant circuits do not produce over-voltagesfollowing disruptive discharges since they de-tune whenever the loadimpedance changes


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INPT-R&DTesting transformer requirement

They are designed to withstand frequent short circuits when the test object

fails or experiences flashover

They should have higher impedance than typical HV power transformer.Typical range is 20-30%. Too high is not good for regulation.

Short circuit current should be

Minimum 0.1 A for dry tests on solid and liquid insulation

On external self restoring insulation 0.1 A for dry tests and 0.5 A for wet tests. 1

A may be necessary for wet test on large specimens.

For artificial pollution test this value should be upto 15 A. Also R/X 0.1

In order to prevent large dip in applied voltage during non-disruptive partial

discharges the total capacitance of test object and any other capacitor in

test circuit should be in the range of 0.5 to 1 nF


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INPT-R&DSingle phase testing transformers

Though power station equipment are three phase, single phase testing

transformer is normally used for testing

Features;

They differ from power transformer in the sense that they have higher short

circuit impedance to withstand frequent short circuit

It can be operated with overload for short duration

Major advantage of this below 200 kVA is less cost

Major disadvantage above 300 kVA is more cost


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INPT-R&DGeneral arrangement


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INPT-R&DTank type


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INPT-R&DTank type


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INPT-R&DTank type


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INPT-R&DTank type


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INPT-R&DCylindrical type

The test transformer are of theinsulating shell design with metallic

cover and base The insulating cylinder is made of

reinforced Fibreglass, covered with amoisture-rejecting paint

It is important to keep the surface of

insulating cylinder dry and dust free allthe time

During high moisture periods infra-redlamps are used to keep the cylindersurface warm and preventcondensation


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INPT-R&DCylindrical type


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INPT-R&D Cascaded Transformer

Cost of insulation for a single unit is square of operating voltage

When the voltage higher than 400 kV cascade is done

as the insulation required is very high in conventional type

Transportation and assembly are easy as the whole unit is divided into smaller

units

Natural cooling is sufficient

The units are enclosed by large size metal rings to prevent corona


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INPT-R&DBlock diagram


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INPT-R&D Construction of Cascaded Transformer

V1V2

3V2V1

V12V2


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INPT-R&DCascaded transformer

First unit is energized from low voltage primary

In the same unit second unit excitation winding is available with the

same no of turns as the primary of the first unit

Second unit primary is fed from the first unit

The potential of Second unit is fixed by the potential of secondary of

the first unit

Secondary of second stage transformer is connected in series with

secondary of first unit

Some times the second transformer unit is grounded at the half of

the potential to reduce the insulation to half


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INPT-R&D Cascaded transformer


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INPT-R&D Cascaded transformer


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INPT-R&DHigh voltage resonant circuits

Series resonant circuit

Consists of an inductor in series with a capacitive test object, or a capacitor in

series with an inductive test object By varying circuit parameters or the supply frequency, the circuit can be tuned to

achieve a voltage considerably greater than that of the source and with asubstantially sinusoidal shape

This circuit is useful when testing objects such as cables, capacitors in which theleakage currents on the external insulation are very small in comparison with thecapacitive currents through the test object

Unsuitable for testing external insulation under contaminated conditions

Parallel resonant circuit

Consists of a capacitive test object in parallel with a variable inductance and ahigh-voltage source

By varying the inductance, the circuit can be tuned, resulting in a considerable

reduction in the current drawn from the high-voltage source


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INPT-R&D

R L

VoV C

Equivalent circuit with Capacitive load

Transformers simplified equivalent circuit can be modeled as shown in figure.

The output voltage, V0 for the circuit is given by the expression

I Vo/ V I = 1/ {( RC)2

+ (2

LC - 1)2

}0.5

For the light loads C is very less.

As the load increases output voltage increases.

Vo = I * Xc

= V/R * Xc

= 1/( CR) * V

= Q V

Q is the quality factor of the circuit. It is designed with the value of 50 to 70

So the output voltage is Q times input voltage applied

INPT R&D


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INPT-R&DHigh voltage resonant circuits

Advantages

Low input power requirements

Negligible harmonic distortion

Fault Current is limited in the series

resonant mode preventing damage to the

test sample

Smaller in size compared to similar rating

of conventional transformer Used for routine and type tests of MV and

HV and general purpose laboratory tests

including wet tests

INPT R&D


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INPT-R&DHigh Voltage Impulse Generation

Transient over voltages due to lightning and switching surges causes fast

rising voltage on transmission tower and hence on electrical equipments

To simulate the service transient condition on the equipment for itswithstand strength it is necessary to generate the impulse voltage

On the basis of the front and tail time following classification is made

Lightning impulse generation

Switching impulse generation

Very fast transient generation

INPT R&D


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INPT-R&DHigh Voltage Impulse Generation

The impulse is usually generated by an

impulse generator consisting of

a number of capacitors that are charged in

parallel from a direct voltage source and then

discharged in series into a circuit that includes

the test object and the measuring system

INPT R&D


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INPT-R&DImpulse generator Block diagram

INPT R&D


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INPT-R&DSingle Stage Marx Circuit

Rs - Front resistor V0 - Rectified voltage

Rp - Tail resistor V(t) - Impulse voltage

C1 - Generator capacitor

C2 - Load capacitance

G - Sphere gap

V0 C1C2

Rs

Rp

G

V (t)

INPT R&D


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INPT-R&DThree stage Marx Circuit

Uo charging voltage

Cs impulse capacitor

f sphere gap

Re discharging resistor

RL charging resistor

RD damping resistorRL>>Re>>RD

INPT R&D


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INPT-R&DLightning impulse waveshape

Impulses with front duration up to 20 s are defined as lightningimpulses

Standard Lightning Impulse 1.2/50 s

1.2 s is front time ( 30%)

50 s is time to half-value( 20%)

Peak value ( 3%)

Nomenclature U Peak value

T1 Front time

T2 Time to half-value (tail time)

T Time between point A and B

(30% and 90% of peak valuerespectively)

O1 Virtual origin

INPT R&DCh d li h i i l h


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INPT-R&DChopped lightning impulse waveshape

A chopped lightning impulse is a prospective full lightning impulse

during which any type of discharge causes a rapid collapse of the

voltage The collapse of the voltage can occur on the front, at the peak, or on

the tail

Tc Time to chopping virtual origin to instant of chopping

INPT-R&DCh d li ht i i l h


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A standard chopped lightning impulse is a standard impulse that is

chopped by an external gap after 2 to 5 s

Other time values for chopping may be specified by the product

standard

Because of practical difficulties in measurement, the virtual duration of

voltage collapse has not been standardized.

INPT-R&DCh d li ht i i l h


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Chopped impulse is defined by

Peak voltage U

Front time T1

Virtual steepness S = U/T1. This is the

slope of a straight line drawn between

points E and F

This front-chopped impulse is considered

linearly rising if the front, from 30%

amplitude up to the instant of chopping, isentirely enclosed between two lines

parallel to the line E-F, but displaced from

it in time by 0.05 T1

INPT-R&DS it hi i l h


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INPT-R&DSwitching impulse waveshape

Impulses with longer front duration (>20 s) are defined as switchingimpulses

Standard switching impulse 250/2500 s

250 s is time to peak ( 20%)

2500 s is time to half-value( 60%)

Peak value ( 3%)

Nomenclature U Peak value

Tp Time to peak

T2 Time to half-value (tail time)

Td Time above 90% of peak

value

INPT-R&DI l t


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INPT R&DImpulse current

Exponential

Defined by the front time T1 and the time to half-value T2

1/20 impulse: front time : 1 s time to half-value : 20 s




Tolerances

Peak value 10%

Front time (T1) 10%

Time to half-value (T2) 10%

INPT-R&DI l t


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INPT R&DImpulse current

Rectangular

Defined by the duration of the peak Td and the total duration Tt

Rectangular impulse currents with durations of the peak of 500 s,

1000 s, or 2000 s and total durations from 2000 s to 3200 s

Tolerances

Peak value +20% - 0%

Duration of peak +20% - 0%

INPT-R&D


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INPT R&D


Generation of high voltages

Measurement of high voltages

Test procedures

Uncertainty

INPT-R&DI t d ti t HV M t


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INPT R&DIntroduction to HV Measurement

Low Voltage system: Voltage < 1 kV

Moving iron and moving coil type instruments are used with series

resistance (multiplier) for low voltage measurement Factors in high voltage measurements

The measurement of voltage and current in the HV tests are difficult byconventional measuring and recording systems

When the voltage increases power consumed by multipliers increases

Reduction of stray capacitance is not easy

The other difficulties are related to large sizes necessary to

control electrical fields

avoid flashover

to control heat dissipation within the circuits

INPT-R&DHV measuring system


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&HV measuring system

Measuring system

Complete set of devices suitable for performing a high-voltage or

impulse-current measurement is called as measuring system

Components of HV measuring systems consists of

Converting device

Transmission device

Recording device

INPT-R&DClassification HV Measurement


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Classification HV Measurement

Direct method

Voltage is measured when the meter is connected directlywith high voltage system

Examples;

Electrostatic voltmeter

Sphere gap / rod-rod gap

Indirect method

Voltage is measured by scaling it down to suitably lowervalue

Examples;

Series resistance Micro ammeter

Voltage transformer Peak volt meter

Potential dividers Oscilloscopes , HV probe

HV probe

10 kV Electrostatic voltmeter

INPT-R&DSphere gaps


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Sphere gaps

IEC 60052 - Voltage measurement by means ofstandard air gaps

A uniform field spark gap will have sparkover

voltage within tolerance under constantatmospheric conditions

By precise experiments, the breakdown voltagevariation with gap spacing, for different diametersand distances, can be measured

Two identical metal spheres made of copper,aluminium or brass is used separated by an air

gap

The potential difference between the spheres israised until a spark appears

Standard values of Diameter of spheres are 6.25,12.5, 25, 50, 75, 100, 150, and 200 cm

INPT-R&DSphere gaps


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Sphere gaps

INPT-R&DEffect of atmosphere


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Effect of atmosphere

The density of the gas (generally air) and humidity affects the spark-over

voltage for a given gap setting

The spark over voltage for a given gap setting under STP must bemultiplied by the correction factor to obtain the actual spark-over voltage

Spark over voltage at NTP, U = KtU0

The atmospheric correction factors have been described earlier

In the uniform field configuration, sparkover voltage is 30 kVpeak/cm in air at

20 0C and 101.3 kPa pressure

INPT-R&DSphere gaps Protection


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Sphere gaps Protection

A series resistance of 100 to 1000 k is connected in series withsphere gap to

limit the break down current as causes pitting of sphere gap

suppress unwanted oscillation in the source voltage when break down

occurs in the case of impulse voltage

INPT-R&DFactors affecting measurement


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Factors affecting measurement

Tolerance on size, shape and conditions of spheres and their surfaces

Nearby earthed objects

Humidity

Irradiation and polarity

Dust particle

Rise time of voltage waveforms

INPT-R&DPeak values disruptive discharge voltages


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Peak values disruptive discharge voltages

Reference: Table 2 from IEC 60052

INPT-R&DSphere gaps


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Sphere gaps

Advantage

The sphere gap method of measuring high voltage is the most reliable

and is used as the standard for calibration purposes

It can be used to measure peak ac voltage up to 1 MHz

Accuracy of measurement is proved as 3 %

Disadvantage

It can not be used for the voltage having rise time of lesser than 0.5 s DC spark over voltage reduction was about 20 % for lesser gap

distances (1.3mm) with irradiation

At long gap spark over voltage is not linear with dc voltage

INPT-R&DRod gap measurement


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Rod gap measurement

The rods shall be made of steel or brass, with a

solid square section, sides between 10 mm and

25 mm and have a common axis. The ends

shall be cut at right angles to the axis leaving

the edges sharp in order to get a reproducible

breakdown mechanism

The clearance from the tip of the high voltage to

earthed objects and walls, other than the ground

plane, shall be not less than 5 m

INPT-R&DRod gap measurement


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Rod gap measurement

The disruptive discharge voltage Uo for positive and negative direct

voltage at standard reference atmosphere is given, for either the

vertical and horizontal gap by; Uo = 2 + 0,534 d

Where, Uo is in kilovolts and d is the gap spacing in millimeters

This equation is valid for gap distances d between 250 mm and 2 500 mm

and for a humidity range h/ between 1 g/m3 and 13 g/m3

Under these conditions, the disruptive discharge voltage Uo has an

estimated uncertainty of 3% for a level of confidence not less than 95%

The rod-rod gap shall not be used as an approved measuring device at

gap spacing less than 250 mm because of the absence of streamer

pre-discharges

INPT-R&DResistance in series with Micro ammeter


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Resistance in series with Micro ammeter

High series resistance (specially designed to withstand high voltage) and

resistance of 20 k /V is used with micro ammeter (having 50 A -movement).

This method is applicable for both ac and dc

A safety gap or neon lamp is connected across the micro-ammeter

INPT-R&DResistance in series with Micro ammeter


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Resistance in series with Micro ammeter

Advantage

By using a stable supply (of accuracy 0.1%) 1% accuracy can be

achieved

Disadvantages

When the above method is used for alternating voltages, there would

be the effect of the distributed capacitances

Their stability of resistances are temperature dependant The two resistors are set by heat dissipation and heat transfer outside

Current limits can be up to 1 to 2 mA

INPT-R&DPeak reading voltmeters


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Peak reading voltmeters

Peak value measurement is important in HV measurement

Types of Peak voltage measurement

Capacitor charging method

Neon Lamp Method

Rectifier-Capacitor current method

Rectifier with divider method - Impulse voltage measurement

INPT-R&DVoltage/potential transformer


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Potential transformer

Inductive voltage transformer

Capacitor voltage transformer

Voltage/potential transformer

Voltage is measured by stepping down the voltage from one side to

another side by Faradays law principle

INPT-R&DInductive voltage transformer


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Inductive voltage transformer

High voltage is measured by stepping down

according to the ratio of turns between the

primary and secondary Very simple design and construction

Error due to Phase angle and ratio

It does not permit fast rising transient

Insulation required for very high voltages morethan 100 kV is more and hence not cost effective

INPT-R&DCapacitor voltage transformer


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Capacitance divider with a inductive voltagetransformer is used

These are field measuring equipment unlikedividers are laboratory measuring equipmentswith very good accuracy

Advantages

The high voltage capacitor can be used in PLCCapplication instead of coupling capacitor

Simpler design and easy installation

Disadvantages

Voltage ratio will vary with temperature

Ferro resonance occurrence in power system

Limited power output

INPT-R&DCapacitor voltage transformer


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INPT-R&DHigh Voltage divider


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Types of Dividers

Resistive dividers

Capacitance dividers

Mixed dividers

High Voltage divider

Potential divider consists of two impedances , HV arm (Z1) and LV

arm (Z2) connected in series

High voltage is applied to HV arm and measuring voltage is taken

from LV arm

The height of the divider depends on the flashover voltage between

the electrodes

Connection between LV arm and and measuring instrument ismade by shielded cable to avoid stray capacitance

INPT-R&DResistive potential divider


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200 kV and 100 kV

Resistive potential divider

Used for the measurement of all kind of high

impulse voltages with steep wave fronts

measurement of front chopped impulses Used when an additional capacitance in the test

circuit is not permissible

A distributed screen of sections and using an

auxiliary potential divider to give fixed potential to

the screens

These are housed in flexi-glass cylinders

containing a matched set of precision metal film

resistors, alternatively anti inductively wound CrNi

wire wound resistors

INPT-R&DResistive potential divider


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Resistive potential divider

Advantages

Resistive Voltage Dividers can measure up to 100 kV ac or 200 kV dc

with accuracy better than 0.5% It is possible to measure the impulse voltage 2 MV by the careful design

of low voltage arm

They are generally used when an additional capacitance in the test

circuit is not permissible due to the slowdown effect on the rise time

Disadvantages

Distributed capacitance significantly affect the resultant ratio.

More than 200 kV resistive divider design is difficult

INPT-R&DCapacitive potential divider


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Capac t e pote t a d de

A single capacitor unit or stack of units can be used forthe measurement

In capacitor voltage divider two capacitances C1

and C2

are used in series, and the measuring system isconnected across the lower arm capacitor

Pure capacitive voltage dividers cannot be used formeasuring impulses due to generation of oscillationscreated by with pure LC circuit

L being stray inductance of lead and C the capacitance ofthe divider

Neglecting the capacitance of the cable (approx. 50pF/m)the effective capacitance of C1 and C2 in series is C1C2/

(C1+C2), and since the charge is the same,

VC2

= C1

/(C1

+C2

) V

1000 kV divider

INPT-R&DCapacitive potential divider


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p p

Advantage

Very good high frequency response for small capacitance and small

dimensional divider

Disadvantage

Pure capacitive dividers are sensitive to input voltage with short rise

time

It forms series resonance circuit with lead inductance in the low voltage

arms

INPT-R&DMixed divider circuit


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Combination of resistor and capacitor are used to

eliminate the effect of distributed stray capacitances.

The distributed capacitors compensate for the currentdrawn by stray capacitances

It can be classified into two types

Parallel Resistive-capacitive voltage divider

Damped capacitive voltage dividers

3 MV 3.5 MV

1 MV

INPT-R&DParallel Resistive-capacitive voltage divider


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p g

To reduce the nonlinear frequency dependant characteristics

resistive divider capacitance is connected in parallel with resistor

This is achieved by selecting equal time constants in both highvoltage and low voltage arms

Advantages

At high frequencies it acts like a capacitive divider and at low frequency

like resistive divider.

Loading effect can be reduced by step by step

compensation

INPT-R&DDamped capacitive voltage dividers


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p p g

Combination of very low resistors and pure

capacitance are connected in series

This used to reduce the voltage oscillationsand reflections due to traveling wave

It is possible to design more than 6.5 MV

voltage measurements

Disadvantage

Pure capacitive dividers are sensitive to inputvoltage with short rise time.

It forms series resonance circuit with lead

inductance in the low voltage arms

6.5 MV outdoor type

INPT-R&DRequirements of divider as per IEC 60-2


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q p

Measuring system should measure Peak impulse voltage with the overall

uncertainty of 3%

Uncertainty of front chopped impulse measurement should be 5% Uncertainty of tail chopped impulse measurement should be 3%

Time parameters should be measured within 10 % accuracy

To reproduce oscillations partial response time should be < 15 ns

Rise time and response time are important so that measuring circuit will transfer

the same voltage to be measured

Eliminating the stray capacitance in the secondary of the divider is important

INPT-R&DSelection of Dividers


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Selection of divider is based on

Maximum value of each voltage type to be measured

Required or specified transfer behavior of the voltage measuring

configuration consisting of high voltage lead/divider, measuring

cable/measuring unit

Adaptability of low voltage arm to measuring instruments and

measuring cable

Capacitive load of test circuit

Application of divider : -indoor, -outdoor, -stationary, -mobile and

-suspended installation at ceiling or wall


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INPT-R&DAccuracy requirements


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Alternating voltage

Total uncertainty of peak or mean value measurement should be < 3 %

Harmonics < 10 %

Direct voltage

Total uncertainty of mean value measurement should be < 3 %

Lesser than 10 % of actual ripple or 1% error of the mean value

whichever is more

INPT-R&DAccuracy requirements


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Lightning and switching impulse voltage

Uncertainty of Peak of full impulse or chopped impulse on the tail < 3 %

Impulse chopped on the front when the chopping time Tc , Tc > 2 s,uncertainty should be 3 %

when the chopping time Tc, 0.5 s Tc 2 s, uncertaintyshould be less than 5 %

Time parameter less than 10 %

If the frequency of oscillation is less than 0.5 MHz and duration of

overshoot is 1 s mean curve should be drawn to see the magnitude.Overshoot is nearer to peak of less than 5 % of peak value

INPT-R&D


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Measurementof high voltages

Test procedures

Uncertainty

INPT-R&DTest with alternating voltage


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Withstand voltage tests

The voltage is applied starting at a value sufficiently low to prevent effects ofover-voltages due to switching transients

It should be raised sufficiently slowly to permit accurate reading of themeasuring instrument, but not so slowly as to cause unnecessarily prolongedstress on the test object at the test voltage

The rate of rise should be 2% per second above 75% of the estimated final testvoltage

The test voltage should be maintained for the specified time and then reduced

It should not be suddenly interrupted as this may generate switching transientsthat could cause damage or erratic test results

The requirements of the test are satisfied if no disruptive discharge occurs onthe test object

Deviations from this recommendation may be specified by the appropriateapparatus standard

INPT-R&DTest with alternating voltage


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Disruptive discharge voltage tests

The voltage should be raised in the manner described in withstand

voltage tests until a disruptive discharge occurs on the test object The value of the test voltage reached at the instant of the disruptive

discharge shall be recorded

Assured disruptive discharge voltage tests

The voltage should be raised in the manner described in withstand

voltage tests until a disruptive discharge occurs on the test object The value of the test voltage reached just prior to the disruptive

discharge should be recorded

The requirements of the test are generally satisfied if this voltage is nothigher than the assured disruptive discharge voltage on each one of aspecified number of voltage applications

INPT-R&DTests with impulse voltages


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There are four methods are specified by IEC 60060-1

Procedure A

This procedure is applicable to the non-self restoring insulation

3 impulses of specified shape and polarity at the rated withstand voltage level are

applied to the test object

If there is no indication of discharges observed, test object passes

Procedure B

15 impulses of the specified shape and polarity at the withstand level are applied

to the test object

Test object passes if not more than two disruptive discharge occurs in the self

restoring part of the insulation and no indication of failure in the non-restoring

insulation



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Procedure C

3 specified shape and polarity at the withstand voltage level is applied

to the test object If no disruptive discharge occurs test object passed the test

If more than one disruptive discharge occurs, test object failed the

test

If one disruptive discharge occurs in the self restoring part of the

insulation, then 9 additional impulses are applied. If no dischargeoccurs test object has passed the test



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Procedure D Statistical method

For self restoring insulation the 10% impulse disruptive discharge voltage U10 is

evaluated by using statistical test procedures

Direct evaluation of U10or U50 and indirect evaluation of U10 can be done.

In direct method number of test voltage are applied to find 10% disruptivedischarge voltage

In indirect method

U10 = U50( 1 - 1.3z ) z= 0.03 in general

U50 can be evaluated by

multiple level method

up-and-down method

Test object is passed if U10 is not less than the specified impulse withstand

voltage

INPT-R&DCombined Voltage Tests


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Simulate conditions where one terminal of the open switch is

energized at the specified power frequency voltage and the other

terminal may be subject to either a lightning or switchingovervoltage

The test voltages are characterized by their amplitude, waveshape,

polarity, and any time delay between the application of the two

voltages

INPT-R&DCombined Voltage Tests


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There is possibility of a disruptive

discharge during the test

suitable protective devices (decouplingresistors, inductors, capacitors,

orprotective gaps) protect the test

sources

Definition of the applied waveshape is

left to the appropriate product standard

Measuring device is based on the

requirements for the fastest and slowest

waveshapes to be observed

In all cases, voltages are measured as

referred to ground

INPT-R&D


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Measurementof high voltages

Test procedures

Uncertainty

INPT-R&DConcept of Uncertainty


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Error

The concept of error is now old and no longer used.

Error is deviation from true value. While calculating error we assumethat the true value is known. However true value is never known.

Uncertainty

This concept says that any quantity is known to exist within a definite

interval (nominal value and a range around that) with a given degree of

confidence.

Thus in this concept any quantity is described to lie within an interval

and the level of confidence associated with it.


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INPT-R&DComponents of uncertainty


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Systematic components

Uncertainty arising out of attributable factors which are known

They are estimated by knowledge of effects of such factors on themeasurement

They represent the accuracy of the system

Random components

These components are ones which cannot be attributed to any known factors

These are evaluated by statistical measurements

They represent the precision of the system

INPT-R&DOld and new ways


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Old way

The measurement is represented by the indicated single value say 10

Volts If a 0.5 class instrument has been used for this measurement we

assume that the true value lies within 10 0.5%.

However the meter is not the only source of error in measurement.

There are other sources like, personnel, ambient conditions which

affect meter performance and various unknown (random) factors

INPT-R&DOld and new ways


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New way

The concept of uncertainty accounts for all these.

The meter when calibrated is characterized with total uncertainty of calibrationaccounting all factors which affected calibration.

Further, when measurement is made the total uncertainty of the measurement isevaluated considering all factors that affect the measurement (including theuncertainty of the calibration)

A typical report of measurement with uncertainty figure is

10 V 0.5 V with 95% confidence level.Here 0.5 V is the total uncertainty in estimating the voltage value. Andthere is 95% confidence that the measurement lies within the range 9.5 to 10.5 V


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