FACULTY OF ENGINEERING

LAB SHEET

HIGH VOLTAGE ENGINEERING

EET 4106

TRIMESTER 2 (2012-2013)

HV 1: TRANSFORMER OIL TESTING

HV 2: ANALYSIS, DESIGN AND SIMULATION OF

IMPULSE GENERATING CIRCUITS

*Note: On-the-spot evaluation may be carried out during or at the end of the experiment.

Students are advised to read through this lab sheet before doing experiment. Your

performance, teamwork effort, and learning attitude will count towards the marks.

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EET 4106 HIGH VOLTAGE ENGINEERING

EXPERIMENT 1

TRANSFORMER OIL TESTING

OBJECTIVES To conduct withstand test of given transformer oil sample.

To find out the breakdown voltage of oil sample.

To compute the standard deviation of the test results.

1. Introduction Transformer Oil is high quality electrical insulating oil. It is manufactured using

specially selected base stocks to help provide protection against oxidation and

sludge formation. It is recommended for use as an electrical insulating oil in

applications such as transformers, oil immersed switch gear, circuit breakers, oil

filled capacitors, tap changers, reclosures and fuses, where an oil meeting the

Australian Standard, British Standards Institution (BSI), International

Electrotechnical Commission (IEC) or other comparable specification is required

by the equipment manufacturer or user.

Good insulating properties, achieved by high dielectric strength and low dielectric

losses are the result of careful control in manufacture and handling. High dielectric

strength ensures good insulation of electrical conductors and prevention of arcing

between electrodes under the voltage stresses encountered in normal insulating oil

service. Low loss tangent minimizes energy loss due to the changing polarity of

the alternating current.

Good heat transfer and fluid flow characteristics are obtained as a result of low

viscosity and pour point. This assures effective cooling of transformer cores and

windings, and ease of operation of switches, circuit breakers, pumps, regulators,

and load tap changer mechanisms. Good oxidation stability minimizes

development of sludge and acidity in storage and service. Sludge and acidity can

have an adverse effect on the electrical properties and cooling ability.

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2. SAMPLING & TESTING

2.1. Causes of Bad Oil

Transformers and switchgear oil may be rendered unsuitable for further use

due to four main reasons

Low dielectric strength

High acidity

High sludge content

Excessive free water content

Low dielectric strength may result form many causes, the most common of

which is foreign particles or fibres and water in combination. Individually their

effect may be relatively small, but together a contamination of only a few parts

in a million can cause considerable lowering of the breakdown voltage of the

oil. High acidity, sludge and free water should not be tolerated, but will not

necessarily reduce the dielectric strength below acceptable or specified levels.

2.2. Appearance of the Sample

Only an experienced person can judge the condition of insulating oil from its

appearance, but a general guide may be obtained from the following

observations: -

A cloudy appearance may indicate that sludge has been formed.

A dark yellow color could be a sign of overheating

A blackish color often results from an arc having taken place with either

carbonization of the oil or of the degradation of insulation within the

equipment.

A green color may be due to copper salts dissolving in the oil

3. PERFORMING THE TEST

3.1. Cleanliness of the Apparatus

The necessity for scrupulous cleanliness in the apparatus and during the

process of sampling cannot be over emphasized. The measurement of the

dielectric strength of the sample is as dependent on the cleanliness of the test

cell and the sampling apparatus as the condition of the oil itself. After cleaning

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the apparatus, it should under no circumstances be wiped, dried or even

handled with a dusty or fibrous cloth as loose dust or fibers

3.2. Precautions necessary during Sampling

Since the oil taken from a drain cock will inevitably contain excess of settled

out solid impurities, and will not necessarily be representative of the bulk of

the oil, it should be run to waste until clear.

Always run off a quantity of oil into a glass bottle or a test tube for an initial

check on the appearance of the oil. Attention to this point may prevent

contamination of a clean test vessel.

If necessary, thoroughly clean and dry the sample point using a suitable

solvent. Any cloth used should be lint free. Open the sample cock and drain to

waste enough oil to ensure that the sample cock is fully flushed and the sample

is representative of the bulk of oil to be tested. This quantity will depend upon

the size of the transformer or main container. About 2 liters is usually suitable.

When sampling from a drum or supply container, the oil should flow at a

steady rate into the test vessel and after being swilled around the sides it should

be discarded. Without altering the rate of flow of the oil the quantity required

for the test should then be run off while taking extreme care to prevent the

ingress of atmospheric dust, cloth fibers or moisture. Do not use a syphon. In

the absence of a useable sample point, use a ‘thief’.

Shield the sample from direct light until ready to be tested.

Turbulence and air bubbles should be avoided when pouring the oil. Relevant

national specifications should be observed.

4. ITEMS REQUIRED

OIL TEST SET OTS60AF/2 (Appendix A)

TEST SAMPLE OF TRANSFORMER OIL

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5. PREPARING THE TEST VESSEL Separate the cover holding the electrode mountings from the container.

Ensure that the vessel is thoroughly clean, both inside and outside.

Mount the appropriate electrodes on the sliding arms, if they are not already in

place. It may be necessary to slacken one or both of the clamp screws at the

electrode supports and move the sliding arms back. The sliding arms have a

threaded stud and the electrodes screw on to these. Screw on and tighten firmly

with finger pressure.

Set the gap between the electrodes according to the requirement of the testing

specification being undertaken. Spacing gauges are provided in the accessory

kit for this purpose. The clamp screws at the bottom of both electrode supports

should be slackened and the sliding arms moved so that the gap is

approximately central between the two supports. Tighten the clamp on one

support to hold one side firmly, then adjust the other so that when the gauge is

passed between the electrodes it touches both simultaneously. Tighten the

clamp screw on the second support. Recheck the gap after it has been set.

A list of electrode shapes and gap spacing for standard testing specifications is

shown in figure. Each test display screen also shows electrode shape; spacing,

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and type of stirring for each pre-programmed test specification, where

appropriate.

Clean the vessel in accordance with the instructions given in the relevant test

specification to be used, then fill the container part of the vessel with the

sample oil until the level is about 12mm from top.

If required, drop in a clean magnetic stirrer (not for ASTM D877 and ASTM

D1816 specifications, or any case where impeller stirring has been selected).

Ensure that the stirrer bar is central in the vessel.

Carefully reassemble the two parts of the vessel.

6. LOADING THE TEST VESSEL

Open the test chamber cover by turning the trapped key interlock knob a ¼

turn anticlockwise and pulling it outward.

Place the test vessel in the chamber so that the ends of the connector arms rest

on the cradle terminals at the top of the support horns. Clean away any spilled

oil.

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Caution: Visually confirm that the two spring earth contacts linking the metal

EMC/discharge barrier on the test chamber cover to the back of the chamber

are intact.

Close the test chamber cover, pushing in and turning the interlock knob

clockwise to secure it.

7. PREPARING THE OIL TEST SET

Adjust the mains voltage selector if necessary and connect the oil test to a

suitable supply. It is recommended that the green/yellow earth terminal is

connected to a known good earth.

Switch on both the supply and the test set. The copyright message appears on

the display for a brief time and is followed by the main programme menu

screen. The options available from the main menu are:-

AS 1767-1976

ASTM D877-87

ASTM D1816-84a

BS 148:1984

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BS 5730a:1979

BS 5874:1980

CEI 10-1-1987

EN60156 – 1996

IEC 156 1995

OPTIONS

IP 295/83

NFC 27-221:1974

ГOCT 6581-75

SABS 555-1985

UNE21-309-89

VDE 0307/84

5 MINUTE TEST

SELF CHECK

CHECK CALIBRATION

CUSTOM

WITHSTAND (PROOF)

The main menu display allocates to the three control keys, as their function

changes. On switching on, these will be (from left to right) ,, and SELECT.

The left and center keys control scrolling up and down through the menu options,

and the SELECT key activates the highlighted option.

Note: The display shows only five menu options at any time and those in view

will depend upon the last option used. When scrolling, the highlighted option is

the one ready for selection.

Select the BS 148/IEC 156 1995 option and follow the display instructions.

Selecting OPTIONS from the main menu

DISPLAY CONTRAST/

DISPLAY BACKLIGHT/

LANGUAGE/ select English

PRINTER CONTROL/

PRINT LAST RESULTS

AUTO PRINT/ select ON

NO. OF COPIES/ select accordingly

BAUD RATE / select for external printer usage

QUIT

INT. PRINTER ON

EXT. PRINTER OFF

TIME/DATE/ adjust Time & Date accordingly

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8. AUTOMATIC TESTING SEQUENCE Choose the appropriate pre-programmed oil testing specification program from the

main menu by scrolling through using ,keys until the required specification is

highlighted. Then press SELECT.

When a test specification has been selected in this way, the display screen changes

to show the name of the test (this remains in view throughout the test sequence)

and gives an option to START the test sequence with the left hand key, or return

to the main menu with the right hand key. For each separate test specification, the

display will indicate appropriate choices of electrode shape and spacing, if the

specification stipulates these. The appropriate method of stirring is also shown. If

the STIR MODE option is displayed, the user may vary the mode of stirring

between MAGNETIC (magnetic stirrer bar), IMPELLER (stirrer test vessel), or

NONE as is appropriate to the selected test.

When started, each test sequence is carried out completely automatically.

Note: The microprocessor software contains routines, which monitor the operation

of the test set. In the unlikely event of a problem occurring (whether caused by the

test set or an external event), the test currently in progress will when resumed

continue from the point at which the problem occurred. If this is not possible the

test set will be placed in a safe condition and a situation status message will be

displayed.

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9. CALCULATIONS After an automatic test sequence is completed, the average is calculated from the

following formula:

Average

x = ∑=

=

ni

i

ixn 1

1 Where n = number of results

x = Mean of results

xi = ith

individual result

Standard Deviation

S = 1

1

22

−

−∑=

=

n

xnxni

i

i

Where S = sample standard deviation

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What are the properties of good Transformer Oil?

What is the schedule for testing of Transformer Oil?

Compare the appearance of Transformer oil before and after testing.

Explain the test procedure of transformer oil with suitable reason.

Explain the results of transformer oil breakdown from the BS148 & IEC

156.

Record the following parameters accurately while performing the test

with stopwatch.

a. Initial stand time

b. Rate of rise of test voltage

c. Intermediate test time

d. Intermediate stand time

e. Number of tests

f. Maximum duration of selected test sequence

APPENDIX A

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TRANSFORMER MINERAL OIL BRITISH STANDARD: BS 148

DESCRIPTION

Transformer Oil is a high quality electrical insulating oil. It is manufactured using specially selected base stocks to help provide protection against oxidation and sludge formation. Careful processing and handling ensure that the oil is stable and free of water and other contaminants and remains so until it reaches the user.

RECOMMENDED FOR

Transformer Oil is recommended for use as an electrical insulating oil in applications such as transformers, oil immersed switchgear, circuit breakers, oil filled capacitors, tap changers, electrical enclosures and fuses, where an oil meeting the Australian Standard, British Standards Institution, International Electro technical Commission or other comparable specification is required by the equipment manufacturer or user.

It should not be used where safety considerations require the use of non-flammable insulating oil.

PRODUCT BENEFITS

Good insulating properties, achieved by high dielectric strength and low dielectric loss tangent (power factor) are the result of careful control in manufacture and handling. High dielectric strength ensures good insulation of electrical conductors and prevention of arcing between electrodes under the voltage stresses encountered in normal insulating oil service. Low loss tangent minimises energy loss due to the changing polarity of the alternating current.

Good heat transfer and fluid flow characteristics as a result of low viscosity and low pour point. This assures effective cooling of transformer cores and windings, and ease of operation of switches, circuit breakers, pumps, regulators, load tap changer mechanisms, etc.

Good oxidation stability minimises development of sludge and acidity in storage and service. Sludge and acidity can have an adverse effect on the electrical properties and cooling ability of the oil and shorten service life.

ELECTRICAL CHARACTERISTICS

The Dielectric Breakdown Strength of an insulating material is a measure of its resistance to electrical breakdown when an electric potential is applied across it.

Immersing two electrodes in a sample of the oil, then applying an AC voltage across the electrodes test electrical insulating oils. The voltage is then increased in a specified manner until electrical breakdown occurs. The various tests used differ in electrode spacing and shape, rate of increase of voltage and thus give different breakdown values for the same oil.

The important consideration, however, is that oils meeting the minimum dielectric breakdown strength

have adequate insulating ability for all normal applications for which they are recommended.

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The dielectric strength of insulating oil is reduced if there is water or solid particles in the oil. Also,

oxidation products, resulting from oil deterioration in service, lower the dielectric strength. As a result

periodic checks of the dielectric strength of an oil can be a useful method of determining whether

contamination or deterioration of oil has occurred.

Any two adjacent conductors form a capacitor. In an ideal capacitor, the phase difference between an

applied AC voltage and the current is 90° and the power dissipated is zero. If the dielectric between the

conductors is less than ideal, the phase difference will be less than 90° and some power dissipation will

occur. In order to keep this loss low, it is desirable to have the dielectric as near to ideal as is practical.

For insulating oils the value for this characteristic is called the power factor, or loss tangent (dissipation

factor) and is expressed as a percentage at a specified temperature. These values are determined

experimentally and represent trigonometric functions of the angle of phase difference. With the

particular functions used, a value of zero would represent a 90° phase difference and the ideal condition,

therefore low values are desirable.

Oxidation and contamination of an oil can cause power factor or loss tangent of an oil to rise, so

determination of this property may provide useful information about a used electrical insulating oil.

Since these values vary with temperatures, comparisons must always be made at the same temperature.

The resistivity of electrical insulating oil is a measure of the resistance to DC current flow between

conductors. The resistivity of mineral insulating oil is naturally high but, as with loss tangent, is very

sensitive to the presence of even minute amounts of suspended water, free ions or ion forming materials

such as acidic oxidation products or polar contaminants. Again, measurement of the resistivity of used

insulating oil may provide useful information as to whether or not the oil is suitable for further service.

SERVICE CONSIDERATIONS

The sensitivity of the electrical insulating oil properties to contaminants, such as water and solid

particles, makes it extremely important to handle and store these products so that the risk of

contamination is minimised.

As a general rule, all shipments of electrical insulating oils should be tested for dielectric strength

before being placed in service. If the dielectric breakdown strength is low, the oil should be polished by

filtration and/or dehydration.

Drums of electrical insulating oils should be stored indoors. If drums must be stored outside, they

should be placed on their sides on racks that will keep them clear of any ground water. The bungs

should be positioned low to prevent them from breathing moisture. The drums should be protected with

a waterproof cover to keep off rain, and to protect the drum markings from being obliterated.

Electrical insulating oils in service should be checked at periodic intervals to determine if they are suitable

for continued service. Proper sampling is critical.

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EET 4106 HIGH VOLTAGE ENGINEERING

EXPERIMENT 2

ANALYSIS, DESIGN AND SIMULATION OF IMPULSE

GENERATING CIRCUITS

1. Objectives

To analyse single-stage impulse generating circuits and derive expressions for front

and tail times.

To design circuit parameters to generate lightning and switching impulses of

specified waveshape.

To simulate the impulse generating circuit using MATLAB simulink (or any other

simulation software) and verify the design.

2. Introduction

Transient overvoltages due to lightning and switching surges cause steep build-up of

voltage on transmission lines and other electrical apparatus. Experimental investigations

showed that impulse voltage is a unidirectional voltage with a rise time of 0.5 to 10 µs and decay time to 50% of the peak value of the order of 30 to 200 µs. The waveshapes

are arbitrary, but could be either positive or negative. The maximum value is called the

peak value of the impulse and the impulse voltage is specified by this value. If an impulse

voltage develops without causing flash over or punctures, it is called a full impulse

voltage. If flash over or puncture occurs, thus causing a sudden collapse of the impulse

voltage, it is called a chopped impulse voltage. The lightning overvoltage can be

represented by equation

V = V0e-αt

– e-βt

(1)

where α and β are constants of microsecond values.

The above equation represents a unidirectional wave, which usually rises rapidly to the

peak value and slowly falls to zero value. The general waveshape is given Fig. 1. A full

impulse voltage is characterized by its peak value and its two time intervals, the wave

front and wave tail time intervals. Thus, 1.2/50 µs, 1000 kV wave represents an impulse

voltage wave with a front time of 1.2 µs, fall time to 50% peak value of 50 µs, and a peak

value of 1000 kV. Hence, the wave front time of an impulse wave is the time taken by the

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wave to reach its maximum value starting from zero value. When impulse waveshapes

are recorded, the initial portion of the wave will not be clearly defined or sometimes will

be missing. Moreover, due to disturbances it may contain superimposed oscillations in

the rising portion. Therefore, the wave front time is specified as 1.25 times the time taken

for the wave to rise from 10% to 90% of its peak value. The wave tail time is given as

time to reach 50% of the peak value on the tail side. The standard impulse wave shape

specified in BSS is a 1.2/50 µs wave i.e. a wave front of 1.2 µs and a wave tail of 50 µs.

A tolerance of not more than ± 50% on the duration of the wave front and 20% on the

time to half value on the wave tail is allowed. The impulse wave is completely specified

as 100 kV, 1.2/50 µs where 100 kV is the peak value of the wave. For the 100 kV, 1.2/50

µs wave the approximate values of the parameters of equation (1) are: V0 = 104 kV, α =

0.01465, and β = 3.218 when time ‘t’ is expressed in µs. α and β control the front and tail

times, respectively.

Fig. 1 General Lightning Impulse Waveshape

3. Analysis of Impulse Generator Circuits

A double exponential waveform may be simulated in the laboratory with a series R-L-C

circuit under over damped conditions or by the combination of two R-C circuits. A

capacitor C1 previously charged to a particular DC voltage is suddenly discharged into

the wave-shaping network by the spark gap (G). The discharge voltage v0 (t) across C2

(test object) gives rise to the desired double exponential wave shape.

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(a) (b)

Fig. 2 Circuits for producing impulse voltages

The two circuits are widely used for generating impulses are shown in Fig. 2. They differ

only in the position of the wave tail control resistance R2. When R2 is on load side of R1

the two resistances form a potential divider which reduces the output voltage but when R2

is on the generator side of R1 this particular loss of output voltage is absent. The impulse

capacitor C1 is charged through a charging resistance to a DC voltage V0 and then

discharged by flashing over the switching gap with a pulse of suitable value. The desired

impulse voltage appears across the load capacitance C2. The value of the circuit elements

determines the shape of the output impulse voltage.

Circuit (a)

An expression for the output voltage across the capacitor C2 can be derived as

(2)

where

211112

111

CRCRCR++=+ βα and

2211 CRCR

1=αβ (3)

From (3) α and β can be computed.

The impulse wave-front time, tail time and peak value, can be computed using the

following expressions.

Wave front time, t1 =

− α

βαβ

ln1

(4)

Once the front time is known, the tail time can be computed as follows:

(5)

Therefore

(6)

Solving (6) iteratively, the tail time can be computed.

Circuit (b)

Similar analysis can be done for circuit in Fig. 2(b)

)()(

)(21

0

tt eeCR

Vtv βα

αβ−− −

−=

)()(

)()()(

5.0)(5.0 2211

21

20

21

10

ttttee

CR

Vtvee

CR

Vtv

βαβα

αβαβ−−−− −

−==−

−×

=

Keeee tttt =−=− −−−− )(5.0)( 1122 βαβα

]ln[1

2

2

teKt

β

α−+−=

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4. Design of Circuit Parameters for a given waveshape

Determination of α and β for given front and tail times for circuit in Fig. 2(a).

From (3) we can write

+=αβ

αβ ln1

1t (7a)

From (5) we can write

where K is defined in (5) (7b)

Solving (7a) and (7b) using Gauss iterative technique α and β for given front and tail

times can be computed.

Determination of R1 and R2 for the specified C1 and C2 and for the given

α and β

Usually the load capacitance C2 will be fixed by the test equipment (load). For the

given value of the load capacitor C2 the source capacitance C1 is normally chosen 5 –

10 times C2 for better voltage efficiency. Hence we have to design R1 and R2 for the

given α and β, and C1 and C2.

Let

211112

111

CRCRCRa ++=+= βα and

2211 CRCR

1== αβb

From these two equations we can derive

+−+

+=

1

21

2

2

21

2

)(4

)(2

1

bC

CC

b

a

b

a

CCR (8)

and 221

1CRbC

1=R (9)

+

= −Ket

t2

1ln

1

2

βα

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5. Simulation of the Impulse Generation Circuit

The following circuit may be used to simulate the circuit to generate the impulse voltage

using MATLAB Simulink.

Figure 3 Circuit for generating impulse voltage

6. Excercise

(i) Compute the front time and tail time for the circuit shown in Fig. 2(a) for C1 = 0.05

µF, C2 = 1000pF, R1 = 300 and R2 = 1500. (Hint: Compute α and β from (2) and (3)

and then compute t1 and t2 from (4) and (6))

(ii) Simulate the circuit and plot the waveform if the initial charging voltage is 500 kV.

What is the peak value of the impulse? From the plot verify your computed front

and tail time.

(iii) Design circuit parameters R1 and R2 for circuit in Fig 2(a) to generate the following

impulse voltages and present your results in the tabular form shown below. Assume

the ratio C1/C2 = 10 and C2 =1000 pF. (Hint: Compute α and β for given t1 and t2

using 7(a) and 7(b) iteratively and then compute R1and R2 using (8) and (9). From

(2) you can observe that the peak voltage is v0(t1) and it is function of source

voltage V. Therefore, for the given peak voltage you can compute the source

voltage V.)

S.No. t1/t2 (µs) α (µs)-1

β (µs)-1

R1 (Ω) R2 (Ω) Source voltage (kV)

1. 0.5/5, 300 kV 2. 1.5/50, 500 kV 3. 1/40, 200 kV 4. 300/3000,

100 kV

(iv) Verify your design for 1.5/50, 500 kV lighting impulse and for 300/3000, 100 kV

switching impulse by simulation and discuss your result.

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(v) Give a brief write up about what you have learned from this experiment.