Dr M A Panneerselvam, Professor, Anna University 1 HIGH VOLTAGE ENGINEERING FOR B.E.(EEE) STUDENTS...

Dr M A Panneerselvam, Professor, Anna University

HIGH VOLTAGE ENGINEERING

B.E.(EEE) STUDENTS

ANNA UNIVERSITY

INTRODUCTION OF THE FACULTY

NAME : Dr. M.A. PANNEERSELVAM

QUALIFICATION : B.E., (ELECTRICAL) M.E (HIGH VOLTAGE

ENGINEERING) Ph.D (HIGH VOLTAGE

ENGINEERING)AREA OF SPECIALISATION : ELECTRICAL MACHINES

& HIGH VOLTAGE

ENGINEERING

NO. OF PAPERS : ABOUT 30 IN BOTH NATIONALPUBLISHED & INTERNATIONAL JOURNALS

NO. OF Ph.D’s PRODUCED : 4

CANDIDATES WORKING FOR Ph.D. CURRENTLY : 4

LIST OF REFERENCES 1. High Voltage Engineering -4 th Edition- M.S. Naidu and V.Kamaraju- Tata Mc.Graw-Hill Publishing Co. Ltd.,- New Delhi- 2009.2. High Voltage Engineering -3 rd Edition- C.L. Wadhwa - New Age International(P) Ltd. Publishers - New Delhi, Bangalore …- 2010.

3. High Voltage Engineering - J.R.Lucas - Sri Lanka - 2001.4. High Voltage Engineering- Kuffel,E and Abdullah,M - Pergomon Press, Oxford-1970.5. High Voltage Engineering Fundamentals - 2 nd Edition - Kuffel,E , Zaengl,W.S and Kuffel,J - Butterworths, London- 2000.

6. High Voltage Measurement Techniques - Schwab,A.J - M.I.T. Press, Cambridge - 1972.7. High voltage Technology - Alston,L.L - Oxford University Press, Oxford-1968.8. High Voltage Laboratory Techniques- Craggs, J.D. and Meek, J.M - Butterworths, London- 1954.

9. Indian Standards Specification on High Voltage Testing of Electrical Apparatus ( IS 1876-1961, IS 2071 Part I-1974, IS 2071 Part II-1974, IS 2071 Part III-1976, IS 2026 Part III- 1981, IS 3070 Part I-1985, IS 2516 Part II/Sec.2-1965 and IS 698 ).

THE SUBJECT DEALS WITH THE FOLLOWING TOPICS:

1.OVERVOLTAGES

2.BREAKDOWN IN GASES, SOLIDS , LIQUIDS AND VACUUM DIELECTRICS

3.GENERATION OF VERY HIGH VOLTAGES AND CURRENTS

4.MEASUREMENT OF VERY HIGH VOLTAGES AND CURRENTS

5.HIGH VOLTAGE TESTING &

INSULATION COORDINATION

UNIT 1 : OVERVOLTAGES

1.0 NATURE OF OVERVOLTAGES

1.External overvoltages / Lightning overvoltages

2.Internal overvoltages / Switching surges 3.Power frequency overvoltages due to system faults 4.DC overvoltages

1.1 LIGHTNING OVERVOLTAGES

+ + + + - - - + + + + - - - -+ + + + - - - - - + + + + - - -

- - - - ++++ - - - -++++- - - - +++++ - - - +++++

Due to lightning and thunder storms overvoltages are injected onto the transmission lines.

DISCHARGE

TYPE - I

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - -- - - - - - - - - -- - - - - -- - - - - - - - - - -- - - - - - - -- - - - - - - -- - - - - - - - - -

DISCHARGE

TYPE - II

+ + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +

- - - - - - - - - - - - -- - - - - - - - - -- - - - - -- - - - - - - - - - -- - - - - - - -- - - - - - - -- - - - - - - - - -

TYPE - III

I AMPSI AMPS

/ 2 / 2 / 2 / 2

ƵƵƵƵ

+++++++++++++++++++ - - - - - - - - - - - - - - - - - -

PROPAGATION OF LIGHTNING CHANNEL

1.1.1 Voltage developed due to lightning stroke:

EQUIVALENT CIRCUIT

For a lightning stroke current of 200 kA and assuming a surge impedance of 400 Ω for overhead line, the voltage developed is equal to( I x Z/ 2 ) = 200 x 103 x 400/2 = 40 x 106 = 40 MV.

1.1.2 Traveling waves on transmission lines :

TRANSMISSION LINE WITH SURGE IMPEDANCE ‘Z’

The velocity of traveling waves on overhead lines is 300 m / μs and on cables is approximately 150 m / μs.

1.1.3 Impulse voltage wave shape:

Specification for impulse voltage: ( AS PER INDIAN STANDARDS )

t1 Time to Front 1.2 s

t2 Time to Tail 50 s

Vp Peak voltage

Tolerances allowed:

For Front time, t1 ± 30%

For Tail time, t2 ± 20%

Oscillations around the peak ,Vp, ± 5 %

1.1.4 Types of impulse voltages :

FULL IMPULSE CHOPPED IMPULSE FRONT OF WAVE

IMPULSE

1.2 INTERNAL OVERVOLTAGES (SWITCHING SURGES)

1.2.1 Reasons for switching surge voltages:

• Sudden opening of a line

• Sudden closing of a line

•Connection of inductance /Capacitance

•Sudden connection and removal of loads , etc.

Any sudden disturbance taking place in a transmission line will cause switching surge .

For the range of values of the inductance and capacitance of overhead lines the frequency of the switching surges are generally in the range of kc/s and they exist for a duration of milliseconds.

1.2.2 Switching surges on transmission lines: Ex.1 Opening of an unloaded OH line:

Simply opening of an unloaded line transmission line may result in switching surge as shown in the fig.

Assume the switch ‘AB’ is opened

at time, t = 0, when the AC voltage is at its peak. During the next half cycle the voltage at terminal ‘A’ changes to negative peak of AC voltage , wheras the voltage at terminal ‘B’ remains at positive peak. Hence the voltage across the

switch becomes 2 Vp .If the switch is unable withstand this voltage it breaks down and a switching surge

of magnitude 2 Vp travels on the line. At the terminations it gets reflected and refracted and builds up further to a higher level.

Another example for generation of switching surge is the operation of a circuit breaker as shown in the next figure.

Ex.2 Operation of a circuit breaker:

RE STRIKING VOLTAGE

RECOVERY VOLTAGE

ARC VOLTAGE

FAULT CURRENT

RESTRIKING VOLTAGE ACROSS A CIRCUIT BREAKERThe maximum voltage across the breaker contacts = 2 Vp =2√2 VRMS

The voltage after reflection and refraction at the terminals of the transmission line may reach a maximum of 5 t0 6 times the system voltage.

1.3 PF OVERVOLTAGES DUE TO LOCAL SYSTEM FAULTS

1.3.1 Local faults in the systems are : Line to ground fault (3) Double line to ground fault (3)

Double line fault (3) Triple line fault (1) Triple line to ground fault (1)

Of the total 11 faults above, a double line to ground fault is more dangerous with respect to

overvoltages developed.Coefficient of earthing (COE) of a system is defined as the ratio of the

voltage of the healthy phase to

ground to that of the line voltage

in the event of a double line to ground fault.

The value of COE varies between 1/√3 to 1.0(i.e., 0.59 to 1.0) depending upon the neutral impedance.

When the value of ‘COE” is less than 70 % ,the system is said to be an effectively or solidly earthed system.

When the ‘COE’ is more than 70 %, the system is said to be a non effectively earthed system.

Systems above 230 kV are generally effectively earthed.

For System ratings above 230 kV the Switching surge voltages attain very high values and become more severe than impulse voltages.

Hence,the insulation design (i.e., insulation coordination) is based on switching surges rather than impulse voltages.

1.4 DC OVERVOLTAGES

During the past 2 to 3 decades HVDC systems came into existence.

HVDC systems have converters and inverters at the sending end and receiving end respectively employing thyristers. Switching surges are produced due to thyristers’ operation.

1.5 TRAVELLING WAVES ON TRANSMISSION LINES:

LONG TRANSMISSION LINE

Assuming a lossless line (i.e., R=0,G=0) when the wave has travelled a distance ‘x’ after a time ‘t’, the electrostatic flux associated with the voltage wave is, q = CxV ------------(1)

The current is given by the rate of charge flow , I = dq/dt = VC dx/dt---(2)

Here dx/dt is the velocity of the travelling wave represented by, I = VC v -------------------(3)

Similarly, the electromagnetic flux associated with the current wave, Φ = Lx I ---------------------(4)

The voltage is the rate of change of flux linkages, V = LI dx/dt = LIv --------------------(5) Dividing Eqn.(5) by (3), V/I= LIv/VCv = LI/CV V2/I2=L/C. i.e.,V/I=Z=√(L/C) --------(6)

Next multiplying Eqn. (5) and (6),

VI = VCv x LIv = VILC v2 v2 = VI / VILC = 1/ (LC) v = 1 / √ (LC) -----------------------(7)

Substituting the values for ‘L’ and ‘C’ of overhead lines we get,

v = 1 / ((2x10-7 ln d/r x 2πε/(ln d/r)) = 3x108 m/sec. = 300 m/μ sec.

which is the velocity of light.

Hence, travelling waves travel with velocity of light on overhead lines.

In cables, since εr >1, the velocity of travelling waves is lesser than overhead lines and is approximately 150 m/ μ sec.

Open ended line:

The voltage wave and current waves travelling towards the open end are related by, V / I = Z. Since the current at the open end is zero, the electromagnetic energy vanishes and is transformed into electrostatic energy:

i.e.,½ L(dx) I2 = ½ C(dx)e2 i.e.,(e/I)2 = L/C = Z2 . i.e., e=IZ=V. Hence, the potential at the open end is raised by ‘V’ volts and becomes V+V=2V. The incident wave = V, the reflected wave = V and the refracted (transmitted) wave = V+V= 2V

The refracted(transmitted) wave = Incident wave + Reflected wave.

For an open ended line the reflection coefficient for voltage wave is +1 and the reflection coefficient for current wave is -1.

VOLTAGE AND CURRENT WAVES OPEN ENDED LINE

Short circuited line:

For a short circuited line , the reflection coefficient for voltage wave is -1 and for current wave is +1.

VOLTAGE AND CURRENT WAVES FOR SHORT CIRCUITED LINE

Reflection and transmission coefficients for line terminated with impedance ‘R’ :

Let the incident voltage and current waves be V and I, the reflected waves V’ and I’ and the transmitted waves V’’ and I’’. It is seen in the earlier sections that whatever be the value of terminating impedance,whether it is open or short circuited , either the current wave or

voltage wave is reflected back with negative sign, i.e., I’ = - V’/Z

I=V/Z , I’=-V’/Z and I’’=V’’/ R. Since I’’=I+I’ and V’’= V+V’, we have, V’’/R = V/Z – V’/Z =V/Z – (V’’-V)/Z = 2V/Z – V’’/Z.

V’’(1/R +1/Z)= V’’(R+Z)/RZ = 2V/Z V’’= V 2R/(R+Z) and I’’ = I 2Z/(R+Z)

Hence , the refraction coefficients for voltage and current waves for open ended line respectively are:

2R / R+Z and 2Z / R+Z

Similarly, the reflection coefficients for voltage and current for open ended line are respectively:

(R-Z) / (R+Z) and - (R-Z) / (R+Z)

Bewley’s Lattice Diagram:

1.6 COMPARISON OF DIFFERENT TYPES OF OVERVOLTAGES

1 Lightning overvoltage :

Lightning overvoltage is an external overvoltage as it is independent of the system parameters. It injects

current on to the transmission lines producing a voltage ranging from kV to MV . It has a wave shape of 1.2/50 μs and exists for a period of microseconds. The very high rate of rise of the impulse voltage striking the line is

equivalent to applying a voltage at very high frequency of the order of Mc/s.

2 Switching Surge :

Switching surges are internal overvoltages as they are dependant upon the system

parameters (i.e., the voltage level, the values of R,L and C of the line). Their magnitudes range from 4 to 6 times the system voltage and they have damped oscillations of kc/s and exist for durations of milliseconds.

3 Power frequency overvoltage :

Due to local system faults such as ‘double line to ground faults’ the voltage of the healthy phase to ground will increase from phase voltage to line voltage depending upon the neural earthing impedance of the system.

1.7 PROTECTION OF TRANSMISSION LINES AGAINST

OVERVOLTAGES

1.5.0 Transmission lines are protected from lightning and switching surges by adopting the following methods :

1.Use of shielding wires

2. Reduction of tower footing resistance and use of counter poises

3.Using spark gaps ( sphere gap and horn gap )

4.Connection of surge absorbers

5.Overhead lines connected to cables

6.Using protector tubes ( Expulsion Arresters )

7.Using non-linear resister lightning arresters ( Valve Arresters)

1.7.1 Shielding wires:

Shielding wires are ground wires connected above phase wires.

The shielding angle should be less than 300 for effective protection of the transmission line against lightning stroke.

69 SHIELDING ARRANGEMENT OF TRANSMISSION LINES

1.7.2 Reduction of tower footing resistance and use of counter

poises :

ARRANGEMENT SHOWING COUNTER POISES

1.7.3 Spark gaps :

When Spark gaps are connected between phase to ground the gaps breakdown due to lightning overvoltage and lightning energy is diverted to ground through gaps.

Spark gaps are of the following types:

Rod gapsHorn gaps Sphere gaps

Sphere gaps are generally preferred as they have ,

Consistency in breakdown

Less affected by humidity and other atmospheric conditions.

Lesser impulse ratio

ROD GAP HORN GAP

IMPULSE HORN GAP SPHERE GAP

IMPULSE RATIO:

Impulse ratio is defined as the ratio of peak impulse breakdown voltage to that of peak power frequency breakdown voltage of a given insulation.

Sphere gaps have impulse ratio around unity and hence they offer better protection against lightning overvoltages and helps in reduction of insulation of equipment connected in the system.

1.7.4 Surge absorbers :•Power loss takes place due to corona at excess overvoltages and helps in the reduction of such overvoltages.

•In addition the front time of the impulse voltage is increased resulting in reduced stress on the equipment.

IMPULSE VOLTAGE AT DIFFERENT TIMES ON A TRANSMISSION LINE

Connection of resistance in series and Ferranti’s surge absorber :

FERRANTI’S SURGE ABSORBER

1.7.5 Connection of UG cable to overhead line :

The reflection coefficient, R = ZC – ZL / ZC + ZL .

Taking ZL as 400 ohms and Zc as 60 ohms

The reflection coefficient , R

= 60 – 400 / 60 + 400 = -340 / 460 = -0.739The voltage transmitted into the cable 1.0 - 0.739 = 0.261 pu = 26 % of the incident voltage.

1.7.6 Protector tubes ( Expulsion Arresters ) :

Spark gaps have the following draw backs:

They offer protection against overvoltages by diverting the lightning energy to ground but

they cannot arrest the power follow currents.

They are always used as secondary protection except for very small system voltages.

The drawbacks of expulsion arrestors are:They require certain minimum energy to produce gas to quench the arc.

For very high current values they may explode due to very high pressure of gas generated.

EXPULSION ARRESTER

1.7.7 Non linear resister lightning arresters( Valve Arresters ) :

These arresters act as valve in the sense that they offer very low impedance for lightning voltages and offer very high impedance for power frequency currents.

88 VALVE ARRESTER

CHARACTERISTIC CURVES FOR VALVE ARRESTORS

VOLTAGE TIME CHARACTERISTICS RESIDUAL VOLTAGE

REPRESENTATIVE PHOTOGRAPHS

LIGHTNING DISCHARGE

Dr M A Panneerselvam, Professor, Anna University 1 HIGH VOLTAGE ENGINEERING FOR B.E.(EEE) STUDENTS...

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