Dr M A Panneerselvam, Professor, Anna University

Name: Dr M A Panneerselvam, Professor, Anna University
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Dr M A Panneerselvam, Professor, Anna University
HIGH VOLTAGE ENGINEERING FOR B.E.(EEE) STUDENTS OF ANNA UNIVERSITY Dr M A Panneerselvam, Professor, Anna University

INTRODUCTION OF THE FACULTY
NAME : Dr. M.A. PANNEERSELVAM QUALIFICATION : B.E., (ELECTRICAL) M.E (HIGH VOLTAGE ENGINEERING) Ph.D (HIGH VOLTAGE AREA OF SPECIALISATION : ELECTRICAL MACHINES & HIGH VOLTAGE ENGINEERING Dr M A Panneerselvam, Professor, Anna University

NO. OF PAPERS : ABOUT 30 IN BOTH NATIONAL PUBLISHED & INTERNATIONAL JOURNALS NO. OF Ph.D’s PRODUCED : 4 CANDIDATES WORKING FOR Ph.D. CURRENTLY : 4 Dr M A Panneerselvam, Professor, Anna University

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 2. High Voltage Engineering -3 rd Edition- C.L. Wadhwa - New Age International(P) Ltd. Publishers - New Delhi, Bangalore … Dr M A Panneerselvam, Professor, Anna University

3. High Voltage Engineering - J.R.Lucas - Sri Lanka 4. High Voltage Engineering- Kuffel,E and Abdullah,M - Pergomon Press, Oxford-1970. 5. High Voltage Engineering Fundamentals nd Edition - Kuffel,E , Zaengl,W.S and Kuffel,J - Butterworths, London Dr M A Panneerselvam, Professor, Anna University

6. High Voltage Measurement Techniques - Schwab,A.J - M.I.T. Press, Cambridge 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 Dr M A Panneerselvam, Professor, Anna University

9. Indian Standards Specification on High Voltage Testing of Electrical Apparatus ( IS , 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 and IS 698 ). Dr M A Panneerselvam, Professor, Anna University

THE SUBJECT DEALS WITH THE FOLLOWING TOPICS: 1.OVERVOLTAGES 2.BREAKDOWN IN GASES, SOLIDS , LIQUIDS AND VACUUM DIELECTRICS Dr M A Panneerselvam, Professor, Anna University

3.GENERATION OF VERY HIGH VOLTAGES AND CURRENTS 4.MEASUREMENT OF VERY HIGH VOLTAGES AND CURRENTS 5.HIGH VOLTAGE TESTING & INSULATION COORDINATION Dr M A Panneerselvam, Professor, Anna University

UNIT 1 : OVERVOLTAGES 1.0 NATURE OF OVERVOLTAGES
1.External overvoltages / Lightning overvoltages Dr M A Panneerselvam, Professor, Anna University

2.Internal overvoltages / Switching surges 3.Power frequency overvoltages due to system faults 4.DC overvoltages Dr M A Panneerselvam, Professor, Anna University

1.1 LIGHTNING OVERVOLTAGES
Due to lightning and thunder storms overvoltages are injected onto the transmission lines. CLOUD DISCHARGE Dr M A Panneerselvam, Professor, Anna University TYPE - I

DISCHARGE CLOUD CLOUD TYPE - II Dr M A Panneerselvam, Professor, Anna University

I AMPS I AMPS  / 2  / 2  / 2  / 2 Ƶ Ƶ Ƶ Ƶ TYPE - III Dr M A Panneerselvam, Professor, Anna University

PROPAGATION OF LIGHTNING CHANNEL Dr M A Panneerselvam, Professor, Anna University

1.1.1 Voltage developed due to lightning stroke: EQUIVALENT CIRCUIT Dr M A Panneerselvam, Professor, Anna University

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 : Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

Impulse voltage wave shape: Dr M A Panneerselvam, Professor, Anna University

Specification for impulse voltage: ( AS PER INDIAN STANDARDS )
t1  Time to Front  1.2 s t2  Time to Tail  50 s Vp  Peak voltage Dr M A Panneerselvam, Professor, Anna University

Tolerances allowed: For Front time, t1  ± 30% For Tail time, t2  ± 20% Oscillations around the peak ,Vp, ± 5 % Dr M A Panneerselvam, Professor, Anna University

1.1.4 Types of impulse voltages : FULL IMPULSE CHOPPED IMPULSE FRONT OF WAVE IMPULSE Dr M A Panneerselvam, Professor, Anna University

1.2 INTERNAL OVERVOLTAGES (SWITCHING SURGES)
1.2.1 Reasons for switching surge voltages: Sudden opening of a line Sudden closing of a line Dr M A Panneerselvam, Professor, Anna University

Connection of inductance / Capacitance Sudden connection and removal of loads , etc. Any sudden disturbance taking place in a transmission line will cause switching surge . Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

Assume the switch ‘AB’ is opened Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

B 2 Vp t = 0 A switch becomes 2 Vp .If the switch is unable withstand this voltage it breaks down and a switching surge Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

Ex.2 Operation of a circuit breaker:
RE STRIKING VOLTAGE 2Vp RECOVERY VOLTAGE ARC VOLTAGE FAULT CURRENT Dr M A Panneerselvam, Professor, Anna University

RESTRIKING VOLTAGE ACROSS A CIRCUIT BREAKER
The 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. Dr M A Panneerselvam, Professor, Anna University

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) Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

overvoltages developed. Coefficient of earthing (COE) of a system is defined as the ratio of the Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

Hence,the insulation design (i.e., insulation coordination) is based on switching surges rather than impulse voltages DC OVERVOLTAGES During the past 2 to 3 decades HVDC systems came into existence. Dr M A Panneerselvam, Professor, Anna University

HVDC systems have converters and inverters at the sending end and receiving end respectively employing thyristers. Switching surges are produced due to thyristers’ operation. Dr M A Panneerselvam, Professor, Anna University

1.5 TRAVELLING WAVES ON TRANSMISSION LINES: LONG TRANSMISSION LINE Dr M A Panneerselvam, Professor, Anna University

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) Dr M A Panneerselvam, Professor, Anna University

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) Dr M A Panneerselvam, Professor, Anna University

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), Dr M A Panneerselvam, Professor, Anna University

VI = VCv x LIv = VILC v 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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

Open ended line: Dr M A Panneerselvam, Professor, Anna University

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: Dr M A Panneerselvam, Professor, Anna University

i.e.,½ L(dx) I2 = ½ C(dx)e 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 Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

VOLTAGE AND CURRENT WAVES OPEN ENDED LINE Dr M A Panneerselvam, Professor, Anna University

Short circuited line: For a short circuited line , the reflection coefficient for voltage wave is -1 and for current wave is +1. Dr M A Panneerselvam, Professor, Anna University

VOLTAGE AND CURRENT WAVES FOR SHORT CIRCUITED LINE Dr M A Panneerselvam, Professor, Anna University

Reflection and transmission coefficients for line terminated with impedance ‘R’ : Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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: Dr M A Panneerselvam, Professor, Anna University

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) Dr M A Panneerselvam, Professor, Anna University

Bewley’s Lattice Diagram: Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

1.7 PROTECTION OF TRANSMISSION LINES AGAINST OVERVOLTAGES
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 Dr M A Panneerselvam, Professor, Anna University

5.Overhead lines connected to cables 6.Using protector tubes ( Expulsion Arresters ) 7.Using non-linear resister lightning arresters ( Valve Arresters) Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

SHIELDING ARRANGEMENT OF TRANSMISSION LINES Dr M A Panneerselvam, Professor, Anna University

Reduction of tower footing resistance and use of counter poises : ARRANGEMENT SHOWING COUNTER POISES Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

Spark gaps are of the following types: Rod gaps Horn gaps Sphere gaps Dr M A Panneerselvam, Professor, Anna University

Sphere gaps are generally preferred as they have , Consistency in breakdown Less affected by humidity and other atmospheric conditions. Lesser impulse ratio Dr M A Panneerselvam, Professor, Anna University

ROD GAP HORN GAP Dr M A Panneerselvam, Professor, Anna University

IMPULSE HORN GAP SPHERE GAP Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

IMPULSE VOLTAGE AT DIFFERENT TIMES ON A TRANSMISSION LINE
Dr M A Panneerselvam, Professor, Anna University

Connection of resistance in series and Ferranti’s surge absorber : FERRANTI’S SURGE ABSORBER Dr M A Panneerselvam, Professor, Anna University

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 Dr M A Panneerselvam, Professor, Anna University

The reflection coefficient , R = 60 – 400 / = -340 / 460 = The voltage transmitted into the cable = pu = 26 % of the incident voltage. Dr M A Panneerselvam, Professor, Anna University

Protector tubes ( Expulsion Arresters ) : Spark gaps have the following draw backs: They offer protection against overvoltages by diverting the lightning energy to ground but Dr M A Panneerselvam, Professor, Anna University

they cannot arrest the power follow currents. They are always used as secondary protection except for very small system voltages. Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

EXPULSION ARRESTER Dr M A Panneerselvam, Professor, Anna University

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. Dr M A Panneerselvam, Professor, Anna University

VALVE ARRESTER Dr M A Panneerselvam, Professor, Anna University

CHARACTERISTIC CURVES FOR VALVE ARRESTORS VOLTAGE TIME CHARACTERISTICS RESIDUAL VOLTAGE Dr M A Panneerselvam, Professor, Anna University

REPRESENTATIVE PHOTOGRAPHS
OF LIGHTNING DISCHARGE Dr M A Panneerselvam, Professor, Anna University

Dr M A Panneerselvam, Professor, Anna University

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