## HIGH VOLTAGE ENGINEERING -2 Marks Questions with Answer, Anna University Question Bank for High Voltage Engineering ...

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DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING
07EE021 - HIGH VOLTAGE ENGINEERING
TWO MARKS QUESTION AND ANSWERS
PREPARED BY
Mr.S.SARAVANAN M.E.,(Ph.D)
Assistant Professor /EEE
Mr.M.SURESH M.E.,
Assistant Professor /EEE
2 MARKS QUESTION AND ANSWERS
OVER VOLTAGES IN ELECTRICAL POWER SYSTEMS
1. What are the chief causes of over voltages in electric power system?
1) Lightning over voltages (Natural causes)
2) Switching over voltages (system oriented causes)
2. How are switching over voltages originated in a power system?
Switching over voltages originate in the system itself by the connection and
disconnection of circuit breaker contacts or due to initiation or interruption of faults.
3. What are switching over voltages?
Switching over voltages are highly damped short duration over voltages. They are
temporary over voltages of power frequency or its harmonic frequencies.
Ø They are sustained or weakly damped
Ø They originate in switching and fault clearing process.
4. For ultra high voltages, perhaps, switching surges may be the chief condition for design
considerations. Why?
The magnitudes of lighting voltages appearing on a transmission line do not depend on
line design hence lightning performance does not improve with increasing insulation level, that
is, the system voltage. On the other hand switching over voltages is proportional to operating
voltage. Hence for ultra high voltages switching surges may he the chief condition for
consideration.
5. State the parameters and characteristics of the lightning strokes.
1. Amplitude of currents
2. The rate of rise.
3. The probability distribution
4. Wave shape of the lightning voltage and current.
5. Time to peak value.
6. How are lightning strokes on transmission lines classified?
1) Direct strokes
2) Inducted strokes
Direct stroke:
When thunder cloud directly discharges on to a transmission line tower or line wires, it is
called direct stroke. This is the most severe form and this occurs rarely.
Inducted Stroke:
When thunder storm generates negative charges at its ground end. The transmission line
and Tower develop induced positive charges. Normally lines are unaffected, because they are
insulated by string insulators. However, because of the high field gradients involved, the positive
charge leak from the Tower along the insulator surfaces to the live conductors, after a few micro
seconds, (say). When the cloud discharges through some earthed objects other than the
transmission line, huge concentration of positive charge is left with.
Ø The transmission line and earth act as a huge capacitor.
Ø This may result in a stroke and hence the name inducted lightning stroke.
7. What is Back Flashover?
Some times when a direct lightning stroke occurs on tower if the tower footing resistance
is considerable, the potential of the tower rises to a large value, in view of the huge lightning
stroke current, steeply with respect to the line and consequently a flash over may take place,
along the insulator string. This is known as Back Flashover.
8. Give the mathematical Model for lightning?
Let Io – lightning current (current source)
Zo – source impedance (of the cloud )
Z - object Impedance
V - Voltage built across the object
Then
V = I.Z
= Io (Zo/Z + Zo)Z
= Io. /(1 + (Z / Zo))
Zo = 1000 to 3000 ohms (generally)
Z = object Impedance
Tr line : 300 to 500 ohms
Ground wire : 100-150ohms
Tower : 10-50 ohms
Therefore Z/Zo=less and can be neglected.
Therefore V= Io.Z
Where Io = lightning stroke current
Z = surge impedance.
9. A lightning stroke 10KA strikes a line of 400 ohms surge Impedance.
(I) What is the over Voltage caused?
(II) If a direct stroke occurs over the top of the unshielded Tr line what is the over voltage?
Case :I
V = Io . Z
= 10KA .400 = 4000 kv
Case II
V = Io (Z/2) = (10 x 400 KV)/2 =2000KV
10. What is Thunder storm days?
Thunder storm days (TD) (is known as the Iso Keraunic level) is defined as the number
of days in a year when thunder is heard or recorded in a particular location,
Ø The incidence of lightning strikes on Tr. Line / substation in related to T.D.
Ø T.D is =5 to 10 in Brittan
30 to 50 in USA
30 t0 50 in India
11. What are the Causes for Switching surges?
(1) Making and Breaking of electric circuits.
(2) Initiation or termination of faults.
(3) Energisation and de energisation of cables, capacitors, transformer,Reactors, load etc.
12. What are the effects Switching surges on power system?
Power system has large Inductance and capacitance.
Ø Switching surges may create abnormal over voltages (six times)
Ø Switching surges with a high rate of rise of voltage may cause repeated restriking
of the arc between the CB contacts and damage the contacts.
Ø They have high Natural frequency components and damped normal frequency
component.
13) Explain the Mechanism of Switching over voltage in EHV system?
Switching over voltage are generated when there is a sudden release of internal energy
stored in either in the electrostatic form (in the capacitance) or in the electromagnetic from in the
inductance). This happens where
(1) Low inductive current is interrupted (Transformers and reactors)
(2) Small capacitive current interrupted (Unloaded lines)
(3) Ferro resonance condition
(4) Energisation long EHV lines.
14. What are the measures taken to control/ reduce the switching over voltages?
1. One step or multi step energisation of lines by pre insertion of resistors
2. Phase controlled closing of circuit Breakers with proper systems.
3. Drainage of Trapped charges on long lines (by discharging) before the closing of the
lines.
4. Limiting over voltage by surge diverters.
15. What are the causes for power frequency over voltage in a system?
1. Sudden loss of loads.
2. Disconnection of inductive loads.
3. Ferranti effects and unsymmetrical faults.
4. Saturation in Transformers
16. Name the various methods for protection of Transmission lines against lightning over
voltages?
1. Shielding the over head line using ground wires.
2. Using ground rods and counterpoise wires
3. Using protective devices like expulsion gap, protector tubes, surge diverters at
appropriate places.
17. What is a ground wire in a Transmission System?
Ø Ground wire is a conductor run parallel to the main conductors of the transmission line
supported on the same tower and earthed every equally and regularly spaced towers.
Ø It is run above the main conductors.
Ø It shields the line conductors from induced charges and lightning discharges.
Ø The shielding angle should be less than 30.
18. Distinguish between an expulsion gap and a protector tube/ value type LAS?
In the expulsion gap type there is no non linear resistance.
19. What is the purpose of Insulation coordination?
1. Electric Power supply should ensure reliability and continuity
2. At the same time cost should be low
3. A gradation of system Insulation and protective devices operation is needed, keeping
in view of the importance and cost of equipments, duration of interruption etc. Hence the need
for insulation coordination.
20. The volt ampere characteristics of a non linear resistor used in a surge arrester is given
by:
V = K In
Where K and n are constants
n = 0.5 to 0.6 for silicon carbide
n = 0.02 to 0.03 for ZnO
For Silicon carbide n=0.5 to 0.6, which is not enough to limit the power frequency follow
on current within limit. Hence spark gaps are used. Where as for ZnO, the characteristics is such
that even without the spark gap, the current value can be limited within the value (gapless
lightning arrestors)
21. Give the wave shape of a standard lightening impulse and a standard switching impulse
voltage?
Standard lightning impulse voltage
1) Vp = Peak value, Tolerance ±3%
2) Tf = front time1.2 μs ±30%
3) Tt = tail time: 50 μs ±20%
Standard switching impulse voltage
1) Vp : Tolerance ±3%
2) Tf : 250 ± 20%
3) Tt : 2500 ± 60%
22. For proper protection how should the ground wire be positioned?
1. They should be positioned at a height above line conductors such that they intercept
the lightning stroke.
2. The phase Conductor should be in the protected Zone: within a quarter circle with the
radius = the ground clearance and centre at ground wire
3. The shielding angle should be <300
4. There should be no side Flash over
5. Tower footing resistance should be low to prevent back Flesh over.
23. What are counter poise wires ?
Horizontal wires buried at a depth of 1m in the ground, they may be parallel to the
conductors or radial from the tower footing They are to reduce tower footing resistance.
24. What are ground rods?
Additional rods provided driven into the ground near the tower footing and connected to
the tower footing to reduce the tower footing resistance [15 mm dia, 3.0 m long, 10 to 16 rods]
25. What are the characteristics of an ideal surge diverter?
1) When the line voltage is less than the limiting value the leakage current should be
zero.
2) When the line voltage exceeds the limit, it should offer zero impedance irrespective of
the wave shape, so that the surge voltage is by passed.
3) Immediately after the passing of surge, and immediately after Normal voltage is
returned, it should act again as a perfect insulator.
26. What are the design considerations for LAS for EHV application?
1. Rate of rise of voltage
2. The type of system, whether effectively earthed or grounded through an insulator etc.
3. The operating condition of the arrester.
27. What is valving off voltage?
Below this voltage, the LAS will not conduct. Valving off voltage should be greater than
the Normal voltage (power frequency).Otherwise there will be continuous flow of power
frequency run current and hence heat and destruction.
28) What is the Mechanism of generation of switching over voltage?
Ø Making and breaking of electric circuits of large capacitance andInductance for example
Transmission line.
Ø De energisation of reactive loads like power transformer (unloaded) reactors in
(inductance)
Ø The above constitute, sudden release of internal energy stored in Electrostatic form ( in
capacitance) and in Electromagnetic form (inb Inductance) and causes switching surge
Amplitude : 2 to 3.3 pu
Duration : 1 to 10 ms
The over voltage has high Natural frequency component and damped normal frequency
component.
29. What are the purposes of providing ground wire protection for transmission lines?
1. Ground wires are placed above Transmission line, suitable shielding angle is provided,
when the thunder clouds come near the Tr line, since the base of the cloud has –ve charges, +ve
charges are induced in the ground wire. These induced charges are drawn to the ground since
ground wires are earthed periodically. Consequently potential rise will be small and hence
induced lightning stroke can be prevented. Hence the frequency of lightning stroke is reduced.
2. When lightning strokes, takes place traveling waves are generated, they move.
Correspondingly over voltages are induced in the ground wires, which will moppose the
traveling waves and Flattening of the High voltage curve on the transmission line.
30. Why ground rods are provided?
1) To reduce Tower footing Resistance
2) A number of rods of 15mm dia , 2.5 m to 3.0 m long driven to the ground up to say
50m, in hard soils
3) They are interconnected and connected to the tower footing
4) Reduction in tower footing resistance reduces the surge impedance of the Tower and
back flash over.
31. Give the equivalence circuit of a surge diverter
VTh - Open circuit voltage at junctions.
Zth - Thevinins equivalent Impedance
S - Surge diverter.
32. What are the disadvantages of spark gap surge diverter?
1. Depends on atmosphere conditions
2. Arc cleaning to be done after surge flow.
3. For the same voltage peak, the gap to be set for lightning over voltage is lesser than the
gap to be set for switching over voltage and hence if we set a spark gap surge diverter for
lightning over voltage, frequent flashover occurs for switching surge even if the peak voltage is
lesser than the set value.
33. What are the characteristics of an ideal surge diverter?
1. When the line voltage is less than the limiting value, leakage current should be zero.
Perfect insulator and impedance is infinite.
2. When the line voltage exceeds the limit, it should offer zero impedance and
irrespective of the voltage shape it should by pass the voltage.
3. After the surge is bypassed & immediately after normal voltage is restored, once again
is should act as an insulator.
34) How are the above characteristics are obtained in practice?
One or more air gaps with a suitable non linear resistance can produce the above
characteristics.
ELECTRICAL BREAKDOWN IS GASES, SOLIDS& LIQUIDS
1. Name a few gases used as insulation medium?
N2, CO2, CC2F2 (Freon), SF6 (Sulphur Hexa Fluoride)
2. Name the theories explaining B.D in gaseous insulation?
1) Town sends Theory
2) Streamer Theory.
3. What are the physical conditions governing ionization mechanism in gases dielectrics?
1) Pressure
2) Temperature
3) Electrode configuration
4) Nature of electrode surface
5) Availability of initial conducting particles
4) What is primary ionization?
Electron produced at the cathode by some external means, during its travel towards the
anode due to the field applied, make collisions with neutral atoms/molecules and liberate
electrons & positive ions. The liberated ions make future collisions and the process continue. The
electrons and the ions constitute current. This process is called primary ionization.
5) What is secondary ionization?
Ø The librated positive ions, during the primary ionization process migrate towards
cathode bombard and emit secondary electrons from the cathode.
Ø The excited atoms/molecules, got excited during the collision of initial electrons,
emit photons which bombard the cathode & emit secondary electrons
Ø Metastable (excited particles) bombard the cathode metal surface & producesecondary
electrons
The secondary electrons released as above make ionization collisions & produce additional
electrons. The electrons again produce ionization collisions & the process repeats. This is called
& secondary ionization. The discharge is self sustained because once the secondary electrons are
formed. They take care of the situation. Where there are initial electrons at cathode or not
ionization proceeds. That is called self sustained discharge.
6. Define primary ionization co-efficient .(Town-sends Ist ionization co-efficient)?
The average number of ionizing collisions made by an electron per centimeter travel of
the electron in the direction of the field is called Town-sends Ist ionization co-efficient .It
depends on the gas pressure and E/P
7. What is Town-sends secondary ionization co-efficient ?
It is the net number of secondary ions produced per incident positive ion (γ1) or photon
(γ2) or metastable particle (γ 3)
γ = (γ 1 + γ 2+ γ 3)
γ = f (E/P)
8) What is Town-sends condition for Breakdown?
Town-sends current growth equation is
I = Io e αd/1- γ (eαd – 1)
Town-sends criterion for BD
γ (e αd -1) = 1
Since e αd >>>1
The criterion becomes
γ e αd = 1
Where γ = Town-sends secondary ionization coefficient
α = Town-sends primary ionization coefficient
d = gap
9. What is Spark voltage sparking distance?
We have Town-sends criterion for BD
γ e αd = 1
The voltage applied which creates the above breakdown condition is called spark voltage
Vs and the corresponding gap d is called sparking distance.
10) Demerits of Town-sends theory?
1. Beyond a p.d > 1000 torr cm, this theory does net explain correctly.
2. Town sends theory says that current growth depends on ionization. But actually it
depends on gas pressure and geometry of gap.
3. Town sends mechanism predicts time lag of 10-5 sec. But actually the time lag is 10-8
sec.
4. The discharge form is not as the one predicted by Town-sends theory. It is filamentary
& irregular and not “diffused form” as predicted by town-sends.
11) Streamer theory is based on what?
Ø Streamer theory considers the influence of space charge on the applied field.
Ø Secondary avalanches are produced from the gap
Ø Transformation from avalanche to streamer occurs when the length of avalanche exceeds
a certain value.
Ø Streamer theory overcomes the demerits of Town-sends theory.
12. Explain why Electronegative gas has high BD value?
Ø The molecules of (SF6 gas) electro neg. gases have the property of electron attachment,
(i.e., the outermost orbit of the molecules has holes)
Ø There molecules attach the electrons in the gap to become negative ions
Ø Negative ions have lesser mobility than electron
Ø This attachment plays an effective role of removing electrons which otherwise have led
to current growth and break down
Ø Number of attaching electrons made by one electron drifting 1 cm in the direction of the
field is called attachment coefficient.
13. Distinguish between BD in uniform field and BD in Non uniform field?
1. In the uniform field, increase in applied voltage produces a Breakdown in the gap in
the form of a spark with out any preliminary discharge.
2. In the non uniform field, an increase in applied field, first cause a discharge inm the
gas around the points where the field is the highest. (Eg. Sharp Points, Curves of electrode). This
from of discharge is called corona discharge, which extends finally as the field is increased and
bridges the gap between the electrodes ultimately & cause BD.
14. What are the characteristics of corona discharge?
1. It has bluish luminescence.
2. It produces hissing noise.
3. Air surrounding the corona becomes converted to ozone.
4. Creates loss of Power.
5. Create radio interference.
6. It causes deterioration of the insulation surface.
15. What is corona inception field?
The voltage gradient required to produce visual ac corona in air at a conductor surface is
called corona inception field.
16. Nature of corona on certain configuration of Electrodes?
1. Transmission line D.C
a. When the voltage is +ive
Bluish white sheath over the entire surface of this conductor.
b. When the voltage is –ive
Reddish glowing spots distributed along the length
2. Point to plane configuration
a. When the point is positive
Corona current increases steadily with voltage, after a point current
becomes mpulsed with repetitive frequency of 1 KHz, the burst composing of
small bursts, burst corona, ultimately leading to BD.
b. When the point is -ive
Corona appears as current pulse called Trichel pulses. The reception
frequency is proportional to applied voltage and inversely proportion to pressure.
3. Sphere-Plane Configuration:
a. For small space – Uniform field
b. For fairly large spacing – The field is non uniform
c. For larger spacing - The field is non uniform
Ø Corona inception field is proportional to the diameter of the
Ø sphere.
Ø B.D precedes corona.
Ø Corona is controlled by spacing.
4. Rod to Rod
1. BD Voltage higher when –ive
2. BD voltage depends on humidity of air.
3. The field is highly non uniform.
5. Sphere to sphere gap
1. Field is uniform up to the paint gap <d2
2. BDV does not depend on humidity and voltage wave form.
3. Formative time lag is small.
4. Used for HV measurement.
17. What is Paschens Law?
Paschens law explains the relationship between the Break Down voltage and the product
of pressure (p) and gap (d), in the case of Breakdown in gas.
It states that,
V = f (p.d)
The Breakdown voltage is a function of p.d.
Derivation
We Know
Condition for BD as per Town sends theory is
γ (e αd -1) = 1
We know
α = f1 (E/p)
of
γ = f2 (E/p)
E = v/d
Substituting we have
f2 (E/p) [e f1(E/p] – 1 = 1
f2 (V/pd) [e f1(V/pd] – 1 = 1 ------- eq.1
eq.1 shows the relationship between V and pd.
ie V = f (p.d)
18. What is Vaccum?
Atmospheric Pressure = 760 torr
High Vacuum = 1 x 10-3 to 1 x 10-6 torr
Very high Vacuum = 1 x 10-6 to 1 x 10-8 torr
Ultra Vacuum = 10 x 10-8 torr & below
For electrical Insulation purposes
Vacuum => High Vacuum
=> 1 x 10-3 torr to 1 x 10-6 torr.
19. Basic of BD in Vacuum?
Ø There is no gas molecule in vacuum
Ø No collision – the initial electron crosses the gap without any collision.
Ø Hence BD not possible, (theoretically)
Ø But actually when applied voltage is very high somehow beyond a very high applied
voltage due to some or other reasons gases are librated inside the chamber causing
BD.
20. What are the various factors affecting B.D. in vacuum medium?
Ø Gap length
Ø Geometry & material of electrode.
Ø Surface uniformity of the electrode
Ø Treatment of the surface (Surface treatment)
Ø Presence of extraneous particles & residual gas pressure in the gap.
21. Name the various mechanisms explaining Vacuum Break Down?
Ø Particle Exchange Mechanism
Ø Field emission Mechanism
• Anode heating Mechanism
• Cathode heating Mechanism
Ø Clump theory
22. What is Time lag for Break Down?
The time difference between the instant of applied voltage and the occurrence of
breakdown.
23. What are the requirement of gases for insulation purposes?
1) High dielectric strength
2) High thermal stability
24. What property of SF6 gas is not favorable in electrical approach?
It is not environmentally friendly and it causes global warming. Hence SF6 is used along
with Air or other suitable gases.
25. Distinguish between the BD in pure liquid & commercial liquid?
Pure liquid
Ø Theoretically only possible (hypothetical).
Ø BD is due to Electronic breakdown involving commission of electrons at fields
greater than 100 KV/cm. Town-sends type of primary ionization & secretary
ionization can be applicable.
Commercial liquid contains suspended particles, bubbles of air or liquid etc. BD Mechanisms
are influenced by these impurities.BD depends on several factors
Ø Nature & condition of electrodes.
Ø Physical properties of liquid.
Ø The impurities present in the liquid.
Ø No single theory can explain the BD.
26. What are the parameters that alter the BD strength of liquid dielectrics?
Ø Physical properties like pressure, temperature.
Ø Dissolved impurities
Ø Suspended particles.
Ø Nature & conditions of electrodes
27. Name a few liquid dielectrics?
1) Transformer oil
2) Synthetic hydro carbons – (Polyolefin’s)
3) Chlorinated hydro carbons: P.C.B. (Toxic)
4) Silicone oils. Alternative to PCB
5) Esters
1) Natural Esters : Castor oil
2) Organic Ester & Phosphate esters (synthetic Esters)
6) Hydrocarbons tetrachloro ethylene & per fluro poly ether.
28. Qualities of good dielectrics (liquid)?
1. High heat transfer capacity
2. Good dielectric strength
3. Good chemical satiety
29. BDV of pure liquid depends on what factors?
BDV of pure liquid depends on
Ø Field applied
Ø Gap separation
Ø Cathode work function
Ø Temperature
Ø Density
Ø Viscosity
Ø Temperature of liquid
Ø Molecular structure
30. What are the various theories of BD of commercial liquids?
Ø Suspended particle mechanism
Ø Cavitations and bubble mechanism
Ø Thermal mechanism of breakdown
Ø Stressed oil volume theory
31. What is the principle of stressed oil volume Theory in Breakdown liquids?
The BDV of liquid dielectric depends on the region which is subjected to the highest
stress and the volume of liquid contained in the region.
32. What are the characteristics of a good solid dielectric?
1. low dielectric loss
2. high mechanical strength
3. free from gaseous inclusions
4. free from moisture
5. resistance to thermal & chemical degradation
6. High BD Strength.
33. How can solid dielectrics be classified?
1. Organic dielectric
e.g. Paper, Wood, rubber
2. In organic dielectric
e.g. mica, glass, porcelain, p v c, epoxy resins, Perspex.
34. What are the various BD Mechanisms for solid dielectrics?
1. Assuming no external influences
1. Intrinsic BD
o Electronic BD
o Avalanche BD
2. Electro Mechanical Fracture Mechanism
3. Thermal BD
2. Considering the External Influence
1. Chemical BD
2. BD due to Tracking & Treeing
3. BD due to internal discharge.
35. The usual Mechanism of BD in solid dielectric?
The usual Mechanism is Thermal BD.
36. What is the cause for long term deterioration & BD in solid dielectrics?
The long term deterioration & BD in solid dielectrics is due to Internal discharges.
37. What is meant by Intrinsic strength of a solid dielectric?
All extraneous influences have to be isolated and the BD value which depends on the
structure of the materials and the temperature is called intrinsic BD strength of solid dielectric.
eg. Poly vinyl Alcohol
at – 1960c : 15 MV/cm (Intrinsic)
&
at Normal. Temp : 5 MV/ cm to 10 MV/cm
38. What is ‘TRACKING’ and ‘TREEING’ is solid dielectric B.D?
TRACKING:
Formation of a continuous conduction path across the surface of the insulation mainly
due to surface erosion under voltage application is called ‘Tracking’. Water -Conduction pathheat
-Drying - Conduction film - Carbonization -B.D
TREEING
The spreading of spark channels during tracking in the form of the branches of tree is
called Treeing.
GENERATION OF HIGH VOLTAGES AND CURRENTS
1. What is the necessity for generating high voltages?
Ø Applications like electric microscope, X rays, particle accelerators, Electro
static precipitators etc.
Ø Testing power apparatuses.
Ø Insulation testing.
2. What are the various methods available for generating High DC voltage?
1. Half & full wave rectifiers.
2. Voltage multiplayer circuits.
Ø Cockcraft Walten Circuit
Ø Delta-tran/Engi-tran
3. Van de graaff generators.
4. Electro static generators.
3. What is the expression for average ripple in a Cock- craft Walten voltage multipler?
δ v = I[ 2n(2n+1)]/ƒC 2
Where
δ v = total ripple
I = the current
n = number stages
f = frequency of input voltage.
4. What is the expression for regulation (drop in voltage) Δv in a cock croft Walten
method?
PREPARED BY S.SARAVANAN AND M.SURESH, KONGU ENGINEERING COLLEGE, PERUNDURAI Page 27
Δv = 2/ ƒc [(2n3 /3) + (n2 /2) – (n/6)]
Where
Δv = voltage drop
f = frequency
c = capacitance
n = number of stages
5. Give the expression for optimum number of stages in a cock croft Walter voltage
multiplier?
Where
V max = Maximum of voltage
f = frequency
c = capacitance
I = current
6. State the principle of Van de Graaff generator ?
Mechanical energy is directly converted into electrostatic, electrical energy (without any
electromagnetic conversion, as in the case of an electromagnetic machine like synchronous
generator).
7. Express lighting over voltage in exponential form?
V = V0 [ e-α t- e- βτ]
(where α, β are constants of μs value)
8. How Impulse voltages are produced in the lab?
Capacitors previously charged to DC voltage is discharged into a wave shaping network
(LR, R1 R2, R3 or other combination) by closing a switch. This gives the desired output (double
exponential wave).
9. What is the principle of Marx circuit?
A bank of capacitors are charged in parallel and then discharged in series into a wave
shaping network to produce a lighting impulse voltage, double exponential fast rising & slow
decaying voltage.
10. How switching Impulse voltage can be produced in the lab?
1. Impulse generator circuits can be used by suitably modifying the R1 & R2.
2. Power Tr or Testing Tr, excited by dc voltages giving oscillatory wave (Tesla tal)
MEASUREMENT OF HIGH VOLTAGES AND CURRENTS
1. What is the Specialty of high voltage / current measurement?
1. Safety of men & materials.
2. Accuracy
3. Induction of over voltage, due to stray coupling.
4. Proper location.
5. Linear extrapolation not valid.
6. Electro magnetic interference.
2. Different devices used for High DC voltages?
1. Series resistance micro ammeter.
2. Resistance potential dividers
3. Generating of Voltmeters
4. Sphere gap & Spark gaps.
3. What are the various methods used for measurement of power frequency AC voltages?
1. Series impedance ammeter.
2. Potential dividers, resistance or capacitive Type.
3. Potential Transformers electromagnetic or C. V. T.
4. Electrostatic voltmeters.
5. Sphere gap.
4. What is the method available for measurement & High frequency AC voltages or
Impulse voltages or other rapidly rising voltages?
1. Potential dividers, resistance Type or capacitance Type with CRO.
2. Peak Voltmeter.
3. Sphere gaps.
5. What are the various methods available for measurement of High direct currents?
1. Resistive shunt with mille ammeter
2. Hall effect generators,
3. Magnetic links
6. What are the methods available for measurement of High alternating current?
1. Resistive shunts with milli ammeters
2. Electro magnetic current Transformers.
7. What are the various methods available for measurement of High Impulse currents or
High frequency ac or fast rising ac?
1. Resistive shunts
2. Magnetic pot cut meter.
3. Magnetic links
4. Hall Effect generators.
5. Faraday generators.
8. What are the limitations of resistance potential dividers?
1. Power dissipation
2. Source loading
3. Temperature effect & long term stately
4. Sensitivity to Mechanical strain.
5. Direct connection to HV terminals.
9. What is the principle of generating voltmeters?
It is a variable capacitance electrostatic voltage generator, generating current proportional
to the applied voltage. It does not absorb power from the voltage measuring source. It is driven
by external synchronous / constant speed motor.
Q = charge
C = capacitance
V = voltage applied
I = current
Q = C.V
i = dq/dt = d/dt (CV) = c (dv/dt) +v (dc/dt)
For a dc voltage, V is constant
Then i = V (dc/dt)
Capacitance is changed by rotating the dielectric materials with a suitable shape by a
constant speed motor.
10. What are the advantages of generating voltmeters?
1. No source loading
2. No direct contact with HV terminals.
3. Scale is linear & extension easy.
4. Very convenient, to measure voltages for Van de graaff generators.
11. What are the limitations of generating voltmeters?
1. They require calibrations
2. Careful construction necessary
3. Disturbance in mounting make calibrations invalid.
12. What is the condition for using a generator volt meter to measure power frequency
High voltage?
[The speed of the synchronous motor = Speed of the supply voltage.
Or = ½ the speed of the supply voltage). Here speed means angular velocity
13. What is the principle of electrostatic voltmeter?
In an electrostatic field in a parallel plate configuration consisting of two electrodes,
when one of the electrodes is free to move, it will experience a force when voltage is applied and
the force is proportional to the squire of the applied voltage. The force can be measured & from
the force, the voltage applied can be calculated using the calibration chart.
F = - δW/δs
= - δ/δs (1/2 CV2) = - 1/2 V2 (δC/δs)
= - 1/2 V2 (δ/δs) (Aε0/s) = -1/2 V2 ε0A (1/s2)
F α(V/s) 2
Since F is proportional to square of voltage applied, it can be used for ac & dc voltage
measurement.
14. What is the principle of a ‘Sphere gap’ for measurement of High voltages?
A uniform field sphere gap will always have a spark over voltage within known tolerance
under constant atmospheric conditions. Hence it can be used for measurement of peak value of
the voltage. It is independent of the voltage wave form and hence suitable for all types of wave
forms, from dc to Impulse voltages of short rise times (Rise time < 0.5 μs) and ac voltages up to
1MHz frequency.
15. What are the factors influencing the spark over voltage of a sphere gap?
1. Nearby earthed object
2. Atmospheric conditions and humidity
3. irradiation
4. Polarity and rise time of voltage wave forms.
16. What is the principle of Regowski Coil?
If a coil is placed surrounding a current carrying conductor, the voltage signal induced
nin the coil.
V2(t) = M d/dt ( I(t) )
When V2(t) is passed through an Integrating network then
Vm(t) = M (1/RC)∫dI(t)/dt = M I (t)/RC
When R & C are those of the Integrating circuits.
i.e. Vm(t) = K I (t)
Thus by measuring Vm(t), we can calculate I(t), the current.
17. What are the qualities of a CRO used for measurement of impulse voltage
measurements?
1. Sealed tube, hot cathode Type
2. Photo graphic arrangements for recording wave forms.
3. Input voltage range from 5mv/cm to 20v/cm.
4. Probes and attenuators to handle signals up to 600v.
5. Band with and rise time of the CRO should be adequate.
6. Rise time 5 n.s. & band width as high as 500 MHz. may be necessary.
7. Oscilloscopes are fitted with good cameras for recoding purposes, with writing speed
9cm/n.s.
8. Normally provided with internal & external triggering facility.
9. It is necessary that oscilloscope time base should be started before the voltage reaches
the deflecting plates. Such facility should be available.
18. What are the qualities of instrument lead and management of Test circuits during
Impulse measurement?
The instrument leads and connections etc should be so made that the induced voltage due
to stray Pick ups, due to electro magnetic influences are avoided.
19. Compare the performance of a potential Divider and a generating Voltmeter
for measurement of DC Voltages?
Potential Dividers
Ø Direct contact with HV
Ø Source loading
Ø Power dissipation.
Ø 4They require calibration
Generating Voltmeter
Ø No direct contact
Ø No source loading
Ø Does not absorb power from voltage measuring source
Ø Scale is linear & extension range easy.
Ø They require calibration
Ø Careful construction necessary
Ø Disturbance in mounting make calibration invalid.
20. What are the advantages of CVT, (capacitance voltage transformer)?
1. Simple & easy to install
2. Can be used for relaying also.
3. Can be used as a coupling capacitor for PLCC & relaying.
4. Frequency independency (unlike a conventional electromagnetic potential
Transformer).
5. Provides isolation for HV & LV.
21. What are the demerits of a CVT?
1. Introduces Ferro resonance in Power system.
2. Voltage ratio is susceptible to Temperature variations.
22. What are the different types of shunts used for impulse voltage measurement?
1. Bifilar flat strip
2. Coaxial shunt
3. Squirrel cage shunt
23. What is the principle of a Faraday generator in measurement of high current?
Where a linearly polarized light beam passes through a transparent crystal in the presence
of a magnets field, the plane of polarization of the light beam undergoes rotation. The angle of
rotation is proportional to the magnetic field
α = V B. l
Where
α = Rotation of the plane
V = Constant
B = Magnetic Field
l = Length of the crystal
Where
B is made to be produced by the current to be measured I (t).
α = K I (t)
24. Explain the function of a delay cable in the HV measurements using a CRO?
With rapidly changing signals, the CRO time base should be started before the original
signal reaches the CRO y plates otherwise, the signed may be missed by the CRO. Therefore
while measurement, using a CRO, the Triggering of the CRO time base is done directly (X plate)
immediately and the signal proportional to voltage /current to be measured is sent through a
delay cable to the vertical (Y) plates so that the required delay is obtained. Delay cable may be a
long inter connecting cable 20 to 50m long.
25. What are the criteria required to assess the potential dividers used for High Impulse
voltages?
1. The shape of the voltage in the test arrangement should be transferred with out any
distortion to the LV side.
2. Simple determination of the transfer function is to be ensured.
3. They should be suitable for Multi purposes, namely: power frequency, switching
Impulse, lighting Impulse voltages etc. That is the dividers should have broad band widths.
HIGH VOLTAGE TESTING OF ELECTRICAL APPARATUS AND
INSULATION CO ORDINATION
1. What are the necessities of High voltage testing?
1. To check whether they are as per the design and as per specifications and
standards.
2. To ensure that the HV equipment is able to withstand over voltages produced
naturally or within the system.
2. What is the specialty of HV Testing?
1. The H.V. lab requires higher space.
2. Special equipments are required.
3. Special Techniques are required.
3. Name how standards for HV Testing
1. B I S - Bureau of Indian Standards.
2. I E C - International Electro Tech. Commission.
3. B S I - British Standard Institution.
4. I E E E - Instituting Electrical & Electronics Engineering.
5. I S O P - International Standards Organization.
6. A N S I - American Standards Institute
7. C I G R E - International council on large electrical system.
8. ISS - Indian Standard Specifications
4. Definitions:
1. What is disruptive discharge voltage?
The Voltage that produces loss of dielectric strength of equipment is called
disruptive discharge voltage. In solid-it is called puncture. In liquid or air-it is called
Flashover.
2. What is Flashover?
When a loss of dielectric strength occurs inside a liquid or gaseous insulation or
along the surface of a solid Insulation, it is called flashover.
3. What is Puncture?
When a loss of dielectric strength occurs inside a solid it is called puncture.
4. What are self restoring and Non self restoring insulation?
Insulation which completely regains its dielectric strength after a disruptive
Discharge is called a self restoring insulation. Insulation which does not regain its
insulating property after a disruptive discharge is a Non self restoring insulation.
5. What is withstand voltage
Withstand Test is a Test in which the specified voltage is applied to the test object
under specified conditions to check whether the equipment withstands W/o. any
discharge/ flash over . The test voltage which is applied to a Test object in a withstand
Test is called withstand voltage. It is the voltage that the equipment is capable of
withstanding under specified conditions.
6. 50% Flashover voltage
The Test voltage which has 50% probability for flashover is called 50% flashover
voltage.
7. 100% Flashover voltage
The test voltage which causes flashover of the test object at each of its
application.
8. Creepage Distance:
It is the shortest distance on the contour of the external surface of the insulator
that is between the two metal fittings on the insulator
9. AC Test Voltage
Alternating current voltage of frequency 40 to 60 Hz, approximately sinusoidal
(7% deviation is permitted) is called AC Test voltage.
10. Impulse voltage
It is a fast rising slow decaying voltage, characterized by its peak value, time to
front and time to half value.
Standard Impulse Voltage
1. Peak : Tolerance ±3%
2. Time to Front : T f 1.2μsec ±30%
3. Time to half value : Tt 50μsec ±20%
Standard Switching Voltage
1. Peak : Tolerance ±3%
2. Time to Front : 250 μs ± 20%
3. Time to half value : 2500μs ± 60%
11. How are the Testing of insulators classified?
1. Type Test
Done whenever a new brand is introduced and a new design is adopted.
2. Routine Test
Whenever the quality of the individual equipment is to be established say at the
time of purchase.
12. What are the various High voltage Tests done on insulators?
Ø Power frequency Flashover Test Dry
Ø Power frequency Flashover Test Wet
Ø Power frequency Withstand Test (One Minute) Dry
Ø Power frequency Withstand Test (One Minute) Wet
Ø Impulse withstand Test Dry
Ø Impulse Flashover TestDry
Ø Pollution Testing (Power Frequency)
Ø Partial discharge Test
Ø Radio Interference Test.
13. What is meant by atmospheric correction with reference to High Voltage Testing?
Normally HV Tests are done under Normal Temperature, pressure & humidity conditions
and then the values are corrected to the following conditions.
Temp : 27oC
Pressure : 1013 Millibar 760 torr
Absolute humidity : 17gram/m3
This is done by applying the following correction factors.
h = humidity correction factor
d = air density correction factor
If
Va = Voltage under Test conditions
& Vs = Voltage under reference atmospheric candidate
Then
Vs = Va x h/d
d = 0.289 b/ (273+t)
Where
b = atmospheric Pressure in millibar
t = atmospheric temp in degree C.
h = Can be obtained form graph. (Humidity / Dry bulb thermometer reading)
14. What are the various HV Test done on Bushings?
1. Power frequency Tests
Ø Power factor Test
Ø Partial Discharge Test
Ø 1 Minute W.S. Test
Ø Visible discharge Test
2. Impulse Voltage Test
Ø Impulse with stand Test – Full wave (Positive & Negative Polarity)
Ø Impulse with stand Test – Chopped wave (Positive & Negative polarity)
Ø Switching surge Flashover Test
Ø Impulse Flash over Test under oil.
15. What are the steps for Impulse withstand Test on Power Transformer?
1. Apply one full Impulse of 75% BIL of Power Transformer
2. Apply one full Impulse of 100% BIL of Power Transformer
3. Apply Two chopped wave of 100% BIL
4. Apply one full wave of 100% BIL
5. Apply one full wave of 75% BIL
The Power Tr should stand. Then, it passes the Test.
16. What are the various HV Tests done one circuit Breakers?
1. Power frequency WS Test, 1 minute, dry.
2. Power frequency WS Test, wet
3. Impulse voltage WS Test dry.
4. Switching impulse WS Test
(The above Tests are done with both circuit Breaker Open & closed condition).
17. What are the various Tests (HV Tests) done on surge diverters?
1. Insulation withstand Test, power frequency both dry and wet.
2. Power frequency voltage spark over Test.
3. Standard Impulse voltage spark over Test.
4. Front of wave voltage spark over Test.
5. Switching Impulse voltage spark over Test.
6. Residual voltage Test.
7. Current Impulse withstand Test.
o High current
o Long duration.
8. Pressure relief Test (When fitted)
9. Pollution Tests
18. What is the necessity for measurement of RIV?
Sometimes electrical equipment like power Transformer, conductors, rotating machines
etc. produce unwanted electrical signals in the radio frequency range of 150k Hz to 30 M Hz,
where as the power frequency being 50 Hz. These signals affect the communication systems &
should be prevented. Hence RIV measurement is necessary.
19. What is meant by insulation co-ordination in EHV power system?
Insulation co-ordination is the grading of the insulation level of
(1) Various equipments in a power system
(2) Various parts of the equipments
(3) Protection devices in such a way that, in the event of a serious over voltage, less vital,
less important, less costlier, easy to repair equipment/part of equipment breaks down first
and thereby avoiding major breakdown & interruption to consumers, cost of replacement
etc.
For e.g.
1. In the event of an over voltage, a string insulator on Transmission line should
breakdown before the bushing of a power Transformer.
2. The bushing of the power Tr. should breakdown first before the Breakdown of the
winding of the Power Transformer.
20. What is system protection level and its selection depend on what factors?
In the power system, system protection level is established considering the,
1. Location of the station
2. Protection level of arrester
3. Line shielding
21. What is BIL?
The basic insulation levels are reference levels fixed by standards for each voltage levels.
Basic impulse levels are reference levels expressed in terms of impulse crest voltage(Vp) with a
standard lightning impuse voltage(1.2/50 micro seconds wave) for any apparatus the insulation
level as demonstated by suitable tests should be greater than or equal to the BIL
22. While selecting an equipment for a power system what should he its BIL when
compared to the system protection level?
For any equipment insulation level should be more than the BIL.For proper insulation
coordination its insulation level should be greater than the system protection level over the
margin determined by the following factorS
1. Atmospheric Condition
2. Station Location
3. Protection level of arresters.
4. Importance of the equipments etc.
Hence the system protection level will be less than BIL
**********ALL THE BEST*********
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