Week-High voltage engineering presentstion

1. Flashover of non-uniform (NU) gaps: the polarity effect

2. V-I characteristics of electrical discharge
• V-I graph is strictly valid for DC discharges in gas discharge tubes
• however, similar phenomenon occur in case of AC flashover in air
• not realizable in one single setup
• explains different stages of discharge with I ranging from nA to kA

3. V-I characteristics of electrical discharge
• electrons from cosmic radiation accelerate & cause
ionization
• electron avalanche—>secondary emission—> Townsend
discharges
• space charge effect is initially negligible
• with increasing current, space charge becomes important
• space charge sheaths formed & transition to glow occurs

4. V-I characteristics of electrical discharge
• glow transition is often referred as ‘breakdown’
• With increase in current, transition from a normal glow to an
abnormal glow occurs
• plasma heating & transition to an arc discharge occurs
• Arc is marked by -ve resistance region (low V and high I)

5. Corona discharges
• In uniform gaps, V at which ionisation starts = BDV (3 kV/mm
for air)
• in non uniform gaps, the Emax occurs near electrodes of
smaller ‘r’; ionization threshold gets exceeded only in these
areas
• Partial discharge or ‘corona’ occurs thereafter
• Corona is a self- sustaining discharge (f = 20 Hz to 20 kHz)
• it is visible as bluish/violet colour and audible with a hissing
noise

6. Corona discharges
• Corona is a luminous discharge formed due to ionization of
the air surrounding an electrode, caused by a voltage gradient
exceeding a certain critical value.
• These discharges generate light, audible noise, radio noise,
and energy loss among other things.
• undesirable effects of corona
‣ pre-cursor to flashover
‣ radio interference
‣ additional power loss
‣ insulation degradation due to UV radiation from corona

7. Corona discharges
More details
A variety of forms of corona discharge, from
various metal objects. Notice, especially in the
last two pictures, how the discharge is
concentrated at the points on the objects.

8. Corona on cylindrical conductors
a : 2cm, b: 12 cm, V: 150 kV
- gap between conductors: 10 cm
- avg. E: 15 kV/cm
- actual field variation Ea is
field strength exceeds 30 kV/cm
in the shaded region
- In this region the air gets ionized
- partial breakdown or corona
starts

9. Peek’s empirical equation for visible corona onset at the surface of
the HV conductor in coaxial cylinders:
E =E0mδ(1+K (δ.a)−s) kV/cm
where E0 = 30 (units: kV/cm)
m = surface roughness factor (0.7 to 1)
a = radius of the inner conductor
K and s are constants (K=0.3, s=0.5)
and δ is air density factor given by
where,
p: ambient pressure
p0: normal pressure (1 bar)
t: ambient temperature
t0: normal temperature (20°C)
Corona at sharp points
A sharp point can be
modelled as a small
sphere (radius a)
If ground is very far from
the sharp point, then
Esharp point = V/a

10. Problems caused by corona
Corona in air appears as bluish luminous discharge with ozone
formation
Corona Interference (Radio Interference Voltage, RIV):
- Corona current pulses produce magnetic and electrostatic fields
- the fields in turn induce high frequency voltage pulses in nearby
radio antennas (Radio interference in the range 0.2 to 10 MHz)
Corona Losses:
- Corona current causes power loss on the line
- During rain, corona forms on droplets on the conductor
- losses of tens of MW can occur on a 500 kV line

11. Measures to curb Corona
Corona is caused by field intensification at sharp points
(small r)
- Sharp edges and points on HV hardware must be avoided
- For EHV lines bundled conductors (4 per phase) are used
- On the 800 kV lines (UHV) 6 or 8 conductors are used
- bundle conductors result in large radius thus minimizing
losses
- Grading rings fitted at insulator-Line joints reduces corona

12. Measures to curb Corona

13. Breakdown in Vacuum as Electrical Insulator
Conductive matter that enables breakdown in vacuum
insulation can come from several sources
• Semi-conductive surface oxides from the inner surfaces
of the vacuum chamber
• Impurity concentrations, such as, adsorbates, dust
• Organic vapours from, e.g., grease and rubber O-rings
The breakdown strength of vacuum depends strongly
upon the magnitude of pressure.

14. Breakdown characteristics with ac power frequency voltage, measured at
different pressures for an electrode system with weakly non-uniform field

15. Generation of High Voltages (AC)in lab
- HVAC is generated in lab for insulation testing purpose
- primary source of power is at 230 V (120 V), 50 Hz (60 Hz)
- load impedances involved are in Mega ohm range and currents
< 1 A
- hence, High Voltage testing need not require high power ratings

16. Methods to generate high AC voltages
are:
• Cascade transformers (power freq)
• Resonant transformer (power freq)
• Resonant transformer (high freq:
Tesla Coil)
Generation of High Voltages (AC)in lab

17. Cascade Transformers
- for V < 400kV, a single transformer can be used for testing
- for voltage > 400 kV, single transformer is usually avoided
‣ insulation problems
‣ expensive
‣ Transportation and assembly become difficult
- Cascade or series connection of transformers is a better choice

18. Cascade Transformers
• 2 or 3 identical transformers in
cascade
• secondary HV windings in series

20. - Each transformer has LV, HV and tertiary or excitation windings
- tank of 1st transformer, T1, is at ground potential
‣ Excitation winding of T1 is connected to LV winding of T2
‣ Ratings of Exc.winding (prev T) and LV winding (next T) are
similar
‣ HV winding and excitation winding are taken through a bushing
Circuit configuration

21. Circuit configuration
- HV of T1 and secondary of T2 connected to tank of T2
- Tank potential of T2 is V and hence, it is kept on an insulator
- HV of T2 and secondary of T3 connected to tank of T3
- Tank potential of T3 is 2V and kept on a much higher
insulator
- All three HV windings are in series
- The output Voltage at T3 secondary is 3V w.r.t. ground

22. If P is the rated KVA of the cascade set, then P = 3VI
each of the HV windings will thus carry a current of, I = P/3V
Each of the secondary windings carry 1/3rd power i.e. P/3
Load distribution in Cascade
Transformer

23. - Primary of T3 is loaded with P/3 —>supplied by exc winding of T2
- Next, primary of T2 supplies 2/3rd P (P/3 of sec + P/3 of exc)
- Finally, primary of T1 supplies full power P
- i.e. heavy loading of primary and excitation windings for lower
stages; a factor to be considered in designing the cascade set
- Load distribution in Cascade Transformer

24. ( R) x Mea su r ed volt a ge a t T1
- o/p voltage at T3 =
Cascade ratio
secondary (kV)
- In the three stage transformer case discussed, R=3 (practically,
slightly >3)
- Usually, load that comes on cascade set is capacitive
- for capacitive load, full load voltage > no-load voltage
- for a cascade set, this “loading effect” will be more
Output Voltage of Cascade Transformer: ratio effect
Unity PF
Lag
Lead

25. Advantages and Disadvantages of Cascade Transformer
• Disadvantage
- Expensive and requires more space
• Advantages
- Natural cooling is sufficient
- The transformers are light and compact
- Transportation and assembly is easy
- Construction is identical for individual units

26. High Voltage Lab, IISc
Each transformer is rated at 50 Hz, 2.3 kV/350 kV
Secondary of each transformer has a measurement winding (1:1000)
An alternator (1 MVA, 2.3 kV) feeds primary of first transformer
1963 2023

27. Cascade Transformer connection at High Voltage Lab, IISc

28. Generation of High Alternating Voltages in Lab
Methods to generate high AC voltages are:
• Cascade transformers
• Resonant transformer (power freq)
• Resonant transformer (high freq: TESLA COIL)

29. Circuit basically consists of
• an HV transformer (test transformer)
• adjustable inductance
• Shunt capacitance across output terminal (bushing + test
object)
Series Resonant Transformer

30. Series Resonant Transformer
Generally used for
• testing cables
• dielectric loss measurement
• capacitive load tests

31. Series Resonant Transformer
• Series resonance occurs at power frequency if ωL = 1/ ωC
• Current is limited by circuit resistance
• Voltage across test object will be purely sinusoidal

32. Series Resonant Transformer
• where R is the series resistance of the circuit
• Inductance can be varied over a wide range depending on
load C
• Under resonance, current in the circuit is V/R
• Vout = Vc =(V/R)(1/ωC)
• where V = applied voltage
• Since at resonance, ωL = 1/ ωC
• Vout = Vc = (V/R)(1/ωC) = (V/R)ωL = V(1/R)(sqrt(L/C))
= V Qf
• where Qf = quality factor of the circuit (40 < Q < 80)
—>eqt ckt —>

33. Series ResonantTransformer
we have Vout =V Qf
- Vout can be varied by varying input voltage V
- Vout can be varied by varying the inductance (thus Qf)
- if Qf = 40, then Vout = 40 times input AC voltage; HVAC is achieved
Typically for a rated voltage of 500 kV
, current of 5A and
Capacitive load of 1000-60,000 pF
L will be about 470-9500 H

34. Series Resonant Transformer
we have at resonance Vout = V Qf and Qf = XL/R = Xc/R
- Current in test object is in phase with source voltage (since circuit
is in resonance)
- Power required from source is Pin = V I = apparent power=real
power
- (at resonance, apparent kVA is = real power dissipated)
- (Reactive) Power supplied to test object is
Pout = VoutI = QfVI = Qf Pin
- Qf is, therefore, = Pout / Pin = stored energy/dissipated energy
- if Qf = 40, then reactive kVA of load is 40 times the apparent kVA
of input transformer
‣ i.e. small power rating of input transformer is sufficient
Phasor diagram for the power
at resonance
S = Apparent power P = Real
power
Q = Reactive power

35. Series Resonant Transformer
some points to remember
- Qf of series resonant circuit is = Reactive power / Real power
- Qf signifies how much energy is stored compared to that dissipated
- at resonance, inductive reactor supplies capacitive reactor power
- real power dissipated in resistance
= (1/Qf)(Capacitance reactive power)
- input transformer only needs to meet real power losses, which, at
resonance equals the apparent power

36. Series Resonant Transformer
Advantages
- Input power requirements in kV
A = kV
Aload/Quality factor
- pure sine wave output; suppression of harmonics
- if test object fails, resonance is lost & no HV across test object
- simple test arrangement
- lesser weight considerations compared to cascade set
‣ cascade transformer —>10 to 20 kg/KV
A
‣ series resonant circuit —>3 to 6 kg/KV
A

37. Generation of High Alternating Voltages in Lab
Methods to generate high AC voltages are:
• Cascade transformers
• Resonant transformer (power freq)
• Resonant transformer (high freq: TESLA COIL)

38. • Tesla coil is also known as High frequency resonant transformer
• High frequency HV is sometimes necessary (few kHz to MHz)
- to test the power apparatus under switching surges
- to test the insulator flashover
- but HFHV —> causes dielectric loss and heating —> insulation failure
- need to produce damped high frequency voltages
Tesla Coil

39. Main Features……..
- absence of iron core and hence saving in cost and size
- Pure sine wave output
- Slow build-up of voltage over a few cycles and hence no
damage due to switching surges
Tesla Coil

40. Tight mag coupling b/w pri & sec
V
oltage gain —>by turns ratio
Iron core —>low frequencies
Secondary is closed
Standard Transformer Vs Tesla Coil
Relatively loose coupling
V
oltage gain —>mainly by resonance
Air-core —>higher frequencies
Secondary is open
Standard Transformer Tesla Coil

41. Basic circuit of Tesla coil
Tesla Coil

42. Basic circuit of Tesla coil
Tesla Coil

43. Tesla coil in Nikola Tesla Museum, Belgrade, Serbia

44. Basic circuit
Tesla Coil
50 Hz HV
AC
source
Cs = total secondary capacitance to ground
S. gap = spark gap
Modified circuit
- Primary and secondary circuits are RLC circuits with very low R
- dotted lines are not directly visible
- “top-load” is just one plate of capacitor Cs ( a distributed one)

46. Tesla Coil
- charging of primary capacitor Cp by 50 Hz HVAC source
- Capacitor is so chosen that it gets fully charged in every half cycle
- +ve/-ve charging of capacitor will not affect operation of Tesla coil
Cp

47. Tesla Coil
- When Cp is fully charged, spark occurs closing the primary circuit
- Spark gap is adjusted to fire exactly when Cp voltage reaches peak
- charge and discharge of Cp takes place twice in one voltage cycle

48. Tesla Coil
- Cp & Lp form parallel resonant circuit —>HF oscillation results
- Energy dissipated in spark gap causes HF oscillation to decay
- Hundreds of these “damped oscillations” occur per second
- natural resonant frequency is usually between 50 kHz to 400 kHz
- What happens in secondary winding?

49. Tesla Coil
- Primary and secondary circuits are magnetically coupled
- Oscillations in primary will induce similar ones in secondary
- Cs and Ls result in another parallel resonant circuit

50. - Cp & Lp form parallel resonant circuit —> HF oscillation results
- energy transfer occurs from Cp to Lp & back to Cp & back to Lp…due to resonance
- electric field in Cp changes to magnetic field in Lp and back to electric field in Cp at a
rate determined by Lp x Cp
- Ls picks up some energy from Lp each time Lp charges up
- note that primary and secondary ckts resonate at same frequency
- Cs gets electrically charged from Ls as and when Ls discharges at a rate determined by Ls x Cs
- Energy in Ls builds up little by little from Lp in each cycle —> called resonant rise
- Terminal voltage at “top-load” or Cs gets higher on each cycle till a breakdown occurs

51. Tesla Coil – slightly different connection

52. Tesla Coil
- Resonant frequency of the primary must be = that of the secondary
- Oscillations in the primary, induce emf in secondary
- weak magnetic coupling is desirable between pri & sec i.e. the
coupling constant is between 0.05 - 0.2
- Several oscillations will therefore be required to transfer energy
- Strong coupling causes fast voltage rise in secondary, causing inter-turn
spark s

53. - Energy gets transferred →primary to secondary resonant circuit
- Over several cycles, am plitude of prima ry oscillations decreases
and that of secondary oscillation increases
- Decay of primary oscillation →"Primary Ring-down"
- S t a r t of secondary oscillation is →"Secondary Ring-up"
primary and secondary
voltages
Tesla Coil

54. TESLA COIL
- Spark gap in primary stops firing due to decrease in voltage
- Primary ckt is now open & energy is trapped in secondary
- When secondary voltage is high enough, sparks occur at “top load”
- secondary voltage will be a damped oscillation
- Oscillation decays exponentially as charge decays due to sparks

55. Tesla Coil
- Primary capacitor begins to charge again from HV supply
- Whole process of energy transfer repeats
- Energy transfer takes place several hundred times per second

56. Tesla Coil
How does a Tesla coil generates such high voltages?
- HV gain of Tesla Coil lies in transfer of energy from:
‣ Large primary capacitance —->small secondary capacitance
i.e. Energy (pri) = 0.5 Cp Vp² = 0.5 CsVs2 = Energy (secondary)
- for example, if primary capacitor is 47nF and it is charged to 20kV
then stored energy is
Ep =0.5 x 47nF x (20000)² = 9.4 Joules

57. Tesla Coil
- we have, Ep = 0.5 x 47nF x (20000)² = 9.4 Joules
- Secondary stray capacitance is typically around 25 pF
- assuming no loss of energy from primary to secondary, we have
- Ep =Es =9.4J =0.5 x 25pF x Vs²
Vs² = 9.4 / (0.5 x 25pF)
Vs = 867 kV

58. i.e. CpLp =CsLs or Cp/Cs =Ls/Lp
and
V
oltage Gain = sqrt (Cp / Cs) = sqrt (Ls / Lp)
Voltage gain of Tesla coil
- since all primary energy goes into the secondary, we have
Energy (pri) = Energy (sec) = 0.5 Cp Vp² = 0.5 CsVs2
or Vs/Vp = V
oltage Gain = sqrt (Cp / Cs)
further,
- primary and secondary circuits share same resonant frequency fr

59. Tesla Coil
In practice, ‘top load’ voltage << theoretically calculated values
because:
- Energy loss in winding resistances
- Energy loss in primary spark gap (as light, heat & sound)
- EM radiation loss from primary and secondary coils (antennas)
- Corona from the “Top load” to nearby grounded objects

60. Tesla Coil
Distribution of secondary capacitance
The total secondary capacitance Cs is:
Cs =Ct +Cb +Ce
where,
Ct: Top load to Ground capacitance with air as dielectric
Cb: Inter-turn capacitance of secondary winding
Ce: Capacitance between top load and nearby objects/walls
(Significant for long drawn sparks)
All put together Cs will be few tens of pF……but still plays a
crucial role in voltage gain

61. Generation of High Impulse Voltages in Lab
- Disturbances in electric system →transient voltages
- Transient or Impulse voltage magnitudes >> power freq AC voltages
‣ Lightning impulses (shorter duration - 1.2/50 us)
‣ Switching impulses (longer duration - 250/2500 us)
- Power apparatus need to be tested for these impulse voltages
(in addition to power freq AC voltages)
- Therefore, a need arises to generate impulse voltages in lab

62. • Impulse voltage is normally unidirectional voltage
- rises rapidly to a peak value and then falls less rapidly to zero
• Impulse voltage can be
- a full wave
‣ wavefor m appear s completely with out causin g flash over or
puncture on the load side
- ‘tail chopped or front chopped’ wave
‣ flash-over occurs causing the voltage to fall extremely rapidly
‣ used for detection of winding/turn faults in transformers