HVDC generation methods

1. GENERATION OF HIGH DIRECT
CURRENT VOLTAGES
1

2. Unit 1
 Generation and transmission of electric
energy, voltage stress, testing voltages
 Generationof direct voltages – AC to DC
conversion – single phase rectifier circuits –
cascade circuits
 Voltage multiplier circuits – Cockroft-Walton
circuit – voltage regulation – ripple factor
 Electrostatic generators
2

3. Generation and transmission
of electric energy
V kV 400 700 1000 1200 1500
P MW 640 2000 4000 5800 9000
3

4. Major a.c systems in chronological order of their
installations
4

5. HVDC TRANSMISSION
 The first public power station was put into service in
1882 in London (Holborn)
 The first major a.c. power station was commissioned in
1890 at Deptford, supplying power to central London
over a distance of 28 miles at 10 000 V.
 The longest HVDC link in the world is currently the
Xiangjiaba–Shanghai 2,071 km
 Various HVDC links in INDIA are:
 ± 500 kV , 1500 MW Rihand – Delhi HVDC,814km
 ± 500 kV ,2000 MW, HVDC Talchar – Kolar
Transmission Link, 1450km

5

6. HVDC IN INDIA
Back-to-Back
HVDC LINK CONNECTING
REGION
CAPACITY
(MW)
Vindyachal North – West 2 x 250
Chandrapur West – South 2 x 500
Vizag – I East – South 500
Sasaram East – North 500
Vizag – II East – South 500
6

7. 7

8. Latest HVDC systems
 In Northern India, using ±800 kV 3,000 MW Ultra High Voltage
Direct Current (UHVDC) technology (1,365 km transmission
line-to connect Champa (State of chattisgarh), Central India, to
Khurukshetra (State of Haryana))
 The main Alstom units involved in the project will be the HVDC
Centre of Excellence in Stafford (UK), and the Alstom Grid
India units located at Noida, Hosur, Padappai, Pallavaram and
Vadodara in India.
 In India, Alstom has supplied HVDC systems for Vizag (State
of Andhra Pradesh), Chandrapur (State of Maharashtra) and
Sasaram (State of Bihar)
8

9. ABB transformers and other key equipment to enable
Changji-Guquan link to transmit 12,000 megawatts of
electricity over 3,000 kilometers at 1.1 million volts,
setting new world records on voltage level, transmission
capacity and distance
Pioneer of HVDC Solns (Alstom) Ningdong-Shandong
660 kV transmission scheme in China and is currently
working on the Rio Madeira project in Brazil, the world’s
longest HVDC link
9

10. ADVANTAGES
Advantages of dc
transmission
Technical
Advantages
Economic
Advantages
10

11. Advantages Of HVDC
Technical advantages:
 No requirement of reactive power
 Practical absence of transmission line length limitations
 No system stability problems
 Interconnection of asynchronously operated power
systems
 No production of charging current
 No increase of short circuit power at the connection point
11

12.  Independent control of AC systems
 Fast change of energy flow i.e. Ability of
quick and bidirectional control of energy flow
 Lesser corona loss and radio interference
 Greater reliability
 Increase of transmission capacity
 Can be used for submarine and underground
transmission
12

13. Economic Advantages:
 Low cost of DC lines and cables
 Simple in construction
 Low cost for insulators and towers
 Less Line losses
 Transmission line can be built in stages
13

14. Disadvantages Of HVDC
 Use of converters ,filters etc increases the overall cost
 DC circuit breakers are more expensive
 HVDC converters have low overloading capacity
 More maintenance is required for insulators
 Voltage transformation is possible only on AC side
14

15. 15

16. Comparison between the prices of AC
& DC
Transmission
16

17. Major d.c. systems in chronological
order of their installations
17

18. Major HVDC schemes
18

19. Voltage stress
 Operating voltage determines the dimensions of the
insulation which forms part of the generation,
transmission and distribution equipment
 The voltage stresses on power systems arise from
various over voltages. These may be of external or
internal origin
 Their magnitude depends on the rated voltage, the
instance at which a change in operating conditions
occurs, the complexity of the system and so on
19

20. In designing the system’s insulation the two
areas of specific importance are:
 Determination of the voltage stresses which
the insulation must withstand, and
 Determination of the response of the insulation
when subjected to these voltage stresses
20

21. Testing voltages
 Highest voltage of a particular system
 The magnitude and type of test voltage varies with
the rated voltage of a particular apparatus
 All types of apparatus for alternating voltages, direct
voltages, switching impulse voltages and lightning
impulse voltages
 Are laid down in the relevant national and
international standards
21

22. Types of Testing voltages
 Testing with power frequency voltages
 Testing with lightning impulse voltages
 Testing with switching impulses
 Testing with very low-frequency voltage
 D.C. voltages
22

23. Testing with power frequency
voltages
 To assess the ability of the apparatus’s insulation
withstand
 under the system’s power frequency voltage the
apparatus is subjected
 to the 1-minute test under 50 Hz or 60 Hz
depending upon the country
23

24. Testing with lightning impulse
voltages
 The standard impulse voltage has been accepted as
an aperiodic impulse that reaches its peak value in 1.2
μsec and then decreases slowly (in about 50 μsec) to
half its peak value
24

25. Testing with switching
impulses
 The recommended switching surge voltage has been
designated to have a front time of about 250 μsec and
half value time of 2500 μsec
 For GIS (gas-insulated switchgear) on-site testing,
oscillating switching impulse voltages are
recommended for obtaining higher efficiency of the
impulse voltage generator
25

26. Testing with very low-frequency
voltage
 Paper-insulated lead covered cables (PILC)
on-site testing (4–4.5V0)
 Insulation cables (5–8V0)
 On-site testing of cables under very low
frequency (VLF) of approx. 0.1Hz
26

27.  High DC voltages are required
 In the research work of pure and applied physics
 In insulation tests on cables and capacitors
 In impulse generator charging units
 Electronic valve rectifiers are used for generation of d.c voltages up to 100
kV with output currents about 100 mA
Rectifier circuits
Half
wave
Full wave
Voltage
doubler type
27

28. Full wave and half wave rectifier
circuit
• Rectifiers may
be electron tube
or a solid state
device
• Single electron tube of PIV up to 200 kV and semiconductor
diodes of PIV up to 20 kV are available
28

29.  HALF WAVE RECTIFIER:
 Capacitor charged to Vmax , maximum secondary voltage of the hv
transformer
 Peak inverse rating should be at least 2 Vmax
 FULL WAVE RECTIFIER
 Rectifier A and B conducts at either of the half cycles
 Source transformer requires a centre tapped secondary with a rating
of 2V
29

30.  Most commonly preferred diodes for high voltage rectifiers:
 Silicon diodes with PIV of 1 kV to 2 kV
 Selenium element stacks with PIV of up to 500 kV for lab applications
 Both full and half wave rectifiers produce dc voltages less than the
maximum ac voltage
 Ripple or voltage fluctuations present should be kept within reasonable
limits by means of filters
30

31. Ripple Voltage with Half-Wave and
Full-Wave Rectifiers
 With smoothing capacitor, the voltage on no-load for any rectifier
equals the maximum ac voltage
 When loaded, fluctuation δV appears in output dc voltage called the
ripple. With proper choice of filter capacitor, ripple gets reduced
 Ripple depends on :
 1. Supply voltage frequency
 2. Time constant
 3. Reactance of the supply transformer
31

32. Input and output voltage waveforms of half-
and full-wave rectifier circuits
32

33. Voltage doubler circuits
 Rectifiers are rated to a peak inverse voltage of 2 Vmax
 Cascaded voltage doublers are used when larger output voltages are
needed without changing the input transformer level
 Isolating Transformers are used to provide an insulation in case of
cascaded circuits
 Arrangement of filament transformers, capacitors and rectifiers
becomes cumbersome if more than 4V is needed with cascaded steps
33

34. Simple voltage doubler Cascaded voltage doubler
34

35. Waveforms of ac voltage and dc output voltage on no-
load of the voltage doubler
35

36. VOLTAGE MULTIPLIER CIRCUITS
 CASCADED
RECTIFIER
UNIT WITH
PULSE
GENERATOR
• Ripple < 1% ; Load current of about 150 μA
• Pulse frequency = 500 to 1000 Hz
36

37. Schematic
current
waveforms
across the first
and the last
capacitors of
cascaded
voltage
multiplier circuit
37

38. Voltage waveforms
across the first and
the last capacitors of
cascaded voltage
multiplier circuit
38

39. Ripple voltage δV and voltage drop ∆V
in a cascaded voltage multiplier circuit
With load, the output voltage of the cascaded rectifiers is
less than 2n Vmax , where n is the number of stages
39

40. Cockroft – Walton Voltage Multiplier circuit
• First stage i.e., D1,D2,C1,C2 and the
transformer T are identical as the
voltage doubler
• For higher output voltages, the circuit
is repeated with cascade connection
• D1, D3, D2n-1 conduct during +ve half
cycle
• D2, D4, D2n conduct during -ve half
cycle
40

41. Ripple in cascaded voltage
multiplier circuits
 When load current I1 is supplied from capacitor C2 to load RL
during the non conduction period t2, the charge transferred
per cycle is related as follows
41

42. On no-load, the voltages between stages of a cascaded circuit are raised
by 2Vmax , giving an output of 2nVmax for n stages
42

43. Ripple can be reduced if the capacitances C1,C2 are made nC and C3,
C4 are made (n-1)C and so on so that, total ripple =
43

44. Voltage drop on no-load and Regulation
 The change of average voltage across the load from no-load
theoretical value expressed as a percentage of no-load is
called the regulation
 Capacitor C2 is charged to
 Total Voltage drop will be
44

45. Most of the voltage drop is at the lower stage capacitors
45

46.  For larger values of n ( ≥ 5 ) , terms are small and hence
neglected.
 Therefore, the optimum number of stages for a minimum voltage drop
may be expressed as,
 For generation of high dc voltages, one-phase, two-pulse, voltage
multiplier circuit, three-phase, six-pulse , voltage multiplier circuits can
also be used
46

47. A Cockroft–Walton d.c. generator for voltages up to
900 kV/10 mA with fast polarity reversal
47

48. ENGETRON OR DELTATRON
CIRCUITS FOR VERY HIGH
VOLTAGES
DELTRATRON
UNIT
48

49.  Circuit consists primarily of a series connection of transformers, which do
not have any iron core.
 The whole chain of cascaded transformers is loaded by a terminating
resistor; thus the network acts similarly to a terminated transmission line
along which the a.c. voltage remains nearly constant and has a phase shift
between input (high-frequency power supply) and output (termination)
 The disadvantage is the procedure to change polarity, as all modules have
to be reversed
49

50. ELECTROSTATIC
MACHINES
 BASIC PRINCIPLE: In electrostatic machines, charged bodies are
moved in an electric field against the electrostatic field in order that
mechanical energy is converted into an electric energy
 Let, charge density of an insulated belt = δ ; Electric field = E(x) ;
separation between electrodes = s ; then,
50

51.  If belt moves with the velocity , v, then the mechanical power P required
to move the belt is
 Current, I, in the system is given as
 Potential difference, V, between the electrodes is
 Mechanical power P = F. v is converted into electric power P = V.I
assuming that there are no losses in the system
 One such electrostatic generator is Van de graff generator which produces
very high output voltages with small output currents
51

52. Van-de-graff Generator
Outline of belt driven electrostatic generator
52

53.  Generator
 Enclosed in an earthed metallic cylindrical vessel
 Operated under pressure or in vacuum
 Charge is sprayed onto an insulating moving belt from corona points and
is removed and collected from the belt connected to the inside of an
insulating metal electrode. Belt is driven by a motor
 Potential of the high voltage electrode above the earth at any instant is
where, Q =charge stored ; C = capacitance of the high voltage electrode to
earth
53

54.  Rate of rise of potential of high voltage electrode is given as
where I = net charging current
 High voltage electrode attains a steady potential when load current and
leakage current are equal to charging current. Shape of the electrode is
nearly spherical to avoid local discharges
 Charging of the belt is done by lower spray points
 Charge is transferred onto the high voltage electrode from the belt by the
collecting points. Belt returns with charge dropped and fresh charges are
sprayed onto the belt by lower spray point
54

55.  Self charging system is obtained by means of connecting the upper pulley
to the collector needle and hence, higher potential is maintained at high
voltage terminal
 Charges on the returning belt are neutralized by second row of corona
points connected to the inside of the high voltage terminal. Thus for a
given belt speed, rate of charging is doubled
 Van de Graff generators are useful for very high voltage and low current
applications. Extremely flexible and precise machines for voltage control
55

56. 25-MV electrostatic tandem
accelerator
56

57. ELECTROSTATIC
GENERATORS
 Constant voltage variable capacitor machines
 Electrostatic generator consists of a stator with interleaved
rotor vanes forming a variable capacitor and operates in
vacuum
57

58.  Current through a variable capacitor is given by,
where, C is a capacitor charged to a potential V
 Power input to the circuit at any instant is
 If dC/dt is negative, mechanical energy is converted to electrical energy
 Capacitor charged with dc voltage
 Hence, the power output will be
 As the rotor rotates, capacitance C decreases and the voltage across C
increases
58

59. Diagrammatic
cross-section of the
Felici generator
• Suitable for use with particle
accelerator, electrostatic paint
spray equipment, electrostatic
precipitator, X-ray purposes
and testing hv cables
59

60. Sames electrostatic generator
• Vacuum-insulated ‘varying
capacitance machine’.
• Provides a high voltage in the
range up to about 1MV and/or
high power in the range of
megawatts
• Only a reference to this type of
generator might be useful
60

61. REGULATION OF DC
VOLTAGES
 Output voltage of a d.c source changes with the load current as well as
with the input voltage variations. Hence regulator circuit is essential to
maintain a constant voltage
 Allowable tolerance: 0.1% to 0.01%
 Essential Parts of a d.c voltage regulator:
 Detecting element
 Controlling element
61

62. Schematic
diagram of
voltage
stabilizers
• If ∆E0 is the change in E0 as a result of a change of ∆Ei in Ei, then
the stabilization ratio S is defined as,
62

63.  If R0 is the effective internal resistance of the generator, which is given as
where, ∆I0 is the change in load current with the input voltage remaining
constant
63

64. Series type regulator and degenerative
feedback system of voltage regulation
Series d.c voltage regulator
64

65.  V3 provides the reference voltage to keep the cathode potential of the tube V2
constant
 Current through the divider R2, R4 and R5 changes, when E0 changes and hence
the bias voltage of the tube V2 changes in the opposite manner
 R6 together with R5 provides a voltage input to the d.c amplifier in proportion to
fluctuations in the input voltage, in such a way to reduce the effect of variations in
the output voltage.
 Output voltage fluctuations are measured by a potential divider of ratio
 Convenient method of regulation is the degenerative feedback system
65

66. Degenerative feedback system of voltage
regulation
66

67. Output voltage E0 is given as,
67

68. Stabilization ratio can be increased by making β as large as possible
68

69. LINE PROTECTION
69

70. Transmission Line Protection
Transmission lines can vary in length from several hundred feet to
several hundred miles, and in voltage (line-to-line) from 46KV to 750KV.
Construction can be simple, such as a single wood pole with insulators
atop a cross arm, with little spacing between the conductors and from
the conductors to ground. At the other end of the scale are metal lattice
structures with bundled conductors (2 or more conductors per phase) with
large spacing between conductors and between conductors and ground.
.
70

71. Transmission LineProtection

72. Transmission Line
Protection
72

73. Transmission Line Protection
Ice Storm
What Can Go Wrong?
FAULTS (Short Circuits)
Some causes of faults:
●Trees
● Lightning
● Animals (birds, squirrels, snakes)
● Weather (wind, snow, ice)
● Natural Disasters (earthquakes,
floods)
● Faulty equipment (switches,
insulators, clamps, etc.)
73

74. Transmission Line Protection
74

75. Faults
“Faults come uninvited and seldom go away voluntarily.”
Fault Types:
●Single line-to-ground
● Line-to-line
● Three Phase
● Line-to-line-to-ground
75
Transmission Line Protection

76. How Do We Protect Transmission Lines?
A. Overcurrent
B. Directional Overcurrent
C. Distance (Impedance)
D. Pilot
1. DCB (Directional Comparison Blocking
2. POTT (Permissive Overreaching Transfer Trip)
E. Line Current Differential
76
3 Transmission Line Protection

77. Transmission Line Protection
Overcurrent Protection
Non-Directional
Relay responds to overcurrent condition
Instantaneous (IOC) device #50
No intentional time delay
Time Overcurrent (TOC) device #51
Various curve types, including inverse, very
inverse, extremely inverse
77

78. Transmission Line Protection
Over current Line Protection
78

79. Transmission Line Protection
AC Schematic
79

80. 3 Transmission Line Protection
Time Overcurrent Curves

81. Transmission Line Protection
Directional Over current Protection
--Relay responds to overcurrent condition in the
forward direction only (device #67, 67N, 67NT) Will not
respond to reverse faults.
--Compares the current in the line versus a known
reference that will always be the same (such as a
voltage or polarizing current source).
81

82. Transmission Line Protection
82

83. Transmission Line Protection
Directional Overcurrent Example
83

84. Distance Protection
A distance relay measures the impedance of a line using
the voltage applied to the relay and the current
applied to the relay.
When a fault occurs on a line, the current rises
significantly and the voltage collapses significantly.
The distance relay (also known as impedance relay)
determines the impedance by Z = V/I. If the
impedance is within the reach setting of the relay, it
will operate.
84
Transmission Line Protection

85. Transmission Line Protection
Distance Relay
CT and PT
Connections
85

86. Transmission Line Protection
86

87. Transmission Line Protection
Distance Protection
Typical zone reach
settings
87

88. Transmission Line Protection
Distance Protection
When a fault occurs on a
transmission line, the current
increases and the angle of the
current with respect to the
voltage changes to a lagging
angle, usually between 60 to 85
degrees.
88

89. Transmission Line Protection
Distance Protection
The most common characteristic (or
protection shape) of distance relays is
the mho characteristic, a circular type
reach characteristic.
Distance relays have a settable
maximum torque angle (mta), which is
the angle of the current compared to
the angle of the voltage at which the
relay is most sensitive. In the drawing
on the right, the mta is approximately
75 degrees.
89