General Overview of HVDC
Transmission System
WHY HVDC ?
 Asynchronous connection (enables to connect two
different electrical networks having different frequency
& voltage)
 Power flow control (enables the stability of electrical
network)
 Added benefit to the transmission like stability, power
quality etc.
WHY HVDC?
 Environmental advantages (lesser right of way
requirement)
 Lower line losses compared to AC line (no corona &
charging current)
 Economical (only two conductor for transmission &
lesser tower height)
Comparison With HVAC
ITEM
HVAC
1.
Power Transmission Capability Low
2.
Distance
3.
System Connection
4.
5.
6.
Right of Way requirements
Power Control
Features – Frequency Control,
Reactive Power Control,
Damping of Oscillations etc.
HVDC
Limited by Stability
considerations. Switching
Stations required.
Synchronous
High (e.g. 3000 MW
Bipole)
No Limitation. Cheaper
alternative for Long
Distances
Asynchronous
High
No
Not Available
Low
Yes
Available
Comparison With HVAC
ITEM
HVAC
HVDC
7. Tapping of Power Connection
Simple
Costly, Multi-terminal
Scheme required
8. Economical Alternative for
Low to Medium distance,
Medium Power Range.
Contributes to System
SCL
Long Distance Bulk
Power Transmission
Does not Contribute to
System SCL
Relatively Lesser
More Pronounced
Higher insulator creepage
distance is required
9. System SCL (for consideration
in developed AC systems due
to high fault currents)
10 Pollution Effects
.
SO WHY HVDC RATHER THAN HVAC ?

Long distances make HVDC cheaper

Improved link stability

Fault isolation

Asynchronous link

Control of load flow (DC voltage can be exactly
controlled)
Comparison of right of way
400 kV AC Lines
96 m
500 kV DC Line
46 m
Cost comparison of ac and dc
transmission
Cost of AC Line
Cost
Break even
distance
Cost of DC Line
Cost of DC terminal
Cost of AC terminal
 500 – 700 km
Distance in km
Types of HVDC Transmission system
Mono Polar System:
 One pole, one conductor for transmission and current
return path is through earth.
 Mainly used for submarine cable transmission.
Types of HVDC Transmission system
 Bipolar System:
 Two poles, two conductors in transmission line, one positive with
respect to earth & other negative
 The mid point of Bi-poles in each terminal is earthed via an electrode
line and earth electrode.
 In normal condition power flows through lines & negligible current
through earth electrode. (in order of less than 10 Amps.)
Types of HVDC Transmission system
 Homo Polar System:
 Two poles at same polarity & current return path is
through ground.
 This system was used earlier for combination of cable &
over head transmission.
Types of HVDC Transmission system
 Back to Back HVDC Coupling:
 Usually bipolar without earth return.
 Converter & inverters are located at the same place.
 No HVDC Transmission line.
 Provides Asynchronous tie between two electrical
network
 Improves system stability
 Power transfer can be in either direction.
Types of HVDC Transmission system
 Multi Terminal HVDC System:
 Three or more terminal connected in parallel, some feed
power and some receive power from HVDC Bus.
 Provides Inter connection between the three or more AC
network.
 System stability of AC network can be improved.
HOW HVDC WORKS
POWER FLOW EQUATIONS
 FOR AC TRANSMISSION:
POWER(P) =
V1 V2
X
Sinδ
POWER FLOW EQUATIONS
 FOR DC TRANSMISSION:
Vdr (Vdr-Vdi)
POWER(P) =
R
HVDC Transmission Normal Power Direction
Rectifier
Id
Invert
er
Note! Only a small voltage difference
Page 28
HVDC Transmission Reverse Power Direction
Inverter
Note that the current flow is in
the same direction.
Only the polarity is reversed.
(changing a).
Id
Rectifier
Note! Only a small voltage difference
Page 29
Basic Diagram of HVDC System
DC
AC SYSTEM A
TERMINAL A
Ld
TRANSMISSION
LINE
Id
P d = V d Id
TERMINAL B
Ld
Vd
FILTER
FILTER
AC SYSTEM B
6-Pulse Convertor Bridge
Ld
1
E1
Ls
iA
Ls
iB
Ls
iC
3
5
Vd
V'd
4
6
Id
2
Id
12-Pulse Convertor Bridge
Y

12-Pulse Valve Group With AC Filters
Filter
BIPOLE HVDC
MODES OF OPERATION
BASIC HVDC Single Line Diagram
Smoothing Reactor
Thyristor
Valves
DC Filter:
DT 12/24
DT 12/36
DC OH Line
Smoothing Reactor
DC Filter:
DT 12/24
DT 12/36
Converter
Transformer
Converter
Transformer
DC Filter:
DT 12/24
DT 12/36
400 kV
AC Bus
AC Filters
Thyristor
Valves
DC Filter:
DT 12/24
DT 12/36
400 kV
AC Bus
AC Filters
Modes of Operation
Bipolar
Smoothing Reactor
Thyristor
Valves
DC OH Line
Smoothing Reactor
Thyristor
Valves
Current
Converter
Transformer
Converter
Transformer
Current
400 kV
AC Bus
AC Filters,
Reactors
400 kV
AC Bus
AC Filters
Modes of Operation
Monopolar Ground Return
Smoothing Reactor
DC OH Line
Thyristor
Valves
Thyristor
Valves
Converter
Transformer
400 kV
AC Bus
AC Filters,
Reactors
Smoothing Reactor
Current
Converter
Transformer
400 kV
AC Bus
AC Filters
Modes of Operation
Monopolar Metallic Return
Smoothing Reactor
DC OH Line
Thyristor
Valves
Thyristor
Valves
Converter
Transformer
400 kV
AC Bus
AC Filters,
Reactors
Smoothing Reactor
Current
Converter
Transformer
400 kV
AC Bus
AC Filters
HVDC Thyristor Valves
Modern Thyristor Valves
 Valve Modular Design
 Suspended Installation
 Water Cooled
Valve Hall Equipments & Thyristors at
Vindhyachal
THYRISTOR VALVE:
 AIR INSULATED
 WATER COOLED
 DESIGNED FOR INDOOR USE
 OCTUPLE UNIT
(Each Physical Structure Contain
08 Valve Functions)
Valve Hall Equipments & Thyristors at
Vindhyachal

Each single valve has 04 Thyristor
modules connected in series

Each Thyristor Module has 06
thyristors with Voltage Divider &
Control circuits

Each Thyristor Module in a Valve is
series connected to a Reactor

Total no of Thyristors in one valve
hall : 576

Each valve has 1 Thyristor
redundant out of 24
VALVE HALL EQUIPMENTS & THYRISTOR
Thyristor
TCU
Heat sinks
C1
C2
 CONFIGURATION OF ONE THYRISTOR LEVEL
HVDC Vindhyachal Thyristor Rating
 CURRENT RATING:



RATED DIRECT CURRENT: 3600 A
MAX. DIRECT CURRENT AT RATED POWER: 3700 A
MAX. DIRECT CURRENT AT OVERLOAD:
(i) Id MAX FOR 2HR IN 12 HRS. : 4150 A
(ii) Id MAX FOR 5 SEC. IN 5 MIN. : 4650 A
 VOLTAGE RATING:


NON REPETITIVE REVERSE VOLTAGE: 5350 V
NON REPETITIVE FORWARD VOLTAGE: 4850 V
Valve Reactor
Thyristor
Thyristors



The Thyristor is a solid state
semiconductor switching device
with four layers of alternating P
and N type material.
It has three terminals; Anode,
Cathode & Gate
The
Thyristor
continue
to
conduct as long as they are
forward biased ( that is as long
as the voltage across the device
has not reversed).
Anode
P
J1
N
Gate
J2
P
J3
N
Cathode
Thyristor vs Diode
 Like the Diode, Thyristor is also unidirectional
device that blocks current flow from cathode to
anode.
 Unlike the Diode, a Thyristor also blocks
current flow from anode to cathode until it is
triggered by a proper gate signal between gate
& cathode terminals.
Static I-V Characteristics of a
Thyristor
+Ia
Forward conduction mode
Latching current
Reverse leakage current
Holding current
Vbr
-Va
Reverse blocking mode
Forward blocking
mode
V bo
+Va
Forward leakage current
Vbo = Forward break over voltage
Vbr = Reverse breakdown voltage
-Ia
Effect of Gate Current on Forward
Breakover Voltage
+Ia
Ig3>Ig2>Ig1

The effect of gate current on the
forward break over voltage of
thyristor can be understood by the
figure alongside.

For Ig = 0, forward break over
voltage is Vbo

For Ig1>Ig, forward break over
voltage is V1<Vbo


For Ig2>Ig1, forward break over
voltage is V2<V1<Vbo
For Ig3>Ig2>Ig1, forward break over
voltage is V3<V2<V1<Vbo
Ig1
Ig2
Ig3
Ig=0
V3
V2
V1
Vbo
+Va
V3<V2<V1
Cooling System
Basic Control Concepts
Rectifier
α
Current Reference
DC Line
Inverter
γ
Voltage Reference
HVDC control system
To inverter
Porder
Power
control
Pmod
Iorder
Current
control
amplifier
Converter
unit
firing
control
Iresponse
Ud response
Voltage
measuring
system
Id
Basic Control Concepts
Ud
(p.u.)
Rectifier
Characteristics
Inverter
Characteristics
1.0
Oper.
Point
Rect Id Cont
1.0
Id (p.u.)
Converter Characteristics
Basic Control Concepts
Ud
(p.u.)
Rectifier
Characteristics
Inverter
Characteristics
1.0
Inv Id Cont
Oper.
Point
Rect Id Cont
1.0
Id (p.u.)
Converter Characteristics
AC Side Harmonics
iy
iy + i 
iy
i
iy + i
i
Y Y
Y 
%
iy
in
10
5
iy + i 105
in
5 7
11 13 17 19
23 25
11 13
23 25
Transformer function in HVDC system
•Supply of AC voltages into two separate circuits feeding the
rectifier bridges with a phase shift of 30 electrical degrees for
reduction of low order harmonics esp. 5th & 7th harmonics.
•As a galvanic barrier between AC and DC systems to prevent
DC potential entering into the AC system
•Reactive Impedance in the AC supply to reduce short circuit
currents and to control the rate of rise in valve current during
commutation.
Converter Transformers
Type of Connections
No. of design X
No. of units
Spares
required
3 Phase Star-Delta &
Star-Star
2X2
2
Single phase 2 winding
2X6
2
Singe phase 3 winding
2x3
1
Extended deltaconnection
2X2
1
3 Phase 3 winding
2X2
1
DC High Speed Switches
Transfer between configurations
Transfer between
configurations by
means of MRTB
and MRS(GRTS)
DC MEASURING DEVICES




Measurement on DC side for control, monitoring and Protection
AC CTs cannot be used on DC side – saturation
DC current measuring devices –
 DC shunt – low value resistor
 mV drop from the shunt will be taken for determining the current
 To solve insulation problems – electrical signals are converted to optical at the
shunt and at control system converted to electrical
 Supply for the conversion process is obtained from the control panels in the form
of optical power
DC voltage divider
 Capacitive & resistor divider circuit
 Drop across the resistor scaled for determining the voltage
 Optical conversion process is same as the current measuring device
Smoothing Reactor  Connected in series in each converter with each pole
 Decreases harmonic voltages and currents in the DC line
 Smoothen the ripple in the DC current and prevents the
current from becoming discontinuous at light loads
 Limits crest current (di/dt) in the rectifier due to a short
circuit on DC line
 Limits current in the bypass valve firing due to the
discharge of the shunt capacitances of the dc line
DC Filters
 To reduce the magnitude of the harmonic currents
circulating in the HVDC transmission line to avoid
unacceptable interferences.
 DC filters are needed for HVDC transmission with Bipole
Link and overhead line.
 In the case of back-to-back and cable transmission
systems, there are no requirements for dc filters.
Thank You