unit 5 hvdc
TRANSCRIPT
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CONVENTIONALLY POWER TRANSMISSION IS EFFECTED
THROUGH HVAC SYSTEMS ALL OVER THE WORLD.
HVAC TRANSMISSION IS HAVING SEVER LIMITATIONS LIKE LINE
LENGTH , UNCONTROLLED POWER FLOW, OVER/LOW
VOLTAGES DURING LIGHTLY / OVER LOADED
CONDITIONS,STABILITY PROBLEMS,FAULT ISOLATION ETC
CONSIDERING THE DISADVANTAGES OF HVAC SYSTEM ANDTHE ADVANTAGES OF HVDC TRANSMISSION , POWERGRID HAS
CHOOSEN HVDC TRANSMISSION FOR TRANSFERRING 2000 MW
FROM ER TO SR
COMPARISION OF HVAC & HVDC SYSTEMS
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HVDC: USE less current
Direct current : Rollalong the line ;opposing force friction(electrical resistance )
AC current willstruggle againstinertia in the line(100times/sec)-cuurent inertiainductance-reactivepower
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Better Voltage utilisation rating
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DC has Greater Reach
Distance as well as
amount of POWER
determine the choice
of DC over AC
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DC
The alternating current in a cable leaks current (charging
movements) in the same manner as a pulsating pressure
would be evened out in an elastic tube.
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DIRECT CURRENT CONSERVES FOREST
AND SAVES LAND
Fewer support TOWER, less losses
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CONTROLLING or BEING
CONTROLLED
By raising the level in tank ;controlled water flow
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CONTROLLING or BEING
CONTROLLED
ZERO IF Vr=VI=10V
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HVDC provides increase powerbut does not increase the short
circuit POWER
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ADVANTAGES OF HVDC OVER HVAC TRANSMISSION
CONTROLLED POWER FLOW IS POSSIBLE
VERY PRECISELY
ASYNCHRONOUS OPERATION POSSIBLE
BETWEEN REGIONS HAVING DIFFERENT
ELECTRICAL PARAMETERS
NO RESTRICTION ON LINE LENGTH AS NO
REACTANCE IN DC LINES
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ADVANTAGES OF HVDC OVER HVAC TRANSMISSION
STABILISING HVAC SYSTEMS -DAMPENING OF POWER
SWINGS AND SUB SYNCHRONOUS FREQUENCIES OF
GENERATOR.
FAULTS IN ONE AC SYSTEMS WILL NOT EFFECT THE OTHER
AC SYSTEM.
CABLE TRANSMISSION
.
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ADVANTAGES OF HVDC OVER HVAC TRANSMISSION
CHEAPER THAN HVAC SYSTEM DUE TO LESS TRANSMISSION
LINES & LESS RIGHT OF WAY FOR THE SAME AMOUNT OF
POWER TRANSMISSION
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COST: AC vs DC Transmission
Terminal Cost AC
Terminal Cost DC
Line Cost DC
Line Cost AC
Break Even Distance
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HVDC BIPOLAR TRANSMISSION SYSTEM
2 DOUBLE CIRCUIT HVAC TRANSMISSION SYSTEMS
2000 MW HVDC VIS- A- VIS HVAC SYSTEMS
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AC
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DC
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DC
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HVDC BIPOLAR LINKS IN INDIA
NER
ER
SR
NRNER
ER
SR
NR
RIHAND-DELHI -- 2*750 MW
CHANDRAPUR-PADGE 2* 750 MW
TALCHER-KOLAR 2*1000 MW
ER TO SR
SILERU-BARASORE - 100 MW
EXPERIMENTAL PROJECTER SR
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HVDC IN INDIA
BipolarHVDC LINK CONNECTING
REGION
CAPACITY
(MW)
LINE
LENGTH
Rihand Dadri
North-North 1500 815
Chandrapur -
Padghe
West - West 1500 752
Talcher
Kolar
East South 2500 1367
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ASYNCHRONOUS LINKS IN INDIA
NER
ER
SR
NRNER
ER
SR
NR
VINDYACHAL (N-W) 2*250 MW
CHANDRAPUR (W-S) 2*500 MW
VIZAG (E-S) - 2*500 MW
SASARAM (E-N) - 1*500 MW
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HVDC IN INDIA
Back-to-Back
HVDC LINK CONNECTINGREGION
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
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BASIC PRINCIPLES
OF
HVDC TRANSMISSION
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AC Transmission Principle
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HVDC Transmission Principle
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6-Pulse Convertor Bridge
3
6
CiLs
4
E1 Ls
Ls
Bi
iA
1
2
I
V'd
5
Vd
IddL
d
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Voltage and Current of an Ideal
Diode 6 Pulse Converter
Alpha = 0
Overlap = 0
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Operation of Converter
Each thyristor conducts for 120
Every 60 one Thyristor from +ve limb and one Thyristorfrom ve limb is triggered
Each thyristor will be triggered when voltage across itbecomes positive
Thyristor commutates the current automatically when the
voltage across it becomes ve. Hence, this process is callednatural commutation and the converters are called LineCommutated converters
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Triggering can be delayed from this point and this is called firing angle
Output voltage of the converter is controlled by controlling the
Rectifier action
If > 90 negative voltage is available across the bridge Inverteraction
Due to finite transformer inductance, current transfer from onethyristor valve to the other cannot take place instantly
This delay is called over lap angle and the reactance calledcommutating reactance. This also causes additional drop in the voltage
Operation of Converter
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Effect of Control Angle
B
A
2
C
1
u u
Vd
u
3
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RECTIFIER VOLTAGE
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INVERTER VOLTAGE
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DC Terminal Voltage
120
RECTIFICATION
0240 180 300 120 60 180
0.866E . 2
LL
E . 2LL
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DC Terminal Voltage
120
INVERSION
0240 180 300 120 60 180
0.866E . 2LLE . 2
LL
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DC Voltage Verses Firing Angle
-1
-0.8
-0.6
-0.4
-0.2
0
0.2
0.4
0.6
0.8
1
0 30 60 90 120 150 180
Vd
alpha
Vd=Vac*1.35 *(cos alpha-uk/2)
Valve Voltage and Valve
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Valve Voltage and Valve
Current
120 180A
u0.866
240120
u
60
FC
D
B E
180A
u
60 60
K
G J L
H
N
M
3000
P
u
S
E . 2LL
60R
Q
RECTIFICATION
=15
+u E . 2LL
Valve Voltage and Valve
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Valve Voltage and Valve
Current
M Q
120 180 R
N
Pu
240 120R
Q
180
u
0
B
F
SA
C
E
DH
60
J
K
G L
INVERSION=15
6060
u u
60
0.866E . 2LL
E . 2LL
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12-Pulse Convertor Bridge
Y
Commonly Used in HVDC systems
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Commonly adopted in all HVDC applications
Two 6 pulse bridges connected in series
30 phase shift between Star and Delta
windings of the converter transformer
Due to this phase shift, 5thand 7thharmonicsare reduced and filtering higher order
harmonics is easier Higher pulse number than 12 is not
economical
12-Pulse Convertor Bridge
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DC VOLTAGE AT = 15
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DC VOLTAGE AT = 90
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DC VOLTAGE AT = 165
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HVDC Link Voltage Profile
I R
DC CABLE or O/H LINE
I Ed rd
RECTIFIER
dio RV
I X2
d c
cos
rI Ed
L I X
2
d c
cos
VdioI
INVERTER
VdR=VdioR cos-Id Xc+Er VdI=VdioI(cos-Id Xc+Er
2 2
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Control of DC Voltage
V 1 V 3 V 5
V 2V 6V 4
Phase A
Ud
Phase B
Phase C
Id
Power FlowAC System DC System
V 1 V 3 V 5
V 2V 6V 4
Phase A
Ud
Phase B
Phase C
Id
AC System DC SystemPower Flow
30 60 90 120 150 180
0
+Ud
-Ud
160
5
Rectifier
Operation
Inverter
Operation
Rectifier Operation Inverter Operation
R l i hi f DC V l Ud d Fi i
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Relationship of DC Voltage Ud and Firing
Angle
30 60 90 120 150 180
0
+Ud
-Ud
160
Limit Inv
5
Limit Rect.
RectifierOperation
InverterOperation
tw
o
60=
Ud
o
30=o
0=
o
90= o
120= o
150=
-Ud
tw
Ud
Ud
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NORMAL POWER DIRECTION
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REVERSE POWER OPERATION
Schematic of HVDC
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Schematic of HVDC
M d f O ti
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Modes of Operation
DC OH Line
Converter
Transformer
Thyristor
Valves
400 kVAC Bus
AC Filters,Reactors
Smoothing Reactor
Converter
Transformer
Thyristor
Valves
400 kVAC Bus
AC Filters, shuntcapacitors
Smoothing Reactor
Bipolar
Current
Current
Modes of Operation
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Modes of Operation
DC OH Line
Converter
Transformer
Thyristor
Valves
400 kVAC Bus
AC Filters,Reactors
Smoothing Reactor
Converter
Transformer
Thyristor
Valves
400 kVAC Bus
AC Filters
Smoothing Reactor
Monopolar Ground Return
Current
Modes of Operation
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Modes of Operation
DC OH Line
Converter
Transformer
Thyristor
Valves
400 kVAC Bus
AC Filters,Reactors
Smoothing Reactor
Converter
Transformer
Thyristor
Valves
400 kVAC Bus
AC Filters
Smoothing Reactor
Monopolar Metallic Return
Current
TALCHERTALCHER KOLARSCHEMATIC
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Kolar
Chintamani
Cudappah
HoodyHosur
Salem
Udumalpet
MadrasBlore
+/- 500 KV DC line
1370 KM
Electrode
Station
Electrode
Station
TALCHER
400kv System
220kv system
KOLAR
SCHEMATIC
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Sharing of Talcher Power
Tamil Nadu - 636 MW
A.P. - 499 MW
Karnataka - 466 MW
Kerala - 330 MW
Pondicherry - 69 MW
32%
23%
17% 3%
25%
T.N. A.P.Karnataka Kerala
Pondy
KOLAR SINGLE LINE DIAGRAM
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Project Highlights
FOR TRANSMITTING 2000 MW OF POWER FROM NTPC TALCHER
STPS -II AND FOR SHARING AMOGEST SOUTHERN STATES THE
2000 MW HVDC BIPOLAR TRANSMISSION SYSTEM IS ENVISAGED
AS
EAST SOUTH INTERCONNECTOR II (ESICON II).
THIS IS THE LARGEST TRANSMISSION SYSTEM TAKEN UP IN
THE COUNTRY SO FAR
THE PROJECT SCHEDULE IS QUITE CHALLENGING AGAINST THE 50 MONTHS FOR SUCH PROJECTS, THE
PROJECT SCHEDULE IS ONLY 39 MONTHS
SCHEDULED COMPLETION BY JUNE 2003
TACLHER-KOLAR 500 kV HVDC TRANSMISSION SYTEM
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Project Highlights
KEY DATES
AWARD OF HVDC TERMINAL STATION PKG -
14TH MAR 2000
AWARD OF HVAC PACKAGE -
27TH APR 2000
APPROVED PROJECT COST - RS. 3865.61 CR
THIS IS THE FIRST OF SUCH SYSTEM WHERE THE ENTIRE
GENERATION IN ONE REGION IS EARMARKED TO
ANOTHER REGION.
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Salient Features Rectifier Talcher, Orissa
Inverter Kolar, Karnataka
Distance
1370 km
Rated Power 2000 MW
Operating Voltage 500 kV DC
Reduced Voltage 400 kV DC
Overload
Long time, 40C 1.25 pu per pole
Half an hour 1.3 pu per pole
Five Seconds 1.47 pu per pole
SYSTEM CAPACITIES
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SYSTEM CAPACITIES
BIPOLAR MODE OF OPERATION -- 2000 MW
MONO POLAR WITH GROUND RETURN --- 1000 MW
MONO POLAR WITH METALLIC RETURN MODE --- 1000 MW
DEBLOCKS EACH POLE AT P min 100 MW
POWER DEMAND AT DESIRED LEVEL
POWER RAMP RATE -- 1 300 MW /MIN
POWER REVERSAL IN OFF MODE
SYSTEM CAPACITIES
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SYSTEM CAPACITIES
OVER LOAD CAPACBILITIES
RATED POWER -- 2000 MW
LONG TIME OVER LOAD POWER 8/10 HOURS -- 2500 MW
SHORT TIME OVER LOAD 5 SEC- 3210 MW
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HARMONIC FILTERS
AT TALCHER
TOTAL FILTERS 14
DT 12/24 FILTERS EACH 120 MVAR - 7 NOS
DT 3/36 FILTERS EACH 97 MVAR - 4 NOS
SHUNT REACTORS 138 MVAR- 2 NOS
SHUNT CAPCITORS 138 MVAR- 1 NOSDC FILTERS DT 12/24 & DT 12/36 1 No per pole.
AT KOLAR
TOTAL FILTERS 17DT 12/24 FILTERS EACH 120 MVAR - 8 NOS
DT 3/36 FILTERS EACH 97 MVAR - 4 NOS
SHUNT CAPCITORS 138 MVAR- 5 NOS
DC FILTERS DT 12/24 & DT 12/36 1 each pole
SYSTEM CAPACITIES
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MONOPOLAR GROUND RETURN - 1000 MW POWER CANBE TRANSMITTED THROUGH THIS MODE WHERE THERETURN PATH IS THROUGH THE GROUND WHICH ISFACILITATED THROUGH A EARTH ELECTRODE STATIONSITUATED AT ABOUT 35 KMS FROM THE TERMINALS ANDCONNECTED BY A DOUBLE CIRCUIT TRANSMISSION LINE.
MONOPOLAR METALLIC RETURN - 1000 MW POWER CANBE TRANSMITTED THROUGH THIS MODE WHERE THERETURN PATH IS THE TRANSMISSION LINES OF OTHERPOLE.
BALANCED BIPOLAR MODE 2000 MW CAN BETRANSMITTED THROUGH THIS MODE WHERE WITH ONE+VE AND OTHER VE .
SYSTEM CAPACITIES
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HIGH VOLTAGE DIRECT
CURRENT TRANSMISSION
(HVDC)
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Advantages In a number of applications HVDC is more effective than AC
transmission. Examples include:
Undersea cables, where high capacitance causes additional AClosses. (e.g. 250 km Baltic Cable between Sweden and Germany)
Long power transmission without intermediate taps, for example,in remote areas
Power transmission and stabilization between unsynchronized ACdistribution systems
Connecting a remote generating plant to the distribution grid
Reducing line cost: 1) fewer conductors 2) thinner conductors
since HVDC does not suffer from the skin effect Facilitate power transmission between different countries that use
AC at differing voltages and/or frequencies
Synchronize AC produced by renewable energy sources
Di d t
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Disadvantages The disadvantages of HVDC are in conversion,
switching and control. Expensive inverters with limited overload capacity
Higher losses in static inverters at smaller transmissiondistances
The cost of the inverters may not be offset by reductionsin line construction cost and lower line loss.
High voltage DC circuit breakers are difficult to build
because some mechanism must be included in the circuitbreaker to force current to zero, otherwise arcing andcontact wear would be too great to allow reliableswitching.
C t f HVDC Tr mi i
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Cost of HVDC Transmission Costs vary widely depending on power rating, circuit length,
overhead vs. underwater route, land costs, and AC networkimprovements required at either terminal.
For example, for an 8 GW, 40 km link laid under the EnglishChannel, the following are approximate primary equipmentcosts for a 2 GW, 500 kV bipolar conventional HVDC link is:
Converter stations ~$170 M
Subsea cable + installation ~$1.5 M/km
So for an 8 GW capacity between England and France in four links,
little change is left from ~$1.2B for the installed works. Add another$300$450M for the other works depending on additional onshoreworks required
HVDC System Configurations and Components
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HVDC System Configurations and Components
HVDC links can be broadly classified into:
Basic links
Monopolar links
Bipolar links
Homopolar links
Extended links
Back-to-back links Multiterminal links
Monopolar Links
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Monopolar Links
It uses one conductor
The return path is provided by ground or water
Use of this system is mainly due to cost considerations
A metallic return may be used where earth resistivity is too high
This configuration type is the first step towards a bipolar link
Bipolar Links
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Bipolar Links It uses two conductors, one positive and the other negative
Each terminal has two converters of equal rated voltage,connected in series on the DC side
The junctions between the converters is grounded
Currents in the two poles are equal and there is no ground current
If one pole is isolated due to fault, the other pole can operate with
ground and carry half the rated load (or more using overloadcapabilities of its converter line)
Homopolar Links
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Homopolar Links
It has two or more conductors all having the same
polarity, usually negative Since the corona effect in DC transmission lines is
less for negative polarity, homopolar link is usuallyoperated with negative polarity
The return path for such a system is through ground
Components of HVDC Transmission Systems
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Components of HVDC Transmission Systems
1. Converters
2. Smoothing reactors
3. Harmonic filters
4. Reactive power supplies
5. Electrodes
6. DC lines
7. AC circuit breakers
Components of HVDC Transmission Systems
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Components of HVDC Transmission Systems
Converters
They perform AC/DC and DC/AC conversion
They consist of valve bridges and transformers Valve bridge consists of high voltage valves connected in a
6-pulse or 12-pulse arrangement
The transformers are ungrounded such that the DC systemwill be able to establish its own reference to ground
Smoothing reactors
They are high reactors with inductance as high as 1 H inseries with each pole
They serve the following: They decrease harmonics in voltages and currents in DC lines
They prevent commutation failures in inverters
Prevent current from being discontinuous for light loads
Harmonic filters
Converters generate harmonics in voltages and currents.These harmonics may cause overheating of capacitors andnearby generators and interference with telecommunicationsystems
Harmonic filters are used to mitigate these harmonics
Components of HVDC Transmission Systems contd.
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p y
Reactive power supplies
Under steady state condition conditions, the reactivepower consumed by the converter is about 50% of the
active power transferred Under transient conditions it could be much higher
Reactive power is, therefore, provided near the converters
For a strong AC power system, this reactive power isprovided by a shunt capacitor
Electrodes
Electrodes are conductors that provide connection to theearth for neutral. They have large surface to minimizecurrent densities and surface voltage gradients
DC lines
They may be overhead lines or cables
DC lines are very similar to AC lines
AC circuit breakers
They used to clear faults in the transformer and for takingthe DC link out of service
They are not used for clearing DC faults
DC faults are cleared by converter control more rapidly
Converter Theory and Performance
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Converter Theory and Performance
Multiple Bridge Converters
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Multiple Bridge Converters
Two or more bridges are connected in seriesto obtain as a high a direct voltage asrequired
These bridges are series on the DC side,parallel on the AC side
A bank of transformers is connectedbetween the AC source and the bridges
The ratio of the transformers are adjustableunder load
Multiple bridge converters are used in evennumbers and arranged in pairs for 12-pulsearrangement
Multiple Bridge Converters
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Multiple Bridge Converters
Two banks of transformers, one connected in Y-
Y and the other Y-are used to supply each pair
of bridges
The three-phase voltage supplied at one bridge is
displaced from the other by 30 degrees
These AC wave shapes for the two bridges addup to produce a wave shape that is more
sinusoidal than the current waves of each of the
6-pulse bridges
This 12-pulse arrangement effectively eliminates
5th and 7th harmonics on the AC side. Thisreduces the cost of harmonic filters
This arrangement also reduces ripple in the DC
voltage
Control of HVDC Systems
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Control of HVDC Systems
Objectives of Control
Efficient and stable operation
Maximum flexibility of power control without
compromising the safety of equipment
Content
Principle of operation of various control systems
Implementation and their performance during normaland abnormal system conditions
Basic principles of control
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Basic principles of control
Direct current from the rectifier to the
inverter
Power at the rectifier terminal
Power at the inverter terminal
Basic means of control
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Basic means of control
Internal voltages, Vdorcosand Vdoicos, can used becontrolled to control the voltages at any point on the line
and the current flow (power)
This can be accomplished by:
Controlling firing angles of the rectifier and inverter (for fast
action)
Changing taps on the transformers on the AC side (slow
response)
Power reversal is obtained by reversal of polarity of
direct voltages at both ends
Basis for selection of control
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Basis for selection of control
Following considerations influence theselection of control characteristics:
Prevention of large fluctuation in DCvoltage/current due to variation In AC side voltage
Maintaining direct voltage near rated value
Power factor at the receiving and sending endsshould be as high as possible
Control implementation
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Control implementation
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Control implementation
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Control implementation
Power control
To transmit a scheduled power, the correspondingcurrent order is determined by:
Iord
=Po/Vd
Bridge/converter unit control
Determines firing angles and
sets their limits
Pole control It coordinates the conversion
of current order to a firing
angle order, tap changer
control and other protection
sequences
Multiterminal HVDC network
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Multiterminal HVDC network
Successful application of two-terminal DC systems led to the
development of multi-terminal networks
There are two possible connection schemes for MTDC systems:
Constant voltage parallel scheme
Constant current series scheme