© 2017 Electric Power Research Institute, Inc. All rights reserved. Dr. Ram Adapa Technical Executive, EPRI [email protected] HVDC Lines and Cables Course June 12, 2017 Basics of HVDC: AC compared to DC
Jan 21, 2020
© 2017 Electric Power Research Institute, Inc. All rights reserved.
Dr. Ram Adapa
Technical Executive, EPRI
HVDC Lines and Cables Course
June 12, 2017
Basics of HVDC:
AC compared to DC
2© 2017 Electric Power Research Institute, Inc. All rights reserved.
Increased Benefits of Long Distance Transmission
Carrying energy from cheap generation sources which are
far away from the load centers.
Long distance transmission increases competition in new
wholesale electricity markets
Long distance electricity trade could include across nations
or multiple areas within a nation and allows arbitrage of price
differences
Long distance transmission allows interconnection of
networks and thus reducing the reserve margins across all
networks.
More stable long distance transmission is needed to meet
contractual obligations
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Transmitting Fuel versus Transmitting Energy
Load centers can be served by:
– Long distance transmission with remote generation
– Transmitting fuel to the local generation facilities
Bottom line is Economics to see which option is better
Depends on many factors
– Type of fuel – coal can be transported, hydro can’t
– Cost of transporting fuel to local generators
– Availability of generation facilities close to load centers
– Allowable pollution levels at the local gen. facilities
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Long Distance Transmission – AC versus DC
AC versus DC debate goes back to beginnings of Electricity
– DC was first (Thomas Edison)
– AC came later (Tesla / Westinghouse)
AC became popular due to transformers and other AC
equipment
Long Distance Transmission
– AC versus DC - based on economics and technical requirements
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5
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Long Distance AC Transmission
Allows step up and step down of voltages
Intermediate substations are possible to serve load
Reduces current & losses at high voltages
Limited maximum MW capability due to steady state stability
limits (surge impedance loading limits) & transient stability
limits
Series capacitor compensation can increase loading on the
lines but sub synchronous resonance issues need to be
addressed
Needs reactive power support (shunt capacitors, SVCs,
STATCOMs) to keep acceptable voltages
Lines operating at ratings lot lower than the thermal
capability of the lines
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Long Distance DC Transmission
Converts AC to DC, transmits dc power over long distances,
and inverts DC to AC
Controls the power flow on the DC line to a desired value
Most economical for long distance transmission
Can operate the DC lines close to thermal limits
DC can provide direct control between regional AC grids
DC converter stations are more expensive than AC
substations
Intermediate substations require multi-terminal DC which is
not prevalent in use because of complexity
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HVDC Opportunities
The potential for long distance transmission for bulk power
transfer
The potential for asynchronous interconnection. For
example, it allows for connecting networks of 50 Hz and 60
Hz frequencies.
Higher system controllability with at least one HVDC link
embedded in an AC grid.
– In the deregulated environment, the controllability feature is
particularly useful where control of energy trading is needed.
Lower overall investment cost.
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HVDC Advantages
Lower losses. Typically, because HVDC comprises active
power flow only, it causes 20% lower losses than HVAC
lines, which comprise active and reactive power flow.
Less expensive circuit breakers, simpler bus-bar
arrangements in switchgear, and simpler safety
arrangements because HVDC links do not increase the short
circuit currents, as converters ensure that the current added
never exceeds a preset value.
Increased stability and improvements in power quality.
Enhanced environmental solutions.
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HVDC Benefits
Management of congestion
Increasing transmission capacity
Frequency control following loss of generation
Voltage stability control, recovery following faults
Capability of providing emergency power and black start
during grid restoration following major transmission
contingencies
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HVDC Benefits
Power oscillation damping
Avoidance of cascading blackouts
Precise power transfer control between interconnected
transmission areas during emergencies
Rating of HVDC systems as determined only by the real
power demand of transmission capacity (versus HVAC
system ratings as determined by both real and reactive
powers)
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Relative Cost of AC versus DC
For equivalent transmission capacity, a DC line has lower
construction costs than an AC line:
– A double HVAC three-phase circuit with 6 conductors is needed to get
the reliability of a two-pole DC link
– DC requires less insulation
– For the same conductor, DC losses are less, so other costs, and
generally final losses too, can be reduced.
– An optimized DC link has smaller towers than an optimized AC link of
equal capacity.
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HVDC has lower losses than AC for the same power
transfer (1200 MW Example)
HVDC line has lower losses than AC line for same power
Converter losses are extra (~ 0.6% of total power)
Total HVDC System losses are lower than AC system losses
Source: ABB (2003)
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Typical Tower Structures
Typical tower structures and
rights-of-way for alternative
transmission systems of 2,000
MW capacity.
Source: Arrillaga
(1998)
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AC versus DC (Continued)
Right-of-way for an AC Line designed to carry 2,000 MW is
more than 70% wider than the right-of-way for a DC line of
equivalent capacity.– This is particularly important where land is expensive or permitting is a
problem.
HVDC cables can reduce land and environmental costs, but
is more expensive per km than overhead line.
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AC versus DC (Continued)
The remaining costs also differ:– The need to convert to and from AC implies the terminal stations for a
DC line cost more.
– There are extra losses in DC/AC conversion relative to AC voltage
transformation.
– Operation and maintenance costs are lower for an optimized HVDC
than for an equal capacity optimized AC system.
The cost advantage of HVDC increases with the length, but
decreases with the capacity, of a link.
For both AC and DC, design characteristics trade-off fixed
and variable costs, but losses are lower on the optimized DC
link.
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17
• The cost of a DC link depends on:
the cost of the substations
the cost of the line or cable
• HVDC is more economical than
AC when the transmission distance :
is >300 miles for Overhead lines
Is>30 miles for underground cables
DC
SubstationBreak Even
Distance
Cost
DC
AC
Substation
AC
Transmission distance
Note: Assume right-of-way costs same for AC or DC
AC versus DC: Break Even Distances
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AC versus DC: Typical Breakeven distances
Source: Arrillaga (1998)
This graph is based on late 1990s technologies – old numbers are 500 miles but present
breakeven distances are estimated as 300 miles for 2000 MW power transfer
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AC versus DC: Cost Comparison
When comparing costs for AC and DC, the following need
to be considered:
DC Converter / AC substation costs
Line costs
Corridor costs
Operation & Maintenance costs
Costs associated with losses (e.g. DC losses are lower
than AC)
Bottom line – Complete life cycle cost should be
considered over an estimated life span (30 to 40 years)
of the equipment.
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$0
$1,000
$2,000
$3,000
$4,000
$5,000
$6,000
$7,000
$8,000
0 50 100 150 200 250 300
Line Length - Miles
Tra
nsm
issio
n C
ost
- $/M
W-M
ile
Series1
Series2
Series3
Series4
Series11
138 kV
230 kV
345 kV
500 kV
765 kV
A Broader look: Example AC Transmission Costs in
North America
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HVDC Transmission System Costs
HVDC Converter costs
HVDC Line costs & Transmission Corridor costs
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HVDC Converter Cost Structure
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AC versus DC Transmission Costs – Consider
cost of losses - Reduces break even distance
AC
AC
Million Euros
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Source: ABB
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HVDC is particularly suited to undersea transmission, where
the losses from AC cables are large.– First commercial HVDC link (Gotland 1 Sweden, in 1954) was an
undersea one.
Back-to-back converters are used to connect two AC
systems with different frequencies – as in Japan – or two
regions where AC is not synchronized – as in the US.
Special Applications of HVDC
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HVDC links can stabilize AC system frequencies and
voltages, and help with unplanned outages.– A DC link is asynchronous, and the conversion stations include
frequency control functions.
– Changing DC power flow rapidly and independently of AC flows can
help control reactive power.
– HVDC links designed to carry a maximum load cannot be overloaded
by outage of parallel AC lines.
Special Applications (continued)
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Principle of AC Transmission
Schematic of AC system
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Basic HVDC Transmission
DC link
Transformer
F F
Harmonic Filter
(Reactive Power)
Receiving
End
Sending
End
Idc
RT
t
Idci
t
i
Iac
t
i
Iac
InverterRectifier
V1 V2
2T1DC RRR
2V1VI
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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
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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 system will be able
to establish its own reference to ground
Smoothing reactors
They are high reactors with inductance as high as 1 H in series 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 and nearby generators
and interference with telecommunication systems
Harmonic filters are used to mitigate these harmonics
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Reactive power supplies
Under steady state condition, the reactive power 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 is provided by a
shunt capacitor
Electrodes
Electrodes are conductors that provide connection to the earth for
neutral. They have large surface to minimize current 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 taking the DC link
out of service
They are not used for clearing DC faults
DC faults are cleared by converter control more rapidly
Components of HVDC Transmission Systems….
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HVDC Scheme Types
AC ACDC 1 Station
• Back-to-Back
− frequency changing
− asynchronous connection
AC
DCStation 2
AC
Station 1• Point-to-Point Overhead
Line
− bulk transmission
− overland
AC
DC Station 2
AC
Station 1
Submarine Cables
• Point-to-Point Submarine Cable
− bulk transmission
− underwater or underground
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Decrease voltage at station B or increase voltage at station A. power flows from A B Normal
direction
Decrease voltage at station B or increase voltage at station A. power flows from A B Normal
direction
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Power reversal is obtained by reversal of polarity of direct voltages at both ends.
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• Monopolar links
• Bipolar links
• Homopolar links
• Symmetrical Monopolar links
• Multiterminal links
• DC Grids
HVDC links can be broadly classified into:
HVDC System Configurations
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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.
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Bipolar Links
Each terminal has two converters of equal rated voltage,
connected in series on the DC side.
The junctions between the converters is grounded.
If one pole is isolated due to fault, the other pole can operate
with ground and carry half the rated load (or more using
overload capabilities of its converter line).
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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 usually operated with
negative polarity.
The return path for such a system is through ground.
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Symmetrical Monopolar Link
An alternative is to use two high-voltage conductors,
operating at ± half of the DC voltage, with only a single
converter at each end. In this arrangement, known as
the symmetrical monopole, the converters are earthed only
via a high impedance and there is no earth current. The
symmetrical monopole arrangement is uncommon with line-
commutated converters (the NorNed interconnection being a
rare example) but is very common with Voltage Sourced
Converters when cables are used.
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HVDC Converter Technology: LCC Versus VSC
Line Commutated Converter
(or Current Source Converter )
• Thyristor based
• Switches on-off one time per cycle
Voltage Source Converter
• IGBT Based
(Insulated Gate Bipolar Transistor)
• Switches on-off many times per cycle
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LCC Switch turn on by gate pulse but external circuit needed to turn off
– VSC has turn on and turn off capability without external circuit due to self commutation
LCC suffers commutation failures as a result of a sudden drop in the amplitude or phase shift in the AC voltage, which result in dc temporal over-current
– Ability to turn on and off switches means VSC does not suffer from commutation failures
Existing HVDC largely point to point
Multi – terminal being talked about more and more – few installations exist
– Multi terminal LCC problematic due to difficulty changing polarity
– VSC more suitable for multi terminal operation
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Line Commutated Converters
Large filters required due to low order harmonics generated
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Voltage Source Converters2 Level
• Most simple VSC design
• Requires high harmonic
filters
• High switching frequency
required
• 1st generation VSC
• Uses PWM
3 Level
• Slightly more refined than 2
level
• Still requires filtering but
lower harmonics
• Used in some installations
but surpassed by MMC
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VSC : Recent new Topology (MMC)
~
=
~
=
~
=
1
2
n
1
2
n
SM electronics
1
2
IGBT2 D2
D1IGBT1
• Modular Multilevel Converter
• In this case the converter arms
are constructed from identical
sub-modules that are
individually controlled to obtain
the desired ac voltage.
Half-Chain Links shown here.
Full-Chain Links can be used to
reduce fault currents on DC side
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Modular Multi Level Converters
• Much more complex control
• Almost no requirement for AC
filters
• Most expensive and complex
topology
• Lower losses due to lower
switching frequency per
switch
• Inherent redundancy
• Modular design
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VSC Short History
• First introduced in 1997 with the 3MW, +/-10 kV
dc technology demonstrator at Hellsjön,
Sweden
• In 2007 Cross Sound cable having a rating of
330 MW and ±150 kV dc
• Awarded projects not in operation yet –
• France to Spain 320 kV, two bipoles
(2x1000 MW), using underground extruded
cable of 64 km (40 miles)
• Skagerak 4 (one pole) at 500 kV, 700 MW
by 2014 using DC submarine cable (140
km)+land cable(104 km) between Norway &
Denmark .
VSC
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VSC
Currently all the HVDC VSC systems are designed with solid
extruded cables XLPE cables, with the exception of the
Caprivi HVDC inter-connector in Namibia, where the
technology is applied to an overhead line. The project is
rated at 300 MW at 350 kV.
The use of VSC is being expanded to overhead lines and dc
voltage can be increased to higher levels (above 320 kV
because there is no limit of dc cable voltage)
One of the important applications of HVDC VSC converters
is integration of off-shore wind farms.
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Function LCC VSCSemi-Conductor
Device
Thyristors currently 6
inch, 8.5 kV and 5000 Amps. No controlled
turn off capability
IGBTs with anti-parallel free wheeling diode, with
controlled turn-off capability. Current rating 4.5 to 6
kV and turn off current of 1200 Amps.
DC transmission
voltage
Up to +/- 800 kV
bipolar operation. 1000 kV under
consideration in China
Up to +/- 320 kV to 400 kV currently limited by
HVDC cable if extruded XLPE cable is used.
Up to +/- 350 kV with Overhead line, can go higher
DC power Currently in the range
of 6000 MW per
bipolar system
Currently in the range of 600 to 1000 MW per pole
Reactive Power
requirements
Consumes reactive
power up to 60% of
its rating
Does not consume any reactive power and each
terminal can independently control its reactive power.
Filtering Requires large filter
banksRequires moderate size filter banks or no filters at all.
Black start Limited application Capable of black start and feeding passive loads
HVDC Converter Technology: LCC vs. VSC
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Function LCC VSC
Commutation failure
performance
Fails commutation for ac
disturbances
Does not fail commutation
Over load capability Available if designed for up to
any required design value
Does not have any overload
capability
Application with overhead lines Can be applied and dc line faults
can be cleared by converter
control
Can be applied but dc line faults
are cleared by trip of ac breaker,
or the use of a dc circuit breaker.
Currently one application of
overhead line. It has mostly been
applied with cables
Small taps Not economic and affects the
performance
Economic and seems not affect
the performance
Load rejection over voltage Large and has to be mitigated
because of the large reactive
power support
not large because of small size of
filters if required.
HVDC Converter Technology: LCC vs. VSC
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Function LCC VSC
Foot print Can be large Small for the
comparable rating to
an LCC
Off shore wind farms Can be applied with
some dynamic
voltage control
Straight forward
application
Power losses Typically 0.8% per
converter station at
rated power
Typically 0.8 to 1.0%
per terminal with
multilevel converters
HVDC Converter Technology: LCC vs. VSC
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Trans Bay VSC DC Cable
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HVDC IN NORTH AMERICA
Blackwater (200 MW)
Artesia (200 MW)Sidney
(200 MW)
Stegall
(110 MW)
Rapid City DC
(200 MW)
Miles City
(200 MW)
IPP (2400 MW)
PDCI (3100 MW)
TBC (400 MW)
McNeill
(150 MW)
Eel River
(320 MW)
Square Butte
(500 MW)Nelson River
(1620 MW)
Nelson River II
(1800 MW)
Coal Creek
(1000 MW)
Oklaunion
(200 MW)
Madawaska
(350 MW)
Highgate
(200 MW)
Quebec –
New England
(2000 MW)
Welsh
(600 MW)
Eagle Pass
(36 MW)
Cross Sound
Cable (300 MW)
Lamar (210 MW)Neptune
(600 MW)
Sharyland
(150 MW)
Chateauguay
(1000 MW)
LCC HVDC
VSC HVDC
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Examples of HVDC Projects Around the World
Nelson River 2
CU-project
Vancouver Island
Pole 1
Pacific Intertie
Pacific Intertie
Upgrading
Pacific Intertie
Expansion
Intermountain
Blackwater
Itaipu
Inga-Shaba
Cahora Bassa
Brazil-ArgentinaInterconnection I
English
ChannelDürnrohrSardinia-ItalyItaly-Greece
Highgate
Chateauguay
Quebec-New
England
Skagerrak 1&2
Skagerrak 3
Konti-Skan 1
Konti-Skan 2
Baltic Cable
Fenno-Skan
Gotland 1
Gotland 2
Gotland 3
Kontek
SwePol
Chandrapur-
Padghe
Rihand-Delhi
Vindhyachal
Sakuma
Gezhouba-
Shanghai
Leyte-Luzon
Broken Hill
New Zealand 1
New Zealand 2
Three Gorges -
Changzhou
Brazil-ArgentinaInterconnection II
Gotland
Murraylink
Directlink
Moselstahlwerke
Cross Sound Cable
Eagle Pass
Tjæreborg
Hällsjön
Hagfors
HVDC Classic Converters
CCC Converters
HVDC Light (VSC) Converters
Source: ABB
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DC Grids – The Future of DC Transmission
– DC Grids for Offshore Wind– Considered more in Europe than in other
countries– Need to resolve many issues Power & Voltage control DC circuit breakers Standard DC voltages Communication needs
– CIGRE/IEEE WGs Two Topologies
DC Node
AC Node
DC Line
(a)
(b)
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DC Grid Configurations: Offshore Development – Point to Point System
Source: ALSTOM
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DC Grid Configurations: Offshore Grid System
Source: ALSTOM
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Macro Grid
HVDC Network Concept
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59
Overlay DC Grid Gives Access to Renewable
Sources within Europe
• Interconnection of remote
renewable energy sources
• Overcoming “bottlenecks” in the
existing AC grids
• Low loss (HVDC) transmission
systems
• Controllable power flows over a
wide area
• Avoidance of synchronisation over
a wide area
• Less environmental impact than
AC reinforcement
3000k
m
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Cigrè B4-52: HVDC Grid Feasibility Study
1 Introduction
2HVDC grids – concepts and lessons learned from history
3Available Converter Technologies, VSC and LCC Comparison
4Motivation of an HVDC grid
5HVDC grid Configurations
6Fault Performance
7Protection Requirements
8New components in HVDC grid – Including Questionnaires to manufacturers
9Power Flow Control in DC Grids
10The Requirements on an HVDC grid – Security and Reliability
11Needed Standardization
12New working groups within the HVDC grid area
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DC Grid Standardisation Activities
• Cigrè have started five further DC grid working groups;
– B4-56: Guidelines for the preparation of “connection agreements” or “Grid Codes” for HVDC grids
– B4-57: Guide for the development of models for HVDC converters in a HVDC grid
– B4-58: Devices for load flow control and methodologies for direct voltage control in a meshed HVDC Grid
– B4-59: Protection of Multi-terminal HVDC Grids
– B4-60: Designing HVDC Grids for Optimal Reliability and Availability performance
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A Sample of European Proposals
G. Asplund, B. Jacobson, B. Berggren, K.
Lindén ”Continental Overlay HVDC-Grid”, Cigré
conference, B4-109, Paris, 2010
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Atlantic Wind Connection
http://atlanticwindconnection.com/download/AtlanticWindConnection_Brochure.pdf
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Atlantic Wind Connection Project(see: www.atlanticwindconnection.com/ferc/2010-12-filing/Petition_for_Declaratory_Order.pdf)
What
A sub-sea HVDC backbone
transmission system
Where
Extending from northern New Jersey to
southern Virginia.
Who
Marubeni
Good Earth
Elia
Why
Serve as an efficient collector of ac
power from offshore wind farms
Relieve transmission congestion on the
eastern ac grid
Improve regional system reliability.
2010 P
S36A
65© 2017 Electric Power Research Institute, Inc. All rights reserved.CENELEC meeting 29.06.11 P 65
Comparison of AC and DC parameters
AC PARAMETER DC PARAMETER
Frequency
Target DC Voltage
Vdc
Voltage Change
))sin(V(
Voltage Change
V
Impedance of Connection
)X(
Resistance of Connection
R
Real Power
XsinVV
Real Power
RVV
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When closed the DC breaker must have very low losses
• optimum solution mechanical switch
DC Breakers
Main switch
Unlike an AC breaker the DC
current never experiences a
current zero. Hence, to
interrupt the DC current the DC
breaker must drive the load
current to zero.
AC
DC
Modular hybrid solution to drive current to zero
• critical component is the mechanical switch as it has
to operate VERY fast to minimise the peak current to
be interrupted by the auxiliary branch
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HVDC Circuit Breaker Developments
– Many ideas are explored– Fast growing area– Numerous R&D projects– Minimize size, cost, & interruption time
Solid State Circuit
Breaker
V
VV
IG IS
IV
New Hybrid Circuit Breaker
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New HVDC Circuit Breaker Developments –
Hot of the Press (as of November 7, 2012)
ABB develops world’s first circuit breaker for HVDC
November 7, 2012
By PennEnergy Editorial Staff
Source:ABBABB (NYSE: ABB), the leading power and automation technology group, has announced a breakthrough in the
ability to interrupt direct current, solving a 100-year-old electrical engineering puzzle and paving the way for a
more efficient and reliable electricity supply system.
After years of research, ABB has developed the world’s first circuit breaker for high voltage direct current (HVDC).
It combines very fast mechanics with power electronics, and will be capable of ‘interrupting’ power flows
equivalent to the output of a large power station within 5milliseconds- that is thirty times faster than the blink of a
human eye.
The breakthrough removes a 100-year-old barrier to the development of DC transmission grids, which will enable
the efficient integration and exchange of renewable energy. DC grids will also improve grid reliability and enhance
the capability of existing AC (alternating current) networks. ABB is in discussions with power utilities to identify
pilot projects for the new development.
ABB has written a new chapter in the history of electrical engineering,” said Joe Hogan, CEO of ABB. “This
historical breakthrough will make it possible to build the grid of the future. Overlay DC grids will be able to
interconnect countries and continents, balance loads and reinforce the existing AC transmission networks. “
The Hybrid HVDC breaker development has been a flagship research project for ABB, which invests over $1
billion annually in R&D activities. The breadth of ABB’s portfolio and unique combination of in-house
manufacturing capability for power semiconductors, converters and high voltage cables (key components of
HVDC systems) were distinct advantages in the new development.
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Current State of HVDC versus HVAC
Many Existing HVDC systems are old (30 - 50 years old)– Life extension is taking place
Highest DC Voltage is UHVDC at +/- 800 kV in China & India– South Africa & Brazil are also considering
– For long distances over 3000 km
– For Bulk Power Transfer ( 3000 to 6000 MW)
UHVDC of +/- 1000 to 1100 kV is planned in Asia for up to 8000 MW - China VSC HVDC is increasing (+/- 320 kV up to 1000 MW)
Max AC Voltage in North America is 765 kV (EHVAC)UHVAC (1000 kV to 1200 kV) is considered in China
(highest in the world)
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For transfers of above 6,000 MW over 4,000 km, the
optimum voltage rises to 1,000–1,200 kV.– Technological developments in LCC converter stations seem to be
ready to handle these voltages.
HVDC and HVAC overlays for regional interconnections
Segmenting AC grids with DC back-to-backs for improved
reliability
Growth of VSC DC applications – more dc cable projects
DC Grids for renewable integration
Future Trends in HVDC
71© 2017 Electric Power Research Institute, Inc. All rights reserved.
Together…Shaping the Future of Electricity