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Institute for High-Voltage Engineering and Systems Management
High Voltage Engineering For
Modern Transmission Networks
Michael MUHR
O.Univ.-Prof. Dipl.-Ing. Dr.techn. Dr.h.c.
Institute for High-Voltage Engineering and Systems Management
Graz University of Technology
Austria
Michael MUHR
High Voltage Engineering For Modern Transmission Networks
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Institute for High-Voltage Engineering and Systems Management
Content
1. Introduction
2. High Voltage AC Transmission (HVAC)
3. High Voltage DC Transmission (HVDC)
4. Future Developments & Trends
5. Transmission Lines
6. Overhead Lines
7. Cable Lines
8. Gas-Insulated Lines
9. Technical Developments
10. Summary
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1. Introduction
Essential changes in the framework:
 Liberalisation of the electricity market
 Increasing of electricity transportation / transit
 Renewable Energies are on the rise
 Maintenance and modernisation / replacement
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Source: IEA; UN; Siemens PG CS4 - 08/2002
Development of the world population and the power consumption
between 1980 and 2020
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2. High Voltage AC Transmission (HVAC)
 Economical environmentally friendly and low-losses only
with the usage of high voltage
 Voltage levels for HVAC in Austria and major parts of
Europe: 110 kV, 220 kV and 380 kV
 Advantage: Easy transformation of energy between the
different voltage levels, convenient and safe handling
(application)
 Unfavourable: Transmission and compensation of reactive
power, stability problems, frequency effects can cause
voltage differences and load angle issues at long lines
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In Discussion: China 1000 kV
Japan 1100 kV
India 1200 kV
Source: SIEMENS
Development of Voltage Levels for HVAC
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Control of active power flow
 Phase Shifter Transformer (PST)
 Flexible AC Transmission Systems (FACTS)
FACTS – Elements:
 Elements controllable with power electronics
 System is more flexible and is able to react fast to changes
in the grid
 Control of power flow and compensation of reactive power
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Phase shift transformers (PST)
 Distribution of current depends on Impedances only
 Unequal distribution Implementation of additional voltage
w/o PST
sources
i
1
X1
itotal
i2
X2
UPST
itotal
~
i1+Δi
i2-Δi
with PST
X1
X2
 Control of active power flow
 Additional voltage with 90° shift of phase voltage
 PST implements a well-defined phase-shift between
primary and secondary part of the transformer
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3. High Voltage DC Transmission (HVDC)
 Transmission of high amounts of electrical power over long
lines (> 1000 km)
 Sub-sea power links (submarine cables)
No compensation of reactive power necessary
 Coupling of grids with different network frequency
 Asynchronous operation
 Low couple - power
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Advantages of HVDC
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No (capacitive) charging currents
Grid coupling (without rise of short-circuit current)
No stability problems (frequency)
Higher power transfer
No inductive voltage drop
No Skin-Effect
High flexibility and controllability
Disadvantages of HVDC
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Additional costs for converter station and filters
Harmonics
requires reactive power
Expensive circuit breakers
Low overload capability
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4. Future Trends
Source: SIEMENS PTD SE NC - 2002
Costs of a high voltage transmission system
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Possibilities for Transmission Systems
for high power
Alternating Current (AC)
Direct Current (DC)
Hybrid AC / DC - Connection
Hybrid Connection
Source: SIEMENS
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Transmission Line Systems
AC
DC
Maximum voltage
in operation
kV 800
+/- 600
Maximum voltage
under development
kV 1000
+/- 800
Maximum power
per line in
operation
MW 2000
3150
Maximum power
per line under
development
MW 4000
6400
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Prof. S. Gubanski / Chalmers University of Technology
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Network Stability
 Separation of large and heavy meshed networks to prevent
mutual influences and stability issues
 Usage of HVDC close couplings
 Fast control of frequency and transfer power possible
 Limitation of short-circuit power
 Improvement of transient network stability
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5. Transmission Lines
 Liberalisation of the Electricity Market
 Renewable Energy is on the rise
 Increased environmental awareness
Possibilities for
Transmission Lines
in High Voltage Networks:
Overhead Line
Cable Line
Gas Insulated Line
Decision Criteria
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Framework
 Economic necessity
 Transmission capacity
 Voltage level
 Comply with (n-1) – criteria
 Reliability of supply
 Operational conditions
 Environmental requirements
 (Civil) engineering feasibility
 Economics
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6. Overhead Lines
 Insulating Material: Air
 High voltages are easy to handle with sufficient
distances/clearances and lengths
 Permitted phase wire temperature of phase wires is
determined by mechanical strength
 Overhead lines are defined by their natural power PNat
 Thermal Power limit is a multiple of PNat
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6. Overhead Lines – Advantages
 Simple and straightforward layout
 (Relatively) easy and fast to erect and to repair
 Good operating behaviour
 Long physical life
 Large load capacity and overload capability
 Lowest (capacitive) reactive power of all systems
 Longest operational experience
 Lowest unavailability
 Lowest investment costs
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6. Overhead Lines – Disadvantages
 High failure rate (most failure are arc failures without
consequences)
 Impairment of landscape (visibility)
 Low electromagnetic fields can be achieved through
distances and arrangements
 Highest losses
 Highest operational costs because of current-dependent
losses
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7. Cable Lines
 Insulating Materials
 Plastics/Synthetics (PE, XLPE)
 Oil – Paper
 Polypropylene Laminated Paper (PPLP): reduced power loss and higher electrical
strength than oil-paper cables
 Synthetic cables are environmental friendly, dielectrics undergo an ageing
process, voltage levels are currently limited to about 500 kV
 Cables have a high capacitance  large capacitive currents  limits
maximum (cable) line length  compensation
 Transferable power is limited by:
 permitted temperature of the dielectric
 high thermal resistances of accessories & auxiliary equipment
 soil condition
 Thermal Power Stherm is essential for continuous rating/operation
 High voltage cables have a much higher Pnat than Stherm (of about 2...6)
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7. Cable Lines – Advantages
 Large load capacity possible with thermal foundation and
cross-bonding
 Lower impedances per unit length when compared to
overhead lines
 Lower failure rate than overhead lines
 No electrical field on the outside
 Losses are only 50% of an overhead line
 Operational costs (including losses) are about half of the
costs of an overhead line
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7. Cable Lines - Disadvantages
 High requirements to purity of synthetic insulation and watertightness
 Overload only temporary possible  influences lifespan of
insulation
 High reactive power, compensation necessary
 PD-Monitoring on bushings, temperature monitoring
 Unavailability is notable higher when compared to overhead
lines (high repairing efforts)
 Lifespan: 30 to 40 years (assumed)
 Extensive demand of space, drying out of soil, only very limited
usage of line route possible
 threshold value for the magnetic field (100 µT) can be exceeded
 3-6 times investment costs compared to overhead lines
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8. Gas-Insulated Lines (GIL)
 Insulating Material: SF6 and N2: Currently 80% N2 and 20 % SF6;
pressure: 3 to 6 bar
 Currently no buried lines; laying only in tunnels or openly
 Many flanges necessary
 Compensation of (axial) thermal expansion of ducts
 SF6: Environmental compatibility ?
 Gas monitoring
 Easy conversion from other line systems to GIL
 High transmission capacity
 large overload capability
 Minimal dielectric losses
 Low mutual capacitance  low charging current / power
 Good heat dissipation to the environment
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8. Gas-Insulated Lines – Advantages
 Large transmission capacity
 High load capacity
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High overload capability
Lower impedance per unit length than overhead lines
Low failure rates
High lifespan expected (Experience with GIS)
 No ageing
 Lowest electro-magnetically fields
 Lower losses than cables
 Lower operational costs (including losses) than cable lines
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8. Gas-Insulated Lines – Disadvantages
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High Requirements to purity and gas-tightness
Higher reactive power than overhead lines
Gas monitoring, failure location, PD-monitoring
Higher unavailability than cables because of long period of
repair
 Short operational experience, only short distances in
operation
 Large sections necessary, only limited usage of soil
possible, issues with SF6
 Investment costs 7-12 times higher when compared to
overhead lines
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9. Technical Development
 High Temperature Superconductivity (HTS)
 Cable Technology: New developments are applied to
medium voltage networks
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Reduced losses
Reduced weight
Compact systems
Temperature currently 138 K (- 135 °C)
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Structural Elements of Mono-Core Power Cable
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Structural Elements of 3-in-1 Power Cable
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Nanotechnology
 Nanotechnology for cables for medium and high voltage
applications (voltage level up to about 500 kV)
 Advantages:
 Reduction of space charge
 Improved partial discharge behaviour
 Increase of the electric field strength for the dielectric breakdown
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Institute for High-Voltage Engineering and Systems Management
Nanotechnology
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10. Summary – Energy Transmission
Energy Losses
 Joule Effect – Heating of conductors
 Magnetic losses – Energy in the magnetic field
 Dielectric losses – Energy in the insulating materials
Remedies
 Transformers with reduced losses
 Transformers with superconductivity
 High temperature superconductivity (HTS) - Cables
 Nanotechnology
 Direct Current Transmission (HVDC)
 Ultra High Voltage (UHV)
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Transmission Systems (1)
Alternating Current Transmission (HVAC)
 All 3 Systems possible
 Overhead lines up to 1500 kV (multiple conductor wires)
 Cable lines up to 500 kV
 GIL currently up to 550 kV, higher voltages possible
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Transmission Systems (2)
Direct Current Transmission (HVDC)
 Overhead lines up to 1000 kV possible
 Oil-Paper cables up to 500 kV
 Cables with synthetic materials up to 200 kV (space
charges), with nanotechnology higher values are possible
(~ 500 kV)
 GIL is currently under research
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Institute for High-Voltage Engineering and Systems Management
Transmission Systems (3)
 In general, overhead-, cable- and gas-insulated lines are
suitable for alternating current transmission systems
 Cables and GIL are currently only applied for short lengths
 specifically for example in urban areas, tunnels, undercrossings, etc.  Therefore no operational experience nor
actual costs can be given for long sections
 In a macro-economical point of view, overhead lines are the
most favourable system (the capital value of cables 2 to 3
times and GIL 4 to 6 higher)
 Currently overhead lines are from the technical and
economical point of view the best solution
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Thank you for your attention!
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