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

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

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

Source: IEA; UN; Siemens PG CS4 - 08/2002 Development of the world population and the power consumption between 1980 and 2020

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

Development of Voltage Levels for HVAC In Discussion: China 1000 kV Japan 1100 kV India 1200 kV Source: SIEMENS Development of Voltage Levels for HVAC

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

Phase shift transformers (PST) Distribution of current depends on Impedances only Unequal distribution Implementation of additional voltage sources 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 itotal i2 i1 w/o PST X1 X2 UPST ~ i2-Δi i1+Δi with PST

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

Advantages of HVDC Disadvantages of HVDC 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 Additional costs for converter station and filters Harmonics requires reactive power Expensive circuit breakers Low overload capability

Costs of a high voltage transmission system 4. Future Trends Source: SIEMENS PTD SE NC - 2002 Costs of a high voltage transmission system

Possibilities for Transmission Systems for high power Alternating Current (AC) Direct Current (DC) Hybrid AC / DC - Connection Hybrid Connection Source: SIEMENS

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

Prof. S. Gubanski / Chalmers University of Technology

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

in High Voltage Networks: 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

Framework Economic necessity Transmission capacity Voltage level Comply with (n-1) – criteria Reliability of supply Operational conditions Environmental requirements (Civil) engineering feasibility Economics

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

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

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

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)

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

7. Cable Lines - Disadvantages High requirements to purity of synthetic insulation and water- tightness Overload only temporary possible ð 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

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

8. Gas-Insulated Lines – Advantages Large transmission capacity High load capacity 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

8. Gas-Insulated Lines – Disadvantages 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

9. Technical Development High Temperature Superconductivity (HTS) Cable Technology: New developments are applied to medium voltage networks Reduced losses Reduced weight Compact systems Temperature currently 138 K (- 135 °C)

Structural Elements of Mono-Core Power Cable

Structural Elements of 3-in-1 Power Cable

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

Nanotechnology

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)

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

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

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, under- crossings, 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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