michael muhr 1 high voltage engineering for modern transmission networks institute for high-voltage...

Michael MUHR

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High Voltage Engineering For Modern Transmission Networks

Institute for High-Voltage Engineering and Systems Management


Michael MUHRO.Univ.-Prof. Dipl.-Ing. Dr.techn. Dr.h.c.O.Univ.-Prof. Dipl.-Ing. Dr.techn. Dr.h.c.

Institute for High-Voltage Engineering and Systems ManagementGraz University of Technology

Austria


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Michael MUHR High Voltage Engineering For Modern Transmission Networks

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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Development of the world population and the power consumption between 1980 and 2020

Source: IEA; UN; Siemens PG CS4 - 08/2002


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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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Development of Voltage Levels for HVAC

In Discussion: China 1000 kVJapan 1100 kVIndia 1200 kV

In Discussion: China 1000 kVJapan 1100 kVIndia 1200 kV

Source: SIEMENS


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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

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

i1w/o PST

X1

X2

UPST

~itotal

i2-Δi

i1+Δi with PSTX1

X2


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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 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


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4. Future Trends

Costs of a high voltage transmission system

Source: SIEMENS PTD SE NC - 2002


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Possibilities for Transmission Systems for high power

Hybrid Connection

Alternating Current (AC)

Direct Current (DC)

Hybrid AC / DC - Connection

Source: SIEMENS


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Transmission Line SystemsAC DC

Maximum voltagein operation

kV 800 +/- 600

Maximum voltageunder development

kV 1000 +/- 800

Maximum powerper line in operation

MW 2000 3150

Maximum powerper 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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Overhead LineOverhead Line

5. Transmission Lines

Liberalisation of the Electricity Market Renewable Energy is on the rise Increased environmental awareness

Liberalisation of the Electricity Market Renewable Energy is on the rise Increased environmental awareness

Possibilities forTransmission Lines

in High Voltage Networks:

Possibilities forTransmission Lines

in High Voltage Networks:

Decision CriteriaDecision Criteria

Cable LineCable Line Gas Insulated LineGas Insulated Line


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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 water- tightness

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 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

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

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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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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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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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

Michael MUHR

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Thank you for your attention!

michael muhr 1 high voltage engineering for modern transmission networks institute for high-voltage...

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