Christian Ohler, ABB Switzerland Corporate Research Physics of Electric Power Systems ABB Group August 1, 2012 Slide 1
Purpose of this Presentation Describe power systems from a physicists point of view (1) Overhead transmission lines (2) Transformers (3) Generators (4) Circuit breakers Focus is on the components and their design ( = materials + geometry): Why do they look like this? August 1, 2012 Slide 2
Motivation Large power grids provide lowest cost electricity Electricity provides 15% of the final energy consumption uses 30% of the primary energy Energy: ~ 10% of GDP Electricity: ~ 3% of GDP World GDP (2010): ~ 60 000 BUSD Sources: IEA, IMF August 1, 2012 Slide 3
Motivation Large power grids provide highest reliability electricity The weighted average interruption duration («SAIDI») is about 15-60 minutes per year (0.005% of the time) in Europe and US Source: R. Munoz et al., Overview of the Quality of Electricity Supply in Spain, ICREPQ 11 Photo: Joachim Kruse August 1, 2012 Slide 4
(1) Overhead Transmission Lines August 1, 2012 Slide 5
Overhead Transmission Lines August 1, 2012 Slide 6
Overhead Transmission Lines Details August 1, 2012 Slide 7
Overhead Transmission Lines Observations Tower (High Voltage) Three lines per circuit Bundles of Aluminum Conductors Steel Reinforced (ACSR) Support and suspension insulators with sheds and shield electrodes August 1, 2012 Slide 8
We need high voltage overhead lines to limit the transmission losses Transferred power is voltage times current: P trans = U x I Resistive losses go with the square of current: P loss = R x I 2 Example: Aluminum conductor with 10 cm 2 cross section, one phase Resistance 0.03 /km At maximum current of 2 600 A, P loss = 200 kw/km At 380 V P trans = 1 MW, P loss = 20% per km At 380 kv P trans = 1 GW, P loss = 0.02% per km August 1, 2012 Slide 9
High voltage insulation is a fight against the risk of destructive breakdowns Breakdown field strength Dry air at 1 bar: 3 kv/mm Sulfur hexafluoride at 6 bar: 50 kv/mm Porcelain / Epoxy resin: 30 kv/mm Depends on pressure (for gases), temperature, electrode geometry, duration, purity, ageing Statistical character explains need for large safety margins in practical designs August 1, 2012 Slide 10
Three symmetric phases provide constant power flow with 50% less conductor material I1 N V1~ I1+I2+I3 = 0 V2~ V3~ I2 Complex phasor diagram August 1, 2012 Slide 11
Conductor bundles reduce skin effect losses, parasitic inductance, and Corona discharges Bundle approximates hollow conductor of larger diameter Lower electric and magnetic fields at conductor surface Impedance from self inductance about ten times bigger than resistance H Skin effect Field lines around bundle August 1, 2012 Slide 12
Overhead transmission lines get their design from minimizing conductor and tower material cost August 1, 2012 Slide 13
(2) Transformers August 1, 2012 Slide 14
Transformers August 1, 2012 Slide 15
Transformers Details ABB Group 28 th March 2012 Slide 16
Transformers Details ABB Group 28 th March 2012 Slide 17
Transformers Details Steel core Windings August 1, 2012 Slide 18
Transformers Observations Many sizes Cooling and fluid insulation Three phase Coaxial low voltage and high voltage windings per phase Laminated core steel August 1, 2012 Slide 19
Transformers allow for adequate voltage levels for generation, transmission, distribution, and consumption Application ABB transformers 1 Power generation 6 2 1 Power transformers 2 Wind & Solar Power generation 2 3 3 1 2 Dry or Special transformers 3 Transmission and distribution 4 1 3 Power and distribution transformers 4 Commercial and residential usage 1 3 3 4 Dry and distribution transformers 5 5 Railway application 1 Dry or Special transformers (Traction) 6 5 4 6 Oil & Gas application 2 Subsea transformers ABB Group 28 th March 2012 Slide 20
Edison (DC) lost the «War of Currents» against Tesla and Westinghouse (AC) because of transformers DC and AC Technology were competing evenly (~1880). In the end, AC won the battle. Why? Big losses in transmission: Voltage transformation required. AC transformers then easily available. Edison DC voltage conversion then clumsy (combination of motors and generators) DC required local generation (close to consumers) and different lines for different purposes (motors / lights). Westinghouse Tesla ABB Corporate Research February 1 st 2012 Slide 21
Voltage and current ratio are determined by the turn ratio U I N U I N 1 2 1 2 1 2 Magnetic core i 1 Electric Load i 2 u 2 N2 Windings N 1 u 1 ~ AC Voltage Source =2000 =1 ABB Group 28 th March 2012 Slide 22
Three phase transformers = 0 August 1, 2012 Slide 23
Coaxial windings reduce leakage flux transformers often need to be inductive for short circuit limitation H leakage =1 =2000 August 1, 2012 Slide 24
No-load transformer losses are limited by use of thin laminated sheets of silicon steel Eddy-current losses in the core depend on lamination thickness and the skin-depth = 2 Laminate blocks eddy currents I2 I1 ABB Group 28 th March 2012 Slide 25 Top view of laminated core
Transformers have losses between 0.3% and 3% of the nominal power Ohmic loss in conductors Hysteresis losses in core Eddy current losses in the core Losses in structural steel No load loss very relevant for partial load
Transformers need dedicated cooling Conservator Radiators Fans Tank August 1, 2012 Slide 27
Transformers get their design from minimizing core and conductor size ABB Group 28 th March 2012 Slide 28
(3) Synchronous Generators August 1, 2012 Slide 29
Synchronous Generators August 1, 2012 Slide 30
Synchronous Generators Details Contact ring for carbon brushes Stator winding in two planes August 1, 2012 Slide 31
Synchronous generators control the real power by adjusting the rotor angle by adjusting the torque Operating point August 1, 2012 Slide 32
Synchronous generators control the reactive power and hence voltage by adjusting the rotor field excitation Capacitive current Overexcitation Inductive current Underexcitation August 1, 2012 Slide 33
Synchronous generators control real power and reactive power August 1, 2012 Slide 34
(4) Circuit breakers August 1, 2012 Slide 35
Grids are heavily interconnected. August 1, 2012 Slide 36
Why do we need circuit breakers? Anything that can go wrong, will go wrong somewhere. ABB Group August 1, 2012 Slide 37
Circuit breakers can interrupt short circuits and disconnect grid segments with a fault HV Circuit Breakers GENERATION TRANSMISSION DISTRIBUTION 12-24kV 6000-24000 A 50-500kA HIGH VOLTAGES 72-800kV 2500-4000 A 25-63kA MEDIUM VOLTAGE 1-36 kv LOW VOLTAGE < 1 KV The existence of high voltage circuit breakers is another advantage of AC over DC August 1, 2012 Slide 38
Circuit Breakers are automatically-operated electrical switches that protect a circuit from overload damage A circuit-breaker Device used to open/close electric circuits Ideal conductor in close position Ideal insulator in open position Why using a circuit breaker? 1. Fault current switching Unplanned events initiated by the network s protection system 2. Load current switching Planned events initiated by the system operator August 1, 2012 Slide 39
How does a Circuit Breaker interrupt? A V Recovery voltage 5 Voltage across the Circuit Breaker 1 Short circuit current 4 Current across the Circuit Breaker fault 2 3 1. Fast separate electrical contacts to «draw» a switching arc. Circuit Breaker in fully closed open position 2. Use part of the arc energy and/or a piston to build up pressure in dedicated volumes. 3. Force a gas flow which cools the arc as it approaches a current zero crossing. 4. Interrupt the current at the current zero crossing. 5. Recover the dielectric strenght of the gas faster than the voltage rise from the network. ABB Group August 1, 2012 Slide 40
Research in Gas Circuit Breakers Many interconnected physical disciplines Material Properties Arc physics Radiation transport Turbulent transport Heat transfer Dielectrics Gas dynamics Diagnostics Mechanics and control Beforecurrentzero At zero After zero August 1, 2012 Slide 41
Circuit breakers can interrupt short circuits and disconnect grid segments with a fault August 1, 2012 Slide 42
Summary Power components enable the reliable grid operation High voltage overhead transmission lines enable long distance power transmission with minimal conductor and tower material Transformers allow for adequate voltage levels for generation, transmission, distribution, and consumption Synchronous generators control real power and reactive power Circuit breakers can interrupt short circuits and disconnect grid segments with a fault August 1, 2012 Slide 43
We have not treated the control aspects August 1, 2012 Slide 44
Interconnected power grids are a remarkable engineering success and achievement of our societies The continental European grid is capacity-wise the largest synchronous power grid of the world August 1, 2012 Slide 45