Christian Ohler, ABB Switzerland Corporate Research Physics of Electric Power Systems. ABB Group August 1, 2012 Slide 1

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1 Christian Ohler, ABB Switzerland Corporate Research Physics of Electric Power Systems ABB Group August 1, 2012 Slide 1

2 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

3 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): ~ 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.

4 Motivation Large power grids provide highest reliability electricity The weighted average interruption duration («SAIDI») is about 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

5 (1) Overhead Transmission Lines August 1, 2012 Slide 5

6 Overhead Transmission Lines August 1, 2012 Slide 6

7 Overhead Transmission Lines Details August 1, 2012 Slide 7

8 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

9 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 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

10 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

11 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

12 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

13 Overhead transmission lines get their design from minimizing conductor and tower material cost August 1, 2012 Slide 13

14 (2) Transformers August 1, 2012 Slide 14

15 Transformers August 1, 2012 Slide 15

16 Transformers Details ABB Group 28 th March 2012 Slide 16

17 Transformers Details ABB Group 28 th March 2012 Slide 17

18 Transformers Details Steel core Windings August 1, 2012 Slide 18

19 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

and consumption Application ABB transformers 1

Wind & Solar Power generation 2 3 3 1 2 Dry or

distribution 4 1 3 Power and distribution

Special transformers (Traction) 6 5 4 6 Oil &

20 Transformers allow for adequate voltage levels for generation, transmission, distribution, and consumption Application ABB transformers 1 Power generation Power transformers 2 Wind & Solar Power generation Dry or Special transformers 3 Transmission and distribution Power and distribution transformers 4 Commercial and residential usage Dry and distribution transformers 5 5 Railway application 1 Dry or Special transformers (Traction) 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

21 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

22 Voltage and current ratio are determined by the turn ratio U I N U I N 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

23 Three phase transformers = 0 August 1, 2012 Slide 23

24 Coaxial windings reduce leakage flux transformers often need to be inductive for short circuit limitation H leakage =1 =2000 August 1, 2012 Slide 24

25 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

26 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

27 Transformers need dedicated cooling Conservator Radiators Fans Tank August 1, 2012 Slide 27

28 Transformers get their design from minimizing core and conductor size ABB Group 28 th March 2012 Slide 28

29 (3) Synchronous Generators August 1, 2012 Slide 29

30 Synchronous Generators August 1, 2012 Slide 30

31 Synchronous Generators Details Contact ring for carbon brushes Stator winding in two planes August 1, 2012 Slide 31

32 Synchronous generators control the real power by adjusting the rotor angle by adjusting the torque Operating point August 1, 2012 Slide 32

33 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

34 Synchronous generators control real power and reactive power August 1, 2012 Slide 34

35 (4) Circuit breakers August 1, 2012 Slide 35

36 Grids are heavily interconnected. August 1, 2012 Slide 36

37 Why do we need circuit breakers? Anything that can go wrong, will go wrong somewhere. ABB Group August 1, 2012 Slide 37

38 Circuit breakers can interrupt short circuits and disconnect grid segments with a fault HV Circuit Breakers GENERATION TRANSMISSION DISTRIBUTION 12-24kV A kA HIGH VOLTAGES kV 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

39 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

Short circuit current 4 Current across the Circuit Breaker

Fast separate electrical contacts to «draw» a switching arc.

Use part of the arc energy and/or a piston to build up

Force a gas flow which cools the arc as it approaches a

40 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 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

Turbulent transport Heat transfer Dielectrics Gas dynamics

41 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

42 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

43 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

44 We have not treated the control aspects August 1, 2012 Slide 44

45 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

Power System Analysis I TRANSMISSION LINE PARAMETERS

Power System Analysis I TRANSMISSION LINE PARAMETERS Components of Overhead Lines Transmission Line Parameters An overhead transmission line consists of conductors, insulators, support structures, and,