The electrical system: the very fundamentals The material of this presentation is entirely based on teaching material developed by prof. Thierry Van Cutsem
Objectives Show the overall structure of an electric power system Highlight a few important features of power system operation Illustrate those on the Belgian and European system Present some orders of magnitude it is important to have in mind Introduce some terminology 2
A large-scale system In modern society, electricity has become a commodity definition : marketable good or service whose instances are treated by the market as equivalent with no regard to who produced them behind the power outlet there is a complex industrial process electric energy systems are the largest systems ever built by man thousands of km of overhead lines and underground cables, of transformers tens/hundreds of power plants + a myriad of distributed energy sources devices to (dis)connect elements: substations, circuit breakers, isolators protection systems: to eliminate faults real-time measurements : active and reactive power flows, voltage magnitudes, current magnitudes, energy counters, phasor measurement units controllers: distributed (e.g. in power plant) or centralized (control center) etc. unlike most other complex systems built by man, power systems are exposed to external aggressions (rain, wind, ice, storm, lightning, etc.) 3
Low-probability but high-cost failures In spite of those disturbances, modern electric power systems are very reliable Example : typical duration of power supply interruption 0.5 hour / year availability =. = 99.994 %! however, the cost of unserved energy is high average cost used by CREG (Belgian regulator) to estimate the impact of forced load curtailment : 8 300 /MWh (source: Bureau fédéral du plan) varies with time of the day : between 6 000 and 9 000 /MWh varies with type of consumer : 2 300 /MWh for domestic, much higher for industrial higher average cost considered elsewhere : e.g. 26 000 /MWh in France! large-scale failures (blackouts) have tremendous societal consequences next two slides: examples of blackouts and their impacts 4
USA-Canada blackout, Aug. 2003 source : North American Electric Reliability Council (NERC) 50 million people disconnected initially 61 800 MW of load cut in USA & Canada cost in USA : 4 to 10 billion US $ in Canada : 18.9 million working hours lost 265 power plants shut down restoration time : few hours up to 4 days 5
Italian blackout, Sept. 2003 cascade tripping of interconnection lines separation of Italy from rest of UCTE system deficit of 6 660 MW imported in Italian system, causing frequency to collapse in Italy 340 power plants shut down 55 million people disconnected initially - 26 000 MW lost (blackout occurred during night) estimated cost of disruption 139 million US $ restoration time : up to 15 hours source: Union for the Co-ordination of Transmission of Electricity (UCTE) which is now part of ENTSOe 6
Network : from early DC to present high-voltage AC End of 19 th century : Gramme, Edison devised the first generators, which produced Direct Current (DC) under relatively low voltages impossibility to transmit large powers with direct current: if voltage cannot be increased, the current must be but increasing the current wastes energy and requires large sections of conductors impossible to interrupt a large DC (no zero crossing), e.g. after a short-circuit changing for Alternating Current (AC) voltage increased and lowered thanks to the transformer standardized values of frequency : 50 and 60 Hz (other values used at a few places) larger nominal voltages have been used progressively up to 400 kv in Western Europe up to 765 kv in North America experimental lines at 1100 kv or 1200 kv (Kazakhstan, Japan, etc.) 7
Structure of Belgian electric network transmission meshed structure large power plant 10-25 kv (sub-transmission) large power plant 10-25 kv 10-25 kv very high voltage 400 & 225 kv high voltage 70 & 150 kv interconnection with another country large industrial customer distribution radial structure feeder feeder Note. In Belgium there are 30 and 36 kv underground cable networks, in Brussels and Antwerp areas. These are not distribution networks because they are meshed and play the role of subtransmission residential customers low voltage tertiary sector 0.4 1 kv small industrial customer dispersed generation 8
380 (and 220 kv) grid in Belgium and interconnections 9
Length of network by voltage level and type in Belgium sources : ELIA, SYNERGRID December 2014 Very High Voltage High Voltage Medium Voltage Low Voltage Nominal voltage (kv) Underground cables (km) Transmission Overhead lines (km) Total (km) 380 891 891 220 5 300 305 150 474 1 993 2 467 70 295 2 346 2 641 36 1 954 8 1 962 30 124 22 146 Total 2 852 5 560 8 412 Distribution ( 1 ) 1 < 30 69 532 5 754 75 286 < 1 ( 2 ) 79 044 49 076 128 120 ( 1 ) 570 connection points between T & D ( 2 ) does not include public lighting Total 148 576 54 830 203 406 Total number of transformers : 74 271 10
Electrical energy balance over the year 2013 in Belgium generation 62.4 losses 1.4 import 17.2 Transmission 7.6 export All values in TWh 51.3 1 TWh = 10 3 GWh = 10 6 MWh = 10 9 kwh generation 5.4 9.0 Total consumption = 26.5 + 1.8 + 55.6 = 83.9 TWh 17.5 26.5 1.8 pumped storage Distribution 54.2 1.4 autoproduction loads autoproduction loads 55.6 losses Yearly average consumption of a family (4 persons) 3500 4000 kwh 2.5 source : SYNERGRID 11
Network losses Belgium 370 & & 1.4 2.5 62.4 17.2 5.4 4.6 % 12
Production of electrical energy in Belgium Type Gener. capacity Dec. 2013 Gener. capacity June 2015 Energy produced in 2013 Capacity factor MW MW % total GWh % total % Nuclear 5 926 5 620 27.7 40 632 51.9 78 Fossil : total 7 500 6 570 32.4 26 917 34.4 Gas 6 880 5 860 28.9 19 985 25.5 33 Coal 410 480 2.4 2 352 3.0 65 Oil 210 230 1.1 0 0 Other / mixed fuels??? 4 580 5.8 Hydro : total 1 430 1 380 6.8 1 672 2.1 Run-of-river 120 180 0.9 357 0.5 34 Renewable non hydro: total 5 740 6 680 33.0 9 128 11.7 Wind 1 720 2 120 10.5 3 563 4.5 24 Photovoltaic 2 680 3 160 15.6 2 424 3.1 10 Biomass 1 340 1 400 6.9 3 141 4.0 27 TOTAL 20 596 20 250 100 78 349 100 sources : ELIA, ENTSOe 13
Comments on the previous slide Nuclear generation capacity involves all units, even those which were shut down due to technical issues hydro power : consists mainly of pumped stored gas power plants includes small CHP (Combined Heat Power) units same for biomass main hydro generation is by pumped storage plants Capacity Factor = ( ) ( ) usually close to 90 % for nuclear, but technical issues made some units unavailable note the low value for photovoltaic sources! Trends : shutdown of some gas units (cannot compete on electricity market) natural hydro resources saturated in Belgium off-shore wind farms have a higher capacity factor than on-shore ones: wind is more steady in the sea; great potential available public opposition to construction of on-shore wind farms in densely populated areas: Not In My BackYard (NIMBY) 14
Examples of variability of wind and photovoltaic generation (Germany) hour of the year 2013 hour of the year 2013 15
The power balance primary energy sources conversion electrical generators AC network stored losses consumers Conservation of Energy over an infinitesimal time : Introducing the corresponding powers at time.... the consumers decide how much power they want to consume! this demand fluctuates at any time 16
Network elements which store electrical energy : inductors and capacitors Example of inductor = 2 cos = (1 + cos 2) In sinusoidal steady state, the power in an inductor or a capacitor reverts every quarter of a period, and is zero on the average In balanced three-phase operation, the sum of the powers in the inductors/capacitors of the three phases is zero at any time! Hence, electrical energy cannot be stored in the AC network! = / to be stored, electrical energy has to be converted into another form of energy mechanical: e.g. potential energy in the upper reservoir of a pumping station, flywheels, etc. chemical: batteries (energy amounts are still very small) 17
losses mainly due to Joule effects depend on currents in components kept as small as possible, not really controllable Conclusion the variations of load power have to be compensated by the generators however, the conversion primary electrical energy is not instantaneous: example: changing the flow of steam or water in a turbine takes a few seconds an energy buffer is needed to quickly compensate power imbalances this is provided by the rotating masses of synchronous generators a deficit (resp. excess) of generation wrt load results in a decrease (resp. increase) of speed of rotation speeds (and hence, frequency) in a synchronous generator and its turbine, kinetic energy = maximum power of the generator produced during 2 to 5 seconds controlling the power balance in a power system without rotating machines (only power electronic interfaces) would be a challenge (still at research level)! larger variations in load (e.g. during the day) require starting up/shutting down power plants ahead of time 18
12500 MW Consumed power in Belgium max load of the day min load of the day 6000 MW 2014 2015 12600 MW y monotonic diagram relative to one month (similarly for one year) source: ELIA during x hours the load power has been greater than y MW 31x24=744 hours 7000 MW x 19
12600 MW Consumed power in Belgium holidays source: ELIA 7100 MW evolution of load in January 2015 w-e w-e w-e w-e w-e evolution of load in the first week of January 2015 holidays w-e 20
Consumed power in Europe and Belgium Monthly power in ENTSOe networks source: ENTSOe Peak loads recorded on the Belgian transmission system Year Date Time Day Power (MW) 2010 Dec 14 18:00 Tue 14 200 2011 Feb 1 18:15 Tue 13 000 2012 Feb 7 18:30 Tue 13 144 2013 Jan 17 18:00 Thu 13 255 2014 Dec 4 18:00 Thu 12 692 source: SYNERGRID 21
Large AC interconnections Motivations : mutual support between partners to face the loss of generation units each partner would have to set up a larger reserve if it would operate isolated larger diversity of energy sources available within the interconnection allows exploiting complementarity of nuclear, hydro and wind power plants allows partners to sell/buy energy, to create a large electricity market. Constraints : if one partner is unable to properly contain a major incident, the effects may propagate to the other partners networks a transaction from one point to another cannot be forced to follow a contractual path; it distributes over parallel paths ( wheeling ) (see example on next slide). Partners not involved in the transaction undergo the effects of the power flow in large AC interconnections, there may be emergence of badly damped, slow (0.1 to 0.5 Hz frequency) interarea oscillations rotors of synchronous generators in one area oscillate against the rotors of generators located in another area it may not be possible to connect two networks with different power quality standards 22
Example of paths followed by a transaction Paths taken by a production increment of 100 MW in Belgium covered by a load increase of 100 MW in Italy (variation of losses neglected) 23
European networks ENTSOe : European Network of Transmission System Operators (TSO) for electricity 41 TSOs from 34 countries www.entsoe.eu 24
The synchronous grids of Europe RG = Regional Group source : ENTSOe 25
Yearly energy exchange of the countries members of ENTSOe 26
The come-back of Direct Current Advances in power electronics rectifiers and inverters able to carry larger currents through higher voltages transmission applications made possible 1 st use : power transmission over longer distances through overhead lines backbone transmission system of Hydro-Québec dark blue: AC transmission at 735 kv light blue: HVDC link : 1018 km ± 450 kv 2000 MW 27
The come-back of Direct Current 2 nd use : power transmission in submarine cables DC more attractive than AC for distances above 50 km : owing to capacitive effects of AC cables existing links in Europe : see slide No. 24 projects involving Belgium: Nemo with England, Alegro with Germany : see slide No. 9 connection of off-shore wind parks : AC and DC connections of off-shore wind parks in North Sea to the grid of the Tennet German TSO (links under construction shown with dotted lines) source : ENTSOe 28
The come-back of Direct Current 3 rd use : back-to-back connection of : two networks with different nominal frequencies connection of 50 and 60 Hz systems in Japan connection of Brazil at 60 Hz with Argentina at 50 Hz two networks that have the same nominal frequency but cannot be merged into a single AC network, e.g. for stability reasons UCTE and Russian (IPS/UPS) system Eastern - Western interconnections in North-America 29
Main technological challenge : Direct Current circuit breaker (to clear short-circuits in DC grid without shutting down the whole grid) 30
Appendix. Some characteristics of power plants Type of power plant Classic thermal (coal, gas, oil) Combined-cycle gas unit Efficiency wrt primary energy source Cost of fuel + operation & maintenance /MWh Cost of installed power (construction) /kw Average starting time 40 % 45 1 500 1 h if unit is warm 8 h if unit is cold Up to 60 % 40 750 2 to 6 h Nuclear 33 % 10 2 900 (without waste treatment) Hydro 90 % 0 (water) + 15 Pumped storage 85 % 0 (water) + 15 Gas turbine (peaking unit) Depends a lot on the type of plant 24 h 5 min 900 2 to 4 min 40 % 65 500 Less than 10 min Wind 50 % 0 (wind) + 15 (on-shore) / 30 (off-shore) photovoltaic 15 % 0 (sun) + 15 1 500 (on-shore) 4 000 (off-shore) A few min 2 000 negligible 31