Samuel NGUEFEU Le rseau de transport d'lectricit (RTE) DAY 1: SMART - PowerPoint PPT Presentation

European cooperation Network on Energy Transition in Electricity Samuel NGUEFEU Le réseau de transport d'électricité (RTE) DAY 1: SMART GRIDS TABLE 1: TECHNOLOGICAL CHALLENGES RELATED WITH SMART GRIDS DEVELOPMENT INTERNATIONAL SUMMER SCHOOL “SMART GRIDS AND SMART CITIES” Barcelona, 6-8 June 2017

• 01. HVAC versus HVDC in Power Transmission • 02. Two technologies: LCC and VSC • 03. Cost comparison: DC against AC • 04. What about MVDC? 2

■ The battle of currents • Thomas Edison: believes in DC � 1878: incandescent lamp � 1882: 1 st DC distribution grid Lucien Gaulard • Georges Westinghouse: in favor of AC • 1885 : Importing transformer from L. Gaulard & J. Gibbs and AC generator from Siemens. 1886 : 1 st AC grid, hydro generator 500 V, • transformer 3000 V, network of 100 V bulbs. • 1887 : 30 other electric lighting systems based on AC are installed. • 1888 : AC meters (O. Shallenger) et AC motors (N. Tesla) make the victory of AC. Nikola Tesla 3 3

• AC transformers help reducing losses while transmitting energy at high voltages • Easy development of the network to connect new loads • Easy elimination of faulty grid sections thanks to AC Breakers A D B E Public Grid Limit Substation Substation C Connection of a new load onto an existing HVAC line 4

• Better control of power flows in meshed networks. • Stable connection of asynchronous networks i.e. with a large phase difference. • Connection of AC networks operating at different frequencies. • Transmission capacity of lines and cables is saved in DC thanks to the absence of reactive power. • 30% less copper in the conductors [DC] to transmit same power as in AC. • DC overhead/Underground infrastructure have smaller environmental impacts. Dimensions of 1000 MW towers 5

Advantages of DC Drawbacks of DC • • More power per conductor (2 Converter stations are expensive • conductors instead of 3) Converters create harmonics • Smaller transmission towers • Converters generate additional losses • (30% cost saving in line building) Converters are additional sources of failures • • Lower transmission losses Converters increase the weight of offshore • No charging current in steady state stations • • No reactive power transfer Additional components may reduce reliability / • availaibility No skin effect • • DC grids are not easy to operate No transmission length limitation • • No mature DC breakers exist today No synchronous operation required • • Only 200 GW of installed DC transmission Asynchronous connections are possible capacity • Isolation and even mitigation of • Multi-voltage level i.e. connection involving electromechanical oscillations different DC voltage levels needs DC/DC • Controllability of active power converters • Isolation from AC faults 6

Line Commutated current source Converters Light Triggered Thyristor ON state OFF state OFF state Electrical Triggered Thyristor 7

U RS U RT U ST U SR U TR U TS U RS U U RS RT Normal operation Prohibited configuration Commutation phenomenon 8

Gate sequence every 20 ms is as follows: 1 2’ 2 3’ 3 4’ 4 5’ 5 6’ 6 1’ The thyristor bridge is basically a current source: it converts the DC current I d into a 3-phase alternating current (I R , I s , I T ) in the presence of (U R , U s , U T . 9

Mitigation of AC side harmonics T/4 T/2 3T/4 i 1 i 2 i 1 Phase currents, lagging ( α α α α ) phase i 1 + i 2 Y Y voltages � � reactive � � i 1 + i 2 power absorption. Y ∆ ∆ ∆ ∆ [%] I n i 2 10 I 1 5 n 5 7 11 13 17 19 23 25 [%] Σ I n 5 i 1 + i 2 n 11 13 23 25 10

3 Quadrivalves 500 kV DC suspended valves - Upper trace: Transformer voltage - Middle trace: Valve voltage - Lower trace: DC voltage 11

Air core smoothing reactor: Smoothing reactor in oil: 270mH, 150mH, 500kV et 1800A 500kV et 3000A 12

13

Power (MW) Thyristor 100 GTO 10 IGBT 1 Voltage Source Converters 0.1 f (kHz) 0.01 0.01 0.1 1 10 IGBT 14

VSC: self-commutated Voltage Source Converters • Bi-directional Valves (anti- parallel diodes) • No need for cumbersome AC filters and capacitor banks • Voltages across IGBT valves are always positive • 2 IGBT-based converter stations – 1 controls DC voltage – 1 controls DC power (or DC current) • Power reversal is managed by changing the DC current direction : no voltage polarity reversal across the cables • High frequency Commutation (1 – 2 kHz) • Forced Commutation, to turn on and turn off the IGBTs 15

X I P VSC R δ P = 3V c Icos ϕ V r V a Q jXI Q = 3V c Isin ϕ V r ϕ V a I RI Convention assumed for VSC: positive P & Q are supplied, resistance R is neglected U ⋅ U ⋅ sin δ P = a r U U a ⋅ X U dc I ⋅ r dc_max X ( ) U 2 U U cos δ U ⋅ ⋅ − − a Q = r a r 3 Ur I ⋅ ⋅ X X max - 2 2 S = P + Q ≤ S = 3 ⋅ U ⋅ I r max max - U dc I ⋅ dc_max 2 2  2  U U ⋅ U   ( ) 2 cos 2 sin 2 1 _ _ _ _ :  Q  P 0 + = ⇒ + a + + = a r for a given grid voltage   δ δ   X X     As for any synchronous machine, the typical PQ diagram of a VSC exhibits some limitations 16

• PWM ~2 kHz minimises the size of AC filters. • but increases commutation losses up to 3% per converter! Phase-to-phase Phase-to-neutral voltage voltage DC voltage PWM frequency Length Hällsjön 1997 Sweden 3 MW ±10 kV 10 km overhead lines 1950 Hz Gotland 1999 Sweden 50 MW ±80 kV 70 km submarine cables 1950 Hz Directlink 2000 Australia 3 x 60 MW ±80 kV 59 km underground cables 1950 Hz Tjaereborg 2000 Denmark 7.2 MW ±9 kV 4.4 km submarine cables 1950 Hz 17

• Higher DC voltages are possible without increasing stress on off- state valves. • PWM ~1350 Hz reduces losses to 1.8% per converter. • The waves harmonic content is also improved Phase-to-phase Phase-to-neutral voltage voltage PWM frequency Eagle Pass 2000 USA 36 MW ±15.9 kV Back-to-Back 1500 Hz Cross Sound 2002 USA 330 MW ±150 kV 40 km submarine cables 1260 Hz Murray Link 2002 Australia 220 MW ±150 kV 180 km underground cables 1350 Hz 18

OPWM : u U dc commutations 2 are stopped to further 0 reduce the 3 2 t π π π π ω 2 2 losses U dc − 2 α α α 1 2 3 Tension phase-neutre • OPWM ~2 kHz brings the losses to 1.6% per converter station and simplifies the control with respect to generation 2. • The wave quality is slightly poorer (OPWM frequency = 1950 Hz) Troll A 2005 Norway 2 x 41 MW ±60 kV 67 km submarine cables Estlink 2006 Est –Fin 350 MW ±150 kV 105 km underground cables Caprivi Link 2010 Namibia 300 MW 350 kV 970 km overhead lines Valhall 2011 Norway 78 MW ±150 kV 292 km submarine cables Nord E.ON 1 2012 Germany 400 MW ±150 kV 203 km sub. & underg. cables East West 2012 Ireland- 500 MW ±200 kV 261 km sub. & underg. cables interconnector Wales 19

The number of contributing modules is adjusted at each instant in each half-arm of the converter to create the sinusoidal voltage Va having the desired amplitude and phase angle I dc Vdc = + 320 kV Single phase equivalent diagram V sup V max + P, Q V a Xt Xv Va Ia Xv V inf V r_400 m V r =m V r_400 I dc + − Vdc = − 320 kV Losses: ~ 2,2% for both stations i.e. 1.1 % per converter station The v oltage s ource c onverter structurally transforms the DC voltage 2V dc into a set of three-phase balanced alternating voltages (V a , V b , V c ). 20

LCC HVDC VSC HVDC Semi conductor Thyristors, "natural" commutation IGBT, forced commutation Equivalent Current Source Voltage Source Up to 8 GW ± 800kV (bipole); About 1.2 GW per symmetrical monopole Power Ratings 10 GW ± 1.1MV 3333 km commissioned 2017 1.4 GW ± 515 kV 730 km commissioned 2020 Power reversal By polarity reversal of the DC voltage Reversal of the flowing current direction Overhead lines or mass-impregnated OHL or XLPE cables – cost effective and Line or Cable cables or oil filled cables environmentally friendly Reactive power Absorbs Q (~0,6 P) Supplies/Absorbs an adjustable Q Costs 190 M€ ± 20% / 2 converter stations 1000 MW 180 M€ ± 20% / 2 converter stations 1000 MW Losses ~1,4 % (2 LCC substations) ~ 2.2 % (2 MMC substations) Footprint 3 to 5 hectares for 1000 MW 50% less i.e. about 2 ha Power strength Scc Scc network > 2.5 P N No minimum ac grid’ Short Circuit Level LCC is sensitive to AC sags and can cause AC grid sensitivity Lesser sensitivity to AC sags AC swells (energization of AC filters) DC faults clearing Quick clearing without breakers Low clearing due to AC breakers operation Miscellaneous Communication mandatory between stations Blackstart possible ABB, General Electric, SIEMENS (EU) ABB, General Electric, SIEMENS, Manufacturers Mitsubishi/Toshiba/Hitachi (JP), NARI (CN) Mitsubishi/Toshiba/Hitachi, NARI 21