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

SLIDE 1

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

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

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SLIDE 3

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■ The battle of currents

Thomas Edison: believes in DC

1878: incandescent lamp 1882: 1st DC distribution grid

1885 : Importing transformer from L. Gaulard

& J. Gibbs and AC generator from Siemens.

1886 : 1st AC grid, hydro generator 500 V,

transformer 3000 V, network of 100 V bulbs.

1887 : 30 other electric lighting systems based
n AC are installed.
1888 : AC meters (O. Shallenger) et AC motors

(N. Tesla) make the victory of AC.

Georges Westinghouse: in favor of AC

Lucien Gaulard Nikola Tesla

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Connection of a new load onto an existing HVAC line

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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

Substation Substation Public Grid Limit A B D E C

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5

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.

Dimensions of 1000 MW towers

DC overhead/Underground infrastructure have smaller

environmental impacts.

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Advantages of DC Drawbacks of DC

More power per conductor (2

conductors instead of 3)

Smaller transmission towers

(30% cost saving in line building)

Lower transmission losses
No charging current in steady state
No reactive power transfer
No skin effect
No transmission length limitation
No synchronous operation required
Asynchronous connections are possible
Isolation and even mitigation of

electromechanical oscillations

Controllability of active power
Isolation from AC faults
Converter stations are expensive
Converters create harmonics
Converters generate additional losses
Converters are additional sources of failures
Converters increase the weight of offshore

stations

Additional components may reduce reliability /

availaibility

DC grids are not easy to operate
No mature DC breakers exist today
Only 200 GW of installed DC transmission

capacity

Multi-voltage level i.e. connection involving

different DC voltage levels needs DC/DC converters

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Line Commutated current source Converters

Light Triggered Thyristor Electrical Triggered Thyristor

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ON state OFF state OFF state

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Prohibited configuration Normal operation

U RS U RT U ST U SR U TR U TS U RS U RS RT U

Commutation phenomenon

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SLIDE 9

The thyristor bridge is basically a current source: it converts the DC current Id into a 3-phase alternating current (IR, Is, IT) in the presence of (UR, Us, UT.

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Gate sequence every 20 ms is as follows: 1 2’ 2 3’ 3 4’ 4 5’ 5 6’ 6 1’

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T/4 T/2 3T/4

[%] [%] 10 5 5 5 7 11 13 17 19 23 25 11 13 23 25

n n

i1 i2

Σ I n i1 + i2

I I

n 1

Y Y Y ∆ ∆ ∆ ∆

i1 + i 2 i2 i1 i1 + i 2

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Mitigation of AC side harmonics

Phase currents, lagging (α α α α) phase voltages

reactive

power absorption.

SLIDE 11

500 kV DC suspended valves 3 Quadrivalves

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Upper trace: Transformer voltage
Middle trace: Valve voltage
Lower trace: DC voltage

SLIDE 12

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

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SLIDE 13

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SLIDE 14

Voltage Source Converters

IGBT

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0.01 0.1 1 10 100 0.01 0.1 1 10 f (kHz) Thyristor GTO IGBT Power (MW)

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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

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Convention assumed for VSC: positive P & Q are supplied, resistance R is neglected

X Vr Va Vr

Va

I

jXI

δ

Q P

VSC R RI

ϕ P = 3VcIcosϕ Q = 3VcIsinϕ

X

sinδ U U P

r a

⋅ ⋅ =

max r max 2 2

I U 3 S Q P S ⋅ ⋅ = ≤ + =

( )

2 r a 2 2 2 a 2 2

X U U P X U Q : _ _ _ _ 1 δ sin δ cos       ⋅ = + +         + ⇒ = + voltage grid given a for X U2

a

−

X U U

r a ⋅

max

I Ur 3 ⋅ ⋅

dc_max dc I

U ⋅

dc_max dc I

U

⋅

As for any synchronous machine, the typical PQ diagram of a VSC exhibits some limitations

( )

X U cosδ U U Q

r a r

− ⋅ ⋅ =

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Phase-to-neutral voltage Phase-to-phase voltage DC voltage

Length

PWM frequency

Hällsjön 1997 Sweden 3 MW ±10 kV 10 km

verhead 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

PWM ~2 kHz minimises

the size of AC filters.

but increases

commutation losses up to 3% per converter!

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Phase-to-neutral voltage Phase-to-phase 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

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

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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

verhead 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 interconnector 2012 Ireland- Wales 500 MW ±200 kV 261 km

sub. & underg. cables

2 π π 2 π 3 π 2 t ω

1

α

2

α

3

α

2 Udc 2 Udc − u

OPWM : commutations are stopped to further reduce the losses

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Losses: ~ 2,2% for both stations i.e. 1.1 % per converter station

Single phase equivalent diagram −Vdc = − 320 kV Vdc = + 320 kV Xt

Vr_400 Ia m Vr =m Vr_400 P, Q

Va Vmax

+

Idc Idc Va

+

Vsup Vinf

Xv Xv

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 The voltage source converter structurally transforms the DC voltage 2Vdc into a set of three-phase balanced alternating voltages (Va, Vb, Vc).

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LCC HVDC VSC HVDC

Semi conductor Thyristors, "natural" commutation IGBT, forced commutation Equivalent Current Source Voltage Source Power Ratings Up to 8 GW ± 800kV (bipole); 10 GW ± 1.1MV 3333 km commissioned 2017 About 1.2 GW per symmetrical monopole 1.4 GW ± 515 kV 730 km commissioned 2020 Power reversal By polarity reversal of the DC voltage Reversal of the flowing current direction Line or Cable Overhead lines or mass-impregnated cables or oil filled cables OHL or XLPE cables – cost effective and 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 Sccnetwork > 2.5 PN No minimum ac grid’ Short Circuit Level AC grid sensitivity LCC is sensitive to AC sags and can cause AC swells (energization of AC filters) Lesser sensitivity to AC sags DC faults clearing Quick clearing without breakers Low clearing due to AC breakers operation Miscellaneous Communication mandatory between stations Blackstart possible Manufacturers ABB, General Electric, SIEMENS (EU) Mitsubishi/Toshiba/Hitachi (JP), NARI (CN) ABB, General Electric, SIEMENS, Mitsubishi/Toshiba/Hitachi, NARI

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Converter stations significantly increase DC

transmission costs when the length of the connection is short.

With AC transmission, reactive

compensation devices increase the costs, while pushing away the break-even distance.

At a given length and active power, the

construction of AC OverHead Lines is 30 % less expensive than DC (smaller towers and 2 conductors instead of 3).

Break-even distances beyond which DC

becomes cheaper than AC are around 40 km for cables and 500 km for OHL.

These thresholds move towards 50/600 km if

transmission losses are ignored.

Costs Transmission distances Break-even distances AC terminals AC line AC losses AC/DC substations DC line DC losses

U (kV) Qcâble (Mvar/km)

Typical shunt reactors per end Maximal length min max Average

63

0.2 0.66 0.43

– – 90

0.4 1 0.7

2x64 Mvar

< 300 km

150

– 2 –

– – 225

2 4.5 3.3

2x80 Mvar 80 km 400

6.3 11.3 8.8

2x125 Mvar 50 km Possible lengths for Cable equipped at both ends with devices dedicated to 60% shunt compensation

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Costs Transmission distances Break-even distances AC terminals AC line AC losses AC/DC substations DC line DC losses

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From point-to-point links to meshed HVDC grids

Specific DC grid Components

AC-DC converter structures
Breakers, disconnectors, limiters
Current Flow Controllers
DC-DC converter structures
Prototypes + low scale testing

Control and Protection

f/P, Udc/P, Udc/Idc droop controllers
Fault detection algorithms
System Protection strategy

DC grid topologies

Large enough (> 10 substations)
CAPEX and OPEX comparisons

Investigation methods and tools

Offline Static/dynamic simulations
Real-time interoperability (SMARte)
Contribution to pre-standardisation

SLIDE 25

25 A few words on Medium Voltage Direct Current (MVDC)

Majority of progress in DC-based techno has occurred in either HVDC or LVDC.
Microgrids and urban grids typically operate at medium voltage.
Some consultants expects DC-distribution-network revenue will grow froim $2.8 billion

in 2015 to $5.1 billion in 2024.

Advantages of MVDC
Reduce the number of components in the field
Reduce the amount of labor in the field
Reduce the energy losses in the power conversion systems
Reduce CAPEX: DC in data center results in 15% cost savings (no AC conversion)
Ready to accomodate storage systems
Drawbacks of MVDC
A whole new ball game for construction and O&M: equipment are not on the shelf

certification needed that could take some time