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Mini-Course on Future Electric GridsPart 2 of 2
Dirk Van Hertem — Dirk.VanHertem@ieee.org
Electric power systemsEKC2, Controllable power systemsElectrical engineering department
Royal Institute of Technology, Sweden
March 8, 2010
K.U.Leuven (Belgium) KTH, Stockholm (Sweden)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 1 / 47
Introduction Course overview
Who am I?
Master in engineering from KHK Geel, Belgium
Master of science in engineering from K.U.Leuven, Belgium
PhD in engineering from K.U.Leuven, Belgium
Currently Post-Doc researcher at the Royal Institute of Technology,Stockholm, Sweden
Program manager controllable power systems group of the Swedishcenter of excellence for electric power systems (EKC2)
Active member of both IEEE and Cigré
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 2 / 47
Introduction Course overview
Course overview and objectives
Overview Part 1New situation in the power system
1 Liberalization of the market2 Increased penetration of smaller, variable energy sources3 No single authority in Europe4 Lacking investments in the transmission system
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 3 / 47
Introduction Course overview
Course overview and objectives
Overview Part 2International coordination in the power system
How this coordination is evolving (Coreso)
Power flow controllers
Coordination and power flow controllers
The future “supergrid”. . .
. . . and the road towards it
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 3 / 47
Introduction Course overview
What it is about and what not
Not the grid of 2050
Main focus is Europe
Not about smart grids (or not specifically)
About transmission and not distribution
Mainly from a grid operator point of view
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 4 / 47
Introduction Course overview
1 IntroductionCourse overview
2 Coordination in the power systemSituation sketchInformation exchange between TSOsSteps towards increased coordination: Coreso example
3 Power flow controllersIntroductionControlling PFC in an international contextExample: Losses in a gridNeed for coordinationHow to coordinate?
4 SupergridsA supergrid?Technology requirements for the supergridControlling the supergridTechno-Economic approach to a supergrid
5 Conclusions
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 5 / 47
Coordination in the power system
1 IntroductionCourse overview
2 Coordination in the power systemSituation sketchInformation exchange between TSOsSteps towards increased coordination: Coreso example
3 Power flow controllersIntroductionControlling PFC in an international contextExample: Losses in a gridNeed for coordinationHow to coordinate?
4 SupergridsA supergrid?Technology requirements for the supergridControlling the supergridTechno-Economic approach to a supergrid
5 Conclusions
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 6 / 47
Coordination in the power system Situation sketch
Power system control before liberalization
Vertically integrated companies
Generator company and grid operator are one company
Power system operator controls the power system:Unit dispatch is done by system operatorsTopology changes: Line switchingReactive power: capacitor switching and VAr control of generatorsInternational/-zonal redispatch (at cost)
All generation is centrally controlled
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 7 / 47
Coordination in the power system Situation sketch
Now: different involved parties
Unbundling separated generator, transmission, distribution andsuppliers
Power exchanges were introduced
Renewables were introduced
Generation no longer directly controlled by transmission systemoperator
Operator controls the transmission system:Unit dispatch can be requested by system operators at a costTopology changes: Line switchingReactive power: capacitor switching, but VAr control of generators?International/-zonal redispatch (at cost)A significant increase of power flow controlling devices is noticed
Less stable pattern due to market: high volatility
Need for firm capacity for the market participants
⇒ Higher need for control with less “free” means
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 8 / 47
Coordination in the power system Information exchange between TSOs
Interconnected power system: information exchange
The different zones are interconnected (synchronous zones)
Operated independently
International market operation
Operation of the system effects the system cross-border
Information is exchanged:Grid status (important outages)Day-ahead congestion forecastsExpected available capacitiesAny emergency with possible effects outside of the zone
Not everything is exchangedNot all the generation data (aggregated)Grid data on a “need-to-know” basis
Quite good working system
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 9 / 47
Coordination in the power system Information exchange between TSOs
DACF: Day-ahead congestion forecasts
ProcedureEstimated zonal grid (cut at the borders) is provided
Together with expected aggregated load/generationpatterns
The planned state of devices such as on-load tapchangers and capacitors is provided
Sum of generation, load and losses equals theplanned exchange
Exchange is set in the interconnections (X-nodes)
Reactive power is set to a sensible amount
Local load flow is run
Data file is uploaded and merged
Merged load flow is run and returned to TSO
In case of congestion: TSOs negotiate appropriateactions
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 10 / 47
Coordination in the power system Information exchange between TSOs
Still some problems
Unexpected loop flows
Uncertainty in the system remains high
Black-outs or near black-outs due to lack off coordination and orcommunication
August 2003: Italian black-out:Stopping pumped hydro (or reverse) might have helpedMiscommunication was one of the main problems
November 2006: UCTE near black-outCommunication between operators failedSequence of events that could have been avoided
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 11 / 47
Coordination in the power system Information exchange between TSOs
Limitations in cooperation
Unforeseen events may occur
Not everything is known
With higher uncertainties and less control options, the system operatorhas limited tools available
Some problems might be easily solved in another zone instead of costlylocal actions
System-wide security assessments are not performed/updated duringthe day
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 12 / 47
Coordination in the power system Steps towards increased coordination: Coreso example
Steps towards increased coordination: Coreso example
What is Coreso?The first Regional Technical Coordination Service Center (created Dec.2008, in operation since Feb. 2009)
Independent company, located in Brussels (www.coreso.eu)
Shareholders are TSOs (founders Elia and RTE, and National grid),open to others
Coreso does not operate the grid, but acts as a coordinated supervisionfor its members
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 13 / 47
Coordination in the power system Steps towards increased coordination: Coreso example
Steps towards increased coordination: Coreso example
Service provider for TSOsType of services:
Pro-active assessment of the safety level of the network (day ahead andclose to real time forecast)Proposing to the TSOs the implementation of optimized coordinatedactions to master these risksRelaying significant information and coordinating the agreement onremedial actionsContributing to ex-post analysis and experience reviews of significantoperating events for the appropriate areaProviding D-2 capacity forecast
Focus on:Supra national view on the networkCross-border impacts between TSOsImproved regional integration of renewable energy
Area of interest: participating TSOs
Security analysis extends to CWE (Benelux, France and Germany)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 13 / 47
Power flow controllers
1 IntroductionCourse overview
2 Coordination in the power systemSituation sketchInformation exchange between TSOsSteps towards increased coordination: Coreso example
3 Power flow controllersIntroductionControlling PFC in an international contextExample: Losses in a gridNeed for coordinationHow to coordinate?
4 SupergridsA supergrid?Technology requirements for the supergridControlling the supergridTechno-Economic approach to a supergrid
5 Conclusions
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 14 / 47
Power flow controllers Introduction
What is power flow control
Bending the laws of KirchhoffIn normal systems, power flows according to the laws of Kirchhoff
Power flows in meshed networks depend on the relative impedance ofthe lines
Using power flow controlling devices, these flows can be influenced
Simplified: PFC work as a valve
Overloaded lines can be relieved
System can be adjusted to the situation: day-night, summer-winter,import-export, maintenance situations,. . .
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 15 / 47
Power flow controllers Introduction
Power flow control
Power flow equations for a simple transmission line:
Active power: PR = | ~US | · | ~UR |X · sin(δ)
Reactive power: QR = | ~US | · | ~UR |X · cos(δ)− | ~UR |2
X
Receiving end power can be altered through voltage,impedance and angle
Different technologies exist: mechanically switched,thyristor based and fast switches
Subset of FACTS (flexible AC transmission systems)
~IS ~IR
~UR~US
~X
~UR
·~I ·X
~I
δ~US
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
Power flow controllers Introduction
Power flow control
Power flow equations for a simple transmission line:
Active power: PR = | ~US | · | ~UR |X · sin(δ)
Reactive power: QR = | ~US | · | ~UR |X · cos(δ)− | ~UR |2
X
Receiving end power can be altered through voltage,impedance and angle
Different technologies exist: mechanically switched,thyristor based and fast switches
Subset of FACTS (flexible AC transmission systems)
~IS ~IR
~UR~US
~X
~UR
·~I ·X
~I
δ~US
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
Power flow controllers Introduction
Power flow control
Power flow equations for a simple transmission line:
Active power: PR = | ~US | · | ~UR |X · sin(δ)
Reactive power: QR = | ~US | · | ~UR |X · cos(δ)− | ~UR |2
X
Receiving end power can be altered through voltage,impedance and angle
Different technologies exist: mechanically switched,thyristor based and fast switches
Subset of FACTS (flexible AC transmission systems)
~IS ~IR
~UR~US
~X
Voltage
~UR
~US
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
Power flow controllers Introduction
Power flow control
Power flow equations for a simple transmission line:
Active power: PR = | ~US | · | ~UR |X · sin(δ)
Reactive power: QR = | ~US | · | ~UR |X · cos(δ)− | ~UR |2
X
Receiving end power can be altered through voltage,impedance and angle
Different technologies exist: mechanically switched,thyristor based and fast switches
Subset of FACTS (flexible AC transmission systems)
~IS ~IR
~UR~US
~X
Impedance
~I
~UR
~US
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
Power flow controllers Introduction
Power flow control
Power flow equations for a simple transmission line:
Active power: PR = | ~US | · | ~UR |X · sin(δ)
Reactive power: QR = | ~US | · | ~UR |X · cos(δ)− | ~UR |2
X
Receiving end power can be altered through voltage,impedance and angle
Different technologies exist: mechanically switched,thyristor based and fast switches
Subset of FACTS (flexible AC transmission systems)
~IS ~IR
~UR~US
~X
Angle~UR
~US
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 16 / 47
Power flow controllers Introduction
PFC devices: examples
Phase shifting transformerMechanically switched device
Basic principle of a transformer
How it works: Injects a part ofthe line voltage of opposingphases in series with the phasevoltage to create an angledifference
Different types: direct/indirectand symmetrical/asymmetrical
Cheap, robust, efficient andslow
~UR
~UM3~UM2
~UM1
k · ~UM23
k · ~UM23
~UM1
~∆U1 = 2·k · ~UM23
~UR1~US1
~UM23 ~UM23
~UM31~UM12
~UM3~UM2
~US
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 17 / 47
Power flow controllers Introduction
PFC devices: examples
TSSC ↔ TCSC
TSSC/TCSC: Thyristor switched/controlledseries capacitor
Compensate the natural series inductance oftransmission lines
Especially used for longer lines
Possible to use for dynamic power systemoscillation damping
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 17 / 47
Power flow controllers Introduction
HVDC: High Voltage Direct Current
LCC HVDCLine commutated converter HVDC
Exists for over 50 years
High ratings, relative low losses
Needs a strong AC grid to connect to
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���
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�� �� ��
�� ��
�� ��
����
����
����
����
����
Converter
DC reactor
DC filter
Y/∆
AC filter
AC switchyard
Y/Y
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
Power flow controllers Introduction
HVDC: High Voltage Direct Current
CIGRE B4-37 / VSC Transmission Topologies
4-4
0 90 180 270 360
1
0
1
Degree
Lin
e-to
-neu
tral
vol
tage
(pu
)
Figure 4.2: Single-phase ac voltage output for 2-level converter with PWM switching at 21 times fundamental frequency
4.2.3 Three-Level Neutral Point Clamped Converter A three phase converter consisting of three 3-level phase units is illustrated in Figure 4.3. The single-phase output voltage waveform, assuming fundamental frequency switching, is also shown in Figure 4.3. The converter has three dc terminals to connect to a split or centre-tapped dc source. As seen, there are twice as many valves used as in the 2-level phase unit, and additional diodes are required to connect to the dc supply centre-tap, which is the reference zero potential. However, with identical valve terminal-to-terminal voltage rating, the total dc supply voltage can be doubled so that the output voltage per valve remains the same.
Ud
UL1
UL2
+Ud
-Ud�
Ud
Neutral(mid-) point
UL3
+
-
Figure 4.3: Three-phase 3-level NPC converter and associated ac voltage waveform for one phase
The ac waveform shown in the figure is the phase-to-neutral voltage, assuming fundamental frequency switching of the valves. The neutral voltage is the voltage at the midpoint of the dc capacitor. As illustrated in Figure 4.3, the output voltage of the 3-level phase unit can be positive, negative, or zero. Positive output is produced by gating on both upper valves in the phase unit, while negative output is produced by gating on both lower valves. Zero output is produced when the upper and lower middle valves, connecting the centre tap of the dc supply via the two diodes to the output, are gated on. At zero output, positive current is conducted by the upper-middle controllable device and the upper centre-tap diode, and negative current by the lower-middle controllable and the lower centre-tap diode.
Figure: Scheme of a 3-level 3-phase VSC
CIGRE B4-37 / VSC Transmission Topologies
4-5
As indicated in the figure, the relative duration of the positive (and negative) output voltage with respect to the duration of the zero output is a function of control parameter �, which defines the conduction interval of the top upper, and the bottom lower valves. The magnitude of the fundamental frequency component of the output voltage produced by the phase unit is a function of parameter �. When � equals zero degrees it is maximum, while at � equals 90 degrees it is zero. Thus, one advantage of the 3-level phase unit is that it has an internal capability to control the magnitude of the output voltage without changing the number of valve switchings per cycle. The operating advantages of the 3-level phase unit can only be fully realised with some increase in circuit complexity, as well as more rigorous requirements for managing the proper operation of the converter circuit. These requirements are related to executing the current transfers (commutation) between the four (physically large) valves, with well-constrained voltage overshoot, while maintaining the required di/dt and dv/dt for the semiconductors without excessive losses. An additional requirement is to accommodate the increased ac ripple current with a generally high triplen harmonic content flowing through the mid-point of the dc supply. This may necessitate the use of a larger dc storage capacitor or the employment of other means to minimise the fluctuation of the mid-point voltage. However, once these problems are solved, the 3-level phase unit provides a useful building block to structure high power converters, particularly when rapid ac voltage control is needed. The conduction periods for the inner and the outer valves is different, and therefore it is possible to use two different designs of a VSC valve for the two positions. By switching the valves more frequently, it is possible to eliminate more harmonics. A typical PWM switched waveform, using a carrier based control method with a frequency of 21 times fundamental frequency, is given in Figure 4.4. For the purpose of this illustration, the dc capacitor has been assumed to have an infinite capacitance (i.e., no dc voltage ripple).
0 90 180 270 360
1
0
1
Degree
Lin
e-to
-neu
tral
vol
tage
(pu
)
Figure 4.4 Single-phase ac voltage output for 3-level NPC converter with PWM switching at 21 times fundamental frequency
4.2.4 Multi-Level Neutral Point Clamped Converter In order to further reduce the harmonic content of the ac output voltage, the basic 3-level phase unit can be extended to a multi-level, 2n+1 phase unit (n=1,2,3,�) configuration. 2n dc supplies, provided by 2n dc storage capacitors (which are common to all three-phase units of a complete three-phase converter), are connected in series, providing 2n+1 discrete voltage levels. Four times n valves are required with 4n-2 diodes to selectively connect the 2n+1 voltage levels to the output. A three-phase converter using 5-level converter phase units with the corresponding single-phase output voltage waveform, in which, as an example, the 3rd, 5th, and 7th harmonics are absent, is shown in Figure 4.5. However, it should be remembered that in practice one degree of freedom would be needed
Figure: Voltage waveform of a 3-level 3-phaseVSC with single phase output voltage(fswitch = 21× fn)
VSC HVDCVoltage source converter
Quite new
Fast switching (PWM)
Highly dynamic
Makes its own rotating field
Relative high losses
Only two manufactures (ABB andSiemens)
(→ Source: Cigré Tech. Rep. 269)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
Power flow controllers Introduction
HVDC: High Voltage Direct Current
HVDC is a special power flow controllerAllows full, independent active power flow control
VSC HVDC also provides independent reactive power flow control
The ultimate power flow controller, yet not a true power flow controller
BA
HVDC as a single link between two independent networks, no possibility foractive power flow control (flow is equal to the imbalance in the zones)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
Power flow controllers Introduction
HVDC: High Voltage Direct Current
HVDC is a special power flow controllerAllows full, independent active power flow control
VSC HVDC also provides independent reactive power flow control
The ultimate power flow controller, yet not a true power flow controller
HVDC as part of the meshed AC power system, HVDC can be operated as aPFC, with a flow independent on the rest of the system
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
Power flow controllers Introduction
HVDC: High Voltage Direct Current
HVDC is a special power flow controllerAllows full, independent active power flow control
VSC HVDC also provides independent reactive power flow control
The ultimate power flow controller, yet not a true power flow controller
BA
Two meshed networks are connected through multiple HVDC. HVDC can beused as PFC when there is coordination
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 18 / 47
Power flow controllers Introduction
Power flow controlling devices: classification
Series & Shunt
AC Network controller
Conventional(Switched)
FACTS Devices(Fast, static)
R, L, C
Transformer Valves
Voltage Source
Shunt devices
Series devices
CombinedPhase Shifting
Switched Series
Compensation:
L and C
Switched Shunt
Compensation:
L and C
Thyristor
Static VAr Controller
(SVC)
Thyristor Controlled
Reactors (TCR),. . .
Thyristor Controlled
and Thyristor Switched
Series Compensator
(TCSC and TSSC).
Thyristor Controlled
Phase Angle
Regulators (TCPST) VSC HVDC
Controller (UPFC)
Unified Power Flow
(SSSC)
Static Synchronous
(STATCOM)
Compensator
Static Synchronous
Series Compensator
LCC HVDC
Transformer(PST)
Convertor (IGBT)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 19 / 47
Power flow controllers Introduction
Existing/planned power flow controllers in the Benelux
u
u
uu uu
uuuu?
UK-Fr-Meeden �Diele
-Gronau
��Monceau
-Norned
��9XXy
Van Eyck-Zandvliet
BritNed
(source: UCTE)
1 HVDC interconnector UK-FR
2 Meeden PSTs (2×)
3 Gronau PST
4 Monceau PST
5 Norned HVDC
6 Van Eyck PSTs
7 Zandvliet PST
8 Diele
9 BritNed (2011?)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 20 / 47
Power flow controllers Introduction
Existing/planned power flow controllers in the Benelux
uu NEMOu
u
uu uu
uuuu?
UK-Fr-Meeden �Diele
-Gronau
��Monceau
-NornedCobra and/or Norned 2
��9XXy
Van Eyck-Zandvliet
u uBE-DE
BritNed
(source: UCTE)
1 HVDC interconnector UK-FR
2 Meeden PSTs (2×)
3 Gronau PST
4 Monceau PST
5 Norned HVDC
6 Van Eyck PSTs
7 Zandvliet PST
8 Diele
9 BritNed (2011?)
10 NEMO (2013?)
11 Belgium Germany (?)
12 Cobra and/or Norned 2 (?)
Most are less than 10 years old
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 20 / 47
Power flow controllers Controlling PFC in an international context
Control of PFCLocally controlled
The investment is normally done by a TSOsTherefore control is done by the TSO to fulfill his own objectivesPayed for by the local market participants,so “revenues” should be returned to the local market as well
Optimal use of the transmission systemMinimum lossesMaximum securityMaximum transmission capacity
Effects are not localDevices are mostly placed on the borderThe effects of active power flow control can reach far into neighboringsystemsSome control actions are intended to influence “external” powers
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 21 / 47
Power flow controllers Controlling PFC in an international context
Multiple zones, multiple PFC
(A) (B) (C) (D)
20 % 80 %50 % 50 % 50 % 50 %-10 % 110 %
β α αα
A
B
C
Gen
Load
D
A
B
C
Gen
Load
D
A
B
C
Gen
Load
D
A
B
C
Gen
Load
D
Example of possible problems with power flow control in multiple zones
A: Generation in the south, load in the north, equal flow distribution
B: Zone B invest in a power flow controller: power flow is shifted
C: Overcompensation by B (following schedules, optimizing for zone B)
D: D also invests in a power flow controller: two investments, no advantage
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 22 / 47
Power flow controllers Example: Losses in a grid
System losses with power flow control
Higher losses in one line 6= higher system losses0.1 pu R and 0.1 pu X in parallelPloss =R1 · I2
1 +R2 · I22 =R1 · I2
1⇒ shift power to the line with X
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������������������������������
R = 0.1 pu
X = 0.1 pu
I2
I1
A PFC can lower losses by pushing the current towards lines with lowerresistanceIn case of a constant X/R ratio, the use of a PFC increases the overalllosses in the systemBut also lowering local losses (while having higher system losses)Example IEEE39-bus system as test grid
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 23 / 47
Power flow controllers Example: Losses in a grid
Example: Three zone system, two PFCGenerators are circles, load busses are square
Green lines are PFC
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 24 / 47
Power flow controllers Example: Losses in a grid
Losses within multiple zones, two PST
Phase shifter 1 (degree)
Pha
se s
hifte
r 2
(deg
ree)
−25 −20 −15 −10 −5 0 5 10 15 20 25
−20
−10
0
10
20
Losses in the 3 zonesdependent on the settings of the two PSTs.
Contour plot of the lossesin the 3 zones
Zone 1, Zone 2 and Zone3: 3 optima
PST 1 is controlled by zone2
PST 2 is controlled by zone1 or 3 (interconnector)(example: 1)
Initial control zone is “bad”for zone 2
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
Power flow controllers Example: Losses in a grid
Losses within multiple zones, two PST
Phase shifter 1 (degree)
Pha
se s
hifte
r 2
(deg
ree)
−25 −20 −15 −10 −5 0 5 10 15 20 25
−20
−10
0
10
20
Losses in the 3 zonesdependent on the settings of the two PSTs.
@@R
@@R ��Contour plot of the lossesin the 3 zones
Zone 1, Zone 2 and Zone3: 3 optima
PST 1 is controlled by zone2
PST 2 is controlled by zone1 or 3 (interconnector)(example: 1)
Initial control zone is “bad”for zone 2
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
Power flow controllers Example: Losses in a grid
Losses within multiple zones, two PST
Phase shifter 1 (degree)
Pha
se s
hifte
r 2
(deg
ree)
−25 −20 −15 −10 −5 0 5 10 15 20 25
−20
−10
0
10
20
Losses in the 3 zonesdependent on the settings of the two PSTs.
Contour plot of the lossesin the 3 zones
Zone 1, Zone 2 and Zone3: 3 optima
PST 1 is controlled by zone2
PST 2 is controlled by zone1 or 3 (interconnector)(example: 1)
Initial control zone is “bad”for zone 2
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
Power flow controllers Example: Losses in a grid
Losses within multiple zones, two PST
Phase shifter 1 (degree)
Pha
se s
hifte
r 2
(deg
ree)
−25 −20 −15 −10 −5 0 5 10 15 20 25
−20
−10
0
10
20
Losses in the 3 zonesdependent on the settings of the two PSTs.
Contour plot of the lossesin the 3 zones
Zone 1, Zone 2 and Zone3: 3 optima
PST 1 is controlled by zone2
PST 2 is controlled by zone1 or 3 (interconnector)(example: 1)
Initial control zone is “bad”for zone 2
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
Power flow controllers Example: Losses in a grid
Losses within multiple zones, two PST
Phase shifter 1 (degree)
Pha
se s
hifte
r 2
(deg
ree)
−25 −20 −15 −10 −5 0 5 10 15 20 25
−20
−10
0
10
20
Losses in the 3 zonesdependent on the settings of the two PSTs.
Contour plot of the lossesin the 3 zones
Zone 1, Zone 2 and Zone3: 3 optima
PST 1 is controlled by zone2
PST 2 is controlled by zone1 or 3 (interconnector)(example: 1)
Initial control zone is “bad”for zone 2
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
Power flow controllers Example: Losses in a grid
Losses within multiple zones, two PST
Suboptimal optimization3 zones, 3 optimal phase shifter settings
Phase shifters are not mutually controlled or coordinated
Good for one can be bad for another
Nash-equilibrium?
Best solution for the system is not achieved
Angle (PST1, PST2)Losses (MW) (−13◦,0◦) (−5◦,9◦) (0◦,2◦) (−5◦,6◦)Zone 1 11.4 13.2 12.3 12.4Zone 2 11.6 8.72 9.8 8.91Zone 3 12.0 9.18 9.17 9.23Total 35.0 31.1 31.3 30.6
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 25 / 47
Power flow controllers Need for coordination
Need for coordination. . .
Different objectivesMinimize local losses, not foreignMaximize export capacity to “B”, not import from “C”
Objectives can be excludingWhat is good for zone “A”, is not necessary good for “B”And vice-versa
Global objective is generally not reached when there are multipleobjectives
TSOs are no competitors, but each has his own objectiveRather unwillingly obstructing other TSOs or grid users
PFC control has financial repercussions
Communication is key
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 26 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
Solving local problem(no coordinationneeded)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
Local objective
Do not take actions ofneighbor into account
Coordinate only forsafety
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
Optimize, knowingneighboring systems
Different objectives
Nash-equilibrium
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
PFC control = money
Include in the marketmechanism?
PFC and flow basedmarket coupling?
6
?
Zone 1
Zone 2
Zone 1+2
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
PFC influence is limitedin distance
Possibilities toimplement in the currentframework
Coreso is taking firststeps
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
Optimize social welfare
Additional organization:difficult
ISO: who will invest?
TSO: national assets willhave to merge
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Possible control regimes of PFC for the European system
Local, single control objective
Every party on its own
Uncoordinated operation
PFC coordination in a market environment
Regional coordination ⇒ most realistic first step
Full system coordinationNew organizationSingle ISO approachSingle TSO approach
Optimize social welfare
Additional organization:difficult
ISO: who will invest?
TSO: national assets willhave to merge
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 27 / 47
Power flow controllers How to coordinate?
Regulatory framework
Current frameworkPFCs are generally left out of the regulations
UCTE operation handbook mentions PSTs as possible means ofguaranteeing security
No special required agreements exist to enforce PFC coordination
Proposed changesFor the TSOs/operators:⇒ Increased communication
Future European regulationPFCs and their effects should not be forgotten in forthcoming regulationsAim for more coordination through effective regulations
Not only TSOs but also for regulators
First step towards further integration, and insufficient on a long term
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 28 / 47
Supergrids
1 IntroductionCourse overview
2 Coordination in the power systemSituation sketchInformation exchange between TSOsSteps towards increased coordination: Coreso example
3 Power flow controllersIntroductionControlling PFC in an international contextExample: Losses in a gridNeed for coordinationHow to coordinate?
4 SupergridsA supergrid?Technology requirements for the supergridControlling the supergridTechno-Economic approach to a supergrid
5 Conclusions
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 29 / 47
Supergrids A supergrid?
A supergrid?
What is a supergrid?A popular definition: a supergrid is an overlay grid connecting differentgeneration and load centers over larger distances
It serves as a backbone
Adds reliability and security of supply to the system
A grid offers redundancy
Sometimes also called “hypergrid”
New?Recurring issue
Electric transmission started from 1 generator to several local loads
Grids became interconnected, at increasingly higher voltages
The 400 kV grid became the supergrid of the 50’s
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 30 / 47
Supergrids A supergrid?
A supergrid?
Early idea of a supergrid (after WW2)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 30 / 47
Supergrids A supergrid?
A supergrid?
Early idea of a supergrid (after WW2)Implemented as a 400 kV AC grid
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 30 / 47
Supergrids A supergrid?
Supergrid to connect remote renewable energy sourcesThere is plenty of renewable energy available
Solar from the Sahara, wind from the North Sea and hydro from Norwayto balance
(source: desertec)Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 31 / 47
Supergrids A supergrid?
Supergrid to connect remote renewable energy sourcesThere is plenty of renewable energy available
Solar from the Sahara, wind from the North Sea and hydro from Norwayto balance
±1 km between mills(1/km2)
take 10 MW/mill (future)
UCTE: 600 GW generation
Capacity factor 1/3
Required surface to replaceUCTE generation:600 ·103×3
1×10 = 180·103km2
square of 430 km×430 km
or 100 km wide, 1800 kmlong coastal track (Germanyhas about 2300 kmcoastline)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 31 / 47
Supergrids A supergrid?
Supergrids: current “proposals”
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 32 / 47
Supergrids Technology requirements for the supergrid
Technology for the supergrid
RequirementsHigh power transfer capabilities
Long distances
High transmission efficiency
Cheap
Offshore connections
High reliability
Compatible with the current infrastructure
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 33 / 47
Supergrids Technology requirements for the supergrid
Technology for the supergridPotential technologies
Overhead lines AC connectionsOHL has high power ratingsAllows long distances, but at high lossesNo offshore connectionsOHL are difficult to get permissions
AC cablesLimited length and ratingDifficult system operation
LCC HVDC (thyristor based)Current source inverterParallel connecting of multiple terminals is troublesomeSeries connection gives reliability problemsCables are possible although limited capacity
VSC HVDC (Fast switches)Voltage source converter: straightforward parallel connectionsConverter ratings are limited (but rising)Cables are possible although limited capacityWeak grids are possible
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 33 / 47
Supergrids Technology requirements for the supergrid
Technology for the supergrid
Conclusion⇒ No perfect solution.
VSC HVDC for offshore supergridAC OHL when possible?
For Europe, VSC HVDC seems most appropriateAC system on shore is already quite strongMany load centers are located relatively close to the sea
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 33 / 47
Supergrids Technology requirements for the supergrid
Ratings
“Super”grid needs to be biggerthan existing 400 kV ACsystems
Existing AC: ≈ 2 GVA/circuit
⇒ 5 GW? – 10 GW?
New developments are needed,especially if cables are used
0 1 2 3 40
200
400
600
800
{VSC HVDCXLPE cable1100 MW
{VSC/LCC HVDCMI cable 2000 MW
{VSC/LCC HVDCOil filled cable2000 MW
{VSC HVDCOHL2000 MW
LCC HVDCOHL6400 MW }
IDC [kA]
UDC [kV ]
Figure: Current possible ratings for HVDC systems (UDCrefers here to the pole voltage, in a bipolar setup,P = 2·UDC · IDC ).
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 34 / 47
Supergrids Technology requirements for the supergrid
Standards
Similar to the AC system, standards are needed
Standard voltagesOnce chosen, it is difficult to changeWhat with the integration existing/upcoming lines?
Different manufacturers must be able to connect to the same DCsystem (no vendor lock-in)
The control systems of different manufacturers/owners must operatetogether and without detriment to the AC system
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 35 / 47
Supergrids Technology requirements for the supergrid
How should the grid look like?
DC Grid
AC Grid
Option 1Multi-terminal withoutredundancy
DC and AC system form eachothers redundancy
Injections and thus DC flowsare controlled
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 36 / 47
Supergrids Technology requirements for the supergrid
How should the grid look like?
DC Grid
AC Grid
Option 2Grid of point-to-point DC lines
Converter at both ends
Some lines in the AC grid arereplaced by DC lines
Full control
AC connections and thereforeAC protection devices
Many expensive and lossyconverters
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 36 / 47
Supergrids Technology requirements for the supergrid
How should the grid look like?
DC Grid
AC Grid
Option 3Meshed DC grid
Redundant lines
Only converters at interfacebetween AC and DC grid
Reduced losses
DC flows can not be directlycontrolled
Cigré workgroup B4-52considers only this a real DCgrid
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 36 / 47
Supergrids Technology requirements for the supergrid
Connecting to the existing AC system
The current AC system has not many infeed/withdrawal points for> 5 GW
Reinforcements are needed in the existing AC system as well
The complete grid build-up/orientation might changeOriginally from generation centers (near mines, mountains,. . . ) to loadcentersWith supergrid: to from the nearest supergrid terminal (near the shore) toinland load centers
SecurityN-1 connection: Serious disturbance in the system when a terminal isdisconnected1 or 2 connections per zone?What rating and how many connections to smaller synchronous zones:Ireland (7.8 GW installed capacity), Nordel (61 GW installed capacity),. . .
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 37 / 47
Supergrids Technology requirements for the supergrid
Protection
Current VSC HVDC protectionInterrupting DC currents is difficult
AC protection is easy
⇒ Opening the AC system, disconnecting the complete DC circuit
PS
Figure: Protection system (PS) in existing VSC HVDC systems
NOT USEFUL for supergrid
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
Protection
Current VSC HVDC protectionInterrupting DC currents is difficult
AC protection is easy
⇒ Opening the AC system, disconnecting the complete DC circuit
PS
Figure: Protection system (PS) in existing VSC HVDC systems
NOT USEFUL for supergrid
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
ProtectionSupergrid protection boundaries
Fault causes rapidly changing currents in all lines
Selectivity: Only the affected DC line must be switched
IGBTs cannot withstand high overloads
Fast enough (DC: no inductance XL to limit the current)
Only in case of DC fault and not during load change or AC fault
ConsequencesFault location (branch) detection within a few milliseconds
Too fast for communication between measurement devices
Independent detection systems
Opening at both sides of the faulted line
No opening of other branches
Backup in case this fails
New superfast DC breakers must be developedWaiting longer results in more difficult switching and is lethal for the IGBTs
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
Protection
Example: 4 terminal MT HVDC system
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
Protection
Fault occurs in the DC circuit (t = 0)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
Protection
Rapidly changing currents throughout the system
VDC =L ·di
dt+R · i
i(t)= VDC
R+
(I0 − VDC
R
)·e− R
L · t
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
Protection
Protection system must indicate the faulted line
PS
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Technology requirements for the supergrid
Protection
Opening of the faulted line (t < 5ms)
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 38 / 47
Supergrids Controlling the supergrid
Power balance and flows
At any time, the power balance must be zero: (∑
i PAC→DC)−Ploss = 0
Injections can be fully controlled (DC) but compensation for losses isneeded
Slack bus or distributed slack bus
Power flows are according to the laws of Kirchhoff
Redispatching of DC injections might be needed to change DC flowsand avoid congestion
The DC system flows are determined by the DC voltages at theconverter side
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 39 / 47
Supergrids Controlling the supergrid
Interaction between AC and DC system
DC system will have a profound influence on AC system flows
Changing the power injections between nodes can have importantconsequences
How the interaction will/should be is not trivial, especially with multiplezones and multiple synchronous zones
A VSC HVDC terminal is highly dynamicOperation may not jeopardize AC system security (interactions betweenAC and DC controls)Operation of electrically close terminals may interferePotential to increase stability and damping in the system
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 40 / 47
Supergrids Controlling the supergrid
Segmenting the AC system?
In synchronous AC systems, events propagate throughout the system
By subdividing current synchronous zones in different smaller zones,this can be limited
Part of the synchronizing power would be lost as well
Might be an option for currently loosely or non-synchronized systems(USA?)
DC Grid
AC Grid
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 41 / 47
Supergrids Techno-Economic approach to a supergrid
Potential benefits of a supergrid
Income: 4 clear economic benefits1 Access to remote energy sources2 Higher penetration of renewable energy sources by improved balancing3 Improved grid security4 Reduced congestion in the system
Costs: expensive installationHVDC terminals and cables are expensive
There are other resources besides renewables (generation mix)
Radial HVDC links to shore are possible as well
AC system upgrades might be sufficient for many years
Pay-back timeIs it interesting from an economic point of view to install a supergrid?
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 42 / 47
Supergrids Techno-Economic approach to a supergrid
Regulations and ownershipMany operational questions remain
Who will own/invest in the supergrid?TSOs (ENTSO-E?)Governments/EUGenerator companiesPrivate investors
The investor wants a return on investment!
The owner determines how the grid will look likeHow many connectionsWhich connection points
How is the combined AC and DC power system operated?
How will money be earned?Regulated marketMerchant gridConnection charges for offshore generators
Who will be the regulating authority?
Multi-zonal regulations?
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 43 / 47
Conclusions
1 IntroductionCourse overview
2 Coordination in the power systemSituation sketchInformation exchange between TSOsSteps towards increased coordination: Coreso example
3 Power flow controllersIntroductionControlling PFC in an international contextExample: Losses in a gridNeed for coordinationHow to coordinate?
4 SupergridsA supergrid?Technology requirements for the supergridControlling the supergridTechno-Economic approach to a supergrid
5 Conclusions
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 44 / 47
Conclusions
Conclusions 1: coordination
In the multi-zonal transmission system, coordination is not trivial
Cooperation exists, but can be better
Coreso is a new and promising initiative
Power flow controlling devices are increasingly present in the grid
PFCs influence losses, transmission capacity, security,. . .
PFCs influence the operation of the local transmission system. . . also that of neighbors
Make coordination even more important
Different manners of coordination are possibleUntil now, no true coordination exists
First step: communicate
Second step: implemented in the regional initiatives framework/coreso
Optimum would be full coordination, with a single European TSO?
The current situation is not ideal nor a full implementation of the IEM
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 45 / 47
Conclusions
Conclusions 2: supergrid
The DC supergrid is often seen as the ultimate solution to integratingrenewable energy sources
The potential is great
But many challenges remain
Technical:Ratings are currently insufficientProtection is an issueOffshore grid will not solve all problems
Operation and control:The power balance must be controlledThe new system must remain secure (N-1)The combined AC and DC system interact
Economic:What is the rate of return? and who will pay?What about regulations?Who and how will the supergrid be controlled?
⇒ A supergrid? Yes, but not tomorrow. . .
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 46 / 47
Conclusions
Questions
?
1
Dirk Van Hertem (Electric Power Systems, KTH) Mini-course on Future Electric Grids (2/2) 8/03/2010 47 / 47
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