future electricity grids 2/2

Mini-Course on Future Electric GridsPart 2 of 2

Dirk Van Hertem — [email protected]

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



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



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



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



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


Coordination in the power system





5 Conclusions


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


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


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



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



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



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


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


www.coreso.eu

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)


Power flow controllers





5 Conclusions


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,. . .



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



Power flow control




X




~IS ~IR

~UR~US

~X

Voltage

~UR

~US



Power flow control




X




~IS ~IR

~UR~US

~X

Impedance

~I

~UR

~US



Power flow control




X




~IS ~IR

~UR~US

~X

Angle~UR

~US



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



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



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

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

��

Converter

DC reactor

DC filter

Y/∆

AC filter

AC switchyard

Y/Y




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)




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)







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







BA

Two meshed networks are connected through multiple HVDC. HVDC can beused as PFC when there is coordination



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),. . .


and Thyristor Switched

Series Compensator

(TCSC and TSSC).


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)



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?)



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


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


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


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

��

��

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



Example: Three zone system, two PFCGenerators are circles, load busses are square

Green lines are PFC



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





Pha

se s

hifte

r 2

(deg

ree)

−25 −20 −15 −10 −5 0 5 10 15 20 25

−20

−10

0

10

20


@@R

@@R ��Contour plot of the lossesin the 3 zones









Pha

se s

hifte

r 2

(deg

ree)

−25 −20 −15 −10 −5 0 5 10 15 20 25

−20

−10

0

10

20


Contour plot of the lossesin the 3 zones








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


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


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)










Local objective

Do not take actions ofneighbor into account

Coordinate only forsafety










Optimize, knowingneighboring systems

Different objectives

Nash-equilibrium










PFC control = money

Include in the marketmechanism?

PFC and flow basedmarket coupling?

6

?

Zone 1

Zone 2

Zone 1+2










PFC influence is limitedin distance

Possibilities toimplement in the currentframework

Coreso is taking firststeps










Optimize social welfare

Additional organization:difficult

ISO: who will invest?

TSO: national assets willhave to merge








Regional coordination ⇒ most realistic first step


Optimize social welfare

Additional organization:difficult

ISO: who will invest?

TSO: national assets willhave to merge



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


Supergrids





5 Conclusions


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



A supergrid?

Early idea of a supergrid (after WW2)



A supergrid?

Early idea of a supergrid (after WW2)Implemented as a 400 kV AC grid



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


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)



Supergrids: current “proposals”


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



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



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



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 ).



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



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




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




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



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),. . .



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



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



Protection

Example: 4 terminal MT HVDC system



Protection

Fault occurs in the DC circuit (t = 0)



Protection

Rapidly changing currents throughout the system

VDC =L ·di

dt+R · i

i(t)= VDC

R+

(I0 − VDC

R

)·e− R

L · t



Protection

Protection system must indicate the faulted line

PS



Protection

Opening of the faulted line (t < 5ms)


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



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



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


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?


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?


Conclusions





5 Conclusions


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


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. . .


Conclusions

Questions

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1


future electricity grids 2/2

Technology

electric power systemsekc2

coreso power

reactive power

power system1liberalization

future electric grids

company power system

coreso example3 power

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