Post on 14-Jun-2018
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Conclusions / Hypothesisyp• A meshed HVDC grid has the potential to improve
it f l !security of supply!• A multi-terminal HVDC grid in the North Sea can
effectively integrate the synchronous interconnections (UK, UCTE and Nordic)
• Can be operated as ONE control area (if desirable)
R ( i d d ) b h d• Reserves (primary and secondary) can be shared without “technical constraints”
• Fast control and protection will enable network splitting to avoid risk of cascading outages and complete blackouts
• Fully integrate the power markets across the2
Fully integrate the power markets across the asynchronous areas.
O tliOutline• Introduction: Why HVDC – VSC HVDC
– Multi-terminal HVDC– Multi-terminal HVDC• Power system security and Control
objectives• Modelling and control designModelling and control design• Examples (illustrating security control
t )aspects)
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HVDC Why?y1. To reduce total transmission loss for long distance
power linespower lines
Break even distance for onshore application is
4about 600km and less than 100km for subseatransmissions
Cont…2. AC transmission becomes weak (unstable) for very large
Cont…( ) y g
distances3. Subsea power transmission3. Subsea power transmission
- Large capacitive currents in AC cables severly limit the transmission capacity of long distance AC cables
- The most common application area of HVDC is for subsea power transmission
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P t itPower system security
Security standards: Deterministic (N-1) or risk based
Ability to manage contingencies / outages:
Availability of reserves
Availability of transmission capacityAvailability of transmission capacity
Stability and control
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Main challenges in operation andMain challenges in operation and control
• Primary control: Less primary reserves if new generation provide lessLess primary reserves if new generation provide less
frequency response• Secondary control:• Secondary control: More need for secondary reserves with more variable
generationgeneration• Tertiary control: Benefits with larger control areas and exchange of
reserves.
New possibilities with an offshore Multi-terminal HVDC grid!
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HVDC grid!
Control objectives: DesiredControl objectives: Desired operational capabilitiesp p
• Balancing of offshore wind power variation• Resilience, e.g. to loss of a VSC-HVDC
terminal (≈load/generation loss) or line/cableterminal ( load/generation loss) or line/cable • Frequency response enhancement of AC grids• Market integrated operation• AC and DC fault handling capabilitiesAC and DC fault handling capabilities
C t ll h ld b d i d t tControllers should be designed to meet the requirements specified above.
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the requirements specified above.
Modelling:Active and Passive AC grid Connections
Passive AC grid connection:-AC voltage control at PCC
Time-average VSC model
AC voltage control at PCC-no synchronization-no control of currentno control of current
, ,2 2 2
1
a dc b dc c dca b c
m U m U m UV V V Active AC grid connection:-Grid synchronization
12o a a b b c cI m i m i m i -control of current flow
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Control of VSC Connected to PassiveControl of VSC Connected to Passive AC Grid
AC voltage controlAC voltage control at PCC, |V|
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Control of VSC Connected to ActiveControl of VSC Connected to Active AC grid g
Two options:
• |m|, δ control: uses phasor measurements
• decoupled axes (dq) control : usesinstantaneous measurementsinstantaneous measurements
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|m|, δ control
• Suitable for modelling VSC control in phasor basedl t i i l ti t l ( DI SILENT)electric power simulation tools (eg: DIgSILENT).
• Voltage and current phasor measurements introduceadditional time constant into the controllers
0o
+-
|m|*additional time constant into the controllers
δ*
+ cos+
ma*ωt ×
PIδ*
P* +
120o
+-
cosωt m *×
|m|*PI
P-
PI+ |m|*
|V|* or |Q|*δ*
cos+
ωt mb×
-
|V| or |Q|
240o
+-
cosωt *
|m|*
15δ*
cos+
ωt mc×
Decoupled axes (dq) controlDecoupled axes (dq) control
• Involves abc/dq transf.• Fast control responses
Mostl sed in practice• Mostly used in practicefor VSC control
• Modelling is possibleg pwith electromagnetictransient softwares such
PSCADas PSCAD
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Outer Controllers• Set the references to active & reactive (inner) current controllers• Active power and/or DC bus voltage control (there types shown below)• Reactive power and/or AC voltage control (not shown here)
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Safe operating area of a VSCSafe operating area of a VSCController actions are limited within safe operating areaController actions are limited within safe operating area
Safe operating area of a VSC: (a) U vs I safe operating region (b) U vs P safe operating region
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(b) U vs P safe operating region
Constant DC voltage control within the safe operating region
U* = DC voltage referenceU* = DC voltage reference
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DC voltage droop control within the safe operating region
U* = DC voltage referenceU* = DC voltage reference
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Two terminal VSC-HVDC ControlHVDC
S L
Rectifier(Source converter)
Inveter(Load converter)
transmission
PS PL
Control modes (3x3=9 combinations) RemarksRectifier Inverter
Fixed power Fixed power X (Not viable)Fixed power DC Droop √ (With risk of DC overvoltage)Fixed power DC Droop √ (With risk of DC overvoltage)Fixed power Fixed DC voltage √ (With risk of DC overvoltage)DC Droop Fixed power √ (Good P control by Inv )DC Droop Fixed power √ (Good, P control by Inv.)DC Droop DC Droop √ (Good, P control by Both)DC Droop Fixed DC voltage √ (Good P control by Rect )DC Droop Fixed DC voltage √ (Good, P control by Rect.)Fixed DC voltage Fixed power √ (Good, P control by Inv.)Fi ed DC oltage DC Droop √ (Good P control b In )
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Fixed DC voltage DC Droop √ (Good, P control by Inv.)Fixed DC voltage Fixed DC voltage X (Not viable)
DC Voltage Control HVDC
S L
Rectifier(Source converter)
Inveter(Load converter)
Responses(Primary & secondary DC voltage control)
HVDC
transmission
( y y g )PS PL
UDCPL0
Operatingpoint PL1
UDC PL0
PL1
UDC PL0
C
PL1
PS
PS1
B
A
C
PS0
PS1
A
C
PS0
A
B
P P
PS0
399
400
401
ge
399
400
401
e
399
400
401 P
BS0
395
396
397
398
DC voltag
Rectifier (source)terminal DC bus voltage
395
396
397
398
DC voltage
Rectifier (source)terminal DC bus voltage
395
396
397
398
DC voltage
Rectifier (source)terminal DC bus voltage
10 20 30 40 50 60 70
950
1000
1050
)
950
1000
1050
W)
10 20 30 40 50 60 70395
950
1000
1050
10 20 30 40 50 60 70395
800
850
900
Power (M
W)
Power taken byinverter (load) terminal
800
850
900
Power (M
W
Power taken byinverter (load) terminal
800
850
900
Power (M
W)
Power taken byinverter (load) terminal
2310 20 30 40 50 60 70
750
Time (s)
10 20 30 40 50 60 70
750
Time (s)
10 20 30 40 50 60 70
750
Time (s)
(a) (b) (c)
Power flow control in DC grid :Power flow control in DC grid :achieved by DC voltage droop
No need for communication between terminals No need for communication between terminals Many converter terminals contribute to DC voltage regulation DC analogy to distributed frequency droop control in AC
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DC analogy to distributed frequency droop control in AC systems
Similarities of AC grid frequency droop control d DC lt d t l f MTDCand DC voltage droop control of MTDC
Steady state characteristic of: (a)synchronous generator (b)VSC-HVDC terminal (The dots at the end of the characteristic lines
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HVDC terminal (The dots at the end of the characteristic lines show tripping points.)
Test model: ”North Sea DC grid”Test model: North Sea DC gridOffshore load
(Oil/gas platform)
Prated=250 MWC t t P
All cable resistances: r=0.01 Ω/kmAll bl i 5 F/k
Prated=450 MW Prated=1000 MWDC d d
l45=250
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Constant Power terminal
All cable capacitances:c=5 μF/kmBipolar DC transmission for all cases
Rated DC voltage =+/-200 kV
DC droop mode DC droop mode
l14=500 km
50 Km
1 4NORDEL GridScotland
l12=300 km l34=700 km
l23=600 km
UCTE Grid
UK National Grid
Prated=800 MWDC droop mode
2 3Prated=750 MWDC droop=∞ 6
l26=120 km
England
Prated=600 MWVariable (wind) power
26Offshore windfarm
S it l iSecurity analysisExamples illustrating security aspects p g y p
related to operation and control
• Managing normal wind variations• Outage of DC connections• Outage of DC connections• Outage of generation (wind farm tripping)
P i t l t id• Primary control response to ac grid contingency (exchange of primary reserves)
• Need for secondary control
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Variations of DCVariations of DC voltage with
fluctuating wind 460
480
500
W)
fluctuating wind power and DC droop control
380
400
420
440
Power (M
W Generated wind power from offshore WF
presponses 10 20 30 40 50 60 70 80 90 100 110
360
380
402
404
DC bus 1 (Scotland)
396
398
400
402
voltage (kV)
DC bus‐1 (Scotland)
DC bus‐2 (England)
DC bus‐3 (UCTE)
DC bus‐4 (NORDEL)
l45 =250 Km
10 20 30 40 50 60 70 80 90 100 110392
394
396DC
600
DC bus‐5 (Oil/gas platform)
DC bus‐6 (Offshore Windfarm)
m
400
500
600
(MW)
From Scotland (via Conv‐1)
To England (via Conv‐2)
To UCTE (via Conv‐3)
l26 =120 km
200
300Power To UCTE (via Conv‐3)
From NORDEL (via Conv‐4)
To oil/gas platform (via Conv‐5)
From offshore WF (via Conv‐6)
2810 20 30 40 50 60 70 80 90 100 110
Time (s)
Outage of DC line 1-2
Terminal-1 (Conv-1) ti t d 5
200
250
MW)
Power via Line 1‐2
Power via Line 1‐3continues to draw power from DC grid via lines 1-3 and 1-4 when 50
100
150
Line Pow
er (M
Power via Line 1‐4
Power via Line 2‐3
line 1-2 is disconnected. 85 90 95 100 1050
L
402
Power via Line 4‐3
398
400
ltage (kV)
DC bus‐1 (Scotland)
DC bus‐2 (England)
DC bus‐3 (UCTE)
DC bus 4 (NORDEL)
l45 =250 Km
394
396DC vol DC bus‐4 (NORDEL)
DC bus‐5 (Oil/gas platform)
DC bus‐6 (Offshore Windfarm)
85 90 95 100 105
450
500
550
W)
From Scotland (via Conv‐1)
To England (via Conv‐2)
l26=120 km
250
300
350
400
Power (M
W To UCTE (via Conv‐3)
From NORDEL (via Conv‐4)
To oil/gas platform (via Conv‐5)
29 85 90 95 100 105150
200
Time (s)
From offshore WF (via Conv‐6)
Outage of connection to windfarmOutage of connection to windfarm
500
550
From Scotland
400
450
500 From Scotland(via Conv‐1)
To England(via Conv 2)
300
350
00
W)
(via Conv‐2)
To UCTE(via Conv‐3)
l45 =250 Km 200
250
Power (M
W
From NORDEL(via Conv‐4)
100
150To oil/gasplatform(via Conv‐5)
l26 =120 km
0
50 From offshore WF(via Conv‐6)
Terminals 1, 2 and 4 compensate for lost power
83 84 85 86 87 88 89 90 91 92
Time (s)
30flow from offshore wind farm.
Frequency response enhancement of AC grids by VSC HVDCof AC grids by VSC-HVDC
Frequency droop control implemetation on VSC-HVDC with: ( ) t t t l (b) DC lt d t l(a) constant power control (b) DC voltage droop control
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Frequency response enhancement of AC grids by MTDCof AC grids by MTDC
1.001
u.)
UK national grid
0.998
0.999
1
frequency (p.u
UCTE grid
NORDEL grid
Oil/gas platform
l45 =250 Km
402100 110 120 130 140 150 160 170 180
0.997
0.998
Grid f
Oil/gas platform
Offshore WF
l26 =12
398
400
402
age (kV)
DC bus‐1 (Scotland)
DC bus‐2 (England)
DC bus‐3 (UCTE)
120 km
392
394
396DC volta DC bus‐4 (NORDEL)
DC bus‐5 (Oil/gas platform)
DC bus‐6 (Offshore Windfarm)
UCTE grid frequency500
600
W)
From Scotland (via Conv‐1)
To England (via Conv‐2)
100 110 120 130 140 150 160 170 180392
UCTE grid frequency supported by NORDEL and UK (via two HVDC
i l d300
400
Power (M
W
To UCTE (via Conv‐3)
From NORDEL (via Conv‐4)
To oil/gas platform (via Conv‐5)
F ff h WF ( i C 6)
34stations: one at England and the other in Scotland) 100 110 120 130 140 150 160 170 180
200
Time (s)
From offshore WF (via Conv‐6)
Comparison of grid responses (UCTE grid): with and without frequency support from DC gridand without frequency support from DC grid
0.9995
1
0.999
)
0.998
0.9985
equency (p.u.
With frequency support by MTDCWithout frequency support by MTDC
0.997
0.9975
Grid fre
0.9965
0.997
100 110 120 130 140 150 160
0.996
Time (s)
35Frequency response improves with presence of frequency support from DC grid.
( )
Precise control of power flowSchedule/ dispatch Control
typeTerminal No
PDC(MW) UDC (kV)No. (MW)
1 600 - Droop
2 - 400 Droop
3 -750 - Fixed P
4 550 - Droop
5 -900 - Fixed P
Control references Control typeTerminal
NoPDC
(MW) UDC (kV)No. (MW) DC ( )
1 600 400 Droop
2 500 400 Droopp
3 -750 400 Fixed P
4 550 400 Droop
N t i !36
5 -900 400 Fixed P Not precise!
Cntd…Schedule/ dispatch Control
typeTerminal No.
PDC(MW) UDC (kV)( )
1 600 - Droop2 - 400 Droop3 750 Fi d P3 -750 - Fixed P4 550 - Droop5 -900 - Fixed P
DC Power flow analyisis
Control references Control tTermina PDC
flow analyisis
typeTerminal No.
PDC(MW) UDC (kV)
1 600.00 399.550 Droop
2 535.94 400.000 Droop
3 -750.00 396.613 Fixed P
4 550 00 396 928 D Precise!37
4 550.00 396.928 Droop
5 -900.00 385.247 Fixed P
Precise!(Desired power flow achieved)
F t k MTDCFuture works on MTDC• Protection schemes
F lt d t ti d l li ti• Fault detection and localization algorithms
• Communication based (?)Communication based (?)
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Future worksFuture works…Impact of wide area MTDCImpact of wide area MTDC (Stability, operational, …)
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