Control and Operation of Multi-terminal
VSC-HVDC
Temesgen Haileselassie
j
Uhlen
Kjetil
1
Conclusions / Hypothesis
yp
• A meshed HVDC grid has the potential to improve
security
it off supply!
l !
• 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
Reserves ((primary
i
and
d secondary)
d ) can b
be shared
h d
without “technical constraints”
• Fast control and protection will enable network splitting
to avoid risk of cascading outages and complete
blackouts
2
• Fully integrate the power markets across the
asynchronous areas.
O tli
Outline
• Introduction: Why HVDC – VSC HVDC
– Multi-terminal HVDC
• Power system security and Control
objectives
• Modelling and control design
• Examples (illustrating security control
aspects)
t )
3
HVDC Why?
y
1. To reduce total transmission loss for long distance
power lines
4
Break even distance for onshore application is
about 600km and less than 100km for subsea
transmissions
Cont…
2. AC transmission becomes weak ((unstable)) for very
y large
g
distances
3. 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
5
Limits of transmission capacity,
Submarine HVAC cables (ref Nexans)
6
Why multiterminal VSC-HVDC (MTDC) in the
N th S
North
Sea?
?
7
Grid integration
8
P
Power
system
t
security
it
 Security standards: Deterministic (N-1) or risk based
 Ability to manage contingencies / outages:
 Availability of reserves
 Availability of transmission capacity
 Stability and control
9
Main challenges in operation and
control
• Primary control:
 Less primary reserves if new generation provide less
frequency response
• Secondary control:
 More need for secondary reserves with more variable
generation
• Tertiary control:
 Benefits with larger control areas and exchange of
reserves.
 New possibilities with an offshore Multi-terminal
HVDC grid!
10
Control objectives: Desired
operational
p
capabilities
p
• Balancing of offshore wind power variation
• Resilience, e.g. to loss of a VSC-HVDC
terminal (≈load/generation
( load/generation loss) or line/cable
• Frequency response enhancement of AC grids
• Market integrated operation
• AC and DC fault handling capabilities
Controllers
C
t ll should
h ld be
b designed
d i d to
t meett
the requirements specified above.
11
Modelling:
Active and Passive AC grid Connections
Time-average VSC model
Passive AC grid connection:
AC voltage control at PCC
-AC
-no synchronization
-no
no control of current
Active AC grid connection:
-Grid synchronization
-control of current flow
12
maU dc
mU
, Vb  b dc ,
2
2
1
I o   ma ia  mbib  mcic 
2
Va 
Vc 
mcU dc
2
Control of VSC Connected to Passive
AC Grid
AC voltage control
at PCC, |V|
13
Control of VSC Connected to Active
AC grid
g
Two options:
• |m|, δ control: uses phasor measurements
• decoupled axes (dq) control : uses
instantaneous measurements
14
|m|, δ control
• Suitable for modelling VSC control in phasor based
electric
l t i power simulation
i l ti tools
t l (eg:
(
DI SILENT)
DIgSILENT).
• Voltage and current phasor measurements introduce
additional time constant into the controllers
|m|*
0o
ωt
*
P +
PI
δ
δ*
120o
P
ωt
*
|V| or |Q|
+
-
|m|
ma*
+
|m|*
cos
×
mb*
|m|*
240o
|V| or |Q|
ωt
15
×
+
δ*
*
PI
cos
+
*
-
*
+
+
+
δ*
cos
×
mc*
Decoupled axes (dq) control
•
•
•
•
16
Involves abc/dq transf.
Fast control responses
Mostl used
Mostly
sed in practice
for VSC control
Modelling
g is p
possible
with electromagnetic
transient softwares such
as PSCAD
Outer Controllers
•
•
•
17
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)
Safe operating area of a VSC
Controller 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
18
Constant power control within the safe
operating region
P* = power reference
19
Constant DC voltage control within the
safe operating region
U* = DC voltage reference
20
DC voltage droop control within the safe
operating region
U* = DC voltage reference
21
Two terminal VSC-HVDC Control
Rectifier
(Source converter) S
HVDC
Inveter
L (Load converter)
transmission
PS
PL
Control modes (3x3=9 combinations)
Rectifier
22
Remarks
Inverter
Fixed power
Fixed power
X (Not viable)
Fixed power
DC Droop
√ (With risk of DC overvoltage)
Fixed power
Fixed DC voltage
√ (With risk of DC overvoltage)
DC Droop
Fixed power
√ (Good,
(Good P control by Inv
Inv.))
DC Droop
DC Droop
√ (Good, P control by Both)
DC Droop
Fixed DC voltage
√ (Good,
(Good P control by Rect.)
Rect )
Fixed DC voltage
Fixed power
√ (Good, P control by Inv.)
Fi ed DC voltage
Fixed
oltage
DC Droop
√ (Good,
(Good P control by
b Inv.)
In )
Fixed DC voltage
Fixed DC voltage
X (Not viable)
DC Voltage Control
Responses
Rectifier
(Source converter) S
transmission
((Primary
y & secondary
y DC voltage
g control))
UDC Operating
point PL0
UDC
PL1
PL0
L
HVDC
PS
PL
UDC
PL1
PL0
PL1
C
PS
A
A
PS1
B
PS1
PS0
B
P
401
400
400
400
399
399
399
397
Rectifier (source)
terminal DC bus voltage
395
10
20
30
40
50
60
DC voltage 401
398
398
397
Rectifier (source)
terminal DC bus voltage
396
395
10
70
20
30
40
50
60
398
397
395
10
70
1000
1000
950
950
950
Power taken by
inverter (load) terminal
800
750
10
20
30
40
Time (s)
(a)
50
60
900
850
Power taken by
inverter (load) terminal
800
70
Power (MW)
1000
Power (MW
W)
1050
Power (MW))
1050
850
750
10
20
30
40
Time (s)
(b)
50
60
Rectifier (source)
terminal DC bus voltage
396
1050
900
PS0
P
401
DC voltagee DC voltagge P
23
C
A
B
396
Inveter
(Load converter)
20
30
50
60
70
900
850
Power taken by
inverter (load) terminal
800
70
40
750
10
20
30
40
Time (s)
(c)
50
60
70
Power flow control in DC grid :
achieved by DC voltage droop
24
 No need for communication between terminals
 Many converter terminals contribute to DC voltage regulation
 DC analogy to distributed frequency droop control in AC
systems
Similarities of AC grid frequency droop control
and
d DC voltage
lt
d
droop control
t l off MTDC
25
Steady state characteristic of: (a)synchronous generator (b)VSCHVDC terminal (The dots at the end of the characteristic lines
show tripping points.)
Test model: ”North
North Sea DC grid”
grid
Offshore load
(Oil/gas platform)
Prated=250 MW
C t t Power
Constant
P
terminal
All cable resistances: r=0.01 Ω/km
All cable
bl capacitances:c=5
i
5 μF/km
F/k
Bipolar DC transmission for all cases
Rated DC voltage =+/-200 kV
5
50
=2
l 45
UK National
Grid
Km
Scotland
Prated=450 MW
DC droop mode
1 l =500 km
14
l12=300 km
Prated=1000 MW
DC ddroop mode
d
NORDEL Grid
4
l34=700 km
UCTE Grid
l23=600 km
20
=1
l 26
England
6
km
2
Prated=800 MW
DC droop mode
3
Prated=750 MW
DC droop=∞
Prated=600 MW
Variable (wind) power
Offshore windfarm
26
S
Security
it analysis
l i
Examples
p
illustrating
g security
y aspects
p
related to operation and control
•
•
•
•
Managing normal wind variations
Outage of DC connections
Outage of generation (wind farm tripping)
Pi
Primary
control
t l response tto ac grid
id
contingency (exchange of primary reserves)
• Need for secondary control
27
500
Power (MW
W)
480
460
Generated wind power
from offshore WF
440
420
400
380
360
10
404
DC voltage (kV)
Variations of DC
voltage with
fluctuating wind
power and DC
droop
p control
responses
20
30
40
50
60
70
DC bus 1 (Scotland)
DC bus‐1 (Scotland)
400
DC bus‐2 (England)
398
DC bus‐3 (UCTE)
100
110
DC bus‐4 (NORDEL)
396
=2
l 45
392
10
600
DC bus‐5 (Oil/gas platform)
DC bus‐6 (Offshore Windfarm)
50
20
30
40
50
60
70
80
90
100
110
Km
From Scotland (via Conv‐1)
500
=1
l 26
20
Power (MW)
90
402
394
To England (via Conv‐2)
400
To UCTE (via Conv‐3)
To UCTE (via Conv‐3)
From NORDEL (via Conv‐4)
300
km
To oil/gas platform (via Conv‐5)
From offshore WF (via Conv‐6)
200
10
28
80
20
30
40
50
60
Time (s)
70
80
90
100
110
Outage of DC line 1-2
Terminal-1 (Conv-1)
continues
ti
to
t draw
d
power from DC grid via
lines 1-3 and 1-4 when
line 1-2 is disconnected.
L
Line Power (M
MW)
250
Power via Line 1‐2
200
Power via Line 1‐3
150
5
Power via Line 1‐4
100
Power via Line 2‐3
50
Power via Line 4‐3
0
85
90
95
100
105
=2
l 45
DC volltage (kV)
402
DC bus‐1 (Scotland)
400
DC bus‐2 (England)
398
DC bus‐3 (UCTE)
DC bus 4 (NORDEL)
DC bus‐4 (NORDEL)
396
DC bus‐5 (Oil/gas platform)
50
394
Km
DC bus‐6 (Offshore Windfarm)
=1
l 26
20
km
Power (MW
W)
85
550
90
95
500
From Scotland (via Conv‐1)
450
To England (via Conv‐2)
400
To UCTE (via Conv‐3)
350
To oil/gas platform (via Conv‐5)
250
150
85
105
From NORDEL (via Conv‐4)
300
From offshore WF (via Conv‐6)
200
29
100
90
95
Time (s)
100
105
Outage of connection to windfarm
550
From Scotland
From
Scotland
(via Conv‐1)
500
450
To England
(via Conv 2)
(via Conv‐2)
400
00
5
=2
l 45
Power (MW
W)
350
To UCTE
(via Conv‐3)
300
From NORDEL
(via Conv‐4)
250
0
Km
200
To oil/gas
platform
(via Conv‐5)
150
100
20
=1
l 26
km
From offshore WF
(via Conv‐6)
50
0
83
84
85
86
87
88
89
90
91
Time (s)
30
Terminals 1, 2 and 4 compensate for lost power
flow from offshore wind farm.
92
Frequency response enhancement
of AC grids by VSC-HVDC
VSC HVDC
Frequency droop control implemetation on VSC-HVDC with:
( ) constant
(a)
t t power control
t l (b) DC voltage
lt
droop
d
control
t l
31
Frequency response enhancement
example: two terminal VSC-HVDC
32
Effect of frequency droop control (in VSCHVDC ) on AC grid
HVDCs)
id ffrequency responses
33
Frequency response enhancement
of AC grids by MTDC
50
=2
l 45
Grid ffrequency (p.u
u.)
1.001
UK national grid
1
UCTE grid
0.999
NORDEL grid
Oil/gas platform
0.998
Offshore WF
0.997
Km
100
110
120
130
140
150
160
170
180
=1
l 26
20
km
DC voltaage (kV)
402
DC bus‐1 (Scotland)
400
DC bus‐2 (England)
398
DC bus‐3 (UCTE)
396
DC bus‐4 (NORDEL)
DC bus‐5 (Oil/gas platform)
394
392
100
600
DC bus‐6 (Offshore Windfarm)
110
120
130
140
150
34
Power (MW
W)
170
180
From Scotland (via Conv‐1)
500
UCTE grid frequency
supported by NORDEL
and UK (via two HVDC
stations:
i
one at England
l d
and the other in Scotland)
160
To England (via Conv‐2)
To UCTE (via Conv‐3)
400
From NORDEL (via Conv‐4)
300
To oil/gas platform (via Conv‐5)
F
From offshore WF (via Conv‐6)
ff h
WF ( i C
6)
200
100
110
120
130
140
Time (s)
150
160
170
180
Comparison of grid responses (UCTE grid): with
and without frequency support from DC grid
1
0.9995
Grid freequency (p.u.)
0.999
0.9985
With frequency support by MTDC
Without frequency support by MTDC
0.998
0.9975
0.997
0.9965
0.996
100
110
120
130
Time (s)
( )
35
140
150
160
Frequency response improves with presence of frequency
support from DC grid.
Precise control of power flow
Schedule/ dispatch
Control
Terminal PDC
UDC (kV) type
No
No.
(MW)
1
600
Droop
2
-
400
Droop
3
-750
-
Fixed P
4
550
-
Droop
5
-900
-
Fixed P
Control references
Control
Terminal PDC
U ((kV)) type
No
No.
(MW) DC
1
600
400
Droop
36
2
500
400
p
Droop
3
-750
400
Fixed P
4
550
400
Droop
5
-900
400
Fixed P
N t precise!
Not
i !
Cntd…
Schedule/ dispatch
Control
Termina PDC
U (kV) type
l No. ((MW)) DC
1
600
Droop
2
400
Droop
3
-750
750
Fi d P
Fixed
4
550
Droop
5
-900
Fixed P
DC Power
flow analyisis
Control references
Control
Termina PDC
t
UDC (kV) type
l No. (MW)
1
600.00 399.550 Droop
37
2
535.94 400.000 Droop
3
-750.00 396.613 Fixed P
4
550 00 396.928
550.00
396 928 Droop
D
Precise!
5
-900.00 385.247 Fixed P
(Desired power flow achieved)
F t
Future
works
k on MTDC
•
•
•
38
Protection schemes
F lt detection
Fault
d t ti and
d localization
l
li ti
algorithms
Communication based (?)
Future works
works…
Impact of wide area MTDC
(Stability, operational, …)
39
Thank you!
40