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A DC Voltage Control Strategy for
MMC MTDC Grids incorporating
Multiple Master Stations
C. Lin, X. Wu State Grid Smart Grid
Research Institute
1
C. E. Spallarossa,
T. C. Green Imperial College London
Paper 14TD0422
MTDC Grids
2
Future North Sea MTDC Grid
Yang ShanSi Jiao
Da Qu
Dai
ShanZhoushan
Ben Dao
Power Grid
HVDC
AC line
AC line
VSC-HVDC
Zhoushan Project, South East China
5 terminals (VSC MMC)
Research question:
Control of MTDC
(effectiveness, reliability,
robustness)?
MTDC Control Strategy
3
1. Constant Vdc Control : Master and Slave
2. P-Vdc Droop Control : Peer to Peer
• 2 Masters: proper P sharing
and stable V profile
• Vdc ref modified via a droop
γ considering Idc
• In case of outage of one of
the slacks, the other keeps
Vdc within the limits (±5%)
3. Master Droop Control
Five-terminal DC Grid
4
AC side DC side DC cables(100km)
Vac 220 kV Vdc +/-320 kV R 0.0127 Ω/km
Prated 1000MW Ldc 14 mH L 0.93 mH/km
SCR 20 Rdc 3e7 Ω C 12.74 nF/km
Dynamic Operations
5
• Assess the effectiveness of the MTDC control strategy proposed
(Master Droop Control).
• Evaluation of β and γ droop constants defined through sensitivity
analysis approach.
1) P-Vdc Droop Control
Outage at Conv 1
Conv 1: Master (Vdc-Q)
Conv 2, 3, 5: P-Q + P-Vdc
droop
Conv 4: Vac-f
2) Master Droop Control
Outage at Conv 1
Conv 1, 2: Masters (Vdc-Q)+
Master droop
Conv 3, 5: P-Q + P-Vdc droop
Conv 4: Vac-f
1. P-Vdc Droop Control
6
0.5 1 1.5 20.9
1
1.1
1.2
1.3
1.4
time, s
V, p
.u.
B1
B2
B3=B*
β1=0.5
β2=0.8
β3=1.2
• Outage of Conv1 at t = 1s
• Sensitivity analysis for β : βmin < β < βmax
For β < βmin (=β1) system unstable
For β > βmax(=β3) P-Vdc negligible
Post-fault: Vdc rise
above acceptable
limits (+1.05 p.u.; -
0.95 p.u.) for every β
β3 : best voltage
profile (optimal value
for P-Vdc droop β*=β3)
1. P-Vdc Droop Control
7
Power sharing:
P1 reduced to 0 p.u.;
P2, P3, P5
modulated for
mismatch sharing
and power balance
0.5 1 1.5 2-0.75
-0.5
-0.25
0
0.25
0.5
0.75
time,s
P, p
.u.
P3
P1
P5
P4
P2
P-Vdc droop control:
correct power sharing
BUT Vdc out of the limits.
2. Multiple Masters
8
0.5 1 1.5 20.9
1
1.1
1.2
1.3
1.4
time, s
V, p
.u.
y2=y*
ymax
ymin
γmin=0.01
γ2 = 0.05
γmax=0.1 Post fault: if β=β* for
P-Vdc droop, Vdc within
acceptable limits for
γmin and γ2 (optimal
value: γmin=γ*)
• Outage at Conv 1 at t = 1s
• Sensitivity analysis for γ : γ min< γ < γmax.
For γ < γ min (=0.01) ineffective
For γ > γ max(=0.1) instability
2. Multiple Masters
9
0.5 1 1.5 2-1
-0.75
-0.5
-0.2
0
0.25
0.5
0.75
time, s
P, p
.u.
P1
P2
P3
P5
P4
Master Droop Control:
Balanced power sharing
along with fast voltage recovery.
Power sharing (β* and γ*):
Small influence of P-Vdc
droop
Second master (Conv 2)
mainly compensates for
power mismatch
Conclusion
10
• Alternative DC voltage control strategy for correct
power sharing along with voltage stability in case of
an outage in the slack converter.
• It surpasses the limitations associated with the other
control methods thanks to the coexistence of two
master converters.
Back Up Slides
11
MMC Modelling
12
MMC model realized with OPAL-RT Artemis Block (MMC IP Cell)
MMC Control : traditional structure
• outer and inner control in the upper level control,
• capacitor voltage balancing (BCA), modulation algorithm and
circulating current suppression control (CCSC) in lower level control
MMC Properties
Prated 1000 MW Rcell 22 kΩ Ccell 1.1 mF
Vac 220 kV V0cap 0 V Larm 0.138 H
Vdc ±320 kV Ncell/phase 60 Rarm 1 Ω
DFIG Wind Farm
13
DFIG Wind Turbine
Pnom per unit 2.2 MW
N of unit 50
V 575 V
Nnom 1500 rpm
• DFIG model from OPAL-RT
Artemis Library
• Wind Farm: 5 clusters of 12
units in radial topology
MTDC Control Strategy
• Constant Vdc Control : Master and Slave
14
• Centralized control strategy
• Master (Vdc-Q): control of Vdc
• Slaves (P-Q): control of P transfers
• Not robust in case of outage of master station.
Basic scheme for Vdc control. Basic scheme for P control.
MTDC Control Strategy
• P-Vdc Droop Control : Peer to Peer
15
• Coordinated control strategy.
• Regulation of P according to Vdc variations.
• β droop defines the sharing of the power imbalance
• If converter trips: proper P sharing at cost of jeopardizing V
stability.