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.