Modelling DC

ERC Starting Grant, 2011-2015

May 2018

Dragan Jovcic

d.jovcic@abdn.ac.uk

School of Engineering

University of Aberdeen

1. DC transmission networks are required for large-scale offshore energy evacuation,

2. DC grids will be fundamentally different from AC transmission systems,

3. Technical challenges include:• DC voltage stepping

• DC fault isolation

• DC grid control and dynamics

4. DC/DC and DC hubs will play key role in DC grids. They do not exist.

5. Modeling challenges • Numerous converters

• High-frequency dynamics

• Coupling between DC and AC dynamics,

6. Control/stability challenges • DC grid dynamics are two orders of magnitude faster than AC grids

• Distributed control is preferred

1. Background

2

2. DC/DC converters

3Figure 1. Connecting two existing HVDC using a DC/DC converter.

Using DC/DC converter to connect two HVDC lines

• Power trading between two DC lines of different voltage levels,

• Improved operating flexibility,

• Two protection zones, DC faults are not transferred across DC/DC,

• Two HVDC of different manufacturers. DC/DC resolves multivendor issues,

• DC/DC becomes:

• transformer (DC voltage stepping) ,

• power flow regulator,

• DC Circuit Breaker.

DC/DC

+/-400kV

+/-300kV

DC cable (200km)

DC cable

(20-100km)

500MW

DC cable (500km)

+/-300kV

DC cable (300km) DC cable (200km)

2. DC/DC converters

4

Fig.2. 10-terminal DC grid with 3 local radial DC systems and 3 DC/DC

=

Ba-A0

Ba-B0

= AC/DC Converter Station

SA0

SB0

Ba-A1

Ba-B1

Ba-B3

Bm-A1

Bb-B2

Bm-B2 Bm-B3

Bm-F1

Bm-E1

Bb-B1-1

Bb-B4

Bm-C1

Bb-C2

Bb1-D1-1

Cm-A1

Cb1-A1-1

Cb-B2Cm-B2

Cm-B3

Cm-F1

Cm-E1

Cb1-D1-1

Cb-C2

Cm-C1

Bo-C2

Bo-D1

Bo-E1

Bo-F1

800MVA

800MVA

800MVA

800MVA

200MVA

800MVA1200MVA

800MVA

1200MVA

50km

200km

200km

200k

m

200k

m

200km

200km

200km 50km

800M

W

210km

800MW

DC/DC Converter

300k

m80

0MW

280k

m200M

W

200km

800MW

200km

1400MW

300k

m12

00M

W

300k

m12

00M

W

200km1200MW/200km

1200

MW

447k

m

M

DC Cable

DC Overhead line

=DC

DC=

1200MW

450km

1200

MW

730k

m

790

175

324

0

89

86

392

971

324

500

188

103

1200

1200

450

100

4970

750

100

1300

900

618107

0

800MW

500

309

309

1000500

1000

500497

100

500

600

392

800

±400kV±400kV

±400kV±400kV

±400kV

±400kV

±200kV

±200kV

±200kV

±200kV

±400kV

971

447k

m12

00M

W

=

=

=

=DC D

C=

±400kV

=DCDC=

=

=

=

=

1200MVA

1200MVA

790

=

=

800MVA=

500

=DC DC

=

300k

m80

0MW

500

=

=

=

1200MW/200km

750

750

7501200MVA

=

971

1200MVA

1200MVA

971

±400kV

448

783

783

Bb-B2-1 Bb-B2-2

Bb-B1-2

Bb1-D1-2

Cb1-D1-2

Cb1-A1-2

Bb1-A1-1 Bb1-A1-2

Hybrid DC CB

Objectives of Multi-port DC hub in the DC Grid

• Acts like an electronic substation, connecting numerous DC lines,

• Enable any port to trade power with any other port in the DC hub,

• Ability to connect DC lines of different dc voltage ratings, and different manufacturers,

•

• Ability to control power in each DC line,

• Connect/disconnect any DC line “on the fly” without affecting operation of the grid,

• Isolate the faulted DC cable and provide undisturbed operation of the remaining grid,

• DC Circuit breaker may not be required.

3. DC Hubs

5

AC1

DC

HUB

AC2

AC3

1

2

3

~

~

±60kV

±80kV

±120kV

±320kV

±400kV

VSC1

VSC2

VSC3

VSC4

VSC5

AC4

AC5

~

~

~

DC Cable 1

DC Cable 2

DC Cable 3

DC Cable 4

DC Cable 5

Fig.3. 5-terminal DC grid with a 5-port DC hub

DC fault tolerance and high security/redundancy of DC hubs

• Any port is readily disconnected for DC faults,

• Any phase is readily disconnected in case of an internal fault,

(graceful degradation),

• Use redundant phase to meet N-1 criterion (substitute faulted

phase with a spare),

6

V1dc

Port 1

C1

L1 CB1

Port 3

C3

L3CB3

Bus_A

Bus_B

Bus_C

Bus_D

Bus_G

vcA

Port 2

C2

L2 CB2

Port 4ignd

vcB

vcC

vcD

Inner LCL

circuit

V1dc

V4dc

V4dc

V3dc

V3dcC4

L4CB4V2dc

V2dc

Fig.4. 4-port, 4-phase DC hub

3. DC Hubs

-4

-3

-2

-1

0

1

2

3

4

0.9 0.95 1 1.05 1.1 1.15 1.2

Pid

c(p

u)

Time(s)

P1dc

P2dc

P3dc

P4dc

-6

-4

-2

0

2

4

6

0.45 0.5 0.55 0.6 0.65

Pid

c(p

u)

Time(s)

P1dc

P2dc

P3dc

P4dc

7

Advantages:

•DC fault is only a local disturbance

•DC hub inherently reduces fault current,

•DC fault is readily isolated

•No need for fast protection, reliability is high

•Each DC line can have different voltage

level

•Each DC line can have different HVDC

technologies

3. DC Hubs

Fig.5. North Sea DC grid with 4 DC hubs

8

4. DC Grid demonstrators

Fig.6. 5-terminal, 900V, DC Grid demonstrator at Aberdeen HVDC research centre

9

4. DC Grid demonstrators

Fig.7. Testing 30kW, 200V/900V LCL DC/DC converter

a. DC/DC step up operation b. DC/DC step down operation

c. HV side DC fault d. LV side DC fault

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5. DC Grid modelling

DC Grid modeling challenges:

• Model of N-terminal DC grid is substantially mode complex than N-node AC grid,

• DC grids will include numerous MMC AC/DC, DC/DC converters,

• Simulation based on average modeling (PSCAD,EMTP) is still very slow,

• CIGRE 10 terminal DC grid model, 20s of real time takes 4 hours simulation using average model (20µs).

• Standard simulation platforms (PSCAD,EMTP) support only trial and error in time domain,

• Eigenvalue studies or frequency domain studies are required but not possible,

• Medium frequency (300Hz-1000Hz) will be used in the dc/dc converter to reduce size,

• Simulation step needs to be small to comply with the fastest sampling in the dc/dc converter,

• Multiple DQ frames are required at different frequencies,

• Non-linear elements can not be directly transferred to DQ frame,

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5. DC Grid modelling

Fig. 8. MMC 10th order DQ analytical model validation.

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5. DC Grid modelling

Original system

(Kp_PLL=30, Ki_PLL=500)

System with increased PLL gains

(Kp_PLL=300, Ki_PLL=5000)

-14.56 ± j313.2

-17.82± j129.5

-6.98 ± j317

-33.74± j101.3

System 1

(KP_CCSC=0.5, KI_ CCSC =50)

System 2

(KP_CCSC=10, KI_CCSC=50)

-20.1 ± j122.2

-155.0± j637.8

-6.2 ± j129.7

-165.2± j681.6

DQ frame average linearized model enables eigenvalue studies

• High PLL gains cause subsynchronous frequency instability at 45Hz.

• High gains of circulating current suppression controller deteriorate stability at 20Hz

• Status of converter average modelling

Converter Analytical, linearised model

MMC AC-DC 10th order model Developed

MMC AC-DC (blocked state) Challenging

MMC isolated DC-DC (dual active bridge) Complex model

MMC DC-DC Challenging

Line commutated AC-DC converter Developed

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6. DC Grid control

DC grid control challenges:

•DC grid is more difficult to control than traditional AC systems,

•There is no common frequency, which indicates power unbalance,

•DC voltage indicates global power unbalance but it also changes with local power flow,

•DC grid dynamics are 2 orders of magnitude faster than AC grid dynamics,

•DC grid components have low (overcurrent/undervolatge) tolerances (trip at 0.85pu),

•There are no passive loads with stabilising feedback (lower voltage still draws same current),

•All components are controllable. Numerous control loops,

•No inertia. GW powers should be balanced within 1-2ms.

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6. DC Grid control

Fig. 10. DC Grid Dispatcher Controller

DC Grid controller demands:

•Grid must be stable without communication with despatcher. Distributed primary response,

•Converters should contribute grid power balancing for a disturbance. Secondary response ensures power

balance (automatic),

•Optimisation and re-dispatching by dispatcher. Tertiary response (human intervention),

Primary/secondary response is critical but may require trade-offs:

•Droop based control enables power balancing but dynamic stability may not be good,

•Fast DC voltage control is required at each terminal, but may not be optimal,

Fig. 9. 3-level controller for DC grid terminals

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6. DC Grid control

CIGRE DC Grid:

• 5 Offshore VSC terminals,

• 6 onshore VSC terminals,

• 2 DC/DC converters,

• 2 separate DC systems

• One DC system is bipolar,

• Meshed DC lines,

• Onshore AC systems,

Control system requirements:

• automatic power balance,

• optimal operating point,

• stable recovery for large

disturbances,

Fig. 11. CIGRE 11-terminal DC grid benchmark

16

6. DC Grid control

Fig. 12. DC voltages after terminal Cb-A1 outage (1GW loss),

Simulation of an outage of a 1GW VSC terminal:

•Primary response maintains stability. All variables are

within operating limits. No VSC tripping.

•Secondary response balances power. A new operating

point is established within 0.5s.

•Tertiary response re-establishes 1pu average DC

voltage within 2s.

Further research work is required:

1. DC/DC converters as multifunctional DC grid components,• Role in DC grids (voltage stepping, fault isolation, power flow control)

• Topologies, optimisation of losses, reliability, ...

2. DC Hubs (electronics DC substations), • Role in DC grids (voltage stepping, fault isolation, power flow control)

• Topologies, optimisation of losses, reliability, ...

• Control, modelling,

3. Modelling DC grids,• Simulation of grids with many converters,

• Analytical modeling (for eigenvalue studies),

4. Control/stability challenges, • Grid control layers,

• Distributed and centralized control,

• Robustness, flexibility,

5. Hardware (low power) demonstrations,

7. Conclusion – further research

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