Modelling DC

ERC Starting Grant, 2011-2015

February 2020

Dragan Jovcicd.jovcic@abdn.ac.uk

School of EngineeringUniversity of Aberdeen

While AC power transmission was dominant in 20th century, DC transmission will be prevailing in the 21st.

Motivation1.DC transmission networks are required for large-scale offshore energy evacuation,2. DC grids will be fundamentally different from AC transmission systems. Technical challenges include:

• DC voltage stepping • DC fault isolation• DC grid control and dynamics

3. Typical (mid-size) DC grid shown in Figure 1:• How many DC/DC are required and where should they be located?• Is it better to use many DC CBs or few DC/DC?• Takes 4 hours of simulation on PSCAD for 1s of real time,• Not possible to determine eigenvalues,• Takes a PhD project to tune controls for stable operation,

Goals of Modelling DC project:1.Analyse role and topologies for DC/DC in large DC grids. 2. Analyse role and topologies for DC hubs in large DC grids.3. Develop Models for DC/DC and DC hubs, 4. Understand DC grid Control/stability challenges,

1. Background

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Hybrid DC CB

Figure 1. 11 terminal DC grid (based on CIGRE DC grid) with 3 DC/DC converters.

2. DC/DC converters

3Figure 2. Connecting two existing DC systems using a DC/DC converter.

Using DC/DC converter in DC grids

• Power trading between two DC systems 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

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DC cable (200km)

DC cable (20-100km)

500MW

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

• Interconnect multiple DC systems of different DC voltages,• Power flow in possible on each port. Any port is readily disconnected,• DC faults are not propagated to other ports,• No need for DC circuit Breakers,• 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 stand-by phase),

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V1dc

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C1

L1 CB1

Port 3

C3

L3CB3

Bus_A

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vcB

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Inner LCL circuit

V1dc

V4dc

V4dc

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Fig.3. 4-port, 4-phase DC hub. Phase D is in stand by.

3. DC Hubs

-4-3-2-101234

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Advantages:•DC fault is only a local disturbance •DC hub inherently reduces fault current,•DC fault is readily isolated•No need for DC CBs or 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.4. North Sea DC grid with 4 DC hubs

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4. DC Grid demonstrators

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

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5. DC Grid modelling

Average value, simulation based, DC Grid modeling: • Average value (type 5) model for each converter is fastest approach with reasonable accuracy,• 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 study in time domain,

Small-signal, linearized, state space, analytical models:• Multiple DQ frames are required at different frequencies and harmonics,• Non-linear elements can not be directly transferred to DQ frame,• Requires manual modeling with elementary equations for each converter, • Medium frequency (300Hz-1000Hz) circuits in the dc/dc converter complicate modeling,• Eigenvalue studies or frequency domain studies are possible, • Parametric studies are fast even for very complex DC grids.

Key eigenvalues original system -14.56 ± j313.2

-17.82± j129.5

Key eigenvalues with increased

PLL gains

-6.98 ± j317-33.74± j101.3

Fig. 6. MMC 10th order DQ analytical model validation, and eigenvalue study example.

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6. DC Grid control

Fig. 8. DC Grid Dispatcher Controller

Primary/secondary response is decentralised:•DC grid dynamics are 2 orders of magnitude faster than AC grid dynamics.•No inertia. GW powers should be balanced within 1-2ms.•Each converter contributes to DC voltage control.•Control interactions within and between converters are a challenge.

DC Grid central controller:•Secondary response ensures power balance (automatic),•Optimisation of secondary response requires communication and a dispatcher.•Slow, Average DC voltage regulation is proposed, •Tertiary response (human intervention),

Fig. 7. 3-level controller for DC grid terminals

Further research/development work:

1. DC/DC converters as multifunctional DC grid components,• Have not been developed for GW powers.• DC grid planning with DC/DC (architecture, location, flexibility, security),• Complementary/interaction with DC Circuit Breakers, • Optimisation of losses, cost, size, reliability, ...• Adoption of MMC approach,

2. DC Hubs (electronics DC substations), • Have not been developed for GW powers.• DC grid planning with DC/DC (architecture, location, flexibility, security),• Optimisation of losses, cost, size, reliability ...• Control, modelling,

3. Simulation/Modelling of DC grids,• Simulation is very slow and model is too complex if number of converters is high (over 10),• Eigenvalue and parametric study capability is needed.• Faster analytical modeling is required for complex systems.

4. DC grid Control/stability challenges, • Generic control framework does not exist. • Grid control layers and interactions,• Bandwidth segmentation. Temporal and spatial interactions. Distributed fast, and centralized slow control, • Robustness, flexibility, interoperability, interaction with AC grids,

5. Hardware (low power) demonstrations, • De-risking of new converters and grid topologies,

7. Conclusion – further research

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