rms vs emt: the need for new simulation tools€¦ · rms vs emt: the need for new simulation tools...

Matas Dijokas, DTU

RMS vs EMT: the need for new

simulation tools

RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU

Offshore AC grid and HVDC links

218 September 2019

• 5 wind power parks (Psum = 3.5 GW)

• Hub configuration: 220kV AC

• 5 HVDC links

• Hub connected to 3 synchronous

areas


VSC modelComponents and phasors

3

• Converter of MMC type

• Phase reactor – leakage inductance and series resistance of transformer

• Unity transformer ratio assumed

• Two converter control strategies:

– Grid following control

– Grid forming control

18 September 2019


Grid following control (Onshore VSC)

418 September 2019


Grid following control (Onshore VSC)

5

• Converter controls DC link voltage and

reactive power exchange at onshore station

• Outer and inner control loops use PI control

• Decoupled current control

18 September 2019


Grid forming control (Offshore VSC)

618 September 2019


Grid forming control (Offshore VSC)

7

• Converter controls AC hub voltage and

active power sharing

• Two P-f control strategies:

– Different frequency droop values

– Same frequency droop values

• The frequency droop value kf selected

considering:

– Capability available on VSC

– Inertia of the onshore grids

• Converter equipped with Virtual

Impedance Control

18 September 2019

𝑃𝑖 = 𝑃𝑖𝑜 +

𝑘𝑓𝑖σ𝑗=1𝑛 𝑘𝑓𝑗

∙ ∆𝑃𝑤

Converter

Virtual Impedance Controller

Active Power Controller

Is RMS still good enough to

analyse the dynamics of

zero-inertia systems?


Time domain simulationsStudy case introduction

9

Active power change:

1. Power request from onshore station

2. Wind power increase

Fault:

3. DC-link outage

18 September 2019


Case 1: Power request from onshore station

10

• Good match of RMS and EMT simulation regarding active power and frequency responses

• Slight mismatch between RMS and EMT model regarding hub voltage. EMT is able to detect high frequency

oscillations in hub voltage

• Overall, RMS model is appropriate when it comes to power exchange between the different onshore grids.

No need to consider the transmission cable dynamics

18 September 2019

Frequency VoltageActive power


Case 2: Wind power increase

11

• Good match of RMS and EMT simulations regarding the active power and frequency responses

• RMS model is unable to detect the maximum voltage deviation and high frequency oscillations in Hub-

voltage

• Overall, for wind power variation RMS model is appropriate when the focus is on active power and frequency

response. However, it fails to detect the maximum Hub-voltage deviation

18 September 2019



Case 3: DC-link outage

12

• The active power from the wind is exported from the hub

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13


• DC-link outage simulated and isolated with two breakers

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14


• DC-link outage simulated and isolated with two breakers

• Power redirected to remaining VSC links and determined by

frequency droop values kf

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15

• No visible frequency mismatch between RMS and EMT models

• RMS model is unable to detect the maximum deviation and high frequency oscillations in Hub-voltage/active

power

• overall, for large disturbances (link-outage), RMS model fails to capture the dominant modes that

characterize the power and voltage responses

18 September 2019



Main takeaways

For the :

1. The simulations are able to to

capture the after a

2. The simulation reflects power changes and faults accurately, but requires

1618 September 2019

Need for new tools to capture the dynamics of zero-inertia

systems under fault, but remain computationally efficient

for large scale systems

Thank You

rms vs emt: the need for new simulation tools€¦ · rms vs emt: the need for new simulation tools...

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