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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
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
Grid following control (Onshore VSC)
418 September 2019
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
Grid forming control (Offshore VSC)
618 September 2019
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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?
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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
Frequency VoltageActive power
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
Case 3: DC-link outage
12
• The active power from the wind is exported from the hub
18 September 2019
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
Case 3: DC-link outage
13
• The active power from the wind is exported from the hub
• DC-link outage simulated and isolated with two breakers
18 September 2019
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
Case 3: DC-link outage
14
• The active power from the wind is exported from the hub
• DC-link outage simulated and isolated with two breakers
• Power redirected to remaining VSC links and determined by
frequency droop values kf
18 September 2019
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
Case 3: DC-link outage
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
Frequency VoltageActive power
RMS vs EMT: the need for new simulation tools / Matas Dijokas ,DTU
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