IEEE Power Electronics Magazine - June 2023 - 38

used, 2) conventional STATCOM with
GFL control, and 3) E-STATCOM with
GFM control [14].
Figure 4 shows the simulation
waveforms, where the active power
output of WPP and the voltage magnitudes
at the POI of WPP are given
in Figure 4 (a) and (b), respectively.
In case 1, the POI voltage is severely
degraded due to inadequate reactive
power compensation, which contravenes
grid codes and limits the
active power output of WPP. Further,
both voltage and active power
present obvious oscillations around 35 Hz (lower than
synchronous frequency 50/60 Hz), i.e., sub-synchronous
resonance, which sustains over 300 ms, indicating insufficient
damping in the WPP when connected to a weak grid.
In case 2, the conventional STATCOM effectively regulate
the POI voltage through the provision of reactive power
support. Yet, the improvement of dynamic performance
is limited. The WPP still lacks damping. Unlike the previous
cases, in case 3, the E-STATCOM not only improves
voltage stiffness (as evidenced by the significantly lower
voltage variation shown in Figure 4b), but it also actively
provides the desired damping through GFM control to
mitigate resonance (only small oscillations) and stabilize
the WPP under weak grid interconnection conditions.
2) Inertial Response
Figure 5 gives a comparison between the inertial
responses of an E-STATCOM and a synchronous condenser
Moreover, the GFM
control allows for
flexibly designing the
damping effect of the
E-STATCOM, thereby
smoothening the
frequency variations.
with a flywheel (SC-FW) storage system,
both with identical inertia constants.
The WPP is not involved in the
inertial response. The grid frequency
in Figure 5(a) changes linearly with a
RoCoF of −1 Hz/s. The E-STATCOM
exhibits similar inertial response to
the SC-FW within the first tens of milliseconds
as depicted in Figure 5(b)
[5]. This fast response is critical to
mitigate the RoCoF and reduce the
frequency nadir. Moreover, the GFM
control allows for flexibly designing
the damping effect of the E-STATCOM,
thereby smoothening the frequency variations.
3) Selective Harmonic Damping
Figure 6 presents a comparison of the performances
without and with the selective harmonic voltage damping
function of E-STATCOM under an ultra-weak grid condition
[13]. The harmonics injected by the offshore WPP are emulated
by a paralleled harmonic current source, according to
the IEC harmonic model of WT [15]. The injected harmonics
are 5th, 7th, 11th, and 13th.
The voltages at POI are clearly distorted when the
selective harmonic damping function is disabled, and
the total harmonic distortion (THD) is 4.3%, as shown
in the fast Fourier transform (FFT) result in Figure 6(a).
Yet, when the selective harmonic damping control is
enabled, the concerned harmonic voltages are dampened
effectively. The voltage THD at POI is reduced to 1.2%, as
shown in Figure 6(b).
FIG 4 Comparison of step responses [14]. (a) Active power output of WPP. (b) Voltage magnitude at POI.
FIG 5 Comparison of inertial response [7]. (a) Linear change of grid frequency. (b) Active power output of E-STATCOM.
38 IEEE POWER ELECTRONICS MAGAZINE z June 2023
IEEE Power Electronics Magazine - June 2023

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