IEEE Power & Energy Magazine - May/June 2019 - 57

Study Examples With INELFE Replicas
High-Frequency Resonance Investigations

An unusual incident occurred on the inelFe hVdc link on
22 september 2015. A high-frequency oscillation (around
1.7 khz) was recorded at the pcc, followed by the tripping
of the hVdc links. The event resulted from an interaction
between the hVdc link and the surrounding ac network. The
harmonic interactions were successfully reproduced and analyzed with replicas, as shown in Figure 11, which compares
the on-site and replica voltage measurements. The main points
from the event investigation lead us to note the following:
✔ Accurate eMT representation of ac grids is needed to
capture high-frequency behavior.
✔ it is necessary to use physical replicas of actual controls
to examine their exact behavior. it was not possible to
replicate this incident with the offline model provided
by the manufacturer because this offline model reflected control simplifications and/or outdated versions of
the controls that have since been updated.
✔ network configurations can lead to harmonic interactions between ac grid and hVdc controls. These highfrequency resonances depend on several network parameters, including an ac line series, shunt impedances,
the short circuit level, and the ac network configuration.

✔ Stage 2 (5 s < t < 55 s): The opening of BrK1 at this

stage results in an oscillation around 0.155 hz between pcc1 and pcc2.
✔ Stage 3 (t > 55 s): At this stage, there is load transfer,
opening of BrK2, and closing of BrK3. This leads to
increased damping of the oscillations.

(a)

Investigations of Interarea Oscillations

This section considers a practical interarea oscillation case
and simulation using the inelFe replicas. The test case
centers on the east-center-west interarea oscillation mode
based on the sequence of events that occurred on 1 december
2016. The real-time simulator configuration does not model
a detailed and realistic network; rather, it mimics the interarea oscillation of approximately 0.155 hz resulting from the
sequence of events. Therefore, the purpose of this study is
to validate the control behavior once the power electronics
system is in service and to investigate potential future control
system improvements and validate software modifications on
the inelFe control cubicles.
Based on the sequence of events, one can represent the
equivalent network as in Figure 12. Zline1 and Zline2 are the
equivalent ac lines between France and spain. Two generic
synchronous machine models (h1 and h2) reproduce the
interarea oscillation mode. The inelFe link is operating
in ac line emulation mode, which responds similarly to the
behavior of an ac line by increasing/decreasing the hVdc
link active power flow as a function of the angular differences between the ac converter buses.
The real-time simulator model is a representation of the
system equivalent shown in Figure 12. The simulator models the following sequence of events to mimic the interarea
oscillation event.
✔ Stage 1 (t < 5 s): All circuit breakers are closed, except
the BrK3, which is open. A 2-Gw exchange takes
place between pcc1 and pcc2.
may/june 2019

(b)

figure 11. The INELFE ac voltage at the PCC, illustrating
the high-frequency oscillation issue: (a) the on-site results
and (b) the INELFE replicas.

PCC1

PCC2

BRK1 Zline1
Zline2
INELFE Link

PTotal

H1
10 GW
L1
8 GW

PHVdc Station 1 Station 2

Zline3 BRK3 BRK2
L3
1 GW

SM
H2
10 GW

L2
11 GW

figure 12. The equivalent grid around the HVdc INELFE
link. SM: synchronous machine; BRK: circuit breaker.
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IEEE Power & Energy Magazine - May/June 2019

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