IEEE Power & Energy Magazine - November/December 2019 - 92

Ancillary services or other market-based approaches should
be developed to assign value to the system benefits from GFM
technology, similarly to frequency support and black start services.
frequency (RoCoF) after a large generation or load disconnection, thereby avoiding a cascading disconnection of SGs,
particularly gas turbines. The second is to arrest the frequency decay and raise the frequency nadir after a generation
trip or arrest the frequency increase and lower the frequency
zenith after a load trip.
This minimum level of synchronous inertia can be re duced, but not fully substituted, with the fast frequency
response (FFR) that can be provided by GFL IBPSs. Additional inertia above the minimum level is required to form
a viable island in case of system separation and to maintain power system security. This inertia can be provided by
additional synchronous machines or supplemented through
FFR from IBPSs. Generally, a response time of several hundred milliseconds is sought. Research and several practical
examples from small island system applications have demonstrated that a minimum inertia level requirement can be
eliminated if a certain share of inverters is GFM with a sufficient energy buffer.

Disturbance Ride-Through
High- and Low-Voltage Ride-Through

In addition to remaining connected to the power system,
IBPSs are expected to support system recovery by injecting
an active and reactive current of appropriate magnitude in
a timely manner. The need for fast active power recovery is
not a major issue in highly interconnected power systems but
is critical in islanded or weakly interconnected ones. Unless
they are designed with a significant overcurrent capability,
IBPSs may have a limited ability to provide high active and
reactive current injections simultaneously. A further concern
with GFL IBPSs is the potential necessity to intentionally
slow down injection rates in low system strength conditions. State-of-the-art GFL converter control system designs
assist with mitigating this concern. However, the response
inherently depends on various real-time system conditions.
The stable response of a GFM IBPS does not depend on
the available system strength. A higher or faster injection
of active or reactive current will not therefore destabilize
its performance, provided that it can be sustained by the
power system to which the GFM IBPS is connected.
Step Changes in the Voltage Phase Angle

Transient changes in the source voltage phase angle could
result in the incorrect operation of control systems used in
GFL IBPSs. Not only is the depth of voltage dip important
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but also the instantaneous change in phase. This implies the
need to study the impact of a range of disturbance types
and locations rather than focusing only on severe close-in
faults. GFM IBPSs are inherently able to prevent fast angular change and improve system security for more severe system disturbances.

Adverse System Interaction
Adverse interactions among multiple power plants have
been experienced for several decades, including subsynchronous resonance and subsynchronous torsional interactions. These interactions are generally associated with
certain operating conditions that can be studied and mitigated during project design. Control interactions involving
the control systems of several IBPSs are generally more
complex to identify and analyze than those pertaining to
SG because the frequency at which the interactions occur
could vary substantially, spanning the sub- and supersynchronous frequency ranges. No particular operating scenarios or outages necessarily initiate these interactions,
and they can occur without any disturbances. The stable
operation of GFM IBPSs does not depend on the available
system strength; with properly designed controls, GFM
IBPSs will be less susceptible to adverse interactions under
reduced system strength conditions.

Protection System Impact
A combination of the declining number of online SGs and the
limited short circuit current capability of IBPSs leads to an
overall reduction in the system fault current levels. In addition, most IBPSs were historically designed to provide
positive-sequence current injections only. Transmission-level
protection systems rely on negative- or zero-sequence quantities in addition to the positive-sequence component, including
an appropriate magnitude of these components as well as the
expected phase relationship. In scenarios with a high penetration of IBPSs, unbalanced faults have led to incorrectly
operated distance protection. As the penetration of IBPSs
increases, these issues may continue. German grid codes
already require the deliberate injection of a negative-sequence
current component, which addresses this concern to some
extent. Some commercially available GFM IBPSs provide a
fault current contribution of 200% of the rated value at the
inverter terminals. Compared to the existing GFL IBPSs, this
higher current injection capability would allow the simultaneous injection of both positive- and negative-sequence
current components, alleviating the concern discussed.
november/december 2019

IEEE Power & Energy Magazine - November/December 2019

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