IEEE Power & Energy Magazine - July/August 2019 - 79

2) A momentary overvoltage occurred during one disturbance and caused the loss of generation from an entire
PV plant instead of just one cluster of PV arrays.
3) Many inverters appear to be set to temporarily shut
down if voltage is outside the normal operating range
of 0.90-1.10 per unit. These inverters remain offline
until the voltage returns to within the normal operating range.
4) The IBRs have very different response characteristics
than the traditional rotating machinery, and these new
response characteristics should be incorporated into
the ride-through capabilities.
5) The NERC PRC-024-2 standard currently allows inverters to instantaneously trip if the system conditions
are outside a defined set of boundaries; this is an area
that should be reevaluated because the existing PRC024-2 standard was created in the context of traditional rotating machinery.
6) To protect the inverters, a "momentary cessation" mode
is used during system disturbances. In this mode,
solar plants are "technically" still connected to the system and have not tripped (in compliance with NERC
reliability standards), but they do not inject current into
the system during and immediately after the fault to
support frequency, voltage, and short circuit duty. This
momentary cessation mode should not be used for newly interconnecting resources to the bulk power systems
(BPSs) and should be eliminated to the greatest extent
possible for existing resources on the BPS because of
the reliability risk that the operating mode poses.
In general, the ride-through and mandatory operation
requirements set forth by IEEE 1547-2018 should be sufficient to avoid tripping large amounts of DERs because of disturbances related to inverter issues; this assumes that voltage
and frequency measurements are done properly.

Planning and Operational Needs
Impacts on DERs are typically identified during interconnection studies, along with suitable mitigation measures
that ensure adequate integration considerations. Most of
these studies focus on steady-state aspects at the feeder/circuit or distribution system level; there is not a readily available vehicle or tool set for performing integrated T&D interconnection studies, where the aggregate amount of DERs
on an interface point basis is considered. As DER penetration increases, impacts may affect the operation of BPSs.
Therefore, it is important to better understand and model
the dynamic behaviors of DERs. There are strong incentives designed to integrate DERs into wholesale market
operations, but there are no definitive studies of the impact
of wholesale market participation, especially in ancillary
services such as regulation of the distribution system. This
type of study will require developing suitable models of
T&D systems and using advanced simulation tools, including time-series and probabilistic analyses. Jointly modeljuly/august 2019

ing T&D systems, and accounting for all of these aspects,
is quickly becoming an important study area for the industry. For instance, according to the recommendations issued
by NERC, PV plant owners are required to provide transmission operators with dynamic model updates. This will
allow for model benchmarks against actual events to validate accuracy and then to test the transmission system with
more severe disturbances to determine if the grid is stable.
As the industry continues to interconnect larger DERs, this
dynamic system performance analysis must become a part
of the review and interconnection process in the future.
A very important aspect of integrating IBRs to improve
system reliability and performance is reserving active power
headroom for operating reserve margin. This is a market
mechanism challenge, rather than a technical obstacle. There
are many discussions on the negative impact of IBRs on inertia, which limits the frequency changes immediately after
disturbances. However, the practical issue is not about assuring "inertia" but about attendant potential issues in balancing
load and generation to provide frequency response. Conventional rotating generators, when they have operating reserve
margins, naturally respond to imbalance between generation
and load through governor action. Smart inverters can react
immediately and much faster than conventional generation in
balancing load and generation, but they need to have reserve
margin with which to operate. This may not be as easy as relying on natural, slower response by synchronous generation,
which acts to slow down frequency response. Smart inverters
will require new functionalities to facilitate their autonomous
response. Therefore, a new NERC standard is needed, which
clarifies and expands current reliability requirements for
IBRs to support the grid during system disturbances. Those
standards are required to price the ERSs provided.
In summary, IBRs bring new features and operational
challenges to power grid planning, operation, and control.
Three technical domains are expected to have an immediate
reliability impact on BPSs: voltage/reactive power support,
frequency response and control, and power system protection coordination with a low fault current. A useful reference
document is the IEEE report to NERC, "Impact of Inverter
Based Generation on Bulk Power System Dynamics and
Short-Circuit Performance," which identifies solutions to
these potential conditions and provides guidance to users for
addressing these issues reliably.

Energy Storage
Energy storage resources (ESRs) are emerging as a nonwire alternative (NWA) that mitigates the effects of renewable DER variability, addresses transmission congestion,
improves the utilization of T&D infrastructures, and
improves resilience and reliability by providing end users
with the ability to self-supply during contingency conditions. This has led to ongoing debates regarding the future
of power delivery systems and the relevance of T&D grids.
Despite the increase of DERs and ESRs, T&D grids will
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IEEE Power & Energy Magazine - July/August 2019

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