IEEE Power & Energy Magazine - November/December 2020 - 83

Unlike synchronous generators, solar inverters act as a positivesequence source and, depending on design, have insignificant
negative- and zero-sequence fault current.
-Laboratories (United States) have considered IEEE Stan- protection-grade DTT scheme. It is important that relays are
dard 1547-2018 grid support functions and fault ride- set to respect DER plant ride-through requirements.
In another recent survey, we asked for current practices/
through requirements.
These studies found that some detection methods work relay settings at the DER plants. As shown in Figure 11,
better than others. The key outcomes were the definitions of there were two main responses.
generic islanding-detection methods. The studies addressed
✔✔ The recloser is used for overvoltage/undervoltage and
the performance evaluation of each method, including senfrequency trips, disconnection for momentary power
sitivity to DER mix, system loading, and feeder characterisloss, monitoring plant status, and ensuring time delay
tics. Ride-through and grid support functions were found to
for re-energizing (48%).
have a limited impact on the performance of active islanding
✔✔ There is limited use of the recloser, mainly for on/off
detection methods.
by the operator, not tripping in the case of grid momenRide-through requirements have made previously develtary events or power loss, and to allow for immediate
oped Sandia guidelines for the assessment of unintended
energization with the restoration of grid voltage (30%).
islanding risk less applicable with increasing DER penetraA good example of coordinating site recloser relays with
tion levels. In the past, 67% of generation/load and 1% var DER site intertie protection functions is provided in Figure 12,
(reactive power) unbalance were reasonable criteria. With courtesy of Southern Company Service. Tie-line reclosers
IEEE 1547-2018 category III ride-through, simple screen- are required by Southern for any plant of 1 MW or more (a
ing would require a conservative 25% of generation/load very common threshold). When the DERs are connected via
and 7% var unbalance criteria.
a low-side delta transformer or a zigzag GT, the DTT might
This work indicates that two things need to happen also be integrated with the site recloser. For all cases, the
for the acceptance of onboard protection as an alternative settings allow for overvoltage/undervoltage and frequency
to DTT. The first is for inverter manufacturers to identify ride-through.
which generic detection methods they are using. The second
Finally, when DERs are placed into grid service, they
is for utilities to update island risk-assessment methods to become a functioning part of the grid. This implies that
account for new ride-through requirements. Certifica- the utility system depends on the DERs. The accuracy and
tion is necessary but not sufficient. Assessments need to consider plant location and DER type,
Pick a Description That Best Describes Your Utility's
load/generation balance, number
Practices for Controlling Reclosers Connecting DER Plants
and type of grid loss events, and
Recloser Is Used for O/U V and
detection method details.

Coordination of Plant Site
Recloser Practices
Directly related to DTT and DER
ride-through, there is the need
to coordinate relays that control plant-to-grid interconnection
reclosers. Plant site multifunction
relays installed with reclosers
have the full range of protection
setting options. Typically, these
reclosers are required for operational on/off control of larger
DER facilities. We found that, in
many cases, they are also used
as the disconnection means for a
november/december 2020

F Trip; Disconnect for Feeder Outages
and Time Delay on Re-Energization

Recloser Is Used for O/U V and F Trip;
Disconnect for Feeder Outages

Typically Limit the Use of Recloser to
On/Off Switch for Operator and/or DTT

C 11%

Only Require Recloser If DTT
Is Specified
Avoid DER Reclosers Requiring
Utility Control

48%

30%

E 11%

figure 11. The practices applied to DER site reclosers, with 76 utilities responding.
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IEEE Power & Energy Magazine - November/December 2020

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