IEEE Power & Energy Magazine - November/December 2020 - 80

Detection Opportunity
Inverter-connected DERs can detect primary-side, open-phase
events using negative-sequence components. EPRI has shown
that such inverter detection methods are effective depending
on the plant transformer connection and relative generation
to load levels. Inverter detection may alert system operators for follow-up corrective measures if communication is
added, as shown in Figure 7.
So far, with proper design, there can be no conflict between
DER ride-through and open-phase detection; however, a
delayed response may be needed to allow ride-through during
unbalanced events. This delay, lasting up to 5 min, needs to be
allowed during DER commissioning tests. Both passive and
active detection methods are depicted in Figure 8. The relative effect of the interconnection transformer configuration is
shown in Table 3.

Islanding-Prevention and Site
Reclosing Practices
Should we let go of direct-transfer trip (DTT)? With deployment levels exceeding minimum loading in many locations,
utilities must consider the effects of reverse power and the
risk of unintended DER islanding when a grid connection is
lost. Unintended islanding has been a long-standing concern
in the United States, beginning with the U.S. Public Utility
Regulatory Policies Act (PURPA) in 1978. Although interconnection rules for qualifying facilities have come a long
way, the idea of unsupervised DER operation is still a key
consideration for grid safety and reliability.
Since PURPA and, for the most part, still today, utilitygrade relaying and communications have been required for
synchronous machine-based and large PV plants. Conventional relay protections passively detect island conditions

table 2. The open-phase voltage with inverter ceases to energize.
Open-Phase Voltage

Transformer Type

Yg/Yg (5 leg)

Yg/Y (5 leg)

Yg/T

T/Yg

With GT and capacitance

Primary/secondary

1.2/1.18 p.u.

1.11/1.08 pu

N/A

1.38/1.21 pu

Without GT and with
capacitance

Primary/secondary

Ferroresonance

Ferroresonance

~1 p.u.

Ferroresonance

Without GT and capacitance

Primary/secondary

0.6 p.u.

0.7 p.u.

1 p.u.

0.4 pu

N/A: not applicable.

Island

Opens on
Detection
DER
Breaker/
Fuses
Open Phase

Substation

figure 7. An open-phase incident (midfeeder) detected by the downstream PV plant. (Source: EPRI.)
80	

ieee power & energy magazine	

november/december 2020



IEEE Power & Energy Magazine - November/December 2020

Table of Contents for the Digital Edition of IEEE Power & Energy Magazine - November/December 2020

Contents
IEEE Power & Energy Magazine - November/December 2020 - Cover1
IEEE Power & Energy Magazine - November/December 2020 - Cover2
IEEE Power & Energy Magazine - November/December 2020 - Contents
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IEEE Power & Energy Magazine - November/December 2020 - Cover3
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