IEEE Power & Energy Magazine - November/December 2020 - 81

Vneg
Ineg
Inverter
Trips

Inverter
Trips

Vneg_trip
td
t

Vneg_trip

Open Phase Event
Vneg_db

(a)

Vneg

(b)

figure 8. Open-phase passive detection by (a) monitoring the reduction in negative-sequence voltage or (b) actively
injecting negative-sequence current.

and are limited to voltage and/or frequency measurements
without positive feedback.

Island-Prevention Practices
The common practice has been to augment local detection
with DTT when an upstream protective device opens. DTT
(or transfer-trip) systems are used to quickly disconnect
DERs when a feeder breaker opens, usually at the substation; see Figure 9.
Island prevention using utility-controlled DTT is reliable and effective, but it is typically expensive for individual
sites. It can be seriously impractical for high numbers of
DER sites. Communications alternatives to copper and fiber
are being considered, such as power line carrier and cellular.
Also, practices extending the time for reclosing a main-line
device are used in some areas with high DER penetration.
Current practices to address DER islanding risk vary among
utilities, as shown in Figure 10.
Pacific Gas and Electric Company (PG&E) and Dominion
Energy participated in the survey, and the authors provided
these additional details on their practices.
✔✔ In California, all investor-owned utilities follow California's Rule 21 for screening islanding concerns,
even though risk varies among utilities, plant types,
and feeder characteristics. For PG&E, supervisory
control and data acquisition (SCADA) system-controlled reclosers have been used in cases of larger
individual PV plants connected to a 12.5-kV distribution system. Although this approach is slower than
conventional DTT, it provides cost-effective telemetry
and plant disconnection for islanding. Reclosing is delayed for 10 s to coordinate with the SCADA system
and reduce the risk of an out-of-phase connection. In
addition, voltage reclose blocking might be applied on
some devices. In this case, the line-voltage magnitude
is checked to verify that it is de-energized, and the
november/december 2020

table 3. The effectiveness before and after IEEE
Standard 1547-2018 depends on the transformer.
Yg/Yg
Open-Phase
Voltage
Cases

Transformer
Type
5 leg

Passive
detection

IEEE
1547A-2014

Active
detection

Yg/Y

5 leg

Yg/T

T/Yg

Good

Poor

Good

IEEE
1547-2018

Poor

IEEE
1547A-2014

Better

Good Better

IEEE
1547-2018

Good

Good Good

auto-reclosing is blocked if the voltage is ≥15% of the
nominal voltage during the auto-reclosing attempt.
PG&E also evaluates aggregate DERs at substations and transmission line sections to ensure that the
total does not present an islanding risk. This has been
a primary concern because of high aggregate DER
penetration and back-feeding into the transmission
system (with related GFO concerns). DTT is normally
required for all synchronous machines. These are common to power plants in the San Joaquin Valley and
small hydropower units in the Sierra Nevada. For PV
plants with onboard islanding-detection systems and
certified to UL 1741SA, DTT is not required in most
cases. These criteria may be updated based on ongoing review efforts as described in the Working Group 4
Final Report (see "For Further Reading").
✔✔ In U.S. states Virginia and North Carolina, Dominion Energy has experienced the deployment of many
large PV plant connections on its 34.5-kV distribution
feeder system. Islanding prevention is a high priority,
ieee power & energy magazine

IEEE Power & Energy Magazine - November/December 2020

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