IEEE Power & Energy Magazine - November/December 2020 - 79

The Role of DERs in
Open-Phase Events

plant layout where voltage on the open phase reached 1.2 per
unit (p.u.) when inverters ceased to energize, leaving the ac filter connected. This is not unusual for installations requiring
GTs and where MV cables add capacitance to the open circuit.
Modeling with GTs and cable capacitance indicates that
steady-state, open-phase voltage reached 1.1-1.2 p.u. Without
the cable capacitance and grounding, voltage on the open phase
is typically lower, approximately 0.5-1 p.u. Table 2 summarizes
the expected primary and secondary voltages for four common
interconnection transformer winding and core types, assuming
no load. The table shows the results both with and without GTs,
without inverter and cable capacitance.
✔✔ Yg primary/Yg secondary case: With a local GT, steady
voltage might appear on the open phase. The level depends on circuit capacitance and GT reactance. Without
a GT and at low load, cable capacitance and no-load conditions can create a nonlinear resonance (ferroresonance).
✔✔ Yg primary/Y secondary case: Ferroresonance can be
eliminated with an MV side GT.
✔✔ Yg primary/T secondary case: Effective grounding
recreates 1-p.u. voltage on the open phase.
✔✔ T primary/Yg secondary case: Ferroresonance is
more likely and severe.

Although open-phase incidents are a concern without DERs,
the addition of three-phase PV plants has led to an increase
in reported incidents. One explanation is the increased
number of fuses and plant-scale generation sources. Plant
design, control of back-feed, and transformer connections
all play a role. Behaviors vary among underground cables,
different transformer windings, and the addition of supplemental grounding.
IEEE Standard 1547-2018 prohibits DERs from backfeeding into abnormal grid conditions, including open-phase
events; however, detecting a plant high-side, open-phase
event from the DER terminals can be challenging. Depending on the interconnection transformer windings (T, Y, or
Yg), DER terminal voltage might remain within the normal
operating range. Utilities have reported plant commissioning delays as a result of failures of open-phase field tests.
Table 1 shows field experience results from the commissioning of PV plants reported in EPRI 3002015949 in April
2019. These plants were built prior to the new IEEE 1547
standard. Typical solutions are the addition of primary-side,
negative-sequence relays or inverter firmware changes. The
referenced section of IEEE Standard 1547 requires DERs to "cease
to energize and trip" immediately
table 1. A summary of open-phase test results showing 34 failures
upon detection. A trip requires a
at 82 large PV plant sites.
5-min delay time after voltage
Reasons for
Ceased but restarted
Did not
Cease
Open-phase Other
is restored.
Failures
in <5 min
cease to
delayed voltage >1.1
reasons
generate
>2 s
per unit
Feeder open-phase incidents
are problematic for PV plants,
Number of
1
2
16
3
12
with or without DER back-feed.
Failures
A voltage on the open phase can
IEEE Standard 4.10.35
6.2.26
-
be recreated via plant transformer
1547 Section
windings and in the case of cable
(Source: Xcel Energy; used with permission.)
capacitance. Figure 6 illustrates a

V
Inverter ac
Filter Capacitor

Line
Impedance

Interconnection
Transformer

BK3

UG Cable
Impedance

BK1, Phase C
Open

BK2
Closed Breaker
V

Voltage Meter

figure 6. A plant layout, from the feeder main line on the right to the PV plant on the left, with its protection devices (BK)
and underground (UG) cable connection.
november/december 2020

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IEEE Power & Energy Magazine - November/December 2020

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