IEEE Power & Energy Magazine - November/December 2020 - 77

Overvoltages exceeding equipment withstand capability
could lead to medium-voltage arrester and instrument
transformer failures as well as load equipment damage.
Equipment failure incidents associated with PV plant LRO
have been reported, and choosing plant interconnection
equipment capable of handling transient overvoltages is a recommended practice. For example, heavy-duty-rated surge
arresters and potential transformers with higher basic insulation levels are more likely to tolerate LRO.
In the meantime, inverter overvoltage limits were applied in
Hawaii and have been added to IEEE Standard 1547-2018.
These were confirmed by National Renewable Energy Laboratory and Southern California Edison lab testing of inverters. EPRI is also conducting research for the New York
State Energy Research and Development Authority to better understand how LRO combines with GFO during unbalanced faults. This work included the role of island load and
MV surge arresters.
Looking ahead, inverter LRO and GFO are expected to
be largely mitigated with the application of the new IEEE
Standard 1547-2018. An inverter certified under the standard
"shall not cause fundamental frequency line-to-ground voltage to exceed 138% for a duration exceeding one cycle."

generators for all three sequence components. This illustrates why inverters need to be treated differently in any
short circuit modeling and simulation platform.
The collective efforts of the industry have greatly
im-p roved the modeling techniques of inverter-based
generation. Currently, most short circuit simulation platforms-including ASPEN, CAPE, and ETAP-provide
for inverter modeling. Solar inverters are modeled as a
voltage-controlled current source. By populating a table
listing fault-current magnitude and angles at various terminal voltages, fault characteristics are incorporated into
the analysis.
Still, precautions are required for solar inverter modeling.
In addition to inherent current limits, solar inverters respond
to faults by preprogrammed control modes. For example,
unity power factor control mode will behave differently than
reactive support control mode. Therefore, it is recommended
that utilities verify inverter settings during the commissioning process and validate model performance through ongoing field measurements.

Fault and Arc-Flash Contributions

An accurate short circuit representation is key to protection
coordination and relay settings. Unlike synchronous generators, solar inverters act as a positive-sequence source and,
depending on design, have insignificant negative- and zerosequence fault current. Figure 3 shows a single-phase-toground fault near a 20-MW solar PV power plant. The faulted
phase dropped to 35% of the nominal voltage. In response,
the inverters contributed to the location fault current.
Figure 3 shows the symmetrical component fault currents
from the solar inverters. It is noted that the fault current was
dominated by a positive-sequence current (I1). Negative and
zero-sequence components (I2 and I0) are less than 5% of the
rated current. The point-of-interconnection (POI) recloser
opened eventually and eliminated the inverters' contribution.
Throughout the event, the inverters' fault-current magnitude
did not significantly increase, and limited by sunlight, the
inverters' fault current reached only 80% of the rated current.
The observed solar inverter fault contributions are relatively low and very different from those of synchronous
november/december 2020

VA
VB
VC

30
Voltage (kV)

PV Plant Fault Contribution

35
25
20
15
10
5
0

0.05

0.1
Time (s)

0.15

0.2

(a)
350
300
Current (A)

PV plants have limited fault current but may extend faultclearing time. Supplemental grounding can add to fault current and arc flash. A closer look is needed to inform lineworker practices.

I1
I2
I0

250
200
150
100
50
0

0.05

0.1
Time (s)

0.15

0.2

(b)

figure 3. The solar inverter fault (a) phase voltage and (b)
symmetrical component current contribution.
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IEEE Power & Energy Magazine - November/December 2020

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