IEEE Power & Energy Magazine - November/December 2020 - 76

Voltage (kV)

significantly different fault behavior. When supplemental grounding
is required, it is via the interconnecGrounded Wye/Grounded Wye
tion or a special grounding transformer (GT). A reactor may also be
required in the transformer neutral
No Preference
for protection coordination.
Figure 1 shows the results of
Grounded Wye/Delta
a 2013 Electric Power Research
(Primary-Side Grounded)
Institute (EPRI) survey identifying
the Yg-Yg transformer configuDelta/Grounded Wye
ration as the most common DER
(Primary-Side Delta)
connection. A 2020 EPRI survey
provided similar results. Given that
Grounded Wye/Delta (With
the Yg-Yg connection does not proa Reactively Grounded
Primary-Side Wye)
vide effective grounding, the question of supplemental requirements
Delta/Grounded Wye (With
for PV plants is raised. The current
a Reactively Grounded Wye
landscape is mixed, as some utilion the DG Side)
ties require grounding and some
No Interconnection
do not. If required, details on how
Transformer Required
to achieve effective grounding vary
for differences in distribution sys0
10
20
30
40
50
tems but also for differences in
(%)
opinions and traditional practices.
In the most recent 2020 survey,
figure 1. The preferred transformer connections for inverter-based systems. (Source:
EPRI interviewed 20 large utilities
EPRI 3002001277.)
regarding system grounding practices and PV plants. There was a
50/50 split on requiring supplemental grounding for inverterbased DERs. Three of 10 utilities that now require DER supVA
50
plemental grounding may consider grounded load as effecVB
tive for inverters in the future. This inverter consideration
VC
is proposed in IEEE/ANSI Standard C62.92-6 and IEEE
Power & Energy Society Technical Report 21. This new face
0
on an old issue is expected to continue evolving with more
inverter-interfaced PV and energy storage.
Another overvoltage concern for inverters is their response
to
a
sudden loss of load. Load-rejection overvoltage (LRO) hap-50
pens
when a grid-following inverter is disconnected, e.g.,
13.5 13.55 13.6 13.65 13.7 13.75 13.8 13.85
due
to
breaker operation. Overvoltages occur because there is a
Time (s)
temporary surplus of generation. A recent study by Dominion
Energy on utility-scale solar power plants recorded elevated
figure 2. The LRO measured at a utility-scale solar power
plant. Grid disconnection happened at 13.5 s.
voltage levels upon the sudden disconnection of PV plants.
Figure 2 shows an example captured at a typical utilityscale,
5-MW plant at 34.5 kV. The transient overvoltage at
phases in a four-wire system. Effective grounding is intended
to limit the line-to-ground overvoltage to 0.8 Vl-l. Maintain- this plant reached 159% of the nominal voltage and lasted
ing this coefficient of grounding is a classic design objective 12 cycles. Such a phenomenon is not uncommon with rapid
in three-phase power systems and defined in IEEE/ANSI load rejection. Among 1,128 load-rejection events captured,
12% exhibited root mean square voltage magnitudes above
Standard C62.92.
For distributed generation (DG), the concern for GFO is 140% of the nominal, and most events lasted between one
a key factor in selecting the grid connection transformer. In and six cycles.
Overvoltages exceeding equipment withstand capabilthe case of synchronous machine DERs, connections are normally required to provide supplemental system grounding. For ity could lead to medium-voltage (MV) arrester and instruinverter-connected DERs, the need is less clear because of ment transformer failures as well as load equipment damage.
76

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IEEE Power & Energy Magazine - November/December 2020

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