IEEE Electrification Magazine - March 2020 - 28

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I E E E E l e c t r i f i cati o n M agaz ine / MARCH 2020

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Voltage (kV)

to achieve the RVC based on the
standards. Field measurements
within the microgrid showed the
voltage sag within the microgrid to
be in the 0.52-0.98-p.u. range durMicrogrid Ungrounded During
ing the grounding transformer
~200 ms After PCC Opens
energization using a gang-operated
Grounding Transformer
15-kV breaker (Figure 3).
Energized
Another factor that must be
monitored during the transformer
energization is the RVC powerquality factor. According to the
standard, RVC should not exceed
Grid Voltage
Microgrid Voltage
3% per second averaged over a
period of 1 s if the DER is located at
medium voltage. In the past, severFigure 3. The voltage sag during the grounding transformer energization. PCC: point of comal approaches have been implemon coupling.
mented to reduce the transformer
simulations, a grounding transformer with lower
inrush current. Among others, these include
1) inserting a current-limiting device (resistor, reactor,
impedance tends to increase the possibility of ferroand so on)
resonance within the microgrid in an open phase
2) high- or low-voltage premagnetization
condition. Since there currently are no industry3) an individual pole breaker closing-time optimization
based standards that outline the steps for designing
based on the point of wave switching or transformer
grounding transformer within microgrids with
residual magnetism
inverter-based DERs, a new internally developed sim4) using a voltage source inverter in parallel to inject the
ulations-based approach was used to determine the
current in the opposite direction
grounding transformer characteristics. This new
5) changing the residual flux by some means.
approach will be presented to the IEEE C62.92 StanThree approaches were implemented to reduce the
dards Committee for further evaluation.
grounding transformer inrush current at the microgrid
Lesson 3: Mitigation of the Transformer
facility (Figure 4). The first approach (indicated by 1 in the
Inrush Current
figure) was based on the measurement of the residual
Any transformer connected to a power grid experiences a
magnetism of grounding transformer using a sophistilarge inrush current during the energization. In a steadycated controller along with a single pole magnetically
state condition, the transformer operating point is typicalactuated breaker. The second approach (indicated by 2 in
ly below the knee point of the saturation curve. During the
the figure) was based on using a high-side premagnetizainrush, transformer flux increases and reaches into the
tion method, in this case composed of two medium-voltsaturation region, where the magnetizing current values
age breakers and two back-to-back transformers with a
are significantly higher than the steady-state values. Sevmuch smaller rating. A premagnetization method should
eral factors affect the value of the transformer inrush
have a smaller transformer of the grounding transformcurrent: transformer size, magnetic core residual flux,
er, in the 1% range. In this microgrid, the 500-kVA
transformer hysteresis curve, voltage angle at the moment
grounding transformer would require a 5-kVA premagneof switching, and the X/R ratio of the supplying source.
tization transformer. Since 15-kV class transformers are
Why is this a specific challenge in a microgrid?
not available in a 5-kVA rating, dual 75-kVA transformers
There are two reasons. First, if the transition to island
(the smallest 3Ø rating available) were used. Upon premode is seamless, the grounding transformer needs to
magnetization of the grounding transformer through
become connected to the microgrid in the islanded mode.
75-kVA transformers, a gang-operated breaker connectSecond, if the transition to island is not seamless and the
ed the grounding transformer to the microgrid. Followmicrogrid operates using the BESS in black-start mode,
ing, both breakers in the premagnetization circuit
the total connected MVA rating of all transformers could
were tripped. The third approach (indicated by 3 in the
be too high when it is added to the load that also needs to
figure) was based on low-side premagnetization and
be energized for the black start to occur successfully. If
used an internally developed scheme with a soft-switchinverter-based DERs are used as the main source of volting device.
age and frequency, then most of the reactive power needFull testing of all three proposed solutions is still
ed for the transformer energization will come from the
ongoing. To date, some of the lessons learned include
BESS, which can cause large voltage sags and the inability
the following.
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7.04
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5.54
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4.04
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IEEE Electrification Magazine - March 2020

Table of Contents for the Digital Edition of IEEE Electrification Magazine - March 2020