IEEE Electrification Magazine - March 2020 - 34

challenging. The biggest difference between a seamless
transition and a black start is the amount of reactive power
needed to supply the required transformer inrush current.
This does not exist during a seamless transition because
transformers are already energized, except for the smaller
grounding transformer.
During a black-start operation, the total amount of connected transformer MVA rating can easily be in 2-8 times
the maximum DER output. Since the transformer inrush
current is several times higher than the maximum rated
current, this issue becomes even more challenging. If a
synchronous generator is available as part of the DER mix,
then the black-start operation could be more successful,
depending on its size. Note that energizing the microgrid
from the grid takes about 60 ms and most of the loads
built today assume step function during energization.
However, if inverter-based DERs (such as BESS, fuel cells,
or microturbines) are the only generation sources available for black starts, then the energization becomes more
challenging. First, there are no industry standards that
govern the black start with inverter-based DERs. Further,

81D6D = 10 Cycles

81D6P = 62 Hz

81D4P = 60.5 Hz

all loads expect the voltage change from 0 to the nominal
level as quickly as possible (60 ms at the microgrid site
based on the field measurements) and inverter-based
DERs are not capable of providing that step voltage function due to a lack of inertia. Figure 12 portrays what happens during a successful microgrid black start with an
inverter-based DER, in this case a BESS.
The recording shown in Figure 12 is based on the primary voltage (12.47 kV) as obtained from the relay on the
high side of the BESS transformer with synchophasor data.
First, when the BESS is black started, the voltage is zero,
because the ac breaker inside BESS is open. When this
breaker closes, the voltage immediately goes from 0 to 1.12 p.u.
This voltage is a step function, but in this case the BESS
does not pick up any load; it essentially only energizes the
primary voltage section of the BESS up to the medium voltage breaker. Even though the 1.12-p.u. voltage value would
cause the microgrid to trip, this does not happen for two
reasons. First, protection settings within the microgrid
relays must be torque controlled for the black start. Second, this voltage is not the actual phase-to-ground voltage.

81D5D = 4,800 Cycles

81D5P = 61 Hz
81D4D = 5 Cycles

81D4D = 5 Cycles

NFREQ = 60 Hz

81D1P = 59.3 Hz

81D1D = 5 Cycles

81D1D = 5 Cycles
81D2P = 58.5 Hz

81D3P = 57 Hz

81D2D = 4,800 Cycles

81D3D = 10 Cycles

Closed
PCC Status

Open
Energized

t = 28-32 ms

Grounding
Transformer

t=0s

t=2s

Grid Synchronization
Command Issued
Figure 11. The frequency settings during the transition to grid.

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

De-Energized

t = 4-12 s

Grid Synchronization
Completed and PCC Is Closed

Grounding
Transformer
De-Energized

Grid Connected
Steady State

IEEE Electrification Magazine - March 2020

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