IEEE Power Electronics Magazine - December 2019 - 45

sponds to 120 V root mean square (RMS). The corresponding
current of PV Inverter I is shown in Figure 6(a) as well. The
current has some transients initially, mainly caused by the
initialization of the isolation transformer, but it quickly settles down. At around t = 6 s, PV Inverter II synchronizes with
the home grid and then connects to it. After the connection
of PV Inverter II, the current of PV Inverter I decreases, and
the current of PV Inverter II increases to the same value of
PV Inverter I, due to the power sharing mechanism. Figure 6(b) shows the zoomed-in results at around t = 10 s. The
two inverters have very consistent voltages and currents and
work together to stabilize the grid. There are some harmonics in the output currents of both inverters that is caused by
the nonlinearity of the isolation transformer and the
household loads. Note that both inverters share the

functions of the real power with respect to the voltage and
the reactive power with respect to the frequency to regulate
the voltage and the frequency. The system can also blackstart without the public grid. With the voltage feedback into
the droop design, accurate load sharing of both real power
and reactive power, and power balance, can be achieved. In
the set mode, SP and SQ are OFF, and SC is at position g, the
desired power Pset and Qset can be exchanged with the public
grid with accurate power regulation.
The same technology is adopted for the public-grid side
VSM of the SYNDEM energy bridge, making it possible to
synchronize the energy bridge with the public grid while the
home-grid loads are in operation and to achieve seamless
mode change between grid-tied operation and islanded operation for the home grid. To achieve the maximum power acquisition from the renewables, maximum power point tracking
algorithms are embedded into the solar and wind inverters.

Results From Field Operations
The home grid reliably operates to support the smart home
under different scenarios. Four typical cases are selected
and presented to demonstrate the autonomous operation of
the system.

Black-Start and Grid-Forming
The black-start and grid-forming capabilities are demonstrated with the results shown in Figure 6. The isolation
transformer and some household loads, e.g., lights, laptops,
and so on are initially connected to the home grid. At t = 2 s,
PV inverter I starts to form the grid to supply electricity to
household loads, as shown in Figure 6(a). The peak-to-peak
output voltage of PV Inverter I is around 340 V, which corre-

FIG 4 The backbone of the home grid that contains five SYNDEM
inverters, one energy bridge, and battery packs.

E∗
SP
Ke

RMS

Pset
1
s

P
-

vr
ω∗
-

1
s
ωt + δ

ωd

m
K
s

vo
Calculation

Vd
E

is
i

1
Ls + R

Sc s
g

Qset
SQ

FIG 5 The self-synchronized universal droop controller [16].

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IEEE Power Electronics Magazine - December 2019

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