IEEE Power Electronics Magazine - December 2019 - 46

harmonic currents well. At t = 15 s, a large resistive load is
added to the system, the currents of both inverters increase,
as shown in Figure 6(a). The zoomed-in results at around t =
18 s are shown in Figure 6(c). With the connection of the
resistive load, the current quality is improved. The voltage
quality is improved as well. Both inverters continue sharing
the current well after the load change.

From Islanded to Grid-Tied Operation
The connection of the energy bridge to the public grid is
demonstrated with the results shown in Figure 7. The public
grid is always ON. At t = 10 s, the energy bridge starts to synchronize with the public grid. There is some initial current in
the energy-bridge current (i.e., the inductor current) during
the synchronization stage, as shown in Figure 7(a), which is
caused by the filter capacitor. The energy bridge is connected to the public grid at about t = 10.4 s. Simultaneously,
the energy bridge also connects and delivers power to the
home grid, causing an increase of the energy-bridge current
because some power is sent to the home grid. The corresponding public-grid current increases as well. The zoomedin results at around t = 18 s are shown in Figure 7(b). The

energy-bridge voltage at the public-grid side is synchronized
with the public-grid voltage with almost the same phase
angle due to the intrinsic synchronization mechanism of the
VSM. The public-grid current is similar to the energy-bridge
current because the filter capacitor is small.

Seamless Mode Change When the Public
Grid Is Lost and Recovered
The results are shown in Figure 8. Initially, the home grid is in
continuous operation in the islanded mode with the public
grid ON. At t = 2 s, the energy bridge starts to connect the
home grid to the public grid and the home grid enters into the
grid-tied operation mode, as shown in Figure 8(a). It absorbs
power from the public grid and delivers power to the home
grid, with the energy-bridge output current at the home-grid
side determined by the load on the home grid according to the
droop function. The public-grid current increases accordingly
to keep the power balance of the common dc bus of the backto-back converters. At about t = 8 s, the public grid is lost, and
the energy bridge immediately shuts down. Both the publicgrid current and the energy-bridge current become zero. The
home grid enters into the islanded mode and is not affected by
the loss of the public grid. When the public grid is recovered at
about t = 14 s, the energy bridge is resynchronized with the
public grid and the home grid before autonomously connecting the home grid to the public grid. Figure 8(b) shows the
zoomed-in results at around t = 18 s. The home-grid voltage is
different from the public-grid voltage, demonstrating the independent operation of the home grid from the public grid. The
energy-bridge output current has some harmonics that are
caused by the split-phase isolation transformer and the

(a)

(a)

(b)

(b)
(c)
FIG 6 Black-start and grid-forming capabilities: (a) whole process
and zoomed-in results at around (b) t = 10 s and (c) t = 18 s.

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IEEE POWER ELECTRONICS MAGAZINE

z	December 2019

FIG 7 The connection of the energy bridge from islanded to
grid-tied operation: (a) whole process and (b) zoomed-in results
at around t = 18 s.
IEEE Power Electronics Magazine - December 2019

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