IEEE Power Electronics Magazine - September 2019 - 43

the inertia of the dc microgrid and to restrain fluctuations of
1) The charge process only starts in light-load conditions,
the dc bus voltage. In a conventional synchronous generator,
since there should be excess generation capacities from
the rotational rotor provides the inertia to deal with instant
renewable energy sources.
power fluctuations. The virtual inertia is implemented by
2) The discharge process automatically begins when
leveraging the dc side capacitor since it acts as the buffer for
renewable sources cannot or can only marginally meet
sourcing or sinking energy. Therefore, adequate emulated
the load demands.
inertia is capable of preventing abrupt changes of the dc bus
However, since the bus signals often vary due to the
voltage. Then, with the help of dc output current feedforward
intermittent distributed sources and the load fluctuations,
control, the BIC can quickly extract or inject the current from
the current-controlled BICs frequently switch operation
or to the dc microgrid, thus maintaining the voltage stability
modes. This leads to excessive transmission power loss
of the dc microgrid. However, a tradeoff exists. When stabiin BICs and cables for the unnecessary power exchange
lizing the dc side, the ac side of the BIC
between ac/dc sides (e.g., when both ac
must be stable, or the ac side perforand dc microgrids can be locally balmance must be sacrificed to stabilize
anced). Even worse, frequent charge/
the dc bus voltage.
discharge of energy storage with load
BICs and distributed
In the second method, the system
fluctuations under the current control
renewable energy
inertia can also be emulated by the
scheme may result in obvious degsources in dc and ac
dc-link capacitors in the dc terminals
radation of battery life. To solve this
of BICs without modifying the hardproblem, a multisegment current-conmicrogrids are all
ware. The difference is that the stabitrolled scheme was proposed in [7].
fast-response
lization of ac frequency will be at the
With the presented control scheme,
expense of dc bus voltage. The advanthe fluctuation range of the dc bus voltdevices, resulting in
tage of a current-controlled BIC with
age and ac bus frequency is divided
decreased power
virtual inertia is that it establishes
into three segments by two thresholds.
a specific relationship between virWhen the system power can be balsystem inertia.
tual inertia and dc-link capacitors as
anced by regulating the output of the
well as other design parameters. The
distributed sources or state-of-charge
control algorithm can also provide
of energy storage within an individual
a dynamically adaptive virtual inertia during frequency
microgrid, the BICs are in an off state. However, once either
events. Hence, the nadir of the ac bus frequency will be
the ac bus frequency or the dc bus voltage has exceeded the
smaller, and changes of bus frequency will be lower.
set threshold, the BICs will automatically start to operate to
Generally, these two methods each need sufficiently
avoid a deviation that is too large for any single microgrid.
large capacitors, which are relatively expensive. MoreAlthough the multisegment control target is clear, and
over, when operation is stable with less need for power
the frequent charge or discharge of energy storage with
exchange, these large capacitors are idle, which is a waste
load variation under the BIC control scheme is avoided, the
of a costly resource.
nonlinear control system is complicated and prone to oscillate near each segment point.

Voltage Control Techniques for BICs
Inertia Loss
BICs and distributed renewable energy sources in dc and ac
microgrids are all fast-response devices, resulting in
decreased power system inertia. Under this condition, the
dc bus voltage and ac frequency are vulnerable to power
fluctuations arising from the intermittent distributed
sources or variations of loads. During extreme events, the
dc bus voltage and ac frequency can easily go outside a stable range, leading to undesirable load shedding, cascading
failures, or even large-scale blackouts [1]. To deal with this
inertia issue, the concept of virtual inertia control for BICs
[similar to the concept of the current-controlled virtual synchronous generator (VSG)] is proposed to enhance the inertia of the hybrid system [9], [10].
Generally, such methods can be divided into two types:
one that stabilizes the dc bus voltage and one that stabilizes the ac bus frequency. For the first type, particular
virtual inertia control is often added to the BIC to enhance

The voltage-controlled method has its own calculated voltage reference, which can work as "voltage-frequency transformers." It can stabilize the ac bus voltage with or without
depending on ac bus bar support and thus enhance system
stability. The most commonly used voltage-controlled techniques are often based on dual loop control, droop control,
and virtual synchronous control [11], [12]. The hybrid
microgrid in Zhejiang has used a voltage-controlled BIC
based on droop control.
The typical voltage-controlled structure is shown in Figure 5, where the normalized ac/dc bus signals are also sent to
the active power controller, but with their own voltage reference and generated phases. While intended for the reactive
power, an independent reactive power controller is employed
to calculate the voltage magnitude reference according to the
detected and rated reactive power. Generally, it often consists of a power loop to regulate the active and reactive coordinated power though the BIC, a voltage loop to control the
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IEEE Power Electronics Magazine - September 2019

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