IEEE Power Electronics Magazine - September 2019 - 46

caused by line cables will inevitably
strategy or the virtual impedancelead to the difference in their termibased decoupling methods cannot be
Another important
nal voltage. This causes circulation
applied to solve this problem.
issue with voltageproblems between parallel BICs.
Table 1 provides a summary of the
In addition, since the BIC is usucurrent-controlled and voltage-concontrolled BICs is the
ally located between the ac bus and
trolled BIC techniques. The aforemenactive and reactive
the dc bus, the circulation level will
tioned content mainly describes the
be large with a wide damage area.
issues under steady state. The islanding
coordination power
In [15], a real-time circulating curdetection of a hybrid microgrid may
coupling problem.
rent reduction method for parallel
vary under the current-controlled
harmonic-elimination pulsewidth
and voltage-controlled BICs. This is
modulation (HEPWM) BICs is probecause all of the existing communicaposed. This method does not affect
tion and the active-based and passivethe line voltages of HEPWM BICs and can easily enable
based islanding detection methods need bus information [16].
or disable the added control system to provide bumpless
However, the ac bus and dc bus are coupled through BICs.
power transfer. However, this method eliminates the cirAt the time of the disconnection of the utility grid, instantaculating current at the modulation index around unity,
neous bus bar variables may change under different BIC conwhereas at the medium or low modulation index, perfortrol types. Whether these will have influence on the islanding
mance cannot be guaranteed.
detection method still needs to be investigated.
Another important issue with voltage-controlled
BICs is the active and reactive coordination power couConclusions
pling problem. In current-controlled BICs, the reference
This article discusses control techniques of the current-conacquisition and closed-loop regulation are performed
trolled and voltage-controlled BICs in a hybrid ac/dc
directly in the dq coordinate system, the d-axis refmicrogrid. We draw the following conclusions:
erence is usually set to zero since there is no reactive
1) The current-controlled BIC is simple and easy to deploy.
power on the dc side. Hence, its coupling is small. By
It is easy to decouple its active and reactive power concontrast, in the voltage-controlled BICs, there is specific
trol and has few circulation problems. However, it highly
voltage closed-loop control, accompanied by the influrelies on ac bus voltage support and often has poor operence of line cables. This means the active and reactive
ating characteristics in a weak grid.
power will be bound to couple together. Under this cir2) The voltage-controlled BIC usually has its own voltage
cumstance, even when the given reference is zero, there
reference and can operate with or without the support of
can still be reactive power injection from the dc bus,
ac side bus bars. It also depends little on PLLs. Thereresulting in the dc voltage variations and in turn indifore, from the perspective of the system stability, it is
rectly affecting the ac bus frequency, dc bus voltage, and
more suitable than current-controlled BICs for hybrid
active power control balance. Extra transmission losses
microgrids when in islanded mode or connected to a
or even system instability may occur as well. Taking
weak grid. Nonetheless, power coupling and parallel cirthe example of the currently widely proposed voltageculation problems are hard to avoid.
controlled BIC analog with VSG, its active and reactive
3) There is no one-size-fits-all solution. The best control
power is fully coupled through the virtual flux linkage.
strategy will depend on the application requirements
and topology selections.
However, even the conventional rotation angle decoupling

Table 1. A comparison and summary.
Classification

Application Areas

Advantages

Disadvantages

Current-controlled
techniques

1) Grid-tied operation
mode
2) With energy storage
support

1) Easy implementation
2) Fewer circulation problems
3) Easy to decouple active and
reactive power

1) High dependence on ac bus bar support
and PLL units
2) Frequent charge or discharge modes,
switch of energy storage, and operation
mode switch of BICs
3) Poor operating characteristics in
weak utility grid

Voltage-controlled
techniques

1) Grid-tied or islanded
operation mode
2) Weak ac microgrid
3) High demand for ac and
dc bus support

1) Low dependence on ac bus
2) Avoids control mode switch
3) Similar droop characteristic when in
parallel operation
4) Easy cooperation with hierarchical
control within microgrids

1) Easy circulating current between BICs
2) Easy active and reactive power coupling
3) Power sharing easily affected by
line impedance
4) Few considerations for reactive
power coordination

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IEEE Power Electronics Magazine - September 2019

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